Muscle lactate transport studied in sarcolemmal giant vesicles: dependence on fibre type and age

Decreased Blood Glucose and Lactate: Is a Useful Indicator of Recovery Ability in Athletes?

During low-intensity exercise stages of the lactate threshold test, blood lactate concentrations gradually diminish due to the predominant utilization of total fat oxidation. However, it is unclear why blood glucose is also reduced in well-trained athletes who also exhibit decreased lactate concentrations. This review focuses on decreased glucose and lactate concentrations at low-exercise intensity performed in well-trained athletes. During low-intensity exercise, the accrued resting lactate may predominantly be transported via blood from the muscle cell to the liver/kidney. Accordingly, there is increased hepatic blood flow with relatively more hepatic glucose output than skeletal muscle glucose output. Hepatic lactate uptake and lactate output of skeletal muscle during recovery time remained similar which may support a predominant Cori cycle (re-synthesis). However, this pathway may be insufficient to produce the necessary glucose level because of the low concentration of lactate and the large energy source from fat. Furthermore, fatty acid oxidation activates key enzymes and hormonal responses of gluconeogenesis while glycolysis-related enzymes such as pyruvate dehydrogenase are allosterically inhibited. Decreased blood lactate and glucose in low-intensity exercise stages may be an indicator of recovery ability in well-trained athletes. Athletes of intermittent sports may need this recovery ability to successfully perform during competition.

Lactate metabolism: historical context, prior misinterpretations, and current understanding

Lactate (La^-) has long been at the center of controversy in research, clinical, and athletic settings. Since its discovery in 1780, La^- has often been erroneously viewed as simply a hypoxic waste product with multiple deleterious effects. Not until the 1980s, with the introduction of the cell-to-cell lactate shuttle did a paradigm shift in our understanding of the role of La^- in metabolism begin. The evidence for La^- as a major player in the coordination of whole-body metabolism has since grown rapidly. La^- is a readily combusted fuel that is shuttled throughout the body, and it is a potent signal for angiogenesis irrespective of oxygen tension. Despite this, many fundamental discoveries about La^- are still working their way into mainstream research, clinical care, and practice. The purpose of this review is to synthesize current understanding of La^- metabolism via an appraisal of its robust experimental history, particularly in exercise physiology. That La^- production increases during dysoxia is beyond debate, but this condition is the exception rather than the rule. Fluctuations in blood [La^-] in health and disease are not typically due to low oxygen tension, a principle first demonstrated with exercise and now understood to varying degrees across disciplines. From its role in coordinating whole-body metabolism as a fuel to its role as a signaling molecule in tumors, the study of La^- metabolism continues to expand and holds potential for multiple clinical applications. This review highlights La^-'s central role in metabolism and amplifies our understanding of past research.

Importance of pH homeostasis in metabolic health and diseases: crucial role of membrane proton transport

Protons dissociated from organic acids in cells are partly buffered. If not, they are transported to the extracellular fluid through the plasma membrane and buffered in circulation or excreted in urine and expiration gas. Several transporters including monocarboxylate transporters and Na(+)/H(+) exchanger play an important role in uptake and output of protons across plasma membranes in cells of metabolic tissues including skeletal muscle and the liver. They also contribute to maintenance of the physiological pH of body fluid. Therefore, impairment of these transporters causes dysfunction of cells, diseases, and a decrease in physical performance associated with abnormal pH. Additionally, it is known that fluid pH in the interstitial space of metabolic tissues is easily changed due to little pH buffering capacitance in interstitial fluids and a reduction in the interstitial fluid pH may mediate the onset of insulin resistance unlike blood containing pH buffers such as Hb (hemoglobin) and albumin. In contrast, habitual exercise and dietary intervention regulate expression/activity of transporters and maintain body fluid pH, which could partly explain the positive effect of healthy lifestyle on disease prognosis.

Sex differences in the modulation of vasomotor sympathetic outflow during static handgrip exercise in healthy young humans

Sex differences in sympathetic neural control during static exercise in humans are few and the findings are inconsistent. We hypothesized women would have an attenuated vasomotor sympathetic response to static exercise, which would be further reduced during the high sex hormone [midluteal (ML)] vs. the low hormone phase [early follicular (EF)]. We measured heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA) in 11 women and 10 men during a cold pressor test (CPT) and static handgrip to fatigue with 2 min of postexercise circulatory arrest (PECA). HR increased during handgrip, reached its peak at fatigue, and was comparable between sexes. BP increased during handgrip and PECA where men had larger increases from baseline. Mean ± SD MSNA burst frequency (BF) during handgrip and PECA was lower in women (EF, P < 0.05), as was ΔMSNA-BF smaller (main effect, both P < 0.01). ΔTotal activity was higher in men at fatigue (EF: 632 ± 418 vs. ML: 598 ± 342 vs. men: 1,025 ± 416 a.u./min, P < 0.001 for EF and ML vs. men) and during PECA (EF: 354 ± 321 vs. ML: 341 ± 199 vs. men: 599 ± 327 a.u./min, P < 0.05 for EF and ML vs. men). During CPT, HR and MSNA responses were similar between sexes and hormone phases, confirming that central integration and the sympathetic efferent pathway was comparable between the sexes and across hormone phases. Women demonstrated a blunted metaboreflex, unaffected by sex hormones, which may be due to differences in muscle mass or fiber type and, therefore, metabolic stimulation of group IV afferents.

