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First published online July 20, 2006
Journal of Experimental Biology 209, 2813-2827 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02345
Review Article |
Intestinal anion exchange in marine fish osmoregulation
RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA
e-mail: mgrosell{at}rsmas.miami.edu
Accepted 23 May 2006
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Summary |
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 TOP Summary Osmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
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Key words: hypertonic water absorption, Cl-/HCO3- exchange, secondary active Cl- and HCO3- transport, acidic absorbate
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Osmoregulation in marine fish |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
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). (2)
Osmoconformity and ionoregulation are seen in marine elasmobranchs
(Hazon et al., 2003) and in
the lobe-finned coelacanth (Griffith et
al., 1974), which maintains plasma osmolality at or slightly above
that of the surrounding seawater but NaCl concentrations at approximately
of ambient levels. (3) The most common strategy is osmoregulation,
found in all teleosts (Marshall and
Grosell, 2005) and marine lampreys
(Morris, 1958), achieved by
the regulation of the main extracellular electrolyte (Na+ and
Cl-) levels at approximately of full strength
seawater.
Under steady state conditions at constant salinities, hagfish,
elasmobranchs and coelacanths presumably do not need to drink to maintain
water balance. It was shown, however, that the unavoidable renal and
extra-renal fluid loss to the hypertonic marine environment in
hypo-osmoregulating fish was compensated for by ingestion of seawater with
subsequent water absorption across the gastro-intestinal tract
(Smith, 1930). More recently,
it was demonstrated that even the osmoconforming elasmobranchs display
transient drinking when exposed to elevated ambient salinity, and evidence for
components of the rennin-angiotensin system (RAS), even in the elasmobranchs
(Anderson et al., 2002;
Hazon et al., 2003) and
hagfish (Cobb et al., 2004) as
well as in lamprey (Brown et al.,
2005), continues to accumulate. The drinking reflex is at least in
part controlled by RAS and it thus appears that the ability to regulate
ingestion of seawater and thereby the magnitude of intestinal fluid absorption
is an ancestral osmoregulatory trait among fishes.
The ingestion and processing of the imbibed seawater for osmoregulatory
purposes have, at least in teleost fish, received much attention for three
quarters of a century since the first classic studies by
Smith published in 1930
(Smith, 1930). It is now well
established that an initial desalinization of the ingested seawater occurs in
the esophagus, which absorbs Na+ and Cl- through both
passive and active transport pathways, although the cellular mechanisms remain
to be fully elucidated. The esophageal absorption of salt combined with a
limited water permeability (Hirano and
Mayer-Gostan, 1976; Parmelee
and Renfro, 1983) of this gastro-intestinal segment results in a
reduction of osmotic pressure in the fluids entering the anterior portion of
the intestine (Marshall and Grosell,
2005). This reduction in osmotic pressure allows for fluid
absorption by the more distal segments of the gastro-intestinal tract. Fluid
absorption appears to occur along the entire length of the intestine at rates
of 2-6 µl cm-2 h-1
(Grosell et al., 1999;
Grosell et al., 2001;
Grosell et al., 2005;
Grosell and Jensen, 1999;
Wilson et al., 2002) despite
slight osmotic uphill gradients (McDonald
and Grosell, 2006) and is driven by Na+ and
Cl- transport (Ando et al.,
1986; Mackay and Lahlou,
1980; Skadhauge,
1974; Usher et al.,
1991). The preferential absorption of Na+,
Cl- and water leaves Mg2+ and
SO2-4 highly concentrated in the intestinal fluids such
that MgSO4 accounts for the majority of the osmotic pressure
(Grosell et al., 2001;
Wilson et al., 2002). The
chemical composition of the intestinal fluids is unusual in containing these
high concentrations of divalent ions and also very high levels of
HCO3-, and by being alkaline with pH in some cases
reaching values higher than 9 (Wilson et
al., 1996; Wilson,
1999).
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;
Wilson et al., 2002). In
addition, the net composition of the fluid absorbed by the marine teleost
intestine is discussed as it appears to deviate from the isotonicity typical
of water absorption across other vertebrate leaky epithelia, including the
gall bladder, the renal tubule and small intestine of terrestrial vertebrates
(Larsen, 2000;
Larsen et al., 2002;
Nedergaard et al., 1999). |
Intestinal transport processes |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
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;
Skadhauge, 1974;
Usher et al., 1991). The
Na+ absorption is ultimately fueled by the basolateral
Na+/K+-ATPase (NKA), which extrudes 3Na+ in
exchange for 2K+ and thereby establishes a strong cytosolic
negative membrane potential and low intracellular Na+
concentrations (Skou, 1990;
Skou and Esmann, 1992)
(Fig. 1). The NKA activity is
generally high in marine fish intestine, even when compared to the gill
(Grosell et al., 1999;
Hogstrand et al., 1999), and
is higher in euryhaline fish acclimated to seawater than in freshwater
acclimated individuals (Colin et al.,
1985; Fuentes et al.,
1997; Jampol and Epstein,
1970; Kelly et al.,
1999; Madsen et al.,
1994), illustrating the osmoregulatory importance of this enzyme.
Furthermore, NKA gene expression is elevated in the intestine of euryhaline
teleost fish following transfer from freshwater to seawater, demonstrating the
significance of this enzyme for successful marine osmoregulation
(Cutler et al., 2000;
Jensen et al., 1998;
Seidelin et al., 2000).
Apical co-transporters
The electrochemical Na+ gradient established by the basolateral
NKA provides the energy necessary for the thermodynamic uphill transport of
K+ and Cl- from the intestinal lumen across the apical
membrane via two parallel cotransport systems:
Na+:Cl- (NC) and
Na+:K+:2Cl- (NKCC) co-transporters
(Field et al., 1980;
Frizzell et al., 1979;
Halm et al., 1985;
Musch et al., 1982)
(Fig. 1). As for NKA, the NKCC
gene exhibits increased expression in euryhaline fish intestine when
transferred from freshwater to seawater
(Cutler and Cramb, 2002),
illustrating the importance of NKCC for seawater osmoregulation. A large
number of studies have demonstrated that net Cl- absorption rates
exceed corresponding net Na+ absorption rates
(Table 1), which may in part be
attributed to the stoichiometry of NKCC. However, considering the relatively
low K+ concentration in seawater (10 mmol l-1 compared
to 
430 mmol l-1 Na+), NKCC cannot account for the
often large excess of net Cl- absorption (see `Missing cationic
charge' section, below). Recently, much of the excess net Cl-
absorption has been linked to anion exchange processes in the apical membrane
of the marine teleost intestinal epithelium
(Grosell et al., 2005).
