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5

Messenger Targets for Drug Action II

Hormones and their receptors

5.1 OVERVIEW OF RELEVANT ANATOMY AND

PHYSIOLOGY OF HORMONES

In 1902, Bayliss and Starling demonstrated that the effect exerted by the duodenum on
pancreatic secretion was due to a bloodborne factor that seemingly acted as a “chemi-
cal messenger.” Their research led to the identification of this chemical messenger,
which they called secretin. They went on to suggest that secretin was not unique,
hypothesizing that many chemical agents are secreted by various cells throughout the
body and that these agents, upon distribution by the bloodstream and circulation, influ-
ence the function of organs that are located some distance away. They coined the term
hormoneto describe such chemical messengers that are synthesized in one organ
system and distributed via the circulation to distant organ systems to elicit an altered
biochemical response. Insulin is the prototypic example of a hormone; it is biosynthe-
sized in the pancreas and then distributed via the bloodstream to organs and tissues
throughout the body where it influences carbohydrate, protein, and fat metabolism. The
medical discipline of endocrinologydeals with the diagnosis and treatment of diseases
related to hormones and hormonally responsive systems.
Hormones are central to homeostasis since they facilitate chemical control over meta-
bolic and biochemical processes throughout the entire body. The endocrine system,
which exerts control over chemical processesvia hormones, is crucial to homeostasis
over an intermediate time scale. The endocrine system is distinct from the nervous
system, which employs neurotransmitters to control electrical and electrochemical
processesand to influence homeostasis over a shorter time scale. The endocrine system
is also distinct from the immune system, which employs immunomodulators to control
cellular processesand to influence homeostasis over a longer-term time scale. There is,
of course, a large amount of overlap among these three control systems.
The endocrine system is composed of hormone-producing organs within the body.
Probably the most important endocrine organ is the pituitary gland, located at the base
of the skull and intimately associated with the hypothalamus; both the pituitary and
the hypothalamus secrete a wide variety of peptidic hormones and are crucial to the