The pH-dependent effect of mexiletine on IVS4 sodium channel mutants R1448H/C

The effects of extracellular pH (6.2, 7.4 and 8.2) and 0.1mM mexiletine, a channel blocker of the lidocaine type, are studied on two sodium channel mutations of the fourth voltage sensor, R1448H/C. The fast inactivated channel state to which mexiletine preferentially binds is destabilized by the mutations. In contrast to the expected low response of R1448H/C carriers, mexiletine is particularly effective in preventing exercise-induced stiffness and paralysis from which these patients suffer. Our measurements performed in the whole-cell mode on stably transfected HEK cells show for the first time that the mutations strikingly accelerate closed-state inactivation and, as steady-state fast inactivation is shifted to more negative potentials, stabilize the fast inactivated channel state in the potential range around the resting potential. At pH 7.4 and 8.2, the phasic mexiletine block is larger for R1448C (55%) and R1448H (47%) than for wild type (WT) channels (31%) due to slowed recovery from block (520 ms for R1448C versus 270 ms for WT at pH 7.4) although the recovery from inactivation is slightly faster for the mutants (1.9 ms for R1448C versus 3.8 ms for WT at pH 7.4). At pH 6.2, the effect of mexiletine is smaller and shows a relatively fast recovery from block. We conclude that enhanced closed-state inactivation expands the concept of a mutation-induced uncoupling of channel inactivation from activation to a new potential range and that the higher mexiletine efficacy in R1448H/C carriers than other myotonic patients, offering a pharmacogenetic strategy for mutation-specific treatment.