Does condition of Atlantic cod (Gadus morhua) have a greater impact upon swimming performance at Ucrit or sprint speeds?
1 Université Laval, Département de Biologie, Québec,
G1K 7P4, Canada
2 Ministère des Pêches et des Océans, Institut
Maurice-Lamontagne, 850 Route de la Mer, C.P. 1000, Mont-Joli, Québec,
G5H 3Z4, Canada
* Author for correspondence (e-mail: helga.guderley{at}bio.ulaval.ca)
Accepted 14 June 2004
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Summary |
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Key words: white muscle, red muscle, condition, Ucrit swimming, sprint swimming, aerobic metabolism, anaerobic metabolism, Atlantic cod, Gadus morhua.
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Introduction |
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In temperate habitats, fish frequently encounter marked seasonal variations
in food availability that may modify their performance. When reductions in
prey abundance are coupled with seasonal decreases in temperature, fish may
survive using the reserves accumulated during periods of high productivity.
However, generalised decreases in prey availability can cause extended periods
of starvation from which fish have difficulty recovering. In Atlantic cod,
hepatic lipid reserves are the first to be mobilised during starvation,
followed by glycogen from liver and muscle and finally by muscle protein
(Black and Love, 1986). Such
decreases in macromolecular content lead to marked increases in tissue water,
from 15 to 80% in the liver and from 77 to 92% in white muscle
(Dutil et al., 1995
; Lambert
and Dutil,
1997a
,b
).
Accordingly, prolonged starvation markedly modifies the metabolic capacities
of cod tissues (Guderley et al.,
1996
; Dutil et al.,
1998
). Just as cod white muscle is more affected by starvation
than red muscle, starvation has a lesser impact upon the aerobic than the
glycolytic capacity in both fibre types
(Martínez et al.,
2003
). The activities of four glycolytic enzymes
[phosphofructokinase (PFK), lactate dehydrogenase (LDH), pyruvate kinase (PK)
and creatine kinase (CK)] decline markedly in red and white muscle with
starvation, while the activities of two mitochondrial enzymes [cytochrome
c oxidase (CCO) and citrate synthase (CS)] are less affected. These
changes in muscle metabolic capacity are likely to reduce swimming capacity,
in particular all behaviours relying upon glycolytic capacity.
The sparing of muscle aerobic capacity during starvation
(Johnston and Goldspink, 1973;
Beardall and Johnston, 1983
;
Loughna and Goldspink, 1984
)
should favour the maintenance of routine locomotor activities at the detriment
of sprint swimming. In cod, endurance at speeds above
Ucrit is reduced 70% by a decrease in condition. As the
number of burst-coast movements is the best predictor of endurance at these
speeds, glycolytic fibres provide much, if not most, of the needed power
(Martínez et al.,
2003
). Ucrit tests assess the speed a fish can
sustain during the experimental swimming period (typically 20-40 min). While
swimming near Ucrit is supported both by oxidative and
glycolytic fibres, sprint swimming is only powered by glycolytic fibres. Given
these differences in the muscles participating in sprint and
Ucrit swimming, the loss of glycolytic capacity occurring
with a decrease in condition should have a stronger impact upon sprint than
Ucrit swimming.
Biomechanical studies of fish swimming clearly show that muscle fibres work
differently according to their position along the trunk
(van Leeuwen et al., 1990;
Altringham et al., 1993
;
Rome et al., 1993
;
Wardle and Videler, 1993
;
Jayne and Lauder 1994
;
Thys, 1997
). In Atlantic cod,
caudal white muscle fibres transmit the force generated in the rostral and
middle myotomes towards the caudal fin
(Altringham et al., 1993
;
Davies et al., 1995
;
Johnston et al., 1995
). Caudal
fibres are primarily active during lengthening and only produce positive work
towards the end of their activity. In Atlantic cod, enzymatic activities in
white and red fibres change longitudinally (Martínez et al.,
2000
,
2003
), with the highest
mass-specific activities being found caudally. While these longitudinal
patterns are intriguing, particularly in light of the longitudinal differences
in performance during swimming, it is unknown whether the metabolic capacities
of the muscle fibres at different positions in the cod are correlated with the
capacities for sprint and Ucrit swimming.
