| Heat stress affects the feed intake,decreases the production performance and reproductive performance in animals,and results in great economic loss for the livestock.One of the reasons that body damage caused by heat stress is that heat stress induces the cellular oxidative stress.So far,all kinds of anti-heat stress drugs were added to alleviate damages of heat stress in the livestock.Zinc(Zn)is generally known to be a feed additive with growth-promoting and anti-oxidant properties.Some studies reported that supplement Zn in the diet could relieve heat stress,but its mechanism is still unclear.And studies have shown that Zn could alleviate liver oxidative damage induced by diabetes and alcohol.Liver is the main organ of the zinc metabolism,and also is sensitive to heat shock.Hence,studying the influence of Zn on hepatic damage in the heat-stressed animals will contribute to alleviate heat stress in the livestock.The study intends to investigate effects of Zn on hepatic damage in the heat-stressed mice and Bama mini pigs,aiming to reveal the mechanism of anti-stress and the impacts of Zn in the heat-stressed liver.1 Effect of Zn on hepatic oxidative damage and Nrf2 antioxidant pathway in heat-exposed mice24 Eight-week-old male mice were divided into control(TN,0),heat treatment(HT,0),Zn(TN,250)and Zn plus heat treatment(HT,250)groups,and were fed basal diets containing 0,0,250 or 250 mg/Kg Zn(zinc sulfate),respectively.After 30 days of pre-feeding,thermo-neutral groups(containing control group and zinc group)were maintained at a controlled temperature(25 ℃),whereas the two heat-treated groups(containing heat treatment group and Zn plus heat treatment group)were exposed to an elevated ambient temperature(40-42 ℃)for 2 hours each day.After heat exposure for eight consecutive days,sera and liver tissues were collected.The heat-treated mice exhibited reduction of body weights,liver weights and hepatosomatic indices.Histological observation revealed that the hepatocytes of the heat treatment group were subjected to serious damage and exhibited irregular arrangements and nuclear pyknosis.Moreover,heat treatment significantly increased the hepatic malondialdehyde(MDA)levels,significantly reduced the serum alkaline phosphatase(AKP)levels,hepatic copper/zinc-superoxide dismutase(CuZn-SOD)and glutathione peroxidase(GSH-PX)activities.However,supplement 250 mg/kg Zn significantly increased the Zn levels in serum and liver.In the Zn plus heat treatment group,the histomorphology of the liver was restored;the serum aspartate aminotransferase(AST)level was significantly decreased,the serum AKP level was slightly increased;the hepatic CuZn-SOD and GSH-PX activities were significantly increased compared to the heat treatment group.Furthermore,supplement 250 mg/kg Zn significantly increased expressions of the hepatic Nrf2 protein and Nrf2,Keapl genes.In the heat treatment condition,supplement 250 mg/kg Zn also significantly increased hepatic NQO1 gene expression.Hence,our results showed that supplement 250 mg/kg Zn might increase hepatic Nrf2 expression level,Zn in this concentration might alleviate heat-induced hepatic injury as revealed by restored histomorphology and AST level.Moreover,our results further suggested that Zn might exert its protective effects via the activation of the Nrf2-antioxidant pathway.2 Effects of Zn on hepatic oxidative stress and P62/Nrf2 pathway in the heat-stressed Bama mini pigs24 male Bama miniature pigs aged 6 months were sorted by BW and randomly divided into control group(TN,0),heat treatment group(HT,0),zinc group(TN,1500)and zinc plus heat treatment group(HT,1500).Two groups(TN and HT)received each of the two diets(0 and 1500).After pretreatment with two zinc sulfate levels for 30 days,two thermo-neutral groups(TN)remained at the same temperature(25 ℃).The heat treatment groups(HT,also consuming 0 and 1500 mg/kg zinc diets)were kept in controlled temperature(40 ℃)for 5 hours per day.The protocol was continued for 8 d(from 31 d to 38 d).The results showed that at d 31,d 34 and d 38,heat treatment significantly increased the breathing rate,body surface temperature and rectum temperature in Bama miniature pigs.Heat treatment could significantly reduce serum zinc level and Zn supplemention could significantly increase zinc contents in serum and liver.Histological observation showed that heat treatment leads to the accumulation of inflammatory cytokines in liver tissues,meanwhile liver cell boundaries were unclear;zinc group showed hepatic nuclear enrichment and accumulation of inflammatory;hepatocytes in the zinc plus heat treatment group exhibited irregular arrangement and appeared less mitochondria.Increased serum ALT and AST levels in zinc group and reduced serum AKP level in the heat treatment group further revealed that the damage in the liver may result from both high dose of Zn and heat treatment.Supplement 1500 mg/kg Zn significantly increased hepatic MDA concentration,while heat treatment significantly increased hepatic antioxidant enzymes CuZn-SOD and GSH-PX activities.RT-PCR and Western Blot results showed that heat treatment increase hepatic Nrf2 protein level and upregulate Nrf2 and SOD1 gene levels.Compared with the control group,the hepatic NQO1 gene level was also significantly increased in heat treatment group.Supplement 1500 mg/kg Zn not only significantly increased in hepatic Nrf2,P62 and NQO1 gene levels,but also upregulated hepatic Nrf2 and P62 protein levels.These results indicated that the heat resistance of Bama miniature pigs might be related to activation of the inner antioxidant system.Supplement 1500 mg/kg Zn could improve growth performance of Bama miniature pigs,but presenting low toxicity to the liver,causing the hepatic oxidative damage and activating P62/Nrf2 antioxidant pathway.And supplement 1500 mg/kg Zn didn’t enhance the heat-stress resistance of Bama miniature pigs,and its mechanism needs further study. |
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