Ionic permeabilities of the cell membranes of sheep tracheal epithelium.

1. The ionic permeabilities of the apical and basolateral membranes of sheep tracheal epithelial cells have been evaluated by studying the effect of ion replacements on the apical (Va) and basolateral (Vb) membrane potentials and apical voltage divider ratio (fa). 2. The results provide evidence for the existence of conductances for Cl-, Na+ and K+ at the apical membrane, and of an important K+ conductance at the basolateral membrane. These characteristics agree with the model for Cl- secretion and Na+ absorption proposed for tracheal epithelium of other species. 3. The short-circuit current (Isc) across this tissue was found to increase significantly when HCO3- and CO2 were added to the bathing solutions. The addition of 0.5 mM-SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid) to the serosal bathing solution decreased Isc only if HCO3- and CO2 were present in the bathing solutions. These results suggest the existence of a HCO3- secretion in sheep tracheal epithelium under short-circuit conditions. 4. The results obtained in HCO3- replacement studies are compatible with an electrically silent HCO3- entry across the basolateral membrane and an electrogenic exit at the apical membrane of the cells.