Interrelations of muscle functional MRI,diffusion‐weighted MRI and 31P‐MRS in exercised lower back muscles

Exercise‐induced changes of transverse proton relaxation time (T₂), tissue perfusion and metabolic turnover were investigated in the lower back muscles of volunteers by applying muscle functional MRI (mfMRI) and diffusion‐weighted imaging (DWI) before and after as well as dynamic ³¹P‐MRS during the exercise. Inner (M. multifidus, MF) and outer lower back muscles (M. erector spinae, ES) were examined in 14 healthy young men performing a sustained isometric trunk‐extension. Significant phosphocreatine (PCr) depletions ranging from 30% (ES) to 34% (MF) and Pi accumulations between 95% (left ES) and 120%–140% (MF muscles and right ES) were observed during the exercise, which were accompanied by significantly decreased pH values in all muscles (?pH ≈ –0.05). Baseline T₂ values were similar across all investigated muscles (approximately 27 ms at 3 T), but revealed right–left asymmetric increases (T₂,_inc) after the exercise (right ES/MF: T₂,_inc = 11.8/9.7%; left ES/MF: T₂,_inc = 4.6/8.9%). Analyzed muscles also showed load‐induced increases in molecular diffusion D (p = .007) and perfusion fraction f (p = .002). The latter parameter was significantly higher in the MF than in the ES muscles both at rest and post exercise. Changes in PCr (p = .03), diffusion (p < .01) and perfusion (p = .03) were strongly associated with T_2,inc, and linear mixed model analysis revealed that changes in PCr and perfusion both affect T_2,inc (p < .001). These findings support previous assumptions that T₂ changes are not only an intra‐cellular phenomenon resulting from metabolic stress but are also affected by increased perfusion in loaded muscles. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamic multivoxel‐localized 31P MRS during plantar flexion exercise with variable knee angle

Exercise studies investigating the metabolic response of calf muscles using ³¹P MRS are usually performed with a single knee angle. However, during natural movement, the distribution of workload between the main contributors to force, gastrocnemius and soleus is influenced by the knee angle. Hence, it is of interest to measure the respective metabolic response of these muscles to exercise as a function of knee angle using localized spectroscopy. Time‐resolved multivoxel ³¹P MRS at 7 T was performed simultaneously in gastrocnemius medialis and soleus during rest, plantar flexion exercise and recovery in 12 healthy volunteers. This experiment was conducted with four different knee angles. PCr depletions correlated negatively with knee angle in gastrocnemius medialis, decreasing from 79±14 % (extended leg) to 35±23 %(～40°), and positively in soleus, increasing from 20±21 % to 36±25 %; differences were significant. Linear correlations were found between knee angle and end‐exercise PCr depletions in gastrocnemius medialis (R²=0.8) and soleus (R²=0.53). PCr recovery times and end‐exercise pH changes that correlated with PCr depletion were consistent with the literature in gastrocnemius medialis and differences between knee angles were significant. These effects were less pronounced in soleus and not significant for comparable PCr depletions. Maximum oxidative capacity calculated for all knee angles was in excellent agreement with the literature and showed no significant changes between different knee angles. In conclusion, these findings confirm that plantar flexion exercise with a straight leg is a suitable paradigm, when data are acquired from gastrocnemius only (using either localized MRS or small surface coils), and that activation of soleus requires the knee to be flexed. The present study comprises a systematic investigation of the effects of the knee angle on metabolic parameters, measured with dynamic multivoxel ³¹P MRS during muscle exercise and recovery, and the findings should be used in future study design.     

Hyperpolarized 13C‐lactate to 13C‐bicarbonate ratio as a biomarker for monitoring the acute response of anti‐vascular endothelial growth factor (anti‐VEGF) treatment

