title: Inhibition of arachidonate release by secretagogue-stimulated pancreatic islets suppresses both insulin secretion and the rise in beta-cell cytosolic calcium ion concentration.
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.
authors.affiliation: Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: S
authors.initials: S
authors.lastname: Ramanadham
authors.forename: R W
authors.initials: RW
authors.lastname: Gross
authors.forename: X
authors.initials: X
authors.lastname: Han
authors.forename: J
authors.initials: J
authors.lastname: Turk
mesh_terms: D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D015232:Dinoprostone / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D009281:Naphthalenes / Q000494:pharmacology*; D010741:Phospholipases A / Q000037:antagonists & inhibitors*; D054467:Phospholipases A2; D011189:Potassium Chloride / Q000494:pharmacology; D011753:Pyrones / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D007328:Insulin; D009281:Naphthalenes; D011753:Pyrones; D016718:Arachidonic Acid; D011189:Potassium Chloride; C068314:6-(bromomethylene)tetrahydro-3-(1-naphthaleneyl)-2H-pyran-2-one; D002217:Carbachol; D010741:Phospholipases A; D054467:Phospholipases A2; D005947:Glucose; D015232:Dinoprostone; D002118:Calcium