Hormones and the Endocrine System Chapter 45 Hormones and the Endocrine System

YOU MUST KNOW Two ways hormones affect target organs. The secretion, target, action, and regulation of at least three hormones. An illustration of both positive and negative feedback in the regulation of homeostasis by hormones.

The Body’s Long-Distance Regulators The endocrine system and the nervous system act individually and together in regulating an animal’s physiology. The endocrine system of an animal is the sum of all its hormone-secreting cells and tissues. Endocrine glands are ductless and secrete hormones directly into body fluids. Hormones are chemical signals that cause a response in target cells. Positive and negative feedback regulates most endocrine secretion. 3

Control Pathways and Feedback Loops Example Stimulus Low blood glucose Receptor protein Pancreas secretes glucagon ( ) Endocrine cell Blood vessel Liver Target effectors Response Suckling Sensory neuron Hypothalamus/ posterior pituitary Neurosecretory Posterior pituitary secretes oxytocin ( ) Smooth muscle in breast Milk release Hypothalamic neurohormone released in response to neural and hormonal signals Hypothalamus secretes prolactin- releasing hormone ( ) Anterior pituitary prolactin ( ) Mammary glands Milk production (c) Simple neuroendocrine pathway (b) Simple neurohormone pathway (a) Simple endocrine pathway Glycogen breakdown, glucose release into blood Figure 45.2a–c There are three types of hormonal control pathways: simple endocrine, simple neurohormone, and simple neuroendocrine. In each pathway, a receptor/sensor (blue) detects a change in some internal or external variable (the stimulus) and informs the control center (gold). The control center sends out an efferent signal, either a hormone (red circles) or neurohormone (red squares). An endocrine cell carries out BOTH the receptor and control center functions.

Mechanisms of Hormone Action Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses. There are basically two mechanisms of hormone action: Cell Surface Receptors: bind the hormone, and a signal transduction pathway is triggered, eliciting a response to the signal. Ex: The binding of epinephrine to liver cells causes a cascade that leads to the conversion of glycogen to glucose. Intracellular Receptors: bound by hormones that are lipid-soluble. The receptor then acts as a transcription factor, causing a change in gene expression. Ex: Testosterone and estrogen enter the nuclei of target cells, bind the DNA, and stimulate transcription of certain genes. Reception (the target cell’s detection of a signal coming from outside the cell – signal is detected when it binds to a cellular protein on the cell’s surface). Signal transduction (binding changes receptor protein in some way – initiating transduction which converts the signal to a form that can bring about a cellular response). Response (transduced signal triggers a specific cellular response).

Varying Degrees of Hormonal Effect The hormone epinephrine has multiple effects in mediating the body’s response to short-term stress Different receptors different cell responses Epinephrine a receptor b receptor Vessel constricts dilates Glycogen breaks down and glucose is released from cell (a) Intestinal blood vessel (b) Skeletal muscle blood vessel (c) Liver cell Different intracellular proteins Glycogen deposits Note: hormones in the body can affect one tissue, a few tissues, or most of the tissues in the body (as with sex hormones). Or, they may affect other endocrine glands (tropic 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 Figure 45.4a–c

Paracrine Signaling by Local Regulators In a process called paracrine signaling Various types of chemical signals elicit responses in nearby target cells Paracrine signaling involves local regulators – they convey messages between neighboring cells (as opposed to long-distance endocrine signaling by hormones). These can elicit cell responses more quickly than hormones. http://www.sinauer.com/neuroscience4e/animations7.1.html Local regulators have various functions and include Cytokines and growth factors Immune responses and cell differentiation Nitric oxide Relaxes smooth muscle cells which dilates vessels and improves blood flow Prostaglandins Several regulatory functions – including smooth muscle contraction in female uterine helping sperm to reach egg

The Major Human Endocrine Glands Hypothalamus Pineal gland Pituitary gland Thyroid gland Parathyroid glands Adrenal glands Pancreas Ovary (female) Testis (male) Figure 45.7 Hypothalamus Neurosecretory cells of the hypothalamus Axon Anterior pituitary Posterior HORMONE ADH Oxytocin TARGET Kidney tubules Mammary glands, uterine muscles http://bcs.whfreeman.com/thelifewire/content/chp42/4202002.html The hypothalamus and pituitary integrate many functions of the vertebrate endocrine system: The HYPOTHALAMUS receives information from nerves throughout the body and from other parts of the brain and then initiates endrocrine signals in response. The POSTERIOR PITUITARY is an extension of the hypothalamus that stores and secretes two hormones: Oxytocin causes contraction of the uterine muscles in childbirth and ejection of milk during nursing. Antidiuretic hormone (ADH) makes the collecting tubules of the kidney more permeable to water, increasing water retention.

