Contribution of plasmin to sex differences in platelet aggregation in the rat

Platelet aggregation was induced more strongly in male than in female 5, 12, and 45 week-old rats by both collagen and arachidonic acid. This is in agreement with our previous reports which suggested that the sex differences in platelet aggregation may be a primary characteristic of rat platelets. In plasma, plasmin-like activity was higher in male than in female rats. Plasmin alone induced aggregation, and low concentrations of plasmin synergistically enhanced collagen-induced aggregation in both male and female rats. Platelets potentiated plasmin generation by plasminogen activator at various Ca2+ concentrations in both male and female rats. Platelets from males displayed more efficient plasmin generation in 2 mM extracellular Ca2+ than those from females. If platelets were activated by abnormal causes in plasma, generated plasmin could make a greater contribution to the potentiated effect of platelet aggregation in male than in female rats. This study suggests that plasmin may be a partial cofactor in sex differences in platelet aggregation in the rat.