The roles of ion fluxes in skeletal muscle fatigue

Resting membrane potential and intracellular [Na+] at rest, during fatigue and during recovery in rat soleus muscle fibres in situ

Large trans-sarcolemmal ionic shifts occur with fatiguing exercise or stimulation of isolated muscles. However, it is unknown how resting membrane potential (EM) and intracellular sodium concentration ([Na+]i) change with repeated contractions in living mammals. We investigated (i) whether [Na+]i (peak, kinetics) can reveal changes of Na+-K+ pump activity during brief or fatiguing stimulation and (ii) how resting EM and [Na+]i change during fatigue and recovery of rat soleus muscle in situ. Muscles of anaesthetised rats were stimulated with brief (10 s) or repeated tetani (60 Hz for 200 ms, every 2 s, for 30 s or 300 s) with isometric force measured. Double-barrelled ion-sensitive microelectrodes were used to quantify resting EM and [Na+]i. Post-stimulation data were fitted using polynomials and back-extrapolated to time zero recovery. Mean pre-stimulation resting EM (layer 2-7 fibres) was -71 mV (surface fibres were more depolarised), and [Na+]i was 14 mM. With deeper fibres, 10 s stimulation (2-150 Hz) increased [Na+]i to 38-46 mM whilst simultaneously causing hyperpolarisations (7.3 mV for 2-90 Hz). Fatiguing stimulation for 30 s or 300 s led to end-stimulation resting EM of -61 to -53 mV, which recovered rapidly (T1/2, 8-22 s). Mean end-stimulation [Na+]i increased to 86-101 mM with both fatigue protocols and the [Na+]i recovery time-course (T1/2, 21-35 s) showed no difference between protocols. These combined findings suggest that brief stimulation hyperpolarises the resting EM, likely via maximum Na+-induced stimulation of the Na+-K+ pump. Repeated tetani caused massive depolarisation and elevations of [Na+]i that together lower force, although they likely interact with other factors to cause fatigue. [Na+]i recovery kinetics provided no evidence of impaired Na+-K+ pump activity with fatigue. KEY POINTS: It is uncertain how resting membrane potential, intracellular sodium concentration ([Na+]i), and sodium-potassium (Na+-K+) pump activity change during repeated muscle contractions in living mammals. For rat soleus muscle fibres in situ, brief tetanic stimulation for 10 s led to raised [Na+]i, anticipated to evoke maximal Na+-induced stimulation of the Na+-K+ pump causing an immediate hyperpolarisation of the sarcolemma. More prolonged stimulation with repeated tetanic contractions causes massive elevations of [Na+]i, which together with large depolarisations (via K+ disturbances) likely reduce force production. These effects occurred without impairment of Na+-K+ pump function. Together these findings suggest that rapid activation of the Na+-K+ pump occurs with brief stimulation to maintain excitability, whereas more prolonged stimulation causes rundown of the trans-sarcolemmal K+ gradient (hence depolarisation) and Na+ gradient, which in combination can impair contraction to contribute to fatigue in living mammals.

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.

Assessment of acute neuromuscular fatigue manifestations and functional performances after heavy resistance exercise

BACKGROUND

This study aimed to assess neuromuscular fatigue after heavy resistance exercise in rugby players.

METHODS

Twelve male rugby players performed five sets of knee extension exercise lifting 80% of their one repetition maximum until failure, with 3min of rest in-between. Maximal voluntary contraction (MVC) and surface electromyographic activity from quadriceps muscles, as well as ions (i.e., Na+, K+, and Cl-) and metabolic responses (i.e., blood lactate and ammonia concentrations) were measured before and after exercise. Maximum repetitions performance and both peripheral (RPEp) and overall body (RPEo) rating of perceived exertion were recorded following each set.

RESULTS

The number of maximum repetitions decreased significantly across sets (P<0.001). Both RPEp and RPEo increased significantly across sets (P<0.001) with higher RPEp values after each set (P<0.001). Both RPEp (r=-0.98, P<0.01) and RPEo (r=-0.99, P<0.001) were negatively correlated with the changes in the number of maximum repetitions. MVC (P<0.001), root mean square (P<0.05), and neuromuscular efficiency (P<0.01) as well as Na+ (P<0.01), Cl- (P<0.001) and blood concentrations of lactate (P<0.001) and ammonia (P<0.001) decreased significantly after the exercise. However, K+ (P<0.001) increased after the resistance exercise.

CONCLUSIONS

Heavy resistance exercise affected both objective (i.e., neuromuscular and biochemical parameters) and subjective (i.e., RPE) aspects of neuromuscular fatigue.

Post-exercise Supplementation of Sodium Bicarbonate Improves Acid Base Balance Recovery and Subsequent High-Intensity Boxing Specific Performance

The aim of this study was to assess the effects of post-exercise sodium bicarbonate (NaHCO₃) ingestion (0.3 g.kg⁻¹ body mass) on the recovery of acid-base balance (pH, HCO3-, and the SID) and subsequent exercise performance in elite boxers. Seven elite male professional boxers performed an initial bout of exhaustive exercise comprising of a boxing specific high-intensity interval running (HIIR) protocol, followed by a high-intensity run to volitional exhaustion (T_LIM1). A 75 min passive recovery then ensued, whereby after 10 min recovery, participants ingested either 0.3 g.kg⁻¹ body mass NaHCO₃, or 0.1 g.kg⁻¹ body mass sodium chloride (PLA). Solutions were taste matched and administered double-blind. Participants then completed a boxing specific punch combination protocol, followed by a second high-intensity run to volitional exhaustion (T_LIM2). Both initial bouts of T_LIM1 were well matched between PLA and NaHCO₃ (ICC; r = 0.94, p = 0.002). The change in performance from T_LIM1 to T_LIM2 was greater following NaHCO₃ compared to PLA (+164 ± 90 vs. +73 ± 78 sec; p = 0.02, CI = 45.1, 428.8, g = 1.0). Following ingestion of NaHCO₃, pH was greater prior to T_LIM2 by 0.11 ± 0.02 units (1.4%) (p < 0.001, CI = 0.09, 0.13, g = 3.4), whilst HCO3- was greater by 8.8 ± 1.5 mmol.l⁻¹ (26.3%) compared to PLA (p < 0.001, CI = 7.3, 10.2, g = 5.1). The current study suggests that these significant increases in acid base balance during post-exercise recovery facilitated the improvement in the subsequent bout of exercise. Future research should continue to explore the role of NaHCO₃ supplementation as a recovery aid in boxing and other combat sports.

