Endocrinology: nitric oxide-mediated insulin secretion in response to citrulline in islet beta-cells

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.

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

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.

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.

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.