Calcium inactivation in skeletal muscle fibres of the stick insect, Carausius morosus.

Abstract

Inactivation of Ca currents in skeletal muscle fibers of the stick insect, C. morosus, was studied using a 3-electrode voltage-clamp method. The extent of inactivation showed a voltage-dependence similar to that of the Ca current, inactivation being absent in the absence of a Ca current, maximal at potentials where Ca currents are largest, and reduced at potentials close to ECa. Ca currents inactivated along a double exponential time course, both when measured from the decline of Ca current during a single pulse and when measured using a 2 pulse protocol. In 20 mM-Ca-Ringer the fast time constant of inactivation had a mean value of 27 ms and that of the slow time constant was 134 ms, at 0 mV and 5.degree. C. The rate of inactivation was slowed, and its extent reduced, in low [Ca]0, where Ca currents were smaller. Similarly, inactivation was faster and more complete in high-Ca-Ringer. The rate of recovery from inactivation also followed a double exponential time course, with time constants of 638 ms and 4 s following a 500 ms inactivating of Ca entry during the inactivating pulse, being slower in high [Ca]0 and following longer inactivating pulses. Inactivation was slowed and reduced in extent when Ba2+ or Sr2+ carried current. Inactivation in Ba solutions may be due to depletion of Ba2+ from the lumen of the transverse tubules. Ba2+ does not compete with Ca2+ for the inactivation mechanism. Inactivation of Ca currents in stick insect muscle fibers is primarily mediated by Ca2+ entry.