Skeletal muscle myofibrillar protein metabolism in heart failure: relationship to immune activation and functional capacity

Heart Failure Severity Stratification Beyond INTERMACS Profiles: A Step Toward Optimal Left Ventricular Assist Device Timing

The purpose of this analysis is to determine whether pectoralis muscle measures quantified on pre left ventricular assist device (LVAD) computerized tomography (CT) scans can identify subgroups of patients with differential disease severity within each Interagency Registry for Mechanical Circulatory Support (INTERMACS) profile. Patients with chest CTs performed ≤3 months before LVAD implantation at University of Minnesota (n = 143) and Houston Methodist Hospital (n = 133) were identified from the larger LVAD cohorts (University of Minnesota n = 353, Houston Methodist =278). Unilateral Pectoralis muscle mass indexed to body surface area and pectoralis muscle attenuation were measured on preoperative chest CT scans. Patients within each INTERMACS profile were separated into HIGH and LOW PEC muscle groups. Kaplan-Meier and multivariable cox regression analyses were performed to compare mortality among INTERMACS profiles by HIGH and LOW PEC muscle groups. INTERMACS 3 and 4 patients in the HIGH PEC groups had the highest survival on LVAD support (1 year survival 85% vs. 68%, log rank P = 0.0001). Being in this group was associated with a 60% reduction in the hazards rate (HR) of death after LVAD (adjusted HR 0.40, 95% confidence interval 0.25-0.62). Additionally, renal function deterioration in the year before LVAD was associated with lower INTERMACS profiles and lower measured pectoralis muscle tissue attenuation at the time of LVAD implantation. INTERMACS 3 and 4 patients with the highest pectoralis muscle measures had the best survival after LVAD. The association between renal function deterioration and sarcopenia suggests these muscle changes are progressive. Computerized tomography quantification of sarcopenia may help identify optimal LVAD implantation timing.

The Effect of Wheel Exercise on Functional Indices of Cachexia in Tumor-bearing Mice

INTRODUCTION

Cancer-related fatigue and muscle wasting have received significant attention over the last few decades with the goal of establishing interventions that can improve cancer patient life quality and survival. Increased physical activity has shown to reduce cancer-associated fatigue and has been proposed as a promising therapeutic to attenuate cancer-induced wasting. However, significant gaps remain in our understanding of how physical activity affects the compositional and functional changes that initiate muscle wasting. The purpose of the current study was to determine the effect of wheel exercise on body composition and functional indices of cancer cachexia before the development of significant wasting.

METHODS

Thirteen-week-old male Apc (MIN) and C57BL/6 (B6) mice were given free wheel access (W) or a locked wheel (Sed) for 5 wk.

RESULTS

Wheel activity was reduced in the MIN compared with B6; however, wheel access increased complex II expression in isolated skeletal muscle mitochondria regardless of genotype. Wheel access had no effect on tumor burden or plasma interleukin-6 in the MIN. MIN-W increased body weight and lean mass compared with MIN-Sed, and there was a direct correlation between wheel distance and lean mass change. MIN-W increased grip strength and treadmill time to fatigue compared with MIN-Sed. Within MIN-W mice, skeletal muscle fatigability was only improved in high runners (>60 min·d).

CONCLUSIONS

Our results suggest that there were therapeutic benefits of increased activity related to body composition, behavior, and whole-body function that were not dependent on exercise duration; however, there was an exercise threshold needed to improve skeletal muscle fatigability in tumor-bearing mice. Interestingly, wheel access was able to improve compositional and functional outcomes without mitigating tumor number or size.

Functional Capacity and Inflammatory Mediators in Elderly Residents of Counties with Different Human Development Index

A number of studies have indicated that certain factors, including socioeconomic status and education, are associated with the functional health status of the elderly. Another relevant factor in aging is chronic subliminal inflammation, with increased levels of circulating inflammatory cytokines such as interleukin-6 (IL-6), tumor necrosis factor α (TNF-α), and soluble tumor necrosis factor receptor 1 (sTNFR-1), commonly seen in the elderly. High levels of these inflammatory mediators could impair the functional capacity. In this respect, the aim of this cross-sectional study was to compare plasma levels of inflammatory mediators and functional capacity of older women living in three Brazilian counties with different Human Development Index. We evaluated 154 women aged ≥65 years, regardless of race and/or social status. IL-6 and sTNFR-1 plasma levels were measured by ELISA and the functional capacity by the Short Physical Performance Battery (SPPB) test. Comparison among groups was performed using one-way ANOVA with Bonferroni post hoc correction, Kruskal–Wallis, and Mann–Whitney U tests. Women that lived in one of the counties with high HDI had lower functional capacity (p < 0.001). The population from the county with the highest HDI had lower plasma levels of sTNFR-1 (p < 0.05). There was no significant difference in plasma levels of IL-6 (p > 0.05). Besides this, women from the counties with lower HDI had a higher number of self-reported diseases and higher income (p < 0.05). Women that lived in the county with the highest HDI had a higher average education level (p < 0.05). The results showed differences in functional capacity and plasma levels of sTNFR-1 between the counties. In addition, the level of education, family income, and number of self-reported diseases show regional diversities in the aging process, suggesting these factors having an influence on inflammatory mediators and functional capacity.

Skeletal muscle fiber characteristics in patients with chronic heart failure: impact of disease severity and relation with muscle oxygenation during exercise

INTRODUCTION

Skeletal muscle function in patients with heart failure and reduced ejection fraction (HFrEF) greatly determines exercise capacity. However, reports on skeletal muscle fiber dimensions, fiber capillarization, and their physiological importance are inconsistent.

METHODS

Twenty-five moderately-impaired patients with HFrEF and 25 healthy control (HC) subjects underwent muscle biopsy sampling. Type I and type II muscle fiber characteristics were determined by immunohistochemistry. In patients with HFrEF, enzymatic oxidative capacity was assessed, and pulmonary oxygen uptake (VO₂) and skeletal muscle oxygenation during maximal and moderate-intensity exercise were measured using near-infrared spectroscopy.

