Lactate and force production in skeletal muscle

Implementation of 3D printing technologies to electrochemical and optical biosensors developed for biomedical and pharmaceutical analysis

Three-dimensional (3D) printing technology has been applied in many areas. In recent years, new generation biosensorshave been emerged with the progress on 3D printing technology (3DPT) . Especially in the development of optical and electrochemical biosensors, 3DPT provides many advantages such as low cost, easy to manufacturing, being disposable and allow point of care testing. In this review, recent trends in the development of 3DPT based electrochemical and optical biosensors with their applications in the field of biomedical and pharmaceutical are examined. In addition, the advantages, disadvantages and future opportunities of 3DPT are discussed.

Nelumbinis Stamen Ameliorates Chronic Restraint Stress-Induced Muscle Dysfunction and Fatigue in Mice by Decreasing Serum Corticosterone Levels and Activating Sestrin2

Nelumbo nucifera Gaertn. is an important aquatic vegetable, and its dried stamen (Nelumbinis stamen, NS) is a valuable nutraceutical usually used as a herbal tea. Here, we used ultrahigh-performance liquid chromatography (UPLC)-quadrupole time-of-flight mass spectrometry and high-performance liquid chromatography (HPLC) to chemically profile NS and quantify their main constituent flavonoids, respectively. In total, 44 components were identified, including organic acids, flavonoids, monoterpene glycosides, and fatty acids. Experimental mice were induced with fatigue by exposure to chronic restraint stress (CRS) for 8 h daily for 15 days and then treated with an aqueous extract of NS (0.5 and 1 g/kg) via gavage. NS significantly mitigated CRS-induced skeletal muscle dysfunction and fatigue in mice possibly by lowering serum corticosterone levels and restoring Sestrin2 expression in the gastrocnemius to regulate metabolism, preserve mitochondrial homeostasis, and promote antioxidant capacity. These results demonstrate that NS can be used as a nutraceutical or supplement for controlling stress-induced muscle dysfunction and fatigue.

Systematic Investigations on the Metabolic and Transcriptomic Regulation of Lactate in the Human Colon Epithelial Cells

Lactate, primarily produced by the gut microbiota, performs as a necessary “information transmission carrier” between the gut and the microbiota. To investigate the role of lactate in the gut epithelium cell–microbiota interactions as a metabolic signal, we performed a combinatory, global, and unbiased analysis of metabolomic and transcriptional profiling in human colon epithelial cells (Caco-2), using a lactate treatment at the physiological concentration (8 mM). The data demonstrated that most of the genes in oxidative phosphorylation were significantly downregulated in the Caco-2 cells due to lactate treatment. Consistently, the levels of fumarate, adenosine triphosphate (ATP), and creatine significantly decreased, and these are the metabolic markers of OXPHOS inhibition by mitochondria dysfunction. The one-carbon metabolism was affected and the polyol pathway was activated at the levels of gene expression and metabolic alternation. In addition, lactate significantly upregulated the expressions of genes related to self-protection against apoptosis. In conclusion, lactate participates in gut–gut microbiota communications by remodeling the metabolomic and transcriptional signatures, especially for the regulation of mitochondrial function. This work contributes comprehensive information to disclose the molecular mechanisms of lactate-mediated functions in human colon epithelial cells that can help us understand how the microbiota communicates with the intestines through the signaling molecule, lactate.

Lower Muscle and Blood Lactate Accumulation in Sickle Cell Trait Carriers in Response to Short High-Intensity Exercise

It remains unclear whether sickle cell trait (SCT) should be considered a risk factor during intense physical activity. By triggering the polymerization-sickling-vaso-occlusion cascade, lactate accumulation-associated acidosis in response to high-intensity exercise is believed to be one of the causes of complications. However, our understanding of lactate metabolism in response to high-intensity exercise in SCT carriers is incomplete. Thirty male SCT carriers (n = 15) and healthy subjects (n = 15) with and without α-thalassemia performed a 2-min high-intensity exercise. Blood and muscle lactate concentrations were measured at exercise completion. Time courses of blood lactate and glucose concentrations were followed during the subsequent recovery. Additional biochemical analyses were performed on biopsies of the vastus lateralis muscle. SCT was associated with lower blood and muscle lactate concentrations in response to the short high-intensity exercise. Compared to controls, the muscle content among SCT carriers of lactate transporter MCT4 and β2-adrenergic receptor were higher and lower, respectively. During recovery, the lactate removal ability was higher in SCT carriers. In the present study, no effect of α-thalassemia was observed. The lower blood and muscle lactate accumulations in SCT carriers may, to some extent, act as protective mechanisms: (i) against exercise-related acidosis and subsequent sickling, that may explain the relatively rare complications observed in exercising SCT carriers; and (ii) against the deleterious effects of intracellular lactate and associated acidosis on muscle function, that might explain the elevated presence of SCT carriers among the best sprinters.

Potassium-induced potentiation of subtetanic force in rat skeletal muscles: influences of β2-activation, lactic acid, and temperature

Moderate elevations of extracellular K+ concentration ([K+]o) occur during exercise and have been shown to potentiate force during contractions elicited with subtetanic frequencies. Here, we investigated whether lactic acid (reduced chloride conductance), β2-adrenoceptor activation, and increased temperature would influence the potentiating effect of potassium in slow- and fast-twitch muscles. Isometric contractions were elicited by electrical stimulation at various frequencies in isolated rat soleus and extensor digitorum longus (EDL) muscles incubated at normal (4 mM) or elevated K+, in combination with salbutamol (5 μM), lactic acid (18.1 mM), 9-anthracene-carboxylic acid (9-AC; 25 μM), or increased temperature (30-35°C). Elevating [K+]o from 4 mM to 7 mM (soleus) and 10 mM (EDL) potentiated isometric twitch and subtetanic force while slightly reducing tetanic force. In EDL, salbutamol further augmented twitch force (+27 ± 3%, P < 0.001) and subtetanic force (+22 ± 4%, P < 0.001). In contrast, salbutamol reduced subtetanic force (-28 ± 6%, P < 0.001) in soleus muscles. Lactic acid and 9-AC had no significant effects on isometric force of muscles already exposed to moderate elevations of [K+]o. The potentiating effect of elevated [K+]o was still well maintained at 35°C. Addition of salbutamol exerts a further force-potentiating effect in fast-twitch but not in slow-twitch muscles already potentiated by moderately elevated [K+]o, whereas lactic acid, 9-AC, or increased temperature does not exert any further augmentation. However, the potentiating effect of elevated [K+]o was still maintained in the presence of these, thus emphasizing the positive influence of moderately elevated [K+]o for contractile performance during exercise.

