Gregory E Morley, Ph.D.

Unique Properties of the ATP-Sensitive K+ Channel in the Mouse Ventricular Cardiac Conduction System
Bao, Li; Kefaloyianni, Eirini; Lader, Joshua; Hong, Miyoun; Morley, Gregory; Fishman, Glenn I; Sobie, Eric A; Coetzee, William A
2011 Dec 1;4(6):926-935, Circulation. Arrhythmia & electrophysiology
Background- The specialized cardiac conduction system (CCS) expresses a unique complement of ion channels that confer a specific electrophysiological profile. ATP-sensitive potassium (K(ATP)) channels in these myocytes have not been systemically investigated. Methods and Results- We recorded K(ATP) channels in isolated CCS myocytes using Cntn2-EGFP reporter mice. The CCS K(ATP) channels were less sensitive to inhibitory cytosolic ATP compared with ventricular channels and more strongly activated by MgADP. They also had a smaller slope conductance. The 2 types of channels had similar intraburst open and closed times, but the CCS K(ATP) channel had a prolonged interburst closed time. CCS K(ATP) channels were strongly activated by diazoxide and less by levcromakalim, whereas the ventricular K(ATP) channel had a reverse pharmacological profile. CCS myocytes express elevated levels of Kir6.1 but reduced Kir6.2 and SUR2A mRNA compared with ventricular myocytes (SUR1 expression was negligible). SUR2B mRNA expression was higher in CCS myocytes relative to SUR2A. Canine Purkinje fibers expressed higher levels of Kir6.1 and SUR2B protein relative to the ventricle. Numeric simulation predicts a high sensitivity of the Purkinje action potential to changes in ATP:ADP ratio. Cardiac conduction time was prolonged by low-flow ischemia in isolated, perfused mouse hearts, which was prevented by glibenclamide. Conclusions- These data imply a differential electrophysiological response (and possible contribution to arrhythmias) of the ventricular CCS to K(ATP) channel opening during periods of ischemia
— id: 148727, year: 2011, vol: 4, page: 926, stat: Journal Article,

Spinal cord stimulation prevents tachypacing-induced atrial fibrillation
Bernstein S.A.; Wong B.; Holmes D.S.; Kuznekoff L.M.; Rooke R.; Alvstrand M.; Vasquez C.; Bharmi R.; Shah R.; Rosenberg S.P.; Farazi T.G.; Chinitz L.; Morley G.E.
2011 ;8(5 SUPPL 1):S330-S330, Heart rhythm
Introduction: Spinal cord stimulation (SCS) has been shown to modulate atrial electrophysiology and confer protection against ischemia and ventricular arrhythmias. We hypothesized that SCS may reduce susceptibility to tachypacing (TP) induced atrial fibrillation (AF). Methods: Spinal cord leads (Octrode, St. Jude Medical) were implanted in the upper thoracic spine (T1-T5) of canines and connected to pulse generators (EonC, St. Jude Medical). The AV node was ablated and atrial effective refractory period (AERP) was measured at baseline and with SCS (n=10). In separate animals the AV node was ablated and endocardial RA and RV pacing leads were connected to dual chamber pacemakers for ambulatory AF induction. Custom firmware provided continuous 30s periods of atrial TP followed by 6s sense windows. TP was interrupted by detection of AF (atrial rate >250 bpm) and resumed upon return to sinus rhythm. AF Index was defined as the fraction of time the animal did not receive TP relative to the total allowable TP time. The effect of SCS delivered intermittently for 6 hr/day (SCS ON; n=3) on AF index was followed for 8 weeks and compared to control (SCS OFF; n=3). Results: Right (p=0.002) and left (p=0.009) AERP were significantly longer during SCS (168+/-15.1, 168+/-14.8 ms) compared to baseline (130+/-8.7, 152+/-10.3 ms). AF Index was significantly decreased in the SCS ON compared to SCS OFF (p<0.0001). AF Index was >70% in the SCS OFF group and <5% in the SCS ON animals starting at week 3 (Figure). Conclusions: These data demonstrate that SCS prolongs AERP and prevents TP-induced AE (Graph presented)
— id: 131860, year: 2011, vol: 8, page: S330, stat: Journal Article,

Spatiotemporal electrophysiological changes in a murine ablation model
Bernstein SA; Duggirala S; Floberg M; Elfvendal P; Kuznekoff LM; Lader JM; Vasquez C; Morley GE
2011 Oct;13(10):1494-1500, Europace
Aims High recurrence rates after complex radiofrequency ablation procedures, such as for atrial fibrillation, remain a major clinical problem. Local electrophysiological changes that occur following cardiac ablation therapy are incompletely described in the literature. The purpose of this study was to determine whether alterations in conduction velocity, action potential duration (APD), and effective refractory period resolve dynamically following cardiac ablation. Methods and results Lesions were delivered to the right ventricle of mice using a subxiphoid approach. The sham-operated control group (SHAM) received the same procedure without energy delivery. Hearts were isolated at 0, 1, 7, 30, and 60 days following the procedure and electrophysiological parameters were obtained using high-resolution optical mapping with a voltage-sensitive dye. Conduction velocity was significantly decreased at the lesion border in the 0, 7, and 30 day groups compared to SHAM. APD(70) at the lesion border was significantly increased at all time points compared to SHAM. Effective refractory period was significantly increased at the lesion border at 0, 1, 7, and 30 days but not at 60 days post-ablation. This study demonstrated that post-ablation electrophysiological changes take place immediately following energy delivery and resolve within 60 days. Conclusions Cardiac ablation causes significant electrophysiological changes both within the lesion and beyond the border zone. Late recovery of electrical conduction in individual lesions is consistent with clinical data demonstrating that arrhythmia recurrence is associated with failure to maintain bi-directional conduction block
— id: 135529, year: 2011, vol: 13, page: 1494, stat: Journal Article,

Remodeling of Atrial ATP-Sensitive Potassium Channels in a Model of Salt-induced Elevated Blood Pressure
Lader JM; Vasquez C; Bao L; Maass K; Qu J; Kefalogianni E; Fishman G; Coetzee WA; Morley GE
2011 Sep;301(3):H964-H974, American journal of physiology. Heart & circulatory physiology
Background: Hypertension is associated with the development of atrial fibrillation, however the electrophysiological consequences of this condition remain poorly understood. K(ATP) channels, which contribute to ventricular arrhythmias, are also expressed in the atria. We hypothesized that salt-induced elevated blood pressure leads to atrial K(ATP) channel activation and increased arrhythmia inducibility. Methods and Results: Elevated blood pressure was induced in mice with a high salt diet (HS) for four weeks. High resolution optical mapping was used to measure atrial arrhythmia inducibility, effective refractory period (ERP) and action potential duration (APD(90)). Excised patch clamping was performed to quantify K(ATP) channel properties and density. K(ATP) channel protein expression was also evaluated. Atrial arrhythmia inducibility was 22% higher in HS compared to control hearts. ERP and APD(90) were significantly shorter in the RAA and LAA of HS compared to control hearts. Perfusion with 1 muM glibenclamide or 300 muM tolbutamide significantly decreased arrhythmia inducibility and prolonged APD(90) in HS hearts compared to untreated HS hearts. K(ATP) channel density was 156% higher in myocytes isolated from HS compared to control animals. SUR1 protein expression was increased in the HS LAA (415% of NS) and RAA (372% of NS). Conclusion: K(ATP) channel activation provides a mechanistic link between salt-induced elevated BP and increased atrial arrhythmia inducibility. The findings of this study have important implications for the treatment and prevention of atrial arrhythmias in the setting of hypertensive heart disease and may lead to new therapeutic approaches
— id: 135528, year: 2011, vol: 301, page: H964, stat: Journal Article,

Mice With Cardiac Overexpression of Peroxisome Proliferator-Activated Receptor gamma Have Impaired Repolarization and Spontaneous Fatal Ventricular Arrhythmias
Morrow, John P.; Katchman, Alexander; Son, Ni-Huiping; Trent, Chad M.; Khan, Raffay; Shiomi, Takayuki; Huang, Haiyan; Amin, Vaibhav; Lader, Joshua M.; Vasquez, Carolina; Morley, Gregory E.; D'Armiento, Jeanine; Homma, Shunichi; Goldberg, Ira J.; Marx, Steven O.
2011 DEC 20 ;124(25):2812-U123, Circulation
Background-Diabetes mellitus and obesity, which confer an increased risk of sudden cardiac death, are associated with cardiomyocyte lipid accumulation and altered cardiac electric properties, manifested by prolongation of the QRS duration and QT interval. It is difficult to distinguish the contribution of cardiomyocyte lipid accumulation from the contribution of global metabolic defects to the increased incidence of sudden death and electric abnormalities. Methods and Results-In order to study the effects of metabolic abnormalities on arrhythmias without the complex systemic effects of diabetes mellitus and obesity, we studied transgenic mice with cardiac-specific overexpression of peroxisome proliferator-activated receptor gamma 1 (PPAR gamma 1) via the cardiac alpha-myosin heavy-chain promoter. The PPAR gamma transgenic mice develop abnormal accumulation of intracellular lipids and die as young adults before any significant reduction in systolic function. Using implantable ECG telemeters, we found that these mice have prolongation of the QRS and QT intervals and spontaneous ventricular arrhythmias, including polymorphic ventricular tachycardia and ventricular fibrillation. Isolated cardiomyocytes demonstrated prolonged action potential duration caused by reduced expression and function of the potassium channels responsible for repolarization. Short-term exposure to pioglitazone, a PPAR gamma agonist, had no effect on mortality or rhythm in WT mice but further exacerbated the arrhythmic phenotype and increased the mortality in the PPAR gamma transgenic mice. Conclusions-Our findings support an important link between PPAR gamma activation, cardiomyocyte lipid accumulation, ion channel remodeling, and increased cardiac mortality. (Circulation. 2011;124:2812-2821.)
— id: 149881, year: 2011, vol: 124, page: 2812, stat: Journal Article,

