Calf muscle adaptation in intermittent claudication. Side-differences in muscle metabolic characteristics in patients with unilateral arterial disease

Impaired microcirculatory function, mitochondrial respiration, and oxygen utilization in skeletal muscle of claudicating patients with peripheral artery disease

Peripheral artery disease (PAD) is an atherosclerotic disease that impairs blood flow and muscle function in the lower limbs. A skeletal muscle myopathy characterized by mitochondrial dysfunction and oxidative damage is present in PAD; however, the underlying mechanisms are not well established. We investigated the impact of chronic ischemia on skeletal muscle microcirculatory function and its association with leg skeletal muscle mitochondrial function and oxygen delivery and utilization capacity in PAD. Gastrocnemius samples and arterioles were harvested from patients with PAD (n = 10) and age-matched controls (Con, n = 11). Endothelium-dependent and independent vasodilation was assessed in response to flow (30 μL·min-1), acetylcholine, and sodium nitroprusside (SNP). Skeletal muscle mitochondrial respiration was quantified by high-resolution respirometry, microvascular oxygen delivery, and utilization capacity (tissue oxygenation index, TOI) were assessed by near-infrared spectroscopy. Vasodilation was attenuated in PAD (P < 0.05) in response to acetylcholine (Con: 71.1 ± 11.1%, PAD: 45.7 ± 18.1%) and flow (Con: 46.6 ± 20.1%, PAD: 29.3 ± 10.5%) but not SNP (P = 0.30). Complex I + II state 3 respiration (P < 0.01) and TOI recovery rate were impaired in PAD (P < 0.05). Both flow and acetylcholine-mediated vasodilation were positively associated with complex I + II state 3 respiration (r = 0.5 and r = 0.5, respectively, P < 0.05). Flow-mediated vasodilation and complex I + II state 3 respiration were positively associated with TOI recovery rate (r = 0.8 and r = 0.7, respectively, P < 0.05). These findings suggest that chronic ischemia attenuates skeletal muscle arteriole endothelial function, which may be a key mediator for mitochondrial and microcirculatory dysfunction in the PAD leg skeletal muscle. Targeting microvascular dysfunction may be an effective strategy to prevent and/or reverse disease progression in PAD.NEW & NOTEWORTHY Ex vivo skeletal muscle arteriole endothelial function is impaired in claudicating patients with PAD, and this is associated with attenuated skeletal muscle mitochondrial respiration. In vivo skeletal muscle oxygen delivery and utilization capacity is compromised in PAD, and this may be due to microcirculatory and mitochondrial dysfunction. These results suggest that targeting skeletal muscle arteriole function may lead to improvements in skeletal muscle mitochondrial respiration and oxygen delivery and utilization capacity in claudicating patients with PAD.

Effects of fasting on isolated murine skeletal muscle contractile function during acute hypoxia

Stored muscle carbohydrate supply and energetic efficiency constrain muscle functional capacity during exercise and are influenced by common physiological variables (e.g. age, diet, and physical activity level). Whether these constraints affect overall functional capacity or the timing of muscle energetic failure during acute hypoxia is not known. We interrogated skeletal muscle contractile properties in two anatomically distinct rodent hindlimb muscles that have well characterized differences in energetic efficiency (locomotory- extensor digitorum longus (EDL) and postural- soleus muscles) following a 24 hour fasting period that resulted in substantially reduced muscle carbohydrate supply. 180 mins of acute hypoxia resulted in complete energetic failure in all muscles tested, indicated by: loss of force production, substantial reductions in total adenosine nucleotide pool intermediates, and increased adenosine nucleotide degradation product-inosine monophosphate (IMP). These changes occurred in the absence of apparent myofiber structural damage assessed histologically by both transverse section and whole mount. Fasting and the associated reduction of the available intracellular carbohydrate pool (~50% decrease in skeletal muscle) did not significantly alter the timing to muscle functional impairment or affect the overall force/work capacities of either muscle type. Fasting resulted in greater passive tension development in both muscle types, which may have implications for the design of pre-clinical studies involving optimal timing of reperfusion or administration of precision therapeutics.

Regeneration of Mitochondrial Function in Gastrocnemius Muscle in Peripheral Arterial Disease After Successful Revascularisation

OBJECTIVE

Myopathy, characterised by altered mitochondrial function, is a central part of the pathophysiology of peripheral arterial disease and the aim of this study was to investigate the effect of revascularisation on mitochondrial function.

METHODS

High resolution respirometry was used to investigate mitochondrial respiration and the results were normalised to citrate synthase activity (CSA), a marker of mitochondrial content. Ten patients with symptomatic peripheral arterial disease (study group) and 10 subjects without ischaemia (control group) were included. Ankle brachial index and ultrasound imaging were performed before and after vascular intervention to confirm technically successful revascularisation. Within the study group, muscle biopsies from the gastrocnemius muscle were taken before vascular intervention and six weeks after revascularisation. Within the control group, tissue was harvested once.

RESULTS

There were no significant group differences regarding anthropometric data. CSA showed a significant increase after successful revascularisation (CSA pre-operative 281.4 (252.4-391.8) nmol/min/mg protein vs. CSA post-operative 438.5 (361.4-471.3) nmol/min/mg protein; p = .01) with post-operative return of values to the range of control subjects (CSA control 396.6 (308.2-435.9)). Mitochondrial respiration normalised to CSA in oxidative phosphorylation (P) as well as in electron transfer (E) capacity were significantly reduced post-operatively when compared with pre-operative values (P pre-operative 0.218 (0.196-0.266) pmol/(sec×mg) per CSA vs. post-operative 0.132 (0.116-0.150) pmol/(sec×mg) per CSA, p = .007; E pre-operative 0.230 (0.195-0.279) pmol/(sec×mg) per CSA vs. post-operative 0.129 (0.120-0.154) pmol/(sec×mg) per CSA, p = .005) meaning a post-operative return of values to within the range of control subjects (P control 0.124 (0.080-0.155) pmol/(sec×mg) per CSA; E control 0.121 (0.079-0.125) pmol/(sec×mg) per CSA).

CONCLUSION

With these results, it has been shown that the initially impaired mitochondrial function and content can normalise after revascularisation.

