The Endocrine System
319
The other catecholamine hormone, dopamine, is synthe-
sized by neurons in the hypothalamus. Dopamine is released
into a special circulatory system called a portal system (see
Section B), where it acts to regulate the activity of certain cells
in the pituitary gland.
Peptide and Protein Hormones
Most hormones are either peptides or proteins. They range in
size from small peptides having only three amino acids to small
proteins (some of which are glycoproteins). For convenience,
we will refer to all these hormones as
peptide hormones.
In many cases, peptide hormones are initially synthe-
sized on the ribosomes of endocrine cells as larger proteins
known as preprohormones, which are then cleaved to
prohor-
mones
by proteolytic enzymes in the rough endoplasmic retic-
ulum (
Figure 11–3
). The prohormone is then packaged into
secretory vesicles by the Golgi apparatus. In this process, the
prohormone is cleaved to yield the active hormone and other
peptide chains found in the prohormone. Therefore, when the
cell is stimulated to release the contents of the secretory vesi-
cles by exocytosis, the other peptides are secreted along with
the hormone. In certain cases they, too, may exert hormonal
effects. In other words, instead of just one peptide hormone,
the cell may be secreting multiple peptide hormones that dif-
fer in their effects on target cells.
As mentioned in Chapters 5 and 6, many peptides serve
as both neurotransmitters (or neuromodulators) and as hor-
mones. For example, some of the hormones secreted by the
endocrine glands in the gastrointestinal tract are also pro-
duced by neurons in the brain, where they may function as
neuromodulators.
Steroid Hormones
The third family of hormones is the steroids, the lipids whose
ringlike structure was described in Chapter 2.
Steroid hor-
mones
are primarily produced by the adrenal cortex and the
Table 11–1
Summary of the Hormones
(continued)
Site Produced
(Endocrine Gland)
Hormone
Major Function* Is Control Of:
Pituitary glands:
Anterior pituitary
Growth hormone (somatotropin)
Growth, mainly via local production of IGF-1; protein,
carbohydrate, and lipid metabolism
Thyroid-stimulating hormone
(thyrotropin)
Thyroid gland activity and growth
Adrenocorticotropic hormone
(corticotropin)
Adrenal cortex activity and growth
Prolactin
Milk production in breast
Gonadotropic hormones:
Follicle-stimulating hormone
M
a
l
e
s
G
am
e
t
e
p
r
o
d
u
c
t
i
o
n
Females
Ovarian follicle growth
Luteinizing hormone:
Males
Testicular production of testosterone
Females
Ovarian production of estradiol; ovulation
β
-lipotropin and
β
-endorphin
Possibly fat mobilization and analgesia during stress
Posterior pituitary
$
Oxytocin
Milk let-down; uterine motility
Vasopressin (antidiuretic hormone, ADH)
Blood pressure; water excretion by the kidneys
Placenta
Human chorionic gonadotropin (hCG)
Secretion of progesterone and estrogen by corpus luteum
Estrogens
See Gonads: ovaries
Progesterone
See Gonads: ovaries
Human placental lactogen (hPL)
Breast development; organic metabolism
Thymus
Thymopoietin
T-lymphocyte function
Thyroid
Thyroxine (T
4
) and triiodothyronine (T
3
)
Metabolic rate; growth; brain development and function
Calcitonin
Plasma calcium in some vertebrates (role unclear in humans)
Multiple cell types
Growth factors
(e.g., epidermal growth
factor)
Growth and proliferation of specifi c cell types
Other (produced in blood)
Angiotensin II
Blood pressure; production of aldosterone from adrenal
cortex
*This table does not list all functions of all hormones.
The names and abbreviations in parentheses are synonyms.
Some classifi cations include the cytokines under the category of growth factors.
$
The posterior pituitary stores and secretes these hormones; they are made in the hypothalamus.
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