Minimum Information about a Cardiac Electrophysiology Experiment (MICEE): Standardised reporting for model reproducibility, interoperability, and data sharing
Quinn, T. A.; Granite, S.; Allessie, M. A.; Antzelevitch, C.; Bollensdorff, C.; Bub, G.; Burton, R. A. B.; Cerbai, E.; Chen, P. S.; Delmar, M.; DiFrancesco, D.; Earm, Y. E.; Efimov, I. R.; Egger, M.; Entcheva, E.; Fink, M.; Fischmeister, R.; Franz, M. R.; Garny, A.; Giles, W. R.; Hannes, T.; Harding, S. E.; Hunter, P. J.; Iribe, G.; Jalife, J.; Johnson, C. R.; Kass, R. S.; Kodama, I.; Koren, G.; Lord, P.; Markhasin, V. S.; Matsuoka, S.; McCulloch, A. D.; Mirams, G. R.; Morley, G. E.; Nattel, S.; Noble, D.; Olesen, S. P.; Panfilov, A. V.; Trayanova, N. A.; Ravens, U.; Richard, S.; Rosenbaum, D. S.; Rudy, Y.; Sachs, F.; Sachse, F. B.; Saint, D. A.; Schotten, U.; Solovyova, O.; Taggart, P.; Tung, L.; Varro, A.; Volders, P. G.; Wang, K.; Weiss, J. N.; Wettwer, E.; White, E.; Wilders, R.; Winslow, R. L.; Kohl, P.
2011 OCT ;107(1):4-10, Progress in biophysics & molecular biology
Cardiac experimental electrophysiology is in need of a well-defined Minimum Information Standard for recording, annotating, and reporting experimental data. As a step towards establishing this, we present a draft standard, called Minimum Information about a Cardiac Electrophysiology Experiment (MICEE). The ultimate goal is to develop a useful tool for cardiac electrophysiologists which facilitates and improves dissemination of the minimum information necessary for reproduction of cardiac electrophysiology research, allowing for easier comparison and utilisation of findings by others. It is hoped that this will enhance the integration of individual results into experimental, computational, and conceptual models. In its present form, this draft is intended for assessment and development by the research community. We invite the reader to join this effort, and, if deemed productive, implement the Minimum Information about a Cardiac Electrophysiology Experiment standard in their own work. (C) 2011 Elsevier Ltd. All rights reserved
— id: 141785, year: 2011, vol: 107, page: 4, stat: Journal Article,

Phosphatase-resistant gap junctions inhibit pathological remodeling and prevent arrhythmias
Remo, Benjamin F; Qu, Jiaxiang; Volpicelli, Frank M; Giovannone, Steven; Shin, Daniel; Lader, Joshua; Liu, Fang-Yu; Zhang, Jie; Lent, Danielle S; Morley, Gregory E; Fishman, Glenn I
2011 Jun 10;108(12):1459-1466, Circulation research
Rationale: Posttranslational phosphorylation of connexin43 (Cx43) has been proposed as a key regulatory event in normal cardiac gap junction expression and pathological gap junction remodeling. Nonetheless, the role of Cx43 phosphorylation in the context of the intact organism is poorly understood. Objective: To establish whether specific Cx43 phosphorylation events influence gap junction expression and pathological remodeling. Methods and Results: We generated Cx43 germline knock-in mice in which serines 325/328/330 were replaced with phosphomimetic glutamic acids (S3E) or nonphosphorylatable alanines (S3A). The S3E mice were resistant to acute and chronic pathological gap junction remodeling and displayed diminished susceptibility to the induction of ventricular arrhythmias. Conversely, the S3A mice showed deleterious effects on cardiac gap junction formation and function, developed electric remodeling, and were highly susceptible to inducible arrhythmias. Conclusions: These data demonstrate a mechanistic link between posttranslational phosphorylation of Cx43 and gap junction formation, remodeling, and arrhythmic susceptibility
— id: 134445, year: 2011, vol: 108, page: 1459, stat: Journal Article,

The cardiac fibroblast: functional and electrophysiological considerations in healthy and diseased hearts
Vasquez, Carolina; Benamer, Najate; Morley, Gregory E
2011 Apr;57(4):380-388, Journal of cardiovascular pharmacology
Cardiac fibrosis occurs in a number of cardiovascular diseases associated with a high incidence of arrhythmias. A critical event in the development of fibrosis is the transformation of fibroblasts into an active phenotype or myofibroblast. This transformation results in functional changes including increased proliferation and changes in the release of signaling molecules and extracellular matrix deposition. Traditionally, fibroblasts have been considered to affect cardiac electrophysiology indirectly by physically isolating myocytes and creating conduction barriers. There is now increasing evidence that cardiac fibroblasts may play a direct role in modulating the electrophysiological substrate in diseased hearts. The purpose of this review is to summarize the functional changes associated with fibroblast activation, the membrane currents that have been identified in adult cardiac fibroblasts, and describe recent studies of fibroblast-myocyte electrical interactions with emphasis on the changes that occur with cardiac injury. Further analysis of fibroblast membrane electrophysiology and their interactions with myocytes will lead to a more complete understanding of the arrhythmic substrate. These studies have the potential to generate new therapeutic approaches for the prevention of arrhythmias associated with cardiac fibrosis
— id: 130903, year: 2011, vol: 57, page: 380, stat: Journal Article,

Enhanced fibroblast-myocyte interactions in response to cardiac injury
Vasquez, Carolina; Mohandas, Poornima; Louie, Karen L; Benamer, Najate; Bapat, Ashwini C; Morley, Gregory E
2010 Oct 15;107(8):1011-1020, Circulation research
RATIONALE: A critical event in the development of cardiac fibrosis is the transformation of fibroblasts into myofibroblasts. The electrophysiological consequences of this phenotypic switch remain largely unknown. OBJECTIVE: Determine whether fibroblast activation following cardiac injury results in a distinct electrophysiological phenotype that enhances fibroblast-myocyte interactions. METHODS AND RESULTS: Neonatal rat myocyte monolayers were treated with media (CM) conditioned by fibroblasts isolated from normal (Fb) and infarcted (MI-Fb) hearts. Fb and MI-Fb were also plated on top of myocyte monolayers at 3 densities. Cultures were optically mapped after CM treatment or fibroblast plating to obtain conduction velocity and action potential duration (APD(70)). Intercellular communication and connexin43 expression levels were assessed. Membrane properties of Fb and MI-Fb were evaluated using patch clamp techniques. MI-Fb CM treatment decreased conduction velocity (11.1%) compared to untreated myocyte cultures. APD(70) was reduced by MI-Fb CM treatment compared to homocellular myocyte culture (9.4%) and Fb CM treatment (6.4%). In heterocellular cultures, MI-Fb conduction velocities were different from Fb at all densities (+29.8%, -23.0%, and -16.7% at 200, 400, and 600 cells/mm(2), respectively). APD(70) was reduced (9.6%) in MI-Fb compared to Fb cultures at 200 cells/mm(2). MI-Fb had more hyperpolarized resting membrane potentials and increased outward current densities. Connexin43 was elevated (134%) in MI-Fb compared to Fb. Intercellular coupling evaluated with gap fluorescence recovery after photobleaching was higher between myocytes and MI-Fb compared to Fb. CONCLUSIONS: These data demonstrate cardiac injury results in significant electrophysiological changes that enhance fibroblast-myocyte interactions and could contribute to the greater incidence of arrhythmias observed in fibrotic hearts
— id: 113942, year: 2010, vol: 107, page: 1011, stat: Journal Article,

Hyperlipidemia has chamber specific effects on electrophysiological parameters
Bapat A.; Morley G.
2009 ;6(5 SUPPL 1):S358-S359, Heart rhythm
Introduction: Hyperlipidemia has long been associated with arrhythmias and sudden death, generally due to ischemic infarction. However, recent evidence suggests cholesterol levels may also directly modify membrane ion channel function. We hypothesized that feeding with a Western-type diet (WD) will directly alter epicardial conduction velocity (CV) and action potential durations (APD). Methods: Control C57BL/6 and ApoE knockout hearts were extracted, Langerdorff perfused and optically mapped. APD50, APD70 and CV were determined for the right atrium (RA), left atrium (LA), right ventricle (RV) and left ventricle (LV) while pacing at 10 Hz. The ApoE knockout mice were either fed a normal diet (ND) or WD for 1, 2, or 4 weeks. Control mice received ND. Results: There were no significant differences in APD50, APD70, or CV between control and ND ApoE knockout mice for any of the chambers (p>0.05). APD70 (ms) was significantly (p<0.05) increased at 1 week (13.8+/- 0.8 RA; 14+/-1 LA), 2 weeks (14.1+/-0.96 RA; 20.1+/-2.7 LA), and 4 weeks (15.2+/-1.8 RA; 15.2+/-0.9 LA) of WD in both atria compared to controls (10.4+/-0.38 RA; 9.5+/-0.37 LA). APD50 values showed similar changes. RV CV (mm/s) was significantly slower in the ApoE mice fed WD at 1 week (0.51+/-0.02 RV), 2 weeks (0.45+/-0.05 RV), and 4 weeks (0.47+/-0.06 RV) compared to controls (0.62+/-0.03). LV CV was significantly slower in ApoE mice on WD for 2 weeks (0.355+/-0.02) compared to control (0.53+/-0.02). Conclusions: WD resulted in chamber specific changes in CV and APD. These results suggest that elevated cholesterol levels may be arrhythmogenic in the absence of myocardial ischemia (Figure presented)
— id: 131856, year: 2009, vol: 6, page: S358, stat: Journal Article,

Altered KATP channel function: A mechanism for atrial fibrillation in hypertension
Lader J.M.; Vasquez C.; Morley G.E.
2009 ;6(5 SUPPL 1):S170-S171, Heart rhythm
Introduction: Hypertension (HTN) is associated with the depletion of myocardial energy stores and the development of atrial fibrillation (AF). The electrophysiological alterations in the setting of HTN are poorly understood. We hypothesized that chronic HTN leads to the activation of atrial KATP channels and the creation of an arrhythmogenic substrate. Methods: HTN was induced in mice by administration of a high salt diet (HS) for 2-4 weeks. Two control groups were studied: one was given a normal salt diet (NS) and the second was given the high salt diet with the diuretic hydrochlorothiazide (HCTZ). The development of HTN and changes in cardiac function were assessed with tail-cuff plethysmography and echocardiography. Optical mapping was used to evaluate electrophysiological parameters and AF inducibility. Results: Systolic blood pressure was significantly increased in HS (121.2+/-1.3mmHg) compared to NS (103.4+/-1.0mmHg) animals. Average fractional shortening was not significantly different. ERP was unchanged in the RAA and significantly decreased in the LAA of HS (11.8+/-1.6ms) compared to NS animals (26.5+/-2.5ms). Average LAA (10.8+/-0.7ms) and RAA (15.5+/-1.6ms) APD90 was significantly shorter in HS compared to NS animals (14.4+/-1.0 and 19.9+/-0.9ms, respectively). Importantly, AF inducibility was higher in the HS (N=6/16) compared to the NS group (N=0/12). Perfusion with the KATP antagonist glybenclamide (GLY; 10muM) restored LAA ERP (31.0+/-2.7ms) and APD90 (20.7+/-1.7ms) in the HS group. AF inducibility following GLY perfusion decreased in the HS (N=1/11) and was unchanged in the NS (N=0/12) group. No differences were observed between NS and HCTZ groups, suggesting that the elevated salt intake did not play a significant role in the observed differences. In addition, no differences in CV or interstitial fibrosis were observed between HS and NS animals, suggesting changes in refractoriness are responsible for the increased vulnerability to AF. Conclusions: These data indicate HTN leads to regionally specific activation of atrial KATP channels and an increase in the vulnerability to induction of AF, making IK(ATP) an attractive therapeutic target
— id: 131853, year: 2009, vol: 6, page: S170, stat: Journal Article,

