No matter which way you look at it, fish are leaky; either soaking up water and losing ions in freshwater environments, or desiccating in seawater. Yet most fish that have adapted to survive in one of these environments have overcome the challenges of their watery worlds. Freshwater species scavenge ions to replace the salts they lose, while seawater species replenish the lost fluid by drinking and excreting the salt. In both cases, the salts are pumped across the fish's gills, taking them up when in freshwater and excreting in saltwater. But how about the species that choose to move between the two; how do they deal with transition' Patricia Schulte wonders? They must be able to reverse the gill's ion pumps as they move from one environment to another. Knowing that fish which are capable of moving between fresh and salt water change the levels of the sodium/potassium ATPase ion pump in some tissues,Schulte decided to take a closer look at the ion pump as rainbow trout transferred from fresh to salt water.

Na+/K+ ATPases are not simple proteins. Each active pump is built up from three components; α, β and γ, and mammals have several different isoforms of each component. It's a mystery why there are so many isoforms' explains Schulte. She also knew that the ATPase's activity depends on the relative proportions of each component's isoform in the pump. Could fish have multiple isoforms too? Working with Jeffrey Semple,and Jason Bystriansky, Schulte decided to investigate whether the rainbow trout produced several different isoforms of the ATPase's α subunit. Working with fish acclimated to freshwater, Semple found five, expressed at different levels throughout the fishes' bodies, with four produced in the gill.

But how did the fish react to a sudden change in salinity? Would they switch between isoforms to regulate the pump's activity? Jeff Richards began testing the levels of each α isoform as he abruptly moved them from freshwater to 40%, or 80% sea water. Over a period of days, Richards collected the fishes' gills as they adjusted to the salty conditions, looking at the levels of each isoform in the gill. The results were clearcut. In freshwater,the levels of α1a in the gill were high, but as soon as Richards transferred them to dilute seawater, the protein's level dropped. After a few days, Richards noticed that the levels of the α1b isoform began rising. The trout had switched from the freshwater α1a isoform, to the saltspecific α1b isoform.

And when the team constructed the component's phylogentic tree, theα1a and α1b isoforms were right next to each other. This suggests a very recent duplication' explains Schulte. So the fish's ability to migrate from rivers to the ocean and back again, courtesy of the specialised αisoforms, is probably a specific salmonid adaptation' she adds.

Having identified two candidate ATPase isoforms that could account for the trout's amazing ability to migrate between rivers and oceans, the team are keen to know how the gill deploys the pump switching isoforms. Does the gill tissue produce different cell types as they move location? Or do they regulate the expression levels of each isoform in the gill as they migrate between the two? But it could take a few more migration seasons before Richards can answer those questions and solve the mystery of the trout mobility.

Richards, J. G., Semple, J. W., Bystriansky J. S. and Schulte,P. M. (
2003
). Na+/K+-ATPaseα-isoform switching in gills of rainbow trout (Oncorhynchus mykiss) during salinity transfer.
J. Exp. Biol.
206
,
4475
-4486.