Sodium citrate ingestion protocol impacts induced alkalosis, gastrointestinal symptoms, and palatability

To compare the effect of 500 mg·kg-1 body mass (BM) sodium citrate ingested in solution or capsules on induced alkalosis, gastrointestinal symptoms and palatability. Twenty-four healthy and active participants completed two testing sessions, ingesting 500 mg·kg-1 BM sodium citrate within solution or capsules. Capillary blood samples were collected pre-ingestion, and every 30-min for 240-min post-ingestion; samples were analyzed for blood pH and [HCO3- ]. A validated questionnaire was used to quantify gastrointestinal symptoms at the same 30-min intervals. Palatability was quantified immediately after ingestion using a validated scale. There was a greater peak and change from baseline for capsules versus solution for blood pH (P < 0.001) and [HCO3- ] (P = 0.013). Blood pH and [HCO3- ] time to peak was 199 and 204 min, respectively, after capsule ingestion, both significantly later than after solution (P = 0.034, P = 0.001). Gastrointestinal symptoms were significantly elevated above baseline for both ingestion modes at each time point between 30 and 120 min after ingestion (P = 0.003), with no differences between modes at any time point (P = 0.644). Capsules were significantly more palatable than solution (P < 0.001). We recommend 500 mg·kg-1 BM sodium citrate ingestion in capsules, at least 200 min before exercise, to achieve greater alkalosis, minimize gastrointestinal symptoms, and maximize.

Molecular stressors underlying exercise training-induced improvements in K+ regulation during exercise and Na+ ,K+ -ATPase adaptation in human skeletal muscle

Despite substantial progress made towards a better understanding of the importance of skeletal muscle K⁺ regulation for human physical function and its association with several disease states (eg type-II diabetes and hypertension), the molecular basis underpinning adaptations in K⁺ regulation to various stimuli, including exercise training, remains inadequately explored in humans. In this review, the molecular mechanisms essential for enhancing skeletal muscle K⁺ regulation and its key determinants, including Na⁺ ,K⁺ -ATPase function and expression, by exercise training are examined. Special attention is paid to the following molecular stressors and signaling proteins: oxygenation, redox balance, hypoxia, reactive oxygen species, antioxidant function, Na⁺ ,K⁺ , and Ca²⁺ concentrations, anaerobic ATP turnover, AMPK, lactate, and mRNA expression. On this basis, an update on the effects of different types of exercise training on K⁺ regulation in humans is provided, focusing on recent discoveries about the muscle fibre-type-dependent regulation of Na⁺ ,K⁺ -ATPase-isoform expression. Furthermore, with special emphasis on blood-flow-restricted exercise as an exemplary model to modulate the key molecular mechanisms identified, it is discussed how training interventions may be designed to maximize improvements in K⁺ regulation in humans. The novel insights gained from this review may help us to better understand how exercise training and other strategies, such as pharmacological interventions, may be best designed to enhance K⁺ regulation and thus the physical function in humans.

Limit to steady-state aerobic power of skeletal muscles

Like any other kind of cell, muscle cells produce energy by oxidizing the fuel substrate that they absorb together with the needed oxygen from the surroundings. Oxidation occurs entirely within the cell. It means that the reactants and products of reaction must at some time be dissolved in the cell's cytosol. If a cell operates at steady state, its cytosol composition remains constant. Therefore, the cytosol in a muscle that produces work at steady state must contain a constant amount of fuel, oxygen, and product of reaction dissolved in it. The greater the power produced, the higher the concentration of these solutes. There is a limit, however, to the maximum amount of solutes that the cytosol can contain without damaging the cell. General thermodynamic arguments, which are reviewed in this paper, help relate this limit to the dehydration and overhydration limits of the cell. The present analysis shows that the same limits entail a limit to the maximum power that a muscle can produce at steady state. This limit depends on the composition of the fuel mixture used by the muscle. The analysis also determines the number of fuel carbon atoms that must be oxidized in parallel within a cell to produce a given power. It may well happen that a muscle cannot reach the maximum attainable power because it cannot activate all the parallel oxidation paths that are needed to produce it. This may be due to a series of reasons ranging from health issues to a lack of training. The paper shows how the methods of indirect calorimetry can provide all the experimental data needed to determine the actual number of parallel oxidation paths that at steady state must be active in a muscle in a given exercise. A diagram relating muscle power to the number of parallel oxidation paths and fuel composition is finally presented. It provides a means to assess the power capacity of animal muscles and can be applied to evaluate their fitness, stamina, margins for improvement, and athletic potential.

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.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

Effects of gas exchange on acid-base balance

This paper describes the interactions between ventilation and acid-base balance under a variety of conditions including rest, exercise, altitude, pregnancy, and various muscle, respiratory, cardiac, and renal pathologies. We introduce the physicochemical approach to assessing acid-base status and demonstrate how this approach can be used to quantify the origins of acid-base disorders using examples from the literature. The relationships between chemoreceptor and metaboreceptor control of ventilation and acid-base balance summarized here for adults, youth, and in various pathological conditions. There is a dynamic interplay between disturbances in acid-base balance, that is, exercise, that affect ventilation as well as imposed or pathological disturbances of ventilation that affect acid-base balance. Interactions between ventilation and acid-base balance are highlighted for moderate- to high-intensity exercise, altitude, induced acidosis and alkalosis, pregnancy, obesity, and some pathological conditions. In many situations, complete acid-base data are lacking, indicating a need for further research aimed at elucidating mechanistic bases for relationships between alterations in acid-base state and the ventilatory responses.

Pulmonary gas exchange and acid-base balance during exercise

As the first step in the oxygen-transport chain, the lung has a critical task: optimizing the exchange of respiratory gases to maintain delivery of oxygen and the elimination of carbon dioxide. In healthy subjects, gas exchange, as evaluated by the alveolar-to-arterial PO2 difference (A-aDO2), worsens with incremental exercise, and typically reaches an A-aDO2 of approximately 25 mmHg at peak exercise. While there is great individual variability, A-aDO2 is generally largest at peak exercise in subjects with the highest peak oxygen consumption. Inert gas data has shown that the increase in A-aDO2 is explained by decreased ventilation-perfusion matching, and the development of a diffusion limitation for oxygen. Gas exchange data does not indicate the presence of right-to-left intrapulmonary shunt developing with exercise, despite recent data suggesting that large-diameter arteriovenous shunt vessels may be recruited with exercise. At the same time, multisystem mechanisms regulate systemic acid-base balance in integrative processes that involve gas exchange between tissues and the environment and simultaneous net changes in the concentrations of strong and weak ions within, and transfer between, extracellular and intracellular fluids. The physicochemical approach to acid-base balance is used to understand the contributions from independent acid-base variables to measured acid-base disturbances within contracting skeletal muscle, erythrocytes and noncontracting tissues. In muscle, the magnitude of the disturbance is proportional to the concentrations of dissociated weak acids, the rate at which acid equivalents (strong acid) accumulate and the rate at which strong base cations are added to or removed from muscle.

