SKELETAL MUSCLE: METABOLISM Flashcards
Describe the overall process of energy usage and the available sources of fuel used during exercise
- See flowchart from lecture
- Sources of fuel: Diet - food entering blood in absorption phase (2-3 hrs after a meal); release of stored fuel during postabsorptive stage(especially during sleep or during a fast or prolonged exercise)
- Fuel is ingested and broken down to glucose, free fatty acids, amino acids, and ketone bodies. Fuels are oxidized. 60% is converted to heat and 40% isconverted into ATP. Note that ATP is at a constant universal amount and not stored. ATP is used for energy for structural integrity, differentiated function, growth and division, and in response to stimuli and stress.
- Result: 60-70% used for resting metabolic rate; 25-30% used for movement
Describe the different energy sources during exercise and their general timeline
- ATP + creatine phosphate (0-2 minutes)
- Anaerobic glycolysis (0-5 minutes): muscle glycogen
- Aerobic oxidation (3 minutes - 4+ hours): plasmic free fatty acids and adipose tissue triglycerides
- Aerobic oxidation (3 minutes - 4+ hours): muscle glycogen, plasma glucose, liver glyxogen
Skeletal muscle fibres have enough stored ATP for about ___ of contraction
3 seconds
Why is skeletal muscle fibre energy storage not very effective?
not very energy dense, is an ineffective energy store for longer duration exercise
- remember, only stores enough ATP for 3s of contraction
What is the role of creatine phosphate (PCr) in muscle fibres?
can be quickly broken down to make more ATP
Can supply energy needs of contraction for ~10s but is regenerated
How does the liver play a role in blood glucose regulation?
- Liver glucose contributes to blood glucose levels when needed
- Gycogen synthases adds glucose to glycogen. The liver stores ~100g of glycogen
- Glycogen phosphorylase removes glucose molecules from glycogen.
The liver expresses Glucose-6-phosphatase. Thus, glucose from the liver glycogen can directly contribute to blood glucose levels.
What is the role of muscle glycogen?
It is mobilized (converted to glucose) for energy use during exercise. Muscle glycogen can be broken down into glucose monomers via glycogenolysis. Requires glycogen phosphorylase
- muscle can store ~400g of glycogen.
Describe glycolysis
Glycolysis is an important metabolic pathway
- ~12 chemical reactions occur. Note this is not as quick as creatine phosphate is in ATP generation
- Provides enough energy for ~90s of contraction
- Anaerobic (does not require O2)
- Lactic acidic is a by-product which causes fatigue and potentially muscle soreness?
Describe aerobic respiration
- slowest route to regenerate ATP
- supply ATP for several hours (if fuel lasts)
occurs in mitochondria
Q: Why are there differences in oxidative and glycolytic metabolism capaicties across muscle fibre types?
Different MHC isoforms lead to differences in ATPase activity and kinetics
Describe the difference across muscle fibres in terms of
- oxidative capacity, mitochondrial content, capillary density, myoglobin
- glycolytic capacity
Type I slow twitch muscle fibers:
- High oxidative capacity, mitochondrial content, capillary density, myoglobin
- Moderate glycolytic capacity
Type IIb
- Low oxidative capacity, mitochondrial content, capillary density, myoglobin
- High glycolytic capacity
Type IIa
- Very high oxidative capacity, mitochondrial content, capillary density, myoglobin
- High glycolytic capacity
Describe the process of generating ATP from free fatty acids
- Free fatty acids released from adipose tissue by lipolysis circulate in blood bound to albumin
- Free fatty acids transport proteins move free fatty acids into muscle cells
- Free fatty acids bypass glycolysis and enter the Kreb’s cycle through conversion to various intermediates (e.g. acetyl CoA)
- Free fatty acids require carnitine palmityltransferases (CPTs) to cross the mitochondrial membrane
- Each cycle of Beta-oxidation removes two carbons and generates one acetyl CoA
- Free fatty acids products participate in oxidative phosphorylation (Kreb’s, ETC)
- Also produces FADH2 (1) and NADH (1)
- Yields up to 17 molecules of ATP making it a more efficient source of energy compared to carbohydrates/amino acids
9kcal/g (lipids) vs 4kcal/gram (carbs/protein)
Describe the process of generating ATP from amino acids
- Amino acids are the last resort as a source of fuel. The body will maintain proteins as long as possible
- Amino acids can be oxidized after transamination (transfer of amino group to another molecule)
- The amino group must be removed because it can give rise to ammonia, which is highly toxic
- Bypasses glycolysis: carbon skeletons of amino acids enter Kreb’s cycle via conversion to various intermediates (e.g. pyruvate, acetyl CoA)
- The use of amino acids for energy is coupled to the urea cycle in the liver (converts ammonia to urea, we won’t discuss the details here)
Describe energy sources at rest
- Circulating fatty acids are primary energy source
- O2 is abundant, and aerobic metabolism is used
- Glucose taken in is stored as glycogen
- Phosphocreatine reserves are built up
Describe energy sources at intense activity
- Engaged in your favorite high-intensity sport
- Muscles lack O2 to support mitochondria
- Muscles rely on glycolysis for ATP
- Pyruvic acid build up, is converted to lactic acid
