Platelet aggregation and haemolysis induced in rats by intravenous infusion of ADP - effect of potentially antithrombotic drugs

Platelet hypoaggregability in rats exposed to SART stress (repeated cold stress)

A prolongation of bleeding time accompanied by thrombocytopenia and abnormalities in coagulation-fibrinolysis systems has been observed in laboratory animals exposed to a chronic form of environmental stress induced by severe fluctuations of air temperature, known as SART (specific alternation of rhythm in temperature) stress. In order to clarify the hemostatic profile under SART stress in more detail, the present study examined platelet aggregability in vitro as well as in vivo in stressed rats. During exposure to stress, thrombocytopenia developed from day 5, and remained up to at least day 14. In vitro aggregation of platelets stimulated by ADP or collagen was markedly decreased in stressed rats, compared with unstressed rats. Furthermore, stressed rats exhibited in vivo hypoaggregability of platelets, as estimated by the magnitude of the drop in circulating platelet counts following intravenous injection of ADP and collagen. Protein and cholesterol content in platelets remained constant after stress exposure. These results indicate that SART-stressed rats exhibit platelet dysfunctions in addition to thrombocytopenia. Considering the previous findings, the hemostatic system under SART stress appears to show a general tendency toward hemorrhage.

Impaired thromboxane production by newly formed platelets after aspirin administration to thrombocytopenic rats

The in vivo inhibitory effect of aspirin on platelet cyclo-oxygenase is irreversible and lasts for the entire platelet life-span. Reappearance of cyclo-oxygenase activity in blood after aspirin has been proposed as a measure of the formation of new platelets and as an indirect indicator of platelet survival. A delay of 24--72 h in recovery, however, has been observed and it has been suggested that aspirin might also inhibit megakaryocyte cyclo-oxygenase. To test this possibility, aspirin (100 mg/kg) or saline were administered i.p. to rats made thrombocytopenic 2 h later (platelet count less than 5% of basal value) by a specific antiplatelet antiserum. Malondialdehyde (MDA) and thromboxane B2 (TxB2) production by platelets was measured by spectrophotometry and radioimmunoassay respectively, during the period of platelet count restoration. By 24 h after thrombocytopenia was induced, platelet count was about 15% of basal values in control and aspirin-treated rats. However, while in controls MDA and TxB2 production was restored to about 20% of basal values, in aspirin-treated rats less than 5% return of activity was detected. A marked difference between the two groups was still found 96 h after induction of thrombocytopenia, when platelet count restoration was similar. Since aspirin disappeared very rapidly from the circulation, the delay in recovery of cyclo-oxygenase activity supports the hypothesis of a megakaryocyte effect of this drug.

Effect of pretreatment with acetylsalicylate on surgical bleeding and peroperative mortality in rats undergoing kidney transplantation

42 rats were pretreated with L-ASA before kidney transplantation, 43 rats acted as controls. 9 rats with L-ASA, but no control rats, died with i.p. haemorrhage. However, in animals surviving the operation, the intraoperative blood loss did not differ significantly between the 2 groups.