Volume regulation in mammalian skeletal muscle: the role of sodium-potassium-chloride cotransporters during exposure to hypertonic solutions

Controversy exists as to whether mammalian skeletal muscle is capable of volume regulation in response to changes in extracellular osmolarity despite evidence that muscle fibres have the required ion transport mechanisms to transport solute and water in situ. We addressed this issue by studying the ability of skeletal muscle to regulate volume during periods of induced hyperosmotic stress using single, mouse extensor digitorum longus (EDL) muscle fibres and intact muscle (soleus and EDL). Fibres and intact muscles were loaded with the fluorophore, calcein, and the change in muscle fluorescence and width (single fibres only) used as a metric of volume change. We hypothesized that skeletal muscle exposed to increased extracellular osmolarity would elicit initial cellular shrinkage followed by a regulatory volume increase (RVI) with the RVI dependent on the sodium–potassium–chloride cotransporter (NKCC). We found that single fibres exposed to a 35% increase in extracellular osmolarity demonstrated a rapid, initial 27–32% decrease in cell volume followed by a RVI which took 10-20 min and returned cell volume to 90–110% of pre-stimulus values. Within intact muscle, exposure to increased extracellular osmolarity of varying degrees also induced a rapid, initial shrinkage followed by a gradual RVI, with a greater rate of initial cell shrinkage and a longer time for RVI to occur with increasing extracellular tonicities. Furthermore, RVI was significantly faster in slow-twitch soleus than fast-twitch EDL. Pre-treatment of muscle with bumetanide (NKCC inhibitor) or ouabain (Na+,K+-ATPase inhibitor), increased the initial volume loss and impaired the RVI response to increased extracellular osmolarity indicating that the NKCC is a primary contributor to volume regulation in skeletal muscle. It is concluded that mouse skeletal muscle initially loses volume then exhibits a RVI when exposed to increases in extracellular osmolarity. The rate of RVI is dependent on the degree of change in extracellular osmolarity, is muscle specific, and is dependent on the functioning of the NKCC and Na+, K+-ATPase.

Control of cell volume in skeletal muscle

Regulation of cell volume is a fundamental property of all animal cells and is of particular importance in skeletal muscle where exercise is associated with a wide range of cellular changes that would be expected to influence cell volume. These complex electrical, metabolic and osmotic changes, however, make rigorous study of the consequences of individual factors on muscle volume difficult despite their likely importance during exercise. Recent charge-difference modelling of cell volume distinguishes three major aspects to processes underlying cell volume control: (i) determination by intracellular impermeant solute; (ii) maintenance by metabolically dependent processes directly balancing passive solute and water fluxes that would otherwise cause cell swelling under the influence of intracellular membrane-impermeant solutes; and (iii) volume regulation often involving reversible short-term transmembrane solute transport processes correcting cell volumes towards their normal baselines in response to imposed discrete perturbations. This review covers, in turn, the main predictions from such quantitative analysis and the experimental consequences of comparable alterations in extracellular pH, lactate concentration, membrane potential and extracellular tonicity. The effects of such alterations in the extracellular environment in resting amphibian muscles are then used to reproduce the intracellular changes that occur in each case in exercising muscle. The relative contributions of these various factors to the control of cell volume in resting and exercising skeletal muscle are thus described.

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.

The in vivo and in vitro effects of L-carnitine supplementation on the erythrocyte membrane acetylcholinesterase, Na+, K+-ATPase and Mg2+-ATPase activities in basketball players

BACKGROUND

We investigated whether the activities of erythrocyte membrane acetylcholinesterase (AChE), Na+, K+-ATPase and Mg2+-ATPase are modulated in basketball players pre- vs. post-forced training with or without L-carnitine (L-C) supplementation.

METHODS

Blood was obtained from 10 male players pre-game (group A) and post-game (group B) and after 1 month L-C supplementation (2 g/24 h orally) pre-training (group C) and post-training (group D). Lactate, pyruvate and total antioxidant status (TAS) were measured with commercial kits, catecholamines with HPLC and the enzyme activities spectrophotometrically.

RESULTS

Lactate, pyruvate, AChE, Na+, K+-ATPase and catecholamines were increased (p<0.001) and TAS was decreased (p<0.001) in group B. In contrast, TAS remained unaltered and the all enzyme activities were reduced (p<0.001) in group D at the same time of study. Mg2+-ATPase activity remained unchanged. In vitro incubation of the modulated AChE and Na+, K+-ATPase with L-C (25 microM) from group B and group D resulted in a non-significant reduction of the enzymes in group B and complete restoration of their activities in group D.

CONCLUSIONS

The increase of AChE and Na+, K+-ATPase activities may be due to the elevation of catecholamines in group B. Carnitine utilization by the muscles during training may result in a reduction of the enzyme activities (group D). The latter is supported by the recovery of the enzyme activities after incubation of the membranes from group D with L-C.

Alterações hidroeletrolíticas agudas ocorridas no Triatlon Ironman Brasil

O Triatlon Ironman caracteriza-se por ser uma atividade de resistência constituída por 3,8km de natação, 180km de ciclismo e 42,2km de corrida, no qual o atleta exercita-se, em média, por cerca de 13 horas. Neste contexto, o atleta exposto a tal carga de esforço e adversidades ambientais, experimenta alterações orgânicas agudas em seus sistemas biológicos, incluindo os distúrbios hidroeletrolíticos. O objetivo deste estudo é descrever as alterações hídricas e eletrolíticas encontradas em atletas de triatlon Ironman. De 2002 a 2005 foram avaliados 109 atletas voluntários antes e imediatamente após as provas realizadas em Florianópolis-SC Brasil, com análise sanguínea dos eletrólitos sódio, e potássio, e medida de massa corporal. Os dados do sódio sérico de 89 atletas foram correlacionados com o grau de desidratação e modificações percentuais de peso corporal. Dados de 77 atletas, quanto ao potássio sérico, foram avaliados isoladamente de forma descritiva. Seis atletas (6,7%) apresentaram-se euhidratados ou superhidratados ao final da prova, 50 atletas desidrataram de 0 a 3% (56,2%), 29 de 3 a 6% (32,6%) e 4 atletas (4,5%) desidrataram mais que 6%. Houve uma tendência a ocorrer hiponatremia entre aqueles que desidrataram menos ou ganharam peso. O potássio teve um comportamento dentro dos limites da normalidade em toda amostra. Conclui-se que os distúrbios hidroeletrolíticos (hiponatremia e desidratação) são incidentes nesta modalidade esportiva, sendo a superhidratação a etiologia provável da hiponatremia denotada pelo ganho ou perdas discretas de peso.

