DISTINCT REGULATION OF BETA-1-ADRENERGIC AND BETA-2-ADRENERGIC RECEPTORS IN CHINESE-HAMSTER FIBROBLASTS

The agonist-induced reduction of beta-adrenergic receptor (beta-AR) cell surface density is a well documented phenomenon. The mechanisms responsible for this regulation have been well characterized for the beta(2)AR. They include a rapid sequestration of the receptor away from the cell surface in a vesicular compartment and a longer term down-regulation of the total beta(2)AR number. In contrast, very little is known about the cell surface regulation of the beta(1)AR. In the present study, we have compared the agonist-mediated regulation of beta(1)- and beta(2)AR in Chinese hamster fibroblasts transfected with the cDNA encoding either beta-AR subtype. Cells expressing similar numbers of the two beta-AR subtypes were selected for the study. The expressed receptors exhibit typical beta(1)- and beta(2)AR selectivity for agonists and antagonists, as assessed by radioligand binding. Both receptors were found to be positively coupled to the adenylyl cyclase stimulatory pathway, but marked differences in the receptor regulation profiles were observed. Treatment of the cells expressing the beta(2)AR with the agonist isoproterenol leads to a rapid sequestration of > 30% of the receptors away from the cell surface into a light vesicular fraction, where they are inaccessible to the hydrophilic ligand CGP-12177. In contrast, virtually no agonist-induced sequestration is observed in the cells expressing the beta(1)AR. Longer exposure of the cells to isoproterenol leads to a time-dependent reduction in the total number of beta-ARs in both beta(1)- and beta(2)AR- expressing cell lines. However, this down-regulation is significantly slower in the cells expressing the beta(1)AR. In fact, no appreciable down-regulation of the beta(1)ARs is detected in the first 4 hr of agonist treatment, compared with a down-regulation of > 50% of the beta(2)ARs for the same period. After a 24-hr treatment with isoproterenol, < 20% of the original number of beta(2)ARs remain, whereas 60% of the beta(1)ARs are still present after the same treatment. These results, therefore, suggest that, when expressed in an identical cell line, beta(1)AR and beta(2)AR follow distinct patterns of regulation. In fact, both agonist-induced sequestration and down-regulation are considerably blunted for the beta(1)AR, compared with the beta(2)AR.