[Comparative study of the pre and post-synaptic alpha blocking activity of thymoxamine and its metabolites in the rat vas deferens (author's transl)].

1. The action of thymoxamine and its two metabolites at pre- and post-synaptic alpha-adrenoceptors have been studied on rat vas deferent. 2. Low frequency electrical stimulation of the rat isolated vas deferens produced regular contractions that were inhibited by low concentrations of clonidine. The inhibition of the vas deferens produced by clonidine was presynaptic in origin and involved alpha-adrenoceptors. 3. Presynaptic alpha-adrenoceptor antagonist activity was assessed by studying the effect of increasing concentrations of the antagonists on cumulative clonidine dose-response curves on the stimulated vas deferens. 4. Postsynaptic alpha-adrenoceptor antagonist activity was assessed by comparison of control cumulative noradrenaline dose-response curves with those in the presence of increasing concentrations of antagonists in the isolated vas deferens. 5. The results indicate that thymoxamine and its metabolites block postsynaptic alpha-adrenoceptors preferentially. Phentolamine is more potent in blocking presynaptic than postsynaptic alpha-adrenoreceptors. 6. At postsynaptic alpha-adrenoceptors thymoxamine and deacetylthymoxamine present nearly the same blocking activity, N-demethyl déacétylthymoxamine is less active. At presynaptic alpha-adrenoceptors their activity is practically negligible. 7. The findings support the view that pre- and post-synaptic alpha-adrenoceptors in the rat vas deferens differ in their sensitivity to alpha adrenoceptor antagonists.