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chapter
2
Fuel for Exercising
Muscle:
Metabolism and
Hormonal Control
Learning Objectives
• Learn how our bodies change the food we eat into ATP
to provide our muscles with the energy they need to
move
• Examine the three metabolic systems that generate
ATP
• Learn how the endocrine system monitors and
responds to changes in the body’s systems during
exercise
Metabolism and Bioenergetics
• Nutrients from foods are the substrates for metabolism
and are provided and stored as:
– Carbohydrate
– Fat
– Protein
• Each cell contains chemical pathways that convert
these substrates to usable energy, a process called
bioenergetics
• The energy we derive from food is stored in cells in the
form of adenosine triphosphase (ATP)
• ATP serves as the immediate source of energy for most
body functions including muscle contraction
Kilocalorie
• Energy in biological systems is measured in kilocalories
• All energy eventually degrades to heat
• One kilocalorie is the amount of heat energy needed to
raise 1 kg of water from 1 °C to 15 °C
Energy Sources
• At rest, the body uses carbohydrate and fat almost
equally for energy
• Protein provides little energy for cellular activity, but
serves as the building blocks for the body’s tissues
• During intense short-duration muscular effort, the body
relies mostly on carbohydrate to generate ATP
• Longer, less intense exercise utilizes carbohydrates
and fat for sustained energy production
Carbohydrate
• All dietary carbohydrate is ultimately converted to
glucose
• Glucose is taken up by muscles and liver and
converted to the complex sugar molecule called
glycogen
• Glycogen is stored in the cytoplasm of muscle cells,
where it can be quickly used to form ATP
• Glycogen is also stored in the liver, where it is
converted back to glucose as needed and transported
by the blood to the muscles to form ATP
Fat
• Provides substantial energy during prolonged, lowintensity activity
• Body stores of fat are larger than carbohydrate
reserves
• Fat is less readily available for cellular metabolism
compared to carbohydrate
• Fat is stored as triglycerides and must be broken down
to free fatty acids (FFAs) to be used in metabolism
• More energy is derived from breaking down fat (9.4
kcal/g) compared to carbohydrate (4.1 kcal/g)
Protein
• Protein can be used as a minor source of energy, but it
must be converted to glucose via glucogenesis
• Proteins can generate FFAs during starvation through
lipogenesis
• Protein can supply up to 5-10% of energy during
prolonged exercise
• Proteins must be broken down to their basic units—
amino acids—to be used for energy
The Lock-and-Key Action of Enzymes
in the Catabolism of Compounds
• Enzymes control the rate
of free energy release
from substrates
• Enzyme names end with
the suffix -ase
Energy for Cellular Metabolism
Key Points
• We derive energy from food and store it as the highenergy compound ATP
– Carbohydrate (4.1 kcal/g)
– Fat (9.4 kcal/g)
– Protein (4.1 kcal/g)
• Carbohydrate is stored as glycogen in muscles and
the liver and is more accessible than protein or fat
• Glucose, directly from food or broken down from
glycogen, is the usable form of carbohydrate
• Fat is stored as triglycerides in adipose tissue and
broken down to FFA
ATP Is Generated
Through 3 Energy Systems
1. ATP-PCr system
2. Glycolytic system
3. Oxidative system
ATP Is Generated
Through 3 Energy Systems
ATP-PCr System
• Cells store small amounts of ATP, and
phosphocreatine (PCr), which is broken down to
regenerate ATP
• Release of ATP from PCr is facilitated by the enzyme
creatine kinase
• This process does not require oxygen (anaerobic)
• ATP and PCr sustain the muscle’s energy needs for 315 sec during an all-out sprint
• 1 mole of ATP is produced per one mole of PCr
ATP-Phosphocreatine System
ATP-PCr
Although ATP is being used at a very high rate, the energy from PCr
is used to resynthesize ATP, preventing the ATP level from
decreasing. At exhaustion, both ATP and PCr concentrations are low.
Glycogen Breakdown
and Synthesis
Glycolysis is the breakdown of glucose; it may be
anaerobic or aerobic.
Glycogenesis is the process by which glycogen is
synthesized from glucose to be stored in the liver or
muscle.
Glycogenolysis is the process by which glycogen is
broken down into glucose-1-phosphate to be used for
energy production.
