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

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.

Need for speed: Human fast-twitch mitochondria favor power over efficiency

OBJECTIVE

Human skeletal muscle consists of a mixture of slow- and fast-twitch fibers with distinct capacities for contraction mechanics, fermentation, and oxidative phosphorylation. While the divergence in mitochondrial volume favoring slow-twitch fibers is well established, data on the fiber type-specific intrinsic mitochondrial function and morphology are highly limited with existing data mainly being generated in animal models. This highlights the need for more human data on the topic.

METHODS

Here, we utilized THRIFTY, a rapid fiber type identification protocol to detect, sort, and pool fast- and slow-twitch fibers within 6 h of muscle biopsy sampling. Respiration of permeabilized fast- and slow-twitch fiber pools was then analyzed with high-resolution respirometry. Using standardized western blot procedures, muscle fiber pools were subsequently analyzed for control proteins and key proteins related to respiratory capacity.

RESULTS

Maximal complex I+II respiration was 25% higher in human slow-twitch fibers compared to fast-twitch fibers. However, per mitochondrial volume, the respiratory rate of mitochondria in fast-twitch fibers was approximately 50% higher for complex I+II, which was primarily mediated through elevated complex II respiration. Furthermore, the abundance of complex II protein and proteins regulating cristae structure were disproportionally elevated in mitochondria of the fast-twitch fibers. The difference in intrinsic respiratory rate was not reflected in fatty acid-or complex I respiration.

CONCLUSION

Mitochondria of human fast-twitch muscle fibers compensate for their lack of volume by substantially elevating intrinsic respiratory rate through increased reliance on complex II.

High-Intensity Training Represses FXYD5 and Glycosylates Na,K-ATPase in Type II Muscle Fibres, Which Are Linked with Improved Muscle K+ Handling and Performance

Na+/K+ ATPase (NKA) comprises several subunits to provide isozyme heterogeneity in a tissue-specific manner. An abundance of NKA α, β, and FXYD1 subunits is well-described in human skeletal muscle, but not much is known about FXYD5 (dysadherin), a regulator of NKA and β1 subunit glycosylation, especially with regard to fibre-type specificity and influence of sex and exercise training. Here, we investigated muscle fibre-type specific adaptations in FXYD5 and glycosylated NKAβ1 to high-intensity interval training (HIIT), as well as sex differences in FXYD5 abundance. In nine young males (23.8 ± 2.5 years of age) (mean ± SD), 3 weekly sessions of HIIT for 6 weeks enhanced muscle endurance (220 ± 102 vs. 119 ± 99 s, p < 0.01) and lowered leg K+ release during intense knee-extensor exercise (0.5 ± 0.8 vs. 1.0 ± 0.8 mmol·min–1, p < 0.01) while also increasing cumulated leg K+ reuptake 0–3 min into recovery (2.1 ± 1.5 vs. 0.3 ± 0.9 mmol, p < 0.01). In type IIa muscle fibres, HIIT lowered FXYD5 abundance (p < 0.01) and increased the relative distribution of glycosylated NKAβ1 (p < 0.05). FXYD5 abundance in type IIa muscle fibres correlated inversely with the maximal oxygen consumption (r = –0.53, p < 0.05). NKAα2 and β1 subunit abundances did not change with HIIT. In muscle fibres from 30 trained males and females, we observed no sex (p = 0.87) or fibre type differences (p = 0.44) in FXYD5 abundance. Thus, HIIT downregulates FXYD5 and increases the distribution of glycosylated NKAβ1 in type IIa muscle fibres, which is likely independent of a change in the number of NKA complexes. These adaptations may contribute to counter exercise-related K+ shifts and enhance muscle performance during intense exercise.

