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(v)1.5 currents in homozygous RyR2-P2328S (RyR2(S/S)) mice with slowed conduction and increased arrhythmogenicity. Yet clinically, and in murine models, the Na(v)1.5 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 Ca(2+) release. This contrasts with the increased arrhythmic incidence and reduced I(Na) and λ with flecainide in WT.