Hyperpolarized [1‐¹³C]pyruvate MRS provides a unique imaging opportunity to study the reaction kinetics and enzyme activities of in vivo metabolism because of its favorable imaging characteristics and critical position in the cellular metabolic pathway, where it can either be reduced to lactate (reflecting glycolysis) or converted to acetyl‐coenzyme A and bicarbonate (reflecting oxidative phosphorylation). Cancer tissue metabolism is altered in such a way as to result in a relative preponderance of glycolysis relative to oxidative phosphorylation (i.e. Warburg effect). Although there is a strong theoretical basis for presuming that readjustment of the metabolic balance towards normal could alter tumor growth, a robust noninvasive in vivo tool with which to measure the balance between these two metabolic processes has yet to be developed. Until recently, hyperpolarized ¹³C‐pyruvate imaging studies had focused solely on [1‐¹³C]lactate production because of its strong signal. However, without a concomitant measure of pyruvate entry into the mitochondria, the lactate signal provides no information on the balance between the glycolytic and oxidative metabolic pathways. Consistent measurement of ¹³C‐bicarbonate in cancer tissue, which does provide such information, has proven difficult, however. In this study, we report the reliable measurement of ¹³C‐bicarbonate production in both the healthy brain and a highly glycolytic experimental glioblastoma model using an optimized ¹³C MRS imaging protocol. With the capacity to obtain signal in all tumors, we also confirm for the first time that the ratio of ¹³C‐lactate to ¹³C‐bicarbonate provides a more robust metric relative to ¹³C‐lactate for the assessment of the metabolic effects of anti‐angiogenic therapy. Our data suggest a potential application of this ratio as an early biomarker to assess therapeutic effectiveness. Furthermore, although further study is needed, the results suggest that anti‐angiogenic treatment results in a rapid normalization in the relative tissue utilization of glycolytic and oxidative phosphorylation by tumor tissue. Copyright © 2016 John Wiley & Sons, Ltd.     

In vivo measurement of aldehyde dehydrogenase‐2 activity in rat liver ethanol model using dynamic MRSI of hyperpolarized [1‐13C]pyruvate

To date, measurements of the activity of aldehyde dehydrogenase‐2 (ALDH2), a critical mitochondrial enzyme for the elimination of certain cytotoxic aldehydes in the body and a promising target for drug development, have been largely limited to in vitro methods. Recent advancements in MRS of hyperpolarized ¹³C‐labeled substrates have provided a method to detect and image in vivo metabolic pathways with signal‐to‐noise ratio gains greater than 10 000‐fold over conventional MRS techniques. However aldehydes, because of their toxicity and short T₁ relaxation times, are generally poor targets for such ¹³C‐labeled studies. In this work, we show that dynamic MRSI of hyperpolarized [1‐¹³C]pyruvate and its conversion to [1‐¹³C]lactate can provide an indirect in vivo measurement of ALDH2 activity via the concentration of NADH (nicotinamide adenine dinucleotide, reduced form), a co‐factor common to both the reduction of pyruvate to lactate and the oxidation of acetaldehyde to acetate. Results from a rat liver ethanol model (n = 9) show that changes in ¹³C‐lactate labeling following the bolus injection of hyperpolarized pyruvate are highly correlated with changes in ALDH2 activity (R² = 0.76). Copyright © 2012 John Wiley & Sons, Ltd.     

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 ³¹P‐magnetic resonance spectroscopy (³¹P‐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. Copyright © 2013 John Wiley & Sons, Ltd.     

Adaptation of skeletal muscles to training

Based on the myosin ATPase reaction, human skeletal muscles are composed of two main fibre types, named slow (ST) and fast (FT) twitch fibres, respectively. With few exceptions, ST and FT fibres are evenly represented in the muscles, however with a large interindividual variation. Endurance athletes tend to have a predominance of ST fibres while sprinters have a predominance of FT fibres. The ST fibres are surrounded by 3-4 capillaries, and they have the largest potential for terminal oxidation and the smallest for glycolysis. Of the FT fibres, two subtypes may be distinguished (a and b), of which no FTb fibres are seen in the endurance trained muscles of athletes. Training also results in an increase in the number of capillaries for all fibre types. FTa fibres have a metabolic potential which is intermediate to that of the ST and FTb fibres. With endurance training, the potential for terminal oxidation increases, resulting in a larger ability to use fat as a fuel during submaximal exercise and in a reduced production of lactate. Thus, training has a glycogen sparing effect and endurance increases. Human intercostal muscles appear to have approximately 60% ST fibres. In the external intercostal muscles, the number of capillaries and the occurrence of FTb fibres is similar to the findings in untrained muscles. In contrast, the internal intercostal muscles placed in the mid-axillary line have no FTb fibres and relatively many capillaries. Thus, these (expiratory) muscles appear to be extensively used.     

