ENDOTHELIN-1 INHIBITS INDUCTION OF NITRIC-OXIDE SYNTHASE AND GTP CYCLOHYDROLASE-I IN RAT MESANGIAL CELLS

To investigate the interaction between endothelin (ET) and the nitric oxide system, we examined the effects of ET-1 and ET-3 on the inductio n of inducible nitric oxide synthase (iNOS) and guanosine triphosphate cyclohydrolase I (GTP:CHI), the rate-limiting enzyme of de novo synth esis of the cofactor tetrahydrobiopterin (BH4), in rat mesangial cells . ET-1 inhibited the nitrite accumulation induced by a combination of interleukin-1 beta, tumor necrosis factor-alpha, and lipopolysaccharid e in a concentration-dependent manner. The inhibitory effect of ET-3 w as less potent than that of ET-1. A selective ET(A) antagonist, BQ-485 , and an ET(A) and ET(B) antagonist, TAK-044, abolished the inhibitory effects of ET-1, whereas the selective ET(B) antagonist BQ-788 had no effect on the inhibition produced by ET-1. These observations indicat e that ET-1 inhibits cytokine-stimulated nitrite accumulation through the ET(A) receptor. Western blot analysis showed that the suppression of nitrite accumulation was accompanied by a decrease in iNOS protein. Northern blot analysis showed that ET-1 inhibited the expression of b oth iNOS and GTP:CHI mRNA. In conclusion, ET-1 inhibits cytokine-stimu lated nitric oxide production through the ET(A) receptor by suppressin g the expression of iNOS and GTP:CHI mRNA in rat mesangial cells.