Activation of cationic amino acid transport through system y+ correlates with expression of the T-cell early antigen gene in human lymphocytes

Lysine influx (2 microM) into activated human B and T lymphocytes through transport systems y+ and y+L was measured. In activated T cells very substantial activation of system y+ was detected; system y+L was also activated in these cells but with a slower time course and to a smaller extent. No stimulation of either system was found in activated B cells. The time course of activation of system y+ precisely matched the expression of the T cell early antigen gene described by MacLeod et al, 1990. The functional significance of these observations with respect to L-arginine transport and nitric oxide synthesis in activated T lymphocytes is discussed.

Identification of a new transport system (y+L) in human erythrocytes that recognizes lysine and leucine with high affinity

1. The effect of neutral amino acids on the transport of L-lysine across the human erythrocyte membrane was studied. 2. All neutral amino acids tested (range 0.3-5 mM) inhibit the influx of L-[14C]lysine (1 microM). The inhibition pattern is biphasic, and tends to reach a maximum at approximately 50% of the original flux. The concentrations that give 25% inhibition are (mM): L-cysteine (2.7), L-alanine (1.3), L-serine (0.9), L-isoleucine (0.6), L-phenylalanine (0.35), L-methionine (< 0.3), L-leucine (< 0.3). L-lysine and L-arginine completely inhibit the rate at the highest concentration. 3. These results can be explained by assuming that L-lysine transport occurs through two independent transporters that differ in their affinity for neutral amino acids. A detailed kinetic analysis of the effect of L-leucine on L-lysine entry is consistent with this hypothesis. 4. Using a new experimental strategy, the substrate and inhibitor transport parameters for the two systems were determined. The half-saturation constants for lysine (+/- S.E.M.) are found to be: KmA, 0.014 +/- 0.002 mM and KmB, 0.112 +/- 0.017 mM. The maximum rates differ by a factor of 8.2 (VmaxB/VmaxA). The leucine inhibition constants are: KiA, 0.022 +/- 0.003 mM and KiB, 30.36 +/- 7.9 mM. If the sodium in the incubation medium is replaced by potassium, the apparent affinity for leucine (1/KiA) is reduced approximately 30-fold. 5. The maximum inhibition caused by leucine decreases as the lysine concentration is raised, showing that leucine acts upon the higher affinity system. 6. When added to the trans side, L-leucine, L-phenylalanine and L-isoleucine do not cause inhibition, but stimulate the flux by approximately 30%. This indicates that these analogues are also transported. 7. In conclusion, in the concentration range 1-100 microM, lysine crosses the red cell membrane through two distinct transport systems, one of which recognizes both neutral and cationic amino acids with high affinity.

Bile acid transport by basal membrane vesicles of human term placental trophoblast

The aim of this work was to investigate the first step in the vectorial translocation of bile acids from the fetus to the mother, which is the transfer across the basal (i.e., fetal-facing) plasma membrane of the trophoblast. Thus, the uptake of [14C]taurocholate by basal plasma membrane vesicles obtained from normal human term placentas was studied. Taurocholate retention into vesicles was studied using a rapid filtration technique that was modified to reduce the taurocholate binding to the filters and to the external surface of the vesicles. Using 100 mumol/L substrate, the membrane vesicles showed a temperature-dependent, Na(+)-independent transport of taurocholate into an osmotically reactive intravesicular space. The initial rate of taurocholate influx in the presence of 100 mmol/L KNO3 followed saturation kinetics (apparent Km for taurocholate = 670 +/- 128 mumol/L; Vmax = 1.86 +/- 0.28 nmol/mg protein.60 s at 37 degrees C). Over the 6.9-7.9 pH range neither internal nor external pH nor inward nor outward proton gradients affected the uptake of taurocholate. When the electrical potential difference across the basal membrane was manipulated by external anion replacement (Cl-, SCN-, SO4(2-), or NO3-) or by valinomycin-induced K(+)-diffusion potential (vesicle inside negative), taurocholate uptake was not significantly modified. Taurocholate uptake was cis-inhibited in the presence of 1 mmol/L glycocholate, 0.5 mmol/L 4,4'-diisothiocyanostilbene-2,2'-disulfonate and 0.5 mmol/L sulfobromophthalein. However, 1 mmol/L probenecid or 0.5 mmol/L p-aminohippurate had no effect. Moreover, preloading the vesicles with 100 mmol/L HCO3- (but not with 100 mmol/L Cl- or 50 mmol/L SO4(2-) induced a significant enhancement in the initial rate of taurocholate uptake. In summary, these findings provide strong evidence for the presence of an electroneutral transport system for taurocholate in the basal plasma membrane of human chorionic trophoblast. They also suggest that this is likely to be an anion-exchange system.

