The action of flecainide on the wild-type cardiac ryanodine receptor

The genetic disorder catecholaminergic polymorphic ventricular tachycardia (CPVT) causes the function of the sarcoplasmic reticulum (SR) Ca2+ release channel (RyR2) to be altered and induces fatal arrhythmias during stress or exercise. Flecainide, a class 1C sodium channel (Nav1.5) inhibitor and anti-arrhythmic agent, was recently observed to be effective against CPVT arrhythmias. Controversially, it was suggested that flecainide acted directly on RyR2, alongside its action on Nav1.5. The present study sought to establish whether flecainide affected RyR2 activity to prevent pro-arrhythmic Ca2+ waves in wild type (WT) cardiomyocytes and if so, to elucidate the mechanisms responsible. Flecainide or flecainide-FITC was applied to intact or saponin permeabilised cardiomyocytes isolated from WT rat ventricle. Confocal microscopy was used to image SR Ca2+ release after application of flecainide or trans-sarcolemmal movement of fluorescent flecainide-FITC. In intact myocytes, flecainide decreased pro-arrhythmic Ca2+ wave frequency although Ca2+ spark properties were unchanged, indicating no effect on RyR2 despite prolonged flecainide exposure (45 min). Flecainide-FITC traversed the sarcolemma over a period of hours and primarily accumulated in the mitochondria. In permeabilised myocytes, where flecainide had immediate access to RyR2 and Nav1.5 was non-functional, a 10-20% decrease in wave frequency was apparent, accompanied by sustained changes in Ca2+ spark properties. The effect on waves was potentiated when the SR counter-current was inhibited by substitution of K+ with Cs+. These results suggest that flecainide has an anti-arrhythmic effect on RyR2 in permeabilised WT cardiomyocytes. However, slow cytosolic accumulation and the requirement for an increased drug concentration may explain the absence of an effect on RyR2 in intact cells. Nevertheless, these findings support the concept that RyR2 channel activity can be pharmacologically manipulated and that RyR2 represents a potential pharmacological target in patients with arrhythmia.