Control of calcium efflux from sarcoplasmic reticulum vesicles by external calcium.

Factors modifying calcium efflux from rabbit skeletal muscle sarcoplasmic reticulum vesicles were examined after phosphate-supported calcium uptake reached steady state. Increasing external Ca”+ promoted calcium efflux by a mechanism that exhibited Michaelis-Menten kinetics; V,,, at 25” with 5 mM MgATP as substrate and 50 mM phosphate as calcium-precipitating anion was -400 nmol/mg/min and the level of Ca2+ outside the vesicles at which calcium efflux velocity is half-maximal, Kc,, was 0.8 to 1.0 PM. Replacing 120 mM KC1 with 120 mM NaCl did not appreciably change calcium efflux. Elevation of Mg*+ in the range 0.10 to 1.13 rnM markedly inhibited calcium efflux, Mg*+ acting as a competitive inhibitor with Ca2+ for binding to external sites. This inhibitory effect of Mg2+ resembles that observed for “Ca’+-triggered calcium release” in “skinned” muscle fibers. Variations of the calcium ion concentration inside the microsomal vesicles (Cai) in the range 150 to 750 pM altered the maximal calcium efflux velocity but did not significantly influence Kc-, and reduction of Ca, to 4 to 8 FM by replacing phosphate with 2.5 to 5 mM oxalate as the calciumprecipitating anion did not lower K,,, suggesting that the interaction of calcium-sensitive sites on the external surface of the membranes with Ca*+ is independent of the interaction of the internal sites with Ca2+. Substitution of acetyl phosphate for ATP increased K,, approximately lo-fold, but did not abolish the ability of increasing external Ca”+ to promote calcium efflux. The calcium-activated calcium efflux thus cannot be attributed to reversal of the calcium pump, which is inhibited by high external Ca*+ and ATP, and requires ADP. A number of similarities were found between factors that influence calcium transport via the calcium pump and those that modify calcium efflux.