Types of Chemical
Messengers
A hormone is a chemical that is
secreted into extracellular fluid and
carried by the blood
-Can therefore act at a
distance from source
-Only targets with
receptor can respond
Paracrine regulators do not travel
in blood
-Allow cells of organ to
regulate each other (cytokines)
Pheromones are chemicals
released into the environment to
communicate among individuals of
a single species
Some neurotransmitters are
distributed by the blood and act as
neurohormones
-Norepinephrine
coordinates the activity of heart,
liver and blood vessels during
stress
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Endocrine System
The endocrine system includes all
the organs and tissues that produce
hormones
-Includes endocrine
glands, which are specialized to
secrete hormones
-Also
organs, like the liver, that secrete
hormones in addition to
other functions
Exocrine glands secrete their
products, such as saliva or milk,
into a duct for transport to the
outside
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Classes of Hormones
Molecules that function as
hormones must exhibit
two basic 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
Three chemical classes
meet these requirements
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
-Bind to intracellular receptors
-Hydrophilic (polar) = water-soluble
-All other hormones
-Bind to extracellular receptors
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Paracrine Regulators
Paracrine regulation occurs in
most organs
-Growth factors are
proteins that promote
growth and cell division in
specific organs
-Epidermal growth
factor = Skin
-Nerve growth factor
= Neurons
-Insulin-like growth
factor = Bone
-Cytokines = Immune
system
The endothelium of blood
vessels is a rich source of
paracrine regulators
-Nitric oxide (NO) which
promotes vasodilation
-Endothelin which
stimulates vasoconstriction
-Bradykinin which
promotes vasodilation
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Paracrine Regulators
Prostaglandins
-A diverse group of fatty acids that are
produced in almost every organ
-Regulate a variety of functions including:
-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 (derived from cholesterol) and
the thyroid hormones (tyrosine + iodine)
-As well as the retinoids, or vitamin A
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Lipophilic Hormones
These hormones circulate in the blood bound
to transport proteins
-Dissociate from carrier at target cells
-Pass through the cell membrane and
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
Hydrophilic hormones include the peptide,
protein and catecholamine hormones
-Too large or polar to cross cell membrane
Hormones bind to extracellular receptors
-Initiate signal transduction pathways
1. Activation of protein kinases
2. 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
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Second-messenger
systems
Receptors are linked to a
second-messengergenerating enzyme via
membrane proteins called G
proteins
-Are thus called G-proteincoupled receptors (GPCP)
-When the G protein
activates the enzyme, the
second-messenger
molecules increase
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The Pituitary Gland
The pituitary gland is also known
as the hypophysis
-It hangs by a stalk from the
hypothalamus
The pituitary gland consists of two
parts:
-Anterior pituitary
(adenohypophysis)
-Appears glandular
-Posterior pituitary
(neurohypophysis)
-Appears fibrous
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The Posterior Pituitary
The posterior pituitary develops from growth
of the brain
-It remains directly connected to the
hypothalamus by a tract of axons
It stores and releases two hormones, that are
actually produced by neuron cell bodies in
the hypothalamus
-Neuroendocrine reflex
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The Posterior Pituitary
Antidiuretic hormone (ADH)
-A peptide hormone that stimulates water
reabsorption by the kidney, and thus inhibits
diuresis (urine production)
Oxytocin
-Like ADH, it is 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
The anterior pituitary develops from a pouch
of epithelial tissue of the embryo’s mouth
It produces at least seven essential hormones
-Collectively called tropic hormones or
tropins
-Can be categorized into three families
-Peptide hormones, protein hormones
and glycoprotein hormones
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The Anterior Pituitary
Peptide hormones
-Fewer than 40 amino acids in size
-Adrenocorticotropic hormone (ACTH)
-Melanocyte-stimulating hormone (MSH)
Protein hormones
-A single chain of about 200 amino acid
-Growth hormone (GH)
-Prolactin (PRL)
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The Anterior Pituitary
Glycoprotein hormones
-Dimers, containing alpha (a) and beta (b)
subunits, each around 100 amino acids
-Thyroid-stimulating hormone (TSH)
-Luteinizing hormone (LH)
-Follicle-stimulating hormone (FSH)
