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

Joint torques and powers are reduced during ambulation for both limbs in patients with unilateral claudication

OBJECTIVES

Symptomatic peripheral arterial disease (PAD) results in significant gait impairment. In an attempt to fully delineate and quantify these gait alterations, we analyzed joint kinematics, torques (rotational forces), and powers (rotational forces times angular velocity) in patients with PAD with unilateral claudication for both the affected and nonaffected legs.

METHODS

Twelve patients with unilateral PAD (age, 61.69 +/- 10.53 years, ankle-brachial index [ABI]: affected limb 0.59 +/- 0.25; nonaffected limb 0.93 +/- 0.12) and 10 healthy controls (age, 67.23 +/- 12.67 years, ABI >1.0 all subjects) walked over a force platform to acquire gait kinetics, while joint kinematics were recorded simultaneously. Data were collected for the affected and nonaffected limbs during pain free (PAD-PF) and pain induced (PAD-P) trials. Kinetics and kinematics were combined to quantify torque and powers during the stance period from the hip, knee, and ankle joints.

RESULTS

The affected limb demonstrated significantly (P <.05) reduced ankle plantar flexion torque compared to controls during late stance in both PAD-PF and PAD-P trials. There were significant reductions in ankle plantar flexion power generation during late stance for both the affected (P <.05) and nonaffected limbs (P <.05) compared to control during PAD-PF and PAD-P trials. No significant differences were noted in torque comparing the nonaffected limbs in PAD-PF and PAD-P conditions to control for knee and hip joints throughout the stance phase. Significant reductions were found in knee power absorption in early stance and knee power generation during mid stance for both limbs of the patients with PAD as compared to control (P <.05).

CONCLUSION

Patients with PAD with unilateral claudication demonstrate significant gait impairments in both limbs that are present even before they experience any claudication symptoms. Overall, our data demonstrate significantly reduced ankle plantar flexion torque and power during late stance with reduced knee power during early and mid stance for the affected limb. Further studies are needed to determine if these findings are dependent on the location and the severity of lower extremity ischemia and whether the changes in the nonaffected limb are the result of underlying PAD or compensatory changes from the affected limb dysfunction.

Effective training for patients with intermittent claudication

OBJECTIVE

Current guidelines for treatment of intermittent claudication (IC) do not include a specific recommendation for the intensity of exercise therapy. Thus, the purpose of this study was to determine the relative effectiveness of high versus low intensity exercise for patients with IC, and further to study the effect of such training on blood flow to the legs during exercise.

DESIGN

The effect of eight weeks of supervised endurance training was examined in 16 patients with IC. The patients were randomly assigned to training at intensities corresponding to either 60% or 80% of their peak oxygen consumption (VO2peak), respectively.

RESULTS

VO2peak and time to exhaustion increased significantly (9% and 16%, respectively) more in the high intensity group (p<0.05). Blood flow to the legs did not change after training in any of the groups.

CONCLUSION

High intensity training gave larger improvements in VO2peak and time to exhaustion than low intensity training. As blood flow did not change after the exercise program, it is likely that the observed different increase of VO2peak was due to changed mitochondrial oxidative capacity and/or skeletal muscle diffusive capacity.

Gait variability is altered in patients with peripheral arterial disease

OBJECTIVE

Claudication is the most common presentation of peripheral arterial disease (PAD), producing significant ambulatory compromise. Claudicating patients, most of whom are elderly, have reduced mobility and poor health outcomes, including an increased risk of falls. The gait of elderly fallers is characterized by increased variability. Increase in the variability of the locomotor system makes the gait more noisy and unstable. The purpose of this study is to investigate gait variability in patients with PAD.

METHODS

Nineteen symptomatic PAD patients (age, 63.6 +/- 9.8 years; body mass, 82.1 +/- 18.5 kg; height, 1.71 +/- 0.06 m) walked on a treadmill in the absence of pain or claudication symptoms while joint flexion and extension kinematics were captured. Results were compared with results obtained from 17 matched healthy controls (age, 65.2 +/- 12.5 years; body mass, 82.0 +/- 25.9.5 kg; height, 1.73 +/- 0.08 m). Relative joint angles were calculated for the ankle, knee, and hip flexion/extension, and the stride-to-stride variability of joint flexion and extension was calculated from at least 30 consecutive footfalls. Variability was expressed using the largest Lyapunov exponent, standard deviation, and coefficient of variation. Independent t tests were used to compare gait variability between groups.

