Hormones are biologically active substances. They are produced by endocrine glands in small amounts; their aim is to control organism functions, their regulation and coordination.
The chemical nature of almost all hormones is known. Due to the fact that the chemical formulas that reflect the structure of the hormones are bulky, their trivial names are used. Modern classification of hormones is based on their chemical nature. There are three groups of true hormones:
- peptide and protein hormones;
- amino acid derivatives;
- steroid hormones.
Eicosanoids are hormone-like substances that have a local effect.
For peptide and protein hormones, which include 250 or more amino acid residues are hormones of the hypothalamus and pituitary and pancreatic hormones. The hormones derived from amino acid are mainly tyrosine, as well as adrenaline and noradrenaline. The hormones of steroid nature are hormones of adrenal cortex (corticosteroids), sex hormones (estrogens and androgens), as well as hormonal form of vitamin D. To eicosanoids we can refer arachidonic acid: prostaglandins, thromboxanes and leukotrienes.
Humans have 2 regulatory systems with the help of which an organism adapts to constant inside and outside changes. One of them is nervous system which acts very fast and in the form of impulses it transmit signals through the net of nerves and nerve cells. The other system which is endocrine system performs chemical regulation with the help of hormones which are transported by blood and influence places which are located far from tissues and organs of their secretion. Endocrine system interacts with nervous system. This interaction can be made through some hormones which function as mediators between nervous system and organs which are responsible for their influence. In this case we can speak about neuro-endocrine regulation. At normal state there is a balance between endocrine glands activity, state of the nervous system and target-tissues response. Violations in any of these elements can lead to violations from norm. Excess (hyperfunction of the endocrine gland) or insufficient (hypofunction of endocrine glands) production of hormones leads to various diseases that are accompanied by profound biochemical changes in the body.
The physiological effects of hormones designed to: provide humoral, i.e. carried through the blood, regulation of biological processes, and maintaining the integrity and constancy of the internal environment; harmonious interaction between the cellular components of the body; regulate processes of growth, maturation and reproduction.
Hormones regulate the activity of all body cells. They affect the sharpness of mind and physical mobility, build and height; determine the growth of hair, the tone of voice, sexual drive and behavior. Due to the endocrine system, people can adapt to strong temperature fluctuations, excess or deficiency of food, to physical and emotional stress. Hormones regulate the sexual and reproductive function and psycho-emotional condition of the body.
Endocrine glands are represented in the human pituitary gland, thyroid and parathyroid glands, adrenal glands, pancreas, gonads (testes and ovaries), placenta, and the hormone-producing areas of the gastrointestinal tract. Some compounds with hormone action are synthesized in the organism. For example, the hypothalamus secretes a number of substances (liberins) required for the release of pituitary hormones. These releasing factors or liberins are delivered to the pituitary gland through the blood vessels.
Hormone may have several target organs, and changes they cause can affect a number of organism functions. Hormones sometimes work together, so the effect of a hormone can depend on the presence of some other hormones. Growth hormone, for example, is ineffective in the absence of thyroid hormone.
Hormonal action is based on two basic mechanisms: hormones which can not penetrate the cell (water soluble) act through receptors which are located on cell membrane; hormones which can penetrate the cell membrane (fat soluble) act through receptors which are located in cell cytoplasm. Only the presence of specific receptor-protein determines cell sensibility for this hormone, in other words it make the cell to be a target cell. A hormone could have several target organs.
The first mechanism of hormone action is that the hormone binds to its specific receptors on the cell surface; binding triggers a series of reactions. As a result so-called mediators are formed, which have a direct impact on cellular metabolism. These intermediaries are usually cAMP and / or calcium ions, which are released from intracellular structures or enter the cell from the outside. Both cAMP and calcium ions are used to transmit the external signal into the cell. Some membrane receptors, including insulin receptor, go on a shorter way: they penetrate through the membrane, and as part of their molecule binds the hormone on the cell surface, while another part begins to function as an active enzyme on the side facing into the cell, it provides a manifestation of the hormonal effect.
The second mechanism of action – through the cytoplasmic receptors – is characteristic of steroid hormones (hormones of the adrenal cortex and sexual hormones), and thyroid hormones (T3 and T4). Penetrating into the cell containing the appropriate receptor hormone forms hormone-receptor complex with it. This complex undergoes activation (by ATP), and then penetrates into the cell nucleus where the hormone has a direct effect on the expression of certain genes by stimulating the synthesis of specific RNA and proteins. These newly formed proteins are usually short-lived. They are responsible for those changes which constitute the physiological effects of the hormone.