THRIFTY: a novel high-throughput method for rapid fibre type identification of isolated skeletal muscle fibres

Fibre type-specific analyses are required for broader understanding of muscle physiology, but such analyses are difficult to conduct due to the extreme time requirements of dissecting and fibre typing individual fibres. Investigations are often confined to a small number of fibres from few participants with low representativeness of the entire fibre population and the participant population. To increase the feasibility of conducting large-scale fibre type-specific studies, a valid and rapid method for high-throughput fibre typing of individually dissected fibres was developed and named THRIFTY (for high-THRoughput Immunofluorescence Fibre TYping). Employing THRIFTY, 400 fibre segments were fixed onto microscope slides with a pre-printed coordinated grid system, probed with antibodies against myosin heavy chain (MyHC)-I and MyHC-II and classified using a fluorescence microscope. The validity and speed of THRIFTY was compared to a previously validated protocol (dot blot) on a fibre-to-fibre basis. Fibre pool purity was evaluated using 'gold standard' SDS-PAGE and silver staining. A modified THRIFTY-protocol using fluorescence western blot equipment was also validated. THRIFTY displayed excellent agreement with the dot blot protocol, κ = 0.955 (95% CI: 0.928, 0.982), P < 0.001. Both the original and modified THRIFTY protocols generated type I and type II fibre pools of absolute purity. Using THRIFTY, 400 fibres were typed just under 11 h, which was approximately 3 times faster than dot blot. THRIFTY is a novel and valid method with high versatility for very rapid fibre typing of individual fibres. THRIFTY can therefore facilitate the generation of large fibre pools for more extensive mechanistic studies into skeletal muscle physiology. KEY POINTS: Skeletal muscle is composed of different fibre types, each with distinct physiological properties. To fully understand how skeletal muscle adapts to external cues such as exercise, nutrition and ageing, fibre type-specific investigations are required. Such investigations are very difficult to conduct due to the extreme time requirements related to classifying individually isolated muscle fibres. To bypass this issue, we have developed a rapid and reliable method named THRIFTY which is cheap as well as versatile and which can easily be implemented in most laboratories. THRIFTY increases the feasibility of conducting larger fibre type-specific studies and enables time-sensitive assays where measurements need to be carried out in close connection with tissue sampling. By using THRIFTY, new insights into fibre type-specific muscle physiology can be gained which may have broad implications in health and disease.

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

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

Moving human muscle physiology research forward: an evaluation of fiber type-specific protein research methodologies

Human skeletal muscle is a heterogeneous tissue composed of multiple fiber types that express unique contractile and metabolic properties. While analysis of mixed fiber samples predominates and holds value, increasing attention has been directed toward studying proteins segregated by fiber type, a methodological distinction termed “fiber type-specific.” Fiber type-specific protein studies have the advantage of uncovering key molecular effects that are often missed in mixed fiber homogenate studies but also require greater time and resource-intensive methods, particularly when applied to human muscle. This review summarizes and compares current methods used for fiber type-specific protein analysis, highlighting their advantages and disadvantages for human muscle studies, in addition to recent advances in these techniques. These methods can be grouped into three categories based on the initial processing of the tissue: 1) muscle-specific fiber homogenates, 2) cross sections of fiber bundles, and 3) isolated single fibers, with various subtechniques for performing fiber type identification and protein quantification. The relative implementation for each unique methodological approach is analyzed from 83 fiber type-specific studies of proteins in live human muscle found in the literature to date. These studies have investigated several proteins involved in a wide range of cellular functions that are important to muscle tissue. The second half of this review summarizes key findings from this ensemble of fiber type-specific human protein studies. We highlight examples of where this analytical approach has helped to improve understanding of important physiological topics such as insulin sensitivity, muscle hypertrophy, muscle fatigue, and adaptation to different exercise programs.

Resistance training upregulates skeletal muscle Na+, K+-ATPase content, with elevations in both α1 and α2, but not β isoforms

PURPOSE

The Na⁺, K⁺-ATPase (NKA) is important in regulating trans-membrane ion gradients, cellular excitability and muscle function. We investigated the effects of resistance training in healthy young adults on the adaptability of NKA content and of the specific α and β isoforms in human skeletal muscle.

METHODS

Twenty-one healthy young males (22.9 ± 4.6 year; 1.80 ± 0.70 m, 85.1 ± 17.8 kg, mean ± SD) underwent 7 weeks of resistance training, training three times per week (RT, n = 16) or control (CON, n = 5). The training program was effective with a 39% gain in leg press muscle strength (p = 0.001). A resting vastus lateralis muscle biopsy was taken before and following RT or CON and assayed for NKA content ([³H]ouabain binding site content) and NKA isoform (α₁, α₂, β_1, β₂) abundances.

