Activated protein C inhibits thrombus formation in a system with flowing blood

We studied the effect of increasing concentrations of protein C (PC) and activated protein C (APC) on haemostasis in an in vitro thrombosis model. Blood from healthy donors was anticoagulated with citrate-phosphate-dextrose (final citrate concentration 19 mM) or a low molecular weight heparin (LMWH, 20 IU/ml). Enzymatically denuded rabbit aorta segments were exposed to flowing blood for 10 min in an annular perfusion chamber. PC and APC were added to the perfusate immediately prior to exposure. In citrated blood at a shear rate of 800/s, PC and APC induced a statistically significant decrease in platelet deposit at 16 micrograms/ml and 32 micrograms/ml. In perfusions performed with blood anticoagulated with LMWH, there was no effect on platelet deposition at 16 and 32 micrograms/ml either at shear rates of 300/s or 800/s. Addition of PC showed no effect on fibrin deposition at a shear rate of 300/s; in contrast, a nonstatistically significant 40% reduction was seen at a shear rate of 800/s, compared to controls. Addition of APC caused a 100% reduction in fibrin formation at 16 and 32 micrograms/ml at both shear rates studied. PC and APC inhibited platelet deposition on the exposed subendothelial surface, in a dose-dependent manner. Effects of PC and APC on platelet function might be mediated through inhibition of thrombin generation at the platelet microenvironment.