Calcium Overload Reduces Flecainide Efficacy Against Spontaneous Calcium Release

Abstract

The antiarrhythmic drug flecainide is effective in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), a disease caused by mutations in RyR2 Ca-release channels or calsequestrin (Casq2). Flecainide inhibits RyR2 channels and blocks spontaneous Ca-release (SCR) in Casq2-knockout (KO) cardiomyocytes, a CPVT model. However, a recent report failed to find flecainide efficacy against SCRs in cardiomyocytes from another CPVT model, RyR2R4496C+/- mice. To address this controversy, we compare the effect of flecainide in Casq2-KO and RyR2-R4496C myocytes under identical conditions. Method/Results: Ventricular myocytes from Casq2-KO and RyR2R4496C+/- mice were loaded with Fura2-AM to measure intracellular [Ca]. After 30min exposure to flecainide (FLEC, 6μM) or vehicle (VEH), myocytes were field-stimulated at 1Hz in presence of isoproterenol (1μM). SCRs were quantified during a 40s pause post-pacing. The flecainide effect was strongly dependent on extracellular [Ca]. At 2mM Ca, only Casq2-KO myocytes exhibited SCRs, which were strongly suppressed by flecainide (Figure). At 3mM Ca, both groups had frequent SCRs. Flecainide had reduced efficacy in Casq2-KO but was highly effective in RyR-R4496C myocytes (Figure). Conclusion: Flecainide inhibits SCRs in all CPVT models tested. However, flecainide efficacy is reduced by Ca overload, which could explain the divergent literature reports.