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Anterior Pituitary
The activity of the anterior
pituitary is controlled by
hormones of the
hypothalamus
-Neurons secrete
releasing hormones and
inhibiting hormones,
which diffuse into blood
capillaries at the
hypothalamus’ base
-Each hormone
delivered by the
hypothalamohypophysial
portal system regulates a
specific anterior pituitary
hormone
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The Anterior Pituitary
The hypothalamus and the anterior pituitary
are partially controlled by the very hormones
whose secretion they stimulate
-This is termed negative feedback or
feedback inhibition
Positive feedback is uncommon because it
causes deviations from homeostasis
-One example is the control of ovulation
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The Anterior Pituitary
The anterior pituitary is sometimes referred to
as the “master gland”
-Its effects may be direct or indirect
-Direct: activation of nonendocrine targets
-Indirect: activation of other endocrine
glands
Hypophysectomy is the surgical removal of
the pituitary
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Anterior Pituitary Disorders
Growth hormone stimulates growth of
muscles and connective tissue
-It also promotes the production of insulinlike growth factors
-Stimulate cell division in the epiphyseal
growth plates, and thus bone elongation
Gigantism is caused by an excessive
secretion of growth hormone in a child
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Anterior Pituitary Disorders
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Anterior Pituitary Disorders
In contrast, pituitary dwarfism is caused by a
deficiency in GH secretion during childhood
GH can no longer cause an increase in height
in adults because human skeletal plates
transform from cartilage into bone at puberty
-Excessive GH secretion in an adult results
in acromegaly
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The Thyroid &
Parathyroid Glands
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
-It secretes:
-Thyroid hormones
Thyroxine
Triiodothyronine
Calcitonin
-A peptide hormone
-Stimulates the uptake of
calcium (Ca2+) into bones
-Thus, lowering its levels in
the blood
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The Thyroid Gland
Thyroid hormones bind to nuclear receptors
-Regulate carbohydrate & lipid metabolism
-Adults with hypothyroidism have low
production of thyroxine
-Reduced metabolism and overweight
-Adults with hyperthyroidism have high
production of thyroxine
-High metabolism and weight loss
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-Trigger metamorphosis in amphibians
The Parathyroid Glands
The parathyroid glands are four small
glands attached to the thyroid
-Produce parathyroid hormone (PTH) in
response to falling levels of Ca2+ in blood
-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
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The Parathyroid Glands
PTH also indirectly leads to the absorption of
Ca2+ from food in the intestines by its
activation of vitamin D
-Vitamin D is produced in the skin from a
cholesterol derivative in response to UV light
-It diffuses into blood in an inactive form
-Activated by an enzyme that is
stimulated by PTH
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The Adrenal Glands
The adrenal glands are located
just above each kidney
-Medulla = Inner portion
-Stimulated by the
sympathetic division of the
autonomous nervous system
-Cortex = Outer portion
-Stimulated by the
anterior pituitary, through the
hormone ACTH
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The Adrenal Glands
The medulla secretes the catecholamines
epinephrine and norepinephrine
-These trigger “alarm” responses helping
the body prepare for extreme efforts
-Lead to an increase in: heart rate,
blood pressure, blood glucose level, and
blood flow to heart and muscle
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The Adrenal Glands
The cortex secretes steroid hormones called
corticosteroids
-Glucocorticoids (such as cortisol)
maintain glucose homeostasis and modulate
some aspects of the immune response
-Mineralocorticoids (such as aldosterone)
regulate mineral balance by stimulating the
kidneys to reabsorb Na+ and excrete K+
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The Pancreas
The pancreas is located adjacent to stomach
-Connected to the duodenum of the small
intestine by the pancreatic duct
The 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
The 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
The pineal gland is located in the roof of the
third ventricle of the brain
-Functions as an endocrine gland by
secreting the hormone melatonin
-Reduces dispersal of melanin granules
-Regulates biological clocks
-Synchronizes various body
processes to a circadian rhythm
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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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