Regulation of hormone secretion is conducted by several interconnected mechanisms. For example, the production of cortisol is regulated by a feedback mechanism that operates at the level of the hypothalamus. Once in the blood the concentration of cortisol decreases, the hypothalamus secretes corticoliberin (CRH) – a factor that stimulates the secretion of pituitary corticotropin (ACTH). Increasing levels of ACTH, in turn, stimulates the secretion of cortisol in the adrenal glands, resulting in increases of cortisol in the blood. Increased level of cortisol suppresses CRH secretion by feedback mechanism and cortisol level in the blood is decreased. The secretion of cortisol is regulated not only by a feedback mechanism. For example, stress causes the release of CRH and, consequently, the entire series of reactions that increase the secretion of cortisol. In addition, cortisol secretion is subject to circadian rhythm, it's very high on awakening, but is gradually decreased to a minimum during sleep. The mechanisms of control are also the metabolic rate of the hormone and the loss of their activity. Similar regulation systems operate in relation to other hormones.
Main human hormones.
Anterior pituitary hormones. The gland tissue of the anterior pituitary produces: growth hormone (GH) or somatotropin which influences all organism tissues increasing their anabolic activity (in other words, the processes of organism tissue component synthesis and energy resources increase); melanocyte stimulating hormone (MSH) which increases pigment production in some skin cells (melanocytes and melanophores); thyrotropin hormone (TH) or thyroid stimulating hormone; follicle stimulating hormone (FSH) and luteinizing hormone (LH) which refers to gonadotropins, their action is aimed at sex glands; prolactin is a hormone stimulating formation of mammal glands and lactation.
Posterior pituitary hormones are vasopressin and oxytocin. Both of them are produced in hypothalamus but they are stored and liberated in the posterior pituitary which is placed a bit lower than hypothalamus. Vasopressin supports the tone of blood vessels and is an anti-diuretic hormone which influences water balance. Oxytocin stimulates uterine contraction and milk ejection.
Thyroid and parathyroid hormones. Main hormones of the thyroid gland are thyroxine (T4) and triiodothyronine (T3). Coming into the blood stream they bind with specific plasma proteins and thus they are not so fast to be liberated, they act slowly and for a long period of time. Thyroid hormones stimulate protein metabolism and food substances degradation with heat and energy discharge. It is expressed in the increased utilization of O2. These hormones also influence carbohydrate metabolism and regulate the rate of free fatty acids mobilization from adipose tissue. Increase production of thyroid hormones can result in thyro toxicosis. Their deficiency could lead to hypothyreosis (miksidema). Thyroid gland secretes calcitonin which participates in the process of phosphor and calcium metabolism, and secretes powerful thyroid stimulator g-globuline, causing hyperthyroid state.
Parathyroid gland hormone is parathormone (PTH). It keeps the constant level of calcium in blood. When it is decreased parathyroid hormone is released and activates the transfer of calcium from the bones into the blood as long as the content of calcium does not return to normal. Parathormone increased production leads to bone diseases, stones in kidney, calcification of the renal tubules. Its deficiency is accompanied by a significant lowering of calcium level in blood and is expressed in increased nerve-muscle excitement, convulsions and craps.
Adrenal hormones are adrenaline (epinephrine) and nor-adrenaline (norepinephrine). Adrenaline is considered to be a metabolic hormone or survival hormone as it provides organism reaction on some sudden danger. At danger adrenaline is secreted into blood and mobilizes carbohydrates for quick energy discharge, it increases muscle strength, leads to pupil enlargement and constriction of peripheral blood vessels. Adrenaline stimulates production of ACTH which, in its turn, stimulates adrenal cortisole as a result of which protein transformation into glucose increases. Glucose is necessary to enrich the amount of glycogen in liver and muscles. Glycogen is used when there is a case of danger.
Nor-adrenaline is vasoconstrictor, it constricts blood vessels and increases arterial pressure.
Adrenal cortex secretes three main groups of hormones: mineralocorticoids, glucocorticoids and sex steroids (androgens and estrogens).
Mineralocorticoid hormons are aldosterone and deoxycorticosterone. Their activity is connected with salt balance maintenance.
Glucocorticoids influence carbohydrate, protein and fat metabolism and immune mechanisms. The most important among them are cortisole and corticosterone. Sex steroids which play secondary role are similar to those which are synthesized in gonads. These are dehydroepiandrosterone sulfate, Δ4 which is androstendion, dihydroepiandrosterone and some estrogens.
Excess of cortisol leads to disruption of metabolism, causing hyperglukoneogenesis, ie excessive conversion of protein to carbohydrates. This condition is known as Cushing's syndrome, characterized by loss of muscle mass, decrease of glucose in the tissue. It is manifested by anomalous increase in the concentration of sugar in the blood when it comes with food, as well as bone demineralization. Hypoadrenalism occurs in acute and chronic forms. Its cause is a severe, rapidly developing bacterial infection: it can damage the adrenal glandular tissue and lead to a profound shock. In chronic pathological process due to the partial destruction of the adrenal gland Addison's disease develops, characterized by severe weakness, weight loss, low blood pressure, gastrointestinal disorders, increased demand for salt, and pigmentation of the skin.