RESULTS

Symptomatic PAD patients had significantly higher largest Lyapunov exponent values and coefficient of variation values for all joints, and higher standard deviation values at the ankle and the hip (P < .05).

CONCLUSION

Symptomatic PAD patients have increased gait variability at the ankle, knee, and hip joints at baseline ambulation in the absence of claudication pain. Our findings indicate significant baseline deterioration in the locomotor system of symptomatic PAD patients. This deterioration results in increased noise and instability of gait and is a potential contributing factor to the falls and mobility problems experienced by symptomatic PAD patients.

Peripheral arterial disease affects kinematics during walking

OBJECTIVE

Claudication is the most common manifestation of peripheral arterial disease (PAD) producing significant ambulatory compromise. The purpose of this study was to use advanced biomechanical analysis to characterize the kinematic ambulatory pattern of claudicating patients. We hypothesized that compared with control subjects, claudicating patients have altered kinematic gait patterns that can be fully characterized utilizing advanced biomechanical analysis.

METHODS

The study examined fourteen PAD patients (age: 58 +/- 3.4 years; weight: 80.99 +/- 15.64 kg) with clinically diagnosed femoro-popliteal occlusive disease (Ankle Brachial Index (ABI): 0.56 +/- 0.03, range 0.45 to 0.65) and five healthy controls (age: 53 +/- 3.4 years; weight: 87.38 +/- 12.75 kg; ABI >or= 1). Kinematic parameters (hip, knee, and ankle joint angles in the sagittal plane) were evaluated during gait in patients before and after the onset of claudication pain and compared with healthy controls. Joint angles were calculated during stance time. Dependent variables were assessed (maximum and minimum flexion and extension angles and ranges of motion) and mean ensemble curves were generated. Time to occurrence of the discrete variables was also identified.

RESULTS

Significantly greater ankle plantar flexion in early stance and ankle range of motion during stance was observed in PAD patients (P < .05). Time to maximum ankle plantar flexion was shorter and time to maximum ankle dorsiflexion was longer in PAD patients (P < .05). These differences were noted when comparing PAD patients prior to and after the onset of claudication with healthy controls. The analysis of the kinematic parameters of the knee and the hip joints revealed no significant differences between PAD patients and controls.

CONCLUSION

PAD patients with claudication demonstrate significant gait alterations in the ankle joint that are present prior to the onset of claudication pain. In contrast, the joint motion of the hip and knee did not differ in PAD patients when compared with controls. Further research is needed to verify our findings and assess the impact of more proximal disease in PAD patients as well as the effect of revascularization on joint kinematics.

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.

Mitochondriopathy of peripheral arterial disease

The signs and symptoms of peripheral arterial occlusive disease (PAD), including claudication, rest pain, and tissue loss, are consequences of compromised bioenergetics and oxidative tissue injury within the affected lower extremities. Compromised bioenergetics is the result of a combination of low blood flow through diseased arteries and diminished adenosine triphosphate production by dysfunctional mitochondria. The tissue injury appears to be secondary to increased production of reactive oxygen species by dysfunctional mitochondria and by inflammation, in association with ischemia and ischemia/reperfusion. In this review, we present the current histomorphologic, physiologic, and biochemical evidence defining the nature of this mitochondriopathy and discuss its contribution to the pathogenesis and clinical manifestations of PAD.

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.

Antioxidants attenuate oxidative damage in rat skeletal muscle during mild ischaemia

We have previously shown oxidative stress and oedema, caused by both xanthine oxidase-derived oxidants and infiltrating neutrophils, within skeletal muscle after contractile-induced claudication. The purpose of this study was to determine whether supplementation with antioxidant vitamins attenuates the oxidative stress, neutrophil infiltration and oedema associated with an acute bout of contractile-induced claudication. Rats received vehicle, vitamin C, vitamin E or vitamin C + E for 5 days prior to contractile-induced claudication. Force production was significantly reduced in the claudicant limbs of all groups compared with the control (sham) limb of control animals. Contractile-induced claudication caused a significant increase in protein oxidation, lipid peroxidation, neutrophil infiltration and oedema compared with sham muscles. Supplementation with vitamin C, E or C + E prevented the increases in each of these, and there were no differences between groups. These findings suggest that, in an animal model of exercise-induced claudication, neutrophil chemotaxis is caused by oxidizing species and that antioxidant supplementation can prevent oxidative damage, neutrophil infiltration and oedema following an acute bout of contractile-induced claudication.