Glycogen resynthesis and exercise performance with the addition of fenugreek extract (4-hydroxyisoleucine) to post-exercise carbohydrate feeding

The purpose of this study was to determine the effect of adding fenugreek extract (FG) to post-exercise carbohydrate feeding on glycogen resynthesis and subsequent exercise performance in normoglycemic male endurance athletes. A muscle biopsy sample was obtained from the vastus lateralis from subjects prior to exercise for 5 h at 50% of peak cycling power (52.1 +/- 3.3% of VO(2) peak). A second muscle biopsy sample was obtained immediately after exercise. Immediately after and 2 h after the second biopsy subjects ingested either an oral dose of dextrose (GLU) (1.8 g x kg BW(-1)) or GLU with FG containing 1.99 +/- 0.20 mg x kg(-1) 4-hydroxyisoleucine (GLU + FG) in a randomized, cross-over, double blind design. At 4 h post-exercise a third biopsy was taken and subjects received a standardised meal along with FG or a placebo capsule. At 15 h post-exercise subjects underwent their final muscle biopsy before completing a simulated 40 km cycling time trial. There was no difference in muscle glycogen at any time between GLU and GLU + FG. Additionally, 40 km time trial performance was similar for average power output (221 +/- 28 vs. 213 +/- 16 watts) and for time to completion (69.7 +/- 3.7 vs. 70.5 +/- 2.2 min) for the GLU and GLU + FG, respectively. Despite earlier data to the contrary, the present results do not support an effect of fenugreek supplementation on glycogen resynthesis, even though this may have been the result of differences in experimental protocol.

An in vivo and in vitro study of erythrocyte membrane acetylcholinesterase, (Na+, K+)-ATPase and Mg2+-ATPase activities in basketball players on α-tocopherol supplementation. The role of l-carnitine

BACKGROUND & AIMS

Vitamin E (alpha-tocopherol, alpha-Te) and carnitine reduce lipid peroxidation. THE AIM WAS TO: To investigate the erythrocyte membrane acetylcholinesterase (AChE), Na+, K+-ATPase and Mg2+-ATPase activities in basketball players with or without alpha-Te supplementation, before and after training. In vitro, we aimed to find out any additional effect of L-carnitine (L-C) on the modulated enzyme activities.

SUBJECTS AND METHODS

Blood was obtained from 10 players before (group A), after exercise (group B) and after 1 month on alpha-Te (200 mg/24 h orally) supplementation before (group C) and after the game (group D). Lactate, pyruvate, muscle enzyme activities and total antioxidant status (TAS) were measured with commercial kits. Catecholamines and alpha-Te were determined with HPLC methods and membrane enzyme activities spectrophotometrically.

RESULTS

Lactate, pyruvate, muscle enzymes and catecholamine levels were increased (P<0.001) in all groups after training. Alpha-Te levels and Mg2+-ATPase activity remained unaltered before and after exercise. TAS was decreased in the groups after the game. AChE activity was increased in group B (P<0.01) and decreased in group D (P<0.01). After the exercise, Na+, K+-ATPase activity was increased in group B and remained unaltered in group D. In vitro incubation of membranes from group D with L-C resulted in a partially restoration of the membrane AChE activity, whereas Na+, K+-ATPase and Mg2+-ATPase activities were found unchanged.

CONCLUSIONS

Alpha-Te supplementation in basketball players results in an increase of TAS and AChE activity, whereas the other enzyme activities were found unchanged. L-C addition may restore AChE activity, which was modulated by training in players on alpha-Te.

L-Cysteine supplementation prevents exercise-induced alterations in human erythrocyte membrane acetylcholinesterase and Na+,K+-ATPase activities

BACKGROUND

L-Cysteine (L-Cys) is implicated in the reduction of free radical production. The aim of this study was to investigate whether L-Cys supplementation prevents modulation of the activities of erythrocyte membrane acetylcholinesterase (AChE), Na(+),K(+)-ATPase and Mg(2+)-ATPase induced by free radicals in basketball players during training.

METHODS

Blood was obtained from 10 basketball male players before (group A) and after a game (group B) and after 1 week of L-Cys (0.5 g/24 h orally) supplementation before (group C) and after training (group D). Lactate, pyruvate and total antioxidant status (TAS) were measured using commercial kits and the enzyme activities were determined spectrophotometrically.

RESULTS

Both lactate and pyruvate levels remarkably increased after exercise. In contrast, TAS levels significantly decreased in group B, increased in group C and then declined (group D), reaching those of group A. AChE activity was statistically increased post-exercise (3.98+/-0.04 Delta OD/min x mg protein) compared with pre-training (2.90+/-0.05 Delta OD/min x mg protein, p<0.01). Na(+),K(+)-ATPase activity was also higher post-exercise (1.27+/-0.05 micromol Pi/h x mg protein) than that pre-exercise (0.58+/-0.04 micromol Pi/h x mg protein, p<0.001). When the players were supplemented with L-Cys, both AChE and Na(+),K(+)-ATPase activities remained unaltered post-exercise. Mg(2+)-ATPase activities were unchanged in all groups studied.

CONCLUSIONS

L-Cys supplementation may protect the enzyme activities studied against stimulation induced by free radical production during training in athletes by ameliorating their total antioxidant capacity.

Changes in tissue water content measured with multiple-frequency bioimpedance and metabolism measured with 31P-MRS during progressive forearm exercise

Multiple-frequency bioimpedance analysis (MFBIA) has been used to determine the cellular water composition in the human body. It is noninvasive and has demonstrated good correlations with other invasive measures of tissue water. However, the ability of this method to study transient changes in tissue water in specific muscle groups has not been explored. In this study, MFBIA was used to assess changes in forearm intracellular water (ICW), extracellular water (ECW), and total water (TW) in seven healthy volunteers during and after a progressive wrist flexion exercise protocol. In an identical trial, (31)P magnetic resonance spectroscopy ((31)P-MRS) was used to assess changes in intracellular pH and phosphocreatine (PCr). At the completion of exercise, forearm ICW increased 12.6% (SD 0.07, P = 0.003), TW increased 10.1% (SD 0.06, P = 0.005), and no significant changes were recorded for ECW. A significant correlation was found between the changes in intracellular pH and changes in ICW during exercise (r = -0.84, P = 0.018). With the use of regression analysis, average changes in P(i), PCr, and pH were found to predict changes in ICW (R(2) = 0.98, P = 0.005). In conclusion, MFBIA was sensitive enough to measure transient changes in the exercising forearm muscle. The changes seen were consistent with the hypothesis that intracellular acidification and PCr hydrolysis are important mediators of cellular osmolality and therefore may be responsible for the increased volume of water in the intracellular space that is often recorded after short-term high-intensity exercise.

Effect of repetitive stimulation on cell volume and its relationship to membrane potential in amphibian skeletal muscle

The effect of electrical stimulation on cell volume, V (c), and its relationship to membrane potential, E (m), was investigated in Rana temporaria striated muscle. Confocal microscope xz-plane scanning and histology of plastic sections independently demonstrated significant and reversible increases in V (c) of 19.8+/-0.62% (n=3) and 27.1+/-8.62% (n=3), respectively, after a standard stimulation protocol. Microelectrode measurements demonstrated an accompanying membrane potential change, DeltaE (m), of +23.6+/-0.98 mV (n=3). The extent to which this DeltaE (m) might contribute to the observed changes in V (c) was explored in quiescent muscle exposed to variations in extracellular potassium concentration, [K(+)](e). E (m) and V (c) varied linearly with log [K(+)](e) and [K(+)](e), respectively, in the range 2.5-15 mM (R (2)=0.99 and 0.96), and these results were used to reconstruct an approximately linear relationship between V (c) and E (m) (DeltaV (c)=0.85E (m)+68.53; R (2)=0.99) and hence derive the DeltaV (c) expected from the DeltaE (m) during stimulation. This demonstrated that both the time course and magnitude of the increase and recovery of V (c) observed in active muscles could be reproduced by the corresponding [K(+)](e)-induced depolarisation in quiescent muscles, suggesting that the depolarisation associated with membrane activity makes a substantial contribution to the cell swelling during exercise. Furthermore, conditions of Cl(-) deprivation abolished the relationship between E (m) and V (c), supporting a mechanism in which the depolarisation of E (m) drives a passive redistribution of Cl(-) and hence cellular entry of Cl(-) and K(+) and an accompanying, osmotically driven, increase in V (c).

