Acute agonist-mediated desensitization of the human α1a-adrenergic receptor is primarily independent of carboxyl terminus regulation -: Implications for regulation of α1aAR splice variants

Despite important roles in myocardial hypertrophy and benign prostatic hyperplasia, little is known about acute effects of agonist stimulation on alpha(1a)-adrenergic receptor (alpha(1a)AR) signaling and function. Regulatory mechanisms are likely complex since 12 distinct human alpha(1a)AR carboxyl-terminal splice variants have been isolated. After determining the predominance of the alpha(1a-1)AR isoform in human heart and prostate, we stably expressed an epitope-tagged alpha(1a-1)AR cDNA in rat-1 fibroblasts and subsequently examined regulation of signaling, phosphorylation, and internalization of the receptor. Human alpha(1a-1)AR-mediated inositol phosphate signaling is acutely desensitized in response to both agonist and phorbol 12-myristate 13-acetate (PMA) exposure. Concurrent with desensitization, alpha(1a)ARs in (32)Pi- labeled cells are rapidly phosphorylated in response to both NE and PMA stimulation. Despite the ability of PKC to desensitize alpha(1a)ARs when directly activated with PMA inhibitors of PKC have no effect on agonist-mediated desensitization. In contrast, involvement of GRK kinases is suggested by the ability of GRK2 to desensitize alpha(1a)ARs. Internalization of cell surface alpha(1a)ARs also occurs in response to agonist stimulation (but not PKC activation), but is initiated more slowly than receptor desensitization. Significantly, deletion of the alpha(1a)AR carboxyl terminus has no effect on receptor internalization or either agonist-induced or GRK-mediated receptor desensitization. Because mechanisms underlying acute agonist-mediated regulation of human alpha(1a)ARs are primarily independent of the carboxyl terminus, they may be common to all functional alpha(1a)AR isoforms.