In the present study, we established two groups of Atlantic cod, differing
widely in condition, to evaluate (1) whether sprint or
Ucrit swimming is more sensitive to differences in
condition and (2) to obtain a wide range of muscle metabolic capacities to
evaluate metabolic correlates of sprint and Ucrit swimming
performance. During 12 weeks, the cod in one group were fed every second day
whereas those in the other group were starved. We then determined the sprint
and Ucrit swimming performance of the cod. Sprint swimming
was measured as described by Martínez et al.
(2002). The
Ucrit was measured using a Blazka swimming respirometer
following Reidy et al. (2000
).
As cod can differ in the extent to which they rely upon unsteady swimming
while reaching Ucrit (Nelson et al.,
1994
,
1996
), we filmed the fish to
quantify burst-coast movements. A white muscle biopsy was taken directly after
the Ucrit test to determine lactate levels. We assessed
the metabolic capacities of white and red muscle sampled at three locations
along the body (behind the head, at the middle of the body and at the caudal
peduncle). We estimated the total amount of red and white muscle by
dissection. In white muscle, we measured glycolytic enzymes (PFK, PK, CK and
LDH), mitochondrial enzymes (CCO and CS) and a biosynthetic enzyme [nucleoside
diphosphate kinase (NDPK)]. In red muscle, we measured PFK, LDH, CCO, CS and
pyruvate dehydrogenase (PDH), a mitochondrial enzyme controlling rates of
pyruvate oxidation (Richards et al.,
2002
). These data were used to examine the links between sprint
and Ucrit swimming, the speed at repeated burst-coasting,
condition, anatomic characteristics, enzyme, protein and water levels in white
and red muscles along the cod body.
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Materials and methods |
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Sprint and Ucrit swimming protocols
Swim performance was measured at Institut Maurice-Lamontagne. We used a
swimming flume based on Nelson et al.
(2002), modified and used as
described by Martínez et al.
(2002
) to measure sprint
performance. Statistical analysis used the maximum velocity recorded in the
first six sections of the tunnel as a measure of a fish's sprint performance.
Ucrit swimming tests were performed in a Blazka-type
respirometer (BII Consulting, Burlington, ON, Canada) equipped with an
electrical grid at the downstream end. The respirometer had a total volume of
109 litres, a swimming section of 94 cm length and 24 cm in diameter, and was
powered by a motor with micrometric adjustment of the power.
Ucrit tests were based on Brett
(1964
) as modified by Nelson et
al. (1994
) without the
overnight acclimation. First, the fish had a 30 min acclimation in the swim
tunnel at a speed of 15 cm s-1. The speed was then gradually
increased to 25 cm s-1 (an increase of 1 cm s-1
min-1 over 10 min), the speed at which the test started. Fish swam
for 20 min intervals, between which the speed was increased by 10 cm
s-1 in 10 min (1 cm s-1 min-1 increment)
until the fish was exhausted and the test terminated. The fish was considered
exhausted when it would stay against the back grid and would not react to
electrical stimuli. An electrical stimulus was given to a fish only if it
remained against the back grid for at least 5 s. The exact swimming speeds
were established using a speed-position relationship determined in preliminary
tests. The Ucrit speed was calculated as described by
Brett (1964
), and a solid
blocking effect correction was performed according to equation 94 of Webb
(1978
). For the starved cod,
this correction was between 4 and 6% whereas for fed cod it ranged between 7
and 11.5%. Fish were filmed during the Ucrit swimming test
to establish their position in the chamber (for application of the
speed-position relationship) as well as the number of burst-coast
movements.
Dissection and tissue sampling
Hematocrit was measured immediately after death according to Klawe et al.
