Comparative studies of high performance swimming in sharks II. Metabolic biochemistry of locomotor and myocardial muscle in endothermic and ectothermic sharks
1 Center for Marine Biotechnology and Biomedicine and Marine Biology
Research Division, Scripps Institution of Oceanography, University of
California San Diego, La Jolla, CA 92093-0204, USA
2 Department of Biological Science, California State University, Fullerton,
CA 92834-6850, USA
* Author for correspondence at present address: Department of Zoology, Weber State University, Ogden, UT 84408-2505, USA (e-mail: dbernal{at}weber.edu)
Accepted 8 May 2003
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Summary |
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Key words: Lamnidae, shark, elasmobranch, muscle biochemistry, endothermy, metabolic biochemistry, locomotor muscle, cardiac muscle, aerobic capacity, anaerobic capacity, temperature, citrate synthase, creatine phosphokinase, lactate dehydrogenase
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Introduction |
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The first paper in this series (Bernal
et al., 2003) compared the position and quantity of RM in lamnids,
other sharks and tunas and showed the presence and relative development of
ultrastructural and biochemical features augmenting the aerobic capacity of
lamnid RM. The present study investigates the biochemical basis for
high-performance swimming by comparing aerobic and anaerobic enzyme activities
in the myotomal muscles and myocardium of endothermic lamnids with those of
actively swimming ectothermic sharks.
Fish RM powers sustained aerobic swimming, and the fast-twitch, glycolytic
white muscle (WM) is specialized for short-duration accelerations and bursts
powered by anaerobic metabolism (Johnston,
1981; Bone, 1988
).
In most fish species, RM and WM are morphologically distinct and easily
differentiated, making it possible to isolate homogenous samples of each fiber
type and quantify their metabolic enzyme activities. Comparative assessments
of tissue metabolic capacity have generally focused on the activities of the
enzymes citrate synthase (CS), creatine phosphokinase (CPK) and lactate
dehydrogenase (LDH). CS catalyzes the first reaction of the Krebs citric acid
cycle and correlates with tissue mitochondrial density; CPK catalyzes the
transfer of a high-energy phosphate group from intracellular creatine
phosphate to ADP, rapidly producing ATP; LDH catalyzes the reversible
reaction, pyruvate + NADH
lactate + NAD+, which maintains
redox balance during anaerobic glycolysis.
Phylogenetically based comparisons of tunas and their ectothermic sister
taxa (mackerels, Spanish mackerels and bonitos) show that, when measured at
20°C, tunas have higher CS, LDH and CPK activities in WM but similar CS
activities in the RM (Dickson,
1988,
1995
,
1996
;
Freund, 1999
;
Korsmeyer and Dewar, 2001
).
However, when the elevated RM temperature of the tunas is taken into account,
tunas also have higher RM CS activities than do the ectothermic scombrids
(Dickson, 1996
). For the heart
ventricle (HV; the heart is not thermally isolated by a counter-current heat
exchanger and therefore operates at ambient temperature), some aerobic enzymes
are present at higher activities in tunas than in their ectothermic relatives
(reviewed by Dickson, 1995
,
1996
) but others are not
significantly higher in tunas (Freund,
1999
).
The objectives of our study were to make a comparable assessment of tissue
enzyme activities in the endothermic lamnid sharks relative to those of other
active ectothermic sharks. Dickson et al.
(1993) conducted an initial
study of this type and concluded that the pattern of enzymatic activities in
the locomotor muscle in one lamnid, the shortfin mako shark (Isurus
oxyrinchus), relative to ectothermic sharks paralleled the pattern
documented for tunas relative to ectothermic scombrids. They reported higher
CS and LDH activities in mako WM but similar CS activities in both the RM and
HV of mako and ectothermic sharks. However, there were three limitations of
their study: (1) limited access to mako specimens and delays in obtaining
tissue samples that may have compromised tissue quality, (2) limited
comparative material [i.e. the blue shark Prionace glauca (order
Carcharhiniformes) was the only obligate swimming, ectothermic shark available
and it had a surprisingly low WM LDH activity] and (3) all assays were
conducted at 20°C, and the effect of elevated muscle temperatures on RM
and WM enzyme activities was not measured.
