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First published online May 21, 2007
Journal of Experimental Biology 210, 1935-1943 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.005371
Responses to hypoxia and recovery: repayment of oxygen debt is not associated with compensatory protein synthesis in the Amazonian cichlid, Astronotus ocellatus
1 Ocean Sciences Centre, Memorial University of Newfoundland, St. John's,
NL, A1C 5S7, Canada
2 Laboratory of Ecophysiology and Molecular Evolution, Instituto Nacional de
Pesquisas da Amazônia, Alameda Cosme Ferreira, 1756, 69.083-000, Manaus,
Amazonas, Brazil
* Author for correspondence (e-mail: jmlewis{at}mun.ca)
Accepted 5 March 2007
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Summary |
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Key words: hypoxia, recovery, routine metabolic rate, protein synthesis, lactate production, Astronotus ocellatus
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; Lutz and Nilsson,
1997). The latter approach allows survival for longer periods of
hypoxia/anoxia because of the conservation of energy and the limited
accumulation of toxic end products, such as lactate. However, there is a
trade-off to this approach as deep metabolic depression impairs the animal's
ability to respond to external stimuli and leaves the animal vulnerable to
predators.
The majority of successful oxyconformers are ectotherms that survive short
bouts of hypoxia at warm temperatures, but require seasonal or behaviourally
regulated decreases in environmental/body temperature to survive extended
anoxia. Such animals are the crucian carp (Carassius carassius),
goldfish (Carassius auratus), common frog (Rana temporaria)
and two species of freshwater turtle (Chrysemys picta bellii and
Trachemys scripta elegans)
(Boutilier, 2001). In these
animals a depression of metabolic rate by 70–95% occurs during
hypoxia/anoxia, based on oxygen consumption rates or calorimetry
(Van Waversveld et al., 1989;
Jackson, 1968). This
depression at the whole animal level is accompanied by tissue-specific
decreases in protein synthesis of 50% in crucian carp
(Smith et al., 1996) and 70 to
>90% in freshwater turtles (Land et
al., 1993; Bailey and Driedzic,
1996; Fraser et al.,
2001). Protein synthesis is one of the major energy consuming
processes, accounting for 18–26% of cellular energy expenditure
(Hawkins, 1991). As such, the
downregulation of protein turnover is one of the major contributing factors to
the depression in ATP turnover and metabolic depression at the whole animal
level (Guppy et al.,
1994).
Animals that are exposed to a prolonged period of oxygen deprivation
accumulate an oxygen debt that is repaid during recovery by a substantial
increase in oxygen consumption. This oxygen debt has been shown to occur at
both the whole animal and tissue levels in goldfish after extended hypoxia
exposure (Van den Thillart and Verbeek,
1991; Johansson et al.,
1995). Johansson et al.
(Johansson et al., 1995)
predicted that a substantial increase in protein turnover would accompany the
repayment of the oxygen debt, but a consistent pattern in protein synthesis
during recovery from hypoxia has not been found. For example, an in
vitro study on turtle hepatocytes exposed to 12 h of anoxia showed a
significant overshoot in protein synthesis rates to 160% of normoxic levels
during recovery (Land et al.,
1993). However, in vivo studies on crucian carp and a
freshwater turtle species did not show hyperactivation of protein synthesis
during post-anoxic recovery (Smith et al.,
1996; Fraser et al.,
2001).
The Amazonian cichlid, the oscar or acará-açu Astronotus
ocellatus, is an ideal species to study hypoxia-induced metabolic
depression without the confounding variable of decreased temperature. During
periods of high water, Amazon várzeas become flooded and the surfaces
of the lakes become densely covered with floating marcrophytes
(Val and Almeida-Val, 1995).
The dense surface vegetation causes extreme diurnal variation in dissolved
oxygen levels, with supersaturation occurring at midday when photosynthesis is
at its maximum and levels dropping close to zero during the night
(MacCormack et al., 2003).
