Endothelin-1 inhibits the expression of inducible nitric oxide synthase

Because nitric oxide (NO.) and endothelin (ET)-1 frequently have opposing effects on physiological and inflammatory processes, we sought to determine whether ET-1 regulates NO. synthesis by the inducible isoform of NO. synthase (iNOS). L2 cells are a rat lung epithelial cell line that synthesizes ET-1 and in which ET-1 has an autocrine role. In the current study, we demonstrate that L2 cells generate the oxidative products of NO., nitrite and nitrate, after exposure to tumor necrosis factor-alpha, lipopolysaccharide, and interferon-gamma. Exposure to these cytokines also dramatically increases the expression of iNOS mRNA. NG-monomethyl-L-arginine, dexamethasone, and cycloheximide prevent the cytokine-mediated increase in NO. oxidative products, demonstrating that iNOS accounts for their generation. Because L2 cells synthesize ET-1, to test the effect of removing endogenous ET-1, we used phosphoramidon (an ET-converting enzyme inhibitor) or BQ-123 (an ET receptor A antagonist). Removal of endogenous ET-1 with either phosphoramidon or BQ-123 significantly augments cytokine-stimulated NO. synthesis by approximately 20%. To further test the effect of ET-1 on iNOS, we treated cells with phosphoramidon to inhibit endogenous ET-1 synthesis and then administered ET-1 (10(-9) to 10(-7) M). In this setting, ET-1 significantly decreases inducible NO. production by 33% and iNOS mRNA by 50%. We conclude that ET-1 can decrease inducible NO. synthesis by cytokine-stimulated lung epithelial cells.