31P magnetic resonance spectroscopy study of phosphocreatine recovery kinetics in skeletal muscle: the issue of intersubject variability

3T 31P/1H calf muscle coil for 1H and 31P MRI/MRS integrated with NIRS data acquisition

PURPOSE

Our aim was to design and build a 3T 31P/1H calf coil that is capable of providing both good 31P and 1H transmit and receive performance, as well as being capable of accommodating a near-infrared spectroscopy (NIRS) device for simultaneous NIRS data and MRI/MRS acquisition.

METHOD

In this work, we propose a new 3T 31P/1H birdcage combination design consisting of two co-centrically positioned birdcages on the same surface to maximize transmit efficiency and sensitivity for both nuclei. The 31P birdcage is a high-pass birdcage, whereas the 1H birdcage is a low-pass one to minimize coupling. The diameter of the 31P/1H birdcage combination was designed to be large enough to accommodate a NIRS device for simultaneous NIRS data and MRI/MRS acquisition.

RESULTS

The one-layer coil structure of the birdcage combination significantly streamlines the mechanical design and coil assembly process. Full-wave simulation results show that the 31P and 1H are very well decoupled with each other, and the 1H and 31P SNR surpasses that of their standalone counterparts in the central area. Experiment results show that the inclusion of a NIRS device does not significantly affect the performance of the coil, thus enabling simultaneous NIRS and MRI readouts during exercise.

CONCLUSION

Our findings demonstrate the feasibility and effectiveness of this dual-tuned coil design for combined NIRS and MRS measurements, offering potential benefits for studying metabolic and functional changes in the skeletal muscle in vivo.

Correlation between skeletal muscle acetylcarnitine and phosphocreatine metabolism during submaximal exercise and recovery: interleaved 1H/31P MRS 7 T study

Acetylcarnitine is an essential metabolite for maintaining metabolic flexibility and glucose homeostasis. The in vivo behavior of muscle acetylcarnitine content during exercise has not been shown with magnetic resonance spectroscopy. Therefore, this study aimed to explore the behavior of skeletal muscle acetylcarnitine during rest, plantar flexion exercise, and recovery in the human gastrocnemius muscle under aerobic conditions. Ten lean volunteers and nine overweight volunteers participated in the study. A 7 T whole-body MR system with a double-tuned surface coil was used to acquire spectra from the gastrocnemius medialis. An MR-compatible ergometer was used for the plantar flexion exercise. Semi-LASER-localized 1H MR spectra and slab-localized 31P MR spectra were acquired simultaneously in one interleaved exercise/recovery session. The time-resolved interleaved 1H/31P MRS acquisition yielded excellent data quality. A between-group difference in acetylcarnitine metabolism over time was detected. Significantly slower τPCr recovery, τPCr on-kinetics, and lower Qmax in the overweight group, compared to the lean group was found. Linear relations between τPCr on-kinetics, τPCr recovery, VO2max and acetylcarnitine content were identified. In conclusion, we are the first to show in vivo changes of skeletal muscle acetylcarnitine during acute exercise and immediate exercise recovery with a submaximal aerobic workload using interleaved 1H/31P MRS at 7 T.

Effects of Hydroxyurea on Skeletal Muscle Energetics and Function in a Mildly Anemic Mouse Model

Hydroxyurea (HU) is a ribonucleotide reductase inhibitor most commonly used as a therapeutic agent in sickle cell disease (SCD) with the aim of reducing the risk of vaso-occlusion and improving oxygen transport to tissues. Previous studies suggest that HU may be even beneficial in mild anemia. However, the corresponding effects on skeletal muscle energetics and function have never been reported in such a mild anemia model. Seventeen mildly anemic HbAA Townes mice were subjected to a standardized rest-stimulation (transcutaneous stimulation)-protocol while muscle energetics using 31Phosphorus magnetic resonance spectroscopy and muscle force production were assessed and recorded. Eight mice were supplemented with hydroxyurea (HU) for 6 weeks while 9 were not (CON). HU mice displayed a higher specific total force production compared to the CON, with 501.35 ± 54.12 N/mm3 and 437.43 ± 57.10 N/mm3 respectively (+14.6%, p < 0.05). Neither the total rate of energy consumption nor the oxidative metabolic rate were significantly different between groups. The present results illustrated a positive effect of a HU chronic supplementation on skeletal muscle function in mice with mild anemia.

Association of Mitochondrial Function, Substrate Utilization, and Anaerobic Metabolism With Age-Related Perceived Fatigability

Previous work has shown that poorer mitochondrial function is associated with age-related perceived fatigability. However, whether glucose oxidation and anaerobic metabolism are intermediate factors underlying this association remains unclear. We examined the total cross-sectional association between mitochondrial function and perceived fatigability in 554 adults aged 22-99 years. Mitochondrial function was assessed by skeletal muscle oxidative capacity (kPCr) using 31P magnetic resonance spectroscopy. Perceived fatigability was measured by rating of perceived exertion after a 5-minute (0.67 m/s) treadmill walk. The intermediate role of glucose oxidation (measured by the rate of change of respiratory exchange ratio [RER change rate] during the 5-minute treadmill walk) and anaerobic metabolism (measured by ventilatory threshold [VeT] during a maximal treadmill test) was evaluated by examining their cross-sectional associations with kPCr and perceived exertion. For each 0.01/s lower kPCr, perceived fatigability was 0.47 points higher (p = .002). A 0.01/s lower kPCr was also associated with 8.3 L/min lower VeT (p < .001). Lower VeT was associated with higher fatigability at lower levels of kPCr but not at higher kPCr levels (β for interaction = 0.017, p = .002). kPCr and RER change rate were not significantly associated (p = .341), but a 0.01/min higher RER change rate was associated with 0.12-point higher fatigability (p = .001). Poorer mitochondrial function potentially contributes to higher perceived fatigability through higher glucose oxidation and higher anaerobic metabolism. Future studies to further explore the longitudinal mechanisms between these metabolic changes and fatigability are warranted.

Interleaved 31 P MRS/1 H ASL for analysis of metabolic and functional heterogeneity along human lower leg muscles at 7T

PURPOSE

MR offers the unique possibility to noninvasively investigate cellular energy metabolism via 31P MRS, while blood perfusion, which provides oxygen and substrates to the tissue, is accessible by arterial spin labeling (ASL) 1H MRI. Because metabolic and hemodynamic parameters are linked, it would be desirable to study them simultaneously. A 3D-resolved method is presented that allows such measurements with high spatiotemporal resolution and has the potential to discern differences along an exercising muscle.

METHODS

Multi-voxel localized ³¹ P MRS was temporally interleaved with multi-slice pASL 1H MRI. Phosphorus spectra were collected from two adjacent positions in gastrocnemius medialis (GM) during rest, submaximal plantar flexion exercise and recovery, while perfusion and T 2 * -weighted axial images were acquired at the same time. Seventeen healthy volunteers (9 f / 8 m) were studied at 7 T.

RESULTS

An increase of postexercise perfusion and T 2 * -weighted signal in GM positively correlated with end-exercise PCr depletion and pH drop. At proximal positions functional and metabolic activity was higher than distally, that is, perfusion increase and peak T 2 * -weighted signal, end-exercise PCr depletion, end-exercise pH, and PCr recovery time constant were significantly different. An NOE-induced SNR increase of approximately 20 % (P < .001), at rest, was found in interleaved ³¹ P spectra, when comparing to ³¹ P-only acquisitions.

CONCLUSIONS

A technique for fast, simultaneous imaging of muscle functional heterogeneity in ASL, T 2 * and acquisition of time-resolved ³¹ P  MRS data is presented. These single exercise recovery experiments can be used to investigate local variations during disease progression in patients suffering from vascular or muscular diseases.

