Biomedical subjects

C C Michel

Publications and source records attributed to C C Michel.

At least 37 records · Page 2Linked to original sources

PubMed1997

Endothelial gaps: time course of formation and closure in inflamed venules of rats.

In the rat trachea, substance P causes rapid but transient plasma leakage. We sought to determine how closely the number, morphology, and size of endothelial gaps correspond to the time course of this leakage. Endothelial gaps were examined by scanning electron microscopy (EM), by transmission EM, or by light microscopy after silver nitrate staining. Substance P-induced leakage of the particulate tracer Monastral blue peaked at 1 min but decreased with a half-life of 0.3 min. The number of silver-stained gaps also peaked at 1 min then decreased significantly more slowly (half-life 1.9 min) than the leakage. Scanning EM revealed two types of endothelial gaps, designated vertical gaps and oblique slits. Vertical gaps predominated at peak leakage, whereas oblique slits became more common as the leakage diminished. Measurements of the mean diameter of vertical gaps made by light microscopy, scanning EM, and transmission EM were all in the range of 0.36-0.47 micron. Fingerlike endothelial cell processes that appeared during gap formation became shorter as the leakage diminished (mean length: 1.44 microns at 1 min compared with 1.06 microns at 3 min after substance P), suggesting a role in gap closure. We conclude that the plasma leakage occurring immediately after an inflammatory stimulus results from the rapid formation of endothelial gaps. Multiple factors, including alterations in gap morphology, gap closure, and changes in driving force, are likely to participate in the rapid decrease in the leakage.

Animals↗

PubMed1997

Effects of anastomoses on solute transcapillary exchange in countercurrent systems.

OBJECTIVE: To investigate effects of anastomoses between the descending vasa recta (DVR) and the ascending vasa recta (AVR) on the distribution of small solutes in the interstitial fluid of the renal medulla. METHODS: Countercurrent capillary loops, surrounded by a secretory epithelium, were used to model microvessels in the renal medulla. Anastomoses between the DVR and AVR were modeled as a decrease in cross-sectional area of the vessels and a decrease in flow velocity from the base to the tip of the capillary loop. When experimental data were used to evaluate parameters of the model, it was seen that diffusive transport of solute in the axial direction of the capillary was negligible, and the equations could be greatly simplified. RESULTS: General formulae of the solute concentration distribution were derived for different degrees of shunting between the two limbs of the capillary loop. Analytical solutions for the steady-state solute distribution were obtained when the sizes of capillaries and flows in them were assumed to decrease linearly with the distance from the base to the tip of the capillary loop. When the effects of reduction in the size of the limbs were compared with the effects of reduction of flow velocities on solute distribution, it was found that, in the presence of anastomoses, change in flow velocity increases the axial gradient of the solute concentration more than change in the cross-sectional area. The combined effects of a decrease in flow velocity and cross-sectional area can easily double the axial gradient of the solute concentration for a modest degree of anastomotic shunting. CONCLUSIONS: In this study, we separated effects of anastomotic flow between the DVR and AVR from other factors affecting the complicated countercurrent solute exchanges in the renal medulla. Results from the model show that anastomoses increase the solute concentration in the medullary interstitium and also the axial gradient of the solute concentration there.

Animals↗

PubMed1996

Openings in frog microvascular endothelium induced by high intravascular pressures.

