The Endocrine System

Chapter 46

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

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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

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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 brain4

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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 characteristics1.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

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3 Classes of Hormones

1. Peptides and proteins– Glycoproteins

2. Amino acid derivatives– Catecholamines– Thyroid hormones– Melatonin

3. Steroids– Sex steroids– Corticosteroids

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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

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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

system11

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

• Endothelin stimulates vasoconstriction

• Bradykinin promotes vasodilation

• Paracrine regulation supplements autonomic nervous system

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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 anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen

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Lipophilic Hormones

• Lipophilic hormones include the steroid hormones and the thyroid hormones

• Also retinoids, or vitamin A

• Can enter cells through plasma membrane

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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

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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 messengers17

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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

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Hydrophilic Hormones

• Second-messenger systems– Many hydrophilic hormones work through

second messenger systems– One involving cyclic adenosine

monophosphate (cAMP)

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Hydrophilic Hormones• Second-messenger systems

– Receptors are linked to a second-messenger-generating 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 second-messenger-

generating system– Single hormone can have distinct actions in 2 different cells

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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

– Different embryonic origins– Different hormones

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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– Neuroendocrine reflex

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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

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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 hormones26

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)

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The Anterior Pituitary

• Glycoprotein hormones– Dimers, containing alpha () and beta ()

subunits, each around 100 amino acids – Thyroid-stimulating hormone (TSH)– Luteinizing hormone (LH)– Follicle-stimulating hormone (FSH)– FSH and LH

• Function in both men and women• Referred to as gonadotropins

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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)

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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

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Goiters are caused by a lack of iodine

in the diet

The Anterior Pituitary

• Pituitary gland was referred to as the “master gland”– Hypophysectomy caused a number of deficits

• Effects may be direct or indirect– Direct: activation of nonendocrine targets

• Growth hormone, prolactin, and MSH

– Indirect: activation of other endocrine glands• Tropic hormones ACTH, TSH, LH, and FSH

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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

• Acromegaly34

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 melanin35

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

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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

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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

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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+

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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

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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

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The Pancreas

• Insulin – Secreted by beta () 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 () cells of the islets– Promotes the hydrolysis of glycogen in the

liver and fat in adipose tissue45

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Diabetes Mellitus

• Diabetics cannot take up glucose from blood

• Type I (insulin-dependent diabetes)– Individuals lack insulin-secreting 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

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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

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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

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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

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