Monocarboxylate transport in erythrocytes

Red blood cell lactate transport in sickle disease and sickle cell trait

This study determined and compared rates and mechanisms of lactate transport in red blood cells (RBCs) of persons with 1) sickle cell disease (HbSS), 2) sickle cell trait (HbAS), and 3) a control group (HbAA). Blood samples were drawn from 30 African-American volunteers (10 HbSS, 10 HbAS, 10 HbAA). Lactate influx into RBCs was measured by using [14C]lactate at six (2, 5, 10, 15, 25, and 40 mM) unlabeled lactate concentrations. The monocarboxylate transporter pathway was blocked by p-chloromercuriphenylsulfonic acid to determine its percent contribution to total lactate influx. Generally, total lactate influx into RBCs from the HbSS group was significantly greater than influx into RBCs from HbAS or HbAA, with no difference between HbAS and HbAA. Faster influx into HbSS RBCs was attributed to increased monocarboxylate transporter activity [increased apparent Vmax (V'max)]. V'max (4.7 +/- 0.6 micromol x ml(-1) x min(-1)) for HbSS RBCs was significantly greater than V'max of HbAS RBCs (2.9 +/- 1.5 micromol x ml(-1) x min(-1)) and HbAA RBCs (2.0 +/- 0.5 micromol x ml(-1) x min(-1)). Km (42.8 +/- 8 mM) for HbSS RBCs was significantly greater than Km (27 +/- 12 mM) for HbAA RBCs. We suspect that elevated erythropoietin levels in response to chronic anemia and/or pharmacological treatment (erythropoietin injections, hydroxyurea ingestion) is the underlying mechanism for increased lactate transport capacity in HbSS RBCs.

Does exercise-induced hypoxemia modify lactate influx into erythrocytes and hemorheological parameters in athletes?

This study investigated 1) red blood cells (RBC) rigidity and 2) lactate influxes into RBCs in endurance-trained athletes with and without exercise-induced hypoxemia (EIH). Nine EIH and six non-EIH subjects performed a submaximal steady-state exercise on a cyclo-ergometer at 60% of maximal aerobic power for 10 min, followed by 15 min at 85% of maximal aerobic power. At rest and at the end of exercise, arterialized blood was sampled for analysis of arterialized pressure in oxygen, and venous blood was drawn for analysis of plasma lactate concentrations and hemorheological parameters. Lactate influxes into RBCs were measured at three labeled [U-14C]lactate concentrations (1.6, 8.1, and 41 mM) on venous blood sampled at rest. The EIH subjects had higher maximal oxygen uptake than non-EIH ( P < 0.05). Total lactate influx was significantly higher in RBCs from EIH compared with non-EIH subjects at 8.1 mM (1,498.1 ± 87.8 vs. 1,035.9 ± 114.8 nmol·ml−1·min−1; P < 0.05) and 41 mM (2,562.0 ± 145.0 vs. 1,618.1 ± 149.4 nmol·ml−1·min−1; P < 0.01). Monocarboxylate transporter-1-mediated lactate influx was also higher in EIH at 8.1 mM ( P < 0.05) and 41 mM ( P < 0.01). The drop in arterial oxygen partial pressure was negatively correlated with total lactate influx measured at 8.1 mM ( r = −0.82, P < 0.05) and 41 mM ( r = −0.84, P < 0.05) in the two groups together. Plasma lactate concentrations and hemorheological data were similar in the two groups at rest and at the end of exercise. The results showed higher monocarboxylate transporter-1-mediated lactate influx in the EIH subjects and suggested that EIH could modify lactate influx into erythrocyte. However, higher lactate influx in EIH subjects was not accompanied by an increase in RBC rigidity.

Injections of recombinant human erythropoietin increases lactate influx into erythrocytes

Previous studies showed that erythropoietin not only increases erythrocyte production but is also essential in both the synthesis and the good functioning of several erythrocyte membrane proteins, including band 3. It is still unknown whether anion and/or H(+) fluxes are modified by erythropoietin. This study aimed to evaluate the effect of recombinant human erythropoietin (rHuEPO) injections on lactate transport into erythrocytes via band 3 and H(+)-monocarboxylate transporter MCT-1, two proteins involved in lactate exchange. Nine athletes received subcutaneous rHuEPO (50 U/kg body mass 3 times a week for 4 wk), and seven athletes received a saline solution (placebo group). All subjects were also supplemented with oral iron and vitamins B(9) and B(12). Lactate transport into erythrocytes was studied before and after the rHuEPO treatment at different lactate concentrations (1.6, 8.1, 41, and 81.1 mM). After treatment, MCT-1 lactate uptake was increased at 1.6, 41 (P < 0.01), and 81.1 mM lactate concentration (P < 0.001) although lactate uptake via band 3 and nonionic diffusion were unchanged. MCT-1 maximal velocity increased in the rHuEPO group (P < 0.05), reaching higher values than in the placebo group (P < 0.05) after treatment. Our results show that rHuEPO injections increased MCT-1 lactate influx at low and high lactate concentrations. The increase in MCT-1 maximal velocity suggests that rHuEPO may stimulate MCT-1 synthesis during erythrocyte formation in bone marrow.

The SLC16 gene family-from monocarboxylate transporters (MCTs) to aromatic amino acid transporters and beyond

The monocarboxylate cotransporter (MCT) family now comprises 14 members, of which only the first four (MCT1-MCT4) have been demonstrated experimentally to catalyse the proton-linked transport of metabolically important monocarboxylates such as lactate, pyruvate and ketone bodies. SLC16A10 (T-type amino-acid transporter-1, TAT1) is an aromatic amino acid transporter whilst the other members await characterization. MCTs have 12 transmembrane domains (TMDs) with intracellular N- and C-termini and a large intracellular loop between TMDs 6 and 7. MCT1 and MCT4 require a monotopic ancillary protein, CD147, for expression of functional protein at the plasma membrane. Lactic acid transport across the plasma membrane is fundamental for the metabolism of and pH regulation of all cells, removing lactic acid produced by glycolysis and allowing uptake by those cells utilizing it for gluconeogenesis (liver and kidney) or as a respiratory fuel (heart and red muscle). The properties of the different MCT isoforms and their tissue distribution and regulation reflect these roles.

The loop between helix 4 and helix 5 in the monocarboxylate transporter MCT1 is important for substrate selection and protein stability

Transport of lactate, pyruvate and the ketone bodies acetoacetate and beta-hydroxybutyrate, is mediated in most mammalian cells by members of the monocarboxylate transporter family (SLC16). A conserved signature sequence has been identified in this family, which is located in the loop between helix 4 and helix 5 and extends into helix 5. We have mutated residues in this signature sequence in the rat monocarboxylate transporter (MCT1) to elucidate the significance of this region for monocarboxylate transport. Mutation of R143 and G153 resulted in complete inactivation of the transporter. For the MCT1(G153V) mutant this was explained by a failure to reach the plasma membrane. The lack of transport activity of MCT1(R143Q) could be partially rescued by the conservative exchange R143H. The resulting mutant transporter displayed reduced stability, a decreased V (max) of lactate transport but not of acetate transport, and an increased stereoselectivity. Mutation of K137, K141 and K142 indicated that only K142 played a significant role in the transport mechanism. Mutation of K142 to glutamine resulted in an increase of the K (m) for lactate from 5 mM to 12 mM. In contrast with MCT1(R143H), MCT1(K142Q) was less stereoselective than the wild-type. A mechanism is proposed that includes all critical residues.