Ischaemia-induced muscle metabolic abnormalities are poorly alleviated by endurance training in a mouse model of sickle cell disease

NEW FINDINGS

What is the central question of this study? The aim of this study was to evaluate the potential beneficial effects of endurance training during an ischaemia-reperfusion protocol in a mouse model of sickle cell disease (SCD). What is the main finding and its importance? Endurance training did not reverse the metabolic defects induced by a simulated vaso-occlusive crisis in SCD mice, with regard to intramuscular acidosis, mitochondrial dysfunction or anatomical properties. Our results suggest that endurance training would reduce the number of vaso-occlusive crises rather than the complications related to vaso-occlusive crises.

ABSTRACT

The aim of this study was to investigate whether endurance training could limit the abnormalities described in a mouse model of sickle cell disease (SCD) in response to an ischaemia-reperfusion (I/R) protocol. Ten sedentary (HbSS-SED) and nine endurance-trained (HbSS-END) SCD mice were submitted to a standardized protocol of I/R of the leg, during which ATP, phosphocreatine and inorganic phosphate concentrations and intramuscular pH were measured using magnetic resonance spectroscopy. Forty-eight hours later, skeletal muscles were harvested. Oxidative stress markers were then measured. Although the time course of protons accumulation was slightly different between trained and sedentary mice (P < 0.05), the extent of acidosis was similar at the end of the ischaemic period. The initial rate of phosphocreatine resynthesis measured at blood flow restoration, illustrating mitochondrial function, was not altered in trained mice compared with sedentary mice. Although several oxidative stress markers were not different between groups (P > 0.05), the I/R-related increase of uric acid concentration observed in sedentary SCD mice (P < 0.05) was not present in the trained group. The spleen weight, generally used as a marker of the severity of the disease, was not different between groups (P > 0.05). In conclusion, endurance training did not limit the metabolic consequences of an I/R protocol in skeletal muscle of SCD mice, suggesting that the reduction in the severity of the disease previously demonstrated in the basal state would be attributable to a reduction of the occurrence of vaso-occlusive crises rather than a decrease of the deleterious effects of vaso-occlusive crises.

Exacerbated metabolic changes in skeletal muscle of sickle cell mice submitted to an acute ischemia-reperfusion paradigm

Sickle cell disease (SCD) is characterized by painful vaso-occlusive crisis. While there are several metabolic abnormalities potentially associated with muscular ischemia-reperfusion cycles that could be harmful in the context of SCD, the metabolic consequences of such events are still unknown. Ten controls (HbAA), thirteen heterozygous (HbAS), and ten homozygous (HbSS) SCD mice were submitted to a standardized protocol of rest-ischemia-reperfusion of the left leg during which adenosine triphosphate, phosphocreatine, and inorganic phosphate concentrations as well as intramuscular pH were measured using phosphorous magnetic resonance spectroscopy (MRS). Forty-eight hours later, skeletal muscles were harvested. Oxidative stress markers were then measured on the tibialis anterior. At the end of the ischemic period, HbSS mice had a lower pH value as compared with the HbAA and HbAS groups (P<0.01). During the reperfusion period, the initial rate of phosphocreatine resynthesis was lower in HbSS mice as compared with HbAA (P<0.05) and HbAS (P<0.01) animals. No significant difference among groups was observed regarding oxidative stress markers. HbSS mice displayed a higher intramuscular acidosis during the ischemic period while their mitochondrial function was impaired as compared with their HbAA and HbAS counterparts. These metabolic abnormalities could worsen the complications related to the pathology of SCD.

Impaired Muscle Efficiency but Preserved Peripheral Hemodynamics and Mitochondrial Function With Advancing Age: Evidence From Exercise in the Young, Old, and Oldest-Old

Muscle weakness in the elderly has been linked to recurrent falls and morbidity; therefore, elucidating the mechanisms contributing to the loss of muscle function and mobility with advancing age is critical. To this aim, we comprehensively examined skeletal muscle metabolic function and hemodynamics in 11 young (23 ± 2 years), 11 old (68 ± 2 years), and 10 oldest-old (84 ± 2 years) physical activity-matched participants. Specifically, oxidative stress markers, mitochondrial function, and the ATP cost of contraction as well as peripheral hemodynamics were assessed during dynamic plantar flexion exercise at 40 per cent of maximal work rate (WRmax). Both the PCr recovery time constant and the peak rate of mitochondrial ATP synthesis were not significantly different between groups. In contrast, the ATP cost of dynamic contractions (young: 1.5 ± 1.0, old: 3.4 ± 2.1, oldest-old: 6.1 ± 3.6 mM min-1 W-1) and systemic markers of oxidative stress were signficantly increased with age, with the ATP cost of contraction being negatively correlated with WRmax (r = .59, p < .05). End-of-exercise blood flow per Watt rose significantly with increasing age (young: 37 ± 20, old: 82 ± 68, oldest-old: 154 ± 93 mL min-1 W-1). These findings suggest that the progressive deterioration of muscle contractile efficiency with advancing age may play an important role in the decline in skeletal muscle functional capacity in the elderly.

Evidence of a metabolic reserve in the skeletal muscle of elderly people

The purpose of the present study was to determine whether mitochondrial function is limited by O₂ availability or the intrinsic capacity of mitochondria to synthesize ATP in elderly individuals. To this aim, we examined, in comparison to free-flow conditions (FF), the effect of superimposing reactive hyperemia (RH), induced by a period of brief ischemia during the last min of exercise, on O₂ availability and mitochondrial function in the calf muscle. 12 healthy, untrained, elderly subjects performed dynamic plantar flexion exercise and phosphorus magnetic resonance spectroscopy (³¹P-MRS), near-infrared spectroscopy (NIRS), and Doppler ultrasound were used to assess muscle metabolism and peripheral hemodynamics. Limb blood flow [area under the curve (AUC), FF: 1.5±0.5L; RH: 3.2±1.1L, P<0.01] and convective O₂ delivery (AUC, FF: 0.30±0.13L; RH: 0.64±0.29L, P<0.01) were significantly increased in RH in comparison to FF. RH was also associated with significantly higher capillary blood flow (P<0.05) and this resulted in a 33% increase in estimated peak mitochondrial ATP synthesis rate (FF: 24±11 mM.min⁻¹; RH: 31±7 mM.min⁻¹, P<0.05). These results document a hemodynamic reserve in the contracting calf muscle of the elderly accessible by superimposing reactive hyperemia. Furthermore, this increase in O₂ availability enhanced mitochondrial function thus indicating a skeletal muscle metabolic reserve despite advancing age and low level of physical activity.

Sex-specific impact of aging on the blood pressure response to exercise

An exaggerated blood pressure (BP) response to exercise has been linked to cardiovascular disease, but little is known about the impact of age and sex on this response. Therefore, this study examined the hemodynamic and skeletal muscle metabolic response to dynamic plantar flexion exercise, at 40% of maximum plantar flexion work rate, in 40 physical activity-matched young (23 ± 1 yr, n = 20) and old (73 ± 2 yr, n = 20), equally distributed, male and female subjects. Central hemodynamics and BP (finometer), popliteal artery blood flow (Doppler ultrasound), and skeletal muscle metabolism (³¹P-magnetic resonance spectroscopy) were measured during 5 min of plantar flexion exercise. Popliteal artery blood flow and high-energy phosphate responses to exercise were not affected by age or sex, whereas aging, independent of sex, attenuated stroke volume and cardiac output responses. Systolic BP and mean arterial pressure responses were exaggerated in old women (Δ42 ± 4 and Δ28 ± 3 mmHg, respectively), with all other groups exhibiting similar increases in systolic BP (old men: Δ27 ± 8 mmHg, young men: Δ27 ± 3 mmHg, and young women: Δ22 ± 3 mmHg) and mean arterial pressure (old men: Δ15 ± 4 mmHg, young men: Δ19 ± 2 mmHg, and young women: Δ17 ± 2 mmHg). Interestingly, the exercise-induced change in systemic vascular resistance in old women (∆0.8 ± 1.0 mmHg·l^-1·min^-1) was augmented compared with young women and young and old men (∆-2.8 ± 0.5, ∆-1.6 ± 0.6, and ∆-3.18 ± 1.4 mmHg·l^-1·min^-1, respectively, P < 0.05). Thus, in combination, advancing age and female sex results in an exaggerated BP response to exercise, likely the result of a failure to reduce systemic vascular resistance. NEW & NOTEWORTHY An exaggerated blood pressure response to exercise has been linked to cardiovascular disease; however, little is known about how age and sex impact this response in healthy individuals. During dynamic exercise, older women exhibited an exaggerated blood pressure response driven by an inability to lower systemic vascular resistance.

