Conduction Slowing Contributes to Spontaneous Ventricular Arrhythmias in Intrinsically Active Murine RyR2-P2328S Hearts

Cardiac arrhythmogenesis: roles of ion channels and their functional modification

Cardiac arrhythmias cause significant morbidity and mortality and pose a major public health problem. They arise from disruptions in the normally orderly propagation of cardiac electrophysiological activation and recovery through successive cardiomyocytes in the heart. They reflect abnormalities in automaticity, initiation, conduction, or recovery in cardiomyocyte excitation. The latter properties are dependent on surface membrane electrophysiological mechanisms underlying the cardiac action potential. Their disruption results from spatial or temporal instabilities and heterogeneities in the generation and propagation of cellular excitation. These arise from abnormal function in their underlying surface membrane, ion channels, and transporters, as well as the interactions between them. The latter, in turn, form common regulatory targets for the hierarchical network of diverse signaling mechanisms reviewed here. In addition to direct molecular-level pharmacological or physiological actions on these surface membrane biomolecules, accessory, adhesion, signal transduction, and cytoskeletal anchoring proteins modify both their properties and localization. At the cellular level of excitation-contraction coupling processes, Ca2+ homeostatic and phosphorylation processes affect channel activity and membrane excitability directly or through intermediate signaling. Systems-level autonomic cellular signaling exerts both acute channel and longer-term actions on channel expression. Further upstream intermediaries from metabolic changes modulate the channels both themselves and through modifying Ca2+ homeostasis. Finally, longer-term organ-level inflammatory and structural changes, such as fibrotic and hypertrophic remodeling, similarly can influence all these physiological processes with potential pro-arrhythmic consequences. These normal physiological processes may target either individual or groups of ionic channel species and alter with particular pathological conditions. They are also potentially alterable by direct pharmacological action, or effects on longer-term targets modifying protein or cofactor structure, expression, or localization. Their participating specific biomolecules, often clarified in experimental genetically modified models, thus constitute potential therapeutic targets. The insights clarified by the physiological and pharmacological framework outlined here provide a basis for a recent modernized drug classification. Together, they offer a translational framework for current drug understanding. This would facilitate future mechanistically directed therapeutic advances, for which a number of examples are considered here. The latter are potentially useful for treating cardiac, in particular arrhythmic, disease.

Arrhythmogenic mechanism of a novel ryanodine receptor mutation underlying sudden cardiac death

AIMS

The ryanodine receptor 2 (RyR2) is essential for cardiac muscle excitation-contraction coupling; dysfunctional RyR2 participates in the development of inherited arrhythmogenic cardiac disease. In this study, a novel RyR2 mutation A690E is identified from a patient with family inheritance of sudden cardiac death, and we aimed to investigate the pathogenic basis of the mutation.

METHODS AND RESULTS

We generated a mouse model that carried the A690E mutation. Mice were characterized by adrenergic-induced ventricular arrhythmias similar to clinical manifestation of the patient. Optical mapping studies revealed that isolated A690E hearts were prone to arrhythmogenesis and displayed frequency-dependence calcium transient alternans. Upon β-adrenoceptor challenge, the concordant alternans was shifted towards discordant alternans that favour triggering ectopic beats and Ca2+ re-entry; similar phenomenon was also found in the A690E cardiomyocytes. In addition, we found that A690E cardiomyocytes manifested abnormal Ca2+ release and electrophysiological disorders, including an increased sensitivity to cytosolic Ca2+, an elevated diastolic RyR2-mediated Ca2+ leak, and an imbalance between Ca2+ leak and reuptake. Structural analyses reveal that the mutation directly impacts RyR2-FK506 binding protein interaction.

CONCLUSION

In this study, we have identified a novel mutation in RyR2 that is associated with sudden cardiac death. By characterizing the function defects of mutant RyR2 in animal, whole heat, and cardiomyocytes, we demonstrated the pathogenic basis of the disease-causing mutation and provided a deeper mechanistic understanding of a life-threatening cardiac arrhythmia.

Cardiomyocyte electrophysiology and its modulation: current views and future prospects

Normal and abnormal cardiac rhythms are of key physiological and clinical interest. This introductory article begins from Sylvio Weidmann's key historic 1950s microelectrode measurements of cardiac electrophysiological activity and Singh & Vaughan Williams's classification of cardiotropic targets. It then proceeds to introduce the insights into cardiomyocyte function and its regulation that subsequently emerged and their therapeutic implications. We recapitulate the resulting view that surface membrane electrophysiological events underlying cardiac excitation and its initiation, conduction and recovery constitute the final common path for the cellular mechanisms that impinge upon this normal or abnormal cardiac electrophysiological activity. We then consider progress in the more recently characterized successive regulatory hierarchies involving Ca2+ homeostasis, excitation-contraction coupling and autonomic G-protein signalling and their often reciprocal interactions with the surface membrane events, and their circadian rhythms. Then follow accounts of longer-term upstream modulation processes involving altered channel expression, cardiomyocyte energetics and hypertrophic and fibrotic cardiac remodelling. Consideration of these developments introduces each of the articles in this Phil. Trans. B theme issue. The findings contained in these articles translate naturally into recent classifications of cardiac electrophysiological targets and drug actions, thereby encouraging future iterations of experimental cardiac electrophysiological discovery, and testing directed towards clinical management. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Advances in basic and translational research in atrial fibrillation

Atrial fibrillation (AF) is a very common cardiac arrhythmia with an estimated prevalence of 33.5 million patients globally. It is associated with an increased risk of death, stroke and peripheral embolism. Although genetic studies have identified a growing number of genes associated with AF, the definitive impact of these genetic findings is yet to be established. Several mechanisms, including electrical, structural and neural remodelling of atrial tissue, have been proposed to contribute to the development of AF. Despite over a century of exploration, the molecular and cellular mechanisms underlying AF have not been fully established. Current antiarrhythmic drugs are associated with a significant rate of adverse events and management of AF using ablation is not optimal, especially in cases of persistent AF. This review discusses recent advances in our understanding and management of AF, including new concepts of epidemiology, genetics and pathophysiological mechanisms. We review the current status of antiarrhythmic drug therapy for AF, new potential agents, as well as mechanism-based AF ablation. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

Guidelines for assessment of cardiac electrophysiology and arrhythmias in small animals

Cardiac arrhythmias are a major cause of morbidity and mortality worldwide. Although recent advances in cell-based models, including human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM), are contributing to our understanding of electrophysiology and arrhythmia mechanisms, preclinical animal studies of cardiovascular disease remain a mainstay. Over the past several decades, animal models of cardiovascular disease have advanced our understanding of pathological remodeling, arrhythmia mechanisms, and drug effects and have led to major improvements in pacing and defibrillation therapies. There exist a variety of methodological approaches for the assessment of cardiac electrophysiology and a plethora of parameters may be assessed with each approach. This guidelines article will provide an overview of the strengths and limitations of several common techniques used to assess electrophysiology and arrhythmia mechanisms at the whole animal, whole heart, and tissue level with a focus on small animal models. We also define key electrophysiological parameters that should be assessed, along with their physiological underpinnings, and the best methods with which to assess these parameters.

