Asymmetry in the transport of lactate by basolateral and brush border membranes of rat kidney cortex

Mechanisms of basolateral base transport in the renal proximal tubule

Renal proximal tubules absorb HCO3- by secretion of H+ into the tubular lumen. This paper focuses on the mechanisms of HCO3- exit across the basolateral cell membrane. The major exit pathway is rheogenic sodium bicarbonate co-transport. This system transports Na+ and HCO3-, but not Cl-, in obligatory coupling at a fixed overall stoichiometry of three HCO3- to one Na+. The fact that HCO3- flux is reduced after inhibition of cytoplasmic and/or membrane-bound peritubular carbonic anhydrase indicates that HCO3- is not transported as such but is split during permeation into its buffer subspecies from which it is regenerated on the other side of the membrane. Since flow of OH- or of H+ (in opposite directions) can be excluded, it appears most likely that one HCO3- and one CO3(2-) move together with one Na+. Besides carbonic anhydrase inhibitors, disulphonic stilbenes and harmaline are known to block the co-transporter. In addition to rheogenic Na+ (HCO3-)3 co-transport, Na+-dependent and Na+-independent electroneutral Cl-/HCO-3 exchange have been identified. The latter mechanisms are particularly important in S3 segments of proximal tubule where Na+ (HCO3-)3 co-transport is missing. Further mechanisms which operate in parallel, but at lower rates, are electroneutral SO4(2-)/HCO3- exchange and, in some species, lactate/HCO3- exchange. Moreover, there may be some uncoupled OH- flux and it is reasonable to assume that OH- (or H+) flux is involved in the transport of dicarboxylic acids across the basolateral cell membrane.

The human tumour suppressor gene SLC5A8 expresses a Na+-monocarboxylate cotransporter

The orphan cotransport protein expressed by the SLC5A8 gene has been shown to play a role in controlling the growth of colon cancers, and the silencing of this gene is a common and early event in human colon neoplasia. We expressed this protein in Xenopus laevis oocytes and have found that it transports small monocarboxylic acids. The electrogenic activity of the cotransporter, which we have named SMCT (sodium monocarboxylate transporter), was dependent on external Na(+) and was compatible with a 3 : 1 stoichiometry between Na(+) and monocarboxylates. A portion of the SMCT-mediated current was also Cl(-) dependent, but Cl(-) was not cotransported. SMCT transports a variety of monocarboxylates (similar to unrelated monocarboxylate transport proteins) and most transported monocarboxylates demonstrated K(m) values near 100 microm, apart from acetate and d-lactate, for which the protein showed less affinity. SMCT was strongly inhibited by 1 mm probenecid or ibuprofen. In the absence of external substrate, a Na(+)-independent leak current was also observed to pass through SMCT. SMCT activity was strongly inhibited after prolonged exposure to high external concentrations of monocarboxylates. The transport of monocarboxylates in anionic form was confirmed by the observation of a concomitant alkalinization of the cytosol. SMCT, being expressed in colon and kidney, represents a novel means by which Na(+), short-chain fatty acids and other monocarboxylates are transported in these tissues. The significance of a Na(+)-monocarboxylate transporter to colon cancer presumably stems from the transport of butyrate, which is well known for having anti-proliferative and apoptosis-inducing activity in colon epithelial cells.

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.

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.

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.

Lactate transport is mediated by a membrane-bound carrier in rat skeletal muscle sarcolemmal vesicles