Comparative NMR and NIRS analysis of oxygen-dependent metabolism in exercising finger flexor muscles

Muscle contraction requires the physiology to adapt rapidly to meet the surge in energy demand. To investigate the shift in metabolic control, especially between oxygen and metabolism, researchers often depend on near-infrared spectroscopy (NIRS) to measure noninvasively the tissue O₂ Because NIRS detects the overlapping myoglobin (Mb) and hemoglobin (Hb) signals in muscle, interpreting the data as an index of cellular or vascular O₂ requires deconvoluting the relative contribution. Currently, many in the NIRS field ascribe the signal to Hb. In contrast, ¹H NMR has only detected the Mb signal in contracting muscle, and comparative NIRS and NMR experiments indicate a predominant Mb contribution. The present study has examined the question of the NIRS signal origin by measuring simultaneously the ¹H NMR, ³¹P NMR, and NIRS signals in finger flexor muscles during the transition from rest to contraction, recovery, ischemia, and reperfusion. The experiment results confirm a predominant Mb contribution to the NIRS signal from muscle. Given the NMR and NIRS corroborated changes in the intracellular O₂, the analysis shows that at the onset of muscle contraction, O₂ declines immediately and reaches new steady states as contraction intensity rises. Moreover, lactate formation increases even under quite aerobic condition.

Mitochondrial function assessed by 31P MRS and BOLD MRI in non-obese type 2 diabetic rats

The study aims to characterize age‐associated changes in skeletal muscle bioenergetics by evaluating the response to ischemia‐reperfusion in the skeletal muscle of the Goto‐Kakizaki (GK) rats, a rat model of non‐obese type 2 diabetes (T2D). ³¹P magnetic resonance spectroscopy (MRS) and blood oxygen level‐dependent (BOLD) MRI was performed on the hindlimb of young (12 weeks) and adult (20 weeks) GK and Wistar (control) rats. ³¹P‐MRS and BOLD‐MRI data were acquired continuously during an ischemia and reperfusion protocol to quantify changes in phosphate metabolites and muscle oxygenation. The time constant of phosphocreatine recovery, an index of mitochondrial oxidative capacity, was not statistically different between GK rats (60.8 ± 13.9 sec in young group, 83.7 ± 13.0 sec in adult group) and their age‐matched controls (62.4 ± 11.6 sec in young group, 77.5 ± 7.1 sec in adult group). During ischemia, baseline‐normalized BOLD‐MRI signal was significantly lower in GK rats than in their age‐matched controls. These results suggest that insulin resistance leads to alterations in tissue metabolism without impaired mitochondrial oxidative capacity in GK rats.

Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice

The purpose of this study was to investigate the effects of a partial suppression of monocarboxylate transporter (MCT)-1 on skeletal muscle pH, energetics, and function (MCT1^+/- mice). Twenty-four MCT1^+/- and 13 wild-type (WT) mice were subjected to a rest-exercise-recovery protocol, allowing assessment of muscle energetics (by magnetic resonance spectroscopy) and function. The study included analysis of enzyme activities and content of protein involved in pH regulation. Skeletal muscle of MCT1^+/- mice had lower MCT1 (-61%; P < 0.05) and carbonic anhydrase (CA)-II (-54%; P < 0.05) contents. Although intramuscular pH was higher in MCT1^+/- mice at rest (P < 0.001), the mice showed higher acidosis during the first minute of exercise (P < 0.01). Then, the pH time course was similar among groups until exercise completion. MCT1^+/- mice had higher specific peak (P < 0.05) and maximum tetanic (P < 0.01) forces and lower fatigability (P < 0.001) when compared to WT mice. We conclude that both MCT1 and CAII are involved in the homeostatic control of pH in skeletal muscle, both at rest and at the onset of exercise. The improved muscle function and resistance to fatigue in MCT1^+/- mice remain unexplained.-Chatel, B., Bendahan, D., Hourdé, C., Pellerin, L., Lengacher, S., Magistretti, P., Fur, Y. L., Vilmen, C., Bernard, M., Messonnier, L. A. Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice.

Impaired cardiac and skeletal muscle bioenergetics in children, adolescents, and young adults with Barth syndrome

Barth syndrome (BTHS) is an X-linked condition characterized by altered cardiolipin metabolism and cardioskeletal myopathy. We sought to compare cardiac and skeletal muscle bioenergetics in children, adolescents, and young adults with BTHS and unaffected controls and examine their relationships with cardiac function and exercise capacity. Children/adolescents and young adults with BTHS (n = 20) and children/adolescent and young adult control participants (n = 23, total n = 43) underwent 31P magnetic resonance spectroscopy (31P-MRS) of the lower extremity (calf) and heart for estimation of skeletal muscle and cardiac bioenergetics. Peak exercise testing (VO2peak) and resting echocardiography were also performed on all participants. Cardiac PCr/ATP ratio was significantly lower in children/adolescents (BTHS: 1.5 ± 0.2 vs.

CONTROL

2.0 ± 0.3, P < 0.01) and adults (BTHS: 1.9 ± 0.2 vs.

CONTROL

2.3 ± 0.2, P < 0.01) with BTHS compared to Control groups. Adults (BTHS: 76.4 ± 31.6 vs.

CONTROL

35.0 ± 7.4 sec, P < 0.01) and children/adolescents (BTHS: 71.5 ± 21.3 vs.

CONTROL

31.4 ± 7.4 sec, P < 0.01) with BTHS had significantly longer calf PCr recovery (τPCr) postexercise compared to controls. Maximal calf ATP production through oxidative phosphorylation (Qmax-lin) was significantly lower in children/adolescents (BTHS: 0.5 ± 0.1 vs.

CONTROL

1.1 ± 0.3 mmol/L per sec, P < 0.01) and adults (BTHS: 0.5 ± 0.2 vs.

CONTROL

1.0 ± 0.2 mmol/L sec, P < 0.01) with BTHS compared to controls. Blunted cardiac and skeletal muscle bioenergetics were associated with lower VO2peak but not resting cardiac function. Cardiac and skeletal muscle bioenergetics are impaired and appear to contribute to exercise intolerance in BTHS.

Accuracy and precision of quantitative 31P-MRS measurements of human skeletal muscle mitochondrial function

Although theoretically sound, the accuracy and precision of (31)P-magnetic resonance spectroscopy ((31)P-MRS) approaches to quantitatively estimate mitochondrial capacity are not well documented. Therefore, employing four differing models of respiratory control [linear, kinetic, and multipoint adenosine diphosphate (ADP) and phosphorylation potential], this study sought to determine the accuracy and precision of (31)P-MRS assessments of peak mitochondrial adenosine-triphosphate (ATP) synthesis rate utilizing directly measured peak respiration (State 3) in permeabilized skeletal muscle fibers. In 23 subjects of different fitness levels, (31)P-MRS during a 24-s maximal isometric knee extension and high-resolution respirometry in muscle fibers from the vastus lateralis was performed. Although significantly correlated with State 3 respiration (r = 0.72), both the linear (45 ± 13 mM/min) and phosphorylation potential (47 ± 16 mM/min) models grossly overestimated the calculated in vitro peak ATP synthesis rate (P < 0.05). Of the ADP models, the kinetic model was well correlated with State 3 respiration (r = 0.72, P < 0.05), but moderately overestimated ATP synthesis rate (P < 0.05), while the multipoint model, although being somewhat less well correlated with State 3 respiration (r = 0.55, P < 0.05), most accurately reflected peak ATP synthesis rate. Of note, the PCr recovery time constant (τ), a qualitative index of mitochondrial capacity, exhibited the strongest correlation with State 3 respiration (r = 0.80, P < 0.05). Therefore, this study reveals that each of the (31)P-MRS data analyses, including PCr τ, exhibit precision in terms of mitochondrial capacity. As only the multipoint ADP model did not overstimate the peak skeletal muscle mitochondrial ATP synthesis, the multipoint ADP model is the only quantitative approach to exhibit both accuracy and precision.

