Tissue-specific and isoform-specific changes in MCT1 and MCT4 in heart and soleus muscle during a 1-yr period

Sex-specific differences in the metabolic enzyme activity and transporter levels in mouse skeletal muscle during postnatal development

Although sex-associated differences in energy metabolism in adults are well-characterized, developmental sex-specific changes in skeletal muscle metabolism are largely unknown. This study investigated sex differences in high-energy phosphate, glycolytic, and mitochondrial enzyme activities and metabolite transporter protein levels in mouse skeletal muscles during the early postnatal period (day 10), post-weaning (day 28), sexual maturity (day 56), and adult life (day 140). No significant sex-specific differences were observed on days 10 and 28, except for glucose transporter (GLUT) 4 level. The hexokinase, phosphofructokinase, and lactate dehydrogenase activities of skeletal muscle were higher and the citrate synthase, cytochrome c oxidase, and β-hydroxyacyl-CoA dehydrogenase activities were lower in female mice than those in male mice on days 56 and 140. The GLUT4 and FAT/CD36 protein levels were higher and the monocarboxylate transporter 4 level was lower in the skeletal muscles of female mice than those of male mice, particularly on days 56 and 140. At 140 days of age, the respiratory exchange ratio during treadmill running (15 m/min, 60 min) was lower in females than that in males, despite no sex differences at rest. In summary, sex differences were not evident in the early postnatal and post-weaning periods but became apparent after the mice reached sexual maturity. These findings indicate that sexually mature animals are a better model for investigating sex differences, particularly in the context of studying energy metabolism in mice.

Monocarboxylate Transporters (SLC16): Function, Regulation, and Role in Health and Disease

The solute carrier family 16 (SLC16) is comprised of 14 members of the monocarboxylate transporter (MCT) family that play an essential role in the transport of important cell nutrients and for cellular metabolism and pH regulation. MCTs 1-4 have been extensively studied and are involved in the proton-dependent transport of L-lactate, pyruvate, short-chain fatty acids, and monocarboxylate drugs in a wide variety of tissues. MCTs 1 and 4 are overexpressed in a number of cancers, and current investigations have focused on transporter inhibition as a novel therapeutic strategy in cancers. MCT1 has also been used in strategies aimed at enhancing drug absorption due to its high expression in the intestine. Other MCT isoforms are less well characterized, but ongoing studies indicate that MCT6 transports xenobiotics such as bumetanide, nateglinide, and probenecid, whereas MCT7 has been characterized as a transporter of ketone bodies. MCT8 and MCT10 transport thyroid hormones, and recently, MCT9 has been characterized as a carnitine efflux transporter and MCT12 as a creatine transporter. Expressed at the blood brain barrier, MCT8 mutations have been associated with an X-linked intellectual disability, known as Allan-Herndon-Dudley syndrome. Many MCT isoforms are associated with hormone, lipid, and glucose homeostasis, and recent research has focused on their potential roles in disease, with MCTs representing promising novel therapeutic targets. This review will provide a summary of the current literature focusing on the characterization, function, and regulation of the MCT family isoforms and on their roles in drug disposition and in health and disease. SIGNIFICANCE STATEMENT: The 14-member solute carrier family 16 of monocarboxylate transporters (MCTs) plays a fundamental role in maintaining intracellular concentrations of a broad range of important endogenous molecules in health and disease. MCTs 1, 2, and 4 (L-lactate transporters) are overexpressed in cancers and represent a novel therapeutic target in cancer. Recent studies have highlighted the importance of MCTs in glucose, lipid, and hormone homeostasis, including MCT8 in thyroid hormone brain uptake, MCT12 in carnitine transport, and MCT11 in type 2 diabetes.

Molecular Characteristics, Regulation, and Function of Monocarboxylate Transporters

Lactate transporters play an important role in the glutamate recycling. Here their kinetics and tissue distribution with emphasis on the brain are addressed. Recent evidence shows their participation in important brain functions that involve intercellular communication, such as hypothalamic glucose sensing. Furthermore, we describe the regulation of their expression and some animal models that have allowed clarification of their functions.

