The filled pore approximation: a theoretical framework for solute
-solvent coupling in narrow water channels.
Welling, Dan J., Paul A. Welling, and Larry W. Welling.
RESEARCH SERVICE, VETERANS ADMINISTRATION MEDICAL CENTER, KANSAS
CITY, MO 64128; AND DEPARTMENTS OF PATHOLOGY AND PHYSIOLOGY,
UNIVERSITY OF KANSAS MEDICAL CENTER, KANSAS CITY, KS 66103; AND
DEPARTMENT OF PHYSIOLOGY, UNIVERSITY OF MARYLAND SCHOOL OF MEDICINE,
BALTIMORE, MD 21201
APStracts 2:0350C, 1995.
A phenomenological model is presented of water and solute transport
that is applicable to water pores with radii less than approximately
2 [angstrom]a. This includes such examples as Gramicidin A, the
proximal tubule basolateral membrane, and the Aquaporin-1 (CHIP-28)
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, in spite of 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 to
its frictional interaction with neighboring waters. This is
consistent with values previously reported for sodium chloride in
cell membranes of proximal tubule. The model predicts a minimum
effective pore radius for a water channel of 1.78 [angstrom]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.
Received 21 October 1994; accepted in final form 18 September
1995.
APS Manuscript Number C632-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95