RESULTS

While muscle fiber cross-sectional area (CSA) was not different between patients with HFrEF and HC, percentage of type I fibers was higher in HC (46±15% versus 37±12%, respectively, P=0.041). Fiber type distribution and CSA were not different between patients in New York Heart Association (NYHA) class II and III. Type I muscle fiber capillarization was higher in HFrEF compared with controls (capillary-to-fiber perimeter exchange (CFPE) index: 5.70±0.92 versus 5.05±0.82, respectively, P=0.027). Patients in NYHA class III had slower VO₂ and muscle deoxygenation kinetics during onset of exercise, and lower muscle oxidative capacity than those in class II (P<0.05). Also, fiber capillarization was lower, but not compared with HC. Higher CFPE index was related to faster deoxygenation (r_spearman=-0.682, P=0.001), however, not to muscle oxidative capacity (r=-0.282, P=0.216).

CONCLUSIONS

Type I muscle fiber capillarization is higher in HFrEF compared with HC, but not in patients with greater exercise impairment. Greater capillarization may positively affect VO₂ kinetics by enhancing muscle oxygen diffusion.

Whole-Body Protein Metabolism in Chronic Heart Failure: Relationship to Anabolic and Catabolic Hormones

BACKGROUND

Patients with chronic heart failure frequently experience profound wasting during the course of the disease, a condition termed cardiac cachexia. Although protein is the primary structural and functional component of most tissues, few studies have examined the effect of heart failure on protein metabolism. Moreover, no study has assessed the relationship of protein turnover to hormonal alterations thought to promote cachexia. Thus, our goal was to determine if whole-body protein metabolism is altered in heart failure patients and to assess the relationship of protein kinetics to circulating levels of anabolic and catabolic hormones.

METHODS

We measured whole-body protein metabolism using 13C-leucine, body composition, and circulating anabolic and catabolic hormone levels in 10 patients with chronic heart failure and 11 elderly controls.

RESULTS

No differences in leucine rate of appearance, oxidation, or nonoxidative disposal were noted between heart failure patients and controls. However, in a subgroup of patients characterized by increased resting energy expenditure for their metabolic body size (n = 4; > or = 20% above that predicted from fat-free mass), leucine rate of appearance (mean +/- SE; 146 +/- 6 micromol/min), an index of protein breakdown, tended to be higher compared with patients with normal resting energy expenditure (n = 5; 120 +/- 8 micromol/min) and controls (127 +/- 4 micromol/min; p = .06). Alterations in anabolic/catabolic hormone balance did not explain increased protein breakdown in this subgroup, and no correlations were found between hormone levels and protein breakdown in the heart failure group as a whole. In contrast, increased circulating interleukin-6 soluble receptor (r = 0.829; p < .01) and reduced insulin-like growth factor-I (r =-.751; p < .05) levels were related to greater rates of leucine oxidation in heart failure patients.

CONCLUSION

Our results demonstrate that, although increased protein turnover is not a generalized feature of heart failure, there is a subgroup of patients characterized by resting hypermetabolism and increased protein breakdown. Moreover, hormonal alterations related to the heart failure syndrome were related to increased protein oxidation.

Adiponectin resistance in skeletal muscle: pathophysiological implications in chronic heart failure

Skeletal muscle wasting is a common complication of chronic heart failure (CHF) and linked to poor patient prognosis. In recent years, adiponectin was postulated to be centrally involved in CHF-associated metabolic failure and muscle wasting. This review discusses current knowledge on the role of adiponectin in CHF. Particular emphasis will be given to the complex interaction mechanisms and the intracellular pathways underlying adiponectin resistance in skeletal muscle of CHF patients. In this review, we propose that the resistance process is multifactorial, integrating abnormalities emanating from insulin signalling, mitochondrial biogenesis, and ceramide metabolism.

Sarcopenic obesity and the pathogenesis of exercise intolerance in heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) is the most common form of heart failure (HF) in older adults. The primary chronic symptom in patients with HFpEF, even when well compensated, is severe exercise intolerance. Cardiac and peripheral functions contribute equally to exercise intolerance in HFpEF, though the latter has been the focus of fewer studies. Of note, multiple studies with exercise training have shown that exercise intolerance can improve significantly in the absence of improvements in exercise cardiac output, indicating a role of peripheral, noncardiac adaptations. In addition, clinical drug trials performed to date in HFpEF, all of which have focused on influencing cardiovascular function, have not been positive on primary clinical outcomes and most have not improved exercise capacity. Mounting evidence indicates that sarcopenic obesity, characterized by the coexistence of excess fat mass and decreased muscle mass, could contribute to the pathophysiology of exercise intolerance in older HFpEF patients and may provide avenues for novel treatments.

Exercise intolerance in heart failure with preserved ejection fraction: more than a heart problem

Heart failure (HF) with preserved ejection fraction (HFpEF) is the most common form of HF in older adults, and is increasing in prevalence as the population ages. Furthermore, HFpEF is increasing out of proportion to HF with reduced EF (HFrEF), and its prognosis is worsening while that of HFrEF is improving. Despite the importance of HFpEF, our understanding of its pathophysiology is incomplete, and optimal treatment remains largely undefined. A cardinal feature of HFpEF is reduced exercise tolerance, which correlates with symptoms as well as reduced quality of life. The traditional concepts of exercise limitations have focused on central dysfunction related to poor cardiac pump function. However, the mechanisms are not exclusive to the heart and lungs, and the understanding of the pathophysiology of this disease has evolved. Substantial attention has focused on defining the central versus peripheral mechanisms underlying the reduced functional capacity and exercise tolerance among patients with HF. In fact, physical training can improve exercise tolerance via peripheral adaptive mechanisms even in the absence of favorable central hemodynamic function. In addition, the drug trials performed to date in HFpEF that have focused on influencing cardiovascular function have not improved exercise capacity. This suggests that peripheral limitations may play a significant role in HF limiting exercise tolerance, a hallmark feature of HFpEF.

Heart failure with preserved ejection fraction in the elderly: scope of the problem

Heart failure with preserved ejection fraction (HFpEF) is the most common form of heart failure (HF) in older adults, particularly women, and is increasing in prevalence as the population ages. With morbidity and mortality on par with HF with reduced ejection fraction, it remains a most challenging clinical syndrome for the practicing clinician and basic research scientist. Originally considered to be predominantly caused by diastolic dysfunction, more recent insights indicate that HFpEF in older persons is typified by a broad range of cardiac and non-cardiac abnormalities and reduced reserve capacity in multiple organ systems. The globally reduced reserve capacity is driven by: 1) inherent age-related changes; 2) multiple, concomitant co-morbidities; 3) HFpEF itself, which is likely a systemic disorder. These insights help explain why: 1) co-morbidities are among the strongest predictors of outcomes; 2) approximately 50% of clinical events in HFpEF patients are non-cardiovascular; 3) clinical drug trials in HFpEF have been negative on their primary outcomes. Embracing HFpEF as a true geriatric syndrome, with complex, multi-factorial pathophysiology and clinical heterogeneity could provide new mechanistic insights and opportunities for progress in management. This article is part of a Special Issue entitled CV Aging.

