Download Hormones and Their Actions

Animal Hormones
Animal Hormones
• Introduction
• Hormones and Their Actions
• Hormonal Control of Molting and Development in
Insects
• Vertebrate Endocrine Systems
• Hormone Actions: The Role of Signal Transduction
Pathways
Hormones and Their Actions
• Hormone-secreting cells are called endocrine
cells.
• Cells receiving the hormonal message are called
target cells and must have appropriate receptors.
• The binding of the receptor activates a response.
• The distance over which the signal operates
distinguishes hormone groups; some act close to
the release site, others at distant body locations.
Hormones and Their Actions
• Hormones can be classified into three main groups:
 Peptides or proteins. They are water soluble and
transported by vesicles out of the cell that made
them.
 Steroid hormones are lipid-soluble and can
diffuse out of the cell that made them but in the
blood they must be bound to carrier proteins.
 Amine hormones are derivatives of the amino
acid tyrosine. Some are water-soluble and some
are lipid-soluble.
Hormones and Their Actions
• The receptors for lipid-soluble hormones are
inside cells, either in the cytoplasm or in the
nucleus.
• The action of lipid-soluble hormones is mediated
by intracellular hormone receptors that usually
alter gene expression.
Hormones and Their Actions
• The receptors for water-soluble proteins are large
glycoproteins on the cell surface with three
domains:
 A binding domain projecting outside the plasma
membrane
 A transmembrane domain that anchors the
receptor in the membrane
 A cytoplasmic domain that extends into the
cytoplasm of the cell
• The cytoplasmic domain initiates the target cell’s
response by activating protein kinases or protein
phosphatases.
Hormones and Their Actions
• Some hormones act locally.
• Autocrine hormones act on the secreting cell
itself.
• Paracrine hormones act on cells near the site of
release.
• Paracrine hormones are released in tiny amounts,
or are inactivated rapidly by enzymes, or are
taken up efficiently by local cells. They never get
into the circulatory system.
Figure 42.1 Chemical Signaling Systems
Hormones and Their Actions
• Growth factors, which stimulate growth and
differentiation of cells, are a major class of
paracrine hormones.
• Growth factors also act as autocrine hormones:
Some of the hormone influences the cell that
secreted it, preventing the cell from secreting too
much hormone.
• Neurons may also be considered to be paracrine
cells because they use chemicals called
neurotransmitters to send messages to another
cell.
Hormones and Their Actions
• Most hormones diffuse into the blood, which
distributes them throughout the body.
• When the hormone message encounters a cell with
the proper receptor, it binds and triggers a
response.
• The same hormone can cause different responses
in different types of cells.
• An example is epinephrine. The nervous system
reacts to an emergency very quickly and stimulates
adrenal cells to secrete epinephrine. The result is
the fight-or-flight response.
Hormones and Their Actions
• The epinephrine acts on different cells in the body:
 In the heart, it stimulates faster and stronger
heartbeat.
 Blood vessels in some areas constrict to send
more blood to muscles.
 In the liver, glycogen is broken down to glucose
to provide quick energy.
 In fat tissue, fats are mobilized as another
energy source.
Hormones and Their Actions
• Endocrine refers to cells or glands that do not
have ducts leading to the outside of the body;
they secrete their products directly into the
extracellular fluid.
• Some endocrine cells are single cells within a
tissue.
• Digestive hormones, for example, are secreted by
isolated endocrine cells in the wall of the stomach
and small intestine.
• Some endocrine cells aggregate into secretory
organs called endocrine glands.
• In vertebrates, nine major endocrine glands make
up the endocrine system.
Figure 42.2 The Endocrine System of Humans
Hormonal Control of
Molting and Development in Insects
• Hormonal control is more complex in insects
having complete metamorphosis.
• An example is the silkworm. The egg hatches into
a larva that has a high amount of juvenile hormone
in its body.
• As long as the level of juvenile hormone stays high,
larvae molt into larvae; when the juvenile hormone
level wanes, pupae are formed.
• No juvenile hormone is found in the pupae, so they
molt into adults.
Figure 42.4 Complete Metamorphosis
Animation
Vertebrate Endocrine Systems
• The posterior pituitary releases two hormones:
antidiuretic hormone and oxytocin.
• They are made by neurons in the hypothalamus,
are called neurohormones, and are packaged in
vesicles.
• The vesicles are transported down the axons of
the neurons that made them and are stored in the
posterior pituitary.
• This movement of the vesicles is achieved by
kinesin proteins, powered by ATP, that “walk”
down the microtubules of the axon.
Figure 42.5 The Posterior Pituitary Releases Neurohormones
Animation
Vertebrate Endocrine Systems
• The anterior pituitary releases four tropic
hormones, which control activities of other
endocrine glands.
