Other hormonal responses during aerobic exercise

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The sympathetic nervous system and adrenaline also bring about the mobilisation of stored fuels (discussed in detail below). Other hormones respond to aerobic exercise, and are involved in the regulation of fuel availability. Both growth

Rest

Cardiac output, 5 litres/min

Fig. 8.11 Schematic drawing of the distribution of blood flow between various organs and tissues at rest (bottom) and during strenuous exercise (top) (distribution shown by the area of the black squares). Adipose tissue is not shown, but accounts for about 5-10% of cardiac output at rest, about 1% during exercise. Reproduced from Astrand, P.-O. & Rodahl, K. (1977) Textbook of Work Physiology, 2nd edn. With permission of McGraw-Hill Inc and the authors.

Rest

Cardiac output, 5 litres/min

Fig. 8.11 Schematic drawing of the distribution of blood flow between various organs and tissues at rest (bottom) and during strenuous exercise (top) (distribution shown by the area of the black squares). Adipose tissue is not shown, but accounts for about 5-10% of cardiac output at rest, about 1% during exercise. Reproduced from Astrand, P.-O. & Rodahl, K. (1977) Textbook of Work Physiology, 2nd edn. With permission of McGraw-Hill Inc and the authors.

hormone and Cortisol are secreted in response to exercise, rising in concentration in the plasma gradually over the first 30 minutes to 1 hour (Fig. 8.12) - i.e. these are relatively slow responses, and are likely to be involved particularly in the release of stored fuels during prolonged exercise. The plasma glucose concentration may rise or fall during exercise (discussed below), but the insulin concentration falls somewhat during endurance exercise (Fig. 8.13). This represents a-adrenergic inhibition of its secretion from the pancreas, brought about by the increased circulating adrenaline concentrations. Glucagon secretion may also increase, although this is not a major change except with very strenuous, prolonged exercise. The increase in adrenaline, glucagon, growth hormone and cortisol concentrations is a typical 'stress' response (see Fig. 7.6). Since the major effects of glucagon are on the liver, and liver metabolism may not be dominant during exercise because of restricted blood flow, there may be little role for glucagon in this situation.

Fig. 8.12 Plasma concentrations of growth hormone (top panel) and cortisol (lower panel) during aerobic exercise at about 60% of maximal aerobic power. The exercise, on a bicycle, began with a 'warm-up' (shown as 0-10 minutes) and then carried on for 60 minutes (until 70 min on the X-axis). Based on Hodgetts, V., Coppack, S.W., Frayn, K.N. & Hockaday, T.D.R. (1991) Factors controlling fat mobilization from human subcutaneous adipose tissue during exercise. JAppl Physiol 71: 445-451. With permission of the American Physiological Society.

Fig. 8.12 Plasma concentrations of growth hormone (top panel) and cortisol (lower panel) during aerobic exercise at about 60% of maximal aerobic power. The exercise, on a bicycle, began with a 'warm-up' (shown as 0-10 minutes) and then carried on for 60 minutes (until 70 min on the X-axis). Based on Hodgetts, V., Coppack, S.W., Frayn, K.N. & Hockaday, T.D.R. (1991) Factors controlling fat mobilization from human subcutaneous adipose tissue during exercise. JAppl Physiol 71: 445-451. With permission of the American Physiological Society.

Fig. 8.13 Plasma glucose (top panel) and insulin (lower panel) concentrations during aerobic exercise at about 60% of maximal aerobic power. The protocol was the same as in Fig. 8.12. Note how the plasma insulin concentration falls during exercise, despite a rise in the plasma glucose concentration. The plasma lactate concentration is also shown (middle panel); it increases at the beginning of exercise and then subsides. Based on Hodgetts, V., Coppack, S.W., Frayn, K.N. & Hockaday, T.D.R. (1991) Factors controlling fat mobilization from human subcutaneous adipose tissue during exercise. J Appl Physiol 71: 445-451. With permission of the American Physiological Society.

Fig. 8.13 Plasma glucose (top panel) and insulin (lower panel) concentrations during aerobic exercise at about 60% of maximal aerobic power. The protocol was the same as in Fig. 8.12. Note how the plasma insulin concentration falls during exercise, despite a rise in the plasma glucose concentration. The plasma lactate concentration is also shown (middle panel); it increases at the beginning of exercise and then subsides. Based on Hodgetts, V., Coppack, S.W., Frayn, K.N. & Hockaday, T.D.R. (1991) Factors controlling fat mobilization from human subcutaneous adipose tissue during exercise. J Appl Physiol 71: 445-451. With permission of the American Physiological Society.

Fig. 8.14 Coordination of metabolism by the nervous system during endurance exercise. Adrenaline secreted from the adrenal medulla may be responsible, or may reinforce the effects of the sympathetic nerves, for increased lipolysis and for suppression of insulin secretion.

The way in which the somatic and sympathetic nervous systems coordinate physiological and metabolic changes during endurance exercise is illustrated in Fig. 8.14.

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