The myopathy of peripheral arterial occlusive disease: part 1. Functional and histomorphological changes and evidence for mitochondrial dysfunction

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 (PAD). These works provide evidence that a state of repetitive cycles of exercise-induced ischemia followed by reperfusion at rest operates in PAD patients 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, we highlight the currently available evidence that characterizes the nature and mechanisms responsible for this myopathy. In part 1, the authors review the functional and histomorphological characteristics of the myopathy and focus on the biochemistry and bioenergetics of its mitochondriopathy. In part 2, they then review accumulating evidence that oxidative stress related to ischemia reperfusion is probably the major operating mechanism of PAD myopathy. 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.

Peripheral arterial disease affects ground reaction forces during walking

OBJECTIVE

Claudication is the most common manifestation of peripheral arterial disease (PAD), producing significant ambulatory compromise. The gait of claudicating patients has been evaluated using primarily temporal and spatial parameters. With the present study, we used advanced biomechanical measures to further delineate the ambulatory impairment of claudicating patients. We hypothesized that the claudicating legs of PAD patients have an altered kinetic gait pattern compared with normal legs from control subjects.

METHODS

Ambulation kinetics (ground reaction forces) were evaluated in claudicating patients and compared with age-matched healthy controls. Forces were analyzed in the vertical, anterior-posterior, and medial-lateral directions. Time from heel touch-down to toe-off (stance time) and time spent in double-limb support were also evaluated.

RESULTS

The study recruited 14 PAD patients (age, 58 +/- 3.4 years; weight, 80.99 +/- 15.64 kg) with femoropopliteal occlusive disease (ankle-brachial index [ABI], 0.56 +/- 0.03) and five controls (age, 53 +/- 3.4 years; weight, 87.38 +/- 12.75 kg; ABI, >or=1.00). Vertical force curve evaluation demonstrated significant flattening in claudicating patients resulting in a lower and less fluctuant center of mass when ambulating. In the anterior-posterior direction, claudicating patients demonstrated significantly decreased propulsion forces. In the medial-lateral direction, they had significantly increased forces consistent with wider steps and an inability to swing their legs straight through. Claudicating patients demonstrated a greater stance time and time in double limb support compared with healthy controls. Most importantly, gait abnormalities were present before the onset of claudication, with gait worsening after the onset of claudication.

CONCLUSION

Claudicating patients demonstrate significant gait impairments that are present even before they experience any limb discomfort. These alterations may make them feel more stable and secure while attempting to minimize use of the affected limb. Advanced biomechanical analysis, using ambulation kinetics, permits objective and quantitative evaluation of the gaits of claudicating patients. Such evaluation may point to new rehabilitation strategies and provide objective measurement of functional outcomes after medical and surgical therapy.

Peripheral arterial disease assessment: wall, perfusion, and spectroscopy

INTRODUCTION

Peripheral arterial disease (PAD) is characterized by lower limb arterial obstruction due to atherosclerosis and is increasingly common. Presently used methods for diagnosis and follow-up as well as for assessment of novel therapies are limited.

MATERIALS AND METHODS

Three distinct magnetic resonance examinations were developed. The first was high-resolution black-blood atherosclerotic plaque imaging of the superficial femoral artery using a surface coil and flow saturation. Second, first-pass contrast-enhanced dual-contrast perfusion imaging of the calf muscle was performed at peak exercise using a magnetic resonance (MR)-compatible pedal ergometer. Lastly, (31)P MR spectroscopy was also performed at peak exercise to measure phosphocreatine (PCr) recovery kinetics.

RESULTS

Seventeen patients (age, 63 +/- 10 yrs) with mild to moderate PAD were studied with black-blood atherosclerotic plaque imaging. Mean atherosclerotic plaque volume measured was 7.27 +/- 3.73 cm(3). Eleven patients (age, 61 +/- 11 yrs) with mild to moderate symptomatic PAD and 22 normal control subjects were studied with first-pass contrast-enhanced perfusion imaging. Perfusion index was stepwise increased from patients to normal subjects with matched workload to normal subjects at maximal exercise. For PCr recovery kinetics, 20 patients with mild to moderate PAD and 14 controls were studied. The median recovery time constant of PCr was 34.7 seconds in the controls and 91.0 seconds in the PAD patients (P < 0.0001).

