An investigation of presynaptic α‐adrenoceptor subtypes in the pithed rat heart and in the rat isolated vas deferens

Abstract

The presynaptic cardio‐inhibitory effects of the α‐adrenoceptor agonists xylazine, cirazoline and amidephrine and their interaction with the antagonists yohimbine and prazosin were investigated in the pithed rat. The presynaptic inhibitory effects of the α2‐selective agonist xylazine were antagonized by the α2‐antagonist yohimbine but not by the α2‐antagonist prazosin, thus demonstrating the lack of α2‐adrenoceptor antagonism by prazosin. The presynaptic inhibitory effects of cirazoline were antagonized equally by yohimbine and prazosin, and the presynaptic inhibitory effects of the selective α1‐agonist amidephrine were antagonized by prazosin more potently than by yohimbine. In the nifedipine‐treated isolated epididymal portion of the rat vas deferens, both xylazine and amidephrine produced concentration‐dependent inhibition of the isometric contraction to single pulse electrical stimulation. The α2‐antagonist rauwolscine antagonized the inhibitory effects of xylazine but not of amidephrine. It is concluded that inhibitory α1‐adrenoceptors, as well as the already established α2‐receptors, are present presynaptically in the pithed rat heart and in the rat vas deferens. 1984 British Pharmacological Society