Norepinephrine- and Phorbol Ester-induced Phosphorylation of α1a-Adrenergic Receptors: FUNCTIONAL ASPECTS

Abstract

Maximal adrenergic responses in Rat-1 fibroblasts expressing alpha(1a)-adrenergic receptors are not blocked by activation of protein kinase C. In contrast, activation of protein kinase C induces the phosphorylation of alpha(1b)-adrenoreceptors and blocks their actions. The effect of norepinephrine and phorbol esters on alpha(1a)-adrenoreceptor phosphorylation and coupling to G proteins were studied. Both stimuli lead to dose-dependent receptor phosphorylation. Interestingly, protein kinase C activation affected to a much lesser extent the actions of alpha(1a)-adrenergic receptors than those of the alpha(1b) subtype (norepinephrine elicited increases in calcium in whole cells and [(35)S]GTPgammaS binding to membranes). Basal phosphorylation of alpha(1a)-adrenergic receptors was much less than that observed with the alpha(1b) subtype. The carboxyl terminus seems to be the main domain for receptor phosphorylation. Therefore, chimeric receptors, where the carboxyl-terminal tails of alpha(1a) and alpha(1b) adrenergic receptors were exchanged, were constructed and expressed. alpha(1a)-Adrenoreceptors wearing the carboxyl tail of the alpha(1b) subtype had a high basal phosphorylation and displayed a strong phosphorylation in response to norepinephrine and phorbol esters. Our results demonstrate that stimulation of alpha(1a)-adrenergic receptor, or activation of protein kinase C, leads to alpha(1a)-adrenergic receptor phosphorylation. alpha(1a)-Adrenoreceptors are affected to a much lesser extent than alpha(1b)-adrenoreceptors by protein kinase C activation.