Unpacking the renal system component of the "structure and function" core concept of physiology by an Australian team.

An Australia-wide consensus was reached on seven core concepts of physiology, one of which was "structure and function" with the descriptor "Structure and function are intrinsically related to all levels of the organism. In all physiological systems, the structure from a microscopic level to an organ level dictates its function." As a framework for the structure and function core concept, the renal system was unpacked by a team of 5 Australian Physiology educators from different universities with extensive teaching experience into hierarchical levels, with 5 themes and 25 subthemes up to 3 levels deep. Within theme 1, the structures that comprise the renal system were unpacked. Within theme 2, the physiological processes within the nephron such as filtration, reabsorption, and secretion were unpacked. Within theme 3, the processes involved in micturition were unpacked. In theme 4, the structures and processes involved in regulating renal blood flow and glomerular filtration were unpacked; and within theme 5, the role of the kidney in red blood cell production was unpacked. Twenty-one academics rated the difficulty and importance of each theme/subtheme, and results were analyzed using a one-way ANOVA. All identified themes were validated as "essential" to "important"/"moderately important" and rated between "difficult" to "not difficult." A similar framework consisting of structure, physiological processes, physical processes, and regulation can be used to unpack other body systems. Unpacking of the body systems will provide a list of what students should be taught in curricula across Australian universities and inform assessment and learning activities.NEW & NOTEWORTHY This is the first attempt to unpack and validate the "structure and function" core concept in physiology with all Australian educators. We unpacked the renal system into themes with hierarchical levels, which were validated by an experienced team of Australian physiology educators. Our unpacking of the "structure and function" core concept provides a specific framework for educators to apply this important concept in physiology education.

Six weeks of N-acetylcysteine antioxidant in drinking water decreases pathological fiber branching in MDX mouse dystrophic fast-twitch skeletal muscle.

Introduction: It has been proposed that an increased susceptivity to oxidative stress caused by the absence of the protein dystrophin from the inner surface of the sarcolemma is a trigger of skeletal muscle necrosis in the destructive dystrophin deficient muscular dystrophies. Here we use the mdx mouse model of human Duchenne Muscular Dystrophy to test the hypothesis that adding the antioxidant NAC at 2% to drinking water for six weeks will treat the inflammatory phase of the dystrophic process and reduce pathological muscle fiber branching and splitting resulting in a reduction of mass in mdx fast-twitch EDL muscles. Methods: Animal weight and water intake was recorded during the six weeks when 2% NAC was added to the drinking water. Post NAC treatment animals were euthanised and the EDL muscles dissected out and placed in an organ bath where the muscle was attached to a force transducer to measure contractile properties and susceptibility to force loss from eccentric contractions. After the contractile measurements had been made the EDL muscle was blotted and weighed. In order to assess the degree of pathological fiber branching mdx EDL muscles were treated with collagenase to release single fibers. For counting and morphological analysis single EDL mdx skeletal muscle fibers were viewed under high magnification on an inverted microscope. Results: During the six-week treatment phase NAC reduced body weight gain in three- to nine-week-old mdx and littermate control mice without effecting fluid intake. NAC treatment also significantly reduced the mdx EDL muscle mass and abnormal fiber branching and splitting. Discussion: We propose chronic NAC treatment reduces the inflammatory response and degenerative cycles in the mdx dystrophic EDL muscles resulting in a reduction in the number of complexed branched fibers reported to be responsible for the dystrophic EDL muscle hypertrophy.

Skeletal Muscle and Kidney Crosstalk in Chronic Kidney Disease.

The functioning of complex organisms requires a constant and delicate balance of processes both between and within cells, tissues, and organ systems. There is growing appreciation for the role of signalling crosstalk connecting different organ systems of the body, even from tissues traditionally classified as "inert" in terms of their capacity to produce chemical signals that can act on other organ systems. Many of these secreted molecules have been shown to contribute to, or exacerbate, a variety of functions and diseases in other organ systems, even if the two organs are not functionally linked. For example, there is a strong association with skeletal muscle atrophy and dysfunction in patients with chronic kidney disease (CKD). Identification of molecules produced and secreted by skeletal muscle has existed for some time, and there is emerging evidence that skeletal muscle may directly affect kidney function. Conversely, factors produced and secreted by the kidneys in various models of CKD have been shown to contribute to reduced muscle functionality. This review will focus on crosstalk in both directions between skeletal muscle and the kidneys. The emphasis will be on direct interaction between these organs using examples of secreted factors that are produced by the muscle or kidneys (including activin A, myostatin, microRNA's, irisin and mitsugumin 53), often under pathophysiological conditions. Our understanding of how the kidneys and skeletal muscle interact with each other is key to elucidating the pathophysiology processes that drive health and disease.