Peripheral artery disease, calf skeletal muscle mitochondrial DNA copy number, and functional performance

In people without lower extremity peripheral artery disease (PAD), mitochondrial DNA copy number declines with aging, and this decline is associated with declines in mitochondrial activity and functional performance. However, whether lower extremity ischemia is associated with lower mitochondrial DNA copy number and whether mitochondrial DNA copy number is associated with the degree of functional impairment in people with PAD is unknown. In people with and without PAD, age 65 years and older, we studied associations of the ankle-brachial index (ABI) with mitochondrial DNA copy number and associations of mitochondrial DNA copy number with functional impairment. Calf muscle biopsies were obtained from 34 participants with PAD (mean age: 73.5 years (SD 6.4), mean ABI: 0.67 (SD 0.15), mean 6-minute walk distance: 1191 feet (SD 223)) and 10 controls without PAD (mean age: 73.1 years (SD 4.7), mean ABI: 1.14 (SD 0.07), mean 6-minute walk distance: 1387 feet (SD 488)). Adjusting for age and sex, lower ABI values were associated with higher mitochondrial DNA copy number, measured in relative copy number (ABI<0.60: 914, ABI 0.60-0.90: 731, ABI 0.90-1.50: 593; p trend=0.016). The association of mitochondrial DNA copy number with the 6-minute walk distance and 4-meter walking velocity differed significantly between participants with versus without PAD ( p-value for interaction=0.001 and p=0.015, respectively). The correlation coefficient between mitochondrial DNA copy number and the 6-minute walk distance was 0.653 ( p=0.056) among people without PAD and -0.254 ( p=0.154) among people with PAD and ABI < 0.90. In conclusion, lower ABI values are associated with increased mitochondrial DNA copy number. Associations of mitochondrial DNA copy number with the 6-minute walk distance and 4-meter walking velocity significantly differed between people with versus without PAD, with stronger positive associations observed in people without PAD than in people with PAD. The cross-sectional and exploratory nature of the analyses precludes conclusions regarding causal inferences. ClinicalTrials.gov Identifier: NCT02246660.

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.

A systematic review of muscle morphology and function in intermittent claudication

OBJECTIVE

Intermittent claudication (IC) is frequently associated with deterioration in walking capacity and physical function, and it can often result in an impairment in balance. Whereas supervised exercise is recommended by the National Institute for Health and Care Excellence as the first-line treatment, the mechanism behind walking improvement is poorly understood. The existing literature suggests that there may be some physiologic change to the skeletal muscle contributing to the functional impairment, but these data are conflicting. We therefore sought to undertake a systematic review to clarify the muscle properties of patients with IC.

METHODS

A systematic review of randomized and nonrandomized trials that investigated the role of muscle function in patients diagnosed with IC was undertaken using MEDLINE, Cochrane Central Register of Controlled Trials, and Embase databases. The searches were limited from 1947 to June 2016 in the English language.

RESULTS

The search yielded a total of 506 articles, of which 206 were duplicate articles. Of the remaining 300, a total of 201 were excluded from full-text analysis; 99 full-text articles were assessed for eligibility, with 30 articles deemed appropriate for inclusion in the review. There were four main categories of functional outcome measures: muscle strength, muscle size, muscle fiber type, and muscle metabolism. A total of 2837 patients were included in the study. Nine studies reported on muscle strength, incorporating isometric, concentric, eccentric, and endurance measures. Eight studies reported on muscle size, incorporating circumference, computed tomography scans, and ultrasound imaging techniques. Eleven studies reported on muscle fibers, incorporating fiber type proportions, fiber size, and capillarity measures. Seven papers reported on muscle metabolism, incorporating adenosine diphosphate recovery and phosphocreatine recovery measures.

CONCLUSIONS

Previous literature has found clear evidence that strength (of the calf and thigh musculature) and calf characteristics are related to mortality and functional declines. However, this review has demonstrated the vast array of muscle groups assessed and multiple methods employed to determine strength; therefore, it is unclear exactly what measure of "strength" is impaired. Furthermore, the underlying morphologic causes of potential changes in strength are unclear. This information is essential for designing optimal exercise interventions. The data acquired during this systematic review are heterogeneous, with a substantial lack of high-quality intervention-based studies. Future research should endeavor to establish standardized testing procedures and to implement randomized controlled trials for targeted therapeutic interventions.

Mitochondrial Bioenergetics in the Metabolic Myopathy Accompanying Peripheral Artery Disease

Peripheral artery disease (PAD) is a serious but relatively underdiagnosed and undertreated clinical condition associated with a marked reduction in functional capacity and a heightened risk of morbidity and mortality. The pathophysiology of lower extremity PAD is complex, and extends beyond the atherosclerotic arterial occlusion and subsequent mismatch between oxygen demand and delivery to skeletal muscle mitochondria. In this review, we evaluate and summarize the available evidence implicating mitochondria in the metabolic myopathy that accompanies PAD. Following a short discussion of the available in vivo and in vitro methodologies to quantitate indices of muscle mitochondrial function, we review the current evidence implicating skeletal muscle mitochondrial dysfunction in the pathophysiology of PAD myopathy, while attempting to highlight questions that remain unanswered. Given the rising prevalence of PAD, the detriment in quality of life for patients, and the associated significant healthcare resource utilization, new alternate therapies that ameliorate lower limb symptoms and the functional impairment associated with PAD are needed. A clear understanding of the role of mitochondria in the pathophysiology of PAD may contribute to the development of novel therapeutic interventions.

Skeletal muscle metabolic changes in peripheral arterial disease contribute to exercise intolerance: a point-counterpoint discussion

Patients with claudication have a marked impairment in exercise performance. Several factors contribute to this limitation, including reductions in large vessel blood flow and oxygen delivery as well as metabolic abnormalities in skeletal muscle. The relative contribution of these factors and their role in the pathophysiology of the exercise limitation is discussed using a point-counterpoint approach. Future directions for research conclude the discussion.

Capillary ultrastructure and mitochondrial volume density in skeletal muscle in relation to reduced exercise capacity of patients with intermittent claudication

Intermittent claudication (IC) is the most commonly reported symptom of peripheral arterial disease (PAD). Impaired limb blood flow is a major casual factor of lower exercise tolerance in PAD but cannot entirely explain it. We hypothesized that IC is associated with structural changes of the capillary-mitochondria interface that could contribute to the reduction of exercise tolerance in IC patients. Capillary and mitochondrial morphometry were performed after light and transmission electron microscopy using vastus lateralis muscle biopsies of 14 IC patients and 10 age-matched controls, and peak power output (PPO) was determined for all participants using an incremental single-leg knee-extension protocol. Capillary density was lower (411 ± 90 mm−2 vs. 506 ± 95 mm−2; P ≤ 0.05) in the biopsies of the IC patients than in those of the controls. The basement membrane (BM) around capillaries was thicker (543 ± 82 nm vs. 423 ± 97 nm; P ≤ 0.01) and the volume density of mitochondria was lower (3.51 ± 0.56% vs. 4.60 ± 0.74%; P ≤ 0.01) in the IC patients than the controls. In the IC patients, a higher proportion of capillaries appeared with collapsed slit-like lumen and/or swollen endothelium. PPO was lower (18.5 ± 9.9 W vs. 33.5 ± 9.4 W; P ≤ 0.01) in the IC patients than the controls. We suggest that several structural alterations in skeletal muscle, either collectively or separately, contribute to the reduction of exercise tolerance in IC patients.

