Different sodium chloride cotransport systems in the apical membrane of rabbit gallbladder epithelial cells

Summary

The kinetics of Cl⁻ influx from the lumen to the cell and the paracellular pathway was examined in isolated rabbit gallbladder by measuring³⁶Cl uptake (45 s) and by correcting it for the extracellular space with³H-sucrose. The paracellular fraction of the influx was studied by incubating the tissue in Na⁺-free saline or in solutions containing 25mm SCN⁻; the kinetics turned out to be hyperbolic. The cellular fraction of the influx comprised three components. The first was immediately Na⁺-dependent and insensitive both to exogenous and endogenous cell bicarbonate; its sigmoidal kinetics revealed the presence of a carrier with three Cl⁻ binding sites cooperating positively with one another, with strong interaction factors. The second cellular component was immediately Na⁺-dependent and sensitive to endogenous cell bicarbonate; the kinetics was hyperbolic with a maximum at 20mm Cl⁻ concentration and a substrate inhibition from 20 to 130mm; it was completely inhibited by 10⁻⁴ m acetazolamide. The third cellular component was slowly Na⁺-dependent and slowly sensitive to exogenous bicarbonate; its kinetics was hyperbolic, without substrate inhibition in the tested Cl⁻ concentration range. On this basis, the presence of three Na⁺−Cl⁻ cotransports is suggested: i) on a single carrier without any exchange with H⁺ and HCO ⁻₃ , ii) on a single carrier with an exchange with H⁺ and HCO ⁻₃ , and iii) on two separate carriers in exchange with H⁺ and HCO ⁻₃ .