Development and mechanisms of fetal hypoxia in severe fetal growth restriction.

Severe fetal growth restriction (FGR) is often associated with hypoxia. We studied FGR hypoxia in an experimental model which is produced by exposing pregnant ewes to a hyperthermic environment. The study utilized simultaneous measurements of several relevant factors, e.g., uterine and umbilical blood flows and O(2) uptakes. Sixteen ewes were divided equally into control (C) and hyperthermic (HT) groups. Hyperthermia (40 degrees C for 12h/35 degrees C for 12h; approximately 35% relative humidity, RH) was maintained for 80 days commencing at approximately 38 days gestational age (dGA term 147+/-3 days). All ewes were then placed in a control environment ( approximately 21 degrees C, 24h; approximately 30% RH) and studied at approximately 134 dGA. Mean HT placental and fetal weights were 39% and 45% of C, respectively (p<0.0001), umbilical O(2) uptake/kg fetus was 76% of C (p<0.01) and umbilical venous PO(2) was reduced (20.2 vs. 29.7 Torr, p<0.001). Contrary to the hypothesis that FGR hypoxia is due to maternal placental hypoperfusion, uterine flow was not reduced in relation to O(2) uptake. The uterine-umbilical venous PO(2) difference was enlarged (38 vs. 23 Torr, p<0.0001). This difference is the expression of a balance between developmental changes in placental structure and oxidative metabolism, which have opposite effects in terms of fetal oxygenation. We postulate that FGR hypoxia results from disproportionate underdevelopment of those changes which allow for a progressive increase in umbilical O(2) uptake.

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.

Umbilical threonine uptake during maternal threonine infusion in sheep.

OBJECTIVE: The infusion into the maternal circulation of amino acid solutions failed to increase umbilical threonine (THR) uptake above normal even when THR was present in the infusate at a relatively high concentration. The purpose of the present study was to determine whether umbilical THR uptake can be increased by infusing a THR solution that does not contain any other amino acids. STUDY DESIGN: Five pregnant sheep (130+/-1.0 days after conception) were infused for 2h with a threonine solution (4.4+/-0.2 micromol.kg(-1).min(-1)). Plasma amino acids, glucose and lactate, hematocrit, blood O(2) content in maternal arterial, uterine venous, umbilical arterial and venous blood were measured. Uterine and umbilical blood flows were measured before and during the infusion and were used to calculate uterine and umbilical uptakes. Maternal and foetal plasma insulin and glucagon concentrations were also measured. RESULTS: The THR infusion increased maternal plasma THR (904 vs 236 microM, P< 0.001), foetal plasma THR (539 vs 334 microM, P< 0.01), and both uterine (20.4 vs 4.7 micromol.min(-1).kg(-1)(fetalweight), P< 0.05) and umbilical (8.6 vs 3.8 micromol.min(-1).kg(-1)(fetalweight), P< 0.001) THR uptakes. The uterine-umbilical THR uptake difference increased significantly (11.8 vs 0.9 micromol.min(-1).kg(-1)(fetalweight), P< 0.05). There were significant (P< 0.001) decreases in the foetal arterial plasma concentrations of tyrosine and the branched chain amino acids, as well as in isoleucine umbilical uptake (P< 0.05). There was a significant increase in maternal plasma glucagon (P< 0.01). CONCLUSION: A maternal THR infusion that causes a 3.8-fold increase in maternal plasma THR concentration above normal, with no significant increase in the concentration of other amino acids, leads to a 2.3-fold increase in umbilical THR uptake. This contrasts with the absence of a significant increase in umbilical THR uptake when THR was infused as part of an amino acid mixture in previous studies. The evidence supports the hypothesis that, in vivo, THR flux from placenta to foetus is mediated by a saturable, rate limiting transport system which is subject to inhibition by other neutral amino acids.

Induction of glutamate dehydrogenase in the ovine fetal liver by dexamethasone infusion during late gestation.

