Ion transport by the placenta: a review of membrane transport systems

Deciphering the Epigenetic Landscape: Placental Development and Its Role in Pregnancy Outcomes

The placenta stands out as a unique, transitory, and multifaceted organ, essential to the optimal growth and maturation of the fetus. Functioning as a vital nexus between the maternal and fetal circulatory systems, it oversees the critical exchange of nutrients and waste. This exchange is facilitated by placental cells, known as trophoblasts, which adeptly invade and remodel uterine blood vessels. Deviations in placental development underpin a slew of pregnancy complications, notably fetal growth restriction (FGR), preeclampsia (PE), recurrent spontaneous abortions (RSA), and preterm birth. Central to placental function and development is epigenetic regulation. Despite its importance, the intricate mechanisms by which epigenetics influence the placenta are not entirely elucidated. Recently, the scientific community has turned its focus to parsing out the epigenetic alterations during placental development, such as variations in promoter DNA methylation, genomic imprints, and shifts in non-coding RNA expression. By establishing correlations between epigenetic shifts in the placenta and pregnancy complications, researchers are unearthing invaluable insights into the biology and pathophysiology of these conditions. This review seeks to synthesize the latest findings on placental epigenetic regulation, spotlighting its crucial role in shaping fetal growth trajectories and development. Through this lens, we underscore the overarching significance of the placenta in the larger narrative of gestational health.

Feeding rumen-protected lysine prepartum alters placental metabolism at a transcriptional level

Rumen-protected Lys (RPL) fed to Holstein cows prepartum resulted in a greater intake and improved health of their calves during the first 6 wk of life. However, whether increased supply of Lys in late gestation can influence placental tissue and, if so, which pathways are affected remain to be investigated. Therefore, we hypothesize that feeding RPL during late gestation could modulate placental metabolism, allowing for improved passage of nutrients to the fetus and thus influencing the offspring development. Therefore, we aimed to determine the effects of feeding RPL (AjiPro-L Generation 3, Ajinomoto Health and Nutrition North America) prepartum (0.54% DM of TMR) on mRNA gene expression profiles of placental samples of Holstein cows. Seventy multiparous Holstein cows were randomly assigned to 1 of 2 dietary treatments, consisting of TMR top-dressed with RPL (PRE-L) or without (control, CON), fed from 27 ± 5 d prepartum until calving. After natural delivery (6.87 ± 3.32 h), placentas were rinsed with physiological saline (0.9% sodium chloride solution) to clean any dirtiness from the environment and weighed. Then, 3 placentomes were collected, one from each placental region (cranial, central, and caudal), combined and flash-frozen in liquid nitrogen to evaluate the expression of transcripts and proteins related to protein metabolism and inflammation. Placental weights did not differ from cows in PRE-L (15.5 ± 4.03 kg) and cows in CON (14.5 ± 4.03 kg). Feeding RPL prepartum downregulated the expression of NOS3 (nitric oxide synthase 3), involved in vasodilation processes, and SOD1, which encodes the enzyme superoxide dismutase, involved in oxidative stress processes. Additionally, feeding RPL prepartum upregulated the expression of transcripts involved in energy metabolism (SLC2A3, glucose transporter 3; and PCK1, phosphoenolpyruvate carboxykinase 1), placental metabolism and cell proliferation (FGF2, fibroblast growth factor 2; FGF2R, fibroblast growth factor 2 receptor; and PGF, placental growth factor), Met metabolism (MAT2A, methionine adenosyltransferase 2-α), and tended to upregulate IGF2R (insulin-like growth factor 2 receptor). Placental FGF2 and LRP1 (low-density lipoprotein receptor-related protein 1) protein abundance were greater for cows that received RPL prepartum than cows in CON. In conclusion, feeding RPL to prepartum dairy cows altered uteroplacental expression of genes and proteins involved in cell proliferation, and in metabolism and transport of glucose. Such changes are illustrated by increased expression of SLC2A3 and PCK1 and increased protein abundance of FGF2 and LRP1 in uteroplacental tissue of cows consuming RPL.

Hypernatremia in Newborn Due to Uncontrolled Maternal Diabetes Insipidus

Neonatal hypernatremia often occurs in poorly breastfeeding infants, and rehydration without complications is the primary treatment. However, the etiologies and management for neonates who present with hypernatremia immediately after birth are not well reported. In this case report, we describe a neonate with hypernatremia and hyperosmolality, born to a mother with oligohydramnios of unknown etiology. Meticulous fluid therapies considering the time to physiologic diuresis were successfully performed, and the underlying cause was determined to be an uncontrolled maternal central diabetes insipidus.

Early-onset neonatal hyperkalemia associated with maternal hypermagnesemia: a case report

BACKGROUND

Neonatal nonoliguric hyperkalemia (NOHK) is a metabolic abnormality that occurs in extremely premature neonates at approximately 24 h after birth and is mainly due to the immature functioning of the sodium (Na⁺)/potassium (K⁺) pump. Magnesium sulfate is frequently used in obstetrical practice to prevent preterm labor and to treat preeclampsia; this medication can also cause hypermagnesemia and hyperkalemia by a mechanism that is different from that of NOHK. Herein, we report the first case of very early-onset neonatal hyperkalemia induced by maternal hypermagnesemia.

CASE PRESENTATION

A neonate born at 32 weeks of gestation developed hyperkalemia (K⁺ 6.4 mmol/L) 2 h after birth. The neonate's blood potassium concentration reached 7.0 mmol/L 4 h after birth, despite good urine output. The neonate and his mother had severe hypermagnesemia caused by intravenous infusion of magnesium sulfate given for tocolysis due to pre-term labor.

CONCLUSION

The early-onset hyperkalemia may have been caused by the accumulation of potassium ions transported through the placenta, the shift of potassium ions from the intracellular to the extracellular space in the infant due to the malfunctioning of the Na⁺/K⁺ pump and the inhibition of renal distal tube potassium ion secretion, there is a possibility that these mechanisms were induced by maternal and fetal hypermagnesemia after maternal magnesium sulfate administration. Because neonatal hyperkalemia poses a significant risk for the development of life-threatening cardiac arrhythmia, this case highlights the necessity of maternal blood magnesium monitoring during magnesium sulfate administration and neonatal blood potassium monitoring when there is severe maternal hypermagnesemia at delivery.

Cytotoxicant-induced trophoblast dysfunction and abnormal pregnancy outcomes: role of zinc and metallothionein

Normal trophoblast function, including implantation, hormone production, and formation of the selectively permeable maternofetal barrier, is essential for the establishment and maintenance of the fetoplacental unit and proper fetal development. Maternal cytotoxicant exposure causes the destruction of these cells, especially the terminally differentiated syncytiotrophoblasts, and results in a myriad of poor pregnancy outcomes. These outcomes range from intrauterine growth retardation and malformation to spontaneous abortion or stillbirth. There is recent evidence that the metal-binding protein, metallothionein, is involved in the protection of human trophoblastic cells from heavy metal-induced and severe oxidative stress-induced apoptosis. Metallothionein, with its unique biochemical structure, can both bind essential metal ions, such as the transcription modulator zinc, and yet allow their ready displacement by toxic nonessential metal ions or damaging free radicals. These properties suggest that metallothionein may be responsible not only for sequestering the cytotoxic agents, but also for altering signal transduction in the affected cells. Here, we review several identified causes of adverse pregnancy outcomes (specifically, prenatal exposure to cigarette smoke and alcohol, gestational infection, and exposure to environmental contaminants), discuss the role of zinc in modulating the cellular response to these toxic insults, and then propose how metallothionein may function to mediate this protective response.

