Time course of the response of mitochondria from oxidative muscle during thermal acclimation of rainbow trout, Oncorhynchus mykiss
Département de Biologie, Université Laval, Québec, Canada, G1K 7P4
* Author for correspondence (e-mail: helga.guderley{at}bio.ulaval.ca)
Accepted 2 July 2003
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Summary |
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Key words: mitochondria, thermal acclimation, oxidative muscle, enzyme activity, thermal compensation, rainbow trout, Oncorhynchus mykiss
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Introduction |
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In rainbow trout (Oncorhynchus mykiss), cold acclimation and
acclimatisation decrease the ratio of phosphatidylcholine to
phosphatidylethanolamine (PC/PE) in membranes from liver mitochondria
(Miranda and Hazel, 1996),
increase the capacity of skeletal muscle mitochondria to oxidise pyruvate and
acyl carnitines and increase polyunsaturation of mitochondrial phospholipids
(Guderley et al., 1997
). Cold
acclimation and acclimatisation also increase the activity of some
mitochondrial enzymes: ß-hydroxyacyl CoA dehydrogenase
(Guderley and Gawlicka, 1992
),
cytochrome c oxidase (CCO), citrate synthase (CS) and carnitine
palmitoyl transferase (CPT) (St. Pierre et
al., 1998
). Furthermore, the cristae surface density of
mitochondria (St. Pierre et al.,
1998
) and the total mitochondrial volume in oxidative muscle
fibres increase at low acclimatisation temperature
(Egginton et al., 2000
).
However, cold acclimation of rainbow trout does not increase the proportion of
oxidative fibre volume occupied by mitochondria
(St. Pierre et al., 1998
;
Egginton et al., 2000
).
Little is known about the time course of thermal acclimation and, in
particular, the time course of changes in mitochondrial capacities. During
cold acclimation (from 15°C to 5°C) of goldfish (Carassius
auratus), enzymatic activities initially decrease (between 6 h and 12 h)
and subsequently stabilise between 48 h and 72 h
(Lehmann, 1970). The PC/PE in
plasma membranes of rainbow trout kidney decreases after 8 h of cold
acclimation (from 20°C to 5°C;
Hazel and Landrey, 1988a
). The
proportions of saturated and monounsaturated fatty acids change most rapidly
(16-48 h), while long-chain polyunsaturated fatty acids only increase after
10-21 days of thermal acclimation (Hazel
and Landrey, 1988b
). The beginning of warm acclimation (from
9°C to 28°C) of white sucker (Catostomus commersoni) leads to
an induction of heat-shock protein 70 in glycolytic muscle, suggesting
degradation of protein (Hardewig et al.,
2000
). The concomitant decrease of CS activity may reflect the
lack of protection against degradation in the mitochondrial matrix.
Our study examines the time course of changes in oxidative capacities of
mitochondria from oxidative muscle of trout during warm (5°C to 15°C)
and cold (15°C to 5°C) acclimation. Trout were studied during the
initial thermal change and during acclimation to the new temperature. Although
these temperatures are well within the range naturally experienced by trout
(Thibault et al., 1997), their
best performance is observed after acclimatisation to intermediate
temperatures (Taylor et al.,
1996
). By examining mitochondrial substrate oxidation at 5°C
and 15°C, as well as the concentrations of mitochondrial components, we
sought to evaluate the potential mechanisms by which oxidative capacities
change. Thus, we measured (1) the levels of ADP-ATP translocase (ANT), the
inner mitochondrial membrane carrier of nucleotides that constitutes a point
of control of mitochondrial respiration
(Groen et al., 1982
), (2) the
concentration of an integral component of the electron transport chain,
cytochrome b, (3) activities of CS, CPT and CCO and (4) phospholipid
and protein contents. We also measured these enzyme activities in oxidative
muscle to assess the time course of changes in the aerobic capacity of muscle
during thermal acclimation.
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Materials and methods |
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Warm acclimation (cold to warm) began in February 2001 at 5°C. Fifteen
fish were used to assess the oxidative capacities of these winter-acclimatised
trout (field temperature, 4°C). Then, water temperature was raised by
4°C over 2 days, and 12 specimens were studied during the subsequent 3
days. The water temperature was then warmed by a further 4°C over 2 days,
and 12 specimens were studied during the subsequent 3 days. During the third
and final thermal change, the water temperature was increased by 2°C over
1 day to reach 15°C. Another 12 fish were studied during the first 3 days
at 15°C. Four weeks after the beginning of the temperature increments,
eight trout were examined. Finally, 8 weeks after the beginning of the thermal
change, when we assumed the trout were completely warm acclimated, 14 trout
were used to assess final oxidative capacities.