Effects of short-term training on plasma acid-base balance during incremental exercise in man

The present study examined the effect of short-term submaximal training on plasma acid-base balance during exercise. The influence of water and ion exchange between plasma, active muscles and erythrocytes in the response to training were also studied. The contributions of independent physicochemical variables (i.e. strong ion difference ([SID]), total concentration of weak acids ([Atot]) and PO2) to changes in arterial (a) and femoral venous (v) plasma [H+] were examined in six subjects (age 24+/-1.5 years; maximum oxygen consumption rate (VO2,max), 3.67+/-0.24 l min(-1)) during steady-state cycling for 15 min at each of 30, 65 and 75% of VO2,max before (pre) and after (post) training for 7 days on a cycle ergometer (2 h daily at 60 % VO2,max). The rise in [H+]a during exercise was attenuated post-training by 3 and 5 nequiv l(-1) (P<0.05) at 65 and 75% VO2,max, respectively, due first to less decrease in [SID]a, secondary to lower [Cl-]a and [Lac-]a; and second, to a reduction in [Atot]a, due to greater plasma volume and less plasma water flux (Jv) into leg muscle (P<0.05). The rise in [H+]v was also less in post-training by 4.5 and 6 nequiv l(-1) (P<0.05) at 65 and 75% VO2,max, respectively, and attributed solely to lower [Atot]v (P<0.05). Attenuation of exercise induced decreases in plasma [SID]a and [SID]v from rest to 75 % VO2,max was accompanied by reductions in erythrocyte Lac- and Cl- uptake (P<0.05), and smaller increases in erythrocyte K+ release (P<0.05). We conclude that the training-induced attenuation of the rise in plasma [H+]a and [H+]v during incremental exercise resulted from adaptive changes within muscles (less Lac- production and less water uptake) and erythrocytes (less uptake of Lac-, Cl- and K+), leading to greater [SID] and lower [Atot] in both arterial and femoral venous plasma.

Na+-dependent transport of pyruvate in erythrocytes of the Pacific hagfish (Eptatretus stouti)

We investigated the mechanisms of pyruvate transport in the erythrocytes of an ancient marine agnathan, the Pacific hagfish (Eptatretus stouti), and a sedentary euryhaline teleost, the starry flounder (Platichthys stellatus). Uptake of [14C]pyruvate (50 µM, 10°C) by flounder erythrocytes was slow (t1/2 (half-life) ~ 30 min), nonconcentrative, and mediated by the band 3 Cl-/HCO-3 exchanger in combination with a process similar to the H+/monocarboxylate symporter present in freshwater teleosts and mammalian erythrocytes. In contrast, pyruvate uptake by hagfish erythrocytes (50 µM, 10°C) was rapid (t1/2 ~ 1.5 min) and, in 10 min, reached an intracellular concentration more than 20-fold higher than that present in the extracellular medium. Pyruvate accounted for almost 90% of the accumulated intracellular radioactivity, the remaining label being incorporated into tricarboxylic acid cycle intermediates and glutamate. Influx of pyruvate was saturable (apparent Km = 12 mM) and inhibited by p-chloromercuriphenylsulphonate (PCMBS) (Ki = 71 µM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) (Ki = 0.49 mM). Transport was inhibited poorly by α-cyano-4-hydroxycinnamate (CIN) (Ki > 4 mM) and was not coupled to the movement of protons. Instead, the influx of pyruvate was Na+ dependent. A sigmoidal relationship between pyruvate transport and extracellular Na+ concentration was observed, suggesting a Na+:pyruvate coupling ratio greater than 1:1. In contrast with previously described Na+-dependent monocarboxylate transport activities in mammalian renal and intestinal epithelia, the hagfish erythrocyte system did not transport lactate.

Uptake of lactate by the liver: effect of red blood cell carriage

Multiple-indicator dilution experiments with labeled lactate were performed in the livers of anesthetized dogs. A mixture of (51)Cr-labeled erythrocytes, [(3)H]sucrose, and L-[1-(14)C]lactate or a mixture of (51)Cr-labeled erythrocytes, [(14)C]sucrose, and L-[2-(3)H]lactate was injected into the portal vein, and samples were obtained from the hepatic vein. Data were evaluated using a model comprising flow along sinusoids, exchange of lactate between plasma and erythrocytes and between plasma and hepatocytes, and, in the case of L-[1-(14)C]lactate, metabolism to H[(14)C]O(-)(3) within hepatocytes. The coefficient for lactate efflux from erythrocytes was 0.62 +/- 0.24 s(-1), and those for influx into and efflux from hepatocytes were 0.44 +/- 0.13 and 0.14 +/- 0.07 s(-1), respectively. The influx permeability-surface area product of the hepatocyte membrane for lactate (P(in)S, in ml x s(-1) x g(-1)) varied with total flow rate (F, in ml s(-1) x g(-1)) according to P(in)S = (3.1 +/- 0.5)F + (0.021 +/- 0.014). Lactate in plasma, erythrocytes, and hepatocytes was close to equilibrium, whereas lactate metabolism was rate limiting.

An endogenous monocarboxylate transport in Xenopus laevis oocytes

We investigated the existence of an endogenous system for lactate transport in Xenopus laevis oocytes. (36)Cl-uptake studies excluded the involvement of a DIDS-sensitive anion antiporter as a possible pathway for lactate movement. L-[(14)C]lactate uptake was unaffected by superimposed pH gradients, stimulated by the presence of Na(+) in the incubating solution, and severely reduced by the monocarboxylate transporter inhibitor p-chloromercuribenzenesulphonate (pCMBS). Transport exhibited a broad cation specificity and was cis inhibited by other monocarboxylates, mostly by pyruvate. These results suggest that lactate uptake is mediated mainly by a transporter and that the preferred anion is pyruvate. [(14)C]pyruvate uptake exhibited the same pattern of functional properties evidenced for L-lactate. Kinetic parameters were calculated for both monocarboxylates, and a higher affinity for pyruvate was revealed. Various inhibitors of monocarboxylate transporters reduced significantly pyruvate uptake. These studies demonstrate that Xenopus laevis oocytes possess a monocarboxylate transport system that shares some functional features with the members of the mammalian monocarboxylate cotransporters family, but, in the meanwhile, exhibits some particular properties, mainly concerning cation specificity.

Helix 8 and helix 10 are involved in substrate recognition in the rat monocarboxylate transporter MCT1

Transport of lactate, pyruvate, and the ketone bodies, acetoacetate and beta-hydroxybutyrate, is mediated in many mammalian cells by the monocarboxylate transporter MCT1. To be accepted as a substrate, a carboxyl group and an unpolar side chain are necessary. Site-directed mutagenesis of the rat MCT1 was used to identify residues which are involved in substrate recognition. Helices 8 and 10 but not helix 9 were found to contain critical residues for substrate recognition. Mutation of arginine 306 to threonine in helix 8 resulted in strongly reduced transport activity. Concomitantly, saturable transport was transformed into a nonsaturable dependence of transport activity on lactate concentration, suggesting that binding of the substrate was strongly impaired. Furthermore, proton translocation in the mutant was partially uncoupled from monocarboxylate transport. Mutation of phenylalanine 360 to cysteine in helix 10 resulted in an altered substrate side chain recognition. In contrast to the wild-type transporter, monocarboxylates with more bulky and polar side chains were recognized by the mutated MCT1. Mutation of selected residues in helix 9 and helix 11 (C336A, H337Q, and E391Q) did not cause alterations of the transport properties of MCT1. It is suggested that substrate binding occurs in the carboxy-terminal half of MCT1 and that helices 8 and 10 are involved in the recognition of different parts of the substrate.

Characterization of the high-affinity monocarboxylate transporter MCT2 in Xenopus laevis oocytes

Observations on lactate transport in brain cells and cardiac myocytes indicate the presence of a high-affinity monocarboxylate transporter. The rat monocarboxylate transporter isoform MCT2 was analysed by expression in Xenopus laevis oocytes and the results were compared with the known characteristics of lactate transport in heart and brain. Monocarboxylate transport via MCT2 was driven by the H(+) gradient over the plasma membrane. Uptake of lactate strongly increased with decreasing pH, showing half-maximal stimulation at pH 7.2. A wide variety of monocarboxylates and ketone bodies, including lactate, pyruvate, beta-hydroxybutyrate, acetoacetate, 2-oxoisovalerate and 2-oxoisohexanoate, were substrates of MCT2. All substrates had a high affinity for MCT2. For lactate a K(m) value of 0.74+/-0.07 mM was determined at pH 7.0. For the other substrates, K(i) values between 100 microM and 1 mM were measured for inhibition of lactate transport, which is about one-tenth of the corresponding values for the ubiquitously expressed monocarboxylate transporter isoform MCT1. Monocarboxylate transport via MCT2 could be inhibited by alpha-cyano-4-hydroxycinnamate, anion-channel inhibitors and flavonoids. It is suggested that cells which express MCT2 preferentially use lactate and ketone bodies as energy sources.