Growth and function of the normal human placenta

The placenta is the highly specialised organ of pregnancy that supports the normal growth and development of the fetus. Growth and function of the placenta are precisely regulated and coordinated to ensure the exchange of nutrients and waste products between the maternal and fetal circulatory systems operates at maximal efficiency. The main functional units of the placenta are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. After implantation, trophoblast cells proliferate and differentiate along two pathways described as villous and extravillous. Non-migratory, villous cytotrophoblast cells fuse to form the multinucleated syncytiotrophoblast, which forms the outer epithelial layer of the chorionic villi. It is at the terminal branches of the chorionic villi that the majority of fetal/maternal exchange occurs. Extravillous trophoblast cells migrate into the decidua and remodel uterine arteries. This facilitates blood flow to the placenta via dilated, compliant vessels, unresponsive to maternal vasomotor control. The placenta acts to provide oxygen and nutrients to the fetus, whilst removing carbon dioxide and other waste products. It metabolises a number of substances and can release metabolic products into maternal and/or fetal circulations. The placenta can help to protect the fetus against certain xenobiotic molecules, infections and maternal diseases. In addition, it releases hormones into both the maternal and fetal circulations to affect pregnancy, metabolism, fetal growth, parturition and other functions. Many placental functional changes occur that accommodate the increasing metabolic demands of the developing fetus throughout gestation.

Calcium channels activated by endothelin-1 in human trophoblast

Ca2+ transfer across the syncytiotrophoblast (ST) of the human placenta is essential for normal fetal development. However, the nature of Ca2+ conductance in the ST and the mechanisms by which it is regulated are poorly understood. With the major signal transduction pathway of endothelin-1 (ET1) acting via phospholipase C (PLC) and Ca2+, we used ET1 to analyse the nature of Ca2+ channels on cultured trophoblastic cells by means of cytofluorimetric analysis using the ratiometric Ca2+ indicator Indo-1. Results indicate that ET1 (10(-7) M) stimulates a biphasic (transient and sustained) increase in [Ca2+]i in trophoblastic cells. This response is mediated by the endothelin receptor B (ETB) coupled to PLC, since treatment with BQ788 (10(-6) M) or U73122 (2 microM) totally abolished the response. Persistence of the rapid transient rise in [Ca2+]i in Ca2+-free extracellular medium confirms the release of Ca2+ from intracellular stores in response to ET1 stimulation. Furthermore, abolition of the sustained increase in [Ca2+]i in Ca2+-free extracellular medium argues in favour of the entry of Ca2+ during the plateau phase. Abolition of this plateau phase by Ni2+ (1 mM) in the presence of extracellular Ca2+ confirmed the existence of an ET1-induced Ca2+ entry. No evidence for the presence of voltage-operated channels was demonstrated during ET1 action since nifedipine (10(-6) M) did not reduce the Ca2+ response and depolarization with a hyper-potassium solution had no effect. Pharmacological studies using the imidazole derivatives SK&F96365 (30 microM) and LOE 908 (10 microM) partially inhibited the ET1-evoked Ca2+ response, thus providing evidence for the presence of both store-operated Ca2+ channels and non-selective cationic channels in the human ST.

Calcium fluxes in human trophoblast (BeWo) cells: calcium channels, calcium-ATPase, and sodium-calcium exchanger expression

Although placental transfer of maternal calcium (Ca(2+)) is a crucial process for fetal development, the biochemical mechanisms are poorly understood. In the current study, we have investigated the characteristics of Ca(2+) fluxes in relation with cell Ca(2+) homeostasis in the human placental trophoblast cell line BeWo. Time-courses of Ca(2+) uptake by BeWo cells displayed rapid initial entry (initial velocity (V(i)) of 3.42 +/- 0.35 nmol/mg protein/min) and subsequent establishment of a plateau. Ca(2+) efflux studies with (45)Ca(2+)-loaded cells also showed rapid declined of cell-associated (45)Ca(2+) with a V(i) of efflux (Ve(i)) of 3.30 +/- 0.08 nmol/mg protein/min. Further identification of membrane gates for Ca(2+) entry in BeWo cells was carried out. Expression of Ca(2+) transporter/channel CaT1 and L-type alpha(1S) subunit was showed by RT-PCR. However, mRNA for CaT2 channel and L-type alpha(1C) and alpha(1D) subunits were not revealed. Membrane systems responsible for intracellular Ca(2+) extrusion from BeWo cells were also investigated. Plasma membrane Ca(2+)-ATPases (PMCA) and Na/Ca exchangers (NCX) were detected by Western blot in BeWo cells. Expression of specific isoforms of PMCA and NCX was further investigated by RT-PCR. Messenger RNAs of four isoforms of PMCA (PMCA 1-4) were detected. The presence of messenger RNAs of two NCX isoforms (NCX1 and NCX3) was observed. Ca(2+) flux studies in Na-free incubation medium indicated that NCX played a minimal role in the cell Ca(2+) fluxes. Inorganic ions such as cadmium and manganese did not modify the Ca(2+) fluxes, however, barium increased cell-associated (45)Ca(2+) by, in part, by reducing radiolabel exit.

Expression of calcium channels along the differentiation of cultured trophoblast cells from human term placenta

Placental transfer of maternal calcium (Ca2+) is carried out in vivo by the syncytiotrophoblast layer. Although this process is crucial for fetal development, it remains poorly understood. Cytotrophoblasts isolated from human term placenta undergo spontaneous syncytiotrophoblast-like morphological and biochemical differentiation in vitro and are thought to reflect in vivo syncytiotrophoblast. In the present study, we characterized the Ca2+ uptake potential and the expression of several Ca2+ channels by human trophoblasts during differentiation in vitro for up to 6 days. Secretion of hCG (specific differentiation marker) and uptake of Ca2+ by trophoblasts increased gradually as a function of days in culture. Both hCG secretion and Ca2+ uptake were maximal on Day 4 and declined on Days 5-6. Expression of the Ca2+ transporter proteins CaT1 and CaT2 was revealed by reverse transcription-polymerase chain reaction in cytotrophoblasts freshly isolated from human term placenta. In addition, messengers for two L-type Ca2+ channel isoforms (alpha(1C) and alpha(1D)) were also detected. Levels of CaT1, CaT2, and L-type Ca2+ channel mRNA increased gradually during culture, reaching a maximum between Days 2 and 3. In contrast to CaT1 and CaT2 expression that declined thereafter to levels observed on Day 1, L-type channel expression decreased by 50% but remained above the expression level of Day 1. Our results indicate that the pattern of CaT1 and CaT2 expression correlates with the Ca2+ uptake potential along the differentiation of cultured human trophoblasts isolated from term placenta. This correlation provides circumstantial evidence for a role of this family of channels in basal Ca2+ uptake by the syncytiotrophoblast.

Calcium uptake and calcium transporter expression by trophoblast cells from human term placenta

Placental transfer of maternal calcium (Ca(2+)) is a crucial step for fetal development although the biochemical mechanisms responsible for this process are largely unknown. This process is carried out in vivo by the placental syncytiotrophoblast layer. The aim of this study was to define the membrane gates responsible for the syncytiotrophoblast Ca(2+) entry, the first step in transplacental transfer. We have investigated the basal Ca(2+) uptake by primary culture of human term placenta syncytiotrophoblast. Kinetic studies revealed an active extracellular Ca(2+) uptake by cultured human syncytiotrophoblast. We demonstrated by Northern blot the presence of transcript for calcium transporter type 1 (CaT1) in cultured human syncytiotrophoblast and CaT1 expression was further confirmed by reverse transcription polymerase chain reaction (RT-PCR). In addition, the expression of calcium transporter type 2 (CaT2) was revealed by RT-PCR in cultured human syncytiotrophoblast. It has been reported that the activity of this family of Ca(2+) channels is voltage-independent, and is not sensitive to L-type Ca(2+) channels agonist and antagonist. Interestingly, modulation of membrane potential by extracellular high potassium concentration and valinomycin had no effect on the basal Ca(2+) uptake of human syncytiotrophoblast. Moreover, the addition of L-type Ca(2+) channel modulators (Bay K 8644 and nitrendipine) to the incubation medium had also no effect on the basal Ca(2+) uptake, suggesting that the process is mainly voltage-independent and does not involved L-type Ca(2+) channels. On the other hand, we observed that two known blockers of CaT-mediated Ca(2+) transport, namely extracellular magnesium (Mg(2+)) and ruthenium red, dose-dependently inhibited Ca(2+) uptake by cultured human syncytiotrophoblast. Therefore, our results suggest that basal Ca(2+) uptake of human syncytiotrophoblast may be assured by CaT1 and CaT2.