(1963). A sample of white
muscle (approximately 1 g) was rapidly removed from below the dorsal fin for
lactate measurements. Muscle samples for enzyme measurements (3-5 g and 0.5-2
g for white and red muscle, respectively) were taken at three sites along the
cod body: (1) behind the head (rostral), (2) at the middle of the body and (3)
in the caudal region. White muscle samples were taken above the lateral line,
while red muscle was dissected just under the lateral line. Care was taken
during these dissections to avoid contamination by pink fibres. All tissues
sampled were immediately frozen in liquid nitrogen, then stored at -70°C
before transport on dry ice to Université Laval, where they were stored
at -70°C until biochemical assay. We quantified the total mass of white
and red muscle by dissection. Intermediate (pink) fibres were included with
red muscle in this dissection. All tissues (±0.1 g) and gonads were
weighed to calculate the hepatosomatic index (liver mass x somatic
mass-1) and the gonadosomatic index (gonad mass x somatic
mass-1), respectively. Somatic mass was calculated by subtracting
the mass of the gonad and stomach contents from the total mass of the
fish.
Tissue extraction
Extracts were prepared by homogenising samples in 10 volumes of 50 mmol
l-1 imidazole HCl, 2 mmol l-1 MgCl2, 5 mmol
l-1 ethylene diamine tetraacetic acid (EDTA), 1 mmol l-1
reduced glutathione, 0.1% Triton X-100, pH 7.5, using a Polytron (Brinkman
Instruments, Rexdale, Ontario, Canada) for three 20-s periods. The samples
were maintained on ice during and between the periods of homogenisation.
Enzyme activity assays
CCO, CS, NDPK, PFK, PK and LDH were measured according to Couture et al.
(1998). For CK (E.C. 2.7.3.2),
the assay was as follows: 75 mmol l-1 Tris-HCl, 5 mmol
l-1 MgCl2.6H2O, 4 mmol l-1
glucose, 0.75 mmol l-1 ADP, 5 mmol l-1 AMP, 0.3 mmol
l-1 NADP, 24 mmol l-1 phosphocreatine (omitted for
control), hexokinase and glucose-6-phosphate dehydrogenase in excess, pH 7.6.
For PDH (E.C. 1.2.4.1), the assay followed Thibault et al.
(1997
) as follows: 50 mmol
l-1 Tris-HCl, 0.5 mmol l-1 EDTA, 0.2% Triton X-100, 2.5
mmol l-1 NAD, 0.1 mmol l-1 coenzyme A, 1 mmol
l-1 MgCl2, 0.1 mmol l-1 oxalate, 1 mg
ml-1 bovine serum albumin, 0.6 mmol l-1
p-iodonitrotetrazolium violet (INT), 6 U lipoamide dehydrogenase, 0.2 mmol
l-1 thiamine pyrophosphate, 5 mmol l-1 pyruvate (omitted
for control), pH 7.8. Enzyme activities were expressed in international units
(µmol substrate transformed to product min-1) per gram tissue
wet mass. Muscle enzyme activities were measured in the order: PFK, PK, CK,
LDH, CS, CCO and NDPK in white muscle, and PFK, PDH, CS, CCO, LDH in red
muscle. All assays were run in duplicate.
Muscle protein, lactate and water contents
Protein concentration was measured using bicinchoninic acid
(Smith et al., 1985) in white
and red muscle. Muscle water content was determined after drying 2 g of muscle
for 48 h at 60°C. Lactate content was determined using the method of
Gutmann and Wahlefeld (1974
).
Red muscle water content was measured in the rostral sample.
Chemicals and biochemicals were purchased from Sigma Chemical Co. (St Louis, MO, USA), Boehringer Mannheim Co. (Montréal, Canada), Fisher Scientific Co. (Montréal, Canada) or ICN Pharmaceuticals Inc. (Montréal, Canada).
Calculated parameters and statistical analyses
Individual growth rates were calculated using the following equation:
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where growth rate is measured in % day-1, W1 represents the initial mass in g or fork length in cm, W2 represents the final mass in g or fork length in cm, and t is the number of days of the experimental period.
