491
CHAPTER THIRTEEN
Endocrine System
in cells but are synthesized just before they are released.
They are rapidly inactivated.
Some prostaglandins regulate cellular responses to hor-
mones. For example, different prostaglandins can either
activate or inactivate adenylate cyclase in cell membranes,
thereby controlling production of cAMP and altering the
cell’s response to a hormone.
Prostaglandins produce a variety of effects. Some pros-
taglandins can relax smooth muscle in the airways of the
lungs and in the blood vessels, dilating these passageways.
Yet other prostaglandins can contract smooth muscle in the
walls of the uterus, causing menstrual cramps and labor
contractions. They stimulate secretion of hormones from
the adrenal cortex and inhibit secretion of hydrochloric acid
from the wall of the stomach. Prostaglandins also infl
uence
movements of sodium ions and water in the kidneys, help
regulate blood pressure, and have powerful effects on both
male and female reproductive physiology. When tissues are
injured, prostaglandins promote infl
ammation (see chapter
16, p. 627).
Understanding prostaglandin function has medical applications.
Drugs such as aspirin and certain steroids that relieve the joint pain
of rheumatoid arthritis inhibit production of prostaglandins in the
synovial F
uid of a±
ected joints. Daily doses of aspirin may reduce the
risk of heart attack by altering prostaglandin activity. Prostaglandins
may be used as drugs to dilate constricted blood vessels to relieve
hypertension.
PRACTICE
7
What are prostaglandins?
8
Describe one possible function of prostaglandins.
9
What are e±
ects of prostaglandins?
13.4
CONTROL OF HORMONAL
SECRETIONS
The body must be able to turn processes on and off. (Recall
how acetylcholine is removed from the neuromuscular junc-
tion to end muscle contraction.) In the case of hormones,
half-life measures removal. Half-life is the time for half of
the hormone molecules to be removed from the plasma. For
example, a hormone with a half-life of ten minutes would
start out at 100% of its blood concentration, and if secre-
tion were to stop, it would drop to 50% in ten minutes, 25%
in another ten minutes, 12.5% in another ten minutes, and
so on. Hormones with short half-lives (a few minutes) con-
trol body functions that turn on and off quickly, whereas the
effects of hormones with longer half-lives, such as thyroid
hormone and steroids, may last for days.
follicle-stimulating hormone (FSH), and luteinizing hormone
(LH) from the anterior pituitary gland; antidiuretic hormone
(ADH) from the posterior pituitary gland; parathyroid hor-
mone (PTH) from the parathyroid glands; norepinephrine
and epinephrine from the adrenal glands; calcitonin from the
thyroid gland; and glucagon from the pancreas.
An abnormality in cAMP-mediated signaling can lead to symptoms
from many endocrine glands. In McCune-Albright syndrome, for exam-
ple, a defect in the G protein that activates adenylate cyclase results in
conversion of ATP to cAMP even without hormonal stimulation. As a
result, cells in the pituitary, thyroid, gonads, and adrenal glands secrete
hormones in excess. One symptom is precocious puberty. Infant girls
menstruate, and boys as young as six years produce mature sperm.
Symptoms vary widely because the syndrome results from a mutation
that is not present in all cells of an individual.
Certain nonsteroid hormones use second messengers
other than cAMP. For example, a second messenger called
diacylglycerol (DAG), like cAMP, activates a protein kinase
leading to a cellular response.
In another mechanism, a hormone binding its recep-
tor increases calcium ion concentration in the cell. Such a
hormone may stimulate transport of calcium ions inward
through the cell membrane or induce release of calcium ions
from cellular storage sites via a second messenger called
inositol triphosphate (IP3). The calcium ions combine with
the protein
calmodulin
(see chapter 9, p. 301), altering its
molecular structure in a way that activates the molecule.
Activated calmodulin can then interact with enzymes, alter-
ing their activities and thus eliciting diverse responses.
Still another hormonal mechanism uses
cyclic guanosine
monophosphate
(cyclic GMP, or cGMP). Like cAMP, cGMP
is a nucleotide derivative and functions in much the same
manner as a second messenger.
Cellular response to a steroid hormone (and thyroid hor-
mone) is directly proportional to the number of hormone-
receptor complexes that form. In contrast, response to a
hormone operating through a second messenger is greatly
ampli±
ed. This is possible because many second messen-
ger molecules can be activated in response to just a few
hormone-receptor complexes. Cells are highly sensitive
to changes in the concentrations of nonsteroid hormones
because of such ampli±
cation.
PRACTICE
4
How does a steroid hormone act on its target cells?
5
How does a nonsteroid hormone act on its target cells?
6
What is a second messenger?
Prostaglandins
Prostaglandins are paracrine substances, acting locally, that
are potent and present in small amounts. They are not stored
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