Protein kinase C-mediated phosphorylation and desensitization of human alpha(1b)-adrenoceptors

Human alpha(1b)-adrenoceptors stably expressed (B-max similar to 800 fmol/m g membrane protein) in mouse fibroblasts were able to increase intracellula r Ca2+ and inositol phosphate production in response to noradrenaline. Acti vation of protein kinase C desensitized the alpha(1b)-adrenergic-mediated a ctions but did not block the ability of the cells to respond to lysophospha tidic acid. Inhibition or downregulation of protein kinase C also blocked t he action of the tumor promoter on the adrenergic effects. Photolabeling ex periments indicated that the receptor has an apparent molecular weight of s imilar to 80 kDa. The receptors were phosphorylated in the basal state and such phosphorylation was increased when the cells were incubated with phorb ol myristate acetate or noradrenaline. Incubation of the cells with phorbol myristate acetate or noradrenaline blocked noradrenaline-promoted [S-35]GT P-gamma-S binding to membranes, suggesting receptor-G protein uncoupling. T he results indicate that activation of protein kinase C blocked/desensitize d human alpha(1b)-adrenoceptors and that such effect was associated to rece ptor phosphorylation. (C) 1999 Elsevier Science B.V. All rights reserved.