Receptor density and cAMP accumulation: Analysis in CHO cells exhibiting stable expression of a cDNA that encodes the beta₂-adrenergic receptor

The relationship between hormone receptor number and hormone-stimulated cAMP accumulation was probed in CHO cells that were transfected with the cDNA encoding the beta-adrenergic receptor under the control of the SV40 early promoter (expression vector pSV2BAR). CHO cells were cotransfected with pSV2BAR and expression vector pHOMER that directs the expression of a neomycin-resistance gene, and stable transfectants were selected. Clones expressing receptor at levels from 30 (wild-type) to 6000 fmol/mg membrane protein were isolated and further characterized for receptor mRNA content (measured by solution hybridization with a single-stranded cDNA probe), steady-state expression of receptor (measured by immunoblotting and indirect immunofluorescence), and their ability to accumulate intracellular cAMP in response to a beta-adrenergic agonist. Receptor mRNA content and the steady-state level of receptor protein and its expression at the cell surface were found to increase with receptor density as measured by radioligand binding. Over a 200-fold range of receptor expression, CHO transfectants displayed increasing efficacy of agonist-stimulated cAMP accumulation and increasing maximal cAMP accumulation in response to agonist. These data provide for the first time an analysis of the relationship between the density of a G-protein-linked receptor and a receptor-mediated response under conditions where the levels of G-proteins and adenylate cyclase are unaltered.