Localization within a thin optical section of fluorescent blood platelets flowing in a microvessel

Cell-resolved blood flow simulations of saccular aneurysms: effects of pulsatility and aspect ratio

We study the effect of pulsatile flow on the transport of red blood cells (RBCs) and platelets into aneurysm geometries with varying dome-to-neck aspect ratios (AR). We use a validated two-dimensional lattice Boltzmann model for blood plasma with a discrete element method for both RBCs and platelets coupled by the immersed boundary method. Flow velocities and vessel diameters were matched with measurements of cerebral perforating arteries and flow was driven by a synthetic heartbeat curve typical for such vessel sizes. We observe a flow regime change as the aspect ratio increases from a momentum-driven regime in the small aspect ratio to a shear-driven regime in the larger aspect ratios. In the small aspect ratio case, we see the development of a re-circulation zone that exhibits a layering of high (greater than or equal to 7 s) and low (less than 7 s) residence cells. In the shear-driven regime, we see high and low residence cells well mixed, with an increasing population of cells that are trapped inside the aneurysm as the aspect ratio increases. In all cases, we observe aneurysms that are platelet-rich and red blood cell-poor when compared with their respective parental vessel populations. Pulsatility also plays a role in the small aspect ratio as we observe a smaller population of older trapped cells along the aneurysm wall in the pulsatile case when compared with a steady flow case. Pulsatility does not have a significant effect in shear-driven regime aspect ratios.

Application of a strain rate gradient microfluidic device to von Willebrand's disease screening

Von Willebrand's disease (VWD) is the most common inherited bleeding disorder caused by either quantitative or qualitative defects of von Willebrand factor (VWF). Current tests for VWD require relatively large blood volumes, have low throughput, are time-consuming, and do not incorporate the physiologically relevant effects of haemodynamic forces. We developed a microfluidic device incorporating micro-contractions that harnesses well-defined haemodynamic strain gradients to initiate platelet aggregation in citrated whole blood. The microchannel architecture has been specifically designed to allow for continuous real-time imaging of platelet aggregation dynamics. Subjects aged ≥18 years with previously diagnosed VWD or who presented for evaluation of a bleeding disorder, where the possible diagnosis included VWD, were tested. Samples were obtained for device characterization as well as for pathology-based testing. Platelet aggregation in the microfluidic device is independent of platelet amplification loops but dependent on low-level platelet activation, GPIb/IX/V and integrin α_IIbβ₃ engagement. Microfluidic output directly correlates with VWF antigen levels and is able to sensitively detect aggregation defects associated with VWD subtypes. Testing demonstrated a strong correlation with standard clinical laboratory-based tests. Head-to-head comparison with PFA100® demonstrated equivalent, if not improved, sensitivity for screening aggregation defects associated with VWD. This strain rate gradient microfluidic prototype has the potential to be a clinically useful, rapid and high throughput-screening tool for VWD as well as other strain-dependent platelet disorders. In addition, the microfluidic device represents a novel approach to examine the effects of high magnitude/short duration (ms) strain rate gradients on platelet function.

Effects of shear rate, confinement, and particle parameters on margination in blood flow

The effects of flow and particle properties on margination of particles in red blood cell (RBC) suspensions is investigated using direct numerical simulation (DNS) of cellar blood flow. We focus on margination of particles in the flow of moderately dense suspensions of RBCs. We hypothesize that margination rate in nondilute suspensions is mainly driven by the RBC-enhanced diffusion of marginating particles in the RBC-filled region. We derive a scaling law for margination length in a straight channel. Margination length increases cubically with channel height and is independent of shear rate. We verify this scaling law for margination length by DNS of flowing RBCs and marginating particles. We also show that rigidity and size both lead to particle margination with rigidity having a more significant effect compared to size within the range of parameters in this study.

Fluid Mechanics of Blood Clot Formation

Intravascular blood clots form in an environment in which hydrodynamic forces dominate and in which fluid-mediated transport is the primary means of moving material. The clotting system has evolved to exploit fluid dynamic mechanisms and to overcome fluid dynamic challenges to ensure that clots that preserve vascular integrity can form over the wide range of flow conditions found in the circulation. Fluid-mediated interactions between the many large deformable red blood cells and the few small rigid platelets lead to high platelet concentrations near vessel walls where platelets contribute to clotting. Receptor-ligand pairs with diverse kinetic and mechanical characteristics work synergistically to arrest rapidly flowing cells on an injured vessel. Variations in hydrodynamic stresses switch on and off the function of key clotting polymers. Protein transport to, from, and within a developing clot determines whether and how fast it grows. We review ongoing experimental and modeling research to understand these and related phenomena.