Glucocorticoids near term are known to upregulate many important enzyme systems prior to birth. Glutamate dehydrogenase (GDH) is a mitochondrial enzyme that catalyzes both the reversible conversion of ammonium nitrogen into organic nitrogen (glutamate production) and the oxidative deamination of glutamate resulting in 2-oxoglutarate. The activity of this enzyme is considered to be of major importance in the development of catabolic conditions leading to gluconeogenesis prior to birth. Ovine hepatic GDH mRNA expression and activity were determined in near-term (130 days of gestation, term 147 +/- 4 days) control and acutely dexamethasone-treated (0.07 mg(-1) hr(-1) for 26 hr) fetuses. Dexamethasone infusion had no effect on placental or fetal liver weights. Dexamethasone infusion for 26 hr significantly increased hepatic GDH mRNA expression. This increased GDH mRNA expression was accompanied by an increase in hepatic mitochondrial GDH activity, from 30.0 +/- 7.4 to 58.2 +/- 8.1 U GDH/U CS (citrate synthase), and there was a significant correlation between GDH mRNA expression and GDH activity. The generated ovine GDH sequence displayed significant similarity with published human, rat, and murine GDH sequence. These data are consistent with the in vivo studies that have shown a redirection of glutamine carbon away from net hepatic glutamate release and into the citric acid cycle through the forward reaction catalyzed by GDH, i.e., glutamate to oxoglutarate.

Placental expression of VEGF, PlGF and their receptors in a model of placental insufficiency-intrauterine growth restriction (PI-IUGR).

Placental development requires adequate and organized interaction of vascular growth factors and their receptors, including vascular endothelial growth factor (VEGF) and placental growth factor (PlGF). Both VEGF and PlGF, acting through the tyrosine kinase receptors VEGFR-1 and VEGFR-2, have been implicated in playing a role in ovine placental vascular development. The present studies describe the placental expression of components of the VEGF family at two maturational time points (55 and 90 days post coitus, dpc) in a hyperthermic-induced ovine model of placental insufficiency-intrauterine growth restriction (PI-IUGR). Both caruncular and cotyledonary VEGF and PlGF mRNA concentration increased with gestational age (P< 0.05), whereas only cotyledonary VEGF and PlGF protein concentration increased over gestation (P< 0.002). At 55 dpc, VEGF mRNA concentration was elevated in hyperthermic (HT) ewes, compared to control thermoneutral (TN) animals (TN; 0.52+/-0.08 vs HT; 1.27+/-0.17 VEGF/GAPDH, P< 0.001). At 90 dpc, expression of PlGF and VEGF mRNA was not altered by the HT treatment. Both TN cotyledonary VEGFR-1 and VEGFR-2 mRNA expression levels rose significantly over the period studied (P< 0.05 and P< 0.01 respectively). Receptor mRNA concentration in HT cotyledonary tissue was significantly reduced at 90 dpc (VEGFR-1; TN 0.21+/-0.02 vs HT 0.11+/-0.01 VEGFR-1/actin, P< 0.05, VEGFR-2; TN 0.18+/-0.05 vs HT 0.07+/-0.01 VEGFR-2/actin, P< 0.01). Soluble VEGFR-1 (sVEGFR-1) mRNA was not detected in these tissues. These alterations in growth factor and growth factor receptor mRNA expression, as a result of environmental heat stress early in placental development, could impair normal placental vascular development. Furthermore, alterations in VEGF, VEGFR-1 and VEGFR-2 mRNA expression, during the period of maximal placental growth, may contribute to the development of placental insufficiency, and ultimately intrauterine growth restriction.

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.

Cotyledon and binucleate cell nitric oxide synthase expression in an ovine model of fetal growth restriction.

Heat exposure early in ovine pregnancy results in placental insufficiency and intrauterine growth restriction (PI-IUGR). We hypothesized that heat exposure in this model disrupts placental structure and reduces placental endothelial nitric oxide synthase (eNOS) protein expression. We measured eNOS protein content and performed immunohistochemistry for eNOS in placentas from thermoneutral (TN) and hyperthermic (HT) animals killed at midgestation (90 days). Placental histomorphometry was compared between groups. Compared with the TN controls, the HT group showed reduced delivery weights (457 +/- 49 vs. 631 +/- 21 g; P < 0.05) and a trend for reduced placentome weights (288 +/- 61 vs. 554 +/- 122 g; P = 0.09). Cotyledon eNOS protein content was reduced by 50% in the HT group (P < 0.03). eNOS localized similarly to the vascular endothelium and binucleated cells (BNCs) within the trophoblast of both experimental groups. HT cotyledons showed a reduction in the ratio of fetal to maternal stromal tissue (1.36 +/- 0.36 vs. 3.59 +/- 1.2; P< or = 0.03). We conclude that eNOS protein expression is reduced in this model of PI-IUGR and that eNOS localizes to both vascular endothelium and the BNC. We speculate that disruption of normal vascular development and BNC eNOS production and function leads to abnormal placental vascular tone and blood flow in this model of PI-IUGR.

