Biomedical subjects

A R Pries

Publications and source records attributed to A R Pries.

At least 37 records · Page 2Linked to original sources

PubMed2000

The endothelial surface layer.

The endothelial lining of blood vessels presents a large surface area for exchange of materials between blood and tissues, and is critically involved in many other processes such as regulation of blood flow, inflammatory responses and blood coagulation. It has long been known that the luminal surface of the endothelium is lined with a glycocalyx, a layer of membrane-bound macromolecules which has been determined by electron microscopy to be several tens of nanometers thick. However, investigations in vivo have indicated the presence of a much thicker endothelial surface layer (ESL), with an estimated thickness ranging from 0.5 microm to over 1 microm, that restricts the flow of plasma and can exclude red blood cells and some macromolecular solutes. The evidence for the existence of the ESL, hypotheses about its composition and biophysical properties, its relevance to physiological processes, and its possible clinical implications are considered in this review.

Animals↗

PubMed2000

Evidence for modulation of genes involved in vascular adaptation by prolonged exposure of endothelial cells to shear stress.

OBJECTIVE: Shear stress is known to modulate gene expression. However, the molecular link between blood flow and long-time vessel adaptation is still unclear. In this study, the variations of gene expression by prolonged shear stress exposure was investigated in order to identify genes possibly involved in flow dependent vascular adaptation. METHODS: Human umbilical vein endothelial cells (HUVECs) were exposed to laminar shear stress (6 dyn/cm(2); 24 h) and analyzed by differential display (DDRT-PCR). Flow-modulation of differentially expressed genes by different exposure times (4, 24, 48 h) and in human cardiac microvascular endothelial cells (HCMECs) (24 h exposure) was analyzed by RT-PCR and northern blotting. RESULTS: DDRT-PCR analysis displayed 13 down- and 20 up-regulated products in response to flow. Four known genes were identified: Angiopoietin-2, a protein reported to reduce vessel stability, was progressively (4-48 h) down-regulated by shear stress. The induction of the anti-angiogenic metalloproteinase METH-1 was maximal after 4 h exposure and sustained over the time (24-48 h). Growth arrest-specific mRNA 3 (gas3) and calpactin 1 light chain (p11) were up-regulated only by prolonged exposure (24-48 h). Analysis of the expression of angiopoietin-2, METH-1, gas3, and p11 in shear stress exposed (24 h) HCMECs showed modulation patterns comparable to those observed in HUVECs. CONCLUSION: Since angiopoietin-2 and METH-1 are known to be involved in vessel regression/stabilization, the reported modulation of these genes by prolonged shear stress exposure strongly suggests their participation in flow-dependent vascular adaptation.

ADAM Proteins↗

PubMed2000

Influence of the endothelial glycocalyx on cerebral blood flow in mice.

The endothelial surface layer (glycocalyx) of cerebral capillaries may increase resistance to blood flow. This hypothesis was investigated in mice by intravenous administration of heparinase (2500 IU/kg body weight in saline), which cleaves proteoglycan junctions of the glycocalyx. Morphology was investigated by transmission electron microscopy. Cerebral perfusion velocity was recorded before and during heparinase or saline treatment using laser-Doppler flowmetry. In addition, cerebral blood flow (CBF) was measured 10 minutes after heparinase or saline treatment using the iodo[14C]antipyrine method. Laser-Doppler flowmetry and CBF measurements were performed during normocapnia and severe hypercapnia (PCO2: 120 mm Hg). After heparinase, morphology showed a reduced thickness of the glycocalyx in cortical microvessels by 43% (P < 0.05) compared with saline-treated controls. Under normocapnic conditions, a 15% (P < 0.05) transient increase of cerebral flow velocity occurred 2.5 to 5 minutes after heparinase injection. Laser-Doppler flow and CBF returned to control values ten minutes after the injection. However, during severe hypercapnia, heparinase treatment resulted in a persisting increase in laser-Doppler flow (6%, P < 0.05) and CBF (30%, P < 0.05). These observations indicate the existence of a flow resistance in cerebral capillaries exerted by the glycocalyx. The transient nature of the CBF increase during normocapnia may be explained by a vascular compensation that is exhausted during severe hypercapnia.

