598
UNIT 3
Regulation and Integration of the Body
16
Antagonism
occurs when one hormone opposes the action of
another hormone. For example, insulin, which lowers blood
glucose levels, is antagonized by glucagon, which raises blood
glucose levels. How does antagonism occur? Antagonists
may compete for the same receptors, act through different
metabolic pathways, or even, as noted in the progesterone-
estrogen interaction at the uterus, cause down-regulation of
the receptors for the antagonistic hormone.
Check Your Understanding
4.
Name the two major chemical classes of hormones. Which
class consists entirely of lipid-soluble hormones? Name
the only hormone in the other chemical class that is lipid
soluble.
5.
Consider the signaling mechanisms of water-soluble and
lipid-soluble hormones. In each case, where are the receptors
found and what is the final outcome?
6.
What are the three types of stimuli that control hormone
release?
For answers, see Appendix H.
The Pituitary Gland
and Hypothalamus
Describe structural and functional relationships between
the hypothalamus and the pituitary gland.
Discuss the structure of the posterior pituitary, and describe
the effects of the two hormones it releases.
List and describe the chief effects of anterior pituitary
hormones.
Securely seated in the sella turcica of the sphenoid bone, the tiny
pituitary gland
, or
hypophysis
(hi-pof
9
ĭ-sis; “to grow under”),
secretes at least eight hormones. Usually said to be the size and
shape of a pea, this gland is more accurately described as a pea
on a stalk. Its stalk, the funnel-shaped
infundibulum
, connects
the gland to the hypothalamus superiorly as shown in
Focus on
Hypothalamus and Pituitary Interactions
(Figure 16.5)
.
In humans, the pituitary gland has two major lobes. One lobe
is neural tissue and the other is glandular.
Te
posterior pituitary
(lobe) is composed largely of neu-
ral tissue such as pituicytes (glia-like supporting cells) and
nerve fibers. It releases
neurohormones
(hormones secreted
by neurons) received ready-made from the hypothalamus.
Consequently, this lobe is a hormone-storage area and not a
true endocrine gland that manufactures hormones. Te pos-
terior lobe plus the infundibulum make up the region called
the
neurohypophysis
(nu
0
ro-hi-pof
9
ĭ-sis), a term commonly
used (incorrectly) to indicate the posterior lobe alone.
Te
anterior pituitary
(lobe), or
adenohypophysis
(ad
0
ĕ-
no-hi-pof
9
ĭ-sis), is composed of glandular tissue (
adeno
5
gland). It manufactures and releases a number of hormones
(
Table 16.1
on pp. 602–603).
Half-Life, Onset, and Duration
of Hormone Activity
Hormones are potent chemicals, and they exert profound ef-
fects on their target organs even at very low concentrations.
Hormones circulate in the blood in two forms—free or bound
to a protein carrier. In general, lipid-soluble hormones (steroids
and thyroid hormone) travel in the bloodstream attached to
plasma proteins. Most others circulate without carriers.
Te concentration of a circulating hormone in blood at any
time reflects (1) its rate of release, and (2) the speed at which
it is inactivated and removed from the body. Some hormones
are rapidly degraded by enzymes in their target cells. However,
most hormones are removed from the blood by the kidneys or
liver, and the body excretes their breakdown products in urine
or, to a lesser extent, in feces. As a result, the length of time for a
hormone’s blood level to decrease by half, referred to as its
half-
life
, varies from a fraction of a minute to a week. Water-soluble
hormones have the shortest half-lives.
How long does it take for a hormone to have an effect? It var-
ies. Some hormones provoke target organ responses almost im-
mediately, while others, particularly steroid hormones, require
hours to days before their effects are seen. Additionally, some
hormones are secreted in a relatively inactive form and must be
activated in the target cells.
Te duration of hormone action is limited, ranging from 10
seconds to several hours, depending on the hormone. Effects
may disappear rapidly as blood levels drop, or they may persist
for hours even at very low levels. Because of these many varia-
tions, hormonal blood levels must be precisely and individually
controlled to meet the continuously changing needs of the body.
Interaction of Hormones at Target Cells
Understanding hormonal effects is a bit more complicated than
you might expect because multiple hormones may act on the
same target cells at the same time. In many cases the result of
such an interaction is not predictable, even when you know the
effects of the individual hormones. Here we will look at three
types of hormone interaction—permissiveness, synergism, and
antagonism.
Permissiveness
is the situation in which one hormone can-
not exert its full effects without another hormone being
present. For example, reproductive system hormones largely
regulate the development of the reproductive system, as we
might expect. However, thyroid hormone is also necessary
(has a permissive effect) for normal
timely
development of
reproductive structures. Lack of thyroid hormone delays re-
productive development.
Synergism
occurs when more than one hormone produces
the same effects at the target cell and their combined effects
are amplified. For example, both glucagon (produced by the
pancreas) and epinephrine cause the liver to release glucose
to the blood. When they act together, the amount of glucose
released is about 150% of what is released when each hor-
mone acts alone.
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