Antihistamines Flashcards
histamines
chemicals produced in specialized cells through body
role in allergic reaction and regulation of gastric acid secretion
histamine effects
Vascular system:
-dilates small blood vessels
- increases capillary permeability
Bronci
- constriction of smooth muscle
Stomach
- secretion of gastric acid
CNS
- acts as a neurotransmitter
distribution of histamine
most tissues
- high in skin, lungs, gi tract
- low in plasma
synthesis and storage of histamines
mast cells ( in tissues)
basophils (in blood)
stored in secretory granules
release of histamine causes
-Allergic and non-Allergic reactions
allergic release of histamines
initial requirement
- production of IgE antibodies
- antibodies attach to mast cells and basophils
subsequent
- allergen binds to 2 adjacent IgE (creates a bridge)
- binding causes histamine granules to fuse with cell membrane and release
**REQUIRES PRIOR EXPOSURE
Non-Allergic release histamine
Several agents can act directly on mast cells to release histamine
non prior sensitization is required
What receptors does Histamine act upon
Histamine 1 (H1)
Histamine 2 (H2)
Stimulation of H1 receptors (5)
vasodilation:
dilation of arterioles and venules
- skin becomes warm and flushed
- BP drops
Increased capillary permeability:
capillary endothelial tissue contacts
pores increase in size
fluids, protein and platelets escape
Bronchoconstriction
- constriction of bronchi
CNS
- role in cognition, memory and sleeping/waking cycles
Others
itching and pain
Histamine 2 stimulation
activation of GI tract parietal cells
- secretion of gastric acid
what are mild allergy symptoms caused by?
Stimulation of H1 receptors
Severe Allergic reactions are called
Treatment?
Anaphalactic Shock: bronchoconstriction and hypotension
Treatment: EPINEPHERINE principal mediators
Classification of H1 Antagonists
First- generation (highly sedative)
Second-generation antihistamines (not sedative)
H1 antagonists mechanism
H1 blockers bind SELECTIVELY to H1 bistamine receptors
- block histamine
H1 blocker also bind to non-histamine receptors (muscarinic receptors)
- side effects
H1 antagonists pharmacological effects (6)
Major therapeutic effect of H1 antagonists
Peripheral:
blocking of histamine at H1 receptors
Arterioles and venules
- localized flushing
Capillary beds:
- reduces increased permeability
=REDUCED EDEMA
Sensory Nerves
- reduces itching and pain
Mucous membranes
- suppresses secretion of mucus
CNS effects
- therapeutic doses: CNS depression (slow rxn time)
- Overdose: CNS stimulation ( convulsions)
H1 therapeutic uses
Mild Allergy:
- reduce symptoms
Motion Sickness
- block muscarinic receptors in the neural pathway from the inner ear to the medulla vomiting center
Insomnia
- cause drowsiness
- every OTC sleep aid contains H1 antagonists
H1 antagonist blocks mast cells from releasing histamines. T/F
False- they prevent the binding of histamines to H1 receptors
Do H1 antagonists help with the common cold
NO
Adverse effects H1 antagonists
Sedation
- most common side-effect
-IMPAIRED LEARNING AND MEMORY
–> impairment when driving is equal to blood levels over legal limit
GI effect
- nausea, vomiting
- administer with food to help
Anticholinergic effect
- antimuscarinic action
–> dry mouth and throat
–> urinary urgency
–> constipation
Second-Generation H1 antagonists drug name
Fexofenadine (F)
Second-Generation H1 antagonists
sedation effects
What effects Absorption
Cost
Dont produce sedation
- can’t cross BBB well
- low affinity to CNS H1 receptors
Some fruit juices reduce absorption
- no juices 4 hr before dosing
Relatively expensive
