Role of Na+,K+-pumps and transmembrane Na+,K+-distribution in muscle function. The FEPS lecture - Bratislava 2007

A century of exercise physiology: effects of muscle contraction and exercise on skeletal muscle Na+,K+-ATPase, Na+ and K+ ions, and on plasma K+ concentration-historical developments

This historical review traces key discoveries regarding K+ and Na+ ions in skeletal muscle at rest and with exercise, including contents and concentrations, Na+,K+-ATPase (NKA) and exercise effects on plasma [K+] in humans. Following initial measures in 1896 of muscle contents in various species, including humans, electrical stimulation of animal muscle showed K+ loss and gains in Na+, Cl- and H20, then subsequently bidirectional muscle K+ and Na+ fluxes. After NKA discovery in 1957, methods were developed to quantify muscle NKA activity via rates of ATP hydrolysis, Na+/K+ radioisotope fluxes, [3H]-ouabain binding and phosphatase activity. Since then, it became clear that NKA plays a central role in Na+/K+ homeostasis and that NKA content and activity are regulated by muscle contractions and numerous hormones. During intense exercise in humans, muscle intracellular [K+] falls by 21 mM (range - 13 to - 39 mM), interstitial [K+] increases to 12-13 mM, and plasma [K+] rises to 6-8 mM, whilst post-exercise plasma [K+] falls rapidly, reflecting increased muscle NKA activity. Contractions were shown to increase NKA activity in proportion to activation frequency in animal intact muscle preparations. In human muscle, [3H]-ouabain-binding content fully quantifies NKA content, whilst the method mainly detects α2 isoforms in rats. Acute or chronic exercise affects human muscle K+, NKA content, activity, isoforms and phospholemman (FXYD1). Numerous hormones, pharmacological and dietary interventions, altered acid-base or redox states, exercise training and physical inactivity modulate plasma [K+] during exercise. Finally, historical research approaches largely excluded female participants and typically used very small sample sizes.

Potassium homeostasis - Physiology and pharmacology in a clinical context

Membrane voltage controls the function of excitable cells and is mainly a consequence of the ratio between the extra- and intracellular potassium concentration. Potassium homeostasis is safeguarded by balancing the extra-/intracellular distribution and systemic elimination of potassium to the dietary potassium intake. These processes adjust the plasma potassium concentration between 3.5 and 4.5 mmol/L. Several genetic and acquired diseases but also pharmacological interventions cause dyskalemias that are associated with increased morbidity and mortality. The thresholds at which serum K+ not only associates but also causes increased mortality are hotly debated. We discuss physiologic, pathophysiologic, and pharmacologic aspects of potassium regulation and provide informative case vignettes. Our aim is to help clinicians, epidemiologists, and pharmacologists to understand the complexity of the potassium homeostasis in health and disease and to initiate appropriate treatment strategies in dyskalemic patients.

Different K+-release in distal myogenic and neurogenic muscular weakness during non-ischemic exercise

INTRODUCTION

In myotonic dystrophy, an increased potassium release upon ischemic forearm exercise has been previously described. However, it remains unclear whether this is specific for myotonic dystrophies or just due to distal muscular weakness.

METHODS

Non-ischemic forearm test (NIFET) was performed and venous K⁺ concentration was measured at rest and at three different force levels (20-30%, 50-60%, 70-80%) related to maximal contraction force (MCF) in patients with distal myogenic (n = 7), neurogenic (n = 7) muscular weakness and healthy volunteers (n = 12). The specific K⁺ release was defined as K⁺ increase related to workload as force-time-integral during repetitive contraction.

RESULTS

Workload was lower at all force levels in both disease groups compared to the control group. With increasing workload, the K⁺ concentrations increased in all study groups. Analysing individual force levels related to the maximum contraction force (MCF), a higher specific K⁺ release was measured at low force levels in myopathies (20-30% MCF) in comparison to higher force levels (p = 0.02). At 20-30% MCF, the specific K⁺ release was significantly higher in myogenic compared to neurogenic muscular weakness (p = 0.005). At 50-60% and 70-80% MCF, the specific K⁺ values converged and did not significantly differ between the three groups (p = 0.09 and p = 0.37).

DISCUSSION

At low force levels, K⁺ efflux related to workload is higher in patients with myogenic in comparison to neurogenic distal paresis. Our results indicate a different regulation of K⁺ balance in neurogenic and myogenic muscular weakness possibly due to a different recruitment behaviour of motor units and the firing rate of motor neurons.

A single oral glucose load decreases arterial plasma [K+ ] during exercise and recovery

AIM

We investigated whether acute carbohydrate ingestion reduced arterial potassium concentration ([K+ ]) during and after intense exercise and delayed fatigue.

