Fluctuations of euglobulin lysis time, tissue plasminogen activator, and free and total plasminogen activator inhibitor levels in plasma in daytime.

Blood was taken from healthy 15 males and 10 females at 9:30 h, 10:00 h and 12:30 h. The euglobulin fraction was prepared and clotted by the addition of human thrombin. The clot lysis time shortened significantly from 9:30 h to 10:00 h (p less than 0.01) and further to 12:30 h (p less than 0.01). Plasma levels of tissue plasminogen activator (t-PA) antigens did not change from 9:30 h to 10:00 h, but slightly and significantly to 12:30 h (p less than 0.01). Plasma levels of free plasminogen activator inhibitor-1 (PAI-1) and complex of t-PA-PAI-1 decreased from 9:30 h to 10:00 h (p less than 0.02) and to 12:30 h (p less than 0.01). Plasma levels of total PAI-1 (free plus complex) decreased from 9:30 h to 10:00 h (p less than 0.01) and to 12:30 h (p less than 0.01). These results suggest that a major factor contributing to the enhanced fibrinolytic activity of the euglobulin fraction may be a level of PAI-1 (free and total).