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

The kinetics of Cl- influx from the lumen to the cell and the paracellular pathway was examined in isolated rabbit gallbladder by measuring 36Cl uptake (45 s) and by correcting it for the extracellular space with 3H-sucrose. The paracellular fraction of the influx was studied by incubating the tissue in Na+-free saline or in solutions containing 25 mM 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 20 mM Cl- concentration and a substrate inhibition from 20 to 130 mM; 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 HCO3-, ii) on a single carrier with an exchange with H+ and HCO-, and, and iii) on two separate carriers in exchange with H+-and HCO3-.