Measurement of spin-lattice relaxation times and kinetic rate constants in rat muscle using progressive partial saturation and steady-state saturation transfer

³¹P spin-lattice relaxation times (T₁) of metabolites in rat calf muscle at 1.9 Tesla and the forward rate through the creatine kinase (CK) reaction have been measured using a new method based on modeling progressive saturation explicitly incorporating the effect of chemical exchange. In a separate series of experiments, we compared our method with inversion recovery both in vitro and in vivo, finding agreement between the techniques. We found that the T₁ values of phosphocreatine (PCr) (6.6 ± 0.3 s), γ-ATP (2.6 ± 0.6 s), α-ATP (2.4 ± 0.4 s) and β-ATP (2.2 ± 0.2 s) are unchanged by stimulation of sufficient intensity to induce a 32% drop in PCr level. The errors in T₁ 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 γ-ATP under typical conditions. Use of longer repetition times results in larger errors in measured values of T₁. This source of error can be effectively eliminated by use of sufficiently short repetition times. We found that the rate constant of the forward CK reaction was increased 60% by stimulation, from 0.20 ± 0.03 s^-1 to 0.32 ± 0.05 s^{- 1}, but that the phosphorus flux did not change.