A difference in the affinity of some selective alpha 2-adrenoceptor antagonists when compared on isolated vasa deferentia of rat and rabbit.

The alpha 2-adrenoceptor antagonist potencies of the benzoquinolizines (Wy 26 703, Wy 25 309, Wy 26 392), the benzodioxans (RX 781 094, RS 21 361), yohimbine and rauwolscine have been compared at presynaptic alpha 2-adrenoceptors in the isolated vasa deferentia of the rat and rabbit. Yohimbine and rauwolscine are of equal potency as antagonists in both the rat and rabbit against the agonists clonidine or UK 14304. The benzoquinolizines and benzodioxans are very weak antagonists of clonidine or UK 14304 at the presynaptic alpha 2-adrenoceptors of the rabbit vas deferens when compared to their potency at the presynaptic alpha 2-adrenoceptors of the rat vas deferens. This suggests that the presynaptic alpha 2-adrenoceptors present in the rat vasa deferentia may be different from those present in the rabbit vasa deferentia.