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Background: Plakophilin-2 (PKP2) is classically defined as a desmosomal protein. Mutations in PKP2 associate with most cases of gene-positive arrhythmogenic right ventricular cardiomyopathy. A better understanding of PKP2 cardiac biology can help elucidate the mechanisms underlying arrhythmic and cardiomyopathic events consequent to PKP2 deficiency. Here, we sought to capture early molecular/cellular events that can act as nascent arrhythmic/cardiomyopathic substrates. Methods: We used multiple imaging, biochemical and high-resolution mass spectrometry methods to study functional/structural properties of cells/tissues derived from cardiomyocyte-specific, tamoxifen-activated, PKP2 knockout mice (PKP2cKO) 14 days post-tamoxifen injection, a time point preceding overt electrical or structural phenotypes. Myocytes from right or left ventricular free wall were studied separately. Results: Most properties of PKP2cKO left ventricular myocytes were not different from control; in contrast, PKP2cKO right ventricular (RV) myocytes showed increased amplitude and duration of Ca 2+ transients, increased Ca 2+ in the cytoplasm and sarcoplasmic reticulum, increased frequency of spontaneous Ca 2+ release events (sparks) even at comparable sarcoplasmic reticulum load, and dynamic Ca 2+ accumulation in mitochondria. We also observed early- and delayed-after transients in RV myocytes and heightened susceptibility to arrhythmias in Langendorff-perfused hearts. In addition, ryanodine receptor 2 in PKP2cKO-RV cells presented enhanced Ca 2+ sensitivity and preferential phosphorylation in a domain known to modulate Ca 2+ gating. RNAseq at 14 days post-tamoxifen showed no relevant difference in transcript abundance between RV and left ventricle, neither in control nor in PKP2cKO cells. Instead, we found an RV-predominant increase in membrane permeability that can permit Ca 2+ entry into the cell. Connexin 43 ablation mitigated the membrane permeability increase, accumulation of cytoplasmic Ca 2+ , increased frequency of sparks and early stages of RV dysfunction. Connexin 43 hemichannel block with GAP19 normalized [Ca 2+ ] i homeostasis. Similarly, protein kinase C inhibition normalized spark frequency at comparable sarcoplasmic reticulum load levels. Conclusions: Loss of PKP2 creates an RV-predominant arrhythmogenic substrate (Ca 2+ dysregulation) that precedes the cardiomyopathy; this is, at least in part, mediated by a Connexin 43-dependent membrane conduit and repressed by protein kinase C inhibitors. Given that asymmetric Ca 2+ dysregulation precedes the cardiomyopathic stage, we speculate that abnormal Ca 2+ handling in RV myocytes can be a trigger for gross structural changes observed at a later stage.

Cardiac pathologies associated with arrhythmic activity are often accompanied by inflammation. The contribution of inflammatory cells to the electrophysiological properties of injured myocardium is unknown. Myocardial scar cell types and intercellular contacts were analyzed using a three-dimensional reconstruction from serial blockface scanning electron microscopy data. Three distinct cell populations were identified: inflammatory, fibroblastic and endocardial cells. While individual fibroblastic cells interface with a greater number of cells, inflammatory cells have the largest contact area suggesting a role in establishing intercellular electrical connections in scar tissue. Optical mapping was used to study the electrophysiological properties of scars in fetal liver chimeric mice generated using connexin43 knockout donors (bmpKO). Voltage changes were elicited in response to applied current pulses. Isopotential maps showed a steeper pattern of decay with distance from the electrode in scars compared with uninjured regions, suggesting reduced electrical coupling. The tissue decay constant, defined as the distance voltage reaches 37% of the amplitude at the edge of the scar, was 0.48 ± 0.04 mm (n = 11) in the scar of the bmpCTL group and decreased 37.5% in the bmpKO group (n = 10). Together these data demonstrate inflammatory cells significantly contribute to scar electrophysiology through coupling mediated at least partially by connexin43 expression.

Plakophilin-2 (PKP2) is a component of the desmosome and known for its role in cell–cell adhesion. Mutations in human PKP2 associate with a life-threatening arrhythmogenic cardiomyopathy, often of right ventricular predominance. Here, we use a range of state-of-the-art methods and a cardiomyocyte-specific, tamoxifen-activated, PKP2 knockout mouse to demonstrate that in addition to its role in cell adhesion, PKP2 is necessary to maintain transcription of genes that control intracellular calcium cycling. Lack of PKP2 reduces expression of Ryr2 (coding for Ryanodine Receptor 2), Ank2 (coding for Ankyrin-B), Cacna1c (coding for CaV1.2) and Trdn (coding for triadin), and protein levels of calsequestrin-2 (Casq2). These factors combined lead to disruption of intracellular calcium homeostasis and isoproterenol-induced arrhythmias that are prevented by flecainide treatment. We propose a previously unrecognized arrhythmogenic mechanism related to PKP2 expression and suggest that mutations in PKP2 in humans may cause life-threatening arrhythmias even in the absence of structural disease., It is believed that mutations in desmosomal adhesion complex protein plakophilin 2 (PKP2) cause arrhythmia due to loss of cell-cell communication. Here the authors show that PKP2 controls the expression of proteins involved in calcium cycling in adult mouse hearts, and that lack of PKP2 can cause arrhythmia in a structurally normal heart.