Crucial residue involved in L-lactate recognition by human monocarboxylate transporter 4 (hMCT4)

Background

Monocarboxylate transporters (MCTs) transport monocarboxylates such as lactate, pyruvate and ketone bodies. These transporters are very attractive therapeutic targets in cancer. Elucidations of the functions and structures of MCTs is necessary for the development of effective medicine which targeting these proteins. However, in comparison with MCT1, there is little information on location of the function moiety of MCT4 and which constituent amino acids govern the transport function of MCT4. The aim of the present work was to determine the molecular mechanism of L-lactate transport via hMCT4.

Experimental approach

Transport of L-lactate via hMCT4 was determined by using hMCT4 cRNA-injected Xenopus laevis oocytes. hMCT4 mediated L-lactate uptake in oocytes was measured in the absence and presence of chemical modification agents and 4,4′-diisothiocyanostilbene-2,2′-disulphonate (DIDS). In addition, L-lactate uptake was measured by hMCT4 arginine mutants. Immunohistochemistry studies revealed the localization of hMCT4.

Results

In hMCT4-expressing oocytes, treatment with phenylglyoxal (PGO), a compound specific for arginine residues, completely abolished the transport activity of hMCT4, although this abolishment was prevented by the presence of L-lactate. On the other hand, chemical modifications except for PGO treatment had no effect on the transport activity of hMCT4. The transporter has six conserved arginine residues, two in the transmembrane-spanning domains (TMDs) and four in the intracellular loops. In hMCT4-R278 mutants, the uptake of L-lactate is void of any transport activity without the alteration of hMCT4 localization.

Conclusions

Our results suggest that Arg-278 in TMD8 is a critical residue involved in substrate, L-lactate recognition by hMCT4.

Metabolomic profiling analysis reveals chamber-dependent metabolite patterns in the mouse heart

Energy of the cardiac muscle largely depends on fatty acid oxidation. It is known that the atrium and ventricle have chamber-specific functions, structures, gene expressions, and pathologies. The left ventricle works as a high-pressure chamber to pump blood toward the body, and its muscle wall is thicker than those of the other chambers, suggesting that energy utilization in each of the chambers should be different. However, a chamber-specific pattern of metabolism remains incompletely understood. Recently, innovative techniques have enabled the comprehensive analysis of metabolites. Therefore, we aimed to clarify differences in metabolic patterns among the chambers. Male C57BL6 mice at 6 wk old were subject to a comprehensive measurement of metabolites in the atria and ventricles by capillary electrophoresis and mass spectrometry. We found that overall metabolic profiles, including nucleotides and amino acids, were similar between the right and left ventricles. On the other hand, the atria exhibited a distinct metabolic pattern from those of the ventricles. Importantly, the high-energy phosphate pool (the total concentration of ATP, ADP, and AMP) was higher in both ventricles. In addition, the levels of lactate, acetyl CoA, and tricarboxylic acid cycle contents were higher in the ventricles. Accordingly, the activities and/or expression levels of key enzymes were higher in the ventricles to produce more energy. The present study provides a basis for understanding the chamber-specific metabolism underlining pathophysiology in the heart.

Effect of growth on monocarboxylate transporters and indicators of energy metabolism in the gluteus medius muscle of Thoroughbreds

OBJECTIVE

To examine the changes in monocarboxylate transporter (MCT) 1 and MCT4 content and in indicators of energy metabolism in the gluteus medius muscle (GMM) of Thoroughbreds during growth.

