REGULATION OF NITRIC-OXIDE PRODUCTION BY RAT ALVEOLAR MACROPHAGES IN RESPONSE TO SILICA EXPOSURE

In the present study, it was confirmed that in vivo exposure of rats t o silica significantly increases nitric oxide (NO) production by bronc hoalveolar lavage cells (BALC), a population of cells that includes al veolar macrophages. Possible mechanisms whereby NO production could be upregulated by rat alveolar macrophages following silica exposure wer e examined to determine ii there is a direct effect of silica on alveo lar macrophage NO production or if other factors are involved. BALC we re obtained from normal male rats and cultured for 2 h. Nonadherent ce lls were then removed and the enriched alveolar macrophage cell popula tions were exposed to test agents for 18-20 h. Media nitrate and nitri te (NOx) concentrations were used to assess NO production, and, in som e cases, inducible NO synthase mRNA levels were indexed. In vitro expo sure to silica (0.1-100 mu g/ml) had no significant effect on basal NO levels. Furthermore, NO generation was not additionally increased abo ve levels induced by interferon gamma (IFN), lipopolysaccharide (LPS), or other cytokines during simultaneous incubations with silica and IF N, a 2-h pretreatment with silica followed by IFN, or preincubation wi th IFN, LPS, and/or other cytokines before the addition of silica. To evaluate whether cell-cell interactions might be required for the indu ction of NO production during silica challenge, alveolar macrophages w ere cultured with splenic lymphocytes or blood-derived polymorphonucle ar leukocytes. Coculture of splenic lymphocytes with alveolar macropha ges resulted in media NOx levels that were greater than the additive l evels from each cell type. However, the presence of silica was without additional effect on NO production by either of these cell types. Fur thermore, it was found that conditioned media, derived from adherent B ALC following silica treatment in vivo, could induce NO production by naive alveolar macrophages. In summary, the collective results from th ese experiments suggest that cell-cell communication factors, involvin g the interaction of pneumocytes following in vivo silica exposure, ar e necessary for the induction of NO by alveolar macrophages.