Endocrine System
 Overview
 Hypothalamus
and pituitary
gland
 Hormones,
receptors and
their actions
– Second
messengers
Cell-Cell Communication


Necessary for
integration of body
activities
Mechanisms
– neurotransmitters

released from neurons
to travel across gap to
2nd cell
– gap junctions

holes in cell
membranes between
cells: links cells
electrically and
metabolically
– paracrine (local)
hormones

secreted into tissue
fluids to affect nearby
cells
Components of Endocrine System

Endocrine glands
– produce hormones

Hormone
– chemical messenger
secreted into bloodstream,
stimulates response in
another tissue or organ

Target cells
– have receptors
for hormone
Endocrine
cell
Endocrine
tissue
Hormone
Interstitial
space
Blood
Target tissue
Differences in Nervous and Endocrine
Systems

Means of communication
– nervous system has both
electrical and chemical
methods
– endocrine system has only
chemical methods

Speed and persistence of
response
– nervous system reacts quickly
(1 - 10 msec) and stops
quickly
– endocrine system reacts
slowly (hormone release in
seconds or days), effect may
continue for weeks

Area of effect
– nervous system effects are
targeted and specific (one
organ or tissue)
– endocrine system may have
general, widespread effects on
many organs
Similarities in Nervous and Endocrine
Systems

Several chemicals function as both hormones and
neurotransmitters
– norepinephrine, epinephrine, dopamine

Some hormones secreted by “neuroendocrine”
cells (neurons)
– oxytocin and antidiuretic hormone

Overlapping effects on same target cells
– norepinephrine and glucagon cause glycogen hydrolysis
in liver

Systems regulate each other
– neurons trigger hormone secretion
– hormones stimulate or inhibit neurons
Endocrine Organs
Hypothalamohypophyseal Axis


Hypothalamus
Regulates primitive
functions from water
balance to sex drive
Many functions carried
out by pituitary gland


Pituitary Gland
(Hypophysis)
Suspended from
hypothalamus by stalk
(infundibulum)
Posterior and anterior
parts
Hypothalamohypophyseal
tract
Hypothalamohypophyseal
portal system
Hypothalamohypophyseal Portal System
Gonadotropin- releasing hormone
controls FSH + LH release
Thyrotropin- releasing hormone
Corticotropin- releasing hormone
Prolactin- releasing hormone
Prolactin- inhibiting hormone
GH- releasing hormone
Somatostatin


Hormones secreted by hypothalamus, travel in portal system to anterior pituitary
Hormones (red box) secreted by anterior pituitary (under control of hypothalamic
releasers and inhibitors)
Pituitary Hormones - Anterior Lobe

Tropic hormones target
other endocrine glands
– gonadotropins target
gonads, FSH (follicle
stimulating hormone)
and LH (luteinizing
hormone)
– TSH (thyroid stimulating
hormone)
– ACTH
(adrenocorticotropic
hormone)


PRL (prolactin)
GH (growth hormone )
Pituitary Hormone Actions
Anterior Lobe Hormones

FSH
– ovaries, stimulates development of eggs and follicles
– testes, stimulates production of sperm

LH
– females, stimulates ovulation and corpus luteum to secrete
progesterone and estrogen
– males, stimulates interstitial cells of testes to secrete testosterone

TSH
– stimulates growth of gland and secretion of thyroid hormone (TH)

ACTH or corticotropin
– regulates response to stress, stimulates adrenal cortex to secrete of
corticosteroids that regulate glucose, fat & protein metabolism

PRL
– female, milk synthesis after delivery
– male,  LH sensitivity, thus  testosterone secretion

GH or somatotropin
– promotes tissue growth
Pituitary Hormones - Posterior Lobe
 Stores
and releases
oxytocin and ADH
 ADH
– targets kidneys to 
water retention,
reduce urine
 Oxytocin
– labor contractions,
lactation
– possible role sperm
transport, emotional
bonding
Hormone Chemistry

Steroids
– derived from cholesterol
– lipid soluble; water insoluble;
hydrophobic
– act via intracellular receptors
– turn on gene transcription

sex steroids, corticosteroids
Hormone Chemistry

Steroids

Polypeptides and proteins
– small proteins made by
ER/GN
– often processed and
modified
– water soluble; hydrophilic
– act via membrane-bound
receptors
– modify existing metabolic
proteins

Oxy, ADH; all releasing and
inhibiting hormones of
hypothalamus; most of
anterior pituitary hormones
Hormone Chemistry