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.

Behavioral and neurochemical changes associated with chronic exposure to low-level concentration of pesticide mixtures

In order to assess behavioral and neurochemical changes resulting from pesticide exposure, food-restricted male weanling rats were exposed for 90 d to low doses (1 ppb-10,000 ppb range) of individual pesticides (aldicarb, metribuzin, or methomyl) or mixtures of them. During exposure, rats were trained to run a T-maze and tested for spatial discrimination reversal learning. At sacrifice, three brain regions (cortex, hippocampus, and neostriatum) were assayed for the neurotransmitters dopamine, acetylcholine, and serotonin. Animals treated with a mixture of two insecticides and one herbicide were found to have slower speeds in maze-running (motor control) and also had altered levels of choline in their neostriatums. Rats treated with one herbicide compound (metribuzin) took longer to learn on two reversals; this group also had a significantly lower acetylcholine/choline ratio in their hippocampus.

Whole-cell K+ currents in type II pneumocytes freshly isolated from rat lung: pharmacological evidence for two subpopulations of cells

The patch clamp technique was used to record whole cell K+ currents in type II pneumocytes freshly isolated from adult rats. Depolarizing voltage steps evoked outward K+ currents which were distinguished into low and high threshold types, only one type being apparent in any one cell. Low-threshold (LT) currents were activated at test potentials of -40 mV to -20 mV and were reduced in amplitude by 20 mM tetraethylammonium (TEA). High-threshold (HT) currents were activated only at test potentials positive to -20 mV and current noise was always greater than for LT currents. HT currents were also significantly more sensitive than were LT currents to block by TEA. Quinine (1 mM) blocked LT currents reversibly at all activating test potentials. HT currents were also reversibly blocked by 1 mM quinine, but in a voltage-dependent manner, the degree of block increasing with increasing test potential. 4-Aminopyridine (2 mM) further distinguished the two current types: it was virtually without effect on HT currents but caused large reductions in LT current amplitudes, apparently by acting on the open channels underlying this current. These data clearly distinguish type II pneumocytes into two subpopulations and suggest that they may play separate roles in the functioning of the intact alveolar epithelium.

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.

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.

The determination of kinetic parameters for carrier-mediated transport of non-labelled substrate analogues: a general method applied to the study of divalent anion transport in placental membrane vesicles

A method is described that allows the determination of kinetic parameters for carrier-mediated transport of unlabelled compounds. The ability of these compounds to relieve the inhibition of labelled substrate efflux produced by addition of an external competitive inhibitor is studied. The method is of general applicability and does not depend upon any intrinsic asymmetry in the ratio of the rates of translocation of the loaded and unloaded carrier. In this paper it is used in a study of the human placental sulphate transporter. Experiments on brush-border membrane vesicles show the method to predict quantitatively kinetic parameters for unlabelled sulphate entry. Further analysis shows this carrier to have a broad specificity for other oxyanions (selenate, tung-state, molybdate and chromate) with the following selectivity sequence: for rate of translocation, SO4, SeO4 greater than WO4 greater than MoO4 much greater than CrO4; for binding affinity, CrO4 greater than MoO4, WO4 greater than SO4, SeO4.

