Fig. 2. (A ) Quantification of human adenosine 1 receptors by characterization of [³H]-8-cyclopentylxanthine (CPX) binding to membranes prepared from transfected Chinese hamster ovary cells (20 μg protein/tube). Saturation isotherm and Scatchard plot (inset ) conform to a single-site model with a dissociation constant, K^d, of 2.42 ± 0.21 nm and a B^maxof 2,755 ± 91 fmol/mg protein. Nonspecific (NS) binding was measured in the presence of excess 8-cyclopentyltheophylline (100 μm). Each point  is the mean of triplicate determinations. Bound units for the Scatchard plot are expressed as fmol per assay tube. (B ) Human adenosine 1 receptors expressed in Chinese hamster ovary cells were functionally active, thus decreasing cAMP levels on binding of the agonist N⁶-cyclopentyladenosine (CPA) (10⁻¹²–10⁻⁵M). In agreement with the action of a competitive antagonist, addition of CPX (1 μm) resulted in a right shift of the dose–response curve, increasing EC⁵⁰values nearly 1,000-fold.