Arrhythmogenic potential of activated fibroblasts
Vasquez C.; Feig J.E.; Mohandas P.; Fisher E.A.; Morley G.E.
2009 ;6(5 SUPPL 1):S458-S458, Heart rhythm
Introduction: A critical event in the development of cardiac fibrosis is the transformation of fibroblasts into myofibroblasts. Fibroblasts isolated from healthy hearts and grown under standard tissue culture conditions express alpha-SMA and have been referred to as myofibroblasts. However, recent data suggest the in vitro transformation does not fully replicate the in vivo activation process. The purpose of this study was to investigate the potential of activated fibroblasts to contribute to an arrhythmogenic substrate through paracrine and direct coupling effects. Methods: Confluent neonatal rat myocyte monolayers were treated with media (CM) conditioned by cardiac fibroblasts isolated from ventricles of healthy (Fb) and infarcted (MI-Fb) hearts and optically mapped 16-20 hours later. To study the combined paracrine and direct coupling effect, Fb and MI-Fb were plated on top of myocyte monolayers. Results: Treatment with both Fb CM (16.6+/-0.4 cm/s) and MIFb CM (15.8+/-0.4 cm/s) significantly decreased conduction velocity (CV) compared to homocellular myocyte monolayers (Myo; 19.7+/-0.7 cm/s). Action potential duration (APD70) was significantly reduced by MI-Fb CM (143.6+/-1.7 ms) treatment compared to Myo (159.4+/-4.0 ms) and Fb CM (153.4+/-2.7 ms). In heterocellular cultures, Fb significantly decreased (17.0+/-0.5 cm/s) and MI-Fb increased (22.0+/-0.6 cm/s) average CV compared to Myo. In addition, CV was significantly faster with MI-Fb compared to Fb (p=1.95E-8). Fb (145.0+/-3.9 ms) and MIFb (131.1+/-3.7 ms) significantly reduced APD70 compared to Myo (159.4+/-4.0 ms), and APD70 was significantly shorter with MI-Fb compared to Fb (p=0.01). Analysis of Cx43 levels showed a significant upregulation of Cx43 in MI-Fb compared to Fb. Conclusions: These data demonstrate Fb exert predominantly paracrine effects while MI-Fb affect myocyte electrophysiology through a combination of paracrine and direct coupling mechanisms. Moreover, APD shortening and increased Cx43 levels in MI-Fb could contribute to the greater incidence of arrhythmias observed in fibrotic hearts. These findings may lead to the development of new anti-arrhythmic therapeutic approaches targeting the fibroblast activation process
— id: 131858, year: 2009, vol: 6, page: S458, stat: Journal Article,

Electrical remodeling contributes to complex tachyarrhythmias in connexin43-deficient mouse hearts
Danik, Stephan B; Rosner, Gregg; Lader, Joshua; Gutstein, David E; Fishman, Glenn I; Morley, Gregory E
2008 Apr;22(4):1204-1212, FASEB journal
Loss of connexin43 (Cx43) gap junction channels in the heart results in a marked increase in the incidence of spontaneous and inducible polymorphic ventricular tachyarrhythmias (PVTs). The mechanisms resulting in this phenotype remain unclear. We hypothesized that uncoupling promotes regional ion channel remodeling, thereby increasing electrical heterogeneity and facilitating the development of PVT. In isolated-perfused control hearts, programmed electrical stimulation elicited infrequent monomorphic ventricular tachyarrhythmias (MVT), and dominant frequencies (DFs) during MVT were similar in the right ventricle (RV) and left ventricle (LV). Moreover, conduction properties, action potential durations (APDs), and repolarizing current densities were similar in RV and LV myocytes. In contrast, PVT was common in Cx43 conditional knockout (OCKO) hearts, and arrhythmias were characterized by significantly higher DFs in the RV compared to the LV. APDs in OCKO myocytes were significantly shorter than those from chamber-matched controls, with RV OCKO myocytes being most affected. APD shortening was associated with higher levels of sustained current in myocytes from both chambers as well as higher levels of the inward rectifier current only in RV myocytes. Thus, alterations in cell-cell coupling lead to regional changes in potassium current expression, which in this case facilitates the development of reentrant arrhythmias. We propose a new mechanistic link between electrical uncoupling and ion channel remodeling. These findings may be relevant not only in cardiac tissue but also to other organ systems where gap junction remodeling is known to occur.-Danik, S. B., Rosner, G., Lader, J., Gutstein, D. E., Fishman, G. I., Morley, G. E. Electrical remodeling contributes to complex tachyarrhythmias in connexin43-deficient mouse hearts
— id: 75197, year: 2008, vol: 22, page: 1204, stat: Journal Article,

Dyssynchronous activation in heterozygous Cx43 germline knockout mice induces steady-state potassium current remodelling and sustained dyssynchrony
Kontogeorgis, A; Kaba, RA; Li, X; Wit, AL; Morley, GE; Peters, NS; Gutstein, DE
2008 JUL ;94(3):A1-A1, Heart (British Cardiac Society)
— id: 86971, year: 2008, vol: 94, page: A1, stat: Journal Article,

Decreased connexin43 expression in the mouse heart potentiates pacing-induced remodeling of repolarizing currents
Kontogeorgis, Andrianos; Li, Xiaodong; Kang, Eunice Y; Feig, Jonathan E; Ponzio, Marc; Kang, Guoxin; Kaba, Riyaz A; Wit, Andrew L; Fisher, Edward A; Morley, Gregory E; Peters, Nicholas S; Coetzee, William A; Gutstein, David E
2008 Nov;295(5):H1905-H1916, American journal of physiology. Heart & circulatory physiology
Gap junction redistribution and reduced expression, a phenomenon termed gap junction remodeling (GJR), is often seen in diseased hearts and may predispose towards arrhythmias. We have recently shown that short-term pacing in the mouse is associated with changes in connexin43 (Cx43) expression and localization, but not with increased inducibility into sustained arrhythmias. We hypothesized that short-term pacing, if imposed on murine hearts with decreased Cx43 abundance, could serve as a model for evaluating the electrophysiologic effects of GJR. We paced wildtype (normal Cx43 abundance) and heterozygous Cx43 knockout mice (Cx43(+/-), 66% mean reduction in Cx43) for six hours at 10-15% above their average sinus rate. We investigated the electrophysiologic effects of pacing on the whole animal using programmed electrical stimulation, and in isolated ventricular myocytes with patch clamp studies. Cx43(+/-) myocytes had significantly shorter action potential durations (APD) and increased steady state and inward rectifier potassium currents (Iss and IK1, respectively) compared to wildtype littermate cells. In Cx43(+/-) hearts, pacing resulted in significant prolongation of ventricular effective refractory period and action potential duration, and significant diminution of Iss compared to unpaced Cx43(+/-) hearts. However, these changes were not seen in paced wildtype mice. These data suggest that Cx43 abundance plays a critical role in regulating currents involved in myocardial repolarization and their response to pacing. Our study may aid in understanding how dyssynchronous activation of diseased, Cx43-deficient myocardial tissue can lead to electrophysiologic changes which may contribute to the worsened prognosis often associated with pacing in the failing heart. Key words: Connexin43, ventricular myocytes, mouse, gap junction
— id: 116200, year: 2008, vol: 295, page: H1905, stat: Journal Article,

Connexin40 imparts conduction heterogeneity to atrial tissue
Leaf, David E; Feig, Jonathan E; Vasquez, Carolina; Riva, Pamela L; Yu, Cindy; Lader, Joshua M; Kontogeorgis, Andrianos; Baron, Elvera L; Peters, Nicholas S; Fisher, Edward A; Gutstein, David E; Morley, Gregory E
2008 Oct 24;103(9):1001-1008, Circulation research
Impulse propagation in cardiac tissue is a complex process in which intercellular coupling through gap junction channels is a critical component. Connexin40 (Cx40) is an abundant gap junction protein that is expressed in atrial myocytes. Alterations in the expression of Cx40 have been implicated in atrial arrhythmogenesis. The purpose of the current study was to assess the role of Cx40 in atrial impulse propagation. High-resolution optical mapping was used to study conduction in the right and left atrial appendages of isolated Langendorff-perfused murine hearts. Wild-type (Cx40(+/+)), heterozygous (Cx40(+/-)), and knockout (Cx40(-/-)) mice, both adult and embryonic, were studied to assess the effects of reduced Cx40 expression on sinus node function and conduction velocity at different pacing cycle lengths (100 and 60 ms). In both adult and late-stage embryonic Cx40(+/+) mice, heterogeneity in CV was found between the right and left atrial appendages. Either partial (Cx40(+/-)) or complete (Cx40(-/-)) deletion of Cx40 was associated with the loss of conduction heterogeneity in both adult and embryonic mice. Additionally, sinus node impulse initiation was found to be ectopic in Cx40(-/-) mice at 15.5 days postcoitus, whereas Cx40(+/+) mice showed normal activation occurring near the crista terminalis. Our findings suggest that Cx40 plays an essential role in establishing interatrial conduction velocity heterogeneity in the murine model. Additionally, we describe for the first time a developmental requirement for Cx40 in normal sinus node impulse initiation at 15.5 days postcoitus
— id: 93330, year: 2008, vol: 103, page: 1001, stat: Journal Article,

Leaky Ca2+ release channel/ryanodine receptor 2 causes seizures and sudden cardiac death in mice
Lehnart, SE; Mongillo, M; Bellinger, A; Lindegger, N; Chen, BX; Hsueh, W; Reiken, S; Wronska, A; Drew, LJ; Ward, CW; Lederer, WJ; Kass, RS; Morley, G; Marks, AR
2008 JUN ;118(6):2230-2245, Journal of clinical investigation
The Ca2+ release channel ryanodine receptor 2 (RyR2) is required for excitation-contraction coupling in the heart and is also present in the brain. Mutations in RyR2 have been linked to exercise-induced sudden cardiac death (catecholaminergic polymorphic ventricular tachycardia [CPVT]). CPVT-associated RyR2 mutations result in 'leaky' RyR2 channels due to the decreased binding of the calstabin2 (FKBP12.6) subunit, which stabilizes the closed state of the channel. We found that mice heterozygous for the R2474S mutation in Ryr2 (Ryr2-R2474S mice) exhibited spontaneous generalized tonic-clonic seizures (which occurred in the absence of cardiac arrhythmias), exercise-induced ventricular arrhythmias, and sudden cardiac death. Treatment with a novel RyR2-specific compound (S107) that enhances the binding of calstabin2 to the mutant Ryr2-R2474S channel inhibited the channel leak and prevented cardiac arrhythmias and raised the seizure threshold. Thus, CPVT-associated mutant leaky Ryr2-R2474S channels in the brain can cause seizures in mice, independent of cardiac arrhythmias. Based on these data, we propose that CPVT is a combined neurocardiac disorder in which leaky RyR2 channels in the brain cause epilepsy, and the same leaky channels in the heart cause exercise-induced sudden cardiac death
— id: 86957, year: 2008, vol: 118, page: 2230, stat: Journal Article,

Response to letters regarding article, "Abnormal conduction and morphology in the atrioventricular node of mice with atrioventricular canal targeted deletion of Alk3/Bmpr1a receptor"
Stroud, DM; Yu, C; Fishman, GI; Morley, GE; Gaussin, V; Burch, JBE; Mishina, Y; Schneider, MD
2008 AUG 5 ;118(6):E107-E107, Circulation
— id: 86825, year: 2008, vol: 118, page: E107, stat: Journal Article,

