Letter to the editor: “Interpretation of 31P NMR saturation transfer experiments: do not forget the spin relaxation properties”

Increasing mitochondrial ATP synthesis with butyrate normalizes ADP and contractile function in metabolic heart disease

Metabolic heart disease (MHD), which is strongly associated with heart failure with preserved ejection fraction, is characterized by reduced mitochondrial energy production and contractile performance. In this study, we tested the hypothesis that an acute increase in ATP synthesis, via short chain fatty acid (butyrate) perfusion, restores contractile function in MHD. Isolated hearts of mice with MHD due to consumption of a high fat high sucrose (HFHS) diet or on a control diet (CD) for 4 months were studied using 31 P NMR spectroscopy to measure high energy phosphates and ATP synthesis rates during increased work demand. At baseline, HFHS hearts had increased ADP and decreased free energy of ATP hydrolysis (ΔG~ATP ), although contractile function was similar between the two groups. At high work demand, the ATP synthesis rate in HFHS hearts was reduced by over 50%. Unlike CD hearts, HFHS hearts did not increase contractile function at high work demand, indicating a lack of contractile reserve. However, acutely supplementing HFHS hearts with 4mM butyrate normalized ATP synthesis, ADP, ΔG~ATP and contractile reserve. Thus, acute reversal of depressed mitochondrial ATP production improves contractile dysfunction in MHD. These findings suggest that energy starvation may be a reversible cause of myocardial dysfunction in MHD, and opens new therapeutic opportunities.

31 P magnetic resonance spectroscopy in skeletal muscle: Experts' consensus recommendations

Skeletal muscle phosphorus-31 31 P MRS is the oldest MRS methodology to be applied to in vivo metabolic research. The technical requirements of 31 P MRS in skeletal muscle depend on the research question, and to assess those questions requires understanding both the relevant muscle physiology, and how 31 P MRS methods can probe it. Here we consider basic signal-acquisition parameters related to radio frequency excitation, TR, TE, spectral resolution, shim and localisation. We make specific recommendations for studies of resting and exercising muscle, including magnetisation transfer, and for data processing. We summarise the metabolic information that can be quantitatively assessed with 31 P MRS, either measured directly or derived by calculations that depend on particular metabolic models, and we give advice on potential problems of interpretation. We give expected values and tolerable ranges for some measured quantities, and minimum requirements for reporting acquisition parameters and experimental results in publications. Reliable examination depends on a reproducible setup, standardised preconditioning of the subject, and careful control of potential difficulties, and we summarise some important considerations and potential confounders. Our recommendations include the quantification and standardisation of contraction intensity, and how best to account for heterogeneous muscle recruitment. We highlight some pitfalls in the assessment of mitochondrial function by analysis of phosphocreatine (PCr) recovery kinetics. Finally, we outline how complementary techniques (near-infrared spectroscopy, arterial spin labelling, BOLD and various other MRI and 1 H MRS measurements) can help in the physiological/metabolic interpretation of 31 P MRS studies by providing information about blood flow and oxygen delivery/utilisation. Our recommendations will assist in achieving the fullest possible reliable picture of muscle physiology and pathophysiology.

A simple approach to evaluate the kinetic rate constant for ATP synthesis in resting human skeletal muscle at 7 T

Inversion transfer (IT) is a well-established technique with multiple attractive features for analysis of kinetics. However, its application in measurement of ATP synthesis rate in vivo has lagged behind the more common saturation transfer (ST) techniques. One well-recognized issue with IT is the complexity of data analysis in comparison with much simpler analysis by ST. This complexity arises, in part, because the γ-ATP spin is involved in multiple chemical reactions and magnetization exchanges, whereas Pi is involved in a single reaction, Pi → γ-ATP. By considering the reactions involving γ-ATP only as a lumped constant, the rate constant for the reaction of physiological interest, kPi→γATP , can be determined. Here, we present a new IT data analysis method to evaluate kPi→γATP using data collected from resting human skeletal muscle at 7 T. The method is based on the basic Bloch-McConnell equation, which relates kPi→γATP to m˙Pi, the rate of Pi magnetization change. The kPi→γATP value is accessed from m˙Pi data by more familiar linear correlation approaches. For a group of human subjects (n = 15), the kPi→γATP value derived for resting calf muscle was 0.066 ± 0.017 s(-1) , in agreement with literature-reported values. In this study we also explored possible time-saving strategies to speed up data acquisition for kPi→γATP evaluation using simulations. The analysis indicates that it is feasible to carry out a (31) P IT experiment in about 10 min or less at 7 T with reasonable outcome in kPi→γATP variance for measurement of ATP synthesis in resting human skeletal muscle. We believe that this new IT data analysis approach will facilitate the wide acceptance of IT to evaluate ATP synthesis rate in vivo. Copyright © 2015 John Wiley & Sons, Ltd.