Micro-PTV measurement of the fluid shear stress acting on adherent leukocytes in vivo

Leukocyte adhesion is determined by the balance between molecular adhesive forces and convective dispersive forces. A key parameter influencing leukocyte adhesion is the shear stress acting on the leukocyte. This measure is indispensable for determining the molecular bond forces and estimating cell deformation. To experimentally determine this shear stress, we used microparticle tracking velocimetry analyzing more than 24,000 images of 0.5 microm fluorescent microbeads flowing within mildly inflamed postcapillary venules of the cremaster muscle in vivo. Green fluorescent protein, expressed under the lysozyme-M promoter, made leukocytes visible. After applying stringent quality criteria, 3 of 69 recordings were fully analyzed. We show that endothelial cells, but not leukocytes, are covered by a significant surface layer. The wall shear rate is nearly zero near the adherent arc of each leukocyte and reaches a maximum at the apex. This peak shear rate is 2-6-fold higher than the wall shear rate in the absence of a leukocyte. Microbead trajectories show a systematic deviation toward and away from the microvessel axis upstream and downstream from the leukocyte, respectively. The flow field around adherent leukocytes in vivo allows more accurate estimates of bond forces in rolling and adherent leukocytes and improved modeling studies.

Platelet P-selectin is required for pulmonary eosinophil and lymphocyte recruitment in a murine model of allergic inflammation

Platelets are necessary for lung leukocyte recruitment in a murine model of allergic inflammation, and platelet–leukocyte aggregates are formed in circulating blood of patients with asthma after allergen exposure. However, it is unknown how platelets induce pulmonary leukocyte recruitment in asthma. Here, we have investigated the importance of platelet adhesion molecule expression on pulmonary eosinophil and lymphocyte recruitment and on leukocyte CD11b and very late antigen (VLA)–4 expression in mice. Pulmonary leukocyte recruitment in platelet-depleted mice (sensitized and exposed to ovalbumin) transfused with fixed, unstimulated platelets (FUSPs) was abolished, whereas transfusion with platelets stimulated and fixed (FSPs), expressing P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1), restored eosinophil and lymphocyte recruitment. Transfusion with platelets from P-selectin–deficient mice, or with FSPs stimulated in the presence of a blocking anti–P-selectin antibody, were unable to restore pulmonary leukocyte recruitment. Flow cytometric analysis revealed increased expression of CD11b and VLA-4 on leukocytes attached to platelets after allergen exposure, and CD11b expression on leukocytes was suppressed in thrombocytopenic mice but was restored with the transfusion of FSPs, but not FUSPs, a phenomenon concurrent with the formation of platelet–leukocyte complexes. P-selectin expression on the surfaces of platelets is a major requirement for pulmonary eosinophil and lymphocyte recruitment, allowing circulating platelets to bind to and stimulate leukocytes for endothelial attachment.

A novel mouse-driven ex vivo flow chamber for the study of leukocyte and platelet function

Various in vitro and in vivo techniques exist for study of the microcirculation. Whereas in vivo systems impress with their physiological fidelity, in vitro systems excel in the amount of reduction that can be achieved. Here we introduce the autoperfused ex vivo flow chamber designed to study murine leukocytes and platelets under well-defined hemodynamic conditions. In our model, the murine heart continuously drives the blood flow through the chamber, providing a wide range of physiological shear rates. We used a balance of force approach to quantify the prevailing forces at the chamber walls. Numerical simulations show the flow characteristics in the chamber based on a shear-thinning fluid model. We demonstrate specific rolling of wild-type leukocytes on immobilized P-selectin, abolished by a blocking MAb. When uncoated, the surfaces having a constant shear rate supported individual platelet rolling, whereas on areas showing a rapid drop in shear platelets interacted in previously unreported grapelike conglomerates, suggesting an influence of shear rate on the type of platelet interaction. In summary, the ex vivo chamber amounts to an external vessel connecting the arterial and venous systems of a live mouse. This method combines the strengths of existing in vivo and in vitro systems in the study of leukocyte and platelet function.

