Bufo marinus bladder H-K-ATPase carries out electroneutral ion transport

Bufo marinus bladder H-K-ATPase belongs to the Na-K-ATPase and H-K-ATPase s ubfamily of oligomeric P-type ATPases and is closely related to rat and hum an nongastric H-K-ATPases. It has been demonstrated that this ATPase transp orts K+ into the cell in exchange for protons and sodium ions, but the stoi chiometry of this cation exchange is not yet known. We studied the electrog enic properties of B. marinus bladder H-K-ATPase expressed in Xenopus laevi s oocytes. In a HEPES-buffered solution, K+ activation of the H-K-ATPase in duced a slow-onset inward current that reached an amplitude of similar to 2 0 nA after 1-2 min. When measurements were performed in a solution containi ng 25 mM HCO3- at a PCO2 of 40 Torr, the negative current activated by K+ w as reduced. In noninjected oocytes, intracellular alkalization activated an inward current similar to that due to B. marinus H-K-ATPase. We conclude t hat the transport activity of the nongastric B. marinus H-K-ATPase is not i ntrinsically electrogenic but that the inward current observed in oocytes e xpressing this ion pump is secondary to intracellular alkalization induced by proton transport.