The Glycolytic System
• Requires 10-12 enzymatic reactions to break down
glycogen to lactic acid, producing ATP
• Occurs in the cytoplasm
• Glycolysis does not require oxygen (anaerobic)
• Without oxygen present, pyruvic acid produced by
glycolysis becomes lactic acid
• 1 mole of glycogen produces 3 moles of ATP; 1
mole of glucose produces 2 moles of ATP because
1 mole is used to convert glucose to glucose-6phosphate
• ATP-PCr and glycolysis provide the energy for ~2
min of all-out activity
Glycolysis
ATP-PCr and Glycolytic Systems
Key Points
• ATP-PCr system
– Pi is separated from PCr by creatine kinase
– Pi is combined with ADP to form ATP
– Energy yield: 1 mole of ATP per 1 mole of PCr
• Glycolytic system
– Glucose or glycogen is broken down to pyruvic acid
– Without oxygen, pyruvic acid is converted to lactic
acid
– 1 mole of glucose yields 2 moles of ATP
– 1 mole of glycogen yields 3 moles of ATP
Energy Sources for the Early Minutes
of Intense Exercise
The combined actions of the ATP-PCr and glycolytic
systems allow muscles to generate force in the
absence of oxygen; thus these two energy systems
are the major energy contributors during the early
minutes of high-intensity exercise.
The Oxidative System
• The oxidative system uses oxygen to generate
energy from metabolic fuels (aerobic)
• Oxidative production of ATP occurs in the
mitochondria
• Can yield much more energy (ATP) than anaerobic
systems
• The oxidative system is slow to turn on
• Primary method of energy production during
endurance events
Aerobic Glycolysis and the Electron
Transport Chain
Krebs Cycle
Oxidation of Carbohydrate
1. In the presence of oxygen, pyruvic acid from
glycolysis is converted to acetyl coenzyme A (acetyl
CoA)
2. Acetyl CoA enters the Krebs cycle and forms 2 ATP,
carbon dioxide, and hydrogen
3. Hydrogen ion created during glycolysis and through
the Krebs cycle combines with two coenzymes
(NAD and FAD)
4. NAD and FAD carry hydrogen ions to the electron
transport chain
(NAD and FAD → NADH and FADH)
The Electron Transport Chain
1. The electron transport chain splits NADH and FADH,
producing hydrogen ions which are recombined with
oxygen to produce water
2. Electrons produced from the split of NADH and FADH
provide the energy for the phosphorylation of ADP to
ATP
3. One molecule of glycogen can generate up to 37-39
molecules of ATP
Oxidative Phosphorylation:
The Electron Transport Chain
Oxidation of Fat
• Lipolysis is the breakdown of triglycerides into
glycerol and free fatty acids (FFAs)
• FFAs travel via blood to muscle fibers and are
broken down by enzymes in the mitochondria into
acetic acid, which is converted to acetyl CoA
through β-oxidation
• Acetyl CoA enters the Krebs cycle and the electron
transport chain
• Fat oxidation requires more oxygen compared with
glucose because a FFA molecule contains more
carbon
Oxidation of Protein
• Body uses little protein during rest and exercise (510% to sustain prolonged exercise)
• Some amino acids can be converted into glucose or
intermediates of oxidative metabolism
• Energy yield from protein is difficult to determine
• The nitrogen in amino acids is converted into urea,
which requires ATP
Common Pathways for the Metabolism of
Fat, Carbohydrate, and Protein
Interaction of the Energy Systems
Oxidative Capacity of Muscle
.
• Oxidative capacity of muscle (QO2) is a measure of its
maximal capacity to use oxygen
• Representative enzymes to measure oxidative capacity
– Succinate dehydrogenase
– Citrate synthase
Oxidative Capacity in Muscle
Oxidative Metabolism
Key Points
• The oxidative system involves the breakdown of
substrates in the presence of oxygen
• Oxidation of carbohydrate involves glycolysis, the
Krebs cycle, and the electron transport chain, resulting
in the formation of H2O, CO2, and 38-39 molecules of
ATP
• Fat oxidation involves β-oxidation of free fatty acids,
the Krebs cycle, and the electron transport chain to
produce more ATP than carbohydrate
• The maximum rate of ATP formation from fat oxidation
is too low to match the rate of ATP utilization during
high intensity exercise
(continued)
Oxidative Metabolism (continued)
Key Points
• Protein contributes little to energy production, and its
oxidation is complex because amino acids contain
nitrogen, which cannot be oxidized
• The oxidative capacity of muscle fibers depends on
their oxidative enzyme levels, fiber-type composition,
and oxygen availability
Hormonal Control
• Hormones significantly affect metabolism
• Hormones are secreted directly into the blood and act
as chemical signals to their target cells
• Hormones travel away from the cells that secrete them
and specifically affect the activities of other cells and
organs
• 2 types of hormones: steroid and nonsteroid
• Hormone secretion is regulated by negative feedback
• Hormone receptors can increase (upregulation) or
decrease (downregulation)
Locations of the Major
Endocrine Organs
Steroid Hormones
•
•
•
•
•
•
Formed from cholesterol
Lipid soluble
Capable of direct gene activation
Receptors are in the cytoplasm or in the nucleus
