Diffusion barriers to passive lipid uptake of canine posterior tracheal epithelium

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.