Exercise PhysiologyMPB 326

David Wasserman, PhD

Light Hall Rm 823

3-7336

The Remarkable Thing about Exercise

The Great Debate

• Top-down

• Feedback control

Energy Metabolism and the Three Principles of Fuel

Utilization

The need for energy starts when calcium is released from the sarcoplasmic reticulum of contracting muscle

The Working Muscle

Energy for Contraction

Muscle relaxation requires energy too!

Where does this ATP come from?

Sources of ATP

Stored in muscle cell (limited)

Synthesized from macronutrients

Common Processes for ATP productionAnaerobic System

a. ATP-PC (Phosphagen system)  b. Anaerobic glycolysis (lactic acid system)

Aerobic Systema. Aerobic glycolysisb. Fatty acid oxidationc. TCA Cycle

1.Stored in the muscle cells (PCr > ATP)

2.ATP + H2O ADP + Pi + E (ATPase hydrolysis)

3.PCr + ADP ATP + Cr (creatine kinase reaction)

4.ADP + ADP ATP + AMP (adenylate kinase)

5.PCr represents the most rapidly available source of ATPa) Does not depend on long series of reactionsb) No O2 transportation required

c) Limited storage, readily depleted ~ 10 s

ATP-PCr (Phosphagen system)

Glycolysis

Glucose + 2 ADP + 2 Pi + 2 NAD+

2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Lactate Dehydrogenase

Pyruvate + CoA + NADH + H+

Lactate + NAD+

Hypoxic conditions

Pyruvate Dehydrogenase

Pyruvate + CoA + NADP+

Acetyl-CoA + CO2 + NADPH

Lots of Oxygen

Pyruvate Dehydrogenase

Pyruvate + CoA + NADP+

Acetyl-CoA + CO2 + NADPH

Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2H20

CoASH + 3 NADH + 3H+ + FADH2 + GTP + 2CO2

TCA Cycle

Beta Oxidation of Fatty Acids

7 FAD + 7 NAD+ + 7 CoASH + 7 H2O +

H(CH2CH2)7CH2CO-SCoA

8 CH3CO-SCoA + 7 FADH2 + 7 NADH + 7 H+

Summary of ATP Production via Lipid Oxidation

ATP Balance Sheet for Palmitic Acid (16 carbon) ATP

• Activation of FA chain -1

• ß oxidation (16 Carbons / 2) –1 = 7 (at 5 ATP each) 35

• Acetyl-CoA (16 Carbons / 2) = 8 (at 12 ATP each) 96

Total per chain 130

Electrochemical Energy and ATP Synthesis

Energy for “Burst” and Endurance Activities

How long Can it Last?• phosphagen system...8 to 10 sec• anaerobic glycolysis…1.3 to 1.6 min • aerobic system.........unlimited time (as long as nutrients last)

Rate of ATP Production (M of ATP/min)• phosphagen system ..............4 • anaerobic glycolysis..………2.5 • aerobic system.......................1

Aerobic Energy

• During low intensity exercise, the majority of energy is provided aerobically

• Energy produced aerobically requires O2

• Therefore, O2 uptake can be used as a measure for energy use

Exercise Testing in Health and DiseaseExercise Testing in Health and Disease

Oxygen Uptake and Exercise Domains

4

2

150 Work Rate (Watts)Work Rate (Watts)

INCREMENTAL

ModerateModerate

HeavyHeavy

300

VOVO 22 (l/min)

(l/min)

SevereSevere

00

Heart Disease

Anaerobic Threshold Concept

250

Exercise

(watts)

0

5

10

15

Exercise

20015010050

Rest Period

Onset of lactic acidosis

Blood

Lactate

mM

Athlete

Anaerobic Threshold in Some Elite Long

Distance Athletes can be close to Max

100

Oxygen Uptake

(% maximum)

Exercise

80604020Basal Oxygen Uptake

Onset of lacticacidosis

0

5

10

15

Blood

Lactate

mM

BillRodgers

Oxygen Deficit and Debt

Oxygen Uptake and Exercise Domains

2

00 1212Time (minutes)

