Na,K-ATPase activity in mouse muscle is regulated by AMPK and PGC-1α

AMPK and glucose deprivation exert an isoform-specific effect on the expression of Na+,K+-ATPase subunits in cultured myotubes

In skeletal muscle, Na+,K+-ATPase (NKA), a heterodimeric (α/β) P-type ATPase, has an essential role in maintenance of Na+ and K+ homeostasis, excitability, and contractility. AMP-activated protein kinase (AMPK), an energy sensor, increases the membrane abundance and activity of NKA in L6 myotubes, but its potential role in regulation of NKA content in skeletal muscle, which determines maximum capacity for Na+ and K+ transport, has not been clearly delineated. We examined whether energy stress and/or AMPK affect expression of NKA subunits in rat L6 and primary human myotubes. Energy stress, induced by glucose deprivation, increased protein content of NKAα1 and NKAα2 in L6 myotubes, while decreasing the content of NKAα1 in human myotubes. Pharmacological AMPK activators (AICAR, A-769662, and diflunisal) modulated expression of NKA subunits, but their effects only partially mimicked those that occurred in response to glucose deprivation, indicating that AMPK does not mediate all effects of energy stress on NKA expression. Gene silencing of AMPKα1/α2 increased protein levels of NKAα1 in L6 myotubes and NKAα1 mRNA levels in human myotubes, while decreasing NKAα2 protein levels in L6 myotubes. Collectively, our results suggest a role for energy stress and AMPK in modulation of NKA expression in skeletal muscle. However, their modulatory effects were not conserved between L6 myotubes and primary human myotubes, which suggests that coupling between energy stress, AMPK, and regulation of NKA expression in vitro depends on skeletal muscle cell model.

Quantitative proteomics reveals the neurotoxicity of trimethyltin chloride on mitochondria in the hippocampus of mice

Trimethyltin chloride (TMT) is a potent neurotoxin widely used as a constituent of polyvinyl chloride plastic in the industrial and agricultural fields. However, the underlying mechanisms by which TMT leads to neurotoxicity remain elusive. In the present study, we constructed a dose and time dependent neurotoxic mouse model of TMT exposure to explore the molecular mechanisms involved in TMT-induced neurological damage. Based on this model, the cognitive ability of TMT exposed mice was assessed by the Morris water maze test and a passive avoidance task. The ultrastructure of hippocampus was analyzed by the transmission electron microscope. Subsequently, proteomics integrated with bioinformatics and experimental verification were employed to reveal potential mechanisms of TMT-induced neurotoxicity. Gene ontology (GO) and pathway enrichment analysis were done by using Metascape and GeneCards database respectively. Our results demonstrated that TMT-exposed mice exhibited cognitive disorder, and mitochondrial respiratory chain abnormality of the hippocampus. Proteomics data showed that a total of 7303 proteins were identified in hippocampus of mice of which 224 ones displayed a 1.5-fold increase or decrease in TMT exposed mice compared with controls. Further analysis indicated that these proteins were mainly involved in tricarboxylic acid (TCA) cycle and respiratory electron transport, proteasome degradation, and multiple metabolic pathways as well as inflammatory signaling pathways. Some proteins, including succinate-CoA ligase subunit (Suclg1), NADH dehydrogenase subunit 5 (Nd5), NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 4-like 2 (Ndufa4l2) and cytochrome c oxidase assembly factor 7 (Coa7), which were closely related to mitochondrial respiratory electron transport, showed TMT dose and time dependent changes in the hippocampus of mice. Moreover, apoptotic molecules Bax and cleaved caspase-3 were up-regulated, while anti-apoptotic Bcl-2 was down-regulated compared with controls. In conclusion, our findings suggest that impairment of mitochondrial respiratory chain transport and promotion of apoptosis are the potential mechanisms of TMT induced hippocampus toxicity in mice.

Transcriptome analysis of the mantle tissue of Pinctada fucata with red and black shells under salinity stress

To understand the molecular responses of Pinctada fucata with different shell colors to salinity stress, we used transcriptome sequencing on the mantle of P. fucata with a black shell and red shell color under the salinity of 20, 35, and 50. The 414 and 2371 differentially expressed genes (DEGs) in P. fucata with a black shell under low- or high-salt stress, while there were 588 and 3009 DEGs in P. fucata with a red shell. KEGG pathway enrichment analysis showed that, under low salt stress, the DEGs of P. fucata with the black shell were significantly enriched in pathways MAPK signaling pathway, protein processing in endoplasmic reticulum, vitamin B6 metabolism, longevity regulating pathway-multiple species, estrogen signaling pathway and antigen processing and presentation, the DEGs of P. fucata with a red shell were significantly enriched in pathways vitamin B6 metabolism. Under high salt stress, the DEGs of P. fucata with a red shell were significantly enriched in pathways arginine biosynthesis. 11 DEGs were randomly selected for quantitative real-time PCR, and the results were consistent with the RNA-seq. In addition, under high salt stress, DEGs were enriched into some pathways related to osmotic regulation and immune defense of P. fucata with black shell and red shell, such as Glycolysis / Gluconeogenesis, AMPK signaling pathway, Beta-Alanine metabolism, Glycine, serine and threonine metabolism, MAPK signaling pathway and Phagosome. The study showed that high salt stress had a greater influence on P. fucata with two shell colors, and P. fucata with a black shell made a positive immune defense response. Our results will improve to further understand the salt tolerance mechanism of P. fucata with different shell colors.