Skeletal muscle myofilament adaptations to aging, disease, and disuse and their effects on whole muscle performance in older adult humans

Skeletal muscle contractile function declines with aging, disease, and disuse. In vivo muscle contractile function depends on a variety of factors, but force, contractile velocity and power generating capacity ultimately derive from the summed contribution of single muscle fibers. The contractile performance of these fibers are, in turn, dependent upon the isoform and function of myofilament proteins they express, with myosin protein expression and its mechanical and kinetic characteristics playing a predominant role. Alterations in myofilament protein biology, therefore, may contribute to the development of functional limitations and disability in these conditions. Recent studies suggest that these conditions are associated with altered single fiber performance due to decreased expression of myofilament proteins and/or changes in myosin-actin cross-bridge interactions. Furthermore, cellular and myofilament-level adaptations are related to diminished whole muscle and whole body performance. Notably, the effect of these various conditions on myofilament and single fiber function tends to be larger in older women compared to older men, which may partially contribute to their higher rates of disability. To maintain functionality and provide the most appropriate and effective countermeasures to aging, disease, and disuse in both sexes, a more thorough understanding is needed of the contribution of myofilament adaptations to functional disability in older men and women and their contribution to tissue level function and mobility impairment.

Skeletal muscle abnormalities and exercise intolerance in older patients with heart failure and preserved ejection fraction

Heart failure (HF) with preserved ejection fraction (HFPEF) is the most common form of HF in older persons. The primary chronic symptom in HFPEF is severe exercise intolerance, and its pathophysiology is poorly understood. To determine whether skeletal muscle abnormalities contribute to their severely reduced peak exercise O2 consumption (Vo2), we examined 22 older HFPEF patients (70 ± 7 yr) compared with 43 age-matched healthy control (HC) subjects using needle biopsy of the vastus lateralis muscle and cardiopulmonary exercise testing to assess muscle fiber type distribution and capillarity and peak Vo2. In HFPEF versus HC patients, peak Vo2 (14.7 ± 2.1 vs. 22.9 ± 6.6 ml·kg(-1)·min(-1), P < 0.001) and 6-min walk distance (454 ± 72 vs. 573 ± 71 m, P < 0.001) were reduced. In HFPEF versus HC patients, the percentage of type I fibers (39.0 ± 11.4% vs. 53.7 ± 12.4%, P < 0.001), type I-to-type II fiber ratio (0.72 ± 0.39 vs. 1.36 ± 0.85, P = 0.001), and capillary-to-fiber ratio (1.35 ± 0.32 vs. 2.53 ± 1.37, P = 0.006) were reduced, whereas the percentage of type II fibers was greater (61 ± 11.4% vs. 46.3 ± 12.4%, P < 0.001). In univariate analyses, the percentage of type I fibers (r = 0.39, P = 0.003), type I-to-type II fiber ratio (r = 0.33, P = 0.02), and capillary-to-fiber ratio (r = 0.59, P < 0.0001) were positively related to peak Vo2. In multivariate analyses, type I fibers and the capillary-to-fiber ratio remained significantly related to peak Vo2. We conclude that older HFPEF patients have significant abnormalities in skeletal muscle, characterized by a shift in muscle fiber type distribution with reduced type I oxidative muscle fibers and a reduced capillary-to-fiber ratio, and these may contribute to their severe exercise intolerance. This suggests potential new therapeutic targets in this difficult to treat disorder.

Myofilament protein alterations promote physical disability in aging and disease

Skeletal muscle contractile function declines with age and age-associated diseases. Although muscle atrophy undoubtedly contributes to this decrease, recent findings suggest that reduced myofilament protein content and function also may participate. Based on these data, we propose that age- and disease-related alterations in myofilament proteins represent one molecular mechanism contributing to the development of physical disability.

Myofibrillar protein overdegradation in overweight patients with chronic heart failure: the relationship to serum potassium levels

OBJECTIVES

Muscle release of the amino acid 3-methyl-histidine (3MH) is a sensitive index of myofibrillar protein overdegradation (MPO). We hypothesized that patients with chronic heart failure (CHF) could have increased muscle release of 3MH, which in turn reflects MPO, and that serum electrolyte sodium (Na(+)) and potassium (K(+)) levels may be associated with this 3MH muscle release.

METHODS

Thirty-one overweight outpatients (body mass index, 27 ± 4.4 kg/m(2); 22 men and 9 women; age, 56 ± 8.7 y) with clinically stable CHF were studied. After a 24-hour meat-free diet and overnight fasting, patients underwent blood sampling from a cannulated arm vein (V) and concomitantly from the arterial artery (A) to determine plasma 3MH levels and to calculate the A-V difference. Serum levels of Na(+) and K(+) in the venous blood were determined, and the Na(+)/K(+) ratio was calculated. Ten healthy subjects who were matched for gender, age, and body mass index served as controls and underwent the same protocol as the patients with CHF.

RESULTS

The patient group had higher arterial (P = 0.02) and venous (P = 0.005) 3MH levels but a similar A-V 3MH difference (P = 0.28) as compared with the controls. Within the CHF group, 67.7% of patients released 3MH, which resulted in a negative A-V value (P < 0.02 as compared with controls). In patients with CHF, the A-V 3MH difference correlated positively with the serum K(+) level (r = 0.62; P = 0.0002) and negatively with Na(+)/K(+) ratio (r = -0.55; P = 0.002). No association was found between the A-V 3MH difference and the Na(+) level.

CONCLUSIONS

The study demonstrated the existence of MPO in resting overweight patients with CHF, thereby suggesting that low serum levels of K(+) may contribute to MPO.

