Ca2+ modulation of sarcoplasmic reticulum Ca2+ release in rat skeletal muscle fibers.
Ca2+ transients and the rate of Ca2+ release (dCaREL/dt) from the sarcoplasmic reticulum (SR) in voltage-clamped, fast-twitch skeletal muscle fibers from the rat were studied with the double Vaseline gap technique and using mag-fura-2 and fura-2 as Ca2+ indicators. Single pulse experiments with different returning potentials showed that Ca2+ removal from the myoplasm is voltage independent. Thus, the myoplasmic Ca2+ removal (dCaREM/dt) was studied by fitting the decaying phase of the Ca2+ transient (Melzer, Ríos & Schneider, 1986) and dCaREL/dt was calculated as the difference between dCa/dt and dCaREM/dt. The fast Ca2+ release decayed as a consequence of Ca2+ inactivation of Ca2+ release. Double pulse experiments showed inactivation of the fast Ca2+ release depending on the prepulse duration. At constant interpulse interval, long prepulses (200 msec) induced greater inactivation of the fast Ca2+ release than shorter depolarizations (20 msec). The correlation (r) between the myoplasmic [Ca2+]i and the inhibited amount of Ca2+ release was 0.98. The [Ca2+]i for 50% inactivation of dCaREL/dt was 0.25 microM, and the minimum number of sites occupied by Ca2+ to inactivate the Ca2+ release channel was 3.0. These data support Ca2+ binding and inactivation of SR Ca2+ release.
Delbono
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Department of Physiology, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, N.C. 27157, USA.
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D016257:Fura-2; D017136:Ion Transport; D018485:Muscle Fibers, Skeletal / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D002118:Calcium; D016257:Fura-2