We previously demonstrated that high levels of ROS in endothelial cells activate calpain, a calcium-dependent protease [21] . We therefore investigated the role of calpain in PM-mediated ZO-1 degradation. PM (100 μM, 1 hr) induced a significant increase in calpain activity in ECs, which was inhibited by selective calpain inhibitors ALLN (30 μM, 1 hr pretreatment) and calpeptin (10 μM, 1 hr pretreatment) (Figure 2A ). PM-induced calpain activation was also inhibited by BAPTA-AM (50 μM, 1 hr pretreatment), a calcium chelator, and by NAC (5 mM, 1 hr pretreatment) (Figure 2A ). We next determined the role of calpain in PM-induced EC barrier disruption. PM-induced reduction in TER was partially inhibited by calpeptin or ALLN ( Figure 2B ). In parallel, ALLN and calpeptin also significantly prevented PM-induced ZO-1 degradation, as did chelation of intracellular calcium via BAPTA-AM ( Figure 2C ).