Modifying actions of calcium ionophores on insulin release

Beta-Cell-rich pancreatic islets were microdissected from noninbred ob/obmice and exposed to the calcium ionophores X-537A and A-23187. X-537A differed from A-23187 in being a potent insulin secretagogue at non-stimulating glucose concentrations. Both ionophores inhibited the stimulation of insulin release obtained after adding 20 mM glucose to the incubation medium. The latter observation is consistent with the idea of a reduced beta-cell function when the Ca-2+ in the functionally important intracellular pool (s) exceeds a certain concentration. The ionophore inhibition of the glucose-stimulated insulin release may at least in part result from decreased formation of cyclic AMP, since X-537A proved to be as effective as L-epinephrine in reducing the islet content of this nucleotide in the presence of a phosphodiesterase inhibitor. The secretagogic action of X-537A at a low glucose concentration persisted when different ions were omitted from the incubation medium and was actually considerably enhanced in the absence of extracellular Ca-2+. The insulin-releasing action of X-537A was neither influenced by 3-O-methyglucose nor by drugs blocking the alpha or beta-adrenergic receptor sites. Exposure of the pancreatic beta-cells to metabolic inhibitors in concentrations which significantly reduced the secretory response to glucose, potentiated stimulation of insulin release by X-537A, suggesting that this effect may in part be accounted for by intracellular dissolution of secretory granules.