Mechanisms involved in the effect of nitric oxide synthase inhibition on L-arginine-induced insulin secretion

1 A constitutive nitric oxide synthase (NOSc) pathway negatively controls L-arginine-stimulated insulin release by pancreatic beta cells. We investigated the effect of glucose on this mechanism and whether it could be accounted for by nitric oxide production. 2 NOSc was inhibited by N-omega-nitro-L-arginine methyl ester (L-NAME), and sodium nitroprusside (SNP) was used as a palliative NO donor to test whether the effects of L-NAME resulted from decreased NO production. 3 In the rat isolated perfused pancreas, L-NAME (5 mM) strongly potentiated L-arginine (5 mM)-induced insulin secretion at 5 mM glucose, but L-arginine and L-NAME exerted only additive effects at 8.3 mM glucose. At 11 mM glucose, L-NAME significantly inhibited L-arginine-induced insulin secretion. Similar data were obtained in rat isolated islets. 4 At high concentrations (3 and 300 mu M), SNP increased the potentiation of arginine-induced insulin output by L-NAME, but not at lower concentrations (3 or 30 nM). 5 L-Arginine (5 mM) and L-ornithine (5 mM) in the presence of 5 mM glucose induced monophasic beta cell responses which were both significantly reduced by SNP at 3 nM but not at 30 nM; in contrast, the L-ornithine effect was significantly increased by SNP at 3 mu M. 6 Simultaneous treatment with L-ornithine and L-arginine provoked a biphasic insulin response. 7 At 5 mM glucose, L-NAME (5 mM) did not affect the L-ornithine secretory effect, but the amino acid strongly potentiated the alteration by L-NAME of L-arginine-induced insulin secretion. 8 L-Citrulline (5 mM) significantly reduced the second phase of the insulin response to L-NAME (5 mM) + L-arginine (5 mM) and to L-NAME + L-arginine + SNP 3 mu M. 9 The intermediate in NO biosynthesis, N-G-hydroxy-L-arginine (150-300 mu M) strongly counteracted the potentiation by L-NAME of the secretory effect of L-arginine at 5 mM glucose. 10 We conclude that the potentiation of L-arginine-induced insulin secretion resulting from the blockade of NOSc activity in the presence of a basal glucose concentration (1) is strongly modulated by higher glucose concentrations, (2) is not due to decreased NO production but (3) is probably accounted for by decreased levels of N-G-hydroxy-L-arginine or L-citrulline, resulting in the attenuation of an inhibitory effect on arginase activity.