We used JMP 3.2.1 (SAS Institute Inc., Duxbury Press, Belmont, CA, USA) to
examine the effect of feeding treatment (starvation and feeding) upon anatomic
parameters and general characteristics. We further examined the impact of
feeding treatment and site of sampling (with repeated measures in individual
fish) on enzyme activity, protein and water content. In these models, feeding
treatment was a factor, sampling site was nested within fish and fish was
nested within feeding treatment. An interaction term between feeding treatment
and sampling site was also included. Enzyme activity, total protein and water
content were the variables. Data met the criteria of normality and homogeneity
of variances. Differences were considered significant at =0.05. The
a posteriori `add contrasts' function was used to establish specific
differences due to feeding treatment at each sampling site. Longitudinal
differences in enzyme activity were also examined with separate models for
each feeding group. In these models, site of sampling (repeated measures in
individual fish) was the factor, and a posteriori application of the
`add contrasts' function was used to establish specific differences.
A forward stepwise analysis was used to determine which variables best explain the variability of sprint and Ucrit performance as well as the speed at the start of repeated burst-coast movements. In addition to the biochemical variables, the following morphological variables were included in the analysis: somatic mass (g), length (cm), somatic condition factor, sex, gonadosomatic index and hepatosomatic index, as well as the mass (g) of gonads, liver, stomach, intestine, heart, gill arches, carcass, total white muscle and total red muscle. The criterion of entry and removal of a variable into the regression was based on the level of significance of the R2 (P<0.05 to stay in the model). Once the forward stepwise analysis was finished, we plotted the significant variables from the forward stepwise model versus the predicted performance variable (sprint, Ucrit or speed at repeated burst-coasts) to see how they correlated. As length is one of the major factors determining maximum speed in fish, it was added to the last model. Once the final models were built, we checked the collinearity between the remaining variables using the VIF (variance inflation factor). When a variable showed marked collinearity with the others, it was removed from the model (statistical VIF index >15). Models used the forward stepwise analysis and the VIF option in JMP 3.2.1 (SAS Institute Inc.).
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Results |
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Effects of feeding regime on biochemical parameters
As expected for such a large difference in condition factor, starvation
significantly decreased the activity of all enzymes measured in white muscle
(PFK, PK, CK, LDH, CS, CCO and NDPK; P<0.0001;
Fig. 1). The activity of the
glycolytic enzymes increased in the rostral-caudal direction (PFK, PK and LDH,
P<0.0001; CK, P=0.046), as did the activity of CS and CCO
(P=0.007). NDPK levels decreased in the rostralcaudal direction
(P<0.0001). At each site of sampling, starved cod had
significantly lower levels of all enzymes measured (P<0.0001),
with the greatest differences being apparent for PFK, PK and LDH.
|
In red muscle, the activity of all of the enzymes measured was significantly lower in starved than fed cod at each site of sampling (P<0.0001, except for PDH in the middle of the body where P=0.0003; Fig. 2). As in white muscle, the glycolytic enzymes were more affected than the mitochondrial enzymes, with the exception of CCO behind the head. Enzyme activity in red muscle showed longitudinal variation within each group. Mitochondrial enzymes increased in the rostral-caudal direction in both fed and starved cod. A rostral-caudal increase in glycolytic enzyme activities was also apparent in starved cod. However, in fed cod, glycolytic enzyme activities in red muscle were higher in rostral than caudal samples.
|
While starved cod had a lower protein content in white and red muscle than fed cod (P<0.001; Figs 1, 2), site of sampling did not systematically affect this parameter. Only in starved cod did sampling site affect protein contents: in red muscle, the rostral sample had the lowest protein content, whereas in white muscle the rostral sample had the highest protein content. Starved cod had water contents of approximately 85% in red and white muscle; these were 5-6% higher than in fed cod. In white muscle, water content showed a significant interaction between feeding regime and the site of sampling (P<0.0001): rostral samples had the lowest water content in fed cod while in starved cod, the caudal samples were lowest. Starvation did not significantly change the hematocrit (P=0.38; Table 1).
Sprint swimming performance and its physiological correlates
The sprint performance of cod expressed as absolute (cm s-1) or
relative speeds [body length s-1 (bl s-1)] was
significantly affected by starvation (P<0.0001 and
P<0.05, respectively; Fig.
