The Endocrine System
317
SECTION A
Principles of Hormonal Control Systems
Hormone Structures and Synthesis
Hormones fall into three major chemical classes: (1) amines,
(2) peptides and proteins, and (3) steroids.
Amine Hormones
The
amine hormones
are all derivatives of the amino acid
tyrosine. They include the
thyroid hormones,
the catechol-
amines
epinephrine
and
norepinephrine
(produced by the
adrenal medulla), and
dopamine
(produced by the hypothal-
amus). The structure and synthesis of the iodine-containing
thyroid hormones will be described in detail in Section C of
this chapter. For now, their structures are shown in
Figure
11–2
. Chapter 6 described the steps of catecholamine syn-
thesis; the structures of the amine hormones are reproduced
here in Figure 11–2.
There are two adrenal glands, one on top of each kid-
ney. Each
adrenal gland
is composed of an inner
adrenal
medulla,
which secretes amine hormones, and a surrounding
adrenal cortex,
which secretes steroid hormones. As Chapter
6 described, the adrenal medulla is really a modifi ed sympa-
thetic ganglion whose cell bodies do not have axons. Instead,
they release their secretions into the blood, thereby fulfi
lling a
criterion for an endocrine gland.
The adrenal medulla secretes mainly two amine hor-
mones, epinephrine and norepinephrine. Recall from Chapter
6 that these molecules constitute, with dopamine, the chemi-
cal family of catecholamines. In humans, the adrenal medulla
secretes approximately four times more epinephrine than nor-
epinephrine. This is because the adrenal medulla expresses
high amounts of an enzyme called phenyl-N-methyltransfer-
ase (PNMT), which catalyzes the step that converts norepi-
nephrine to epinephrine. Epinephrine and norepinephrine
exert actions similar to those of the sympathetic nerves,
which, because they do not express PNMT, make only norepi-
nephrine. These actions are described in various chapters and
summarized in Section B.
Blood vessel
Secretion
Epithelial cell
Duct
Exocrine
gland
Endocrine
gland
Figure 11–1
Exocrine gland secretions enter ducts, whereas hormones secreted
by endocrine (ductless) glands diffuse directly into the blood.
3, 5, 3', 5'– Tetraiodothyronine (thyroxine, T
4
)
3, 5, 3'– Triiodothyronine (T
3
)
HO
O
3
5
3'
5'
O
CH
2
CH
OH
C
NH
2
H
OH
HO
O
3
5
3'
5'
O
CH
2
CH
OH
C
NH
2
HO
HO
C
H
H
C
H
H
C
H
H
C
H
H
C
NH
2
Norepinephrine
H
OH
HO
HO
C
H
CH
3
N
Epinephrine
HO
HO
NH
2
Dopamine
Figure 11–2
Chemical structures of the amine hormones: thyroxine,
triiodothyronine, norepinephrine, epinephrine, and dopamine. The
two thyroid hormones differ by only one iodine atom, a difference
noted in the abbreviations T
3
and T
4
. The position of the carbon
atoms in the two rings of T
3
and T
4
are numbered.
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