Impact of angiotensin II on skeletal muscle metabolism and function in mice: contribution of IGF-1, Sirtuin-1 and PGC-1α

Activation of the renin-angiotensin-aldosterone system and increased levels of angiotensin II (Ang-II) occurs in numerous cardiovascular diseases such as chronic heart failure (CHF). Another hallmark in CHF is a reduced exercise tolerance with impaired skeletal muscle function. The aim of this study was to investigate in an animal model the impact of Ang-II on skeletal muscle function and concomitant molecular alterations. Mice were infused with Ang-II for 4 weeks. Subsequently, skeletal muscle function of the soleus muscle was assessed. Expression of selected proteins was quantified by qRT-PCR and Western blot. Infusion of Ang-II resulted in a 33% reduction of contractile force, despite a lack of changes in muscle weight. At the molecular level an increased expression of NAD(P)H oxidase and a reduced expression of Sirt1, PGC-1α and IGF-1 were noticed. No change was evident for the ubiquitin E3-ligases MuRF1 and MafBx and α-sarcomeric actin expression. Cytophotometrical analysis of the soleus muscle revealed a metabolic shift toward a glycolytic profile. This study provides direct evidence of Ang-II-mediated, metabolic deterioration of skeletal muscle function despite preserved muscle mass. One may speculate that the Ang-II-mediated loss of muscle force is due to an activation of NAD(P)H oxidase expression and a subsequent ROS-induced down regulation of IGF-1, PGC-1α and Sirt1.

Skeletal muscle protein metabolism in human heart failure

PURPOSE OF REVIEW

This review considers evidence that the clinical condition of heart failure alters skeletal muscle protein synthesis and/or breakdown to promote skeletal muscle wasting and functional decrements that ultimately contribute to the symptomology of the disease.

RECENT FINDINGS

Advanced HF is frequently accompanied by muscle atrophy and a cachectic phenotype. Protein metabolic derangements that promote this phenotype are understudied and poorly understood. Instead, most investigations have evaluated regulatory hormones/signaling pathways thought to be reflective of protein synthesis and breakdown. Several of these recent studies have provided exciting data suggesting that the dysfunctional myocardium releases catabolic agents that could promote the skeletal muscle myopathic phenotype either directly or through modulation of other regulatory systems (e.g., energy balance).

SUMMARY

Although our understanding of skeletal muscle atrophy and dysfunction in heart failure is limited, recent studies have provided clues about the nature and timing of protein metabolic dysfunction. More specifically, skeletal muscle protein metabolic derangements likely evolve during periods of disease-related stress (i.e., acute disease exacerbation and hospitalization) and potentially derive in part, from signals promoted in the damaged/dysfunctional myocardium. Despite these compelling studies, there is a surprising lack of data regarding the nature or timing of specific protein metabolic defects in heart failure.

Skeletal muscle molecular alterations precede whole-muscle dysfunction in NYHA Class II heart failure patients

Background

Heart failure (HF), a debilitating disease in a growing number of adults, exerts structural and neurohormonal changes in both cardiac and skeletal muscles. However, these alterations and their affected molecular pathways remain uncharacterized. Disease progression is known to transform skeletal muscle fiber composition by unknown mechanisms. In addition, perturbation of specific hormonal pathways, including those involving skeletal muscle insulin-like growth factor-1 (IGF-1) and insulin-like growth factor-binding protein-5 (IGFB-5) appears to occur, likely affecting muscle metabolism and regeneration. We hypothesized that changes in IGF-1 and IGFB-5 mRNA levels correlate with the transformation of single–skeletal muscle fiber myosin heavy chain isoforms early in disease progression, making these molecules valuable markers of skeletal muscle changes in heart failure.

Materials and methods

To investigate these molecules during “early” events in HF patients, we obtained skeletal muscle biopsies from New York Heart Association (NYHA) Class II HF patients and controls for molecular analyses of single fibers, and we also quantified isometric strength and muscle size.

Results

There were more (P < 0.05) single muscle fibers coexpressing two or more myosin heavy chains in the HF patients (30% ± 7%) compared to the control subjects (13% ± 2%). IGF-1 and IGFBP-5 expression was fivefold and 15-fold lower in patients with in HF compared to control subjects (P < 0.05), respectively. Strikingly, there was a correlation in IGF-1 expression and muscle cross-sectional area (P < 0.05) resulting in a decrease in whole-muscle quality (P < 0.05) in the HF patients, despite no significant decrease in isometric strength or whole-muscle size.

Conclusion

These data indicate that molecular alterations in myosin heavy chain isoforms, IGF-1, and IGFB-5 levels precede the gross morphological and functional deficits that have previously been associated with HF, and may be used as a predictor of functional outcome in patients.

Effects of testosterone on cardiomyocyte calcium homeostasis and contractile function in female rats

The role of testosterone (T) in the regulation of cardiovascular function in females is not well understood. Our goal was to examine the effect of T on cardiomyocyte biology by measuring sarcomere shortening/relaxation and intracellular calcium cycling in adult female Sprague-Dawley rats. The rats were divided into the following four groups: (1) sham operated; (2) ovariectomized (OVX); (3) OVX plus T; and (4) OVX + T plus an aromatase inhibitor (AI). The final group was added to rule out effects from bioconversion of T to oestradiol. Sarcomere/calcium dynamics were measured after 4 weeks at 2 and 6 Hz, then at 6 Hz following exposure to 300 nm isoprenaline. Additionally, the acute (i.e. non-genomic) effects of T were evaluated in sham-operated and OVX + T + AI rats. There were no group differences, nor was there evidence for an effect of T on frequency or isoprenaline response. Additionally, there were no findings to indicate an acute, non-genomic T effect. Moreover, the relative α- and β-myosin heavy chain isoform complement was unchanged by OVX or T replacement. Our results argue against acute or chronic effects of T on cardiomyocyte shortening dynamics, calcium cycling or myosin heavy chain expression, arguing against any direct effect of T on cardiomyocyte function in adult females.