ANIMALS

6 Thoroughbreds (3 males and 3 females).

PROCEDURES

Samples of GMM were obtained when horses were 2, 6, 12, and 24 months old. Muscle proteins were separated via SDS-PAGE; amounts of MCT1 and MCT4 and peroxisome proliferator-activated receptor-γ coactivator-1α content were determined by use of western blotting. Muscle activities of phosphofructokinase and citrate synthase were measured biochemically; lactate dehydrogenase isoenzymes were separated by agarose gel electrophoresis and quantified.

RESULTS

Compared with findings when horses were 2 months old, MCT1 protein content in GMM samples obtained when the horses were 24 months old was significantly higher; however, MCT4 protein content remained unchanged throughout the study period. Peroxisome proliferator-activated receptor-γ coactivator-1α content was significantly increased at 24 months of age and citrate synthase activity was increased at 6 and 24 months of age, compared with findings at 2 months. Phosphofructokinase activity remained unaltered during growth. The percentage contributions of lactate dehydrogenase 1 and 2 isoenzymes to the total amount of all 5 isoenzymes at 12 and 24 months of age were significantly higher than those at 2 months of age.

CONCLUSIONS AND CLINICAL RELEVANCE

Changes in protein contents of MCTs and the lactate dehydrogenase isoenzyme profile in GMM samples suggested that lactate usage capacity increases with growth and is accompanied by an increase in the oxidative capacity in Thoroughbreds.

PGC-1alpha increases skeletal muscle lactate uptake by increasing the expression of MCT1 but not MCT2 or MCT4

We examined the relationship between PGC-1alpha protein; the monocarboxylate transporters MCT1, 2, and 4; and CD147 1) among six metabolically heterogeneous rat muscles, 2) in chronically stimulated red (RTA) and white tibialis (WTA) muscles (7 days), and 3) in RTA and WTA muscles transfected with PGC-1alpha-pcDNA plasmid in vivo. Among rat hindlimb muscles, there was a strong positive association between PGC-1alpha and MCT1 and CD147, and between MCT1 and CD147. A negative association was found between PGC-1alpha and MCT4, and CD147 and MCT4, while there was no relationship between PGC-1alpha or CD147 and MCT2. Transfecting PGC-1alpha-pcDNA plasmid into muscle increased PGC-1alpha protein (RTA +23%; WTA +25%) and induced the expression of MCT1 (RTA +16%; WTA +28%), but not MCT2 and MCT4. As a result of the PGC-1alpha-induced upregulation of MCT1 and its chaperone CD147 (+29%), there was a concomitant increase in the rate of lactate uptake (+20%). In chronically stimulated muscles, the following proteins were upregulated, PGC-1alpha in RTA (+26%) and WTA (+86%), MCT1 in RTA (+61%) and WTA (+180%), and CD147 in WTA (+106%). In contrast, MCT4 protein expression was not altered in either RTA or WTA muscles, while MCT2 protein expression was reduced in both RTA (-14%) and WTA (-10%). In these studies, whether comparing oxidative capacities among muscles or increasing their oxidative capacities by PGC-1alpha transfection and chronic muscle stimulation, there was a strong relationship between the expression of PGC-1alpha and MCT1, and PGC-1alpha and CD147 proteins. Thus, MCT1 and CD147 belong to the family of metabolic genes whose expression is regulated by PGC-1alpha in skeletal muscle.

Organ growth functions in maturing male Sprague-Dawley rats based on a collective database