The main mechanisms of trimethyltin chloride-induced neurotoxicity: Energy metabolism disorder and peroxidation damage

Trimethyltin chloride (TMT) is a by-product in the synthesis of organotin, a plastic stabilizer. With the rapid development of industry, the occupational hazards caused by TMT cannot be ignored. TMT is a typical neurotoxicant, which mainly damages the limbic system and brainstem of the nervous system. Previous studies have demonstrated that the neurotoxicity induced by TMT is linked to the inhibition of energy metabolism, but the underlying mechanism remains elusive. In order to investigate the mechanism of TMT-induced inhibition of energy metabolism, C57BL/6 male mice were administered by IP injection in different TMT doses (0 mg/kg, 1.00 mg/kg, 2.15 mg/kg and 4.64 mg/kg) and times (1d, 3d and 6d), and then the changes of superoxide dismutase (SOD) activity, malondialdehyde (MDA) level and Na+-K+-ATPase activity in cerebral cortex, cerebellum, hippocampus, pons, medulla oblongata of mice, the expressions of Na+-K+-ATPase protein, AMP-activated protein kinase (AMPK), phosphorylated AMP-activated protein kinase(p-AMPK)and peroxisome proliferator-activated receptor γ coactivator-1 α (PGC-1α) in hippocampus and medulla oblongata were measured; the effects of TMT on the viability, the activity of SOD, glutathione (GSH) and Na+-K+-ATPase, MDA level, and the expression of PGC-1α and Na+-K+-ATPase protein in N2a cells were measured by different TMT doses and times, in order to verify the experiments in vivo. Our results found that most of the mice showed depression, tremor, epilepsy, spasm and other symptoms after TMT exposure. Moreover, with the increase of TMT dose, the activity of Na+-K+-ATPase and the expressions of AMPK protein in the hippocampus and medulla oblongata of mice decreased, and the expressions of p-AMPK protein increased. Peroxidative damage was evident in hippocampus, medulla oblongata of mice and N2a cells, and the expression of PGC-1α and Na+-K+-ATPase protein was significantly down-regulated. Therefore, it is reasonable to believe that TMT-induced neurotoxic symptoms and inhibition of energy metabolism may be related to p-AMPK and down-regulation of PGC-1α in the hippocampus and medulla oblongata.

Effect of differentiation, de novo innervation, and electrical pulse stimulation on mRNA and protein expression of Na+,K+-ATPase, FXYD1, and FXYD5 in cultured human skeletal muscle cells

Denervation reduces the abundance of Na⁺,K⁺-ATPase (NKA) in skeletal muscle, while reinnervation increases it. Primary human skeletal muscle cells, the most widely used model to study human skeletal muscle in vitro, are usually cultured as myoblasts or myotubes without neurons and typically do not contract spontaneously, which might affect their ability to express and regulate NKA. We determined how differentiation, de novo innervation, and electrical pulse stimulation affect expression of NKA (α and β) subunits and NKA regulators FXYD1 (phospholemman) and FXYD5 (dysadherin). Differentiation of myoblasts into myotubes under low serum conditions increased expression of myogenic markers CD56 (NCAM1), desmin, myosin heavy chains, dihydropyridine receptor subunit α_1S, and SERCA2 as well as NKAα2 and FXYD1, while it decreased expression of FXYD5 mRNA. Myotubes, which were innervated de novo by motor neurons in co-culture with the embryonic rat spinal cord explants, started to contract spontaneously within 7–10 days. A short-term co-culture (10–11 days) promoted mRNA expression of myokines, such as IL-6, IL-7, IL-8, and IL-15, but did not affect mRNA expression of NKA, FXYDs, or myokines, such as musclin, cathepsin B, meteorin-like protein, or SPARC. A long-term co-culture (21 days) increased the protein abundance of NKAα1, NKAα2, FXYD1, and phospho-FXYD1^Ser68 without attendant changes in mRNA levels. Suppression of neuromuscular transmission with α-bungarotoxin or tubocurarine for 24 h did not alter NKA or FXYD mRNA expression. Electrical pulse stimulation (48 h) of non-innervated myotubes promoted mRNA expression of NKAβ2, NKAβ3, FXYD1, and FXYD5. In conclusion, low serum concentration promotes NKAα2 and FXYD1 expression, while de novo innervation is not essential for upregulation of NKAα2 and FXYD1 mRNA in cultured myotubes. Finally, although innervation and EPS both stimulate contractions of myotubes, they exert distinct effects on the expression of NKA and FXYDs.

The role of AMPK in regulation of Na+,K+-ATPase in skeletal muscle: does the gauge always plug the sink?

