What do magnetic resonance-based measurements of Pi→ATP flux tell us about skeletal muscle metabolism?

Mitochondrial involvement in sarcopenia

Sarcopenia lowers the quality-of-life for millions of people across the world, as accelerated loss of skeletal muscle mass and function contributes to both age- and disease-related frailty. Physical activity remains the only proven therapy for sarcopenia to date, but alternatives are much sought after to manage this progressive muscle disorder in individuals who are unable to exercise. Mitochondria have been widely implicated in the etiology of sarcopenia and are increasingly suggested as attractive therapeutic targets to help restore the perturbed balance between protein synthesis and breakdown that underpins skeletal muscle atrophy. Reviewing current literature, we note that mitochondrial bioenergetic changes in sarcopenia are generally interpreted as intrinsic dysfunction that renders muscle cells incapable of making sufficient ATP to fuel protein synthesis. Based on the reported mitochondrial effects of therapeutic interventions, however, we argue that the observed bioenergetic changes may instead reflect an adaptation to pathologically decreased energy expenditure in sarcopenic muscle. Discrimination between these mechanistic possibilities will be crucial for improving the management of sarcopenia.

Use of 31 P magnetisation transfer magnetic resonance spectroscopy to measure ATP changes after 670 nm transcranial photobiomodulation in older adults

Mitochondrial function declines with age, and many pathological processes in neurodegenerative diseases stem from this dysfunction when mitochondria fail to produce the necessary energy required. Photobiomodulation (PBM), long-wavelength light therapy, has been shown to rescue mitochondrial function in animal models and improve human health, but clinical uptake is limited due to uncertainty around efficacy and the mechanisms responsible. Using 31 P magnetisation transfer magnetic resonance spectroscopy (MT-MRS) we quantify, for the first time, the effects of 670 nm PBM treatment on healthy ageing human brains. We find a significant increase in the rate of ATP synthase flux in the brain after PBM in a cohort of older adults. Our study provides initial evidence of PBM therapeutic efficacy for improving mitochondrial function and restoring ATP flux with age, but recognises that wider studies are now required to confirm any resultant cognitive benefits.

Nonequilibrium thermodynamics and mitochondrial protein content predict insulin sensitivity and fuel selection during exercise in human skeletal muscle

Introduction: Many investigators have attempted to define the molecular nature of changes responsible for insulin resistance in muscle, but a molecular approach may not consider the overall physiological context of muscle. Because the energetic state of ATP (ΔGATP) could affect the rate of insulin-stimulated, energy-consuming processes, the present study was undertaken to determine whether the thermodynamic state of skeletal muscle can partially explain insulin sensitivity and fuel selection independently of molecular changes. Methods: 31P-MRS was used with glucose clamps, exercise studies, muscle biopsies and proteomics to measure insulin sensitivity, thermodynamic variables, mitochondrial protein content, and aerobic capacity in 16 volunteers. Results: After showing calibrated 31P-MRS measurements conformed to a linear electrical circuit model of muscle nonequilibrium thermodynamics, we used these measurements in multiple stepwise regression against rates of insulin-stimulated glucose disposal and fuel oxidation. Multiple linear regression analyses showed 53% of the variance in insulin sensitivity was explained by 1) VO2max (p = 0.001) and the 2) slope of the relationship of ΔGATP with the rate of oxidative phosphorylation (p = 0.007). This slope represents conductance in the linear model (functional content of mitochondria). Mitochondrial protein content from proteomics was an independent predictor of fractional fat oxidation during mild exercise (R2 = 0.55, p = 0.001). Conclusion: Higher mitochondrial functional content is related to the ability of skeletal muscle to maintain a greater ΔGATP, which may lead to faster rates of insulin-stimulated processes. Mitochondrial protein content per se can explain fractional fat oxidation during mild exercise.

Mitochondrial Dysfunction in Intensive Care Unit-Acquired Weakness and Critical Illness Myopathy: A Narrative Review

Mitochondria are key structures providing most of the energy needed to maintain homeostasis. They are the main source of adenosine triphosphate (ATP), participate in glucose, lipid and amino acid metabolism, store calcium and are integral components in various intracellular signaling cascades. However, due to their crucial role in cellular integrity, mitochondrial damage and dysregulation in the context of critical illness can severely impair organ function, leading to energetic crisis and organ failure. Skeletal muscle tissue is rich in mitochondria and, therefore, particularly vulnerable to mitochondrial dysfunction. Intensive care unit-acquired weakness (ICUAW) and critical illness myopathy (CIM) are phenomena of generalized weakness and atrophying skeletal muscle wasting, including preferential myosin breakdown in critical illness, which has also been linked to mitochondrial failure. Hence, imbalanced mitochondrial dynamics, dysregulation of the respiratory chain complexes, alterations in gene expression, disturbed signal transduction as well as impaired nutrient utilization have been proposed as underlying mechanisms. This narrative review aims to highlight the current known molecular mechanisms immanent in mitochondrial dysfunction of patients suffering from ICUAW and CIM, as well as to discuss possible implications for muscle phenotype, function and therapeutic approaches.

Hyperpolarized NMR study of the impact of pyruvate dehydrogenase kinase inhibition on the pyruvate dehydrogenase and TCA flux in type 2 diabetic rat muscle

The role of pyruvate dehydrogenase in mediating lipid-induced insulin resistance stands as a central question in the pathogenesis of type 2 diabetes mellitus. Many researchers have invoked the Randle hypothesis to explain the reduced glucose disposal in skeletal muscle by envisioning an elevated acetyl CoA pool arising from increased oxidation of fatty acids. Over the years, in vivo NMR studies have challenged that monolithic view. The advent of the dissolution dynamic nuclear polarization NMR technique and a unique type 2 diabetic rat model provides an opportunity to clarify. Dynamic nuclear polarization enhances dramatically the NMR signal sensitivity and allows the measurement of metabolic kinetics in vivo. Diabetic muscle has much lower pyruvate dehydrogenase activity than control muscle, as evidenced in the conversion of [1-¹³C]lactate and [2-¹³C]pyruvate to HCO₃^- and acetyl carnitine. The pyruvate dehydrogenase kinase inhibitor, dichloroacetate, restores rapidly the diabetic pyruvate dehydrogenase activity to control level. However, diabetic muscle has a much larger dynamic change in pyruvate dehydrogenase flux than control. The dichloroacetate-induced surge in pyruvate dehydrogenase activity produces a differential amount of acetyl carnitine but does not affect the tricarboxylic acid flux. Further studies can now proceed with the dynamic nuclear polarization approach and a unique rat model to interrogate closely the biochemical mechanism interfacing oxidative metabolism with insulin resistance and metabolic inflexibility.

Origin of the 31 P MR signal at 5.3 ppm in patients with critical limb ischemia

An unknown intense signal (P_un ) with a mean chemical shift of 5.3 ppm was observed in ³¹ P MR spectra from the calf muscles of patients with the diabetic foot syndrome. The aim of the study was to identify the origin of this signal and its potential as a biomarker of muscle injury. Calf muscles of 68 diabetic patients (66.3 ± 8.6 years; body mass index = 28.2 ± 4.3 kg/m² ) and 12 age-matched healthy controls were examined by (dynamic) ³¹ P MRS (3 T system, ³¹ P/¹ H coil). Phantoms (glucose-1-phosphate, P_i and PCr) were measured at pH values of 7.05 and 7.51. At rest, P_un signals with intensities higher than 50% of the P_i intensity were observed in 10 of the 68 examined diabetic subjects. We tested two hypothetical origins of the P_un signal: (1) phosphorus from phosphoesters and (2) phosphorus from extra- and intracellular alkaline phosphate pools. 2,3-diphosphoglycerate and glucose-1-phosphate are the only phosphoesters with signals in the chemical shift region close to 5.3 ppm. Both compounds can be excluded: 2,3-diphosphoglycerate due to the missing second signal component at 6.31 ppm; glucose-1-phosphate because its chemical shifts are about 0.2 ppm downfield from the P_i signal (4.9 ppm). If the P_un signal is from phosphate, it represents a pH value of 7.54 ± 0.05. Therefore, it could correspond to signals of P_i in mitochondria. However, patients with critical limb ischemia have rather few mitochondria and so the P_un signal probably originates from interstitia. Our data suggest that the increased P_un signal observed in patients with the diabetic foot syndrome is a biomarker of severe muscular damage.

