The effects of bovine serum albumin and a form of cationised ferritin upon the molecular selectivity of the walls of single frog capillaries

The glycocalyx as a permeability barrier: basic science and clinical evidence

Preclinical studies in animals and human clinical trials question whether the endothelial glycocalyx layer is a clinically important permeability barrier. Glycocalyx breakdown products in plasma mostly originate from 99.6–99.8% of the endothelial surface not involved in transendothelial passage of water and proteins. Fragment concentrations correlate poorly with in vivo imaging of glycocalyx thickness, and calculations of expected glycocalyx resistance are incompatible with measured hydraulic conductivity values. Increases in plasma breakdown products in rats did not correlate with vascular permeability. Clinically, three studies in humans show inverse correlations between glycocalyx degradation products and the capillary leakage of albumin and fluid.

The endothelial glycocalyx: Structure and function in health and critical illness

OBJECTIVE

To conduct a narrative review of the current literature in reference to the structure and function of the endothelial glycocalyx (EG) and its contribution to the pathophysiology of conditions relevant to the veterinary emergency and critical care clinician. Novel therapies for restoring or preserving the EG will also be discussed.

DATA SOURCES

Online databases (PubMed, CAB abstracts, Scopus) were searched between January 1st 2017 and May 1st 2017 for English language articles without publication date restriction. Keywords included EG, endothelial surface layer, degradation, syndecan-1, heparan sulfate, critical illness, sepsis, trauma, and therapeutics.

DATA SYNTHESIS

The EG is a complex and important structure located on the luminal surface of all blood vessels throughout the body. It plays an important role in normal vascular homeostasis including control of fluid exchange across the vascular barrier. Loss or degradation of the EG has an impact on inflammation, coagulation, and vascular permeability and tone. These changes are essential components in the pathophysiology of many conditions including sepsis and trauma. A substantial body of experimental animal and human clinical research over the last decade has demonstrated increased circulating concentrations of EG degradation products in these conditions. However, veterinary-specific research into the EG and critical illness is currently lacking. The utility of EG degradation products as diagnostic and prognostic tools continues to be investigated and new therapies to preserve or improve EG structure and function are under development.

CONCLUSIONS

The recognition of the presence of the EG has changed our understanding of transvascular fluid flux and the pathophysiology of many conditions of critical illness. The EG is an exciting target for novel therapeutics to improve morbidity and mortality in conditions such as sepsis and trauma.

Plasma for burn shock resuscitation: is it time to go back to the future?

Patients with burn shock can be challenging to resuscitate. Burn shock produces a variety of physiologic derangements: Patients are hypovolemic from volume loss, have a increased systemic vascular resistance, and may have a depressed cardiac output depending on the extent of the thermal injury. Additionally, the burn wound produces a significant inflammatory cascade of events that contributes to the shock state. Fluid resuscitation is foundational for the initial treatment of burn shock. Typical resuscitation is with intravenous lactated Ringer's in accordance with well-established formulas based on burn wound size. In the past century, as therapies to treat thermal injuries were being developed, plasma was the fluid used for burn resuscitation; in fact, plasma was used in World War II and throughout the 1950s and 1960s. Plasma was abandoned because of infectious risks and complications. Despite huge strides in transfusion medicine and the increased safety of blood products, plasma has never been readopted for burn resuscitation. Over the past 15 years, there has been a paradigm shift in trauma resuscitation: Less crystalloid and more blood products are used; this strategy has demonstrated improved outcomes. Plasma is a physiologic fluid that stabilizes the endothelium. The endotheliopathy of trauma has been described and is mitigated by transfusion strategies with a 1:1 ratio of RBCs to plasma. Thermal injury also results in endothelial dysfunction: the endotheliopathy of burns. Plasma is likely a better resuscitation fluid for patients with significant burn wounds because of its capability to restore intravascular volume status and treat the endotheliopathy of burns.

