Effect of sarcoplasmic reticulum (SR) calcium content on SR calcium release elicited by small voltage-clamp depolarizations in frog cut skeletal muscle fibers equilibrated with 20 mM EGTA.
Cut muscle fibers from Rana temporaria (sarcomere length, 3.5-3.9 micro(m); 14-16 degreesC) were mounted in a double Vaseline-gap chamber and equilibrated with an external solution that contained tetraethyl ammonium- gluconate and an internal solution that contained Cs as the principal cation, 20 mM EGTA, and 0 Ca. Fibers were stimulated with a voltage-clamp pulse protocol that consisted of pulses to -70, -65, -60, -45, and -20 mV, each separated by 400-ms periods at -90 mV. The change in total Ca that entered into the myoplasm (Delta[CaT]) and the Ca content of the SR ([CaSR]) were estimated with the EGTA/phenol red method (Pape, P.C., D.-S. Jong, and W.K. Chandler. 1995. J. Gen. Physiol. 106:259-336). Fibers were stimulated with the pulse protocol, usually every 5 min, so that the resting value of [CaSR] decreased from its initial value of 1,700-2, 300 microM to values near or below 100 microM after 18-30 stimulations. Three main findings for the voltage pulses to -70, -65, and -60 mV are: (a) the depletion-corrected rate of Ca release (release permeability) showed little change when [CaSR] decreased from its highest level (>1,700 microM) to approximately 1,000 microM; (b) as [CaSR] decreased below 1,000 microM, the release permeability increased to a maximum level when [CaSR] was near 300 microM that was on average about sevenfold larger than the values observed for [CaSR] > 1,000 microM; and (c) as [CaSR] decreased from approximately 300 microM to <100 microM, the release permeability decreased, reaching half its maximum value when [CaSR] was approximately 110 microM on average. It was concluded that finding b was likely due to a decrease in Ca inactivation, while finding c was likely due to a decrease in Ca-induced Ca release.
Département de physiologie et biophysique, Université de Sherbrooke Faculté de Médecine, Sherbrooke, Québec J1H5N4, Canada. p.pape@courrier.usherb.ca
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D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D015220:Calcium Channels / Q000502:physiology; D002614:Chelating Agents / Q000494:pharmacology*; D004533:Egtazic Acid / Q000494:pharmacology*; D007425:Intracellular Membranes / Q000502:physiology; D015640:Ion Channel Gating / Q000502:physiology; D008856:Microscopy, Fluorescence / Q000295:instrumentation; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000737:chemistry / Q000502:physiology* / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000737:chemistry / Q000166:cytology* / Q000648:ultrastructure; D055095:Optics and Photonics / Q000295:instrumentation; D018408:Patch-Clamp Techniques; D011896:Rana temporaria; D012519:Sarcoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D013997:Time Factors
D015220:Calcium Channels; D002614:Chelating Agents; D004533:Egtazic Acid; D002118:Calcium