Magnetic coupling of creatine/phosphocreatine protons in rat skeletal muscle, as studied by H-1-magnetization transfer MRS

Off-resonance saturation caused a reduction of the 3.04 ppm NMR signal from the methyl protons of creatine in rat hindleg skeletal muscle. H-1-NMR spe ctra were recorded over a 200 kHz range of off-resonance saturation frequen cies. The span of frequencies over which the creatine signal was reduced gr eatly exceeded that expected for direct saturation by the off-resonance RF- field. This suggests that there is a motionally restricted proton pool whic h exchanges magnetization with the free creatine pool. The experimental dat a were fitted to characterize the immobilized proton pool and the exchange kinetics, using a two-pool exchange model. The immobile pool was estimated to amount to ca. 2.5% of the mobile pool of free creatine, while the rate o f exchange between the mobile and immobile configurations is ca. 2.3 sec(-1 ), After depletion of phosphocreatine by termination of the animal, the MT effect on the creatine methyl protons remained unchanged. This indicates th at phosphocreatine and creatine both contribute to the MT phenomenon. Selec tive saturation of the mobile water pool also led to a reduction in the int ensity of the total creatine methyl signal, suggesting that water and creat ine are magnetically coupled via a macromolecular interface, The precise me chanism responsible for and the biological significance of the pronounced c reatine magnetization transfer effect in rat skeletal muscle remains to be established. Magn Reson Med 42:665-672, 1999. (C) 1999 Wiley-Liss, Inc.