Importance of pH regulation and lactate/H+transport capacity for work production during supramaximal exercise in humans

We examine the influence of the cytosolic and membrane-bound contents of carbonic anhydrase (CA; CAII, CAIII, CAIV, and CAXIV) and the muscle content of proteins involved in lactate and proton transport [monocarboxylate transporter (MCT) 1, MCT4, and Na+/H+exchanger 1 (NHE1)] on work capacity during supramaximal exercise. Eight healthy, sedentary subjects performed exercises at 120% of the work rate corresponding to maximal oxygen uptake (Ẇmax) until exhaustion in placebo (Con) and metabolic alkalosis (Alk) conditions. The total (Wtot) and supramaximal work performed (Wsup) was measured. Muscle biopsies were obtained before and immediately after standardized exercises (se) at 120% Ẇmaxin both conditions to determine the content of the targeted proteins, the decrease in muscle pH (ΔpHm), and the muscle lactate accumulation ([Lac]m) per joule of Wsup(ΔpHm/Wsup-seand Δ[Lac]m/Wsup-se, respectively) and the dynamic buffer capacity. In Con, Wsupwas negatively correlated with ΔpHm/Wsup-se, positively correlated with Δ[Lac]m/Wsup-seand MCT1, and tended to be positively correlated with MCT4 and NHE1. CAII + CAIII were correlated positively with ΔpHm/Wsup-seand negatively with Δ[Lac]m/Wsup-se, while CAIV was positively related to Wtot. The changes in Wsupwith Alk were correlated positively with those in dynamic buffer capacity and negatively with Wsupin Con. Performance improvement with Alk was greater in subjects having a low content of proteins involved in pH regulation and lactate/proton transport. These results show the importance of pH regulating mechanisms and lactate/proton transport on work capacity and the role of the CA to delay decrease in pHmand accumulation in [Lac]mduring supramaximal exercise in humans.

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.

Lactate--a signal coordinating cell and systemic function

Since its first documented observation in exhausted animal muscle in the early 19th century, the role of lactate (lactic acid) has fascinated muscle physiologists and biochemists. Initial interpretation was that lactate appeared as a waste product and was responsible in some way for exhaustion during exercise. Recent evidence, and new lines of investigation, now place lactate as an active metabolite, capable of moving between cells, tissues and organs, where it may be oxidised as a fuel or reconverted to form pyruvate or glucose. The questions now to be asked concern the effects of lactate at the systemic and cellular level on metabolic processes. Does lactate act as a metabolic signal to specific tissues, becoming a metabolite pseudo-hormone? Does lactate have a role in whole-body coordination of sympathetic/parasympathetic nerve system control? And, finally, does lactate play a role in maintaining muscle excitability during intense muscle contraction? The concept of lactate acting as a signalling compound is a relatively new hypothesis stemming from a combination of comparative, cell and whole-organism investigations. It has been clearly demonstrated that lactate is capable of entering cells via the monocarboxylate transporter (MCT) protein shuttle system and that conversion of lactate to and from pyruvate is governed by specific lactate dehydrogenase isoforms, thereby forming a highly adaptable metabolic intermediate system. This review is structured in three sections, the first covering pertinent topics in lactate's history that led to the model of lactate as a waste product. The second section will discuss the potential of lactate as a signalling compound, and the third section will identify ways in which such a hypothesis might be investigated. In examining the history of lactate research, it appears that periods have occurred when advances in scientific techniques allowed investigation of this metabolite to expand. Similar to developments made first in the 1920s and then in the 1980s, contemporary advances in stable isotope, gene microarray and RNA interference technologies may allow the next stage of understanding of the role of this compound, so that, finally, the fundamental questions of lactate's role in whole-body and localised muscle function may be answered.

Metabolic support of moderate activity differs from patterns seen after extreme behavior in the desert iguana Dipsosaurus dorsalis

This study examined glucose and lactate metabolism in an iguanid lizard, Dipsosaurus dorsalis, during rest and after activity patterned on field behavior (15 s of running at 1 m/s). Metabolite oxidation and incorporation into glycogen by the whole animal, the liver, and oxidative and glycolytic muscle fibers were measured using (14)C- and (13)C-labeled compounds. Results showed that lactate metabolism is more responsive to changes that occurred between rest and recovery, whereas glucose appears to play a more steady state role. After activity, lactate oxidation produced 57 times as much ATP during 1 h of recovery than did glucose oxidation. However, lactate oxidation rates were elevated for only 30 min after activity, while glucose oxidation remained elevated beyond 1 h. Lactate was the primary source for glycogen synthesis during recovery, and glucose was the main glycogenic substrate during rest. This study supports previous research showing that brief activity in D. dorsalis is primarily supported by glycolysis and phosphocreatine breakdown, but it also suggests that there may be less of a reliance on glycolysis and a greater reliance on phosphocreatine than previously shown. The findings presented here indicate that the metabolic consequences of the behaviorally relevant activity studied are less severe than has been suggested by studies using more extreme activity patterns.

