Differences in airway reactivity in normal and allergic sheep after exposure to sulfur dioxide

The effect of breathing 5 ppm sulfur dioxide (SO2) on airway reactivity was studied in both normal and allergic conscious sheep. Allergic sheep were defined as animals in which inhalation of Ascaris suum extract resulted in bronchospasm as evidenced by an increase in mean pulmonary flow resistance (RL), hyperinflation, and a fall in dynamic compliance. Airway reactivity was assessed by measuring the increase of RL after 18 breaths of 0.25% carbachol (c), from an initial RL value obtained after 18 breaths of buffered saline (s) [RL(c-s)]. RL and RL(c-s) were determined prior to, immediately after, and 24 h after exposure to 5 ppm SO4 for 4 h. In both groups RL remained unchanged after SO2 exposure. Prior to exposure, RL(c-s) was not significantly different in seven normal (0.3 +/- 0.1) and seven allergic sheep [0.4 +/- 0.2 (SD) cmH2O X l-1 X s], and there was no significant change in RL (c-s) immediately after SO2 exposure in either group. Twenty-four h later, RL(c-s) RL(c-s) increased to 0.7 +/- 0.8 (P less than 0.2) in normal and to 1.8 +/- 0.9 cmH2O X l-1 X s (P less than 0.01) in allergic sheep. Because the increase in RL(c-s) after 24 h was greater (P less than 0.01) in allergic than in normal sheep, we conclude that SO2 exposure increased airway reactivity more in the former than in the latter.