Deterministic inactivation of calcium release channels in mammalian skeletal muscle.
Enzymatically dissociated fibres from the extensor digitorum communis muscle of rats were mounted into a double Vaseline gap chamber. The rate of calcium release (R(rel)) from the sarcoplasmic reticulum (SR) and changes in SR permeability to Ca2+ (PSR) were calculated from measured changes in intracellular calcium concentration. Calcium release during a prepulse attenuated the inactivating component of PSR of the subsequent test pulse. The suppression was graded, larger release causing greater suppression, as expected from a calcium-dependent inactivation process. However, if the dissociation constant of the putative inhibitory calcium binding site (Kd) was estimated using different test pulses different affinities were obtained: a smaller test pulse yielded a smaller Kd. Comparing the suppression of the inactivatable component of PSR during the test pulse (suppression) with the inactivatable component during the prepulse (pre-inactivation) revealed a linear relationship with a regression coefficient close to unity. Lowering intracellular magnesium by decreasing its concentration to 25 microM in the internal solution altered the time course of PSR. The maximal peak-to-steady-level ratio was increased to 6.3 +/- 0.4 (n = 10, mean +/- s.e.m.) from a control value of 3.0 +/- 0.2 (n = 19). Despite the apparent change in steady-state inactivation, suppression remained equal to that pre-inactivation. Our results support the view that a depolarizing pulse always recruits the same set of calcium release channels and a portion of these channels undergoes a deterministic inactivation process.
Szentesi
P
P
Department of Physiology and Cell Physiology Research Group of the Hungarian Academy of Sciences, Medical and Health Sciences Centre, Medical School, University of Debrecen, Hungary.
Kovács
L
L
Csernoch
L
L
D000818:Animals; D015220:Calcium Channels / Q000502:physiology*; D004558:Electric Stimulation; D005552:Forelimb; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008564:Membrane Potentials; D018482:Muscle, Skeletal / Q000378:metabolism*; D009994:Osmolar Concentration; D051381:Rats
D015220:Calcium Channels; D008274:Magnesium