Skeletal Muscle Bioenergetics in Critical Limb Ischemia and Diabetes

INTRODUCTION

Mitochondrial dysfunction is implicated in the metabolic myopathy accompanying peripheral artery disease (PAD) and critical limb ischemia (CLI). Type-2 diabetes mellitus (T2DM) is a major risk factor for PAD development and progression to CLI and may also independently be related to mitochondrial dysfunction. We set out to determine the effect of T2DM in the relationship between CLI and muscle mitochondrial respiratory capacity and coupling control.

METHODS

We studied CLI patients undergoing revascularization procedures or amputation, and non-CLI patients with or without T2DM of similar age. Mitochondrial respiratory capacity and function were determined in lower limb permeabilized myofibers by high-resolution respirometry.

RESULTS

Fourteen CLI patients (65 ± 10y) were stratified into CLI patients with (n = 8) or without (n = 6) T2DM and were compared to non-CLI patients with (n = 18; 69 ± 5y) or without (n = 19; 71 ± 6y) T2DM. Presence of CLI but not T2DM had a marked impact on all mitochondrial respiratory states in skeletal muscle, adjusted for the effects of sex. Leak respiration (State 2, P < 0.025 and State 4o, P < 0.01), phosphorylating respiration (P < 0.001), and maximal respiration in the uncoupled state (P < 0.001), were all suppressed in CLI patients, independent of T2DM. T2DM had no significant effect on mitochondrial respiratory capacity and function in adults without CLI.

CONCLUSIONS

Skeletal muscle mitochondrial respiratory capacity was blunted by ∼35% in patients with CLI. T2DM was not associated with muscle oxidative capacity and did not moderate the relationship between muscle mitochondrial respiratory capacity and CLI.

Pathophysiological Aspects of Muscle Atrophy and Osteopenia Induced by Chronic Constriction Injury (CCI) of the Sciatic Nerve in Rats

Skeletal muscle atrophy is a condition characterized by a loss of muscle mass and muscle strength caused by an imbalance between protein synthesis and protein degradation. Muscle atrophy is often associated with a loss of bone mass manifesting as osteoporosis. The aim of this study was to evaluate if chronic constriction injury (CCI) of the sciatic nerve in rats can be a valid model to study muscle atrophy and consequent osteoporosis. Body weight and body composition were assessed weekly. Magnetic resonance imaging (MRI) was performed on day zero before ligation and day 28 before sacrifice. Catabolic markers were assessed via Western blot and Quantitative Real-time PCR. After the sacrifice, a morphological analysis of the gastrocnemius muscle and Micro-Computed Tomography (Micro-CT) on the tibia bone were performed. Rats that underwent CCI had a lower body weight increase on day 28 compared to the naive group of rats (p < 0.001). Increases in lean body mass and fat mass were also significantly lower in the CCI group (p < 0.001). The weight of skeletal muscles was found to be significantly lower in the ipsilateral hindlimb compared to that of contralateral muscles; furthermore, the cross-sectional area of muscle fibers decreased significantly in the ipsilateral gastrocnemius. The CCI of the sciatic nerve induced a statistically significant increase in autophagic and UPS (Ubiquitin Proteasome System) markers and a statistically significant increase in Pax-7 (Paired Box-7) expression. Micro-CT showed a statistically significant decrease in the bone parameters of the ipsilateral tibial bone. Chronic nerve constriction appeared to be a valid model for inducing the condition of muscle atrophy, also causing changes in bone microstructure and leading to osteoporosis. Therefore, sciatic nerve constriction could be a valid approach to study muscle-bone crosstalk and to identify new strategies to prevent osteosarcopenia.

Exosome-mediated regulatory mechanisms in skeletal muscle: a narrative review

Skeletal muscle plays a paramount role in physical activity, metabolism, and energy balance, while its homeostasis is being challenged by multiple unfavorable factors such as injury, aging, or obesity. Exosomes, a subset of extracellular vesicles, are now recognized as essential mediators of intercellular communication, holding great clinical potential in the treatment of skeletal muscle diseases. Herein, we outline the recent research progress in exosomal isolation, characterization, and mechanism of action, and emphatically discuss current advances in exosomes derived from multiple organs and tissues, and engineered exosomes regarding the regulation of physiological and pathological development of skeletal muscle. These remarkable advances expand our understanding of myogenesis and muscle diseases. Meanwhile, the engineered exosome, as an endogenous nanocarrier combined with advanced design methodologies of biomolecules, will help to open up innovative therapeutic perspectives for the treatment of muscle diseases.

[Pathohistological characteristics of muscles in patients with chronic lower limb ischemia]

Despite the widespread occurrence of ischemic diseases of the lower extremities, including atherosclerosis and diseases with an autoimmune component of their pathogenesis, the pathohistological signs of damage and concomitant chronic ischemia, compensatory tissue responses as intracellular and cellular regeneration remain out of the field of vision in researchers.

OBJECTIVE

To assess the signs of damage (the extent of necrosis and apoptosis, capillary density (CD)) and regeneration (the cross-sectional muscle fiber area (CSMFA), the proportion of centrinucleated muscle fibers (CNMFs), and that of connective tissue), by using the gastrocnemius medial head biopsy specimens obtained from patients with heterogeneous forms of chronic lower limb obliterating diseases (CLLODs).

