Heparin and a low molecular weight fraction enhances thrombolysis and by this pathway exercises a protective effect against thrombosis.

In human volunteers unfractionated heparin and a low molecular weight fraction of heparin (LMWH) caused an increase in plasma plasminogen activator (PA) which peaked at 3 hours after subcutaneous injection. Using a perfused isolated rabbit ear model the enhancement of PA activity was confirmed and was related to the anti-Xa activity of both products infused. Using a modified rabbit Wessler model for thrombus formation it was found that, when using doses of heparin and LMWH sufficient to give a 100% antithrombotic effect, antifibrinolytic drugs (eg. epsilon-ACA and aprotinin), negated this protective effect. It is concluded that the effect of heparin and LMWH on haemostasis is mediated in part through the enhancement which these drugs have on fibrinolysis, the latter being arguably a major defence against fibrin formation during thrombosis.