Ca-ATPases in pancreatic islets.

Mg-independent Ca-ATPase activity was measured in secretory granules, mitochondria and microsomes from albino mouse islets and in secretory granules from noninbred ob/ob mouse islets. The enzyme existed in a high-affinity (Km for Ca2+ approx. 10(-7) M) and a low-affinity (Km approx. 10(-5) M) form. In all subfractions the high-affinity Ca-ATPase was inhibited by cyclic AMP, caffeine and Na+. Alloxan stimulated the microsomal Ca-ATPase by 25%, but had no effect on Ca-ATPase activity in granules and mitochondria. Glucose and glucose metabolites had no effect on Ca-ATPase in the secretory granule fraction from ob/ob mouse islets, whereas NADH inhibited the enzymes by 35%. The secretory granule Ca-ATPase was also inhibited by pCMBS (43%), chlorpromazin (87%) and ruthenium red (23%). 45Ca uptake was studied in secretory granules isolated from ob/ob mouse islets. The uptake was accelerated by addition of ATP, the maximum effect being found at 1 to 2 mM ATP. Omission of MgCl2 decreased the uptake by 25%. 45Ca uptake was abolished in the presence of pCMBS and chlorpromazine, whereas caffeine had no effect. The importance of Ca-ATPase in 45Ca transport and regulation of insulin release is discussed.