Quantitation of metabolites of isolated chicken enterocytes using NMR spectroscopy

Isolated enterocytes prepared from chicken small intestine were studied by proton nuclear magnetic resonance (NMR). Perchloric acid extracts of cells were prepared from duodenum, jejunum, and ileum, and concentrations of 19 different metabolites were determined after the virtually complete assignment of all peaks in the aliphatic region of the spectrum. High concentrations of serine ethanolamine phosphodiester (SEP), creatine, taurine, and acidic amino acids were found in all segment extracts. Relatively high concentrations of SEP (approximately 12 mM) and acidic amino acids (approximately 3.5 mM) were found in the ileum, whereas creatine (3 mM) and lactate (6.5 mM) were found at higher concentrations in jejunum. Taurine (approximately 8 mM), choline (0.5 mM), and betaine (approximately 0.5 mM) were evenly distributed throughout the segments. Fasting (40 h) led to substantial increases in the concentrations of pyruvate, succinate, SEP, and taurine, while neutral hydrophobic amino acid concentrations fell appreciably. The significance of the findings is discussed, and the possible contributions of SEP, taurine, and choline to membrane lipid homeostasis are considered.

Pulmonary glucose transport in the fetal sheep

1. In the chronically catheterized sheep fetus between 122 and 143 days gestation the concentration of D-glucose in lung liquid was very low (usually less than 0.01 mM, the lower limit of detection of the analytical method) whereas the mean plasma concentration was 0.19 mM (S.E.M. 0.4, n = 13). 2. When the lung liquid concentration of D-glucose was raised to 1.67-5.00 mM, rapid uptake was observed until the concentration had fallen to its preceding low level. The uptake showed saturation kinetics (Vmax = 2.29-8.78 mumol/min, increasing with gestation; mean Km = 0.14 +/- 0.02 mM, n = 11, no change with gestation). This active uptake of glucose was blocked by phloridzin (10(-4) M). It was associated with a decrease in lung liquid secretion rate from which a change in net sodium flux could be inferred of an order suggesting one-to-one glucose-sodium co-transport. 3. Radiolabelled 3-O-methyl-D-glucose (3-O-meG) - a monosaccharide which is transported but not metabolized - was taken up rapidly from lung liquid and this rapid uptake was inhibited by D-glucose with 50% inhibition at 0.35 mM (+/- 0.08, n = 9). It was also inhibited by phloridzin (10(-4) M). 4. Radiolabelled 2-deoxy-D-glucose - a monosaccharide which is not a substrate for sodium-coupled transport - was taken up only very slowly from lung liquid; the rate of uptake was appropriate for passive diffusional transport and it was unaffected by the addition of D-glucose or phloridzin to lung liquid. 5. Intravenous infusion of D-glucose caused no detectable increase in the concentration of glucose in lung liquid unless phloridzin was added, when a slow increase was observed. 6. In two experiments with active transport blocked by phloridzin in lung liquid (10(-4) M), the rate of entry of labelled 3-O-meG from plasma to lung liquid was measured during intravenous infusion of this tracer for 29 and 23 h. The rates of entry were similar to the rate of efflux of the tracer from lung liquid when uptake was blocked by phloridzin or D-glucose, and similar to the rate expected for a metabolically inert tracer (i.e. it was some two orders of magnitude less than efflux from lung liquid in the absence of an inhibitor).(ABSTRACT TRUNCATED AT 400 WORDS)

A new form of asymmetry in epithelia: kinetics of apical and basal sulphate transport in human placenta

Sulphate transport into and out of membrane vesicles prepared from both the brush-border and basal surfaces of the human placental trophoblast has been studied. For both surfaces of this epithelium clear evidence of trans-stimulation of influx is seen but for efflux, acceleration of labelled sulphate movement by addition of external sulphate is seen only in basal and not in brush-border membrane vesicles. This kinetic asymmetry may underlie the previously observed, but unexplained, finding that the concentration of sulphate is greater in the fetal than in the maternal circulation.

