['Increased osmolarity of the airway surface has been shown to abolish the airway relaxant effects of inhaled nitric oxide in rabbits in vivo and in guinea-pig trachea in vitro.', 'In this study, we used a guinea-pig tracheal perfusion method to investigate whether superoxide anions, which rapidly react with nitric oxide, could be responsible for the reduced effect of nitric oxide in hyperosmolar airways.', 'Guinea-pig tracheas were constricted with carbachol (CCh) and then subjected to the nitric oxide donor sodium nitroprusside (SNP) under isoosmolar or hyperosmolar conditions. Hyperosmolarity was created by increasing the NaCl concentration of the buffer on the epithelial side of the airway.', 'The relaxation produced by SNP was significantly less following hyperosmolar challenge, with a relaxation by 31 +/- 7% in hyperosmolar conditions as compared to 53 +/- 6% under normal isoosmolar conditions (P<0.05). The experiment was then performed in the presence of superoxide dismutase (SOD) that reduces levels of superoxide anions. SOD restored the relaxing potency of SNP in hyperosmolar conditions back to normal, to 46 +/- 5%.'] Does hyperosmolarity decrease the relaxing potency of sodium nitroprusside on guinea-pig trachea by the release of superoxide anions?