Relief from glucose-induced over-stimulation sensitizes the adenylate cyclase-cAMP system of rat pancreatic islets

Hyperglycemia impairs beta -cen function. This effect is partly exerted by beta -cell over-stimulation by mechanisms that are not completely clarified , We have presently investigated whether over-stimulation alters the respon siveness of the islet adenylate cyclase-cAMP system. Effects of over-stimul ation were assessed from comparisons in rat pancreatic islets after stimula tion by culture for 22 h with high (27 mM) glucose or after the additional presence of diazoxide which reversibly blocks secretion. Islet ATP levels w ere similar under both conditions. Forskolin increased islet cAMP levels do se-dependently after culture under both conditions; however. the cAMP respo nses to forskolin were enhanced by the previous co-presence of diazoxide: b y 354, 183 and 168% respectively in the presence of 0.1, 1.0 and 25 muM for skolin (P<0.05) or less for the effect of diazoxide, Enhancement was not di minished by Ca2+ omission during final incubations, nor by blocking Gi prot eins with pertussis toxin (0.1 <mu>g/ml). Enhancement was dependent on the glucose concentration during culture, i.e. co-culture with diazoxide at a n on-stimulatory concentration of glucose (6.0 mM) failed to affect the subse quent cAMP response to forskolin. Acute administration of glucose (16.7 mM) failed to increase islet cAMP content after culture at high glucose only, whereas a modest (about 20%) but significant stimulation was seen after co- culture with diazoxide. Go-culture with diazoxide left-shifted the insulin dose-response to a cAMP analogue 5,6-dichloro-1-beta -D-riboiuranosyl-benzi nlidazole- -3',5'-cyclic monophosphorothioate. We conclude that over-stimul ation importantly modifies the generation of cAMP, and also affects the ins ulin-releasing effect of the cyclic nucleotide.