Effects And Mechanisms Of Heat Stress On Lactose Synthesis Of Holstein Cows

This research focused on the effects of heat stress on milk performance and health of Holstein cows.The study consists of three parts:1 Effects of heat stress on milk performance,antioxidant function and cytokine release of lactating cowsHolstein cows(n=4)were utilized in a 2x2 crossover design during two experimental periods(each period lasted 10 d and were separated by 30 d)while housed in environmentally control chambers.Blood and milk samples were collected on d 1,4,7 and 10 d during both periods.Glucose tolerance test(GTT)was performed on 6 d of each period.Heat stress(HS)significantly reduced the daily milk yield of lactating dairy cows by 27.39%,from 27.3 kg to 21.43 kg.The content of milk protein,milk fat and lactose showed no difference,but the yield of lactose reduced 300g/d(P<0.05).The circulating glucose and the stimulated glucose following GTT decreased during heat stress(P<0.05).Serum MDA increased significantly in the HS group(P<0.05).Serum catalase CAT activity increased significantly(P = 0.05)and SOD had a tendency to increase(P = 0.07)in the HS group.Serum GSH-PX showed no significant difference between the two groups(P>0.05).The serum IgG,IgM and IL-6 of heat stress group had no significant difference(P>0.05)between the two groups.The serum IgA and TNF reduced significantly(P<0.05).Hyperthermia and humidity induced heat stress of lactating cows,leading to changes of circulating and stimulated glucose,oxidative stress and immune function,especially the decrease of milk and lactose yield.During heat stress,the utilization of glucose extra-mammary increased,lowering the blood glucose,which may account for the reduced lactose yield and milk production.2Effects of heat stress on hepatic injury,apoptosis and the protective function of HSPs in lactating cowsHolstein cows(n=4)were utilized in a 2×2 crossover design during two experimental periods(each period lasted 10 d and were separated by 30 d)while housed in environmentally control chambers.Bloodsamples were collected on d 1,4,7 and 10 d during both periods.Liver tissues were collected on 10 d of each period.Liver function-related enzymes and the gene expression of gluconeogenesis,inflammation,apoptosis and HSPs were detected.Serum GOT and LDH of heat stress group increased significantly(P<0.05).Serum GPT and AKP had no significant difference between the two groups(P>0.05).Hepatic gluconeogenesis key enzymes:pyruvate carboxylase(PC)gene expression had a tendency to decrease(P = 0.09)and phosphoenolpyruvate carboxykinase 1(PEPCK)showed no significant difference between the two groups(P>0.05).The gene expression of liver pro-inflammatory factors NF-?B,TNF-? and IL-1? increased significantly(P<0.05)in the HS group.The promoting apoptotic genes JNK,Caspase-3,Caspase-9 and the anti-apoptotic gene Bcl-2 were up-regulated significantly in the HS group(P<0.05).The pro-apoptotic gene Bax had a tendency to increase in the HS group(P?0.06).HSP27 gene expression increased significantly in the HS group(P<0.05)and HSP70,HSP90,and HSF-1 showed no significant difference between the two groups(P>0.05).The gene expression of inflammation,apoptosis and HSPs in the liver reinforced,while hepatic gluconeogenesis weakened.The results suggested that hepatic glucose production did not decrease,but anti-apoptosis and anti-injury process enhanced glucose consumption.Namely,glucose allocated to the mammary reduced and lactose synthesis was blocked during short-term heat stress.3 Effects of heat stress on mammary injuty and apoptosis and the protective function of HSPs in lactating cowsHolstein cows(n=4)were utilized in a 2×2 crossover design during two experimental periods(each period lasted 10 d and were separated by 30 d)while housed in environmentally control chambers.Blood samples were collected on d 1,4,7 and 10 d during both periods.Mammary gland collected on 10 d of each period.The gene expression of oxidative stress,inflammation,apoptosis and HSPs of mammary tissue were measured.In vitro,cell viability,mitochondrial membrane potential and the gene expression of HSPs and oxidative stress were detected in the 0 h,0.5 h,1 h,1.5 h,2 h,4 h,8 h and 12 h heat-stressed(42 ?)bovine mammary epithelial cells.In vivo results showed that heat stress significantly up-regulated the gene expression of mammary HSF-1 and HSP70(P<0.05).HSP27 gene expression decreased significantly in the HS group(P<0.05).The gene expression of HSP27 was significantly down-regulated(P<0.05)and HSP90 showed no significant difference(P>0.05).The mRNA expression of anti-oxygen Keapl,Nrf2,NQO1,HO-1,proinflammatory factors NF-?B,TNF,IL-1?,and pro-apoptotic gene JNK,Caspase-3 and Caspase-9 showed no significant difference between the two groups(P>0.05).In vitro results showed that different heat time significantly reduced mammary epithelial cell viability(P<0.05)and damaged mitochondrial membrane potential.Cell viability gradually recovered from 2 h to 12 h(P<0.05).The gene expression of HSP27 decreased significantly(P<0.05),while HSP70 increased significantly(P<0.05).The gene expression of HSP90 increased significantly in 0-4 h(P<0.05),and returned to normal level after then.The gene expression of HSF-1 increased significantly in some points(P<0.05).The gene expression of Keapl showed no significant difference(P<0.05),while Nrf2 and NQO1 increased significantly(P<0.05).During short-term heat stress,the gene expression of HSPs and HSF-1 up-regulated to protect the mammary tissue.In vitro,heat stress significantly reduced the cell viability and mitochondrial membrane potential of bovine mammary epithelial cells.Meanwhile,oxidative stress and early apoptosis were activated and some HSPs up-regulated to protect the cell.