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 approximately 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 microm(2)/ms, which is 61% of the aqueous Na(+) free diffusion coefficient (D(free)) at 37 degrees C (1.9 microm(2)/ms), while the ADC for brain water is 28% of the water D(free) at 37 degrees C (3 microm(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.