Background: About eighteen percent of cirrhotic patients come along with decreased systemic arterial oxygenation and expansion of pulmonary venous plexus which triggered by nitric oxide. The level of nitrate and iNOS significantly increase in the cirrhotic patients. However, the localization of nNOS and iNOS in the lung tissue has not yet been clarified.
Objective: The present study, therefore, aimed to demonstrate the sites of expansion of pulmonary blood vessels and to localize nNOS and iNOS in the lung tissue of cirrhotic rat models induced by thioacetamide (TAA).
Material and Method: The rats were divided into 5 groups. The first group was the control. The other four groups were treated with 200 mg/kg body weight of TAA 3 times per week for 1, 2, 3, or 4 month(s), respectively. At the end of each month rats in each treated group were sacrificed. Lung histology and pulmonary NOS expression was studied by light microscope and immunohistochemical technique, respectively.
Results: It was found that diameter of blood vessels were highest increased in the right lower lobe of the 4-months TAA-treated group. In addition, iNOS and nNOS expression was localized at epithelium of respiratory tract, endothelium of pulmonary vessel and macrophage at this age.
Conclusion: The present study demonstrated that the pulmonary blood vessels at the right lower lobe with cirrhotic background got enormous dilatation. iNOS and nNOS were immunostained at epithelium of respiratory tract, pulmonary endothelium and macrophages. Our observations suggested that enhanced NOS expression is important in the development of systemic hyperdynamic circulatory abnormalities in cirrhosis. As appearance of vasodilatation at right lower lobe of lung, it could, therefore, be evidence confirming that there was a real connection between inferior pulmonary vein and azygos vein at the embryonic period but obliterated later.

Keywords: Nitric oxide synthase, Hepatopulmonary syndrome, Cirrhosis, Thioacetamide