Figure 12. Open-channel block in chloramine-T-treated hNav1.4 Na⁺ channels by mexiletine.

A, superimposed Na⁺ currents were recorded at various concentrations of mexiletine. Before these records were taken, the cell was treated with 0.5 mm chloramine-T for 2.5 min and then washed with drug-free bath solution for 5 min. The Na⁺ currents were evoked by a 400 ms test pulse to +30 mV every 30 s. A steady state at each mexiletine concentration was established before application of the next solution. TTX at 1 μm was also applied to identify hNav1.4 Na⁺ currents. Treatment with chloramine-T was limited to 2.5 min since prolonged incubation of this oxidant increased the non-linear leak currents. B, the dose–response curve for the open-channel block (relative block at the end of the 400 ms test pulse) was constructed using the data set as shown in A. All pulses were delivered at 30 s intervals. The amplitudes of Na⁺ current were measured, normalized to the amplitude of the control, and plotted against the mexiletine concentration. Leak currents after 1 μm TTX were subtracted from the current measurements. Continuous lines represent fits to the data with the Hill equation. IC₅₀ values ±s.e.m[Hill coefficients ±s.e.m.] are 7.5 ± 0.8 μm[1.0 ± 0.1] for open-channel block (n= 5). Cells were cotransfected with the β1 subunit.