Affinities of Adrenergic Drugs for α ₂-Adrenoceptors in Dog Saphenous Vein in Comparison with Those in Rat Brain or Human Platelets

Abstract

The affinities of drugs for the postsynaptic α₂-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 α₂-adrenoceptors in human platelets or in rat brain were determined by radioligand binding assay using ³H-yohimbine (YOH) or ³Hclonidine (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 α₂-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 α₂-adrenoceptor-mediated effects on vascular tissues primarily through binding to a low-affinity form of the α₂-adrenoceptors.