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

Endocrine Mechanisms Connecting Exercise to Brown Adipose Tissue Metabolism: a Human Perspective

PURPOSE OF REVIEW

To summarize the state-of-the-art regarding the exercise-regulated endocrine signals that might modulate brown adipose tissue (BAT) activity and/or white adipose tissue (WAT) browning, or through which BAT communicates with other tissues, in humans.

RECENT FINDINGS

Exercise induces WAT browning in rodents by means of a variety of physiological mechanism. However, whether exercise induces WAT browning in humans is still unknown. Nonetheless, a number of protein hormones and metabolites, whose signaling can influence thermogenic adipocyte's metabolism, are secreted during and/or after exercise in humans from a variety of tissues and organs, such as the skeletal muscle, the adipose tissue, the liver, the adrenal glands, or the cardiac muscle. Overall, it seems plausible to hypothesize that, in humans, exercise secretes an endocrine cocktail that is likely to induce WAT browning, as it does in rodents. However, even if exercise elicits a pro-browning endocrine response, this might result in a negligible effect if blood flow is restricted in thermogenic adipocyte-rich areas during exercise, which is still to be determined. Future studies are needed to fully characterize the exercise-induced secretion (i.e., to determine the effect of the different exercise frequency, intensity, type, time, and volume) of endocrine signaling molecules that might modulate BAT activity and/or WAT browning or through which BAT communicates with other tissues, during exercise. The exercise effect on BAT metabolism and/or WAT browning could be one of the still unknown mechanisms by which exercise exerts beneficial health effects, and it might be pharmacologically mimicked.

Redox Signaling in Widespread Health Benefits of Exercise

Significance: Exercise-induced reactive oxygen species (ROS) production activates multiple intracellular signaling pathways through genomic and nongenomic mechanisms that are responsible for the beneficial effects of exercise in muscle. Beyond the positive effect of exercise on skeletal muscle cells, other tissues such as white and brown adipose, liver, central nervous system, endothelial, heart, and endocrine organ tissues are also responsive to exercise. Recent Advances: Crosstalk between different cells is essential to achieve homeostasis and to promote the benefits of exercise through paracrine or endocrine signaling. This crosstalk can be mediated by different effectors that include the secretion of metabolites of muscle contraction, myokines, and exosomes. During the past 20 years, it has been demonstrated that contracting muscle cells produce and secrete different classes of myokines, which functionally link muscle with nearly all other cell types. Critical Issues: The redox signaling behind this exercise-induced crosstalk is now being decoded. Many of these widespread beneficial effects of exercise require not only a complex ROS-dependent intramuscular signaling cascade but simultaneously, an integrated network with many remote tissues. Future Directions: Strong evidence suggests that the powerful beneficial effect of regular physical activity for preventing (or treating) a large range of disorders might also rely on ROS-mediated signaling. Within a contracting muscle, ROS signaling may control exosomes and myokines secretion. In remote tissues, exercise generates regular and synchronized ROS waves, creating a transient pro-oxidative environment in many cells. These new concepts integrate exercise, ROS-mediated signaling, and the widespread health benefits of exercise.

Blood Biomarkers in Sports Medicine and Performance and the Future of Metabolomics

The field of sports medicine and performance has undergone an important transformation in the past years where the scientific approach is becoming increasingly more important for teams and athletes. Physical and physiological fitness, nutrition, fatigue and recovery, as well as injury prevention are key elements of the scientific monitoring of athletes nowadays. Many different methods are used nowadays as part of the scientific monitoring and testing of the competitive athlete. Among them, physiological and metabolic testing, biomechanical and movement assessments, GPS-based tracking systems, heart rate monitors, power meters, and training software are an integrative part of the scientific monitor program of many teams and athletes.Blood biomarkers through traditional blood analysis have been used for over three decades (mainly in Europe) to monitor athletic performance. In the same manner that different cells in the body respond to the stress of an infection or a disease, cells in athletes respond to the stress of competition and training. Nowadays, the area of blood biomarkers is an emerging field in the US offering important level of possibilities to monitor athletes. The field of metabolomics can offer a significantly higher level of blood biomarkers for sports medicine and performance monitoring.

Monitoring Exercise-Induced Muscle Fatigue and Adaptations: Making Sense of Popular or Emerging Indices and Biomarkers

Regular exercise with the appropriate intensity and duration may improve an athlete’s physical capacities by targeting different performance determinants across the endurance–strength spectrum aiming to delay fatigue. The mechanisms of muscle fatigue depend on exercise intensity and duration and may range from substrate depletion to acidosis and product inhibition of adenosinetriphosphatase (ATPase) and glycolysis. Fatigue mechanisms have been studied in isolated muscles; single muscle fibers (intact or skinned) or at the level of filamentous or isolated motor proteins; with each approach contributing to our understanding of the fatigue phenomenon. In vivo methods for monitoring fatigue include the assessment of various functional indices supported by the use of biochemical markers including blood lactate levels and more recently redox markers. Blood lactate measurements; as an accompaniment of functional assessment; are extensively used for estimating the contribution of the anaerobic metabolism to energy expenditure and to help interpret an athlete’s resistance to fatigue during high intensity exercise. Monitoring of redox indices is gaining popularity in the applied sports performance setting; as oxidative stress is not only a fatigue agent which may play a role in the pathophysiology of overtraining syndrome; but also constitutes an important signaling pathway for training adaptations; thus reflecting training status. Careful planning of sampling and interpretation of blood biomarkers should be applied; especially given that their levels can fluctuate according to an athlete’s lifestyle and training histories.

