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Na
+
/K
+
-ATPase were reduced after 4 days at a
p
CO
2
of 20 000 μatm, while enzyme activity remained
constant (Seidelin
et al.
2001). In the gills of Japanese
l ounder, activity increased during exposure to 10
000 and 50 000 μatm (unpublished data, cited in
Ishimatsu
et al.
2005). Recent studies in two marine
teleosts found an increase in the capacity of
branchial Na
+
/K
+
-ATPase during acclimation to
hypercapnia at 10 000 μatm (Deigweiher
et al.
2008 ;
Melzner
et al.
2009b). In the common eelpout,
Zoarces viviparus
, the specii c contents of Na
+
/K
+
-
ATPase mRNA and functional protein as well as its
maximum activity rose rapidly with the onset of
hypercapnia, and increased up to twofold above
control levels during 6 weeks of exposure to a
p
CO
2
of 10 000 μatm (Deigweiher
et al
. 2008 ). Long-term
acclimation (4 to 12 months) of Atlantic cod (
Gadus
morhua
) also led to increased Na
+
/K
+
-ATPase activi-
ties and protein concentrations at a
p
CO
2
of 6000
μatm (Melzner
et al.
2009b ). Changes in enzyme
activity were small at 3000 μatm, indicating that
capacities under control conditions were sufi cient
to cope with the additional ion-regulatory effort
required under moderate hypercapnia. It remains
to be explored to what extent this statement not
only holds close to the thermal optimum of the spe-
cies but also at extreme temperatures.
Teleosts with high regulatory capacities may also
draw from a functional reserve that can be activated
upon demand, e.g. by protein modii cation through
phosphorylation (see Ramnanan and Storey 2006).
This mechanism may be important during the early
regulatory phase and may partly explain any dis-
crepancy between changing mRNA, protein, and
functional levels. Long-term acclimation may then
respond to the requirement for higher functional
rates of ion and acid-base regulation at higher con-
centrations of the respective transporter.
The electrochemical gradient provided by Na
+
/
K
+
-ATPase is exploited by gradient-dependent
transporters and channels such as NHE, Na
+
/HCO
3
-
cotransporter (NBC), and Cl
-
/HCO
3
-
exchanger
(AE). NBC and AE are both solute carrier 4 bicarbo-
nate transporters, which were i rst characterized in
gills of freshwater i shes (Wilson
et al.
2000 ; Hirata
et al.
2003 ; Perry
et al.
2003 ). Recent i ndings indicate
that they also play an important role during hyper-
capnic acclimation of marine teleosts (Deigweiher
et al.
2008). Similar to transcriptional changes of
AE1 and two NHE1 isoforms, mRNA levels of NBC
decreased transiently during early acclimation to
acute elevations in CO
2
. However, levels were
enriched about threefold at the end of the 6-week
exposure period, whereas AE1 and NHE1s were
restored to control levels (Deigweiher
et al.
2008 ). In
freshwater i sh, the level of NBC1 mRNA increased
immediately after external acidii cation in
Tribolodon
hakonensis
( Hirata
et al.
2003 ) and under environ-
mental hypercapnia in
Oncorhynchus mykiss
( Perry
et al.
2003). During long-term acclimation, higher
levels of the NBC1 transporter may thus support
high levels of extracellular HCO
3
-
, which is instru-
mental during the compensation of acidosis. The
i nding of constant Na
+
levels over time suggests
that the enhanced activity of NBC1 might be coun-
teracted by mechanism(s) that extrude Na
+
.
The difference in timing between the responses of
NBC1 in freshwater and seawater may relate to the
more 'relaxed' situation for acid-base regulation in
seawater, which is characterized by high levels of
bicarbonate and counter-ions that support ion and
acid-base regulation. The putative freshwater ori-
gin of teleost i shes (Fyhn
et al.
1999 ) and the exist-
ence of extant anadromous and katadromous teleost
species corroborate the conclusion that i shes are
characterized by highly l exible ion and acid-base
regulation systems that are well adapted to handle
the largely different ionic compositions of their
respective ambient media.
The pathways contributing to acid-base regula-
tion may be more complicated than discussed here
owing to the large number of transporters and chan-
nels present in gill epithelia, their heterogeneous
localization and orientation in apical and basolateral
membranes, and also due to the presence of different
cell types within gill epithelia (see, e.g., the discus-
sion in Perry
et al.
2003 ). Extensive transcriptomic
studies may be useful in the identii cation and char-
acterization of the transporters and channels
involved. Building on current mechanistic knowl-
edge of marine teleost ion and pH regulation (Fig.
8.2 ) Deigweiher
et al.
( 2008 ) carried out a comprehen-
sive analysis of transcriptomic changes in gill ion
exchange mechanisms under hypercapnia. They
found an apical Na
+
/H
+
-exchanger isoform 2
(NHE2), two V-type H
+
-ATPase (HA) isoforms, as
