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Hormones and Their Mechanisms of Action, Lecture notes of Anatomy

Louisiana Tech UniversityAnatomy

A comprehensive overview of the different types of hormones and their mechanisms of action within the body. It covers the classification of hormones into steroid hormones, thyroid hormones, and non-steroid hormones, as well as the various signaling pathways and second messengers involved in hormone-mediated cellular responses. The document also delves into the specific actions and regulation of key hormones such as growth hormone, prolactin, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, aldosterone, cortisol, insulin, and glucagon. The information presented in this document would be highly relevant for students studying topics related to endocrinology, physiology, and biochemistry at the university level.

Typology: Lecture notes

2023/2024

Uploaded on 05/17/2024

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CHAPTER 13
ENDOCRINE SYSTEM
13.1 GENERAL CHARACTERISTICS OF THE ENDOCRINE SYSTEM
A. Endocrine System ( endocrine means “internal secretion”) Figure 13.1
1. Secrete substances into internal environment.
a. hormones – act upon target cells
b. paracrine secretions – affect neighboring cells
1) an example is the release of histamine from certain white blood cells that
causes dilation of nearby blood vessels
c. autocrine secretions – affect only the secreting cell itself
1) an example is liver cells stimulating themselves to release stored iron
2. Cells communicate using chemical signals that bind to receptor molecules. Fig. 13.3
a. receptors are proteins or glycoproteins with binding sites for a specific hormone
3. Hormones help regulate metabolic processes:
a. control rates of certain chemical reactions
b. aid in transporting substances through membranes
c. regulate water balance, electrolyte balance, and blood pressure
d. vital roles in reproduction, development, and growth
13.2 Hormone Action
A. Chemistry of Hormones Table 13.3
1. Most are either steroids synthesized from cholesterol or are nonsteroids, including
amines, peptides, proteins or glycoproteins.
2. Steroid Hormones Figure 13.4a
a. Lipids that include complex rings of carbon and hydrogen atoms.
b. Derived from cholesterol.
1) testosterone, estrogens, aldosterone, cortisol
2) vitamin D is a modified steroid – when converted to the active form in the
kidneys and liver, becomes the hormone calcitriol
3. Nonsteroid Hormones
a. Amines, including norepinephrine and epinephrine, derived from the amino
acid, tyrosine Figure 13.4b
1) norepinephrine and epinephrine synthesized in the adrenal medulla
2) thyroxine synthesized in thyroid gland – composed of tyrosine
3) melatonin synthesized from tryptophan
b. Protein hormones composed of long chains of amino acids. Figure 13.4c
1) glycoproteins consist of proteins joined to carbohydrates
2) peptide hormones are short chains of amino acids Figure 13.4d
c. Prostaglandins are paracrine substances. Figure 13.4e
1) lipids ( 20-carbon fatty acids that include 5-carbon rings ) – synthesized from
the fatty acid arachidonic acid found in cell membranes
2) produced in cells of the liver, kidneys, heart, lungs, thymus gland, pancreas,
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CHAPTER 13

