Placental transport and fetal and placental metabolism of amino acids

The mechanisms and regulation of placental amino acid transport to the human foetus

The mechanisms by which amino acids are transferred across the human placenta are fundamental to our understanding of foetal nutrition. Amino acid transfer across the human placenta is dependent on transport across both the microvillous and basal plasma membranes of the placental syncytiotrophoblast, and on metabolism within the syncytiotrophoblast. Although the principles underlying uptake of amino acids across the microvillous plasma membrane are well understood, the extent to which amino acids are metabolised within human placenta and the mechanisms by which amino acids are transported out of the placenta across the basal plasma membrane are not well understood. Understanding the mechanisms and regulation of amino acid transport is necessary to understand the causes of intrauterine growth restriction in human pregnancy.

Ovine feto-placental metabolism

Fetal growth depends on the transplacental nutrient supply, which, in turn, is determined partially by the consumption and production of nutrients by the uteroplacental tissues. In fetal sheep, the rates of growth and umbilical glucose uptake decline coincidently towards term in parallel with the normal prepartum rise in plasma cortisol. While cortisol is known to reduce growth in fetal sheep, its effects on the uteroplacental handling and delivery of nutrients remain unknown. Hence, this study, quantified the rates of umbilical uptake and uteroplacental consumption of nutrients in preterm fetuses infused with cortisol for 5 days to mimic the prepartum cortisol surge. Umbilical uptakes of glucose and lactate, but not oxygen, were significantly lower in cortisol- than saline-infused fetuses, irrespective of whether values were expressed as absolute or weight-specific rates. The rate of uteroplacental consumption of glucose, but not oxygen, was significantly higher in cortisol- than saline-infused animals. Absolute rates of uteroplacental lactate production were lower in cortisol-infused animals. When all data were combined, fetal plasma cortisol levels were positively correlated to uteroplacental glucose consumption and inversely related to umbilical glucose uptake. Cortisol treatment had no apparent effect on placental mRNA expression for the glucose transporters, GLUT-1 and GLUT-3. The results demonstrate that cortisol is physiological regulator of uteroplacental metabolism and nutrient delivery to the sheep fetus. These observations have important implications for fetal growth both in late gestation and during adverse intrauterine conditions, which raise fetal cortisol levels earlier in gestation.

15N-tracer studies in formula-fed preterm infants: the role of glycine supply in protein turnover

In preterm infants, protein-turnover rates obtained by [15N]glycine as a tracer are known to be overestimated. This may reflect the insufficient supply of dietary glycine. In this randomized study, the influence of a glycine-rich diet on whole body protein turnover rates in eight male preterm infants (29-32 weeks, 1,200-2,540 g birthweight) using the 15N-tracer technique on days 21 and 28 of life was investigated to evaluate the necessity of supplementing preterm infant formulas with proteins rich in glycine. Before and during the study, the infants were alternately fed with a commercial available preterm infant formula (I, 2% protein, 40 mg glycine/dl) and a variety of this formulation with glycine-rich proteins (II, 2% protein, 130 mg glycine/dl). The protein-turnover rates were computed after 15N-single-pulse labeling with the help of the three-compartment model (TCM) and the urinary ammonia end-product method (AEPM). The tracer used was [15N]glycine (dosage: 2 mg 15N/kg). For the determination of 15N-excess-excretion kinetics, fractionated urine specimens were collected over a 36-h period. The protein-turnover rate calculated by TCM was 8.8 +/- 1.6 g/kg/day (formula I) and 7.7 +/- 2.0 g/kg/day (formula II); using AEPM, the rate was 8.7 +/- 2.5 g/kg/day and 7.5 +/- 1.5 g/kg/day, respectively. We conclude that the presaturation of the precursor pool by an adequate glycine intake minimizes drawbacks that may arise when using [15N]-glycine as a tracer in preterm infants, and a protein concentration of 2%, as in formula I, and consequently, a 170% glycine content when compared with the same human milk volume, meets the glycine requirement.

The role of the placenta in fetal nutrition and growth

The placenta plays a key role in the nutrition of the fetus. It mediates the active transport of nutrients and metabolic wastes across the barrier separating maternal and fetal compartments, as well as modifying the composition of some nutrients through its own metabolic activity. The function of the placenta is essential to the growth of a healthy fetus; it is becoming apparent that the activities of the placenta are in turn modulated by signals originating from the fetus. Communication between placenta and fetus is especially critical in intrauterine growth retardation. The importance of the interaction of factors like insulin-like growth factor and epidermal growth factor with their receptors is becoming increasingly clear.

