Page 271 - Anatomy and Physiology of Farm Animals, 8th Edition

256 / Anatomy and Physiology of Farm Animals

Cells within target organs are capable of
the cyclooxygenase (COX) enzyme fam- recognizing and responding to a given
VetBooks.ir ily and it is important to remember that hormone because they contain specific
the same enzymes that are involved
receptors capable of binding, or forming a
in the synthesis pathways for prosta-
glandins are also used to synthesize chemical union, with the hormone. As
thromboxanes. Therefore, the side described in Chapter 2, these cellular
effects of these drugs (e.g., gastric ulcer- receptors may be components of the cell
ation and kidney injury) may occur due membrane and have a binding site exposed
to the indiscriminate inhibition of both to the extracellular fluid, or they may be
undesirable effects (i.e., inflammation) contained within the cytoplasm or nucleus
and important desirable effects (e.g., of cells. In either case, a receptor for a spe­
maintenance of blood flow to stomach cific hormone must be present for a cell to
and kidneys). NSAID use in animals, respond to the hormone and the sensitivity
including injectable or oral Banamine® of that cell to the hormone increases as
(flunixin meglumine) and phenylbuta- the number of available receptors for the
zone, requires veterinary prescription hormone increases in the cell.
because of the additional consideration The presence and number of receptors
of withdrawal periods prior to slaughter within target cells may change in certain
and species specificity in dosing and conditions. Such changes are one way that
use; whereas aspirin and ibuprofen are the biologic effect of a given hormone can
over‐the‐counter NSAIDs widely used be regulated. For example, levels of estro-
by humans. Some newer NSAIDs have gen, a reproductive hormone produced by
been developed to selectively inhibit the ovary prior to ovulation as well as by
only certain enzymatic pathways to stop the placenta during pregnancy, increases
the production of prostaglandins associ- in circulation shortly before birth (parturi­
ated with inflammation (COX‐2) rather tion). The increase in estrogen stimulates
than those that are more protective and an increase of oxytocin receptors in the
associated with activating platelets or smooth muscle of the uterus. The increase
protecting the stomach (COX‐1). in oxytocin receptors prepares the uterus
Aspirin is particularly effective at inhib- so that oxytocin can promote uterine con­
iting the synthesis of thromboxanes for tractions that are essential during parturi­
blood clotting, and this is part of the tion. Without the increase in oxytocin
rationale for low‐dose aspirin use in receptors stimulated by estrogen, oxytocin
people to reduce the potential for clot release itself would not provide adequate
formation. uterine contractions for normal parturi­
tion. An increase in receptors on target
cells is termed up‐regulation, and a
Hormone Receptors decrease is termed down‐regulation.

Only certain specific populations of cells
respond to an individual hormone. The Cellular Effects of Peptide
term target organ is used to identify the Hormones
tissue whose cells will be affected by a
given hormone. Some hormones have The receptors for peptide hormones are
multiple target organs, for they affect cells found in the cell membrane. Peptides can­
in several sites. For example, both skeletal not freely diffuse through the lipid bilayer
muscle and liver are among the target of the cell membrane, so their receptors
organs for insulin. Table 13‐1 lists the must be in the outer cell membrane to be
major endocrine glands, their hormones, available to the hormones in the extracel­
and their general effects on their target lular fluid. As described in Chapter 2,
organs. the binding of the hormone with the