Ontogeny of amino acid transport system A in rat placenta

Regulation of placental amino acid transport in health and disease

Abnormal fetal growth, i.e., intrauterine growth restriction (IUGR) or fetal growth restriction (FGR) and fetal overgrowth, is associated with increased perinatal morbidity and mortality and is strongly linked to the development of metabolic and cardiovascular disease in childhood and later in life. Emerging evidence suggests that changes in placental amino acid transport may contribute to abnormal fetal growth. This review is focused on amino acid transport in the human placenta, however, relevant animal models will be discussed to add mechanistic insights. At least 25 distinct amino acid transporters with different characteristics and substrate preferences have been identified in the human placenta. Of these, System A, transporting neutral nonessential amino acids, and System L, mediating the transport of essential amino acids, have been studied in some detail. Importantly, decreased placental Systems A and L transporter activity is strongly associated with IUGR and increased placental activity of these two amino acid transporters has been linked to fetal overgrowth in human pregnancy. An array of factors in the maternal circulation, including insulin, IGF-1, and adiponectin, and placental signaling pathways such as mTOR, have been identified as key regulators of placental Systems A and L. Studies using trophoblast-specific gene targeting in mice have provided compelling evidence that changes in placental Systems A and L are mechanistically linked to altered fetal growth. It is possible that targeting specific placental amino acid transporters or their upstream regulators represents a novel intervention to alleviate the short- and long-term consequences of abnormal fetal growth in the future.

Maternal intermittent fasting during pregnancy induces fetal growth restriction and down-regulated placental system A amino acid transport in the rat

During Ramadan, many pregnant Muslim women fast between dawn and sunset. Although the impacts of prolonged maternal intermittent fasting (IF) on fetal growth and placental function are under-researched, reported effects include reduced placental weight and birth weight. In the present study, pregnant Wistar rats were used to model repeated cycles of IF on fetal development and placental function and to examine sex-specific effects. In the IF group, food was withdrawn daily from 17:00 to 09:00 over 21 days of gestation, while the control group received food ad libitum. Both groups had free water access. IF dams consumed less food, had significantly reduced weight compared with controls, with reduced plasma glucose and amino acids. Both fetal sexes were significantly lighter in the IF group with reduced fetal plasma amino acids. Placental weights and morphology were unchanged. The profile of placental metabolites was altered in the IF group with sex-specific responses evident. Transplacental flux of 14C-methylaminoisobutyric acid (14C-MeAIB), a system A amino acid transporter substrate, was significantly reduced in both fetal sexes in the IF group. Sodium-dependent 14C-MeAIB uptake into isolated placental plasma membrane vesicles was unchanged. The gene expression of system A transporter Slc38a1, Slc38a2 and Slc38a4 was up-regulated in IF male placentas only. No changes were observed in placental SNAT1 and SNAT2 protein expression. Maternal IF results in detrimental impacts on maternal physiology and fetal development with changes in the placental and fetal metabolite profiles. Reduced placental system A transporter activity may be responsible for fetal growth restriction in both sexes.