Effects of erythrocyte aggregation and venous network geometry on red blood cell axial migration

Axial migration of red blood cells in small glass tubes can cause blood viscosity to be effectively independent of shear rate. However, this phase separation may not occur to the same degree in the venous network due to infusion of cells and aggregates at branch points. To investigate this hypothesis, we followed trajectories of fluorescently labeled red blood cells in the venular network of the rat spinotrapezius muscle at normal and reduced flow with and without red blood cell aggregation. Cells traveling near the wall of an unbranched venular segment migrated approximately 1% of the longitudinal path length without aggregation and migrated slightly more with aggregation. Venular segment length between branch points averaged three to five times the diameter. Cells in the main vessel were shifted centrally by up to 20% of diameter at branch points, reducing the migration rate of cells near the opposite wall to <1% even in the presence of aggregation. We conclude that formation of a cell-free marginal layer in the venular network is attenuated due to the time dependence of axial migration and the frequent branching of the network.

Effect of erythrocyte aggregation on velocity profiles in venules

A recent whole organ study in cat skeletal muscle showed that the increase in venous resistance seen at reduced arterial pressures is nearly abolished when the muscle is perfused with a nonaggregating red blood cell suspension. To explore a possible underlying mechanism, we tested the hypothesis that red blood cell aggregation alters flow patterns in vivo and leads to blunted red blood cell velocity profiles at reduced shear rates. With the use of fluorescently labeled red blood cells in tracer quantities and a video system equipped with a gated image intensifier, we obtained velocity profiles in venous microvessels (45-75 microm) of rat spinotrapezius muscle at centerline velocities between 0.3 and 14 mm/s (pseudoshear rates 3-120 s(-1)) under normal (nonaggregating) conditions and after induction of red blood cell aggregation with Dextran 500. Profiles are nearly parabolic (Poiseuille flow) over this flow rate range in the absence of aggregation. When aggregation is present, profiles are parabolic at high shear rates and become significantly blunted at pseudoshear rates of 40 s(-1) and below. These results indicate a possible mechanism for increased venous resistance at reduced flows.

Rheological influences on thrombosis

Blood flow plays important roles in the localization and morphology of thrombosis within the circulation. Blood flow properties (rheological variables) are associated with thrombotic risk factors and thrombotic risk; conversely their modification may reduce thrombotic risk.

Platelet microparticles promote platelet interaction with subendothelial matrix in a glycoprotein IIb/IIIa-dependent mechanism

BACKGROUND

Platelets, on activation, release vesicular particles called platelet microparticles. Despite their procoagulant activity, their functional role in platelet-vessel wall interactions is not known.

METHODS AND RESULTS

We examined the binding of microparticles to vessel wall components in vitro and in vivo. Microparticles bound to fibrinogen-, fibronectin-, and collagen-coated surfaces. Compared with activated platelets, we observed minimal binding of microparticles to vitronectin and von Willebrand factor. The glycoprotein IIb/IIIa (GP IIb/IIIa) inhibitors abciximab and eptifibatide (Integrilin) inhibited the binding to fibrinogen and fibronectin but had minimal effect on binding to collagen. Furthermore, monoclonal antibodies to GP Ib or anionic phospholipid-binding proteins (beta2-glycoprotein I or annexin V) had no effect in these interactions. Microparticles did not bind to monolayers of resting or stimulated human umbilical vein endothelial cells (HUVECs), even in the presence of fibrinogen or von Willebrand factor. However, under similar conditions, microparticles bound to extracellular matrix produced by cultured HUVECs. Abciximab inhibited this interaction by approximately 50%. In a rabbit model of arterial endothelial injury, the infusion of 51Cr-labeled microparticles resulted in a 3- to 5-fold increase of microparticle adhesion to the injured site compared with the uninjured site (P<0.05%). Furthermore, activated platelets bound to surface-immobilized microparticles in a GP IIb/IIIa-dependent mechanism. This binding increased in the presence of fibrinogen by approximately 30%.

CONCLUSIONS

Platelet microparticles bind to subendothelial matrix in vitro and in vivo and can act as a substrate for further platelet binding. This interaction may play a significant role in platelet adhesion to the site of endothelial injury.

