The relationship between cardiac conduction velocity (CV) and extracellular potassium (K+) is biphasic, with modest hyperkalemia increasing CV and severe hyperkalemia slowing CV. Recent studies from our group suggest that elevating extracellular sodium (Na+) and calcium (Ca2+) can enhance CV by an extracellular pathway parallel to gap junctional coupling (GJC) called ephaptic coupling that can occur in the gap junction adjacent perinexus. However, it remains unknown whether these same interventions modulate CV as a function of K+. We hypothesize that Na+, Ca2+, and GJC can attenuate conduction slowing consequent to severe hyperkalemia. Elevating Ca2+ from 1.25 to 2.00 mM significantly narrowed perinexal width measured by transmission electron microscopy. Optically mapped, Langendorff-perfused guinea pig hearts perfused with increasing K+ revealed the expected biphasic CV-K+ relationship during perfusion with different Na+ and Ca2+ concentrations. Neither elevating Na+ nor Ca2+ alone consistently modulated the positive slope of CV-K+ or conduction slowing at 10-mM K+; however, combined Na+ and Ca2+ elevation significantly mitigated conduction slowing at 10-mM K+. Pharmacologic GJC inhibition with 30-μM carbenoxolone slowed CV without changing the shape of CV-K+ curves. A computational model of CV predicted that elevating Na+ and narrowing clefts between myocytes, as occur with perinexal narrowing, reduces the positive and negative slopes of the CV-K+ relationship but do not support a primary role of GJC or sodium channel conductance. These data demonstrate that combinatorial effects of Na+ and Ca2+ differentially modulate conduction during hyperkalemia, and enhancing determinants of ephaptic coupling may attenuate conduction changes in a variety of physiologic conditions.

Many cardiac pathologies are associated with reduced gap junction (GJ) coupling, an important modulator of cardiac conduction velocity (CV). However, the relationship between phenotype and functional expression of the connexin GJ family of proteins is controversial. For example, a 50% reduction of GJ coupling has been shown to have little impact on myocardial CV due to a concept known as conduction reserve. This can be explained by the ephaptic coupling (EpC) theory whereby conduction is maintained by a combination of low GJ coupling and increased electrical fields generated in the sodium channel rich clefts between neighboring myocytes. At the same time, low GJ coupling may also increase intracellular charge accumulation within myocytes, resulting in a faster transmembrane potential rate of change during depolarization (dV/dt_max) that maintains macroscopic conduction. To provide insight into the prevalence of these two phenomena during pathological conditions, we investigated the relationship between EpC and charge accumulation within the setting of GJ remodeling using multicellular simulations and companion perfused mouse heart experiments. Conduction along a fiber of myocardial cells was simulated for a range of GJ conditions. The model incorporated intercellular variations, including GJ coupling conductance and distribution, cell-to-cell separation in the intercalated disc (perinexal width—WP), and variations in sodium channel distribution. Perfused heart studies having conditions analogous to those of the simulations were performed using wild type mice and mice heterozygous null for the connexin gene Gja1. With insight from simulations, the relative contributions of EpC and charge accumulation on action potential parameters and conduction velocities were analyzed. Both simulation and experimental results support a common conclusion that low GJ coupling decreases and narrowing WP increases the rate of the AP upstroke when sodium channels are densely expressed at the ends of myocytes, indicating that conduction reserve is more dependent on EpC than charge accumulation during GJ uncoupling.

Background: Cardiac voltage-gated sodium channel gain-of-function prolongs repolarization in the Long-QT Syndrome Type 3 (LQT3). Previous studies suggest that narrowing the perinexus within the intercalated disc, leading to rapid sodium depletion, attenuates LQT3-associated action potential duration (APD) prolongation. However, it remains unknown whether extracellular sodium concentration modulates APD prolongation during sodium channel gain-of-function. We hypothesized that elevated extracellular sodium concentration and widened perinexus synergistically prolong APD in LQT3. Methods and Results: LQT3 was induced with anemone toxin type II (ATXII) in Langendorff-perfused guinea pig hearts (n=20). Sodium concentration was increased from 145 to 160 mM. Perinexal expansion was induced with mannitol or the sodium channel β1-subunit adhesion domain antagonist (βadp1). Epicardial ventricular action potentials were optically mapped. Individual and combined effects of varying clefts and sodium concentrations were simulated in a computational model. With ATXII, both mannitol and βadp1 significantly widened the perinexus and prolonged APD, respectively. The elevated sodium concentration alone significantly prolonged APD as well. Importantly, the combination of elevated sodium concentration and perinexal widening synergistically prolonged APD. Computational modeling results were consistent with animal experiments. Conclusions: Concurrently elevating extracellular sodium and increasing intercalated disc edema prolongs repolarization more than the individual interventions alone in the LQT3. This synergistic effect suggests an important clinical implication that hypernatremia in the presence of cardiac edema can markedly increase LQT3-associated APD prolongation. Therefore, this is the first study to provide evidence of a tractable and effective strategy to mitigate LQT3 phenotype by managing patient sodium levels and preventing cardiac edema.

