Bioenergetics of adaptation to a salinity transition in euryhaline teleost (Oreochromis mossambicus) brain.

Freshwater (FW) teleosts are capable of acclimating to seawater (SW) following such a transfer from FW. However, their osmoregulating mechanisms are still unclear, particularly those in the brain. The present study was conducted to examine acute changes that occur in brain Na(+)-K(+)-ATPase activity, creatine kinase (CK) activity, creatine, creatinine contents, and ATP levels of tilapia (Oreochromis mossambicus) in response to this transition. After transfer to SW (25 ppt), the Na(+)-K(+)-ATPase activity was maintained for 8 hr at higher levels than that in FW. In contrast, in 35 ppt SW, Na(+)-K(+)-ATPase was maintained at a even higher level than in FW for the first 2 hr. Brain Na(+)-K(+)-ATPase contents in both the 25 and 35 ppt SW groups were significantly elevated within 1 and 0.5 hr after transfer from FW, respectively. Interestingly, brain CK activities and content (homodimer of the B subunit [BB] form) in both the 25 and 35 ppt SW groups were significantly elevated within 1 hr after transfer from FW. The ATP contents in 35 ppt SW increased abruptly within 0.5 hr, and then gradually decreased during the next 2 hr. Unlike the 35 ppt group that declined in ATP contents, the 25 ppt group leveled off within 24 hr. The elevations in CK activity and creatine levels after transfer from FW to SW imply that abrupt salinity changes alter phosphocreatine/CK ratio. Such changes are needed to satisfy the increases in the energetic requirement of the cotransport mechanisms mediating osmoregulation.