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Anion exchange |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
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), excluding a strictly digestive role of this phenomenon.
Almost four decades later, the original observations by Smith were confirmed
for rainbow trout acclimated to different salinities
(Shehadeh and Gordon, 1969).
Although total CO2 was not measured directly in these studies on
rainbow trout, a large anion:cation gap was observed by comparing
concentrations of the major cations (Ca2+, K+,
Mg2+ and Na+) to concentrations of major anions
(Cl- and SO42-) and was attributed to high
concentrations of HCO-3 and
CO32-(Shehadeh and
Gordon, 1969). The study on rainbow trout reported direct
measurements of pH in the intestinal or rectal fluids ranging between 8.1 and
9.0, depending on salinity and from which segment of the intestine the fluid
samples were obtained. These pH values agree well with the first reports of pH
increasing in rectal fluids from 7 to 9 in eels during seawater acclimation
(Cordier and Maurice, 1956). It
was recognized (Shehadeh and Gordon,
1969) that the likely source of HCO3- and
CO32- was endogenous and further, that
Cl-/HCO3- exchange might account for both
secretion of HCO3- and absorption of Cl-.
Observations of a higher carbonate content in intestinal fluids and higher
intestinal Cl- absorption rates in trout acclimated to higher
salinities (Shehadeh and Gordon,
1969) in these early studies had already suggested a role for
Cl-/HCO3- exchange in marine osmoregulation.
In the first study of the mechanisms by which HCO3- is
secreted into the intestine, a direct requirement for Cl- as well
as DIDS sensitivity was demonstrated in the goby and it was suggested that
basolateral Cl-/HCO3- exchange might assist
in Cl- transport across the basolateral membrane
(Dixon and Loretz, 1986).
These findings using the goby also implied a role for intestinal anion
exchange in marine osmoregulation. Subsequently, evidence for an apical, DIDS
sensitive Cl-/HCO3- exchange process in the
seawater acclimated eel intestine (Ando and
Subramanyam, 1990) further supported a role for anion exchange in
intestinal Cl- absorption. Also supporting the role for intestinal
anion exchange in osmoregulation were observations of rectal secretion of
solid carbonate pellets (see `Alkaline precipitation' section below) in gulf
toadfish held in 100% seawater but not in 25% seawater (hypo-osmotic)
(Walsh et al., 1991). In
agreement with Shehadeh and Gordon
(Shehadeh and Gordon, 1969),
Walsh and co-workers concluded from their investigation of the gulf toadfish
that the source of total CO2 found in the intestinal lumen of
toadfish was most likely endogenous, and further, that intestinal epithelial
respiration was the likely source of CO2. This latter suggestion
was not empirically tested until much later (see `Mechanisms of intestinal
anion exchange' section below), but was supported by the high density of
mitochondria observed in the intestinal epithelial cells
(Walsh et al., 1991),
indicating a high level of CO2 production.
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).
This was in contrast to post-smolt individuals where rectal fluids were
circum-neutral 24 h post transfer and remained less alkaline than fluids
obtained from smolts until 7 days post transfer, at which time rectal fluids
from both smolts and post-smolts were highly alkaline.
Recent demonstrations of intestinal HCO3- secretion
in elasmobranchs stimulated by dehydration following transfer to a
hyperosmotic environment (Taylor and
Grosell, 2006a) further support the suggested osmoregulatory role
of intestinal anion exchange. These observations, combined with observations
of elevated intestinal HCO3- concentrations in sturgeon
exposed to hyperosmotic conditions, suggest that a role for intestinal
HCO3- secretion in osmoregulation is perhaps an
ancestral trait, which may be common to all fish (and perhaps other animals)
that need to drink seawater (Taylor and
Grosell, 2006a).
The substantial base output via the continuous release of highly
alkaline rectal fluids in marine teleost fish led to the proposal by Wilson
and coworkers of a role for intestinal anion exchange in acid-base balance
(Wilson et al., 1996;
Wilson, 1999). However, more
recent studies have revealed that induced alkalosis does not stimulate
intestinal base secretion (Wilson et al.,
2002), and it thus appears that although base released with rectal
fluids makes up a substantial exchange of basic equivalents with the
environment, intestinal anion exchange does not play a role in dynamic
regulation of acid-base balance. In fact, reports of total CO2 and
pH in intestinal fluids from freshwater and seawater acclimated rainbow trout,
European eel and European flounder revealed that the occurrence of alkaline
intestinal fluids is salinity dependent
(Wilson, 1999), strongly
suggesting a role in osmoregulation. The salinity dependence of intestinal
anion exchange is also illustrated by total CO2 concentrations in
intestinal fluids obtained from freshwater and seawater acclimated tilapia
(Fig. 2).
Examining intestinal transport of water, Na+, Cl- and
HCO3- in the lemon sole using an in situ
intestinal perfusion approach demonstrated that approximately 50% of
intestinal Cl- uptake occurred in exchange for
HCO3- secretion while the remaining 50% was accompanied
by Na+ absorption (Grosell et
al., 1999). These observations were the first to quantify the
significance of intestinal anion exchange for Cl- and thus water
absorption, and highlight the anion exchange process as being quantitatively
important for marine osmoregulation. Later, intestinal Cl- and
water absorption in the absence of luminal Na+ was demonstrated
using the Pacific sanddab (Grosell et al.,
2001). Notably, Cl- absorption under these conditions
was matched by HCO3- secretion, suggesting that the
Na+ independent fraction of Cl- absorption was
accomplished by anion exchange. Furthermore, it was noted that Cl-
absorption via this anion exchange system must be secondary active in
nature since it occurred against an electrochemical gradient even in the
absence of Na+.