Increased ATP synthesis might counteract hepatic lipid accumulation in acromegaly

Patients with active acromegaly (ACRO) exhibit low hepatocellular lipids (HCL), despite pronounced insulin resistance (IR). This contrasts the strong association of IR with nonalcoholic fatty liver disease in the general population. Since low HCL levels in ACRO might be caused by changes in oxidative substrate metabolism, we investigated mitochondrial activity and plasma metabolomics/lipidomics in active ACRO. Fifteen subjects with ACRO and seventeen healthy controls, matched for age, BMI, sex, and body composition, underwent 31P/1H-7-T MR spectroscopy of the liver and skeletal muscle as well as plasma metabolomic profiling and an oral glucose tolerance test. Subjects with ACRO showed significantly lower HCL levels, but the ATP synthesis rate was significantly increased compared with that in controls. Furthermore, a decreased ratio of unsaturated-to-saturated intrahepatocellular fatty acids was found in subjects with ACRO. Within assessed plasma lipids, lipidomics, and metabolomics, decreased carnitine species also indicated increased mitochondrial activity. We therefore concluded that excess of growth hormone (GH) in humans counteracts HCL accumulation by increased hepatic ATP synthesis. This was accompanied by a decreased ratio of unsaturated-to-saturated lipids in hepatocytes and by a metabolomic profile, reflecting the increase in mitochondrial activity. Thus, these findings help to better understanding of GH-regulated antisteatotic pathways and provide a better insight into potentially novel therapeutic targets for treating NAFLD.

31 P magnetic resonance spectroscopy in skeletal muscle: Experts' consensus recommendations

Skeletal muscle phosphorus-31 31 P MRS is the oldest MRS methodology to be applied to in vivo metabolic research. The technical requirements of 31 P MRS in skeletal muscle depend on the research question, and to assess those questions requires understanding both the relevant muscle physiology, and how 31 P MRS methods can probe it. Here we consider basic signal-acquisition parameters related to radio frequency excitation, TR, TE, spectral resolution, shim and localisation. We make specific recommendations for studies of resting and exercising muscle, including magnetisation transfer, and for data processing. We summarise the metabolic information that can be quantitatively assessed with 31 P MRS, either measured directly or derived by calculations that depend on particular metabolic models, and we give advice on potential problems of interpretation. We give expected values and tolerable ranges for some measured quantities, and minimum requirements for reporting acquisition parameters and experimental results in publications. Reliable examination depends on a reproducible setup, standardised preconditioning of the subject, and careful control of potential difficulties, and we summarise some important considerations and potential confounders. Our recommendations include the quantification and standardisation of contraction intensity, and how best to account for heterogeneous muscle recruitment. We highlight some pitfalls in the assessment of mitochondrial function by analysis of phosphocreatine (PCr) recovery kinetics. Finally, we outline how complementary techniques (near-infrared spectroscopy, arterial spin labelling, BOLD and various other MRI and 1 H MRS measurements) can help in the physiological/metabolic interpretation of 31 P MRS studies by providing information about blood flow and oxygen delivery/utilisation. Our recommendations will assist in achieving the fullest possible reliable picture of muscle physiology and pathophysiology.

Shortening of apparent transverse relaxation time of inorganic phosphate as a breast cancer biomarker

Phosphorus MRS offers a non-invasive tool for monitoring cell energy and phospholipid metabolism and can be of additional value in diagnosing cancer and monitoring cancer therapy. In this study, we determined the transverse relaxation times of a number of phosphorous metabolites in a group of breast cancer patients by adiabatic multi-echo spectroscopic imaging at 7 T. The transverse relaxation times of phosphoethanolamine, phosphocholine, inorganic phosphate (P_i ), glycerophosphocholine and glycerophosphatidylcholine were 184 ± 8 ms, 203 ± 17 ms, 87 ± 8 ms, 240 ± 56 ms and 20 ± 10 ms, respectively. The transverse relaxation time of P_i in breast cancer tissue was less than half that of healthy fibroglandular tissue. This effect is most likely caused by an up-regulation of glycolysis in breast cancer tissue that leads to interaction of P_i with the GAPDH enzyme, which forms part of the reversible pathway of exchange of P_i with gamma-adenosine tri-phosphate, thus shortening its apparent transverse relaxation time. As healthy breast tissue shows very little glycolytic activity, the apparent T₂ shortening of P_i due to malignant transformation could possibly be used as a biomarker for cancer.

Modular 31 P wideband inversion transfer for integrative analysis of adenosine triphosphate metabolism, T1 relaxation and molecular dynamics in skeletal muscle at 7T

PURPOSE

For efficient and integrative analysis of de novo adenosine triphosphate (ATP) synthesis, creatine-kinase-mediated ATP synthesis, T₁ relaxation time, and ATP molecular motion dynamics in human skeletal muscle at rest.

METHODS

Four inversion-transfer modules differing in center inversion frequency were combined to generate amplified magnetization transfer (MT) effects in targeted MT pathways, including Pi ↔ γ-ATP, PCr ↔ γ-ATP, and ³¹ P_γ(α)ATP ↔ ³¹ P_βATP . MT effects from both forward and reverse exchange kinetic pathways were acquired to reduce potential bias and confounding factors in integrated data analysis.

RESULTS

Kinetic data collected using 4 wideband inversion modules (8 minutes each) yielded the forward exchange rate constants, k_{PCr →γ ATP} = 0.31 ± 0.05 s^-1 and k_{Pi →γ ATP} = 0.064 ± 0.012 s^-1 , and the reverse exchange rate constants, k_γATP→Pi = 0.034 ± 0.006 s^-1 and k_γATP→PCr = 1.37 ± 0.22 s^-1 , respectively. The cross-relaxation rate constant, σ_{γ(α) ↔ βATP} was -0.20 ± 0.03 s^-1 , corresponding to ATP rotational correlation time τ_c of 0.8 ± 0.1 × 10^-7 seconds. The intrinsic T₁ relaxation times were Pi (9.2 ± 1.4 seconds), PCr (6.2 ± 0.4 seconds), γ-ATP (1.8 ± 0.1 seconds), α-ATP (1.4 ± 0.1 seconds), and β-ATP (1.1 ± 0.1 seconds). Muscle ATP T₁ values were found to be significantly longer than those previously measured in the brain using a similar method.

CONCLUSION

A combination of multiple inversion transfer modules provides a comprehensive and integrated analysis of ATP metabolism and molecular motion dynamics. This relatively fast technique could be potentially useful for studying metabolic disorders in skeletal muscle.

Mitoproteomics: Tackling Mitochondrial Dysfunction in Human Disease

Mitochondria are highly dynamic and regulated organelles that historically have been defined based on their crucial role in cell metabolism. However, they are implicated in a variety of other important functions, making mitochondrial dysfunction an important axis in several pathological contexts. Despite that conventional biochemical and molecular biology approaches have provided significant insight into mitochondrial functionality, innovative techniques that provide a global view of the mitochondrion are still necessary. Proteomics fulfils this need by enabling accurate, systems-wide quantitative analysis of protein abundance. More importantly, redox proteomics approaches offer unique opportunities to tackle oxidative stress, a phenomenon that is intimately linked to aging, cardiovascular disease, and cancer. In addition, cutting-edge proteomics approaches reveal how proteins exert their functions in complex interaction networks where even subtle alterations stemming from early pathological states can be monitored. Here, we describe the proteomics approaches that will help to deepen the role of mitochondria in health and disease by assessing not only changes to mitochondrial protein composition but also alterations to their redox state and how protein interaction networks regulate mitochondrial function and dynamics. This review is aimed at showing the reader how the application of proteomics approaches during the last 20 years has revealed crucial mitochondrial roles in the context of aging, neurodegenerative disorders, metabolic disease, and cancer.

31 P T2 s of phosphomonoesters, phosphodiesters, and inorganic phosphate in the human brain at 7T

PURPOSE

To determine the phosphorus-31 T2 s of phosphomonoesters, phosphodiesters, and inorganic phosphate in the healthy human brain at 7T.

METHODS

A 3D chemical shift imaging multi-echo sequence with composite block pulses for refocusing was used to measure one free induction decay (FID) and seven full echoes with an echo spacing of 45 ms on the brain of nine healthy volunteers (age range 22-45 years; average age 27 ± 8 years). Spectral fitting was used to determine the change in metabolic signal amplitude with echo time.

RESULTS

The average apparent T2 s with their standard deviation were 202 ± 6 ms, 129 ± 6 ms, 86 ± 2 ms, 214 ± 10 ms, and 213 ± 11 ms for phosphoethanolamine, phosphocholine, inorganic phosphate, glycerophosphoethanolamine, and glycerophosphocholine, respectively.

