Study On The Relationship Between Cadmium Accumulation And Toxic Effects In The Juvenile Southern Catfish(Silurus Meridionalis Chen)

Cadmium（Cd）is one of the most toxic metals to fish and other aquatic organisms.Due to the development of smelting,the excessive combustion of minerals,the extensive use of pesticides and accidental discharge of mine wastes and natural weathering of rocks,the aquatic ecosystem continues polluted by Cd.Cd pollution has negative impacts on the fitness and survival of fish and other aquatic organisms.Any toxicity manifested by aquatic organisms,at the first level,results from accumulation of metals.However,the relationship between the lethal toxicity of Cd to fish and its accumulation in fish remains controversial.Based on reviewing literatures,we found that the previous studies suffered from some shortcomings as they:1)did not give the exact threshold value of Cd toxicity in tissues,2)did not differentiate the detoxified and metabolically available Cd,3)did not analyze the Cd content of toxic action sites in target organs,4)did not know whether the lethal toxicity of Cd be related to Cd content at the subcellular level.In order to reveal the relationship between Cd accumulation and lethal effect in organ,tissue and subcellular level,a series of experiments were conducted on the juvenile southern catfish（Silurus meridionalis Chen）（body weight:50.0±0.2 g）,an important commercial fish in China.Two series of toxicological effects of acute and subacute exposure to fish in water carried out in artificial soft water with a hardness of 25.0±0.2 mg Ca CO₃/L at 27.5±0.1℃.In the acute exposure experiment,fish exposed to Cd concentrations of 0,3,4,5,6,7,8,9,10,11,12,13,and 14mg/L for 96 hours.Survival status were checked hourily during the exposure and the dead fish was taken out immediately to minimize the influences on the amounts of Cd²⁺,Ca²⁺,and Na⁺in the dead fish.To explore mechanisms in response to the acute lethal Cd toxicity,the content of Cd in the tissues of dead and surviving fish,as well as the carcass Ca²⁺and Na⁺content in of dead and surviving fish was measured.The quantitative relationship between Cd exposure concentration and Cd content in tissues was anaylized.The content of Cd,Ca²⁺and Na⁺in tissues were compared between dead and surviving fish.In the subacute experiment,fish exposed to five nominal Cd treatments at concentrations of 0,100,200,400,and 800μg/L.During the exposure period,fish fed with a ration level of 2%body weight,and the food intake and mortality of the fish were recorded.On each sampling date,the resting metabolic rate,growth performance,the content of Cd,Ca²⁺,Na⁺,K⁺and Mg²⁺in gills,liver,kidneys and the carcass were measured.The content of Cd in gills,liver and kidneys at the subcellular level,as well as the metabolic function of liver mitochondria were measured.The accumulation and distribution of Cd at the tissues and subcellular level of surviving and dead fish were compared,as well as the differences of Ca²⁺,Na⁺,K⁺and Mg²⁺content,to investigate the lethal mechanism caused by subacute Cd exposure at the tissues and subcellular level.The main results obtained in this study were as follows:Ⅰ.The results of Cd acute exposure to fish listed as follows.1.The 96 hours median lethal concentration(96h LC₅₀)was 6.42 mg/L.2.There was a positive linear correlation between the Cd concentrations in the gills,kidneys,heart,and carcass of dead fish and the Cd exposure concentration of the whole fish（P<0.05）.The Cd concentrations in all tissues and in surviving whole fish positively and linearly correlated with Cd exposure concentration（P<0.05）.The slope of the content of Cd in the gills,kidneys,heart,carcass,and whole body of dead fish with Cd exposure concentrations was significantly higher than that for the surviving fish（P<0.001）.3.The liver Cd content of dead southern catfish firstly increased with the increasing Cd exposure concentration and then tended to be stable,staying within a small range of change.4.The Cd accumulation rates in the gills,liver,kidneys,heart,carcass,and the whole body of dead and surviving fish linearly and positively correlated with the Cd exposure concentrations（P<0.05）.The slope of the Cd accumulation rate in different tissues and the whole body of dead fish with the Cd exposure concentration was significantly higher than that for the surviving fish（P<0.001）.5.There was a clear threshold between the Cd concentration of gills,kidney,carcass and whole fish between the dead and surviving fish.6.Although the content of Na⁺of all the surviving fish after Cd exposure was significantly higher than that of all the dead fish,the Na⁺content was significantly lower than that of the control.7.The Ca²⁺content of all the dead fish after Cd exposure was significantly higher than that of all the surviving fish,but significantly lower than the carcass Ca²⁺content of all controls.Ⅱ.The results of Cd subacute exposure to fish listed as follows.1.The resting metabolic rate increased at the concentration of 100μg/L all the time.However,he resting metabolic rate of the experimental fish significantly decreased,then increased,and subsequently decreased at concentrations of 200,400,and 800μg/L with duration.2.The specific weight growth rates（SGR）of Cd-treated fish were significantly lower than that of the control group（P<0.05）.After exposure to Cd（100,200,and 400μg/L）at day 21,the condition factors of the Cd-exposed groups were significantly lower than that of the control group（P<0.05）,while the condition factors were significantly decreased after 7days of exposure to 800μg/L.The hepatic index of experimental fish in all exposure groups was significantly lower than that of the control group（P<0.05）.3.The inhibition of mitochondrial state III respiratory rate and the respiratory control rates（RCR）,as well as the cytochrome c oxidase（CCO）activity in the liver of Cd-exposed fish was affected by exposure concentration(F_4,139=91.17,P<0.001)and