Lower extremity manifestations of peripheral artery disease: the pathophysiologic and functional implications of leg ischemia

Lower extremity peripheral artery disease (PAD) is frequently underdiagnosed, in part because of the wide variety of leg symptoms manifested by patients with PAD and in part because of the high prevalence of asymptomatic PAD. In primary care medical practices, 30% to 60% of patients with PAD report no exertional leg symptoms and ≈45% to 50% report exertional leg symptoms that are not consistent with classic intermittent claudication. The prevalence and extent of functional impairment and functional decline in PAD may also be underappreciated. Functional impairment and functional decline are common in PAD, even among those who are asymptomatic. Lower extremity ischemia is also associated with pathophysiologic changes in calf skeletal muscle, including smaller calf muscle area, increased calf muscle fat content, impaired leg strength, and impaired metabolic function. People with severe PAD have poorer peroneal nerve conduction velocity compared with people with mild PAD or no PAD. The degree of ischemia-related pathophysiologic changes in lower extremity muscles and peripheral nerves of people with PAD are associated with the degree of functional impairment. New interventions are needed to improve functional performance and prevent mobility loss in the large number of patients with PAD, including in those who are asymptomatic or who have exertional leg symptoms other than claudication.

Reliability of contrast-enhanced ultrasound for the assessment of muscle perfusion in health and peripheral arterial disease

We investigated the reliability of contrast-enhanced ultrasound (CEUS) in assessing calf muscle microvascular perfusion in health and disease. Response to a post-occlusive reactive hyperaemia test was repeated on two occasions >48 h apart in healthy young (28 ± 7 y) and elderly controls (70 ± 5 y), and in peripheral arterial disease patients (PAD, 69 ± 7 y; n = 10, 9 and 8 respectively). Overall, within-individual reliability was poor (coefficient of variation [CV] range: 15-87%); the most reliable parameter was time to peak (TTP, 15-48% CV). Nevertheless, TTP was twice as long in elderly controls and PAD compared to young (19.3 ± 10.4 and 22.0 ± 8.6 vs. 8.9 ± 6.2 s respectively; p < 0.01), and area under the curve for contrast intensity post-occlusion (a reflection of blood volume) was ∼50% lower in elderly controls (p < 0.01 versus PAD and young). Thus, CEUS assessment of muscle perfusion during reactive hyperaemia demonstrated poor reliability, yet still distinguished differences between PAD patients, elderly and young controls.

Endothelial FoxO1 is an intrinsic regulator of thrombospondin 1 expression that restrains angiogenesis in ischemic muscle

Peripheral artery disease (PAD) is characterized by chronic muscle ischemia. Compensatory angiogenesis is minimal within ischemic muscle despite an increase in angiogenic factors. This may occur due to the prevalence of angiostatic factors. Regulatory mechanisms that could evoke an angiostatic environment during ischemia are largely unknown. Forkhead box O (FoxO) transcription factors, known to repress endothelial cell proliferation in vitro, are potential candidates. Our goal was to determine whether FoxO proteins promote an angiostatic phenotype within ischemic muscle. FoxO1 and the angiostatic matrix protein thrombospondin 1 (THBS1) were elevated in ischemic muscle from PAD patients, or from mice post-femoral artery ligation. Mice with conditional endothelial cell-directed deletion of FoxO proteins (Mx1Cre (+), FoxO1,3,4 (L/L) , referred to as FoxOΔ) were used to assess the role of endothelial FoxO proteins within ischemic tissue. FoxO deletion abrogated the elevation of FoxO1 and THBS1 proteins, enhanced hindlimb blood flow recovery and improved neovascularization in murine ischemic muscle. Endothelial cell outgrowth from 3D explant cultures was more robust in muscles derived from FoxOΔ mice. FoxO1 overexpression induced THBS1 production, and a direct interaction of endogenous FoxO1 with the THBS1 promoter was detectable in primary endothelial cells. We provide evidence that FoxO1 directly regulates THBS1 within ischemic muscle. Altogether, these findings bring novel insight into the regulatory mechanisms underlying the repression of angiogenesis within peripheral ischemic tissues.

Myocyte vascular endothelial growth factor is required for exercise-induced skeletal muscle angiogenesis

We have previously shown, using a Cre-LoxP strategy, that vascular endothelial growth factor (VEGF) is required for the development and maintenance of skeletal muscle capillarity in sedentary adult mice. To determine whether VEGF expression is required for skeletal muscle capillary adaptation to exercise training, gastrocnemius muscle capillarity was measured in myocyte-specific VEGF gene-deleted (mVEGF(-/-)) and wild-type (WT) littermate mice following 6 wk of treadmill running (1 h/day, 5 days/wk) at the same running speed. The effect of training on metabolic enzyme activity levels and whole body running performance was also evaluated in mVEGF(-/-) and WT mice. Posttraining capillary density was significantly increased by 59% (P < 0.05) in the deep muscle region of the gastrocnemius in WT mice but did not change in mVEGF(-/-) mice. Maximal running speed and time to exhaustion during submaximal running increased by 20 and 13% (P < 0.05), respectively, in WT mice after training but were unchanged in mVEGF(-/-) mice. Training led to increases in skeletal muscle citrate synthase (CS) and phosphofructokinase (PFK) activities in both WT and mVEGF(-/-) mice (P < 0.05), whereas β-hydroxyacyl-CoA dehydrogenase (β-HAD) activity was increased only in WT mice. These data demonstrate that skeletal muscle capillary adaptation to physical training does not occur in the absence of myocyte-expressed VEGF. However, skeletal muscle metabolic adaptation to exercise training takes place independent of myocyte VEGF expression.

Muscle-specific VEGF deficiency greatly reduces exercise endurance in mice

Vascular endothelial growth factor (VEGF) is required for vasculogenesis and angiogenesis during embryonic and early postnatal life. However the organ-specific functional role of VEGF in adult life, particularly in skeletal muscle, is less clear. To explore this issue, we engineered skeletal muscle-targeted VEGF deficient mice (mVEGF-/-) by crossbreeding mice that selectively express Cre recombinase in skeletal muscle under the control of the muscle creatine kinase promoter (MCKcre mice) with mice having a floxed VEGF gene (VEGFLoxP mice). We hypothesized that VEGF is necessary for regulating both cardiac and skeletal muscle capillarity, and that a reduced number of VEGF-dependent muscle capillaries would limit aerobic exercise capacity. In adult mVEGF-/- mice, VEGF protein levels were reduced by 90 and 80% in skeletal muscle (gastrocnemius) and cardiac muscle, respectively, compared to control mice (P < 0.01). This was accompanied by a 48% (P < 0.05) and 39% (P < 0.05) decreases in the capillary-to-fibre ratio and capillary density, respectively, in the gastrocnemius and a 61% decrease in cardiac muscle capillary density (P < 0.05). Hindlimb muscle oxidative (citrate synthase, 21%; beta-HAD, 32%) and glycolytic (PFK, 18%) regulatory enzymes were also increased in mVEGF-/- mice. However, this limited adaptation to reduced muscle VEGF was insufficient to maintain aerobic exercise capacity, and maximal running speed and endurance running capacity were reduced by 34% and 81%, respectively, in mVEGF-/- mice compared to control mice (P < 0.05). Moreover, basal and dobutamine-stimulated cardiac function, measured by transthoracic echocardiography and left ventricular micromanomtery, showed only a minimal reduction of contractility (peak +dP/dt) and relaxation (peak -dP/dt, tau(E)). Collectively these data suggests adequate locomotor muscle capillary number is important for achieving full exercise capacity. Furthermore, VEGF is essential in regulating postnatal muscle capillarity, and that adult mice, deficient in cardiac and skeletal muscle VEGF, exhibit a major intolerance to aerobic exercise.

