Effects Of Waterborne Cadmium On Tilapia(Orechromis Niloticus) And Approach To Mechanism Of The Effects

Contamination of fresh water systems by heavy metal cadmium(Cd) has been recognized as a global environmental problem, exposure of fish to even low Cd²⁺ concentrations can have severe toxic effects on growth, development and reproduction. The transfer of Cd²⁺ through food chains also endangers the human health. The effects of pollutants on fish are similar to that of mammalian(including human being) due to the semblable physiological structure. Thus, the deep investigation on the toxic effects of Cd²⁺ on fish is quite important for the realization of sustained development of aquaculture, environmental protection and human health. In current study, the variations of tilapia Oreochromis niloticus cultured worldwide in endocrine, osmo-and ion-regulation and energy metabolism as well as the GSH metabolism have been investigated, the possible mechanisms for the endocrine disruption and the induction of reactive oxygen species(ROS) have been intensively discussed. The main contents are as following:1 The sensitivity of tilapia O. niloticus to waterborne Cd²⁺The acute toxicity of Cd²⁺ to tilapia O. niloticus was studied with the static test method. The intoxication symptom and the LC₅₀(median lethal concentration) value through linear regression based on the recording mortality rate were investigated. The results demonstrated that the fish exposed to the highest Cd²⁺ concentration showed abnormal behavior initially. The main symptoms showed that the swimming ability of fish was affected heavily, the fish hastily swim up and down at the beginning of intoxication, subsequently, fish loss body balance and swam slowly and responses obtuse, with the prolonged exposure period the fish swam spirally and gradually lose the swimming ability and died finally. The intoxication symptom appeared after a longer exposure period in the fish exposed to lower Cd²⁺ concentrations, however, the pathological changes were similar in spite of the different exposure concentrations. The results of dissection showed that the fish were characteristic of dull-redness blood, purple gill covered with much mucous and swollen abdominal cavity filled with yellow jelly materials. The LC₅₀ of 24, 48, 72 and 96h for Cd²⁺ to juvenile tilapia are 27.73mg/L, 16.87mg/L, 12.43mg/L and 9.11mg/L, respectively. The safe concentration of Cd²⁺ to juvenile tilapia is 0.91 mg/L.2 Kinetics of cadmium accumulation, distribution and elimination in tilapiaThe Cd²⁺ accumulation and elimination in the tilapia after exposure to sublethal concentrations(0.05, 0.5 mg l^-1 Cd²⁺) for 60 days followed by a 30-day depuration under a simulated pollution had been investigated. The results showed that Cd²⁺ exposure resulted in an increased Cd uptake in tissues with the exposure periods and concentrations, and the patterns of accumulation varied with the organs. For gill, liver, and kidney Cd²⁺concentrations increased sharply. However, Cd²⁺ level in muscle was increased slowly. After 60 days Cd²⁺ exposure, the order of Cd²⁺ accumulation was in organs kidney>liver>gill>muscle. During the elimination days, the loss of accumulated Cd²⁺ was rapid and immediate in gill whereas the Cd²⁺ concentration increased slightly in kidney and remained constant in liver and muscle. At the end of the cleaning period, the elimination rate in muscle was about 59.37 % at 0.5mg/L Cd²⁺ and 80.44 % at 1.0mg/LCd²⁺. Hence, it is expected that the tilapia contaminated by waterborne Cd can reach the edible standard after elimination in the cleaning water.3 The effects of waterborne Cd²⁺ on the plasma osmolality and ion compositionJuvenile tilapia were exposed to different Cd²⁺ concentrations(0.5, 1.5mg/L) for 20 days. The plasma osmolality and ion composition as well as the regulation system were investigated after the exposure of 5, 10 and 20 days. The results showed that the exposure ot 0.5mg/L Cd²⁺ for 5 days caused to an increase in the microdige of pavement cells(PC) and cortisl(Cor), adrenocorticotrophic hormone(ACTH) concentrations in plasma as well as the adenylate cyclase(AC) activity and cAMP contents in head kidney but no variation in chloride cells(CC) and ATPase. After 10 days of exposure, the microdige of PC becomes irrgeluar and the surface area of CC was elevated without significant changes in the number, ATPase activities werer enhanced and other indicators return to normal level. Subsequently, the amounts were enhanced with a decreas in the denstity of granule in the CCs while the surface area of partial CCs were increased and showed hillock prominence after 20-day exposure, meanwhile, ATPase and AC activities and ACTH, Cor, cAMP concentrations were higher significantly than the control. There was no significant change in the plasma osmolality and ion composition during the whole exposure period. On the other hand, there was an obvious increase in the microridge of the squamous PCs, and the individual was discriminable during the whole exposure period at 1.5mg/LCd²⁺, while the CC amount and ATPase activities showed an increase trend after 5-day exposure without significant difference, however, ACTH, Cor, cAMP, K⁺ concentration as well as AC were elevated significantly, in addition, the osmolality and Ca²⁺ concentration reamined at normoral level whereas Na⁺ and Cl^-concentrations were decreased significantly(p<0.05). After 10 days of exposure, CC amounts increased and the density of granule decrease obviously, ATPase were increased significantly with normal value in other indicators assayed. A similar change to that of 5-day exposure in K⁺, Na⁺ and Cl^-concentrations was detected