Differential regulation of human α1-adrenoceptor subtypes

Abstract

We have compared the agonist-induced downregulation of human α_1A-, α_1B- and α_1D-adrenoceptors upon stable expression in rat-1 fibroblasts. During a 24-h incubation the agonist phenylephrine downregulated α_1A- and α_1B-adrenoceptors in a concentration-dependent manner. While maximum downregulation was similar for both subtypes, the threshold concentration for significant reductions was markedly higher for α_1A- than for α_1B-adrenoceptors (10 μM vs. 100 nM). The downregulation of both subtypes by 100 μM phenylephrine was time-dependent, and significant reductions were observed already after 2– 4 h. In contrast, incubation of α_1D-adrenoceptor-expressing cells with phenylephrine increased receptor number in a time- and concentration-dependent manner. The downregulation of α_1B-adrenoceptors by 100 μM phenylephrine for 24 h was accompanied by a matching reduction in mRNA abundance, but no such reduction was seen for α_1A-adrenoceptors. These treatment conditions also caused a functional desensitization of agonist-stimulated inositol phosphate formation for α_1A- and α_1B- but not for α_1D-adrenoceptors. Treatment with the phorbol ester phorbol-12-myristate-13-acetate did not change receptor density or mRNA abundance and did not cause functional desensitization. We conclude that human α₁-adrenoceptor subtypes are differentially regulated by agonist treatment even if they are expressed in the same cell line.