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.

Near-infrared spectroscopy/imaging for monitoring muscle oxygenation and oxidative metabolism in healthy and diseased humans

Near-infrared spectroscopy (NIRS) was initiated in 1977 by Jobsis as a simple, noninvasive method for measuring the presence of oxygen in muscle and other tissues in vivo. This review honoring Jobsis highlights the progress that has been made in developing and adapting NIRS and NIR imaging (NIRI) technologies for evaluating skeletal muscle O(2) dynamics and oxidative energy metabolism. Development of NIRS/NIRI technologies has included novel approaches to quantification of the signal, as well as the addition of multiple source detector pairs for imaging. Adaptation of NIRS technology has focused on the validity and reliability of NIRS measurements. NIRS measurements have been extended to resting, ischemic, localized exercise, and whole body exercise conditions. In addition, NIRS technology has been applied to the study of a number of chronic health conditions, including patients with chronic heart failure, peripheral vascular disease, chronic obstructive pulmonary disease, varying muscle diseases, spinal cord injury, and renal failure. As NIRS technology continues to evolve, the study of skeletal muscle function with NIRS first illuminated by Jobsis continues to be bright.

Delayed contrast agent kinetics in ischemic skeletal muscle

PURPOSE

To detect skeletal muscle ischemia with first-pass gadolinium (Gd) kinetics after exercise.

MATERIALS AND METHODS

Eleven subjects with intermittent claudication performed a symptom-limited bilateral plantar flexion exercise in the magnet. Regional ROIs were placed bilaterally in the gastrocnemius and soleus muscles, and a signal intensity (SI) time-curve analysis was performed. Induced ischemia was validated prior to the MRI with the systolic ankle-arm blood pressure index (AAI) measured after a symptom-limited treadmill exercise.

RESULTS

Exercise induced ischemic pain in 16 of 22 legs with a significantly reduced AAI (0.31 +/- 0.15). The time to contrast arrival (TCA) was delayed in symptomatic ischemic legs vs. asymptomatic legs (16.3 +/- 6.9 seconds vs. 11.1 +/- 2.7 seconds, P < 0.05). The maximum SI during recovery was higher in the soleus muscle than in the gastrocnemius muscle in ischemic legs (1.55 +/- 0.1 vs. 1.44 +/- 0.1, P < 0.05). Symptomatic regions had a less steep upslope than asymptomatic regions (43 +/- 15 vs. 63 +/- 14, P < 0.001), with a graded upslope response to ischemia. However, a normal upslope was found in 10 of 29 ischemic regions, and some of the regions showed delayed contrast arrival, suggesting a pseudonormal upslope in ischemic regions.

CONCLUSION

Exercise-induced ischemia was detected with the use of an SI time-curve analysis. However, disregarding the arterial input function and distribution volume of the tracer may lead to misinterpretation of some ischemic regions.

Shortened T2 after exercise in ischemic skeletal muscle

PURPOSE

To detect skeletal muscle ischemia with transverse relaxometry after ischemic exercise.

MATERIALS AND METHODS

Ten subjects with intermittent claudication were studied. T2 was measured in the gastrocnemius and soleus muscles (m. gastrocnemius and m. soleus) at rest and repeatedly after exercise during 45 minutes of recovery. Prior to MRI a symptom-limited treadmill exercise was performed, and the ankle-arm blood pressure index (AAI) was measured at rest and after exercise.

RESULTS

In the 14 legs with ischemic pain, a diverging response was found in the calf: T2 increased in m. gastrocnemius by 5.6% +/- 4.9%, but decreased in m. soleus by -1.2% +/- 4.4% (P < 0.001). Moreover, 13 regions in legs with ischemic pain and reduced AAI (from 0.7 +/- 0.2 at rest to 0.31 +/- 0.15 after exercise) had shortened T2 (-3.6% +/- 1.8%) immediately after exercise. This finding was most frequent in m. soleus and two regions of m. gastrocnemius. Recovery was delayed in the latter two regions.

CONCLUSION

T2 may identify ischemic muscles after hypoxic exercise. Shortened T2 suggests a reduced water content (e.g., distribution volume of water) and may affect the upslope kinetics of an extravascular perfusion tracer. The different responses to ischemia by the soleus and gastrocnemius muscle may be due in part to their different fiber type compositions.

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.

Metabolic inertia in contracting skeletal muscle: a novel approach for pharmacological intervention in peripheral vascular disease

Peripheral vascular disease (PVD) is generally accepted to result in the failure of skeletal muscle blood flow to increase adequately at the onset of muscular work. There are currently no routine pharmacological interventions towards the treatment of PVD, however, recent Phase III trials in the USA have demonstrated the clinical potential of the phosphodiesterase III inhibitor Cilostazol for pain-free and maximal walking distances in patients with intermittent claudication. PVD is characterized by a marked reliance on oxygen-independent routes of ATP regeneration (phosphocreatine hydrolysis and glycolysis) in skeletal muscle during contraction and the rapid onset of muscular pain and fatigue. The accumulation of metabolic by-products of oxygen-independent ATP production (hydrogen and lactate ions and inorganic phosphate) has long been associated with an inhibition in contractile function in both healthy volunteers and PVD patients. Therefore, any strategy that could reduce the reliance upon ATP re-synthesis from oxygen-independent routes, and increase the contribution of oxygen-dependent (mitochondrial) ATP re-synthesis, particularly at the onset of exercise, might be expected to improve functional capacity and be of considerable therapeutic value. Historically, the increased contribution of oxygen-independent ATP re-synthesis to total ATP generation at the onset of exercise has been attributed to a lag in muscle blood flow limiting oxygen delivery during this period. However, recent evidence suggests that limited inertia is present at the level of oxygen delivery, whilst considerable inertia exists at the level of mitochondrial enzyme activation and substrate supply. In support of this latter hypothesis, we have reported on a number of occasions that activation of the pyruvate dehydrogenase complex, using pharmacological interventions, can markedly reduce the dependence on ATP re-synthesis from oxygen-independent routes at the onset of muscle contraction. This review will focus on these findings and will highlight the pyruvate dehydrogenase complex as a novel therapeutic target towards the treatment of peripheral vascular disease, or any other disease state where premature muscular fatigue is prevalent due to metabolite accumulation.

Abnormal mitochondrial respiration in skeletal muscle in patients with peripheral arterial disease

OBJECTIVE

Discrete morphologic, enzymatic and functional changes in skeletal muscle mitochondria have been demonstrated in patients with peripheral arterial disease (PAD). We examined mitochondrial respiration in the gastrocnemius muscle of nine patients (10 legs) with advanced PAD and in nine control patients (nine legs) without evidence of PAD.