BACKGROUND: Plakophilin-2 (PKP2) is classically defined as a desmosomal protein. Mutations in PKP2 associate with most cases of gene-positive arrhythmogenic right ventricular cardiomyopathy (ARVC). A better understanding of PKP2 cardiac biology can help elucidate the mechanisms underlying arrhythmic and cardiomyopathic events consequent to PKP2 deficiency. Here, we sought to capture early molecular/cellular events that can act as nascent arrhythmic/cardiomyopathic substrates. METHODS: We used multiple imaging, biochemical and high-resolution mass spectrometry methods to study functional/structural properties of cells/tissues derived from cardiomyocyte-specific, tamoxifen-activated, PKP2 knockout mice (“PKP2cKO”) 14 days post-tamoxifen (post-TAM) injection, a time point preceding overt electrical or structural phenotypes. Myocytes from right or left ventricular free wall were studied separately. RESULTS: Most properties of PKP2cKO left ventricular (PKP2cKO-LV) myocytes were not different from control; in contrast, PKP2cKO right ventricular (PKP2cKO-RV) myocytes showed increased amplitude and duration of Ca(2+) transients, increased [Ca(2+)] in the cytoplasm and sarcoplasmic reticulum (SR), increased frequency of spontaneous Ca(2+) release events (sparks) even at comparable SR load, and dynamic Ca(2+) accumulation in mitochondria. We also observed early- and delayed-after transients in RV myocytes and heightened susceptibility to arrhythmias in Langendorff-perfused hearts. In addition, RyR2 in PKP2cKO-RV cells presented enhanced Ca(2+) sensitivity and preferential phosphorylation in a domain known to modulate Ca(2+) gating. RNAseq at 14 days post-TAM showed no relevant difference in transcript abundance between RV and LV, neither in control nor in PKP2cKO cells. Instead, we found an RV-predominant increase in membrane permeability that can permit Ca(2+) entry into the cell. Cx43 ablation mitigated the membrane permeability increase, accumulation of cytoplasmic Ca(2+), increased frequency of sparks and early stages of RV dysfunction. Cx43 hemichannel block with GAP19 normalized [Ca(2+)](i) homeostasis. Similarly, PKC inhibition normalized spark frequency at comparable SR load levels. CONCLUSIONS: Loss of PKP2 creates an RV-predominant arrhythmogenic substrate (Ca(2+) dysregulation) that precedes the cardiomyopathy; this is, at least in part, mediated by a Cx43-dependent membrane conduit and repressed by PKC inhibitors. Given that asymmetric Ca(2+) dysregulation precedes the cardiomyopathic stage, we speculate that abnormal Ca(2+) handling in RV myocytes can be a trigger for gross structural changes observed at a later stage.

OBJECTIVE: Cardiac catheter cryoablation is a safer alternative to radiofrequency ablation for arrhythmia treatment, but electrophysiological (EP) effects during and after freezing are not adequately characterized. The goal of this study was to determine transient and permanent temperature induced EP effects, during and after localized tissue freezing. METHODS: Conduction in right (RV) and left ventricles (LV) was studied by optical activation mapping during and after cryoablation in paced, isolated Langendorff-perfused porcine hearts. Cryoablation was performed endocardially (n=4) or epicardially (n=4) by a cryoprobe cooled to −120 °C for 8 minutes. Epicardial surface temperature was imaged with an infrared camera. Viability staining was performed after ablation. Motion compensation and co-registration was performed between optical mapping data, temperature image data, and lesion images. RESULTS: Cryoablation produced lesions 14.9 +/− 3.1 mm in diameter and 5.8 +/− 1.7 mm deep. A permanent lesion was formed in tissue cooled below −5 +/− 4 °C. Transient EP changes observed at temperatures between 17 and 37 °C during cryoablation surrounding the frozen tissue region directly correlated with local temperature, and include action potential (AP) duration prolongation, decrease in AP magnitude, and slowing in conduction velocity (Q10=2.0). Transient conduction block was observed when epicardial temperature reached 17 °C). SIGNIFICANCE: The observed changes explain effects observed during clinical cryoablation, including transient increases in effective refractory period, transient conduction block, and transient slowing of conduction. The presented quantitative data on temperature dependence of EP effects may enable the prediction of the effects of clinical cryoablation devices.

