Ca 2+ potentiates corticotropin-induced, but not isoproterenol-induced, [ 3H]guanosine diphosphate release in rat adipocyte membranes

Abstract

EGTA abolished corticotropin (ACTH)-stimulated adenylate cyclase activity in rat adipocyte membranes. In contrast, the potency of guanosine triphosphate (GTP) stimulation of adenylate cyclase activated with ACTH was greater in the presence of Ca 2+ (1 mmol/L). EGTA (1 mmol/L) powerfully inhibited ACTH-stimulated [ 3H]guanosine diphosphate (GDP) release from membranes prelabeled with [ 3H]GTP in the presence of isoproterenol (ISO) or ACTH, whereas Ca 2+ significantly increased it. In contrast, neither EGTA nor Ca 2+ affected ISO-stimulated [ 3H]GDP release. These data clearly show that Ca 2+ is necessary for the binding of ACTH to its receptor, and that Ca 2+ stimulates the interaction of the ACTH-occupied receptor with GTP-binding proteins.