The Effects of Thiamine Tetrahydrofurfuryl Disulfide on Physiological Adaption and Exercise Performance Improvement

Thiamine, named as vitamin B1, is an important cofactor for the critical enzymes regarding to glucose metabolism, like transketolase, pyruvate dehydrogenase, and alpha-ketoglutarate dehydrogenase. The thiamine tetrahydrofurfuryl disulfide (TTFD) is a derivative of thiamine with higher bioavailability and solubility than thiamine and has been widely applied to health maintenance and disease therapy. Higher physical activities are associated with higher thiamine supplements for efficient energy metabolism. Furthermore, the effective dose of TTFD, beneficial to exercise physiological adaption and performance, still be further validated and the safety evaluation were also an important issue to be considered for potential application. ICR (Institute of Cancer Research) strain mice were allocated as 0, 50, 100, and 500 mg/kg dose groups and administrated by oral gavage consecutively for 6 weeks. Physical activities including grip strength and aerobic endurance were measured. Various fatigue-associated biochemical variables such as lactate, glucose, blood urine nitrogen (BUN) or creatine kinase (CK), were also assessed. The levels of liver and muscle glycogen were measured as an indicator of energy storage at the end of the experiment. Toxicity assessments for long-term supplementation were also further evaluated for safety consideration. TTFD supplementation significantly increased the endurance and grip strength and demonstrated beneficial effects on lactate production and clearance rate after an acute exercise challenge. The TTFD supplementation significantly mitigated the BUN and CK indexes after extended exercise and elevated the glycogen content in the liver and muscle tissues. According to body composition, biochemical and histopathological data, daily administration of TTFD for over 6 weeks (subacute toxicity) also demonstrated reasonable safety results for long-term and adequate supplementation. The toxicity of TTFD were also considered as safety for long-term supplementation with indicated doses. Furthermore, the TTDF could be applied to not only the health promotion but also improvement of exercise physiological adaption and the TTFD could be further considered as potential ergogenic aids combined with different nutrient strategy.

Chloride Channels Take Center Stage in Acute Regulation of Excitability in Skeletal Muscle: Implications for Fatigue

Initiation and propagation of action potentials in muscle fibers is a key element in the transmission of activating motor input from the central nervous system to their contractile apparatus, and maintenance of excitability is therefore paramount for their endurance during work. Here, we review current knowledge about the acute regulation of ClC-1 channels in active muscles and its importance for muscle excitability, function, and fatigue.

Anti-fatigue activity of sea cucumber peptides prepared from Stichopus japonicus in an endurance swimming rat model

BACKGROUND

Sea cucumber (Stichopus japonicus) is a well-known nutritious and luxurious seafood in Asia which has attracted increasing attention because of its nutrition and bioactivities in recent years. In this study, the anti-fatigue activity of sea cucumber peptides (SCP) prepared from S. japonicus was evaluated in a load-induced endurance swimming model.

RESULTS

The SCP prepared in this study was mainly made up of low-molecular-weight peptides (<2 kDa). The analysis result of amino acid composition revealed that SCP was rich in glycine, glutamic acid and proline. The endurance capability of rats to fatigue was significantly improved by SCP treatment. Meanwhile, the remarkable alterations of energy metabolic markers, antioxidant enzymes, antioxidant capacity and oxidative stress biomarkers were normalized. Moreover, administration of SCP could modulate alterations of inflammatory cytokines and downregulate the overexpression of TRL4 and NF-κB.

CONCLUSION

SCP has anti-fatigue activity and it exerted its anti-fatigue effect probably through normalizing energy metabolism as well as alleviating oxidative damage and inflammatory responses. © 2017 Society of Chemical Industry.

High Dietary Fructose: Direct or Indirect Dangerous Factors Disturbing Tissue and Organ Functions

High dietary fructose is a major contributor to insulin resistance and metabolic syndrome, disturbing tissue and organ functions. Fructose is mainly absorbed into systemic circulation by glucose transporter 2 (GLUT2) and GLUT5, and metabolized in liver to produce glucose, lactate, triglyceride (TG), free fatty acid (FFA), uric acid (UA) and methylglyoxal (MG). Its extrahepatic absorption and metabolism also take place. High levels of these metabolites are the direct dangerous factors. During fructose metabolism, ATP depletion occurs and induces oxidative stress and inflammatory response, disturbing functions of local tissues and organs to overproduce inflammatory cytokine, adiponectin, leptin and endotoxin, which act as indirect dangerous factors. Fructose and its metabolites directly and/or indirectly cause oxidative stress, chronic inflammation, endothelial dysfunction, autophagy and increased intestinal permeability, and then further aggravate the metabolic syndrome with tissue and organ dysfunctions. Therefore, this review addresses fructose-induced metabolic syndrome, and the disturbance effects of direct and/or indirect dangerous factors on the functions of liver, adipose, pancreas islet, skeletal muscle, kidney, heart, brain and small intestine. It is important to find the potential correlations between direct and/or indirect risk factors and healthy problems under excess dietary fructose consumption.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Anti-fatigue effect of Myelophil in a chronic forced exercise mouse model

This study was performed to evaluate the anti-fatigue effects of Myelophil. ICR male mice (10 weeks old) were forced to run for 1 hour, 5 days/week for 4 weeks. Each running session was followed by administration of distilled water, Myelophil (50 or 100 mg/kg), or ascorbic acid (100 mg/kg) 1h later. Equal proportions of Astragali Radix and Salviae Miltiorrhizae Radix were extracted using 30% ethanol, and formulated into Myelophil. To evaluate the anti-fatigue effects of Myelophil, exercise tolerance and forced swimming tests were conducted. Underlying mechanisms, including oxidant-antioxidant balance, inflammatory response, and energy metabolism, were investigated by analyzing skeletal muscle tissues and/or sera. Myelophil significantly increased exercise ability and latency times, and decreased the number of electric shocks and immobility time on exercise tolerance and forced swimming tests compared with control group. Myelophil also significantly ameliorated fatigue-induced alterations in oxidative stress biomarkers, antioxidant enzymes and antioxidant capacity, as measured by multiple assays, including enzyme activity assays and western blotting, as well as alterations in pro- and anti-inflammatory cytokines in skeletal muscle. Furthermore, Myelophil normalized alterations in energy metabolic markers in sera. These findings suggest that Myelophil reduces the effects of chronic fatigue, likely by attenuating oxidative and inflammatory responses and normalizing energy metabolism. Consequently, this study provides evidence for the clinical relevance of Myelophil.