CONCLUSIONS

Three distinct MR examinations of different aspects of peripheral arterial disease have been developed and tested and shown to differentiate patients with mild to moderate PAD from normal controls. Taken together, these tests are potential quantitative end points for clinical trials of novel therapies in PAD.

Plantar Flexion as an Alternative to Treadmill Exercise for Evaluating Patients with Intermittent Claudication

OBJECTIVE

The aim of this study was to examine whether the plantar flexion test could adequately replace treadmill testing in patients who were unable to exercise.

DESIGN

Prospective observational study.

PATIENTS

Twenty-seven patients with intermittent claudication secondary to peripheral arterial disease (PAD).

METHODS

Patients performed two treadmill tests and two plantar flexion tests. Ankle pressure, near infrared spectroscopy (NIRS) data, heart rate and blood pressures were monitored along with pain-free and maximum walking distances for treadmill, pain-free and maximum exercise time for plantar flexion.

RESULTS

Maximum exercise time and walking distance were well correlated (R=0.74). Eleven patients (41%) developed non-claudicating symptoms during the treadmill test but not during the flexion test. Rate pressure product was significantly higher after the treadmill but not after the plantar flexion.

CONCLUSIONS

Plantar flexion test showed good reliability and correlation. Plantar flexion may serve as an alternative to treadmill testing in evaluating muscle pain in patients with intermittent claudication.

High-energy phosphate metabolism in the calf muscle of healthy humans during incremental calf exercise with and without moderate cuff stenosis

It is known that the relevance of a peripheral stenosis for muscle function increases with exercise. Our intention was to investigate the impact of a moderate cuff stenosis (CS) at 120 mmHg of the superficial femoral artery on high-energy phosphate (HEP) metabolism during isotonic, incremental calf exercise. Serial phosphorus 31 magnetic resonance spectroscopy (31P MRS) and velocity-encoded phase-contrast MR imaging (VEPC MRI) were carried out in each leg of ten healthy male volunteers. Each leg underwent four increments of calf exercise (2, 3, 4 and 5 W) followed by recovery during separate exercise sessions with and without a CS at 120 mmHg. The serial 31P MRS measurements had a time resolution of 10 s. VEPC MRI was performed at the end of each increment during separate sessions. During all increments, we detected significant differences (P < 0.05) in the phosphocreatine (PCr) time constants and the amount of PCr hydrolysis between the sessions without and with CS. Regarding the time courses of the PCr, inorganic phosphate (Pi) and pH level, we observed significant differences (P < 0.002) during exercise and recovery. During both conditions, the end-increment PCr levels as well as blood flow correlated significantly with the mechanical power. The PCr time constants during exercise significantly correlated with the intramuscular pH, but not with blood flow or mechanical power. However, the PCr recovery time constants correlated significantly with blood flow and end-exercise pH. Our study shows that reduction of blood flow due to a peripheral stenosis results in a prolongation of PCr time constants, decreased PCr and pH level as well as increased Pi level during exercise. We believe that 31P MRS during incremental exercise might provide additional information for assessing the relevance of a peripheral stenosis and its impact on muscle function.

31P MR spectroscopy and in vitro markers of oxidative capacity in type 2 diabetes patients

BACKGROUND

Skeletal muscle mitochondrial function in type 2 diabetes (T2D) is currently being studied intensively. In vivo (31)P magnetic resonance spectroscopy ((31)P MRS) is a noninvasive tool used to measure mitochondrial respiratory function (MIFU) in skeletal muscle tissue. However, microvascular co-morbidity in long-standing T2D can interfere with the (31)P MRS methodology.

AIM

To compare (31)P MRS-derived parameters describing in vivo MIFU with an in vitro assessment of muscle respiratory capacity and muscle fiber-type composition in T2D patients.

METHODS

(31)P MRS was applied in long-standing, insulin-treated T2D patients. (31)P MRS markers of MIFU were measured in the M. vastus lateralis. Muscle biopsy samples were collected from the same muscle and analyzed for succinate dehydrogenase activity (SDH) and fiber-type distribution.

