Rationale:
The class Ic antiarrhythmic drug flecainide prevents ventricular tachyarrhythmia in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), a disease caused by hyperactive cardiac ryanodine receptor (RyR2) calcium (Ca) release. Although flecainide inhibits single RyR2 channels
in vitro
, reports have claimed that RyR2 inhibition by flecainide is not relevant for its mechanism of antiarrhythmic action and concluded that sodium channel block alone is responsible for flecainide’s efficacy in CPVT.
Objective:
To determine whether RyR2 block independently contributes to flecainide’s efficacy for suppressing spontaneous sarcoplasmic reticulum (SR) Ca release and for preventing ventricular tachycardia
in vivo
.
Methods and Results:
We synthesized
N
-methyl flecainide analogues (QX-FL and NM-FL) and showed that
N
-methylation reduces flecainide’s inhibitory potency on RyR2 channels but not on cardiac sodium channels. Antiarrhythmic efficacy was tested utilizing a calsequestrin knockout (Casq2-/-) CPVT mouse model. In membrane-permeabilized Casq2-/- cardiomyocytes — lacking intact sarcolemma and devoid of sodium channel contribution — flecainide, but not its analogues, suppressed RyR2-mediated Ca release at clinically relevant concentrations. In voltage-clamped, intact Casq2-/- cardiomyocytes pretreated with tetrodotoxin (TTX) to inhibit sodium channels and isolate the effect of flecainide on RyR2, flecainide significantly reduced the frequency of spontaneous SR Ca release, while QX-FL and NM-FL did not.
In vivo
, flecainide effectively suppressed catecholamine-induced ventricular tachyarrhythmias in Casq2-/- mice, whereas NM-FL did not, despite comparable sodium channel block.
Conclusions:
Flecainide remains an effective inhibitor of RyR2-mediated arrhythmogenic Ca release even when cardiac sodium channels are blocked. In mice with CPVT, sodium channel block alone was not enough to prevent arrhythmias. Hence, RyR2 inhibition by flecainide is critical for its mechanism of antiarrhythmic action.
SAP97 and Dystrophin Macromolecular Complexes Determine Two Pools of Cardiac Sodium Channels Na
v
1.5 in Cardiomyocytes
