Effects of tensile spatial shear stress gradients on porcine iliac artery permeability and endothelial cell biology

The apparent tendency of atherosclerotic lesions to form in complex blood flow environments has led to many theories regarding the importance of hemodynamic forces in endothelium-mediated atherosusceptibility. The effects of one such aspect of complex flow fields, the spatial shear stress gradient, on in vivo endothelial permeability and in vitro endothelial gene transcription were examined in this study. The flow fields in porcine iliac arteries were characterized, and permeability to macromolecules, as measured using uptake of dye-labeled albumin, was assessed throughout the arteries at locations in which both spatial shear stress gradient and shear stress magnitude were calculated. It was found that a composite function of gradient and magnitude best correlated with permeability in these arteries. The ranges of physiological gradients and magnitudes, as determined in the animal studies, were used to design converging-width flow chambers capable of imparting such flow fields on cultured porcine aortic endothelial cells. Experiments were designed such that the effect of gradient at constant levels of magnitude, and vice versa, could be studied. Cells attributed to fields of interest were either collected for analysis of expression levels of various genes by real time PCR or immunostained to quantify protein levels of endothelial nitric oxide synthase (eNOS). From the gene analysis, VCAM-1 expression was insensitive to either variable, and the remaining four genes each had unique sensitivities. eNOS showed a strong positive dependence upon magnitude only. The expression of c-jun was weakly correlated with magnitude and gradient, but not on their interaction. MCP-1 expression correlated inversely with gradient and also depended on the interaction of gradient with magnitude. ICAM-1 expression also depended on the interaction term, and it increased with magnitude. Protein levels for eNOS were different than mRNA levels and were sensitive to gradient. This study is the first in which the effect of spatial gradient has been isolated from that of shear stress magnitude and in which interactions between the two have been studied. The results support the notion that cells are able to sense each hemodynamic force independently and that signaling pathways evoked by them may be partially shared.