Skeletal muscle mitochondrial density, gene expression, and enzyme activities in human heart failure: minimal effects of the disease and resistance training

Impaired skeletal muscle energetics could adversely affect physical and metabolic function in patients with heart failure (HF). The effect of HF on aspects of mitochondrial structure and function, independent of muscle disuse and other disease-related confounding factors, however, is unclear. Moreover, no study has evaluated whether resistance exercise training, a modality that increases functional capacity, might derive its benefits through modulation of mitochondrial structure and function. Thirteen HF patients and 14 age- and physical activity-matched controls were evaluated for skeletal muscle mitochondrial size/content, gene expression, and enzyme activity before and after an 18-wk resistance exercise-training program. At baseline, HF patients and controls had similar mitochondrial fractional areas, although HF patients had larger average mitochondrion size (P < 0.05) and a trend toward a reduced number of mitochondria (P ≤ 0.10). No differences in the expression of transcriptional regulators or cytochrome oxidase subunits or the activity of mitochondrial and cytosolic enzymes were noted. Relationships among transcriptional regulators suggested that networks controlling mitochondrial content and gene expression are intact. Resistance training increased (P < 0.01) mitochondrial transcription factor A expression in patients and controls, and this increase was related to improvements in muscle strength (P = 0.05). Training did not, however, alter mitochondrial size/content, enzyme activities, or expression of other transcriptional regulators. In conclusion, our results suggest that the HF syndrome has minimal effects on skeletal muscle mitochondrial biology when the confounding effects of muscle disuse and other disease-related factors are removed. Moreover, the beneficial effects of resistance training on physical function in HF patients and controls are likely not related to alterations in mitochondrial biology.

Resistance training alters skeletal muscle structure and function in human heart failure: effects at the tissue, cellular and molecular levels

Reduced skeletal muscle function in heart failure (HF) patients may be partially explained by altered myofilament protein content and function. Resistance training increases muscle function, although whether these improvements are achieved by correction of myofilament deficits is not known. To address this question, we examined 10 HF patients and 14 controls prior to and following an 18 week high-intensity resistance training programme. Evaluations of whole muscle size and strength, single muscle fibre size, ultrastructure and tension and myosin-actin cross-bridge mechanics and kinetics were performed. Training improved whole muscle isometric torque in both groups, although there were no alterations in whole muscle size or single fibre cross-sectional area or isometric tension.Unexpectedly, training reduced the myofibril fractional area of muscle fibres in both groups. This structural change manifested functionally as a reduction in the number of strongly bound myosin-actin cross-bridges during Ca²⁺ activation. When post-training single fibre tension data were corrected for the loss of myofibril fractional area, we observed an increase in tension with resistance training. Additionally, training corrected alterations in cross-bridge kinetics (e.g. myosin attachment time) in HF patients back to levels observed in untrained controls. Collectively, our results indicate that improvements in myofilament function in sedentary elderly with and without HF may contribute to increased whole muscle function with resistance training. More broadly, these data highlight novel cellular and molecular adaptations in muscle structure and function that contribute to the resistance-trained phenotype.

Effect of resistance training on physical disability in chronic heart failure

PURPOSE

Patients with chronic heart failure (CHF) report difficulty performing activities of daily living. To our knowledge, however, no study has directly measured performance in activities of daily living in these patients to systematically assess their level of physical disability. Moreover, the contribution of skeletal muscle weakness to physical disability in CHF remains unclear. Thus, we measured performance in activities of daily living in CHF patients and controls, its relationship to aerobic capacity and muscle strength, and the effect of resistance exercise training to improve muscle strength and physical disability.

METHODS

Patients and controls were assessed for performance in activities of daily living, self-reported physical function, peak aerobic capacity, body composition, and muscle strength before and after an 18-wk resistance training program. To remove the confounding effects of several disease-related factors (muscle disuse, hospitalization, acute illness), we recruited controls with similar activity levels as CHF patients and tested patients >6 months after any disease exacerbation/hospitalization.

RESULTS

Performance in activities of daily living was 30% lower (P < 0.05) in CHF patients versus controls and was related to both reduced aerobic capacity (P < 0.001) and muscle strength (P < 0.01). Moreover, resistance training improved (P < 0.05 to P < 0.001) physical function and muscle strength in patients and controls similarly, without altering aerobic capacity.

CONCLUSIONS

CHF patients are characterized by marked physical disability compared with age- and physical activity-matched controls, which is related to reduced aerobic capacity and muscle strength. CHF patients respond to resistance training with normal strength/functional adaptations. Our results support muscle weakness as a determinant of physical disability in CHF and show that interventions that increase muscle strength (resistance training) reduce physical disability.

Changes in metabolic profile and population of skeletal muscle fibers of mice overexpressing calsequestrin: influence of losartan

In heart failure, exertional fatigue of skeletal muscles can occur. A transgenic mouse overexpressing calsequestrin can be regarded as an animal model of heart failure. The aims of the present study were to investigate, whether at the time of cardiac failure the composition of fiber types of skeletal muscles was altered, what kind of alterations in glycolytic and oxidative enzyme activities occurred in different muscle fiber types and whether these were affected by the administration of the angiotensin II receptor blocker, losartan. Hemodynamic parameters were determined using a working heart preparation. Four groups of mice were investigated: wild-type (WT) mice and transgenic (TG) mice overexpressing calsequestrin, with and without losartan treatment. Enzyme activities were measured in homogenates of Rectus femoris muscle and in muscle fibers, which were typed by their metabolic profile. Calcineurin expression was measured by Western blotting. Succinate dehydrogenase activity was increased by 275% in R. femoris muscle homogenates of TG compared to WT mice. This was due to a 57% increase in slow oxidative fibers, which was accompanied by an increased calcineurin expression in TG muscles. This increase was attenuated by losartan treatment. With respect to glycerol-3-phosphate-dehydrogenase (GPDH), no difference was evident comparing WT and TG. Treatment with losartan resulted in a down-regulation of GPDH in WT and TG. In conclusion, changes in skeletal muscles occur in this mouse model of heart failure and these changes were antagonized by losartan. In contrast to heart failure patients, in the mouse model a shift to the oxidative phenotype of skeletal muscle was noted, possibly due to enhanced calcineurin expression.

Chronic heart failure reduces Akt phosphorylation in human skeletal muscle: relationship to muscle size and function

Patients with chronic heart failure (HF) frequently lose muscle mass and function during the course of the disease. A reduction in anabolic stimuli to the muscle has been put forth as a potential mechanism underlying these alterations. The present study examined the hypothesis that skeletal muscle tissue from HF patients would show reduced IGF-1 expression and phosphorylation of signaling molecules downstream of receptor activation. To isolate the unique effect of HF on these variables, we limited the confounding effects of muscle disuse and/or acute disease exacerbation by recruiting controls (n = 11) with similar physical activity levels as HF patients (n = 11) and by testing patients at least 6 mo following any bouts of disease exacerbation/hospitalization. IGF-1 expression in skeletal muscle was similar between patients and controls. Despite this, HF patients were characterized by reduced levels of phospho-Akt/Akt (S473; -43%; P < 0.05), whereas no differences were found in total Akt protein content or phospho- or total protein content of mammalian target of rapamycin (mTOR; S2448), glycogen synthase kinase-3β (GSK-3β; S9), eukaryotic translation initiation factor 4E binding protein-1 (eIF4E-BP; T37/46), p70 ribosomal S6 kinase (p70 S6K; T389), or eIF2Bε (S540). Reduced phospho-Akt/Akt levels and phospho-mTOR/mTOR were related to decreased skeletal muscle myosin protein content (r = 0.602; P < 0.02) and knee extensor isometric torque (r = 0.550; P < 0.05), respectively. Because patients and controls were similar for age, muscle mass, and physical activity, we ascribe the observed alterations in Akt phosphorylation and its relationship to myosin protein content to the unique effects of the HF syndrome.