Clinical Characteristics, Genetic Findings and Arrhythmic Outcomes of Patients with Catecholaminergic Polymorphic Ventricular Tachycardia from China: A Systematic Review

INTRODUCTION

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare inherited cardiac ion channelopathy. The present study aims to examine the clinical characteristics, genetic basis, and arrhythmic outcomes of CPVT patients from China to elucidate the difference between CPVT patients in Asia and Western countries.

METHODS

PubMed and Embase were systematically searched for case reports or series reporting on CPVT patients from China until 19 February 2022 using the keyword: "Catecholaminergic Polymorphic Ventricular Tachycardia" or "CPVT", with the location limited to: "China" or "Hong Kong" or "Macau" in Embase, with no language or publication-type restriction. Articles that did not state a definite diagnosis of CPVT and articles with duplicate cases found in larger cohorts were excluded. All the included publications in this review were critically appraised based on the Joanna Briggs Institute Critical Appraisal Checklist. Clinical characteristics, genetic findings, and the primary outcome of spontaneous ventricular tachycardia/ventricular fibrillation (VT/VF) were analyzed.

RESULTS

A total of 58 unique cases from 15 studies (median presentation age: 8 (5.0-11.8) years old) were included. All patients, except one, presented at or before 19 years of age. There were 56 patients (96.6%) who were initially symptomatic. Premature ventricular complexes (PVCs) were present in 44 out of 51 patients (86.3%) and VT in 52 out of 58 patients (89.7%). Genetic tests were performed on 54 patients (93.1%) with a yield of 87%. RyR2, CASQ2, TERCL, and SCN10A mutations were found in 35 (71.4%), 12 (24.5%), 1 (0.02%) patient, and 1 patient (0.02%), respectively. There were 54 patients who were treated with beta-blockers, 8 received flecainide, 5 received amiodarone, 2 received verapamil and 2 received propafenone. Sympathectomy (n = 10), implantable cardioverter-defibrillator implantation (n = 8) and ablation (n = 1) were performed. On follow-up, 13 patients developed VT/VF.

CONCLUSION

This was the first systematic review of CPVT patients from China. Most patients had symptoms on initial presentation, with syncope as the presenting complaint. RyR2 mutation accounts for more than half of the CPVT cases, followed by CASQ2, TERCL and SCN10A mutations.

Effects of azumolene on arrhythmia substrate in a model of recurrent long-duration ventricular fibrillation

BACKGROUND

Proarrhythmic risk of conventional anti-arrhythmic agents is linked to unintended modulation of membrane voltage dynamics. We have demonstrated that the anti-fibrillatory effect of azumolene is mediated via stabilization of the hyperphosphorylated ryanodine receptor (RyR2), leading to attenuation of diastolic calcium leak. However, the concomitant effects on membrane voltage dynamics have not been evaluated yet.

METHODS

After baseline optical mapping, Langendorff-perfused rabbit hearts treated with azumolene, or vehicle, were subjected to global ischemia-reperfusion (I/R) followed by two episodes of long-duration ventricular fibrillation (LDVF). Simultaneous dual epicardial calcium transient (CaT) and voltage dynamics were studied optically.

RESULTS

Pre-treatment with azumolene was associated with higher CaT amplitude alternans ratios (0.94 ± 0.02 vs. 0.78 ± 0.03 in control hearts, at 6 Hz; p = 0.005; and action potential amplitude alternans ratio (0.95 ± 0.02 vs. 0.78 ± 0.04 at 6.0 Hz; p = 0.02), and reduction of action potential duration (APD₈₀) dispersion (9.0 ± 4.8 msec vs. 19.3 ± 6.6 msec at 6.0 Hz p = 0.02) and optical action potential upstroke rise time (26.3 ± 2.6 msec in control vs. 13.8 ± 0.6 msec at 6.0 Hz, p = 0.02) after LDVF. No change in action potential duration (APD) was noted with azumolene treatment.

CONCLUSION

In a model of ischemic recurrent LDVF, treatment with azumolene led to reduction of cardiac alternans, i.e., calcium and voltage alternans. Unlike conventional anti-arrhythmic agents, reduction of action potential upstroke rise time and preservation of action potential duration following azumolene treatment may reduce the proarrhythmia risk.

Ca2+ mishandling and mitochondrial dysfunction: a converging road to prediabetic and diabetic cardiomyopathy

Diabetic cardiomyopathy is defined as the myocardial dysfunction that suffers patients with diabetes mellitus (DM) in the absence of hypertension and structural heart diseases such as valvular or coronary artery dysfunctions. Since the impact of DM on cardiac function is rather silent and slow, early stages of diabetic cardiomyopathy, known as prediabetes, are poorly recognized, and, on many occasions, cardiac illness is diagnosed only after a severe degree of dysfunction was reached. Therefore, exploration and recognition of the initial pathophysiological mechanisms that lead to cardiac dysfunction in diabetic cardiomyopathy are of vital importance for an on-time diagnosis and treatment of the malady. Among the complex and intricate mechanisms involved in diabetic cardiomyopathy, Ca²⁺ mishandling and mitochondrial dysfunction have been described as pivotal early processes. In the present review, we will focus on these two processes and the molecular pathway that relates these two alterations to the earlier stages and the development of diabetic cardiomyopathy.

Ca2+-dependent modulation of voltage-gated myocyte sodium channels

Voltage-dependent Na⁺ channel activation underlies action potential generation fundamental to cellular excitability. In skeletal and cardiac muscle this triggers contraction via ryanodine-receptor (RyR)-mediated sarcoplasmic reticular (SR) Ca²⁺ release. We here review potential feedback actions of intracellular [Ca²⁺] ([Ca²⁺]_i) on Na⁺ channel activity, surveying their structural, genetic and cellular and functional implications, translating these to their possible clinical importance. In addition to phosphorylation sites, both Nav1.4 and Nav1.5 possess potentially regulatory binding sites for Ca²⁺ and/or the Ca^2+-sensor calmodulin in their inactivating III–IV linker and C-terminal domains (CTD), where mutations are associated with a range of skeletal and cardiac muscle diseases. We summarize in vitro cell-attached patch clamp studies reporting correspondingly diverse, direct and indirect, Ca²⁺ effects upon maximal Nav1.4 and Nav1.5 currents (I_max) and their half-maximal voltages (V_1/2) characterizing channel gating, in cellular expression systems and isolated myocytes. Interventions increasing cytoplasmic [Ca²⁺]_i down-regulated I_max leaving V_1/2 constant in native loose patch clamped, wild-type murine skeletal and cardiac myocytes. They correspondingly reduced action potential upstroke rates and conduction velocities, causing pro-arrhythmic effects in intact perfused hearts. Genetically modified murine RyR2-P2328S hearts modelling catecholaminergic polymorphic ventricular tachycardia (CPVT), recapitulated clinical ventricular and atrial pro-arrhythmic phenotypes following catecholaminergic challenge. These accompanied reductions in action potential conduction velocities. The latter were reversed by flecainide at RyR-blocking concentrations specifically in RyR2-P2328S as opposed to wild-type hearts, suggesting a basis for its recent therapeutic application in CPVT. We finally explore the relevance of these mechanisms in further genetic paradigms for commoner metabolic and structural cardiac disease.

