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First published online August 8, 2008
Journal of Experimental Biology 211, 2559-2565 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018119
The ontogeny of aerobic and diving capacity in the skeletal muscles of Weddell seals
1 Department of Biology, Colorado State University, Fort Collins, CO 80523-1878,
USA
2 School of Kinesiology and Health Science, York University, Toronto, ON, Canada
M3J 1P3
3 Department of Biology, University of Michigan Flint, Flint, MI 48502,
USA
4 Biological Sciences Department, California State Polytechnic University,
Pomona, CA 91768, USA
5 Lillehei Heart Institute, University of Minnesota, Minneapolis, MN 55455,
USA
6 Department of Ecology and Evolutionary Biology, University of California at
Santa Cruz, Santa Cruz, CA 95060, USA
7 Department of Marine Biology, Texas A&M University at Galveston,
Galveston, TX 77551, USA
* Author for correspondence (e-mail: kanatous{at}lamar.colostate.edu)
Accepted 4 June 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: diving, hypoxia, ontogeny, seals, skeletal muscle
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Davis et al., 1991;
Kanatous et al., 1999;
Kanatous et al., 2001;
Kanatous et al., 2002;
Polasek et al., 2006).
The ontogeny of these skeletal muscle adaptations has been poorly described
in diving mammals. By contrast, numerous studies involving terrestrial mammals
have described changes in fiber type and metabolic profile of skeletal muscles
as a function of exercise and ontogeny. For example, detailed transcriptional
analyses have been undertaken to define upstream activation motifs including a
CCAC box, A/T element, nuclear factor of activated T cells (NFAT) response
element and E boxes that are necessary for muscle-specific transcription of
the oxidative fiber program that includes the transcription of myoglobin
(Chin, 2004;
Chin, 2005;
Chin et al., 1997;
Chin et al., 1998;
Chin et al., 2003). Following
differentiation, myoglobin and oxidative fiber expression are coordinately
regulated by neural and muscular activities that stimulate calcium signaling
within the cell; specifically through calcium-induced calcium release through
the interaction between cell surface l-type calcium channels and the ryanodine
receptors of the sarcoplasmic reticulum. Stimuli that enhance intracellular
calcium levels increase calcineurin (a Ca2+/calmodulin-dependent
serine phosphatase) activity and gene expression. Upon activation, calcineurin
dephosphorylates the transcription factor NFAT, which translocates to the
nucleus and combinatorially interacts with other transcription factors to
regulate myoglobin and oxidative fiber gene expression during ontogeny of
exercise in terrestrial mammals. Since myoglobin concentrations and oxidative
capacities are important adaptations in the skeletal muscles of divers, the
present study hypothesizes that the calcium calcineurin pathway will play an
important role in the ontogeny of the skeletal muscles in Weddell seals.
Smaller, juvenile animals are less capable divers than adults, partly
because of their higher mass-specific metabolic rate and proportionately
smaller blood and muscle oxygen stores
(Burns, 1996;
Burns et al., 1999;
Kooyman et al., 1983). In
addition, the muscle adaptations (as described above) that enhance diving
performance may not completely develop until a young animal is several years
old. As a result, they can neither dive as long nor as deep as adults. Young
Weddell seals are therefore at a disadvantage in their ability to forage on
deep-living prey such as Antarctic silverfish (Pleuragramma
antarcticum), and this appears to influence survival during the initial
years of life (Testa, 1987).
Therefore, the ontogeny of muscle aerobic capacity, lipid metabolism and
oxygen stores in the skeletal muscles is important for diving ability, yet we
have only recently begun to describe the development of these physiological
variables. In the present study, we investigate the ontogeny of skeletal
muscle adaptations that ultimately determine the diving capabilities of
Weddell seals and hypothesize a shift towards increased aerobic fibers and
enzymes as Weddell seals develop.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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7 years, mean mass 385±13 kg)
were captured with a purse string net along natural tidal cracks in McMurdo
Sound. The ages of all the seals were determined from flipper tags using the
data provided by personal communication with R. Garrott, J. Rotella and D.
Siniff. The seals were sedated with ketamine [1.5 mg kg–1
(Davis et al., 1983;
Davis et al., 1999)], weighed
with a hanging digital scale (accuracy ±0.5 kg), and muscle biopsies
were taken under local anesthesia. In order to standardize our sampling, all
biopsies were taken from the mid-belly of the muscle and at the same location
in all age classes (one-third of the body length from the tail). Pups were
returned to their mothers within 30 min of recovery from mild sedation.
