Ligand Efficacy and Potency at Recombinant α2 Adrenergic Receptors: Agonist-Mediated [35s]gtpγs Binding

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