Α2-Adrenoceptor Agonists Stimulate High-Affinity GTPase Activity in a Receptor Subtype-Selective Manner

Transfected Chinese hamster ovary cells expressing human alpha(2a)-, alpha(2B)- and alpha(2C)-adrenoceptor subtypes were used to monitor alpha(2)-adrenoceptor-stimulated GTP hydrolysis. Incubation with 100 mu M (-)-adrenaline resulted in stimulation of pertussis toxin-sensitive GTPase by 380% after activation of the alpha(2A)-subtype, by 320% after activation of the alpha(2B)-subtype and by 110% after activation of the alpha(2C)-subtype. The agonists dexmedetomidine, UK14,304 (5-bromo-6-[2-imidazoline-2-ylamino]quinoxaline) and oxymetazoline showed subtype-dependent efficacy. Dexmedetomidine was a full agonist at the alpha(2B)-subtype and a partial agonist at the alpha(2A)- and the alpha(2C)-subtypes. UK14,304 was a full agonist at the alpha(2A)-subtype and a partial agonist at the other two. Oxymetazoline showed strong partial agonism at the alpha(2B)-subtype (63% of adrenaline), but did not significantly activate the alpha(2A)- and the alpha(2C)-subtypes. These results agreed with cAMP accumulation experiments carried out with cell lines endogenously expressing the alpha(2A)-subtype (human erythro-leukemia, HEL) or the alpha(2B)-subtype (neuroblastoma-glioma, NG108-15). The GTPase assay may thus provide a valuable tool for the identification of subtype-selective alpha(2)-adrenoceptor agonists. (C) 1999 Elsevier Science B.V. All rights reserved.