Juveniles and adults were detained for less than one hour post-biopsy and were
released near the site of capture once the animals had regained full voluntary
locomotion.
Muscle biopsies
Three muscle samples of approximately 50 mg each were collected with a 6-mm
biopsy cannula (Depuy, Warsaw, IN, USA) from the swimming (m. longissimus
dorsi) muscle. Control samples were collected from the m. soleus, a
predominantly slow oxidative muscle of laboratory rats (Sprague Dawley) and
mice (C57/Bl/6) euthanized by cervical dislocation after 2–3 min of
carbon dioxide anesthesia. Muscle samples were placed either into 2%
glutaraldehyde fixative or frozen in liquid nitrogen immediately upon
collection for western immunoblot analysis, measurement of enzyme activities
and myoglobin concentration. Due to the extremely cold environmental
temperatures at the time of sampling (–10 to –40°C), the
muscle samples for fiber typing were split to determine the best
cryopreservation technique under these conditions. One half were mounted on
foil in gum tragacanth and tissue freezing compound and frozen in liquid
nitrogen-cooled isopentane for 15–30 s. The other half were fixed in 4%
paraformaldehyde overnight, incubated in a sucrose/glycerol-based
cryoprotectant for 30 min to minimize freeze artifact, mounted on foil in gum
tragacanth and tissue freezing compound and frozen in liquid nitrogen-cooled
isopentane for 15–30 s. Frozen samples were stored at –80°C
until analysis for western immunoblot analysis, fiber typing, enzyme activity
and myoglobin concentration.
Measurement of enzyme activities and myoglobin concentrations
Muscle samples were thawed, weighed and homogenized at 0°C in buffer
containing 79% phosphate-buffered saline (PBS), 20% glycerol, 1% TWEEN 20, 1
mmoll–1 DL-dithiothreitol and protease inhibitor
cocktail. The homogenates were spun for 4–5 min at 10,000
g, and the supernatant was divided into aliquots and stored at
–80°C until used for the assays. The enzymes assayed were as
follows: citrate synthase (CS), important in the citric acid cycle; cytochrome
c oxidase (COX), as a measure of the flux though the electron
transport chain; β-hydroxyacyl CoA dehydrogenase (HAD), an indicator for
the β-oxidation of fatty acids; lactate dehydrogenase (LDH), needed for
the conversion of pyruvate to lactate in anaerobic glycolysis; total
intramuscular lipase, important for the uptake of fatty acids from circulating
triglycerides (TAG) in lipoproteins (e.g. chylomicrons and VLDL). Activities
of CS and COX were measured with a Beckman Coulter DU800 Series
spectrophotometer (Fullerton, CA, USA). Temperature was maintained at 37°C
using a Beckman Peltier Temperature Controller. Activities of LDH, HAD and
lipase were measured with a BioTek Synergy HT Multi-Detection microplate
reader (Winooski, VT, Canada) at 37°C. The assay conditions were as
follows. LDH: 50 mmoll–1 imidazole; 0.15
mmoll–1 NADH, pH 7.0 at 37°C; 1 mmoll–1
pyruvate; 
A340, millimolar extinction coefficient
(
340)=6.22. HAD: 50 mmoll–1 imidazole, 1
mmoll–1 EDTA, 0.1 mmoll–1 acetoacetyl CoA,
and 0.15 mmoll–1 NADH, pH 7.0 at 37°C;
A340, 340=6.22. CS: 50
mmoll–1 imidazole; 0.25 mmoll–1
5,5'-dithiobis(2-nitrobenzoic acid) (DTNB); 0.4 mmoll–1
acetyl CoA; 0.5 mmoll–1 oxaloacetate, pH 7.5 at 37°C;
A412, 412=13.6. COX: 0.1
mmoll–1 DTT; 0.22 mmoll–1
ferrocytochrome-c, 10 mmoll–1 Tris-HCl pH 7.0 with
120 mmoll–1 KCl; A550 550=21.84.