Chronic heart failure decreases cross-bridge kinetics in single skeletal muscle fibres from humans

Skeletal muscle function is impaired in heart failure patients due, in part, to loss of myofibrillar protein content, in particular myosin. In the present study, we utilized small-amplitude sinusoidal analysis for the first time in single human skeletal muscle fibres to measure muscle mechanics, including cross-bridge kinetics, to determine if heart failure further impairs contractile performance by altering myofibrillar protein function. Patients with chronic heart failure (n = 9) and controls (n = 6) were recruited of similar age and physical activity to diminish the potentially confounding effects of ageing and muscle disuse. Patients showed decreased cross-bridge kinetics in myosin heavy chain (MHC) I and IIA fibres, partially due to increased myosin attachment time (t(on)). The increased t(on) compensated for myosin protein loss previously found in heart failure patients by increasing the fraction of the total cycle time myosin is bound to actin, resulting in a similar number of strongly bound cross-bridges in patients and controls. Accordingly, isometric tension did not differ between patients and controls in MHC I or IIA fibres. Patients also had decreased calcium sensitivity in MHC IIA fibres and alterations in the viscoelastic properties of the lattice structure of MHC I and IIA fibres. Collectively, these results show that heart failure alters skeletal muscle contraction at the level of the myosin-actin cross-bridge, leading to changes in muscle mechanics which could contribute to impaired muscle function. Additionally, we uncovered a unique kinetic property of MHC I fibres, a potential indication of two distinct populations of cross-bridges, which may have important physiological consequences.

Impaired muscle protein anabolic response to insulin and amino acids in heart failure patients: relationship with markers of immune activation

Patients with chronic HF (heart failure) experience muscle atrophy during the course of the disease. The mechanisms underlying muscle atrophy in HF, however, are not understood. Thus we evaluated leg phenylalanine balance and kinetics in HF patients and controls following a brief fast (24 h) and under euglycaemic-hyperinsulinaemic-hyperaminoacidaemic conditions to determine whether HF increases muscle protein catabolism in response to nutritional deprivation and/or diminishes the anabolic response to meal-related stimuli (insulin and amino acids) and whether alterations in protein metabolism correlate to circulating cytokine levels. No differences in phenylalanine balance, rate of appearance or rate of disappearance were found between patients and controls under fasting conditions. However, the anabolic response to hyperinsulinaemia-hyperaminoacidaemia was reduced by more than 50% in patients compared with controls. The diminished anabolic response was due to reduced suppression of the leg phenylalanine appearance rate, an index of protein breakdown, in HF patients; whereas no group difference was found in the increase in the leg phenylalanine disappearance rate, an index of protein synthesis. The diminished responses of both phenylalanine balance and appearance rate to hyperinsulinaemia-hyperaminoacidaemia were related to greater circulating IL-6 (interleukin-6) levels. Our results suggest that, following a brief period of nutritional deprivation, HF patients demonstrate an impaired muscle protein anabolic response to meal-related stimuli, due to an inability to suppress muscle proteolysis, and that this diminished protein anabolic response correlates with markers of immune activation. The inability to stimulate muscle protein anabolism following periods of nutritional deficiency may contribute to muscle wasting in HF patients.

Comparative biomechanics of thick filaments and thin filaments with functional consequences for muscle contraction

The scaffold of striated muscle is predominantly comprised of myosin and actin polymers known as thick filaments and thin filaments, respectively. The roles these filaments play in muscle contraction are well known, but the extent to which variations in filament mechanical properties influence muscle function is not fully understood. Here we review information on the material properties of thick filaments, thin filaments, and their primary constituents; we also discuss ways in which mechanical properties of filaments impact muscle performance.

Mechanisms underlying skeletal muscle weakness in human heart failure: alterations in single fiber myosin protein content and function

BACKGROUND

Patients with chronic heart failure (HF) frequently experience skeletal muscle weakness that limits physical function. The mechanisms underlying muscle weakness, however, have not been clearly defined.

METHODS AND RESULTS

This study examined the hypothesis that HF promotes a loss of myosin protein from single skeletal muscle fibers, which in turn reduces contractile performance. Ten patients with chronic HF and 10 controls were studied. Muscle atrophy was not evident in patients, and groups displayed similar physical activity levels, suggesting that observed differences reflect the effects of HF and not muscle atrophy or disuse. In single muscle fibers, patients with HF showed reduced myosin heavy chain protein content (P<0.05) that manifested as a reduction in functional myosin-actin cross-bridges (P<0.05). No evidence was found for a generalized loss of myofilament protein, suggesting a selective loss of myosin. Accordingly, single muscle fiber maximal Ca(2+)-activated tension was reduced in myosin heavy chain I fibers in patients (P<0.05). However, tension was maintained in myosin heavy chain IIA fibers in patients because a greater proportion of available myosin heads were bound to actin during Ca(2+) activation (P<0.01).

CONCLUSIONS

Collectively, our results show that HF alters the quantity and functionality of the myosin molecule in skeletal muscle, leading to reduced tension in myosin heavy chain I fibers. Loss of single fiber myosin protein content represents a potential molecular mechanism underlying muscle weakness and exercise limitation in patients with HF.

Reduced knee extensor function in heart failure is not explained by inactivity

BACKGROUND

The goal of this study was to determine if heart failure alters knee extensor muscle torque, power production or contractile velocity.

METHODS

Heart failure patients (n=11; 70.4±4.3 yrs) and controls (n=11; 70.3±3.4 yrs) matched for age and sex were evaluated for knee extensor contractile performance under isometric and isokinetic conditions and body composition by dual energy X-ray absorptiometry. Additionally, we recruited sedentary to minimally active elderly controls to match heart failure patients for habitual physical activity and assessed activity levels using accelerometry.

