Study On The Oxidative Stress Of Nitrobenzene On The Grass Carp(Ctenopharyngodon Idellus)

The oxidative stress of nitrobenzene (NB) on the grass carp, Ctenopharyngodon idellus was investigated in this study by the way of intraperitoneal injection. The NB doses were 300,600,1200,2400 and 4800 mg/kg according to the body weight of grass carp, and the times after injection were the 4th,8th and 12th day, respectively. With malondialdehyde(MDA), total antioxidative capacity(T-AOC), glutathione S-transferase (GST), catalase (CAT) and glutathione peroxidase (GPX) as characteristic indexes, the effects of NB on the antioxidant defense system in liver, kidney, heart and brain of grass carp were studied and the toxic mechanisms were discussed. The results were shown as follows.1. The effects of NB on the liver of grass carp at different doses and times after injection were shown as follows:(1) On the 4th day, MDA content increased significantly in the exposure groups of 600,1200,2400 and 4800 mg/kg (p<0.05) except that the group of 300 mg/kg showed no significant change compared to the oil control group. There was a significant positive linear correlation between the MDA content in the liver of grass carp and NB dose in all experiment groups (MDA=0.0008D+0.367, R2=0.784,p<0.05). On the 8th and 12th day, the MDA content in all the experiment groups increased with increasing NB dose, and it showed significant difference in five NB-treated groups compared with that in the oil control group (p<0.05). It was suggested that NB could significantly induce the lipid peroxidation of grass carp at the dose as low as 300 mg/kg with the extending of time. With the increasing of NB dose, the T-AOC in the liver of grass carp at three times after injection increased firstly, the greatest T-AOC appeared in 600 mg/kg group on the 4th day and in 1200 mg/kg group on the 8th and 12th day, and then decreased with further increasing NB dose. The T-AOC in the group of 4800 mg/kg had been lower than that in the control group, and this indicated that the antioxidants in the grass carp began to decline even be impaired. With the increasing of NB dose, the general tendency of GST, CAT and GPX activities in the liver of grass carp was as follows:On the 4th day, the GST and CAT activities were significantly induced in the five NB-treated groups, and the GPX activity was induced in various degrees. On the 8th day, GST and GPX activities in all the NB-treated groups basically decreased with the increasing NB dose, while CAT activity was induced firstly and then had a tendency to decrease. On the 12th day, the activities of GST and GPX were firstly induced and then inhibited, whereas the CAT activity had no significant changes in all the NB-treated groups. (2) In general, the MDA content in the liver of the grass carp increased with the increasing of time and the T-AOC decreased with the extending of time. It indicated that a greater degree of lipid peroxidation had occurred in grass carp with the extended time after injection, whereas the total antioxidative capacity of grass carp had a recession. The NB dose which induced the highest level of the T-AOC showed general tendency of increasing with the extending of time, and it increased from 600 mg/kg on the 4th day to 1200 mg/kg on the 8th and 12th day. It seemed that grass carp probably began to have a certain degree of adaptability and tolerance to nitrobenzene. Generally, with the post-exposure time prolonged, the GST, CAT and GPX activities in the liver of grass carp were initially induced and then inhibited in various degrees in all the NB-treated groups, and this changing trend was similar to the T-AOC. However, the responding sensitivity of the three antioxidative enzymes in the liver of grass carp to the stress of NB was different. CAT showed the most sensitive reaction, which could be induced even in the lowest NB-treated group (300 mg/kg) on the 4th and 8th day, whereas GST and GPX in the liver of grass carp showed a high sensitivity at the higher dose groups (1200,2400 and 4800 mg/kg).2. The MDA content and T-AOC level in the kidney of grass carp had a tendency to decrease with the NB dose increasing on the 4th day, which suggested no more oxidation damage occurred in it. However, the MDA content and T-AOC level had a tendency to increase with the NB increasing on the 8th and 12th day, which showed a greater degree of oxidation had occurred in the kidney of grass carp because the increased T-AOC failed to prevent and remove excessive generation and accumulation of reactive oxygen species, which would lead to more oxidative damage of grass carp.3. The MDA content in the heart of grass carp had a tendency to decrease with the NB dose increasing on the 4th and 8th day, while the T-AOC level was not modified significantly after NB exposure in heart. On the 12th day, the MDA content and T-AOC level showed an increasing trend but not significant, which showed the heart of grass carp was significantly unaffected in the 12 days of treatment.4. The MDA content and T-AOC level in the brain of grass carp were not modified significantly with the NB dose increasing all the time, which showed the responding sensitivity of the brain to the NB was so little that the oxidative damage occurred almost impossibly in the brain of the grass carp.These results showed that the exposure of nitrobenzene disrupted the dynamic equilibrium between the natural oxidation and antioxidant defense system of grass carp. The total antioxidative capacity including GST, CAT, GPX and so on showed various degrees of abnormal fluctuation, which indicated that a greater degree of lipid peroxidation and various degrees of oxidative damage had occurred in grass carp. Moreover, the tissue specialty of the oxidative damage showed that the liver of grass carp occurred the worst oxidative damage, and the kidney occurred some oxidative damage, while the heart and the brain occurred little oxidative damage. This study provided some basic data for the further study including oxidative damage of nitrobenzene compounds on fish, the responsiveness of fish to the nitrobenzene compounds and the biological monitoring of organic pollutants in water environment.