Baboon erythrocyte ghosts contain β-adrenergic receptors

We have used the β-adrenergic antagonist [³H]dihydroalprenolol ([³H]DHA) to identify binding sites on the erythrocyte membrane of the primate Papio ursinus. Analysis of the saturation isotherm revealed binding to be saturable with a maximal number of binding sites of 499 fmol/mg protein. [³H]DHA binds specifically to the erythrocyte ghosts with an apparent dissociation constant (K(d)) of 0.57 ± 0.06 nM. A similar value for K(d) (0.46 ± 0.07 nM) was evaluated from the rate constants of association (0.013 ± 0.003 · nM^-1· min^-1) and dissociation (0.006 ± 0.001 · min^-1). β-adrenergic agonists compete for the binding sites with an order of potency (dl-isoproterenol > l-epinephrine > l-norepinephrine) typical of a β₂-adrenergic receptor. Binding was shown to be stereospecific with l-stereoisomers being more potent than their corresponding d-stereoisomers in causing half-maximal inhibition. Isoproterenol stimulated the production of intracellular adenosine 3',5'-cyclic monophosphate (cAMP) in a concentration-dependent manner, maximal levels (1.130 ± 0.358 pmol cAMP/10⁸ cells) being four times the basal levels. The results demonstrate the existence of a large number of β-adrenergic receptors on baboon erythrocyte ghosts.