METHODS

In a randomized, double-blind crossover design, eight males ingested 300 ml water containing 75 g glucose (CHO) or placebo (CON); rested for 60 min, then performed high-intensity intermittent cycling (HIIC) at 130% V ˙ O 2peak , comprising three 45-s exercise bouts (EB), then a fourth EB until fatigue. Radial arterial (a) and antecubital venous (v) blood was sampled at rest, before, during and after HIIC and analyzed for plasma ions and metabolites, with forearm arteriovenous differences (a-v diff) calculated to assess inactive forearm muscle effects.

RESULTS

Glucose ingestion elevated [glucose]a and [insulin]a above CON (p = .001), being, respectively, ~2- and ~5-fold higher during CHO at 60 min after ingestion (p = .001). Plasma [K+ ]a rose during and declined following each exercise bout in HIIC (p = .001), falling below baseline at 5 min post-exercise (p = .007). Both [K+ ]a and [K+ ]v were lower during CHO (p = .036, p = .001, respectively, treatment main effect). The [K+ ]a-v diff across the forearm widened during exercise (p = .001), returned to baseline during recovery, and was greater in CHO than CON during EB1, EB2 (p = .001) and EB3 (p = .005). Time to fatigue did not differ between trials.

CONCLUSION

Acute oral glucose ingestion, as used in a glucose tolerance test, induced a small, systemic K+ -lowering effect before, during, and after HIIC, that was detectable in both arterial and venous plasma. This likely reflects insulin-mediated, increased Na+ ,K+ -ATPase induced K+ uptake into non-contracting muscles. However, glucose ingestion did not delay fatigue.

Effects of natural polyphenol-rich pomegranate juice supplementation on plasma ion and lipid profiles following resistance exercise: a placebo-controlled trial

Background

Pomegranate juice (POMj) contains abundant soluble polyphenolic antioxidant compounds and is recommended for its cardioprotective/atheroprotective properties. However, very few studies have investigated the efficacy of POMj supplementation to alter physiological responses during intensive physical exercise. This placebo-controlled study aimed to examine whether supplementation with natural polyphenol-rich-POMj could influence the ionic or lipid responses to an intensive resistance training session in elite athletes.

Methods

Nine elite weightlifters (21 ± 1 years) performed two Olympic-weightlifting sessions after ingesting placebo and POMj supplements. Venous blood samples were collected at rest and 3 min after each session for assessment of plasma sodium ([Na⁺]), potassium ([K⁺]), chloride ([Cl^-]), calcium ([Ca²⁺]), triglyceride ([TG]) and high-density lipoprotein ([HDL-C]), low-density lipoprotein ([HDL-C]) and total ([TC]) cholesterol concentrations.

Results

Plasma [K⁺] and [TG] were lowered post-exercise compared to resting values in the PLA condition (p = 0.03 for K⁺ and p = 0.02 for TG) with no pre-to-post exercise differences in the other plasma ion and lipid markers (p > 0.05). Compared to rest, plasma [Na⁺] and [Cl^-] were increased (p = 0.04, %change = 4.10% for Na⁺ and p = 0.02, %change = 4.44% for Cl^-), but there were no differences in the other plasma ion or lipid markers post-exercise after POMj supplementation (p > 0.05). Post-exercise plasma [Na⁺], [Cl^-], and [HDL-C] were greater following POMj supplementation compared to PLA (p = 0.01 for Cl- and HDL-C, p = 0.02 for Na+, and p = 0.04 for TC), with no between-supplement post-exercise differences in the other ion and lipid markers (p > 0.05).

Conclusion

In conclusion, supplementation with POMj has the potential to attenuate the acute imbalance of plasma [K⁺] and to improve blood lipid responses (i.e., HDL-C) following resistance exercises in elite weightlifters. However, further large research in both athletic and non-athletic populations is needed to corroborate these preliminary observations and to elucidate the potential underlying mechanisms and translational potential of our novel observations.

Trial registration

Name of the registry:ClinicalTrials.gov PRSThe registration number:NCT02697903.Date of Registry: 03/03/2016 'Retrospectively registered'.The registration title: Pomegranate Improve Biological Recovery Kinetics in Elite Weightlifter.

