Presence Of Shear Stress Research Articles

Direct demonstration of radiolabeled von Willebrand factor binding to platelet glycoprotein Ib and IIb-IIIa in the presence of shear stress.

In this study it is demonstrated for the first time that shear stress induces the binding of exogenous von Willebrand factor (vWF) multimers to platelets. The vWF preparations used were: 125I-vWF purified from human cryoprecipitate (and including all vWF multimers present in normal plasma); and 35S-cysteine-vWF secreted by human umbilical vein endothelial cells (HUVECs) (and containing unusually large vWF forms, as well as all plasma-type vWF multimers). Direct shear-induced binding to washed platelets (300-360 x 10(3)/microliters) of radiolabeled vWF was maximum at 60-120 dynes/cm2 evaluated at 30 sec and was in extent about one-quarter of the binding stimulated by ristocetin after 3 min of incubation. The shear-induced binding of only a small percentage of added radiolabeled vWF was sufficient to initiate aggregation. Radiolabeled vWF attached to both glycoprotein (GP) Ib and GPIIb-IIIa receptors in the shear field, with complete inhibition of binding occurring with simultaneous blockade of both receptors. Binding was potentiated by ADP released from sheared platelets.

Study of red cell aggregation in pulsatile flow from ultrasonic Doppler power measurements.

Human red cell aggregability and disaggregability represent important hemorheological parameters of blood. Several techniques have been proposed to evaluate the tendency of red cells to form aggregates and to disrupt in the presence of shear stress. One of the most recent approaches is based on the characterization of the intensity of ultrasonic scattered signals. A pulsatile flow loop model is used in the present study to demonstrate the potential applicability of Doppler ultrasound to detect and characterize the hemodynamic behavior of red cell aggregates. Porcine whole blood specimens collected from 20 different pigs were circulated in the flow model (tube diameter of 0.476 cm) at different mean velocities and pulsation rates. At a pulsation of 70 beats/min for mean velocities of 13 cm/sec and 63 cm/sec, no cyclic variation of the Doppler power was observed, suggesting the absence of rouleaux build-up and rouleaux disruption. At a pulsation of 20 beats/min and mean velocities of 11 cm/sec and 38 cm/sec, statistically significant cyclic variations (p < 0.01) were measured. It is suggested that aggregate size enlargement, rouleaux orientation with the flow field and the effect of shear stress on rouleaux disruption are possible causes for the observed cyclic variation of the Doppler power within the flow cycle at a pulsation of 20 beats/min. A discussion of the potential application of this technique for in vivo study in large vessels is given.

Nonequilibrium molecular dynamics calculation of self-diffusion in a non-Newtonian fluid subject to a Couette strain field

Molecular dynamics simulations have been performed on a simple fluid in the non-Newtonian regime in order to study the nature of diffusion in the presence of shear stress. The nonequilibrium molecular dynamics (NEMD) sllod algorithm is used to generate a homogeneous boundary driven Couette flow. The elements of the tensor of diffusion coefficients have been evaluated by three different routes: through Einstein relations with the corresponding elements of the tensor of molecule displacements, through Green–Kubo expressions involving tensor of momentum autocorrelation functions and cross-correlation functions, and by utilizing an NEMD color field method. All three routes to the tensor of diffusion coefficients are shown to yield consistent results. A simple spherically symmetric Lennard-Jones fluid at its triple point was used in the simulation study. We find that in a Couette strain field of sufficiently large strain rate, diffusion in all directions is enhanced substantially. We propose and verify a new asymptotic relationship between the diagonal elements of the tensor of diffusion coefficients and the strain rate.

