Seawater-acclimation abates cold effects on Na+, K+-ATPase activity in gills of the juvenile milkfish, Chanos chanos

The milkfish (Chanos chanos) is an aquaculture species that can be cultivated in ponds with different salinities. Low temperatures seriously affect the homeostasis of the milkfish because it is an ectothermic species. In the present study, seawater (SW)- and freshwater (FW)-acclimated milkfish were raised at the normal temperature (28 °C) and the non-lethal low temperature (18 °C), respectively, for one week to illustrate salinity effects on cold tolerance as well as cold effects on branchial ionoregulatory mechanisms of the milkfish. The SW-acclimated milkfish exhibited better cold tolerance than the FW individuals. The low temperature (18 °C) led to differences in the ionoregulatory responses in gills between the SW and FW milkfish, and these responses were elucidated. The Na+ concentration decreased in the SW milkfish when exposed to 18 °C, while the Cl− concentration increased in the FW fish at 18 °C. In addition, the cold effects on gill Na+; K+-ATPase (NKA) activities were greater in the FW/18 °C group than in the SW/18 °C group. In contrast to the constant NKA expression, the relative protein amounts of gill heat shock protein 70 (HSP70) increased in both the SW and FW milkfish under cold stress. Compared with hyper-salinity, hypo-salinity was more stressful for euryhaline milkfish at low temperature. In summary, this study demonstrates that under cold stress gill NKA activity plays critical roles in maintenance of homeostasis of the milkfish.