Download Skeletal muscle substrate metabolism

International Journal of Obesity (1999) 23, Suppl 3, S7±S10
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Skeletal muscle substrate metabolism
H Hoppeler1*
1
Institute of Anatomy, University of Berne, Berne, Switzerland
Endurance power of muscles is determined largely by the capacities to oxidize substrates in mitochondria in the
process of making ATP by oxidative phosphorylation. This review explores physiological and morphological factors
that may cause limitation of carbohydrate and fat utilization by muscle cells. The pathways for oxygen and substrates
converge in muscle mitochondria. In mammals, a structural limitation of carbohydrate and lipid transfer
from the
microvascular system to muscle cells is reached at a moderate work intensity (that is, at 40 ± 50% of VO2max). At
higher work rates intracellular substrate stores must be used for oxidation. Because of the importance of these
intracellular stores for aerobic work we ®nd larger intramyocellular substrate stores in endurance trained athletes.
The transfer limitations for carbohydrates and lipids on the level of the sarcolemma implies that the design of the
respiratory cascade from lungs to muscle mitochondria re¯ects primarily oxygen demand.
Keywords: mitochondria; lipid; carbohydrate; muscle
What governs substrate metabolism
in muscle cells?
Muscle mitochondria set the
demand for oxygen and substrates
In order to understand substrate selection of working
muscle cells we must consider how the pathways for
oxygen and substrates converge at the mitochondria
and how substrate ¯uxes intervene with setting
oxygen consumption. The O2 pathway from the environment to muscle mitochondria is simple: it follows a
single path without branches and there are only very
limited O2 stores in myoglobin. The substrate pathways are more complex. Both carbohydrate and lipid
pathways branch to form four parallel pathways that
converge on the mitochondria (Figure 1). Proteins can
be ignored because in well-fed subjects their contribution to aerobic metabolism is minimal. During exercise, when rates of substrate oxidation are highest,
blood is shunted away from the gut and rates of
substrate uptake are lowest. At this point, stores
supply most of the fuel for oxidation. Organismic
substrate stores outside muscle are in the liver and in
the adipose tissue. Inside the muscle cells carbohydrates are stored as glycogen granules, and fatty
acids as lipid droplets (Figure 2). We will have to
delineate the importance of the parallel substrate
pathways as a function of exercise intensity and
duration. We would also like to know whether
limiting factors are primarily structural or functional.
When humans exercise at their maximal rate of oxygen
consumption, the mitochondria located in their skeletal
muscles consume more than 90% of the oxygen and
substrates.1 Even at lower work intensities the main
substrate consumer is muscle tissue. We are therefore
justi®ed in concentrating on the mitochondria in this
particular tissue. The mitochondrion is bounded by a
continuous outer membrane and contains a second
inner membrane that forms multiple infoldings, the
cristae (Figure 2). Between the membranes we ®nd the
`inter-membrane space'; the innermost compartment is
the mitochondrial matrix. In the matrix, substrates are
broken down in the Krebs cycle generating reducing
equivalents. The beta-oxidation of fatty acids is also
located in the matrix. Thus, the matrix volume is the
appropriate structural parameter to characterize substrate catabolism, assuming a constant density of
enzymes in the matrix space. In most mammals,
matrix volume appears to be proportional to mitochondrial volume and the ratio between mitochondrial
volume and matrix volume is also invariant under
conditions of endurance exercise training.2 It follows
from these constant relationships that the volume of
mitochondria in skeletal muscle is a good measure of
matrix enzymes, as well as the quantity of respiratory
chain present.
The supply of substrates from
cellular stores to mitochondria
*Correspondence: Dr. H. Hoppeler, Institute of Anatomy,
University of Berne, BuÈhlstrasse 26, CH-3000 Berne 9,
Switzerland
When exercising at VO2max, substrates must be
transported into the mitochondria at a rate suf®cient
Substrate metabolism in muscle
H Hoppeler
S8
Figure 1 Model for structure function relationship of oxygen
and intracellular substrate supply to the mitochondria of skeletal
muscle cells. Dots indicate oxygen, open circles indicate fatty
acids, squares indicate glucose; row of squares indicates glycogen and triangles indicate acetyl CoA. The horizontal arrows
indicate the pathways of intracellular substrate breakdown from
the intracellular stores to the terminal oxidase in the mitochondrial membrane (black square). The vertical arrows indicate the
supply routes of oxygen and substrates from the capillaries, with
dotted arrows for the supply route to intracellular stores (temporally split from the phase of oxidation; modi®ed from 6).
CHOic ˆ intracellular glucose stores, glycogen Gls ˆ glycolysis
KC= ˆ Krebscycle b OX ˆ beta oxidation FATic ˆ interacellular
lipid stores.
