alpha(2)-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)-adre noceptor-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 agon ists dexmedetomidine, UK14,304 (5-bromo-6-[2-imidazoline-2-ylamino]quinoxal ine) and oxymetazoline showed subtype-dependent efficacy. Dexmedetomidine w as a full agonist at the alpha(2B)-subtype and a partial agonist at the alp ha(2A)- and the alpha(2C)-subtypes. UK14,304 was a full agonist at the alph a(2A)-subtype and a partial agonist at the other two. Oxymetazoline showed strong partial agonism at the alpha(2B)-subtype (63% of adrenaline), but di d not significantly activate the alpha(2A)- and the alpha(2C)-subtypes. The se 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 GTPas e assay may thus provide a valuable tool for the identification of subtype- selective alpha(2)-adrenoceptor agonists. (C) 1999 Elsevier Science B.V. Al l rights reserved.