Acute atrial arrhythmogenesis in murine hearts following enhanced extracellular Ca(2+) entry depends on intracellular Ca(2+) stores

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'.

Modulated Calcium Homeostasis and Release Events Under Atrial Fibrillation and Its Risk Factors: A Meta-Analysis

Background: Atrial fibrillation (AF) is associated with calcium (Ca²⁺) handling remodeling and increased spontaneous calcium release events (SCaEs). Nevertheless, its exact mechanism remains unclear, resulting in suboptimal primary and secondary preventative strategies.

Methods: We searched the PubMed database for studies that investigated the relationship between SCaEs and AF and/or its risk factors. Meta-analysis was used to examine the Ca²⁺ mechanisms involved in the primary and secondary AF preventative groups.

Results: We included a total of 74 studies, out of the identified 446 publications from inception (1982) until March 31, 2020. Forty-five were primary and 29 were secondary prevention studies for AF. The main Ca²⁺ release events, calcium transient (standardized mean difference (SMD) = 0.49; I² = 35%; confidence interval (CI) = 0.33–0.66; p < 0.0001), and spark amplitude (SMD = 0.48; I² = 0%; CI = −0.98–1.93; p = 0.054) were enhanced in the primary diseased group, while calcium transient frequency was increased in the secondary group. Calcium spark frequency was elevated in both the primary diseased and secondary AF groups. One of the key cardiac currents, the L-type calcium current (I_CaL) was significantly downregulated in primary diseased (SMD = −1.07; I² = 88%; CI = −1.94 to −0.20; p < 0.0001) and secondary AF groups (SMD = −1.28; I² = 91%; CI = −2.04 to −0.52; p < 0.0001). Furthermore, the sodium–calcium exchanger (I_NCX) and NCX1 protein expression were significantly enhanced in the primary diseased group, while only NCX1 protein expression was shown to increase in the secondary AF studies. The phosphorylation of the ryanodine receptor at S2808 (pRyR-S2808) was significantly elevated in both the primary and secondary groups. It was increased in the primary diseased and proarrhythmic subgroups (SMD = 0.95; I² = 64%; CI = 0.12–1.79; p = 0.074) and secondary AF group (SMD = 0.66; I² = 63%; CI = 0.01–1.31; p < 0.0001). Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) expression was elevated in the primary diseased and proarrhythmic drug subgroups but substantially reduced in the secondary paroxysmal AF subgroup.

Conclusions: Our study identified that I_CaL is reduced in both the primary and secondary diseased groups. Furthermore, pRyR-S2808 and NCX1 protein expression are enhanced. The remodeling leads to elevated Ca²⁺ functional activities, such as increased frequencies or amplitude of Ca²⁺ spark and Ca²⁺ transient. The main difference identified between the primary and secondary diseased groups is SERCA expression, which is elevated in the primary diseased group and substantially reduced in the secondary paroxysmal AF subgroup. We believe our study will add new evidence to AF mechanisms and treatment targets.

Disruption of cardiac cholinergic neurons enhances susceptibility to ventricular arrhythmias

The parasympathetic nervous system plays an important role in the pathophysiology of atrial fibrillation. Catheter ablation, a minimally invasive procedure deactivating abnormal firing cardiac tissue, is increasingly becoming the therapy of choice for atrial fibrillation. This is inevitably associated with the obliteration of cardiac cholinergic neurons. However, the impact on ventricular electrophysiology is unclear. Here we show that cardiac cholinergic neurons modulate ventricular electrophysiology. Mechanical disruption or pharmacological blockade of parasympathetic innervation shortens ventricular refractory periods, increases the incidence of ventricular arrhythmia and decreases ventricular cAMP levels in murine hearts. Immunohistochemistry confirmed ventricular cholinergic innervation, revealing parasympathetic fibres running from the atria to the ventricles parallel to sympathetic fibres. In humans, catheter ablation of atrial fibrillation, which is accompanied by accidental parasympathetic and concomitant sympathetic denervation, raises the burden of premature ventricular complexes. In summary, our results demonstrate an influence of cardiac cholinergic neurons on the regulation of ventricular function and arrhythmogenesis.

Catheter ablation is a common therapy for atrial fibrillation but disrupts cardiac cholinergic neurons. Here the authors report that cholinergic neurons innervate heart ventricles and show that their ablation leads to increased susceptibility to ventricular arrhythmias in mouse models and in patients.

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.

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.

Imaging atrial arrhythmic intracellular calcium in intact heart

Abnormalities in intracellular Ca(2+) signaling have been proposed to play an essential role in the pathophysiology of atrial arrhythmias. However, a direct observation of intracellular Ca(2+) in atrial myocytes during atrial arrhythmias is lacking. Here, we have developed an ex vivo model of simultaneous Ca(2+) imaging and electrocardiographic recording in cardiac atria. Using this system we were able to record atrial arrhythmic intracellular Ca(2+) activities. Our results indicate that atrial arrhythmias can be tightly linked to intracellular Ca(2+) waves and Ca(2+) alternans. Moreover, we applied this strategy to analyze Ca(2+) signals in the hearts of WT and knock-in mice harboring a 'leaky' type 2 ryanodine receptor (RyR2-R2474S). We showed that sarcoplasmic reticulum (SR) Ca(2+) leak increases the susceptibility to Ca(2+) alternans and Ca(2+) waves increasing the incidence of atrial arrhythmias. Reduction of SR Ca(2+) leak via RyR2 by acute treatment with S107 reduced both Ca(2+) alternans and Ca(2+) waves, and prevented atrial arrhythmias.

