Lactate and pyruvate transport is dominated by a pH gradient-sensitive carrier in rat skeletal muscle sarcolemmal vesicles

MLKL overexpression leads to Ca2+ and metabolic dyshomeostasis in a neuronal cell model

The necroptotic effector molecule MLKL accumulates in neurons over the lifespan of mice, and its downregulation has the potential to improve cognition through neuroinflammation, and changes in the abundance of synaptic proteins and enzymes in the central nervous system. Notwithstanding, direct evidence of cell-autonomous effects of MLKL expression on neuronal physiology and metabolism are lacking. Here, we tested whether the overexpression of MLKL in the absence of cell death in the neuronal cell line Neuro-2a recapitulates some of the hallmarks of aging at the cellular level. Using genetically-encoded fluorescent biosensors, we monitored the cytosolic and mitochondrial Ca2+ levels, along with the cytosolic concentrations of several metabolites involved in energy metabolism (lactate, glucose, ATP) and oxidative stress (oxidized/reduced glutathione). We found that MLKL overexpression marginally decreased cell viability, however, it led to reduced cytosolic and mitochondrial Ca2+ elevations in response to Ca2+ influx from the extracellular space. On the contrary, Ca2+ signals were elevated after mobilizing Ca2+ from the endoplasmic reticulum. Transient elevations in cytosolic Ca2+, mimicking neuronal stimulation, lead to higher lactate levels and lower glucose concentrations in Neuro-2a cells when overexpressing MLKL, which suggest enhanced neuronal glycolysis. Despite these alterations, energy levels and glutathione redox state in the cell bodies remained largely preserved after inducing MLKL overexpression for 24-48 h. Taken together, our proof-of-concept experiments are consistent with the hypothesis that MLKL overexpression in the absence of cell death contributes to both Ca2+ and metabolic dyshomeostasis, which are cellular hallmarks of brain aging.

Role of Lactate in the Regulation of Transcriptional Activity of Breast Cancer-Related Genes and Epithelial-to-Mesenchymal Transition Proteins: A Compassion of MCF7 and MDA-MB-231 Cancer Cell Lines

The Warburg Effect is characterized by accelerated glycolytic metabolism and lactate production and under fully aerobic conditions is a hallmark of cancer cells. Recently, we have demonstrated the role of endogenous, glucose-derived lactate as an oncometabolite which regulates gene expression in the estrogen receptor positive (ER+) MCF7 cell line cultivated in glucose media. Presently, with the addition of a triple negative breast cancer (TNBC) cell line, MDA-MB-231, we further confirm the effect of lactate on gene expression patterns and extend results to include lactate effects on protein expression. As well, we report effects of lactate on the expression of E-cadherin and vimentin, proteins associated with epithelial-to-mesenchymal transition (EMT). Endogenous lactate regulates the expression of multiple genes involved in carcinogenesis. In MCF7 cells, lactate increased the expression of EGFR, VEGF, HIF-1a, KRAS, MIF, mTOR, PIK3CA, TP53, and CDK4 as well as decreased the expression of ATM, BRCA1, BRCA2, E2F1, MET, MYC, and RAF mainly after 48h of exposure. On the other hand, in the MDA-MB-231 cell line, lactate increased the expressions of PIK3CA, VEGF, EGFR, mTOR, HIF-1α, ATM, E2F1, TP53 and decreased the expressions of BRCA1, BRCA2, CDK4, CDK6, MET, MIF, MYC, and RAF after 48h of exposure. In response to endogenous lactate, changes in protein expression of representative genes corroborated changes in mRNA expressions. Finally, lactate exposure decreased E-cadherin protein expression in MCF7 cells and increased vimentin expression in MDA-MB-231 cells. Furthermore, by genetically silencing LDHA in MCF7 cells, we show suppression of protein expression of EGFR and HIF-1α, while full protein expression occurred under glucose and glucose + exogenous lactate exposure. Hence, endogenous, glucose-derived lactate, and not glucose, elicited changes in gene and protein expression levels. In this study, we demonstrate that endogenous lactate produced under aerobic conditions (Warburg Effect) elicits important changes in gene and protein expression in both ER+ and TNBC cell lines. The widespread regulation of multiple genes by lactate and involves those involved in carcinogenesis including DNA repair, cell growth, proliferation, angiogenesis, and metastasis. Furthermore, lactate affected the expression of two relevant EMT biomarkers, E-cadherin and vimentin, which could contribute to the complex process of EMT and a shift towards a more mesenchymal phenotype in the two cancer cell lines studied.

History and Function of the Lactate Receptor GPR81/HCAR1 in the Brain: A Putative Therapeutic Target for the Treatment of Cerebral Ischemia

GPR81 is a G-protein coupled receptor (GPCR) discovered in 2001, but deorphanized only 7 years later, when its affinity for lactate as an endogenous ligand was demonstrated. More recently, GPR81 expression and distribution in the brain were also confirmed and the function of lactate as a volume transmitter has been suggested since then. These findings shed light on a new function of lactate acting as a signaling molecule in the central nervous system, in addition to its well-known role as a metabolic fuel for neurons. GPR81 seems to act as a metabolic sensor, coupling energy metabolism, synaptic activity, and blood flow. Activation of this receptor leads to Gi-mediated downregulation of adenylyl cyclase and subsequent reduction in cAMP levels, regulating several downstream pathways. Recent studies have also suggested the potential role of lactate as a neuroprotective agent, mainly under brain ischemic conditions. This effect is usually attributed to the metabolic role of lactate, but the underlying mechanisms need further investigation and could be related to lactate signaling via GPR81. The activation of GPR81 showed promising results for neuroprotection: it modulates many processes involved in the pathophysiology of ischemia. In this review, we summarize the history of GPR81, starting with its deorphanization; then, we discuss GPR81 expression and distribution, signaling transduction cascades, and neuroprotective roles. Lastly, we propose GPR81 as a potential target for the treatment of cerebral ischemia.

What the Lactate Shuttle Means for Sports Nutrition

The discovery of the lactate shuttle (LS) mechanism may have two opposite perceptions, It may mean very little, because the body normally and inexorably uses the LS mechanism. On the contrary, one may support the viewpoint that understanding the LS mechanism offers immense opportunities for understanding nutrition and metabolism in general, as well as in a sports nutrition supplementation setting. In fact, regardless of the specific form of the carbohydrate (CHO) nutrient taken, the bodily CHO energy flux is from a hexose sugar glucose or glucose polymer (glycogen and starches) to lactate with subsequent somatic tissue oxidation or storage as liver glycogen. In fact, because oxygen and lactate flow together through the circulation to sites of utilization, the bodily carbon energy flow is essentially the lactate disposal rate. Consequently, one can consume glucose or glucose polymers in various forms (glycogen, maltodextrin, potato, corn starch, and fructose or high-fructose corn syrup), and the intestinal wall, liver, integument, and active and inactive muscles make lactate which is the chief energy fuel for red skeletal muscle, heart, brain, erythrocytes, and kidneys. Therefore, if one wants to hasten the delivery of CHO energy delivery, instead of providing CHO foods, supplementation with lactate nutrient compounds can augment body energy flow.

