Postprandial increases in nitrogenous excretion and urea synthesis in the giant mudskipper Periophthalmodon schlosseri
1 Department of Biological Science, National University of Singapore, Kent
Ridge, Singapore 117543, Republic of Singapore
2 Natural Sciences, National Institute of Education, Nanyang Technological
University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore
* Author for correspondence (e-mail: dbsipyk{at}nus.edu.sg)
Accepted 10 June 2004
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Summary |
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Key words: amino acids, ammonia, ammonia excretion, feeding, glutamine, mudskipper, nitrogen metabolism, ornithine-urea cycle, Periophthalmodon schlosseri, urea
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Introduction |
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Mudskippers (Periophthalmus spp., Boleophthalmus spp., Scartelaos spp. and Periopthalmodon spp.) are euryhaline and amphibious gobioid teleosts (Order: Perciformes, and Family Gobiidae) usually found in mangrove swamps and estuaries.
They are highly adaptable to different environmental conditions
(Clayton, 1993;
Chew et al., 2004
). The mud
deposited by the river at the estuary forms a suitable habitat for these
mudskippers to thrive and build their burrows. During the breeding season, the
female lays eggs inside the burrow, and the male stays therein to take care of
the developing embryos.
The giant mudskipper Periophthalmodon schlosseri can be found on
muddy shores in estuaries and in the tidal zone of rivers in Singapore
(Ip et al., 1990), Indonesia,
New Guinea, India, Peninsular Malaysia, Sarawak and Thailand
(Murdy, 1989
). It is
carnivorous and can grow up to 27 cm in length. Periophthalmodon
schlosseri is the only species of mudskipper that has not been found
outside the tropics. It can survive aerial exposure much better than other
mudskippers (Ip et al., 1993
;
Kok et al., 1998
), in part due
to its specialized gill morphology and morphometry (Low et al.,
1988
,
1990
; Wilson et al.,
1999
,
2000
). While other mudskipper
species build burrow on soft mud and disappear into the burrow twice a day
during high tides, P. schlosseri builds its burrow on high ground and
usually swims along the water's edge when the tide is high.
Gordon et al. (1969,
1978
) reported that when the
mudskipper Periophthalmus sorbinus was exposed to terrestrial
conditions for 12 h, urea production increased more than threefold. However,
Gregory (1977
) could not
detect activity of some ornithine-urea cycle (OUC) enzymes, including
carbamoyl phosphate synthetase (CPS), argininosuccinate synthetase and
argininosuccinate lyase from the liver of Periophthalmus expeditionium,
Periophthalmus gracilis, and Scartelaos histophorous. It was
therefore concluded that urea was produced in livers of these mudskippers
through uricolysis, involving urate oxidase, allantoinase and allantoicase.
The activities of arginase and urate oxidase in their livers are high enough
to account for the rate of urea excretion
(Gregory, 1977
). Working on
the mudskippers Periophthalmus modestus (as P. cantonensis)
and Boleophthalmus pectinitrostris, Morii
(1979
) and Morii et al.
(1978
,
1979
) reported that ammonia
was not detoxified to urea in these mudskippers during aerial exposure. Iwata
et al. (1981
) and Iwata
(1988
) also reported that urea
production remained unchanged in P. modestus exposed to environmental
ammonia or terrestrial conditions. When P. modestus was exposed to
15N-labelled ammonia, urea-N was only slightly labelled
(Iwata and Deguichi, 1995
).
Recently, Lim et al. (2001
)
confirmed that no N-acetylglutamate activated CPS activity could be detected
(detection limit=0.001 µmol min-1 g-1) from the liver
mitochondria of Boleophthalmus boddaerti. Taking all these results
together, it can be concluded that urea synthesis de novo may not
occur in Periophthalmus spp., Scartelaos spp. or
Boleophthalmus spp.