Graphical abstract

Moderately elevated extracellular [K+] potentiates submaximal force and power in skeletal muscle via increased [Ca2+]i during contractions

The extracellular K+ concentration ([K+]o) increases during physical exercise. We here studied whether moderately elevated [K+]o may increase force and power output during contractions at in vivo-like subtetanic frequencies and whether such potentiation was associated with increased cytosolic free Ca2+ concentration ([Ca2+]i) during contractions. Isolated whole soleus and extensor digitorum longus (EDL) rat muscles were incubated at different levels of [K+]o, and isometric and dynamic contractility were tested at various stimulation frequencies. Furthermore, [Ca2+]i at rest and during contraction was measured along with isometric force in single mouse flexor digitorum brevis (FDB) fibers exposed to elevated [K+]o. Elevating [K+]o from 4 mM up to 8 mM (soleus) and 11 mM (EDL) increased isometric force at subtetanic frequencies, 2–15 Hz in soleus and up to 50 Hz in EDL, while inhibition was seen at tetanic frequency in both muscle types. Elevating [K+]o also increased peak power of dynamic subtetanic contractions, with potentiation being more pronounced in EDL than in soleus muscles. The force-potentiating effect of elevated [K+]o was transient in FDB single fibers, reaching peak after ~4 and 2.5 min in 9 and 11 mM [K+]o, respectively. At the time of peak potentiation, force and [Ca2+]i during 15-Hz contractions were significantly increased, whereas force was slightly decreased and [Ca2+]i unchanged during 50-Hz contractions. Moderate elevation of [K+]o can transiently potentiate force and power during contractions at subtetanic frequencies, which can be explained by a higher [Ca2+]i during contractions.

Synchronization Modulation of Na/K Pumps Induced Membrane Potential Hyperpolarization in Both Physiological and Hyperkalemic Conditions

The capability of the synchronization modulation (SM) technique in enhancing the function of Na/K pumps has been demonstrated in various cells and tissues, including cardiomyocytes, a monolayer of cultured MDCK kidney cells, peripheral blood vessels, and frog skeletal muscles. This study characterized the membrane potential hyperpolarization induced by SM in both physiological and high [K⁺]_o conditions on single skeletal muscle fibers. The results showed that SM could consistently induce membrane potential hyperpolarization by a few millivolts, and this hyperpolarization was not possible in the presence of ouabain. In contrast, the same electrical pulses but with random frequencies, constant frequencies, or synchronization with backward-modulation could not hyperpolarize the membrane potential. Prolonged field application and higher field intensity enhanced the effects of SM-induced hyperpolarization. Finally, the effect of SM was tested on skeletal muscle fibers incubated in a solution with high external potassium. Results showed that the SM electric field could hyperpolarize the membrane potential even if the external K⁺ concentration was higher than the normal, which implied the therapeutic effects of the SM electric field on the hyperkalemic situation.

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.

Neuromuscular manifestations of work-related myalgia in women specific to extensor carpi radialis brevis

This study assessed neuromuscular function in the extensor carpi radialis brevis (ECRB) of female workers diagnosed with work-related myalgia (WRM, n = 14, age 45.2 ± 1.9 years) and the ECRB of healthy controls (CON, n = 10, age 34.6 ± 2.5 years). Groups were compared on voluntary and electrically evoked functional responses at rest (Pre), immediately following a 5 min repetitive task (Post-0) performed at 60% maximal voluntary contraction (MVC), and after 5 min of recovery (Post-5). Despite near complete motor unit activation (MUA) (CON 98% ± 1% vs. WRM 99% ± 1%), at Pre, WRM produced 26% less (P < 0.05) MVC force than CON. Following an MVC, twitch force was increased (P < 0.05) by 94% ± 13% and 54% ± 11% in CON and WRM, respectively (CON vs. WRM; P < 0.05). The peak force and the maximal rates of force development and decline of electrically evoked contractions (10-100 Hz) were generally depressed (P < 0.05) at Post-0 and Post-5 relative to Pre. The response pattern to increasing frequencies of stimulation was not different (P > 0.05) between groups and MUA was not impaired (CON 97% ± 1% vs. WRM 97% ± 1%; P > 0.05). In conclusion, the peripheral weakness observed in the ECRB in WRM at rest does not result in abnormal fatigue or recovery responses after performing a task controlled for relative demand (60% MVC).

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.

Specialized Functional Diversity and Interactions of the Na,K-ATPase

Na,K-ATPase is a protein ubiquitously expressed in the plasma membrane of all animal cells and vitally essential for their functions. A specialized functional diversity of the Na,K-ATPase isozymes is provided by molecular heterogeneity, distinct subcellular localizations, and functional interactions with molecular environment. Studies over the last decades clearly demonstrated complex and isoform-specific reciprocal functional interactions between the Na,K-ATPase and neighboring proteins and lipids. These interactions are enabled by a spatially restricted ion homeostasis, direct protein-protein/lipid interactions, and protein kinase signaling pathways. In addition to its "classical" function in ion translocation, the Na,K-ATPase is now considered as one of the most important signaling molecules in neuronal, epithelial, skeletal, cardiac and vascular tissues. Accordingly, the Na,K-ATPase forms specialized sub-cellular multimolecular microdomains which act as receptors to circulating endogenous cardiotonic steroids (CTS) triggering a number of signaling pathways. Changes in these endogenous cardiotonic steroid levels and initiated signaling responses have significant adaptive values for tissues and whole organisms under numerous physiological and pathophysiological conditions. This review discusses recent progress in the studies of functional interactions between the Na,K-ATPase and molecular microenvironment, the Na,K-ATPase-dependent signaling pathways and their significance for diversity of cell function.

