Heterogeneous response of traction force at focal adhesions of vascular smooth muscle cells subjected to macroscopic stretch on a micropillar substrate

Traction force generated at focal adhesions (FAs) of cells plays an essential role in regulating cellular functions. However, little is known about how the traction force at each FA changes during cell stretching. Here we investigated dynamic changes in traction force at FAs during macroscopic stretching of porcine aortic smooth muscle cells (SMCs) cultured on elastic micropillar substrates. SMCs were cultured on polydimethylsiloxane (PDMS)-based substrates with a micropillar array, and stretched approximately in the direction of their major axis and then released by stretching and relaxing the substrates. This stretch–release cycle was repeated twice with cell strain rates of 0.3%/15 s up to a 3% strain, and the deflection of the PDMS micropillars was measured simultaneously to obtain the traction force at each FA F, total force in the cell's major axis direction Fall, and whole-cell strain εcell. Traction forces of SMCs during stretching varied widely with location: their changes at some pillars synchronized well with the applied strain εcell, but others did not synchronized. Whole-cell stiffness estimated as the slope of the loading limb of the Fall–εcell curves was ∼10 nN/%, which was the same order of magnitude of the reported stiffness of cultured SMCs obtained in a tensile test. Interestingly, Fall at a zero-strain state (pretension at the whole-cell level) actively increased in some cells following the loading/unloading process, as did whole-cell stiffness. Such a change did not occur in cultured SMCs in the tensile test in which cells were held with a pair of micropipettes coated with nonspecific adhesive. These results indicate that SMCs showed a myogenic response when stretched through their multiple FAs, but not through nonspecific adhesions on their membrane. SMCs may behave differently depending on the sites through which they are stretched.