42 Animal Hormones 42 Animal Hormones Introduction Hormones

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42 Animal Hormones

42 Animal Hormones

42 Animal Hormones • Introduction • Hormones and Their Actions • Hormonal Control of

42 Animal Hormones • Introduction • Hormones and Their Actions • Hormonal Control of Molting and Development in Insects • Vertebrate Endocrine Systems • Hormone Actions: The Role of Signal Transduction Pathways

42 Introduction • Chemical messages, or hormones, produce and coordinate anatomical, physiological, and behavioral

42 Introduction • Chemical messages, or hormones, produce and coordinate anatomical, physiological, and behavioral changes in an animal. • An example are the cichlid fish that live in shallow pools around Lake Tanganyika, Africa. • Dominant male cichlids are brightly colored and aggressive. • Nondominant male cichlids look like females. • Russell Fernald showed that hormones control the production of characteristics of a dominant male.

42 Hormones and Their Actions • Control and regulation require information. • In multicellular

42 Hormones and Their Actions • Control and regulation require information. • 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 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.

42 Hormones and Their Actions • Hormone-secreting cells are called endocrine cells. • Cells

42 Hormones and Their Actions • Hormone-secreting cells are called endocrine cells. • Cells receiving the hormonal message are called target cells and must have appropriate receptors. • The binding of the receptor activates a response. • The distance over which the signal operates distinguishes hormone groups; some act close to the release site, others at distant body locations.

42 Hormones and Their Actions • Hormones can be classified into three main groups:

42 Hormones and Their Actions • Hormones can be classified into three main groups: § Peptides or proteins. They are water soluble and transported by vesicles out of the cell that made them. § Steroid hormones are lipid-soluble and can diffuse out of the cell that made them but in the blood they must be bound to carrier proteins. § Amine hormones are derivatives of the amino acid tyrosine. Some are water-soluble and some are lipid-soluble.

42 Hormones and Their Actions • The receptors for lipid-soluble hormones are inside cells,

42 Hormones and Their Actions • The receptors for lipid-soluble hormones are inside cells, either in the cytoplasm or in the nucleus. • The action of lipid-soluble hormones is mediated by intracellular hormone receptors that usually alter gene expression.

42 Hormones and Their Actions • The receptors for water-soluble proteins are large glycoproteins

42 Hormones and Their Actions • The receptors for water-soluble proteins are large glycoproteins on the cell surface with three domains: § A binding domain projecting outside the plasma membrane § A transmembrane domain that anchors the receptor in the membrane § A cytoplasmic domain that extends into the cytoplasm of the cell • The cytoplasmic domain initiates the target cell’s response by activating protein kinases or protein phosphatases.

42 Hormones and Their Actions • Some hormones act locally. • Autocrine hormones act

42 Hormones and Their Actions • Some hormones act locally. • Autocrine hormones act on the secreting cell itself. • Paracrine hormones act on cells near the site of release. • Paracrine hormones are released in tiny amounts, or are inactivated rapidly by enzymes, or are taken up efficiently by local cells. They never get into the circulatory system.

Figure 42. 1 Chemical Signaling Systems

Figure 42. 1 Chemical Signaling Systems

42 Hormones and Their Actions • Growth factors, which stimulate growth and differentiation of

42 Hormones and Their Actions • Growth factors, which stimulate growth and differentiation of cells, are a major class of paracrine hormones. • Growth factors also act as autocrine hormones: Some of the hormone influences the cell that secreted it, preventing the cell from secreting too much hormone. • Neurons may also be considered to be paracrine cells because they use chemicals called neurotransmitters to send messages to another cell.

42 Hormones and Their Actions • Most hormones diffuse into the blood, which distributes

42 Hormones and Their Actions • Most hormones diffuse into the blood, which distributes them throughout the body. • When the hormone message encounters a cell with the proper receptor, it binds and triggers a response. • The same hormone can cause different responses in different types of cells. • An example is epinephrine. The nervous system reacts to an emergency very quickly and stimulates adrenal cells to secrete epinephrine. The result is the fight-or-flight response.

