MR-compatible pedal ergometer for reproducible exercising of the human calf muscle

A pneumatic MR-compatible pedal ergometer was designed to perform dynamic contraction exercises of the human calf muscle in a whole-body 3 T MR scanner. The set-up includes sensors for monitoring mechanical parameters, such as pedal angle, cadence as well as applied force and power. Actual parameter values during the exercise were presented to the volunteer as a visual feedback to enable real-time self-adjustment of pedal deflection and cadence to the target reference value. Time-resolved dynamic ³¹P-MR spectroscopic measurements of phosphocreatine (PCr), inorganic phosphate (Pi) and pH were performed in a pilot experiment before, during, and after the exercise by a single volunteer. Two different load strengths were applied in these experiments (15% and 25% of the maximum voluntary contraction, MVC). As expected, mechanical and metabolic parameters differed for the two load levels. Small variations of the cadence, power and metabolic changes (time constants of PCr depletion and Pi accumulation) during the experiments demonstrate a highly reproducible mechanical output by the volunteer mediated by the ergometer.     

In vivo observation of quadrupolar splitting in 39K magnetic resonance spectroscopy of human muscle tissue

The purpose of this work was to explore the origin of oscillations of the T^*₂ decay curve of ³⁹K observed in studies of ³⁹K magnetic resonance imaging of the human thigh. In addition to their magnetic dipole moment, spin‐³/₂ nuclei possess an electric quadrupole moment. Its interaction with non‐vanishing electrical field gradients leads to oscillations in the free induction decay and to splitting of the resonance. All measurements were performed on a 7T whole‐body MRI scanner (MAGNETOM 7T, Siemens AG, Erlangen, Germany) with customer‐built coils. According to the theory of quadrupolar splitting, a model with three Lorentzian‐shaped peaks is appropriate for ³⁹K NMR spectra of the thigh and calf. The frequency shifts of the satellites depend on the angle between the calf and the static magnetic field. When the leg is oriented parallel to the static magnetic field, the satellites are shifted by about 200 Hz. In the thigh, rank‐2 double quantum coherences arising from anisotropic quadrupolar interaction are observed by double‐quantum filtration with magic‐angle excitation. In addition to the spectra, an image of the thigh with a nominal resolution of (16 × 16 × 32) mm³ was acquired with this filtering technique in 1:17 h. From the line width of the resonances, ³⁹K transverse relaxation time constants T^*_{2, fast} = (0.51 ± 0.01) ms and T^*_{2, slow} = (6.21 ± 0.05) ms for the head were determined. In the thigh, the left and right satellite, both corresponding to the short component of the transverse relaxation time constant, take the following values: T^*_{2, fast} = (1.56 ± 0.03) ms and T^*_{2, fast} = (1.42 ± 0.03) ms. The centre line, which corresponds to the slow component, is T^*₂, _slow = (9.67 ± 0.04) ms. The acquisition time of the spectra was approximately 10 min. Our results agree well with a non‐vanishing electrical field gradient interacting with ³⁹K nuclei in the intracellular space of muscle tissue. Copyright © 2016 John Wiley & Sons, Ltd.     

High-intensity exercise decreases muscle buffer capacity via a decrease in protein buffering in human skeletal muscle

We have previously reported an acute decrease in muscle buffer capacity (βm_{in vitro}) following high-intensity exercise. The aim of this study was to identify which muscle buffers are affected by acute exercise and the effects of exercise type and a training intervention on these changes. Whole muscle and non-protein βm_{in vitro} were measured in male endurance athletes (VO_2max = 59.8 ± 5.8 mL kg⁻¹ min⁻¹), and before and after training in male, team-sport athletes (VO_2max = 55.6 ± 5.5 mL kg⁻¹ min⁻¹). Biopsies were obtained at rest and immediately after either time-to-fatigue at 120% VO_2max (endurance athletes) or repeated sprints (team-sport athletes). High-intensity exercise was associated with a significant decrease in βm_{in vitro} in endurance-trained males (146 ± 9 to 138 ± 7 mmol H⁺·kg d.w.⁻¹·pH⁻¹), and in male team-sport athletes both before (139 ± 9 to 131 ± 7 mmol H⁺·kg d.w.⁻¹·pH⁻¹) and after training (152 ± 11 to 142 ± 9 mmol H⁺·kg d.w.⁻¹·pH⁻¹). There were no acute changes in non-protein buffering capacity. There was a significant increase in βm_{in vitro} following training, but this did not alter the post-exercise decrease in βm_{in vitro}. In conclusion, high-intensity exercise decreased βm_{in vitro} independent of exercise type or an interval-training intervention; this was largely explained by a decrease in protein buffering. These findings have important implications when examining training-induced changes in βm_{in vitro}. Resting and post-exercise muscle samples cannot be used interchangeably to determine βm_{in vitro}, and researchers must ensure that post-training measurements of βm_{in vitro} are not influenced by an acute decrease caused by the final training bout.     

