HUMAN BIOCHEMISTRY - L. V. Kapilevich - 2016

PART 1. SPORTS BIOCHEMISTRY

HORMONES

Hormones are Organic compounds of diverse structures that exert a regulatory influence on METABOLISM and the physiological Functions of Organs.

Hormones are secreted by Endocrine glands, which lack excretory ducts and release hormones directly into the bloodstream.

Mechanism of hormone Action

Hormones do not directly affect cellular reactions themselves. A hormone triggers a specific effect only after binding to a designated, highly specific receptor.

Hormones are classified into Water-soluble and lipid-soluble categories. This Classification determines their MECHANISM OF ACTION. Specifically, lipid-soluble hormones easily cross The Cell membrane—which is composed primarily of a lipid bilayer—whereas water-soluble hormones cannot. Consequently, the receptors for water-soluble and lipid-soluble hormones are located in different cellular compartments (the membrane versus the Cytoplasm). Upon binding to a membrane receptor, a hormone initiates a cascade of intracellular reactions without affecting genetic material. Conversely, a cytoplasmic receptor-hormone complex can interact with nuclear receptors and induce Changes in the genetic apparatus, leading to the synthesis of new Proteins.

Hormone effects can be altered by Metabolic Disorders, changes in the physicochemical parameters of the body (Temperature, acidity, osmotic pressure), fluctuations in key substrate concentrations associated with diseases, and physical exertion. As a result, The Influence of hormones on their target organs may be either enhanced or diminished.

Classification of hormones

1. Protein-based hormones (proteins and Polypeptides): Hypothalamic and pituitary hormones, thyroid Calcitonin, parathyroid hormone, and Pancreatic Hormones.

2. Amino acid-derived hormones (Tyrosine derivatives): iodine-containing THYROID HORMONES and Adrenal Medullary Hormones.

3. Steroid Hormones: Hormones of the adrenal cortex and Gonads.

Regulation of Hormone Production

The Synthesis and Secretion of hormones into the Blood are controlled by The Nervous system. In simplified terms, the relationship between the hormonal (endocrine) and nervous systems can be described as follows. When the body is exposed to external factors, or when changes occur in the blood and various organs, the relevant information is transmitted via afferent (sensory) nerves to the Central Nervous System (CNS). In response, the Hypothalamus (a region of the Diencephalon) produces BIOLOGICALLY ACTIVE SUBSTANCES (Hypothalamic hormones) that travel to the Pituitary Gland (hypophysis), where they stimulate or inhibit the secretion of trophic hormones (Anterior Pituitary Hormones). Trophic hormones are released from the pituitary into the bloodstream, transported to the endocrine glands, and stimulate the synthesis and secretion of specific hormones that subsequently affect target organs. Thus, the body operates through an integrated neurohormonal or neurohumoral regulatory system.

All endocrine glands function in a coordinated manner and exert mutual influence on one another. The Introduction of hormones into the body not only affects the function of the gland that naturally produces that hormone, but can also negatively disrupt the entire neurohormonal regulation system. Therefore, The Use of hormonal preparations as doping agents is hazardous to athletes' health.

Endocrine Glands and Their Secreted Hormones

Hypothalamus. Secretes liberins (releasing hormones), which stimulate the pituitary gland to produce hormones, and statins (inhibiting hormones), which suppress pituitary hormone production.

Pituitary Gland (Hypophysis). Comprises three lobes, each secreting its own specific hormones.

The anterior lobe secretes Growth Hormone (somatotropin), thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, and prolactin.

Somatotropin is involved in regulating body GROWTH AND DEVELOPMENT. By enhancing Cartilage tissue synthesis in the epiphyseal regions of bones, this hormone stimulates longitudinal body growth during childhood, while the activation of periosteal growth increases bone thickness and width. Tissue mass increases within Muscle and Connective Tissue, accompanied by the enlargement of Internal Organs. The primary effects of somatotropin are mediated through its impact on metabolism, leading to: 1) enhanced lipolysis and a reduction in adipose tissue mass; 2) increased amino acid uptake and Protein Synthesis, resulting in body mass gain through non-adipose tissue; 3) increased Gluconeogenesis and elevated blood sugar levels. At the same time, most of the growth-promoting effects of the hormone are mediated by specialized humoral factors (hormones) originating in the Liver, Kidneys, and Bone tissue, known as somatomedins.

