³¹P and ²³Na NMR spectroscopy of normal and ischemic rat skeletal muscle. Use of a shift reagent in vivo

²³Na NMR spectroscopy was used 1, to define the distribution of the shift reagent for cations, triethylenetetraminehexaacetatedysprosium(III), DyTTHA³⁻, in the living rat; 2, to define the characteristics of the Na resonances reporting intra‐ and extracellular Na⁺ in skeletal muscle in vivo; and 3, to calculate the Na⁺ concentrations in the intra‐ and extracellular spaces of the gastrocnemius muscle during well‐perfused and ischemic conditions. The concentration of DyTTHA³⁻ infused intravenously into the jugular vein of the living rat reached a maximum value of 8–9 mM in the extracellular space of the muscle after ca 40 min of infusion. This allowed excellent discrimination of extra‐ and intracellular Na signals (Na_o and Na_i, respectively) and did not spoil the resolution of concurrent ³¹P NMR spectra. Infusion of shift reagent changed neither hemodynamic performance of the rat nor the high‐energy phosphate content of skeletal muscle. Shift reagent enters ca 15% (v/w) of the rat body weight; this corresponds to almost all of the “fast” or rapidly permeable extracellular space. It is excreted from the body with a pseudo‐first order rate constant of 0.0158 min⁻¹. In resting muscle, we estimate that [Na⁺]_i is 3–5 mM and, in muscle perfused with the sodium salt of the shift reagent, that [Na⁺]_o in the fast exchangeable extracellular space is 166 mM. During 11 h of ischemia at 37°C, the area of the Na_i⁺ signal area monotonically increased sixfold. Based on estimates for maximum changes in fluid shifts reported by the decrease in the area of the Na_o signal area, we calculate that the lower limit for [Na⁺]_i after 11 h of ischemia is 27 mM. The NMR‐visibility factors for the extracellular and intracellular Na⁺ signals are essentially the same. This study demonstrates that the shift reagent DyTTHA³⁻ is acutely non‐toxic and that the ²³Na NMR spectra obtained can be used to quantitate [Na⁺]_o and [Na⁺]_i in tissues in vivo. Using this technique, we found that the transmembrane sodium gradient fell from ca 35 in well‐perfused skeletal muscle to <6 during prolonged ischemia.