AMP-activated protein kinase (AMPK) is a cellular energy gauge and a major regulator of cellular energy homeostasis. Once activated, AMPK stimulates nutrient uptake and the ATP-producing catabolic pathways, while it suppresses the ATP-consuming anabolic pathways, thus helping to maintain the cellular energy balance under energy-deprived conditions. As much as ~ 20-25% of the whole-body ATP consumption occurs due to a reaction catalysed by Na⁺,K⁺-ATPase (NKA). Being the single most important sink of energy, NKA might seem to be an essential target of the AMPK-mediated energy saving measures, yet NKA is vital for maintenance of transmembrane Na⁺ and K⁺ gradients, water homeostasis, cellular excitability, and the Na⁺-coupled transport of nutrients and ions. Consistent with the model that AMPK regulates ATP consumption by NKA, activation of AMPK in the lung alveolar cells stimulates endocytosis of NKA, thus suppressing the transepithelial ion transport and the absorption of the alveolar fluid. In skeletal muscles, contractions activate NKA, which opposes a rundown of transmembrane ion gradients, as well as AMPK, which plays an important role in adaptations to exercise. Inhibition of NKA in contracting skeletal muscle accentuates perturbations in ion concentrations and accelerates development of fatigue. However, different models suggest that AMPK does not inhibit or even stimulates NKA in skeletal muscle, which appears to contradict the idea that AMPK maintains the cellular energy balance by always suppressing ATP-consuming processes. In this short review, we examine the role of AMPK in regulation of NKA in skeletal muscle and discuss the apparent paradox of AMPK-stimulated ATP consumption.

Low Ouabain Doses and AMP-Activated Protein Kinase as Factors Supporting Electrogenesis in Skeletal Muscle

Many motor disorders are associated with depolarization of the membrane of skeletal muscle fibers due to the impaired functioning of Na,K-ATPase. Here, we studied the role of ouabain (specific Na,K-ATPase ligand) and AMP-activated protein kinase (key regulator of muscle metabolism) in the maintenance of muscle electrogenesis; the levels of these endogenous factors are directly related to the motor activity. After 4-day intraperitoneal administration of ouabain (1 µg/kg daily), a hyperpolarization of sarcolemma was registered in isolated rat diaphragm muscles due to an increase in the electrogenic activity of Na,K-ATPase. In acute experiments, addition of nanomolar ouabain concentrations to the bathing solution resulted in the muscle membrane hyperpolarization within 15 min. The effect of ouabain reversed to membrane depolarization with the increase in the external potassium concentration. It is possible that Na,K-ATPase activation by ouabain may be regulated by such factors as specific subcellular location, interaction with molecular partners, and changes in the ionic balance. Preventive administration of the AMP-activated protein kinase activator AICAR (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside; 400 mg/kg body weight daily for 7 days) in chronic experiments resulted in the stabilization of the endplate structure and abolishment of depolarization of the rat soleus muscle membrane caused by the motor activity cessation. The obtained data can be useful for creating approaches for correction of muscle dysfunction, especially at the early stages, prior to the development of muscle atrophy.

Stimulation of Na+/K+-ATPase with an Antibody against Its 4th Extracellular Region Attenuates Angiotensin II-Induced H9c2 Cardiomyocyte Hypertrophy via an AMPK/SIRT3/PPARγ Signaling Pathway

Activation of the renin-angiotensin system (RAS) contributes to the pathogenesis of cardiovascular diseases. Sodium potassium ATPase (NKA) expression and activity are often regulated by angiotensin II (Ang II). This study is aimed at investigating whether DR-Ab, an antibody against 4^th extracellular region of NKA, can protect Ang II-induced cardiomyocyte hypertrophy. Our results showed that Ang II treatment significantly reduced NKA activity and membrane expression. Pretreatment with DR-Ab preserved cell size in Ang II-induced cardiomyopathy by stabilizing the plasma membrane expression of NKA and restoring its activity. DR-Ab reduced intracellular ROS generation through inhibition of NADPH oxidase activity and protection of mitochondrial functions in Ang II-treated H9c2 cardiomyocytes. Pharmacological manipulation and Western blotting analysis demonstrated the cardioprotective effects were mediated by the activation of the AMPK/Sirt-3/PPARγ signaling pathway. Taken together, our results suggest that dysfunction of NKA is an important mechanism for Ang II-induced cardiomyopathy and DR-Ab may be a novel and promising therapeutic approach to treat cardiomyocyte hypertrophy.

Cycling with blood flow restriction improves performance and muscle K+ regulation and alters the effect of anti-oxidant infusion in humans

KEY POINTS

Training with blood flow restriction (BFR) is a well-recognized strategy for promoting muscle hypertrophy and strength. However, its potential to enhance muscle function during sustained, intense exercise remains largely unexplored. In the present study, we report that interval training with BFR augments improvements in performance and reduces net K⁺ release from contracting muscles during high-intensity exercise in active men. A better K⁺ regulation after BFR-training is associated with an elevated blood flow to exercising muscles and altered muscle anti-oxidant function, as indicated by a higher reduced to oxidized glutathione (GSH:GSSG) ratio, compared to control, as well as an increased thigh net K⁺ release during intense exercise with concomitant anti-oxidant infusion. Training with BFR also invoked fibre type-specific adaptations in the abundance of Na⁺ ,K⁺ -ATPase isoforms (α₁ , β₁ , phospholemman/FXYD1). Thus, BFR-training enhances performance and K⁺ regulation during intense exercise, which may be a result of adaptations in anti-oxidant function, blood flow and Na⁺ ,K⁺ -ATPase-isoform abundance at the fibre-type level.

