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First published online December 16, 2008
Journal of Experimental Biology 212, ii (2009)
Copyright © 2009 The Company of Biologists Limited
doi: 10.1242/jeb.027631
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Inside JEB |
GILLS SWITCH ATPASE DOMAIN TO SWITCH PUMP DIRECTION
kathryn{at}biologists.com
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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
(
and β) and the subunit can be expressed in different
forms (isoforms) in the gill. Knowing that the gill seems to switch expression
of the 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
subunit isoforms dominated the transcription pattern and that the fish seemed
to switch from 1a transcription in freshwater to
1b in seawater.
But where did these changes happen in the fish's gill? Tracking the
location of subunit expression in the fish gills in freshwater, Madsen
found high levels of 1a mRNA in the lamellae and filament.
However, when the fish adjusted to their new saltwater home, the
1a transcript retreated to deep within the filament while
the previously restricted 1b spread throughout the
filament.
Madsen is very excited that the gill switches between the 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 1a subunit
consumes significantly more ATP per pumped sodium ion than the
1b subunit, Madsen suggests that this allows
1a-rich gill cells to maintain a greater sodium gradient
than cells packed with the 1b 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 the
1b subunit could be sufficient to rid salmon of ions that
seeped into their blood from seawater.
References
Madsen, S. S., Kiilerich, P. and Tipsmark, C. K.
(2009). Multiplicity of expression of
Na+,K+–ATPase -subunit isoforms in the gill
of Atlantic salmon (Salmo salar): cellular localisation and absolute
quantification in response to salinity change. J. Exp.
Biol. 212,78
-88.
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Related articles in JEB:
-subunit isoforms in the gill of Atlantic salmon (Salmo salar): cellular localisation and absolute quantification in response to salinity change
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