Hydrophysiology NMR reveals mechanisms of steady-state water exchange in neural tissue. Williamson & Ravin et al.
Description
Water molecules exchange incessantly across cell membranes and among different environments within the cell, but what the dominant transport pathways are and whether they are active or passive is not known. While most studies report exchange to be passively driven by random thermal motion, some studies report that water actively cycles with ions across membranes. However, active ion transport and cell volume are intertwined, and it is not clear which is affecting exchange. Here we perform experiments controlling for osmotic pressures (i.e., tonicity), to perturb active transport independent of cell volume. We utilize realtime NMR hydrophysiology methods to study steady-state water exchange and diffusion in viable ex vivo neonatal mouse spinal cord samples. We find that water exchange is primar- ily passive but is linked to tonicity maintained by active trans- port. Exchange slows following sodium–potassium pump inhi- bition but recovers to a normal rate after adding extracellular osmolytes. Though exchange alone cannot distinguish normal from inactive samples, combining it with apparent water dif- fusion coefficients (ADC) does. Data and modeling suggest a multisite exchange mechanism in which tonicity modulates the dominant apparent exchange pathway between fast transmem- brane exchange and slow intracellular exchange. The trans- membrane pathway has a high activation energy but does not require ions, suggesting that in this pathway exchange likely oc- curs through the lipid bilayer rather than through channels or cotransporters. These methods may one day be translated to clinical MRI scanners to determine features of the cellular state in vivo.
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