Comparisons of acute inflammatory responses of nose-only inhalation and intratracheal instillation of ammonia in rats

Objective: To establish a rat model with respiratory and pulmonary responses caused by inhalation exposure to non-lethal concentrations of ammonia (NH3) that can be used for evaluation of new medical countermeasure strategies for NH3-induced acute lung injury (ALI). This is of great value since no specific antidotes of NH3-induced injuries exist and medical management relies on supportive and symptomatically relieving efforts. Methods: Female Sprague-Dawley rats (8-9 weeks old, 213g +/- 2g) were exposed to NH3 using two different exposure regimens; nose-only inhalation or intratracheal instillation. The experiment was terminated 5 h, 24 h, 14 and 28 days post-exposure. Results: Nose-only inhalation of NH3 (9000-15 000 ppm) resulted in increased salivation and labored breathing directly post-exposure. Exposure did not increase inflammatory cells in bronchoalveolar lavage fluid but exposure to 12 000 ppm NH3 during 15 min reduced body weight and induced coagulation abnormalities by increasing serum fibrinogen levels. All animals were relatively recovered by 24 h. Intratracheal instillation of NH3 (1%) caused early symptoms of ALI including airway hyperresponsiveness, neutrophilic lung inflammation and altered levels of coagulation factors (increased fibrinogen and PAI-1) and early biomarkers of ALI (IL-18, MMP-9, TGF beta) which was followed by increased deposition of newly produced collagen 14 days later. Histopathology analysis at 5 h revealed epithelial desquamation and that most lesions were healed after 14 days. Conclusions: This study demonstrates that intratracheal instillation can reproduce several early hallmarks of ALI. Our findings therefore support that the intratracheal instillation exposure regimen can be used for new medical countermeasure strategies for NH3-induced ALI.