State-dependent block of wild-type and inactivation-deficient Na+ channels by flecainide

The antiarrhythmic agent flecainide appears beneficial for painful congenital myotonia and LQT-3/ DeltaKPQ syndrome. Both diseases manifest small but persistent late Na+ currents in skeletal or cardiac myocytes. Flecainide may therefore block late Na+ currents for its efficacy. To investigate this possibility, we characterized state-dependent block of flecainide in wild-type and inactivation-deficient rNav1.4 muscle Na+ channels (L435W/ L437C/A438W) expressed with beta1 subunits in Hek293t cells. The flecainide-resting block at -140 mV was weak for wild-type Na+ channels, with an estimated 50% inhibitory concentration (IC50) of 365 muM when the cell was not stimulated for 1,000 s. At 100 muM flecainide, brief monitoring pulses of +30 mV applied at frequencies as low as 1 per 60 s, however, produced an similar to70% use-dependent block of peak Na+ currents. Recovery from this use-dependent block followed an exponential function, with a time constant over 225 s at -140 mV. Inactivated wildtype Na+ channels interacted with flecainide also slowly at -50 mV, with a time constant of 7.9 s. In contrast, flecainide blocked the open state of inactivation-deficient Na+ channels potently as revealed by its rapid time-dependent block of late Na+ currents. The IC50 for flecainide open-channel block at +30 mV was 0.61 muM, right within the therapeutic plasma concentration range; on-rate and off-rate constants were 14.9 muM(-1)s(-1) and 12.2 s(-1), respectively. Upon repolarization to -140 mV, flecainide block of inactivation-deficient Na+ channels recovered, with a time constant of 11.2 s, which was similar to20-fold faster than that of wild-type counterparts. We conclude that flecainide directly blocks persistent late Na+ currents with a high affinity. The fast-in activation gate, probably via its S6 docking site, may further stabilize the flecainide-receptor complex in wild-type Na+ channels.