Tracing the lactate shuttle to the mitochondrial reticulum

Isotope tracer infusion studies employing lactate, glucose, glycerol, and fatty acid isotope tracers were central to the deduction and demonstration of the Lactate Shuttle at the whole-body level. In concert with the ability to perform tissue metabolite concentration measurements, as well as determinations of unidirectional and net metabolite exchanges by means of arterial–venous difference (a-v) and blood flow measurements across tissue beds including skeletal muscle, the heart and the brain, lactate shuttling within organs and tissues was made evident. From an extensive body of work on men and women, resting or exercising, before or after endurance training, at sea level or high altitude, we now know that Organ–Organ, Cell–Cell, and Intracellular Lactate Shuttles operate continuously. By means of lactate shuttling, fuel-energy substrates can be exchanged between producer (driver) cells, such as those in skeletal muscle, and consumer (recipient) cells, such as those in the brain, heart, muscle, liver and kidneys. Within tissues, lactate can be exchanged between white and red fibers within a muscle bed and between astrocytes and neurons in the brain. Within cells, lactate can be exchanged between the cytosol and mitochondria and between the cytosol and peroxisomes. Lactate shuttling between driver and recipient cells depends on concentration gradients created by the mitochondrial respiratory apparatus in recipient cells for oxidative disposal of lactate.

Studies using isotope tracer technologies have significantly improved understanding of how lactate, a metabolite produced as fuel during normal metabolism and in response to exercise, moves or ‘shuttles’ throughout the body. George Brooks and colleagues at the University of California, Berkeley, USA, reviewed the history of the understanding of lactate shuttling, which has largely been informed by human studies using isotope tracer infusions during rest and exercise. Such research highlights continuous organ–organ, cell–cell, and intracellular lactate shuttling. Lactate moves between producer cells such as skeletal muscle cells and consumer cells in tissues including the heart and brain, where it is preferred over glucose as an energy source. Shuttling depends on lactate concentration gradients created by mitochondrial networks in recipient cells. Lactate is disposed of via oxidation.

Lactate and Myocadiac Energy Metabolism

The myocardium is capable of utilizing different energy substrates, which is referred to as “metabolic flexibility.” This process assures ATP production from fatty acids, glucose, lactate, amino acids, and ketones, in the face of varying metabolic contexts. In the normal physiological state, the oxidation of fatty acids contributes to approximately 60% of energy required, and the oxidation of other substrates provides the rest. The accumulation of lactate in ischemic and hypoxic tissues has traditionally be considered as a by-product, and of little utility. However, recent evidence suggests that lactate may represent an important fuel for the myocardium during exercise or myocadiac stress. This new paradigm drives increasing interest in understanding its role in cardiac metabolism under both physiological and pathological conditions. In recent years, blood lactate has been regarded as a signal of stress in cardiac disease, linking to prognosis in patients with myocardial ischemia or heart failure. In this review, we discuss the importance of lactate as an energy source and its relevance to the progression and management of heart diseases.

Role of the Heart in Lactate Shuttling

After almost a century of misunderstanding, it is time to appreciate that lactate shuttling is an important feature of energy flux and metabolic regulation that involves a complex series of metabolic, neuroendocrine, cardiovascular, and cardiac events in vivo. Cell–cell and intracellular lactate shuttles in the heart and between the heart and other tissues fulfill essential purposes of energy substrate production and distribution as well as cell signaling under fully aerobic conditions. Recognition of lactate shuttling came first in studies of physical exercise where the roles of driver (producer) and recipient (consumer) cells and tissues were obvious. One powerful example of cell–cell lactate shuttling was the exchange of carbohydrate energy in the form of lactate between working limb skeletal muscle and the heart. The exchange of mass represented a conservation of mass that required the integration of neuroendocrine, autoregulatory, and cardiovascular systems. Now, with greater scrutiny and recognition of the effect of the cardiac cycle on myocardial blood flow, there brings an appreciation that metabolic fluxes must accommodate to pressure-flow realities within an organ in which they occur. Therefore, the presence of an intra-cardiac lactate shuttle is posited to explain how cardiac mechanics and metabolism are synchronized. Specifically, interruption of blood flow during the isotonic phase of systole is supported by glycolysis and subsequent return of blood flow during diastole allows for recovery sustained by oxidative metabolism.

Lactate in contemporary biology: a phoenix risen

After a century, it's time to turn the page on understanding of lactate metabolism and appreciate that lactate shuttling is an important component of intermediary metabolism in vivo. Cell‐cell and intracellular lactate shuttles fulfil purposes of energy substrate production and distribution, as well as cell signalling under fully aerobic conditions. Recognition of lactate shuttling came first in studies of physical exercise where the roles of driver (producer) and recipient (consumer) cells and tissues were obvious. Moreover, the presence of lactate shuttling as part of postprandial glucose disposal and satiety signalling has been recognized. Mitochondrial respiration creates the physiological sink for lactate disposal in vivo. Repeated lactate exposure from regular exercise results in adaptive processes such as mitochondrial biogenesis and other healthful circulatory and neurological characteristics such as improved physical work capacity, metabolic flexibility, learning, and memory. The importance of lactate and lactate shuttling in healthful living is further emphasized when lactate signalling and shuttling are dysregulated as occurs in particular illnesses and injuries. Like a phoenix, lactate has risen to major importance in 21st century biology.

The tortuous path of lactate shuttle discovery: From cinders and boards to the lab and ICU

Once thought to be a waste product of oxygen limited (anaerobic) metabolism, lactate is now known to form continuously under fully oxygenated (aerobic) conditions. Lactate shuttling between producer (driver) and consumer cells fulfills at least 3 purposes; lactate is: (1) a major energy source, (2) the major gluconeogenic precursor, and (3) a signaling molecule. The Lactate Shuttle theory is applicable to diverse fields such as sports nutrition and hydration, resuscitation from acidosis and Dengue, treatment of traumatic brain injury, maintenance of glycemia, reduction of inflammation, cardiac support in heart failure and following a myocardial infarction, and to improve cognition. Yet, dysregulated lactate shuttling disrupts metabolic flexibility, and worse, supports oncogenesis. Lactate production in cancer (the Warburg effect) is involved in all main sequela for carcinogenesis: angiogenesis, immune escape, cell migration, metastasis, and self-sufficient metabolism. The history of the tortuous path of discovery in lactate metabolism and shuttling was discussed in the 2019 American College of Sports Medicine Joseph B. Wolffe Lecture in Orlando, FL.

Noninvasive analysis and identification of an intramuscular fluid collection by postmortem 1H-MRS in a case of a fatal motor vehicle accident

In a case of a fatal traffic accident, a suspicious finding was identified in the muscular tissue of the left thigh by whole-body postmortem computed tomography. To better interpret the finding, the lower extremities were investigated by magnetic resonance imaging (MRI) and proton magnetic resonance spectroscopy (¹H-MRS). MRI revealed the presence of an evenly distributed intramuscular fluid and ¹H-MRS of a volume within the fluid detected concentrations of acetate and lactate. The fluid was assumed to be an extravasation of an intraosseous infusion, erroneously administered to the intermediate vastus of the left thigh during resuscitation, which was later confirmed when access to resuscitation protocols was granted. Further ex situ ¹H-MRS investigations of five different infusion fluids showed the possible discrimination of the fluids and further indicated the unknown fluid to be a Ringer's acetate solution. This paper presents the case-based application of postmortem intramuscular 1H-MRS and introduces the possibility of its use to differentiate exo- and endogenic fluids for forensic interpretation. Further research for this method regarding problems in forensic pathology is needed.

The Science and Translation of Lactate Shuttle Theory

Once thought to be a waste product of anaerobic metabolism, lactate is now known to form continuously under aerobic conditions. Shuttling between producer and consumer cells fulfills at least three purposes for lactate: (1) a major energy source, (2) the major gluconeogenic precursor, and (3) a signaling molecule. "Lactate shuttle" (LS) concepts describe the roles of lactate in delivery of oxidative and gluconeogenic substrates as well as in cell signaling. In medicine, it has long been recognized that the elevation of blood lactate correlates with illness or injury severity. However, with lactate shuttle theory in mind, some clinicians are now appreciating lactatemia as a "strain" and not a "stress" biomarker. In fact, clinical studies are utilizing lactate to treat pro-inflammatory conditions and to deliver optimal fuel for working muscles in sports medicine. The above, as well as historic and recent studies of lactate metabolism and shuttling, are discussed in the following review.

