Download Muscle Energetics and Fatigue - Dr. Feher

Muscle Energetics and Fatigue
Joseph Feher, Ph.D.
LECTURE OUTLINE:
I.
II.
III.
IV.
Rate of ATP consumption by active muscle depends on the load.
A.
The free energy of ATP hydrolysis is the chemical source of mechanical
work.
B.
ATP hydrolysis is linked to mechanical work through the actomyosin
ATPAse.
C.
The rate of cross-bridge cycling and ATPase activity depend on the load.
D.
ATP hydrolysis is also needed for control reactions.
Rate and amount of ATP consumption varies with the intensity and duration of
exercise
Muscle consumption of ATP is always fast
A.
Normal activation of muscle is by trains of impulses
B.
In vigorous exercise, frequency of muscle activation increases
C.
Each activation of the muscle requires fast ATP hydrolysis
D.
Intensity of exercise changes the rest period for metabolism to recover
energy reserves
Metabolism regenerates ATP in different time scales and capacities
A.
B.
Direct phosphorylation regenerates ATP fastest.
1.
Creatine phosphate provides the first buffer for ATP in muscle, but
it increases Pi.
2.
Myokinase can regenerate ATP from ADP.
Glycolysis provides a rapid but low capacity supply of ATP for fast twitch
fibers.
C.
Oxidative phosphorylation provides a slower but high capacity supply of
ATP.
V.
The fuel used by muscle varies with intensity and duration of exercise
VI.
At high intensity, glucose and glycogen is the preferred fuel for muscle
VII.
VIII.
A.
Carbohydrates are stored as glycogen and mobilized by glycogenolysis
B.
Glycolysis uses muscle glycogen and blood glucose
C.
Glycolysis produces pyruvate, ATP and NADH without requiring oxygen
D.
Generation of lactic acid regenerates cytoplasmic NAD+
Muscle and muscle fibers can be classified by metabolic properties
A.
Muscles can be classified as SO, FOG and FG
B.
Whole muscles are mixtures of muscle fiber types
C.
Muscle fiber types differ in the isomers of many different proteins
D.
Muscle fiber types differ in the relative amount of organelles
E.
Muscle fiber types are more continuously distributed than suggested by
the classification schemes
Mismatch of lactic acid production and oxidation determines the rate of lactic acid
release into the blood
A.
Blood lactate levels rise progressively with increases in exercise intensity
B.
Mitochondria can metabolize lactic acid
C.
Lactic acid forms a carrier system for NADH oxidation
D.
Lactic acid is produced by fully oxygenated tissue
E.
Three lactic acid shuttles remove lactate from its site of production
1.
the intracellular shuttle removes lactate to the mitochondria
2.
The cell-cell shuttle removes lactate to adjacent oxidative fibers
3.
F.
IX.
X.
The Cori cycle shuttles lactate to the liver for gluconeogenesis
The “anaerobic threshold” really has to do with mismatch of lactic acid
production and oxidation
1.
The rest periods between contractions become shorter
2.
The fast glycolytic fibers are increasingly recruited over oxidative
fibers
3.
Sympathetic stimulation increases the rate of glyocogenolysis
Exercise increases glucose transporters in the muscle sarcolemma
A.
Glucose uptake by muscle depends on the number and activity of GLUT4
transporters
B.
Insulin increases the number of GLUT4 transporters by recruiting latent
transporters
C.
Exercise increases the number of active GLUT4 transporters independent
of insulin
D.
Mechanism of GLUT4 recruitment is probably through AMPK and
CAMK
The site of Fatigue depends on the muscle and intensity of exercise.
A.
Fatigue is a reduction in developed force.
B.
Metabolic fatigue occurs with repetitive stimulation
C.
Sites of fatigue could be in any process from brain to contractile element
D.
Fatigue during maximum sustained contractions is not in the brain.
E.
Pi and H+ in muscle interferes with force development by actomyosin
ATPase.
F.
Fatigue at submaximal work rates depends on glycogen stores.
1.
Fatigue accompanies glycogen depletion
2.
XI.
The importance of glycogen in exercise is evident from muscle glyogenoses
XII.
Training regimens are designed to increase strength of endurance
A.
The most rapid training involves training the brain
B.
Training for strength induces muscle hypertrophy
C.
XIII.
XIV.
Fatigue can be delayed by glycogen supercompensation
1.
Muscle strength depends on muscle size
2.
Training sends a signal for muscle growth
a.
Myostatin inhibits muscle growth
b.
Insulin-like growth factor-1 and muscle growth factor also
influence muscle size
Endurance training uses repetitive movements
Our ability to switch muscle fiber types is limited
A.
Muscle capabilities are determined by the mixture of muscle fiber types
B.
Single muscle fibers can be hybrids
C.
It is unlikely that muscle types interconvert in human training regimens
Practice Questions
OBJECTIVES:
1. Describe how intensity of exercise differs in heavy resistance vs. endurance
exercise
2. Explain why ATP hydrolysis in active muscle is always fast
3. List the sources of ATP generation in order of their rapidity
4. List the sources of ATP generation in order of their capacity
5. Describe how fuel use varies with intensity and duration of exercise
6. Explain how lactic acid production allows for more rapid glycolysis
7. List the types of muscle fibers based on metabolic properties
8. List the metabolic fates of lactic acid
9. Explain why exercise reduces diabetic’s need for insulin
10. Identify the likely site of fatigue in high-intensity, short-duration exercise
11. Identify the likely site of fatigue in moderate-intensity, long-duration exercise
12. Describe the overall changes that occur during hypertrophy
13. Describe the function of myostatin and the consequence of its lack
14. Explain how muscle hybrid types can arise
15. Describe our limited ability to switch muscle types
Suggested Reading: Berne and Levy, pp. 237-241
I.
