Thrombin-stimulated platelet aggregation is inhibited by kallikrein in a time- and concentration-dependent manner.

Many in vitro studies have shown that activation of prekallikrein (PK) to kallikrein (KAL) in normal plasma triggers rapid activation of the coagulation cascade. In agreement, the coagulation activation is impaired in PK-deficient plasma. Paradoxically, PK-deficient patients show a tendency to thrombosis. To investigate the discrepancy between the in vitro and in vivo findings, we analyzed the effect of KAL on the rate of platelet aggregation. For this research, physiologic concentrations of washed human platelets were incubated for 5 and/or 10 min with approximately 2.2 to 88 nM human plasma KAL (< 1/100 to approximately 1/3 of PK concentrations in plasma) prior to the addition of high concentrations of alpha-thrombin (54 nM) or fibrinogen plus ADP. KAL concentrations were arbitrarily selected on the assumption that concentrations of free KAL (the enzymatically active species) were minute in normal plasma and higher when KAL production was enhanced, and/or inhibitors were depleted. Full platelet aggregation was that seen in the absence of KAL or PK. Inhibition of platelet aggregation stimulated by thrombin was markedly increased with increased KAL concentrations and incubation times. The degree of inhibition by KAL was smaller when ADP was the agonist. The data suggest that KAL may play a role in the modulation of platelet aggregation in vivo under normal conditions as well as when prolonged, high concentrations of KAL occur in blood. The data may also help to explain the intriguing observation that PK-deficient patients show a tendency to thrombotic episodes and myocardial infarction whereas in vitro assays predict bleeding.