Net amino acid flux across the fetal liver and placenta during spontaneous ovine parturition.

The uptake and/or release of amino acids across the fetal liver and the placenta were studied in 8 pregnant sheep during the 5 days preceding delivery of the lamb. During spontaneous parturition, there was a significant decrease in fetal hepatic glutamine uptake, in fetal hepatic glutamate release and in placental glutamate uptake. The fetal plasma concentrations of glutamate, leucine, isoleucine, valine, phenylalanine, serine and tyrosine also decreased significantly in the 5 days preceding delivery. There was no significant net output of glucose from the fetal liver nor any change in net lactate uptake by the liver. During this same time period there was a significant increase in the fetal plasma cortisol concentration and a decrease in progesterone output by the pregnant uterus. The results are compared to the previously reported amino acid changes during dexamethasone-induced parturition.

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.

Effect of dexamethasone on fetal hepatic glutamine-glutamate exchange.

Intravenous infusion of dexamethasone (Dex) in the fetal lamb causes a two- to threefold increase in plasma glutamine and other glucogenic amino acids and a decrease of plasma glutamate to approximately one-third of normal. To explore the underlying mechanisms, hepatic amino acid uptake and conversion of L-[1-(13)C]glutamine to L-[1-(13)C]glutamate and (13)CO(2) were measured in six sheep fetuses before and in the last 2 h of a 26-h Dex infusion. Dex decreased hepatic glutamine and alanine uptakes (P < 0.01) and hepatic glutamate output (P < 0.001). Hepatic outputs of the glutamate (R(Glu,Gln)) and CO(2) formed from plasma glutamine decreased to 21 (P < 0.001) and 53% (P = 0.009) of control, respectively. R(Glu,Gln), expressed as a fraction of both outputs, decreased (P < 0.001) from 0.36 +/- 0.02 to 0.18 +/- 0.04. Hepatic glucose output remained virtually zero throughout the experiment. We conclude that Dex decreases fetal hepatic glutamate output by increasing the routing of glutamate carbon into the citric acid cycle and by decreasing the hepatic uptake of glucogenic amino acids.

Altered arterial concentrations of placental hormones during maximal placental growth in a model of placental insufficiency.

Pregnant ewes were exposed chronically to thermoneutral (TN; 20+/-2 degrees C, 30% relative humidity; n=8) or hyperthermic (HT; 40+/-2 degrees C 12 h/day, 35+/-2 degrees C 12 h/day, 30% relative humidity, n=6) environments between days 37 and 93 of pregnancy. Ewes were killed following 56 days of exposure to either environment (days in treatment (dit)), corresponding to 93+/-1 day post coitus (dpc). Maternal core body temperatures (CBT) in HT ewes were significantly elevated above the TN ewes (HT; 39.86+/-0.1 degrees C vs TN; 39.20+/-0.1 degrees C; P<0.001). Both groups of animals displayed circadian CBT, though HT ewes had elevated amplitudes (HT; 0.181+/-0.002 degrees C vs TN; 0.091+/-0.002 degrees C; P<0.001) and increased phase shift constants (HT; 2100 h vs TN; 1800 h; P<0.001). Ewes exposed to chronic heat stress had significantly reduced progesterone and ovine placental lactogen (oPL) concentrations from 72 and 62 dpc respectively (P<0.05), corresponding to approximately 30 dit. However, when compared with the TN ewes, HT cotyledonary tissue oPL mRNA and protein concentrations were not significantly different (P>0.1). Prolactin concentrations rose immediately upon entry into the HT environment, reaching concentrations approximately four times that of TN ewes, a level maintained throughout the study (HT; 216.31+/-32.82 vs TN; 54. 40+/-10.0; P<0.0001). Despite similar feed intakes and euglycemia in both groups of ewes, HT fetal body weights were significantly reduced when compared with TN fetuses (HT; 514.6+/-48.7 vs TN; 703. 4+/-44.8; P<0.05), while placental weights (HT; 363.6+/-63.3 vs TN; 571.2+/-95.9) were not significantly affected by 56 days of heat exposure. Furthermore, the relationship between body weight and fetal length, the ponderal index, was significantly reduced in HT fetuses (HT; 3.01+/-0.13 vs TN; 3.57+/-0.18; P<0.05). HT fetal liver weights were also significantly reduced (HT; 27.31+/-4.73 vs TN; 45.16+/-6.16; P<0.05) and as a result, the brain/liver weight ratio was increased. This study demonstrates that chronic heat exposure lowers circulating placental hormone concentrations. The observation that PL mRNA and protein contents are similar across the two treatments, suggests that reduced hormone concentrations are the result of impaired trophoblast cell development, specifically trophoblast migration. Furthermore, the impact of heat exposure during maximal placental growth is great enough to restrict early fetal development, even before the fetal maximal growth phase (100 dpc-term). These data highlight that intrauterine growth retardation (IUGR) may result primarily from placental trophoblast cell dysfunction, and secondarily from later reduced placental size.