CONCLUSION

The determined apparent T2 for phosphoethanolamine, glycerophosphocholine, and glycerophosphoethanolamine is approximately 200 ms. The lower apparent T2 value for phosphocholine is attributed to the overlap of this resonance with the 3-phosphorous resonance of 2,3-diphosphoglycerate from blood, with an apparent shorter T2 . Omitting the FID signal and the first echo of phosphocholine leads to a T2 of 182 ± 7 ms, whereas a biexponential analysis leads to 203 ± 4 ms. These values are more in line with phosphoethanolamine and the phosphodiesters. The short T2 of inorganic phosphate is subscribed to the fast reversible exchange with γ-adenosine triphosphate, which is mediated by glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase within the glycolytic pathway. Magn Reson Med 80:29-35, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Mitochondrial respiratory capacity remains stable despite a comprehensive and sustained increase in insulin sensitivity in obese patients undergoing gastric bypass surgery

AIM

It has been proposed, but not yet demonstrated by convincing evidence in published articles, that insulin resistance and mitochondrial respiratory function are causally related physiological phenomena. Here, we tested the prediction that weight loss-induced increase in insulin sensitivity will correlate with a corresponding change in mitochondrial respiratory capacity over the same time period.

METHODS

Insulin sensitivity was evaluated using the hyperinsulinaemic-euglycaemic clamp technique, and skeletal muscle mitochondrial respiratory capacity was evaluated by high-resolution respirometry in 26 patients with obesity. Each experiment was performed ~2 months and 1-2 weeks before, and ~4 and ~19 months after Roux-en-Y gastric bypass (RYGB) surgery.

RESULTS

A substantial weight loss was observed in all patients, and insulin sensitivity increased in all patients over the 21-months time period of the study. In contrast, skeletal muscle mitochondrial respiratory capacity, intrinsic mitochondrial respiratory capacity and mitochondrial content remained unchanged over the same time period.

CONCLUSION

Among obese patients with and without type 2 diabetes undergoing RYGB surgery, intrinsic mitochondrial respiratory capacity in skeletal muscle is not correlated with insulin sensitivity before or after the surgical intervention. Mitochondrial respiratory function may not be germane to the pathophysiology and/or aetiology of obesity and/or type 2 diabetes.

Dominant and sensitive control of oxidative flux by the ATP-ADP carrier in human skeletal muscle mitochondria: Effect of lysine acetylation

The adenine nucleotide translocase (ANT) of the mitochondrial inner membrane exchanges ADP for ATP. Mitochondria were isolated from human vastus lateralis muscle (n = 9). Carboxyatractyloside titration of O₂ consumption rate (J_o) at clamped [ADP] of 21 μM gave ANT abundance of 0.97 ± 0.14 nmol ANT/mg and a flux control coefficient of 82% ± 6%. Flux control fell to 1% ± 1% at saturating (2 mM) [ADP]. The KmADP for J_o was 32.4 ± 1.8 μM. In terms of the free (-3) ADP anion this KmADP was 12.0 ± 0.7 μM. A novel luciferase-based assay for ATP production gave KmADP of 13.1 ± 1.9 μM in the absence of ATP competition. The free anion KmADP in this case was 2.0 ± 0.3 μM. Targeted proteomic analyses showed significant acetylation of ANT Lysine23 and that ANT1 was the most abundant isoform. Acetylation of Lysine23 correlated positively with KmADP, r = 0.74, P = 0.022. The findings underscore the central role played by ANT in the control of oxidative phosphorylation, particularly at the energy phosphate levels associated with low ATP demand. As predicted by molecular dynamic modeling, ANT Lysine23 acetylation decreased the apparent affinity of ADP for ANT binding.

Assessing tissue metabolism by phosphorous-31 magnetic resonance spectroscopy and imaging: a methodology review

Many human diseases are caused by an imbalance between energy production and demand. Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide the unique opportunity for in vivo assessment of several fundamental events in tissue metabolism without the use of ionizing radiation. Of particular interest, phosphate metabolites that are involved in ATP generation and utilization can be quantified noninvasively by phosphorous-31 (31P) MRS/MRI. Furthermore, 31P magnetization transfer (MT) techniques allow in vivo measurement of metabolic fluxes via creatine kinase (CK) and ATP synthase. However, a major impediment for the clinical applications of 31P-MRS/MRI is the prohibitively long acquisition time and/or the low spatial resolution that are necessary to achieve adequate signal-to-noise ratio. In this review, current 31P-MRS/MRI techniques used in basic science and clinical research are presented. Recent advances in the development of fast 31P-MRS/MRI methods are also discussed.

Assessing Metabolism and Injury in Acute Human Traumatic Brain Injury with Magnetic Resonance Spectroscopy: Current and Future Applications

Traumatic brain injury (TBI) triggers a series of complex pathophysiological processes. These include abnormalities in brain energy metabolism; consequent to reduced tissue pO₂ arising from ischemia or abnormal tissue oxygen diffusion, or due to a failure of mitochondrial function. In vivo magnetic resonance spectroscopy (MRS) allows non-invasive interrogation of brain tissue metabolism in patients with acute brain injury. Nuclei with “spin,” e.g., ¹H, ³¹P, and ¹³C, are detectable using MRS and are found in metabolites at various stages of energy metabolism, possessing unique signatures due to their chemical shift or spin–spin interactions (J-coupling). The most commonly used clinical MRS technique, ¹H MRS, uses the great abundance of hydrogen atoms within molecules in brain tissue. Spectra acquired with longer echo-times include N-acetylaspartate (NAA), creatine, and choline. NAA, a marker of neuronal mitochondrial activity related to adenosine triphosphate (ATP), is reported to be lower in patients with TBI than healthy controls, and the ratio of NAA/creatine at early time points may correlate with clinical outcome. ¹H MRS acquired with shorter echo times produces a more complex spectrum, allowing detection of a wider range of metabolites.³¹ P MRS detects high-energy phosphate species, which are the end products of cellular respiration: ATP and phosphocreatine (PCr). ATP is the principal form of chemical energy in living organisms, and PCr is regarded as a readily mobilized reserve for its replenishment during periods of high utilization. The ratios of high-energy phosphates are thought to represent a balance between energy generation, reserve and use in the brain. In addition, the chemical shift difference between inorganic phosphate and PCr enables calculation of intracellular pH.¹³ C MRS detects the ¹³C isotope of carbon in brain metabolites. As the natural abundance of ¹³C is low (1.1%), ¹³C MRS is typically performed following administration of ¹³C-enriched substrates, which permits tracking of the metabolic fate of the infused ¹³C in the brain over time, and calculation of metabolic rates in a range of biochemical pathways, including glycolysis, the tricarboxylic acid cycle, and glutamate–glutamine cycling. The advent of new hyperpolarization techniques to transiently boost signal in ¹³C-enriched MRS in vivo studies shows promise in this field, and further developments are expected.

In-vivo31P-MRS of skeletal muscle and liver: A way for non-invasive assessment of their metabolism

In addition to direct assessment of high energy phosphorus containing metabolite content within tissues, phosphorus magnetic resonance spectroscopy (³¹P-MRS) provides options to measure phospholipid metabolites and cellular pH, as well as the kinetics of chemical reactions of energy metabolism in vivo. Even though the great potential of ³¹P-MR was recognized over 30 years ago, modern MR systems, as well as new, dedicated hardware and measurement techniques provide further opportunities for research of human biochemistry. This paper presents a methodological overview of the ³¹P-MR techniques that can be used for basic, physiological, or clinical research of human skeletal muscle and liver in vivo. Practical issues of ³¹P-MRS experiments and examples of potential applications are also provided. As signal localization is essential for liver ³¹P-MRS and is important for dynamic muscle examinations as well, typical localization strategies for ³¹P-MR are also described.

Mitochondrial Coupling and Contractile Efficiency in Humans with High and Low V˙O2peaks

INTRODUCTION

Endurance training elicits tremendous adaptations of the mitochondrial energetic capacity. Yet, the effects of training or physical fitness on mitochondrial efficiency during exercise are still unclear. Accordingly, the purpose of the present study was to examine in vivo the differences in mitochondrial efficiency and ATP cost of contraction during exercise in two groups of adults differing in their aerobic capacity.

