Vol. 292, Issue 2, 664-671, February 2000

Bidirectional Allosteric Effects of Agonists and GTP at _2A/D-Adrenoceptors¹

Wang-Ni Tian, Duane D. Miller and Richard C. Deth

Department of Pharmaceutical Sciences, Northeastern University, Boston, Massachusetts (W.-T.T., R.C.D.); and Department of Pharmaceutical Sciences, University of Tennessee, Memphis, Tennessee (D.D.M.).

Agonists and GTP exert reciprocal effects on the stability of the G protein-coupled receptor/G protein complex, implying bidirectional control over the receptor/G protein interface. To investigate this relationship, we compared the ability of a series of hydroxyl-substituted phenethylamine and imidazoline agonists to stimulate [³⁵S]guanosine 5'-O-(3-thio)triphosphate ([³⁵S]GTPS) binding in membranes from _2A/D-adrenergic receptor-transfected PC12 cells with the magnitude of the GTP-induced reduction in agonist affinity in [³H]rauwolscine-binding studies. Agents previously described as full and partial agonists in functional studies showed similar relative efficacies in promoting GTP binding (r = 0.97) as well as similar relative potencies (r = 0.94). Efficacy among agonists for promotion of [³⁵S]GTPS binding was closely correlated with the relative influence of GTPS on agonist binding (r = 0.97), consistent with a bidirectional allosteric influence by agonists and GTP on receptor/G protein complexation. In an additional series of tolazoline derivatives, a range in efficacy from full agonism to strong inverse agonism was observed, depending on the presence or absence of hydroxyl substituents. Together these results suggest that agonist-induced repositioning of transmembrane helices via their hydroxyl interactions is a critical determinant of the stability of the receptor/G protein complex and therefore of agonist efficacy.