Pharmacological characterisation of the presynaptic alpha-adrenoceptor in the rat vas deferens.

The interactions between alpha-adrenoceptor agonists and antagonists on the twitch response of the rat isolated vas deferens to low frequency motor nerve stimulatioh have been examined. Oxymetazoline, clonidine, naphazoline and BAY-1470 caused a concentration-dependent inhibition of the twitch response at concentrations lower than those causing smooth muscle stimulation. The twitch-inhibitory effect of these compounds is thought to result from stimulation of presynaptically located alpha-adrenoceptors. In contrast, phenylephrine and methoxamine exerted little inhibitory effect at concentrations less than those which produced postsynaptic stimulation. The inhibitory effect of clonidine was antagonised by phentolamine, piperoxan, yohimbine and tolazoline, but not by thymoxamine. These results characterise the presynaptic alpha-adrenoceptors in the rat vas deferens as being of the same type as those present in the rabbit pulmonary artery and rat heart, but different from those located post-synaptically in sympathetically innervated tissues.