1. We have investigated the effects of microvascular pressures, high enough to rupture the vessel wall, upon the ultrastructure and permeability of mesenteric capillaries in pithed frogs. 2. The vessels were perfused via micropipettes with Ringer solutions containing bovine serum albumin (1 mg ml-1) and a few human red cells. After a perfused section of vessel had been closed downstream, pressure (applied via the micropipette) was raised in a series of steps of 10 mmHg, each lasting approximately 10 s, until the vessel ruptured. Fluid filtration through the vessel wall prior to rupture was estimated from the movements of the red cells. 3. Seven vessels were fixed in glutaraldehyde immediately after rupture and prepared for electron microscopy. The electron micrographs revealed openings in the vessel walls and thirty-six of these gaps were completely defined in runs of serial sections made on four of the vessels. Twenty-nine of these gaps passed through the endothelial cells (transcellular) and seven were intercellular. 4. The pressure at which a vessel ruptured, Pb, was measured in twenty-nine vessels and had a mean +/- S.E.M. value of 79.6 +/- 5.0 cmH2O. In ten of these vessels, which had a mean +/- S.E.M. Pb of 84.2 +/- 6.5 cmH2O, microvascular pressure was lowered immediately after the initial rupture and the vessel perfused at a pressure of 20 cmH2O. Pb was then remeasured and found to be 69.9 +/- 8.4 cmH2O, which was not significantly different from its initial value. 5. Hydraulic permeability (Lp) was measured in six vessels over the range of 15-30 cmH2O before and 10 min after the vessel wall ruptured at high pressure. Mean values were 5.5 x 10(-7) and 4.0 x 10(-7) cms-1 cmH2O-1 and were not significantly different. 6. At pressures equal to and 10 mmHg below Pb, small short-lived increases in filtration rate were observed. It is suggested that these may correspond to the increased permeability to fluid and macromolecules observed at high microvascular pressures in intact capillary beds.

Animals↗

PubMed1996

Theoretical studies of steady-state transcapillary exchange in countercurrent systems.

OBJECTIVE: As a first step in modeling microvascular exchange in the renal medulla, we developed mathematical models to explore the effects of blood flow, permeability, and anatomical arrangement of microvessels on the steady-state distribution of solute in the blood and the interstitial fluid (ISF). METHODS: Single capillaries and countercurrent capillary loops were used to model microvessels that were surrounded by a secretory epithelium over either the whole or part of the capillary length. Solute concentration in the vessels and the ISF were derived analytically. We also derived approximate solutions that ignored axial diffusion of solute. RESULTS: The full and approximate solutions were in good agreement with data based on measurements in the renal medulla. Model results revealed that concentration in the ISF falls rapidly with distance beyond the region of solute secretion and equilibrates with the concentration in capillaries, even with countercurrent exchange between the two limbs of the capillary loop. The ratio of the product of the permeability and area to the flow of the afferent limb, gamma 1, is an important parameter. When gamma 1 > 4, countercurrent exchange in a capillary loop facilitates a greater ISF concentration gradient than with a single capillary. Changes in flow also have a greater effect on this gradient. CONCLUSION: The model of countercurrent exchange presented here not only demonstrates the sensitivity of interstitial concentration gradients of solute to flow through capillary loops but also reveals the importance of the absolute value of gamma 1 in determining the magnitude and direction of these gradients.

Animals↗

PubMed1996

Perfusion of single tumor microvessels: application to vascular permeability measurement.

OBJECTIVE: To develop a new method for determining the relative importance of convection versus diffusion in macromolecular transport across tumor microvessel walls. METHODS: The human colon adenocarcinoma LS174T was transplanted in the dorsal skinfold chamber in a severe combined immunodeficient (SCID) mouse. The vasculature at the tumor surface was exposed by carefully removing the glass window of the chamber. A tumor microvessel was randomly selected, which was approximately 20-40 microns in diameter, embedded in the connective tissue 10-12 microns below the surface of the tumor. The vessel was cannulated with a micropipette and perfused with fluorescein isothiocyanate (FITC)-labeled bovine serum albumin (BSA) at different perfusion pressures. The fluorescence intensity was recorded on videotapes via a video system attached to the fluorescence microscope for offline analysis. The apparent vascular permeability was determined based on the time-dependence of fluorescence intensity and the vessel diameter. RESULTS: The apparent vascular permeability of single vessels to FITC-labeled BSA was quantified at perfusion pressures of 20-45 cmH2O. The pressure dependence of vascular permeability in LS174T tumors was heterogeneous. On average, there was no correlation between the apparent vascular permeability and the perfusion pressure in the range of 20-35 cmH2O (p = 0.73), even though the apparent permeability increased significantly when the pressure was increased from 20 to 45 cmH2O (p = 0.008). CONCLUSIONS: These results indicate that convection in the transvascular transport of albumin is not significant in non-peripheral regions of solid tumors in which the pressure difference across the vessel wall is small or even negligible. In addition to the permeability studies, this preparation can be used to study cell-cell interactions in single tumor vessels under defined flow conditions.

Adenocarcinoma↗

PubMed1996

Transport of macromolecules through microvascular walls.