Thyroid-stimulating hormone promotes The production of thyroid hormones.

Adrenocorticotropic hormone (ACTH) stimulates the secretion of Adrenal Cortex Hormones.

Follicle-stimulating hormone regulates the maturation of Germ Cells.

Lactogenic hormone (prolactin) stimulates milk production in women postpartum.

The intermediate lobe produces melanocyte-stimulating hormone, which enhances pigment production by certain Skin cells (melanocytes and melanophores).

The posterior lobe produces Vasopressin and Oxytocin.

Vasopressin (antidiuretic hormone) maintains blood pressure via baroreceptors and a direct effect on the vascular wall, acts as one of the regulators of adrenocorticotropic hormone secretion, increases the release of thyroid-stimulating hormone from the pituitary gland, increases prostaglandin synthesis by the interstitial Cells of the renal medulla, induces contraction of glomerular mesangial cells, exerts a mitogenic effect, causes platelet aggregation, and promotes the release of coagulation factors—von Willebrand factor, factor VIII, and tissue-type plasminogen activator—while also participating in central nervous system processes, particularly memory functions.

Oxytocin affects smooth muscle, particularly the musculature of the pregnant Uterus. Under the influence of oxytocin, the membrane permeability to potassium ions increases, its Resting Potential decreases, and its excitability rises. Oxytocin also increases milk secretion by enhancing the production of the lactogenic hormone. In addition, it can induce the rapid ejection (letdown) of milk from the mammary gland through its action on contractile elements.

The Thyroid Gland secretes iodine-containing hormones (thyroxine and triiodothyronine) and calcitonin (thyrocalcitonin).

Upon reaching its target organs, thyroxine is converted into triiodothyronine, which acts directly on the cell. Thyroid hormones promote growth and development, and stimulate tissue growth and differentiation. They increase tissue oxygen demand, as well as systemic blood pressure and Heart rate and contractility. Furthermore, they elevate the level of alertness, mental energy, and activity, accelerate the flow of cognitive associations, and raise motor activity, body temperature, and basal metabolic rate.

Calcitonin takes part in regulating phosphorus-calcium Metabolism in the Body, as well as the balance of osteoclast and osteoblast activity. It lowers plasma calcium and phosphate levels by enhancing calcium and phosphate uptake by osteoblasts, and also stimulates the development and functional activity of osteoblasts. Simultaneously, thyrocalcitonin inhibits the development and functional activity of osteoclasts and the processes of bone resorption.

The Parathyroid glands produce parathyroid hormone.

Parathyroid hormone increases the release of the readily soluble fraction of calcium from bone, but its primary effect consists in accelerating the synthesis of Enzymes that drive The breakdown of the bone matrix. Under the influence of parathyroid hormone, the bone framework undergoes resorption, and Calcium Ions are released into the blood. In the kidneys, parathyroid hormone enhances the urinary excretion of phosphates and increases calcium reabsorption, thereby reducing its urinary loss. In addition, parathyroid hormone promotes the elimination of sodium and potassium from the body while reducing The excretion of magnesium. Another effect of parathyroid hormone is The conversion of vitamin D from its inactive form into the active one.

The Pancreas produces Insulin, Glucagon, and Somatostatin.

The primary Action of Insulin is to lower blood glucose levels, which is achieved mainly through three mechanisms: 1) inhibiting glucose production in the liver; 2) inhibiting the breakdown of Glycogen (a glucose polymer that the body can convert into glucose when needed) in The Liver and Muscles; and 3) stimulating glucose utilization by Tissues. Inadequate insulin secretion or

its elevated neutralization by autoantibodies leads to high blood glucose levels and The Development of Diabetes Mellitus.

The main effect of glucagon is to raise blood glucose levels by stimulating its production in the liver.

Somatostatin suppresses pentagastrin-induced gastric secretion, as well as pancreozymin- and secretin-induced pancreatic secretion.

