Prosser C G, McLaren R D
AgResearch, Ruakura Research Centre, Hamilton, New Zealand.
J Dairy Sci. 1997 Jul;80(7):1281-7. doi: 10.3168/jds.S0022-0302(97)76057-0.
The objective of this study was to determine the response of individual milk proteins to a reduction in amino acid (AA) availability induced by atropine and to determine whether the response was different in cows with different beta-lactoglobulin (LG) phenotypes. Six cows that were homozygous for the A variant of beta-LG and six cows that were homozygous for the B variant of beta-LG were each given a single subcutaneous injection of saline or 20 mg of atropine. In both groups of cows, atropine decreased milk yield by 30% and reduced the concentration of alpha-lactalbumin (LA) by 25 to 30% at 8 h following injection. Eight hours after atropine injection, yield of beta-LG was 41% lower than it was following saline injection, and yield of beta-casein (CN) after atropine injection declined 16% relative to saline. Concentrations of BSA and the ratio of gamma-CN to beta-CN, which reflects plasmin activity in milk, were significantly increased after administration of atropine. Although the response to atropine tended to be more pronounced in cows that were homozygous for beta-LG B, they were not significantly different from the response of cows that were homozygous for beta-LG B, they were not significantly different from the response of cows that were homozygous for beta-LG A. The differential response of individual proteins to a reduction in AA concentrations in whole blood suggested that susceptibility to restriction in substrate availability differed for individual proteins. The concentration of lactose in plasma did not change, which implied that the integrity of the mammary epithelial barrier was not compromised when AA derived from blood were diminished. The consistent concentration of lactose combined with the minimal increase in total yield of BSA in milk following atropine treatment indicated that the increased concentration in milk of proteins derived from serum was due to the concentrating effect of lower milk volume.
本研究的目的是确定个体乳蛋白对阿托品诱导的氨基酸（AA）可利用性降低的反应，并确定不同β-乳球蛋白（LG）表型的奶牛的反应是否存在差异。6头β-LG A变体纯合的奶牛和6头β-LG B变体纯合的奶牛分别皮下注射一次生理盐水或20mg阿托品。在两组奶牛中，阿托品使产奶量降低30%，并在注射后8小时使α-乳白蛋白（LA）浓度降低25%至30%。注射阿托品8小时后，β-LG的产量比注射生理盐水后低41%，注射阿托品后β-酪蛋白（CN）的产量相对于生理盐水下降了16%。注射阿托品后，牛血清白蛋白（BSA）的浓度以及反映牛奶中纤溶酶活性的γ-CN与β-CN的比值显著增加。尽管β-LG B纯合的奶牛对阿托品的反应往往更明显，但它们与β-LG A纯合的奶牛的反应没有显著差异。个体蛋白质对全血中AA浓度降低的不同反应表明，个体蛋白质对底物可利用性限制的敏感性不同。血浆中乳糖浓度没有变化，这意味着当血液来源的AA减少时，乳腺上皮屏障的完整性没有受到损害。乳糖浓度的持续稳定以及阿托品处理后牛奶中BSA总产量的最小增加表明，牛奶中血清来源蛋白质浓度的增加是由于牛奶体积减少的浓缩作用。
