Sodium ion apparent diffusion coefficient in living rat brain

Abstract

The apparent diffusion coefficient (ADC) of Na+ was determined in live rat brain. The brain extracellular-to-intracellular Na+ content ratio is ∼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 μm2/ms, which is 61% of the aqueous Na+ free diffusion coefficient (Dfree) at 37°C (1.9 μm2/ms), while the ADC for brain water is 28% of the water Dfree at 37°C (3 μm2/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 Dfree 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/Dfree ratios in living brain tissue may be attributable to the extracellular environment hindering molecular displacements twofold less than the intracellular environment. © 2005 Wiley-Liss, Inc.