Tolerance to sulfur dioxide-induced bronchoconstriction in subjects with asthma

Abstract

A study to determine whether the bronchoconstriction induced by low concentration of sulfur dioxide in subjects with asthma decreases with repeated exposure was undertaken. Eight subjects with asthma performed 3 min of voluntary eucapnic hyperpnea with 0.5 ppm of SO 2 in humidified filtered air three times at 30-min intervals and we measured specific airway resistance ( SR aw) before and after each period of hyperpnea. Specific airway resistance increased significantly more after the first exposure to SO 2 [(from 7.6 ± 1.7 to 15.5 ± 2.0 L × cm H 2O/liter/sec (mean ± SEM)] than after the second (from 8.1 ± 1.3 to 10.8 ± 1.6) or third (from 7.6 ± 1.6 to 10.1 ± 1.9) exposures ( P < 0.025). When seven subjects repeated hyperpnea with SO 2 24 hr and 7 days later, SR aw increased as much as it had after the first exposure (from 8.2 ± 2.5 to 15.5 ± 4.5 at 24 hr and from 6.6 ± 1.4 to 15.4 ± 2.1 at 7 days). In four subjects repeated exposure to SO 2 caused short-term inhibition of the bronchomotor response to SO 2 but did not inhibit the bronchomotor response to histamine aerosol. It was concluded that repeated exposures to a low concentration of SO 2 over a short period (on 1 day) can induce tolerance to the bronchomotor effects of SO 2 in subjects with asthma. Tolerance to the bronchomotor effects of SO 2 is not caused by decreased responsiveness of airway smooth muscle or a generalized decrease in the responsiveness of vagal reflex pathways since the bronchomotor response to histamine is preserved.