Total lipase activity was determined using a long-wavelength fluorescent assay
kit from MGT Incorporated (product M1214; Eugene, OR, USA). Specific enzyme
activities (µmol min–1 g–1 wet mass
muscle) were calculated from the rate of change of the assay absorbance at the
maximal linear slope. Aliquots from the supernatant after centrifugation were
used for myoglobin assays. Myoglobin aliquots were diluted with phosphate
buffer (0.04 moll–1, pH 6.6) and the resulting mixture
centrifuged for 50 min at 28,000 g at 4°C. As previously
described (Kanatous et al.,
1999) the method of Reynafarje (Reynafarje, 1963) was adapted and
used to determine myoglobin concentration. In brief, the supernatant was
bubbled with 99.9% carbon monoxide (CO) for 3 min to convert the myoglobin to
carboxymyoglobin. The absorbance of the supernatant at 538 and 568 nm was
measured using a Bio-Tek PowerWave 340x microplate reader. A myoglobin
standard (horse myoglobin, Sigma-Aldrich, St Louis, MO, USA) was included with
each set of samples. The myoglobin concentrations were calculated as described
previously (Reynafarje, 1963) and expressed in mg g–1 fresh
tissue.
Immunohistochemical fiber typing
Cross sections of each muscle sample were cut into serial thin sections
(7–9 µm) with a Shandon cryotome (Waltham, MA, USA) maintained at
–20°C. Sections were placed onto glass slides, four serial sections
per slide. Transverse orientation was verified using a standard light
microscope. Fiber type distribution was determined and verified utilizing two
methods.
In the first method, sections were stained using a metachromatic ATPase
staining protocol modified from (Ogilvie
and Feeback, 1990). Briefly, the procedure was as follows: (1)
ATPase pre-incubation for 8 min (pH 4.5) at room temperature, (2) three 2-min
rinses in Tris buffer (pH 7.8), (3) incubation with ATP for 25 min (pH 9.4),
(4) three calcium chloride rinses, (5) counterstaining in 0.1% Toluidine Blue
for 1 min, (6) dehydration in ethanol and (7) clearing in xylene. The
proportion of slow oxidative (Type I), fast-twitch oxidative (Type IIA) and
fast-twitch glycolytic (Type IIB) fibers was determined by standard point
counting procedure and is presented as a percentage relative to the total
number of fibers.
In the second method, slides were fixed in ice-cold alcohol–formalin
acetic acid fixative in a Coplin jar, washed with PBS, and a proteinaceous
blocking agent was applied to each section to minimize non-specific antibody
binding. A series of monoclonal antibodies specific to myosin heavy chain
isoforms Type I, Type IIA and Type IIB was applied to one section on each of
the slides and incubated overnight in a humidity chamber at 4°C. Serial
amplification of the primary antibody was accomplished using an incubation of
biotinylated secondary antibody for 20 min, followed by a series of PBS
washes, followed by a 20-min incubation with alkaline-phosphotase streptavidin
conjugate. After washing with PBS, Fast Red substrate or DAB
(3,3'-diaminobenzidine tetrahydrochloride) chromagen was applied to the
slides. When adequate color development was seen, the slides were washed in
water or a peroxidase to stop the reaction. The slides were counterstained
with Mayer's hematoxylin, washed in water, and a coverslip was mounted onto
the slide with Dako glycergel. A sample of approximately 200–400
artifact-free fibers showing good staining was counted from each section using
a camera-mounted microscope attached to a PC loaded with BIOQUANT software
(Bioquant, Nashville, TN, USA). Fibers showing a reaction to a specific
antibody were considered to have that myosin heavy chain isoform. Percentages
of Type I, Type IIA, and Type IIB fibers were generated for each sample
(Kanatous et al., 2002). Since
there were no differences in the results from the metachromatic or
immunohistochemical staining techniques, the results for the metachromatic
stain were reported.
Western blot analysis to define the changes in the expression of calcium regulatory and responsive proteins
Western blot analysis was performed according to a previously published
protocol (Garry et al., 1998;
Wu et al., 2000) to determine
changes in the expression of proteins. Rabbit anti-myoglobin serum (1:3000;
DAKO, Carpenteria, CA, USA), mouse anti-calsequestrin (1:1000; Affinity
Bioreagents, Golden, CA, USA), mouse anti-calcineurin B (1:250; Affinity
Bioreagents), mouse anti-InsP3 (1:2000; Affinity
Bioreagents) and mouse anti-SERCA2 ATPase (1:2500; Affinity Bioreagents) were
used as the primary antiserum, which was detected using a horseradish
peroxidase (HRP)-conjugated secondary antiserum. Protein bands were visualized
using a chemiluminescent reagent (Pierce Supersignal reagent, Rockford, IL,
USA) and intensity was quantified using a computerized digital analysis
program (Scion Image 1.62c; Scion Corp., Frederick, MO, USA).