Effects of 8-week, interval-based inspiratory muscle training and breathing retraining in patients with generalized myasthenia gravis

STUDY OBJECTIVE

To assess the effect of interval-based inspiratory muscle training (IMT) combined with breathing retraining (BR) in patients with generalized myasthenia gravis (MG) in a partial home program.

DESIGN

A randomized controlled trial with blinding of outcome assessment.

SETTING

A secondary-care respiratory clinic.

PATIENTS

Twenty-seven patients with generalized MG were randomized to a control group or a training group.

INTERVENTIONS

The training group underwent interval-based IMT associated with BR (diaphragmatic breathing [DB] and pursed-lips breathing [PLB]) three times a week for 8 weeks. The sessions included 10 min each of DB, interval-based IMT, and PLB. Interval-based IMT consisted of training series interspersed with recovery time. The threshold load was increased from 20 to 60% of maximal inspiratory pressure (P(Imax)) over the 8 weeks.

MEASUREMENTS AND RESULTS

Lung function, respiratory pattern, respiratory muscle strength, respiratory endurance, and thoracic mobility were measured before and after the 8 weeks. The training group improved significantly compared to control group in P(Imax) (p = 0.001), maximal expiratory pressure (P(Emax)) [p = 0.01], respiratory rate (RR)/tidal volume (V(T)) ratio (p = 0.05), and upper chest wall expansion (p = 0.02) and reduction (p = 0.04). Significant differences were seen in the training group compared to baseline P(Imax) (p = 0.001), P(Emax) (p = 0.01), maximal voluntary ventilation (p = 0.02), RR/V(T) ratio (p = 0.003), Vt (p = 0.02), RR (p = 0.01), total time of RR (p = 0.01), and upper chest wall expansion (p = 0.005) and reduction (p = 0.005). No significant improvement was seen in lower chest wall or lung function.

CONCLUSIONS

The partial home program of interval-based IMT associated with BR is feasible and effective in patients with generalized MG. Improvements in respiratory muscle strength, chest wall mobility, respiratory pattern, and respiratory endurance were observed.

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.

Mechanisms and management of fatigue in health and disease: symposium introduction

Exercise intolerance is a condition commonly experienced by both the healthy and those with disease. Yet we have only a limited understanding of the underlying mechanisms and, consequently, the management of this condition. In this Symposium, a major objective was to address the role of the muscle cell in weakness and fatigue. We have focused on addressing the advances made in characterizing the basis of muscle cell contractility with particular respect to the processes and proteins involved in excitation and contraction, and how these processes can be modified during repetitive activity. Three reviews are provided on this subject. Each addresses a specific link in the cascade of events from neural activation of the muscle to the generation of force. In the first review the processes involved in signal transduction in the sarcolemma and T-tubule, and which regulate membrane excitability, are examined. The second review analyzes the sarcoplasmic reticulum regulation of the intracellular messenger that controls the myofibrillar complex, namely free calcium. The final review in this series deals with the events regulating actin-myosin behaviour and the mechanical response. All reviews place special emphasis on how different sites can be modified by repetitive activity and, as a consequence, how they can represent a potential source of fatigue. Since it is important to understand the nature, manifestations, and measurement of weakness and fatigue, a comprehensive review on these topics is also provided.

Model for the behaviour of compartmental CO(2) stores during incremental exercise

The respiratory exchange ratio (RER) is a valid method for determining fat and carbohydrate oxidation during exercise when the exchange of respiratory gas is in a state of steady flux between the tissue and fluid compartments and the alveoli. However, under incremental intensity or heavy exercise conditions, the movement of electrolytes, fluids, and CO(2) between body-fluid compartments is accentuated, leading to increased hydrogen-ion concentration ([H(+)]), decreased bicarbonate-ion concentration ([HCO(3) (-)]) and CO(2) stores, and the excretion of additional CO(2) at the alveoli (i.e. H(+)+HCO(3) (-) --> CO(2)+H(2)O) elevating the CO(2) minute volume. This non-respiratory CO(2) excretion can invalidate use of the RER for determination of fat and carbohydrate oxidation. Direct measurement of the labile CO(2) store and non-respiratory CO(2) excretion during exercise is difficult. Therefore, physicochemical models were derived to illustrate the likely behaviour of compartmental CO(2) stores during 8 W.min(-1) incremental cycling exercise to formulate correction factors to the RER for the non-respiratory CO(2) component. From these models, a polynomial regression equation was derived to describe the change in the total labile CO(2) store volume during incremental exercise from the relationship with blood HCO(3) (-) content: CO(2) volume (ml) = -17x(2)+464x+650, where x is the arterialised blood standard HCO(3) (-) concentration (mmol.l(-1)), relative to resting conditions. Non-respiratory CO(2) excretion (ml.min(-1)) was then determined from the rate of change in CO(2) volume. The modelling method could allow for straightforward calculation of the non-respiratory CO(2) excretion rate for future validation work.

Biochemistry of exercise-induced metabolic acidosis

The development of acidosis during intense exercise has traditionally been explained by the increased production of lactic acid, causing the release of a proton and the formation of the acid salt sodium lactate. On the basis of this explanation, if the rate of lactate production is high enough, the cellular proton buffering capacity can be exceeded, resulting in a decrease in cellular pH. These biochemical events have been termed lactic acidosis. The lactic acidosis of exercise has been a classic explanation of the biochemistry of acidosis for more than 80 years. This belief has led to the interpretation that lactate production causes acidosis and, in turn, that increased lactate production is one of the several causes of muscle fatigue during intense exercise. This review presents clear evidence that there is no biochemical support for lactate production causing acidosis. Lactate production retards, not causes, acidosis. Similarly, there is a wealth of research evidence to show that acidosis is caused by reactions other than lactate production. Every time ATP is broken down to ADP and Pi, a proton is released. When the ATP demand of muscle contraction is met by mitochondrial respiration, there is no proton accumulation in the cell, as protons are used by the mitochondria for oxidative phosphorylation and to maintain the proton gradient in the intermembranous space. It is only when the exercise intensity increases beyond steady state that there is a need for greater reliance on ATP regeneration from glycolysis and the phosphagen system. The ATP that is supplied from these nonmitochondrial sources and is eventually used to fuel muscle contraction increases proton release and causes the acidosis of intense exercise. Lactate production increases under these cellular conditions to prevent pyruvate accumulation and supply the NAD+needed for phase 2 of glycolysis. Thus increased lactate production coincides with cellular acidosis and remains a good indirect marker for cell metabolic conditions that induce metabolic acidosis. If muscle did not produce lactate, acidosis and muscle fatigue would occur more quickly and exercise performance would be severely impaired.