This review addresses the long-standing controversy over the principal mechanisms of transport of macromolecules through the endothelium of microvessels of 'normal' permeability. Two types of mechanism have been proposed: convective transport through 'large pores' in the endothelium: transport via vesicles (transcytosis). The different techniques for estimating microvascular permeability to macromolecules are described and values for microvascular permeability to serum albumin in different tissues are tabulated. Whereas the evidence for convective transport when obtained from experiments on perfused microvascular beds remains convincing, attention is drawn to recent measurements using the tracer uptake technique which suggest that transport in the intact circulation from blood to tissues may not be coupled to fluid movement. Direct evidence for the involvement of endothelial vesicles in transendothelial transport has been reported relatively recently but the mechanisms whereby macromolecules are conveyed through the vesicular system have yet to be established. The possibility of convective transport through transient transendothelial channels formed by the fusion of vesicles is discussed.

Animals↗

PubMed1996

[General principles of oxygen transport in the venous wall].

OBJECTIVE: To review the mechanisms of oxygen transport in the walls of arteries and veins and to consider whether oxygen transport might limit oxygen consumption in the venous wall. METHODS: (i) To review evidence for the contributions of transport from the vascular lumen and transport from the vasa vasorum to the supply of oxygen to the walls of arteries. (ii) To use the general principles of oxygen diffusion through tissues to extrapolate the findings of investigations of oxygen transport into arterial walls to transport into the walls of the larger veins. RESULTS AND DISCUSSION: The magnitude of oxygen transport from the vessel lumen into the vessel wall is dependent on the partial pressure of oxygen (PO2) in the vessel lumen. While luminal PO2 is constant and high in arteries, it is a physiological variable in veins of healthy people. In larger veins, there is evidence that flow in the vasa vasorum increases as PO2 in the vessel lumen falls. The extent to which this regulatory action of the vasa vasorum may be comprised by raised venous pressure and venous distension has not been defined.

Animals↗

PubMed1995

Transcellular gaps in microvascular walls of frog and rat when permeability is increased by perfusion with the ionophore A23187.

1. The experiments described in this paper aimed to determine whether the gaps which develop in microvascular endothelium in association with increases in permeability are located in the intercellular clefts or are openings passing through the endothelial cells. 2. Hydraulic permeability (Lp) was estimated in frog mesenteric capillaries and single rat venules using a microperfusion-micro-occlusion technique before and during perfusion with solutions containing the ionophore A23187 at a concentration of 10 microM. When Lp was seen to have increased, the tissues were fixed in situ with 2.5% glutaraldehyde. 3. The increases in Lp varied considerably from vessel to vessel. In six frog vessels Lp increased from 2.6 +/- 0.9 x 10(-7) to 266 +/- 159 x 10(-7) cm s-1 cmH2O-1 and in three rat venules Lp rose from 0.94 +/- 0.09 x 10(-7) to 16.4 +/- 4.9 x 10(-7) cm s-1 cmH2O-1 (means +/- S.E.M.). 4. Forty openings or gaps were completely reconstructed from electron micrographs of serial ultrathin sections of the six frog vessels. Thirty-nine of these gaps passed through the endothelial cells and did not communicate with the intercellular clefts; one was intercellular. Similarly, fifteen out of sixteen gaps reconstructed from electron micrographs of the rat venules were transcellular and clearly separated from the intercellular clefts. 5. The increased Lp and associated ultrastructural changes induced by A23187 were reversed by perfusion with ionophore-free solutions.

Animals↗

PubMed1995

Fluid uptake from the renal medulla into the ascending vasa recta in anaesthetized rats.

