Two cationic amino acid transport systems in human placental basal plasma membranes

Change in Cationic Amino Acid Transport System and Effect of Lysine Pretreatment on Inflammatory State in Amyotrophic Lateral Sclerosis Cell Model

Amyotrophic lateral sclerosis (ALS) is a lethal neurological disorder characterized by the deterioration of motor neurons. The aim of this study was to investigate alteration of cationic amino acid transporter (CAT-1) activity in the transport of lysine and the pretreatment effect of lysine on pro-inflammatory states in an amyotrophic lateral sclerosis cell line. The mRNA expression of cationic amino acid transporter 1 was lower in NSC-34/hSOD1^G93A (MT) than the control cell line (WT), lysine transport is mediated by CAT-1 in NSC-34 cell lines. The uptake of [³H]L-lysine was Na⁺-independent, voltage-sensitive, and strongly inhibited by inhibitors and substrates of cationic amino acid transporter 1 (system y⁺). The transport process involved two saturable processes in both cell lines. In the MT cell line, at a high-affinity site, the affinity was 9.4-fold higher and capacity 24-fold lower than that in the WT; at a low-affinity site, the capacity was 2.3-fold lower than that in the WT cell line. Donepezil and verapamil competitively inhibited [³H]L-lysine uptake in the NSC-34 cell lines. Pretreatment with pro-inflammatory cytokines decreased the uptake of [³H]L-lysine and mRNA expression levels in both cell lines; however, the addition of L-lysine restored the transport activity in the MT cell lines. L-Lysine exhibited neuroprotective effects against pro-inflammatory states in the ALS disease model cell lines. In conclusion, studying the alteration in the expression of transporters and characteristics of lysine transport in ALS can lead to the development of new therapies for neurodegenerative diseases.

Taurine Transport Into Fetal Cord Blood Cells: Inhibition by Cyclosporine A

OBJECTIVES

Pregnant women undergoing long-term organ transplant treatment have an increased incidence of delivering infants with intrauterine growth restriction (IUGR). Cyclosporine A is used as an immunosuppressant in such women and indirect evidence suggests that IUGR might result from an effect of cyclosporine A on amino acid transport by the placenta. In this study we tested the hypothesis that the transport of an essential amino acid, taurine, by fetal tissue other than the placenta is modulated by cyclosporine A.

METHODS

Cord blood cells (CBCs) were used to test this hypothesis as an easily obtainable fetal tissue. Transport of taurine into CBCs was measured using standard tracer flux assays.

RESULTS

Uptake of [(3)H] taurine by CBCs was linear over 15 minutes (76.2 +/- 16.6 fmol/10(6) cells/min, mean +/- SEM, n = 6) and inhibitable by 10 mM beta-alanine, a substrate of the system-beta taurine transport protein (6.7 +/- 1.0 fmol/10(6) cells/min, n = 6, P <.05, paired Student t test). Pre-incubation with cyclosporine A (5 microM) inhibited [(3)H] taurine uptake by 29.3%-5.3% (n = 8, P <.05, paired Student t test).

CONCLUSIONS

These data show that amino acid transport via system-beta can be measured in CBCs and may be a useful model for amino acid transport studies in fetal cells. We also show that system-beta was inhibited by the immunosuppressant, cyclosporine A. This suggests that the increased incidence of IUGR reported in mothers treated with cyclosporine A may be due partially to effects on taurine uptake into fetal cells outside the placenta.

Erythrocytes L-arginine y+ transporter inhibition by N-ethylmaleimide in ice-bath

Erythrocytes L: -arginine uptake is conveyed by y+ and y+L membrane transport systems. Pre-incubation with N-ethylmaleimide for 10 min at 37°C inhibits the y+ system. The aim of this study was to determine the ideal pre-incubation temperature in evaluating y+ and y+L systems. Cells were pre-incubated with or without N-ethylmaleimide for 10 min at 4°C and 37°C. L: -Arginine uptake was quantified by radioisotope and standard erythrocytes membrane flux methodology. Results demonstrate that erythrocytes L: -arginine content is depleted by pre-incubation at 37°C for 10 min, thus changing the V (max) measurement. The inhibitory effect of N-ethylmaleimide pre-incubation was temperature independent and already complete after 1 min of incubation. No significant difference in kinetic parameters was detected between cells pre-incubated at 37°C or 4°C, under zero-trans conditions. In conclusion, we suggest that measurement of erythrocytes L: -arginine uptake by y+ and y+L systems could be carried out without N-ethylmaleimide pre-incubation at 37°C.

Hepatic and adipose tissue depot-specific changes in lipid metabolism in Late-onset Obese (LOB) rats

Transgenic Late-onset OBesity (LOB) rats slowly develop a male-specific, autosomal dominant, obesity phenotype with a specific increase in peri-renal white adipose tissue (WAT) depot and preserved insulin sensitivity (Bains et al. in Endocrinology 145:2666-2679, 2004). To better understand the remarkable phenotype of these rats, the lipid metabolism was investigated in male LOB and non-transgenic (NT) littermates. Total plasma cholesterol (C) levels were normal but total plasma triacylglycerol (TAG) (2.8-fold) and hepatic TAG content (25%) was elevated in LOB males. Plasma VLDL-C and VLDL-TAG levels were higher while plasma apoB levels were 60% lower in LOB males. Increased hepatic TAG secretion explained the increased VLDL levels in LOB males. The hepatic gene expression of FAS, SCD-1, mitochondrial (mt)GPAT, and DGAT2 was up-regulated in both old obese and young non-obese LOB rats. Lipoprotein lipase (LPL) activity in heart and epididymal white adipose tissue (WAT) was unchanged, while LPL activity was increased in peri-renal WAT (30%) and decreased in soleus muscle (40%). Moreover, FAS, SCD-1 and DGAT2 gene expression was increased in peri-renal, but not in epididymal WAT. Basal lipolysis was reduced or unchanged and beta-adrenergic stimulated lipolysis was reduced in WAT from both old obese and young non-obese LOB rats. To summarize, the obese phenotype of LOB male rats is associated with increased hepatic TAG production and secretion, a shift in LPL activity from skeletal muscle to WAT, reduced lipolytic response in WAT depots and a specific increase in expression of genes responsible for fatty acid and TAG synthesis in the peri-renal depot.

