Study On The Oxidative Stress Of The Grass Carp(Ctenopharyngodon Idellus)after Exposure To The Water-borne Cadmium(Cd)

The oxidative stress of the grass carp (Ctenopharyngodon idellus) after exposure to the water-borne cadmium (Cd) was investigated in artificial soft water (28CaCO3mg/L) at (27.5±0.5)℃. Two series of experiments were conducted under the laboratory condition. The first experiment is about acute toxicity experiment of Cd exposure. According to the results of preliminary experiment, the Cd exposure concentration in acute toxicity experiment was carried out at0,1.91,2.45,3.14,4.03,4.03and5.17mg/L. The median lethal concentration in96hours (96h LC50) and safe concentration (SC) of grass carp were determined. The second experiment is subacute toxicity experiment of Cd exposure. Based on the result of96h hours safe concentration (SC), the Cd exposure concentrations in subacute toxicity experiment was designed as0,13.27,39.80,19.40,358.20and1074.60μg/L. And then, the effects of different water-borne Cd concentrations on oxidative stress and antioxidant defense system of grass carp were studied. The main results were shown as follows. The results in the first experiment were shown as follows:The mortality of grass carp increased with increasing Cd concentration in the96-hour acute toxicity experiment. Through the method of linear regression analysis of SPSS, the median lethal concentration (3.98mg/L) and the safe concentration (0.0398mg/L) of Cd to grass carp were obtained. The results in the second experiment were shown as follows:1. The effects of Cd exposure on the liver of the grass carp①In the4-day and8-day experiments, the content of metallothionein (MT) in the liver of grass carp increased with increasing Cd exposure concentration showed a downward trend after the first rise, the highest MT appeared in119.40μg/L group, other groups were significantly higher than control, except for13.27μg/L group,(p<0.05).②In the4-day and8-day experiments，there was a significantly positive linear correlation between the malondialdehyde (MDA) content in the liver of grass carp and Cd exposure concentration in each group. The content of MDA were increased significantly in each groups except that the group of13.27μg/Lshowed no significant change compared to control，(p<0.05).③In the4-day and8-day experiments, the total antioxidant capacity (T-AOC) in each group increased with increasing Cd exposure concentrationshowed a downward trend after the first rise, the highest T-AOC appeared in119.40μg/L group, but on the8th day the group of1074.60μg/L were significantly lower than control group(p<0.05). In the4d and8d’s experiment,the CAT activity in the liver of grass carp is similar to the level of T-AOC, but the CAT activity in the group of13.27μg/L were significantly higher than control group(p<0.05).2. The effects of Cd exposure on the kidney and the gill of the grass carpIn the4-day and8-day experiments, the contents of MDA, T-AOC level and CAT activity in the kidney and the gill of the grass carp were similar to that of liver with the variation trend of Cd exposure concentration. In the4d and8d’s experiment, the contents of MDA of the gill in the13.27μg/L group were significantly higher than control group. On the8th day the content MDA of the kidney in the13.27μg/L group was significantly higher than in the control group (p<0.05). In the same concentration and at the same time Cd exposure kidney T-AOC level and CAT activity were lower than that of liver totally. However, the T-AOC level and CAT activity in gill were lower than that in liver and kidney.3. The effects of Cd exposure on the brain of the grass carp①In the4-day and8-day experiments, the activity of acetylcholin esterase (AChE) in brain of the grass carp had a downward tendency with the Cd concentration increasing, AChE activity in the1074.60μg/L group were significantly lower than control (p<0.05), and the inhibition ratio was75.64%. ②In the4-day and8-day experiments, The T-AOC level and CAT activity in the brain of grass carp showed a first increased and then decreased with Cd concentration increasing, and then their activity in the1074.60μg/L group were not significantly than in the control (p>0.05).The conclusions suggested in this study were shown as follows:1. Compared with fish toxicity classification standard and the majority of the fish in the reports and references, Cd in the water-borne belongs to the category of high toxic heavy metal to the96h LC50of the grass carp. This showed Cd has a high poisonousness to the grass carp.2. Water-brone Cd concentration has a significant linear positive correlation with Oxidative damage degree of the grass carp. It is suggested that content determination of MDA of the grass carp is considered as a biological indicator which is used as a biological monitoring for metal of water pollution.3. In the Cd exposure experimen, the MT content, the T-AOC and CAT of the grass carp appeared different degree of volatility. Under stress of the low concentration, organism showed hormesis by increasing the ability of antioxidant to fight against the poison caused by Cd exposure. But, the ability of increasing antioxidant is limited, and with stress of high concentration especially over time, hormesis was weakening and even showed inhibiting effection. So dynamic equilibrium of defense system of oxidation and antioxidation was destroyed and grass carp would suffer from more oxidative damage.4. The active reaction of AChE in brain of grass carp is very sensitive to exposure to Cd, it may be used as a biochemical criterion to show the degree of heavy metal poisoning indirectly.5. Under the same concentrationand the same time of Cd exposure, the order of the degree of oxidative damage ingrass carp is:gill>liver>kidney, and then the ability of antioxidation is:liver>kidney>gill. Oxidative damage and ability of antioxidation of grass carp have tissue specificity, which may be relative to the morphological structure of organs and physiological function.