Reversibility of PRKAG2 glycogen-storage cardiomyopathy and electrophysiological manifestations
Wolf, Cordula M; Arad, Michael; Ahmad, Ferhaan; Sanbe, Atsushi; Bernstein, Scott A; Toka, Okan; Konno, Tetsuo; Morley, Gregory; Robbins, Jeffrey; Seidman, J G; Seidman, Christine E; Berul, Charles I
2008 Jan 15;117(2):144-154, Circulation
BACKGROUND: PRKAG2 mutations cause glycogen-storage cardiomyopathy, ventricular preexcitation, and conduction system degeneration. A genetic approach that utilizes a binary inducible transgenic system was used to investigate the disease mechanism and to assess preventability and reversibility of disease features in a mouse model of glycogen-storage cardiomyopathy. METHODS AND RESULTS: Transgenic (Tg) mice expressing a human N488I PRKAG2 cDNA under control of the tetracycline-repressible alpha-myosin heavy chain promoter underwent echocardiography, ECG, and in vivo electrophysiology studies. Transgene suppression by tetracycline administration caused a reduction in cardiac glycogen content and was initiated either prenatally (Tg(OFF(E-8 weeks))) or at different time points during life (Tg(OFF(4-16 weeks)), Tg(OFF(8-20 weeks)), and Tg(OFF(>20 weeks))). One group never received tetracycline, expressing transgene throughout life (Tg(ON)). Tg(ON) mice developed cardiac hypertrophy followed by dilatation, ventricular preexcitation involving multiple accessory pathways, and conduction system disease, including sinus and atrioventricular node dysfunction. CONCLUSIONS: Using an externally modifiable transgenic system, cardiomyopathy, cardiac dysfunction, and electrophysiological disorders were demonstrated to be reversible processes in PRKAG2 disease. Transgene suppression during early postnatal development prevented the development of accessory electrical pathways but not cardiomyopathy or conduction system degeneration. Taken together, these data provide insight into mechanisms of cardiac PRKAG2 disease and suggest that glycogen-storage cardiomyopathy can be modulated by lowering glycogen content in the heart
— id: 135319, year: 2008, vol: 117, page: 144, stat: Journal Article,

Student research at NYU: some things to consider
Morley, Gregory E
2007 ;1:9-10, Probe: the publication of research on biomedical endeavors
— id: 75312, year: 2007, vol: 1, page: 9, stat: Journal Article,

Examination of t-wave alternans in wild type mice
Rosner G; Bullinga J; Shai A; Morley GE
2007 ;1:34-34, Probe: the publication of research on biomedical endeavors
— id: 75325, year: 2007, vol: 1, page: 34, stat: Journal Article,

The contribution of TTX sensitive current to heart rate and epicardial conduction in the intact murine heart
Rosner, Gregg F; Morley, Gregory E
2007 ;1:35-35, Probe: the publication of research on biomedical endeavors
— id: 75326, year: 2007, vol: 1, page: 35, stat: Journal Article,

Abnormal conduction and morphology in the atrioventricular node of mice with atrioventricular canal targeted deletion of Alk3/Bmpr1a receptor
Stroud, Dina Myers; Gaussin, Vinciane; Burch, John B E; Yu, Cindy; Mishina, Yuji; Schneider, Michael D; Fishman, Glenn I; Morley, Gregory E
2007 Nov 27;116(22):2535-2543, Circulation
BACKGROUND: The atrioventricular (AV) node is essential for the sequential excitation and optimized contraction of the adult multichambered heart; however, relatively little is known about its formation from the embryonic AV canal. A recent study demonstrated that signaling by Alk3, the type 1a receptor for bone morphogenetic proteins, in the myocardium of the AV canal was required for the development of both the AV valves and annulus fibrosus. To test the hypothesis that bone morphogenetic protein signaling also plays a role in AV node formation, we investigated conduction system function and AV node morphology in adult mice with conditional deletion of Alk3 in the AV canal. METHODS AND RESULTS: High-resolution optical mapping with correlative histological analysis of 28 mutant hearts revealed 4 basic phenotypic classes based on electrical activation patterns and volume-conducted ECGs. The frequency of AV node conduction and morphological abnormalities increased from no detectable anomalies (class I) to severe defects (class IV), which included the presence of bypass tracts, abnormal ventricular activation patterns, fibrosis of the AV node, and twin AV nodes. CONCLUSIONS: The present findings demonstrate that bone morphogenetic protein signaling is required in the myocardium of the AV canal for proper AV junction development, including the AV node
— id: 75196, year: 2007, vol: 116, page: 2535, stat: Journal Article,

Gap junctions and propagation of the cardiac action potential
Bernstein, Scott A; Morley, Gregory E
2006 ;42:71-85, Advances in cardiology
Pacemaker cells in the heart generate periodic electrical signals that are conducted to the working myocardium via the specialized conduction system. Effective cell-to-cell communication is critical for rapid, uniform conduction of cardiac action potentials-- a prerequisite for effective, synchronized cardiac contraction. Local circuit currents form the basis of the depolarization wave front in the working myocardium. These currents flow from cell to cell via gap junction channels. In this chapter, we trace the path of the action potential from its generation in the sinus node to propagation through the working myocardium, with a detailed discussion of the role of gap junctions. First, we review the transmembrane ionic currents and the basic principles of conduction of the action potential to the working myocardium via the specialized tissues of the heart. Next, we consider the relative contribution of cell geometry, size, and gap junction conductance. These factors are examined in terms of their source-to-sink relationships. Lastly, we will discuss new insights into the importance of gap junctions in cardiac conduction in health and disease which have been gained from high resolution optical mapping in connexin-deficient mice
— id: 64668, year: 2006, vol: 42, page: 71, stat: Journal Article,

Consequences of Cardiac Myocyte-Specific Ablation of KATP channels in Transgenic Mice expressing Dominant Negative Kir6 Subunits
Tong, XiaoYong; Porter, Lisa M; Liu, GongXin; Dhar-Chowdhury, Piyali; Srivastava, Shekhar; Pountney, David J; Yoshida, Hidetada; Artman, Michael; Fishman, Glenn I; Yu, Cindy; Iyer, Ramesh; Morley, Gregory E; Gutstein, David E; Coetzee, William A
2006 Aug;291(2):H543-H551, American journal of physiology. Heart & circulatory physiology
Cardiac KATP channels are formed by Kir6.2 and SUR2A subunits. We produced transgenic mice which express dominant negative Kir6.x pore-forming subunits (Kir6.1-AAA or Kir6.2-AAA) in cardiac myocytes by driving their expression with the alpha-myosin heavy chain promoter. Weight gain and development after birth of these mice were similar to wild-type mice, but an increased mortality was noted after the age of 4-5 months. Transgenic mice lacked cardiac KATP channel activity as assessed with patch clamp techniques. Consistent with a decreased current density observed at positive voltages, the action potential duration was increased in these mice. Some myocytes developed early afterdepolarizations following isoproterenol treatment. Hemodynamic measurements revealed no significant effects on ventricular function (apart from a slightly elevated heart rate) whereas in-vivo electrophysiological recordings revealed a prolonged ventricular effective refractory period in transgenic mice. The transgenic mice tolerated stress less well as evident from treadmill stress tests. The pro-arrhythmogenic features and lack of adaptation to a stress response in transgenic mice suggests that these features are intrinsic to the myocardium and that KATP channels in the myocardium have an important role in protecting the heart from lethal arrhythmias and adaptation to stress situations
— id: 63616, year: 2006, vol: 291, page: H543, stat: Journal Article,

Altered right atrial excitation and propagation in connexin40 knockout mice
Bagwe, Suveer; Berenfeld, Omer; Vaidya, Dhananjay; Morley, Gregory E; Jalife, Jose
2005 Oct 11;112(15):2245-2253, Circulation
BACKGROUND: Intercellular coupling via connexin40 (Cx40) gap junction channels is an important determinant of impulse propagation in the atria. METHODS AND RESULTS: We studied the role of Cx40 in intra-atrial excitation and propagation in wild-type (Cx40(+/+)) and knockout (Cx40(-/-)) mice using high-resolution, dual-wavelength optical mapping. On ECG, the P wave was significantly prolonged in Cx40(-/-) mice (13.4+/-0.5 versus 11.4+/-0.3 ms in Cx40(+/+)). In Cx40(+/+) hearts, spontaneous right atrial (RA) activation showed a focal breakthrough at the junction of the right superior vena cava, sulcus terminalis, and RA free wall, corresponding to the location of the sinoatrial node. In contrast, Cx40(-/-) hearts displayed ectopic breakthrough sites at the base of the sulcus terminalis, RA free wall, and right superior vena cava. Progressive ablation of such sites in 4 Cx40(-/-) mice resulted in ectopic focus migration and cycle length prolongation. In all Cx40(-/-) hearts the focus ultimately shifted to the sinoatrial node at a very prolonged cycle length (initial ectopic cycle length, 182+/-20 ms; postablation sinus cycle length, 387+/-44 ms). In a second group of experiments, epicardial pacing at 10 Hz revealed slower conduction in the RA free wall of 5 Cx40(-/-) hearts than in 5 Cx40(+/+) hearts (0.61+/-0.07 versus 0.94+/-0.07 m/s; P<0.05). Dominant frequency analysis in Cx40(-/-) RA demonstrated significant reduction in the area of 1:1 conduction at 16 Hz (40+/-10% versus 69+/-5% in Cx40(+/+)) and 25 Hz (36+/-11% versus 65+/-9% in Cx40(+/+)). CONCLUSIONS: This is the first demonstration of intra-atrial block, ectopic rhythms, and altered atrial propagation in the RA of Cx40(-/-) mice
— id: 62133, year: 2005, vol: 112, page: 2245, stat: Journal Article,

Impulse initiation and conduction in the murine atria: a basis for future investigation of sinus node dysfunction
Baron, Elvera; Morley, Gregory
2005 ;1:118-121, Conference Proceedings (IEEE Engineering in Medicine & Biology Society)
The prevalence of atrial conduction defects and sinus node dysfunction increases with age. These age-related changes may play a critical role in establishing the substrate for the development of atrial fibrillation (AF), the most common sustained arrhythmia. Despite the association between atrial arrhythmias and age, little is known of the mechanisms that underlie changes in atrial electrophysiological function. Ongoing studies in our laboratory are focused on determining the mechanisms of atrial conduction defects associated with aging and disease. The purpose of this report is to present some initial studies of the murine sinus node and the approach we have taken to quantify conduction at the site of impulse initiation
— id: 105901, year: 2005, vol: 1, page: 118, stat: Journal Article,

