PAEC were acutely exposed to shear stress (20 dyn/cm2, 4 h) in the presence or absence of the combined ET receptor antagonist, tezosentan (5μM). The levels of H2O2 were determined in the media by H2DCFDA oxidation reaction and whole cell extracts (20μg) were subjected to Western blot analysis in order to determine changes in catalase protein levels. Catalase activity (H2O2 degraded/min/mg protein) was also determined. The shear mediated increase in H2O2 was enhanced by tezosentan (A) and attenuated by PEG-catalase (100U/ml, A). Neither shear nor tezosentan changed catalase protein levels (B). However, acute shear stress decreased catalase activity, which was further attenuated by tezosentan (C). Data are mean± SEM; n =3-6. *P <0.05 vs. no shear, †P <0.05 vs. shear alone, ‡P<0.05 vs. shear and tezosentan.