Fluid mechanics of arterial stenosis: relationship to the development of mural thrombus

In this study, we analyzed blood flow through a model stenosis with Reynolds numbers ranging from 300 to 3,600 using both experimental and numerical methods. The jet produced at the throat was turbulent, leading to an axisymmetric region of slowly recirculating flow. For higher Reynolds numbers, this region became more disturbed and its length was reduced. The numerical predictions were confirmed by digital particle image velocimetry and used to describe the fluid dynamics mechanisms relevant to prior measurements of platelet deposition in canine blood flow (R.T. Schoephoerster et al., Atherosclerosis and Thrombosis 12:1806-1813, 1993). Actual deposition onto the wall was dependent on the wall shear stress distribution along the stenosis, increasing in areas of flow recirculation and reattachment. Platelet activation potential was analyzed under laminar and turbulent flow conditions in terms of the cumulative effect of the varying shear and elongational stresses, and the duration platelets are exposed to them along individual platelet paths. The cumulative product of shear rate and exposure time along a platelet path reached a value of 500, half the value needed for platelet activation under constant shear (J.M.. Ramstack et al., Journal of Biomechanics 12: 113-125, 1979).

Steady flow in an aneurysm model: correlation between fluid dynamics and blood platelet deposition

Laminar and turbulent numerical simulations of steady flow in an aneurysm model were carried out over Reynolds numbers ranging from 300 to 3600. The numerical simulations are validated with Digital particle Image Velocimetry (DPIV) measurements, and used to study the fluid dynamic mechanisms that characterize aneurysm deterioration, by correlating them to in vitro blood platelet deposition results. It is shown that the recirculation zone formed inside the aneurysm cavity creates conditions that promote thrombus formation and the viability of rupture. Wall shear stress values in the recirculation zone are around one order of magnitude less than in the entrance zone. The point of reattachment at the distal end of the aneurysm is characterized by a pronounced wall shear stress peak. As the Reynolds number increases in laminar flow, the center of the recirculation region migrates toward the distal end of the aneurysm, increasing the pressure at the reattachment point. Under fully turbulent flow conditions (Re = 3600) the recirculation zone inside the aneurysm shrinks considerably. The wall shear stress values are almost one order of magnitude larger than those for the laminar cases. The fluid dynamics mechanisms inferred from the numerical simulation were correlated with measurements of blood platelet deposition, offering useful explanations for the different morphologies of the platelet deposition curves.

An estimated shape function for drift in a platelet-transport model

Prior work has shown that concentration profiles of platelets in flowing whole blood and of platelet-sized beads in flowing blood suspensions can include near-wall excesses. A model to describe this phenomenon was built about a single-component convective diffusion equation. To incorporate redistribution to preferred sites by shear flows of red cell suspensions, the model used a drift shape function (in addition to the commonly used augmented diffusion coefficient). This paper reports experiments that provide an average concentration profile from which the shape function for that model is calculated; the experiments and shape function are for the particular conditions of 40% hematocrit, platelet-sized latex beads (2.5 microns diameter), tube ID of 217 microns, and a wall shear rate of 555 s-1. Less precise estimates of the shape function obtained from data of previous studies indicate that the shape function is similar for the hematocrit of 15%.

Influence of dextrans on platelet distribution in arterioles and venules

Dextrans bind to the surface of platelets, red blood cells and endothelium. We investigated whether a low doses (30 mg/kg IV) of 40-kDa (Dx40), neutral, 500-kDa (Dx500) or sulphated, 500-kDa (Dx500S) dextrans influence platelet distribution in rabbit mesenteric arterioles and venules (diameter 17-33 microns). Intravital fluorescence videomicroscopy was used to visualize platelets labelled in vivo with acridine red. Their concentration distribution determined within a thin optical section about the median vessel plane was expressed relative to the mean concentration in that vessel. In arterioles, Dx500 and Dx500S increased the relative platelet concentration in the centre [radial position (R): 0.0-0.4 R] from 0.60 to 1.07 (P < 0.001) and 1.20 (P < 0.003), and reduced it near the wall (0.8-0.9 R) from 1.59 to 0.93 (P < 0.02) and 0.95 (P < 0.03) respectively. In venules a similar, but non-significant, effect was observed. Dx40 did not change platelet distribution in arterioles, but decreased their concentration in venules in the centre from 1.08 to 0.71 (P < 0.03) and increased it at the wall from 0.89 to 1.27 (P < 0.04). The deformability of red blood cells was unchanged, but their aggregation tendency increased approximately two-fold after Dx500 and Dx500S injection, while Dx40 had no influence. Leucocyte margination in venules did not affect platelet distribution. Dextran injection did not change microvascular flow velocity or plasma viscosity, suggesting that the observed changes in arteriolar platelet distribution were caused by binding of dextran to the surface of platelets and/or red blood cells.