SCN5A encodes the α subunit of the major cardiac sodium channel Na(V)1.5. Mutations in SCN5A are associated with conduction disease and ventricular fibrillation (VF); however, the mechanisms that link loss of sodium channel function to arrhythmic instability remain unresolved. Here, we generated a large-animal model of a human cardiac sodium channelopathy in pigs, which have cardiac structure and function similar to humans, to better define the arrhythmic substrate. We introduced a nonsense mutation originally identified in a child with Brugada syndrome into the orthologous position (E558X) in the pig SCN5A gene. SCN5A(E558X/+) pigs exhibited conduction abnormalities in the absence of cardiac structural defects. Sudden cardiac death was not observed in young pigs; however, Langendorff-perfused SCN5A(E558X/+) hearts had an increased propensity for pacing-induced or spontaneous VF initiated by short-coupled ventricular premature beats. Optical mapping during VF showed that activity often began as an organized focal source or broad wavefront on the right ventricular (RV) free wall. Together, the results from this study demonstrate that the SCN5A(E558X/+) pig model accurately phenocopies many aspects of human cardiac sodium channelopathy, including conduction slowing and increased susceptibility to ventricular arrhythmias.

Fever is a highly conserved systemic response to infection dating back over 600 million years. Although conferring a survival benefit, fever can negatively impact the function of excitable tissues, such as the heart, producing cardiac arrhythmias. Here we show that mice lacking fibroblast growth factor homologous factor 2 (FHF2) have normal cardiac rhythm at baseline, but increasing core body temperature by as little as 3 °C causes coved-type ST elevations and progressive conduction failure that is fully reversible upon return to normothermia. FHF2-deficient cardiomyocytes generate action potentials upon current injection at 25 °C but are unexcitable at 40 °C. The absence of FHF2 accelerates the rate of closed-state and open-state sodium channel inactivation, which synergizes with temperature-dependent enhancement of inactivation rate to severely suppress cardiac sodium currents at elevated temperatures. Our experimental and computational results identify an essential role for FHF2 in dictating myocardial excitability and conduction that safeguards against temperature-sensitive conduction failure., Fever is a defence mechanism against infection, but it may also cause abnormal heart rhythm via unknown mechanism. Here the authors identify FHF2 protein as a key regulator of myocardial excitability that protects the heart against conduction failure in response to an increase in body temperature.

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. © 2011 Elsevier Ltd.

Studies have demonstrated non-myocytes, including fibroblasts, can electrically couple to myocytes in culture. However, evidence demonstrating current can passively spread across scar tissue in the intact heart remains elusive. We hypothesize electrotonic conduction occurs across non-myocyte gaps in the heart and is partly mediated by Connexin43 (Cx43). We investigated whether non-myocytes in ventricular scar tissue are electrically connected to surrounding myocardial tissue in wild type and fibroblast-specific protein-1 driven conditional Cx43 knock-out mice (Cx43fsp1KO). Electrical coupling between the scar and uninjured myocardium was demonstrated by injecting current into the myocardium and recording depolarization in the scar through optical mapping. Coupling was significantly reduced in Cx43fsp1KO hearts. Voltage signals were recorded using microelectrodes from control scars but no signals were obtained from Cx43fsp1KO hearts. Recordings showed significantly decreased amplitude, depolarized resting membrane potential, increased duration and reduced upstroke velocity compared to surrounding myocytes, suggesting that the non-excitable cells in the scar closely follow myocyte action potentials. These results were further validated by mathematical simulations. Optical mapping demonstrated that current delivered within the scar could induce activation of the surrounding myocardium. These data demonstrate non-myocytes in the scar are electrically coupled to myocytes and coupling depends on Cx43 expression.

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 γ 1 (PPARγ1) via the cardiac α-myosin heavy-chain promoter. The PPARγ 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 PPARγ agonist, had no effect on mortality or rhythm in WT mice but further exacerbated the arrhythmic phenotype and increased the mortality in the PPARγ transgenic mice. Conclusions— Our findings support an important link between PPARγ activation, cardiomyocyte lipid accumulation, ion channel remodeling, and increased cardiac mortality.

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. APD70 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.

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.

Ventricular preexcitation, which characterizes Wolff-Parkinson-White syndrome, is caused by the presence of accessory pathways that can rapidly conduct electrical impulses from atria to ventricles, without the intrinsic delay characteristic of the atrioventricular (AV) node. Preexcitation is associated with an increased risk of tachyarrhythmia, palpitations, syncope, and sudden death. Although the pathology and electrophysiology of preexcitation syndromes are well characterized, the developmental mechanisms are poorly understood, and few animal models that faithfully recapitulate the human disorder have been described. Here we show that activation of Notch signaling in the developing myocardium of mice can produce fully penetrant accessory pathways and ventricular preexcitation. Conversely, inhibition of Notch signaling in the developing myocardium resulted in a hypoplastic AV node, with specific loss of slow-conducting cells expressing connexin-30.2 (Cx30.2) and a resulting loss of physiologic AV conduction delay. Taken together, our results suggest that Notch regulates the functional maturation of AV canal embryonic myocardium during the development of the specialized conduction system. Our results also show that ventricular preexcitation can arise from inappropriate patterning of the AV canal–derived myocardium.

