energy metabolism during exercise

Question 1

describe energy metabolism for muscle contraction? 6

Answer 1

• ATP is the direct fuel for contraction, supplying the ATPase activity of myosin
• Therefore, ATP is needed to support muscle contraction, with ATP utilisation increasing more than 100-fold in ms
• At rest muscle has approximately 5mmol of ATP per kg wet weight
• During vigorous contraction, this last <2 seconds
• In short term, muscle can increase its rate of production of ATP by 20-100-fold
• Initially the main furl for this is glycogen stored within the muscle itself, however as time goes on, other tissues need to co-operate to provide fuel for energy production

Question 2

describe muscle metabolism is rest (post-absorptive state)? 3

Answer 2

• In resting muscle, glycogen stores are maintained/replenished
• Oxidative metabolism of fatty acids provides energy for the muscle
• There is little lactate produced as pyruvate is used for glycogen synthesis or used in the TCA cycle as there is sufficient oxygen present in the muscle

Question 3

describe muscle metabolism at the onset of exercise? 4

Answer 3

• Glycogenolysis provides the source of fuel
• Increase in oxygen consumption for oxidative phosphorylation
• Increased blood flow to muscles due to local mediators (NO) and beta-adrenergic stimulation of vascular smooth muscles
• The amount of energy derived from glycolysis and ox phos is dependent on the intensity and duration of exercise

Question 4

what is phosphorylase activated by? 2

Answer 4

• AMP, which acts allosterically

- phosphorylation in response to stress hormones, increased cytoplasmic CA2+

Question 5

what is glycogen mobilisation in the muscle controlled by? 3

Answer 5

• CA2+ levels in the cytoplasm of muscle cells, levels
• levels of AMP
• adrenaline

Question 6

what is glycogen synthase activated by?

inactivated by?

Answer 6

• allosterically by glucose-6-phosphate which is low during exercise
• by phosphorylation in response to stress hormones and increased cytoplasmic CA2+

Question 7

what inhibits phosphofructokinase-1?
what activates it?
what does this mean for this molecule?

Answer 7

• allosterically by ATP
• activated by AMP and Fry-2,6-P
• it is an important sensor of energy availability and needs during exercise

Question 8

describe the role of Ca2+ in muscle contraction? 3

Answer 8

• Increase in Ca2+ concentration is the signal for muscle contraction
• Also increases muscle glycogen breakdown by activating glycogen phosphorylase to supply the energy required
• Stimulates the production of nitric oxide which causes vasodilation of the blood vessels and increased blood flow

Question 9

what are the characteristics of skeletal muscle blood flow? 4

Answer 9

• Skeletal muscle accounts for 20% of cardiac output at rest, it can increase to more than 80% during extreme physical exertion
• Coordinated rhythmical contractions (running) enhance blood flow by means of the skeletal muscle pump mechanism
• Blood flow is strongly determined by local regulatory (tissue and endothelial) factors such as tissue hypoxia, adenosine, K+, CO2, H+ and NO
• Vascular beta2-adrenoceptors result in vasodilation when stimulated by agonists such as adrenaline

Question 10

describe the 3 systems for forming ATP in muscle?

• fuel
• O2 required?
• speed
• relative ATP production

Answer 10

• anaerobic ATP-PC
• phosphocreatine
• No
• fastest
• few; limited
• anaerobic lactic acid
• glycogen
• no
• fast
• few; limited
• aerobic oxygen system
• glycogen, fats, proteins
• yes
• slow
• many; unlimited

Question 11

what is phosphocreatine? 4

Answer 11

• An extra source of energy in muscle cells
• First top-up source for muscle ATP
• At rest, the muscle has about 100 mmol creatine phosphate per kg dry weight. During vigorous contraction this lasts approximately 16 seconds
• This may be long enough for a 100-200m sprint

Question 12

what is anaerobic glycolysis? 4

Answer 12

• Glycogen breakdown and glycolysis are greatly stimulated during contraction
• If the increased rate of metabolism outstrips the oxygen supply, glycolysis can proceed anaerobically
• Much less ATP is produced and lactate builds
• Even when the oxygen supply is sufficient, pyruvate may be formed faster than it can be oxidised. This also leads to an accumulation of lactate

Question 13

describe lactate metabolism via the cori cycle?

