Figure 1. Activation of hNav1.4 wild-type channels in the absence and presence of 100 μm mexiletine.

Currents were evoked by 5 ms test pulses from –120 to +50 mV in 10 mV increments without (A) or with (B) 100 μm mexiletine. The inward current evoked by a pulse to –50 mV and the outward current evoked by a pulse to +50 mV are labelled. C, normalized membrane conductance (g_m) was plotted against the corresponding membrane voltage. g_m was determined from the equation g_m=I_Na/(E_m–E_Na), where I_Na is the peak current, E_m is the amplitude of the voltage step, and E_Na is the estimated reversal potential of the Na⁺ current. Plots were fitted with a Boltzmann function, y= 1/{1 + exp[(V_0.5–V)/k]}. The average midpoint voltage (V_0.5) and slope (k) for hNav1.4 wild-type (○, n= 7; fitted value ±s.e.m. of the fit) were –29.4 ± 0.7 mV and 10.1 ± 0.7 mV, respectively, and –34.5 ± 0.5 mV and 7.6 ± 0.4 mV for the mexiletine-treated cell (•, n= 5; P < 0.05), respectively. Cells were cotransfected with the β1 subunit. Holding potential was set at –140 mV and the time interval between pulses was 10 s.