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/ΔKPQ 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 β1 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 μM when the cell was not stimulated for 1,000 s. At 100 μM flecainide, brief monitoring pulses of +30 mV applied at frequencies as low as 1 per 60 s, however, produced an ∼70% 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 wild-type 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 μM, right within the therapeutic plasma concentration range; on-rate and off-rate constants were 14.9 μM⁻¹s⁻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 ∼20-fold faster than that of wild-type counterparts. We conclude that flecainide directly blocks persistent late Na⁺ currents with a high affinity. The fast-inactivation gate, probably via its S6 docking site, may further stabilize the flecainide-receptor complex in wild-type Na⁺ channels.