Our objectives were to surmount the limitations of the Dickson et al.
(1993) study and to expand the
phylogenetic basis for comparing the biochemical indices of aerobic and
anaerobic capacity in the locomotor muscle and HV of lamnids and other
species. In addition to acquiring tissues from several active ectothermic
sharks, we used a larger number of mako sharks sampled immediately after
capture and obtained data on another lamnid, the salmon shark (Lamna
ditropis). We also sampled tissues from the common thresher shark
(Alopias vulpinus; family Alopiidae, order Lamniformes). Thresher
sharks are more closely related to the lamnids and are similar to them in
having a central and anterior position of the RM, which is served by a lateral
circulation and a small putative heat exchanger
(Bone and Chubb, 1983
;
Block and Finnerty, 1994
;
Bernal et al., 2003
). Although
thresher shark endothermy remains undocumented
(Carey et al., 1971
),
anatomical studies (Fudge and Stevens,
1996
) and ongoing field studies suggest that A. vulpinus
can elevate its RM temperature (D. Bernal and C. Sepulveda, unpublished). In
addition to these comparisons, we tested the hypotheses that lamnids, like
tunas, benefit from RM endothermy by having higher RM enzyme activities at
in vivo temperatures, that lamnid WM adjacent to RM has a higher
metabolic capacity because it is warmed by thermal conduction from the RM and
that enzymes from the different regions of the WM differ in thermal
sensitivity, reflecting their typical thermal range.
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Materials and methods |
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Immediately after capture, sharks were euthanized, and samples of RM, WM and HV (separating, when possible, compact and spongy myocardium) were obtained. WM and RM were sampled at 4050% fork length (FL; i.e. at the position of the first dorsal fin) in mako, salmon, thresher and blue sharks. Samples from the hammerhead, bonnethead, Atlantic sharpnose and Atlantic blacknose sharks were collected at a more posterior body position (6080% FL; at the level of the second dorsal fin). To test for the possible effect of proximity to RM (i.e. an increase in tissue temperature) on WM enzyme activities in mako and salmon sharks, samples were removed from three body depths (close to the skin, midway between the skin and the RM, and close to the RM) at the 4550% FL body position. All tissue samples were immediately placed in cryogenic vials and frozen in a liquid nitrogen dry shipper (Thermolyne 10) or on dry ice and then stored (124 months) at -80°C. Initial tests verified that neither freezer storage times of 124 months at -80°C nor homogenate centrifugation time (15900 s) affected enzyme activities.
Enzyme assays
Samples of RM, WM and HV were first cleared of attached connective tissue
and any tissue dehydrated by freezing. Approximately 0.2 g of frozen tissue
was then taken from each sample and homogenized on ice in a Kontes Duall
ground-glass homogenizer with 9x the volume of buffer [2 mmol
l-1 EDTA, 3 mmol l-1 MgCl2, 10 mmol
l-1 NaCl, 150 mmol l-1 KCl, 200 mmol l-1
trimethylamine n-oxide (TMAO), 400 mmol l-1 urea, 80 mmol
l-1 imidazole buffer (pH 7 at 20°C)]. Homogenates were
centrifuged (12 000 g) for 15 min at 4°C and the
supernatants were removed and kept on ice, without further purification, until
their use in enzyme assays. Supernatants were diluted in homogenization buffer
[CS (5x), LDH (4181x) and CPK (2576x)] just
before assays were run to ensure that the enzymes were not substrate
limited.