Ten different organ weights (liver, spleen, kidneys, heart, lungs, brain, adrenals, testes, epididymes, and seminal vesicles) of male Sprague-Dawley (S-D) rats of different ages (1-280 d) were extracted based on a thorough literature survey database. A generalized Michaelis-Menten (GMM) model, used to fit organ weights versus age in a previous study (Schoeffner et al., 1999) based on a limited data, was used to find the best fit model for the present expanded data compilation. The GMM model has the functional form: Wt = (Wt(o).K(gamma) + Wt(max).Age(gamma))/(K(gamma) + Age(gamma)) where Wt is organ/tissue weight at a specified age, Wt(o) and Wt(max) are weight at birth and maximal growth, respectively, and K and gamma are constants. Organ weights were significantly correlated with their respective ages for all organs and tissues. GMM-derived organ growth and percent body weight (%BW) fractions of different tissues were plotted against animal age and compared with experimental values as well as previously published models. The GMM-based organ growth and %BW fraction profiles were in general agreement with our empirical data as well as with previous studies. The present model was compared with the GMM model developed previously for six organs--liver, spleen, kidneys, heart, lungs, and brain--based on a limited data, and no significant difference was noticed between the two sets of predictions. It was concluded that the GMM models presented herein for different male S-D rats organs (liver, spleen, kidneys, heart, lungs, brain, adrenals, testes, epididymes, and seminal vesicles) are capable of predicting organ weights and %BW ratios accurately at different ages.

Testosterone increases lactate transport, monocarboxylate transporter (MCT) 1 and MCT4 in rat skeletal muscle

We have examined the effects of administration of testosterone for 7 days on monocarboxylate transporter (MCT) 1 and MCT4 mRNAs and proteins in seven metabolically heterogeneous rat hindlimb muscles and in the heart. In addition, we also examined the effects of testosterone treatment on plasmalemmal MCT1 and MCT4, and lactate transport into giant sarcolemmal vesicles prepared from red and white hindlimb muscles and the heart. Testosterone did not alter MCT1 or MCT4 mRNA, except in the plantaris muscle. Testosterone increased MCT1 (20%-77%, P < 0.05) and MCT4 protein (29%-110%, P< 0.05) in five out of seven muscles examined. In contrast, in the heart MCT1 protein was not increased (P> 0.05), and MCT 4 mRNA and protein were not detected. There was no correlation between the testosterone-induced increments in MCT1 and MCT4 proteins. Muscle fibre composition was not associated with testosterone-induced increments in MCT1 protein. In contrast, there was a strong positive relationship between the testosterone-induced increments in MCT4 protein and the fast-twitch fibre composition of rat muscles. Lactate transport into giant sarcolemmal vesicles was increased in red (23%, P< 0.05) and white muscles (21%, P< 0.05), and in the heart (58%, P< 0.05) of testosterone-treated animals (P< 0.05). However, plasmalemmal MCT1 protein (red, +40%, P< 0.05; white, +39%, P< 0.05) and plasmalemmal MCT4 protein (red, +25%, P< 0.05; white, +48%, P< 0.05) were increased only in skeletal muscle. In the heart, plasmalemmal MCT1 protein was reduced (-20%, P< 0.05). In conclusion, these studies have shown that testosterone induces an increase in both MCT1 and MCT4 proteins and their plasmalemmal content in skeletal muscle. However, the testosterone-induced effect was tissue-specific, as MCT1 protein expression was not altered in the heart. In the heart, the testosterone-induced increase in lactate transport cannot be explained by changes in plasmalemmal MCT1 content, but in skeletal muscle the increase in the rate of lactate transport was associated with increases in plasmalemmal MCT1 and MCT4.

Distribution of monocarboxylate transporters MCT1-MCT8 in rat tissues and human skeletal muscle