1. We have investigated fluid movements between superficial ascending vasa recta (AVR) and the interstitium of exposed papillae of the renal medullae in 15-day-old Sprague-Dawley rats anaesthetized with Hypnorm and Hypnovel. 2. Using a development of the red cell micro-occlusion technique, fluid filtration and reabsorption rates per unit area of vessel wall (Jv/A) were determined in 54 single perfused AVR at known microvascular pressures (Pc). The relation between Jv/A and Pc was non-linear suggesting hydraulic permeabilities (Lp) of 50-100 x 10(-7) cm s-1 cmH2O-1 when Pc was between 0-10 cmH2O and 150-200 x 10(-7) cm s-1 cmH2O-1 when Pc was 10-15 cmH2O. 3. Rates of fluid reabsorption into the AVR estimated by a densitometric technique in a further fourteen vessels were consistent with Lp values of 50-100 x 10(-7) cm s-1 cmH2O-1 when Pc was -2 to 0 cmH2O. 4. The effective oncotic pressures of perfusates containing bovine serum albumin (BSA) were consistent with minimum values for the reflection coefficients of the walls of the AVR to BSA of between 0.59 and 0.72. 5. The concentration of native serum albumin in the papillary interstitial fluid was 9.1 +/- 0.6 mg ml-1 (mean +/- S.E.M., n = 16, from 9 rats), which is approximately 25% of the plasma level. 6. After their microinjection into the medullary interstitium, Patent Blue V and Evans Blue-albumin cleared within 1 min. There was no evidence of preferential movement of either dye towards the base of the exposed renal medulla. 7. Because Lp of the AVR is high, mean pressures of only approximately 3 cmH2O are necessary to account for the total clearance of fluid from the medullary interstitium into the AVR. From published data and from our own observations, it appears that differences in hydrostatic and oncotic pressure across the walls of the AVR are more than sufficient to provide this driving force. The possibility of the clearance of protein from the interstitium into the AVR is discussed.

Absorption↗

PubMed1995

Subatmospheric closing pressures in individual microvessels of rats and frogs.

1. We have investigated the hypothesis that ascending vasa recta (AVR) in the rat renal medulla are able to remain open when the external pressure is greater than the internal. 2. Individual vasa recta were cannulated in anaesthetized rats with Evans Blue albumin solution and then occluded downstream prior to the first branchpoint. When the intraluminal pressure was lowered, the lumina collapsed at a mean pressure of approximately -4.0 cmH2O for both AVR and descending vasa recta. 3. The studies were extended to include microvessels from rat spinotrapezius muscle and mesentery and frog mesentery; mean closing pressures were -3.2, -4.2 and -5.3 cmH2O, respectively. 4. Subatmospheric closing pressures may allow small differences in hydrostatic pressure alone to drive fluid uptake into the AVR.

Animals↗

PubMed1995

The measurement of permeability in single rat venules using the red cell microperfusion technique.

The red cell microperfusion-micro-occlusion technique has been used to measure fluid filtration and reabsorption (Jv/A) at known microvascular pressures (Pc) in single mesenteric venules of anaesthetized rats. The relation between Jv/A and Pc is linear over the range of Pc from 15 to 50 cmH2O and its slope is the hydraulic permeability, Lp. Reproducible estimates of Lp can be made in the same venule in separate microperfusions. The value of Pc at Jv/A = 0 varies linearly with perfusate oncotic pressure and is the effective oncotic pressure of the perfusate, sigma delta pi, when the hydrostatic pressure in the pericapillary fluid is zero. The mean value for Lp (+/- S.E.M.) in forty venules was 2.43 (+/- 0.2) x 10(-7) cm s-1 cmH2O-1. Two potential errors of the micro-occlusion technique (vessel distensibility and marker red cell size) were investigated. It was found that the effects of vessel distensibility had little effect on red cell movements at times later than 2 s after a step change in Pc. Red cell size had a potentially large effect on estimates of the absolute values of Lp. Cooling the mesenteric tissues from 37 to 7 degrees C reduced Lp in proportion to the change in the reciprocal of water viscosity with temperature. Rat venular permeability was shown to be sensitive to histamine, with Lp increasing and sigma delta pi falling in a concentration-dependent fashion with histamine concentrations of 1.6 x 10(-5) to 3.3 x 10(-4) mol l-1 in the perfusate.

Animals↗

PubMed1995

Renal medullary microcirculation: architecture and exchange.

The architecture of the renal medullary microcirculation is highly specialized. Consistent with their role in countercurrent exchange, the vessels (the vasa recta and the intervening capillaries) have high permeabilities to fluid and small hydrophilic solutes. The urea permeability of the continuous endothelium of the descending vasa recta (DVR) in the outer medulla is greatly enhanced by a urea transporter. Aquaporin channels have also been identified in these vessels. In spite of the absence of lymphatics from the inner medulla, fluid uptake from the interstitial fluid (ISF) through the fenestrated endothelium of the ascending vasa recta (AVR) appears to be driven by differences in hydrostatic and oncotic pressure. Because the AVR have high Lp's [10(-5) cm s-1 (cm H2O)-1] and are mechanically linked to surrounding structures, small increments of ISF pressures above the pressure within the AVR can drive significant volumes of fluid into AVR if ISF volume expands. The lower reflection coefficients to serum albumin of the AVR as compared with the DVR may be important in the clearance of interstitial plasma protein. Recent work on isolated DVR from the outer medulla has revealed that these vessels are capable of vasoconstriction and thus of regulating medullary blood flow.