Metabolic Effects of Long-Term Reduction in Free Fatty Acids With Acipimox in Obesity: A Randomized Trial

CONTEXT

Increased circulating free fatty acids (FFAs) have been proposed to contribute to insulin resistance in obesity. Short-term studies have investigated the effects of acipimox, an inhibitor of hormone-sensitive lipase, on glucose homeostasis, but longer-term studies have not been performed.

OBJECTIVE

To test the hypothesis that long-term treatment with acipimox would reduce FFA and improve insulin sensitivity among nondiabetic, insulin-resistant, obese subjects.

DESIGN, SETTING, PATIENTS, AND INTERVENTION

At an academic medical center, 39 obese men and women were randomized to acipimox 250 mg thrice-daily vs identical placebo for 6 months.

MAIN OUTCOME MEASURES

Plasma lipids, insulin sensitivity, adiponectin, and mitochondrial function via assessment of the rate of post-exercise phosphocreatine recovery on (31)P-magnetic resonance spectroscopy as well as muscle mitochondrial density and relevant muscle gene expression.

RESULTS

Fasting glucose decreased significantly in acipimox-treated individuals (effect size, -6 mg/dL; P = .02), in parallel with trends for reduced fasting insulin (effect size, -6.8 μU/mL; P = .07) and HOMA-IR (effect size, -1.96; P = .06), and significantly increased adiponectin (effect size, +668 ng/mL; P = .02). Acipimox did not affect insulin-stimulated glucose uptake, as assessed by euglycemic, hyperinsulinemic clamp. Effects on muscle mitochondrial function and density and on relevant gene expression were not seen.

CONCLUSION

These data shed light on the long-term effects of FFA reduction on insulin sensitivity, other metabolic parameters, and muscle mitochondrial function in obesity. Reduced FFA achieved by acipimox improved fasting measures of glucose homeostasis, lipids, and adiponectin but had no effect on mitochondrial function, mitochondrial density, or muscle insulin sensitivity.

Impaired mitochondrial function and reduced energy cost as a result of muscle damage

PURPOSE

Although it has been largely acknowledged that isometric neuromuscular electrostimulation (NMES) exercise induces larger muscle damage than voluntary contractions, the corresponding effects on muscle energetics remain to be determined. Voluntary exercise-induced muscle damage (EIMD) has been reported to have minor slight effects on muscle metabolic response to subsequent dynamic exercise, but the magnitude of muscle energetics alterations for NMES EIMD has never been documented.

METHODS

³¹P magnetic resonance spectroscopy measurements were performed in 13 young healthy males during a standardized rest-exercise-recovery protocol before (D0) and 2 d (D2) and 4 d (D4) after NMES EIMD on knee extensor muscles. Changes in kinetics of phosphorylated metabolite concentrations (i.e., phosphocreatine [PCr], inorganic phosphate [Pi], and adenosine triphosphate [ATP]) and pH were assessed to investigate aerobic and anaerobic rates of ATP production and energy cost of contraction (Ec).

RESULTS

Resting [Pi]/[PCr] ratio increased at D2 (+39%) and D4 (+29%), mainly owing to the increased [Pi] (+43% and +32%, respectively), whereas a significant decrease in resting pH was determined (-0.04 pH unit and -0.03 pH unit, respectively). PCr recovery rate decreased at D2 (-21%) and D4 (-23%) in conjunction with a significantly decreased total rate of ATP production at D4 (-18%) mainly owing to an altered aerobic ATP production (-19%). Paradoxically, Ec was decreased at D4 (-21%).

CONCLUSION

Overall, NMES EIMD led to intramuscular acidosis in resting muscle and mitochondrial impairment in exercising muscle. Alterations of noncontractile processes and/or adaptive mechanisms to muscle damage might account for the decreased Ec during the dynamic exercise.

FNDC5 relates to skeletal muscle IGF-I and mitochondrial function and gene expression in obese men with reduced growth hormone

OBJECTIVE

To investigate the relationship of skeletal muscle FNDC5 mRNA expression and circulating irisin to the GH/IGF-I axis and to skeletal muscle mitochondrial function and mitochondria-related gene expression in obese men.

DESIGN

Fifteen abdominally obese men with reduced growth hormone received 12weeks of recombinant human GH (rhGH). Before and after treatment, they underwent (31)P-magnetic resonance spectroscopy to evaluate phosphocreatine (PCr) recovery as a measure of mitochondrial function and skeletal muscle biopsy to assess expression of mitochondrial-related genes. Serum irisin and IGF-I and skeletal muscle FNDC5 and IGF-I mRNA were measured.

RESULTS

At baseline, skeletal muscle FNDC5 mRNA was significantly and positively associated with IGF-I mRNA (ρ=0.81, P=0.005) and rate of PCr recovery (ρ=0.79, P=0.006). Similar relationships of circulating irisin to IGF-I mRNA (ρ=0.63, P=0.05) and rate of PCr recovery (ρ=0.48, P=0.08) were demonstrated, but were not as robust as those with muscle FNDC5 expression. Both serum irisin and skeletal muscle FNDC5 mRNA were significantly associated with PPARγ (ρ=0.73, P=0.02 and ρ=0.85, P=0.002), respectively. In addition, FNDC5 mRNA was correlated with skeletal muscle PGC-1α (ρ=0.68, P=0.03), NRF1 (ρ=0.66, P=0.04) and TFAM (ρ=0.79, P=0.007) mRNA. Neither serum irisin nor muscle mRNA expression of FNDC5 changed with rhGH treatment.

CONCLUSION

These novel data in skeletal muscle demonstrate that local expression of FNDC5 is associated with mRNA expression of IGF-I and mitochondrial function and mitochondria-related gene expression in obese subjects with reduced growth hormone and suggest a potential role for FNDC5 acting locally in muscle in a low GH state. Further studies are needed to clarify the relationship between the GH/IGF-I axis and irisin.

Relationship between serum IGF-1 and skeletal muscle IGF-1 mRNA expression to phosphocreatine recovery after exercise in obese men with reduced GH

CONTEXT

GH and IGF-1 are believed to be physiological regulators of skeletal muscle mitochondria.

OBJECTIVE

The objective of this study was to examine the relationship between GH/IGF-1 and skeletal muscle mitochondria in obese subjects with reduced GH secretion in more detail.

DESIGN

Fifteen abdominally obese men with reduced GH secretion were treated for 12 weeks with recombinant human GH. Subjects underwent (31)P-magnetic resonance spectroscopy to assess phosphocreatine (PCr) recovery as an in vivo measure of skeletal muscle mitochondrial function and percutaneous muscle biopsies to assess mRNA expression of IGF-1 and mitochondrial-related genes at baseline and 12 weeks.