Flecainide Paradoxically Activates Cardiac Ryanodine Receptor Channels under Low Activity Conditions: A Potential Pro-Arrhythmic Action

Cardiac ryanodine receptor (RyR2) mutations are implicated in the potentially fatal catecholaminergic polymorphic ventricular tachycardia (CPVT) and in atrial fibrillation. CPVT has been successfully treated with flecainide monotherapy, with occasional notable exceptions. Reported actions of flecainide on cardiac sodium currents from mice carrying the pro-arrhythmic homozygotic RyR2-P2328S mutation prompted our explorations of the effects of flecainide on their RyR2 channels. Lipid bilayer electrophysiology techniques demonstrated a novel, paradoxical increase in RyR2 activity. Preceding flecainide exposure, channels were mildly activated by 1 mM luminal Ca²⁺ and 1 µM cytoplasmic Ca²⁺, with open probabilities (P_o) of 0.03 ± 0.01 (wild type, WT) or 0.096 ± 0.024 (P2328S). Open probability (P_o) increased within 0.5 to 3 min of exposure to 0.5 to 5.0 µM cytoplasmic flecainide, then declined with higher concentrations of flecainide. There were no such increases in a subset of high P_o channels with P_o ≥ 0.08, although P_o then declined with ≥5 µM (WT) or ≥50 µM flecainide (P2328S). On average, channels with P_o < 0.08 were significantly activated by 0.5 to 10 µM of flecainide (WT) or 0.5 to 50 µM of flecainide (P2328S). These results suggest that flecainide can bind to separate activation and inhibition sites on RyR2, with activation dominating in lower activity channels and inhibition dominating in more active channels.

Finite element analysis predicts Ca2+ microdomains within tubular-sarcoplasmic reticular junctions of amphibian skeletal muscle

A finite element analysis modelled diffusional generation of steady-state Ca²⁺ microdomains within skeletal muscle transverse (T)-tubular-sarcoplasmic reticular (SR) junctions, sites of ryanodine receptor (RyR)-mediated SR Ca²⁺ release. It used established quantifications of sarcomere and T-SR anatomy (radial diameter [Formula: see text]; axial distance [Formula: see text]). Its boundary SR Ca²⁺ influx densities,[Formula: see text], reflected step impositions of influxes, [Formula: see text] deduced from previously measured Ca²⁺ signals following muscle fibre depolarization. Predicted steady-state T-SR junctional edge [Ca²⁺], [Ca²⁺]_edge, matched reported corresponding experimental cytosolic [Ca²⁺] elevations given diffusional boundary efflux [Formula: see text] established cytosolic Ca²⁺ diffusion coefficients [Formula: see text] and exit length [Formula: see text]. Dependences of predicted [Ca²⁺]_edge upon [Formula: see text] then matched those of experimental [Ca²⁺] upon Ca²⁺ release through their entire test voltage range. The resulting model consistently predicted elevated steady-state T-SR junctional ~ µM-[Ca²⁺] elevations radially declining from maxima at the T-SR junction centre along the entire axial T-SR distance. These [Ca²⁺] heterogeneities persisted through 10⁴- and fivefold, variations in D and w around, and fivefold reductions in d below, control values, and through reported resting muscle cytosolic [Ca²⁺] values, whilst preserving the flux conservation ([Formula: see text] condition, [Formula: see text]. Skeletal muscle thus potentially forms physiologically significant ~ µM-[Ca²⁺] T-SR microdomains that could regulate cytosolic and membrane signalling molecules including calmodulin and RyR, These findings directly fulfil recent experimental predictions invoking such Ca²⁺ microdomains in observed regulatory effects upon Na⁺ channel function, in a mechanism potentially occurring in similar restricted intracellular spaces in other cell types.

Mitochondrial Dysfunction Increases Arrhythmic Triggers and Substrates; Potential Anti-arrhythmic Pharmacological Targets

Incidence of cardiac arrhythmias increases significantly with age. In order to effectively stratify arrhythmic risk in the aging population it is crucial to elucidate the relevant underlying molecular mechanisms. The changes underlying age-related electrophysiological disruption appear to be closely associated with mitochondrial dysfunction. Thus, the present review examines the mechanisms by which age-related mitochondrial dysfunction promotes arrhythmic triggers and substrate. Namely, via alterations in plasmalemmal ionic currents (both sodium and potassium), gap junctions, cellular Ca²⁺ homeostasis, and cardiac fibrosis. Stratification of patients' mitochondrial function status permits application of appropriate anti-arrhythmic therapies. Here, we discuss novel potential anti-arrhythmic pharmacological interventions that specifically target upstream mitochondrial function and hence ameliorates the need for therapies targeting downstream changes which have constituted traditional antiarrhythmic therapy.

Protein expression profiles in murine ventricles modeling catecholaminergic polymorphic ventricular tachycardia: effects of genotype and sex

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is associated with mutations in the cardiac ryanodine receptor (RyR2). These result in stress-induced ventricular arrhythmic episodes, with clinical symptoms and prognosis reported more severe in male than female patients. Murine homozygotic RyR2-P2328S (RyR2^S/S ) hearts replicate the proarrhythmic CPVT phenotype of abnormal sarcoplasmic reticular Ca²⁺ leak and disrupted Ca²⁺ homeostasis. In addition, RyR2^S/S hearts show decreased myocardial action potential conduction velocities (CV), all features implicated in arrhythmic trigger and substrate. The present studies explored for independent and interacting effects of RyR2^S/S genotype and sex on expression levels of molecular determinants of Ca²⁺ homeostasis (CASQ2, FKBP12, SERCA2a, NCX1, and Ca_V 1.2) and CV (Na_V 1.5, Connexin (Cx)-43, phosphorylated-Cx43, and TGF-β1) in mice. Expression levels of Ca²⁺ homeostasis proteins were not altered, hence implicating abnormal RyR2 function alone in disrupted cytosolic Ca²⁺ homeostasis. Furthermore, altered Na_V 1.5, phosphorylated Cx43, and TGF-β1 expression were not implicated in the development of slowed CV. By contrast, decreased Cx43 expression correlated with slowed CV, in female, but not male, RyR2^S/S mice. The CV changes may reflect acute actions of the increased cytosolic Ca²⁺ on Na_V 1.5 and Cx43 function.