To date, the only mudskipper that possesses a full complement of hepatic
OUC enzymes, in spite of uncertainty on the type of mitochondrial CPS present,
is the giant mudskipper P. schlosseri
(Lim et al., 2001). However,
similar to other mudskipper species, detoxification of ammonia to urea does
not occur in P. schlosseri confronted with adverse environmental
conditions such as aerial exposure (Ip et
al., 1993
; Lim et al.,
2001
), alkaline environmental pH
(Chew et al., 2003
) and
environmental ammonia (Peng et al.,
1998
; Randall et al.,
1999
). Instead, P. schlosseri adopts other strategies to
defend against ammonia toxicity (Ip et al.,
2001a
,
in press
;
Randall et al., 2004
;
Chew et al., 2004
). It is
capable of actively excreting NH4+ against an ammonia
concentration (Randall et al.,
1999
; Ip et al.,
2004a
) or in a medium with alkaline pH
(Chew et al., 2003
),
manipulating the pH of the external environment
(Chew et al., 2003
;
Ip et al., 2004a
), and
altering the phospholipid composition of its skin to reduce the influx of
NH3 during environmental ammonia exposure
(Ip et al., in press
). In
addition, it can detoxify ammonia to glutamine when exposed to high
concentrations of environmental ammonia
(Peng et al., 1998
), and
reduce ammonia production and undergo partial amino acid catabolism during
aerial exposure (Ip et al.,
2001b
, Lim et al.,
2001
). With the development of all these mechanisms, it remains an
enigma as to why there is still the need to express the OUC in the liver of
adult P. schlosseri.
Although the expression of OUC is known to occur in fish embryos
(Depeche et al., 1979;
Wright, 1995
;
Chadwick and Wright, 1999
;
Terjesen et al., 2000
), the
presence of a functional OUC in the liver of adult teleosts is rare, except
for the Lake Magadi tilapia Alcolapia grahami
(Randall et al., 1989
), the
gulf toadfish Opsanus beta
(Mommsen and Walsh, 1989
;
Anderson and Walsh, 1995
) and
certain catfishes (Heteropneustes fossilis and Clarias
batrachus) from India (Saha and
Ratha, 1994
; Saha et al.,
1997
,
1999
; for a contrary view on
C. batrachus, see Ip et al.,
2004b
; Chew et al.,
2004
). Being the only mudskipper that is carnivorous (other
species are either herbivorous or omnivorous), we suspected that the presence
of the OUC in P. schlosseri could be related to its high protein diet
(mangrove crabs and small fishes), and is involved in the fish's defence
against postprandial ammonia toxicity. A postprandial surge in plasma ammonia
level is known to occur in several fish species
(Kaushik and Teles, 1985
;
Wicks and Randall, 2002
).
Therefore, this study was undertaken to determine the effects in P.
schlosseri of feeding on nitrogen (N) excretion and metabolism, with
special emphasis on the role of urea synthesis in ammonia detoxification. The
hypothesis tested was that feeding would induce increased urea synthesis and
urea excretion in this mudskipper.
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Materials and methods |
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Feed analysis
The wet mass of guppies was obtained to the nearest mg. Samples of guppies
were then freeze-dried and the dry mass recorded. Subsequently, they were
analyzed for nitrogen (N) and carbon (C) using a Eurovector EA3011 Elemental
Analyzer (Milan, Italy) equipped with the Callidus software. BBOT
(C26H26N2O2S) standard obtained
from Eurovector was used as a standard for comparison. In addition some
samples were extracted in 70% ethanol for 24 h to remove non-protein
N-compounds, before freeze-drying for nitrogen and carbon analyses. The
difference in values between samples with and without ethanol extraction
revealed the combined contribution of ammonia, urea, free amino acids (FAAs),
purines and pyrimidines to the N and C contents of the guppy.
To determine the content of ammonia, urea, FAAs and protein-bound amino acids (PAAs) in guppies, samples were weighed, ground to a powder in liquid nitrogen, and homogenized using an Ultra-Turrax homogenizer (Janke and Kundel, Staufeni, Staufen, Germany) in 5 volumes (w/v) of 6% trichloroacetic acid (TCA) at 24 000 r.p.m. three times for 20 s each, with a 10 s interval between each homogenization. The homogenate was centrifuged at 10 000 g at 4°C for 15 min to obtain the supernatant and precipitated proteins.
The pH of the supernatant was adjusted to 5.5-6.0 with 2 mol l-1
KHCO3. Ammonia was assayed using the method of Bergmeyer and
Beutler (1985). Urea was
determined colorimetrically by the method of Jow et al.
(1999
). For FAA analysis, the
supernatant obtained was adjusted to pH 2.2 with 4 mol l-1 lithium
hydroxide and diluted appropriately with 0.2 mol l-1 lithium
citrate buffer (pH 2.2). FAAs were analyzed using a Shimadzu LC-6A amino acid
analysis system (Kyoto, Japan) with a Shim-pack ISC-07/S1504 Li-type column
(Kyoto, Japan).