Considering the simultaneous measurements of intestinal net Na+ and Cl- absorption in different species of marine teleosts summarized in Table 1, it is clear that Cl- absorption consistently exceeds Na+ absorption regardless of species. In the first five studies listed, HCO3- secretion rates were measured in addition to NaCl absorption and a good correlation between the magnitude of the difference between Na+ and Cl- absorption and the HCO3- secretion is evident. This correlation strongly suggests that the excess Cl- absorption can be attributed to Cl-/HCO3- exchange. Considering all the studies in Table 1 and assuming that anion exchange can account for the excess Cl- absorption in cases where HCO3- secretion was not measured, anion exchange consistently accounts for a substantial portion of Cl- absorption, in some cases up to 71%.
Observations of Cl- absorption in the absence of Na+
absorption (no Na+ on the luminal side of the intestinal
epithelium) was first reported for the winter flounder
(Field et al., 1978) and later
for the Pacific sanddab (Grosell et al.,
2001) and the European flounder
(Grosell et al., 2005). In the
study on the European flounder, Cl- and water absorption was
observed across flounder intestine despite lack of net Na+
absorption even when Na+ was present in the intestinal lumen. The
salines used in the study on the European flounder were designed to mimic
in vivo conditions characterized by a substantial uphill
electrochemical gradient for Cl- absorption. Under various
experimental manipulations of Cl- and HCO3-
transport, a co-variation between net Cl- uptake rates and
HCO3- secretion rates was observed
(Grosell et al., 2005). Thus,
in the absence of net Na+ uptake, the intestinal anion exchange is
clearly capable of active Cl- absorption, which in turn provides
the driving force for water absorption. Furthermore, it was demonstrated that
the high HCO3- concentrations found in intestinal fluids
of marine teleost fish are the product of active HCO3-
secretion across the intestinal epithelium mediated by
Cl-/HCO3- exchange. Additional components are
required to account for active HCO3- secretion processes
mediated by Cl-/HCO3- exchange, which warrant
a discussion of the epithelial transport mechanisms responsible for both
thermodynamic uphill HCO3- and Cl-
transport.
Mechanisms of intestinal anion exchange
Source of substrate and energy for active transport
Active luminal secretion of HCO3- was recently
demonstrated for European flounder
(Grosell et al., 2005) and has
been confirmed for gulf toadfish, which displays a Q10 of 1.8-3.0
for intestinal HCO3- secretion
(Grosell and Genz, 2006). A
limited number of studies have addressed the exact mechanistic nature of the
intestinal anion exchange system in marine fish. These studies include pH-stat
type experiments on the goby and the Japanese eel
(Ando and Subramanyam, 1990;
Dixon and Loretz, 1986) and
more recent experiments including both pH-stat techniques and isolated
intestinal sac procedures (Grosell et al.,
2001; Grosell et al.,
2005; Grosell and Genz,
2006; Grosell and Jensen,
1999; Wilson et al.,
2002; Wilson and Grosell,
2003). From these studies, an understanding of the transport
mechanisms responsible for the active HCO3- secretion is
emerging, but numerous questions remain to be addressed and it seems that
substantial species-specific differences may exist.
Apical anion exchange
Substantial evidence for an apical
Cl-/HCO3- exchanger has accumulated since the
first documentation of HCO3- secretion being dependent
on luminal Cl- in the goby
(Dixon and Loretz, 1986), and
includes luminal Cl- dependence in Japanese eel
(Ando and Subramanyam, 1990),
rainbow trout (Wilson et al.,
1996), pacific sanddab
(Grosell et al., 2001) and
European flounder (Grosell et al.,
2005). Further evidence is reduced luminal
HCO3- secretion in the presence of DIDS in the luminal
fluids of Japanese eel (Ando and
Subramanyam, 1990), the European flounder
(Grosell and Jensen, 1999) and
the pacific sanddab (Grosell et al.,
2001). Additional compelling evidence is cross-reactivity of an
anion exchanger (AE1) antibody with the apical surface of the brackish water
mudskipper and seawater acclimated coho salmon
(Wilson et al., 2002). In
agreement with intestinal HCO3- secretion being most
pronounced in seawater versus freshwater acclimated fish, AE1
cross-reactivity with the apical region of coho salmon intestinal epithelium
is strong in seawater acclimated individuals but absent in freshwater
acclimated conspecifics (Wilson et al.,
2002).
Although evidence is strong for an apical anion exchanger in marine teleost
intestinal epithelium, two reports of no luminal DIDS effects on
HCO3- secretion
(Dixon and Loretz, 1986;
Wilson et al., 1996) disagree
with the presence of an apical anion exchanger. However, both these studies
employed relatively low concentrations of DIDS and showed mucosal
Cl- dependence, still supporting the presence of an apical anion
exchange process.
Source of HCO3-
Transepithelial HCO3- transport
Evidence for transepithelial HCO3- transport comes
from observations of reduced luminal HCO3- secretion
when serosal HCO3- is replaced with
PO2-4 in the Japanese eel
(Ando and Subramanyam, 1990)
and from observations of mucosal HCO3- secretions
correlating with serosal HCO3- concentrations in the
European flounder (Grosell et al.,
2005). Furthermore, luminal HCO3- secretion
is reduced when serosal HCO3- is replaced with Hepes in
the gulf toadfish (Grosell and Genz,
2006). It should be noted, however, that substantial
HCO3- secretion persists even in the absence of serosal
HCO3- and that alternative sources of
HCO3- appear to be able to sustain approximately 30-60%
of the luminal HCO3- secretion at least in the goby,
European flounder and gulf toadfish (Dixon
and Loretz, 1986; Grosell et
al., 2005; Grosell and Genz,
2006). Ando and Subramanyam, working on the Japanese eel, reported
a larger contribution of serosal HCO3- to luminal
HCO3- secretion but employed 24.9 mmol l-1
HCO3- in serosal salines
(Ando and Subramanyam, 1990).
While this HCO3- concentration is consistent with
mammalian extracellular fluid values, it is far above the 2-10 mmol
l-1 normally seen in teleost fish, including eels
(Marshall and Grosell, 2005),
and might explain highly elevated luminal HCO3-
secretion rates.
Nevertheless, transepithelial HCO3- transport
requires absorption of HCO3- by the intestinal
epithelial cells from the relatively low extracellular concentrations across
the basolateral membrane, which exhibits a cytosolic negative potential
difference of 80 mV (Loretz,
1995). Luminal HCO3- secretion has been
demonstrated to be dependent on serosal Na+ in Japanese eel,
Pacific sanddab and gulf toadfish (but apparently not in European flounder;
M.G., personal observations) (Ando and
Subramanyam, 1990; Grosell et
al., 2005; Grosell and Genz,
2006) and is sensitive to DIDS in serosal salines in the Japanese
eel (Ando and Kobayashi, 1978)
and the goby (Loretz, 1995).
