An H-1-MRS evaluation of the phosphocreatine/creatine pool (tCr) in human muscle

The human gastrocnemius was examined with and without creatine supplementat ion under the conditions of rest, ischemic fatigue (IF), and recovery to pe rturb the pool sizes and equilibrium between phosphocreatine (PCr) and crea tine (Cr). H-1- and P-31-magnetic resonance spectroscopy (MRS) were used to examine the total creatine (tCr) pool in each of the metabolic states. P-3 1-MRS monitored the depletion of the PCr peak during IF to <5% of that at r est. H-1-MRS focused on the tCr methyl peak at 3.02 ppm (dipolar coupled tr iplet), at which point it was expected that the triplet peak intensity woul d be similar both in IF and rest. Initial H-1-MRS data showed the peak inte nsity during IF decreased, suggesting a change in tCr pool size. Subsequent studies of transverse relaxation time (T-2) revealed that this decline was primarily due to a more rapid T-2 decay of the tCr peak in IF (T-2<similar to>40 ms) compared with at rest (T(2)similar to 162 ms). Because Cr is the major contributor to tCr in IF, it is possible that there is a pool of Cr displaying reduced mobility in vivo. Moreover, the residual dipolar coupled triplet observed at rest collapsed into a broad singlet during IF, suggest ive of significant changes in the ordered environment experienced at rest f or PCr compared with when it is converted to Cr during IF. In addition, the se data suggest that in H-1-MRS studies whose goals include quantitative es timates of tCr pool sizes, standardized metabolic conditions or careful T-2 evaluations will be required.