METHODS

Mitochondrial respiratory rates were determined with a Clark electrode in an oxygraph cell containing saponin-skinned muscle bundles. Muscle samples were obtained from the anteromedial aspect of the gastrocnemius muscle, at a level 10 cm distal to the tibial tuberosity. Mitochondria respiratory rate, calculated as nanoatoms of oxygen consumed per minute per milligram of noncollagen protein, were measured at baseline (V(0)), after addition of substrates (malate and glutamate; (V(SUB)), after addition of adenosine diphosphate (ADP) (V(ADP)), and finally, after adenine nucleotide translocase inhibition with atractyloside (V(AT)). The acceptor control ratio, a sensitive indicator of overall mitochondrial function, was calculated as the ratio of the respiratory rate after the addition of ADP to the respiratory rate after adenine nucleotide translocase inhibition with atractyloside (V(ADP)/ V(AT)).

RESULTS

Respiratory rate in muscle mitochondria from patients with PAD were not significantly different from control values at baseline (0.31 +/- 0.06 vs 0.55 +/- 0.12; P =.09), but V(sub) was significantly lower in patients with PAD compared with control subjects (0.43 +/- 0.07 vs 0.89 +/- 0.20; P <.05), as was V(ADP) (0.69 +/- 0.13 vs 1.24 +/- 0.20; P <.05). Respiratory rates after atractyloside inhibition in patients with PAD were no different from those in control patients (0.47 +/- 0.07 vs 0.45 +/- P =.08). Compared with control values, mitochondria from patients with PAD had a significantly lower acceptor control ratio (1.41 +/- 0.10 vs 2.90 +/- 0.20; P <.001).

CONCLUSION

Mitochondrial respiratory activity is abnormal in lower extremity skeletal muscle in patients with PAD. When considered in concert with the ultrastructural and enzymatic abnormalities previously documented in mitochondria of chronically ischemic muscle, these data support the concept of defective mitochondrial function as a pathophysiologic component of PAD.

Morphological, histochemical, and interstitial pressure changes in the tibialis anterior muscle before and after aortofemoral bypass in patients with peripheral arterial occlusive disease

BACKGROUND

Morphological and electrophysiological studies of ischemic muscles in peripheral arterial disease disclosed evidence of denervation and fibre atrophy. The purpose of the present study is to describe morphological changes in ischemic muscles before and after reperfusion surgery in patients with peripheral occlusive arterial disease, and to provide an insight into the effect of reperfusion on the histochemistry of the reperfused muscle.

METHODS

Muscle biopsies were obtained from the tibialis anterior of 9 patients with chronic peripheral arterial occlusive disease of the lower extremities, before and after aortofemoral bypass, in order to evaluate the extent and type of muscle fibre changes during ischemia and after revascularization. Fibre type content and muscle fibre areas were quantified using standard histological and histochemical methods and morphometric analysis. Each patient underwent concentric needle electromyography, nerve conduction velocity studies, and interstitial pressure measurements.

RESULTS

Preoperatively all patients showed muscle fibre atrophy of both types, type II fibre area being more affected. The mean fibre cross sectional area of type I was 3,745 microm2 and of type II 4,654 microm2. Fibre-type grouping, great variation in fibre size and angular fibres were indicative of chronic dennervation-reinnervation, in the absence of any clinical evidence of a neuropathic process. Seven days after the reperfusion the areas of both fibre types were even more reduced, being 3,086 microm2 for type I and 4,009 microm2 for type II, the proportion of type I fibres, and the interstitial pressure of tibialis anterior were increased.

CONCLUSIONS

The findings suggest that chronic ischemia of the leg muscles causes compensatory histochemical changes in muscle fibres resulting from muscle hypoxia, and chronic dennervation-reinnervation changes, resulting possibly from ischemic neuropathy. Reperfusion seems to bring the oxidative capacity of the previously ischemic muscle closer to normal.

Muscle adaptation to altitude: tissue capillarity and capacity for aerobic metabolism

Prolonged exposure to high altitude leads to reduced muscle mass and performance. The fall in muscle mass follows a reduction in fiber size, which at first was believed to be accompanied by increased fiber capillarization and aerobic enzymes. Subsequent studies showed that hypoxia alone does not alter capillary number and geometry in skeletal muscles of mammals at altitude. It was also found that alterations in fiber size and aerobic enzymes depend on a number of additional factors, including animal activity and the level of hypoxia (e.g., moderate vs. extreme altitude). With training at altitude, fiber capillary number and aerobic enzymes are increased, indicating that muscle potential for plasticity is conserved in hypoxia. Recent studies have also shown that capillary number and geometry are altered in muscles of several species of birds native or exposed to higher altitude; that is, that capillary growth can occur in skeletal muscle in response to chronic exposure to high altitude. In this mini review, we summarize these data and current knowledge on muscle capillary to fiber structural relationships and their implications for muscle aerobic function at altitude.

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.

Exercise training and peripheral vascular disease

BACKGROUND

Conservative management is advocated as a treatment of choice for patients with intermittent claudication. This is a review of the mechanisms behind the improvement following an exercise rehabilitation programme.

METHODS

All Medline articles from the National Library of Medicine, USA containing the text words 'claudication' or 'peripheral vascular disease' and 'exercise' were reviewed. Cross-referencing from relevant articles was carried out.

RESULTS AND CONCLUSION

The poor physical status of a patient with intermittent claudication is not solely due to a reduction in blood flow to the lower limbs; associated factors, such as metabolic inefficiency, poor cardiorespiratory reserve and exercise-induced inflammation contribute. An exercise programme frequently improves both the physical aspect and quality of life, and the success of such exercise is multifactorial. An increase in the blood flow to the lower extremity is uncommon. Other factors, such as a redistribution of blood flow, changes in oxidative capacity of the skeletal muscles and greater utilization of oxygen, occur and the associated metabolic dysfunction of the skeletal muscles is rectified. Following exercise training, blood rheology improves and exercise-induced inflammation is ameliorated; cardiorespiratory status also benefits and the oxygen cost of exercise decreases.

Phosphorus 31 nuclear magnetic resonance spectroscopy suggests a mitochondrial defect in claudicating skeletal muscle

OBJECTIVE

Decreased oxygen supply is generally accepted as the primary cause of muscle dysfunction in patients with peripheral arterial occlusive disease (PAOD) and intermittent claudication, although reported morphologic changes in the mitochondria of claudicating muscle suggest that impaired energy utilization may also play a role. With the measurement of the phosphate-rich compounds of muscle energy metabolism (adenosinetriphosphate [ATP], adenosinediphosphate [ADP], and phosphocreatine [PCr]) and pH, phosphorus P 31 magnetic resonance spectroscopy ((31)P MRS) provides a unique, noninvasive method to investigate this hypothesis further.

METHODS

Calf muscle bioenergetics were studied in 12 men with moderate claudication (ankle-brachial index >/=0.5 and </=0.8) and 14 normal control subjects with the use of (31)P MRS and standard treadmill testing. Phosphorus MRS evaluation of the superficial posterior calf muscles was carried out with a 90-second submaximal isometric plantar flexion exercise. This mild exercise was chosen to permit in-magnet testing and to allow study of intrinsic mitochondrial efficiency under conditions of unchallenged blood flow. Phosphocreatine and ADP recovery time constants (t.c.), two very sensitive measures of oxidative mitochondrial function, as well as intracellular pH and ATP production via anaerobic glycolysis were determined during three exercise sessions and the results averaged and compared to known values obtained from a control population.

