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

Placental Lactate Transporter Activity and Expression in Intrauterine Growth Restriction

OBJECTIVES

To compare lactate uptake in the microvillous plasma membrane (maternal facing [MVM]) in term and preterm placentas in intrauterine growth restriction (IUGR) and appropriate weight for gestational age (AGA) controls, and in the basal plasma membrane (fetal facing [BM]) at term. In addition, we examine the expression of monocarboxylate transporters (MCT1 and MCT4).

METHODS

We measured [14C] L-lactate uptakes into vesicles prepared from MVM and BM, stimulated by an inwardly directed H+ gradient. MCT expression was examined by Western blotting.

RESULTS

In term placentas, mean (+/- SE) [14C] L-lactate uptake into MVM vesicles of the IUGR (n = 6) and AGA (n = 11) groups at initial rate was similar (15.4 +/- 2.3 versus 15.0 +/- 1.1 pmol/mg protein/20 s). In preterm placentas, in IUGR (n = 3) and AGA (n = 3) groups, [14C] l-lactate uptake into MVM was also not significantly different. In BM vesicles from term placentas, [14C] L-lactate uptake was significantly lower in IUGR (n = 5) than in AGA (n = 6) controls (3.6 +/- 0.4 versus 5.6 +/- 0.6 pmol/mg protein/20 s, P <.05). MCT1 and MCT4 were expressed in BM vesicles, but there was no difference in expression between the IUGR and AGA groups.

CONCLUSIONS

These findings suggest that in IUGR placental lactate transport capacity in the BM is reduced, which may adversely affect placental lactate clearance.

Disposition of glycolic acid into rat and rabbit embryos in vitro

High dose gavage administration of ethylene glycol (EG) induces teratogenicity in rodents, but not in rabbits, resulting from saturation of intermediate EG metabolism and glycolic acid (GA) accumulation. In vivo, rat embryos sequester GA 2-4-fold higher than maternal blood, a phenomenon absent in rabbits and proposed not to occur in humans. This research explored the mechanisms of GA disposition into rat and rabbit conceptuses using whole embryo culture (WEC). Rat and rabbit embryos concentrated GA from the culture medium. In vitro to in vivo discordance in the rabbit plausibly stemmed from anatomical differences between these models. GA sequestration was attenuated at 4°C in both species. Rat embryos further demonstrated pH-dependence of GA sequestration and inhibition by D-lactic acid. These data suggest GA disposition into rat and rabbit embryos is energy- and pH-dependent, and carrier-mediated. Anatomical and maternal-to-conceptal pH gradient differences likely underlie the lack of enhanced GA disposition in non-rodent species.

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

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

Carrier-mediated uptake of Levofloxacin by BeWo cells, a human trophoblast cell line

OBJECTIVE

Placental transfer of Levofloxacin (LF), a broad spectrum fluoroquinolone antibiotic, and its inhibition was investigated in BeWo cells, a human trophoblast cell line.

METHODS

The experiments of LF uptake by BeWo cells were performed after preincubation and in the presence of the P-glycoprotein inhibitors (Cyclosporin A, Verapamil and Quercetin), the organic anion/cation transporter inhibitor (Cimetidine) and the MCT substrates (lactic acid and salicylic acid).

RESULTS

P-glycoprotein inhibitors increased the uptake of LF by BeWo cells. The increase in LF accumulation by Cyclosporin A, Verapamil and Quercetin was by 30, 90 and 80%, respectively. Cimetidine, the organic cation inhibitor, increased the transport of LF by 48%. Lactic acid and salicylic acid, the MCT substrates, initially decreased the accumulation of LF by 30% and subsequently increased the uptake of LF by 500 and 53%, respectively.

CONCLUSIONS

The uptake of LF by human trophoblast cells is mediated by multiple transporters as well as passive diffusion.

