PROTON PUMPS IN THE FISH GILL AND KIDNEY

The proton pump or vacuolar type H+-ATPase is an oligomeric protein re sponsible for electrogenic H+ secretion in a variety of acid-secreting epithelia. Recently, the proton pump was identified in both the gill and kidney of freshwater-adapted rainbow trout (Oncorhynchus mykiss). Using immunocytochemistry, H+-ATPase has been localized in the pavemen t cells and chloride cells of the lamellar epithelium. During periods of internal acidosis, there is a marked increase in the expression of the branchial proton pump as identified by Western analysis, immunocyt ochemistry and in situ hybridization. This augmented expression of pro ton pumps occurs concomitantly with a marked increase in branchial aci d excretion and Na+ uptake. Immunocytochemical studies suggest that th e pavement cell, rather than the chloride cell, is the predominant sit e of acid excretion during periods of acidosis. These findings are con sistent with the notion that in freshwater teleosts, Na+ uptake and H excretion are linked via the coupling of the electrogenic proton pump to apical membrane Naf channels. This mechanism may be controlled by hormones including cortisol and/or growth hormone. The fish kidney pla ys an important role in regulating acidosis via the reabsorption of fi ltered HCO3-. Recently, we have demonstrated using Western analysis an d immunocytochemistry, the presence of proton pump in rainbow trout ki dney and observed increased H+-ATPase expression during respiratory ac idosis. These new findings suggest a role for the renal proton pump in acid-base regulation.