Skeletal muscle aging in F344BN F1-hybrid rats: II. Improved contractile economy in senescence helps compensate for reduced ATP-generating capacity

We used a pump-perfused rat hind-limb preparation to compare young adult (YA: 8-9- month-old), late middle-aged (LMA: 28-29-month-old), and senescent (SEN: 36-month-old) rats at similar rates of convective O(2) delivery during a 4-minute contraction bout. We hypothesized that not only would VO(2) and lactate production be reduced, but also that contractile economy would be altered with aging. Peak tension was lower in LMA (42%) and SEN (71%) versus YA. VO(2) and lactate efflux was progressively lower with increasing age. Estimated adenosine triphosphate per N of force was increased in LMA (35%) and reduced in SEN (31%) versus YA. Myosin heavy chain (MHC) analysis by sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed a lower MHC type IIb and higher MHC type IIa/IIx in SEN versus YA. Therefore, whereas contractile economy is impaired in LMA, it is improved in SEN, and this latter effect may be due in part to reduced type IIb MHC.

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 selection for growth rate on myosin heavy chain temporal and spatial localization during turkey breast muscle development

The temporal and spatial localization of the heavy chain fast form of myosin was studied during turkey pectoralis major muscle development in a randombred control line (RBC2), a subline (F) of RBC2 selected only for increased 16-wk BW, a commercial sire line (B), and reciprocal crosses of the F and B lines. Pectoralis major muscle samples were obtained from three females and three males from each group in a manner to avoid contraction. After fixing and sectioning, the muscle samples were stained with a monoclonal antibody to determine the temporal and spatial localization of the heavy chain fast form of myosin. The percentage of muscle fibers at 25 d of incubation and 1 wk posthatch expressing the fast form of myosin heavy chain was calculated. The average percentage of muscle fibers expressing the fast form of myosin heavy chain for all genetic lines combined at embryonic d 25 for males was 76.1 and for females 66.6, whereas at 1 wk posthatch the average percentage for males was 24.2 and 36.1 for females. No interaction of sex and genetic group was noted at either age. At 25 d of embryonic development and at 1 wk of age, additive and nonadditive genetic effects were important in the inheritance of the fast form of myosin heavy chain. Heterosis was negative at both ages but significant at 1 wk of age. By 4 wk posthatch, all the muscle fibers in each genetic group were expressing the myosin heavy chain fast form, and no sex differences were observed. At 16 wk posthatch muscle fiber fragmentation was noted in the samples having reduced endomysial spacing. In the fragmenting muscle fiber areas, expression of the heavy chain fast form of myosin was observed. These muscle fiber changes were predominant in the growth selected F-line suggesting that growth selection may be associated with muscle damage.

Influence of selection for breast muscle mass on myosin isoform composition and metabolism of deep pectoralis muscles of male and female turkeys

Advances in genetic selection and nutrition have resulted in rapid growth rates and increased muscle mass, predisposing turkeys to muscle disorders such as deep pectoral myopathies and increasing the incidence of pale, soft, and exudative muscle. The objective of this study was to determine if selection for breast muscle mass created an increase in anaerobic capacity of the deep pectoralis muscle. A total of 67, 18-wk-old, male and female turkeys from two male (tom) lines and one female (hen) line were used. Each bird was anesthetized and one deep pectoralis muscle was electrically stimulated via the pectoral nerve. Muscle pH was recorded every 30 s for 4 min of stimulation and every 1 min for a 10-min recovery period. Non-stimulated muscles, contralateral to the stimulated side, were assayed for lactate dehydrogenase (LDH) and glyceraldehyde phosphate dehydrogenase (GAPDH). Myosin isoforms were resolved with SDS-PAGE. Line or gender had no effect on rate of pH decline during or after stimulation. Declines in pH during stimulation were greater than during the recovery period (0.06 vs. 0.02 U/min). The lightweight male line (LM) had the greatest breast muscle mass as a percentage of body weight (P < 0.05) and the greatest LDH [293 mmol nicotinamide adenine dinucleotide (NADH) min(-1)microg(-1); P < 0.0001] and GAPDH (0.4452 mmol NADH min(-1)microg(-1); P < 0.05) activities. Hens had greater percentages breast weight than males (P < 0.05) and a tendency for increased enzyme activities. The LM line had the largest ratio (2.33:1) (P < 0.05) of adult-to-neonatal myosin. Genetic selection for breast muscle mass resulted in an increased ratio of adult-to-neonatal myosin and increased anaerobic capacity. This effect on myosin isoform composition and anaerobic capacity supports handling modifications that are line specific to minimize meat quality defects.

Evidence for facilitated lactate uptake in lizard skeletal muscle

To understand more fully lactate metabolism in reptilian muscle, lactate uptake in lizard skeletal muscle was measured and its similarities to the monocarboxylate transport system found in mammals were examined. At 2 min, uptake rates of 15 mmol l–1 lactate into red iliofibularis (rIF) were 2.4- and 2.2-fold greater than white iliofibularis (wIF) and mouse soleus, respectively. α-Cyano-4-hydroxycinnamate (15 mmol l–1) caused little inhibition of uptake in wIF but caused a 42–54 % reduction in the uptake rate of lactate into rIF, suggesting that much of the lactate uptake by rIF is via protein-mediated transport. N-ethymaleimide (ETH) (10 mmol l–1) also caused a reduction in the rate of uptake, but measurements of adenylate and phosphocreatine concentrations show that ETH had serious effects on rIF and wIF and may not be appropriate for transport inhibition studies in reptiles. The higher net uptake rate by rIF than by wIF agrees with the fact that rIF shows much higher rates of lactate utilization and incorporation into glycogen than wIF. This study also suggests that lactate uptake by reptilian muscle is similar to that by mammalian muscle and that, evolutionarily, this transport system may be relatively conserved even in animals with very different patterns of lactate metabolism.

