Evaluation of extra‐ and intracellular apparent diffusion coefficient of sodium in rat skeletal muscle: Effects of prolonged ischemia

The mechanism of water and sodium apparent diffusion coefficient (ADC) changes in rat skeletal muscle during global ischemia was examined by in vivo 1 H and 23 Na magnetic resonance spectroscopy (MRS). The ADCs of Na + and water are expected to have similar characteristics because sodium is present as an aqua‐cation in tissue. The shift reagent, TmDOTP 5‐ , was used to separate intra‐ and extracellular sodium (Na   i +and Na   e + , respectively) signals. Water, total tissue sodium (Na   t + ), Na   i + , and Na   e +ADCs were measured before and 1, 2, 3, and 4 hr after ischemia. Contrary to the general perception, Na   i +and Na   e +ADCs were identical before ischemia. Thus, ischemia‐induced changes in Na   t +ADC cannot be explained by a simple change in the size of relative intracellular or extracellular space. Na   t +and Na   e +ADCs decreased after 2–4 hr of ischemia, while water and Na   i +ADC remained unchanged. The correlation between Na   t +and Na   e +ADCs was observed because of high Na   e +concentration. Similarly, the correlation between water and Na   i +ADCs was observed because cells occupy 80% of the tissue space in the skeletal muscle. Ischemia also caused an increase in the Na   i +and an equal decrease in Na   e +signal intensity due to cessation of Na + /K + ‐ATPase function. Magn Reson Med 59:485–491, 2008. © 2008 Wiley‐Liss, Inc.