Detection of metabolite changes in response to a varying visual stimulation paradigm using short‐TE 1H MRS at 7 T

The two‐fold benefit of ¹H magnetic resonance spectroscopy (MRS) at high B₀ fields – enhanced sensitivity and increased spectral dispersion – has been used previously to study dynamic changes in metabolite concentrations in the human brain in response to visual stimulation. In these studies, a strong visual on/off stimulus was combined with MRS data acquisition in a voxel location in the occipital cortex determined by an initial functional magnetic resonance imaging experiment. However, 1) to exclude the possibility of systemic effects (heartbeat, blood flow, etc.), which tend to be different for on/off conditions, a modified stimulation condition not affecting the target voxel needs to be employed, and 2) to assess important neurotransmitters of low concentration, in particular γ‐aminobutyric acid (GABA), it may be advantageous to analyze steady‐state, rather than dynamic, conditions. Thus, the aim of this study was to use short‐TE ¹H MRS methodology at 7 T to detect differences in steady‐state metabolite levels in response to a varying stimulation paradigm in the human visual cortex. The two different stimulation conditions were termed voxel and control activation. Localized MR spectra were acquired using the SPECIAL (spin‐echo full‐intensity acquired localized) sequence. Data were analyzed using LCModel. Fifteen individual metabolites were reliably quantified. On comparison of steady‐state concentrations for voxel versus control activation, a decrease in GABA of 0.05 mmol/L (5%) and an increase in lactate of 0.04 mmol/L (7%) were found to be the only significant effects. The observed reduction in GABA can be interpreted as reduced neuronal inhibition during voxel activation, whereas the increase in lactate hints at an intensification of anaerobic glycolysis. Differences from previous studies, notably the absence of any changes in glutamate, are attributed to the modified experimental conditions. This study demonstrates that the use of advanced ¹H MRS methodology at 7 T allows the detection of subtle changes in metabolite concentrations involved in neuronal activation and inhibition.     

Peak blood ammonia and lactate after submaximal,maximal and supramaximal exercise in sprinters and long-distance runners

Summary The purpose of this study was to elucidate the difference in peak blood ammonia concentration between sprinters and long-distance runners in submaximal, maximal and supramaximal exercise. Five sprinters and six long-distance runners performed cycle ergometer exercise at 50% maximal, 75% maximal, maximal and supramaximal heart rates. Blood ammonia and lactate were measured at 2.5, 5, 7.5, 10 and 12.5 min after each exercise. Peak blood ammonia concentration at an exercise intensity producing 50% maximal heart rate was found to be significantly higher compared to the basal level in sprinters (P < 0.01) and in long-distance runners (P < 0.01). The peak blood ammonia concentration of sprinters was greater in supramaximal exercise than in maximal exercise (P < 0.05), while there was no significant difference in long-distance runners. The peak blood ammonia content after supramaximal exercise was higher in sprinters compared with long-distance runners (P < 0.01). There was a significant relationship between peak blood ammonia and lactate after exercise in sprinters and in long-distance runners. These results suggest that peak blood ammonia concentration after supramaximal exercise may be increased by the recruitment of fast-twitch muscle fibres and/or by anaerobic training, and that the processes of blood ammonia and lactate production during exercise may be strongly linked in sprinters and long-distance runners.     

Improved spectral resolution and high reliability of in vivo 1H MRS at 7 T allow the characterization of the effect of acute exercise on carnosine in skeletal muscle

