energy metabolism during exercise Flashcards
what is the fuel for contraction?
ATP
how much does ATP utilisation increase during muscle contraction?
100-fold in milliseconds
how much ATP does the muscle have at rest?
5mmol per kg of wet weight
how much does ATP production increase in the short term (muscle)?
20-100 fold
what is the initial fuel for muscle contraction?
glycogen
in the post-absorptive state what provides energy for the muscle?
oxidative metabolism of fatty acids
what are the 3 stages of muscle metabolism at the onset of exercise?
1) glycogenolysis provides fuel source
2) increase in oxygen consumption of oxidative phosphorylation
3) increased blood flow to muscles due to local mediators (NO) and beta adrengeric stimulation of vascular smooth muscles
what is the amount of energy derived from glycolysis and oxidative phosphorylation dependent on?
the intensity and duration of exercise
what is phosphorylase activated by?
- activated by AMP which acts allosterically
- activated by phosphorylation in response to stress hormones increased cytoplasmic calcium
what is glycogen synthase activated by?
activated allosterically by glucose-6-phosphate which is low during exercise
what is glycogen synthase inactivated by?
inactivated by phosphorylation in response to stress hormones, increased cytoplasmic calcium
what is phosphofructokinase-1 inhibited by?
inhibited allosterically by ATP
what is phosphofructokinase-1 activated by?
activated by AMP and Fru-2,6-P2
what controls glycogen mobilisation in the muscle?
- calcium levels in the cytoplasm of muscle cells
- levels of AMP
- adrenaline
what is the role of calcium in muscle contraction?
- increase in calcium 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
what are the characteristics of skeletal muscle blood flow?
- skeletal muscle accounts for about 20% of cardiac output at rest; can increase to more than 80% during extreme physical exertion
- coordinated, rhythmical contractions enhance blood flow by means of the skeletal muscle pump mechanism
- blood flow is strongly determined by regulatory factors such as tissue hypoxia, adenosine, K+, CO2, H+ and nitric oxide
- vascular beta 2 adrenoreceptors result in vasodilation when stimulated by agonists such as adrenaline
what happens to the levels of adrenaline during physical work?
increases
what does adrenaline do during physical work?
promotes glycogen and lipid mobilisation
what happens to the levels of insulin and glucagon during physical work?
- decreases insulin
- increases glucagon
what do insulin and glucagon do during physical work?
- promotes glycogenolysis and gluconeogenesis
- insulin inhibits gluconeogenesis
what does muscle contraction activate?
glut 4 even in the absence of insulin
how does anaerobic energy pathways compare to aerobic pathways with muscle contraction?
anaerobic energy pathways have a much higher power (rate of ATP production) but smaller capacity (total ATP produced) that aerobic pathways
what happens with aerobic metabolism for muscle contraction?
carbohydrate oxidation has a high power output but a lower capacity that fat oxidation
what does phosphocreatine (creatine phosphate) do?
it is an extra source of energy in muscle cells
what are the key characteristics of phosphocreatine (creatine phosphate)?
- first top up source for muscle ATP
- at rest, muscle has about 100 mol creatine phosphate perks dry weight
- during vigorous contraction this last approximately 16 seconds
- may be enough for 100-200m sprint
what happens with anaerobic glycolysis?
- 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 up
- even when oxygen supply is sufficient, pyruvate may be formed faster than it can be oxidised which also causes the accumulation of lactate
what happens with lactate metabolism during the cori cycle?
- 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
what happens with aerobic metabolism during exercise?
- in addition to glycogen, muscle also uses fatty acids from the adipose tissue to maintain ATP levels
- however, aerobic metabolism of glucose and fatty acid oxidation are dependent of an adequate oxygen supply to the muscle
- oxygen is necessary for any ATP production via fatty acid oxidation
what is fatigue?
the inability to maintain desired power output
when does fatigue occur?
when the rate of ATP utilisation exceeds its rate of synthesis
what happens with fatigue?
- accumulation of pyruvate and lactic acid in the contracting muscle result in a decline in force generated due to a decrease in muscle pH
- glycolysis inhibited by H+ from lactic acid
what are the 2 fuels used in short sprints to replenish ATP?
- phosphocreatine
- anaerobic glycogen breakdown to lactate
what happens to the fuels when 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
what happens to the fuels during a marathon?
the muscles are reliant on oxidative metabolism of glycogen and also glucose from the liver and fatty acids from the adipose tissue
what happens to muscle metabolism during a sprint?
Fuel: PC and anaerobic glycolysis
- catecholamines stimulate glycogen breakdown in muscle which is converted anaerobically to lactate
- phosphocreatine is converted to creatine within 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 proceeds which the liver can use to maintain blood glucose levels via gluconeogenesis
what happens to muscle metabolism for a middle distance run?
- as the distance increases aerobic oxidation of glycogen makes up to 30% of the ATP required to support contraction
- some of the oxygen required may come from 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 to 5% and essentially zero over 1500m
what happens to muscle metabolism during the first 10 minutes of a marathon?
- muscle glycogen and glucose from the liver are used to power muscles mainly through glycolysis
- increased vasodilation in the muscles increases oxygen supply increasing the aerobic glycogen utilisation and ATP production
- glycogen breakdown stimulated by increased AMP and adrenaline release
- fatty acids are mobilised by the release of adrenaline to allow the liver to maintain blood glucose levels by the provision of energy and glycerol backbone
what happens to muscle metabolism during 30 mins - 2 hours of a marathon?
- ATP generation via oxidation and fatty acids
- increased reliance on fatty acid oxidation over this longer timespan of relatively intense exercise
- lactate, glycerol and muscle amino acids used to support glucose production by the liver, energy vein derived from fatty acid oxidation
what happens with the oxidation of carbohydrate or fat in endurance exercise?
- fats alone can supply most of the needs for resting muscle, but exercising muscle have an absolute require for some glucose
- fatty acid breakdown during extended periods of exercise proceeds on a continual background levels of glucose metabolism
- this requirement for a background level of glucose metabolism increases with exercise intensity
what happens with muscle metabolism in the finishing stages of a marathon?
- about 90% of liver glycogen used
- insulin levels remain very low and glucagon levels elevated
- ketone bodies produced by the liver and may be used by muscles to generate atm
what does ‘hitting the wall’ mean?
exercise intensity drops because of a lack of available ATP
what happens with muscle metabolism in the hitting the wall stage of a marathon?
- a marathon requires about 700g of glycogen whereas muscle and liver normally contain only 500g therefore glycogen stores are largely depleted after 20 miles
- body 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 depleted
why does hypoglycaemia often occur in the final stages of a marathon?
with prolonged high intensity exercise, liver glucose output may fall below muscle glucose uptake resulting in hypoglycaemia
what are the typical symptoms of hypoglycaemia?
- confusion
- lack of cognitive function
- lactic acidosis
- exhaustion
