Calcium inactivation in skeletal muscle fibres of the stick insect, Carausius morosus.
Ashcroft FM., Stanfield PR.
1. Inactivation of Ca currents in skeletal muscle fibres of the stick insect, Carausius morosus, was studied using a three-electrode voltage-clamp method. 2. 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. 3. 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 two pulse protocol. In 20 mM-Ca-Ringer the fast time constant of inactivation had a mean value of 27 msec and that of the slow time constant was 134 msec, at O mV and 5 degrees C. 4. The rate of inactivation was slowed, and its extent reduced, in low [Ca]o, where Ca currents are smaller. Similarly, inactivation was faster and more complete in high-Ca-Ringer. 5. The rate of recovery from inactivation also followed a double exponential time course, with time constants of 638 msec and 4 sec following a 500 msec inactivating pulse in 20 mM-Ca-Ringer at 5 degrees C. Recovery appeared to be related to the amount of Ca entry during the inactivating pulse, being slower in high [Ca]o and following longer inactivating pulses. 6. 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. 7. Ba2+ does not compete with Ca2+ for the inactivation mechanism. 8. It is concluded that inactivation of Ca currents in stick insect muscle fibres is primarily mediated by Ca2+ entry.