Effect of a myocardial volume overload on lactate transport in skeletal muscle sarcolemmal vesicles

This study sought to determine the effect of a myocardial volume overload (MVO) on sarcolemmal (SL) lactate (La(-)) transport and the aerobic profile of skeletal muscle. SL vesicles were obtained from female rats 10 wk after either a MVO was induced by creation of an infrarenal fistula (n = 10), or sham surgeries were performed (n = 11). Influx of (14)C-labeled L(+)-La(-) was measured at various unlabeled La(-) concentrations under zero-trans conditions. La(-) transport kinetics were determined using a Michaelis-Menten equation with an added linear component to discriminate between carrier-mediated and diffusional transport. Although heart and lung weights were significantly increased (P < 0.0001) in the MVO group, left ventricular function was only modestly altered (P < 0.05). A significant reduction in type I myosin heavy chain (MHC) in the soleus and a strong trend (P = 0.06) for a reduced type IIx MHC in the plantaris were observed in MVO rats, but no differences in citrate synthase activity or monocarboxylate transporter proteins (MCT)-1 expression were noted in any muscle. Carrier-mediated La(-) influx into SL vesicles was similar between sham and MVO (K(m) = 12 +/- 1 and 18 +/- 3 mM; apparent V(max) = 772 +/- 99 and 827 +/- 80 nmol. mg(-1). min(-1), respectively). Total influx at 100 mM was lower in MVO, and this was due to a 30% reduction in membrane diffusion. In conclusion, a 10-wk MVO did not alter MCT-mediated La(-) transport or protein expression but was associated with modest changes in myofibrillar proteins and impaired SL diffusive properties.

Role of plasma membrane transporters in muscle metabolism

Muscle plays a major role in metabolism. Thus it is a major glucose-utilizing tissue in the absorptive state, and changes in muscle insulin-stimulated glucose uptake alter whole-body glucose disposal. In some conditions, muscle preferentially uses lipid substrates, such as fatty acids or ketone bodies. Furthermore, muscle is the main reservoir of amino acids and protein. The activity of many different plasma membrane transporters, such as glucose carriers and transporters of carnitine, creatine and amino acids, play a crucial role in muscle metabolism by catalysing the influx or the efflux of substrates across the cell surface. In some cases, the membrane transport process is subjected to intense regulatory control and may become a potential pharmacological target, as is the case with the glucose transporter GLUT4. The goal of this review is the molecular characterization of muscle membrane transporter proteins, as well as the analysis of their possible regulatory role.

Lactate transporters (MCT proteins) in heart and skeletal muscles

Lactate traverses the cell membranes of many tissues, including the heart and skeletal muscle via a facilitated monocarboxylate transport system that functions as a proton symport and is stereoselective for L-lactate. In the past few years, seven monocarboxylate transporters have been cloned. Monocarboxylate transporters are ubiquitously distributed among many tissues, and the transcripts of several monocarboxylate transporters are present within many of the same tissues. This complicates the identification of their metabolic function. There is also evidence that that there is some species specificity, with differences in MCT tissue distributions in hamsters, rats, and humans. MCT1 and MCT3-M/MCT4 are present in rat and human muscles, and MCT1 expression is highly correlated with the oxidative capacity of skeletal muscles and with their capacity to take up lactate from the circulation. MCT1 is also present in heart and is located on the plasma membrane (in subdomains), T-tubules, and in caveolae. With training, MCT1 is increased in rat and human muscle, and in rat hearts, resulting in an increased uptake of lactate from the buffers perfused through these tissues and an increase in lactate efflux out of purified vesicles. In humans, the training-induced increases in MCT1 are associated with an increased lactate efflux out of muscle. MCT3-M/MCT4 is not correlated with the muscles' oxidative capacities but is equally abundant in Type IIa and IIb muscles, whereas it is markedly lower in slow-twitch (Type I) muscles. Clearly, we are at the threshold of a new era in understanding the regulation of lactate movement into and out of skeletal muscle and cardiac cells.

Muscle as a consumer of lactate

Historically, muscle has been viewed primarily as a producer of lactate but is now considered also to be a primary consumer of lactate. Among the most important factors that regulate net lactate uptake and consumption are metabolic rate, blood flow, lactate concentration ([La]), hydrogen ion concentration ([H+]), fiber type, and exercise training. Muscles probably consume more lactate during steady state exercise or contractions because of increased lactate oxidation since enhancements in lactate transport due to acute activity are small. For optimal lactate consumption, blood flow should be adequate to maintain ideal [La] and [H+] gradients from outside to inside muscles. However, it is not clear that greater than normal blood flow will enhance lactate exchange. A widening of the [La] gradient from outside to inside muscle cells along with an increase in muscle [La] enhances both lactate utilization and sarcolemmal lactate transport. Similarly, a significant outside to inside [H+] gradient will stimulate sarcolemmal lactate influx, whereas an increased intramuscular [H+] may stimulate exogenous lactate utilization by inhibiting endogenous lactate production. Oxidative muscle fibers are metabolically suited for lactate oxidation, and they have a greater capacity for sarcolemmal lactate transport than do glycolytic muscle fibers. Endurance training improves muscle capacity for lactate utilization and increases membrane transport of lactate probably via an increase in Type I monocarboxylate transport protein (MCT1) and perhaps other MCT isoforms as well. The future challenge is to understand the regulatory roles of both lactate metabolism and membrane transport of lactate.