In the past decade, a family of monocarboxylate transporters (MCTs) have been identified that can potentially transport lactate, pyruvate, ketone bodies, and branched-chain ketoacids. Currently, 14 such MCTs are known. However, many orphan transporters exist that have transport capacities that remain to be determined. In addition, the tissue distribution of many of these MCTs is not well defined. Such a cataloging can, at times, begin to suggest the metabolic role of a particular MCT. Recently, a number of antibodies against selected MCTs (MCT1, -2, -4, and -5 to -8) have become commercially available. Therefore, we examined the protein expression of these MCTs in a large number of rat tissues (heart, skeletal muscle, skin, brain, testes, vas deferens, adipose tissue, liver, kidney, spleen, and pancreas), as well as in human skeletal muscle. Unexpectedly, many tissues coexpressed 4-5 MCTs. In particular, in rat skeletal muscle MCT1, MCT2, MCT4, MCT5, and MCT6 were observed. In human muscle, these same MCTs were present. We also observed a pronounced MCT7 signal in human muscle, whereas a very faint signal occurred for MCT8. In rat heart, which is an important metabolic sink for lactate, we confirmed that MCT1 and -2 were expressed. In addition, MCT6 and -8 were also prominently expressed in this tissue, although it is known that MCT8 does not transport aromatic amino acids or lactate. This catalog of MCTs in skeletal muscle and other tissues has revealed an unexpected complexity of coexpression, which makes it difficult to associate changes in monocarboxylate transport with the expression of a particular MCT. The differences in transport kinetics for lactate and pyruvate are only known for MCT1, -2 and -4. Transport kinetics remain to be established for many other MCTs. In conclusion, this study suggests that in skeletal muscle, as well as other tissues, lactate and pyruvate transport rates may not only involve MCT1 and -4, as other monocarboxylate transporters are also expressed in rat (MCT2, -5, -6) and human skeletal muscle (MCT2, -5, -6, -7).

Tissue-Specific and Fatty Acid Transporter-Specific Changes in Heart and Soleus Muscle Over a 1-yr Period

Rates of fatty acid oxidation increase rapidly in both rat heart and skeletal muscle in the early postnatal period. Therefore, we examined in heart and soleus muscle, (a) whether there were rapid changes in fatty acid transporter (FAT/CD36, FABPpm) mRNA and protein expression early in life (days 10 -36) and thereafter (days 84, 160, 365), and (b) whether the rates of fatty acid transport and the plasmalemmal content of FAT/CD36 and FABPpm were altered. Protein expression was altered rapidly from day 10-36 in both heart (FAT/CD36 only, +21%, P < 0.05)) and soleus muscle (FAT/CD36 + 100%, P < 0.05; FABPpm -20%, P < 0.05), with no further changes thereafter (P < 0.05). Rates of fatty acid transport (day 10 vs day 160) were increased in heart (+33%, P < 0.05) and muscle (+85%, P < 0.05), and were associated with concomitant increases in plasmalemmal FABPpm (+44%, P < 0.05) and FAT/CD36 (+16%, P < 0.05) in the heart, and only plasmalemmal FAT/CD36 in muscle (+90%, P < 0.05). Therefore, known changes in the rates of fatty acid oxidation in heart and muscle early in life appear to be accompanied by a concurrent upregulation in the rates of fatty acid transport and the expression of FAT/CD36 in heart and muscle, as well as an increase in plasmalemmal FAT/CD36 and FABPpm in the heart, and only plasmalemmal FAT/CD36 in soleus muscle. We speculate that the rapid upregulation of fatty acid transport rates in heart and muscle are needed to support the increased rates of fatty oxidation that have been previously observed in these tissues.

MCT-4, A511/Basigin and EF5 expression patterns during early chick cardiomyogenesis indicate cardiac cell differentiation occurs in a hypoxic environment

We have identified the presence of the hypoxia marker EF5 in the stage 4/5 chick heart fields. This suggests that cardiac cell differentiation occurs in a relatively anaerobic environment. Monocarboxylate transporter (MCT) studies in adult cardiac myocytes have demonstrated that MCTs catalyze proton-linked pyruvate and lactate transport activity. 5A11/Basigin is an ancillary protein that targets MCTs to the plasma membrane for their function. MCT-4 expression is most evident in cells with a high glycolytic rate associated with hypoxic energy production. Subsequent to the immunohistochemical localization of EF5 in the early heart field, we continued in our analysis during stages 5 to 12 for the expression of indicators of cellular glycolytic metabolism in the developing heart, such as MCT-4, MCT-1, and 5A11 (Basigin/CD147). Our observations indicate that MCT-4 and 5A11/Basigin are expressed early, in a differential left-right pattern, in the bi-lateral plate mesoderm, as the cardiac compartment is forming. At stage 11, MCT-4/5A11 continues to be highly expressed in the myocardial wall of the looping heart, but not in the dorsal mesocardium. RT-PCR analyses for MCT-1, -4, and 5A11 indicate that MCT-4 and 5A11 are expressed throughout precardiac, embryonic, and fetal stages in the heart. MCT-1 is first detected in the heart on embryonic day 3 and then remains expressed throughout development to hatching. These results indicate that cardiac precursor cells are equipped for differentiating in a hypoxic environment using anaerobic metabolism for energy production.

