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Endocrine System

Endocrine System

Growth Hormone

Growth hormone (GH) or somatotropin is produced by the anterior pituitary. GH promotes cell division, protein synthesis, and cellular proliferation by increasing amino acid transport through cell membranes and increasing RNA production. GH also decreases carbohydrate utilization and increases the mobilization and utilization of fats as an energy source. GH production increases in response to increasing exercise levels and appears to be controlled by neural factors. The response for GH to exercise training appears to be minimal, and limited evidence suggests resting GH levels are unaffected.

Corticotropin

Corticotropin or adrenocorticotropic hormone (ACTH) is also produced by the anterior pituitary and regulates hormonal secretion of the adrenal cortex. ACTH enhances fat mobilization, increases the rate of gluconeogenesis, and stimulates protein catabolism throughout the body. ACTH directly increases the production of the adrenal cortex hormone cortisol, which increases during exercise. Physical training appears to increase the levels of ACTH during exercise.

FSH, LH, and Testosterone

In the female, follicle-stimulating hormone (FSH) initiates follicle growth in the ovaries and stimulates estrogen secretion. Luteinizing hormone (LH) also causes estrogen secretion and causes rupture of the follicle. In the male, FSH promotes development of the sperm, while LH increases the secretion of testosterone by the testes. Neither LH or FSH levels appear to change in response to exercise. With training, however, females may have depressed levels of FSH and altered LH levels which can lead to menstral dysfunction. Testosterone promotes male sex characteristics, increases red blood cells, increases muscle hypertrophy, and decreases body fat. Testosterone increases in response to exercise; however, exercise trained individuals have depressed levels of testosterone.

Vasopressin/Antidiuretic Hormone

Antidiuretic hormone is released by the posterior pituitary and controls excretion of water by the kidneys. Antidiuretic hormone levels increase during and after intense exercise resulting in reabsorption of water by the kidneys and limiting dehydration. There does not appear to be a significant change in antidiuretic hormone levels in response to training.

Thyroid Hormones

Thyroxine (T4) and triiodothyronine (T3) are secreted by the thyroid gland and increase the metabolic rate of all cells. The thyroid hormones also increase fat and carbohydrate metabolism and can increase the basal metabolic rate (BMR) as much as four times. Blood levels of free thyroxin increase in response to exercise. With exercise training the turnover rate of T4 and T3 is increased; however, appreciable changes in resting BMR are rare.

Catecholamines

The catecholamines epinepherine and norepinepherine are produced and secreted by the adrenal medulla and are regulators of the sympathetic nervous system. Norepinepherine increases 2 to 6-fold during moderate to intense exercise while epinepherine increases dramatically only with intense exercise. Increased sympathetic output is related to increased cardiac output, distribution of blood flow, liver glycogenolysis, and fat utilization. With exercise training, the adrenal-sympathetic response to submaximal levels of exercise is reduced.

Glucocorticoids

The primary glucocorticoid secreted by the adrenal cortex is cortisol. Cortisol stimulates protein catabolism, accelerates fat utilization, and inhibits glucose uptake. The response of glucocorticoid levels to exercise is not fully understood; however cortisol concentrations increase during and following intense exercise. In response to moderate exercise training, increases in cortisol output are tempered. With continual high intensity exercise, however, cortisol output and adrenal gland hypertrophy can occur.

Pancreatic Hormones

Pancreatic Hormones Glucagon, which is produced and secreted from the pancreas increases in response to exercise. The primary effect of increased glucagon levels is to increase blood plasma glucose levels by stimulating liver glucogenolysis and gluconeogenesis. The pancreatic hormone insulin decreases blood glucose levels, promotes carbohydrate transport into cells, and increases glycogen and fat synthesis. Insulin levels progressively decrease with increasing levels of exercise intensity and duration. With training, the levels of insulin and glucagon are maintained closer to resting values. It is believed this reduced response to exercise is related to an increased sensitivity to insulin. Conversely, inactive individuals exhibit hyperinsulinemia or a reduced sensitivity to insulin.

References

McArdle, W. D., F. I. Katch, and V. L. Katch. Exercise Physiology: Energy, Nutrition, and Human Performance. Phila., PA: Lea & Febiger, 1986.

Salem, G J & R F Zernicke. Moderate-exercise-related adaptations in mechanics and matrix composition od immature femoral neck and lumbar vertebra. J. Biomech. Logical effects of physical conditioning. Med. Sci. Sports 1:50 56, 1969.

Wheeler, G. D. Reduced serum testosterone and prolactin levels in male distance runners. JAMA, 252: 514, 1984.