Carbohydrate utilization by exercising muscle following pre-exercise glucose ingestion

The effect on exercising muscle metabolism of prior ingestion of 200 g glucose was examined in six healthy subjects during 40 min leg exercise at 30% of maximal oxygen uptake. Leg glucose uptake during exercise was on average two- to three-fold higher after glucose (E + G) compared to exercise without glucose (E) and could account for 44-48% of the oxidative leg metabolism (control value: 19%, P less than 0.05-0.01). In contrast to E, which was associated with a significant release of leg lactate, pyruvate and alanine, E + G gave no leg production of lactate or alanine and an uptake of pyruvate. The respiratory exchange ratios (R) were higher during G + E and corresponded to a carbohydrate oxidation of 54-69% as against 46-49% (P less than 0.05-0.01) during E. Estimated from R-values and leg oxygen and glucose uptakes, carbohydrate oxidation during G less than E was almost completely accounted for by blood glucose. During E, on the other hand, carbohydrate oxidation exceeded leg glucose uptake, indicating a small but significant muscle glycogen breakdown (P less than 0.01). The rate of glycogen utilization during E or G + E was too small to be detected by direct measurements of muscle glycogen content. The results demonstrate that glucose ingestion prior to light exercise is followed by increased uptake and more efficient oxidation of glucose, as well as by insignificant muscle glycogen degradation by exercising muscle. Although the present findings suggest a glycogen-conserving effect of glucose ingestion under these conditions, the main fuel shift is from fat to glucose oxidation.     

Metabolism of exercising skeletal muscle during beta 1-selective adrenoceptor blockade

Concentrations of glycogen, glucose, glucose-6-phosphate and lactate in the lateral vastus muscle were measured in seven subjects before and after dynamic muscle exercise at a work load of 75% of each subject's maximal working capacity, and with and without intravenous administration of the beta 1-selective beta-adrenoceptor blocking agent, atenolol. Pulmonary oxygen uptake was measured during exercise. Heart rate and arterial blood pressure were measured throughout the study. Arterial concentrations of glucose, lactate and free fatty acids were measured at rest and during exercise. The muscle concentration of glycogen and the extent of glycogen depletion with exercise were not influenced by the beta 1-adrenoceptor blocker. Similarly, there was no change in the muscle concentrations of glucose, glucose-6-phosphate and lactate. Heart rate decreased at rest and during exercise. Arterial blood pressure was not influenced by beta-blockade. Pulmonary oxygen uptake decreased by 6.5%. The exercise induced rise in arterial blood concentration of free fatty acids was abolished by beta 1-selective beta-blockade. It is concluded that the decrease in lactate release from exercising muscles during beta 1-adrenoceptor blockade seen in other studies cannot be explained by an impaired breakdown of muscle glycogen. It may be inferred, however, that a reduced availability of free fatty acids in the exercising muscles during beta 1-selective (and non-selective) beta-blockade may enhance the combustion of pyruvic acid and thereby decrease the production of lactate.     

FAWSETS perfusion measurements in exercising skeletal muscle

Arterial spin labeling (ASL) techniques are now recognized as valid tools for providing accurate measurements of cerebral and cardiac perfusion. The labeling process used with most ASL techniques creates two problems, magnetization transfer (MT) effects and arterial transit time effects, that require compensation. The compensation process limits time resolution and hinders absolute quantification. MT effects are particularly problematic in skeletal muscle because they are large and change rapidly during exercise. The protocol presented here was developed specifically for quantification of perfusion in exercising skeletal muscle. The ASL technique that was implemented, FAWSETS, eliminates MT effects and arterial transit times. Localized, single-voxel perfusion measurements were acquired from rat hind limbs at rest, during ischemia and during three different levels of stimulated exercise. The results demonstrate sufficient sensitivity to determine the time constants for perfusion changes at onset of, and during recovery from, exercise and to distinguish the differences in the amplitude of the perfusion response to different levels of exercise. Additional measurements were conducted to demonstrate insensitivity to MT effects. The exercise protocol is easily adaptable to phosphorous magnetic resonance measurements, allowing the possibility to acquire local measurements of perfusion and metabolism from the same tissue in future experiments.     

