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 microcirculatory maintenance. We examined the question whether ET-1 may affect NO formation by controlling the expression of the inducible isoform of the NO synthase (iNOS) in cultured rat glomerular mesangial cells (MCs), as induced by the inflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha) plus interleukin-1 beta (IL-1 beta). We found that ET-1 in MCs markedly reduced cytokine-induced NO production (measured as stable NO2-) and inhibited the expression of iNOS mRNA (Northern blot analysis) and of iNOS protein (Western blotting). Inhibition of cytokine-stimulated iNOS mRNA expression by ET-1 was almost complete at the level of gene transcription while post-transcriptional effects were not detected. The ETA receptor antagonist BQ-123 blocked the inhibitory effect of ET1. The ETA agonist sarafotoxin 6b (S6b) inhibited, while the ETB agonist-sarafotoxin 6c (S6c) did not inhibit cytokine-initiated iNOS transcription in MCs. The results demonstrate that ET-1 can strongly inhibit cytokine induction of iNOS and formation of NO in cultured MCs, and that this action is mediated via the ETA receptor. While the precise mechanism(s) and biological relevance of this ET-1 effect are presently unclear, it is conceivable that down-regulation of iNOS by the vasopressor ET-1 may serve in vivo to prevent massive NO build-up and subsequent vasomotor collapse in the glomerular capillary tuft. 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 overproduction of NO.