Comparative modeling of combined transport of water and graded-size molecules across the glomerular capillary wall

Chronic kidney disease is a common and growing problem worldwide that necessitates recognition of individual risk and appropriate laboratory testing before its progression to end-stage renal failure, requiring dialysis or transplantation for survival. Clearance studies using various graded-size probe molecules established that the passage of molecules/proteins across the glomerular capillary barrier of mammalian kidneys is increasingly restricted as their size increase. Few mathematical models were developed to describe the dynamics of the size-selective functions of macromolecules across membranes and gelatins. In the present study, we compare the behavior of three mathematical descriptions for the Fiber Matrix theory, an Extended Fiber Matrix theory, and an Alternative Statistical Physics analysis to describe the size-selective function of the glomerular capillary barrier; using mainly its hemodynamic, morphometric and hydrodynamic variables; in two experimental rat models. The glomerular basement membrane was represented as a homogeneous three-dimensional network of fibers of uniform length (Lf), radius (Rf), total fractional solid volume of fibers (Vf) and characteristic Darcy permeability. The models were appropriate for simulating in vivo fractional clearance data of neutral Dextran and Ficoll macromolecules from two experimental rat models. We believe that the Lf, Rf and Vf best-fit numerical values may signify new insights for the diagnosis of human nephropathies.

Nutritional and other types of oedema, albumin, complex carbohydrates and the interstitium - a response to Malcolm Coulthard's hypothesis: Oedema in kwashiorkor is caused by hypo-albuminaemia

The various types of oedema in man are considered in relation to Starling's hypothesis of fluid movement from capillaries, with the main emphasis on nutritional oedema and the nephrotic syndrome in children. It is concluded that each condition has sufficient anomalous findings to render Starling's hypothesis untenable. The finding that the endothelial glycocalyx is key to control of fluid movement from and into the capillaries calls for complete revision of our understanding of oedema formation. The factors so far known to affect the function of the glycocalyx are reviewed. As these depend upon sulphated proteoglycans and other glycosaminoglycans, the argument is advanced that the same abnormalities will extend to the interstitial space and that kwashiorkor is fundamentally related to a defect in sulphur metabolism which can explain all the clinical features of the condition, including the formation of oedema.

Albumin modulates S1P delivery from red blood cells in perfused microvessels: mechanism of the protein effect

Removal of plasma proteins from perfusates increases vascular permeability. The common interpretation of the action of albumin is that it forms part of the permeability barrier by electrostatic binding to the endothelial glycocalyx. We tested the alternate hypothesis that removal of perfusate albumin in rat venular microvessels decreased the availability of sphingosine-1-phosphate (S1P), which is normally carried in plasma bound to albumin and lipoproteins and is required to maintain stable baseline endothelial barriers (Am J Physiol Heart Circ Physiol 303: H825-H834, 2012). Red blood cells (RBCs) are a primary source of S1P in the normal circulation. We compared apparent albumin permeability coefficients [solute permeability (Ps)] measured using perfusates containing albumin (10 mg/ml, control) and conditioned by 20-min exposure to rat RBCs with Ps when test perfusates were in RBC-conditioned protein-free Ringer solution. The control perfusate S1P concentration (439 ± 46 nM) was near the normal plasma value at 37 °C and established a stable baseline Ps (0.9 ± 0.4 × 10(-6) cm/s). Ringer solution perfusate contained 52 ± 8 nM S1P and increased Ps more than 10-fold (16.1 ± 3.9 × 10(-6) cm/s). Consistent with albumin-dependent transport of S1P from RBCs, S1P concentrations in RBC-conditioned solutions decreased as albumin concentration, hematocrit, and temperature decreased. Protein-free Ringer solution perfusates that used liposomes instead of RBCs as flow markers failed to maintain normal permeability, reproducing the "albumin effect" in these mammalian microvessels. We conclude that the albumin effect depends on the action of albumin to facilitate the release and transport of S1P from RBCs that normally provide a significant amount of S1P to the endothelium.

Loss of the endothelial glycocalyx links albuminuria and vascular dysfunction

Patients with albuminuria and CKD frequently have vascular dysfunction but the underlying mechanisms remain unclear. Because the endothelial surface layer, a meshwork of surface-bound and loosely adherent glycosaminoglycans and proteoglycans, modulates vascular function, its loss could contribute to both renal and systemic vascular dysfunction in proteinuric CKD. Using Munich-Wistar-Fromter (MWF) rats as a model of spontaneous albuminuric CKD, multiphoton fluorescence imaging and single-vessel physiology measurements revealed that old MWF rats exhibited widespread loss of the endothelial surface layer in parallel with defects in microvascular permeability to both water and albumin, in both continuous mesenteric microvessels and fenestrated glomerular microvessels. In contrast to young MWF rats, enzymatic disruption of the endothelial surface layer in old MWF rats resulted in neither additional loss of the layer nor additional changes in permeability. Intravenous injection of wheat germ agglutinin lectin and its adsorption onto the endothelial surface layer significantly improved glomerular albumin permeability. Taken together, these results suggest that widespread loss of the endothelial surface layer links albuminuric kidney disease with systemic vascular dysfunction, providing a potential therapeutic target for proteinuric kidney disease.

