RT Journal Article SR Electronic T1 Inverse Agonist Activity at the α2A-Adrenergic Receptor JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 532 OP 542 DO 10.1124/mol.59.3.532 VO 59 IS 3 A1 Susan M. Wade A1 Keng-Li Lan A1 Dorian J. Moore A1 Richard R. Neubig YR 2001 UL http://molpharm.aspetjournals.org/content/59/3/532.abstract AB Constitutive activation of G protein-coupled receptors (GPCRs) is now well recognized and many classical GPCR antagonists have been found to be inverse agonists. For the α2A-adrenergic receptor (AR) we determine the relative inverse efficacies of a series of antagonists and utilize the extended ternary complex model to estimate the fraction of constitutively active mutant (CAM) receptors in the active state. Stable Chinese hamster ovary cell lines expressing the porcine α2A-AR in its wild-type (WT) and constitutively activated (CAM-T373K) form were isolated. Activation of both Gi and Gs was enhanced for CAM receptors. cAMP production was suppressed in cells with the CAM α2A-AR and this suppression was reversed by α2-adrenergic antagonists with an order of inverse efficacy of rauwolscine > yohimbine > RX821002 > MK912, whereas phentolamine and idazoxan were essentially neutral antagonists. This striking difference in inverse efficacy between idazoxan and RX821002 may account for in vivo pharmacological differences between these two α2-adrenergic antagonists. Agonist binding affinity to the non-G protein-coupled CAM receptor was 3- to 9-fold higher than to WT, whereas binding of the most efficacious inverse agonists, yohimbine and rauwolscine, was 1.7- and 2.1-fold weaker. Analysis of this difference by the extended ternary complex model indicates that approximately 50% of the CAM α2A-AR is in the active (R*) state although there is no detectable constitutive activity of the WT receptor in the absence of agonist.