Objective To evaluate the role of c-AMP-protein kinase A (cAMP-PKA) on the up-regulation of heme oxygenase-1 (HO-1) expression during lipopolysaccharide (LPS)-induced acute lung injury (ALI) in rats.Methods Forty-eight healthy male Sprague-Dawley rats,weighing 180-220 g,aged 2.5-3.0 months,were randomly divided into 4 groups (n =12 each)∶ normal control group (group C),ALI group (group ALI),H89 +ALI group (group H + ALI) and H89 group (group H).In group C,normal saline (solvent for LPS) 0.5 ml was injected via the femoral vein and normal saline (solvent for H89) 0.5 ml was injected subcutaneously 2 h later.In group ALI,10 mg/kg LPS 0.5 ml was injected via the femoral vein and normal saline 0.5 ml was injected subcutaneously 2 h later.In group H +ALI,10 mg/kg LPS 0.5 ml was injected via the femoral vein and 5 mg/kg H89 0.5ml was injected subcutaneously 2 h later.In group H,normal saline 0.5 ml was injected via the femoral vein and 5 mg/kg H89 0.5 ml was injected subcutaneously 2 h later.The rats were then sacrificed at 6 h after iv injection of LPS and the lungs were removed for microscopic examination and lung water content.The pathological changes of the lung were scored.The expression of HO-1 and PKA (by Western blot) and HO-1 mRNA (by RT-PCR) was detected.Results Compared with group C,the pathological score and lung water content were significantly increased,and the expression of HO-1,PKA and HO-1 mRNA was up-regulated in groups ALI and H +ALI (P ＜0.05),and no significant change was found in the parameters mentioned above in group H (P ＞ 0.05).The pathological score and lung water content were significantly higher,and the expression of HO-1,AP-1 and HO-1 mRNA was significantly lower in group H + ALI than in group ALI (P ＜ 0.05).Conclusion Activation of signaling pathway c-AMP-PKA is involved in the up-regulation of HO-1 expression during LPS-induced ALI in rats.