Does intracellular metabolite diffusion limit post-contractile recovery in burst locomotor muscle?
1 Department of Biology and Marine Biology, University of North Carolina
Wilmington, 601 South College Road, Wilmington, NC 28403-5915, USA
2 Department of Chemical and Biomedical Engineering, FAMU-FSU College of
Engineering, Tallahassee, FL 32310-6046, USA
* Author for correspondence (e-mail: kinseys{at}uncw.edu)
Accepted 12 May 2005
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Summary |
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Key words: muscle fiber, fiber growth, diffusion, metabolic modeling, reactiondiffusion, exercise, metabolism, scaling, crustacean, blue crab, Callinectes sapidus, phosphagen, arginine phosphate, mitochondria
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Introduction |
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While the value of purely kinetic analyses of muscle energy metabolism is
readily apparent, the conditions under which diffusive flux may be important
in either limiting the net rate of aerobic processes or influencing the
evolution of metabolic pathways are unresolved
(Suarez, 2003). The principal
hurdle to understanding the role of diffusion and metabolic organization is
that most metabolic measurements constitute weighted-averages over an entire
cell or tissue, making it difficult to observe localized intracellular events
or concentration gradients. However, several studies that employed
reactiondiffusion mathematical modeling of aerobic metabolism found
theoretical evidence for concentration gradients in high-energy phosphate
molecules during steady-state contraction in muscle (Mainwood and Rakusan,
1982; Meyer et al., 1984
;
Hubley et al., 1997
;
Aliev and Saks, 1997
;
Kemp et al., 1998
;
Vendelin et al., 2000
;
Saks et al., 2003
). The
intracellular diffusive flux of high-energy phosphates is largely mediated by
phosphagen kinases, such as creatine kinase (CK) and arginine kinase (AK),
although the mechanistic details are still the subject of study (reviewed by
Walliman et al., 1992
;
Ellington, 2001
).
In an effort to understand the role of diffusion and metabolic organization
on the control of metabolism, we have been examining metabolic processes in an
extreme anaerobic muscle model system. The muscles that power burst swimming
in the blue crab, Callinectes sapidus, grow hypertrophically, and
during post-metamorphic development the diameter of fibers increases from
<60 µm in juveniles to >600 µm in adults
(Boyle et al., 2003). Moreover,
the distribution of mitochondria changes dramatically during development. In
small anaerobic fibers, mitochondria are uniformly distributed throughout the
cell, whereas in large fibers the mitochondria are largely clustered at the
sarcolemmal membrane, forming an oxidative cylinder at the periphery of the
cell (Boyle et al., 2003
).
Thus, the average distance between mitochondria in small fibers is several
microns, while in large fibers there may be hundreds of microns between
mitochondrial clusters. The potentially limiting rate of diffusive flux of
metabolites over such large distances is exacerbated by intracellular barriers
in muscle that lead to a time-dependent reduction in metabolite diffusion
coefficients for movement in the direction perpendicular to the fiber axis
(D
) (Kinsey et
al., 1999
; De Graaf et al., 2001;
Kinsey and Moerland, 2002
).
This means that over the short diffusion distances characteristic of small
anaerobic fibers, the D
is about 2-fold higher than
the D
for the long diffusion distances that typify
large fibers. While the burst contraction function of these muscles should not
be impacted by intracellular diffusion, the aerobic recovery process may be
compromised by the extreme size of the fibers in adult animals. There are, in
fact, substantial size-dependent differences in the recovery of the anaerobic
fibers following burst contraction. Small anaerobic fibers accumulate lactate
and modestly deplete glycogen during burst contraction, and both of these
metabolites recover to resting levels relatively quickly following an exercise
bout (Boyle et al., 2003
;
Johnson et al., 2004
). The
large fibers similarly accumulate lactate and deplete glycogen during
contraction, but following exercise they continue to accumulate large amounts
of lactate and further deplete glycogen. Full aerobic recovery of these
metabolites requires several hours in adult blue crabs
(Milligan et al., 1989
;
Henry et al., 1994
;
Boyle et al., 2003
;
Johnson et al., 2004
).