RESULTS

During the (31)P MRS protocol, the end exercise intracellular pH (7.11 +/- 0.01 vs 7.11 +/- 0.01) and ATP production by anaerobic glycolysis (0.13 +/- 0.05 vs 0.14 +/- 0.03 mmol/L per second) were no different in PAOD patients versus control subjects, confirming that the protocol exercise did not significantly reduce oxygen supply. Phosphocreatine and ADP recovery t.c. (137 +/- 41 vs 44 +/- 3 seconds and 60 +/- 10 vs 29 +/- 2 seconds, respectively) were significantly slower than normal (P <.05, t test). There was, however, no correlation between these measures of mitochondrial function and any treadmill parameter (P >.5, Pearson moment correlation).

CONCLUSIONS

Phosphorus 31 MRS provides the first direct evidence of defective energy metabolism in the mitochondria of claudicating calf muscle. This defect appears to be independent of both arterial flow and the severity of occlusive disease in patients with mild to moderate claudication. Coupled with documented ultrastructural and DNA abnormalities in the mitochondria of claudicating skeletal muscle, these data provide evidence for a secondary cause of muscle dysfunction in intermittent claudication.

Acquired skeletal muscle metabolic myopathy in atherosclerotic peripheral arterial disease

Peripheral arterial disease (PAD) is associated with an increased risk of overall cardiovascular mortality, and substantial morbidity resulting from claudication. While the initial disease process is clearly the result of atherosclerosis in the arterial circulation of the limb, altered hemodynamics do not completely explain the pathophysiology of claudication. Work from several laboratories has demonstrated secondary changes in the skeletal muscle of patients with PAD which are consistent with the presence of an acquired metabolic myopathy in these patients. Key findings include an alteration in the expression of mitochondrial enzymes, the accumulation of metabolic intermediates, altered regulation of mitochondrial respiration, increased oxidative stress, and the presence of somatic mutations in the mitochondrial genome. Understanding the metabolic changes associated with PAD is important in understanding the pathophysiology of claudication and in the development of novel therapeutic strategies.

Vascular growth in hypoxic skeletal muscle

The critical role of skeletal muscle capillaries is the supply of oxygen to skeletal muscle fibers during conditions of maximal aerobic work. The supply of substrates under these conditions is not limited by the vascular bed but rather by the capacity of the sarcolemmal transporter systems. Because of this dominant role of oxygen supply in muscle tissue, hypoxia has generally been considered to be an important stimulus for capillary neo-formation in skeletal muscle. Early morphometric work seemed to indicate that animals exposed to permanent hypoxia had in fact a significantly improved vascular supply in muscle tissue. Later work questioned these early findings and it was concluded that hypoxia per se was not a sufficient stimulus for capillary neo-formation but that additional stimuli such as cold-exposure needed to be present. In humans exposed to severe hypoxia during simulated or real ascents to Mt. Everest an increase in capillary density was in fact found. However, this increase could be shown to result from a reduction of muscle fiber volume and not from capillary growth. Broadly compatible results were obtained in animal experiments in which changes in capillarity were assessed in muscles with limited blood supply which were exposed to chronic electrical stimulation. Recently we have shown that endurance exercise training in humans results in a rise in mRNA of vascular endothelial growth factor (VEGF) only when carried out vigorously and in hypoxia. These results indicate that molecular techniques will allow in the near future to delineate the role played by hypoxia in capillary neo-formation.

Oxygen uptake kinetics during exercise are slowed in patients with peripheral arterial disease

Patients with peripheral arterial disease (PAD) have arterial occlusions that limit peripheral blood flow. This study evaluated the dynamic response in O(2) consumption (VO(2)) at the onset of constant-load exercise (VO(2) kinetics) in patients with PAD. Eight patients with bilateral PAD, seven patients with unilateral PAD, nine age-matched nonsmoking controls, and seven smoking controls performed graded treadmill exercise to assess peak VO(2). Subjects also performed constant-load exercise tests at 2.0 miles/h at 0 and 4% grade to determine VO(2) kinetics. Peak VO(2) was reduced 50% in patients with PAD compared with both control groups (P < 0.05). At 4% grade, phase 2 VO(2) kinetics were significantly slowed for the PAD groups compared with controls (60.1 +/- 15.7 and 58.7 +/- 8.3 s, unilateral and bilateral PAD groups, respectively; compared with 28. 4 +/- 19.3 and 27.9 +/- 8.1 s, nonsmoking and smoking controls, respectively; P < 0.05). No relationship was found between VO(2) kinetics and disease severity. These data demonstrate that VO(2) kinetics are markedly slowed in patients with PAD. The impairment in VO(2) kinetics is not related to smoking status or arterial disease severity and therefore may reflect altered control of skeletal muscle metabolism.

Skeletal muscle mitochondrial DNA injury in patients with unilateral peripheral arterial disease

BACKGROUND

Patients with peripheral arterial disease (PAD) have exercise limitation due to claudication-limited pain and metabolic alterations in skeletal muscle. PAD is also associated with oxidative stress, which is a known cause of mitochondrial DNA (mtDNA) injury. The present study was designed to test the hypothesis that PAD is associated with mtDNA injury, as reflected by an increased frequency of a specific 4977-base pair (bp) mtDNA deletion mutation.

METHODS AND RESULTS

The deletion frequency was quantified in gastrocnemius muscle of 8 patients with unilateral PAD and 10 age-matched control subjects with the use of polymerase chain reaction methodologies. Muscle from the hemodynamically unaffected (less affected) PAD limb showed an 8-fold increased deletion frequency and the hemodynamically affected (worse affected) PAD limb had a 17-fold increased deletion frequency compared with muscle from control subjects. The frequency of the 4977-bp deletion in the worse-affected limb was positively correlated with the age of the patients but not the claudication-limited exercise performance of the patients. Total mtDNA content, citrate synthase activity, and cytochrome c oxidase activity were not different in the muscle from the 3 limb populations. However, the ratio of citrate synthase to cytochrome c oxidase was higher in the worse- versus less-affected limbs of PAD patients.

CONCLUSIONS

The present study demonstrates a large increase in the frequency of the mtDNA 4977-bp deletion in patients with PAD but in a distribution not limited to the hemodynamically affected limb.

