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Handout8newest - HANDOUT 8 HORMONES A. General Properties...

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HANDOUT 8 HORMONES A. General Properties of Hormones: 1. Secretion : Hormones can be secreted from the classic endocrine glands (such as hypothalamus, pituitary, pineal, thyroid and parathyroid, thymus, adrenals, pancreas, ovaries/testis) or diffuse endocrine systems (some particular cells of adipose tissue, skin, stomach and small intestine, liver, kidney, and heart) or neurons (these hormones are called neurohormones), or cells of the immune system (these hormones are called cytokines) 2. Secretion pathway : Majority of hormones are secreted into the blood stream, but some of them are secreted into the external environment ( these hormones are called ectohormone) Example of ectohormone is pheromone: ant releases trail pheromone to mark the route to food source. 3. Transportation: traditional hormones have to travel a long distance to reach and affect their target. On the other hand, candidate hormones sometimes referred to as “factor” act locally on proximal targets. If these factors affect the same cell type as the cell secreting them, these factors will be called autocrines. If they affect different cell types than the type secreting them, these factors will be called paracrines. Some hormones can be traditional hormones if secreted from one location, and can also be candidate hormones/factors if secreted from a different location. Example: cholecystokinin (CCK) 4. Concentration: hormones only require a very low concentration to bring about their effects. Molecules that require high concentration to exert their effects will not be qualified as hormones. 5. Receptors: all hormones need to bind to their receptors to affect the cells. Also, one hormone can have different effects if binding to receptors at different locations. Example: insulin: in muscle and adipose tissues: alter glucose transport proteins and enzymes for glucose metabolism; in liver and brain: only change glucose metabolism (remember: in brain glucose metabolism completely depends on insulin) 6. Duration: hormones have limited duration and short half-life (the amount of time required to reduce hormone concentration by one half). If not body will not be able to maintain homeostasis, and may result in death. Termination of hormone actions can occur by: - reducing hormone secretion - degradation of hormones in blood stream by enzymes located in liver and kidneys. Hormones will be broken down into inactive metabolites which be excreted in the bile or urine - terminate activity of hormones already bound to the receptors: by enzymes found in the plasma, or by endocytosis of hormones into the cell and digesting them in cell’s lysosomes. B. Classifications of hormones: The popular scheme to classify hormones is based on chemical classes:
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3 chemical classes: -peptide/protein hormones: composed of linked amino acids -steroid hormones: derived from cholesterols ( remember: cholesterol is a lipid found in the cell membranes of all tissues, important for the protection of nerves, make cell
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Handout8newest - HANDOUT 8 HORMONES A. General Properties...

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