Monocarboxylate transporters in subsarcolemmal and intermyofibrillar mitochondria

Whether subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria contain monocarboxylate transporters (MCTs) is controversial. We have examined the presence of MCT1, 2, and 4 in highly purified SS and IMF mitochondria. These mitochondria were not contaminated with plasma membrane, sarcoplasmic reticulum or endosomal compartments, as the marker proteins for these sub-cellular compartments (Na+-K+-ATPase, Ca2+-ATPase, and the transferrin receptor) were not present in SS or IMF mitochondria. MCT1, MCT2, and MCT4 were all present at the plasma membrane. However, MCT1 and MCT4 were associated with SS mitochondria. In contrast, the IMF mitochondria were completely devoid of MCT1 and MCT4. However, MCT2 was associated with both SS and IMF mitochondria. These observations suggest that SS and IMF mitochondria have different capacities for metabolizing monocarboxylates. Thus, the controversy as to whether mitochondria can take up and oxidize lactate will need to take account of the different distribution of MCTs between SS and IMF mitochondria.

Relationship between skeletal muscle MCT1 and accumulated exercise during voluntary wheel running

We examined whether the quantity of exercise performed influences the expression of monocarboxylate transporter (MCT) 1 and MCT4 in mouse skeletal muscles (plantaris, tibialis anterior, soleus) and heart. Wheel running exercise (1, 3, and 6 wk) was used, which results in marked variations in self-selected running activity. Differences in muscle MCT1 and MCT4 among animals, before the initiation of running, were not related to the quantity of exercise performed on the first day of wheel running. No changes in MCT4 were observed over the course of the study (P > 0.05). After 6 wk of running, were there significant increases in heart (50%; P < 0.05) and muscle MCT1 (31-60%; P < 0.05) but not after 1 and 3 wk (P > 0.05). Because skeletal muscle MCT1 and running distances varied considerably, we examined the relationship between these two parameters. Within the first week of training, MCT1 was negatively correlated with the accumulated running distance (r = -0.70, P < 0.05). On further analysis, it appears that, in the first week, excessive running (>20 km/wk) represses MCT1 (-16.1%; P < 0.05), whereas more modest amounts of running (<20 km/wk) increase MCT1 (+37%; P < 0.05). After 3 wk of running, a positive relationship was observed between MCT1 and running distance (r = +0.76), although there is a threshold that must be exceeded before an increase over the control animals occurs. Finally, in week 6, when MCT1 was increased in the tibialis anterior and plantaris muscles, there were no correlations with the accumulated running distances. These studies have shown that mild exercise training fails to increase MCT4 and that changes in MCT1 are complex, depending not only the accumulated exercise but also on the stage of training.

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.

