Correcting human heart 31P NMR spectra for partial saturation. Evidence that saturation factors for PCr/ATP are homogeneous in normal and disease states

Increased adiposity is associated with altered skeletal muscle energetics

The objective of this pilot study was to characterize relationships between skeletal muscle energy metabolism and body composition in healthy adults with varied amounts and distribution of adipose tissue. In vivo muscle energetics were quantified using dynamic 31P magnetic resonance spectroscopy with knee extension exercise standardized to subject lean body mass. Spearman's correlation analysis examined relationships between muscle metabolism indices and measures of adiposity including body mass index (BMI), total body fat, and quadriceps intermuscular adipose tissue (IMAT). Post hoc partial correlations were examined controlling for additional body composition measures. Kruskal-Wallis tests with Dunn-Sidak post hoc comparisons evaluated group differences in energy metabolism based on body composition profiles (i.e., lean, normal-weight obese, and overweight-obese) and IMAT tertiles. BMI negatively correlated with end-exercise muscle pH after correcting for IMAT and total body fat (r = -0.46, P = 0.034). Total adiposity negatively correlated with maximum oxidative capacity after correcting for IMAT (r = -0.54, P = 0.013). IMAT positively correlated with muscle proton buffering capacity after correcting for total body fat (r = 0.53, P = 0.023). Body composition groups showed differences in end-exercise fall in [PCr] with normalized workload (P = 0.036; post hoc: overweight-obese < lean, P = 0.029) and maximum oxidative capacity (P = 0.021; post hoc: normal-weight obese < lean, P = 0.016). IMAT tertiles showed differences in end-exercise fall in [PCr] with normalized workload (P = 0.035; post hoc: 3rd < 1st, P = 0.047). Taken together, these results support increased adiposity is associated with reduced muscle energetic efficiency with more reliance on glycolysis, and when accompanied with reduced lean mass, is associated with reduced maximum oxidative capacity.NEW & NOTEWORTHY Skeletal muscle energy production is influenced by both lean body mass and adipose tissue but the effect of their distribution on energy metabolism is unclear. This study examined variations in quadriceps muscle energy metabolism in healthy adults with varied relative amounts of lean and adipose tissue. Results suggest increased adiposity is associated with reduced muscle energetic efficiency with more reliance on glycolysis, and when accompanied with reduced lean mass, is associated with reduced maximum oxidative capacity.

Assessing the Feasibility of Dynamic 31P Spectroscopy for Metabolic Studies with a 1.0T Extremity Scanner

Objective: The feasibility of conducting in vivo non-localized ³¹P Magnetic Resonance Spectroscopy (MRS) with a 1.0T extremity scanner and the potential to increase accessibility of this important diagnostic tool for low cost applications is revisited. Methods: This work presents a custom transmit-only quadrature birdcage, four-element receive coil array, and spectrometer interfaced to a commercial ONI 1.0T magnet for enabling multi-channel, non-1H frequency capabilities. A custom, magnetic resonance compatible plantar flexion-extension exercise device was also developed to enable exercise protocols. The coils were assessed with bench measurements and ³¹P phantom studies before an in vivo demonstration. Results: In pulse and acquire spectroscopy of a phantom, the array was found to improve the signal-to-noise ratio (SNR) by a factor of 1.31 and reduce the linewidth by 13.9% when compared to a large loop coil of the same overall size. In vivo testing results show that two averages and a four second repetition time for a temporal resolution of eight seconds was sufficient to obtain phosphocreatine recovery values and baseline pH levels aligned with expected literature values. Conclusion: Initial in vivo human skeletal muscle ³¹P MRS allowed successful monitoring of metabolic changes during an 18-minute exercise protocol. Significance: Adding an array coil and multinuclear capability to a commercial low-cost 1.0T extremity scanner enabled the observation of characteristic ³¹P metabolic information, such as the phosphocreatine recovery rate and underlying baseline pH.