Fluorescence correlation spectroscopy can probe albumin dynamics inside lung endothelial glycocalyx

The endothelial glycocalyx is believed to play a major role in capillary permeability by functioning as a macromolecular barrier overlying the intercellular junction. Little is known about the functional attributes of the glycocalyx (i.e., porosity and permeability) or which constituents contribute to its overall structure-function relationship. In this report, we demonstrate the utility of fluorescence correlation spectroscopy (FCS) to measure albumin diffusion rates and concentration profiles above the cell surface and overlying the intercellular junctions of lung capillary endothelial cells. Albumin diffusion rates and concentration profiles were obtained before and after enzymatic digestion of the glycocalyx with pronase, heparanase, or hyaluronidase. The results suggest a structure interacting with albumin located from 1.0 to 2.0 microm above the cell membrane capable of reducing albumin diffusion by 30% while simultaneously increasing albumin concentration fivefold. Digestion of the glycocalyx with pronase or heparanase resulted in only modest changes in albumin diffusion and concentration profiles. Hyaluronidase digestion completely eliminated albumin-glycocalyx interactions. These data also suggest that hyaluronan is a major determinant for albumin interactions with the lung endothelial glycocalyx. Confocal images of heparan sulfate and hyaluronan confirm a cell-surface layer 2-3 mum in thickness, thus supporting FCS measurements. In summary, we report the first use of FCS to probe extracellular structures and further our understanding of the structure-function relationship of the lung microvascular endothelial glycocalyx.

The Aqueous Outflow System as a Mechanical Pump

PURPOSE

To describe a new aqueous outflow model involving a mechanical pump.

MATERIALS AND METHODS

Laboratory materials include human and monkey eyes; methods include the dissecting microscope, light microscopy, scanning electron microscopy, transmission electron microscopy, and tracer studies. Clinical methods involve human subject slit lamp, gonioscopy, and operating microscope examination.

RESULTS

Laboratory evidence demonstrates that aqueous outflow tissues are responsive to intraocular pressure induced deformation. Deformation occurs in response to small pressure gradients. Laboratory evidence also demonstrates the presence of valves discharging aqueous to Schlemm's canal. The laboratory model predicts pulsatile aqueous discharge in vivo. Clinical in vivo evidence demonstrates pulsatile aqueous flow from the anterior chamber into Schlemm's canal, from Schlemm's canal into collector channels, and from Schlemm's canal into aqueous and episcleral veins, all synchronous with the ocular pulse.

CONCLUSIONS

Aqueous outflow tissue deformation caused by normal intraocular pressure transients induces pulsatile one-way discharge of aqueous to the vascular system. The model identifies biomechanical coupling of intraocular pressure with aqueous outflow tissue deformation and also sites of high flow capable of inducing shear stress. These mechanotransduction mechanisms, well characterized as a means of controlling pressure and flow in the vascular system, also provide a means of regulatory feedback to control intraocular pressure and aqueous flow.

Endothelial barriers: from hypothetical pores to membrane proteins

The anatomical counterpart of the physiologically defined small pore system of capillary endothelia has proved difficult to establish. In non-brain continuous capillaries, the contributions of caveolar and transmembrane pathways are likely to be small and paracellular clefts are probably the dominant routes. Analogy with epithelial paracellular pathways suggests that tight junctions may be the most restrictive elements. However, structural features of tight junction-based models are incompatible with physiological data; it is more likely that the tight junction acts as a shutter limiting the available cleft area. Proposed molecular sieves elsewhere in the paracellular pathway include the glycocalyx and the cadherin-based complexes of the adherens junctions. The molecular architecture of tight junctions and adherens junctions is moderately well defined in terms of molecular species, and there are differences at both sites between the endothelial and epithelial spectra of protein expression. However, definition of the size-restricting pore remains elusive and may require structural biology approaches to the spatial arrangements and interactions of the membrane molecular complexes surrounding the endothelial paracellular clefts.

