494
UNIT THREE
PRACTICE
12
Where is the pituitary gland?
13
List the hormones that the anterior and posterior lobes of the
pituitary gland secrete.
14
Explain how the hypothalamus controls the actions of the
pituitary gland.
Anterior Pituitary Hormones
The anterior lobe of the pituitary gland is enclosed in a dense
capsule of collagenous connective tissue and largely consists
of epithelial tissue organized in blocks around many thin-
walled blood vessels. The epithelial tissue has F
ve types of
secretory cells. They are
somatotropes
that secrete GH,
mam-
matropes
that secrete PRL,
thyrotropes
that secrete TSH,
cor-
ticotropes
that secrete ACTH, and
gonadotropes
that secrete
┬▒SH and LH
(f
gs. 13.14
and
13.15)
. In males, LH (luteiniz-
ing hormone) is known as ICSH (interstitial cell-stimulating
hormone) because it affects the interstitial cells of the testes
(see chapter 22, p. 845).
Growth hormone,
also called
somatotropin
(STH), is
a protein that stimulates cells to enlarge and more rapidly
divide. It enhances the movement of amino acids through
cell membranes and increases the rate of protein synthesis.
GH also decreases the rate at which cells use carbohydrates
and increases the rate at which they use fats.
Growth hormone secretion varies during the day, peak-
ing during sleep. Two biochemicals from the hypothalamus
control its secretion. They are released alternately, exerting
opposite effects.
Growth hormone-releasing hormone
(GHRH)
stimulates secretion of GH, and
somatostatin
(SS) inhibits
secretion.
Nutritional state can affect control of GH. More GH is
released during periods of protein deF ciency and abnormally
low blood glucose concentration. Conversely, when blood
protein and glucose concentrations increase, growth hor-
mone secretion decreases. Apparently the hypothalamus can
sense changes in the concentrations of certain blood nutri-
ents and it releases GHRH in response to some of them.
Upon reaching the anterior lobe of the pituitary, each of
the hypothalamic releasing hormones acts on a speciF
c pop-
ulation of cells. Some of the resulting actions are inhibitory
(prolactin release-inhibiting hormone and somatostatin), but
most stimulate the anterior pituitary to release hormones
that stimulate the secretions of peripheral endocrine glands.
In many of these cases, important negative feedback rela-
tionships regulate hormone levels in the bloodstream.
Figure 13.13
shows this general relationship.
Receptors
Hormone control
mechanism senses
change.
Receptors
Hormone control
mechanism senses
change.
too high
too low
Normal
hormone
levels
Control center
Endocrine gland
inhibited.
Control center
Endocrine gland
stimulated.
Stimulus
Hormone levels rise or
controlled process
increases.
Effectors
Hormone secretion
decreased.
Effectors
Hormone secretion
increased.
Response
Hormone levels
return toward
normal.
Stimulus
Hormone levels drop or
controlled process
decreases.
Response
Hormone levels
return toward
normal.
FIGURE 13.10
Hormone secretion is under negative feedback
control.
Increasing concentration
of hormone
Average
concentration
of hormone
Actual
concentration
of hormone
Time
FIGURE 13.11
As a result of negative feedback, hormone
concentrations remain relatively stable, although they may F
uctuate
slightly above and below average concentrations.
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