Phospholipase A2 is important for glucose induction of rhythmic Ca2+ signals in pancreatic β cells

Objectives: Pancreatic beta cells respond to glucose stimulation with pulses of insulin release generated by oscillatory rises of the cytoplasmic Ca2+ concentration ([Ca2+](i)). The observation that exposure to external ATP and other activators of cytoplasmic phospholipase A(2) (cPLA(2)) rapidly induces rises of [Ca2+](i) similar to ordinary oscillations made it important to analyze whether suppression of the cPLA(2) activity affects glucose-induced [Ca2+](i) rhythmicity in pancreatic beta cells. Methods: Ratiometric fura-2 technique was used for measuring [Ca2+](i) in single beta cells and small aggregates prepared from ob/ob mouse islets. Results: Testing the effects of different inhibitors of cPLA(2) in the presence of 20 mM glucose, it was found that N-(p-amylcinnamoyl) anthranilic acid (ACA) removed the oscillations at a concentration of 25 mu M, arachidonyl trifluoromethyl ketone (AACOCF3) at 10 mu M and bromoenol lactone (BEL) at 10 to 15 mu M. Withdrawal of ACA and BEL resulted in reappearance of the oscillations. Suppression of the arachidonic acid production by addition of 5 mu M of the diacylglycerol lipase inhibitor 1,6-bis-(cyclohexyloximino-carbonylamino)-hexane (RHC 80267) effectively removed the [Ca2+](i) oscillations, an effect reversed by removal of the inhibitor or addition of 100 mu M tolbutamide. Suppression of the arachidonic acid production had a restrictive influence also on the transients of [Ca2+](i) supposed to synchronize the beta-cell rhythmicity. Although less sensitive than the oscillations, most transients disappeared during exposure to 50 mu M ACA or 35 mu M RHC 80267. Conclusions: The results support the idea that cyclic variations of cPLA(2) activity are important for the generation and synchronization of the beta-cell [Ca2+](i) oscillations responsible for pulsatile release of insulin.