Voltage and cosubstrate dependence of the Na-HCO3 cotransporter kinetics in renal proximal tubule cells

The voltage dependence of the kinetics of the sodium bicarbonate cotransporter was studied in proximal tubule cells. This electrogenic cotransporter transports one Na+, three HCO3-, and two negative charges. Cells were grown to confluence on a permeable support, mounted on a Ussing-type chamber, and permeabilized apically to small monovalent ions with amphotericin B. The steady-state, di-nitro-stilbene-di-sulfonate-sensitive current was shown to be sodium and bicarbonate dependent and therefore was taken as flux through the cotransporter, Voltage-current relations were measured as a function of Na+ and HCO3- concentrations between -160 and +160 mV under zero-trans and symmetrical conditions, The kinetics could be described by a Michaelis-Menten behavior with a Hill coefficient of 3 for HCO3- and 1 for Na+. The data were fitted to six-state ordered binding models without restrictions with respect to the rate-limiting step. All ordered models could quantitatively account for the observed current-voltage relationships and the transinhibition by high bicarbonate concentration. The models indicate that 1) the unloaded transporter carries a positive charge; 2) the binding of cytosolic bicarbonate to the transporter "senses" 12% of the electric field in the membrane, whereas its translocation across the membrane "senses" 88% of the field; 3) the binding of Na+ to the cotransporter is voltage independent.