RESULTS

After RT, each of NKA content (12%, 311 ± 76 vs 349 ± 76 pmol g wet weight^-1, p = 0.01), NKA α₁ (32%, p = 0.01) and α₂ (10%, p < 0.01) isoforms were increased, whereas β₁ (p = 0.18) and β₂ (p = 0.22) isoforms were unchanged. NKA content and isoform abundances were unchanged during CON.

CONCLUSIONS

Resistance training increased muscle NKA content through upregulation of both α₁ and α₂ isoforms, which were independent of β isoform changes. In animal models, modulations in α₁ and α₂ isoform abundances in skeletal muscle may affect fatigue resistance during exercise, muscle hypertrophy and strength. Whether similar in-vivo functional benefits of these NKA isoform adaptations occurs in human muscle with resistance training remains to be determined.

Inactivity and exercise training differentially regulate abundance of Na+-K+-ATPase in human skeletal muscle

Physical inactivity is a global health risk that can be addressed through application of exercise training suitable for an individual's health and age. People's willingness to participate in physical activity is often limited by an initially poor physical capability and early onset of fatigue. One factor associated with muscle fatigue during intense contractions is an inexcitability of skeletal muscle cells, reflecting impaired transmembrane Na⁺/K⁺ exchange and membrane depolarization, which are regulated via the transmembranous protein Na⁺-K⁺-ATPase (NKA). This short review focuses on the plasticity of NKA in skeletal muscle in humans after periods of altered usage, exploring NKA upregulation with exercise training and downregulation with physical inactivity. In human skeletal muscle, the NKA content quantified by [³H]ouabain binding site content shows robust, yet tightly constrained, upregulation of 8-22% with physical training, across a broad range of exercise training types. Muscle NKA content in humans undergoes extensive downregulation with injury that involves substantial muscular inactivity. Surprisingly, however, no reduction in NKA content was found in the single study that investigated short-term disuse. Despite clear findings that exercise training and injury modulate NKA content, the adaptability of the individual NKA isoforms in muscle (α_1-3 and β_1-3) and of the accessory and regulatory protein FXYD1 are surprisingly inconsistent across studies, for exercise training as well as for injury/disuse. Potential reasons for this are explored. Finally, we provide suggestions for future studies to provide greater understanding of NKA regulation during exercise training and inactivity in humans.

A fast, reliable and sample-sparing method to identify fibre types of single muscle fibres

Many skeletal muscle proteins are present in a cell-specific or fibre-type dependent manner. Stimuli such as exercise, aging, and disease have been reported to result in fibre-specific responses in protein abundances. Thus, fibre-type-specific determination of the content of specific proteins provides enhanced mechanistic understanding of muscle physiology and biochemistry compared with typically performed whole-muscle homogenate analyses. This analysis, however, is laborious and typically not performed. We present a novel dot blotting method for easy and rapid determination of skeletal muscle fibre type based on myosin heavy chain (MHC) isoform presence. Requiring only small amounts of starting muscle tissue (i.e., 2–10 mg wet weight), muscle fibre type is determined in one-tenth of a 1–3-mm fibre segment, with the remainder of each segment pooled with fibre segments of the same type (I or II) for subsequent protein quantification by western blotting. This method, which we validated using standard western blotting, is much simpler and cheaper than previous methods and is adaptable for laboratories routinely performing biochemical analyses. Use of dot blotting for fibre typing will facilitate investigations of fibre-specific responses to diverse stimuli, which will advance our understanding of skeletal muscle physiology and biochemistry.

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.