Evaluation of absorption of heparin-DOCA conjugates on the intestinal wall using a surface plasmon resonance

We validated the application of the surface plasmon resonance (SPR) technique to reliably determine adhesion of drugs to the intestinal wall using heparin-DOCA conjugates, developed to enhance the oral absorption of poorly absorbed heparin. In this study, heparin conjugates, or deoxycholyl-heparin (H-DOCA) and bisdeoxycholyl-heparin (H-bis-DOCA), were synthesized by covalently coupling the synthesized succinimido deoxycholate (DOCA-NHS) or succinimido bis-deoxycholyl-L-lysine (DOCA-bis-NHS) to amine groups of heparin, and their physicochemical and biological properties were evaluated. To mimic the duodenal and ileal surfaces, duodenal and ileal brush border membrane (BBM) vesicles isolated from Sprauge-Dawley (SD) rats were immobilized onto a biosensor chip composed of dextran derivatives with modified lipophilic residues. The adhesion of heparin conjugates on the BBM surface was evaluated by measuring the SPR response signal. The adhesion of heparin conjugates was significantly dependent on the conjugated DOCA molecules: that is, they showed higher adhesion signal on the ileal BBM surface than that on the duodenal BBM surface. In particular, the solubilized heparin conjugates in DMSO solution presented significantly increased adhesion affinity on the ileal BBM surface. The adhesion of heparin conjugates on the intestinal surfaces was successfully assayed using the surface plasmon resonance technique with the sensor chip on which BBM vesicles were immobilized.

Lactate utilization by brain cells and its role in CNS development

We studied the role played by lactate as an important substrate for the brain during the perinatal period. Under these circumstances, lactate is the main substrate for brain development and is used as a source of energy and carbon skeletons. In fact, lactate is used actively by brain cells in culture. Neurons, astrocytes, and oligodendrocytes use lactate as a preferential substrate for both energy purposes and as precursor of lipids. Astrocytes use lactate and other metabolic substrates for the synthesis of oleic acid, a new neurotrophic factor. Oligodendrocytes mainly use lactate as precursor of lipids, presumably those used to synthesize myelin. Neurons use lactate as a source of energy and as precursor of lipids. During the perinatal period, neurons may use blood lactate directly to meet the need for the energy and carbon skeletons required for proliferation and differentiation. During adult life, however, the lactate used by neurons may come from astrocytes, in which lactate is the final product of glycogen breakdown. It may be concluded that lactate plays an important role in brain development.

Conditionally immortalized syncytiotrophoblast cell lines as new tools for study of the blood-placenta barrier

Syncytiotrophoblasts play an essential role in restriction of drug delivery through the blood-placenta barrier (BPB). Conditionally immortalized syncytiotrophoblast cell lines, TR-TBTs, were established at gestational day 18 from pregnant transgenic rats (Tg-rats) harboring the temperature-sensitive SV 40 (ts SV40) large T-antigen. TR-TBTs exhibit temperature-sensitive cell growth due to the expression of SV 40 large T-antigen, and thus the cell growth can be regulated by changing the culture temperature. TR-TBTs exhibit typical properties of syncytiotrophoblast cells, such as syncytium-like morphology, the expression of cytokeratins and hormones, and polarized expression of glucose transporter 1 (GLUT1) and GLUT3. TR-TBTs express in vivo influx and efflux transporters, such as taurine transporter (TauT), betaine/GABA transporter (BGT-1), amino acid transporter 2 (ATA2), organic anion transporting polypeptide 2 (oatp2), organic cation/carnitine transporter (OCTN2), P-glycoprotein (P-gp), and breast cancer resistance protein (BCRP/ABCG2). Moreover, TR-TBTs exhibit taurine, GABA, and DHEA-S uptake activity via TauT, BGT-1, and oatp2, respectively. Therefore, TR-TBTs can be used for the analysis of these functions and would be a good in vitro models for investigating carrier-mediated transport functions at the BPB.

Polarized lactate transporter activity and expression in the syncytiotrophoblast of the term human placenta

We investigated the polarization of l-lactate transport in human syncytiotrophoblast by measuring uptake of [(14)C] l-lactate by both microvillous (maternal-facing; MVM) and basal (fetal-facing; BM) plasma membranes. [(14)C] l-lactate uptake by MVM and BM was stimulated in the presence of an inwardly directed H(+)gradient, with a significantly higher uptake in MVM than in BM at initial rate (15.4+/-2.3 vs 5.6+/-0.6 pmol/mg protein/20 sec). Stereospecific inhibition was observed in MVM, with a higher affinity for l-lactate compared with d-lactate. In BM, there was no difference in the inhibition by these two stereoisomers. Inhibition of lactate uptake in both MVM and BM by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of monocarboxylate transporter (MCT) activity, indicated MCT-mediated mechanisms across both membranes. Kinetic modelling supported a two-transporter model as the best fit for both MVM and BM, the K(m)of the major component being 6.21 mm and 25.01 mm in MVM and BM respectively. Western blotting and immunolocalization examining the distribution of MCT1 and MCT4, showed that MCT expression was polarized, MCT1 being predominantly localized to BM and MCT4 showing greater abundance on MVM. CD147, a chaperone protein for MCT1 and MCT4, was equally expressed by both membranes. These studies demonstrate that the opposing plasma membranes of human syncytiotrophoblast are polarized with respect to both MCT activity and expression.

