Amplification of Insulin Secretion by Lipid Messengers

D-glucose induces a rise in pancreatic islet β-cell cytosolic [Ca²⁺] by processes requiring both glucose metabolism and Ca²⁺ entry from the extracellular space, and this Ca²⁺ signal is thought to be critical to the induction of insulin secretion. Insulin secretagogues also induce phospholipid hydrolysis and accumulation of phospholipid-derived mediators in islets, including the lipid messengers DAG, nonesterified arachidonic acid, and arachidonate 12-LO products. This study offers the following viewpoints on potential roles of these lipid messengers in insulin secretion as working hypotheses: 1) the Ca²⁺ signal provided to the β-cell by D-glucose induces insulin secretion only in the context of amplifying background signals provided by the β-cell content of messengers including DAG; 2) muscarinic receptor agonists amplify glucose-induced insulin secretion in part by altering the β-cell content of DAG; 3) the Ca²⁺ signal provided by metabolism of D-glucose is amplified by the level of nonesterified arachidonic acid in β-cell membranes, which acts to facilitate Ca²⁺ entry; 4) metabolism of glucose induces accumulation of nonesterified arachidonate in beta-cells via activation of a recently identified ASCI-PLA₂ enzyme, which may be a component of the β-cell fuel sensor apparatus; and 5) arachidonate 12-LO metabolites are potential candidates as adjunctive modulators of β-cell K⁺-channel activity.