Phasic effects of glucose, phospholipase A2, and lysophospholipids on insulin secretion

Arachidonic acid and lysophospholipids generated by glucose stimulation of phospholipase A2 may be related to the biphasic pattern of insulin secretion. Therefore, we examined the effects of glucose, exogenous phospholipase A2, lysophospholipids, and pharmacological agents which perturb the reesterification or oxygenation of arachidonic acid in superfused monolayer cultures of neonatal rat islet cells. Nordihydroguaiaretic acid (20 microM), an inhibitor of islet lipoxygenase, significantly decreased phasic glucose-stimulated insulin secretion, especially during phase 1, suggesting that stimulatory lipoxygenase metabolites of arachidonic acid contribute to the effects of glucose stimulation. Treatment with exogenous phospholipase A2 (10 mU/ml) or melittin (1.0 or 2.0 micrograms/ml) to generate arachidonic acid and lysophospholipids de novo caused a monophasic release of insulin, followed by a gradual decline in insulin secretion despite the continued presence of these agonists. Conversely, p-hydroxymercuribenzoate (15, 30, and 50 microM), which blocks the reacylation of lysophospholipids with arachidonic acid, evoked a concentration-dependent biphasic stimulation of insulin secretion which was reversible. Lipoxygenase inhibition had no effect upon phase 1 secretion by p-hydroxymercuribenzoate, although it did partially reduce phase 2 secretion. However, lysophosphatidylcholine (50, 75, and 100 micrograms/ml) also caused a concentration-dependent biphasic stimulation of insulin secretion which resembled that seen with p-hydroxymercuribenzoate, suggesting that lysophospholipids were mediating the effects of p-hydroxymercuribenzoate. We speculate that during glucose stimulation of the islet, the following three phospholipase A2-initiated changes may be important: generation of lysophospholipid to stimulate directly insulin secretion, generation of arachidonic acid and lipoxygenase-mediated arachidonate metabolites, which positively modulate insulin secretion, and generation of cyclooxygenase-mediated arachidonate metabolites, which negatively modulate insulin secretion.