RESULTS

Groups did not differ for total or regional body composition or average daily physical activity level. Despite similar muscle size and use, heart failure patients exhibited 21-29% lower (P<0.05 to P<0.01) isometric knee extensor torque throughout a range of knee angles, 15-33% lower (P=0.05 to P<0.01) peak concentric torque measured at various isokinetic speeds and corresponding reductions (P=0.05 to P<0.01) in peak power output. Expression of peak isokinetic torque data relative to isometric torque eliminated group differences, suggesting that impaired contractile function under dynamic conditions is explained by deficits in the force generating capacity of muscle. No group differences were found in the time required to reach target velocity during isokinetic contractions, an index of contractile velocity.

CONCLUSION

Because group differences in muscle torque were independent of age, sex, physical activity level and muscle size, our results suggest that muscle contractile dysfunction in these patients is likely attributable to the heart failure syndrome.

Skeletal muscle contractile protein function is preserved in human heart failure

Skeletal muscle weakness is a common finding in patients with chronic heart failure (CHF). This functional deficit cannot be accounted for by muscle atrophy alone, suggesting that the syndrome of heart failure induces a myopathy in the skeletal musculature. To determine whether decrements in muscle performance are related to alterations in contractile protein function, biopsies were obtained from the vastus lateralis muscle of four CHF patients and four control patients. CHF patients exhibited reduced peak aerobic capacity and knee extensor muscle strength. Decrements in whole muscle strength persisted after statistical control for muscle size. Thin filaments and myosin were isolated from biopsies and mechanically assessed using the in vitro motility assay. Isolated skeletal muscle thin-filament function, however, did not differ between CHF patients and controls with respect to unloaded shortening velocity, calcium sensitivity, or maximal force. Similarly, no difference in maximal force or unloaded shortening velocity of isolated myosin was observed between CHF patients and controls. From these results, we conclude that skeletal contractile protein function is unaltered in CHF patients. Other factors, such as a decrease in total muscle myosin content, are likely contributors to the skeletal muscle strength deficit of heart failure.

Smoking impairs muscle protein synthesis and increases the expression of myostatin and MAFbx in muscle

Smoking causes multiple organ dysfunction. The effect of smoking on skeletal muscle protein metabolism is unknown. We hypothesized that the rate of skeletal muscle protein synthesis is depressed in smokers compared with non-smokers. We studied eight smokers (> or =20 cigarettes/day for > or =20 years) and eight non-smokers matched for sex (4 men and 4 women per group), age (65 +/- 3 and 63 +/- 3 yr, respectively; means +/- SEM) and body mass index (25.9 +/- 0.9 and 25.1 +/- 1.2 kg/m(2), respectively). Each subject underwent an intravenous infusion of stable isotope-labeled leucine in conjunction with blood and muscle tissue sampling to measure the mixed muscle protein fractional synthesis rate (FSR) and whole body leucine rate of appearance (Ra) in plasma (an index of whole body proteolysis), the expression of genes involved in the regulation of muscle mass (myostatin, a muscle growth inhibitor, and MAFBx and MuRF-1, which encode E3 ubiquitin ligases in the proteasome proteolytic pathway) and that for the inflammatory cytokine TNF-alpha in muscle, and the concentration of inflammatory markers in plasma (C-reactive protein, TNF-alpha, interleukin-6) which are associated with muscle wasting in other conditions. There were no differences between nonsmokers and smokers in plasma leucine concentration, leucine rate of appearance, and plasma concentrations of inflammatory markers, or TNF-alpha mRNA in muscle, but muscle protein FSR was much less (0.037 +/- 0.005 vs. 0.059 +/- 0.005%/h, respectively, P = 0.004), and myostatin and MAFBx (but not MuRF-1) expression were much greater (by approximately 33 and 45%, respectivley, P < 0.05) in the muscle of smokers than of nonsmokers. We conclude that smoking impairs the muscle protein synthesis process and increases the expression of genes associated with impaired muscle maintenance; smoking therefore likely increases the risk of sarcopenia.

Oxygen delivery by blood determines the maximal VO2 and work rate during whole body exercise in humans: in silico studies

It has been proposed by Saltin (J Exp Biol 115: 345-354, 1985) that oxygen delivery by blood is limiting for maximal work and oxygen consumption in humans during whole body exercise but not during single-muscle exercise. To test this prediction quantitatively, we developed a static (steady-state) computer model of oxygen transport to and within human skeletal muscle during single-muscle (quadriceps) exercise and whole body (cycling) exercise. The main system fluxes, namely cardiac output and oxygen consumption by muscle, are described as a function of the "primary" parameter: work rate. The model is broadly validated by comparison of computer simulations with various experimental data. In silico studies show that, when all other parameters and system properties are kept constant, an increase in the working muscle mass from 2.5 kg (single quadriceps) to 15 kg (two legs) causes, at some critical work intensity, a drop in oxygen concentration in muscle cells to (very near) zero, and therefore oxygen supply by blood limits maximal oxygen consumption and oxidative ATP production. Therefore, the maximal oxygen consumption per muscle mass is significantly higher during single-muscle exercise than during whole body exercise. The effect is brought about by a distribution of a limited amount of oxygen transported by blood in a greater working muscle mass during whole body exercise.

Abnormalities of whole body protein turnover, muscle metabolism and levels of metabolic hormones in patients with chronic heart failure

OBJECTIVE

It is well known that chronic heart failure (CHF) is associated with insulin resistance and cachexia, but little is known about the underlying substrate metabolism. The present study was undertaken to identify disturbances of basal glucose, lipid and protein metabolism.

DESIGN

We studied eight nondiabetic patients with CHF (ejection fraction 30 +/- 4%) and eight healthy controls. Protein metabolism (whole body and regional muscle fluxes) and total glucose turnover were isotopically assayed. Substrate oxidation were obtained by indirect calorimetry. The metabolic response to exercise was studied by bicycle ergometry exercise.

