Hormones Study Documents

MADE BY APHRODITE PASCOE HORMONES The coordination in mammals is achieved by the neuroendocrine system; it involves a. Nervous system ​ ​ ➢ Electrical signals originates in the neuron and travel very rapidly to the axon tip where neurotransmitters are released ➢ Neurotransmitter may travel about a micrometer across the synaptic cleft to the next neuron. b. Endocrine system ​ ​ ➢ Hormones are secreted to the blood stream and carried to target tissues that are away from the secreting cell. What is a hormone? • Hormones are chemical • messengers which are extremely potent and are produced in very small amounts As hormone concentration is very low, a high affinity interact ion with the receptor is necessary. • Receptor is a cellular protein that binds the hormone. • The binding ligands to receptors are: o Never covalent o Always reversible o Specific and saturable. • Target cell is any cell in which the hormone binds to its receptor o Endocrine hormones are released into the blood and carried to target cells throughout o o • the body (e.g. Insulin) Paracrine hormones are released into the extracellular space and diffuse to neighboring target cells (e.g eicosanoid hormones) Autocrine hormones are released by and affect the same cell, binding to receptors on the cell surface (interleukin) Hormone is synthesized and secreted by the endocrine glands. The major endocrine systems and their target tissues Hormone Classification 1. Mechanism of Action ​ ​ The mechanism of action- Group II hormones i. Polypeptide-protein and glycoprotein hormones. ✓ It has more than 40 hormones ✓ Synthesized as part of a larger molecule (preprohormone) on ribosones that contains a leader sequence at its amino terminal end ✓ Signal peptide is removed in the ER resuting protein is prohormone. ✓ Prohormone undergoes posttranslational processing to produce mature hormone. ✓ Prohormones are packaged into secretory vesicles while mature hormones are released by exocytosis. ii. Catecholamine hormones ✓ Produced by the brain, nerve tissue and adrenal glands ✓ They act as both neurotransmitters and hormones vital to the maintenance of homeostasis through the autonomic nervous system. ✓ Dopamine, norepinephrine and epinephrine are physiologically active molecules known as catecholamine. ✓ It is released in response to emotional or physical stress. Hence, responsible for “fight or flight” response; they are also highly concentrated within secretory vesicles and released by exocytosis. iii. Eicosanoids ✓ They are paracrine hormones secreted into the interstitial fluid and act on nearby cells. ✓ Synthesized in almost all tissues except erythrocytes; they are not stored and produced when needed. ✓ Examples include: o Prostaglandins – promote contraction of smooth muscle (intestine and uterus); mediates pain and inflammation; short half-life. o Thromboxanes – exits as thromboxane A2 and thromboxane B2. It is a vasoconstrictor and a potent hypertensive agent which regulates platelet function and blood clotting. o Leukotrienes – stimulate contraction of smooth muscle in the intestine, pulmonary airways and trachea: they are also mediators of severe immune response. General Mechanism of action – Group I hormones ✓ They are lipophilic ✓ Their receptors are typically intracellular (nuclear receptors) i. Steroid hormones: ✓ Controls metabolism, inflammation, immune functions, salt and water balance, development of sexual characteristics, and the ability to withstand illness and injury. ✓ Natural steroid hormones are synthesized from cholesterol in the gonads and adrenal glands. Example include: o o o Sex hormones: androgen and estrogens Minerolocorticoids Glucocorticoids ii. Thyroid hormones ✓ Thyroglobulin is the precursor of thyroid hormones ✓ Thyroid hormones are stored in colloid in the follicular space and released by thyroglobulin by hydrolysis inside the thyroid cell. ✓ Stimulate energy yielding metabolism especially in liver and muscle. ✓ Increase the expression of genes encoding key catabolic enzymes iii. Vitamin D hormone = Calcitriol (1,25-dihydroxycholecalciferol) ✓ Calcitriol is the active form of Vit. D. ✓ The hormone is obtained from diet or by photolysis of 7-dehydrocholesterol in the skin. iv. Retinoid hormones ✓ They are potent hormones which regulate growth, survival and differentiation of cells. ✓ The prohormone retinol is synthesized from vitamin A primarily in liver and many tissues convert retinol to the hormone retinoic acid. Classes of hormones Mechanism of synthesis ✓ Ribosomal o Peptide-protein and glycoprotein hormones ✓ Enzymatic o Amino acid derivatives o Steroid hormones o Eicosanoids ✓ Both ribosomal and enzymatic o Thyroid hormones Negative and Positive Feedback Control Mechanism Binding proteins ✓ Carriers are necessary for lipophilic hormones that are not soluble in blood. Examples: -Tyroxine binding globulin (TBG) ✓ Binds T4 with greater affinity than T3 ✓ The plasma half-life of T4 is longer than T3 -Corticosteroid binding globulin (CBG) ✓ Binds cortisol -Sex-hormone binding globulin (SHBG) ✓ Binds testosterone and estradiol ✓ Binds testosterone with a higher affinity than estradiol 2. Chemical structure ​ i. Amino acid derivatives ​Derived from amino acids through simple modifications; it includes, catecholamines (dopamine, norepinephrine and epinephrine), serotonin, melatonin, and the thyroid hormones. ii. Peptide, protein or glycoprotein hormones iii. Steroid hormones iv. eicosanoids 3. Site of synthesis ✓ Ovary o Estradiol o Progesterone ✓ Testis o Testosterone ✓ Gastrointestinal system (GIS) o Gastrin o Secretin o cholecystokinin 4. Function ✓ Hormones regulating glucose metabolism o Hypoglycemic – insulin o Hyperglycemic – glucagon, cortisol, adrenaline, noradrenaline ✓ Hormones regulating C and Pi metabolism o PTH stimulates reabsorption and increase absorption of Ca and secretion of P by ++ o o kidney and intestine Vitamin D: Increase absorption of Ca by kidney and stimulates its release from bone. Calcitonin has the opposite effect of PTH ✓ Hormones regulating water and salt metabolism, eg, ADH, aldosterone, natriuretic factors ✓ Anabolic hormones eg insulin, GH, testosterone ✓ Catabolic and lipopytic hormones eg adrenaline, thyroid hormones ✓ Stress hormones eg glucagon, adrenaline, noradrenaline and cortisol 5. Mode of storage and secretion ➢ No storage- steroid hormones, Vit D and eicosanoids ➢ Storage out of cell – thyroid hormones ➢ Storage in intracellular vesicles- catecholamines, peptide and protein hormones