3). The maximal burst swimming speed expressed as cm
s-1 decreased by 30% whereas the speed expressed in bl
s-1 decreased by 21.5%. Most cod traversed the tunnel in less than
2 s and reached their maximal speed early in the tunnel. Sex did not affect
the sprint swimming performance in either group (P>0.05).
|
Of all anatomic and biochemical variables measured, the activities of CCO and NDPK in white muscle behind the head were the strongest correlates of sprint performance expressed as cm s-1 (Table 2). Both variables were positively correlated with the absolute speed. Because length markedly affects swimming speeds and is positively correlated with sprint swimming, we constructed a model in which length was forced to stay. In this model, the same variables (CCO and NDPK in the rostral white muscle) and the protein level in white muscle at the middle of the body were correlated with absolute sprint swimming (Table 2). The activity of CCO in rostral white muscle was the primary correlate of relative sprint swimming speed (Table 2). When length was forced to stay in the model, both the activity of CCO in white muscle and PDH in rostral red muscle were correlated with sprint performance (Table 2). Overall, the activity of CCO in rostral white muscle was the strongest correlate of sprint swimming performance of cod.
|
Ucrit swimming and its physiological correlates
At the end of the Ucrit test, lactate concentration in
white muscle was 13-fold higher in fed than starved cod
(Fig. 4). Starvation markedly
decreased Ucrit in absolute or relative speeds
(P<0.0001 and P=0.003, respectively). The
Ucrit expressed in cm s-1 decreased by 38%
whereas the relative speed decreased by 28%. Cod changed their swimming
pattern towards the end of the tests, going from steady swimming to
burst-coast movements. Although some cod used occasional burst-coast movements
earlier, it was only near the end of the test that repeated burst-coasting
occurred. Accordingly, the speed at which repeated burst-coast movements
appeared was similar to the Ucrit, for absolute as well as
for relative speeds (Fig. 4).
The total number of burst-coast movements made during a test was decreased by
starvation (Wilcoxon test, P=0.02) but was quite variable, with
burst-coasts ranging from 1 to 83 in the starved group and from 1 to 187 in
the fed group. Lactate generation during burst-coasts was considerably lower
in starved than fed cod (0.62±0.2 vs 2.9±1.2 µmol
lactate g-1 burst-coast-1; Wilcoxon test,
P=0.001).
|
Of all the anatomic, biochemical and behavioural parameters we measured, the same four variables were consistently retained as correlates of Ucrit performance and explained as much as 80% of variance (P<0.0001; Table 3). The strongest correlate was always the speed at which regular burst-coast movements started (P<0.0001); the other three variables were CK activity in rostral white muscle, NDPK activity in the white muscle sampled in the middle of the body and hematocrit.
|
To identify the biochemical or anatomic correlates of Ucrit swimming, we omitted the speed at which regular burst-coasting started, given its proximity to the Ucrit. For Ucrit expressed as absolute speed, this model explained 56% of the variability (P<0.0001; Table 4). PDH activity in central red muscle and water content in central white muscle were the strongest correlates (P=0.0004), followed by CS activity in caudal red muscle (P=0.01) and CS activity in central white muscle (P=0.025). When performance was expressed as relative speeds, these variables explained 41% of the variability.
|
Given that speed at which regular burst-coasting begins is an externally visible estimator of Ucrit, we also sought its biochemical and anatomic correlates. When this speed was expressed in absolute speeds, PDH level in red muscle and total protein concentration in white muscle sampled at the caudal peduncle were significant correlates and explained 52% of the variability (Table 5). When this speed was expressed as bl s-1, these variables explained 35% of the variability.
|
Good athlete/bad athlete vs specialised swimming styles
To evaluate which of these patterns better describes the swimming
capacities of the cod we used, we examined the correlations between individual
sprint and Ucrit performance, using both relative and
absolute speeds. Weak positive correlations were found between the sprint and
Ucrit speeds expressed in cm s-1
(R2adj=0.11; P=0.005) as well as with
the speed at which repeated burst-coast movements started in cm s-1
(R2adj=0.14; P=0.002). Analysis of the
correlations of residuals from the length-speed relationships did not reveal
stronger relationships.