A low conductance, non-selective cation channel from human placenta

Non-selective cation channels have been identified in the plasma membranes of many different cells. Previous research using fluorescent techniques has demonstrated the presence of cation conductances in membranes from human trophoblast. The purpose of this work was to explore, by electrophysiological methods, a non-selective cation channel in apical membranes from human placenta. Human placental apical membranes were purified by differential centrifugation and reconstituted in giant liposomes. These giant liposomes were then used for electrophysiological studies and were probed for the presence of cation channels by the patch-clamp method. The channel identified had a linear current-potential relationship with a conductance of around 16 pS in symmetrical Na(+) solution. Under asymmetrical conditions the reversal potential was close to the reversal potential for Na(+). The channel was equally permeable to sodium and potassium and the permeability sequence was NH+4>Cs(+) approximately Rb(+)>Na(+) approximately K(+)>Li(+). The channel also showed permeability to calcium and barium. The channel was insensitive to calcium but was blocked by millimolar concentration of Mg(2+). We have demonstrated the presence of a low conductance, non-selective cation channel in placental apical membranes. These channels share some properties with non-selective cation channels previously described in other different cells. The precise role of these channels in placental physiology has yet to be determined.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Characteristics of calcium uptake by BeWo cells, a human trophoblast cell line

Placental transfer of maternal calcium (Ca(2+)) is a crucial process for fetal development although the biochemical mechanisms responsible for this transfer are largely unknown. We have investigated the characteristics of Ca(2+)uptake by the human placental trophoblast cell line BeWo. The kinetics studies revealed an active extracellular Ca(2+)uptake by BeWo cells, which was rapid in the first 2 min (initial velocity (V(i)) of 4.17+/-0.25 nmol/mg/min), and showed a subsequent plateau. Uptake experiments performed at V(i)with increasing concentrations of Ca(2+)resulted in a typical saturation curve (K(m)of 0.54+/-0.07 m m and V(max)of 7.07+/-0.28 nmol/mg protein/min). Lowering the pH of the incubation medium from 7.4 to 5.5 led to Ca(2+)uptake inhibition of 40-50 per cent. The presence of voltage-sensitive (l -type) Ca(2+)channels in BeWo cells was demonstrated by Western blot. Therefore, the implication of such channels in basal Ca(2+)uptake of BeWo cells was investigated. Cell depolarization with extracellular high potassium concentration (40 m m), and hyperpolarization with extracellular high chloride concentration (60 m m) or with valinomycin (10 microm) did not influence the basal Ca(2+)uptake of BeWo cells. The L-type Ca(2+)channel modulators (Bay K 8644 and Nitrendipine) had no effect on the Ca(2+)uptake. An antagonist of receptor-mediated, store-operated and voltage-gated Ca(2+)channels (SKF-96365) also did not modulate the Ca(2+)uptake of BeWo cells. Therefore, our results indicate that the basal Ca(2+)uptake of BeWo cells is inhibited by lowering pH of the incubation medium, is voltage independent, and is not influenced by l -type Ca(2+)channel and capacitative Ca(2+)conductance modulators.

Corticotropin-releasing hormone binding to the syncytiotrophoblast membranes

The human placenta secretes large amounts of corticotropin-releasing hormone (CRH) which was thought to exert a paracrine action in the placenta. We have recently characterized high-affinity binding sites for CRH in the human placenta. However, our studies utilized whole placental membranes, which did not identify the site of binding of CRH in the plasma membrane. In this study we investigated the characteristics of CRH binding to purified mother-facing, brush border membranes (BBM) and fetus-facing, basal plasma membranes (BPM) of the syncytiotrophoblast. The two membranes were separated by a series of differential and density-gradient centrifugations. The purity of the membranes was determined by measuring alkaline phosphatase, as a marker of BBM and Na+/K+ATPase as a marker of BPM. Each membrane showed specific and high-affinity binding. Scatchard analysis revealed a high-affinity binding site for CRH with Kd of 1.0 +/- 0.15 and 1.3 +/- 0.176 for BBM and BPM, respectively. The maximal number of binding sites was significantly different (P < 0.01) in the two plasma membranes: Bmax of 79 +/- 6.4 fmol/mg protein for BBM and 23 +/- 3.9 fmol/mg protein for BPM. Both the mother-facing and fetus-facing membranes of the syncytiotrophoblast contain binding proteins for CRH, with significantly more binding sites on the mother-facing membranes. The functional consequences of CRH binding could be different for the two polar membranes due to differential localization of second messenger systems between the two membrane types. It is proposed that partial purification of BBM and BPM provides a better system to study CRH action in the placenta, than whole placental membrane preparations.

Implication of ATP and sodium in arachidonic acid incorporation by placental syncytiotrophoblast brush border and basal plasma membranes in the human

The human placental syncytiotrophoblast is the main site of exchange of nutrients and minerals between the mother and her fetus. In order to characterize the placental transport of some fatty acids, we studied the incorporation of arachidonic acid, a fetal primordial fatty acid, in purified bipolar syncytiotrophoblast brush border (BBM) and basal plasma membranes (BPM) from human placenta. The basal arachidonic acid incorporation in BBM and BPM was time dependent and reached maximal values of 0.75+/-0.10 and 0.48+/-0.18 pmol/mg protein, respectively, after 2.5 min. The presence of adenosine triphosphate (ATP) (3 m m) increases significantly the maximal incorporation of arachidonic acid by sixfold (4.75+/-0.35 pmol/mg) and ninefold (4.40+/-0.84 pmol/mg) in BBM and BPM, respectively. Moreover, an increase in the arachidonic acid incorporation was also obtained in the presence of sodium where the values achieved 7.68+/-0.98 (10x) and 6.53 pmol/mg (13.6x) for BBM and BPM, respectively. We also showed that the combination of both Na(+)and ATP increases significantly the maximal incorporation of arachidonic acid in BPM to 7.89+/-0.15 pmol/mg protein, while in BBM it did not modify its incorporation (8.18+/-0.25 pmol/mg protein), as compared to the presence of sodium alone. Our results demonstrate that arachidonic acid is incorporated by both placental syncytiotrophoblast membranes, and is ATP and sodium-linked. However, different mechanisms seem to be involved in this fatty acid incorporation through BBM and BPM, since the presence of Na(+)or ATP increased it, while the association of these two elements increased it only in BPM. We also demonstrated by osmolarity experiments that both membranes bind arachidonic acid, potentially involving one or more fatty acids binding proteins.

The clinical chemistry of inorganic sulfate

Although inorganic sulfate is an essential and ubiquitous anion in human biology, it is infrequently assayed in clinical chemistry today. Serum sulfate is difficult to measure accurately without resorting to physicochemical methods, such as ion chromatography, although many other techniques have been described. It is strongly influenced by a variety of physiological factors, including age, diet, pregnancy, and drug ingestion. Urinary excretion is the principal mechanism of disposal for the excess sulfate produced by sulfur amino acid oxidation, and the kidney is the primary site of regulation. In renal failure, sulfoesters accumulate and hypersulfatemia contributes directly to the unmeasured anion gap characteristic of the condition. In contrast, sulfate in urine is readily assayed by a number of means, particularly nephelometry after precipitation as a barium salt. Sulfate is most commonly assayed today as part of the clinical workup for nephrolithiasis, because sulfate is a major contributor to the ionic strength of urine and alters the equilibrium constants governing saturation and precipitation of calcium salts. Total sulfate deficiency has hitherto not been described, although genetic defects in sulfate transporters have been associated recently with congenital osteochondrodystrophies that may be lethal. New insights into sulfate transport and its hormonal regulation may lead to new clinical applications of sulfate analysis in the future.

