Metabolic regulation during aerobic exercise

In Section 8.4.1, anaerobic and aerobic exercise were described as the two ex-

Stimulation time (sec)

Fig. 8.7 Concentrations of ATP and of phosphocreatine (PCr) in Type II fibres in human muscle during contractions brought about by electrical stimulation. After 6 contractions (each 1.6 sec long; i.e. at ~10 sec) and after 12 contractions (~20 sec) a muscle biopsy was taken and rapidly frozen, and later the Type I and Type II fibres were separated for analysis. With repeated contractions, the force generated decreases slightly, the PCr concentration falls sharply, but the concentration of ATP remains almost constant. The implication is that ATP is being rapidly resynthesised at the expense of PCr. Adapted from Soderlund, K., Greenhaff, P.L. & Hultman, E. (1992) Energy metabolism in type I and type II human muscle fibres during short term electrical stimulation at different frequencies. Acta Physiol Scand 144: 15-22. With permission of the Scandinavian Physiological Society. Following Maughan et al. (1997).

Stimulation time (sec)

Fig. 8.7 Concentrations of ATP and of phosphocreatine (PCr) in Type II fibres in human muscle during contractions brought about by electrical stimulation. After 6 contractions (each 1.6 sec long; i.e. at ~10 sec) and after 12 contractions (~20 sec) a muscle biopsy was taken and rapidly frozen, and later the Type I and Type II fibres were separated for analysis. With repeated contractions, the force generated decreases slightly, the PCr concentration falls sharply, but the concentration of ATP remains almost constant. The implication is that ATP is being rapidly resynthesised at the expense of PCr. Adapted from Soderlund, K., Greenhaff, P.L. & Hultman, E. (1992) Energy metabolism in type I and type II human muscle fibres during short term electrical stimulation at different frequencies. Acta Physiol Scand 144: 15-22. With permission of the Scandinavian Physiological Society. Following Maughan et al. (1997).

treme forms of exercise. Many forms of exercise consist of a combination of the two. Games such as tennis and soccer require moments of intense power output (serving, kicking), accompanied by endurance performance (running about the court or pitch for 90 minutes or more). In running events, the 100 m sprint is virtually completely anaerobic: it is said that the elite sprinter has no need to draw breath during it. (Most of us would doubtless need several breaths!) The 400 m run is a combination of both anaerobic and aerobic exercise, and with increasing distance, the aerobic component becomes more dominant. The marathon run (42.2 km, 26.2 miles) is often taken as an example of almost pure aerobic exercise.

The characteristic of aerobic exercise is that it can be sustained for long periods. Of necessity, this means that stored fuels other than those in the muscles must be used, and must be completely oxidised so that partial breakdown products such as lactic acid do not build up. Complete oxidation of substrates also gives a much higher energy yield than partial breakdown: for instance, complete oxidation of 1 molecule of glucose gives rise to 31 molecules of ATP,2 whereas anaerobic glycolysis to 2 molecules of lactate generates 3 molecules of ATP. Not surprisingly, then, the muscle fibres most involved in aerobic exercise are the more oxidative, slow-twitch Type I fibres (see Section 4.3.2). In order for these muscles to produce external work at a high rate over a long period, they must be supplied with substrates (including O2), and the products of me-

Metabolic Changes During Exercise

Fig. 8.8 Coordinated regulation of glycogenolysis and contraction by Ca2+ ions in skeletal muscle. Elevation of the concentration of Ca2+ ions in the sarcoplasm occurs in response to the arrival of a nerve impulse (see Box 8.3), and is responsible for the initiation of contraction. As discussed in Box 2.4, Fig. 2.4.2, Ca2+ ions can also activate phosphorylase kinase (independently of its phosphorylation state). Therefore glycogen breakdown is initiated by arrival of the nerve impulse. The figure also shows the activation of phosphorylase by adrenaline (as in liver). It has been suggested that the anticipation of exercise may 'prime' the system by an increase in adrenaline. Glycogen cannot be broken down until there is an increase in the concentration of inorganic phosphate (P), which is released as soon as contraction begins. The regulation of glycogen breakdown by Ca2+ ions is not confined to muscle; it can also occur in liver, and accounts for activation of glycogenolysis by catecholamines acting via a1 adrenoceptors. However, the physiological significance is not known.

Fig. 8.8 Coordinated regulation of glycogenolysis and contraction by Ca2+ ions in skeletal muscle. Elevation of the concentration of Ca2+ ions in the sarcoplasm occurs in response to the arrival of a nerve impulse (see Box 8.3), and is responsible for the initiation of contraction. As discussed in Box 2.4, Fig. 2.4.2, Ca2+ ions can also activate phosphorylase kinase (independently of its phosphorylation state). Therefore glycogen breakdown is initiated by arrival of the nerve impulse. The figure also shows the activation of phosphorylase by adrenaline (as in liver). It has been suggested that the anticipation of exercise may 'prime' the system by an increase in adrenaline. Glycogen cannot be broken down until there is an increase in the concentration of inorganic phosphate (P), which is released as soon as contraction begins. The regulation of glycogen breakdown by Ca2+ ions is not confined to muscle; it can also occur in liver, and accounts for activation of glycogenolysis by catecholamines acting via a1 adrenoceptors. However, the physiological significance is not known.

tabolism such as CO2 must be removed, at a sufficiently high rate. This necessitates coordinated changes in the circulatory system.

It is illuminating to consider how much ATP is needed for endurance exercise - for instance, running a marathon. An approximate calculation is given in Box 8.5. A marathon runner will use almost his or her own body weight of ATP. Clearly this ATP was not all stored at the beginning of exercise! In fact the total amount of ATP is probably almost the same at the end of the race as at the beginning (see from Fig. 8.7 how the ATP content of muscle is maintained even during intense, anaerobic contractions). In other words, the ATP pool must be continuously resynthesised, to the extent that about 60 -70 kg of ATP are synthesised during the race. The question now becomes: what are the metabolic fuels used for ATP resynthesis during aerobic exercise?

The major fuels used in aerobic exercise vary with the intensity of the exercise and with the duration. In relatively light exercise most of the energy required comes from non-esterified fatty acids delivered from adipose tissue.

Box 8.4 Activation of the pathway of glycolysis at the start of anaerobic exercise

At the start of intense anaerobic exercise, the net flux through the glycolytic pathway may increase about 1000-fold. In the text and Fig. 8.8 the link between contraction and glycogen breakdown is explained. Nevertheless, the enzymes of the pathway itself must be activated in order to allow this increase in flux. Regulation of the enzyme phosphofructokinase (PFK) is best understood and will be discussed here as an illustration.

Allosteric regulation

Regulation by fructose 2,6-bisphosphate, important in the liver, is probably not a major factor in exercising muscle. However, a number of intermediates act as allosteric effectors of PFK. These include:

Activators Inhibitors

AMP ATP

Pi Citrate*

Fructose 1,6-bisphosphate Phosphocreatine*

Fructose 6-phosphate H+*

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