Cecilia Teng

Leucine disposal and oxidation rates in the fetal lamb

To verify previous indirect evidence suggesting an important role of amino acids as substrates of fetal oxidative metabolism, leucine disposal and oxidation rates were measured in chronically catheterized fetal lambs during the last month of gestation. Under normal physiologic conditions the leucine oxidation rates were 6.43 +/- 1.02 mumol min-1 for fetuses with an average weight of 3.22 +/- 0.07 kg and comparable in magnitude to the fetal rate of leucine accretion. In seven animals studied before and during fasting, the fetal leucine oxidation rate increased with fasting from 5.8 +/- 1.0 to 10.8 +/- 1.3 mumol min-1. These data demonstrate that there is rapid oxidative degradation of leucine by the fetus and that the rate of this process increases in response to maternal fasting.

Production and utilization of amino acids by ovine placenta in vivo

Uterine and umbilical uptakes of plasma amino acids were measured simultaneously in eighteen singleton pregnant ewes at 130 ± 1 days gestation for the purpose of establishing which amino acids are produced or used by the uteroplacenta under normal physiological conditions and at what rates. The branched-chain amino acids (BCAA) had uterine uptakes significantly greater than umbilical uptakes. Net uteroplacental BCAA utilization was 8.0 ± 2.5 μmol ⋅ kg fetus-1 ⋅ min-1( P < 0.005) and represented 42% of the total BCAA utilization by fetus plus uteroplacenta. There was placental uptake of fetal glutamate (4.2 ± 0.3 μmol ⋅ kg fetus-1 ⋅ min-1, P < 0.001) and no uterine uptake of maternal glutamate. Umbilical uptake of glutamine was ∼61% greater than uterine uptake, thus demonstrating net uteroplacental glutamine production of 2.2 ± 0.9 μmol ⋅ kg fetus-1 ⋅ min-1( P < 0.021). In conjunction with other evidence, these data indicate rapid placental metabolism of glutamate, which is in part supplied by the fetus and in part produced locally via BCAA transamination. Most of the glutamate is oxidized, and some is used to synthesize glutamine, which is delivered to the fetus. There was net uteroplacental utilization of maternal serine and umbilical uptake of glycine produced by the placenta. Maternal serine utilization and glycine umbilical uptake were virtually equal (3.14 ± 0.50 vs. 3.10 ± 0.46 μmol ⋅ kg fetus-1 ⋅ min-1). This evidence supports the conclusion that the ovine placenta converts large quantities of maternal serine into fetal glycine.Uterine and umbilical uptakes of plasma amino acids were measured simultaneously in eighteen singleton pregnant ewes at 130 +/- 1 days gestation for the purpose of establishing which amino acids are produced or used by the uteroplacenta under normal physiological conditions and at what rates. The branched-chain amino acids (BCAA) had uterine uptakes significantly greater than umbilical uptakes. Net uteroplacental BCAA utilization was 8.0 +/- 2.5 mumol.kg fetus-1.min-1 (P < 0.005) and represented 42% of the total BCAA utilization by fetus plus uteroplacenta. There was placental uptake of fetal glutamate (4.2 +/- 0.3 mumol.kg fetus-1.min-1, P < 0.001) and no uterine uptake of maternal glutamate. Umbilical uptake of glutamine was approximately 61% greater than uterine uptake, thus demonstrating net uteroplacental glutamine production of 2.2 +/- 0.9 mumol.kg fetus-1.min-1 (P < 0.021). In conjunction with other evidence, these data indicate rapid placental metabolism of glutamate, which is in part supplied by the fetus and in part produced locally via BCAA transamination. Most of the glutamate is oxidized, and some is used to synthesize glutamine, which is delivered to the fetus. There was net uteroplacental utilization of maternal serine and umbilical uptake of glycine produced by the placenta. Maternal serine utilization and glycine umbilical uptake were virtually equal (3.14 +/- 0.50 vs. 3.10 +/- 0.46 mumol.kg fetus-1.min-1). This evidence supports the conclusion that the ovine placenta converts large quantities of maternal serine into fetal glycine.

Ontogenetic changes in the rates of protein synthesis and leucine oxidation during fetal life.