ABSTRACT

We examined whether blood flow restriction (BFR) augments training-induced improvements in K⁺ regulation and performance during intense exercise in men, and also whether these adaptations are associated with an altered muscle anti-oxidant function, blood flow and/or with fibre type-dependent changes in Na⁺ ,K⁺ -ATPase-isoform abundance. Ten recreationally-active men (25 ± 4 years, 49.7 ± 5.3 mL kg^-1  min^-1 ) performed 6 weeks of interval cycling, where one leg trained without BFR (control; CON-leg) and the other trained with BFR (BFR-leg, pressure: ∼180 mmHg). Before and after training, femoral arterial and venous K⁺ concentrations and artery blood flow were measured during single-leg knee-extensor exercise at 25% (Ex1) and 90% of thigh incremental peak power (Ex2) with i.v. infusion of N-acetylcysteine (NAC) or placebo (saline) and a resting muscle biopsy was collected. After training, performance increased more in BFR-leg (23%) than in CON-leg (12%, P < 0.05), whereas K⁺ release during Ex2 was attenuated only from BFR-leg (P < 0.05). The muscle GSH:GSSG ratio at rest and blood flow during exercise was higher in BFR-leg than in CON-leg after training (P < 0.05). After training, NAC increased resting muscle GSH concentration and thigh net K⁺ release during Ex2 only in BFR-leg (P < 0.05), whereas the abundance of Na⁺ ,K⁺ -ATPase-isoform α₁ in type II (51%), β₁ in type I (33%), and FXYD1 in type I (108%) and type II (60%) fibres was higher in BFR-leg than in CON-leg (P < 0.05). Thus, training with BFR elicited greater improvements in performance and reduced thigh K⁺ release during intense exercise, which were associated with adaptations in muscle anti-oxidant function, blood flow and Na⁺ ,K⁺ -ATPase-isoform abundance at the fibre-type level.

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.

AMP-Activated Protein Kinase as a Key Trigger for the Disuse-Induced Skeletal Muscle Remodeling

Molecular mechanisms that trigger disuse-induced postural muscle atrophy as well as myosin phenotype transformations are poorly studied. This review will summarize the impact of 5′ adenosine monophosphate -activated protein kinase (AMPK) activity on mammalian target of rapamycin complex 1 (mTORC1)-signaling, nuclear-cytoplasmic traffic of class IIa histone deacetylases (HDAC), and myosin heavy chain gene expression in mammalian postural muscles (mainly, soleus muscle) under disuse conditions, i.e., withdrawal of weight-bearing from ankle extensors. Based on the current literature and the authors’ own experimental data, the present review points out that AMPK plays a key role in the regulation of signaling pathways that determine metabolic, structural, and functional alternations in skeletal muscle fibers under disuse.

Isoform-specific role of Na/K-ATPase α1 in skeletal muscle

The distribution of Na/K-ATPase α-isoforms in skeletal muscle is unique, with α1 as the minor (15%) isoform and α2 comprising the bulk of the Na/K-ATPase pool. The acute and isoform-specific role of α2 in muscle performance and resistance to fatigue is well known, but the isoform-specific role of α1 has not been as thoroughly investigated. In vitro, we reported that α1 has a role in promoting cell growth that is not supported by α2. To assess whether α1 serves this isoform-specific trophic role in the skeletal muscle, we used Na/K-ATPase α1-haploinsufficient (α1^+/-) mice. A 30% decrease of Na/K-ATPase α1 protein expression without change in α2 induced a modest yet significant decrease of 10% weight in the oxidative soleus muscle. In contrast, the mixed plantaris and glycolytic extensor digitorum longus weights were not significantly affected, likely because of their very low expression level of α1 compared with the soleus. The soleus mass reduction occurred without change in total Na/K-ATPase activity or glycogen metabolism. Serum analytes including K⁺, fat tissue mass, and exercise capacity were not altered in α1^+/- mice. The impact of α1 content on soleus muscle mass is consistent with a Na/K-ATPase α1-specific role in skeletal muscle growth that cannot be fulfilled by α2. The preserved running capacity in α1^+/- is in sharp contrast with previously reported consequences of genetic manipulation of α2. Taken together, these results lend further support to the concept of distinct isoform-specific functions of Na/K-ATPase α1 and α2 in skeletal muscle.

