Inhibition of arachidonate release by secretagogue-stimulated pancreatic islets suppresses both insulin secretion and the rise in .beta.-cell cytosolic calcium ion concentration

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Abstract
Fuel secretagogues induce hydrolysis of esterified arachidonic acid from pancreatic islet cell phospholipids and accumulation of nonesterified arachidonate at concentrations up to 35 microM. Exogenous arachidonate (5-30 microM) amplifies depolarization-induced insulin secretion from islets. Fuel secretagogue-induced hydrolysis of arachidonate from islet phospholipids occurs in Ca(2+)-free medium, suggesting the possible involvement of a Ca(2+)-independent phospholipase. In the companion paper [Gross et al. (1993) Biochemistry (preceding paper in this issue)], we demonstrated that the major islet phospholipase A2 is Ca(2+)-independent, ATP-stimulated, and inhibited by the haloenol lactone suicide substrate (HELSS) (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one. Here we demonstrate that HELSS suppressed both release of the arachidonate metabolite prostaglandin E2 and insulin secretion from islets stimulated with D-glucose and the muscarinic agonist carbachol. Both prostaglandin E2 release and insulin secretion were suppressed with similar concentration profiles and time courses. Islet oxidation of [14C]-glucose to [14C]CO2, activities of islet lactate dehydrogenase and alanine and aspartate aminotransferases, and carbachol-induced inositol phosphate accumulation in islets were all unaffected by HELSS. Depolarization of isolated beta-cells with 40 mM KCl induced a rise in cytosolic [Ca2+] that was also unaffected by HELSS. In contrast, the 17 mM D-glucose-induced rise in beta-cell [Ca2+] was inhibited by HELSS in a concentration-dependent manner, but that induced by exogenous arachidonate (15 microM) was not. These results suggest that fuel secretagogues activate the islet Ca(2+)-independent phospholipase A2, resulting in release of nonesterified arachidonate, which facilitates Ca2+ entry into beta-cells and promotes insulin secretion.