ABSTRACT. Studies of fetal leucine metabolism and protein synthetic rate, using L-(1-14)leucine as tracer, were carried out in 12 pregnant ewes at midgestation and compared with similar studies in late gestation. The disposal rate of fetal plasma leucine ranged between 3.07 and 9.06 μmol/min and was correlated (r = 0.89) to fetal dry weight. The fluxes to CO2 excretion and to protein synthesis were 18.6 ± 2.6 and 37.2 ± 2.6% of disposal rate, respectively. The flux of leucine molecules into the placenta was relatively large and correlated to the placental/fetal dry weight ratio (r = 0.84). The mean fractional protein synthetic rate was 0.216 ± 0.01 day-1. Comparison with late gestation data showed that fractional protein synthetic rate is inversely correlated (r = 0.87) to gestational age and that fetal protein synthetic rate (PRs, g/day) is related to fetal dry weight (DW, g) by the allometric equation:log PRs = -0.503 + 0.754 log DWThe 0.754 exponent is similar to the exponent relating fetal oxygen consumption to dry weight (0.729). This indicates that protein synthesis and energy metabolism per g dry weight decrease during fetal growth at approximately the same rate so that the protein synthesis/oxygen consumption ratio tends to remain constant.

Fetal and maternal non-glucose carbohydrates and polyols concentrations in normal human pregnancies at term.

The objective of the present investigation was to determine fetal and maternal plasma concentrations of nonglucose carbohydrates and polyols in normal human pregnancies at term. Uncomplicated human pregnancies (n = 50) were studied at ≥37 wk gestation. Blood samples were obtained from umbilical artery, umbilical vein, and maternal peripheral blood at the time of elective cesarean section. Plasma concentrations of inositol, glycerol, erythritol, sorbitol, and mannose were determined by HPLC analysis. Differences between umbilical venous, umbilical arterial, and maternal concentration were tested by the two-tailed t test for paired samples. Correlations between umbilical and maternal concentration and between umbilical venoarterial concentration difference and umbilical arterial concentration were assessed by Pearsons correlation and multiple regression analysis. All newborns were appropriate for gestational age, and oxygenation and acid-base balance were within the normal range for all fetuses studied. For most of the polyols (inositol, sorbitol, and erythritol), the fetal concentration was significantly higher than the maternal concentration. The umbilical venoarterial concentration difference for inositol was −10.5 ± 3.6 μM, for glycerol was 10 ± 1.7 μM, for sorbitol was 3.8 ± 0.5 μM (p < 0.001), and for mannose was 7.6 ± 0.7 μM. There was a significant correlation between maternal concentration and umbilical venous concentration of mannose (UVMAN = 15.38 + 0.69 MMAN; R2 = 0.46; p < 0.001). These results indicate that in normal human pregnancies at term, inositol is produced by the fetus, sorbitol is produced by the placenta, and there is a significant umbilical uptake of mannose from the maternal circulation.

Uptake and Transport by the Ovine Placenta of Neutral Nonmetabolizable Amino Acids with Different Transport System Affinities

Placental uptake and transport of three nonmetabolizable amino acids with different reactivities for transport systems were studied in sheep under normal physiologic conditions. Methylaminoisobutyric acid (MeAIB), which has specific affinity for the sodium-dependent A system transporters, demonstrated placental concentrative uptake from the uterine and the umbilical circulations, but virtually no transport from mother to fetus. By contrast, aminoisobutyric acid (AIB) and aminocyclopentane-1-carboxylic acid (ACP), which have affinity for both sodium-dependent and sodium-independent transporters, demonstrated both concentrative uptake and transport from mother to fetus. ACP transport rate to the fetus was approximately twice the AIB transport rate. It is concluded that a neutral amino acid which interacts almost exclusively with the weakly reversible system A transporters may be transported rapidly into the placenta and may attain high concentrations within this organ but cannot escape from placenta to fetus down its own concentration gradient because the exit route is controlled by reversible amino acid transporters at the fetal surface of the placenta. Conversely, high affinity for reversible Na-independent transporters may be a necessary condition for the rapid transport of an amino acid from placenta to fetus.

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.

The tissue and plasma concentration of polyols and sugars in sheep intrauterine growth retardation.