β2-adrenergic stimulation enhances Ca2+ release and contractile properties of skeletal muscles, and counteracts exercise-induced reductions in Na+-K+-ATPase Vmax in trained men

The aim of the present study was to examine the effect of β2-adrenergic stimulation on skeletal muscle contractile properties, sarcoplasmic reticulum (SR) rates of Ca(2+) release and uptake, and Na(+)-K(+)-ATPase activity before and after fatiguing exercise in trained men. The study consisted of two experiments (EXP1, n = 10 males, EXP2, n = 20 males), where β2-adrenoceptor agonist (terbutaline) or placebo was randomly administered in double-blinded crossover designs. In EXP1, maximal voluntary isometric contraction (MVC) of m. quadriceps was measured, followed by exercise to fatigue at 120% of maximal oxygen uptake (V̇O2, max ). A muscle biopsy was taken after MVC (non-fatigue) and at time of fatigue. In EXP2, contractile properties of m. quadriceps were measured with electrical stimulations before (non-fatigue) and after two fatiguing 45 s sprints. Non-fatigued MVCs were 6 ± 3 and 6 ± 2% higher (P < 0.05) with terbutaline than placebo in EXP1 and EXP2, respectively. Furthermore, peak twitch force was 11 ± 7% higher (P < 0.01) with terbutaline than placebo at non-fatigue. After sprints, MVC declined (P < 0.05) to the same levels with terbutaline as placebo, whereas peak twitch force was lower (P < 0.05) and half-relaxation time was prolonged (P < 0.05) with terbutaline. Rates of SR Ca(2+) release and uptake at 400 nm [Ca(2+)] were 15 ± 5 and 14 ± 5% (P < 0.05) higher, respectively, with terbutaline than placebo at non-fatigue, but declined (P < 0.05) to similar levels at time of fatigue. Na(+)-K(+)-ATPase activity was unaffected by terbutaline compared with placebo at non-fatigue, but terbutaline counteracted exercise-induced reductions in maximum rate of activity (Vmax) at time of fatigue. In conclusion, increased contractile force induced by β2-adrenergic stimulation is associated with enhanced rate of Ca(2+) release in humans. While β2-adrenergic stimulation elicits positive inotropic and lusitropic effects on non-fatigued m. quadriceps, these effects are blunted when muscles fatigue.

Glycolysis in energy metabolism during seizures

Studies have shown that glycolysis increases during seizures, and that the glycolytic metabolite lactic acid can be used as an energy source. However, how lactic acid provides energy for seizures and how it can participate in the termination of seizures remains unclear. We reviewed possible mechanisms of glycolysis involved in seizure onset. Results showed that lactic acid was involved in seizure onset and provided energy at early stages. As seizures progress, lactic acid reduces the pH of tissue and induces metabolic acidosis, which terminates the seizure. The specific mechanism of lactic acid-induced acidosis involves several aspects, which include lactic acid-induced inhibition of the glycolytic enzyme 6-diphosphate kinase-1, inhibition of the N-methyl-D-aspartate receptor, activation of the acid-sensitive 1A ion channel, strengthening of the receptive mechanism of the inhibitory neurotransmitter γ-minobutyric acid, and changes in the intra- and extracellular environment.

Recent insights into the molecular basis of muscular fatigue

The cause of muscle fatigue has been studied for more than 100 yr, yet its molecular basis remains poorly understood. Prevailing theories suggest that much of the fatigue-induced loss in force and velocity can be attributed to the inhibitory action of metabolites, principally phosphate (Pi) and hydrogen ions (H, i.e., acidosis), on the contractile proteins, but the precise detail of how this inhibition occurs has been difficult to visualize at the molecular level. However, recent technological developments in the areas of biophysics, molecular biology, and structural biology are enabling researchers to directly observe the function and dysfunction of muscle contractile proteins at the level of a single molecule. In fact, the first direct evidence that high levels of H and Pi inhibit the function of muscle's molecular motor, myosin, has recently been observed in a single molecule laser trap assay. Likewise, advances in structural biology are taking our understanding further, providing detail at the atomic level of how some metabolites might alter the internal motions of myosin and thereby inhibit its ability to generate force and motion. Finally, new insights are also being gained into the indirect role that muscle regulatory proteins troponin (Tn) and tropomyosin (Tn) play in the fatigue process. In vitro studies, incorporating TnTm, suggest that a significant portion of the decreased force and motion during fatigue may be mediated through a disruption of the molecular motions of specific regions within Tn and Tm. These recent advances are providing unprecedented molecular insight into the structure and function of the contractile proteins and, in the process, are reshaping our understanding of the process of fatigue.

Development and evaluation of a multi-frequency bioelectrical impedance analysis analyzer for estimating acupoint composition

The purpose of this study was to suggest a new method of estimating acupoint compositions by using a multi-frequency bioelectrical impedance analysis (MF-BIA) method at 5 kHz, 50 kHz and 200 kHz within 2 cm of acupoints divided into local segments. To verify the system developed, we confirmed the stable occurrence of a constant current at every frequency, regardless of the impedance connected to the electrodes. Moreover, we found left and right distal bicep brachii aponeurosis to be identical by using ultrasound imaging, and we analyzed the repeatability of the findings by making 10 consecutive sets of measurements (p > 0.05). To evaluate the practical use of the acupoint composition, we used the MF-BIA analyzer to measure the left and right LU3, LU4, and LU9 at the lung meridian. We confirmed that the potentials generated were equal to the changes in the cell membrane function, which were caused by the applied frequency (p < 0.01). We also verified that the MF-BIA analyzer measurements corresponded to the acupoint components by comparing the left and right potentials generated (p > 0.05). Hence, we conclude that the MF-BIA analyzer can be used to estimate the acupoint composition based on the acupoint state.

