PARALLEL EFFECTS OF ARACHIDONIC-ACID ON INSULIN-SECRETION, CALMODULIN-DEPENDENT PROTEIN-KINASE ACTIVITY AND PROTEIN-KINASE-C ACTIVITY IN PANCREATIC-ISLETS

A potential role of arachidonic acid in the modulation of insulin secr etion was investigated by measuring its effects on calmodulin-dependen t protein kinase and protein kinase C in islet subcellular fractions. The results were interpreted in the light of arachidonic acid effects on insulin secretion from intact islets. Arachidonic acid could replac e phosphatidylserine in activation of cytosolic protein kinase C (K0.5 of 10-mu-M) and maximum activation was observed at 50-mu-M arachidona te. Arachidonic acid did not affect the Ca2+ requirement of the phosph atidylserine-stimulated activity. Arachidonic acid (200-mu-M) inhibite d (> 90%) calmodulin-dependent protein kinase activity (K0.5 = 50-100- mu-M) but modestly increased basal phosphorylation activity (no added calcium or calmodulin). Arachidonic acid inhibited glucose-sensitive i nsulin secretion from islets (K0.5 = 24-mu-M) measured in static secre tion assays. Maximum inhibition (approximately 70%) was achieved at 50 -100-mu-M arachidonic acid. Basal insulin secretion (3 mM glucose) was modestly stimulated by 100-mu-M arachidonic acid but in a non-saturab le manner. In perifusion secretion studies, arachidonic acid (20-mu-M) had no effect on the first phase of glucose-induced secretion but nea rly completely suppressed second phase secretion. At basal glucose (4 mM), arachidonic acid induced a modest but reproducible biphasic insul in secretion response which mimicked glucose-sensitive secretion. Howe ver, phosphorylation of an 80 kD protein substrate of protein kinase C was not increased when intact islets were incubated with arachidonic acid, suggesting that the small increases in insulin secretion seen wi th arachidonic acid were not mediated by protein kinase C. These data suggest that arachidonic acid generated by exposure of islets to gluco se may influence insulin secretion by inhibiting the activity of calmo dulin-dependent protein kinase but probably has little effect on prote in kinase C activity.