An in vivo Model for the Assessment of Acute Fibrinolytic Capacity of the Endothelium

Abstract

SummaryThe effects on blood flow and plasma fibrinolytic and coagulation parameters of intraarterial substance P, an endothelium dependent vasodilator, and sodium nitroprusside, a control endothelium independent vasodilator, were studied in the human forearm circulation. At subsystemic locally active doses, both substance P (2-8 pmol/min) and sodium nitroprusside (2-8 μg/min) caused dose-dependent vasodilatation (p <0.001 for both) without affecting plasma concentrations of PAI-1, von Willebrand factor antigen or factor VIII:C activity. Substance P caused local increases in t-PA antigen and activity (p <0.001) in the infused arm while sodium nitroprusside did not. At higher doses, substance P increased blood flow and t-PA concentrations in the noninfused arm. We conclude that brief, locally active and subsystemic infusions of intraarterial substance P cause a rapid and substantial local release of t-PA which appear to act via a flow and nitric oxide independent mechanism. This model should provide a useful and selective method of assessing the in vivo capacity of the forearm endothelium to release t-PA acutely.

Bradykinin Stimulates Tissue Plasminogen Activator Release From Human Forearm Vasculature Through B 2 Receptor–Dependent, NO Synthase–Independent, and Cyclooxygenase-Independent Pathway

“…First, in this and previous studies, it has been consistently demonstrated that SNP does not induce tPA release at doses that increase FBF and shear stress. 6,21,22 Second, treatment with L-NMMA and indomethacin selectively modulated vasodilation without altering the tPA response to BK. Regardless, the finding that B 2 receptor antagonism blocks the effects of exogenous BK on tPA release from the human vasculature lays the groundwork for studies elucidating the contribution of endogenous BK to the effects of ACE inhibitors on endothelial tPA release in humans.…”

“…On the other hand, an NO-independent effect of BK on tPA release is further supported by the finding in this and numerous other studies that SNP, an NO donor, does not stimulate endothelial tPA release. 6,21,22 Studies in vitro and in vivo suggest that prostaglandins do not contribute significantly to the vasodilator response to BK in human vasculature. Thus, pretreatment with COX inhibitors such as aspirin or indomethacin does not alter vasodilation to BK in isolated coronary or resistance arteries.…”

Impaired Coronary Tissue Plasminogen Activator Release Is Associated With Coronary Atherosclerosis and Cigarette Smoking

“…2 This has allowed us to show that cigarette smoking is associated with an impairment of acute tPA release in the forearm circulation. 6 We hypothesized that the acute local coronary release of tPA would be influenced by both the extent of coronary atheroma and smoking habit.…”

“…This dynamic aspect of endothelial function and fibrinolytic balance may be directly relevant to the pathogenesis of atherothrombosis, but only recently have robust methods to determine acute tPA release been developed. 2 Small areas of denudation and thrombus deposition are a common finding on the surface of atheromatous plaques and are usually subclinical. 3 In the presence of an imbalance in the fibrinolytic system, however, such microthrombi may propagate, ultimately leading to arterial occlusion.…”

Methods for Evaluating Endothelial Function in Humans

“…82 In this model, different stimuli, such as desmopressin 83 or bradykinin, 84 are used to stimulate endothelial cells to release t-PA. To avoid confounding hemodynamic effects related to the systemic infusion, as well as the activation of the sympathetic nervous system and concomitant release of other mediators, investigators have primarily focused on the measurement of the t-PA release in individual vascular beds. Regional forearm t-PA release in response to intrabrachial infusions has been assessed by using 2 similar methodologies: the arteriovenous technique, 85 based on the differences in plasma concentrations of t-PA between the inflowing arterial and outflowing venous plasma of a single arm, and the venovenous technique, 86 based on the differences in venous plasma concentrations between the 2 arms. Both methods use strain gauge plethysmography to measure forearm blood flow which, multiplied by the difference in the t-PA measured and corrected for hematocrit, gives the net t-PA release in the forearm.…”