Update on antiarrhythmic drug pharmacology

Cardiac arrhythmias constitute a major public health problem. Pharmacological intervention remains mainstay to their clinical management. This, in turn, depends upon systematic drug classification schemes relating their molecular, cellular, and systems effects to clinical indications and therapeutic actions. This approach was first pioneered in the 1960s Vaughan-Williams classification. Subsequent progress in cardiac electrophysiological understanding led to a lag between the fundamental science and its clinical translation, partly addressed by The working group of the European Society of Cardiology (1991), which, however, did not emerge with formal classifications. We here utilize the recent Revised Oxford Classification Scheme to review antiarrhythmic drug pharmacology. We survey drugs and therapeutic targets offered by the more recently characterized ion channels, transporters, receptors, intracellular Ca²⁺ handling, and cell signaling molecules. These are organized into their strategic roles in cardiac electrophysiological function. Following analysis of the arrhythmic process itself, we consider (a) pharmacological agents directly targeting membrane function, particularly the Na⁺ and K⁺ ion channels underlying depolarizing and repolarizing events in the cardiac action potential. (b) We also consider agents that modify autonomic activity that, in turn, affects both the membrane and (c) the Ca²⁺ homeostatic and excitation-contraction coupling processes linking membrane excitation to contractile activation. Finally, we consider (d) drugs acting on more upstream energetic and structural remodeling processes currently the subject of clinical trials. Such systematic correlations of drug actions and arrhythmic mechanisms at different molecular to systems levels of cardiac function will facilitate current and future antiarrhythmic therapy.

Generation of induced pluripotent stem cells (iPSCs) from an infant with catecholaminergic polymorphic ventricular tachycardia carrying the double heterozygous mutations A1855D in RyR2 and Q1362H in SCN10A

Induced pluripotent stem cells (iPSCs) were generated from peripheral blood mononuclear cells (PBMCs) isolated from the peripheral blood of a 4 month-old boy with catecholaminergic polymorphic ventricular tachycardia carrying the double heterozygous mutations RyR2-A1855D and SCN10A-Q1362H. PBMCs were reprogrammed using non-integrative Sendai viral vectors containing reprogramming factors OCT4, SOX2, KLF4 and C-MYC. The iPSCs were shown to express pluripotent markers, have trilineage differentiation potential, carry RyR2-A1855D and SCN10A-Q1362H mutations and have a normal karyotype. They will be useful for studying the pathogenesis of CPVT patients with ≥2 variants.

Ion channel gating in cardiac ryanodine receptors from the arrhythmic RyR2-P2328S mouse

Mutations in the cardiac ryanodine receptor Ca²⁺ release channel (RyR2) can cause deadly ventricular arrhythmias and atrial fibrillation (AF). The RyR2-P2328S mutation produces catecholaminergic polymorphic ventricular tachycardia (CPVT) and AF in hearts from homozygous RyR2^{P2328S/P2328S} (denoted RyR2^S/S) mice. We have now examined P2328S RyR2 channels from RyR2^S/S hearts. The activity of wild-type (WT) and P2328S RyR2 channels was similar at a cytoplasmic [Ca²⁺] of 1 mM, but P2328S RyR2 was significantly more active than WT at a cytoplasmic [Ca²⁺] of 1 µM. This was associated with a >10-fold shift in the half maximal activation concentration (AC₅₀) for Ca²⁺ activation, from ∼3.5 µM Ca²⁺ in WT RyR2 to ∼320 nM in P2328S channels and an unexpected >1000-fold shift in the half maximal inhibitory concentration (IC₅₀) for inactivation from ∼50 mM in WT channels to ≤7 μM in P2328S channels, which is into systolic [Ca²⁺] levels. Unexpectedly, the shift in Ca²⁺ activation was not associated with changes in sub-conductance activity, S2806 or S2814 phosphorylation or the level of FKBP12 (also known as FKBP1A) bound to the channels. The changes in channel activity seen with the P2328S mutation correlate with altered Ca²⁺ homeostasis in myocytes from RyR2^S/S mice and the CPVT and AF phenotypes.This article has an associated First Person interview with the first author of the paper.

Reduced cardiomyocyte Na+ current in the age-dependent murine Pgc-1β-/- model of ventricular arrhythmia

Peroxisome proliferator-activated receptor-γ coactivator-1 deficient (Pgc-1β-/- ) murine hearts model the increased, age-dependent, ventricular arrhythmic risks attributed to clinical conditions associated with mitochondrial energetic dysfunction. These were accompanied by compromised action potential (AP) upstroke rates and impaired conduction velocities potentially producing arrhythmic substrate. We tested a hypothesis implicating compromised Na+ current in these electrophysiological phenotypes by applying loose patch-clamp techniques in intact young and aged, wild-type (WT) and Pgc-1β-/- , ventricular cardiomyocyte preparations for the first time. This allowed conservation of their in vivo extracellular and intracellular conditions. Depolarising steps elicited typical voltage-dependent activating and inactivating inward Na+ currents with peak amplitudes increasing or decreasing with their respective activating or preceding inactivating voltage steps. Two-way analysis of variance associated Pgc-1β-/- genotype with independent reductions in maximum peak ventricular Na+ currents from -36.63 ± 2.14 (n = 20) and -35.43 ± 1.96 (n = 18; young and aged WT, respectively), to -29.06 ± 1.65 (n = 23) and -27.93 ± 1.63 (n = 20; young and aged Pgc-1β-/- , respectively) pA/μm2 (p < 0.0001), without independent effects of, or interactions with age. Voltages at half-maximal current V*, and steepness factors k in plots of voltage dependences of both Na+ current activation and inactivation, and time constants for its postrepolarisation recovery from inactivation, remained indistinguishable through all experimental groups. So were the activation and rectification properties of delayed outward (K+ ) currents, demonstrated from tail currents reflecting current recoveries from respective varying or constant voltage steps. These current-voltage properties directly implicate decreases specifically in maximum available Na+ current with unchanged voltage dependences and unaltered K+ current properties, in proarrhythmic reductions in AP conduction velocity in Pgc-1β-/- ventricles.

Ventricular pro-arrhythmic phenotype, arrhythmic substrate, ageing and mitochondrial dysfunction in peroxisome proliferator activated receptor-γ coactivator-1β deficient (Pgc-1β-/-) murine hearts

INTRODUCTION

Ageing and age-related bioenergetic conditions including obesity, diabetes mellitus and heart failure constitute clinical ventricular arrhythmic risk factors.

MATERIALS AND METHODS

Pro-arrhythmic properties in electrocardiographic and intracellular recordings were compared in young and aged, peroxisome proliferator-activated receptor-γ coactivator-1β knockout (Pgc-1β-/-) and wild type (WT), Langendorff-perfused murine hearts, during regular and programmed stimulation (PES), comparing results by two-way ANOVA.

