Inhibiting effect of ammonia on citric acid-induced cough in pigs: A possible involvement of substance P

The effect of ammonia on the cough response to citric acid and on substance P release from C-fibers involved in this reflex was assessed. For a period from one to four days, piglets were exposed, in an inhalation chamber, to ammonia at a concentration of 15 or 30 ppm. During exposure, cough inductio n tests were done every two days. Recovery of the cough reflex after ammoni a exposure was also determined. In a separate group of piglets exposed for 2 days to 30 ppm ammonia, substance P content was determined in bronchial a nd tracheal lavage fluids and in the tracheal and bronchial mucosa. Ammonia (30 ppm) was found to inhibit coughing significantly (the cough frequency was reduced by 64%) after a two-day exposure. In animals exposed for 4 days to this ammonia concentration, the recovery ranged from 3 to 7 days (mean: 5 days). The same ammonia concentration also caused the substance P conten t to increase significantly in bronchoalveolar lavage fluid (to 432% of its initial value) and tracheal lavage fluid (to 149%) and to decrease signifi cantly in the tracheal mucose (-58%), however the content in bronchial muco sa was not significantly affected (-43%). Exposure to 15 ppm ammonia had no effect on the frequency of citric acid-induced coughing. In conclusion, am monia inhibits citric acid-induced coughing in pigs at concentrations that can be detected in piggeries. This inhibitory effect may be related to subs tance-P depletion in C-fiber endings.