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

Expression of monocarboxylate transporter 1 (MCT1) in the dog intestine

In this study, the expression and distribution of monocarboxyolate transporter 1 (MCT1) along the intestines (duodenum, jejunum, ileum, cecum, colon and rectum) of dogs were investigated at both the mRNA and protein levels. The expression of MCT1 protein and its distribution were confirmed by Western blotting and immunohistochemical staining using the antibody for MCT1. We identified mRNA coding for MCT1 and a 43-kDa band of MCT1 protein in all regions from the duodenum to the rectum. Immunoreactive staining for MCT1 was also observed in epithelial cells throughout the intestines. MCT1 immunoreactivity was greater in the large intestine than in the small intestine. MCT1 protein was predominantly expressed on the basolateral membranes along intestinal epithelial cells, suggesting that MCT1 may play an important role in lactate efflux and transport of short-chain fatty acids (SCFAs) to the bloodstream across the basolateral membranes of the dog intestine.

Cellular expression of monocarboxylate transporters (MCT) in the digestive tract of the mouse, rat, and humans, with special reference to slc5a8

Short-chain fatty acids (SCFA) are monocarboxylates produced by bacterial fermentation that play a crucial role in maintaining homeostasis in the large intestine. Two major transporters for SCFA, monocarboxylate transporter (MCT) and slc5a8 (or SMCT), exist in the digestive tract. The present histochemical study using in situ hybridization and immunohistochemistry revealed the distribution and subcellular localization of the MCT family in the digestive tract of mice, rats, and humans, comparing these with that of slc5a8. The expression of mucosal MCT1 in the mouse and rat was most intense in the cecum, followed by the colon, but low in the stomach and small intestine. Among other MCT subtypes, only MCT2 was detected in the parietal cell region of the gastric mucosa. Slc5a8 had predominant expression sites in the distal half of the large bowel and in the most terminal ileum. The mucosal MCT1 was localized in the basolateral membrane of enterocytes, while slc5a8 was restricted to the apical cell membrane, suggesting the involvement of slc5a8 in the uptake of luminal SCFA, and of MCT1 in the efflux of SCFA and monocarboxylate metabolites towards blood circulation. The large intestine expressed both types of the transporter, but their distribution patterns differed along the longitudinal axis of the intestine and along the perpendicular axis of the mucosa.

Anion antiport mechanism is involved in transport of lactic acid across intestinal epithelial brush-border membrane

Intestinal epithelial membrane transport of L-lactic acid was characterized using rabbit jejunal brush-border membrane vesicles (BBMVs). The uptake of L-[(14)C]lactic acid by BBMVs showed an overshoot phenomenon in the presence of outward-directed bicarbonate and/or inward-directed proton gradients. Kinetic analysis of L-[(14)C]lactic acid uptake revealed the involvement of two saturable processes in the presence of both proton and bicarbonate gradients. An arginyl residue-modifying agent, phenylglyoxal, inhibited L-[(14)C]lactic acid transport by the proton cotransporter, but not by the anion antiporter. The initial uptakes of L-[(14)C]lactic acid which are driven by bicarbonate ion and proton gradients were inhibited commonly by monocarboxylic acids and selectively by anion exchange inhibitor 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid and protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone, respectively. These observations demonstrate that L-lactic acid is transported across the intestinal brush-border membrane by multiple mechanisms, including an anion antiporter and a previously known proton cotransporter.

Identification and characterization of a monocarboxylate transporter (MCT1) in pig and human colon: its potential to transport L-lactate as well as butyrate

1. Oligonucleotide primers based on the human heart monocarboxylate transporter (MCT1) cDNA sequence were used to isolate a 544 bp cDNA product from human colonic RNA by reverse transcription-polymerase chain reaction (RT-PCR). The sequence of the RT-PCR product was identical to that of human heart MCT1. Northern blot analysis using the RT-PCR product indicated the presence of a single transcript of 3.3 kb in mRNA isolated from both human and pig colonic tissues. Western blot analysis using an antibody to human MCT1 identified a specific protein with an apparent molecular mass of 40 kDa in purified and well-characterized human and pig colonic lumenal membrane vesicles (LMV). 2. Properties of the colonic lumenal membrane L-lactate transporter were studied by the uptake of L-[U-14C]lactate into human and pig colonic LMV. L-Lactate uptake was stimulated in the presence of an outward-directed anion gradient at an extravesicular pH of 5.5. Transport of L-lactate into anion-loaded colonic LMV appeared to be via a proton-activated, anion exchange mechanism. 3. L-Lactate uptake was inhibited by pyruvate, butyrate, propionate and acetate, but not by Cl- and SO4(2-). The uptake of L-lactate was inhibited by phloretin, mercurials and alpha-cyano-4-hydroxycinnamic acid (4-CHC), but not by the stilbene anion exchange inhibitors, 4,4'-diisothiocyanostilbene-2, 2'-disulphonic acid (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2, 2'-disulphonic acid (SITS). 4. The results indicate the presence of a MCT1 protein on the lumenal membrane of the colon that is involved in the transport of L-lactate as well as butyrate across the colonic lumenal membrane. Western blot analysis showed that the abundance of this protein decreases in lumenal membrane fractions isolated from colonic carcinomas compared with that detected in the normal healthy colonic tissue.

Fluid, electrolyte, and acid-base responses to exercise in racehorses

During both high-intensity and short-distance exercise, the high rate of expended energy is met by anaerobic oxidation of glucose to lactic acid; this is the main cause of metabolic acidosis observed during racing. In addition, plasma volume decreases because water moves from the vasculature to the intracellular and interstitial spaces at the onset of intense exercise. These fluid shifts, together with active ion-exchange between blood and tissue, cause marked changes in electrolyte concentrations. This article reviews the mechanisms of acid-base disturbances, fluid shifts, and electrolyte changes, and discusses related areas such as buffer capacity, lactic acid distribution, and the effects of training. The influences of health, dietary cation-anion balance, supplements, and medication such as creatine, sodium bicarbonate, and furosemide are emphasized.

Proton-lactate cotransport in basolateral membrane vesicles from rat jejunum

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