The role of the tissue factor pathway in initiation of coagulation.

Three model systems have been used to study the dynamics of the blood clotting process initiated by tissue factor (TF): synthetic plasma mixtures prepared with purified coagulation proteins and inhibitors; mathematical models based on the reaction constants, stoichiometries and thermodynamics of individual catalyst and inhibitor reactions; and contact suppressed whole blood induced to clot in vitro by the addition of exogenous TF. In the three models, the generation of thrombin can be described in terms of an initiation phase in which pmol/l concentrations of the coagulation serine proteases are generated and the cofactor proteins factor V (FV) and FVIII are activated. Subsequently, explosive thrombin generation occurs during a propagation phase. The complementary inhibitory pathways extinguish the generation of thrombin. Tissue factor pathway inhibitor (TFPI), present in low concentrations, primarily influences the duration of the initiation phase and has little influence on the propagation phase. Antithrombin III (ATIII), present in higher concentrations, has little influence during the initiation phase, but decreases the rate of thrombin generation during the propagation phase. The protein C pathway cannot act in the absence of thrombin and therefore only influences the duration of the propagation phase by inactivating activated FV. Thus combinations of TFPI plus ATIII and TFPI plus protein C pathway components contribute to the synergistic inhibitory processes. As a consequence of the roles of pro, and anti-coagulants, the generation of thrombin by the TF pathway becomes a threshold limited process.