Journal of Applied Physiology, Vol 52, Issue 5 1223-1229, Copyright © 1982 by American Physiological Society
Diffusion barriers to passive lipid uptake of canine posterior tracheal epithelium
S. F. Man and A. B. Thomson
In studies of passive uptake of canine posterior tracheal epithelium in
vitro, we examined the effective resistance of the unstirred water layer
(UWL) when the bulk phase of the preparation was stirred and unstirred.
Rates of uptake of fatty acids were corrected for unstirred layer effects.
The incremental change in free energy, derived from the linear relationship
between uptake and chain length of lipids, was -271 cal/mol. Measurement of
the effective thickness (d) and surface area (SW) of the UWL showed that d
fell and SW rose as the rate of stirring increased. SW was less than 15% of
the tracheal epithelial cell membrane's surface area. It is concluded that
1) lipid membrane of the tracheal epithelium and the overlying UWL
represent the major diffusion barriers for lipid uptake across the tracheal
epithelium; 2) failure to correct for the UWL resistance leads to serious
errors in the estimates of permeability properties of the epithelium to
lipids; and 3) the unstirred layer severely limits the proportion of cell
membrane available for transport in vitro.