Metabolomic Research on Newborn Infants With Intrauterine Growth Restriction

To compare differences in metabolites between newborns with intrauterine growth restriction (IUGR) and those who are appropriate for gestational age (AGA) in order to understand the changes in metabolites of newborns with IUGR and to explore the possible metabolic mechanism of tissue and organ damages in patients with IUGR, with the ultimate goal of providing the basis for clinical intervention.A total of 60 newborns with IUGR and 60 AGA newborns who were hospitalized in the neonatal intensive care unit of our hospital between January 2011 and December 2015 and who underwent metabolic disease screening were enrolled in this study. The differences in 21 amino acids and 55 carnitines in peripheral blood, as well as changes in the ratios of free carnitine and acylcarnitine to total carnitine, were compared.Metabolites, particularly alanine, homocysteine, leucine, methionine, ornithine, serine, tyrosine, isovaleryl carnitine, and eicosenoyl carnitine, differed according to newborns' birth weight (<3rd percentile, 3rd-5th percentiles, 5th-10th percentiles, and 10th-90th percentiles), with those with lower birth weight showing the greater difference (P < 0.05). Metabolites also differed by gestational age, and the differences observed were mainly as follows: preterm and full-term newborns showed differences in metabolites, mainly in alanine, proline, cerotoyl carnitine, and tetradecanedioyl carnitine (P < 0.05); preterm and full-term AGA newborns showed differences in metabolites, mainly in alanine, glutamine, homocysteine, pipecolic acid, proline, heptanoyl carnitine, and sebacoyl carnitine (P < 0.05); and preterm and full-term newborns with IUGR showed differences in metabolites, mainly in arginine, glutamic acid, homocysteine, histidine, leucine, isoleucine, ornithine, serine, threonine, tryptophan, valine, heptanoyl carnitine, decanoyl carnitine, linoleyl carnitine, methylmalonyl carnitine, glutarylcarnitine, sebacoyl carnitine, hydroxyacetyl carnitine, and hydroxyhexadecancenyl carnitine (P < 0.05). Among newborns with IUGR, metabolites differed among males and females, mainly in aspartic acid, glutamic acid, and hexacosenoic acid (P < 0.05). Birth weight had no significant effects on free carnitine concentration or on the ratios of free carnitine and acylcarnitine to total carnitine (P < 0.05).IUGR infants exhibit significant abnormalities in amino acid and acylcarnitine metabolism, especially those with birth weight below the third percentile. With increasing birth weight, amino acids and acylcarnitines showed compensatory increases or reductions, and when birth weight reached the 10th percentile, the newborns with IUGR resembled the AGA newborns.

Improving amino acid nutrition to prevent intrauterine growth restriction in mammals

Intrauterine growth restriction (IUGR) is one of the most common concerns in human obstetrics and domestic animal production. It is usually caused by placental insufficiency, which decreases fetal uptake of nutrients (especially amino acids) from the placenta. Amino acids are not only building blocks for protein but also key regulators of metabolic pathways in fetoplacental development. The enhanced demands of amino acids by the developing conceptus must be met via active transport systems across the placenta as normal pregnancy advances. Growing evidence indicates that IUGR is associated with a reduction in placental amino acid transport capacity and metabolic pathways within the embryonic/fetal development. The positive relationships between amino acid concentrations in circulating maternal blood and placental amino acid transport into fetus encourage designing new therapies to prevent or treat IUGR by enhancing amino acid availability in maternal diets or maternal circulation. Despite the positive effects of available dietary interventions, nutritional therapy for IUGR is still in its infancy. Based on understanding of the underlying mechanisms whereby amino acids promote fetal growth and of their dietary requirements by IUGR, supplementation with functional amino acids (e.g., arginine and glutamine) hold great promise for preventing fetal growth restriction and improving health and growth of IUGR offspring.

Regulation of placental nutrient transport and implications for fetal growth

Fetal macronutrient requirements for oxidative metabolism and growth are met by placental transport of glucose, amino acids, and, to a lesser extent that varies with species, fatty acids. It is becoming possible to relate the maternal–fetal transport kinetics of these molecules in vivo to the expression and distribution of specific transporters among placental cell types and subcellular membrane fractions. This is most true for glucose transport, although apparent inconsistencies among data on the roles and relative importance of the predominant placenta glucose transporters, GLUT-1 and GLUT-3, remain to be resolved. The quantity of macronutrients transferred to the fetus from the maternal bloodstream is greatly influenced by placental metabolism, which results in net consumption of large amounts of glucose and, to a lesser extent, amino acids. The pattern of fetal nutrient supply is also altered considerably by placental conversion of glucose to lactate and, in some species, fructose, and extensive transamination of amino acids. Placental capacity for transport of glucose and amino acids increases with fetal demand as gestation advances through expansion of the exchange surface area and increased expression of specific transport molecules. In late pregnancy, transport capacity is closely related to placental size and can be modified by maternal nutrition. Preliminary evidence suggests that placental expression and function of specific transport proteins are influenced by extracellular concentrations of nutrients and endocrine factors, but, in general, the humoral regulation of placental capacity for nutrient transport is poorly understood. Consequences of normal and abnormal development of placental transport functions for fetal growth, especially during late gestation, and, possibly, for fetal programming of postnatal disorders, are discussed.

