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• Two systems coordinate communication throughout the body: the endocrine system and the nervous system • The endocrine system secretes hormones that coordinate slower but longer-acting responses including reproduction, development, energy metabolism, growth, and behavior • The nervous system conveys high-speed electrical signals along specialized cells called neurons; these signals regulate other cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings In multicellular animals, nerve impulses provide electric signals; hormones provide chemical signals. Hormones are secreted by cells, diffuse into the extracellular fluid, and often are distributed by the circulatory system. Hormones reach all parts of the body, but only target cells are equipped to respond Hormones work much more slowly than nerve impulse transmission and are not useful for controlling rapid actions. Hormones coordinate longer-term developmental processes such as reproductive cycles. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 45.1: Hormones and other signaling molecules bind to target receptors, triggering specific response pathways • Signals can be of many kinds light, sound, smell, touch etc. many of which are due to chemicals. • Chemical signals bind to receptor proteins on target cells • Only target cells with the appropriate receptor respond to the signal Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Types of Secreted Signaling Molecules • Secreted chemical signals include – Hormones – Local regulators – Neurotransmitters – Neurohormones – Pheromones Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Local Regulators • Local regulators are chemical signals that travel over short distances by diffusion • Local regulators help regulate blood pressure, nervous system function, and reproduction • Local regulators are divided into two types – Paracrine signals act on cells near the secreting cell – Autocrine signals act on the secreting cell itself Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Signaling by Local Regulators • In paracrine signaling, nonhormonal chemical signals called local regulators elicit responses in nearby target cells • Types of local regulators: – Cytokines and growth factors – Nitric oxide (NO) – Prostaglandins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Long distance signaling involves the chemical being taken by circulation to the target tissues. Usually hormones are released from a special cell or organ called an endocrine cell/organ, travel through the bloodstream, and interact with the receptor or a target cell to cause a physiological response. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-2 Blood vessel Response (a) Endocrine signaling Response (b) Paracrine signaling Response (c) Autocrine signaling Synapse Neuron Response (d) Synaptic signaling Neurosecretory cell Blood vessel (e) Neuroendocrine signaling Response Chemical Classes of Hormones • Three major classes of molecules function as hormones in vertebrates: – Polypeptides (proteins and peptides) – Amines derived from amino acids – Steroid hormones Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Lipid-soluble hormones (steroid hormones) pass easily through cell membranes, while water-soluble hormones (polypeptides and amines) do not • The solubility of a hormone correlates with the location of receptors inside or on the surface of target cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-3 Water-soluble Lipid-soluble 0.8 nm Polypeptide: Insulin Steroid: Cortisol Amine: Epinephrine Amine: Thyroxine Cellular Response Pathways • Water and lipid soluble hormones differ in their paths through a body • Water-soluble hormones are secreted by exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors • Lipid-soluble hormones diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and diffuse through the membrane of target cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Signaling by any of these hormones involves three key events: – Reception – Signal transduction – Response Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-5-1 Fat-soluble hormone Watersoluble hormone Signal receptor Transport protein TARGET CELL (a) Signal receptor NUCLEUS (b) Fig. 45-5-2 Fat-soluble hormone Watersoluble hormone Transport protein Signal receptor TARGET CELL Cytoplasmic response OR Signal receptor Gene regulation Cytoplasmic response (a) NUCLEUS (b) Gene regulation Pathway for Water-Soluble Hormones • Binding of a hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoplasm, enzyme activation, or a change in gene expression Animation: Water-Soluble Hormone Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Multiple Effects of Hormones • The same hormone may have different effects on target cells that have – Different receptors for the hormone – Different signal transduction pathways – Different proteins for carrying out the response • A hormone can also have different effects in different species Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-8-1 Same receptors but different intracellular proteins (not shown) Epinephrine Epinephrine receptor receptor Glycogen deposits Glycogen breaks down and glucose is released. (a) Liver cell Vessel dilates. (b) Skeletal muscle blood vessel Fig. 45-8-2 Same receptors but different intracellular proteins (not shown) Different receptors Epinephrine