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 evidence by an increase in mean pulmonary flow resistance (R(L)), hyperinflation, and a fall in dynamic compliance. Airway reactivity was assessed by measuring the increase of R(L) after 18 breaths of 0.25% carbachol (c), from an initial R(L) value obtained after 18 breaths of buffered saline (s) [R(L)(c-s)]. R(L) and R(L)(c-s) were determined prior to, immediately after, and 24 h after exposure to 5 ppm SO2 for 4 h. In both groups R(L) remained unchanged after SO exposure. Prior to exposure, R(L)(c-s) was not significantly different in seven normal (0.3 ± 0.1) and seven allergic sheep [0.4 ± 0.2 (SD) cmH2O.l-1.s], and there was no significant change in R(L) (c-s) immediately after SO2 exposure in either group. Twenty-four h later, R(L)(c-s) (R(L)(c-s) increased to 0.7 ± 0.8 (P<0.2) in normal and to 1.8 ± 0.9 cmH2O.l-1.s (P<0.01) in allergic sheep. Because the increase in R(L)(c-s) after 24 h was greater (P<0.001) in allergic than in normal sheep, we conclude that SO2 exposure increased airway reactivity more in the former than in the latter.
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