Channel Activity of Cardiac Ryanodine Receptors (RyR2) Determines Potency and Efficacy of Flecainide and R-Propafenone against Arrhythmogenic Calcium Waves in Ventricular Cardiomyocytes

Flecainide blocks ryanodine receptor type 2 (RyR2) channels in the open state, suppresses arrhythmogenic Ca²⁺ waves and prevents catecholaminergic polymorphic ventricular tachycardia (CPVT) in mice and humans. We hypothesized that differences in RyR2 activity induced by CPVT mutations determines the potency of open-state RyR2 blockers like flecainide (FLEC) and R-propafenone (RPROP) against Ca²⁺ waves in cardiomyocytes. Using confocal microscopy, we studied Ca²⁺ sparks and waves in isolated saponin-permeabilized ventricular myocytes from two CPVT mouse models (Casq2^-/-, RyR2-R4496C^+/-), wild-type (c57bl/6, WT) mice, and WT rabbits (New Zealand white rabbits). Consistent with increased RyR2 activity, Ca²⁺ spark and wave frequencies were significantly higher in CPVT compared to WT mouse myocytes. We next obtained concentration-response curves of Ca²⁺ wave inhibition for FLEC, RPROP (another open-state RyR2 blocker), and tetracaine (TET) (a state-independent RyR2 blocker). Both FLEC and RPROP inhibited Ca²⁺ waves with significantly higher potency (lower IC₅₀) and efficacy in CPVT compared to WT. In contrast, TET had similar potency in all groups studied. Increasing RyR2 activity of permeabilized WT myocytes by exposure to caffeine (150 µM) increased the potency of FLEC and RPROP but not of TET. RPROP and FLEC were also significantly more potent in rabbit ventricular myocytes that intrinsically exhibit higher Ca²⁺ spark rates than WT mouse ventricular myocytes. In conclusion, RyR2 activity determines the potency of open-state blockers FLEC and RPROP for suppressing arrhythmogenic Ca²⁺ waves in cardiomyocytes, a mechanism likely relevant to antiarrhythmic drug efficacy in CPVT.