Creatine loading and resting skeletal muscle phosphocreatine flux: a saturation-transfer NMR study

P-31 saturation-transfer nuclear magnetic resonance spectroscopy was used t o study skeletal muscle phosphocreatine (PCr) flux in healthy male voluntee rs. Data analysis included consideration of effects from incomplete saturat ion and radiofrequency spillover. Spectra were recorded from the resting ga strocnemius muscle before and after 6 days of creatine monohydrate (Cr .H2O ) intake (20 g/day). Parallel to an improved muscle performance during maxi mal intermittent exercise following Cr .H2O supplementation, the concentrat ion of PCr increased (P = 0.01) by 23% (34.9 +/- 2.8 mmol/l vs. 28.6 +/- 2. 7 mmol/l), whereas other metabolites were unaffected (inorganic phosphate: 4.3 +/- 1.4 mmol/l, free intracellular Mg2+: 1.1 +/- 0.7 mmol/l, cytosolic pH: 7.04 +/- 0.02). Forward and reverse fluxes through the creatine kinase (CK) reaction did not change significantly from their baseline levels (v(fo r): 11.8 +/- 5.4 mmol/l per second vs. 15.3 +/- 6.8 mmol/l per second. (v(r ev): 9.5 +/- 3.4 mmol/l per second vs. 10.9 +/- 3.7 mmol/l per second). The rate of PCr resynthesis in resting muscle is not limited by the CK reactio n, which is near equilibrium. Consequently, the post-load increase in total creatine has no effect on the unidirectional CK reaction rates. (C) 2001 E lsevier Science B.V. All rights reserved.