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

Dairy cow parity affects relationships among nutritional parameters in the blood of dams, umbilical cords, and calves and placental development at calving

Heifer growth and milk production in lactating cows may diminish the nutrient supply to the fetus. This study aimed to analyze the characteristics of the nutrient supply to the fetus in primiparous and multiparous cows. We investigated maternal, umbilical cord, and calf blood glucose and amino acid levels, as well as placental development in 28 primiparous (PP) and 30 multiparous (MP) Holstein cows. Although the total cotyledonary weight and surface area showed no significant differences, the MP group exhibited larger individual cotyledons (P < 0.01) and fewer medium-sized cotyledons (P < 0.05). Within the PP group, total cotyledonary weight and surface area positively correlated with blood glucose (r = 0.71-0.77; P < 0.01) and total essential amino acid (r = 0.55; P < 0.05) concentrations in the umbilical veins. However, no significant correlation was observed in the MP group. Blood glucose and amino acid concentrations in the umbilical vein, umbilical artery, and calf were significantly lower in the MP group (P < 0.05), although no difference was observed in the dams between the groups. In conclusion, the nutrient status of primiparous cows can alter fetal nutrient supply. Moreover, multiparous cows have larger individual cotyledons as an adaptive response to increased milk production during pregnancy. However, this adaptive response in multiparous cows did not completely restore nutrient supply to the fetus to the same extent as that in primiparous cows. Therefore, the nutritional management of multiparous cows during pregnancy must be reconsidered.

Maternal-fetal cross-talk via the placenta: influence on offspring development and metabolism

Compelling epidemiological and animal experimental data demonstrate that cardiometabolic and neuropsychiatric diseases originate in a suboptimal intrauterine environment. Here, we review evidence suggesting that altered placental function may, at least in part, mediate the link between the maternal environment and changes in fetal growth and development. Emerging evidence indicates that the placenta controls the development and function of several fetal tissues through nutrient sensing, modulation of trophoblast nutrient transporters and by altering the number and cargo of released extracellular vesicles. In this Review, we discuss the development and functions of the maternal-placental-fetal interface (in humans and mice) and how cross-talk between these compartments may be a mechanism for in utero programming, focusing on mechanistic target of rapamycin (mTOR), adiponectin and O-GlcNac transferase (OGT) signaling. We also discuss how maternal diet and stress influences fetal development and metabolism and how fetal growth restriction can result in susceptibility to developing chronic disease later in life. Finally, we speculate how interventions targeting placental function may offer unprecedented opportunities to prevent cardiometabolic disease in future generations.

Nutritional parameters in the blood of dams during late gestation and immediately after calving, in the umbilical vein at calving, and in the blood of calves immediately following birth in Holstein heifers pregnant with either Holstein or beef breed fetuses

Dairy cattle management lacks consideration of fetal breed, the effect of which on fetal growth and nutrition are unclear. We investigated blood parameters in 12 late-pregnant Holstein heifers with similar (Holstein, n = 5) or different (Japanese Black [n = 4] or F1 cross [n = 3]; Holstein × Japanese Black) fetus breeds and in their umbilical cords and calves. Samples were obtained from dams 1 week before calving (-1 week) and immediately after calving, from the umbilical vein at calving, and from calves immediately after birth. Dams with beef fetuses had higher serum glucose levels (-1 week; p < .05) than those with Holstein fetuses. Plasma total amino acid, total essential amino acid, total nonessential amino acid, and other amino acid concentrations were lower in the umbilical veins of dams with calves of the beef breeds than in those of the Holstein breeds (p < .05). Furthermore, serum glucose and plasma amino acid levels were lower in the beef calves than in the Holstein calves (p < .05). Overall, nutrient supply from dams to beef fetuses was lower than that to Holstein fetuses. Our findings may facilitate feeding management of dairy cattle pregnant with beef breeds for appropriate fetal growth and nutrition.

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.

Does maternal-fetal transfer of creatine occur in pregnant sheep?