SUBJECTS AND METHODS

The investigation included the biopsy specimens obtained from 44 men under 65 years of age (their mean age was 54±9.8 years) with Stage IIB-IV chronic limb ischemia (according to the A.V. Pokrovsky-Fontaine classification) with its history of at least six months. The nosological entities were atherosclerotic lesion in 33 patients (distal atherosclerosis n=13), multistage lesion (n=8), and Leriche's syndrome (n=12); autoimmune-mediated vascular injury in 11 patients (Buerger's disease (n=7) and nonspecific aortoarteritis (n=4)). The similar muscle fragments obtained during autopsy from the deceased without obvious signs of cardiovascular system diseases were examined as a control.

RESULTS

It was found that there was a statistically significant difference between the nosological entities, as compared to the control in terms of CD and CSMFA (a decrease), the proportion of CNMFs and that of connective tissue (an increase). No substantial differences were found in the studied parameters between the nosological entities.

CONCLUSION

The findings may suggest the universal mechanism for damage to striated muscle tissue because of circulatory hypoxia, regardless of its etiology and the common character of tissue compensatory-adaptive responses (regeneration).

Temporal Association Between Ischemic Muscle Perfusion Recovery and the Restoration of Muscle Contractile Function After Hindlimb Ischemia

During incomplete skeletal muscle recovery from ischemia, such as that occurs with critical limb ischemia, the temporal relationship between recovery of muscle capillary perfusion and contractile function is poorly defined. We examined this relationship in BALB/cJ mice (N = 24) following unilateral hindlimb ischemia (HLI), which pre-clinically mimics the myopathy observed in critical limb ischemia patients. Specifically, we examined this relationship in two phenotypically distinct muscles (i.e., "oxidative" soleus - Sol and "glycolytic" extensor digitorum longus - EDL) 14- or 56-days after HLI. Although overall limb blood flow (LDPI) reached its' recovery peak (48% of control) by HLI d14, the capillary networks in both the Sol and EDL (whole mount confocal imaging) were disrupted and competent muscle capillary perfusion (perfused lectin⁺μm²/muscle μm²) remained reduced. Interestingly, both Sol and EDL muscles recovered their distinct capillary structures and perfusion (Con Sol; 0.056 ± 0.02 lectin⁺μm²/muscle μm², and Con EDL; 0.039 ± 0.005 lectin⁺μm²/muscle μm²) by HLI d56 (Sol; 0.062 ± 0.011 lectin⁺μm²/muscle μm² and EDL; 0.0035 ± 0.005 lectin⁺μm²/muscle μm²), despite no further improvement in limb blood flow (LDPI). Both muscles suffered severe myopathy, indicated by loss of dystrophin positive immunostaining and the absence of stimulation induced isometric force production at HLI d14. Dystrophin immunofluorescence returned at HLI d56, although neither myofiber CSA (μm²) nor isometric force production (58 and 28% sustained deficits, Sol and EDL, respectively) recovered completely in either muscle. In summary, we reveal that the temporal relationship between the restoration of muscle capillary perfusion and functional ischemic skeletal muscle regeneration favors competent muscle capillary perfusion recovery in BALB/c mice in a phenotypically non-distinct manner.

The effects of RSR13 on microvascular Po2 kinetics and muscle contractile performance in the rat arterial ligation model of peripheral arterial disease

Exercise intolerance and claudication are symptomatic of peripheral arterial disease. There is a close relationship between muscle O2 delivery, microvascular oxygen partial pressure (P mvO2), and contractile performance. We therefore hypothesized that a reduction of hemoglobin-oxygen affinity via RSR13 would maintain a higher P mvO2 and enhance blood-muscle O2 transport and contractile function. In male Wistar rats (12 wk of age), we created hindlimb ischemia via right-side iliac artery ligation (AL). The contralateral (left) muscle served as control (CONT). Seven days after AL, phosphorescence-quenching techniques were used to measure P mvO2 at rest and during contractions (electrical stimulation; 1 Hz, 300 s) in tibialis anterior muscle (TA) under saline ( n = 10) or RSR13 ( n = 10) conditions. RSR13 at rest increased TA P mvO2 in CONT (13.9 ± 1.6 to 19.3 ± 1.9 Torr, P < 0.05) and AL (9.0 ± 0.5 to 9.9 ± 0.7 Torr, P < 0.05). Furthermore, RSR13 extended maintenance of the initial TA force (i.e., improved contractile performance) such that force was not decreased significantly until contraction 240 vs. 150 in CONT and 80 vs. 20 in AL. This improved muscle endurance with RSR13 was accompanied by a greater ΔP mvO2 (P mvO2 decrease from baseline) (CONT, 7.4 ± 1.0 to 11.2 ± 1.3; AL, 6.9 ± 0.5 to 8.6 ± 0.6 Torr, both P < 0.05). Whereas RSR13 did not alter the kinetics profile of P mvO2 (i.e., mean response time) substantially during contractions, muscle force was elevated, and the ratio of muscle force to P mvO2 increased. In conclusion, reduction of hemoglobin-oxygen affinity via RSR13 in AL increased P mvO2 and improved muscle contractile performance most likely via enhanced blood-muscle O2 diffusion.

