Endocrinology: nitric oxide-mediated insulin secretion in response to citrulline in islet beta-cells.
BACKGROUND
Nitric oxide (NO) synthases (NOSs) are expressed in insulin secreting beta-cells. However, physiologic role of NO in insulin release is still controversial. We previously reported that argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL), together with NOS, constitute the citrulline-argininosuccinate-arginine (Cit-AS-Arg) cycle in beta-cells and that this cycle metabolizes citrulline to produce NO and increase cytosolic Ca2+ concentration ([Ca2+]i) in islet beta-cells.


OBJECTIVE
This study examined whether this cycle could be linked to insulin release.


METHODS
Islets of Langerhans were isolated from Wistar rats by collagenase digestion and further dispersed into single beta-cells. [Ca2+]i in beta-cells was measured by dual-wavelength fura-2 microfluorometry combined with digital imaging. NO production was assayed by DAF-2 microfluorometry. Insulin release was determined by ELISA.


RESULTS
Citrulline at a physiologic concentration (0.1 mM) increased insulin release from rat islets and increased [Ca2+]i in rat beta-cells in the presence of 8.3 mmol/l glucose, and they were inhibited by a NOS inhibitor, NG-monomethyl-L-arginine (NMMA). Citrulline induced NO production in rat beta-cells. A NO-donor increased insulin release and [Ca2+]i in rat islet beta-cells.


CONCLUSIONS
The metabolism of physiologic concentrations of citrulline by the Cit-AS-Arg cycle leads to NO production and resultant potentiation of glucose-induced insulin release, in which a Ca2+-mediated pathway could be involved.
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects; D002478:Cells, Cultured; D002956:Citrulline / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004797:Enzyme-Linked Immunosorbent Assay; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors / Q000378:metabolism; D052247:Nitric Oxide Synthase Type II; D051381:Rats; D017208:Rats, Wistar; D019323:omega-N-Methylarginine / Q000494:pharmacology
D004791:Enzyme Inhibitors; D007328:Insulin; D019323:omega-N-Methylarginine; D002956:Citrulline; D009569:Nitric Oxide; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; C496318:Nos2 protein, rat; D005947:Glucose; D002118:Calcium