Adrenergic Stimulation of Regional Plasminogen Activator Release in Rabbits

In vivo release of tissue-type plasminogen activator antigen from the human brachial artery

The rate of secretion of t-PA from vascular endothelial cells has been proposed as a good marker of endothelial function. Our aim was to measure the rate of in vivo t-PA antigen release from the human brachial artery and not from the combined vascular pool of the upper extremity. In 10 healthy male volunteers, we have occluded the forearm by a sphygmomanometer cuff for 5-7 min in order to reduce the blood flow in the brachial artery. Arterial blood samples were taken from the cubital artery above the occlusion and from the contralateral artery that served as the control. The arterial t-PA antigen concentrations were significantly higher after forearm occlusion than in the non-occluded contralateral arteries: median 3.3 (range between first and third quartile 2.1-3.6) vs. 2.5 (2.0-3.2) ng/ml, p=0.03. The PAI-1 antigen concentrations did not change significantly: 6.9 (2.3-18.6) ng/ml in the contralateral artery vs. 5.4 (1.8-8.0) ng/ml after occlusion, p=0.09. The average forward blood flow in the distal 15 cm of the brachial artery that was measured by duplex ultrasound decreased from 107 (97-118) ml/min at baseline to 25 ml/min (19-33) ml/min during occlusion. The release rate of circulating t-PA antigen was calculated as the product of the increment in arterial t-PA concentration and the average plasma flow in the arterial segment of interest with a volume of 2 ml. The median release rate of t-PA antigen under conditions of reduced blood flow in the brachial artery was 3.3 (1.1-11.5) ng/min. We conclude that the secretion of t-PA antigen from arterial endothelium of healthy subjects is substantial, whereas the arterial wall is not an important source of PAI-1 in vivo.

The circulatory regulation of TPA and UPA secretion, clearance, and inhibition during exercise and during the infusion of isoproterenol and phenylephrine

BACKGROUND

Exercise to exhaustion and infusions of isoproterenol and phenylephrine were used to study interactions between plasminogen activator regulation and the control of regional blood flow in 10 healthy males.

METHODS AND RESULTS

Experimental measurements of cardiac output, heart rate, tissue plasminogen activator (TPA), urokinase plasminogen activator (UPA), plasminogen activator inhibitor (PAI-1), C1-inhibitor, and TPA/C1-inhibitor complex during the infusions and exercise were used to develop a comprehensive fluid-phase model of the circulatory regulation of fibrinolysis. alpha- and beta-adrenergic agonists increased TPA and UPA in plasma by different mechanisms: Phenylephrine decreased hepatic blood flow and thus clearance while isoproterenol stimulated increased secretion of TPA and UPA. Exercise to exhaustion increased TPA and UPA through a combination of increased secretion and decreased clearance. The time course of UPA and TPA release were similar, but the magnitude of their secretion responses differed. In vivo, C1-inhibitor bound to TPA at a rate of 553 mol-1.s-1. C1-inhibitor contributed equally with PAI-1 to TPA inhibition when active PAI-1 levels were low (20 to 50 pmol/L) but was less important when active PAI-1 levels were high.

CONCLUSIONS

We conclude that secretion, inhibition, clearance, and regional blood flow effects must all be taken into account when evaluating changes in plasminogen activator levels.