Alk3/Bmpr1a receptor is required for development of the atrioventricular canal into valves and annulus fibrosus
Gaussin, Vinciane; Morley, Gregory E; Cox, Luk; Zwijsen, An; Vance, Kendra M; Emile, Lorin; Tian, Yimin; Liu, Jing; Hong, Chull; Myers, Dina; Conway, Simon J; Depre, Christophe; Mishina, Yuji; Behringer, Richard R; Hanks, Mark C; Schneider, Michael D; Huylebroeck, Danny; Fishman, Glenn I; Burch, John B E; Vatner, Stephen F
2005 Aug 5;97(3):219-226, Circulation research
Endocardial cushions are precursors of mature atrioventricular (AV) valves. Their formation is induced by signaling molecules originating from the AV myocardium, including bone morphogenetic proteins (BMPs). Here, we hypothesized that BMP signaling plays an important role in the AV myocardium during the maturation of AV valves from the cushions. To test our hypothesis, we used a unique Cre/lox system to target the deletion of a floxed Alk3 allele, the type IA receptor for BMPs, to cardiac myocytes of the AV canal (AVC). Lineage analysis indicated that cardiac myocytes of the AVC contributed to the tricuspid mural and posterior leaflets, the mitral septal leaflet, and the atrial border of the annulus fibrosus. When Alk3 was deleted in these cells, defects were seen in the same leaflets, ie, the tricuspid mural leaflet and mitral septal leaflet were longer, the tricuspid posterior leaflet was displaced and adherent to the ventricular wall, and the annulus fibrosus was disrupted resulting in ventricular preexcitation. The defects seen in mice with AVC-targeted deletion of Alk3 provide strong support for a role of Alk3 in human congenital heart diseases, such as Ebstein's anomaly. In conclusion, our mouse model demonstrated critical roles for Alk3 signaling in the AV myocardium during the development of AV valves and the annulus fibrosus
— id: 61382, year: 2005, vol: 97, page: 219, stat: Journal Article,

Focal Gap Junction Uncoupling and Spontaneous Ventricular Ectopy
Gutstein, David E; Danik, Stephan B; Lewitton, Steve; France, David; Liu, Fangyu; Chen, Franklin L; Zhang, Jie; Ghodsi, Newsha; Morley, Gregory E; Fishman, Glenn I
2005 Sep;289(3):H1091-H1098, American journal of physiology. Heart & circulatory physiology
Genetic studies in the mouse have demonstrated that conditional cardiac-resticted loss of connexin43 (Cx43), the major ventricular gap junction protein, is highly arrhythmogenic. However, whether more focal gap junction remodeling, as is commonly seen in acquired cardiomyopathies, influences the propensity for arrhythmogenesis is not known. We examined electrophysiological properties and the frequency of spontaneous and inducible arrhythmias in genetically engineered chimeric mice derived from injection of Cx43-deficient embryonic stem cells into wildtype recipient blastocysts. Chimeric mice had numerous well-circumscribed microscopic Cx43-negative foci in their hearts, comprising ~ 15% of the total surface area as determined by immunohistochemical analysis. Systolic function in the chimeric mice was significantly depressed as measured echocardiographically (19.0% decline in fractional shortening compared with controls, p < 0.05) and by invasive hemodynamics (17.6% reduction in dP/dT, p < 0.01). Chimeras had significantly more spontaneous arrhythmic events than controls (p < 0.01), including frequent runs of non-sustained ventricular tachycardia in some of the chimeric mice. However, in contrast to mice with conditional cardiac-resticted loss of Cx43 in the heart, no sustained ventricular tachyarrhythmias were observed. We conclude that focal areas of uncoupling in the myocardium increase the likelihood of arrhythmic triggers, but more widespread uncoupling is required to support sustained arrhythmias
— id: 56033, year: 2005, vol: 289, page: H1091, stat: Journal Article,

Cardiac-specific loss of N-cadherin leads to alteration in connexins with conduction slowing and arrhythmogenesis
Li, JF; Kostetskii, I; Patel, VV; Xiong, YM; Yu, C; Morley, GE; Molkentin, JD; Radice, GL
2005 NOV 25 ;97(11):1205-1205, Circulation research
— id: 59594, year: 2005, vol: 97, page: 1205, stat: Journal Article,

Cardiac-specific loss of N-cadherin leads to alteration in connexins with conduction slowing and arrhythmogenesis
Li, Jifen; Patel, Vickas V; Kostetskii, Igor; Xiong, Yanming; Chu, Antony F; Jacobson, Jason T; Yu, Cindy; Morley, Gregory E; Molkentin, Jeffery D; Radice, Glenn L
2005 Sep 2;97(5):474-481, Circulation research
The remodeling of ventricular gap junctions, as defined by changes in size, distribution, or function, is a prominent feature of diseased myocardium. However, the regulation of assembly and maintenance of gap junctions remains poorly understood. To investigate N-cadherin function in the adult myocardium, we used a floxed N-cadherin gene in conjunction with a cardiac-specific tamoxifen-inducible Cre transgene. The mutant animals appeared active and healthy until their sudden death approximately 2 months after deleting N-cadherin from the heart. Electrophysiologic analysis revealed abnormal conduction in the ventricles of mutant animals, including diminished QRS complex amplitude consistent with loss of electrical coupling in the myocardium. A significant decrease in the gap junction proteins, connexin-43 and connexin-40, was observed in N-cadherin-depleted myocytes. Perturbation of connexin function resulted in decreased ventricular conduction velocity, as determined by optical mapping. Our data suggest that perturbation of the N-cadherin/catenin complex in heart disease may be an underlying cause, leading to the establishment of the arrythmogenic substrate by destabilizing gap junctions at the cell surface
— id: 62134, year: 2005, vol: 97, page: 474, stat: Journal Article,

Reduced intercellular coupling leads to paradoxical propagation across the Purkinje-ventricular junction and aberrant myocardial activation
Morley, Gregory E; Danik, Stephan B; Bernstein, Scott; Sun, Yanjie; Rosner, Gregg; Gutstein, David E; Fishman, Glenn I
2005 Mar 15;102(11):4126-4129, Proceedings of the National Academy of Sciences of the United States of America
Ventricular tachycardia is a common heart rhythm disorder and a frequent cause of sudden cardiac death. Aberrant cell-cell coupling through gap junction channels, a process termed gap junction remodeling, is observed in many of the major forms of human heart disease and is associated with increased arrhythmic risk in both humans and in animal models. Genetically engineered mice with cardiac-restricted knockout of Connexin43, the major cardiac gap junctional protein, uniformly develop sudden cardiac death, although a detailed electrophysiological understanding of their profound arrhythmic propensity is unclear. Using voltage-sensitive dyes and high resolution optical mapping techniques, we found that uncoupling of the ventricular myocardium results in ectopic sites of ventricular activation. Our data indicate that this behavior reflects alterations in source-sink relationships and paradoxical conduction across normally quiescent Purkinje-ventricular muscle junctions. The aberrant activation profiles are associated with wavefront collisions, which in the setting of slow conduction may account for the highly arrhythmogenic behavior of Connexin43-deficient hearts. Thus, the extent of gap junction remodeling in diseased myocardium is a critical determinant of cardiac excitation patterns and arrhythmia susceptibility
— id: 52629, year: 2005, vol: 102, page: 4126, stat: Journal Article,

Somatic events modify hypertrophic cardiomyopathy pathology and link hypertrophy to arrhythmia
Wolf, Cordula M; Moskowitz, Ivan P G; Arno, Scott; Branco, Dorothy M; Semsarian, Christopher; Bernstein, Scott A; Peterson, Michael; Maida, Michael; Morley, Gregory E; Fishman, Glenn; Berul, Charles I; Seidman, Christine E; Seidman, J G
2005 Dec 13;102(50):18123-18128, Proceedings of the National Academy of Sciences of the United States of America
Sarcomere protein gene mutations cause hypertrophic cardiomyopathy (HCM), a disease with distinctive histopathology and increased susceptibility to cardiac arrhythmias and risk for sudden death. Myocyte disarray (disorganized cell-cell contact) and cardiac fibrosis, the prototypic but protean features of HCM histopathology, are presumed triggers for ventricular arrhythmias that precipitate sudden death events. To assess relationships between arrhythmias and HCM pathology without confounding human variables, such as genetic heterogeneity of disease-causing mutations, background genotypes, and lifestyles, we studied cardiac electrophysiology, hypertrophy, and histopathology in mice engineered to carry an HCM mutation. Both genetically outbred and inbred HCM mice had variable susceptibility to arrhythmias, differences in ventricular hypertrophy, and variable amounts and distribution of histopathology. Among inbred HCM mice, neither the extent nor location of myocyte disarray or cardiac fibrosis correlated with ex vivo signal conduction properties or in vivo electrophysiologically stimulated arrhythmias. In contrast, the amount of ventricular hypertrophy was significantly associated with increased arrhythmia susceptibility. These data demonstrate that distinct somatic events contribute to variable HCM pathology and that cardiac hypertrophy, more than fibrosis or disarray, correlates with arrhythmic risk. We suggest that a shared pathway triggered by sarcomere gene mutations links cardiac hypertrophy and arrhythmias in HCM
— id: 61378, year: 2005, vol: 102, page: 18123, stat: Journal Article,

Transitions in ventricular activation revealed by two-dimensional optical mapping
Chuck, Emil Thomas; Meyers, Kathleen; France, David; Creazzo, Tony L; Morley, Gregory E
2004 Oct;280(2):990-1000, Anatomical record. Part A, Discoveries in molecular, cellular, & evolutionary biology
While cardiac function in the mature heart is dependent on a properly functioning His-Purkinje system, the early embryonic tubular heart efficiently pumps blood without a distinct specialized conduction system. Although His-Purkinje system precursors have been identified using immunohistological techniques in the looped heart, little is known whether these precursors function electrically. To address this question, we used high-resolution optical mapping and fluorescent dyes with two CCD cameras to describe the motion-corrected activation patterns of 76 embryonic chick hearts from tubular stages (stage 10) to mature septated hearts (stage 35). Ventricular activation in the tubular looped heart (stages 10-17) using both calcium-sensitive fluo-4 and voltage-sensitive di-4-ANEPPS shows sequentially uniform propagation. In late looped hearts (stages 18-22), domains of the dorsal and lateral ventricle are preferentially activated before spreading to the remaining myocardium and show alternating regions of fast and slow propagation. During stages 22-26, action potentials arise from the dorsal ventricle. By stages 27-29, action potential breakthrough is also observed at the right ventricle apex. By stage 31, activation of the heart proceeds from foci at the apex and dorsal surface of the heart. The breakthrough foci correspond to regions where putative conduction system precursors have been identified immunohistologically. To date, our study represents the most detailed electrophysiological characterization of the embryonic heart between the looped and preseptated stages and suggests that ventricular activation undergoes a gradual transformation from sequential to a mature pattern with right and left epicardial breakthroughs. Our investigation suggests that cardiac conduction system precursors may be electrophysiologically distinct and mature gradually throughout cardiac morphogenesis in the chick
— id: 62135, year: 2004, vol: 280, page: 990, stat: Journal Article,