External anions relieve DIDS inhibition of SO4 efflux from placental membrane vesicles

Sulphate efflux from human placental microvillus membrane vesicles was inhibited by external DIDS (KI congruent to 10(-6) M). This inhibition was partially reversed on addition of the translocated substrates sulphate or selenate to the external medium: selenite which is not translocated does not protect against DIDS inhibition. These findings show that the mechanism responsible for sulphate efflux can be modified by substrate in the external medium.

Sulphate transport into vesicles prepared from human placental brush border membranes: inhibition by trace element oxides

Inhibitors of SO4(2-) entry into and efflux from vesicles prepared from the brush border membrane of term human placenta have been studied, using 35SO4(2-) and an anion-exchange column assay. Divalent anions were found to be either potent or relatively feeble inhibitors of SO4(2-) uptake. Those which, like SO4(2-) itself, have a tetrahedral configuration, were strongly inhibitory and all of these anions were the metal oxides of elements of group VI (A and B) of the Periodic Table (chromate, molybdate, tungstate, selenate and thiosulphate). Divalent anions which were only weak inhibitors of SO4(2-) uptake were arsenate, phosphate and tetraborate. Chromate and to a lesser extent molybdate were effective inhibitors of SO4(2-) efflux from vesicles pre-loaded with SO4(2-). SO4(2-) efflux was insensitive to the electrical potential across the vesicle membrane. These results are discussed with respect to the mechanism for SO4(2-) transport across this membrane from mother to fetus and in the context of the transport to the feto-placental unit of the essential trace elements, selenium, chromium and molybdenum.

Cation transport in lung epithelial cells derived from fetal, newborn, and adult rabbits

Recent studies done with fetal and adult sheep and with monolayers of cultured rat alveolar type II cells suggest that active transport of Na+ across the lung epithelium may contribute to liquid absorption from air spaces, an essential component of the normal switch from placental to pulmonary gas exchange at birth. The goals of this work were 1) to study the ontogeny of cation transport in lung epithelial cells derived from fetal, newborn, and adult rabbits and 2) to determine the influence of premature birth, air breathing, labor, and postnatal lung maturation on K+ uptake in these cells. We harvested granular pneumonocytes by tracheal instillation of proteolytic enzymes followed by centrifugation of the dispersed cells over a discontinuous density gradient of metrizamide. This procedure yielded 65-90% granular pneumonocytes, of which more than 80% excluded vital dye. Using freshly isolated cells, we measured uptake of 86Rb+, which mimics transmembrane movement of K+, in the presence or absence of 10(-4) M ouabain and in the presence or absence of 5 X 10(-4) M furosemide or bumetanide. In adult rabbit studies, 86Rb+ uptake was twice as fast in lung epithelial cells (98 +/- 7 nmol X 10(6) cells-1 X h-1) as it was in alveolar macrophages (51 +/- 6 nmol X 10(6) cells-1 X h-1). Ouabain inhibited 55-60% of the uptake by pneumonocytes, and "loop" diuretics inhibited an additional 15-20%. The rate of 86Rb+ uptake in fetal cells was less than 10% (6 +/- 1 nmol X 10(6) cells-1 X h-1) of the rate in adult cells; ouabain inhibited 80-85% of 86Rb+ uptake in fetal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Human placental sulphate transport: studies on chorionic trophoblast brush border membrane vesicles