Physiological and biochemical characteristics of skeletal muscles in sedentary and active rats

Laboratory rats are sedentary if housed in conditions where activity is limited. Changes in muscle characteristics with chronic inactivity were investigated by comparing sedentary rats with rats undertaking voluntary wheel running for either 6 or 12 weeks. EDL (type II fibers) and soleus (SOL) muscles (predominantly type I fibers) were examined. When measured within 1-2 h post-running, calcium sensitivity of the contractile apparatus was increased, but only in type II fibers. This increase disappeared when fibers were treated with DTT, indicative of oxidative regulation of the contractile apparatus, and was absent in fibers from rats that had ceased running 24 h prior to experiments. Specific force production was ~ 10 to 25% lower in muscle fibers of sedentary compared to active rats, and excitability of skinned fibers was decreased. Muscle glycogen content was ~ 30% lower and glycogen synthase content ~ 50% higher in SOL of sedentary rats, and in EDL glycogenin was 30% lower. Na⁺, K⁺-ATPase α1 subunit density was ~ 20% lower in both EDL and SOL in sedentary rats, and GAPDH content in SOL ~ 35% higher. There were no changes in content of the calcium handling proteins calsequestrin and SERCA, but the content of CSQ-like protein was increased in active rats (by ~ 20% in EDL and 60% in SOL). These findings show that voluntary exercise elicits an acute oxidation-induced increase in Ca²⁺ sensitivity in type II fibers, and also that there are substantial changes in skeletal muscle characteristics and biochemical processes in sedentary rats.

Cold-water immersion after training sessions: effects on fiber type-specific adaptations in muscle K+ transport proteins to sprint-interval training in men

Effects of regular use of cold-water immersion (CWI) on fiber type-specific adaptations in muscle K⁺ transport proteins to intense training, along with their relationship to changes in mRNA levels after the first training session, were investigated in humans. Nineteen recreationally active men (24 ± 6 yr, 79.5 ± 10.8 kg, 44.6 ± 5.8 ml·kg^-1·min^-1) completed six weeks of sprint-interval cycling, either without (passive rest; CON) or with training sessions followed by CWI (15 min at 10°C; COLD). Muscle biopsies were obtained before and after training to determine abundance of Na⁺, K⁺-ATPase isoforms (α_1-3, β_1-3) and phospholemman (FXYD1) and after recovery treatments (+0 h and +3 h) on the first day of training to measure mRNA content. Training increased ( P < 0.05) the abundance of α₁ and β₃ in both fiber types and β₁ in type-II fibers and decreased FXYD1 in type-I fibers, whereas α₂ and α₃ abundance was not altered by training ( P > 0.05). CWI after each session did not influence responses to training ( P > 0.05). However, α₂ mRNA increased after the first session in COLD (+0 h, P < 0.05) but not in CON ( P > 0.05). In both conditions, α₁ and β₃ mRNA increased (+3 h; P < 0.05) and β₂ mRNA decreased (+3 h; P < 0.05), whereas α₃, β₁, and FXYD1 mRNA remained unchanged ( P > 0.05) after the first session. In summary, Na⁺,K⁺-ATPase isoforms are differently regulated in type I and II muscle fibers by sprint-interval training in humans, which, for most isoforms, do not associate with changes in mRNA levels after the first training session. CWI neither impairs nor improves protein adaptations to intense training of importance for muscle K⁺ regulation. NEW & NOTEWORTHY Although cold-water immersion (CWI) after training and competition has become a routine for many athletes, limited published evidence exists regarding its impact on training adaptation. Here, we show that CWI can be performed regularly without impairing training-induced adaptations at the fiber-type level important for muscle K⁺ handling. Furthermore, sprint-interval training invoked fiber type-specific adaptations in K⁺ transport proteins, which may explain the dissociated responses of whole-muscle protein levels and K⁺ transport function to training previously reported.

Intense interval training in healthy older adults increases skeletal muscle [3H]ouabain-binding site content and elevates Na+,K+-ATPase α2 isoform abundance in Type II fibers

