The effect of holding potential on the asymmetry currents in squid giant axons

Abstract

1. Asymmetry currents were recorded from intracellularly perfused squid axons subjected to an equal number of exactly equal positive and negative voltage clamp pulses. The asymmetry currents consisted of an on-response at the beginning of the pulses and an off-response at the end of the pulses.

2. The asymmetry currents were markedly reduced by 30 mM glutaraldehyde applied internally.

3. Clamp pulses of varying height, superimposed on a holding potential of — 80 to — 100 mV, were used to study the voltage and time dependence of the asymmetry current. The magnitude of the on- and off-response increased with increasing pulse height along a sigmoid curve. The time constants of the on- and off-response depended on the potential during the depolarizing pulses; the time constant of the on-response had a maximum at an internal potential of — 12 mV, the time constant of the off-response was largest at positive internal potentials.

4. Holding the membrane at a potential of — 80 to — 100 mV for several minutes led to a slow increase of the size of the asymmetry current.

5. Changing the holding potential from — 80 or — 100 mV to a less negative potential caused a decrease of the asymmetry current. At holding potentials less negative than — 65 or — 60 mV the asymmetry current reversed its sign: the transient current at the beginning of the pulses turned into an inward current and the transient current at the end of the pulses became outward. No inactivation of the asymmetry current was seen in the range of holding potentials studied (V < — 3 mV).

6. The results are generally consistent with the idea that the asymmetry currents are in some way related to the opening and closing of the Na gates; they suggest, however, that the asymmetrical charge movement does not simply reflect the voltage and time dependence of the m system.