Heterogeneity of L-alanine transport systems in brush-border membrane vesicles from rat placenta during late gestation

Proline, leucine, and alanine transport in placental microvillous membrane vesicles prepared from late gestational rats

To characterize the active transport of amino acids across the placenta, uptakes of proline, leucine, and alanine were kinetically examined in placental microvillous membrane vesicles (PMV) prepared from rats in the late gestational period. Uptake rates of these amino acids in PMV showed saturable hyperbolic curves that obeyed Michaelis-Menten kinetics. Proline, leucine, and alanine transport were demonstrated to be carrier mediated systems with sodium-dependent, -independent, and both manner, respectively. In addition, sodium-dependent L-alanine transport showed two different systems, and new sodium-independent alanine transport system (K(m) of 1.12 mM) was observed in rat placenta. From these results, rat placenta has carrier mediated amino acid transport systems, and possesses at least three different transport systems for alanine.

Neutral amino acid transport in placental plasma membrane vesicles in the late pregnant rat. Evidence for a B0-like transport system

Rat placental plasma membrane vesicles have been used to study both alanine and leucine transport at late gestation. The results presented are consistent with the presence of more than two separate transport systems for neutral amino acids in the rat placenta. One system is clearly Na(+)-independent and transports alanine (KM approximately or = 2 mM; Vmax approximately or = 360 pmol ala/mg prot x 5 s), leucine (KM approximately or = 0.07 mM; Vmax approximately or = 100 pmol leu/mg prot x 5 s), serine, cysteine and 2-amino-2 norbornane carboxylic acid (BCH) showing similar properties to the L system, present in many cell types. The other systems are Na(+)-dependent and transport alanine (a, KM approximately or = 5 mM; Vmax approximately or = 3761 pmol ala/mg prot x 5 s; b, KM approximately or = 0.07 mM; Vmax approximately or = 376 pmol ala/mg prot x 5 s), cysteine, serine and leucine (KM approximately or = 0.2 mM; Vmax approximately or = 112 pmol leu/mg prot x 5 s) with different kinetic behaviour referring affinity and capacity. While one of them is sensitive to inhibition by methylaminoisobutyric acid (MeAIB), the other is a B0-like system similar to that characterized in bovine brush-border enterocyte membrane vesicles.

Effect of fetal growth restriction on system A amino acid transporter activity in the maternal facing plasma membrane of rat syncytiotrophoblast

To determine whether reduced maternofetal transfer of neutral amino acids in growth-restricted fetal rats is due to decreased system A transporter activity, we measured Na(+)-dependent MeAIB uptake by membrane vesicles from placentas of fetuses growth-restricted due to uterine artery ligation and control placentas (sham ligation). Na(+)-dependent uptake of methylamino-isobutyric acid (MeAIB) was linear over 15-60 s in vesicles from both ligated and sham-ligated sides of the uterus. Na(+)-dependent uptake of MeAIB at 30 s did not differ in paired measurements on vesicles from ligated and sham-ligated horns, 0.063 +/- 0.004 versus 0.056 +/- 0.005 nmol/mg of vesicle protein. The kinetics of Na(+)-dependent MeAIB uptake were similar in paired measurements on vesicles from ligated and sham-ligated horns, with overall K(m) = 4.4 +/- 0.5 mM and Vmax = 0.93 +/- 0.08 nmol/mg vesicle protein per 30 s. Uptake of tracer was inhibited 85-95% by known substrates for the system A amino acid transporter (alanine > or = serine > MeAIB > glycine = proline). We conclude that the system A transporter is present in the maternal facing plasma membrane of rat syncytiotrophoblast, but that the activity of this system, per mg of vesicle protein, is unaffected in fetal growth restriction induced by a decrease in maternal placental blood flow in late pregnancy.

Effect of low-protein diet-induced intrauterine growth retardation on rat placental amino acid transport

Given the central role of the placenta in nutrient transport to the fetus, one might propose that maternal nutrition would have a regulatory effect on this nutrient delivery. We have examined the effect of a low-protein adequate-calorie diet on specific amino acid transport processes by the rat placenta. Maternal weight, fetal weight, and placental weight were all significantly reduced in dams fed a low-protein (5% casein), isocaloric diet when compared with dams pair-fed a control (20% casein) diet. Even though maternal serum amino acid levels were maintained in the low-protein animals, fetomaternal serum amino acid ratios were significantly reduced, suggesting a reduction in nutrient transfer to the fetus. Apical and basal membrane vesicles were isolated from the placental trophoblast and were used to examine the amino acid transport capacity of both maternal-facing and fetal-facing membranes, respectively. Na+-dependent neutral amino acid transport mediated by system A was decreased in both membrane preparations, while transport mediated by system ASC was unaffected. The Na+-dependent anionic amino acid uptake by system X(-)AG (EAAC1) was reduced on the basal membrane, while the Na+-independent component was similar between the low-protein and control diet-fed dams. Cationic amino acid uptake was also reduced on both membrane surfaces. A decreased steady-state mRNA content for EAAC1 and CAT1 (system y+) suggests that reduced synthesis of the transporter proteins is responsible for the decrease in transport activity. Taken together, these data support the hypothesis that maternal protein malnutrition affects nutrient delivery to the fetus by downregulation of specific amino acid transport proteins.