MUSCULAR ACTIVITY CONSUMES ATP AT RATES DEPENDENT ON
LOAD
A.
The free energy of ATP hydrolysis is the chemical source of mechanical
work.
In adenosine triphosphate ( ATP), energy is required to attach each of the
phosphates on the ribose moiety. These bonds are less stable than their
starting materials: The γ bond makes ATP less stable than ADP and Pi ;
the β Pi bond makes ADP less stable than AMP and Pi. When these bonds
are hydrolyzed, chemical energy becomes available for work with the
remainder dissipated as heat.
B.
ATP hydrolysis is linked to mechanical work through the myosin ATPase.
The cross-bridge cycle links shortening or force production to the
hydrolysis of ATP through the kinetics of the actin and myosin
interactions. In an intact muscle, macroscopic force results from thousands
of these cross-bridges cycling at furious rates.
Figure 1. Structures of ATP and ADP and the ATP hydrolysis reaction. Under the conditions of the myoplasm
[ATP] = 5 x 10-3M, [ADP] = 0.04 x 10-3 M, [Pi] = 5 x 10-3 M and pH = 7.0, hydrolysis of ATP liberates
approximately 57 kjoules mol-1
C.
The rate of cross-bridge cycling and ATPase activity depend on the load.
Unlike smooth muscle, which can generate force without large amounts of
ATP hydrolysis, skeletal muscle hydrolyzes ATP whether or not it
shortens. Isometric contractions consume ATP because the cross bridges
continue to cycle even when shortening does not occur. Eccentric
contractions also require ATP hydrolysis to resist stretch and thereby
impart rigidity to the joints. But fewer muscle fibers are recruited in
eccentric contractions, because more force results from the eccentric
contractions. Thus, the rate of ATP consumption for whole muscles varies
with the speed and type of contraction.
D.
ATP hydrolysis is also needed for control reactions.
Muscles also use ATP to re-accumulate activator Ca2+ into the SR, for
active ion pumping by the Na+-K+-ATPase , and for maintenance chores.
During activity, the actomyosin ATPase is the main cause of ATP
hydrolysis.
II.
RATE AND AMOUNT OF ATP CONSUMPTION VARIES WITH THE
INTENSITY AND DURATION OF THE EXERCISE
The rate of ATP utilization by the aggregate muscles of the body depends on the
intensity of the exercise. The total amount of ATP used during a bout of exercise
is its rate of utilization times the duration of the event. High intensity exercise can
be sustained only for short periods, whereas moderate intensity exercise can be
endured for long times. The relationship between intensity and sustainable effort
is not linear. Table 1 below lists the approximate rates and amounts of ATP
needed for different track events.
Rate of ATP consumption
Amount of ATP needed
(mol/min)
(mol)
Rest
0.07
---100 m sprint
2.6
0.4
800 m run
2.0
3.4
1500 m run
1.7
6
42200 m marathon
1.0
150
Table 1. Rate and amount of ATP needed for different track events.
Event
Adapted from Hultman, E. and Sjoholm, H. Biochemical causes of fatigue, in “Human Muscle Power” (1986)
Human Kinetics Publishers, Inc, Champaign Illinois.
III.
MUSCLE CONSUMPTION OF ATP IS ALWAYS FAST WHEN THE
MUSCLE IS ‘ON’
A.
Normal activation of muscle is by trains of impulses
The motor neuron carries a single code for activating its muscle fibers:
the temporal pattern of its action potentials. For particular movements,
motor units in groups of muscles are activated in a particular sequence to
coordinate the movement. Figure 2 shows an electromyogram (EMG) of
rat leg muscles obtained at a slow walking speed. The EMG records
electrical activity of the muscle, not force. Each muscle is activated at
appropriate times for a definite length of time, and this activity alternates
with periods of rest.
B.
In vigorous exercise, frequency of muscle activation increases
The EMGs in Fig. 2 illustrate muscle activation in slow walking. In
vigorous walking, the frequency of activation increases and the duration of
each activation decreases. The duty cycle, the fraction of time that the
muscle is activated, increases with increased intensity. Many animals
engage different sets of muscle coordination, called gaits, to vary their
speed of locomotion. In humans, walking and running form distinctive
gaits, but running fast and running slowly differ mainly in the speed and
recruitment rather than the sequence of activation. Weight lifting differs
from running in that it activates a selected few muscles. Increasing the
intensity of weight lifting means increasing the weight, and this is
achieved by increasing recruitment of the muscle fibers until 100%
recruitment is reached at the maximum weight. The maximum weight can
be lifted only slowly, so the muscles must be activated by a train of
impulses that last as long as it takes to lift the weight. This differs from the
activation pattern of repetitive exercise such as walking or running in that
in weight lifting there is no rest phase for the muscle fibers until the
exercise is over. This is the origin of the different kind of fatigue for
heavy resistance exercise versus endurance exercise.
Figure 2. Electromyogram of rat leg muscles at a slow walking speed, 1 mph, on a treadmill. Modified from
G.A. Brooks, T.D. Fahey, T.P. White and K.M. Baldwin, Exercise Physiology, Third Edition, McGraw Hill,
1999.
C.
Each activation of the muscle requires fast ATP hydrolysis
Larger EMG amplitudes (recorded in volts) indicate greater number of
muscle fibers that are firing action potentials. Each muscle fiber, when
activated, is activated completely. The control of force for the entire
muscle is achieved by the temporal recruitment of the fibers: which fibers
are being activated and with what frequency and in what sequence. Each
activation of a muscle fiber entails fast rates of ATP hydrolysis because all
of the actomyosin cross-bridges are activated by the Ca2+ transient with
each action potential on the motor neuron.
D.
