T(1) measurement of (31)P metabolites at rest and during steady-state dynamic exercise using a clinical nuclear magnetic resonance scanner

Age-related structural and functional changes of low back muscles

During aging declining maximum force capacity with more or less unchanged fatigability is observed with the underlying mechanisms still not fully understood. Therefore, we compared morphology and function of skeletal muscles between different age groups. Changes in high-energy phosphate turnover (PCr, Pi and pH) and muscle functional MRI (mfMRI) parameters, including proton transverse relaxation time (T2), diffusion (D) and vascular volume fraction (f), were investigated in moderately exercised low back muscles of young and late-middle-aged healthy subjects with (31)P-MR spectroscopy, T2- and diffusion-weighted MRI at 3T. In addition, T1-weighted MRI data were acquired to determine muscle cross-sectional areas (CSA) and to assess fat infiltration into muscle tissue. Except for pH, both age groups showed similar load-induced MR changes and rates of perceived exertion (RPE), which indicates comparable behavior of muscle activation at moderate loads. Changes of mfMRI parameters were significantly associated with RPE in both cohorts. Age-related differences were observed, with lower pH and higher Pi/ATP ratios as well as lower D and f values in the late-middle-aged subjects. These findings are ascribed to age-related changes of fiber type composition, fiber size and vascularity. Interestingly, post exercise f was negatively associated with fat infiltration with the latter being significantly higher in late-middle-aged subjects. CSA of low back muscles remained unchanged, while CSA of inner back muscle as well as mean T2 at rest were associated with maximum force capacity. Overall, applying the proposed MR approach provides evidence of age-related changes in several muscle tissue characteristics and gives new insights into the physiological processes that take place during aging.

Detection of ATP by "in line" 31P magnetic resonance spectroscopy during oxygenated hypothermic pulsatile perfusion of pigs' kidneys

OBJECT

To demonstrate that adenosine triphosphate (ATP), which provides a valuable biomarker for kidney viability in the context of donation after cardiac death (DCD) transplantation, can be detected by means of (31)P magnetic resonance spectroscopy (MRS) if kidneys are perfused with oxygenated hypothermic pulsatile perfusion (O(2)+HPP).

MATERIALS AND METHODS

Porcine kidney perfusion was carried out using a home made, MR-compatible HPP-machine. Consequently, kidney perfusion could be performed continuously during magnetic resonance imaging and magnetic resonance spectroscopy recording. (31)P MR spectroscopy consisted of 3-dimensional chemical shift imaging (CSI), which allowed for the detection of ATP level in line. (31)P CSI was performed at 3 tesla in 44 min with a nominal voxel size of 6.1 cc.

RESULTS

(31)P CSI enabled the detection of renal ATP when pO(2) was equal to 100 kPa. With pO(2) of 20 kPa, only phosphomonoester, inorganic phosphate and nicotinamide adenine dinucleotide could be found. Semi-quantitative analysis showed that ATP level was 1.3 mM in normal kidney perfused with pO(2) of 100 kPa.

CONCLUSIONS

This combined technology may constitute a new advance in DCD organ diagnostics prior to transplantation, as it allows direct assessment of ATP concentration, which provides a reliable indicator for organ bioenergetics and viability. In this study, kidneys presenting no warm ischemia were tested in order to establish values in normal organs. The test could be easily integrated into the clinical environment and would not generate any additional delay into the transplantation clinical workflow.

Muscle damage alters the metabolic response to dynamic exercise in humans: a31P-MRS study

We used31P-magnetic resonance spectroscopy to test the hypothesis that exercise-induced muscle damage (EIMD) alters the muscle metabolic response to dynamic exercise, and that this contributes to the observed reduction in exercise tolerance following EIMD in humans. Ten healthy, physically active men performed incremental knee extensor exercise inside the bore of a whole body 1.5-T superconducting magnet before (pre) and 48 h after (post) performing 100 squats with a load corresponding to 70% of body mass. There were significant changes in all markers of muscle damage [perceived muscle soreness, creatine kinase activity (434% increase at 24 h), and isokinetic peak torque (16% decrease at 24 h)] following eccentric exercise. Muscle phosphocreatine concentration ([PCr]) and pH values during incremental exercise were not different pre- and post-EIMD ( P > 0.05). However, resting inorganic phosphate concentration ([Pi]; pre: 4.7 ± 0.8; post: 6.7 ± 1.7 mM; P < 0.01) and, consequently, [Pi]/[PCr] values (pre: 0.12 ± 0.02; post: 0.18 ± 0.05; P < 0.01) were significantly elevated following EIMD. These mean differences were maintained during incremental exercise ( P < 0.05). Time to exhaustion was significantly reduced following EIMD (519 ± 56 and 459 ± 63 s, pre- and post-EIMD, respectively, P < 0.001). End-exercise pH (pre: 6.75 ± 0.04; post: 6.83 ± 0.04; P < 0.05) and [PCr] (pre: 7.2 ± 1.7; post: 14.5 ± 2.1 mM; P < 0.01) were higher, but end-exercise [Pi] was not significantly different (pre: 19.7 ± 1.9; post: 21.1 ± 2.6 mM, P > 0.05) following EIMD. The results indicate that alterations in phosphate metabolism, specifically the elevated [Pi] at rest and throughout exercise, may contribute to the reduced exercise tolerance observed following EIMD.

