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First published online October 21, 2004
Journal of Experimental Biology 207, vi (2004)
Copyright © 2004 The Company of Biologists Limited
doi: 10.1242/jeb.01301
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Outside JEB |
NEW ATPASE FOUND IN ATLANTIC STINGRAY
University of Miami
dmcdonald{at}rsmas.miami.edu
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An exciting new discovery has been made in the Atlantic stingray that has
implications for ion regulation in elasmobranchs. In mammals, an active
transport protein (HK
1) is present in the stomach and kidney that
mediates proton (H+ or acid) secretion in exchange for potassium
ions (K+) and functions in digestion, acid–base balance and
ion regulation. Like mammals, elasmobranchs (sharks, skates and rays) are also
believed to secrete acid into their stomachs, but unlike mammals, they
primarily use their gills for acid excretion and ion regulation instead of
their kidneys. Keith Choe, working with David Evans and his team, set out to
determine whether an HK1-like protein exists in the Atlantic stingray
and to determine if it is expressed in the gill.
The group searched for HK1 in the Atlantic stingray gill and stomach
by using an antibody made against pig HK1. With this antibody, the
group identified HK1 in gill cells rich in Na+/K+
ATPase. These cells are important in acid secretion and ion absorption in
fish. HK1 immunoreactivity was not found in cells that stained for
V-type H+-ATPase; cells that are thought to be important in base
secretion in fish. Knowing that HK1 is found in mammalian gastric
glands, the team also found HK1 immunoreactivity in the stomach of the
stingray. Having found that the stingray produces HK1, the next step
was to determine whether its expression is regulated during freshwater
exposure or ion regulation.
Characterising stingray HK1 on a molecular level, Evan's team found
that it is highly similar to rat HK1, and that it is expressed in the
stingray's stomach and gill but, surprisingly, not in the kidney. Using
quantitative, real-time PCR, the team determined that the expression of gill
HK1 does not change with elevated environmental CO2 but is
higher in fish acclimated to freshwater than those acclimated to seawater
suggesting that it plays a role in freshwater acclimation.
That the expression of HK1 in the gill is unaffected by the
respiratory acidosis caused by high environmental CO2 suggests that
this gene is not involved in acid–base balance. However, this finding
does not exclude the involvement of pre-existing gill HK1 proteins in
response to acid–base disturbance. Importantly, HK1 is
upregulated in freshwater-acclimated stingrays, suggesting that it may act as
a mechanism to increase active K+ uptake across the gill when
environmental K+ levels are low, possibly making HK1 vital
for the physiological fitness of this freshwater organism.
This study is the first to provide direct evidence for the presence of
HK1 in any fish. From an evolutionary perspective, stomach acid
secretion first appeared in elasmobranchs, which suggests that the Atlantic
stingray may be one of the oldest living organisms to have HK1.
References
Choe, K. P, Verlander, J. W., Wingo, C. S., and Evans, D. H.
(2004). A putative H+/K+ ATPase in the
Atlantic stingray, Dasyatis sabina: primary sequence and expression
in gills. Am. J. Physiol. Regul. Integr. Comp.
Physiol. 10.1152/ajpregu.00513.2003
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