Uptake and transport by the ovine placenta of neutral nonmetabolizable amino acids with different transport system affinities

Improving pregnancy outcomes in humans through studies in sheep

Experimental studies that are relevant to human pregnancy rely on the selection of appropriate animal models as an important element in experimental design. Consideration of the strengths and weaknesses of any animal model of human disease is fundamental to effective and meaningful translation of preclinical research. Studies in sheep have made significant contributions to our understanding of the normal and abnormal development of the fetus. As a model of human pregnancy, studies in sheep have enabled scientists and clinicians to answer questions about the etiology and treatment of poor maternal, placental, and fetal health and to provide an evidence base for translation of interventions to the clinic. The aim of this review is to highlight the advances in perinatal human medicine that have been achieved following translation of research using the pregnant sheep and fetus.

Regulation of placental nutrient transport and implications for fetal growth

Fetal macronutrient requirements for oxidative metabolism and growth are met by placental transport of glucose, amino acids, and, to a lesser extent that varies with species, fatty acids. It is becoming possible to relate the maternal–fetal transport kinetics of these molecules in vivo to the expression and distribution of specific transporters among placental cell types and subcellular membrane fractions. This is most true for glucose transport, although apparent inconsistencies among data on the roles and relative importance of the predominant placenta glucose transporters, GLUT-1 and GLUT-3, remain to be resolved. The quantity of macronutrients transferred to the fetus from the maternal bloodstream is greatly influenced by placental metabolism, which results in net consumption of large amounts of glucose and, to a lesser extent, amino acids. The pattern of fetal nutrient supply is also altered considerably by placental conversion of glucose to lactate and, in some species, fructose, and extensive transamination of amino acids. Placental capacity for transport of glucose and amino acids increases with fetal demand as gestation advances through expansion of the exchange surface area and increased expression of specific transport molecules. In late pregnancy, transport capacity is closely related to placental size and can be modified by maternal nutrition. Preliminary evidence suggests that placental expression and function of specific transport proteins are influenced by extracellular concentrations of nutrients and endocrine factors, but, in general, the humoral regulation of placental capacity for nutrient transport is poorly understood. Consequences of normal and abnormal development of placental transport functions for fetal growth, especially during late gestation, and, possibly, for fetal programming of postnatal disorders, are discussed.

The transplacental transport of essential amino acids in uncomplicated human pregnancies

OBJECTIVE

The purpose of this study was to assess the placental transport of the essential amino acids (EAAs) in normal pregnancies.

STUDY DESIGN

Nine ((13)C or (2)H) EAAs were infused simultaneously as a bolus into the maternal circulation of 12 patients with uncomplicated pregnancy before cesarean delivery. Maternal samples were collected before and after the bolus; umbilical blood was collected at delivery. The fetal/maternal molar percent enrichment for each EAA was calculated for both the umbilical vein and artery. Plasma amino acids enrichments were analyzed by gas chromatography mass spectrometry and concentrations by high performance liquid chromatography. Data were analyzed with paired and unpaired t-test.

RESULTS

The umbilical arterial enrichments were significantly lower than the venous. Fetal/maternal ratios for leucine, isoleucine, methionine, and phenylalanine were > 0.80, with no significant differences among their molar percent enrichment ratios, whereas fetal/maternal ratios of the other 5 EAAs were significantly lower (< 0.60).

CONCLUSION

The EAAs showed significant umbilical uptake and striking differences in their transport rates in vivo.

