$376 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentat ions

EC intracellular calcium concentration depends strongly on wall shear stress, the modeling results were used to argue that the transport of agonists at the EC surface does not contribute significantly to flow-induced calcium mobilization. We had previously extended the models to incorporate flow-induced ATP release and had demonstrated that if the kinetics of ATP release are sufficiently rapid, then the ATP+ADP concentration at the EC surface becomes a strong function of wall shear stress, indicating that agonist transport may contribute significantly to the EC calcium flow response. In the present study we have investigated the effect of flow disturbance downstream of a backward facing step (BFS) on ATP/ADP concentration under both steady and pulsatile flow conditions. The adenine nucleotide concentration was determined by solving the coupled Navier-Stokes and advection-diffusion equations in the BFS geom- etry. ATP/ADP hydrolysis was assumed to follow irreversible Michaelis-Menten kinetics. The results demonstrate that the ATP+ADP concentration at the EC surface is intricately sensitive to the kinetics of flow-induced ATP release. For both steady and pulsatile flow over a wide range of wall shear stress, the increased nucleotide residence time within the flow separation region immediately downstream of the step leads to considerably lower ATP+ADP concentrations at the EC surface in that region than in areas of undisturbed flow further downstream. Our findings suggest that disturbed and undisturbed flow would be expected to affect EC calcium mobilization differently, which may contribute to the observed endothelial dysfunction in regions of disturbed flow.

4074 Tu, 16:15-16:30 (P25) Shear- induced changes in microvascular hydraul ic conduct iv i ty N.R. Harris 1, M.-H. Kim 1, J.M. Tarbell 2. 1Louisiana State University Health Science Center, Shreveport, Louisiana USA, 2City College of New YorklCUNY, New York, New York USA

The effect of shear forces on microvascular permeability primarily has been examined in micropipette-cannulated vessels or in endothelial monolayers in vitro. Our study was designed to determine whether acute changes in blood flow shear stress might influence measurements of hydraulic conductivity (Lp) in autoperfused microvessels in vivo. Rat mesenteric microvessels were observed via intravital microscopy experiments, in which occlusion of a third- order arteriole with a micropipette was used to divert and increase flow through a non-occluded capillary or fourth-order arteriolar branch. Microvessel red blood cell velocity was measured using an optical Doppler velocimeter, and the velocity was used to calculate shear rate. Transvascular fluid filtration rate in the branching vessel was measured with a Landis technique, in which a glass micropipette tip was lowered onto the vessel by micromanipulator to compress the lumen. In some experiments, NO synthesis was inhibited via tissue exposure to 50 ~tM L-NAME (nitro-L-arginine methyl ester). A 2.03-fold increase in capillary shear rate resulted in a 1.91-fold increase in filtration rate. Using Starling's law of filtration and assuming a 17% increase in hydrostatic pressure, the estimated increase in Lp was 53±4% (mean ± standard error; N = 15 capillaries). A positive correlation was observed between the shear- induced change in Lp and the percentage change in shear rate in the initial 10 seconds of measurement (p < 0.001, r 2 = 71.2%). The shear-induced increase in capillary Lp disappeared within 20-30 s of the removal of the shear (that is, during the measurement occlusion) and could be eliminated with nitric oxide synthase inhibition. The shear-induced increase in Lp (53±4%) was greater in capillaries compared with terminal arterioles (23±4%; N = 11). In summary, an acute change in shear may regulate Lp by a nitric oxide-dependent mechanism that displays heterogeneity within a microvascular network.

4567 Tu, 16:30-16:45 (P25) Computer s imulat ion o f coupled luminal and transmural mass t ranspor t processes in a carotid bi furcat ion model M. Prosi 1 , K. Perktold 2. 1MOX, Department of Mathematics, Politecnico di Milano, Milan, Italy, 2Institute of Mathematics D, Graz University of Technology, Graz, Austria

Numerous investigations indicate a strong correlation between the develop- ment and progress of atherosclerotic lesions and disturbed cardiovascular transport mechanisms. These transport processes take place inside the arterial lumen and the arterial wall. Therefore, the numerical study of the coupled transport in the lumen and the arterial wall layers is important to gain a deeper insight into the disease process. In this study coupled mathematical models for the numerical simulation of low-density lipoprotein (LDL) transport in the arte- rial lumen and the arterial wall are presented. These models differ in the level of complexity of the description of the arterial wall and require a suitable set of physical parameters characterizing the transport properties of the wall layers considered. Based on an electrical analogy for mass transport processes we apply an inverse model to estimate the physical parameters from available concentration measurements. To investigate the effect of complex flow patterns on the mass transport we apply these models to an anatomically correct computational model of a human carotid artery bifurcation. Due to the water- permeable nature of the arterial wall a local accumulation of macromolecules