Cellular properties of extensor carpi radialis brevis and trapezius muscles in healthy males and females

In this study, we sought to determine whether differences in cellular properties associated with energy homeostasis could explain the higher incidence of work-related myalgia in trapezius (TRAP) compared with extensor carpi radialis brevis (ECRB). Tissue samples were obtained from the ECRB (n = 19) and TRAP (n = 17) of healthy males and females (age 27.9 ± 2.2 and 28.1 ± 1.5 years, respectively; mean ± SE) and analyzed for properties involved in both ATP supply and utilization. The concentration of ATP and the maximal activities of creatine phosphokinase, phosphorylase, and phosphofructokinase were higher (P < 0.05) in ECRB than TRAP. Succinic dehydrogenase, citrate synthase, and cytochrome c oxidase were not different between muscles. The ECRB also displayed a higher concentration of Na(+)-K(+)-ATPase and greater sarcoplasmic reticulum Ca(2+) release and uptake. No differences existed between muscles for either monocarboxylate transporters or glucose transporters. It is concluded that the potentials for high-energy phosphate transfer, glycogenolysis, glycolysis, and excitation-contraction coupling are higher in ECRB than TRAP. Histochemical measurements indicated that the muscle differences are, in part, related to differing amounts of type II tissue. Depending on the task demands, the TRAP may experience a greater metabolic and excitation-contraction coupling strain than the ECRB given the differences observed.

The August Krogh Institute: Capillaries and beyond

Bengt Saltin knew very well the history and work of the giants whose shoulders he was standing upon, such as August Krogh and Johannes Lindhard. He was basically a physiologist interested in physical activity and exercise, particularly in the cardiovascular and muscular responses. Some of his major original contributions were (a) the human muscle model in terms of the one-legged, knee extensor quantifying work by the high-precision Krogh ergometer and, using this, challenging Krogh's proposed autoregulation of capillary blood flow during exercise; (b) the electrolyte fluxes quantification on an intra- and extra-cellular level in human muscle during exercise to reveal such changes as possible fatigue mechanisms; and (c) the evidence presented that underlined the health-enhancing effect of physical exercise training from bedside to workplace.

Excitation of skeletal muscle is a self-limiting process, due to run-down of Na+, K+ gradients, recoverable by stimulation of the Na+, K+ pumps

The general working hypothesis of this study was that muscle fatigue and force recovery depend on passive and active fluxes of Na⁺ and K⁺. This is tested by examining the time-course of excitation-induced fluxes of Na⁺ and K⁺ during 5–300 sec of 10–60 Hz continuous electrical stimulation in rat extensor digitorum longus (EDL) muscles in vitro and in vivo using ²²Na and flame photometric determination of Na⁺ and K⁺. 60 sec of 60 Hz stimulation rapidly increases ²²Na influx, during the initial phase (0–15 sec) by 0.53 μmol(sec)⁻¹(g wet wt.)⁻¹, sixfold faster than in the later phase (15–60 sec). These values agree with flame photometric measurements of Na⁺ content. The progressive reduction in the rate of excitation-induced Na⁺ uptake is likely to reflect gradual loss of excitability due to accumulation of K⁺ in the extracellular space and t-tubules leading to depolarization. This is in keeping with the concomitant progressive loss of contractile force previously demonstrated. During electrical stimulation rat muscles rapidly reach high rates of active Na⁺, K⁺-transport (in EDL muscles a sevenfold increase and in soleus muscles a 22-fold increase), allowing efficient and selective compensation for the large excitation-induced passive Na⁺, K⁺-fluxes demonstrated over the latest decades. The excitation-induced changes in passive fluxes of Na⁺ and K⁺ are both clearly larger than previously observed. The excitation-induced reduction in [Na⁺]_o contributes considerably to the inhibitory effect of elevated [K⁺]_o. In conclusion, excitation-induced passive and active Na⁺ and K⁺ fluxes are important causes of muscle fatigue and force recovery, respectively.

