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chapter
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Endocrine Metabolism 1: Introduction
triggered by the same substance clearly manifest the dif-
ferent specialized cell functions that are affected.
Hormones and neurotransmitters may have evolved
from the same, or similar, ancestral prototypes in uni-
cellular organisms. Substances that resemble mammalian
hormones in structure and biological activity serve as neu-
rotransmitters in some invertebrates and lower vertebrates.
Several peptide and steroid hormones long considered to
be evolutionarily advanced mammalian forms have been
found in certain prokaryotes and unicellular eukaryotes.
Several mammalian brain-gut peptides occur in the skin
of amphibians. Specific receptor sites for mammalian hor-
mones are demonstrable at all stages of phyletic evolution.
Thus, the structures of hormones and neurotransmitters
and their functions as chemical messengers may have been
highly conserved during evolution. The fact that many pro-
totypic messengers in unicellular organisms predate the
appearance of nerve cells implies that neurotransmitters
may be a specialized form of hormone. Chemically, hor-
mones are of four types:
1. Hormonal amine,
2. Peptide, protein, or glycoprotein,
3. Steroid, and
4. Eicosanoid.
Amine hormones and some agents of the second type
have counterparts in the nervous system that function as
neurotransmitters.
30.1
Hormonal Amines
Hormonal amines are derived from amino acids and, in
most cases, represent simple modifications of the par-
ent compound. Table 30-1 lists the important hormonal
amines, parent amino acids, major sites of synthesis, and
principal actions. All of these amines except the thyroid
hormones (Chapter 33) are decarboxylated products that
are synthesized both in and out of the nervous system.
Within the nervous system, they are important neurotrans-
mitters; outside the nervous system, the cells that produce
TABLE 30-1
Hormonal Amines
Hormone
Parent
Amino Acid
Major Site
Neurotransmitter in
of Synthesis
Nervous System
Systemic Action
Dopamine
Tyr
Hypothalamus;
adrenal medulla
Yes
Renal vasodilation (D j)
inhibition of anterior
pituitary hormone release
(D2); inhibition of
aldosterone synthesis(D,).
Epinephrine
Tyr
Adrenal medulla
Minor
Cardiovascular and
metabolic effects (via a- and
/
3
-adrenergic receptors).
Norepinephrine
Tyr
Peripheral nerve endings;
adrenal medulla
Yes
Cardiovascular and
metabolic effects (via a- and
/3-adrenergic receptors).
Histamine
His
Mast cells; basophils;
regenerating cells
Yes
Systemic vasodilation
(Hj, H2); increased
capillary permeability
(H |); increased gastric
secretion (H2); increased
heart rate (H2).
Serotonin
Trp
Mast cells; liver;
sympathetic nerve
endings; stored
platelets
Yes
\hsodilation in skin and
skeletal muscle, vaso-
constriction elsewhere;
positive inotropic and
chronotropic effects.
3,5,3'-Triiodothyronine
(T
3 ) 1
Tyr
Peripheral conversion from
thyroxine (T4); thyroid
7
Many effects, e.g., regulation
of metabolism.
1
An amino acid.