Gap junction redistribution and reduced expression, a phenomenon termed gap junction remodeling (GJR), is often seen in diseased hearts and may predispose toward 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 electrophysiological effects of GJR. We paced wild-type (normal Cx43 abundance) and heterozygous Cx43 knockout (Cx43+/−; 66% mean reduction in Cx43) mice for 6 h at 10–15% above their average sinus rate. We investigated the electrophysiological 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 ( Iss) and inward rectifier ( IK1) potassium currents compared with those of wild-type littermate cells. In Cx43+/− hearts, pacing resulted in a significant prolongation of ventricular effective refractory period and APD and significant diminution of Iss compared with unpaced Cx43+/− hearts. However, these changes were not seen in paced wild-type 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 electrophysiological changes, which may contribute to the worsened prognosis often associated with pacing in the failing heart.

Myocardial injuries often lead to fibrotic deposition. This review presents evidence supporting the concept that fibroblasts in the heart electrically couple to myocytes.

Monitoring and controlling cardiac myocyte activity with optogenetic tools offer exciting possibilities for fundamental and translational cardiovascular research. Genetically encoded voltage indicators may be particularly attractive for minimal invasive and repeated assessments of cardiac excitation from the cellular to the whole heart level.To test the hypothesis that cardiac myocyte-targeted voltage-sensitive fluorescence protein 2.3 (VSFP2.3) can be exploited as optogenetic tool for the monitoring of electric activity in isolated cardiac myocytes and the whole heart as well as function and maturity in induced pluripotent stem cell-derived cardiac myocytes.We first generated mice with cardiac myocyte-restricted expression of VSFP2.3 and demonstrated distinct localization of VSFP2.3 at the t-tubulus/junctional sarcoplasmic reticulum microdomain without any signs for associated pathologies (assessed by echocardiography, RNA-sequencing, and patch clamping). Optically recorded VSFP2.3 signals correlated well with membrane voltage measured simultaneously by patch clamping. The use of VSFP2.3 for human action potential recordings was confirmed by simulation of immature and mature action potentials in murine VSFP2.3 cardiac myocytes. Optical cardiograms could be monitored in whole hearts ex vivo and minimally invasively in vivo via fiber optics at physiological heart rate (10 Hz) and under pacing-induced arrhythmia. Finally, we reprogrammed tail-tip fibroblasts from transgenic mice and used the VSFP2.3 sensor for benchmarking functional and structural maturation in induced pluripotent stem cell-derived cardiac myocytes.We introduce a novel transgenic voltage-sensor model as a new method in cardiovascular research and provide proof of concept for its use in optogenetic sensing of physiological and pathological excitation in mature and immature cardiac myocytes in vitro and in vivo.