In vitro enzyme assays for RM, WM and HV were run in a total volume of 2.0 ml using a temperature-controlled (±0.2°C) spectrophotometer (Shimadzu UV 1201S or Hewlett-Packard 8452A diode-array). Enzymatic activity is proportional to the change in absorbance over time and is reported in international units (IU; µmol substrate converted to product per min) per g tissue wet mass. The final conditions for each assay were as follows. Citrate synthase (CS): 0.1 mmol l-1 acetyl-CoA, 0.5 mmol l-1 oxaloacetate, 0.1 mmol l-1 dithiobis nitrobenzoic acid, 3 mmol l-1 MgCl2, 10 mmol l-1 NaCl, 150 mmol l-1 KCl, 200 mmol l-1 TMAO, 400 mmol l-1 urea, 80 mmol l-1 Tris buffer (pH 8 at 20°C); absorbance changes were measured at 412 nm. Creatine phosphokinase (CPK): 12.5 mmol l-1 creatine phosphate, 0.5 mmol l-1 ADP, 0.3 mmol l-1 NADP+, 3.5 mmol l-1 glucose, 10 mmol l-1 AMP, excess hexokinase and glucose-6-phosphate dehydrogenase coupling enzymes, 100 mmol l-1 KCl, 10 mmol l-1 MgCl2, 200 mmol l-1 TMAO, 400 mmol l-1 urea, 50 mmol l-1 imidazole HCl (pH 7.0 at 20°C); absorbance changes were measured at 340 nm. Lactate dehydrogenase (LDH): 0.15 mmol l-1 NADH, 1 mmol l-1 pyruvate, 3 mmol l-1 MgCl2, 10 mmol l-1 NaCl, 150 mmol l-1 KCl, 200 mmol l-1 TMAO, 400 mmol l-1 urea, 80 mmol l-1 imidazole HCl (pH 7 at 20°C); absorbance changes were measured at 340 nm.
Enzyme assays for interspecific comparisons were carried out at 20°C
under saturating substrate conditions, as determined in preliminary tests.
Thermal effects on CS and LDH activities in mako, salmon and thresher shark RM
were determined by measuring enzyme activity at 5°C intervals from 5°C
to 30°C. This same temperature range was used to assess possible
differences in thermal effects on the enzymatic activities of mako and salmon
shark WM positioned at three different body depths between the skin and RM. To
measure thermal sensitivity, the thermal rate coefficient (Q10) was
estimated from the slope of the linear regression of log10(enzyme
activity at Tn) vs
[(Tn-Tn-1)x0.1], where
Tn is the assay temperature (in °C), and the
Q10 is equal to 10slope
(Schmidt-Nielsen, 1993).
Statistical analyses
Tissue-specific in vitro enzyme activities were tested for
significant interspecific differences by using a general linear model (GLM)
analysis of covariance (ANCOVA) or analysis of variance (ANOVA) in Minitab
(version 10.5). The initial GLM ANCOVA model for each enzyme in each tissue
included species, mass, FL (with mass and FL as covariates),
all possible two-way interactions and a three-way interaction term. Data that
were not normally distributed, based on a KolmogorovSmirnov test, or
that did not meet the assumption of homogeneity of variance, as determined
from residuals vs fit plots, were log10-transformed before
ANCOVA. When significant mass effects were found, and if ANCOVA showed a
significant species effect, a linear regression was fitted to a plot of enzyme
activity vs mass for each species, and a Pearson product-moment
correlation coefficient was used to test for significance. When no significant
size effects were found, the mean values for each species were compared by
ANOVA, followed by a post-hoc TukeyKramer multiple comparisons
test. A Student's paired t-test was used to test for differences in
enzyme activities between compact and spongy myocardium. A significance level
of =0.05 was used in all statistical analyses.
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Results |
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Muscle enzyme activities
Red muscle
There was no significant effect of fish mass or FL on RM CS or LDH
at 20°C, and mean enzyme activities were used for comparisons among
species (Table 1). ANOVA
indicated no interspecific differences in mean RM CS activity. There were
interspecific differences in mean RM LDH activities, but the results of the
post-hoc multiple comparisons test differed depending on which
species was used as the basis for comparison. The blue shark had a
significantly lower mean RM LDH activity than all other species studied, but
values for the other species overlapped. When the mako was compared with all
other sharks, its RM LDH activity did not differ from that in the salmon shark
or thresher shark but was significantly greater than in the other three
species. RM LDH activity in the salmon shark did not differ from that of the
mako, common thresher or leopard sharks but was significantly greater than in
the other two species. Common thresher shark RM LDH activity did not differ
from that of any other species except the blue shark.