Adaptation, Physiological↗

PubMed2000

Microcirculatory network structures and models.

Terminal vascular beds exhibit a high degree of heterogeneity. Pertinent parameters are nonlinearly related, and their distributions are not independent. The classical "typical vessel" approach using averaged values for different vessel classes may not lead to a correct understanding of physiology and pathophysiology of terminal vascular beds. Such problems can be avoided by studying microcirculatory functions at the network level using a combination of experiments and theoretical models. In this approach, distributions and relationships of pertinent parameters are measured in vivo, leading to the development of comprehensive databases. Such databases can be analyzed and complemented by suitable mathematical models, permitting estimation of parameters that are difficult to measure, and critical assessment of quantitative theories and hypotheses for microvascular function. This collaborative process between experimentally and theoretically oriented investigators may be facilitated in the future by the development of web-based repositories of experimental data and theoretical models.

Adaptation, Physiological↗

PubMed2000

Determination of microvascular flow pattern formation in vivo.

Blood flow in microvessels differs significantly from that of red blood cells (RBC) flowing through long, straight glass tubes in vitro. The in vivo situation is characterized by the presence of plasma favoring aggregation, by the irregular geometry of vessel segments, and by frequent branching points. Here, a method is presented to characterize flow patterns in microvascular blood flow during intravital microscopy based on Fourier analysis of recorded light intensity patterns. The interpretation of the resulting power spectra in terms of pattern size distribution was validated by model experiments employing artificial textures and by reverse transformation of idealized spectra. The determined size of RBC flow patterns in microvessels ranged from approximately 8 microm in capillaries to approximately 14 microm in vessels of >30 microm. With increasing shear rate above approximately 100 s(-1) pattern size increased, possibly reflecting formation of short-lived flow clusters. Below approximately 100 s(-1) an increase of pattern size with decreasing shear rate was found in experiments using local occlusion and treatment with high-molecular-weight dextran, suggesting the formation of aggregates. The dynamic process of generation and destruction of RBC flow patterns could well contribute to flow resistance in vivo in peripheral vascular beds.

Animals↗

PubMed1999

Structural autoregulation of terminal vascular beds: vascular adaptation and development of hypertension.

-It is widely accepted that the early phase of primary hypertension is characterized by elevated cardiac output, whereas in later stages the increased blood pressure is due to increased peripheral resistance. To study long-term effects of increased blood flow on peripheral resistance, structural adaptation of microvascular networks in response to changes in blood flow was simulated using a previously developed theoretical model. The diameter of each vessel segment was assumed to change in response to local levels of shear stress, transmural pressure, a metabolic stimulus dependent on blood flow rate, and a conducted stimulus. Network morphologies and topologies were derived from intravital microscopy of the rat mesentery. Adaptive responses to the 4 stimuli were quantitatively balanced to yield stable and realistic distributions of vascular diameters and blood flow rates when the total flow rate was set to observed levels. To simulate effects of increased cardiac output, network flow resistance after structural adaptation was determined for a range of flow rates. Resistance increased with increasing flow, and increases in pressure were up to 3-fold greater than proportional to the increases in flow. According to the model, flow-dependent changes of network resistance result mainly from the vascular response to transmural pressure, which also causes arteriovenous asymmetry of diameters and pressure drops. Therefore, in vascular beds that exhibit arteriovenous asymmetry, increased flow may trigger increased flow resistance by a mechanism involving the tendency of vascular segments to reduce their luminal diameters in response to increased transmural pressure.

Adaptation, Physiological↗

PubMed1998

Blood flow resistance during hemodilution: effect of plasma composition.