Pyruvate shuttle in muscle cells: high-affinity pyruvate transport sites insensitive to trans-lactate efflux

The specificity of the transport mechanisms for pyruvate and lactate and their sensitivity to inhibitors were studied in L6 skeletal muscle cells. Trans- and cis-lactate effects on pyruvate transport kinetic parameters were examined. Pyruvate and lactate were transported by a multisite carrier system, i.e., by two families of sites, one with low affinity and high capacity (type I sites) and the other with high affinity and low capacity (type II). The multisite character of transport kinetics was not modified by either hydroxycinnamic acid (CIN) or p-chloromercuribenzylsulfonic acid (PCMBS), which exert different types of inhibition. The transport efficiency (TE) ratios of maximal velocity to the trans-activation dissociation constant (Kt) showed that lactate and pyruvate were preferentially transported by types I and II sites, respectively. The cis-lactate effect was observed with high Ki values for both sites. The trans-lactate effect on pyruvate transport occurred only on type I sites and exhibited an asymmetric interaction pattern (Kt of inward lactate > Kt of outward lactate). The inability of lactate to trans-stimulate type II sites suggests that intracellular lactate cannot recruit these sites. The high-affinity type II sites act as a specific pyruvate shuttle and constitute an essential relay for the intracellular lactate shuttle.

T3 increases lactate transport and the expression of MCT4, but not MCT1, in rat skeletal muscle

Triiodothyronine (T3) regulates the expression of genes involved in muscle metabolism. Therefore, we examined the effects of a 7-day T3 treatment on the monocarboxylate transporters (MCT)1 and MCT4 in heart and in red (RG) and white gastrocnemius muscle (WG). We also examined rates of lactate transport into giant sarcolemmal vesicles and the plasmalemmal MCT1 and MCT4 in these vesicles. Ingestion of T3 markedly increased circulating serum T3 (P < 0.05) and reduced weight gain (P < 0.05). T3 upregulated MCT1 mRNA (RG +77, WG +49, heart +114%, P < 0.05) and MCT4 mRNA (RG +300, WG +40%). However, only MCT4 protein expression was increased (RG +43, WG +49%), not MCT1 protein expression. No changes in MCT1 protein were observed in any tissue. T3 treatment doubled the rate of lactate transport when vesicles were exposed to 1 mM lactate (P < 0.05). However, plasmalemmal MCT4 was only modestly increased (+13%, P < 0.05). We conclude that T3 1) regulates MCT4, but not MCT1, protein expression and 2) increases lactate transport rates. This latter effect is difficult to explain by the modest changes in plasmalemmal MCT4. We speculate that either the activity of sarcolemmal MCTs has been altered or else other MCTs in muscle may have been upregulated.

Effect of high-intensity training on exercise-induced gene expression specific to ion homeostasis and metabolism

Changes in gene expression during recovery from high-intensity, intermittent, one-legged exercise were studied before and after 5.5 wk of training. Genes related to metabolism, as well as Na+, K+, and pH homeostasis, were selected for analyses. After the same work was performed before and after the training period, several muscle biopsies were obtained from vastus lateralis muscle. In the untrained state, the Na+-K+-ATPase alpha1-subunit mRNA level was approximately threefold higher (P < 0.01) at 0, 1, and 3 h after exercise, relative to the preexercise resting level. After 3-5 h of recovery in the untrained state, pyruvate dehydrogenase kinase 4 and hexokinase II mRNA levels were elevated 13-fold (P < 0.001) and 6-fold (P < 0.01), respectively. However, after the training period, only pyruvate dehydrogenase kinase 4 mRNA levels were elevated (P < 0.05) during the recovery period. No changes in resting mRNA levels were observed as a result of training. In conclusion, cellular adaptations to high-intensity exercise training may, in part, be induced by transcriptional regulation. After training, the transcriptional response to an exercise bout at a given workload is diminished.

Exercise training alleviates MCT1 and MCT4 reductions in heart and skeletal muscles of STZ-induced diabetic rats

