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.

Voltage-gated sodium (Nav) channels drive the rising phase of the action potential, essential for electrical signalling in nerves and muscles. The Nav channel α-subunit contains the ion-selective pore. In the cardiomyocyte, Nav1.5 is the main Nav channel α-subunit isoform, with a smaller expression of neuronal Nav channels. Four distinct regulatory β-subunits (β1–4) bind to the Nav channel α-subunits. Previous work has emphasised the β-subunits as direct Nav channel gating modulators. However, there is now increasing appreciation of additional roles played by these subunits. In this review, we focus on β-subunits as homophilic and heterophilic cell-adhesion molecules and the implications for cardiomyocyte function. Based on recent cryogenic electron microscopy (cryo-EM) data, we suggest that the β-subunits interact with Nav1.5 in a different way from their binding to other Nav channel isoforms. We believe this feature may facilitate trans-cell-adhesion between β1-associated Nav1.5 subunits on the intercalated disc and promote ephaptic conduction between cardiomyocytes.

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.