The PLAC1 protein localizes to membranous compartments in the apical region of the syncytiotrophoblast

PLAC1 is a trophoblast-specific gene that maps to a locus on the X-chromosome important to placental development. We have previously shown that PLAC1 gene expression is linked to trophoblast differentiation. The objective of this study was to define the localization of the PLAC1 polypeptide as a prerequisite to understanding its function. Polyclonal antibodies specific for the putative PLAC1 polypeptide were generated. The subcellular localization of PLAC1 in the trophoblast was examined by immunohistochemical analysis of human placenta complemented by immunoblot analysis of subcellular fractions. Brightfield immunohistochemical analysis of placental tissue indicated that the PLAC1 protein localizes to the differentiated syncytiotrophoblast in the apical region of the cell. Deconvlution immunofluorescence microscopy confirmed localization to the apical region of the syncytiotrophoblast. Its distribution included both intracellular compartments as well as loci in close association with the maternal-facing, microvillous brush border membrane (MVM). These findings were supported by immunoblot analysis of subcellular fractions. A 30 kDa band was associated with the microsomal fraction of placental lysates but not the mitochondrial, nuclear, or soluble fractions, suggesting PLAC1 is targeted to a membrane location. Plasma membranes were obtained from the fetal-facing, basal surface (BM) and the maternal-facing, MVM of the syncytiotrophoblast membrane. PLAC1 immunoreactivity was only detected in membrane fractions derived from the apical MVM consistent with immunohistochemical analyses. These data demonstrate that the PLAC1 protein is restricted primarily to the differentiated trophoblast, localizing to intracellular membranous compartment(s) in the apical region of the syncytiotrophoblast and associated with its apical, microvillous membrane surface.

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.

Activity and expression of Na+/H+ exchanger isoforms in the syncytiotrophoblast of the human placenta

The purpose of this study was to compare Na+/H+ exchanger (NHE) activity in the microvillous (MVM) and basal (BM) plasma membrane of the human placental syncytiotrophoblast and to determine the relative contribution of various NHE isoforms to this activity. Uptake of 22Na into isolated MVM vesicles in the presence of a H+ gradient, at initial rate, was four- to fivefold higher than that by BM vesicles (214+/-28 vs. 49+/-9 pmol/mg protein per 30 s, respectively, means+/-SEM, n=8, 6, P<0.001). The 22Na uptake by MVM, but not by BM, was reduced in the absence of a H+ gradient and in the presence of 500 microM amiloride. To determine the contribution of NHE1, NHE2 and NHE3 isoforms to NHE activity in MVM, we investigated the effect of amiloride analogues which show isoform selectivity. HOE 694, an analogue selective for NHE1 at low concentrations, inhibited 22Na uptake with an EC50 of 0.13+/-0.05 microM (n=6), whereas S3226, an analogue selective for NHE3 at low concentrations had an EC50 of 3.01+/-0.85 microM (n=5). To investigate this further, we measured recovery of syncytiotrophoblast intracellular pH (pHi) from an acid load using a H+-selective, fluorescent dye (BCECF) loaded into isolated intact placental fragments. This recovery was blocked in the absence of Na+ and the presence of amiloride (500 microM) and concentrations of HOE 694 and S3226 were comparable to those used in vesicle experiments. Overall these data show that under the conditions used NHE activity in the term placental syncytiotrophoblast is absent from BM. NHE activity in the MVM is attributable predominantly to NHE1.

Overexpression of lipoprotein lipase improves insulin resistance induced by a high-fat diet in transgenic rabbits

AIMS/HYPOTHESIS

Dysfunctions of lipoprotein lipase (LPL) have been found to be associated with dyslipidaemias, atherosclerosis, obesity and insulin resistance. There are two conflicting hypotheses regarding the roles of LPL in glucose metabolism and insulin resistance. Whether systemically increased LPL activity would be beneficial or detrimental to insulin sensitivity is yet to be resolved. To address this issue, we studied transgenic rabbits overexpressing human LPL transgene.

METHODS

LPL transgenic and control rabbits were fed a 10% high-fat diet (HFD) for 16 weeks. To evaluate glucose metabolism, we compared plasma levels of glucose and insulin in transgenic rabbits with control rabbits and performed an intravenous glucose tolerance test. In addition, we measured adipose tissue accumulation in HFD-fed rabbits.

RESULTS

Increased LPL activity in transgenic rabbits resulted in a significant reduction of plasma triglycerides and non-esterified fatty acids, but not in basal levels of glucose and insulin. HFD feeding induced an elevation of plasma glucose levels accompanied by hyperinsulinaemia in control rabbits, but was significantly inhibited in transgenic rabbits. The intravenous glucose tolerance test showed that transgenic rabbits had faster glucose clearance associated with lower levels of insulin secretion than control rabbits. In addition, there was a significant reduction of body adipose tissue in transgenic rabbits compared with in control rabbits fed an HFD. Scanning electron microscopic examination revealed that adipocytes in transgenic rabbits were predominately small cells.

CONCLUSIONS/INTERPRETATION

Our results showed that systemically increased LPL activity improves insulin resistance and reduces adipose accumulation in transgenic rabbits, indicating that systemic elevation of LPL may have potential benefits for the treatment of insulin resistance and obesity.

ATP-sensitive potassium channels in the cerebral circulation

BACKGROUND

In brain blood vessels, electrophysiological studies proving the existence of ATP-sensitive potassium channels (KATP) are scarce. However, numerous pharmacological studies establish the importance of KATP channels in these blood vessels. This review emphasizes the data supporting the importance of vascular KATP in the responses of brain blood vessels.

SUMMARY OF REVIEW

Electrophysiological data show the existence of KATP in smooth muscle and endothelium of brain vessels. A much larger number of studies in virtually all experimental species have shown that classic openers of KATP dilate brain arteries and arterioles. This response can by blocked by glibenclamide, a selective inhibitor of KATP opening. Several physiological or pathophysiological responses are also blocked by glibenclamide. KATP contains a multiplicity of potential sites of interaction with drugs of diverse, sometimes unrelated, structures. Drugs with imidazole or guanidinium groups are particularly likely to have effects on KATP. This complicates interpretation of the actions of such drugs when used as supposedly selective pharmacological probes for other putative targets. A pH-sensitive site on the internal surface of cloned channels may explain the glibenclamide-inhibitable dilation produced by intracellular acidosis and perhaps by CO2. In some situations KATP appears to be involved in either the synthesis/release or action of endothelium-derived mediators of cerebrovascular tone. The importance of KATP may be dependent on the portion of the cerebrovascular tree being studied and on diverse experimental conditions, age, species, and the presence of disease.