Shear-dependent inhibition of granulocyte adhesion to cultured endothelium by dextran sulfate

Adhesion of polymorphonuclear granulocytes (PMNs) in microvessels occurs in the presence of shear forces exerted by the blood flow. To model this in vitro, phorbol myristate acetate (PMA)-activated PMN were exposed to shear stress on cultured human umbilical vein endothelial cells (HUVECs) and on plastic dishes coated with bovine serum albumin (BSA). PMN adhesion to HUVECs averaged 36% of the total PMNs added and was reduced to 21% by shear stress of approximately 1.5 dynes.cm-2. On BSA, adhesion was reduced from 59% to 35%. Dextran sulfate (molecular weight 500,000) inhibited PMN adhesion in a dose-dependent manner when shear stress was applied. At a concentration of 1 mg.ml-1, inhibition was 72% on HUVECs and 76% on BSA. Half-maximal inhibition was reached at approximately 1 microgram.mL-1 dextran sulfate, corresponding to 2 nmol/L. Without shear stress, dextran sulfate had no effect on HUVECs and only a moderate effect on BSA. The murine monoclonal antibody (MoAb) 60.3, recognizing an epitope on the leukocyte adhesion glycoprotein CD18, inhibited PMN adhesion equally well with and without shear. A low dose of MoAb 60.3 enhanced the effect of dextran sulfate without shear stress. Flow cytometry (FACS) did not show inhibition of MoAb 60.3 binding to PMNs by dextran sulfate. These results indicate that a dextran sulfate-inhibitable adhesion process is important for PMN adhesion in the presence of shear stress.

Shear-Dependent Inhibition of Granulocyte Adhesion to Cultured Endothelium by Dextran Sulfate

Adhesion of polymorphonuclear granulocytes (PMNs) in microvessels occurs in the presence of shear forces exerted by the blood flow. To model this in vitro, phorbol myristate acetate (PMA)-activated PMN were exposed to shear stress on cultured human umbilical vein endothelial cells (HUVECs) and on plastic dishes coated with bovine serum albumin (BSA). PMN adhesion to HUVECs averaged 36% of the total PMNs added and was reduced to 21% by shear stress of approximately 1.5 dynes.cm-2. On BSA, adhesion was reduced from 59% to 35%. Dextran sulfate (molecular weight 500,000) inhibited PMN adhesion in a dose-dependent manner when shear stress was applied. At a concentration of 1 mg.ml-1, inhibition was 72% on HUVECs and 76% on BSA. Half-maximal inhibition was reached at approximately 1 microgram.mL-1 dextran sulfate, corresponding to 2 nmol/L. Without shear stress, dextran sulfate had no effect on HUVECs and only a moderate effect on BSA. The murine monoclonal antibody (MoAb) 60.3, recognizing an epitope on the leukocyte adhesion glycoprotein CD18, inhibited PMN adhesion equally well with and without shear. A low dose of MoAb 60.3 enhanced the effect of dextran sulfate without shear stress. Flow cytometry (FACS) did not show inhibition of MoAb 60.3 binding to PMNs by dextran sulfate. These results indicate that a dextran sulfate-inhibitable adhesion process is important for PMN adhesion in the presence of shear stress.

Yield functions that account for the effects of initial and subsequent plastic anisotropy

A descriptive initial yield function is presented from an examination of experimentally determined yield loci, component plastic strain paths and Lode's parameters that indicate either a severe textural anisotropy in a material or a slight departure from the von Mises condition. The transition from the initial function to one that describes a subsequent yield surface which translates with the stress vector is developed and compared with experimental results. Observations on the Bauschinger, Swift and hardening effects in subsequent yield loci, defined at the limit of proportionality, are adequately represented through Ziegler's translation law when modified for non-linear work hardening. It is shown that the marked distortion, particularly apparent, in the presence of shear stress, may be represented by considering separately the translation for each quadrant of the initial yield locus.

Force in static general relativity

The force acting on the matter inside an arbitrary closed surface S is considered for the case of a static space-time galpha beta with a static matter tensor Talpha beta satisfying Talpha beta ; beta =0. Since the matter is in equilibrium, the body force is balanced by the surface force. The component of this force in any chosen direction is defined as the surface integral of an invariant and is transformed into a volume integral. Shrinking S to a point then yields the density of the body force. The author applies this result to special situations. For instance, it is found that in the presence of shear stress the body force can have a non-zero component perpendicular to the gradient of g44.