During high tidal volume mechanical ventilation in patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), regions of the lung are exposed to excessive stretch, causing inflammatory responses and further lung damage. In this study, the effects of mechanical stretch on intracellular Ca(2+) concentration ([Ca(2+)](i)), which regulates a variety of endothelial properties, were investigated in human pulmonary microvascular endothelial cells (HPMVECs). HPMVECs grown on fibronectin-coated silicon chambers were exposed to uniaxial stretching, using a cell-stretching apparatus. After stretching and subsequent unloading, [Ca(2+)](i), as measured by fura-2 fluorescence, was transiently increased in a strain amplitude-dependent manner. The elevation of [Ca(2+)](i) induced by stretch was not evident in the Ca(2+)-free solution and was blocked by Gd(3+), a stretch-activated channel inhibitor, or ruthenium red, a transient receptor potential vanilloid inhibitor. The disruption of actin polymerization with cytochalasin D inhibited the stretch-induced elevation of [Ca(2+)](i). In contrast, increases in [Ca(2+)](i) induced by thapsigargin or thrombin were not affected by cytochalasin D. Increased actin polymerization with sphingosine-1-phosphate or jasplakinolide enhanced the stretch-induced elevation of [Ca(2+)](i). A simple network model of the cytoskeleton was also developed in support of the notion that actin stress fibers are required for efficient force transmission to open stretch-activated Ca(2+) channels. In conclusion, mechanical stretch activates Ca(2+) influx via stretch-activated channels which are tightly regulated by the actin cytoskeleton different from other Ca(2+) influx pathways such as receptor-operated and store-operated Ca(2+) entries in HPMVECs. These results suggest that abnormal Ca(2+) homeostasis because of excessive mechanical stretch during mechanical ventilation may play a role in the progression of ALI/ARDS.