Remarkably, many fish have the ability to move easily between freshwater and seawater, despite the demands these two extreme environments make on their physiology. For example, in freshwater, fish constantly lose salt and gain water from the environment, requiring them to have special mechanisms to take up salts and excrete water in order to maintain salt and water balance. The complete opposite occurs in seawater, where fish continuously gain salt and lose water to the environment. Consequently, marine fish need to actively excrete excess salts and drink water to maintain hydromineral balance. Determining the mechanisms that allow the same fish to acclimate quickly to these two very different environments has been an intense area of research for decades.

To date, the hormone cortisol has been shown to play an important role during acclimation to seawater by causing changes in gill cell morphology and stimulating the enzyme Na+/K+-ATPase to help in the extrusion of salts. The hormone prolactin is important during acclimation to freshwater as it reverses the morphological modifications caused by cortisol and decreases gill permeability. Yet, these two hormones alone cannot account for all the changes that occur in fish during salinity change. In mammals,evidence has accumulated suggesting a role for ouabain, a familiar Na+/K+-ATPase inhibitor, in the regulation of hydromineral balance. S. Kajimura and colleagues decided to examine whether a ouabain-like substance is present in fish and to determine whether ouabain is involved in hydromineral balance during seawater or freshwater acclimation.

Choosing to work with tilapia, a fish that is found in both freshwater and marine environments and moves between the two, the team exposed the fish to a rapid salinity change and then took blood samples at 2, 4, 8 and 24 h after the transfer. The team measured the plasma and tissue concentrations of ouabain using a radioimmunoassay with a specific antibody against ouabain.

Interestingly, ouabain was detected in all of the tissues examined,regardless of salinity. The highest concentration was found in the head kidney, which is the area of cortisol production in fish and is analogous to the adrenal gland in mammals.

When freshwater-acclimated fish were transferred to seawater, the team saw a significant increase in plasma osmolality. Plasma ouabain and cortisol levels also rose and all three increases were significantly correlated. By contrast, seawater-acclimated tilapia that were transferred to freshwater showed a significant decrease in plasma osmolality although there was no significant correlation between plasma osmolality, ouabain or cortisol levels,possibly because there was little change in plasma cortisol and ouabain levels.

Kajimura and the team have shown for the first time measurable ouabain levels within the circulation and tissues of fish. Ouabain may be involved in the regulation of hydromineral balance in tilapia, in conjunction with cortisol, and, if so, could potentially be more important during acclimation to seawater than to freshwater. However, the physiological function of ouabain is still yet to be determined. It will be exciting to learn the implications of this possible new hormone on the physiology of fish!

Kajimura, S., Hirano, R., Moriyama, S., Vakkuri, O., Leppaluoto,J. and Grau, E. G. (
). Changes in plasma concentrations of immunoreactive ouabain in the tilapia in response to changing salinity: is ouabain a hormone in fish?
Gen. Comp. Endocrinol.