Skeletal muscle signaling, metabolism, and performance during sprint exercise in severe acute hypoxia after the ingestion of antioxidants

The aim of this study was to determine if reactive oxygen species (ROS) could play a role in blunting Thr¹⁷²-AMP-activated protein kinase (AMPK)-α phosphorylation in human skeletal muscle after sprint exercise in hypoxia and to elucidate the potential signaling mechanisms responsible for this response. Nine volunteers performed a single 30-s sprint (Wingate test) in two occasions while breathing hypoxic gas ([Formula: see text] = 75 mmHg): one after the ingestion of placebo and another following the intake of antioxidants (α-lipoic acid, vitamin C, and vitamin E), with a randomized double-blind design. Vastus lateralis muscle biopsies were obtained before, immediately after, and 30- and 120-min postsprint. Compared with the control condition, the ingestion of antioxidants resulted in lower plasma carbonylated proteins, attenuated elevation of the AMP-to-ATP molar ratio, and reduced glycolytic rate (P < 0.05) without significant effects on performance or V̇o₂ The ingestion of antioxidants did not alter the basal muscle signaling. Thr¹⁷²-AMPKα and Thr^184/187-transforming growth factor-β-activated kinase 1 (TAK1) phosphorylation were not increased after the sprint regardless of the ingestion of antioxidants. Thr²⁸⁶-CaMKII phosphorylation was increased after the sprint, but this response was blunted by the antioxidants. Ser⁴⁸⁵-AMPKα1/Ser⁴⁹¹-AMPKα2 phosphorylation increased immediately after the sprints coincident with increased Akt phosphorylation. In summary, antioxidants attenuate the glycolytic response to sprint exercise in severe acute hypoxia and modify the muscle signaling response to exercise. Ser⁴⁸⁵-AMPKα1/Ser⁴⁹¹-AMPKα2 phosphorylation, a known mechanism of Thr¹⁷²-AMPKα phosphorylation inhibition, is increased immediately after sprint exercise in hypoxia, probably by a mechanism independent of ROS.NEW & NOTEWORTHY The glycolytic rate is increased during sprint exercise in severe acute hypoxia. This study showed that the ingestion of antioxidants before sprint exercise in severe acute hypoxia reduced the glycolytic rate and attenuated the increases of the AMP-to-ATP and the reduction of the NAD⁺-to-NADH.H⁺ ratios. This resulted in a modified muscle signaling response with a blunted Thr²⁸⁶-CaMKII but similar AMP-activated protein kinase phosphorylation responses in the sprints preceded by the ingestion of antioxidants.

Effect of increased and maintained frequency of speed endurance training on performance and muscle adaptations in runners

The aim of the study was, in runners accustomed to speed endurance training (SET), to examine the effect of increased and maintained frequency of SET on performance and muscular adaptations. After familiarization (FAM) to SET, 18 male ( n = 14) and female ( n = 4) runners (V̇o2max: 57.3 ± 3.4 ml/min; means ± SD) completed 20 sessions of maintained low-frequency (LF; every fourth day; n = 7) or high-frequency (HF; every second day; n = 11) SET. Before FAM as well as before and after an intervention period (INT), subjects completed a series of running tests and a biopsy from m. vastus lateralis was collected. Ten-kilometer performance improved ( P < 0.05) ~3.5% during FAM with no further change during INT. Time to exhaustion at 90% vV̇o2max was 15 and 22% longer ( P < 0.05) during FAM and a further 12 and 16% longer ( P < 0.05) during INT in HF and LF, respectively. During FAM, muscle expression of NHE1 and maximal activity of citrate synthase (CS) and phosphofructokinase (PFK) increased ( P < 0.05), running economy (RE) improved ( P < 0.05), and V̇o2max was unchanged. During INT, both HF and LF increased ( P < 0.05) muscle expression of NKAβ1, whereas maximal activity of CS and PFK, RE, and V̇o2max were unchanged. Furthermore, during INT, muscle expression of FXYD1 and SERCA1, and FXYD1 activity increased ( P < 0.05) in HF, while muscle expression of SERCA2 decreased ( P < 0.05) in LF. Thus increased or maintained frequency of SET leads to further improvements in short-term exercise capacity, but not in 10-km running performance. The better short-term exercise capacity may be associated with elevated expression of muscle proteins related to Na+/K+ transportation and Ca2+ reuptake.

NEW & NOTEWORTHY Ten speed endurance training (SET) sessions improved short-term exercise capacity and 10-km performance, which was followed by further improved short-term exercise capacity, but unchanged 10-km performance after 20 SET sessions performed with either high frequency (4 per 8 days) or continued low frequency (2 per 8 days) in trained runners. The further gain in short-term exercise capacity was associated with changes in muscle expression of proteins of importance for the development of fatigue.

Molecular characterization and expression analysis of AMPK α subunit isoform genes from Scophthalmus maximus responding to salinity stress

AMP-activated protein kinase (AMPK) is a highly conserved and multi-functional protein kinase that plays important roles in both intracellular energy balance and cellular stress response. In the present study, molecular characterization, tissue distribution and gene expression levels of the AMPK α1 and α2 genes from turbot (Scophthalmus maximus) under salinity stress are described. The complete coding regions of the AMPK α1 and α2 genes were isolated from turbot through degenerate primers in combination with RACE using muscle cDNA. The complete coding regions of AMPK α1 (1722 bp) and α2 (1674 bp) encoded 573 and 557 amino acids peptides, respectively. Multiple alignments, structural analysis and phylogenetic tree construction indicated that S. maximus AMPK α1 and α2 shared a high amino acid identity with other species, especially fish. AMPK α1 and α2 genes could be detected in all tested tissues, indicating that they are constitutively expressed. Salinity challenges significantly altered the gene expression levels of AMPK α1 and α2 mRNA in a salinity- and time-dependent manners in S. maximus gill tissues, suggesting that AMPK α1 and α2 played important roles in mediating the salinity stress in S. maximus. The expression levels of AMPK α1 and α2 mRNA were a positive correlation with gill Na⁺, K⁺-ATPase activities. These findings will aid our understanding of the molecular mechanism of juvenile turbot in response to environmental salinity changes.