Enhanced glomerular permeability to macromolecules in the Nagase analbuminemic rat

Plasma albumin restricts capillary water filtration. Accordingly, the glomerular ultrafiltration coefficient is higher in Nagase analbuminemic rats (NAR) than in Sprague-Dawley controls. We investigated whether the glomerular permeability to macromolecules is also enhanced in NAR. SDS-PAGE fractionation of urine proteins showed several bands with molecular masses between 60 and 90 kDa in NAR only. Acute administration of BSA to NAR led to nearly complete disappearance of these proteins from urine, an effect partially reversed when most of the exogenous albumin was cleared from circulation. The fractional clearance of 70-kDa dextran was increased in NAR, indicating a size defect. Binding of cationized ferritin to the glomerular basement membrane was decreased in NAR, suggesting associated depletion of fixed anions. The magnitude of cationic ferritin binding correlated negatively with the fractional clearance of 70-kDa dextran, suggesting that the two abnormalities may share a common pathogenic mechanism. Collectively, these results suggest enhanced glomerular permeability to macromolecules in NAR. Albumin may be necessary to maintain the normal glomerular permselectivity properties.

Quasi-periodic substructure in the microvessel endothelial glycocalyx: a possible explanation for molecular filtering?

The luminal surface of endothelial cells is lined with the glycocalyx, a network structure of glycoproteins probably 50 to 100 nm thick. It has been suggested that a relatively regular fibre-matrix structure may be responsible for the ultrafiltration properties of microvascular walls, both when the endothelium is continuous and when it is fenestrated. Positive structural evidence demonstrating an underlying periodicity in the glycocalyx has been hard to obtain. Here we present structural analysis of glycocalyx samples prepared in a variety of ways for electron microscopy. Using computed autocorrelation functions and Fourier transforms of representative areas of the electron micrograph images, we show that there is an underlying three-dimensional fibrous meshwork within the glycocalyx with characteristic spacings of about 20 nm. Together with a fibre diameter consistent with our observations of about 10-12 nm, the 20-nm spacing provides just the size regime to account satisfactorily for the observed molecular filtering; the observations are consistent with the fibre matrix model. We also show that the fibrous elements may occur in clusters with a common intercluster spacing of about 100 nm and speculate that this may reveal organisation of the glycocalyx by a quasi-regular submembranous cytoskeletal scaffold.

Effects of plasma proteins on sieving of tracer macromolecules in glomerular basement membrane

It was found previously that the sieving coefficients of Ficoll and Ficoll sulfate across isolated glomerular basement membrane (GBM) were greatly elevated when BSA was present at physiological levels, and it was suggested that most of this increase might have been the result of steric interactions between BSA and the tracers (5). To test this hypothesis, we extended the theory for the sieving of macromolecular tracers to account for the presence of a second, abundant solute. Increasing the concentration of an abundant solute is predicted to increase the equilibrium partition coefficient of a tracer in a porous or fibrous membrane, thereby increasing the sieving coefficient. The magnitude of this partitioning effect depends on solute size and membrane structure. The osmotic reduction in filtrate velocity caused by an abundant, mostly retained solute will also tend to elevate the tracer sieving coefficient. The osmotic effect alone explained only about one-third of the observed increase in the sieving coefficients of Ficoll and Ficoll sulfate, whereas the effect of BSA on tracer partitioning was sufficient to account for the remainder. At physiological concentrations, predictions for tracer sieving in the presence of BSA were found to be insensitive to the assumed shape of the protein (sphere or prolate spheroid). For protein mixtures, the theoretical effect of 6 g/dl BSA on the partitioning of spherical tracers was indistinguishable from that of 3 g/dl BSA and 3 g/dl IgG. This suggests that for partitioning and sieving studies in vitro, a good experimental model for plasma is a BSA solution with a mass concentration matching that of total plasma protein. The effect of plasma proteins on tracer partitioning is expected to influence sieving not only in isolated GBM but also in intact glomerular capillaries in vivo.

