OBJECTIVETo evaluate the time-effect of silica on the expression of lung tissue nitric oxide synthase (NOS) in early inflammatory damage stage of silicotic rat.

METHODSAnimal models were established by direct tracheal instillation of silica into rat lungs. Total NOS and induced NOS (iNOS) activities in bronchoalveolar lavage fluid (BALF) were assayed. The expression of iNOS protein in paraffin-embedded lung sections with Streptavidin/peroxidase (SP) immunohistochemistry were measured by tissue microarray and Image-Pro Plus.

RESULTSMost of the expression of iNOS was in the cytoplasm of macrophages and neutrophils. iNOS integrated optical density (IOD) of lung tissue increased 1.47 x 10(5) and 2.73 x 10(5) more respectively in silicatreated rats 3, 7 days after exposure than in control rats (P < 0.05), and decreased 1.11 x 10(5) more 28 days after exposure (P < 0.01). The activities of iNOS in BALF increased by 0.86, 1.89 and 0.92 U/ml respectively 3, 7, 14 days after exposure (P < 0.05 or P < 0.01). The activities of total NOS in BALF increased by 1.43, 2.05, 2.61 and 2.19 U/ml respectively 1, 3, 7, 14 days after exposure (P < 0.05 or P < 0.01).

CONCLUSIONAfter silica instillation, the iNOS-positive cells in rat lung tissue were mostly macrophages and neutrophils. There is a parabolic changing trend in the level of expression of lung iNOS during 1 - 28 day exposure to silica.