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

A method for non-invasive, quantitative P-31 NMR spectroscopic investigation of mitochondrial function in human skeletal muscle in situ is described. High time resolution P-31 NMP 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 (Delta G(p)) were calculated at incremental steady-states of energy balance. The covariation of mitochondrial ATP synthesis flux, J(p)(MOP), and Delta G(p) was quasilinear over the physiological range of free energy values. Curve fit analysis of the covariation yielded a maximal sustainable J(p)(MOP) of 0.24 +/- 0.06 mmol ATP l(-1) s(-1) and a midpoint potential, (Delta G(p))(0.5), of 58.1 +/- 1.2 kJ/mole in the muscle cells.