The aims of this study were to observe the behavior of carnosine peaks in human soleus (SOL) and gastrocnemius (GM) muscles following acute exercise, to determine the relaxation times and to assess the repeatability of carnosine quantification by ¹H MRS at 7 T. Relaxation constants in GM and SOL were measured by a stimulated echo acquisition mode (STEAM) localization sequence. For T₁ measurement, an inversion recovery sequence was used. The repeatability of the measurement and the absolute quantification of carnosine were determined in both muscles in five healthy volunteers. For absolute quantification, an internal water reference signal was used. The effect of acute exercise on carnosine levels and resonance lines was tested in eight recreational runners/cyclists. The defined carnosine measurement protocol was applied three times – before and twice after (approximately 20 and 40 min) a 1‐h submaximal street run and additional toe‐hopping. The measured T₁ relaxation times for the C2‐H carnosine peak at 7 T were 2002 ± 94 and 1997 ± 259 ms for GM and SOL, respectively, and the T₂ times were 95.8 ± 9.4 and 81.0 ± 21.8 ms for GM and SOL, respectively. The coefficient of variation of the carnosine quantification measurement was 9.1% for GM and 6.3% for SOL, showing high repeatability, and the intraclass correlation coefficients (ICCs) of 0.93 for GM and 0.98 for SOL indicate the high reliability of the measurement. Acute exercise did not change the concentration of carnosine in the muscle, but affected the shape of the resonance lines, in terms of the shifting and splitting into doublets. Carnosine measurement by ¹H MRS at 7 T in skeletal muscle exhibits high repeatability and reliability. The observed effects of acute exercise were more prominent in GM, probably as a result of the larger portion of glycolytic fibers in this muscle and the more pronounced exercise‐induced change in pH. Our results support the application of the MRS‐based assessment of carnosine for pH measurement in muscle compartments. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

Depth‐resolved surface coil MRS (DRESS)‐localized dynamic 31P‐MRS of the exercising human gastrocnemius muscle at 7 T

Dynamic ³¹P‐MRS with sufficiently high temporal resolution enables the non‐invasive evaluation of oxidative muscle metabolism through the measurement of phosphocreatine (PCr) recovery after exercise. Recently, single‐voxel localized ³¹P‐MRS was compared with surface coil localization in a dynamic fashion, and was shown to provide higher anatomical and physiological specificity. However, the relatively long TE needed for the single‐voxel localization scheme with adiabatic pulses limits the quantification of J‐coupled spin systems [e.g. adenosine triphosphate (ATP)]. Therefore, the aim of this study was to evaluate depth‐resolved surface coil MRS (DRESS) as an alternative localization method capable of free induction decay (FID) acquisition for dynamic ³¹P‐MRS at 7 T. The localization performance of the DRESS sequence was tested in a phantom. Subsequently, two dynamic examinations of plantar flexions at 25% of maximum voluntary contraction were conducted in 10 volunteers, one examination with and one without spatial localization. The DRESS slab was positioned obliquely over the gastrocnemius medialis muscle, avoiding other calf muscles. Under the same load, significant differences in PCr signal drop (31.2 ± 16.0% versus 43.3 ± 23.4%), end exercise pH (7.06 ± 0.02 versus 6.96 ± 0.11), initial recovery rate (0.24 ± 0.13 mm /s versus 0.35 ± 0.18 mm /s) and maximum oxidative flux (0.41 ± 0.14 mm /s versus 0.54 ± 0.16 mm /s) were found between the non‐localized and DRESS‐localized data, respectively. Splitting of the inorganic phosphate (Pi) signal was observed in several non‐localized datasets, but in none of the DRESS‐localized datasets. Our results suggest that the application of the DRESS localization scheme yielded good spatial selection, and provided muscle‐specific insight into oxidative metabolism, even at a relatively low exercise load. In addition, the non‐echo‐based FID acquisition allowed for reliable detection of ATP resonances, and therefore calculation of the specific maximum oxidative flux, in the gastrocnemius medialis using standard assumptions about resting ATP concentration in skeletal muscle. Copyright © 2014 John Wiley & Sons, Ltd.     

One-legged endurance training: leg blood flow and oxygen extraction during cycling exercise

Aim: As a consequence of enhanced local vascular conductance, perfusion of muscles increases with exercise intensity to suffice the oxygen demand. However, when maximal oxygen uptake (VO₂max) and cardiac output are approached, the increase in conductance is blunted. Endurance training increases muscle metabolic capacity, but to what extent that affects the regulation of muscle vascular conductance during exercise is unknown. Methods: Seven weeks of one‐legged endurance training was carried out by twelve subjects. Pulmonary VO₂ during cycling and one‐legged cycling was tested before and after training, while VO₂ of the trained leg (TL) and control leg (CL) during cycling was determined after training. Results: VO₂max for cycling was unaffected by training, although one‐legged VO₂max became 6.7 (2.3)% (mean ± SE) larger with TL than with CL. Also TL citrate synthase activity was higher [30 (12)%; P < 0.05]. With the two legs working at precisely the same power during cycling at high intensity (n = 8), leg oxygen uptake was 21 (8)% larger for TL than for CL (P < 0.05) with oxygen extraction being 3.5 (1.1)% higher (P < 0.05) and leg blood flow tended to be higher by 16.0 (7.0)% (P = 0.06). Conclusion: That enhanced VO₂max for the trained leg had no implication for cycling VO₂max supports that there is a central limitation to VO₂max during whole‐body exercise. However, the metabolic balance between the legs was changed during high‐intensity exercise as oxygen delivery and oxygen extraction were higher in the trained leg, suggesting that endurance training ameliorates blunting of leg blood flow and oxygen uptake during whole‐body exercise.     