Cold acclimation (15°C to 5°C) began in August 2001. Fifteen fish
were used to assess the oxidative capacities of summer-acclimatised trout
(field temperature, 15°C). Water temperature was decreased by 4°C
over 2 days, and 12 trout were studied during the next 3 days. Another
decrease of 4°C corresponded to the second step of acclimation, and 10
more trout were examined during the subsequent 3 days. Due to some problems
with the cooling system, the decrease to the final temperature (5°C)
occurred 6 weeks after the initial thermal change. Because cold acclimation is
typically slower than warm acclimation
(Cossins et al., 1977
), we
assessed the final capacities 10 weeks after the beginning of the thermal
change. Twelve trout were studied at each of the last two steps.
Tissue sampling
Fish were stunned by a blow to the head and rapidly killed by transection
of the spinal cord behind the head. After measuring body mass and length,
oxidative muscle on both sides was sampled. The intact caudal section,
including the tail, was frozen at -80°C for later analysis of enzyme
activities.
Isolation of mitochondria and respirometry
Mitochondria were isolated according to Guderley et al.
(1997). The mitochondrial
pellet was re-suspended in a volume of reaction buffer corresponding to
one-tenth of the mass of muscle used (i.e. 300 ml of buffer for 3 g of
muscle).
Oxygen consumption was measured at 5°C and 15°C according to
Guderley et al. (1997). For
each assay, malate was added to a final concentration of 0.38 mmol
l-1 to spark the Krebs cycle, and pyruvate or palmitoyl carnitine
was added to a final concentration of 2.38 mmol l-1 or 47.6 mmol
l-1, respectively. Oxidative phosphorylation (state 3) began with
the addition of ADP to a final concentration of 0.93 mmol l-1.
After measurement of state 4 rates, 1 mg ml-1 oligomycin was added
(state 4ol) to evaluate oxygen consumption in the absence of
oxidative phosphorylation (Estabrook,
1967
).
Cytochrome b and ANT concentrations
Cytochrome b contents were evaluated by difference spectra read
after reduction by 2 mmol l-1 succinate with electron flow blocked
between cytochrome b and cytochrome c1 by 2.28
mmol l-1 antimycin (Sherratt et
al., 1988). Difference spectra against the oxidised sample were
obtained with a double-beam spectrophotometer (Varian-Cary 210).
The concentration of ANT was measured in mitochondrial suspensions by
titration with its noncompetitive irreversible inhibitor, carboxyatractyloside
(CAT). Using the polarographic method, oxygen consumption with saturating ADP
levels (3.72 mmol l-1) was inhibited by adding small volumes (10 ml
decreasing to 0.5 ml) of a 0.1 mmol l-1 CAT solution. State 3
respiration was gradually inhibited, and the inhibition was considered
complete when addition of CAT had no further effect on oxygen uptake. The
quantity of ANT in mitochondrial suspensions corresponded to half of the CAT
needed for inhibition, because two CAT molecules bind to one ANT molecule
(Willis and Dallman,
1989).
Protein concentrations
The protein concentration in mitochondrial suspensions was determined by
the bicinchoninic acid method (Smith et
al., 1985), using 2% Triton X-100 to solubilise the membranes.
Mitochondrial protein concentrations in oxidative muscle were calculated using
CS activity in the muscle and mitochondrial preparations (U g-1 and
U mg-1 protein) (St. Pierre et
al., 1998
): mg mitochondrial protein g-1 muscle = U
g-1 muscle/U mg-1 mitochondrial protein.
Enzymatic activities
Enzymes were measured in oxidative muscle dissected from the frozen tails
and in aliquot parts of the mitochondrial preparations that had been frozen at
-80°C. Citrate synthase (CS), cytochrome c oxidase (CCO) and
carnitine palmitoyl transferase (CPT) were measured at 5°C and 15°C
according to the extraction and assay conditions in Thibault et al.
(1997), except that the
extraction buffer included 0.1% Triton X-100 and did not include
fructose-2,6-bisphosphate. Mitochondrial suspensions were diluted in a buffer
containing 50 mmol l-1 imidazole-HCl, 5 mmol l-1 EDTA,
0.1% Triton X-100 and 1 mmol l-1 reduced glutathione, pH 7.5. All
assays were run in duplicate. 1 unit of enzymatic activity (U) corresponds to
1 mmol of substrate transformed to product per minute.
