Transport specificity for neutral and basic amino acids at maternal and fetal interfaces of the guinea-pig placenta

Potential Impact of Metabolic and Gut Microbial Response to Pregnancy and Lactation in Lean and Diet-Induced Obese Rats on Offspring Obesity Risk

SCOPE

Maternal obesity programs metabolic dysfunction in offspring, increasing their susceptibility to obesity and metabolic diseases in later life. Moreover, pregnancy and lactation are associated with many metabolic adaptations, yet it is unclear how diet-induced maternal obesity may interrupt these processes.

METHODS AND RESULTS

¹ H NMR serum metabolomics analysis was performed on samples collected pre-pregnancy and in pregnant and lactating lean and high fat/sucrose (HFS) diet-induced obese Sprague-Dawley rats to identify maternal metabolic pathways associated with developmental programming of offspring obesity. Gut microbial composition was assessed using qPCR. Offspring of HFS dams had nearly 40% higher adiposity at weaning compared to offspring of lean dams. While pregnancy and lactation were associated with distinct maternal metabolic changes common to both lean and obese dams, we identified several metabolic differences, potentially implicating dysregulated one-carbon and mammary gland metabolism in the metabolic programming of obesity. Gut microbial composition was significantly altered with obesity, and both gestation and lactation were accompanied by changes in gut microbiota.

CONCLUSION

Diet-induced maternal obesity and consumption of an obesogenic maternal diet results in differential metabolic and gut microbial adaptations to pregnancy and lactation; these maladaptations may be directly involved in maternal programming of offspring susceptibility to obesity.

Evolution of Placental Function in Mammals: The Molecular Basis of Gas and Nutrient Transfer, Hormone Secretion, and Immune Responses

Placenta has a wide range of functions. Some are supported by novel genes that have evolved following gene duplication events while others require acquisition of gene expression by the trophoblast. Although not expressed in the placenta, high-affinity fetal hemoglobins play a key role in placental gas exchange. They evolved following duplications within the beta-globin gene family with convergent evolution occurring in ruminants and primates. In primates there was also an interesting rearrangement of a cassette of genes in relation to an upstream locus control region. Substrate transfer from mother to fetus is maintained by expression of classic sugar and amino acid transporters at the trophoblast microvillous and basal membranes. In contrast, placental peptide hormones have arisen largely by gene duplication, yielding for example chorionic gonadotropins from the luteinizing hormone gene and placental lactogens from the growth hormone and prolactin genes. There has been a remarkable degree of convergent evolution with placental lactogens emerging separately in the ruminant, rodent, and primate lineages and chorionic gonadotropins evolving separately in equids and higher primates. Finally, coevolution in the primate lineage of killer immunoglobulin-like receptors and human leukocyte antigens can be linked to the deep invasion of the uterus by trophoblast that is a characteristic feature of human placentation.

Transport and metabolism of amino acids in placenta

In all mammalian species, the 20 amino acids of the genetic code are required for net protein accretion. The nutritional supply of amino acids for growth is defined as the net umbilical uptake of amino acids, representing the net transfer from maternal circulation, through the placenta and then to the fetus, of essential and non-essential amino acids. In considering the primary role of the placenta in the delivery of amino acids to the fetus for metabolism, it is important to consider the multiplicity of factors that may affect these overall delivery rates, including the activity and location of amino acid transporter systems, changes in these systems as gestation advances, effects of changes in placental surface area, uteroplacental blood flows, and maternal concentrations of amino acids. In this review, we discuss placental amino acid transport, the systems and their associated proteins, umbilical uptake data in animal and human studies, and amino acid transport in fetal growth restriction. Additionally, we discuss the current pool of thought concerning the mechanisms of placental amino acid transport as generated through in vitro vesicle studies and how they relate to the in vivo fluxes of animal studies. Finally, we discuss fetoplacental amino acid metabolism and specifically interorgan exchange.

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.