Ouabain-sensitive H,K-ATPase: tissue-specific expression of the mammalian genes encoding the catalytic alpha subunit

Human ATP1AL1 and corresponding genes of other mammals encode the catalytic alpha subunit of a non-gastric ouabain-sensitive H,K-ATPases, the ion pump presumably involved in maintenance of potassium homeostasis. The tissue specificity of the expression of these genes in different species has not been analyzed in detail. Here we report comparative RT-PCR screening of mouse, rat, rabbit, human, and dog tissues. Significant expression levels were observed in the skin, kidney and distal colon of all species (with the exception of the human colon). Analysis of rat urogenital organs also revealed strong expression in coagulating and preputial glands. Relatively lower expression levels were detected in many other tissues including brain, placenta and lung. In rabbit brain the expression was found to be specific to choroid plexus and cortex. Prominent similarity of tissue-specific expression patterns indicates that animal and human non-gastric H,K-ATPases are indeed products of homologous genes. This is also consistent with the high sequence similarity of non-gastric H,K-ATPases (including partial sequences of hitherto unknown cDNAs for mouse and dog proteins).

Properties of two multisubstate Cl- channels from human syncytiotrophoblast reconstituted on planar lipid bilayers

We describe the first successful reconstitution of placental ionic channels on planar lipid bilayers. An apical plasma membrane-enriched vesicle fraction from human syncytiotrophoblast at term was prepared by following isotonic agitation, differential centrifugation, and Mg2+-induced selective precipitation of nonapical membranes, and its purity was assessed by biochemical and morphological marker analysis. We have already reported that, unlike previous patch-clamp studies, nonselective cation channels were incorporated in most cases, a result consistent with the higher permeability for cations as compared with Cl- and with the low apical membrane potential difference at term revealed by fluorescent probe partition studies, and microelectrode techniques. In this paper, we report that Cl--selective channels were incorporated in 4% of successful reconstitutions (14 out of 353) and that their analysis revealed two types of activity. One of them was consistent with a voltage-dependent, 100-pS channel while the other was consistent with the lateral association of 47-pS conductive units, giving rise to multibarrelled, DIDS-sensitive channels of variable conductance (300 to 650 pS). The latter displayed a very complex behavior which included cooperative gating of conductive units, long-lived substates, voltage-dependent entry into an apparent inactivated state, and flickering activity. The role of the reported Cl- channels in transplacental ion transport and/or syncytium homeostasis remains to be determined.

Effects of cadmium on trophoblast calcium transport

Maternal exposure to cadmium (Cd) during pregnancy has been linked to low fetal birthweight, which may be attributed to placental damage and/or dysfunction in nutrient transport. Previous studies have suggested that Cd is accumulated in the placenta, and that placental transport of calcium (Ca) and zinc (Zn) is perturbed by Cd. To investigate the mechanism of Cd perturbation of Ca transport, we used JEG-3, a human choriocarcinoma cell line which exhibits trophoblastic properties, to analyse Cd effects in vitro. Treatment with Cd at low, physiologically relevant concentrations (e.g. 0.04 microM) did not result in obvious changes in cell morphology or integrity, whereas higher concentrations (> or = 0.16 microM) affected cell integrity. With lower concentrations of Cd treatment for 24 h, activities of cellular Ca uptake and transport, and Ca2+ binding were decreased, and intracellular [Ca2+] ([Ca2+]i) profile was also altered; however, membrane-associated Ca(2+)-activated ATPase activity remained relatively unchanged. Interestingly, cellular Ca uptake activity was unaffected by short-term (30 min) Cd pretreatment. The 24-h Cd treatment also resulted in elevated expression of the metal-binding protein, metallothionein, whereas the expression of a trophoblast-specific cytosolic Ca(2+)-binding protein (HCaBP) was drastically reduced. These results strongly suggest that Cd exposure significantly compromises the Ca handling ability of trophoblastic cells; this effect is probably not due to perturbations in Ca channel or membrane Ca pump activities, but rather a consequence of alterations in subcellular, cytosolic Ca2+ binding activities.

Calcitonin gene-related peptide receptor in human placental syncytiotrophoblast brush-border and basal plasma membranes

Minerals, such as calcium and potassium, are essential for fetal development, but their transplacental transport, and in particular, the effect of hormones on this process has not been extensively studied. Human alpha-calcitonin gene-related peptide (h alpha CGRP), a hormone constituted of 37 amino acids, is obtained by the alternative splicing of the mRNA from the calcitonin gene, and could be implicated in placental ion transport. In order to study the presence of this receptor, brush-border and basal plasma membranes were purified, and membrane binding studies were conducted using [125I]h alpha CGRP. The initiation of binding of [125I]h alpha CGRP to both membranes was rapid and reached maximal value after 10 min of incubation at 37 degrees C. Scratchard analysis revealed single-affinity binding sites for h alpha CGRP with Kd equal to 4412.45 +/- 604.81 pM and 2673.24 +/- 552.51 pM for brush-border and basal plasma membranes, respectively, which were significantly different. Moreover, the maximal number of receptors was significantly different (P < 0.001) in both membranes, with Bmax of 627.94 +/- 31.40 fmol/mg protein for brush-border membranes and 343.70 +/- 43.52 fmol/mg protein in basal-plasma membranes. Competitive displacement of [125I]h alpha CGRP with other ligands showed the following potencies; h alpha CGRP approximately h beta CGRP approximately Cys (acm)2,7 CGRP > CGRP (8-37), but no competition was observed with human and salmon calcitonin. Half-maximal displacement for human alpha- and beta CGRP was reached at approximately 10(-10)M for brush-border and basal-plasma membranes. alpha- and beta CGRP, and their fragment and analogue, stimulated cyclic AMP production in placental homogenate ranging from 143-163 per cent. Thus, our results show the presence of CGRP-specific receptors in both the syncytiotrophoblast membranes of human placenta. The role(s) of this related peptide in placenta remains to be investigated.

Asymmetrical distribution of G proteins in syncytiotrophoblastic brush-border and basal-plasma membranes of human term placenta

In human placental syncytiotrophoblast brush-border (BBM, facing the mother) and basal-plasma membranes (BPM, facing to fetus) we have recently demonstrated the presence of calcaemic hormone-specific receptors for parathyroid hormone and calcitonin, which could be implicated in calcium transport from the mother to the fetus. It is well recognized that signal transducing G proteins (guanosinc nucleotide-binding proteins) can associate with various transmembrane receptors and effector proteins, and regulate a variety of second-messenger systems and ion channels. In this present paper, we investigated the presence of a variety of alpha and beta subunits of G proteins in both syncytiotrophoblast, BBM and BPM by Western blot technique. For the first time, we were able to demonstrate the presence of G proteins in the bipolar syncytiotrophoblast membranes, which were evaluated by immunoblotting using affinity purified antiserum raised against the alpha subunits of Gi1, Gi1/i2, Gi3, G0, Gq, Gs, G7 and against the beta subunits. In BBM, we identified the alpha subunits of Gi1, Gi3, G0, Gq, Gs (42, 46 kDa), Gz and beta subunits. The same alpha subunits of G proteins were found in BPM, although alpha subunits of Gi1, Gq, Gs (46 kDa) were located predominantly in the BBM, and the alpha subunit of G0 was found preferentially in BPM. Moreover, in BBM and BPM, a purified antisera raised against the alpha subunits of Gi1 and Gs, detected a 105 kDa protein and a 67 kDa protein, respectively. Interestingly, the 67 kDa protein was preferentially located in BBM, and none of these proteins were detectable in membranes prepared from brain (control). The asymmetrical distribution of the alpha subunits of G proteins among the two different placental bipolar membranes might reflect the very specialized function of these syncytiotrophoblast membranes in ions and nutrients transport from the mother to the fetus.