METHOD

We simultaneously assessed the ATP synthesis and O2 fluxes with P-magnetic resonance spectroscopy and pulmonary gas exchange measurements in seven endurance-trained (ET, V˙O2max: 67 ± 8 mL·min⁻¹·kg⁻¹) and seven recreationally active (RA, V˙O2max: 43 ± 7 mL·min⁻¹·kg⁻¹) subjects during 6 min of dynamic moderate-intensity knee extension.

RESULTS

The ATP cost of dynamic contraction was not significantly different between ET and RA (P > 0.05). Similarly, end-exercise O2 consumption was not significantly different between groups (ET: 848 ± 155 mL·min⁻¹ and RA: 760 ± 131 mL·min⁻¹, P > 0.05). During the recovery period, the PCr offset time constant was significantly faster in ET compared with RA (ET: 32 ± 8 s and RA: 43 ± 10 s, P < 0.05), thus indicating an increased mitochondrial capacity for ATP synthesis in the quadriceps of ET. In contrast, the estimated mitochondrial efficiency during exercise was not significantly different (P/O, ET: 2.0 ± 1.0 and RA: 1.8 ± 0.4, P > 0.05). Consequently, the higher mitochondrial capacity for ATP synthesis in ET likely originated from an elevated mitochondrial volume density, mitochondria-specific respiratory capacity, and/or slower postexercise inactivation of oxidative phosphorylation by the parallel activation mechanism.

CONCLUSION

Together, these findings reveal that 1) mitochondrial and contractile efficiencies are unaltered by several years of endurance training in young adults, and 2) the training-induced improvement in mitochondrial energetic capacity appears to be independent from changes in mitochondrial coupling.

Skeletal muscle mitochondria as a target to prevent or treat type 2 diabetes mellitus

Low levels of physical activity and the presence of obesity are associated with mitochondrial dysfunction. In addition, mitochondrial dysfunction has been associated with the development of insulin resistance and type 2 diabetes mellitus (T2DM). Although the evidence for a causal relationship between mitochondrial function and insulin resistance is still weak, emerging evidence indicates that boosting mitochondrial function might be beneficial to patient health. Exercise training is probably the most recognized promoter of mitochondrial function and insulin sensitivity and hence is still regarded as the best strategy to prevent and treat T2DM. Animal data, however, have revealed several new insights into the regulation of mitochondrial metabolism, and novel targets for interventions to boost mitochondrial function have emerged. Importantly, many of these targets seem to be regulated by factors such as nutrition, ambient temperature and circadian rhythms, which provides a basis for nonpharmacological strategies to prevent or treat T2DM in humans. Here, we will review the current evidence that mitochondrial function can be targeted therapeutically to improve insulin sensitivity and to prevent T2DM, focusing mainly on human intervention studies.

A simple approach to evaluate the kinetic rate constant for ATP synthesis in resting human skeletal muscle at 7 T

Inversion transfer (IT) is a well-established technique with multiple attractive features for analysis of kinetics. However, its application in measurement of ATP synthesis rate in vivo has lagged behind the more common saturation transfer (ST) techniques. One well-recognized issue with IT is the complexity of data analysis in comparison with much simpler analysis by ST. This complexity arises, in part, because the γ-ATP spin is involved in multiple chemical reactions and magnetization exchanges, whereas Pi is involved in a single reaction, Pi → γ-ATP. By considering the reactions involving γ-ATP only as a lumped constant, the rate constant for the reaction of physiological interest, kPi→γATP , can be determined. Here, we present a new IT data analysis method to evaluate kPi→γATP using data collected from resting human skeletal muscle at 7 T. The method is based on the basic Bloch-McConnell equation, which relates kPi→γATP to m˙Pi, the rate of Pi magnetization change. The kPi→γATP value is accessed from m˙Pi data by more familiar linear correlation approaches. For a group of human subjects (n = 15), the kPi→γATP value derived for resting calf muscle was 0.066 ± 0.017 s(-1) , in agreement with literature-reported values. In this study we also explored possible time-saving strategies to speed up data acquisition for kPi→γATP evaluation using simulations. The analysis indicates that it is feasible to carry out a (31) P IT experiment in about 10 min or less at 7 T with reasonable outcome in kPi→γATP variance for measurement of ATP synthesis in resting human skeletal muscle. We believe that this new IT data analysis approach will facilitate the wide acceptance of IT to evaluate ATP synthesis rate in vivo. Copyright © 2015 John Wiley & Sons, Ltd.

Mitochondrial involvement in skeletal muscle insulin resistance: A case of imbalanced bioenergetics

Skeletal muscle insulin resistance in obesity associates with mitochondrial dysfunction, but the causality of this association is controversial. This review evaluates mitochondrial models of nutrient-induced muscle insulin resistance. It transpires that all models predict that insulin resistance arises as a result of imbalanced cellular bioenergetics. The nature and precise origin of the proposed insulin-numbing molecules differ between models but all species only accumulate when metabolic fuel supply outweighs energy demand. This observation suggests that mitochondrial deficiency in muscle insulin resistance is not merely owing to intrinsic functional defects, but could instead be an adaptation to nutrient-induced changes in energy expenditure. Such adaptive effects are likely because muscle ATP supply is fully driven by energy demand. This market-economic control of myocellular bioenergetics offers a mechanism by which insulin-signalling deficiency can cause apparent mitochondrial dysfunction, as insulin resistance lowers skeletal muscle anabolism and thus dampens ATP demand and, consequently, oxidative ATP synthesis.

Test-retest analysis of multiple 31 P magnetization exchange pathways using asymmetric adiabatic inversion

PURPOSE

A ³¹ P-MR inversion transfer (IT) method with a short adiabatic inversion pulse is proposed and its test-retest reliability was evaluated for two spectral fitting strategies.

METHODS

Assessment in a test-retest design (3 Tesla, vastus muscles, 12 healthy volunteers, 14 inversion times, 22 ms asymmetric adiabatic inversion pulse, adiabatic excitation); spectral fitting in Fitting Tool for Interrelated Arrays of Datasets (FitAID) and Java Magnetic Resonance User Interface (jMRUI); least squares solution of the Bloch-McConnell-Solomon matrix formalism including all 14 measured time-points with equal weighting.

RESULTS

The cohort averages of k[PCr→γ-ATP] (phosphocreatine, PCr; adenosine triphosphate, ATP) are 0.246 ± 0.050s^-1 versus 0.254 ± 0.050s^-1 , and k[Pi→γ-ATP] 0.086 ± 0.033s^-1 versus 0.066 ± 0.034s^-1 (average ± standard deviation, jMRUI versus FitAID). Coefficients of variation of the differences between test and retest are lowest (9.5%) for k[PCr→γ-ATP] fitted in FitAID, larger (15.2%) for the fit in jMRUI, and considerably larger for k[Pi→γ-ATP] fitted in FitAID (43.4%) or jMRUI (47.9%). The beginning of the IT effect can be observed with magnetizations above 92% for noninverted lines while inversion of the ATP resonances is better than -72%.

CONCLUSION

The performance of the asymmetric adiabatic pulse allows an accurate observation of IT effects even in the early phase; the least squares fit of the Bloch-McConnell-Solomon matrix formalism is robust; and the type of spectral fitting can influence the results significantly. Magn Reson Med 78:33-39, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Alterations of Mitochondrial Function and Insulin Sensitivity in Human Obesity and Diabetes Mellitus

Mitochondrial function refers to a broad spectrum of features such as resting mitochondrial activity, (sub)maximal oxidative phosphorylation capacity (OXPHOS), and mitochondrial dynamics, turnover, and plasticity. The interaction between mitochondria and insulin sensitivity is bidirectional and varies depending on tissue, experimental model, methodological approach, and features of mitochondrial function tested. In human skeletal muscle, mitochondrial abnormalities may be inherited (e.g., lower mitochondrial content) or acquired (e.g., impaired OXPHOS capacity and plasticity). Abnormalities ultimately lead to lower mitochondrial functionality due to or resulting in insulin resistance and type 2 diabetes mellitus. Similar mechanisms can also operate in adipose tissue and heart muscle. In contrast, mitochondrial oxidative capacity is transiently upregulated in the liver of obese insulin-resistant humans with or without fatty liver, giving rise to oxidative stress and declines in advanced fatty liver disease. These data suggest a highly tissue-specific interaction between insulin sensitivity and oxidative metabolism during the course of metabolic diseases in humans.