Minocycline Treatment Reduces Mass and Force Output From Fast-Twitch Mouse Muscles and Inhibits Myosin Production in C2C12 Myotubes.

Minocycline, a tetracycline-class of antibiotic, has been tested with mixed effectiveness on neuromuscular disorders such as amyotrophic lateral sclerosis, autoimmune neuritis and muscular dystrophy. The independent effect of minocycline on skeletal muscle force production and signalling remain poorly understood. Our aim here is to investigate the effects of minocycline on muscle mass, force production, myosin heavy chain abundance and protein synthesis. Mice were injected with minocycline (40 mg/kg i.p.) daily for 5 days and sacrificed at day six. Fast-twitch EDL, TA muscles and slow-twitch soleus muscles were dissected out, the TA muscle was snap-frozen and the remaining muscles were attached to force transducer whilst maintained in an organ bath. In C2C12 myotubes, minocycline was applied to the media at a final concentration of 10 µg/mL for 48 h. In minocycline treated mice absolute maximal force was lower in fast-twitch EDL while in slow-twitch soleus there was an increase in the time to peak and relaxation of the twitch. There was no effect of minocycline treatment on the other contractile parameters measured in isolated fast- and slow-twitch muscles. In C2C12 cultured cells, minocycline treatment significantly reduced both myosin heavy chain content and protein synthesis without visible changes to myotube morphology. In the TA muscle there was no significant changes in myosin heavy chain content. These results indicate that high dose minocycline treatment can cause a reduction in maximal isometric force production and mass in fast-twitch EDL and impair protein synthesis during myogenesis in C2C12 cultured cells. These findings have important implications for future studies investigating the efficacy of minocycline treatment in neuromuscular or other muscle-atrophy inducing conditions.

Glucocorticoid-induced CREB activation and myostatin expression in C2C12 myotubes involves phosphodiesterase-3/4 signaling.

Muscle atrophy in metabolic conditions like chronic kidney disease (CKD) and diabetes are associated with glucocorticoid production, dysfunctional insulin/Akt/FoxO3 signaling and increased myostatin expression. We recently found that CREB, a transcription factor proposed to regulate myostatin expression, is highly phosphorylated in some wasting conditions. Based on a novel Akt-PDE3/4 signaling paradigm, we hypothesized that reduced Akt signaling contributes to CREB activation and myostatin expression. C2C12 myotubes were incubated with dexamethasone (Dex), an atrophy-inducing synthetic glucocorticoid. Akt/CREB signaling and myostatin expression were evaluated by immunoblot and qPCR analyses. Inhibitors of Akt, phosphodiesterase (PDE)-3/4, and protein kinase A (PKA) signaling were used to test our hypothesis. Incubating myotubes with Dex for 3-24 h inhibited Akt phosphorylation and enhanced CREB phosphorylation as well as myostatin mRNA and protein. Inhibition of PI3K/Akt signaling with LY294002 similarly increased CREB phosphorylation. Isobutyl-methylxanthine (IBMX, a pan PDE inhibitor), milrinone (PDE3 inhibitor) and rolipram (PDE4 inhibitor) augmented CREB phosphorylation and myostatin expression. Inhibition of protein kinase A by PKI reverted Dex- or IBMX-induced CREB phosphorylation and myostatin expression. Our study provides evidence supporting a newly identified mechanism by which a glucocorticoid-related reduction in Akt signaling contributes to myostatin expression via CREB activation.

Palmitate-induced ER stress and inhibition of protein synthesis in cultured myotubes does not require Toll-like receptor 4.