24

CONSTANT LOAD

ModerateModerate

HeavyHeavy

SevereSevere

4

Lactate and Exercise

0

6

12

Blood LactatemM

12

Time (minutes)

0 24

Three Principles of Fuel Utilization during Exercise

• Maintaining glucose homeostasis

• Using the fuel that is most efficientStorageMetabolic

• Preserving muscle glycogen core

Glucose homeostasis is usually maintained despite increased glucose uptake by the working muscle

Time (min)

BloodGlucose

(mg/dl)

0

20

40

60

80

100

0

1

2

3

4

5

-30 0 30 60

Rates of GlucoseEntry and

Removal fromthe Blood

(mg•kg-1•min-1)

Entry

ModerateExercise

Removal

Liver Glycogen

BloodGlucose

MuscleGlycogen

Carbohydrate Stores after an Overnight FastSedentary

4 grams100

grams

400grams

Liver Glycogen

BloodGlucose

MuscleGlycogen

Carbohydrate Stores after an Overnight Fast 1 hr of Exercise

4 grams100

grams

400grams

Liver Glycogen

BloodGlucose

MuscleGlycogen

Carbohydrate Stores after an Overnight Fast 2 hr of Exercise

4 grams100

grams

400grams

Liver Glycogen

BloodGlucose

MuscleGlycogen

Carbohydrate Stores after an Overnight Fast 3 hr of Exercise

4 grams100

grams

400grams

Liver Glycogen

BloodGlucose

MuscleGlycogen

Carbohydrate Stores after an Overnight Fast 4 hr of Exercise

4 grams100

grams

400grams

!!!

Contribution of different fuels to metabolism by the working muscle is determined by 3 objectives:

• Maintaining glucose homeostasis

• Using the fuel that is most efficientStorageMetabolic

• Preserving muscle glycogen core

The Most Efficient Fuel depends on Exercise Intensity and Duration

Metabolic EfficiencyCHO is preferred during high intensity exercise because its metabolism yields more energy per liter of O2 than fat metabolism.

kcal/l of O2

CHO 5.05 Fat 4.74

CHO can also produce energy without O2!!!

Storage EfficiencyFat is preferred during prolonged exercise because its metabolism provides more energy per unit mass than CHO metabolism.

kcal/g of fuel

CHO 4.10 Fat 9.45

Fats are stored in the absence of H2O.

Effects of Exercise Intensity

• Plasma FFA (fat from fat cells) is the primary fuel source for low intensity exercise

• As intensity increases, the source shifts to muscle glycogen

From: Powers & Howley. (2007). Exercise Physiology. McGraw-Hill.

Effects of Exercise Duration

From: Powers & Howley. (2007). Exercise Physiology. McGraw-Hill.

Fuel Selection

• As intensity increases carbohydrate use increases, fat use decreases

• As duration increase, fat use increases, carb use decreases

From: Powers & Howley. (2007). Exercise Physiology. McGraw-Hill.

Contribution of different fuels to metabolism by the working muscle is determined by 3 objectives:

• Maintaining glucose homeostasis

• Using the fuel that is most efficientStorageMetabolic

• Preserving muscle glycogen core

Other fuels are utilized to spare muscle glycogen during prolonged exercise thereby delaying exhaustion

GNGGLY

Adipose

As exercise duration increases: • More energy is derived from fats and less from glycogen. • Amino acid, glycerol, lactate and pyruvate carbons are recycled into glucose.

LactatePyruvate

Amino Acids

NEFAGlycerol

NEFA

GlucoseATP

GLY

Muscle

Liver

Contribution of different fuels to metabolism by the working muscle is determined by 3 objectives:

• Maintaining glucose homeostasis

• Using the fuel that is most efficientStorageMetabolic

• Preserving muscle glycogen core

Discussion Question

Can you accommodate all three principles of fuel utilization?

Why not?

What is the Consequence?