Statistical analysis
Statistical analysis was performed by analysis of variance (ANOVA) with
Tukey post-hoc tests (P
0.05, Sigmastat 2.0). Results are
presented as means ± s.e.m.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). The swimming muscles of adult Weddell seals are composed of
a mixed fiber type population consisting primarily of slow-twitch oxidative
fibers (Type I), fast-twitch oxidative fibers (Type IIA) and a near absence of
fast-twitch glycolytic fibers (Type IIB)
(Fig. 1A). The total volume
density of mitochondria was also similar to previously reported values
(Kanatous et al., 2002) and
comparable to sedentary terrestrial mammals of similar body mass
(Fig. 2).
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0.05). These results indicate that the aerobic potential
of pups and juveniles is significantly greater than that of adults and similar
to that of comparably sized athletic terrestrial mammals and short-duration
divers (Fig. 2). To our
knowledge, Weddell seals are the first reported species where there is a shift
from aerobic Type I fibers to an increase in Type IIA associated with a
significant decrease in mitochondrial density as the animals mature.
Age-related changes in myoglobin concentrations and enzyme activities
The changes in the concentration of myoglobin revealed some unexpected
results. Myoglobin assays and western immunoblot analysis (Figs
3,
4) revealed that juveniles had
a significantly greater concentration of myoglobin in their swimming muscles
as compared with both adults and pups (72.4±7 vs
55.9±2.5 and 35.5±3 mg g–1 wet mass muscle,
respectively). In addition, adult myoglobin values were significantly greater
than those in pups (Fig. 3,
55.9±2.5 vs 35.5±3 mg g–1 wet mass
muscle; ANOVA, P0.05).
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There were no significant differences in enzyme activities between the pup, juvenile and adult for either of the aerobic enzymes CS or COX (Table 1). Although not statistically significant, LDH, a marker of anaerobic capacity, tended to be higher in pups than in either juveniles or adults. The pups had significantly higher HAD activity (2.5x) (Table 1) and a greater lipase activity (1.5x) (Table 1) than either the juveniles or adults. There was no statistical difference in any enzyme activity between the adult and juvenile age classes. The CS:HAD ratio, an index of the contribution of fatty acid metabolism to overall aerobic metabolism, was 0.3 in the juveniles and adults and 0.1 in pups. This indicates a greater reliance on fatty acid metabolism for the maintenance of aerobic metabolism in pups than in either juvenile or adult seals.
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Ontogenetic changes in the expression of calcium regulatory and responsive proteins
Western blot gel electrophoresis showed that juvenile Weddell seals had the
highest levels of relative protein expression for all the proteins. As seen
previously with the myoglobin assay, the juveniles had a significantly greater
expression of myoglobin compared with either the pups or adults
(P<0.05). In addition, calcineurin, InsP3
receptor and SERCA2 ATPase were significantly greater (P<0.05) in
the juveniles and pups as compared with the adults
(Fig. 4).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Cobb et al., 1994b;
Dearolf et al., 2000;
Schutt et al., 1994).
The difference in Weddell seal skeletal muscle physiology observed during
this study may arise from the three very distinct stages of their life
history. During the first few weeks of life, Weddell seal pups are a
non-diving terrestrial mammal that must rely on lanugo (natal fur) for
thermoregulation in the extremely harsh environmental conditions of
Antarctica. Together with the thermoregulatory benefits of lanugo, an
increased aerobic capacity may provide additional thermogenesis during
lactation and weaning. Juveniles possess low whole-body oxygen stores and
hence relatively poor diving capacity compared with adults
(Burns et al., 2005;
Burns et al., 2007;
Clark et al., 2006;
Clark et al., 2007;
Fowler et al., 2006;
Weise and Costa, 2007). This
is reflected in both skeletal muscle physiology and mean dive durations
(Burns, 1996) that are
consistent with those of short-duration, shallower divers such as the Steller
sea lion (Fig. 2). As juveniles
continue to mature into elite deep divers, their skeletal muscles are
transformed to a more sedentary state in order to maintain low levels of
aerobic metabolism under the hypoxic conditions associated with long-duration
diving. These results are in contrast to our original hypothesis, which
expected a shift towards more aerobic fibers as Weddell seals matured and
adapted to the hypoxic conditions associated with prolonged diving.