Lactic acid efflux from white skeletal muscle is catalyzed by the monocarboxylate transporter isoform MCT3

The newly cloned proton-linked monocarboxylate transporter MCT3 was shown by Western blotting and immunofluorescence confocal microscopy to be expressed in all muscle fibers. In contrast, MCT1 is expressed most abundantly in oxidative fibers but is almost totally absent in fast-twitch glycolytic fibers. Thus MCT3 appears to be the major MCT isoform responsible for efflux of glycolytically derived lactic acid from white skeletal muscle. MCT3 is also expressed in several other tissues requiring rapid lactic acid efflux. The expression of both MCT3 and MCT1 was decreased by 40-60% 3 weeks after denervation of rat hind limb muscles, whereas chronic stimulation of the muscles for 7 days increased expression of MCT1 2-3-fold but had no effect on MCT3 expression. The kinetics and substrate and inhibitor specificities of monocarboxylate transport into cell lines expressing only MCT3 or MCT1 have been determined. Differences in the properties of MCT1 and MCT3 are relatively modest, suggesting that the significance of the two isoforms may be related to their regulation rather than their intrinsic properties.

Effect of prior eccentric contractions on lactate/H+ transport in rat skeletal muscle

The effect of prior eccentric contractions on skeletal muscle lactate/H+ transport was investigated in rats. Lactate transport was measured in sarcolemmal giant vesicles obtained from soleus and red (RG) and white gastrocnemii (WG) muscles 2 days after intense eccentric contractions (ECC) and from the corresponding contralateral control (CON) muscles. The physiochemical buffer capacity was determined in the three muscle types from both ECC and CON legs. Furthermore, the effect of prior eccentric contractions on release and muscle content of lactate and H+ during and after supramaximal stimulation was examined using the perfused rat hindlimb preparation. The lactate transport rate was lower (P < 0.05) in vesicles obtained from ECC-WG (29%) and ECC-RG (13%) than in vesicles from the CON muscles. The physiochemical buffer capacity was reduced (P < 0.05) in ECC-WG (13%) and ECC-RG (9%) compared with the corresponding CON muscles. There were only marginal effects on the soleus muscle. Muscle lactate concentrations and release of lactate during recovery from intense isometric contractions were lower (P < 0.05) in ECC than in CON hindlimbs, indicating decreased anaerobic glycogenolysis. In conclusion, the sarcolemmal lactate/H+ transport capacity and the physiochemical buffer capacity were reduced in prior eccentrically stimulated WG and RG in rats, suggesting that muscle pH regulation may be impaired after unaccustomed eccentric exercise. In addition, the data indicate that the glycogenolytic potential is decreased in muscles exposed to prior eccentric contractions.

Training intensity-dependent and tissue-specific increases in lactate uptake and MCT-1 in heart and muscle

We investigated the effects of 3 wk of moderate- (21 m/min, 8% grade) and highintensity treadmill training (31 m/min, 15% grade) on 1) monocarboxylate transporter 1 (MCT-1) content in rat hindlimb muscles and the heart and 2) lactate uptake in isolated soleus (Sol) muscles and perfused hearts. In the moderately trained group MCT-1 was not increased in any of the muscles [Sol, extensor digitorum longus (EDL), and red (RG) and white gastrocnemius (WG)] (P > 0.05). Similarly, lactate uptake in Sol strips was also not increased (P > 0.05). In contrast, in the heart, MCT-1 (+36%, P < 0.05) and lactate uptake (+72%, P < 0.05) were increased with moderate training. In the highly trained group, MCT-1 (+70%, P < 0.05) and lactate uptake (+79%, P < 0.05) were increased in Sol. MCT-1 was also increased in RG (+94%, P < 0.05) but not in WG and EDL (P > 0.05). In the highly trained group, heart MCT-1 (+44%, P < 0.05) and lactate uptake (+173%, P < 0.05) were increased. In conclusion, it has been shown that 1) in both heart and skeletal muscle lactate uptake is increased only when MCT-1 is increased; 2) training-induced increases in MCT-1 occurred at a lower training intensity in the heart than in skeletal muscle; 3) in the heart, lactate uptake was increased much more after high-intensity training than after moderate-intensity training, despite similar increases in heart MCT-1 with these two training intensities; and 4) the increases in MCT-1 occurred independently of any changes in the heart's oxidative capacity (as measured by citrate synthase activity).

Muscle pH regulation: role of training

In most cell types, including resting skeletal muscle fibers, internal pH (pHi) is kept constant at a relatively alkaline level. The high pHi is obtained in spite of a chronic acid load resulting from cellular metabolism and passive influx of protons driven by electrochemical forces. Regulation of pHi depends on continuous activity of membrane transport systems that mediate an outflux of H+ (or bicarbonate influx), whereby the acid load is counterbalanced. The transporters involved in muscle pH regulation at rest are the Na+/H+ exchange system as well as the Na+-dependent and Na+-independent Cl- bicarbonate transport systems. The Na+/H+ exchanger seems to be active at resting pHi levels in skeletal muscle. Therefore, pH homeostasis in skeletal muscle most likely involves an equilibrium between counter-directed H+ fluxes. A minor fraction of H+ release during intense exercise is mediated by the Na+/H+ exchanger. The capacity of this system is increased with training and hypoxia in rat skeletal muscle. The dominant acid extruding system associated with intense exercise is the lactate/H+ co-transporter. It has been demonstrated that the capacity of the lactate/H+ co-transporter of rat skeletal muscle is upregulated with training and chronic electrical stimulation, and that it is reduced upon denervation and hindlimb unweighting. Moreover, athletes can have an elevated lactate/H+ co-transport capacity, whereas the thigh muscle of spinal cord-injured individuals has a lower transport capacity than the one of healthy untrained subjects. Thus, it appears that the capacity of the lactate/H+ transporter is affected by the level of muscle activity in both rats and humans. In addition, the rate of H+ release from muscle may also be influenced by capillarization and local blood flow. Finally the resulting pH displacement during acid accumulation is determined by the cellular buffer capacity, which may also undergo adaptive changes.

