Electrical and activation properties of skeletal muscle.

Some properties of the membrane of locust and frog skeletal muscle were investigated. It was found that the resting membrane potential (EM) of the retractor unguis muscle of Schistocerca gregaria was dominated by the K-equilibrium potential (EK) which was negative to EM. EK was calculated in two ways: first using estimates of [K]i from experiments where [K]o was changed and secondly, from measurements of [K]i using ion-selective electrodes. These two methods gave estimates for [K]i of about 140 mM (at room temperature, ca. 20°C, in locust saline). The difference between EM and EK was accounted for using constant-field type equations assuming: 1) a small resting permeability to Na-ions, and 2) Cl-ions to be passively distributed. Measurements using C1-selective electrodes showed that, at room temperature, EC1 was not significantly different to EM. A reversible fall in EM could be produced by cooling (to 5-8°C) or by addition of ouabain. Measurements using K-selective electrodes showed a fall in [K]i on cooling. Voltage-clamp experiments on locust skeletal muscle showed the presence of two outward currents. First, the early outward current which activates at a membrane potential of between -20 and -30 mV and secondly, the delayed outward current which activates at positive values of EM and has a reversal potential close to EK. It seems likely that the early outward current will be responsible for the production of graded responses in these muscles under normal conditions. Little evidence was found from voltage-clamp experiments for an inwardly rectifying K-channel in locust muscle, though an asymmetry in the time-course of the change in EM when the external [K] x [C1] product was raised and lowered was observed. Experiments on frog skeletal muscle showed that both the conductance and the time-dependent properties of the inwardly-rectifying K-channel were better described as being dependent on [K]o and EM, rather than (V-EK) as was previously supposed.