Fetal hepatic oxygen consumption under normal conditions in the fetal lamb.

Previous measurements of fetal hepatic blood flow have relied on microsphere methodology. Estimates of fetal hepatic oxygen consumption (VO2), based on these measurements and the oxygen content difference across the fetal hepatic circulation, have been quite variable. To estimate hepatic VO2 in the fetal lamb by a different methodology, we applied the Fick principle using the steady-state uptake of indocyanine green (ICG) by the fetal liver to measure left hepatic blood flow in 10 pregnant ewes. Sampling catheters were inserted into the fetal external iliac artery, left hepatic vein, and umbilical vein. ICG was infused to steady state (for approximately 60 min) through a fetal brachial vein. Four sets of ICG concentration differences across the circulation of the left hepatic lobe were determined for each animal, and left hepatic lobe blood flow calculated. The oxygen concentration difference was measured simultaneously and VO2 of the left hepatic lobe calculated. In addition, we measured fetal VO2 and calculated the ratio of hepatic to fetal VO2. Left hepatic lobe blood flow was 382.30 ml/min/100 g tissue (COV = 0.32), a result statistically no different than in 4 animals with an independent measurement of hepatic blood flow using an ethanol equilibration method. Hepatic VO2 was 1.74 micromol/min/g tissue (COV = 0.13), and hepatic to fetal VO2 ratio was 18.23% (COV = 0.19). Our results indicate that normal fetal hepatic oxygen uptake per gram of tissue is less variable than previously suggested, and that ICG can be applied in the fetus for the purpose of hepatic blood flow measurement.

Relationship of fetal alanine uptake and placental alanine metabolism to maternal plasma alanine concentration.

Uterine and umbilical uptakes of alanine (Ala) were measured in 10 ewes before (control) and during intravenous infusion of Ala, which increased maternal arterial Ala concentration from 115 +/- 14 to 629 +/- 78 microM (P < 0.001). In 8 of these ewes, placental Ala fluxes were traced by constant intravenous infusion of L-[3,3,3-2H3]Ala in the mother and L-[1-13C]Ala in the fetus. Rates are reported as micromoles per minute per kilogram fetus. Ala infusion increased uterine uptake (2.5 +/- 0.6 to 15.6 +/- 3.1, P < 0.001), umbilical uptake (3.1 +/- 0.5 to 6.9 +/- 0.8, P < 0.001), and net uteroplacental utilization (-0.7 +/- 0.8 to 8.6 +/- 2.7, P < 0.01) of Ala. Control Ala flux to fetus from mother (Rf,m) was much less than the Ala flux to fetus from placenta (Rf,p) (0.17 +/- 0.04 vs. 5. 0 +/- 0.6). Two additional studies utilizing L-[U-13C]Ala as the maternal tracer confirmed the small relative contribution of Rf,m to Rf,p. During maternal Ala infusion, Rf,m increased significantly (P < 0.02) but remained a small fraction of Rf,p (0.71 +/- 0.2 vs. 7.3 +/- 1.3). We conclude that maternal Ala entering the placenta is metabolized and exchanged for placental Ala, so that most of the Ala delivered to the fetus is produced within the placenta. An increase in maternal Ala concentration increases placental Ala utilization and the fetal uptake of both maternal and placental Ala.