RESULTS

Several (31)P MRS parameters of MIFU showed moderate to good correlations with the percentage of type I fibers and type I fiber-specific SDH activity (Pearson's R between 0.70 and 0.75). In vivo and in vitro parameters of local mitochondrial respiration also correlated well with whole-body fitness levels (VO (2peak)) in these patients (Pearson's R between 0.62 and 0.90).

CONCLUSION

Good correlations exist between in vivo and in vitro measurements of MIFU in long-standing insulin-treated T2D subjects, which are qualitatively and quantitatively consistent with previous results measured in healthy subjects. This justifies the use of (31)P MRS to measure MIFU in relation to T2D.

High-Energy Phosphate Metabolism During Calf Ergometry in Patients With Isolated Aorto-Iliac Artery Stenoses

OBJECTIVES

Patients with peripheral arterial disease (PAD) and aorto-iliac atherosclerotic lesions suffer from a broad range of complaints, such as pain at the hip, the thigh, and calf claudication. The purpose of this study was to investigate the high-energy metabolism in the calf muscle of patients with PAD with isolated aorto-iliac stenoses during incremental plantar flexion exercise.

MATERIALS AND METHODS

Using a 1.5 T whole-body magnetic resonance (MR) scanner, 12 patients with PAD with uni- or bilateral aorto-iliac atherosclerotic lesions and 10 healthy male controls underwent serial phosphor-31 MR spectroscopy during incremental exercise at 2, 3, 4, and 5 W. The phosphocreatine (PCr) time constants were calculated for each increment and recovery using a monoexponential model. In the patient group, the run-off resistance was determined on MR angiograms. In both the patients and the controls, the ankle brachial pressure index was measured.

RESULTS

The diseased legs exhibited significantly increased PCr time constants during the second and the third workload increment at 3 and 4 W, but not during the first increment at 2 W and recovery compared with normal controls. Only 3 diseased legs succeeded the last increment at 5 W. We detected significant correlations between the ankle brachial pressure index scores and the PCr time constants when including both the diseased and the control legs. The diseased legs showed a significant correlation with the run-off resistance only during the first increment.

CONCLUSIONS

Our study shows that the impairment of muscle metabolism, expressed by prolonged PCr time constants, occurs with greater work intensities in patients with aorto-iliac disease compared with patients with multisegmental PAD, as recently published, whereas our patients collective exhibited normal PCr recovery time constants. Our findings may help to understand variability of clinical symptoms in aorto-iliac PAD.

Diagnostic Value of Proton MR Spectroscopy in Peripheral Arterial Occlusive Disease: A Prospective Evaluation

OBJECTIVE

The objective of the present study was to determine the detectability of metabolic alterations in patients with peripheral arterial occlusive disease (PAOD) using proton MR spectroscopy (hydrogen-1 MR spectroscopy).

SUBJECTS AND METHODS

Twenty-seven people were included in this study: 10 patients with PAOD and a pain-free walking distance of less than 200 m served as the patient group and 17 young healthy subjects served as a control group. Hydrogen-1 MR spectroscopy was performed on a 1.5-T scanner using an extremity coil and a point-resolved spectroscopy (PRESS) sequence (TR/TE, 1,500/30; 256 repetitions). For the patient group, a voxel was localized in the gastrocnemius muscle of the diseased leg. The data were processed using standard 1H MR spectroscopy tools. The identification of resonances detected on all MR spectra was made: intramyocellular lipids at 1.2 ppm, extramyocellular lipids at 1.6 ppm, lactate at 4.1 ppm, glucose with two main peaks at 3.4 and 3.8 ppm, choline at 3.2 ppm, and creatine at 3.0 and 3.9 ppm. To avoid operator bias, three spectral intensities were measured after correcting baseline and phase of MR spectra each time. The creatine signal was used as an internal reference; thus, all spectra were scaled relative to creatine. We compared the resultant intensity ratios between the two groups using the Mann-Whitney U test.

RESULTS

The lactate-creatine quotient was higher in the patient group, with a ratio of 1.6, than in the control group, with a ratio of 0.6. The glutamate-creatine ratio was higher in the patient group than in the control group (1.3 vs 0.8, respectively). All other ratios were higher in the control group. The best ratio for differentiating between healthy subjects and patients with PAOD was the glucose-lactate ratio. The patient group had a glucose-lactate quotient of 5.4, whereas the control group had a glucose-lactate quotient of 21.5 (p = 0.001).