RESULTS

At baseline, skeletal muscle IGF-1 mRNA expression was significantly associated with PCr recovery (r = 0.79; P = .01) and nuclear respiratory factor-1 (r = 0.87; P = .001), mitochondrial transcription factor A (r = 0.86; P = .001), peroxisome proliferator-activated receptor (PPAR)γ (r = 0.72; P = .02), and PPARα (r = 0.75; P = .01) mRNA expression, and trended to an association with PPARγ coactivator 1-α (r = 0.59; P = .07) mRNA expression. However, serum IGF-1 concentration was not associated with PCr recovery or any mitochondrial gene expression (all P > .10). Administration of recombinant human GH increased both serum IGF-1 (change, 218 ± 29 μg/L; P < .0001) and IGF-1 mRNA in muscle (fold change, 2.1 ± 0.3; P = .002). Increases in serum IGF-1 were associated with improvements in total body fat (r = -0.53; P = .04), trunk fat (r = -0.55; P = .03), and lean mass (r = 0.58; P = .02), but not with PCr recovery (P > .10). Conversely, increase in muscle IGF-1 mRNA was associated with improvements in PCr recovery (r = 0.74; P = .02), but not with body composition parameters (P > .10).

CONCLUSION

These data demonstrate a novel association of skeletal muscle mitochondria with muscle IGF-1 mRNA expression, but independent of serum IGF-1 concentrations.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Evidence of Preserved Oxidative Capacity and Oxygen Delivery in the Plantar Flexor Muscles With Age

Studies examining the effect of aging on skeletal muscle oxidative capacity have yielded equivocal results; however, these investigations may have been confounded by differences in oxygen (O(2)) delivery, physical activity, and small numbers of participants. Therefore, we evaluated skeletal muscle oxidative capacity and O(2) delivery in a relatively large group (N = 40) of young (22 ± 2 years) and old (73 ± 7 years) participants matched for physical activity. After submaximal dynamic plantar flexion exercise, phosphocreatine (PCr) resynthesis ((31)P magnetic resonance spectroscopy), muscle reoxygenation (near-infrared spectroscopy), and popliteal artery blood flow (Doppler ultrasound) were measured. The phosphocreatine recovery time constant (Tau) (young: 33 ± 16; old: 30 ± 11 seconds), maximal rate of adenosine triphosphate (ATP) synthesis (young: 25 ± 9; old: 27 ± 8 mM/min), and muscle reoxygenation rates determined by the deoxyhemoglobin/myoglobin recovery Tau (young: 48 ± 5; old: 47 ± 9 seconds) were similar between groups. Similarly, although tending to be higher in the old, there were no significant age-related differences in postexercise popliteal blood flow (area under the curve: young: 1,665 ± 227 vs old: 2,404 ± 357 mL, p = .06) and convective O(2) delivery (young: 293 ± 146 vs old: 404 ± 191 mL, p = .07). In conclusion, when physical activity and O(2) delivery are similar, oxidative capacity in the plantar flexors is not affected by aging. These findings reveal that diminished skeletal muscle oxidative capacity is not an obligatory accompaniment to the aging process.

The effects of tesamorelin on phosphocreatine recovery in obese subjects with reduced GH

CONTEXT

Few studies have assessed the relationship between GH and mitochondrial function.

OBJECTIVE

The objective of this study was to determine the effects of improving IGF-I using a GHRH analog, tesamorelin, on mitochondrial function assessed by phosphocreatine (PCr) recovery using (31)P magnetic resonance spectroscopy in obese adults with reduced GH.

DESIGN

A total of 39 obese men and women with reduced GH secretion as determined by GHRH-arginine stimulation tests underwent magnetic resonance spectroscopy as part of a 12-month, double-blind, randomized, placebo-controlled trial comparing tesamorelin vs placebo. PCr recovery after submaximal exercise was assessed at baseline and at 12 months.

RESULTS

At baseline, there were no differences in age, sex, race/ethnicity, and GH or PCr parameters between tesamorelin and placebo. After 12 months, tesamorelin treatment led to a significantly greater increase in IGF-I than did placebo treatment (change, 102.9±31.8 μg/L vs 22.8±8.9 μg/L, tesamorelin vs placebo; P=.02). We demonstrated a significant positive relationship between increases in IGF-I and improvements in PCr recovery represented as ViPCr (R=0.56; P=.01). The association between IGF-I and PCr recovery was even stronger among subjects treated with tesamorelin only (ViPCr: R=0.71; P=.03). This association remained significant after controlling for age, sex, race, ethnicity, and parameters of body composition and insulin sensitivity (all P<.05).

CONCLUSIONS

Increases in IGF-I from 12 months of treatment with tesamorelin were significantly associated with improvements in PCr recovery parameters in obese men and women with reduced GH secretion, suggestive of improvements in mitochondrial function.

Interrelation of 31P-MRS metabolism measurements in resting and exercised quadriceps muscle of overweight-to-obese sedentary individuals

Phosphorus magnetic resonance spectroscopy ((31)P-MRS) enables the non-invasive evaluation of muscle metabolism. Resting Pi-to-ATP flux can be assessed through magnetization transfer (MT) techniques, and maximal oxidative flux (Q(max)) can be calculated by monitoring of phosphocreatine (PCr) recovery after exercise. In this study, the muscle metabolism parameters of 13 overweight-to-obese sedentary individuals were measured with both MT and dynamic PCr recovery measurements, and the interrelation between these measurements was investigated. In the dynamic experiments, knee extensions were performed at a workload of 30% of maximal voluntary capacity, and the consecutive PCr recovery was measured in a quadriceps muscle with a time resolution of 2 s with non-localized (31)P-MRS at 3 T. Resting skeletal muscle metabolism was assessed through MT measurements of the same muscle group at 7 T. Significant linear correlations between the Q(max) and the MT parameters k(ATP) (r = 0.77, P = 0.002) and F(ATP) (r = 0.62, P = 0.023) were found in the study population. This would imply that the MT technique can possibly be used as an alternative method to assess muscle metabolism when necessary (e.g. in individuals after stroke or in uncooperative patients).

Effects of exercise-induced intracellular acidosis on the phosphocreatine recovery kinetics: a 31P MRS study in three muscle groups in humans

Little is known about the metabolic differences that exist among different muscle groups within the same subjects. Therefore, we used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to investigate muscle oxidative capacity and the potential effects of pH on PCr recovery kinetics between muscles of different phenotypes (quadriceps (Q), finger (FF) and plantar flexors (PF)) in the same cohort of 16 untrained adults. The estimated muscle oxidative capacity was lower in Q (29 ± 12 mM min(-1), CV(inter-subject) = 42%) as compared with PF (46 ± 20 mM min(-1), CV(inter-subject) = 44%) and tended to be higher in FF (43 ± 35 mM min(-1), CV(inter-subject) = 80%). The coefficient of variation (CV) of oxidative capacity between muscles within the group was 59 ± 24%. PCr recovery time constant was correlated with end-exercise pH in Q (p < 0.01), FF (p < 0.05) and PF (p < 0.05) as well as proton efflux rate in FF (p < 0.01), PF (p < 0.01) and Q (p = 0.12). We also observed a steeper slope of the relationship between end-exercise acidosis and PCr recovery kinetics in FF compared with either PF or Q muscles. Overall, this study supports the concept of skeletal muscle heterogeneity by revealing a comparable inter- and intra-individual variability in oxidative capacity across three skeletal muscles in untrained individuals. These findings also indicate that the sensitivity of mitochondrial respiration to the inhibition associated with cytosolic acidosis is greater in the finger flexor muscles compared with locomotor muscles, which might be related to differences in permeability in the mitochondrial membrane and, to some extent, to proton efflux rates.

Changes in phosphocreatine concentration of skeletal muscle during high-intensity intermittent exercise in children and adults

PURPOSE

The aim of the present study was to test the hypotheses that a greater oxidative capacity in children results in a lower phosphocreatine (PCr) depletion, a faster PCr resynthesis and a lower muscle acidification during high-intensity intermittent exercise compared to adults.

