Cl- transport in apical plasma membrane vesicles isolated from bovine tracheal epithelium

The Cl- transport properties of the luminal border of bovine tracheal epithelium have been investigated using a highly purified preparation of apical plasma membrane vesicles. Transport of Cl- into an intravesicular space was demonstrated by (1) a linear inverse correlation between Cl- uptake and medium osmolarity and (2) complete release of accumulated Cl- by treatment with detergent. The rate of Cl- uptake was highly temperature-sensitive and was enhanced by exchange diffusion, providing evidence for a carrier-mediated transport mechanism. Transport of Cl- was not affected by the 'loop' diuretic bumetanide or by the stilbene-derivative anion-exchange inhibitors SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid) and DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid). In the presence of the impermeant cation, tetramethylammonium (TMA+), uptake of Cl- was minimal; transport was stimulated equally by the substitution of either K+ or Na+ for TMA+. Valinomycin in the presence of K+ enhanced further Cl- uptake, while amiloride reduced Na+-stimulated Cl- uptake towards the minimal level observed with TMA+. These results suggest the following conclusions: (1) the tracheal vesicle membrane has a finite permeability to both Na+ and K+; (2) the membrane permeability to the medium counterion determines the rate of Cl- uptake; (3) Cl- transport is not specifically coupled with either Na+ or K+; and, finally (4) Cl- crosses the tracheal luminal membrane via an electrogenic transport mechanism.