Figure 2 Electron micrograph of longitudinal section of skeletal
muscle tissue. In the center, on the level of the z-line, we ®nd
inter®brillar mitochondria with a lipid droplet immediately adjacent. li ˆ lipid droplet mc ˆ central mitochondria mf ˆ myo®laments marker indicates 0.5 mm.
to fuel oxidative phosphorylation. The oxidation of
one mole of glucose consumes 6 moles of oxygen, and
one mole of fat consumes 23 moles of O2. It has been
clearly established in dogs and goats that circulatory
carbohydrates and lipids can only cover a fraction of
the total substrate demand of mitochondria in working
muscles.3,4 Both carbohydrate and lipid transport from
capillaries into muscle cells appear to be maximal at
quite low exercise intensities, corresponding to
approximately 40% of VO2max and cannot be upregulated at higher work intensities. Instead, the mitochondria depend on obtaining their fuels from
intracellular substrate stores whereby at VO2max
over 80% of the fuel is supplied from glycogen
granules. These results were obtained in a comparative setting of adaptive variation, but they are very
similar to the situation observed in exercising humans
where a heavy reliance on intracellular substrate
stores at higher work intensities has also been demonstrated.5 From this it follows that substrates for
mitochondrial oxidation
at work intensities of
around 80% of VO2max must primarily be supplied
from glycogen granules and lipid droplets inside the
muscle cells, with no more than 20 ± 30% of the fuel
coming from the capillaries. There is evidence that
this is due to a limitation of fuel supply from the
capillary with the sarcolemma as the main barrier.6
What are the structures that determine ¯ow through
the intracellular substrate pathways? On careful observation, one ®nds that all of the lipid droplets are in
direct contact with the outer mitochondrial membrane.6 They form a tight contact which is often
marked by a dent in the mitochondria (Figure 2). It
seems likely that the fatty acids are transported across
the mitochondrial membrane preferentially in these
areas of contact, allowing them to circumvent transport problems associated with their low solubility in
the cytosol. The maximal ¯ow through the cytosolic
pathway is reached at low exercise intensities, and it
can supply at best about 20% of the fuel required at
VO2max . From this it follows that inter®brillar
mitochondria likely depend substantially on lipid
droplets for fatty acid supply.
The importance of intracellular lipid stores for
aerobic performance has long been suspected. Already
in 1973 it was noted that highly endurance trained
subjects had considerably larger substrate stores in the
form of lipid droplets in their muscles.7 Later it was
found that six weeks of endurance exercise training
was suf®cient to increase intracellular lipid concentration by a factor of two from 0.5% to 1% of the
muscle ®ber volume. Highly trained cyclists or endurance runners may have even larger lipid stores in their
trained muscles.8 Moreover, it was demonstrated in
1976 that the lipid (and glycogen) stores disappeared
almost completely after a 100 km run9,10 indicating
that the intracellular lipid droplets represent the metabolically accessible form of substrate reserves in
muscle tissue. In situations in which the reliance on
fatty acid metabolism is reduced, such as in high
altitude residents, we ®nd that intracellular substrate
stores are massively reduced also.11
An additional observation which is likely relevant to
the understanding of lipid oxidation in skeletal muscle
tissue relates to the distribution of mitochondria
Substrate metabolism in muscle
H Hoppeler
within the muscle cell. Whereas a large fraction of
mitochondria is found interspersed between myo®brils, there is a smaller population of mitochondria
found massed under the sarcolemma close to capillaries (subsarcolemmal mitochondria). The location of
these mitochondria puts them in close vicinity to the
vascular supply of both oxygen and substrates. It has
been demonstrated by mathematical modelling that
mitochondrial distribution within muscle cells is not
likely to play a major role for oxygen diffusion
because diffusivity of oxygen is high in muscle
®bers partly related to the myoglobin content of
muscle.12 As a consequence of their location under
the sarcolemma, problems of cytosolic transport of
substrates is minimal for subsarcolemmal mitochondria. It can be argued that this puts these mitochondria
in a favourable position with regard to oxidizing lipids
supplied by capillaries (for a review of lipid transfer to
mitochondria see reference 13). The notion of a
preferential oxidation of vascular lipids by subsarcolemmal mitochondria is further supported by the
circumstantial evidence indicating a greater relative
increase of subsarcolemmal versus inter®brillar mitochondria with endurance exercise training.14 This
®nding can be interpreted as indicating a general
shift towards a larger reliance on fat metabolism
which has two structural components a) an increase
of substrate stores favouring lipid metabolism of
inter®brillar mitochondria and b) a large increase in
subsarcolemmal mitochondria favouring oxidation of
lipids taken from the vasculature.
It is currently unclear what throttles triacylglycerol
oxidation in exercising muscle. Is it the transfer of
fatty acids to the mitochondrial matrix or is it boxidation?13 Likewise, the control of the balance
between lipid and carbohydrate (CHO) metabolism
in muscle cells is poorly understood. In resting muscle
there is some indication that the glucose ± fatty acid
cycle (Randle cycle) is operative, however under
conditions of exercise the mechanism of regulation
is currently debated.15 A possible regulation could
also occur through malonyl-CoA. The increase in the
use of lipids as a substrate with long-term exercise of
low intensity is probably related to the increase in
plasma free fatty acids,5,15. However, again we do not
know how the muscle cell selects between the two
substrates and what governs use of intracellular versus
extracellular substrates.
