ENDOTHELIN-1 INHIBITS THE EXPRESSION OF INDUCIBLE NITRIC-OXIDE SYNTHASE

Because nitric oxide (NO .) and endothelin (ET)-1 frequently have oppo sing effects on physiological and inflammatory processes, we sought to determine whether ET-1 regulates NO . synthesis by the inducible isof orm of NO . synthase (iNOS). L2 cells are a rat lung epithelial cell l ine 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 necrosi s factor-alpha, Lipopolysaccharide, and interferon-gamma. Exposure to these cytokines also dramatically increases the expression of iNOS mRN A. N-G-monomethyl-L-arginine, dexamethasone, and cycloheximide prevent the cytokine-mediated increase in NO . oxidative products, demonstrat ing that iNOS accounts for their generation. Because L2 cells synthesi ze ET-1, to test the effect of removing endogenous ET-1, we used phosp horamidon (an ET-converting enzyme inhibitor) or BQ-123 (an ET recepto r A antagonist). Removal of endogenous ET-1 with either phosphoramidon or BQ-123 significantly augments cytokine-stimulated NO . synthesis b y similar to 20%. To further test the effect of ET-1 on iNOS, we treat ed 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 sig nificantly decreases inducible NO . production by 33% and iNOS mRNA by 50%. We conclude that ET-1 can decrease inducible NO . synthesis by c ytokine-stimulated lung epithelial cells.