Insulin-like growth factor-I (IGF-I), growth hormone (GH), and prolactin (PRL) play important roles in milk protein synthesis, and their plasma concentrations were reported to be affected by dietary protein intake. To investigate the relationship between circulating amino acid (AA) and concentrations of these hormones, 18 Wistar rats aged 14 wk were assigned to a low (LP; 9% protein), standard (SP; 21% protein), or high-protein (HP; 35% protein) diet from parturition through day 15 of lactation. Plasma, liver, pituitary gland, skeletal muscle, and mammary gland samples were collected at the end of treatment. Circulating and hepatic IGF-I concentrations increased linearly with elevated dietary protein concentrations (P < 0.0001). Rats receiving the HP diet had higher circulating GH (P < 0.01) and pituitary PRL concentrations (P < 0.0001) but lower pituitary GH concentration (P < 0.0001) relative to those in rats receiving the LP and SP diets. Pearson correlation test performed on composed data across treatments showed that several circulating AAs were correlated with circulating and tissue concentrations of IGF-I, GH, and PRL. Multiple linear regression analyses identified Leu, Gln, Ala, Gly, and Arg as the main AAs associated with hormone responses (R2 = 0.37 ~ 0.80; P < 0.05). Rats fed the LP and HP diets had greater Igf1 and Ghr gene expression in skeletal muscle than those fed the SP diets (P < 0.01). However, LP treatment decreased Prlr mRNA abundance in mammary glands as compared with the SP and HP treatments (P < 0.05). The HP diets increased AA transporter expression (P < 0.01) but decreased mammalian target of rapamycin (P < 0.05) and 70 kDa ribosomal protein S6 kinase 1 (P < 0.01) phosphorylation in mammary glands as compared with the LP and SP diets. The results of the present study suggested that several circulating AAs mediated the effects of dietary protein supply on concentrations of IGF-I, GH, and PRL, which in turn altered the metabolism status in peripheral tissues including the lactating mammary glands.

中文翻译：

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氨基酸和激素的相互作用调节喂食不同蛋白质含量饮食的哺乳大鼠的生长和乳蛋白合成之间的平衡

胰岛素样生长因子-I (IGF-I)、生长激素 (GH) 和催乳素 (PRL) 在乳蛋白合成中起重要作用，据报道它们的血浆浓度受膳食蛋白质摄入量的影响。为了研究循环氨基酸 (AA) 和这些激素浓度之间的关系，18 只 14 周大的 Wistar 大鼠被分配到低（LP；9% 蛋白质）、标准（SP；21% 蛋白质）或高蛋白组（ HP；35% 蛋白质）从分娩到哺乳第 15 天的饮食。在治疗结束时收集血浆、肝脏、垂体、骨骼肌和乳腺样本。循环和肝脏 IGF-I 浓度随着膳食蛋白质浓度的升高呈线性增加 (P < 0.0001)。接受 HP 饮食的大鼠具有较高的循环 GH (P < 0.01) 和垂体 PRL 浓度 (P & lt; 0.0001) 但与接受 LP 和 SP 饮食的大鼠相比，垂体 GH 浓度较低 (P < 0.0001)。对跨治疗组合数据进行的 Pearson 相关性检验表明，几种循环 AA 与 IGF-I、GH 和 PRL 的循环和组织浓度相关。多元线性回归分析确定 Leu、Gln、Ala、Gly 和 Arg 是与激素反应相关的主要氨基酸（R2 = 0.37 ~ 0.80；P < 0.05）。喂食 LP 和 HP 饮食的大鼠在骨骼肌中的 Igf1 和 Ghr 基因表达高于喂食 SP 饮食的大鼠（P < 0.01）。然而，与 SP 和 HP 处理相比，LP 处理降低了乳腺中 Prlr mRNA 的丰度（P < 0.05）。HP 饮食增加了 AA 转运蛋白的表达（P < 0. 01) 但与 LP 和 SP 饮食相比，哺乳动物的雷帕霉素靶点 (P < 0.05) 和 70 kDa 核糖体蛋白 S6 激酶 1 (P < 0.01) 在乳腺中的磷酸化降低。本研究的结果表明，几种循环 AA 介导了膳食蛋白质供应对 IGF-I、GH 和 PRL 浓度的影响，进而改变了包括泌乳乳腺在内的外周组织的代谢状态。