In Vivo 31P-NMR Diffusion Spectroscopy of ATP and Phosphocreatine in Rat Skeletal Muscle

Abstract

AbstractThe aim of this study was to measure the diffusion of ATP and phosphocreatine (PCr) in intact rat skeletal muscle, using 31P-NMR. The acquisition of the diffusion-sensitized spectra was optimized in terms of the signal-to-noise ratio for ATP by using a frequency-selective stimulated echo sequence in combination with adiabatic radio-frequency pulses and surface coil signal excitation and reception. Diffusion restriction was studied by measuring the apparent diffusion coefficients of ATP and PCr as a function of the diffusion time. Orientation effects were eliminated by determining the trace of the diffusion tensor. The data were fitted to a cylindrical restriction model to estimate the unbounded diffusion coefficient and the radial dimensions of the restricting compartment. The unbounded diffusion coefficients of ATP and PCr were ∼90% of their in vitro values at 37°C. The diameters of the cylindrical restriction compartment were ∼16 and ∼22μm for ATP and PCr, respectively. The diameters of rat skeletal muscle fibers are known to range from 60 to 80μm. The modelling therefore suggests that the in vivo restriction of ATP and PCr diffusion is not imposed by the sarcolemma but by other, intracellular structures with an overall cylindrical orientation