The Mechanism Of Flecainide Action In Cpvt Involves A Direct Effect On Ryr2

The class Ic antiarrhythmic drug flecainide has now become the world-wide standard of care for patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), however there is a controversy regarding its mechanism of action. Several reports have indicated that sodium channel block alone is responsible for its efficacy, whereas data from our group point toward the direct blocking effect of flecainide on RyR2 as the major mechanism for arrhythmia suppression in CPVT. Here we utilize a calsequestrin knockout (Casq2-/-) CPVT mouse model to test the effects of flecainide and fully-charged quaternary flecainide analogue (QX-FL), which is a potent sodium channel blocker but does not affect RyR2 activity, on spontaneous SR calcium release in membrane-permeabilized and intact ventricular myocytes. In membrane-permeabilized cardiomyocytes — lacking intact sarcolemma and devoid of sodium channel contribution — flecainide, but not QX-FL, effectively reduced the frequency of spontaneous Ca waves in a dose-dependent manner. In voltage-clamped intact cardiomyocytes, pretreatment with 30 µM tetrodotoxin (TTX) completely blocked sodium current and reduced frequency of spontaneous calcium waves (vehicle: 21.4±5.6 waves/min.; TTX: 12.8±2.9 waves/min.; p=0.3, 8 cells/group) although the difference was not statistically significant. Treatment with flecainide (6 µM in both pipette and extracellular solutions) dramatically reduced frequency of spontaneous calcium waves regardless of the presence of TTX (Flec.: 4.6±0.8 waves/min. (n=10); Flec.+TTX: 2.5±0.5 waves/min. (n=7); p<0.05 for both groups vs. vehicle), while QX-FL did not (QX-FL: 18.7±3.9 waves/min. (n=8); QX-FL+TTX: 14.2±3.7 waves/min. (n=9); p>0.4 for both groups vs. vehicle). Importantly, the SR Ca content, estimated for each cell by rapid caffeine application (10 mM) at the end of the experiment, was not different between all experimental groups. Our results further support the unique role of flecainide as a direct inhibitor of RyR2-mediated Ca release activity that determines its efficacy in CPVT.