An important and novel finding in our present study is that the
concentration of myoglobin is significantly higher in juveniles than adults.
Previous studies investigating body oxygen stores in diving mammals have
reported neonates with significantly lower total body oxygen stores compared
with adults. This suggests that increases in oxygen stores are triggered by
foraging (Burns et al., 2005;
Clark et al., 2006;
Clark et al., 2007;
Fowler et al., 2006;
Weise and Costa, 2007). Our
findings that the pups had significantly lower concentrations of myoglobin and
hence lower intramuscular stores of oxygen compared with either the adults or
juveniles support these previous studies. The result that juveniles had
increased myoglobin concentrations when compared with adults has not been
shown in aquatic or terrestrial mammals. Although it has been reported that
juvenile seals have significantly shorter dive durations, they are
considerably more active swimmers than adults
(Burns et al., 1999;
Call et al., 2007). Recent work
in our laboratory has found that hypoxia as a lone stimulus was not sufficient
to induce the expression of myoglobin in either cell culture or whole-animal
mouse studies. However, we found that hypoxia in combination with exercise
became a powerful stimulus for the induction of myoglobin in skeletal and
cardiac muscle (S.B.K., unpublished data). This suggests that the elevated
energetic activity underwater in combination with breath-hold diving in
juveniles may be the stimulus for their greater myoglobin expression when
compared with adults. As the animals mature they employ energy-conserving
modes of locomotion and lower their energetic output during diving
(Davis et al., 1999;
Kanatous et al., 2002) as
myoglobin levels simultaneously decrease. In other words, the expression of
myoglobin under hypoxic conditions is directly correlated to activity level in
both terrestrial and diving mammals.
In contrast to total body oxygen stores, which significantly increase as
these seals mature, the aerobic potential of the skeletal muscle is
significantly greater in the muscles of pups than in either juveniles or
adults. This would appear to be a paradox in that the pups do not dive and are
quite sedentary compared with the diving, and therefore relatively more
active, juveniles and adults. However, we hypothesized that, as homeotherms,
Weddell seal pups would be under thermoregulatory stress in order to maintain
a core temperature of 37°C in an environment with temperatures ranging
between –10 and –40°C. Noren et al. recently observed that,
although pups had the greatest proportion of blubber among the three age
classes, their greater surface area to volume ratio and limited ability to
minimize body-to-environment temperature gradients lead to the greatest
calculated mass-specific heat loss (Noren et al., 2008). This implies that
immature seals rely on elevated metabolic heat production to counter heat
loss. This metabolic heat production is also substantiated by fatty acid
analysis among age classes of Weddell seals, where levels of
triglyceride-based fatty acids in the skeletal muscle were greatest in pups
(S.J.T., unpublished data). Interestingly, it has been shown that acute
exercise in humans is accompanied by an increase in muscle triglyceride
breakdown, which increases whole-body fatty acid oxidation for up to 16h,
adding to overall heat production (Schenk
and Horowitz, 2007). Therefore, we believe that this partitioning
of more fatty acids toward triglyceride synthesis within locomotor muscles
especially in pups, can provide a foundation in which thermoregulatory demands
can be met. Moreover, the lack of enhanced aerobic enzyme activities
associated with the significantly greater mitochondrial volume density and
enhanced lipolytic enzyme capacities in the pups suggests a metabolic
uncoupling in favor of heat production through non-shivering thermogenesis
(Duchamp and Barre, 1993;
Dulloo et al., 2002;
Solinas et al., 2004). We
suspect that elevated mitochondrial volume densities in the skeletal muscles
of pups potentially have a greater role in uncoupled non-shivering
thermogenesis (Blix and Steen,
1979; Blix et al.,
1979; Grav and Blix,
1979). As the animals mature, their heat loss to the environment
decreases, reducing the need for additional metabolic heat production leading
to an overall decrease in mitochondrial volume densities.