42 Hormones and Their Actions • The epinephrine acts on different cells in the

42 Hormones and Their Actions • The epinephrine acts on different cells in the body: § In the heart, it stimulates faster and stronger heartbeat. § Blood vessels in some areas constrict to send more blood to muscles. § In the liver, glycogen is broken down to glucose to provide quick energy. § In fat tissue, fats are mobilized as another energy source.

42 Hormones and Their Actions • Endocrine refers to cells or glands that do

42 Hormones and Their Actions • Endocrine refers to cells or glands that do not have ducts leading to the outside of the body; they secrete their products directly into the extracellular fluid. • Some endocrine cells are single cells within a tissue. • Digestive hormones, for example, are secreted by isolated endocrine cells in the wall of the stomach and small intestine. • Some endocrine cells aggregate into secretory organs called endocrine glands. • In vertebrates, nine major endocrine glands make up the endocrine system.

Figure 42. 2 The Endocrine System of Humans

Figure 42. 2 The Endocrine System of Humans

42 Hormonal Control of Molting and Development in Insects • Because insects have rigid

42 Hormonal Control of Molting and Development in Insects • Because insects have rigid exoskeletons, they have episodic growth patterns and must molt periodically. • The growth stage between each molt is called an instar. • Experiments by British physiologist Wigglesworth showed how molting is triggered by a hormone from the brain.

Figure 42. 3 A Diffusible Substance Triggers Molting (Part 1)

Figure 42. 3 A Diffusible Substance Triggers Molting (Part 1)

Figure 42. 3 A Diffusible Substance Triggers Molting (Part 2)

Figure 42. 3 A Diffusible Substance Triggers Molting (Part 2)

42 Hormonal Control of Molting and Development in Insects • Two hormones work together

42 Hormonal Control of Molting and Development in Insects • Two hormones work together in insects to regulate molting: brain hormone and ecdysone. • Cells in the brain produce brain hormone which is stored in a pair of structures attached to the brain called the corpora cardiaca. • After appropriate stimulation (e. g. , a blood meal for Rhodnius), the corpora release brain hormone, which diffuses to an endocrine gland called the prothoracic gland. • The prothoracic gland releases ecdysone which stimulates molting.

42 Hormonal Control of Molting and Development in Insects • Wigglesworth also demonstrated that

42 Hormonal Control of Molting and Development in Insects • Wigglesworth also demonstrated that another hormone is responsible for determining when an insect molts into an adult. • By removing only the front part of the head, it was shown that the rear part, containing the corpora allata, produces a substance preventing a molt to the adult stage. • The substance is called juvenile hormone. If it is present, Rhodnius molts to another juvenile instar. • Normally, during the fifth instar, the corpora allata stop making this hormone and the insect molts to the adult stage.

42 Hormonal Control of Molting and Development in Insects • Hormonal control is more

42 Hormonal Control of Molting and Development in Insects • Hormonal control is more complex in insects having complete metamorphosis. • An example is the silkworm. The egg hatches into a larva that has a high amount of juvenile hormone in its body. • As long as the level of juvenile hormone stays high, larvae molt into larvae; when the juvenile hormone level wanes, pupae are formed. • No juvenile hormone is found in the pupae, so they molt into adults.

Figure 42. 4 Complete Metamorphosis

Figure 42. 4 Complete Metamorphosis

42 Vertebrate Endocrine Systems • The pituitary gland of mammals is a link between

42 Vertebrate Endocrine Systems • 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. • The pituitary gland sits in a depression at the bottom of the skull and is attached to the hypothalamus. • The pituitary is made of two parts: anterior and posterior.

42 Vertebrate Endocrine Systems • The posterior pituitary releases two hormones: antidiuretic hormone and

42 Vertebrate Endocrine Systems • The posterior pituitary releases two hormones: antidiuretic hormone and oxytocin. • They are made by neurons in the hypothalamus, are called neurohormones, and are packaged in vesicles. • The vesicles are transported down the axons of the neurons that made them and are stored in the posterior pituitary. • This movement of the vesicles is achieved by kinesin proteins, powered by ATP, that “walk” down the microtubules of the axon.