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.

Effect of voluntary hypocapnic hyperventilation or moderate hypoxia on metabolic and heart rate responses during high-intensity intermittent exercise

PURPOSE

To investigate the effect of voluntary hypocapnic hyperventilation or moderate hypoxia on metabolic and heart rate responses during high-intensity intermittent exercise.

METHODS

Ten males performed three 30-s bouts of high-intensity cycling [Ex1 and Ex2: constant-workload at 80% of the power output in the Wingate anaerobic test (WAnT), Ex3: WAnT] interspaced with 4-min recovery periods under normoxic (Control), hypocapnic or hypoxic (2500 m) conditions. Hypocapnia was developed through voluntary hyperventilation for 20 min prior to Ex1 and during each recovery period.

RESULTS

End-tidal CO₂ pressure was lower before each exercise in the hypocapnia than control trials. Oxygen uptake ([Formula: see text]) was lower in the hypocapnia than control trials (822 ± 235 vs. 1645 ± 245 mL min^-1; mean ± SD) during Ex1, but not Ex2 or Ex3, without a between-trial difference in the power output during the exercises. Heart rates (HRs) during Ex1 (127 ± 8 vs. 142 ± 10 beats min^-1) and subsequent post-exercise recovery periods were lower in the hypocapnia than control trials, without differences during or after Ex2, except at 4 min into the second recovery period. [Formula: see text] did not differ between the control and hypoxia trials throughout.

CONCLUSIONS

These results suggest that during three 30-s bouts of high-intensity intermittent cycling, (1) hypocapnia reduces the aerobic metabolic rate with a compensatory increase in the anaerobic metabolic rate during the first but not subsequent exercises; (2) HRs during the exercise and post-exercise recovery periods are lowered by hypocapnia, but this effect is diminished with repeated exercise bouts, and (3) moderate hypoxia (2500 m) does not affect the metabolic response during exercise.

High-intensity intermittent exercise training with chlorella intake accelerates exercise performance and muscle glycolytic and oxidative capacity in rats

The purpose of this study was to investigate the effect of chronic chlorella intake alone or in combination with high-intensity intermittent exercise (HIIE) training on exercise performance and muscle glycolytic and oxidative metabolism in rats. Forty male Sprague-Dawley rats were randomly assigned to the four groups: sedentary control, chlorella intake (0.5% chlorella powder in normal feed), HIIE training, and combination of HIIE training and chlorella intake for 6 wk ( n = 10 each group). HIIE training comprised 14 repeats of a 20-s swimming session with a 10-s pause between sessions, while bearing a weight equivalent to 16% of body weight, 4 days/week. Exercise performance was tested after the interventions by measuring the maximal number of HIIE sessions that could be completed. Chlorella intake and HIIE training significantly increased the maximal number of HIIE sessions and enhanced the expression of monocarboxylate transporter (MCT)1, MCT4, and peroxisome proliferator-activated receptor γ coactivator-1α concomitantly with the activities of lactate dehydrogenase (LDH), phosphofructokinase, citrate synthase (CS), and cytochrome- c oxidase (COX) in the red region of the gastrocnemius muscle. Furthermore, the combination further augmented the increased exercise performance and the enhanced expressions and activities. By contrast, in the white region of the muscle, MCT1 expression and LDH, CS, and COX activities did not change. These results showed that compared with only chlorella intake and only HIIE training, chlorella intake combined with HIIE training has a more pronounced effect on exercise performance and muscle glycolytic and oxidative metabolism, in particular, lactate metabolism.

Determinants of curvature constant (W') of the power duration relationship under normoxia and hypoxia: the effect of pre-exercise alkalosis

Purpose

This study investigated the effect of induced alkalosis on the curvature constant (W’) of the power-duration relationship under normoxic and hypoxic conditions.

Methods

Eleven trained cyclists (mean ± SD) Age: 32 ± 7.2 years; body mass (bm): 77.0 ± 9.2 kg; VO_2peak: 59.2 ± 6.8 ml·kg⁻¹·min⁻¹ completed seven laboratory visits which involved the determination of individual time to peak alkalosis following sodium bicarbonate (NaHCO₃) ingestion, an environment specific ramp test (e.g. normoxia and hypoxia) and four x 3 min critical power (CP) tests under different experimental conditions. Participants completed four trials: alkalosis normoxia (ALN); placebo normoxia (PLN); alkalosis hypoxia (ALH); and placebo hypoxia (PLH). Pre-exercise administration of 0.3 g.kg⁻¹ BM of NaHCO₃ was used to induce alkalosis. Environmental conditions were set at either normobaric hypoxia (FiO₂: 14.5%) or normoxia (FiO₂: 20.93%).

Results

An increase in W’ was observed with pre-exercise alkalosis under both normoxic (PLN: 15.1 ± 6.2 kJ vs. ALN: 17.4 ± 5.1 kJ; P = 0.006) and hypoxic conditions (ALN: 15.2 ± 4.9 kJ vs. ALN: 17.9 ± 5.2 kJ; P < 0.001). Pre-exercise alkalosis resulted in a larger reduction in bicarbonate ion (HCO₃⁻) concentrations during exercise in both environmental conditions (p < 0.001) and a greater blood lactate accumulation under hypoxia (P = 0.012).

Conclusion

Pre-exercise alkalosis substantially increased W’ and, therefore, may determine tolerance to exercise above CP under normoxic and hypoxic conditions. This may be due to NaHCO₃ increasing HCO₃⁻ buffering capacity to delay exercise-induced acidosis, which may, therefore, enhance anaerobic energy contribution.

Expression of Monocarboxylate Transporter Isoforms in Rat Skeletal Muscle Under Hypoxic Preconditioning and Endurance Training

Previously, we have reported the regulation of monocarboxylate transporters (MCT)1 and MCT4 by physiological stimuli such as hypoxia and exercise. In the present study, we have evaluated the effect of hypoxic preconditioning and training on expression of different MCT isoforms in muscles. We found the increased mRNA expression of MCT1, MCT11, and MCT12 after hypoxic preconditioning with cobalt chloride and training. However, the expression of other MCT isoforms increased marginally or even reduced after hypoxic preconditioning. Only the protein expression of MCT1 increased after hypoxia preconditioning. MCT2 protein expression increased after training only and MCT4 protein expression decreased both in preconditioning and hypoxic training. Furthermore, we found decreased plasma lactate level during hypoxia preconditioning (0.74-fold), exercise (0.78-fold), and hypoxia preconditioning along with exercise (0.67-fold), which indicates increased lactate uptake by skeletal muscle. The protein-protein interactions with hypoxia inducible factor-1 and MCT isoforms were also evaluated, but no interaction was found. In conclusion, we say that almost all MCTs are expressed in red gastrocnemius muscle at the mRNA level and their expression is regulated differently under hypoxia preconditioning and exercise condition.

