Application of receptor theory provides further evidence for the existence of the epsilon-opioid receptor in rat vas deferens.

In the present study, we analyzed the interaction of concomitantly-acting pairs of opioid agonists in inhibiting the electrically induced contraction of the rat vas deferens (RVD). The results when compared with the predictions of the law of mass action and the receptor theory (1,2) provide evidence for a specific receptor for beta-endorphin (beta-EP) in this organ. This receptor is not occupied by the delta-agonist D-Ala2-D-Leu5-enkephalin (DADLE), or the mu-agonists Sandoz FK-33824, fentanyl and etorphine in the range of concentrations necessary for these to exert their maximal agonistic action in the test. The activity of beta-endorphin in the rat vas deferens is in agreement with the proposed existence (3,4) of the epsilon-type of opioid receptor therein.