Short-term training increases human muscle MCT1 and femoral venous lactate in relation to muscle lactate

We examined the effects of increasing a known lactate transporter protein, monocarboxylate transporter 1 (MCT1), on lactate extrusion from human skeletal muscle during exercise. Before and after short-term bicycle ergometry training [2 h/day, 7 days at 65% maximal oxygen consumption (VO2max)], subjects (n = 7) completed a continuous bicycle ergometer ride at 30% VO2max (15 min), 60% VO2max (15 min), and 75% VO2max (15 min). Muscle biopsy samples (vastus lateralis) and arterial and femoral venous blood samples were obtained before exercise and at the end of each workload. After 7 days of training the MCT1 content in muscle was increased (+18%; P < 0.05). The concentrations of both muscle lactate and femoral venous lactate were reduced during exercise (P < 0.05) that was performed after training. High correlations were observed between muscle lactate and venous lactate before training (r = 0.92, P < 0.05) and after training (r = 0.85, P < 0.05), but the slopes of the regression lines between these variables differed markedly. Before training, the slope was 0.12 +/- 0.01 mM lactate.mmol lactate-1.kg muscle dry wt-1, and this was increased by 33% after training to 0.18 +/- 0.02 mM lactate.mmol lactate-1.kg muscle dry wt-1. This indicated that after training the femoral venous lactate concentrations were increased for a given amount of muscle lactate. These results suggest that lactate extrusion from exercising muscles is increased after training, and this may be associated with the increase in skeletal muscle MCT1.

Chronic electrical stimulation increases MCT1 and lactate uptake in red and white skeletal muscle

We examined whether chronic stimulation of red and white rat muscles increased the concentrations of the monocarboxylate transporter MCT1. Red and white tibialis anterior (RTA and WTA, respectively) and extensor digitorum longus (EDL) muscles were chronically stimulated via the peroneal nerve for 7 days. Stimulated and contralateral control muscles were examined for MCT1 content, L-lactate uptake, lactate dehydrogenase (LDH) isoforms, and muscle fiber composition. MCT1 was 1.5 times greater in stimulated RTA, 3 times greater in stimulated WTA, and 1.9 times greater in stimulated EDL compared with respective control muscles (P < 0.05). L-Lactate uptake increased in all stimulated muscles (P < 0.05), and this was highly correlated with the increase in MCT1 (r = 0.96). The heart-type LDH (H-LDH) subunits also increased in all stimulated muscles (P < 0.05). The H-LDH subunits correlated highly with MCT1 in the muscles (r = 0.83). There was no change in muscle-type LDH subunits (P > 0.05). There were negligible alterations in muscle fiber composition in the stimulated muscles, suggesting that the increase in MCT1 was independent of changes in muscle fiber composition. These studies are the first to demonstrate that chronic muscle contraction increases MCT1 concentrations in both red and white skeletal muscles.

Lactate infusion in anesthetized rats produces insulin resistance in heart and skeletal muscles

Plasma lactate is elevated in many physiological and pathological conditions, such as physical exercise, obesity, and diabetes, in which a reduction of insulin sensitivity is also present. Furthermore, an increased production of lactate from muscle and adipose tissue together with increased gluconeogenic substrate flux to the liver plays a primary role in enhancing hepatic glucose production (HGP) in diabetes. It has been shown that lactate may interfere with the utilization and oxidation of other substrates such as free fatty acids (FFAs). The aim of this study was to investigate if lactate infusion affects peripheral glucose utilization in rats. Animals were acutely infused with lactate to achieve a final lactate concentration of 4 mmol/L. They were then submitted to a euglycemic-hyperinsulinemic clamp to study HGP and overall glucose metabolism (rate of disappearance [Rd]). At the end of the clamp, a bolus of 2-deoxy-[1-3H]-glucose was injected to study insulin-dependent glucose uptake in different tissues. The results show that lactate infusion did not affect HGP either in the basal state or at the end of clamp, whereas glucose utilization significantly decreased in lactate-infused rats (26.6 +/- 1.1 v 19.5 +/- 1.4 mg.kg-1.min-1, P < .01). A reduction in the tissue glucose utilization index was noted in heart (18.01 +/- 4.44 v 46.21 +/- 6.51 ng.mg-1.min-1, P < .01), diaphragm (5.56 +/- 0.74 v 9.01 +/- 0.93 ng.mg-1.min-1, P < .01), soleus (13.62 +/- 2.29 v 34.05 +/- 6.08 ng.mg-1.min-1, P < .01), and red quadricep (4.43 +/- 0.73 v 5.88 +/- 0.32 ng.mg-1.min-1, P < .05) muscle in lactate-infused animals, whereas no alterations were observed in other muscles or in adipose tissue. Therefore, we suggest that acute lactate infusion induces insulin resistance in the heart and some muscles, thus supporting a role for lactate in the regulation of peripheral glucose metabolism.

