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.

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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 β-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 β-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 β-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 β-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.

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.

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