PT  - JOURNAL ARTICLE
AU  - Wang-Ni Tian
AU  - Duane D. Miller
AU  - Richard C. Deth
TI  - Bidirectional Allosteric Effects of Agonists and GTP at α<sub>2A/D</sub>-Adrenoceptors
DP  - 2000 Feb 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 664--671
VI  - 292
IP  - 2
4099  - http://jpet.aspetjournals.org/content/292/2/664.short
4100  - http://jpet.aspetjournals.org/content/292/2/664.full
SO  - J Pharmacol Exp Ther2000 Feb 01; 292
AB  - 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]GTPγS) binding in membranes from α2A/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]GTPγS binding was closely correlated with the relative influence of GTPγS 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. The American Society for Pharmacology and Experimental Therapeutics