Endocrine System Section A (2).pptx

THE ENDOCRINE SYSTEM HPHS222 Dr Lindokuhle Mabuza-Mashaba ([email protected]) INSPIRING GREATNESS  The What, How, and Why The scientific study of hormones and endocrine organs is called endocrinology. INSPIRING GREATNESS ...

THE ENDOCRINE SYSTEM HPHS222 Dr Lindokuhle Mabuza-Mashaba ([email protected]) INSPIRING GREATNESS  The What, How, and Why The scientific study of hormones and endocrine organs is called endocrinology. INSPIRING GREATNESS  Learning Outcomes  Section A: Hormones and hormonal control systems  Hormones and endocrine glands  Hormone structures and synthesis  Hormone transport in the blood  Hormone metabolism and excretion  Mechanisms of hormone action  Inputs that control hormone secretion  Types of endocrine disorders INSPIRING GREATNESS The Major Endocrine Organs INSPIRING GREATNESS  Hormones And Endocrine Glands  Hormones are chemical messengers that enter the blood, which carries them from their site of secretion to target cells.  Exocrine glands secrete their products into a duct, from where the secretions either exit the body (as in sweat) or enter the lumen of another organ (such as in the digestive system)  Endocrine glands are ductless and release hormones directly into the blood, from where they can reach distant target cells. INSPIRING GREATNESS INSPIRING GREATNESS INSPIRING GREATNESS INSPIRING GREATNESS  Hormones Structures And Synthesis Hormones fall into three major structural classes: 1. Amines 2. Peptides and proteins 3. Steroids INSPIRING GREATNESS  Amine Hormones  Amine hormones are derivatives of the amino acid tyrosine.  They include:  Thyroid hormones (produced by the thyroid gland)  Catecholamines - epinephrine and norepinephrine (produced by the adrenal medulla)  Dopamine (produced by the hypothalamus)  There are two adrenal glands, one above each kidney.  The adrenal gland is composed of an inner adrenal medulla which secretes catecholamines and a surrounding adrenal cortex, which secretes steroid hormones. INSPIRING GREATNESS  Peptide And Protein Hormones  Most hormones are polypeptides  Peptides - Short polypeptides with a known function.  Proteins - Longer polypeptides with tertiary structure and a known function.  Hormones in this class range in size from small peptides having only three amino acids to large proteins, some of which contain carbohydrate (glycoproteins). INSPIRING GREATNESS  Synthesis And Secretion Of Peptide Hormones  The synthesis and secretion of peptide hormones are processed by enzymes from preprohormones containing a signal peptide to prohormones  Further processing results in one or more active hormones that are stored in secretory vesicles.  Secretion of stored secretory vesicles occurs by the process of exocytosis. INSPIRING GREATNESS  Steroid Hormones  Steroid hormones make up the third family of hormones.  Steroid hormones are primarily produced by the adrenal cortex and the gonads (testes and ovaries), as well as by the placenta during pregnancy.  Vitamin D is enzymatically converted in the body to an active steroid hormone. INSPIRING GREATNESS  Steroid Hormones 1. Hormones of the Adrenal Cortex  Mineralocorticoid (aldosterone)  Glucocorticoid (cortisol and corticosterone)  Androgens/Sex hormones (dehydroepiandrosterone (DHEA) and androstenedione) INSPIRING GREATNESS  Aldosterone is a mineralocorticoid because its effects are on salt (mineral) balance.  Kidneys’ handling of sodium, potassium, and hydrogen ions.  The production of aldosterone is under the control of another hormone called angiotensin II  Aldosterone enters the circulation and acts on cells of the kidneys to stimulate Na+ and H2O retention, and K+ and H+ excretion in the urine.  Cortisol and corticosterone are called glucocorticoids.  Effects on the metabolism of glucose and other organic nutrients.  Cortisol is the predominant glucocorticoid in humans.  Cortisol exerts many other effects, including facilitation of the body’s responses to stress and regulation of the immune system. INSPIRING GREATNESS  Dehydroepiandrosterone (DHEA) and androstenedione belong to the class of steroid hormones known as androgens.  Major male sex steroid testosterone, produced by the testes.  The adrenal androgens are much less potent than testosterone, and they are usually of little physiological significance in the adult male.  Functions in the adult female and in both sexes in the fetus and at puberty. INSPIRING GREATNESS  Steroid Hormones 2. Hormones of the Gonads  The endocrine cells in the testes express large amounts of an enzyme that converts androstenedione to testosterone, which is the major androgen secreted by the testes.  The ovarian endocrine cells synthesize the female sex hormones estrogens (primarily estradiol and estrone).  Estradiol is the predominant estrogen present during a woman’s lifetime.  The ovarian endocrine cells express large amounts of the enzyme aromatase, which catalyzes the conversion of androgens to estrogens.  The gonads and the adrenal cortex, steroid hormones may undergo further conversion in other organs.  For example, testosterone is converted to estradiol in some of its target cells. INSPIRING GREATNESS  Endocrine cells of the corpus luteum, an ovarian structure that arises following each ovulation, secrete another major steroid hormone, progesterone.  