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Vol. 282, Issue 3, 1600-1607, 1997
Brain Tumor Research Center and CNS Injury & Edema Research Center,
Dept. of Neurological Surgery, University of California, San Francisco,
California
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
NG-monomethyl-L-arginine. When ascorbate was
administered along with sodium nitroprusside to increase nitric oxide
generation, as little as 5 µM sodium nitroprusside was toxic when
superoxide dismutase was inhibited. Whereas concentrations of 50 to 500 µM sodium nitroprusside and 0.4 mM ascorbate caused ~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.
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