Vascular permeability in a human tumour xenograft: molecular charge dependence

Molecular charge is one of the main determinants of transvascular transport. There are, however, no data available on the effect of molecular charge on microvascular permeability of macromolecules in solid tumours. To this end, we measured tumour microvascular permeability to different proteins having similar size but different charge. Measurements were performed in the human colon adenocarcinoma LS174T transplanted in transparent dorsal skinfold chambers in severe combined immunodeficient (SCID) mice. Bovine serum albumin (BSA) and IgG were fluorescently labelled and were either cationized by conjugation with hexamethylenediamine or anionized by succinylation. The molecules were injected i.v. and the fluorescence in tumour tissue was quantified by intravital fluorescence microscopy. The fluorescence intensity and pharmacokinetic data were used to calculate the microvascular permeability. We found that tumour vascular permeability of cationized BSA (pI-range: 8.6-9.1) and IgG (pI: 8.6-9.3) was more than two-fold higher (4.25 and 4.65x10(-7) cm s(-1)) than that of the anionized BSA (pI approximately 2.0) and IgG (pI: 3.0-3.9; 1.11 and 1.93x10(-7) cm s(-1), respectively). Our results indicate that positively charged molecules extravasate faster in solid tumours compared to the similar-sized compounds with neutral or negative charges. However, the plasma clearance of cationic molecules was approximately 2x faster than that of anionic ones, indicating that the modification of proteins enhances drug delivery to normal organs as well. Therefore, caution should be exercised when such a strategy is used to improve drug and gene delivery to solid tumours.

Oxidized lipoproteins degrade the endothelial surface layer : implications for platelet-endothelial cell adhesion

BACKGROUND

Flowing erythrocytes and platelets are separated from the luminal endothelial cell (EC) surface by a 0.5-microm-wide space named the endothelial surface layer. We hypothesized that the disruption of the endothelial surface layer by oxidized low-density lipoproteins (Ox-LDL) contributes to atherogenic increases in vascular wall adhesiveness.

METHODS AND RESULTS

The hamster cremaster muscle preparation was used for intravital microscopic observation of the distance between erythrocytes and the capillary EC surface. Moderate Ox-LDL was prepared by exposing native LDL to CuSO(4) for 6 hours. The dimension of the EC surface layer averaged 0.6+/-0.1 microm during control situations, but a bolus intravenous injection of Ox-LDL (0.4 mg/100 g of body weight) transiently diminished the EC surface layer by 60% within 25 minutes, which correlated with a transient increase in the number of platelet-EC adhesions. Combined administration of superoxide dismutase and catalase completely blocked the effect of Ox-LDL on the dimension of the EC surface layer and inhibited platelet-EC adhesion.

CONCLUSIONS

Oxygen-derived free radicals mediate the disruption of the EC surface layer and increase vascular wall adhesiveness by Ox-LDL.

Microvascular permeability

This review addresses classical questions concerning microvascular permeabiltiy in the light of recent experimental work on intact microvascular beds, single perfused microvessels, and endothelial cell cultures. Analyses, based on ultrastructural data from serial sections of the clefts between the endothelial cells of microvessels with continuous walls, conform to the hypothesis that different permeabilities to water and small hydrophilic solutes in microvessels of different tissues can be accounted for by tortuous three-dimensional pathways that pass through breaks in the junctional strands. A fiber matrix ultrafilter at the luminal entrance to the clefts is essential if microvascular walls are to retain their low permeability to macromolecules. Quantitative estimates of exchange through the channels in the endothelial cell membranes suggest that these contribute little to the permeability of most but not all microvessels. The arguments against the convective transport of macromolecules through porous pathways and for the passage of macromolecules by transcytosis via mechanisms linked to the integrity of endothelial vesicles are evaluated. Finally, intracellular signaling mechanisms implicated in transient increases in venular microvessel permeability such as occur in acute inflammation are reviewed in relation to studies of the molecular mechanisms involved in signal transduction in cultured endothelial cells.