Viral cardiac infection represents a significant clinical challenge encompassing several etiological agents, disease stages, complex presentation, and a substantial lack of mechanistic understanding. Myocarditis is a leading cause of sudden cardiac death in young adults, for which viral infection is the most common cause. Current knowledge in the field is dominated by later phases of disease, and the role of host immune responses in pathogenesis. However, little is known regarding how viral infection can acutely induce an arrhythmogenic substrate in the heart, and how this may create an already dangerous substrate prior to the immune response. Viruses are known to alter gap junction intercellular communication in various cell types, leading us to investigate the impact of infection on cardiomyocyte coupling and electrophysiology. Adenovirus is a leading cause of myocarditis, but due to species-specificity, models of infection are lacking and it is not understood how adenoviruses cause sudden cardiac arrest. Mouse Adenovirus Type-3 (MAdV-3) was recently isolated and reported to be cardiotropic, yet has not been utilized to understand mechanisms of cardiac infection and pathology in the research setting. We have developed MAdV-3 infection as a model to investigate acute cardiac infection and molecular alterations to the infected heart prior to an appreciable immune response or gross cardiomyopathy. By optical mapping we find decreases in conduction velocity concomitant with increased phosphorylation of Cx43-Ser368, a residue known to regulate gap junction channel function. Additional to changes in gap junctions, patch clamping of MAdV-3-infected adult mouse ventricular cardiomyocytes reveals prolonged action potential duration as a result of decreased IK1 and IKs current density. Turning to human systems, we find that human adenovirus type-5 (HAdV-5) increases phosphorylation of Cx43Ser368 and disrupts calcium transients synchrony in human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs), indicating common mechanisms with our mouse whole heart and adult cardiomyocyte data. Together, these findings demonstrate that adenoviral infection creates an arrhythmogenic substrate through direct targeting of gap junction and ion channel function in the heart. Such alterations are known to precipitate arrhythmias and likely contribute to sudden cardiac death in acutely infected patients, while preceding the pathological remodeling of subsequent myocarditis disease progression.

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Myocarditis is responsible for 42% of sudden cardiac death in young adults, yet mechanisms underlying virally induced arrhythmia remain elusive. Adenovirus is a leading etiological agent of myocarditis but species‐specificity has limited development of animal disease models. Normal electrical impulse propagation in the heart is achieved through intercellular coupling via gap junctions, composed primarily of connexin43 (Cx43). Changes in Cx43 expression, localization, and/or function underlie the arrhythmias of sudden cardiac death. Given that gap junctions also propagate innate and adaptive antiviral immune responses, we hypothesized that adenovirus would target Cx43 to facilitate replication. Our prior work has demonstrated that Cx43 expression and function are indeed reduced rapidly during adenoviral infection limiting gap junction communication in human epithelial and cardiac cells. We are now employing a recently described cardiotropic strain of mouse adenovirus, MAdV‐3, to develop our work into whole animal studies and investigate virally‐induced alterations in cardiac electrophysiology and the molecular mechanisms of the resulting arrhythmogenic substrate. Adult mice were inoculated with MAdV‐3 and sampled after 7 days to model acute cardiac infection. We find MAdV‐3 viral genomes are specifically enriched in heart tissue, confirming cardiotropism. No cardiomyopathy was apparent by echocardiography or histopathology, consistent with human acute myocarditis patients. We find reductions in cardiac ion channel and connexin mRNA transcript levels after infection, and at the protein level, Cx43 is phosphorylated at residues known to reduce function. Ex vivo optical mapping experiments illustrate decreased conduction velocity in infected hearts. Turning to primary adult mouse cardiomyocytes, we detect prolonged action potential duration and impaired K+ current in infected cells by patch clamping. Confocal and super‐resolution localization microscopy of infected cardiac tissue and isolated cardiomyocytes reveals remodeling of gap junctions with alterations in complexing with scaffolding proteins and other ion channels at the cell‐cell junction. Finally, employing human iPSC‐derived cardiomyocytes and human adenovirus type‐5, we find increased Cx43 phosphorylation and, using optical mapping, perturbation of intercellular coupling during infection, just as we see with MAdV‐3. Our data demonstrate that reduced cellular coupling and ion channel function during adenoviral infection generates an arrhythmogenic substrate prior to an appreciable immune response or cardiomyopathy development. MAdV‐3 infection therefore provides a novel model of cardiac infection and myocarditis and provides physiologically relevant insight into mechanisms of virally‐induced arrhythmias.

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