Our aim was to determine the disposition of creatine in ovine pregnancy and whether creatine is transferred across the placenta from mother to fetus. Pregnant ewes received either 1) a continuous intravenous infusion of creatine monohydrate or saline from 122 to 131 days gestation, with maternal and fetal arterial blood and amniotic fluid samples collected daily for creatine analysis and fetal tissues collected at necropsy at 133 days for analysis of creatine content, or 2) a single systemic bolus injection of [¹³C]creatine monohydrate at 130 days of gestation, with maternal and fetal arterial blood, uterine vein blood, and amniotic fluid samples collected before and for 4 h after injection and analyzed for creatine, creatine isotopic enrichment, and guanidinoacetic acid (GAA; precursor of creatine) concentrations. Presence of the creatine transporter-1 (SLC6A8) and l-arginine:glycine amidinotransferase (AGAT; the enzyme synthesizing GAA) proteins were determined by Western blots of placental cotyledons. The 10-day creatine infusion increased maternal plasma creatine concentration three- to fourfold (P < 0.05) without significantly changing fetal arterial, amniotic fluid, fetal tissues, or placental creatine content. Maternal arterial ¹³C enrichment was increased (P < 0.05) after bolus [¹³C]creatine injection without change of fetal arterial ¹³C enrichment. SLC6A8 and AGAT proteins were identified in placental cotyledons, and GAA concentration was significantly higher in uterine vein than maternal artery plasma. Despite the presence of SLC6A8 protein in cotyledons, these results suggest that creatine is not transferred from mother to fetus in near-term sheep and that the ovine utero-placental unit releases GAA into the maternal circulation.

Urea production and arginine metabolism are reduced in the growth restricted ovine foetus

Urea production may be impaired in intrauterine growth restriction (IUGR), increasing the risk of toxic hyperammonaemia after birth. Arginine supplementation stimulates urea production, but its effects in IUGR are unknown. We aimed to determine the effects of IUGR and arginine supplementation on urea production and arginine metabolism in the ovine foetus. Pregnant ewes and their foetuses were catheterised at 110 days of gestation and randomly assigned to control or IUGR groups. IUGR was induced by placental embolisation. At days 120 and 126 of gestation, foetal urea production was determined from [14C]-urea kinetics and arginine metabolism was determined from the appearance of radioactive metabolites from [3H]-arginine, both at baseline and in response to arginine or an isonitrogenous mixed amino acid supplementation. Urea production decreased with gestational age in the embolised animals (13.9 ±  3.1 to 11.2 ±  3.0 μmol/kg per min, P ≤ 0.05) but not in the controls (13.3 ±  3.5 to 14.8 ±  6.0 μmol/kg per min). Arginine supplementation increased urea production in both groups, but only at 126 days of gestation (control: 15.0 ±  8.5 to 17.0 ±  9.4 μmol/kg per min; embolised: 11.7 ±  3.1 to 14.3 ±  3.1 μmol/kg per min, P ≤ 0.05). Embolisation reduced foetal arginine concentrations by 20% ( P ≤ 0.05) while foetal arginine consumption was reduced by 27% ( P ≤ 0.05). The proportions of plasma citrulline and hydroxyproline derived from arginine were reduced in the embolised animals. These data suggest that foetal urea production and arginine metabolism are perturbed in late gestation after placental embolisation.

Placental amino acids transport in intrauterine growth restriction

The placenta represents a key organ for fetal growth as it acts as an interface between mother and fetus, regulating the fetal-maternal exchange of nutrients, gases, and waste products. During pregnancy, amino acids represent one of the major nutrients for fetal life, and both maternal and fetal concentrations are significantly different in pregnancies with intrauterine growth restriction when compared to uncomplicated pregnancies. The transport of amino acids across the placenta is a complex process that includes the influx of neutral, anionic, and cationic amino acids across the microvilluos plasma membrane of the syncytiotrophoblast, the passage through the cytoplasm of the trophoblasts, and the transfer outside the trophoblasts across the basal membrane into the fetal circulation. In this paper, we review the transport mechanisms of amino acids across the placenta in normal pregnancies and in pregnancies complicated by intrauterine growth restriction.

Recent advances in our understanding of protein and amino acid metabolism in the human fetus

PURPOSE OF REVIEW

Premature infants often suffer from suboptimal outcome, at least partially due to suboptimal nutrition. Gaining insight into human fetal amino acid metabolism might ultimately lead to an improved nutritional strategy for prematurely born infants. Our aim was, therefore, to discuss recent findings with regard to human fetal amino acid metabolism.

