MASS-SPECTROMETRIC EVIDENCE THAT AGENTS THAT CAUSE LOSS OF CA2-ACID FROM PANCREATIC-ISLET MEMBRANE PHOSPHOLIPIDS BY A MECHANISM THAT DOES NOT REQUIRE A RISE IN CYTOSOLIC CA2+ CONCENTRATION( FROM INTRACELLULAR COMPARTMENTS INDUCE HYDROLYSIS OF ARACHIDONIC)

Stimulation of pancreatic islets with glucose induces phospholipid hyd rolysis and accumulation of nonesterified arachidonic acid, which may amplify the glucose-induced Ca2+ entry into islet beta-cells that trig gers insulin secretion. Ca2+ loss from beta-cell intracellular compart ments has been proposed to induce both Ca2+ entry and events dependent on arachidonate metabolism. We examine here effects of inducing Ca2loss from intracellular sequestration sites with ionophore A23187 and thapsigargin on arachidonate hydrolysis from islet phospholipids. A231 87 induces a decline in islet arachidonate-containing phospholipids an d release of nonesterified arachidonate. A23187-induced arachidonate r elease is of similar magnitude when islets are stimulated in Ca2+-repl ete or in Ca2+-free media or when islets loaded with the intracellular Ca2+ chelator BAPTA are stimulated in Ca2+-free medium, a condition i n which A23187 induces no rise in beta-cell cytosolic [Ca2+]. Thapsiga rgin also induces islet arachidonate release under these conditions. A 23187- or thapsigargin-induced arachidonate release is prevented by a bromoenol lactone (BEL) inhibitor of a beta-cell phospholipase A(2) (i PLA(2)), 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 int racellular compartments thus induce arachidonate hydrolysis from phosp holipids by a EEL-sensitive mechanism that does not require a rise in cytosolic [Ca2+], and a EEL-sensitive enzyme-like iPLA(2) or a related membranous activity may participate in sensing Ca2+ compartment conte nt.