Interaction between SO2 and cold-induced bronchospasm in anesthetized rabbits

In anesthetized, paralyzed and artificially ventilated rabbits, reflex changes in lung resistance induced by cooling the inspired air from 38 to 15 degrees C were studied before and after 45 min periods of SO2 exposure at two different concentrations (0.5 or 5 ppm). Both concentrations of SO2 induced significant increase in RL in intact animals (+16% and +50%, respectively). The effect of 5 ppm SO2 persisted after vagotomy. The cold-induced bronchospasm was halved after exposure to 0.5 ppm SO2 and was no longer significant after exposure to 5 ppm SO2. In both cases, RL recovered to control values 40 min after the end of SO2 exposure and then, the magnitude of cold-induced bronchospasm also recovered. The reflex bronchoconstrictor response to phenyldiguanide (PDG) i.v. disappeared after exposure to 5 ppm SO2. However, the bronchomotor response to histamine i.v., which involved both reflex and direct actions on airway smooth muscle, was not altered. These results show that (1) prolonged increase in RL measured after SO2 exposure does not result from a vagal reflex; (2) the cold-induced bronchospasm, as well as the bronchomotor response to PDG, are reduced or suppressed during the period where the effect of SO2 persisted. This suggests that 45 min exposure to SO2 induces transient alterations in tracheobronchial wall, which reduce the accessibility to nervous receptors in the airways.