The role of carnitine and carnitine supplementation during exercise in man and in individuals with special needs

Carnitine is critical for normal skeletal muscle bioenergetics. Carnitine has a dual role as it is required for long-chain fatty acid oxidation, and also shuttles accumulated acyl groups out of the mitochondria. Muscle requires optimization of both of these metabolic processes during peak exercise performance. Theoretically, carnitine availability may become limiting for either fatty acid oxidation or the removal of acyl-CoAs during exercise. Despite the theoretical basis for carnitine supplementation in otherwise healthy persons to improve exercise performance, clinical data have not demonstrated consistent benefits of carnitine administration. Additionally, most of the anticipated metabolic effects of carnitine supplementation have not been observed in healthy persons. The failure to demonstrate clinical efficacy of carnitine may reflect the complex pharmacokinetics and pharmacodynamics of carnitine supplementation, the challenges of clinical trial design for performance endpoints, or the adequacy of endogenous carnitine content to meet even extreme metabolic demands in the healthy state. In patients with end stage renal disease there is evidence of impaired cellular metabolism, the accumulation of metabolic intermediates and increased carnitine demands to support acylcarnitine production. Years of nutritional changes and dialysis therapy may also lower skeletal muscle carnitine content in these patients. Preliminary data have demonstrated beneficial effects of carnitine supplementation to improve muscle function and exercise capacity in these patients. Peripheral arterial disease (PAD) is also associated with altered muscle metabolic function and endogenous acylcarnitine accumulation. Therapy with either carnitine or propionylcarnitine has been shown to increase claudication-limited exercise capacity in patients with PAD. Further clinical research is needed to define the optimal use of carnitine and acylcarnitines as therapeutic modalities to improve exercise performance in disease states, and any potential benefit in healthy individuals.

Exercise testing and exercise rehabilitation for patients with peripheral arterial disease: status in 1997

Intermittent claudication is a common manifestation of peripheral arterial occlusive disease (PAOD). Patients with claudication are limited in terms of work, housework and leisure activities so that functional status is very impaired. Therefore, the goals for treatment should focus on improving the functional impairment as well as on modifying risk factors. Evaluation of the functional status is of critical importance before beginning any therapy so that any resultant changes can be assessed. A validated graded treadmill protocol and validated questionnaires are used for this purpose. Three questionnaires that are currently used include the Walking Impairment Questionnaire, the PAOD Physical Activity Recall and the Medical Outcomes Study SF-36. Exercise rehabilitation is a method that has been particularly efficacious for treating the functional impairment associated with intermittent claudication. Exercise rehabilitation has been shown to improve pain-free treadmill walking distance by 44% to 300% and absolute walking distance by 25% to 442%. In addition, improvements have also been reported (using questionnaire data) in the ability to walk distances and speeds, in amount of habitual physical activity and in physical functioning. Thus, exercise rehabilitation has caused improvements not only in exercise capacity but also in community-based functional status. Because of the benefits of this treatment, in addition to the low associated morbidity, exercise therapy is recommended as an important treatment option for people with intermittent claudication due to PAOD.

Skeletal muscle metabolism as a target for drug therapy in peripheral arterial disease

Peripheral arterial disease (PAD) is an atherosclerotic disease which modifies lower extremity hemodynamics. There is considerable evidence that skeletal muscle metabolism is altered in PAD. Several studies have demonstrated altered mitochondrial enzyme content in PAD muscle as compared with controls, and enzyme activity may not increase normally in PAD with exercise training. A variety of metabolic intermediates, including acylcarnitines, accumulate in muscle of PAD patients, suggesting incomplete oxidative metabolism. Studies employing 31P-NMR (nuclear magnetic resonance) also suggest a metabolic myopathy in PAD. Strikingly, while hemodynamics do not predict claudication-limited performance, metabolic injury as evidenced by acylcarnitine accumulation is strongly correlated with patients' functional status in PAD. Further, exercise rehabilitation improves claudication-limited performance without modifying large vessel hemodynamics. The stress placed on skeletal muscle during exercise in PAD and the observed evidence of metabolic dysfunction is similar to ischemia/reperfusion injury in cardiac muscle. Recognition of the role of cellular metabolic injury and function in PAD has formed the basis for novel therapeutic strategies in this disease.

Altered surface myoelectric signals in peripheral vascular disease: correlations with muscle fiber composition

Conduction velocity (CV) and median frequency (MDF) during tetanic electrical stimulation of the tibialis anterior muscle were evaluated in patients with uncomplicated peripheral arterial occlusive disease. Results were analyzed with respect to biopsy determination of diameter and proportion of types 1 and 2 muscles fibers. Initial MDF and CV correlated positively with type 2, but not type 1 fiber diameter. Initial MDF was reduced bilaterally in patients with unilateral peripheral arterial occlusive disease as compared to normal subjects, indicating that chronic ischemia alone cannot explain the altered myoelectric signal. Physical training increased pain-free walking distance and raised initial MDF, though CV remained unchanged. Fatigue indices were highly interrelated, but showed no correlation with any of the other evaluation variables. Thus, initial MDF, a correlate of type 2 muscle fiber distribution in chronically ischemic tibialis anterior muscles, is altered in peripheral vascular disease. However, muscle ischemia alone cannot explain all aspects of this abnormality.

Flow-independent angiography for peripheral vascular disease: initial in-vivo results

Flow-independent angiography (FIA), an approach that isolates arterial blood using MR relaxation characteristics rather than flow effects, was evaluated for application in peripheral vascular disease (PVD). First, pilot studies were conducted in which FIA coronal projection images were obtained from controls and symptomatic patients with PVD to assess clinical utility. All control images corresponded to the expected leg arterial anatomy with little interference from deep veins (one of five) and muscle (zero of five). Superficial venous signal was less well suppressed in comparison to deep veins (four of five). Images of symptomatic patients were less consistent with difficulty suppressing muscle and deep venous signal in some cases and edema when present. We then compared T2 values for muscle (T2m, tibialis anterior), arterial blood (femoral and popliteal arteries), and venous blood (femoral, popliteal, and saphenous veins) in controls (n = 8) and symptomatic patients with intermittent claudication (n = 5) or ischemic rest pain (n = 7). Changes in T2 measurements of various tissues accounted for poorer contrast in symptomatic patients. Patients with ischemic rest pain had significantly higher T2m compared with controls (T2m = 39.3 +/- 2.1 (1 standard error of the mean [SEM]) versus 30.9 +/- .4, P < .01). For all measurements, other than saphenous vein, variances were greater in symptomatic patients. To realize the inherent advantages of FIA for this clinical application, additional work on suppression of signals from muscle, veins, and edema is required. One promising approach involves shifting from projection images to three-dimensional acquisitions for improved tissue suppression.

Effect of endurance training under hypoxic condition on oxidative enzyme activity in rat skeletal muscle

The adaptive response of oxidative enzyme activity in the skeletal muscle to training in normoxic and in normobaric hypoxic training was studied. Forty male Wistar rats were divided into 4 groups: normoxia + sedentary (NS, n = 10); hypoxia + sedentary (HS, n = 10); normoxia + training (NT, n = 10); and hypoxia + training (HT, n = 10). Rats in the NT group ran on a treadmill for 30 min a day at 20-30 m.min-1, 4 days a week for 10 weeks in normoxia. Rats in the HT group performed the same training protocol as NT in an ambient FIO2 decreased to 12%. HS rats were exposed to hypoxia in the same degree, duration and frequency as HT without exercise. After the training period, the soleus and the plantaris muscles were removed, and the activities of mitochondrial enzymes, malate dehydrogenase (MDH) and 3-hydroxyacyl-CoA dehydrogenase (HAD) were measured by a spectrophotometer. The normoxic training did not increase MDH or HAD activities, in either the soleus or the plantaris. This absence of change in mitochondrial enzyme activities is considered to be the results of inadequate stimulus of training, including a relatively low amount of exercise. On the other hand, the hypoxic training enhanced the MDH activity in the soleus by 17.5% compared with NS (P < 0.01) and by 20.5% compared with HS (P < 0.01). Also in the plantaris, the MDH activity in HT was higher than that in HS (15.7%, P < 0.05). These findings suggest that even moderate training by which enzyme activity is not increased under normoxic conditions can enhance the oxidative capacity in the skeletal muscle when the training is performed in a hypoxic environment.