Epinephrine Epinephrine receptor receptor receptor Glycogen deposits Glycogen breaks down and glucose is released. (a) Liver cell Vessel dilates. (b) Skeletal muscle blood vessel Vessel constricts. (c) Intestinal blood vessel Hormones • Endocrine signals (hormones) are secreted into extracellular fluids and travel via the bloodstream • Endocrine glands are ductless and secrete hormones directly into surrounding fluid • Hormones mediate responses to environmental stimuli and regulate growth, development, and reproduction Exocrine glands have ducts and secrete substances onto body surfaces or into body cavities (for example, tear ducts) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-10 Major endocrine glands: Hypothalamus Pineal gland Pituitary gland Thyroid gland Parathyroid glands Organs containing endocrine cells: Thymus Heart Adrenal glands Testes Liver Stomach Pancreas Kidney Kidney Small intestine Ovaries Concept 45.2: Negative feedback and antagonistic hormone pairs are common features of the endocrine system • Hormones are assembled into regulatory pathways • A negative feedback loop inhibits a response by reducing the initial stimulus • Negative feedback regulates many hormonal pathways involved in homeostasis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Coordination of Endocrine and Nervous Systems in Invertebrates • In insects, molting and development are controlled by a combination of hormones: – A brain hormone stimulates release of ecdysone from the prothoracic glands – Juvenile hormone promotes retention of larval characteristics – Ecdysone promotes molting (in the presence of juvenile hormone) and development (in the absence of juvenile hormone) of adult characteristics Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-13-3 Brain Neurosecretory cells Corpus cardiacum PTTH Corpus allatum Low JH Prothoracic gland Ecdysone EARLY LARVA Juvenile hormone (JH) LATER LARVA PUPA ADULT Coordination of Endocrine and Nervous Systems in Vertebrates • The hypothalamus receives information from the nervous system and initiates responses through the endocrine system • Attached to the hypothalamus is the pituitary gland composed of the posterior pituitary and anterior pituitary The pituitary gland of mammals is a link between the nervous system and many endocrine glands and plays a crucial role in the endocrine system. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-14 Cerebrum Pineal gland Thalamus Cerebellum Pituitary gland Hypothalamus Spinal cord Hypothalamus Posterior pituitary Anterior pituitary • The posterior pituitary stores and secretes hormones that are made in the hypothalamus:these hormones are called neuroendocrine hormones • The anterior pituitary makes and releases hormones under regulation of the hypothalamus Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-15 Hypothalamus Neurosecretory cells of the hypothalamus Axon Posterior pituitary Anterior pituitary HORMONE ADH Oxytocin TARGET Kidney tubules Mammary glands, uterine muscles ADH The function of antidiuretic hormone (ADH) is to increase water conservation by the kidney. If there is a high level of ADH secretion, the kidneys resorb water. If there is a low level of ADH secretion, the kidneys release water in dilute urine. ADH release by the posterior pituitary increases if blood pressure falls or blood becomes too salty. ADH causes peripheral blood vessel constriction to help elevate blood pressure and is also called vasopressin. • Oxytocin induces uterine contractions and the release of milk • Suckling sends a message to the hypothalamus via the nervous system to release oxytocin, which further stimulates the milk glands • This is an example of positive feedback, where the stimulus leads to an even greater response Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-17 Tropic effects only: FSH LH TSH ACTH Neurosecretory cells of the hypothalamus Nontropic effects only: Prolactin MSH Nontropic and tropic effects: GH Hypothalamic releasing and inhibiting hormones Portal vessels Endocrine cells of the anterior pituitary Posterior pituitary Pituitary hormones HORMONE FSH and LH TSH ACTH Prolactin MSH GH TARGET Testes or ovaries Thyroid Adrenal cortex Mammary glands Melanocytes Liver, bones, other tissues Tropic Hormones • A tropic hormone regulates the function of endocrine cells or glands • The four strictly tropic hormones are – Thyroid-stimulating hormone (TSH) – Follicle-stimulating hormone (FSH) – Luteinizing hormone (LH) – Adrenocorticotropic hormone (ACTH) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Nontropic Hormones • Nontropic hormones target nonendocrine tissues • Nontropic hormones produced by the anterior pituitary are – Prolactin (PRL) – Melanocyte-stimulating hormone (MSH) • Prolactin stimulates lactation in mammals but has diverse effects in different vertebrates • MSH influences skin pigmentation in some vertebrates and fat metabolism in mammals Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Growth Hormone • Growth hormone (GH) is secreted by the anterior pituitary gland and has tropic and nontropic actions • It promotes growth directly and has diverse metabolic effects • It stimulates production of growth factors • An excess of GH can cause gigantism, while a lack of GH can cause dwarfism Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 42.6 Effects of Excess Growth Hormone Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Releasing