Phosphoinositide Hydrolysis and Insulin Secretion in Response to Glucose Stimulation Are Impaired in Isolated Rat Islets by Prolonged Exposure to the Sulfonylurea Tolbutamide*

Isolated rat islets of Langerhans were incubated for 2 h in a [3H]inositol-containing medium supplemented with 7 mM glucose and the sulfonylurea tolbutamide (50-200 microM). After labeling, the ability of these islets to respond during a subsequent perifusion to 20 mM glucose or 15 mM alpha-ketoisocaproate (KIC) was assessed. The following major observations were made. Prior exposure to tolbutamide inhibited [3H]inositol efflux, inositol phosphate accumulation, and the insulin secretory responses of subsequently perifused islets to 20 mM glucose stimulation. When present during the 2-h labeling period, the calcium channel blocker nitrendipine (500 nM), a compound that abolishes tolbutamide-induced increases in PI hydrolysis, blocked these inhibitory effects of tolbutamide. In addition, the diacylglycerol kinase inhibitor monooleoylglycerol (50 microM) restored the impaired second phase insulin secretory response noted after a 2-h tolbutamide exposure. Prior exposure to tolbutamide (200 microM) also desensitized the islet, in terms of [3H] inositol phosphate accumulation, [3H]inositol efflux, and insulin secretory responses, to 15 mM KIC. The inclusion of monooleoylglycerol during the stimulatory period with KIC restored second phase insulin secretion. The results support the conclusion that chronic tolbutamide-induced increases in PI hydrolysis render the beta-cell insensitive to a subsequent 20-mM glucose or 15-mM KIC stimulus. Blocking tolbutamide-induced increases in PI hydrolysis during the labeling period eliminates the adverse effects of the sulfonylurea. The ineffectiveness of glucose and KIC to maintain insulin secretory responses from prior tolbutamide-exposed islets appears to be the result of the inability of these agonists to appropriately activate PI hydrolysis.