Abstract

Whereas agonist-directed differential signaling at a single receptor subtype has become an accepted pharmacological concept, distinct behaviors by ligands that are assumed to be antagonists is less documented. The intrinsic activity and capacity of antagonism for a new series of imidazoline-derived adrenergic ligands analogous to dexefaroxan were investigated by measuring two distinct signaling pathways at the recombinant human α_2A-adrenoceptor (α_2A AR): 1) pertussis toxin-resistant guanosine 5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding responses mediated by either a recombinant G_αoCys³⁵¹Ile or G_αi2Cys³⁵²Ile protein in CHO-K1 cells, and 2) inhibition of cAMP formation in a stably transfected C6-glial cell line. Ligands could be differentiated as inverse agonists [i.e., 2-(4-methoxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851062], neutral antagonists [i.e., 2-(4-hydroxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851057], partial [i.e., 2-(4-chloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821008], and high-efficacy [i.e., 2-(6,7-dichloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821010] agonists at a precoupled α_2A AR state in the copresence of a G_αoCys³⁵¹Ile protein but not G_αi2Cys³⁵²Ile protein by monitoring [³⁵S]GTPγS binding responses. Neither positive nor negative efficacy was observed for these compounds by monitoring the adenylate cyclase pathway at a presumably low-affinity α_2A AR state. The capacity of the dexefaroxan analogs to antagonize the (-)-epinephrine-mediated [³⁵S]GTPγS binding response at a G_αoCys³⁵¹Ile protein was inversely correlated with their magnitude of intrinsic activity and unrelated to their ligand binding affinity for the α_2A AR. On the other hand, their capacity to antagonize either (-)-epinephrine or 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline tartrate (UK 14304)-mediated inhibition of forskolin-stimulated cAMP formation was not related with the rank order of antagonist capacity for the (-)-epinephrine-mediated [³⁵S]GTPγS binding response. In conclusion, these data demonstrate that certain α₂ AR ligands that are assumed to be antagonists, may yield dissimilar pharmacological responses, dependent on the investigated agonist-stimulated effector pathway.