Relation Between the Hypothalamus and Pituitary Gland Other hypothalamic cells produce tropic hormones that are secreted into the blood and transported to the anterior pituitary or adenohypophysis Tropic Effects Only FSH, follicle-stimulating hormone LH, luteinizing hormone TSH, thyroid-stimulating hormone ACTH, adrenocorticotropic hormone Nontropic Effects Only Prolactin MSH, melanocyte-stimulating hormone Endorphin Nontropic and Tropic Effects Growth hormone Neurosecretory cells of the hypothalamus Portal vessels Endocrine cells of the anterior pituitary Hypothalamic releasing hormones (red dots) HORMONE FSH and LH TSH ACTH MSH TARGET Testes or ovaries Thyroid Adrenal cortex Mammary glands Melanocytes Pain receptors in the brain Liver Bones Pituitary hormones (blue dots) Figure 45.8 The ANTERIOR PITUITARY consists of endocrine cells that synthesize and secrete several hormones. Some of these are TROPIC HORMONES, which means they stimulate the activity of other endocrine tissues (FSH, LH, TSH, ACTH). FSH (Follicle-Stimulating Hormone) stimulates development of the ovarian follicles in females and promotes spermatogenesis in males by acting on cells in the seminiferous tubules. Luteinizing Hormone (LH) triggers ovulation in females and stimulates the production of testosterone by the interstitial cells of the testes. 9

Human Endocrine Glands & Their Hormones Table 45.1

Human Endocrine Glands & Their Hormones Table 45.1

Nonpituitary Hormones Nonpituitary hormones help regulate metabolism, homeostasis, development, and behavior Many nonpituitary hormones regulate various functions in the body and include: Thyroid hormones Parathyroid Hormone and Calcitonin Insulin and Glucagon Adrenal Hormones Glucocorticoids, such as cortisol Mineralocorticoids, such as aldosterone Sex hormones

Thyroid Hormones The thyroid gland consists of two lobes located on the ventral surface of the trachea Produces two iodine-containing hormones, triiodothyronine (T3) and thyroxine (T4)

Negative Feedback Loops Control Thyroid Hormones The hypothalamus and anterior pituitary control the secretion of thyroid hormones through two negative feedback loops Hypothalamus Anterior pituitary TSH Thyroid T3 T4 + http://www.biologyinmotion.com/thyroid/index.html The hypothalamus secretes TSH-releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to synthesize and release the thyroid hormones T3 and T4. These hormones exert negative feedback on the hypothalamus and anterior pituitary by inhibiting the release of TRH and TSH. Figure 45.9

Hyperthyroidism The thyroid hormones play crucial roles in stimulating metabolism and influencing development and maturation Figure 45.10 Hyperthyroidism, excessive secretion of thyroid hormones can cause Graves’ disease in humans Tissue behind the eyes can become swollen and fibrous, causing the characteristic of bulging eyes.

Parathyroid Hormone and Calcitonin: The maintainence of blood calcium level is one example of how homeostasis is maintained by negative feedback. Calcitonin Thyroid gland releases calcitonin. Stimulates Ca2+ deposition in bones Reduces Ca2+ uptake in kidneys STIMULUS: Rising blood Ca2+ level Blood Ca2+ level declines to set point Homeostasis: Blood Ca2+ level (about 10 mg/100 mL) level rises Falling blood Ca2+ release from bones Parathyroid gland Increases in intestines Active vitamin D Stimulates Ca2+ uptake in kidneys PTH Figure 45.11 Two antagonistic hormones, parathyroid hormone (PTH) and calcitonin play the major role in calcium (Ca2+) homeostasis in mammals http://bcs.whfreeman.com/thelifewire/content/chp42/4202003.html Calcitonin, secreted by the thyroid gland Stimulates Ca2+ deposition in the bones and secretion by the kidneys, thus lowering blood Ca2+ levels PTH, secreted by the parathyroid glands Has the opposite effects on the bones and kidneys, and therefore raises Ca2+ levels Also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca2+ from food

Maintenance of Glucose Homeostasis Beta cells of pancreas are stimulated to release insulin into the blood. Insulin Liver takes up glucose and stores it as glycogen. Body cells take up more glucose. Blood glucose level declines to set point; stimulus for insulin release diminishes. STIMULUS: Rising blood glucose level (for instance, after eating a carbohydrate- rich meal) Homeostasis: (about 90 mg/100 mL) rises to set point; stimulus for glucagon Dropping blood glucose 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 Figure 45.12 http://bcs.whfreeman.com/thelifewire/content/chp50/5002002.html Two types of cells in the pancreas secrete insulin and glucagon, antagonistic hormones that help maintain glucose homeostasis. Insulin reduces blood glucose levels by Promoting the cellular uptake of glucose Slowing glycogen breakdown in the liver Promoting fat storage Glucagon increases blood glucose levels by Stimulating the conversion of glycogen to glucose in the liver Stimulating the breakdown of fat and protein into glucose

