Biomedical subjects

P Gaehtgens

Publications and source records attributed to P Gaehtgens.

At least 37 records · Page 2Linked to original sources

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↗

PubMed1997

Angiotensin II-induced leukocyte adhesion on human coronary endothelial cells is mediated by E-selectin.

Clinical data suggest a link between the activation of the renin-angiotensin system and cardiovascular ischemic events. Leukocyte accumulation in the vessel wall is a hallmark of early atherosclerosis and plaque progression. E-Selectin, vascular cell adhesion molecule-1 (VCAM-1), and intercellular adhesion molecule-1 (ICAM-1) are adhesion molecules participating in mediating interactions between leukocytes and endothelial cells and have been found to be expressed in athero-sclerotic plaques. We investigated whether angiotensin II, the effector of the renin-angiotensin system, influences the endothelial expression of E-selectin, VCAM-1, and ICAM-1. In coronary endothelial cells derived from explanted human hearts, angiotensin II (10(-11) to 10(-5) mol/L) induced a concentration-dependent increase in E-selectin expression. The effect was measured by cell ELISA and duplex reverse-transcription polymerase chain reaction (RT-PCR) and reached its maximum at 10(-7) mol/L. Angiotensin II induced only a small increase in E-selectin expression in cardiac microvascular endothelial cells. VCAM-1 and ICAM-1 were not affected by angiotensin II stimulation. In addition, the effect of angiotensin II-induced E-selectin expression on leukocyte adhesion was quantified under flow conditions. Angiotensin II (10(-7) mol/L) increased leukocyte adhesion significantly to 67% of the maximal effect by tumor necrosis factor-alpha at a wall shear stress of 2 dyne/cm2. This adhesion was found to be E-selectin dependent, as demonstrated by blocking antibodies. The AT1-receptor antagonist DUP 753 significantly reduced E-selectin-dependent adhesion, whereas the AT2-receptor antagonist PD 123177 had no inhibitory effect. In addition, only AT1-receptor, but not AT2-receptor, mRNA could be detected by RT-PCR in coronary endothelial cells. Therefore, it is suggested that AT1 receptors mediate the effects of angiotensin II on E-selectin expression and leukocyte adhesion on coronary endothelial cells.

Angiotensin II↗

PubMed1997

Activation of the beta2 integrin Mac-1 (CD11b/CD18) by an endogenous lipid mediator of human neutrophils and HL-60 cells.

beta2 integrins (CD11/CD18) play a key role in the adhesion, activation, migration and phagocytosis of human neutrophils. In order to exert their functions, beta2 integrins require activation, which results in an enhancement of ligand affinity. This functional up-regulation is probably due to a conformational change of the beta2 integrins, but the mechanisms of inside-out signaling that trigger this activation are still under investigation. In the present study, the effect of cellular lipids on the affinity state of beta2 integrins was investigated. Lipids were extracted from human neutrophils and HL-60 cells after stimulation with IL-8 or phorbol ester, respectively. The extracts were purified by anion exchange chromatography and/or HPLC fractionation. The lipid extracts induced the adhesion of neutrophils to fibrinogen and, in a cell-free assay system, the binding of C3bi-coated zymosan-particles by purified beta2 integrin Mac-1 (CD11b/CD18). The integrin up-regulating activity was resistant to ester hydrolysis, eluted as one particular HPLC-fraction, and showed an absorption maximum at 194+/-2 nm. Taken together, these data support the concept that activated neutrophils and HL-60 cells can generate an endogenous lipid mediator, which up-regulates ligand binding activity of beta2 integrins.

CD18 Antigens↗

PubMed1996

Beta2 integrins mediate protein tyrosine phosphorylation in human neutrophils.