Intracellular lactate- and pyruvate-interconversion rates are increased in muscle tissue of non-insulin-dependent diabetic individuals

The contribution of muscle tissues of non-insulin-dependent diabetes mellitus (NIDDM) patients to blood lactate appearance remains undefined. To gain insight on intracellular pyruvate/lactate metabolism, the postabsorptive forearm metabolism of glucose, lactate, FFA, and ketone bodies (KB) was assessed in seven obese non-insulin-dependent diabetic patients (BMI = 28.0 +/- 0.5 kg/m2) and seven control individuals (BMI = 24.8 +/- 0.5 kg/m2) by using arteriovenous balance across forearm tissues along with continuous infusion of [3-13C1]-lactate and indirect calorimetry. Fasting plasma concentrations of glucose (10.0 +/- 0.3 vs. 4.7 +/- 0.2 mmol/liter), insulin (68 +/- 5 vs. 43 +/- 6 pmol/liter), FFA (0.57 +/- 0.02 vs. 0.51 +/- 0.02 mmol/liter), and blood levels of lactate (1.05 +/- 0.04 vs. 0.60 +/- 0.06 mmol/liter), and KB (0.48 +/- 0.04 vs. 0.29 +/- 0.02 mmol/liter) were higher in NIDDM patients (P < 0.01). Forearm glucose uptake was similar in the two groups (10.3 +/- 1.4 vs. 9.6 +/ 1.1 micromol/min/liter of forearm tissue), while KB uptake was twice as much in NIDDM patients as compared to control subjects. Lactate balance was only slightly increased in NIDDM patients (5.6 +/- 1.4 vs. 3.3 +/- 1.0 micromol/min/liter; P = NS). A two-compartment model of lactate and pyruvate kinetics in the forearm tissue was used to dissect out the rates of lactate to pyruvate and pyruvate to lactate interconversions. In spite of minor differences in the lactate balance, a fourfold increase in both lactate- (44.8 +/- 9.0 vs. 12.6 +/- 4.6 micromol/min/liter) and pyruvate-(50.4 +/- 9.8 vs. 16.0 +/- 5.0 micromol/min/liter) interconversion rates (both P < 0.01) were found. Whole body lactate turnover, assessed by using the classic isotope dilution principle, was higher in NIDDM individuals (46 +/- 9 vs. 21 +/- 3 micromol/min/kg; P < 0.01). Insights into the physiological meaning of this parameter were obtained by using a whole body noncompartmental model of lactate/pyruvate kinetics which provides a lower and upper bound for total lactate and pyruvate turnover (NIDDM = 46 +/- 9 vs. 108 +/- 31; controls = 21 +/- 3 - 50 +/-13 micromol/min/kg). In conclusion, in the postabsorptive state, despite a trivial lactate release by muscle, lactate- and pyruvate-interconversion rates are greatly enhanced in NIDDM patients, possibly due to concomitant impairment in the oxidative pathway of glucose metabolism. This finding strongly suggest a major disturbance in intracellular lactate/pyruvate metabolism in NIDDM.

Role of the lactate transporter (MCT1) in skeletal muscles

We used an antibody, constructed against the monocarboxylate transporter 1 (MCT1) protein (L. Carpenter, R. C. Poole, and A. P. Halestrap. Biochim. Biophys. Acta 1279: 157-165, 1996), to study the expression and role of MCT1 in rat skeletal muscles. MCT1 was higher in red than in white muscles (P < 0.05) and was highly correlated with the oxidative fiber content (%slow-twitch oxidative + %fast-twitch oxidative glycolytic) of skeletal muscles (r = 0.91). MCT1 was highly related to lactate uptake in skeletal muscles (r = 0.90). Total lactate dehydrogenase (LDH) activity, an index of glycolysis, was negatively correlated with MCT1 in rat muscles (r = -0.80). MCT1 was also strongly correlated with the heart-type forms of LDH (LDH-1 vs. MCT1, r = 0.83; LDH-2 vs. MCT1, r = 0.89). There was no relationship between MCT1 and the muscle form of LDH (LDH-5; P > 0.05). MCT1 was highly correlated with citrate synthase activity, a marker of the oxidative capacity of muscle (r = 0.82). Therefore, MCT1 may have kinetics that favor the uptake of L-lactate into the muscle cell for oxidative metabolism, and MCT1 may be coordinately expressed with the heart forms of LDH and enzymes of oxidative metabolism.

Effects of alpha-cyano-4-hydroxycinnamic acid on fatigue and recovery of isolated mouse muscle

Fatigue and recovery of mouse soleus and extensor digitorum longus muscles were investigated in standard saline and in saline containing the lactate + hydrogen ion transport blocker, alpha-cyano-4-hydroxycinnamic acid (cinnamate). The fatigue protocol was a series of brief isometric tetani which reduced isometric force by about 25%. Recovery was monitored by test tetani during recovery. Both muscles recovered completely in standard saline. Soleus muscle also recovered completely in the presence of cinnamate, whereas extensor digitorum longus hardly recovered at all. Force during fatigue and recovery can be described in a mathematical simulation in which force depends on intracellular inorganic phosphate and pH, and the only effect of cinnamate is to block lactate + hydrogen ion transport. The results of the simulation suggest that during the fatiguing series of tetani pH changes are small and have a negligible effect on force, but pH is a major determinant of the timecourse of recovery in extensor digitorum longus.

