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 congruent to 25 hr, beta 2 congruent to 200 hr. These experiments 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 enhance 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 occupancy of receptors as assessed in radioligand binding studies. Receptor recovery following removal of agonists was blocked by cycloheximide and was much faster than the 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-regulation 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.