Human monocarboxylate transporter 2 (MCT2) is a high affinity pyruvate transporter

The transport of pyruvate and lactate across cellular membranes is an essential process in mammalian cells and is mediated by the H+/monocarboxylate transporters (MCTs). We have molecularly cloned and characterized a novel human monocarboxylate transporter, MCT2. The cDNA is 1,907 base pairs long and encodes a polypeptide of 478 amino acids with 12 predicted transmembrane domains. Human MCT2 is the product of a single gene that mapped to chromosome 12q13 by fluorescence in situ hybridization. The kinetic properties of human MCT2 fulfill the criteria to establish it as a H+/monocarboxylate transporter; however, the unique biochemical feature of human MCT2 is its high affinity for the transport of pyruvate (apparent Km of 25 microM), implying that it is a primary pyruvate transporter in man. Comparison of human MCT1 and MCT2 with regard to tissue distribution and RNA transcript variants disclosed substantial differences. Human MCT2 mRNA expression was restricted in normal human tissues but widely expressed in cancer cell lines, suggesting that MCT2 may be pre-translationally regulated in neoplasia. We found co-expression of human MCT1 and MCT2 at the mRNA level in human cancer cell lines, including the hematopoietic lineages HL60, K562, MOLT-4, and Burkitt's lymphoma Raji, and solid tumor cells such as SW480, A549, and G361. These findings suggest that the two monocarboxylate transporters, MCT1 and MCT2, have distinct biological roles.

Lactate distribution in the blood during steady-state exercise

PURPOSE

The purpose of this investigation was to examine the plasma to red blood cell (RBC) lactate concentration ([La]) gradient and RBC:plasma [La] ratio during 30 min of steady-state cycle ergometer exercise at work rates below lactate threshold (<LT = approximately 40% of peak cycle ergometer O2 uptake ¿VO2peak¿) and above LT (approximately 70% of VO2peak).

METHODS

Eight subjects (cycling VO2peak = 41.6+/-1.6 mL x kg(-1) x min(-1); LT = 57.9 + 1.2% VO2peak) performed 30 min of cycle exercise at intensities < and > LT. Blood samples were taken from a heated forearm vein, immediately cooled to 4 degrees C in a dry-ice ethanol slurry, and centrifuged at 4 degrees C to separate plasma and RBCs.

RESULTS

During >LT, plasma [La] rose to 8.8+/-1.1 mM after 10 min and remained above 6 mM. RBC [La] (4.9+/-0.7 mM) was significantly lower than plasma [La] at 10 min and remained lower throughout exercise. As a result, there was a sizable [La] gradient (approximately 3.5 mM) from plasma to RBC during most of >LT. In <LT, plasma [La] increased only slightly from rest (1.6+/-0.2 mM) after 6 min (2.4+/-0.3 mM) and then declined to approximately 2 mM for the remainder of the trial. The plasma to RBC [La] gradient averaged approximately 0.8 mM throughout <LT. Despite drastically different plasma to RBC [La] gradients in <LT and >LT, the ratio of RBC [La]:plasma [La] was the same for both (0.58+/-0.02) and not significantly different from rest.

CONCLUSIONS

These results refuted our hypothesis that the RBC:plasma [La] ratio would decrease at the onset of >LT exercise because of muscle lactate release exceeding the ability of RBCs to take up the lactate. Instead, there appears to be an equilibrium between plasma [La] and RBC [La] in arterialized venous blood from a resting muscle group as evidenced by the constant RBC [La]:plasma [La] ratio.

Characterization of the monocarboxylate transporter 1 expressed in Xenopus laevis oocytes by changes in cytosolic pH

Several laboratories have investigated monocarboxylate transport in a variety of cell types. The characterization of the cloned transporter isoforms in a suitable expression system is nevertheless still lacking. H+/monocarboxylate co-transport was therefore investigated in monocarboxylate transporter 1 (MCT1)-expressing Xenopus laevis oocytes by using pH-sensitive microelectrodes and [14C]lactate. Superfusion with lactate resulted in intracellular acidification of MCT1-expressing oocytes, but not in non-injected control oocytes. The basic kinetic properties of lactate transport in MCT1-expressing oocytes were determined by analysing the rates of intracellular pH changes under different conditions. The results were in agreement with the known properties of the transporter, with respect to both the dependence on the lactate concentration and the external pH value. Besides lactate, MCT1 mediated the reversible transport of a wide variety of monocarboxylic acids including pyruvate, D,L-3-hydroxybutyrate, acetoacetate, alpha-oxoisohexanoate and alpha-oxoisovalerate, but not of dicarboxylic and tricarboxylic acids. The inhibitor alpha-cyano-4-hydroxycinnamate bound strongly to the transporter without being translocated, but could be displaced by the addition of lactate. In addition to changes in the intracellular pH, lactate transport also induced deviations from the resting membrane potential.

Lactate influx into red blood cells from trained and untrained human subjects

PURPOSE

The purpose of this study was to compare the fractional contributions of the three pathways of lactate transport (band 3 system, nonionic diffusion, and monocarboxylate pathway) into red blood cells (RBC) from trained and untrained humans.

METHODS

Blood samples were obtained from 19 male subjects: 5 untrained, 5 aerobically-trained, 5 competitive collegiate cross-country runners, and 4 competitive collegiate sprinters. The influx of lactate into the RBC was measured by a radioactive tracer technique using [14C]lactate. Discrimination of each pathway of lactate transport was achieved by using PCMBS (1 mM) to block the monocarboxylate pathway and DIDS (0.2 mM) to block the band 3 system. Nonionic diffusion was calculated as the difference between total lactate influx and the sum of band 3 and monocarboxylate lactate influx.

RESULTS

Total lactate influx into the RBC from the more aerobic individuals (trained subjects and cross-country runners) was significantly faster at 1.6 mM lactate concentration ([La]) as compared with the influx into RBC of the untrained subjects. Total influx of lactate was significantly higher (P < 0.05) in the RBC from the sprinters as compared with that in the RBC from the untrained subjects at 41 mM [La]. There were no significant differences among the four groups with regard to the total influx of lactate at 4.1, 8.1, and 20 mM [La]. In general, the percentage of total lactate influx accounted for by each of the three parallel pathways at 1.6, 8.1, and 41.0 mM [La] was not different among the four groups of subjects.

CONCLUSIONS

Overall, the groups were more similar than different with regard to RBC lactate influx.

Comparison of lactate transport in astroglial cells and monocarboxylate transporter 1 (MCT 1) expressing Xenopus laevis oocytes. Expression of two different monocarboxylate transporters in astroglial cells and neurons

The transport of lactate is an essential part of the concept of metabolic coupling between neurons and glia. Lactate transport in primary cultures of astroglial cells was shown to be mediated by a single saturable transport system with a Km value for lactate of 7.7 mM and a Vmax value of 250 nmol/(min x mg of protein). Transport was inhibited by a variety of monocarboxylates and by compounds known to inhibit monocarboxylate transport in other cell types, such as alpha-cyano-4-hydroxycinnamate and p-chloromercurbenzenesulfonate. Using reverse transcriptase-polymerase chain reaction and Northern blotting, the presence of mRNA coding for the monocarboxylate transporter 1 (MCT1) was demonstrated in primary cultures of astroglial cells. In contrast, neuron-rich primary cultures were found to contain the mRNA coding for the monocarboxylate transporter 2 (MCT2). MCT1 was cloned and expressed in Xenopus laevis oocytes. Comparison of lactate transport in MCT1 expressing oocytes with lactate transport in glial cells revealed that MCT1 can account for all characteristics of lactate transport in glial cells. These data provide further molecular support for the existence of a lactate shuttle between astrocytes and neurons.

