604
UNIT 3
Regulation and Integration of the Body
16
GH release, while
growth hormone–inhibiting hormone
(GHIH)
, also called
somatostatin
(so
0
mah-to-stat
9
in), inhibits
it. Recent research shows that the “hunger hormone” ghrelin
(produced by the stomach) also stimulates GH release.
GHIH release is triggered by the feedback of GH and IGFs.
Rising levels of GH also feed back to inhibit its own release.
GHIH is also produced in various locations in the gut, where it
inhibits the release of virtually all gastrointestinal and pancre-
atic secretions—both endocrine and exocrine.
As indicated in Table 16.1, a number of secondary triggers
also influence GH release. Typically, GH secretion has a daily
cycle, with the highest levels occurring during evening sleep.
±e total amount secreted daily peaks during adolescence and
then declines with age.
Homeostatic Imbalance
16.2
Both hypersecretion and hyposecretion of GH may result in
structural abnormalities. Hypersecretion in children results
in
gigantism
because GH targets the still-active epiphyseal
(growth) plates. ±e person becomes abnormally tall, o²en
reaching a height of 2.4 m (8 feet), but has relatively normal
body proportions
(Figure 16.7)
.
If excessive GH is secreted a²er the epiphyseal plates have
closed,
acromegaly
(ak
0
ro-meg
9
ah-le) results. Literally trans-
lated as “enlarged extremities,” this condition is characterized by
overgrowth of bony areas still responsive to GH, namely bones
of the hands, feet, and face. Hypersecretion usually results from
an anterior pituitary tumor that churns out excessive GH. ±e
usual treatment is surgical removal of the tumor, but this surgery
does not reverse anatomical changes that have already occurred.
Hyposecretion of GH in adults usually causes no problems,
but GH deficiency in children slows long bone growth, a condi-
tion called
pituitary dwarfism
(Figure 16.7). Such individuals
attain a maximum height of 1.2 m (4 feet), but usually have
fairly normal body proportions. Lack of GH is o²en accompa-
nied by deficiencies of other anterior pituitary hormones, and
if thyroid-stimulating hormone and gonadotropins are lacking,
the individual will be malproportioned and will fail to mature
sexually as well. Fortunately, human GH is produced commer-
cially by genetic engineering techniques. When pituitary dwarf-
ism is diagnosed before puberty, growth hormone replacement
therapy can promote nearly normal growth.
±e availability of synthetic GH also has a downside. Ath-
letes and the elderly have been tempted to use GH for its
Growth hormone (GH)
Feedback
Inhibits GHRH release
Stimulates GHIH release
Inhibits GH synthesis
and release
Anterior
pituitary
Liver and
other tissues
Indirect actions
(growth-
promoting)
Direct actions
(metabolic,
anti-insulin)
Insulin-like growth
factors (IGFs)
Extraskeletal
Skeletal
Fat
metabolism
Carbohydrate
metabolism
Increased cartilage
formation and
skeletal growth
Increased protein
synthesis, and
cell growth and
proliferation
Increased
fat breakdown
and release
Increased blood
glucose and other
anti-insulin effects
Effects
Effects
Produce
Hypothalamus
secretes growth
hormone–releasing
hormone (GHRH), and
GHIH (somatostatin)
Initial stimulus
Physiological response
Result
Increases, stimulates
Reduces, inhibits
Figure 16.6
Growth-promoting and metabolic actions of growth hormone (GH).
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