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Discussion |
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The similar impact of starvation on sprint and critical swimming speeds
suggests that, particularly in cod, with its paucity of oxidative muscle,
glycolytic muscle makes an important contribution to Ucrit
swimming. Starvation decreases the distance and time swum by Atlantic cod
during prolonged swimming above Ucrit by 70%
(Martínez et al.,
2003). This greater impact of starvation on endurance above
Ucrit was not due to a greater difference in condition
factors. The starved fish had a mean condition of 0.5 in both studies, and the
fed fish had a condition of 0.8 in Martínez et al.
(2003
) compared with 1.0 in
the present study. Rather, we suggest that prolonged swimming at speeds above
Ucrit makes greater demands on white muscle than sprint or
Ucrit swimming. The very brief nature of sprint swimming
does not exhaust white muscle. Repeated burst-coast movements provide a clear
signal that endurance limits are being reached
(Martínez et al., 2003
;
Peake and Farrell, 2004
). The
similarity between the Ucrit and the speed at which cod
began repeated burst-coasting suggests that the contractile contribution of
white muscle to Ucrit swimming is limited. While the
metabolic activation of glycolytic fibres at speeds >70%
Ucrit in rainbow trout
(Burgetz et al., 1998
;
Richards et al., 2002
), and in
smallmouth bass during their transition between steady and unsteady swimming
(Peake and Farrell, 2004
),
suggests a contribution of glycolytic fibres at speeds before repeated
burst-coasting, clearly this contribution will be greater at higher speeds.
Thus, when starvation of Atlantic cod leads to marked loss of white muscle,
the sprint and Ucrit swimming capacities are reduced, but
endurance at speeds above Ucrit falls to a much greater
extent. Given the exclusive contribution of aerobic metabolism to sustained
swimming, comparison of sprint and sustained swimming in starved and fed cod
would be useful.
In contrast to our demonstration of an effect of condition on sprint
performance of cod, previous studies have not always found an effect of
condition (Reidy et al., 2000;
Martínez et al., 2002
).
This contradiction can be explained by the differentiation of the condition of
the experimental groups and by non-linear effects of condition. In the current
study, the condition factor of fed cod was twice as high as that of starved
cod (1.0 versus 0.5). A smaller range in condition factors was used
in previous studies (0.7-1.1, Reidy et
al., 2000
; 0.6-0.8,
Martínez et al., 2002
).
Condition factors between 0.6 and 0.9 are not unusual for cod from northern
Norway and from the northern Gulf of St Lawrence
(Eliassen and Vahl, 1982
;
Lambert and Dutil, 1997a
). A
mean condition near 0.7 is normal for cod from the Gulf of St Lawrence during
the spring, just after spawning (Lambert
and Dutil, 1997a
). However, cod with condition factors between 0.5
and 0.6 have a great risk of mortality
(Dutil and Lambert, 2000
).
Nonetheless, even short-term (5 day) starvation of trout depletes muscle
glycogen, thereby reducing glycogen mobilisation and lactate accumulation
during exhaustive exercise, as well as accelerating the post-exercise
recuperation of phosphocreatine levels
(Scarabello et al., 1991
).
Clearly, many aspects of the physiology and motivation of fish are affected by
starvation, and the effects of condition are unlikely to be linear.
During extensive starvation, the ultrastructure of white muscle is more
affected than that of red muscle (Johnston
and Goldspink, 1973; Patterson
et al., 1974
; Beardall and Johnston,
1983
,
1985
;
Black and Love, 1986
). At the
ultrastructural level, volume fractions of mitochondria and myofibrils, fibre
size and capillary supply are reduced. The increased water content in white
muscle reduces the total volume of muscle fibres occupied by myofibrils and
mitochondria and dilutes cytosolic enzymes
(Beardall and Johnston, 1985
).
We found that starvation led to similar increases (5-6%) in the water content
of red and white muscle but reduced the total volume of white muscle much more
(by 81%) than that of red muscle (up to 56%). Nonetheless, white muscle mass
was not a correlate of sprint or Ucrit swimming in our
multiple regression models, even though a significant partial correlation
exists between white muscle mass and sprint swimming performance.
