1. In the presence of MgATP and increasing amounts of added Ca2+, isolated liver microsomal vesicles accumulate approx. 10 nmol of Ca2+/mg of protein and buffer ambient free Ca2+ at increasing concentrations (0.22-10.9 microM). Under these experimental conditions, microsomal glucose-6-phosphatase activity is unaffected by the concentration of extravesicular free Ca2+. 2. Different levels of intravesicular Ca2+ were obtained by treating microsomes with the Ca2+ ionophore A23187 and by stimulating active microsomal Ca2+ accumulation with Pi (3 mM). In both instances, microsomal glucose-6-phosphatase activity is unaffected by the level of intravesicular Ca2+.