We have previously hypothesized that anaerobic metabolism is recruited
following burst contractions in the large anaerobic fibers to accelerate
certain key phases of recovery that would otherwise be overly slow due to
intracellular diffusion constraints
(Kinsey and Moerland, 2002;
Boyle et al., 2003
;
Johnson et al., 2004
). In the
present study, we tested this hypothesis by examining the fiber size
dependence of the rate of post-contractile arginine phosphate (AP)
resynthesis, and these data were compared to a mathematical
reactiondiffusion model of aerobic metabolism in crab fibers. The
phosphagen, AP, is the initial energy source used during burst contraction,
and its rapid resynthesis following an initial exercise bout allows subsequent
high-force contractions. We predicted (1) that the measured rate of AP
resynthesis would be independent of fiber size, (2) that the predicted rate of
AP resynthesis by aerobic metabolism would be fiber size dependent, with a
considerably lower rate in large fibers than in small fibers, and (3) that the
contributions of anaerobic metabolism would offset intracellular diffusive
flux limitations on AP recovery in the large fibers, which would account for
the expected contradictory results of (1) and (2) above. Our results were
consistent with these predictions, with the exception that intracellular
metabolite diffusion does not appear to be a substantial limiting factor of AP
recovery rate in large fibers. This suggests that the low fiber surface
area:volume ratio (SA:V), which may limit oxygen flux, is a
more important determinant of metabolic rate and/or metabolic design in the
large fibers.
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Materials and methods |
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Exercise protocol
Crabs were induced to undergo a burst swimming response as described
previously (Boyle et al., 2003;
Johnson et al., 2004
). Crabs
were held suspended in the air by a clamp in a manner that allows free motion
of the swimming legs, and small wire electrodes were placed in two small holes
drilled into the mesobranchial region of the dorsal carapace. A Grass
Instruments SD9 physiological stimulator (Astro Med, Inc., West Warwick, RI,
USA) was used to deliver a small voltage (80 Hz, 200 ms duration, 10 V
cm1 between electrodes) to the thoracic ring ganglia, which
elicited a burst swimming response in the 5th periopods for several seconds
following the stimulation. A single pulse was administered every 30 s until
the animal was no longer capable of a burst response, which was evident when
it responded by moving its legs at a notably slower rate. Immediately
following exercise, animals were returned to aerated full-strength seawater
for a recovery period of 0, 15, 30 or 60 min.
Metabolite measurement
At the end of the recovery period, crabs were rapidly cut in half along
their sagittal plane in order to minimize spontaneous burst contraction of the
swimming legs that typically occurs during sacrifice. The dorsal carapace,
reproductive and digestive organs were removed and the basal cavity, which
houses the muscles of the fifth periopod, was exposed. The light levator
muscle was rapidly isolated by cutting away the surrounding muscle and
freeze-clamped while still intact within the animal. The time elapsed from
sacrifice to freeze clamping the muscle was 6090 s. Tissue samples were
immediately homogenized in a 635-fold dilution of chilled 7% perchloric
acid with 1 mmol EDTA using a Fisher Powergen 125 homogenizer and then
centrifuged at 16 000 g for 30 min at 4°C. The supernatant
pH was neutralized with 3 mol l1 potassium bicarbonate in 50
mmol l1 PIPES, stored on ice for 10 min and centrifuged at
16 000 g for 15 min at 4°C. The supernatant was
immediately analyzed by 31P nuclear magnetic resonance (NMR)
spectroscopy. NMR spectra were collected at 162 MHz on a Bruker 400 DMX
spectrometer to determine relative concentrations of AP and inorganic
phosphate (Pi). Spectra were collected using a 90° excitation
pulse and a relaxation delay of 12 s, which ensured that the phosphorus nuclei
were fully relaxed and peak integrals for the metabolites were proportional to
their relative concentrations. Forty-eight scans were acquired for a total
acquisition time of 10 min. The area under each peak was integrated using
Xwin-NMR software to yield relative concentrations of each metabolite. Two-way
analysis of variance (ANOVA) was used to analyze the post-contractile
metabolite concentrations for the interaction between size class and recovery
time. All metabolite data are presented as means ±
S.E.M.
