Insulin secretion in rats: effects of neuropeptide Y and noradrenaline

Neuropeptide Y (NPY) occurs within nerves in the rat pancreas, some of which are adrenergic. Therefore, we examined the influence of NPY alone and together with noradrenaline (NA) on insulin release in the rat. When infused alone for 30 min under basal conditions, NPY increased basal plasma insulin concentrations by 32 +/- 13 microU/ml at the highest dose level tested (68 pmol/min), as compared to +7 +/- 7 microU/ml in the controls (p less than 0.05). In contrast, NPY at 17 pmol/min reduced the plasma insulin response to both glucose (by 11%; p less than 0.001) and to arginine (by 26%; p less than 0.001). Infusion of NA alone (340 pmol/min) significantly elevated the basal plasma insulin levels by 41 +/- 4 microU/ml (p less than 0.001). NPY (17 pmol/min) completely abolished this effect (p less than 0.001). When NA (340 pmol/min) was added to an ongoing glucose infusion, the plasma insulin levels were markedly reduced (by 48%; p less than 0.001). By introducing NPY (17 pmol/min), this inhibitory action occurred more rapidly. However, NPY did not affect the maximal response to NA. In isolated rat islets, both NPY (10(-6) M) and NA (10(-6) M) inhibited glucose-stimulated insulin secretion. However, the maximal inhibitory effect exerted by the two neurotransmitters was not altered by giving them together. We conclude that, in the rat, NPY and NA both elevate basal plasma insulin levels and inhibit stimulated insulin secretion. In combination, NPY also induces a more rapid onset of the inhibitory action of NA on glucose-induced insulin secretion.