ENDOCRINE SYSTEM

13.1 GENERAL CHARACTERISTICS OF THE ENDOCRINE SYSTEM

A. Endocrine System ( endocrine means “internal secretion”) Figure 13.

  1. Secrete substances into internal environment. a. hormones – act upon target cells b. paracrine secretions – affect neighboring cells 1) an example is the release of histamine from certain white blood cells that causes dilation of nearby blood vessels c. autocrine secretions – affect only the secreting cell itself 1) an example is liver cells stimulating themselves to release stored iron
  2. Cells communicate using chemical signals that bind to receptor molecules. Fig. 13. a. receptors are proteins or glycoproteins with binding sites for a specific hormone
  3. Hormones help regulate metabolic processes: a. control rates of certain chemical reactions b. aid in transporting substances through membranes c. regulate water balance, electrolyte balance, and blood pressure d. vital roles in reproduction, development, and growth 13.2 Hormone Action A. Chemistry of Hormones Table 13.
  4. Most are either steroids synthesized from cholesterol or are nonsteroids, including amines, peptides, proteins or glycoproteins.
  5. Steroid Hormones Figure 13.4a a. Lipids that include complex rings of carbon and hydrogen atoms. b. Derived from cholesterol.
  1. testosterone, estrogens, aldosterone, cortisol
  2. vitamin D is a modified steroid – when converted to the active form in the kidneys and liver, becomes the hormone calcitriol
  1. Nonsteroid Hormones a. Amines , including norepinephrine and epinephrine, derived from the amino acid, tyrosine Figure 13.4b
  1. norepinephrine and epinephrine synthesized in the adrenal medulla
  2. thyroxine synthesized in thyroid gland – composed of tyrosine
  3. melatonin synthesized from tryptophan b. Protein hormones composed of long chains of amino acids. Figure 13.4c
  4. glycoproteins consist of proteins joined to carbohydrates
  5. peptide hormones are short chains of amino acids Figure 13.4d c. Prostaglandins are paracrine substances. Figure 13.4e
  6. lipids ( 20-carbon fatty acids that include 5-carbon rings ) – synthesized from the fatty acid arachidonic acid found in cell membranes
  7. produced in cells of the liver, kidneys, heart, lungs, thymus gland, pancreas,

brain, and reproductive organs B. Actions of Hormones

  1. Steroid Hormones and Thyroid Hormones Figure 13.5, Table 13. a. insoluble in water – carried in plasma weakly bound to plasma proteins; soluble in lipids that make up bulk of cell membranes 1) diffuse easily into target cells b. combine with specific protein receptors inside target cell ( usually in nucleus) c. the resulting hormone-receptor complex binds to particular region of DNA and either activates or inhibits specific genes 1) the activated genes are transcribed into mRNA, which then directs the synthesis of specific proteins a) enzymes, transport proteins, hormone receptors d. e.g. aldosterone – in response to aldosterone, cells that form tubules within the kidney begin to synthesize more Na + /K + pumps, the proteins that actively transport these ions across the cell membrane, retaining sodium and excreting potassium – this is a response to low blood pressure because water will follow sodium and maintain blood volume and pressure
  2. Nonsteroid Hormones Figure 13. a. usually combines with specific receptor molecules on the target cell membrane 1) the receptor molecule is a protein that has a binding site and an activity site b. hormone combines with binding site which causes the activity site to interact with other membrane proteins 1) this may alter the function of enzymes or membrane transport mechanisms, changing the concentrations of still other cellular components 2) the hormone that triggers this cascade of biochemical activity is a first messenger 3) biochemicals that induce the changes that are recognized as responses to the hormone are called second messengers 4) the entire process of chemical communication, from outside cells to inside, is called signal transduction c. cyclic adenosine monophosphate ( cylic AMP, or cAMP ) is used by many hormones as a second messenger Table 13. 1) a hormone binds to its receptor 2) the hormone-receptor complex activates a G protein , which activates an enzyme called adenylate cyclase , an integral membrane protein a) this enzyme removes two phosphates from ATP and circularizes it, forming cyclic AMP Figure 13. 3) cyclic AMP activates a set of enzymes called protein kinases – these transfer phosphate groups from ATP molecules to protein substrate molecules – converting them to active forms 4) activated proteins than alter various cellular processes

f. help regulate blood pressure g. powerful effects on both male and female reproductive physiology h. promote inflammation when tissues are injured 13.3 CONTROL OF HORMONAL SECRETIONS A. Removal of Hormones

