Lack of alpha 2-antiplasmin enhances ADP induced platelet micro-aggregation through the presence of excess active plasmin in mice.

BACKGROUND: The role of alpha2-antiplasmin (alpha2-AP) on platelet aggregation was investigated using mice deficient in alpha2-AP (alpha2-AP(-/-)) or using wild type mice (alpha2-AP(+/+)).
METHODS: Blood samples were taken from each mouse under anesthesia with ether and platelet rich plasma (PRP) was prepared. Platelet aggregation induced by various doses of ADP (0.3-30 microM) was detected using a laser-light scattering (LS) system. Aggregated forms were observed using a scanning electron microscopy (SEM).
RESULTS: Dose-dependent platelet aggregation was not different in both types of mice. However, platelet micro-aggregate formation in alpha2-AP(-/-) mice induced by low dose of ADP (1.0 microM) markedly increased compared to the situation in wild type mice. Aggregated form detected by SEM showed supported data from LS analysis. When washed platelets of alpha2-AP(+/+) mice were resuspended in plasma of alpha2-AP(-/-) mice, platelet micro-aggregation was also increased. On the contrary, when washed platelets of alpha2-AP(-/-) mice were suspended in plasma of alpha2-AP(+/+) mice, platelet micro-aggregation did not change. In separate experiments, tPA (1.0 microg/ml) was added to PRP before the stimulation of ADP. tPA had no effect on platelet aggregation in alpha2-AP(+/+) mice, however platelet micro-aggregation in alpha2-AP(-/-) mice was markedly increased by the treatment with tPA. Moreover, the amount of released ATP from stimulated platelets was increased in alpha2-AP(-/-) mice treated with tPA.
CONCLUSION: Lack of alpha2-AP increased platelet micro-aggregation, and plasmin plays an important role in the formation of platelet aggregation when alpha2-AP knockout mice are used. Consequently, the reduction of alpha2-AP could be a risk factor for the activation of platelets resulting in thrombus formation.