OBJECTIVES: To investigate the causes of wide variations in reported effects of hemodilution on flow resistance of vascular beds. METHODS: (a) In a meta-analysis of 28 prior studies, resistance values at hematocrits of zero (R0) and 0.45 (R0.45) were derived. Study design characteristics (presence of vasodilatory reserve or leukocytes, species, tissue, hemodiluent) were tested by ANOVA for their relation to the ratio R0/R0.45. (b) Experiments were performed to determine flow resistance during hemodilution in the rat mesentery with (n = 8) and without (n = 11) pretreatment with heparinase, which modifies the endothelial glycocalyx. (c) A mathematical flow simulation for mesenteric microvascular networks was used to predict resistance effects of hemodilution and of a hypothetical layer on the endothelial surface. RESULTS: (a) In prior studies using native plasma for hemodilution R0 averaged 50 +/- 8% of R0.45, while in studies using artificial solutions R0 averaged 32 +/- 12% of R0.45. The larger reduction of flow resistance upon dilution with artificial media is independent of viscosity and oncotic pressure. Other design characteristics did not show strong significant effects. (b) Present experiments showed large reductions of flow resistance with saline hemodilution which were nearly halved after heparinase pretreatment. (c) Resistance effects of hemodilution with plasma or after heparinase treatment agree with model predictions based on tube flow rheology of blood. The larger resistance effects of dilution with artificial media can be explained by the removal of an endothelial surface layer of approximately 1.5 microns thickness. CONCLUSIONS: The results imply that changes of plasma composition, due to use of artificial infusion media, influence peripheral resistance and tissue perfusion. They are consistent with the hypothesis that interactions between endothelial glycocalyx structures and plasma components lead to formation of a thick layer at the endothelial surface which increases flow resistance.

Animals↗

PubMed1998

A model for red blood cell motion in glycocalyx-lined capillaries.

The interior surfaces of capillaries are lined with a layer (glycocalyx) of macromolecules bound or absorbed to the endothelium. Here, a theoretical model is used to analyze the effects of the glycocalyx on hematocrit and resistance to blood flow in capillaries. The glycocalyx is represented as a porous layer that resists penetration by red blood cells. Axisymmetric red blood cell shapes are assumed, and effects of cell membrane shear elasticity are included. Lubrication theory is used to compute the flow of plasma around the cell and within the glycocalyx. The effects of the glycocalyx on tube hematocrit (Fahraeus effect) and on flow resistance are predicted as functions of the width and hydraulic resistivity of the layer. A layer of width 1 micron and resistivity 10(8) dyn.s/cm4 leads to a relative apparent viscosity of approximately 10 in a 6-micron capillary at discharge hematocrit 45% and flow velocity of approximately 1 mm/s. This is consistent with experimental observations of increased flow resistance in microvessels in vivo, relative to glass tubes with the same diameters.

Capillaries↗

PubMed1998

Structural adaptation and stability of microvascular networks: theory and simulations.

A theoretical model was developed to simulate long-term changes of vessel diameters during structural adaptation of microvascular networks in response to tissue needs. The diameter of each vascular segment was assumed to change with time in response to four local stimuli: endothelial wall shear stress (tauw), intravascular pressure (P), a flow-dependent metabolic stimulus (M), and a stimulus conducted from distal to proximal segments along vascular walls (C). Increases in tauw, M, or C or decreases in P were assumed to stimulate diameter increases. Hemodynamic quantities were estimated using a mathematical model of network flow. Simulations were continued until equilibrium states were reached in which the stimuli were in balance. Predictions were compared with data from intravital microscopy of the rat mesentery, including topological position, diameter, length, and flow velocity for each segment of complete networks. Stable equilibrium states, with realistic distributions of velocities and diameters, were achieved only when all four stimuli were included. According to the model, responses to tauw and P ensure that diameters are smaller in peripheral than in proximal segments and are larger in venules than in corresponding arterioles, whereas M prevents collapse of networks to single pathways and C suppresses generation of large proximal shunts.

