18_LectureOutline

18_LectureOutline - Introduction to the Endocrine System...

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Introduction to the Endocrine System The Endocrine System Regulates long-term processes Growth Development Reproduction Uses chemical messengers to relay information and instructions between cells Homeostasis and Intercellular Communication Direct Communication Exchange of ions and molecules between adjacent cells across gap junctions Occurs between two cells of same type Highly specialized and relatively rare Paracrine Communication Uses chemical signals to transfer information from cell to cell within single tissue Most common form of intercellular communication Endocrine Communication Endocrine cells release chemicals ( hormones ) into bloodstream Alters metabolic activities of many tissues and organs simultaneously Target Cells Are specific cells that possess receptors needed to bind and “read” hormonal messages Hormones Stimulate synthesis of enzymes or structural proteins Increase or decrease rate of synthesis Turn existing enzyme or membrane channel “on” or “off” Hormones Can be divided into three groups Amino acid derivatives
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Peptide hormones Lipid derivatives Circulate freely or bound to transport proteins Secretion and Distribution of Hormones Free Hormones Remain functional for less than 1 hour Diffuse out of bloodstream: bind to receptors on target cells Are broken down and absorbed: by cells of liver or kidney Are broken down by enzymes: in plasma or interstitial fluids Thyroid and Steroid Hormones Remain in circulation much longer Enter bloodstream More than 99% become attached to special transport proteins Bloodstream contains substantial reserve of bound hormones Mechanisms of Hormone Action Hormone Receptor Is a protein molecule to which a particular molecule binds strongly Responds to several different hormones Different tissues have different combinations of receptors Presence or absence of specific receptor determines hormonal sensitivity Hormones and Plasma Membrane Receptors Catecholamines and peptide hormones Are not lipid soluble Unable to penetrate plasma membrane Bind to receptor proteins at outer surface of plasma membrane (extracellular receptors) Bind to receptors in plasma membrane Cannot have direct effect on activities inside target cell Use intracellular intermediary to exert effects First messenger: leads to second messenger may act as enzyme activator, inhibitor, or cofactor results in change in rates of metabolic reactions Important Second Messengers
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Cyclic-AMP (cAMP) Derivative of ATP Cyclic-GMP (cGMP) Derivative of GTP Calcium ions The Process of Amplification Is the binding of a small number of hormone molecules to membrane receptors Leads to thousands of second messengers in cell Magnifies effect of hormone on target cell Down-regulation
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18_LectureOutline - Introduction to the Endocrine System...

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