The SNAT4 isoform of the system A amino acid transporter is functional in human placental microvillous plasma membrane

Placental system A activity is important for the supply of neutral amino acids needed for fetal growth. There are three system A isoforms: SNAT1, SNAT2 and SNAT4, but the contribution of each to system A-mediated transport is unknown. Here, we have used immunohistochemistry to demonstrate that all three isoforms are present in the syncytiotrophoblast suggesting each plays a role in amino acid transport across the placenta. We next tested the hypothesis that the SNAT4 isoform is functional in microvillous plasma membrane vesicles (MVM) from normal human placenta using a method which exploits the unique property of SNAT4 to transport both cationic amino acids as well as the system A-specific substrate MeAIB. The data show that SNAT4 contribution to system A-specific amino acid transport across MVM is higher in first trimester placenta compared to term (approx. 70% and 33%, respectively, P < 0.01). Further experiments performed under more physiological conditions using intact placental villous fragments suggest a contribution of SNAT4 to system A activity in first trimester placenta but minimal contribution at term. In agreement, Western blotting revealed that SNAT4 protein expression is higher in first trimester MVM compared to term (P < 0.05). This study provides the first evidence of SNAT4 activity in human placenta and demonstrates the contribution of SNAT4 to system A-mediated transport decreases between first trimester and term: our data lead us to speculate that at later stages of gestation SNAT1 and/or SNAT2 are more important for the supply of amino acids required for normal fetal growth.

SNAT4 isoform of system A amino acid transporter is expressed in human placenta

The system A amino acid transporter is encoded by three members of the Slc38 gene family, giving rise to three subtypes: Na+-coupled neutral amino acid transporter (SNAT)1, SNAT2, and SNAT4. SNAT2 is expressed ubiquitously in mammalian tissues; SNAT1 is predominantly expressed in heart, brain, and placenta; and SNAT4 is reported to be expressed solely by the liver. In the placenta, system A has an essential role in the supply of neutral amino acids needed for fetal growth. In the present study, we examined expression and localization of SNAT1, SNAT2, and SNAT4 in human placenta during gestation. Real-time quantitative PCR was used to examine steady-state levels of system A subtype mRNA in early (6-10 wk) and late (10-13 wk) first-trimester and full-term (38-40 wk) placentas. We detected mRNA for all three isoforms from early gestation onward. There were no differences in SNAT1 and SNAT2 mRNA expression with gestation. However, SNAT4 mRNA expression was significantly higher early in the first trimester compared with the full-term placenta (P < 0.01). We next investigated SNAT4 protein expression in human placenta. In contrast to the observation for gene expression, Western blot analysis revealed that SNAT4 protein expression was significantly higher at term compared with the first trimester (P < 0.05). Immunohistochemistry and Western blot analysis showed that SNAT4 is localized to the microvillous and basal plasma membranes of the syncytiotrophoblast, suggesting a role for this isoform of system A in amino acid transport across the placenta. This study therefore provides the first evidence of SNAT4 mRNA and protein expression in the human placenta, both at the first trimester and at full term.

Placental uptake and transport of ACP, a neutral nonmetabolizable amino acid, in an ovine model of fetal growth restriction

Reductions in fetal plasma concentrations of certain amino acids and reduced amino acid transport in vesicle studies suggest impaired placental amino acid transport in human fetal growth restriction (FGR). In the present study, we tested the hypothesis of an impairment in amino acid transport in the ovine model of hyperthermia-induced FGR by determining transplacental and placental retention and total placental clearance of a branched-chain amino acid (BCAA) analog, the nonmetabolizable neutral amino acid aminocyclopentane-1-carboxylic acid (ACP), in singleton control (C) and FGR pregnancies at 135 days gestation age (dGA; term 147 dGA). At study, based on the severity of the placental dysfunction, FGR fetuses were allocated to severe (sFGR, n = 6) and moderate FGR (mFGR, n = 4) groups. Fetal (C, 3,801.91 +/- 156.83; mFGR, 2,911.33 +/- 181.35; sFGR, 1,795.99 +/- 238.85 g; P < 0.05) and placental weights (C, 414.38 +/- 38.35; mFGR, 306.23 +/- 32.41; sFGR, 165.64 +/- 28.25 g; P < 0.05) were reduced. Transplacental and total placental clearances of ACP per 100 g placenta were significantly reduced in the sFGR but not in the mFGR group, whereas placental retention clearances were unaltered. These data indicate that both entry of ACP into the placenta and movement from the placenta into fetal circulation are impaired in severe ovine FGR and support the hypothesis of impaired placental BCAA transport in severe human FGR.