Recent insights into muscle fatigue at the cross-bridge level

The depression in force and/or velocity associated with muscular fatigue can be the result of a failure at any level, from the initial events in the motor cortex of the brain to the formation of an actomyosin cross-bridge in the muscle cell. Since all the force and motion generated by muscle ultimately derives from the cyclical interaction of actin and myosin, researchers have focused heavily on the impact of the accumulation of intracellular metabolites [e.g., P(i), H(+) and adenosine diphoshphate (ADP)] on the function these contractile proteins. At saturating Ca(++) levels, elevated P(i) appears to be the primary cause for the loss in maximal isometric force, while increased [H(+)] and possibly ADP act to slow unloaded shortening velocity in single muscle fibers, suggesting a causative role in muscular fatigue. However the precise mechanisms through which these metabolites might affect the individual function of the contractile proteins remain unclear because intact muscle is a highly complex structure. To simplify problem isolated actin and myosin have been studied in the in vitro motility assay and more recently the single molecule laser trap assay with the findings showing that both P(i) and H(+) alter single actomyosin function in unique ways. In addition to these new insights, we are also gaining important information about the roles played by the muscle regulatory proteins troponin (Tn) and tropomyosin (Tm) in the fatigue process. In vitro studies, suggest that both the acidosis and elevated levels of P(i) can inhibit velocity and force at sub-saturating levels of Ca(++) in the presence of Tn and Tm and that this inhibition can be greater than that observed in the absence of regulation. To understand the molecular basis of the role of regulatory proteins in the fatigue process researchers are taking advantage of modern molecular biological techniques to manipulate the structure and function of Tn/Tm. These efforts are beginning to reveal the relevant structures and how their functions might be altered during fatigue. Thus, it is a very exciting time to study muscle fatigue because the technological advances occurring in the fields of biophysics and molecular biology are providing researchers with the ability to directly test long held hypotheses and consequently reshaping our understanding of this age-old question.

The influence of respiratory acid-base changes on muscle performance and excitability of the sarcolemma during strenuous intermittent hand grip exercise

Acidification has been reported to provide protective effects on force production in vitro. Thus, in this study, we tested if respiratory acid-base changes influence muscle function and excitability in vivo. Nine subjects performed strenuous, intermittent hand grip exercises (10 cycles of 15 s of work/45 s of rest) under respiratory acidosis by CO2 rebreathing, alkalosis by hyperventilation, or control. The Pco2, pH, K+ concentration ([K+]), and Na+ concentration were measured in venous and arterialized blood. Compound action potentials (M-wave) were elicited to examine the excitability of the sarcolemma. The surface electromyogram (EMG) was recorded to estimate the central drive to the muscle. The lowest venous pH during the exercise period was 7.24 ± 0.03 in controls, 7.31 ± 0.05 with alkalosis, and 7.17 ± 0.04 with acidosis ( P < 0.001). The venous [K+] rose to similar maximum values in all conditions (6.2 ± 0.8 mmol/l). The acidification reduced the decline in contraction speed ( P < 0.001) but decreased the M-wave area to 73.4 ± 19.8% ( P < 0.001) of the initial value. After the first exercise cycle, the M-wave area was smaller with acidosis than with alkalosis, and, after the second cycle, it was smaller with acidosis than with the control condition ( P < 0.001). The duration of the M-wave was not affected. Acidification diminished the reduction in performance, although the M-wave area during exercise was decreased. Respiratory alkalosis stabilized the M-wave area without influencing performance. Thus, we did not find a direct link between performance and alteration of excitability of the sarcolemma due to changes in pH in vivo.

Lactic acid restores skeletal muscle force in an in vitro fatigue model: are voltage-gated chloride channels involved?

High interstitial K(+) concentration ([K(+)]) has been reported to impede normal propagation of electrical impulses along the muscle cell membrane (sarcolemma) and then also into the transverse tubule system; this is one considered underlying mechanism associated with the development of muscle fatigue. Interestingly, the extracellular buildup of lactic acid, once considered an additional cause for muscle fatigue, was recently shown to have force-restoring effects in such conditions. Specifically, it was proposed that elevated lactic acid (and intracellular acidosis) may lead to inhibition of voltage-gated chloride channels, thereby reestablishing better excitability of the muscle cell sarcolemma. In the present study, using an in vitro muscle contractile experimental setup to study functionally viable rectus abdominis muscle preparations obtained from normal swine, we examined the effects of 20 mM lactic acid and 512 μM 9-anthracenecarboxylic acid (9-AC; a voltage-gated chloride channel blocker) on the force recovery of K(+)-depressed (10 mM K(+)) twitch forces. We observed a similar muscle contractile restoration after both treatments. Interestingly, at elevated [K(+)], myotonia (i.e., hyperexcitability or afterdepolarizations), usually present in skeletal muscle with inherent or induced chloride channel dysfunctions, was not observed in the presence of either lactic acid or 9-AC. In part, these data confirm previous studies showing a force-restoring effect of lactic acid in high-[K(+)] conditions. In addition, we observed similar restorative effects of lactic acid and 9-AC, implicating a beneficial mechanism via voltage-gated chloride channel modulation.

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.

Lactate downregulates the glycolytic enzymes hexokinase and phosphofructokinase in diverse tissues from mice

We examined the effects of lactate on the enzymatic activity of hexokinase (HK), phosphofructokinase (PFK) and pyruvate kinase (PK) in various mouse tissues. Our results showed that lactate inhibited PFK activity in all the analyzed tissues. This inhibitory effect was observed in skeletal muscle even in the presence of insulin. Lactate directly inhibited the phosphorylation of PFK tyrosine residues in skeletal muscle, an important mechanism of the enzyme activation. Moreover, lactate indirectly inhibited HK activity, which resulted from its cellular redistribution, here attributed to alterations of HK structure. PK activity was not affected by lactate. The activity of HK and PFK is directly related to glucose metabolism. Thus, it is conceivable that lactate exposure can induce inhibition of glucose consumption in tissues.

Lactate per se improves the excitability of depolarized rat skeletal muscle by reducing the Cl- conductance

Studies on rats have shown that lactic acid can improve excitability and function of depolarized muscles. The effect has been related to the ensuing reduction in intracellular pH causing inhibition of muscle fibre Cl(-) channels. However, since several carboxylic acids with structural similarities to lactate can inhibit muscle Cl(-) channels it is possible that lactate per se can increase muscle excitability by exerting a direct effect on these channels. We therefore examined the effects of lactate on the function of intact muscles and skinned fibres together with effects on pH and Cl(-) conductance (G(cl)). In muscles where extracellular compound action potentials (M-waves) and tetanic force response to excitation were reduced by (mean ± s.e.m.) 82 ± 4% and 83 ± 2%, respectively, by depolarization with 11 mm extracellular K(+), both M-waves and force exhibited an up to 4-fold increase when 20 mm lactate was added. This effect was present already at 5 mm and saturated at 15 mm lactate, and was associated with a 31% reduction in G(Cl). The effects of lactate were completely blocked by Cl(-) channel inhibition or use of Cl(-)-free solutions. Finally, both experiments where effects of lactate on intracellular pH in intact muscles were mimicked by increased CO₂ tension and experiments with skinned fibres showed that the effects of lactate could not be related to reduced intracellular pH. It is concluded that addition of lactate can inhibit ClC-1 Cl(-) channels and increase the excitability and contractile function of depolarized rat muscles via mechanisms not related to a reduction in intracellular pH.

