Exposing cells to beta-adrenergic agonists results in multiple effects on the beta-adrenergic receptor-coupled adenylyl cyclase. The receptors rapidly become desensitized and less efficient at stimulating adenylyl cyclase; then the receptors become sequestered and inaccessible to hydrophilic ligands; and finally, the receptors are down-regulated as evidenced by a loss of antagonist binding. We previously found that the endogenous beta1-adrenergic receptors expressed by human neurotumor SK-N- MC cells are resistent to desensitization by the beta-adrenergic receptor kinase. The latter has been shown to desensitize the human beta2- adrenergic receptor. We also observed that the beta1 receptors are resistant to down-regulation but undergo sequestration. In order to explore possible differences in the regulation of the two receptor subtypes. we transfected BHK and CHW hamster cells with an expression vector into which we inserted a cDNA encoding one or the other receptor subtypes, and isolated stable receptor-expressing transformants. In addition, we obtained transfected beta1- and beta2- CHO hamster cells and beta2-L mouse cells from other laboratories. The different cell lines were exposed to the agonist isoproterenol for different times and assayed for sequestration, down-regulation and desensitization. In all the transfected cell lines tested, beta2 receptors underwent sequestration faster and more extensively than beta1 receptors. Down-regulation was much more complicated and appeared to be affected by cell type. Although we have only been able to thoroughly assess three cell lines for desensitization, the results are encouraging. When cells were exposed to agonist for 30 min, the two cells lines expressing beta1 receptors exhibited no significant reduction in maximum stimulation of adenylyl cyclase by agonist. By contrast, the one beta2 receptor-expressing cell line tested so far underwent a 43% desensitization. Although these results are incomplete, they suggest differences in agonist-mediated regulation of human beta1- and beta2-adrenergic receptors, particularly sequestration and desensitization. These differences may relate to structural differences in the two receptors, especially their C-termini.
