Insulin release transduction mechanism through acid glucan 1,4-alpha-glucosidase activation is Ca2+ regulated

An important signal involved in glucose-stimulated insulin secretion is transduced through the action of a lysosomal acid, glucan 1,4-alpha-glucosidase. We investigated the Ca2+ dependency of this enzyme activity in relation to insulin release. In isolated islets, increased levels of extracellular Ca2+ induced a large increase in acid glucan 1,4-alpha-glucosidase activity accompanied by a similar increase in insulin release at both substimulatory and stimulatory concentrations of glucose. At low glucose the Ca2+ "inflow" blocker nifedipine unexpectedly stimulated enzyme activity without affecting insulin release. However, nifedipine suppressed 45Ca2+ outflow from perifused islets at low glucose and at Ca2+ deficiency when intracellular Ca2+ was mobilized by carbachol. This nifedipine-induced retention of Ca2+ was reflected in increased acid glucan 1,4-alpha-glucosidase activity. Adding different physiological Ca2+ concentrations or nifedipine to islet homogenates did not increase enzyme activity. Neither selective glucan 1,4-alpha-glucosidase inhibition nor the ensuing suppression of glucose-induced insulin release was overcome by a maximal Ca2+ concentration. Hence, Ca(2+)-induced changes in acid glucan 1,4-alpha-glucosidase activity were intimately coupled to similar changes in Ca(2+)-glucose-induced insulin release. Ca2+ did not affect the enzyme itself but presumably activated either glucan 1,4-alpha-glucosidase-containing organelles or closely interconnected messengers.