Massage impairs postexercise muscle blood flow and "lactic acid" removal

PURPOSE

This study tested the hypothesis that one of the ways sports massage aids muscle recovery from exercise is by increasing muscle blood flow to improve "lactic acid" removal.

METHODS

Twelve subjects performed 2 min of strenuous isometric handgrip (IHG) exercise at 40% maximum voluntary contraction to elevate forearm muscle lactic acid. Forearm blood flow (FBF; Doppler and Echo ultrasound of the brachial artery) and deep venous forearm blood lactate and H+ concentration ([La-], [H+]) were measured every minute for 10 min post-IHG under three conditions: passive (passive rest), active (rhythmic exercise at 10% maximum voluntary contraction), and massage (effleurage and pétrissage). Arterialized [La-] and [H+] from a superficial heated hand vein was measured at baseline.

RESULTS

Data are presented as mean +/- SE. Venoarterial [La-] difference ([La-]v-a) at 30 s of post-IHG was the same across conditions (passive = 6.1 +/- 0.6 mmol x L(-1), active = 5.7 +/- 0.6 mmol x L(-1), massage = 5.5 +/- 0.6 mmol x L(-1), NS), whereas FBF was greater in passive (766 +/- 101 mL x min(-1)) versus active (614 +/- 62 mL x min(-1), P = 0.003) versus massage (540 +/- 60 mL x min(-1), P < 0.0001). Total FBF area under the curve (AUC) for 10 min after handgrip was significantly higher in passive versus massage (4203 +/- 531 vs 3178 +/- 304 mL, P = 0.024) but not versus active (3584 +/- 284 mL, P = 0.217). La(-)- efflux (FBF x [La-]v-a) AUC mirrored FBF AUC (passive = 20.5 +/- 2.8 mmol vs massage = 14.7 +/- 1.6 mmol, P = 0.03, vs active = 15.4 +/- 1.9 mmol, P = 0.064). H+ efflux (FBF x [H+]v-a) was greater in passive versus massage at 30 s (2.2 +/- 0.4e(-5) vs 1.3 +/- 0.2e(-5) mmol, P < 0.001) and 1.5 min (1.0 +/- 0.2e(-5) vs 0.6 +/- 0.09e(-5) mmol, P = 0.003) after IHG.

CONCLUSIONS

Massage impairs La(-) and H+ removal from muscle after strenuous exercise by mechanically impeding blood flow.

Effect of previous exhaustive exercise on metabolism and fatigue development during intense exercise in humans

The present study examined how metabolic response and work capacity are affected by previous exhaustive exercise. Seven subjects performed an exhaustive cycle exercise ( approximately 130%-max; EX2) after warm-up (CON) and 2 min after an exhaustive bout at a very high (VH; approximately 30 s), high (HI; approximately 3 min) or low (LO; approximately 2 h) intensity. Compared with CON, performance during EX2 was reduced (P<0.05) more in HI and LO than in VH (61+/-4% and 68+/-3% vs 35+/-4%). The muscle glycogen before EX2 was lower (P<0.05) in LO than in HI and VH, but the muscle glycogen utilization rates during EX2 were not different. Muscle glycogen concentration before EX2 was related (P<0.05) to the mean rate of muscle glycogen utilization during EX2 in HI and VH, and the mean rate of muscle lactate accumulation in LO. In HI, muscle pH before EX2 was lower (P<0.05) compared with VH and LO, but the same in HI and VH at the end of EX2. In HI, muscle pH before and after EX2 was inversely related (P<0.05) to the decrease in EX2 performance. Thus, muscle glycogen availability and low muscle pH do not per se control but appear to affect the rate of glycogenolysis/glycolysis and fatigue development during a repeated high-intensity exercise lasting 1/2-2 min.

Development of myoglobin concentration and acid buffering capacity in harp (Pagophilus groenlandicus) and hooded (Cystophora cristata) seals from birth to maturity

Pinnipeds rely on muscle oxygen stores to help support aerobic diving, therefore muscle maturation may influence the behavioral ecology of young pinnipeds. To investigate the pattern of muscle development, myoglobin concentration ([Mb]) and acid buffering ability (beta) was measured in ten muscles from 23 harp and 40 hooded seals of various ages. Adult [Mb] ranged from 28-97 to 35-104 mg g tissue(-1) in harp and hooded seals, respectively, with values increasing from the cervical, non-swimming muscles to the main swimming muscles of the lumbar region. Neonatal and weaned pup muscles exhibited lower (approximately 30% adult values) and less variable [Mb] across the body than adults. In contrast, adult beta showed little regional variation (60-90 slykes), while high pup values (approximately 75% adult values) indicate significant in utero development. These findings suggest that intra-uterine conditions are sufficiently hypoxic to stimulate prenatal beta development, but that [Mb] development requires additional postnatal signal such as exercise, and/or growth factors. However, because of limited development in both beta and [Mb] during the nursing period, pups are weaned with muscles with lower aerobic and anaerobic capacities than those of adults.

Do multiple ionic interactions contribute to skeletal muscle fatigue?

During intense exercise or electrical stimulation of skeletal muscle the concentrations of several ions change simultaneously in interstitial, transverse tubular and intracellular compartments. Consequently the functional effects of multiple ionic changes need to be considered together. A diminished transsarcolemmal K(+) gradient per se can reduce maximal force in non-fatigued muscle suggesting that K(+) causes fatigue. However, this effect requires extremely large, although physiological, K(+) shifts. In contrast, moderate elevations of extracellular [K(+)] ([K(+)](o)) potentiate submaximal contractions, enhance local blood flow and influence afferent feedback to assist exercise performance. Changed transsarcolemmal Na(+), Ca(2+), Cl(-) and H(+) gradients are insufficient by themselves to cause much fatigue but each ion can interact with K(+) effects. Lowered Na(+), Ca(2+) and Cl(-) gradients further impair force by modulating the peak tetanic force-[K(+)](o) and peak tetanic force-resting membrane potential relationships. In contrast, raised [Ca(2+)](o), acidosis and reduced Cl(-) conductance during late fatigue provide resistance against K(+)-induced force depression. The detrimental effects of K(+) are exacerbated by metabolic changes such as lowered [ATP](i), depleted carbohydrate, and possibly reactive oxygen species. We hypothesize that during high-intensity exercise a rundown of the transsarcolemmal K(+) gradient is the dominant cellular process around which interactions with other ions and metabolites occur, thereby contributing to fatigue.

