Glucose-stimulated sequestration of Ca2+ in clonal insulin-releasing cells. Evidence for an opposing effect of muscarinic-receptor activation.

Abstract

Net fluxes of Ca2+ and acid production were studied in clonal insulin-releasing cells (RINm5F) by using colour indicators and dual-wavelength spectrophotometry. After equilibration with a medium containing 10-20 microM-Ca2+, only minimal amounts of Ca2+ (0.08 mmol/kg of protein) were released from the cells by subsequent additions of the respiratory blocker antimycin A and the Ca2+ ionophore A23187. The presence of 20 mM-glucose resulted in an almost 5-fold increase of the acid production and in a stimulated net uptake of Ca2+. The latter process was independent of the extracellular Ca2+ concentration and reached saturation after 20 +/- 1 min, when it corresponded to 1.18 +/- 0.07 mmol of calcium/kg of protein. Whereas the thiol reagent iodoacetamide suppressed the acid production, interference with mitochondrial function by using antimycin A or the uncoupler carbonyl cyanide m-chlorophenylhydrazone had the opposite effect. The latter two drugs induced a selective release of Ca2+ from a pool containing 35% of that taken up during glucose exposure. Most of the remaining Ca2+ was liberated by A23187 or iodoacetamide. Carbamoylcholine was also selective in mobilizing glucose-stimulated calcium, but this calcium (17%) appeared to originate from the pool insensitive to mitochondrial poisons. The action of carbamoylcholine was blocked by atropine and did not depend on the presence of extracellular Na+. The opposite effects of glucose and muscarinic-receptor activation on a non-mitochondrial calcium pool are consistent with participation of the endoplasmic reticulum in the glucose-induced sequestration of Ca2+ in pancreatic beta-cells.