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 secretion was investigated by measuring its effects on calmodulin-dependent 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 replace phosphatidylserine in activation of cytosolic protein kinase C (K_0.5 of 10 μM) and maximum activation was observed at 50 μM arachidonate. Arachidonic acid did not affect the Ca²⁺ requirement of the phosphatidylserine-stimulated activity. Arachidonic acid (200 μM) inhibited (> 90%) calmodulin-dependent protein kinase activity (K_0.5 = 50-100 μM) but modestly increased basal phosphorylation activity (no added calcium or calmodulin). Arachidonic acid inhibited glucose-sensitive insulin secretion from islets (K_0.5 = 24 μM) measured in static secretion assays. Maximum inhibition (approximately 70%) was achieved at 50-100 μM arachidonic acid. Basal insulin secretion (3 mM glucose) was modestly stimulated by 100 μM arachidonic acid but in a non-saturable manner. In perifusion secretion studies, arachidonic acid (20 μM) had no effect on the first phase of glucose-induced secretion but nearly completely suppressed second phase secretion. At basal glucose (4 mM), arachidonic acid induced a modest but reproducible biphasic insulin secretion response which mimicked glucose-sensitive secretion. However, 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 with arachidonic acid were not mediated by protein kinase C. These data suggest that arachidonic acid generated by exposure of islets to glucose may influence insulin secretion by inhibiting the activity of calmodulin-dependent protein kinase but probably has little effect on protein kinase C activity.