Track 14. Cardiovascular Mechanics

4951 Mo-Tu, no. 77 (P66) Measurement of dynamic deformability of erythrocyte with counter rotat ing rheoscope S. Hashimoto 1 , T. Furukawa 1 , S. Mochizuki 1 , N. Ogawa 1 , H. Otani 2, H. Imamura 2, T. Iwasaka 2. 1Biomedical Eng, Osaka Inst Tech, Osaka, Japan, 2 Kansai Medical University, Moriguchi, Japan

Dynamic deformability of erythrocytes is an important parameter to maintain blood pulsatile circulation, because cells have to deform to pass through capillary. Before hemolysis, sublethal damage on erythrocytes occurs through artificial organs, blood pumps e.g., which might decrease dynamic deformabil- ity. A parallel-disk type of counter-rotating rheoscope system has been designed and manufactured. In the system, Couette-type shear field is induced in the fluid between two counter-rotating disks, which are made of transparent silica glass. The sinusoidal rotating speed was regulated with a stepping motor, which is controlled by a computer. The shear rate, which is calculated from the velocity deference between two disks, is constant regardless of the distance from the disk. An erythrocyte can be observed microscopically under shear without translational movement, when it is suspended at the stationary plane in the middle part of the shear field between two disks. An erythrocyte deforms from biconcave to ellipsoid in Couette-type of shear field. Deformation of the erythrocyte was quantified with an elongation index (E), which was calculated from dimensions of the major (L) and minor (W) axes by E = (L-W)/(L+W). Before measurement of deformation, the human erythrocytes were classified according to the density by a centrifugation method. Erythrocytes were sus- pended in a dextran aqueous solution of 0.1 Pa s viscosity to separate each other and exposed to the high shear stress (<6 Pa) field at low shear rate at twenty degrees Celsius. The experimental results shows, that the lighter cells are more compliant than heavier cells and follow fast change of shear stress. The advantage of a parallel-disk type of counter-rotating rheoscope system is that an erythrocyte deforms in the shear field without contact to the surface of wall, which gives effects to the deformation of the erythrocyte. This work was supported by JSPS.

6990 Mo-Tu, no. 78 (P66) Valvular interstit ial cell mechanobiology: effects of substrate stiffness A. Throm 1,2, H. Hinds 3, K. Billiar 1,4. 1Dept of Biomedical Engineering, Worcester Polytechnic Institute, Worcester, MA, USA, 2Graduate School of Biomedical Sciences, UMass Medical School, Worcester, MA, USA, 3 Dept of Biology and Biotechnology, Worcester Polytechnic Institute, Worcester, MA, USA, 4Dept of Surgery, UMass Medical School, Worcester, MA, USA

During heart valve remodeling and in many disease states, valvular interstitial cells (VICs) shift to a synthetic and contractile activated myofibroblast phe- notype characterized by increased expression of alpha smooth muscle actin (A-SMA). The goal of this study is to determine if VIC activation is modulated by the stiffness of the surrounding matrix and, if the hypothesized phenotypic shift is found, whether it can be reversed. VICs obtained from porcine aortic and mitral valves were plated on polyacrylamide substrates of various stiffness to represent tissues from normal (compliant) and diseased (stiff) valves and compared to VICs grown on standard in vitro culture substrates (non-compliant glass and plastic). VICs were cultured up to two weeks in the absence of exogenous TGF-beta, a known stimulator of myofibroblast differentiation. For a subset of cells, the substrate stiffness was changed after prolonged culture to assess phenotypic plasticity. Cell morphology, A-SMA expression, and traction force were quantified. Flow cytometry results indicate that a low percentage of VICs from healthy valves are myofibroblasts (A-SMA+) directly after harvesting or after culture on highly compliant substrates, whereas following prolonged culture on stiff substrates the cells are predominantly A-SMA+. Microscopy studies confirm that with increasing substrate stiffness there is an increase in cell spreading and A-SMA expression, as well as demonstrate a concomitant change in traction force exerted by the cells. Further, when A-SMA+ VICs were plated on compliant substrates, there was a decrease in A-SMA expression. Future studies are necessary to determine the mechanism by which VICs sense the stiffness of their surroundings. As VICs are the primary cell type found in heart valves and are responsible for the maintenance and repair of the valve structure, understanding how they respond to their mechanical environment is crucial for understanding valve pathology and for developing rational therapies for diseased valves.