Changes in muscle oxygenation during weight-lifting exercise

The quantitative analysis of haemoglobin oxygenation of contracting human muscle during weight-lifting exercise was studied noninvasively and directly using near-infrared spectroscopy. This method was developed as a three-wavelength method which confirmed the volume changes in oxygenated haemoglobin (oxy-Hb), deoxygenated haemoglobin (deoxy-Hb) and blood volume (total-Hb; Oxy-Hb+deoxy-Hb). Nine healthy adult men with various levels of training experience took part in the study. Ten repetition maximum (10 RM) one-arm curl exercise was performed by all the subjects. Results showed that at the beginning of the 10-RM exercise, rapid increases of deoxy-Hb and decreases of oxy-Hb were observed. In addition, total-Hb gradually increased during exercise. These results corresponded to the condition of arm blood flow experimentally restricted using a tourniquet in contact with the shoulder joint, and they showed the restriction of venous blood flow and an anoxic state occurring in the dynamically contracted muscle. In three sets of lifting exercise with short rest periods, these tendencies were accelerated in each set, while total-Hb volume did not return to the resting state after the third set for more than 90 s. These results would suggest that a training regimen emphasizing a moderately high load and a high number of repetitions, and a serial set with short rest periods such as usually performed by body builders, caused a relatively long-term anoxic state in the muscle.

Functional status and mobility among elderly women with lower extremity arterial disease: the Study of Osteoporotic Fractures

OBJECTIVE

To investigate the relationship between lower extremity arterial disease, functional status, and mobility among elderly women.

DESIGN

Cross-sectional study.

SETTING

Community.

PARTICIPANTS

1492 healthy white women, 65 years of age or older, residing in a rural community, able to walk without the assistance of another person, and enrolled in the Pittsburgh clinic of the multicenter Study of Osteoporotic Fractures. Those with bilateral hip replacement were excluded.

MEASUREMENTS

Ankle/arm index (AAI); instrumental activities of daily living (IADLs); measures of recent physical activity, muscle strength, gait and balance; general demographic, lifestyle, and physical variables.

RESULTS

Women with lower extremity arterial disease (defined as an AAI of 0.9 or less) were more likely to report difficulty with one or more IADLs than were women free of this disease. After adjusting for age and other potential confounders, only difficulty with walking 2-3 blocks remained highly correlated with disease (relative risk (RR) 2.8, 95% confidence interval (CI) 1.6, 4.8). Several measures of physical activity were inversely and independently related to a low AAI. Muscle strength in the hip, arm, knee, and hand and measures of static and dynamic balance were correlated with low AAI in the univariate analysis, but most of these trends were not statistically significant after adjustment for age and other confounders. Exclusion of women with symptomatic arterial disease did not substantially affect the results obtained.

CONCLUSION

Women with mild, predominantly subclinical, lower extremity arterial disease living in the community have decreased functional status and mobility.

Increased capillary density due to atrophy of ischaemic soleus muscle of the rat

This investigation was undertaken to determine whether a severe limitation of blood supply by external iliac artery ligation can change the number and density of capillaries in rat soleus muscle. The external iliac artery in one hindlimb was ligated for 2, 7 or 28 days, and the other, sham-operated, hindlimb was used as a control. Muscle blood flow in the ischaemic soleus muscle at 2, 7 and 28 days after external iliac artery ligation was significantly decreased compared with the control. The muscle fibre area and the ratio of the fibre area to body mass in the ischaemic soleus muscle at 28 days after the external iliac artery ligation were significantly reduced in the ischaemic soleus muscle, but no change in the number of capillaries per fibre was observed. Capillary density per millimetres squared at 28 days and the ratios of the number of capillaries around type I fibres to the fibre area at 7 and 28 days were significantly increased in the ischaemic soleus muscle (P < 0.05). These results suggested that long-term severe limitation of blood flow in the soleus muscle by ligation of the external iliac artery could have induced the increase in capillary density, as a result of atrophy of muscle fibres rather than capillary growth.

Muscle fibre types and enzyme activities after training with local leg ischaemia in man

Eight healthy men performed supine one-legged training on a bicycle ergometer 45 min per leg four times per week for 4 week. The ergometer and lower body were inside a pressure chamber, the opening of which was sealed at the level of the crotch. One leg trained with impeded leg blood flow (I-leg), induced by an increased (50 mmHg) chamber pressure, at the highest tolerable intensity. The contralateral leg trained at the same power under normal pressure (N-leg). Before and after training biopsies were taken from the vastus lateralis of both legs and maximal one-legged exercise tests were executed with both legs. Biopsies were repeated when the subjects had been back to their habitual physical activity for 3 months. Training increased exercise time to exhaustion, but more in the I-leg than in the N-leg. After training, the I-leg had higher activity of citrate synthase (CS), a marker of oxidative capacity, and lower activity of the M-subunit of lactate dehydrogenase isoenzymes. It also had a higher percentage of type-I fibres and a lower percentage of IIB fibres, larger areas of all fibre types and a greater number of capillaries per fibre. It is concluded that ischaemic training changes the muscle metabolic profile in a direction facilitating aerobic metabolism. An altered fibre-type composition may contribute, but is not enough prerequisite for the change.

Effects of ischaemic training on local aerobic muscle performance in man

The aim of the study was to compare the effects of ischaemic and non-ischaemic training on aerobic performance. In 10 subjects, peak oxygen uptake (peak VO2) and time to fatigue (TTF) for one-legged exercise were measured before and after 4 weeks (4 times week-1) of one-legged training. Each training session started with one leg training for 45 min with 20% blood-flow reduction induced by local application of a supra-atmospheric external pressure of 50 mmHg (ischaemic leg; I-leg). We have previously shown that this decreases leg blood flow by about 20%. The contralateral leg (non-restricted-flow leg; N-leg), serving as a control, then trained with an identical power-output profile for 45 min but without flow restriction. In the I-leg the average training-induced increments in TTF and peak VO2 were 27 and 24%, respectively. In the N-trained leg TTF and peak-VO2 increased 10 and 14%, respectively. Both increments were significantly greater (P < 0.05) in the I-trained leg. Moreover, the performance increase in the I-trained leg was exaggerated (P < 0.05) in the ischaemic test condition, i.e. there was a specificity in the training response. In conclusion, ischaemia acts as an additive stimulus to training leading to an exaggerated increase in endurance and peak-VO2 compared to identical training without blood-flow restriction. The main explanation is probably an enhanced local adaptation in the I-trained leg.