Electrophysiological characterization of slow inactivation in a mutant channel Na_VSp1-170C. (A) Use-dependent development of slow inactivation: depolarizations from a holding potential of −120 mV to 20 mV were applied at 0.2 Hz (left, for 200 ms in duration) and 1 Hz (right, for 150 ms in duration) frequencies, and the normalized peak current for each pulse was plotted as a function of the pulse number for Na_VSp1-WT (gray squares) and Na_VSp1-F170C (blue squares). The error bars denote SDs from n = 4 for Na_VSp1-WT and n = 5 for Na_VSp1-F170C. (B) Voltage dependence of inactivation: peak currents were measured during test pulses to 20 mV after a 2-s inactivating pulse to the indicated potentials. Values from individual experiments were normalized to the maximum test pulse currents. Inactivation curves were fit with a Boltzmann equation, 1/1+exp[(V−V_h)/k_h], where V_h is the half-inactivation voltage and k_h is the slope factor. For Na_VSp1-WT, mean V_h was −30.3 ± 1.7 mV (n = 4); for Na_VSp1-F170C (n = 4), V_h was −35.7 ± 1.3 mV. (C) Time constants of inactivation versus voltage curves for Na_VSp1-WT (gray squares, n = 4) and Na_VSp1-F170C (blue squares, n = 4) obtained by fitting a single exponential to the current decay during depolarization to shown potentials from a holding potential of −120 mV.