Characteristics of L-lactic acid transport in basal membrane vesicles of human placental syncytiotrophoblast

The characteristics of L-lactic acid transport across the trophoblast basal membrane were investigated and compared with those across the brush-border membrane by using membrane vesicles isolated from human placenta. The uptake of L-[(14)C]lactic acid into basal membrane vesicles was Na(+) independent, and an uphill transport was observed in the presence of a pH gradient ([H(+)](out) > [H(+)](in)). L-[(14)C]lactic acid uptake exhibited saturation kinetics with a K(m) value of 5.89 +/- 0.68 mM in the presence of a pH gradient. p-Chloromercuribenzenesulfonate and alpha-cyano-4-hydroxycinnamate inhibited the initial uptake, whereas phloretin or 4,4'-diisothiocyanostilbene-2,2'-disulfonate did not. Mono- and dicarboxylic acids suppressed the initial uptake. In conclusion, L-lactic acid transport in the basal membrane is H(+) dependent and Na(+) independent, as is also the case for the brush-border membrane transport, and its characteristics resemble those of monocarboxylic acid transporters. However, there were several differences in the effects of inhibitors between basal and brush-border membrane vesicles, suggesting that the transporter(s) involved in L-lactic acid transport in the basal membrane of placental trophoblast may differ from those in the brush-border membrane.

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

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

Proton gradient-dependent transport of valproic acid in human placental brush-border membrane vesicles

PURPOSE

To investigate the transport mechanism of valproic acid across the human placenta, we used human placental brush-border membrane vesicles and compared them with that of lactic acid.

METHODS

Transport of [3H]valproic acid and [14C]lactic acid was measured by using human placental brush-border membrane vesicles.

RESULTS

The uptakes of [3H]valproic acid and [14C]lactic acid into brush-border membrane vesicles were greatly stimulated at acidic extravesicular pH. The uptakes of [3H]valproic acid and [14C]lactic acid were inhibited by various fatty acids, p-chloromercuribenzene sulfonate, alpha-cyano-4-hydroxycinnamate, and FCCP. A kinetic analysis showed that it was saturable, with Michaelis constants (Kt) of 1.04 +/- 0.41 mM and 1.71 +/- 0.33 mM for [3H]valproic acid and [14C]lactic acid, respectively. Furthermore, lactic acid competitively inhibited [3H]valproic acid uptake and vice versa.

CONCLUSION

These results suggest that the transport of valproic acid across the microvillous membrane of human placenta is mediated by a proton-linked transport system that also transports lactic acid. However, some inhibitors differentially inhibited the uptakes of [3H]valproic acid and [14C]lactic acid, suggesting that other transport systems may also contribute to the elevated fetal blood concentration of valproic acid in gravida.

Interactions among glucose, lactate and adenosine regulate energy substrate utilization in hippocampal cultures

Glucose is the major energy source during normal adult brain activity. However, it appears that glial-derived lactate is preferred as an energy substrate by neurons following hypoxia-ischemia. We examined factors influencing this switch in energetic bias from glucose to lactate in cultured hippocampal neurons, focusing on the effects of the physiological changes in lactate, glucose and adenosine concentrations seen during hypoxia-ischemia. We show that with typical basal concentrations of lactate and glucose, lactate had no effect on glucose uptake. However, at the concentrations of these metabolites found after hypoxia-ischemia, lactate inhibited glucose uptake. Reciprocally, glucose had no effect on lactate utilization regardless of glucose and lactate concentrations. Furthermore, we find that under hypoglycemic conditions adenosine had a small, but significant, inhibitory effect on glucose uptake. Additionally, adenosine increased lactate utilization. Thus, the relative concentrations of glucose, lactate and adenosine, which are indicative of the energy status of the hippocampus, influence which energy substrates are used. These results support the idea that after hypoxia-ischemia, neurons are biased in the direction of lactate rather than glucose utilization and this is accomplished through a number of regulatory steps.