SO4(2-) transport by microvillous brush border membrane vesicles prepared from normal-term human placenta has been studied using an ion-exchange column assay. The uptake of this anion is time dependent and at 20 degrees C reaches a point of equilibrium by 60 min. 4,4'-Diisothiocyanostilbene-2,2'-disulphonate (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2-2'-disulphonate (SITS) (at 10(-4) M) were found to inhibit approximately 80% of uptake. The concentration of DIDS producing half-maximal inhibition was approximately 10(-5) M. In contrast furosemide and probenecid were weak inhibitors of SO4(2-) influx. The anions Cl-, I-, thiocyanate (SCN-) and gluconate (cis-side) caused minimal inhibition of SO4(2-) influx. Salicylate (10 mM) produced 47% inhibition. The divalent anions thiosulphate, tungstate and unlabelled SO4(2-) (10 mM) inhibited the uptake of SO4(2-) (at 1 mM) to the same extent as DIDS. The vesicle membrane potential was altered by varying external K+ concentration using valinomycin. The DIDS-sensitive SO4(2-) influx was not affected by changes in membrane potential. A novel method has been developed for studying solute efflux from pre-loaded vesicles. Both DIDS and a reduction in temperature were effective inhibitors of SO4(2-) efflux. These results are discussed.

Sulphate transport in human placental brush-border membrane vesicles: competitive inhibition by selenate

The effect of selenate on sulphate uptake by human placental brush-border membrane vesicles has been investigated. Selenate added to the incubation medium inhibits 1 mM sulphate uptake in a dose-dependent fashion with a Ki of approx. 2.5 mM. The inhibition by selenate is competitive, suggesting that selenate and sulphate share a common transporter (an anion exchange system) which may be of particular importance for the transport of such essential trace elements to the fetus.

Transport of butyric acid in vascularly perfused anuran small intestine: importance of pH and anion transport

Butyric acid transport was studied in the isolated, vascularly perfused frog small intestine. At luminal butyric acid concentrations of 5-50 mM, absorption was a nonlinear function of the luminal concentration, whereas the relationship of absorption to concentration remained linear at 0-1,000 microM. The most important factor regulating the rate and direction of butyric acid transport was the pH. We used unidirectional flux analysis to determine net transport across the epithelium while the pH of the luminal or vascular compartments was changed. We found a four- to fivefold decrease in butyric acid transport into the portal circulation as the lumen pH was increased from 6.0 to 8.0. The pH of the vascular perfusate influenced the vascular-to-lumen transport of butyric acid in the same proportions. The second important regulatory factor of butyric acid transport was the 4,4'-diisothiocyananostilbene-2,2'-disulfonic acid (DIDS)-sensitive anion transport protein. DIDS added to the lumen at 10(-6) M decreased butyric acid transport by approximately 40% at pH 7.4. DIDS also inhibited butyric acid transport when added to the vascular perfusate or when transport was measured in a vascular-to-lumen direction. We suggest that, at the relatively low pH of the proximal small intestine, butyric acid becomes protonated and lipophilic and is mainly transported directly through the cell membrane. At the more alkaline pH of the distal small intestine butyric acid is in the ionized form and transport by the DIDS-sensitive anion transport protein may predominate.

Chloride transport in human placental microvillus membrane vesicles. I. Evidence for anion exchange

36Cl uptake by microvillus brush border membrane vesicles prepared from normal term human placenta has been studied using an ion exchange column assay and a novel method to correct for vesicle recovery. Uptake of this anion, which was largely into an osmotically active space, was time dependent with a half-time of approximately 4 min at 4 degrees C. DIDS at a maximal inhibitory concentration inhibited 36Cl uptake by approximately 40%; furosemide (10(-4)M) showed a similar degree of inhibition and the effects of the two drugs appeared not to be additive. Anions including Cl, Br, I, NO3, salicylate and SCN reduced 36Cl uptake, the latter two being the most potent; gluconate was without effect. Inhibition by maximal inhibitory concentrations of DIDS and the anions SCN and salicylate was not additive. Neither the DIDS-sensitive nor the SCN-sensitive component of 36Cl uptake was influenced by membrane potential; in contrast the inhibitor-insensitive component was increased by an inside positive membrane potential. These findings suggest that under the conditions employed an electroneutral anion exchange system is responsible for approximately half of the chloride flux across this plasma membrane.