Activation and inactivation curves. Peak Na⁺ conductance (G_Na) was measured during a 25-millisecond depolarization to various test potentials from a holding potential of –120 mV to characterize steady-state activation for WT (filled circles; n = 47), F1490L (filled diamonds; n = 28), M1493I (filled squares; n = 21), and double mutant F1490L-M1493I (open circles; n = 24). G_Nais calculated from the relation G_Na = I_Na/(V – V_rev), where I_Na is the peak inward Na⁺ current during the test depolarization (V) and V_rev is the Na⁺ reversal potential. Data are normalized to maximum peak conductance (G_max) and fit to a two-state Boltzmann distribution: G_Na/G_max = (1 + exp[(V – V_1/2)/k])^–1, where V_1/2 is the test potential for half-maximal Na⁺ activation and k determines the steepness of the voltage dependence. Inactivation curves plotted for Na⁺ currents for WT (filled circles; n = 43), F1490L (filled diamonds; n = 29), M1493I (filled squares; n = 19), and double mutant F1490L-M1493I (open circles; n = 34). Cells were held at –120 mV and subjected to a 200-millisecond conditioning pulse ranging from –120 to 0 mV followed by a 25-millisecond test pulse to 0 mV. Values are mean ± SEM.