Effects of bradykinin receptor antagonists on antigen‐induced respiratory distress, airway hyperresponsiveness and eosinophilia in guinea‐pigs

We examined effects of bradykinin (BK) receptor antagonists on airway hyperresponsiveness and eosinophilia in sensitized guinea‐pigs that had been administered single, as well as repeated (chronic) challenges with inhaled ovalbumin. In addition, the effects of BK antagonists on antigen‐induced respiratory distress during the chronic study were noted. At 24 h following single antigen challenge, guinea‐pigs exhibited airway hyperresponsiveness to the bronchoconstrictor effect of i.v. histamine, characterized by a left shift in the dose‐response curve. In addition, responses to the maximum dose of histamine that could be used were significantly increased in hyperresponsive guinea‐pigs. The percentages of bronchoalveolar fluid, eosinophil and neutrophils also increased. A BK B₁ receptor antagonist, desArg⁹‐[Leu⁸]‐BK, significantly inhibited airway hyperresponsiveness induced by single antigen challenge. A B₂ receptor antagonist, d‐Arg‐[Hyp³, Thi^5,8,d‐Phe⁷]‐BK (NPC 349) had a small, but statistically significant inhibitory effect on responsiveness to the highest histamine dose in challenged animals. DesArg⁹‐[Leu⁸]‐BK significantly inhibited the neutrophilia, whereas NPC 349 inhibited infiltration by both cell types. Chronic antigen challenge also caused airway hyperresponsiveness to i.v. acetylcholine (ACh), distinguished by an increase in the slope of the dose‐response curve. Thus, the magnitude of the bronchoconstrictor responses to the maximum dose of ACh that could be used was significantly increased. No change in sensitivity to ACh was evident. Marked eosinophilia was also noted in the trachea, bronchi and lung parenchyma. Airway hyperresponsiveness and eosinophilia, induced by chronic antigen challenge, were markedly inhibited by the B₂ antagonists, d‐Arg‐[Hyp³,d‐Phe⁷]‐BK (NPC 567) or d‐Arg‐[Hyp³,Thi⁵d‐Tic⁷,Tic⁸]‐BK (NPC 16731). NPC 16731 also abolished antigen‐induced cyanosis, and delayed the onset of dyspnoea, doubling the time taken for animals to exhibit respiratory distress. The ability of BK receptor antagonists to inhibit antigen‐induced airway hyperresponsiveness, in addition to eosinophilia, indicates an important role for endogenous kinins. Moreover, the abrogation of eosinophil infiltration suggests that BK has a significant function in maintaining allergic inflammation of the airways. 1992 British Pharmacological Society