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CHAPTER 46
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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
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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 brain
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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 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
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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
system
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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
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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 antiinflammatory 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 messengers
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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
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)
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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
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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
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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
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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 hormones
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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
– 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
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
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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
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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 (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
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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
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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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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
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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
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