Anion exchanger AE1 as a candidate pathway for taurine transport in rat erythrocytes

Taurine has been shown to act as an osmolyte during the regulatory volume decrease process in a variety of cell types. The nature of the taurine efflux carrier is thought to consist of a diffusional pathway with pharmacological properties similar to a chloride channel or through an anion exchanger. We propose that taurine is a substrate of the anion exchanger AE1, also called band 3. Experiments were performed in rat-erythrocytes, which express large amounts of band 3. Taurine uptake and efflux transport experiments were determined in the presence of inhibitors of anion carriers and chloride channels. Both taurine uptake and efflux were inhibited by band 3 inhibitors 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS), niflumic acid, or furosemide. Moreover, DIDS competes with taurine at a common binding site in the uptake process. Specific inhibitors of the electroneutral cotransport as well as inhibitors of the chloride channels were ineffective in blocking taurine transport. Thus we suggest that band 3 may be the protein responsible for taurine transport in rat erythrocytes.

Lactate distribution in the blood during progressive exercise

The purpose of this study was to examine the effect of increment durations of 1-min and 4-min during progressive incremental exercise tests on: 1) the distribution of lactate between plasma and red blood cells (RBCs), and 2) lactate threshold (LT) detection via three conventional methods using whole blood lactate concentration ([La]) or plasma [La]. Eight males (age, 22.5 +/- 0.6 yr: height, 170.6 +/- 2.3 cm, weight, 76.0 +/- 3.1 kg, and VO2peak, 42.8 +/- 2.0 mL.kg-1.min-1) performed two progressive load tests to volitional fatigue on a cycle ergometer. Work rate was increased 30 W at 1-min or 4-min intervals. All data were normalized to individual LT work rates. For both protocols, whole blood [La], plasma [La], RBC [La], and [La] gradient increased significantly (P < 0.05) after exercise intensity exceeded LT. However, the RBC:plasma [La] ratio remained at the resting value throughout the progressive exercise tests. The increase in [La] gradient after LT, with no change in the RBC:plasma lactate ratio, suggests that given an incremental work rate increase of 30 W, 1 min is adequate for equilibration of lactate between the plasma and RBCs. Also, under the conditions of this investigation, neither blood fraction analyzed nor exercise protocol had any effect on estimations of LT (in terms of VO2) by the Visual and Log-Log methods. However, LT determined by a fixed [La] of 2 mM may underestimate LT when plasma samples are used.

Inhibition of lactate export by quercetin acidifies rat glial cells in vitro

The relationship between glial lactate release and glial intracellular pH (pHi) regulation is studied using C6 glioma cells and rat astrocytes in vitro, and the lactate transport inhibitors quercetin and alpha-cyano-4-hydroxycinnamate (CHC). pHi is measured using 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). The results show that lactate release is mediated partly by a specific lactate transport system inhibitable by quercetin (50 microM), but not by CHC (5 mM). Inhibition by quercetin results in a significant 3-4-fold increase of intracellular lactate and a decrease of intracellular pH to 6.9. A participation of quercetin-inhibitable lactate transport in glial pHi-regulation is suggested by the observation that pHi-homeostasis after acidification by diffusion of undissociated lactic acid into the cell is inhibited by quercetin. The existence of a system controlling lactate release in glial cells may also reflect a function of astrocytes to supply neurons with lactate.

Studies of the membrane topology of the rat erythrocyte H+/lactate cotransporter (MCT1)

1. Hydrophobicity analysis of the monocarboxylate/proton cotransporter MCT1 (lactate transporter) suggests a structure with 12 transmembrane (TM) segments, presumed to be alpha-helical. 2. A series of anti-peptide antibodies have been raised against regions of the MCT1 sequence, which each recognize a polypeptide of approx. 40 kDa in rat erythrocytes. The topology of rat MCT1 was investigated by studying the immunoreactive fragments derived from proteolytic digestion of the protein in intact rat erythrocytes and leaky membranes. 3. Reactivity with an anti-(C-terminus) antibody was prevented on treatment of leaky membranes, but not intact cells, with carboxypeptidase Y, indicating that the C-terminus of the protein is cytoplasmically disposed. 4. Treatment of intact cells in saline buffer with trypsin, chymotrypsin, bromelain and protease K (up to 1 mg/ml) resulted in no degradation of MCT1, indicating the absence of any large exposed extracellular loop. In a buffer of low ionic strength (containing sucrose), cleavage was observed with bromelain at an extracellular site, probably TM9/10.5. Treatment of leaky membranes with low (less than 100 micrograms/ml) concentrations of several proteases resulted in fragmentation of MCT1, reflecting cleavage at the cytoplasmic face of the membrane. These treatments generated N-terminal fragments of apparent molecular mass approx. 17-19 kDa that were resistant to further degradation. The epitopes for the TM6/7 and C-terminal antibodies were either lost from the membrane or destroyed under most of these conditions, indicating that these regions of the protein are located in the cytoplasm. 6. More detailed structural prediction analysis of MCT-related sequences was made assuming the constraints placed upon the possible arrangements by the experimental data outlined above. This analysis provided additional strong evidence for the 12-TM-segment model, with cytoplasmic N- and C-terminal ends and a large internal loop between TM6 and TM7. The predicted helices were assigned moments of hydrophobicity and residue substitution; for a number of TM segments this permitted the prediction of the sides of the helix that faced membrane lipid and the interior of the protein.

Effect of 2-chloropropionate on initial lactate uptake by rat skeletal muscle sarcolemmal vesicles

2-Chloropropionate (2-CP) is a halogenated monocarboxylic acid generally used to decrease blood lactate concentration in various metabolic states. To investigate whether it has an inhibitory effect on sarcolemmal lactate transport, we compared the initial rate of lactate transport in sarcolemmal membrane vesicles purified from 20 male Wistar rats with and without 2-CP. Transport by these vesicles was measured as uptake of L-(+)-[U-14C]lactate under pH gradient-stimulated cis inhibition. The time courses of 1 mM L-(+)-lactate uptake into vesicles both with and without 10 mM 2-CP (L- or D-) displayed saturation kinetics. Lactate uptake values were lower with 10 mM L-2-CP and 10 mM D-2-CP in comparison to the control values. Both 10 mM L-2-CP and 10 mM D-2-CP significantly inhibited 1 mM L-(+)-lactate uptake (55.8 +/- 9.1 and 53.5 +/- 12.1%, respectively; P < 0.001), whereas a smaller inhibition was observed with a higher lactate concentration of 50 mM (40.2 +/- 11.2 and 38.7 +/- 12.4%; P < 0.001 and P < 0.05, respectively). However, a higher D-2-CP concentration (50 mM) increased the inhibition of pH-stimulated 1 mM L-(+)-lactate uptake (77.0 +/- 9.4%; P < 0.001). D-2-CP had a trans-stimulation effect on the initial rate of lactate efflux of 1 mM L-(+)-lactate compared with baseline efflux (9.5 +/- 0.8 vs. 5.1 +/- 0.4 nmol.min-1.mg protein-1; P < 0.05). 2-CP significantly inhibited the initial rate of lactate uptake in skeletal muscle sarcolemmal membrane vesicles. This result suggests that 2-CP is a nonstereoselective substrate of the lactate muscle carrier that impairs lactate transport.

Symmetry and pH dependency of the lactate/proton carrier in skeletal muscle studied with rat sarcolemmal giant vesicles

Muscle lactate transport was studied in rat sarcolemmal giant vesicles, in which the membrane orientation was similar to that of intact cells. The lactate concentration dependency was studied using influx experiments under equilibrium exchange. zero-trans. and infinite-cis conditions. At constant external and internal pH the carrier behaved symmetrically with regard to lactate transport from the two sides of the membrane. Lactate at the trans side stimulated the lactate tracer flux (trans-acceleration). Both in zero-trans influx and efflux experiments the lactate flux was stimulated in a symmetric way by H+ at the cis side, consistent with protons being the substrate for the carrier. In contrast, the lactate flux was symmetrically inhibited by H+ at the trans side (trans-inhibition), the underlying mechanism is likely that the protonated carrier with no lactate bound is unable to reorient in the membrane. These findings have implications both for the kinetic model and for understanding the physiological function of the lactate/proton transporter.

