Am J Physiol Cell Physiol AJP: Renal Physiology
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Am J Physiol Cell Physiol 265: C650-C657, 1993;
0363-6143/93 $5.00
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AJP - Cell Physiology, Vol 265, Issue 3 C650-C657, Copyright © 1993 by American Physiological Society

ARTICLES

Uptake and caffeine-induced release of calcium in fast muscle fibers of Xenopus laevis: effects of MgATP and P(i)

G. J. Stienen, I. A. van Graas and G. Elzinga
Laboratory for Physiology, Free University, Amsterdam, The Netherlands.

To elucidate the origin of the reduction in force during prolonged muscle fatigue, the dependency of Ca2+ uptake and release on MgATP and P(i) concentration was studied in saponin-skinned fast skeletal muscle fibers of the iliofibularis muscle of Xenopus laevis at 3 degrees C. The sarcoplasmic reticulum was loaded with Ca2+ for 5 min at pCa 7.0. The amount of Ca2+ released was derived from the area of the caffeine-induced force response. Ca2+ uptake increased with the MgATP concentration present during loading. It was half maximal at 20 microM and saturated at higher concentrations. The kinetics of Ca2+ release were affected for MgATP concentrations between 0.1 and 0.5 mM or less, but the amount of Ca2+ released by caffeine in ATP-free solutions was substantial. Phosphate (15 mM) only slightly reduced Ca2+ uptake when the loading period was short (1 min). It is unlikely, therefore, that the reduction in MgATP concentration contributes to the depression of Ca2+ released from the sarcoplasmic reticulum during fatigue. The increase in P(i) concentration could play a small role by reducing Ca2+ uptake.


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