Amastatin interferes with the antagonist properties of MEN 10,208 in the rabbit pulmonary artery but not in the hamster trachea.

The tachykinin peptide agonists neurokinin A and [beta Ala8]neurokinin A-(4-10), and the NK2 tachykinin receptor-selective antagonists MEN 10,208, MEN 10,207, MEN 10,282, MEN 10,376 and R396 were assayed in the isolated rabbit pulmonary artery and isolated hamster trachea in the absence and in the presence of the aminopeptidase inhibitor amastatin (10 microM for 30 min). The affinity of MEN 10,208 in the rabbit pulmonary artery was markedly reduced in the presence of amastatin (pKB values from 7.47 to 5.94), while it was unchanged in the hamster trachea. Neither neurokinin A, [beta Ala8]neurokinin A-(4-10), nor the other antagonists were affected by pretreatment with amastatin in either bioassay. The results obtained in the rabbit pulmonary artery show that MEN 10,208 is degraded by local amastatin-sensitive enzymes (possibly aminopeptidase M), which may convert the linear octapeptide MEN 10,208 to the heptapeptide MEN 10,207 by removing the N-terminal Thr from the amino acid sequence of MEN 10,208. The present results are discussed in relation to a previously reported heterogeneity between NK2 receptors of the rabbit and bovine species, and show amastatin to be a new tool for the classification of tachykinin receptors with peptide ligands.