485
CHAPTER THIRTEEN
Endocrine System
The
peptide
hormones are short chains of amino acids (F g.
13.4
d
). This group includes hormones associated with the pos-
terior pituitary gland and some produced in the hypothalamus.
Another group of compounds, called
prostaglandins
(pros
tah-glan
dinz), are paracrine substances. They regulate
neighboring cells. Prostaglandins are lipids (20-carbon fatty
acids that include 5-carbon rings) and are synthesized from
a type of fatty acid (arachidonic acid) in cell membranes
(F
g. 13.4
e
). Prostaglandins are produced in a wide variety of
cells, including those of the liver, kidneys, heart, lungs, thy-
mus gland, pancreas, brain, and reproductive organs.
Table 13.2
lists the names and abbreviations of some of
the hormones discussed in this chapter.
Table 13.3
and F
g-
ure 13.4 summarize the chemical composition of hormones.
Other hormones related to speciF c organ systems are dis-
cussed in their appropriate chapters.
PRACTICE
1
What is a hormone?
2
How do endocrine glands and exocrine glands dif
er?
3
How are hormones chemically classiF
ed?
Actions of Hormones
Hormones exert their effects by altering metabolic processes.
A hormone might change the activity of an enzyme neces-
sary for synthesizing a particular substance or alter the rate at
which particular chemicals are transported through cell mem-
branes. A hormone delivers its message to a cell by uniting
with the binding site of its receptor. The more receptors the
hormone binds on its target cells, the greater the response.
The number of receptors on target cells may change.
Up-regulation
is an increase in the number of receptors on
a target cell, often in response to a prolonged decrease in
the level of a hormone.
Down-regulation
is the opposite,
a decrease in the number of receptors due to a prolonged
increase in hormone levels. Therefore, the number of recep-
tors changes in ways that maintain an appropriate response
to hormone level.
Steroid Hormones and Thyroid Hormones
Steroid hormones and thyroid hormones are insoluble in
water. They are carried in the bloodstream weakly bound to
plasma proteins in a way that they are released in sufF cient
quantity to affect their target cells. However, unlike amine,
peptide, and protein hormones, steroid and thyroid hor-
mones are soluble in the lipids that make up the bulk of cell
membranes. ±or this reason, these hormones can diffuse into
cells relatively easily and may enter any cell in the body.
Once inside a target cell, steroid and thyroid hormones
combine (usually in the nucleus) with specific protein
receptors. The resulting
hormone-receptor complex
binds
to particular DNA sequences, either activating or repress-
ing specific genes. Activated genes are transcribed into
messenger RNA (mRNA), which enters the cytoplasm
Steroid Hormones
Steroids
(ste
roidz) are lipids that include complex rings of
carbon and hydrogen atoms
(f g. 13.4
a
)
. Steroids differ by
the types and numbers of atoms attached to these rings and
the ways they are joined (see F g. 2.16). All steroid hormones
are derived from cholesterol (see chapter 2, p. 64). They
include sex hormones such as testosterone and the estro-
gens, and secretions of the adrenal cortex (the outer portion
of the adrenal gland), including aldosterone and cortisol.
Vitamin D is a modiF ed steroid and can be converted into a
hormone, as is discussed later in this chapter, in the section
entitled “Parathyroid Hormone” (see also chapter 18, p. 711).
Nonsteroid Hormones
Hormones called
amines,
including norepinephrine and epi-
nephrine, are derived from the amino acid tyrosine. These
hormones are also synthesized in the adrenal medulla (the
inner portion of the adrenal gland) (F
g. 13.4
b
).
Protein
hormones, like all proteins, are composed of
long chains of amino acids, linked to form intricate molec-
ular structures (see chapter 2, pp. 65–67 and fig. 13.4
c
).
They include the hormone secreted by the parathyroid gland
and some of those secreted by the anterior pituitary gland.
Certain other hormones secreted from the anterior pituitary
gland are
glycoproteins,
which consist of proteins joined to
carbohydrates.
Hypothalamus
Pituitary gland
Thyroid gland
Thymus
Adrenal gland
Pancreas
Ovary
(in female)
Parathyroid gland
Pineal gland
Kidney
Testis
(in male)
FIGURE 13.3
Locations o± major endocrine glands.
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