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 ∼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 (D^free) at 37°C (1.9 μm²/ms), while the ADC for brain water is 28% of the water D^free 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 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.