Page 278 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 263

The role of GH in the determination of
oxytocin is often slowly titrated because body stature in growing animals was intro­
VetBooks.ir even a small dose can result in an duced in Chapter 5. As was discussed
overwhelming biologic response and
earlier, GH itself has little direct effect on
potentially nonproductive uterine con-
tractions if there are a lot of available cartilage proliferation and bone growth in
oxytocin receptors. young animals. Its growth‐promoting
Antidiuretic hormone (ADH) is pro­ effects are mediated by other peptides,
duced by neurons of the supraoptic nucleus somatomedins (primarily insulin‐like
in the hypothalamus and released from the growth factors 1 and 2 [IGF‐1 and IGF‐2]),
neurohypophysis in response to increases which are released by the liver and cells in
in blood osmolality (concentration of the area of growth plates in bone when
dissolved substances) or severe decreases stimulated by GH. The somatomedins are
in blood pressure, both of which are influ­ the direct stimulators of chondrocytes
enced by the animal’s hydration status. within the growth plates. Somatomedins
The water‐retaining effects of ADH on the also have negative feedback effects on the
kidney to maintain hydration are discussed hypothalamus and adenohypophysis to
in Chapter 23. In most species, the release regulate the release of GH. In addition to
of ADH occurs prior to the stimulation of GH, the secretion of somatomedins by the
thirst for the correction of dehydration by liver of young growing animals is regulated
voluntary oral intake of fluids. ADH also in part by nutrition. Inadequate nutrition
produces constriction of blood vessels, an may retard growth in part because of a
effect that gives the hormone its other suppression of somatomedin secretion.
name, vasopressin or arginine vasopres- Small dogs have lower blood levels of
sin (the form found in most mammals). IGF‐1 than large dogs, suggesting that
within a species body size and IGF‐1 levels
are correlated.
Hormones Excessive GH in young animals leads to
of the Adenohypophysis gigantism. Increases in body size are not
possible in older animals where growth
Growth Hormone plates are closed. Excessive GH (with asso­
ciated IGFs) in mature animals leads to
The release of growth hormone (GH), also acromegaly. Further increases in stature
called somatotropin or somatotropic hor­ are not possible in mature animals, but
mone, from the somatotrope cells of the cartilage proliferation around joints and
adenohypophysis, is regulated by hypotha­ other skeletal locations produces enlarge­
lamic factors that either stimulate (GH‐ ments in these areas and a characteristic
releasing hormone, or GHRH) or inhibit coarseness of facial features. Affected
(GH release–inhibiting hormone, GHIH, adults also have derangements of carbohy­
or somatostatin) release. GH levels are drate and lipid metabolism (increased
highest in young, growing animals, but adult blood levels of glucose and fatty acids)
animals continue to secrete GH. Increases in because of the metabolic effects of exces­
GH secretion in adults occur in response to sive GH.
a variety of stimuli, but probably the most The amino acid sequence of GH var-
important physiologic stimulus is a reduc­ ies among mammalian species, and thus
tion in plasma glucose. In adults, GH func­ GH produced by one species is not
tions as a regulator of metabolism during always biologically effective in a differ-
starvation, deficits in plasma glucose, or ent species. Recombinant DNA technol-
hibernation. GH acts to reduce protein ogy has been used to produce both
breakdown and the use of glucose for energy human recombinant GH and recombi-
in skeletal muscle and to increase the mobi­ nant bovine somatotropin. The human
lization of fatty acids from adipose tissue. product is used clinically to prevent