In an effort to unravel some of the underlying mechanisms regulating the
ontogenetic changes in Weddell seal skeletal muscle physiology, we undertook
western immunoblot analysis of selected calcium regulatory proteins. It has
been well established in terrestrial animals that calcium signaling, as well
as its downstream targets of calcineurin and NFAT, plays an important role in
determining fiber type distribution, aerobic capacity and myoglobin
concentrations in skeletal muscles (Chin
et al., 1998; Schiaffino et
al., 2007; Spangenburg and
Booth, 2003). While numerous calcium regulatory and sensitive
proteins were tested, due to the unique nature of our animal model our
analysis was limited to those antibodies that gave reliable and reproducible
results (calcineurin B, calsequestrin, InsP3 receptor and
SERCA2). As observed in terrestrial mammals, the protein expression pattern of
calcineurin B was similar to the expression pattern of myoglobin
(Chin et al., 1998). More
specifically, these patterns were significantly higher in the juveniles
compared with the adults. However, in contrast to the expression of myoglobin,
calcineurin B was not significantly different in the pup but did show a trend
toward being higher in the juvenile. These changes in calcineurin may aid in
the fiber type conversion that is observed between the three age classes, but
is not likely to be the sole determining factor.
The expression of InsP3 receptor and SERCA2 in skeletal
muscle also shows an age-related pattern where pups and juveniles have
consistently higher expression levels than adults. With respect to
calsequestrin, we found no differences between the age classes in the
expression levels using western immunoblot analysis. These findings are in
contrast to that observed in terrestrial mammals where an increase in
InsP3 receptor, SERCA, ryanodine and calsequestrin
expression is observed between neonates and juveniles
(de Jonge et al., 2006;
Eizema et al., 2007). While
the significance of these findings is limited on their own, the changes in
these calcium regulatory proteins provide some preliminary insight into the
regulation of calcium during the developmental changes within Weddell seal
skeletal muscle.
It has been shown that the regulation of fiber type in terrestrial mammals
is a combination of neural and molecular regulation and is independent of
oxidative capacity (Chin,
2005). While our results support the hypothesis that there is a
shift in fiber type population due to an increase in aerobic capacity and
mitochondrial volume density as the animals become more active, our
aquatically linked model differs from the terrestrial system in that the fiber
type shift was associated with a significant decrease in mitochondrial density
and no change in aerobic enzyme capacity. This would serve to decrease
cellular and, in turn, overall metabolism necessary for increased diving
capacity in deep and long-duration divers while maintaining the metabolic flux
through the citric acid cycle in response to chronically hypoxic episodes
associated with long-duration diving.
In addition to the results revealed here, we have successfully isolated RNA from all of the age classes and transformed it into cDNA for subtractive hybridization analysis. Our initial subtractions between adults and pups have yielded over 20 transcripts that are upregulated in the adult compared with the pup. In addition, the transcripts identified from the subtraction correlate with our physiological results. We have identified transcripts for myoglobin, myosin heavy chain IIa, calcineurin, cytochrome c oxidase and NADH dehydrogenase. The subtractive hybridization analysis further corroborated our physiological analysis indicating that the adults had a significantly greater percentage of fast-oxidative fibers as well as myoglobin concentration. These initial findings also suggest that some of the changes in physiology are regulated at the transcript level, further supporting the role of the calcium/calcineurin pathway in regulating the changes in mammalian skeletal muscle even in diving mammals. While this analysis is far from complete, we believe it is an important first step to be able to find transcripts that are representative of the changes in mammalian physiology.
In summary, newborn Weddell seal pups have an extremely high aerobic potential, with mitochondrial volume densities similar to those found in terrestrial animal athletes and short-duration divers. However, this enhanced aerobic capacity is not an adaptation towards diving but is due to their high fat diet and the need to offset thermoregulatory costs associated with the harsh environment of Antarctica. As the young seals begin to dive and mature into juveniles, their skeletal muscles begin to transform; as juveniles, they initiate the development of fast-oxidative fibers and significantly increase their intramuscular stores of oxygen in the form of oxymyoglobin. As they continue to mature and increase their diving capacity, Weddell seals increase their anaerobic capacity by significantly increasing their percentage of Type IIA fast-oxidative fibers in their skeletal muscles. In addition, their skeletal muscles transform to a relatively more sedentary state in order to maintain low levels of aerobic metabolism under the hypoxic conditions associated with long-duration diving (Fig. 5). The results of this study also indicate that these changes in skeletal muscle physiology are associated with changes in calcineurin expression. In conclusion, the skeletal muscles of Weddell seals undergo a unique transformation from a state of high aerobic potential, to offset the thermoregulatory costs as pups, to lower aerobic potential as adults in order to maintain the low levels of aerobic metabolism associated with long-duration diving.
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Acknowledgments |
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References |
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denotes
significantly different from adults (P<0.05, ANOVA).
