Tissue-type plasminogen activator: a possible candidate of endothelium-derived hyperpolarization factor?

Abstract

It is well established that endothelial cells release endothelium-derived hyperpolarizing factor (EDHF) which dilates blood vessels. However, the identity of EDHF has not yet been clearly understood. Tissue-typeplasminogen activator (tPA) is a serine enzyme secreted by endothelialcells. It plays a critical role in the regulation of hemostasis by convertingplasminogen into plasmin which initiates fibrinolysis and restrictspropagation of the clot beyond the site of vascular injury. Besides, tPA isused in the treatment of acute myocardial infarction because of itsfibrinolytic property. Interestingly, both tPA and EDHF are released fromendothelial cells upon the stimulation by bradykinin and substance P, andthe release occurs in response to the increase in intracellular calciumconcentration. The aim of the present study was to investigate thevasodilatory effect of tPA and whether this effect could account for theEDHF-dependent vasodilation. The results of tissue bath study showedthat exogenously addition of tPA elicited dilation of porcine coronaryarteries in a dose-dependent manner, with an EC50 value of 0.0079 ug/ml.The vasodilatory effect of tPA was the same when the endothelial cellswere removed, suggesting that tPA mainly acted on vascular smoothmuscle cells but not endothelial cells. The vasodilatory effect of tPA wasnot affected by the pre-treatment with iberiotoxin, a large conductancecalcium-activated potassium (BKCa) channel blocker, indicating thatother mechanism(s) might be involved. To study the EDHF response,porcine coronary arteries were pre-treated with L-NAME (which blocknitric oxide synthesis) and indomethacin (which block prostacyclinsynthesis) before dilation by bradykinin. Plasminogen activator inhibitor-1 (PAI-1), an inhibitor of tPA, did not change the EDHF response. Inaddition, enzyme-linked immunosorbent assay (ELISA) study revealedthat the endogenous release of tPA from porcine coronary arterialendothelial cells, upon the stimulation by bradykinin, was in a negligiblelevel and was far below the concentration which could cause vasodilation.We therefore conclude that tPA can dilate porcine coronary arteries.However, our data do not support the hypothesis that tPA is a candidate of EDHF.