We compared the changes in monocarboxylate transporter 1 (MCT1) and 4 (MCT4) proteins in heart and skeletal muscles in sedentary control and streptozotocin (STZ)-induced diabetic rats (3 wk) and in trained (3 wk) control and STZ-induced diabetic animals. In nondiabetic animals, training increased MCT1 in the plantaris (+51%; P < 0.01) but not in the soleus (+9%) or the heart (+14%). MCT4 was increased in the plantaris (+48%; P < 0.01) but not in the soleus muscles of trained nondiabetic animals. In sedentary diabetic animals, MCT1 was reduced in the heart (-30%), and in the plantaris (-31%; P < 0.01) and soleus (-26%) muscles. MCT4 content was also reduced in sedentary diabetic animals in the plantaris (-52%; P < 0.01) and soleus (-25%) muscles. In contrast, in trained diabetic animals, MCT1 and MCT4 in heart and/or muscle were similar to those of sedentary, nondiabetic animals (P > 0.05) but were markedly greater than in the sedentary diabetic animals [MCT1: plantaris +63%, soleus +51%, heart +51% (P > 0.05); MCT4: plantaris +107%, soleus +17% (P > 0.05)]. These studies have shown that 1) with STZ-induced diabetes, MCT1 and MCT4 are reduced in skeletal muscle and/or the heart and 2) exercise training alleviated these diabetes-induced reductions.

A 44-kDa of protein identical to the N-terminal amino acid sequence of MCT1 in human circulation

A family of specific carrier protein designated as monocarboxylate transporter (MCT) has been known to transport the lactate and other moncarboxylates in mammalian cells. We hypothesized the presence of serum protein in human circulation that may works as a lactate carrier and that biochemical structure would possesses common structure with MCT on the plasma membrane. Immunoblot analysis with an anti-MCT1 polyclonal antibody suggested the presence of a 44-kDa protein in human circulation and N-terminal amino acid sequencing exhibited a stretch of 14 amino acids which is completely identical to MCT1. The unbound fractions from the GST-MCTI fusion protein-immobilized glutathione sepharose 4B column demonstrated that lactic acid concentration began to increase with one fraction delay compared to Sepharose 4B and GST-immobilized column. When lactic acid was washed away with PBS, lactic acid concentrations in the effuluent constantly decreased in both Sepharose 4B and GST-immobilized column. However, GST-MCT1-immobilized column showed specific convex curve from fraction approximately 3 mM of lactate and demonstrated wash out delay compared to Sepharose 4B and GST-immobilized column. These observations demonstrated biochemical and immunological similarities between a 44-kDa protein purified from human serum and MCT1 present on the plasma membrane. The studies on MCT1-fusion protein suggested possible functional properties of a 44-kDa protein as a lactate buffer by holding and unhand a lactate according to the lactate concentration in human blood. The experiments described herein have suggested the existence of lactate carrier in human circulation which is free from plasma membrane.

Changes in MCT 1, MCT 4, and LDH expression are tissue specific in rats after long-term hypobaric hypoxia

Little is known about the effect of chronic hypobaric hypoxia on the enzymes and transporters involved in lactate metabolism. We looked at the protein expression of monocarboxylate transporters MCT 1, MCT 2, and MCT 4, along with total lactate dehydrogenase (LDH) and LDH isozymes in skeletal muscle, cardiac muscle, and liver. Expression of these components of the lactate shuttle affects the ability to transport and oxidize lactate. We hypothesized that the expression of MCTs and LDH would increase after acclimation to high altitude (HA). The response to acclimation to HA was, however, tissue specific. In addition, the response was different in whole muscle (Mu) and mitochondria-enriched (Mi) fractions. Heart, soleus, and plantaris muscles showed the greatest response to HA. Acclimation resulted in a 34% increase in MCT 4 in heart and a decrease in MCT 1 (-47%) and MCT 4 (-47%) in plantaris Mu. In Mi fractions, the heart had an increase (+40%) and soleus a decrease (-40%) in LDH. HA also had a significant effect on the LDH isozyme composition of both the Mu and Mi fractions. Mitochondrial density was decreased in both the soleus (-17%) and plantaris (-44%) as a result of chronic hypoxia. We conclude that chronic hypoxia had a tissue-specific effect on MCTs and LDH (that form the lactate shuttle) but did not produce a consistent increase in these components in all tissues.