Gating of myotonic Na channel mutants defines the response to mexiletine and a potent derivative.

BACKGROUND Myotonia and periodic paralysis caused by sodium channel mutations show variable responses to the anti-myotonic drug mexiletine. OBJECTIVE To investigate whether variability among sodium channel mutants results from differences in drug binding affinity or in channel gating. METHODS Whole-cell sodium currents (I(Na)) were recorded in tsA201 cells expressing human wild-type (WT) and mutant skeletal muscle sodium channels (A1156T, hyperkalemic periodic paralysis; R1448C, paramyotonia congenita; G1306E, potassium-aggravated myotonia). RESULTS At a holding potential (hp) of -120 mV, mexiletine produced a tonic (TB, 0.33 Hz) and a use-dependent (UDB, 10 Hz) block of peak I(Na) with a potency following the order rank R1448C > WT approximately equal A1156T > G1306E. Yet, when assayed from an hp of -180 mV, TB and UDB by mexiletine were similar for the four channels. The different midpoints of channel availability curves found for the four channels track the half-maximum inhibitory value (IC50) measured at -120 mV. Thus differences in the partitioning of channels between the closed and fast-inactivated states underlie the different IC50 measured at a given potential. The mexiletine-derivative, Me7 (alpha-[(2-methylphenoxy)methyl]-benzenemethanamine), behaved similarly but was approximately 5 times more potent than mexiletine. Interestingly, the higher drug concentrations ameliorated the abnormally slower decay rate of myotonic I(Na). CONCLUSIONS These results explain the basis of the apparent difference in block of mutant sodium channels by mexiletine and Me7, opening the way to a more rationale drug use and to design more potent drugs able to correct specifically the biophysical defect of the mutation in individual myotonic patients.