These observations are consistent with a
Na+:HCO3- cotransporter (NBC) in the
basolateral membrane, which would allow for HCO3- import
against an electrochemical gradient across the basolateral membrane driven by
the favorable Na+ gradient. However, a role for NBC in intestinal
HCO3- secretion remains to be conclusively demonstrated
and it should be noted that the European flounder does not require serosal
Na+ for luminal HCO3- secretion (M.G.,
personal observations), nor is it sensitive to serosal DIDS
(Grosell and Jensen, 1999),
perhaps illustrating interesting species differences.
Metabolic CO2
As mentioned above, substantial intestinal HCO3-
secretion persists even in the absence of serosal HCO3-,
suggesting that hydration of CO2 within the intestinal epithelium
might provide HCO3- for apical secretion. The source of
CO2 for hydration within the epithelial cells could be
extracellular (which would require diffusion from the extracellular fluids
across the basolateral membrane) or endogenous metabolic CO2 from
within the intestinal epithelium. Evidence for epithelial, endogenous
metabolic CO2 providing the substrate for
HCO3- comes from experiments on both flounder and gulf
toadfish using serosal Hepes buffered salines gassed with 100% O2
(Grosell and Genz, 2006;
Wilson and Grosell, 2003).
Under these conditions, 60-80% of control HCO3-
secretion rates (physiological HCO3- concentrations and
partial pressure of CO2 in serosal fluids) persists, showing a
significant contribution from endogenous epithelial metabolic CO2.
In addition, it appears that extracellular CO2 may provide the
substrate for cellular CO2 hydration and thus
HCO3- secretion at least in the European flounder.
Experiments with elevated (2% compared to controls of 0.5%) serosal
CO2 levels revealed increased luminal HCO3-
secretion as well as elevated net Cl- and water absorption
(Grosell et al., 2005). It
should be noted, however, that reduction of extracellular CO2
concentration did not result in reduced HCO3- secretion,
suggesting that while extracellular super-physiological CO2 levels
may contribute to elevated luminal HCO3- secretion,
other sources of HCO3- are sufficient to sustain basal
control levels of HCO3- secretion.
Accepting that a substantial part of the secreted
HCO3- is derived from endogenous metabolic
CO2 allows for predictions of metabolic rates of marine teleost
intestinal epithelia. Such considerations led to an estimate of intestinal
epithelium metabolic rate being at least five- to eightfold higher than
corresponding mass-specific whole animal consumption rates
(Grosell et al., 2001), which
is supported by the high mitochondrial density in marine teleost intestinal
epithelia (Walsh et al.,
1991). This five- to eightfold higher mass-specific metabolic rate
is estimated assuming that all metabolic CO2 produced translates to
HCO3- secretion. However, it is likely that some
CO2 diffuses from the epithelial cells to the extracellular fluid,
perhaps indicating that the metabolic rate may be even higher than five- to
eightfold that of mass-specific whole animal rates. In addition to the
osmoregulatory role of the intestine, other functions including digestion,
nutrient absorption, endocrine activity and barrier functions
(Mommsen et al., 2003) are
conceivably energetically costly and may explain the need for high abundance
of mitochondria. The hydration of the metabolic waste product, CO2,
and the subsequent exchange of HCO3- for Cl-
effectively exchanges a gas that exerts limited osmotic pressure with a main
osmolyte, Cl-, which in turn provides the osmotic driving force for
cellular water uptake.
Three studies have demonstrated the importance of the enzyme carbonic
anhydrase for intestinal HCO3- secretion in the goby,
the rainbow trout and the gulf toadfish by use of pharmacological inhibitors
(Dixon and Loretz, 1986;
Grosell and Genz, 2006;
Wilson et al., 1996). However,
although carbonic anhydrase mediated CO2 hydration contributes to
overall CO2 hydration within the intestinal epithelium, it should
be noted that relatively low inhibition (30-40%) of
HCO3- secretion was observed for both species, despite
the use of relatively high inhibitor concentrations. These observations may
suggest that non-mediated CO2 hydration can account for some of the
overall basal HCO3- production and thus excretion, but
may also indicate that experimental conditions prevented full carbonic
anhydrase inhibition.
Basolateral H+ extrusion
Regardless of the CO2 source (extracellular versus
endogenous), cellular CO2 hydration produces H+in
addition to HCO3-, and the H+ must be
excreted from the epithelial cells to prevent reversal of the hydration
reaction and thereby allow for accumulation of cellular
HCO3- for anion exchange. Furthermore, it seems clear
that the H+ extrusion must occur across the basolateral membrane
since the intestinal epithelium exhibits substantial net base secretion
(serosal 
mucosal) (Grosell et al.,
2001; Grosell et al.,
2005).
A prediction of similar basolateral H+ extrusion rates and
apical HCO3- secretion rates in the gulf toadfish
intestine was confirmed using pH-stat titration techniques in both mucosal and
serosal salines, conclusively demonstrating that H+ ions arising
from CO2 hydration are extruded across the basolateral membrane
(Grosell and Genz, 2006). The
importance of this basolateral H+ extrusion for apical
Cl-/HCO3- exchange is clearly illustrated in
the gulf toadfish by reversible, H+ concentration dependent
inhibition of luminal HCO3- secretion when serosal pH is
reduced from 7.8 to 7.4, 7.0 and 6.6
(Grosell and Genz, 2006).
Reduction of serosal pH would make H+ gradients across the
basolateral membrane less favorable for H+ extrusion, which
presumably results in reduced cellular HCO3- for apical
anion exchange.
Basolateral H+ extrusion seems to occur via a
Na+/H+ exchange (NHE) mechanism, at least in the gulf
toadfish, as luminal HCO3- secretion relies on serosal
Na+ even in the absence of serosal HCO3-
(Grosell and Genz, 2006).
