Intact cell binding properties of cells expressing altered beta-adrenergic receptors

During the course of equilibrium competition binding assays with intact cells, agonists induce conversion of beta-adrenergic receptors (BARs) from a native form with high affinity for agonists to a form with a markedly lower apparent affinity. The roles of receptor internalization, receptor-Gs coupling, and receptor phosphorylation in this agonist-induced conversion to the low affinity form were investigated. Agonist and antagonist competition for [125I]iodopindolol binding to intact cells was measured in mouse L cells expressing wild-type BARs (C+I+), mutated BARs that do not couple to Gs but do internalize (C-I+), and mutated BARs that do not couple to Gs and do not internalize (C-I-). For C+I+ and C-I+ cells, most of the receptors exhibited apparent affinities for the agonist isoproterenol that were 500-900-fold lower in equilibrium assays with intact cells than in short-time assays with intact cells or in equilibrium assays with isolated membranes, similar to previous results with cells expressing native BARs. The extent of conversion to this lower affinity form for C-I- cells was markedly decreased. Binding properties for the antagonist metoprolol were similar for all three BARs in both short-time and equilibrium assays. Isoproterenol competition in short-time and equilibrium assays also was compared in Chinese hamster fibroblasts expressing wild-type BARs, mutated BARs that lack BAR kinase sites, mutated BARs that lack cAMP-dependent protein kinase sites, and mutated BARs that lack both types of phosphorylation sites. All three BAR phosphorylation mutants showed only small but significant decreases, relative to the wild-type BAR, in the extent of conversion to the low affinity form. These results provide additional evidence that receptor internalization is the major determinant for the conversion of intact cell BARs to the low affinity form. Receptor phosphorylation may play a minor role in conversion to the low affinity form, whereas receptor coupling to Gs is apparently not required.