Permeability characteristics of erythrocyte membrane to okadaic acid and calyculin A

The rates of transport of the protein phosphatase inhibitors okadaic acid and calyculin A through rabbit erythrocyte membranes have been estimated by measuring protein phosphatase type 2A (PP2A) activity in lysates. High concentrations of okadaic acid (100 nM) cause rapid (t 1/2 approximately 10 min) inhibition of PP2A. However, the t 1/2 for okadaic acid influx is much longer because the concentration is much higher than the concentration inhibiting 50% of the maximal response (IC50). The estimated t 1/2 is over 1 h at 37 degrees C and over 4 h at 25 degrees C. The effect of low extracellular pH indicates that the undissociated acid is the permeant species. It takes hours to reverse the effect of okadaic acid, because the efflux must proceed through several half times before the concentration is below the IC50 for PP2A. The permeation of calyculin A in contrast to okadaic acid is too fast to measure at 25 degrees C. Our results indicate that okadaic acid entry into erythrocytes is slower than is generally believed; it is crucial to consider concentration, temperature, pH, and time of exposure to okadaic acid to interpret the effects of this agent on intact cells.

The kinetics, substrate, and inhibitor specificity of the monocarboxylate (lactate) transporter of rat liver cells determined using the fluorescent intracellular pH indicator, 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein

The kinetics of transport of L-lactate, pyruvate, ketone bodies, and other monocarboxylates into isolated hepatocytes from starved rats were measured at 25 degrees C using the intracellular pH-sensitive dye, 2',7'-bis(carboxyethyl)- 5(6)-carboxyfluorescein, to detect the associated proton influx. Transport kinetics were similar, but not identical, to those determined using the same technique for the monocarboxylate transporter (MCT) of Ehrlich Lettré tumor cells (MCT1) (Carpenter, L., and Halestrap, A. P. (1994) Biochem. J. 304, 751-760). Km values for L-lactate (4.7 mM), D-lactate (27 mM), D,L-2-hydroxybutyrate (3.3 mM), L-3-hydroxybutyrate (12.7 mM), and acetoacetate (6.1 mM) were very similar in both cell types, whereas in hepatocytes the Km values were higher than MCT1 for pyruvate (1.3 mM, cf. 0.72 mM), D-3-hydroxybutyrate (24.7 mM, cf. 10.1 mM), D-2-chloropropionate (1.3 mM, cf. 0.8 mM), 4-hydroxybutyrate (18.1 mM, cf. 7.7 mM), and acetate (5.4 mM, cf. 3.7 mM). In contrast, the hepatocyte carrier had lower Km values than MCT1 for glycolate, chloroacetate, dichloroacetate, and 2-hydroxy-2-methylpropionate. Differences in stereoselectivity were also detected; both carriers showed a lower Km for L-lactate than D-lactate, while hepatocyte MCT exhibited a lower Km for D- than L-2-chloropropionate and for L- than D-3-hydroxybutyrate; this is not the case for MCT1. A range of inhibitors of MCT1, including alpha-cyanocinnamate derivatives, phloretin, and niflumic acid, inhibited hepatocyte MCT with K0.5 values significantly higher than for tumor cell MCT1, while stilbene disulfonate derivatives and p-chloromercuribenzene sulfonate had similar K0.5 values in both cell types. The branched chain ketoacids alpha-ketoisocaproate and alpha-ketoisovalerate were also potent inhibitors of hepatocyte MCT with K0.5 values of 270 and 340 microM, respectively. The activation energy of L-lactate transport into hepatocytes was 58 kJ mol-1, and measured rates of transport at 37 degrees C were considerably greater than those required for maximal rates of gluconeogenesis. The properties of the hepatocyte monocarboxylate transporter are consistent with the presence of a distinct isoform of MCT in liver cells as suggested by the cloning and sequencing of MCT2 from hamster liver (Garcia, C. K., Brown, M. S., Pathak, R. K., and Goldstein, J. L. (1995) J. Biol. Chem. 270, 1843-1849).

Characterization of oxalate transport by the human erythrocyte band 3 protein

This paper describes characteristics of the transport of oxalate across the human erythrocyte membrane. Treatment of cells with low concentrations of H2DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) inhibits Cl(-)-Cl- and oxalate-oxalate exchange to the same extent, suggesting that band 3 is the major transport pathway for oxalate. The kinetics of oxalate and Cl- self-exchange fluxes indicate that the two ions compete for a common transport site; the apparent Cl- affinity is two to three times higher than that of oxalate. The net exchange of oxalate for Cl-, in either direction, is accompanied by a flux of H+ with oxalate, as is also true of net Cl(-)-SO4(2-) exchange. The transport of oxalate, however, is much faster than that of SO4(2-) or other divalent anions. Oxalate influx into Cl(-)-containing cells has an extracellular pH optimum of approximately 5.5 at 0 degrees C. At extracellular pH below 5.5 (neutral intracellular pH), net Cl(-)-oxalate exchange is nearly as fast as Cl(-)-Cl- exchange. The rapid Cl(-)-oxalate exchange at acid extracellular pH is not likely to be a consequence of Cl- exchange for monovalent oxalate (HOOC-COO-; pKa = 4.2) because monocarboxylates of similar structure exchange for Cl- much more slowly than does oxalate. The activation energy of Cl(-)-oxalate exchange is about 35 kCal/mol at temperatures between 0 and 15 degrees C; the rapid oxalate influx is therefore not a consequence of a low activation energy. The protein phosphatase inhibitor okadaic acid has no detectable effect on oxalate self-exchange, in contrast to a recent finding in another laboratory (Baggio, B., L. Bordin, G. Clari, G. Gambaro, and V. Moret. 1993. Biochim. Biophys. Acta. 1148:157-160.); our data provide no evidence for physiological regulation of anion exchange in red cells.

Discrimination of transport systems for methylmercury uptake in rat erythrocytes using methylmercury-mercaptalbumin by inhibitors and other factors

This is a continuation of studying the transport system for the uptake of methylmercury (MeHg). The aim of the current study was to study transport systems in rat erythrocyte for the uptake of MeHg while using MeHg-mercaptalbumin (MeHgMASH) complex. The uptake of methylmercury was studied in isolated erythrocytes from rats at 5 degrees C. Different reagents were used to study different transport systems in rat erythrocytes: adenosine 5'-triphosphate (ATP), ouabain and sodium fluoride for the active transport systems; probenecid for the organic anion transport system; 4',4-diisothiocyano-2',2-stilbenedisulphonic acid (DIDS), maleimide and N-ethylmaleimide for Cl- transport system; verapamil for Ca2+ ion transport system; colchicine and vinblastine for the microtubule system; verapamil for Ca2+ ion transport system; colchicine and vinblastine for the microtubule system; valinomycin for the effect of membrane potential; hexanol for the protein-mediated transport system and nonelectrolyte diffusion. The results showed that the uptake of MeHg might be involved in several transport systems: the active transport systems, an organic anion transport system, Cl- ion transport system, and Ca2+ ion transport system. The transport systems were slightly sensitive to the membrane potential. These transport systems seem to share similarities with the transport systems for the uptake of MeHg when using MeHg-cysteine and MeHg-glutathione complexes.

Screening of potential transport systems for methyl mercury uptake in rat erythrocytes at 5 degrees by use of inhibitors and substrates

The current study was designed to screen the potential transport systems for methyl mercury (MeHg) uptake by isolated erythrocytes from rats at 5 degrees. Several inhibitors and substrates were used to test which potential transport system might be involved in MeHg uptake. Probenecid was used to test the organic anion transport system, valinomycin was used to test the effect of the membrane potential, D-glucose and cytochalasin B were used to test the facilitated diffusive D-glucose transport system and colchicine and vinblastine were used to test the microtubule system. The effects of Ca++, Mg++ and Na+ on MeHg uptake have been examined. Ouabain, ATP and glucose were used to test the active transport system, cysteine for the cysteine-facilitated transport system, glycine for system Gly, DL-methionine for system L, and MeHgCl and 4',4-diisothiocyano-2',2-stilbenedisulfonic acid (DIDS) for the Cl- ion transport system. The results showed that MeHg uptake might be involved in the following transport systems at 5 degrees: 1) organic anion transport system; 2) facilitated diffusive D-glucose transport system; 3) cysteine-facilitated transport system; 4) Cl- ion transport system. Moreover, the transport systems for MeHg uptake were sensitive to the membrane potential. Although the mechanisms of interaction of transport systems have not been fully clarified, evidence has been presented which support the existence of several simultaneous transport systems for MeHg uptake.