Figure 42. 5 The Posterior Pituitary Releases Neurohormones

Figure 42. 5 The Posterior Pituitary Releases Neurohormones

42 Vertebrate Endocrine Systems • The function of antidiuretic hormone (ADH) is to increase

42 Vertebrate Endocrine Systems • 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.

42 Vertebrate Endocrine Systems • The function of oxytocin is to stimulate uterine muscle

42 Vertebrate Endocrine Systems • The function of oxytocin is to stimulate uterine muscle contraction for the birth process. • It also stimulates milk flow in the mother’s breasts. Suckling by the baby, or even the sight or sound of the baby, can cause the mother to secrete oxytocin and release milk.

42 Vertebrate Endocrine Systems • The anterior pituitary releases four tropic hormones, which control

42 Vertebrate Endocrine Systems • The anterior pituitary releases four tropic hormones, which control activities of other endocrine glands. • They are peptide and protein hormones; each is produced by a different type of pituitary cell. • The four tropic hormones are: thyrotropin, adrenocorticotropin, luteinizing hormone, and follicle-stimulating hormone.

Figure 42. 7 Hormones from the Hypothalamus Control the Anterior Pituitary

Figure 42. 7 Hormones from the Hypothalamus Control the Anterior Pituitary

42 Vertebrate Endocrine Systems • Other peptide and protein anterior pituitary hormones influence tissues

42 Vertebrate Endocrine Systems • Other peptide and protein anterior pituitary hormones influence tissues that are not endocrine glands. • These include: growth hormone, prolactin, melanocyte-stimulating hormone, endorphins, and enkephalins.

42 Vertebrate Endocrine Systems • Growth hormone (GH) acts on many tissues to promote

42 Vertebrate Endocrine Systems • Growth hormone (GH) acts on many tissues to promote growth. • GH stimulates cells to take up amino acids. • GH also stimulates the liver to produce chemical messages (insulin-like growth factors) that stimulate bone and cartilage growth. • Gigantism is the result of overproduction of GH in children. • Underproduction of GH causes pituitary dwarfism. GH is now produced by genetically engineered bacteria.

Figure 42. 6 Effects of Excess Growth Hormone

Figure 42. 6 Effects of Excess Growth Hormone

42 Vertebrate Endocrine Systems • Prolactin stimulates the production and secretion of milk in

42 Vertebrate Endocrine Systems • Prolactin stimulates the production and secretion of milk in female mammals. • It is also important in pregnancy and, in males, has a role in controlling the endocrine functions of the testes.

42 Vertebrate Endocrine Systems • Endorphins and enkephalins are the body’s natural opiates. In

42 Vertebrate Endocrine Systems • Endorphins and enkephalins are the body’s natural opiates. In the brain, these molecules act as neurotransmitters in pathways. • The production of these hormones, as well as some other anterior pituitary hormones, is governed by one gene. • The gene encodes for a protein called proopiomelanocortin. • This large molecule is later cleaved into several peptides including adrenocorticotropin, melanocyte-stimulating hormone, endorphins, and enkephalins.

42 Vertebrate Endocrine Systems • The anterior pituitary is controlled by neurohormones from the

42 Vertebrate Endocrine Systems • The anterior pituitary is controlled by neurohormones from the hypothalamus. • The hypothalamus obtains data about body conditions and the external environment through both neuronal and hormonal signals. • The hypothalamus and the anterior pituitary are connected by portal blood vessels. • Secretions from hypothalamic nerves are transported by these blood vessels to the anterior pituitary.

Figure 42. 7 Hormones from the Hypothalamus Control the Anterior Pituitary

Figure 42. 7 Hormones from the Hypothalamus Control the Anterior Pituitary

42 Vertebrate Endocrine Systems • Thyrotropin-releasing hormone (TRH) was the first releasing hormone extracted

42 Vertebrate Endocrine Systems • Thyrotropin-releasing hormone (TRH) was the first releasing hormone extracted from the hypothalamus. • It causes anterior pituitary cells to release thyrotropin, which in turn stimulates the thyroid gland. • Gonadotropin-releasing hormone (Gn. RH) causes the anterior pituitary to release tropic hormones that control gonad activity. • Now many more hypothalamic neurohormones are known.