The role of the placenta in fetal nutrition and growth

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

Placental amino acid transport

Normal fetal growth and development depend on a continuous supply of amino acids from the mother to the fetus. The placenta is responsible for the transfer of amino acids between the two circulations. The human placenta is hemomonochorial, meaning that the maternal and fetal circulations are separated by a single layer of polarized epithelium called the syncytiotrophoblast, which is in direct contact with maternal blood. Transport proteins located in the microvillous and basal membranes of the syncytiotrophoblast are the principal mechanism for transfer from maternal blood to fetal blood. Knowledge of the function and regulation of syncytiotrophoblast amino acid transporters is of great importance in understanding the mechanism of placental transport and potentially improving fetal and newborn outcomes. The development of methods for the isolation of microvillous and basal membrane vesicles from human placenta over the past two decades has contributed greatly to this understanding. Now a primary cultured trophoblast model is available to study amino acid transport and regulation as the cells differentiate. The types of amino acid transporters and their distribution between the syncytiotrophoblast microvillous and basal membranes are somewhat unique compared with other polarized epithelia. These differences may reflect the unusual circumstance of this epithelium that is exposed to blood on both sides. The current state of knowledge as to the types of transport systems present in syncytiotrophoblast, their regulation, and the effects of maternal consumption of drugs on transport are discussed.

Uptake of L-leucine and L-phenylalanine across the basolateral cell surface in isolated oxyntic glands

The time course, kinetic, specificity and sodium-dependence of L-leucine and L-phenylalanine uptake by rabbit isolated oxyntic glands were studied in order to identify the systems involved in the transport of branched-chain and aromatic neutral amino acids through the basolateral cell membrane. The uptake was measured directly in the disrupted cells after incubation of the glands with the 3H-labelled amino acid both in a sodium-containing and a sodium-free medium. The uptake of L-leucine was largely carrier-mediated whilst L-phenylalanine was taken up by either carrier-mediated and nonsaturable processes. Both amino acids were taken up by a Na(+)-independent process. The kinetic parameters of L-leucine and L-phenylalanine carrier-mediated influx were, respectively: Kt = 2.71 mM and Jmax = 1390 nmol mg-1 s-1, Kt = 1.03 mM and Jmax = 176 nmol mg-1 s-1. From cross-inhibition studies it can be inferred that L-leucine is primarily transported by a Na(+)-independent system which shows specificity for bulky side chains dipolar amino acids. The system displays similar affinities for L-phenylalanine (Ki = 2.81 mM) and L-isoleucine (Ki = 2.62 mM). Similar results were obtained from self-inhibition experiments: the Ki of the carrier-mediated uptake of L-leucine and L-phenylalanine were 2.12 and 2.40 mM (from a Hanes plot) or 3.2 and 0.8 mM (from a Dixon plot), respectively. It is concluded that a sodium-independent transport system, like Christensen's 'L' type, is shared by branched-chain and aromatic dipolar amino acids, which only shows slight differences in their affinities for the carrier.

Protection against 2-methoxyethanol-induced teratogenesis by serine enantiomers: studies of potential alteration of 2-methoxyethanol pharmacokinetics