Regulation of cellular pH in skeletal muscle fiber types, studied with sarcolemmal giant vesicles obtained from rat muscles

Sarcolemmal giant vesicles obtained from rat hindlimb muscles were used as a model for the study of pH regulation in skeletal muscle. The transport systems involved in the recovery from 40 mM lactate and pHi 6.5 were quantified from both flux measurements of the co-transported ions and counter-ions, and from measurements of the rate of the internal pH change. The diffusion of lactic acid plus the carrier-mediated co-transport of lactate and H+ had the highest capacity to transport protons (240 nmol H+/mg protein per min). These systems are therefore responsible for a large part of the H+ efflux in periods with a high lactate production. The capacity of the HCO(3)- - dependent systems was 47 nmol/mg per min, and the capacity of the Na+/H+ exchange system was 33 nmol/mg per min in vesicles from mixed muscles. The capacity to remove H+ by the lactate/H+ co-transport system and by the bicarbonate-dependent systems was significantly higher in vesicles from predominantly red fibers than in vesicles from white fibers, whereas the distribution of the Na+/H+ exchange system was independent of fiber type. These observations demonstrate that the pH regulation during muscle activity in red muscles is more effective than in white muscles.

L(+)-lactate binding to a protein in rat skeletal muscle plasma membranes

Biochemical studies were conducted to determine the location of a putative lactate transport protein in rat skeletal muscle plasma membranes (PM). PM (50-100 micrograms protein) were incubated with [U-14C] L(+)-lactate, in the presence or absence of unlabeled monocarboxylates or potential inhibitors, after which proteins were separated by SDS-PAGE. Gel slices (2 mm) were cut and analyzed for 14C. [U-14C] L(+)-lactate was bound to plasma membranes in the 30 to 40 kDa molecular mass range. Binding of [U-14C] L(+)-lactate was inhibited by N-ethylmaleimide, unlabeled L-lactate and pyruvate, and in a dose dependent manner by alpha-cyano-4-hydroxycinnamate (r = 0.995), but not by cytochalasin-B. The inhibition of [U-14C] L(+)-lactate binding was similar to the inhibition of lactate transport. Therefore the transport of L(+)-lactate across skeletal muscle plasma membranes involves a polypeptide of 30 to 40 kDa.

Effects of exercise on lactate transport into mouse skeletal muscles

Skeletal muscle lactate transport was investigated in vitro in isolated fast-twitch (EDL) and slow-twitch soleus (Sol) skeletal muscles from control and exercised mice. Exercise (23 m/min, 8% grade) reduced muscle glycogen by 37% in EDL (p < 0.05) and by 35% in Sol muscles (p < 0.05). Lactate transport measurements (45 sec) were performed after 60 min of exercise in intact EDL and Sol muscles in vitro, at differing pH (6.5 and 7.4) and differing lactate concentrations (4 and 30 mM). Lactate transport was observed to be greater in Sol than in EDL (p < 0.05). In the exercised muscles there was a small but significant increase in lactate transport (p < 0.05). Lactate transport was greater when exogenous lactate concentrations were greater (p < 0.05) and more rapid at the lower pH (p < 0.05). These studies demonstrated that lactate transport was increased with exercise.

Lactate and H+ effluxes from human skeletal muscles during intense, dynamic exercise

1. Lactate and H+ efflux from skeletal muscles were studied with the one-legged knee extension model under conditions in which blood flow, arterial lactate and the muscle-blood lactate concentration gradient were altered. Subjects exercised one leg twice to exhaustion (EX1, EX2), separated by a 10 min recovery and a period of intense intermittent exercise. After 1 h of recovery the exercise protocol was repeated with the other leg. Low-intensity exercise was performed with one leg during the recovery periods, while the other leg was passive during its recovery periods. 2. Prior to, and immediately after, EX1 and EX2 and then 3 and 10 min after EX1, a biopsy was taken from the vastus lateralis of the exercised leg for lactate, pH, muscle water and fibre-type determinations. Measurements of leg blood flow and venous-arterial differences for lactate (whole blood and plasma), pH, partial pressure of CO2 (PCO2), haemoglobin, saturation and base excess (BE) were performed at the end of exercise and regularly during the recovery period after EX1. 3. The lactate release was linearly related (r = 0.96; P < 0.05) to the muscle lactate gradient over a range of muscle lactate from 0 to 45 mmol (kg wet wt)-1. The muscle lactate transport was evaluated from the net femoral venous-arterial differences (V-Adiff) for lactate. This rose with increases in the muscle lactate gradients, but as the gradient reached higher levels the V-Adiff lactate responded less than at smaller gradients. Thus, the lactate transport over the muscle membrane appears to be partly saturated at high muscle lactate concentrations. 4. The percentage of slow twitch (%ST) fibres was inversely related to the muscle lactate gradient, but it was not correlated to the lactate release at the end of the exercises. In spite of a significantly higher blood flow during active recovery, the lactate release was the same whether the leg was resting or performed low-intensity exercise in the recovery periods. In several other conditions the muscle lactate and H+ gradients would have predicted that the V-Adiff lactate would have been greater than it actually was. Thus, a variety of factors affect muscle lactate transport, including arterial lactate concentration, muscle perfusion, muscle contraction pattern and muscle morphology. 5. The muscle and femoral venous pH declined during EX1 to 6.73 and 7.14-7.15, respectively, and they increased to resting levels during 10 min of either passive or active recovery.(ABSTRACT TRUNCATED AT 400 WORDS)