Autacoids: Intro, Histamine Agonists & Antagonists Flashcards
Major sites with high concentrations of histamine?
- Mast cells
- Basophils
- Enterochromaffin like cells found in the fundus of the stomach
How is histamine formed in the body?
Formed by decarboxylation of the amino acid histidine by the enzyme histidine decarboxylase.
Outline the roles played by histamine in anaphylactic reactions.
- Contraction of smooth muscles.
- Stimulation of secretions.
- Dilation and increased capillary permeability.
- Stimulation of sensory nerve endings.
The Triple Response to Histamine is characterized by ?
- Wheal ( edematous) - due to increased capillary permeability.
- Reddening- due to dilation of small blood vessels resulting in increased blood flow.
- Flare
List down 2 Antihistamines that act by directly inhibiting the release of histamine.
- Cromolyn
- Nedocromil
Differences between 1st and 2nd generation antihistamines.
- 1st generation antihistamines can penetrate the CNS resulting in sedation while 2nd generation antihistamines have less sedation effects since they can’t cross the BBB.
- 1st generation antihistamines can interact with other receptors resulting in unwanted effects while 2nd generation antihistamines are specific for H1 receptors.
List down the major 1st generation H1 antihistamine drugs.
- Chlorpheniramine
- Cyclizine
- Diphenhydramine
- Dimenhydrinate
- Doxepin
- Doxylamine
- Hydroxyzine
- Meclizine
- Promethazine
- Antazoline
What are the major H1 antihistamines that cause sedation?
- Chlorpheniramine
- Diphenhydramine
- Doxylamine
- Hydroxyzine
- Promethazine
Which antihistamines drugs are recommended in treatment of motion sickness?
- Diphenhydramine
- Dimenhydrinate
- Cyclizine
- Meclizine
- Hydroxyzine
- Scopolamine - DOC but is an anticholinergic drug
List down the major 2nd generation Antihistamines.
C- FLAD
* Cetirizine
* Fexofenadine
* Loratadine
* Acrivastine
* Desloratadine
– Desloratadine, fexofenadine and Loratadine are completely none sedating , the rest have weak sedation potential.
Outline the major clinical uses of H1 antihistamines .
- Used in management of allergic reactions mediated by IgE sensitized mast cells.
- Management of motion sickness & nausea e.g Diphenhydramine & Cyclizine.
- Treatment of insomnia by inducing drowsiness & sedation e.g Diphenhydramine & Doxylamine.
- Treatment of Parkinsonism e.g Diphenhydramine is used to suppress the extra pyramidal system effects.
- Local anaesthesia for patients allergic to conventional anaesthetic drugs like procaine e.g diphenhydramine & promethazine.
List down the major adverse effects associated with Antihistamine intake.
- Drowsiness , dizziness
- Urinary retention
- Tachycardia
- Hypotension
- Vertigo
- Blurred vision
- Tremors
- Dry mouth
- Fatigue
- Increased appetite
- Tinnitus
- Uncoordination
What is the chief clinical use of H2 receptor blockers?
- Used to inhibit gastric acid secretion in treatment of gastric ulcers and gastroesophageal reflux disease.
- MoA - competitively blocks binding of histamine to H2 receptors ( reduced cAMP)
List down 5 drugs commonly used as H2 receptor blockers.
- Cool - Cimetidine
- Rebels - Ranitidine
- Need - Nizatidine
- Freedom - Famotidine
- Too - Tiotidine
State the major locations of the Histamine receptors in the body.
- H1 receptors - smooth muscles, endothelium, brain.
- H2 receptors - gastric mucosa, cardiac muscle, mast cells, brain.
- H3 receptors - brain, myenteric plexus and other neurons.
- H4 receptors - eosinophils, neutrophils, CD4 T- cells , leukocytes in bone marrow & circulating blood.
