The B-cells of pancreatic islets maintain the blood D-glucose concentration within a narrow range by secreting insulin in response to rising D-glucose concentrations in the fluid bathing them. D-glucose must be transported into the B-cell and metabolized there in order to induce insulin secretion. Signals derived from the metabolism of D-glucose within a-cells increase the membrane permeability to Ca2+ and induce a rise in the cytosolic (Ca2+], which is a critical signal in the induction of insulin secretion. Upon stimulation with secretagogues including D-glucose, phospholipid hydrolytic events occur in a-cells and result in the accumulation of a variety of phospholipid-derived mediators including nonesterified arachidonic acid and arachidonate metabolites. A major fraction of the D-glucose-induced hydrolysis of arachidonate from B-cell membrane phospholipids is independent of Ca2+ influx and occurs in Ca2+ -free medium, but D-glucose-induced eicosanoid release from islets does not occur when glucose metabolism is prevented by compounds which inhibit the phosphorylation of D.glucose and its entry into glycolysis. Sufficient nonesterified arachidonate accumulates in D-glucose-stimulated islets to achieve an increment in the (3-cell concentration of at least 30-70 uM, and such concentrations of arachidonate induce a rise in the (3-cell cytosolic (Ca2+] and amplify depolarization-induced insulin secretion. The D-glucose-induced hydrolysis of arachidonate from (3-cell membrane phospholipids is mediated by a phospholipase A2 enzyme whose activity is independent of Ca2+ and stimulated by ATP and which prefers plasmalogen substrates with sn-2 arachidonoyl residues. Selective inhibition of this islet ATP stimulated, Ca2+independent (ASCI) -phospholipase A2 (PLA2) with a haloenol lactone suicide substrate which is sterically similar to plasmalogens suppresses the D-glucose-induced hydrolysis of arachidonate from (3-cell phospholipids, the rise in (3-cell cytosolic (Ca2+], and insulin secretion. Islets contain substantial amounts of plasmenylethanolamine molecular species bearing arachidonoyl residues in the sn-2 position, and these molecules are hydrolyzed more rapidly than diacylphospholipid substrates by islet ASCI-PLA2 in vitro and also undergo hydrolysis in intact, secretagogue-stimulated islets. Islet ASCI-PLA2 appears to constitute an important component of the D-cell D-glucose sensor apparatus.
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