Mathematical modeling
The general modeling approach was the same as that described in Hubley et
al. (1997), with parameters
adjusted to comply with blue crab fibers and the addition of a mitochondrial
reaction boundary condition, a basal rate of ATP consumption, an appropriate
kinetic expression for the phosphagen kinase (AK), and
D
values from crustacean anaerobic fibers that
incorporated the time dependence of diffusion
(Kinsey and Moerland, 2002
).
The diffusion and reaction of ATP, ADP, AP, arginine (Arg) and Pi
were modeled in a one-dimensional system that extended from the surface of a
mitochondrion to a distance (
/2) equal to half of the mean free
spacing between mitochondria or between clusters of mitochondria. Reactions
catalyzed by AK, myosin ATPase and basal ATPase were assumed to occur
homogenously throughout the domain 0
x
/2, where
x is distance from the mitochondrial surface. A burst
contractionrecovery cycle was modeled in the anaerobic, light levator
fibers (so named because they lack the high density of mitochondria that give
the aerobic, dark levator its characteristic pigmentation;
Tse et al., 1983
) using
conditions appropriate for a small (100 µm)-diameter fiber from a juvenile
animal and a large (600 µm)-diameter fiber from an adult. Simulations were
generated using the finite element analysis software, FEMLAB (Comsol, Inc.,
Burlington, MA, USA).
Temporally and spatially dependent concentration profiles of ATP, ADP, AP,
Arg and Pi were calculated according to the molar-species
continuity equation:
![]() | (1) |
The mitochondrial boundary conditions at x=0 balance the fluxes of
ATP and ADP into the bulk phase with the rates of formation and consumption at
the mitochondria and are modeled using Michaelis-Menten kinetics with ADP
activation (Meyer et al.,
1984):
![]() | (2) |
![]() | (3) |
![]() | (4) |
AK catalyzes the reversible phosphoryl-transfer reaction
AP+ADPArg+ATP, and intracellular AP serves as the initial energy source
used during burst contraction in crustacean muscle. The reaction proceeds by a
rapid equilibrium, random mechanism and was modeled according to the kinetic
expression of Smith and Morrison
(1969
):
![]() | (5) |
Myosin ATPase was modeled using Michaelis-Menten kinetics
(Pate and Cooke, 1985;
Hubley et al., 1997
):
![]() | (6) |
For each simulation, myosin ATPase was activated for 7 s at 10 Hz to
simulate burst contraction and was then deactivated during the
post-contractile recovery period, whereas the basal ATPase was active
throughout the entire contractionrecovery cycle. Small fibers (100
µm diameter) were modeled assuming a uniform distribution of mitochondria,
whereas large fibers (600 µm diameter) were assumed to have mitochondria
only at the periphery of the fiber (subsarcolemmal mitochondria) as described
in Boyle et al. (2003). Large
fibers were also modeled assuming a uniform distribution of mitochondria in
order to assess the consequences of the extreme diffusion distances (300
µm) associated with an exclusively subsarcolemmal distribution.
Model input parameters are detailed in
Table 1. Theresting metabolite
concentrations for crustacean anaerobic locomotor fibers were obtained from a
combination of the data in Head and Baldwin
(1986), 31P-NMR
spectra collected by Kinsey and Ellington
(1996
) and calculations using
the AK equilibrium constant (Teague and
Dobson, 1999
). The resting metabolite concentrations were the same
in small and large fibers (Baldwin et al.,
1999
). The D
values for each metabolite
were based both on direct measurements from crustacean anaerobic fibers and
calculations from the relationship of molecular mass and
D
in these fibers
(Kinsey and Moerland, 2002
).
