Effect of shear stress on water and LDL transport through cultured endothelial cell monolayers

Previous animal experiments have shown that the transport of LDL into arterial walls is shear stress dependent. However, little work has probed shear effects on LDL transport invitro where conditions are well defined and mechanisms are more easily explored. Therefore, we measured shear induced water and LDL fluxes across cultured bovine aortic endothelial (BAEC) monolayers invitro and developed a three-pore model to describe the transport dynamics. Cell apoptosis was quantified by TdT-mediated dUTP nick end labeling (TUNEL) assay. We also examined the role of nitric oxide (NO) in shear induced water and LDL fluxes by incubating BAEC monolayers with an NO synthase inhibitor, N^G-monomethyl-l-arginine (l-NMMA). Our results show that direct exposure of endothelial monolayers to 12dyn/cm² shear stress for 3h elicited a 2.37-fold increase in water flux (J_v), a 3.00-fold increase in LDL permeability (P_e), a 1.32-fold increase in LDL uptake, and a 1.68-fold increase in apoptotic rate. l-NMMA treatment of BAEC monolayers blocked shear induced J_v response, but had no significant effect on shear responses of P_e and cell apoptosis. A long time shear exposure (12h) of endothelial monolayers reduced P_e and apoptotic rate close to the baseline. These results suggest that an acute change in shear stress from a static baseline state induces increases in water flux that are mediated by an NO dependent mechanism. On the other hand, the permeability of endothelial monolayers to LDL is enhanced by a short term-shear application and reduced nearly to the baseline level by a longer time shear exposure, positively correlated to the leaky junctions forming around apoptotic cells.