histamine

Question 1

4 known histamine receptors:

Answer 1

1. H1 receptor
2. H2 receptor
3. H3 receptor
4. H4 receptor

Question 2

H1 receptor

Answer 2

o Smooth muscle contraction – bronchoconstriction + ileum contraction
o Systemic vasodilation
o Itching
o Arousal + wakefulness

Question 3

H2 receptor

Answer 3

o Secretion of gastric acid in stomach

o Increase in HR + CO

Question 4

H1 receptor antagonists

Answer 4

o Loratadine
o Certirizine
o Fexodenadine

Question 5

H2 receptor antagonists:

Answer 5

o Ranitidine

o Famotidine

Question 6

“Triple Response”

Answer 6

1. Localised redness of the skin
   - Red line
   - Transient local vasodilation due to histamine, appears in a few seconds
2. Surrounding flare
   - Spreading redness extending beyond the red line
   - Appears slowly
3. Wheal
   - Localised oedema in the region of the red line
   - Increased capillary permeability and venules due to histamine release
   - Appears in 1-2 minute

• Accompanied by itch
• These effects primarily mediated by stimulation of H1 receptors

Question 7

Sedating antihistamines (1st generation)

Answer 7

• Block/inhibit the actions of histamine at H1 receptors
• E.g. Promethazine (Phenergan) – motion sickness
• E.g. Doxylamine (Restavit) – insomnia
• E.g. Polaramine
• They can cross the BBB and cause sedation, drowsiness
• Don’t combine with alcohol, benzos – depressant effect
• Used to treat allergic rhinitis (hayfever), hives, skin rashes, prophylactic use
• Often used in cough/cold preparations

Question 8

Non-sedating antihistamines (2nd generation)

Answer 8

• Block/inhibit the actions of histamine at H1 receptors
• Cardiac safe
• E.g. Fexofenadine (Telfast) – doesn’t cross BBB
• E.g. Loratadine (Claritin)
• E.g. Certirizine (Zyrtec)
• Loratadine and Certirizine DO cross BBB BUT are pumped back out via P-glycoprotein transporters (therefore, they are sedating but less sedating than 1 generations AHs)
• For: allergic rhinitis, hives, skin rashes, prophylactic use