Heparin-Platelet Interaction and Effects on Platelet Aggregation
Confusing data exists in the literature on the effect of heparin on platelet aggregation. Clarification is achieved by the discovery that heparin, itself, produces some platelet aggregation with maximal effect a t 0.6 U/ml heparin with human PRP diluted 1:1 with PO4 buffered saline at pH 7.6. The extra stimulus of platelet activation by heparin (usually unnoticed) increases synergistically the platelet response to most aggregating agents with maximal effect also at 0.6 U/ml of added heparin. With collagen, as with thrombin, however, the addition of heparin decreases platelet aggrega tion due to specific binding of heparin. At high concentrations of heparin (25-50 U/ml) all platelet aggregation is inhibited. Excess heparin, over and above the amount that can bind to the platelets produces electrostatic shielding that prevents the negatively charged platelets from interacting. It is also possible that aggregation is reduced by a loss of fibrinogen and fibronectin from the platelet surface by the formation of soluble heparin complexes.