CONCLUSION

Proton MR spectroscopy has the potential to allow identification of patients who have PAOD on the basis of altered muscle metabolism.

Dynamic MRS and MRI of skeletal muscle function and biomechanics

MR is a powerful technique for studying the biomechanical and functional properties of skeletal muscle in vivo in health and disease. This review focuses on 31P, 1H and 13C MR spectroscopy for assessment of the dynamics of muscle metabolism and on dynamic 1H MRI methods for non-invasive measurement of the biomechanical and functional properties of skeletal muscle. The information thus obtained ranges from the microscopic level of the metabolism of the myocyte to the macroscopic level of the contractile function of muscle complexes. The MR technology presented plays a vital role in achieving a better understanding of many basic aspects of muscle function, including the regulation of mitochondrial activity and the intricate interplay between muscle fiber organization and contractile function. In addition, these tools are increasingly being employed to establish novel diagnostic procedures as well as to monitor the effects of therapeutic and lifestyle interventions for muscle disorders that have an increasing impact in modern society.

Mitochondrial defects and oxidative damage in patients with peripheral arterial disease

Abnormal mitochondrial function is present in patients with peripheral arterial disease and may contribute to its clinical manifestations. However, the specific biochemical mitochondrial defects and their association with increased oxidative stress have not been fully characterized. Gastrocnemius muscle was obtained from peripheral arterial disease patients (n = 25) and age-matched controls (n = 16) and mitochondrial parameters were measured. Complexes I through IV of the electron transport chain were individually evaluated to assess for isolated defects. Muscle was also evaluated for protein and lipid oxidative changes by measuring the levels of protein carbonyls, lipid hydroperoxides, and total 4-hydroxy-2-nonenal binding and for the activities of the antioxidant enzymes superoxide dismutase, catalase, and glutathione peroxidase. Mitochondrial electron transport chain complexes I, III, and IV in arterial disease patients demonstrated significant reductions in enzymatic activities and mitochondrial respiration compared to controls. Oxidative stress biomarker analysis demonstrated significantly increased levels of protein carbonyls, lipid hydroperoxides, and 4-hydroxy-2-nonenal compared to control muscle. Antioxidant enzyme activities were altered, with a significant decrease in superoxide dismutase activity and significant increases in catalase and glutathione peroxidase. Peripheral arterial disease is associated with abnormal mitochondrial function and evidence of significant oxidative stress.

High-energy phosphate metabolism during incremental calf exercise in patients with unilaterally symptomatic peripheral arterial disease measured by phosphor 31 magnetic resonance spectroscopy

BACKGROUND

The treadmill exercise test is the most important examination of the functional ability of patients with intermittent claudication or leg pain during exercise, but it does not provide any metabolic information in the calf muscle. The purpose of this study was to investigate the high-energy metabolism in the calf muscle during incremental progressive plantar flexion exercise of a selected peripheral arterial disease (PAD) patient group.

METHODS

Using a 1.5-T whole-body magnetic resonance scanner, 17 male patients with PAD who had 1 symptomatic and 1 asymptomatic leg and 9 healthy male controls underwent serial phosphor 31 (31P) magnetic resonance spectroscopy during incremental exercise at 2, 3, 4, and 5 W. Furthermore, magnetic resonance angiography was performed, and the ankle-brachial pressure index was determined in the patient group. The runoff resistance (ROR) was separately assessed in each patient's leg.

RESULTS

The symptomatic legs exhibited significantly increased phosphocreatine (PCr) time constants during the first three workload increments (2-4 W) and the recovery phase compared with the asymptomatic legs and the normal controls. Only two symptomatic legs reached the last increment at 5 W. Compared with the normal controls, the asymptomatic legs showed significantly increased PCr time constants only at 5 W. In the patient group, we detected significant correlations between the PCr time constants and the ROR, as well as the ankle-brachial pressure index. Moreover, the symptomatic legs presented significantly lower PCr levels and pH values at the end of exercise compared with the asymptomatic and control legs.

CONCLUSIONS

Our study shows that muscle function in PAD patients can be objectively quantified with the help of 31P magnetic resonance spectroscopy and correlates significantly with hemodynamic parameters such as ROR and ankle-brachial pressure index. Consequently, 31P magnetic resonance spectroscopy seems to be a useful method to monitor the muscle function of PAD patients for evaluation of established therapies or new therapeutic strategies during research trials.

