1. The contractile response to histamine, acetylcholine (ACh), KCl or electrical field stimulation (EFS) was examined in paired tracheal rings (one of each being denuded by mucosal rubbing), which were mounted in muscle chambers filled with a continuously aerated physiological salt solution at 37°C.

2. Removal of the respiratory mucosa increased the sensitivity of airway muscle to ACh, histamine and EFS, but not to KCl. The hypersensitivity of denuded rings to histamine and EFS was greater than to ACh. Atropine reduced the histamine hypersensitivity observed.

3. Pretreating intact preparations with indomethacin augmented their responsiveness to EFS, histamine and ACh.

4. Indomethacin augmentation of histamine- and EFS-induced responses was greater in preparations without epithelium.

5. We conclude that the airway mucosa may be associated with a factor that reduces airway smooth muscle responsiveness to stimulation.

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