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 [betaAla8]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 a bsence and in the presence of the aminopeptidase inhibitor amastatin ( 10 muM for 30 min). The affinity of MEN 10,208 in the rabbit pulmonary artery was markedly reduced in the presence of amastatin (pK(B) value s from 7.47 to 5.94), while it was unchanged in the hamster trachea. N either neurokinin A, [betaAla8]neurokinin A-(4-10), nor the other anta gonists were affected by pretreatment with amastatin in either bioassa y. The results obtained in the rabbit pulmonary artery show that MEN 1 0,208 is degraded by local amastatin-sensitive enzymes (possibly amino peptidase M), which may convert the linear octapeptide MEN 10,208 to t he heptapeptide MEN 10,207 by removing the N-terminal Thr from the ami no acid sequence of MEN 10,208. The present results are discussed in r elation to a previously reported heterogeneity between NK2 receptors o f the rabbit and bovine species, and show amastatin to be a new tool f or the classification of tachykinin receptors with peptide ligands.