Substance P and capsaicin-induced contraction of human bronchi

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 finding may however 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 however indicates the existence of a local non-cholinergic axon-reflex control of bronchial smooth muscle tone by substance P in man.