Nitric oxide- and superoxide-mediated toxicity in cerebral endothelial cells

Nitric oxide and superoxide are free radicals that appear to contribute to the pathogenesis of a number of brain disorders, and cerebral endothelial cells are a potential target of these agents. Because of the capacity for these two agents to combine, it has been suggested that nitric oxide might either enhance or inhibit the toxic effects of superoxide. To establish the effect of the generation of superoxide and nitric oxide alone and in combination, cerebral endothelial cells were exposed to sodium nitroprusside, a source of nitric oxide, and/or paraquat, a source of superoxide. Paraquat enhanced the toxicity of sodium nitroprusside, as did diethyldithiocarbamate, an inhibitor of superoxide dismutase, which supports the hypothesis that enhanced levels of superoxide can combine with nitric oxide to form a more toxic product. Also, the toxicity of paraquat could be partially inhibited by blocking endogenous nitric oxide synthesis using N(G)-monomethyl-L-arginine. When ascorbate was administered along with sodium nitroprusside to increase nitric oxide generation, as little as 5 microM sodium nitroprusside was toxic when superoxide dismutase was inhibited. Whereas concentrations of 50 to 500 microM sodium nitroprusside and 0.4 mM ascorbate caused approximately 100% toxicity, there was no measurable toxicity when these doses were accompanied by 2 mM glutathione or 50 U/ml of catalase; this suggests that peroxides may also contribute to nitric oxide toxicity. These results suggest that the simultaneous generation of nitric oxide and superoxide is synergistic, resulting in enhanced toxicity.