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Exercise Physiology MPB 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 production Anaerobic System a. ATP-PC (Phosphagen system) b. Anaerobic glycolysis (lactic acid system) Aerobic System a. Aerobic glycolysis b. Fatty acid oxidation c. TCA Cycle ATP-PCr (Phosphagen system) 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 ATP a) Does not depend on long series of reactions b) No O2 transportation required c) Limited storage, readily depleted ~ 10 s Glycolysis Glucose + 2 ADP + 2 Pi + 2 NAD+ 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O Lactate Dehydrogenase Hypoxic conditions Pyruvate + CoA + NADH + H+ Lactate + NAD+ Pyruvate Dehydrogenase Lots of Oxygen Pyruvate + CoA + NADP+ Acetyl-CoA + CO2 + NADPH Pyruvate Dehydrogenase Pyruvate + CoA + NADP+ Acetyl-CoA + CO2 + NADPH TCA Cycle Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2H20 CoASH + 3 NADH + 3H+ + FADH2 + GTP + 2CO2 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 ß oxidation (16 Carbons / 2) –1 = 7 (at 5 ATP each) Acetyl-CoA (16 Carbons / 2) = 8 (at 12 ATP each) Total per chain -1 35 96 130 Electrochemical Energy and ATP Synthesis Energy for “Burst” and Endurance Activities Rate of ATP Production (M of ATP/min) • phosphagen system ..............4 • anaerobic glycolysis..………2.5 • aerobic system.......................1 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) 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 Disease Oxygen Uptake and Exercise Domains INCREMENTAL VO2 (l/min) 4 Severe 2 Heavy Moderate 0 150 Work Rate (Watts) 300 Anaerobic Threshold Concept Exercise 15 Blood Lactate mM Heart Disease Onset of lactic acidosis 10 5 Athlete 0 50 Rest Period 150 100 Exercise (watts) 200 250 Anaerobic Threshold in Some Elite Long Distance Athletes can be close to Max Exercise 15 Blood Lactate mM Onset of lactic acidosis 10 Bill Rodgers 5 0 Basal Oxygen Uptake 20 60 40 Oxygen Uptake (% maximum) 80 100 Oxygen Deficit and Debt Oxygen Uptake and Exercise Domains CONSTANT LOAD Severe 4 Heavy 2 0 Moderate 12 Time (minutes) 24 Lactate and Exercise 12 Blood Lactate mM 6 0 0 12 Time (minutes) 24 Three Principles of Fuel Utilization during Exercise • Maintaining glucose homeostasis • Using the fuel that is most efficient Storage Metabolic • Preserving muscle glycogen core Glucose homeostasis is usually maintained despite increased glucose uptake by the working muscle Moderate Exercise 100 80 Blood Glucose (mg/dl) 60 40 20 0 5 Rates of Glucose Entry and Removal from the Blood (mg•kg-1•min -1) 4 Entry 3 2 Removal 1 0 -30 0 30 Time (min) 60 Carbohydrate Stores after an Overnight Fast Sedentary Liver Glycogen Blood Glucose Muscle Glycogen 400 grams 4 grams 100 grams Carbohydrate Stores after an Overnight Fast 1 hr of Exercise Liver Glycogen Blood Glucose Muscle Glycogen 400 grams 4 grams 100 grams Carbohydrate Stores after an Overnight Fast 2 hr of Exercise Liver Glycogen Blood Glucose Muscle Glycogen 400 grams 4 grams 100 grams Carbohydrate Stores after an Overnight Fast 3 hr of Exercise Liver Glycogen Blood Glucose Muscle Glycogen 400 grams 4 grams 100 grams Carbohydrate Stores after an Overnight Fast 4 hr of Exercise Liver Glycogen Blood Glucose Muscle Glycogen 400 grams 4 grams 100 grams !!! Contribution of different fuels to metabolism by the working muscle is determined by 3 objectives: • Maintaining glucose homeostasis • Using the fuel that is most efficient Storage Metabolic • Preserving muscle glycogen core The Most Efficient Fuel depends on Exercise Intensity and Duration Metabolic Efficiency CHO is preferred during high intensity exercise because its metabolism yields more energy per liter of O2 than fat metabolism. kcal/l of O2 CHO Fat 5.05 4.74 CHO can also produce energy without O2!!! Storage Efficiency Fat is preferred during prolonged exercise because its metabolism provides more energy per unit mass than CHO metabolism. kcal/g of fuel CHO Fat Fats are stored in the absence of H2O. 4.10 9.45 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 From: Powers & Howley. (2007). Exercise Physiology. McGraw-Hill. • As intensity increases carbohydrate use increases, fat use decreases • As duration increase, fat use increases, carb use decreases Contribution of different fuels to metabolism by the working muscle is determined by 3 objectives: • Maintaining glucose homeostasis • Using the fuel that is most efficient Storage Metabolic • Preserving muscle glycogen core Other fuels are utilized to spare muscle glycogen during prolonged exercise thereby delaying exhaustion Lactate Pyruvate Amino Acids Adipose NEFA Glycerol NEFA Muscle GLY GNG ATP GLY Glucose Liver 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. Contribution of different fuels to metabolism by the working muscle is determined by 3 objectives: • Maintaining glucose homeostasis • Using the fuel that is most efficient Storage Metabolic • Preserving muscle glycogen core Discussion Question Can you accommodate all three principles of fuel utilization? Why not? What is the Consequence?