Energy Metabolism during Exercise Flashcards
Energy Metabolism for muscle contraction
ATP is the direct fuel for contraction, supplying the ATPase activity of myosin
ATP utilisation increases more than 100-fold in milliseconds
At rest, muscle has approx. 5 mmol of ATP per kg of wet weight
During vigorous contraction, this lasts <2 seconds
In the short term, muscle can increase its rate of production of ATP 20-100 fold
Initially the main fuel 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
Muscle Metabolism at rate (post absorptive state)
In resting muscle, glycogen stores are maintained/replenished
Oxidative metabolism of fatty acids provides energy for the muscle
When resting, little lactate produced as pyruvate is used for glycogen synthesis or used in TCA cycle as sufficient O2 present in muscle
ATP not generated via PC system at rest
Muscle metabolism - onset of exercise
Glycogenolysis provides fuel source
Increase in O2 consumption for ox phos
Increased blood flow to muscles due to local mediators (NO) and beta-adrenergic stimulation of vascular smooth muscles
Amount of energy derived from glycolysis and ox phos is dependent on the intensity and duration of the exercise
Role of Ca2+ in muscle contraction
Glycogen mobilisation in the muscle is controlled by Ca2+ levels in cytoplasm of muscle cells, by levels of AMP and by adrenaline
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 NO which causes vasodilation of blood vessels and increased blood flow
Characteristics of skeletal muscle blood flow
Skeletal muscle accounts for about 20% of cardiac output at rest; but can increase to more than 80% during extreme physical exertion
Coordinated, rhythmical contractions (e.g. 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 nitric oxide
Vascular beta-2 adrenoreceptors result in vasodilation when stimulated by agonists such as adrenaline
Hormonal control of metabolism during exercise
Increased adrenaline – promotes glycogen and lipid mobilisation
Decreased insulin and increased glucagon – promotes glycogenesis and gluconeogenesis
Decreased insulin also important because insulin inhibits gluconeogenesis
(No need for insulin for muscle glucose uptake – muscle contraction activates Glut 4 even in the absence of insulin)
3 systems for forming ATP in muscle
Anaerobic: ATP-PC, using phosphocreatine, fastest, but few ATPs produced; Lactic acid, using glycogen, fast, but few ATPs produced
Aerobic: oxygen system, glycogen, fats and proteins used, O2 required, slow and many/unlimited ATPs produced
Anaerobic energy pathways have higher power (rate) but smaller capacity (total ATP)
With aerobic metabolism, carb oxidation has a higher power output but lower capacity than fat oxidation
Phosphocreatine (or creatine phosphate)
Extra source of energy in muscle
First top-up source for muscle ATP
At rest, muscle has about 100mmol creatine phosphate per kg dry weight. During vigorous contraction, this lasts approx. 16 seconds - may be enough for a 100-200m sprint
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 the oxygen supply is sufficient, pyruvate may be formed faster than it can be oxidised. This also causes the accumulation of lactate
Lactate metabolism via 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
Fatigue
Inability to maintain desired power output
Occurs when rate of ATP utilisation exceeds its rate of synthesis
Accumulation of pyruvate and lactic acid in the contracting muscle result in a decline in force generated
Due to decrease in muscle pH
Glycolysis inhibited by H+ from lactic acid
ATP synthesis - effect of intensity and duration
Only 2 fuels are used in short sprints to replenish ATP - phosphocreatine and anaerobic glycogen breakdown to lactate
As distance increases, phosphocreatine levels are exhausted and muscle relies solely on glycogen breakdown either anaerobically to lactate or aerobically to CO2 via TCA cycle
During a marathon, muscles are reliant on oxidative metabolism of glycogen and also glucose from the liver and fatty acids from the adipose tissue
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 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 proceeds which the liver can use to maintain blood glucose levels via gluconeogenesis
Muscle metabolism during a middle distance run
Aerobic oxidation of glycogen only makes up 30% of the ATP required; some oxygen may come from oxymyoglobin in the muscle
Lactate is still a major end product of glycogen metabolism contributing 65% of ATP required
Contribution of phosphocreatine becomes less and less - at 800m it contributes 5% of ATP, and 0% at over 1500 m
Marathon
First 10 mins: muscle glycogen and glucose from liver used; increased vasodilation = increased O2 supply (more aerobic glycogen utilisation); glycogen breakdown stimulated by AMP and adrenaline release; Fatty acids mobilised by release of adrenaline (allowing liver to maintain blood glucose levels)
30mins-2hours: ATP generated via oxidation of glucose and fatty acids (more fatty acid oxidation); lactate, glycerol and muscle amino acids used to support glucose production by the liver, energy being derived from fatty acid oxidation
2+ hours: 90% of liver glycogen used; insulin levels low and glucagon high; ketone bodies produced by liver and may be used by muscle to generate ATP (in addition to fatty acids)