Phospholipid measurements
We followed Mills et al.
(1984) in extracting total
lipids from mitochondrial suspensions using chloroform:methanol (2:1) and 1
mol l-1 sulphuric acid. The phospholipid content was evaluated by
measuring the phosphorus concentration. Phosphorus reacts with 8.5% ammonium
molybdate and is reduced by 0.2% stannous chloride, forming a blue complex
that was measured at 680 nm. The mass of phospholipid was calculated by
multiplying the mass of phosphorus by 25
(Porter et al., 1996
).
Statistical analysis
We used JMP IN 3.2.1 (SAS Institute Inc., Cary, NC, USA) to perform the
statistical analyses. Analysis of variance (ANOVA) followed by Tukey multiple
comparison a posteriori tests was used, with a level of significance
of =0.05. A logarithmic transformation of phospholipid data was
performed to obtain homogeneity of variances, but untransformed data are
shown.
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Results |
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Muscle aerobic capacities: enzymatic activities
Thermal acclimation led to considerable compensation of CS and CPT
activity. The specific activity of CS and CPT in oxidative muscle decreased by
the end of warm acclimation at both assay temperatures (P<0.05),
while CCO activities at 5°C initially increased and then returned to
initial values (Table 2). By
the end of cold acclimation, the activities of CS at 5°C and of CPT at
both assay temperatures were higher than the initial activity
(P<0.05; Table 2).
CCO activity in homogenates was unchanged by cold acclimation.
|
Changes in maximal mitochondrial capacities during thermal
acclimation
During warm and cold acclimation, the respiratory control ratios (RCR;
state 3/state 4) of isolated mitochondria oxidising pyruvate ranged from 4.5
to 11.2, while those with palmitoyl carnitine ranged from 3.5 to 9.5. For both
substrates at both assay temperatures, RCRs decreased with warm acclimation
and increased with cold acclimation. During warm acclimation, for a given
assay temperature, maximal (state 3) rates of pyruvate oxidation (expressed
over the different denominators; i.e. mg protein, ANT, cytochrome b
and phospholipids) were equivalent to those of palmitoyl carnitine, except at
the end of the experiment when pyruvate was more readily oxidised (week 8;
Figs 1,
2). During cold acclimation,
mitochondria typically oxidised pyruvate at higher rates than palmitoyl
carnitine (Figs 1,
2). The Q10 of
pyruvate oxidation was approximately 2, whereas that for palmitoyl carnitine
was approximately 2.5. Neither changed markedly with thermal acclimation.
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During warm acclimation, maximal oxidative capacities changed in a biphasic fashion at both assay temperatures. Pyruvate and palmitoyl carnitine oxidation per mg mitochondrial protein increased until week 2 (P<0.05) and then decreased markedly until week 8 (P<0.05; Figs 1, 2). When expressed per nmol ANT, state 3 rates again decreased by the end of warm acclimation, but no initial increase was apparent. With nmol cytochrome b as the denominator, the decrease was less pronounced and only significant in the case of palmitoyl carnitine oxidation. When oxidation rates were expressed per mg phospholipids, rates did not change during warm acclimation (Figs 1, 2). During warm acclimation, the fraction of CCO and CS activity needed for maximal rates of pyruvate and palmitoyl carnitine oxidation decreased by week 8 (shown in Fig. 3 for pyruvate oxidation).
|
During cold acclimation, maximal rates of pyruvate oxidation changed in a biphasic pattern with rates at both assay temperatures, first decreasing (P<0.05) and then increasing (P<0.05; Fig. 1). This biphasic pattern was apparent when rates were expressed per mg mitochondrial protein and per nmol ANT. Six weeks of cold acclimation increased rates of pyruvate oxidation per nmol cytochrome b and palmitoyl carnitine oxidation per mg protein, nmol ANT and nmol cytochrome b (P<0.05; Figs 1, 2). Rates expressed relative to mg phospholipids decreased by week 1 and subsequently rose slightly (Figs 1, 2). The proportion of mitochondrial CCO and CS activity used during maximal rates of pyruvate oxidation (state 3/CCO and state 3/CS) initially decreased and then rose slightly during cold acclimation (Fig. 3).
