Role of the Endothelium in the Vascular Effects of the Thrombin Receptor (Protease-Activated Receptor Type 1) in Humans

Objectives
The purpose of this study was to determine the role of the endothelium in the vascular actions of protease-activated receptor type 1(PAR-1) activation in vivo in man.

Background
Thrombin is central to the pathophysiology of atherothrombosis. Its cellular actions are mediated via PAR-1. Protease-activated receptor type 1 activation causes arterial vasodilation, venoconstriction, platelet activation, and tissue-type plasminogen activator release in man.

Methods
Dorsal hand vein diameter was measured in 6 healthy volunteers before and after endothelial denudation. Forearm arterial blood flow, plasma fibrinolytic factors, and platelet activation were measured in 24 healthy volunteers during venous occlusion plethysmography. The effects of inhibition of prostacyclin, nitric oxide (NO), and endothelium-derived hyperpolarizing factor on PAR-1 responses were assessed during coadministration of aspirin, the “NO clamp” (L-NG-monomethyl arginine and sodium nitroprusside), and tetraethylammonium ion, respectively.

Results
Endothelial denudation did not affect PAR-1–evoked venoconstriction (SFLLRN; 0.05 to 15 nmol/min). Although aspirin had no effect, SFLLRN-induced vasodilation (5 to 50 nmol/min) was attenuated by the NO clamp (p < 0.0001) and tetraethylammonium ion (p < 0.05) and abolished by their combination (p < 0.01). The NO clamp augmented SFLLRN-induced tissue-type plasminogen activator and plasminogen activator inhibitor type 1 antigen (p < 0.0001) release, but tetraethylammonium ion and aspirin had no effect. SFLLRN-induced platelet activation was unaffected by NO or prostacyclin inhibition.

Conclusions
Acting via PAR-1, thrombin causes contrasting effects in the human vasculature and has a major interaction with the endothelium. This highlights the critical importance of endothelial function during acute arterial injury and intravascular thrombosis, as occurs in cardiovascular events including myocardial infarction and stroke.

Role of the Endothelium in the Vascular Effects of the Thrombin Receptor (Protease-Activated Receptor Type 1) in Humans

Thrombin is central to the pathophysiology of atherothrombosis and exerts its vascular effects via protease activated receptor type 1 (PAR-1). We report the contrasting role of the endothelium in PAR-1 activation in vivo in man, where it mediates arterial PAR-1–induced vasodilation and tissue plasminogen activator release but does not provide a major contribution to venous tone or plasminogen activator inhibitor type 1 release. Our findings provide evidence of a major interaction between the endothelium and thrombin in vivo. This highlights the critical importance of endothelial function during acute arterial injury and thrombosis, as occurs in acute coronary syndromes.

Ingibjörg J. Gu mundsdóttir, Ninian N. Lang, Nicholas A. Boon, Christopher A. Ludlam, David J. Webb, Keith A. Fox, David E. Newby