Exogenous Arachidonic Acid Promotes Insulin Release From Intact or Permeabilized Rat Islets by Dual Mechanisms: Putative Activation of Ca2+ Mobilization and Protein Kinase C

Abstract

A number of indirect studies suggest a role for endogenous arachidonic acid (AA) in pancreatic islet function. To probe the effects of this fatty acid, AA and other polyunsaturated fatty acids were exogenously provided in Ca2+-free medium to avoid the formation of insoluble or impermeant Ca2+-arachidonate complexes. Concentrations of AA of ≧3 μM induced potent and sustained but reversible 45Ca efflux from prelabeled intact (or digitonin-permeabilized) islets; AA also nduced insulin release at somewhat higher concentrations. Other unsaturated fatty acids (erucic, oleic, linoleic, linolenic, dihomo-γ-linolenic, eicosapentaenoic, docosahexaenoic acids) were generally less active than AA itself, indicating a structure-function relationship. The effects of AA were saturable, were inhibitable by cooling, and were not accompanied in parallel by 51Cr release or trypan blue retention, suggesting a nontoxic mechanism. At low concentrations (3.3–16 μM), at which AA does not stimulate insulin release, AA-induced 45Ca efflux was not reduced by pretreatment with ionomycin (to deplete membrane-bound Ca2+ stores), suggesting stimulation of Ca2+ extrusion through the plasma membrane. At higher concentrations (≧25 μM), at which AA promotes insulin release, further Ca2+ efflux was stimulated, which was blunted by pretreatment with ionomycin (as well as by trifluoperazine). Conversely, pretreatment with AA obliterated the effects of ionomycin (3 μM) on cellular Ca2+ mobilization. Thus, AA also mobilizes Ca2+ from intracellular organelles, leading to a rise in free cytosolic Ca2+ (as previously reported). AA-induced 45Ca efflux and insulin release were independent of the presence of extracellular Na+ and did not require the oxygenation of AA. Dose-response curves comparing45Ca efflux and insulin secretion suggested that AA also stimulates hormone release by at least one other mechanism in addition to Ca2+ mobilization. This second stimulatory effect of AA could be seen in digitonin-permeabilized islets, where changes in cytosolic free Ca2+ concentration were vitiated by EGTA-containing buffers. Such secretion was also saturable and was inhibited by cooling or by spermine (which inhibits protein kinase C in the islet). Furthermore, AA-induced secretion from either intact or permeabilized islets was blunted by prolonged pretreatment of islets with a phorbol ester to deplete them of protein kinase C. Thus, exogenous arachidonic acid seems to be a complete secretagogue, having stimulatory effects both on Ca2+ mobilization and Ca2+-related secretory processes, putatively the activation of protein kinase C.