Selective receptors for β-endorphin on the rat vas deferens

The isolated rat vas deferens, being insensitive to morphine, contains selective binding sites for β-end-orphin. A half-maximal inhibition of twitch tension evoked by electrical stimulation is established with 100 nM β-endorphin, while fragments of β-endorphin, that is, methionine-enkephalin, α- and γ-endorphin, are almost ineffective. The opiate alkaloid etorphine, a powerful inhibitor of guinea-pig ileum and mouse vas deferens, is 100-fold less potent on the rat vas deferens. The unique β-endorphin activity suggests very specific binding sites for this peptide, which cannot be related to the μ- or δ-receptors so far described for opiods on isolated preparations.