White muscle
Mean WM CS activities in the mako and salmon shark are 1.63.2 times
higher than in the other sharks, including the thresher shark
(Table 1). ANCOVA indicated a
significant mass effect and a significant massxspecies interaction for
WM CS activity at 20°C. Follow-up tests for correlations between body mass
and WM CS activity in each species demonstrated significant correlations only
for the mako (negative) and the scalloped hammerhead (positive)
(Fig. 1).
|
The highest WM LDH activities measured were in mako and salmon sharks, but values for the mako overlap those measured in the Atlantic blacknose and Atlantic sharpnose sharks (Fig. 1), and the lowest LDH activities were found in the blue and thresher sharks. The ANCOVA for WM LDH activity at 20°C also showed a significant mass effect and massxspecies interaction, but, when the data for each species were analyzed individually, no significant correlations between enzyme activity and body mass were found (Fig. 1).
No body size effects were found for WM CPK activity, and the only interspecific difference detected for this enzyme was a significantly greater activity in the mako than in the Atlantic blacknose shark (P<0.05, N=26; Table 1).
Myocardial tissue
Body size had no effect on LDH or CS activities (20°C) in the HV (a mix
of both spongy and compact myocardium) of six shark species. The multiple
comparisons test showed equivalent mean LDH activities in the mako, salmon,
common thresher and leopard sharks, which were significantly higher than those
of both the scalloped hammerhead and blue sharks (P<0.05;
Fig. 2). Also, blue shark HV
LDH is significantly less than in the scalloped hammerhead
(Fig. 2).
|
Mean HV CS activity in the mako did not differ from that in the salmon shark but was significantly greater than in all other species studied (P<0.05; Fig. 2). When the salmon shark was the comparison group, its HV CS did not differ from that of the mako, common thresher or scalloped hammerhead sharks but was significantly greater (P<0.05) than in the blue and leopard sharks. The leopard shark had a significantly lower mean HV CS activity than all other species except the blue shark (P<0.05; Fig. 2). Blue shark HV CS activity did not differ significantly from that in the scalloped hammerhead, common thresher and leopard sharks but was significantly lower than that of the two lamnids (P<0.05).
Comparison of isolated spongy and compact myocardial samples from mako, salmon and common thresher sharks (Table 2) shows a significantly higher CS activity in the spongy myocardium (paired t-test; P<0.05, N=9) but no differences between spongy and compact layer LDH activities.
|
Temperature effects on lamniform shark muscle enzyme activities
Red muscle
Comparable Q10 values over 530°C indicate no
interspecific differences in the effect of temperature on the activities of RM
CS and LDH in the mako (N=4), salmon (N=2) and thresher
(N=7) sharks (Fig. 3).
Close agreement among the three species exists in the Q10 range
determined for each enzyme, but the Q10 for LDH (mean, 2.01; range,
1.972.08) is greater than that for CS (1.72; 1.651.76). For each
species, there were no apparent body size effects on the activities of either
CS or LDH or on the Q10 values
(Fig. 3).
|
White muscle
Data for two mako and two salmon sharks document the absence of an effect
of WM proximity to the warm RM (i.e. under the skin vs midway between
skin and RM vs close to the RM) on the thermal sensitivity of CS and
LDH activities. The Q10 values were similar for both species and
for both enzymes (Table 3).
Fig. 4 plots the
temperatureenzyme activity data for one of the makos (49 kg) and one of
the salmon sharks (127 kg) and shows the WM sample positions in relation to
the thermal contour patterns in the salmon shark. The enzyme activities were
similar at all three WM positions sampled.
