Hormones: How do they work in the body Part II

The Hormones and Their Glands

The Pituitary Gland

The pituitary gland releases many hormones. The pituitary gland is an extremely small gland located at the base of the brain and is connected to the hypothalamus. Physiologically it has two distinctive lobes, each of which secretes specific hormones.

1. The posterior lobe is responsible for the secretion of antidiuretic hormone (ADH), or vasopressin, which functions mainly to promote water reabsorption from the collecting tubules of the kidney. The other hormone secreted from the posterior lobe is oxytocin. Its main function are stimulation of milk ejection and contraction of the pregnant uterus.

2. The anterior lobe secretes the following hormones: (a) growth hormone (GH), which stimulates growth and development. (b) thyroid-stimulating hormone (TSH), which stimulates production and release of the thyroid hormones, © adrenocorticotropic hormone (ACTH), which stimulates the production and release of glucocorticoid hormone from the adrenal cortex, (d) follicle-stimulating hormone (FSH), which promotes growth of the ovarian follicle in the female and spermatogenesis in the male. (e) utilizing hormone (LH) , which stimulates ovulation, formation of the corpus luteum, and secretion of interstitial cells in the male, (f) prolactin (PRL), which stimulates secretion of milk after pregnancy, and (g) endorphins, which are related to relief of pain and production of euphoria.

As may be seen, the pituitary gland is a very important endocrine gland and because of its many hormones, it is sometimes referred to as the “master” gland. This title might be challenged from the perspective of the hypothalamus. After all, it produces releasing factors and “hormones” that stimulate or inhibit the release of all hormones produced by the anterior lobe except for the endorphins.

The Adrenal Glands

The adrenal glands, as their name implies, sit on top of the kidneys. Physiologically, the adrenal gland is really two separate endocrine glands, the adrenal medulla (inner portion of the gland); and the adrenal cortex (the outer portion).

1. The adrenal medulla is similar to and under the direct influence of the sympathetic nervous system. Its hormones are also similar to the nervous system in that they secrete epinephrine and nor epinephrine. These two hormones are referred to as catecholamines. The catecholamines are the type of hormones referred to earlier that have effects on all the tissues of the body.

2. The adrenal cortex secretes some forty hormones that belong to the class of compounds known as steroids. They are divided into the following three groups on the basis of their major actions.

(a) The mineral corticoids primarily affect electrolyte metabolism. The most important mineralocortocoid is aldosterone, which functions to increase the reabsoption of sodium from the distal tubules of the kidney. This, in turn, causes the reabsorption of chloride and water.

(b) The glucocorticoids, although named because of their effects on glucose metabolism, also have an effect on protein and fat metabolism. The most important glucocorticoid is cortical. The glucocorticoids (principally cortical promote the increased synthesis of glucose. (gluconeogenesis) from amino acids, depress liver lipogenesis, and mobilize fat in adipose tissue. Two other effects of cortical are maintenance of vascular reactivity (without cortical, the blood vessels are unable to respond to circulating catecholamines) and inhibition of the inflammatory reaction, the normal response of tissues to injury. Because of this latter action, glucocorticoids are often administered in massive pharmacological doses as anti-inflammatory steroids.

C The androgens cause development of male secondary sex characteristics. Androgens are primarily secreted by the testes in males, but production by the adrenals occurs in both sexes. The most important androgen is testosterone. The female counterpart to the androgens are the estrogens.

The Pancreas

The two major hormones secreted by the pancreas are insulin and glucagon. Both of these hormones are secreted by the cells of the islets of langerhans, insulin from the beta cells and glucagon from the alpha cells. Insulin is hypoglycemic (I.e., it lowers the blood glucose levels). It does this by increasing the rate of glucose transport through the membrane of most cells in the body. Essentially, insulin stimulates the process facilitated of glucose. Aside from its effects on cellular glucose uptake, insulin increases fat deposition in the adipocytes (fat cells). A lack of insulin results in diabetes mellitus. Glucagon has effects that are the opposite of insulin. Therefore, secretion of glucagon causes increased blood glucose levels. Glucagon has two major effects on carbohydrate or glucose metabolism: (10 glycogenolysis, or the breakdown of glycogen and (2) increased gluconeogenesis, or the synthesis of glucose from molecules that are not themselves carbohydrates, such as protein or fat.

The Thyroid Gland

The thyroid gland is located on the upper part of the trachea just below the larynx (voice box). Its principal hormone are thyroxin and triiodothyronine, although it also secretes a hormone called calcitonin. Both thyroxin and triiodothryonine requires small amounts of iodine (1 mg per week) for their formation. To prevent iodine deficiency, common table salt is iodized. The release of thyroxin and triiodothyronine is controlled by the thyroid-stimulating hormone (TSH) secreted from the posterior lobe of the pituitary gland.

