The effects of various inhibitors on platelet aggregation induced by polylysine

Abstract

Cationic polymers such as polylysine, aggregate platelets by reducing the negative surface charge and forming bridges between the adjacent platelet membranes. A controversy however, has arisen about the question whether it can elicit the release reaction.In the present study, three types of polylysines were tested in platelet rich plasma (PRP) and in washed platelet suspension (WPS) for the ability to cause aggregation and release reaction. We also studied the effects of various inhibitors including negative charged acid mucopolysaccharide, heparin.All three types of polylysine induced platelet aggregation which showed biphasic aggregation curve in PRP but showed no secondary curve of aggregation in WPS. At the same concentration of polylysine, maximum aggregation was slightly reduced in both system when the lower molecular one was used. Various inhibitors such as NEM, adenosine, aspirin, PGE1, completely inhibited the aggregation in PRP but had no effect on the aggregation in WPS. Heparin inhibited the both aggregation in PRP and WPS and higher concentration of heparin was needed to inhibit aggregation when the larger molecular one of polylysine was used.Polylysine caused release reaction from the 14C-5HT labeled platelets in PRP but could not elicit release reaction in WPS. Although the small amount of radioactivity was detected in surrounding medium in WPS, these were probably due to the increased permeability of membrane because the same amount of radioactivity was also detected when the various inhibitors were added.Electron microscopic observation of aggregated platelets in PRP and WPS showed the evidence supporting the result of the presence or absence of release reaction in two systems assessed by measuring the radioactivity.Polylysine aggregates platelets by reducing the negative surface charge of platelets and can elicit release reaction in the presence of Ca++ or plasma. The aggregation in PRP and WPS was inhibited by heparin probably due to neutralizing the positive charge of polylysine. In contrast to in WPS, inhibitors other than heparin inhibited the first wave of aggregation in PRP but this mechanism remains unexplained.The average distance between plasma membrane of the aggregated platelets in WPS did not vary with the degree of polymerization of these polylysine. In this respect, platelet aggregation differs from aggregation of red blood cells by polylysine in which widening of the intercellular space were observed by others with an increase in molecular size.