Placental transport of threonine and its utilization in the normal and growth-restricted fetus.

Placental transport and fetoplacental utilization of threonine (Thr) were compared at 130 +/- 1 days gestational age between seven control ewes (C) and six ewes in which intrauterine growth restriction (IUGR) had been induced by exposure to high ambient temperature from 33 +/- 1 to 112 +/- 2 days of gestation. The fluxes were measured using simultaneous intravenous infusions of L-[1-13C]Thr into the mother and L-[U-14C]Thr into the fetus. The IUGR group had less fetal weight (1.27 +/- 0.14 vs. 3.10 +/- 0.10 kg, P < 0.01) and placental weight (120 +/- 17 vs. 295 +/- 14 g, P < 0.01) than the C group. The direct flux of maternal Thr into the fetal systemic circulation was less in the IUGR fetuses, both relative to fetal weight (1.40 +/- 0.19 vs. 2.19 +/- 0.18 mumol.min-1.kg fetus-1, P = 0.0107) and placental weight (1.5 +/- 0.2 vs. 2.3 +/- 0.2 mumol.min-1.100 g placenta-1, P = 0.0187). In both groups, there was excretion of CO2 produced from fetal Thr. The rate of CO2 production from fetal plasma Thr carbon by fetus plus placenta was reduced in the IUGR group (1.50 +/- 0.23 vs. 2.86 +/- 0.32 mumol.min-1.kg fetus-1, P = 0.0065). We conclude that the flux of maternal Thr into the IUGR fetus is markedly reduced because of a reduction in placental mass and because of a weight-specific reduction in Thr placental transport. The reduced flux is routed into fetal Thr accretion via a decrease in fetal Thr oxidation.

Metabolic alterations in the fetal hepatic and umbilical circulations during glucocorticoid-induced parturition in sheep.

Fetal hepatic amino acid metabolism has unique features in comparison to postnatal life. Thus, it seemed likely that this metabolism might be changed by the endocrine changes which precede birth. To explore the changes in placental and fetal carbohydrate and amino acid metabolism that occur during parturition, labor was induced in six ewes at 131 +/- 1 d gestation with a fetal infusion of dexamethasone. For purpose of chemical analysis, blood was withdrawn before and approximately 3 and 25 h from the start of the infusion from maternal arterial, uterine venous, umbilical venous, fetal arterial, and left hepatic venous catheters. Fetal oxygenation remained normal. At 25 h, both fetal and maternal arterial plasma glucose concentrations increased (p < 0.01 and p < 0.02, respectively) and umbilical glucose uptake decreased (p < 0.05). Fetal glutamate showed a significant reduction in its hepatic output (p < 0.05) with a concomitant reduction in fetal arterial plasma concentration (p < 0.05) and placental uptake (p < 0.01). Fetal plasma concentrations of several other amino acids were markedly increased. The reduction in placental glutamate uptake was temporally associated with a decline in progesterone release by the pregnant uterus. These data suggest the hypothesis that glutamate plays a role in integrating the complex changes in placental and fetal hepatic metabolism that occur during parturition.

Placental transport and fetal utilization of leucine in a model of fetal growth retardation.

Placental transport and fetal utilization of leucine were studied at 130 days of gestation in six control ewes and in seven ewes in which intrauterine growth retardation (IUGR) had been induced by exposure to heat stress. Leucine fluxes were measured during simultaneous intravenous infusion of L-[1-13C]leucine into the mother and L-[1-14C] leucine into the fetus. In the IUGR group, the following leucine fluxes, expressed as micromol/min/kg fetus, were reduced compared with control: net uterine uptake (3.44 vs. 8.56, P<0.01), uteroplacental utilization (0.0 vs. 4.7, P<0.01), fetal disposal rate (6.4 vs. 8.9, P<0.001), flux from placenta to fetus (5.0 vs. 7.1, P<0.01), direct transport from mother to fetus (1.6 vs. 3.4, P<0.01), flux from fetus to placenta (1.5 vs. 3.2, P<0.001), and oxidation of fetal leucine by fetus plus placenta (2.1 vs. 3.2, P<0.02). Uterine uptake, uteroplacental utilization, and direct transport were also significantly reduced per gram placenta. We conclude that maternal leucine flux into the IUGR placenta is markedly reduced. Most of the reduced flux is routed into fetal metabolism via a decrease in placental leucine utilization and a decrease in the leucine flux from fetus to placenta.