Intensity of exercise changes the rest period for metabolism to recover
energy reserves
Increasing intensity of exercise increases the overall frequency at which
individual muscle fibers are activated. Each activation entails furious rates
of ATP hydrolysis. Increasing the frequency of activation necessarily
decreases the time that the muscle is not activated, and this is the time the
muscle has for metabolism to oxidize substrates produced during the
contractions. Thus, every muscle contraction requires rapid ATP
consumption and regeneration, but at low intensity exercise there is
more time for metabolism to recover the resting state.
IV.
METABOLISM REGENERATES ATP IN DIFFERENT TIME SCALES
AND CAPACITIES (See Figure 3 and Table 2)
A.
Direct phosphorylation re-generates ATP fastest.
1.
Creatine phosphate provides the first buffer for ATP in muscle,
but it increases Pi.
Muscles contain about 15-20 mM creatine phosphate. Creatine
phosphokinase (CPK) catalyzes the phosphorylation of ADP from
creatine phosphate to form ATP. This extremely rapid reaction
helps to “buffer” ATP concentrations near the normal 5 mM in the
muscle fiber myoplasm. When creatine phosphate is used to
regenerate ATP, myoplasmic [Pi] increases. Creatine
phosphokinase is located nearby ATP-utilizing reactions such as
the myosin ATPase and SR Ca-ATPase and may directly transfer
ATP to these enzymes. This is called substrate channeling. These
ATPase enzymes split ATP faster in the presence of CPK. All
contractions of muscle require creatine phosphate regeneration of
ATP for maximum force.
2.
Myokinase can re-generate ATP from ADP.
Another enzyme, myokinase, converts two molecules of ADP into
ATP and AMP. AMP levels may be what is sensed by the “fuel
gauge” of muscle fibers. When it goes up, you are running out of
fuel.
B.
Glycolysis provides a rapid but low capacity supply of ATP for fast twitch
fibers.
Glycolysis begins with glucose that may arise from glycogen or from the
blood, and ends with two pyruvate molecules. Glycolysis requires 2ATP
molecules and generates 4ATP, for a net gain of only 2 ATP molecules
per molecule of glucose. Additional ATP (4 or 6 per molecule of glucose)
can be generated from the NADH produced by the oxidation of
glyceraldehyde-3- phosphate during glycolysis.
C.
Oxidative phosphorylation provides a slower but high capacity supply of
ATP.
Cytosolic pyruvate formed by glycolysis enters the mitochondria to be
converted to acetyl CoA by pyruvate dehydrogenase. In this process, 1
CO2 is released and reducing equivalents as NADH are produced. The
acetyl CoA is converted to 2 more CO2 molecules through the TCA cycle,
which also produces NADH, FADH2 and GTP. In the presence of oxygen,
the NADH and FADH2 is oxidized through the Electron Transport Chain
(ETC). The ETC pumps H+ ions out of the mitochondrial matrix,
establishing a [H+] gradient and an electrical potential across the inner
mitochondrial membrane. This electrochemical gradient for H+ is used by
the mitochondrial ATP synthase to synthesize ATP from ADP and Pi. Net
ATP production from the complete oxidation of pyruvate is 30 ATP
molecules per molecule of glucose. Oxygen is needed as the final electron
acceptor from the ETC. Without oxygen, the ETC remains reduced and
everything backs up. The TCA stops for lack of NAD+, and beta oxidation
of fats stops for the same reason. However, lack of oxygen is pathological
rather than physiological. In normal physiology, the issue is how fast
oxidative phosphorylation is going with respect to ATP consumption.
V.
THE FUEL USED BY MUSCLE VARIES WITH INTENSITY AND
DURATION OF EXERCISE
Muscles can use fats, carbohydrates and proteins as fuels. Which is used at what
rates depends on the type, intensity and duration of exercise. At rest, muscles use
mainly free fatty acids. At moderate exercise (< 50% maximum O2 consumption
(VO2)), muscle use blood glucose and free fatty acids. At higher intensities of
exercise (>50% VO2) the proportion contributed by glycogen becomes
increasingly important so that at 70-80% VO2 aerobic metabolism of glycogen is
predominant.
Figure 3. Overall energy metabolism driving contraction in skeletal muscle. ATP is consumed in a variety of
reactions including the actomyosin cross-bridges and the SR Ca-ATPase pump. ATP is provided by a variety of
routes including glycolysis and complete oxidation of carbohydrates through the TCA cycle and electron
transport chain (ETC) in the mitochondria. The source of glucose for glycolysis can be muscle glycogen or
plasma glucose. The glucose is imported into the muscle by a glucose transporter, GluT4. Plasma glucose
originates from liver and extrahepatic tissues either through glycolysis (liver) or gluconeogenesis (liver,
kidneys, intestine). Fatty acids form acetylCoA through beta oxidation and the acetyl CoA is then completely
oxidized, in the presence of adequate oxygen, in the mitochondria. These fatty acids may derive from muscle or
from adipose stores. When glycolytic flux is rapid and myoplasmic NADH accumulates, glycolysis continues
by the regeneration of NAD+ by converting pyruvate to lactic acid by lactate dehydrogenase (LDH). Production
of lactic acid thereby allows glycolysis to continue. Lactic acid produced in this way is transported into the
blood and from there to the liver where it can be converted to glucose again. This cycle of muscle glucose to
lactate to liver lactate to glucose is the Cori cycle.
Rate of ATP production
Amount of ATP available
(mol/min)
(mol)
ATP and creatine phosphate
4.4
0.7
Glycogen to lactate
2.4
1.6
Muscle glycogen to CO2
1.0
84
Liver glycogen to CO2
0.4
19
Fatty acids to CO2
0.4
4000
Table 2. Rate and amount of ATP available for contraction from various fuel sources
Source of energy
Table 2 shows the rates of ATP production and amounts of ATP available from various
sources. It is important to remember that every muscle contraction utilizes creatine
phosphate and glycogen to lactate, but at low frequency the oxidation of lactate or blood
glucose during the rest period pays to resynthesize glycogen.
