1. chapter 2
Fuel for Exercising Muscle: Metabolism and Hormonal Control

2. 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

3. 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

4. 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

5. 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

6. 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

7. Fat
Provides substantial energy during prolonged, low-intensity 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)

9. 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

10. 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

11. Energy for Cellular Metabolism
Key Points
We derive energy from food and store it as the high- energy 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

12. ATP Is Generated Through 3 Energy Systems
ATP-PCr system
Glycolytic system
Oxidative system

13. ATP Is Generated Through 3 Energy Systems

14. 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 3-15 sec during an all-out sprint
1 mole of ATP is produced per one mole of PCr

15. ATP-Phosphocreatine System

16. 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.

17. 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.

18. The Glycolytic System
Requires 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-6-phosphate
ATP-PCr and glycolysis provide the energy for ~2 min of all-out activity

19. Glycolysis

20. 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

21. 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.

22. 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

23. Aerobic Glycolysis and the Electron Transport Chain

24. Krebs Cycle

25. Oxidation of Carbohydrate
In the presence of oxygen, pyruvic acid from glycolysis is converted to acetyl coenzyme A (acetyl CoA)
Acetyl CoA enters the Krebs cycle and forms 2 ATP, carbon dioxide, and hydrogen
Hydrogen ion created during glycolysis and through the Krebs cycle combines with two coenzymes (NAD and FAD)
NAD and FAD carry hydrogen ions to the electron transport chain
(NAD and FAD → NADH and FADH)

26. The Electron Transport Chain
The electron transport chain splits NADH and FADH, producing hydrogen ions which are recombined with oxygen to produce water
Electrons produced from the split of NADH and FADH provide the energy for the phosphorylation of ADP to ATP
One molecule of glycogen can generate up to molecules of ATP

27. Oxidative Phosphorylation: The Electron Transport Chain

29. 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

31. Oxidation of Protein
Body uses little protein during rest and exercise (5-10% 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

32. Common Pathways for the Metabolism of Fat, Carbohydrate, and Protein

33. Interaction of the Energy Systems

34. 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

35. Oxidative Capacity in Muscle

36. 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 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)

37. 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

38. 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)

39. Locations of the Major Endocrine Organs

40. 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)

41. The Mechanism of Action of a Steroid Hormone

42. 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)

43. The Mechanism of Action of a Nonsteroid Hormone

44. 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)

45. 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

47. 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

48. Changes in Plasma Concentrations of Epinephrine, Norepinephrine, Glucagon, Cortisol and Glucose During 3 h of Cycling at 65% of VO2max .

49. 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

50. Changes in Plasma Concentrations During Cycling at 65% to 70% of VO2max.

51. Regulation of Fat Metabolism During Exercise
Lipolysis is hormonally controlled during exercise by:
Decreased insulin
Epinephrine
Norepinephrine
Cortisol
Growth hormone

52. 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

53. 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

54. 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

55. Mechanism by which ADH Conserves Body Water

56. 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

57. Changes in Plasma Volume and Aldosterone Concentrations During Cycling

58. 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

59. Exercise and Renal Function in the Regulation of Plasma Volume

60. Changes in Plasma Volume During Three Days of Repeated Exercise and Dehydration

61. 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)

62. 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