RESULTS AND DISCUSSION

Young and aged Pgc-1β-/- showed higher frequencies and durations of arrhythmic episodes through wider PES coupling-interval ranges than WT. Both young and old, regularly-paced, Pgc-1β-/- hearts showed slowed maximum action potential (AP) upstrokes, (dV/dt)max (∼157 vs. 120-130 V s-1), prolonged AP latencies (by ∼20%) and shortened refractory periods (∼58 vs. 51 ms) but similar AP durations (∼50 ms at 90% recovery) compared to WT. However, Pgc-1β-/- genotype and age each influenced extrasystolic AP latencies during PES. Young and aged WT ventricles displayed distinct, but Pgc-1β-/- ventricles displayed similar dependences of AP latency upon (dV/dt)max resembling aged WT. They also independently increased myocardial fibrosis. AP wavelengths combining activation and recovery terms paralleled contrasting arrhythmic incidences in Pgc-1β-/- and WT hearts. Mitochondrial dysfunction thus causes pro-arrhythmic Pgc-1β-/- phenotypes by altering AP conduction through reducing (dV/dt)max and causing age-dependent fibrotic change.

Multiple targets for flecainide action: implications for cardiac arrhythmogenesis

Flecainide suppresses cardiac tachyarrhythmias including paroxysmal atrial fibrillation, supraventricular tachycardia and arrhythmic long QT syndromes (LQTS), as well as the Ca2+ -mediated, catecholaminergic polymorphic ventricular tachycardia (CPVT). However, flecainide can also exert pro-arrhythmic effects most notably following myocardial infarction and when used to diagnose Brugada syndrome (BrS). These divergent actions result from its physiological and pharmacological actions at multiple, interacting levels of cellular organization. These were studied in murine genetic models with modified Nav channel or intracellular ryanodine receptor (RyR2)-Ca2+ channel function. Flecainide accesses its transmembrane Nav 1.5 channel binding site during activated, open, states producing a use-dependent antagonism. Closing either activation or inactivation gates traps flecainide within the pore. An early peak INa related to activation of Nav channels followed by rapid de-activation, drives action potential (AP) upstrokes and their propagation. This is diminished in pro-arrhythmic conditions reflecting loss of function of Nav 1.5 channels, such as BrS, accordingly exacerbated by flecainide challenge. Contrastingly, pro-arrhythmic effects attributed to prolonged AP recovery by abnormal late INaL following gain-of-function modifications of Nav 1.5 channels in LQTS3 are reduced by flecainide. Anti-arrhythmic effects of flecainide that reduce triggering in CPVT models mediated by sarcoplasmic reticular Ca2+ release could arise from its primary actions on Nav channels indirectly decreasing [Ca2+ ]i through a reduced [Na+ ]i and/or direct open-state RyR2-Ca2+ channel antagonism. The consequent [Ca2+ ]i alterations could also modify AP propagation velocity and therefore arrhythmic substrate through its actions on Nav 1.5 channel function. This is consistent with the paradoxical differences between flecainide actions upon Na+ currents, AP conduction and arrhythmogenesis under circumstances of normal and increased RyR2 function.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Arrhythmic effects of Epac-mediated ryanodine receptor activation in Langendorff-perfused murine hearts are associated with reduced conduction velocity

Recent papers have attributed arrhythmic substrate in murine RyR2‐P2328S hearts to reduced action potential (AP) conduction velocities (CV), reflecting acute functional inhibition and/or reduced expression of sodium channels. We explored for acute effects of direct exchange protein directly activated by cAMP (Epac)‐mediated ryanodine receptor‐2 (RyR2) activation on arrhythmic substrate and CV. Monophasic action potential (MAP) recordings demonstrated that initial steady (8 Hz) extrinsic pacing elicited ventricular tachycardia (VT) in 0 of 18 Langendorff‐perfused wild‐type mouse ventricles before pharmacological intervention. The Epac activator 8‐CPT (8‐(4‐chlorophenylthio)‐2′‐O‐methyladenosine‐3′,5′‐cyclic monophosphate) (VT in 1 of 7 hearts), and the RyR2 blocker dantrolene, either alone (0 of 11) or with 8‐CPT (0 of 9) did not then increase VT incidence (P>.05). Both progressively increased pacing rates and programmed extrasystolic (S2) stimuli similarly produced no VT in untreated hearts (n=20 and n=9 respectively). 8‐CPT challenge then increased VT incidences (5 of 7 and 4 of 8 hearts respectively; P<.05). However, dantrolene, whether alone (0 of 10 and 1 of 13) or combined with 8‐CPT (0 of 10 and 0 of 13) did not increase VT incidence relative to those observed in untreated hearts (P>.05). 8‐CPT but not dantrolene, whether alone or combined with 8‐CPT, correspondingly increased AP latencies (1.14±0.04 (n=7), 1.04±0.03 (n=10), 1.09±0.05 (n=8) relative to respective control values). In contrast, AP durations, conditions for 2:1 conduction block and ventricular effective refractory periods remained unchanged throughout. We thus demonstrate for the first time that acute RyR2 activation reversibly induces VT in specific association with reduced CV.

Epac-induced ryanodine receptor type 2 activation inhibits sodium currents in atrial and ventricular murine cardiomyocytes

Acute RyR2 activation by exchange protein directly activated by cAMP (Epac) reversibly perturbs myocyte Ca²⁺ homeostasis, slows myocardial action potential conduction, and exerts pro‐arrhythmic effects. Loose patch‐clamp studies, preserving in vivo extracellular and intracellular conditions, investigated Na⁺ current in intact cardiomyocytes in murine atrial and ventricular preparations following Epac activation. Depolarising steps to varying test voltages activated typical voltage‐dependent Na⁺ currents. Plots of peak current against depolarisation from resting potential gave pretreatment maximum atrial and ventricular currents of −20.23 ± 1.48 (17) and −29.8 ± 2.4 (10) pA/μm² (mean ± SEM [n]). Challenge by 8‐CPT (1 μmol/L) reduced these currents to −11.21 ± 0.91 (12) (P < .004) and −19.3 ± 1.6 (11) pA/μm² (P < .04) respectively. Currents following further addition of the RyR2 inhibitor dantrolene (10 μmol/L) (−19.91 ± 2.84 (13) and −26.6 ± 1.7 (17)), and dantrolene whether alone (−19.53 ± 1.97 (8) and −27.6 ± 1.9 (14)) or combined with 8‐CPT (−19.93 ± 2.59 (12) and −29.9 ± 2.5(11)), were indistinguishable from pretreatment values (all P >> .05). Assessment of the inactivation that followed by applying subsequent steps to a fixed voltage 100 mV positive to resting potential gave concordant results. Half‐maximal inactivation voltages and steepness factors, and time constants for Na⁺ current recovery from inactivation in double‐pulse experiments, were similar through all the pharmacological conditions. Intracellular sharp microelectrode membrane potential recordings in intact Langendorff‐perfused preparations demonstrated concordant variations in maximum rates of atrial and ventricular action potential upstroke, (dV/dt)_max. We thus demonstrate an acute, reversible, Na⁺ channel inhibition offering a possible mechanism for previously reported pro‐arrhythmic slowing of AP propagation following modifications of Ca²⁺ homeostasis, complementing earlier findings from chronic alterations in Ca²⁺ homeostasis in genetically‐modified RyR2‐P2328S hearts.