VI.
AT HIGH INTENSITY, GLUCOSE AND GLYCOGEN IS THE
PREFERRED FUEL FOR MUSCLE
A.
Carbohydrates are stored as glycogen and mobilized by glycogenolysis.
Muscle cells burn glucose, but the amount of free glucose in the blood is
limited and cannot fuel muscle activity alone. Muscles and liver store
carbohydrates as glycogen. Glycogen is mobilized through glycogenolysis
to provide glucose for muscle activity. Glycogenolysis is controlled by
sympathetic nervous activity and circulating epinephrine. It is mediated by
phosphorylase, and is controlled by a Gs protein linked to adenylyl
cyclase, the production of cAMP and the activation of protein kinase A.
The rapid utilization of ATP in normal contractions appears to
require glycogenolysis. Glycogen is re-generated during the resting phase
of the muscle between trains of impulses.
B.
Glycolysis uses muscle glycogen and blood glucose.
Glycogen stored in muscle is dedicated to glycolysis because muscle lacks
glucose 6-phosphatase that converts G-6P to glucose. Only glucose can
cross the cell membrane. Ionically charged G-6P cannot. Because it lacks
the enzyme to make free glucose, muscle cannot export significant
glucose. Liver and other tissues produce glucose that can travel to muscle
through the blood. Muscle tissues take up glucose by a transporter, Glut4,
that is sensitive to exercise. The Glut4 transporter is recruited to the cell
membrane by insulin, but exercise also recruits these transporters in the
absence of insulin. This is why diabetics cut back on their insulin when
they exercise.
C.
Glycolysis produces pyruvate, ATP and NADH without requiring oxygen.
Glycolysis produces a net gain of 2 molecules of ATP per molecule of
glucose. It also produces NADH from NAD+ as an obligatory cofactor for
the reaction of glyceraldehyde-3-phosphate to 1,3-diphosphoglycerate. If
the cell runs out of cytoplasmic NAD+, glycolysis will stop. Generating
ATP without requiring oxygen is called anaerobic metabolism.
D.
Generation of lactic acid regenerates cytoplasmic NAD+
As noted above, glycolysis requires NAD+ in order to proceed.
Cytoplasmic NADH produced by glycolysis can be oxidized back to
NAD+ by the mitochondria through shuttle systems (the malate shuttle and
the glycerolphosphate shuttle) that transfer reducing equivalents (NADH)
into the mitochondrial matrix. Conversion of NADH to NAD+ requires an
oxidized electron transport chain. During rapid bursts of glycolysis, the
mitochondria cannot keep up with the NADH generated by glycolysis. In
these rapid bursts, both NADH and pyruvate concentrations momentarily
spike. Lactic dehydrogenase converts pyruvic acid to lactic acid,
simultaneously converting NADH to NAD+. This NAD+ can then be used
to allow glycolysis to proceed at the glyceraldehyde 3-phosphate step.
Thus, lactic acid production allows glycolysis to proceed during rapid
glycolytic bursts of ATP production, but the reaction goes faster in
exercise because the concentrations of its substrate, pyruvate, increases.
VII.
MUSCLES AND MUSCLE FIBERS CAN BE CLASSIFIED BY THEIR
METABOLIC PROPERTIES
A.
Muscles can be classified as slow oxidative (SO), fast glycolytic (FG) and
fast oxidative-glycolytic (FOG).
Muscles can be classified by their mechanical properties (see Skeletal
Muscle Mechanics) and by their myosin staining (see Contractile
Mechanisms). Muscle fibers can also be classified on the basis of their
metabolic capabilities. Peter and coworkers described three types of fibers:
slow oxidative (SO); fast glycolytic (FG) and fast oxidative-glycolytic
(FOG). The three main classification schemes are shown in Table 3.
Muscle Property Used to
Classify Types
Burke
mechanical properties
Brooke
myosin ATPase staining
Peter
metabolic capacity
Table 3. Muscle Fiber Type Classification Schemes.
Classification Scheme
B.
Fiber Types
S, FR, FI, FF
I, IIA, IIB, IIC
SO, FOG, FG
Whole muscles are mixtures of muscle fiber types.
Whole muscles consist of thousands of muscle fibers and these muscle
fibers are distributed among the various muscle fiber types. Specific
muscles may be predominately one type or another, and the distribution of
fiber types in a given muscle varies between individuals.
C.
Muscle fiber types differ in the isoforms of many different proteins.
Brooke’s classification scheme is based on the expression of different
myosin isoforms in skeletal muscle. Many other proteins also can be
expressed as one of several different isoforms. The SERCA Ca-ATPase
has a fast-twitch (SERCA1a) and slow-twitch isoform (SERCA2a);
calsequestrin within the SR lumen has at least two different isoforms, a
“fast” type and a “cardiac” type. The RyR has different isoforms (RyR1 in
skeletal muscle and RyR2 in cardiac muscle). TnC is expressed in
different forms in fast skeletal (TnC2) and slow twitch skeletal muscle and
cardiac muscle (TnC1). Why muscles have some of these isoforms is not
yet clear. Table 4 compares some of the different proteins expressed in
different muscle types and the relative abundance of selected organelles.
D.
Muscle fiber types also differ in the relative amount of organelles.
Oxidative fibers contain a lot of mitochondria compared to glycolytic
fibers. The relative amounts of the SR also vary depending on the speed of
contraction. Slow twitch fibers generally have about one-half as many SR
Ca2+ pumps as fast twitch fibers. The myoplasmic parvalbumin content
correlates well with the speed of the fibers, whereas the myoglobin content
correlates well with its oxidative capacity.
