The influence of voltage conditioning on chloride currents in amphibian muscle membrane: the sigmoid conductance-voltage relation extended into the outward current region.

Sartorius muscle fibres of Xenopus laevis were depolarized in solutions of high K+ content and then voltage clamped in solutions in which K+ was replaced by Rb+ and(or) tetraethylammonium ion (TEA+) at pH 9. A three-microelectrode clamp system was used in which the bath was held at virtual ground using an operational amplifier in a current meter configuration. The holding potential was set at zero membrane current potential (resting potential), close to -25 mV. A two-pulse paradigm was used to test the effects of conditioning the membrane at voltages away from the resting potential on initial currents at positive test potentials. In the absence of TEA+ rapidly rising outward currents were generated at positive test potentials, following hyperpolarizing conditioning. These currents inactivated in time and obscured predicted chloride currents. When TEA+ was added to the solution (60 mequiv./L) the currents at positive potentials rose more slowly and declined either very slowly or not at all. Projection of these current waveforms, by curve fitting, to the instant of potential change gave a sigmoid dependence of test current on conditioning voltage that was predicted from earlier results. Predictably there is a test voltage at which the initial current is independent of conditioning potential: from the data it appears that this is not necessarily the resting potential, but the cause of the shift is not clear. The results also indicated that there is a component of outward current that is very small, apparently carried by cations ("delayed rectifier current"), that does not inactivate, even at potentials more positive than the resting potential.