SHEAR-STRESS MODULATES THE PROLIFERATION RATE, PROTEIN-SYNTHESIS, ANDMITOGENIC ACTIVITY OF ARTERIAL SMOOTH-MUSCLE CELLS

Background. The aim of this study was to determine the correlation bet ween hemodynamic forces and proliferation of smooth muscle cells (SMC) . Methods. Bovine arterial SMC were seeded in a fibronectin-coated pol ystyrene cylinder at 5 X 10(5) cells/tube and allowed to reach conflue nce and to adhere for 48 hours. The experimental groups were subjected to a laminar flow of 150 ml/min (9 dyne/cm2), 100 ml/min (6 dyne/cm2) , and 50 ml/min (3 dyne/cm2) for 24 hours. The control group was subje cted to similar incubation conditions without flow. The cells in the e xperiments remained attached and viable. All experiments were performe d in triplicate or more. Results. Shear stress significantly reduced ( p < 0.001) the 24-hour incorporation of tritiated thymidine and cell p roliferation. This effect was proportional to the level of shear stres s and was still evident 24 hours after flow cessation. Results of flow cytometry confirmed a lower percentage of SMC in S phase with increas ing shear stress. Synthesis of cell-associated proteins was increased twofold (p < 0.0 1) in SMC subjected to laminar flow. SMC subjected to shear stress released a higher quantity of mitogens, including a plat elet-derived growth factor (PDGF)-like substance as detected by immuno logic testing. Fifty percent volume per volume conditioned serum-free medium from SMC subjected to shear stress increased threefold the trit iated thymidine uptake in PDGF receptor-bearing Swiss 3T3 cells as com pared with conditioned serum-free medium from control SMC not subjecte d to shear stress and twelvefold as compared with standard control. Th e release of mitogens was proportional to the level of shear stress an d was still evident 24 hours after flow cessation. The mitogenic activ ity was partially reduced (30%, p < 0.01) by an excess of monospecific anti-PDGF antibody. Conclusions. We conclude that (1) increasing shea r stress inhibits SMC proliferation and stimulates the synthesis of ce ll-associated proteins and the release of mitogens and (2) decreasing shear stress facilitates proliferation of SMC. Thus, in situations of arterial flow separation, the increased release of mitogens from SMC s ubjected to high shear stress and the increased proliferation rate and susceptibility to mitogens of SMC subjected to very low shear stress may generate a critical condition that predisposes to the development of atherosclerosis with early plaque formation in regions of low-flow shear stress.