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

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 methysergide 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.