State 4 and state 4ol rates during thermal
acclimation
Generally, for a given assay temperature, state 4 and state 4ol
rates (per mg protein) of pyruvate oxidation were equivalent to those for
palmitoyl carnitine during warm and cold acclimation
(Fig. 4). State 4ol
rates were roughly 60% of state 4 rates, suggesting that ATPase activity
increases state 4 rates above those due to proton leak. During warm
acclimation, state 4 respiration rates for both substrates at 15°C
increased until week 2 (P<0.05) and afterwards returned to initial
rates (Fig. 4). State
4ol rates showed this pattern at both assay temperatures. During
cold acclimation, state 4 rates tended to decrease initially and then return
to or exceed initial values. This biphasic pattern was most apparent for state
4ol rates (Fig. 4).
Thus, states 3, 4 and 4ol per mg protein showed similar time
courses during thermal acclimation, with initial changes being reversed with
extended acclimation.
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Changes in muscle aerobic capacity during thermal acclimation
During warm acclimation, CS activity in mitochondrial suspensions (U
mg-1 protein) increased until week 2 and afterwards returned to
initial values (Table 3). CPT
levels increased during weeks 1 and 2 and subsequently decreased and
stabilised. CCO activities assayed at 5°C increased at week 2 and then
returned to initial values. During cold acclimation, CS activity in
mitochondrial suspensions increased after two weeks, returning to initial
values by week 10 (Table 3).
CPT activities gradually rose, while CCO activity in mitochondrial suspensions
increased considerably by week 6.
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Mitochondrial protein concentration in oxidative muscle was estimated from CS activity in mitochondrial and muscle extracts (see Materials and methods). During warm acclimation, mitochondrial protein concentration decreased from week 0 to week 2 (P<0.05) and generally remained below initial values until the end of acclimation (Fig. 5). During cold acclimation, mitochondrial protein content did not change (P>0.05). Using oxygen uptake per unit CS and the specific activity of CS in muscle, we calculated the maximal rates of oxygen uptake per g muscle. This calculation indicated that the capacity of muscle for mitochondrial substrate oxidation (nmol O2 min-1 g-1 wet mass muscle) at 15°C decreased during warm acclimation. During cold acclimation, the estimated aerobic capacity at 5°C first decreased and then increased above initial values (Fig. 5). Thus, the biphasic nature of the changes in mitochondrial capacities during warm and cold acclimation was reflected in the overall aerobic capacity of muscle.
|
Mitochondrial concentrations of phospholipids, ANT and cytochrome
b
The mitochondrial phospholipid concentration decreased from approximately
0.5 mg mg-1 protein to 0.27 mg mg-1 protein during warm
acclimation and remained at 0.27 mg mg-1 protein during cold
acclimation. Overall, phospholipids typically represented less than half the
mass of protein in the mitochondrial preparations.
During warm acclimation, ANT levels were approximately 2.5 nmol mg-1 mitochondrial protein and did not change significantly. During cold acclimation, ANT concentrations were somewhat lower (1.5-1.8 nmol mg-1 protein), increased slightly at week 1, decreased at week 6 but did not differ at other times. During both warm and cold acclimation, ANT per mg phospholipid ranged between 5 nmol mg-1 phospholipid and 12 nmol mg-1 phospholipid but showed little significant variation.
Warm acclimation significantly increased cytochrome b content from 0.49 nmol mg-1 mitochondrial protein to 1.0 nmol mg-1 mitochondrial protein, but no changes were apparent during cold acclimation. Cytochrome b remained around 1.8 nmol mg-1 phospholipid during warm and cold acclimation.
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Discussion |
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As the condition factors of the trout did not change during acclimation
experiments, it is unlikely that dietary limitations led to the responses of
muscle mitochondria. Nonetheless, the higher hepatosomatic index in
cold-acclimated and cold-acclimatised trout indicates that trout stored
reserves in their liver when exposed to cold temperatures
(Voss, 1985). This also occurs
in goldfish and lake whitefish (Coregonus clupeaformis)
(Van den Thillart and Smit,
1984
; Blier and Guderley,
1988
).
Biphasic responses during thermal acclimation were apparent for state 3, state 4 and oligomycin-inhibited rates of oxygen uptake, particularly when expressed per mg protein. During warm acclimation, these rates increased markedly at both assay temperatures until week 2. After week 3, mitochondria seemed to adjust to the warming, with rates at both assay temperatures then returning to or falling below initial values. Cold acclimation decreased state 3 rates of pyruvate oxidation at week 1 and increased these rates by week 6; states 4 and 4ol showed similar tendencies. The fact that during warm and cold acclimation biphasic patterns were observed for distinct phases of mitochondrial substrate oxidation suggests a unifying causal mechanism.