Glutamine-glutamate exchange between placenta and fetal liver.

The hypothesis that glutamine shuttles nitrogen between placenta and fetal liver via interconversion with glutamate was explored by infusing L-[1,2-13C2]glutamine in six fetal sheep chronically catheterized for sampling of the umbilical and hepatic circulations. Fetal plasma glutamine disposal rate was 19.9 +/- 1.3 mumol.min-1.kg fetus-1. Entry of glutamine from the placenta accounted for approximately 60% of the total glutamine entry rate in fetal plasma. Glutamine was taken up by fetal liver, and 45.3 +/- 7.9% of the glutamine taken up was released as glutamate. The fetal liver released large quantities of glutamate, as evidenced by a sixfold increase in plasma glutamate concentration in the blood flowing through the left hepatic lobe and a hepatic glutamate output-to-O2 uptake molar ratio of 0.149 +/- 0.013. In conjunction with a previous study of fetal glutamate metabolism, these data demonstrate that glutamine entering the fetal circulation is converted to glutamate by the fetal liver at a rate of approximately 3-4 mumol.min-1.kg fetus-1.

Glutamate metabolism in fetus and placenta of late-gestation sheep.

Glutamate is produced by the fetal liver and taken up by the placenta. To explore the functional meaning of this exchange, the disposal rate (DR), clearance, conversion to glutamine, and decarboxylation rate of fetal plasma glutamate were studied at 129 +/- 2 days of gestation in seven fetal lambs infused via a systemic vein with L-[2,3,3,4,4-2H5]glutamate and L-[1-14C]glutamate. In two experiments, L-[1-13C]glutamate was also infused. The mean glutamate DR and clearance were 11.9 +/- 1.3 mumol.min-1.kg-1 and 200 +/- 8 ml.min-1.kg-1, respectively. The placenta extracted 88.5 +/- 0.8% of the tracer glutamate carried by the umbilical circulation and contributed to 61.3 +/- 3.2% of the glutamate DR. Most of the 14C infused as L-[1-14C]glutamate was converted to 14CO2: 37 +/- 4% by the fetus and 41 +/- 6% by the placenta. Of the labeled glutamate taken up by the placenta, 6.2 +/- 1.5% was returned to the fetus as glutamine. The glutamine-to-glutamate enrichment ratio in fetal arterial plasma was 0.066 +/- 0.008. We conclude that fetal plasma glutamate has an exceptionally high clearance because the flux of glutamate into the placenta is virtually equal to umbilical glutamate delivery rate. The main pathway of fetal plasma glutamate disposal is oxidation by placental and fetal tissues. Placental conversion of glutamate to fetal glutamine is a relatively small component of the placental metabolism of fetal glutamate.

Amino acid uptake by the fetal ovine hindlimb under normal and euglycemic hyperinsulinemic states.

As part of an effort to establish the contribution of different fetal organs to fetal amino acid metabolism, we measured in nine sheep fetuses the uptake of 27 amino acids by the hindlimb under normal conditions and conditions by euglycemic hyperinsulinemia. The fetal hindlimb is representative of nonvisceral tissues, which in the mature fetus account for approximately 70% of fetal weight and 30% of fetal O2 consumption. In the normal condition, there was a significant uptake of 21 amino acids for a net total nitrogen uptake of 132 +/- 21 mg N.day-1 x 100 g-1. The amino acids taken up by the fetal limb included alanine and glutamine. In addition, the fetal limb had significant glutamate and serine uptakes. Because glutamate flows from fetus to placenta and there is no fetal uptake of maternal serine, this indicates production and interorgan transport of these amino acids within the fetus. Insulin infusion significantly decreased the arterial concentration of every amino acid with the exception of cystathionine and significantly increased limb blood flow and glucose uptake. It significantly increased the limb uptake of alanine, asparagine, glycine, isoleucine, methionine, and tyrosine, decreased the uptake of aspartate, and produced no significant change in the net total nitrogen uptake, which remained similar to control (137 +/- 16 mg N.day-1 x 100 g-1).