Potassium and lactate uptake by noncontracting tissue during strenuous exercise

Efflux of potassium (K+) and lactate (lac-) from active skeletal muscle during high intensity exercise leads to increased plasma [K+] and [lac-] in venous and arterial blood. The exercise-induced increases in these ions in human athletes is ameliorated, in part, by uptake by noncontracting tissue, reflected by an increase in the arteriovenous difference across these tissues from rest to exercise. To test the hypothesis that these ions are taken up by noncontracting tissue in strenuously exercising horses, plasma [K+] and [lac-] were measured in blood samples collected simultaneously from the carotid artery (CA), femoral vein (FV), and transverse facial vein (TFV) in 5 unconditioned Standardbred horses performing 4.5 min of strenuous treadmill exercise. Further, the arteriovenous differences ((a-v)diff) in [K+] and [lac-] were calculated across active tissue (CA-FV) and noncontracting tissue (CA-TFV). After 3 min of strenuous exercise, plasma [K+] in FV blood (mean +/- s.e. 7.1 +/- 0.2 mmol/l) was greater (P<0.05) than in CA blood (6.8 +/- 0.2 mmol/l) and both of these concentrations were greater (P<0.05) than a value of 5.7 +/- 0.1 mmol/l in TFV blood. Similarly, plasma [lac-] increased to 20.6 +/- 0.8 and 21.1 +/- 0.8 mmol/l in CA and FV blood, respectively, and these concentrations were greater (P<0.05) than a value of 19.1 +/- 0.8 mmol/l in TFV blood. The magnitude of the (a-v)diff for both [K+] and [lac-] increased (P<0.05) across active and noncontracting tissue during high-speed exercise but the directions were opposite, supporting release by active tissue and uptake by noncontracting tissue. These results confirm that uptake of K+ and lac- by noncontracting tissue contributes to regulation of the plasma concentrations of these ions in strenuously exercising horses.

K+-induced twitch potentiation is not due to longer action potential

The objective of this study was to determine whether an increased duration of the action potential contributes to the K+-induced twitch potentiation at 37 degrees C. Twitch contractions were elicited by field stimulation, and action potentials were measured with conventional microelectrodes. For mouse extensor digitorum longus (EDL) muscle, twitch force was greater at 7-13 mM K+ than at 4.7 mM (control). For soleus muscle, twitch force potentiation was observed between 7 and 11 mM K+. Time to peak and half-relaxation time were not affected by the increase in extracellular K+ concentration in EDL muscle, whereas both parameters became significantly longer in soleus muscle. Decrease in overshoot and prolongation of the action potential duration observed at 9 and 11 mM K+ were mimicked when muscles were respectively exposed to 25 and 50 nM tetrodotoxin (TTX; used to partially block Na+ channels). Despite similar action potentials, twitch force was not potentiated by TTX. It is therefore suggested that the K+-induced potentiation of the twitch in EDL muscle is not due to a prolongation of the action potential and contraction time, whereas a longer contraction, especially the relaxation phase, may contribute to the potentiation in soleus muscle.

Effects of 28-day mechanical and chewing stress on content of bound and diffusible ions in muscles of mastication

Type I and type II muscle fibres have different ion concentrations. Muscles adapt to chronic stress by changing of fibre types and remodelling of the myosin heavy chains in the muscle fibres. The present investigation on ionic change during muscular contraction was carried out on 10-week-old pigs (6 treated animals, 6 controls) over a 28-day period. Six pigs received acrylic build-ups to induce mechanical advancement of the lower jaw and chronic chewing stress. Muscle tissue was taken from the masseter (M1, M2, M3), temporal (TP1, TP2), medial pterygoid (PM) and geniohyoid (GH) muscles by a standardized method. Eighty-four muscle samples were used for histological fibre differentiation with mATPase. Energy-dispersive X-ray microanalysis of muscles was carried out in an environmental scanning electron microscope. Endurance stress in the stressed muscles was seen as an increase of type I fibres (P < 0.001). This histological change and ionic alterations were measured in the anterior region of the masseter (M1 and M2) and in the posterior region of the temporal muscle (TP2). Smaller changes were found in the medial pterygoid muscle. We measured in this muscles increases in potassium, sulphur, chloride (P < 0.05) and even larger increases in phosphate (up to 1.5 mmol/g to 2.3 mmol/g, P < 0.001) and sodium (3-fold, P < 0.001). The results reveal the effects of chronic stress on muscle fibres and ion concentration in the muscle. Chronic stress resulted in an increase of type I fibres and increased ion concentration in the same muscle region. These are considered to be indicators of more efficient contraction. The changes in ion concentration are an important factor in muscle contraction.

Metabolic component of intestinal PCO(2) during dysoxia

The adequacy of intestinal perfusion during shock and resuscitation might be estimated from intestinal tissue acid-base balance. We examined this idea from the perspective of conventional blood acid-base physicochemistry. As the O(2) supply diminishes with failing blood flow, tissue acid-base changes are first "respiratory, " with CO(2) coming from combustion of fuel and stagnating in the decreasing blood flow. When the O(2) supply decreases to critical, the changes become "metabolic" due to lactic acid. In blood, the respiratory vs. metabolic distinction is conventionally made using the buffer base principle, in which buffer base is the sum of HCO(3)(-) and noncarbonate buffer anion (A(-)). During purely respiratory acidosis, buffer base stays constant because HCO(3)(-) cannot buffer its own progenitor, carbonic acid, so that the rise of HCO(3)(-) equals the fall of A(-). During anaerobic "metabolism," however, lactate's H(+) is buffered by both A(-) and HCO(3)(-), causing buffer base to decrease. We quantified the partitioning of lactate's H(+) between HCO(3)(-) and A(-) buffer in anoxic intestine by compressing intestinal segments of anesthetized swine into a steel pipe and measuring PCO(2) and lactate at 5- to 10-min intervals. Their rises followed first-order kinetics, yielding k = 0. 031 min(-1) and half time = approximately 22 min. PCO(2) vs. lactate relations were linear. Over 3 h, lactate increased by 31 +/- 3 mmol/l tissue fluid (mM) and PCO(2) by approximately 17 mM, meaning that one-half of lactate's H(+) was buffered by tissue HCO(3)(-) and one-half by A(-). The data were consistent with a lumped pK(a) value near 6.1 and total A(-) concentration of approximately 30 mmol/kg. We conclude that the respiratory vs. metabolic distinction could be made in tissue by estimating tissue buffer base from measured pH and PCO(2).