Effects of Age on Na(+),K(+)-ATPase Expression in Human and Rodent Skeletal Muscle

The maintenance of transmembrane Na(+) and K(+) concentration gradients and membrane potential is vital for the production of force in skeletal muscle. In aging an inability to maintain ion regulation and membrane potential would have adverse consequences on the capacity for performing repeated muscle contractions, which are critical for everyday activities and functional independence. This short review focusses on the effects of aging on one major and vital component affecting muscle Na(+) and K(+) concentrations, membrane potential and excitability in skeletal muscle, the Na(+),K(+)-ATPase (Na(+),K(+)-pump, NKA) protein. The review examines the effects of age on NKA in both human and rodent models and highlights a distant lack of research in NKA with aging. In rodents, the muscle NKA measured by [(3)H]ouabain binding site content, declines with advanced age from peak values in early life. In human skeletal muscle, however, there appears to be no age effect on [(3)H]ouabain binding site content in physically active older adults between 55 and 76 years compared to those aged between 18 and 30 years of age. Analysis of the NKA isoforms reveal differential changes with age in fiber-types in both rat and humans. The data show considerable disparities, suggesting different regulation of NKA isoforms between rodents and humans. Finally we review the importance of physical activity on NKA content in older humans. Findings suggest that physical activity levels of an individual may have a greater effect on regulating the NKA content in skeletal muscle rather than aging per se, at least up until 80 years of age.

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.

Intensive training and reduced volume increases muscle FXYD1 expression and phosphorylation at rest and during exercise in athletes

The present study examined the effect of intensive training in combination with marked reduction in training volume on phospholemman (FXYD1) expression and phosphorylation at rest and during exercise. Eight well-trained cyclists replaced their regular training with speed-endurance training (10-12 × ∼30-s sprints) two or three times per week and aerobic high-intensity training (4-5 × 3-4 min at 90-95% of peak aerobic power output) 1-2 times per week for 7 wk and reduced the training volume by 70%. Muscle biopsies were obtained before and during a repeated high-intensity exercise protocol, and protein expression and phosphorylation were determined by Western blot analysis. Expression of FXYD1 (30%), actin (40%), mammalian target of rapamycin (mTOR) (12%), phospholamban (PLN) (16%), and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) γ/δ (25%) was higher (P < 0.05) than before the training intervention. In addition, after the intervention, nonspecific FXYD1 phosphorylation was higher (P < 0.05) at rest and during exercise, mainly achieved by an increased FXYD1 Ser-68 phosphorylation, compared with before the intervention. CaMKII, Thr-287, and eukaryotic elongation factor 2 Thr-56 phosphorylation at rest and during exercise, overall PKCα/β, Thr-638/641, and mTOR Ser-2448 phosphorylation during repeated intense exercise as well as resting PLN Thr-17 phosphorylation were also higher (P < 0.05) compared with before the intervention period. Thus, a period of high-intensity training with reduced training volume increases expression and phosphorylation levels of FXYD1, which may affect Na(+)/K(+) pump activity and muscle K(+) homeostasis during intense exercise. Furthermore, higher expression of CaMKII and PLN, as well as increased phosphorylation of CaMKII Thr-287 may have improved intracellular Ca(2+) handling.

Single-fiber expression and fiber-specific adaptability to short-term intense exercise training of Na+-K+-ATPase α- and β-isoforms in human skeletal muscle

The Na(+)-K(+)-ATPase (NKA) plays a key role in muscle excitability, but little is known in human skeletal muscle about fiber-type-specific differences in NKA isoform expression or adaptability. A vastus lateralis muscle biopsy was taken in 17 healthy young adults to contrast NKA isoform protein relative abundance between type I and IIa fibers. We further investigated muscle fiber-type-specific NKA adaptability in eight of these adults following 4-wk repeated-sprint exercise (RSE) training, comprising three sets of 5 × 4-s sprints, 3 days/wk. Single fibers were separated, and myosin heavy chain (I and IIa) and NKA (α1-3 and β1-3) isoform abundance were determined via Western blotting. All six NKA isoforms were expressed in both type I and IIa fibers. No differences between fiber types were found for α1-, α2-, α3-, β1-, or β3-isoform abundances. The NKA β2-isoform was 27% more abundant in type IIa than type I fibers (P < 0.05), with no other fiber-type-specific trends evident. RSE training increased β1 in type IIa fibers (pretraining 0.70 ± 0.25, posttraining 0.84 ± 0.24 arbitrary units, 42%, P < 0.05). No training effects were found for other NKA isoforms. Thus human skeletal muscle expresses all six NKA isoforms and not in a fiber-type-specific manner; this points to their different functional roles in skeletal muscle cells. Detection of elevated NKA β1 after RSE training demonstrates the sensitivity of the single-fiber Western blotting technique for fiber-type-specific intervention effects.