To study the kinetics of lactate transport in an isolated, nonmetabolizing system, skeletal muscle sarcolemmal membrane vesicles were purified from 22 female Sprague-Dawley rats. L(+)-[U-14C] Lactate at 10 concentrations demonstrated saturation kinetics with a Vmax of 139.4 nmol/mg/min, and an apparent Km of 40.1 mM. Threefold higher initial rates of L(+)-lactate uptake were seen at 37 degrees C than at 25 degrees C, indicating temperature sensitivity. Transport was stereospecific for the L(+) isomer: isotopic D(-) uptake rates remained linear at concentrations from 1 to 200 mM, and 1 mM D(-) remained 6-fold lower in net uptake after 60 min than the L(+) isomer. Furthermore, unlabeled 10 mM D(-)-lactate in the external medium could only inhibit 1 mM isotopic (L(+) uptake by 12%, whereas unlabeled 10 mM L(+)-lactate and pyruvate inhibited 82 and 71%, respectively. Additionally, 10 mM beta-hydroxybutyrate and acetoacetate could moderately inhibit (27 and 32%, respectively) 1 mM L(+)-lactate transport, but the unsubstituted aliphatic monocarboxylates (formate, acetate, propionate), tricarboxylic acid cycle intermediates (malate, succinate, oxaloacetate, alpha-ketoglutyrate, citrate), amino acids (alanine, aspartate, glutamate), and palmitate or adenosine in 10-fold excess could not effectively inhibit 1 mM L(+)lactate uptake under cis-transport conditions. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid could inhibit L(+)-lactate transport by only 13%, so that lactate transport does not appear to be affected directly by Cl- or HCO3- fluxes. It was demonstrated that KCl could not evoke a membrane potential-induced overshoot of lactate uptake in the presence or absence of valinomycin. Moreover, gluconate could substitute for Cl-, indicating that Cl- flux does not contribute to a membrane potential-dependent component of the transport mechanism, suggesting an electroneutral translocation process. Protein-modifying reagents significantly inhibited 1 mM L(+)-lactate transport during pH-stimulated conditions (p-chloromercuriphenyl-sulfonic acid, 83%; N-ethylmaleimide, 86%; HgCl2, 56%; mersalyl, 63% inhibition). We conclude that the skeletal muscle lactate transporter is a membrane-bound protein, specifically associated with the sarcolemma, that demonstrates saturation kinetics, competition, stereospecificity, and sensitivity to temperature as well as various ionic cis-inhibitors. The lactate transporter is a potentially important regulator of lactate flux across skeletal muscle, and may help to regulate intracellular pH and intermediary metabolism during lactic acidosis.

Characterization of ischemia-induced loss of epithelial polarity

Total renal ischemia for various time intervals (0-50 min) resulted in the rapid and duration-dependent redistribution of polarized membrane lipids and proteins in renal proximal tubule cells. Following only 15 min of ischemia, apical membrane enrichment of NaK-ATPase, normally a basolateral membrane (BLM) enzyme, had increased (1.6 +/- 0.6 vs. 2.9 +/- 1.2, P less than 0.01). In vivo histochemical localization of NaK-ATPase showed reaction product throughout the apical microvillar region. PTH-stimulatable adenylate cyclase, another BLM protein, was also found in ischemic but not control apical membrane fractions. One dimensional SDS-PAGE showed four bands, present in control BLM and ischemic apical membranes, which could not be found in control apical membrane fractions. Immunohistochemical localization of leucine aminopeptidase (LAP) showed the enzyme was limited to the apical domain in control cells. Following ischemic injury (50 min), LAP staining could be seen within the cell and along the BLM. Following 24 hr of reperfusion, the BLM distribution of LAP was further enhanced. With cellular recovery from ischemic injury (5 days), LAP was again only visualized in the apical membrane. Duration-dependent alterations in apical and BLM lipids were also observed. Apical sphingomyelin and phosphatidylserine and the cholesterol-to-phospholipid ratio decreased rapidly while apical phosphatidylcholine and phosphatidylinositol increased. Taken together, these results indicate renal ischemia causes rapid duration-dependent reversible loss of surface membrane polarity in proximal tubule cells.

Effect of electroneutral luminal and basolateral lactate transport on intracellular pH in salamander proximal tubules