Type I Muscle Type IIa Muscle
Twitch
Slow
Fast
Fatigue
Resistant
Resistant
Metabolism
Oxidative
Oxidative
Mitochondria
+++
++++
SR volume
++
+++
Glycogen
+
+++
Myosin Heavy Chain MHC-I
MHC-IIa
Myosin Light Chain MLC-1aS, -1bS MLC-1f, -3f
SR Ca-ATPase
SERCA2a
SERCA1a
Phospholamban
++
Calsequestrin
fast and cardiac fast
RyR
RyR1
RyR1
Troponin C
TnC1
TnC2
Myglobin
+++
+++
Paravalbumin
+
Table 4. Comparison of different muscle types.
Type IIb Muscle
Fast
Fatigable
Glycolytic
+
++++
++++
MHC-IIb, -IIx
MLC-1f, -3f
SERCA1a
fast
RyR1
TnC2
++
Cardiac Muscle
Resistant
Oxidative
++++
+
++
MHC-α, MHC-β
MLC-1v, -1a
SERCA2a
+
cardiac
RyR2
TnC1
+++
-
VIII.
MISMATCH OF LACTIC ACID PRODUCTION AND LACTIC ACID
OXIDATION DETERMINES THE RATE OF LACTIC ACID RELEASE
A.
Blood lactate levels rise progressively with increases in exercise intensity
Progressive increases in exercise intensity causes a progressive rise in
blood lactic acid, as shown in Fig. 4. The increased circulating lactic acid
is caused by release of lactic acid from the working muscles. Originally
lactic acid was thought to be produced during anaerobic metabolism, so
that the increased levels in the blood was thought to represent increased
reliance on anaerobic metabolism. For this reason, the knee in the curve
was called the “anaerobic threshold”. Newer views suggest that the
tissue is not anaerobic, even though lactic acid production is increased.
Figure 4. Blood lactate concentration as a function of relative
work load. Lactate levels in blood increase only gradually until
about 60% of VO2 max is reached, and then lactate concentration
increases markedly with further increases in exercise intensity.
B.
Mitochondria can metabolize lactic acid
Mitochondria possess a monocarboxylic acid transporter (MCT1) that
transports lactic acid into the mitochondria. Mitochondria also possess
lactic dehydrogenase, LDH, that converts lactic acid to pyruvate. Lactate
enters the mitochondria over MCT1 (Monocarboxylic acid transporter)
and is converted back to pyruvate by mitochondrial LDH. The pyruvate is
then oxidized by the TCA cycle.
C.
Lactic acid forms a carrier system for NADH oxidation
Because lactic acid can enter the mitochondria and be converted back to
pyruvate, it carries cytoplasmic reducing equivalents, as NADH, into the
mitochondria (Fig. 5). NADH is converted to NAD+ by LDH in the
cytoplasm, lactic acid crosses over to the mitochondria and converts
NAD+ to NADH in the matrix. The lactic acid, converted to pyruvate, is
then consumed by the mitochondria.
Figure 5. Lactic acid carries reducing equivalents into the mitochondria. Cytosolic NAD+ is an
obligatory requirement for glycolysis. Conversion of pyruvate to lactic acid in the cytoplasm
regenerates NAD+ so that glycolysis can continue. The lactic acid enters the mitochondria over the
MCT1 carrier (which also transports pyruvate) and is converted back to pyruvate in the
mitochondria by mitochondrial lactate dehydrogenase (LDH). The NADH generated in the
mitochondria can be oxidized back to NAD+ by the electron transport chain.
D.
Lactic acid is produced by fully oxygenated tissue
The classical view is that lactate is produced only under anaerobic
conditions, when muscle PO2 falls below levels that fully energize
mitochondria. This view is now thought to be wrong. The main fact that
doesn’t fit is that lactate is produced by exercising muscles that are
fully oxygenated.
E.
Three lactate shuttles remove lactate from the site of production
1. The intracellular shuttle removes lactate to the mitochondria in the
same cell
Lactate produced in the cytosol moves into the mitochondria of the
cell where the lactate is converted to pyruvate and oxidized by the
TCA cycle coupled to oxidative phosphorylation.
2. The cell-cell shuttle removes lactate to adjacent oxidative cells
The fastest fibers produce lactate at the highest rate. They first become
unable to oxidize all of the lactic acid themselves, and the lactate
enters the blood. Neighboring oxidative fibers, which are generally
smaller than the large glycolytic fibers, take up some of this lactate
and oxidize it. This constitutes the cell-cell shuttle (Fig. 6.)
3. The Cori cycle shuttles lactate to the liver for gluconeogenesis
The liver takes up lactate that is released into the blood by the active
muscles. The liver either metabolizes the lactate for energy or uses it
to make new glucose through gluconeogenesis, and exports the
glucose into the blood. Muscles can then take up this glucose and use
it again for energy. This cycle of blood glucose to muscle lactate to
blood lactate to liver lactate and back to blood glucose is called the
Cori cycle.
F.
The “anaerobic threshold” really has to do with mismatch of lactic acid
production and oxidation
The increases in blood lactate with intensity of exercise is caused by the
release of more lactic acid by the exercising muscles than can be
metabolized by the aggregate tissues of the body. Release outstrips
oxidation plus gluconeogenesis. This has the appearance of an increase of
anaerobic metabolism, and in one sense it is. Every muscle contraction
involves a period of “anaerobic” generation of ATP. In less intense
exercise, there is sufficient rest time for the lactic acid produced during
this period to be oxidized. When exercise intensity increases, three
different things happen:
1. The rest period between contractions becomes shorter
2. The fast glycolytic fibers are increasingly recruited over the oxidative
3. Sympathetic nervous system increases the rate of glycogenolysis,
further increasing the supply of pyruvate, and, by mass action, the
production of lactate
Because of these, lactate release by the active muscles soars with
increased exercise intensity and outstrips the ability of the tissues to
metabolize the lactate. This occurs without gross anaerobic conditions.
The muscle are still fully oxygenated. In this sense there is no
anaerbiosis yet there is increased periods of lactate production by
anaerobic pathways.
