Uptake, retention, and efflux of Ca2+ by mitochondrial preparations from skeletal muscle.
Functionally intact mitochondria, substantially free of contamination, were isolated from rabbit gastrocnemius muscle after protease digestion and their Ca2+-handling properties examined. When judged by their capacity to retain large Ca2+ loads and the magnitude of basal and Na+-stimulated Ca2+ effluxes, the most suitable isolation method was digestion of finely minced muscle in buffered isoosmotic KCl with low levels (0.4 mg/g) of trypsin or the bacterial protease nagarse, followed by differential centrifugation. Polytron disruption of skeletal muscle in both sucrose- and KCl-based media released mitochondria deficient in cytochrome c. Use of the divalent ion chelator EDTA rather than EGTA in the isolation medium sharply reduced Ca2+-dependent respiratory control and tolerance of the mitochondria to Ca2+ loads, probably by removing Mg2+ essential to membrane integrity. ADP-dependent respiratory control was not altered in mitochondria prepared in an EDTA-containing isolation medium. Purification of mitochondria on a Percoll density gradient did not improve their Ca2+-handling ability despite removal of minor contaminants. Mitochondria prepared by the protease method could accumulate micromole loads of Ca2+/mg while maintaining a low basal Ca2+ efflux. Addition of BSA to the assay medium slightly improved Ca2+ retention but was not essential either during isolation or assay. Ca2+-dependent state 3 respiration was maximal at pH 6.5-7.0 while respiratory control and Ca2+/O were optimal at pH 7.0-7.5. Neither Pi nor oxaloacetate induced Ca2+ release from loaded mitochondria when monitored for 30 min after ruthenium red addition. Na+-stimulated Ca2+ efflux had sigmoidal kinetics with a Hill coefficient of 3. Since skeletal muscle mitochondria can be isolated and assayed in simple media, functional deficiencies of mitochondria from diseased muscle are unlikely to be masked.
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002458:Cell Fractionation; D004533:Egtazic Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008931:Mitochondria, Muscle / Q000378:metabolism*; D010101:Oxygen Consumption / Q000187:drug effects; D010710:Phosphates / Q000494:pharmacology; D011817:Rabbits; D012710:Serum Albumin, Bovine / Q000494:pharmacology
D010710:Phosphates; D012710:Serum Albumin, Bovine; D004533:Egtazic Acid; D000244:Adenosine Diphosphate; D002118:Calcium