The D
used for the short diffusion distances
characteristic of small fibers was higher than that for the long distances
found in large fibers due to the time dependence of radial diffusion in muscle
(Kinsey et al., 1999
;
Kinsey and Moerland, 2002
).
Intracellular diffusion distances (
/2) were estimated from the total
mitochondrial fractional area
(Amito/Acell), which was 0.026 in
small fibers and 0.017 in large fibers (recalculated from data collected by
Boyle et al., 2003
), and the
mean area/mitochondrion (amito), which was 0.608
µm2 (Boyle et al.,
2003
), using the relationship:
![]() | (7) |
|
While the model generated temporally and spatially resolved concentrations
of metabolites, our experimental measurements yielded values that were
spatially averaged across the fiber. In order to compare the model results
with the experimental data, some of the model data were mathematically volume
averaged over the domain from x=0 to x=/2:
![]() | (8) |
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Results |
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Examples of 31P-NMR spectra from perchloric acid muscle extracts demonstrate the reciprocal change of AP and Pi during a burst exerciserecovery cycle that results from the stoichiometric coupling of cellular ATPases (including myosin ATPase) and the AK reaction (Fig. 1). The time course of relative changes in AP and Pi concentrations is shown in Fig. 2, where the NMR peak integrals at each time point have been normalized to the mean resting integrals to allow direct comparison of the rate of recovery in small and large animals. A rapid depletion of AP (and increase in Pi) is followed in small and large fibers by a slow recovery that is complete in about 60 min. Despite the large differences in body mass and fiber size between the small and large animals, the rate of recovery was essentially the same for both groups and there was no significant interaction between size class and recovery time for AP (F=0.63, d.f.=3, P=0.60) or Pi (F=1.78, d.f.=3, P=0.16).
|
|
Reactiondiffusion analysis of contraction and recovery
Since the size independence of post-contractile AP resynthesis described
above presumably arises from anaerobic contributions to recovery in the large
fibers (Boyle et al., 2003;
Johnson et al., 2004
), the
reactiondiffusion analysis allows us to test whether this pattern
results from diffusive constraints on the aerobic component of recovery. The
spatially and temporally resolved concentrations of high-energy phosphate
molecules are presented in Fig.
3. The rate of recovery was somewhat faster in the small than in
the large fibers, and there were no intracellular gradients in small fibers,
as expected. However, there were only mild gradients present in the large
fibers, indicating that diffusive flux is fast relative to the mitochondrial
reaction (Fig. 3). This result
is not consistent with intracellular diffusive flux limiting aerobic
metabolism during post-contractile recovery, even though metabolite diffusion
in the large fiber was modeled over a distance of 300 µm. Thus, the
relatively small differences between the small and large fibers in
Fig. 3 result almost
exclusively from differences in mitochondrial density
(Table 1). The model results
were also volume-averaged to allow a comparison of the observed and simulated
recovery rates. The observed and modeled AP recovery data are in agreement for
the small fibers, but in the large fibers it is clear that aerobic metabolism
alone could not account for the relatively high observed rate of
post-contractile recovery (Fig.
4). Thus, anaerobic metabolism appears to accelerate AP recovery
in the large fibers, but in the context of the present model this simply
serves to offset the mass-specific decrease in aerobic capacity that typifies
metabolic scaling in general (Schmidt-Nielson, 1984) and not to compensate for
diffusion limitations.
|
|
If recovery in the large fibers is not substantially constrained by diffusion, then how close are the fibers to being limited by intracellular diffusive flux? Fig. 5 shows the effect of incremental increases in the rate of the mitochondrial boundary reaction. It can be seen that doubling the Vm,mito leads to the formation of only slightly steeper concentration gradients, which means that there is a minimally increased control of aerobic flux by intracellular diffusion, and the concentration gradients grow more substantial as Vm,mito is further increased. However, it is also clear that the metabolic recovery rate increases in proportion to the increases in Vm,mito. Only when unrealistically high rates of Vm,mito are used do steep concentration gradients appear, indicating diffusion limitation of recovery rate. Thus, the mitochondrial reaction rate used in the model fits our data well (Fig. 4) and is considerably below that which would lead to substantial diffusive limitations of aerobic flux in large fibers (Fig. 5).