Cardiomyocyte ionic currents in intact young and aged murine Pgc-1β-/- atrial preparations

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Energetically-deficient Pgc-1β−/− murine atria show age-dependent arrhythmia.

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Voltage clamp studies investigated their underlying membrane current changes.

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Pgc-1β−/− atria showed reduced inward Na⁺ currents with normal voltage-dependences.

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Outward repolarising K+ currents retained normal activation and rectification.

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A resulting slowed action potential conduction explains the arrhythmic phenotype.

Introduction

Recent studies reported that energetically deficient murine Pgc-1β^−/− hearts replicate age-dependent atrial arrhythmic phenotypes associated with their corresponding clinical conditions, implicating action potential (AP) conduction slowing consequent upon reduced AP upstroke rates.

Materials and methods

We tested a hypothesis implicating Na⁺ current alterations as a mechanism underlying these electrophysiological phenotypes. We applied loose patch-clamp techniques to intact young and aged, WT and Pgc-1β^−/−, atrial cardiomyocyte preparations preserving their in vivo extracellular and intracellular conditions.

Results and discussion

Depolarising steps activated typical voltage-dependent activating and inactivating inward (Na⁺) currents whose amplitude increased or decreased with the amplitudes of the activating, or preceding inactivating, steps. Maximum values of peak Na⁺ current were independently influenced by genotype but not age or interacting effects of genotype and age on two-way ANOVA. Neither genotype, nor age, whether independently or interactively, influenced voltages at half-maximal current, or steepness factors, for current activation and inactivation, or time constants for recovery from inactivation following repolarisation. In contrast, delayed outward (K⁺) currents showed similar activation and rectification properties through all experimental groups. These findings directly demonstrate and implicate reduced Na⁺ in contrast to unchanged K⁺ current, as a mechanism for slowed conduction causing atrial arrhythmogenicity in Pgc-1β^−/− hearts.

Pro-arrhythmic atrial phenotypes in incrementally paced murine Pgc1β-/- hearts: effects of age

New Findings

What is the central question of this study?

Can we experimentally replicate atrial pro‐arrhythmic phenotypes associated with important chronic clinical conditions, including physical inactivity, obesity, diabetes mellitus and metabolic syndrome, compromising mitochondrial function, and clarify their electrophysiological basis?

What is the main finding and its importance?

Electrocardiographic and intracellular cardiomyocyte recording at progressively incremented pacing rates demonstrated age‐dependent atrial arrhythmic phenotypes in Langendorff‐perfused murine Pgc1β^−/− hearts for the first time. We attributed these to compromised action potential conduction and excitation wavefronts, whilst excluding alterations in recovery properties or temporal electrophysiological instabilities, clarifying these pro‐arrhythmic changes in chronic metabolic disease.

Atrial arrhythmias, most commonly manifesting as atrial fibrillation, represent a major clinical problem. The incidence of atrial fibrillation increases with both age and conditions associated with energetic dysfunction. Atrial arrhythmic phenotypes were compared in young (12–16 week) and aged (>52 week) wild‐type (WT) and peroxisome proliferative activated receptor, gamma, coactivator 1 beta (Ppargc1b)‐deficient (Pgc1β^−/−) Langendorff‐perfused hearts, previously used to model mitochondrial energetic disorder. Electrophysiological explorations were performed using simultaneous whole‐heart ECG and intracellular atrial action potential (AP) recordings. Two stimulation protocols were used: an S1S2 protocol, which imposed extrasystolic stimuli at successively decremented intervals following regular pulse trains; and a regular pacing protocol at successively incremented frequencies. Aged Pgc1β^−/− hearts showed greater atrial arrhythmogenicity, presenting as atrial tachycardia and ectopic activity. Maximal rates of AP depolarization (dV/dt_max) were reduced in Pgc1β^−/− hearts. Action potential latencies were increased by the Pgc1β^−/− genotype, with an added interactive effect of age. In contrast, AP durations to 90% recovery (APD₉₀) were shorter in Pgc1β^−/− hearts despite similar atrial effective recovery periods amongst the different groups. These findings accompanied paradoxical decreases in the incidence and duration of alternans in the aged and Pgc1β^−/− hearts. Limiting slopes of restitution curves of APD₉₀ against diastolic interval were correspondingly reduced interactively by Pgc1β^−/− genotype and age. In contrast, reduced AP wavelengths were associated with Pgc1β^−/− genotype, both independently and interacting with age, through the basic cycle lengths explored, with the aged Pgc1β^−/− hearts showing the shortest wavelengths. These findings thus implicate AP wavelength in possible mechanisms for the atrial arrhythmic changes reported here.

Effects of ageing on pro-arrhythmic ventricular phenotypes in incrementally paced murine Pgc-1β -/- hearts

A range of chronic clinical conditions accompany cardiomyocyte energetic dysfunction and constitute independent risk factors for cardiac arrhythmia. We investigated pro-arrhythmic and arrhythmic phenotypes in energetically deficient C57BL mice with genetic ablation of the mitochondrial promoter peroxisome proliferator-activated receptor-γ coactivator-1β (Pgc-1β), a known model of ventricular arrhythmia. Pro-arrhythmic and cellular action potential (AP) characteristics were compared in intact Langendorff-perfused hearts from young (12–16 week) and aged (> 52 week), wild-type (WT) and Pgc-1β^−/− mice. Simultaneous electrocardiographic and intracellular microelectrode recordings were made through successive trains of 100 regular stimuli at progressively incremented heart rates. Aged Pgc-1β^−/− hearts displayed an increased incidence of arrhythmia compared to other groups. Young and aged Pgc-1β^−/− hearts showed higher incidences of alternans in both AP activation (maximum AP upshoot velocity (dV/dt)_max and latency), recovery (action potential duration (APD₉₀) and resting membrane potential (RMP) characteristics compared to WT hearts. This was particularly apparent at lower pacing frequencies. These findings accompanied reduced (dV/dt)_max and increased AP latency values in the Pgc-1β^−/− hearts. APs observed prior to termination of the protocol showed lower (dV/dt)_max and longer AP latencies, but indistinguishable APD₉₀ and RMPs in arrhythmic compared to those in non-arrhythmic hearts. APD restitution analysis showed that Pgc-1β^−/− and WT hearts showed similar limiting gradients. However, Pgc-1β^−/− hearts had shortened plateau AP wavelengths, particularly in aged Pgc-1β^−/− hearts. Pgc-1β^−/− hearts therefore show pro-arrhythmic instabilities attributable to altered AP conduction and activation rather than recovery characteristics.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Cardiac disease and arrhythmogenesis: Mechanistic insights from mouse models

