The present investigation was a pilot study to examine the effects of endotoxin in comparison with ozone on bronchial hyperactivity in the guinea pig. A single dose of inhaled endotoxin caused an approximately 30% fall in sGaw, recovery occurring within 10min. Inhalation of the vehicle (saline) caused an increase in sGaw with return to baseline levels within 10min. In contrast to these changes, inhalation of ozone was without effect on sGaw. In naive animals exposure to a challenge dose of carbachol had minimal effects on the sGaw both on a first occasion and 1 week later. Exposure to saline did not alter this response when a challenge dose was given 1h later.

When guinea pigs were challenged with carbachol 1h after the inhalation of either ozone or endotoxin a significant fall in sGaw occurred indicating the presence of hyperactivity. However, 24h after exposure to endotoxin the hyperactivity had disappeared as Judged by response to the carbachol challenge.

No hyperactivity was observed in guinea pigs exposed daily to endotoxin when challenged after five days. Carbachol challenge after a weekend break following I month of daily endotoxin exposure also failed to demonstrate a state of hyperresponiveness of the airway.

These results nay be an indication of the development of tolerance to hyperactivity induced on repeated exposure to endotoxin. However, the development of airway hyporeactivity cannot be ruled out as an explanation of the repeat exposure data. In any event, the findings are of relevance to the role of endotoxin as a putative byssinogenic agent.