Genome-wide analysis of DNA methylation before-and after exercise in the thoroughbred horse with MeDIP-Seq

Athletic performance is an important criteria used for the selection of superior horses. However, little is known about exercise-related epigenetic processes in the horse. DNA methylation is a key mechanism for regulating gene expression in response to environmental changes. We carried out comparative genomic analysis of genome-wide DNA methylation profiles in the blood samples of two different thoroughbred horses before and after exercise by methylated-DNA immunoprecipitation sequencing (MeDIP-Seq). Differentially methylated regions (DMRs) in the pre-and post-exercise blood samples of superior and inferior horses were identified. Exercise altered the methylation patterns. After 30 min of exercise, 596 genes were hypomethylated and 715 genes were hypermethylated in the superior horse, whereas in the inferior horse, 868 genes were hypomethylated and 794 genes were hypermethylated. These genes were analyzed based on gene ontology (GO) annotations and the exercise-related pathway patterns in the two horses were compared. After exercise, gene regions related to cell division and adhesion were hypermethylated in the superior horse, whereas regions related to cell signaling and transport were hypermethylated in the inferior horse. Analysis of the distribution of methylated CpG islands confirmed the hypomethylation in the gene-body methylation regions after exercise. The methylation patterns of transposable elements also changed after exercise. Long interspersed nuclear elements (LINEs) showed abundance of DMRs. Collectively, our results serve as a basis to study exercise-based reprogramming of epigenetic traits.

Relevance of excitable media theory and retinal spreading depression experiments in preclinical pharmacological research

In preclinical neuropharmacological research, molecular, cell-based, and systems using animals are well established. On the tissue level the situation is less comfortable, although during the last decades some effort went into establishing such systems, i.e. using slices of the vertebrate brain together with optical and electrophysiological techniques. However, these methods are neither fast, nor can they be automated or upscaled. By contrast, the chicken retina can be used as a suitable model. It is easy accessible and can be kept alive in vitro for hours up to days. Due to its structure, in addition the retina displays remarkable intrinsic optical signals, which can be easily used in experiments. Also to electrophysiological methods the retina is well accessible. In excitable tissue, to which the brain and the retina belong, propagating excitation waves can be expected, and the spreading depression is such a phenomenon. It has been first observed in the forties of the last century. Later, Martins-Ferreira established it in the chicken retina (retinal spreading depression or RSD). The electrophysiological characteristics of it are identical with those of the cortical SD. The metabolic differences are known and can be taken into account. The experimental advantage of the RSD compared to the cortical SD is the pronounced intrinsic optical signal (IOS) associated with the travelling wave. This is due to the maximum transparency of retinal tissue in the functional state; thus any physiological event will change it markedly and therefore can be easily seen even by naked eye. The theory can explain wave spread in one (action potentials), two (RSDs) and three dimensions (one heart beat). In this review we present the experimental and the excitable media context for the data interpretation using as example the cholinergic pharmacology in relation to functional syndromes. We also discuss the intrinsic optical signal and how to use it in pre-clinical research.

Quantification of Na+,K+ pumps and their transport rate in skeletal muscle: functional significance

During excitation, muscle cells gain Na⁺ and lose K⁺, leading to a rise in extracellular K⁺ ([K⁺]_o), depolarization, and loss of excitability. Recent studies support the idea that these events are important causes of muscle fatigue and that full use of the Na⁺,K⁺-ATPase (also known as the Na⁺,K⁺ pump) is often essential for adequate clearance of extracellular K⁺. As a result of their electrogenic action, Na⁺,K⁺ pumps also help reverse depolarization arising during excitation, hyperkalemia, and anoxia, or from cell damage resulting from exercise, rhabdomyolysis, or muscle diseases. The ability to evaluate Na⁺,K⁺-pump function and the capacity of the Na⁺,K⁺ pumps to fill these needs require quantification of the total content of Na⁺,K⁺ pumps in skeletal muscle. Inhibition of Na⁺,K⁺-pump activity, or a decrease in their content, reduces muscle contractility. Conversely, stimulation of the Na⁺,K⁺-pump transport rate or increasing the content of Na⁺,K⁺ pumps enhances muscle excitability and contractility. Measurements of [³H]ouabain binding to skeletal muscle in vivo or in vitro have enabled the reproducible quantification of the total content of Na⁺,K⁺ pumps in molar units in various animal species, and in both healthy people and individuals with various diseases. In contrast, measurements of 3-O-methylfluorescein phosphatase activity associated with the Na⁺,K⁺-ATPase may show inconsistent results. Measurements of Na⁺ and K⁺ fluxes in intact isolated muscles show that, after Na⁺ loading or intense excitation, all the Na⁺,K⁺ pumps are functional, allowing calculation of the maximum Na⁺,K⁺-pumping capacity, expressed in molar units/g muscle/min. The activity and content of Na⁺,K⁺ pumps are regulated by exercise, inactivity, K⁺ deficiency, fasting, age, and several hormones and pharmaceuticals. Studies on the α-subunit isoforms of the Na⁺,K⁺-ATPase have detected a relative increase in their number in response to exercise and the glucocorticoid dexamethasone but have not involved their quantification in molar units. Determination of ATPase activity in homogenates and plasma membranes obtained from muscle has shown ouabain-suppressible stimulatory effects of Na⁺ and K⁺.

