Response of cultured endothelial cells to mechanical stimulation

Endothelial cells covering the luminal surface of vessels are exposed to at least two different mechanical forces: 1) fluid shear stress produced by the circulation of blood, and 2) periodic stretching and relaxing as a result of the diameter oscillations caused by blood pulsation. In this study we present an apparatus which was constructed to imitate the volume pulse with its typical incisura of the abdominal aorta. Using this apparatus, we exposed cultured endothelial cells to continuously produced cyclic and directional stretching and relaxation for three days. In all experiments cells remained attached and viable when subjected to mechanical stimulation. The vast majority of endothelial cells which underwent mechanical stimulation became elongated and oriented with their longer axis perpendicular to the direction of stretching (angle of cell orientation: α=88.7°±12°; $\bar x$ ±SD), whereas cells on unstretched membranes had a cobblestone-like appearance and remained in random orientation. In the stretched cells, the factor of elongation was f=6.8±1.3; $\bar x$ ±SD; unstretched cells which exhibited a polygonal shape had a factor of elongation of f=1.8±0.8; $\bar x$ ±SD. In addition, the behavior of cytoskeletal components such as microfilaments and microtubules was examined in the process of cell orientation as both are actively involved in alterations of cell shape and cell migration. Actin filaments were oriented in parallel alignment perpendicular to the stretch direction (angle of actin filament orientation: β=90.4°±9°; $\bar x$ ±SD). A distinct orientation of microtubules was not observed, althougn a noticeable number of microtubules was observed to be in parallel alignment. Furthermore, microtubules of cells which underwent mechanical stimulation exhibited a pronounced asymmetric intracellular distribution with strongly fluorescent cytoplasmic areas in which microtubules seemed to be accumulated. The results indicate that endothelial cell elongation and orientation in vitro can be induced by periodic stretching and relaxation comparable to the periodic oscillations of the vessel wall due to blood pulsation in vivo.