Imaging of leukocytes within the rat brain cortex in vivo

Confocal laser scanning microscopy was used in a rat closed cranial window preparation in order to study rhodamin 6G-labeled leukocytes within the brain cortex in vivo. Leukocytes were visualized up to 150 microns beneath the rat brain surface in noninvasive optical sections. In pial venules, leukocytes were seen flowing with the blood stream, rolling along or sticking to the endothelium, and migrating through the vessel wall. Within cerebral capillaries, leukocyte flux, velocities, and leukocyte plugging were measured. After additional intravenous administration of fluorescein, the plasma, leukocytes, and erythrocytes were visualized simultaneously. Based on stacks of optical sections of fluorescein-labeled capillaries, the individual capillaries were localized within the three-dimensional microvascular network. The usefulness of this technique was illustrated in a feasibility study in which leukocyte sticking to the vascular walls of venules, leukocyte extravasation, and intracapillary leukocyte plugging were monitored in a model of global cerebral ischemia.

Model of platelet transport in flowing blood with drift and diffusion terms

A drift term is added to the convective diffusion equation for platelet transport so that situations with near-wall excesses of platelets can be described. The mathematical relationship between the drift and the fully developed, steady-state platelet concentration profile is shown and a functional form of the drift that leads to concentration profiles similar to experimentally determined profiles is provided. The transport equation is numerically integrated to determine concentration profiles in the developing region of a tube flow. With the approximate drift function and typical values of augmented diffusion constant, the calculated concentration profiles have near-wall excesses that mimic experimental results, thus implying the extended equation is a valid description of rheological events. Stochastic differential equations that are equivalent to the convective diffusion transport equation are shown, and simulations with them are used to illustrate the impact of the drift term on platelet concentration profiles during deposition in a tube flow.

The behavior of sonicated albumin microbubbles within the microcirculation: a basis for their use during myocardial contrast echocardiography

The purpose of this study was to determine whether the behavior of sonicated albumin microbubbles accurately mimics red blood cell flow in the microcirculation and is thus consistent with their use as in vivo tracers of red blood cell flow during myocardial contrast echocardiography. Accordingly, microbubbles prepared from fluorescein-conjugated albumin and fluorescently labeled red blood cells were injected intravascularly in eight golden hamsters. Their intravascular distribution, velocities, arteriolar-to-venular transit and flux ratios at branch points were determined in the microcirculation of the cheek pouch. Albumin microbubbles (mean diameter, 4.9 +/- 3.6 microns) and red blood cells displayed a similar frequency of distribution across the arteriolar lumen (33% in the central 20% of the arterioles), and their arteriolar velocities were also similar (2.5 +/- 0.7 mm/sec and 2.3 +/- 0.7 mm/sec,p = NS). The mean velocities of microbubbles correlated well with those of red blood cells at baseline and after adenosine application (r = 0.97 and r = 0.89, respectively), as did the calculated maximum velocity (r = 0.98 and r = 0.80, baseline and adenosine, respectively). The velocity profiles across the lumen of the vessels for albumin microbubbles and red blood cells were similar at baseline and after adenosine-induced velocity changes. The flux ratios at branch points also correlated well (r = 0.92, p less than 0.001). Arteriolar-to-venular transit times of albumin microbubbles were similar to those of red blood cells in vessels ranging in size from 22 microns to 45 microns. We conclude that the behavior of albumin microbubbles in the microcirculation mimics that of red blood cells and supports their use as intravascular tracers of red blood cell flow during myocardial contrast echocardiography.