The precipitated proteins were hydrolyzed with 4 mol l-1
methanesulfonic acid containing 0.2% 3-(2-aminoethyl)indole (Pierce, Rockford,
IL, USA) under vacuum in Pierce hydrolysis tubes at 115°C for 22 h by the
method of Simpson et al.
(1976). The hydrolysate was
centrifuged, adjusted to pH 2.2 with 4 mol l-1 lithium hydroxide
and diluted appropriately with 0.2 mol l-1 lithium citrate buffer
(pH 2.2) for analysis by the Shimadzu LC-6A amino acid analysis system.
Despite performing complete FAA and PAA analyses on the guppy samples, only the contents (µmol g-1 wet mass) of free and protein-bound arginine, total FAA and total PAA are presented here.
Feeding the animals
Specimens were divided into two groups. The first group of control fish was
not fed while the second group of experimental fish was allowed to feed on
guppies to 1.5% of their body mass ad libitum. The experiment was
considered to commence (time = 0 h) when the fish stopped feeding upon
satiation. The fed fish and control fish were gently transferred to individual
tanks containing 800 ml of 50% seawater. The actual mass of feed consumed by
the fish was then calculated by subtracting the mass of any leftover food from
the initial mass of food given to the fish.
Collection of water, tissue samples and feed for analyses
Water samples (3 ml) were collected at 3 h intervals during the subsequent
24 h period post-feeding, acidified with 70 µl of 1 mol l-1 HCl,
and kept at 4°C until analysis. At 0, 3, 6, 12 and 24 h, fish were killed
by a strong blow to the head, and the lateral muscle, liver, gut and brain
quickly excised. The gut was removed, flushed well with water, and divided
into two halves longitudinally. The excised tissues and organs (<1 g) were
immediately freeze-clamped in liquid nitrogen using pre-cooled tongs
(Faupel et al., 1972). Frozen
samples were kept at -80°C until analysis. Blood samples were collected
from the severed caudal peduncle into heparinized capillary tubes, and
centrifuged at 5000 g and 4°C for 5 min to obtain the
plasma. The plasma was deproteinized by the addition of an equal volume (v/v)
of ice-cold 6% TCA and centrifuged at 10 000 g and 4°C for
15 min. The resulting supernatant was kept at -25°C until analysis.
The frozen samples were weighed, ground to a powder in liquid nitrogen, and homogenized using the Ultra-Turrax homogenizer in 5 volumes (w/v) of ice-cold 6% perchloric acid at 24 000 r.p.m. three times, 20 s each, 10 s interval between each homogenization. The homogenate was centrifuged at 10 000 g at 4°C for 30 min, and the supernatant obtained was kept at -25°C until analysis.
Determination of ammonia and urea concentrations in water samples
Ammonia in water samples was determined by the method of Anderson and
Little (1986), and urea content
was analyzed as described by Jow et al.
(1999
). The rates of ammonia
or urea excreted were expressed as µmol N 3 h-1 g-1
wet mass of the fish.
Determination of ammonia, urea and glutamine in tissues samples
The deproteinized tissue samples were adjusted to pH 6.0-6.5 with 2 mol
l-1 KHCO3. Ammonia and urea contents were determined
using the method of Bergmeyer and Beutler
(1985) and Jow et al.
(1999
), respectively. Urea
content in the brain was not determined because of the small size of the brain
sample. Glutamine was determined by the method of Mecke
(1985
). Results were expressed
as µmol g-1 wet mass tissue or µmol ml-1
plasma.
Statistical analyses
Results are presented as means ± standard errors of the mean
(S.E.M.). Two-tail Student's t-test
and one-way analysis of variance followed by Duncan's multiple-range test were
used to evaluate differences between means, where applicable. Arcsine
transformation was applied to percentage data before statistical analysis.
Differences with P<0.05 were regarded as statistically
significant.