Effect of maternal cholestasis on bile acid transfer across the rat placenta-maternal liver tandem

Cholestasis of pregnancy induces alterations in bile acid transport by human trophoblast plasma membrane (TPM) vesicles. We investigated whether maternal cholestasis affects the overall ability of the rat placenta to carry out vectorial bile acid transfer from the fetus to the mother. Complete obstructive cholestasis (OCP) was maintained during the last week of pregnancy and released at term (day 21), before experiments were performed. In situ single-pass perfusion of one placenta per rat with 250 nmol [(14)C]glycocholic acid (GC) revealed an impaired uptake in OCP rats (2.28 vs. 5.53 nmol in control rats). Approximately 100% of GC taken up by control placentas was secreted in maternal bile over 120 minutes (5.38 nmol), whereas this was only 61% (1.40 nmol) of the GC taken up by OCP placentas. When 5 nmol GC was administered through the jugular vein no significant difference between both groups in total GC bile output was found. The efficiency (V(max)/K(M)) of adenosine triphosphate (ATP)-dependent GC transport by vesicles from the maternal side of TPM was decreased (-41%) in OCP. Moreover, histological examination of the placentas suggested a reduction in the amount of functional trophoblast in the OCP group. This was consistent with a lower antipyrine diffusion across the placenta in these animals. In sum, our results indicate that maternal cholestasis affects the ability of the placenta to efficiently carry out bile acid transfer from fetal to maternal blood. Changes in both the structure and the functionality of the chorionic tissue may account for this impairment.

Carrier-mediated transport of monocarboxylic acids in BeWo cell monolayers as a model of the human trophoblast

The monolayer-forming, human choriocarcinoma cell line, BeWo, was used to study the mechanisms of monocarboxylic acid transport across the human trophoblast. Benzoic acid, acetic acid, and lactic acid were used as markers for monocarboxylic acid carrier-mediated transport. The uptake of benzoic acid by BeWo cells was saturable (K(t) = 0.6 +/- 0.3 mM) at higher concentrations and significantly inhibited by typical metabolic inhibitors, sodium azide and 2, 4-dinitrophenol. A selection of different monocarboxylic acids, including a natural substrate lactic acid, also substantially inhibited the uptake of benzoic acid and acetic acid by BeWo cells, whereas dicarboxylic acids did not affect the uptake of either marker. Monocarboxylic acid uptake was pH-dependent and inhibited by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), a protonophore. Kinetic analysis using Lineweaver-Burk plots revealed that monocarboxylic acids competitively inhibited the uptake of benzoic, lactic, and acetic acid by BeWo cells. In transport experiments, the permeation of benzoic acid from apical-to-basolateral side was greater than the permeation from the basolateral-to-apical side, and the transport of benzoic acid from apical-to-basolateral side was inhibited by monocarboxylic acids. The findings obtained in the present study confirm the existence of an asymmetric, carrier-mediated transport system for monocarboxylic acids across the BeWo cell, a representative of the human trophoblast.

Proline, leucine, and alanine transport in placental microvillous membrane vesicles prepared from late gestational rats

To characterize the active transport of amino acids across the placenta, uptakes of proline, leucine, and alanine were kinetically examined in placental microvillous membrane vesicles (PMV) prepared from rats in the late gestational period. Uptake rates of these amino acids in PMV showed saturable hyperbolic curves that obeyed Michaelis-Menten kinetics. Proline, leucine, and alanine transport were demonstrated to be carrier mediated systems with sodium-dependent, -independent, and both manner, respectively. In addition, sodium-dependent L-alanine transport showed two different systems, and new sodium-independent alanine transport system (K(m) of 1.12 mM) was observed in rat placenta. From these results, rat placenta has carrier mediated amino acid transport systems, and possesses at least three different transport systems for alanine.