Free amino acid concentrations in normal and abnormal third trimester placental villi

Homogenates of placental villi were obtained from normotensive pregnancies and from pregnancies complicated by pre-eclampsia and growth retardation. The homogenates were analysed for free amino acid concentrations using high performance liquid chromatography (HPLC). The study shows that there are significantly higher concentrations of two essential amino acids, L-arginine and phenylalanine (P < 0.05), in the growth-retarded group compared to the term normotensive and pre-eclamptic groups. There are significantly higher concentrations of the non-essential amino acids: glutamic acid in the growth retarded group compared to the normotensive group, and of tyrosine in the growth-retarded group compared to the pre-eclamptic group. However, there is no increase in the glycine/valine ratio, a characteristic marker for kwashiorkor or a reduction in total protein concentration in the pre-eclamptic or growth-retarded groups. These findings suggest that there may be abnormalities in placental metabolism, amino acid transfer across the basal membrane into the fetal circulation, and also fetoplacental perfusion in pregnancies complicated by pre-eclampsia and growth retardation.

Linoleic acid transport by human placental syncytiotrophoblast membranes

The placenta syncytiotrophoblast is the site of exchange of nutrients, lipids and minerals between the mother and the fetus. In order to characterize the transport of fatty acids by the placenta, we purified bipolar syncytiotrophoblast brush border and basal plasma membranes from human placenta. These purified brush border and basal plasma membranes enriched 3-fold and 22-fold, respectively, in sodium/potassium-ATPase and 27-fold and 6-fold in alkaline phosphatase activity, compared with the placental homogenates. Fatty acid transport was performed at different fatty acid/albumin ratios to evaluate the optimal uptake conditions. The maximal transport efficiency, for linoleic acid bound to albumin by sonication, was obtained with a 6:1 fatty acid/albumin ratio in brush border and basal plasma membranes. The linoleic acid transport observed with brush border membranes followed Michaelis-Menten kinetics, with a Michaelis constant of 7.89 +/- 0.01 microM and a maximal incorporation rate of 30.80 +/- 6.39 pmol.mg-1.min-1. Linoleic acid transport was very low in basal plasma membranes and we obtained a Michaelis constant of 0.95 +/- 0.01 microM and a maximal incorporation rate of 1.62 +/- 5.06 pmol.mg-1.min-1. In order to show that linoleic acid accumulated within brush border and plasma membrane vesicles, and to eliminate the possibility of a non-specific binding of fatty acid to these membranes, we demonstrated by an osmolarity experiment, the decrease of the linoleic acid transport in brush border and basal plasma membranes obtained in the presence of 455 microM essential fatty acid at 23 degrees C for 180 min. The results presented in this study suggest that linoleic acid is transported significantly by syncytiotrophoblast brush border membranes and basal plasma membranes. Thus, it may represent a unidirectional transport from mother to fetus through the brush border membranes facing the mother, followed by transport at a slower rate through basal plasma membranes facing the fetus.

Na+ transport, H+ concentration gradient dissipation, and system A amino acid transporter activity in purified microvillous plasma membrane isolated from first-trimester human placenta: comparison with the term microvillous membrane

Our purpose was to isolate microvillous plasma membrane from first-trimester placenta and to measure its transport properties with regard to Na+, H+, and a neutral amino acid. Microvillous membrane was isolated from first-trimester (10 to 13 weeks) and term (38 to 42 weeks) placenta and the purity determined. Uptake of 22Na+ was measured in the presence of an outwardly directly H+ gradient in the presence or absence of amiloride (0.5 mmol/L). The rate of dissipation of an H+ concentration gradient was determined with the H(+)-sensitive fluorescent probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. 14C-methylaminoisobutyric acid uptake was measured in the presence and absence of an inwardly directed Na+ gradient. Purity, vesicle volume, vesicle orientation, and electron micrographic appearance of the first-trimester membranes were similar to those obtained from term placenta, but vesicle protein recovery was lower. Amiloride-sensitive Na+ uptake and Na(+)-dependent 14C-methylaminoisobutyric acid uptake was threefold to fourfold lower by first-trimester than by term membranes. The rate of H+ concentration gradient dissipation was greater in the former. The first-trimester microvillous membrane has similar general characteristics to that from term placenta, but its transport activity is quite different.

Transtrophoblast and microvillus membrane potential difference in mature intermediate human placental villi

Single mature intermediate villi from term placentas after normal gestation and vaginal or cesarian delivery were identified microscopically and mounted in a bath for conventional micro-electrode studies. With the application of strict selection criteria, the following observations on electrical potential difference (PD) were made. 1) With the tissue bathed in Earle's medium (37 degrees C) the PD across the syncytiotrophoblast microvillus membrane with respect to the bath was not normally distributed. The median PD was -22 mV (range -12 to -60 mV, n = 200). This fell to -6 mV after prior incubation for 4 h with cyanide (3 mM) or iodoacetate (2 mM) but was not altered by short-term application of these agents or by 0.1 mM ouabain. Substituting Na+ in Earle's medium with choline had no effect on PD, but replacing Cl- with gluconate caused a depolarization of 6 mV (P < 0.002). Increasing KCl in the bath fluid revealed an apparently low microvillus membrane K+ conductance. The low microvillus membrane PD may reflect a low Na(+)-K(+)-ATPase activity and/or a low membrane permeability to K+. 2) The transtrophoblast PD measured by insertion and withdrawal of the electrode was significantly different from zero [P < 0.003, median -3 mV (range 0 to -15 mV), n = 11]; and PD measured by insertion of the electrode into the villus core and beyond was -6 mV (significantly different from zero P < 0.003, range -2.5 to -10 mV, n = 6). If a similar PD were to exist in vivo, it could have a significant influence on ion transport across the placenta at term.

Characterization of an ATP-driven H+ pump in human placental brush-border membrane vesicles

The presence of an ATP-driven H+ pump as measured by H+ uptake upon addition of ATP was not demonstrable in human placental brush-border membrane vesicles when used in their native form, owing to their right-side-out orientation. However, the presence of the H+ pump in these membranes became evident when the membrane vesicles were transiently exposed to 1% cholate, with subsequent removal of the detergent to re-form the vesicles. Apparently, cholate pretreatment reoriented the H+ pump from an inward-facing configuration to outward-facing. Consequently, H+ uptake in response to externally added ATP was easily demonstrable in these cholate-pretreated vesicles by using the delta pH indicator Acridine Orange. In addition, bafilomycin A1-sensitive ATPase activity was measurable in cholate-pretreated vesicles, but not in native intact vesicles, indicating reorientation of the H+ pump. The reoriented H+ pump was electrogenic because H+ uptake was stimulated by an inside-negative anion-diffusion potential or when the vesicles were voltage-clamped. ATP supported H+ uptake with an apparent Km of 260 microM. ITP and GTP supported the pump activity partially, whereas CTP and UTP did not. Mg2+ and Mn2+ were the most preferred bivalent cations. Co2+ and Zn2+ showed partial activity, whereas Ca2+ and Ba2+ showed little or no activity. The pump was inhibited by nanomolar concentrations of bafilomycin A1 and micromolar concentrations of N-ethylmaleimide, p-chloromercuribenzenesulphonate, NN-dicyclohexylcarbodi-imide and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, but was relatively insensitive to oligomycin, vanadate and NaN3. The inhibition by N-ethylmaleimide was protectable by ATP. It is concluded that human placental brush-border membranes possess an ATP-driven H+ pump and that, on the basis of its characteristics, it belongs to the class of vacuolar (V-type) H+ pumps.

Anion transport by human placenta: a study of chloride and sulphate efflux from isolated placental tissue fragments

The efflux of radiolabelled sulphate and chloride from fragments prepared from normal human term placentae has been examined so that a comparison between the transport properties of whole tissue and isolated placental membranes may be made. Sulphate efflux was found via a temperature- and DIDS sensitive mechanism. External chloride and sulphate were able to trans-accelerate the egress of labelled sulphate from tissue fragments via a DIDS-inhibitable route. These results are consistent with sulphate transport being mediated via an anion-exchange process. Chloride efflux from fragments of placenta was via a system which could be trans-stimulated by external Cl-: this moiety of transport was inhibited by DIDS. Several differences between the characteristics of placental tissue anion transport and isolated membrane vesicles were noted and are discussed.

