Vacuolar-type H+-ATPase and Na-, K--ATPase expression in gills of Atlantic salmon (Salmo salar) during isolated and combined exposure to hyperoxia and hypercapnia in fresh water.

Changes in branchial vacuolar-type H+-ATPase B-subunit mRNA and Na+,K+-ATPase alpha- and beta-subunit mRNA and ATP hydrolytic activity were examined in smolting Atlantic salmon exposed to hyperoxic and/or hypercapnic fresh water. Pre-smolts, smolts, and post-smolts were exposed for 1 to 4 days to hyperoxia (100% O-2) and/or hypercapnia (2% CO2). Exposure to hypercapnic water for 4 days consistently decreased gill vacuolar-type H(+)ATPase B-subunit mRNA levels. Salmon exposed to hyperoxia had either decreased or unchanged levels of gill B-subunit mRNA. Combined hyperoxia+hypercapnia decreased B-subunit mRNA levels, although not to the same degree as hypercapnic treatment alone. Hyperoxia generally increased Na+,K+-ATPase alpha- and beta-subunit mRNA levels, whereas hypercapnia reduced mRNA levels in presmolts (beta) and smolts (alpha and beta). Despite these changes in mRNA levels, whole tissue Na+,K+-ATPase activity was generally unaffected by the experimental treatments. We suggest that the reduced expression of branchial vacuolar-type H+-ATPase B-subunit mRNA observed during internal hypercapnic acidosis may lead to reduction of functional V-type H+-ATPase abundance as a compensatory response in order to minimise intracellular HCO3- formation in epithelial cells.