Evidence for a transient potassium membrane current dependent on calcium influx in crab muscle fibre.

1. Voltage-clamp experiments were achieved on crab muscle fibre with the double sucrose-gap technique. 2. The accuracy of the imposed voltage has been controlled with an impaled micro-electrode connected to an external circuit. 3. Step depolarizations elicit two kinds of records. In type I fibres, the initial current exhibits only an inward calcium component. In type II fibres, the initial current exhibits a hump, transient outward current, mixed with the calcium current; these fibres exhibit always action potentials with fast repolarization. 4. A potassium origin is suggested for this outward current, due to its dependence on [K]o and its inhibition by TEA. 5. In fibres with a composite initial current, the voltage dependence of the availability of the measured inward current appears complex. It can be shown to be the sum of a simple calcium inactivation (which is observed alone in TEA solution) and a fast potassium inactivation. This potassium conductance is nearly half-available at the resting membrane potential. 6. The origin of the transient outward current is tentatively described. Consecutive to a transient internal increase of calcium ions (due to the calcium current) its activation curve is shifted in an hyperpolarizing direction resulting in an increased activation for an apparent identical depolarization. 7. This fast outward current which overlaps the calcium inward current can account for the low amplitude and the variability of the electrical activity of crab muscle fibres.