ATP of Skeletal Muscles Energy of Skeletal Muscles Lecture -3 ATP of Skeletal 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 ATP or adenosine triphosphate is the energy currency used by our body everyday to perform a number of tasks: 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+PO4 releases energy. Muscles typically store limited amounts of ATP – enough to power 4-6 seconds of activity So resting muscles must have energy stored in other ways.

Systems of generation of ATP 1- ATP-Ctreatine 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 Immediate Short-term Long-term

Energy Transfer Systems and Exercise 100% % Capacity of Energy System Anaerobic Glycolysis Aerobic Energy System ATP – Creatine Phosphate 10 sec 30 sec 2 min 5+ min Exercise Time

Energy systems for muscular exercise Mole of ATP/min Time to Fatigue Immediate: ATP-CP (ATP & creatine phosphate) 4 5 - 10 sec Short Term: Glycolytic (Glycogen-Lactic Acid) 2.5 1 - 2 min Long Term: Oxidative 1 Unlimited time

Energy requirements The three energy systems often operate simultaneously during physical activity HOWEVER Relative contribution of each system to total energy requirement differs depending on exercise intensity & duration 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) As exercise intensity diminishes & duration extends beyond 4 minutes, energy more dependent on aerobic metabolism

Fatigued muscle no longer contracts due 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 cells is less responsive to motor neuronal stimulation

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

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

Fast Vs. Slow Fibers Type I Type II

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

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

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 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 Resting muscle fibers takes up free fatty acids from blood. Fatty acids are oxidized (in the mitochondria) to produce acetyl CoA & molecules of NADH & FADH2 Acetyl CoA will then enter the citric acid cycle (in the mitochondria) ATP, NADH, FADH2 & CO2 NADH & FADH2 will enter the electron transport chain.  synthesis of ATP

RESTING MUSCLE FIBERS METABOLISM Figure 10–20a

ATP sources in working muscle At the beginning of exercise, muscle fibers immediately use stored ATP 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 cont. After the phosphagen system is depleted, the process of anaerobic glycolysis can maintain ATP supply for about 45-60s Glycogen stored in muscles  Glucose  2 pyruvic acid + 2 ATPs 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 O2 for aerobic metabolism.

WORKING MUSCLE FIBERS METABOLISM Figure 10–20c

ATP sources in working muscle cont. 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 cont. Aerobic metabolism occurs when the respiratory & cardiovascular systems have “caught up with” the working muscles. 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 (made from glucose via glycolysis) & amino acids.

WORKING MUSCLE FIBERS METABOLISM

Aerobic Vs. Anaerobic sources of energy No requires oxygen Source of energy: ONLY Carbohydrate (anaerobic glycolysis) End Products: Lactate & ATP Aerobic Require oxygen Source of energy: mainly fatty acids, then carbohydrate End Products: CO2, H2O & ATP

The Cori cycle & The glucose-alanine cycle

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