Delayed calf muscle phosphocreatine recovery after exercise identifies peripheral arterial disease

OBJECTIVES

In this study we intend to characterize phosphocreatine (PCr) recovery kinetics with phosphorus-31 ((31)P) magnetic resonance spectroscopy in symptomatic peripheral arterial disease (PAD) patients compared with control subjects and determine the diagnostic value and reproducibility of this parameter.

BACKGROUND

Due to the inconsistent relationship between flow and function in PAD, novel techniques focused on the end-organ are needed to assess disease severity and measure therapeutic response.

METHODS

Fourteen normal subjects (5 men, age 45 +/- 14 years) and 20 patients with mild-to-moderate symptomatic PAD (12 men, age 67 +/- 10 years, mean ankle brachial index 0.62 +/- 0.13) were studied. Subjects exercised one leg to exhaustion while supine in a 1.5-T magnetic resonance scanner using a custom-built plantar flexion device. Surface coil-localized, free induction decay acquisition localized to the mid-calf was used. Each 31P spectrum consisted of 25 signal averages at a repetition time of 550 ms. The PCr recovery time constant was calculated by monoexponential fit of PCr versus time, beginning at exercise completion.

RESULTS

Median exercise time was 195.0 s in normal subjects and 162.5 s in PAD patients (p = 0.06). Despite shorter exercise times in patients, the median recovery time constant of PCr was 34.7 s in normal subjects and 91.0 s in PAD patients. Area under the receiver-operating characteristic curve was 0.925 +/- 0.045. Test-retest reliability was excellent.

CONCLUSIONS

The PCr recovery time constant is prolonged in patients with symptomatic PAD compared with normal subjects. The method is reproducible and may be useful in the identification of disease. Further study of this parameter's ability to track response to therapy as well as its prognostic capability is warranted.

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.

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.

Metabolic determinants of the onset of acidosis in exercising human muscle: a 31P-MRS study

Onset of intracellular acidosis during muscular exercise has been generally attributed to activation or hyperactivation of nonoxidative ATP production but has not been analyzed quantitatively in terms of H(+) balance, i.e., production and removal mechanisms. To address this issue, we have analyzed the relation of intracellular acidosis to H(+) balance during exercise bouts in seven healthy subjects. Each subject performed a 6-min ramp rhythmic exercise (finger flexions) at low frequency (LF, 0.47 Hz), leading to slight acidosis, and at high frequency (HF, 0.85 Hz), inducing a larger acidosis. Metabolic changes were recorded using (31)P-magnetic resonance spectroscopy. Onset of intracellular acidosis was statistically identified after 3 and 4 min of exercise for HF and LF protocols, respectively. A detailed investigation of H(+) balance indicated that, for both protocols, nonoxidative ATP production preceded a change in pH. For HF and LF protocols, H(+) consumption through the creatine kinase equilibrium was constant in the face of increasing H(+) generation and efflux. For both protocols, changes in pH were not recorded as long as sources and sinks for H(+) approximately balanced. In contrast, a significant acidosis occurred after 4 min of LF exercise and 3 min of HF exercise, whereas the rise in H(+) generation exceeded the rise in H(+) efflux at a nearly constant H(+) uptake associated with phosphocreatine breakdown. We have clearly demonstrated that intracellular acidosis in exercising muscle does not occur exclusively as a result of nonoxidative ATP production but, rather, reflects changes in overall H(+) balance.

Skeletal muscle mitochondrial ATP production rate and walking performance in peripheral arterial disease

This study tested the hypotheses that skeletal muscle mitochondrial ATP production rate (MAPR) is impaired in patients with peripheral arterial disease (PAD) and that it relates positively to their walking performances. Seven untrained patients, eight exercise-trained patients and 11 healthy controls completed a maximal walking test and had muscle sampled from the gastrocnemius medialis muscle. Muscle was analysed for its MAPR in the presence of pyruvate, palmitoyl-L-carnitine or both, as well as citrate synthase (CS) activity. MAPRs were not different between untrained PAD and controls. In contrast, MAPRs (pyruvate) were significantly higher in trained PAD vs. controls. MAPR (pyruvate combinations) was also significantly higher in trained than untrained PAD muscle. MAPR and CS activity were highly correlated with walking performance in patients, but not in controls. These data do not support the hypothesis that isolated mitochondria are functionally impaired in PAD and demonstrate that the muscle mitochondrial capacity to oxidize carbohydrate is positively related to walking performance in these patients.

