Thrombin-induced platelet activation is inhibited by high- and low-molecular-weight heparin

Background —Thrombin binds to platelet glycoprotein Ib (Gp Ib), and this interaction contributes to platelet activation. Thrombin ligation to Gp Ib was recently shown to be inhibited by heparin, thus raising the hypothesis, investigated in this article, that heparin might inhibit thrombin-induced platelet activation. Methods and Results —Aggregation of gel-filtered platelets by 1 nmol/L thrombin was reduced by both high-molecular-weight (MW) (14 500-Da) and low-MW (4500-Da) heparin, with IC 50 values of 1.65±0.26 and 5.13±0.8 μmol/L, respectively. Homogeneous-MW fractions (16 000- to 13 000-Da range) were used to evaluate the heparin effect on intracytoplasmic calcium release by thrombin. Calcium mobilization by 1 nmol/L thrombin was reduced as a function of heparin concentration, and the inhibitory effect was correlated to the MW of heparin fractions (IC 50 values were 1.9±0.39, 6.07±0.83, and 14.8±0.43 μmol/L for 16 000-, 9000-, and 3000-Da heparin, respectively). Platelet aggregation and calcium mobilization by ADP and by the thrombin receptor–activating peptide were not affected by heparin. The activation of Gp Ib–depleted platelets by α-thrombin was not inhibited by heparin. Moreover, platelet stimulation by heparin binding site phosphopyridoxylated thrombin, which has a severe impairment of Gp Ib ligation, was not affected by heparin. Finally, heparin did not interfere with the hydrolysis by thrombin of the protease-activated receptor 1. Conclusions —These results demonstrated that heparin, by inhibiting the thrombin–Gp Ib interaction, is able to interfere with thrombin-induced platelet activation. The extent of the inhibitory effect is directly related to the MW of heparin fractions.