Effects of traditional vs. alternating whole-body strength training on squat performance

Traditional strength training with 80% of 1 repetition maximum (1RM) uses 2- to 5-minute rest periods between sets. These long rest periods minimize decreases in volume and intensity but result in long workouts. Performing upper-body exercises during lower-body rest intervals may decrease workout duration but may affect workout performance. Therefore, the purpose of this study was to compare the effects of traditional vs. alternating whole-body strength training on squat performance. Twenty male (24 ± 2 years) volunteers performed 2 workouts. The traditional set (TS) workout consisted of 4 sets of squats (SQ) at 80% of 1RM on a force plate with 3-minute rest between sets. The alternating set (AS) workout also consisted of 4 sets of SQ at 80% of 1RM but with bench press, and bench pull exercises performed between squat sets 1, 2 and 3 with between-exercise rest of 50 seconds, resulting in approximately 3-minute rest between squat sets. Sets 1-3 were performed for 4 repetitions, whereas set 4 was performed to concentric failure. Total number of completed repetitions of the fourth squat set to failure was recorded. Peak ground reaction force (GRF), peak power (PP), and average power (AP) of every squat repetition were recorded and averaged for each set. There was no significant interaction for GRF, PP, or AP. However, volume-equated AP was greater during the TS condition (989 ± 183) than the AS condition (937 ± 176). During the fourth squat set, the TS condition resulted in more repetitions to failure (7.5 ± 2.2) than the AS condition (6.5 ± 2.2). Therefore, individuals who aim to optimize squat AP should refrain from performing more than 3 ASs per exercise. Likewise, those who aim to maximize squat repetitions to failure should refrain from performing upper-body multijoint exercises during squat rest intervals.

Hyperpolarized 13C NMR observation of lactate kinetics in skeletal muscle

The production of glycolytic end products, such as lactate, usually evokes a cellular shift from aerobic to anaerobic ATP generation and O2 insufficiency. In the classical view, muscle lactate must be exported to the liver for clearance. However, lactate also forms under well-oxygenated conditions, and this has led investigators to postulate lactate shuttling from non-oxidative to oxidative muscle fiber, where it can serve as a precursor. Indeed, the intracellular lactate shuttle and the glycogen shunt hypotheses expand the vision to include a dynamic mobilization and utilization of lactate during a muscle contraction cycle. Testing the tenability of these provocative ideas during a rapid contraction cycle has posed a technical challenge. The present study reports the use of hyperpolarized [1-(13)C]lactate and [2-(13)C]pyruvate in dynamic nuclear polarization (DNP) NMR experiments to measure the rapid pyruvate and lactate kinetics in rat muscle. With a 3 s temporal resolution, (13)C DNP NMR detects both [1-(13)C]lactate and [2-(13)C]pyruvate kinetics in muscle. Infusion of dichloroacetate stimulates pyruvate dehydrogenase activity and shifts the kinetics toward oxidative metabolism. Bicarbonate formation from [1-(13)C]lactate increases sharply and acetyl-l-carnitine, acetoacetate and glutamate levels also rise. Such a quick mobilization of pyruvate and lactate toward oxidative metabolism supports the postulated role of lactate in the glycogen shunt and the intracellular lactate shuttle models. The study thus introduces an innovative DNP approach to measure metabolite transients, which will help delineate the cellular and physiological role of lactate and glycolytic end products.

Endogenous Nutritive Support after Traumatic Brain Injury: Peripheral Lactate Production for Glucose Supply via Gluconeogenesis

We evaluated the hypothesis that nutritive needs of injured brains are supported by large and coordinated increases in lactate shuttling throughout the body. To that end, we used dual isotope tracer ([6,6-(2)H2]glucose, i.e., D2-glucose, and [3-(13)C]lactate) techniques involving central venous tracer infusion along with cerebral (arterial [art] and jugular bulb [JB]) blood sampling. Patients with traumatic brain injury (TBI) who had nonpenetrating head injuries (n=12, all male) were entered into the study after consent of patients' legal representatives. Written and informed consent was obtained from healthy controls (n=6, including one female). As in previous investigations, the cerebral metabolic rate (CMR) for glucose was suppressed after TBI. Near normal arterial glucose and lactate levels in patients studied 5.7±2.2 days (range of days 2-10) post-injury, however, belied a 71% increase in systemic lactate production, compared with control, that was largely cleared by greater (hepatic+renal) glucose production. After TBI, gluconeogenesis from lactate clearance accounted for 67.1% of glucose rate of appearance (Ra), which was compared with 15.2% in healthy controls. We conclude that elevations in blood glucose concentration after TBI result from a massive mobilization of lactate from corporeal glycogen reserves. This previously unrecognized mobilization of lactate subserves hepatic and renal gluconeogenesis. As such, a lactate shuttle mechanism indirectly makes substrate available for the body and its essential organs, including the brain, after trauma. In addition, when elevations in arterial lactate concentration occur after TBI, lactate shuttling may provide substrate directly to vital organs of the body, including the injured brain.

Cerebral metabolism following traumatic brain injury: new discoveries with implications for treatment

Because it is the product of glycolysis and main substrate for mitochondrial respiration, lactate is the central metabolic intermediate in cerebral energy substrate delivery. Our recent studies on healthy controls and patients following traumatic brain injury (TBI) using [6,6-(2)H2]glucose and [3-(13)C]lactate, along with cerebral blood flow (CBF) and arterial-venous (jugular bulb) difference measurements for oxygen, metabolite levels, isotopic enrichments and (13)CO2 show a massive and previously unrecognized mobilization of lactate from corporeal (muscle, skin, and other) glycogen reserves in TBI patients who were studied 5.7 ± 2.2 days after injury at which time brain oxygen consumption and glucose uptake (CMRO2 and CMRgluc, respectively) were depressed. By tracking the incorporation of the (13)C from lactate tracer we found that gluconeogenesis (GNG) from lactate accounted for 67.1 ± 6.9%, of whole-body glucose appearance rate (Ra) in TBI, which was compared to 15.2 ± 2.8% (mean ± SD, respectively) in healthy, well-nourished controls. Standard of care treatment of TBI patients in state-of-the-art facilities by talented and dedicated heath care professionals reveals presence of a catabolic Body Energy State (BES). Results are interpreted to mean that additional nutritive support is required to fuel the body and brain following TBI. Use of a diagnostic to monitor BES to provide health care professionals with actionable data in providing nutritive formulations to fuel the body and brain and achieve exquisite glycemic control are discussed. In particular, the advantages of using inorganic and organic lactate salts, esters and other compounds are examined. To date, several investigations on brain-injured patients with intact hepatic and renal functions show that compared to dextrose + insulin treatment, exogenous lactate infusion results in normal glycemia.

Lactate kinetics at the lactate threshold in trained and untrained men

To understand the meaning of the lactate threshold (LT) and to test the hypothesis that endurance training augments lactate kinetics [i.e., rates of appearance and disposal (Ra and Rd, respectively, mg·kg−1·min−1) and metabolic clearance rate (MCR, ml·kg−1·min−1)], we studied six untrained (UT) and six trained (T) subjects during 60-min exercise bouts at power outputs (PO) eliciting the LT. Trained subjects performed two additional exercise bouts at a PO 10% lower (LT-10%), one of which involved a lactate clamp (LC) to match blood lactate concentration ([lactate]b) to that achieved during the LT trial. At LT, lactate Ra was higher in T (24.1 ± 2.7) than in UT (14.6 ± 2.4; P < 0.05) subjects, but Ra was not different between UT and T when relative exercise intensities were matched (UT-LT vs. T-LT-10%, 67% V̇o2max). At LT, MCR in T (62.5 ± 5.0) subjects was 34% higher than in UT (46.5 ± 7.0; P < 0.05), and a reduction in PO resulted in a significant increase in MCR by 46% (LT-10%, 91.5 ± 14.9, P < 0.05). At matched relative exercise intensities (67% V̇o2max), MCR in T subjects was 97% higher than in UT ( P < 0.05). During the LC trial, MCR in T subjects was 64% higher than in UT ( P < 0.05), in whom %V̇o2max and [lactate]b were similar. We conclude that 1) lactate MCR reaches an apex below the LT, 2) LT corresponds to a limitation in MCR, and 3) endurance training augments capacities for lactate production, disposal and clearance.

