Islet constitutive nitric oxide synthase and nitric oxide production. Modulatory effects on insulin and glucagon secretion

Abstract: The aim of this thesis was to elucidate the role of nitric oxide (NO) in the transduction signalling of insulin and glucagon release by stimulating or inhibiting islet endogenous NO-production as well as by using NO donors. Insulin release stimulated by L-arginine, the substrate for constitutive NO-synthase (cNOS)-catalyzed NO-production, was increased by the selective NOS-inhibitor NG-nitro-L-arginine methyl ester (L-NAME) and suppressed by the intracellular NO donor hydroxylamine. Insulin release stimulated by glucose, sulphonylurea and the cholinergic agonist carbachol as well as by protein kinase C activation was also increased by NOS-inhibition and suppressed by the NO donor. Insulin release induced by cyclic AMP activators was unaffected by NOS-inhibition and modestly increased by hydroxylamine. The potentiating effect of L-NAME on insulin release stimulated by glucose or carbachol had little influence on Ca2+-fluxes and was still evident in diazoxide-treated, K+-depolarized islets suggesting that the major inhibitory action of NO was exerted independently of membrane depolarization. Data from NO donor experiments permitted a similar conclusion. The L-NAME-induced increase in glucose-stimulated insulin release was accompanied by a corresponding decrease in islet cNOS-activity both in vitro and in vivo. Glucagon release stimulated by different glucagon secretagogues was inhibited by NOS-inhibition both in vitro and in vivo and still evident in K+-depolarized islets. NO-donation potentiated glucagon release. The results suggest a major action of NO on distal events in glucagon release. The present data indicate that NO is a negative modulator of insulin release induced by glucose, sulphonylurea, L-arginine and cholinergic stimulation but not by direct cAMP activation. This inhibitory effect is presumably exerted through S-nitrosylation of "critical" thiol groups. In contrast, NO is a positive modulator of glucagon release induced by all types of secretagogues tested. This effect is presumably exerted in part by NO facilitating the exocytotic process.