CONCLUSIONS

KATP have been shown to mediate a wide range of cerebrovascular response in physiologic or pathologic circumstances in a variety of experimental conditions. Their relevance to cerebrovascular responses in humans remains to be explored.

System y+ localizes to different membrane subdomains in the basolateral plasma membrane of epithelial cells

We report here that the system y+ cationic amino acid transporter ATRC1 localized to clusters within the basolateral membrane of polarized Madin-Darby canine kidney and human embryonic kidney (HEK) cells, suggesting that the transporters are restricted to discrete membrane microdomains in epithelial cells. Based on solubility in nonionic detergents, two populations of ATRC1 molecules existed: approximately half of the total ATRC1 in HEK cells associated with the actin membrane cytoskeleton, whereas another one-fourth resided in detergent-resistant membranes (DRM). In agreement with these findings, cytochalasin D reduced the amount of ATRC1 associated with the actin membrane cytoskeleton. Although some ATRC1 clusters in HEK cells colocalized with caveolin, the majority of ATRC1 did not colocalize with this marker protein for a type of DRM called caveolae. This distribution of ATRC1 is somewhat different from that reported for pulmonary artery endothelial cells in which transporters cluster predominantly in caveolae, suggesting that differences in the proportion of ATRC1 in specific membrane microdomains correlate with differences in the physiological role of the transporter in polarized kidney epithelial vs. vascular endothelial cells.

Down-regulation of adipose tissue lipoprotein lipase during fasting requires that a gene, separate from the lipase gene, is switched on

During short term fasting, lipoprotein lipase (LPL) activity in rat adipose tissue is rapidly down-regulated. This down-regulation occurs on a posttranslational level; it is not accompanied by changes in LPL mRNA or protein levels. The LPL activity can be restored within 4 h by refeeding. Previously, we showed that during fasting there is a shift in the distribution of lipase protein toward an inactive form with low heparin affinity. To study the nature of the regulatory mechanism, we determined the in vivo turnover of LPL activity, protein mass, and mRNA in rat adipose tissue. When protein synthesis was inhibited with cycloheximide, LPL activity and protein mass decreased rapidly and in parallel with half-lives of around 2 h, and the effect of refeeding was blocked. This indicates that maintaining high levels of LPL activity requires continuous synthesis of new enzyme protein. When transcription was inhibited by actinomycin, LPL mRNA decreased with half-lives of 13.3 and 16.8 h in the fed and fasted states, respectively, demonstrating slow turnover of the LPL transcript. Surprisingly, when actinomycin was given to fed rats, LPL activity was not down-regulated during fasting, indicating that actinomycin interferes with the transcription of a gene that blocks the activation of newly synthesized LPL protein. When actinomycin was given to fasted rats, LPL activity increased 4-fold within 6 h, even in the absence of refeeding. The same effect was seen with alpha-amanitin, another inhibitor of transcription. The response to actinomycin was much less pronounced in aging rats, which are obese and insulin-resistant. These data suggest a default state where LPL protein is synthesized on a relatively stable mRNA and is processed into its active form. During fasting, a gene is switched on whose product prevents the enzyme from becoming active even though synthesis of LPL protein continues unabated.

Ovine placental and fetal arginine metabolism at normal and increased maternal plasma arginine concentrations

Arginine (A) may play a significant role in fetal growth, by stimulating insulin secretion and as a precursor for both polyamine synthesis and nitric oxide production. To determine whether increased maternal plasma A concentrations can enhance delivery of A to the fetus, uterine, umbilical, and net uteroplacental (UP) A uptake rates were measured in 12 pregnant ewes at 129.6 +/- 0.4 d gestation (mean +/- SEM) during normal and after 3 h of increased maternal plasma A concentrations. With a 2.7-fold increase in maternal plasma A concentrations (p < 0.001), there were significant increases in uterine A uptake (13.8 +/- 1.0 to 41.3 +/- 7.7 micromol/min, p < 0.005), umbilical A uptake (3.3 +/- 0.5 to 5.2 +/- 0.8 micromol.min(-1).kg(-1) fetus, p < 0.005), UP A uptake (17.8 +/- 6.2 to 89.2 +/- 20.3 micromol.min(-1).kg(-1) placenta, p < 0.01), fetal arterial A concentration (98.7 +/- 6.3 to 137.1 +/- 9.9 microM, p < 0.001), maternal A disposal rate (143.7 +/- 9.4 to 217.0 +/- 6.7 micromol/min, p < 0.001), fetal A disposal rate (7.9 +/- 0.8 to 9.9 +/- 1.1 micromol.min(-1).kg(-1), p < 0.05), fetal A oxidation rate (1.31 +/- 0.24 to 1.84 +/- 0.36 micromol.min(-1).kg(-1), p < 0.05), and plasma insulin concentration in both fetus (16 +/- 2 to 20 +/- 2 microU/mL, p < 0.001) and mother (24 +/- 3 to 32 +/- 4 microU/mL, p < 0.001). Thus, increased maternal plasma A concentration increases maternal, UP, and fetal A net uptake, and increases insulin secretion in mother and fetus. The 4.2-fold larger increase in UP than net fetal A uptake could represent preferential UP A metabolism relative to fetal A metabolism, relatively limited placental-fetal A transport capacity compared with uterine A uptake capacity, or both; responsible mechanisms remain unknown.

Dietary D-psicose, a C-3 epimer of D-fructose, suppresses the activity of hepatic lipogenic enzymes in rats

D-Psicose (D-ribo-2-hexulose), a C-3 epimer of D-fructose, is present in small quantities in commercial carbohydrate complexes or agricultural products. Wistar male rats were fed experimental diets which consisted of 5% D-psicose, cellulose, D-fructose or D-glucose for 28 days. Abdominal adipose tissue weight was significantly lower (P < 0.05) in rats fed the D-psicose diet than in rats fed a D-fructose and D-glucose diets, even though the four dietary groups were offered the same amount throughout the experimental period. Fatty acid synthase and glucose 6-phosphate dehydrogenase activities in the liver were significantly lower (P < 0.05) in rats fed the D-psicose diet than in rats fed the D-fructose and D-glucose diets. However, lipoprotein lipase activities in the heart, soleus muscle and perirenal adipose tissue were the same. These results suggest that a supplement of D-psicose in the diet suppresses hepatic lipogenic enzyme activities. The lower abdominal fat accumulation in rats fed a D-psicose diet might result from lower lipogenesis in the liver.