NEW & NOTEWORTHY This is the first investigation to examine the effect of RSR13 (erythrocyte allosteric effector) on skeletal muscle microvascular oxygen partial pressure kinetics and contractile function using an arterial ligation model of peripheral arterial disease in experimental animals. The present results provide strong support for the concept that reducing hemoglobin-O2 affinity via RSR13 improved tibialis anterior muscle contractile performance most likely via enhanced blood-muscle O2 diffusion.

Muscle strength and control characteristics are altered by peripheral artery disease

OBJECTIVE

Peripheral artery disease (PAD), a common manifestation of atherosclerosis, is characterized by lower leg ischemia and myopathy in association with leg dysfunction. Patients with PAD have impaired gait from the first step they take with consistent defects in the movement around the ankle joint, especially in plantar flexion. Our goal was to develop muscle strength profiles to better understand the problems in motor control responsible for the walking impairment in patients with PAD.

METHODS

Ninety-four claudicating PAD patients performed maximal isometric plantar flexion contractions lasting 10 seconds in two conditions: pain free (patient is well rested and has no claudication symptoms) and pain induced (patient has walked and has claudication symptoms). Sixteen matched healthy controls performed the pain-free condition only. Torque curves were analyzed for dependent variables of muscle strength and motor control. Independent t-tests were used to compare variables between groups, and dependent t-tests determined differences between conditions.

RESULTS

Patients with PAD had significantly reduced peak torque and area under the curve compared with controls. Measures of control differed between PAD conditions only. Load rate and linear region duration were greater in the pain condition. Time to peak torque was shorter in the pain condition.

CONCLUSIONS

This study conclusively demonstrates that the plantar flexor muscles of the PAD patient at baseline and without pain are weaker in patients with PAD compared with controls. With the onset of claudication pain, patients with PAD exhibit altered muscle control strategies and further strength deficits are manifest compared to baseline levels. The myopathy of PAD legs appears to have a central role in the functional deterioration of the calf muscles, as it is evident both before and after onset of ischemic pain.

Sagittal plane joint kinetics during stair ascent in patients with peripheral arterial disease and intermittent claudication

Stair negotiation poses a substantial physical demand on the musculoskeletal system and this challenging task can place individuals at risk of falls. Peripheral arterial disease (PAD) can cause intermittent claudication (IC) pain in the calf and results in altered gait mechanics during level walking. However, whether those with PAD-IC adopt alternate strategies to climb stairs is unknown. Twelve participants with PAD-IC (six bilateral and six unilateral) and 10 healthy controls were recruited and instructed to ascend a five-step staircase whilst 3D kinematic data of the lower-limbs were recorded synchronously with kinetic data from force plates embedded into the staircase on steps two and three. Limbs from the unilateral group and both limbs from the bilateral claudicants were categorised as claudicating (N=18), asymptomatic (N=6) and control (N=10). Claudicants walked more slowly than healthy controls (trend; P=<0.066). Both claudicating- and asymptomatic-limb groups had reduced propulsive GRF (P=0.025 and P=0.002, respectively) and vertical GRF (P=0.005 and P=0.001, respectively) compared to controls. The claudicating-limb group had a reduced knee extensor moment during forward continuance (P=0.060), ankle angular velocity at peak moment (P=0.039) and ankle power generation (P=0.055) compared to the controls. The slower gait speed, irrespective of laterality of symptoms, indicates functional capacity was determined by the limitations of the claudicating limb. Reduced ankle power generation and angular velocity (despite adequate plantarflexor moment) implies velocity-dependent limitations existed in the calf. The lack of notable compensatory strategies indicates reliance on an impaired muscle group to accomplish this potentially hazardous task, highlighting the importance of maintaining plantarflexor strength and power in those with PAD-IC.

Walking performance is positively correlated to calf muscle fiber size in peripheral artery disease subjects, but fibers show aberrant mitophagy: an observational study

BACKGROUND

Patients with lower extremity peripheral artery disease (PAD) have decreased mobility, which is not fully explained by impaired blood supply to the lower limb. Additionally, reports are conflicted regarding fiber type distribution patterns in PAD, but agree that skeletal muscle mitochondrial respiration is impaired.

METHODS

To test the hypothesis that reduced muscle fiber oxidative activity and type I distribution are negatively associated with walking performance in PAD, calf muscle biopsies from non-PAD (n = 7) and PAD participants (n = 26) were analyzed immunohistochemically for fiber type and size, oxidative activity, markers of autophagy, and capillary density. Data were analyzed using analysis of covariance.

RESULTS

There was a wide range in fiber type distribution among subjects with PAD (9-81 % type I fibers) that did not correlate with walking performance. However, mean type I fiber size correlated with 4-min normal- and fastest-paced walk velocity (r = 0.4940, P = 0.010 and r = 0.4944, P = 0.010, respectively). Although intensity of succinate dehydrogenase activity staining was consistent with fiber type, up to 17 % of oxidative fibers were devoid of mitochondria in their cores, and the core showed accumulation of the autophagic marker, LC3, which did not completely co-localize with LAMP2, a lysosome marker.

CONCLUSIONS

Calf muscle type I fiber size positively correlates with walking performance in PAD. Accumulation of LC3 and a lack of co-localization of LC3 with LAMP2 in the area depleted of mitochondria in PAD fibers suggests impaired clearance of damaged mitochondria, which may contribute to reduced muscle oxidative capacity. Further study is needed to determine whether defective mitophagy is associated with decline in function over time, and whether interventions aimed at preserving mitochondrial function and improving autophagy can improve walking performance in PAD.

