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The Endocrine System
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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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 cells3
4
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Adenohypophysis
HypothalamusHypothalamus Pineal gland
Neurohypophysis Pituitary
gland
Thymus
Thyroid gland
Pancreas
Testes
(in males)
Ovaries (in females)
Pineal gland
Parathyroid glands
(behind thyroid)
Adrenal
glands
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6
7
8
9
Hydrophilic
ransport proteins
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Activated4
Target cell
Nontarget
cells
3
Activated
3
Target cellNuclear
receptor
5. Unused,
deactivated
hormones
are removed
by the liver
and kidney.
4. Target cells
possess
receptors,
and are
activated by
hormones.
Lipophilic
3. Nontarget
cells lack
receptors,
and cell
stimulation
does not
occur.
Nontarget
cells
T
1 1
Blood
vessels
Hormones
Secretory
vesicles
Extracellular
fluid
Extracellular
fluid
Endocrine gland B
1. Hormones
secreted into
extracellular
fluid and
diffuse into
bloodstream.Hormones
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Endocrine gland A
Membrane
receptors
2. Hormones
distributed by
blood to all
cells. Diffuse
from blood to
extracellular
fluid.
2
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 10
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
GTPGTP
1. Receptors Function as Kinase Enzymes
γ
αβ
2. Receptors Activate G Proteins
Hormones
GPCR GPCR
Second messenger-
generating enzyme
Active G
protein
Second
messenger
Active
protein kinase
Target
proteins
Inactive
protein kinase
Inactive G
protein
Cellular
response
ATP
Hormone
Receptor
Inactive
Active kinase
domain
ADP
Phosphorylated
proteinTarget
protein
Cellular
response
GDP
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
– 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-messenger-
generating enzyme via membrane proteins called G
proteins
• G protein–coupled receptors (GPCRs)
– 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 14
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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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
• Acromegaly16
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 – metabolic rate and body temperature
• Triiodothyronine
– Calcitonin
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The Pancreas
• Exocrine and endocrine glands
• 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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( – )( – )Blood glucose increased
Stimulus
Blood glucose decreased
Stimulus
After a Meal Between Meals
EffectorEffector
Ne
ga
tive
fe
ed
ba
ck
Ne
ga
tive
fe
ed
ba
ck
Increased glucagon
production
By a cells
Increased insulin
production
by β cells
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ResponseResponse
Glucose moves from
blood into cells,
reducing blood glucose
Glycogen hydrolized to
glucose, then secreted into
blood, increasing blood glucose
( + )( + )
Sensor
Pancreatic
Islets
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 exercise22
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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