In white muscle, contractile activity during burst swimming primarily
depends on ATP generation from phosphocreatine and from anaerobic glycolysis
(Bone, 1966;
Bone et al., 1978
;
Dobson et al., 1987
;
Altringham and Ellerby, 1999
).
During the first few seconds of intense muscular activity, such as sprint
swimming, ATP is maintained at a relatively constant level, but the PCr level
declines steadily as the CK reaction replenishes the depleted ATP. Sprint
swimming can be maintained only for a short duration, just enough for the
activation of anaerobic glycolysis in muscle fibres.
How then could the mitochondrial capacity of white muscle play a role in
setting sprint swimming performance, as the strong correlation between CCO
activity in white muscle and sprint swimming might suggest? This strong
relationship may reflect a role of oxidative phosphorylation in minimising
intracellular gradients in ATP and PCr before exercise. This suggestion is
supported by the study of Hubley et al.
(1997), who examined the
influence of mitochondrial volume density upon spatial gradients in PCr and
ATP in muscle fibres. An increase in the mitochondrial volume density in the
range found in white fibres reduces the severity of temporal-spatial gradients
in PCr and ATP during burst activity. In this fashion, the higher oxidative
capacity in white muscle of fed cod could increase their sprint swimming
performance by making ATP available to a greater proportion of the myofibrils
than in starved cod. The deterioration of white fibres in starved cod would
slow oxidative ATP production, lead to greater intracellular gradients in ATP
and PCr, thus compromising the already diminished contractile function.
Furthermore, higher oxidative capacities would facilitate return to resting
PCr and ATP levels between sprints.
The longitudinal variation in catabolic capacities of white muscle may
reflect the longitudinal variation of muscle work during swimming. Rostrally,
muscle fibres generate power while shortening and then transfer this power
towards the caudal peduncle, while caudally active fibres resist stretching
(negative work) (van Leeuwen et al.,
1990; Rome, 1992
;
Wardle and Videler, 1993
;
Jayne and Lauder, 1995
).
Davies et al. (1995
) found
that rostral white fibres of cod have faster relaxation times than caudal
fibres. Furthermore, these rostral fibres show a faster activation phase than
the caudal fibres. Therefore, maximum power will be produced by the rostral
white fibres, given their relatively short contraction time
(Thys et al., 2001
). Our
results suggest that the positive work by rostral white muscle may be limited
by its oxidative capacities. This may explain, in part, why sprint swimming
performance was positively related with the levels of CCO in the rostral
muscle.
Fed and starved cod did not differ in the speed increment that was
supported by burst-coasting (i.e. the maximal speed attained minus the speed
at which repeated burst-coasting began). This was surprising since fed cod
with higher anaerobic capacity and functionally intact white muscle were
expected to maintain this anaerobically fuelled swimming for longer, thus
leading to a greater separation between Ucrit and the
speed at which repeated burst-coasting began. Examination of the videotapes of
the Ucrit tests showed that even though both fed and
starved cod made extensive use of burst-coast movements
(Fig. 4), starved cod made
weaker burst-coast movements than fed cod. In agreement with this observation,
white muscle lactate levels at Ucrit were about 13-fold
higher in fed than in starved cod (Fig.
4), and lactate generation per burst-coast was 5-fold higher in
fed cod. Glycogen depletion and weak burst-coast movements are typical of
starved fish (Black and Love,
1986; Scarabello et al.,
1991
; Martínez et al.,
2003
) and agree with the low lactate levels in starved cod. When
swimming could no longer be maintained aerobically in the
Ucrit test, starved and fed cod responded differently, but
the end result was the same, virtual exhaustion. In starved cod, the highly
degraded white muscle only had a weak glycolytic swimming capacity (inability
to make strong burst-coast movements), bringing rapid exhaustion, while in fed
cod, the intensity and high energetic cost of burst-coasting with its
concomitant accumulation of lactate also led to rapid exhaustion. So, no
matter what the nutritional status was, the speed at which cod started
burst-coasting repeatedly was similar to their critical swimming speed. The
variability in the number of burst-coast movements used by the cod may reflect
variability in swimming strategies. Some cod may start sporadic use of
burst-coast movements at slower speeds as a way to save energy. Biomechanical
models show that burst-coasting may be 4-6 times more efficient than steady
swimming (Videler and Weihs,
1982
; Winger et al.,
2000
). We examined whether the number of burst-coast movements
could explain variability in Ucrit, as found by Winger et
al. (1999
). Such a
relationship was only apparent for the starved cod taken alone.