• They are peptide and protein hormones; each is
produced by a different type of pituitary cell.
• The four tropic hormones are: thyrotropin,
adrenocorticotropin, luteinizing hormone, and
follicle-stimulating hormone.
Figure 42.7 Hormones from the Hypothalamus Control the Anterior Pituitary
Table 42.2 Releasing and Release-Inhibiting Neurohormones of the Hypothalamus
Figure 42.8 Multiple Feedback Loops Control Hormone Secretion
Animation 1, 2
Vertebrate Endocrine Systems
• The adrenal glands are made up of the adrenal
medulla and the adrenal cortex.
• The medulla produces epinephrine and
norepinephrine.
• The medulla develops from the nervous system
and remains under its control.
• The cortex is under hormonal control, mainly by
adrenocorticotropin (ACTH) from the anterior
pituitary.
Figure 42.10 The Adrenal Gland Has an Outer and an Inner Portion
Vertebrate Endocrine Systems
• The adrenal medulla produces epinephrine
(adrenaline) in response to stress, initiating fightor-flight reactions, such as increased heart and
breathing rates and elevated blood pressure.
• It also produces norepinephrine, a
neurotransmitter involved in physiological
regulation.
• Epinephrine and norepinephrine are amine
hormones. They bind to two types of receptors in
target cells: a-adrenergic and b-adrenergic.
Vertebrate Endocrine Systems
• Norepinephrine acts mostly on the alpha type, so
drugs called beta blockers, which inactivate only
b-adrenergic receptors, can be used to reduce
fight-or-flight responses to epinephrine.
• The beta blockers leave the alpha sites open to
norepinephrine and its regulatory functions.
Vertebrate Endocrine Systems
• Adrenal cortex cells use cholesterol to produce
three classes of steroid hormones called
corticosteroids:
 Glucocorticoids influence blood glucose
concentrations and other aspects of fuel
molecule metabolism.
 Mineralocorticoids influence extracellular
ionic balance.
 Sex steroids stimulate sexual development
and reproductive activity. These are secreted
in only minimal amounts by the adrenal cortex.
Figure 42.11 The Corticosteroid Hormones are Built from Cholesterol
Vertebrate Endocrine Systems
• The main mineralocorticoid, aldosterone,
stimulates the kidney to conserve sodium and
excrete potassium.
• The main glucocorticoid, cortisol, mediates the
body’s response to stress.
• The fight-or-flight response ensures that muscles
have adequate oxygen and glucose for immediate
response.
Vertebrate Endocrine Systems
• Shortly after a frightening stimulus, blood cortisol
rises.
• Cortisol stimulates cells that are not critical to the
emergency to decrease their use of glucose.
• It also blocks the immune system reactions, which
temporarily are less critical.
• Cortisol can therefore be used to reduce
inflammation and allergy.
Vertebrate Endocrine Systems
• Cortisol release is controlled by ACTH from the
anterior pituitary which, in turn is controlled by
adrenocorticotropin-releasing hormone from
the hypothalamus.
• The cortisol response is much slower than the
epinephrine response.
• Turning off the cortisol response is also critical to
avoid the consequences of long-term stress.
• Cortisol has negative feedback effect on brain
cells that decreases the release of
adrenocorticotropin-releasing hormone.
Vertebrate Endocrine Systems
• The gonads (testes and ovaries) produce steroid
hormones synthesized from cholesterol.
• Androgens are male steroids, the dominant one
being testosterone.
• Estrogens and progesterone are female
steroids, the dominant estrogen being estradiol.
• Sex steroids determine whether a fetus develops
into a male or female.
• After birth, sex steroids control maturation of sex
organs and secondary sex characteristics such as
breasts and facial hair.
Vertebrate Endocrine Systems
• Until the seventh week of an embryo’s
development, either sex may develop.
• In mammals, the Y chromosome causes the
gonads to start producing androgens in the
seven-week-old embryo, and the male
reproductive system develops.
• If androgens are not released, the female
reproductive system develops.
• In birds, the opposite rules apply: male features
are produced unless estrogens are present to
trigger female development.
Figure 42.12 The Development of Human Sex Organs
Hormone Actions:
The Role of Signal Transduction Pathways
• Hormones are released in very small amounts,
yet they cause large and very specific responses
in target organs and tissues.
• Strength of hormone action results from signal
transduction cascades that amplify the original
signal.
• Selective action is keyed to appropriate receptors
of cells responding to hormones.
• Specific receptors can also be linked to different
response mechanisms, as is the case with
receptors for epinephrine and norepinephrine.
Figure 42.14 Some Hormones Can Activate a Variety of Signal Transduction Pathways