Ligand efficacy and potency at recombinant α2 adrenergic receptors -: Agonist-mediated [35S]GTPγS binding

Alpha-2 adrenergic receptors (alpha(2) AR) mediate incorporation of guanosine 5'-O-(gamma-thio) triphosphate ([S-35]GTP gamma S) into isolated membranes via receptor-catalyzed exchange of [S-35]GTP gamma S for GDP. In the current study, we used [S-35]GTP gamma S incorporation to characterize the intrinsic activity and potency of agonists and antagonists at the cloned mouse alpha(2a/d) and human alpha(2a), alpha(2b), and alpha(2c) ARs. Full agonists increased [S-35]GTP gamma S binding to membranes by 2- to 3-fold. Antagonists did not increase [S-35]GTP gamma S binding but competitively inhibited agonist-stimulated [S-35]GTP gamma S binding. Compounds with intrinsic activities less than that of the full agonists norepinephrine (NE) or epinephrine (EPI) were capable of antagonizing agonist-stimulated [S-35]GTP gamma S binding. The agonistic properties of a number of alpha(2) AR ligands were characterized at each alpha(2) AR subtype. The rank order of agonist potency for selected compounds at the human receptors (with intrinsic activity compared with NE, defined as 1.0) was: alpha(2a): Dexmedetomidine (0.73) > guanabenz (0.38) > UK-14304 (1.02) > clonidine (0.32) > ST-91 (0.63) > NE (1.00). alpha(2b): Dexmedetomidine (1.10) > clonidine (0.18) > guanabenz (0.71) > NE (1.00) > ST-91 (0.44) > UK-14304 (0.59). alpha(2c): Dexmedetomidine (1.03) > NE (1.00) > UK-14304 (0.75) > ST-91 (0.32) greater than or equal to clonidine (0.23) much greater than guanabenz (0). This report provides a functional characterization of adrenergic receptor ligands at human and mouse alpha(2a/d) AR It also illustrates the utility of [S-35]GTP gamma S incorporation as a functional marker of receptor activation. (C) 1998 Elsevier Science Inc.