Transport of exogenous 5-hydroxytryptamine across the outer plasma membrane of the syncytial tegument ofHymenolepis diminutais by simple diffusion

The uptake of 5-hydroxytryptamine (5HT) from a 10 μM solution of exogenous [3H]5HT into the tegument of Hymenolepis diminuta was linear for the first 20 min of incubation. The rate of transport was 0.04 ± 0.01 pmol∙mg wet mass−1∙min−1, and there were no significant differences in the rate of uptake by the anterior, middle, and posterior regions of the body. The initial uptake was not Na+-dependent, was not saturable at up to 100 μM, was not highly temperature-dependent (Q10~ 1.2), and displayed activation energy of 11.8 kJ∙mol−1. Furthermore, uptake was not inhibited by p-chloromercuriphenyl sulphonic acid, imipramine, amiloride, or 5HT analogues, which collectively support a non-carrier-mediated uptake mechanism. Washing of the tissues with 10 mM 5HT after incubation in 10 μM [3H]5HT displaced less than 10% of the remaining [3H]5HT associated with the tissues, and little radioactivity was extracted by washing in acetone or chloroform. The uptake of [3H]5HT, however, was pH-dependent, the rate of uptake being closely correlated with the proportion of unprotonated 5HT. Only a small portion of the transported [3H]5HT was metabolized to a product associated with 5-hydroxyindoleacetic acid, and metabolism was significantly inhibited by the monoamine oxidase inhibitors iproniazid phosphate, deprenyl, and clorgyline. The present study showed that small amounts of [3H]5HT were taken up by H. diminuta by simple diffusion, little of the [3H]5HT was adsorbed to the surface of the worms or dissolved in the lipid phase of the plasma membrane, and some of the [3H]5HT taken up was metabolized by a monoamine oxidase-like enzyme.

Proton-lactate cotransport in basolateral membrane vesicles from rat jejunum

Proton-coupled lactate transport across the basolateral membrane of rat jejunal enterocyte was studied using well purified membrane vesicles. L-lactate uptake is stimulated by an inwardly directed H+ gradient; the effect of the pH difference is drastically reduced by FCCP and by pCMBS; unlabelled L-lactate causes a strong inhibition, whilst furosemide is uneffective. The H+ gradient-dependent stimulation of L-lactate uptake is significantly inhibited also by SCN-: this finding could explain results recently reported in the literature in which H(+)-lactate symport was not evidenced in basolateral membranes from rat jejunum.

Effect of fetal growth restriction on system A amino acid transporter activity in the maternal facing plasma membrane of rat syncytiotrophoblast

To determine whether reduced maternofetal transfer of neutral amino acids in growth-restricted fetal rats is due to decreased system A transporter activity, we measured Na(+)-dependent MeAIB uptake by membrane vesicles from placentas of fetuses growth-restricted due to uterine artery ligation and control placentas (sham ligation). Na(+)-dependent uptake of methylamino-isobutyric acid (MeAIB) was linear over 15-60 s in vesicles from both ligated and sham-ligated sides of the uterus. Na(+)-dependent uptake of MeAIB at 30 s did not differ in paired measurements on vesicles from ligated and sham-ligated horns, 0.063 +/- 0.004 versus 0.056 +/- 0.005 nmol/mg of vesicle protein. The kinetics of Na(+)-dependent MeAIB uptake were similar in paired measurements on vesicles from ligated and sham-ligated horns, with overall K(m) = 4.4 +/- 0.5 mM and Vmax = 0.93 +/- 0.08 nmol/mg vesicle protein per 30 s. Uptake of tracer was inhibited 85-95% by known substrates for the system A amino acid transporter (alanine > or = serine > MeAIB > glycine = proline). We conclude that the system A transporter is present in the maternal facing plasma membrane of rat syncytiotrophoblast, but that the activity of this system, per mg of vesicle protein, is unaffected in fetal growth restriction induced by a decrease in maternal placental blood flow in late pregnancy.

Rotavirus interaction with isolated membrane vesicles

To gain information about the mechanism of epithelial cell infection by rotavirus, we studied the interaction of bovine rotavirus, RF strain, with isolated membrane vesicles from apical membrane of pig enterocytes. Vesicles were charged with high (quenching) concentrations of either carboxyfluorescein or calcein, and the rate of fluorophore release (dequenching) was monitored as a function of time after mixing with purified virus particles. Purified single-shelled particles and untrypsinized double-shelled ones had no effect. Trypsinized double-shelled virions induced carboxyfluorescein release according to sigmoid curves whose lag period and amplitude were a function of virus concentration and depended on both temperature and pH. The presence of 100 mM salts (Tris Cl, NaCl, or KCl) was required, since there was no reaction in isoosmotic salt-free sorbitol media. Other membrane vesicle preparations such as apical membranes of piglet enterocyte and rat placenta syncytiotrophoblasts, basolateral membranes of pig enterocytes, and the undifferentiated plasma membrane of cultured MA104 cells all gave qualitatively similar responses. Inhibition by a specific monoclonal antibody suggests that the active species causing carboxyfluorescein release is VP5*. Ca2+ (1 mM), but not Mg2+, inhibited the reaction. In situ solubilization of the outer capsid of trypsinized double-shelled particles changed release kinetics from sigmoidal to hyperbolic and was not inhibited by Ca2+. Our results indicate that membrane destabilization caused by trypsinized outer capsid proteins of rotavirus leads to fluorophore release. From the data presented here, a hypothetical model of the interaction of the various states of the viral particles with the membrane lipid phase is proposed. Membrane permeabilization induced by rotavirus may be related to the mechanism of entry of the virus into the host cell.