exposure time(F_5,139=14.23,P<0.001).4.The Cd content in gills,liver,kidneys and carcass of Cd-exposed fish increased with the increasing concentration of Cd and exposure time.During the 14 days of exposure,the Cd content in gill was higher than that of in liver and kidney of Cd-treated fish.There was no obvious differences in Cd accumulation of all tissues and whole fish between the dead fish and the surviving fish.5.After exposured to 100,200 and 400μg/L,the content of Na⁺in gills,carcass and whole fish significantly decreased in the early stage of exposure,and then recovered to the comparable level compared with the control group.The content of Na⁺in gills,kidneys,carcass and whole fish of Cd-exposed fish were significantly lower than that of the control group during the exposure period at a concentration of 800μg/L（P<0.05）.The liver Na⁺content did not significantly reduced after Cd exposure（P>0.05）.There was no obvious differences in Na⁺content of all tissues and whole body between dead fish and the living fish.6.After exposure to 100μg/L,only the carcass Ca²⁺content significantly temporality reduced and then returned to normal at day 7.The carcass and whole fish Ca²⁺firstly decreased,and then recovered to the same level as the control group at day 21 at a concentration of 200μg/L.After exposure to 400μg/L,gills Ca²⁺content significantly reduced（P<0.05）,liver and kidneys Ca²⁺content was not significant change,but carcass and whole fish decreased,and then recovered to the same level as the control group at day21.In addition,the content of Ca²⁺in carcass and whole fish Ca²⁺content firstly decreased,and then recovered to the same level as the control group at day 21.However,gills,carcass and whole fish Ca²⁺content significantly lower than that of the control group under 800μg/L exposure（P<0.05）.There was no obvious differences in Ca²⁺content of all tissues and whole fish between the dead fish and the living fish.7.Cd exposure had no significant effect on the content of K⁺and Mg²⁺in all tissues and whole fish.8.For the gills the distribution order of Cd content in subcellular fractions was as follows:heat stable-protein fraction（HSF）>heat-denatured fraction（HDF）>nuclei and cellular debris fraction（NCD）>mitochondria fraction（Mit）≈microsomes and lysosomes fraction（ML）>metal-rich granules fraction（MRG）.For the liver,the order was HSF≈HDF>ML≈Mit>MRG≈NCD.The order for the kidney was HSF>NCD>MIT≈HDF>ML≈MRG.9.Cd content in the metal-sensitive fraction（MSF:MIT,ML,HDF）of gills,liver and kidneys increased with exposure concentration and duration.However,the increase ranges were different.During the exposure of 14 days,the Cd content in the MSF of the gills of experimental fish was higher than that in the liver and kidneys.The Cd content in the Mit of gills was consistent with the observed pattern in gill.In 800μg/L exposure group,the content of Cd in HDF of gills in Cd-exposed fish increased sharply at day 7 and 21,and was significantly higher than that in liver and kidney.10.Cd concentrations in the metal-detoxified fraction（MDF:HSF,MRG）of the gills increased with exposure concentration and duration（100,200,and 400μg/L）.Cd content in the metal-detoxified fraction of liver and kidneys increased with exposure concentration and duration（100,200,400,and 800μg/L）.In 800μg/L exposure group,the content of Cd in HSF of gills increased and then decreased.11.The proportion of Cd content in MSF of gills increased from 18%to 30%-41%,while the proportion of Cd content in MDF decreased from 61%to 43%-54%.In contrast,the proportion of Cd content in MSF of kidneys increased from 12%to 14%-19%,while the proportion of Cd content in MDF increased from 59%to 62%-69%.However,there was no significant difference between Cd content in MSF and MDF of liver.12.The HDF Cd content in gills,Mit Cd content in liver,as well as Mit and HDF Cd content in kidneys of surviving fish was significantly lower than that of the corresponding subcellular components of dead fish（P<0.05）.There was no significant difference between the content of Cd in the other subcellular components of surviving and the corresponding tissues of dead fish（P>0.05）.The conclusions suggested in this study were as follows:1.After acute Cd exposure,the Cd content and accumulation rate in different tissues and the whole fish of dead individuals were significantly higher than that of surviving individuals of the southern catfish,which indicates that both the Cd content of tissues and Cd accumulation rates correlated with mortality.The saturation of Cd detoxification ability in liver was an important cause of death.2.Between the dead and surviving individuals,there was an obvious boundary of the Cd content in gills,kidneys,carcass and whole fish after acute Cd exposure.The obvious lethal threshold for the Cd content in gills,kidneys,carcass and whole fish were 9.53,37.04,0.84,and 1.38μg/g wet weight,respectively.3.The loss of Na⁺and Ca²⁺in southern catfish was an important mechanism leading to death after acute exposured to Cd.The lethal threshold for Na⁺content was 34.5μmol/g wet weight.Therefore,gills and kidneys were the lethal organs for the southern catfish acute exposured to Cd.4.Subacute exposure to Cd led to the proportion of Cd content in the detoxification fractions（MDF）of gills decreased with duration,while the proportion of Cd content in the sensitive fractions（MSF）increased with duration,which resulted in a decreased ability to regulated Na⁺and Ca²⁺content.Finally,a gradual decrease in fish Na⁺and Ca²⁺content occured.5.In the subacute Cd-exposed southern catfish,Na⁺and Ca²⁺content decreased,but the fish regulated the Na⁺and Ca²⁺content by starting the"damage-repair"mechanism.When Cd exposure concentration was lower,fish regulated the Na⁺and Ca²⁺content above the lethal threshold,and the fish survived.When Cd exposure concentration was higher,fish failed to regulate the Na⁺and Ca²⁺content above the lethal threshold,and the fish died.