Extraction of periodic multivariate signals: mapping of voltage-dependent dye fluorescence in the mouse heart
Sornborger, Andrew; Sirovich, Lawrence; Morley, Gregory
2003 Dec;22(12):1537-1549, IEEE Transactions on medical imaging
In many experimental circumstances, heart dynamics are, to a good approximation, periodic. For this reason, it makes sense to use high-resolution methods in the frequency domain to visualize the spectrum of imaging data of the heart and to estimate the deterministic signal content and extract the periodic signal from background noise in experimental data. In this paper, we describe the first application of a new method that we call cardiac rhythm analysis which uses a combination of principal component analysis and multitaper harmonic analysis to extract periodic, deterministic signals from high-resolution imaging data of cardiac electrical activity, We show that this method significantly increases the signal-to-noise ratio of our recordings, allowing for better visualization of signal dynamics and more accurate quantification of the properties of electrical conduction. We visualize the spectra of three cardiac data sets of mouse hearts exhibiting sinus rhythm, paced rhythm and monomorphic tachycardia. Then, for pedagogical purposes, we investigate the tachycardia more closely, demonstrating the presence of two distinct periodicities in the re-entrant tachycardia. Analysis of the tachycardia shows that cardiac rhythm analysis not only allows for better visualization of electrical activity, but also provides new opportunities to study multiple periodicities in signal dynamics
— id: 62136, year: 2003, vol: 22, page: 1537, stat: Journal Article,

Neuregulin-1 promotes formation of the murine cardiac conduction system
Rentschler, Stacey; Zander, Jennifer; Meyers, Kathleen; France, David; Levine, Rebecca; Porter, George; Rivkees, Scott A; Morley, Gregory E; Fishman, Glenn I
2002 Aug 6;99(16):10464-10469, Proceedings of the National Academy of Sciences of the United States of America
The cardiac conduction system is a network of cells responsible for the rhythmic and coordinated excitation of the heart. Components of the murine conduction system, including the peripheral Purkinje fibers, are morphologically indistinguishable from surrounding cardiomyocytes, and a paucity of molecular markers exists to identify these cells. The murine conduction system develops in close association with the endocardium. Using the recently identified CCS-lacZ line of reporter mice, in which lacZ expression delineates the embryonic and fully mature conduction system, we tested the ability of several endocardial-derived paracrine factors to convert contractile cardiomyocytes into conduction-system cells as measured by ectopic reporter gene expression in the heart. In this report we show that neuregulin-1, a growth and differentiation factor essential for ventricular trabeculation, is sufficient to induce ectopic expression of the lacZ conduction marker. This inductive effect of neuregulin-1 was restricted to a window of sensitivity between 8.5 and 10.5 days postcoitum. Using the whole mouse embryo culture system, neuregulin-1 was shown to regulate lacZ expression within the embryonic heart, whereas its expression in other tissues remained unaffected. We describe the electrical activation pattern of the 9.5-days postcoitum embryonic mouse heart and show that treatment with neuregulin-1 results in electrophysiological changes in the activation pattern consistent with a recruitment of cells to the conduction system. This study supports the hypothesis that endocardial-derived neuregulins may be the major endogenous ligands responsible for inducing murine embryonic cardiomyocytes to differentiate into cells of the conduction system
— id: 32704, year: 2002, vol: 99, page: 10464, stat: Journal Article,

Conditional gene targeting of connexin43: exploring the consequences of gap junction remodeling in the heart
Gutstein DE; Morley GE; Fishman GI
2001 ;8(4-6):345-348, Cell adhesion & communications
Abnormalities in cardiac gap junction expression have been postulated to contribute to arrhythmias and ventricular dysfunction. We investigated the role of cardiac gap junctions by generating a heart-specific conditional knock-out (CKO) of connexin43 (Cx43), the major cardiac gap junction protein. While the Cx43 CKO mice have normal heart structure and contractile function, they die suddenly from spontaneous ventricular arrhythmias. Because abnormalities in gap junction expression in the diseased heart can be focal, we also generated chimeric mice formed from Cx43-null embryonic stem (ES) cells and wildtype recipient blastocysts. Heterogeneous Cx43 expression in the chimeric mice resulted in conduction defects and depressed contractile function. These novel genetic murine models of Cx43 loss of function in the adult mouse heart define gap junctional abnormalities as a key molecular feature of the arrhythmogenic substrate and an important factor in heart dysfunction
— id: 32705, year: 2001, vol: 8, page: 345, stat: Journal Article,

Conduction slowing and sudden arrhythmic death in mice with cardiac-restricted inactivation of connexin43
Gutstein DE; Morley GE; Tamaddon H; Vaidya D; Schneider MD; Chen J; Chien KR; Stuhlmann H; Fishman GI
2001 Feb 16;88(3):333-339, Circulation research
Cardiac arrhythmia is a common and often lethal manifestation of many forms of heart disease. Gap junction remodeling has been postulated to contribute to the increased propensity for arrhythmogenesis in diseased myocardium, although a causative role in vivo remains speculative. By generating mice with cardiac-restricted knockout of connexin43 (Cx43), we have circumvented the perinatal lethal developmental defect associated with germline inactivation of this gap junction channel gene and uncovered an essential role for Cx43 in the maintenance of electrical stability. Mice with cardiac-specific loss of Cx43 have normal heart structure and contractile function, and yet they uniformly (28 of 28 conditional Cx43 knockout mice observed) develop sudden cardiac death from spontaneous ventricular arrhythmias by 2 months of age. Optical mapping of the epicardial electrical activation pattern in Cx43 conditional knockout mice revealed that ventricular conduction velocity was significantly slowed by up to 55% in the transverse direction and 42% in the longitudinal direction, resulting in an increase in anisotropic ratio compared with control littermates (2.1+/-0.13 versus 1.66+/-0.06; P:<0.01). This novel genetic murine model of primary sudden cardiac death defines gap junctional abnormalities as a key molecular feature of the arrhythmogenic substrate
— id: 27670, year: 2001, vol: 88, page: 333, stat: Journal Article,

Heterogeneous expression of Gap junction channels in the heart leads to conduction defects and ventricular dysfunction
Gutstein DE; Morley GE; Vaidya D; Liu F; Chen FL; Stuhlmann H; Fishman GI
2001 Sep 4;104(10):1194-1199, Circulation
BACKGROUND:- Heterogeneous remodeling of gap junctions is observed in many forms of heart disease. The consequent loss of synchronous ventricular activation has been hypothesized to result in diminished cardiac performance. To directly test this hypothesis, we designed a murine model of heterogeneous gap junction channel expression. Methods and Results-- We generated chimeric mice formed from connexin43 (Cx43)-deficient embryonic stem cells and wild-type or genetically marked ROSA26 recipient blastocysts. Chimeric mice developed normally, without histological evidence of myocardial fibrosis or hypertrophy. Heterogeneous Cx43 expression resulted in conduction defects, however, as well as markedly depressed contractile function. Optical mapping of chimeric hearts by use of voltage-sensitive dyes revealed highly irregular epicardial conduction patterns, quantified as significantly greater negative curvature of the activation wave front (-1.86+/-0.40 mm in chimeric mice versus -0.86+/-0.098 mm in controls; P<0.01; n=6 for each group). Echocardiographic studies demonstrated significantly reduced fractional shortening in chimeric mice (26.6+/-2.3% versus 36.5+/-1.6% in age-matched 129/SvxC57BL/6F1 wild-type controls; P<0.05). CONCLUSIONS:- These data suggest that heterogeneous Cx43 expression, by perturbing the normal pattern of coordinated myocardial excitation, may directly depress cardiac performance
— id: 27669, year: 2001, vol: 104, page: 1194, stat: Journal Article,

Understanding conduction of electrical impulses in the mouse heart using high-resolution video imaging technology
Morley GE; Vaidya D
2001 Feb 1;52(3):241-250, Microscopy research & technique
The conduction of electrical impulses in the heart depends on the ability to efficiently transfer excitatory current between individual myocytes. Several recent studies have focused on the use of optical mapping techniques to determine the electrophysiological consequences and the proarrhythmic effects of reducing intercellular coupling in newly developed connexin knockout mice. This work has begun to unravel important questions regarding the role of connexins in intercellular coupling and propagation of electrical impulses in the heart. The purpose of this review is to discuss the techniques and unique issues involved in imaging electrical wave propagation in the heart. In addition, we will review recent experimental studies that address the role of intercellular communication in the development of cardiac arrhythmias
— id: 32708, year: 2001, vol: 52, page: 241, stat: Journal Article,

Visualization and functional characterization of the developing murine cardiac conduction system
Rentschler S; Vaidya DM; Tamaddon H; Degenhardt K; Sassoon D; Morley GE; Jalife J; Fishman GI
2001 May;128(10):1785-1792, Development
The cardiac conduction system is a complex network of cells that together orchestrate the rhythmic and coordinated depolarization of the heart. The molecular mechanisms regulating the specification and patterning of cells that form this conductive network are largely unknown. Studies in avian models have suggested that components of the cardiac conduction system arise from progressive recruitment of cardiomyogenic progenitors, potentially influenced by inductive effects from the neighboring coronary vasculature. However, relatively little is known about the process of conduction system development in mammalian species, especially in the mouse, where even the histological identification of the conductive network remains problematic. We have identified a line of transgenic mice where lacZ reporter gene expression delineates the developing and mature murine cardiac conduction system, extending proximally from the sinoatrial node to the distal Purkinje fibers. Optical mapping of cardiac electrical activity using a voltage-sensitive dye confirms that cells identified by the lacZ reporter gene are indeed components of the specialized conduction system. Analysis of lacZ expression during sequential stages of cardiogenesis provides a detailed view of the maturation of the conductive network and demonstrates that patterning occurs surprisingly early in embryogenesis. Moreover, optical mapping studies of embryonic hearts demonstrate that a murine His-Purkinje system is functioning well before septation has completed. Thus, these studies describe a novel marker of the murine cardiac conduction system that identifies this specialized network of cells throughout cardiac development. Analysis of lacZ expression and optical mapping data highlight important differences between murine and avian conduction system development. Finally, this line of transgenic mice provides a novel tool for exploring the molecular circuitry controlling mammalian conduction system development and should be invaluable in studies of developmental mutants with potential structural or functional conduction system defects
— id: 32707, year: 2001, vol: 128, page: 1785, stat: Journal Article,

