DRESS localized FAST technique at 7T uncovers the relation between mitochondrial capacity and ATP synthase flux in exercising gastrocnemius medialis muscle
Marjeta Tušek Jelenc1,2, Marek Chmelík1,2, Barbara Ukropcová3,4, Wolfgang Bogner1,2, Siegfried Trattnig1,2, Jozef Ukropec4, Martin Krššák1,2,5, and Ladislav Valkovič1,2,6,7

1High Field MR Centre, Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna, Austria, 2Christian Doppler Laboratory for Clinical Molecular MR Imaging, Vienna, Austria, 3Institute of pathophysiology, Faculty of Medicine, Comenius University, Bratislava, Slovakia, 4Obesity section, Diabetes and Metabolic Disease Laboratory, Institute of Experimental Endocrinology, Slovak Academy of Sciences, Bratislava, Slovakia, 5Division of Endocrinology and Metabolism, Department of Internal Medicine III, Medical University of Vienna, Vienna, Austria, 6Department of Imaging Methods, Institute of Measurement Science, Slovak Academy of Sciences, Bratislava, Slovakia, 7University of Oxford Centre for Clinical Magnetic Resonance Research, University of Oxford, John Radcliffe Hospital, Oxford, United Kingdom

Synopsis

The aim of the study was to investigate the relation between the maximum oxidative flux (Qmax), a valid measure of muscular mitochondrial capacity and ATP synthase flux (FATP) measured in exercising gastrocnemius medialis muscle in healthy young and elderly subjects. Furthermore, we explored the possibility of direct measurement of both, Qmax and FATP_ex, in a single experiment. The dynamic experiment consisted of the acquisition of baseline data during two minutes of rest, six minutes of aerobic plantar flexion exercise (during which a 3.5 minutes long FAST measurement was performed), and six minutes of recovery. Our data showed significant correlation between ATP synthase flux in exercising muscle and maximal oxidative flux.

Introduction

Dynamic 31P-MRS is broadly used to measure the skeletal muscle maximum oxidative flux (Qmax) which is a valid measure of muscular mitochondrial capacity [1]. The measurement of Pi-to-ATP (FATP) flux using saturation transfer (ST) technique provides different, more complex parameter of metabolic function [2]. However, if measured during exercise, ST can provide a direct measure of the demand driven, mitochondrial ATP synthesis flux. Recently, it has been shown feasible to determine the FATP under exercise conditions (FATP_ex), by a DRESS localized Four-Angle ST (FAST) technique at 7T [3]. Our aim was to explore the possibility of direct measurement of both, Qmax and FATP_ex, in a single exercise-recovery experiment and to investigate the relations between these two parameters of mitochondrial metabolism in gastrocnemius medialis muscle in healthy young and elderly subjects.

Materials and Methods

Four healthy young (3m/1f; age 29±3 years; BMI = 23±2 kg/m2) and five elderly volunteers (1m/4f; age 62±4.8 years; BMI = 26.5±3.6 kg/m2) underwent 31P MRS on a 7T system (Siemens Healthcare, Erlangen, Germany). Subjects were lying in a supine position on a plantar flexion ergometer (Ergospect, Innsbruck, Austria) with the right calf muscle placed over a double-tuned (31P/1H) surface coil (10cm, Rapid Biomedical, Rimpar, Germany). The measurement protocol is depicted in Figure 1. The DRESS localized FAST measurement block consisted of two experiments, the first using a flip angle of 52° and NA=8, and the second with a flip angle of 15° and NA=24. Both experiments were performed w/o γ-ATP saturation (-2.48ppm, 2.48ppm and 12.52ppm) with the TR=2s, slab thickness=15 mm and preparation scans=4. The total measurement time of the FAST protocol was ~3.5 minutes.

For the dynamic examination, a standard protocol (2 minutes rest, – 6 minutes exercise – and 6 minutes recovery) was used with the plantar flexion performed once every TR (2s) at a work load of ~25% of maximal voluntary contraction force [4]. Two minutes after the onset of exercise, when the steady state of PCr depletion was reached, the DRESS localized FAST experiment during plantar flexion exercise started.

The recovery constant τ, initial recovery VPCr and maximal oxidative flux Qmax were calculated from PCr recovery kinetics. Pi-to-ATP (ATP synthesis) and PCr-to-ATP (CK) reaction rates and corresponding fluxes FATP and FCK were calculated from FAST experiment (as described in [5]), both at rest and during exercise. The relations between maximal oxidative flux Qmax and metabolic flux (FATP_ex) were compared by linear regression. The difference in FATP between resting state and challenged conditions (ΔFATP = FATP_ex – FATP_rest) was also compared to Qmax.