Several simple physiological compounds attenuate the teratogenic effects of 2-methoxyethanol (2-ME) when coadministered with 2-ME to mice. The mechanism of this protective action, however, has not been elucidated. Alteration of the kinetics of 2-ME and its oxidation product 2-methoxyacetic acid (2-MAA), the putative ultimate toxicant, was considered. D-Serine, the most efficacious attenuator, and L-serine (both 16.5 mmol/kg po) were examined for their abilities to mitigate 2-ME teratogenicity and to alter the disposition of an oral or sc bolus dose of 2-ME (3.3 mmol/kg containing 6 microCi 2-[methoxy-14C]ethanol) given to CD-1 mice on Gestation Day 11. L-Serine reduced the incidence of malformed fetuses from greater than or equal to 72% to 26-28%, while only 18 and 9% of fetuses were affected after coadministration of D-serine with sc and po 2-ME, respectively. Changes in the metabolism of orally administered 2-[14C]ME were specific to each enantiomer. D-Serine reduced the amount of 2-methoxy-N-acetylglycine eliminated in the urine to 70-75% of values observed with 2-ME alone, and concurrently increased the amount of urinary 2-MAA. L-Serine induced an initially higher rate of 14CO2 exhalation. Both enantiomers delayed gastrointestinal absorption of 2-ME, and significantly reduced 2-MAA levels in maternal plasma during the first hour after dosing. This resulted in a nonsignificant decrease (10-17%) in total embryonic exposure to 2-MAA. However, when 2-ME was injected sc, maternal plasma 2-ME/2-MAA pharmacokinetics were not affected by serine. In addition, dosing with 2.3 and 1.3 mmol 2-ME/kg sc alone showed that the embryo 2-MAA exposure levels which cause malformations in less than or equal to 35% fetuses were considerably lower than those measured following serine plus 3.3 mmol 2-ME/kg (po or sc). These data infer that serine does not protect against 2-ME-induced teratogenicity by altering 2-ME pharmacokinetics and reducing 2-MAA levels in the embryo.

D-glucose transfer rates derived from single-injection and steady state tracer experiments in the isolated guinea-pig placenta

In isolated guinea-pig placentae the transfer of D-[3H]glucose and L-[14C]glucose was investigated in single injection (SIE) and steady state (SSE, n = 11, placental weight 4.5 +/- 1.14 g) experiments at constant perfusion flow rates (3 ml/min). In SIE, a mixture of D- and L-glucose was injected as a bolus into either the fetal or maternal side of the placenta, uptake curves were obtained and the maximal extraction values Umax were derived. From these the membrane rate constants Kmc and Kfc of either the maternal or fetal side of the trophoblast membrane were calculated. The specific placental transfer rate constants were computed for the maternal-fetal (Kspec,mf) and fetal-maternal (Kspec,fm) direction from the amount of label transferred to the acceptor side in either SIE or in SSE (where labelled D- and L-glucose had been added to the stock solution). The chemical concentration of D-glucose was changed (5, 50 and 100 mmol/l, n = 11) and it was found that all rate constants decreased with increasing D-glucose concentration. At a D-glucose concentration of 5 mmol/l, Kmc was 3.11 +/- 1.59 ml/min (n = 6) and Kfc 2.69 +/- 0.5 ml/min (n = 5), the combined membrane rate constant (which determines the total placental transfer) was estimated to be 1.44 ml/min. This value was not significantly smaller than the mean (both directions) transfer rate constant Kspec in SIE (1.51 +/- 0.89 ml/min) or SSE (1.52 +/- 0.56 ml/min). Thus the results from uptake and transfer experiments are consistent. From the rate constants D-glucose fluxes at different chemical D-glucose concentrations were estimated. Whereas the mean Km values for all fluxes based on the various rate constants were about 30 mmol/l, the maximal flux Vmax was highest at the maternal trophoblast membrane (159.1 +/- 70.2 mumol/min), it was 107.9 +/- 11.6 mumol/min at the fetal side and 67.3 +/- 7.9 mumol/min in SIE or SSE for placental transfer in both directions. It is concluded that D-glucose carriers predominate at the maternal side of the trophoblast, and that maternal-fetal glucose transfer is proportionate to the transplacental glucose concentration difference within the physiological range of glucose concentrations.

