Pacheco D, Tavendale M H, Reynolds G W, Barry T N, Lee J, McNabb W C
Nutrition and Behaviour, AgResearch Ltd, and Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand.
Br J Nutr. 2003 Aug;90(2):271-81. doi: 10.1079/bjn2003886.
The utilisation of essential amino acids (EAA) by the mammary gland of lactating dairy cows fed fresh forages was studied to provide basic information useful in designing strategies to increase the production of milk protein from pasture-fed dairy cows. The relationship between the flux of EAA in the whole body and their uptake by the mammary gland was determined in four cows in early lactation (length of time in milk 44 (SD 14.5) d) producing 21 (SD 4.0) kg milk/d. The cows were maintained in metabolism stalls and fed fresh perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture ad libitum or restricted to 75 % ad libitum intake. The whole-body fluxes of amino acids (AA) were measured using an arterio-venous infusion of universally (13)C-labelled AA. Whole-body fluxes of fourteen AA were estimated. Isotope dilution indicated that mammary utilisation accounted for one-third of the whole-body flux of EAA, with individual AA ranging between 17 and 35 %. Isoleucine, leucine, valine and lysine were the EAA with the greatest partitioning towards the mammary gland (up to 36 % of the whole-body flux), which could reflect a potentially limiting effect on milk protein synthesis. In the case of AA with low partitioning to the mammary gland (for example, histidine), it is suggested that non-mammary tissues may have priority over the mammary gland and therefore the supply of this AA may also limit milk protein synthesis.
为了提供有助于设计提高放牧奶牛乳蛋白产量策略的基础信息，对采食新鲜牧草的泌乳奶牛乳腺对必需氨基酸（EAA）的利用情况进行了研究。在4头处于泌乳早期（产奶时长44（标准差14.5）天）、日产奶量21（标准差4.0）千克的奶牛中，测定了全身必需氨基酸通量与其被乳腺摄取之间的关系。奶牛被饲养在代谢栏中，自由采食新鲜多年生黑麦草（Lolium perenne）和白三叶（Trifolium repens）牧草，或限制采食量为自由采食量的75%。使用普遍标记有（13）C的氨基酸进行动静脉输注来测量氨基酸（AA）的全身通量。估计了14种氨基酸的全身通量。同位素稀释表明，乳腺利用率占必需氨基酸全身通量的三分之一，单个氨基酸的利用率在17%至35%之间。异亮氨酸、亮氨酸、缬氨酸和赖氨酸是向乳腺分配比例最高的必需氨基酸（高达全身通量的36%），这可能反映出对乳蛋白合成存在潜在限制作用。对于向乳腺分配比例较低的氨基酸（例如组氨酸），表明非乳腺组织可能比乳腺具有优先权，因此这种氨基酸的供应也可能限制乳蛋白合成。