after 20 days of exposure, meanwhile a significant decrease in CC amount, Ca²⁺-ATPase, osmolality and Ca²⁺ concentration were observed, whereas, a normal Cor and augmented ACTH concentration in plasma were detected. Furthermore, the MDA levels in kidney were kept at the control levels in two treatment groups. The results showed that the impairment of gill, especial the inhibition of ATPase would lead to the disruption of plasma ion composition and osmolality, however, the fish can cope with the stress through enhancing secretion of Cor, which can increase CC amount and surface and ATPase activity and strengthen the regulation ability. But exposure of fish to the higher concentration for a longer time, the regulation ability of plasma osmolality and ion composition. The normal ACTH concentration and AC activity suggest that impairment of ion regulation and osmolality may be ascribe to the disruption of ACTH signal transduction and/or inhibit the synthesis of Cor, which would lead to a decrease in the repair ability. Hence, the disruption of endocrine signal transduction would be one of the possible mechanisms for Cd²⁺toxicity.4 The effects of waterborne Cd²⁺ on the energy metabolism in the gills of tilapia and the possible mechanismThe activities of succinate dehydrogenase(SDH), cytochrome C oxidase(CCO), superoxide dismutase(SOD) and MDA in mitochondria in the gills of tilapia and phosphorfucto kinase(PFK), glutamate oxaloacetate transaminase(GOT), glutamate pyruvate transaminase(GPT), phosphoenolpyruvate carboxykinase(PEPCK), lactic acid(LD), total protein, free amino acid(FAA), ATP, ADP, AMP in gills and plasma glucose as well as FAA in plasma were determined after exposure to different Cd²⁺ concentrations(0mg/L,0.5mg/L and 2.5mg/L for 7 days. The results showed that 5-day exposure of fish to 0.5mg/L Cd²⁺ led to a constant elevation in SOD, SDH and CCO activites while returned to normal level after 7 days of exposure, meantime a transitional increase in plasma glucose on the third and fifth day were detected. Howver, exposure to 0.5mg/L Cd²⁺ had no significant effect on other index. The exposure of fish to 2.5mg/L lead to damage to the mitochondrion in the gill, and some swollen and blank regions and broken membranes in mitochondria were observed after 7 days of exposure. Meanwhile, the mitochondrial scoring was significant higher than the control. The SOD activities was induced significantly in the initial period and rose to the maximum after 3 days of exposure and showed a decreasing trend and the difference became significant after 7-day exposure. PFK, SDH and CCO activities also were enhanced continuously at the beginning and were up to the highest value and elevated by 58.33%, 37.00%and 59.62% than the control, respectively, at one time, FAA contents and PEPCK, GPT, GOT activities were increased significantly. However, all indicators assayed were decreased significantly apart from the return of PEPCK to normal levels and significant elevation in LD contents after 7 days of exposure. Plasma glucose were increased continuously during the exposure period and reached the highest with 0.97mg/L, which was increased by 67.24%(p<0.005), at the end of the experiment. The significant increase in plasma FAA concentration was detected on the 5th day and 7th day, which was increased by 30.91 % and 60.31 % as compared to the control, respectively.The present results demonstrated that tilpia speed up the protein turnover metabolism and carbohydrate aerobics glycolytic to satisfy the energy requirement after exposure of tilapia to lower concentration of Cd²⁺ for a short time. For the short-term exposure of fish to higher concentration of Cd²⁺, fish also enhance the supply of energy through speeding the carbohydrate aerobics glycolytic and decompose partial protein to cope with the stress, but for a long-term exposure there is obstacle in the supply of energy due to the decrease in PFK activity and the impairment of mitochondrion resulting from the inhibition of SOD. Hence, it can be concluded that the obstacle of energy metabolism was one of the possible mechanism for Cd²⁺ toxicity.5 The variation of hepatic GSH metabolism and the induction of ROS after exposure to waterborne Cd²⁺To explore the effects of Cd²⁺ exposure of tilapia(Oreochromis niloticus) on hepatic glutathione(GSH) metabolism, tilapias were exposed to 3.0 mg/l Cd for 1,5,10, 20 and 40 days. The contents of reduced GSH and oxidized GSH(GSSG) and the activities of enzymes involved in GSH metabolism and the GSSG-GSH ratio were investigated. The results showed that reduced GSH were depleted progressively whereas the GSSG-GSH ratio increased. The activities of selenium-dependent glutathione peroxidase, glutathione S-transferase and glucose-6-phosphate dehydrogenase and GSSG levels increased initially and decreased subsequently but still higher than the controls. Glutathione reductase activity dropped on the 40th day. A transient increase inγ-glutamylcysteine synthetase activity was detected on the 20th day. Mitochondrion scoring was elevated significantly only after 40 days of exposure. These results indicated that sub-chronic exposure to waterborne Cd²⁺ affect the hepatic GSH metabolism. During the initial period, the synthesis and turnover were strengthened to deal with the stress while the response ability weakened for a longer exposure. In addition, the significant elevation in mitochondrion scoring after 40 days of exposure while the GSH was depleted suggested the ROS in the initial period was induced via other routines not mitochondrion impairment, but the depletion of GSH accelerate the formation of ROS due to the impairment of mitochondrion.