Null mutation of connexin43 causes slow propagation of ventricular activation in the late stages of mouse embryonic development
Vaidya D; Tamaddon HS; Lo CW; Taffet SM; Delmar M; Morley GE; Jalife J
2001 Jun 8;88(11):1196-1202, Circulation research
Connexin43 (Cx43) is the principal connexin isoform in the mouse ventricle, where it is thought to provide electrical coupling between cells. Knocking out this gene results in anatomic malformations that nevertheless allow for survival through early neonatal life. We examined electrical wave propagation in the left (LV) and right (RV) ventricles of isolated Cx43 null mutated (Cx43(-/-)), heterozygous (Cx43(+/)(-)), and wild-type (WT) embryos using high-resolution mapping of voltage-sensitive dye fluorescence. Consistent with the compensating presence of the other connexins, no reduction in propagation velocity was seen in Cx43(-/-) ventricles at postcoital day (dpc) 12.5 compared with WT or Cx43(+/)(-) ventricles. A gross reduction in conduction velocity was seen in the RV at 15.5 dpc (in cm/second, mean [1 SE confidence interval], WT 9.9 [8.7 to 11.2], Cx43(+/)(-) 9.9 [9.0 to 10.9], and Cx43(-/-) 2.2 [1.8 to 2.7; P<0.005]) and in both ventricles at 17.5 dpc (in RV, WT 8.4 [7.6 to 9.3], Cx43(+/)(-) 8.7 [8.1 to 9.3], and Cx43(-/-) 1.1 [0.1 to 1.3; P<0.005]; in LV, WT 10.1 [9.4 to 10.7], Cx43(+/)(-) 8.3 [7.8 to 8.9], and Cx43(-/-) 1.7 [1.3 to 2.1; P<0.005]) corresponding with the downregulation of Cx40. Cx40 and Cx45 mRNAs were detectable in ventricular homogenates even at 17.5 dpc, probably accounting for the residual conduction function. Neonatal knockout hearts were arrhythmic in vivo as well as ex vivo. This study demonstrates the contribution of Cx43 to the electrical function of the developing mouse heart and the essential role of this gene in maintaining heart rhythm in postnatal life
— id: 32706, year: 2001, vol: 88, page: 1196, stat: Journal Article,

Early onset heart failure in transgenic mice with dilated cardiomyopathy
Hall DG; Morley GE; Vaidya D; Ard M; Kimball TR; Witt SA; Colbert MC
2000 Jul;48(1):36-42, Pediatric research
In children, dilated cardiomyopathy is due to a variety of etiologies and usually carries a grave prognosis. The purpose of the present study was to carefully follow the progression of events leading to cardiac dilatation and congestive heart failure in a dilated cardiomyopathy model in neonatal and juvenile mice. These initial steps are often not well characterized. Furthermore, the loss of gap junctions and reduced electrical coupling of cardiomyocytes frequently found in human cardiomyopathies are also observed in these early stages. By 2 wk of age, molecular markers associated with hypertrophy were already altered. Cardiomyocyte hypertrophy, reduced connexin43 expression, and decreased conduction velocity were apparent by 4 wk, before overt cardiac dysfunction (decreased shortening fraction and chamber remodeling) that was not present until 12 wk of age. Our results show that in this model cardiomyopathic changes are present by 2 wk after birth and progress rapidly during the subsequent 2 postnatal weeks. Combined with the observations of other models of heart disease, we suggest that the first 2 wk of postnatal life are absolutely critical for normal cardiac development, and events that perturb homeostasis during this period determine whether the heart will continue to develop normally. These animals exhibit early symptoms of disease including reduced connexin43 and conduction defects before impaired cardiac function and demonstrate for the first time a temporal association between decreased connexin43 levels and the initiation of a contractility deficit that ends in heart failure
— id: 32711, year: 2000, vol: 48, page: 36, stat: Journal Article,

Cardiac gap junction remodeling by stretch: is it a good thing?
Morley GE; Jalife J
2000 Aug 18;87(4):272-274, Circulation research
— id: 32710, year: 2000, vol: 87, page: 272, stat: Journal Article,

Characterization of conduction in the ventricles of normal and heterozygous Cx43 knockout mice using optical mapping
Morley GE; Vaidya D; Jalife J
2000 Mar;11(3):375-377, Journal of cardiovascular electrophysiology
— id: 32712, year: 2000, vol: 11, page: 375, stat: Journal Article,

High-resolution optical mapping of the right bundle branch in connexin40 knockout mice reveals slow conduction in the specialized conduction system
Tamaddon HS; Vaidya D; Simon AM; Paul DL; Jalife J; Morley GE
2000 Nov 10;87(10):929-936, Circulation research
Connexin40 (Cx40) is a major gap junction protein that is expressed in the His-Purkinje system and thought to be a critical determinant of cell-to-cell communication and conduction of electrical impulses. Video maps of the ventricular epicardium and the proximal segment of the right bundle branch (RBB) were obtained using a high-speed CCD camera while simultaneously recording volume-conducted ECGs. In Cx40(-/-) mice, the PR interval was prolonged (47.4+/-1.4 in wild-type [WT] [n=6] and 57.5+/-2.8 in Cx40(-/-) [n=6]; P<0.01). WT ventricular epicardial activation was characterized by focused breakthroughs that originated first on the right ventricle (RV) and then the left ventricle (LV). In Cx40(-/-) hearts, the RV breakthrough occurred after the LV breakthrough. Additionally, Cx40(-/-) mice showed RV breakthrough times that were significantly delayed with respect to QRS complex onset (3.7+/-0.7 ms in WT [n=6] and 6.5+/-0.7 ms in Cx40(-/-) [n=6]; P<0.01), whereas LV breakthrough times did not change. Conduction velocity measurements from optical mapping of the RBB revealed slow conduction in Cx40(-/-) mice (74.5+/-3 cm/s in WT [n=7] and 43.7+/-6 cm/s in Cx40(-/-) [n=7]; P<0.01). In addition, simultaneous ECG records demonstrated significant delays in Cx40(-/-) RBB activation time with respect to P time (P-RBB time; 41.6+/-1.9 ms in WT [n=7] and 55.1+/-1.3 ms in [n=7]; P<0.01). These data represent the first direct demonstration of conduction defects in the specialized conduction system of Cx40(-/-) mice and provide new insight into the role of gap junctions in cardiac impulse propagation
— id: 32709, year: 2000, vol: 87, page: 929, stat: Journal Article,

Connexins and impulse propagation in the mouse heart
Jalife J; Morley GE; Vaidya D
1999 Dec;10(12):1649-1663, Journal of cardiovascular electrophysiology
Gap junction channels are essential for normal cardiac impulse propagation. Three gap junction proteins, known as connexins, are expressed in the heart: Cx40, Cx43, and Cx45. Each of these proteins forms channels with unique biophysical and electrophysiologic properties, as well as spatial distribution of expression throughout the heart. However, the specific functional role of the individual connexins in normal and abnormal propagation is unknown. The availability of genetically engineered mouse models, together with new developments in optical mapping technology, makes it possible to integrate knowledge about molecular mechanisms of intercellular communication and its regulation with our growing understanding of the microscopic and global dynamics of electrical impulse propagation during normal and abnormal cardiac rhythms. This article reviews knowledge on the mechanisms of cardiac impulse propagation, with particular focus on the role of cardiac connexins in electrical communication between cells. It summarizes results of recent studies on the electrophysiologic consequences of defects in the functional expression of specific gap junction channels in mice lacking either the Cx43 or Cx40 gene. It also reviews data obtained in a transgenic mouse model in which cell loss and remodeling of gap junction distribution leads to increased susceptibility to arrhythmias and sudden cardiac death. Overall, the results demonstrate that these are potentially powerful strategies for studying fundamental mechanisms of cardiac electrical activity and for testing the hypothesis that certain cardiac arrhythmias involve gap junction or other membrane channel dysfunction. These new approaches, which permit one to manipulate electrical wave propagation at the molecular level, should provide new insight into the detailed mechanisms of initiation, maintenance, and termination of cardiac arrhythmias, and may lead to more effective means to treat arrhythmias and prevent sudden cardiac death
— id: 32713, year: 1999, vol: 10, page: 1649, stat: Journal Article,

Characterization of conduction in the ventricles of normal and heterozygous Cx43 knockout mice using optical mapping
Morley GE; Vaidya D; Samie FH; Lo C; Delmar M; Jalife J
1999 Oct;10(10):1361-1375, Journal of cardiovascular electrophysiology
INTRODUCTION: Gap junction channels are important determinants of conduction in the heart and may play a central role in the development of lethal cardiac arrhythmias. The recent development of a Cx43-deficient mouse has raised fundamental questions about the role of specific connexin isoforms in intercellular communication in the heart. Although a homozygous null mutation of the Cx43 gene (Cx43-/-) is lethal, the heterozygous (Cx43+/-) animals survive to adulthood. Reports on the cardiac electrophysiologic phenotype of the Cx43+/- mice are contradictory. Thus, the effects of a null mutation of a single Cx43 allele require reevaluation. METHODS AND RESULTS: High-resolution video mapping techniques were used to study propagation in hearts from Cx43+/- and littermate control (Cx43+/+) mice. Local conduction velocities (CVs) and conduction patterns were quantitatively measured by determining conduction vectors. We undertook the characterization of ECG parameters and epicardial CVs of normal and Cx43+/- mouse hearts. ECG measurements obtained from 12 Cx43+/+ and 6 Cx43+/- age matched mice did not show differences in any parameter, including QRS duration (14.5 +/- 0.9 and 15.7 +/- 2.3 msec for Cx43+/+ and Cx43+/-, respectively). In addition, using a sensitive method of detecting changes in local CV, video images of epicardial wave propagation revealed similar activation patterns and velocities in both groups of mice. CONCLUSION: A sensitive method that accurately measures local CVs throughout the ventricles revealed no changes in Cx43+/- mice, which is consistent with the demonstration that ECG parameter values in the heterozygous mice are the same as those in wild-type mice
— id: 32714, year: 1999, vol: 10, page: 1361, stat: Journal Article,

Reentry and fibrillation in the mouse heart. A challenge to the critical mass hypothesis
Vaidya D; Morley GE; Samie FH; Jalife J
1999 Jul 23;85(2):174-181, Circulation research
The idea that fibrillation is only possible in hearts exceeding a critical size was introduced by W. Garrey >80 years ago and has since been generally accepted. In ventricular tissue, this critical size was originally estimated to be 400 mm(2). Recent estimates suggest that the critical size required for sustained reentry is approximately 100 to 200 mm(2), whereas 6 times this area is required for ventricular fibrillation. According to these estimates, fibrillation is not possible in the mouse heart, where the ventricular surface area is approximately 100 mm(2). To test whether sustained ventricular fibrillation could be induced in such an area, we used a high-speed video imaging system and a voltage-sensitive dye to quantify electrical activity on the epicardial surface of the Langendorff-perfused adult mouse heart. In 6 hearts, measurements during ventricular pacing at a basic cycle length (BCL) of 120 ms yielded maximum and minimum conduction velocities (CV(max) and CV(min)) of 0.63+/-0.04 and 0.38+/-0.02 mm/ms, respectively. At a BCL of 80 ms, CV(max) and CV(min) changed to 0.55+/-0.03 and 0. 34+/-0.02 mm/ms. Action potential durations (APDs), measured at 70% repolarization at those pacing frequencies were found to be 44.5+/-2. 9 and 40.4+/-2.6 ms, respectively. The wavelengths (CVxAPD) were calculated to be 28.6+/-3.4 mm in the CV(max) direction and 16.8+/-1. 5 mm in the CV(min) direction at BCL 120 ms. Wavelengths were significantly reduced (P<0.05) at BCL 80 ms (CV(max), 22.2+/-1.8 mm; CV(min), 13.7+/-0.9 mm). In 5 hearts, stationary vortex-like reentry organized by single rotors (4 of 5 hearts) or by pairs of rotors (1 of 5 hearts) was induced by burst pacing. In the ECG, the activity manifested as sustained monomorphic tachycardia. Detailed analysis showed that the local CVs were reduced in the vicinity of the rotor center, which allowed the reentry to take place within a smaller area than was calculated from wavelength measurements during pacing. In 4 of 7 hearts, burst pacing resulted in a polymorphic ECG pattern indistinguishable from ventricular fibrillation. These data challenge the critical mass hypothesis by demonstrating that ventricular tissue with an area as small as 100 mm(2) is capable of undergoing sustained fibrillatory activity
— id: 32715, year: 1999, vol: 85, page: 174, stat: Journal Article,