Isolation of plasma membrane vesicles from mouse placenta at term and measurement of system A and system beta amino acid transporter activity

Placental amino acid transport is essential for optimal fetal growth and development, with a reduced fetal provision of amino acids being implicated as a potential cause of fetal growth restriction (FGR). Understanding placental insufficiency related FGR has been aided by the development of mouse models that have features of the human disease. However, to take maximal advantage of these, methods are required to study placental function in the mouse. Here, we report a method to isolate plasma membrane vesicles from mouse placenta near-term and have used these to investigate two amino acid transporters, systems A and β, the activities of which are reduced in human placental microvillous plasma membrane (MVM) vesicles from FGR pregnancies. Plasma membrane vesicles were isolated at embryonic day 18 by a protocol involving homogenisation, MgCl₂ precipitation and centrifugation. Vesicles were enriched 11.3 ± 0.5-fold in alkaline phosphatase activity as compared to initial homogenate, with minimal intracellular organelle contamination as judged by marker analyses. Cytochemistry revealed alkaline phosphatase was localised between trophoblast layers I and II, with intense reaction product deposited on the maternal-facing plasma membrane of layer II, suggesting that vesicles were derived from this trophoblast membrane. System A and system β activity in mouse placental vesicles, measured as Na⁺-dependent uptake of ¹⁴C-methylaminoisobutyric acid (MeAIB) and ³H-taurine respectively confirmed localisation of these transporters to the maternal-facing plasma membrane of layer II. Comparison to human placental MVM showed that system A activity was comparable at initial rate between species whilst system β activity was significantly lower in mouse. This mirrored the lower expression of TAUT observed in mouse placental vesicles. We conclude that syncytiotrophoblast layer II-derived plasma membrane vesicles can be isolated and used to examine transporter function.

The SNAT4 isoform of the system A amino acid transporter is functional in human placental microvillous plasma membrane

Placental system A activity is important for the supply of neutral amino acids needed for fetal growth. There are three system A isoforms: SNAT1, SNAT2 and SNAT4, but the contribution of each to system A-mediated transport is unknown. Here, we have used immunohistochemistry to demonstrate that all three isoforms are present in the syncytiotrophoblast suggesting each plays a role in amino acid transport across the placenta. We next tested the hypothesis that the SNAT4 isoform is functional in microvillous plasma membrane vesicles (MVM) from normal human placenta using a method which exploits the unique property of SNAT4 to transport both cationic amino acids as well as the system A-specific substrate MeAIB. The data show that SNAT4 contribution to system A-specific amino acid transport across MVM is higher in first trimester placenta compared to term (approx. 70% and 33%, respectively, P < 0.01). Further experiments performed under more physiological conditions using intact placental villous fragments suggest a contribution of SNAT4 to system A activity in first trimester placenta but minimal contribution at term. In agreement, Western blotting revealed that SNAT4 protein expression is higher in first trimester MVM compared to term (P < 0.05). This study provides the first evidence of SNAT4 activity in human placenta and demonstrates the contribution of SNAT4 to system A-mediated transport decreases between first trimester and term: our data lead us to speculate that at later stages of gestation SNAT1 and/or SNAT2 are more important for the supply of amino acids required for normal fetal growth.

The effects of dietary protein restriction on chorda tympani nerve taste responses and terminal field organization