Hypertrophy-Promoting Effects of Leucine Supplementation and Moderate Intensity Aerobic Exercise in Pre-Senescent Mice

Several studies have indicated a positive influence of leucine supplementation and aerobic training on the aging skeletal muscle signaling pathways that control muscle protein balance and muscle remodeling. However, the effect of a combined intervention requires further clarification. Thirteen month old CD-1^® mice were subjected to moderate aerobic exercise (45 min swimming per day with 3% body weight workload) and fed a chow diet with 5% leucine or 3.4% alanine for 8 weeks. Serum and plasma were prepared for glucose, urea nitrogen, insulin and amino acid profile analysis. The white gastrocnemius muscles were used for determination of muscle size and signaling proteins involved in protein synthesis and degradation. The results show that both 8 weeks of leucine supplementation and aerobic training elevated the activity of mTOR (mammalian target of rapamycin) and its downstream target p70S6K and 4E-BP1, inhibited the ubiquitin-proteasome system, and increased fiber cross-sectional area (CSA) in white gastrocnemius muscle. Moreover, leucine supplementation in combination with exercise demonstrated more significant effects, such as greater CSA, protein content and altered phosphorylation (suggestive of increased activity) of protein synthesis signaling proteins, in addition to lower expression of proteins involved in protein degradation compared to leucine or exercise alone. The current study shows moderate aerobic training combined with 5% leucine supplementation has the potential to increase muscle size in fast-twitch skeletal muscle during aging, potentially through increased protein synthesis and decreased protein breakdown.

The impact of peripheral arterial disease: A proposal for a new classification

Clasically, intermittent claudication, an intermediate stage in peripheral arterial disease, has been considered as a benign condition when considering only the muscular pain on walking. In this paper our aim is to attract attention about the effects linked to ischemic pain and the oxidative injury resulting from episodes of ischemia/reperfusion. Throughout this process alterations in calcium homeostasis as well as uncontrolled generation of reactive oxygen species, in association with the mitochondrial dysfunction and inflammatory phenomena, could lead to accelerate atherosclerosis, with an increased cardiovascular risk stated by means of a reduced ankle-brachial index. Taking this idea into account we propose a possible new classification for the management of the peripheral arterial disease, combining the Fontaine and Rutherford classifications and thinking about the described systemic effects in order to change the traditional management of peripheral arterial disease.

Pathogenesis of the limb manifestations and exercise limitations in peripheral artery disease

Patients with peripheral artery disease have a marked reduction in exercise performance and daily ambulatory activity irrespective of their limb symptoms of classic or atypical claudication. This review will evaluate the multiple pathophysiologic mechanisms underlying the exercise impairment in peripheral artery disease based on an evaluation of the current literature and research performed by the authors. Peripheral artery disease results in atherosclerotic obstructions in the major conduit arteries supplying the lower extremities. This arterial disease process impairs the supply of oxygen and metabolic substrates needed to match the metabolic demand generated by active skeletal muscle during walking exercise. However, the hemodynamic impairment associated with the occlusive disease process does not fully account for the reduced exercise impairment, indicating that additional pathophysiologic mechanisms contribute to the limb manifestations. These mechanisms include a cascade of pathophysiological responses during exercise-induced ischemia and reperfusion at rest that are associated with endothelial dysfunction, oxidant stress, inflammation, and muscle metabolic abnormalities that provide opportunities for targeted therapeutic interventions to address the complex pathophysiology of the exercise impairment in peripheral artery disease.

Altered Ca(2+) signaling in skeletal muscle fibers of the R6/2 mouse, a model of Huntington's disease

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat within the gene encoding the protein huntingtin. The resulting elongated glutamine (poly-Q) sequence of mutant huntingtin (mhtt) affects both central neurons and skeletal muscle. Recent reports suggest that ryanodine receptor-based Ca(2+) signaling, which is crucial for skeletal muscle excitation-contraction coupling (ECC), is changed by mhtt in HD neurons. Consequently, we searched for alterations of ECC in muscle fibers of the R6/2 mouse, a mouse model of HD. We performed fluorometric recordings of action potentials (APs) and cellular Ca(2+) transients on intact isolated toe muscle fibers (musculi interossei), and measured L-type Ca(2+) inward currents on internally dialyzed fibers under voltage-clamp conditions. Both APs and AP-triggered Ca(2+) transients showed slower kinetics in R6/2 fibers than in fibers from wild-type mice. Ca(2+) removal from the myoplasm and Ca(2+) release flux from the sarcoplasmic reticulum were characterized using a Ca(2+) binding and transport model, which indicated a significant reduction in slow Ca(2+) removal activity and Ca(2+) release flux both after APs and under voltage-clamp conditions. In addition, the voltage-clamp experiments showed a highly significant decrease in L-type Ca(2+) channel conductance. These results indicate profound changes of Ca(2+) turnover in skeletal muscle of R6/2 mice and suggest that these changes may be associated with muscle pathology in HD.

