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Chapter 17
Functional Organization of the
Endocrine System
17-2
17.1 Principles of Chemical
Communication
• Classes of Chemical Messengers
– Autocrine chemical messengers: released by cells and have a
local effect on same cell type from which chemical signals
released; e.g., prostaglandin
– Paracrine chemical messengers: released by cells and affect other
cell types locally without being transported in blood; e.g.,
somatostatin
– Neurotransmitter: produced by neurons and secreted into
extracellular spaces by presynaptic nerve terminals; travels short
distances; influences postsynaptic cells; e.g., acetylcholine.
– Endocrine chemical messengers: type of intercellular signal.
Produced by cells of endocrine glands, enter circulatory system,
and affect distant cells; e.g., estrogen
17-3
17-4
Characteristics of the Endocrine
System
• Glands that secrete chemical messengers
(hormones) into circulatory system
• Hormone characteristics
–
–
–
–
Produced in small quantities
Secreted into intercellular space
Transported some distance in circulatory system
Acts on target tissues elsewhere in body
• Regulate activities of body structures
• Ligands: more general term for chemical signals
17-5
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
17-6
•
Comparison of Nervous and
Endocrine Systems
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
–
Amplitude vs. frequency
17-7
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17-8
17.2 Hormones
•
General Characteristics of Hormones
1. Stability
•
Half-life: The length of time it takes for half a dose of substance to be
eliminated from circulatory system
–
–
Long half-life: regulate activities that remain at a constant rate through
time. Usually lipid soluble and travel in plasma attached to proteins
Short half-life: water-soluble hormones as proteins, epinephrine,
norepinephrine. Have a rapid onset and short duration
2. Communication
•
Interaction with target cell
3. Distribution
•
•
Hormones dissolve in blood plasma and are transported in unbound or
are reversibly bound to plasma proteins.
Hormones are distributed quickly because they circulate in the blood.
17-9
Protein Bound Transport
17-10
17-11
17-12
Patterns of Hormone Secretion
•
Chronic hormone regulation.
Maintenance of relatively
constant concentration of
hormone. Thyroid hormone.
•
Acute hormone regulation.
Epinephrine in response to stress.
•
Episodic (Cyclic) hormone
regulation. Female reproductive
hormones.
17-13
17.3 Control of Hormone Secretion
•
•
Most hormones controlled by negative feedback
systems
Most hormones are not secreted at constant rate,
but their secretion is regulated by three different
methods
1. The action of a substance other than a hormone on an
endocrine gland.
2. Neural control of endocrine gland.
3. Control of secretory activity of one endocrine gland
by hormone or neurohormone secreted by another
endocrine gland
17-14
Control by Humoral Stimuli
17-15
Control by Neural Stimuli
17-16
Control by Hormonal Stimuli
17-17
Negative Feedback
17-18
Positive Feedback
17-19
17.4 Hormone Receptors and
Mechanisms of Action
17-20
Target Tissue Specificity and Response
• Portion of molecule where
hormone binds is called
binding site.
• If the molecule is a receptor
(like in a cell membrane) the
binding site is called a
receptor site
• hormone/receptor site is
specific; e.g., epinephrine
cannot bind to the receptor
site for insulin.
• The purpose of binding to
target tissue is to elicit a
response by the target cell.
17-21
Decrease in Receptor Number
• Normally, receptor molecules are degraded and replaced on a
regular basis.
• 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.
17-22
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. Ovarian cells are now more
sensitive to LH, even if the concentration of LH does not
change. This causes ovulation.
17-23
Classes of Receptors
• Lipid-soluble hormones bind
to nuclear receptors
• Hormones
– Lipid soluble and
relatively small molecules;
pass through the plasma
membrane
– React either with enzymes
in the cytoplasm or with
DNA to cause
transcription and
translation
– Thyroid hormones,
testosterone, estrogen,
progesterone, aldosterone,
and cortisol
17-24
Classes of Receptors
• Water-soluble hormones bind to membrane-bound
receptors: integral proteins with receptor site at
extracellular surface. Interact with hormones that
cannot pass through the plasma membrane.
• Hormones
– Water-soluble or largemolecular-weight
hormones. Attachment of
hormone causes
intracellular reaction.
– Large proteins,
glycoproteins,
polypeptides; smaller
molecules like
epinephrine and
norepinephrine
17-25
17-26
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.
17-27
17-28
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17-29
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
17-30
17-31
Insert table 17.5
17-32
Receptors that Activate G Proteins
17-33
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17-34
G Proteins that open Calcium ion Channels
17-35
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17-36
G Proteins that Interact with
Adenylate Cyclase
17-37
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17-38
G Proteins that Interact with other
Intercellular Mediators
17-39
Receptors That Directly Alter the
Activity of Intracellular Mediator
17-40
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.
17-41
Signal Amplification
17-42
17-43