Impact of nutrition on muscle mass, strength, and performance in older adults

Muscle strength plays an important role in determining risk for falls, which result in fractures and other injuries. While bone loss has long been recognized as an inevitable consequence of aging, sarcopenia-the gradual loss of skeletal muscle mass and strength that occurs with advancing age-has recently received increased attention. A review of the literature was undertaken to identify nutritional factors that contribute to loss of muscle mass. The role of protein, acid-base balance, vitamin D/calcium, and other minor nutrients like B vitamins was reviewed. Muscle wasting is a multifactorial process involving intrinsic and extrinsic alterations. A loss of fast twitch fibers, glycation of proteins, and insulin resistance may play an important role in the loss of muscle strength and development of sarcopenia. Protein intake plays an integral part in muscle health and an intake of 1.0-1.2 g/kg of body weight per day is probably optimal for older adults. There is a moderate [corrected] relationship between vitamin D status and muscle strength. Chronic ingestion of acid-producing diets appears to have a negative impact on muscle performance, and decreases in vitamin B12 and folic acid intake may also impair muscle function through their action on homocysteine. An adequate nutritional intake and an optimal dietary acid-base balance are important elements of any strategy to preserve muscle mass and strength during aging.

Sodium bicarbonate treatment during transient or sustained lactic acidemia in normoxic and normotensive rats

Introduction

Lactic acidosis is a frequent cause of poor outcome in the intensive care settings. We set up an experimental model of lactic acid infusion in normoxic and normotensive rats to investigate the systemic effects of lactic acidemia per se without the confounding factor of an underlying organic cause of acidosis.

Methodology

Sprague Dawley rats underwent a primed endovenous infusion of L(+) lactic acid during general anesthesia. Normoxic and normotensive animals were then randomized to the following study groups (n = 8 per group): S) sustained infusion of lactic acid, S+B) sustained infusion+sodium bicarbonate, T) transient infusion, T+B transient infusion+sodium bicarbonate. Hemodynamic, respiratory and acid-base parameters were measured over time. Lactate pharmacokinetics and muscle phosphofructokinase enzyme's activity were also measured.

Principal Findings

Following lactic acid infusion blood lactate rose (P<0.05), pH (P<0.05) and strong ion difference (P<0.05) drop. Some rats developed hemodynamic instability during the primed infusion of lactic acid. In the normoxic and normotensive animals bicarbonate treatment normalized pH during sustained infusion of lactic acid (from 7.22±0.02 to 7.36±0.04, P<0.05) while overshoot to alkalemic values when the infusion was transient (from 7.24±0.01 to 7.53±0.03, P<0.05). When acid load was interrupted bicarbonate infusion affected lactate wash-out kinetics (P<0.05) so that blood lactate was higher (2.9±1 mmol/l vs. 1.0±0.2, P<0.05, group T vs. T+B respectively). The activity of phosphofructokinase enzyme was correlated with blood pH (R2 = 0.475, P<0.05).

Conclusions

pH decreased with acid infusion and rose with bicarbonate administration but the effects of bicarbonate infusion on pH differed under a persistent or transient acid load. Alkalization affected the rate of lactate disposal during the transient acid load.

The SLC16A family of monocarboxylate transporters (MCTs)--physiology and function in cellular metabolism, pH homeostasis, and fluid transport

The SLC16A family of monocarboxylate transporters (MCTs) is composed of 14 members. MCT1 through MCT4 (MCTs 1-4) are H(+)-coupled monocarboxylate transporters, MCT8 and MCT10 transport thyroid hormone and aromatic amino acids, while the substrate specificity and function of other MCTs have yet to be determined. The focus of this review is on MCTs 1-4 because their role in lactate transport is intrinsically linked to cellular metabolism in various biological systems, including skeletal muscle, brain, retina, and testis. Although MCTs 1-4 all transport lactate, they differ in their transport kinetics and vary in tissue and subcellular distribution, where they facilitate "lactate-shuttling" between glycolytic and oxidative cells within tissues and across blood-tissue barriers. However, the role of MCTs 1-4 is not confined to cellular metabolism. By interacting with bicarbonate transport proteins and carbonic anhydrases, MCTs participate in the regulation of pH homeostasis and fluid transport in renal proximal tubule and corneal endothelium, respectively. Here, we provide a comprehensive review of MCTs 1-4, linking their cellular distribution to their functions in various parts of the human body, so that we can better understand the physiological roles of MCTs at the systemic level.

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.

Transpulmonary pyruvate kinetics

Shuttling of intermediary metabolites, such as pyruvate, contributes to the dynamic energy and biosynthetic needs of tissues. Tracer kinetic studies offer a powerful tool to measure the metabolism of substrates like pyruvate that are simultaneously taken up from and released into the circulation by organs. However, we understood that during each circulatory passage, the entire cardiac output transits the pulmonary circulation. Therefore, we examined the transpulmonary pyruvate kinetics in an anesthetized rat model during an unstimulated (Con), lactate clamp (LC), and epinephrine infusion (Epi) conditions using a primed-continuous infusion of [U-¹³C]pyruvate. Compared with Con and Epi stimulation, LC significantly increased mixed central venous ([v]) and arterial ([a]) pyruvate concentrations (P < 0.05). We hypothesized that the lungs, specifically the pulmonary capillary beds are sites of simultaneous production and removal of pyruvate and contributes significantly to whole body carbohydrate intermediary metabolism. Transpulmonary net pyruvate balances were positive during all three conditions, indicating net pyruvate uptake. Net balance was significantly greater during epinephrine stimulation compared with the unstimulated control (P < 0.05). Tracer-measured pyruvate fractional extraction averaged 42.8 ± 5.8% for all three conditions and was significantly higher during epinephrine stimulation (P < 0.05) than during either Con or LC conditions, that did not differ from each other. Pyruvate total release (tracer measured uptake - net balance) was significantly higher during epinephrine stimulation (400 ± 100 μg/min) vs. Con (30 ± 20 μg/min) (P < 0.05). These data are interpreted to mean that significant pyruvate extraction occurs during circulatory transport across lung parenchyma. The extent of pulmonary parenchymal pyruvate extraction predicts high expression of monocarboxylate (lactate/pyruvate) transporters (MCTs) in the tissue. Western blot analysis of whole lung homogenates detected three isoforms, MCT1, MCT2, and MCT4. We conclude that a major site of circulating pyruvate extraction resides with the lungs and that during times of elevated circulating lactate, pyruvate, or epinephrine stimulation, pyruvate extraction is increased.

Control of MCT1 function in cerebrovascular endothelial cells by intracellular pH

Monocarboxylic Acid Transporter 1 (MCT1) is expressed on the plasma membrane of cerebrovascular endothelial cells where it is the only known facilitator of lactic acid transport across the blood brain barrier. During stroke, brain injury, and certain other brain pathologies, anaerobic glycolysis produces severe lactic acidosis of brain tissue leading to brain cell damage. Therefore, a better understanding of factors that control MCT1 function may be the key to better understanding the origins and treatment of pathological lactic acidosis. In this study, we characterized the effects of intracellular pH in controlling MCT1 function and showed that microtubule disruption targeted this mechanism in rat cerebrovascular endothelial cells. Acidic intracellular pH values were shown to strongly inhibit lactic acid transport into the cytoplasmic space, while alkalinization of the cytoplasm significantly enhanced this transport function. These results support a better understanding of how cerebrovascular endothelial MCT1 may contribute to the development of lactic acidosis in brain pathologies, and suggest targeting it as a novel therapy.

