Autacoids 1: Histamine Flashcards
site of H1 receptor
- BVs
- Smooth muscles
- Skin (sensory nerve endings)
- CNS
Site of H2 receptor
- BVs
- gastric parietal cells
- Heart
- Mast cells
H1 receptor mechanism
Gq
+ causes vasodilatation by releasing Nitric oxide (which increases cAMP)
H2 receptor mechanism
Gs (increases cAMP)
Action of H1 receptor
- Vasodilatation
- Bronchoconstriction & GIT spasm
- Skin: itching, urticaria & pain
- function related to appetite in CNS
Action of H2 receptor
- Vasodilatation
- Increase HCL secretion
- Negative feedback onto mast cells
- Increase Heart rate & contractility
Physiological antagonist of Histamine
Adrenaline
Examples of H2 blockers
- Cimetidine
- Ranitidine
-tidine
Action of H2 blockers
decrease gastric secretions (used for peptic ulcer)
Action of H1 blockers
Antihistaminics
Antihistaminic are divided into ____
- 1st gen
- 2nd gen
Enumerate 1st gen Antihistaminics
- Dimenhydrinate
- Diphenhydramine
- Carbinoxamine
- Chlorpheniramine
- Clemastine
- Cyclizine
- Mecilizine
- Promethazine
- Hydroxyzine
- Cyproheptadine
Enumerate 2nd gen Antihistaminics
Lora, ceti, fexo
* Loratadine
* Cetirizine
* Fexofenadine
difference between 1st gen & 2nd gen antihistaminics
- 1st gen can cross BBB, thus have a strong sedative effect
- 1st gen autonomic receptor blocking effect (atropine like + blocks Alpha receptors)
- 1st gen has short duration
1st gen antihistaminics effects not releated to Histamine receptors
- Sedation
- Atropine like action: urine retention, dry mouth, blurred vision
- Antiemetic: for motion sickness
- Alpha blocker: postural hypotension
- serotonin receptor blocker (Cyproheptadine)
2nd gen antihistaminics effects not releated to Histamine receptors
not present
theraputic uses of Antihistaminics
- Allergic reactions (except for bronchial asthema)
- Motion sickenss (by blocking muscarinic receptors & H1 of vomiting center)
- Carcinoid Syndrome = ectgopic secretion of serotonin (block Serotonin (5HT) receptors)
Explain why Antihistaminics are ineffective in bronchal asthma
- bronchal asthma involves other mediators aside from histamine, mainly Leukotriens
- It’s Atropine like (opposes bronchodilatation)
which 2 antihistaminics are especially used in motion sickness
- Diphenhydramine
- Promethazine
which antihistaminic is especially used in Carcinoid syndrome
Cyproheptidine
side effects of Antihistaminics
- Sedation: so not used by drivers or Children as may cause convulsions
- Atropine like action: dry mouth, urine retention, blurred vision
- Orthostatic hypotension
early 2nd gen antihistaminics may cause prolonged QT interval & serous arrhythmias = torsade de pointes, especially when given with CYP450 inhibitors (the enzyme that metabolizes early 2nd gen antihistaminics)
Drug interaction of 1st gen antihistaminics
can potentiate sedative effect of Hypnotics
Drug interactions of 2nd gen antihistaminics
arrhythmia when given with CYP450 inhibtors