Physiologically based pharmacokinetics of methoxyacetic acid: dose-effect considerations in C57BL/6 mice

Methoxyacetic acid (MAA), a weak acid with a pKa of 3.57, was used to test the broad hypothesis that distribution of weak acids in maternal and fetal tissues is determined principally by the pKa of the acid and the pH values of tissue and fluid compartments and to examine tissue dose-teratogenesis relationships, as well as administered dose-teratogenesis relationships. Five related experimental studies were conducted in pregnant C57BL/6CrIBR mice: a conventional dose-response study of developmental toxicity and transplacental pharmacokinetics in mice, a second dose-response study in which reproductive outcomes in litters from individual dams were related to individual pharmacokinetic behavior, a protein-binding experiment, an embryo tissue localization study, and determination of pH in maternal and embryonic compartments after exposure to MAA. MAA was administered intraperitoneally at 9:00 a.m. on day 10 of gestation, at doses ranging from 88 to 164 mg/kg. Localization within the forelimb bud of the embryo, an MAA target site, was determined by computerized image analysis of the distribution of radiolabeled MAA. The kinetic predictions of a physiologically based model incorporating tissue pH values and MAA pKa agreed well with observed concentrations at the lowest dose. However, at intermediate and higher doses, concentrations in both maternal and embryonic tissues were consistently underestimated. MAA was bound neither to maternal plasma proteins nor to embryonic proteins. Intermediate and higher doses of MAA caused dose-dependent transient depressions in tissue pH, but these were not of sufficient duration to bring predicted tissue concentrations into congruence with the concentrations observed. Distribution of MAA within the forelimb bud was broadly consistent with the pH hypothesis, but MAA concentration was not increased in the distal postaxial sector that is the site of the precursor cells of the missing digits. Internal exposure to MAA, defined as the area under the maternal plasma or embryo concentration curve (AUC), was not proportional to administered dose, but AUC-response relationships generated by the group and individual dose-response studies were comparable. While AUC may be a useful measure of effective MAA dose, it cannot be accurately predicted at teratogenic doses of this agent by the model as it is presently structured.

Characterization of the enhanced transport of L- and D-lactate into human red blood cells infected with Plasmodium falciparum suggests the presence of a novel saturable lactate proton cotransporter

Human erythrocytes parasitized with the malarial protozoan Plasmodium falciparum showed rates of L-lactate, D-lactate, and pyruvate uptake many fold greater than control cells. Thus it was necessary to work at 0 degrees C to resolve true initial rates of transport. Studies on the dependence of the rate of transport on substrate concentration implied the presence in parasitized cells of both a saturable mechanism blocked by alpha-cyano-4-hydroxycinnamate (CHC) and a nonsaturable mechanism insensitive to CHC. The former was dominant at physiological substrate concentrations with Km values for pyruvate and D-lactate of 2.3 and 5.2 mM, respectively, with no stereoselectivity for L- over D-lactate. CHC was significantly less effective as an inhibitor of lactate transport in parasitized erythrocytes than in uninfected cells, whereas p-chloromercuribenzenesulfonate, a potent inhibitor in control cells, gave little or no inhibition of lactate transport into parasitized erythrocytes. Inhibition of transport into infected cells was also observed with phloretin, furosemide, niflumic acid, stilbenedisulfonate derivatives, and 5-nitro-2-(3-phenylpropylamino)benzoic acid at concentrations similar to those that inhibit the lactate carrier of control erythrocytes. These compounds were more effective inhibitors of the rapid transport of chloride into infected cells than of lactate transport, whereas CHC was more effective against lactate transport. This implies that different pathways are involved in the parasite-induced transport pathways for lactate and chloride. The transport of L-lactate into infected erythrocytes was also inhibited by D-lactate, pyruvate, 2-oxobutyrate, and 2-hydroxybutyrate. The intracellular accumulation of L-lactate at equilibrium was dependent on the transmembrane pH gradient, suggesting a protogenic transport mechanism. Our data are consistent with lactate and pyruvate having direct access to the malarial parasite, perhaps via the proposed parasitophorous duct or some close contact between the host cell and parasite plasma membranes, with transport across the latter by both a proton-linked carrier (CHC-sensitive, saturable, and the major route) and free diffusion of the undissociated acid (CHC-insensitive, unsaturable, and a minor route).

L(+)-lactate binding to a protein in rat skeletal muscle plasma membranes

Biochemical studies were conducted to determine the location of a putative lactate transport protein in rat skeletal muscle plasma membranes (PM). PM (50-100 micrograms protein) were incubated with [U-14C] L(+)-lactate, in the presence or absence of unlabeled monocarboxylates or potential inhibitors, after which proteins were separated by SDS-PAGE. Gel slices (2 mm) were cut and analyzed for 14C. [U-14C] L(+)-lactate was bound to plasma membranes in the 30 to 40 kDa molecular mass range. Binding of [U-14C] L(+)-lactate was inhibited by N-ethylmaleimide, unlabeled L-lactate and pyruvate, and in a dose dependent manner by alpha-cyano-4-hydroxycinnamate (r = 0.995), but not by cytochalasin-B. The inhibition of [U-14C] L(+)-lactate binding was similar to the inhibition of lactate transport. Therefore the transport of L(+)-lactate across skeletal muscle plasma membranes involves a polypeptide of 30 to 40 kDa.

The kinetics, substrate and inhibitor specificity of the lactate transporter of Ehrlich-Lettre tumour cells studied with the intracellular pH indicator BCECF

1. Suspensions of cultured Ehrlich-Lettre tumour cells were loaded with the pH-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and changes in intracellular pH upon addition of L-lactate and other monocarboxylates were continuously monitored by fluorimetry using dual-wavelength excitation (450/500 nm) and single-wavelength emission (> 520 nm). 2. The rapid fluorescence changes were analysed by first-order regression analysis, and with suitable calibration procedures this enabled calculation of initial rates of proton uptake associated with monocarboxylate transport. 3. The stoichiometry was shown to be one proton per lactate molecule transported. 4. The kinetics of carrier-mediated transport of a wide range of monocarboxylates were determined at 25 degrees C. The Km values for L-lactate, pyruvate and D-lactate were found to be 4.54, 0.72 and 27.5 mM respectively, similar to values found previously for rat erythrocytes. This similarity was shared with a wide range of variously substituted C2, C3 and C4 monocarboxylates, all of which were transported with similar Vmax. No stereoselectivity was found in the Km values for D- and L-2-chloropropionate (0.75 mM) or D- and L-3-hydroxybutyrate (11 mM), but in the latter case the Vmax. of the D-isomer was twice that of the L-isomer. 5. The temperature-dependence of L-lactate transport demonstrated a transition point, with activation energies of 60 and 109 kJ.mol-1 above and below 19 degrees C respectively The Km for L-lactate below the transition temperature was about half that above it. 6. Inhibition of lactate transport into tumour cells by a wide range of compounds known to inhibit the erythrocyte monocarboxylate carrier was analysed. Patterns of inhibition were similar to those seen in the erythrocyte, but the Ki values were 2-4-fold higher in the tumour cells. 7. It is concluded that tumour cells contain an isoform of the monocarboxylate carrier with functional properties almost identical with that found in erythrocytes. This is probably identical with MCT1, which was recently cloned and sequenced from Chinese Hamster Ovary cells [Kim Garcia, Goldstein, Pathak, Anderson and Brown (1994) Cell 76, 865-873].

N-terminal protein sequence analysis of the rabbit erythrocyte lactate transporter suggests identity with the cloned monocarboxylate transport protein MCT1

An improved purification for the rabbit erythrocyte lactate transporter, using aminoethyl-Sepharose chromatography, is described. The process of purification of the 40-50 kDa transporter, labelled with 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS), was followed by Western blotting with anti-DIDS antibodies [Poole, R. C. and Halestrap, A. P. (1992) Biochem. J. 283, 855-862]. Fractions highly-enriched in transporter were further purified by SDS/PAGE and the 40-50 kDa DIDS-labelled polypeptide was subjected to N-terminal protein sequencing. This analysis identified the first 16 amino acids of the protein. With the exception of one conservative substitution, this protein sequence is identical to the N-terminal protein sequence predicted from a cDNA isolated from Chinese hamster ovary cells that encode a monocarboxylate transporter, MCT1 [Kim Garcia, C., Goldstein, J. L., Pathak, R. K., Anderson, R. G. W. and Brown, M. S. (1994) Cell 76, 865-873]. This observation, along with similarities in functional properties, leads us to conclude that lactate transport in rabbit erythrocytes is mediated by the MCT1 monocarboxylate transporter isoform.