Reciprocal inhibition of umbilical uptake within groups of amino acids

Eight pregnant sheep were infused with two amino acid mixtures of different composition: essential amino acids only and the essentials plus some of the nonessentials. Uterine and umbilical uptakes of amino acids were measured before and during infusion. For most of the amino acids, the infusion increased both maternal plasma concentration and umbilical uptake. However, depending on the infusate composition, the increase in maternal concentration of some amino acids was associated with no change or a significant reduction in umbilical uptake. Data were pooled from this and other, similar studies to test the hypothesis that umbilical uptake of several amino acids can be inhibited by coinfused amino acids. The test consisted of fitting the data, by means of multiple regression analysis, to the linear transformation of a saturation kinetics equation in which uptake is assumed to depend on maternal arterial concentrations. The analysis showed significant inhibitory effects within the neutral essential amino acids group and within the lysine-arginine group, with no demonstrable interaction between the two groups. Uterine uptakes did not show clear evidence of saturability and inhibitory interactions, suggesting a large transport capacity and low transporter affinity on the maternal surface of the trophoblast. We conclude that the transport of any given amino acid from placenta to fetus is a function of both its own maternal concentration and the maternal concentration of inhibitory amino acids.

The effect of an elevated maternal lysine concentration on placental lysine transport in pregnant sheep

OBJECTIVES

In a previous study, the coinfusion into the maternal circulation of lysine and several other amino acids failed to increase significantly lysine umbilical uptake. The purpose of this study was to determine whether umbilical lysine uptake can be increased by infusing a lysine solution that does not contain any other amino acid.

STUDY DESIGN

Six late-gestation ewes were studied on 2 consecutive days. Samples were collected in both the control (first day) and experimental (second day) periods simultaneously from the maternal artery, uterine vein, fetal artery, and umbilical vein. In the control period, L-[1-(13)C] lysine was infused into the maternal circulation. During the experimental period, both L-[1-(13)C] lysine and L-(12)C lysine were infused to increase maternal lysine concentration. Uterine and umbilical blood flows were measured by the steady state diffusion technique. Uterine and umbilical uptake of lysine and of alpha-aminoaminoadipic acid (AAD, a biproduct of lysine oxidation) were calculated.

RESULTS

In response to a 2.7-fold increase in maternal lysine concentration (P<.001), fetal lysine concentration increased approximately 70% (P<.05) and umbilical uptake 50% (P<.05). In the experimental period, there was a significant (P<.05) placental uptake of fetal AAD, and the fetal/maternal plasma (13)C-lysine-specific activity ratio increased from 0.221+/-0.026 to 0.294+/-0.029 (P<.05). In response to the increase in maternal lysine concentration, the maternal and fetal concentrations of several other amino acids were significantly decreased.

CONCLUSION

This study establishes that the umbilical uptake of lysine can be increased by infusing lysine in the maternal circulation. However, the lysine infusion is associated with a decrease in the maternal concentration and umbilical uptake of other essential amino acids. These data, compared with the results of previous studies, indicate that attempts to increase the fetal uptake of an amino acid via maternal infusion may decrease the uptake of other amino acids by decreasing their maternal concentration and by inhibition of placental transport.

Nutrition in early life. How important is it?

While few would argue the importance of nutrition during adult life, temporary excess or deficiency has typically been thought to be of little long-term consequence. Recent data, summarized above, suggests that this may not be the case during in utero life, when alterations in the quantity or quality of nutrients provided may have life-long consequences. Perhaps even more surprisingly, decisions made in the neonatal period, such as whether to breastfeed or bottle feed, may have impacts on later health that, while small individually, have huge public health implications. Clarification of the links between adult health and fetal/neonatal nutrition are clearly required. Prospective studies, though difficult because of the time involved, will play a key role in this process, as will more basic research on the mechanisms underlying both normal and pathologic fetal development.

