Evidence for the epsilon-type of opioid receptor in the rat vas deferens.

The epsilon-opioid receptor type has been postulated to be specific for beta-endorphin on the basis of structure-activity studies in the isolated rat vas deferens (RVD). Since beta-endorphin also displays high affinity for other opioid receptors, the present investigations were conducted to better define the epsilon-receptor in the RVD. Therefore, the interaction of concomitantly acting pairs of opioid agonists was examined and analyzed according to the predictions of the law of mass action and the receptor theory. Our data provide evidence for a specific receptor for beta-endorphin in this organ, a receptor not recognized by either the delta-agonist D-Ala2-D-Leu5-enkephalin, or the mu-agonists FK-33824, fentanyl, and etorphine in the range of concentrations necessary for them to exert maximal inhibition of electrically induced twitch tension. The activity displayed by beta-endorphin in the RVD is in agreement with the existence of the proposed epsilon-type of opioid receptor therein.