Unaccustomed eccentric contractions impair plasma K+ regulation in the absence of changes in muscle Na+,K+-ATPase content

The Na⁺,K⁺-ATPase (NKA) plays a fundamental role in the regulation of skeletal muscle membrane Na⁺ and K⁺ gradients, excitability and fatigue during repeated intense contractions. Many studies have investigated the effects of acute concentric exercise on K⁺ regulation and skeletal muscle NKA, but almost nothing is known about the effects of repeated eccentric contractions. We therefore investigated the effects of unaccustomed maximal eccentric knee extensor contractions on K⁺ regulation during exercise, peak knee extensor muscle torque, and vastus lateralis muscle NKA content and 3-O-MFPase activity. Torque measurements, muscle biopsies, and venous blood samples were taken before, during and up to 7 days following the contractions in six healthy adults. Eccentric contractions reduced peak isometric muscle torque immediately post-exercise by 26±11% and serum creatine kinase concentration peaked 24 h post-exercise at 339±90 IU/L. During eccentric contractions, plasma [K⁺] rose during Set 1 and remained elevated at ∼4.9 mM during sets 4–10; this was despite a decline in work output by Set 4, which fell by 18.9% at set 10. The rise in plasma [K⁺].work⁻¹ ratio was elevated over Set 2 from Set 4– Set 10. Eccentric contractions had no effect on muscle NKA content or maximal in-vitro 3-O-MFPase activity immediately post- or up to 7 d post-exercise. The sustained elevation in plasma [K⁺] despite a decrease in work performed by the knee extensor muscles suggests an impairment in K⁺ regulation during maximal eccentric contractions, possibly due to increased plasma membrane permeability or to excitation-contraction uncoupling.

Excitation–contraction coupling properties in women with work-related myalgia: a preliminary study

We investigated the potential role of selected excitation–contraction coupling processes in females with work-related myalgia (WRM) by comparing WRM with healthy controls (CON) using tissue from extensor carpi radialis brevis (ECRB) and trapezius (TRAP) muscles. For the ECRB, age (mean ± SE) was 29.6 ± 3.5 years for CON (n = 9) and 39.2 ± 2.8 years for WRM (n = 13), while for the TRAP, the values were 26.0 ± 2.1 years for CON (n = 7) and 44.6 ± 2.9 years for WRM (n = 11). For the sarcoplasmic reticulum (SR) of the ECRB, WRM displayed concentrations (nmol·(mg protein)−1·min−1) that were lower (P < 0.05) for Total (202 ± 4.4 vs 178 ± 7.1), Basal (34 ± 1.6 vs 30.1 ± 1.3), and maximal Ca2+-ATPase activity (Vmax, 168 ± 4.9 vs 149 ± 6.3), and Ca2+-uptake (5.06 ± 0.31 vs 4.13 ± 0.29), but not SERCA1a and SERCA2a isoforms, by comparison with CON. When age was incorporated as a co-variant, Total, Basal, and Ca2+-uptake remained different from CON (P < 0.05), but not Vmax (P = 0.13). For TRAP, none of the ATPase properties differed between groups (P > 0.05) either before or following adjustment for age. No differences (P > 0.05) were observed between the groups for Ca2+-release in the SR for either TRAP or ECRB. Similarly, no deficiencies, regardless of muscle, were noted for either the Na+–K+-ATPase content or the α and β subunit isoform distribution in WRM. This preliminary study provides a basis for further research, with expanded numbers, investigating the hypothesis that abnormalities in SR Ca2+-regulation are involved in the cellular etiology of WRM.

Extracellular potassium homeostasis: insights from hypokalemic periodic paralysis

Extracellular potassium makes up only about 2% of the total body's potassium store. The majority of the body potassium is distributed in the intracellular space, of which about 80% is in skeletal muscle. Movement of potassium in and out of skeletal muscle thus plays a pivotal role in extracellular potassium homeostasis. The exchange of potassium between the extracellular space and skeletal muscle is mediated by specific membrane transporters. These include potassium uptake by Na(+), K(+)-adenosine triphosphatase and release by inward-rectifier K(+) channels. These processes are regulated by circulating hormones, peptides, ions, and by physical activity of muscle as well as dietary potassium intake. Pharmaceutical agents, poisons, and disease conditions also affect the exchange and alter extracellular potassium concentration. Here, we review extracellular potassium homeostasis, focusing on factors and conditions that influence the balance of potassium movement in skeletal muscle. Recent findings that mutations of a skeletal muscle-specific inward-rectifier K(+) channel cause hypokalemic periodic paralysis provide interesting insights into the role of skeletal muscle in extracellular potassium homeostasis. These recent findings are reviewed.