Dynamic muscle quality of the plantar flexors is impaired in claudicant patients with peripheral arterial disease and associated with poorer walking endurance

OBJECTIVE

Peripheral arterial disease and intermittent claudication (PAD-IC) negatively affects physical activity and function. There is evidence for plantarflexor muscle dysfunction and weakness; however, the extent to which this dysfunction can be attributed to reduced muscle size or quality, or both, is not yet known. This study investigated whether in vivo plantarflexor muscle quality during static and dynamic contractions is altered by PAD-IC and whether such changes are associated with impaired walking endurance according to initial and absolute claudication distances.

METHODS

The study recruited 22 participants, consisting of 10 healthy controls and 12 claudicant patients with occlusion of the superficial femoral artery (seven unilateral and five bilateral). Muscle quality of the combined gastrocnemius muscles during static contractions was calculated by normalizing the estimated maximal potential muscle force to the physiological cross-sectional area of the lateral and medial gastrocnemius. Muscle quality during dynamic contractions of the combined plantarflexor muscles was calculated as the ratio of peak voluntary concentric plantarflexor power and the summed volume of lateral and medial gastrocnemius.

RESULTS

Dynamic muscle quality was 24% lower in the claudicating-limb and asymptomatic-limb groups compared with controls (P = .017 and P = .023). The differences were most apparent at the highest contraction velocity (180°/s). Dynamic muscle quality was associated with reduced walking endurance (R = 0.689, P = .006 and R = 0.550, P = .042 for initial and absolute claudication distance, respectively). The claudicating-limb group demonstrated a trend toward reduced static muscle quality compared with controls (22%, P = .084). The relative contribution of the soleus muscle to plantarflexion maximum voluntary contraction was significantly higher in the claudicating-limb and asymptomatic-limb groups than in controls (P = .012 and P = .018).

CONCLUSIONS

The muscle strength of the plantarflexors in those with PAD-IC appears to be impaired at high contraction velocities. This may be explained by some reduction in gastrocnemii muscle quality and a greater reliance on the prominently type I-fibered soleus muscle. The reduced dynamic capability of the plantarflexor muscles was associated with disease severity and walking ability; therefore, efforts to improve plantarflexor power through dynamic exercise intervention are vital to maintain functional performance.

Protein concentration and mitochondrial content in the gastrocnemius predicts mortality rates in patients with peripheral arterial disease

OBJECTIVE

This study evaluated the hypothesis that protein concentration and mitochondrial content in gastrocnemius biopsies from patients with peripheral arterial disease (PAD) predict mortality rates.

BACKGROUND

PAD patients experience advancing myopathy characterized by mitochondrial dysfunction, myofiber degradation, and fibrosis in their ischemic legs, along with increased mortality rates.

METHODS

Samples from the gastrocnemius of PAD patients were used for all analyses. Protein concentration was normalized to muscle wet weight, and citrate synthase activity (standard measure of mitochondrial content in cells) was normalized to muscle wet weight and protein concentration. Protein and citrate synthase data were grouped into tertiles and 5-year, all-cause mortality for each tertile was determined with Kaplan-Meier curves and compared by the modified Peto-Peto test. A Cox-regression model for each variable controlled for the effects of clinical characteristics.

RESULTS

Of the 187 study participants, 46 died during a mean follow-up of 23.0 months. Five-year mortality rate was highest for patients in the lowest tertile of protein concentration. Mortality was lowest for patients in the middle tertile of citrate synthase activity when normalized to either muscle wet weight or protein concentration. The mortality hazard ratios (HRs) from the Cox analysis were statistically significant for protein concentration normalized to muscle wet weight (lowest vs middle tertile; HR = 2.93; P = 0.008) and citrate synthase normalized to protein concentration (lowest vs middle tertile; HR = 4.68; P = 0.003; and lowest vs highest tertile; HR = 2.36; P = 0.027).

CONCLUSIONS

Survival analysis of a contemporaneous population of PAD patients identifies protein and mitochondrial content of their gastrocnemius as predictors of mortality rate.

Peripheral arterial disease affects the frequency response of ground reaction forces during walking

BACKGROUND

Walking is problematic for patients with peripheral arterial disease. The purpose of this study was to investigate the frequency domain of the ground reaction forces during walking to further elucidate the ambulatory impairment of these patients.

METHODS

Nineteen bilateral peripheral arterial disease patients and nineteen controls were included in this study. Subjects were matched for age and gait speed. Participants walked over a force plate sampling at 600 Hz. PAD patients were tested before (pain-free condition) after the onset of claudication symptoms (pain). We calculated median frequency, frequency bandwidth, and frequency containing 99.5% of the signal for the vertical and anterior-posterior ground reaction forces.

FINDINGS

Our results showed reduced median frequency in the vertical and anterior-posterior components of the ground reaction forces between the control group and both peripheral arterial disease conditions. We found reduced frequency bandwidth in the anterior-posterior direction between controls and the peripheral arterial disease pain-free condition. There were no differences in median frequency or bandwidth between peripheral arterial disease pain-free and pain conditions, but an increase in the frequency content for 99.5% of the signal was observed in the pain condition.

INTERPRETATION

Reduced frequency phenomena during gait in peripheral arterial disease patients compared to velocity-matched controls suggests more sluggish activity within the neuromotor system. Increased frequency phenomena due to pain in these patients suggest a more erratic application of propulsive forces when walking. Frequency domain analysis thus offers new insights into the gait impairments associated with this patient population.