Testicle hormones. Testis are gland of mixed secretion as they produce sperm (outside secret) and secrete sex hormones (inside secret). Endocrine function of testicles is expressed in Leydig cells which secrete Δ4 – androstendion and testosterone, basic male sex hormone. Leydig cells produce a small amount of estrogen (estradiol). Testis is under gonadotropins control. Gonadotropin FSH stimulates the formation of sperm (spermatogenesis). Under the influence of LH Leydig cells produce testosterone. Spermatogenesis occurs only at sufficient quantities of androgens. Testosterone and other androgens are responsible for development of male secondary sexual characters. Hypogonadism is the decrease of testis function including testosterone secretion and spermatogenesis. Its reason could be testis disease or pituitary functional deficiency. Increased androgens secretion is present at tumor of Leydig cells which can lead to excessive development of male secondary sexual characters especially at teenagers. Sometimes testis tumor produces estrogens and lead to feminization.
Ovaries hormones. Ovaries have two functions: development of ovum and hormone secretion. Ovaries hormones are estrogens, progesterone and Δ4 which is androstendion. Estrogens determine female secondary sexual characters. Estradiol is produced in the cells of growing follicle. As a result of FSH and LH action, follicle becomes mature and breaks liberating the ovum. Broken follicle converts into yellow body which secretes estradiol and progesterone. These hormones prepare endometrium to implantation of fertilized ovum. If there is no fertilization then yellow body is repressed, secretion of estradiol and progesterone stops, endometrium is driven apart leading to menstruation cycle.
Pancreatic hormones. Pancreas is the gland of mixed secretion. Exocrine component are digestive enzymes which in the form of inactive precursors come to duodenum through the ductus pancreaticus as a digestive juice. Endosecretion is provided by islets of Langerhans. α-cells produce hormone which is called glucagon and β-cells produce insulin. Insulin main action is lowering glucose level in blood, carried out in three ways: inhibition of glucose production in the liver, inhibition of the liver and muscle glycogen breakdown, stimulation of glucose utilization by tissues. Insulin secretion deficiency or its increased neutralization by autoantibodies lead to high glucose level in blood and diabetes mellitus development. Glucagon action is aimed at glucose level increase in blood due to stimulation of its production in liver.
Placenta hormones. Placenta is a porous membrane which connects an embryo with uterus. It produces chorionic gonadotropin (CG) and human placenta lactogen (PL). Similar to testis, placenta produces progesterone and some estrogens (estron, estradiol, 16-hydroxy-dehydro-epiandrosterone and estriol). CG keeps the corpus luteum which produces estradiol and progesterone. They keep the wholeness of uterus endometrium. PL is a strong metabolic hormone. Influencing carbohydrate and fat metabolism, they help to keep glucose and nitrogen-containing compounds in mother’s body and provide the fetus by nutritive substances in proper amount. PL helps to mobilize free fatty acids which are the source of energy for mother’s organism.
Gastrointestinal hormones. Hormones of gastro-intestinal tract are gastrin, cholecystokinin, secretin and pancreosimin. They are polypeptides produced by mucous membrane of gastro-intestinal tract as response on specific stimulation. Gastrin stimulates the hydrochloric acid production, cholecystokinin controls excretion from gall-bladder and pancreosimin regulates pancreas juice production.
Neurohormones. This is a group of chemical substances, which are produced by the cells (neurons). They express hormone similar action. They stimulate and repress other cells activity and include release factors and neuro-mediators. Their function is nerve impulse transmission across synapse whole which separates nerve cells from each other. Neuromediators include dophamine, adrenaline, nor-adrenaline, serotonine, histamine, acetylcholine and γ-amino-buteric acid and also neuromediators (endorphins) which have morphine similar and analgetic action. Analgesic effect of morphine and other opiates is determined by their similarity to endorphins which provide their binding with the same pain blocking receptors.
Hormones are widely used as specific medicines. For example, adrenaline is effective at asthma, some skin leisures are treated with glucocorticoids, pediatricians use anabolic steroids and urologists make use of estragens.
1. Give the classification of hormones on their chemical nature.
2. What is the mechanism of hormones action?
3. Describe the pituitary hormones and their effects on metabolism.
4. Describe the hormones of the pancreas.
5. What is the biological role of thyroid hormones?
6. What structure are the hormones of the adrenal medulla? What is their role in metabolism?
7. Give a description of the biological role of parathyroid hormone.
8. What is the biological role and structure of the adrenal cortex hormones?
9. What functions are performed by sex hormones in the organism? What is their chemical nature?