The effects of external potassium and long duration voltage conditioning on the amplitude of sodium currents in the giant axon of the squid, Loligo pealei.

Giant axons were voltage-clamped in solutions of constant sodium concentration (230 mM) and variable potassium concentrations (from 0 to 210 mM). The values of the peak initial transient current, I(p), were measured as a function of conditioning prepulse duration over the range from less than 1 msec to over 3 min. Prepulse amplitudes were varied from E(m) = -20 mv to E(m) = -160 mv. The attenuation of the I(p) values in high [K(o)] was found to vary as a function of time when long duration conditioning potentials were applied. In both high and low [K(o)], I(p) values which had reached a quasi-steady-state level within a few milliseconds following a few milliseconds of hyperpolarization were found to increase following longer hyperpolarization. A second plateau was reached with a time constant of about 100-500 msec and a third with a time constant in the range of 30 to 200 sec. The intermediate quasi-steady-state level was absent in K-free ASW solutions. Sodium inactivation curves, normalized to I(pmax) values obtained at either the first or second plateaus, were significantly different in different [K(o)]. The inactivation curves, however, tended to superpose after about 1 min of hyperpolarizing conditioning. The time courses and magnitudes of the intermediate and very slow sodium conductance restorations induced by long hyperpolarizing pulses are in agreement with those predicted from the calculated rates and magnitudes of [K(+)] depletion in the space between the axolemma and the Schwann layer.