Prenatal dietary sodium restriction produces profound developmental effects on rat functional taste responses and formation of neural circuits in the brainstem. Converging evidence indicates that the underlying mechanisms for these effects are related to a compromised nutritional state and not to direct stimulus-receptor interactions. We explored whether early malnourishment produces similar functional and structural effects to those seen following dietary sodium restriction by using a protein deficient, sodium replete diet. To determine if early dietary protein-restriction affects the development of the peripheral gustatory system, multi-fiber neurophysiological recordings were made from the chorda tympani nerve and anterograde track tracing of the chorda tympani nerve into the nucleus of the solitary tract (NTS) was accomplished in rats fed a protein-restricted or a control diet (6% and 20%, respectively). The dietary regimens began on embryonic day 7 and continued until rats were used for neurophysiological recordings (postnatal days (P) 35-50) or for chorda tympani terminal field labeling (P40-50). Responses to a concentration series of NaCl, sodium acetate, KCl, and to 0.50 M sucrose, 0.03 M quinine-HCl, and 0.01 N HCl revealed attenuated responses (30-60%) to sodium-specific stimuli in rats fed the 6% protein diet compared with those fed the 20% protein diet. Responses to all other stimuli were similar between groups. Terminal field volumes were nearly twofold larger in protein-restricted rats compared with controls, with the differences located primarily in the dorsal-caudal zone of the terminal field. These results are similar to the results seen previously in rats fed a sodium-restricted diet throughout pre- and postnatal development, suggesting that dietary sodium- and protein-restriction share similar mechanisms in altering gustatory development.

Impact of forskolin and amino acid depletion upon System A activity and SNAT expression in BeWo cells

Amino acid transport System A (SysA) plays an important role in mediating the transplacental transfer of neutral amino acids from mother to fetus. Given that prior work has demonstrated that SysA activity is regulated, both over gestation and in response to dietary restriction during pregnancy, we examined the response of SysA activity and sodium-dependent neutral amino acid transporter (SNAT; responsible for SysA activity) expression to cAMP analogues and amino acid deprivation in BeWo cells, an accepted model of placental syncytia. SysA activity was unaffected by forskolin, a cAMP agonist, at 48 and 72 h. Amino acid depletion was associated with an up-regulation of SysA activity, largely mediated through an enhancement of SNAT2 (Slc38a2) expression at both the protein and mRNA level. SNAT1 (Slc38a1) expression did not change in response to amino acid depletion, while SNAT4 (Slc38a4) could not be detected. In summary, SysA activity in BeWo cells responds to amino acid depletion through the differential regulation of SNAT subtypes.

SNAT4 isoform of system A amino acid transporter is expressed in human placenta

The system A amino acid transporter is encoded by three members of the Slc38 gene family, giving rise to three subtypes: Na+-coupled neutral amino acid transporter (SNAT)1, SNAT2, and SNAT4. SNAT2 is expressed ubiquitously in mammalian tissues; SNAT1 is predominantly expressed in heart, brain, and placenta; and SNAT4 is reported to be expressed solely by the liver. In the placenta, system A has an essential role in the supply of neutral amino acids needed for fetal growth. In the present study, we examined expression and localization of SNAT1, SNAT2, and SNAT4 in human placenta during gestation. Real-time quantitative PCR was used to examine steady-state levels of system A subtype mRNA in early (6-10 wk) and late (10-13 wk) first-trimester and full-term (38-40 wk) placentas. We detected mRNA for all three isoforms from early gestation onward. There were no differences in SNAT1 and SNAT2 mRNA expression with gestation. However, SNAT4 mRNA expression was significantly higher early in the first trimester compared with the full-term placenta (P < 0.01). We next investigated SNAT4 protein expression in human placenta. In contrast to the observation for gene expression, Western blot analysis revealed that SNAT4 protein expression was significantly higher at term compared with the first trimester (P < 0.05). Immunohistochemistry and Western blot analysis showed that SNAT4 is localized to the microvillous and basal plasma membranes of the syncytiotrophoblast, suggesting a role for this isoform of system A in amino acid transport across the placenta. This study therefore provides the first evidence of SNAT4 mRNA and protein expression in the human placenta, both at the first trimester and at full term.