Regulation of pH in human skeletal muscle: adaptations to physical activity

Regulation of pH in skeletal muscle is the sum of mechanisms involved in maintaining intracellular pH within the normal range. Aspects of pH regulation in human skeletal muscle have been studied with various techniques from analysis of membrane proteins, microdialysis, and the nuclear magnetic resonance technique to exercise experiments including blood sampling and muscle biopsies. The present review characterizes the cellular buffering system as well as the most important membrane transport systems involved (Na(+)/H(+) exchange, Na-bicarbonate co-transport and lactate/H(+) co-transport) and describes the contribution of each transport system in pH regulation at rest and during muscle activity. It is reported that the mechanisms involved in pH regulation can undergo adaptational changes in association with physical activity and that these changes are of functional importance.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Lactate favours the dissociation of skeletal muscle 6-phosphofructo-1-kinase tetramers down-regulating the enzyme and muscle glycolysis

For a long period lactate was considered as a dead-end product of glycolysis in many cells and its accumulation correlated with acidosis and cellular and tissue damage. At present, the role of lactate in several physiological processes has been investigated based on its properties as an energy source, a signalling molecule and as essential for tissue repair. It is noteworthy that lactate accumulation alters glycolytic flux independently from medium acidification, thereby this compound can regulate glucose metabolism within cells. PFK (6-phosphofructo-1-kinase) is the key regulatory glycolytic enzyme which is regulated by diverse molecules and signals. PFK activity is directly correlated with cellular glucose consumption. The present study shows the property of lactate to down-regulate PFK activity in a specific manner which is not dependent on acidification of the medium. Lactate reduces the affinity of the enzyme for its substrates, ATP and fructose 6-phosphate, as well as reducing the affinity for ATP at its allosteric inhibitory site at the enzyme. Moreover, we demonstrated that lactate inhibits PFK favouring the dissociation of enzyme active tetramers into less active dimers. This effect can be prevented by tetramer-stabilizing conditions such as the presence of fructose 2,6-bisphosphate, the binding of PFK to f-actin and phosphorylation of the enzyme by protein kinase A. In conclusion, our results support evidence that lactate regulates the glycolytic flux through modulating PFK due to its effects on the enzyme quaternary structure.

Locomotor muscle fatigue modifies central motor drive in healthy humans and imposes a limitation to exercise performance

We asked whether the central effects of fatiguing locomotor muscle fatigue exert an inhibitory influence on central motor drive to regulate the total degree of peripheral fatigue development. Eight cyclists performed constant-workload prefatigue trials (a) to exhaustion (83% of peak power output (W(peak)), 10 +/- 1 min; PFT(83%)), and (b) for an identical duration but at 67% W(peak) (PFT(67%)). Exercise-induced peripheral quadriceps fatigue was assessed via changes in potentiated quadriceps twitch force (DeltaQ(tw,pot)) from pre- to post-exercise in response to supra-maximal femoral nerve stimulation (DeltaQ(tw,pot)). On different days, each subject randomly performed three 5 km time trials (TTs). First, subjects repeated PFT(83%) and the TT was started 4 min later with a known level of pre-existing locomotor muscle fatigue (DeltaQ(tw,pot) -36%) (PFT(83%)-TT). Second, subjects repeated PFT(67%) and the TT was started 4 min later with a known level of pre-existing locomotor muscle fatigue (DeltaQ(tw,pot) -20%) (PFT(67%)-TT). Finally, a control TT was performed without any pre-existing level of fatigue. Central neural drive during the three TTs was estimated via quadriceps EMG. Increases in pre-existing locomotor muscle fatigue from control TT to PFT(83%)-TT resulted in significant dose-dependent changes in central motor drive (-23%), power output (-14%), and performance time (+6%) during the TTs. However, the magnitude of locomotor muscle fatigue following various TTs was not different (DeltaQ(tw,pot) of -35 to -37%, P = 0.35). We suggest that feedback from fatiguing muscle plays an important role in the determination of central motor drive and force output, so that the development of peripheral muscle fatigue is confined to a certain level.

Electrostimulation improves muscle perfusion but does not affect either muscle deoxygenation or pulmonary oxygen consumption kinetics during a heavy constant-load exercise

Electromyostimulation (EMS) is commonly used as part of training programs. However, the exact effects at the muscle level are largely unknown and it has been recently hypothesized that the beneficial effect of EMS could be mediated by an improved muscle perfusion. In the present study, we investigated rates of changes in pulmonary oxygen consumption (VO(2p)) and muscle deoxygenation during a standardized exercise performed after an EMS warm-up session. We aimed at determining whether EMS could modify pulmonary O(2) uptake and muscle deoxygenation as a result of improved oxygen delivery. Nine subjects performed a 6-min heavy constant load cycling exercise bout preceded either by an EMS session (EMS) or under control conditions (CONT). VO(2p) and heart rate (HR) were measured while deoxy-(HHb), oxy-(HbO(2)) and total haemoglobin/myoglobin (Hb(tot)) relative contents were measured using near infrared spectroscopy. EMS significantly increased (P < 0.05) the Hb(tot) resting level illustrating a residual hyperaemia. The EMS priming exercise did not affect either the HHb time constant (17.7 +/- 14.2 s vs. 13.1 +/- 2.3 s under control conditions) or the VO(2p) kinetics (time-constant = 18.2 +/- 5.2 s vs. 15.4 +/- 4.6 s under control conditions). Likewise, the other VO(2p) parameters were unchanged. Our results further indicated that EMS warm-up improved muscle perfusion through a residual hyperaemia. However, neither VO(2p) nor [HHb] kinetics were modified accordingly. These results suggest that improved O(2) delivery by residual hyperaemia induced by EMS does not accelerate the rate of aerobic metabolism during heavy exercise at least in trained subjects.

