Time‐dependent effect of inhaled cigarette smoke exposure in the bleomycin‐induced lung injury rat model

Cigarette smoke (CS) substances are known to induce diverse ailments such as cancer, decreased immunity, and lung diseases. Although some studies have been actively conducted to evaluate cigarette toxicity, the current animal exposure methods, that is, exposure of 28‐ or 90‐days, require considerable research cost and lead to obscure results of the CS effects. In a previous study, we compared the effects of CS in a rat model of bleomycin (BLM) and lipopolysaccharide (LPS) induced lung disease. We determined that compared to the LPS‐induced rat model, the BLM‐induced rat model was more sensitive to alterations in secreting cytokines and total cell number. In the current study, we further confirmed the time‐point of effective inhalation exposure by CS in the BLM‐induced lung injury rat model. Using an automatic video instillator, rats were administered a single dose of 2.5 mg/kg BLM (day 1), and subsequently exposed to CS via inhalation (nose‐only) 4 h/day, for 1, 2, 3, and 4 weeks. The bronchoalveolar lavage fluid (BALF) was obtained from the right lung lobes, total cell numbers were counted, and chemokine and cytokine expressions were evaluated using Enzyme‐Linked Immunosorbent Assay. For the 1‐week exposure, we observed a greater increase of neutrophils in the BLM + CS 300 μg/L group than in the BLM or CS 300 μg/L groups. Exposure of CS in the BLM‐induced lung injury rat model enhanced the secretions of chemokines and cytokines, such as CCL2/MCP‐1, CXCL2/MIP‐2 and TNF‐α, at 1 week. Immunohistochemistry and Hematoxylin and Eosin staining of lungs at 1–2 weeks after exposure clearly confirmed this tendency in the increased levels of CCL2/MCP‐1 and TNF‐α. Taken together, these results indicate that the rat model of BLM‐induced lung injury is more sensitive to CS exposure than other rat models, and may be an appropriate model to evaluate the effect of CS exposure at 1–2 weeks.