Steroids

Polypeptides and proteins

Monoamines (biogenic
amines)
– derived from amino acids
(usually tyrosine)
– water soluble; hydrophilic
– act via membrane-bound
receptors
– modify existing metabolic
proteins

catecholamines
(norepinephrine,
epinephrine, dopamine)
and thyroid hormones
Hormone Transport


Monoamines and peptides/proteins are hydrophilic so
mix easily with blood plasma – dissolved
Steroids and thyroid hormone are hydrophobic and
must bind to transport proteins for transport
– bound hormone - hormone attached to transport protein,
(prolongs half-life to weeks, protects from enzymes and
kidney filtration)
– only unbound hormone can leave capillary to reach target
cell (half-life a few minutes)

Transport proteins in blood plasma
Hormone Receptors

Located on plasma
membrane (membranebound) or intracellular
(soluble within cytoplasm
or nucleus)
– sometimes found on the
membrane of mitochondria
and other organelles,
– usually thousands for given
hormone
– turn activities on or off
when hormone binds



metabolic pathways
gene expression
Exhibit specificity and
saturation
Hormone Mode of Action

Hydrophobic
hormones (steroids
and thyroid hormone)
penetrate by diffusion
of the plasma
membrane
– ultimately enter
nucleus

Hydrophilic hormones
(monoamines and
peptides) can not pass
through membrane so
bind to membranebound receptors
– 2nd messenger
activation
Which is it?
Receptor Response to Ligand
Change shape, change function
activate internal proteins, enzymes, transcription factors etc
Membrane Receptor Action
Ion Channel Activation

Hormone binding to receptor results in
depolarization or hyperpolarization of the membrane
Membrane Receptor Action
G-protein Activation

Hormone binding to receptor results in modification
(activation or inhibition) of “downstream” enzymes
and 2nd messenger formation or degradation
E
E
b
g
R
b
a
b
GTP
b
g
g
GDP
E*
g
g a
GDP
GDP
b
R*
E*
R*
a*
GTP
b
GDP
g
R*
a*
GTP
Second Messengers
Cyclic nucleotides
(cAMP, cGMP)
 Diacylglycerol
(DAG)
 Inositol
triphosphate (IP3)
 Calcium

cAMP as a Second Messenger
1) Hormone binding
activates G protein
2) GP activates adenylate
cyclase
3) AC produces cAMP
4) cAMP activates kinases
5) Kinases activate
enzymes
6) Metabolic reactions:
synthesis, secretion,
degradation, altered
membrane potentials
Membrane Receptor Action
Enzyme Receptor Activation

Hormone binding to receptor results in activation of
intrinsic (receptor) enzyme and “downstream”
metabolic changes
Membrane Receptor Action
Intracellular Receptors

Hormone binding to receptor results in activation of
transcription factors and gene expression
Summary of Hormonal Actions
Hormone
Hormone
Membrane -bound
Intracellular
receptor
receptor
Receptor linked
Receptor linked
Receptor linked
Activates
to ion channel
to G protein
to enzyme
genes
Opens or closes
Activates already
Synthesizes
Ion channels
existing enzymes
enzymes
Cell Response
Regulation of Hormone Secretion

Non-hormonal
– Control of release
dependent upon
concentration of other
non-hormonal substance
(i.e., glucose)
Regulation of Hormone Secretion


Non-hormonal
Neuronal
– Secretion or
inhibition of
release influenced
by emotions or
stress
Regulation of Hormone Secretion



Non-hormonal
Neuronal
Hormonal
– Control of release
regulated by
other hormones
Review questions
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12.
What are the different methods of cell to cell signaling? Provide
an example of where/when each method occurs.
Name and approximately locate the main endocrine glands.
Compare and contrast the nervous and endocrine systems.
Describe the relationship between the hypothalamus and the
pituitary gland. What is the tract system? What about the portal
system?
Which hormones are released from the anterior pituitary and
which from the posterior pituitary? Briefly describe the actions of
these hormones on their targets.
What are releasing and inhibiting hormones? Where are they
released from? What is there target (usually)?
Compare and contrast the main classes/types of hormones and
their receptors.
What different ways do membrane-bound receptors act upon their
target cell? Provide an example of each of these types of activity.
Explain how intracellular receptors differ from membrane-bound
receptors (include cellular location and mechanism of action).
Briefly summarize the cellular effect of hydrophilic hormone
activation and hydrophobic hormone activation.
What are second messengers and why are they important? Be
able to provide at least one example of cAMP acting as a second
messenger.
How is hormonal release regulated? Provide an example for each
way.