The pharmacology of sheep tracheobronchial muscle: a relaxant effect of histamine on the isolated bronchi

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Abstract
1 Histamine contracted the tracheal and major bronchial muscles of sheep and relaxed the muscles of the lesser bronchi and bronchioles. 2 The stimulant action of histamine on the trachea was antagonized by mepyramine, whereas the relaxant effect on the bronchial tree was not. 3 5-Hydroxytryptamine contracted the musculature from all parts of the respiratory tract—an effect which was specifically antagonized by both methy-sergide and atropine. 4 Acetylcholine contracted all sheep tracheobronchial muscle and isoprenaline relaxed it. 5 Bradykinin had a very weak stimulant action. 6 It is concluded that histamine relaxes sheep bronchi by a direct excitation of histamine-sensitive receptors which are not blocked by mepyramine. No indirect components through autonomic nervous elements or by way of catecholamine release were evident. 5-Hydroxytryptamine seemed to have a dual mode of action in contracting sheep tracheobronchial muscle (a) directly on its own receptors and (b) indirectly through parasympathetic components.

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