Signaling properties of human alpha(1D)-adrenoceptors lacking the carboxyl terminus: intrinsic activity, agonist-mediated activation, and desensitization.

alpha(1)-Adrenoceptors are differentially regulated by protein kinase C-mediated phosphorylation. The most sensitive member of this family is the alpha(1D)-subtype, which is also characterized by a constitutive activity and a reduced expression at the plasma membrane controlled by the amino terminus. Information on the structural domains that determine the function and regulation of this receptor subtype is scarce. Therefore, the function and phosphorylation of amino terminus-truncated (Delta1-79, (DeltaN)) alpha(1D)-adrenoceptors were studied and compared with those of alpha(1D)-adrenoceptors truncated both at the amino and carboxyl termini (Delta1-79 and Delta441-572, (DeltaN-DeltaC)). These receptors were stably expressed in rat-1 fibroblast, at relatively high density ( approximately 2 pmol/mg of membrane protein), and showed intrinsic activity that was markedly increased by noradrenaline. Interestingly, activation of protein kinase C markedly attenuated (desensitized) the function of both DeltaN and DeltaN-DeltaC alpha(1D)-adrenoceptors. These receptors were photolabeled and immunoprecitated with an antibody directed against an influenza hemagglutinin epitope inserted at the amino termini. Metabolic labeling with radioactive phosphate and receptor immunoprecipitation studies indicated that these receptors are phosphoproteins whose phosphorylation state is increased by noradrenaline and by activation of protein kinase C. Our data indicate that carboxyl terminus-truncated alpha(1D)-adrenoceptors are fully functional and subjected to regulation by phosphorylation. The roles of the carboxyl termini differ among alpha(1)-adrenoceptor subtypes.