Heparin cofactor II significance for the inhibition of thrombin at the injured vessel wall.

The thrombin inhibitory role of antithrombin III (ATIII) and heparin cofactor II (HCII) was studied in vitro using intact and injured rabbit aortae. When intact vessels were loaded with thrombin and then exposed to either heat defibrinogenated human plasma (HDHP) or ATIII the same degree of thrombin inhibition was achieved demonstrating that ATIII was the only plasma component involved in thrombin inhibition on the intact vessel wall. When the media of the vessel wall was loaded with thrombin and then exposed to ATIII or HCII a significantly higher thrombin activity remained on the surface than when it was exposed to defibrinogenated plasma. A mixture of ATIII and HCII resulted in a greater inhibition of thrombin than ATIII or HCII alone. It is concluded that, contrary to what happens on the endothelium, HCII and ATIII inhibit additively thrombin on the injured vessel wall. HCII thus plays an essential role for the inhibition of thrombin at the injured vessel wall. It is also concluded that an additional plasma component participates in thrombin inhibition on the media but its contribution is negligible as compared with ATIII or HCII.