RESULTS

Our data confirm that CHF patients have a decreased lean body mass. CHF patients are characterised by (i) decreased glucose oxidation [glucose oxidation (mg kg(-1) min(-1)): 1.25 +/- 0.09 (patients) vs. 1.55 +/- 0.09 (controls), P < 0.01] and muscle glucose uptake [a - v diff(glucose) (micromol L(-1)): -10 +/- 25 (patients) vs. 70 +/- 22 (controls), P < 0.01], (ii) elevated levels of free fatty acids (FFA) [FFA (mmol L(-1)): 0.72 +/- 0.05 (patients) vs. 0.48 +/- 0.03 (controls), P < 0.01] and 3-hydroxybutyrate and signs of elevated fat oxidation and muscle fat utilization [a - v diff(FFA) (mmol L(-1)): 0.12 +/- 0.02 (patients) vs. 0.05 +/- 0.01 (controls), P < 0.05] and (iii) elevated protein turnover and protein breakdown [phenylalanine flux (micromol kg(-1) h(-1)): 36.4 +/- 1.5 (patients) vs. 29.6 +/- 1.3 (controls), P < 0.01]. Patients had high circulating levels of noradrenaline, glucagon, and adiponectin, and low levels of ghrelin. We failed to observe any differences in metabolic responses between controls and patients during short-term exercise.

CONCLUSIONS

In the basal fasting state patients with CHF are characterized by several metabolic abnormalities which may contribute to CHF pathophysiology and may provide a basis for targeted intervention.

The effect of moderate exercise training on skeletal muscle myosin heavy chain distribution in chronic heart failure

BACKGROUND

The myosin heavy chain (MHC) is altered in chronic heart failure (CHF), but the effect of exercise on MHC expression in CHF patients is not understood. The aim of the present study was to show the effect of aerobic exercise on MHC distribution in patients with CHF.

METHODS

Patients (n=17) with stable NYHA class I-III CHF were randomised into training and control groups. For a period of three months, the training group cycled on an ergometric cycle 3 times a week for 30 min, the control group continued as they did previously. Both a baseline and a final 3 month graded maximal exercise test and exercise endurance test with constant submaximal work load were performed. Muscle samples, obtained from vastus lateralis muscle at baseline and after 3 months from the 8 patients in the training group and the 9 in the control group, were analysed for MHC distribution using SDS-polyacrylamide gel electrophoresis.

RESULTS

Baseline MHC distributions were similar in both groups and training did not alter the MHC distribution. Exercise duration, at constant submaximal work load, improved from 14.9+/-7.1 to 26.9+/-9.6 min (p<0.01 for the change between the groups). Training did not improve peak oxygen consumption.

CONCLUSION

No correlation between the change in exercise capacity and MHC distribution appeared despite the significant improvement of exercise duration.

Skeletal muscle myofibrillar mRNA expression in heart failure: relationship to local and circulating hormones

Chronic heart failure is characterized by changes in skeletal muscle that contribute to exercise intolerance and muscle weakness. To determine whether changes in the quantity and isoform distribution of key myofibrillar proteins are related to altered gene expression, we measured skeletal muscle myofibrillar mRNA abundance in nine heart failure patients (mean +/- SE; 63 +/- 3 yr) and nine controls (70 +/- 3 yr). In addition, we assessed the relationship of circulating levels of anabolic and catabolic hormones, as well as local expression of insulin-like growth factor (IGF)-I, to myofibrillar mRNA abundance. Heart failure patients were characterized by lower abundance of mRNA encoding the myosin heavy chain (MHC) I isoform (P < 0.01), whereas MHC IIa and MHC IIx mRNA did not differ between groups. Actin mRNA was also lower in heart failure patients compared with controls (P < 0.001). The expression of each MHC isoform transcript correlated with its respective protein product (MHC I: r = 0.656, P < 0.01; MHC IIa: r = 0.489, P < 0.05; MHC IIx: r = 0.505, P < 0.05; n = 18 for all). In addition to changes in myofibrillar transcripts, we found lower (P < 0.01) skeletal muscle IGF-1Ea mRNA content in heart failure patients. Myofibrillar mRNA levels were positively associated with skeletal muscle IGF-1Ea transcript levels (range of r values: 0.663-0.765; P values: <0.01 to <0.001) and modestly associated with circulating markers of immune activation (range of r values: -0.487 to -0.555; P values: <0.05 to <0.03). Our findings suggest that alterations in skeletal muscle MHC content and isoform distribution in heart failure may derive, in part, from changes in MHC gene expression. The relationships of myofibrillar mRNA content to both local and circulating hormones further suggest that alterations in the balance between anabolic and catabolic hormones in heart failure patients may influence skeletal muscle myofibrillar protein phenotype by altering gene expression.

Depression of force production and ATPase activity in different types of human skeletal muscle fibers from patients with chronic heart failure

Isometric force production and ATPase activity were determined simultaneously in single human skeletal muscle fibers (n = 97) from five healthy volunteers and nine patients with chronic heart failure (CHF) at 20 degrees C. The fibers were permeabilized by means of Triton X-100 (1% vol/vol). ATPase activity was determined by enzymatic coupling of ATP resynthesis to the oxidation of NADH. Calcium-activated actomyosin (AM) ATPase activity was obtained by subtracting the activity measured in relaxing (pCa = 9) solutions from that obtained in maximally activating (pCa = 4.4) solutions. Fiber type was determined on the basis of myosin heavy chain isoform composition by polyacrylamide SDS gel electrophoresis. AM ATPase activity per liter cell volume (+/-SE) in the control and patient group, respectively, amounted to 134 +/- 24 and 77 +/- 9 microM/s in type I fibers (n = 11 and 16), 248 +/- 17 and 188 +/- 13 microM/s in type IIA fibers (n = 14 and 32), 291 +/- 29 and 126 +/- 21 microM/s in type IIA/X fibers (n = 3 and 5), and 325 +/- 32 and 205 +/- 21 microM/s in type IIX fibers (n = 7 and 9). The maximal isometric force per cross-sectional area amounted to 64 +/- 7 and 43 +/- 5 kN/m(2) in type I fibers, 86 +/- 11 and 58 +/- 4 kN/m(2) in type IIA fibers, 85 +/- 6 and 42 +/- 9 kN/m(2) in type IIA/X fibers, and 90 +/- 5 and 59 +/- 5 kN/m(2) in type IIX fibers in the control and patient group, respectively. These results indicate that, in CHF patients, significant reductions occur in isometric force and AM ATPase activity but that tension cost for each fiber type remains the same. This suggests that, in skeletal muscle from CHF patients, a decline in density of contractile proteins takes place and/or a reduction in the rate of cross-bridge attachment of approximately 30%, which exacerbates skeletal muscle weakness due to muscle atrophy.