Secretion of primary urine by glomeruli of the hagfish kidney

Adrenaline and noradrenaline increased the perfusion pressure (Pperf) and single glomerulus filtration rate (SGFR) of perfused hagfish glomeruli. Small amounts (0.1 % or 0.5 %) of bovine serum albumin (BSA) in perfusion fluids containing Ficoll 70 did not diminish the loss of colloid from hagfish glomerular capillaries as has been reported for other perfused capillaries. However, replacement of Ficoll 70 with an osmotically equivalent amount (3 %) of BSA appreciably reduced colloid loss. It was concluded that adrenaline and colloids enhanced flow through the urine-forming capillaries. Whereas adrenaline elevated the SGFR, colloid lowered the SGFR probably by a direct effect on the fluid permeability of the capillary walls. The flow-enhancing effect of adrenaline was used to ensure the exposure of urine-forming capillaries to two inhibitors of active fluid transport, ouabain and 2,4-dinitrophenol (DNP). Both substances lowered the single glomerulus filtration fraction (SGFF), probably by affecting a fluid secretion mechanism. In addition, DNP diminished the flow-enhancing effect of adrenaline. This study provides relatively unequivocal evidence that fluid secretion underlies the formation of primary urine by the hagfish.

Chronic lymph flow responses to hyperproteinemia

The long-term responses of lymph flow, lymph protein transport, and the permeability-surface area (PS) product to hyperproteinemia have been studied in conscious dogs. Plasma protein concentration (PPC) was increased by daily intravenous infusion of previously collected autologous plasma for 9 days. Lymph flow was determined by collecting lymph chronically from a lymphatic afferent to the popliteal node in the hind leg. Compared with the average value during the normal-PPC period, the following changes occurred during 10 days of high PPC: lymph flow decreased from 12.3 +/- 1.1 to 3.8 +/- 0.6 microl/min, lymph protein transport decreased from 241 +/- 24 to 141 +/- 21 microg/min, PS product decreased from 4.7 +/- 0.5 to 3.0 +/- 0.5 microl/min, PPC increased from 7.1 +/- 0.1 to 8.8 +/- 0.4 g/dl, lymph protein concentration increased from 1.9 +/- 0.1 to 3.8 +/- 0.1 g/dl, plasma colloid osmotic pressure increased from 18. 6 +/- 0.8 to 24.2 +/- 2.1 mmHg, and lymph colloid osmotic pressure increased from 4.8 +/- 0.2 to 10.4 +/- 0.7 mmHg. In conclusion, long-term hyperproteinemia in dogs resulted in chronic decreases in lymph flow, lymph protein transport, and the PS product and chronic increases in lymph protein concentration and lymph colloid osmotic pressure. The marked decrease in lymph flow during hyperproteinemia decreased lymph protein transport and thus contributed to the increase in lymph protein concentration. In addition, the decreases in PS product and lymph protein transport suggest that transcapillary protein flux decreases during hyperproteinemia.

Effect of steady shear stress on fluid filtration through the rabbit arterial wall in the presence of macromolecules

1. Transport properties of the arterial wall in general and endothelial cells in particular are known to be sensitive to fluid flow generated shear stress. 2. The effect of steady wall shear stress was studied in vitro on transmural water filtration in the presence of a control perfusate (10 mg/mL bovine serum albumin) and a test perfusate (50 mg/mL Dextran 500). The latter is a macromolecule with certain physical properties comparable to those of the circulating low density lipoproteins. Dextran 500 is a neutral polysaccharide carrying no charge groups. 3. Male rabbits were anaesthetized with sodium pentobarbital and one of the common carotid arteries was cannulated and excised. 4. Fluid flux across the artery wall was measured at a constant transmural pressure (150 cm H2O) in the presence and absence of luminal flow of the perfusate. 5. Luminal flow produced an average increase by 21% in the filtration flux relative to its value under static conditions with 10 mg/mL bovine serum albumin in the perfusate. In the presence of 50 mg/mL Dextran 500 in the perfusate an average reduction by 36% was observed in fluid flux when luminal flow was imposed in the artery. 6. The increase in flux with the control perfusate may be due to a structural alteration of the glycocalyx fibre matrix caused by flow imposed shear stress. The observed decrease in flux with 50 mg/mL Dextran 500 may be accounted for by the deformation of the dextran polymer chains when subjected to shear stress. Although intact molecules are likely to be excluded by the glycocalyx, parts of the chain may insert themselves in the network and enhance the steric exclusion of water. Also, decreased hydration of the medial interstitium may increase medial resistance to fluid transport.

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.

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.

