Ranolazine block of human Na_V1.4 sodium channels and paramyotonia congenita mutants

The antianginal drug ranolazine exerts voltage- and use-dependent block (UDB) of several Na⁺ channel isoforms, including Na_V1.4. We hypothesized that ranolazine will similarly inhibit the paramyotonia congenita Na_V1.4 gain-of-function mutations, R1448C, R1448H and R1448P that are associated with repetitive action potential firing. Whole-cell Na⁺ current (I_Na) was recorded from HEK293 cells expressing the hNa_V1.4 WT or R1448 mutations. At a holding potential (HP) of -140 mV, ranolazine exerted UDB (10 hz) of WT and R1448 mutations (IC₅₀ = 59-71 μM). The potency for ranolazine UDB increased when the frequency of stimulation was raised to 30 hz (IC₅₀ = 20-27 μM). When the HP was changed to -70 mV to mimic the resting potential of an injured skeletal muscle fiber, the potency of ranolazine to block I_Na further increased; values of ranolazine IC₅₀ for block of WT, R1448C, R1448H and R1448P were 3.8, 0.9, 6.3 and 0.9 μM, respectively. Ranolazine (30 μM) also caused a hyperpolarizing shift in the voltage-dependence of inactivation of WT and R1448 mutations. The effects of ranolazine (30 μM) to reduce I_Na were similar (∼35% I _Na inhibition) when different conditioning pulse durations (2-20 msec) were used. Ranolazine (10 μM) suppressed the abnormal I_Na induced by slow voltage ramps for R1448C channels. In computer simulations, 3 μM ranolazine inhibited the sustained and excessive firing of skeletal muscle action potentials that are characteristic of myotonia. Taken together, the data indicate that ranolazine interacts with the open state and stabilizes the inactivated state(s) of Na_V1.4 channels, causes voltage- and use-dependent block of I_Na and suppresses persistent I_Na. These data further suggest that ranolazine might be useful to reduce the sustained action potential firing seen in paramyotonia congenita.