Figure 6. The lactate shuttles. Lactate is produced in the cytoplasm by LDH acting on pyruvate and NADH.
Lactate can be shuttled from the cytoplasmic compartment to the mitochondrial compartment by importing the
lactate into the mitochondria and linking it to synthesis of pyruvate and generation of NADH in the
mitochondrial matrix. This is the intracellular lactate shuttle. Secondly, lactate can be exported into the blood
where it is taken up by adjacent oxidative muscle fibers and completely oxidized by its mitochondria. This is
the cell-to-cell lactate shuttle. Third, lactate released into the blood when lactic acid production is high can be
taken up by liver cells (also called hepatocytes). The hepatocytes resynthesize glucose from the lactate and
export it back into the blood where it can be taken up by the exercising muscle, for example. This is the Cori
cycle.
IX.
EXERCISE INCREASES GLUCOSE TRANSPORTERS IN THE MUSCLE
SARCOLEMMA
A.
Glucose uptake by muscles depends on the number and activity of GLUT4
transporters
One of the important fuels for muscle is blood glucose, which originates
mainly from the liver, intestine and kidney through glycogenolysis or
gluconeogenesis. Blood glucose enters the muscle fibers through specific
GLUT4 transporters in the muscle fiber membrane. The rate of uptake
depends on the number of these transporters in the membrane and their
activity.
B.
Insulin increases the number of GLUT4 transporters by recruiting latent
transporters
One of the most important effects of insulin is to increase the uptake of
glucose by the peripheral tissues, especially muscle. Insulin increases
glucose uptake by recruiting GLUT4 transporters from latent storage in
vesicles in the muscle fiber. This mechanism is shown in Fig. 7.
C.
Exercise increases the number of active GLUT4 transporters independent
of insulin
Exercise itself exerts an insulin-like effect and increases glucose uptake
by increasing the number of GLUT4 transporters, but without increases in
insulin. Diabetic persons who inject insulin and then exercise could
experience too much glucose removal and possibly suffer from
hypoglycemia. For this reason, diabetics should reduce their injection of
insulin when they anticipate they will exercise.
D.
Mechanism of GLUT4 recruitment is probably through AMPK and
CAMK
The mechanism by which exercise increases GLUT4 transporters is not
yet completely worked out. However, researchers believe than AMPK, a
protein kinase stimulated by AMP, and calmodulin-dependent protein
kinase, CAMK, may be involved. AMPK is stimulated by AMP, which is
produced from ADP by myokinase when ADP concentrations rise during
contractions. The AMPK is thought to be a kind of “fuel gauge” that
senses low fuel levels and then switches off ATP-consuming reactions and
turns on ATP-producing reactions. AMP acts in a negative feed-back
mechanism to restore ATP levels. CAMK, on the other hand, is activated
by Ca2+ when it rises to activate the myofilaments. If it simultaneously
activates GLUT4 transporters, it acts as a feed-forward mechanism to
begin increasing ATP supply in anticipation of its need to support
contraction.
Figure 7. Insulin, AMPK and CAMK increase GLUT4 incorporation into the sarcolemma of muscle
fibers. All three increase the number of sarcolemma GLUT4 from a population of latent transporters
located in vesicles in the cell. The increased numbers of GLUT4 transporters increase glucose uptake and
generation of ATP through glycolysis and oxidation of pyruvate or lactate.
X.
THE SITE OF FATIGUE DEPENDS ON THE MUSCLE AND INTENSITY
OF EXERCISE
A.
Fatigue is a reduction in developed force.
Fatigue is a transient loss of work capacity resulting from preceding
work. Reduction in the maximal force from a muscle, compared to its
rested force, is viewed as fatigue. By this criteria, fatigue sets in very
soon after maximum effort. Usually a person can bench press a maximal
weight only once.
B.
Metabolic fatigue occurs with repetitive stimulation.
A second kind of fatigue follows exercise at submaximal force for many
repetitions. After some time, we tire and eventually become unable to
develop even this submaximal force.
C.
Sites of fatigue could be in any process in the chain from brain to
contractile element.
In principle, impairment of any one of the chain of events starting with the
central nervous system and ending with the contractile elements of muscle
could reduce developed force. These events include excitatory drive to the
higher motor centers (motivation or effort), balance between excitatory
and inhibitory pathways in spinal motor neurons; conduction of action
potential on the motor neuron to the neuromuscular junction; transmission
across the neuromuscular junction; propagation of the muscle action
potential over the SL and into the T-tubules; excitation-contraction
coupling; generation of force at the actin and myosin filaments.
D.
Fatigue during maximum sustained contractions in humans is not in the
brain.
In classic experiments, Merton tested the adequacy of central nervous
system activation of fatiguing muscles by directly stimulating the nerve
leading to a muscle during maximal voluntary contractions. If the
voluntary contraction was maximal, the electrical stimulation would not
increase the force. He found no increase in force upon external stimulation
and concluded that the CNS is not the cause of fatigue. Direct stimulation
of the muscle also did not increase force of fatigued muscle, suggesting
that the neuromuscular junction failure also does not cause fatigue.
E.
Pi and H+ in muscle interferes with force development by actomyosin
ATPase.
In fast twitch fibers activated for short bursts, creatine phosphate
regenerates ATP from ADP. The terminal phosphate comes from the
creatine phosphate, so that regeneration of ATP increases Pi in the
myoplasm. At the same time, activation of anaerobic glycolysis produces
ATP with a build up of lactic acid and H+ ions. During exercise the pH of
muscle can fall from pH 7.0 to pH 6.0. Both Pi and H+ interfere with force
production, by directly inhibiting the acto-myosin ATPase or by making
the myofilaments less sensitive to activator Ca2+. The resulting reduction
in force is perceived as fatigue.
F.
Fatigue at submaximal work rates depends on glycogen stores.