Adaptation, Physiological↗

PubMed1998

Observations on the accuracy of photometric techniques used to measure some in vivo microvascular blood flow parameters.

OBJECTIVE: The accuracy of optical methods used to measure in vivo microvascular blood flow parameters is investigated using measurements made in all vessels of microvascular networks of the rat mesentery. METHODS: The principle of mass conservation was applied to in vivo blood flow rate and discharge hematocrit data, which were determined by photometric methods. One of the several implied assumptions of most interpretations of in vivo optical data is that the vessels are circular in cross-section: to see the impact of vessel lumen shape on one of these measurements, the average velocity of blood flowing through a D-shaped glass capillary tube was measured by the dual-slit method. RESULTS: For in vivo data, significant imbalance exists in a large number of bifurcations, and the correlation between the blood flow imbalance and the red cell flux imbalance is very small (r2 = 0.39), indicating multiple sources of error. Furthermore, the measured discharge hematocrits were consistent with the observed flow directions at bifurcations in only 39% to 46% of the bifurcations in a network. The imbalance at these bifurcations is not simply caused by the inaccuracy of measurements in only a few microvessels that join such bifurcations, i.e., the inaccuracies are evenly distributed among the vessels. The results of the in vitro study of blood velocity measurement in D-shaped tubes indicates that the ratio of dual-slit velocity to the actual average blood velocity is sensitive to the shape of the vessel lumen, and is a function of blood flow rate, hematocrit, vessel lumen shape, and orientation. CONCLUSIONS: Significant inaccuracies exist in the flow and hematocrit data obtained by current methods of interpretation of in vivo photometric measurements. These inaccuracies must be considered when making vessel to-vessel comparisons, or vessel-by-vessel comparisons between in vivo observations and model predictions, even though the inaccuracies are greatly reduced when comparing averaged data.

Animals↗

PubMed1997

Diameter variability and microvascular flow resistance.

Microvessels are known to exhibit irregular shapes, deviating substantially from an idealized cylindrical tube geometry. Such irregularities must be taken into account in calculating microvascular flow resistance and may add to the observation that flow resistance in living microvessels in vivo is about twice that predicted on the basis of tube flow studies in vitro. The present study was aimed at providing a comprehensive database describing the apparent diameter variability for all segments of a complete microvascular network in the rat mesentery and assessing the impact of this variability on segmental flow resistance and the pressure drop across the network. Diameters were estimated by intravital microscopy at axial intervals of 20 microns along the 546 vessel segments of a mesenteric microvessel network, resulting in 6,319 separate diameter measurements. The amplitude of diameter variations in individual vessel segments decreased from approximately 15% of the mean vessel diameter in the smallest segments (approximately 5 microns diam) to approximately 5% in the largest segments (approximately 60 microns diam). Segmental hindrance was estimated to be 10-23% higher than calculated from arithmetic mean diameter, depending on the model used to estimate the hydrodynamically effective segment diameter. The overall pressure drop across the network calculated using a mathematical flow simulation was increased by 7-13.5%. This increase in flow resistance can explain approximately 10% of the observed discrepancy between flow resistance in vivo and in vitro.

Animals↗

PubMed1997

Microvascular blood flow resistance: role of endothelial surface layer.