Imidazoline binding sites in human placenta: evidence for heterogeneity and a search for physiological function

1. An alpha 2-adrenoceptor antagonist, idazoxan, that binds to both alpha 2-adrenoceptors and to imidazoline sites (IR), has been used to characterize human placental IR. Human placenta is shown to be the richest source of IR (1800 +/- 100 fmol mg-1 protein; Kd 38.9 +/- 3.4 nM). 2. Primary cells derived from human placenta and grown in monolayers, also displayed a high density of receptors (3209 +/- 136 fmol mg-1 in cytotrophoblasts and 3642 +/- 144 fmol mg-1 protein in syncytiotrophoblast enriched cell culture). 3. [3H]-idazoxan did not show binding characteristics of alpha 2-adrenoceptors in human placental membranes or human trophoblastic cells, thus making it a ligand of choice to study the imidazoline site. The tissue appeared to be lacking alpha 2-adrenoceptors in that other alpha 2-adrenoceptor ligands, [3H]-rauwolscine and [3H]-clonidine, do not bind to alpha 2-adrenoceptors in human placenta. 4. IRs are localized on the cell surface, as determined by the release of bound [3H]-idazoxan from cells, when washed with high ionic/acidic medium. 5. Imidazoline receptors of human placenta display high affinity for amiloride (72 +/- 27 nM). The high affinity was used as a criterion to classify IR to IRa subtype (placenta, rabbit kidney, rabbit liver and rabbit adipose cells) as opposed to the IRb subtype which display low affinity for amiloride (greater than 2 microM, in all the other tissues).6. Several novel ligands comprising a guanido functional group attached to an aromatic residue (e.g. benziliden-amino-guanidine (BAG), guanido pyrole) display pronounced selectivity for IR over the M2-adrenoceptors as the affinity of BAG is about 40 fold higher (Kd= 18.9 +/- 13.8 nM in human placenta), than the affinity for M2-adrenoceptors (Kd = 768 +/- 299 nM in human platelets). Imidazoline sites bind selectively BAG and other guanido ligands thus indicating a distinct structural requirement at its site of binding.7. K+ channel blockers and monovalent ions (e.g. Cs' and NH4+) interfere with idazoxan binding to IR, indicating a possible involvement of IR in K+ transport.

Carrier-mediated L-lactate transport in brush-border membrane vesicles from rat placenta during late gestation

The mechanism for L-lactate transport across microvillous membrane vesicles prepared from rat placenta was examined. Uptake of L-lactate into these vesicles was mainly the result of transport into the intravesicular (osmotically active) space. The initial rate of L-lactate uptake was not affected by the presence of an inward gradient of either Na+ or K+. In the presence of an inward-directed proton gradient, L-lactate uptake was markedly stimulated, accumulating at concentrations 6-7-fold higher than the equilibrium. Lower transmembrane pH gradients were associated with slower initial uptakes and smaller overshoots. L-Lactate uptake determined under an inside-directed pH gradient was strongly inhibited by p-chloromercuriphenylsulphonic acid, a protein-thiol oxidizing agent. L-Lactate uptake was: (1) saturable as a function of the concentration of L-lactate, (2) inhibited by monocarboxylic acids such as pyruvate, D-lactate, beta-hydroxybutyrate and alpha-cyano-4-hydroxycinnamic acid, and (3) temperature-dependent. When present inside the vesicles, L-lactate, pyruvate and beta-hydroxybutyrate caused trans-stimulation of L-lactate uptake both in the presence and in the absence of an inside-directed pH gradient, indicating that L-lactate transport is a reversible process that can be shared by other monocarboxylic acids. There were no significant changes in maximal initial rate or in the kinetic parameters of L-lactate transport during the last 3 days of gestation.

Sulfate transport in human placenta: further evidence for a sodium-independent mechanism

Sulfate transport in isolated placental brush-border membrane vesicles has properties consistent with an anion exchange process. To ascertain the relevance of this finding to sulfate accumulation by the fetus and placenta in vivo, we examined sulfate transport in human placental tissue slices, comparing sulfate uptake with that of a non-metabolizable amino acid marker, alpha-aminoisobutyrate (AIB). In contrast to AIB, which was actively concentrated from physiological media, sulfate uptake by the placenta slice was concentrative only in the absence of sodium and at low pH. Uptake of sulfate reached a steady state after 60 min. It was blocked by DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonate), a specific inhibitor of anion transport, but not by ouabain. We found no evidence for Na(+)-dependent uptake of sulfate in incubated placental tissue. It seems unlikely that Na(+)-dependent sulfate transport by the placenta can be responsible for net sulfate accumulation by the human fetus.

Salicylate inhibits human placental sulphate transport in vitro

The effect of salicylate on sulphate transport by the human placenta has been studied using isolated brush-border membrane vesicles and placental tissue slices. Sulphate uptake by isolated vesicles was inhibited in a dose dependent fashion (K1 approximately 3 mM) by salicylate. It appears that this drug blocks sulphate accumulation in a non-competitive manner. Sulphate efflux from preloaded vesicles was also found to be markedly reduced by salicylate in a non-competitive fashion. Consistent with the vesicle studies salicylate inhibited sulphate transport by placental tissue slices. The results suggest that salicylate ingestion could compromise feto-placental sulphate homeostasis. In addition we have found that the aspirin-like drug, flufenamic acid, inhibits sulphate transport by isolated microvillus membrane vesicles.

Adenosine transport and nitrobenzylthioinosine binding in human placental membrane vesicles from brush-border and basal sides of the trophoblast

The nucleoside transport activity of human placental syncytiotrophoblast brush-border and basal membrane vesicles was compared. Adenosine and uridine were taken up into an osmotically active space. Adenosine was rapidly metabolized to inosine, metabolism was blocked by preincubating vesicles with 2'-deoxycoformycin, and subsequent adenosine uptake studies were performed in the presence of 2'-deoxycoformycin. Adenosine influx by brush-border membrane vesicles was fitted to a two-component system consisting of a saturable system with apparent Michaelis-Menten kinetics (apparent Km approx. 150 microM) and a linear component. Adenosine uptake by the saturable system was blocked by nitrobenzylthioinosine (NBMPR), dilazep, dipyridamole and other nucleosides. Inhibition by NBMPR was associated with high-affinity binding of NBMPR to the brush-border membrane vesicles (apparent Kd 0.98 +/- 0.21 nM). Binding of NBMPR to these sites was blocked by adenosine, inosine, uridine, thymidine, dilazep and dipyridamole, and the respective apparent Ki values were 0.23 +/- 0.012, 0.36 +/- 0.035, 0.78 +/- 0.1, 0.70 +/- 0.12 (mM), and 0.12 and 4.2 +/- 1.4 (nM). In contrast, adenosine influx by basal membrane vesicles was low (less than 10% of the rate observed with brush-border membrane vesicles under similar conditions), and hence no quantitative studies of adenosine uptake could be performed with these vesicles. Nevertheless, high-affinity NBMPR binding sites were demonstrated in basal membrane vesicles with similar properties to those in brush-border membrane vesicles (apparent Kd 1.05 +/- 0.13 nM and apparent Ki values for adenosine, inosine, uridine, thymidine, dilazep and dipyridamole of 0.14 +/- 0.045, 0.54 +/- 0.046, 1.26 +/- 0.20, 1.09 +/- 0.18 mM and 0.14 and 3.7 +/- 0.5 nM, respectively). Exposure of both membrane vesicles to UV light in the presence of [3H]NBMPR resulted in covalent labeling of a membrane protein(s) with a broad apparent Mr on SDS gel electropherograms of 77,000-45,000, similar to that previously reported for many other tissues, including human erythrocytes. We conclude that the maternal (brush-border) and fetal (basal) surfaces of the human placental syncytiotrophoblast possess broad-specificity, facilitated-diffusion, NBMPR-sensitive nucleoside transporters.

