Shear stress response element

Khachigian LM, Resnick N, Gimbrone MA Jr, Collins T. Nuclear factor-KB interacts functionally with the platelet-derived growth factor B-chain shear-stress response element in vascular endothelial cells exposed to fluid shear stress. / Clin Invest. 1995 96 1169-1175. 

Resnick N, Collins T, Atkinson W, Bonthron DT, Dewey CF Jr, Gimbron MA. Platelet-derived growth factor B chain promoter contains cis-acting fluid shear-stress responsive element. Proc Natl Acad Sci USA. 1993 90 7908. 

P450 8A1 is constitutively expressed in human endothelial cells . The human CYP8A1 gene (chromosome 20) has 10 exons and has consensus sequences for Spl, AP-2, an interferon--/ response element, GATA NF, B, a CACCC box, glucocorticoid receptor, and a shear stress responsive element (GAGACC) . Whether or not all of these are functional and how they interact to maintain constitutive expression is not well understood yet. 

NOS III is found to be constitutively expressed in endothelial cells and some other cell types (see below). Some mechanisms that also regulate the expression of the NOS III gene have been described. Shear stress produced by the flowing blood not only increases endothelial NO release acutely (Lamontagne etal., 1992) but also up-regulates NOS III expression (Nishida et al., 1992). A putative shear stress-responsive element has been described... 

Resnick, N., Collins, T., Atkinson, W., Bonthron, D. T., Dewey, C. F., Jr., and Gimbrone, M. A., Jr. (1993). Platelet-derived growth factor (3 chain promoter contains a cis-acting fluid shear-stress-responsive element. Proc. Natl. Acad. Sci. U.S.A. 90, 4591-4595. 

In mechanical terms, the Kelvin model is a parallel combination of a dashpot and a spring as illustrated in Fig. 11. The extension of dashpot and spring is always equal, as is the induced strain in both elements. If suddenly a shear stress is applied, the dashpot can only respond slowly but continuously. The response of the spring is hence inhibited, and its maximum elongation is only reached with delay. The viscosity of the dashpot determines the response time, i.e., the higher... 

In some colloidal dispersions, the shear rate (flow) remains at zero until a threshold shear stress is reached, termed the yield stress (ry), and then Newtonian or pseudoplastic flow begins. A common cause of such behaviour is the existence of an inter-particle or inter-molecular network, which initially acts like a solid and offers resistance to any positional changes of the volume elements. In this case, flow only occurs when the applied stress exceeds the strength of the network and what was a solid becomes a fluid. Examples include oil well drilling muds, greases, lipstick, toothpaste and natural rubber polymers. An illustration is provided in Figure 6.13. Here, the flocculated structures are responsible for the existence of a yield stress. Once disrupted, the nature of the floe break-up process determines the extent of shear-thinning behaviour as shear rate increases. 

The shear stress sensor for turbulent flow needs to accurately capture the complete turbulent fluctuation spectrum. Therefore, the shear stress sensor should possess a large bandwidth with flat and minimum frequency-phase relationship. For direct measurement, i.e., floating point sensors, the resonant frequency of the floating element and the fluidic damping determines the usable bandwidth. For the thermal sensor, the thermal inertia of the sensor element and the frequency-dependent heat conduction to the substrate influence the usable bandwidth. It is complicated to analytically predict the frequency response of the thermal sensor. Therefore, dynamic calibration is essential to characterize the frequency response of the sensor. 

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