PDH activity in red muscle at the caudal peduncle was the strongest
correlate of the speed at which repeated burst-coasting started. Glycogen is
utilised at virtually all sustained swimming speeds
(Richards et al., 2002). By
its transformation of pyruvate to acetyl-CoA, PDH transfers substrates from
glycolysis to the Krebs cycle. In the initial phases of prolonged swimming
tests such as the Ucrit, glycogen is slowly depleted
during its oxidation to CO2 and H2O. Our data indicate
that cod with higher PDH activity in caudal red muscle can swim faster
aerobically, delaying use of burst-coast movements and attaining a better
performance in the Ucrit tests. Thus, PDH activity in red
muscle may set the rate at which oxidative muscle can break down pyruvate. The
caudal musculature, both red and white, consistently showed the highest levels
of mitochondrial enzymes (present study;
Martínez et al., 2003
),
suggesting its force transmission is critical during aerobic activity. PDH
activity was not only a good indicator of the speed at which repeated
burst-coasting started, but, given the limited extension provided by
burst-coasting, it also influenced the Ucrit of cod. In
fact, when the speed at first burst-coast was omitted in our stepwise
analysis, PDH activity in red muscle (this time in the middle of the body) was
again the variable that best explained variability in
Ucrit (Table
4).
Whereas Ucrit and sprint swimming were affected to a
similar extent by starvation, cod with high Ucrit values
were not consistently strong sprinters. At first glance, the use of white
muscle during the Ucrit protocol would suggest that such
fish have a greater glycolytic capacity and should be stronger sprinters.
While this tendency is apparent in comparison of the mean performances of the
fed and starved cod, analysis of the inter-individual variability in these
swimming behaviours does not reveal similar hierarchies of
Ucrit and sprint performance. We have previously reported
that inter-individual differences in sprint performance were maintained
despite considerable changes in condition index
(Martínez et al.,
2002). Given the short duration of sprints, inter-individual
differences in the sensitivity to the stimulus may condition sprint swimming
as much as muscle metabolic status, explaining the weak correlation between
sprint and Ucrit swimming performance.
The energetic condition of wild cod in some populations is rather poor in
the spring after prolonged starvation during winter. The present study and
Martínez et al. (2003)
indicate that winter starvation will decrease sprint,
Ucrit and endurance during swimming at speeds above
Ucrit through its effects on muscle mass and metabolic
capacities. Sprint and Ucrit swimming speeds were reduced
by 21-38% by starvation but, to exceed their Ucrit,
food-deprived cod were limited to burst-coast movements that seemed less
powerful than those of well-fed cod, leading to a greater reduction in
performance. In the wild, seasonal and inter-individual variation in swimming
capacity are likely to occur as cod vary in condition. Cod weakened by a
winter fast may be at risk in terms of natural selection just when they must
carry out long migrations to replenish their reserves during the short summer
period.
In summary, we used cod in a range of conditions and examined their swimming performance and its potential metabolic determinants. Multiple regression analysis allowed us to look at the inter-individual variation in Ucrit, the speed at which repeated burst-coast movements began and sprint swimming and to pinpoint the physiological and biochemical parameters most closely linked to this inter-individual variation. The speed at which regular burst-coasting began was closely related to Ucrit. The strong link between PDH activity in red muscle (both caudal and central positions) and the speed at the start of repeated burst-coasting shows that muscle mitochondrial capacity is an interesting correlate of whole animal performance. Of the various parameters we measured, both anatomic and biochemical, this best predicted Ucrit performance. Sprint swimming was also best predicted by white muscle activity of a mitochondrial enzyme, CCO, suggesting that aerobic preparation/recuperation of muscle for rapid contraction is central for this brief activity. Finally, despite the considerably greater loss of white than red muscle during starvation, sprint and Ucrit swimming differed by virtually the same extent between starved and fed cod.
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