  1. Removal of hormones is measured by half-life, which is the time it takes for half of the hormone molecules to be removed from the plasma. a. some have half-lives of a few minutes, whereas others, such as thyroid and steroids, may last for days b. continually excreted in urine and broken down by enzymes, primarily in the liver B. Control Sources
  2. Three ways of control, all which employ negative feedback. Figures 13.8, 13. a. hypothalamus controls the anterior pituitary gland’s release of tropic hormones , which are hormones that stimulate other endocrine glands to release hormones b. nervous system stimulates some glands directly
  1. epinephrine and norepinephrine c. another group of glands responds directly to changes in the composition of the internal environment
  2. blood glucose levels
  1. In a few cases, positive feedback contributes to hormone secretion control. a. oxytocin is a hormone responsible for milk secretion from the breast
  2. Hormone levels in the bloodstream remain relatively stable. Figure 13. 13.4 PITUITARY GLAND A. Structure Figure 13.11, 13.
  3. About 1 cm. in diameter – located at the base of brain.
  4. Attached to the hypothalamus by the infundibulum – lies in the sella turcica of the sphenoid bone.
  5. Two distinct portions: anterior lobe ( adenohypophysis ) and a posterior lobe ( neurohypophysis ). a. the cells of the posterior lobe do not synthesize any hormones – neurosecretory cells of the hypothalamus synthesize two important hormones that are released by the posterior lobe b. in the fetus, the intermediate lobe is present – produces the hormone melanocyte-stimulating hormone (MSH) - lobe not present in adults but the secretory cells persist in the other two lobes
  6. Releasing hormones from the hypothalamus control secretions from the anterior lobe. Figures 13.13, 13. a. releasing hormones travel in the hypophyseal portal system to the capillary bed in the anterior lobe b. the releasing hormones act upon a specific population of cells – some actions

are inhibitory, but most stimulate the anterior lobe to release stimulating hormones B. Anterior Pituitary Hormones Figure 13.15, Table 13.

  1. This lobe is enclosed in a dense capsule of collagenous connective tissue and largely consists of epithelial tissue organized in blocks around many thin-walled blood vessels. Five types of secretory cells: a. somatotropes secrete GH b. mammatropes secrete PRL c. thyrotropes secrete TSH d. corticotropes secrete ACTH e. gonadotropes secrete FSH and LH
  2. Growth hormone , also called somatotropin , is a protein that stimulates cells to enlarge and more rapidly divide. a. enhances movement of amino acids through the cell membranes b. increases rate of protein synthesis c. decreases rate at which cells use carbohydrates d. increases rate at which cells use fats e. secreted in pulses, especially during sleep – release controlled by two biochemicals from the hypothalamus 1) growth hormone-releasing hormone (GHRH) stimulates secretion of GH 2) somatostatin (SS) inhibits secretion of GH f. more GH is released during periods of protein deficiency and abnormally low blood glucose concentration g. GH can stimulate elongation of bone tissue directly – its effect on cartilage requires a biochemical from the liver called insulin-like factor-I (IGF-I), a somatomedin. IGF-I promotes growth of cartilage.
  3. Prolactin is a protein that promotes breast-milk production. a. excess PRL in males decreases secretion of LH, a hormone necessary for production of androgens – PRL may help maintain normal sperm production b. secretion under inhibitory control by dopamine from the hypothalamus, also called prolactin-release inhibiting hormone ( PIH )
  4. Thyroid-stimulating hormone , thyrotropin , is a glyocoprotein. a. controls secretion of certain hormones from the thyroid gland 1) can also stimulate growth of thyroid – possibly leading to a goiter b. hypothalamus partially regulates TSH secretion by producing thyrotropin- releasing hormone (TRH) c. as blood concentrations of thyroid hormones increase, secretions of TRH and TSH decline Figure 13. d. external factors influence release of TRH and TSH 1) exposure to extreme cold – increases secretion 2) emotional stress – increases or decreases secretion
  5. Adrenocorticotropic hormone is a peptide that controls manufacture and secretion of certain hormones from the adrenal cortex. Also called corticotropin.
  1. oxytocin has no established function in males although it is present. a) may stimulate movement of sperm and fluids in the male reproductive tract during sexual activity 13.5 THYROID GLAND A. Structure and Location Figures 13.19, 13.
  1. Located just below the larynx on either side and anterior to the trachea. Very vascular structure consisting of two lobes connected by an isthmus.
  2. Made up of many secretory parts called follicles. a. cavities of follicles are lined with simple cuboidal cells and filled with a clear viscous colloid , which consists primarily of a glycoprotein called thyroglobulin. B. Thyroid Hormones Table 13.
  3. Thyroxine ( tetraiodothyronine, or T 4 because it contains four atoms of iodine ) and triiodothyronine ( T 3 because it contains three atoms of iodine ) are produced by the follicular cells and affect cellular metabolic rates. Figure 13. a. increase the rate at which cells release energy from carbohydrates b. enhance the rate of protein synthesis c. stimulate breakdown and mobilization of lipids d. essential for normal growth and development and for maturation of nervous system e. major factor determining how many calories the body must consume at rest to maintain life – measured as the basal metabolic rate f. TSH controls levels of both hormones
  4. Thyroglobulin is secreted by follicular cells and is rich in tyrosine molecules. Tyrosine binds with iodine and the thyroglobulin coils into its tertiary structure, creating potential thyroid hormones waiting to be released.
  5. Calcitonin is synthesized by extrafollicular cells called C cells. Helps control blood calcium and phosphate ion concentrations. a. helps lower concentrations of calcium and phosphate ions by decreasing the rate at which they leave bones by inhibiting osteoclasts b. increases the rate at which calcium and phosphate are deposited in bone matrix by stimulating osteoblasts c. increases excretion of calcium ions and phosphate ions by kidneys d. its actions help protect bones from resorption during pregnancy and lactation, when calcium is needed for growth of fetus and synthesis of breast milk 13.6 PARATHYROID GLANDS A. Structure and Location Figures 13.25, 13.
  6. Usually four glands located on the posterior surface of the thyroid gland. The body of the gland consists of many tightly packed secretory cells that are closely associated with capillary networks. B. Parathyroid Hormone (PTH)
  7. A protein hormone also called parathormone that increases blood calcium ion