|
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Discussion |
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It is well established that some crustacean muscles produce lactate
following contraction, and it has been speculated that this leads to an
increased rate of metabolic recovery
(Ellington, 1983;
Head and Baldwin, 1986
;
Kamp, 1989
;
Henry et al., 1994
;
Baldwin et al., 1999
;
Morris and Adamczewska, 2002
;
Johnson et al., 2004
). We
first described the fiber size-dependence of post-exercise glycogen depletion
(Boyle et al., 2003
) and
lactate production (Johnson et al.,
2004
) in crustacean muscle and attributed the observed pattern to
the long intracellular diffusion distances and/or the low
SA:V associated with the large developmental increase in
fiber size. While the studies cited above suggested that post-contractile
recovery was accelerated by anaerobic metabolism, the present study is, to our
knowledge, the first demonstration in crustacean muscle of a metabolic
recovery process (AP resynthesis) that is faster in the large fibers as a
result of anaerobic contributions.
In our view, the patterns of recovery reported previously
(Boyle et al., 2003;
Johnson et al., 2004
) and
herein are clearly related to fiber size. It was therefore surprising that the
model results did not indicate a limitation of aerobic flux by intracellular
metabolite diffusion, considering that AP and arginine, which are the key
diffusing species (Ellington and Kinsey,
1998
), can traverse the
/2 distance in small fibers in
<30 ms, while needing 16 000 times longer (nearly 8 min) to cover the
distance modeled in large fibers (Kinsey
and Moerland, 2002
). Implicit in this finding is that kinetic
expressions alone (no diffusion component) would have been nearly sufficient
to simulate the differences between small and large fibers in
Fig. 3. This is at odds with
some previous reactiondiffusion mathematical analyses in burst
anaerobic muscle. Hubley et al.
(1997
) found substantial
concentration gradients for PCr and the free energy of ATP hydrolysis
(
GATP) in fish white muscle during contraction,
while Boyle et al. (2003
)
applied the reactiondiffusion model of Mainwood and Rakusan (1982) to
blue crab light levator muscle and likewise found dramatic concentration
gradients for AP and
GATP. However, both of these
models assumed higher rates of steady-state ATP demand and perfect buffering
of high-energy phosphate concentrations at the mitochondrial membrane, which
means that rates of ATP chemical flux were always high relative to the rates
of diffusive flux. By contrast, the present study used a simple kinetic
expression for the mitochondrial boundary reaction and reasonable maximal
rates of ATP production. Further, no additional ATP demand was applied during
recovery beyond the thermodynamic drive to restore the resting steady-state
metabolite concentrations.
It could be argued that we underestimated the Vm,mito
and post-contractile ATP demand and therefore misjudged the effect of
diffusion. Thus, the approach used herein represents a conservative analysis
of the potential for diffusion limitation in these muscle fibers. It should be
noted, however, that the model results for AP recovery paralleled our
observations in the small fibers (Fig.
4), which rely exclusively on aerobic metabolism for recovery
(Boyle et al., 2003;
Johnson et al., 2004
), and the
low Vm,mito values are consistent with observations that
complete aerobic recovery from exercise in blue crabs occurs over many hours
(Booth and McMahon, 1985
;
Milligan et al., 1989
;
Henry et al., 1994
;
Boyle et al., 2003
;
Johnson et al., 2004
). Our
results are also consistent with the generalized analysis of diffusion
limitation described by Weisz
(1973
), which relates the
observed rate of the catalytic process to rates of diffusive flux. Applying
this approach to the present case, we can conclude that even if
Vm,mito and post-contractile ATPase rates were
underestimated, the observed rate of AP recovery is simply too slow to be
limited by diffusive flux (Weisz,
1973
).
