Mass Spectrometric Evidence That Agents That Cause Loss of Ca2+ from Intracellular Compartments Induce Hydrolysis of Arachidonic Acid from Pancreatic Islet Membrane Phospholipids by a Mechanism That Does Not Require a Rise in Cytosolic Ca2+ Concentration**This work was supported by U.S. Public Health Service grants PO1-HL57278, P41-RR-00954, and S10-RR-11260 and by an American Diabetes Association Career Development Award (S.R.).

Abstract

Abstract Stimulation of pancreatic islets with glucose induces phospholipid hydrolysis and accumulation of nonesterified arachidonic acid, which may amplify the glucose-induced Ca2+ entry into isletβ -cells that triggers insulin secretion. Ca2+ loss fromβ -cell intracellular compartments has been proposed to induce both Ca2+ entry and events dependent on arachidonate metabolism. We examine here effects of inducing Ca2+ loss from intracellular sequestration sites with ionophore A23187 and thapsigargin on arachidonate hydrolysis from islet phospholipids. A23187 induces a decline in islet arachidonate-containing phospholipids and release of nonesterified arachidonate. A23187-induced arachidonate release is of similar magnitude when islets are stimulated in Ca2+-replete or in Ca2+-free media or when islets loaded with the intracellular Ca2+ chelator BAPTA are stimulated in Ca2+-free medium, a condition in which A23187 induces no rise in β-cell cytosolic [Ca2+]. Thapsigargin also induces islet arachidonate release under these conditions. A23187- or thapsigargin-induced arachidonate release is prevented by a bromoenol lactone (BEL) inhibitor of a β-cell phospholipase A2 (iPLA2), which does not require Ca2+ for catalytic activity and which is negatively modulated by and physically interacts with calmodulin by Ca2+-dependent mechanisms. Agents that cause Ca2+ loss from islet intracellular compartments thus induce arachidonate hydrolysis from phospholipids by a BEL-sensitive mechanism that does not require a rise in cytosolic [Ca2+], and a BEL-sensitive enzyme-like iPLA2 or a related membranous activity may participate in sensing Ca2+ compartment content.