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Results |
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The mean body mass of P. schlosseri (N=17) and the wet mass of the guppies ingested were 95.8 ± 6.9 g and 0.76 ± 0.06 g, respectively. After feeding, a significant increase in the ammonia excretion rate occurred immediately between 0 and 6 h. The ammonia excretion rate of the experimental animals was greatest between 0 and 3 h (3.18 µmol N 3 h-1 g-1 wet mass fish), and was approximately 1.7-fold greater than the corresponding control value (Fig. 1). Overall, the amount of ammonia excreted within the 24 h period for fed fish (19.5 µmol N 24 h-1 g-1 wet mass) was significantly greater than the unfed control (13.8 µmol N 24 h-1 g-1 wet mass). The rate of urea excretion also increased immediately after feeding, and the increase lasted for 12 h (Fig. 2). The greatest urea excretion rate was observed at 12 h, reaching 0.714 µmol N 3 h-1 g-1 wet mass, which was 2.14-fold greater than the corresponding control value (Fig. 2). The urea excretion rates between 12 and 21 h were comparable to the corresponding controls, but increased significantly again at 21-24 h post-feeding (Fig. 2). The amount of urea excreted within the 24 h period for fed specimens (3.74 µmol N 24 h-1 g-1 wet mass) was significantly greater than in the unfed control (2.08 µmol N 24 h-1 g-1 wet mass). Specifically between 6 and 12 h, the amount of total-N excreted as urea-N increased to 26%, which was approximately 1.60-fold greater than the corresponding control value (Fig. 3).
|
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Ammonia content of the muscle remained relatively unchanged during the 24 h period post-feeding (Fig. 4). In contrast, there was a 2.2-fold increase in ammonia content in the liver at 6 h(Fig. 4), and a slight but significant increase in ammonia content in the gut at 3 h and 6 h (Fig. 4). In the brain, there were significant decreases in the ammonia content throughout the 24 h period post-feeding (Fig. 4). The ammonia concentration in the plasma decreased significantly at 3 h and returned to the normal level thereafter (Fig. 4).
|
The urea contents of the muscle and liver of P. schlosseri at 3 h post-feeding were significantly lower than the corresponding 0 h control value, but by 6 h had increased significantly (Fig. 5). There was also a significant increase in plasma urea concentration in fed fish at 6 h (Fig. 5). By contrast, the urea content of the gut remained relatively constant throughout the 24 h period post-feeding (Fig. 5).
|
Feeding had no significant effect on the glutamine content in muscle of P. schlosseri, but the glutamine content in the liver significantly decreased at 3 h and 6 h. In addition, brain glutamine content significantly decreased in the fed fish at 12 h and 24 h post-feeding (Fig. 6). By contrast, there was a significant increase in the glutamine content in the gut at 3 h post-feeding (Fig. 6).
|
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Discussion |
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In rainbow trout Onchorhynchus mykiss, the plasma ammonia
concentration increases to 37 µg ml-1 or 2.07 µmol
ml-1 8 h after feeding (Wicks
and Randall, 2002). By contrast, the rates of ammonia and urea
excretion in the giant mudskipper increased by 1.70- and 1.92-fold,
respectively, within the first 3 h post-feeding. In fact, the greatest rate of
ammonia excretion was observed at 0-3 h and returned back to normal at 6-9 h.
Since there were significant decreases in ammonia levels in plasma and brain
of P. schlosseri at 3 h post-feeding, these results indicate that
P. schlosseri had unloaded the ammonia originally present in some of
its tissues immediately after feeding (probably initiated by the feeding
action involved), in anticipation of ammonia being released by catabolism of
excess amino acids. In addition, there was a 1.4-fold increase in ammonia
excretion over the 24 h period, with the majority excreted between 12 h and 21
h. This unique pattern of ammonia excretion is part of a novel phenomenon:
unlike in other fish species (Kaushik and
Teles, 1985
; Wicks and
Randall, 2002
), there was no postprandial surge in ammonia
concentration in the plasma of P. schlosseri during the 24 h
post-feeding period. Other factors contributing to this novel phenomenon were
increased synthesis and excretion of urea (see below) in P.
schlosseri after feeding.
A significant increase in the rate of urea excretion in P. schlosseri also occurred immediately after feeding, and lasted for 12 h. At 3 h post-feeding, the urea contents in the muscle and liver decreased by 36.6% and 74.7%, respectively. Taken together, these results indicate that feeding induced an instantaneous increase in the rate of nitrogenous excretion (ammonia + urea) in P. schlosseri.