We used microelectrodes to examine the effects of organic substrates, particularly lactate (Lac-), on the intracellular pH (pHi) and basolateral membrane potential (Vbl) in isolated, perfused proximal tubules of the tiger salamander. Exposure of the luminal and basolateral membranes to 3.6 mM Lac- caused pHi to increase by approximately 0.2, opposite to the decrease expected from nonionic diffusion of lactic acid (HLac) into the cell. Addition of Lac- to only the lumen also caused alkalinization, but only if Na+ was present. This alkalinization was not accompanied by immediate Vbl changes, which suggests that it involves luminal, electroneutral Na/Lac cotransport. Addition of Lac- to only the basolateral solution caused pHi to decrease by approximately 0.08. The initial rate of this acidification was a saturable function of [Lac-], was not affected by removal of Na+, and was reversibly reduced by alpha-cyano-4-hydroxycinnamate (CHC). Thus, the pHi decrease induced by basolateral Lac- appears to be due to the basolateral entry of H+ and Lac-, mediated by an H/Lac cotransporter (or a Lac-base exchanger). Our data suggest that this transporter is electroneutral and is not present at the luminal membrane. A key question is how the addition of Lac- to the lumen increases pHi. We found that inhibition of basolateral H/Lac cotransport by basolateral CHC reduced the initial rate of pHi increase caused by luminal Lac-. On the other hand, luminal CHC had no effect on the luminal Lac(-)-induced alkalinization. These data suggest that when Lac- is present in the lumen, it enters the cell from the lumen via electroneutral Na/Lac cotransport and then exists with H+ across the basolateral membrane via electroneutral H/Lac cotransport. The net effect is transepithelial Lac- reabsorption, basolateral acid extrusion, and intracellular alkalinization.

Evidence for OH-/H+ permeation across the peritubular cell membrane of rat renal proximal tubule in HCO3(-)-free solutions

Membrane potentials and intracellular pH were measured on rat renal proximal tubular cells in vivo to test whether sodium-bicarbonate cotransport across the peritubular cell membrane accepts OH- (or H+ in opposite direction) or whether it requires the CO2, HCO3-, CO3= buffer to operate. It was found that step changes of peritubular pH in nominally HCO3(-)-free and CO2-free solutions produced qualitatively similar initial potential responses and cell pH responses as changes in peritubular HCO3- concentrations. These responses, however, were considerably smaller and they were neither reduced in Na+-free solutions nor inhibited by the stilbene derivative SITS which is known to block Na+ (HCO3-)n cotransport completely. We conclude that the cotransporter requires the CO2, HCO3-, CO3= buffer for its physiological operation but that high rates of OH- or H+ can also be transferred across the peritubular cell membrane in HCO3(-)-free solutions, probably through a separate transport system.

Contraluminal transport of small aliphatic carboxylates in the proximal tubule of the rat kidney in situ

In order to study the characteristic of contraluminal transport of hydrophylic small fatty acids the in situ stopped flow microperfusion technique [12] has been applied. By measuring with 4 s contact time the decrease in the contraluminal concentration of the respective radiolabelled substances the concentration dependence of the influx into the cortical cells was tested. The 4 s decrease in contraluminal concentration of chloroacetate, L-lactate, D-lactate, 3-hydroxybutyrate and acetoacetate was between 26% and 31%. For each substance the percent decrease was the same, no matter whether it was offered in a concentration of 0.1 or 10 mmol/l. Contraluminal disappearance of 0.1 mmol/l L-lactate was not influenced by 5 mmol/l H2DIDS, probenecid, phloretin, mersalyl or cyanocinnamate, but it was significantly (37%) inhibited by 5-nitro-2-(phenyl-propyl-amino) benzoate, a blocker of the nonspecific anion channel. The percent decrease in propionate uptake was somewhat larger - between 36% and 39% - but again not different at 0.01, 0.1, 1.0 and 10 mmol/l. With pyruvate the contraluminal decrease was 20% at 0.1 mmol/l and 31% at 10 mmol/l. The percent disappearance of the aromatic pyrazinoate was 38% and 34% at 0.1 and 10 mmol/l and for nicotinate 42% and 22%, respectively. The disappearance of nicotinate (0.1 mmol/l) was significantly inhibited by 10 mmol/l pyrazinoate and paraaminohippurate (PAH). The data are in agreement with the hypothesis that the hydrophilic small fatty acids traverse the contraluminal cell side by simple diffusion, possibly via the unspecific anion channel [14], pyruvate via the dicarboxylic acid pathway in a cooperative manner and pyrazinoate, as well as nicotinate, via the PAH pathway.