Co-Polarized [1-13C]Pyruvate and [1,3-13C2]Acetoacetate Provide a Simultaneous View of Cytosolic and Mitochondrial Redox in a Single Experiment

Cellular redox is intricately linked to energy production and normal cell function. Although the redox states of mitochondria and cytosol are connected by shuttle mechanisms, the redox state of mitochondria may differ from redox in the cytosol in response to stress. However, detecting these differences in functioning tissues is difficult. Here, we employed ¹³C magnetic resonance spectroscopy (MRS) and co-polarized [1-¹³C]pyruvate and [1,3-¹³C₂]acetoacetate ([1,3-¹³C₂]AcAc) to monitor production of hyperpolarized (HP) lactate and β-hydroxybutyrate as indicators of cytosolic and mitochondrial redox, respectively. Isolated rat hearts were examined under normoxic conditions, during low-flow ischemia, and after pretreatment with either aminooxyacetate (AOA) or rotenone. All interventions were associated with an increase in [P_i]/[ATP] measured by ³¹P NMR. In well-oxygenated untreated hearts, rapid conversion of HP [1-¹³C]pyruvate to [1-¹³C]lactate and [1,3-¹³C₂]AcAc to [1,3-¹³C₂]β-hydroxybutyrate ([1,3-¹³C₂]β-HB) was readily detected. A significant increase in HP [1,3-¹³C₂]β-HB but not [1-¹³C]lactate was observed in rotenone-treated and ischemic hearts, consistent with an increase in mitochondrial NADH but not cytosolic NADH. AOA treatments did not alter the productions of HP [1-¹³C]lactate or [1,3-¹³C₂]β-HB. This study demonstrates that biomarkers of mitochondrial and cytosolic redox may be detected simultaneously in functioning tissues using co-polarized [1-¹³C]pyruvate and [1,3-¹³C₂]AcAc and ¹³C MRS and that changes in mitochondrial redox may precede changes in cytosolic redox.

Suppression of skeletal muscle signal using a crusher coil: A human cardiac (31) p-MR spectroscopy study at 7 tesla

Purpose

The translation of sophisticated phosphorus MR spectroscopy (³¹P‐MRS) protocols to 7 Tesla (T) is particularly challenged by the issue of radiofrequency (RF) heating. Legal limits on RF heating make it hard to reliably suppress signals from skeletal muscle that can contaminate human cardiac ³¹P spectra at 7T. We introduce the first surface‐spoiling crusher coil for human cardiac ³¹P‐MRS at 7T.

Methods

A planar crusher coil design was optimized with simulations and its performance was validated in phantoms. Crusher gradient pulses (100 μs) were then applied during human cardiac ³¹P‐MRS at 7T.

Results

In a phantom, residual signals were 50 ± 10% with BISTRO (B₁‐insensitive train to obliterate signal), and 34 ± 8% with the crusher coil. In vivo, residual signals in skeletal muscle were 49 ± 4% using BISTRO, and 24 ± 5% using the crusher coil. Meanwhile, in the interventricular septum, spectral quality and metabolite quantification did not differ significantly between BISTRO (phosphocreatine/adenosine triphosphate [PCr/ATP] = 2.1 ± 0.4) and the crusher coil (PCr/ATP = 1.8 ± 0.4). However, the specific absorption rate (SAR) decreased from 96 ± 1% of the limit (BISTRO) to 16 ± 1% (crusher coil).

Conclusion

A crusher coil is an SAR‐efficient alternative for selectively suppressing skeletal muscle during cardiac ³¹P‐MRS at 7T. A crusher coil allows the use of sequence modules that would have been SAR‐prohibitive, without compromising skeletal muscle suppression. Magn Reson Med 75:962–972, 2016. © 2015 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.

Human cardiac 31P magnetic resonance spectroscopy at 7 Tesla

Purpose

Phosphorus magnetic resonance spectroscopy (³¹P-MRS) affords unique insight into cardiac energetics but has a low intrinsic signal-to-noise ratio (SNR) in humans. Theory predicts an increased ³¹P-MRS SNR at 7T, offering exciting possibilities to better investigate cardiac metabolism. We therefore compare the performance of human cardiac ³¹P-MRS at 7T to 3T, and measure T₁s for ³¹P metabolites at 7T.