Evidence for a K(ATP) ion channel link in the inhibition of hypercapnic dilation of pial arterioles by 7-nitroindazole and tetrodotoxin

7-Nitroindazole, an inhibitor of neuronal nitric oxide synthase, reportedly inhibits hypercapnic dilation, but tetrodotoxin, an inhibitor of neuronal transmission, reportedly does not. Thus, evidence does not uniformly support the hypothesis of a neurogenic link to the hypercapnic response. Others suggest the hypercapnic response is mediated by a K(ATP) ion channel. In the following studies, we observed that topically administered tetrodotoxin inhibited dilations produced by hypercapnia. In addition, topical tetrodotoxin and either topical or intraperitoneal 7-nitroindazole, inhibited dilations produced by the K(ATP) channel openers, cromakalim and pinacidil. Inhibition of hypercapnic dilation and inhibition of dilation by the openers of the K(ATP) channel was immediately reversed by either L-lysine or L-arginine, amino acids previously shown to facilitate opening of the channel. The data strongly supports the previous conclusion that there is a K(ATP) ion channel link in the response of pial arterioles to hypercapnia. The location of the channel is not established by these data, nor is it known whether the action of tetrodotoxin on the channel was direct or indirect.

Amino acid transporters in the human placenta

Amino acid transport across the human placenta is active, mediated by specific transporters in syncytiotrophoblast plasma membranes. Using functional criteria such as substrate specificity and sodium dependence, approximately 15 transport systems have been identified in the human placenta. Recently, the area of molecular biology of amino acid transporters has evolved rapidly and at least 25 cDNA clones coding for mammalian amino acid transporters or transporter subunits have been identified. The primary objective of this review is to integrate the available functional data on placental amino acid transport systems with recent molecular information on mammalian amino acid transporters. Furthermore, models for the mechanisms for net materno-fetal transfer of amino acids are discussed. Finally, the evidence to suggest that alterations in placental amino acid transport systems may play a crucial role in the regulation of fetal growth are presented briefly.

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.

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.

Development and polarization of cationic amino acid transporters and regulators in the human placenta

We have investigated L-arginine transport systems in the human placental syncytiotrophoblast across gestation using purified microvillous (MVM) and basal (BM) plasma membrane vesicles. In MVM from first-trimester and term placentas, L-arginine transport was by systems y(+) and y(+)L. In BM (term placentas), however, there was evidence for system y(+)L only. The Michaelis constant of system y(+)L was significantly lower (P < 0.05) in first-trimester compared with term MVM and lower in term MVM compared with BM (P < 0.05). There was no functional evidence for system b(0+) in term MVM or BM. Cationic amino acid transporter (CAT) 1, CAT 4, and 4F2hc were detected using RT-PCR in placentas throughout gestation. rBAT was not detected in term placentas. An approximately 85-kDa and an approximately 135-kDa protein was detected by Western blotting in MVM under reducing and nonreducing conditions, respectively, consistent with the 4F2hc monomer and the 4F2hc-light chain dimer, and their expression was significantly higher (P < 0.05) in term compared with first-trimester MVM. These proteins were not detected in BM despite functional evidence for system y(+)L. These data suggest different roles for 4F2hc in the development and polarization of cationic amino acid transporters in the syncytiotrophoblast.

IGF binding protein-1 (IGFBP-1) is preferentially associated with the fetal-facing basal surface of the syncytiotrophoblast in the human placenta

IGF receptors are expressed in a spatially polarized manner on the syncytiotrophoblast cell membrane. We therefore examined the hypothesis that IGFBPs expressed at the maternal-fetal interface interact with distinct surfaces of the syncytiotrophoblast membrane to modulate IGF function. Membrane vesicles were prepared specifically from the maternal-facing, microvillous membrane (MVM) and the fetal-facing, basal membrane (BM) surfaces of the syncytiotrophoblast. The association of IGFBPs with each membrane preparation was determined by ligand blot analysis. A doublet migrating at 38/42 kD was detected in both MVM and BM preparations. Selective immunoprecipitation followed by ligand blot analysis identified this IGF binding species as IGFBP-3. Additionally, a protein migrating at approximately 29 kD was associated primarily with the BM. This protein was identified as IGFBP-1 by both immunoprecipitation and ligandblotting techniques. Non-denaturing PAGE revealed five distinct bands corresponding to different degrees of phosphorylation. The phosphorylation pattern of BM-associated IGFBP-1 was identical to that of native IGFBP-1 in amniotic fluid. Immunohistological analysis of term placenta revealed IGFBP-1-specific staining of the syncytiotrophoblast and the fetal capillary/pericapillary bed. The localization of IGFBP-1 to a distinct compartment within the fetal placenta, not in proximity to the syncytiotrophoblast type I IGF receptor, suggests it may play a role in regulating/targeting IGF activity within the stromal compartment or by exerting IGF-independent effects on the basal surface of the syncytiotrophoblast. The nature of its binding to the BM has not been determined.

Placental anionic and cationic amino acid transporter expression in growth hormone overexpressing and null IGF-II or null IGF-I receptor mice

The role of growth hormone (GH), insulin-like growth factor (IGF)-II and the IGF-I receptor (IGF-Ir) in the regulation of the in vivo expression of Na(+)-coupled anionic [System X-AG; GLAST1 (EAAT1), GLT1 (EAAT2), EAAC1 (EAAT3), EAAT4; where the human homologues of amino acid transport proteins first cloned in the rat are given in parentheses] and Na(+)-independent cationic (System y(+);CAT1) amino acid transport proteins was evaluated by comparing transporter expression in day 17 placentae of mice that overexpressed bovine GH (GH+) or that carried null gene mutations for IGF-II or IGF-Ir. Northern analysis revealed no apparent difference in the mRNA content of GLAST1 (EAAT1), EAAC1 (EAAT3), or EAAT4, in homogenates of GH+ placentae, but levels of GLT1 (EAAT2) and CAT1 mRNA were increased. Immunoblot analysis revealed that whole-placental steady-state GLAST1 (EAAT1), EAAC1 (EAAT3), and EAAT4 protein levels were not affected by GH+, whereas GLT1 (EAAT2) levels were increased. Immunohistochemical analysis showed that the cell-specific expression of the anionic and CAT1 transporters was not affected by overexpression of GH. Similar analyses of null IGF-II placentae demonstrated increases in GLAST1 (EAAT1), EAAT4 and CAT1 mRNAs. Parallel immunoblot analysis demonstrated decreased expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3) protein, but an increased expression of EAAT4. In null IGF-II and IGF-Ir placentae, however, GLT1 (EAAT2) and EAAC1 (EAAT3) protein content was decreased in junctional zone cells, whereas CAT1 content was increased in junctional and labyrinth zone cells. These data indicate that an excess level of GH stimulates GLT1 (EAAT2) expression and that a normal level of IGF-II is required for typical expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3), but that IGF-II downregulates the expression of EAAT4 and CAT1.

