An electrogenic Na(+)-HCO(-)(3) cotransporter (NBC) with a novel COOH-terminus, cloned from rat brain

We screened rat brain cDNA libraries and used 5'rapid amplification of cDNA ends to clone two electrogenicNa⁺-HCO₃ cotransporter(NBC) isoforms from rat brain (rb1NBC and rb2NBC). At the amino acidlevel, one clone (rb1NBC) is 96% identical to human pancreas NBC. Theother clone (rb2NBC) is identical to rb1NBC except for 61 uniqueCOOH-terminal amino acids, the result of a 97-bp deletion near the3' end of the open-reading frame. Using RT-PCR, we confirmed thatmRNA from rat brain contains this 97-bp deletion. Furthermore, wegenerated rabbit polyclonal antibodies that distinguish between theunique COOH-termini of rb1NBC (rb1NBC) and rb2NBC (rb2NBC).rb1NBC labels an ~130-kDa protein predominantly from kidney, andrb2NBC labels an ~130-kDa protein predominantly from brain.rb2NBC labels a protein that is more highly expressed in corticalneurons than astrocytes cultured from rat brain; rb1NBC exhibits theopposite pattern. In expression studies, applying 1.5%CO₂/10 mM HCO₃ toXenopus oocytes injected with rb2NBC cRNA causes 1)pH_i to recover from the initial CO₂-inducedacidification and 2) the cell to hyperpolarize. Subsequently,removing external Na⁺ reverses the pH_i increaseand elicits a rapid depolarization. In the presence of 450 µM DIDS,removing external Na⁺ has no effect on pH_i andelicits a small hyperpolarization. The rate of the pH_idecrease elicited by removing Na⁺ is insensitive toremoving external Cl. Thus rb2NBC is aDIDS-sensitive, electrogenic NBC that is predominantly expressed inbrain of at least rat.