Encyclopedia of Health and Medicine

126 The Endocrine System
In the brain, epinephrine is an active and abundant
neurotransmitter in numerous neurologic
functions, including heightened alertness and cognitive
function during stressful situations. Epinephrine
is also a pharmaceutical DRUG used to
treat CARDIAC ARREST and cardiovascular SHOCK.
When injected intravenously to produce blood
levels significantly higher than normal in the
bloodstream, epinephrine causes accelerated
actions such as those it initiates as an endogenous
hormone in the stress response. It also stabilizes
the electrical activity of the heart to normalize the
heart’s rhythm (antiarrhythmic). In other pharmaceutical
applications epinephrine blocks the
body’s inflammatory response in severe allergic
reactions and anaphylactic shock, and can reduce
bleeding and extend the effectiveness of injected
local anesthetics.
EPINEPHRINE, ADRENALINE, OR ADRENALIN?
Epinephrine, adrenaline, and Adrenalin are the
same chemical. Adrenalin (no e) is a proprietary
DRUG trademarked in the United States. Epinephrine
and adrenaline designate either the endogenous
chemical (HORMONE or NEUROTRANSMITTER) or
the GENERIC DRUG. Only the United States uses
the term epinephrine. Other countries follow the
international standard terminology, which uses
the term adrenaline. This is because in the
United States trademark protections prevent
alternate names for trademarked products that
could be confused with the trademark.
For further discussion of epinephrine within
the context of the endocrine system’s structure
and function please see the overview section “The
Endocrine System.”
See also DOPAMINE; NOREPINEPHRINE.
estrogens A collective term for the “female” sex
hormones, including prohormones (chemical precursors
the body converts to hormones) and
metabolites (byproducts of HORMONE METABOLISM).
Estrogens are among the steroid hormones the
body synthesizes from cholesterol. Estrogens derive
from ANDROGENS (the “male” sex hormones).
Common use applies the singular term estrogen
to refer to any or all of the three endogenous (naturally
occurring within the body) estrogen hormones:
estradiol, estriol, and estrone. Estradiol is
the most potent and most biochemically active of
the estrogens. Estrone is very similar in chemical
structure to estradiol though exerts a weaker
response. Estriol is a metabolite of both estradiol
and estrone.
In a woman’s body the levels of these closely
related hormones change at MENARCHE and at
MENOPAUSE, and fluctuate within the menstrual
cycle and with PREGNANCY. Estradiol is the predominant
estrogen during the years of FERTILITY, with
the less-potent estrone moving into dominance
after menopause. In a man’s body estrogen levels
remain fairly constant. The HYPOTHALAMUS’s secretion
of GONADOTROPIN-RELEASING HORMONE (GNRH)
regulates the hormonal cascade for production
and release of the estrogens. In women this cascade
is cyclic, establishing the monthly menstrual
cycle during the four decades or so a woman is
fertile.
Men and women alike have estrogens (just as
both sexes also have androgens). The OVARIES in
women, the TESTES in men, and the adrenal cortex
of the ADRENAL GLANDS in men and women synthesize
(produce) most of the estrogens in the BLOOD
circulation. During pregnancy the PLACENTA produces
estrogens as well. Adipose (fat) cells and the
LIVER in both sexes, and the breasts in women, also
synthesize small amounts of estrogens.
In women the estrogens establish secondary
sex characteristics and fertility, and maintain pregnancy.
The estrogens are essential in both sexes
for cholesterol metabolism, BONE calcium content
and density, thyroid function, SKIN health, and collagen
maintenance. The estrogens also have roles
in mood and emotion, probably in both men and
women though more pronounced in women
because estrogen levels fluctuate with the menstrual
cycle.
Various endocrine disorders may result in estrogen
levels that are too high or too low, with consequences
for fertility in women and for
cholesterol metabolism in men and women. Doctors
use pharmaceutical preparations of estrogens
and estrogen analogs (drugs that bind with estrogen
receptors though do not have estrogen activity)
for a diverse array of therapeutic applications
including CONTRACEPTION (birth control pills), treatment
for HORMONE-DRIVEN CANCERS (notably PROSTATE