Evaluation of in vivo mitochondrial bioenergetics in skeletal muscle using NMR and optical methods

It is now clear that mitochondria are involved as either a cause or consequence of many chronic diseases. This central role of the mitochondria is due to their position in the cell as important integrators of cellular energetics and signaling. Mitochondrial function affects many aspects of the cellular environment such as redox homeostasis and calcium signaling, which then also exert control over mitochondrial function. This complex dynamic between mitochondrial function and the cellular environment highlights the value of examining mitochondria in vivo in the intact physiological environment. This review discusses NMR and optical approaches used to measure mitochondria ATP and oxygen fluxes that provide in vivo measures of mitochondrial capacity and quality in animal and human models. Combining these in vivo measurements with more traditional ex vivo analyses can lead to new insights into the importance of the cellular environment in controlling mitochondrial function under pathological conditions. Interpretation and underlying assumptions for each technique are discussed with the goal of providing an overview of some of the most common approaches used to measure in vivo mitochondrial function encountered in the literature.

31P magnetization transfer measurements of Pi→ATP flux in exercising human muscle

Fundamental criticisms have been made over the use of (31)P magnetic resonance spectroscopy (MRS) magnetization transfer estimates of inorganic phosphate (Pi)→ATP flux (VPi-ATP) in human resting skeletal muscle for assessing mitochondrial function. Although the discrepancy in the magnitude of VPi-ATP is now acknowledged, little is known about its metabolic determinants. Here we use a novel protocol to measure VPi-ATP in human exercising muscle for the first time. Steady-state VPi-ATP was measured at rest and over a range of exercise intensities and compared with suprabasal oxidative ATP synthesis rates estimated from the initial rates of postexercise phosphocreatine resynthesis (VATP). We define a surplus Pi→ATP flux as the difference between VPi-ATP and VATP. The coupled reactions catalyzed by the glycolytic enzymes GAPDH and phosphoglycerate kinase (PGK) have been shown to catalyze measurable exchange between ATP and Pi in some systems and have been suggested to be responsible for this surplus flux. Surplus VPi-ATP did not change between rest and exercise, even though the concentrations of Pi and ADP, which are substrates for GAPDH and PGK, respectively, increased as expected. However, involvement of these enzymes is suggested by correlations between absolute and surplus Pi→ATP flux, both at rest and during exercise, and the intensity of the phosphomonoester peak in the (31)P NMR spectrum. This peak includes contributions from sugar phosphates in the glycolytic pathway, and changes in its intensity may indicate changes in downstream glycolytic intermediates, including 3-phosphoglycerate, which has been shown to influence the exchange between ATP and Pi catalyzed by GAPDH and PGK.

Skeletal muscle alkaline Pi pool is decreased in overweight-to-obese sedentary subjects and relates to mitochondrial capacity and phosphodiester content

Defects in skeletal muscle energy metabolism are indicative of systemic disorders such as obesity or type 2 diabetes. Phosphorus magnetic resonance spectroscopy ((31)P-MRS), in particularly dynamic (31)P-MRS, provides a powerful tool for the non-invasive investigation of muscular oxidative metabolism. The increase in spectral and temporal resolution of (31)P-MRS at ultra high fields (i.e., 7T) uncovers new potential for previously implemented techniques, e.g., saturation transfer (ST) or highly resolved static spectra. In this study, we aimed to investigate the differences in muscle metabolism between overweight-to-obese sedentary (Ob/Sed) and lean active (L/Ac) individuals through dynamic, static, and ST (31)P-MRS at 7T. In addition, as the dynamic (31)P-MRS requires a complex setup and patient exercise, our aim was to identify an alternative technique that might provide a biomarker of oxidative metabolism. The Ob/Sed group exhibited lower mitochondrial capacity, and, in addition, static (31)P-MRS also revealed differences in the Pi-to-ATP exchange flux, the alkaline Pi-pool, and glycero-phosphocholine concentrations between the groups. In addition to these differences, we have identified correlations between dynamically measured oxidative flux and static concentrations of the alkaline Pi-pool and glycero-phosphocholine, suggesting the possibility of using high spectral resolution (31)P-MRS data, acquired at rest, as a marker of oxidative metabolism.

Feasibility and repeatability of localized (31) P-MRS four-angle saturation transfer (FAST) of the human gastrocnemius muscle using a surface coil at 7 T

Phosphorus ((31) P) MRS, combined with saturation transfer (ST), provides non-invasive insight into muscle energy metabolism. However, even at 7 T, the standard ST method with T1 (app) measured by inversion recovery takes about 10 min, making it impractical for dynamic examinations. An alternative method, i.e. four-angle saturation transfer (FAST), can shorten the examination time. The aim of this study was to test the feasibility, repeatability, and possible time resolution of the localized FAST technique measurement on an ultra-high-field MR system, to accelerate the measurement of both Pi -to-ATP and PCr-to-ATP reaction rates in the human gastrocnemius muscle and to test the feasibility of using the FAST method for dynamic measurements. We measured the exchange rates and metabolic fluxes in the gastrocnemius muscle of eight healthy subjects at 7 T with the depth-resolved surface coil MRS (DRESS)-localized FAST method. For comparison, a standard ST localized method was also used. The measurement time for the localized FAST experiment was 3.5 min compared with the 10 min for the standard localized ST experiment. In addition, in five healthy volunteers, Pi -to-ATP and PCr-to-ATP metabolic fluxes were measured in the gastrocnemius muscle at rest and during plantar flexion by the DRESS-localized FAST method. The repeatability of PCr-to-ATP and Pi -to-ATP exchange rate constants, determined by the slab-selective localized FAST method at 7 T, is high, as the coefficients of variation remained below 20%, and the results of the exchange rates measured with the FAST method are comparable to those measured with standard ST. During physical activity, the PCr-to-ATP metabolic flux decreased (from FCK  = 8.21 ± 1.15 mM s(-1) to FCK  = 3.86 ± 1.38 mM s(-1) ) and the Pi -to-ATP flux increased (from FATP  = 0.43 ± 0.14 mM s(-1) to FATP  = 0.74 ± 0.13 mM s(-1) ). In conclusion, we could demonstrate that measurements in the gastrocnemius muscle are feasible at rest and are short enough to be used during exercise with the DRESS-localized FAST method at 7 T.

Mitochondrial Function and Diabetes: Consequences for Skeletal and Cardiac Muscle Metabolism

SIGNIFICANCE

An early hallmark in the development of type 2 diabetes is the resistance to the effect of insulin in skeletal muscle and in the heart. Since mitochondrial function was found to be diminished in patients with type 2 diabetes, it was suggested that this defect might be involved in the etiology of insulin resistance. Although several hypotheses were suggested, yet unclear is the mechanistic link between these two phenomena.

RECENT ADVANCES

Herein, we review the evidence for disturbances in mitochondrial function in skeletal muscle and the heart in the diabetic state. Also the mechanisms involved in improving mitochondrial function are considered and, whenever possible, human data is cited.

CRITICAL ISSUES

Reported evidence shows that interventions that improve skeletal muscle mitochondrial function also improve insulin sensitivity in humans. In the heart, available data from animal studies suggests that enhancement of mitochondrial function can reverse aging-induced changes in heart function, and can be protective against cardiomyopathy and heart failure.

FUTURE DIRECTIONS

Mitochondria and their functions can be targeted with the aim of improving skeletal muscle insulin sensitivity and cardiac function. However, human clinical intervention studies are needed to fully substantiate the potential of mitochondria as a target to prevent cardiometabolic disease.

Amplification of the effects of magnetization exchange by (31) P band inversion for measuring adenosine triphosphate synthesis rates in human skeletal muscle

PURPOSE

The goal of this study was to amplify the effects of magnetization exchange between γ-adenosine triphosphate (ATP) and inorganic phosphate (Pi) for evaluation of ATP synthesis rates in human skeletal muscle.

METHODS

The strategy works by simultaneously inverting the (31) P resonances of phosphocreatine (PCr) and ATP using a wide bandwidth, adiabatic inversion radiofrequency pulse followed by observing dynamic changes in intensity of the noninverted Pi signal versus the delay time between the inversion and observation pulses. This band inversion technique significantly delays recovery of γ-ATP magnetization; consequently, the exchange reaction, Pi ↔ γ-ATP, is readily detected and easily analyzed.

RESULTS

The ATP synthesis rate measured from high-quality spectral data using this method was 0.073 ± 0.011 s(-1) in resting human skeletal muscle (N = 10). The T1 of Pi was 6.93 ± 1.90 s, consistent with the intrinsic T1 of Pi at this field. The apparent T1 of γ-ATP was 4.07 ± 0.32 s, about two-fold longer than its intrinsic T1 due to storage of magnetization in PCr.