METHODS

Sixteen children (9.4 ± 0.5 years) and 16 adults (26.1 ± 0.3 years) completed a protocol consisting of a dynamic plantar flexion (10 bouts of 30-s exercise at 25 % of one repetition maximum separated by 20-s recovery), followed by 10 min of passive recovery. Changes of PCr, ATP, inorganic phosphate, and phosphomonoesters were measured by means of (31)Phosphorous-magnetic resonance spectroscopy during and post-exercise.

RESULTS

Average PCr (percentage of [PCr] at initial rest (%[PCr]i)) at the end of the exercise (adults 17 ± 12 %[PCr]i, children 38 ± 17 %[PCr]i, P < 0.01) and recovery periods (adults 37 ± 14 %[PCr]i, children 57 ± 17 %[PCr]i, P < 0.01) was significantly lower in adults compared to children, induced by a stronger PCr decrease during the first exercise interval (adults -73 ± 10 %[PCr]i, children -55 ± 15 %[PCr]i, P < 0.01). End-exercise pH was significantly higher in children compared to adults (children 6.90 + 0.20, -0.14; adults 6.67 + 0.23, -0.15, P < 0.05).

CONCLUSIONS

From our results we suggest relatively higher rates of oxidative ATP formation in children's muscle for covering the ATP demand of high-intensity intermittent exercise compared to adults, enabling children to begin each exercise interval with significantly higher PCr concentrations and leading to an overall lower muscle acidification.

Reduced muscle oxidative capacity is independent of O2 availability in elderly people

Impaired O2 transport to skeletal muscle potentially contributes to the decline in aerobic capacity with aging. Thus, we examined whether (1) skeletal muscle oxidative capacity decreases with age and (2) O2 availability or mitochondrial capacity limits the maximal rate of mitochondrial ATP synthesis in vivo in sedentary elderly individuals. We used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to examine the PCr recovery kinetics in six young (26 ± 10 years) and six older (69 ± 3 years) sedentary subjects following 4 min of dynamic plantar flexion exercise under different fractions of inspired O2 (FiO2, normoxia 0.2; hyperoxia 1.0). End-exercise pH was not significantly different between old (7.04 ± 0.10) and young (7.05 ± 0.04) and was not affected by breathing hyperoxia (old 7.08 ± 0.08, P > 0.05 and young 7.05 ± 0.03). Likewise, end-exercise PCr was not significantly different between old (19 ± 4 mM) and young (24 ± 5 mM) and was not changed in hyperoxia. The PCr recovery time constant was significantly longer in the old (36 ± 9 s) compared to the young in normoxia (23 ± 8 s, P < 0.05) and was not significantly altered by breathing hyperoxia in both the old (35 ± 9 s) and young (29 ± 10 s) groups. Therefore, this study reveals that the muscle oxidative capacity of both sedentary young and old individuals is independent of O2 availability and that the decline in oxidative capacity with age is most likely due to limited mitochondrial content and/or mitochondrial dysfunction and not O2 availability.

Short-term training alters the control of mitochondrial respiration rate before maximal oxidative ATP synthesis

AIM

Short-term exercise training may induce metabolic and performance adaptations before any changes in mitochondrial enzyme potential. However, there has not been a study that has directly assessed changes in mitochondrial oxidative capacity or metabolic control as a consequence of such training in vivo. Therefore, we used (31) P-magnetic resonance spectroscopy ((31) P-MRS) to examine the effect of short-term plantar flexion exercise training on phosphocreatine (PCr) recovery kinetics and the control of respiration rate.

METHOD

To this aim, we investigated 12 healthy men, experienced with this exercise modality (TRA), and 7 time-control subjects (TC).

RESULTS

After 5 days of training, maximum work rate during incremental plantar flexion exercise was significantly improved (P < 0.01). During the recovery period, the maximal rate of oxidative adenosine triphosphate synthesis (PRE: 28 ± 13 mm min(-1) ; POST: 26 ± 15 mm min(-1) ) and the PCr recovery time constant (PRE: 31 ± 19 s; POST: 29 ± 16) were not significantly altered. In contrast, the Hill coefficient (nH ) describing the co-operativity between respiration rate and ADP was significantly increased in TRA (PRE: nH = 2.7 ± 1.4; POST: nH = 3.4 ± 1.9, P < 0.05). Meanwhile, there were no systematic variations in any of these variables in TC.

CONCLUSION

This study reveals that 5 days of training induces rapid adaptation in the allosteric control of respiration rate by ADP before any substantial improvement in muscle oxidative capacity occurs.

Mitochondrial function and increased convective O2 transport: implications for the assessment of mitochondrial respiration in vivo

Although phosphorus magnetic resonance spectroscopy (31P-MRS)-based evidence suggests that in vivo peak mitochondrial respiration rate in young untrained adults is limited by the intrinsic mitochondrial capacity of ATP synthesis, it remains unknown whether a large, locally targeted increase in convective O2 delivery would alter this interpretation. Consequently, we examined the effect of superimposing reactive hyperemia (RH), induced by a period of brief ischemia during the last minute of exercise, on oxygen delivery and mitochondrial function in the calf muscle of nine young adults compared with free-flow conditions (FF). To this aim, we used an integrative experimental approach combining 31P-MRS, Doppler ultrasound imaging, and near-infrared spectroscopy. Limb blood flow [area under the curve (AUC), 1.4 ± 0.8 liters in FF and 2.5 ± 0.3 liters in RH, P < 0.01] and convective O2 delivery (AUC, 0.30 ± 0.16 liters in FF and 0.54 ± 0.05 liters in RH, P < 0.01), were significantly increased in RH compared with FF. RH was also associated with significantly higher capillary blood flow (P < 0.05) and faster tissue reoxygenation mean response times (70 ± 15 s in FF and 24 ± 15 s in RH, P < 0.05). This resulted in a 43% increase in estimated peak mitochondrial ATP synthesis rate (29 ± 13 mM/min in FF and 41 ± 14 mM/min in RH, P < 0.05) whereas the phosphocreatine (PCr) recovery time constant in RH was not significantly different (P = 0.22). This comprehensive assessment of local skeletal muscle O2 availability and utilization in untrained subjects reveals that mitochondrial function, assessed in vivo by 31P-MRS, is limited by convective O2 delivery rather than an intrinsic mitochondrial limitation.

Oxygenation and flow in the limbs: Novel methods to characterize peripheral artery disease

Peripheral arterial disease (PAD) affects approximately 8 million Americans and is associated with high morbidity and increased mortality. Current therapies for PAD are limited and development of new therapeutic agents is needed. Present diagnostic methods for PAD are insensitive to the subtle microvascular and metabolic changes that occur beyond macrovacular stenosis and therefore may be less useful endpoints for clinical trials. Phosphorus-31 magnetic resonance (MR) spectroscopy, MR muscle perfusion, and MR oximetry are novel methods capable of evaluating both the macrovascular and microvascular changes that occur in PAD patients.

³¹P-magnetization transfer magnetic resonance spectroscopy measurements of in vivo metabolism

Magnetic resonance spectroscopy offers a broad range of noninvasive analytical methods for investigating metabolism in vivo. Of these, the magnetization-transfer (MT) techniques permit the estimation of the unidirectional fluxes associated with metabolic exchange reactions. Phosphorus (³¹P) MT measurements can be used to examine the bioenergetic reactions of the creatine-kinase system and the ATP synthesis/hydrolysis cycle. Observations from our group and others suggest that the inorganic phosphate (P(i)) → ATP flux in skeletal muscle may be modulated by certain conditions, including aging, insulin resistance, and diabetes, and may reflect inherent alterations in mitochondrial metabolism. However, such effects on the P(i) → ATP flux are not universally observed under conditions in which mitochondrial function, assessed by other techniques, is impaired, and recent articles have raised concerns about the absolute magnitude of the measured reaction rates. As the application of ³¹P-MT techniques becomes more widespread, this article reviews the methodology and outlines our experience with its implementation in a variety of models in vivo. Also discussed are potential limitations of the technique, complementary methods for assessing oxidative metabolism, and whether the P(i) → ATP flux is a viable biomarker of metabolic function in vivo.

