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

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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.