血液生化学与1H磁共振波谱分析在兔肝纤维化中的诊断效能及相关性分析

目的探讨血清肝纤维化标志物和1H磁共振波谱分析（1H-MRS）指标在兔肝纤维化中的诊断效能及相互关系。方法健康新西兰大白兔45只,随机分为6组：第1组为对照组,5只;第2-6组为实验组,每组8只,共40只。对以四氯化碳（CC14）诱导产生的肝纤维化实验组兔和对照组兔行血清肝纤维化标志物和lH．MRS检查。血清学指标包括透明质酸（HA）、Ⅲ型前胶原（PCⅢ）、Ⅳ型胶原（C．Ⅳ）和层黏连蛋白（LN）,1H-MRS参数为胆碱峰高choAmp、胆碱面积choAre、胆碱／脂质峰高比值（cho／lipid）Amp和胆碱／脂质面积比值（cho／lipid）Are。以病理学肝纤维化分期S≥1为判断阳性的标准,应用受试者工作特性曲线（ROC）评价两者各诊断指标在肝纤维化中的诊断效能,同时行血清学和1H-MRS指标间的相关性分析。结果ROC曲线下面积血清HA（0．893）〉C-Ⅳ（0．804）〉LN（0．732）〉PCⅢ（0．643）,1H-MRS中依次为（cho／lipid）Amp（0．753）〉（cho／lipid）Are（0．742）〉choAre（0．544）〉choAmp（0．497）;除choAmp外,所有指标的曲线下面积、灵敏度和特异度均在0．5-0．9之间。choAmp、choAre与C-IV之间存在显著正相关（P〈0．01）,choAre、（cho／lipid）Are与LN存在正相关（P〈0．05）,（Cho／lipid）Amp与血清学各指标间无相关性。结论血清肝纤维化标志物和1H．MRS在诊断肝纤维化中能力相仿．但无灵敏度和特异度俱佳的诊断指标。两者之间存在的相关关系可能与间质纤维化影响代谢物的交换和排出有关。     

Non-invasive in vivo localized 1H spectroscopy of human astrocytoma implanted in rat brain: regional differences followed in time

Human astrocytoma cells were cultured and inoculated into the rat brain. From the pre-clinical to the terminal state, tumour growth was monitored by in vivo MR imaging and by localized water-suppressed 1H spectroscopy (0.12-0.15 cm3 volumes) and spectroscopic imaging (0.01 cm3 voxels) employing the ACE localization technique. The MR experiments were conducted completely non-invasively, leaving the scalp intact. Brain spectra were obtained, showing distinct resonances for more than five different brain metabolites; they were not contaminated with lipid signals because of the adequate localization. Tumour progression, monitored in a selected volume of interest, was reflected in the corresponding spectra by decreasing intensities for resonances of N-acetyl aspartate and (phospho)creatine and increasing intensities for resonances of choline compounds and lactate. From spectroscopic imaging experiments metabolic heterogeneity could be deduced within the tumorous region. At particular times during tumour development spectra were obtained greatly resembling localized 1H MR spectra obtained from patients with astrocytomas by the use of similar localization methods. This emphasizes the relevance of animal model study for the evaluation of MR spectroscopic investigations in human brain tumour diagnosis and therapy evaluation.     

Absolute quantification of carnosine in human calf muscle by proton magnetic resonance spectroscopy

Carnosine has been shown to be present in the skeletal muscle and in the brain of a variety of animals and humans. Despite the various physiological functions assigned to this metabolite, its exact role remains unclear. It has been suggested that carnosine plays a role in buffering in the intracellular physiological pHi range in skeletal muscle as a result of accepting hydrogen ions released in the development of fatigue during intensive exercise. It is thus postulated that the concentration of carnosine is an indicator for the extent of the buffering capacity. However, the determination of the concentration of this metabolite has only been performed by means of muscle biopsy, which is an invasive procedure. In this paper, we utilized proton magnetic resonance spectroscopy (1H MRS) in order to perform absolute quantification of carnosine in vivo non-invasively. The method was verified by phantom experiments and in vivo measurements in the calf muscles of athletes and untrained volunteers. The measured mean concentrations in the soleus and the gastrocnemius muscles were found to be 2.81 +/- 0.57/4.8 +/- 1.59 mM (mean +/- SD) for athletes and 2.58 +/- 0.65/3.3 +/- 0.32 mM for untrained volunteers, respectively. These values are in agreement with previously reported biopsy-based results. Our results suggest that 1H MRS can provide an alternative method for non-invasively determining carnosine concentration in human calf muscle in vivo.     

