Histamine H1 receptor
Location and effect when activated
- Bronchiolar smooth muscle – bronchoconstriction, increased bronchial mucus production
- Nasal smooth muscle – increased nasal mucus production
- Heart – slower conduction through AV node (longer PR interval)
- Endothelial cells – increased capillary dilatation and permeability (hypotension, edema)
Main purposes of H1 antihistamines
- Decreased vascular permeability – useful in treating angioedema, urticaria and hypotension seen early in anaphylaxis
- Inhibiting bronchoconstriction – useful in anaphylaxis (alongside or while waiting for epinephrine)
- Useful in pruritus as well, surprisingly not useful in nasal congestion with the exception of cetirizine and desloratadine
Do not delay administration of epinephrine for the delivery of anti-histamine. Anti-histamine is a lower priority than epinephrine!
Examples
- 1st generation
- Chlorpheniramine (Chlortripolon)
- Diphenhydramine (Benadryl)
- Dimenhydrinate (Gravol)
- 2nd generation
- Bilastine (Blexten)
- Cetirizine (Reactine)
- Useful in nasal congestion
- Fexofenadine (Allegra)
- Loratadine (Claritin)
- Rupatadine (Rupall)
- Desloratadine (Aerius)
- Active metabolite of loratadine
- Useful in nasal congestion
2nd generation antihistamines are less sedating, have less anticholinergic effects and are longer acting and are therefore over 1st generation antihistamines!
Histamine H2 receptor
Location and effect when activated
- Parietal cells of stomach – increased gastric acid secretion
- Heart – positive inotropic effect and enhanced automaticity
- Vascular smooth muscle – relaxation and vasodilation (hypotension)
- Neutrophils – inhibit neutrophil activation
Main purposes of H2 antihistamines
- Decreased gastric acid secretion – useful in patients with GERD
Examples
- Cimetidine (Tagamet)
- Famotidine (Pepcid)
- Nizatidine (Axid)
- Ranitidine (Zantac) in 2019 was recalled
To really simplify: H1 = anti-allergic, H2 = anti-acid!