Low‐volume muscle endurance training prevents decrease in muscle oxidative and endurance function during 21‐day forearm immobilization

Aim: To examine the effects of low‐volume muscle endurance training on muscle oxidative capacity, endurance and strength of the forearm muscle during 21‐day forearm immobilization (IMM‐21d). Methods: The non‐dominant arm (n = 15) was immobilized for 21 days with a cast and assigned to an immobilization‐only group (Imm‐group; n = 7) or an immobilization with training group (Imm+Tr‐group; n = 8). Training comprised dynamic handgrip exercise at 30% of pre‐intervention maximal voluntary contraction (MVC) at 1 Hz until exhaustion, twice a week during the immobilization period. The duration of each exercise session was 51.7 ± 3.4 s (mean ± SE). Muscle oxidative capacity was evaluated by the time constant for phosphocreatine recovery (τ_offPCr) after a submaximal handgrip exercise using ³¹phosphorus‐magnetic resonance spectroscopy. An endurance test was performed at 30% of pre‐intervention MVC, at 1 Hz, until exhaustion. Results: τ _offPCr was significantly prolonged in the Imm‐group after 21 days (42.0 ± 2.8 and 64.2 ± 5.1 s, pre‐ and post‐intervention respectively; P < 0.01) but did not change for the Imm+Tr‐group (50.3 ± 3.0 and 48.8 ± 5.0 s, ns). Endurance decreased significantly for the Imm‐group (55.1 ± 5.1 and 44.7 ± 4.6 s, P < 0.05) but did not change for the Imm+Tr‐group (47.9 ± 3.0 and 51.7 ± 4.0 s, ns). MVC decreased similarly in both groups (P < 0.01). Conclusions: Twice‐weekly muscle endurance training sessions, each lasting approx. 50 s, effectively prevented a decrease in muscle oxidative capacity and endurance; however, there was no effect on MVC decline with IMM‐21d.     

Endurance training attenuates exercise-induced oxidative stress in erythrocytes in rat

The aim of this study was to investigate whether endurance training reduces exercise-induced oxidative stress in erythrocytes. Male rats (n=54) were divided into trained (n=28) and untrained (n=26) groups. Both groups were further divided equally into two groups where the rats were studied at rest and immediately after exhaustive exercise. Endurance training consisted of treadmill running 1.5 h·day^–1, 5 days a week for 8 weeks, reaching the speed of 2.1 km·h^–1 at the fourth week. For acute exhaustive exercise, graded treadmill running was conducted reaching the speed of 2.1 km·h^–1 at the 95th min, 10% uphill, and was continued until exhaustion. Acute exhaustive exercise increased the erythrocyte malondialdehyde level in sedentary but not in trained rats compared with the corresponding sedentary rest and trained rest groups, respectively. While acute exhaustive exercise decreased the erythrocyte superoxide dismutase activity in sedentary rats, it increased the activity of this enzyme in trained rats. On the other hand, acute exhaustive exercise increased the erythrocyte glutathione peroxidase activity in sedentary rats; however, it did not affect this enzyme activity in trained rats. Erythrocyte glutathione peroxidase activity was higher in trained groups compared with untrained sedentary group. Neither acute exhaustive exercise nor treadmill training affected the erythrocyte total glutathione level. Treadmill training increased the endurance time in trained rats compared with sedentary rats. The results of this study suggest that endurance training may be useful to prevent acute exhaustive exercise-induced oxidative stress in erythrocytes by up-regulating some of the antioxidant enzyme activities and may have implications in exercising humans.     

