Effect of food restriction on lactate sarcolemmal transport

Muscle MCT4 Content Is Correlated with the Lactate Removal Ability during Recovery Following All-Out Supramaximal Exercise in Highly-Trained Rowers

The purpose of this study was to test if the lactate exchange (γ₁) and removal (γ₂) abilities during recovery following short all-out supramaximal exercise correlate with the muscle content of MCT1 and MCT4, the two isoforms of the monocarboxylate transporters family involved in lactate and H⁺ co-transport in skeletal muscle. Eighteen lightweight rowers completed a 3-min all-out exercise on rowing ergometer. Blood lactate samples were collected during the subsequent passive recovery to assess an individual blood lactate curve (IBLC). IBLC were fitted to the bi-exponential time function: La(t) = [La](0) + A₁(1 − e-γ1t) + A₂(1 − e-γ2t) where [La](0) is the blood lactate concentration at exercise completion and the velocity constants γ₁ and γ₂ denote the lactate exchange and removal abilities, respectively. An application of the bi-compartmental model of lactate distribution space allowed estimation of the lactate removal rate at exercise completion [LRR(0)]. Biopsy of the right vastus lateralis was taken at rest to measure muscle MCT1 and MCT4 content. Fiber type distribution, activity of key enzymes and capillary density (CD) were also assessed. γ₁ was correlated with [La](0) (r = −0.54, P < 0.05) but not with MCT1, MCT4 or CD. γ₂ and LRR(0) were correlated with MCT4 (r = 0.63, P < 0.01 and r = 0.73, P < 0.001, respectively) but not with MCT1 or cytochrome c oxidase activity. These findings suggest that the lactate exchange ability is highly dependent on the milieu so that the importance of the muscle MCT1 and MCT4 content in γ₁ was hidden in the present study. Our results also suggest that during recovery following all-out supramaximal exercise in well-trained rowers, MCT4 might play a significant role in the distribution and delivery of lactate for its subsequent removal.

Monocarboxylate transporters: new players in body weight regulation

Over the last two decades, several genes have been identified that appear to play a role in the regulation of energy homeostasis and body weight. For a small subset of them, a reduction or an absence of expression confers a resistance to the development of obesity. Recently, a knockin mouse for a member of the monocarboxylate transporter (MCT) family, MCT1, was demonstrated to exhibit a typical phenotype of resistance to diet-induced obesity and a protection from its associated metabolic perturbations. Such findings point out at MCTs as putatively new therapeutic targets in the context of obesity. Here, we will review what is known about MCTs and their possible metabolic roles in different organs and tissues. Based on the description of the phenotype of the MCT1 knockin mouse, we will also provide some insights about their putative roles in weight gain regulation.

Effects of acute and chronic exercise on sarcolemmal MCT1 and MCT4 contents in human skeletal muscles: current status

Two lactate/proton cotransporter isoforms (monocarboxylate transporters, MCT1 and MCT4) are present in the plasma (sarcolemmal) membranes of skeletal muscle. Both isoforms are symports and are involved in both muscle pH and lactate regulation. Accordingly, sarcolemmal MCT isoform expression may play an important role in exercise performance. Acute exercise alters human MCT content, within the first 24 h from the onset of exercise. The regulation of MCT protein expression is complex after acute exercise, since there is not a simple concordance between changes in mRNA abundance and protein levels. In general, exercise produces greater increases in MCT1 than in MCT4 content. Chronic exercise also affects MCT1 and MCT4 content, regardless of the initial fitness of subjects. On the basis of cross-sectional studies, intensity would appear to be the most important factor regulating exercise-induced changes in MCT content. Regulation of skeletal muscle MCT1 and MCT4 content by a variety of stimuli inducing an elevation of lactate level (exercise, hypoxia, nutrition, metabolic perturbations) has been demonstrated. Dissociation between the regulation of MCT content and lactate transport activity has been reported in a number of studies, and changes in MCT content are more common in response to contractile activity, whereas changes in lactate transport capacity typically occur in response to changes in metabolic pathways. Muscle MCT expression is involved in, but is not the sole determinant of, muscle H(+) and lactate anion exchange during physical activity.

Effect of weight loss on lactate transporter expression in skeletal muscle of obese subjects

The effects of weight loss on skeletal muscle lactate transporter [monocarboxylate transporter (MCT)] expression in obese subjects were investigated to better understand how lactate transporter metabolism is regulated in insulin-resistant states. Ten obese subjects underwent non-macronutrient-specific energy restriction for 15 wk. Anthropometric measurements and a needle biopsy of the vastus lateralis muscle before and after the weight loss program were performed. Enzymatic activity, fiber type distribution, and skeletal muscle MCT protein expression were measured. Muscle from nonobese control subjects was used for comparison of MCT levels. The program induced a weight loss of 9.2 ± 1.6 kg. Compared with controls, muscle from obese subjects showed a strong tendency ( P = 0.06) for elevated MCT4 expression (+69%) before the weight loss program. MCT4 expression decreased (−7%) following weight loss to reach levels that were not statistically different from control levels. There were no differences in MCT1 expression between controls and obese subjects before and after weight loss. A highly predictive regression model ( R2= 0.93), including waist circumference, citrate synthase activity, and percentage of type 1 fibers, was found to explain the highly variable MCT1 response to weight loss in the obese group. Therefore, in obesity, MCT1 expression appears linked both to changes in oxidative parameters and to changes in visceral adipose tissue content. The strong tendency for elevated expression of muscle MCT4 could reflect the need to release greater amounts of muscle lactate in the obese state, a situation that would be normalized with weight loss as indicated by decreased MCT4 levels.