Although HCO3- secretion is insensitive to even high
concentrations of amiloride and EIPA
[5-(N-ethyl-N-isopropyl) amiloride] added to the serosal
medium, dependence on Na+ gradients is clearly demonstrated by
experiments with the NKA inhibitor ouabain. When NKA is inhibited and
Na+ gradients are partly depleted, luminal
HCO3- secretion is greatly reduced, presumably because
H+ extrusion via an NHE-like mechanism is reduced
(Grosell and Genz, 2006).
These recent observations of ouabain sensitivity confirm similar observations
on the goby (Dixon and Loretz,
1986) and observations of luminal HCO3-
secretion being dependent on serosal Na+ in the Japanese eel
(Ando and Subramanyam, 1990).
However, it should be noted that in both the studies on the goby and the eel,
serosal salines contained HCO3- and that the apparent
dependence on Na+ gradients could also be explained by basolateral
HCO3- import via NBC.
However, for the gulf toadfish where an NHE-like transporter is critical for luminal HCO3- secretion, and also for goby and eel where NBC or NHE appear to play a role in HCO3- secretion, Na+ gradients are important. Thus, HCO3-secretion, at least in gulf toadfish, goby and eel, rely on electrochemical gradients established by NKA, which ultimately fuel basolateral H+ extrusion and thereby the secondary active HCO3- and Cl- transport by the apical anion exchanger.
An apparent lack of serosal Na+ dependence in the European flounder (M.G., personal observation) may imply that species differences in transport mechanisms exist. An alternative mechanism might include a basolateral H+-pump that could fuel basolateral H+ extrusion and thereby the active HCO3- secretion across the apical membrane independently of Na+ gradients.
Alkaline precipitation
In addition to the more direct role for anion exchange in Cl-
and thereby water absorption, an indirect but possibly quantitatively
important consequence of highly alkaline intestinal fluids is the
precipitation of Ca2+ and Mg2+ carbonates within the
intestinal lumen. The presence of macroscopic light colored solids in the
intestinal lumen even of starved fish had already been noted by Smith in his
classic study published in 1930 (Smith,
1930) and has since been noted to be most pronounced at higher
salinities (Shehadeh and Gordon,
1969). Walsh and coworkers were the first to report that these
intestinal solids consisted primarily of Mg2+ and Ca2+
carbonates (Walsh et al.,
1991) but the overall importance of this precipitation within the
intestinal lumen has only recently been recognized.
Carbonate precipitates account for 20% of rectal base excretion in
marine fish under control conditions
(Wilson et al., 1996;
Wilson et al., 2002;
Wilson and Grosell, 2003),
whereas 30-65% of the rectal Ca2+ excretion can be accounted for by
the precipitates (Shehadeh and Gordon,
1969; Wilson and Grosell,
2003). Considering that only a modest fraction of Ca2+
ingested with seawater is absorbed across the intestinal epithelium and that
as much as 85% of the ingested seawater is absorbed, intestinal fluid
Ca2+ concentrations could be expected to be approximately sixfold
higher than corresponding seawater Ca2+ concentrations (10
mmol l-1). However, numerous studies report intestinal fluid
Ca2+ concentrations at 5 mmol l-1
(Marshall and Grosell, 2005),
which in earlier reports was erroneously interpreted to be the result of
substantial intestinal Ca2+ uptake
(Evans, 1993;
Hickman, 1968;
Karnaky, 1998). The formation
of carbonate precipitates, which is a direct consequence of the high
HCO3- concentration and alkaline conditions, account for
the majority of the Ca2+ lost from the intestinal fluids and has
important consequences for the osmotic pressure of the intestinal fluid. An
estimated reduction of 70 mOsm resulting from the precipitation of
Ca2+ and CO32- in the intestinal fluids
(Wilson et al., 2002) is an
obvious benefit for intestinal fluid absorption and has also been demonstrated
to be important for Ca2+ homeostasis
(Wilson and Grosell,
2003).
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In vitro versus in vivo discrepancies |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
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). These two observations appear to be in contrast to the
situation in vivo, where intestinal fluid Cl-
concentrations always exceed Na+ concentrations and where a
substantial increase of HCO3- concentrations in
intestinal fluids is seen in the anterior region with little or no further
increase along the gastro-intestinal tract. These two discrepancies between
in vitro and in vivo observations will be discussed
below.
Considering first the issue of Cl- versus
Na+ uptake rates, the apparent discrepancy between in
vitro and in vivo observations can be attributed to different
concentrations of Cl- and Na+ in seawater. In general,
Cl- concentrations exceed Na+ concentrations in seawater
by 70 mmol l-1, which means that seawater ingestion results in
higher gastrointestinal intake of Cl- than Na+. Further,
desalinization in the esophagus occurs via both passive and active
equimolar Na+ and Cl- absorption
(Kirsch and Meister, 1982;
Parmelee and Renfro, 1983;
Smith, 1930;
Wilson et al., 1996). With
little or no transport across the gastric mucosa in starved fish, the
consequence of the higher concentrations of Cl- than Na+
in seawater and equal molar Na+ and Cl- absorption in
the esophagus is that fluids entering the intestine contain higher
concentrations of Cl- than Na+. However, while
intestinal fluid Cl- concentrations remain higher than the
corresponding Na+ concentrations as fluids move along the
intestine, the difference between Na+ and Cl-
concentrations is reduced and can be as low as 10-30 mmol l-1
(Grosell et al., 2001;
Marshall and Grosell, 2005;
Taylor and Grosell, 2006a).
The reduced difference between Na+ and Cl-
concentrations in intestinal fluids compared to seawater must be the result of
intestinal Cl- absorption in excess of Na+ absorption.
Using the gulf toadfish as an example, gastro-intestinal seawater intake and
processing is illustrated in Fig.
3A,B. Intake of Na+ and Cl- with seawater
(Fig. 3A) was calculated
assuming a drinking rate of 2 ml kg-1 h-1 and seawater
concentrations of 489 and 420 mmol l-1 Cl- and
Na+, respectively. The amount of Na+ and Cl-
passing the esophagus is estimated from Na+ and Cl-
concentrations measured in stomach fluids of starved fish
(Marshall and Grosell, 2005)
and an assumption of no esophageal water absorption. The amount of
Na+ and Cl- present in various segments of the intestine
(Fig. 3A) is estimated from
Na+ and Cl- concentrations measured in toadfish
intestinal fluids (Taylor and Grosell,
2006a) and an assumption of 20% fluid absorption in each of the
four intestinal segments, yielding a total fractional fluid absorption of 80%
(Marshall and Grosell, 2005).
