Effect of calcium upon sodium inactivation in the giant axon of Loligo pealei.

Giant axons of Loligo pealei were voltage clamped in artificial seawater solutions containing varying concentrations of calcium from 10 to 100 mM, and the sodium conductance inactivation was measured with a series of two-pulse experiments. The h infinity vs. voltage curve showed a shift of about 10 mV in the depolarizing direction on the voltage axis for a tenfold increase in external calcium without substantial alteration in the slope of the voltage dependence. The kinetics of the inactivation process were found to be exponential for hyperpolarizing prepulses, but showed some indication of a sigmoidal decay for depolarizing prepulses in all calcium concentrations employed. Increasing calcium increased the delay in the sigmoidal response. The inactivation time constant tauh increased as a function of calcium concentration over the potential range studied, -10 to -90 mV. The values of the rate constants alphah and betah are decreased with an increase in calcium and these effects are not consistent with parallel shifts of the rate constant vs. voltage curves along the voltage axis for changes in calcium concentration. Magnesium does not behave as an equimolar substitute for calcium. The effect of a solution containing 10 mM calcium and 50 mM magnesium is intermediate to that of solutions containing 10 and 30 mM calcium alone. Predictions of a recent model for the sodium conductance (Moore, J.W., Cox, E.B., 1976 Biophys. J. 16:171) which employs calcium binding were compared with the experimental data.