Effect Of Nitrite On Thyroid Hormone Metabolism In Grass Carp Ctenopharyngodon Idellus

In recent years,the main reason for the reported increase in the nitrite level is high density farming,with excess feeding leading to high levels of residual protein based feed and nitrogen excretion.When the residual protein based feed supply and nitrogen excretion outstrips the metabolic capability of indigenous flora in aquaculture water,the nitrite accumulates continuously.Because of its potential toxicity to aquatic organisms,nitrite becomes the focus of modern aquaculture.In order to investigate whether nitrite has a potential interference effect on the thyroid hormone metabolism in fish,the juvenile grass carp(Ctenopharyngodon idellus)were used to assess the interference of the thyroid follicle morphology,the THs levels,the iodothyronine deiodinases(IDs)activities and the gene transcription related to TH metabolism.We exposed healthy fish with a mass of 65.8 ± 5.5 g and a body length of13.2 ± 1.2 cm to five concentrations of sodium nitrite(0 mg/L,0.5 mg/L,1 mg/L,4 mg/L,and 16 mg/L,respectively),and the samples were taken at 0 h,12 h,24 h,48 h,and 96 h after exposure.We exposed healthy fish with a mass of 98.7 ± 12.4 g and a body length of 15.26±1.94 cm to five concentrations of sodium nitrite(0 mg/L,0.2 mg/L,0.4 mg/L,0.6 mg/L,and 0.8 mg/L,respectively),and the samples were taken at 0 d,12 d,24 d,36 d,and 48 d after exposure.Results and conclusions are as follows:(1)Nitrite caused the death of grass carp.In acute exposure,no fish died in the control and low does groups(0.5 and 1 mg/L)during the experimental period and mortalities of 4.76 and 9.52% were found in the 4 and 16 mg/L groups,respectively,at 96 h after exposure.In chronic exposure,no fish were dead in not only the control group but also treatment groups during the experimental period.(2)Nitrite altered the morphology of thyroid follicles.In acute exposure,the thyroid follicles in the control group had varying sizes,linked the simple cuboidal epithelium,and were filled with colloid within the lumen.The follicles typically had a slightly oval appearance after 96 h exposure.In the 16 mg/L group,the amount of colloid was diminished in the thyroid follicles,and a lot of vacuoles were found in the edge of the follicle after 96 h exposure.In chronic exposure,the hyperplasia of thyroid follicle epithelial cells was observed in teatment groups after 48 d exposure,and few vacuoles were found in the edge of the follicle in 0.8 mg/L group.(3)Nitrite altered the level of thyroid-stimulating hormone(TSH).In acute exposure,serum TSH level decreased significantly as the nitrite concentration increase,the maximal reduction was observed at 48 h after exposure.In chronic exposure,serum TSH level increased,the maximal augment was observed at 36 d after exposure.(4)Nitrite altered the level of THs.In acute exposure,thyroxine(T4)level was increased in the 0.5 mg/L,1 mg/L groups and decreased in the 4 mg/L and 16 mg/L groups.The level of free thyroxine(FT4),triiodothyronine(T3),free triiodothyronine(FT3)and the ratio of T3/T4 were significantly decreased.However,the level of 3,3',5'-triiodothyronine(r T3)was increased after exposure.In chronic exposure,the level of T4,FT4,T3,FT3 and the ratio of T3/T4 were significantly decreased.The level of r T3 was significantly decreased in 0.8 mg/L group after 24 d and 96 d exposure and significantly increased in 0.2 mg/L,0.4 mg/L groups after 36 d exposure.(5)Nitrite altered the activity of iodothyronine deiodinases(IDs).In acute exposure,the type ? iodothyronine deiodinases(ID1)activity increased significantly in a dose-dependent manner as the nitrite concentration increased.The type ?iodothyronine deiodinases(ID2)activity was decreased,in the first 24 h of exposure,ID2 activity was significantly decreased.After 48 h exposure,the activity of ID2 returned to the control level.Whereas the type ? iodothyronine deiodinases(ID3)activity was increased after exposure,ID3 activity increased first and then decreased with the enhancement of exposure concentration.In chronic exposure,ID1 activity increased,and significantly enhanced in 0.8 mg/L group at every point.The activity of ID2 was changed.After exposure to nitrite for 24 d,the activity of ID2 was significantly increased in 0.6 mg/L group.After exposure to nitrite for 36 and 48 d,the activity of ID2 was significantly attenuated in 0.2 mg/L group.The activity of ID3 was enhanced after 12 d of nitrite exposure,and significantly enhanced in 0.2 and 0.4 mg/L of group.After exposure to nitrite for 24,36 and 48 d,the activity of ID3 were significantly enhanced in 0.8 mg/L group.(6)Nitrite altered the transcription of genes related to TH metabolism.In acute exposure,the gene transcription of corticotropin-releasing hormone(crh),thyroid-stimulating hormone(tsh)and type II iodothyronine deiodinase(id2)were significantly down-regulated,whereas the gene transcription of type I iodothyronine deiodinase(id1)was significantly up-regulated after exposure.In the first 48 h of exposure,the gene transcription of type III iodothyronine deiodinase(id3)was significantly down-regulated,then up-regulated after 96 h exposure.In the first 24 h of exposure,the gene transcription of transthyretin(ttr)was significantly up-regulated.The gene transcription of thyroid hormone receptor-?(tr-?)was significantly up-regulated after 96 h exposure,but significantly down-regulated in the first 48 h of exposure.The gene transcription of thyroid hormone receptor-?(tr-?)was significantly down-regulated after 48 h and 96 h exposure,and significantly up-regulated in the first 24 h of exposure.In chronic exposure,the id1 ? id2 genes expression were significantly elevated,the maximal augment was observed at 24 d after exposure.The crh?tsh??ttr?tr-? genes expression were significantly elevated,the maximal augment was observed at 36 d after exposure.The id3 gene expression was significantly elevated,the maximal augment was observed at 48 d after exposure.Whereas the tr-? genes expression were significantly depressed in the first 36 d,then elevated after 48 d exposure.The results indicated that nitrite altered the morphology of thyroid follicles,the serum levels of TSH and THs,the activity of IDs,the expression of thyroid endocrine system-related genes in grass carp,disturbed the balance of thyroid hormone metabolism,leaded to the hypothyroidism of juvenile grass carp,resulted in the death of grass carp.