Extra-gastric expression of the proton pump H+/K+ -ATPase in the gills and kidney of the teleost Oreochromis niloticus.

Potassium regulation is essential for the proper functioning of excitable tissues in vertebrates. The H+/ K+-ATPase (HKA), which is comprised of the HKα1 (gene: atp4a) and HKβ (gene: atp4b) subunits, has an established role in potassium and acid-base regulation in mammals and is well known for its role in gastric acidification. However, the role of the HKA in extra-gastric organs such as the gill and kidney is less clear, especially in fishes. In the present study in Nile tilapia Oreochromis niloticus uptake of the K+ surrogate flux marker rubidium (Rb+) has been demonstrated in vivo; however, this uptake was not inhibited with omeprazole, a potent inhibitor of the gastric HKA. This contrasts with gill and kidney ex vivo preparations where tissue Rb+ uptake was significantly inhibited by omeprazole and SCH28080, another gastric HKA inhibitor. The cellular localization of this pump in both gills and kidney was demonstrated using immunohistochemical techniques using custom made antibodies specific for Atp4a and Atp4b. Antibodies against both subunits showed the same apical ionocyte distribution pattern in gill and collecting tubules/ducts in kidney. Atp4a antibody specificity was confirmed by Western blotting. RT-PCT was used to confirm the expression of both subunits in gill and kidney. Taken together these results indicate for the first time K+ (Rb+) uptake in O. niloticus and that the HKA can be implicated by the ex vivo uptake inhibition by omeprazole and SCH28080, verifying a role for HKA in K+ absorption in the gill9s ionocytes and collecting tubule/duct segments of the kidney.