Fig. 3.

A complete BTX-resistant phenotype in 5AA-rNav1.4 mutant channels. (A) Superimposed traces of 5AA mutant Na⁺ currents were recorded during repetitive pulses (+50 mV for 20 ms at 2 Hz). The pipette solution contained 5 μM BTX. The arrows correspond to the current traces after 1, 500, 1,000, 1,500 and 2,000 pulses. The residual maintained current after 1,500 pulses was small (1.1 ± 0.3% of the peak amplitude, n = 16). (B) For the current–voltage relationship, peak Na⁺ currents were recorded at various voltages in 10-mV increments from −120 to +80 mV for 10 ms. Peak Na⁺ currents were measured, converted to conductance, normalized with respect to conductance at +80 mV, and plotted against voltage. Conductance was determined by the equation g_m = I_Na/(E_m − E_Na), where I_Na is the peak current amplitude; E_m is the voltage applied, and E_Na is the estimated reversal potential (11). The plot was fitted with a Boltzmann function, which gave a midpoint voltage (V_0.5) and a slope factor (k_a) of −7.4 ± 1.2 mV and 15.2 ± 1.0 mV, respectively (n = 16). (C) The steady-state inactivation (h_∞) was measured by a conventional 2-pulse protocol (11). Currents were evoked by 5-ms test pulses to +50 mV after 100-ms conditioning pulses ranging from −150 to −30 mV in 5-mV increments. Peak currents at the test pulse were measured, normalized with respect to the maximal peak current, and plotted against the conditioning voltage. The plot was fitted with a Boltzmann function; the fitted midpoint (h_0.5) and slope factor (k_h) were −83.9 ± 0.2 mV and 6.2 ± 0.1 mV (n = 16), respectively. p, pulse(s).