Effects of proteins on the permeability of monolayers of cultured bovine arterial endothelium

1. Monolayers of arterial endothelium on porous membranes were exposed to a pressure of 15 cmH2O at 37 degrees C, or of 30 cmH2O at 0 degree C. At constant pressure, the rate of liquid flow per unit area (Jv/A) through each monolayer decreased with time, in the way previously described for cultured endothelium. This phenomenon has been called sealing. After Jv/A stabilized, the pressure was reduced and the hydraulic permeability (Lp) of the endothelium was calculated from the relationship between Jv/A and pressure. Endothelium was seen to be damaged after some experiments at 37 degrees C, but appeared undamaged after experiments at 0 degree C. 2. Bovine serum albumin (BSA) did not influence the Lp of cultured endothelium. At 37 degrees C, the mean (+/- S.E.M.) endothelial Lp was 47.2 +/- 7.3 x 10(-7) cm s-1 cmH2O-1 (n = 10) in the presence of BSA (5 g (100 ml)-1). This is not significantly different from the mean (+/- S.E.M.) Lp of 53.4 +/- 9.0 x 10(-7) cm s-1 cmH2O-1 (n = 9) in the absence of added protein (P greater than 0.10). At 0 degree C also, there was no significant difference between mean Lps in the presence of BSA (0.1 g (100 ml)-1) and in the absence of added protein. 3. Solutions of BSA (5 g (100 ml)-1 or of the neutral polymer Ficoll 70 (4 g (100 ml)-1) did not exert any effective osmotic pressure across endothelium at 37 or 0 degrees C, respectively. 4. BSA (0.1 g (100 ml)-1) did not enable solutions of Ficoll 70 (4 g (100 ml)-1) to exert an effective osmotic pressure across endothelium at 0 degree C. 5. The mean Lp of endothelium at 0 degree C was significantly lower in the presence of cationized ferritin (CF; 0.1 g (100 ml)-1) than in the absence of added protein (P less than 0.001). Native ferritin (NF; 0.1 g (100 ml)-1) had no effect on Lp. 6. In the presence of CF (0.1 g (100 ml)-1), solutions of Ficoll 70 (4 g (100 ml)-1) exerted a mean effective osmotic pressure of 27.7 cmH2O (n = 5) across endothelium at 0 degree C. The mean effective osmotic pressure exerted across endothelium by solutions of Ficoll 70 (4 g (100 ml)-1) plus NF (0.1 g (100 ml)-1) was 1.2 cmH2O (n = 4).

Plasma proteins modify the endothelial cell glycocalyx of frog mesenteric microvessels

1. We have investigated the interaction of plasma proteins with the endothelial cell using cationized ferritin as a marker of the cell surface glycocalyx. 2. Single microvessels of the frog mesentery were sequentially perfused using glass micropipettes with solutions containing cationized ferritin (CF, 6.7 mg ml-1) in 0.10 M-NaCl and then with either frog plasma or bovine serum albumin (BSA; 50 or 10 mg ml-1), or protein-free Ringer solution, before suffusion fixation in 2.5% glutaraldehyde. 3. A layer of CF, usually two to four molecules thick, was associated with the luminal endothelial cell surface. In vessels post-flushed with protein-free Ringer solution the CF layer was closely adherent to all regions of the luminal endothelium, including the plasma membrane, vesicle diaphragms, coated pits and the entrances to clefts. However, when plasma was present during fixation the CF layer was separated from the cell surface by up to 100 nm over all regions. In vessels post-flushed with BSA the CF layer was also separated from the membrane but the effect was less striking. 4. The association of cationized ferritin with the endothelial cell surface was assessed quantitatively using electron micrographs of transverse sections (approximately 50 nm thick) of the perfused vessels, and expressed in terms of the depth of the layer of CF associated with the endothelial cell surface, its separation from the plasma membrane of the luminal endothelium, and the concentration of CF in the layer. The mean (+/- S.D.) separation in the presence of plasma, 32.3 +/- 10.5 nm (n = 12), was significantly greater (P less than 0.01) than that with either protein-free Ringer solution, 3.0 +/- 1.4 nm (n = 9), or BSA in Ringer solution, 8.3 +/- 3.0 nm (n = 8). The separation seen with BSA in Ringer was also significantly greater than that measured with a final Ringer solution perfusion (P less than 0.01). The effects of 10 and 50 mg ml-1 BSA were not different from one another. The total glycocalyx thickness, defined as the sum of the separation layer and depth of CF layer, with plasma present, 56.2 +/- 13.7 nm, was twice the value seen with Ringer solution, 28.0 +/- 9.1 nm (P less than 0.01), while the total thickness with BSA, 30.9 +/- 5.4 nm, was not different from the Ringer solution value.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of varying albumin concentration and hydrostatic pressure on hydraulic conductivity and albumin permeability of cultured endothelial monolayers