The Adrenal Glands. The adrenal medulla secretes catecholamines. The adrenal cortex secretes glucocorticoids, Steroids, and mineralocorticoids.

Catecholamines (epinephrine and norepinephrine). Epinephrine mediates the response to sudden danger. Upon its onset, epinephrine is released into the bloodstream and mobilizes carbohydrate reserves for the rapid liberation of energy, increases muscle strength, causes pupil dilation, and narrows peripheral Blood Vessels. Thus, reserve forces are directed toward fight or flight; moreover, blood loss is reduced due to vasoconstriction and rapid blood clotting. Epinephrine also stimulates the secretion of ACTH, which in turn stimulates the adrenal cortex to release cortisol, resulting in increased conversion of proteins into glucose, which is necessary to replenish the glycogen reserves in the liver and muscles expended during the alarm reaction. Norepinephrine produces similar effects, though they are less potent.

Glucocorticoids affect carbohydrate, protein, and fat metabolism, as well as immunological defense mechanisms. The most important glucocorticoids are cortisol and corticosterone.

Sex steroids, which play an auxiliary role, are similar to those synthesized in the gonads; they include dehydroepiandrosterone sulfate, D4-androstenedione, dehydroepiandrosterone, and certain estrogens.

Mineralocorticoids are aldosterone and deoxycorticosterone. Their action is primarily associated with the maintenance of salt balance.

The gonads. The Testes have a dual nature, functioning as glands of both exocrine and endocrine secretion. As exocrine glands, they produce sperm, while their endocrine function is carried out by the Leydig cells contained within them, which secrete Male Sex Hormones (androgens), particularly D4-androstenedione and testosterone, the primary male hormone. Leydig cells also produce small amounts of estrogen (estradiol). Spermatogenesis occurs only in the presence of adequate amounts of androgens. Androgens, particularly testosterone, are responsible for the development of secondary male sex characteristics.

The Ovaries have two functions: the development of ova and the secretion of hormones. Ovarian Hormones include estrogens, progesterone, and D4-androstenedione.

Estrogens determine the development of secondary female sex characteristics. The ovarian estrogen, estradiol, is produced in the cells of the growing follicle—the sac that surrounds the developing oocyte. Under the influence of both follicle-stimulating hormone and the lactogenic hormone, the follicle matures and ruptures, releasing the oocyte. The ruptured follicle then transforms into the corpus luteum, which secretes both estradiol and progesterone. Acting in concert, these hormones prepare the uterine lining (endometrium) for the implantation of the fertilized egg. If Fertilization does not occur, the corpus luteum undergoes regression, halting the secretion of estradiol and progesterone, while the endometrium sloughs off, causing menstruation.

The Thymus produces thymosin, thymopoietin, and thymulin. Thymic hormones influence the synthesis of CELLULAR RECEPTORS FOR Neurotransmitters and hormones, stimulate the breakdown of acetylcholine at neuromuscular synapses, regulate carbohydrate, protein, and calcium metabolism as well as the Functions of the thyroid and sex glands, modulate the effects of glucocorticoids and thyroxine (suppressing them) and growth hormone (enhancing them).

The Pineal Gland secretes melatonin, which regulates the biorhythms of endocrine functions and metabolism to adapt the Organism to varying light conditions. The primary Physiological Effect of melatonin consists in the inhibition of gonadotropin secretion both at the adenohypophysis level and indirectly via the suppression of hypothalamic liberin neurosecretion. In addition, it reduces, albeit to a lesser extent, the secretion of other pituitary hormones—corticotropin, thyrotropin, and growth hormone. Melatonin secretion is subject to a circadian rhythm, which determines the periodicity of gonadotropic effects and reproductive function, including the duration of the Menstrual cycle in women.

Self-Assessment Questions

1. In which organs are hormones produced?

2. What is The Mechanism of the selective Action of Hormones on target organs?

3. Under what conditions can The Effect of hormones on organs change?

4. Into what groups are hormones divided according to their chemical Structure?

5. Why is the use of hormonal preparations as doping agents dangerous to health?

6. Which human organ regulates The activity of the endocrine glands?

7. What hormones are secreted by the adrenal glands?