Kinetics and specificity of L-alanine transport across the basolateral cell surface in isolated oxyntic glands

The time course, kinetics, specificity and sodium-dependence of alanine uptake by isolated oxyntic glands were studied. The uptake of alanine by the hydrolyzed cells was measured directly, after incubation of the glands with L-[3H]alanine. L-Alanine total influx was saturable and apparently mediated by a single entry system (Kt = 7.93 mM and Vmax = 8.0 mumol.mg-1.30s-1). The Kt was comparable to previously reported values for L-alanine transport in other epithelial cells. Kinetic studies performed in the presence and absence of Na+ suggest L-alanine uptake is mainly mediated by a Na(+)-dependent carrier system, but in addition, a minor diffusional component has been detected. Cross inhibition experiments performed over a wide range of concentrations (1 to 100 mM) suggest that the Na(+)-dependent transport system for alanine resembles system A and displays higher affinity for L-serine (Ki = 1.81 mM) than for L-alanine (K't = 4.86 mM); a lower affinity was found for L-cysteine (Ki = 16.30 mM). Results obtained with MeAIB support the hypothesis that system A is present at the basolateral membrane of the gland cells.

Two-sided culture of human placental trophoblast. Morphology, immunocytochemistry and permeability properties

We describe the culture of human term placental trophoblast cells on cell-free amniotic membrane, with medium on both sides. Over the course of 2 days, the isolated cells, initially simple, mononucleated and probably cytotrophoblast, form a confluent layer of multinucleated syncytial cells with morphological and immunocytochemical properties of syncytiotrophoblast. This layer becomes polarized with respect to morphology, alkaline phosphatase distribution and hCG secretion. Contamination with amnion cells, and with other cell types that are present in placental tissue, was less than 1 per cent. A preliminary investigation of the permeability properties of the preparation showed that the trophoblast cell layer, rather than the amniotic membrane, was rate-limiting to transtrophoblast transfer, but that possible effects of the supporting membrane should be considered. The transtrophoblast transfer of D-glucose and the non-metabolisable analogue, 3-O-methyl-D-glucose (3OMG), had saturable and non-saturable/leak components in both directions, indicating that carrier-mediated processes were involved. The non-metabolisable amino acid 2-aminoisobutyrate (AIB) was both accumulated within the trophoblast cells, and transferred by saturable and non-saturable processes from the microvillous side, but no saturable accumulation or transfer was observed from the basal side, at the concentrations tried. The results suggest that this model may prove suitable for studies of transtrophoblast transfer.

Transport of amino acids by the human placenta: predicted effects thereon of maternal hyperphenylalaninaemia

Brush border and basal plasma membrane vesicles prepared from normal term human placental syncytiotrophoblast have been used to study amino acid transport. Such studies are reviewed and novel results presented which confirm that saturation of placental transport by phenylalanine is unlikely to limit delivery of this amino acid to the fetus even with grossly raised maternal concentrations. Such raised maternal levels of phenylalanine are, however, likely to severely embarrass the delivery to the fetus across the placental brush border membrane of L-tyrosine and, to a lesser extent, of L-tryptophan. Reasons for thinking that this may be relevant to the fetal damage found in maternal PKU are discussed.

Alanine transport systems in isolated basal plasma membrane of human placenta

Concentrative transfer of amino acids from mother to fetus is affected by transport across both microvillous (maternal-facing) and basal (fetal-facing) plasma membranes of the human placental syncytiotrophoblast. Isolated basal plasma membrane vesicles were used to elucidate transport systems for neutral amino acids across this membrane. The concentration dependence and inhibition of zero-trans-alanine uptake were studied and four pathways for alanine uptake were defined as follows: 1) a sodium-dependent system shared by methylaminoisobutyric acid, which has the characteristics of an A system; 2) a sodium-dependent system resistant to inhibition by methylaminoisobutyric acid, which has the characteristics of an ASC system; 3) a sodium-independent system which may resemble an L system; 4) nonsaturable uptake. The microvillous membrane of the syncytiotrophoblast possesses systems similar to 1 and 3, but system 2 is unique to the basal plasma membrane. Active and passive transport of amino acids across both microvillous and basal plasma membranes may contribute to trophoblast amino acid uptake and nutrition and to the transfer of amino acids to the fetus.

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.

