Abstract
When rats were exposed to 10 ppm NO 2 for 7 days, the succinate-cytochrome c reductase activity, the rate-limiting step of mitochondrial succinoxidase, of the liver decreased progressively, reaching 77% ( P < 0.01) of the control level by the fifth day. By contrast, this activity was gradually increased by a 10-day exposure to 4 ppm NO 2 and reached 1.14-fold that of the control level at the seventh day. A reduction in the components of electron-transport systems in liver microsomes was found during exposure to NO 2 at both concentrations. This reduction was preferential for components of drug-metabolizing systems. The NADPH-cytochrome c reductase activity and the cytochrome P-450 content were decreased to 82% ( P < 0.05) and 76% ( P < 0.05), respectively, at the fifth day of exposure to 10 ppm NO 2. Exposure to 4 ppm NO 2 also caused a significant decrease in the NADPH-cytochrome c reductase and the cytochrome P-450 content, which were lowered to 84% ( P < 0.05) of the control level at the fourth and seventh days, respectively. Reduction in these components appears to occur even at the concentration of 0.4 ppm at the seventh day. Alterations in the components of mitochrondria and microsomes of the lung were considerably different from those of the liver. The protein content of the lung increased to 1.18 ( P < 0.05)- and 1.14 ( P < 0.05)-fold that of the control during exposure to 10 and 4 ppm NO 2, respectively. The succinate-cytochrome c reductase activity showed a reduced value, which was 73% ( P < 0.01) of the control level, 1 day after exposure to 10 ppm NO 2. Subsequently, the activity increased to 1.14-fold that of the control during exposure to 10 (at the fifth day) and 4 ppm (at the fourth day) NO 2. Components of microsomal electron-transport systems also showed a slightly elevated value during exposure to 4 ppm NO 2. Seven days after exposure, the cytochrome P-450 content was decreased to 61% ( P < 0.01) of the control level, while other components were retained at control levels. A significant reduction in the cytochrome P-450 content was also observed at the exposure concentration of 1.2 ppm, but not of 0.4 ppm, at the seventh day.
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