Affinities of Adrenergic Drugs for α₂-Adrenoceptors in Dog Saphenous Vein in Comparison with Those in Rat Brain or Human Platelets

The affinities of drugs for the postsynaptic alpha 2-adrenoceptors in the dog saphenous vein were examined and compared with those in human platelets or in rat brain. The pA2 or pD2 values in the vein were determined in the presence of phenoxybenzamine. The pKi values (dissociation constant) of drugs for binding to alpha 2-adrenoceptors in human platelets or in rat brain were determined by radioligand binding assay using 3H-yohimbine (YOH) or 3H-clonidine (CLO) as ligands. The binding of agonists to 3H-YOH binding sites was carried out in the presence of GTP. The pA2 or pD2 values in the vein had a good correlation with pKi values for YOH sites but not with those for CLO sites in rat brain. The pKi values for YOH sites in human platelets had a good correlation with those for YOH sites in the brain or with pA2 or pD2 values in the vein. These results suggested that the affinities of adrenergic drugs for postsynaptic alpha 2-adrenoceptors in the dog saphenous vein are similar to those in brain and in platelets, and it is probable that the adrenergic drugs produce their alpha 2-adrenoceptor-mediated effects on vascular tissues primarily through binding to a low-affinity form of the alpha 2-adrenoceptors.