Plasma and platelet
factor Va represent different substrates for activated
protein C (APC). In this study, we have measured platelet-dependent
APC resistance and the effect of
aspirin and a
platelet glycoprotein IIbIIIa antagonist (
GR144053F) on this phenomenon. In platelet rich plasma (PRP), progressive
APC resistance was observed with increasing platelet activation. APC sensitivity ratios of 1.8, 1.7, and 1.4 were observed after platelet activation with
thrombin receptor activating peptide (TRAP),
collagen, and
A23187, respectively. Ultracentrifugation at 77,000g for 1 hour abolished
APC resistance indicating that the phenotype is associated exclusively with the platelet membrane.
APC resistance was not observed in the presence of
phosphatidylcholine-
phosphatidylserine (PCPS) vesicles or purified human plasma
lipoproteins.
APC resistance was observed in the presence of platelet-derived microparticles, but to a lesser degree than that in the presence of activated platelets. The platelet-dependent
APC resistance phenotype was also observed when endogenous APC was generated by
Protac (American Diagnostica, Inc, Greenwich, CT). In vitro inhibition of platelet activation with
aspirin had no effect, but the
fibrinogen receptor antagonist,
GR144053F, inhibited platelet-dependent
APC resistance. These results indicate that platelet activation results in an APC-resistant phenotype comparable to that observed in the plasma of patients with
factor V gene mutations affecting critical APC cleavage sites. This suggests that platelet activation at the site of endothelial damage downregulates a critical natural
anticoagulant mechanism. The antithrombotic effect of
aspirin may be due to an indirect effect on platelet-dependent
APC resistance with reduced platelet retention within a developing
thrombus. The more potent antithrombotic effect of
glycoprotein IIbIIIa antagonists may in addition be the result of reduced platelet
factor Va expression and modulation of the platelet-dependent
APC resistance phenotype.