Novel mutations in human and mouse SCN4A implicate AMPK in myotonia and periodic paralysis

Mutations in the skeletal muscle channel (SCN4A), encoding the Nav1.4 voltage-gated sodium channel, are causative of a variety of muscle channelopathies, including non-dystrophic myotonias and periodic paralysis. The effects of many of these mutations on channel function have been characterized both in vitro and in vivo. However, little is known about the consequences of SCN4A mutations downstream from their impact on the electrophysiology of the Nav1.4 channel. Here we report the discovery of a novel SCN4A mutation (c.1762A>G; p.I588V) in a patient with myotonia and periodic paralysis, located within the S1 segment of the second domain of the Nav1.4 channel. Using N-ethyl-N-nitrosourea mutagenesis, we generated and characterized a mouse model (named draggen), carrying the equivalent point mutation (c.1744A>G; p.I582V) to that found in the patient with periodic paralysis and myotonia. Draggen mice have myotonia and suffer from intermittent hind-limb immobility attacks. In-depth characterization of draggen mice uncovered novel systemic metabolic abnormalities in Scn4a mouse models and provided novel insights into disease mechanisms. We discovered metabolic alterations leading to lean mice, as well as abnormal AMP-activated protein kinase activation, which were associated with the immobility attacks and may provide a novel potential therapeutic target.

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.

Fibre type-specific change in FXYD1 phosphorylation during acute intense exercise in humans

The aim of the present study was to examine fibre type-specific Na(+)-K(+) pump subunit expression and exercise-induced alterations in phospholemman (FXYD1) phosphorylation in humans. Segments of human skeletal muscle fibres were dissected and fibre typed, and protein expression was determined by Western blotting. The protein expression of the Na(+)-K(+) pump α2 isoform was lower in type I than in type II fibres (0.63 ± 0.04 a.u. vs. 1.00 ± 0.07 a.u., P < 0.001), while protein expression of the Na(+)-K(+) pump α1 and β1 isoforms was not different. Protein expression of the ATP-dependent potassium channel Kir6.2 was higher in type I compared with type II fibres. In both type I (P < 0.01) and type II fibres (P < 0.001) the AB_FXYD1 signal was lower after exercise compared with rest, indicating an increase in unspecific FXYD1 phosphorylation. The FXYD1 serine 68 phosphorylation was higher (P < 0.001) after exercise compared with rest in type II fibres (1.90 ± 0.25 vs. 1.00 ± 0.08) and not changed in type I fibres. Total FXYD1 was not expressed in a fibre type-specific manner. Expression of phosphofructokinase was lower (P < 0.001) in type I than in type II fibres, whereas citrate synthase and 3-hydroxyacyl-CoA dehydrogenase were more abundant (P < 0.001) in type I fibres. In conclusion, FXYD1 phosphorylation at serine 68 increased after an acute bout of intense exercise in human type II fibres, while AB_FXYD1 signal intensity was lower in both type I and type II fibres, indicating fibre type-specific differences in FXYD1 phosphorylation on serine 63, serine 68 and threonine 69. This, together with the observation of a higher abundance of the Na(+)-K(+) pump α2 isoform protein in type II fibres, is likely to have importance for the exercise-induced human Na(+)-K(+) pump activity in the different fibre types.

Critical role for free radicals on sprint exercise-induced CaMKII and AMPKα phosphorylation in human skeletal muscle

The extremely high energy demand elicited by sprint exercise is satisfied by an increase in O2 consumption combined with a high glycolytic rate, leading to a marked lactate accumulation, increased AMP-to-ATP ratio, and reduced NAD(+)/NADH.H(+) and muscle pH, which are accompanied by marked Thr(172) AMP-activated protein kinase (AMPK)-α phosphorylation during the recovery period by a mechanism not fully understood. To determine the role played by reactive nitrogen and oxygen species (RNOS) on Thr(172)-AMPKα phosphorylation in response to cycling sprint exercise, nine voluntary participants performed a single 30-s sprint (Wingate test) on two occasions: one 2 h after the ingestion of placebo and another after the intake of antioxidants (α-lipoic acid, vitamin C, and vitamin E) in a double-blind design. Vastus lateralis muscle biopsies were obtained before, immediately postsprint, and 30 and 120 min postsprint. Performance and muscle metabolism were similar during both sprints. The NAD(+)-to-NADH.H(+) ratio was similarly reduced (84%) and the AMP-to-ATP ratio was similarly increased (×21-fold) immediately after the sprints. Thr(286) Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and Thr(172)-AMPKα phosphorylations were increased after the control sprint (with placebo) but not when the sprints were preceded by the ingestion of antioxidants. Ser(485)-AMPKα1/Ser(491)-AMPKα2 phosphorylation, a known inhibitory mechanism of Thr(172)-AMPKα phosphorylation, was increased only with antioxidant ingestion. In conclusion, RNOS play a crucial role in AMPK-mediated signaling after sprint exercise in human skeletal muscle. Antioxidant ingestion 2 h before sprint exercise abrogates the Thr(172)-AMPKα phosphorylation response observed after the ingestion of placebo by reducing CaMKII and increasing Ser(485)-AMPKα1/Ser(491)-AMPKα2 phosphorylation. Sprint performance, muscle metabolism, and AMP-to-ATP and NAD(+)-to-NADH.H(+) ratios are not affected by the acute ingestion of antioxidants.

