Anyone who's spent too long in the bath knows that too much time in water isn't good. But fish spend their entire lives immersed in fluid, and for fish that migrate from freshwater to saltwater, the problem is even more complex. They have to maintain a stable body salt concentration (∼320 mmol l–1), regardless of the external salt concentration. Steffen Madsen from the University of Southern Denmark explains that salmon, which spend part of their lives in rivers, have to continually reabsorb lost ions while in fresh water. But as soon as they relocate to the sea, they have to start pumping ions out of their bodies as they seep in. According to Madsen,fish excrete or absorb salts through specialised cells in their gills, and one of the key proteins involved in ion transport is the Na+,K+-ATPase. Madsen explains that Na+,K+-ATPases power the majority of ion movement by consuming ATP to pump sodium out of ion-transporting cells to establish a sodium gradient that ultimately powers other ion transporters. Wondering how the gill reverses its pumps as a fish moves from fresh to salt water, Madsen began monitoring the expression levels of key Na+,K+-ATPase components to find out how the pump responds to freshwater and saltwater conditions(p. 78).
Transferring young salmon from freshwater to seawater, Pia Kiilerich took samples of the fish's gills until the animals had adjusted to the new conditions 7 days later. Monitoring the Na+,K+-ATPase activity, Madsen could see that the enzyme's activity increased significantly as the fish became acclimated to the salty conditions. The fish needed to pump more ions in the salty conditions.
Next, Madsen began investigating changes in the enzyme's composition in response to the environmental change. According to Madsen, the intact Na+,K+-ATPase protein is composed of two subunits(α and β) and the α subunit can be expressed in different forms (isoforms) in the gill. Knowing that the gill seems to switch expression of the α isoforms in response to salinity changes, Madsen decided to quantify the amount of each isoform's mRNA in the fish's gill. Measuring the mRNA levels, Madsen and Christian Tipsmark realised that two of the αsubunit isoforms dominated the transcription pattern and that the fish seemed to switch from α1a transcription in freshwater toα 1b in seawater.
But where did these changes happen in the fish's gill? Tracking the location of α subunit expression in the fish gills in freshwater, Madsen found high levels of α1a mRNA in the lamellae and filament. However, when the fish adjusted to their new saltwater home, theα 1a transcript retreated to deep within the filament while the previously restricted α1b spread throughout the filament.
Madsen is very excited that the gill switches between the α subunits in response to the environmental change. He explains that the osmotic gradient between the fish's tissues and its surroundings is 20 times greater in freshwater than in saltwater. Suspecting that the α1a subunit consumes significantly more ATP per pumped sodium ion than theα 1b subunit, Madsen suggests that this allowsα 1a-rich gill cells to maintain a greater sodium gradient than cells packed with the α1b subunit. The steeper sodium gradient could then power ion transport into the fish's blood from dilute freshwater, while the shallower sodium gradient generated by theα 1b subunit could be sufficient to rid salmon of ions that seeped into their blood from seawater.