CONCLUSION

Band inversion provides an effective method to amplify the effects of magnetization transfer between γ-ATP and Pi. The resulting data can be easily analyzed to obtain the ATP synthesis rate using a two-site exchange model.

Targeting skeletal muscle mitochondria to prevent type 2 diabetes in youth

The prevalence of type 2 diabetes (T2D) has increased dramatically over the past two decades, not only among adults but also among adolescents. T2D is a systemic disorder affecting every organ system and is especially damaging to the cardiovascular system, predisposing individuals to severe cardiac and vascular complications. The precise mechanisms that cause T2D are an area of active research. Most current theories suggest that the process begins with peripheral insulin resistance that precedes failure of the pancreatic β-cells to secrete sufficient insulin to maintain normoglycemia. A growing body of literature has highlighted multiple aspects of mitochondrial function, including oxidative phosphorylation, lipid homeostasis, and mitochondrial quality control in the regulation of peripheral insulin sensitivity. Whether the cellular mechanisms of insulin resistance in adults are comparable to that in adolescents remains unclear. This review will summarize both clinical and basic studies that shed light on how alterations in skeletal muscle mitochondrial function contribute to whole body insulin resistance and will discuss the evidence supporting high-intensity exercise training as a therapy to circumvent skeletal muscle mitochondrial dysfunction to restore insulin sensitivity in both adults and adolescents.

Use of genetically encoded sensors to monitor cytosolic ATP/ADP ratio in living cells

ATP is not only recognized as the universal energy "currency" in most cells but also plays a less well-known role as an intracellular and extracellular messenger. Here, we review novel approaches for measuring free ATP (or ATP/ADP ratios) in living mammalian cells by using genetically encoded sensors. We also discuss the key technical aspects of routine real-time ATP/ADP monitoring using as a model one of the last-generation fluorescent probes, a fusion protein commonly known as "Perceval." Finally, we present detailed guidelines for the simultaneous measurement of cytosolic ATP/ADP ratios and Ca(2+) concentrations alongside electrical parameters in individual pancreatic β cells, in which energy metabolism is tightly linked to plasma membrane excitability to control the secretion of insulin. With appropriate variations, this approach can be adapted to the study of cytosolic ATP/ADP ratios and Ca(2+) concentrations in malignant cells, two important aspects of oncometabolism.

Comparison of (31)P saturation and inversion magnetization transfer in human liver and skeletal muscle using a clinical MR system and surface coils

(31)P MRS magnetization transfer ((31)P-MT) experiments allow the estimation of exchange rates of biochemical reactions, such as the creatine kinase equilibrium and adenosine triphosphate (ATP) synthesis. Although various (31)P-MT methods have been successfully used on isolated organs or animals, their application on humans in clinical scanners poses specific challenges. This study compared two major (31)P-MT methods on a clinical MR system using heteronuclear surface coils. Although saturation transfer (ST) is the most commonly used (31)P-MT method, sequences such as inversion transfer (IT) with short pulses might be better suited for the specific hardware and software limitations of a clinical scanner. In addition, small NMR-undetectable metabolite pools can transfer MT to NMR-visible pools during long saturation pulses, which is prevented with short pulses. (31)P-MT sequences were adapted for limited pulse length, for heteronuclear transmit-receive surface coils with inhomogeneous B1 , for the need for volume selection and for the inherently low signal-to-noise ratio (SNR) on a clinical 3-T MR system. The ST and IT sequences were applied to skeletal muscle and liver in 10 healthy volunteers. Monte-Carlo simulations were used to evaluate the behavior of the IT measurements with increasing imperfections. In skeletal muscle of the thigh, ATP synthesis resulted in forward reaction constants (k) of 0.074 ± 0.022 s(-1) (ST) and 0.137 ± 0.042 s(-1) (IT), whereas the creatine kinase reaction yielded 0.459 ± 0.089 s(-1) (IT). In the liver, ATP synthesis resulted in k = 0.267 ± 0.106 s(-1) (ST), whereas the IT experiment yielded no consistent results. ST results were close to literature values; however, the IT results were either much larger than the corresponding ST values and/or were widely scattered. To summarize, ST and IT experiments can both be implemented on a clinical body scanner with heteronuclear transmit-receive surface coils; however, ST results are much more robust against experimental imperfections than the current implementation of IT.

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.

Modular organization of cardiac energy metabolism: energy conversion, transfer and feedback regulation

To meet high cellular demands, the energy metabolism of cardiac muscles is organized by precise and coordinated functioning of intracellular energetic units (ICEUs). ICEUs represent structural and functional modules integrating multiple fluxes at sites of ATP generation in mitochondria and ATP utilization by myofibrillar, sarcoplasmic reticulum and sarcolemma ion-pump ATPases. The role of ICEUs is to enhance the efficiency of vectorial intracellular energy transfer and fine tuning of oxidative ATP synthesis maintaining stable metabolite levels to adjust to intracellular energy needs through the dynamic system of compartmentalized phosphoryl transfer networks. One of the key elements in regulation of energy flux distribution and feedback communication is the selective permeability of mitochondrial outer membrane (MOM) which represents a bottleneck in adenine nucleotide and other energy metabolite transfer and microcompartmentalization. Based on the experimental and theoretical (mathematical modelling) arguments, we describe regulation of mitochondrial ATP synthesis within ICEUs allowing heart workload to be linearly correlated with oxygen consumption ensuring conditions of metabolic stability, signal communication and synchronization. Particular attention was paid to the structure-function relationship in the development of ICEU, and the role of mitochondria interaction with cytoskeletal proteins, like tubulin, in the regulation of MOM permeability in response to energy metabolic signals providing regulation of mitochondrial respiration. Emphasis was given to the importance of creatine metabolism for the cardiac energy homoeostasis.

Effects of acute lipid overload on skeletal muscle insulin resistance, metabolic flexibility, and mitochondrial performance

We hypothesized that acute lipid-induced insulin resistance would be attenuated in high-oxidative muscle of lean trained (LT) endurance athletes due to their enhanced metabolic flexibility and mitochondrial capacity. Lean sedentary (LS), obese sedentary (OS), and LT participants completed two hyperinsulinemic euglycemic clamp studies with and without (glycerol control) the coinfusion of Intralipid. Metabolic flexibility was measured by indirect calorimetry as the oxidation of fatty acids and glucose during fasted and insulin-stimulated conditions, the latter with and without lipid oversupply. Muscle biopsies were obtained for mitochondrial and insulin-signaling studies. During hyperinsulinemia without lipid, glucose infusion rate (GIR) was lowest in OS due to lower rates of nonoxidative glucose disposal (NOGD), whereas state 4 respiration was increased in all groups. Lipid infusion reduced GIR similarly in all subjects and reduced state 4 respiration. However, in LT subjects, fat oxidation was higher with lipid oversupply, and although glucose oxidation was reduced, NOGD was better preserved compared with LS and OS subjects. Mitochondrial performance was positively associated with better NOGD and insulin sensitivity in both conditions. We conclude that enhanced mitochondrial performance with exercise is related to better metabolic flexibility and insulin sensitivity in response to lipid overload.

The effect of neoadjuvant chemoradiotherapy on whole-body physical fitness and skeletal muscle mitochondrial oxidative phosphorylation in vivo in locally advanced rectal cancer patients--an observational pilot study

Background

In the United Kingdom, patients with locally advanced rectal cancer routinely receive neoadjuvant chemoradiotherapy. However, the effects of this on physical fitness are unclear. This pilot study is aimed to investigate the effect of neoadjuvant chemoradiotherapy on objectively measured in vivo muscle mitochondrial function and whole-body physical fitness.

Methods

We prospectively studied 12 patients with rectal cancer who completed standardized neoadjuvant chemoradiotherapy, recruited from a large tertiary cancer centre, between October 2012 and July 2013. All patients underwent a cardiopulmonary exercise test and a phosphorus magnetic resonance spectroscopy quadriceps muscle exercise-recovery study before and after neoadjuvant chemoradiotherapy. Data were analysed and reported blind to patient identity and clinical course. Primary variables of interest were the two physical fitness measures; oxygen uptake at estimated anaerobic threshold and oxygen uptake at Peak exercise (ml.kg⁻¹.min⁻¹), and the post-exercise phosphocreatine recovery rate constant (min⁻¹), a measure of muscle mitochondrial capacity in vivo.