mRNA activation promotes protein synthesis
Examples
–
–
–
–
Adrenal cortex (cortisol and aldosterone)
Ovaries (estrogen and progesterone)
Testes (testosterone)
Placenta (estrogen and progesterone)
The Mechanism of Action
of a Steroid Hormone
Nonsteroid Hormones
• Protein or peptide and amino acid-derived
• Not lipid soluble
• Triggers a series of intracellular events through second
messenger systems, cAMP
–
–
–
–
–
Activates cellular enzymes
Changes membrane permeability
Promotes protein synthesis
Changes cellular metabolism
Stimulates cellular secretions
• Examples
– Thyroid gland (thyroxine and triiodothyronine)
– Adrenal medulla (epinephrine and norepinephrine)
The Mechanism of Action
of a Nonsteroid Hormone
Hormonal Control
Key Points
• Hormones are secreted into the blood and then circulate
to target cells where they bind to receptors on the target
tissue
• Steroid hormones pass through the cell membrane to
bind with receptors in the cell to directly activate genes
causing protein synthesis
• Nonsteroid hormones bind to receptors in the cell
membrane and activate second messenger systems
which trigger numerous cellular processes
(continued)
Hormonal Control (continued)
Key Points
• A negative feedback system regulates secretion of
most hormones
• The number of hormone receptors can be altered
– Upregulation—increase in the number of available
receptors
– Downregulation—decrease in the number of
available receptors
Regulation of Glucose Metabolism
During Exercise
Glucose concentration during exercise is a balance
between glucose uptake by the exercising muscles and
its release by the liver
– ↑ Glucagon: promotes liver glycogen breakdown and
glucose formation from amino acids
– ↑ Epinephrine: promotes glycogenolysis
– ↑ Norephinephrine: promotes glycogenolysis
– ↑ Cortisol: promotes protein catabolism
Changes in Plasma Concentrations of Epinephrine,
Norepinephrine, Glucagon, Cortisol and
. Glucose During
3 h of Cycling at 65% of VO2max
Glucose Uptake by Muscle
• Plasma insulin concentrations decrease during
prolonged submaximal exercise
• Exercise may enhance insulin’s binding to receptors on
the muscle fiber, reducing the need for high
concentrations of plasma insulin to transport glucose
Changes in Plasma Concentrations
During
.
Cycling at 65% to 70% of VO2max
Regulation of Fat Metabolism
During Exercise
Lipolysis is hormonally controlled during
exercise by:
–
–
–
–
–
Decreased insulin
Epinephrine
Norepinephrine
Cortisol
Growth hormone
Hormonal Control of Metabolism
During Exercise
Key Points
• Plasma glucose is increased by the combined actions
of glucagon, epinephrine, norepinephrine, and cortisol
• Insulin helps glucose enter the cell, but declines during
prolonged exercise
• When carbohydrate reserves are low, the body turns to
more fat oxidation, and lipolysis is increased, which is
facilitated by decreased insulin and increased
epinephrine, norepinephrine, cortisol, and growth
hormone
Hormonal Regulation of Fluid and
Electrolyte Balance During Exercise
• Fluid balance during exercise is critical for optimal
metabolic, cardiovascular, and thermoregulatory
function
• The endocrine system plays a major role in monitoring
fluid levels and correcting imbalances
– Regulates electrolyte balance (especially Na+)
– Antidiuretic hormone (ADH)
– Aldosterone
Antidiuretic Hormone
• Released from the posterior pituitary
• Hemoconcentration during exercise, increased plasma
osmolality, and low plasma volume stimulate the
release of ADH
• ADH promotes water retention in the kidney in an effort
to dilute plasma electrolyte concentrations back to
normal
Mechanism by which ADH
Conserves Body Water
Aldosterone
• Mineralcorticoid hormone
• Secretion is stimulated by:
– ↓ plasma sodium
– ↓ blood volume
– ↓ pressure
– ↑ plasma potassium concentration
• Promotes renal reabsorption of sodium, causing the
body to retain sodium
Changes in Plasma Volume and
Aldosterone Concentrations During
Cycling
Kidneys
• The kidneys strongly influence the maintenance of
plasma volume and blood pressure regulation through
the release of renin
• Renin initiates the renin-angiotensin-aldosterone
mechanism
Exercise and Renal Function in the
Regulation of Plasma Volume
Changes in Plasma Volume During Three
Days of Repeated Exercise and
Dehydration
Hormone Regulation of Fluid and
Electrolyte Balance During Exercise
Key Points
• The two primary hormones involved in the regulation of
fluid balance are ADH and aldosterone
• ADH is released from the posterior pituitary in response
to increased plasma osmolality and low blood volume
• ADH acts on the kidney, promoting water conservation
• When plasma volume or blood pressure decreases, the
kidneys release renin, which converts angiotensinogen
to angiotensin I, which later becomes angiotensin II
(continued)
Hormone Regulation of Fluid and
Electrolyte Balance During Exercise
(continued)
Key Points
• Angiotensin II increases peripheral resistance,
increasing blood pressure
• Angiotensin II triggers the release of aldosterone from
the adrenal cortex, which promotes sodium
reabsorption in the kidney