Inactivation of the asymmetrical displacement current in giant axons of Loligo forbesi.

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Abstract
Asymmetrical displacement currents (gating currents) were recorded in intracellularly perfused squid giant axons by averaging the currents associated with depolarizing and hyperpolarizing pulses. The relation between gating current and Na inactivation was studied by investigating the effect of pulse duration and conditioning pulses. Increasing the pulse duration from 0.3-1 ms to 10-20 ms reduced the off-response of the gating current to 50-70% of its normal size; the time constant was 5 ms at +20 mV and 8.degree. C. The decrease of Na current during a 10-20 ms pulse was stronger and faster; it decayed to 10-26% with a time constant of 1.35 ms. The effect of pulse duration could be demonstrated by using only depolarizing pulses. The charge displacement at the end of single or averaged depolarizing pulses was smaller for long pulse durations than for short. A long depolarizing pulse was followed by a small long-lasting tail of inward current. A conditioning depolarizing pulse of 10-20 ms duration to a potential of -30 or +10 mV, followed by a short recovery period at -70 mV, decreased the on-response of the gating current. Its size was reduced to 46-71% and 61-94%, respectively, for a recovery interval of 1.75 and 5 ms at 2-3.degree. C. The reduction of Na current, measured under similar conditions, was more pronounced; the Na current was decreased to less than 50% of its normal value. Part of the asymmetrical charge displacement was inactivated during a 10-20 ms depolarization.