The molecular mechanisms behind the accumulation of hyaluronan during bleomycin-induced lung injury in rats were investigated. The stimulatory effects of bronchoalveolar lavage fluid (BALF) and alveolar macrophage (AM)-conditioned media on hyaluronan synthesis in normal rat lung fibroblast cultures were studied as well as the hyaluronan binding activity on AM. BALF obtained on days 1 and 5 after bleomycin instillation exhibited hyaluronan stimulatory activity similar to that of 10% fetal serum; the activity returned to control values on day 14 after bleomycin treatment. Conditioned media from cultures of AM obtained from bleomycin-treated rats exhibited stimulatory effects higher than that of media from AM of control rats and equal to or higher than that of 10% fetal calf serum. The stimulatory activity in BALF was significantly inhibited by neutralizing antibodies against transforming growth factor-beta; the activity in AM-conditioned media was only partially affected. Neutralizing antibodies against platelet-derived growth factor-BB or -AA had no such inhibiting effect. Interestingly, AM from bleomycin-treated rats exhibited low hyaluronan binding activity. [3H]Hyaluronan binding by AM on days 1 and 5 after bleomycin administration was about 2-fold and 4-fold lower, respectively, compared with that by AM derived from saline-treated rats. This decrease was normalized 14 days after bleomycin treatment. In conclusion, our results indicate that factors with high potential to stimulate hyaluronan synthesis in rat lung fibroblasts are accumulated in BALF from bleomycin-treated rats and that AM are likely to be one source of such stimulatory factors.(ABSTRACT TRUNCATED AT 250 WORDS)