PHARMACOLOGICAL CHARACTERIZATION OF RAT ALPHA-2-ADRENERGIC RECEPTORS

We described previously the molecular characterization of a rat (alpha-2B-adrenergic receptor and have shown also that the rat genome contains three closely related alpha-2-adrenergic receptor genes. To characterize the ligand-binding properties of these receptor gene products, we expressed the DNAs encoding these receptors individually in COS-1 cells and studied their binding to a wide variety of typical and atypical adrenergic ligands. The receptors displayed high affinity binding to the radioligand [H-3]rauwolscine, with equilibrium dissociation constants ranging from 1.4 to 28 nM. Kinetic analysis of the binding of [H-3]rauwolscine to membranes from transfected cells was in very good agreement with data obtained from saturation analysis. We examined the ability of a number of agents to compete for the binding of [H-3]rauwolscine to the alpha-2-adrenergic receptor-transfected membranes. Whereas one of these receptors displayed a pharmacological profile typical of an alpha-2A-adrenergic receptor, the other two receptors showed similar pharmacological properties characteristic of an alpha-2B-adrenergic receptor. The two alpha-2B-like adrenergic receptors differed, however, in the ratios of K(i) values for oxymetazoline and prazosin, as well as the K(i) ratio of prazosin and yohimbine. In addition, the two alpha-2B-like adrenergic receptors had a 9-fold difference in affinity for chlorpromazine. The pharmacological characterization of the three rat alpha-2-adrenergic receptor gene products is consistent with the known pharmacology of alpha-2-adrenergic receptors, as documented using tissues and cell lines.