A hypothetical P. schlosseri weighing 70 g would have consumed 0.53 g of guppies or 0.84 mmol of N. By 24 h post-feeding, a total of 0.52 mmol (calculated from Figs 1 and 2) or 62.7% of the ingested N would have been excreted by this 70 g fish. Out of this 0.52 mmol N excreted, 22.6% (0.116 mmol) would be urea-N, whereas in control (unfed) fish only 13.1% of the waste-N would be excreted as urea-N. The percentage of urea-N actually excreted increased to 26% between 6 and 12 h in the fed fish, and since there were significant increases in urea content in the muscle and liver at 6 h after the initial `unloading' of urea at 3 h, it can be deduced that an increase in urea production had occurred in P. schlosseri between 3 h and 6 h post-feeding.
Increased urea synthesis in P. schlosseri after feeding
The urea excretion rate increased approximately twofold in P.
schlosseri at 6-12 h post-feeding. In addition, the urea contents of
muscle, liver and plasma increased 1.39-, 2.17- and 1.62-fold, respectively,
at 6 h, before returning back to control values at 12 h. These results suggest
that urea production increased in P. schlosseri after feeding.
Furthermore, it is apparent that the increased rate of urea production must
have been greater than that of urea excretion, in order for urea to be
accumulated in the tissues of the fed fish.
To maintain the concentration of urea in the body of the control animal at a steady state, the rate of urea excretion must balance the rate of urea production. This implies that the hourly rate of urea production in P. schlosseri at 0 h was 0.036 µmol h-1 g-1 [0.216 urea-N/(3 x 2 N); from Fig. 2], or 2.52 µmol h-1 for a hypothetical 70 g fish.
Upon feeding, a 70 g fish would have excreted 20.1 µmol urea between 3 h
and 6 h (0.57 urea-N g-1 x 70 g/2 N; from
Fig. 2). Based on values of 42
g muscle, 2 g liver and 2 ml plasma in a 70 g fish
(Lim et al., 2001), the excess
amount of urea accumulated between 3 h and 6 h can be calculated as (0.47
µmol g-1 x 42 g) + (1.18 µmol g-1 x 2
g) + (0.90 µmol ml-1 x 2 ml) or 23.9 µmol 70
g-1 fish (from Fig.
5). Thus, the amount of urea produced by a 70 g fish during this 3
h period post-feeding is equal to the sum of the amount excreted and the
amount accumulated in the body, which is 20.1 + 23.9 or 44 µmol. The hourly
rate of urea production in a 70 g specimen between 3 h and 6 h post-feeding is
therefore 44 µmol 3 h-1 or 14.7 µmol h-1, which
means that the urea production rate increased 5.8-fold (=14.7 µmol
h-1 vs. 2.52 µmol h-1), with the production
of 36.5 µmol urea in excess, within this 3 h period.
Urea can be produced via uricolysis, argininolysis or the OUC
(Campbell, 1973), but only
production via the OUC can be regarded as a synthetic process. Uric
acid can be produced via purine catabolism; however, purine, together
with other ethanol-extractable nitrogenous compounds, only had a minor
contribution to the total-N in guppies (0.32% of dry mass, or 30.2 µmol N
per 0.53 g guppies, of which at least 23.3 µmol N was contributed by FAA).
Therefore, degradation of purine from ingested guppies apparently could not
account for the amount of 58.1 µmol urea produced during the 24 h period
post-feeding (see above). The amounts of arginine present as FAA and PAA in
0.53 g of guppies consumed by a 70 g P. schlosseri were 0.28 and 38.7
µmol, respectively. Since one mole of arginine gives rise to one mole of
urea, at most 39 µmol of urea could be produced through argininolysis,
based on the highly unlikely assumption that both free and protein-bound
arginine were selectively and completely catabolized in preference to other
amino acids. Even then, this (39 µmol) could account for only 67% of the
58.1 µmol urea produced during the 24 h period. Therefore, it can be
concluded that a major portion of the urea produced by P. schlosseri
after feeding was actually synthesized de novo via the OUC. Urea
synthesis is likely to have occurred in the liver because the greatest
increase in urea content was observed therein. Since there is 2 g of liver in
a 70 g fish, the rate of urea synthesis in the liver between 3 h and 6 h
post-feeding is equal to 14.7 µmol h-1/(60 min x 2 g) or
0.123 µmol min-1 g-1, which is close to the highest
CPS activities reported for the liver of P. schlosseri (0.117 µmol
min-1 g-1; Lim et
al., 2001
).