Diabetes Mellitus Diabetes mellitus, perhaps the best-known endocrine disorder Is caused by a deficiency of insulin or a decreased response to insulin in target tissues Is marked by elevated blood glucose levels Type I diabetes mellitus (insulin-dependent diabetes) Is an autoimmune disorder in which the immune system destroys the beta cells of the pancreas Type II diabetes mellitus (non-insulin-dependent diabetes) Is characterized either by a deficiency of insulin or, more commonly, by reduced responsiveness of target cells due to some change in insulin receptors

Adrenal Hormones: Response to Stress The adrenal glands are adjacent to the kidneys and are actually made up of two glands: the adrenal medulla and the adrenal cortex The adrenal medulla secretes epinephrine and norepinephrine Hormones which are members of a class of compounds called catecholamines These hormones Are secreted in response to stress-activated impulses from the nervous system Mediate various fight-or-flight responses

Stress Hormones from the Adrenal Cortex Hormones from the adrenal cortex also function in the body’s response to stress and fall into three classes of steroid hormones: Glucocorticoids, such as cortisol Influence glucose metabolism and the immune system Mineralocorticoids, such as aldosterone Affect salt and water balance Sex hormones Are produced in small amounts

Stress and the Adrenal Gland Nerve signals Spinal cord (cross section) Hypothalamus Releasing hormone Nerve cell Anterior pituitary Blood vessel Adrenal medulla secretes epinephrine and norepinephrine. Nerve cell Adrenal cortex secretes mineralocorticoids and glucocorticoids. ACTH Adrenal gland Kidney http://bcs.whfreeman.com/thelifewire/content/chp42/4202002.html Stressful stimuli cause the hypothalamus to activate the adrenal medulla via nerve impulses and the adrenal cortex via hormonal signals. The adrenal medulla mediates short-term responses to stress by secreting the catecholamine hormones epinephrine and norepinephrine. The adrenal cortex controls more prolonged responses by secreting steroid hormones. (a) Short-term stress response (b) Long-term stress response Effects of epinephrine and norepinephrine: Effects of mineralocorticoids: Effects of glucocorticoids: 1. Glycogen broken down to glucose; increased blood glucose 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 pressure 3. Increased breathing rate 4. Increased metabolic rate 2. Increased blood volume and blood pressure 5. Change in blood flow patterns, leading to increased alertness and decreased digestive and kidney activity 2. Immune system may be suppressed Figure 45.13a,b

Gonadal Sex Hormones The gonads (testes and ovaries) produce most of the body’s sex hormones: androgens, estrogens, and progestins The testes primarily synthesize androgens, the main one being testosterone - which stimulate the development and maintenance of the male reproductive system Testosterone causes an increase in muscle and bone mass and is often taken as a supplement to cause muscle growth, which carries many health risks Estrogens, the most important of which is estradiol, are responsible for the maintenance of the female reproductive system and the development of female secondary sex characteristics In mammals, progestins, which include progesterone are primarily involved in preparing and maintaining the uterus

Melatonin and Biorhythms The pineal gland, located within the brain secretes melatonin Release of melatonin Is controlled by light/dark cycles The primary functions of melatonin Appear to be related to biological rhythms associated with reproduction

Invertebrate Regulatory Systems In insects, molting and development are controlled by three main hormones: Brain Neurosecretory cells Corpus cardiacum Corpus allatum EARLY LARVA LATER LARVA PUPA ADULT Prothoracic gland Ecdysone Brain hormone (BH) Juvenile hormone (JH) Low JH Neurosecretory cells in the brain produce brain hormone (BH), which is stored in the corpora cardiaca (singular, corpus cardiacum) until release. 1 BH signals its main target organ, the prothoracic gland, to produce the hormone ecdysone. 2 Ecdysone secretion from the prothoracic gland is episodic, with each release stimulating a molt. 3 Juvenile hormone (JH), secreted by the corpora allata, determines the result of the molt. At relatively high concen- trations of JH, ecdysone-stimulated molting produces another larval stage. JH suppresses metamorphosis. But when levels of JH fall below a certain concentration, a pupa forms at the next ecdysone-induced molt. The adult insect emerges from the pupa. 4 http://bcs.whfreeman.com/thelifewire/content/chp42/4202001.html Invertebrate regulatory systems also involve endocrine and nervous system interactions. Diverse hormones regulate different aspects of homeostasis in invertebrates Brain hormone is produced by neurosecretory cells and stimulates the release of ecdysone from the prothoracic glands Ecdysone promotes molting and the development of adult characteristics Juvenile hormone promotes the retention of larval characteristics Figure 45.15