Age- and sex-related differences in muscle phosphocreatine and oxygenation kinetics during high-intensity exercise in adolescents and adults

The aim of this investigation was to examine the adaptation of the muscle phosphates (e.g. phosphocreatine (PCr) and ADP) implicated in regulating oxidative phosphorylation, and oxygenation at the onset of high intensity exercise in children and adults. The hypotheses were threefold: primary PCr kinetics would be faster in children than adults; the amplitude of the PCr slow component would be attenuated in children; and the amplitude of the deoxyhaemoglobin/myoglobin (HHb) slow component would be reduced in children. Eleven children (5 girls, 6 boys, 13 +/- 1 years) and 11 adults (5 women, 6 men, 24 +/- 4 years) completed two to four constant work rate exercise tests within a 1.5 T MR scanner. Quadriceps muscle energetics during high intensity exercise were monitored using (31)P-MRS. Muscle oxygenation was monitored using near-infrared spectroscopy. The time constant for the PCr response was not significantly different in boys (31 +/- 10 s), girls (31 +/- 10 s), men (44 +/- 20 s) or women (29 +/- 14 s, main effects: age, p = 0.37, sex, p = 0.25). The amplitude of the PCr slow component relative to end-exercise PCr was not significantly different between children (23 +/- 23%) and adults (17 +/- 13%, p = 0.47). End-exercise [PCr] was significantly lower, and [ADP] higher, in females (18 +/- 4 mM and 53 +/- 16 microM) than males (23 +/- 4 mM, p = 0.02 and 37 +/- 11 microM, p = 0.02), but did not differ with age ([PCr]: p = 0.96, [ADP]: p = 0.72). The mean response time for muscle tissue deoxygenation was significantly faster in children (22 +/- 4 s) than adults (27 +/- 7 s, p = 0.01). The results of this study show that the control of oxidative metabolism at the onset of high intensity exercise is adult-like in 13-year-old children, but that matching of oxygen delivery to extraction is more precise in adults.

Muscle phosphocreatine kinetics in children and adults at the onset and offset of moderate-intensity exercise

The splitting of muscle phosphocreatine (PCr) plays an integral role in the regulation of muscle O2 utilization during a "step" change in metabolic rate. This study tested the hypothesis that the kinetics of muscle PCr would be faster in children compared with adults both at the onset and offset of moderate-intensity exercise, in concert with the previous demonstration of faster phase II pulmonary O2 uptake kinetics in children. Eighteen peri-pubertal children (8 boys, 10 girls) and 16 adults (8 men, 8 women) completed repeated constant work-rate exercise transitions corresponding to 80% of the Pi/PCr intracellular threshold. The changes in quadriceps [PCr], [Pi], [ADP], and pH were determined every 6 s using 31P-magnetic resonance spectroscopy. No significant (P>0.05) age- or sex-related differences were found in the PCr kinetic time constant at the onset (boys, 21+/-4 s; girls, 24+/-5 s; men, 26+/-9 s; women, 24+/-7 s) or offset (boys, 26+/-5 s; girls, 29+/-7 s; men, 23+/-9 s; women 29+/-7 s) of exercise. Likewise, the estimated theoretical maximal rate of oxidative phosphorylation (Qmax) was independent of age and sex (boys, 1.39+/-0.20 mM/s; girls, 1.32+/-0.32 mM/s; men, 2.36+/-1.18 mM/s; women, 1.51+/-0.53 mM/s). These results are consistent with the notion that the putative phosphate-linked regulation of muscle O2 utilization is fully mature in peri-pubertal children, which may be attributable to a comparable capacity for mitochondrial oxidative phosphorylation in child and adult muscle.