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.

Effect of chronic cocaine administration on amino acid uptake in rat placental membrane vesicles

This study evaluated the effects of chronic exposure to cocaine during pregnancy on amino acid uptake in placental membrane vesicles. Pregnant rats received 62 mg/kg of cocaine hydrochloride by intraperitoneal (IP) injection as a divided daily dose on gestation days 8-19 inclusive. Fetal body weights were significantly decreased by 19% in the cocaine group, while placental weights were unchanged. Placental apical membrane vesicles were prepared from control and cocaine-treated animals, and marker enzyme enrichments for alkaline phosphatase and [3H]-dihydroalprenolol binding did not differ between cocaine and control groups. Rates of uptake (10 sec) of selected radiolabeled amino acids were measured utilizing a rapid filtration technique. Na(+)-dependent apical membrane [3H]-glutamine transport (50 microM) was reduced by 95% (p < 0.05) in cocaine-treated compared to control placentas. Uptake of 50 microM [3H]-methyl aminoisobutyric acid (MeAIB) into apical membranes was also decreased by 43% (p < 0.05) in cocaine membranes. Na(+)-independent [3H]-arginine transport (10 microM), however, did not differ between control or cocaine-treated groups. In summary, chronic cocaine administration selectively inhibited the transport of glutamine and MeAIB into apical membrane vesicles, but had minimal effect on arginine transport. We postulate that this diminution in uptake may contribute to the fetal growth retardation noted in our model.

H(+)-coupled alpha-methylaminoisobutyric acid transport in human intestinal Caco-2 cells

Transepithelial apical-to-basal transport and cellular uptake of the non-metabolisable amino acid alpha-methylaminoisobutyric acid (MeAIB) across confluent monolayers of the human intestinal epithelial cell line Caco-2 are enhanced by a transepithelial pH gradient (apical pH 6.0, basolateral pH 7.4). In Na(+)-free conditions (apical pH 7.4, basolateral pH 7.4), net absorption (120 +/- 58 pmol/cm2 per h, n = 13) and uptake across the apical membrane (cell/medium ratio 0.56 +/- 0.06, n = 13) are low. However, in Na(+)-free conditions with apical pH 6.0, net absorption (685 +/- 95 pmol/cm2 per h, n = 15) and intracellular accumulation (cell/medium ratio 3.63 +/- 0.29, n = 14) were marked. Continuous monitoring of intracellular pH (pHi) in BCECF (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein)-loaded Caco-2 cell monolayers indicated that apical addition of MeAIB (20 mM) was associated with H(+)-flow across the apical membrane in both Na+ and Na(+)-free conditions. This transport process is rheogenic in Na(+)-free media, stimulating an inward short-circuit current in voltage-clamped Caco-2 cell monolayers. On the basis of competition for MeAIB accumulation and pHi experiments, L-proline, glycine, L-alanine and beta-alanine are also substrates for H(+)-linked transport at the apical membrane of Caco-2 cells but L-valine, L-leucine and L-phenylalanine are not. These data are consistent with the expression, in the apical brush-border membrane of Caco-2 cells, of a H(+)-coupled, Na(+)-independent MeAIB carrier.

Influence of hydroxylation and conjugation in cross-inhibition of bile acid transport across the human trophoblast basal membrane

Taurocholate (TC) transport across the basal plasma membrane of the human trophoblast is a carrier-mediated process, whose specificity is probably not restricted to TC. The aim of this work was to gain further insight into the role of hydroxylation and conjugation in the behavior of the carrier system vs. bile acid (BA) species. Radiolabeled TC transport by basal plasma membrane (BPM) vesicles obtained from human term placenta was measured by a rapid filtration technique. Glycocholate (GC), taurochenodeoxycholate (TCDC) and taurodeoxycholate (TDC) inhibited TC binding to BPM. These bile acids compete with TC for the binding sites. Symmetry properties for GC- and TCDC-induced inhibition of TC transport was found in experiments where GC or TCDC were at the cis-side of the membrane (uptake and efflux experiments). GC and TCDC-induced inhibition seems to be of mixed type. By contrast, TDC was observed to affect TC transport differently, depending on whether the experiments addressed uptake or efflux. At the intracellular side of the membrane (uptake), TDC induced a marked increase in both Vmax and Kt. However, at the fetal side (efflux) a significant reduction in both Vmax and Kt was found. In spite of these peculiarities, the values for Ki were very close for GC, TCDC and TDC at the intracellular side but not at the fetal side, where the decreasing order for Ki was GC > TCDC > TDC. TC uptake by BPM vesicles was not modified in the presence of a wide range of estrone sulfate concentrations (0.002-1.0 mM). In summary, these results indicate that a particular bile acid molecular structure is necessary for steroid-related compounds to interact with the bile acid carrier located in BPM. They also suggest that changes in the number and position of hydroxy groups, as well as in the amino-acid moiety in amidated bile acids modify the behavior of the carrier, which may play an important role in the net vectorial transfer of bile acids across the placenta.