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

Abstract

1. A constitutive nitric oxide synthase (NOSc) pathway negatively controls L-arginine-stimulated insulin release by pancreatic β 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^ω-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 μM), 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 β 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 μM.

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 μM.

9. The intermediate in NO biosynthesis, N^G-hydroxy-L-arginine (150–300 μM) 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.

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