In vivo characteristics of placental amino acid transport and metabolism in ovine pregnancy--a review

The placental transport of amino acids which is nutritionally important is the net entry rate into the fetal circulation (the umbilical uptake). This entry rate is a function of transport across cell membranes, the effect of competition among amino acids for transport, particularly across the fetal surface of the trophoblast, and their metabolism and interconversion within the placenta. The result of these different interactive fluxes is that the relationship between maternal concentration and fetal supply of an amino acid differs for each amino acid. For some amino acids there are relatively large bidirectional fluxes at both the fetal and maternal surfaces of the placenta. These fluxes can be measured in vivo utilizing stable isotope methodology. There is an important interorgan exchange of amino acids between the placenta and fetal liver. This exchange is, at least in part, a function of the absence of gluconeogenesis in the fetal liver. Both glutamate and serine, which are released from the fetal liver, are taken up by the placenta from the fetal circulation and metabolized within the placenta.

Physiological importance of system A-mediated amino acid transport to rat fetal development

Fetal growth and development are dependent on the delivery of amino acids from maternal amino acid pools to the fetal blood. This is accomplished via transfer across the apical and basal plasma membrane of the placental syncytiotrophoblast. The aim of this study was to determine whether inhibition of system A (amino acid transporter) was associated with a decrease in fetal weight in the rat. System A is a ubiquitous Na(+)-dependent amino acid transporter that actively transports small zwitterionic amino acids. In brief, system A was inhibited by infusing a nonmetabolizable synthetic amino acid analog, 2-(methylamino)isobutyric acid from days 7-20 of gestation. On day 20, the rats were killed and tissues (maternal liver, fetuses, and placentas) were collected for analysis. The degree of system A inhibition was determined, as was the impact of said inhibition on fetal and maternal weights, system A-mediated placental transport, and placental system A-mediated transporter expression. Our results suggest that when system A is inhibited, fetal weight is diminished [control group: -3.55 +/- 0.04 g (n = 113), experimental group: -3.29 +/- 0.04 g (n = 128)], implying an integral role for system A transport in fetal growth and development in the rat.

Placental transport of leucine, phenylalanine, glycine, and proline in intrauterine growth-restricted pregnancies

L-[1-13C]Leucine, [1-13C]glycine, L-[1-13C]phenylalanine, and L-[1-13C]proline were infused as a bolus into the maternal circulation of seven appropriate for gestational age at 30.3 +/- 3.0 wk and 7 intrauterine growth-restricted pregnancies at 26.5 +/- 1.0 wk gestation to investigate placental transport in vivo. Umbilical venous samples were obtained at the time of in utero fetal blood sampling at 450 +/- 74 sec from the bolus injection. In normal pregnancies the fetal/maternal (F/M) enrichment ratios for leucine (0.76 +/- 0.06) and phenylalanine (0.77 +/- 0.06) were higher (P < 0.01) than the F/M ratios for glycine (0.18 +/- 0.04) and proline (0.22 +/- 0.02). This suggests that these two essential amino acids rapidly cross the placenta in vivo. Compared with the essentials, both glycine and proline had significantly lower F/M enrichment ratios, which were not different from each other. The results support the hypothesis that amino acids with high affinity for exchange transporters cross the placenta most rapidly. In intrauterine growth-restricted pregnancies, the F/M enrichment ratio was significantly lower (P < 0.01) for L-[1-13C]leucine (0.76 +/- 0.06 vs. 0.48 +/- 0.07) and for L-[1-13C]phenylalanine (0.77 +/- 0.06 vs. 0.46 +/- 0.07) compared with appropriate for gestational age pregnancies reflecting impaired transplacental flux. The F/M enrichment ratio did not differ for [1-13C]glycine (0.18 +/- 0.04 vs. 0.17 +/- 0.03), and L-[1-13C]proline (0.22 +/- 0.02 vs. 0.18 +/- 0.04).

Effects of branched-chain amino acids on placental amino acid transfer and insulin and glucagon release in the ovine fetus

OBJECTIVE

Competition for placental amino acid transporters can affect the fetal supply of amino acids. Specifically, the branched-chain amino acids-isoleucine, leucine, and valine-may inhibit the transfer of other amino acids. This study was undertaken to determine the effect of branched-chain amino acids on the umbilical uptake of amino acids.

