Voltage-dependence of Na⁺ channel inactivation. (a) Inactivation time constants were obtained with a protocol of conditioning and test pulses and plotted against conditioning voltage (n = 6–8 for hH1; n = 3–9 for ΔΚ1500). At potentials of –140, –120, and –100 mV, conditioning was effected by trains of twenty 40-ms pulses to –20 mV, followed by 20-ms test pulses. At potentials of –90, –80, –70, and –60 mV, a single conditioning pulse preceded each test pulse. (b) The relaxation phase of the Na⁺ current was fit with a single exponential function and plotted against the test potential (n = 10 for hH1; n = 11 for ΔΚ1500). Both measures show a reduction in the voltage-dependence of inactivation in the ΔΚ1500 mutant channels.