title: Uptake and caffeine-induced release of calcium in fast muscle fibers of Xenopus laevis: effects of MgATP and P(i).
abstract: 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.
authors.affiliation: Laboratory for Physiology, Free University, Amsterdam, The Netherlands.
authors.forename: G J
authors.initials: GJ
authors.lastname: Stienen
authors.forename: I A
authors.initials: IA
authors.lastname: van Graas
authors.forename: G
authors.initials: G
authors.lastname: Elzinga
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D009132:Muscles / Q000378:metabolism*; D010710:Phosphates / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D010710:Phosphates; D002110:Caffeine; D000255:Adenosine Triphosphate; D002118:Calcium