Influence of phosphagen concentration on phosphocreatine breakdown kinetics. Data from human gastrocnemius muscle

At the onset of a square-wave exercise of moderate intensity, in the absence of any detectable lactate production, the hydrolysis of phosphocreatine (PCr) fills the gap between energy requirement and energy yield by oxidative pathways, thus representing a readily available source of energy for the muscle. We verified experimentally the relationships between high-energy phosphates and/or their changes and the time constant of PCr concentration ([PCr]) kinetics in humans (tau(PCr)). High-energy phosphate concentration (by (31)P-NMR spectroscopy) in the calf muscles were measured during three repetitions of the rest-to-work transition of moderate aerobic square-wave exercise on nine healthy volunteers, while resting [PCr] was estimated from the appropriate spectroscopy data. PCr concentration decreased significantly (22 +/- 6%) from rest to steady-state exercise, without differences among the three repetitions. Absolute resting [PCr] and tau(PCr) were consistent with literature values, amounting to 27.5 +/- 2.2 mM and 23.9 +/- 2.9 s, respectively. No significant relationships were detected between individual tau(PCr) and mechanical power, fraction or absolute amount of PCr hydrolyzed, or change in ADP concentration. On the contrary, individual tau(PCr) (s) was linearly related to absolute resting [PCr] (mM), the relationship being described by: tau(PCr) = 0.656 + 0.841.[PCr] (n = 9, R = 0.708, P < 0.05). These data support the view that in humans PCr concentration sets the time course of the oxidative metabolism in skeletal muscle at the start of exercise, being one of the main controllers of oxidative phosphorylation.

Muscle phosphocreatine and pulmonary oxygen uptake kinetics in children at the onset and offset of moderate intensity exercise

To further understand the mechanism(s) explaining the faster pulmonary oxygen uptake (p(VO)(2)) kinetics found in children compared to adults, this study examined whether the phase II p(VO)(2) kinetics in children are mechanistically linked to the dynamics of intramuscular PCr, which is known to play a principal role in controlling mitochondrial oxidative phosphorylation during metabolic transitions. On separate days, 18 children completed repeated bouts of moderate intensity constant work-rate exercise for determination of (1) PCr changes every 6 s during prone quadriceps exercise using (31)P-magnetic resonance spectroscopy, and (2) breath by breath changes in p(VO)(2) during upright cycle ergometry. Only subjects (n = 12) with 95% confidence intervals <or=+/-7 s for all estimated time constants were considered for analysis. No differences were found between the PCr and phase II p(VO)(2) time constants at the onset (PCr 23 +/- 5 vs. p(VO)(2) 23 +/- 4 s, P = 1.000) or offset (PCr 28 +/- 5 vs. p(VO)(2) 29 +/- 5 s, P = 1.000) of exercise. The average difference between the PCr and phase II p(VO)(2) time constants was 4 +/- 4 s for the onset and offset responses. Pooling of the exercise onset and offset responses revealed a significant correlation between the PCr and p(VO)(2) time constants (r = 0.459, P = 0.024). The close kinetic coupling between the p(VO)(2) and PCr responses at the onset and offset of exercise in children is consistent with our current understanding of metabolic control and suggests that an age-related modulation of the putative phosphate linked controller(s) of mitochondrial oxidative phosphorylation may explain the faster p(VO)(2) kinetics found in children compared to adults.

Mitochondrial coupling in humans: assessment of the P/O2 ratio at the onset of calf exercise

Coupling of oxidation to ATP synthesis (P/O2 ratio) is a critical step in the conversion of carbon substrates to fuel (ATP) for cellular activity. The ability to quantitatively assess mitochondrial coupling in vivo can be a valuable tool for basic research and clinical purposes. At the onset of a square wave moderate exercise, the ratio between absolute amount of phosphocreatine split and O2 deficit (corrected for the amount of O2 released from the body O2 stores and in the absence of lactate production), is the mirror image of the P/O2 ratio. To calculate this value, cardiac output (Q), whole body O2 uptake (VO2), O2 deficit (O2(def)) and high-energy phosphates concentration (by 31P-NMR spectroscopy) in the calf muscles were measured on nine healthy volunteers at rest and during moderate intensity plantar flexion exercise (3.44 +/- 0.73 W per unit active muscle mass). Q and VO2 increased (from 4.68 +/- 1.56 to 5.83 +/- 1.59 l min(-1) and from 0.28 +/- 0.05 to 0.48 +/- 0.09 l min(-1), respectively), while phosphocreatine (PCr) concentration decreased significantly (22 +/- 6%) from rest to steady-state exercise. For each volunteer, "gross" O2(def) was corrected for the individual changes in the venous blood O2 stores (representing 49.9 +/- 9.5% of the gross O2(def)) yielding the "net" O2(def). Resting PCr concentration was estimated from the appropriate spectroscopy data. The so calculated P/O2 ratio amounted on average to 4.24 +/- 0.13 and was, in all nine subjects, very close to the literature values obtained directly on intact skeletal muscle. This unfolds the prospect of a non-invasive tool to quantitatively study mitochondrial coupling in vivo.