Neutral amino acid transport systems of microvillous membrane of human placenta

Placental transport produces concentrations of amino acids in fetal blood greater than those of maternal blood. Competitive inhibition studies of zwitterionic amino acid transport in isolated vesicles from the microvillous (maternal facing) plasma membranes of syncytiotrophoblast defined three transport systems: 1) a sodium-dependent system that supports methylaminoisobutyric acid (MeAIB) transport and has the characteristics of an A system; 2) a sodium-independent system with a high affinity for leucine and other amino acids with branched or aromatic side chains; and 3) a sodium-independent system with a preference for alanine as a substrate. The two sodium-independent systems could be further discriminated by marked specificity for trans stimulation with alanine or with leucine. System ASC, known to be present in whole placenta, and the neutral brush-border or imino systems of other polarized epithelia were apparently absent. Kinetic characteristics of the A system make it the probable primary driving force for concentrative transfer of its substrate amino acids to the fetus. Characteristics of the high-affinity leucine system demonstrated that it is saturated by normal serum leucine concentrations. Regulation of either system has the potential to alter placental amino acid uptake and transfer to the fetus.

Characterization of tryptophan transport in human placental brush-border membrane vesicles

The characteristics of tryptophan uptake in isolated human placental brush-border membrane vesicles were investigated. Tryptophan uptake in these vesicles was predominantly Na+-independent. Uptake of tryptophan as measured with short incubations occurred exclusively by a carrier-mediated process, but significant binding of this amino acid to the membrane vesicles was observed with longer incubations. The carrier-mediated system obeyed Michaelis-Menten kinetics, with an apparent affinity constant of 12.7 +/- 1.0 microM and a maximal velocity of 91 +/- 5 pmol/15 s per mg of protein. The kinetic constants were similar in the presence and absence of a Na+ gradient. Competition experiments showed that tryptophan uptake was effectively inhibited by many neutral amino acids except proline, hydroxyproline and 2-(methylamino)isobutyric acid. The inhibitory amino acids included aromatic amino acids as well as other system-1-specific amino acids (system 1 refers to the classical L system, according to the most recent nomenclature of amino acid transport systems). The transport system showed very low affinity for D-isomers, was not affected by phloretin or glucose but was inhibited by p-azidophenylalanine and N-ethylmaleimide. The uptake rates were only minimally affected by change in pH over the range 4.5-8.0. Tryptophan uptake markedly responded to trans-stimulation, and the amino acids capable of causing trans-stimulation included all amino acids with system-1-specificity. The patterns of inhibition of uptake of tryptophan and leucine by various amino acids were very similar. We conclude that system t, which is specific for aromatic amino acids, is absent from human placenta and that tryptophan transport in this tissue occurs via system 1, which has very broad specificity.

Characterization of choline transport at maternal and fetal interfaces of the perfused guinea-pig placenta

