Aerobic & Anaerobic Metabolism in Muscles

Objectives Recognize the importance of ATP as energy source in skeletal muscle. Understand how skeletal muscles derive and utilize ATP for energy. Differentiate between energy metabolism in red and white muscle fibers.

ATP or adenosine triphosphate is the energy currency used by our body everyday to perform a number of tasks: ATP Maintain body temperature Repair damaged cells Digestion of food Mechanical work – movement ATP ↔ ADP + Energy

ATP ATP is the most important form of chemical energy stored in cells Breakdown of ATP into ADP+PO 4 releases energy. Muscles typically store limited amounts of ATP – enough to power 4-6s of activity So resting muscles must have energy stored in other ways. ATP

Systems of generation of ATP 1- ATP-Creatine Phosphate system 2- Glycolytic system 3- Oxidative system The process that facilitates muscular contraction is entirely dependent on body’s ability to provide & rapidly replenish ATP

INTERACTION OF ENERGY SYSTEMS ImmediateShort-termLong-term

100% Capacity of Energy System % Capacity of Energy System 10 sec30 sec2 min5 + min Energy Transfer Systems and Exercise Aerobic Energy System Anaerobic Glycolysis ATP – Creatine Phosphate Exercise Time

Energy systems for muscular exercise Energy Systems Mole of ATP/min Time to Fatigue Immediate: ATP-CP (ATP & creatine phosphate) sec Short Term: Glycolytic (Glycogen-Lactic Acid) min Long Term: Oxidative 1 Unlimited time

Energy requirements The three energy systems often operate simultaneously during physical activity. duration Relative contribution of each system to total energy requirement differs depending on exercise intensity & duration. anaerobic sources Magnitude of energy from anaerobic sources depends on person’s capacity and tolerance for lactic acid accumulation (Athletes are trained so that they will have better tolerance for lactic acid). aerobic metabolism As exercise intensity diminishes & duration extends beyond 4 minutes, energy more dependent on aerobic metabolism

Aerobic Vs. Anaerobic sources of energy Aerobic Aerobic Requires oxygen Source of energy: mainly fatty acids, then carbohydrate End Products : CO 2, H 2 O & ATP Anaerobic Anaerobic does not require oxygen does not require oxygen Source of energy: ONLY Carbohydrate (anaerobic glycolysis) End Products: Lactate & ATP

What factors contribute to muscle fatigue?

Fatigued muscle no longer contracts due to: 1- Accumulation of lactic acid (low pH of sarcoplasm) 2- Exhaustion of energy resources (  ADP &  ATP) 3- Ionic imbalance: Muscle Fatigue

How would a fatigued muscle be able again to contract ? Recovery period: begins immediately after activity ends Oxygen debt (excess post-exercise oxygen consumption) i.e. the amount of oxygen required during resting period to restore muscle to normal conditions

What are the mechanisms by which muscle fibers obtain energy to power contractions?

Muscles and fiber types White muscle White muscle : (glycolytic) fast mostly fast fibers pale (e.g. chicken breast) Red muscle Red muscle: (oxidative) slow mostly slow fibers dark (e.g. chicken legs) Most human muscles are: Mixed fibers pink

Fast Vs. Slow Fibers Type I Type II

mitochondria Abundant mitochondria capillary Extensive capillary supply myoglobin High concentrations of myoglobin Can contract for long periods of time Can contract for long periods of time Fatigue resistant Fatigue resistant fatty acids oxidation, Obtain their ATP mainly from fatty acids oxidation, TCA cycle & ETC (oxidative phosphorylation) Slow fibers

glycogen Large glycogen reserves few mitochondria Relatively few mitochondria Produce rapid, powerful contractions of short Produce rapid, powerful contractions of short duration duration Easily fatigued Easily fatigued anaerobic glycolysis Obtain their ATP mainly from anaerobic glycolysis Fast fibers

ENERGY REQUIREMENTS AND SOURCE OF ENERGY FOR SKELETAL MUSCLE ( Resting vs. Working)

ATP use in the resting muscle cell During periods of muscular rest ATP is required for: 1- Glycogen synthesis (glycogenesis) i.e. storage form of glucose to be used during muscular exercise Creatine phosphate production 2- Creatine phosphate production i.e. energy storage compound to be used at the beginning of muscular contraction

Source of ATP in resting muscle fibers free fatty acids Resting muscle fibers takes up free fatty acids from blood. Fatty acids mitochondria acetyl CoA NADHFADH 2 Fatty acids are oxidized (in the mitochondria) to produce acetyl CoA & molecules of NADH & FADH 2 Acetyl CoAcitric acid cycle mitochondria Acetyl CoA will then enter the citric acid cycle (in the mitochondria)  ATP, NADH, FADH 2 & CO 2 NADH FADH 2 the electron transport chain. NADH & FADH 2 will enter the electron transport chain. ATP  synthesis of ATP

Figure 10–20a RESTING MUSCLE FIBERS METABOLISM

ATP sources in working muscle stored ATP At the beginning of exercise, muscle fibers immediately use stored ATP next 15 seconds creatine phosphate For the next 15 seconds, muscle fibers turn to the creatine phosphate. This system dominates in events such as the 100m dash or lifting weights.

ATP sources in working muscle ATP sources in working muscle con t. anaerobic glycolysis about 45-60s. After the phosphagen system is depleted, the process of anaerobic glycolysis can maintain ATP supply for about 45-60s. pyruvic acid + Glycogen stored in muscles  Glucose  2 pyruvic acid + 2 ATPs lactic acid 2 Pyruvic acid  2 lactic acid Lactic acid diffuses out of muscles  blood  liver  Glucose (by gluconeogenesis)  blood  muscles * * It usually takes a little time for the respiratory and cardiovascular systems to catch up with the muscles and supply O 2 for aerobic metabolism.

Figure 10–20c WORKING MUSCLE FIBERS METABOLISM

Anaerobic metabolism is inefficient: 1- Large amounts of glucose are used for very small ATP returns. 2- Lactic acid is produced leading to muscle fatigue Type of sports uses anaerobic metabolism? Sports that requires bursts of speed and activity e.g. basketball. ATP sources in working muscle ATP sources in working muscle con t.

Aerobic metabolism Aerobic metabolism Occurs when the respiratory & cardiovascular systems have “caught up with” the working muscles. restlight to moderate During rest and light to moderate exercise, aerobic metabolism contributes 95% of the necessary ATP. Compounds which can be aerobically metabolized include: Fatty acids, pyruvic acid amino acids Fatty acids, pyruvic acid (made via glycolysis) & amino acids. ATP sources in working muscle ATP sources in working muscle con t.

WORKING MUSCLE FIBERS METABOLISM

The Cori cycle & The glucose-alanine cycle

The Cori cycle Liverlactateglucosegluconeogenesis Liver converts lactate into glucose via gluconeogenesis The newly formed glucose is transported to muscle to be used for energy again

The glucose-alanine cycle Muscles produce Muscles produce: Pyruvate 1- Pyruvate from glycolysis during exercise 2- NH2 2- NH2 produced from normal protein degradation Pyruvate + NH 2  Alanine Alanine Alanine is transported through the blood to liver Liver converts alanine back to pyruvate Alanine – NH 2 = Pyruvate Pyruvate Pyruvate is converted to glucose (gluconeogenesis). Glucose Glucose is transported to muscle to be used for energy again. Liver converts NH2 to urea for excretion (urea cycle)