Influence of G protein type on agonist efficacy

An agonist at a specific G protein-coupled receptor may exhibit a range of efficacies for any given response in a cell-specific manner.:We report that the relationship between different states of: agonism is regulated by the type of G protein expressed in the cell. In NIH-3T3 alpha(2)-adrenergic rec eptor (AR) transfectants, the alpha(2)-AR agonists clonidine, oxymetazoline , UK-14304, and epinephrine increased [S-35]guanosine-5'-O-(3-thio)triphosp hate binding in a dose-dependent manner from a basal Value of 101.2 +/- 6.5 fmol/mg to a maximal response (100 mu M) of 196.6 +/- 9.8, 182.3 +/- 2, 32 8.1 +/- 11.2, and 340.6 +/- 3 fmol/mg, respectively. Thus, clonidine and ox ymetazoline behaved as-partial agonists. Receptor-mediated activation of G proteins in membrane preparations was blocked by cell pretreatment with per tussis toxin, indicating receptor coupling to the subgroup of pertussis tox in-sensitive G protein (Gi alpha 2,3) expressed in NIH-3T3 cells. Ectopic e xpression of Goal but not Gi alpha 1 increased the relative efficacy of clo nidine and oxymetazoline such that the two ligands now behaved as close to full agonists in this assay system. The relationship between full and parti al agonists in the different genetic backgrounds was. not altered by progre ssive reduction in the amount of G protein: available for coupling to recep tor. The increased efficacy observed for clonidine in the Goal transfectant s was not due;:to changes in the relative affinities or amounts of high-aff inity, Gpp(NH)p-sensitive binding of agonist. These data suggest that there is little difference in the ability of clonidine to interact with or stabi lize alpha(2)-AR-Gi alpha 2/Gi alpha 3 versus alpha(2)-AR-Go alpha 1 comple xes, but that the subsequent step of signal transfer from receptor to G pro tein is more readily achieved for the clonidine/alpha(2)-AR/Go alpha 1 comp lex. Such observations have important implications for receptor theory and drug development.