Non-invasive quantitative 31P MRS assay of mitochondrial function in skeletal muscle in situ

A method for non-invasive, quantitative 31P NMR spectroscopic investigation of mitochondrial function in human skeletal muscle in situ is described. High time resolution 31P NMR measurements of phosphocreatine, inorganic phosphate and ATP resonances were conducted on human forearm flexor muscle during involuntary twitch contraction at eight different frequencies. Mitochondrial and glyco(geno)lytic ATP synthesis fluxes, and the cytosolic free energy of ATP hydrolysis (ΔGp) were calculated at incremental steady-states of energy balance. The covariation of mitochondrial ATP synthesis flux, JpMOP, and ΔGp was quasi-linear over the physiological range of free energy values. Curve-fit analysis of the covariation yielded a maximal sustainable pMOPof 0.24 ± 0.06 mmol ATP l-1·s-1 and a midpoint potential, (ΔGp)0.5, of 58.1 ± 1.2 kJ/mole in the muscle cells. (Mol Cell Biochem 174: 17–22, 1997)