PLATELET - ENDOTHELIAL-CELL INTERACTIONS IN MEDIATING THE RELEASE OF PLASMINOGEN-ACTIVATOR INHIBITOR-1

The present study was undertaken to examine the release mechanism(s) o f PAI-1 from human platelets (P) and endothelial cells (EC), and furth ermore to examine the interactions between P and EC in mediating the r elease of PAI-1. Platelets release PAI-1, mainly in the latent form, i n response to various platelet activators including thrombin, collagen , adenosine diphosphate, arachidonic acid, and epinephrine, but not in response to transforming growth factor-beta beta (tGF beta) or other growth factors. In contrast, human umbilical vein or human aortic EC r elease PAI-1, mainly in the active form, in response to incubation wit h platelets or platelet releasates, tGF beta, endotoxins, protein kina se C activators, and PAF but not in response to the different platelet activators. The stimulating effect of platelets or their releasates o n the release of PAI-1 from ECs was shown to be rapid in onset, and re ached its maximum effect after 24 hours of incubation. In conclusion, the study suggests: a) different mechanisms for the release of PAI-1 f rom P as compared to EC, and b) significant interactions between P and EC in the mediation of active PAI-1 release by EC.