Platelets and phospholipids in tissue factor-initiated thrombin generation

The influence of platelets on tissue factor (TF)-initiated thrombin generat ion in a reconstituted model of blood coagulation and in whole blood was ev aluated. No thrombin generation was observed over 15 min in the reconstitut ed model when either TF or platelets and phospholipids were omitted. At 25 pM TF, the rates of thrombin generation were platelet and PCPS concentratio n-dependent and achieved maximum (1.0 nM/s) in the physiological range of p latelet concentration. Similar rates were achieved in the absence of platel ets when 1-2 muM phospholipid was used. However, the maximum rates of throm bin generation (5.2-6.0 nM/s) and the shortest initiation phase (1 min) wer e attained between 25 and 100 muM phospholipid. In the reconstituted model, an increase in platelet concentration from 0.125 X 10(8)/ml to 0.5 X 10(8) /ml decreased the duration of the initiation phase (in the absence of phosp holipids) from 4.3 min to 2 min. Further increases in platelet concentratio n did not affect this phase. Sequential whole blood studies were conducted in blood of a chemotherapy patient who developed reduced platelet counts. T he TF (12.5 pM) initiated clotting of patient's blood was accelerated from similar to 10 min to 5 min when the platelet concentration increased from 0 .05 X 10(8)/ml to 0.11 X 10(8)/ml. Clotting times were essentially unchange d for platelet concentrations exceeding 0.5 X 10(8)/ml (range 0.5-3.1 X 10( 8)/ml). Similarly, clotting of whole blood obtained from healthy volunteers was not affected by the platelet count, which varied from 1.5 X 10(8)/ml t o 3.1 X 10(8)/ml (4.0 +/- 0.5 min). The data obtained in both models are co nsistent with in vivo observations that clinical bleeding is most likely to occur at platelet counts <0.1 X 10(8)/ml.