Vascular Endothelial Cells Generate Peroxynitrite in Response to Carbon Monoxide Exposure

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Abstract
Carbon monoxide causes a perivascular oxidative injury in animals, and we tested the hypothesis that endothelial cells could be a source of the injurious oxidants. Studies were undertaken to assess whether exposure to carbon monoxide would cause cultured bovine pulmonary artery endothelial cells to liberate reactive species. Concentrations of carbon monoxide between 11 and 110 nM caused progressively higher concentrations of nitric oxide to be released by endothelial cells based on measurements of nitrite and nitrate. Intracellular production of peroxynitrite was indicated by elevated concentrations of nitrotyrosine, and extracellular liberation of peroxynitrite was indicated by oxidation of p-hydroxyphenylacetic acid and dihydrorhodamine-123. Carbon monoxide did not disturb mitochondrial function based on the rate of oxygen consumption, intracellular production of hydrogen peroxide, and the ability of cells to reduce 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Carbon monoxide also did not alter arginine transport by cells or nitric oxide synthase activity, but it was found to increase steady state levels of nitric oxide by competing for intracellular binding sites. Acute cytotoxicity from carbon monoxide, assessed as radioactive chromium leakage, was due to nitric oxide-derived oxidants. A delayed cell death, whose mechanism is not entirely clear, was also demonstrated by chromium leakage and uptake of vital stain. These findings offer a possible mechanism for adverse health effects caused by carbon monoxide at concentrations ranging from the relatively low levels in polluted environments to levels typically encountered with life-threatening poisoning. Carbon monoxide causes oxidative stress by a novel mechanism involving a competition for intracellular binding sites which increases steady state levels of nitric oxide and allows for generation of peroxynitrite by endothelium.