Mitochondrial and plasma membrane lactate transporter and lactate dehydrogenase isoform expression in breast cancer cell lines

We hypothesized that dysregulation of lactate/pyruvate (monocarboxylate) transporters (MCT) and lactate dehydrogenase (LDH) isoforms contribute to the Warburg effect in cancer. Therefore, we assayed for the expression levels and the localizations of MCT (1, 2, and 4), and LDH (A and B) isoforms in breast cancer cell lines MCF-7 and MDA-MB-231 and compared results with those from a control, untransformed primary breast cell line, HMEC 184. Remarkably, MCT1 is not expressed in MDA-MB-231, but MCT1 is expressed in MCF-7 cells, where its abundance is less than in control HMEC 184 cells. When present in HMEC 184 and MCF-7 cells, MCT1 is localized to the plasma membrane. MCT2 and MCT4 were expressed in all the cell lines studied. MCT4 expression was higher in MDA-MB-231 compared with MCF-7 and HMEC 184 cells, whereas MCT2 abundance was higher in MCF-7 compared with MDA-MB-231 and HMEC 184 cells. Unlike MCT1, MCT2 and MCT4 were localized in mitochondria in addition to the plasma membrane. LDHA and LDHB were expressed in all the cell-lines, but abundances were higher in the two cancer cell lines than in the control cells. MCF-7 cells expressed mainly LDHB, while MDA-MB-231 and control cells expressed mainly LDHA. LDH isoforms were localized in mitochondria in addition to the cytosol. These localization patterns were the same in cancerous and control cell lines. In conclusion, MCT and LDH isoforms have distinct expression patterns in two breast cancer cell lines. These differences may contribute to divergent lactate dynamics and oxidative capacities in these cells, and offer possibilities for targeting cancer cells.

Involvement of the cystic fibrosis transmembrane conductance regulator in the acidosis-induced efflux of ATP from rat skeletal muscle

The present study was performed to investigate the effect of acidosis on the efflux of ATP from skeletal muscle. Infusion of lactic acid to the perfused hindlimb muscles of anaesthetised rats produced dose-dependent decreases in pH and increases in the interstitial ATP of extensor digitorum longus (EDL) muscle: 10 mM lactic acid reduced the venous pH from 7.22 ± 0.04 to 6.97 ± 0.02 and increased interstitial ATP from 38 ± 8 to 67 ± 11 nM. The increase in interstitial ATP was well-correlated with the decrease in pH (r(2) = 0.93; P < 0.05). Blockade of cellular uptake of lactic acid using α-cyano-hydroxycinnamic acid abolished the lactic acid-induced ATP release, whilst infusion of sodium lactate failed to depress pH or increase interstitial ATP, suggesting that intracellular pH depression, rather than lactate, stimulated the ATP efflux. Incubation of cultured skeletal myoblasts with 10 mM lactic acid significantly increased the accumulation of ATP in the bathing medium from 0.46 ± 0.06 to 0.76 ± 0.08 μM, confirming the skeletal muscle cells as the source of the released ATP. Acidosis-induced ATP efflux from the perfused muscle was abolished by CFTR(inh)-172, a specific inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR), or glibenclamide, an inhibitor of both K(ATP) channels and CFTR, but it was not affected by atractyloside, an inhibitor of the mitochondrial ATP transporter. Silencing of the CFTR gene using an siRNA abolished the acidosis-induced increase in ATP release from cultured myoblasts. CFTR expression on skeletal muscle cells was confirmed using immunostaining in the intact muscle and Western blotting in the cultured cells. These data suggest that depression of the intracellular pH of skeletal muscle cells stimulates ATP efflux, and that CFTR plays an important role in the release mechanism.

Lactate distribution in culture medium of human myometrial biopsies incubated under different conditions

It is generally believed that a relationship exists between muscle fatigue and intracellular accumulation of lactate. This reasoning is relevant to obstetrical issues. Myocytes in uterus work together during labor, and the contractions need to be strong and synchronized for a child to be delivered. At labor dystocia, the progress of labor becomes slow or arrested after a normal beginning. It has been described that, during labor dystocia, when the force of the contractions is low, the uterus is under hypoxia, and anaerobic conditions with high levels of lactate in amniotic fluid dominate. The purpose of this study was to examine whether myometrial cells are involved in the production of lactate in amniotic fluid and whether there are differences in production and distribution of lactate in cells incubated under aerobic and anaerobic conditions. We also wanted to elucidate the involvement of specific membrane-bound lactate carriers. Women undergoing elective caesarean section were included. Myometrial biopsies from uteri were collected and subjected to either immunohistochemistry to identify lactate carriers or in vitro experiments to analyze production of lactate. The presence of lactate carriers named monocarboxylate transporters 1 and 4 was verified. Myometrial cells produced lactate extracellularly, and the lactate carriers operated differently under anaerobic and aerobic conditions; while being mainly unidirectional under anaerobic conditions, they became bidirectional under aerobic conditions. Human myometrial cells produced and delivered lactate to the extracellular medium under both anaerobic and aerobic conditions. The delivery was mediated by lactate carriers.

Cell-cell and intracellular lactate shuttles

Once thought to be the consequence of oxygen lack in contracting skeletal muscle, the glycolytic product lactate is formed and utilized continuously in diverse cells under fully aerobic conditions. 'Cell-cell' and 'intracellular lactate shuttle' concepts describe the roles of lactate in delivery of oxidative and gluconeogenic substrates as well as in cell signalling. Examples of the cell-cell shuttles include lactate exchanges between between white-glycolytic and red-oxidative fibres within a working muscle bed, and between working skeletal muscle and heart, brain, liver and kidneys. Examples of intracellular lactate shuttles include lactate uptake by mitochondria and pyruvate for lactate exchange in peroxisomes. Lactate for pyruvate exchanges affect cell redox state, and by itself lactate is a ROS generator. In vivo, lactate is a preferred substrate and high blood lactate levels down-regulate the use of glucose and free fatty acids (FFA). As well, lactate binding may affect metabolic regulation, for instance binding to G-protein receptors in adipocytes inhibiting lipolysis, and thus decreasing plasma FFA availability. In vitro lactate accumulation upregulates expression of MCT1 and genes coding for other components of the mitochondrial reticulum in skeletal muscle. The mitochondrial reticulum in muscle and mitochondrial networks in other aerobic tissues function to establish concentration and proton gradients necessary for cells with high mitochondrial densities to oxidize lactate. The presence of lactate shuttles gives rise to the realization that glycolytic and oxidative pathways should be viewed as linked, as opposed to alternative, processes, because lactate, the product of one pathway, is the substrate for the other.

Lactate, fructose and glucose oxidation profiles in sports drinks and the effect on exercise performance

Exogenous carbohydrate oxidation was assessed in 6 male Category 1 and 2 cyclists who consumed CytoMax™ (C) or a leading sports drink (G) before and during continuous exercise (CE). C contained lactate-polymer, fructose, glucose and glucose polymer, while G contained fructose and glucose. Peak power output and VO₂ on a cycle ergometer were 408±13 W and 67.4±3.2 mlO₂·kg⁻¹·min⁻¹. Subjects performed 3 bouts of CE with C, and 2 with G at 62% VO₂peak for 90 min, followed by high intensity (HI) exercise (86% VO₂peak) to volitional fatigue. Subjects consumed 250 ml fluid immediately before (−2 min) and every 15 min of cycling. Drinks at −2 and 45 min contained 100 mg of [U-¹³C]-lactate, -glucose or -fructose. Blood, pulmonary gas samples and ¹³CO₂ excretion were taken prior to fluid ingestion and at 5,10,15,30,45,60,75, and 90 min of CE, at the end of HI, and 15 min of recovery. HI after CE was 25% longer with C than G (6.5±0.8 vs. 5.2±1.0 min, P<0.05). ¹³CO₂ from the −2 min lactate tracer was significantly elevated above rest at 5 min of exercise, and peaked at 15 min. ¹³CO₂ from the −2 min glucose tracer peaked at 45 min for C and G. ¹³CO₂ increased rapidly from the 45 min lactate dose, and by 60 min of exercise was 33% greater than glucose in C or G, and 36% greater than fructose in G. ¹³CO₂ production following tracer fructose ingestion was greater than glucose in the first 45 minutes in C and G. Cumulative recoveries of tracer during exercise were: 92%±5.3% for lactate in C and 25±4.0% for glucose in C or G. Recoveries for fructose in C and G were 75±5.9% and 26±6.6%, respectively. Lactate was used more rapidly and to a greater extent than fructose or glucose. CytoMax significantly enhanced HI.