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Discussion |
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Even though this study has expanded both the number of shark species and tissue samples that have been analyzed, the difficulty in obtaining pelagic shark specimens has reduced our capacity for robust interspecific comparisons because of small sample sizes for some species and interspecific differences in both body size range and the types of tissues sampled. We did not, for example, have access to all tissues in all species (i.e. only WM tissue samples were obtained from Atlantic sharpnose, Atlantic blacknose and bonnethead sharks; Table 1). Also, we obtained samples from only two salmon sharks and, while findings for this species were similar to those for the mako, the small sample size limited the utility of the salmon shark data in interspecific comparisons. The range of fish sizes available for study also differed among the shark species sampled, thereby limiting interspecific comparisons of enzyme activity scaling relationships. Nevertheless, and despite these impediments, our study provides new insight into both the functional significance of shark endothermy and the comparative physiology of the lamnidtuna convergence.
Lamnid and tuna comparisons
Tunas and lamnids have similar specializations for continuous, sustained
swimming (Bernal et al.,
2001a), and species in both groups frequently make vertical
excursions through the water column or undertake extensive seasonal migrations
(Carey et al., 1978
,
1981
;
Paust and Smith, 1989
;
Casey and Kohler, 1992
;
Holts and Bedford, 1993
;
Anderson and Goldman, 2001
;
Gunn and Block, 2001
;
Klimley et al., 2002
).
Underlying the tunalamnid convergence in adaptations for
high-performance swimming are similar RM positions within the body
(Graham et al., 1983
;
Carey et al., 1985
; Bernal et
al., 2001a
,
2003
) and the capacity to
augment RM and other systemic functions through regional endothermy
(Block and Finnerty, 1994
;
Brill, 1996
;
Bernal et al., 2001a
;
Graham and Dickson, 2001
).
Endothermy raises the temperature of RM and WM, which, by speeding the
contractionrelaxation cycle, increases muscle power production
(Johnston and Brill, 1984
;
Altringham and Block, 1997
).
Endothermy also stabilizes tissue temperatures, thereby conserving metabolic
function in the face of ambient temperature reductions (caused by moving below
the thermocline or into higher latitudes), and has probably contributed to
niche expansion by both groups (Block et
al., 1993
; Block and Finnerty,
1994
; Graham and Dickson,
2000
; Bernal et al.,
2001a
,b
;
Boustany et al., 2002
).
Despite the similarities in muscle metabolic patterns of the tunas and
lamnids when compared with related species, the activities of all enzymes
measured in the lamnids are lower than they are in tunas and are more similar
to the activities in ectothermic scombrids and other active teleosts. This is
the case even when tunas and lamnids of similar masses are compared. The lower
muscle metabolic enzyme activities in lamnids relative to tunas parallels the
finding that structural modifications for enhanced oxygen delivery
(viz. capillary manifolds) are less prominent in mako RM than in
skipjack tuna (Katsuwonus pelamis) RM
(Bernal et al., 2003). Lamnid
sharks also have less RM than do tunas (23% vs 413% of
body mass; Graham et al.,
1983
; Carey et al.,
1985
; Bernal et al.,
2003
), but the total volume of mitochondria and myoglobin
concentration is similar in mako and tuna RM fibers
(Bernal et al., 2003
).
Endothermy and lamnid red muscle aerobic capacity
The thermal effects measured for RM CS activity
(Fig. 3) can be used to
estimate the magnitude of increase that occurs due to endothermy in the mako,
salmon and common thresher sharks. The mako, for example, frequents 20°C
water (Table 1) and has an RM
temperature of 2527°C (Carey
and Teal, 1969a; Carey et al.,
1985
). Based on Fig.
3, we estimate a 48% greater RM CS activity at the mako's warmer
in vivo temperature relative to what it would be at ambient
temperature. For the salmon shark, which occurs in 810°C water
(Table 1) and has an RM
temperature of 2426°C (Rhodes
and Smith, 1983
; Anderson and
Goldman, 2001
; Bernal et al.,
2001a
), RM CS activity would be enhanced by 123%. Assuming that
the common thresher shark is an endotherm [this species commonly occurs in
1820°C water (Table
1), and RM temperatures of 2226°C have been measured
(D. Bernal and C. Sepulveda, unpublished)], its RM CS activity would be
elevated by as much as 48%. Use of the increased aerobic capacity resulting
from endothermy requires an increased supply of both O2 and aerobic
fuels to the RM, and both tunas and lamnids have cardiorespiratory
specializations that increase the uptake of O2 at the gills and its
delivery to the RM (Bernal et al.,
2001a
).
