To clarify a possible involvement of the vasoconstrictive peptide endothelin in the regulation of endothelial cell-mediated fibrinolytic system, confluent cultures of vascular endothelial cells from human umbilical vein were incubated in serum-free medium in the presence of endothelin-1 at 100 nM and below, and tissue plasminogen activator antigen (t-PA:Ag) in the medium was determined by enzyme immunoassay. Endothelin-l at 1 nM and above significantly decreased the release of t-PA:Ag from the endothelial cells after a 24 h incubation. The t-PA:Ag release was also decreased by either endothelin-2 or endothelin-3 at 10 nM. The activity of lactate dehydrogenase in the medium was not changed by endothelin-l at 100 nM and below, suggesting that the peptide did not cause nonspecific cell damage. The decrease in the t-PA: Ag release induced by endothelin-1 occurred in the presence or absence of 8-bromo cyclic AMP, which is an active congener of cyclic AMP; 3-isobutyl-l-methylxanthine, which is an inhibitor of phosphodiesterase; and forskolin, which is a stimulator of adenylate cyclase. These results strongly indicated that cyclic AMP which is known to down-regulate t-PA;Ag release was not involved in the endothelin-l effect. However, endothelin-l failed to decrease the t-PA:Ag release in the presence of either calcium ionophore A23187 or EGTA; the ionophore itself markedly decreased the release. The cytosolic calcium accumulation was significantly increased by endothelin-l. These results suggest that endothelin-l decreases the release of t-P A:Ag from human endothelial cells through an excess accumulation of intracellular, especially cytosolic calcium which would be mediated by an extracellular, calcium-dependent mechanism.