Pulmonary fibrosis in L-NAME-treated mice is dependent on an activated endothelin system

Activation of the endothelin (ET) system promotes vasoconstriction, inflammation, and fibrosis in various tissues, including the lung. Therefore, ET-1 transgenic mice overexpressing ET-1 develop pulmonary fibrosis in a slow, age-dependent manner. In vivo, NO is the most important counterregulatory mediator of the ET system and decreases ET-1 promoter activity. The aim of our study was to elucidate the impact on pulmonary inflammation and fibrosis of the interaction between NO and the ET system in young ET-1 transgenic mice before the onset of pulmonary fibrosis. Male ET-1 transgenic mice and wild-type littermates at the age of 8 weeks were randomly allocated to the following 6 groups: WT (n = 11), wild-type animals without treatment; WT + L-NAME (n = 14), wild-type animals receiving L-NAME, an inhibitor of NO synthase; WT + L-NAME + LU (n = 13), wild-type animals receiving L-NAME and LU 302872, a dual ETA/ETB-receptor antagonist; ET1tg (n = 10), ET-1 transgenic mice; ET1tg + L-NAME (n = 13); and ET1tg + L-NAME + LU (n = 13). After 6 weeks, animals were euthanized, and hearts and lungs were harvested for histology and immunohistochemistry. No differences in pulmonary inflammation, as indicated by macrophage infiltration, or in interstitial fibrosis were observed between WT and ET1tg mice at baseline; however, inflammation and interstitial fibrosis were significantly enhanced in ET1tg mice, but not in WT groups, after L-NAME treatment. The combined ETA/ETB-receptor antagonist LU 302872 abolished inflammation and interstitial fibrosis in L-NAME-treated ET1tg mice. Perivascular fibrosis and media/lumen ratio of pulmonary bronchi and arteries did not differ between all study groups. In our study L-NAME induced pulmonary fibrosis and inflammation only in young ET1tg mice. Additional treatment with LU 302872 abolished these effects. We thus conclude that an imbalance between an activated ET system and a suppressed NO system contributes to pulmonary inflammation and fibrosis.