Important for maintaining pregnancy.  Progesterone is also synthesized in other parts of the body—notably, the placenta in pregnant women and the adrenal cortex in both males and females. INSPIRING GREATNESS Synthesis of Adrenal Cortex Hormones INSPIRING GREATNESS Synthesis of Gonadal Hormones INSPIRING GREATNESS  Hormone Transport in the Blood  Peptide and catecholamine hormones  Water-soluble  Dissolved in plasma  Steroid and thyroid hormones  Poorly soluble  Mostly bound to plasma proteins  Small concentrations dissolved in plasma (“free hormones”)  Only free hormones interact with target cells INSPIRING GREATNESS Hormone Transport in the Blood INSPIRING GREATNESS  Hormone Metabolism And Excretion  A hormone’s concentration in the plasma depends upon:  Its rate of secretion by the endocrine gland.  Its rate of removal from the blood.  Removal, or “clearance,” of the hormone occurs either by excretion or by metabolic transformation.  The liver and the kidneys are the major organs that metabolize or excrete hormones. INSPIRING GREATNESS  Mechanisms of Hormone Action  Hormone receptors  The ability of a cell to respond to a hormone depends upon the presence of receptors for that hormone on or in the target cell.  An increase in the number of receptors for a hormone is called up-regulation.  Often resulting from a prolonged exposure to a low concentration of the hormone. This has the effect of increasing target-cell responsiveness to the hormone  A decrease in the number of receptors for a hormone is called down-regulation.  Often from exposure to high concentrations of the hormone. This temporarily decreases target-cell responsiveness to the hormone, thereby preventing overstimulation. INSPIRING GREATNESS Permissive Actions of Hormones INSPIRING GREATNESS  Events Elicited by Hormone–Receptor Binding  Effects of Peptide Hormones and Catecholamines  When activated by hormone binding, the receptors trigger one or more of the signal transduction pathways for plasma membrane receptors. That is, the activated receptors directly influence: (1) enzyme activity that is part of the receptor, (2) activity of cytoplasmic janus kinases associated with the receptor, or (3) G proteins coupled in the plasma membrane to effector proteins—ion channels and enzymes—that generate second messengers such as cAMP and Ca2+.  Effects of Steroid and Thyroid Hormone  The binding of hormone to its receptor leads to the activation (or in some cases, inhibition) of the transcription of particular genes, causing a change in the synthesis rate of the proteins coded for by those genes. INSPIRING GREATNESS Mechanisms of hormone action INSPIRING GREATNESS  Pharmacological Effects of Hormones  Administration of large quantities of a hormone for medical purposes may have effects in an individual that are not usually seen in a healthy person.  Efficacy: measure of the amount of bound ligand required to produce a given response  Potency: a measure of the concentration of ligand required to produce a given response  The measure of potency is called the EC50, the concentration of a ligand required to produce a half- maximal response  Potency is a drug's concentration, and efficacy is a drug's ability. INSPIRING GREATNESS Inputs That Control Hormone Secretion INSPIRING GREATNESS Inputs That Control Hormone Secretion INSPIRING GREATNESS  Example: Hormonal Stimuli (Control by other Hormones)  A hormone that stimulates the secretion of another hormone is often referred to as a tropic hormone.  The tropic hormones usually stimulate not only secretion but also the growth of the stimulated gland. Many hypothalamic hormones stimulate the anterior pituitary to release hormones. The anterior pituitary hormones then stimulate other target organs (thyroid, adrenal and gonads) to secrete still other hormones. INSPIRING GREATNESS  Example: Humoral Stimuli (Control by plasma concentrations of organic nutrient) INSPIRING GREATNESS  Example: Neural Stimuli (Control by neurons) INSPIRING GREATNESS  Types of Endocrine Disorders  Hyposecretion: the secretion of too little hormone: primary & secondary hyposecretion  Hypersecretion: the secretion of too much hormone  Hypo-responsiveness: decreased responsiveness of the target cells to hormone (occurs when the target cells do not respond properly to a hormone).  Hyperresponsiveness: increased responsiveness of the target cells to hormone (occurs when the target cells do not respond properly to a hormone). INSPIRING GREATNESS  Did You Get It? 1. Walking barefoot, you step on a piece of broken glass and immediately pull your foot back. Why is it important that the signal triggering this motion come from the nervous system and not from the endocrine system? 2. What do you understand about the endocrine system? 3. What is a hormone? What does target organ mean? 4. What is the differences between endocrine and exocrine glands? 5. What are the three major structural classes of hormones and the hormones that fall under each class? (Tabulate) 6. Why is cAMP called a second messenger? 7. How are hormones transported in the blood? 8. What do you understand about permissive actions of hormones? 9. What are three ways in which endocrine glands are stimulated to secrete their hormones? INSPIRING GREATNESS