There are other possible size-dependent effects that could confound our
analysis. For instance, size-dependent differences in AP hydrolysis during
dissection, freeze clamping or perchloric acid extraction could conceivably
bias our AP recovery curves. However, in resting animals, the AP/Pi
ratios in extracts were always similar to previous values observed in intact,
superfused crustacean white muscle (Kinsey
and Ellington, 1996), and there were no significant differences in
the AP/Pi ratios between size classes (data not shown). Therefore,
AP hydrolysis during the dissection and/or extraction was minimal and not size
dependent. It is also possible that differences in intracellular pH
(pHi) or free Mg2+ between large and small animals could
alter the AK equilibrium constant and therefore the AP recovery rate. While
lactate accumulation during contraction is the same in both size classes,
post-contractile lactate accumulation is greater in the large fibers
(Johnson et al., 2004
), and
this could lead to a reduced pHi in large fibers that would slow AP
recovery. In addition, the low SA:V in large fibers may
hinder compensatory acid/base equivalent exchange and exacerbate cellular
acidosis, again leading to slower AP recovery. However, it should be noted
that both the post-contractile lactate production and potential effects of
SA:V still fall within the realm of fiber size effects,
which is consistent with our conclusions. In addition, intracellular buffering
capacity in white muscle of crustaceans is greater in larger animals
(Baldwin et al., 1999
), which
may offset the pHi effects described above.
The findings in the present study are somewhat paradoxical. If it is
assumed that a relatively rapid post-contractile recovery in burst muscle is
beneficial, which is apparently the case since large fibers use anaerobic
metabolism to speed up recovery, and if intracellular diffusive flux does not
limit recovery, then why do the large fibers not simply increase the
mitochondrial density to accelerate recovery rather than relying on anaerobic
processes that put them further in oxygen debt? It is clear from Figs
3 and
5 that doubling the
mitochondrial density leads to a near doubling of recovery rate, with only
mild limitation by diffusion. We propose that, in blue crabs, the low
SA:V associated with large fiber size is more important in
limiting aerobic metabolism and/or driving metabolic design than is
intracellular metabolite diffusion. The most compelling evidence in support of
this argument is the dramatic shift in the distribution of mitochondria toward
the periphery of the fiber as the light levator muscle fibers grow
(Boyle et al., 2003). This
distributional change places more mitochondria at the sarcolemmal membrane
near the source of O2 at the expense of increased intracellular
diffusion distances. In our model analysis, there was a very slight advantage
associated with a uniform, instead of subsarcolemmal, distribution of
mitochondria in the large fibers (data not shown). However, the fact that the
developmental shift in mitochondria occurs anyway indicates that O2
flux (which was not included in the model) drives mitochondrial distribution
more than intracellular diffusive flux. This view has been suggested
previously to explain mitochondrial clustering at the sarcolemma in non-giant
mammalian (Mainwood and Rakusan, 1982) and crustacean muscle
(Stokes and Josephson,
1992
).
In addition to the above argument, the partial pressure of oxygen
(O2) in
crustacean blood (including blue crabs) is low relative to that of muscle from
active vertebrate species (Gannon and
Wheatly, 1995
; Forgue et al.,
2001
). This leads to relatively shallow
O2 gradients
across the sarcolemma that, when coupled to the low SA:V of
large fibers, would be expected to promote very low rates of O2
flux into the fiber. The lack of myoglobin (Mb) in the light levator muscle
amplifies this effect, since Mb-less fibers require a higher extracellular
O2 to support a
given rate of O2 consumption compared with muscles with Mb
(Groebe and Thews, 1990
). This
view is consistent with recent observations in isolated Xenopus
laevis skeletal muscle fibers, which are also relatively large and lack
Mb, that low intracellular
O2 limits the
rate of NAD(P)H oxidation by the electron transport system during steady-state
contraction (Hogan et al.,
2005
). Further, the modeled differences between the recovery rate
in small and large fibers are modest, due to the relatively small differences
in oxidative potential (Figs 3,
4;
Table 1), but the measured
differences in post-contractile lactate production among size classes are
dramatic; far greater than would be necessary to accelerate AP resynthesis by
the relatively small amount indicated in
Fig. 4
(Johnson et al., 2004
). If
fiber SA:V limits aerobic metabolism then the size
dependence of aerobic recovery may be much more substantial than is shown in
Fig. 3, which would explain the
strong size dependence of post-contractile lactate production.
