ATP production and mechanical work in exercising skeletal muscle: A theoretical analysis applied to 31P magnetic resonance spectroscopic studies of dialyzed uremic patients

³¹P magnetic resonance spectroscopy (³¹P MRS) can yield much information about bioenergetics in skeletal muscle. During mixed aerobic/glycolytic exercise, changes in phos-phocreatine (PCr) concentration and pH may be abnormal because of reduced muscle mass or reduced efficiency (which the authors combine here as “effective muscle mass”) or because of reduced oxidative capacity. The authors show how these can be distinguished by calculating the nonoxidative and oxidative costs of mechanical work, and also of work per unit of effective muscle mass (measured using the initial rate of ATP turnover). These quantities are substantially time-independent during incremental exercise, and so can be used to compare exercise studies of differing duration. The authors illustrate this analysis by showing that in dialyzed patients with chronic renal failure, the substantial exercise abnormalities seen by ³¹P MRS are due mainly to a decrease in effective muscle mass, which outweighs the oxidative defect implied by the abnormal PCr recovery kinetics.     

In vivo 31P magnetic resonance spectroscopy using a needle microcoil

Batch fabrication methods have been used to produce low‐cost microcoils for magnetic resonance spectroscopy (MRS) that could be discarded after applications such as insertion into tissue during interventional surgery. Needle‐shaped microcoils were constructed using electroplated conductors buried in shafts formed with different combinations of silicon and plastic and used to acquire in vivo ³¹P spectra of rat thigh muscle at 81 MHz. The designs in this study achieved a maximum signal‐to‐noise ratio (SNR) for phosphocreatine (PCr) of 10.4 in a 10‐min acquisition, with the three adenosine triphosphate (ATP) multiplets also clearly visible. An average 20% reduction in PCr occurred over a 60‐min period, and intracellular pH was estimated to be 6.6, which are both evidence of ischemia. A needle microcoil design could have applications in real‐time MRS of tumors or in evaluating pathology in general during surgical investigations. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

In vivo 31P magnetic resonance spectroscopy of human brain at 7 T: an initial experience.

In vivo (31)P spectra were acquired from the human primary visual cortex at 7 T. The relaxation times of the cerebral metabolites, intracellular pH, rate constant (k(f)) of the creatine kinase (CK) reaction, and nuclear Overhauser enhancement (NOE) on the detected phosphorus moieties from irradiation of the water spins were measured from normal subjects. With a 5-cm-diameter surface coil, 3D (31)P chemical shift imaging was performed with a spatial resolution of 7.5 ml and an acquisition resolution of 8 min, resulting in a signal-to-noise ratio (SNR) for phosphocreatine (PCr) resonance of 32. The apparent T(1) and T(2) of PCr measured at 7 T were 3.37 +/- 0.29 s and 132.0 +/- 12.8 ms, respectively, which were considerably longer than those of adenosine triphosphate (ATP) (T(1): 1.02-1.27 s; T(2): 25-26 ms). The NOE measured in this study was 24.3% +/- 1.6% for PCr, and 10% for ATP. The k(f) measured in the human primary visual cortex was 0.24 +/- 0.03 s(-1). The results from this study suggest that ultra-high-field strength is advantageous for performing in vivo (31)P magnetic resonance spectroscopy (MRS) in the human brain.     

Evaluation of muscle diseases using artificial neural network analysis of 31P MR spectroscopy data

Dermatomyositis is an autoimmune disease characterized by an erythematous rash and severe muscle weakness. ³¹P Magnetic resonance spectroscopy (MRS) provides quantitative data for longitudinal monitoring of disease status and responses to immunosuppressive therapy. A disease variant, amyopathic dermatomyositis, presents with a typical rash but no clinical muscle weakness. However, metabolic abnormalities in the oxidative capacity of muscles of amyopathic patients during exercise were detected with ³¹P MRS. Because MRS provided the best quantitative data for evaluating dermatomyositis, the ³¹P metabolic parameters derived from the MR spectra were further processed using an artificial neural network (XERION). The neural network analyses provided additional clinical information from the weighted correlations of multiple ³¹P parameters, namely, inorganic phosphate, phos-phocreatine, ATP, phosphodiesters, and selected ratios. This investigation analyzes the relative importance of the various metabolic parameters for accurate patient characterization and provides insights into the pathogenesis of the disease.     

