HIROI, J.*; MCCORMICK, S.D.: Effect of salinity on Na,K-ATPase and Na,K,2Cl-cotransporter in three salmonids with varying degrees of salinity tolerance

Chloride cells in the gills of teleosts are the major site of ionic regulation. We compared seawater tolerance and chloride cell morphology of three salmonids, lake trout, brook trout and smolt of Atlantic salmon. Three species were transferred from 0 ppt to 10 ppt, 20 ppt and 30 ppt at 1-week intervals to examine gill Na,K-ATPase activity and immunocytochemistry with anti-Na,K-ATPase and anti-Na,K,2Cl-cotransporter. During the transfer, little mortality was observed and plasma ion levels remained stable in brook trout and Atlantic salmon. However, 20% of lake trout died and plasma ion levels increased after transfer to 30 ppt. In lake trout and brook trout, gill Na,K-ATPase activity was low in 0 ppt and increased 2.5-fold after 3 weeks in 30 ppt. Gill Na,K-ATPase activity of Atlantic salmon was already high in 0 ppt and remained high after seawater transfer. Na,K-ATPase and Na,K,2Cl-cotransporter were co-localized to gill chloride cells on both the primary filaments and secondary lamellae in all three species in 0 ppt. After seawater transfer, both the number and staining intensity of chloride cells increased in all three species, but the distributional pattern of the cells varied among the species: chloride cells were still distributed in both filaments and lamellae in lake trout and brook trout; whereas, lamellar chloride cells of Atlantic salmon decreased in number and finally disappeared. These results indicate a direct role of Na,K-ATPase and Na,K,2Cl-cotransporter in seawater acclimation and the importance of filament chloride cells in seawater.