Pre-synaptic and post-synaptic effects of xylazine and naphazoline on the bisected rat vas deferens

The effect of the selective alpha 2-adrenoceptor agonists, naphazoline and xylazine, was studied on the field stimulated bisected rat vas deferens. Xylazine inhibited the twitch response to field stimulation in both the prostatic (ID50 = 0.10 microM) and epididymal (ID50 = 0.08 microM) halves of the rat vas deferens. This effect was antagonized by yohimbine (0.01-0.10 microM). Naphazoline also inhibited the response to field stimulation in the prostatic (ID50 = 0.12 microM) but had no such action on the epididymal half of the rat vas deferens. Indeed, low concentrations of naphazoline (threshold, 0.05 microM) contracted the epididymal vas deferens preparation. These contractions were competitively antagonized by prazosin suggesting an action on post-synaptic alpha 1-adrenoceptors. Neither pre-synaptic nor post-synaptic actions of either drug were affected by cocaine (10 microM) or beta-oestradiol (10 microM) added to the Krebs' solution. The results provide further evidence for the existence of two types of post-synaptic alpha 1-adrenoceptors and suggest a different anatomical localization of these receptors between the two ends of the rat vas deferens.