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Adrenergic Stimulants |
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Structure |
Use, Receptor Site |
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alpha |
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Vassopressor, bronchodilator, nasal decongestant
alpha or beta
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Neo-Synephrine vasopressor nasal decongestant
alpha
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Propadrine,
nasal decongestant
alpha or beta
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vasopressor, bronchodilator, nasal decongestant
alpha or beta
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Sudafed vasopressor, bronchodilator, nasal decongestant
alpha or beta
|
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Afrin, Duration, nasal decongestant |
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Adrenergic Drugs II
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Adrenergic Stimulants:
Adrenergic drugs stimulate the adrenergic nerves directly
by mimicking the action of norepinephrine. Therapeutically, these
drugs are used in nasal decongestants as the main example given
here.
The structures of the stimulants closely mimic the structure
of the neurotransmitters and are thus able to interact with the
receptor site. Adrenergic stimulants may have three modes of
action: direct interaction with specific receptors (examples
are epinephrine and phenylephrine); indirect action by stimulating
release of neurotransmitters; or a mixed action involving both
of the above (examples are phenylpropanolamine and ephedrine).
1. Bronchodilators:
Epinephrine and ephedrine are mainly used in the treatment
of bronchial asthma. This respiratory disease results from a
spasmodic contraction of the smooth muscles of the bronchi. In
addition there is an increase in bronchial secretions of mucus.
In an acute asthmatic attack, manifested by great difficulties
in breathing, epinephrine or isoproternol is administered by
injection or inhalation. These drugs act directly on beta receptors
to relax the smooth muscles (bronchodilation). In addition epinephrine
by an alpha stimulating action, constricts the bronchial mucosa
and reduces secretions.
Ephedrine, although 100 times less potent than epinephrine,
has prolonged bronchodilatory effects after oral administration.
This action as well as giving relief from nasal congestion make
it a valuable drug for the prevention of asthmatic attacks. Pseudoephedrine
is the (-) optical isomer of (+) ephedrine. The activities of
the optical isomers of ephedrine show the rather precise nature
of the receptor-drug interactions. Pseudoephedrine is much less
potent than ephedrine. See the graphic below for details of the
interaction with the receptor site.
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2. Nasal Decongestants and Vasopressors:
The swelling of blood vessels of the mucous membranes of the
nasal passage causes nasal discharge and obstruction. Vasoconstrictor
agents or nasal decongestants act on local sympathetic nerves
and shrink swollen nasal membranes. Decongestants are either
topically administered as drops and inhalants or are orally ingested.
Although topically administered nasal decongestants do shrink
membranes, the relief is only temporary and is followed by "rebound
congestion". Phenylephrine and ephedrine are used topically
and provide relief for 3-4 hours. The response to rebound congestion
may lead to repeated use, damage to nasal mucosa, and chronic
congestion. Xylometazoline and oxymetazoline have a longer lasting
effect (10-12 hours).
Oral administration of nasal decongestants such as phenylpropanolamine,
pseudoephedrine, and phenylephrine provide a more effective means
of reaching the nerves via the blood stream. This route does
not disrupt the mucosa environment, is longer acting, and reduces
the incidence of "rebound congestion".
3. Cardiac Activation:
Ephinephrine can be injected directly into the heart to stimulate
it after it as stopped beating due to drowning, suffocation,
shock, electrocution, and anesthesia. The epinephrine dramatically
restores the heart beat. In cases of shock, norepinephrine has
been used to restore and maintain sufficient blood pressure and
ensure adequate blood flow to vital organs.
When local anesthetics are used to reduce or eliminate pain
in a specific area, epinephrine is frequently used in conjunction
with these agents to constrict the blood vessels at the area
and prevent drug diffusion from that area.
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