Heart metabolic capacity
Profiles of tuna heart enzyme activities indicate heightened aerobic
capacities relative to other active fish species (reviewed by
Dickson, 1995), which suggests
an enhanced oxidative function (i.e. the pumping of blood). An elevated HV CS
activity in mako and salmon sharks (Fig.
2) is consistent with the expectation of a greater cardiac pumping
capacity and further supports the tunalamnid convergence.
Data indicating a greater aerobic capacity (CS activity) for spongy than
for compact myocardium in the three lamniform sharks, but similar LDH activity
in the two layers (Table 2),
also parallel what has been documented for tunas
(Tota, 1983;
Moyes et al., 1992
). The
difference in CS activity may correlate with differences in the quantity of
O2 available to the two different myocardial layers (i.e. compact
is nourished by the O2-rich coronary arteries, whereas spongy
receives a high percentage of hypoxic venous blood). However, additional work
is needed to verify this and to determine the extent of other biochemical and
physiological differences between the two tissue layers and how their function
compares with that of tunas (Tota,
1978
; Dickson,
1995
; Bernal et al.,
2001a
).
White muscle metabolic capacity
The finding of higher WM CS and LDH activities in lamnids relative to the
other shark species (Table 1)
parallels the differences documented for tunas relative to non-tunas (Dickson,
1995,
1996
). The scaling coefficient
[-0.11±0.09 (95% C.I.), from the allometric equation
y=aMb-1, where y is mass-specific enzyme
activity, a is a constant, M is fish mass, and b-1
is the scaling coefficient] for mako shark WM CS is within the range reported
for oceanic pelagic teleosts (-0.37 to -0.06;
Childress and Somero, 1990
).
Although we found a significant positive scaling relationship for scalloped
hammerhead WM CS activity (Fig.
1), a relationship of this type is not consistent with data from
other fish species (Childress and Somero,
1990
). Because most scaling relationships are based on data
extending across a size range of at least one order of magnitude, our opinion
is that this result is without biological significance and is attributable to
the small size range (0.50.8 kg) of hammerhead sharks examined.
The two lamnid species had the highest WM LDH activities, indicative of a high anaerobic capacity, but the values overlap with those measured in the Atlantic blacknose and Atlantic sharpnose sharks (Fig. 1). Because there was an overall increase in WM LDH activity with fish mass but no significant mass correlations within a species, and because the Atlantic blacknose and Atlantic sharpnose sharks were all smaller than the lamnids, we cannot rule out the possibility that the WM LDH activity of larger Atlantic blacknose and Atlantic sharpnose sharks would be similar to that of the lamnids. However, at similar sizes, the lamnids did have higher WM LDH activities than did the common thresher and blue sharks (Fig. 1).
The finding of a low WM LDH activity in the thresher shark was unexpected
because threshers, like makos, are large and can jump clear of the water,
which requires very high exit speeds and WM burst power
(Carey and Teal, 1969a;
Wu, 1977
;
Goolish, 1995
). Furthermore,
because the common thresher is, among the species we studied, more closely
related to the lamnids and because it may also be endothermic, we expected its
enzyme activities to be similar to those of the lamnids. The difference may be
related to feeding habits. Unlike the mako and salmon sharks, threshers use
the long upper lobe of the caudal fin to herd prey (e.g. sardines and
anchovies) into tight groups for feeding
(Gubanov, 1972
), a behavior we
inferred from having captured threshers in the present study by hooking them
by the tail, which was also reported by Gruber and Compagno
(1981
). The use of the tail
for herding prey and feeding may require fewer or shorter bouts of burst
swimming than the predatory behavior of the lamnids. Thus, we propose that the
high WM LDH of the two lamnid species is related to their use of anaerobic
glycolysis during feeding and not specifically to endothermy.
