Norepinephrine- and phorbol ester-induced phosphorylation of alpha(1a)-adrenergic receptors - functional aspects

Maximal adrenergic responses in Rat-1 fibroblasts expressing alpha(1a)-adre nergic receptors are not blocked by activation of protein kinase C, In cont rast, activation of protein kinase C induces the phosphorylation of cu,,adr enoreceptors and blocks their actions, The effect of norepinephrine and pho rbol esters on alpha(1a)-adrenoreceptor phosphorylation and coupling to G p roteins were studied. Both stimuli lead to dose-dependent receptor phosphor ylation, Interestingly, protein kinase C activation affected to a much less er extent the actions of alpha(1a)-adrenergic receptors than those of the c u,, subtype (norepinephrine elicited increases in calcium in whole cells an d [S-35]GTP gamma S binding to membranes). Basal phosphorylation of alpha(1 a)-adrenergic receptors was much less than that observed with the alpha(1b) subtype. The carboxyl terminus seems to be the main domain for receptor ph osphorylation. Therefore, chimeric receptors, where the carboxyl-terminal t ails of alpha(1a) and alpha(1b) adrenergic receptors were exchanged, were c onstructed and expressed. alpha(1a)-Adrenoreceptors wearing the carboxyl ta il 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 p hosphorylation, alpha(1a)-Adrenoreceptors are affected to a much lesser ext ent than alpha(1b)-adrenoreceptors by protein kinase C activation.