Propionate-initiated changes in intracellular pH in rabbit colonocytes

BACKGROUND/AIMS

Because regulation of intracellular pH (pHi) is critical to basic cell functions, most cells have evolved mechanisms to closely regulate intracellular acid-base balance. Short-chain fatty acids (SCFAs), the predominant luminal anion in the colon, acidify the cell interior in several cell systems, but their effect on their "natural target," the colonocytes, has not been examined thoroughly.

METHODS

We monitored the pHi response to a model SCFA, propionate, in isolated cells and epithelial sheets from rabbit proximal colon loaded with the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and -6)carboxyfluorescein.

RESULTS

SCFAs induced a characteristic pHi response curve in colonocytes: an immediate acidification and a recovery phase returning to baseline in 100-200 seconds. Acidification was altered by increasing concentrations of SCFAs, by increasing SCFA chain length, extracellular osmolarity, and intracellular pH, and finally, Na+ removal. The recovery phase was slowed by amiloride and 4-alpha-OH cinnamate, an inhibitor of proton-monocarboxylate cotransport.

CONCLUSIONS

Physiological concentrations of SCFAs have profound effects on intracellular pH. Simple diffusion of the SCFA may not explain the complexities of propionate-induced protonated acidification; the pH recovery phase may involve multiple processes including Na(+)-H+ exchange and H(+)-SCFA cotransport. Luminal constituents such as SCFAs may have significant effects on the intracellular pH and function of colonocytes.

Lactate transport in mammalian ventricle. General properties and relation to K+ fluxes

Net cellular L-lactate efflux associated with accelerated anaerobic glycolysis has been implicated as a potential cause of the marked cellular K+ loss contributing to lethal cardiac arrhythmias in ischemic heart and to impaired function of fatigued skeletal muscle. To examine the mechanisms of transsarcolemmal L-lactate movement in the heart, isolated guinea pig ventricular myocytes were loaded with the fluorescent H+ or K+ indicators, carboxy SNARF-1 or PBFI, respectively, under whole-cell patch-clamp conditions. With H+ as the only permeable monovalent cation, a rapid increase in extracellular L-lactate concentration ([L-]o) from 0 to 30 mmol/L at constant pHo (7.35) caused an intracellular acidification averaging 0.18 +/- 0.02 pH units in 60 seconds (n = 7), reflecting L-lactate influx in association with H+ influx (or OH- efflux). Under voltage-clamp conditions, no significant electrogenic current was associated with H(+)-coupled L-lactate influx, and membrane potential (-75 to +75 mV) had no effect on the degree of acidification produced by 30 mmol/L [L-]o, indicating that L-lactate influx was predominantly nonelectrogenic. Acidification in response to increased [L-]o was saturable (Km, approximately 5 mmol/L), partially stereospecific for L-lactate over D-lactate, and inhibited by 55 +/- 7% and 82 +/- 7% by the monocarboxylate carrier inhibitors alpha-cyano-4-hydroxycinnamate and mersalyl acid, respectively, consistent with a carrier-mediated transport mechanism. Extracellular K+ inhibited H(+)-coupled L-lactate influx by 36 +/- 2%, suggesting that K+ either inhibited or substituted for H+ in cotransport with L-lactate. However, in myocytes loaded with PBFI, no significant increase in [K+]i was detected during exposure to 30 mmol/L [L-]o, suggesting that only a minor component, if any, of L-lactate influx was cotransported or codiffused with K+.

Evidence for a lactate-anion exchanger in the rat jejunal basolateral membrane

BACKGROUND/AIMS

The mechanism by which lactate, absorbed from the intestinal lumen or generated within the epithelium, crosses the basolateral membrane of the enterocyte and enters the bloodstream has not previously been characterized in detail.

METHODS

L-lactate uptake into and efflux from isolated jejunal basolateral membrane vesicles was investigated at room temperature using rapid filtration techniques.

RESULTS

Furosemide sensitive uptake of L-lactate was unaffected by cis sodium or proton gradients but could be stimulated by a trans gradient of bicarbonate and chloride. Kinetic analysis showed uptake to consist of a saturable component with a Michaelis constant (Km) of 3.2 mmol/L and a maximum velocity (Vmax) of 67 pmol.mg protein-1 x s-1 and a nonsaturable alpha-4-hydroxy-cinnamic acid insensitive component. Pyruvate, butyrate, acetate, valerate, and propionate competitively inhibited lactate uptake into the vesicles. Efflux of lactate from preloaded vesicles was furosemide sensitive and accelerated by a trans bicarbonate gradient as well as by 10 mmol/L acetate, butyrate, and pyruvate.

CONCLUSIONS

It is concluded that there is a short chain-fatty acid carrier system in the intestinal basolateral membrane, which operates as an anion exchanger.

Characterisation of erythrocyte transmembrane exchange of trifluoroacetate using 19F-NMR: evidence for transport via the monocarboxylate transporter

The transport of trifluoroacetate (TFA) and difluorophosphate (DFP) into and out of human and sheep erythrocytes was measured using 19F-NMR. The pathways for the transport in human erythrocytes were characterised by differentiating between the transport inhibition caused by different reagents. (1) Pre-treatment of human erythrocytes with N-ethylmaleimide (10 mM) caused a decrease of the membrane-permeability coefficients for TFA influx and efflux to 0.74 +/- 0.05 and 0.83 +/- 0.09-times, respectively, of those determined in the absence of inhibition. Concomitantly there was no apparent effect on the band-3-mediated transport of DFP. Thus, the decrease of the permeability of TFA is consistent with the inhibition being that of the monocarboxylate transporter. (2) Inhibition of TFA and DFP exchange was also seen in human erythrocytes treated with p-chloromercuriphenylsulfonate (pCMBS). The extent of inhibition reached a maximum value for the pCMBS concentrations beyond which further inhibition was not achieved and there was substantial residual exchange of the two solutes. (3) Residual flux of TFA was found in the presence of high concentrations of the inhibitors, alpha-cyano-4-hydroxycinnamate (> or = 4 mM) or 4,4'-dinitrostilbene-2,2'-disulfonate (> or = 1 mM) when each compound was used alone. (4) Complete inhibition of TFA uptake was obtained when human erythrocytes were treated with both alpha-cyano-4-hydroxycinnamate (4 mM) and a stilbene disulfonate. It was, therefore, concluded that simple diffusion of TFA via the lipid bilayer was negligible in human erythrocytes and that incomplete inhibition of the monocarboxylate transporter occurred when the compounds were used alone.

Regional blood-brain lactate influx

Regional blood-to-brain lactate transport was studied in chloral hydrate anesthetized rats using the single pass, dual-label, indicator fractionation, right atrial injection method. Lactate influx was resolved into two components, a saturable, stereospecific (to the L-enantiomer) component and a non-saturable, non-stereospecific diffusional component. The saturable component was found to have a low efficiency and moderate capacity with transport affinity coefficients between 6 and 14 mM and transport maxima of 23-40 mumol/100 g/min in the various brain regions. Lactate transport was not inhibited by probenecid. The diffusional component was determined from D-lactate influx measurements and the regional linear diffusion coefficients ranged from 0.020 to 0.036 ml/g/min. At the usual levels of plasma lactate (1-1.5 mM) these two influx components were about equal. The relative contribution of the non-stereospecific diffusional component was increased at higher plasma lactate concentrations. Lactate clearance, estimated by the total apparent permeability x surface area products was between 6 and 8 ml/100 g/min.

