Four multi-catheterized lactating goats were used in a 4 × 4 Latin square experiment to investigate the responses of amino acid metabolism in portal-drained viscera (PDV), liver, and mammary glands to short-term varying supplies of methionine (Met). During the last 45 h in each experimental period, goats were fasted for 12 h and then abomasally infused with an amino acid (AA) mixture plus glucose for 33 h. Treatments consisted of graded removal of Met from an infused AA mixture to achieve Met content in the infusate of 100% (complete), 60%, 30%, or 0% that in casein. Graded Met removal decreased the production of milk, milk protein, lactose, and fat linearly whilst also decreasing arterial Met concentration linearly (P < 0.05). Meanwhile, net PDV uptake and liver removal of Met decreased linearly (P < 0.05) due to decreased Met affinity of PDV and liver (P < 0.05). Net mammary uptake of Met (P > 0.1) was maintained as Met supply declined. This was achieved through increased mammary affinity (P < 0.05) and increased mammary blood flow (P < 0.05) totally offsetting the negative effect of decreased circulating Met concentration. Graded removal of Met from the infusate linearly decreased mammary uptake-to-milk output ratios of Met (P < 0.05) and tended to decrease essential amino acid (EAA) linearly (0.05 < P < 0.1). Treatments also linearly decreased circulating concentration of prolactin and linearly increased insulin concentration (P < 0.05). In conclusion, results of the present study indicated there were several mechanisms used to mitigate a Met deficiency, including reduced catabolism of Met in PDV, liver, and peripheral tissue (including mammary glands) and a linear increase in mammary blood flow. The observed decreases in milk protein production as Met supply decreased appear to be a result of regulatory events which may have been driven by decreased circulating prolactin, rather than as a result of decreased mammary Met uptake.

在 4 × 4 拉丁方实验中使用了四只多导管泌乳山羊，以研究门静脉引流内脏 (PDV)、肝脏和乳腺中氨基酸代谢对短期变化的甲硫氨酸 (Met) 供应的反应。在每个实验期的最后 45 小时，山羊禁食 12 小时，然后真胃注入氨基酸 (AA) 混合物和葡萄糖 33 小时。处理包括从输注的 AA 混合物中分级去除 Met，以达到输注液中 Met 含量为酪蛋白中的 100%（完全）、60%、30% 或 0%。分级去除蛋氨酸降低了牛奶、乳蛋白、乳糖和脂肪的产量，同时也线性降低了动脉蛋氨酸浓度（P  < 0.05）。同时，PDV 的净摄取量和 Met 的肝脏去除率呈线性下降（P  < 0.05) 由于 PDV 和肝脏的 Met 亲和力降低 ( P  < 0.05)。随着蛋氨酸供应下降，乳腺对蛋氨酸的净摄取量 ( P  > 0.1) 得以维持。这是通过增加乳腺亲和力 ( P  < 0.05) 和增加乳腺血流量 ( P  < 0.05) 来实现的，这完全抵消了循环 Met 浓度降低的负面影响。从输注液中分级去除 Met 会线性降低 Met 的乳腺摄取与乳汁输出比率 ( P  < 0.05)，并倾向于线性降低必需氨基酸 (EAA) (0.05 <  P  < 0.1)。治疗还线性降低催乳素的循环浓度和线性增加胰岛素浓度（P < 0.05)。总之，本研究的结果表明有几种机制可用于减轻 Met 缺乏，包括减少 PDV、肝脏和外周组织（包括乳腺）中的 Met 分解代谢以及乳腺血流量的线性增加。由于 Met 供应减少，观察到的牛奶蛋白产量减少似乎是调节事件的结果，这可能是由循环催乳素减少驱动的，而不是由于乳腺 Met 摄取减少。