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

The distribution of rest periods affects performance and adaptations of energy metabolism induced by high-intensity training in human muscle

The effect of the distribution of rest periods on the efficacy of interval sprint training is analysed. Ten male subjects, divided at random into two groups, performed distinct incremental sprint training protocols, in which the muscle load was the same (14 sessions), but the distribution of rest periods was varied. The 'short programme' group (SP) trained every day for 2 weeks, while the 'long programme' group (LP) trained over a 6-week period with a 2-day rest period following each training session. The volunteers performed a 30-s supramaximal cycling test on a cycle ergometer before and after training. Muscle biopsies were obtained from the vastus lateralis before and after each test to examine metabolites and enzyme activities. Both training programmes led to a marked increase (all significant, P < 0.05) in enzymatic activities related to glycolysis (phosphofructokinase - SP 107%, LP 68% and aldolase - SP 46%, LP 28%) and aerobic metabolism (citrate synthase - SP 38%, LP 28.4% and 3-hydroxyacyl-CoA dehydrogenase - SP 60%, LP 38.7%). However, the activity of creatine kinase (44%), pyruvate kinase (35%) and lactate dehydrogenase (45%) rose significantly (P < 0.05) only in SP. At the end of the training programme, SP had suffered a significant decrease in anaerobic ATP consumption per gram muscle (P < 0.05) and glycogen degradation (P < 0.05) during the post-training test, and failed to improve performance. In contrast, LP showed a marked improvement in performance (P < 0.05) although without a significant increase in anaerobic ATP consumption, glycolysis or glycogenolysis rate. These results indicate that high-intensity cycling training in 14 sessions improves enzyme activities of anaerobic and aerobic metabolism. These changes are affected by the distribution of rest periods, hence shorter rest periods produce larger increase in pyruvate kinase, creatine kinase and lactate dehydrogenase. However, performance did not improve in a short training programme that did not include days for recovery, which suggests that muscle fibres suffer fatigue or injury.

Effects of exercise on muscle transverse relaxation determined by MR imaging and in vivo relaxometry

The purpose of this study was to determine the effects of intense exercise on the proton transverse (T(2)) relaxation of human skeletal muscle. The flexor digitorium profundus muscles of 12 male subjects were studied by using magnetic resonance imaging (MRI; 6 echoes, 18-ms echo time) and in vivo magnetic resonance relaxometry (1,000 echoes, 1.2-ms echo time), before and after an intense handgrip exercise. MRI of resting muscle produced a single T(2) value of 32 ms that increased by 19% (P < 0.05) with exercise. In vivo relaxometry showed at least three T(2) components (>5 ms) for all subjects with mean values of 21, 40, and 137 ms and respective magnitudes of 34, 49, and 14% of the total magnetic resonance signal. These component magnitudes changed with exercise by -44% (P < 0.05), +52% (P < 0.05), and +23% (P < 0.05), respectively. These results demonstrate that intense exercise has a profound effect on the multicomponent T(2) relaxation of muscle. Changes in the magnitudes of all the T(2) components synergistically increase MRI T(2), but changes in the two shortest T(2) components predominate.

K+ transport in resting rat hind-limb skeletal muscle in response to paraxanthine, a caffeine metabolite

This study tested the hypothesis that paraxanthine, a caffeine metabolite, stimulates skeletal muscle potassium (K+) transport by an increase in Na+-K+ ATPase activity. The unidirectional transport of K+ into muscle (JinK) was studied using a perfused rat hind limb technique. Using 12 hind limbs, we examined the response to 20 min of paraxanthine perfusion (0.1 mM), followed by 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 5) to irreversibly inhibit Na+-K+ ATPase activity. Paraxanthine stimulated JinK by 23 ± 5% within 20 min. Ouabain abolished the paraxanthine-induced stimulation of JinK, suggesting the increase in K+ uptake was due to activation of the Na+-K+ ATPase. To confirm the role of the Na+-K+ ATPase, 14 hind limbs were perfused for 20 min with 5 mM ouabain prior to 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 6). Ouabain alone resulted in a 41 ± 7% decrease in JinK within 15 min. Inhibition of ouabain-sensitive JinK prevented the paraxanthine-induced increase in JinK. Hind limbs (n = 3) were also perfused with 0.1 mM paraxanthine for 60 min to examine the response to longer duration paraxanthine perfusion. The paraxanthine-induced increase in JinK continued for the entire 60 min. In another series, hind limbs were perfused with 0.01 (n = 9), 0.1 (n = 9), or 0.5 (n = 6) mM paraxanthine for 15 min. There was no concentration-dependent relationship between JinK and paraxanthine concentration, and 0.01, 0.1, and 0.5 mM paraxanthine increased JinK similarly (25 ± 5, 22 ± 4, and 27 ± 6%, respectively). The effect of paraxanthine on JinK could not be reversed by subsequent perfusion with paraxanthine-free perfusate. Caffeine (0.05-1.0 mM) had no effect on K+ transport. It is concluded that paraxanthine increases JinK in resting skeletal muscle by stimulating ouabain-sensitive Na+-K+ ATPase activity.Key words: caffeine, methylxanthine, ouabain, potassium transport, sodium pump, Na-K ATPase, VO2, glycolysis.

Skeletal muscle Na(+)-K(+)-ATPase and K+ homeostasis during exercise: effects of short-term training

The objective of this study was to determine the effects of 10 consecutive days of moderate intensity training on 1) the concentration of middle gluteal muscle Na(+)-K(+)-ATPase as determined by vanadate-facilitated 3H[ouabain binding; and 2) plasma potassium regulation before, during and after exercise at 100% of the pre-training maximum rate of oxygen consumption (VO2max). Six mature, unfit Thoroughbred horses completed both incremental (for determination of VO2max) and high-intensity exercise protocols before (HI1) and after (HI2) training. There additional horses undertook no training or exercise tests and served as controls for determination of middle gluteal muscle Na(+)-K(+)-ATPase concentration. Training consisted of 10 consecutive days of running at 55% VO2max for 60 min per day (13-14 km/day). For each high intensity exercise protocol, horses completed a 10 min warm-up at 50% VO2max, followed by exercise at 100% of pre-training VO2max (6 degrees incline, mean speed 9.8 m/s) until fatigue. Training resulted in a 13.8% increase in resting plasma volume (pre: 20.9 +/- 0.8 l; post: 23.8 +/- 0.9 l; P = 0.03), and an 8.9% increase in VO2max (pre: 142 +/- 4 ml/kg/min; post: 155 +/- 4 ml/kg/min; P = 0.004) during HI. Peak values for plasma potassium concentration and content during exercise decreased by 13% (P = 0.02) and 7% (P = 0.0002), respectively, after training whereas training had no effect on increases in packed cell volume, plasma total solids, and erythrocyte K+ concentration and content during exercise. Following training, there was also a significant (23%) increase in Na(+)-K(+)-ATPase concentration in biopsies of middle gluteal muscle, as measured by vanadate-facilitated 3H[ouabain binding. We conclude that 10 days of moderate intensity exercise results in increases in skeletal muscle Na(+)-K(+)-ATPase and attenuation in the elevation in plasma K+ during high intensity exercise at the same absolute workload. The increase in middle gluteal muscle Na(+)-K(+)-ATPase concentration is consistent with decreases in K+ efflux from working muscle during exercise.

