The relative potency of enkephalins and beta-endorphin in guinea-pig ileum, mouse vas deferens and rat vas deferens after the administration of peptidase inhibitors

Previous studies have shown that three distinct enzymes, amastatin-sensitive aminopeptidase, captopril-sensitive peptidyl dipeptidase A, and phosphoramidon-sensitive endopeptidase-24.11, played a critical role in the inactivation of enkephalins in isolated preparations. In the present study, therefore, the rank order of the potency of three endogenous opioid peptides, [Met5]-enkephalin, [Leu5]-enkephalin, and beta-endorphin, in three isolated preparations, guinea-pig ileum, mouse vas deferens, and rat vas deferens, was estimated in the presence of the mixture of three peptidase inhibitors, amastatin, captopril, and phosphoramidon. [Met5]-Enkephalin was approximately three-fold more potent than [Leu5]-enkephalin and four-fold more potent than beta-endorphin in guinea-pig ileum in which three opioid peptides were indicated to act on mu-receptors. Additionally, [Met5]-enkephalin was slightly but significantly more potent than [Leu5]-enkephalin and approximately twenty-fold more potent than beta-endorphin at delta-receptor sites in mouse vas deferens. Moreover, [Met5]-enkephalin was approximately three-fold more potent than [Leu5]-enkephalin, but sixty-fold less potent than beta-endorphin in rat vas deferens in which the opioid-receptor type interacting with enkephalins could not be determined. In conclusion, the well-known rank order of the potency of three endogenous opioid peptides was shown to be altered in both guinea-pig ileum and mouse vas deferens but not in rat vas deferens by the pretreatment of the preparations with the mixture of three peptidase inhibitors.