H1.1 State that hormones are chemical messengers
secreted by endocrine glands into the blood and
transported by the blood to specific target cells
• A hormone is a regulatory chemical that is secreted in
to the blood by an endocrine gland or an organ of the body
exhibiting an endocrine function
• The blood carries the hormone to every cell in the
body, but only target cells for a given hormone can respond
to it
H1.2 State that hormones can be steroids, peptides,
and tyrosine derivatives, and provide one example of
each
• Hormones secreted by endocrine glands belong to
four different chemical categories:
1. polypeptides
 composed of chains of amino acids that are
shorter than about 100 amino acids
 examples include insulin and antidiuretic
hormone (ADH)
 lipophobic or polar (water-soluble)
2. glycoproteins (modified proteins)
composed of a polypeptide much longer than
100 amino acids to which is attached a
carbohydrate
 include follicle stimulating hormone (FSH)
and luteinizing hormone (LH)
 lipophobic or polar (water-soluble)

3.
amines

derived from the amino acids tyrosine and
tryptophan

include the hormones from the adrenal medulla
(epinephrine and norepinephrine), thyroid (thyroxine
and calcitonin), and pineal (melatonin) glands

epinephrine and norepinephrine are known as
catacholamines and are derived from tyrosine

thyroxine which is secreted by the thyroid gland is
also derived from tyrosine and is lipophilic (fat-soluble)

all the others are polar, or water-soluble

melatonin is derived from tryptophan
4. steroids
lipids derived from cholesterol that are lipophilic

include testosterone, estradiol (estrogen),
progesterone, aldosterone, and cortisol

 may be subdivided into sex steroids secreted by
the testes, ovaries, placenta, and adrenal cortex
(estrogens, progestins, and androgens)
 also subdivided into corticosteroids secreted by
the adrenal cortex (cortisol) which regulates
glucose balance, and mineralocorticoids
(aldosterone) which regulates salt balance
H1.3 Distinguish between the mode of action
between steroid hormones and protein (peptide)
hormones
• Lipophilic (fat-soluble) and lipophobic or polar (watersoluble) hormones regulate their target cells by different means
• Lipophilic hormones which include all of the steroid
hormones and thyroxine, can easily enter the cell because the
lipid portion of the plasma membrane does not present a barrier
to entry since they are chemically similar
• Because these hormones are not water-soluble, they will
not dissolve in plasma and travel through the blood attached to
protein carriers
• At the target cells, they detach from their carriers and
pass through the plasma membrane
• Some steroid hormones bind to specific receptor
proteins in the cytoplasm forming a hormone-receptor
complex which then moves into the nucleus
• Other steroids travel directly into the nucleus before
finding their receptor (T4)
• Once the hormone receptor is activated through
attachment of the hormone, it binds to specific regions of the
DNA (hormone response elements)
• This directly affects the level of transcription at that
site by activating or deactivating transcription
• Activation produces mRNA which then codes for
specific proteins which will change the metabolism of the
target cell
• The thyroid hormone’s mechanism of action is
similar
• Thyroxine contains 4 iodines (T4) and upon entering
the cell is changed into triiodothryonine with 3 iodines (T3)
which then enters the nucleus and binds to nuclear receptors
• This hormone-receptor complex binds to regions of
the DNA and stimulates gene transcription by regulating
genes in the target cell
• Hormones that are too large or too polar to cross
plasma membranes include the peptide and glycoprotein
hormones as well as the catecholamine hormones
epinephrine and norepinephrine (amine hormones derived
from tyrosine)
• They bind to receptor proteins on the outside surface
of the plasma membrane and do not enter the cell; they are
the “first messenger”
• A number of different molecules inside the cell serve
as “second messengers” to produce the effects of the
hormone
• An example of a second messenger is cyclic AMP
(cAMP) See Fig.47.7 pg. 998
• The interaction between the hormone and its receptor
activates mechanisms that increase the concentration of the
second messengers within the target cell cytoplasm
• The binding of the hormone to its receptor is reversible
and brief
• After it activates the second messenger, it detaches
from the receptor and may travel in the blood to another
target cell somewhere else
• Eventually liver enzymes break down the hormone
into inactive derivatives
• Some of the larger polar hormones act by causing a
change in the shape of membrane proteins called ion
channels
• A change in shape opens normally closed channels
allowing a particular ion to enter or leave the cell or the
change closes normally opened channels
• Examples of this type of second messenger include
inositol triphosphate (IP3) and calcium ions (Ca++)
• In many cases, the second messengers activate
previously inactive enzymes
H1.4 Outline the relationship between the
hypothalamus and the pituitary gland
• The hypothalamus controls the production and
secretion of the anterior pituitary hormones
• The secretion of the anterior pituitary product is
hormonally controlled, instead of through nerve axon
stimulus
• Neurons in the hypothalamus secrete hormones
that are carried by short blood vessels (primary
capillaries which connect to portal venules) directly to
the anterior pituitary
• These hormones will either stimulate or inhibit the
secretion of anterior pituitary hormones
•
The hormones secreted by some endocrine glands feed
back to inhibit the secretion of hypothalamic releasing hormones
and anterior pituitary tropic hormones
• The posterior pituitary contains axons that originate in
cell bodies within the hypothalamus and extend along the stalk of
the pituitary as a tract of fibers
• This relationship results from the way the posterior
pituitary is formed during embryonic development
• Neural tissue from the hypothalamus grows downward
during development to produce the posterior pituitary resulting
in the hypothalamus and posterior pituitary remaining
interconnected by a tract of axons
• See Text Fig 47.12 pg. 1003
H1.5 Explain the control of ADH (vasopressin)
secretion by negative feedback
• The posterior pituitary gland contains axons originating from neurons
in the hypothalamus
• These neurons produce ADH and oxytocin which are stored and
released from the posterior pituitary in response to neural stimulation from the
hypothalamus
• In response to increased blood plasma osmolality by osmoreceptors in
the hypothalamus, ADH is released by the posterior pituitary into the blood
• ADH promotes water retention by the kidneys and increases
vasoconstriction which will raise blood pressure
• This works as a negative feedback loop to correct the initial
disturbance of homeostasis
• See Fig. 47.9 pg. 1000