Bremmer D R, Trower S L, Bertics S J, Besong S A, Bernabucci U, Grummer R R
Department of Dairy Science, University of Wisconsin, Madison 53706, USA.
J Dairy Sci. 2000 Oct;83(10):2239-51. doi: 10.3168/jds.S0022-0302(00)75108-3.
We conducted three experiments to determine the effects of nutritional and hormonal status on microsomal triglyceride transfer protein (MTP) activity and mass. In experiment 1, 18 nonlactating Holstein cows, 75 d before expected calving date, in their second gestation or greater were monitored from d 75 to 55 prepartum. Cows were fed a control diet from d 75 to 62 prepartum for covariable measurements. From d 61 to 55 prepartum, six cows continued to receive the control diet, six cows were restricted to 2.3 kg of grass hay/d, and six cows were fed the control diet plus 1.8 kg of concentrate/d and 500 ml of propylene glycol given 2 times/d as an oral drench. Plasma glucose and serum insulin concentrations were highest in cows that received propylene glycol and lowest in feed restricted cows. Plasma nonesterified fatty acids (NEFA) and liver triglyceride (TG) concentrations were highest in feed restricted cows and not different between cows that received the control diet and cows that received propylene glycol. Hepatic MTP activity and mass were not affected by treatment in experiment 1. In experiment 2, bovine hepatocytes isolated from the caudate process of five preruminating Holstein bull calves were incubated with either 0, 0.5, 1.0, or 2.0 mM NEFA for 48 h. Intracellular TG increased linearly as NEFA concentration in the media increased. Concentration of NEFA in the incubation media had no effect on MTP activity or mass. There was a quadratic effect of concentration of NEFA in the incubation media on MTP mRNA. In experiment 3, bovine hepatocytes isolated from the caudate process of five preruminating Holstein bull calves were incubated with 2 mM [1-14C]oleate for 24 h to accumulate TG, followed by a 36-h period of TG depletion, during which hepatocytes were incubated with no hormone, 10 nM insulin, or 10 nM glucagon. There was no effect of insulin or glucagon on intracellular TG, MTP activity or mass. Cells incubated with no hormone had higher levels of MTP mRNA compared to cells incubated with insulin or glucagon during the depletion period. Results suggest that hepatic MTP mRNA may be affected by TG accumulation, insulin, and glucagon in vitro. However, hepatic MTP activity and mass are not affected by nutritional status of nonlactating dairy cows, TG accumulation in vitro, or insulin and glucagon in vitro.
我们进行了三项实验，以确定营养和激素状态对微粒体甘油三酯转运蛋白（MTP）活性和含量的影响。在实验1中，对18头处于第二胎或更高胎次、预期产犊日期前75天的非泌乳荷斯坦奶牛，从产前75天至55天进行监测。在产前75天至62天，给奶牛饲喂对照日粮以进行协变量测量。从产前61天至55天，6头奶牛继续接受对照日粮，6头奶牛限制饲喂2.3千克禾本科干草/天，6头奶牛饲喂对照日粮加1.8千克精料/天，并每天两次口服500毫升丙二醇。接受丙二醇的奶牛血浆葡萄糖和血清胰岛素浓度最高，限饲奶牛最低。限饲奶牛的血浆非酯化脂肪酸（NEFA）和肝脏甘油三酯（TG）浓度最高，接受对照日粮的奶牛和接受丙二醇的奶牛之间无差异。实验1中，处理对肝脏MTP活性和含量没有影响。在实验2中，从5头反刍前荷斯坦公牛犊的尾叶分离的牛肝细胞，分别与0、0.5、1.0或2.0毫摩尔NEFA孵育48小时。细胞内TG随着培养基中NEFA浓度的增加呈线性增加。培养基中NEFA浓度对MTP活性或含量没有影响。培养基中NEFA浓度对MTP mRNA有二次效应。在实验3中，从5头反刍前荷斯坦公牛犊的尾叶分离的牛肝细胞，先与2毫摩尔[1-14C]油酸孵育24小时以积累TG，然后进行36小时的TG消耗期，在此期间，肝细胞分别与无激素、10纳摩尔胰岛素或10纳摩尔胰高血糖素孵育。胰岛素或胰高血糖素对细胞内TG、MTP活性或含量没有影响。在消耗期，与无激素孵育的细胞相比，与胰岛素或胰高血糖素孵育的细胞MTP mRNA水平更高。结果表明，体外肝脏MTP mRNA可能受TG积累、胰岛素和胰高血糖素影响。然而，非泌乳奶牛的营养状况、体外TG积累或体外胰岛素和胰高血糖素对肝脏MTP活性和含量没有影响。
