Figure 1: Nav1.4-WCW severely disrupts fast inactivation and eliminates inactivation from closed states.

(a) Whole-cell current responses to 20 ms depolarizing voltage steps for rat Nav1.4 (left, only 10 ms shown) and rat Nav1.4-WCW (right) channels at room temperature. Cells were held at −70 mV and hyperpolarized to −120 mV for 20 ms before and following a 20 ms depolarizing pulse from −120 to +50 mV (Nav1.4) or +80 mV (Nav1.4-WCW) in 5–10 mV steps (see inset). (b) Normalized peak conductance–voltage (G–V) relation from recordings as shown in a (mean±s.e.m.). The voltage at which the conductance was half maximal (V_1/2) and the effective charge (z) from single Boltzmann fits to the G–V from individual cells were, for Nav1.4 (mean±s.e.m.): V_1/2=-32.6±1.9 mV, z=3.8±0.4 e⁻, n=5; and for Nav1.4-WCW: V_1/2=-24.9±0.8 mV, z=4.7±0.2 e⁻, n=8. Single Boltzmann fits to the mean were for Nav1.4: V_1/2=-33.3 mV, z=3.4 e⁻, and for Nav1.4-WCW: V_1/2=-24.4 mV, z=4.2 e⁻. (c) Summary of the fraction of the peak current remaining after 10 ms for Nav1.4 and Nav1.4-WCW (mean±s.e.m.). ***t-Test, P<0.001. (d) Nav1.4-WCW whole-cell current responses to a steady-state inactivation protocol consisting of a 20 ms test pulse to −20 mV to assay the fraction of available (that is, non-slow inactivated) channels after a 1 s preconditioning pulse from −80 to 0 mV at room temperature (see inset). Holding potential was –120 mV and a 1 ms hyperpolarizing pulse to −140 mV preceded each test pulse. (e) The normalized peak current response during the test pulse is plotted against the voltage during the preconditioning pulse for five cells (open circles, mean±s.e.m., n=5) fit with a single Boltzmann plus an added constant (solid line; V_1/2=-29.9 mV, z=4.9 e⁻, constant=0.27). The G–V relation for Nav1.4-WCW from b is shown inverted for comparison (dashed line).