1. Fatigue accompanies glycogen depletion
The performance times at high but submaximal work loads depends on
the size of the glycogen stores before exercise. Fatigue appears when
glycogen levels fall but before they are zero. This has led to the
hypothesis of the “glycogen shunt” in which glycogenolysis is
necessary to maintain ATP during contraction, and is resynthesized
during the rest period between contractions. When glycogen becomes
low, it can no longer sustain ATP levels during contraction and force
falls, even though glycogen is not completely used up.
2. Fatigue can be postponed by glycogen supercompensation
Glycogen stores can be increased by a combination of exercise and
carbohydrate consumption. This is referred to as carbohydrate
loading. It is usually accomplished by exhaustive exercise followed
within 2 hours by a high-carbohydrate meal. Under these conditions
the glycogen stores supercompensate and store larger than normal
amounts of glycogen. For serious athletic contests such as marathons
the race preparation is more complicated, taking place over the two
weeks prior to the race.
XI.
THE IMPORTANCE OF GLYCOGEN IN EXERCISE IS EVIDENT
FROM MUSCLE GLYCOGENOSIS (Figure 8)
There are 11 well known hereditary disorders of glycogen or carbohydrate
metabolism that affect muscle alone of together with other tissues. These
disorders cause two major clinical syndromes:
A.
acute, recurrent, reversible muscle dysfunction manifesting as exercise
intolerance, myalgia upon exercise, and cramps, often culminating in
muscle breakdown and myoglobinuria
B.
fixed, often progressive weakness, sometimes simulating dystrophic,
inflammatory or even neurogenic processes
Fig. 8 illustrates the points in metabolism characterized by these glycogenoses.
Figure 8. Muscle Glycogenoses.
XII.
TRAINING REGIMENS ARE DESIGNED TO INCREASE STRENGTH
OR ENDURANCE
A.
The most rapid training involves training the brain.
During initial training (the first few weeks) the maximal voluntary
contraction increases whereas the maximal evoked contraction (produced
by direct and maximal stimulation of the motor nerve) does not. This
suggests that trainees learn to activate their muscles more fully or they
improve coordination of the voluntary contraction. Increases in maximal
evoked contraction require a longer training period.
B.
Training for strength induces muscle hypertrophy.
1. Muscle strength depends on muscle size.
The maximum force that can be exerted by a muscle depends on its
cross-sectional area and architecture such as pinnation. Strength
training employs contractions against large resistances with few
repetitions. It is called resistance training.
2. Training sends a signal for muscle fiber growth.
Beginning training is associated with delayed onset muscle
soreness, or DOMS. Part of the soreness could be due to
microscopic tears in muscle fiber membranes or connective tissue
or to the stretching of sensory neurons from edema or mechanical
stretch. The exercise signals muscle hypertrophy: the diameter of
the fibers increases but the number of fibers stays the same.
Although the dogma states that training increases the size of
muscle fibers but not their number, some electron micrographs of
muscles appear to show muscle fibers splitting length wise. Some
limited amount of hyperplasia (increase in cell number) may also
occur. Both type I and type II fibers hypertrophy in response to
resistance training.
Hypertrophy occurs two ways: muscle fibers make more
myofibrils and satellite cells within the muscle are recruited to
fuse with existing muscle fibers to help control the extra
cytoplasm. During development, satellite cells are recruited to
form myotubes that further differentiate to become muscle fibers.
a. Myostatin inhibits muscle growth
Stretch, hypoxia, and intracellular [Ca2+] signal muscle cell
nuclei to produce a host of transcription factors that
increase synthesis of myofibrillar proteins. One product
that inhibits muscle differentiation and growth is
myostatin. Myostatin is an autocrine and paracrine
hormone produced by muscle cells that is a negative
regulator of muscle mass. Watch the newspapers for
athletic abuse of myostatin inhibitors. People with nonfunctioning myostatin mutations have gross muscle
hypertrophy (N Engl J Med 350:2682-2688, 2004).
b. Insulin-like growth factor-1 and muscle growth factor
(MGF) also influence muscle size
Other muscle growth factors include insulin-like growth
factor-1 (IGF-1) and muscle growth factor (MGF). Muscles
have receptors for IGF-1 that activate the cascade leading
from phosphatidyl inositol 3-kinase (PI3K) to activation of
protein kinase B (PCB-ACK) and mammalian target of
rapamycin (mTOR). This is inhibited by cAMP-dependent
protein kinase when AMP levels rise during hypoxia, for
example. Figure 9 illustrates the likely signaling pathways
for muscle hypertrophy.
3. Strength training decreases relative mitochondrial volume.
Heavy resistance training results in a reduction in the
mitochondrial volume density and the ratio of mitochondrial to
myofilament volume. Strength training appears to have no effect
on the muscle capillarity.
C.
Endurance training uses repetitive movements.
Endurance training increases the capillarity of muscles and tunes the
muscles’ metabolic capabilities. Concentrations of myoglobin and TCA
cycle enzymes are increased as well as both the size and number of
mitochondria. Muscles of endurance trained subjects use fats as the
primary fuel for moderate exercise, thereby sparing glycogen for bursts of
high intensity activity.
Figure 9. The signaling events in muscle hypertrophy. Muscle grows in response to stretch, increases in the
integrated cytoplasmic [Ca2+], androgens and glucocorticoids and other cytokines. Calcium activates
calcineurin, a protein phosphatase that dephosphorylates NFAT (nuclear factor of activated T cells) and
activates it. Myostatin produced by muscle inhibits satellite cell division and differentiation. Muscle cells also
respond to IGF-1 (insulin-like growth factor-1) and MGF (muscle growth factor).
XIII.
OUR ABILITY TO SWITCH MUSCLE FIBERS TYPES IS LIMITED
A.
