Transcript Chapter 46
CHAPTER 46 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Types of Chemical Messengers • Hormone – Regulatory chemical that is secreted into extracellular fluid and carried by the blood – Can act at a distance from source • Endocrine system – Organs and tissues that produce hormones • Only targets with receptor can respond 2 Types of Chemical Messengers • Paracrine regulators do not travel in blood – Allow cells of organ to regulate each other • Pheromones are chemicals released into the environment to communicate among individuals of a single species – Not involved in normal metabolic regulation within an animal 3 Types of Chemical Messengers • Some neurotransmitters are distributed by the blood and act as a hormone – Norepinephrine coordinates the activity of heart, liver, and blood vessels during stress • Neurons can also secrete a class of hormones called neurohormones that are carried by blood – Antidiuretic hormone is secreted by neurons of the brain 4 5 Endocrine System • The endocrine system includes all the organs that secrete hormones – Endocrine – product secreted into extracellular fluid and carried in blood – Exocrine – secrete product into a duct • 2 basic hormone characteristics 1. Must be sufficiently complex to convey regulatory information to their target cells 2. Must be adequately stable to resist destruction before reaching their target cells 6 7 3 Classes of Hormones 1. Peptides and proteins – Glycoproteins 2. Amino acid derivatives – Catecholamines – Thyroid hormones – Melatonin 3. Steroids – Sex steroids – Corticosteroids 8 Classes of Hormones • Hormones may be categorized as: • Lipophilic (nonpolar) – fat-soluble – – – – Steroid hormones and thyroid hormones Travel on transport proteins in blood Bind to intracellular receptors Tend to act over brief time period • Hydrophilic (polar) – water-soluble – – – – All other hormones Freely soluble in blood Bind to extracellular receptors Tend to have much longer active period 9 10 Paracrine Regulators • Paracrine regulation occurs in most organs • Growth factors – Proteins that promote growth and cell division in specific organs • Epidermal growth factor – Activates mitosis in skin • Nerve growth factor – Stimulates growth and survival of neurons • Insulin-like growth factor – Stimulates cell division in developing bone • Cytokines – Specialize in control of cell division and differentiation in immune system 11 Paracrine Regulators • Paracrine regulation of blood vessels • Nitric oxide (NO) – Function as neurotransmitter – Produced by endothelium of blood vessels • Dilates arteries to control blood pressure • Paracrine regulation supplements autonomic nervous system 12 Paracrine Regulators • Prostaglandins – Diverse group of fatty acids that are produced in almost every organ – Regulate a variety of functions • Smooth muscle contraction, lung function, labor, and inflammation – Synthesis is inhibited by nonsteroidal antiinflammatory drugs (NSAIDs) such as aspirin and ibuprofen 13 Lipophilic Hormones • Lipophilic hormones include the steroid hormones and the thyroid hormones • Also retinoids, or vitamin A • Can enter cells through plasma membrane 14 Lipophilic Hormones • Circulate in the blood bound to transport proteins • Dissociate from carrier at target cells • Pass through the cell membrane • Bind to an intracellular receptor, either in the cytoplasm or the nucleus • Hormone-receptor complex binds to hormone response elements in DNA • Regulate gene expression 15 16 Hydrophilic Hormones • Peptide, protein, glycoprotein, and catecholamine hormones • Too large or polar to cross cell membrane • Bind to receptors on plasma membrane • Initiate signal transduction pathways • Activation of protein kinases – Activate or deactivate intracellular proteins by phosphorylation • Production of second messengers 17 18 Hydrophilic Hormones • Receptor kinases – For some peptide hormones (like insulin) the receptor itself is a kinase • Can directly phosphorylate intracellular proteins that alter cellular activity – For other peptide hormones (like growth hormone) the receptor itself is not a kinase • Rather, it activates intracellular kinases 19 Hydrophilic Hormones • Second-messenger systems – Many hydrophilic hormones work through second messenger systems – Two have been described • One involving cyclic adenosine monophosphate (cAMP) • One that generates 2 lipid messengers: inositol triphosphate (IP3) and diacyl glycerol (DAG) 20 Hydrophilic Hormones • Second-messenger systems – Receptors are linked to a second-messengergenerating enzyme via membrane proteins called G proteins • G protein–coupled receptors (GPCR) – When the G protein activates the enzyme, the second-messenger molecules increase • Cellular response depends on the type of G protein activated – Some activate while others inhibit their secondmessenger-generating system – Single hormone can have distinct actions in 2 different cells 21 The Pituitary Gland • Also known as the hypophysis • Hangs by a stalk from the hypothalamus • Consists of two parts – Anterior pituitary (adenohypophysis) • Appears glandular – Posterior pituitary (neurohypophysis) • Appears fibrous 22 The Posterior Pituitary • Appears fibrous because it contains axons that originate in cell bodies within the hypothalamus and that extend along the stalk of the pituitary as a tract of fibers – Develops from outgrowth of the brain • Stores and releases two hormones – Both are actually produced by neuron cell bodies in the hypothalamus 23 The Posterior Pituitary • Antidiuretic hormone (ADH) – Peptide hormone that stimulates water reabsorption by the kidney, and thus inhibits diuresis (urine production) • Oxytocin – Like ADH, composed of 9 amino acids – In mammals, it stimulates the milk ejection reflex and uterine contractions during labor, and it regulates reproductive behavior 24 25 The Anterior Pituitary • Develops from a pouch of epithelial tissue of the embryo’s mouth – Not part of the nervous system • Produces at least 7 essential hormones • Tropic