These simple considerations reveal that the majority of the ingested
Na+ and Cl- are absorbed in the esophagus and the
anterior intestine. From differences in concentrations of Na+ and
Cl- among various gastro-intestinal segments, regional uptake rates
of Na+ and Cl- can be estimated as illustrated in
Fig. 3B. From this illustration
it becomes apparent that esophageal salt absorption also appears to be
equimolar with respect to Na+ and Cl- in vivo,
but that the same is not the case for the intestine. In all segments of the
toadfish intestine, in vivo absorption of Cl- exceeds the
corresponding Na+ absorption, with the anterior intestine
displaying the largest absolute difference between Na+ and
Cl- absorption. In this example
(Fig. 3A,B), based on toadfish,
Cl- absorption exceeds Na+ absorption by 12-57%, which
is generally lower than values for isolated or perfused intestinal epithelia
(Table 1) where Cl-
absorption can be up to 71% higher than Na+ transport. Although the
relative contribution of excess Cl- absorption appears to be lower
in vivo than in vitro or in situ, it is still of
quantitative significance especially in the anterior region of the intestine.
In agreement with the argument presented above that
Cl-/HCO3- exchange accounts for this excess
Cl- absorption, both Cl- absorption
(Fig. 3B) and intestinal
HCO3- secretion (Fig.
4) are most pronounced in the anterior segment of the intestine
in vivo.
|
|
The second discrepancy between in vivo and in vitro
observations is the relatively silent in vivo
Cl-/HCO3- exchange activity in distal
segments of the intestine despite the obvious capacity for
HCO3- secretion in these segments in vitro. As
illustrated in Fig. 4, all
intestinal segments, when isolated from the gulf toadfish, display high
HCO3- secretion rates, which is in agreement with
observations from European flounder, Pacific sanddab and lemon sole
(Grosell et al., 1999;
Grosell et al., 2001;
Grosell and Jensen, 1999). The
high secretion rates in the distal segments of the intestine are in contrast
to the limited or lack of an increase in luminal HCO3-
concentrations in vivo as fluids are moving along the intestine of
all four species. This discrepancy is likely related to the in vitro
conditions employed for measurement of HCO3- transport
rates, which include the use of HCO3- free luminal
salines of identical composition in all intestinal segments. The reduction in
HCO3- concentrations in luminal salines is necessary to
avoid CaCO3 precipitation but is obviously different from in
vivo situations where HCO3- builds up to high
concentrations, especially in the more distal segments. Further, in
vitro measurements to date have used the same Cl-
concentrations for anterior, mid and posterior intestinal segments, which
again contrast with the in vivo situation where Cl-
concentrations in intestinal fluids generally become lower in more distal
segments of the intestine. The likely explanation for the low
HCO3- secretion rates in distal intestinal segments
in vivo, despite the obvious HCO3- transport
capacity seen in vitro, is that Cl- and
HCO3- gradients become less favorable for the anion
exchange process as fluids are moving through, and are being processed by the
intestine.
This argument leads to examination of the thermodynamics of intestinal
anion exchange to assess the potential contribution of this transport process
to active Cl- absorption in different intestinal segments in
vivo. The Nernst equation (below) allows for calculation of the
equilibrium potentials (Veq, in V) associated with ion
gradients across cell membranes and thereby to evaluate if ionic gradients
display equilibrium or are the product of active transport:
 |
where [Xo] and [Xi] refer to the concentrations of the ion in question outside and inside the cell respectively and R,T,F,z have their usual meaning. Note that the natural logarithm is replaced with the 10-based logarithm by applying 2.303 as correction factor.
The electrochemical potential (Vec, in V) acting on the ion in question is simply the difference between the relevant membrane potential (Vm) and Veq.
Considering Cl- distribution across the apical membrane in the
anterior intestine with a luminal Cl- concentration of 93 mmol
l-1 (Taylor and Grosell,
2006a), an intracellular Cl- concentration of 30 mmol
l-1 (Duffey, 1979;
Stewart et al., 1980) and an
apical Vm of -100 mV
(Loretz, 1995), V for
Cl-ec is -71 mV at room temperature. Thus, cellular
Cl- is far above thermodynamic equilibrium across the apical
membrane and apical Cl- absorption must occur via
(secondary) active transport.
The feasibility of the combined movement of two or more ions via the same transport protein can be evaluated by considering the sum of Vec values for the ions in question. Thus apical equimolar anion exchange, transport in the direction of Cl- absorption and HCO3- secretion, can occur when V (Cl-ec)+[-Vec(HCO3-)]>0. Note that the contribution from Vec(HCO3-) is negative since transport is in the opposite direction to Cl-.
At present, there are no measurements of cytosolic
HCO3- concentrations in intestinal epithelial cells but
all other components of the equation are available (Vm,
luminal Cl- and HCO3- and cytosolic
Cl- concentration). With this information and the observation that
HCO3- secretion occurs in vivo in the anterior
and to some extent in the middle segment of the toadfish intestine, cytosolic
concentrations of HCO3- required for activity of apical
anion exchange [Vec(anion exchange)>0] can be
estimated. Calculated electrochemical potentials for apical anion exchange
determined for cytosolic HCO3- concentrations ranging
from 1.5 to 40 mmol l-1 are presented in
Fig. 5. From these calculations
it is obvious that the cytosolic HCO3- concentration
must be >10 mmol l-1 for anion exchange to occur in the gulf
toadfish in vivo in the anterior region of the intestine and 40
mmol l-1 for anion exchange to occur in the mid region.
|
These values agree fairly well with the estimated cytosolic
HCO3- concentration of >9.1 mmol l-1
required to sustain HCO3- secretion in the Pacific
sanddab (Grosell et al.,
2001). The calculations presented in
Fig. 5 are based on the
assumption that cytosolic Cl- and HCO3-
concentrations remain constant along the intestine and demonstrate why there
is no further increase in luminal HCO3- concentrations
from the anterior and mid segments to the more distal segments. At any given
cytosolic HCO3- concentration, the gradual depletion of
luminal Cl- concentrations, combined with the build up of
HCO3- concentrations in the more distal segments, shifts
Vec(anion exchange) to being less favorable for
Cl- absorption and HCO3- secretion. The
experimental conditions used for in vitro measurements are different
in having low HCO- concentrations and identical
Cl-3 concentrations in the lumen of all intestinal
segments. These in vitro conditions impose fewer thermodynamical
constraints on apical anion exchange than is the case in vivo,
especially for the distal segments of the intestine, and explain why distal
segments display high HCO3- secretion rates in
vitro but not in vivo.
