Vorapaxar, but not parmodulin 2, induces endothelial dysfunction upon prolonged exposures. (A) HUVECs were exposed to vehicle alone, 10 μM parmodulin 2, or 0.3 μM vorapaxar, as indicated, for 48 hours and subsequently stained for apoptosis using YO-PRO-1. (B) HUVECs were incubated with the indicated concentrations of either vorapaxar (blue) or parmodulin 2 (red) for either 24 hours (solid lines) or 48 hours (dashed lines) and assayed for apoptosis. Data are presented as means ± SEM (n = 5). (C) Mock-transfected (mock) and PAR1 siRNA–transfected (siPAR1) HUVECs were incubated with either vehicle (black) or 0.3 μM vorapaxar (blue) for either 24 or 48 hours. Samples were subsequently assayed for apoptosis. In each condition, addition of vorapaxar led to a significant increase in apoptosis compared with the unexposed sample (P < .001). Knockdown of PAR1 also increased apoptosis. ***P < .001. Data are presented as means ± SEM (n = 5). (D) HMVEC barrier function was continuously monitored by transendothelial resistance for 24 hours after exposure to either 10 μM parmodulin 2 (red) or 0.3 μM vorapaxar (blue). ***P < .001.