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

Phosphorus (³¹P) 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 P_i‐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, P_i‐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 P_i‐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 F_CK = 8.21 ± 1.15 mM s^?1 to F_CK = 3.86 ± 1.38 mM s^?1) and the P_i‐to‐ATP flux increased (from F_ATP = 0.43 ± 0.14 mM s^?1 to F_ATP = 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. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

Age‐dependent changes in metabolite profile and lipid saturation in dystrophic mice

Duchenne Muscular Dystrophy (DMD) is a fatal X‐linked genetic disorder. In DMD, the absence of the dystrophin protein causes decreased sarcolemmal integrity resulting in progressive replacement of muscle with fibrofatty tissue. The effects of lacking dystrophin on muscle and systemic metabolism are still unclear. Therefore, to determine the impact of the absence of dystrophin on metabolism, we investigated the metabolic and lipid profile at two different, well‐defined stages of muscle damage and stabilization in mdx mice. We measured NMR‐detectable metabolite and lipid profiles in the serum and muscles of mdx mice at 6 and 24 weeks of age. Metabolites were determined in muscle in vivo using ¹H MRI/MRS, in isolated muscles using ¹H‐HR‐MAS NMR, and in serum using high resolution ¹H/¹³C NMR. Dystrophic mice were found to have a unique lipid saturation profile compared with control mice, revealing an age‐related metabolic change. In the 6‐week‐old mdx mice, serum lipids were increased and the degree of lipid saturation changed between 6 and 24 weeks. The serum taurine‐creatine ratio increased over the life span of mdx, but not in control mice. Furthermore, the saturation index of lipids increased in the serum but decreased in the tissue over time. Finally, we demonstrated associations between MRI‐T₂, a strong indicator of inflammation/edema, with tissue and serum lipid profiles. These results indicate the complex temporal changes of metabolites in the tissue and serum during repetitive bouts of muscle damage and regeneration that occur in dystrophic muscle.     

Muscle MRS detects elevated PDE/ATP ratios prior to fatty infiltration in Becker muscular dystrophy

Becker muscular dystrophy (BMD) is characterized by progressive muscle weakness. Muscles show structural changes (fatty infiltration, fibrosis) and metabolic changes, both of which can be assessed using MRI and MRS. It is unknown at what stage of the disease process metabolic changes arise and how this might vary for different metabolites. In this study we assessed metabolic changes in skeletal muscles of Becker patients, both with and without fatty infiltration, quantified via Dixon MRI and ³¹P MRS. MRI and ³¹P MRS scans were obtained from 25 Becker patients and 14 healthy controls using a 7 T MR scanner. Five lower‐leg muscles were individually assessed for fat and muscle metabolite levels. In the peroneus, soleus and anterior tibialis muscles with non‐increased fat levels, PDE/ATP ratios were higher (P < 0.02) compared with controls, whereas in all muscles with increased fat levels PDE/ATP ratios were higher compared with healthy controls (P ≤ 0.05). The P_i/ATP ratio in the peroneus muscles was higher in muscles with increased fat fractions (P = 0.005), and the PCr/ATP ratio was lower in the anterior tibialis muscles with increased fat fractions (P = 0.005). There were no other significant changes in metabolites, but an increase in tissue pH was found in all muscles of the total group of BMD patients in comparison with healthy controls (P < 0.05). These findings suggest that ³¹P MRS can be used to detect early changes in individual muscles of BMD patients, which are present before the onset of fatty infiltration. Copyright © 2014 John Wiley & Sons, Ltd.     

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

Phosphorus magnetic resonance spectroscopy (³¹P‐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 ³¹P‐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). Copyright © 2013 John Wiley & Sons, Ltd.     

Real‐time assessment of 13C metabolism reveals an early lactate increase in the brain of rats with acute liver failure

Intracranial hypertension is a severe complication of acute liver failure (ALF) secondary to brain edema. The pathogenesis of cerebral edema in ALF is not clear, but seems to be related to energy metabolism in which lactate may have an important role. The aim of this study was to follow the synthesis of brain lactate using a novel in vivo metabolic technology in a rat model of ALF. Time‐resolved ¹³C MRS of hyperpolarized ¹³C₁‐pyruvate was used to quantitatively follow the in vivo conversion of pyruvate to its substrates in a model of devascularized ALF in rats. Rats with ALF showed a significant increase in the lactate to pyruvate ratio from 36% to 69% during the progression of liver disease relative to rats with portocaval anastomosis. Rats with ALF also showed a significant increase in the alanine to pyruvate ratio from 72% to 95%. These increases were detectable at very early stages (6 h) when animals had no evident disease signs in their behavior (without loss of righting or corneal reflexes). This study shows the dynamic consequences of cerebral in vivo ¹³C metabolism at real time in rats with ALF. The early detection of the de novo synthesis of lactate suggests that brain lactate is involved in the physiopathology of ALF. Hyperpolarization is a potential non‐invasive technique to follow the in vivo metabolism, and both the development and optimization of ¹³C‐labeled substrates can clarify the mechanism involved in ALF. Copyright © 2014 John Wiley & Sons, Ltd.     

