Fusion proteins were constructed between either a wild-type or mutant Thr370Lys alpha2B-adrenoceptor (alpha2B AR) and a mouse Galpha15 protein to analyze ligand-receptor interactions at a receptor/Galpha15 protein density ratio of 1. Activation of the wild-type alpha2B AR-Galpha15 fusion protein in CHO-K1 cells by (-)-adrenaline induced a time- and concentration-dependent (pEC50 = 7.37+/-0.13) increase in the intracellular Ca2+ concentration, which could be antagonized by RX 811059 (pK(B) = 7.55+/-0.15). Whereas d-medetomidine and oxymetazoline were as efficacious agonists as (-)-adrenaline, the following ligands displayed partial agonist properties: BRL 44408 < atipamezole < clonidine < UK 14304 < BHT 920. A comparison with the mutant Thr370Lys alpha2B AR-Galpha15 fusion protein displayed similar Ca2+ kinetics and a ligand-mediated receptor activation profile characterized by higher potencies and greater maximal Ca2+ responses for the ligands being investigated, including the putative antagonists dexefaroxan and idazoxan. RX 811059 and RX 821002 remained silent. Similar conclusions could be made on enhancement of the ligands' intrinsic activities by coexpression of the mutant Thr370Lys alpha2B AR with either a Galpha15 or Galphao Cys351Ile protein. The Thr370Lys alpha2B AR-Galpha protein interactions may modify the tertiary structure of the mutant receptor in such a way that some putative alpha2 AR antagonists are capable of stabilizing an active receptor conformation, thereby generating positive efficacy.