MEASUREMENT OF SPIN-LATTICE RELAXATION-TIMES AND KINETIC RATE CONSTANTS IN RAT MUSCLE USING PROGRESSIVE PARTIAL SATURATION AND STEADY-STATESATURATION-TRANSFER

P-31 spin-lattice relaxation times (T-1) of metabolites in rat calf mu scle at 1.9 Tesla and the forward rate through the creatine kinase (CK ) reaction have been measured using a new method based on modeling pro gressive saturation explicitly incorporating the effect of chemical ex change. In a separate series of experiments, we compared our method wi th inversion recovery both in vitro and in vivo, finding agreement bet ween the techniques. We found that the T-1 values of phosphocreatine ( PCr) (6.6 +/- 0.3 s), gamma-ATP (2.6 +/- 0.6 s), alpha-ATP (2.4 +/- 0. 4 s) and beta-ATP (2.2 +/- 0.2 s) are unchanged by stimulation of suff icient intensity to induce a 32% drop in PCr level. The errors in T-1 values which arise when chemical exchange is neglected are calculated. These are found to be on the order of 20% for PCr and 30-50% for gamm a-ATP under typical conditions. Use of longer repetition times results in larger errors in measured values of T-1. This source of error can be effectively eliminated by use of sufficiently short repetition time s. We found that the rate constant of the forward CK reaction was incr eased 60% by stimulation, from 0.20 +/- 0.03 s(-1) to 0.32 +/- 0.03 s( -1), but that the phosphorus flux did not change.