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The aim of this study was to evaluate the effects of niacin in diet on the expression of nitric oxide synthase (NOS) in rat lungs of the animal model of silicosis established by direct tracheal instillation of silica particles into rat lungs surgically. The niacin concen- tration in serum was analyzed by high performance liquid chromatography (HPLC). The expression of inducible nitric oxide synthase (iN- OS) protein in paraffin-embedded lung sections was determined by streptavidin/peroxidase (SP) staining. Quantitative analysis by Image- Pro Plus was also performed on the expression of iNOS. The results showed that niacin concentration in serum of the niacin-treated rats was significantly higher than that in the control and silica-treated rats. After 7 days of silica instillation, iNOS integrated optical density (IOD) in rat lungs and total NOS and iNOS activities in bronchoalveolar lavage fluid (BALF) in silica-treated rats rose by 273420.75, 2.61 units! mL and 1.89 units/mL respectively, when compared with those in the control rats. Niacin treatment significantly reduced silica-induced iNOS IOD in rat lung tissues and total NOS and iNOS activities in BALF supernatant by 248292.35, 1.50 units/ml and 0.91 units nil, respectively, as compared with those in silica-treated rats. Therefore, niacin can effectively attenuate the pathological expression of NOS in rat lung tissues induced by silica particles. [ABSTRACT FROM AUTHOR]