A. ocellatus undergoes a significant decrease in routine metabolic
rate (RoMR; 
30%) when oxygen levels in the water reach 20% saturation and
only reverts to anaerobic metabolism once oxygen levels drop below 6%
saturation consumption, which is accompanied by a decrease in RoMR of
approximately 60% (Muusze et al.,
1998). These results suggest that A. ocellatus, like the
crucian carp and freshwater turtles, is able to maintain aerobic metabolism in
situations of oxygen deprivation by decreasing the rate of ATP turnover until
near anoxic conditions are reached. Until recently, little was known about the
cellular mechanisms behind the hypoxia-induced metabolic depression in A.
ocellatus and of its response during recovery from severe hypoxia beyond
the behavioural and physiological responses of A. ocellatus to
hypoxia (Muusze et al., 1998;
Sloman et al., 2006). Recent
studies have begun investigation into the various ATP-consuming processes that
contribute to the whole animal metabolic depression. These studies have shown
significant reduction in Na+,K+-ATPase in gill and
kidney during hypoxia exposure (Richards
et al., 2007) which is accompanied by a reduction of ion exchange
at the gills and an overall reduction in metabolic nitrogenous waste
production (urea and ammonia) (Wood et
al., 2007). The decrease in these ATP consuming processes are not
accompanied by changes in concentration of ATP
(Richards et al., 2007),
indicating that A. ocellatus is able to successfully tolerate
extended hypoxia exposure because of the reduction in key ATP-consuming
processes.
The objectives of this study were to expand our knowledge of the biochemical processes behind the hypoxia-induced metabolic depression and post hypoxia recovery in A. ocellatus through investigation of the tissue-specific protein synthesis rates in relation to the whole animal metabolic depression. In addition, the present study is the first to obtain measurements of whole animal metabolic rate and protein synthesis under similar experimental conditions on the same population of fish.
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Measurement of routine metabolic rate
Individual fish were transferred to a specially designed Plexiglas®
respirometer (15x20x40 cm; 11.875 l) supplied with
oxygen-saturated water (80–100%) from a 100 l reservoir and allowed to
acclimate for 48 h before the beginning of the experiment. Water from the
reservoir was continuously pumped through the respirometer using a submersible
pump (model NK-1, Little Giant Co., Vernon Hills, IL, USA). Water temperature
and oxygen concentration were monitored through a circuit composed of tubing
with extremely low gas permeability (Tygon® Food & LFL, Cole Palmer,
Inc., Oklahoma City, OK, USA) using a peristaltic pump (Masterflex L/S model
77200-12, Cole-Palmer) and flow-through chambers (D201, WTW, Weilheim,
Germany) containing oxygen probes (model CellOx 325, WTW) positioned in the
inflow and outflow tubing of the respirometer. Measurements of water oxygen
levels and water flow rate (range 0.8–1.0 l min–1) were
taken at hourly intervals during the experiment (i.e. during normoxia, hypoxia
and recovery from hypoxia), as well as before the placement of the fish and
immediately after the removal of the fish from the respirometer in order to
correct for bacterial O2 consumption. Bacterial O2
consumption was consistently less than 2% of the RoMR of the fish and was
therefore considered to be negligible. The RoMR of each fish was calculated at
each measurement interval as:
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w is the water
flow rate through the respirometer (l min–1) and
Mb is the mass of fish (kg) [modified from Cech
(Cech, 1990)].
Protein synthesis
Normoxia
Twelve fish were removed from the holding tank, weighed, tagged for
individual recognition and transferred to a separate experimental tank under
identical environmental conditions. After 48 h, fish were injected
intraperitoneally, without anaesthetic, with 1.0 ml 100 g–1
of [2,3-3H]phenylalanine (Amersham International) solution. This
injection solution consisted of 135 mmol l–1 phenylalanine in
a solution containing 125 mmol l–1 NaCl, 3 mmol
l–1 KCl, 1 mmol l–1
MgSO4.7H2O, 1.5 mmol l–1
CaCl2, 5 mmol l–1 Hepes (sodium salt), 5 mmol
l–1 glucose, 2 mmol l–1
Na2HPO4, pH 7.8 at 28°C, in addition to sufficient
[2,3-3H]phenylalanine to ensure a dosage of 100 µCi
ml–1. Following injection, fish were returned immediately to
the experimental tank and after an incubation time of 1, 2 or 3 h, groups of
four fish were killed by a blow to the head and immediate severing of the
spine. Brain, heart, liver, white muscle and gill tissue were excised in that
order, and frozen in liquid nitrogen. All samples were kept at –70°C
until analysis.
Hypoxia
In this treatment, 12 fish were weighed, tagged and transferred to the
experimental tank. After a 48-h acclimation period, fish were exposed to a
stepwise decrease in dissolved oxygen levels as described previously. Fish
were injected immediately once water oxygen saturation reached 10%, and four
fish were sampled (as above) at 1, 2 and 3 h after injection with DO levels
maintained at 10% for the 3 h hypoxia exposure.