It has been proposed for teleost fishes that species with a high WM LDH
activity also have correspondingly high WM CS activities to process lactate
during the post-burst recovery period (Dickson,
1995,
1996
;
Gleeson, 1996
;
Mollet and Cailliet, 1996
). We
found a significant positive correlation between WM LDH and WM CS activities
for the six shark species in which both activities were measured
(Fig. 5). The high CS activity
in lamnid WM suggests that, after repeated bouts of burst swimming, these
fishes are able to process lactate quickly and recover rapidly, as has been
hypothesized for tunas (Arthur et al.,
1992
; Dickson,
1995
).
|
Endothermy effects on lamnid WM enzyme activities
The activities of lamnid WM CS and LDH are even higher when estimated at
in vivo WM temperatures (Table
4; Fig. 4). Unlike
the small mass of deeply positioned RM, which can be assumed to have a uniform
temperature, quantification of the thermal enhancement on WM enzyme activities
requires the integration of the lamnid WM isotherms determined by Carey and
Teal (1969a) with a lamnid WM
distribution map (D. Bernal, unpublished) to estimate the relative amounts of
WM occurring at different temperatures
(Table 4). For example, a
salmon shark swimming in 10°C water would be expected to have an RM
temperature of 24°C (Anderson and
Goldman, 2001
; Bernal et al.,
2001a
). While the WM adjacent to the RM would have nearly the same
temperature, the temperature of WM just below the skin would be close to that
of the ambient water (Carey and Teal,
1969a
). A 148-kg salmon shark has 74 kg of WM (50% of body mass;
D. Bernal, unpublished). Much of this WM (32.6 kg) occurs in sufficient
proximity to the skin to not have an elevated temperature, but approximately
17.4 kg of it occurs within isotherms that are 24°C warmer than
ambient seawater, and 0.80 kg of it is 1012°C warmer than ambient
(Table 4). When the LDH
activity is adjusted for these regional in vivo temperature
elevations (using a Q10 of 1.70;
Table 3), total WM LDH activity
is 13.6% greater than if all the WM was at ambient temperature
(Table 4). A similar increase
would occur in WM CS activity, given the similar Q10 values
(Table 3). Although this
increase is much less than the 123% thermal enhancement calculated for salmon
shark RM CS activity, it could still contribute to the aerobic and anaerobic
performance of the WM.
Our results do not support the hypothesis that enzymes of WM positioned in
more peripheral (and thermally more variable) body regions are less sensitive
to changes in temperature than are enzymes in WM adjacent to the RM. Rather,
the finding of no significant thermal compensation in lamnid WM CS or LDH
activities (Fig. 4) is similar
to recent results for the Atlantic northern bluefin tuna (Thunnus
thynnus) in which WM enzyme activities did not show thermal compensation
along a similar thermal gradient (Fudge et
al., 2001). This suggests that the in vivo WM thermal
gradients in endothermic fishes are either too small or too unstable to have
induced changes in enzyme activity or the expression of different enzyme
isoforms.
In summary, CS activities in the WM and HV were higher in lamnid sharks when compared with active ectothermic sharks at a common reference temperature (20°C). RM CS activity at 20°C did not differ interspecifically, and activities of the other enzymes measured in the two lamnid species overlapped with those of the other shark species studied. In the mako, salmon and thresher sharks, the HV spongy myocardium had a significantly higher CS activity than the compact myocardium, but LDH activity did not differ in the two HV layers. When the enzyme activities in RM and WM of the endothermic sharks were estimated at in vivo temperatures, CS and LDH activities were elevated relative to what they would be at ambient temperature. Within the heterothermic WM of endothermic sharks, there were no detectable regional differences in WM CS or LDH activities or in thermal sensitivity of the enzymes. These findings parallel the general pattern demonstrated in previous studies for tissue metabolic enzyme activities in the endothermic tunas relative to their ectothermic sister species, substantiating the extent of convergence between the tunas and lamnid sharks.
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Acknowledgments |
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References |
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