Acid-base regulation in fishes: Cellular and molecular mechanisms

The mechanisms underlying acid-base transfers across the branchial epithelium of fishes have been studied for more than 70 years. These animals are able to compensate for changes to internal pH following a wide range of acid-base challenges, and the gill epithelium is the primary site of acid-base transfers to the water. This paper reviews recent molecular, immunohistochemical, and functional studies that have begun to define the protein transporters involved in the acid-base relevant ion transfers. Both Na+/H+ exchange (NHE) and vacuolar-type H+-ATPase transport H+ from the fish to the environment. While NHEs have been thought to carry out this function mainly in seawater-adapted animals, these proteins have now been localized to mitochondrial-rich cells in the gill epithelium of both fresh and saltwater-adapted fishes. NHEs have been found in the gill epithelium of elasmobranchs, teleosts, and an agnathan. In several species, apical isoforms (NHE2 and NHE3) appear to be up-regulated following acidosis. In freshwater teleosts, H+-ATPase drives H+ excretion and is indirectly coupled to Na+ uptake (via Na+ channels). It has been localized to respiratory pavement cells and chloride cells of the gill epithelium. In the marine elasmobranch, both branchial NHE and H+-ATPase have been identified, suggesting that a combination of these mechanisms may be utilized by marine elasmobranchs for acid-base regulation. An apically located Cl-/HCO(3) over bar anion exchanger in chloride cells may be responsible for base excretion in fresh and seawater-adapted fishes. While only a few species have been examined to date, new molecular approaches applied to a wider range of fishes will continue to improve our understanding of the roles of the various gill membrane transport processes in acid-base balance. (C) 2002 Wiley-Liss, Inc.