Recovery
To assess changes in protein synthesis rates during recovery from hypoxia,
20 fish were exposed to an acute hypoxia challenge, as described above. After
holding fish at 10% DO for 3 h, air was bubbled into the experimental tank
allowing the dissolved O2 level to return to normoxic levels
(80–100%). Groups of five fish were injected at hourly intervals,
starting when O2 saturation levels returned to normoxic levels
(group 1) and ending 4 h after O2 returned to normoxic levels
(group 4). Each group of fish was sampled 1 h post-injection allowing protein
synthesis to be tracked over a 4-h time period during the post-hypoxic
recovery. Tissues were excised and stored as previously described.
Blood sampling for lactate
Blood samples were obtained from as many fish as possible during the
protein synthesis experiment, resulting in N=4, normoxic;
N=8, hypoxic; and N=7, recovery. Blood was drawn from the
caudal vein with a heparinized syringe prior to sampling the fish for protein
synthesis analysis. Blood samples were centrifuged and plasma was stored at
–70°C for lactate analysis.
Sample preparation and scintillation counting
The protocol used for the analysis of protein synthesis followed that of
Treberg et al. (Treberg et al.,
2005), with techniques modified from the original paper that
presents the flooding dose approach to measure rates of protein synthesis
(Garlick et al., 1980). Samples
were homogenized with a Polytron homogenizer (Brinkmann Instruments, Westbury,
NY, USA) in nine volumes of 6% perchloric acid (PCA) except for liver, which
was homogenized in four volumes of PCA. Homogenized samples were left on ice
for 10–15 min and a 1 ml aliquot was transferred to a microcentrifuge
tube. Excess homogenate (liver, muscle and in some cases brain samples), was
stored at –70°C for the analysis of lactate. The 1 ml aliquot of
homogenate for protein synthesis was centrifuged for 5 min at 15,600
g, after which the supernatant was removed and frozen at
–20°C for analysis of the total free phenylalanine content and its
radioactivity.
The protein pellet was washed by manually re-suspending the pellet in 1.0 ml of 6% PCA, vortexing, centrifuging as described above, and then discarding the supernatant. This wash step was repeated until the radioactivity in the discarded supernatant was at background levels to ensure only protein bound [3H]phenylalanine was being measured in the protein pellet. After sufficient washing, 1.0 ml of 0.3 mol l–1 NaOH was added to the tube containing the protein pellet. The protein pellet was incubated in a water bath held at 37°C until fully dissolved. The dissolved protein was stored at –20°C until analysis for protein content and protein-bound radioactivity.
Aliquots of the original supernatant from the homogenized tissue and the
dissolved protein were added to 10 ml of Ultima GoldTM liquid
scintillation cocktail and counted on a Beckman Coulter LS6500 liquid
scintillation counter to obtain the [2,3-3H]phenylalanine content
of the free and protein-bound phenylalanine pools of the tissues,
respectively. Phenylalanine specific activity was considered to be
c.p.m./phenylalanine content (nmol). This assumes negligible conversion of
radiolabelled phenylalanine to other radiolabelled components, which has been
proved with HPLC analysis in a species of mollusc
(Pakay et al., 2002). This
assumption is considered to be acceptable for the current study given the
elevated and constant c.p.m./phenylalanine ratio in all tissues over the time
course of the study and the linear rate of incorporation of radiolabel into
the protein pool for all tissues.
Biochemical assays
Free pool phenylalanine content was measured from the PCA extraction
supernatant and phenylalanine standards in 6% PCA using a fluorometric assay
following the protocol described in McCaman and Robins
(McCaman and Robins, 1962).
Protein content of the tissue was determined from the NaOH-solubilized protein
pellet by using the BioRad Dc kit (Bio-Rad Laboratories, Hercules,
CA, USA) using standards made from bovine serum albumin. Lactate was measured
in standards in 6% PCA, plasma, liver, white muscle and brain tissue
via the reduction of NAD+ to NADH at 340 nm using a Sigma
diagnostics kit.
