ATP-SENSITIVE K-CHANNEL MODULATION BY PRODUCTS OF PLA(2) ACTION IN THE INSULIN-SECRETING HIT CELL-LINE

Insulin secretion from the islets of Langerhans may be initiated or potentiated by increased phospholipase A(2) (PLA(2)) activity. This patch-clamp study of the insulin-secreting HIT tumor cell line assessed whether inhibition of the ATP-sensitive potassium (K-ATP) channel, which modulates the secretion-associated beta-cell electrical activity, contributes to the secretory response to PLA(2). Exogenous PLA(2) (100-1,000 mU/ml) reversibly suppressed K-ATP channel activity in excised inside-out patches. Similarly, mellitin (0.5-5 mu g/ml), a bee venom component that increases phospholipid susceptibility to metabolism by PLA(2), suppressed K-ATP channel activity, suggesting that PLA(2) is present in excised patches. Adding low concentrations of particular lysophospholipids or arachidonic acid also reduced K-ATP channel activity in excised inside-out patches. In cell-attached patches, the lysophospholipids had a similar effect, whereas arachidonic acid caused channel stimulation; this latter effect was reversed by cyclooxygenase inhibitors. A recently identified ATP-stimulated PLA(2) in beta-cells has been proposed as an important mediator of stimulus-secretion coupling in response to nutrients. The present data illustrating that initial products of PLA(2) action on membrane phospholipids reduce K-ATP channel activity indicate a mechanism that may contribute an early stimulatory signal in this pathway. The observation that metabolism of arachidonate via the cyclooxygenase pathway causes K-ATP channel stimulation demonstrates a potential counterregulatory mechanism.