The role of gray and white matter segmentation in quantitative proton MR spectroscopic imaging

Since the brain's gray matter (GM) and white matter (WM) metabolite concentrations differ, their partial volumes can vary the voxel's ¹H MR spectroscopy (¹H‐MRS) signal, reducing sensitivity to changes. While single‐voxel ¹H‐MRS cannot differentiate between WM and GM signals, partial volume correction is feasible by MR spectroscopic imaging (MRSI) using segmentation of the MRI acquired for VOI placement. To determine the magnitude of this effect on metabolic quantification, we segmented a 1‐mm³ resolution MRI into GM, WM and CSF masks that were co‐registered with the MRSI grid to yield their partial volumes in approximately every 1 cm³ spectroscopic voxel. Each voxel then provided one equation with two unknowns: its i‐ metabolite's GM and WM concentrations C_i^GM, C_i^WM. With the voxels' GM and WM volumes as independent coefficients, the over‐determined system of equations was solved for the global averaged C_i^GM and C_i^WM. Trading off local concentration differences offers three advantages: (i) higher sensitivity due to combined data from many voxels; (ii) improved specificity to WM versus GM changes; and (iii) reduced susceptibility to partial volume effects. These improvements made no additional demands on the protocol, measurement time or hardware. Applying this approach to 18 volunteered 3D MRSI sets of 480 voxels each yielded N‐acetylaspartate, creatine, choline and myo‐inositol C_i^GM concentrations of 8.5 ± 0.7, 6.9 ± 0.6, 1.2 ± 0.2, 5.3 ± 0.6mM, respectively, and C_i^WM concentrations of 7.7 ± 0.6, 4.9 ± 0.5, 1.4 ± 0.1 and 4.4 ± 0.6mM, respectively. We showed that unaccounted voxel WM or GM partial volume can vary absolute quantification by 5–10% (more for ratios), which can often double the sample size required to establish statistical significance. Copyright © 2012 John Wiley & Sons, Ltd.     

Proton magnetic resonance spectroscopy of brain metabolic shifts induced by acute administration of 2‐deoxy‐d‐glucose and lipopolysaccharides

In vivo proton magnetic resonance spectroscopy (¹H MRS) of outbred stock ICR male mice (originating from the Institute of Cancer Research) was used to study the brain (hippocampus) metabolic response to the pro‐inflammatory stimulus and to the acute deficiency of the available energy, which was confirmed by measuring the maximum oxygen consumption. Inhibition of glycolysis by means of an injection with 2‐deoxy‐d ‐glucose (2DG) reduced the levels of gamma‐aminobutyric acid (GABA, p < 0.05, in comparison with control, least significant difference (LSD) test), N‐acetylaspartate (NAA, p < 0.05, LSD test) and choline compounds, and at the same time increased the levels of glutamate and glutamine. An opposite effect was found after injection with bacterial lipopolysaccharide (LPS) – a very common pro‐inflammatory inducer. An increase in the amounts of GABA, NAA and choline compounds in the brain occurred in mice treated with LPS. Different metabolic responses to the energy deficiency and the pro‐inflammatory stimuli can explain the contradictory results of the brain ¹H MRS studies under neurodegenerative pathology, which is accompanied by both mitochondrial dysfunction and inflammation. The prevalence of the excitatory metabolites such as glutamate and glutamine in 2DG treated mice is in good agreement with excitation observed during temporary reduction of the available energy under acute hypoxia or starvation. In turn, LPS, as an inducer of the sickness behavior, which was manifested as depression, sleepiness, loss of appetite etc., shifts the brain metabolic pattern toward the prevalence of the inhibitory neurotransmitter GABA. Copyright © 2014 John Wiley & Sons, Ltd.