Chapter 18
Autocoids and Antihistamines
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Histamine
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There are two types of histamine
receptors. Histamine 1 receptors are
involved with allergic response.
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Histamine is released from mast cells when an allergic
reaction occurs.
Histamine release causes vasodilation, increased
capillary permeability, bronchoconstriction, and pain or
itching in cutaneous nerve endings.
Stimulation of histamine 2 receptors is involved with
increased gastric acid secretion.
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Adverse Reactions
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Anaphylaxis is a major adverse reaction of
histamine release.
It is characterized by difficulty in breathing,
seizures, lapses into unconsciousness, and
death.
Other symptoms include apprehension,
paresthesia, urticaria, edema, choking, cyanosis,
coughing, and wheezing.
Histamine is used in the diagnosis of achlorhydria
and pheochromocytoma.
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Antihistamines
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Antihistamines block histamine 1 receptors
and are used to treat allergic rhinitis,
allergic reaction, nausea and vomiting, and
preoperative sedation.
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Antihistamines can also be used as over-thecounter sleep aids, and as local anesthesia.
 Antihistamines are categorized as either
sedating or nonsedating.
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Antihistamines
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Pharmacologic Effects
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By blocking histamine response these drugs decrease
capillary permeability.
Vascular smooth muscle dilation is also blocked.
These drugs cause bronchial relaxation.
They suppress pain and itching by blocking histamine
actions at cutaneous nerve endings.
These drugs also cause sedation.
They have weak anticholinergic effects.
They have antiemetic and antimotion sickness effect.
They provide some local anesthetic effects.
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Sedating Antihistamines
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Adverse reactions of antihistamines are
the same as their pharmacologic effects.
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The adverse reactions vary in relative amounts
among the different drugs.
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Sedation is the most common side effect of older
antihistamines.
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Dizziness, incoordination, and fatigue can occur.
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Stimulation or excitement often occurs in children,
the elderly, and in those who use high doses.
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Some drugs can cause nausea, vomiting, and
constipation.
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Xerostomia often occurs with older drugs.
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Peripheral (Nonsedating)
H1-Receptor Antagonists
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These drugs do not cross the blood-brain
barrier.
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As a result they are less likely to cause
sedation.
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Prostaglandins and
Thromboxanes
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They are members of a biologically active
group of agents called eicosanoids.
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They are found in most body tissues and fluids.
They are produced in the body in response to
many different stimuli.
These agents can cause vascular smooth
muscle relaxation or constriction depending on
the specific prostaglandin released.
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Prostaglandins and
Thromboxanes
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Thromboxane stimulates platelet
aggregation and prostaglandin1 inhibits
platelet aggregation.
Prostaglandins stimulate uterine
contractions.
Prostaglandins stimulate the release of
growth hormone, increase body
temperature, and stimulate the release of
female sex hormones.
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Prostaglandins and
Thromboxanes
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Prostaglandins have been implicated in
periodontal disease.
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They appear to have a role in the inflammation
that is associated with periodontal disease.
Prostaglandins also prevent the synthesis of
new bone by inhibiting osteoblastic activity.
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Other Autocoids
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Leukotrienes
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Leukotrienes are a group of autocoids derived from
arachidonic acid.
They cause powerful bronchoconstriction.
They also contract the uterus and gastrointestinal tract.
Kinins
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Kinins may play a role in dental disease.
Plasma kinins may be involved with shock and acute or
chronic allergic or inflammatory conditions.
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Other Autocoids
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Substance P
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This peptide is thought to function as a
neurotransmitter in the CNS and as a local
hormone in the gastrointestinal tract.
 It is a vasodilator and produces hypotension.
 It causes secretion in the salivary glands.
 It increases sodium and water excretion from
the kidneys.
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