Downregulation of the renal outer medullary K(+) channel ROMK by the AMP-activated protein kinase

The 5'-adenosine monophosphate-activated serine/threonine protein kinase (AMPK) is stimulated by energy depletion, increase in cytosolic Ca(2+) activity, oxidative stress, and nitric oxide. AMPK participates in the regulation of the epithelial Na(+) channel ENaC and the voltage-gated K(+) channel KCNE1/KCNQ1. It is partially effective by decreasing PIP(2) formation through the PI3K pathway. The present study explored whether AMPK regulates the renal outer medullary K(+) channel ROMK. To this end, cRNA encoding ROMK was injected into Xenopus oocytes with and without additional injection of constitutively active AMPK(γR70Q) (AMPK(α1)-HA+AMPK(β1)-Flag+AMPKγ1(R70Q)), or of inactive AMPK(αK45R) (AMPK(α1K45R)+AMPK(β1)-Flag+AMPK(γ1)-HA), and the current determined utilizing two-electrode voltage-clamp and single channel patch clamp. ROMK protein abundance was measured utilizing chemiluminescence in Xenopus oocytes and western blot in whole kidney tissue. Moreover, renal Na(+) and K(+) excretion were determined in AMPK(α1)-deficient mice (ampk ( -/- )) and wild-type mice (ampk ( +/+ )) prior to and following an acute K(+) load (111 mM KCl, 30 mM NaHCO(3), 4.7 mM NaCl, and 2.25 g/dl BSA) at a rate of 500 μl/h. As a result, coexpression of AMPK(γR70Q) but not of AMPK(αK45R) significantly decreased the current in ROMK1-expressing Xenopus oocytes. Injection of phosphatidylinositol PI((4,5))P(2) significantly increased the current in ROMK1-expressing Xenopus oocytes, an effect reversed in the presence of AMPK(γR70Q). Under control conditions, no significant differences between ampk ( -/- ) and ampk ( +/+ ) mice were observed in glomerular filtration rate (GFR), urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentrations as well as absolute and fractional Na(+) and K(+) excretion. Following an acute K(+) load, GFR, urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentration were again similar in both genotypes, but renal absolute and fractional Na(+) and K(+) excretion were higher in ampk ( -/- ) than in ampk ( +/+ ) mice. According to micropuncture following a K(+) load, delivery of Na(+) to the early distal tubule but not delivery of K(+) to late proximal and early distal tubules was increased in ampk (-/-) mice. The upregulation of renal ROMK1 protein expression by acute K(+) load was more pronounced in ampk (-/-) than in ampk ( +/+ ) mice. In conclusion, AMPK downregulates ROMK, an effect compromising the ability of the kidney to excrete K(+) following an acute K(+) load.

Purinergic activation of rat skeletal muscle membranes increases Vmax and Na+ affinity of the Na,K-ATPase and phosphorylates phospholemman and α1 subunits

Muscle activity is associated with an increase in extracellular purines (ATP, ADP), which are involved in signalling mechanisms. The present study investigates the effect of purines on the function of Na,K-ATPase (Na,K-pump) in rat skeletal muscle. Na,K-ATPase activity was quantified by measuring the release of inorganic phosphate in the presence of ATP and variable Na(+) concentrations. In membranes purified from glycolytic muscle fibres, purinergic stimulation increases V (max) and decreases the K (m) (higher Na(+) affinity) of the Na,K-ATPase. Stimulatory effects were obtained using ATP, ADP, 2-methylthio-ADP and UPT, but not UDP and adenosine. The effect of ADP on V (max) can be inhibited by the non-specific P2Y receptor antagonists, suramin and PPADS. Moreover, the P2Y(13) receptor antagonist MRS 2211 strongly inhibited the response to ADP, whereas the specific P2Y(1) receptor antagonist MRS 2500 had less effect. Based on results from these agonists and antagonists, we conclude that P2Y(13) receptors mediate the main effects observed, that P2Y1 receptors are also involved and that some P2Y(2)/P2Y(4) receptors also appear to be involved. Receptor antagonists had no effect on ADP-induced subunit (phospholemman and α1) phosphorylation and changes in K (m) (Na(+) affinity). Thus, the stimulatory effects of purines are mediated by two independent mechanisms: P2Y receptor-mediated increase in Na,K-ATPase capacity (increased V (max)) and P2Y receptor-independent phosphorylation of Na,K-ATPase phospholemman and α1 subunits, which induce changes in ion affinity. These mechanisms may contribute to up-regulation of Na,K-ATPase during muscle activity.