BLEEDING-TIME IN LABORATORY-ANIMALS .3. DO TAIL BLEEDING TIMES IN RATS ONLY MEASURE A PLATELET DEFECT - (THE ASPIRIN PUZZLE)

The template bleeding time tested was not prolonged in rats given aspirin (single doses ranging from 1-200 mg/kg body wt). Bleeding after the transection of tails immersed in saline at 37.degree. C was significantly longer in aspirin-treated than in control rats. This prolongation was maximal 1 h after aspirin (200 mg/kg) and was back to normal within 24 h, when platelet malondialdehyde formation was still completely inhibited. Both template and transection bleeding times were significantly prolonged in thrombocytopenic and thrombocytopathic rats and in normal rats given a pyrimido-pyrimidine compound (SH-869, dipyridamole). Platelets are important in the hemostatic process of the rat, but the platelet defect induced by aspirin is not detectable by the bleeding time measurement techniques used. These may be sensitive not only to platelet dysfunction but also to modifications of vascular tone and blood flow and to blood coagulation and fibrinolysis systems. The complex and contrasting effects of aspirin on hemostatic factors other than platelets (e.g., on vascular prostacyclin and fibrinolysis) may influence bleeding times measured by different techniques and contribute to the puzzling results obtained so far with this drug.