The reproducibility of 31-phosphorus MRS measures of muscle energetics at 3 Tesla in trained men

Objective

Magnetic resonance spectroscopy (MRS) provides an exceptional opportunity for the study of in vivo metabolism. MRS is widely used to measure phosphorus metabolites in trained muscle, although there are no published data regarding its reproducibility in this specialized cohort. Thus, the aim of this study was to assess the reproducibility of ³¹P-MRS in trained skeletal muscle.

Methods

We recruited fifteen trained men (VO₂peak = 4.7±0.8 L min⁻¹/58±8 mL kg⁻¹ min⁻¹) and performed duplicate MR experiments during plantar flexion exercise, three weeks apart.

Results

Measures of resting phosphorus metabolites were reproducible, with 1.7 mM the smallest detectable difference in phosphocreatine (PCr). Measures of metabolites during exercise were less reliable: exercising PCr had a coefficient of variation (CV) of 27% during exercise, compared with 8% at rest. Estimates of mitochondrial function were variable, but experimentally useful. The CV of PCr_1/2t was 40%, yet much of this variance was inter-subject such that differences of <20% were detectable with n = 15, given a significance threshold of p<0.05.

Conclusions

31-phosphorus MRS provides reproducible and experimentally useful measures of phosphorus metabolites and mitochondrial function in trained human skeletal muscle.

The effect of higher ATP cost of contraction on the metabolic response to graded exercise in patients with chronic obstructive pulmonary disease

To better understand the metabolic implications of a higher ATP cost of contraction in chronic obstructive pulmonary disease (COPD), we used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to examine muscle energetics and pH in response to graded exercise. Specifically, in six patients and six well-matched healthy controls, we determined the intracellular threshold for pH (T(pH)) and inorganic phosphate-to-phosphocreatine ratio (T(Pi/PCr)) during progressive dynamic plantar flexion exercise with work rate expressed as both absolute and relative intensity. Patients with COPD displayed a lower peak power output (WRmax) compared with controls (controls 25 ± 4 W, COPD 15 ± 5 W, P = 0.01) while end-exercise pH (controls 6.79 ± 0.15, COPD 6.76 ± 0.21, P = 0.87) and PCr consumption (controls 82 ± 10%, COPD 70 ± 18%, P = 0.26) were similar between groups. Both T(pH) and T(Pi/PCr) occurred at a significantly lower absolute work rate in patients with COPD compared with controls (controls: 14.7 ± 2.4 W for T(pH) and 15.3 ± 2.4 W for T(Pi/PCr); COPD: 9.7 ± 4.5 W for T(pH) and 10.0 ± 4.6 W for T(Pi/PCr), P < 0.05), but these thresholds occurred at the same percentage of WRmax (controls: 63 ± 11% WRmax for T(pH) and 67 ± 18% WRmax for T(Pi/PCr); COPD: 59 ± 9% WRmax for T(pH) and 61 ± 12% WRmax for T(Pi/PCr), P > 0.05). Indexes of mitochondrial function, the PCr recovery time constant (controls 42 ± 7 s, COPD 45 ± 11 s, P = 0.66) and the PCr resynthesis rate (controls 105 ± 21%/min, COPD 91 ± 31%/min, P = 0.43) were similar between groups. In combination, these results reveal that when energy demand is normalized to WRmax, as a consequence of higher ATP cost of contraction, patients with COPD display the same metabolic pattern as healthy subjects, suggesting that skeletal muscle energy production is well preserved in these patients.

Skeletal muscle mitochondrial function is associated with longitudinal growth velocity in children and adolescents

CONTEXT

Periods of rapid growth require an increase in energy use and substrate formation. Mitochondrial function contributes to each of these and therefore may play a role in longitudinal growth.

METHODS

Twenty-nine children and adolescents of ages 8-15 yr were enrolled in a comprehensive longitudinal assessment of glucose homeostasis and mitochondrial function. Fasting laboratory studies and an estimate of mitochondrial function (as assessed by the time to recovery of phosphocreatine (PCr) concentration after submaximal quadriceps extension/flexion exercise using (31)P magnetic resonance spectroscopy) were obtained at baseline and annually for 2 yr.

RESULTS

Data were complete for 23 subjects. Subjects were 11.3 ± 1.9 (sd) yr old at the beginning of the study; 61% were male. Average annualized growth velocity at 1 yr for boys was 7.1 ± 1.5 cm/yr and for girls 6.5 ± 1.7 cm/yr. More rapid recovery of PCr concentration, suggestive of greater skeletal muscle oxidative phosphorylation capacity at baseline, was associated with faster growth velocity in the subsequent year (r(2) = 0.29; P = 0.008). In multivariate modeling, baseline mitochondrial function remained significantly and independently associated with growth (R(2) for model = 0.51; P = 0.05 for effect of phosphocreatine recovery time constant), controlling for age, gender, Tanner stage, body mass index Z-score, and height Z-score.

CONCLUSIONS

We report a novel association between time to recovery of PCr concentration after submaximal exercise and faster annual linear growth in healthy children. Future studies are needed to determine the physiological mechanisms and clinical consequences of this observation.

Muscle metabolic alterations assessed by 31-phosphorus magnetic resonance spectroscopy in mild Becker muscular dystrophy

Although the molecular defect causing Becker muscular dystrophy (BMD) has been identified, the biochemical mechanisms that lead to muscle necrosis remain unclear. Exercise-related muscle metabolism in 9 mildly affected BMD patients was assessed by muscle 31-phosphorus magnetic resonance spectroscopy ((31)P MRS) during an incremental workload. Compared with normal controls, BMD patients showed deregulation of resting pH and intramuscular membrane breakdown. We also observed increased reliance upon anaerobic metabolism during sustained submaximal contraction and maintenance of oxidative function during recovery.

Comparative determination of energy production rates and mitochondrial function using different 31P MRS quantitative methods in sedentary and trained subjects

Muscle energetics has been largely and quantitatively investigated using (31)P MRS. Various methods have been used to estimate the corresponding rate of oxidative ATP synthesis (ATP(ox)); however, potential differences among methods have not been investigated. In this study, we aimed to compare the rates of ATP production and energy cost in two groups of subjects with different training status using four different methods: indirect method (method 1), ADP control model (method 2) and phosphate potential control model (method 3). Method 4 was a modified version of method 3 with the introduction of a correction factor allowing for similar values to be obtained for the end-exercise oxidative ATP synthesis rate inferred from exercise measurements and the initial recovery phosphocreatine resynthesis rate. Seven sedentary and seven endurance-trained subjects performed a dynamic standardised rest-exercise-recovery protocol. We quantified the rates of ATP(ox) and anaerobic ATP synthesis (ATP(ana)) using (31)P MRS data recorded at 1.5 T. The rates of ATP(ox) over the entire exercise session were independent of the method used, except for method 4 which provided significantly higher values in both groups (p < 0.01). In addition, methods 1-3 were cross-correlated, thereby confirming their statistical agreement. The rate of ATP(ana) was significantly higher with method 1 (p < 0.01) and lower with method 4 (p < 0.01). As a result of the higher rate of ATP(ox), EC (method 4) calculated over the entire exercise session was higher and initial EC (method 1) was lower in both groups compared with the other methods. We showed in this study that the rate of ATP(ox) was independent of the calculation method, as long as no corrections (method 4) were performed. In contrast, results related to the rates of ATP(ana) were strongly affected by the calculation method and, more exactly, by the estimation of protons generated by ATP(ox). Although the absolute EC values differed between the methods, within- or between-subject comparisons are still valid given the tight relationships between them.

The association of growth hormone parameters with skeletal muscle phosphocreatine recovery in adult men

CONTEXT

Previous studies have suggested a relationship between GH and mitochondrial function. However, little is known about the relationship of specific GH indices and in vivo measures of mitochondrial function in humans.

