Charge movement and mechanical repriming in skeletal muscle.

Abstract

Muscles from Rana temporaria were placed in a solution which depolarized the membrane to -30 to -20 mV so that mechanical activation was made refractory. Mechanical repriming and the recovery of voltage dependent charge movement were studied using a voltage clamp technique. Mechanical repriming was investigated by determining the duration of a hyperpolarizing pulse required to elicit a just-visible contraction for various post-pulse potentials. As the post-pulse potential was made more positive shorter repriming times were required to produce a threshold contraction. The relationship approached a minimum repriming time for very positive post-pulse potentials. Hyperpolarization apparently gradually removes some component of the activation mechanism from a refractory state and the effectiveness of the amount which has recovered depends on the post-pulse potential. A quantitative explanation is given using a simple model in which the essential component is assumed to be the charge movement process. The rate of repriming contraction is voltage dependent; at -160 mV the rate is about twice that at -120 mV. Between 4 and 10.degree. C the rate has a Q10 of about 9. Recovery of charge movement was studied using a repriming duration less than that required to produce a threshold contraction. The observed charge movement increased linearly with repriming time, consistent with the approximately linear initial segment of a slow exponential recovery process. Extrapolation of the recovery curve indicated that 2-5 nC/.mu. farads of charge is reprimed in the time necessary to reprime a threshold contraction. The charge which recovers during a subthreshold repriming pulse is distributed according to membrane potential in the same way as a fully reprimed charge. These results are consistent with the hypothesis that voltage dependent charge movement is an intermediate step in excitation-contraction coupling. The characteristics of a 2nd type of charge movement are also described.