Evaluation of the biological effects of low-molecular-weight heparins

In experiments with a laser thrombosis model in rat mesenteric venules, where laser beams induce defined injuries at the vessel endothelium, antithrombotic potency of low-molecular-weight (LMW) heparins was calculated by the number of laser injuries required to induce vessel occluding thrombi. The antithrombotic effect of various LMW heparins was measured at defined time intervals after subcutaneous application. Simultaneously, rat blood was collected and aXa and aIIa plasma levels were determined by chromogenic substrate assays. The antithrombotic effect calculated in the animal model lasted longer than 48 h after a single subcutaneous heparin injection whereas the anticoagulative effect measured as aXa or aIIa activity was measurable only within the first 6 h after subcutaneous application. LMW heparins showed an antithrombotic effect at dosages where no influence on the coagulation system could be demonstrated.