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

Abstract

Factors modifyingcalcium 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