Mexiletine block of wild-type and inactivation-deficient human skeletal muscle hNav1.4 Na+ channels

Mexiletine is a class lb antiarrhythmic drug used for ventricular arrhythmias but is also found to be effective for paramyotonia congenita, potassium-aggravated myotonia, long QT-3 syndrome, and neuropathic pain. This drug elicits tonic block of Na+ channels when cells are stimulated infrequently and produces additional use-dependent block during repetitive pulses. We examined the state-dependent block by mexiletine in human skeletal muscle hNav1.4 wild-type and inactivation-deficient mutant Na+ channels (hNav1.4-L443C/A444W) expressed in HEK293t cells with a beta1 subunit. The 50% inhibitory concentrations (IC50) for the inactivated-state block and the resting-state block of wild-type Na+ channels by mexiletine were measured as 67.8+/-7.0 mum and 431.2+/-9.4 mum, respectively (n = 5). In contrast, the IC50 for the block of open inactivation-deficient mutant channels at +30 mV by mexiletine was 3.3+/-0.1 mum (n = 5), which was within the therapeutic plasma concentration range(2.8+/-11 mum). Estimated on- and off-rates for the open-state block by mexiletine at+ 30 mV were 10.4 muM(-1) s(-1) and 54.4 s(-1), respectively. Use-dependent block by mexiletine was greater in inactivation-deficient mutant channels than in wild-type channels during repetitive pulses. Furthermore, the IC50 values for the block of persistent late hNav1.4 currents in chloramine-T-pretreated cells by mexiletine was 7.5+/-0.8 mum (n = 5) at +30 mV. Our results together support the hypothesis that the in vivo efficacy of mexiletine is primarily due to the open-channel block of persistent late Na+ currents, which may arise during various pathological conditions.