It is well known that endothelial cells (ECs) respond to the fluid imposed shear I stress and change their shapes and functions. We have focused on the importance of cytoplasmic micro-mechanical strain in mechano-sensing mechanism of ECs. To this end, the cytosolic Ca2+ responses of ECs to the mechanical stimulus by laser tweezers that can apply the micro-mechanical force to nano/micro-organisms without any physical contact, were investigated. When the laser spot focused on the nucleus of EC was slightly moved, the cytosolic Ca2+ increased immediately in the same EC, whereas there was no Ca2+ increase without laser spot movement. In the absence of extracellular Ca2+ in the medium or the blockade of stretch activated ion channels, there was also an increase of Ca2+ in stimulated ECs. Therefore, the increased Ca2+ in stimulated ECs is considered to be derived from intracellular Ca2+ store. The heterogeneous Ca2+ propagation from the stimulated EC to surrounding ECs was also observed. Two types of Ca2+ wave propagation were observed, the fast one that the velocity was more than 20μm/sec, and the slow one that the velocity was less than 1μm/sec. The micro-stress induced by the micro-movement of the nucleus can be a trigger of the cytosolic Ca2+ increase and the cytoplasmic micro-mechanical strain may play an important role in mechano-sensing mechanism of ECs.
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