Na⁺ currents during pulse-train stimulation in the wild-type and ΔΚ1500 mutant Na⁺ channels. Trains of 40-ms pulses were applied from a holding potential of –100 mV to a test potential of –20 mV. (a) Normalized currents are plotted against pulse number. The interval between pulses was 50 ms (cycle length 90 ms). The data are from four experiments with the wild-type channel and ten with the ΔΚ1500 mutant. (b) Currents obtained with the ΔΚ1500 mutant channel using 40-ms pulses. (c) The results of one of four additional experiments with the ΔΚ1500 mutant using 100-ms pulses with 50-ms recovery intervals (cycle length 150 ms). P1, first pulse of train; P40, 40th pulse of train.