14.11 Mechanobiology of Vascular Walls and Cells $621

7720 Mo-Tu, no. 79 (P66) KAT P channels mediate the NO-buffering capacity on low-frequency var iabi l i ty o f arterial pressure via the baroref lex N. lida. Department of Biomedical System Engineering, Graduate School of Information Science and Technology, Hokkaido University, Sapporo, Japan

The aim of this study was to examine the interdependent effects of nitric oxide (NO) and ATP-sensitive K + (KATP) channels on low-frequency variability (<1.0 Hz) of arterial pressure (AP) to understand the hemodynamic effects of AP oscillations on the cardiovascular system. Experiments were performed on conscious Wistar rats (n =35), 10-13wks of age. A polyethylene catheter for AP and heart rate (HR) measurements was inserted from the right femoral artery into the terminal abdominal aorta under anesthesia. Another catheter for injection of drugs was inserted into the right external jugular vein. Hexam- ethonium C6, L-NAME, L-arginine, glibenclamide, cromakalim, and prazosin were injected. The effects of the drugs on AP variability and HR variability were analyzed using power spectral techniques with a sampling interval of 0.05 seconds. Effects of NO and KAT P channels on low-frequency variability of AP were investigated in very Iow-(VLF: 0.0-0.2 Hz), Iow-(LF: 0.2-0.6 Hz), and high-(HF: 0.6-2.5 Hz) frequency bands. The AP oscillations in the VLF and LF bands originated from sympathetic activity, whereas the variability in HF may be derived from respiratory activity and be mediated vagally. The AP oscillations in the VLF and LF bands were buffered by NO, but not those in the HF band. The power spectra in the LF band were decreased by infusion of L-NAME, while the power spectra in the VLF band were increased. In contrast, L-Arg increased the variability in LF and decreased the variability in VLR NO may buffer the fluctuations around mean AP via two-pathways. One is the local buffering mechanism: NO buffers AP variability by counteracting sympathetic activity through reduction of cytoplasmic Ca 2+ in smooth muscle cells. The other is the central buffering mechanism: NO buffers AP variability by hyperpolarizing the KAT P channels of baroreceptors. We concluded that KAT P channels may a mechanical factor mediating the NO-buffering capacity via the baroreflex.

6688 Mo-Tu, no. 80 (P66) Fluid mot ion in an orbi t ing culture dish R.E. Berson 1, M.R. Purcell 1, M.K. Sharp 2. 1Department of Chemical Engineering, 2 Biofluid Mechanics Laboratory, Department of Mechanical Engineering, University of Louisville, KY, USA

Endothelial and other cells have been shown to respond to fluid shear in a number of ways. A convenient technique for exposing cells to shear is to grow them on the bottom of a fluid-filled culture dish and to place the dish on an orbiting (shaker) table. However, questions exist regarding the magnitude and distribution of shear to which the cells are subjected and how shear depends on the experimental parameters. This problem has been approached by first delineating the regimes of flow, then developing analytical solutions for some of the limiting cases and computational solutions for some of the transitions. If surface tension is neglected, the resulting flow and the shear on the bottom of the dish is influenced by seven independent parameters, including gravitational acceleration, fluid density and viscosity, dish radius, mean fluid height, and orbital radius and speed. These dimensional parameters can be reduced to a set of four dimensionless parameters -Reynolds number, Froude number, Stokes number and a slope parameter. Reynolds number dictates transition from laminar to turbulent flow. Froude number reflects the steepness of the traveling wave front. Stokes number compares the unsteady boundary layer thickness to the fluid height. The slope parameter is the ratio of steady-state free surface slope and mean aspect ratio of the fluid (fluid height to dish radius). Attention was focused initially on laminar (low Reynolds) flows and low slope parameter. For low Froude and high Stokes, an extension of Stokes 2 nd problem to orbital plate motion provides a solution for flow not too close to the vertical walls of the dish. For low Froude and low Stokes, a Poiseuille-like solution applies in the same region. For the transition in Stokes for low Froude, increasing phase lag of the free surface wave relative to dish motion has been confirmed by computational methods. For the transition in Froude for low Stokes, steepening of the wave front has been documented computationally. Computational results for shear were as low as about 1/3 of that predicted by a previously published equation, while dimensional analysis identified the reason for the limitations of the equation.

6759 Mo-Tu, no. 81 (P66) Effects o f plane shock wave on the endothelial cells in vitro using shock tube M. Tamagawa, S. Iwakura, S. Suetsugu, I. Yamanoi. Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Japan

Recently shock wave phenomena in living tissues are being widely applied in the fields of medical engineering and tissue engineering. In the field of tissue engineering, the bone therapy to regenerate the bone by extracorporeal shock