H+-ATPase ACTIVITY IN CRUDE HOMOGENATES OF FISH GILL TISSUE: INHIBITOR SENSITIVITY AND ENVIRONMENTAL AND HORMONAL REGULATION

N-ethymaleimide-sensitive ATPase activity was measured in crude homogenates of gill tissue from rainbow trout using a coupled-enzyme ATPase assay in the presence of EGTA, ouabain and azide. This NEM-sensitive ATPase activity, determined to be about 1.5 mmolmg21 protein h21 at 15 ̊C for freshwater trout, is also inhibited by other H+ATPase blockers such as DCCD, DES, PCMBS and bafilomycin. It is concluded, therefore, that the NEM-sensitive ATPase activity was generated by a protontranslocating ATPase. Since this NEM-sensitive ATPase was also sensitive to the plasma membrane ATPase inhibitor vanadate, we conclude that the H+-ATPase in fish gill is of the plasma membrane type. The major role of the H+-ATPase in the gill epithelium is to facilitate Na+ uptake from fresh water. Sodium concentration in the external medium was the primary regulator of the H+-ATPase in fish gills, with low sodium levels being associated with high H+-ATPase activity. High external calcium concentration had a marked stimulatory effect on H+-ATPase activity in fish gills when the sodium level was low. Environmental hypercapnia induced a 70% increase in the H+-ATPase activity in fish gills. H+-ATPase activity was also elevated in freshwater fish after chronic cortisol infusion.