AJP - Cell Physiology, Vol 270, Issue 4 C1246-C1254, Copyright © 1996 by American Physiological Society
Filled pore approximation: a theoretical framework for solute-solvent coupling in narrow water channels
D. J. Welling, P. A. Welling and L. W. Welling
Research Service, Veterans Affairs Medical Center, Kansas City, Missouri 64128, USA.
A phenomenological model is presented of water and solute transport that is
applicable to water pores with radii less than approximately 2 A. This
includes such examples as gramicidin A, the proximal tubule basolateral
membrane, and the aquaporin 1 (CHIP28) water channel. The model differs
from the conventional single-file model by allowing for a variation of
unoccupied volume within the pores. It is shown that the accessible or free
portion of the unoccupied volume can be related to the mechanical
frictional coefficients and thereby to the filtration and diffusive
permeabilities by the filled pore approximation. In general, the smallness
of the unoccupied volume represents the compactness of the molecules within
the pore and is indicative of the single-file character of the motion of
water and solute moving together. When that volume is equal to a single
water volume, the results are identical to the conventional single-file
model. An important result is that, despite very low diffusive
permeabilities, the reflection coefficient of a solute can remain at
approximately 0.5 if its frictional interaction with the channel walls is
comparable with its frictional interaction with neighboring water
molecules. This is consistent with values previously reported for NaCl in
cell membranes of proximal tubule. The model predicts a minimum effective
pore radius for a water channel of 1.78 A and corresponds to a maximum
filtration-to-diffusion permeability ratio that is proportional to the
length of the effective pore or channel. This limiting condition
corresponds to a water channel completely filled by water and may be
applicable to the aquaporin 1 water channel.