Statistical analyses
Comparison of oxygen consumption data was carried out by using a repeated
measures ANOVA followed by a Dunnett's post-hoc test, to compare all
values with the normoxic (control) value. Lactate concentrations for normoxia,
hypoxia and recovery treatments were compared using a one-way ANOVA, with
Tukey's post-hoc test for multiple comparisons. In the protein
synthesis experiment, mean tissue phenylalanine content and specific activity
over the incubation time were compared using a one-way ANOVA with Tukey's
post-hoc test for multiple comparisons, and the incorporation of
radioactivity into protein was examined by linear regression. Once data were
confirmed to fit the validation criteria, phenylalanine incorporation rates
for each tissue were compared using a one-way ANOVA followed by a Tukey's
post-hoc test. In all cases P<0.05 was considered
significant.
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Lactate concentration
Lactate concentrations under normoxic conditions in the various tissues
were 0.04±0.03 µmol ml–1 for plasma (N=4)
and 0.25±0.06, 0.84±0.11 and 2.63±0.30 µmol
g–1 tissue for liver (N=11), brain (N=12)
and white muscle (N=6), respectively
(Fig. 2). As there were no
significant differences in lactate concentration in any of the tissues during
the 3-h hypoxia and 4-h recovery periods, results were pooled within each
treatment to give a mean value for hypoxic and recovery samples. During the
3-h hypoxic exposure, only plasma exhibited a significant increase in lactate
concentration (1.13±0.27 µmol ml–1, N=8).
During the post-hypoxic recovery period, plasma lactate returned to levels
(0.42±0.30 µmol ml–1, N=7) that were not
significantly different from pre-hypoxic values. Finally, significant
decreases in lactate concentrations in liver (0.02±0.01 µmol
g–1 tissue, N=20) and brain 0.55±0.06 µmol
g–1 tissue, N=20) occurred during the recovery
period, whereas lactate concentration in white muscle was maintained at
similar concentrations over all three treatments.
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Elevation of free phenylalanine pool content
The concentration of free pool phenylalanine in the various tissues was not
significantly different between the three treatments or between sample times
within treatments. Therefore, results for normoxic, hypoxic and post-hypoxic
fish were pooled and referred to as injected fish (N=44). Injected
fish had free phenylalanine levels of 0.55±0.04, 0.16±0.01,
0.08±0.01, 0.20±0.01 and 0.19±0.01 mmol phe
g–1 fresh tissue (N=44) for liver, white muscle,
brain, heart and gill, respectively. When compared to levels of free
phenylalanine in un-injected fish (0.12, 0.08, 0.03, 0.05, 0.09 mmol
phenylalanine g–1 fresh tissue; N=1), levels were
twofold higher in brain, white muscle and gill, and fivefold higher in liver
and heart tissue. The baseline levels of free phenylalanine from un-injected
oscars and the increase achieved via the flooding dose of
phenylalanine are comparable to levels measured in crucian carp
(Smith et al., 1999).
Intracellular free pool phenylalanine specific activity
Intracellular specific activity of the free phenylalanine pool for both
normoxic and hypoxic fish was elevated 1 h post-injection, and remained
constant over the 3 h that protein synthesis was measured
(Fig. 3). On average, the
specific activity for normoxic fish was 1039±79, 641±27,
711±37, 788±61 and 656±22 c.p.m. nmol–1
phenylalanine for liver, white muscle, brain, heart and gill, respectively
(N=12). For hypoxia-exposed fish, the specific activity for the same
tissues was 708±44, 642±17, 696±26, 622±34 and
620±14 c.p.m. nmol–1 phenylalanine (N=12). As
there was no significant difference in the specific activity between
post-hypoxia recovery sampling times, results were pooled for each tissue to
give mean values of 830±23, 641±12, 702±7.0,
665±10 and 672±17 c.p.m. nmol–1 phenylalanine
for liver, white muscle, brain, heart and gill, respectively
(N=20).
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). At the lowest level of hypoxia tested in the present study
(10% DO), lactate levels were only significantly increased in plasma. However,
this increase only brought lactate levels to one-fifth of levels obtained in
A. ocellatus at 6% DO (Muusze et
al., 1998). These results, in combination with the absence of
lactate accumulation in white muscle, indicate anaerobic metabolism is only
beginning to be employed to supplement energy demands at this level of oxygen
deprivation, and metabolic depression is an effective way of conserving ATP
until A. ocellatus is faced with almost anoxic conditions. In other
studies with comparable lengths of hypoxia exposure, levels of lactate
increased to a greater extent in plasma [4.9-fold
(Richards et al., 2007);
11–16-fold (Wood et al.,
2007)] and white muscle [2–3-fold
(Richards et al., 2007;
Wood et al., 2007)] than in
the current study. This discrepancy in lactate accumulation during hypoxic
exposure is most likely a result of the quick entry into hypoxia (1 h)
for these two studies as compared to the gradual transition into hypoxia of
our study (5 h).
