Malandro M S, Beveridge M J, Kilberg M S, Novak D A
Department of Biochemistry, University of Florida College of Medicine, Gainesville 32610, USA.
Am J Physiol. 1996 Jul;271(1 Pt 1):C295-303. doi: 10.1152/ajpcell.1996.271.1.C295.
Given the central role of the placenta in nutrient transport to the fetus, one might propose that maternal nutrition would have a regulatory effect on this nutrient delivery. We have examined the effect of a low-protein adequate-calorie diet on specific amino acid transport processes by the rat placenta. Maternal weight, fetal weight, and placental weight were all significantly reduced in dams fed a low-protein (5% casein), isocaloric diet when compared with dams pair-fed a control (20% casein) diet. Even though maternal serum amino acid levels were maintained in the low-protein animals, fetomaternal serum amino acid ratios were significantly reduced, suggesting a reduction in nutrient transfer to the fetus. Apical and basal membrane vesicles were isolated from the placental trophoblast and were used to examine the amino acid transport capacity of both maternal-facing and fetal-facing membranes, respectively. Na+-dependent neutral amino acid transport mediated by system A was decreased in both membrane preparations, while transport mediated by system ASC was unaffected. The Na+-dependent anionic amino acid uptake by system X(-)AG (EAAC1) was reduced on the basal membrane, while the Na+-independent component was similar between the low-protein and control diet-fed dams. Cationic amino acid uptake was also reduced on both membrane surfaces. A decreased steady-state mRNA content for EAAC1 and CAT1 (system y+) suggests that reduced synthesis of the transporter proteins is responsible for the decrease in transport activity. Taken together, these data support the hypothesis that maternal protein malnutrition affects nutrient delivery to the fetus by downregulation of specific amino acid transport proteins.
鉴于胎盘在向胎儿转运营养物质方面的核心作用，有人可能会提出母体营养会对这种营养物质输送产生调节作用。我们研究了低蛋白充足热量饮食对大鼠胎盘特定氨基酸转运过程的影响。与成对喂食对照（20%酪蛋白）饮食的母鼠相比，喂食低蛋白（5%酪蛋白）等热量饮食的母鼠的母体体重、胎儿体重和胎盘重量均显著降低。尽管低蛋白动物的母体血清氨基酸水平保持正常，但母胎血清氨基酸比值显著降低，这表明向胎儿的营养物质转运减少。从胎盘滋养层分离出顶端和基底膜囊泡，分别用于检测面向母体和面向胎儿的膜的氨基酸转运能力。两种膜制剂中，由A系统介导的Na+依赖性中性氨基酸转运均降低，而由ASC系统介导的转运不受影响。基底膜上由X(-)AG（EAAC1）系统介导的Na+依赖性阴离子氨基酸摄取减少，而低蛋白饮食组和对照饮食组母鼠之间的非Na+依赖性成分相似。两个膜表面的阳离子氨基酸摄取也减少。EAAC1和CAT1（y+系统）的稳态mRNA含量降低，这表明转运蛋白合成减少是转运活性降低的原因。综上所述，这些数据支持以下假设：母体蛋白质营养不良通过下调特定氨基酸转运蛋白来影响向胎儿的营养物质输送。