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.

Conducted signals within arteriolar networks initiated by bioactive amino acids

Our purpose was to determine the specificity of L-arginine (L-Arg)-induced conducted signals for intra- vs. extracellular actions of L-Arg. Diameter and red blood cell velocities were measured for arterioles [18 +/- 1.6 (SE) micrometer] in the cremaster muscle of pentobarbital sodium-anesthetized (Nembutal, 70 mg/kg) hamsters (n = 53). Remote (conducted) responses were viewed approximately 1,000 micrometer upstream from the local (micropipette) application. Six amino acids were tested: L-arginine, L-cystine, L-leucine, L-lysine, L-histidine, and L-aspartate (100 microM each). Only L-Arg induced a remote dilation; L-lysine and L-aspartate had no effect, and the others each induced a significant remote constriction. There is a second conducted signal initiated by L-arginine that preconditions the arteriolar network and upregulates a direct response of L-arginine to dilate the remote site. This was blocked by inhibition of L-arginine uptake at the local (preconditioning) site (100 microM L-histidine or 1 mM phenformin). Arginine-glycine-aspartate (100 microM)-induced remote dilations (+3. 2 +/- 0.3 micrometer) were not mimicked by a peptide control and were prevented by anti- integrin alphav monoclonal antibody. Remote dilations were greater in animals with a higher wall shear stress for arginine-glycine-aspartate (r2 = 0.92) but not for L-arginine (r2 = 0.12). Thus L-arginine initiates separate conducted signals related to system y+ transport, integrins, and baseline flow.

In vitro modulation of L-arginine transport in trophoblast cells by glucose

BACKGROUND

The uptake of the semi-essential amino acid, L-arginine, into trophoblast cells was measured with the aim of determining the effect of different glucose concentrations on L-arginine influx kinetics.

METHODS

This study used a novel superfused microcarrier culture system of BeWo cells (an established choriocarcinoma cell line with many characteristics of normal human trophoblast) and a rapid, paired-tracer dilution technique to measure unidirectional influx into the cells.

RESULTS

At 10 mmol L-1 D-glucose, L-arginine unidirectional influx across the microvillous border of the cells was saturable with a Km of 1.14 +/- 0.14 mmol L-1 and a Vmax of 121. 36 +/- 5.89 nmol mg-1 protein min-1. When cells were preincubated for 24 h in the presence of 30 mmol L-1 D-glucose, there was a significant increase in the Vmax for L-arginine of nearly 30%. Similarly, preincubation in the presence of 1 mmol L-1 D-glucose and 12.5 mIU mL-1 human insulin reduced the Km for L-arginine influx by over 55%.

CONCLUSION

These data suggest that the modulation of placental transport of L-arginine by glucose and insulin could contribute to the fetal macrosomia observed in diabetic mothers.

Placental transport of leucine and lysine is reduced in intrauterine growth restriction

Intrauterine growth restriction (IUGR) is characterized by a reduction in fetal plasma concentrations of a number of essential amino acids. Whether this is caused by impaired placental transport is unknown. We studied transport of leucine and lysine in syncytiotrophoblast microvillous (MVM) and basal membrane (BM) vesicles isolated from uncomplicated (control) and IUGR pregnancies. In addition, we investigated the possibility that leucine uptake is stimulated by an outwardly directed glycine gradient. Uptake of 3H-L-lysine (0.1 microM) and 3H-L-leucine (0.25 microM) was studied at 37 degrees C using rapid filtration techniques. In IUGR, mediated uptake of lysine was reduced by 44% (p < 0.05) in BM and uptake of leucine was lower in both MVM (-46%, p < 0.05) and BM (-38%, p < 0.05) compared with control vesicles. Intravesicular glycine (2 mM) increased the uptake of leucine by 98% in MVM (p < 0.05). These data suggest that the activity of placental transporters for cationic and neutral amino acids is reduced in IUGR. We speculate that a reduced glycine gradient in the placenta in IUGR, due to reduction in system A activity, will impair leucine transport to the fetus, providing an additional mechanism for reduced placental transport of leucine in IUGR.

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.

Sodium-independent lysine uptake by the BeWo choriocarcinoma cell line

Transport of L-lysine by a cultured placental trophoblast cell line was investigated by characterization of L-[3H]lysine uptake. In the mononuclear form of the BeWo clone b30 choriocarcinoma cell, at least two sodium-independent systems are present. Concentration dependence data were fitted by a two system model with Km values (+/- s.e.) of 2 +/- 0.7 and 94 +/- 31 microM and Vmax values (+/- s.e.) of 0.7 +/- 0.3 and 25 +/- 6.0 nM/mg DNA/min. A portion of sodium-independent uptake was inhibited by the sulphydryl modifying reagent N-ethylmaleimide (NEM). Following NEM treatment, the data were fitted by a single system with Km = 10 +/- 2 microM AND Vmax = 5.1 +/- 0.8 nM/mg DNA/min. In the absence of sodium, NEM-resistant uptake was sensitively inhibited by leucine whereas NEM-sensitive uptake was not inhibited by leucine. It is concluded that like placental basal membrane, the mononuclear BeWo cell possesses two sodium-independent L-lysine transport systems. The high-capacity, NEM-sensitive, leucine-insensitive system resembles the widespread system y+. The high-affinity, NEM-resistant, leucine-sensitive system resembles system b(0,+).

Transporters for cationic amino acids in animal cells: discovery, structure, and function

The structure and function of the four cationic amino acid transporters identified in animal cells are discussed. The systems differ in specificity, cation dependence, and physiological role. One of them, system y+, is selective for cationic amino acids, whereas the others (B[0,+], b[0,+], and y+ L) also accept neutral amino acids. In recent years, cDNA clones related to these activities have been isolated. Thus two families of proteins have been identified: 1) CAT or cationic amino acid transporters and 2) BAT or broad-scope transport proteins. In the CAT family, three genes encode for four different isoforms [CAT-1, CAT-2A, CAT-2(B) and CAT-3]; these are approximately 70-kDa proteins with multiple transmembrane segments (12-14), and despite their structural similarity, they differ in tissue distribution, kinetics, and regulatory properties. System y+ is the expression of the activity of CAT transporters. The BAT family includes two isoforms (rBAT and 4F2hc); these are 59- to 78-kDa proteins with one to four membrane-spanning segments, and it has been proposed that these proteins act as transport regulators. The expression of rBAT and 4F2hc induces system b[0,+] and system y+ L activity in Xenopus laevis oocytes, respectively. The roles of these transporters in nutrition, endocrinology, nitric oxide biology, and immunology, as well as in the genetic diseases cystinuria and lysinuric protein intolerance, are reviewed. Experimental strategies, which can be used in the kinetic characterization of coexpressed transporters, are also discussed.

