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AIM: To investigate the roles of nitric oxide/L-arginine(NO/L-Arg) pathway and urotensin-Ⅱ(UⅡ) in the development of pulmonary hypertension induced by chronic hypoxia-hypercapnia in rats.METHODS: Forty male Sprague-Dawley rats were randomly divided into four groups(n=10): normal control group(A),hypoxia-hypercapnia+saline group(B),hypoxia-hypercapnia+L-Arg liposome group(C) and hypoxia-hypercapnia+N-nitro-L-arginine methyl ester(L-NAME) group(D).Contents of UⅡ,UⅡ mRNA and receptor of UⅡ(UT) mRNA in pulmonary arterioles were measured with immunohistochemistry analysis and in situ hybridization,respectively.Change of small pulmonary vascular microstructure was also investigated.RESULTS:(1) The mean pulmonary artery pressure(mPAP) and the weight ratio of right ventricle to left ventricle plus septum [RV/(LV+S)] in B and D groups were all higher than those in A group(respectively,P