Inhibitor action on placental calcium transport

Human term placental lobules were dually perfused with Krebs Ringer solution at 37 degrees C under open circuit conditions. Provided that perfusate Ca2+ concentrations were between 2.33 and 2.55 mM, there was a steady release of Ca2+ into the fetal circulation and uptake of Ca2+ from the maternal circulation. There was no significant calcium (Ca) protein binding in the perfusates. Addition of dinitrophenol altered the release of Ca2+ to an uptake on the fetal circuit and enhanced Ca2+ uptake on the maternal circuit. It also produced a release of potassium (K)+ and an uptake of Na+ on both sides of the placenta. Ouabain had no significant effect on Ca movements although it produced a marked release of K+ into the fetal perfusate. The effect of cooling on the fetal circuit was similar to that of dinitrophenol (DNP), although it did not produce significant changes in either Ca2+ or K+ movements on the maternal side of the lobule. Both DNP and cooling reduced the Ca concentration ratio between fetal and maternal outflows to unity. Replacement of Na+ by choline Ringer had only transient effect on the extraction of 45Ca from fetal perfusate. These observations indicate that a Ca2+/Na (sodium)+ exchanger does not make a major contribution to the transplacental movement of Ca2+ from mother to fetus and that this process is more probably associated with membrane-bound ATPases.

Preparation of plasma membrane vesicles from the rat placenta at term and measurement of Na+ uptake

Cytochemistry revealed that alkaline phosphatase was localized predominantly to the maternal facing plasma membrane of syncytiotrophoblast layer II in the haemotrichorial rat placenta at term. Plasma membrane vesicles prepared from term rat placenta by homogenization, treatment with MgCl2, and differential centrifugation were enriched 14-fold in alkaline phosphatase activity as compared to homogenate. These vesicles were mainly oriented right side out, as shown by a lack of effect of saponin treatment on alkaline phosphatase activity. Na+ uptake into the vesicles under equilibrium exchange conditions was significantly stimulated (P less than 0.01) nearly threefold in the presence of an outwardly directed H+ gradient as compared to no gradient. The stimulation by a H+ gradient was abolished in the presence of 0.5 mM amiloride. Measurements with the pH-sensitive dye BCECF in the absence of Na+ showed that 95.2 +/- 0.6 per cent of a 1.2 pH unit H+ gradient was dissipated from the vesicles in 2 min, but the remaining gradient was maintained for up to 15 min. These experiments therefore provide evidence that vesicles derived mainly from the maternal facing plasma membrane of syncytiotrophoblast layer II of the rat placenta possess a Na+/H+ exchanger.

Human placental L-tyrosine transport: a comparison of brush-border and basal membrane vesicles

1. The mechanisms responsible for L-tyrosine transport at both the maternal-facing and fetal-facing surfaces of the human full-term placenta have been studied using isolated brush-border and basal membrane vesicles under conditions where a direct comparison of the transport properties of the two membranes can be made. 2. Brush-border vesicle uptake of L-tyrosine was substantially into an osmotically active space. Transport was Na(+)-independent, N-ethylmaleimide-sensitive (half-maximal inhibition, Ki = 1.1 mM), and insensitive to pH over the range 5.5-8.5. The initial rate of brush-border L-tyrosine uptake as a function of concentration showed saturation and obeyed Michaelis-Menten kinetics with Michaelis constant (Km) and maximum velocity (Vmax) values of 54.2 microM and 1.28 pmol (mg protein)-1 s-1, respectively. Influx of L-tyrosine was stereospecific and was virtually completely abolished by L-phenylalanine, L-tryptophan, L-leucine or by 2-aminobicycloheptane-2-carboxylic acid. These properties suggest that system L is responsible for brush-border L-tyrosine transport. 3. Basal membrane transport of L-tyrosine was more complex and uptake was slower than that found in the brush border. Although, as in the brush-border membranes, uptake was completely Na(+)-independent, N-ethylmaleimide was a less effective inhibitor, there was stimulation of transport at more alkaline pH and uptake did not show marked stereospecificity. An apparent Km of 168.9 microM and a Vmax of 0.31 pmol (mg protein)-1 s-1 were calculated for basal L-tyrosine transport. There was clear inhibition by L- and D-tyrosine, L-phenylalanine and L-tryptophan. 2-Aminobicycloheptane-2-carboxylic acid was not as effective. 4. These findings suggest the existence of non-identical carrier-mediated transport systems for L-tyrosine in brush-border and basal membranes. Brush-border transport resembles that by system L; L-tyrosine transport at the basal membrane may be via system t.

Characterization of amino acid transport systems in human placental basal membrane vesicles

The amino acid transport systems have been characterized in basal membrane vesicles prepared from human full-term placental syncytiotrophoblasts. Transport of amino acids across basal membranes occurred via passive diffusion and Na(+)-independent and Na(+)-dependent carrier-mediated systems. Passive diffusion was responsible for a substantial fraction of transport. L-Glutamate and alpha-(methylamino)isobutyrate were transported only Na(+)-independently, while the transport of L-alanine was dependent solely on an Na+ gradient from the outside to the inside of the vesicles. L-Methionine, L-leucine, glycine and L-proline transport were supported by both Na(+)-independent and Na(+)-dependent systems. L-Lysine transport was decreased in the presence of cations, an inwardly directed Na+ gradient was much more effective than a K+ gradient at slowing L-lysine transport. A cross-inhibition analysis of these amino acids indicates that at least three Na(+)-independent and five Na(+)-dependent carrier-mediated systems exist in the human placental syncytiotrophoblast basal membranes. One Na(+)-independent system interacts with all substrates tested. Another Na(+)-independent system carries glycine, L-methionine, L-leucine and L-lysine; it is sensitive to L-glutamate, but not to L-proline or alpha-(methylamino)isobutyrate. The third system is selective for L-lysine, which is inhibited by L-methionine, glycine and L-leucine, but inaccessible to L-glutamate, L-proline and alpha-(methylamino)isobutyrate. One Na(+)-dependent system carries L-alanine, glycine, L-methionine and L-leucine, and it is sensitive to L-proline. The second system mediates transport of L-alanine, glycine, L-methionine and L-proline, but is not sensitive to L-leucine. The third system carries L-alanine, glycine and L-proline, and is inaccessible to L-methionine and L-leucine. The fourth system is responsible for L-methionine and L-leucine; it is sensitive to L-alanine and glycine, but not to L-proline. The fifth system is selective for L-proline.

Expression of a cell surface antigen with potential Ca2+-sensor/receptor function in rat placenta and uterus

The reactivities of monoclonal anti-parathyroid antibodies identifying a cell surface 'calcium sensing' mechanism were investigated in the placenta and uterus of the pregnant rat. The rat placenta showed positively stained cells at the end of pregnancy. Staining was confined exclusively to the columnar epithelial cells lining the sinuses of Duval. In the uterus, positive staining of the epithelium lining the uterine lumen was obtained exclusively prior to and during implantation.

Dicarboxylate transport in human placental brush-border membrane vesicles

Pathways for transport of dicarboxylic acid metabolites by human placental epithelia were investigated using apical membrane vesicles isolated by divalent cation precipitation. The presence of Na+/dicarboxylate cotransport was assessed directly by [14C]succinate tracer flux measurements and indirectly by fluorescence determinations of voltage sensitive dye responses. The imposition of an inwardly directed Na+ gradient stimulated vesicle uptake of succinate achieving levels approximately 5-fold greater than those observed at equilibrium. The increased succinate uptake was specific for Na+ as no stimulation was observed in the presence of Li+, K+ or choline+ gradients. In addition to concentrative accumulation of succinate, a direct coupling of Na+/succinate cotransport was suggested by the absence of a sizeable conductive pathway for succinate uptake and decreased succinate uptake levels associated with a more rapid decay of an imposed Na+ gradient. Na+ gradient-driven succinate uptake was not the result of parallel Na+/H+ and succinate/OH- exchange activities but was reduced by the Na+-coupled inhibitor harmaline. The voltage sensitivity of Na+ gradient-driven succinate uptake suggests Na+/succinate cotransport is electrogenic occurring with net transfer of positive charge. Substrate-specificity studies suggest the tricarboxylic acid cycle intermediates as candidates for transport by the Na+-coupled pathway. Decreasing pH increased the citrate-induced inhibition of succinate uptake suggesting divalent citrate as the preferred substrate for transport. Initial rate determinations of succinate uptake indicate succinate interacts with a single saturable site (Km 33 microM) with a maximal transport rate of 0.5 nmol/mg per min.