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.

Identification of two leucine-sensitive lysine transport activities in human placental basal membrane

The recent demonstration of multiple high-affinity leucine-sensitive cationic transport systems prompted this investigation of their role in lysine uptake in basal cell membrane. Transport of lysine by basal membrane was saturable at both 22 and 37 degrees C and linear in time to 1 min and 30 sec, respectively. At 22 degrees C, at least two systems were active. The portion of uptake inhibited by the sulphydryl binding reagent N-ethylmaleimide (NEM) but not by leucine in the absence of sodium had a high K(m) and high Vmax and was attributed to system y+. NEM-insensitive uptake was fitted by a one-system model with K(m) (+/- s.e.) of 4 +/- 1 microM and a Vmax of 0.9 +/- 0.1 pmol/mg protein/min. This component was completely inhibited by leucine in the absence of sodium but not by glutamine in the presence of sodium. Therefore, it was attributed to system bo,+. At 37 degrees C, at least three systems were active. For essentially the same reasons as above the NEM inhibitable uptake was attributed to system y+. NEM-insensitive uptake was fitted by a one-system model with K(m) of 26 +/- 7 microM and Vmax of 11.1 +/- 2.8 pmol/mg protein/30 sec. Inhibition studies, however, indicated its heterogeneity. NEM-insensitive saturable uptake was only partially inhibited by either leucine in the absence of sodium (system bo,+) or by glutamine in the presence of sodium (system y+L). It is concluded that the NEM-insensitive portion of lysine uptake at 37 degrees C represents activity of both system bo,+ and the temperature-sensitive system y+L. As a previous investigation indicates, only one of these (system y+L) is present in the more specialized microvillous membrane. The demonstration of functional differences in the high affinity leucine transporters of basal and microvillous membrane in this and our previous investigations suggest that the two membranes possess different transport or modifier proteins.

Glutamine transport in human and rat placenta

Glutamine plays an important role in fetal nutrition. This study explored the transport of [3H]glutamine into apical and basal predominant membrane vesicles derived from rat and human placenta. Na+-dependent glutamine transport was present in both apical and basal predominant vesicles derived from 20- and, to a lesser degree, 14-day gestation rat placenta. Amino-acid transport systems A, ASC-like, B(o,+) (in apical membrane vesicles) and, perhaps, y+L were involved in Na+-dependent glutamine transport. Na+-dependent glutamine uptake into human placental microvillus and basolateral membrane vesicles also occurred via several distinct transport activities. Glutamine transport via system N was not detected in either rat or human placental preparations. Na+-dependent glutamine transport in the rat was more pronounced in basal as compared to apical membrane vesicles. Conversely, in the human preparations, activity was significantly higher in microvillus as compared to basolateral membrane vesicles. It is concluded that Na+-dependent glutamine transport occurs through a variety of transport agencies in both the rat and human placenta. Transport varies with ontogeny and between species.

Spatial polarization of insulin-like growth factor receptors on the human syncytiotrophoblast

The expression of IGF receptors on the maternal-facing, microvillous membrane (MVM) surface and the fetal-facing, basal membrane (BM) surface of the syncytiotrophoblast was studied using standard ligand binding assays, covalent cross-linking techniques, and immunoblot analysis. Scatchard analysis of [125I]IGF-I and -II binding revealed the presence of both high and low affinity binding sites associated with each membrane preparation that did not clearly distinguish between the two membrane preparations. Cross-linking analysis, however, demonstrated type I and type II IGF receptors associated primarily with MVM, suggesting that nonreceptor binding sites may contribute to total membrane binding. Ligand blot analysis revealed that BM are uniquely associated with 29- and 24-kD IGF binding proteins (IGFBPs). [125I]QAYL-IGF-I, having reduced affinity for IGFBPs, was therefore used to study receptor-specific binding. Approximately 5-fold more type I IGF receptors were shown to be associated with MVM than BM by Scatchard and cross-linking analyses. This was confirmed by immunoblot analysis. By contrast, immunoblot analysis revealed approximately 50-100% more type II IGF receptor protein associated with BM, whereas cross-linking to [125I]IGF-II revealed a MVM predominance. In the presence of 5 mM mannose 6-phosphate, however, a substantial increase in [125I]IGF-II cross-linked to the type II IGF receptor was observed in BM but not MVM consistent with immunoblot analysis. These data demonstrate that type 1 and unoccupied type II IGF receptors are expressed primarily on the maternal-facing. MVM surface of the syncytiotrophoblast.

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.

Evidence for an interaction between cationic and neutral amino acids at the blood-facing aspect of the lactating rat mammary epithelium

The transport of lysine by perfused lactating rat mammary tissue has been examined using a rapid, paired-tracer dilution technique. This experimental approach allowed the characteristics of lysine transport across the blood-facing aspect of the mammary epithelium to be studied. The clearance of lysine from the perfusate was influenced by the extracellular lysine concentration in a fashion consistent with the presence of carrier-mediated transport. Replacing extracellular Na+ with N-methyl-D-glucamine had no significant effect on lysine transport. Lysine uptake was inhibited by extracellular leucine and glutamine but not by alpha-(methylamino)isobutyric acid. Leucine interacted with lysine transport under Na+(-)free conditions. It appears that the system for cationic acid transport which is situated in the blood-facing aspect of the lactating rat mammary epithelium may also accept neutral amino acids as substrates.

