Endocrine System
17-1
I. Introduction to endocrine system
• A. Classes of Chemical Messengers
– 1. Autocrine chemical messengers: released by cells and have a
local effect on same cell type from which chemical signals
released; e.g., prostaglandin
– 2. Paracrine chemical messengers: released by cells and affect
other cell types locally without being transported in blood; e.g.,
somatostatin
– 3. Neurotransmitter: produced by neurons and secreted into
extracellular spaces by presynaptic nerve terminals; travels short
distances; influences postsynaptic cells; e.g., acetylcholine.
– 4. Endocrine chemical messengers: type of intercellular signal.
Produced by cells of endocrine glands, enter circulatory system,
and affect distant cells; e.g., estrogen
17-2
17-3
B. Characteristics of the Endocrine System
• 1. Glands that secrete chemical messengers (hormones)
into circulatory system
• 2. Hormone characteristics
– Produced in small quantities
– Secreted into intercellular space
– Transported some distance in circulatory system
– Acts on target tissues elsewhere in body
• 3. Regulate activities of body structures
17-4
C. Comparison of Nervous and Endocrine
Systems
•
Similarities
1. Both systems associated with the brain
• Hypothalamus
• Epithalamus
2. May use same chemical messenger as neurotransmitter and
hormone.
• Epinephrine
3. Two systems are cooperative
• Nervous system secretes neuroendocrine peptides, or
neurohormones, into circulatory system
• Some parts of endocrine system innervated directly by
nervous system
17-5
Differences
1. Mode of transport
– Axon
– Blood
2. Speed of response
– Nervous – instant/milliseconds
– Endocrine – delayed/seconds
3. Duration of response
– Nervous – milliseconds/seconds
– Endocrine – minutes/days
4. Amplitude vs. frequency
17-6
II. General characteristics of hormones
A. Stability
• 1. Half-life: The length of time it takes for half a dose of substance to
be eliminated from circulatory system
2. Long half-life: regulate activities that remain at a constant rate
through time. Usually lipid soluble and travel in plasma attached to
proteins
3. Short half-life: water-soluble hormones as proteins, epinephrine,
norepinephrine. Have a rapid onset and short duration
B. Communication
• 1. Interaction with target cell
• 2. Lipid soluble hormones pass through cell membrane and usually
travel to nucleus
• 3. Water soluble hormones generally attach to a receptor site on cell
membrane
C. Distribution
• 1. Hormones dissolve in blood plasma and are transported in
unbound or are reversibly bound to plasma proteins.
• 2. Hormones are distributed quickly because they circulate in the
blood.
17-7
• D. Lipid soluble hormones
•
1. must connect to binding proteins in blood or would be catabolized quickly
•
2. long half lives as they are protected
•
3. tend to have more constant blood levels and regulate basal activity
•
3. the liver can attach water soluble radicals to these enzymes so that the
kidneys can excrete the hormone
17-8
•
4. process called conjugation
• E. Water soluble hormones
•
1. travel freely in circulatory system
•
2. larger and don’t diffuse through membranes easily
•
3. fenestrated target tissues
•
4. short half lives due to proteases circulating in blood
•
5. concentrations tend to change rapidly and they regulate activities with rapid
onset and short duration
17-9
17-10
17-11
III. Patterns of Hormone Secretion
•
A. Chronic hormone regulation.
1. Maintenance of relatively
constant concentration of
hormone. Thyroid hormone.
•
B. Acute hormone regulation.
Epinephrine in response to stress.
•
C. Episodic (Cyclic) hormone
regulation. Female reproductive
hormones.
17-12
IV. Control of Hormone Secretion
•
•
•
•
•
Most hormones controlled by negative feedback systems
In negative feedback, the eventual product turns off the
sensor which initiated the products synthesis-self limiting
Common example is the heating of your house
In positive feedback, the product stimulates the sensor to
increase its own production-self propagating
Most hormones are not secreted at constant rate, but their
secretion is stimulated by three different methods
1. The action of a substance other than a hormone on an
endocrine gland-humoral control-pth and Ca ion
2. Neural control of endocrine gland-ie adrenal cortex
3. Control of secretory activity of one endocrine gland by
hormone or neurohormone secreted by another endocrine
gland-pituitary gland and thyroid
17-13
Control by Humoral Stimuli
17-14
Control by Neural Stimuli
17-15
Control by Hormonal Stimuli
17-16
17.4 Hormone Receptors and
Mechanisms of Action
17-17
V. Target Tissue Specificity and Response
• A. Portion of molecule where
hormone binds is called binding
site.