The major action of the thyroid hormone is a general increase in the metabolic rate. Some specific functions associated with the increased metabolism are (1) an increased protein synthesis making the thyroid hormones necessary for normal growth and development in the young; (2) an increased quantity of intracellular enzymes; (3) an increased size and number of mitochondria; 94) an increased cellular uptake of glucose, and enhanced glycolysis and gluconeogenesis; and (5) an increased mobilization and oxidation of fatty acids.

Calcitonin causes a decrease in the blood calcium level. This hormone works in conjunction with the parathyroid hormone, which will be discussed next.

The Parathyroid Glands

The parathyroid glands are tiny glands embedded in the dorsal surface of the thyroid gland. Parathyroid hormone (PTH) is the hormone they secrete. This hormone, together with calcitonin, regulates the calcium equilibrium in the body. PTH causes more calcium to be absorbed from the digestive tract; therefore, less calcium is lost through the feces and urine. This, along with its action of removing calcium from the bone, causes an increase in the blood calcium level. Calcium acts in the opposite way from PTH; in other words it causes a decrease in blood calcium levels by preventing the removal of calcium from the bone.

The Ovaries and the Testes

An endrocrine glands, the ovaries (female) and the testes (male) produce the sex hormones, androgens in the male and estrogens and progesterone in the female. The most important adrogens are testosterone. You may recall that the production and release of the sex hormones are under the control of utilizing hormone (LH) from the anterior lobe of the pituitary gland. The androgens promote secondary sex characteristics and are recognized as promoting protein anabolism (synthesis) and reducing protein catabolism (breakdown). As will be pointed out in the next chapter, excessive dosages of anabolic steroids cause harmful side effects.

Estrogens from the ovaries have actions in that female comparable to those of androgens in the male. They are responsible for the development and function of the uterus, uterine tubes, and vagina and promote the secondary sex characteristics of the female. The estrogens and progesterone work, along with FSH and LH, to regulate the menstrual cycles of women. In addition, estrogens are thought to provide protection against atherosclerosis and thus coronary heart disease.

Progesterone is secreted in large quantities only after ovulation. It promotes further development of the uterus and mammary glands.

Other production sites

A few substances with hormone-like qualities are produced by various tissues of the body other than the endrocrine glands. For example, under hypoxic conditions the kidney, synthesizes erythropoietin, which in turn stimulates bone marrow to increase production of red blood cells. Because erythropoietin is produced at one location and is carried via the blood to a second location and is carried via the blood to a second location where it exerts its biological effect, it meets most of the definitional qualifications for a hormone. Likewise, prostaglandin are produced by a variety of body tissues including the blood vessels, skeletal muscles, and the heart. Prostaglandins vary in their makeup and actions but primarily influence blood flow regulation through their ability to produce vasodialation. Finally, the somatomedins are a class of substances produced by the liver and several other tissues. These substances stimulate the growth of muscle and cartilage through a turn on of various phases of protein synthesis. The somatomedins themselves are comprised of amino acid chains that are produced as a result of increased growth hormone release from the anterior lobe of the pituitary gland. Consequently, they are often viewed as factors that support growth hormone action rather than as hormone in their own right.

Hormonal Responses to Exercise and Training

Exercise and training cause blood levels of some of the hormones previously mentioned to either increase or decrease in comparison to resting values. The increases or decreases often directly reflect adjustments in the rate of hormone secretion by an endocrine gland.. You should be aware, however, that changes in blood levels also may reflect changes in metabolic turnover rates or clearance rates and hemoconcentration plasma concentration (level) of a given hormone during exercise might be due to an increased rate of secretion, a reduced turnover or clearance of hormone, a reduction in plasma volume due to water losses in sweat, or a combination of one or more of these factors. Some other factors that might affect blood levels of hormones are training status and psychological state, as well as the intensity of the work load and hypoxia. Although the physiological significance of many of these changes is not presently known, the fact that they even respond to exercise is in itself significant.

Future Research on Hormonal Response Combinations

Researchers are beginning to look more at hormone responses to exercise in terms of systems and clusters, such as the hypothalamic-pituitary-adrenal-axis, the reninangiotensin-aldosterone axis, and the anabolic/catabolic steroid hormones. There also are studies of the fuel substrate regulatory and counter regulatory hormones that work to regulate blood levels of carbohydrates and lipids. For example, no differences in male versus female subjects were found during performance of endurance exercise for such regulatory hormones as adrenalin, noradrenalin, growth hormone, insulin, or cortical. Another example is the finding that glucagon to insulin molar ratios, but not cortical levels, were lower in subjects who drank a high concentration carbohydrate solution during prolonged intermittent exercise. The effects of exercise on these counter regulatory hormones would not be so well known if they were simply studied in isolation. More of this type of research will be done in the future.

Daryl Conant, M.Ed