Molecular imaging of angiogenic therapy in peripheral vascular disease with alphanubeta3-integrin-targeted nanoparticles

Noninvasive molecular imaging of angiogenesis could play a critical role in the clinical management of peripheral vascular disease patients. The alpha(nu)beta(3)-integrin, a well-established biomarker of neovascular proliferation, is an ideal target for molecular imaging of angiogenesis. This study investigates whether MR molecular imaging with alpha(nu)beta(3)-integrin-targeted perfluorocarbon nanoparticles can detect the neovascular response to angiogenic therapy. Hypercholesterolemic rabbits underwent femoral artery ligation followed by no treatment or angiogenic therapy with dietary L-arginine. MR molecular imaging performed 10 days after vessel ligation revealed increased signal enhancement in L-arginine-treated animals compared to controls. Furthermore, specifically targeted nanoparticles produced two times higher MRI signal enhancement compared to nontargeted particles, demonstrating improved identification of angiogenic vasculature with biomarker targeting. X-ray angiography performed 40 days postligation revealed that L-arginine treatment increased the development of collateral vessels. Histologic staining of muscle capillaries revealed a denser pattern of microvasculature in L-arginine-treated animals, confirming the MR and X-ray imaging results. The clinical application of noninvasive molecular imaging of angiogenesis could lead to earlier and more accurate detection of therapeutic response in peripheral vascular disease patients, enabling individualized optimization for a variety of treatment strategies.

Chronically ischemic mouse skeletal muscle exhibits myopathy in association with mitochondrial dysfunction and oxidative damage

A myopathy characterized by mitochondrial pathology and oxidative stress is present in patients with peripheral arterial disease (PAD). Patients with PAD differ in disease severity, mode of presentation, and presence of comorbid conditions. In this study, we used a mouse model of hindlimb ischemia to isolate and directly investigate the effects of chronic inflow arterial occlusion on skeletal muscle microanatomy, mitochondrial function and expression, and oxidative stress. Hindlimb ischemia was induced by staged ligation/division of the common femoral and iliac arteries in C57BL/6 mice, and muscles were harvested 12 wk later. Muscle microanatomy was examined by bright-field microscopy, and mitochondrial content was determined as citrate synthase activity in muscle homogenates and ATP synthase expression by fluorescence microscopy. Electron transport chain (ETC) complexes I through IV were analyzed individually by respirometry. Oxidative stress was assessed as total protein carbonyls and 4-hydroxy-2-nonenal (HNE) adducts and altered expression and activity of manganese superoxide dismutase (MnSOD). Ischemic muscle exhibited histological features of myopathy and increased mitochondrial content compared with control muscle. Complex-dependent respiration was significantly reduced for ETC complexes I, III, and IV in ischemic muscle. Protein carbonyls, HNE adducts, and MnSOD expression were significantly increased in ischemic muscle. MnSOD activity was not significantly changed, suggesting MnSOD inactivation. Using a mouse model, we have demonstrated for the first time that inflow arterial occlusion alone, i.e., in the absence of other comorbid conditions, causes myopathy with mitochondrial dysfunction and increased oxidative stress, recapitulating the muscle pathology of PAD patients.

The myopathy of peripheral arterial occlusive disease: Part 2. Oxidative stress, neuropathy, and shift in muscle fiber type

In recent years, an increasing number of studies have demonstrated that a myopathy is present, contributes, and, to a certain extent, determines the pathogenesis of peripheral arterial occlusive disease. These works provide evidence that a state of repetitive cycles of exercise-induced ischemia followed by reperfusion at rest operates in patients with peripheral arterial occlusive disease and mediates a large number of structural and metabolic changes in the muscle, resulting in reduced strength and function. The key players in this process appear to be defective mitochondria that, through multilevel failure in their roles as energy, oxygen radical species, and apoptosis regulators, produce and sustain a progressive decline in muscle performance. In this 2-part review, the currently available evidence that characterizes the nature and mechanisms responsible for this myopathy is highlighted. In part 1, the functional and histomorphological characteristics of the myopathy were reviewed, and the main focus was on the biochemistry and bioenergetics of its mitochondriopathy. In part 2, accumulating evidence that oxidative stress related to ischemia reperfusion is probably the major operating mechanism of peripheral arterial occlusive disease myopathy is reviewed. Important new findings of a possible neuropathy and a shift in muscle fiber type are also reviewed. Learning more about these mechanisms will enhance our understanding of the degree to which they are preventable and treatable.

Skeletal muscle phenotype is associated with exercise tolerance in patients with peripheral arterial disease

OBJECTIVE

To better understand the association between skeletal muscle and exercise intolerance in peripheral arterial disease (PAD), we assessed treadmill-walking performance and gastrocnemius muscle phenotype in healthy control subjects and in patients with PAD. We hypothesized that gastrocnemius muscle characteristics would be altered in PAD compared with control subjects and that exercise tolerance in patients PAD would be related to muscle phenotype.

METHODS

Sixteen patients with PAD and intermittent claudication and 13 healthy controls of the same age participated. Each subject completed a graded treadmill-walking test and underwent a resting muscle biopsy. Muscle biopsy samples were obtained from the medial gastrocnemius muscle of the most ischemic limb in PAD and a limb chosen at random in controls. Samples were analyzed for fiber type and cross-sectional area, capillary-to-fiber ratio, the number of capillaries in contact with each fiber type, and the optical density of glycogen within each fiber by using histochemical procedures. Total muscle glycogen content was determined biochemically.

