Activation of Protease-Activated Receptors 3 and 4 Accelerates Tissue Factor–Induced Thrombin Generation on the Surface of Vascular Smooth Muscle Cells

Objective—To determine factors regulating human aortic smooth muscle cells (HASMC) supported tissue factor-induced thrombin generation. Methods and Results—The addition of nonlipidated tissue factor and Ca2+ to HASMCs maintained in reptilase-treated platelet-poor plasma resulted in the robust formation of thrombin after a lag phase of approximately 6 minutes. Pretreatment with low concentrations of α-thrombin before the addition of tissue factor and Ca2+ accelerated the rate of thrombin generation (time to reach half of peak thrombin was reduced by [mean±SD] 42.0±2.2%; P<0.05) but had no effect on the amount of peak thrombin generated. Protease-activated receptor (PAR) 3 activating peptides (APs) or PAR-4 APs accelerated thrombin generation without affecting peak thrombin levels (time to half of peak thrombin decreased by 17.4±5.6% and 21.7±3.5%; P<0.05 with PAR-3 AP and PAR-4 AP, respectively). The addition of PAR-3 AP and PAR-4 AP together had an additive effect, with a reduction in time to half of peak...