hormones or tropins – Act on other endocrine glands • Can be categorized into three families – Peptide hormones, protein hormones, and glycoprotein hormones 26 The Anterior Pituitary • Peptide hormones – Cleaved from a single precursor protein • Fewer than 40 amino acids in size – Adrenocorticotropic hormone (ACTH) – Melanocyte-stimulating hormone (MSH) • Protein hormones – A single chain of about 200 amino acids – Growth hormone (GH) – Prolactin (PRL) 27 The Anterior Pituitary • Glycoprotein hormones – Thyroid-stimulating hormone (TSH) – Luteinizing hormone (LH) – Follicle-stimulating hormone (FSH) – FSH and LH • Function in both men and women • Referred to as gonadotropins 28 The Anterior Pituitary • Anterior pituitary is controlled by hormones from hypothalamus • Neurons secrete releasing hormones and inhibiting hormones, which diffuse into blood capillaries at the hypothalamus’ base • Each hormone delivered by the hypothalamohypophyseal portal system regulates a specific anterior pituitary hormone – Portal system has 2 capillary beds (not 1) 29 30 The Anterior Pituitary • The hypothalamus and the anterior pituitary are partially controlled by the very hormones whose secretion they stimulate • Negative feedback or feedback inhibition – Acts to maintain relatively constant levels of the target cell hormone • Positive feedback not as common – Causes deviations from homeostasis – Control of ovulation 31 Goiters are caused by a lack of iodine in the diet 32 Anterior Pituitary Disorders • Growth Hormone – Stimulates protein synthesis and growth of muscles and connective tissues – Stimulates production of insulin-like growth factors that stimulate cell division in epiphyseal growth plates – elongation of bone • Gigantism vs pituitary dwarfism – Also functions in adults to regulate protein, lipid, and carbohydrate metabolism • Acromegaly 33 Other anterior pituitary hormones • Prolactin – Acts on glands that are not endocrine glands – Actions appear diverse • Milk production in mammals, “crop milk” and brood patch in birds, electrolyte balance in kidneys • • • • TSH stimulates thyroid ACTH stimulates only adrenal cortex FSH and LH act only on the gonads MSH regulates melanophores or melanocytes that contain melanin 34 The Thyroid Gland • In humans, the thyroid gland is shaped like a bow tie, and lies just below the Adam’s apple in the front of the neck • Secretes – Thyroid hormones • Thyroxine • Triiodothyronine – Calcitonin 35 The Thyroid Gland • Thyroid hormones bind to nuclear receptors • Regulates enzymes controlling carbohydrate and lipid metabolism – Hypothyroidism vs hyperthyroidism in adults • Often functions synergistically with other hormones • Trigger metamorphosis in tadpoles 36 37 The Thyroid Gland • Calcitonin – Peptide hormone – Stimulates the uptake of calcium (Ca2+) into bones – lowering blood Ca2+ levels – Appears less important in the day-to-day regulation of Ca2+ levels in adult humans 38 The Parathyroid Glands • • • • 4 small glands attached to the thyroid Produce parathyroid hormone (PTH) Raises blood Ca2+ levels Stimulates osteoclasts to dissolve calcium phosphate crystals in the bone matrix and release Ca2+ into blood • Stimulates the kidneys to reabsorb Ca2+ from the urine • Vitamin D activated by a PTH controlled enzyme – Stimulates the intestinal absorption of Ca2+ 39 40 The Adrenal Glands • Medulla (inner portion) – Stimulated by the sympathetic division of the autonomic nervous system – Secretes the catecholamines epinephrine and norepinephrine • Cortex (outer portion) – Stimulated by anterior pituitary hormone ACTH – Corticosteroids • Glucocorticoids (like cortisol) act on various cells to maintain glucose homeostasis • Aldosterone (mineralocorticoid) helps regulate mineral balance 41 42 The Pancreas • Exocrine and endocrine gland • Connected to the duodenum of the small intestine by the pancreatic duct • Islets of Langerhans are scattered clusters of cells throughout the pancreas • These govern blood glucose levels through two hormones with antagonistic functions 43 The Pancreas • Insulin – Secreted by beta (b) cells of the islets – Stimulates cellular uptake of blood glucose and its storage as glycogen in the liver and muscle cells, or as fat in fat cells • Glucagon – Secreted by alpha (a) cells of the islets – Promotes the hydrolysis of glycogen in the liver and fat in adipose tissue 44 45 Diabetes Mellitus • Diabetics cannot take up glucose from blood • Type I (insulin-dependent diabetes) – Individuals lack insulin-secreting b cells – Treated by daily injections of insulin • Type II (noninsulin-dependent diabetes) – Most patients have this form – Very low number of insulin receptors – Treated by diet and exercise 46 The Gonads • Ovaries and testes in vertebrates • Produce sex steroids that regulate reproductive development • Estrogen and progesterone – “Female” hormones • Androgens – “Male” hormones – Testosterone and its derivatives 47 The Pineal Gland • Located in the roof of the third ventricle of the brain • Secretes hormone melatonin • Functions of melatonin – Reduces dispersal of melanin granules – Synchronizes various body processes to a circadian rhythm • Secretion of melatonin activated in the dark 48 Other Hormones • Some hormones are secreted by organs that are not exclusively endocrine glands • Atrial natriuretic hormone is secreted by the right atrium of the heart – Promotes salt and water excretion • Erythropoietin is secreted by the kidney – Stimulates the bone marrow to produce red blood cells 49 Insect Hormones • Insects undergo two types of transformations during postembryonic development • Molting – Shedding of old exoskeleton and secretion of a new larger one • Metamorphosis – Radical transformation from the larval to the adult form 50 Insect Hormones • Hormonal secretions influence both molting and metamorphosis • Brain hormone stimulates prothoracic gland to produce ecdysone, or molting hormone – High levels cause molting • Corpora allata produces juvenile hormone – Low levels result in metamorphosis 51 52