Dynamic ³¹P-MRS is broadly used to measure the skeletal muscle maximum oxidative flux (Q_max) which is a valid measure of muscular mitochondrial capacity [1]. The measurement of Pi-to-ATP (F_ATP) 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 F_ATP under exercise conditions (F_{ATP_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, Q_max and F_{ATP_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.

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 V_PCr and maximal oxidative flux Q_max were calculated from PCr recovery kinetics. Pi-to-ATP (ATP synthesis) and PCr-to-ATP (CK) reaction rates and corresponding fluxes F_ATP and F_CK were calculated from FAST experiment (as described in [5]), both at rest and during exercise. The relations between maximal oxidative flux Q_max and metabolic flux (F_{ATP_ex}) were compared by linear regression. The difference in F_ATP between resting state and challenged conditions (ΔF_ATP = F_{ATP_ex} – F_{ATP_rest}) was also compared to Q_max.

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 ΔPCr_ex) 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 Q_max and F_{ATP_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 Q_max and ΔF_ATP (r²=0.77, p=0.00004) (Figure 3b). The underlying details for this observation require further investigation.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 F_ATP under exercise condition also correlates with the mitochondrial capacity.