Objective To investigate the protective effects of imipramine on alveolar epithelial barrier functiou in mice against LPS-induced acute lung injury (ALI),and explore the possible mechanisms.Methods Total of 32 SPF male Balb/c mice were randomly (random number) divided into four groups:control group,Imipramine group,LPS group,LPS + Imipramine group.To establish an animal model of ALI,mice were administered intraperitoneally with LPS in 20 mg/kg.Mice were treated with imipramine in 25 mg/kg 30 min prior to LPS administration.FITC-FD4 was administered in mice via the tail vein with FITC-FD4 10 min before mice sacrificed under anesthesia at 12 hours after LPS administration,then bronchoalveolar lavage fluid (BALF) and lung tissue were obtained.HE staining was used to observe histopathological changes,and pathology scores;lung tissue wet-to-dry weight ratio and BALF/serum FD4 ratio were used to assess pulmonary edema and alveolar epithelial permeability.Real-time PCR,western blot and immunochemistry were employed to detect the mRNA expressions and protein levels of Occludin,Claudin-4 and ZO-1.Data were analyzed with SPSS 16.0 software,one way analysis of variance (ANOVA) was used to compare multiple sets of variables,the intergroup comparisons were analyzed by the least-significant-difference (LSD) tests with P ＜ 0.05 for the statistically significant difference.Results Compared with LPS group,LPS + lmipramine group had a statistically significant decrease in pathological score [(9.22 ± 0.21) vs.(11.23 ± O.55),P ＜ O.05);the wet-to-dry weight ratio in LPS + Imipramine group was less than that in LPS group and the difference was statistically significant (P ＜ 0.05);compared with LPS group,the ratio of BALF/serum FD4 in LPS + Imipramine was less and the difference was statistically significant (P ＜ 0.05);compared with LPS group,the mRNA expressions and protein levels of Occludin,Claudin-4 and ZO-1 in LPS + Imipramine group were significantly increased (mRNA:Occludin:P ＜ 0.05;Claudin-4:P ＜ 0.05;ZO-1:P ＜ 0.05 . western blot:Occludin:P ＜ 0.05;Claudin-4:P ＜ 0.05;ZO-1:P ＜ 0.05).Immunochemistry showed that Occludin and Claudin-4 were present mainly in alveolar epithelial cell membrane,Z0-1 was found mainly in cytoplasm of alveolar epithelial cell.In control group and Imipramine group,tight junction proteins were obviously expressed.Compared with control group,protein levels in LPS group were significantly decreased (Occludin:P ＜ 0.05;Claudin-4:P ＜ 0.05;ZO-1:t =6.59,P ＜ 0.05);compared with LPS group,the tight junction proteins in LPS + Imipramine group were significantly increased (Occludin:P ＜ 0.05;Claudin-4:P ＜ 0.05;ZO-1:P ＜ 0.05).Conclusion The protective effects of imipramine on alveolar epithelial barrier function by up-regulating tight junction proteins expression in murine LPS-induced ALI.