Potassium-stimulated 45Ca2+ uptake by subcellular particles of pancreatic beta-cells.

Accumulation of 45Ca2+ by a microsomal fraction from pancreatic beta-cells of obese-hyperglycemic (ob/ob) mice was stimulated by Mg2+ in a concentration-dependent manner when 2 mM ATP was present. Maximum effect was reached at 2 mM Mg2+. Increasing the Na+ concentration from 20 to 115 mM had no significant effect, but K+ (10-123 mM) stimulated 45Ca2+ accumulation markedly when tested in the presence of 2 mM Mg2+. This effect was not due to changes of osmolarity, ionic strength, or the concentration of Cl-. Stimulation by K+ was drastically reduced on omission of Mg2+ from the incubation medium. K+ did not significantly affect 45Ca2+ uptake by a mitochondria-rich fraction of ob/ob mouse islets. It is suggested that K+ may be involved in modifying the subcellular distribution of calcium ions in pancreatic beta-cells.