The effect of cimetidine on platelet function: a study involving gastric fluid measurements

Gastric fluid samples were aspirated 30 and 60 minutes after the ingestion of two 200 mg tablets of cimetidine. The concentration of cimetidine in these samples was measured and their effect on platelet aggregation assessed in vitro. Gastric fluid samples significantly inhibited adrenaline- and ADP-induced platelet aggregation in vitro. In a further series of experiments, cimetidine solutions, at concentrations found in gastric fluid, inhibited platelet aggregation and thromboxane A₂ (TXA₂) release, in vitro. Ranitidine, another H₂-receptor antagonist, was a more potent inhibitor of platelet aggregation than cimetidine. Since ranitidine is also the more potent H₂-receptor antagonist which, unlike cimetidine, does not include an imidazole group (which is known to inhibit TXA₂ synthesis) in its structure, we conclude that H₂ blockade mediates the observed inhibition of aggregation. This platelet anti-aggregatory effect may be relevant to haemostatic mechanisms involved in bleeding peptic ulcers or gastric erosions exposed to high local concentrations of H₂-receptor antagonists.