Gating of skeletal and cardiac muscle sodium channels in mammalian cells

1 Sodium channel ionic current (INa) and gating current (Ig) were compared for rat skeletal (rSkM1) and human heart Na+ channels (hH1a) heterologously expressed in cultured mammalian cells at ∼13 °C before and after modification by site‐3 toxins (Anthopleurin A and Anthopleurin B). 2 For hH1a Na+ channels there was a concordance between the half‐points (V½) of the peak conductance‐voltage (G–V) relationship and the gating charge‐voltage (Q–V) relationship with no significant difference in half‐points. In contrast, the half‐point of the Q–V relationship for rSkM1 Na+ channels was shifted to more negative potentials compared with its G–V relationship with a significant difference in the half‐points of −8 mV. 3 Site‐3 toxins slowed the decay of INa in response to step depolarizations for both rSkM1 and hH1a Na+ channels. The half‐point of the G–V relationship in rSkM1 Na+ channels was shifted by −8.0 mV while toxin modification of hH1a Na+ channels produced a smaller hyperpolarizing shift of the V½ by −3.7 mV. 4 Site‐3 toxins reduced maximal gating charge (Qmax) by 33% in rSkM1 and by 31% in hH1a, but produced only minor changes in the half‐points and slope factors of their Q–V relationships. In contrast to measurements in control solutions, after modification by site‐3 toxin the half‐points of the G–V and the Q–V relationships for rSkM1 Na+ channels demonstrated a concordance similar to that for hH1a. 5 Q max vs. G max for rSkM1 and hH1a Na+ channels exhibited linear relationships with almost identical slopes, as would be expected if the number of electronic charges (e−) per channel was comparable. 6 We conclude that the faster kinetics in rSkM1 channels compared with hH1a channels may arise from inherently faster rate transitions in skeletal muscle Na+ channels, and not from major differences in the voltage dependence of the channel transitions.