The oxidative capacities of mitochondria are set by their protein content,
the types of fatty acids and phospholipid head groups in their membranes,
their inner membrane surface area (cristae surface density) as well as the
relative levels of proteins involved in electron transport, oxidative
phosphorylation and substrate breakdown. Control of oxidative phosphorylation
by liver mitochondria is shared among several systems, including nucleotide
translocation, substrate entry and electron transport
(Groen et al., 1982). In our
study, state 3 respiration rates expressed per mg mitochondrial protein and
per nmol ANT changed almost in parallel with both substrates at both assay
temperatures, particularly during cold acclimation. This observation is
compatible with a shift in inner membrane phospholipid composition that could
modulate the transport activity of ANT without altering its concentration.
Effectively, cold adaptation of rats leads to a 20% increase in
Vmax of ANT and a 20% decrease in Km
for external ADP while shifting the phospholipid composition of mitochondrial
membranes, decreasing 18:2, increasing 20:4, as well as increasing PE and
decreasing PC (Mak et al.,
1983
).
The biphasic responses of mitochondrial oxidative capacities during thermal
acclimation may reflect modifications of the lipid portion of the
mitochondrial membrane (Hulbert and Else,
2000). A similar mechanism may underlie the parallel changes in
mitochondrial CPT and CCO activity. Interestingly, thermal transfer of rainbow
trout led to biphasic changes in various lipid components of kidney plasma
membranes (Hazel and Landrey,
1988a
,b
).
A marked short-term change in phospholipid head groups (increased PC/PE with
transfer to 20°C and a decrease with transfer to 5°C) was followed by
a gradual return to intermediate values
(Hazel and Landrey, 1988a
).
Transfer to 5°C led to a rapid increase in monounsaturated fatty acids in
PC, with the inverse occurring during transfer to 20°C. These rapid
changes were reversed during longer exposure to the new temperature.
Polyunsaturated long-chain species gradually increased during cold acclimation
(Hazel and Landrey, 1988b
).
Alternatively, the parallel changes of oxidative capacity per mg protein and
per nmol ANT could be influenced by changes in the proportion of other
rate-limiting elements.
Beyond the underlying biphasic response to thermal transfer, mitochondrial
capacities seem to be modified by different mechanisms during warm and cold
acclimation. During warm acclimation, the lack of change of state 3/cytochrome
b coupled with a twofold increase of the levels of cytochrome
b/mitochondrial protein suggests that cytochrome b levels
help to set oxidative capacities. By contrast, during cold acclimation,
mitochondrial oxidative capacities expressed per mg protein, per nmol ANT and
per nmol cytochrome b showed the same time course, and cytochrome
b/mitochondrial protein did not change. As warm and cold acclimation
could stimulate different functional responses, notably on the level of
chaperonin expression (Hardewig et al.,
2000), differences in the underlying mechanisms may be
expected.
Mitochondria from trout oxidative muscle have at least 10-fold higher capacities of CCO and CS (U mg-1 mitochondrial protein) than are used at maximal rates of pyruvate or palmitoyl carnitine oxidation. The capacity of these enzymes was exploited most completely in winter-acclimatised trout, where state 3 rates accounted for approximately 10% of CS activity and 6.5% of CCO activity. During both warm and cold acclimation, this percentage decreased, suggesting capacity limitations at other loci. Expressing mitochondrial oxidation rates relative to the maximal capacities of CCO and CS indicates considerable excess capacity at the level of these enzymes and argues against a major role limiting rates of oxidative phosphorylation.
Although CPT activities in mitochondria and oxidative muscle were 40-fold
lower than the corresponding activities of CS and CCO, trout muscle
mitochondria oxidised pyruvate and palmitoyl carnitine at similar rates. The
explanation of this paradox may lie in the fact that CPT consists of two
membrane-bound enzymes: CPT I, on the outer mitochondrial membrane, and CPT
II, on the inner mitochondrial membrane. Palmitoyl carnitine is formed by CPT
I, it then enters mitochondria through the carnitine carrier and is converted
to palmitoyl CoA by CPT II. Therefore, the forward reaction of CPT I
(palmitoyl-S-CoA + carnitine a palmitoyl carnitine + CoASH, as used in our
activity measurements) is not needed during mitochondrial oxidation of
palmitoyl carnitine. That rates of mitochondrial palmitoyl carnitine oxidation
are fourfold the activity of mitochondrial CPT suggests that the catalytic
rate of CPT II must be higher than that of CPT I. In mitochondria from rat
heart, the formation of palmitoyl CoA by CPT II is about 10-fold faster than
the formation of palmitoyl carnitine by CPT I
(Palmer et al., 1977).
