CHAPTER
Endocrine Metabolism I: Introduction
Survival of multicellular organisms depends on integra-
tion and coordination of differentiated cell functions and
the ability to react appropriately to internal and external
influences that threaten to disrupt homeostatic conditions.
These requirements are fulfilled by a form of intercellu-
lar communication in which chemical signals (messen-
gers) released by one cell evoke a receptor-mediated res-
ponse in another. There are two types of chemical
messenger: neurotransmitters and hormones. Neurotrans-
mitters convey signals from one neuron to another or from
a neuron to an effector cell, travel very short distances
to reach their target sites, and function within the spe-
cialized regions of synapses and junctions. Hormones are
usually defined as messengers that are transported by the
blood to distal target cells. Because they are released into
the interstitial space and thence into blood, they are called
“endocrine” (ductless; “secreted within”) secretions to dis-
tinguish them from those which are released into the ex-
ternal environment (“exocrine” or ductal secretions).
This terminology and these definitions, though not
always appropriate and possibly misleading, are used for
want of better alternatives. The term “hormone” comes
Endocrine topics not discussed in Chapters 30-34 that are covered
elsewhere in the text are as follows: gastrointestinal hormones, Chapter
12; eicosanoids, Chapter 18; pancreatic hormones, Chapter 22; parathy-
roid hormone and vitamin D, Chapter 37; renin-angiotensin system and
antidiuretic hormone, Chapters 32 and 39. A list of expanded acronyms
appears in Appendix VIII.
from the Greek
hormaein,
“to excite.” This is a misnomer
because some hormones have primarily inhibitory effects,
and others may stimulate or inhibit, depending on the tar-
get cell type. The term
endocrine
is misleading because
some hormones (steroids, prostaglandins, growth factors)
are released via ducts to the external environment, where
they affect reproductive activities or wound healing. The
designation
pheromones
may apply to some of these hor-
mones (steroids, prostaglandins) because they act on cells
in another organism or individual. The usual definition
of a hormone excludes those messengers which affect
cells in the vicinity of their release and which do not rely
on transport in the blood to reach their destination. For
these, the term
parahormone
(paracrine secretion, local
hormone, autacoid) is commonly used. However, if pro-
duced in sufficiently large amounts, parahormones dif-
fuse into the vascular compartment and behave as authen-
tic hormones (e.g., somatomedins). Conversely, hormones
frequently assume paracrine functions. For example, an-
drogens produced by Leydig cells of the testes diffuse and
exert their effect on nearby Sertoli cells. Parahormones,
therefore, should be regarded as authentic hormones that
exert mainly local effects. Finally, neurotransmitters and
hormones differ in respect to the systems that use them.
Thus, the nervous and endocrine systems share some
messengers
(norepinephrine,
dopamine,
enkephalins,
endorphins, cholecystokinin, bombesin, etc.) that exert
receptor-mediated effects. The different events that are
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