Characterization of excessive collagen production during development of pulmonary fibrosis induced by chronic silica inhalation in rats.

The activation of collagen synthesis during development of silicotic fibrosis was studied in rats exposed, in dusting chambers, to respirable SiO2 for periods of 2, 4, 6 or 12 months. Control animals were exposed similarly to clean air or TiO2. Development of fibrosis was followed by histological examination, measurement of lung weight and determination of lung collagen content (as hydroxyproline). A steady increase in lung weight and collagen content together with changes in cellularity and metabolic activity of the lungs, as ascertained by chemical determination of DNA and RNA, were measured in the lungs of the SiO2-exposed animals. Hybridization of total lung RNA, extracted at each time point, with cDNA probes specific for type I and type III procollagen mRNA levels showed that the development of fibrosis was associated with increased levels, as compared to age matched controls, of pulmonary procollagen mRNAs. Interestingly, the highest levels of procollagen mRNAs were observed in young (pretreatment control) animals, suggesting that during pulmonary development collagen metabolism in lungs is even greater than during development of fibrosis. In rats exposed to SiO2 the increase in type III procollagen mRNA occurred earlier than the increase in type I procollagen mRNAs. These observations demonstrate both age-dependent and silicosis-related changes in pulmonary procollagen mRNA levels. The results suggest that development of silicosis is associated with an altered capacity of the lungs to regulate collagen accumulation.