Nonetheless, Palmer et al.
(1977
) found no difference
between rates of mitochondrial oxidation of palmitoyl-l-carnitine and
palmitoyl CoA. CPT activities in muscle and mitochondria from trout were
similar to those in striped bass (Morone saxatilis) and in rat heart
(Rodnick and Sidell, 1994
;
Palmer et al., 1977
). While
malonyl CoA inhibition of the forward reaction suggests that CPT I and CPT II
have similar activities in striped bass muscle
(Rodnick and Sidell, 1994
), we
know of no evaluations of the rates of the forward and reverse CPT reactions
for fish muscle.
Thermal acclimation of trout led to only a partial compensation of
mitochondrial oxidative capacities but to greater compensation of the capacity
of muscle for substrate oxidation. Typically, mitochondrial respiration rates
measured at 5°C in 5°C-acclimated individuals were slower than the
same rates measured at 15°C in 15°C-acclimated fish. Furthermore, cold
acclimation seemed incomplete even after 10 weeks, as mitochondrial oxidative
capacities after 10 weeks of cold acclimation were weaker than rates for
winter-acclimatised trout. On the other hand, after 8 weeks of warm
acclimation, mitochondrial oxidative capacities resembled those for
warm-acclimatised trout. The capacity of muscle for pyruvate oxidation (nmol
O2 min-1 g-1 muscle) showed marked thermal
compensation during warm and cold acclimation, as did the capacity for
palmitoyl carnitine oxidation during cold acclimation. These estimates
combined maximal rates of mitochondrial substrate oxidation per unit CS with
CS activities in muscle. CS and CPT activities in mitochondria and in
oxidative muscle decreased during warm acclimation and increased during cold
acclimation. However, cold acclimation led to marked changes of CCO activity
per mg mitochondrial protein (present study) without changing muscle CCO
activities (present study; Guderley and
Gawlicka, 1992). The distinct patterns observed in mitochondrial
and muscle enzyme activities could reflect distinct populations of
mitochondria, one more easily extracted than the other, or could reflect
differential extraction of the mitochondrial enzymes from oxidative
muscle.
Phospholipid concentrations per mg mitochondrial protein changed little
during thermal acclimation. The lack of significant differences in
phospholipid concentrations and in respiration rates per mg phospholipid
during thermal acclimation is consistent with literature reports. Studies on
goldfish brain (Roots, 1968),
goldfish gill mitochondria (Caldwell and
Vernberg, 1970
), carp (Cyprinus carpio) liver
mitochondria (Wodtke, 1978
),
carp oxidative muscle mitochondria
(Wodtke, 1981
) and heart and
liver mitochondria and microsomes from sea bass (Dicentrarchus
labrax; Trigari et al.,
1992
) indicate a constant stoichiometry of phospholipid/protein
with thermal acclimation. The levels of ANT and cytochrome b per mg
phospholipid changed little during thermal acclimation. Thus, the basic
membrane protein/phospholipid stoichiometry seems to change little during
thermal acclimation, despite changes in fatty acid composition. Mitochondrial
oxidative capacities (states 3, 4 and 4ol per mg mitochondrial
protein for both substrates and assay temperatures) consistently changed more
during the first three steps of warm acclimation than cold acclimation. This
is consistent with literature reports that warm acclimation occurs more
quickly than cold acclimation (Cossins et
al., 1977
) and with the underlying thermal dependence of
biological processes. The more pronounced cold compensation in
winter-acclimatised trout (St. Pierre et
al., 1998
; Guderley and St.
Pierre, 1999
) compared with cold-acclimated trout
(Guderley and Gawlicka, 1992
;
present study) may reflect the additional influence of photoperiod.
In conclusion, we showed that thermal acclimation caused a biphasic response in many mitochondrial oxidative capacities, with the pattern observed during cold acclimation being an inverted image of that observed during warm acclimation. Particularly during cold acclimation, mitochondrial oxidative capacities expressed per ANT followed much the same time course as oxidative capacities expressed per mg mitochondrial protein. Fewer differences were observed when rates were expressed relative to cytochrome b. These responses are compatible with a major influence of the phospholipid and fatty acid composition of the inner membrane and of the concentration of proteins in the electron transport system upon mitochondrial oxidative capacities.
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Acknowledgments |
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References |
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