The mouse is the second mammalian species, after the human, in which substantial amount of the genomic information has been analyzed. With advances in transgenic technology, mutagenesis is now much easier to carry out in mice. Consequently, an increasing number of transgenic mouse systems have been generated for the study of cardiac arrhythmias in ion channelopathies and cardiomyopathies. Mouse hearts are also amenable to physical manipulation such as coronary artery ligation and transverse aortic constriction to induce heart failure, radiofrequency ablation of the AV node to model complete AV block and even implantation of a miniature pacemaker to induce cardiac dyssynchrony. Last but not least, pharmacological models, despite being simplistic, have enabled us to understand the physiological mechanisms of arrhythmias and evaluate the anti-arrhythmic properties of experimental agents, such as gap junction modulators, that may be exert therapeutic effects in other cardiac diseases. In this article, we examine these in turn, demonstrating that primary inherited arrhythmic syndromes are now recognized to be more complex than abnormality in a particular ion channel, involving alterations in gene expression and structural remodelling. Conversely, in cardiomyopathies and heart failure, mutations in ion channels and proteins have been identified as underlying causes, and electrophysiological remodelling are recognized pathological features. Transgenic techniques causing mutagenesis in mice are extremely powerful in dissecting the relative contributions of different genes play in producing disease phenotypes. Mouse models can serve as useful systems in which to explore how protein defects contribute to arrhythmias and direct future therapy.

Slowed depolarization and irregular repolarization in catecholaminergic polymorphic ventricular tachycardia: a study from cellular Ca2+ transients and action potentials to clinical monophasic action potentials and electrocardiography

AIMS

Spontaneous Ca²⁺ release leads to afterdepolarizations and triggered arrhythmia in catecholaminergic polymorphic ventricular tachycardia (CPVT). Irregular Ca²⁺ release is hypothesized to manifest as slowed depolarization and irregular repolarization. Our goal was to study depolarization and repolarization abnormalities in CPVT, as they remain largely uninvestigated.

METHODS AND RESULTS

We studied intracellular Ca²⁺ handling and action potentials (APs) in an induced pluripotent stem cell (iPSC) model of CPVT. Induced pluripotent stem cell cardiomyocytes from a RyR2-P2328S patient showed increased non-alternating variability of Ca²⁺ transients in response to isoproterenol. β-Agonists decreased AP upslope velocity in CPVT cells and in monophasic AP recordings of CPVT patients. We compared 24 h electrocardiograms (ECGs) of 19 CPVT patients carrying RyR2 mutations and 19 healthy controls. Short-term variability (STV) of the QT interval was 6.9 ± 0.5 ms in CPVT patients vs. 5.5 ± 0.4 ms in controls (P < 0.05) and associated with a history of arrhythmic events. Mean T-wave alternans (TWA) was 25 ± 1.4 µV in CPVT patients vs. 31 ± 2.0 µV in controls (P < 0.05). Older CPVT patients showed lower maximal upslope velocity of the ECG R-spike than control patients.

CONCLUSION

Catecholaminergic polymorphic ventricular tachycardia patients show higher STV of repolarization but lower TWA on the 24 h ECG than control patients, which is likely to reflect increased non-alternating variability of Ca²⁺ release by mutant RyR2s as observed in vitro. β-Agonists slow depolarization in RyR2-mutant cells and in CPVT patients. These findings may constitute a marker of arrhythmogenicity.

The RyR2-P2328S mutation downregulates Nav1.5 producing arrhythmic substrate in murine ventricles

Catecholaminergic polymorphic ventricular tachycardia (CPVT) predisposes to ventricular arrhythmia due to altered Ca²⁺ homeostasis and can arise from ryanodine receptor (RyR2) mutations including RyR2-P2328S. Previous reports established that homozygotic murine RyR2-P2328S (RyR2^S/S) hearts show an atrial arrhythmic phenotype associated with reduced action potential (AP) conduction velocity and sodium channel (Na_v1.5) expression. We now relate ventricular arrhythmogenicity and slowed AP conduction in RyR2^S/S hearts to connexin-43 (Cx43) and Na_v1.5 expression and Na⁺ current (I_Na). Stimulation protocols applying extrasystolic S2 stimulation following 8 Hz S1 pacing at progressively decremented S1S2 intervals confirmed an arrhythmic tendency despite unchanged ventricular effective refractory periods (VERPs) in Langendorff-perfused RyR2^S/S hearts. Dynamic pacing imposing S1 stimuli then demonstrated that progressive reductions of basic cycle lengths (BCLs) produced greater reductions in conduction velocity at equivalent BCLs and diastolic intervals in RyR2^S/S than WT, but comparable changes in AP durations (APD₉₀) and their alternans. Western blot analyses demonstrated that Cx43 protein expression in whole ventricles was similar, but Na_v1.5 expression in both whole tissue and membrane fractions were significantly reduced in RyR2^S/S compared to wild-type (WT). Loose patch-clamp studies similarly demonstrated reduced I_Na in RyR2^S/S ventricles. We thus attribute arrhythmogenesis in RyR2^S/S ventricles resulting from arrhythmic substrate produced by reduced conduction velocity to downregulated Na_v1.5 reducing I_Na, despite normal determinants of repolarization and passive conduction. The measured changes were quantitatively compatible with earlier predictions of linear relationships between conduction velocity and the peak I_Na of the AP but nonlinear relationships between peak I_Na and maximum Na⁺ permeability.

Flecainide exerts paradoxical effects on sodium currents and atrial arrhythmia in murine RyR2-P2328S hearts

Aims

Cardiac ryanodine receptor mutations are associated with catecholaminergic polymorphic ventricular tachycardia (CPVT), and some, including RyR2-P2328S, also predispose to atrial fibrillation. Recent work associates reduced atrial Na_v1.5 currents in homozygous RyR2-P2328S (RyR2^S/S) mice with slowed conduction and increased arrhythmogenicity. Yet clinically, and in murine models, the Na_v1.5 blocker flecainide reduces ventricular arrhythmogenicity in CPVT. We aimed to determine whether, and how, flecainide influences atrial arrhythmogenicity in RyR2^S/S mice and their wild-type (WT) littermates.

Methods

We explored effects of 1 μm flecainide on WT and RyR2^S/S atria. Arrhythmic incidence, action potential (AP) conduction velocity (CV), atrial effective refractory period (AERP) and AP wavelength (λ = CV × AERP) were measured using multi-electrode array recordings in Langendorff-perfused hearts; Na⁺ currents (I_Na) were recorded using loose patch clamping of superfused atria.

Results

RyR2^S/S showed more frequent atrial arrhythmias, slower CV, reduced I_Na and unchanged AERP compared to WT. Flecainide was anti-arrhythmic in RyR2^S/S but pro-arrhythmic in WT. It increased I_Na in RyR2^S/S atria, whereas it reduced I_Na as expected in WT. It increased AERP while sparing CV in RyR2^S/S, but reduced CV while sparing AERP in WT. Thus, RyR2^S/S hearts have low λ relative to WT; flecainide then increases λ in RyR2^S/S but decreases λ in WT.

