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Calcium inactivation in skeletal muscle fibres of the stick insect, Carausius morosus.
Author(s) -
Ashcroft F M,
Stanfield P R
Publication year - 1982
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1982.sp014345
Subject(s) - chemistry , pulse (music) , calcium , biophysics , time constant , skeletal muscle , voltage clamp , exponential decay , sodium , reaction rate constant , kinetics , anatomy , membrane potential , biochemistry , voltage , biology , physics , organic chemistry , quantum mechanics , nuclear physics , electrical engineering , engineering
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.