and Release inhibiting Neurohormones of the Hypothalumus Anterior Pituitary Hormones • Hormone production in the anterior pituitary is controlled by releasing and inhibiting hormones from the hypothalamus • For example, the production of thyrotropin releasing hormone (TRH) in the hypothalamus stimulates secretion of the thyroid stimulating hormone (TSH) from the anterior pituitary Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hormone Cascade Pathways • A hormone can stimulate the release of a series of other hormones, the last of which activates a nonendocrine target cell; this is called a hormone cascade pathway • The release of thyroid hormone results from a hormone cascade pathway involving the hypothalamus, anterior pituitary, and thyroid gland • Hormone cascade pathways are usually regulated by negative feedback Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-18-3 Pathway Example Stimulus Cold Sensory neuron – Hypothalamus secretes thyrotropin-releasing hormone (TRH ) Neurosecretory cell Blood vessel – Negative feedback Anterior pituitary secretes thyroid-stimulating hormone (TSH or thyrotropin ) Thyroid gland secretes thyroid hormone (T3 and T4 ) Target cells Response Body tissues Increased cellular metabolism Thyroid Hormone: Control of Metabolism and Development • The thyroid gland consists of two lobes on the ventral surface of the trachea • It produces two iodine-containing hormones: triiodothyronine (T3) and thyroxine (T4) Two forms of thyroxine, T3 and T4, are made from tyrosine. T3 (triiodothyronine) has three iodine atoms. T4 has four iodine atoms. More T4 is produced in thyroid, but it can be converted to T3 by an enzyme in the blood. T3 is the more active form of the hormone. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Thyroxine has many roles in regulating metabolism. It stimulates the transcription of many genes in nearly all cells in the body. These include genes for enzymes of energy pathways, transport proteins, and structural proteins. It elevates metabolic rates in most cells and tissues. It promotes the use of carbohydrates over fats for fuel. It promotes amino acid uptake and protein synthesis and so is critical for metabolism, growth and development. Insufficient thyroxine may result in cretinism. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Hyperthyroidism, excessive secretion of thyroid hormones, causes high body temperature, weight loss, irritability, and high blood pressure • Hypothyroidism, low secretion of thyroid hormones, causes weight gain, lethargy, and intolerance to cold Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • A goiter is an enlarged thyroid gland associated with either very low (hypothyroidism) or very high (hyperthyroidism) levels of thyroxine. Hyperthyroid goiter results when the negative feedback mechanism fails even though blood levels of thyroxine are high. A common cause is an autoimmune disease in which an antibody to the TSH receptor is produced. This antibody binds the TSH receptor, causing the thyroid cells to release excess thyroxine. (Graves’ disease) The thyroid remains maximally active and grows larger, causing symptoms associated with high metabolic rates. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hypothalamus Anterior pituitary TSH Thyroid T3 + T4 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hypothyroid goiter results when there is insufficient thyroxine to turn off TSH production. The most common cause is a deficiency of dietary iodine. With high TSH levels, the thyroid gland continues to produce nonfunctional thyroxine and becomes very large. The body symptoms of this condition are low metabolism, cold intolerance, and physical and mental sluggishness. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Parathyroid Hormone and Vitamin D: Control of Blood Calcium • Two antagonistic hormones regulate the homeostasis of calcium (Ca2+) in the blood of mammals – Parathyroid hormone (PTH) is released by the parathyroid glands – Calcitonin is released by the thyroid gland Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems- Calcium regulation Calcium levels in the blood must be regulated within a narrow range. Small changes in blood calcium levels have serious effects. Most calcium in the body is in the bones (99%). About 1% is in the cells, and only 0.1% is in the extracellular fluids. Blood calcium levels are regulated by: Deposition and absorption of bone Excretion of calcium by the kidneys Absorption of calcium from the digestive tract Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hormonal control of calcium homeostasis in mammals Thyroid gland releases calcitonin. Calcitonin Reduces Ca2+ uptake in kidneys Stimulates Ca2+ deposition in bones Blood Ca2+ level declines to set point STIMULUS: Rising blood Ca2+ level Homeostasis: Blood Ca2+ level (about 10 mg/100 mL) STIMULUS: Falling blood Ca2+ level Blood Ca2+ level rises to set point Stimulates Ca2+ release from bones Parathyroid gland PTH Increases Ca2+ uptake in intestines Active vitamin D Stimulates Ca2+ uptake in kidneys Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Calcitonin, released by the thyroid gland, acts to lower calcium levels in the blood. Bone is constantly remodeled by absorption of old bone and production of new bone. Osteoclasts break down bone and release calcium. Osteoblasts use circulating calcium to build new bone. Calcitonin decreases osteoclast activity and stimulates the osteoblasts to take up calcium for bone growth. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Blood calcium decrease triggers release of parathyroid hormone (PTH), from the parathyroid glands which are embedded in the posterior surface of the thyroid gland. This in turn causes the osteoclasts to dissolve bone and release calcium. Parathyroid hormone also promotes calcium resorption by the kidney to prevent loss in the urine. It also promotes vitamin D activation, which stimulates the gut to absorb calcium from food. Parathyroid hormone and calcitonin act antagonistically to regulate blood calcium levels. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems In the kidneys, vitamin D acts with PTH to decrease calcium loss in urine. In bone vitamin D acts like PTH to stimulate bone turnover and the liberation of calcium The overall action of vitamin D is to raise blood calcium levels, which promotes bone deposition. Vitamin D also acts in a negative feedback loop to inhibit transcription of the PTH gene in the parathyroid glands. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Bone minerals have both calcium and phosphate. When PTH stimulates the release of calcium from bone, phosphate is also released. Normal levels of calcium and phosphate in the blood are close to the concentration that could cause them to precipitate as calcium phosphate salts. Calcium phosphate salts are involved in the formation of kidney stones and hardening of artery walls. PTH acts on the kidneys to increase the elimination of phosphate to reduce the possibility of calcium phosphate salt precipitation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems –Glucose Regulation Insulin and glucagon are antagonistic hormones that help maintain glucose homeostasis Insulin is produced in the pancreas in cell clusters called islets of Langerhans. Several cell types have been identified in the islets: Beta () cells produce and secrete insulin. Alpha () cells produce and secrete glucagon (antagonist of insulin). Delta (d) cells produce somatostatin. The remainder of the pancreas acts as an exocrine gland with digestive functions. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems After a meal, blood glucose levels rise and stimulate the cells to release insulin. Insulin stimulates cells to use glucose and to convert it to glycogen and fat. When blood glucose levels fall, the pancreas stops releasing insulin, and cells switch to using glycogen and fat for energy. If blood glucose falls too low, the cells release glucagon which stimulates the liver to convert glycogen back to glucose. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Maintenance of glucose homeostasis Body cells take up more glucose. Insulin Beta cells of pancreas are stimulated to release insulin into the blood. Liver takes up glucose and stores it as glycogen. STIMULUS: Rising blood glucose level (for instance, after eating a carbohydraterich meal) Blood glucose level declines to set point; stimulus for insulin release diminishes. Homeostasis: Blood glucose level (about 90 mg/100 mL) Blood glucose level rises to set point; stimulus for glucagon release diminishes. STIMULUS: Dropping blood glucose level (for instance, after skipping a meal) Alpha cells of pancreas are stimulated to release glucagon into the blood. Liver breaks down glycogen and releases glucose into blood. Glucagon Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Target Tissues for Insulin and Glucagon • Insulin reduces blood glucose levels by – Promoting the cellular uptake of glucose – Slowing glycogen breakdown in the liver – Promoting fat storage Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Glucagon increases blood glucose levels by – Stimulating conversion of glycogen to glucose in the liver – Stimulating breakdown of fat and protein into glucose Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Diabetes mellitus is a disease caused by a lack of the protein hormone insulin (Type I) or a lack of insulin receptors on target cells (Type II). Insulin binds to receptors on the cell membrane and allows glucose uptake. Without insulin or the receptors, glucose accumulates in the blood until it is lost in urine. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems High glucose levels in the blood cause water to move from the cells into the blood by osmosis. The kidneys then increase urine output to get rid of the fluid excess. Cells not taking up glucose use fat and protein for fuel, resulting in the body’s wasting away and tissue and organ damage. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Adrenal Hormones: Response to Stress • The adrenal glands are adjacent to the kidneys • Each adrenal gland actually consists of two glands: the adrenal medulla (inner portion) and adrenal cortex (outer portion) The medulla produces epinephrine and norepinephrine. The medulla develops from the nervous system and remains under its control. The cortex is under hormonal control, mainly by adrenocorticotropin (ACTH) from the anterior pituitary. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Catecholamines from the Adrenal Medulla • The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine (noradrenaline) • These hormones are members of a class of compounds called catecholamines • They are secreted in response to stressactivated impulses from the nervous system • They mediate various fight-or-flight responses Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Epinephrine and norepinephrine – Trigger the release of glucose and fatty acids into the blood – Increase oxygen delivery to body cells – Direct blood toward heart, brain, and skeletal muscles, and away from skin, digestive system, and kidneys • The release of epinephrine and norepinephrine occurs in response to nerve signals from the hypothalamus Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Epinephrine and norepinephrine are amine hormones. They bind to two types of receptors in target cells: adrenergic and -adrenergic Norepinephrine acts mostly on the alpha type, so drugs called beta blockers, which inactivate only adrenergic receptors, can be used to reduce fight-orflight responses to epinephrine. The beta blockers leave the alpha sites open to norepinephrine and its regulatory functions. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-21 Stress Spinal cord Nerve signals Hypothalamus Releasing hormone Nerve cell Anterior pituitary Blood vessel ACTH Adrenal medulla Adrenal cortex Adrenal gland Kidney (a) Short-term stress response Effects of epinephrine and norepinephrine: 1. Glycogen broken down to glucose; increased blood glucose 2. Increased blood pressure 3. Increased breathing rate 4. Increased metabolic rate 5. Change in blood flow patterns, leading to increased alertness and decreased digestive, excretory, and reproductive system activity (b) Long-term stress response Effects of mineralocorticoids: Effects of glucocorticoids: 1. Retention of sodium 1. Proteins and fats broken down ions and water by and converted to glucose, leading kidneys to increased blood glucose 2. Increased blood volume and blood pressure Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 2. Possible suppression of immune system Fig. 45-21b Adrenal medulla Adrenal gland Kidney (a) Short-term stress response Effects of epinephrine and norepinephrine: 1. Glycogen broken down to glucose; increased blood glucose 2. Increased blood pressure 3. Increased breathing rate 4. Increased metabolic rate 5. Change in blood flow patterns, leading to increased alertness and decreased digestive, excretory, and reproductive system activity Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 45-21c Adrenal cortex Adrenal gland Kidney (b) Long-term stress response Effects of mineralocorticoids: Effects of glucocorticoids: 1. Retention of sodium ions and water by kidneys 1. Proteins and fats broken down and converted to glucose, leading to increased blood glucose 2. Increased blood volume and blood pressure 2. Possible suppression of immune system Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Adrenal cortex cells use cholesterol to produce three classes of steroid hormones called corticosteroids: Glucocorticoids influence blood glucose concentrations and other aspects of fuel molecule metabolism. Mineralocorticoids influence extracellular ionic balance. Sex steroids stimulate sexual development and reproductive activity. These are secreted in only minimal amounts by the adrenal cortex. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 42.11 The Corticosteroid Hormones are Built from Cholesterol Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems The main mineralocorticoid, aldosterone, stimulates the kidney to conserve sodium and excrete potassium. The main glucocorticoid, cortisol, mediates the body’s response to stress. The fight-or-flight response ensures that muscles have adequate oxygen and glucose for immediate response. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Shortly after a frightening stimulus, blood cortisol rises. Cortisol stimulates cells that are not critical to the emergency to decrease their use of glucose. It also blocks the immune system reactions, which temporarily are less critical. Cortisol can therefore be used to reduce inflammation and allergy. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Cortisol release is controlled by ACTH from the anterior pituitary which, in turn is controlled by adrenocorticotropin-releasing hormone from the hypothalamus. The cortisol response is much slower than the epinephrine response. Turning off the cortisol response is also critical to avoid the consequences of long-term stress. Cortisol has negative feedback effect on brain cells that decreases the release of adrenocorticotropinreleasing hormone. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems- Reproduction The gonads (testes and ovaries) produce steroid hormones synthesized from cholesterol. Androgens are male steroids, the dominant one being testosterone. Estrogens and progesterone are female steroids, the dominant estrogen being estradiol. Sex steroids determine whether a fetus develops into a male or female. After birth, sex steroids control maturation of sex organs and secondary sex characteristics such as breasts and facial hair. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Until the seventh week of an embryo’s development, either sex may develop. In mammals, the Y chromosome causes the gonads to start producing androgens in the seven-week-old embryo, and the male reproductive system develops. If androgens are not released, the female reproductive system develops. In birds, the opposite rules apply: male features are produced unless estrogens are present to trigger female development. