Protamine-induced platelet aggregation and clotting investigated by ultrasound.

Using high-resolution real-time ultrasound to monitor platelet aggregation and plasma clotting, the effect of protamine on platelets was evaluated in a dynamic system of plasma in vitro. Protamine induced platelet aggregation preceding plasma clotting at both low (1.6 s-1) and moderate (22.6 s-1) shear rates. The onset of aggregation and clotting was accelerated at the higher shear rate. Low-shear clots were heterogeneous on ultrasonic imaging, whereas moderate-shear clots were more densely homogeneous. Protamine-induced platelet aggregation was reversed by additional heparin. Pretreatment of plasma with prostacyclin prevented protamine-induced aggregation, but clot formation occurred. However, such clots were less echogenic. EGTA and hirudin inhibited both aggregation and clotting. Our ultrasonic aggregometry showed that platelet aggregation was induced by protamine before clots formed, and that not only shear rate but platelet activation might affect the rate and composition of plasma clots.