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First published online November 10, 2003
Inside JEB |
TROUT PRIMES PUMP FOR SALINE SWITCH
kathryn{at}biologists.com
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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; 
, ß and 
, 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 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 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 1a 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 1b isoform began rising.
The trout had switched from the freshwater 1a isoform, to the
saltspecific 1b isoform.
And when the team constructed the component's phylogentic tree, the
1a and 1b 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
isoforms, 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.
References
Richards, J. G., Semple, J. W., Bystriansky J. S. and Schulte,
P. M. (2003). Na+/K+-ATPase
-isoform switching in gills of rainbow trout (Oncorhynchus
mykiss) during salinity transfer. J. Exp. Biol.
206,4475
-4486.
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