Results

Median age was 67 years (IQR 64–75). Differences (95%CI) in all three primary variables were significantly negative post-NACRT: Oxygen uptake at estimated anaerobic threshold −2.4 ml.kg⁻¹.min⁻¹ (−3.8, −0.9), p = 0.004; Oxygen uptake at Peak −4.0 ml.kg⁻¹.min⁻¹ (−6.8, −1.1), p = 0.011; and post-exercise phosphocreatine recovery rate constant −0.34 min⁻¹ (−0.51, −0.17), p<0.001.

Conclusion

The significant decrease in both whole-body physical fitness and in vivo muscle mitochondrial function raises the possibility that muscle mitochondrial mechanisms, no doubt multifactorial, may be important in deterioration of physical fitness following neoadjuvant chemoradiotherapy. This may have implications for targeted interventions to improve physical fitness pre-surgery.

Trial Registration

Clinicaltrials.gov registration NCT01859442

Ectopic lipid storage in non-alcoholic fatty liver disease is not mediated by impaired mitochondrial oxidative capacity in skeletal muscle

Non-alcoholic fatty liver disease (NAFLD), characterized by lipid deposition within the liver [intrahepatocellular lipid (IHCL)], is associated with insulin resistance and the metabolic syndrome (MS). It has been suggested that impaired skeletal muscle mitochondrial function may contribute to ectopic lipid deposition, and the associated MS, by altering post-prandial energy storage. To test this hypothesis, we performed a cross-sectional study of 17 patients with NAFLD [mean±S.D.; age, 45±11 years; body mass index (BMI), 31.6±3.4 kg/m2] and 18 age- and BMI-matched healthy controls (age, 44±11 years; BMI, 30.5±5.2 kg/m2). We determined body composition by MRI, IHCL and intramyocellular (soleus and tibialis anterior) lipids (IMCLs) by proton magnetic resonance spectroscopy (1H-MRS) and skeletal muscle mitochondrial function by dynamic phosphorus magnetic resonance spectroscopy (31P-MRS) of quadriceps muscle. Although matched for BMI and total adiposity, after statistical adjustment for gender, patients with NAFLD (defined by IHCL ≥ 5.5%) had higher IHCLs (25±16% compared with 2±2%; P<0.0005) and a higher prevalence of the MS (76% compared with 28%) compared with healthy controls. Despite this, the visceral fat/subcutaneous fat ratio, IMCLs and muscle mitochondrial function were similar between the NAFLD and control groups, with no significant difference in the rate constants of post-exercise phosphocreatine (PCr) recovery (1.55±0.4 compared with 1.51±0.4 min-1), a measure of muscle mitochondrial function. In conclusion, impaired muscle mitochondrial function does not seem to underlie ectopic lipid deposition, or the accompanying features of the MS, in patients with NAFLD.

Feasibility of mapping unidirectional Pi-to-ATP fluxes in muscles of the lower leg at 7.0 Tesla

PURPOSE

To assess the feasibility of mapping the kinetics and unidirectional fluxes of inorganic phosphate (Pi) to adenosine triphosphate (ATP) reactions in the entire volume of the lower leg muscles using a three-dimensional saturation transfer (ST) phosphorus (³¹ P) imaging sequence.

THEORY AND METHODS

We imaged the lower leg muscles of five healthy subjects at 7.0 Tesla. The total experimental time was 45 min. We quantified muscle-specific forward reaction rate constants (k^'_f ) and metabolic fluxes (V_f ) of the Pi-to-ATP reaction in the tibialis anterior, the gastrocnemius, and the soleus.

RESULTS

In the tibialis anterior, k^'_f and V_f were 0.11 s^-1  ± 0.03 (mean ± standard deviation) and 0.34 mM s^-1  ± 0.10, respectively. In the gastrocnemius, k^'_f was 0.11 s^-1  ± 0.04 and V_f was 0.37 mM s^-1  ± 0.11, while in the soleus muscle k^'_f was 0.10 s^-1  ± 0.02 and V_f was 0.36 mM s^-1  ± 0.14.

CONCLUSION

Our results suggest that mapping the kinetics and unidirectional fluxes from Pi-to-ATP in both the anterior and posterior muscles of the lower leg is feasible at ultra-high field and may provide useful insights for the study of insulin resistance, diabetes and aging. Magn Reson Med 74:225-230, 2015. © 2014 Wiley Periodicals, Inc.

Exchange kinetics by inversion transfer: integrated analysis of the phosphorus metabolite kinetic exchanges in resting human skeletal muscle at 7 T

PURPOSE

To develop an inversion pulse-based, chemical exchange saturation transfer-like method for detection of (31) P magnetization exchanges among all nuclear magnetic resonance visible metabolites suitable for providing an integrated kinetic analysis of phosphorus exchange reactions in vivo.

METHODS

The exchange kinetics by inversion transfer (EKIT) sequence includes application of a frequency-selective inversion pulse arrayed over the range of relevant (31) P frequencies, followed by a constant delay and a hard readout pulse. A series of EKIT spectra, each given by a plot of Z-magnetization for each metabolite of interest versus frequency of the inversion pulse, can be generated from this single data set.

RESULTS

EKIT spectra reflect chemical exchange due to known biochemical reactions, cross-relaxation effects, and relayed magnetization transfers due to both processes. The rate constants derived from EKIT data collected on resting human skeletal muscle were: ATP synthesis via ATP synthase (0.050 ± 0.016 s(-1) ), ATP synthesis via creatine kinase (0.264 ± 0.023 s(-1) ), and cross-relaxation between neighboring spin pairs within ATP (0.164 ± 0.022 s(-1) ).

CONCLUSION

EKIT provides a simple, alternative method to detect chemical exchange, cross relaxation, and relayed magnetization transfer effects in human skeletal muscle at 7 T.

MITOCHONDRIA: investigation of in vivo muscle mitochondrial function by 31P magnetic resonance spectroscopy

The most important function of mitochondria is the production of energy in the form of ATP. The socio-economic impact of human diseases that affect skeletal muscle mitochondrial function is growing, and improving their clinical management critically depends on the development of non-invasive assays to assess mitochondrial function and monitor the effects of interventions. 31P magnetic resonance spectroscopy provides two approaches that have been used to assess in vivo ATP synthesis in skeletal muscle: measuring Pi→ATP exchange flux using saturation transfer in resting muscle, and measuring phosphocreatine recovery kinetics after exercise. However, Pi→ATP exchange does not represent net mitochondrial ATP synthesis flux and has no simple relationship with mitochondrial function. Post-exercise phosphocreatine recovery kinetics, on the other hand, yield reliable measures of muscle mitochondrial capacity in vivo, whose ability to define the site of functional defects is enhanced by combination with other non-invasive techniques.

Mitochondrial uncoupling reduces exercise capacity despite several skeletal muscle metabolic adaptations

The effects of mitochondrial uncoupling on skeletal muscle mitochondrial adaptation and maximal exercise capacity are unknown. In this study, rats were divided into a control group (CTL, n = 8) and a group treated with 2,4-dinitrophenol, a mitochondrial uncoupler, for 28 days (DNP, 30 mg·kg−1·day−1in drinking water, n = 8). The DNP group had a significantly lower body mass ( P < 0.05) and a higher resting oxygen uptake (V̇o2, P < 0.005). The incremental treadmill test showed that maximal running speed and running economy ( P < 0.01) were impaired but that maximal V̇o2(V̇o2max) was higher in the DNP-treated rats ( P < 0.05). In skinned gastrocnemius fibers, basal respiration (V0) was higher ( P < 0.01) in the DNP-treated animals, whereas the acceptor control ratio (ACR, Vmax/V0) was significantly lower ( P < 0.05), indicating a reduction in OXPHOS efficiency. In skeletal muscle, DNP activated the mitochondrial biogenesis pathway, as indicated by changes in the mRNA expression of PGC1-α and -β, NRF-1 and −2, and TFAM, and increased the mRNA expression of cytochrome oxidase 1 ( P < 0.01). The expression of two mitochondrial proteins (prohibitin and Ndufs 3) was higher after DNP treatment. Mitochondrial fission 1 protein (Fis-1) was increased in the DNP group ( P < 0.01), but mitofusin-1 and -2 were unchanged. Histochemical staining for NADH dehydrogenase and succinate dehydrogenase activity in the gastrocnemius muscle revealed an increase in the proportion of oxidative fibers after DNP treatment. Our study shows that mitochondrial uncoupling induces several skeletal muscle adaptations, highlighting the role of mitochondrial coupling as a critical factor for maximal exercise capacities. These results emphasize the importance of investigating the qualitative aspects of mitochondrial function in addition to the amount of mitochondria.

