Inhibition of inducible nitric oxide synthase expression by endothelin in rat glial cells prepared from the neonatal rat brain

In primary cultured rat glial cells, a combination of inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta) stimulates production of nitrite via expression of the inducible form of nitric oxide synthase (iNOS), In these cells, simultaneous addition of endothelin (ET) decreased iNOS expression and nitrite accumulation induced by TNF-alpha/IL-1 beta. The inhibitory effect of ET on TNF-alpha/IL-1 beta-stimulated iNOS expression appears to be mediated by ETB receptors, because (1) both ET-I and ET-3 inhibited the effects of TNF-alpha/IL-1 beta on iNOS expression and nitrite accumulation, (2) a selective ETB receptor agonist, Suc-[Glu(9),Ala(11,15)]-ET-1 (8-21) (IRL1620), decreased the effects of TNF-alpha/IL-1 beta, and (3) a selective ETB receptor antagonist, N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarbonyltryptophanyl-D-norleucine, abolished the inhibitory effects of ETs and IRL1620. Incubation of glial cells with lipopolysaccharide (LPS) caused an increase in iNOS expression. Simultaneous addition of ET-3 decreased the effects of LPS (10 and 100 ng/ml) on iNOS expression. Furthermore, cyclic AMP-elevating agents (dibutyryl cyclic AMP and forskolin) inhibited TNF-alpha/IL-1 beta-induced and LPS-induced iNOS expression and nitrite accumulation. These findings suggest that ETs can decrease TNF-alpha/IL-1 beta-induced and LPS-induced iNOS expression via ETB receptors and that cyclic AMP may be involved in this process.