Differential properties of two charge components in frog skeletal muscle

Abstract

Charge movement in frog twitch muscle fibres bathed in a moderately hypertonic solution (containing 350 mM-sucrose) showed a main component (Q beta) and a secondary hump (Q gamma). Assuming that Q beta decays with a single exponential and Q gamma follows the time course of a symmetrical bell, a mathematical model was developed to separate Q gamma from Q beta. The activation curves of Q beta and Q gamma can be individually fitted by the two-state model of Schneider & Chandler (1973). The voltage distribution of Q gamma is approximately twice as steep as that of Q beta and the maximum moveable amount of Q beta about four times that of Q gamma. The decay rate constant of Q beta, expressed as a function of potential, follows the familiar U-shaped curve whereas the reciprocal of the time-to-peak of Q gamma rises linearly with increasing potentials. Application of the mathematical dissection technique to Qon values in solutions of varying concentrations of tetracaine yielded the dose dependence of the suppression of Q gamma by tetracaine. Q gamma inactivates more steeply than Q beta as the holding potential is made less negative. Each of the steady-state inactivation curves of Q beta and Q gamma forms a mirror image with the respective activation curve. It is speculated that Q gamma might be more directly involved than Q beta in triggering calcium release and activating contraction.