Flecainide paradoxically activates cardiac ryanodine receptor channels under low activity conditions: A potential pro‐arrhythmic action

Cardiac ryanodine receptor (RyR2) mutations are implicated in the potentially fatal cate-cholaminergic polymorphic ventricular tachycardia (CPVT) and in atrial fibrillation. CPVT has been successfully treated with flecainide monotherapy, with occasional notable exceptions. Reported actions of flecainide on cardiac sodium currents from mice carrying the pro‐arrhythmic homozygotic RyR2‐P2328S mutation prompted our explorations of the effects of flecainide on their RyR2 chan-nels. Lipid bilayer electrophysiology techniques demonstrated a novel, paradoxical increase in RyR2 activity. Preceding flecainide exposure, channels were mildly activated by 1 mM luminal Ca²⁺ and 1 μM cytoplasmic Ca²⁺, with open probabilities (Po) of 0.03 ± 0.01 (wild type, WT) or 0.096 ± 0.024 (P2328S). Open probability (Po) increased within 0.5 to 3 min of exposure to 0.5 to 5.0 μM cytoplasmic flecainide, then declined with higher concentrations of flecainide. There were no such increases in a subset of high Po channels with Po ≥ 0.08, although Po then declined with ≥5 μM (WT) or ≥50 μM flecainide (P2328S). On average, channels with Po < 0.08 were significantly activated by 0.5 to 10 μM of flecainide (WT) or 0.5 to 50 μM of flecainide (P2328S). These results suggest that flecainide can bind to separate activation and inhibition sites on RyR2, with activation dominating in lower activity channels and inhibition dominating in more active channels.