Electrical properties of sodium bicarbonate symport in kidney epithelial cells (BSC-1)

Intracellular potentials of BSC-1 kidney epithelial cells known to express a Na+-HCO3- symport ranged between -40 and -70 mV, mean value Vm = -55.1 +/- 10.1 mV. Lowering HCO3- at constant partial pressure of CO2 (PCO2) or complete removal of HCO3-/CO2, rapidly depolarized, whereas readding HCO3- hyperpolarized Vm (40 +/- 8 mV/decade HCO3- at constant PCO2). This response was largely reduced by 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (which depolarized Vm to about -20 mV) and in Na+-free medium, but Ba2+ had no effect. In HCO3(-)-Ringer, Na+ removal rapidly depolarized Vm (by 32 +/- 7 mV), and readdition hyperpolarized Vm. This was reduced in HCO3(-)-free medium or by 1 mM DIDS, but not by amiloride (10(-5) and 10(-3) M) or ouabain (10(-4) M). In the absence of HCO3- and/or Na+, steady-state Vm was reduced to -12 +/- 5 mV. Cl- removal had no effect on the responses to Na+ and/or HCO3- and led to a slow steady-state depolarization. Both in the presence and absence of HCO3-, raising pHi (changed by NH4Cl or butyrate) depolarized, whereas lowering pHi hyperpolarized Vm. Lowering pHo in HCO3(-)-free Ringer depolarized Vm (23 +/- 4 mV/decade H+). The slope conductance for K+ is only 6 +/- 2 mV/decade. Thus BSC-1 cells display typical electrical characteristics of Na+-HCO3- symport. In contrast to other systems, the data are compatible with a net electrogenic inward transport of Na+ and HCO3-. There might be an additional H+-(OH-) conductance operating also under nominally bicarbonate-free conditions.