OMEGA-CONOTOXIN INHIBITS GLUCOSE-INDUCED AND ARACHIDONIC ACID-INDUCED RISES IN INTRACELLULAR [CA2+] IN RAT PANCREATIC-ISLET BETA-CELLS

Earlier studies suggest that the accumulation of non-esterified arachidonic acid (AA) in islets following stimulation with glucose participates in the glucose-induced secretion of insulin. A possible role for AA might include the facilitation of Ca2+ influx into islet beta-cells. Recently, we demonstrated that AA induces Ca2+ Influx into purified rat pancreatic islet beta-cells, prepared by fluorescence-activated cell sorting (FACS). This effect was abolished in the presence of the Ca2+-chelator EGTA, but was only partially reduced by the dihydropyridine (DHP) L-type Ca2+-channel blocker, nifedipine. This raised the possibility that DHP-insensitive Ca2+ entry mechanisms may exist in pancreatic beta-cells, in addition to the known DHP-sensitive L-type Ca2+ channels. Here we report that omega-conotoxin (CTX), which blocks omega-type Ca2+-channels, inhibits AA-induced Ca2+ influx by a magnitude similar to that of nifedipine and that the combination of omega-CTX and nifedipine results in a nearly additive decrement in AA-induced increases in beta-cell cytosolic [Ca2+]. We further demonstrate that bovine serum albumin, which complexes free AA and prevents AA-induced increases in cytosolic [Ca2+](i) also inhibits the glucose-induced increase in beta-cell [Ca2+]. These results suggest that rat pancreatic FACS-purified islet beta-cells express omega-type (DHP-insensitive) Ca2+-channels, in addition to DHP-sensitive Ca2+-channels. They further suggest that the glucose-induced accumulation of non-esterified AA in the membranes of beta-cells serves to amplify glucose-mediated Ca2+ influx into the beta-cells.