Magnetic resonance imaging and spectroscopy of the Xenopus oocyte: Insights into intracellular apparent diffusion

Diffusion-weighted MRI, in which contrast is based on the water apparent diffusion coefficient (ADC), is widely used for the early detection of stroke. Water ADC decreases within minutes of the insult; however, the mechanism of the decrease is unknown. The prevailing hypothesis is based on the assumption that intracellular water has a smaller ADC than extracellular, and injury is associated with cell swelling, which leads to a net shift of water from the extracellular to intracellular space. A competing hypothesis suggests that an ADC decrease of specifically intracellular water drives the decrease in total water ADC. Elucidation of the correct mechanism has proven difficult because intra- and extracellular water MR signals are difficult to resolve in brain. The Xenopus oocyte provides a promising model system to study this phenomenon because its intracellular water ADC can be measured directly using methods unsuitable for investigations in brain.; First, the MR properties of the normal oocyte were extensively characterized. Water MR parameters (T₁, T₂, and ADC) showed great spatial heterogeneity, which was related to the lipid distribution of the cell. The total intracellular water ADC value, 0.9 μm2/ms is close to that for total water in mammalian brain. Water ADC values showed no dependence on diffusion time, but some dependence on direction. Both quickly and slowly diffusing water populations were identified for oocyte intracellular water; the ratio of the two ADCs and the signal fractions for these two populations were quite similar to those documented for total mammalian brain water. This result suggests that assignment of these populations in brain to extra- and intracellular water, as has been done by others, may be incorrect. A model for the dependence of diffusional membrane permeability on the now well-characterized intracellular diffusion was developed.; The effects of events related to cell injury on water ADC were examined. Water ADC was proportional to cell volume when cells were shrunken or swollen in hyperosmolar or hypoosmolar media, respectively. After treatment with antimycin A, an inhibitor of ATP production, oocytes showed increased volume; however, intracellular water ADC was decreased 30% in treated cells when compared to volume-change-matched controls. Water ADC was not affected by polymerization or depolymerization of tubulin stores. Further, water ADC did not decrease with breakdown of the cell nucleus in otherwise healthy cells. The measured ADC values of small molecules and ions within the intracellular space were dependent on species size, indicating that intracellular water motion in the oocyte is mainly Brownian displacement with little or no role for any ATP-driven motion that could be disrupted with injury. Taken together, these results suggest that the brain water ADC decrease with stroke may be due to a decrease in the ADC of intracellular water. This decrease may be unrelated to, and in fact in spite of, the cell-swelling event. Additional studies in this system may shed light on the mechanism(s) for the brain water ADC decrease in stroke.