The level of metabolic depression achieved by A. ocellatus is
similar to that of goldfish and crucian carp, both of which decrease metabolic
rate by approximately 70% under anoxia
(Van Waversveld et al., 1989),
but not as great as demonstrated by freshwater turtles (90–95%
reduction) (Jackson, 1968).
Metabolic depression is less in teleosts as a result of maintenance of ion
exchange with the environment and low levels of predator avoidance. For
example, in their natural environment A. ocellatus are susceptible to
predation from air-breathing fish and aerial predators, and laboratory
experiments show that they split their time equally between unprotected
normoxic environments and sheltered hypoxic environments
(Sloman et al., 2006).
Turtles, however, are essentially a closed system as they retreat into their
protective shell and enter a comatose-like state during periods of oxygen
deprivation.
Protein synthesis
The use of the flooding dose methodology to measure in vivo
protein synthesis requires that several validation criteria be fulfilled. The
results from this study show that the injection dosage used successfully
flooded the free phenylalanine pool during both normoxia and 3 h of hypoxic
exposure, causing a two- to fivefold increase in phenylalanine concentration
in the various tissues. As well, the specific activity of the free
phenylalanine pool was elevated 1 h post-injection and remained stable at this
level for the 3 h over which protein synthesis was measured. The final
validation criterion requires the incorporation of radiolabelled phenylalanine
into the tissues to be linear post-injection. This was shown for all tissues
in both normoxia- and hypoxia-exposed fish, except for white muscle
(Fig. 4). The radioactivity of
protein-bound phenylalanine in white muscle was below detectable levels,
indicating rates of protein synthesis in this tissue to be minimal. Given that
rates of protein synthesis in fish white muscle are extremely low as compared
to mammals (Fauconneau et al.,
1995), and A. ocellatus has a much lower mass specific
oxygen uptake than other teleosts, including tropical species
(Almeida-Val et al., 2006), it
is not surprising that protein synthesis was undetectable in the white muscle
of A. ocellatus.
The role of protein synthesis in hypoxia-induced metabolic depression in
ectothermic animals has been previously described in freshwater turtles
(specifically Trachemys scripta elegans and Chrysemys picta
bellii) and in the crucian carp (Carassius carassius), and these
studies show that the extent to which protein synthesis is depressed is
positively linked with the degree to which activity is curtailed. For example,
rates of protein synthesis were suppressed by approximately 70% in the heart
of T. scripta elegans (Bailey and
Driedzic, 1996) and by >95% in various tissues in C. picta
bellii; both these species enter a comatose-like state during anoxia
(Land et al., 1993;
Fraser et al., 2001). By
contrast, the crucian carp, which maintains low levels of activity during
hypoxia/anoxia exposure, exhibits a depression in protein synthesis of
approximately 50% in heart and white muscle, 95% in liver tissue, but no
significant depression in the brain (Smith
et al., 1996). Similar to the crucian carp, A. ocellatus
exhibited tissue-specific depression in protein synthesis when exposed to
acute hypoxia exposure. Rates of protein synthesis in liver, heart and gill
were depressed by 50–60%, whereas rates of protein synthesis in the
brain were only depressed by 27%. Thus, our results reinforce the idea that
fish need to maintain protein synthesis in the brain to prevent damage to
neural tissue, and to sustain appropriate brain functions so that predators
can be effectively avoided. The depression of protein synthesis in gills is
accompanied by a simultaneous decrease in Na+ pumping and leak
rates in the gills, as shown by measurement of
Na+,K+-ATPase activity and Na+ flux
(Richards et al., 2007;
Wood et al., 2007).