Conclusions

Flecainide (1 μm) rescues the RyR2-P2328S atrial arrhythmogenic phenotype by restoring compromised I_Na and λ, changes recently attributed to increased sarcoplasmic reticular Ca²⁺ release. This contrasts with the increased arrhythmic incidence and reduced I_Na and λ with flecainide in WT.

Toward panoramic in situ mapping of action potential propagation in transgenic hearts to investigate initiation and therapeutic control of arrhythmias

To investigate the dynamics and propensity for arrhythmias in intact transgenic hearts comprehensively, optical strategies for panoramic fluorescence imaging of action potential (AP) propagation are essential. In particular, mechanism-oriented molecular studies usually depend on transgenic mouse hearts of only a few millimeters in size. Furthermore, the temporal scales of the mouse heart remain a challenge for panoramic fluorescence imaging with heart rates ranging from 200 min⁻¹ (e.g., depressed sinus node function) to over 1200 min⁻¹ during fast arrhythmias. To meet these challenging demands, we and others developed physiologically relevant mouse models and characterized their hearts with planar AP mapping. Here, we summarize the progress toward panoramic fluorescence imaging and its prospects for the mouse heart. In general, several high-resolution cameras are synchronized and geometrically arranged for panoramic voltage mapping and the surface and blood vessel anatomy documented through image segmentation and heart surface reconstruction. We expect that panoramic voltage imaging will lead to novel insights about molecular arrhythmia mechanisms through quantitative strategies and organ-representative analysis of intact mouse hearts.

Arrhythmic substrate, slowed propagation and increased dispersion in conduction direction in the right ventricular outflow tract of murine Scn5a+/- hearts

Aim

To test a hypothesis attributing arrhythmia in Brugada Syndrome to right ventricular (RV) outflow tract (RVOT) conduction abnormalities arising from Na_v1.5 insufficiency and fibrotic change.

Methods

Arrhythmic properties of Langendorff-perfused Scn5a+/− and wild-type mouse hearts were correlated with ventricular effective refractory periods (VERPs), multi-electrode array (MEA) measurements of action potential (AP) conduction velocities and dispersions in conduction direction (CD), Na_v1.5 expression levels, and fibrotic change, as measured at the RVOT and RV. Two-way anova was used to test for both independent and interacting effects of anatomical region and genotype on these parameters.

Results

Scn5a+/− hearts showed greater arrhythmic frequencies during programmed electrical stimulation at the RVOT but not the RV. The Scn5a+/− genotype caused an independent increase of VERP regardless of whether the recording site was the RVOT or RV. Effective AP conduction velocities (CV†s), derived from fitting regression planes to arrays of observed local activation times were reduced in Scn5a+/− hearts and at the RVOT independently. AP conduction velocity magnitudes derived by averaging MEA results from local vector analyses, CV*, were reduced by the Scn5a+/− genotype alone. In contrast, dispersions in conduction direction, were greater in the RVOT than the RV, when the atrioventricular node was used as the pacing site. The observed reductions in Na_v1.5 expression were attributable to Scn5a+/−, whereas increased levels of fibrosis were associated with the RVOT.

Conclusions

The Scn5a+/− RVOT recapitulates clinical findings of increased arrhythmogenicity through reduced CV† reflecting reduced CV* attributable to reduced Na_v1.5 expression and increased CD attributable to fibrosis.

Loss of Nav1.5 expression and function in murine atria containing the RyR2-P2328S gain-of-function mutation

AIMS

Recent studies reported slowed conduction velocity (CV) in murine hearts homozygous for the gain-of-function RyR2-P2328S mutation (RyR2(S/S)) and associated this with an increased incidence of atrial and ventricular arrhythmias. The present experiments determined mechanisms contributing to the reduced atrial CV.

METHODS AND RESULTS

The determinants of CV were investigated in murine RyR2(S/S) hearts and compared with those in wild-type (WT) and slow-conducting Scn5a(+/-) hearts. Picrosirius red staining demonstrated increased fibrosis only in Scn5a(+/-) hearts. Immunoblot assays showed similar expressions of Cx43 and Cx40 levels in the three genotypes. In contrast, Nav1.5 expression was reduced in both RyR2(S/S) and Scn5a(+/-) atria. These findings correlated with intracellular microelectrode and loose-patch-clamp studies. Microelectrode measurements showed reduced maximum rates of depolarization in Scn5a(+/-) and RyR2(S/S) atria compared with WT, despite similar diastolic membrane potentials. Loose-patch-clamp measurements demonstrated reduced peak Na(+) currents (INa) in the Scn5a(+/-) and RyR2(S/S) atria relative to WT, with similar normalized current-voltage relationships. In WT atria, reduction in INa could be produced by treatment with high extracellular Ca(2+), caffeine, or cyclopiazonic acid, each expected to produce an acute increase in [Ca(2+)]i.

CONCLUSION

RyR2(S/S) atria show reduced levels of Nav1.5 expression and Na(+) channel function. Reduced Na(+) channel function was also observed in WT atria, following acute increases in [Ca(2+)]i. Taken together, the results suggest that raised [Ca(2+)]i produces both acute and chronic inhibition of Na(+) channel function. These findings may help explain the relationship between altered Ca(2+) homeostasis, CV, and the maintenance of common arrhythmias such as atrial fibrillation.

Abnormal Ca(2+) homeostasis, atrial arrhythmogenesis, and sinus node dysfunction in murine hearts modeling RyR2 modification

Ryanodine receptor type 2 (RyR2) mutations are implicated in catecholaminergic polymorphic ventricular tachycardia (CPVT) thought to result from altered myocyte Ca(2+) homeostasis reflecting inappropriate "leakiness" of RyR2-Ca(2+) release channels arising from increases in their basal activity, alterations in their phosphorylation, or defective interactions with other molecules or ions. The latter include calstabin, calsequestrin-2, Mg(2+), and extraluminal or intraluminal Ca(2+). Recent clinical studies additionally associate RyR2 abnormalities with atrial arrhythmias including atrial tachycardia (AT), fibrillation (AF), and standstill, and sinus node dysfunction (SND). Some RyR2 mutations associated with CPVT in mouse models also show such arrhythmias that similarly correlate with altered Ca(2+) homeostasis. Some examples show evidence for increased Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of RyR2. A homozygotic RyR2-P2328S variant demonstrates potential arrhythmic substrate resulting from reduced conduction velocity (CV) in addition to delayed afterdepolarizations (DADs) and ectopic action potential (AP) firing. Finally, one model with an increased RyR2 activity in the sino-atrial node (SAN) shows decreased automaticity in the presence of Ca(2+)-dependent decreases in I Ca, L and diastolic sarcoplasmic reticular (SR) Ca(2+) depletion.