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Sex steroid production increases rapidly at puberty, or sexual maturation, in humans. Control of sex steroids (both male and female) is under the anterior pituitary tropic hormones called luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These gonadotropins are controlled by the hypothalamic gonadotropin-releasing hormone (GnRH). Before puberty, the hypothalamus produces low levels of GnRH. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems At puberty GnRH release increases, stimulating gonadotropin production and, hence, sex steroid production. In females, increased LH and FSH at puberty induce the ovaries to begin female sex hormone production to initiate sexually mature body traits. In males, increased LH stimulates cells in the testes to make androgens which induce changes associated with adolescence. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Synthetic androgens (anabolic steroids) can exaggerate body strength and muscle development. Negative side effects in females include more masculine body features, such as shrinking the uterus and causing an irregular menstrual cycle. In males, the negative side effects include shrinking of the testes, enlarged breasts, and sterility. Continued use of anabolic steroids may increase risk of heart disease, some cancers, kidney damage, and personality disorders. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Melatonin and Biorhythms • The pineal gland, located in the brain, secretes melatonin • Light/dark cycles control release of melatonin • Primary functions of melatonin appear to relate to biological rhythms associated with reproduction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Vertebrate Endocrine Systems Many other body organs, such as the gut and the heart, produce hormones. When blood pressure stretches the heart wall, its cells release atrial natriuretic hormone. This hormone increases sodium ion and water excretion by the kidney, lowering blood pressure and blood volume. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hormone Actions: The Role of Signal Transduction Pathways Hormones are released in very small amounts, yet they cause large and very specific responses in target organs and tissues. Strength of hormone action results from signal transduction cascades that amplify the original signal. Selective action is keyed to appropriate receptors of cells responding to hormones. Specific receptors can also be linked to different response mechanisms, as is the case with receptors for epinephrine and norepinephrine. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hormone Actions: The Role of Signal Transduction Pathways The abundance of hormone receptors can be under feedback control. Continuous high levels of a hormone can decrease the number of its receptors, a process called downregulation. High levels of insulin in type II diabetes mellitus result in a loss of insulin receptors. Upregulation of receptors is a positive feedback mechanism and is less common than downregulation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 42.16 Dose-Response Curves Quantify Response to a Hormone Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hormone Actions: The Role of Signal Transduction Pathways Anything that changes the responsiveness of a system to a hormone is reflected in the dose– response curve. These may include: The number of receptors in the responding cells Changes in signaling pathways Changes in rate-limiting enzymes The availability of substrates or cofactors Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hormone Actions: The Role of Signal Transduction Pathways Hormone responses may also vary in their time course. The length of time for the concentration of a hormone to drop to one-half of the maximum is called its half-life. Epinephrine’s half-life in the blood is only 1–3 minutes. Cortisol or thyroxine half-lives are on the order of days. Half-life is partially determined by degradation and elimination processes. Most hormones are broken down in the liver, removed from the blood by the kidney, and excreted in the urine. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Table 45-1b Table 45-1c Table 45-1d You should now be able to: 1. Distinguish between the following pairs of terms: hormones and local regulators, paracrine and autocrine signals 2. Describe the evidence that steroid hormones have intracellular receptors, while water-soluble hormones have cell-surface receptors 3. Explain how the body regulates general metabolism, Glucose and Calcium homeostatis. 4. How does the body responds to long tern stress? 5. Distinguish between hypothyroid and hyperthyroid goitre Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 1. Explain how the hypothalamus and the pituitary glands interact and how they coordinate the endocrine system 2. Explain the role of tropic hormones in coordinating endocrine signaling throughout the body 3. List and describe the functions of hormones released by the following: anterior and posterior pituitary lobes, thyroid glands, parathyroid glands, adrenal medulla, adrenal cortex, gonads, pineal gland. 4. Explain how these hormones are regulated. 5. Explain the general mechanisms of hormone action. 6. Distinguish signaling by the nervous system vs. the endocrine system Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings