Page 280 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 265

renin–angiotensin system, another group gluconeogenesis by the liver. These meta­
bolic effects are particularly important
of chemical messengers discussed in more
VetBooks.ir detail in Chapter 18. during starvation.
Glucocorticoids (primarily cortisol and
Glucocorticoids are often used thera-
corticosterone) are the major secretory peutically to inhibit inflammatory and
product of both the zona fasciculata and immune responses. The doses used for
zona reticularis, and ACTH is the major these effects produce blood levels that
regulator (stimulator) of their secretion. are much higher than those seen in nor-
Without ACTH the fasciculata and reticu­ mal animals, even when they are
laris both atrophy, but the glomerulosa responding to stress. Such levels and
remains intact. The inner zones of the effects are described as supraphysiologic
adrenal cortex can also be a minor source or pharmacologic. Among the many
of steroid hormones typically associated components of the inflammatory pro-
with the gonads (androgens and estrogens) cess that are inhibited by glucocorticoids
because the synthesis of glucocorticoids are the synthesis pathways for prosta-
from pregnenolone requires these other glandins, leukotrienes, and thrombox-
hormones as intermediaries. The secretion anes. The use of pharmacologic doses of
of these “sex steroids” from the adrenal glucocorticoids results in substantial
glands are typically not clinically relevant negative feedback to the hypothalamus
as they are unnecessary for either normal and pituitary gland, significantly
reproductive behavior or function. decreasing endogenous CRH and ACTH
The single most important regulator of production. Sudden discontinuation of a
ACTH release is the hypothalamic peptide course of glucocorticoid therapy can
CRH. However, the regulation of CRH, result in a medical crisis because the
hence ACTH release, is extremely complex patient’s adrenal gland is no longer pro-
and affected by a wide variety of stimuli. ducing its own glucocorticoids.
ACTH increases are considered to be a
classic sign of stress, and plasma levels of Thyroid‐Stimulating Hormone
ACTH or cortisol are often used in experi­
mental settings to evaluate the overall
stress placed on an animal by any type of The target cells for thyroid‐stimulating
hormone (TSH), also called thyrotropin,
physical or emotional stimulus (e.g., are endocrine cells of the thyroid that pro­
restraint, starvation, presence of a preda­ duce and release thyroxine (T ) and triio­
tor). Both ACTH and glucocorticoids have dothyronine (T ) when stimulated by TSH.
4
negative feedback effects on the pituitary Both are considered amine hormones, for
3
and the hypothalamus to maintain normal each consists of a linkage of two iodinated
resting blood levels of ACTH and gluco­ tyrosine residues (Fig. 13‐6). The 3 and 4
corticoids, but stressful stimuli can over­ refer to the number of iodine atoms in their
ride these effects. molecules. These hormones are necessary
Glucocorticoids have many target tis­
sues throughout the body. In general their for normal growth and development in
young animals, and they regulate basal
effects on these target tissues would seem metabolic rate in the adult.
an appropriate response to counteract
stressful stimuli. For example, glucocorti­ Thyroid Gland. The thyroid gland is
coids increase the rate of gluconeogenesis associated with the proximal part of the
(glucose formation) by the liver and trachea near the thyroid cartilage of the
increase the rate of fatty acid mobilization larynx. Its appearance varies widely among
from lipid tissue. In skeletal muscle, pro­ species, with the thyroid gland of most
tein synthesis is reduced and protein animals possessing two distinct lobes,
degradation is increased, which means variably connected across the midline by a
that more amino acids are available for strip of thyroid tissue called the isthmus. In