AGONIST-MEDIATED REGULATION OF ALPHA-1-ADRENERGIC AND BETA-2-ADRENERGIC RECEPTOR METABOLISM IN A MUSCLE-CELL LINE, BC3H-1

Abstract

We have compared the metabolism of .alpha.1- and .beta.2-adrenergic receptors which are both expressed in BC3H-1 muscle cells. During growth of the cells to confluence, the number of .alpha.1-receptors per mg of membrane protein increases, whereas that of the .beta.2-receptors remains constant. Experiments using cycloheximide and irreversible .alpha.1- and .beta.2-receptor antagonists, phenoxybenzamine and N-[2-hydroxy-3-(1-naphthoxy)-propyl]-N''-bromoacetylethylenediamine, respectively, yield disparate turnover rates (t1/2) for the two receptors: .alpha.1 .simeq. 25 hr, .beta.2 .simeq. 200 hr. These experiment suggest that synthesis of .beta.2-receptors virtually ceases in confluent cells. Maximally effective doses of agonists down-regulated both receptor types 80-90% and enhanced the rates of loss of both receptors (t1/2 = 1-5 hr). The rates of down-regulation were not affected by cycloheximide, implying that agonists enhanced receptor clearance rather than decrease receptor appearance. The rank orders of potencies of agonists for promoting receptor down-regulation were those characteristic of .alpha.1- and .beta.2-receptors. However, concentrations of agonists that resulted in down-regulation of each receptor subtype were 10- to 100-fold lower than those required for occupany of receptors as assessed in radioligand binding studies. Receptor recovery following removal of agonists was blocked by cycloheximide and was much faster than recovery that followed treatment of cells with irreversible antagonists. Therefore, protein synthesis (but perhaps not receptor synthesis per se) appears necessary for recovery from down-regulation. In addition, the rates of recovery of .alpha.1- and .beta.2-receptor-mediated functions (phosphatidylinositol turnover and cyclic AMP synthesis, respectively) following receptor down-regulator or irreversible blockade parallel the rates of receptor recovery. These data indicate that basal metabolism of .alpha.1- and .beta.2-receptors in BC3H-1 cells is substantially different, but that agonist-mediated changes in metabolism of the two receptor subtypes are similar. Thus, common mechanisms appear to mediate the regulation by agonists of .alpha.1- and .beta.2-receptors in these cells.