31P MR Spectroscopy and Computational Modeling Identify a Direct Relation between Pi Content of an Alkaline Compartment in Resting Muscle and Phosphocreatine Resynthesis Kinetics in Active Muscle in Humans

Abstract

The assessment of mitochondrial properties in skeletal muscle is important in clinical research, for instance in the study of diabetes. The gold standard to measure mitochondrial capacity non-invasively is the phosphocreatine (PCr) recovery rate after exercise, measured by 31P Magnetic Resonance spectroscopy (31P MRS). Here, we sought to expand the evidence base for an alternative method to assess mitochondrial properties which uses 31P MRS measurement of the Pi content of an alkaline compartment attributed to mitochondria (Pi2; as opposed to cytosolic Pi (Pi1)) in resting muscle at high magnetic field. Specifically, the PCr recovery rate in human quadriceps muscle was compared with the signal intensity of the Pi2 peak in subjects with varying mitochondrial content of the quadriceps muscle as a result of athletic training, and the results were entered into a mechanistic computational model of mitochondrial metabolism in muscle to test if the empirical relation between Pi2/Pi1 ratio and the PCr recovery was consistent with theory. Localized 31P spectra were obtained at 7T from resting vastus lateralis muscle to measure the intensity of the Pi2 peak. In the endurance trained athletes a Pi2/Pi1 ratio of 0.07 ± 0.01 was found, compared to a significantly lower (p<0.05) Pi2/Pi1 ratio of 0.03 ± 0.01 in the normally active group. Next, PCr recovery kinetics after in magnet bicycle exercise were measured at 1.5T. For the endurance trained athletes, a time constant τPCr 12 ± 3 s was found, compared to 24 ± 5s in normally active subjects. Without any parameter optimization the computational model prediction matched the experimental data well (r 2 of 0.75). Taken together, these results suggest that the Pi2 resonance in resting human skeletal muscle observed at 7T provides a quantitative MR-based functional measure of mitochondrial density.

1H NMRS of carnosine combined with 31P NMRS to better characterize skeletal muscle pH dysregulation in Duchenne muscular dystrophy

“…20 Moreover, the hypothesis of an increased mitochondrial density in patients with DMD is unrealistic. It should also be remembered that endurance training is accompanied by vascular adaptations 52 and increased extracellular space, 53 which could have contributed to the increased P i,b observed in the study by van Oorschot et al 51 In addition, the P i,b /P i,tot ratios determined in this work are consistent with P i concentrations in the extracellular 54 (i.e. 0.5-1mM) and intracellular 55 (i.e.…”

“…A mitochondrial origin has been suggested on the basis, amongst others, of the very short T 1 relaxation time of this P i,b peak . This P i,b resonance was measured to be higher in endurance‐trained as opposed to untrained healthy subjects, and was proposed as a marker of mitochondrial density . The P i,b resonance seen in dystrophic muscle is, however, unlikely to be of the same origin.…”

“…50 This P i,b resonance was measured to be higher in endurance-trained as opposed to untrained healthy subjects, and was proposed as a marker of mitochondrial density. 51 The P i,b resonance seen in dystrophic muscle is, however, unlikely to be of the same origin. It is of much higher amplitude and its T 1 relaxation time was found to be identical to the neutral P i,a in the GRMD canine model.…”

In-vivo 31P-MRS of skeletal muscle and liver: A way for non-invasive assessment of their metabolism

“…A good agreement between the measured relation between Pi 2 /Pi and the recovery time constant, τ PCr , and a model prediction was also found [95]. In addition, there was also a positive correlation reported between [Pi 2 ] and Q max , as well as Pi 2 /Pi and Q max [92].…”

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

“…, van Oorschot et al . ). MMMF respond to training in longitudinal studies in healthy subjects (Forbes et al .…”