Our previous studies have shown that the inhibition of neutral endopeptidase, an enzyme which degrades tachykinins, increases anaphylactic contraction of guinea pig tracheal smooth muscle. Anaphylactic release of tachykinin-like substances was indicated. To investigate this observation further, we examined the effects of phosphoramidon, an inhibitor of a neutral endopeptidase, on contraction induced by mediators of anaphylaxis. Phosphoramidon significantly increased histamine- and leukotriene D4-induced contractions of tracheal rings from unsensitized animals (by 14 and 48%, respectively), but failed to alter the contractile responses to prostaglandins D2 and F2 alpha. In tracheal rings preincubated with tachykinin antagonist-[D-Pro(4), D-Trp(7,9)]-substance P(4-11), or in capsaicin-desensitized tracheal rings, phosphoramidon did not change histamine- and leukotriene D4-induced contractions. In the second part of the study, performed on tracheal rings obtained from ovalbumin-sensitized guinea pigs, we examined the effects of phosphoramidon on contractile responses to histamine and leukotrienes which are released after antigen challenge. The incubation of tracheal rings with H-1-histamine receptor antagonist (diphenhydramine HCl) or leukotriene receptor antagonist (ICI 198.615) prevented a phosphoramidon-dependent increase of antigen-induced contraction. These results indicate that histamine and leukotrienes may be involved in the anaphylactic release of tachykinin-like substances or other neutral endopeptidase substratum.