Ischemia induces partial loss of surface membrane polarity and accumulation of putative calcium ionophores

To determine if ischemia induces alterations in renal proximal tubule surface membranes, brush border (BBM) and basolateral membranes (BLM) were isolated simultaneously from the same cortical homogenate after 50 min of renal pedicle clamping. Ischemia caused a selective decrease in the specific activity of BBM marker enzymes leucine aminopeptidase and alkaline phosphatase, but did not effect enrichment (15 times). Neither specific activity nor enrichment (10 times) of BLM NaK-ATPase was altered by ischemia. Contamination of BBM by intracellular organelles was also unchanged, but there was an increase in the specific activity (41.1 vs. 60.0, P less than 0.01) and enrichment (2.3 vs. 4.3, P less than 0.01) of NaK-ATPase in the ischemic BBM fraction. Ischemia increased BLM lysophosphatidylcholine (1.3 vs. 2.5%, P less than 0.05) and phosphatidic acid (0.4 vs. 1.3%, P less than 0.05). Ischemia also decreased BBM sphingomyelin (38.5 vs. 29.6%, P less than 0.01) and phosphatidylserine (16.1 vs. 11.4%, P less than 0.01), and increased phosphatidylcholine (17.2 vs. 29.7%, P less than 0.01), phosphatidylinositol (1.8 vs. 4.6%, P less than 0.01), and lysophosphatidylcholine (1.0 vs. 1.8%, P less than 0.05). The large changes in BBM phospholipids did not result from new phospholipid synthesis, since the specific activity (32P dpm/nmol Pi) of prelabeled individual and total phospholipids was unaltered by ischemia. We next evaluated if these changes were due to inability of ischemic cells to maintain surface membrane polarity. Cytochemical evaluation showed that while NaK-ATPase could be detected only in control BLM, specific deposits of reaction product were present in the BBM of ischemic kidneys. Furthermore, using continuous sucrose gradients, the enzymatic profile of ischemic BBM NaK-ATPase shifted away from ischemic BLM NaK-ATPase and toward the BBM enzymatic marker leucine aminopeptidase. Taken together, these data suggest that NaK-ATPase activity determined enzymatically and cytochemically was located within ischemic BBM. We propose that ischemia impairs the ability of cells to maintain surface membrane polarity, and also results in the accumulation of putative calcium ionophores.

Evidence for a lactate transport system in the sarcolemmal membrane of the perfused rabbit heart: kinetics of unidirectional influx, carrier specificity and effects of glucagon

The kinetics and specificity of L-lactate transport into cardiac muscle were studied during a single transit through the isolated perfused rabbit heart using a rapid (15 s) paired-tracer dilution technique. Kinetic experiments revealed that lactate influx was highly stereospecific and saturable with an apparent Kt = 19 +/- 6 mM and a Vmax = 8.4 +/- 1.5 mumol/min per g (mean +/- S.E., n = 14 hearts). At high perfusate concentrations (10 mM), the inhibitors alpha-cyano-4-hydroxycinnamate (Ki = 7.3 mM), pyruvate (Ki = 6.5 mM), acetate (Ki = 19.4 mM) and chloroacetate (Ki = 28 mM) reduced L-lactate influx, and Ki values were estimated assuming a purely competitive interaction of the inhibitors with the monocarboxylate carrier. The monocarboxylic acids [14C]pyruvate and [3H]acetate were themselves transported, and sarcolemmal uptakes of respectively 38 +/- 1% and 70 +/- 8% were measured relative to D-mannitol. Perfusion of hearts for 10-30 min with 0.15 or 1.5 microM glucagon increased myocardial lactate production and simultaneously inhibited tracer uptake of lactate, pyruvate and acetate. It is concluded that a stereospecific lactate transporter exhibiting an affinity for other substituted monocarboxylic acids is operative in the sarcolemmal plasma membrane of the rabbit myocardium.

Renal cortical brush-border and basolateral membranes: cholesterol and phospholipid composition and relative turnover