Characterization of the inhibition by stilbene disulphonates and phloretin of lactate and pyruvate transport into rat and guinea-pig cardiac myocytes suggests the presence of two kinetically distinct carriers in heart cells

1. The kinetics of transport of pyruvate (Km 0.20 mM), L-lactate (Km 2.2 mM) and D-lactate (Ki 10.2 mM) into rat cardiac myocytes were studied and compared with those for guinea-pig heart cells [Poole, Halestrap, Price and Levi (1989) Biochem. J. 264, 409-418] whose equivalent values were 0.07, 2.3 and 6.6 mM respectively. Maximal rates of transport were about 5-fold higher in the rat heart cells. 2. 4,4'-Dibenzamidostilbene-2,2'-disulphonate (DBDS), a powerful inhibitor of monocarboxylate transport into erythrocytes [Poole & Halestrap (1991) Biochem. J. 275, 307-312], was found to be a potent but apparently partial inhibitor of lactate and pyruvate transport, with an apparent Ki value at 0.5 mM L-lactate of about 16 microM in both species. Maximal inhibition was 50% and 80% in rat and guinea-pig cells respectively. 3. The maximal extent of inhibition and apparent Ki values were dependent on both the substrate transported and its concentration. Maximum inhibition was less and the Ki was greater at higher substrate concentrations. 4. A variety of other stilbene disulphonates were studied which showed different Ki values and maximal extents of inhibition. 5. Phloretin was a significantly less potent inhibitor of transport into both rat (Ki 25 microM) and guinea-pig (Ki 16 microM) heart cells than into rat erythrocytes (Ki 1.4 microM). In the rat but not the guinea-pig heart cells, inhibition appeared partial (maximal inhibition 84%). 6. We demonstrate that our results can be explained by the presence of two monocarboxylate carriers in heart cells, both with Km values for L-lactate of about 2 mM and inhibited by alpha-cyano-4-hydroxycinnamate, but with different affinities for other substrates and inhibitors. One carrier is sensitive to inhibition by stilbene disulphonates and has lower Km values for pyruvate (0.05-0.10 mM) and D-lactate (5 mM), whereas the other has higher Km values for pyruvate (0.30 mM) and D-lactate (25 mM), and is relatively insensitive to stilbene disulphonates. Rat heart cells possess more of the latter carrier and guinea-pig heart cells more of the former. 7. The significance of these results for the study of lactate transport in the perfused heart is discussed.

Substrate and inhibitor specificity of the lactate carrier of human neutrophils

The substrate and inhibitor specificity of the lactic acid (Lac) transport system of human neutrophils was investigated. The ability of a variety of compounds to inhibit the influx of [14C]lactate, presumably reflecting competition by substrate analogues for binding at the external translocation site, was taken as an index of affinity for the Lac carrier. pH-state techniques were utilized to assess transportability. Results indicate a relatively low order of selectivity, the neutrophil H+(+)lactate- cotransport system demonstrating a broad acceptance of short-chain unsubstituted and substituted alkyl monocarboxylates as well as aromatic monocarboxylates. There was a slight preference for oxo, Cl, and OH substituents over other groups at the two-position of short chain alkyl fatty acids: all were readily transported across the plasma membrane at rates approaching that of L-lactate itself. Aromatic acids were not transported inward by the carrier although these compounds did permeate via simple nonionic diffusion. The neutrophil Lac carrier can be blocked by a number of cyanocinnamate derivatives, the classical inhibitors of monocarboxylate transport in mitochondria, and by dithiol compounds and sulfhydryl-reactive agents. This constellation of biochemical properties is similar to the features that characterize other well described H+(+)lactate- cotransport systems in red blood cells, Ehrlich ascites tumor cells, hepatocytes, and cardiac sarcolemmal vesicles, although significant differences exist when comparisons are made to the Na(+)-dependent lactate transporter of the kidney proximal tubule.

A basolateral lactate/H+ co-transporter in Madin-Darby Canine Kidney (MDCK) cells

Monolayers of Madin-Darby Canine Kidney (MDCK) cells grown on permeable filters generated lactate aerobically and accumulated it preferentially in the basolateral compartment, suggesting the presence of a lactate carrier. The mechanism of lactate transport across apical and basolateral membranes was examined by determining intracellular pH (pHi) microspectrofluorimetrically after addition of lactate to the extracellular solutions and by measuring uptake of [14C]lactate. Addition of 20 mM lactate to the apical compartment produced no change in pHi, whereas lactate added to the basolateral compartment rapidly and reversibly lowered pHi. Pyruvate produced similar results. Inhibitors of lactate/H+ co-transporters, alpha-cyano-4-hydroxycinnamate (CnCN) and quercetin, partially inhibited the fall in pHi produced by basolateral lactate. In contrast, the disulphonic stilbene. DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid) produced no inhibition at 0.5 mM. Kinetic analysis was performed by applying basolateral lactate at various concentrations and measuring the rate of entry (delta pHi/min) in the presence and absence of CnCN. Lactate flux was shown to occur by both non-ionic diffusion and a alpha-cyano-4-hydroxycinnamate-sensitive component (carrier). The latter has a Km of approximately 7 mM for the lactate anion. Propionate, but not formate, lowered pHi to the same degree as did equimolar lactate, but the propionate effect was not inhibited by CnCN. Influx of [14C]lactate was substantially greater across the basolateral membrane than across the apical membrane and occurred in the absence of Na+. We conclude that MDCK cells grown on permeable filters generate lactate aerobically and transport it across the basolateral membrane by way of a lactate/H+ cotransporter.

Effects of cinnamic acid derivatives on in vitro growth of Plasmodium falciparum and on the permeability of the membrane of malaria-infected erythrocytes

Cinnamic acid derivatives (CADs) are known inhibitors of monocarboxylate transport across plasma and mitochondrial membranes. All derivatives were found to inhibit the growth of intraerythrocytic Plasmodium falciparum in culture, which is in correlation with their hydrophobic character. Parasites at the ring and trophozoite stages were equally susceptible to the different derivatives. This result could be attributed to their inhibition of the transport of lactate, the major product of parasite energy metabolism. However, unexpectedly, it was found that all derivatives also inhibit the translocation of carbohydrates and amino acids across the new permeability pathways induced in the host cell membrane by the parasite. This impediment correlated strictly with CADs' effect on parasite growth. Parasites residing in cells permeabilized by means of Sendai virus were less susceptible to the different drugs, a result which implies that in addition to the direct effect on parasite viability, the drugs may have inhibited some process in the host cell whose function may be vital for parasite growth. The effect of CADs on the ATP levels in infected cells, in virus-treated cells, and in the two cellular compartments of the infected cell revealed that the drugs caused a significant decline in ATP level in the parasite compartment, while they provoked only a small effect on ATP level in the intact cells and the host cell compartment. These observations suggest that CADs inhibit ATP production in the parasite and its utilization by the host cell.

Identification and partial purification of the erythrocyte L-lactate transporter

1. Intact erythrocytes were incubated with 100 microM-4,4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS), a concentration sufficient to inhibit lactate transport irreversibly by 65%. DIDS-labelled proteins were detected by immunoblotting of erythrocyte membrane proteins with an anti-DIDS antibody. Labelled polypeptides of 35-45 kDa in rat erythrocytes, and of 40-50 kDa in rabbit and guinea pig erythrocytes, were detected by this technique. In human erythrocytes, which have 10-fold less transport activity, no labelled polypeptide in this molecular mass range was detected. 2. Labelling of these 35-50 kDa polypeptides was decreased markedly in the presence of the specific inhibitors of lactate transport alpha-cyano-4-hydroxycinnamate and 4,4'-dibenzamidostilbene-2,2'-disulphonate (DBDS), which compete with DIDS for binding to the transporter. However, the weakly bound inhibitor 4,4'-dinitrostilbene-2,2'-disulphonate (DNDS) afforded little protection against labelling by DIDS. 3. The lactate transporter from rat erythrocytes was solubilized with decanoyl-N-methyl glucamide (MEGA-10) and partially purified by Mono-Q anion-exchange chromatography, with transport activity eluting at 0.1-0.15 M-NaCl. The 35-45 kDa DIDS-labelled polypeptide from rat erythrocytes was eluted in the same peak of protein as lactate transporter activity during Mono-Q chromatography. 4. These observations provide strong evidence that the lactate transporter is a polypeptide of 35-45 kDa in rat erythrocytes and of 40-50 kDa in rabbit and guinea pig erythrocytes.