Excitatory role of axon reflex in bradykinin-induced contraction of guinea pig tracheal smooth muscle.

To elucidate the role of the axon reflex in the airways, we studied the effects of atropine (10(-6) M) and tetrodotoxin (10(-7) M) on the bradykinin-induced contraction of guinea pig tracheal smooth muscle, with or without pretreatment of the animals with capsaicin. The concentration-response curves to bradykinin (10(-9) to 10(-5) M) were measured in the presence of both indomethacin and propranolol. In the guinea pigs not given capsaicin pretreatment, baseline tension values did not differ before versus after the application of atropine or tetrodotoxin. Tetrodotoxin reduced the bradykinin-induced contraction significantly, but atropine did not change the contraction induced by bradykinin. These observations indicate that bradykinin-induced contraction is potentiated by a neurally mediated action, but that is not mediated by acetylcholine released from the efferent vagal nerve terminals. The contractile response to bradykinin was significantly reduced in the animals treated with capsaicin as compared with those administered vehicle only. Furthermore, in the animals treated with capsaicin, tetrodotoxin did not affect the response to bradykinin. These observations indicate that bradykinin-induced airway smooth muscle contraction is mediated in part by tachykinins released from C-fiber endings, presumably via an axon reflex.