Saturated fatty acids, such as palmitate, are elevated in metabolically dysfunctional conditions like type 2 diabetes mellitus. Palmitate has been shown to impair insulin sensitivity and suppress protein synthesis while upregulating proteolytic systems in skeletal muscle. Increased sarco/endoplasmic reticulum (ER) stress and subsequent activation of the unfolded protein response may contribute to the palmitate-induced impairment of muscle protein synthesis. In some cell types, ER stress occurs through activation of the Toll-like receptor 4 (TLR4). Given the link between ER stress and suppression of protein synthesis, we investigated whether palmitate induces markers of ER stress and protein synthesis by activating TLR4 in cultured mouse C2C12 myotubes. Myotubes were treated with vehicle, a TLR4-specific ligand (lipopolysaccharides), palmitate, or a combination of palmitate plus a TLR4-specific inhibitor (TAK-242). Inflammatory indicators of TLR4 activation (IL-6 and TNFα) and markers of ER stress were measured, and protein synthesis was assessed using puromycin incorporation. Palmitate substantially increased the levels of IL-6, TNF-α, CHOP, XBP1s, and ATF 4 mRNAs and augmented the levels of CHOP, XBP1s, phospho-PERK and phospho-eIF2α proteins. The TLR4 antagonist attenuated both acute palmitate and LPS-induced increases in IL-6 and TNFα, but did not reduce ER stress signaling with either 6 h or 24 h palmitate treatment. Similarly, treating myotubes with palmitate for 6 h caused a 43% decline in protein synthesis consistent with an increase in phospho-eIF2α, and the TLR4 antagonist did not alter these responses. These results suggest that palmitate does not induce ER stress through TLR4 in muscle, and that palmitate impairs protein synthesis in skeletal muscle in part by induction of ER stress.

Docosahexaenoic acid counteracts palmitate-induced endoplasmic reticulum stress in C2C12 myotubes: Impact on muscle atrophy.

Lipid accumulation in skeletal muscle results in dysregulation of protein metabolism and muscle atrophy. We previously reported that treating C2C12 myotubes with palmitate (PA), a saturated fatty acid, increases the overall rate of proteolysis via activation of the ubiquitin-proteasome and autophagy systems; co-treatment with the omega-3 polyunsaturated fatty acid docosahexaenoic acid (DHA) prevents the PA-induced responses. Others have reported that PA induces endoplasmic reticulum (ER) stress which initiates the unfolded protein response (UPR), a collective group of responses that can lead to activation of caspase-mediated proteolysis and autophagy. Presently, we tested the hypothesis that DHA protects against PA-induced ER stress/UPR and its atrophy-related responses in muscle cells. C2C12 myotubes were treated with 500 µmol/L PA and/or 100 µmol/L DHA for 24 h. Proteins and mRNA associated with ER stress/UPR, autophagy, and caspase-3 activation were evaluated. PA robustly increased the phosphorylation of protein kinase R (PKR)-like ER kinase (PERK) and eukaryotic initiation factor 2α (eIF2α). It also increased the mRNAs encoding activating transcription factor 4 (ATF4), spliced X-box binding protein 1 (XBP1s), C/EBP homologous protein (CHOP), and autophagy-related 5 (Atg5) as well as the protein levels of the PERK target nuclear factor erythroid 2-related factor (Nrf2), CHOP, and cleaved (i.e., activated) caspase-3. Co-treatment with DHA prevented all of the PA-induced responses. Our results indicate that DHA prevents PA-induced muscle cell atrophy, in part, by preventing ER stress/UPR, a process that leads to activation of caspase-mediated proteolysis and an increase in expression of autophagy-related genes.

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.

Salbutamol effects on systemic potassium dynamics during and following intense continuous and intermittent exercise.

PURPOSE: Salbutamol inhalation is permissible by WADA in athletic competition for asthma management and affects potassium regulation, which is vital for muscle function. Salbutamol effects on arterial potassium concentration ([K+]a) during and after high-intensity continuous exercise (HIcont) and intermittent exercise comprising repeated, brief sprints (HIint), and on performance during HIint are unknown and were investigated. METHODS: Seven recreationally active men participated in a double-blind, randomised, cross-over design, inhaling 1000 µg salbutamol or placebo. Participants cycled continuously for 5 min at 40 % [Formula: see text]O2peak and 60 % [Formula: see text]O2peak, then HIcont (90 s at 130 % [Formula: see text]O2peak), 20 min recovery, and then HIint (3 sets, 5 × 4 s sprints), with 30 min recovery. RESULTS: Plasma [K+]a increased throughout exercise and subsequently declined below baseline (P < 0.001). Plasma [K+]a was greater during HIcont than HIint (P < 0.001, HIcont 5.94 ± 0.65 vs HIint set 1, 4.71 ± 0.40 mM); the change in [K+]a from baseline (Δ[K+]a) was 2.6-fold greater during HIcont than HIint (P < 0.001). The Δ[K+] throughout the trial was less with salbutamol than placebo (P < 0.001, treatment main effect, 0.03 ± 0.67 vs 0.22 ± 0.69 mM, respectively); and remained less after correction for fluid shifts (P < 0.001). The Δ[K+] during HIcont was less after salbutamol (P < 0.05), but not during HIint. Blood lactate, plasma pH, and the work output during HIint did not differ between trials. CONCLUSIONS: Inhaled salbutamol modulated the [K+]a rise across the trial, comprising intense continuous and intermittent exercise and recovery, lowering Δ[K+] during HIcont. The limited [K+]a changes during HIint suggest that salbutamol is unlikely to influence systemic [K+] during periods of intense effort in intermittent sports.