Alanine transport across the human placental brush border membrane and the role of SH groups in carrier function

We have determined the kinetic characteristics of alanine transport into brush border membrane vesicles (BBMV) of human full term placenta and identified functional groups of the carrier proteins that are important for transport function. Alanine influx into BBMV was found to be mediated by two transport systems with different kinetic features and distinct substrate specificities. An uphill operating electrogenic Na(+)-dependent cotransport system could be kinetically separated from a Na(+)-independent facilitated diffusion system. The Na(+)-dependent transporter mediates Na(+)-alanine cotransport with a 1:1 flux coupling ratio (Hill coefficient 1.13 +/- 0.12) and a Km for alanine of 0.45 +/- 0.06 mmol/l. Half-maximal stimulation of Na(+)-dependent alanine influx was observed at a Na+ concentration (NaCl) of 51.4 +/- 1.3 mmol/l. A variety of group specific reagents were used to identify functional groups in the transport proteins. Only compounds reacting with SH-residues (NEM, DTNB, PCMBS) or NH2-groups (PITC) were found to affect Na+ dependent and Na+ independent alanine transport. The EC50 value for inhibition of alanine influx by PCMBS was 450 +/- 48 mumol/l. Chemical modifications of SH-groups by PCMBS caused a significant reduction (p < 0.005) in the Vmax for Na(+)-dependent alanine influx from 0.57 +/- 0.06 to 0.16 +/- 0.05 nmol.mg protein-1.10s-1 without affecting significantly the Km value. Inhibition by PCMBS was reversed by treatment of BBMV with DTT. When the substrate binding site of the transporter was protected by alanine or leucine, PCMBS still, blocked transport function, indicating that the crucial SH groups are not located within the substrate binding site of the transport proteins.

Placental supply of energy and protein substrates to the fetus

Table 1 shows an approximate metabolic balance sheet for the fetal sheep at late gestation. The metabolic balance in humans has not been determined but is estimated to be similar, except for a greater caloric requirement for fat deposition, adding about 33 kcal/kg/day to the total fetal caloric intake (34). The total fetal metabolic rate accounts for about 58% of caloric uptake. This percentage is close to the sum of the measured oxidation percentages of the principal energy substrates; this discrepancy cannot yet be explained.

Concentrations of amino acids in plasma from 45- to 47-week gestation mares and foetuses (Equus caballus)

Concentrations of 16 of 24 amino acids in plasma of foetuses were significantly higher, while four of 24 were lower, than their concentration in maternal plasma. The higher foetal concentrations of amino acids in plasma are similar to other species, with some exceptions, and suggest that equine placenta actively transports and concentrates amino acids into the umbilical circulation. Concentrations of nine of 24 amino acids were significantly lower in plasma from the umbilical artery compared to plasma from the umbilical vein, while no significant differences were present between maternal artery and vein plasma. The umbilical venous-arterial difference in concentrations of amino acids in plasma suggests the foetus extracts amino acids from the umbilical circulation for catabolism or protein synthesis, as in other species.

Fetal serine fluxes across fetal liver, hindlimb, and placenta in late gestation

Eleven studies of fetal serine fluxes were performed in chronically catheterized fetal lambs by continuous infusion of [1-13C]- and [U-14C]serine into a fetal brachial vein. At tracer serine steady state, samples were collected from the fetal abdominal aorta, umbilical vein, fetal hepatic vein, and fetal femoral vein and from the maternal femoral artery and uterine vein. Analyses were performed for plasma serine and glycine concentration, for serine and glycine 13C mole percent enrichment, and for whole blood 14CO2 and O2 concentrations. Uterine and umbilical blood flows were also measured. The placenta had a significant net uptake of fetal serine (2.1 +/- 0.5 mumol.min-1.kg-1, P < 0.01). Fetal plasma serine disposal rate (DR) was 42.5 +/- 3.9 mumol.min-1.kg-1.CO2 production from decarboxylation of fetal plasma serine represented 7.9 +/- 0.5% of DR, or 10.1 +/- 1.2 mumol CO2.min-1.kg-1. Fetal plasma glycine enrichment was 59.7 +/- 4.9% of fetal plasma serine enrichment. There was a significant loss of tracer serine from the fetal circulation into the placenta accounting for approximately 45% of infused tracer. Fifteen percent of this was converted to glycine and released into the umbilical circulation. There was a significant uptake of tracer serine by both fetal liver and fetal hindlimb with a significant CO2 production by both sites with serine oxidation predominantly in the carcass. These results indicate a high fetal serine disposal rate in the lamb, with rapid fetoplacental serine exchange, resulting in a net uptake of fetal serine by the placenta.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic aspects of fetal and neonatal growth

This review covers the transplacental transport of amino acids to the fetus and the role of placental metabolism and fetal metabolism in the utilization of amino acids. Particular attention is paid to the non-essential amino acids and to their rates of production within the fetus or placenta. The supply of amino acids is compared with their requirements for accretion in protein and for their use as metabolic fuels. Recent studies of protein synthesis in relation to gestational age are also reviewed.

Amino acid nutrition across the placenta

The placenta participates by metabolism, and not simply by transport to and from the fetus, in providing fetal needs for several amino acids, including leucine, glutamine, glutamate, serine, and glycine.