Table 42. 2 Releasing and Release-Inhibiting Neurohormones of the Hypothalamus

Table 42. 2 Releasing and Release-Inhibiting Neurohormones of the Hypothalamus

42 Vertebrate Endocrine Systems • The anterior pituitary cells are also under negative feedback

42 Vertebrate Endocrine Systems • The anterior pituitary cells are also under negative feedback control by the hormones of the glands that they stimulate. • For example, cortisol is produced by the adrenal gland in response to adrenocorticotropin. It returns to the pituitary in the blood, and inhibits further release of adrenocorticotropin. • Cortisol also exerts negative feedback control on the hypothalamus, inhibiting release of adrenocorticotropin-releasing hormone.

Figure 42. 8 Multiple Feedback Loops Control Hormone Secretion

Figure 42. 8 Multiple Feedback Loops Control Hormone Secretion

42 Vertebrate Endocrine Systems • The thyroid gland, located near the trachea, is an

42 Vertebrate Endocrine Systems • The thyroid gland, located near the trachea, is an example of an endocrine gland that is controlled by negative feedback. • The thyroid gland produces the hormone thyroxine in specialized structures called follicles. • Two forms of thyroxine, T 3 and T 4, are made from tyrosine. T 3 (triiodothyronine) has three iodine atoms. T 4 has four iodine atoms. • More T 4 is produced, but it can be converted to T 3 by an enzyme in the blood. T 3 is the more active form of the hormone.

In-Text Art p. 809(1)

In-Text Art p. 809(1)

In-Text Art p. 809(2)

In-Text Art p. 809(2)

42 Vertebrate Endocrine Systems • Thyroxine has many roles in regulating metabolism. § It

42 Vertebrate Endocrine Systems • 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 growth and development. Insufficient thyroxine may result in cretinism.

42 Vertebrate Endocrine Systems • Thyrotropin (or thyroid-stimulating hormone, TSH) from the anterior pituitary

42 Vertebrate Endocrine Systems • Thyrotropin (or thyroid-stimulating hormone, TSH) from the anterior pituitary activates thyroid gland cells to produce thyroxine. • Thyrotropin-releasing hormone (TRH) from the hypothalamus activates TSH-producing cells in the anterior pituitary. • In a negative feedback loop, thyroxine inhibits the response of pituitary cells to TRH. • Therefore, less TSH is released when thyroxine levels are high, and more is released when levels are low.

42 Vertebrate Endocrine Systems • A goiter is an enlarged thyroid gland associated with

42 Vertebrate Endocrine Systems • A goiter is an enlarged thyroid gland associated with either very low (hypothyroidism) or very high (hyperthyroidism) levels of thyroxine. • A thyroid follicle is a layer of epithelial cells surrounding a mass of glycoprotein called thyroglobulin. • Thyroglobulin consists of iodinated tyrosines and is digested by the epithelial cells to make thyroxine. • If there is no iodine present when thyroglobulin is made, the released molecules will be neither T 3 nor T 4 and will not bind to appropriate receptors.

42 Vertebrate Endocrine Systems • Goiter occurs when thyroglobulin production is above normal and

42 Vertebrate Endocrine Systems • Goiter occurs when thyroglobulin production is above normal and the follicles are enlarged. • 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. • The thyroid remains maximally active and grows larger, causing symptoms associated with high metabolic rates.

42 Vertebrate Endocrine Systems • Hypothyroid goiter results when there is insufficient thyroxine to

42 Vertebrate Endocrine Systems • 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.

42 Vertebrate Endocrine Systems • Calcium levels in the blood must be regulated within

42 Vertebrate Endocrine Systems • 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

42 Vertebrate Endocrine Systems • Calcitonin, released by the thyroid gland, acts to lower

42 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.

42 Vertebrate Endocrine Systems • The four parathyroid glands are embedded in the posterior

42 Vertebrate Endocrine Systems • The four parathyroid glands are embedded in the posterior surface of the thyroid gland. • Blood calcium decrease triggers release of parathyroid hormone (PTH), which 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.