A fungal metabolite that eliminates motion artifacts
Jalife J; Morley GE; Tallini NY; Vaidya D
1998 Dec;9(12):1358-1362, Journal of cardiovascular electrophysiology
— id: 32716, year: 1998, vol: 9, page: 1358, stat: Journal Article,

Structure of connexin43 and its regulation by pHi
Morley GE; Ek-Vitorin JF; Taffet SM; Delmar M
1997 Aug;8(8):939-951, Journal of cardiovascular electrophysiology
Electrical coupling in the heart provides an effective mechanism for propagating the cardiac action potential efficiently throughout the entire heart. Cells within the heart are electrically coupled through specialized membrane channels called gap junctions. Studies have shown that gap junctions are dynamic, carefully regulated channels that are important for normal cardiogenesis. We have recently been interested in the molecular mechanisms by which intracellular acidification leads to gap junction channel closure. Previous results in this lab have shown that truncation of the carboxyl terminal (CT) of connexin43 (Cx43) does not interfere with functional channel expression. Further, the pH-dependent closure of Cx43 channels is significantly impaired by removal of this region of the protein. Other studies have shown that the CT is capable of interacting with its receptor even when not covalently attached to the channel protein. From these data we have proposed a particle-receptor model to explain the pH-dependent closure of Cx43 gap junction channels. Detailed analysis of the CT has revealed interesting new information regarding its possible structure. Here we review the most recent studies that have contributed to our understanding of the molecular mechanisms of regulation of the cardiac gap protein Cx43
— id: 32717, year: 1997, vol: 8, page: 939, stat: Journal Article,

PH regulation of connexin43: molecular analysis of the gating particle
Ek-Vitorin JF; Calero G; Morley GE; Coombs W; Taffet SM; Delmar M
1996 Sep;71(3):1273-1284, Biophysical journal
Gap junction channels allow for the passage of ions and small molecules between neighboring cells. These channels are formed by multimers of an integral membrane protein named connexin. In the heart and other tissues, the most abundant connexin is a 43-kDa, 382-amino acid protein termed connexin43 (Cx43). A characteristic property of connexin channels is that they close upon acidification of the intracellular space. Previous studies have shown that truncation of the carboxyl terminal of Cx43 impairs pH sensitivity. In the present study, we have used a combination of optical, electrophysiological, and molecular biological techniques and the oocyte expression system to further localize the regions of the carboxyl terminal that are involved in pH regulation of Cx43 channels. Our results show that regions 261-300 and 374-382 are essential components of a pH-dependent 'gating particle,' which is responsible for acidification-induced uncoupling of Cx43-expressing cells. Regions 261-300 and 374-382 seem to be interdependent. The function of region 261-300 may be related to the presence of a poly-proline repeat between amino acids 274 and 285. Furthermore, site-directed mutagenesis studies show that the function of region 374-382 is not directly related to its net balance of charges, although mutation of only one amino acid (aspartate 379) for asparagine impairs pH sensitivity to the same extent as truncation of the carboxyl terminal domain (from amino acid 257). The mutation in which serine 364 is substituted for proline, which has been associated with some cases of cardiac congenital malformations in humans, also disrupts the pH gating of Cx43, although deletion of amino acids 364-373 has no effect on acidification-induced uncoupling. These results provide new insight into the molecular mechanisms responsible for acidification-induced uncoupling of gap junction channels in the heart and in other Cx43-expressing structures
— id: 32718, year: 1996, vol: 71, page: 1273, stat: Journal Article,

Intramolecular interactions mediate pH regulation of connexin43 channels
Morley GE; Taffet SM; Delmar M
1996 Mar;70(3):1294-1302, Biophysical journal
We have previously proposed that acidification-induced regulation of the cardiac gap junction protein connexin43 (Cx43) may be modeled as a particle-receptor interaction between two separate domains of Cx43: the carboxyl terminal (acting as a particle), and a region including histidine 95 (acting as a receptor). Accordingly, intracellular acidification would lead to particle-receptor binding, thus closing the channel. A premise of the model is that the particle can bind its receptor, even if the particle is not covalently bound to the rest of the protein. The latter hypothesis was tested in antisense-injected Xenopus oocyte pairs coexpressing mRNA for a pH-insensitive Cx43 mutant truncated at amino acid 257 (i.e., M257) and mRNA coding for the carboxyl terminal region (residues 259-382). Intracellular pH (pHo) was recorded using the dextran form of the proton-sensitive dye seminaphthorhodafluor (SNARF). Junctional conductance (Gj) was measured with the dual voltage clamp technique. Wild-type Cx43 channels showed their characteristic pH sensitivity. M257 channels were not pH sensitive (pHo tested: 7.2 to 6.4). However, pH sensitivity was restored when the pH-insensitive channel (M257) was coexpressed with mRNA coding for the carboxyl terminal. Furthermore, coexpression of the carboxyl terminal of Cx43 enhanced the pH sensitivity of an otherwise less pH-sensitive connexin (Cx32). These data are consistent with a model of intramolecular interactions in which the carboxyl terminal acts as an independent domain that, under the appropriate conditions, binds to a separate region of the protein and closes the channel. These interactions may be direct (as in the ball-and-chain mechanism of voltage-dependent gating of potassium channels) or mediated through an intermediary molecule. The data further suggest that the region of Cx43 that acts as a receptor for the particle is conserved among connexins. A similar molecular mechanism may mediate chemical regulation of other channel proteins
— id: 32719, year: 1996, vol: 70, page: 1294, stat: Journal Article,

Effect of basolateral or apical hyposmolarity on apical maxi K channels of everted rat collecting tubule
Stoner LC; Morley GE
1995 Apr;268(4 Pt 2):F569-F580, American journal of physiology
We are able to evert and perfuse rat cortical collecting tubules (CCT) at 37 degrees C. Patch-clamp techniques were used to study high-conductance potassium channels (maxi K) on the apical membrane. Under control conditions (150 mM Na+ and 5 mM K+ in pipette and bathing solutions), the slope conductance averaged 109.8 +/- 6.6 pS (12 channels), and reversal potential (expressed as pipette voltage) was +26.3 +/- 2.4 mV. The percent of time the channel spends in the open state and unitary current when voltage was clamped to 0 mV were 1.4 +/- 0.7% and 3.12 +/- 0.42 pA, respectively. In six patches voltage clamped to 0 mV, the isosmotic solution perfused through the everted tubule (basolateral surface) was exchanged for one made 70 mosmol/kgH2O hyposmotic to the control saline. Open probability increased from 0.019 to 0.258, an increase of 0.239 +/- 0.065 (P < 0.005). In four patches where a maxi K channel was evident, no increase in open probability was observed when a hyposmotic saline was placed on the apical surface. However, when vasopressin was present on the basolateral surface, apical application of hyposmotic saline resulted in a series of bursts of channel activity. The average increase in open probability during bursts was (0.055 +/- 0.017, P < 0.005). We conclude that one function of the maxi K channel located in the apical membrane of the rat CCT may be to release intracellular solute (potassium) during a volume regulatory decrease induced by placing a dilute solution on the basolateral surface or when the apical osmolarity is reduced in the presence of vasopressin. These data are consistent with the hypothesis that the physiological role of the channel is to regulate cell volume during water reabsorption
— id: 32720, year: 1995, vol: 268, page: F569, stat: Journal Article,

Effects of 2,4-dinitrophenol or low [ATP]i on cell excitability and action potential propagation in guinea pig ventricular myocytes
Morley GE; Anumonwo JM; Delmar M
1992 Oct;71(4):821-830, Circulation research
Inhibition of aerobic metabolism leads to a major disruption of cardiac cell homeostasis. The purpose of the present study was twofold: 1) We determined the relative importance of junctional and nonjunctional membrane resistance (Rj and Rm, respectively) in the development of propagation failure during inhibition of aerobic metabolism in guinea pig ventricular cell pairs. 2) We used the patch-action potential clamp technique in single ventricular myocytes to study some of the properties of the membrane channels that are responsible for shortening of action potential duration and eventual failure of cell excitation after metabolic blockade. In most experiments, whole-cell patch pipettes were filled with a solution containing 1 mM EGTA, 5 mM HEPES, and 5 mM ATP. Our results in cell pairs showed that pharmacological inhibition of aerobic metabolism with the mitochondrial uncoupler 2,4-dinitrophenol (DNP) led to a drop in Rm followed by an increase in Rj. The increase in Rj was not sufficient to cause a measurable delay in cell-to-cell propagation, whereas the drop in Rm consistently led to failure of cell excitation. Similar results were obtained in additional experiments in which the EGTA concentration in the pipette was reduced to 50 microM. Similar results were also obtained by loading the recording patch pipettes with a solution containing only 0.1 mM ATP. Our patch-action potential clamp experiments, on the other hand, revealed that DNP induced the opening of time- and voltage-independent membrane channels, with a unitary conductance of 23 pS. The channels allowed for the passage of outward current in the voltage range of the action potential, and the increase in membrane patch conductance correlated with the observed shortening of action potential duration during DNP superfusion. Our experiments provide the first simultaneous recordings of action potentials and DNP-induced channel currents in guinea pig ventricular myocytes. Overall, the data provide new evidence for the understanding of the cellular and subcellular mechanisms involved in the development of slow conduction velocity and propagation block after metabolic blockade
— id: 32721, year: 1992, vol: 71, page: 821, stat: Journal Article,

Dynamics of the inward rectifier K+ current during the action potential of guinea pig ventricular myocytes
Ibarra J; Morley GE; Delmar M
1991 Dec;60(6):1534-1539, Biophysical journal
The potassium selective, inward rectifier current (IK1) is known to be responsible for maintaining the resting membrane potential of quiescent ventricular myocytes. However, the contribution of this current to the different phases of the cardiac action potential has not been adequately established. In the present study, we have used the action potential clamp (APC) technique to characterize the dynamic changes of a cesium-sensitive (i.e., Ik1) current which occur during the action potential. Our results show that (a) Ik1 is present during depolarization, as well as in the final phase of repolarization of the cardiac action potential. (b) The current reaches the zone of inward-going rectification before the regenerative action potential ensues. (c) The maximal outward current amplitude during repolarization is significantly lower than during depolarization, which supports the hypothesis that in adult guinea pig ventricular myocytes, Ik1 rectification is accentuated during the action potential plateau. Our results stress the importance of Ik1 in the modulation of cell excitability in the ventricular myocyte
— id: 32722, year: 1991, vol: 60, page: 1534, stat: Journal Article,