1. Examples: cimetidine, ranitidine
  1. Remember that the parietal cell has receptors for gastrin, histamine and ACh. Gastrin is released from the antral G-cells into the blood. This causes an increase in cytosolic Ca2+, which stimulates acid secretion from the H/K ATPase (the proton pump) on the canalicular surface.
  2. The H2 receptor antagonists are competitive antagonists of the H2 receptor. They are highly selective and do not affect H1 or H3 receptors.
  3. When given in usual doses, they inhibit 60-70% of 24 hour gastric secretion.
  4. Indications as follows:
    • GORD
    • Peptic ulcer disease
    • Gastritis.
  5. Adverse effects (with regards to cimetidine)
    • Cimetidine inhibits binding of dihydrotestosterone to androgen receptors. This inhibits metabolism of estradiol and increases serum prolactin levels. It may cause gynecomastia or impotence in men, and galactorrhea in women.
  1. In particular, cimetidine is a strong inhibitor of hepatic CYP450 pathways. The T ½ of drugs metabolized by these pathways may be prolonged.
    • Ranitidine binds less avidly to CYP systems.