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

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Abstract
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 (Nao and Nai, 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 Nai+ signal area monotonically increased sixfold. Based on estimates for maximum changes in fluid shifts reported by the decrease in the area of the Nao 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.

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