Molecular imaging of brown adipose tissue in health and disease

PURPOSE

Brown adipose tissue (BAT) has transformed from an interfering tissue in oncological (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to an independent imaging research field. This review takes the perspective from the imaging methodology on which human BAT research has come to rely on heavily.

METHODS

This review analyses relevant PubMed-indexed publications that discuss molecular imaging methods of BAT. In addition, reported links between BAT and human diseases such as obesity are discussed, and the possibilities for imaging in these fields are highlighted. Radiopharmaceuticals aiming at several different biological mechanisms of BAT are discussed and evaluated.

RESULTS

Prospective, dedicated studies allow visualization of BAT function in a high percentage of human subjects. BAT dysfunction has been implicated in obesity, linked with diabetes and associated with cachexia and atherosclerosis. Presently, (18)F-FDG PET/CT is the most useful tool for evaluating therapies aiming at BAT activity. In addition to (18)F-FDG, other radiopharmaceuticals such as (99m)Tc-sestamibi, (123)I-metaiodobenzylguanidine (MIBG), (18)F-fluorodopa and (18)F-14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid (FTHA) may have a potential for visualizing other aspects of BAT activity. MRI methods are under continuous development and provide the prospect of functional imaging without ionizing radiation.

CONCLUSION

Molecular imaging of BAT can be used to quantitatively assess different aspects of BAT metabolic activity.

Skeletal muscle ATP kinetics are impaired in frail mice

The interleukin-10 knockout mouse (IL10(tm/tm)) has been proposed as a model for human frailty, a geriatric syndrome characterized by skeletal muscle (SM) weakness, because it develops an age-related decline in SM strength compared to control (C57BL/6J) mice. Compromised energy metabolism and energy deprivation appear to play a central role in muscle weakness in metabolic myopathies and muscular dystrophies. Nonetheless, it is not known whether SM energy metabolism is altered in frailty. A combination of in vivo (31)P nuclear magnetic resonance experiments and biochemical assays was used to measure high-energy phosphate concentrations, the rate of ATP synthesis via creatine kinase (CK), the primary energy reserve reaction in SM, as well as the unidirectional rates of ATP synthesis from inorganic phosphate (Pi) in hind limb SM of 92-week-old control (n = 7) and IL10(tm/tm) (n = 6) mice. SM Phosphocreatine (20.2 ± 2.3 vs. 16.8 ± 2.3 μmol/g, control vs. IL10(tm/tm), p < 0.05), ATP flux via CK (5.0 ± 0.9 vs. 3.1 ± 1.1 μmol/g/s, p < 0.01), ATP synthesis from inorganic phosphate (Pi → ATP) (0.58 ± 0.3 vs. 0.26 ± 0.2 μmol/g/s, p < 0.05) and the free energy released from ATP hydrolysis (∆G ∼ATP) were significantly lower and [Pi] (2.8 ± 1.0 vs. 5.3 ± 2.0 μmol/g, control vs. IL10(tm/tm), p < 0.05) markedly higher in IL10(tm/tm) than in control mice. These observations demonstrate that, despite normal in vitro metabolic enzyme activities, in vivo SM ATP kinetics, high-energy phosphate levels and energy release from ATP hydrolysis are reduced and inorganic phosphate is elevated in a murine model of frailty. These observations do not prove, but are consistent with the premise, that energetic abnormalities may contribute metabolically to SM weakness in this geriatric syndrome.

Mitochondrial respiratory capacity and content are normal in young insulin-resistant obese humans

Considerable debate exists about whether alterations in mitochondrial respiratory capacity and/or content play a causal role in the development of insulin resistance during obesity. The current study was undertaken to determine whether such alterations are present during the initial stages of insulin resistance in humans. Young (∼23 years) insulin-sensitive lean and insulin-resistant obese men and women were studied. Insulin resistance was confirmed through an intravenous glucose tolerance test. Measures of mitochondrial respiratory capacity and content as well as H(2)O(2) emitting potential and the cellular redox environment were performed in permeabilized myofibers and primary myotubes prepared from vastus lateralis muscle biopsy specimens. No differences in mitochondrial respiratory function or content were observed between lean and obese subjects, despite elevations in H(2)O(2) emission rates and reductions in cellular glutathione. These findings were apparent in permeabilized myofibers as well as in primary myotubes. The results suggest that reductions in mitochondrial respiratory capacity and content are not required for the initial manifestation of peripheral insulin resistance.

Defective mitochondrial function in vivo in skeletal muscle in adults with Down's syndrome: a 31P-MRS study

Down's syndrome (DS) is a developmental disorder associated with intellectual disability (ID). We have previously shown that people with DS engage in very low levels of exercise compared to people with ID not due to DS. Many aspects of the DS phenotype, such as dementia, low activity levels and poor muscle tone, are shared with disorders of mitochondrial origin, and mitochondrial dysfunction has been demonstrated in cultured DS tissue. We undertook a phosphorus magnetic resonance spectroscopy ((31)P-MRS) study in the quadriceps muscle of 14 people with DS and 11 non-DS ID controls to investigate the post-exercise resynthesis kinetics of phosphocreatine (PCr), which relies on mitochondrial respiratory function and yields a measure of muscle mitochondrial function in vivo. We found that the PCr recovery rate constant was significantly decreased in adults with DS compared to non-DS ID controls (1.7 ± 0.1 min(-1) vs 2.1 ± 0.1 min(-1) respectively) who were matched for physical activity levels, indicating that muscle mitochondrial function in vivo is impaired in DS. This is the first study to investigate mitochondrial function in vivo in DS using (31)P-MRS. Our study is consistent with previous in vitro studies, supporting a theory of a global mitochondrial defect in DS.

Chemistry and biochemistry of 13C hyperpolarized magnetic resonance using dynamic nuclear polarization

The study of transient chemical phenomena by conventional NMR has proved elusive, particularly for non-(1)H nuclei. For (13)C, hyperpolarization using the dynamic nuclear polarization (DNP) technique has emerged as a powerful means to improve SNR. The recent development of rapid dissolution DNP methods has facilitated previously impossible in vitro and in vivo study of small molecules. This review presents the basics of the DNP technique, identification of appropriate DNP substrates, and approaches to increase hyperpolarized signal lifetimes. Also addressed are the biochemical events to which DNP-NMR has been applied, with descriptions of several probes that have met with in vivo success.

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).

Application of electron paramagnetic resonance spectroscopy to comparative examination of different groups of free radicals in thermal injuries treated with propolis and silver sulphadiazine

Different groups of free radicals expressed in burn wounds treated with propolis and silver sulphadiazine were examined. The thermal effect forms major types of free radicals in a wound because of the breaking of chemical bonds. Free radicals, located in the heated skin, were tested after 21 days of treating by these two substances. The aim of this work was to find the method for determination of types and concentrations of different groups of free radicals in wound after high temperature impact during burning. The effects of the therapy by propolis and silver sulphadiazine on free radicals were studied. Since the chemical methods of free radicals studies are destructive, the usefulness of the electron paramagnetic resonance spectroscopy was tested in this work. The electron paramagnetic resonance spectra measured with the microwave power of 2.2 mW were numerically fitted by theoretical curves of Gaussian and Lorentzian shapes. The experimental electron paramagnetic resonance spectra of tissue samples are best fitted by the sum of one Gauss and two Lorentz lines. An innovatory numerical procedure of spectroscopic skin analysis was presented. It is very useful in the alternative medicine studies.

Methods for assessing mitochondrial function in diabetes

A growing body of research is investigating the potential contribution of mitochondrial function to the etiology of type 2 diabetes. Numerous in vitro, in situ, and in vivo methodologies are available to examine various aspects of mitochondrial function, each requiring an understanding of their principles, advantages, and limitations. This review provides investigators with a critical overview of the strengths, limitations and critical experimental parameters to consider when selecting and conducting studies on mitochondrial function. In vitro (isolated mitochondria) and in situ (permeabilized cells/tissue) approaches provide direct access to the mitochondria, allowing for study of mitochondrial bioenergetics and redox function under defined substrate conditions. Several experimental parameters must be tightly controlled, including assay media, temperature, oxygen concentration, and in the case of permeabilized skeletal muscle, the contractile state of the fibers. Recently developed technology now offers the opportunity to measure oxygen consumption in intact cultured cells. Magnetic resonance spectroscopy provides the most direct way of assessing mitochondrial function in vivo with interpretations based on specific modeling approaches. The continuing rapid evolution of these technologies offers new and exciting opportunities for deciphering the potential role of mitochondrial function in the etiology and treatment of diabetes.