Technical Abstract: INTRODUCTION: Although both fluid shear stress and mass transport of atherogenic substances into the vascular wall are known to be important factors in atherogenesis, there has been little research on the effect of shear stress on vascular permeability. Therefore, the effects of shear stress on the permeability of arteries and the expression of the endothelial cell tight junction molecule occludin, an important regulator of vascular permeability, were investigated. METHODS: Porcine carotid arteries were perfusion cultured ex vivo with low (1.5 dyne/cm(2)) or physiologic (15 dyne/cm(2)) shear stress and 100 mmHg pressure for 24 hours. Subsequently, 20 nm gold particles in solution were infused into the lumen of vessels at 100 mmHg for 30 minutes. Frozen sections were then cut and stained for gold particles. Image analysis was used to determine the density of the particles in the vessel walls. The expression of endothelial cell occludin mRNA and protein were determined using reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting, respectively. RESULTS: Permeability results showed a 2.8-fold increase in the apparent permeability of vessels cultured with low versus physiologic shear stress. RT-PCR and Western blotting results showed significant decreases in occludin mRNA and protein expression at 12 and 24 hours in vessels cultured with low versus physiologic shear stress. CONCLUSIONS: These results demonstrate that low shear stress increases vascular permeability in porcine carotid arteries, possibly owing to decreased occludin expression. These results may have implications in the preferential formation of atherosclerotic vascular disease adjacent to branches and bifurcations where low mean shear stresses may occur.