Skeletal muscle expression of LDH and monocarboxylate transporters in growing rats submitted to protein malnutrition

BACKGROUND

In different circumstances such as infant malnutrition, old age or chronic disease, decline of muscular strength, particularly anaerobic power, is shown. In this context, our laboratory, has demonstrated a decrease in anaerobic glycolytic power in pre-pubertal Bolivian children living at low and high altitude and suffering from marginal protein malnutrition.

AIM OF THE STUDY

To bring molecular support to the relationship between protein malnutrition and anaerobic glycolytic metabolism, we studied the impact of prolonged protein malnutrition on lactate metabolism in different muscles of growing rats. Lactate dehydrogenase (LDH), monocarboxylate transporters (MCT1, MCT4) and membrane protein CD147 were chosen as specific markers of anaerobic glycolytic metabolism.

METHODS

Two groups of 10 weaning male rats were fed for 10 weeks either ad libitum with a well-balanced diet containing 18% protein or an isocaloric-diet containing 8% protein. LDH activity and mRNA amounts of LDH isoforms, MCT, CD147 were measured.

RESULTS

Protein deprivation during rat growth induced a decrease of LDH specific activity in skeletal muscles (mean value of -41%), accompanied by isoform distribution modifications in soleus, but not in glycolytic muscles (extensor digitorum longus (EDL) or plantaris). A reduction in mRNA amounts encoding the LDH A and B subunits was observed in EDL. A decrease in LDH B mRNA amounts was monitored in plantaris, whereas no modification in both LDH isoform mRNA quantities was observed in soleus. MCT1 mRNA quantities were decreased in EDL but MCT4 mRNA quantities remained stable. CD147 mRNA amounts were unchanged except for EDL with a 42% increase.

CONCLUSIONS

The global decreases of LDH activity, LDH and MCT gene expressions in growing rat skeletal muscles support the observed alterations of lactate metabolism associated with lowered muscular anaerobic performances in protein malnutrition.

Role of hypoxia-induced anorexia and right ventricular hypertrophy on lactate transport and MCT expression in rat muscle

To dissect the independent effects of altitude-induced hypoxemia and anorexia on the capacity for cardiac lactate metabolism, we examined the effects of 21 days of chronic hypobaric hypoxia (CHH) and its associated decrease in food intake and right ventricle (RV) hypertrophy on the monocarboxylate transporter 1 and 4 (MCT) expression, the rate of lactate uptake into sarcolemmal vesicles, and the activity of lactate dehydrogenase isoforms in rat muscles. In comparison with control rats (C), 1 mmol/L lactate transport measured on skeletal muscle sarcolemmal vesicles increased by 33% and 58% in hypoxic (CHH, barometric pressure = 495 hPa) and rats pair-fed an equivalent quantity of food to that consumed by hypoxic animals, respectively. The increased lactate transport was higher in PF than in CHH animals ( P < .05). No associated change in the expression of MCT1 protein was observed in skeletal muscles, whereas MCT1 mRNA decreased in CHH rats, in comparison with C animals (42%, P < .05), partly related to caloric restriction (30%, P < .05). MCT4 mRNA and protein increased during acclimatization to hypoxia only in slow-oxidative muscles (68%, 72%, P < .05, respectively). The MCT4 protein content did not change in the plantaris muscle despite a decrease in transcript levels, related to hypoxia and caloric restriction. In both the left and right ventricles, the MCT1 protein content was unaffected by ambient hypoxia or restricted food consumption. These results suggest that MCT1 and MCT4 gene expression in fast-glycolytic muscles is mainly regulated by posttranscriptional mechanisms. Moreover, the results emphasize the role played by caloric restriction on the control of gene expression in response to chronic hypoxia and suggest that hypoxia-induced right ventricle hypertrophy failed to alter MCT proteins.

Inhibition of calcineurin increases monocarboxylate transporters 1 and 4 protein and glycolytic enzyme activities in rat soleus muscle

1. The present study was designed to examine the role of calcineurin in muscle metabolic components by the administration of the specific calcineurin inhibitor cyclosporine A (CsA) to rats. 2. Male Wistar rats were divided into either a CsA-treated group (CT) or a vehicle-treated group (VT). Cyclosporine A was administered subcutaneously to rats at a rate of 25 mg/kg bodyweight per day for 10 successive days. Thereafter, changes in muscle enzyme activities and glucose transporter (GLUT)-4 and monocarboxylate transporter (MCT)-1 and MCT-4 proteins in the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles were examined. 3. There was a significant increase in MCT-1 and MCT-4 proteins in the soleus muscle in the CT group, but not in the EDL muscle. The activities of hexokinase, pyruvate kinase and lactate dehydrogenase in the soleus muscle also increased significantly in the CT group, but a similar increase in enzyme activity was not seen in EDL muscle. The activities of citrate synthase or malate dehydrogenase and the GLUT-4 protein content were not altered by CsA treatment in either the soleus or EDL muscles. 4. These results seem to imply that calcineurin negatively regulates the components of glucose/lactate metabolism, except for GLUT-4, especially in slow-twitch muscle.