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 (α1-3, ß1-2) and in single fibers (α1-3, ß1). 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 α1-3 and ß1-2 isoform abundances were unchanged with ULLS or RT. However, in single muscle fibers, NKA α3 in type I (-66%, P = 0.006) and ß1 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 ß1 (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.

Muscle atrophy in patients with Type 2 Diabetes Mellitus: roles of inflammatory pathways, physical activity and exercise.

Muscle atrophy is caused by an imbalance in contractile protein synthesis and degradation which can be triggered by various conditions including Type 2 Diabetes Mellitus (T2DM). Reduced muscle quality in patients with T2DM adversely affects muscle function, the capacity to perform activities of daily living, quality of life and ultimately may increase the risk of premature mortality. Systemic inflammation initiated by obesity and prolonged overnutrition not only contributes to insulin resistance typical of T2DM, but also promotes muscle atrophy via decreased muscle protein synthesis and increased ubiquitin-proteasome, lysosomal-proteasome and caspase 3- mediated protein degradation. Emerging evidence suggests that the inflammation-sensitive Nuclear Factor κ B (NF-κB) and Signal Transducer and Activator of Transcription 3 (STAT3) pathways may contribute to muscle atrophy in T2DM. In contrast, exercise appears to be an effective tool in promoting muscle hypertrophy, in part due to its effect on systemic and local (skeletal muscle) inflammation. The current review discusses the role inflammation plays in muscle atrophy in T2DM and the role of exercise training in minimising the effect of inflammatory markers on skeletal muscle. We also report original data from a cohort of obese patients with T2DM compared to age-matched controls and demonstrate that patients with T2DM have 60% higher skeletal muscle expression of the atrophy transcription factor FoxO1. This review concludes that inflammatory pathways in muscle, in particular, NF-κB, potentially contribute to T2DM-mediated muscle atrophy. Further in-vivo and longitudinal human research is required to better understand the role of inflammation in T2DM-mediated atrophy and the anti-inflammatory effect of exercise training under these conditions.

Effect of N-acetylcysteine infusion on exercise-induced modulation of insulin sensitivity and signaling pathways in human skeletal muscle.

-Reactive oxygen species (ROS) produced in skeletal muscle may play a role in potentiating the beneficial responses to exercise; however, the effects of exercise-induced ROS on insulin action and protein signaling in humans has not been fully elucidated. Seven healthy, recreationally active participants volunteered for this double-blind, randomized, repeated-measures crossover study. Exercise was undertaken with infusion of saline (CON) or the antioxidant N-acetylcysteine (NAC) to attenuate ROS. Participants performed two 1-h cycling exercise sessions 7-14 days apart, 55 min at 65% V̇o2peak plus 5 min at 85%V̇o2peak, followed 3 h later by a 2-h hyperinsulinemic euglycemic clamp (40 mIU·min(-1)·m(2)) to determine insulin sensitivity. Four muscle biopsies were taken on each trial day, at baseline before NAC infusion (BASE), after exercise (EX), after 3-h recovery (REC), and post-insulin clamp (PI). Exercise, ROS, and insulin action on protein phosphorylation were evaluated with immunoblotting. NAC tended to decrease postexercise markers of the ROS/protein carbonylation ratio by -13.5% (P = 0.08) and increase the GSH/GSSG ratio twofold vs. CON (P < 0.05). Insulin sensitivity was reduced (-5.9%, P < 0.05) by NAC compared with CON without decreased phosphorylation of Akt or AS160. Whereas p-mTOR was not significantly decreased by NAC after EX or REC, phosphorylation of the downstream protein synthesis target kinase p70S6K was blunted by 48% at PI with NAC compared with CON (P < 0.05). We conclude that NAC infusion attenuated muscle ROS and postexercise insulin sensitivity independent of Akt signaling. ROS also played a role in normal p70S6K phosphorylation in response to insulin stimulation in human skeletal muscle.

