Background: We previously reported that Nω-nitro-L-arginine methyl ester (L-NAME) enhances airway responsiveness to inhaled serotonin in cats treated with atropine and propranolol. Objective: To further elucidate the role of nitric oxide (NO) in airway responsiveness, we investigated whether L-NAME induces airway hyperresponsiveness to serotonin and acetylcholine (ACh) in animals with intact innervation. Methods: Cats were anesthetized with pentobarbital sodium (50 mg/kg, i.p.), and mechanically ventilated. To assess airway responsiveness, we measured increase in total pulmonary resistance (RL) produced by delivering serotonin or ACh aerosol to the airway, and determined PC200 (the concentration which caused a 200% increase in RL). Results: The following results were obtained: (1) Airway responsiveness to serotonin was significantly enhanced by the administration of L-NAME (100 mg/kg) in animals treated with atropine and propranolol. (2) Airway responsiveness to serotonin was also significantly enhanced by L-NAME in animals with intact innervation. (3) In contrast, airway responsiveness to ACh was not changed by the addition of L-NAME in cats with intact innervation. Conclusion: These results suggest that NO modulates non-specific airway responsiveness in animals with intact innervation, presumably by a reflex mechanism.
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