concentration and decreases blood phosphate ion concentration.

  1. PTH stimulates bone resorption by osteoclasts and inhibits osteoblast activity. a. causes kidneys to conserve blood calcium ions and to excrete more phosphate ions in urine b. indirectly stimulates absorption of calcium ions from food in intestine by influencing metabolism of vitamin D Figure 13. 1) cholesterol is converted to provitamin D by intestinal enzymes and the provitamin is stored in skin – exposure to UV sunlight changes it to vitamin D ( cholecalciferol ) 2) the liver changes vitamin D to hydroxycholecalciferol, which is carried in the bloodstream or stored in tissues – when PTH is present, this compound can be changed in the kidneys into an active form of vitamin D ( dihyroxychole- calciferol), which controls absorption of calcium ions from the intestines
  2. A negative feedback mechanism operating between the parathyroid glands and the blood calcium ion concentration regulates PTH secretion. Figure 13. 13.7 ADRENAL GLANDS A. Structure and Location Figure 13.
  3. The adrenal glands are shaped like pyramids and sit atop the kidneys embedded in a mass of adipose tissue that encloses the kidney.
  4. Are very vascular and consist of two parts: a central medulla and an outer cortex. a. the adrenal medulla consists of irregularly shaped cells grouped around blood vessels and the cells are intimately connected with the sympathetic nervous system – these cells are modified sympathetic postganglionic neurons, and preganglionic sympathetic nerve fibers control their secretions b. the adrenal cortex is composed of closely packed masses of epithelial cells that form three layers – the zona glomerulosa, the zona fasciculata, and zona reticularis Figure 13.29, 13. B. Hormones of the Adrenal Medulla Figure 13.
  5. Chromaffin cells produce, store, and secrete two closely related hormones – epinephrine ( adrenalin ) and norepinephrine ( noradrenalin ). Both are a type of amine called a catecholamine. Epinephrine is synthesized from norepinephrine.
  6. Synthesis of hormones: Figure 13. tyrosine dopa dopaminenorepinephrineepinephrine
  7. Effects of hormones: Table 13. a. increased heart rate and increased force of cardiac muscle contraction b. elevated blood pressure c. increased breathing rate d. decreased digestive activity
  8. Hormonal effects last up to ten times longer than neurotransmitter effects because they are removed from the tissues slowly.
  9. Certain effectors respond differently to the two hormones, with norepinephrine having a greater effect on alpha receptors. Table 13.
  1. inhibiting prostaglandin synthesis g. can be used to treat autoimmune disorders, allergies, asthma, and recepients of organ transplants or tissue grafts
  1. Adrenal sex hormones produced in the inner zone ( zona reticularis ) include dehydrpepiandrosterone ( DHEA) , a hormone that certain tissues convert to the more potent forms of androgens, such as testosterone. a. the adrenal androgens, primarily estradiol , are also important in post- menopausal women; may also play a role in the female sex drive 13.8 PANCREAS A. Structure and Location Figure 13.
  2. An elongated, somewhat flattened organ that is posterior to the stomach and behind the parietal peritoneum. Attached to the duodenum by a duct, which transports digestive juice into the intestine.
  3. Endocrine portion consists of groups of cells, pancreatic islets ( islets of Langerhans ) , consisting of three distinct types of hormone-secreting cells – alpha cells which secrete glucagon, beta cells which secrete insulin, and delta cells which secrete somatostatin. Figure 13. B. Hormones of the Pancreatic Islets
  4. Glucagon is a protein that stimulates the liver to break down glycogen into glucose ( glycogenolysis ), and to convert noncarbohydrates, such as amino acids, into glucose ( gluconeogenesis ). a. also stimulates breakdown of fats into fatty acids and glycerol b. low concentration of blood glucose stimulates glucagon release from alpha cell
  5. Insulin is a protein that simulates the liver to form glycogen from glucose and inhibits the conversion of noncarbohydrates into glucose. a. promotes facilitated diffusion of glucose through the membranes of cells bearing insulin receptors
  1. cardiac muscle, adipose tissues, resting skeletal muscles b. decreases the concentration of blood glucose, promotes the transport of amino acids into cells, increases protein synthesis, and stimulates adipose cells to synthesize and store fat
  1. Nerve cells obtain glucose by a facilitated diffusion mechanism that is not dependent on insulin, but rather only on the blood glucose concentration.
  2. Insulin and glucagon function together to maintain a relatively constant blood glucose concentration. Figure 13.
  3. Somatostatin ( similar to the hypothalamic hormone ) helps regulate glucose metabolism by inhibiting secretion of glucagon and insulin. 13.9 OTHER ENDOCRINE GLANDS A. Pineal Gland
  4. Small, oval structure located deep between the cerebral hemispheres, attached to