Young adults typically adapt to intense exercise training with an increased skeletal muscle Na⁺,K⁺-ATPase (NKA) content, concomitant with reduced extracellular potassium concentration [K⁺] during exercise and enhanced exercise performance. Whether these changes with longitudinal training occur in older adults is unknown and was investigated here. Fifteen older adults (69.4 ± 3.5 years, mean ± SD) were randomized to either 12 weeks of intense interval training (4 × 4 min at 90-95% peak heart rate), 3 days/week (IIT, n = 8); or no exercise controls (n = 7). Before and after training, participants completed an incremental cycle ergometer exercise test until a rating of perceived exertion of 17 (very hard) on a 20-point scale was attained, with measures of antecubital venous [K⁺]_v Participants underwent a resting muscle biopsy prior to and at 48-72 h following the final training session. After IIT, the peak exercise work rate (25%), oxygen uptake (16%) and heart rate (6%) were increased (P < 0.05). After IIT, the peak exercise plasma [K⁺]_v tended to rise (P = 0.07), while the rise in plasma [K⁺]_v relative to work performed (nmol.L^-1J^-1) was unchanged. Muscle NKA content increased by 11% after IIT (P < 0.05). Single fiber measurements, increased in NKA α₂ isoform in Type II fibers after IIT (30%, P < 0.05), with no changes to the other isoforms in single fibers or homogenate. Thus, intense exercise training in older adults induced an upregulation of muscle NKA, with a fiber-specific increase in NKA α₂ abundance in Type II fibers, coincident with increased muscle NKA content and enhanced exercise performance.

Changes in contractile and metabolic parameters of skeletal muscle as rats age from 3 to 12 months

Laboratory rats are considered mature at 3 months despite that musculoskeletal growth is still occurring. Changes in muscle physiological and biochemical characteristics during development from 3 months, however, are not well understood. Whole muscles and single skinned fibres from fast-twitch extensor digitorum longus (EDL) and predominantly slow-twitch soleus (SOL) muscles were examined from male Sprague-Dawley rats (3, 6, 9, 12 months). Ca²⁺ sensitivity of contractile apparatus decreased with age in both fast- (~ 0.04 pCa units) and slow-twitch (~ 0.07 pCa units) muscle fibres, and specific force increased (by ~ 50% and ~ 25%, respectively). Myosin heavy chain composition of EDL and SOL muscles altered to a small extent with age (decrease in MHCIIa proportion after 3 months). Glycogen content increased with age (~ 80% in EDL and 25% in SOL) and GLUT4 protein density decreased (~ 35 and 20%, respectively), whereas the glycogen-related enzymes were little changed. GAPDH protein content was relatively constant in both muscle types, but COXIV protein decreased ~ 40% in SOL muscle. Calsequestrin (CSQ) and SERCA densities remained relatively constant with age, whereas there was a progressive ~ 2-3 fold increase in CSQ-like proteins, though their role and importance remain unclear. There was also ~ 40% decrease in the density of the Na⁺, K⁺-ATPase (NKA) α1 subunit in EDL and the α2 subunit in SOL. These findings emphasise there are substantial changes in skeletal muscle function and the density of key proteins during early to mid-adulthood in rats, which need to be considered in the design and interpretation of experiments.

Preservation of skeletal muscle mitochondrial content in older adults: relationship between mitochondria, fibre type and high-intensity exercise training

KEY POINTS

Ageing is associated with an upregulation of mitochondrial dynamics proteins mitofusin 2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) in human skeletal muscle with the increased abundance of Mfn2 being exclusive to type II muscle fibres. These changes occur despite a similar content of mitochondria, as measured by COXIV, NDUFA9 and complexes in their native states (Blue Native PAGE). Following 12 weeks of high-intensity training (HIT), older adults exhibit a robust increase in mitochondria content, while there is a decline in Mfn2 in type II fibres. We propose that the upregulation of Mfn2 and MiD49 with age may be a protective mechanism to protect against mitochondrial dysfunction, in particularly in type II skeletal muscle fibres, and that exercise may have a unique protective effect negating the need for an increased turnover of mitochondria.

ABSTRACT

Mitochondrial dynamics proteins are critical for mitochondrial turnover and maintenance of mitochondrial health. High-intensity interval training (HIT) is a potent training modality shown to upregulate mitochondrial content in young adults but little is known about the effects of HIT on mitochondrial dynamics proteins in older adults. This study investigated the abundance of protein markers for mitochondrial dynamics and mitochondrial content in older adults compared to young adults. It also investigated the adaptability of mitochondria to 12 weeks of HIT in older adults. Both older and younger adults showed a higher abundance of mitochondrial respiratory chain subunits COXIV and NDUFA9 in type I compared with type II fibres, with no difference between the older adults and young groups. In whole muscle homogenates, older adults had higher mitofusin-2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) contents compared to the young group. Also, older adults had higher levels of Mfn2 in type II fibres compared with young adults. Following HIT in older adults, MiD49 and Mfn2 levels were not different in whole muscle and Mfn2 content decreased in type II fibres. Increases in citrate synthase activity (55%) and mitochondrial respiratory chain subunits COXIV (37%) and NDUFA9 (48%) and mitochondrial respiratory chain complexes (∼70-100%) were observed in homogenates and/or single fibres. These findings reveal (i) a similar amount of mitochondria in muscle from young and healthy older adults and (ii) a robust increase of mitochondrial content following 12 weeks of HIT exercise in older adults.