the initial phase of cardiac repolarization (phase 1 in humans) is mediated by the transient outward K+ current (Ito). Ito is composed of the Ca2+-independent K+ current (Ito1) and a Ca2+-activated chloride current (Ito2), both of which are voltage dependent (3). Ito1 is further subdivided into fast (Ito,f) and slow (Ito,s) (3). Ito,f is mediated by Kv4 channels that form complexes with K+ channel-interacting protein 2 (KChIP2) proteins in the majority of cardiomyocytes. Kv4 is the pore-forming unit, and KChIP2 is an accessory K+ channel-interacting protein residing on the cytoplasmic side (10, 12). The discovery of KChIP2 proteins in neurons (1) led to the generation of knockout mouse models of KChIP2 to study the role of this protein in cardiac electrophysiology through loss of function (9). Removal of KChIP2 in mice leads to disappearance of Ito,f. More recently, it was discovered that KChIP2 proteins also bind to Cav1.2, which is responsible for the L-type Ca2+ current (ICa,L) (15). In KChIP2−/− mouse cardiomyocytes, ICa,L is reduced but Cav1.2 protein levels are unchanged (15). Therefore, KChIP2 proteins are capable of modulating various ionic currents that determine the action potential plateau and regulate contractility. Action potential duration (APD) prolongation is widely reported to occur in many forms of heart failure. A reduction in Ito has been suggested to be a main contributing factor responsible for the prolongation of APD. KChIP2 expression has been also reported to be significantly reduced in human and animal models of cardiac hypertrophy and heart failure. Changes in KChIP2 expression have been reported to increase the trafficking of Ito channels to the cell membrane and shift the voltage dependence of activation to more negative values. The net result of these changes indicates that KChIP2 can modulate the amplitude of Ito more than eightfold. On the other hand, studies that have been designed to determine the ability of Ito to modulate APD have been somewhat conflicting. Depending on ICa,L levels, decreases in Ito can result in either increased or decreased APD (16). While other studies have reported increases in Ito have little or no effect on APD (11). Changes in Ito that occur with cardiac disease are often accompanied by changes in ICa,L. One potential explanation for the varied effects of Ito on APD could be related to complex interactions of these currents where the net effect on APD can vary. It is also possible that KChiP2 interactions with the channels responsible for Ito and ICa,L contribute to the net effects on cardiac function and APD. In a study by Grubb and colleagues (5) the role of KChIP2 in determining cardiomyocyte Ca2+ transients and how this affects cardiac function were investigated. Specifically, the study focused on finding the mechanisms responsible for the maintenance of cardiac function in the absence of KChIP2, given that both Ito,f and ICa,L are reduced. The authors confirmed previous reports that cardiac function is unchanged in KChIP2 knockout mice (9, 13) and that absence of KChIP2 in isolated myocytes decreased the peak inward Ca2+ current without changing the expression levels of the protein or kinetics responsible for ICa,L (15). Given that reduction in ICa,L might lead to decreased intracellular Ca2+ concentration transients and decreased contraction (7), the authors turned to computational and experimental studies to determine how KChIP2 deficiency might affect Ca2+-induced Ca2+-release (CICR). They addressed the role CICR by measuring synchronization between Ca2+ channels and ryanodine receptor (RyR) coupling, RyR sensitivity to Ca2+, sarcoplasmic reticulum Ca2+ load, fractional Ca2+ release, and rate of decay. Surprisingly none of those was decreased in KChIP−/− myocytes; furthermore, they report increased Ca2+ release with β-adrenergic stimulation, suggesting augmented sensitivity of the RyR. In view of these findings and on simulation data, the authors attribute preserved contractility to a prolongation in APD secondary to the loss of Ito,f. APD prolongation would provide sufficient time for intracellular Ca2+ concentration to rise in the presence of reduced ICa,L. Although this study provided new information on the effects of KChIP2 activity and Ca2+ transients, a few important issues worth mentioning remain unresolved. In particular, conflicting APD data have been reported in the KChIP2−/− mice. Initially it was reported that myocytes isolated from KChIP2−/− mice had increased APD (9). More recently, it was shown that differences in APD between isolated wild-type and KChIP2−/− myocytes were only observed at room temperature but not at physiological temperatures due to an increase in Ito,s (2, 9, 14). These results are in agreement with Thomsen et al. (14) who observed prolonged APD only in KChIP2−/− myocytes that had blocked Ito,s with 4-aminopyridine. In the current study, APD was measured at physiological temperature in isolated myocytes and was longer than wild-type controls. Microelectrode measurements obtained from the epicardial left ventricular surface of intact hearts were performed by Grubb et al. (6) in a previous study and were also prolonged in KChIP2−/− compared with wild-type mice. It remains to be determined if differences observed in KChIP2 effects on APD are due to a heterogeneity of the cardiomyocyte population being assessed. Another important issue is related to the proteins that interact with Kv4. Many other proteins have been found in ventricular tissue that coimmunoprecipitate with Kv4 and modulate either its localization at the membrane surface and/or its channel kinetics (10). Interestingly, these include Ca2+/calmodulin-dependent kinase 2 (CaMKII), which has been shown to have increased activity when Kv4 or KChiP2 are reduced (8). In this study, Grubb et al. report an increase in RyR sensitivity to β-adrenergic stimulation, which remains unexplained. One possible explanation might be the increased CaMKII activity, given that it has recently been shown that CaMKII increases RyR sensitivity to β-adrenergic stimulation (4). Finally, another component of CICR that might be involved in Ca2+ handling in pathological hearts and might influence RyR Ca2+ release is the Na+-Ca2+ exchanger (NCX). NCX influences [Ca2+] in the dyadic cleft, and it has been proposed that at highly positive membrane potentials NCX and ICa,L might act synergistically to trigger Ca2+ release (7). Overall the Grubb et al. study provides us with important information on the role of KChIP2 in modulating Ca2+ transients. In addition, these data help to emphasize the complex interactions between electrophysiological changes and their effects on cardiac function.

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.

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.

Abstract —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 +/ − 8.7 [8.1 to 9.3], and Cx43 −/− 1.1 [0.1 to 1.3; P +/ − 8.3 [7.8 to 8.9], and Cx43 −/− 1.7 [1.3 to 2.1; P

Abstract —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

Abstract —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 –/– 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 –/– mice (74.5±3 cm/s in WT [n=7] and 43.7±6 cm/s in Cx40 –/– [n=7]; P –/– 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 –/– mice and provide new insight into the role of gap junctions in cardiac impulse propagation.