the upper portion of the thalamus near the roof of the third ventricle.

  1. Consists of specialized pineal cells and supportive neuroglial cells
  2. Secretes melatonin , which is synthesized from serotonin, in the absence of light. a. part of the regulation of circadian rhythms 1) sleep/wake rhythms 2) seasonal cycles of fertility in many mammals b. inhibits secretion of gonadotropins from anterior pituitary and helps regulate the female reproductive cycle – may control onset of puberty B. Thymus Gland
  3. Lies in mediastinum posterior to the sternum and between the lungs.
  4. Secretes a group of hormones called thymosins , that affect production and differentiation of certain white blood cells.
  5. Plays an important role in immunity. C. Reproductive Glands
  6. The ovaries produce estrogens and progesterone; the placenta produces estrogens, progesterone, and a gonadotropin; and the testes produce testosterone. D. Digestive Glands
  7. Associated with the linings of the stomach and small intestine. E. Other Hormone-Producing Organs
  8. The heart secretes atrial natriuretic peptide which stimulates urinary sodium excretion and the kidneys secrete erythropoietin, a hormone that stimulates red blood cell production. 13.10 STRESS AND ITS EFFECTS A. Types of Stress
  9. Stressors are factors that lead to a loss of homeostasis and produce the condition called stress.
  10. Physical stress threatens tissues. a. extreme heat or cold b. decreased oxygen concentration c. infections d. prolonged heavy exercise e. loud sounds f. injuries
  11. Psychological stress results from thoughts about real or imagined dangers, personal losses, unpleasant social interactions, or any factors that threaten a person. a. feelings of fear, anger, grief, anxiety, depression, guilt b. pleasant stimuli such as friendly social contact, feelings of joy or happiness, sexual arousal B. Responses to Stress Figure 13.
  12. Hypothalamus controls response to stress, termed the general stress ( or general adaptation ) syndrome. This response maintains homeostasis. a. the “fight or flight” response is activated