Tyrosine kinases play a prominent role in various intracellular signal transduction pathways. In this study, beta2 integrin (CD11/CD18)-mediated adhesive interactions of human neutrophils (PMN) were mimicked by antibody cross-linking of CD11/CD18. Cross-linking of CD18 induced tyrosine phosphorylation of several proteins with apparent molecular masses of approximately 120, 78, 72, 65, and 56 kDa, respectively, as shown by anti-phosphotyrosine immunoblotting of whole cell lysates. Cross-linking of the alpha-subunits of the beta2 integrins demonstrated that only cross-linking of Mac-1 but not LFA-1 or gp150/95 triggered tyrosine signaling. The tyrosine phosphorylations showed a rapid and transient time course. Comparison of the CD18-mediated tyrosine phosphorylation patterns with those induced by chemoattractants gave similar results. The observed tyrosine phosphorylation was specific, since binding and non-binding irrelevant primary antibodies had no effect. Furthermore, F(ab')2 fragments of the anti-CD18 antibody IB4 and addition of F(ab')2 fragments of secondary antibody were sufficient to induce tyrosine phosphorylation. Inhibition of tyrosine kinases by herbimycin A resulted in partial inhibition of the CD18-mediated tyrosine phosphorylation of the 120- and 65-kDa proteins and in complete inhibition of tyrosine phosphorylation of the 78- and 56-kDa proteins. In unstimulated PMN, the tyrosine phosphatase inhibitor sodium orthovanadate had no effect on tyrosine phosphorylation of the 120-kDa protein, but induced phosphorylation of the 78-, 72-, 65-, and 56-kDa proteins. These results indicate that the beta2 integrin-mediated intracellular signaling cascade involves different tyrosine phosphorylation (and dephosphorylation) events, which may play a role in the regulation of PMN functions during inflammation.

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

The leukocyte integrin Mac-1 (CD11b/CD18) contributes to binding of human granulocytes to collagen.

Adhesion of polymorphonuclear granulocytes (PMN) to extracellular matrix proteins has been shown to be important for their migration in vitro and is thought to participate in PMN recruitment to sites of inflammation. Isolated human PMN stimulated with PMA were found to adhere best to microtiter wells coated with the novel ECM glycoprotein undulin (27 +/- 3% of PMNs added), followed by fibrinogen (25 +/- 2%), collagen type VI (18 +/- 2%), fibronectin (16 +/- 2%), and laminin (15 +/- 3%). PMN adhesion to other collagens ranged between 3 and 11%. Monoclonal antibodies recognizing CD18 and CD11b subunits of Mac-1 inhibited adhesion of PMN to collagens by an order of magnitude more effectively than to all noncollagenous substrates. F(ab')2 fragments of the anti-CD18 antibody were also able to block adhesion to collagens. Anti-LFA-1 (CD11a) and anti-CD44 antibodies did not significantly reduce adhesion. PMN adhesion was also inhibited by soluble collagens type II and VI (ID50 approximately 75 micrograms/ml). Binding of soluble radiolabeled collagens type II and VI to PMNs was specific and saturable with apparent dissociation constants of 2.2 and 1.9 nM, respectively, and specific binding of collagens type II and VI was almost completely inhibited by anti-CD18, but not by control antibodies. These data indicate that Mac-1 function is required for binding of human PMN to collagens.

Antibodies, Monoclonal↗

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↗

PubMed1995

Postural vascular response vs. sympathetic vasoconstriction in human skin during orthostasis.

Sympathetic activation and local vascular smooth muscle reactions to vessel distension contribute to the increase in vascular resistance in the skin during orthostasis. The relative contribution of these two mechanisms to the changes of skin blood flow along the body axis on standing was investigated in healthy male subjects by laser-Doppler (LD) fluxmetry. Compared with recumbency, LD flux (LDF) in the standing subjects was reduced by -19.6 +/- 7.2% at the forehead and by -69.6 +/- 9.6% in the leg. In the absence of hydrostatic pressure changes, the LDF changes on standing averaged -29 +/- 13%, independent of skin region, reflecting the effect of vasoconstriction due to sympathetic activation. The postural vascular response, elicited by lowering the arm or the leg from heart level, was significantly attenuated in orthostasis compared with recumbency. The vessel reaction to local alteration of transmural pressure was studied in the skin of the forehead and lower leg by application of external pressure in supine subjects. No difference in vessel responsiveness to changes of transmural pressure was found between these skin sites. The findings suggest that the changes of skin perfusion in orthostasis result from a nonadditive interaction of height-dependent and -independent mechanisms.

Adult↗

PubMed1995

Structure and hemodynamics of microvascular networks: heterogeneity and correlations.