The rate at which glycogen stores can supply
pyruvate to the Krebs cycle or to further degradation
to lactate is not really limited. Glycogen stores supply
pyruvate up to the limit of oxidation by the mitochondria, and then can achieve a further several-fold
increase in break down in order to fuel anaerobic
glycolysis. Clearly, therefore, oxidation in the
mitochondria is not limited by the supply of fuel but
rather by the supply of oxygen from the capillaries Ð
or by the amount of mitochondria that can perform
oxidative phosphorylation.
Microcirculatory supply of oxygen
and substrates
Oxygen diffuses from the capillaries to the mitochondria. In general terms, the maximal conductance must
be related to the total volume of capillaries in skeletal
muscles, a structural parameter that has been measured. Capillary volume is directly proportional to the
capillary surface area available for diffusion of
oxygen out of the blood, since the diameter of
capillaries does not change with both size or adaptation. It has been shown that there is a reasonably good
match between structure and the oxygen supply function at the level of the capillaries. It is generally found
that the higher rates of oxygen consumption are
matched by corresponding differences in the amount
of capillary structure.16
What about the transport of substrates out of the
capillaries? The maximum rate appears to be reached
at very low exercise intensities, as noted above. Even
at 40% of VO2max, only about 20 ± 30% of the
substrates are supplied from the circulating blood,
and the absolute rate does not increase with increasing
intensity. Very similar results have been obtained in
exercising humans as well as in exercising dogs and
goats.5,17 At work intensities above 40% VO2max
intracellular substrate stores, in particular glycogen,
are used to fuel aerobic work, as noted above.
The transport of glucose from capillaries to myocytes has been studied by comparing `athletic' dogs to
`sedentary' goats.6 The transfer of glucose from the
plasma to the interstitial ¯uid is assumed to occur
through the small pore system at the endothelial
intercellular junction. Calculations indicate that
neither the pore system nor the interstitial space
between the capillary and the myocyte introduce a
major resistance to glucose ¯ux across the sarcolemma. Based on morphologic evidence it is concluded that the glucose transporters of the sarcolemma
are likely saturated at low to moderate exercise
intensities and are responsible for the major part of
the resistance in the carbohydrate pathway.
The transport and utilization of fatty acids in
muscle has recently been reviewed extensively.13
Based on data published in many studies these authors
come to the conclusion that it is the delivery rather
than the oxidative metabolism in myoytes that is the
rate ± limiting factor in lipid oxidation during exercise. However, many uncertainties remain and lipid
oxidation could be regulated at various steps in the
route of transport as well as within the metabolic
pathways. The general consensus seems to be that
endurance exercise training increases the importance
of fatty acids as a source of aerobic energy. Likewise,
recent evidence suggests that high fat diets do have a
potential for shifting metabolism towards a higher
reliance on fatty acid metabolism.13 These general
observations could be of importance for athletes
S9
Substrate metabolism in muscle
H Hoppeler
S10
competing in endurance events, in particular in very
long term endurance events where intensities are low
and the importance of fat oxidation high. In this
context it may be noted that for certain long term
endurance events lasting over days or weeks, such as
multi stage bicycle races, the concept of maximal
sustained metabolism may be more important than
that of VO2max.18 In these events racers are limited in
their performance by the need of having to balance
work output to substrate uptake. For humans the limit
of energy uptake with nutrition seem to be approximately 7 000 kcal=d. Therefore, professional cyclists
are capable of elevating basal metabolism acutely by
some 20-fold (aerobic scope) while average 24 h
metabolism (maximal sustained metabolism) can
only be increased 4-fold.
Reviewing the transfer processes for oxygen and
substrates from capillaries to myocytes we can conclude that muscle microvasculature is optimally
designed for the transfer of oxygen, but not for the
transfer of substrates. Because of the lack of substantial oxygen stores, oxygen has to be supplied by the
circulation so as to match the demand of the contracting muscle cells. By contrast, only a limited quantity
of substrates is supplied from the vasculature during
exercise. Animals
and humans working at intensities
above 30% of VO2max rely increasingly on intracellular substrate stores mainly on glycogen at high work
intensities. These stores will eventually be exhausted
during exercise leading to muscle fatigue. The intracellular stores can then be replenished during periods
of rest at low transfer rates.
Conclusions
Our analysis shows that human endurance capacity is
limited mainly by structural constraints. The entire
respiratory system from lung to muscle mitochondria
is built on the constraint of supplying oxygen rather
than substrates to active muscle mitochondria. Carbohydrate and lipid supply rates are likely throttled by
transport processes at the level of the sarcolemma. In
order to ascertain adequate substrate supply at high
work loads both lipids and carbohydrates are stored
within muscle cells. These substrate stores are replenished at low ¯ux rates during periods of rest to reach a
size adequate for high rates of combustion in exercise.
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