In an ovine model of placental insufficiency-induced intrauterine growth retardation (PI-IUGR), characterized by hypoxia, hypoglycemia and a significant reduction in fetal weight, we assessed alterations in fetal and placental polyols. Arterial maternal–fetal concentration differences of glucose and mannose were greater in the PI-IUGR fetus; glucose: C (n = 7), 2.68 ± 0.14 mmol/L versus PI-IUGR (n = 9), 3.18 ± 0.16 mmol/L (P < 0.02) and mannose: C, 42.9 ± 8.1 μmol/L versus PI-IUGR, 68.5 ± 19.1 μmol/L (P < 0.001). For PI-IUGR fetuses, fetal arterial plasma myo-inositol concentrations were significantly increased (P < 0.001). The concentrations of sorbitol, glucose and fructose were significantly reduced (P < 0.03, 0.01, 0.02, respectively). The cotyledons of IUGR placentas had a significantly increased concentration of myo-inositol (P < 0.003) and decreased concentrations of sorbitol, fructose and glycerol (P < 0.01, 0.02, 0.01, respectively). Fetal hepatic concentrations of sorbitol (P < 0.001) and fructose (P < 0.03) were also significantly reduced. These profound changes in both placental and fetal concentrations of polyols and sugars in sheep PI-IUGR pregnancies support the conclusion that within the PI-IUGR placenta there is an increased flux through the glucose 6-P:inositol 1-P cyclase system and decreased flux through the polyol dehydrogenase system, leading to increased placental myo-inositol production and decreased sorbitol production. The decreased placental supply of sorbitol to the fetal liver may lead to decreased fetal hepatic fructose production. These observations highlight that, in association with hypoxic and hypoglycemic PI-IUGR fetuses, there are major placental and fetal alterations in polyol production. The manner in which these alterations in fetoplacental carbohydrate metabolism contribute to the pathophysiology of PI-IUGR is currently unknown.

Determination of the NFAT5/TonEBP transcription factor in the human and ovine placenta.

Osmotic stress results in the accumulation of osmolytes in tissues. Synthesis of these osmolytes is mediated by the transcription factor NFAT5/TonEBP in the human kidney. We tested for the presence of NFAT5 mRNA and protein in the human and ovine placenta and confirmed sorbitol and inositol osmolyte concentrations in these tissues. To determine NFAT5 protein, human and ovine placenta were tested for inositol, sorbitol and glucose using high performance liquid chromatography (HPLC). Additionally, RNA was extracted and cDNA was made from these tissues. PCR was performed and products were sequenced. Western blotting was used to assess the expression of the NFAT5 protein. Human and ovine placenta demonstrated: 1) high concentrations of sorbitol and inositol, 2) presence of NFAT5 mRNA, 3) confirmation by NFAT5 sequence identity, and 4) presence of NFAT5 protein. NFAT5 is present in the ovine and human placenta at the RNA and protein levels that suggest a role for this protein in the induction of these osmolytes. Further trophoblast studies of osmotic stress effects on osmolytes are planned.

Human breast milk sugars and polyols over the first 10 puerperium days.

The transition from colostrum to mature breast milk during early puerperium is associated with significant concentration changes of numerous compounds. However, it is not known whether the free sugars, aminohexoses, and polyols are affected. Therefore, in this study, we aimed to determine their concentrations in human colostrum and milk during the first 10 days postpartum.

Main Routes of Plasma Lactate Carbon Disposal in the Midgestation Fetal Lamb

The turnover rates of plasma lactate, normalized for O2 consumption rate, are higher in the fetus than in the adult. This occurs despite very low rates of fetal gluconeogenesis which preclude the recycling of lactate carbon into glucose. In an effort to establish the main routes of disposal of fetal plasma lactate, 12 midgestation ovine fetuses (age 74 +/- 1 days) were infused intravenously at constant rate with L-[U-14C]lactate for a 4-hour period. At the end of the infusion, the amounts of 14C retained by the fetus and by the placenta, and the distribution of the retained 14C in free and protein-bound amino acids and in lipids were measured. Of the total 14C infused, 17.0 +/- 1.4% was recovered in the placenta, 4.0 +/- 0.3% in the fetal liver, and 15.0 +/- 0.8% in the extrahepatic fetal tissues. Of the retained radioactive carbon, 45-57% was recovered in the free and protein-bound amino acid fractions and 11-17% in the lipid fractions. Approximately 90% of the 14C in the free amino acid fractions was present as glutamate/glutamine, serine, glycine, and alanine carbon. In conjunction with data on fetal CO2 production from lactate carbon, these results demonstrate that the main routes of fetal lactate disposal are oxidation and synthesis of nonessential amino acids and lipids.