Animals↗

PubMed1993

Effects of hydroxyethylrutosides on the permeability of microvessels in the frog mesentery.

1. We have investigated the effects of a standardised mixture of hydroxyethylrutosides (HR, Venoruton), a mixture of five of its main components (M) and each of the five components separately (7-mono-HR, 7,4'-di-HR, 7,3',4'-tri-HR, 5,7,3',4'-tetra-HR and 7,3'4'-tri HQ) upon the permeability of single perfused capillaries and venules in the mesenteries of pithed frogs. 2. In each experiment, the hydraulic permeability (Lp) of a single perfused microvessel and the effective osmotic pressure (sigma delta pi) exerted by macromolecules across its walls were estimated by a microcclusion technique, first during control perfusion and then in the presence of a known concentration of test substance. 3. HR, M and 7,4'-di-HR reduced Lp in a similar concentration-dependent manner over the range of 1 microgram ml-1 to 1 mg ml-1 (maximum reduction was to 40% of control Lp at 1 mg ml-1). At perfusate concentrations greater than 1 mg ml-1, these substances reduced Lp to a lesser extent. While the four other test substances reduced Lp significantly when their perfusate concentrations equalled or exceeded 100 micrograms ml-1, they were all less potent than 7,4'-di-HR. 4. The reduction in Lp induced by the mixture of flavonoids was only slightly reversed by subsequent perfusion with flavonoid-free solutions. 5. When permeability was increased by perfusing with protein-free solutions, both HR and 7,4'-di-HR reduced and then reversed the increase in Lp in a concentration-dependent manner over the range of 1 microgram ml-1 to 100 micrograms ml-1. None of the other component flavonoids was effective in restoring Lp under these conditions.

Animals↗

PubMed1993

Pathways through the intercellular clefts of frog mesenteric capillaries.

1. The three-dimensional ultrastructure of endothelial intercellular clefts of frog mesenteric capillaries of known hydraulic permeability (Lp) has been investigated in the absence and presence of lanthanum ions as tracers of extracellular solute. 2. Experiments were carried out on the exposed mesenteries of pithed frogs and Lp of a chosen microvessel perfused with a Ringer solution containing serum albumin (10-40 mg ml-1) was determined. In some experiments the mesentery was fixed in situ with 2.5% glutaraldehyde immediately after Lp had been measured. In other experiments, measurement of Lp was followed by brief microperfusion (10-20 s) with a second Ringer solution containing 1% lanthanum nitrate as a tracer before in situ fixation of the tissue. The tissue was prepared for electron microscopy using standard techniques. The perfused capillary was identified in the block and serial transverse sections were cut along its length over regions where Lp had been measured. 3. In six capillaries where the tissues were fixed immediately after measurement of Lp, Lp had a mean value (+/- S.E.M.) of 4 (+/- 0.5) x 10(-7) cm s-1 (cmH2O)-1. Serial (30-40 nm) sections of these vessels revealed that a single short narrow region of the intercellular clefts ran almost continuously from section to section. Additional tight regions were regularly seen, but they usually extended for relatively few sections. In 13.36 microns of reconstructed cleft, there were three interruptions of the tight region of 0.14, 0.14 and 0.17 microns respectively. In the region of these discontinuities, the wide region was uninterrupted from luminal to abluminal surface. 4. Examination of the tight junction on a tilting stage revealed that the outer leaflets of the adjacent cells were not fused, but separated by a gap of mean width (+/- S.E.M.) 2.3 (+/- 0.1) nm. 5. In four capillaries perfused with lanthanum nitrate before fixation, mean Lp (+/- S.E.M.) was 6.5 (+/- 0.02) x 10(-7) cm s-1 (cmH2O)-1. Segments of intercellular clefts, totalling 23.56 microns in length, were reconstructed from serial sections and throughout these, electron-dense deposits of lanthanum were observed to fill the luminal parts of the intercellular clefts up to the tight region. Lanthanum deposits filled the entire cleft to the abluminal surface at eleven sites, which accounted for a length of 2.52 microns out of the 23.56 microns. Only five of these regions were delimited within a continuous series of sections and their mean length (+/- S.E.M.) was 0.16 (+/- 0.063) microns.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Transcellular openings through microvascular walls in acutely inflamed frog mesentery.

