Substance P and capsaicin‐induced contraction of human bronchi

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Abstract
Substance P induced a dose-dependent contraction of human segmental bronchi in vitro with a threshold dose of about 10-6 M. These preparations were obtained from patients undergoing lung tumor surgery. The substance P-induced contractions were resistant to mepyramine and atropine, suggesting a direct effect on the bronchial smooth muscle. Capsaicin (10-5 M) also induced a slowly developing strong atropine-resistant contraction of human bronchi in vitro. A rapid tachyphylaxis developed for the response to capsaicin. Both substance P and capsaicin were less potent than acetylcholine and histamine in inducing contractions of human bronchi. This may be partly due to the experimental conditions, and both substance P and capsaicin were comparatively much more potent in guinea pig preparations. Transmural field stimulation of the bronchial preparations in man resulted in contractions that were largely sensitive to atropine. The presence of capsaicin-induced bronchial contractions indicates the existence of a local noncholinergic axon-reflex control of bronchial smooth muscle tone by substance P in man.