Metabolism of lysophospholipids in intact rat islets. The insulin secretagogue p-hydroxymercuribenzoic acid impairs lysophosphatidylcholine catabolism and permits its accumulation

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Abstract
Although recent studies implicate lysophospholipids (lyso-PLs) in stimulus-secretion coupling in the pancreatic islet, almost no data on lyso-PL metabolism therein exist. Therefore, intact rat islets were loaded with insulinotropic and non-toxic concentrations of I-[14C]palmitoyl-lysophosphatidylcholine (lyso-PC) via transbilayer movement, and its metabolic fate was studied. The time-dependent hydrolysis of lyso-PC to fatty acid (lysophospholipase activity) its coversion to phosphatidylcholine (putative acyltransferase activity) and, to a lesser degree, the appearance of label in phosphatidylethanolamine (putative transacylase or base exchange activity) were observed. p-Hydroxymercuribenzoic acid (PHMB) at 100 .mu.M (a concentration previously demonstrated to elicit potent exocytotic insulin release) inhibited all three activities (by 56, 46 and 75%, respectively) and led to the intracellular accumulation of lyso-PC. Antimycin A inhibited phosphatidylcholine formation but not lysophospholipase activity; lyso-PC did not accumulate, implying that blockade of both of the major metabolic pathways is required to induce a detectable increment in lyso-PC levels. Calculations derived from data using the lowest effective insulinotropic concentration of lyso-PC suggested that increments in lyso-PC accumulation at critical membrane sites of less than 10-15% above basal values are sufficient to trigger insulin release. Since PHMB elicited increments of 50-100% in lyso-PC after its translocation into islets, support is provided for the earlier contention that lyso-PLs mediate the insulinotropic effect of PHMB. In addition, these studies may provide a more precise experimental paradigm for future studies of islet lyso-PL metabolism.