Noninvasive monitoring of lactate dynamics in human forearm muscle after exhaustive exercise by 1H‐magnetic resonance spectroscopy at 7 tesla

Despite its importance in energy metabolism, lactate in human skeletal muscle has been difficult to detect by noninvasive ¹H‐magnetic resonance spectroscopy mainly due to interference from large water and lipid signals. Long echo‐time acquisitions at 7 T effectively attenuates the water and lipid signals in forearm muscle allowing direct observation of both lactate resonances, the methine at 4.09 ppm and the methyl at 1.31 ppm. Using this approach, we were able to monitor lactate dynamics at a temporal resolution of 32 s. While lactate was not detectable at rest, immediately after an acute period of exercise to fatigue the forearm muscle, lactate rose to a level comparable to that of creatine (～30 mmol/kg wet weight). In a typical ¹H‐magnetic resonance spectrum collected using a echo‐time of 140 ms, the lactate methine and methyl resonances both appear as doublets with an unusually large splitting of ～20 Hz due to residual dipolar coupling. During muscle recovery following exercise, the lactate signals decay rapidly with a time constant of t_½ = 2.0 ± 0.6 min (n = 12 subjects). This fast and simple lactate detection method may prove valuable for monitoring lactate metabolism in cancer and in sports medicine applications. Magn Reson Med 70:610–619, 2013. © 2012 Wiley Periodicals, Inc.     

Optimized detection of changes in glucose-6-phosphate levels in human skeletal muscle by 31P MR spectroscopy.

As glucose-6-phosphate (G6P) plays a central role in muscle energy metabolism, the possibility to observe changes in the tissue level of this compound in vivo is very relevant. G6P can be detected noninvasively by (31)P MR spectroscopy, but its visibility in vivo is severely hampered due to low tissue levels and spectral overlap with other, stronger phosphomonoester signals. To optimize the observation of changes in G6P levels in human calf muscle by (31)P MR spectroscopy at 1.5 T, we implemented an approach involving a new RF probe and a postacquisition correction method. An anatomically shaped circularly polarized (31)P coil was designed for high intrinsic sensitivity. Together with an additional (1)H coil and (1)H blocking circuits this allowed the application of NOE and (1)H decoupling to further enhance sensitivity. A hyperglycemic hyperinsulinemic clamp was used to increase G6P levels. The spectra were corrected for frequency and phase drift due to scanner instability and leg movements using an automated phase and frequency correction method. Difference (31)P spectroscopy was applied to detect changes of the G6P signal. The result, in five healthy subjects, demonstrated that the combination of sensitivity optimization with automated drift correction enabled a robust detection of G6P changes in time series experiments down to a resolution of 10 min.     

Efficient in vivo 31P magnetization transfer approach for noninvasively determining multiple kinetic parameters and metabolic fluxes of ATP metabolism in the human brain.

ATP metabolism is controlled mainly by ATP synthase (ATP(ase)) and creatine kinase (CK) reactions that regulate cerebral ATP production, transportation, and utilization. These coupled reactions constitute a chemical exchange metabolic network of PCr<-->ATP<-->Pi characterized by two forward and two reverse reaction fluxes, which can be studied noninvasively by in vivo (31)P MRS combined with magnetization transfer (MT). However, it is still debated whether current MT approaches can precisely determine all of these fluxes. We developed and tested a modified in vivo (31)P MT approach based on a multiple single-site saturation (MSS) technique to study the entire PCr<-->ATP<-->Pi network in human occipital lobe at 7T. Our results reveal that 1) the MSS MT approach can explicitly determine all four reaction fluxes with a minimal number of (31)P spectra; 2) the three-spin exchange model accurately determines reverse reaction fluxes, resulting in equal forward and reverse fluxes for both CK and ATP(ase) reactions; and 3) the ATP synthesis rate (8.8 +/- 1.9 micromol/g/min, N = 11) measured in the human brain reflects cerebral oxidative phosphorylation. The MSS MT approach should provide an important modality for noninvasively studying the essential roles of ATP metabolism in brain bioenergetics, function, and diseases.     

