A chloride conductance exhibiting bicarbonate conductivity in renal inner medullary collecting duct cells

The anion conductance in primary cultures of rat inner medullary collecting duct cells was studied using perforated-patch whole-cell clamp technique. Depolarizations above 0 mV induced an outward anionic current with a time-dependent activation (I-ovt) exhibiting a similar conductivity to Cl- and HCO3-. I-ovt showed half-maximal activation around 32 mV with a slope factor of 23 mV, and showed a voltage-dependent activation time course that was well fitted by a sum of two exponential functions. I-ovt was potentiated when external pH values or external Ca2+ concentration was increased and was blocked by external DIDS, DPC and furosemide. These characteristics of I-ovt resemble that of the ClC-K1 channels-mediated currents; however, anion substitution studies showed that I-ovt exhibits a Br- > Cl- > I- > NO3- conductivity sequence, different from that observed in the ClC-K1 channels-mediated conductance. We suggest that, in inner medullary collecting duct cells, ClC-K channels of an unidentified type give rise to this Cl- and HCO3- conductance. This is the first study of a channel-mediated HCO3- current in kidney tubular cells.