RESULTS

Exercise capacity measured on the incremental walking test in the PAD group was only 30% to 40% of that observed in controls. The PAD group had a lower proportion of type I muscle fibers (P < .05), fewer capillaries per muscle fiber (P < .05), and tended to have smaller fiber areas (P = .08). The relative area of type I fibers, the capillary-to-fiber ratio, capillary contacts with type I and IIa fibers, and the optical density of glycogen in type I fibers were all positively correlated with exercise tolerance in the PAD group (P < .05) but not controls.

CONCLUSIONS

These data suggest that muscle phenotype is altered in PAD and that such alterations are associated with the exercise intolerance in these patients. In light of these findings, therapies such as resistance training or electrical stimulation that target skeletal muscle in PAD may prove beneficial, and further investigation of such therapies is warranted.

Vascular Endothelial Growth Factor Overexpression in Ischemic Skeletal Muscle Enhances Myoglobin Expression In Vivo

Therapeutic angiogenesis using vascular endothelial growth factor (VEGF) is considered a promising new therapy for patients with arterial obstructive disease. Clinical improvements observed consist of improved muscle function and regression of rest pain or angina. However, direct evidence for improved vascularization, as evaluated by angiography, is weak. In this study, we report an angiogenesis-independent effect of VEGF on ischemic skeletal muscle, ie, upregulation of myoglobin after VEGF treatment. Mice received intramuscular injection with adenoviral VEGF-A or either adenoviral LacZ or PBS as control, followed by surgical induction of acute hindlimb ischemia at day 3. At day 6, capillary density was increased in calf muscle of Ad.VEGF-treated versus control mice ( P <0.01). However, angiographic score of collateral arteries was unchanged between Ad.VEGF-treated and control mice. More interestingly, an increase in myoglobin was observed in Ad.VEGF-treated mice. Active myoglobin was 1.5-fold increased in calf muscle of Ad.VEGF-treated mice ( P ≤0.01). In addition, the number of myoglobin-stained myofibers was 2.6-fold increased in Ad.VEGF-treated mice ( P =0.001). Furthermore, in ischemic muscle of 15 limb amputation patients, VEGF and myoglobin were coexpressed. Finally, in cultured C2C12 myotubes treated with rhVEGF, myoglobin mRNA was 2.8-fold raised as compared with PBS-treated cells ( P =0.02). This effect could be blocked with the VEGF receptor tyrosine kinase inhibitor SU5416. In conclusion, we show that VEGF upregulates myoglobin in ischemic muscle both in vitro and in vivo. Increased myoglobin expression in VEGF-treated muscle implies an improved muscle oxygenation, which may, at least partly, explain observed clinical improvements in VEGF-treated patients, in the absence of improved vascularization.

Different effects on human skeletal myosin heavy chain isoform expression: strength vs. combination training

Myosin heavy chain (MHC) isoform expression changes with physical training. This may be one of the mechanisms for muscular adaptation to exercise. We aimed to investigate the effects of different strength-training protocols on MHC isoform expression, bearing in mind that alpha- MHC(slow) (newly identified MHC isoform) mRNA may be upregulated in response to training. Twelve volunteers performed a 6-wk strength training with maximum contractions (Max group), and another 12 of similar age performed combination training of maximum contractions and ballistic and stretch-shortening movements (Combi group). Muscle samples were taken from triceps brachii before and after training. MHC isoform composition was determined by SDS-PAGE silver staining, and mRNA levels of MHC isoforms were determined by RT-PCR. In Max group, there was an increase in MHC(2A) (49.4 to 66.7%, P < 0.01) and a decrease in MHC(2X) (33.4 to 19.5%, P < 0.01) after training, although there was no significant change in MHC(slow). In Combi group, there was also an increase in MHC(2A) (47.7 to 62.7%, P < 0.05) and a decrease in MHC(slow) (18.2 to 9.2%, P < 0.05) but no significant change in MHC(2X). An upregulation of alpha-MHC(slow) mRNA was, therefore, found in both groups as a result of training. The strength training with maximum contractions led to a shift in MHC isoform composition from 2X to 2A, whereas the combined strength training produced an MHC isoform composition shift from slow to 2A.

HSP70 expression in skeletal muscle of patients with peripheral arterial occlusive disease

OBJECTIVES

heat shock protein (HSP70) has been studied in the ischaemic myocardium and proven to provide protection against ischaemia. However, HSP70 in ischaemic skeletal muscle in patients with peripheral arterial occlusive disease (PAOD) has not been reported.

METHODS

thirty-four patients with PAOD (Fontaine's criteria: stage II: 15; III: 9 and IV: 10, respectively) and ten non-PAOD controls were enrolled in the study. Calf muscle samples were taken. HSP70 was quantitated by SDS-PAGE using ultrasensitive silver staining with reference to a series of standard HSP70, and HSP70 mRNA was estimated using RT-PCR.

RESULTS

in comparison with the controls [median with range: 24.8 (14.1-35.6) ng in 2.5 microg total protein], HSP70 was increased significantly in PAOD [stage II: 93.1 (62.7-114.3); stage III: 110.1 (89.7-134.5) and stage IV: 77.4 (67.3-101.1)]. Similar results were obtained with HSP70 mRNA.

