ATP of Skeletal Muscles

Energy of Skeletal Muscles Lecture -3

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 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.

ATP

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

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10 sec 30 sec 2 min 5+ 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)

4 5 - 10 sec

Short Term: Glycolytic(Glycogen-Lactic Acid)

2.5 1 - 2 min

Long Term: Oxidative1 Unlimited time

Energy requirements

The three energy systems often operate simultaneously during physical activity

HOWEVERRelative 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:

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

Muscle Fatigue

What are the mechanisms by which muscle fibers obtain energy to

power contractions?

Muscles and fiber types

White muscle : (glycolytic)mostly fast fiberspale (e.g. chicken breast)

Red muscle: (oxidative)mostly slow fibersdark (e.g. chicken legs)

Most human muscles are:

Mixed fiberspink

Fast Vs. Slow Fibers

Type I

Type II

• 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)

Slow fibers

• Large glycogen reserves• Relatively few mitochondria• Produce rapid & powerful contractions BUT of short duration• Easily fatigued• 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

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

Figure 10–20a

RESTING MUSCLE FIBERS METABOLISM

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.

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 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.

ATP sources in working muscle cont .

WORKING MUSCLE FIBERS METABOLISM

Aerobic Vs. Anaerobic sources of energy

Aerobic

Require oxygen

Source of energy :mainly fatty acids, then

carbohydrate

End Products:CO2, H2O & ATP

Anaerobic

No requires oxygen

Source of energy :ONLY Carbohydrate

(anaerobic glycolysis)

End Products:Lactate & 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)