Figure 42. 9 Hormonal Regulation of Calcium (Part 1)

Figure 42. 9 Hormonal Regulation of Calcium (Part 1)

Figure 42. 9 Hormonal Regulation of Calcium (Part 2)

Figure 42. 9 Hormonal Regulation of Calcium (Part 2)

42 Vertebrate Endocrine Systems • Vitamin D is not a true vitamin. It is

42 Vertebrate Endocrine Systems • Vitamin D is not a true vitamin. It is made in skin cells by ultraviolet light, circulates in the blood, and acts on distant cells; therefore it is a hormone. • Vitamin D is not very active, but in the liver it receives an –OH group and in the kidneys another –OH to become (1, 25) Dihydroxyvitamin D, the active form. • PTH stimulates the final step in the kidneys. • The active form binds to cytoplasmic receptors and forms transcription factors. In the digestive tract the transcription factors act to increase synthesis of calcium pumps, calcium channels, and calciumbinding proteins, promoting uptake of calcium.

42 Vertebrate Endocrine Systems • In the kidneys, vitamin D acts with PTH to

42 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.

42 Vertebrate Endocrine Systems • Bone minerals have both calcium and phosphate. • When

42 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.

42 Vertebrate Endocrine Systems • Diabetes mellitus is a disease caused by a lack

42 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.

42 Vertebrate Endocrine Systems • High glucose levels in the blood cause water to

42 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.

42 Vertebrate Endocrine Systems • Insulin is produced in the pancreas in cell clusters

42 Vertebrate Endocrine Systems • Insulin is produced in the pancreas in cell clusters called islets of Langerhans. • Several cell types have been identified in the islets: § Beta (b) cells produce and secrete insulin. § Alpha (a) 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.

42 Vertebrate Endocrine Systems • After a meal, blood glucose levels rise and stimulate

42 Vertebrate Endocrine Systems • After a meal, blood glucose levels rise and stimulate the b 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 a cells release glucagon which stimulates the liver to convert glycogen back to glucose.

42 Vertebrate Endocrine Systems • In the pancreas, somatostatin has paracrine functions of inhibiting

42 Vertebrate Endocrine Systems • In the pancreas, somatostatin has paracrine functions of inhibiting release of both insulin and glucagon. • Outside of the pancreas, it slows gut activities to extend the time of nutrient absorption. • It also acts as a hypothalamic neurohormone inhibitor for the release of GH and thyrotropin by the pituitary.

42 Vertebrate Endocrine Systems • The adrenal glands are made up of the adrenal

42 Vertebrate Endocrine Systems • The adrenal glands are made up of the adrenal medulla and the adrenal cortex. • 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.

Figure 42. 10 The Adrenal Gland Has an Outer and an Inner Portion

Figure 42. 10 The Adrenal Gland Has an Outer and an Inner Portion

42 Vertebrate Endocrine Systems • The adrenal medulla produces epinephrine (adrenaline) in response to

42 Vertebrate Endocrine Systems • The adrenal medulla produces epinephrine (adrenaline) in response to stress, initiating fightor-flight reactions, such as increased heart and breathing rates and elevated blood pressure. • It also produces norepinephrine, a neurotransmitter involved in physiological regulation. • Epinephrine and norepinephrine are amine hormones. They bind to two types of receptors in target cells: a-adrenergic and b-adrenergic.

42 Vertebrate Endocrine Systems • Norepinephrine acts mostly on the alpha type, so drugs

42 Vertebrate Endocrine Systems • Norepinephrine acts mostly on the alpha type, so drugs called beta blockers, which inactivate only b-adrenergic receptors, can be used to reduce fight-or-flight responses to epinephrine. • The beta blockers leave the alpha sites open to norepinephrine and its regulatory functions.

42 Vertebrate Endocrine Systems • Adrenal cortex cells use cholesterol to produce three classes

42 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.

Figure 42. 11 The Corticosteroid Hormones are Built from Cholesterol

Figure 42. 11 The Corticosteroid Hormones are Built from Cholesterol

42 Vertebrate Endocrine Systems • The main mineralocorticoid, aldosterone, stimulates the kidney to conserve

42 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.

42 Vertebrate Endocrine Systems • Shortly after a frightening stimulus, blood cortisol rises. •

42 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.

42 Vertebrate Endocrine Systems • Cortisol release is controlled by ACTH from the anterior

42 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 adrenocorticotropin-releasing hormone.

