Goodman James A, Kroenke Christopher D, Bretthorst G Larry, Ackerman Joseph J H, Neil Jeffrey J
Department of Chemistry, Washington University, St. Louis, MO 63110, USA.
Magn Reson Med. 2005 May;53(5):1040-5. doi: 10.1002/mrm.20444.
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.
在活体大鼠脑中测定了Na⁺的表观扩散系数（ADC）。脑内细胞外与细胞内的Na⁺含量比约为8:2，这与这些空间中水的比例相反。因此，Na⁺的ADC主要受细胞外空间中运动的影响，并且Na⁺可被视为该空间中运动的报告分子。同样，水的ADC主要受细胞内运动的支配。脑Na⁺的ADC为1.15±0.09μm²/ms，这是37℃时水相中Na⁺自由扩散系数（D(free)）的61%（1.9μm²/ms），而脑水的ADC是37℃时水的D(free)的28%（3μm²/ms）。这表明细胞外空间中分子的ADC大约是细胞内空间中分子的两倍。在死后的脑中，Na⁺和水的ADC均降至各自D(free)值的17%。这些结果与死后脑中Na⁺和水的ADC值具有相同的生物物理决定因素一致。在活脑组织中观察到的Na⁺和水的ADC/D(free)比值差异可能归因于细胞外环境对分子位移的阻碍比细胞内环境小两倍。
