T2 L7: Energy metabolism during exercise

• How is muscle metabolism at rest like?

  glycogen stores maintained/replenished

  oxidative metabolism of fatty acids provides energy

  little lactate produced as pyruvate used for glycogen synthesis / used in TCA cycle

  due to sufficient oxygen in muscle
• How is muscle metabolism at onset of exercise like?

  glycogenolysis for fuel

  increase oxygen consumption for OXPHOS

  increase blood flow to muscle due to local mediators (NO) & beta adrenergic stimulation of vascular smooth muscle
• What is the amount of energy derived from glycolysis and OXPHOS dependent on?

  intensity and duration of exercise
• What is glycogen mobilization in muscle controlled by?

  • Ca2+ levels in muscle cell cytoplasm
  • AMP
  • Adrenaline
• What are the roles of Ca2+ in muscle contraction?

  signal for muscle contraction

  activates glycogen phosphorylase (increase glycogen breakdown)

  stimulate NO production (vasodilation - increased blood flow)
• What are the characteristics of skeletal muscle blood flow?

  coordinated, rhytmical contractions enhance blood flow via skeletal muscle pump mechanism

  blood flow strongly determined by local regulatory factors eg tissue hypoxia, adenosine, NO, CO2, H+

  adrenaline -> vascular beta2 adrenoreceptors -> vasodilation
• How is the hormonal control of metabolism during exercise achieved?

  • increased adrenaline - promotes glycogenolysis & lipid mobilisation
  • increased glucagon & decreased insulin - promotes glycogenolysis & gluconeogenesis
• Why is decreased levels of insulin important for the homonal control of metabolism during exercise?

  insulin inhibits gluconeogenesis
• How is glucose still able to be transported into muscle cells even though insulin levels are low during exercise?

  muscle contraction activates GLUT4 even in absence of insulin
• Anaerobic energy pathways have a much higher power (rate of ATP production) but smaller capacity (total ATP produced) than aerobic pathways.
• Carbohydrate oxidation has a higher power output but a lower capacity than fat oxidation.
• How does phosphocreatine metabolism work?

  phosphocreatine + ADP -> ATP + creatine

  reaction reversible by creatine kinase
• What is the difference between phosphate-related enzymes?

  phosphorylase: adds phosphate

  phosphatase: takes away phosphate

  kinase: takes phosphate from ATP and adds it to something else
• What is phosphocreatine?

  first top-up source of muscle ATP

  during vigorous contraction lasts approx 16 secs
• What happens during anaerobic glycolysis?

  much less ATP produced

  lactate builds up
• What is the Cori cycle?

  glycogen in muscle broken down to G6P

  converted to pyruvate

  anaerobic - produces lactate

  lactate in bloodstream delivered to liver

  in liver lactate converted to G6P via gluconeogenesis. Either:

  • into bloodstream and transported back to muscle
  • glycogen synthesis
• What is the mechanism of fatigue?

  rate of ATP utilization exceeds rate of synthesis

  accumulation of pyruvate and lactic acid -> decrease muscle pH -> H+ inhibits glycolysis -> decline in force generated in muscle
• How is the fuel continuum during exercise?

  short distance: PC system (anaerobic)

  Mid distance: Anaerobic glycolysis

  Long distance: aerobic metabolism
• How is muscle metabolism like during a sprint?

  fuel: PC & anaerobic glycolysis

  • cathecolamines -> stimulate glycogen breakdown -> lactate production -> lactic acid -> liver -> gluconeogenesis
  • phosphocreatine + ADP -> creatine + ATP
• How is muscle metabolism like for a middle distance run?

  as distance increases aerobic oxidation of glycogen increases

  some of the oxygen required may come from oxymyoglobin in muscle

  Lactate still major end product

  contribution of PC becomes less and less as distance increases
• How is muscle metabolism like during long periods of intense exercies (30 min - 2 hours)?

  ATP generated via OXPHOS

  increased reliance on fatty acid oxidation

  lactate, glycerol and muscle amino acids used to support gluconeogenesis
• How is muscle metabolism like during the finishing stages of the marathon?

  after 2 hrs about 90% of liver glycogen used

  insulin levels remain very low and glucagon levels elevated

  ketone bodies produced by liver for ATP
• What is 'hitting the wall'?

  glycogen stores largely depleted

  body switches to fatty acids as main source of energy

  pace decreases as fatty acids generate 50% of max power output
• What is a potential risk during the final stages of a marathon?

  hypoglycaemia

  typical symtpoms: confusion, lack of cognitive function, lactic acidosis, exhaustion

  less likely with training
• Oxidation of carbohydrate or fat in endurance exercise?

  exercising muscle has absolute requirement for some glucose

  so glucose always metabolised in the background

  this requirement increases with exercise intensity