Metabolism in exercising arm vs. leg muscle

Arm and leg metabolism were compared by arterial and venous catheterization and blood flow measurements (by dye dilution techniques) in two groups of subjects performing 30-min continuous arm or leg exercise of increasing intensity corresponding to approximately 30, 50 and 80% of max oxygen uptake for arm or leg exercise. The absolute work-loads were 2.5-3 times higher during leg compared to arm exercise. Heart rates were the same in both types of exercise. r-Values were 0.97-1.07 during arm exercise. Arterial noradrenaline and adrenaline levels became higher during leg compared to arm exercise (P less than 0.05-0.01). Arterial lactate concentration was 50% higher for arm exercise at the two lower intensities (P less than 0.001) and the same at the highest intensity compared to leg exercise. Arm lactate release was three times higher (P less than 0.01) or the same as leg lactate output at corresponding exercise intensities. Arm and leg glucose uptake during exercise were of the same magnitude at the lower intensities. In contrast to the leg substrate exchange, arm lactate output was higher than the simultaneous glucose uptake (P less than 0.05-0.001), indicating a relatively higher rate of glycogen degradation. In conclusion, exercising arm compared to leg muscles working at the same relative intensities utilize more carbohydrate, mainly muscle glycogen resulting in higher lactate release by the exercising extremity. This cannot solely be explained on the basis of differences in the degree of training and occurs with lower catecholamine levels compared to leg exercise.     

Non-invasive quantitation of human liver metabolites using image-guided 31P magnetic resonance spectroscopy

Phosphorus-containing metabolites in normal human liver have been quantitated non-invasively with 31P magnetic resonance spectroscopy using surface coils. The location of the volume of interest (VOI) was defined by 1H magnetic resonance imaging. Subsequently, a modified three-dimensional localization technique (ISIS) was used to acquire 31P magnetic resonance spectra from the VOI. To account for partial saturation produced by rapid signal averaging, the spin/lattice relaxation times (T1) of all hepatic phosphorus resonances were measured. The corrected resonance integrals were used to derive absolute molar concentrations for the following hepatic metabolites (mmol/kg wet weight): ATP, 2.0; inorganic phosphate, 2.1; phosphodiesters, 5.4; and phosphomonoesters, 0.9. These values are compared with previously reported values for humans using freeze-clamping techniques, and provide a basis for comparison with studies of hepatic disease in this laboratory.     

Metabolic modulation of sympathetic vasoconstriction in exercising skeletal muscle

The tight coupling of oxygen supply and utilization in exercising skeletal muscle is the result of complex interactions between local mechanisms that control muscle blood flow and substrate utilization and systemic mechanisms that control cardiac output and arterial pressure. The role of the sympathetic nervous system in the integration of these responses, specifically the interaction between sympathetic activation and local vasodilator mechanisms in exercising muscle, has been an active area of research for many years yet remains incompletely understood. The functional consequence of sympathetic activation in exercising skeletal muscle has been the subject of considerable debate. Previous studies in animals and humans have suggested that sympathetic vasoconstricton in active muscle is (a) well maintained and serves to limit active hyperaemia, thereby preventing muscle blood flow from outstripping cardiac output in order to preserve blood pressure and vital organ perfusion or (b) greatly attenuated in order to optimize muscle perfusion, a concept that has been termed 'functional sympatholysis'. Studies performed over the past 70 years have provided conflicting evidence regarding the relative importance of sympathetic vasoconstriction vs. functional sympatholysis in exercising skeletal muscle. The focus of this review is mainly on recent studies in anaesthetized animal preparations and in conscious humans that have provided evidence for the metabolic modulation of sympathetic vasoconstriction in contracting skeletal muscle and have identified a number of key underlying mechanisms that extend the initial concept of sympatholysis.     

Pacifier-produced visual buffering in human infants

Non-nutritive sucking in human infants aged 9–13 weeks produces buffering by reducing visual scanning movements in the presence of movement-invoking cinematic visual displays of both representational and abstract types. Scanning is correlated with movement in a display without the presence of sucking to buffer it.