Significance of thrombin in bleomycin-induced pulmonary fibrosis

To clarify the role of thrombin in fibroblast growth and the development of pulmonary fibrosis in bleomycin-induced interstitial lung disease, we examined the relationship of thrombin activity to fibroblast growth-stimulating activity (FGA) in bronchoalveolar lavage fluid (BALF) from bleomycin-treated rats. Male Wistar rats were given a single intratracheal injection of bleomycin, bronchoalveolar lavage was performed 2, 6 and 15 days later, and the BALF was assayed for thrombin activity and FGA. Higher FGA than the control value was detected in the BALF from rats on Day 6 after bleomycin administration. In bleomycin-treated rats, thrombin activity in the BALF was significantly elevated on Day 2, and maximal on Day 6. The FGA of the BALF from bleomycin-treated rats on Day 6 was significantly decreased by its treatment with various thrombin inhibitors, such as alpha 1-antitrypsin, antithrombin III, hirudin and MD-805. On ammonium sulfate fractionation most of the thrombin activity was recovered in the fraction of 35-50% saturation in which most of the FGA was detected. These results suggest that the FGA of the BALF from bleomycin-treated rats was at least partly due to thrombin, and that thrombin is responsible, at least in part, for fibroblast growth and pulmonary fibrosis in bleomycin-induced interstitial lung disease.