Results and discussion

PCr recovery parameters, metabolic fluxes and forward reaction rate constants from both experiments are summarized in Table 1. The parameters calculated from the DRESS localized FAST experiment at rest and during exercise are in good agreement with previously published results obtained via localized ST experiments [3, 5, 6]. The effect of exercise is visible in Figure 2 as a depletion of PCr (highlighted by ΔPCrex) and as an increase in Pi signal intensity. The increment of Pi-to-ATP flux during exercise found in our work (from 0.59 ± 0.12 mMs-1 to 0.78 ± 0.12 mMs-1 in the young group), is in agreement with a recent dynamic study [7]. The muscular mitochondrial capacity measured by PCr post-exercise recovery are comparable to that in other similar studies [1, 4]. We found a strong linear correlation between Qmax and FATP_ex measured during exercise (Figure 3a) (r2=0.76, p=0.025), which to the best of our knowledge has not been reported before. Furthermore, we also noticed a strong correlation between Qmax and ΔFATP (r2=0.77, p=0.00004) (Figure 3b). The underlying details for this observation require further investigation.

Conclusion

We have shown a significant correlation between ATP synthase flux measured in exercising muscle and the maximal oxidative flux in the same muscle during the recovery. The increase in FATP under exercise condition also correlates with the mitochondrial capacity.

Acknowledgements

We thank all of the participants for their efforts and patience during data collection. This study was supported by the ÖNB Jubiläumsfond (grant #15455 to L.V., grant #16133 to W.B. and grant #15363 to M.K.), by Christian Doppler Society – Clinical Molecular MR Imaging (MOLIMA) and as well by grants from the Agency of the Slovak Academy of Science, VEGA 2/0013/14.

References

[1] Valkovic, L., M. Chmelik, I. Just Kukurova, M. Jakubova, M.C. Kipfelsberger, P. Krumpolec, M. Tusek Jelenc, W. Bogner, M. Meyerspeer, J. Ukropec, I. Frollo, B. Ukropcova, S. Trattnig, and M. Krssak, Depth-resolved surface coil MRS (DRESS)-localized dynamic (31)P-MRS of the exercising human gastrocnemius muscle at 7 T. NMR Biomed. 2014; 27(11): 1346-52.

[2] Kemp, G.J. and K.M. Brindle, What do magnetic resonance-based measurements of Pi-->ATP flux tell us about skeletal muscle metabolism? Diabetes. 2012; 61(8): 1927-34.

[3] Tušek-Jelenc, M., M. Chmelik, W. Bogner, M. Krssak, S. Trattnig, and L. Valkovic, Determination of ATP synthesis and CK exchange rate constants in gastrocnemius muscle at rest and during exercise by DRESS localized 31P-MRS FAST at 7T. ESMRMB. 2015.

[4] Valkovic, L., B. Ukropcova, M. Chmelik, M. Balaz, W. Bogner, A.I. Schmid, I. Frollo, E. Zemkova, I. Klimes, J. Ukropec, S. Trattnig, and M. Krssak, Interrelation of 31P-MRS metabolism measurements in resting and exercised quadriceps muscle of overweight-to-obese sedentary individuals. NMR Biomed. 2013; 26(12): 1714-22.

[5] Bottomley, P.A., R. Ouwerkerk, R.F. Lee, and R.G. Weiss, Four-angle saturation transfer (FAST) method for measuring creatine kinase reaction rates in vivo. Magn Reson Med. 2002; 47(5): 850-63.

[6] Parasoglou, P., D. Xia, and R.R. Regatte, Feasibility of mapping unidirectional Pi-to-ATP fluxes in muscles of the lower leg at 7.0 Tesla. Magn Reson Med. 2015; 74(1): 225–230.

[7] Sleigh, A., 31P magnetisation transfer measurements of Pi - ATP flux in exercising human muscle. ESMRMB. 2013.

Figures

Figure 1: Schematic representation of the measurement protocol. The measurement preparation was followed by the FAST measurement at rest and by the dynamic experiment. Dynamic experiment consisted of the acquisition of baseline data during two minutes of rest, six minutes of aerobic plantar flexion exercise (including FAST), and six minutes of recovery.

Table 1: Comparison of the parameters between young and elderly group measured by FAST at rest and during exercise and by exercise-recovery experiment. Calculated parameters are given as mean ± standard deviation.

Figure 2: 31P spectra showing FAST experiment, measured with FA = 52° (a) at rest and (b) during exercise with saturated γ-ATP (black line) and with control saturation (blue line).

Figure 3: Plots showing the correlations of maximal oxidative flux (Qmax) with ATP flux (FATP_ex) measured in exercising muscle (a) and with ΔFATP (FATP_ex – FATP_rest) (b).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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