OBJECTIVE: To investigate whether heparin has a beneficial effect on lipopolysaccharide (LPS)-induced acute lung injury (ALI) in rats, and to explore the possible underlying mechanisms. METHODS: Thirty-two adult Sprague-Dawley (SD) rats were randomly assigned into the control, heparin control, model, and heparin treatment groups, with 8 in each group. ALI rat model was reproduced by intratracheal instillation of LPS at a dose of 1 mg/kg. The rats in the control and heparin control groups received an equal volume of normal saline at the same times. The rats in the heparin control and heparin treatment groups were intravenously received 50 U/kg heparin every 1 hour after the induction of ALI. Animals were sacrificed 24 hours after LPS challenge. Bronchoalveolar lavage fluid (BALF) and lung tissue samples were collected. Histopathological evaluation, lung wet/dry (W/D) ratio, malondialdehyde (MDA), nitric oxide (NO) and myeloperoxidase (MPO) were analyzed. Enzyme-linked immunosorbent assay (ELISA) was used to measure the concentration of inflammatory factor in BALF. Expression of inducible nitric oxide synthase (iNOS) mRNA in the lung of rats was measured by reverse transcription-polymerase chain reaction (RT-PCR). Western Blot was used to determine the expression of transforming growth factor-β1 (TGF-β1) and phosphorylation of Smad in the lung tissues. The expression of iNOS in lung was determined by immunohistochemistry. RESULTS: In the control and heparin control groups, lung tissue showed a normal structure and clear pulmonary alveoli under a light microscope. In the model group, ALI characters such as extensive thickening of the alveolar wall, significant infiltration of inflammatory cells, demolished structure of pulmonary alveoli, and hemorrhage were found. In the heparin treatment group, heparin treatment markedly alleviated LPS-induced these pathological changes in lung. Compared with control and heparin control groups, lung W/D ratio, lung MDA, NO and MPO levels, and tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in BALF in the model group were increased significantly. Compared with the model group, lung W/D ratio, lung MDA, NO and MPO levels, and TNF-α and IL-6 in BALF in the heparin treatment group were significantly decreased [W/D ratio: 7.54 ± 0.17 vs. 10.69 ± 0.15,MDA (mmol/mg): 2.01 ± 0.30 vs. 2.51 ± 0.25, NO (μmol/L): 3.07 ± 0.21 vs. 3.89 ±0.14,MPO (U/g): 1.94 ± 0.09 vs. 2.74 ± 0.20, TNF-α (μg/L): 201.80 ± 0.27 vs. 297.53 ± 0.34,IL-6 (μg/L): 38.41 ± 0.25 vs. 46.31 ± 0.31,all P<0.05]. RT-PCR showed that the expression of iNOS mRNA in the heparin treatment group was significantly lower than that in the model group (2 (-Δ ΔCt): 3.04 ± 0.18 vs. 4.37 ± 0.15, P < 0.05). Western Blot showed that compared with control group, the protein expressions of iNOS and TGF-β1, and phosphorylation of Smad2 and Smad3 were significantly increased, and the heparin could inhibit the protein expressions compared with model group. Immunohistochemistry showed that positive expressions of iNOS in alveolar epithelial cell and capillary endothelial cell in the heparin treatment group were significantly lower than those in the model group. CONCLUSIONS: Heparin significantly ameliorated the lung injury induced by LPS in rats via the inhibition of nitric oxide synthase expression and the TGF-β/Smad pathway.