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have a brief lifetime and act near their site of synthesis – Local hormones
- hormones act beyond their site of production carried by blood stream for selective action elsewhere in the body
histamine stored in
- mast cells
- basophils
histamine acts on
- H1: mediates allergic reactions (gets blocked by antihistamines); also a neurotransmitter
- H2: promotes gastric acid secretion
IgE synthesized in
peripheral lymphoid tissue
- antibody that binds to mast cells and basophils and triggers release of histamine
while stored (before exocytosis) histamine is bound to either
heparin sulfate or chondroitin sulfate E
histamine allergic response
- Exposure to allergen prompts synthesis of IgE from peripheral lymphoid tissue
- Allergen binds to IgE on mast cells, promotes release of various mediators of anaphylaxis
- Leukotrienes, Prostaglandins, Histamine, etc released by exocytosis
drugs that promote histamine release by displacing histamine from the heparin-protein complex in the mast cell
- Opioids (morphine)
- NMJ competitive blocks (tubocurarine)
Drugs that prevent histamine release
- Cromolyn (aka sodium cromoglycate)
minor:
- Theophylline/Aminophylline
- Beta-2 agonists (bronchodilators)
Cromolyn mechanism
- Mast cell stabilizer (prevents degranulation (exocytosis) of the mast cell
- can be used for Allergic rhinitis (nose spray), Allergic conjunctivitis (eye drops), asthma (inhaler)
Actions of histamine at H1 receptors (classic allergic response)
- bronchoconstriction
- pain, itching
- reduction in vascular resistance: decreased blood pressure, increased heart rate (heart tries to compensate by increasing HR)
- relaxation and contraction of smooth muscle in GI tract
- contraction of gallbladder
H1 receptor antagonists (antihistamines)
- blocking H1 receptors helps with itch
- sedation (if drug crosses blood brain barrier)
- regular basis for managing seasonal allergies vs Acute anaphylactic attacks
Managing severe anaphylaxis
- antihistamine useless - histamine already bound - need to reverse effects not prevent
- B-agonist like epinephrine causes bronchodilation and vasoconstriction
Routes of administration for antihistamines
- Oral
- Topical (Dermal/Ophthalmic)
2 types of antihistamine
- First generation (sedating)
- Diphenhydramine (Benadryl®)
- Second generation (non-sedating) - do not cross the blood brain barrier - Loratadine (Claritin®)
side effects of antihistamines
- sedation
- anticholinergic effects (dry mouth)
arachidonic acid cascade
- phospholipase A2 breaks down membrane phospholipids into arachidonic acid
- arachidonic acid has the cyclooxygenase pathway and the lipoxygenase pathway (makes leukotrienes)
- NSAIDs inhibit the COX pathway (generation of prostaglandins, prostacyclins, thromboxane A2)
- LT receptor antagonists are used in the management of asthma
