热应激状态下泌乳奶牛通过激活GHIGF-I轴增强糖异生变化

【目的】选取分娩1周后的泌乳期荷斯坦奶牛6头,提前适应期1周后,正式饲喂从2013年6月29日至8月5日,总共35 d(5周),使泌乳奶牛处于热应激状态。进而检测泌乳奶牛乳产量及乳蛋白含量,血液中生长激素(GH)、胰岛素样生长因子(IGF-I)、葡萄糖以及肝脏中热休克蛋白70(HSP70)和糖异生作用的变化情况,拟从GH-IGF-I轴的角度阐明泌乳奶牛发生热应激时对糖异生作用及乳品质下降的机制。为进一步揭示奶牛热应激的发生机理及控制奶牛热应激的发生提供理论依据。【方法】分别统计第1—5周泌乳奶牛的产奶量及分析乳蛋白含量,并采集泌乳奶牛颈静脉血液和进行活体采取肝脏组织的方法,检测血液中葡萄糖和GH、IGF-I的含量,采用实时荧光定量PCR(q RT-PCR)技术对奶牛肝脏组织中HSP70和糖异生的关键酶丙酮酸羧化酶(PC)、磷酸烯醇式丙酮酸羧激酶(PEPCK)以及生长激素受体(GHR)、胰岛素样生长因子受体(IGFR)进行检测。【结果】在35 d的饲喂过程中,日间平均气温在32℃以上的持续时间达25 d,且最高温度为38℃,高温持续时间大于72 h,即此气候条件下奶牛处于一个热应激状态。随着泌乳奶牛热应激程度的不断加深,从第1周到第5周产奶量和乳蛋白含量都有不同程度的下降。通过比较第5周和第1周泌乳奶牛肝脏中HSP70的表达,发现第5周HSP70的表达量极显著高于第1周。检测血液中GH、IGF-I以及葡萄糖的含量,发现在第5周的时候其含量均高于第1周且差异显著(P0.05);检测泌乳奶牛肝脏组织中PC和PEPCK的表达水平,发现第5周显著高于第1周(P0.05);通过检测第5周与第1周肝脏组织中GH和IGF-I受体的表达水平,发现GHR和IGFR同样上调,其中IGFR显著上调(P0.05)。【结论】随着泌乳奶牛热应激的程度的不断加深,血液中的葡萄糖含量显著升高,其可能是由于垂体分泌的GH刺激肝脏产生更多的IGF-I,即通过GHIGF-I轴上调肝脏糖异生途径关键酶的表达,使糖异生途径处于激活状态。而乳中乳蛋白含量的下降可能是由于其前体物被过多的用来进行糖异生作用,增加血液中葡萄糖含量,维持机体正常供能所致。

Lactating Dairy Cows Under Heat Stress Enhanced Gluconeogenesis by Activating the GHIGF-I Axis

【Objective】Six Holstein cows in early lactation after delivery were chosen to be in heat stress,which were fed 35 days from June 29 in 2013 for detecting milk yield and protein content, growth hormone (GH), insulin like growth factor-I(IGF-I), glucose and liver heat shock protein 70 (HSP70) and gluconeogenesis change in blood to illuminate glucose gluconeogenesis and milk quality decline mechanism when lactating dairy in heat stress from the GHIGF-I axis angle and further revealing the cows heat stress mechanism and control of dairy cows heat stress.【Method】Glucose and GH, and IGF-I contents in blood were detected by collecting jugular venous blood and in vivo taking liver tissue, furthermore milk yield and protein content in lactating cows were counted and analyzed, respectively, before fifth week. In addition, HSP70, PC, PEPCK, GHR, and IGFR were detected by qRT-PCR.【Result】Dairy cows were in heat stress when the average daytime temperature above 32℃ lasted for 25 days and the highest temperature 38℃ lasted for more than 72 h continuously during 35 days feeding period. The milk yield and protein content generally showed a decline with the degree of heat stress in lactating dairy cows deepening from the first week to the fifth week. The expression of HSP70 in the fifth week was significantly higher than that in the first week, so were the expression of PC and PEPCK and GHR and IGFR in liver tissues (P＜0.05) through detecting their contents.【Conclusion】Blood glucose was significantly increased, in the period of study which may be due to the secretion of pituitary GH stimulates the liver to produce more IGF-I, namely through GHIGF-I axis up-regulation of hepatic gluconeogenesis pathway key enzyme expression, gluconeogenesis way was activated with the degree of heat stress in lactating dairy cows deepening. Milk protein content decline may be caused by its precursors was too much for gluconeogenesis to make levels of glucose increase in blood to maintain body’s normal energy supply.