42 Vertebrate Endocrine Systems • The gonads (testes and ovaries) produce steroid hormones synthesized

42 Vertebrate Endocrine Systems • 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.

42 Vertebrate Endocrine Systems • Until the seventh week of an embryo’s development, either

42 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 sevenweek-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.

Figure 42. 12 The Development of Human Sex Organs

Figure 42. 12 The Development of Human Sex Organs

42 Vertebrate Endocrine Systems • Sex steroid production increases rapidly at puberty, or sexual

42 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 (Gn. RH). • Before puberty, the hypothalamus produces low levels of Gn. RH.

42 Vertebrate Endocrine Systems • Puberty starts when the hypothalamus becomes less sensitive to

42 Vertebrate Endocrine Systems • Puberty starts when the hypothalamus becomes less sensitive to negative feedback by the sex steroids. • Gn. RH 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.

42 Vertebrate Endocrine Systems • Synthetic androgens (anabolic steroids) can exaggerate body strength and

42 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.

42 Vertebrate Endocrine Systems • Melatonin hormone is produced by the pineal gland, located

42 Vertebrate Endocrine Systems • Melatonin hormone is produced by the pineal gland, located between the cerebral hemispheres of the brain. • Melatonin release occurs in the dark, marking the length of night. Exposure to light inhibits melatonin release. • Melatonin is involved in biological rhythms, including photoperiodicity. • In many animals, increasing day length signals the onset of reproductive behavior. • Humans are not photoperiodic, but melatonin may be involved in daily rhythms of the body (light/dark cycles).

Figure 42. 13 The Release of Melatonin Regulates Seasonal Changes

Figure 42. 13 The Release of Melatonin Regulates Seasonal Changes

42 Vertebrate Endocrine Systems • Many other body organs, such as the gut and

42 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.

42 Hormone Actions: The Role of Signal Transduction Pathways • Hormones are released in

42 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.

Figure 42. 14 Some Hormones Can Activate a Variety of Signal Transduction Pathways

Figure 42. 14 Some Hormones Can Activate a Variety of Signal Transduction Pathways

42 Hormone Actions: The Role of Signal Transduction Pathways • The abundance of hormone

42 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.

42 Hormone Actions: The Role of Signal Transduction Pathways • The concentration of hormones

42 Hormone Actions: The Role of Signal Transduction Pathways • The concentration of hormones and receptors can be determined by a technique called immunoassay. • A saturating concentration of a labeled hormone is mixed with an antibody until all the binding sites of the antibody are used. • A sample of unlabeled hormone is then added to the mixture. • The unlabeled hormone displaces some of the labeled hormone from the antibody. • The ratio of labeled to unlabeled antibody is a measure of the amount of unlabeled hormone.

Figure 42. 15 An Immunoassay Measures Hormone Concentration (Part 1)

Figure 42. 15 An Immunoassay Measures Hormone Concentration (Part 1)

Figure 42. 15 An Immunoassay Measures Hormone Concentration (Part 2)

Figure 42. 15 An Immunoassay Measures Hormone Concentration (Part 2)

42 Hormone Actions: The Role of Signal Transduction Pathways • Factors other than hormone

42 Hormone Actions: The Role of Signal Transduction Pathways • Factors other than hormone and receptor concentrations can influence physiological responses to hormones. • A dose–response curve shows the threshold dose of a hormone needed to get a response and the dose that produces the maximum response. • The dose that stimulates half the maximum response is the sensitivity of the cell, tissue, organ, or animal to the hormone.

Figure 42. 16 Dose–Response Curves Quantify Response to a Hormone

Figure 42. 16 Dose–Response Curves Quantify Response to a Hormone

42 Hormone Actions: The Role of Signal Transduction Pathways • Anything that changes the

42 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

42 Hormone Actions: The Role of Signal Transduction Pathways • Hormone responses may also

42 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.

42 Hormone Actions: The Role of Signal Transduction Pathways • The ability of the

42 Hormone Actions: The Role of Signal Transduction Pathways • The ability of the hormone to leave the blood affects its half-life. • If the hormone is bound to carrier proteins, it will have a longer half-life. • The variation in time course allows hormone signaling systems to have temporal features that match their functions. • The nature of the target cell response also affects the time course of hormone action. • For body developmental effects, the time course may be months or even a lifetime.