SNAT expression in rat placenta

Amino acid transport System A (SysA) activity is present within the rodent and human placentas. Inhibition of this transport system is associated with fetal growth retardation. Several cDNAs encoding SysA transport proteins have been discovered, and their presence documented within the human placenta. We have demonstrated the presence of mRNA encoding three of these transporters, SNAT1, 2, and 4 within the rat placenta over the final third of gestation. Abundance of these mRNA species increases from day 14 to day 20 of gestation. Immunohistochemistry demonstrates the presence of SNAT1 and 2 within the placental labyrinth at both days 14 and 20. Transport proteins are also present within marginal giant cells and, for SNAT1, within fetal endothelium. In conclusion, several proteins capable of SysA transport activity are present within the rodent placenta. mRNA expression increases over the final third of gestation, coincident with the period of greatest need for fetal amino acid delivery.

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.

Isoforms of amino acid transporters in placental syncytiotrophoblast: plasma membrane localization and potential role in maternal/fetal transport

Many cell proteins exist as isoforms arising either from gene duplication or alternate RNA splicing. There is growing evidence that isoforms with different, but closely related, functional characteristics are often directed to discrete cellular locations. Thus, specialized functions may be carried out by proteins of similar evolutionary origin in different membrane compartments. In polarized epithelial cells, this mechanism allows the cell to control amino acid transport independently at each of its specialized apical and basolateral plasma membrane domains. Investigations of isoform localization in these membranes have generally been performed in epithelia other than the placental trophoblast.This review of placental amino acid transporter isoforms first provides an overview of their properties and preliminary plasma membrane localization. We then discuss studies suggesting various roles of isoform localization in trophoblast function. To provide insights into the molecular basis of this localization in trophoblast, we present a review of current knowledge of plasma membrane protein localization as derived from investigations with a widely used epithelial model cell line. Finally, we discuss a potential approach using cultured trophoblast-derived cells for studies of transporter isoform localization and function. We hope that this review will stimulate investigation of the properties of trophoblast transporter isoforms, their membrane localization and their contribution to the cellular mechanism of maternal-fetal nutrient transport.

Effects of epidermal growth factor on placental amino acids uptake in pregnant rats

To evaluate the effects of epidermal growth factor (EGF) on placental amino acids uptake, transport activities for L-proline, L-leucine, and L-alanine were kinetically examined in placental microvillous vesicles(PMV) obtained from pregnant rats administered with EGF(100 and 200 microg/kg/day) from day 18 to 21 of pregnancy. The Vmax of Na(+)-dependent proline uptake remarkably increased with a dose-dependent manner of EGF, while Km did not change. In contrast, Vmax and Km values of Na(+)-dependent and -independent alanine, and Na(+)-independent leucine uptake were not affected. These results suggested that EGF enhanced proline transport activity in placental microvillous membranes, resulting in an increase of proline concentration in the fetal blood. The selective up-regulation of proline uptake was considered to contribute to fetal growth by EGF.

Physiological importance of system A-mediated amino acid transport to rat fetal development

Fetal growth and development are dependent on the delivery of amino acids from maternal amino acid pools to the fetal blood. This is accomplished via transfer across the apical and basal plasma membrane of the placental syncytiotrophoblast. The aim of this study was to determine whether inhibition of system A (amino acid transporter) was associated with a decrease in fetal weight in the rat. System A is a ubiquitous Na(+)-dependent amino acid transporter that actively transports small zwitterionic amino acids. In brief, system A was inhibited by infusing a nonmetabolizable synthetic amino acid analog, 2-(methylamino)isobutyric acid from days 7-20 of gestation. On day 20, the rats were killed and tissues (maternal liver, fetuses, and placentas) were collected for analysis. The degree of system A inhibition was determined, as was the impact of said inhibition on fetal and maternal weights, system A-mediated placental transport, and placental system A-mediated transporter expression. Our results suggest that when system A is inhibited, fetal weight is diminished [control group: -3.55 +/- 0.04 g (n = 113), experimental group: -3.29 +/- 0.04 g (n = 128)], implying an integral role for system A transport in fetal growth and development in the rat.

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.

Nutrient transport across the placenta

The placenta forms a selective barrier that functions to transport nutrients that are of critical use to the fetus. Nutrient transport across the placenta is regulated by many different active transporters found on the surface of both maternal and fetal facing membranes of the placenta. The presence of these transporters in the placenta has been implicated in the facilitation of nutrient diffusion and proper fetal growth. In this review, recent developments concerning nutrient transporters that regulate glucose, amino acid, fatty acid, and nucleoside transplacental movement are discussed.