The objective of this study was to quantify the heterogeneity of topological, morphological, and hemodynamic parameters in microvascular networks and to identify functionally relevant correlations among these parameters. Seven networks in the rat mesentery (383-913 vessel segments per network) were examined, and measurements were made of segment generation, diameter, length, and hematocrit in all segments (n = 3,129) and of flow velocity (only in 3 networks, 1,321 segments). In addition, hematocrit, flow rate, and pressure were derived for all segments from a mathematical simulation. All parameters obtained exhibit heterogeneous distributions with coefficients of variation ranging from 0.28 (capillary diameter) to > 1.5 (volume flow and pressure gradient). Several strong correlations exist between parameters, e.g., discharge hematocrit increases with vessel diameter, and shear rate increases with intravascular pressure. Because of such correlations, the extrapolation from average values for "typical vessels" to network properties can lead to substantial errors. For example, the mean network transit time estimated based on averaged quantities is 6.5 s, which is about 60% higher than the true value (4.08 s). Simplified models of the vascular bed may therefore be inadequate to describe functional properties of the microcirculation.

Animals↗

PubMed1995

Design principles of vascular beds.

Hemodynamic parameters were determined in each vessel segment of six complete microvascular networks in the rat mesentery by using a combination of experimental measurements and theoretical stimulations. For a total number of 2592 segments, a strong unified dependence of wall shear stress on intravascular pressure for arterioles, capillaries, and venules was obtained. All three types of segments exhibit an essentially identical variation of shear stress from high to low values (from approximately 100 to 10 dyne/cm2) as intravascular pressure falls from 70 to 15 mm Hg. On the basis of these observations, it is proposed that vascular beds grow and adapt so as to maintain the shear stress in each vessel at a level that depends on local transmural pressure. In contrast to Murray's classic 'minimum-cost' hypothesis, which implies uniformity of wall shear rate throughout the vasculature, the proposed design principle provides an explanation for the functionally important arteriovenous asymmetry of wall shear rates and flow resistance in the circulation.

Animals↗

PubMed1995

Fluid, electrolyte, and packed cell volume shifts in racing greyhounds.

Arterial blood samples were obtained at rest, just before, and 5 minutes after a 704-m race, to quantify changes in hematologic variables, plasma electrolyte and protein concentrations, osmolality, and acid/base variables. Changes in plasma volume were estimated from the change in plasma protein concentration. Immediately prior to the race, plasma volume decreased by 10% from rest and total circulating RBC volume increased by 60%, attributable to increased RBC number rather than size. Increases in blood volume (VB) by 24% and PCV by 29% also were detected before the race. Five minutes after the race, plasma volume was 21% below the resting value and total circulating RBC volume had increased 73% above the resting value, resulting in a 40% increase in PCV. Contraction of the spleen appeared responsible for increased PCV and VB before the race and maintenance of VB after the race. Plasma chloride concentration was the same before and after the race; the chloride content of the plasma decreased by the same fraction (22%) as did the plasma volume, indicating Cl- loss from the plasma. Plasma Na+ content decreased by a smaller fraction (13%), causing Na+ concentration to increase from 151 mEq/L at rest to 167 mEq/L after the race. Assuming that Na+ concentration was the same throughout the extracellular fluid, H2O likely moved into the intracellular compartment. As a consequence of these changes, the inorganic strong ion difference in plasma increased by about 16 mEq/L, tending to minimize the acid/base disturbance induced by the 33 mEq/L increase in lactate concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid-Base Equilibrium↗

PubMed1994

Cross-linking of CD18 in human neutrophils induces an increase of intracellular free Ca2+, exocytosis of azurophilic granules, quantitative up-regulation of CD18, shedding of L-selectin, and actin polymerization.

