Inhibition of neutral endopeptidase potentiates compound 48/80-induced constriction of guinea-pig tracheal smooth muscle.

Our previous studies have shown that the inhibition of neutral endopeptidase, an enzyme which degrades tachykinins, increases anaphylactic construction of guinea-pig tracheal smooth muscle. To investigate this observation further, we examined the effects of phosphoramidon, an inhibitor of a neutral endopeptidase, on constriction induced by the non-immunological mast cell degranulator-compound 48/80. Phosphoramidon produced significant leftward shift of the compound 48/80 concentration-response curve with corresponding decrease in the EC50 value from 51 (28-80) micrograms/ml to 42 (20-72) micrograms/ml. When added during the compound 48/80-induced constriction, phosphoramidon significantly increased the magnitude of this constriction by 69.7% after 30 min, and 78.9% after 45 min. Phosphoramidon was ineffective in tracheal rings from tachykinin-depleted guinea pigs. The incubation of tracheal rings with H1-histamine receptor antagonist (diphenhydramine HCl, 10 microM) and leukotriene receptor antagonist (ICI 198.615, 5 microM) significantly diminished the contractile response to compound 48/80 and prevented a phosphoramidon-dependent increase of this constriction. These results suggest that compound 48/80 induces the release of tachykinins by the stimulatory activity of histamine and leukotrienes. Anaphylactic release of tachykinins would therefore not depend directly on the antigen-antibody reaction.