The near-wall excess of platelet-sized particles in blood flow: its dependence on hematocrit and wall shear rate

Methods involving microscopy were used to obtain concentration profiles of platelet-sized beads during flow through glass channels. Suspensions of fluorescent latex beads (2.38 microns diam) and washed red blood cells were made from an isotonic albumin-dextrose solution. A syringe pump regulated the suspension flow through glass channels, which were either 50 or 100 microns wide; most experiments used a wall shear rate of 1630 sec-1. Via stroboscopic epifluorescence microscopy, photographs were collected on image planes parallel to the channel wall. Profiles of the bead concentration in the narrow channel direction were made by assembling counts of the focused bead images in the photographs. The results showed that a near-wall excess of the beads occurred when the suspension contained a significant fraction of red cells (over 7%). For hematocrits of 15 to 45% (the highest studied), the excess was above five times the concentration in the central region. Experiments with channels of both widths showed the region of excess beads was 5 to 8 micron thick. A series of experiments with 50-micron channels, with a suspension hematocrit of 15%, and with wall shear rates from 50 to 3180 sec-1 showed that near-wall excesses were large only for wall shear rates of 430 sec-1 and above. This work demonstrated the effects of wall shear rate (flow rate) and hematocrit on the number of platelet-sized beads near a surface and hence illustrated physical (rheological) factors that act in blood-surface interaction.

Velocity profiles of blood platelets and red blood cells flowing in arterioles of the rabbit mesentery

Velocity profiles were determined in rabbit mesenteric arterioles (diameter 17-32 micron). A good spatial resolution was obtained by using the blood platelets as small and natural markers of flow, providing for the first time in vivo detailed, quantitative information about the shape of the velocity profiles in microvessels. In some experiments red blood cell velocity profiles were recorded as well. Easy detection of the cells of interest could be achieved by labelling them selectively with a fluorescent dye and visualizing them by intravital fluorescence video microscopy, using flashed illumination. Pairs of flashes were given with a short, preset time interval between both flashes, yielding in one TV picture two images of the same cell displaced over a certain distance for the given time interval. Velocity and mean radial position of cells, flowing within an optical section around the median plane of the vessel, were determined. The shape of the velocity profiles of platelets and red blood cells was similar. The profiles were flattened as compared to a parabola, both in systole and diastole. Vessel diameter did not change measurably during the cardiac cycle. As an index of the degree of blunting of the profiles, the ratio of the maximal and mean velocity of the profile was used, which is 2 for a parabola and 1 for complete plug flow. The index ranged from 1.39 to 1.54 (median 1.50), and increased with vessel diameter. Calculations showed that the blunting of the profiles cannot be explained by an influence of the finite depth of the optical section.

Regional platelet concentration in blood flow through capillary tubes

Platelet concentration was measured in samples from the various components of a bloodflow circuit, including the reservoir, the tube (with i.d. between 50 and 210 micron), and the discharge. The tube sample was collected by halting the flow and then flushing out a length of tube; thus, this sample collected equally from all radial locations. As the discharge sample was well mixed, it reflected the velocity field in the tube. Each reservoir sample was a traditional bulk collection. To ensure that the results represented the physical effects of flow on regional platelet concentration and could be interpreted with simple mass balance relationships, strong anticoagulation (sodium citrate and heparin) and platelet inhibition (prostaglandin E1) were used. Results for all tube diameters and for reservoir hematocrits from 5.5 to 77% and wall shear rates from 80 to 8000 sec-1 show that tubular platelet concentration is greater than reservoir or discharge platelet concentrations, which are equal. For platelet-rich plasma the tubular platelet concentration is decreased compared to the reservoir or discharge values. Mass balances show that the elevated tubular platelet concentration is due to an excess of platelets in radial locations with below average speeds; coupled with the need for red cells, this suggests that excess platelets have a near-wall location. Nonparametric statistical tests show that wall shear rate is a significant variable at a 0.05 confidence level; inner diameter is not found to be a significant variable, probably because of the limited diameter range studied and the experimental errors involved in determining platelet concentrations.

Influence of ADP, blood flow velocity, and vessel diameter on the laser-induced thrombus formation

