Transport of L-leucine hydroxy analogue and L-lactate in rabbit small-intestinal brush-border membrane vesicles

Monocarboxylate transporter 1 mediates DL-2-Hydroxy-(4-methylthio)butanoic acid transport across the apical membrane of Caco-2 cell monolayers

The methionine hydroxy analogue DL-2-hydroxy-(4-methylthio)butanoic acid (DL-HMB) is a supplementary source of methionine commonly added to commercial animal diets to satisfy the total sulfur amino acid requirement. In this study, we characterized DL-HMB transport across the apical membrane of Caco-2 cells to identify the transport mechanism involved in the intestinal absorption of this methionine source. DL-HMB transport induced a significant decrease in intracellular pH (pH(i)) and was inhibited in the presence of the protonophore carbonyl cyanide 4-(trifluoromethoxy)-phenylhydrazone. Moreover, both Na(+) removal and 5-(N-ethyl-N-isopropyl)amiloride, an inhibitor of apical Na(+)/H(+) exchanger (NHE3), significantly reduced substrate uptake and pH(i) recovery, suggesting cooperation between H(+)-dependent DL-HMB transport and NHE3 activity. cis-Inhibition experiments with L-Ala, beta-Ala, D-Pro, betaine, or glycyl-sarcosine excluded the participation of systems proton amino acid transporter 1 and peptide transporter 1. In contrast, alpha-cyano-4-hydroxycinnamate, phloretin, L-lactate, beta-hydroxybutyrate, butyrate, and pyruvate, inhibitors and substrates of monocarboxylate transporter 1 (MCT1), significantly reduced DL-HMB uptake. Dixon plot analysis of L-lactate transport in the presence of DL-HMB revealed a competitive interaction (inhibition constant, 17.5 +/- 0.11 mmol/L), confirming the participation of system MCT1. The kinetics of DL-HMB uptake was described by a model involving passive diffusion and a single low-affinity, high-capacity transport mechanism (K(D), 1.9 nL/microg protein; K(m), 13.1 +/- 0.04 mmol/L; and V(max), 43.6 +/- 0.14 pmol/microg protein) compatible with MCT1 kinetic characteristics. In conclusion, the methionine hydroxy analogue is transported in Caco-2 cell apical membrane by a transport mechanism with functional characteristics similar to those of MCT1.

Intestinal brush-border membrane transport of monocarboxylic acids mediated by proton-coupled transport and anion antiport mechanisms

Intestinal brush-border membrane transport of monocarboxylic acids was investigated by using rabbit intestinal brush-border membrane vesicles (BBMVs) and isolated intestinal tissues mounted on Ussing-type chambers. [3H]Mevalonic acid uptake by BBMVs showed an overshoot phenomenon in the presence of an inwardly directed proton gradient, but not in the presence of an inwardly directed sodium gradient or an outwardly directed HCO3- or chloride gradient. Initial uptake of mevalonic acid was saturable in the presence of a proton gradient. Uptake of [3H]mevalonic acid was inhibited by various monocarboxylic acids, including acetic acid, benzoic acid, lactic acid, nicotinic acid, pravastatin, salicylic acid and valproic acid, but not by dicarboxylic acid or amino acids. Acetic acid, which is transported by both anion antiport and proton-coupled transport systems, induced serosal bicarbonate-dependent alkalinization in the mucosal-side bathing solution of rabbit jejunal tissues, when examined in Ussing-type chambers. Pravastatin, which is a structural analogue of mevalonic acid and is absorbed via proton-coupled transport like mevalonic acid, did not. The result demonstrates that acetic acid is transported by the bicarbonate-dependent anion antiport system, whereas pravastatin is not. So, it is suggested that monocarboxylic acids are transported by at least two independent transporters, namely, a proton-coupled transporter for most monocarboxylic acids, including mevalonic acid, pravastatin and acetic acid, and an anion antiporter for acetic acid, but not for mevalonic acid or pravastatin. Activation of anion antiporter can induce HCO3- secretion in intact intestine.

Potassium activation of Na(+)-dependent leucine transport in brush-border membrane vesicles from rat jejunum

Na(+)-dependent leucine uptake was greater in potassium loaded brush-border membrane vesicles compared with controls. This effect was not mediated by an electrical potential difference, since it was still present in voltage-clamped conditions. Inhibition experiments indicate the same Na(+)-dependent leucine transport activity in the presence or in the absence of potassium. The affinity of sodium for the cotransporter was identical at 10 or 100 mM potassium. Leucine kinetics at different potassium concentrations showed a maximum 2.4-fold increase in Vmax, while Km was unaffected. The secondary plots of the kinetic results were not linear. This kinetic behavior suggests that K+ acts as a non-essential activator of Na(+)-dependent leucine cotransport. A charge compensation of sodium-leucine influx is most probably a component of the potassium effect in the presence of valinomycin.

Partial purification and reconstitution of the sarcolemmal L-lactate carrier from rat skeletal muscle

Purified sarcolemmal membranes from mixed rat hindlimb muscle were solubilized with octylglucoside and the extract subjected to hydroxylapatite (HA) chromatography. Following protein elution with a sodium phosphate gradient and detergent removal by dialysis, the HA eluate was reconstituted into asolectin liposomes using a freeze-thaw procedure. Specific L-[14C]lactate transport activity eluting from the 0.2 M sodium phosphate fraction was 30-fold higher compared with native sarcolemmal vesicles (31.64 versus 1.06 nmol/min per mg). The reconstituted carrier exhibited Michaelis-Menten saturation kinetics with Km and Vmax. values of 46.2 +/- 6.6 mM and 498.7 +/- 17.2 nmol/15 s per mg respectively. L-Lactate transport activity was inhibited 57% by preincubation of proteoliposomes with 10 mM alpha-cyano-4-hydroxycinnamate, a known inhibitor of lactate transport. Analysis of the HA eluates by SDS/PAGE showed the presence of a 34 kDa band corresponding to lactate transport activity. Reconstitution of lactate transport activity eluting from the HA column, together with SDS/PAGE analysis suggests the presence of a 34 kDa polypeptide mediating sarcolemmal lactate exchange in rat skeletal muscle.

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.

Transport of L-leucine hydroxy analogue and L-lactate in human small intestinal brush border membrane vesicles

Nitrogen load can be reduced by substituting dietary protein intake with keto or hydroxy analogues of amino acids. To investigate their intestinal absorption, human jejunal brush border membrane vesicles were used to measure uptake of L-leucine hydroxy analogue (L-LHA) and L-lactate. Uptake assays were performed under voltage-clamped conditions in the presence of valinomycin and K+ inside and outside. Both inward directed H(+)- and Na(+)-gradients stimulated uptake of both substrates. The H(+)-gradient was the major driving force and led to an increased rate of L-LHA or L-lactate transport. The proton ionophore FCCP abolished the H(+)-gradient-driven but not the Na(+)-gradient-driven uptakes of both substrates. The H(+)-gradient-driven uptake of both substrates was trans-stimulated by L-LHA, D-LHA, L-valine hydroxy analogue or L-lactate, respectively. In the presence of a Na(+)-gradient the uptake of tracer L-lactate was trans-stimulated only after preloading the vesicles with L-lactate but not by L-LHA. It can be concluded that the H(+)-gradient-driven transport of L- or D-hydroxy analogues of branched chain amino acids and L-lactate across the human intestinal brush border membrane is mediated by the same carrier.