STUDY DESIGN

Six late-gestation ewes were infused sequentially for 2 hours with 3 different mixtures of amino acids: (1) one that was comparable to commercial parenteral nutrition preparations, (2) the same solution without branched-chain amino acids, and (3) branched-chain amino acids alone. Maternal and fetal blood samples were collected simultaneously for the determination of uterine and umbilical uptake values of amino acids, and for concentrations of arterial insulin, glucagon, glucose, and lactate before (control) and during (experimental) infusion.

RESULTS

Umbilical uptake of branched-chain amino acids increased significantly when they were present in the infusates. The fetal uptake of several other amino acids could be increased by increasing their maternal concentrations. Inhibition of umbilical uptake by branched-chain amino acids could be shown for threonine and methionine. The infusion of branched-chain amino acids alone did not affect maternal and fetal insulin or glucagon concentrations.

CONCLUSIONS

In late-gestation sheep, an increase in maternal plasma concentration of branched-chain amino acids led to increased branched-chain amino acid umbilical uptake, but branched-chain amino acids can also inhibit the transport of some amino acids to the fetus. Changes in fetal plasma concentration and uptake of branched-chain amino acid appear to have no significant effect on fetal insulin or glucagon.

Protein/amino acid metabolism and nutrition in very low birth weight infants

A large number of studies in recent years have described protein and nitrogen metabolism in the neonate. However, the majority of these data are difficult to interpret because of a number of confounding variables, particularly in very low birth weight (VLBW) infants. In contrast, application of state-of-the-art tracer isotopic and molecular biology methods in isolated cell system and whole animals has resulted in major advances in our understanding of the regulation of protein breakdown, synthesis, and protein accretion. The following workshop summary reviews the recent developments in basic physiology of protein metabolism in cellular and animal models in relation to human preterm infants, and identifies the important areas toward which future basic and clinical research should be directed to provide for optimal nitrogen accretion and growth of the VLBW infant.

Placental transport and metabolism of amino acids

This review examines the placental transport and metabolism of amino acids, with a special emphasis on unifying and interpreting in-vivo and in-vitro data. For a variety of technical reasons, in-vivo studies, which quantify placental amino-acid fluxes and metabolism, have been relatively limited, in comparison to in-vitro studies using various placental preparations. Following an introduction to placental amino-acid uptake and transfer to the fetus, the review attempts to reconcile in-vitro placental transport data with in-vivo placental data. Data are discussed with reference to the measured delivery rates of amino acids into the fetal circulation and the contribution of placental metabolism to this rate for many amino acids. The importance of exchange transporters in determining efflux from the placenta into the fetal circulation is presented with special reference to in-vivo studies of non-metabolizable and essential amino acids. The data which illustrate the interconversion and nitrogen exchange of three groups of amino acids, glutamine-glutamate, BCAAs and serine-glycine, within the placenta are discussed in terms of the potential role such pathways may serve for other placenta functions. The review also presents comparisons of the sheep and human placentae in terms of their in-vivo amino-acid transport rates.

An in vivo study of ovine placental transport of essential amino acids

Under normal physiological conditions, essential amino acids (EA) are transported from mother to fetus at different rates. The mechanisms underlying these differences include the expression of several amino acid transport systems in the placenta and the regulation of EA concentrations in maternal and fetal plasma. To study the relation of EA transplacental flux to maternal plasma concentration, isotopes of EA were injected into the circulation of pregnant ewes. Measurements of concentration and molar enrichment in maternal and fetal plasma and of umbilical plasma flow were used to calculate the ratio of transplacental pulse flux to maternal concentration (clearance) for each EA. Five EA (Met, Phe, Leu, Ile, and Val) had relatively high and similar clearances and were followed, in order of decreasing clearance, by Trp, Thr, His, and Lys. The five high-clearance EA showed strong correlation (r(2) = 0.98) between the pulse flux and maternal concentration. The study suggests that five of the nine EA have similar affinity for a rate-limiting placental transport system that mediates rapid flux from mother to fetus, and that differences in transport rates within this group of EA are determined primarily by differences in maternal plasma concentration.