Transport of lactate and other monocarboxylates across mammalian plasma membranes

Transport of L-lactate across the plasma membrane is of considerable importance to almost all mammalian cells. In most cells a specific H(+)-monocarboxylate cotransporter is largely responsible for this process; the capacity of this carrier is usually very high, to support the high rates of production or utilization of L-lactate. The best characterized H(+)-monocarboxylate transporter is that of the erythrocyte membrane, which transports L-lactate and a wide range of other aliphatic monocarboxylates, including pyruvate and the ketone bodies acetoacetate and beta-hydroxybutyrate. This carrier is inhibited by alpha-cyanocinnamate derivatives and some stilbene disulfonates and has been identified as a protein of 35-50 kDa on the basis of purification and specific labeling experiments. Other cells possess similar alpha-cyanocinnamate-sensitive H(+)-linked monocarboxylate transporters, but in some cases there are significant differences in the properties of these systems, sufficient to suggest the existence of a family of such carriers. In particular, cardiac muscle and tumor cells have transporters that differ in their Km values for certain substrates (including stereoselectivity for L- over D-lactate) and in their sensitivity to inhibitors. Mitochondria, bacteria, and yeast also possess H(+)-monocarboxylate transporters that share some properties in common with those in the mammalian plasma membrane but are adapted to their specific roles. However, there are distinct Na(+)-monocarboxylate cotransporters on the luminal surface of intestinal and kidney epithelia, which enable active uptake of lactate, pyruvate, and ketone bodies in these tissues. This article reviews the properties of these transport systems and their role in mammalian metabolism.

Heterogeneity of L-alanine transport systems in brush-border membrane vesicles from rat placenta during late gestation

The placental uptake of L-alanine was studied by using purified brush-border membrane vesicles from rat trophoblasts. Saturation curves were carried out at 37 degrees C in buffers containing 100 mM (zero-trans)-NaSCN, -NaCl, -KSCN, -KCl, or -N-methyl-D-glucamine gluconate. The uncorrected uptake results were fitted by non-linear regression analysis to an equation involving one diffusional component either one or two saturable Michaelian transport terms. In the presence of NaCl, two distinct L-alanine transport systems were distinguished, named respectively System 1 (S-1; Vm1 about 760 pmol/s per mg of protein; KT1 = 0.5 mM) and System 2 (S-2; Vm2 about 1700 pmol/s per mg; KT2 = 9 mM). In contrast, in the presence of K+ (KCl = KSCN) or in the absence of any alkali-metal ions (N-methyl-D-glucamine gluconate), only one saturable system was apparent, which we identify as S-2. When Na+ is present, S-1, but not S-2, appears to be rheogenic, since its maximal transport capacity significantly increases in the presence of an inside-negative membrane potential, created either by replacing Cl- with the permeant anion thiocyanate (NaSCN > NaCl) or by applying an appropriate K+ gradient and valinomycin. alpha-(Methylamino)isobutyrate (methyl-AIB) appears to be a substrate of S-1, but not of S-2. For reasons that remain to be explained, however, methyl-AIB inhibits S-2. We conclude that S-1 represents a truly Na(+)-dependent mechanism, where Na+ behaves as an obligatory activator, whereas S-2 cannot discriminate between Na+ and K+, although its activity is higher in the presence of alkali-metal ions than in their absence (Na+ = K+ > N-methyl-D-glucammonium ion). S-2 appears to be fully developed 2 days before birth, whereas S-1 undergoes a capacity-type activation between days 19.5 and 21.5 of gestation, i.e. its apparent Vmax. nearly doubles, whereas its KT remains constant.