PHARMACOLOGICAL ROLE AND DEGRADATION PROCESSES OF NEUROMEDIN-N IN THEGASTROINTESTINAL-TRACT - AN IN-VITRO AND IN-VIVO STUDY

Neuromedin N (NN) induced a concentration-dependent contraction (ED(50 ) = 2.3 +/- 0.2 mu M) of the isolated longitudinal smooth muscle from guinea pig ileum. This effect was drastically enhanced (ED(50) = 0.06 mu M) by the aminopeptidases M and B inhibitor bestatin (10 mu M), whi ch elicited a 40-fold increase in NN potency. HPLC analysis indicated that the main NN catabolite generated by membranes from guinea pig lon gitudinal smooth muscle homogenate corresponded to des-Lys(1)-NN, whic h results from removal of the N-terminal lysyl residue of NN. The fact that the formation of des-Lys'-NN was fully prevented by bestatin (10 mu M) further supports the involvement of aminopeptidases in NN degra dation. We examined the catabolic fate of NN in vivo in the vascularly perfused dog ileum. Bolus administration or continuous infusion of th e peptide led to rapid disappearance of NN. This was prevented by prio r treatment of ileal segments with bestatin (10 mu M) but not with arp hamenine B (0.5 mu M), which indicated that aminopeptidase M but not a minopeptidase B participated in NN proteolysis in vivo. We showed that 1 and 10 nmol NN trigger the release of 28 +/- 5 acid 59 +/- 1 pmol, respectively, of endogenous vasoactive intestinal polypeptide-like imm unoreactivity after infusion in the vascularly perfused dog ileum. Thi s release was virtually doubled by prior treatment with 10 mu M bestat in but not with 0.5 mu M arphamenine B. Altogether, our data indicate that aminopeptidase WI is largely responsible for NN degradation in vi tro and in vivo in the gastrointestinal tract and could be considered the physiological inactivator of NN in the gut.