Lactate: link between glycolytic and oxidative metabolism

Once thought to be the consequence of oxygen lack in contracting skeletal muscle, the glycolytic product lactate is formed and utilised continuously under fully aerobic conditions. 'Cell-cell' and 'intracellular lactate shuttle' concepts describe the roles of lactate in delivery of oxidative and gluconeogenic substrates as well as in cell signalling. Examples of cell-cell shuttles include lactate exchanges (i) between white-glycolytic and red-oxidative fibres within a working muscle bed; (ii) between working skeletal muscle and heart; and (iii) between tissues of net lactate release and gluconeogenesis. Lactate shuttles exist in diverse tissues including in the brain, where a shuttle between astrocytes and neurons is linked to glutamatergic signalling. Because lactate, the product of glycogenolysis and glycolysis, is disposed of by oxidative metabolism, lactate shuttling unites the two major processes of cellular energy transduction. Lactate disposal is mainly through oxidation, especially during exercise when oxidation accounts for 70-75% of removal and gluconeogenesis the remainder. Lactate flux occurs down proton and concentration gradients that are established by the mitochondrial lactate oxidation complex. Marathon running is a power activity requiring high glycolytic and oxidative fluxes; such activities require lactate shuttling. Knowledge of the lactate shuttle is yet to be imparted to the sport.

Hyperventilation-induced hypocapnic alkalosis slows the adaptation of pulmonary O2 uptake during the transition to moderate-intensity exercise

The effect of voluntary hyperventilation-induced hypocapnic alkalosis (RALK) on pulmonary O2 uptake (VO2) kinetics and muscle deoxygenation was examined in young male adults (n=8) during moderate-intensity exercise. Subjects performed five repetitions of a step-transition in work rate from 20 W cycling to a work rate corresponding to 90% of the estimated lactate threshold during control (CON; PET,CO2, approximately 40 mmHg) and during hyperventilation (RALK; PET,CO2, approximately 20 mmHg). was measured breath-by-breath and relative concentration changes in muscle deoxy- (DeltaHHb), oxy- (DeltaO2Hb) and total (DeltaHbtot) haemoglobin were measured continuously using near-infrared (NIR) spectroscopy (Hamamatsu, NIRO 300). The time constant for the fundamental, phase 2, VO2 response (tau VO2) was greater (P<0.05) in RALK (48+/-11 s) than CON (31+/-9 s), while tauHHb was similar between conditions (RALK, 12+/-4 s; CON, 11+/-4 s). The DeltaHb(tot) was lower (P<0.05) in RALK than CON, prior to (RALK, -3+/-5 micromol l(-1); CON, -1+/-4 micromol l(-1)) and at the end (RALK, 1+/-6 micromol l(-1); CON, 5+/-5 micromol l(-1)) of moderate-intensity exercise. Although slower adaptation of during RALK may be related to an attenuated activation of PDH (and other enzymes) and provision of oxidizable substrate to the mitochondria (i.e. metabolic inertia), the present findings also suggest a role for a reduction in local muscle perfusion and O2 delivery.

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.

Lactate sensitive transcription factor network in L6 cells: activation of MCT1 and mitochondrial biogenesis

We hypothesized that in addition to serving as a fuel source and gluconeogenic precursor, lactate anion (La-) is a signaling molecule. Therefore, we screened genome-wide responses of L6 cells to elevated (10 and 20 mM) sodium-La- added to buffered, high-glucose media. Lactate increased reactive oxygen species (ROS) production and up-regulated 673 genes, many known to be responsive to ROS and Ca2+. The induction of genes encoding for components of the mitochondrial lactate oxidation complex was confirmed by independent methods (PCR and EMSA). Specifically, lactate increased monocarboxylate transporter-1 (MCT1) mRNA and protein expression within 1 h and cytochrome c oxidase (COX) mRNA and protein expression in 6 h. Increases in COX coincided with increases in peroxisome proliferator activated-receptor gamma coactivator-1alpha (PGC1alpha) expression and the DNA binding activity of nuclear respiratory factor (NRF)-2. We conclude that the lactate signaling cascade involves ROS production and converges on transcription factors affecting mitochondrial biogenesis.

Colocalization of MCT1, CD147, and LDH in mitochondrial inner membrane of L6 muscle cells: evidence of a mitochondrial lactate oxidation complex

Results of previous studies suggested a role of mitochondria in intracellular and cell-cell lactate shuttles. Therefore, by using a rat-derived L6 skeletal muscle cell line and confocal laser-scanning microscopy (CLSM), we examined the cellular locations of mitochondria, lactate dehydrogenase (LDH), the lactate-pyruvate transporter MCT1, and CD147, a purported chaperone protein for MCT1. CLSM showed that LDH, MCT1, and CD147 are colocalized with the mitochondrial reticulum. Western blots showed that cytochrome oxidase (COX), NADH dehydrogenase, LDH, MCT1, and CD147 are abundant in mitochondrial fractions of L6 cells. Interactions among COX, MCT1, and CD147 in mitochondria were confirmed by immunoblotting after immunoprecipitation. These findings support the presence of a mitochondrial lactate oxidation complex associated with the COX end of the electron transport chain that might explain the oxidative catabolism of lactate and, hence, mechanism of the intracellular lactate shuttle.

Immunohistochemical analysis of MCT1, MCT2 and MCT4 expression in rat plantaris muscle

We addressed the need for histological assessment of myocellular domains occupied by monocarboxylate transporters (MCT1, MCT2 and MCT4). From the perspective of lactate shuttle hypotheses we posited that MCT1 would be highly expressed in oxidative fibres, whereas MCT4 would be found in highly glycolytic fibres. Furthermore, we hypothesized that MCT1 would be detected at interfibrillar as well as at subsarcolemmal and sarcolemmal cell domains, whereas MCT2 and MCT4 abundances would be most prominent at the sarcolemma. To test these hypotheses, we examined cellular locations of MCT1, MCT2 and MCT4 transporter proteins in different fibre types (slow oxidative, SO; fast oxidative glycolytic, FOG; fast glycolytic, FG) in rat plantaris muscles by the avidin-biotin complex (ABC) as well as other methods. The plantaris was used as it is a mixed fibre skeletal muscle. MCTs, glucose transporter (GLUT4) protein, and mitochondrial constituent cytochrome oxidase (COX) abundances were assessed by immunohistochemistry and Western blotting using affinity-purified antibodies. The staining method was specific and stable, which allowed for semiquantitative assessment of MCT expression. As well, confocal laser scanning microscopy assessed MCT isoform localizations. Findings of the present study were: (1) MCT1 is located at the sarcolemma and throughout the cell interior in SO and FOG fibres where the mitochondrial reticulum was present; (2) in contrast, MCT4 was highly expressed in the sarcolemmal domain of FG and FOG fibres but poorly expressed in SO fibres; and (3) confocal laser-scanning microscopy demonstrated that MCT1 and COX are co-localised at both interfibrillar and subsarcolemmal cell domains, whereas MCT2 is only faintly detected at the sarcolemma of oxidative fibres. MCTs and associated proteins are positioned to facilitate the function of the lactate shuttles.