Openings in the endothelium of frog mesenteric microvessels associated with acute inflammation following mild thermal injury have been examined by reconstruction from serial ultrathin sections. While all the openings lay close to the intercellular junctions only a minority (seven out of thirty-eight) were continuous with the intercellular clefts. The majority of gaps or openings (thirty-one out of thirty-eight) were transcellular and passed through the peripheral cytoplasm of one endothelial cell, separated from the intercellular cleft by a cellular process which was usually less than 2 microns wide.

Animals↗

PubMed1992

The transport of albumin: a critique of the vesicular system in transendothelial transport.

Although albumin circulating in the plasma can exchange with the contents of the extensive vesicular system of microvascular endothelium, the mechanism and quantitative significance of vesicular transport remains unclear. It has recently been shown that transport of electron opaque tracers can occur via the vesicular system, but the detailed ultrastructure is inconsistent with the transcytotic shuttling of single vesicles. Although most physiologic measurements show a major degree of convective coupling in the microvascular permeation of perfused tissues, recent reports suggest that convective coupling is low when albumin escapes from the intact circulation. Because albumin clearance in these studies is consistent with values calculated from ferritin-labeling of vesicles in perfused microvessels, the possibility remains that macromolecular transendothelial transport occurs through transient communications within the vesicular system.

Albumins↗

PubMed1989

The effects of ionophore A23187 on permeability of the frog mesentery microvasculature.

The effects of the divalent cation ionophore A23187, upon microvascular permeability have been investigated in single perfused capillaries and venules in the mesenteries of pithed frogs. In all experiments, the vessels were perfused with Ringer solutions containing the macromolecules Ficoll 70 (40 mg ml-1) and bovine serum albumin (10 mg ml-1). Hydraulic permeability (Lp) of the vessel walls and the effective osmotic pressure (sigma delta pi) exerted across them by the perfusate macromolecules were estimated by the method of Michel (1980). In eleven experiments addition of A23187 to the perfusate raised Lp from a mean value (+/- S.E.M.) of 4.76 (+/- 1.05) x 10(-7) cm s-1 cmH2O-1 to one of 11.97 (+/- 1.96) x 10(-7) cm s-1 cmH2O-1 and reduced sigma delta pi from a mean (+/- S.E.M.) of 16.5 (+/- 2.01) cmH2O to one of 7.6 (+/- 1.93) cmH2O. The effects of A23187 on Lp and sigma delta pi were reversible. A23187 was found to increase Lp and reduce sigma delta pi after the mesentery had been exposed to compound 48/80 to degranulate the mast cells two or more hours previously. A23187 also appeared to be effective in raising Lp and lowering sigma delta pi in the absence of Ca2+ ions from the perfusate and superfusate. The ionophore was found to reduce permeability, however, when Mg2+ ions were removed from the perfusate and superfusate both in the absence and in the presence of Ca2+. This reduction in permeability could be converted into an increase in Lp and fall in sigma delta pi if Mg2+ were restored to the superfusate. It is concluded that A23187 increases vascular permeability in a reversible manner and that Mg2+ ions in the extracellular fluids are necessary for its action.

Animals↗

PubMed1988

The ultrastructure of frog microvessels following perfusion with the ionophore A23187.

The ultrastructure of frog mesenteric capillaries and venules has been examined after the permeability of these vessels to fluid and macromolecules has been increased to a measured extent by perfusion with the ionophore A23187. An average 4.5-fold increase in hydraulic permeability in thirteen vessels was associated with the presence of gaps between the endothelial cells and marked attenuation of endothelial cytoplasm with the appearance of many fenestrations. The changes in ultrastructure suggested much larger increments in permeability than had been measured in these same vessels in vivo.

Animals↗