Characterization of alpha(2)-adrenoceptors mediating contraction of dog saphenous vein: identity with the human alpha(2A) subtype

1 In the dog saphenous vein alpha(1)- and alpha(2)-adrenoceptors mediate noradrenaline-induced contractions in vitro. In order to study the alpha(2)-adrenoceptor in isolation, alpha(2)-adrenoceptors were inactivated by treatment of tissues with the alkylating agent phenoxybenzamine (3.0 mu M for 30 min) in the presence of rauwolscine (1 mu M) to protect alpha(2)-adrenoceptors. 2 Noradrenaline-induced contractions of tissues treated with phenoxybenzamine were antagonized competitively by the selective alpha(2)-adrenoceptor antagonist rauwolscine, pK(B)=8.63+/-0.07 (means+/-s.e.-mean; n=3), consistent with an interaction at alpha(2)-adrenoceptors. 3 Noradrenaline was a full agonist at alpha(2)-adrenoceptors in dog saphenous vein. By use of the method of partial receptor alkylation and analysis of concentration-effect curve data by direct, operational model fitting methods, the affinity (pK(A)) and efficacy (tau) were 5.74+/-0.07 and 7.50+/-1.05, respectively (n=6). Nine other agonists which were examined each had affinities higher than noradrenaline, but with the exception of the imidazoline, A-54741 (5,6-dihydroxy-1,2,3,4-tetrahydro-1-naphthyl-imidazoline) had relatively lower efficacies. 4 To compare the alpha(2)-adrenoceptor in dog saphenous vein to the human recombinant subtypes, the affinities of twenty-one compounds were estimated in functional studies in the dog saphenous vein and in radioligand binding studies for the human alpha(2A), alpha(2B) and alpha(2C) receptor subtypes expressed in Chinese hamster lung (CHL) cells. 5 Of twenty-one compounds examined in ligand binding studies, only nine had greater than ten fold selectivity for one human receptor subtype over either of the other two. These compounds were A-54741, oxymetazoline, guanfacine, guanabenz, prazosin, spiroxatrine, tolazoline, WB 4101 and idazoxan. In dog saphenous vein, their affinities (pK(A) and pK(B) for agonists and antagonists respectively) were: A-54741 (pK(A)=8.03+/-0.05), oxymetazoline (pK(A)=7.67+/-0.09), guanfacine (pK(A)=6.79+/-0.03); guanabenz (pK(A)=7.02+/-0.13); prazosin (pK(B)=5.19+/-0.08), spiroxatrine (pK(B)=6.59+/-0.04), tolazoline (pK(B)=6.21+/-0.07), WE 4101 (pK(B)=7.42+/-0.09) and idazoxan (pK(B)=7.11+/-0.08). 6 Comparisons of affinity estimates for these nine compounds at the receptor in dog saphenous vein and at the human recombinant subtypes suggest that the vascular receptor is most similar to the h alpha(2A) subtype; correlation coefficients (r) were 0.82 (h alpha(2A)), 0.24 (h alpha(2B)) and 0.04 (h alpha(2C)).