Nonelectrolyte permeability of canine tracheal epithelium

We examined the nonelectrolyte permeability characteristics of canine tracheal epithelium in vitro and confirmed that the transepithelial potential difference was 26.3 +/- 2.2 mV, lumen negative. Exposure of the epithelium to a sucrose osmotic load resulted in a streaming potential (SP); a linear relationship was noted between osmotic load and SP. The presence of an osmotic load did not change the short-circuit current and the SP disappeared after removal of the osmotic load. The SP developed with urea, thiourea, D-xylose, and l-glycine were similar to the SP developed for equimolar concentrations of sucrose. The urea and insulin spaces were similar magnitude. When the bulk phase was stirred at 600 rpm, the effective thickness of the unstirred water layer (UWL) external to the epithelial surface was 144 +/- 12 micron. These results support the suggestion that the Staverman reflection coefficients (sigma) of these probe molecules are similar, the estimates of sigma are valid despite the presence of an UWL, and the tracheobronchial epithelium has a pore size smaller than the hydrodynamic radius of urea.