The electrogenic sodium bicarbonate cotransporter (NBCe1) is present in many epithelial and glial cells where it plays a crucial role in intra- and extracellular H+ buffering. In the present study we heterologously expressed the human NBCe1 in Xenopus oocytes either injected or co-expressed with carbonic anhydrase isoform II (CAII). To evaluate the activity of NBCe1 and CAII we measured the intracellular H+ and Na+ concentration using ion-selective microelectrodes and the membrane current in voltage-clamp. Injection of CAII increased the rate of rise of the intracellular H+ concentration during application of CO2/HCO3-, and altered the membrane current and the rate of rise of the intracellular Na+ concentration mediated by NBC activity. In CO2/HCO3--buffered saline CAII added an additional membrane conductance of 2.6 mS to NBC expressing oocytes. Application of 6-ethoxy-2-benzothiazolesulfonamide not only blocked the CAII- mediated increase of the rate of rise in H+ concentration, but also suppressed the increase in the membrane current and the rate of rise of the intracellular Na+ concentration. Co-expressing CAII together with the NBCe1 confirmed the results obtained with direct CAII injection. Our results indicate that the catalytic activity of CAII enhances the transport capacity of the NBCe1.

Supported by the DFG (De 231/16-4 and GRK 845/1)