Membrane potential dependence of the kinetics of cationic amino acid transport systems in human placenta

1. Mediated influx of L-lysine into human placental brush-border membrane vesicles occurs through two systems, one of lower affinity but high capacity, the other of very high affinity but low capacity. These transporters have features characteristic of systems y+ (the classical system) and y+L (recently described in the erythrocyte), respectively. 2. In solutions containing sodium the entry of lysine through the high-affinity system (y+L) is inhibited by the neutral amino acids L-leucine, L-methionine and L-glutamine with comparable high affinity. The removal of sodium reduces the affinity but not the maximal extent of this inhibition. Leucine and methionine, but apparently not glutamine, inhibit lysine entry through system y+ with a much lower affinity. 3. The influx of lysine through system y+ changes markedly in response to alterations of membrane potential. In the presence of an inwardly directed negative diffusion potential created by an inwardly directed thiocyanate (SCN-) gradient, the influx of lysine through this route is accelerated; with an inwardly directed positive potassium diffusion potential, lysine influx through this route is reduced. The influx of lysine through system y+L is much less sensitive to such alterations of potential. 4. Analysis of the kinetic constants characterizing system y+ shows that with a change of potential from zero to negative (approximately -60 mV) the maximum velocity (Vmax) is roughly doubled and the half-saturation constant (Km) halved leading to a 4-fold increase in permeability. For system y+L smaller changes are seen and Km does not change; the resulting increase in y+L permeability is 1.5-fold. 5. These findings are discussed with respect both to the mechanism of membrane transport and placental epithelial function.

Lysine transport in lactating rat mammary tissue: evidence for an interaction between cationic and neutral amino acids

The transport of lysine by the lactating rat mammary gland has been examined to determine whether there is an interaction between cationic and neutral amino acids. Lysine uptake was time dependent and unaffected by replacing Na+ with choline. In the presence of Na+, lysine influx was inhibited by cationic amino acids (arginine, homoarginine, ornithine and lysine) and by a range of neutral amino acids (methionine, glutamine, leucine, phenylalanine, alanine, asparagine, alpha-aminoisobutyric acid (AIB), 2-aminobicyclo [2,2,1] heptane-2-carboxylic acid (BCH), proline and tryptophan). Leucine and glutamine also inhibited lysine influx in the absence of Na+ but phenylalanine and proline did not. Lysine efflux from mammary tissue was trans-accelerated by various cationic amino acids (lysine, arginine, homoarginine and ornithine). In addition, leucine and glutamine were capable of trans-stimulating lysine efflux in the presence and absence of Na+. It appears that cationic and neutral amino acids stimulated lysine efflux at a single locus.

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.

Poly(A)+ RNA from the mucosa of rat jejunum induces novel Na(+)-dependent and Na(+)-independent leucine transport activities in in oocytes of Xenopus laevis

Complementary DNA clones have been isolated recently from rat (D2) and rabbit kidney (rBAT) which induce increased Na(+)-independent Leu and Lys transport activities (System b0, +) when expressed in oocytes of Xenopus laevis. These cDNAs encode type II membrane glycoproteins which show significant homology to the heavy chain of the human and mouse 4F2 surface antigen (4F2hc). Injection of human 4F2hc cRNA into oocytes also results in induction of Leu/Lys transport activity, but with differing cation requirements for the two amino acids (Na(+)-dependent for Leu, Na(+)-independent for Lys: system y+L). System y+L is a newly discovered zwitterionic/cationic amino acid transporter first described in human erythrocytes. Here we have examined the characteristics of Leu transport in Xenopus oocytes microinjected with mRNA from the mucosa of rat jejunum. L-Leu uptake during 10 min (0.2 mM, 20 degrees C) reached 20 pmol/oocyte compared with endogenous uptake by water-injected oocytes of typically 3-4 pmol/oocyte. The expressed transport activity was 80% Na(+)-dependent. The Na(+)-dependent component of the expressed flux was saturable (Km app 0.20 mM) and inhibited by Lys, but not by Ala or Phe. The minor Na(+)-independent component of expressed Leu transport activity was also saturable (Km app 0.10 mM). Amino acid inhibition studies resolved this flux into two main components, one of which was inhibited by Lys, Ala and Phe and another which was only inhibited by Lys. There was a small residual component of Na(+)-independent Leu transport which was insensitive to inhibition by Lys. Experiments utilizing polymerase chain reaction (PCR) demonstrated the presence of both D2 and 4F2hc message in rat jejunum. Hybrid-depletion of jejunal mRNA with an antisense oligonucleotide complementary to D2 had no effect on the expression of Na(+)-linked Leu transport activity, but reduced the smaller Na(+)-independent component of Leu transport by 40%, suggesting only a minor role of D2 in the expression of rat intestinal Leu transport activity. Although the properties of Na(+)-dependent Leu transport were, with the exception of a lack of inhibition by Ala and Phe, consistent with erythrocyte y+L, hybrid-depletion of jejunal mRNA with an antisense oligonucleotide complementary to 4F2hc had no detectable effect on the expressed transport activity. We conclude, therefore, that mRNA from rat jejunum encodes novel Na(+)-dependent and Na(+)-independent transport activities unrelated to the D2/4F2hc glycoproteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Riboflavin uptake in microvillous and basal membrane vesicles isolated from full-term human placentas

Riboflavin uptake was characterized using membrane vesicles isolated from the apical (maternal-facing) and basal (fetal-facing) membranes of the syncytiotrophoblast from full-term human placentas. Equilibrium [3H]riboflavin uptake was insensitive to variations in incubation medium osmolarity in contrast to [3H]alanine uptake into an osmotically sensitive space. Osmotic insensitivity suggested riboflavin binding to a membrane component. The dissociation constant of riboflavin binding was similar in microvillous (Kd = 2 microM) and basal membrane vesicles (Kd = 1 microM). Binding capacity was significantly higher in microvillous membranes (Bmax = 11.9 pmol/mg protein). The relatively high affinity binding to the membrane vesicles may represent a first step in riboflavin transport.

N-ethylmaleimide discriminates between two lysine transport systems in human erythrocytes

