Neuromedin N (NN) induced a concentration-dependent contraction (ED50 = 2.3 +/- 0.2 microM) of the isolated longitudinal smooth muscle from guinea pig ileum. This effect was drastically enhanced (ED50 = 0.06 microM) by the aminopeptidases M and B inhibitor bestatin (10 microM), which elicited a 40-fold increase in NN potency. HPLC analysis indicated that the main NN catabolite generated by membranes from guinea pig longitudinal smooth muscle homogenate corresponded to des-Lys1-NN, which results from removal of the N-terminal lysyl residue of NN. The fact that the formation of des-Lys1-NN was fully prevented by bestatin (10 microM) further supports the involvement of aminopeptidases in NN degradation. We examined the catabolic fate of NN in vivo in the vascularly perfused dog ileum. Bolus administration or continuous infusion of the peptide led to rapid disappearance of NN. This was prevented by prior treatment of ileal segments with bestatin (10 microM) but not with arphamenine B (0.5 microM), which indicated that aminopeptidase M but not aminopeptidase B participated in NN proteolysis in vivo. We showed that 1 and 10 nmol NN trigger the release of 28 +/- 5 and 59 +/- 1 pmol, respectively, of endogenous vasoactive intestinal polypeptide-like immunoreactivity after infusion in the vascularly perfused dog ileum. This release was virtually doubled by prior treatment with 10 microM bestatin but not with 0.5 microM arphamenine B. Altogether, our data indicate that aminopeptidase M is largely responsible for NN degradation in vitro and in vivo in the gastrointestinal tract and could be considered the physiological inactivator of NN in the gut.