Figure 2.

Five different modes for effective HCO₃⁻ extrusion over the basolateral membrane of the proximal tubule [adapted, with permission, from (39)]. The authors ruled out the first four modes because the transport was known to be Na⁺ dependent and electrogenic. Although this leaves option 5 (e) as the correct transport mode, it is still not decided which of the four different transport modes of (e), that is, which ion species, are actually transported by the transporter. This is not only the case for the Na⁺-dependent transport of HCO₃⁻ or a related species over the basolateral membrane of the proximal tubule, but indeed for all Na⁺-dependent transporters discussed in this article.