Unidirectional influx and efflux of choline into the syncytiotrophoblast were investigated from both maternal and fetal circulations of the perfused guinea-pig placenta by using a single-circulation paired-tracer (extracellular reference and test substrate) dilution technique. Cellular uptake of [3H]choline at 0.05 mM was (mean percentage +/- S.E. of mean, n = 14 placentae) 51 +/- 2 and 49 +/- 2, on maternal and fetal sides, respectively. Kinetics of unidirectional influx (0.05-4.0 mM-choline) indicated the existence of saturable and non-saturable components on both sides: on maternal and fetal interfaces the Km (mM) values were respectively, 0.12 and 0.13, the Vmax (mumol min-1 g-1) values, 0.08 and 0.07 and the apparent linear transfer constants (min-1 g-1) 0.11 and 0.12. Efflux of [3H]choline from the placenta back into the ipsilateral circulation (backflux) was generally fast (20-60% in 5-6 min) and asymmetric with the fetal: maternal ratio usually above unity. Transplacental specific choline transfer in the dually perfused placenta, when observed, was small (less than 10% of the injected dose) following tracer injections in either direction based on the 5-6 min collection of the contralateral circulation (at 0.05 mM-choline). Placental retention of [3H]choline at the end of the 5-6 min period was about 25% of the injected dose when the tracers were injected from either circulation. Analogues of choline such as hemicholinium-3, thiamine, ethanolamine and N,N-dimethylethanolamine inhibited choline unidirectional influx, whereas betaine and acetate had no effect. The absence of the normal sodium gradient (perfusate sodium was replaced by Tris or by lithium) did not inhibit choline transport. The metabolic inhibitors dinitrophenol (1.0 mM) and potassium cyanide (1.0 mM) were essentially ineffective (up to 40 min perfusion). The sulphydryl reagent N-ethylmaleimide did not appear to inhibit the influx, in comparison with its effect on [3H]choline backflux which was greatly accelerated, resulting in a dramatic reduction in placental net uptake of the label. Our findings show that choline transport into the placenta is a rapid carrier-mediated process occurring at both maternal and fetal sides of the trophoblast, at physiological blood concentrations. This cellular uptake is possibly related to the synthesis of acetylcholine, which is known to occur in human placental tissue. Specific transplacental transfer of choline was a very slow process under the conditions of our experiments and this contrasted with the observed fast and high uptake into the trophoblast.

Asymmetric calcium influx and efflux at maternal and fetal sides of the guinea-pig placenta: kinetics and specificity

Unidirectional influx of calcium across maternal and fetal sides of the syncytiotrophoblast was investigated in the guinea-pig placenta by using a rapid (less than 30 s) paired-tracer dilution technique. Experiments were performed in an in situ placenta artificially perfused through the umbilical vessels or in an isolated placenta in which both the maternal and fetal circulations were perfused. At equimolar Ca2+ concentrations, unidirectional calcium influx was always significantly lower on the maternal side than on the fetal side. Saturation kinetics were observed: on the fetal side the estimated Km was 1.8 +/- 0.7 mM and Vmax was 1.66 +/- 0.28 mumol/min X g (mean +/- S.E. of mean) and on the maternal side Km ranged from 0.18 to 1.15 mM and Vmax ranged from 0.12 to 0.59 mumol/min X g. When the inhibition of calcium influx was investigated on the fetal side of the trophoblast by using competing cations, the following sequence was observed: Ba2+ greater than Ca2+ congruent to Ni2+ greater than Sr2+ greater than Mg2+ congruent to Li+. Efflux of 45Ca2+ from the trophoblast into the ipsilateral circulation (backflux) was rapid (20-100% in 6 min) and asymmetric since the fetal:maternal ratio was 1.35 +/- 0.11 (mean +/- S.E. of mean) in the presence of 0.1 mM-Ca2+. In the dually perfused placenta, transplacental transfer (6 min) of 45Ca2+ varied over a wide range (0-80%); however, it was similar to that of the extracellular reference tracer, 22Na+, in either maternal-to-fetal or fetal-to-maternal directions. It is suggested that this is a consequence of the 'leakiness' of the dually perfused placenta since the transplacental transfer of 22Na+ and D-[3H]mannitol (or L-[14C]glucose) measured simultaneously were also variable but similar. Transplacental transfer of 45Ca2+ could not be used to characterize specific calcium-transport mechanisms, whereas highly sensitive trophoblast uptake measurements were provided by the single-circulation, paired-tracer technique. Our findings suggest the presence of a specific carrier-mediated transport system for calcium on both maternal and fetal surfaces of the trophoblast. The asymmetries in unidirectional influx into the trophoblast and rapid backflux indicate a mechanism by which the net transfer of calcium from the maternal to the fetal circulation is maintained in favour of the fetus.