Cytosolic HCO3- concentrations of 10-40 mmol
l-1 seem high compared to the 1.5 mmol l-1
HCO3- estimated from the assumed intracellular pH of
7.4, PCO2 of 2.3 mmHg (307 Pa) and no CO2
diffusion limitation (Grosell et al.,
2001). However, based on trout white muscle, which displays 4-5
times higher PCO2
(Wang, 1998), it seems
reasonable to assume that intracellular PCO2 might be also
be substantially higher in the intestinal epithelium and that this might
result in higher cytosolic HCO3- concentrations. In
addition, production of endogenous CO2 may occur at high rates in
this metabolically active tissue, acting to further increase
PCO2 and thus HCO3- concentrations.
Based on these considerations alone, cytosolic HCO3-
concentrations of 10 mmol l-1 may not seem unrealistic.
Local, high HCO3- concentrations in
micro-environments in close proximity of the apical anion exchanger may
explain Cl-/HCO3- exchange under conditions
where >>10 mmol l-1 cytosolic HCO3- is
required and could be the result of recently discovered physical interactions
between carbonic anhydrase II (CAII) and members of the anion exchanger family
(AE1, AE2, AE3 and DRA) (Sterling et al.,
2001; Sterling et al.,
2002). Binding of functional CAII to the COOH terminus of AE1,
AE2, AE3 and DRA has been demonstrated by coimmunoprecipitation and is
required for maximal HCO3- transport activity in mammals
(Sterling et al., 2001;
Sterling et al., 2002). The
direct interaction between AE and CAII, effectively forming a metabolon, is
believed to accelerate HCO3- production and transport by
minimizing the diffusional distance between CAII and the anion exchanger
(Sterling et al., 2002), and
has been reported to stimulate CAII activity directly
(Scozzafava and Supuran,
2002).
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Absorbate composition |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
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Assuming no anion secretion, the `missing' cation(s) in the absorbed fluid
must be equal in charge to the difference between the calculated
concentrations of Na+ and Cl-. The calculation of
osmotic pressure in fluids absorbed (Table
2) by marine fish intestines took the concentration of the missing
cation(s) into account, and assumed mono-valence and an osmotic coefficient of
1 for solutes in the absorbate. Under these assumptions, with a single
exception (Fundulus heteroclitus), absorbate is highly hyperosmotic
with respect to both luminal and the extracellular fluids, which is in marked
contrast to the iso-osmotic fluid absorbed by other vertebrate epithelia, the
gall bladder, the renal proximal tubule and the small intestine in tetrapods
(Larsen, 2000;
Larsen et al., 2002;
Nedergaard et al., 1999).
Admittedly, the osmotic coefficient of NaCl is less than 1 and would be
even lower if the absorbed fluids were dominated by MgSO4 (0.91 and
0.56, respectively) (Taylor and Grosell,
2006b). However, considering that NaCl likely is the dominating
osmolyte in fluids absorbed by the intestine and a conservative osmotic
coefficient of 0.8, the calculated osmotic pressure of fluids absorbed by a
range of marine fish (Table 2)
still appears to be hyperosmotic. If the difference between Cl- and
Na+ absorption is accounted for by absorption of divalent rather
than a monovalent cations, the predicted osmotic pressure would be reduced
because a lower molar concentration would be required to offset the apparent
charge imbalance. Similarly, considering that part of the difference between
Cl- and Na+ absorption could be accounted for by
transepithelial HCO3- secretion, the true anion-cation
Gab would be less than assumed and a `missing cation' would thus contribute
less to the osmotic pressure of absorbed fluids.
While the result of these three factors (reduced osmotic coefficient, absorption of divalent cations and transepithelial HCO3- secretion) may reduce the estimated osmotic pressure listed in Table 2, absorbate still appears hypertonic in most cases. This is clear in at least 6 of 10 cases in which Na+ concentrations were calculated to be substantially higher than 150 mmol l-1, which is typical of iso-osmotic fluids transported by other vertebrate epithelia.
The reason for this difference between the marine teleost intestine and other vertebrate leaky epithelia involved in water absorption is unknown but may be related to the unique challenges faced by the marine fish intestinal epithelium.
In terrestrial vertebrates, the gall bladder and the renal proximal tubules
in vivo are exposed to solutions of similar composition on the
luminal and serosal epithelial surfaces, although this is not necessarily the
case for the intestine of these organisms. Intestinal fluids in terrestrial
vertebrates are dominated by monovalent electrolytes, like Na+,
Cl- and K+ and, to some extent,
HCO3- (Karasov and
Hume, 1997). In contrast to these epithelia, the marine intestine
is exposed to a rather unique luminal environment dominated by divalent ions,
especially Mg2+ and SO2-4, which are
effectively excluded by the intestinal epithelium. Marine fish are in excess
of both Mg2+ and SO2-4 and the main
homeostatic control of these divalent electrolytes appears to be renal
secretion (Marshall and Grosell,
2005). However, only a modest fraction of Mg2+ and
SO2-4 ingested with seawater is absorbed via
the intestine (Marshall and Grosell,
2005). This relatively low uptake of Mg2+ and
SO2-4 is remarkable considering the up to >100-fold
concentration difference across the intestinal epithelium of both ions and the
substantial fluid absorption, and likelihood of solvent drag, displayed by
this organ. Thus, in contrast to other epithelia displaying iso-osmotic fluid
absorption, the marine fish intestine absorbs a NaCl rich solution from a
solution strongly dominated by Mg2+ and
SO2-4. Whether high luminal concentrations of divalent
ions necessitate absorption of hyper-osmotic fluid remains to be tested.
Missing cationic charge
While it appears that fluids absorbed by the marine fish intestine likely
are hyper-osmotic, the missing cation(s) required for charge balance remains
to be conclusively identified. The quantitative importance of this illusive
ion is illustrated by calculated concentration differences between
Na+ and Cl- in the absorbed fluids ranging from 30-274
mmol l-1. No study to date has evaluated the simultaneous net
transport across the intestinal epithelium of water and all electrolytes
present in luminal and serosal fluids, preventing a firm conclusion about the
identity of the ions present in absorbed fluids to be drawn. However, the
available reports appear to provide some insight into this question.