Experimental design of 31P MRS assessment of human forearm muscle function: Restrictions imposed by functional anatomy

The restrictions imposed by the functional anatomy of the finger flexor muscles on the experimental design of ³¹P MRS assessment of human forearm muscle function employing surface coil localization and voluntary exercise were investigated. It was found that ³¹P MRS metabolic data of finger flexor muscle should be correlated with mechanical data of combined flexion of only the ring and little fingers, rather than all four fingers as has been commonly the case in previously reported studies.     

Simultaneous 31P NMR spectroscopy and EMG in exercising and recovering human skeletal muscle: Technical aspects

The bioenergetics of human skeletal muscle can be studied by ³¹P NMR spectroscopy (³¹P-MRS) and by surface electromyography (SEMG). Simultaneous ³¹P-MRS and SEMG permit accurate and noninvasive studies of the correlation between metabolic and electrical changes in exercising and recovering human skeletal muscle, a relationship that is still poorly understood. This study describes the optimization of skeletal muscle ³¹P-MRS in a whole-body magnet, involving surface coil design, utilization of adiabatic radio frequency pulses and advanced time-domain fitting, to the technical design of SEMG. A nonmagnetic ergometer was used for ankle dorsi-flexions that activated only the anterior tibia1 muscle as verified by post exercise imaging. The coil design and the adiabatic sechltanh pulse improved sensitivity by 45% and 56% respectively, compared with standard techniques. Simultaneous electromyographic recordings did not deteriorate the NMR spectra. The VARPRO time domain fitting routine was very suitable for estimating ³¹P muscle spectra. With these methods it was possible to accurately estimate parameters describing metabolic and electrical changes during rest, exercise and the entire recovery period with a 20-s time resolution on a standard 1.5 T whole-body NMR scanner.     

In vivo and in vitro 31P magnetic resonance spectroscopic studies of the hepatic response of healthy rats and rats with acute hepatic damage to fructose loading

The hepatic response to a fructose challenge for control rats, and rats subjected to an acute sublethal dose of carbon tetrachloride (CCI₄) or bromobenzene (BB), was compared using dynamic in vivo ³¹P MRS. Fructose loading conditions were used in which control rats showed only a modest increase in hepatic phosphomonoester (PME), and a small decrease in ATP, P_i, and intracellular pH after fructose administration. Both CCI₄, and BB-treated rats showed a much greater fructose-induced accumulation of PME than did controls. Trolox C, a free radical scavenger, prevented most of this PME increase. BB-treated rats, given sufficient time to recover from the hepa-totoxic insult, responded to the fructose load similarly to controls. Liver aldolase activities of control, toxicant-treated rats, and toxicant plus Trolox C-treated rats correlated inversely with PME accumulation after fructose loading (correlation coefficient: ?0.834, P < 0.05). Perchloric acid extracts of rat livers studied by in vitro ³¹P MRS confirmed that the PME accumulation after fructose loading is mainly due to an increase in fructose 1-phosphate. These studies are consistent with the aldolase-catalyzed cleavage of fructose 1-phosphate being rate-limiting in hepatic fructose metabolism, and that the CCI, and BB treatment modify and inactivate the aldolase enzyme.     

