Studies on the acute release of tissue-type plasminogen activator from human endothelial cells in vitro and in rats in vivo: evidence for a dynamic storage pool

Abstract

The process of acute release of tissue-type plasminogen activator (tPA) is important in locally speeding up fibrinolysis. Using a sensitive enzyme-linked immunosorbent assay for tPA, we investigated the acute release of tPA from cultured human umbilical vein endothelial cells. The addition of thrombin (0.003 to 3 NIH U/mL) caused the dose-dependent release of noncomplexed, enzymatically active tPA into the medium. The amount of tPA released into the medium by thrombin was similar to the difference in the amounts of tPA present in extracts from thrombin-treated cells and control cells. The process of acute release of tPA was complete in 1 minute, whereas the concomitant release of von Willebrand factor into the medium was slightly slower (maximum after 3 minutes). By increasing (c.q. decreasing) tPA synthesis, it was found that the amount of tPA constitutively secreted, the amount acutely released, and the amount in cell extracts were increased (c.q. decreased) to the same extent. The same relation was found in vivo. When rats were pretreated with cholera toxin or retinoic acid to increase tPA synthesis, plasma levels of tPA were increased, whereas acute release of tPA, as induced by bradykinin, was increased to the same extent. Acutely released tPA and constitutively secreted tPA were liberated from different pathways in human umbilical vein endothelial cells; tPA had, relative to the in vivo situation, a short residence time in the acutely releasable pathway.

Plasminogen on the surfaces of fibrin clots prevents adhesion of leukocytes and platelets

“…Furthermore, plasma levels of t-PA are low (1-9 ng mL )1 ) under normal physiologic conditions [33,34] and become high only after venous occlusion or after the administration of agents that induce its acute secretion from endothelial cells [34]. As isolated blood monocytes and platelets bind t-PA with low affinity (K d values in the range 0.3-0.9 lM) [35,36]) these cells are unlikely to bind t-PA under basal conditions and/or retain it after cell isolation.…”

Systemic levels and preportal organ release of tissue‐type plasminogen activator are enhanced by PEEP in the pig

“…Tissue-type plasminogen activator, t-PA, is the key enzyme initiating the intravascular fibrinolysis (4)(5)(6). t-PA is secreted from the vascular endothelium by two main pathways: a) a basal constitutive secretion of t-PA regulated through the cellular protein synthesis and b) an acute, receptor-mediated short-term release from a storage pool (4,7,8). Circulating t-PA exists in a free, active form and in a complex-bound inactive form (4,7), both of which are eliminated mainly by the liver (10,11).…”

Plasminogen activators play an essential role in extracellular-matrix invasion by lymphoblastic T cells

“…(Meissauer et al, 1991;Alizadeh et al, 1995), should stimulate more detailed studies on the role of t-PA in invasive processes in general. The finding that a significant part of t-PA is cell-associated suggests that, in T-leukemia cells, t-PA is either bound to a cell-surface receptor (Hajjar, 1995) or is present in a t-PA storage granule (Van den Eijnden-Schrauwen et al, 1995) as was observed for human endothelial cells. Which of these possibilities applies and to what extent the cell-associated fraction of t-PA is important for cell migration remains to be established.…”