Characteristics of a cationic amino acid transport system in the basolateral membrane of the cat salivary epithelium

The specificity and kinetics of L-lysine influx across the basolateral surface of the cat salivary epithelium have been investigated in the perfused cat submandibular gland using a high-resolution, paired-tracer dilution technique. L-lysine influx was measured at several different perfusate concentrations (0.05-2.5 mM) and was found to be saturable. A Michaelis-Menten analysis based on a single entry site gave a Km of 0.49 +/- 0.08 mM and a Vmax of 231 +/- 20 nmol/min X g. The uptake of L-lysine was highly stereospecific and markedly inhibited by L-arginine (0.25-2.5 mM). The inhibitor constant (Ki) was 0.23 mM, suggesting that the carrier had a greater affinity for L-arginine than L-lysine. When the inhibitory effects of L-histidine (0.5-10 mM) were examined the Ki, estimated at 10 mM, was 4.6 mM. Nine other neutral amino acids (L-alanine, L-serine, L-cysteine, glycine, L-proline, L-homoserine, L-leucine, L-phenylalanine and L-glutamine), and an acidic amino acid (L-aspartate) were also tested at 10 mM and, although several caused inhibition, the Ki was always at least 20 times higher than the measured Km for L-lysine. It is concluded the carrier is highly specific for the L-form of the basic amino acids. The sodium dependence of L-lysine influx was investigated over a range of L-lysine concentrations (0.05-1 mM), and total removal of sodium from the perfusate had no effect on L-lysine influx. In the presence of sodium, L-homoserine, an amino acid not normally present in animal tissues, inhibited L-lysine influx (Ki = 13 mM). This inhibition was not observed in the absence of sodium, and contrasts with the observation that the inhibitory action of L-histidine was sodium independent. The present data suggest that a specific cationic amino acid transport system is operative in the basolateral membrane of the cat salivary epithelium. The properties of this system appear to be similar to the system y+ which has been described in several other cell types.

Discrimination of parallel neutral amino acid transport systems in the basolateral membrane of cat salivary epithelium

Transport of short-chain and long-chain neutral amino acids across the basolateral membrane of the epithelium in the perfused cat salivary gland has been studied using a rapid (less than 30 s) single circulation paired-tracer dilution technique. Amino acid uptake was measured by comparing the venous dilution profiles for a tritiated amino acid and D-[14C]mannitol (extracellular reference) following a bolus intra-arterial injection of a mixture containing both molecules. Unidirectional influx (v) was estimated from the maximal tracer uptake (Umax), the perfusate flow (F) and the perfusate amino acid concentration (Ca): v = [-F . ln (1-Umax)] . Ca. L-alanine influx was saturable and apparently mediated by a single entry system (Km = 0.83 +/- 0.11 mM and Vmax = 655 +/- 32 nmol/min . g). These kinetic constants were considerably lower than our previously reported values for L-phenylalanine: Km = 6.4 mM and Vmax = 1719 nmol/min . g. In cross-inhibition experiments performed over a wide range of concentrations (0.05-24 mM), influx of L-alanine and L-phenylalanine could be further discriminated, since both L-phenylalanine (Ki = 22 mM) and L-alanine (Ki = 19 mM) behaved as poor competitors. Removal of Na+ from the perfusate resulted in a selective inhibition of L-alanine and L-serine influx, whereas influx of the long-chain neutral amino acids L-leucine, L-phenylalanine and L-tryptophan remained unaffected. Although prolonged perfusion of glands with dinitrophenol (0.8 mM for 20-30 min) caused a variable but net inhibition of unidirectional uptake, it markedly enhanced the tracer efflux of L-leucine, L-phenylalanine, L-tyrosine and the basic amino acid L-lysine. It appears that at least two separate neutral amino acid transport systems are operative at the blood-tissue interface of the salivary epithelium: (i) a Na+-dependent alanine-serine-cysteine preferring type of carrier exhibiting a high affinity for amino acids with short, polar or linear side chains and (ii) a Na+-independent leucine preferring type of carrier selective for large neutral amino acids.