Studies on the mechanism of inhibition of glucose-stimulated insulin secretion by noradrenaline in rat islets of Langerhans.

Noradrenaline (norepinephrine) was shown to be a potent inhibitor of glucose-induced insulin release from rat pancreatic islets, with half-maximal inhibition of the secretory response to 20 mM-glucose occurring at approx. 0.3 microM, and complete suppression of the response occurring at 4 microM-noradrenaline. Inhibition of insulin secretion by noradrenaline was antagonized by the alpha 2-adrenergic antagonist yohimbine (half maximally effective dose approximately 1 microM), but was largely unaffected by the alpha 1-adrenergic antagonist prazosin at concentrations up to 50 microM, suggesting that the response was mediated by alpha 2-adrenergic receptors. Noradrenaline significantly reduced the extent of 45Ca2+ accumulation in glucose-stimulated islets, although as much as 5 microM-noradrenaline was required for 50% inhibition of this response. The ability of noradrenaline to inhibit islet-cell 45Ca2+ uptake was totally abolished in media containing 1 mM-dibutyryl cyclic AMP, suggesting that the response may have been secondary to lowering of islet cyclic AMP. Under these conditions, however, noradrenaline was still able to inhibit insulin secretion maximally. The data suggest that the site(s) at which noradrenaline acts to mediate inhibition of insulin secretion in rat islets lies distal to both islet-cell cyclic AMP accumulation and Ca2+ uptake.