Stimulation by d-Glucose of Cyclic Adenosine 3′ : 5′-Monophosphate Accumulation and Insulin Release in Isolated Pancreatic Islets of the Rat

Abstract

Abstract In collagenase isolated rat pancreatic islet incubates, d-glucose stimulated the accumulation of cyclic adenosine 3' : 5'-monophosphate (cAMP) as measured both by incorporation of [3H]adenine into cAMP and by assay of the total amount of nucleotide by the protein kinase binding method. l-Glucose failed to stimulate [3H]cAMP accumulation in islets. d-Glucose had no effect on [3H]cAMP accumulation in rat adipocytes. It is therefore concluded that the action of glucose is specific in the islet. Stimulation by d-glucose (5 mg per ml) was detectable after 45 to 135 s. [3H]cAMP accumulated rapidly during the first 10 min of incubation, after which a sustained but lower rate of accumulation was observed up to 120 min. 3-Isobutyl-1-methylxanthine (IBMX), 0.1 mm, markedly potentiated the stimulatory effect of a high glucose concentration, while [3H]cAMP accumulation in the presence of low glucose (0.6 mg per ml) was only slightly raised. Stimulation of cAMP accumulation induced by high glucose in the presence of IBMX was abolished within 3 min when the glucose concentration of the medium was rapidly decreased. Efflux of radioactive cyclic nucleotide was undetectable under basal conditions. In incubates stimulated by high glucose with or without IBMX, elevated contents of [3H]cAMP could be measured from 30 min on, the nucleotide continuing to accumulate in the medium up to 120 min of incubation. Over a range of glucose concentrations (0.6 to 7.0 mg per ml), insulin release after a 60-min incubation was well correlated with medium [3H]cAMP content both in the presence and absence of IBMX. Stimulation of both cAMP accumulation and insulin release were observed with small changes in medium glucose level such as increasing it from 0.9 to 1.2 mg per ml. In the incubation system used here, cAMP accumulation seems to precede the initiation of insulin release. The results presented in this report are compatible with the hypothesis that insulin release induced by glucose may be mediated by cAMP in the β-cell.