Page 270 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 255

CH 3 H
VetBooks.ir H C CH 2 C

CH 3
CH 2 CH 2 CH 3
CH
CH 2
CH 3
CH 2 C CH 2
CH 3
CH 2 CH CH CH 2
CH 2 C CH

CH C CH 2
HO
CH
CH 2
Figure 13-2. The structure of cholesterol.

significant post‐translational modification prostaglandins, each with a slight differ­
with carbohydrates (glycosylation). Protein ence in chemical structure, and may have
hormones are hydrophilic and typically multiple regulatory functions in a variety
bind to cell surface receptors on their of tissues. Furthermore, one prostaglandin
target cell. may have opposite physiological effects in
different organs. In general terms, prosta­
glandins are implicated in regulation of
Eicosanoids blood vessel diameter, inflammation, blood
clotting, uterine contraction, and ovula­
Eicosanoid is a general term for com­ tion, among many others. Prostaglandins
pounds that are chemical derivatives of are very important in many reproductive
long‐chain fatty acids. Prostaglandins, functions, therefore these substances will
thromboxanes, and leukotrienes are be revisited in Chapter 27.
eicosanoids that function as chemical mes­ Leukotrienes are similar in structure
sengers. Arachidonic acid, a component of to prostaglandins as they are also pro­
cell membranes, is the precursor fatty acid duced from arachidonic acid but via a
in most cases. While these agents are not different enzymatic pathway. There are
classic hormones associated with a single several families of related leukotrienes,
tissue or organ, they are important chemi­ each with specific functions. Leukotrienes
cal messengers involved in the regulation of are produced primarily by monocytes
vastly different physiologic functions in and mast cells and are usually associated
many different body systems. In most cases, with allergic reactions. Release of leukot­
prostaglandins, thromboxanes, and leukot­ rienes increases vascular permeability
rienes act as paracrine agents, for they and induces constriction of airways; in
function near their site of origin and are humans, these substances have been
rapidly metabolized after entering the implicated in producing some of the most
bloodstream. This rapid metabolism in the prolonged manifestations of asthmatic
blood is true for many paracrine agents and attacks.
is a factor that contributes to their charac­ The prominent role for prostaglan-
teristic localized effect. dins is as mediators of inflammation.
Prostaglandins have been isolated from Drugs that inhibit prostaglandin
nearly every tissue of the animal body, synthesis are therefore considered anti‐
although they derive their name from the inflammatory and are referred to as
prostate gland from which they were origi­ nonsteroidal anti‐inflammatory drugs,
nally isolated. There are many different or NSAIDs. These drugs are inhibitors of