One of the suggested mechanisms controlling the depression of protein
synthesis, and therefore depression of metabolic rate, is a decrease in pH
(Hochachka and Somero, 2002; Richards et
al., 2007). The reduction of protein synthesis has been linked to
an increase in recombinant elongation factor 2 kinase (EF2K) caused by
exposure to low pH (Dorovkov et al.,
2002). The significant reduction of protein synthesis in liver,
brain, heart and gill tissues in hypoxia-exposed A. ocellatus,
observed in the present study, combined with the decreases in extracellular
and intracellular pH in A. ocellatus exposed to comparable hypoxic
conditions (Richards et al.,
2007) strengthens the argument of Richards et al., that pH may
have a direct effect on protein synthesis and therefore, metabolic rate in
A. ocellatus.
Recovery from acute hypoxia exposure
A significant overshoot in oxygen consumption to 270% of normoxic rates was
observed during the first hour of recovery, indicating that the 3-h hypoxic
exposure was substantial enough to cause the fish to accumulate an oxygen
debt. Crucian carp have also been shown to accumulate a substantial oxygen
debt during periods of hypoxia (Van den
Thillart and Verbeek, 1991), and it has been suggested the
hyperactivation of metabolic rate during anoxic/severe hypoxic recovery is
associated with the restoration of phosphocreatine, the conversion of lactate
into glycogen, and possibly an increase in protein synthesis
(Johansson et al., 1995).
Although there was no accumulation of lactate in the white muscle in the
present study, Richards et al. (Richards
et al., 2007) have shown a 30% decrease in creatine phosphate in
white muscle after a 4 h exposure to hypoxia, which returned to pre-hypoxia
exposure levels during recovery. As such, the substantial increase in oxygen
consumption seen in our study during the first hour of recovery may be linked
to the restoration of phosphocreatine. An in vitro study on turtle
hepatocytes exposed to 12 h of anoxia has shown a significant increase in
protein synthesis (to 160% of normoxic rates) during the first hour of
recovery (Land et al., 1993).
However, the present study, which measured in vivo protein synthesis
rates, did not show any hyperactivation of protein synthesis in the various
tissues during the recovery period. These results agree with other in
vivo studies showing a hyperactivation of protein synthesis does not
occur in anoxic exposed turtles (Fraser et
al., 2001) or crucian carp
(Smith et al., 1996). There
were two distinct patterns observed in post-hypoxic phenylalanine
incorporation in A. ocellatus. In tissues that are a main source for
protein synthesis, liver and gill, phenylalanine incorporation returned to
pre-hypoxic rates by 1 h post-hypoxia. By contrast, protein synthesis in brain
and heart take longer than 4 h post-hypoxia to fully recover. The slow
recovery in brain tissue is particularly interesting as its hypoxia-induced
reduction in protein synthesis is half of that shown by the other tissues. The
reasons for this remain elusive; however, it may be linked to the removal of a
dietary source of amino acids (due to the cessation of feeding), requiring
A. ocellatus to rely on the recycling of existing protein (i.e.
protein turnover) to replenish diminished supplies due to the decrease of
protein synthesis during metabolic depression.
Conclusions
The present study was successful in furthering the insight into the
biochemical adaptations of A. ocellatus to conditions of extreme low
oxygen to include a description of the role protein synthesis plays in
contributing to the whole animal metabolic depression. The response of A.
ocellatus to acute hypoxia and subsequent recovery, at both the
physiological and biochemical level, was similar to that of the well studied
anoxia-tolerant teleost, the crucian carp. However, there are tissue-specific
differences in the magnitude of the hypoxia-induced depression of protein
synthesis (brain 20%, other tissues 50–60%), which suggest that brain
function is maintained during hypoxia to facilitate active predator avoidance.
As well, this study demonstrated that an acute (3 h) exposure to severe
hypoxia is substantial enough to cause A. ocellatus to accumulate an
oxygen debt, but the repayment of this oxygen debt is not accompanied by a
compensatory hyperactivation in protein synthesis. This latter finding
indicates the high metabolic rate A. ocellatus during the first hour
of recovery is attributed to cellular processes other than the assimilation of
protein. Combining the results from the current work with recent discoveries
from comparable studies on the behavioural, physiological and biochemical
adaptations of A. ocellatus
(Muusze et al., 1998;
Almeida-Val et al., 2000;
Sloman et al., 2006;
Richards et al., 2007;
Wood et al., 2007) it can now
be concluded the metabolic depression observed behaviourally is achieved
through a decrease in physical activity, activation of anaerobic metabolism
and the reduction of energy consuming processes (nitrogenous waste production,
ion exchange and protein synthesis). These biochemical adaptations enable
Astronotus ocellatus to maintain stable ATP levels, and therefore
extend survival time when faced with conditions of extreme low oxygen.
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Acknowledgments |
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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O2)
measurements are shown as means ± s.e.m., N=8 fish.
*Significant differences in RoMR from time 0 (normoxia) values
(P<0.05).
