Sodium ion apparent diffusion coefficient in living rat brain

The apparent diffusion coefficient (ADC) of Na+ was determined in live rat brain. The brain extracellular-to-intracellular Na+ content ratio is similar to 8:2, which is the inverse of that for water in these spaces. Consequently, the ADC of Na+ is primarily affected by motion in the extracellular space, and Na+ can be viewed as a reporter molecule for motion in that space. Likewise, water ADC is dominated by intracellular motion. The brain Na+ ADC was 1.15 +/- 0.09 mu m(2)/ms, which is 61% of the aqueous Na+ free diffusion coefficient (D-free) at 37 degrees C (1.9 mu m(2)/ms), while the ADC for brain water is 28% of the water D-free at 37 degrees C (3 mu m(2)/ms). This suggests that the ADC of molecular species within the extracellular space is roughly twofold that within the intracellular space. In postmortem brain, both Na+ and water decrease to 17% of the respective D-free values. These results are consistent with Na+ and water ADC values sharing the same biophysical determinants in postmortem brain. The observed difference between Na+ and water ADC/D-free ratios in living brain tissue may be attributable to the extracellular environment hindering molecular displacements twofold less than the intracellular environment. (c) 2005 Wiley-Liss, Inc.