17-18. Exercise Metabolism Flashcards
What factors effect the metabolic response to exercise? Which are most important?
- Intensity**
- Duration**
- Fitness
- Exercise modality
- Environmental factors
- Nutritional factors (diet)
What are the 3 main objectives of substrate metabolism?
- Maintain glucose homeostasis
- Metabolize the most effective substrate based on ATP demand
- Spare muscle glycogen
What hormones are responsible for regulating glucose at rest? Which one predominates after 12 hrs of fasting? Following a meal?
- Insulin and glucagon
- Fasting = glucagon
- Following a meal = insulin
What are the factors acting to decrease blood glucose?
- Insulin (decreases w/ exercise)
- Contraction stimulates increased muscle uptake of blood glucose during exercise in active muscle
What are the factors acting to increase blood glucose?
- Epinephrine/Norepinephrine = increases insulin levels; decreases glucagon levels
- Glucagon = activates gluconeogenesis (synthesis of glucose from liver)
- Cortisol
- Growth hormone
What accounts for the increase in glucose uptake by skeletal muscle w/ exercise?
Contraction (Ca2+) stimulates increased muscle uptake of blood glucose during exercise in active muscle
What 2 factors does hormone response during exercise depend on?
Intensity & duration of exercise
What organ is responsible for maintaining blood glucose levels?
Liver
What is the source of blood glucose?
Glycogen
What can one do during exercise to assist in maintaining blood glucose levels?
Use fat metabolism instead of CHO metabolism
-fatty acid conc increases –> muscle will start preferentially utilizing fatty acids instead of glucose
What does the choice of energy source depend on?
Intensity & duration of exercise
What are the primary energy sources used during high-intensity, short-duration exercise?
- CP
- Phosphagens
- Glycogen
As exercise intensity increases, the concentration of catecholamines in the blood increases. How does this influence glycogen breakdown and utilization?
Catecholamines function to spare glucose utilization
How does the choice of energy source change as intensity & duration of exercise change?
Low to high intensity; High to low duration
- LIpids
- CHO, aerobic
- CHO, anaerobic
- CP
Whenever exercise intensity increases suddenly, what is the immediate response?
To maintain ATP levels through the rapid and full activation of CK and rapid depletion of CP
What is the role of membrane transporters?
Help regulate glucose uptake
Name 2 transporters that are recruited to the plasma membrane of the fiber during exercise.
- GLUT 4
- CPT
What accounts for the increase in FFA conc in the blood following the onset of exercise?
- Selective recruitment of oxidative fibers
- Hormonal & non hormonal factors increase lipolysis, FFA release & uptake by the muscle
- Activation of enzymes that favor oxidation of FFA over oxidation of pyruvate
During low intensity exercise, why are FFAs the predominant energy source for oxidation rather than CHOs?
Glucose utilization is inhibited when FAs are available as alternate fuel
Why is the availability of plasma FFA reduced w/ increase in exercise intensity?
- Intensity increases –> start to restrict blood flow to gut & adipose sites which release FFA
- Blood vessels constrict –> release of FFA into blood decreases
Why do FG fibers produce a larger amount of lactate than FOG fibers when recruited?
- FOG fiber oxidize substrate –> do aerobic glycolysis
- FG fibers don’t have oxidative capacity –> do anaerobic glycolysis to produce lactate
As exercise intensity increases, there is a crossover in fuel utilization from primarily fat to CHOs. What is responsible for the increase in glycogen utilization w/ increases in exercise intensity?
- Increase demand for ATP
- Increase rate of ATP utilization
- Increase epinephrine release & increase levels of inorganic phosphate, ADP, AMP in muscle –> lead to increase phosphorylase and PFK activity, increase glycogen breakdown, increase glycolytic flux
- Increase recruitment of fibers that rely primarily on CHO (FG fibers)
- Decrease in fat oxidation
How does diet influence fuel use & RQ during prolonged submaximal steady state exercise? (high carb vs. normal vs. high fat diet)
HIGH CARB: -high RQ to start --> drops drastically w/ increased duration of exercise -Fuel = predominantly fat MIXED DIET: -RQ relatively constant -Fuel = predominantly fat HIGH FAT: -RQ starts low, but stays relatively constant -Fuel = predominantly fat
Under what circumstances is there a crossover in fuel use from mostly CHO to mostly fat?
- Exercise is prolonged (>2-3 hrs)
- Exercise is submaximal & steady state
- RQ is close to 1.0 at start of exercise –> CHO is predominant substrate initially
What can cause an increase in fat metabolism?
- Increase in adipose tissue lipolysis, FA release from AT, FFA delivery to muscle
- Increase in FFA movement across muscle membrane
- Increase in movement of FAs across mitochondrial membrane due to activation of CPT
- Activation of Krebs cycle and ETC in mitochondria
What does enhanced delivery of glucose to contracting muscle depend on?
- Increased blood flow and capillary recruitment
- Increased transport across plasma membrane
- Increase activation of glycolytic and oxidative pathways responsible for glucose metabolism
What are the precursors for liver gluconeogenesis? Where do they come from?
- Lactate from anaerobic glycolysis
- Amino acids from muscle
What 2 cycles allow gluconeogenesis precursors to return to the liver?
- Cori (lactate cycle)
- Glucose-alanine (amino acid) cygle
The capacity to oxidize pyruvate depends on:
- Aerobic capacity = # of mitochondria
- availability of oxygen
- total activity of LDH
How does an acceleration in rate of glycolysis lead to lactate production?
Rate of ATP demand increases –> rate of glycolysis increases –> chooses anaerobic glycolysis (lactate production) to keep up with ATP demands
How does the recruitment of FG fibers lead to lactate production?
FG don’t have oxidative capacity –> do anaerobic glycolysis –> produce lactate
What does the Randle Theory state?
Inhibition of glucose utilization when FAs are available as an alternate fuel
What are the mechanisms by which glycolysis is inhibited?
- Suppression of PDH by increases in acetyl CoA & NADH
- Inhibition of PFK by citrate leading to G6P which suppresses hexokinase & phosphorylase
- Reduced allosteric activation of phosphorylase due to decreased levels of AMP, ADP, and Pi
What does absolute work rate or energy flux determine?
Total quantity of fuel required by the working muscle
What does relative work rate dictate?
Proportion of CHO and fat based fuels for oxidation phosphorylation
How does fuel selection & utilization correlate to exercise intensity (VO2max)?
80% VO2max = energy from CHO exceeds fat
At higher work intensities, muscle glycogen can provide more than half the total energy used by the muscle. How?
- Increase in SNS drive
- Greater abundance of glycolytic as opposed to lipolytic enzymes in skeletal muscle
- Increase recruitment of FG fibers that rely on glycogen
Plasma concentration is maintained through what 4 processes?
- Mobilize glucose from liver glycogen stores
- Mobilize plasma FFA from adipose tissue to spare plasma glucose
- Synthesize new glucose in the liver (gluconeogenesis) from amino acids, lactate, and glycerol
- Block glucose entry into cells to force the substitution of FFA as a fuel