Attempts had been made previously to elucidate the mechanisms adopted by
P. schlosseri to ameliorate ammonia toxicity during exposure to
terrestrial conditions (Ip et al.,
1993,
2001b
;
Lim et al., 2001
), alkaline
environmental pH (Chew et al.,
2003
) or environmental ammonia
(Peng et al., 1998
;
Randall et al., 1999
;
Ip et al., 2004a
). Despite
possessing all the enzymes for urea synthesis de novo in its liver,
P. schlosseri is apparently incapable of detoxifying ammonia to urea
when exposed to these experimental conditions. Thus, our results represent the
first report on the involvement of increased urea synthesis and excretion in
the defense against postprandial ammonia toxicity in the giant mudskipper.
Since all previous studies (Peng et al.,
1998
; Randall et al.,
1999
; Chew et al.,
2003
; Ip et al.,
1993
,
2001b
;
Lim et al., 2001
) on P.
schlosseri were performed using fasted specimens, our results suggest
that, perhaps, an ample supply of energy resources, e.g. after feeding, is a
prerequisite for the induction of urea synthesis.
P. schlosseri can survive aerial exposure much better than other
species of mudskippers (Ip et al.,
1993; Kok et al.,
1998
) partly because of its specialized gill morphology and
morphometry (Low et al., 1988
,
1990
; Wilson et al.,
1999
,
2000
); but fusions of
secondary lamellae would impose inefficiency in the branchial excretion of
ammonia in water. Therefore, in addition to an increase in the rate of ammonia
excretion after feeding, increased urea synthesis is essential in preventing a
postprandial surge of ammonia. However, P. schlosseri remained
ammonotelic throughout the 24 h period post-feeding. With the excess urea
accumulated in the body at 6 h being completely excreted between 6 and 12 h,
the percentage of waste-N excreted as urea-N increased significantly to 26%,
but never exceeded 50%, the criterion for ureotely. In this respect, P.
schlosseri is different from the ammonotelic, but ureogenic, slender
African lungfish Protopterus dolloi, which becomes ureotelic after
feeding (Lim et al., in
press
).
Ammonia and glutamine contents in brain of P. schlosseri decreased significantly after feeding
The mechanisms involved in defense against postprandial ammonia toxicity in
P. schlosseri were so effective that the ammonia level in the brain
decreased throughout the 24 h period. Consequently, P. schlosseri
exhibited a phenomenon different from other fishes with respect to the
response of brain glutamine content to feeding. It is well known that fish
brains are protected from ammonia toxicity by glutamine synthetase
(Mommsen and Walsh, 1991), and
that ammonia levels in the fish brain increases significantly after feeding,
leading to increased glutamine synthesis and its accumulation therein
(Wicks and Randall, 2002
;
Lim et al., in press
). By
contrast, feeding led to a significant decrease in the brain glutamine level
of P. schlosseri between 12 h and 24 h. This observation is
consistent with the fact that there was an absence of any postprandial ammonia
surge in the plasma throughout the 24 h period post-feeding.
Conclusion
The rates of ammonia and urea excretion increased significantly in P.
schlosseri after feeding. It would appear that P. schlosseri was
capable of unloading ammonia and urea from its body (over a 0-3 h period
post-feeding) in anticipation of an increase in ammonia production from
catabolism of excess amino acid. In addition, the rate of urea synthesis
increased 5.8-fold between 3 h and 6 h. These adaptations effectively
prevented a postprandial surge of ammonia in P. schlosseri, as has
been reported previously for other fish species. Consequently, there were
significant decreases in the ammonia content in the brain of P.
schlosseri throughout the 24 h period post-feeding. In addition, unlike
in other fish species, the brain glutamine content decreased significantly
after feeding (12-24 h). Similar to P. schlosseri, certain adult
teleosts such as the largemouth bass Micropterus salmoides and the
plainfin midshipman Porichthys notatus are known to be ureogenic (for
a review, see Anderson, 2001),
although it has been suggested that the OUC and CPS in these ammonotelic
fishes may not have a significant physiologically function
(Anderson, 2001
). Perhaps
future work should aim to elucidate if urea synthesis de novo plays
an important role in defense against postprandial ammonia toxicity in these
ureogenic fishes as reported herein for the giant mudskipper P.
schlosseri.
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References |
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