Fetal and placental glucose and amino acid uptake in the spontaneously hypertensive rat

Until late in gestation, fetal spontaneously hypertensive rat (SHR) is growth retarded. Fetal growth rate increases after placental hypertrophy occurs between fetal days 18 and 20. The increase in placental mass may result in improved transfer of macro nutrients to the fetus and thus stimulate growth. In this study, fetal and placental uptake of the glucose analog 3-O-methyl glucose (3MG) and the amino acid analog amino-isobutyric acid (AIB) from the maternal circulation were compared in the SHR and Wistar-Kyoto (WKY) on days 16, 20 and 22 of gestation. Placental 3MG uptake (d/min/g tissue wet weight) was decreased in the SHR on days 20 and 22 but no differences were observed in fetal 3MG uptake. Increased placental mass in the SHR meant that total placental 3MG uptake was greater in the SHR. Placental uptake of AIB (d/min/g tissue wet weight) was much lower in the SHR (on days 16, 20 and 22) and the decrease was not compensated for by increased placental mass. Fetal uptake of AIB was decreased on days 20 and 22 (P<0.05). AIB uptake (d/min/g tissue wet weight) by the carcass and the internal organs (brain, heart, kidney, liver and lung) was also lower in the SHR. These findings indicate that although fetal growth in the SHR increases rapidly in late gestation following placental hypertrophy, it does so despite a pronounced deficit in amino acid uptake.

Activity and protein localization of multiple glutamate transporters in gestation day 14 vs. day 20 rat placenta

Concentrative absorption of glutamate by the developing placenta is critical for proper fetal development. The expression of GLAST1, GLT1, EAAC1, and EAAT4, known to be capable of D-aspartate-inhibitable and Na(+)-coupled glutamate transport (system X-AG), was evaluated in day 14 vs. day 20 rat chorioallantoic placenta. Steady-state mRNA levels were greater at day 20 for all transporters. Immunohistochemistry determined that the expression of GLAST1, GLT1, and EAAC1 was greater throughout the day 20 placenta and was asymmetric with respect to cellular localization. EAAT4 protein was not detected. System X-AG activity was responsible for most of the Na(+)-dependent glutamate uptake and was greater in day 20 than in day 14 apical and basal membrane subdomains of the labyrinth syncytiotrophoblast. Greater quantities of EAAC1 and GLAST1 protein were identified on day 20, and quantities were greater in basal than in apical membranes. GLT1 expression, unchanged in apical membranes, was decreased in basal membranes. These data correlate transporter mRNA and protein content with transport activity and demonstrate an increasing capacity for glutamate absorption by the developing placenta.

Demonstration of system y+L activity on the basal plasma membrane surface of rat placenta and developmentally regulated expression of 4F2HC mRNA

Na(+)-independent cationic amino acid transport in the rat placenta occurs by leucine-sensitive and leucine-insensitive pathways. The ontogeny of these transport mechanisms within the rat placenta has been described recently. To assign the leucine-inhibitable portion of uptake definitively the uptake of [3H]arginine was studied in the presence of both BCH (to inhibit system Bo,+) and varied concentrations of leucine. Uptake of arginine into basal-enriched membrane vesicles derived from rat placenta was, in the presence of sodium, inhibited by micromolar concentrations of leucine, consistent with assignment of this activity to system y+L. In contrast, the majority of arginine uptake into apical-enriched membrane vesicles was leucine insensitive. Messenger RNA derived from rat placenta at days 14, 16, 18 and 20 of gestation was hybridized with full-length rat cDNA probes against NBAT and 4F2HC (thought to encode proteins associated with system bo,+ and y+L activities, respectively). No NBAT mRNA was detected, whereas 4F2HC mRNA was present at all gestational stages, increasing 12-fold over the last third of gestation. It is concluded that system y+L is present in the basal plasma membrane of the rat placenta syncytium and is subject to developmental regulation by a mechanism that alters the steady content of 4F2HC mRNA.