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

Abstract

1 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. 2 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. 3 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. 4 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. 5 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.