Polymorphonuclear leukocytes (PMNs) exert most of their physiological functions while adherent to surfaces rather than in suspension. PMN adhesion is largely dependent on the function of the beta 2 integrins, CD11a,b,c/CD18. We mimicked engagement of beta 2 integrins by antibody cross-linking of CD18 on isolated human PMNs using both intact monoclonal antibody and F(ab')2 fragments. Within seconds of CD18 cross-linking, we observed a significant, transient rise of intracellular free Ca2+ concentration by 200-300 nM, which was largely due to Ca2+ mobilization from intracellular stores. The Ca2+ signal was blocked after pretreatment with phorbol myristate acetate, an activator of protein kinase C, but not with herbimycin A, a potent inhibitor of tyrosine kinases. In addition to the rise of intracellular free Ca2+ concentration, CD18 cross-linking induced exocytosis of azurophilic granules (release of 26% of total PMN elastase), which was significantly inhibited by herbimycin A. Moreover, 2.2-fold up-regulation of CD18 antigen and significant down-regulation of surface expression of the granulocyte adhesion molecule L-selectin were induced. Granulocyte F-actin content as measured by nitrobenzoxadiazole-phallacidin increased significantly 1 min after CD18 cross-linking. By contrast, CD18 cross-linking by soluble antibodies did not induce superoxide production, but PMNs bound to immobilized monoclonal antibodies against CD18 released significant amounts of superoxide. Initial signaling through beta 2 integrins does not appear to be mediated by a phospholipase C isoform activated through tyrosine phosphorylation, because the Ca2+ signal was not altered by herbimycin A. However, more complex cellular responses including exocytosis were found to require tyrosine phosphorylation. We show that engagement of beta 2 integrins provides an important stimulatory signal to PMNs inducing degranulation, modulation of L-selectin, and cytoskeletal changes.

Actins↗

PubMed1994

Resistance to blood flow in microvessels in vivo.

Resistance to blood flow through peripheral vascular beds strongly influences cardiovascular function and transport to tissue. For a given vascular architecture, flow resistance is determined by the rheological behavior of blood flowing through microvessels. A new approach for calculating the contribution of blood rheology to microvascular flow resistance is presented. Morphology (diameter and length), flow velocity, hematocrit, and topological position were determined for all vessel segments (up to 913) of terminal microcirculatory networks in the rat mesentery by intravital microscopy. Flow velocity and hematocrit were also predicted from mathematical flow simulations, in which the assumed dependence of flow resistance on diameter, hematocrit, and shear rate was optimized to minimize the deviation between measured and predicted values. For microvessels with diameters below approximately 40 microns, the resulting flow resistances are markedly higher and show a stronger dependence on hematocrit than previously estimated from measurements of blood flow in narrow glass tubes. For example, flow resistance in 10-microns microvessels at normal hematocrit is found to exceed that of a corresponding glass tube by a factor of approximately 4. In separate experiments, flow resistance of microvascular networks was estimated from direct measurements of total pressure drop and volume flow, at systemic hematocrits intentionally varied from 0.08 to 0.68. The results agree closely with predictions based on the above-optimized resistance but not with predictions based on glass-tube data. The unexpectedly high flow resistance in small microvessels may be related to interactions between blood components and the inner vessel surface that do not occur in smooth-walled tubes.

Animals↗

PubMed1994

Haemorrheology and long term exercise.

In general, the small number of haemorrheological studies concerning acute changes in long term exercise reveals remarkable small effects, given the substantial changes of pertinent cardiorespiratory parameters. To a large extent, this appears to result from careful maintenance of extra- and intravascular water balance and adequate control of electrolytes. Haemorrheological alterations during long term exercise depend on haematological parameters, especially haematocrit and plasma protein levels, which are acutely changed. During exercise, only small deviations of haematocrit from resting values are seen in the short term. This is explained by the usually constant plasma volume which depends on the amount of fluid intake during exercise. In contrast, the exercise-induced elevation of total intravascular protein content significantly increases the levels of plasma proteins, with the exception of fibrinogen. Although this leads to an increase of plasma viscosity, the absence of substantial haematocrit changes accounts for the remarkably small alterations, if any, of blood viscosity which are observed during long term exercise. Endurance training causes haemodilution by expansion of plasma volume, thus resulting in a reduction of blood and plasma viscosity. Red cell deformability is variable with training as well as during exercise. This appears to be related to methodological problems of measurement as well as to the type of endurance exercise performed. Physiological considerations suggest that the importance, for aerobic work capacity, of haemorrheological changes occurring in long term (submaximal) exercise may be limited. By contrast, this may be different during maximal exercise activity, when the entire cardiovascular reserve has been fully recruited.

Blood Viscosity↗