The laser-induced thrombus model in rat mesenteric arterioles and venules represents a reliable and reproducible in vivo method. It is suitable for basic investigations concerning factors involved in thrombus formation as well as for testing antithrombotic effects of drugs. The laser-induced thrombus formation depends significantly on the presence of ADP, as ADP-utilizing enzymes inhibit thrombosis in the animal model. The instrumental test set-up consists of a 4 W Argon laser (Spectra Physics, Mountain View, CA, USA), a ray adaptation and adjusting device (BTG, Munich, FRG), a microscope (ICM 405, Zeiss, Oberkochen, FRG), and a video system (Sony, Japan). RBC velocity data were recorded with the help of a modified dual-slit technique (acc. to Wayland and Johnson). Results were expressed as number of laser injuries necessary to produce a defined thrombus (minimum size: 1/4 of the vessel diameter) under constant conditions (effective capacity: 30 mW, exposure time: 1/5 sec). The number of laser lesions necessary to induce a defined thrombus decreased with an increase in arteriole diameter (10 to 20 micron) but increased again in larger arterioles and small arteries (greater than 25 micron). On the arteriolar side there are significant correlation coefficients between vessel diameter and RBC velocity (r = 0.69), vessel diameter and No. of laser injuries (r = 0.70), and RBC velocity and No. of laser injuries (r = 0.71). Due to relative low flow conditions in the venules, the number of laser injuries required to induce a defined thrombus does not significantly depend on the vessel diameter.

Platelet-mediated vascular permeability in the rat: a predominant role for 5-hydroxytryptamine

The intradermal injection in rat skin of washed, thrombin-activated platelets produces an increase in vascular permeability, the intensity of which increments with the platelet concentration. Pretreatment of the recipient animals with serotonergic antagonists, including the specific 5-HT2 receptor blocker ketanserin, potently inhibits the platelet-mediated and the 5-HT-induced vascular defect. Amine depletion of platelets or skin tissues with reserpine reduces the response to platelets. Platelet prostanoid and lipoxygenase derivatives play no major role in the vascular response to platelet. The permeability increase induced by exogenous 5-HT and by activated platelets is reduced by alpha 1-adrenergic stimulation with noradrenaline or phenylephrine and by beta 2-stimulation with terbutaline or isoprenaline, and is potentiated by adenosine; this points to a modulation of permeability by blood flow changes and to a direct beta-adrenergic effect at the endothelial cell membrane. This study demonstrates a predominant role for 5-HT in the platelet-mediated vascular permeability increase in a sensitive species like the rat.

Radial distribution of white cells during blood flow in small tubes

The radial distribution of white blood cells (WBC) in blood flowing through glass tubes (i.d. 69 micron) was studied as a function of wall shear stress (range 0.1-2.5 Pa) and suspending medium (plasma, buffered saline, high-molecular-weight dextran solution). It was found that, irrespective of the choice of suspending medium, the highest leukocyte flux at high shear stresses was found in the tube center. WBC redistribution was seen upon lowering the shear stresses: A significant shift of WBC flux toward the marginal fluid layers occurred at the expense of the axial region. After replacement of plasma by other media the flow-dependent redistribution of WBCs was qualitatively unaffected. However, suspension of cells in dextran solution (inducing strong red cell aggregation) resulted in enhanced WBC margination, while in saline (no red cell aggregation) axial accumulation was accentuated. The results support the concept of size-dependent radial distribution of particles in flow of mixed suspensions. If applied to the living microcirculation, the data serve to explain WBC margination in microvessels (the first step in the series of events leading to emigration) in terms of a hydrodynamic phenomenon resulting from red cell/white cell interaction. The pronounced flow dependence of WBC margination results primarily from the effect of shear on red cell aggregation which leads to an alteration of the effective particle size distribution in the flowing blood.

Fluorescent labeling of blood platelets in vivo

In rabbits, a clear microscopic image of individual, fluorescent blood platelets flowing in a mesenteric microvessel could be obtained over the whole cross-sectional area of the vessel, following an intravenous injection of a solution containing 5-15 mg of the fluorochrome Acridine Red. Most of the circulating platelets were labeled. Activation of the platelets by the injected dye was not seen. A fall in platelet count or hematocrit following injection did not occur. Electron microscopy revealed no gross ultrastructural changes. In vitro, the dye reduced platelet aggregation in a dose dependent way. In vivo, aggregation and adhesion of platelets as induced by laser injury or transection of an arteriole was observed in all cases, even following multiple injections of Acridine Red. Primary hemostatic plug formation times as measured in mesenteric arterioles were normal after the first and second injection of 5 mg Acridine Red, but prolonged after subsequent injections. Bleeding times as measured on the ear were prolonged following injection of 15 mg of the dye. It is concluded that this labeling procedure allows the study of the rheological behavior of the platelets in vivo. Whether this technique can also be used to study functional platelet behavior in vivo, needs further investigation.