| Selenium(Se)is an essential mineral element for aquatic animals.Selenium supplementation can promote the growth of animals,enhance the antioxidant capacity,antagonize the toxicity of heavy metals.Se exerts its biological activity mainly through the synthesis of selenoproteins.Deiodinase(DIO)is one of the most important selenoproteins,its main function is to catalyze the deiodination of thyroxine(T4)to triiodothyronine(T3).Studies have shown that Se supplementation can promote the activities of DIOs and regulate the content of thyroid hormone(TH).Cadmium(Cd)is a harmful heavy metal.Many studies have shown that Se can reduce the harm of cadmium by scavenging free radicals in animals and repairing antioxidant system,and selenoprotein can combine with cadmium to form an inert complex to reduce its toxicity.In order to explore the effects of Se supplementation on growth and TH metabolism and the effect of dietary Se on the antagonism of cadmium toxicity in grass carp(Ctenopharyngodon idellus),180 healthy grass carp were divided into three groups and fed with diets of 0.141,0.562 and 1.044 mg Se/kg respectively for 60 days to study the effects of Se supplementation on growth,HPT axis and GH-IGF axis of grass carp.The acute cadmium stress experiment was conducted after the feeding experiment of dietary Se.The antagonistic effect of dietary Se on heavy metal toxicity was evaluated by measuring malondialdehyde(MDA)content,antioxidant enzyme activities,the relative transcription levels of heavy metal scavenging genes and apoptosis related genes in liver of grass carp.The results are as follows:1.Dietary Se supplementation improves the weight gain and specific growth rate of grass carp.The activities of three types DIOs in selenium-supplemented groups were significantly higher than those in the control group.The serum levels of T3,FT3 and TSH were significantly increased,while the serum levels of T4 and FT4 were significantly decreased in selenium-supplemented groups.Histological observation of thyroid showed that selenium deficiency led to hypertrophy of follicular epithelial cells and colloid vacuolation.In addition,the relative expression levels of dio1,dio2 and dio3 increased with the rise of dietary selenium concentration.HPT axis-related genes(crh,tsh-?,ttr,tr-s,tpo,nis)and GH/IGF-1 related genes(gh,ghr,igf1,igf1r)were significantly up-regulated in selenium-supplemented groups.No significant differences in the above indicators were observed between 0.562 mg Se/kg diet group and 1.044 mg Se/kg diet group except T3 content and dio1 relative expression ratio.In summary,dietary Se supplementation improves the hepatic DIO activities,thyroid hormone metabolism and regulates the transcription levels of HPT and GH/IGF axes-related genes,which may be responsible for the growth promotion in grass carp.2.After acute cadmium exposure,the MDA content in 0.562 mg Se/kg group and1.044 mg Se/kg group significantly decreased than that in control group.The activities of SOD,CAT,and GSH-Px increased first and then decreased with the extend of cadmium stress time.The activities of GSH-Px of selenium-supplemented groups significantly increased.The transcription level of abcc2 in 1.044 mg Se/kg group significantly decreased.The relative m RNA transcription level of mt2 in the grass carp liver of selenium-supplemented groups significantly decreased.Cadmium induced the expression level of caspase3,jnk,caspase8,bax,and caspase9.The results showed that selenium supplementation could inhibit cell apoptosis induced by cadmium through improve bcl-2and decreasing the relative expression level of jnk,bax,caspase3,caspase8,caspase9.In addition,TUNEL results of selenium-supplemented groups in grass carp liver cells showed that the apoptotic index significantly decreased.Comprehensive analysis shows that dietary selenium supplementation can effectively reduce the oxidative damage caused by enhancing the activity of GSH-Px and inhibit the apoptosis of liver cells by inhibiting JNK-Caspase pathway. |
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