Influence of cAMP and Calcium on [³H]Inositol Efflux, Inositol Phosphate Accumulation, and Insulin Release From Isolated Rat Islets

The influence of cyclic AMP (cAMP) and extracellular calcium on phosphoinositide (PI) hydrolysis in isolated islets was assessed and related to insulin output. Three stimulants were chosen to activate the β-cell: sulfated cholecystokinin (CCK-8S, 200 nM), high-level glucose (20 mM), and the sulfonylurea tolbutamide (200 μM). The insulin secretory response to all three agonists was amplified by forskolin (which increases cAMP levels) and reduced by nitrendipine (which decreases calcium influx). All three stimulants increased the hydrolysis of inositol-containing phospholipids, an event monitored by an increase in [³H]inositol efflux from [³H]inositol-prelabeled islets and by the accumulation of labeled inositol phosphates. Forskolin, despite its positive impact on insulin secretion, reduced [³H]inositol efflux and inositol phosphate accumulation in response to all agonists. A similar inhibitory effect on these parameters was noted with nitrendipine; however, nitrendipine abolished secretion in response to all agonists. These findings support the following conclusions: 1) an increase in cellular cAMP levels reduces the quantitative impact of various agonists on these indices of PI hydrolysis; 2) despite this inhibitory effect, cAMP amplifies the insulin secretory response to these agonists; and 3) extracellular calcium is a crucial determinant of both PI hydrolysis and the ensuing insulin secretory response.