Abstract
1. Several recent in vitro studies have suggested that production of nitric oxide (NO) from the islet NO system may have an important regulatory influence on the secretion of insulin and glucagon. In the present paper we have investigated, mainly with an in vivo approach, the influence and specificity of the NO synthase (NOS) blocker NG-nitro-L-arginine methyl ester (L-NAME) on L-arginine-induced secretion of insulin and glucagon. 2. In freely fed mice, L-NAME pretreatment (1.2 mmol kg-1) influenced the dynamics of insulin and glucagon release following an equimolar dose of L-arginine, the specific substrate for NOS activity, in that the NOS inhibitor enhanced the insulin response but suppressed the glucagon responses. This was reflected in a large decrease in the plasma glucose levels of the L-NAME pretreated animals. 3. L-NAME pretreatment did not influence the insulin and glucagon secretory responses to the L-arginine-enantiomer D-arginine, which cannot serve as a substrate for NOS activity. 4. Replacing L-NAME pretreatment by pretreatment with D-arginine or L-arginine itself, which both carry the same cationic change and are devoid of NOS inhibitory properties, did not mimic the effects of L-NAME on L-arginine-induced hormone release. 5. Fasting the animals for 24 h totally abolished the L-NAME-induced potentiation of L-arginine stimulated insulin release suggesting that the sensitivity of the beta-cell secretory machinery to NO-production is greatly changed in the fasting state. However, the L-NAME-induced suppression of L-arginine stimulated glucagon release was unaffected by starvation. 6. In isolated islets from freely fed mice, L-arginine (5 mM) stimulated insulin release was greatly enhanced and glucagon release markedly suppressed by the presence of the NOS inhibitor L-NAME in the incubation medium. These effects were abolished in isolated islets taken from 24 h fasted mice. 7. Our present results, which showed that the NOS inhibitor L-NAME markedly enhances insulin release but suppresses glucagon release induced by L-arginine in the intact animal, give strong support to our previous hypothesis that the islet NO system is a negative modulator of insulin secretion and a positive modulator of glucagon secretion. Additionally, we observed that the importance of the beta-cell NO-production for secretory mechanisms, as evaluated by the effect of L-NAME on L-arginine-induced insulin release, was greatly changed after starvation, an effect less prominent with regard to glucagon release.