Plasminogen activator release during venous stasis and exercise as determined by a new specific assay

A method for the specific determination of tissue plasminogen activator in plasma samples has been developed. The method is based on a recently described parabolic rate assay for tissue plasminogen activator (Rånby and Wallén, 1980) measuring plasmin activity by a chromogenic tripeptide-paranitroanilide substrate. The potential interference by plasmin inhibitors in plasma is overcome by acidification of the plasma samples to pH 4 for 15 min prior to the analysis. The baseline activity (after 10 min of rest) in 22 healthy individuals was determined to be 0.05 +/- 0.03 (SD) IU/ml (0.2 +/- 0.1 ng/ml). After venous occlusion for 10 min at 100 mm Hg the activator concentrations had increased to 1.2 +/- 1.2 (SD) IU/ml (5.2 +/- 5.2 ng/ml). Furthermore, prior to venous occlusion almost no plasmin-alpha 2-antiplasmin complex was found in the samples, whereas a pronounced increase in this complex was noted in 90% of the subjects after venous occlusion. During exercise, a gradual work-load dependent increase of tissue plasminogen activator concentration was observed. At maximal work-load the concentration was 2.3 +/- 1.9 (SD) IU/ml (9.9 +/- 8.1 ng/ml active enzyme), which after 10 min rest decreased to 26 +/- 12 (SD) % of the values at maximal work-load. In these samples only small amounts of plasmin-alpha 2-antiplasmin complex were detected, although the activator content was of the same order of magnitude as after venous occlusion.