OBJECTIVE

The objective of this study was to determine the association between GH, IGF-I, and phosphocreatine (PCr) recovery, a measure of mitochondrial function, in otherwise healthy adults.

DESIGN

Thirty-seven healthy men and women were studied at a single university medical center. Subjects underwent GH stimulation testing with GH releasing hormone-arginine and measurement of IGF-I. Mitochondrial function was determined by PCr recovery after submaximal exercise by (31)Phosphorous magnetic resonance spectroscopy. Subjects underwent assessment of lean and fat mass with use of dual energy X-ray absorptiometry.

RESULTS

There were no differences in PCr recovery between men and women (men 20.7±1.5 vs. women 24.8±1.4 mM/min; P > 0.05). IGF-I (r = 0.33; P = 0.04) was associated with PCr recovery in all subjects. Among men, IGF-I (r = 0.69; P = 0.003), peak stimulated GH (r = 0.52; P = 0.04), and GH area under the curve (AUC) (r = 0.53; P = 0.04) were significantly associated with PCr recovery. However, neither IGF-I, peak stimulated GH, nor GH AUC (all P > 0.05) were associated with PCr recovery in women. After adjusting for age, race, and physical activity, IGF-I remained significantly associated with PCr recovery (β = 0.10; P = 0.02) among men.

CONCLUSIONS

IGF-I, peak stimulated GH, and GH AUC are associated with skeletal muscle PCr recovery in men.

Evidence that a higher ATP cost of muscular contraction contributes to the lower mechanical efficiency associated with COPD: preliminary findings

Impaired metabolism in peripheral skeletal muscles potentially contributes to exercise intolerance in chronic obstructive pulmonary disease (COPD). We used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to examine the energy cost and skeletal muscle energetics in six patients with COPD during dynamic plantar flexion exercise compared with six well-matched healthy control subjects. Patients with COPD displayed a higher energy cost of muscle contraction compared with the controls (control: 6.1 ± 3.1% of rest·min(-1)·W(-1), COPD: 13.6 ± 8.3% of rest·min(-1)·W(-1), P = 0.01). Although, the initial phosphocreatine resynthesis rate was also significantly attenuated in patients with COPD compared with controls (control: 74 ± 17% of rest/min, COPD: 52 ± 13% of rest/min, P = 0.04), when scaled to power output, oxidative ATP synthesis was similar between groups (6.5 ± 2.3% of rest·min(-1)·W(-1) in control and 7.8 ± 3.9% of rest·min(-1)·W(-1) in COPD, P = 0.52). Therefore, our results reveal, for the first time that in a small subset of patients with COPD a higher ATP cost of muscle contraction may substantially contribute to the lower mechanical efficiency previously reported in this population. In addition, it appears that some patients with COPD have preserved mitochondrial function and normal energy supply in lower limb skeletal muscle.

Increased skeletal muscle phosphocreatine recovery after sub-maximal exercise is associated with increased carotid intima-media thickness

BACKGROUND

The association between skeletal muscle mitochondrial function and CVD risk in healthy subjects is unknown.

METHODS

Forty subjects were evaluated for CVD risk with lipid profile, oral glucose tolerance test and measurement of carotid intima-media thickness (cIMT). Skeletal muscle mitochondrial function was determined by phosphocreatine recovery after sub-maximal exercise with (31)Phosphorous-MRS and represented as τPCr.

RESULTS

τPCr was positively associated with age (r=+0.41; P=0.009) and cIMT (r=+0.50; P=0.001) on univariate analyses. In multivariate regression analysis controlling for age, the association between τPCr and cIMT remained significant (β=0.003; P=0.03). This association remained significant after controlling for traditional risk factors for CVD including age, gender, tobacco use, BMI, blood pressure, cholesterol and fasting glucose in a combined model (β=0.003; P=0.04; R(2)=0.53; P=0.008 for overall model).

CONCLUSIONS

These data suggest a novel association between skeletal muscle τPCr and increased cIMT, independent of age or traditional CVD risk factors.

Muscle energetics changes throughout maturation: a quantitative 31P-MRS analysis

We quantified energy production in 7 prepubescent boys (11.7 ± 0.6 yr) and 10 men (35.6 ± 7.8 yr) using (31)P-magnetic resonance spectroscopy to investigate whether development affects muscle energetics, given that resistance to fatigue has been reported to be larger before puberty. Each subject performed a finger flexions exercise at 0.7 Hz against a weight adjusted to 15% of their maximal voluntary strength for 3 min, followed by a 15-min recovery period. The total energy cost was similar in both groups throughout the exercise bout, whereas the interplay of the different metabolic pathways was different. At the onset of exercise, children exhibited a higher oxidative contribution (50 ± 15% in boys and 25 ± 8% in men, P < 0.05) to ATP production, whereas the phosphocreatine breakdown contribution was reduced (40 ± 10% in boys and 53 ± 12% in men, P < 0.05), likely as a compensatory mechanism. The anaerobic glycolysis activity was unaffected by maturation. The recovery phase also disclosed differences regarding the rates of proton efflux (6.2 ± 2.5 vs. 3.8 ± 1.9 mM · pH unit(-1) · min(-1), in boys and men, respectively, P < 0.05), and phosphocreatine recovery, which was significantly faster in boys than in men (rate constant of phosphocreatine recovery: 1.3 ± 0.5 vs. 0.7 ± 0.4 min(-1); V(max): 37.5 ± 14.5 vs. 21.1 ± 12.2 mM/min, in boys and men, respectively, P < 0.05). Our results obtained in vivo clearly showed that maturation affects muscle energetics. Children relied more on oxidative metabolism and less on creatine kinase reaction to meet energy demand during exercise. This phenomenon can be explained by a greater oxidative capacity, probably linked to a higher relative content in slow-twitch fibers before puberty.

Bio-energetic impairment in human calf muscle in thyroid disorders: a 31P MRS study

Mitochondrial metabolism particularly oxidative phosphorylation is greatly influenced by thyroid hormones. Earlier studies have described neuromuscular symptoms as well as impaired muscle metabolism in hypothyroid and hyperthyroid patients. In this study, we intend to look in to the muscle bioenergetics including phosphocreatine recovery kinetics based oxidative metabolism in thyroid dysfunction using in vivo (31)P nuclear magnetic resonance spectroscopy (MRS). (31)P MRS was carried out at resting state on 32 hypothyroid, 10 hyperthyroid patients and 25 control subjects. Nine out of 32 hypothyroid patients and 17 out of 25 control subjects under went exercise protocol for oxidative metabolism study and performed plantar flexion exercise while lying supine in 1.5 T magnetic resonance scanner using custom built exercise device. MRS measurements of inorganic phosphate (Pi), phosphocreatine (PCr), phosphodiesters (PDE) and adenosine triphosphate (ATP) of the calf muscle were acquired during rest, exercise and recovery phase. PCr recovery rate constant (k(PCr)) and oxidative capacity were calculated by monoexponential fit of PCr versus time (t) at the beginning of recovery. During resting condition in hypothyroid patients, PCr/Pi ratio was reduced whereas PDE/ATP and Pi/ATP were increased. However, in case of hyperthyroidism, an increased PCr/Pi ratio and reduced PDE/ATP and Pi/ATP were observed. The results confirmed differential energy status of the muscle due to increased or decreased levels of thyroid hormone. Our results also demonstrate reduced oxidative metabolism in hypothyroid patients based on PCr recovery kinetics. PCr recovery kinetics study after exercise revealed decreased PCr recovery rate constant (k(PCr)) in hypothyroid patients compared to controls that resulted in decrease in oxidative capacity of muscle by 50% in hypothyroids. These findings are consistent with a defect of high energy phosphate mitochondrial metabolism in thyroid dysfunction.