Negative Feedback Mechanisms: Evidence that Desensitization of Pineal α₁-Adrenergic Responses Involves Protein Kinase-C

Abstract

alpha 1-Adrenergic stimulation of the pinealocyte translocates protein kinase-C, which, in turn, has an important positive effect on pineal cell function; translocation amplifies beta-adrenergic stimulation of both cAMP and cGMP. In the present report negative feedback effects of protein kinase-C are described, including inhibition of alpha 1-adrenergically induced increases in cytosolic Ca2+ ([Ca2+]i), phosphatidylinositol hydrolysis, and cGMP in beta-adrenergically stimulated cells. Time-course studies of cGMP and [Ca2+]i responses indicated that the onset of inhibition by the protein kinase-C activator 4 beta-phorbol 12-myristate 13-acetate (PMA) is rapid (less than 5 min). In contrast, PMA has no inhibitory effect on norepinephrine stimulation of cAMP accumulation or the induction of arylalkylamine N-acetyltransferase activity, a cAMP-dependent enzyme. This is consistent with the finding that PMA substitutes for the positive effect of alpha 1-activation and directly potentiates beta-adrenergic stimulation of cAMP production. Although PMA does inhibit alpha 1-adrenergic potentiation of cGMP in beta-adrenergically treated cells, it does not inhibit the potentiation of the cGMP response in beta-adrenergically stimulated cells produced by high K+, A23187, and ouabain, agents that translocate protein kinase-C secondary to elevation of [Ca2+]i. This suggests that translocation of protein kinase-C does not block an effect of Ca2+, but probably blocks an earlier step in adrenergic activation, presumably the alpha 1-adrenergic stimulation of [Ca2+]i. Finally, pretreatment of cells with an alpha 1-agonist markedly reduced cAMP and cGMP responses to subsequent beta-adrenergic stimulation. The data indicate that the following negative feedback mechanism is present in the pinealocyte: alpha 1-adrenoceptor-dependent elevation of [Ca2+]i----protein kinase-C translocation----inhibition of alpha 1-adrenergic dependent elevation of [Ca2+]i. This mechanism appears to function physiologically to provide a negative feedback signal which limits adrenergic responses that are dependent on an increase in [Ca2+]i, including the cAMP and cGMP increases.