Isolated human bronchial muscle was stimulated to contract by carbachol, acetylcholine, and histamine. The effects of adrenaline and the alpha- and beta-adrenergic blocking drugs on these contractions were then observed. Phentolamine and phenoxybenzamine potentiated the relaxant effect of adrenaline. Phenoxybenzamine alone caused relaxation of bronchial muscle and inhibited the response to the various spasmogens. Propranolol alone caused a slow increase in muscle tone and antagonized the relaxant effect of adrenaline on carbachol-induced contraction. Phentolamine and phenoxybenzamine added before, with, and after propranolol failed to affect the antagonism of adrenaline by propranolol. The results suggest that the adrenergic receptors in human bronchi are of the beta type.
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