Lysine and alanine transport in the perfused guinea-pig placenta

The characteristics of L-lysine transport were investigated at brush-border (maternal) and basal (fetal) sides of the syncytiotrophoblast in the term guinea-pig placenta artificially perfused either through the umbilical vessels in situ or through both circulations simultaneously. Cellular uptake, efflux and transplacental transfer were determined using a single-circulation paired-tracer dilution technique. Unidirectional L-[3H]lysine uptake (%) (perfusate lysine 50 microM) was high on maternal (M = 87 +/- 1) and fetal (F = 73 +/- 2) sides. L-[3H]Lysine efflux back into the ipsilateral circulation was asymmetrical (F/M ratio = 2.3) and transplacental flux occurred in favour of the fetal circulation. Unidirectional lysine influx kinetics (0.05-8.00 mM) gave Km values of 1.75 +/- 0.70 mM and 0.90 +/- 0.25 mM at maternal and fetal sides, respectively; corresponding Vmax values were 1.95 +/- 0.38 and 0.87 +/- 0.10 mumol.min-1.g-1. At both sides, lysine influx (50 microM) could be inhibited (about 60-80%) by 4 mM L-lysine and L-ornithine and less effectively (about 10-40%) by L-citrulline, L-arginine, D-lysine and L-histidine. At the basal side: (i) lysine influx kinetics were greatly modified in the presence of 10 mM L-alanine (Km = 6.25 +/- 3.27 mM; Vmax = 2.62 +/- 0.94 mumol.min-1.g-1), but unchanged by equimolar L-phenylalanine or L-tryptophan; (ii) in the converse experiments, lysine (10 mM) did not affect the kinetic characteristics for either L-alanine or L-phenylalanine; (iii) L-lysine and L-alanine influx kinetics were not dependent on the sodium gradient; (iv) the inhibition of L-[3H]lysine uptake by 4 mM L-homoserine was partially (60%) Na+-dependent. At the maternal side the kinetic characteristics for alanine influx were highly Na+-dependent, while lysine influx was partially Na+-dependent only at low concentrations (0.05-0.5 mM). Bilateral perfusion with 2,4-dinitrophenol (1 mM) reduced L-[3H]lysine uptake into the trophoblast and abolished transplacental transfer. It is suggested that lysine transport in the guinea-pig placenta is mediated by a specific transport system (y+) for cationic amino-acids. The asymmetry in the degree of sodium-dependency at both trophoblast membranes may in part explain the maternal-to-foetal polarity of placental amino-acid transfer in vivo.

Carrier-mediated transport system for cephalexin in human placental brush-border membrane vesicles

The uptake of cephalosporin antibiotics, cephalexin, was studied with brush-border microvillous plasma membrane vesicles prepared and purified from human full-term placental syncytiotrophoblasts. The uptake of cephalexin by the membrane vesicles was not stimulated in the presence of an Na+ gradient from the outside to the inside of the vesicles, whereas alpha-(methylamino)isobutyrate uptake into the vesicles of the same preparation was stimulated by an Na+ gradient. The equilibrium level of cephalexin uptake decreased with increasing osmolarity of the medium, which indicates that cephalexin is transported into the membrane vesicles. When cephalexin concentrations were varied, the initial rate of uptake obeyed Michaelis-Menten kinetics with Km and Vmax values of 2.29 mM and 2.98 nmol/mg of protein per 60 s, respectively. The uptake of cephalexin was inhibited by structural analogues and sulfhydryl modifying reagents. These results indicate the existence of a carrier-mediated transport system for cephalexin in the human placental brush-border membranes.

Purification and Na+ uptake by human placental microvillus membrane vesicles prepared by three different methods

Three methods were used to prepare microvillus membrane vesicles from each of six human placentas. Two of these incorporated an agitation stage to preferentially remove microvilli and either Ca2+ (Method 1) or Mg2+ (Method 2) aggregation of non-microvillus membrane. The third method involved homogenisation of the tissue followed by Mg2+ aggregation of non-microvillus membrane (Method 3). Enrichment of alkaline phosphatase activity (27.6 +/- 1.9, 25.3 +/- 2.7) and ouabain binding (5.9 +/- 2.6, 5.3 +/- 2.2, respectively) was similar in vesicles prepared by Methods 1 and 2, respectively. Method 3 vesicles showed a significantly (P less than 0.01) lower alkaline phosphatase enrichment (18.1 +/- 1.2), but ouabain binding enrichment (6.3 +/- 1.3) was not different and vesicle protein recovery (mg/g placenta) was 5-fold greater. Na+ uptake in the presence of an outwardly directed proton gradient was significantly inhibited in all microvillus membrane vesicles by amiloride (0.5 mM). However, the amiloride sensitive component of Na+ uptake was 3-6-fold greater in Method 3 vesicles than in Method 1 and 2 vesicles, and showed overshoot above equilibrium in the former but not the latter. Further experiments using the pH sensitive dye, 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein suggested that the proton gradient dissipated faster from Method 1 than from Method 3 vesicles. Thus methodological differences can have a marked effect on transport processes in microvillus membrane vesicles prepared from the human placenta.

Transport and metabolism of adenosine in the perfused guinea-pig placenta

1. Uptake and metabolism of adenosine were investigated from both maternal (M) and fetal (F) circulations of the isolated, dually perfused guinea-pig placenta by using a single-circulation paired-tracer [( 14C]sucrose as extracellular reference, and [3H]adenosine) dilution technique. 2. Maximal [3H]adenosine uptakes (percentage of dose) from adenosine-free perfusates were 75 +/- 1 and 87 +/- 2% (mean +/- S.E. of mean) at maternal and fetal blood-tissue interfaces respectively. Rapid backflux (percentage of influx) of tritium (labelled adenosine and/or adenosine derivatives) from the placental tissue into the ipsilateral circulation was higher at the fetal (24 +/- 2%) than at the maternal side (11 +/- 2%). 3. Tritium uptakes were reduced to 50 +/- 4 (M) and 60 +/- 6% (F) when the perfusion medium contained 100 microM-unlabelled adenosine; backflux was highly stimulated (44% M and 84% F). Neither uptake nor backflux were affected by inosine, uridine, adenine or hypoxanthine present in the perfusion medium (1 mM). 4. Tissue sequestration of tritium (5-6 min) was approximately 60% of the injected dose when perfusates were adenosine-free and 20% or less in the presence of 100 microM-adenosine. 5. Cellular uptake of [3H]adenosine at both sides of the placenta was markedly reduced by the nucleoside transport inhibitors dipyridamole (DIP, 10 microM) and nitrobenzylthioinosine (NBMPR, 5 microM). 6. Thin-layer chromatographic separation of [3H]inosine, [3H]hypoxanthine and [3H]phosphorylated derivatives in venous effluents following a bolus arterial injection of [3H]adenosine showed a greater fraction of metabolites at the fetal side (about 0.75) than at the maternal side (about 0.50). The percentage of [3H]inosine increased when perfusates contained 100 microM-adenosine and the effect was more marked in the fetal circulation. In the presence of DIP and NBMPR the fractional recovery of 3H-labelled metabolites was greatly reduced. 7. During steady-state perfusion of [3H]adenosine (100 microM) a maintained (5-60 min) tritium uptake of about 55% was observed and all the effluent activity was 3H-labelled metabolites [( 3H]adenosine was only 2.8 +/- 0.2%). Under these conditions high-performance liquid chromatography (HPLC) showed that effluents contained xanthine and urate at 16 +/- 1 and 23 +/- 2 microM respectively. 8. Transplacental transfer (6 min) of tritiated compounds (of which only 10-20% was [3H]adenosine) was often less than that of the extracellular marker [14C]sucrose in both maternal-to-fetal and fetal-to-maternal directions.(ABSTRACT TRUNCATED AT 400 WORDS)