(LDL) at the lumen/endothelium interface develops. It is demonstrated that this concentration polarization effect strongly depends on geometrical properties. At sites of separated flow (outer walls of the bifurcation region) as well as in zones of stagnating inplane motion high surface concentration polarization of LDL occurs. However the LDL-distribution in the wall mainly depends on the transport parameters used and is minor affected by the geometry. The numerical results show typically U-shaped concentration profiles across the arterial wall which is in good correspondence to measured data and confirms the usability of the estimation procedure for the transport parameters proposed in this work.

4548 Tu, 16:45-17:00 (P25) Effects o f a shear f low and water f i l t rat ion on t ranspor t o f LDL from f lowing f luid to and prol i ferat ion o f the cells o f a model of an arterial wall T. Karino, X. He, J. Sakai. Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan

It is suspected that concentration polarization of LDL at the luminal surface of an arterial wall plays an important role in the pathogenesis and localization of intimal hyperplasia and atherosclerotic lesions in man. To investigate such a possibility, we studied the effects of a laminar shear flow and water filtration on (1) concentration of LDL at the luminal surface, (2) uptake of LDL by the cells, and (3) proliferation of the cells of an arterial wall, by using a model of an arterial wall (model artery) prepared by co-culturing bovine aortic endothelial cells and smooth muscle cells on a porous membrane or an expanded PTFE vascular graft. It was found that in the presence of water filtration, concentration of LDL occurred at the luminal surface of a model artery. The amount of LDL taken up by the cells was directly related to the surface concentration of LDL and it was the highest in the model artery exposed to the lowest shear flow. The thickness of the cell layer was the greatest in the model artery prepared under the condition of a slow flow hence low wall shear stress (2dynes/cm 2) and in the presence of water filtration (4 10 -6 cm/sec). These results strongly support our hypothesis that, in our arterial system, due to the presence of a filtration flow of water, concentration of LDL occurs at the luminal surface in regions of slow flow (hence low wall shear stress), thus augmenting the uptake of LDL by the cells and accumulation of LDL in subendothelial spaces, eventually leading to localized genesis and development of intimal hyperplasia and atherosclerosis at such sites.

4357 Tu, 17:00-17:15 (P25) In vi t ro s tudy o f LDL t ranspor t under convect ive condi t ions

L.M. Cancel 1 , A. Fitting 2, J.M. Tarbell 1 . 1City College of New York, Department of Biomedical Engineering, New York, NY, USA, 2pennsylvania State University, Department of Chemical Engineering, PA, USA

It is difficult to assess the transport pathways that carry low density lipoprotein (LDL) into the artery wall in vivo, and there has been no previous in vitro study that examined transendothelial transport under physiologically relevant pressurized (convective) conditions. We measured water, albumin, and LDL fluxes across bovine aortic endothelial cell (BAEC) monolayers exposed to a 10cm H20 pressure differential using an automated fluorometer system. Using a three-pore transport model, we determined the relative contributions of vesicles, paracellular transport through "breaks" in the tight junction, and "leaky" junctions associated with dying or dividing cells. The model predicts that the break in the tight junction is the dominant pathway for water (77.7%) and albumin (53%), while most of the LDL (90.9%) is carried through the "leaky" junction. The vesicular pathway accounts for 20 and 9.1% of the transport of albumin and LDL, respectively. To further explore the leaky junction pathway, TNF,J, and cycloheximide were used to induce apoptosis in BAEC monolayers and the fluxes of water and LDL were measured. BAEC monolayers had a baseline apoptosis rate of 0.14%. TNF,J,/cycloheximide treated monolayers had an apoptosis rate of 3.3%, a 24-fold increase. Despite this large increase in apoptosis rate, the convective LDL permeability of treated monolayers increased only by a factor of 2.7, while the water flux increased by a factor of 2.8. These results support the hypothesis that apoptosis rates dictate, at least in part, the permeability of the endothelium to LDL and demonstrate the potential of manipulating endothelial monolayer permeability by altering the rate of apoptosis pharmacologically.