Where is the pituitary gland?
List the hormones that the anterior and posterior lobes of the
pituitary gland secrete.
Explain how the hypothalamus controls the actions of the
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
that secrete GH,
that secrete PRL,
that secrete TSH,
that secrete ACTH, and
±SH and LH
. 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).
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
Growth hormone-releasing hormone
stimulates secretion of GH, and
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
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.
shows this general relationship.
Hormone levels rise or
Hormone levels drop or
Hormone secretion is under negative feedback
As a result of negative feedback, hormone
concentrations remain relatively stable, although they may F
slightly above and below average concentrations.