Observations of blood flow in microvascular networks have shown that the resistance to blood flow is about twice that expected from studies using narrow glass tubes. The goal of the present study was to test the hypothesis that a macromolecular layer (glycocalyx) lining the endothelial surface contributes to blood flow resistance. Changes in flow resistance in microvascular networks of the rat mesentery were observed with microinfusion of enzymes targeted at oligosaccharide side chains in the glycocalyx. Infusion of heparinase resulted in a sustained decrease in estimated flow resistance of 14-21%, hydrodynamically equivalent to a uniform increase of vessel diameter by approximately 1 micron. Infusion of neuraminidase led to accumulation of platelets on the endothelium and doubled flow resistance. Additional experiments in untreated vascular networks in which microvascular blood flow was reduced by partial microocclusion of the feeding arteriole showed a substantial increase of flow resistance at low flow rates (average capillary flow velocities < 100 diameters/s). These observations indicate that the glycocalyx has significant hemodynamic relevance that may increase at low flow rates, possibly because of a shear-dependent variation in glycocalyx thickness.

Animals↗

PubMed1996

Relationship between structural and hemodynamic heterogeneity in microvascular networks.

The relationship between structural and hemodynamic heterogeneity of microvascular networks is examined by analyzing the effects of topological and geometric irregularities on network hemodynamics. Microscopic observations of a network in the rat mesentery provided data on length, diameter, and interconnection of all 913 segments. Two idealized network structures were derived from the observed network. In one, the topological structure was made symmetric; in another a further idealization was made by assigning equal lengths and diameters to all segments with topologically equivalent positions in the network. Blood flow through these three networks was simulated with a mathematical model based on experimental information on blood rheology. Overall network conductance and pressure distribution within the network were found to depend strongly on topological heterogeneity and less on geometric heterogeneity. In contrast, mean capillary hematocrit was sensitive to geometric heterogeneity but not to topological heterogeneity. Geometric and topological heterogeneity contributed equally to the dispersion of arteriovenous transit time. Hemodynamic characteristics of heterogeneous microvascular networks can only be adequately described if both topological and geometric variability in network structure are taken into account.

Animals↗

PubMed1996

Biophysical aspects of blood flow in the microvasculature.

The main function of the microvasculature is transport of materials. Water and solutes are carried by blood through the microvessels and exchanged, through vessel walls, with the surrounding tissues. This transport function is highly dependent on the architecture of the microvasculature and on the biophysical behavior of blood flowing through it. For example, the hydrodynamic resistance of a microvascular network, which determines the overall blood flow for a given perfusion pressure, depends on the number, size and arrangement of microvessels, the passive and active mechanisms governing their diameters, and on the apparent viscosity of blood flowing in them. Suspended elements in blood, especially red blood cells, strongly influence the apparent viscosity, which varies with several factors, including vessel diameter, hematocrit and blood flow velocity. The distribution of blood flows and red cell fluxes within a network, which influences the spatial pattern of mass transport, is determined by the mechanics of red cell motion in individual diverging bifurcations. Here, our current understanding of the biophysical processes governing blood flow in the microvasculature is reviewed, and some directions for future research are indicated.

Animals↗

PubMed1995

Flow-dependent regulation of arteriolar diameter in rat skeletal muscle in situ: role of endothelium-derived relaxing factor and prostanoids.

1. Arteriolar diameter in the resting rat spinotrapezius muscle was studied by intravital video microscopy before and after blockade of the L-arginine-EDRF (NG-nitro-L-arginine, L-NNA) or the cyclo-oxygenase-prostacyclin (indomethacin) pathway. Blockade of either pathway leads to a decrease of arteriolar diameter of 25-40%, while the combined blockade of both results in vasoconstriction of 50-60%. 2. Alteration of blood flow velocity elicited by partial micropipette occlusion induces corresponding changes of vessel diameter. The flow-dependent diameter response is reduced by about 80% by L-NNA. By contrast, blockade of prostanoid production shows no significant influence on vessel response to blood flow alteration in the range tested. 3. Transient overshooting vasodilatation is seen for about 1 min following the sudden restoration of flow velocity subsequent to occlusion. In contrast to the initial phase of this response, the late phase is blocked by L-NNA. 4. The findings suggest that basal release of endothelium-derived relaxing factor (EDRF) and prostanoids leads to additive and independent dilator effects, and that flow-dependent diameter changes are primarily mediated by EDRF. 5. If present data are compared with literature reports, it appears that arterial flow sensitivity is most pronounced in the smallest vessels. In such vessels, flow-dependent dilatation will amplify even small changes of volume flow by more than four times.