A new procedure for the rapid isolation of renal cortical brush-border and basolateral membranes from the same homogenate is described. Brush-border membranes isolated using Mg2+-EGTA precipitation were enriched 18-fold for leucine aminopeptidase and had a recovery of 32.5%. Basolateral membrane fractions were isolated using a discontinuous sucrose gradient and showed an enrichment of 10.7-fold and recovery of 12.8% using (Na+,K+)-ATPase as a marker enzyme. Lipid analysis using two-dimensional TLC separation of phospholipids and gas liquid chromatography for cholesterol showed marked differences in the lipid composition of the brush-border and basolateral membranes. The brush-border membrane had increased sphingomyelin, phosphatidylserine, ethanolamine plasmalogens, and an increased cholesterol-to-phospholipid and sphingomyelin-to-phosphatidylcholine ratio compared to the basolateral membrane. The relative turnover of total membrane and individual phospholipid species using a double isotope ratio method was carried out. Phospholipids were labeled with either phosphorus 32 and 33 or acetate (3H, 1-14C). The relative turnover of phospholipid species and cholesterol differed strikingly. Phosphatidylcholine showed a high turnover, phosphatidylethanolamine and phosphatidylinositol had intermediate values and sphingomyelin, phosphatidylserine and cholesterol had low relative turnover rates. The order of phospholipid class relative turnover was independent of the labeled precursor used. The brush-border membrane had a significantly reduced relative turnover rate for total membrane phospholipids, sphingomyelin and cholesterol compared to the basolateral membrane. These data show marked differences in the lipid composition and relative turnover rates of the phospholipid species of the brush-border and basolateral membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate utilization in the isolated perfused cortical thick ascending limb of rabbit nephron

Isolated segments of cortical thick ascending limbs (cTAL) of rabbit kidney were perfused in vitro and the equivalent short circuit current (Isc) was measured. In a first series all substrates were removed on either side. Isc fell rapidly to 50 +/- 12% after 3 min and to 27 +/- 6% (n = 5) after 10 min. This indicates that in cTAL segments Isc is strictly dependent on the presence of substrates. In series two it was tested what substrates can be utilized by the cTAL segment, and from which epithelial side [bath (b) or lumen (1)] the substrates are taken up. From the 1-side only butyrate (10 mmol X 1(-1) sustained the Isc at 95 +/- 2% (n = 7). All other tested substrates (10 mmol X 1(-1): pyruvate, acetate, beta-OH-butyrate, D-glucose, and L-lactate lead to a marked decline in Isc. From the b-side several substrates (5--10 mmol X 1(-1) sustained the Isc: D-glucose, D-mannose, butyrate, beta-OH-butyrate, acetoacetate, L-lactate, acetate and pyruvate. Other compounds (1--10 mmol X 1(-1): citrate, alpha-ketoglutarate, succinate, glutamate, glutamine, propionate, caprylate and oleate did not sustain Isc. In the third series the mechanism of substrate utilization from the basolateral cell side was studied. It was shown that the Isc is a saturable function of the D-glucose, L-lactate, acetate, pyruvate or beta-OH-butyrate concentration with apparent Km's between 0.05--1.0 mmol X 1(-1). Several known inhibitors of sugar and of anion transport were tested at the bath side: phlorrhizin was without effect. Phloretin (500 mumol X 1(-1) inhibited Isc by 96%, yet its effect was not dependent on the presence of substrates on the b-side since inhibition occurred also if the b-perfusate contained no substrate and Isc was driven by luminal butyrate. Also SITS (5 mmol X 1(-1) exerted only a small inhibitory effect which was not specific since it was also observed with luminal butyrate. alpha-Cyano-m-OH-cinnamate (10 mmol X 1(-1) inhibited the Isc specifically when L-lactate was the bath substrate. Probenecid (1 mmol X 1(-1) had a similar yet less marked inhibitory effect. The D-glucose uptake from the b-side was specifically inhibited by cytochalasin B at 5 X 10(-6) mol X 1(-1). We conclude that the cTAL segment of the rabbit utilizes D-glucose and/or small anions such as pyruvate or L-lactate or acetate to energize salt reabsorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Bicarbonate uptake by basolateral membrane vesicles from rat jejunum

Basolateral membranes from rat jejunal enterocytes have been obtained by self-orienting Percoll-gradient centrifugation. Bicarbonate and L-glucose uptake into osmotically active basolateral membrane vesicles has been studied by a rapid filtration technique. In closed vessels and at pH 8.2 the uptake kinetics of both [14C]bicarbonate and L[3H]glucose have been followed for 30 min at 18 degrees C. Bicarbonate uptake seems to be fast and in efflux experiments SITS and DIDS effect is negligible. This work demonstrates that it is possible to determine bicarbonate flux across basolateral membrane vesicles at pH and temperature values close to usual experimental conditions.