ENDOTHELIN-1 INHIBITS CYTOKINE-STIMULATED TRANSCRIPTION OF INDUCIBLE NITRIC-OXIDE SYNTHASE IN GLOMERULAR MESANGIAL CELLS

Endothelin-1 (ET-1) is a potent vasoconstrictor while nitric oxide (NO ) has strong vasodilatory effects. Recent studies have indicated that vasoconstrictors and NO may mutually modulate their production and/or activity, thus regulating each other in the context of microcirculator y maintenance. We examined the question whether ET-1 may affect NO for mation by controlling the expression of the inducible isoform of the N O synthase (iNOS) in cultured rat glomerular mesangial cells (MCs), as induced by the inflammatory cytokines, tumor necrosis factor-alpha (T NF-alpha) plus interleukin-1 beta (IL-1 beta). We found that ET-1 in M Cs markedly reduced cytokine-induced NO production (measured as stable NO2-) and inhibited the expression of iNOS mRNA (Northern blot analys is) acid of iNOS protein (Western blotting). Inhibition of cytokine-st imulated iNOS mRNA expression by ET-1 was almost complete at the level of gene transcription while post-transcriptional effects were not det ected. The ET(A) receptor antagonist BQ-123 blocked the inhibitory eff ect of ET-1. The ET(A) agonist sarafotoxin 6b (S6b) inhibited, while t he ET(B) agonist sarafotoxin 6c (S6c) did not inhibit cytokine-initiat ed iNOS transcription in MCs. The results demonstrate that ET-1 can st rongly inhibit cytokine induction of iNOS and formation of NO in cultu red MCs, and that this action is mediated via the ET(A) receptor. Whil e the precise mechanism(s) and biological relevance of this ET-1 effec t are presently unclear, it is conceivable that down-regulation of iNO S by the vasopressor ET-1 may serve in vivo to prevent massive NO buil d-up and subsequent vasomotor collapse in the glomerular capillary tuf t. This could help to maintain glomerular ultrafiltration in states of endotoxin excess as well as during glomerular formation and action of TNF-alpha and IL-1 beta causing iNOS induction and subsequent overpro duction of NO.