Cardiac metabolism remains altered for an extended period of time after myocardial infarction. Studies have shown fibroblasts from normal hearts express KATP channels in culture. It is unknown whether fibroblasts from infarcted hearts express KATP channels and whether these channels contribute to scar and border zone electrophysiology. KATP channel subunit expression levels were determined in fibroblasts isolated from normal hearts (Fb), and scar (sMI-Fb) and remote (rMI-Fb) regions of left anterior descending coronary artery (LAD) ligated rat hearts. Whole cell KATP current density was determined with patch clamp. Action potential duration (APD) was measured with optical mapping in myocyte-only cultures and heterocellular cultures with fibroblasts with and without 100 μmol/l pinacidil. Whole heart optical mapping was used to assess KATP channel activity following LAD ligation. Pinacidil activated a potassium current (35.4 ± 7.5 pA/pF at 50 mV) in sMI-Fb that was inhibited with 10 μmol/l glibenclamide. Kir6.2 and SUR2 transcript levels were elevated in sMI-Fb. Treatment with Kir6.2 short interfering RNA decreased KATP currents (87%) in sMI-Fb. Treatment with pinacidil decreased APD (26%) in co-cultures with sMI-Fb. APD values were prolonged in LAD ligated hearts after perfusion with glibenclamide. KATP channels are present in fibroblasts from the scar and border zones of infarcted hearts. Activation of fibroblast KATP channels could modulate the electrophysiological substrate beyond the acute ischemic event. Targeting fibroblast KATP channels could represent a novel therapeutic approach to modify border zone electrophysiology after cardiac injury.

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.

Fibroblasts play a major role in normal cardiac physiology and in the response of the heart to injury and disease. Cardiac electrophysiological research has primarily focused on the mechanisms of remodeling that accompany cardiac disease with an emphasis on myocyte electrophysiology. Recently, there has been increasing interest in the potential role of fibroblasts in cardiac electrophysiology. This review focuses on the arrhythmia mechanisms involving interactions between myocytes and fibroblasts. We also discuss the available evidence supporting the contribution of intracardiac and extracardiac sources to the fibroblast and myofibroblast populations in diseased hearts.

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.

Hypertension is associated with the development of atrial fibrillation; however, the electrophysiological consequences of this condition remain poorly understood. ATP-sensitive K+ (KATP) channels, which contribute to ventricular arrhythmias, are also expressed in the atria. We hypothesized that salt-induced elevated blood pressure (BP) leads to atrial KATP channel activation and increased arrhythmia inducibility. Elevated BP was induced in mice with a high-salt diet (HS) for 4 wk. High-resolution optical mapping was used to measure atrial arrhythmia inducibility, effective refractory period (ERP), and action potential duration at 90% repolarization (APD90). Excised patch clamping was performed to quantify KATP channel properties and density. KATP channel protein expression was also evaluated. Atrial arrhythmia inducibility was 22% higher in HS hearts compared with control hearts. ERP and APD90 were significantly shorter in the right atrial appendage and left atrial appendage of HS hearts compared with control hearts. Perfusion with 1 μM glibenclamide or 300 μM tolbutamide significantly decreased arrhythmia inducibility and prolonged APD90 in HS hearts compared with untreated HS hearts. KATP channel density was 156% higher in myocytes isolated from HS animals compared with control animals. Sulfonylurea receptor 1 protein expression was increased in the left atrial appendage and right atrial appendage of HS animals (415% and 372% of NS animals, respectively). In conclusion, KATP 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.

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.

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.

Cardiac cells contract and are also normally exposed to the mechanical events in their surroundings. It is now well established that both cardiac gene expression and protein synthesis are subject to regulation by mechanical forces, including stretch. For example, mechanical stretch is known to be one of the most important stimuli leading to cardiac hypertrophy,1 2 3 4 5 and recent studies indicate that cardiac myocyte hypertrophy is stimulated in vitro by specific directions and degrees of stretch.6 Similarly, certain stretch-sensitive sarcolemmal ion channels and exchangers have been found in cardiac myocytes7 and have been implicated in the mechanism of stretch-induced arrhythmias.8 However, whereas signal transduction induced by mechanical stretch involves activation of a wide variety of second messenger systems,9 it remains to be determined which molecules are directly affected by stretch and which are the processes whereby mechanical stimuli trigger intracellular signaling pathways to activate protein kinase cascades and produce changes in function. The process of filling and ejecting blood subjects the cells of the heart to repetitive pulsatile stress. Our understanding of the basic electrophysiology underlying the cardiac action potential and its propagation across cells is largely on the basis of patch clamp data and isolated tissue experiments in the absence of mechanical stress. On the other hand, whole-heart electrophysiological mapping studies are often carried out in in situ functioning hearts. In either case, the role of mechanical stress in impulse initiation and propagation has not been adequately addressed. In addition, although stretch is thought to play an important role in cardiac remodeling that is associated with heart failure, very little is known about its role in normal electrical function. The study by Zhuang et al10 in this issue of Circulation Research , which is the result of a successful collaboration between …

We would like to thank Drs Gourdie and Sedmera for their comments about our recent study on the role of bone morphogenic protein (BMP) signaling in atrioventricular (AV) node function.1 We agree that the mechanism responsible for the “split” or “twin” AV nodes we observed in Class IV hearts is as yet unknown. Our data suggest that disruption of BMP signaling affects normal fibrotic deposition, but it was not our contention that this was the primary means of abnormal AV node morphology. In the Discussion section we presented 2 alternative mechanisms (outlined on pages 2541 to 2542), “… BMP signaling is required to maintain proper continuity between the AV node and …

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.