Investigating the relationship between intra-operative electrolyte abnormalities (sodium and potassium) with post-operative complications of coronary artery bypass surgery

Background:

Generally, the electrolyte abnormalities are seen in many hospitalized patients, and this problem increases in ones with heart diseases. The purpose of this study is determination of the prevalence of electrolyte abnormalities during the coronary artery bypass surgery (CABG) and detecting the relationship between these abnormalities with the complications after the surgeries.

Materials and Methods:

This is a cross-sectional study, which is done in Chamran hospital, the medical and educational center of Isfahan, Iran, in 2011. The target population included the patients who have undergone CABG in this hospital. In this study, 100 patients who had been candidates for CABG were selected, and we extracted their recorded intra-operative electrolyte information. The collected data was entered into the computer and analyzed by SPSS software. The Chi-square and t student tests were used for data analysis.

Results:

The mean ± SD of sodium during CABG was 137.95 ± 4.6 (range 127-152) mg\dl. Also, the mean ± SD of potassium was 4.65 ± 0.9 (range: 2.9-7.4). According to these results, 48 patients (48% of all) had electrolyte imbalance and 52 patients (52% of all) were normal. Sodium level in 71% of patients was normal, and in 29% of them was abnormal. Potassium level in 73% of individuals was normal, and in 27% of them was abnormal.

Conclusion:

Giving an attention to electrolyte abnormalities in patients who have undergone CABG surgery is a considerable necessity for them, and sufficient arrangements are needed to prevent such abnormalities.

Excitation-induced exchange of Na+, K+, and Cl- in rat EDL muscle in vitro and in vivo: physiology and pathophysiology

In skeletal muscle, excitation leads to increased [Na(+)](i), loss of K(+), increased [K(+)](o), depolarization, and Cl(-) influx. This study quantifies these changes in rat extensor digitorum longus (EDL) muscles in vitro and in vivo using flame photometric determination of Na(+) and K(+) and (36)Cl as a tracer for Cl(-). In vitro, 5-Hz stimulation for 300 s increased intracellular Na(+) content by 4.6 ± 1.2 µmol/g wet wt (P < 0.002) and decreased intracellular K(+) content by 5.5 ± 2.3 µmol/g wet wt (P < 0.03). This would increase [K(+)](o) by 28 ± 12 mM, sufficient to cause severe loss of excitability as the result of inactivation of Na(+) channels. In rat EDL, in vivo stimulation at 5 Hz for 300 s or 60 Hz for 60 s induced significant loss of K(+) (P < 0.01), sufficient to increase [K(+)](o) by 71 ± 22 mM and 73 ± 15 mM, respectively. In spite of this, excitability may be maintained by the rapid and marked stimulation of the electrogenic Na(+),K(+) pumps already documented. This may require full utilization of the transport capacity of Na(+),K(+) pumps, which then becomes a limiting factor for physical performance. In buffer containing (36)Cl, depolarization induced by increasing [K(+)](o) to 40-80 mM augmented intracellular (36)Cl by 120-399% (P < 0.001). Stimulation for 120-300 s at 5-20 Hz increased intracellular (36)Cl by 100-188% (P < 0.001). In rats, Cl(-) transport in vivo was examined by injecting (36)Cl, where electrical stimulation at 5 Hz for 300 s or 60 Hz for 60 s increased (36)Cl uptake by 81% (P < 0.001) and 84% (P < 0.001), respectively, indicating excitation-induced depolarization. Cl(-) influx favors repolarization, improving K(+) clearance and maintenance of excitability. In conclusion, excitation-induced fluxes of Na(+), K(+), and Cl(-) can be quantified in vivo, providing new evidence that in working muscles, extracellular accumulation of K(+) is considerably higher than previously observed and the resulting depression of membrane excitability may be a major cause of muscle fatigue.

Unchanged [3H]ouabain binding site content but reduced Na+-K+ pump α2-protein abundance in skeletal muscle in older adults