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.

Modifications of mechanoelectric feedback induced by 2,3-butanedione monoxime and Blebbistatin in Langendorff-perfused rabbit hearts

AIM

Myocardial stretching is an arrhythmogenic factor. Optical techniques and mechanical uncouplers are used to study the mechanoelectric feedback. The aim of this study is to determine whether the mechanical uncouplers 2,3-butanedione monoxime and Blebbistatin hinder or modify the electrophysiological effects of acute mechanical stretch.

METHODS

The ventricular fibrillation (VF) modifications induced by acute mechanical stretch were studied in 27 Langendorff-perfused rabbit hearts using epicardial multiple electrodes and mapping techniques under control conditions (n = 9) and during the perfusion of 2,3-butanedione monoxime (15 mM) (n = 9) or Blebbistatin (10 μm) (n = 9).

RESULTS

In the control series, myocardial stretch increased the complexity of the activation maps and the dominant frequency (DF) of VF from 13.1 ± 2.0 Hz to 19.1 ± 3.1 Hz (P < 0.001, 46% increment). At baseline, the activation maps showed less complexity in both the 2,3-butanedione monoxime and Blebbistatin series, and the DF was lower in the 2,3-butanedione monoxime series (11.4 ± 1.2 Hz; P < 0.05). The accelerating effect of mechanical stretch was abolished under 2,3-butanedione monoxime (maximum DF = 11.7 ± 2.4 Hz, 5% increment, ns vs baseline, P < 0.0001 vs. control series) and reduced under Blebbistatin (maximum DF = 12.9 ± 0.7 Hz, 8% increment, P < 0.01 vs. baseline, P < 0.0001 vs. control series). The variations in complexity of the activation maps under stretch were not significant in the 2,3-butanedione monoxime series and were significantly attenuated under Blebbistatin.

CONCLUSION

The accelerating effect and increased complexity of myocardial activation during VF induced by acute mechanical stretch are abolished under the action of 2,3-butanedione monoxime and reduced under the action of Blebbistatin.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

Dihydropyridine (DHPR) and ryanodine receptors (RyRs) are central to transduction of transverse (T) tubular membrane depolarisation initiated by surface action potentials into release of sarcoplasmic reticular (SR) Ca2+ in skeletal muscle excitation-contraction coupling. Electronmicroscopic methods demonstrate an orderly positioning of such tubular DHPRs relative to RyRs in the SR at triad junctions where their membranes come into close proximity. Biochemical and genetic studies associated expression of specific, DHPR and RyR, isoforms with the particular excitation-contraction coupling processes and related elementary Ca2+ release events found respectively in skeletal and cardiac muscle. Physiological studies of intramembrane charge movements potentially related to voltage triggering of Ca2+ release demonstrated a particular qγ charging species identifiable with DHPRs through its T-tubular localization, pharmacological properties, and steep voltage-dependence paralleling Ca2+ release. Its nonlinear kinetics implicated highly co-operative conformational events in its transitions in response to voltage change. The effects of DHPR and RyR agonists and antagonists upon this intramembrane charge in turn implicated reciprocal rather than merely unidirectional DHPR-RyR interactions in these complex reactions. Thus, following membrane potential depolarization, an orthograde qγ-DHPR-RyR signaling likely initiates conformational alterations in the RyR with which it makes contact. The latter changes could then retrogradely promote further qγ-DHPR transitions through reciprocal co-operative allosteric interactions between receptors. These would relieve the resting constraints on both further, delayed, nonlinear qγ-DHPR charge transfers and on RyR-mediated Ca2+ release. They would also explain the more rapid charging and recovery qγ transients following larger depolarizations and membrane potential repolarization to the resting level.

Acute atrial arrhythmogenicity and altered Ca(2+) homeostasis in murine RyR2-P2328S hearts

Aims

The experiments explored for atrial arrhythmogenesis and its possible physiological background in recently developed hetero-(RyR2^+/S) and homozygotic (RyR2^S/S) RyR2-P2328S murine models for catecholaminergic polymorphic ventricular tachycardia (VT) for the first time. They complement previous clinical and experimental reports describing increased ventricular arrhythmic tendencies associated with physical activity, stress, or catecholamine infusion, potentially leading to VT and ventricular fibrillation.

Methods and results

Atrial arrhythmogenic properties were compared at the whole animal, Langendorff-perfused heart, and single, isolated atrial myocyte levels using electrophysiological and confocal fluorescence microscopy methods. This demonstrated that: (i) electrocardiographic parameters in intact anaesthetized wild-type (WT), RyR2^+/S and RyR2^S/S mice were statistically indistinguishable both before and after addition of isoproterenol apart from increases in heart rates. (ii) Bipolar electrogram and monophasic action potential recordings showed significantly higher incidences of arrhythmogenesis in isolated perfused RyR2^S/S, but not RyR2^+/S, relative to WT hearts during either regular pacing or programmed electrical stimulation. The addition of isoproterenol increased such incidences in all three groups. (iii) However, there were no accompanying differences in cardiac anatomy or action potential durations at 90% repolarization and refractory periods. (iv) In contrast, episodes of diastolic Ca²⁺ release were observed under confocal microscopy in isolated fluo-3-loaded RyR2^S/S, but not RyR2^+/S or WT, atrial myocytes. The introduction of isoproterenol resulted in significant diastolic Ca²⁺ release in all three groups.

Conclusions

These findings establish acute atrial arrhythmogenic properties in RyR2-P2328S hearts and correlate these with altered Ca²⁺ homeostasis in an absence of repolarization abnormalities for the first time.