The effects of knee injury on skeletal muscle function, Na+, K+-ATPase content, and isoform abundance.

While training upregulates skeletal muscle Na(+), K(+)-ATPase (NKA), the effects of knee injury and associated disuse on muscle NKA remain unknown. This was therefore investigated in six healthy young adults with a torn anterior cruciate ligament, (KI; four females, two males; age 25.0 ± 4.9 years; injury duration 15 ± 17 weeks; mean ± SD) and seven age- and BMI-matched asymptomatic controls (CON; five females, two males). Each participant underwent a vastus lateralis muscle biopsy, on both legs in KI and one leg in CON. Muscle was analyzed for muscle fiber type and cross-sectional area (CSA), NKA content ([(3)H]ouabain binding), and α1-3 and ß1-2 isoform abundance. Participants also completed physical activity and knee function questionnaires (KI only); and underwent quadriceps peak isometric strength, thigh CSA and postural sway assessments in both injured and noninjured legs. NKA content was 20.1% lower in the knee-injured leg than the noninjured leg and 22.5% lower than CON. NKA α2 abundance was 63.0% lower in the knee-injured leg than the noninjured leg, with no differences in other NKA isoforms. Isometric strength and thigh CSA were 21.7% and 7.1% lower in the injured leg than the noninjured leg, respectively. In KI, postural sway did not differ between legs, but for two-legged standing was 43% higher than CON. Hence, muscle NKA content and α2 abundance were reduced in severe knee injury, which may contribute to impaired muscle function. Restoration of muscle NKA may be important in rehabilitation of muscle function after knee and other lower limb injury.

The effects of osteoarthritis and age on skeletal muscle strength, Na+-K+-ATPase content, gene and isoform expression.

Knee osteoarthritis (OA) is a debilitating disorder prevalent in older populations that is accompanied by declines in muscle mass, strength, and physical activity. In skeletal muscle, the Na(+)-K(+) pump (NKA) is pivotal in ion homeostasis and excitability and is modulated by disuse and exercise training. This study examined the effects of OA and aging on muscle NKA in 36 older adults (range 55-81 yr), including 19 with OA (69.9 ± 6.5 yr, mean ± SD) and 17 asymptomatic controls (CON, 66.8 ± 6.4 yr). Participants completed knee extensor strength testing and a physical activity questionnaire. A vastus lateralis muscle biopsy was analyzed for NKA content ([(3)H]ouabain binding sites), α1-3- and ß1-3-isoform protein abundance (immunoblotting), and mRNA (real-time RT-PCR). The association between age and NKA content was investigated within the OA and CON groups and in pooled data. The NKA content was also contrasted between subgroups below and above the median age of 68.5 yr. OA had lower strength (-40.8%, P = 0.005), but higher NKA α2- (∼34%, P = 0.006) and α3-protein (100%, P = 0.016) abundance than CON and performed more incidental physical activity (P = 0.035). No differences were found between groups for NKA content, abundance of other NKA isoforms, or gene expression. There was a negative correlation between age and NKA content within OA (r = -0.63, P = 0.03) and with both groups combined (r = -0.47, P = 0.038). The NKA content was 25.5% lower in the older (69-81 yr) than in the younger (55-68 yr) subgroup. Hence older age, but not knee OA, was related to lowered muscle NKA content in older adults.

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 α(1)-, α(2)-, α(3)-, ß(1)-, ß(2)-, and ß(3)-isoform gene expression (real-time RT-PCR), protein abundance (immunoblotting), and NKA total content ([(3)H]ouabain binding sites). The Elderly had lower peak oxygen consumption (-36.7%, P = 0.000), strength (-36.3%, P = 0.001), NKA α(2)- (-24.4%, 11.9 ± 4.4 vs. 9.0 ± 2.7 arbitrary units, P = 0.049), and NKA ß(3)-protein abundance (-23.0%, P = 0.041) than Young. The ß(3)-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 [(3)H]ouabain binding site content. Thus skeletal muscle in elderly individuals was characterized by decreased NKA α(2)- and ß(3)-protein abundance, but unchanged α(1) abundance and [(3)H]ouabain binding. The latter was likely caused by reduced α(2) abundance with aging, preventing an otherwise higher [(3)H]ouabain binding that might occur with a greater membrane density in smaller muscle fibers. Further study is required to verify reduced muscle NKA α(2) with aging and possible contributions to impaired exercise capability and daily living activities.