Regulation of uterine and umbilical amino acid uptakes by maternal amino acid concentrations

We tested the hypothesis that decreased fetal amino acid (AA) supply, produced by maternal hypoaminoacidemia (low AA) during hyperglycemia (HG), is reversible with maternal AA infusion and regulates fetal insulin concentration ([I]). We measured net uterine and umbilical AA uptakes during maternal HG/low AA concentration ([AA]) and after maternal intravenous infusion of a mixed AA solution. After 5 days HG, all maternal [AA] except glycine were decreased >50%, particularly essential [AA] (P < 0.00005). Most fetal [AA] also were decreased, especially branched-chain AA (P < 0.001). Maternal AA infusion increased net uterine uptakes of Val, Leu, Ile, Met, and Ser and net umbilical uptakes of Val, Leu, Ile, Met, Phe, and Arg but did not change net uteroplacental uptake of any AA. Fetal [I] increased 55 +/- 14%, P < 0.001, with correction of fetal [AA], despite the lack of change in fetal glucose concentration. Thus generalized maternal hypoaminoacidemia decreases uterine and umbilical uptakes of primarily the essential AA and decreases fetal branched-chain [AA]. These changes are reversed with correction of maternal [AA], which also increases fetal [I].

Glucose metabolism is elevated and vascular resistance and maternofetal transfer is normal in perfused placental cotyledons from severely growth-restricted fetuses

We hypothesized that placental resistance was elevated and transfer reduced in cotyledons from intrauterine growth-restricted (IUGR) fetuses. We perfused 10 cotyledons from term, normally grown fetuses, six from preterm, normally grown fetuses with normal umbilical arterial end-diastolic velocities (EDV), and six from preterm IUGR fetuses (<3rd centile) with absent or reversed umbilical arterial EDV. Perfused cotyledons were pressure-fixed, and villi were observed by scanning electron microscopy. The groups did not differ in fetoplacental resistance at baseline; neither did they differ in the change in resistance that followed the administration of nitroglycerin or angiotensin II. The increase in resistance during hypoxia was similar in the two preterm groups but greater in the term than in the preterm normally grown group (p < 0.05). Groups did not differ in net maternofetal transfer of oxygen or glucose, or in clearance of aminoisobutyric acid or antipyrine. However, glucose consumption was doubled in cotyledons of preterm IUGR versus preterm normally grown fetuses (p < 0.05). Terminal villi of perfused cotyledons from preterm IUGR fetuses displayed less terminal villous branching and budding than preterm controls, as anticipated from previous work. IUGR fetuses with absent or reversed umbilical arterial EDV in vivo may have high placental resistance due to a vasoconstrictive rather than anatomic abnormality and an elevated placental glucose consumption that may impair glucose transfer.

Fetal supply of amino acids and amino nitrogen after maternal infusion of amino acids in pregnant sheep

OBJECTIVE

The objective of the study was to determine whether a prolonged maternal infusion of amino acids would increase the umbilical uptake of amino acids and uteroplacental ammonia production.

STUDY DESIGN

Six pregnant sheep (134.5 2.3 days after conception) were infused for 12 hours overnight with an amino acid solution. Uterine and umbilical blood flows were measured with the ethanol steady-state diffusion technique before (control) and during (experimental) infusion. Plasma amino acid and whole-blood ammonia concentrations were measured.

RESULTS

After infusion, despite an increase in maternal arterial amino acid concentration, umbilical uptakes increased significantly only for branched-chain amino acids. Fetal ammonia concentrations and uteroplacental ammonia production increased moderately. Fetal nitrogen supply did not increase. Uterine nitrogen uptake represented 36% of the maternal nitrogen intake in the control period and 14% in the experimental period.

CONCLUSION

Prolonged maternal infusion of an amino acid solution was a relatively ineffective method of increasing fetal amino acid supply.