Haemodynamic limitations and exercise performance in peripheral arterial disease

It has been frequently argued that haemodynamic limitations are poor predictors of exercise performance in people with peripheral arterial disease (PAD) and intermittent claudication. This review has tried to address this argument through a review of published data that appears to support or counterbalance it, brief consideration of some of the methodological limitations associated with these data, as well as some other considerations. The main argument rests primarily upon data about the resting ankle-brachial index (ABI) and/or blood flow after calf exercise or an ischaemic challenge; whereas the counter argument rests mainly on data about blood flow during walking or cyding exercise. Consideration of the limitations of all methods suggests that the measurement of blood flow during exercise has the greatest value in explaining differences in exercise performance amongst claudicants; whereas the other methods are relatively limited in their explanatory value. This strengthens the counter argument and undermines the main argument proposed by others. Consequently, asserting that haemodynamic limitations are poor predictors of exercise performance in claudicants is not justified in light of available evidence.

Pentoxifylline reverses oxidative mitochondrial defect in claudicating skeletal muscle

OBJECTIVE

Previous morphologic studies and phosphorus nuclear magnetic resonance spectroscopy (31P MRS) have suggested a primary mitochondrial defect in claudicating skeletal muscle. We hypothesized that pentoxifylline may alleviate this defect.

METHODS

The response of calf muscle bioenergetics to pentoxifylline was evaluated in 10 male, nondiabetic claudicants with 31P MRS and standard treadmill testing before and after 12 weeks of pentoxifylline therapy. Phosphocreatine (PCr) and adenosinodiphosphate (ADP) recovery rate constants, two very sensitive measures of oxidative mitochondrial function, were measured.

RESULTS

Seven of the 10 subjects had abnormal baseline PCr (<0.015 s(-1)) and ADP (<0.024 s(-1)) recovery rate constants. These 7 had significant improvement in mitochondrial function with pentoxifylline; their PCr recovery rate constants increased from 0.009 +/- 0.002 to 0.013 +/- 0.002 s(-1) (P = 0.013) and their ADP recovery rate constants increased from 0.015 +/- 0.002 to 0.022 +/- 0.002 s(-1) (P = 0.004). The remaining 3 patients had normal baseline constants and demonstrated no improvement after pentoxifylline therapy. Baseline PCr and ADP recovery rate constants inversely correlated with their corresponding percentage of improvement after pentoxifylline (P < 0.05). In addition the percentage of improvement in the PCr and ADP recovery rate constants correlated with the percentage of improvement in initial claudication distance and maximum walking capacity (P < 0.05).

CONCLUSIONS

Pentoxifylline improves the mitochondriopathy of claudicating muscle, producing the most improvement in limbs with the worse baseline mitochondrial function. These results point to a potential new mode of action for pentoxifylline in the treatment of claudication and identify a subgroup of patients with the best potential for improvement with treatment.

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.

Laboratory approach to mitochondrial diseases

Dysfunction in mitochondrial processes has been related to several pathologies. In these disorders, the cell suffers oxidative imbalance that is mostly due to defects in pyruvate metabolism, mitochondrial fatty acids oxidation, the citric acid cycle or electron transport by the mitochondrial respiratory chain. These metabolic alterations produce mitochondrial diseases that have been related to inherited syndromes, such as MERRF or MELAS. The main affected organs are brain, skeletal muscle, kidney, heart and liver, because of the high energetic demand and the oxidative metabolism. Moreover, the relationship between mitochondrial dysfunction and neurodegenerative processes, such as Parkinson disease or Alzheimer disease, as well as ageing, has been shown. Because mitochondrias are the target of several xenobiotics, such as aspirin, AZT or alcohol consumption, mitochondrial impairment has also been proposed as a mechanism of toxicity. Most laboratory tests that are available in the diagnosis of mitochondrial illness are assayed in tissue biopsies and are usually difficult to interpret. Recently, it has been shown that non-invasive techniques, such as nuclear magnetic resonance or the 2-keto[1-(13)C]isocaproic acid breath test, may be useful to assess mitochondrial function. This article attempts to show the laboratory approach to mitochondrial diseases, reviewing new techniques that could be of great value in the research of mitochondrial function, such as the 2-keto[1-(13)C]isocaproic breath test.