1. The sulfhydryl reagent N-ethylmaleimide (NEM) was shown to inactivate the low affinity lysine transporter in human erythrocytes (system y+) without affecting the high affinity transporter (system y+L). 2. Pre-treatment of the cells with NEM reduced the rate of entry of L-[14C]lysine (1 microM) by approximately 50% (maximum effect). 3. NEM (0.2 mM) inhibited the NEM-sensitive component of the flux with mono-exponential kinetics. The inactivation rate constant (k, +/- S.E.M.) was 0.53 +/- 0.027 min-1 (25 degrees C). The substrate did not protect against inactivation. 4. Lysine self-inhibition experiments revealed two transport systems in untreated cells (half-saturation constants Km; +/- S.E.M.), 12.0 +/- 1.7 microM and 109 +/- 15.6 microM) and only one high affinity system in NEM-treated cells (Km 9.5 +/- 0.67 microM), indicating that NEM inactivates system y+. 5. The NEM-insensitive L-[14C]lysine influx (system y+L) was inhibited with high affinity by unlabelled neutral amino acids. The inhibition constant for L-leucine in sodium medium (Ki +/- S.E.M.) was 10.7 +/- 0.72 microM (37 degrees C). The system was also strongly inhibited by L-methionine, L-glutamine and with less affinity by L-phenylalanine and L-serine. N-methyl-L-leucine, L-proline and 2-amino-2-norbornane-carboxylic acid, a bicyclic analogue of leucine, did not exert a significant effect. 6. Lysine transport through system y+L occurred at the same rate in Na+, K+ or Li+ medium and the binding of lysine to the transporter was unaffected by Na+ replacement. 7. The interaction of system y+L with neutral amino acids was dependent on the cation present in the medium. The inhibition constant for leucine and glutamine increased approximately 90- and 60-fold respectively when Na+ was replaced by K+. Li+ was shown to be a very good substitute for Na+.

Stimulation of system y(+)-like amino acid transport by the heavy chain of human 4F2 surface antigen in Xenopus laevis oocytes

A kidney cortex cDNA clone (rBAT) has recently been isolated, which upon in vitro transcription and capping complementary RNA (cRNA) and injection into Xenopus laevis oocytes induces a system b0,(+)-like amino acid transport activity. This cDNA encodes a type II membrane glycoprotein that shows significant homology to another type II membrane glycoprotein, the heavy chain of the human and mouse 4F2 surface antigen (4F2hc). Here we demonstrate that injection of human 4F2hc cRNA into oocytes results in the activation of a cation-preferring amino acid transport system that appears to be identical to the y(+)-like transport already present in the oocyte. This is based on the following results: (i) Injection of in vitro transcripts from 4F2hc cDNA (4F2hc cRNA) into oocytes stimulates up to 10-fold the sodium-independent uptake of L-arginine and up to 4.1-fold the sodium-dependent uptake of L-leucine. In contrast, 4F2hc cRNA does not increase the basal sodium-independent uptake of L-leucine. (ii) Basal and 4F2hc cRNA-stimulated sodium-independent uptake of L-arginine is completely inhibited by L-leucine in the presence of sodium. Similarly, the basal and 4F2hc cRNA-stimulated sodium-dependent uptake of L-leucine is entirely inhibited by L-arginine. (iii) The stimulation of sodium-independent uptake of L-arginine and the stimulation of sodium-dependent uptake of L-leucine induced by injection of 4F2hc cRNA are both completely inhibited by dibasic L amino acids and to a lesser extent by D-ornithine. (iv) Both basal and 4F2hc cRNA-stimulated sodium-independent uptake of L-arginine show two additional characteristics of the system y+ transport activity: inhibition of L-arginine uptake by L-homoserine only in the presence of sodium and an increase in the inhibition exerted by L-histidine as the extracellular pH decreased. Our results allow us to propose that an additional family of type II membrane glycoproteins (composed by rBAT and 4F2hc) is involved in amino acid transport, either as specific activators or as components of amino acid transport systems.

Mediated Na+-independent transport of l-glutamate and l-cysine in 1- and 2-cell mouse conceptuses

L-Glutamate and L-cystine appeared to compete for transport via a mediated Na(+)-independent transport process in 1- and 2-cell conceptuses. Not only did these substances competitively inhibit each others' uptake by conceptuses, but their Ki values for inhibition were about equal to their Km values for transport in 1-cell conceptuses. Moreover, the transport process interacted strongly with L-amino acids that had 3-6 atoms in a chain between their negatively charged groups, whereas it interacted weakly or not at all with amino acids that did not have these characteristics or that were N-methylated. Transport of anionic amino acids was not altered greatly by pH in the range 4.5-8.0, but transport of L-cystine was much faster at higher pH values. The slower cystine transport at lower pH values was due primarily to protonation of its second amino group. A small increase in the degree of deprotonation of cystine's carboxyl groups also probably contributed slightly to its faster transport at higher pH values. By all of these criteria, the transport process in conceptuses appears to be a form of amino acid transport system xc-. System xc- activity was several-fold higher in 1- than in 2-cell conceptuses. Similarly, L-glutamate uptake by unfertilized eggs was relatively rapid, whereas it was much slower in immature, fully-grown oocytes. System xc- activity in 1-cell conceptuses also appeared to increase in response to the oxidative stress of culture, whereas no such increase was observed for 2-cell conceptuses. We suggest that this transient increase in the activity of system xc- activity during development of 2-cell conceptuses from immature, fully-grown oocytes could help protect unfertilized and fertilized eggs from oxidative stresses in situ.

Human placental syncytiotrophoblast expresses two pharmacologically distinguishable types of Na(+)-H+ exchangers, NHE-1 in the maternal-facing (brush border) membrane and NHE-2 in the fetal-facing (basal) membrane

We investigated whether highly purified preparations of basal (fetal-facing) membrane isolated from normal term human placentas possess Na(+)-H+ exchanger activity. Uptake of Na+ into basal membrane vesicles was stimulated many-fold by an outwardly directed H+ gradient. This H(+)-gradient-dependent uptake was inhibitable by amiloride and its analogues. Na+ uptake in these vesicles did not occur via a Na+ channel, as it was not influenced by changes in membrane potential and, in addition, was inhibited by benzamil only at high micromolar concentrations. The results indicate that the human placental basal membrane possesses Na(+)-H+ exchanger activity. We then studied whether this exchanger is similar to or distinct from the Na(+)-H+ exchanger described in brush border (maternal-facing) membrane preparations. For this purpose, we compared the pharmacological characteristics of the basal membrane Na(+)-H+ exchanger with those of the brush border membrane Na(+)-H+ exchanger. The basal membrane exchanger was about 20-fold less sensitive to inhibition by amiloride and about 70-fold less sensitive to inhibition by dimethylamiloride than was the brush border membrane exchanger. The exchanger activity in both membrane preparations was inhibitable by clonidine and cimetidine, but the inhibition patterns with these compounds were markedly different between basal and brush border membrane preparations. These data demonstrate that the basal membrane Na(+)-H+ exchanger is distinct from the brush border membrane Na(+)-H+ exchanger. The pharmacological profiles of these exchangers indicate that the human placental brush border membrane possesses the housekeeping or non-epithelial type Na(+)-H+ exchanger (NHE-1), whereas the basal membrane possesses the epithelial or apical type Na(+)-H+ exchanger (NHE-2).