Activation of protein kinase C modulates alpha2-adrenergic signalling in rat pancreatic islets

Treatment of rat pancreatic islets with 4beta-phorbol-myristate-acetate (PMA) caused a significant reduction in the ability of the alpha2-adrenoceptor agonist noradrenaline to inhibit glucose-induced insulin secretion. This effect was most evident when low concentrations of the catecholamine were used (less than 1 microM) and was lost when the noradrenaline concentration was increased to 10 microM. The effect was probably mediated by activation of protein kinase C, because the ability of PMA to desensitise islets to noradrenaline was prevented by a selective inhibitor of calcium-dependent isoforms of the enzyme, Gö6976. The response to PMA was reproduced when islet protein kinase C was activated by a receptor-mediated mechanism involving incubation with the muscarinic agonist carbachol. In parallel with desensitisation of the inhibitory control of insulin secretion by noradrenaline, PMA treatment also reduced the ability of a low concentration of noradrenaline (0.1 microM) to inhibit islet cAMP formation. The loss of sensitivity to catecholamine, induced by PMA in rat islets, was not caused by any change in the levels of alpha2-adrenoceptor expression or in their ligand-binding affinity. It was, however, associated with a marked increase in the extent of phosphorylation of members of the Gi/Go, family of pertussis toxin-sensitive G proteins in PMA-treated islets. Immunoprecipitation of Gi alpha2 and Galpha o from 32P-labelled islets after treatment with PMA revealed that both G proteins are substrates for protein kinase C. Overall, the results indicate that activation of protein kinase C leads to phosphorylation of islet Gi and Go causing their uncoupling from alpha2-adrenoceptors. We propose that this mechanism may form an important component of a physiological system designed to limit the tendency for catecholamines to inhibit insulin secretion under conditions in which the parasympathetic innervation of the islets is activated.