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.

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 α-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.

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. Conclusion— 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.

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.

Consequences of cardiac myocyte-specific ablation of KATP channels in transgenic mice expressing dominant negative Kir6 subunits. Am J Physiol Heart Circ Physiol 291: H543–H551, 2006. First published February 24, 2006; doi:10.1152/ajpheart.00051.2006.—Cardiac ATP-sensitive K+ (KATP) channels are formed by Kir6.2 and SUR2A subunits. We produced transgenic mice that express dominant negative Kir6.x pore-forming subunits (Kir6.1-AAA or Kir6.2-AAA) in cardiac myocytes by driving their expression with the α-myosin heavy chain promoter. Weight gain and development after birth of these mice were similar to nontransgenic mice, but an increased mortality was noted after the age of 4–5 mo. 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 EADs after 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 proarrhythmogenic features and lack of adaptation to a stress response in transgenic mice suggest 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.

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 −/− 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.

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 ≈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.

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. ispartof: Circulation Research vol:97 issue:3 pages:219-26 ispartof: location:United States status: published

Genetic studies in the mouse have demonstrated that conditional cardiac-restricted 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 wild-type 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 change of pressure over time, P < 0.01). Chimeras had significantly more spontaneous arrhythmic events than controls ( P < 0.01), including frequent runs of nonsustained 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.

Atrial fibrillation (AF) is the most common supraventricular tachycardia, affecting over 2 million patients, and is associated with 100,000 new strokes per year.1 Data from large, long-term epidemiologic studies such as the Framing-ham Heart Study show a strong link between AF, stroke, congestive heart failure, and mortality.2 In addition, the expected rise in the elderly population in Western countries will cause an increase in the clinical burden of AF.3 Historically, AF has been treated by pharmacological rate and/or rhythm control in combination with chronic oral anticoagulation medication to reduce the risk of embolic phenomena. Large-scale randomized control trials have shown that rhythm control with antiarrhythmic drugs is not superior to a strategy of rate control, most likely owing to the proarrhythmic risk and poor efficacy of contemporary antiarrhythmic drugs.4 These findings have increased the need for better pharmacological approaches for the treatment of AF. Current pharmacological options for the management of AF span all Vaughn-Williams classes of antiarrhythmic drugs. For patients who tolerate AF, rate control is typically achieved with beta-blockers (class II) and calcium channel blockers (class IV). Symptomatic patients with AF routinely require the addition of sodium channel blockers (class I) or potassium channel blockers (class III) to achieve rhythm control. The proarrhythmic risk profile of class I and class III agents has largely limited their use in patients with structural heart disease or baseline QT interval prolongation. Of these agents, amiodarone has consistently been the most effective in maintaining sinus rhythm.5,6 However, the risks of long-term amiodarone use are substantial and include numerous drug-drug interactions and end-organ damage, particularly pulmonary, hepatic, and thyroid toxicity. The need for more effective rhythm control agents with improved safety profiles led to the development of dronedarone. Dronedarone is a class III antiarrhythmic drug that was recently approved as an alternative to amiodarone for the treatment of AF and atrial flutter. Structural modifications to dronedarone, including the lack of an iodine moiety and the addition of a methanesulfonyl group, were hypothesized to decrease the thyroid and pulmonary toxicities associated with amiodarone.7 The antiarrhythmic mechanisms of action remain largely unclear. Similar to amiodarone, dronedarone is a multichannel blocker that affects several outward currents including the rapid (IKr) and slow (IKs) components of the delayed rectifier and the acetylcholine sensitive potassium currents (IACh) and may also reduce the inward currents including the sodium (INa) and L-type calcium currents (ICaL). Initial studies demonstrated the superiority of dronedarone over placebo in the maintenance of sinus rhythm in patients with paroxysmal and persistent AF or atrial flutter.8–10 As a rate control agent, dronedarone has been shown to reduce the mean 24-hour ventricular rate response during recurrent or permanent AF, and the ventricular rate response during peak exercise.8,10,11 The mechanisms responsible for the reduction in ventricular rate remain unclear. However, recent data indicating dronedarone suppresses pacemaker currents including the funny current (If) in the sinoatrial node have emerged. If is an inward current that is found throughout the specialized conduction system and contributes to atrioventricular (AV) nodal conduction. Determining whether dronedarone has similar effects on If in the AV node could provide important information on the mechanisms responsible for heart rate slowing during AF. In this issue of HeartRhythm, Verrier et al12 provide a series of new and interesting experiments exploring the effects of dronedarone on baseline electrophysiological parameters and ventricular rates during AF by using a porcine model. The authors studied intact anesthetized animals and induced AF with a combination of acetylcholine infusion and burst pacing. Consistent with the known effects of dronedarone, at baseline the drug increased PR and QT intervals as well as atrial and ventricular refractory periods. The authors then performed a series of studies comparing the effects of the If blocker, ivabradine alone, and a combination of ivabradine and dronedarone on ventricular rates during induced AF. Both drugs were shown to have no effect on mean arterial pressure, AF duration, or AF dominant frequencies. The authors reported that ivabradine significantly reduces ventricular rates during AF than do ventricular rates obtained during AF at baseline. These data support the interesting concept that blocking AV nodal If slows AV conduction, resulting in lower ventricular rates during AF. The authors then tested whether a combination of ivabradine and dronedarone would cause any additional reduction in ventricular rates during AF. The data obtained during the combined ivabradine and dronedarone administration were not significantly different compared with the ivabradine-alone studies. Since the authors did not find either drug-affected AF dynamics, the reduction of ventricular rate appears to be due to slowing of conduction within the AV node. Although indirect, these data for the first time implicate AV nodal If as an important mechanism in mediating the heart rate slowing effects of dronedarone during AF. In addition to the limitations mentioned by the authors, there are a few additional comments that are worth considering. Although ivabradine has potent If blocking properties, it should be noted that it is not a pure If blocker. Studies have shown that ivabradine can partially block the delayed rectified and L-type calcium currents. These effects tend to be more prominent at concentrations higher than those used by Verrier et al.12 Thus, the potential involvement of other AV nodal ionic currents in reducing ventricular rates during AF should not be excluded. The authors also did not report the effects of dronedarone or ivabradine on AV nodal electrophysiological parameters such as Wenckebach cycle length, A–H intervals, and AV nodal effective refractory periods. These data could have added information implicating AV nodal conduction and possibly If more directly. Finally, as the authors indicated in their Introduction, it should be emphasized that several studies have demonstrated that dronedarone is not suitable for all patients with AF. The ANDROMEDA trial demonstrated increased morality with the use of dronedarone in patients with AF and significant heart failure.13 Furthermore, the results of the PALLAS study, which evaluated dronedarone as a rate control agent in high-risk patients with permanent AF, showed increased cardiovascular events in the dronedarone arm.14 These results prompted the Food and Drug Administration to issue a black box warning that dronedarone is contraindicated in patients with New York Heart Association class IV heart failure or New York Heart Association class II–III heart failure with recent decompensation requiring hospitalization and in patients with permanent AF. Finally, an intriguing finding of the Verrier et al12 study is that the heart rate slowing properties of ivabradine during AF is substantial and may be equivalent to dronedarone. Recent studies have suggested that the suppression of If in the pulmonary veins can suppress atrial ectopic activity and possibly AF. Given the additional evidence provided by Verrier et al12 that If blockade also significantly reduces ventricular rates during AF, it may be worth considering whether If blocking drugs may be effective for both rate and rhythm control. Overall, Verrier et al12 should be congratulated for their excellent study and for adding important new information on the antiarrhythmic properties of dronedarone.

