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Physiology 1021

Endocrinology Specificity of Receptors - only cells with a receptor that recognizes the chemical messenger will respond - receptors are proteins - given receptor only binds to one type or a limited number of molecules with a similar 3-D structure Location of Receptors - receptor in cytosol - receptor in nucleus - membrane bound receptors o ion channels o function as an enzyme, insulin o activate a cytoplasmic protein, stimulates reaction o activate a membrane-bound G protein, GTP instead of ATP turns on certain enzymes in the cell - lipophillic and lipophobic signal molecules Terminology - regulation of receptors o up regulation- increase in number of receptors in response to low exposure to signal o down regulation- decrease in the number of receptors in response to high exposure to signal - Messenger interactions o Synergism- combination of two messengers yields a result that is greater than the sum of each messengers action o Permissiveness- one messenger cannot fully be active unless accompanied by another o Antagonism- blocks the actual signal, a messenger that does not produce a response Local Cell Communication - neurotransmitter -> Receptor -> Autocrine( cell secretes signal and has receptor for that signal) and Paracrine (signal diffuses to neighbouring cells) signals Distant Cell Communication - Endocrine- chemical secreted into blood stream, effects distant cells Function of the Endocrine System 1) Regulates homeostasis 2) With the nervous system, integrates, coordinates and controls functions of other organs and systems Major Components of the Human Endocrine System - Hypothalamus, Pituitary gland, Thyroid and Parathyroid gland, Adrenal gland, Pancreas, Ovaries, Testes Hormones Regulate Various Processes - salt and water balance - growth and development - generation, use and storage of metabolic energy - reproduction - Hormones carry out these actions by altering the activity of one or more proteins in the target cell. Proteins have structural or enzymatic functions Chemical Classes of Hormones 1) Amines tyrosine, T4 T3, dopamine, norepinephrine, epinephrine 2) Peptide Hormones- peptides, polypeptides, Proteins, Glycoproteins 3) Steroid Hormones- cholesterol, pregnenolone, progesterone, DHEA, Aldosterone, cortisol, testosterone, estradiol - 5 families of steroids- glucocorticoid ( sugar regulation), androgen (LH) mineralocorticoid (mineral regulation) progestin and estrogen Hormone Properties Catecholamines, Proteins Steroid, Thyroid Hormones - circulate in blood in free state, hydrophillic - circulate bound to plasma, hydrophobic - cannot diffuse through cell membrane - can diffuse easily through cell membrane - receptors in outer cell membrane surface - receptors inside target cells - rapid action - slow action - removed from circulation quickly - removed more slowly from circulation Hormone Transport Lipophobic hormones are carried in the blood by transport proteins: - facilitate solubility, protect from catabolism and excretion - transport proteins vary in specificity - only free hormone enters/ leaves the circulation Hormone Inactivation - may be degraded by enzymes in the liver or enzymes in the blood - liver binds some lipophillic hormones to water soluble acids which are secreted to bile or excreted by kidneys - small hormones are excreted directly by kidneys - target cell may engulf the hormone-receptor complex by endocytosis - in some cases, metabolism increases the hormones activity Endocrine Disorders Primary Disorders - hyper/hyposecretion- excessive/ deifficiency secretion of a hormone resulting in exaggerated/ diminished effects - Hyper/ Hypo reponsiveness- alteration in the number or structure, alteration in post-receptor events or alteration in the activation/ inactivation of hormone Secondary/ Tertiary Disorders - disruption in the system that regulates the hormones secretion, or, disruption in the system that regulates the hormones regulatory system Negative Feedback Regulation - T3 and T4 hypothalamus TSH Pituitary TSH- Thyroid The Pituitary Gland Location of the Pituitary Gland Below hypothalamus, near optic chiasm and infundibulum, anterior made of gland cells, posterior made of neural cells Posterior Pituitary Gland (Neurohypophsis) - paropatricula nucleus - supra optic nucleus - these send signals to post. Pit. Gland, consist of nerve terminals, made by neurons, travel in blood stream - vasopressin( ADH) oxytocin ADH - control system - ADH is released by high plasma osmolarity, low blood pressure Oxytocin - reflex system - see 21 Anterior Pituitary Gland - ACTH - TSH - GH - LH - FSH - Prolactin - All are proteins, all except prolactin control the secretion of another hormone. All have a growth promoting effect on target tissue Regulation of the Anterior Pituitary Gland - Neurosecretory neurons in hypothalamus - Hypothalamic-hypophyseal portal system Thyroid Gland Anatomy - not needed for life, but four parathyroid glands are - on trachea, below larynx - Calcium regulation Iodinated Compounds - Tyrosine, MIT, DIT, T3 and T4 - Thyroid hormone is T3 and T4, T4 usually secreted more, T3 more potent Synthesis, Storage and Secretion of T3 and T4 - See 28 Regulation of TH secretion - TSh has a trophic effect on the thyroid gland and stimulates most steps in hormone synthesis Effects of Thyroid Hormone - very slow onset, long lasting activity, acts in cells of most tissues - permissive to catcholamines - crucial for normal neural development in utero; required for normal alertness, responsivenss, cognition and emotional tone at all ages - most important regulator of metabolic rate: increase Basal metabolic Rate (BMR)- oxygen consumption and energy expenditure under resting conditions Thyroid Hormone increases BMR - intracellular effects in most cells of the body: o increase Sodium/ Potassium ATPase o increase enzymes o increase substrate use - Whole Body Effects o Increase thermogenesis
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