Methods

Matched ³¹P-MRS data were acquired at 3T and 7T, on nine normal volunteers. A novel Look-Locker CSI acquisition and fitting approach was used to measure T₁s on six normal volunteers.

Results

T₁s in the heart at 7T were: phosphocreatine (PCr) 3.05 ± 0.41s, γ-ATP 1.82 ± 0.09s, α-ATP 1.39 ± 0.09s, β-ATP 1.02 ± 0.17s and 2,3-DPG (2,3-diphosphoglycerate) 3.05 ± 0.41s (N = 6). In the field comparison (N = 9), PCr SNR increased 2.8× at 7T relative to 3T, the Cramer-Ráo uncertainty (CRLB) in PCr concentration decreased 2.4×, the mean CRLB in PCr/ATP decreased 2.7× and the PCr/ATP SD decreased 2×.

Conclusion

Cardiac ³¹P-MRS at 7T has higher SNR and the spectra can be quantified more precisely than at 3T. Cardiac ³¹P T₁s are shorter at 7T than at 3T. We predict that 7T will become the field strength of choice for cardiac ³¹P-MRS. Magn Reson Med 72:304–315, 2014. © 2013 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited.

Xanthine oxidase inhibitors improve energetics and function after infarction in failing mouse hearts

After myocardial infarction, ventricular geometry and function, as well as energy metabolism, change markedly. In nonischemic heart failure, inhibition of xanthine oxidase (XO) improves mechanoenergetic coupling by improving contractile performance relative to a reduced energetic demand. However, the metabolic and contractile effects of XO inhibitors (XOIs) have not been characterized in failing hearts after infarction. After undergoing permanent coronary ligation, mice received a XOI (allopurinol or oxypurinol) or matching placebo in the daily drinking water. Four weeks later, 1H MRI and 31P magnetic resonance spectroscopy (MRS) were used to quantify in vivo functional and metabolic changes in postinfarction remodeled mouse myocardium and the effects of XOIs on that process. End-systolic (ESV) and end-diastolic volumes (EDV) were increased by more than sixfold after infarction, left ventricle (LV) mass doubled ( P < 0.005), and the LV ejection fraction (EF) decreased (14 ± 9%) compared with control hearts (59 ± 8%, P < 0.005) at 1 mo. The myocardial phosphocreatine (PCr)-to-ATP ratio (PCr/ATP) was also significantly decreased in infarct remodeled hearts (1.4 ± 0.6) compared with control animals (2.1 ± 0.5, P < 0.02), in agreement with prior studies in larger animals. The XOIs allopurinol and oxypurinol did not change LV mass but limited the increase in ESV and EDV of infarct hearts by 50%, increased EF (23 ± 9%, P = 0.01), and normalized cardiac PCr/ATP (2.0 ± 0.5, P < 0.04). We conclude that XOIs improve ventricular function after infarction and normalize high-energy phosphate ratio in heart failure. Thus XOI therapy offers a new and potentially complementary approach to limit the adverse contractile and metabolic consequences after infarction.

An increase in the myocardial PCr/ATP ratio in GLUT4 null mice

ATP and creatine phosphate (PCr) are prime myocardial high-energy phosphates. Their relative concentrations are conserved among mammalian species and across a range of physiologic cardiac workloads. The cardiac PCr/ATP ratio is decreased with several pathologic conditions, such as ischemia and heart failure, but there are no reports of an increase in the cardiac PCr/ATP ratio in any species or with interventions. We studied the in vivo energetics in transgenic mice lacking expression of the glucose transport protein GLUT4 (G4N) and observed a significant 60% increase in the myocardial PCr/ATP ratio in G4N that was confirmed in three different experimental settings including intact animals. The higher PCr/ATP in G4N is cardiac-specific and is due to higher total cardiac creatine (CR) concentrations in G4N than in wild-type (WT). However, [ATP], [ADP], and -DG(-ATP) did not differ between the strains. Expression of the creatine transport protein (CreaT) that is responsible for creatine uptake in myocytes was preserved in G4N cardiac tissue. These observations demonstrate, for the first time to our knowledge, that G4N manifest a unique increase in the cardiac PCr/ATP ratio, which suggests a novel genetic strategy for increasing myocardial creatine levels.