Muscle capabilities are determined by the mixture of muscle fiber types
Earlier we saw that muscles are heterogeneous mosaics of different muscle
fiber types, and that the overall mechanical and metabolic performance of
the muscles is a consequence of the composition of that mosaic. One basis
of this heterogeneity is the expression of specific myosin isoforms. There
are at least 20 structurally distinct classes of myosin heavy chains. Eleven
of these are expressed in adult mammalian muscles, but some are specific
to one muscle. The most common isoforms are MHCIb, MHCIIa, MHCIIb
and MHC IId.
B.
Single muscle fibers can be hybrids
Immunohistochemistry reveals that some muscle fibers are “pure” types
that express only a single myosin heavy chain (MHC) isoform. However,
some muscle fibers contain two or more isoforms and are “hybrids”. How
can this be? Recall that each muscle fiber contains many nuclei, typically
located at the periphery of the cell near the sarcolemma, as shown in Fig.
10. Each nucleus controls a volume of cytoplasm or surface of the fiber
called its “nuclear domain”. A separate population of nuclei congregate
near the neuromuscular junction. During transitions between fiber types, it
is possible that some of these nuclei receive different signals than others,
and therefore transcribe different genes for the expression of myosin. In
this way, hybrid muscle fiber types arise. The existence of hybrid muscle
fibers allows a more continuous gradation between muscle types, as
shown in Table 5.
Figure 10. Nuclear domains in muscle fibers. Muscle fiber nuclei are located in the
periphery of the fiber, nearly aligned in rows with more or less regular spacing, with
some 35-80 nuclei per mm of fiber. Each nucleus controls protein expression in a volume
or surface element called its domain.
Muscle Fiber Type
Type I Pure Fiber
Myosin Heavy Chain
Expression
MHCI
MHCI>MHCIIa
Hybrid
MHCIIa>MHCI
Type IIa Pure Fiber
MHCIIa
MHCIIa>MHCIIx
Hybrid
MHCIIx>MHCIIa
Muscle Fiber Description
Slow
Fast Fatiguable
Type IIb Pure Fiber
MHCIIx
Fast Fatiguable
Table 5. The muscle fiber type continuum. Hybrid muscle fibers allow transitional forms
intermediate between Types I, IIa, and IIb. Humans do not make MHCIIb as in experimental
animals, so the human form in fast fatiguable muscles is named MHCIIx.
C.
It is unlikely that muscle types interconvert in human training regimens.
Is it possible for humans to convert a type I slow fiber into a type II fast
fiber, or vice-versa? In cross-innervation experiments in animals, a fasttwitch muscle is removed from its bed and transplanted to a slow-twitch
muscle bed, and a slow-twitch muscle is transplanted to a fast-twitch bed.
The muscles in these cases are converted part way from slow twitch to fast
twitch, and vice-versa. This demonstrates that it is the pattern of neural
stimulation that determines muscle type. Chronic low frequency
stimulation of fast twitch fibers increases the expression of proteins
normally expressed only by slow-twitch fibers. Denervation or muscle
unloading increase the levels of proteins normally expressed by fast-twitch
fibers. The evidence for human transformation of muscle types is
inconclusive. It appears that the stimulation of muscle necessary to
transform the fiber types is so severe that no human can train that hard.
The scientific consensus is that the transformation of muscle types is
limited in part by the original position of the muscle on the muscle fiber
type continuum. The transformation by exercise is always towards a
slower type of muscle, but frank conversion of Type IIB fiber to a Type I
fiber does not occur.
D.
The mechanism of muscle type switching appears to involve calcineurin
Calcineurin is a calcium-dependent protein phosphatase that is located in
the Z disks of skeletal muscle. With continued activity, the integrated
[Ca2+] in muscle cells increases, which may signal calcineurin activation.
It cleaves a phosphate off NFAT (nuclear factor of activated T cells) that
migrates to the nucleus and begins switching off transcription of DNA
coding for MHCIIa mRNA, and switches on the gene for MHCI. The
result is a switching of muscle fiber types. (See Fig. 11).
Figure 11. Mechanism of fiber type switching.
XIV.
PRACTICE QUESTIONS
1.
2.
The main source of ATPase activity in exercising muscle is
A.
The SR Ca-ATPase
B.
The Na-K-ATPase
C.
Acto-myosin ATPase
D.
Myokinase
E.
Creatine kinase
Because of lack of glucose 6 phosphatase
A.
Muscles can import glucose from the blood
B.
Liver cells can contribute to blood glucose homeostasis
3.
4.
5.
C.
The Cori cycle can occur
D.
Muscles cannot contribute substantially to blood glucose
homeostasis
E.
Muscles cannot participate in glycolysis
Fatigue in high-intensity, brief exercise is thought to be caused by
A.
A build up of Pi and H+ in the myoplasm
B.
Lactic acid
C.
Failure of T-tubule transmission
D.
Depletion of glycogen
E.
Failure of neuromuscular transmission
When people with insulin-dependent diabetes exercise, they reduce their
insulin shots because
A.
Exercise increases glucose uptake by muscle
B.
Exercise increases insulin secretion
C.
Exercise increases glycogenolysis
D.
Exercise increases lactic acid production
E.
They do not do this, tricky question person
Lactic acid levels in blood
A.
Increases dramatically at 60-70% VO2 max because muscles
become anaerobic
B.
Increases only when blood supply to muscle is compromised
C.
Is normally close to zero
D.
Explains most of short-term, high intensity fatigue
E.
Increases at 60-70% VO2 max because glycolysis increases
dramatically and lactic oxidation cannot keep up
6.
Persons with low levels of myostatin should
A.
Have predominately Type I muscle fibers
B.
Show exercise intolerance
C.
Have predominately Type II muscle fibers
D.
Have larger muscles than normal
E.
Have smaller muscle than normal
Answers: 1C; 2D; 3A; 4A; 5E; 6D