Smooth muscle cells (SMC) are the major cellular component of the blood vessel wall and are continuously exposed to cyclic stretch due to pulsatile blood flow. This study examined the effects of a physiologically relevant level of cyclic stretch on rat aortic vascular SMC proliferation. Treatment of static SMC with serum, platelet-derived growth factor, or thrombin stimulated SMC proliferation, whereas exposure of SMC to cyclic stretch blocked the proliferative effect of these growth factors. The stretch-mediated inhibition in SMC growth was not due to cell detachment or increased cell death. Flow cytometry analysis revealed that cyclic stretch increased the fraction of SMC in the G(0)/G(1) phase of the cell cycle. Stretch-inhibited G(1)/S phase transition was associated with a decrease in retinoblastoma protein phosphorylation and with a selective increase in the cyclin-dependent kinase inhibitor p21, but not p27. These results demonstrate that cyclic stretch inhibits SMC growth by blocking cell cycle progression and suggest that physiological levels of cyclic stretch contribute to vascular homeostasis by inhibiting the proliferative pathway of SMC.