Animals↗

PubMed1995

Effect of oxygen tension on regulation of arteriolar diameter in skeletal muscle in situ.

Skeletal muscle arterioles are known to constrict upon elevation of ambient PO2. While several studies have shown that the endothelium plays an important role in this response, it is not clear how this response is mediated. We examined the oxygen-induced constriction of arterioles in the rat spinotrapezius muscle. Elevation of superfusion solution PO2 from about 15 to 150 mm Hg caused arteriolar constriction by 25% (+/- 3%, n = 18). Inhibition of prostaglandin synthesis by superfusion of indomethacin (30 microM) produced vasoconstriction by 28% (+/- 9.5%, n = 5), but left the PO2 response unaffected. Blockade of the synthesis of endothelium-derived relaxing factor (EDRF) by NG-nitro-L-arginine (L-NNA, 35 mg/kg i.v.) caused arteriolar constriction by 31% (+/- 8%, n = 8). During application of L-NNA, the constrictor response to PO2 elevation was reduced to 3 +/- 2%. Administration of superoxide dismutase (SOD, 80,000 U/kg i.v.) did not affect the PO2 response. It is concluded that in small arterioles of skeletal muscle both EDRF and prostanoids sustain a significant basal dilatation. The dilatory effects of EDRF but not of prostaglandins are strongly dependent on PO2. The vasoconstriction in response to high ambient PO2 is not due to EDRF breakdown during its diffusion from endothelial to smooth muscle cells.

Animals↗

PubMed1995

Does luminal release of EDRF contribute to downstream microvascular tone?

The contribution of luminally released endothelium-derived relaxing factor (EDRF) transported with the flowing blood to the control of smooth muscle tone of downstream arterioles was studied. The dilatory response of arterioles in the spinotrapezius muscle of the rat to superfusion of acetylcholine (ACH) was investigated by intravital microscopy before and after blockade of blood flow by micropipette occlusion upstream of the site of observation. Vessels were studied without treatment, after topical application of indomethacin (INDO, 3 10(-5) M) to inhibit prostanoid production and after local treatment with NG-nitro-L-arginine (LNNA, 2.9 10(-3) M) to suppress local production of EDRF. In untreated vessels and after INDO, responses to ACH were not significantly different in the presence or absence of blood flow. After LNNA, the responses to ACH were reduced to about 50%, but were again similar with or without flow. These results indicate that luminally transported EDRF does not play a significant role in controlling arteriolar tone in small skeletal muscle arterioles.

Acetylcholine↗

PubMed1995

Transient rheological behavior of blood in low-shear tube flow: velocity profiles and effective viscosity.

Velocity profiles of human blood flowing through vertical and horizontal glass tubes (25-100 microns ID) were measured as a function of time following a sudden reduction of wall shear stress (tau w) from a high value to values ranging from 2 to 100 mPa. Cell velocities at various radial positions were determined off-line from video recordings by digital image analysis. In vertical tubes, symmetric velocity profiles were obtained that developed increasing bluntness with time, particularly at lower tau w and in smaller tubes. In horizontal tubes, velocity profiles developed strong asymmetry as a function of time. Red blood cell (RBC) sedimentation was associated with uniform low flow velocities in the concentrating cell sediment, whereas faster flow and almost parabolic profiles were observed in the supernatant plasma region. Calculations of effective blood viscosity showed a decrease with time at low tau w in vertical tubes but an increase in horizontal tubes. The differences between profile shape and effective viscosity in vertical and horizontal tubes disappeared at tau w > 50 mPa. These findings are related to the cross-sectional distribution of RBC, which depends on RBC aggregation and sedimentation.

Blood Flow Velocity↗