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Hormone physiology All the things you forgot or never learned from Science collage Dr. Maha Daghestani Today lecture Introduction to endocrinology Know and understand what a hormone is Mechanism of hormone action Regulation of hormone secretion Types of endocrine disorder Endocrine System A regulatory system, like the nervous system. Uses chemical messengers instead of nerve impulses. Goes into blood stream, travels to effectors and causes an effect. Endocrine Glands Compare exocrine with endocrine glands. The body contains two types of glands: Exocrine glands secrete their products into ducts or directly onto epithelial surfaces. Endocrine glands secrete their products called hormones, into the extracellular fluid around the secretory cells. The secretion then diffuses into the blood for distribution throughout the body. Which organs are endocrine only? Hypothalamus Pituitary Pineal gland Thymus gland Parathyroid Thyroid Name other organs that have some endocrine function Pancreas Kidneys small intestine gonads (2) stomach heart placenta The pituitary (hypophysis) attached to the underside of the brain Pineal gland The thyroid, situated in the neck close to the larynx The Parathyroids, associated with the thyroid but comprising separate structures Thymus Thymus gland gland The adrenals, situated on top of each kidney The endocrine portion of the pancreas The ovaries The testes Testes anatomy and histology Non-hormonal cell-to-cell communication Paracrine example: histamine released by injured cells causes local capillary walls to become more permeable, allowing more white blood cells and fluid to leak into injured area. Autocrine example: various cytokines involved in development and cell differentiation act both as autocrines and as paracrines. Endocrine system — signaling molecule needs to be carried around the body by the blood stream to be considered a "true hormone." Example: insulin, from pancreatic beta cells Example: acetylcholine — neuromuscular junction, heart, various gastrointestinal functions Example: epinephrine (adrenaline), released by adrenal medulla (essentially a large cluster of sympathetic nerve cells) to affect heart rate and contractility, blood vessel diameters, and short-term energy usage Hormones "A chemical substance, synthesized by a specific organ or tissue and secreted into the blood stream, in which it is carried to other specific sites in the body, where it exerts its action(s)". Chemically, hormones may be conveniently divided into the following classes of organic compound: proteins (eg. insulin) and peptides (eg. oxytocin) amino acid derivatives (eg. thyroxine) steroids (eg. cortisol) Hormones are usually transported in the blood bound to plasma proteins or to proteins specific to the particular hormone Blood Levels of Hormones will depend on: The rate of secretion, which may be divided into: the rate of synthesis The rate of removal Hormone Overview Protein/peptide hormones – – – Preprohormone synthesis is directed by mRNA on endoplasmic reticulum Peptides cleaved from preprohormone to produce a prohormone, after which more peptides are cleaved by the Golgi apparatus to form a hormone. Hormone is then stored in secretory granules for later release Hormone overview Steroid hormones – derivatives of cholesterol Amine hormones – derivatives of tyrosine (thyroid hormone, epinephrine, norepinephrine) Regulation of hormone secretion 1- Negative feedback – – Most common mechanism of hormone secretion regulation Hormone has effects that directly or indirectly down regulate further secretion 2- Positive feedback – self perpetuating (uncommon). 3-Mineral ions concentration. 4-Concentration of organic nutrients Mechanisms of Hormone Action Though hormones are able to reach where ever the blood circulates, they are only able to act upon specific cells that possess receptors specific to that particular hormone. Such cells are termed target cells (target tissues, or target organ). TARGET CELL A TARGET CELL is a cell that responds to a hormone. Target cells must have receptors for a hormone. - A RECEPTOR is a protein able to bind with a hormone No receptor ————> not a target cell - The number of receptors on a target cell is constantly changing. - 2000-100,000 receptors per cell Proteins, peptides and amines Proteins, peptides and amines are not lipid soluble thus they cannot pass across the plasma membranes of cells. The receptors for such hormones are present on the outside of the cell membrane. Binding of the hormone with its membrane receptor causes the production within the cell of a second messenger compound which then exerts the hormone's actions within the cell. The second messenger The second messenger modifies activities within the cell. These activities could include changing enzyme action, altering protein synthesis (through messenger RNA) or opening or closing membrane channels. Examples of second messengers include: - cyclic adenosine monophosphate (cAMP), -cyclic guanosine monophosphate -(cGMP),inositol triphosphate (IP3) and -calcium ions. Proteins freely circulating dissolved in the blood travel on their own very available effect lasts minutes to an hour (immediate response, short term) Cell membrane Cell membrane Membrane protein Protein actions Steroid/Thyroid Hormones transported bound to special transport proteins when a hormone is bound to a transport protein it is "unavailable" small amount available (dissolved in blood) 99% is stored on transport proteins "in reserve" Steroid/Thyroid Hormones when the 1 % binds to the receptor , the hormone level in the blood drops and another 1% is released, from the transport proteins and then replenished by the secretory glands and cells this increases the time of availability of the hormone - hours to days————> weeks offers a gradual, long term response Endocrine disorder Hyposecretion A- primary hyposecretion B- Secondary hyposecretion Hypersecretion A- primary hypersecretion B- Secondary hypersecretion Hyporesponsiveness Relation between hormones Antagonism (Insulin,Glucagon) Antagonist Synergism (estrogen, progesterone) Permissive action( GH, Epinephrine, cortisol). 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