Connexin43 (Cx43), the predominant ventricular gap junction protein, is critical for maintaining normal cardiac electrical conduction, and its absence in the mouse heart results in sudden arrhythmic death. The mechanisms linking reduced Cx43 abundance in the heart and inducibility of malignant ventricular arrhythmias have yet to be established. In this report, we investigate arrhythmic susceptibility in a murine model genetically engineered to express progressively decreasing levels of Cx43. Progressively older cardiac-restricted Cx43 conditional knockout (CKO) mice were selectively bred to produce a heart-specific Cx43-deficient subline (“O-CKO” mice) in which the loss of Cx43 in the heart occurs more gradually. O-CKO mice lived significantly longer than the initial series of CKO mice but still died suddenly and prematurely. At 25 days of age, cardiac Cx43 protein levels decreased to 59% of control values ( P

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.

Programmed electrical stimulation (PES) is a crucial aspect of the evaluation of the risk of arrhythmias in cardiac patients and provides a powerful tool for understanding the mechanisms of arrhythmia in experimental models. Whereas PES in the mouse is well characterized, the procedures allowing for follow-up studies in the same animal have not been developed. In this report, we describe a novel subdiaphragmatic approach that allows for repeat electrophysiological studies in the mouse. Under inhaled anesthesia, PES was performed in 36 wild-type mice via a stimulating electrode introduced through an epigastric incision and placed directly into the diaphragmatic surface of the heart. The procedure was repeated 7 days later. Ventricular effective refractory periods (VERP) did not change significantly between the initial and follow-up trials. Chronic treatment with amiodarone, however, was associated with a 70% prolongation in VERP from initial to follow-up studies ( P ≤ 0.001). In addition, PES of a genetically modified strain with sudden cardiac death, the connexin43 conditional knockout mouse consistently induced lethal polymorphic ventricular tachycardia. Thus sequential PES in mice is feasible with the use of a subdiaphragmatic approach, yields reproducible VERP values, and can be used to follow pharmacologically induced changes in VERP and identify mice at risk of lethal ventricular arrhythmias.

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.

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.

Abstract —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 ≈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 ≈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 (CV×APD) 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 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.