Text Box: ROLE OF ENDOGENOUS CARBON MONOXIDE/HEME OXYGENASE (CO/HO) SYSTEM IN THE DEVELOPMENT OF HYPOXIC PULMONARY HYPERTENSION SHI Yun, DU Junbao, ZENG Chaomei First Hospital, Beijing University, Beijing, China Objective: To study the changing regularity of endogenous CO/HO system under chronic hypoxic condition, and the role of CO/HO on the development of hypoxic pulmonary hypertension. Methods: Forty-two Wistar rats were randomly divided into five groups, hypoxia 0,1,3,7,14 days respectively. Another eighteen Wistar rats were randomly divided into control group, hypoxic group and hypoxic with Znic proporphyrin IX (ZnPP) group. Pulmonary artery mean pressure (mPAP) of each rat was evaluated using right cardiac catheterization. The right-to-left heart weight ratio was determined. The indirect production of CO in plasma and pulmonary tissue homogenates was detected by the double-wavelength spectrophotometer. The expressions and localization of HO-1 were studied by immunohistochemistry. Results: Steady pulmonary hypertension and right ventricular hypertrophy were detected after 7 to 14 days of hypoxia compared with control (P<0.01). The production of CO in the lung tissue homogenates and plasma significantly increased after exposure to hypoxic condition 1 day and 14 days respectively (P<0.01). After 7 and 14 days of hypoxia, HO-1 staining presented in the thickened smooth muscle layer in variety of small pulmonary arteries was increased significantly (P<0.001). Treatment with ZnPP, an inhibitor of HO, markedly enhanced the mPAP in hypoxic rats. Conclusion: The endogenous CO/HO system in pulmonary arteries under chronic hypoxic condition increased in a time-dependent two-peak manner. Endogenous CO/HO serves as an important regulatory system in hypoxic pulmonary hypertension. Role of endogenous carbon monoxide/heme oxygenase(CO/HO) system in the development of hypoxic pulmonary hypertension SHI Yun,DU Junbao,Zeng Chaomei. Pediatric Cardiology Department of Pediatrics,First Hospital,Beijing University, Xi-An Men Street, No.1,West District Beijing China 100034 68314422-3113 62399501 shiyun99@263.net . Shi Yun(石云) 2319