Physiology of body fluids in the horse

This article presents a brief overview of the physiology of homeostasis in the horse and discusses the units of measurement by which dissolved electrolytes are assessed. Differences in electrolyte composition between compartments of the body and the manner in which water is exchanged between them are reviewed. The mechanisms by which alterations in osmolality and effective circulating volume are detected and the physiological responses to those alterations are presented.

Sprint training enhances ionic regulation during intense exercise in men

1. This study investigated the effects of 7 weeks of sprint training on changes in electrolyte concentrations and acid-base status in arterial and femoral venous blood, during and following maximal exercise for 30 s on an isokinetic cycle ergometer. 2. Six healthy males performed maximal exercise, before and after training. Blood samples were drawn simultaneously from brachial arterial and femoral venous catheters, at rest, during the final 10 s of exercise and during 10 min of recovery, and analysed for whole blood and plasma ions and acid-base variables. 3. Maximal exercise performance was enhanced after training, with a 13% increase in total work output and a 14% less decline in power output during maximal cycling. 4. The acute changes in plasma volume, ions and acid-base variables during maximal exercise were similar to previous observations. Sprint training did not influence the decline in plasma volume during or following maximal exercise. After training, maximal exercise was accompanied by lower arterial and femoral venous plasma [K+] and [Na+] across all measurement times (P < 0.05). Arterial plasma lactate concentration ([Lac-]) was greater (P < 0.05), but femoral venous plasma [Lac-] was unchanged by training. 5. Net release into, or uptake of ions from plasma passing through the exercising muscle was assessed by arteriovenous concentration differences, corrected for fluid movements. K+ release into plasma during exercise, and a small net K+ uptake from plasma 1 min post-exercise (P < 0.05), were unchanged by training. A net Na+ loss from plasma during exercise (P < 0.05) tended to be reduced after training (P < 0.06). Release of Lac- into plasma during and after exercise (P < 0.05) was unchanged by training. 6. Arterial and venous plasma strong ion difference ([SID]; [SID] = [Na+] + [K+] - [Lac-] - [Cl-]) were lower after training (mean differences) by 2.7 and 1.8 mmol l-1, respectively (P < 0.05). Arterial and femoral venous CO2 tensions and arterial plasma [HCO3-] were lower after training (mean differences) by 1.7 mmHg, 4.5 mmHg and 1.2 mmol l-1, respectively (P < 0.05), with arterial plasma [H+] being greater after training by 2.2 nmol l-1 (P < 0.05). 7. The acute changes in whole blood volume and ion concentrations during maximal exercise were similar to previous observations: Arterial and femoral whole blood [K+] and [Cl-] were increased, whilst [Na+] was lower, across all observation times after training (P < 0.05). 8. Net uptake or release of ions by exercising muscle was assessed by arteriovenous whole blood concentration differences, corrected for fluid movements. A net K+ uptake by muscle occurred at all times, including exercise, but this was not significantly different after training. An increased net Na+ uptake by muscle occurred during exercise (P < 0.05) with greater Na+ uptake after training (P < 0.05). Net muscle Lac- release and Cl- uptake occurred at all times (P < 0.05) and were unchanged by training. 9. Sprint training improved muscle ion regulation, associated with increased intense exercise performance, at the expense of a greater systemic acidosis. Increased muscle Na+ and K+ uptake by muscle during the final seconds of exercise after training are consistent with a greater activation of the muscle Na(+) - K+ pump, reduced cellular K+ loss and the observed lesser rate of fatigue. The greater plasma acidosis found after sprint training was caused by a lower arterial plasma [SID] due to lower plasma [K+] and [Na+], and higher plasma [Lac-].

Water and ion losses during the cross-country phase of eventing

Loss of total body water and ions during prolonged exercise can predispose the horse to health and performance problems. This study examined total body water (TBW) losses and extracellular (ECF) ion losses during the cross-country (XC) phase of Preliminary, Intermediate and Advanced Horse Trials and CCI level 3-day-events. Jugular venous blood samples and body mass (BM) were collected on 49 horses at rest, Pre-XC, Post-XC and following 30 min of recovery. Plasma was separated from blood cells within 10 min of collection. Plasma was analysed for [Na+], [K+], [Cl-], ionised [Ca+2], [glucose], [lactate] and packed cell volume (PCV) and plasma [protein] ([PP]). Distances ranged from 3000-6000 m for the XC phase with speeds of 500-570 m/min. In general, losses of TBW, Na+ and Cl- increased with increasing level of difficulty. Loss of TBW Post-XC ranged from 2-6.1% of resting values or 8.9-12.6 litres for the Preliminary level to mean +/- s.e. 20.4 +/- 1.8 litres for CCI. Losses of ECF ions ranged from 0.5-6.4% for Na+, 1.2-7.7% for Cl-, gains of 8% to losses of 23% for K+, and gains of 7% to losses of 11.7% for Ca+2 at 30 min Post-XC. There was little recovery following 30 min and deficits of 5.3 +/- 2.5 litres persisted overnight in horses that were tested. Plasma protein and PCV increased Post-XC and PCV remained elevated above resting values at 30 min recovery. It is concluded that significant water and ion losses occur and, in general, increase with increased level of difficulty. There was little change with 30 min of recovery and for some horses losses persisted overnight.

Relation between extracellular [K+], membrane potential and contraction in rat soleus muscle: modulation by the Na+-K+ pump

An increased extracellular K+ concentration ([K+]0) is thought to cause muscle fatigue. We studied the effects of increasing [K+]0 from 4mM to 8-14mM on tetanic contractions in isolated bundles of fibres and whole soleus muscles from the rat. Whereas there was little depression of force at a [K+]0 of 8-9mM, a further small increase in [K+]0 to 11-14mM resulted in a large reduction of force. Tetanus depression at 11mM [K+]0 was increased when using weaker stimulation pulses and decreased with stronger pulses. Whereas the tetanic force/resting membrane potential (EM) relation showed only moderate force depression with depolarization from -74 to -62mV, a large reduction of force occurred when EM fell to-53mV. The implications of these relations to fatigue are discussed. Partial inhibition of the Na+-K+ pump with ouabain (10(-6 )M) caused additional force loss at 11mM [K+]0. Salbutamol, insulin, or calcitonin gene-related peptide all stimulated the Na+-K+ pump in muscles exposed to 11mM [K+]0 and induced an average 26-33% recovery of tetanic force. When using stimulation pulses of 0.1ms, instead of the standard 1.0-ms pulses, force recovery with these agents was 41-44% which was significantly greater (P < 0.025). Only salbutamol caused any recovery of EM (1.3mV). The observations suggest that the increased Na+ concentration difference across the sarcolemma, following Na+-K+ pump stimulation, has an important role in restoring excitability and force.