Band inversion amplifies 31 P-31 P nuclear overhauser effects: Relaxation mechanism and dynamic behavior of ATP in the human brain by 31 P MRS at 7 T

PURPOSE

To develop an improved method to measure the ³¹ P nuclear Overhauser effect (NOE) for evaluation of adenosine triphosphate (ATP) dynamics in terms of correlation time (τ_c ), and contribution of dipole-dipole (DD) and chemical shift anisotropy (CSA) mechanisms to T₁ relaxation of ATP in human brain.

METHODS

The NOE of ATP in human brain was evaluated by monitoring changes in magnetization in the β-ATP signal following a band inversion of all downfield ³¹ P resonances. The magnetization changes observed were analyzed using the Bloch-McConnell-Solomon formulation to evaluate the relaxation and motion dynamic parameters that describe interactions of ATP with cellular solids in human brain tissue.

RESULTS

The maximal transient NOE, observed as a reduction in the β-ATP signal, was 24 ± 2% upon band inversion of γ- and α-ATP, which is 2-3-fold higher than achievable by frequency-selective inversion of either γ- or α-ATP. The rate of ³¹ P-³¹ P cross relaxation (0.21 ± 0.02 s^-1 ) led to a τ_c value of (9.1 ± 0.8) × 10^-8 s for ATP in human brain. The T₁ relaxation of β-ATP is dominated by CSA over the DD mechanism (60%: 40%).

CONCLUSIONS

The band inversion method proved effective in amplifying ³¹ P NOE, and thus facilitating ATP τ_c and relaxation measurements. This technique renders ATP a potentially useful reporter molecule for cellular environments. Magn Reson Med 77:1409-1418, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Feasibility of mapping unidirectional Pi-to-ATP fluxes in muscles of the lower leg at 7.0 Tesla

PURPOSE

To assess the feasibility of mapping the kinetics and unidirectional fluxes of inorganic phosphate (Pi) to adenosine triphosphate (ATP) reactions in the entire volume of the lower leg muscles using a three-dimensional saturation transfer (ST) phosphorus (³¹ P) imaging sequence.

THEORY AND METHODS

We imaged the lower leg muscles of five healthy subjects at 7.0 Tesla. The total experimental time was 45 min. We quantified muscle-specific forward reaction rate constants (k^'_f ) and metabolic fluxes (V_f ) of the Pi-to-ATP reaction in the tibialis anterior, the gastrocnemius, and the soleus.

RESULTS

In the tibialis anterior, k^'_f and V_f were 0.11 s^-1  ± 0.03 (mean ± standard deviation) and 0.34 mM s^-1  ± 0.10, respectively. In the gastrocnemius, k^'_f was 0.11 s^-1  ± 0.04 and V_f was 0.37 mM s^-1  ± 0.11, while in the soleus muscle k^'_f was 0.10 s^-1  ± 0.02 and V_f was 0.36 mM s^-1  ± 0.14.

CONCLUSION

Our results suggest that mapping the kinetics and unidirectional fluxes from Pi-to-ATP in both the anterior and posterior muscles of the lower leg is feasible at ultra-high field and may provide useful insights for the study of insulin resistance, diabetes and aging. Magn Reson Med 74:225-230, 2015. © 2014 Wiley Periodicals, Inc.

Exchange kinetics by inversion transfer: integrated analysis of the phosphorus metabolite kinetic exchanges in resting human skeletal muscle at 7 T

PURPOSE

To develop an inversion pulse-based, chemical exchange saturation transfer-like method for detection of (31) P magnetization exchanges among all nuclear magnetic resonance visible metabolites suitable for providing an integrated kinetic analysis of phosphorus exchange reactions in vivo.

METHODS

The exchange kinetics by inversion transfer (EKIT) sequence includes application of a frequency-selective inversion pulse arrayed over the range of relevant (31) P frequencies, followed by a constant delay and a hard readout pulse. A series of EKIT spectra, each given by a plot of Z-magnetization for each metabolite of interest versus frequency of the inversion pulse, can be generated from this single data set.

RESULTS

EKIT spectra reflect chemical exchange due to known biochemical reactions, cross-relaxation effects, and relayed magnetization transfers due to both processes. The rate constants derived from EKIT data collected on resting human skeletal muscle were: ATP synthesis via ATP synthase (0.050 ± 0.016 s(-1) ), ATP synthesis via creatine kinase (0.264 ± 0.023 s(-1) ), and cross-relaxation between neighboring spin pairs within ATP (0.164 ± 0.022 s(-1) ).

CONCLUSION

EKIT provides a simple, alternative method to detect chemical exchange, cross relaxation, and relayed magnetization transfer effects in human skeletal muscle at 7 T.

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.