While intracellular diffusive fluxes of high-energy phosphate metabolites
do not appear to exert substantial control over the rate of aerobic metabolism
in blue crab giant anaerobic fibers, based on our current one-dimensional
model, there may be other cell types where diffusion is limiting. These are
likely to include systems with relatively high rates of ATP
production/consumption and distant sites of ATP utilization, such as in some
muscle fibers with a higher aerobic capacity than examined here
(Meyer et al., 1984;
Stokes and Josephson, 1992
;
Vendelin et al., 2000
;
Saks et al., 2003
;
Suarez, 2003
) or in the
flagellum of spermatozoa, which has been the subject of many
reactiondiffusion analyses (e.g. Nevo and Rikmenspoel, 1969;
Tombes and Shapiro, 1985
;
Van Dorsten et al., 1997
;
Ellington and Kinsey, 1998
).
However, it is possible that in most cases neither intracellular metabolite
diffusion nor sarcolemmal O2 flux limit aerobic metabolism per
se, but even if this is true it is still likely that the interaction
between diffusive processes and ATP demand has shaped the evolution of
cellular design. For instance, if there are advantages to having large fibers
then the principles of symmorphosis
(Taylor and Weibel, 1981
)
dictate that other metabolic properties, such as mitochondrial density and
distribution, would be adjusted to match rates of O2 and substrate
delivery, thereby avoiding diffusion limitation.
What then are the potential advantages associated with large muscle fibers?
Rome and Lindstedt (1998) have
characterized the manner in which muscle fiber volume is devoted to metabolic
or contractile machinery in relation to muscle function. It is possible that a
burst contractile muscle composed of relatively few large fibers may yield a
greater percentage of total muscle volume that is devoted to myofibrils, and
therefore improve contractile force, compared with muscle with a much larger
number of small fibers. Johnston et al.
(2003
,
2004
) proposed that in certain
cold-water fishes white muscle fibers attain a size that is just below that
which would be diffusion-limited in order to minimize sarcolemmal surface area
over which ionic gradients must be maintained, thus lowering metabolic rates.
A similar argument could be made for blue crab anaerobic fibers, with the
additional consideration that a low mitochondrial content may also constitute
an energy-saving strategy to avoid the costs of mitochondrial biogenesis and
the maintenance of electrochemical gradients across the inner membrane. Forgue
et al. (2001
) have made
complimentary arguments that the low blood
O2 in
crustaceans limits resting metabolic rate to reduce costs during periods of
inactivity. These proposed energy-saving measures are linked; if the capacity
to produce ATP is strategically lowered, then there is no negative consequence
to the low SA:V and long diffusion distances associated with
large fibers. Similarly, if SA:V is lowered to minimize
ionic transport costs, then there is no further consequence to lowering
aerobic capacity, since high rates of mitochondrial respiration would be
limited by low O2 flux in large fibers.
The implication of the hypothesis that selective pressure to lower maintenance costs favors large fiber size is that the benefits of a rapid aerobic recovery following a burst contraction are outweighed by long-term energetic savings. Blue crabs have large chelipeds and highly effective defensive behavior, and they also have the capacity to rapidly bury themselves to avoid predators. These characteristics may obviate the need for additional high-force contractions following an initial bout of burst swimming and may explain why the juvenile crabs do not also employ anaerobic metabolism to accelerate recovery. Large fibers might be particularly important in reducing metabolic costs in cases where anaerobic muscle constitutes a large fraction of the total body mass and is used infrequently, but must maintain a polarized sarcolemma at all times. Additional examples may include lobster abdominal muscle that is used for tail-flip escape maneuvers, or fish white muscle in species that infrequently undergo burst swimming. At present, however, the benefits of large fiber size, if any, in crustaceans and other groups are not known.
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Acknowledgments |
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