Hematological and acid-base changes in men during prolonged exercise with and without sodium-lactate infusion

An emerging technique used for the study of metabolic regulation is the elevation of lactate concentration with a sodium-lactate infusion, the lactate clamp (LC). However, hematological and acid-base properties affected by the infusion of hypertonic solutions containing the osmotically active strong ions sodium (Na+) and lactate (Lac−) are a concern for clinical and research applications of LC. In the present study, we characterized the hematological and plasma acid-base changes during rest and prolonged, light- to moderate-intensity (55% V̇o2 peak) exercise with and without LC. During the control (Con) trial, subjects were administered an isotonic, isovolumetric saline infusion. During LC, plasma lactate concentration ([Lac−]) was elevated to 4 meq/l during rest and to 4–7 meq/l during exercise. During LC at rest, there were rapid and transient changes in plasma, erythrocyte, and blood volumes. LC resulted in decreased plasma [H+] (from 39.6 to 29.6 neq/l) at the end of exercise while plasma [HCO3−] increased from 26 to 32.9 meq/l. Increased plasma strong ion difference [SID], due to increased [Na+], was the primary contributor to decreased [H+] and increased [HCO3−]. A decrease in plasma total weak acid concentration also contributed to these changes, whereas Pco2 contributed little. The infusion of hypertonic LC caused only minor volume, acid-base, and CO2 storage responses. We conclude that an LC infusion is appropriate for studies of metabolic regulation.

Effects of Acute Ingestion of Sodium Citrate on Metabolism and 5-km Running Performance: A Field Study

The purpose of the study was to (a) assess the effects of sodium citrate ingestion on metabolism and performance capacity in a 5-km competitive outdoor stadium run in trained male runners, and (b) elucidate the potential relationship between citrate-induced changes in plasma volume, body mass, and performance. Ten subjects (age 22.1 ± 2.5 yrs, body mass 74.1 ± 6.1 kg, height 180.1 ± 5.7 cm, [Formula: see text] 60.8 ± 5.5 ml kg−1•min−1) participated in the study. There was no effect of treatment on 5-km running time: 1100.0 ± 79.1 and 1082.7 ± 62.0 s in citrate (CIT) and in placebo (PLC) trials, respectively, p = 0.09. Blood pH increased from 7.34 ± 0.07 to 7.49 ± 0.07 (p = 0.002) as a result of administering sodium citrate in the amount of 0.5 g•kg−1 body mass in 1.5 litres of solution but remained stable while the equal volume of placebo drink was consumed: 7.40 ± 0.04 and 7.44 ± 0.09. The relative change in plasma volume after administering the drink was -1.99 ± 3.49% in the PLC and 9.75 ± 6.51% in the CIT trial (p = 0.001). Body mass did not differ before drinking; however, before the start the subjects were heavier in the CIT trial (74.2 ± 6.1 kg) vs. the PLC trial (73.4 ± 6.2 kg, p = 0.048). The shifts in plasma volume and body mass were not related to changes in performance. The results suggest that ingestion of sodium citrate induces an increase in water retention, plasma volume, and blood pH before exercise but does not improve performance in a 5-km competitive run infield conditions in trained male runners. Key words: buffering solution, endurance performance, plasma volume, body mass

Exercise rapidly increases expression of the monocarboxylate transporters MCT1 and MCT4 in rat muscle

We examined the effect of a single exercise session on the protein and mRNA expression of the monocarboxylate transporters MCT1 and MCT4 in rat soleus (SOL), and red (RG) and white gastrocnemius (WG) muscles. Muscle samples were obtained at rest before 2 h of treadmill exercise (21 m min(-1), 15% grade) and immediately after exercise, as well as 5, 10 and 24 h after exercise. During the 2 h exercise bout, MCT1 proteins in RG (+60%) and WG (+56%) were increased (P < 0.05). MCT1 protein was further increased thereafter, with peak increments occurring 10 h after exercise in RG (+157%), WG (+193%) and SOL (+179%) (P < 0.05). Twenty-four hours after exercise, MCT1 protein was still up-regulated in WG (+100%) and SOL (+55%) (P < 0.05), but not in RG. MCT1 mRNA was up-regulated during exercise in RG (+53%) and WG (+98%) and remained elevated until 24 h post-exercise in RG (P < 0.05), but in WG, MCT1 mRNA decreased transiently to pre-exercise levels at 5 and 10 h after exercise, before increasing again at 24 h (+150%) (P < 0.05). MCT4 protein and mRNA were not increased in WG muscle during and after exercise (P > 0.05). In contrast, during exercise, in RG (+41%) and SOL (+98%) MCT4 protein was increased (P < 0.05). Peak increases in MCT4 protein were observed 10 h after exercise in RG (+131%) and SOL (+323%) (P < 0.05). MCT4 protein was still up-regulated 24 h after exercise (RG: +106%; SOL +225%) (P < 0.05). MCT4 mRNA in RG was not increased until 10 (+132%) and 24 h after exercise (+55%) (P < 0.05). These studies have shown that MCT1 and 4 proteins are transiently up-regulated by a single bout of exercise, involving post-transcriptional and transcriptional mechanisms. Thus, MCT1 and MCT4 belong to a class of selected metabolic genes that are very rapidly up-regulated with an exercise stimulus.

Metabolic effects of induced alkalosis during progressive forearm exercise to fatigue

Metabolic alkalosis induced by sodium bicarbonate (NaHCO3) ingestion has been shown to enhance performance during brief high-intensity exercise. The mechanisms associated with this increase in performance may include increased muscle phosphocreatine (PCr) breakdown, muscle glycogen utilization, and plasma lactate (Lac-pl) accumulation. Together, these changes would imply a shift toward a greater contribution of anaerobic energy production, but this statement has been subject to debate. In the present study, subjects ( n = 6) performed a progressive wrist flexion exercise to volitional fatigue (0.5 Hz, 14–21 min) in a control condition (Con) and after an oral dose of NaHCO3 (Alk: 0.3 g/kg; 1.5 h before testing) to evaluate muscle metabolism over a complete range of exercise intensities. Phosphorus-31 magnetic resonance spectroscopy was used to continuously monitor intracellular pH, [PCr], [Pi], and [ATP] (brackets denote concentration). Blood samples drawn from a deep arm vein were analyzed with a blood gas-electrolyte analyzer to measure plasma pH, Pco2, and [Lac-]pl, and plasma [Formula: see text] was calculated from pH and Pco2. NaHCO3 ingestion resulted in an increased ( P < 0.05) plasma pH and [Formula: see text] throughout rest and exercise. Time to fatigue and peak power output were increased ( P < 0.05) by ∼12% in Alk. During exercise, a delayed ( P < 0.05) onset of intracellular acidosis (1.17 ± 0.26 vs. 1.28 ± 0.22 W, Con vs. Alk) and a delayed ( P < 0.05) onset of rapid increases in the [Pi]-to-[PCr] ratio (1.21 ± 0.30 vs. 1.30 ± 0.30 W) were observed in Alk. No differences in total [H+], [Pi], or [Lac-]pl accumulation were detected. In conclusion, NaHCO3 ingestion was shown to increase plasma pH at rest, which resulted in a delayed onset of intracellular acidification during incremental exercise. Conversely, NaHCO3 was not associated with increased [Lac-]pl accumulation or PCr breakdown.