CONCLUSIONS

HSP70 is increased in the ischaemic skeletal muscle in patients with PAOD, and HSP70 expression is different with regard to clinical stages, and the upregulation of HSP70 mRNA implies that the expression of HSP70 seems to be regulated at transcriptional level.

Muscle fiber characteristics in patients with peripheral arterial disease

PURPOSE

There have been conflicting reports of muscle fiber type changes in patients with peripheral arterial disease (PAD). The purpose of this study was to examine the myosin heavy chain (MHC) expression as well as histochemical changes in the gastrocnemius muscle in patients with symptomatic PAD.

METHODS

Needle biopsy specimens were obtained from the medial gastrocnemius of 14 subjects with PAD (mean age (+/- SD), 69.7 +/- 4.8 yr) and eight activity-matched control subjects (mean age, 65.1 +/- 6.6 yr). Ankle-brachial index was assessed using Doppler ultrasound to determine the hemodynamic status of the patients, and maximal walking performance was determined during a graded treadmill test. Expression of MHC isoforms was determined by SDS-PAGE.

RESULTS

The proportion of MHC I was significantly smaller in PAD than in the controls (45.6 +/- 9.1% vs 58.8 +/- 15.0%). The proportion of MHC IIx was also larger in the subjects with PAD compared with the controls (22.9 +/- 9.1% vs 16.0 +/- 11.3%). In addition, there was a significant decrease in the cross-sectional area of the type I and type IIA fibers in the subjects with PAD as well as enhanced capillary density.

CONCLUSIONS

This study showed a significant modification in the expression of MHC isoforms and muscle fiber type in the gastrocnemius in patients with symptomatic PAD. These results suggest that muscle ischemia resulting from PAD is an important factor in causing the adaptations in the contractile apparatus of the muscle.

Mitochondrial function and oxygen supply in normal and in chronically ischemic muscle: a combined 31P magnetic resonance spectroscopy and near infrared spectroscopy study in vivo

PURPOSE

We used (31)P magnetic resonance spectroscopy (MRS) and near-infrared spectroscopy (NIRS) as a means of quantifying abnormalities in calf muscle oxygenation and adenosine triphosphate (ATP) turnover in peripheral vascular disease (PVD).

METHODS

Eleven male patients with PVD (mean age, 65 years; range, 55-76 years) and nine male control subjects of similar age were observed in a case-control study in vascular outpatients. Inclusion criteria were more than 6 months' calf claudication (median, 1.5 years; range, 0.6-18 years); proven femoropopliteal or iliofemoral occlusive or stenotic disease; maximum treadmill walking distance (2 km/h, 10 degrees gradient) of 50 to 230 m (mean, 112 m); ankle-brachial pressure index of 0.8 or less during exercise (mean, 0.47; range, 0.29-0.60). Exclusion criteria included diabetes mellitus, anemia, and magnet contraindications. Simultaneous (31)P MRS and NIRS of lateral gastrocnemius was conducted during 2 to 4 minutes of voluntary 0.5 Hz isometric plantarflexion at 50% and 75% maximum voluntary contraction force (MVC), followed by 5 minutes recovery. Each subject was studied three times, and the results were combined.

RESULTS

Compared with control subjects, patients with PVD showed (1) normal muscle cross-sectional area, MVC, ATP turnover, and contractile efficiency (ATP turnover per force/area); (2) larger phosphocreatine (PCr) changes during exercise (ie, increased shortfall of oxidative ATP synthesis) and slower PCr recovery (47% +/- 7% [mean +/- SEM] decrease in functional capacity for oxidative ATP synthesis, P = .001); (3) faster deoxygenation during exercise and slower postexercise reoxygenation (59% +/- 7% decrease in rate constant, P = .0009), despite reduced oxidative ATP synthesis; (4) correlation between PCr and NIRS recovery rate constants (P < .02); and (5) correlations between smaller walking distance, slower PCr recovery, and reduced MVC (P < .001). The precision of the key measurements (rate constants and contractile efficiency) was 12% to 18% interstudy and 30% to 40% intersubject.

CONCLUSION

The primary lesion in oxygen supply dominates muscle metabolism. Reduced force-generation in patients who are affected more may protect muscle from metabolic stress.

Invited review: Adaptive responses of skeletal muscle to intermittent hypoxia: the known and the unknown

Intermittent hypoxia (IH) describes conditions of repeated, transient reductions in O2 that may trigger unique adaptations. Rest periods during IH may avoid potentially detrimental effects of long-term O2 deprivation. For skeletal muscle, IH can occur in conditions of obstructive sleep apnea, transient altitude exposures (with or without exercise), intermittent claudication, cardiopulmonary resuscitation, neonatal blood flow obstruction, and diving responses of marine animals. Although it is likely that adaptations in these conditions vary, some patterns emerge. Low levels of hypoxia shift metabolic enzyme activity toward greater aerobic poise; extreme hypoxia shifts metabolism toward greater anaerobic potential. Some conditions of IH may also inhibit lactate release during exercise. Many related cellular phenomena could be involved in the response, including activation of specific O2 sensors, reactive oxygen and nitrogen species, preconditioning, hypoxia-induced transcription factors, regulation of ion channels, and influences of paracrine/hormonal stimuli. The net effect of a variety of adaptive programs to IH may be to preserve contractile function and cell integrity in hypoxia or anoxia, a response that does not always translate into improvements in exercise performance.