Answer 13

• Lactate is used by the liver to regenerate glucose which can be transferred back to the muscle for energy production. If there is insufficient blood flow through the muscle, lactic acid builds up in the muscle

Question 14

aerobic metabolism during exercise? 3

Answer 14

• In addition to glycogen, muscle also uses fatty acids from the adipose tissue to maintain ATP levels
• However, aerobic metabolism of glucose and fatty acids are dependent on an adequate oxygen supply to the muscle
• Oxygen is necessary for any ATP production via fatty acid oxidation

Question 15

what is fatigue? 5

Answer 15

• Inability to maintain the desired power output
• Occurs when the rate of ATP utilisation exceeds its rate of synthesis
• Accumulation of pyruvate and lactic acid in the contracting muscle results in a decline in force generated
• Due to a decrease in muscle pH
• Glycolysis is inhibited by H+ from lactic acid

Question 16

describe ATP synthesis: effect of intensity and duration of exercise? 3

Answer 16

• Only 2 fuels are used in short sprints to replenish ATP, phosphocreatine and anaerobic glycogen breakdown to lactate
• As the distance increases, phosphocreatine levels are exhausted, and the muscle relies solely on glycogen breakdown either anaerobically to lactate or aerobically to CO2 via the TCA cycle
• During the marathon, the muscles are reliant on oxidative metabolism of glycogen and also glucose from the liver and fatty acids from the adipose tissue

Question 17

describe muscle metabolism during a sprint? 5

Answer 17

• Fuel= phosphocreatine and anaerobic glycolysis
• Catecholamines stimulate glycogen breakdown in muscle which is converted anaerobically to lactate.
• Phosphocreatine is converted to creatine with the transfer of Pi to ADP to form ATP
• Blood vessels are compressed during sprinting isolating the cells from the blood supply, making the muscles reliant on anaerobic energy production from glycogen
• Large quantities of lactic acid produced as glycolysis precedes which the liver can use to maintain blood glucose levels via gluconeogenesis

Question 18

describe muscle metabolism for a middle-distance run? 3

Answer 18

• As the distance increases, aerobic oxidation of glycogen makes up 30% of the ATP required to support contraction. Some of the oxygen required may come from the oxymyoglobin in the muscle
• Lactate is still a major end product of glycogen metabolism, contributing 65% of the ATP required
• The contribution of phosphocreatine to the ATP required becomes less and less as the distance increases. At 800m it contributes 5% and essentially 0 over 1500m

Question 19

describe a marathon: the first 10 minutes? 4

Answer 19

• Muscle glycogen and glucose from the liver are used to power muscles, mainly through glycolysis
• Increased vasodilation in the muscles increases O2 supply increasing the aerobic glycogen utilisation and ATP production
• Glycogen breakdown stimulated by increased AMP and adrenalin release
• Fatty acids are mobilised by the release of adrenalin to allow liver to maintain blood glucose levels by the provision of energy and the glycerol backbone

Question 20

describe a marathon: 30 mins-2 hours? 3

Answer 20

• ATP generated via oxidation of glucose and fatty acids.
• Increased reliance on fatty acid oxidation over this longer timespan of relatively intense exercise
• Lactate, glycerol and muscle amino acids are used to support glucose production by the liver, energy being derived from fatty acid oxidation

Question 21

describe oxidation of carbohydrate or fat in endurance exercise? 3

Answer 21

• Fats alone can supply most of the needs for resting muscle, but exercising muscle have an absolute requirement for some glucose
• Fatty acids breakdown during extended periods of exercise proceeds on a continual background level of glucose metabolism
• This requirement for a background level of glucose metabolism increases with exercise intensity

Question 22

describe the finishing stages of a marathon? 2

Answer 22

• About 90% of liver glycogen has been used. Insulin levels remain very low and glucagon levels are elevated
• Ketone bodies are produced by the liver and may be used by the muscle to generate ATP (in addition to fatty acids)

Question 23

describe hitting the wall in a marathon? 4

Answer 23

• Hitting the wall means that exercise intensity drops because of a lack of available ATP
• A marathon requires 700g of glycogen, whereas muscle and liver normally only contain 500g, after 20 miles, glycogen stores are largely depleted
• Boy switches to fatty acids as main source of energy with little glucose metabolism
• Fatty acid oxidation only generates sufficient ATP for 50% of maximum power output and pace decreases as glycogen is depleted

Question 24

describe the potential for hypoglycaemia in the final stages of a marathon? 2

Answer 24

• With prolonged high-intensity exercise, liver glucose output may fall below muscle glucose uptake this resulting in hypoglycaemia
• Typical symptoms of confusion, lack of cognitive function, lactic acidosis and exhaustion may occur (less likely with training)