VPT 311: General and Systemic Veterinary Pharmacology (2+1)

Histamine H2 receptor antagonists

H₂ RECEPTOR ANTAGONISTS

• Endogenous histamine released from paracrine enterochromaffin-like cells or mast cells stimulates parietal H₂ receptors.
• Histamine activates a cyclic AMP pathway, activates H^+_ K⁺-ATPase and leads to increased H⁺ in gastric lumen. H₂ receptor antagonists can block this histamine induced secretion and are helpful reducing acid secretion during ulcers.
• Examples of H₂ receptor antagonists include cimetidine, ranitidine, famotidine, nizatidine etc.
• In general, these drugs are well tolerated with few side effects (less than 3% with cimetidine and <1% with other drugs).
• Cimetidine exhibits antiandrogenic properties leading to decreased sperm count, gynecomastia and also inhibits hepatic drug metabolism.
• Inhibition of cytochrome P450 activity by cimetidine leads to potential drug interactions.
• This feature is not seen with ranitidine, famotidine and nizatidine.