Osmoregulation and salinity effects on the expression and activity of Na+,K(+)-ATPase in the gills of European sea bass, Dicentrarchus labrax (L.).

The European sea bass, Dicentrarchus labrax, tolerates salinities ranging from freshwater (FW) to hypersaline conditions. In two experiments, we analysed changes in plasma ions, muscle water content (MWC), gill Na+,K(+)-ATPase activity, and alpha-subunit mRNA expression during the course of acclimation from 15 ppt salt water to FW or high salinity seawater (HSSW). In Experiment 1, fish (6.2 +/- 1.1 g) were acclimated from 15 ppt to either FW, 5, 15, 25, 50, or 60 ppt SW and sampled after 10 days. Gill Na+,K(+)-ATPase activity was stimulated in FW- and in 50 and 60 ppt SW-groups relative to the 15 ppt control group. In Experiment 2, subgroups of fish (89 +/- 7 g) were transferred from 15 ppt SW to FW or 50 ppt SW, and sampled 1, 2, 4, and 10 days later. Plasma osmolality, [Na+] and [Cl-] decreased in the FW-group and increased in the HSSW-group one day after transfer and lasting until day 10. This was accompanied by a pronounced increase in MWC in the FW-group and an insignificant decrease in the HSSW-group. The plasma [Na+]:[Cl-]-ratio increased markedly in the FW-group and decreased slightly in the HSSW-group, suggesting acid-base balance disturbances after transfer. Gill Na+,K(+)-ATPase activity was unchanged in 15 ppt SW but doubled in FW- and HSSW-groups after transfer. In both groups, this was preceded by a 2- to 5-fold elevation of the gill alpha-subunit Na+,K(+)-ATPase mRNA level. Thus increased expression of alpha-subunit mRNA is part of the molecular mechanism of both FW and SW acclimation in sea bass. Gill Na+,K(+)-ATPase Na(+)-, K(+)-, and ouabain-affinity were similar in fish acclimated to FW, 15 ppt, and HSSW, suggesting that identical isoforms of the catalytic subunit of the enzyme are expressed irrespective of salinity.