Comparing localized and nonlocalized dynamic 31P magnetic resonance spectroscopy in exercising muscle at 7T

By improving spatial and anatomical specificity, localized spectroscopy can enhance the power and accuracy of the quantitative analysis of cellular metabolism and bioenergetics. Localized and nonlocalized dynamic ³¹P magnetic resonance spectroscopy using a surface coil was compared during aerobic exercise and recovery of human calf muscle. For localization, a short echo time single‐voxel magnetic resonance spectroscopy sequence with adiabatic refocusing (semi‐LASER) was applied, enabling the quantification of phosphocreatine, inorganic phosphate, and pH value in a single muscle (medial gastrocnemius) in single shots (T_R = 6 s). All measurements were performed in a 7 T whole body scanner with a nonmagnetic ergometer. From a series of equal exercise bouts we conclude that: (a) with localization, measured phosphocreatine declines in exercise to a lower value (79 ± 7% cf. 53 ± 10%, P = 0.002), (b) phosphocreatine recovery shows shorter half time (t_1/2 = 34 ± 7 s cf. t_1/2 = 42 ± 7 s, nonsignificant) and initial postexercise phosphocreatine resynthesis rate is significantly higher (32 ± 5 mM/min cf. 17 ± 4 mM/min, P = 0.001) and (c) in contrast to nonlocalized ³¹P magnetic resonance spectroscopy, no splitting of the inorganic phosphate peak is observed during exercise or recovery, just an increase in line width during exercise. This confirms the absence of contaminating signals originating from weaker‐exercising muscle, while an observed inorganic phosphate line broadening most probably reflects variations across fibers in a single muscle. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Quantitative 31P magnetic resonance spectroscopy of the human breast at 7 T

This study presents quantified levels of phosphorylated metabolites in glandular tissue of human breast using (31)P magnetic resonance spectroscopy at 7 T. We used a homebuilt (1)H/(31)P radiofrequency coil to obtain artifact-free (31)P MR spectra of glandular tissue of healthy females by deploying whole breast free induction decay (FID) detection with adiabatic excitation and outer volume suppression. Using progressive saturation, the estimated apparent T(1) relaxation time of (31)P spins of phosphocholine and phosphoethanolamine was 4.4 and 5.7 s, respectively. Quantitative measures for phosphocholine and phosphoethanolamine levels in glandular tissue were established based on MR imaging. We used a 3D (1)H image of the breast to segment the glandular tissue; this was matched to a 3D (31)P image of the B1- field of the (31)P coil to correct for differences in glandular tissue volume and B(1) inhomogeneity of the (31)P coil. The (31)P MR spectra were calibrated using a phantom with known concentration. Average levels of phosphocholine and phosphoethanolamine in 11 volunteers were 0.84 ± 0.21 mM and 1.18 ± 0.41 mM, respectively. In addition, data of three patients with breast cancer showed higher levels of phosphocholine and phosphoethanolamine compared with healthy volunteers. This may indicate a potential role for the use of (31)P magnetic resonance spectroscopy for characterization, prognosis, and treatment monitoring in breast cancer.     

Phosphorus-31 magnetic resonance spectroscopy of experimentally induced arthritis in rats

Phosphorus-31 magnetic resonance spectroscopy (MRS) of the hind paws of rats was performed at 1.5 Tesla before and during the course of an experimentally-induced inflammatory arthritis. Arthritis was induced by daily subcutaneous administration of 6-sulfanilamidoindazole, an antibacterial sulfa known to produce an acute, self-limited arthritis and periarthritis in the hind paws of rats.Phosphorus-31 spectra obtained after the development of clinical arthritis showed a significant (p<0.01) increase in the intensity of a group of resonances occurring downfiled from phosphocreatine as compared to spectra obtained before treatment. In all rats, this increase correlated well with histological evidence of inflammation as well as with the degree of inflammation judged clinically (r=0.89, p<0.001), and was present before roentgenographic evidence of bony involvement. The use of ³¹P MRS may permit evaluation of the severity of an inflammatory arthritis with greater accuracy than the bony changes definable by plain roentgenograms.This article is the award-winning Resident Paper of the International Skeletal Society for the year 1986. Support for its presentation at the annual meeting was contributed by the Department of Radiology of the University of British Columbia