Aging is associated with reduced muscle mass, weakness, and increased fatigability. In skeletal muscle, the Na+-K+ pump (NKA) is important in regulating Na+-K+ gradients, membrane excitability, and thus contractility, but the effects of aging on muscle NKA are unclear. We investigated whether aging is linked with reduced muscle NKA by contrasting muscle NKA isoform gene expression and protein abundance, and NKA total content in 17 Elderly (66.8 ± 6.4 yr, mean ± SD) and 16 Young adults (23.9 ± 2.2 yr). Participants underwent peak oxygen consumption assessment and a vastus lateralis muscle biopsy, which was analyzed for NKA α1-, α2-, α3-, β1-, β2-, and β3-isoform gene expression (real-time RT-PCR), protein abundance (immunoblotting), and NKA total content ([3H]ouabain binding sites). The Elderly had lower peak oxygen consumption (−36.7%, P = 0.000), strength (−36.3%, P = 0.001), NKA α2- (−24.4%, 11.9 ± 4.4 vs. 9.0 ± 2.7 arbitrary units, P = 0.049), and NKA β3-protein abundance (−23.0%, P = 0.041) than Young. The β3-mRNA was higher in Elderly compared with Young ( P = 0.011). No differences were observed between groups for other NKA isoform mRNA or protein abundance, or for [3H]ouabain binding site content. Thus skeletal muscle in elderly individuals was characterized by decreased NKA α2- and β3-protein abundance, but unchanged α1 abundance and [3H]ouabain binding. The latter was likely caused by reduced α2 abundance with aging, preventing an otherwise higher [3H]ouabain binding that might occur with a greater membrane density in smaller muscle fibers. Further study is required to verify reduced muscle NKA α2 with aging and possible contributions to impaired exercise capability and daily living activities.

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.

Muscle fatigue and excitation-contraction coupling responses following a session of prolonged cycling

AIM

The mechanisms underlying the fatigue that occurs in human muscle following sustained activity are thought to reside in one or more of the excitation-contraction coupling (E-C coupling) processes. This study investigated the association between the changes in select E-C coupling properties and the impairment in force generation that occurs with prolonged cycling.

METHODS

Ten volunteers with a peak aerobic power (VO(2peak)) of 2.95 ± 0.27 L min(-1) (mean ± SE), exercised for 2 h at 62 ± 1.3%. Quadriceps function was assessed and tissue properties (vastus lateralis) were measured prior to (E1-pre) and following (E1-post) exercise and on three consecutive days of recovery (R1, R2 and R3).

RESULTS

While exercise failed to depress the maximal activity (V(max) ) of the Na(+) ,K(+) -ATPase (P = 0.10), reductions (P < 0.05) were found at E1-post in V(max) of sarcoplasmic reticulum Ca(2+) -ATPase (-22%), Ca(2+) -uptake (-26%) and phase 1(-33%) and 2 (-38%) Ca(2+) -release. Both V(max) and Ca(2+) -release (phase 2) recovered by R1, whereas Ca(2+) -uptake and Ca(2+) -release (phase 1) remained depressed (P < 0.05) at R1 and at R1 and R2 and possibly R3 (P < 0.06) respectively. Compared with E1-pre, fatigue was observed (P < 0.05) at 10 Hz electrical stimulation at E1-post (-56%), which persisted throughout recovery. The exercise increased (P < 0.05) overall content of the Na(+), K(+)-ATPase (R1, R2 and R3) and the isoforms β2 (R1, R2 and R3) and β3 (R3), but not β1 or the α-isoforms (α1, α2 and α3).

CONCLUSION

These results suggest a possible direct role for Ca(2+)-release in fatigue and demonstrate a single exercise session can induce overlapping perturbations and adaptations (particularly to the Na(+), K(+)-ATPase).

Clearance of extracellular K+ during muscle contraction--roles of membrane transport and diffusion

Excitation of muscle often leads to a net loss of cellular K⁺ and a rise in extracellular K⁺ ([ K⁺ ]_o), which in turn inhibits excitability and contractility. It is important, therefore, to determine how this K⁺ is cleared by diffusion into the surroundings or by reaccumulation into the muscle cells. The inhibitory effects of the rise in [K⁺ ]_o may be assessed from the time course of changes in tetanic force in isolated muscles where diffusional clearance of K⁺ is eliminated by removing the incubation medium and allowing the muscles to contract in air. Measurements of tetanic force, endurance, and force recovery showed that in rat soleus and extensor digitorum longus (EDL) muscles there was no significant difference between the performance of muscles contracting in buffer or in air for up to 8 min. Ouabain-induced inhibition of K⁺ clearance via the Na⁺,K⁺ pumps markedly reduced contractile endurance and force recovery in air. Incubation in buffer containing 10 mM K⁺ clearly inhibited force development and endurance, and these effects were considerably reduced by stimulating Na⁺,K⁺ pumps with the β₂-agonist salbutamol. Following 30–60 s of continuous stimulation at 60 Hz, the amount of K⁺ released into the extracellular space was assessed from washout experiments. The release of intracellular K⁺ per pulse was fourfold larger in EDL than in soleus, and in the two muscles, the average [K⁺ ]_o reached 52.4 and 26.0 mM, respectively, appreciably higher than previously detected. In conclusion, prevention of diffusion of K⁺ from the extracellular space of isolated working muscles causes only modest interference with contractile performance. The Na⁺,K⁺ pumps play a major role in the clearance of K⁺ and the maintenance of force. This new information is important for the evaluation of K⁺-induced inhibition in muscles, where diffusional clearance of K⁺ is reduced by tension development sufficient to suppress circulation.