One obvious thought is that K+, which is absorbed via
NKCC (see above), accounts for (some of) the missing cationic charge in the
absorbed fluids. However, considering that seawater K+
concentrations are 10 mmol l-1 and assuming this is also the
case for fluids moving from the stomach into the intestine, it is clear that
K+ alone cannot account for the high concentrations of cationic
charge missing from the absorbed fluids. Assuming a 2 ml kg-1
h-1 drinking rate and seawater K+ concentrations, an 80%
fractional fluid absorption and a K+ concentration of 1 mmol
l-1 in the rectal fluids
(Marshall and Grosell, 2005;
Taylor and Grosell, 2006a),
the K+ concentration in absorbed fluids can be calculated to be:
(0.002 l 10 mmol l-1 K+-0.0004 l 1 mmol l-1
K+)/0.016 l=12 mmol l-1. This value falls short of
accounting for the concentrations of cationic charge presented in
Table 2. Similar considerations
based on information in table 6.3 in a recent review
(Marshall and Grosell, 2005)
reveals that the concentrations of Mg2+ and Ca2+ in
absorbed fluids are 12.5 and 2.5 mmol l-1, respectively. Valence
considered, K+, Mg2+ and Ca2+ combined may
thus account for a total of 42 mEquiv cationic charge, which seems sufficient
to offset the charge imbalance in a few of the cases summarized in
Table 2. However, it should be
recognized that some SO2-4 (25 mmol l-1 or 50
mEquiv anionic charge (Marshall and
Grosell, 2005) is likely to be present in the absorbed fluids,
which would effectively cancel out the contribution by K+,
Mg2+ and Ca2+. It should also be noted that the combined
cationic charge accounted for by K+, Mg2+ and
Ca2+ falls far short of accounting for the charge imbalance seen in
most cases (Table 2).
A more likely, but perhaps surprising, cation accounting for the apparent
imbalance is H+ arising from the CO2 hydration fueling
the apical anion exchange process. Arguments have previously been made to
suggest that basolateral H+ secretion from intestinal epithelial
cells is necessary to sustain the substantial net base secretion displayed by
the marine fish intestine (Grosell et al.,
2001; Grosell et al.,
2005). Most recently, it was demonstrated experimentally that
significant basolateral H+ secretion occurred in the gulf toadfish
intestine and that this secretion was necessary for luminal
HCO3- secretion
(Grosell and Genz, 2006).
Measured basolateral H+ secretion rates and water absorption in the
anterior intestine were found to be 0.30±0.02 µmol cm-2
h-1 and 0.92 µl cm-2 h-1, respectively,
equating to an H+ concentration of 326 mmol l-1 in the
absorbed fluids (Grosell and Genz,
2006). Such H+ concentration easily accounts for the
missing cationic charge illustrated in
Table 2 and strongly suggests
simultaneous absorption of water and protons by the intestinal epithelium. The
derived and theoretical pH of the intestinal absorbate in toadfish is highly
acidic (pH<1), which may be a common feature for most marine fish.
Absorption of such acidic fluids would appear to pose a challenge for the
paracellular space and the interstitium, but fluids contained in these
compartments will buffer the acidic absorbate at least to some extent and it
is unlikely that such extremely low pH values can actually be found in the
paracellular space or the interstitium.
It is worth noting that the measurements of serosal H+ secretion
in the gulf toadfish were performed using pH-stat techniques, which arguably
favor apical HCO3- secretion and therefore basolateral
H+ excretion. This in turn may result in an overestimation of
H+ concentrations in fluids absorbed by the intestine under in
vivo-like conditions. However, similar conclusions of a likely acidic
absorbate were derived from measurements using gut sacs in which
HCO3- was allowed to accumulate in the lumen during
measurements (Grosell et al.,
2005).
While intestinal HCO3- secretion is not directly
involved in dynamic acid-base balance, adjustment of acidic fluid absorption
and transepithelial HCO3- transport from the
extracellular fluids to the intestinal lumen, in response to changing
environmental salinity, must have consequences for systemic acid-base balance.
Indeed, experimentally induced elevations in intestinal
HCO3- secretion in vivo result in elevated
branchial secretion of acid-equivalents and reduction of plasma total
CO2 levels in the European flounder
(Wilson and Grosell,
2003).
|
Conclusions |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
|---|
; Wilson,
1999; Wilson et al.,
2002). Apical anion exchange explains high luminal
HCO3- concentrations and alkaline conditions in the
marine teleost intestine and is responsible for a considerable portion of
intestinal Cl- uptake. Hydration of endogenous metabolic
CO2, partly mediated by CA, provides HCO3- as
cellular substrate for the anion exchange process by effectively exchanging
the metabolic waste product (CO2) for uptake of an osmolyte, which
allows for cellular water uptake. Basolateral extrusion of H+
arising from the hydration reaction is critical for luminal
HCO3- secretion and the basolateral NKA provides energy
for both secondary active uptake of Cl- and the secretion of
HCO3- by establishing conditions favorable for
basolateral Na+:H+ exchange, presumably via a
NHE-like protein. Transepithelial HCO3- transport
contributes to intestinal HCO3- secretion and may be
mediated by a basolateral NBC-like carrier. Thermodynamic considerations
predict high cellular HCO3- concentrations in the
intestinal epithelium, which may in part be accounted for by high metabolic
activity and by a close association of AE and CA. Although all intestinal segments when isolated exhibit the capacity for anion exchange it seems that predominantly the anterior segment performs HCO3- secretion and Cl- absorption via anion exchange in vivo. Altered chemistry of luminal content, as fluids are moving along the intestine, pose thermodynamic restraints on the anion exchange process in the distal portion of the intestine. An unavoidable consequence of the intestinal anion exchange is absorption of what appears to be an acidic fluid, which also is also likely hyperosmotic.
|
Future directions |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
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|
Acknowledgments |
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|
References |
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TOPSummaryOsmoregulation in marine fishIntestinal transport processesAnion exchangeIn vitro versus in...Absorbate compositionConclusionsFuture directionsReferences |
|---|
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