A neuronal isoform of nitric oxide synthase expressed in pancreatic beta-cells controls insulin secretion

Evidence is presented showing that a neuronal isoform of nitric oxide synth ase (NOS) is expressed in rat pancreatic islets and INS-1 cells. Sequencing of the coding region indicated a 99.8% homology with rat neuronal NOS (nNO S) with four mutations, three of them resulting in modifications of the ami no acid sequence. Double-immunofluorescence studies demonstrated the presen ce of nNOS in insulin-secreting beta -cells. Electron microscopy studies sh owed that nNOS was mainly localized in insulin secretory granules and to a lesser extent in the mitochondria and the nucleus. We also studied the mech anism involved in the dysfunction of the beta -cell response to arginine an d glucose after nNOS blockade with N-G-nitro-L-arginine methyl ester, Our d ata show that miconazole, an inhibitor of nNOS cytochrome c reductase activ ity, either alone for the experiments with arginine or combined with sodium nitroprusside for glucose, is able to restore normal secretory patterns in response to the two secretagogues. Furthermore, these results were corrobo rated by the demonstration of a direct enzyme-substrate interaction between nNOS and cytochrome c, which is strongly reinforced in the presence of the NOS inhibitor. Thus, we provide immunochemical and pharmacological evidenc e that beta -cell nNOS exerts, like brain nNOS, two catalytic activities: a nitric oxide production and an NOS nonoxidating reductase activity, both o f which are essential for normal beta -cell function. In conclusion, we sug gest that an imbalance between these activities might be implicated in beta -cell, dysregulation involved in certain pathological hyperinsulinic state s.