Flow and the Inhibition of Prothrombinase by Antithrombin I11 and Heparin

Inhibition of prothrombinase by antithrombin 111 (ATIII) and heparin was investigated in a continuous-flow system. Phospholipid-coated capillaries, containing phospholipid-bound factor Xa and factor Va, were perfused with 1.0 pmol/L prothrombin and 0.5 nmol/L factor Va. At 25°C and a flow rate of 32 pL/min (shear rate 28 seconds-‘) the steady-state rates of prothrombin conversion depended linearly on the surface concentration of prothrombinase up to 2 fmol/cm*. The rate of thrombin generation was 952 2 43 (SE) mol/min/ mol prothrombinase. When ATlll was included in the perfusate for 10 minutes, the free thrombin concentration at the outlet of the capillary was markedly reduced: a 50% neutralization was obtained at 0.7 pmol/L ATIII. However, the prothrombinase activity was not inhibited, as could be established after a subsequent perfusion with prothrombin and factor Va. At an ATlll concentration typical of normal plasma (2 pmol/L) a slight neutralization of prothrombinase was observed: 10% neutralization following a IO-minute perfusion. During a perfusion with ATlll in the absence of prothrombin, or in its presence with hirudin (2 kmol/L) also