RECENT FINDINGS

Human fetal protein and amino acid metabolism can be studied in vivo using stable isotope techniques. To date, however, only a few studies employing these techniques have been performed. For one, it was shown in vivo that essential amino acids are transported at different rates across the human placenta. In addition, tyrosine appears not to be a conditionally essential amino acid in the fetus at term, as phenylalanine is hydroxylated into tyrosine at considerable rates. Furthermore, albumin is synthesized at very high rates at two-thirds of gestation; higher than prematurely born infants do at a neonatal intensive care unit. This could indicate that postnatal nutrition of very immature infants can be improved.

SUMMARY

Although technically challenging, more studies regarding human fetal amino acid metabolism should be performed. Premature infants could then benefit from this knowledge from new nutritional strategies.

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.

Immunosuppression routed via the kynurenine pathway: a biochemical and pathophysiologic approach

In the past years, it has been shown that kynurenines pathway is a regulator of both the innate and the adaptive immune responses. Particularly, the initial enzyme of this pathway, indoleamine 2,3-dioxygenase (IDO), is implicated in maintaining tolerance during pregnancy, and also can be expressed in tumors to avoid the immune attack. In this chapter, we will describe how the kynurenine pathway affects the immune system with important implications both in physiology and in pathology. The incorrect activation or blockade suppressive properties of the kynurenine pathway are also implicated in a number of other diseases such as AIDS or autoimmune diseases.

Placental transport: a function of permeability and perfusion

Studies in ovine fetus and placenta have pointed to an interaction between the fetal liver and the placenta. The supply of amino acids and carbohydrates depends on this interaction. These studies have led to clinical studies in normal and high-risk pregnancies. The objective of the present review was to compare changes in fetal circulation, in terms of both velocimetry and actual blood flow measurements, and to couple such data with data on the placental transport of amino acids. Flow studies were carried out on the umbilical vein with measurements of time-averaged velocity and venous diameter. A similar approach was used for measurements of ductus venosus flow. Stable-isotope-labeled amino acids were used to study placental transport by the non-steady state approach. The studies of flow showed a marked reduction in umbilical blood flow even when expressed per kilogram fetal body weight in fetal-growth-restricted pregnancies. This may be coupled with an increased ductus venosus shunt, the combination leading to a marked reduction in fetal hepatic blood flow. The placental transport of some amino acids is reduced in fetal-growth-restricted pregnancies. Furthermore, nonglucose carbohydrates and polyols are found in fetal blood, some in concentrations higher than maternal concentrations. There is significant uptake of several polyols and of mannose across the umbilical circulation in normal pregnancies. In conclusion, both perfusion and permeability can now be studied in both normal and high-risk pregnancies.

Fetoplacental transport and utilization of amino acids in IUGR — a review

Amino acids have multiple functions in fetoplacental development. The supply of amino acids to the fetus involves active transport across and metabolism within the trophoblast. Transport occurs through various amino acid transport systems located on both the maternal and fetal facing membranes, many of which have now been documented to be present in rat, sheep and human placentas. The capacity of the placenta to supply amino acids to the fetus develops during pregnancy through alterations in such factors as surface area and specific time-dependent transport system expression. In intrauterine growth restriction (IUGR), placental surface area and amino acid uptakes are decreased in human and experimental animal models. In an ovine model of IUGR, produced by hyperthermia-induced placental insufficiency (PI-IUGR), umbilical oxygen and essential amino acid uptake rates are significantly reduced in the most severe cases in concert with decreased fetal growth. These changes indicate that severe IUGR is likely associated with a shift in amino acid transport capacity and metabolic pathways within the fetoplacental unit. After transport across the trophoblast in normal conditions, amino acids are actively incorporated into tissue proteins or oxidized. In the sheep IUGR fetus, however, which is hypoxic, hypoglycemic and hypoinsulinemic, there appear to be net effluxes of amino acids from the liver and skeletal muscle, suggesting changes in amino acid metabolism. Potential changes may be occurring in the insulin/IGF-I signaling pathway that includes decreased production and/or activation of specific signaling proteins leading to a reduced protein synthesis in fetal tissues. Such observations in the placental insufficiency model of IUGR indicate that the combination of decreased fetoplacental amino acid uptake and disrupted insulin/IGF signaling in liver and muscle account for decreased fetal growth in IUGR.

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.

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.

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.