Activation, inactivation and recovery in the sodium channels of the squid giant axon dialysed with different solutions

Abstract

Comparisons were made between families of ion currents recorded in voltage-clamped squid axons dialysed with 20 mM NaF and 330 mM CsF or TMAF, and bathed in a solution in which four fifths of the Na was replaced by Tris. The permeability coefficient P Na,fast for the fast-inactivating current in the initial open state was calculated as a function of test potential from the size of the initial peak of I Na . The permeability coefficient P Na,non for the non-inactivating open state was calculated from the steady-state I Na that persisted until the end of the test pulse. Dialysis with TMA had no direct effect on the Qv curve for gating charge. The reversal potential for I Na,non was always lower than that for I Na,fast , the mean difference being about — 9 mV when dialysing with Cs, but only about — 1 mV with TMA. Except close to threshold, P Na,fast was roughly halved by dialysis with TMA as compared with Cs, but P Na,non was substantially increased. The time constant τ h for inactivation of the sodium system was slightly increased during dialysis with TMA in place of Cs, and there were small shifts in the steady-state inactivation curve, but the rate of recovery from inactivation was not measurably altered. The flattening off of the τ h curve at increasingly positive test potentials corresponded to a steady reduction of the apparent inactivation charge until a value of about 0.2 e was reached for pulses to 100 mV. The instantaneous I-V relationship in the steady state was also investigated. The results have a useful bearing on the effects of dialysis with TMA, on the differences between the initial and steady open states of the sodium channel, and on the relative voltage-dependences of the transitions in each direction between the resting and inactivated states.