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Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Some material was previously published.
Mechanisms of Hormonal Regulation
Chapter 17
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The Endocrine System
• Functions Differentiation of reproductive and CNS in fetus Stimulation of growth and development Coordination of the male and female reproductive
system Maintenance of internal environment Adaptation to emergency demands of body
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The Endocrine System
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Hormones
• General characteristics Specific rates and rhythms of secretion
• Diurnal, pulsatile and cyclic, and patterns depending on circulating substances
Operate within feedback systems Affect only target cells with appropriate receptors The liver inactivates hormones, rendering the
hormones more water soluble for renal excretion
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Regulation of Hormone Release
• Hormones are released: In response to an alteration in the cellular environment To maintain a regulated level of certain substances or
other hormones
• Hormones are regulated by chemical, hormonal, or neural factors
• Negative feedback
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Feedback
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Hormone Transport
• Hormones are released into the circulatory system by endocrine glands Water-soluble hormones circulate in free, unbound
forms• Short-acting response• Bind to surface receptors
Lipid-soluble hormones are primarily circulating bound to a carrier
• Rapid and long-lasting response• Bind to plasma membrane
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Cellular Mechanism of Hormone Action
• Target cell
• Up-regulation
• Down-regulation
• Hormone effects Direct effects Permissive effects
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Cellular Mechanism of Hormone Action
• Hormone receptors Located in the plasma membrane or in the intracellular
compartment of the target cell
• Water-soluble hormones High molecular weight Cannot diffuse across the plasma membrane
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Cellular Mechanism of Hormone Action
• Lipid-soluble hormones Easily diffuse across the plasma membrane and
bind to cytosolic or nuclear receptors
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Cellular Mechanism of Hormone Action
• Water-soluble hormones First messenger
• Signal transduction Second-messenger molecules
• Calcium• Cyclic adenosine monophosphate (cAMP)• Cyclic guanosine monophosphate (cGMP)• Tyrosine kinase system
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Cellular Mechanism of Hormone Action
• Lipid-soluble hormones Steroid hormones
• Androgens, estrogens, progestins, glucocorticoids, mineralocorticoids, thyroid hormones, vitamin D, retinoid
Diffuse across the plasma membrane• Bind to cytoplasmic or nuclear receptors
Activate• DNA transcription and translation
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Lipid-Soluble Hormones
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Structure and Function of the Endocrine Glands
• Hypothalamic-pituitary axis Hypothalamus Pituitary gland
• Anterior pituitary (adenohypophysis)– Pars distalis– Pars tuberalis– Intermedia
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Structure and Function of the Endocrine Glands
Pituitary gland • Posterior pituitary (neurohypophysis)
– Median eminence– Pituitary stalk– Pars nervosa
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The Pituitary Gland
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Posterior Pituitary Hormones
• Synthesized with their binding proteins in the supraoptic and paraventricular nuclei of the hypothalamus
• Secreted by the posterior pituitary Antidiuretic hormone (ADH)
• Controls plasma osmolality Oxytocin
• Uterine contractions and milk ejection in lactating women
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Anterior Pituitary Hormones
• Adrenocorticotropic hormone (ACTH)
• Melanocyte-stimulating hormone (MSH)
• Somatotropic hormone
• Prolactin
• Thyroid-stimulating hormone
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Anterior Pituitary Hormones
• Luteinizing hormone
• Follicle-stimulating hormone
• Beta-lipotropin
• Beta-endorphins
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Anterior Pituitary Hormones
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Thyroid and Parathyroid Glands
• Thyroid gland Two lobes lie on either side of the trachea Isthmus Follicles (follicle cells surrounding colloid) Parafollicular cells (C cells)
• Secrete calcitonin Regulation of thyroid hormone secretion
• Thyrotropin-releasing hormone and thyroid-stimulating hormone
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Thyroid and Parathyroid Glands
• Thyroid hormones 90% T4 and 10% T3
Bound to thyroxine-binding globulin, thyroxine-binding prealbumin, or albumin
Affect growth and maturation of tissues, cell metabolism, heat production, and oxygen consumption
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Thyroid and Parathyroid Glands
• Parathyroid glands Small glands located behind the upper and lower
poles of the thyroid gland Produce parathyroid hormone
• Regulator of serum calcium• Antagonist of calcitonin
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Thyroid and Parathyroid Glands
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Endocrine Pancreas
• The pancreas is both an endocrine and an exocrine gland
• Houses the islets of Langerhans Secretion of glucagon and insulin Cells
• Alpha—glucagon • Beta—insulin • Delta—somatostatin and gastrin• F cells—pancreatic polypeptide
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Endocrine Pancreas
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Endocrine Pancreas
• Insulin Synthesized from proinsulin Secretion is promoted by increased blood glucose
levels Facilitates the rate of glucose uptake into the cells
of the body Anabolic hormone
• Synthesis of proteins, lipids, and nucleic acids Amylin
• Peptide hormone co-secreted with insulin
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Endocrine Pancreas
• Glucagon Secretion is promoted by decreased blood glucose
levels Stimulates glycogenolysis, gluconeogenesis, and
lipolysis
• Somatostatin Possible involvement in regulating alpha- and beta-
cell secretions
• Gastrin and pancreatic peptides
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Adrenal Glands
• Adrenal cortex 80% of an adrenal gland’s total weight Zona glomerulosa Zona fasciculata Zona reticularis
• Adrenal medulla Innervated by the sympathetic and parasympathetic
nervous systems
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Adrenal Glands
• Adrenal cortex Stimulated by adrenocorticotropic hormone (ACTH) Glucocorticoid hormones
• Direct effects on carbohydrate metabolism• Anti-inflammatory and growth-suppressing effects• Influence awareness and sleep habits• Most potent naturally occurring glucocorticoid is
cortisol
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Adrenal Glands
• Adrenal cortex Mineralocorticoid hormones
• Affect ion transport by epithelial cells– Increase the activity of the sodium pump of the epithelial cells– Cause sodium retention and potassium and hydrogen loss
• Most potent naturally occurring mineralocorticoid is aldosterone
– Regulated by the renin-angiotensin system
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Adrenal Glands
• Adrenal cortex Adrenal estrogens and androgens
• Estrogen secretion by the adrenal cortex is minimal• The adrenal cortex secretes weak androgens
– Androgens are converted by peripheral tissues to stronger androgens such as testosterone
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Adrenal Glands
• Adrenal medulla Chromaffin cells (pheochromocytes)
• Chromaffin cells secrete the catecholamines epinephrine (majority) and norepinephrine
Release of catecholamines has been characterized as a “fight or flight” response
Catecholamines promote hyperglycemia
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Neuroendocrine Response to Stress
• The endocrine system reacts with the nervous system to respond to stressors
• The stress response also involves the immune system
• Influenced by corticotropin-releasing hormone from the hypothalamus
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Tests of Endocrine Function
• Radioimmunoassay (RIA)
• Enzyme-linked immunosorbent assay (ELISA)
• Bioassay
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Aging and the Endocrine System
• Thyroid gland Glandular atrophy, fibrosis, nodularity, and increased
inflammatory infiltrates
• Parathyroid glands Related to alterations in calcium balance
• Inadequate intake, malabsorption, or renal changes
• Adrenal glands Decreased clearance of cortisol