498
UNIT THREE
SufF
cient concentration of ADH contracts certain smooth
muscles, including those in the walls of blood vessels. As a
result, vascular resistance and blood pressure may increase.
(This is why ADH is also called vasopressin.) Although ADH
is seldom abundant enough to cause high blood pressure, its
secretion increases following severe blood loss. In this situ-
ation, ADH’s vasopressor effect may help to minimize the
drop in blood pressure that results from profuse bleeding
and return blood pressure toward normal.
ADH’s two effects—vasoconstriction and water retention—
are possible because the hormone binds two different receptors
on target cells. The binding of ADH to V1 receptors increases
the concentration of the second messenger inositol triphos-
phate, which increases intracellular calcium ion concentration,
leading to vasoconstriction. The second receptor, V2, is on parts
of the kidneys’ microscopic tubules called collecting ducts.
ADH binding there activates the cAMP second messenger sys-
tem, which ultimately causes collecting duct cells to reabsorb
water that would otherwise be excreted as urine.
The hypothalamus regulates secretion of ADH. Certain
neurons in this part of the brain, called
osmoreceptors,
sense
changes in the concentration of body fl
uids. ±or example,
if a person is dehydrating due to a lack of water intake, the
solutes in blood become more concentrated. The osmorecep-
tors, sensing the resulting increase in osmotic pressure, sig-
nal the posterior pituitary to release ADH, which causes the
kidneys to retain water.
On the other hand, if a person drinks a large volume of
water, body fl
uids become more dilute, which inhibits the
release of ADH. Consequently, the kidneys excrete more
dilute urine until the concentration of body fl
uids returns
to normal.
Blood volume also affects ADH secretion. Increased
blood volume stretches the walls of certain blood vessels,
stimulating volume receptors that signal the hypothalamus
to inhibit release of ADH. However, if hemorrhage decreases
blood volume, these receptors are stretched less and there-
fore send fewer inhibiting impulses. As a result, ADH secre-
tion increases, and as before, ADH causes the kidneys to
conserve water. This helps prevent further volume loss.
The baby f
rst displayed symptoms at f
ve months oF age—he drank
huge volumes oF water. By thirteen months, he had become severely
dehydrated, although he drank nearly continuously. His parents were
constantly changing his wet diapers. Doctors f
nally diagnosed a Form
oF
diabetes insipidus,
which impairs ADH regulation oF water balance.
The boy was drinking su±
cient ²
uids, but his kidneys could not retain
the water. ADH V2 receptors on the kidney collecting ducts were
deFective. The hormone could bind, but the receptor Failed to trig-
ger cAMP Formation. The boy’s ADH was still able to constrict blood
vessels because the V1 receptors were una³
ected. A high-calorie diet
and providing lots oF water preserved the boy’s mental abilities, but
he remained small For his age. Tumors and injury a³
ecting the hypo-
thalamus and posterior pituitary can also cause diabetes insipidus.
Posterior Pituitary Hormones
Unlike the anterior lobe of the pituitary gland, which is pri-
marily composed of glandular epithelial cells, the posterior
lobe largely consists of nerve F bers and neuroglia (
pituicytes
).
The neuroglia support the nerve F bers that originate in the
hypothalamus. The hypothalamic cells that give rise to these
F bers are called neurosecretory cells because their secretions
function not as neurotransmitters but as hormones.
Specialized neurons in the hypothalamus produce the two
hormones associated with the posterior pituitary—
antidiuretic
(an
tı˘-di
u-ret
ik)
hormone
(also known as
vasopressin
) and
oxytocin
(ok
sı˘-to
sin) (see F g. 13.12). These hormones travel
down axons through the pituitary stalk to the posterior pitu-
itary and are stored in vesicles (secretory granules) near the
ends of the axons. The hormones are released into the blood
in response to nerve impulses coming from the hypothala-
mus. Thus, posterior pituitary hormones are synthesized in
the hypothalamus, but they are named for where they enter
the bloodstream.
Antidiuretic hormone and oxytocin are short polypeptides
with similar sequences
(f g. 13.17)
. A
diuretic
is a chemical
that increases urine production. An
antidiuretic,
then, is a
chemical that decreases urine formation. ADH produces its
antidiuretic effect by reducing the volume of water that the
kidneys excrete. In this way, ADH plays an important role in
regulating the concentration of body fl uids (see chapter 20,
pp. 794–795).
´requent and copious urination oFten Follows drinking alcoholic
beverages. This is because alcohol (ethyl alcohol) inhibits ADH
secretion. A person must replace the lost body Fluid to maintain
normal water balance. Although it seems counterintuitive, drink-
ing too much beer can lead to dehydration because the body loses
more ²
uid than it replaces.
Cys
Cys
Asp
Glu
Ile
Pro
Leu
Gly
Tyr
S
S
Oxytocin
Cys
Cys
Asp
Glu
Phe
Pro
Arg
Gly
Tyr
S
S
Antidiuretic hormone
FIGURE 13.17
The structure oF oxytocin di³
ers From that oF ADH by
only two amino acids, yet they Function di³
erently.
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