Hydrophysiology NMR reveals mechanisms of steady-state water exchange in neural tissue. Williamson & Ravin et al.

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.