Dissociation between short-term unloading and resistance training effects on skeletal muscle Na+,K+-ATPase, muscle function, and fatigue in humans

Physical training increases skeletal muscle Na⁺,K⁺-ATPase content (NKA) and improves exercise performance, but the effects of inactivity per se on NKA content and isoform abundance in human muscle are unknown. We investigated the effects of 23-day unilateral lower limb suspension (ULLS) and subsequent 4-wk resistance training (RT) on muscle function and NKA in 6 healthy adults, measuring quadriceps muscle peak torque; fatigue and venous [K⁺] during intense one-legged cycling exercise; and skeletal muscle NKA content ([³H]ouabain binding) and NKA isoform abundances (immunoblotting) in muscle homogenates (α_1-3, β_1-2) and in single fibers (α_1-3, β₁). In the unloaded leg after ULLS, quadriceps peak torque and cycling time to fatigue declined by 22 and 23%, respectively, which were restored with RT. Whole muscle NKA content and homogenate NKA α_1-3 and β_1-2 isoform abundances were unchanged with ULLS or RT. However, in single muscle fibers, NKA α₃ in type I (-66%, P = 0.006) and β₁ in type II fibers (-40%, P = 0.016) decreased after ULLS, with other NKA isoforms unchanged. After RT, NKA α₁ (79%, P = 0.004) and β₁ (35%, P = 0.01) increased in type II fibers, while α₂ (76%, P = 0.028) and α₃ (142%, P = 0.004) increased in type I fibers compared with post-ULLS. Despite considerably impaired muscle function and earlier fatigue onset, muscle NKA content and homogenate α₁ and α₂ abundances were unchanged, thus being resilient to inactivity induced by ULLS. Nonetheless, fiber type-specific downregulation with inactivity and upregulation with RT of several NKA isoforms indicate complex regulation of muscle NKA expression in humans.

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.

Cell specific differences in the protein abundances of GAPDH and Na(+),K(+)-ATPase in skeletal muscle from aged individuals

Na(+), K(+)-ATPase (NKA) isoforms (α1,α2,α3,β1,β2,β3) are involved in the maintenance of membrane potential and hence are important regulators of cellular homeostasis. Given the age-related decline in skeletal muscle function, we investigated whether the natural physiological process of aging is associated with altered abundance of NKA isoforms (α1,α2,α3,β1,β2,β3) or of the commonly used control protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Importantly, measurements were made in both whole muscle or specific fiber types obtained from skeletal muscle biopsies. Seventeen healthy older (AGED, 69.4 ± 3.5 years, mean ± SD) and 14 younger (YOUNG, 25.5 ± 2.8 years) adults underwent a muscle biopsy for biochemical analyses. Comparing homogenates from AGED and YOUNG individuals revealed higher β3 isoform (p<0.05) and lower GAPDH (p<0.05). Analysis of individual fibers in muscle from YOUNG individuals, showed greater α3 and β2 isoforms, and more GAPDH in Type II compared with Type I fibers (p<0.05). In the AGED, GAPDH was higher in Type II compared with Type I fibers (p<0.05), there were no fiber type differences in the NKA isoforms (p>0.05). Compared with the same fiber type in YOUNG, α1 was greater (Type I) and α3 lower (Type II), while in both fiber types, β2 was lower, β3 greater and GAPDH lower, in muscle from AGED individuals (all p<0.05). Overall, we demonstrate that (i) GAPDH is an inappropriate choice of protein for normalization in all skeletal muscle research and (ii) full understanding of the role of NKA isoforms in human skeletal muscle requires consideration of age and muscle fiber type.