Comparison of pulmonary and pleural responses of rats and hamsters to inhaled refractory ceramic fibers

The present study was designed to determine whether pleural fiber burdens o r subchronic pleural fibroproliferative and inflammatory changes can help e xplain the marked interspecies differences in pleural fibrosis and mesothel ioma that are observed following long-term inhalation of RCF-1 ceramic fibe rs by rats and hamsters. Fischer 344 rats and Syrian golden hamsters were e xposed to RCF-1 for 4 h per day, 5 days per week, for 12 consecutive weeks. Lung and pleural fiber burdens were characterized during and after exposur e, For all time points, approximately 67% of fibers associated with lung ti ssues from both rats and hamsters were longer than 5 mu m in length. In com parison, fibers longer than 5 mu m recovered from the pleural compartment, following a 12-week exposure and 12 weeks of recovery, accounted for 13% (h amsters) and 4% (rats) of the distribution. In the 12 weeks after the cessa tion of exposure, the number of fibers longer than 5 mu m in length remaine d constant in the hamster at approximately 150 fibers per cm(2) pleura, Thi s was 2 to 3 times the corresponding fiber surface density in the rat. Sign ificant pulmonary and pleural inflammation was detected at all time points and for both species. DNA synthesis by pleural mesothelial cells was quanti fied by bromodeoxyuridine uptake following 3 days of labeling. Labeling ind ices were higher in hamsters than in rats, both for RCF-1-exposed and filte red air-control animals and was highest for the parietal surface of the ple ura. Significantly greater collagen deposition was measured in the visceral pleura of hamsters 12 weeks post-exposure but was not significantly elevat ed in rats, These findings demonstrate that subchronic inhalation exposure to RCF-1 induces pleural inflammation, mesothelial-cell turnover, pleural f ibrosis, and an accumulation of fibers with a length greater than 5 mu m in the hamster. The accumulation of long fibers in the pleural space may cont ribute to the pathology observed in the hamster following chronic inhalatio n of RCF-1, whereas the presence of short, thin fibers may play a role in t he acute-phase biological response seen in both species.