Experimental analysis of the relationship between charge movement components in skeletal muscle of Rana temporaria.

Abstract

Experiments were performed to ascertain whether the monotonic (q.beta.) and delayed (q.gamma.) components of nonlinear charge in skeletal msucle membranes form a sequential system or are the result of separate, independent processes. The non-linear capacitance studied in a large number of fibers increased with fiber diameter. This dependence was attributable to tetracaine-sensitive (q.gamma.) but not to tetracaine-resistant (q.beta. and q.alpha.) charge. The kinetic and total quantity of q.gamma. charge moving in response to voltage steps from varying pre-pulse potentials to a fixed probe potential remained constant despite variations in the size of the early q.beta. decay. The kinetics of the delayed (q.gamma.) charging current obtained from a single 20 mV depolarizing step were compared with the sum of the responses to two 10 mV steps adding to the same voltage excursion. The respective transients superimposed only if one of the 10 mV steps did not reach the voltage at which q.gamma. first appears. In the 2 preceding experiments, total charge was conserved. These results are consistent with separate and functionally independent q.beta. and q.gamma. systems of potential-dependent charge, with q.gamma. residing in the transverse tubules and q.beta. on surface membrane. The findings can be discussed in terms of a contractile activator with a steep sensitivity to voltage that begins only with depolarization beyond a level close to the actual mechanical threshold.