Regulation of Na+–K+ homeostasis and excitability in contracting muscles: implications for fatigue

The performance of skeletal muscles depends on their ability to initiate and propagate action potentials along their outer membranes in response to motor signals from the central nervous system. This excitability of muscle fibres is related to the function of Na+ and K+ and Cl– channels and to steep chemical gradients for the ions across the cell membranes, i.e., the sarcolemma and T-tubular membranes. At rest, the chemical gradients for Na+ and K+ are maintained within close limits by the action of the Na+–K+ pump. During contractile activity, however, the muscles lose K+, which causes an increase in the concentration of K+ in the extracellular compartments of the body, the magnitude of which depends on the intensity of the exercise and the size of the muscle groups involved. Since the ensuing reduction in the chemical K+ gradient can have adverse effects on muscle excitability, it has repeatedly been suggested that, during intense exercise, the loss of K+ from muscle fibres can contribute to the complex set of mechanisms that leads to the development of muscle fatigue. In this review, aspects of the regulation of Na+–K+ homeostasis and excitability in contracting muscles is discussed within this context, together with the implications for the contractile function of skeletal muscles.

Additive protective effects of the addition of lactic acid and adrenaline on excitability and force in isolated rat skeletal muscle depressed by elevated extracellular K+

During strenuous exercise, extracellular K(+) ([K(+)](o)) is increased, which potentially can reduce muscle excitability and force production. In addition, exercise leads to accumulation of lactate and H(+) and increased levels of circulating catecholamines. Individually, reduced pH and increased catecholamines have been shown to counteract the depressing effect of elevated K(+). This study examines (i) whether the effects of addition of lactic acid and adrenaline on the excitability of isolated muscles are caused by separate mechanisms and are additive and (ii) whether the effect of adding lactic acid or increasing CO(2) is related to a reduction of intra- or extracellular pH. Rat soleus muscles were incubated at a [K(+)](o) of 15 mM, which reduced tetanic force by 85%. Subsequent addition of 20 mM lactic acid or 10(-5) M adrenaline led to a small recovery of force, but when added together induced an almost complete force recovery. Compound action potentials showed that the force recovery was associated with recovery of muscle excitability. The improved excitability after addition of adrenaline was associated with increased Na(+)-K(+) pump activity resulting in hyperpolarization and an increase in the chemical Na(+) gradient. In contrast, addition of lactic acid had no effect on the membrane potential or the Na(+)-K(+) pump activity, but most likely increased excitability via a reduction in intracellular pH. It is concluded that the protective effects of acidosis and adrenaline on muscle excitability and force took place via different mechanisms and were additive. The results suggest that circulating catecholamines and development of acidosis during exercise may improve the tolerance of muscles to elevated [K(+)](o).

Performance predicting factors in prolonged exhausting exercise of varying intensity

Several endurance sports, e.g. road cycling, have a varying intensity profile during competition. At present, few laboratory tests take this intensity profile into consideration. Thus, the purpose of this study was to examine the prognostic value of heart rate (HR), lactate (La(-1)), potassium (K(+)), and respiratory exchange ratio (RER) performance at an exhausting cycling exercise with varying intensity. Eight national level cyclists performed two cycle tests each on a cycle ergometer: (1) a incremental test to establish VO(2max), maximum power (W (max)), and lactate threshold (VO(2LT)), and (2) a variable intensity protocol (VIP). Exercise intensity for the VIP was based upon the VO(2max) obtained during the incremental test. The VIP consisted of six high intense (HI) workloads at 90% of VO(2max) for 3 min each, interspersed by five middle intense (MI) workloads at 70% of VO(2max )for 6 min each. VO(2 )and HR were continuously measured throughout the tests. Venous blood samples were taken before, during, and after the test. Increases in HR, La(-), K(+), and RER were observed when workload changed from MI to HI workload (P < 0.05). Potassium and RER decreased after transition from HI to MI workloads (P < 0.05). There was a negative correlation between time to exhaustion and decrease in La(-) concentration during the first MI (r = -0.714; P = 0.047). Furthermore, time to exhaustion correlated with VO(2LT )calculated from the ramp test (r = 0.738; P = 0.037). Our results suggest that the magnitude of decrease of La(-1) between the first HI workload and the consecutive MI workload could predict performance during prolonged exercise with variable intensity.

Lactic acid and exercise performance : culprit or friend?

This article critically discusses whether accumulation of lactic acid, or in reality lactate and/or hydrogen (H+) ions, is a major cause of skeletal muscle fatigue, i.e. decline of muscle force or power output leading to impaired exercise performance. There exists a long history of studies on the effects of increased lactate/H+ concentrations in muscle or plasma on contractile performance of skeletal muscle. Evidence suggesting that lactate/H+ is a culprit has been based on correlation-type studies, which reveal close temporal relationships between intramuscular lactate or H+ accumulation and the decline of force during fatiguing stimulation in frog, rodent or human muscle. In addition, an induced acidosis can impair muscle contractility in non-fatigued humans or in isolated muscle preparations, and several mechanisms to explain such effects have been provided. However, a number of recent high-profile papers have seriously challenged the 'lactic acid hypothesis'. In the 1990s, these findings mainly involved diminished negative effects of an induced acidosis in skinned or intact muscle fibres, at higher more physiological experimental temperatures. In the early 2000s, it was conclusively shown that lactate has little detrimental effect on mechanically skinned fibres activated by artificial stimulation. Perhaps more remarkably, there are now several reports of protective effects of lactate exposure or induced acidosis on potassium-depressed muscle contractions in isolated rodent muscles. In addition, sodium-lactate exposure can attenuate severe fatigue in rat muscle stimulated in situ, and sodium lactate ingestion can increase time to exhaustion during sprinting in humans. Taken together, these latest findings have led to the idea that lactate/H+ is ergogenic during exercise. It should not be taken as fact that lactic acid is the deviant that impairs exercise performance. Experiments on isolated muscle suggest that acidosis has little detrimental effect or may even improve muscle performance during high-intensity exercise. In contrast, induced acidosis can exacerbate fatigue during whole-body dynamic exercise and alkalosis can improve exercise performance in events lasting 1-10 minutes. To reconcile the findings from isolated muscle fibres through to whole-body exercise, it is hypothesised that a severe plasma acidosis in humans might impair exercise performance by causing a reduced CNS drive to muscle.

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.