Islet constitutive nitric oxide synthase and glucose regulation of insulinrelease in mice

We have studied, by a combined in vitro and in vivo approach, the relation between the inhibitory action of N-G-nitro-L-arginine methyl ester (L-NAME) , a selective inhibitor of nitric oxide synthase (NOS), on the activity of islet constitutive NOS (cNOS) and glucose regulation of islet hormone relea se in mice. The cNOS activity in islets incubated in vitro at 20 mM glucose was not appreciably affected by 0.05 or 0.5 mM L-NAME, but was greatly sup pressed (-60%) by 5 mM L-NAME. Similarly, glucose-stimulated insulin releas e was unaffected by the lower concentrations of L-NAME but greatly enhanced in the presence of 5 mM of the NOS inhibitor. In incubated islets inhibiti on of cNOS activity resulted in a modestly enhanced insulin release in the absence of glucose, did not display any effect at physiological or subphysi ological glucose concentrations, but resulted in a markedly potentiated ins ulin release at hyperglycaemic glucose concentrations. In the absence of gl ucose, glucagon secretion was suppressed by L-NAME. The dynamics of glucose -induced insulin release and Ca-45(2+) efflux from perifused islets reveale d that L-NAME caused an immediate poten tiation of insulin release, and a s light increase in Ca-45(2+) efflux. In islets depolarized with 30 mM K+ in the presence of the K-ATP(+) channel opener, diazoxide, L-NAME still greatl y potentiated glucose-induced insulin release. Finally, an i.v. injection o f glucose to mice pretreated with L-NAME was followed by a markedly potenti ated insulin response, and an improved glucose tolerance. In accordance, is lets isolated directly ex vivo after L-NAME injection displayed a markedly reduced cNOS activity. In conclusion, we have shown here, for the first tim e, that biochemically verified suppression of islet cNOS activity, induced by the NOS inhibitor L-NAME, is accompanied by a marked potentiation of glu cose-stimulated insulin release both in vitro and in vivo. The major action of NO to inhibit glucose-induced insulin release is probably not primarily linked to changes in Ca2+ fluxes and is exerted mainly independently of me mbrane depolarization events.