Multiphasic stress relaxation response of freshly isolated and cultured vascular smooth muscle cells measured by quasi-in situ tensile test.

Vascular smooth muscle cells (SMCs) undergo a phenotypic change from a contractile to a synthetic state under pathological conditions, such as atherogenesis and restenosis. Although the viscoelastic properties of SMCs are of particular interest because of their role in the development of these vascular diseases, the effects of phenotypic changes on their viscoelastic properties are unclear at this stage. We performed the stress relaxation test at constant strain (ε=30%) for the freshly isolated contractile SMCs (FSMCs) and the cultured synthetic SMCs (CSMCs) maintaining in situ cell shape and cytoskeletal integrity. We also investigated the effect of extracellular Ca2+ on their viscoelastic behaviors. FSMCs and CSMCs exhibited multiphasic stress relaxation, which consisted of rapid relaxation, occurring on a time scale of several seconds and several 10 seconds, and slow relaxation occurring on a time scale of 1000 seconds. The estimated elastic modulus of CSMCs was less than one-half that of FSMCs, that was associated with a decreased of amount of actin stress fibers (SFs) during the transition from contractile to synthetic phenotypes. FSMCs showed a conservation of tension with extracellular Ca2+ following rapid stress relaxation. In contrast, CSMCs showed a consecutive decrease in tension independent of Ca2+. This suggests that the decrease in tension in a long time scale may be involved in mechanical remodeling of SFs induced through a Rho-dependent pathway, which is Ca2+-independent and become predominant in the transition from contractile to synthetic phenotypes.