Inhibition of Arachidonate Release by Secretagogue-Stimulated Pancreatic Islets Suppresses both Insulin Secretion and the Rise in β-Cell Cytosolic Calcium Ion Concentration

Fuel secretagogues induce hydrolysis of esterified arachidonic acid from pancreatic islet cell phospholipids and accumulation of nonesterified arachidonate at concentrations up to 35 μM. Exogenous arachidonate (5-30 μM) amplifies depolarization-induced insulin secretion from islets. Fuel secretagogueinduced hydrolysis of arachidonate from islet phospholipids occurs in Ca²+-free medium, suggesting the possible involvement of a Ca²⁺-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 Ca²⁺-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 β-cells with 40mM KCl induced a rise in cytosolic [Ca²⁺] that was also unaffected by HELSS. In contrast, the 17 mM d-glucose-induced rise in β-cell [Ca²⁺] was inhibited by HELSS in a concentration-dependent manner, but that induced by exogenous arachidonate (15 μM) was not. These results suggest that fuel secretagogues activate the islet Ca²⁺-independent phospholipase A₂, resulting in release of nonesterified arachidonate, which facilitates Ca²⁺ entry into β-cells and promotes insulin secretion.