Active calcium uptake by islet-cell endoplasmic reticulum

Active calcium uptake was demonstrated in a subcellular fraction of islets which was enriched in endoplasmic reticulum. Calcium uptake was stimulated by ATP in a magnesium-dependent manner. The rate of calcium accumulation was sustained by oxalate (10 mM) and uptake was prevented or reversed by addition of the calcium ionophore A23187. This calcium uptake process was not affected by azide or ruthenium red. Direct comparison of calcium uptake by endoplasmic reticulum-enriched and plasma membrane-enriched fractions indicated that the uptake was not due to contamination of the fraction with plasma membrane vesicles. These factors as well as the purity of the fraction indicate that the calcium uptake system resides in the endoplasmic reticulum. The properties of the islet endoplasmic reticulum calcium uptake system are similar to properties reported for endoplasmic reticulum derived from other cell types. These properties include stimulation by potassium and a Km for ionized calcium of 1.5 +/- 0.3 microM. The islet-cell endoplasmic reticulum may play a critical role in cellular calcium homeostasis and contribute to the regulation of insulin secretion.