Insulin secretion. Interrelationships of glucose, cyclic adenosine 3:5-monophosphate, and calcium

Glucose elevates both cyclic adenosine 3:5-monophosphate (cyclic AMP) and insulin secretion rapidly and in a parallel dose-dependent fashion in perifused rat islets. Theophylline stimulates cyclic AMP much more than glucose, yet secretion is much less. When the two agents are combined, cyclic AMP is similar to theophylline alone yet secretion is augmented synergistically. Glucose-induced cyclic AMP generation and insulin secretion are dependent on extracellular calcium. Theophylline-induced insulin secretion is also extracellular calcium-dependent; however, theophylline-induced cyclic AMP elevation is independent of extracellular calcium. Thus, extracellular calcium has multiple effects on insulin secretion, some of which appear unrelated to a terminal secretory process. When glucose is combined with theophylline at physiologic levels of extracellular calcium, both the first and second phases of secretion are prominent. At extracellular calcium levels of 0.05 mM, only the second phase is prominent whereas at 10 nM extracellular calcium (ethylene glycol bis(beta-aminoethyl ether)-N,-tetraacetic acid) only the first phase is prominent. A divalent cation ionophore (a23187, Eli Lilly), which transports calcium and magnesium ions across biological membranes, was used to elucidate further the role of calcium and magnesium. If the ionophore (10 muM) is perifused for 5 min at low extracellular calcium and magnesium, and physiologic calcium is then added, a sudden spike of insulin release occurs in the absence of cyclic AMP generation. Similar results were obtained with magnesium. When the ionophore is perifused for 30 min at low calcium and magnesium, insulin secretion again occurs in the absence of cyclic AMP generation. Electron microscopic examination of the B cells following perifusion with the ionophore shows no specific alterations. These observations suggest that: (a) glucose elevates cyclic AMP, but the latter acts primarily as a positive feed-forward modulator of glucose-induced insulin release; and (b) extracellular calcium has multiple effects on insulin secretion both upon, and independent of, the cyclic AMP system.