• B. If the molecule is a receptor (like
in a cell membrane) the binding site
is called a receptor site
• C. hormone/receptor site is
specific; e.g., epinephrine cannot
bind to the receptor site for insulin.
• D. The purpose of binding to target
tissue is to elicit a response by the
target cell.
• E. Diagram shows what happens
with larger water soluble hormone
which binds to a receptor on the
membrane surface
17-18
F. Changes in Receptor Number
• 1. Normally, receptor molecules are degraded and replaced on a
regular basis.
• 2. Down-regulation
– Rate at which receptors are synthesized decreases in some cells after the
cells are exposed to a hormone.
– Combination of hormones and receptors can increase the rate at which
receptor molecules are degraded. This combined form is taken into the cell
by phagocytosis and then broken down.
– Explains the desensitization that can occur to some drugs
17-19
3. Increase in Receptor Number- Up regulation
– Some stimulus causes increase in synthesis of receptors for a
hormone, thus increases sensitivity to that hormone
– For example, FSH stimulation of the ovary causes an
increase of LH receptors.
– Prepares the ovarian cells’ membranes to prepare for the LH
surge that stimulates ovulation
17-20
VI. Classes of Receptors
A. Lipid soluble hormones
• 1. Lipid-soluble hormones bind to
nuclear receptors
• 2. Lipid soluble hormones are
relatively small molecules; pass
through the plasma membrane
• 3. React either with enzymes in the
cytoplasm or with DNA to cause
transcription and translation
• 4. Examples include thyroid
hormones, testosterone, estrogen,
progesterone, aldosterone, and
cortisol
• 5. because of transcription and
translation processes, there is a lag
time between hormone binding and
having its effect exerted
17-21
B. Water-soluble hormones
• 1. Water-soluble hormones
bind to membrane-bound
receptors
• 2. integral proteins with
receptor site at extracellular
surface.
• 3. Interact with hormones
that cannot pass through the
plasma membrane.
• 4. Attachment of hormone
causes intracellular reaction
• 5. often binding of hormone
causes the production of a
second molecule that
activates existing internal
systems
• 6. adrenalin acts this way
• 7. faster acting and shorter
lived affects when compared
to lipid soluble hormones
17-22
17-23
Action of Nuclear Receptors
• Proteins in cytoplasm or nucleus
• Hormones bind with intracellular receptor and
receptor-hormone complex activate certain genes,
causes transcription of mRNA and translation.
These proteins (enzymes) produce the response of
the target cell to the hormone
• Latent period of several hours because time is
required to produce mRNA and protein
• Processes limited by breakdown of receptorhormone complex
• Estrogen and testosterone produce different
proteins in cells that cause the differing secondary
sexual characteristics of females and males.
17-24
17-25
Membrane-Bound Receptors
•Intracellular mediators: ions or molecules that
enter cell or are produced in cell
•Can be produced because of G protein activation
•Regulate intracellular enzyme activities
17-26
17-27
Insert table 17.5
17-28
Receptors that Activate G Proteins
17-29
G Proteins that open Calcium ion Channels
17-30
G Proteins that Interact with
Adenylate Cyclase
17-31
G Proteins that Interact with other
Intercellular Mediators
17-32
Receptors That Directly Alter the
Activity of Intracellular Mediator
17-33
Receptors That Phosphorylate
Intracellular Proteins
• Hormones bind to
membrane-bound
receptors.
• Part of receptor protein on
inside of membrane acts
as an enzyme to
phosphorylate proteins
• E.g., insulin receptors
bound to insulin cause
phosphorylation of
proteins and cell responds
to presence of insulin.
17-34
Signal Amplification
17-35
17-36