Competitive blockade of adrenergic α-receptors and histamine receptors by thymoxamine

Abstract The adrenergic α-receptor blocking activity of 4-(2-dimethylaminoethoxy)-5-isopropyl-2-methylphenyl acetate, thymoxamine, was quantitatively investigated on the vas deferens of the guinea-pig and on arterial strips from guinea-pigs, rabbits, cats and dogs. The blockade fulfilled the established criteria for competitive antagonism of noradrenaline. On these various tissues the pA2 was about 7.0; thymoxamine was more potent than piperoxan and less potent than dihydroergotamine. There was no evidence of β-receptor or 5-hydroxytryptamine receptor blocking activity. Against histamine as the agonist on the guinea-pig ileum, thymoxamine was a competitive antagonist with a pA2 of about 6.5.