An in vitro model of the endothelial transport barrier was developed using bovine aortic endothelial cell monolayers cultured on a porous polycarbonate substrate. Hydraulic conductivity (Lp) was measured by a bubble tracking technique at varying pressure differentials and albumin concentrations. The effective albumin permeability (Pc) was determined by measuring the flux of fluorescent-labeled albumin across monolayers at varying hydrostatic pressures. Lp determined at pressure differentials between 5.0 and 10 cm H2O demonstrated a strong dependence on albumin concentration, decreasing approximately 10-fold from 21.3 x 10(-7) +/- 3.18 x 10(-7) cm/sec/cm H2O (mean +/- SEM) at 0.0 g/dl to 2.35 x 10(-7) +/- 0.20 x 10(-7) cm/sec/cm H2O at 1.0 g/dl albumin. Increasing the albumin concentration from 1.0 to 4.0 g/dl reduced Lp by an additional 16% to 1.97 x 10(-7) +/- 0.17 x 10(-7) cm/sec/cm H2O. Furthermore, Lp was moderately dependent on the pressure differential, increasing by about a factor of two with a doubling of the pressure differential. The effective permeability (Pc) was also dependent on the pressure differential. At an albumin concentration of 4.0 g/dl, Pc increased from 1.37 x 10(-6) +/- 0.26 x 10(-6) cm/sec at 0.0 cm H2O to 5.06 x 10(-6) +/- 1.92 x 10(-6) cm/sec at 10 cm H2O. Analysis of Pc and Jv data, however, demonstrates that water and albumin do not share a common pathway in crossing the endothelial monolayer. These data suggest the existence of a large pore pathway for albumin. Thus, the in vitro system has many of the transport characteristics of intact vessels in vivo and should be useful for physiological studies of the endothelial transport barrier.

Permeability of frog mesenteric capillaries after partial pronase digestion of the endothelial glycocalyx

1. The proteolytic enzyme pronase, which degrades the endothelial cell glycocalyx, was perfused through single capillaries of frog mesentery. Hydraulic conductivity (Lp) of each vessel was determined before and after pronase perfusion. In three vessels in which Lp increased, the ultrastructure of interendothelial clefts was examined. In a separate group of frogs the effect of pronase on the endothelial glycocalyx was assessed by using cationized ferritin to label the capillary luminal surface. 2. Control Lp was 2.0 x 10(-7) cm s-1 cmH2O-1 (10 mg ml-1 bovine serum albumin, BSA, in frog Ringer solution). Vessels were then perfused with a solution containing 0.1 mg ml-1 pronase and 10 mg ml-1 BSA for 1 min. Lp measured in the same eleven vessels increased to 4.9 x 10(-7) cm s-1 cmH2O-1 (P less than 0.005). 3. Transverse sections of three of these vessels were examined by transmission electron microscopy at eight sites along each vessel. In these sections a total of 156 interendothelial cell clefts were found and photographed. No morphological features, such as fenestrations, transendothelial channels, or intercellular gaps associated with inflammation, were found which might account for the increases in Lp. 4. Measurement of cleft dimensions yielded a harmonic mean cleft depth (delta x) of 0.32 microns and an arithmetic mean cleft depth of 0.64 microns. Mean width (w) of the clefts outside the tight regions was 0.012 microns and the cleft length per unit area (L) was 1330 cm-1. The mean fractional pore area of vessel wall per unit cleft depth, Ap/delta x, calculated as Lw/delta x, was 48.7 cm-1. 5. There was less cationic ferritin (CF) labelling of the luminal glycocalyx in pronase-perfused than in control capillaries. On average, the proportion of the luminal surface covered by CF was 85% in controls and 42% in pronase-treated capillaries (P less than 0.01). In some vessels the CF pattern was greatly disrupted, indicating large changes in the glycocalyx structure. 6. It is concluded that the moderate increases in Lp induced by pronase perfusion are associated with partial digestion of the endothelial glycocalyx but are not accompanied by changes in the dimensions of the intercellular cleft. These observations support the fibre matrix hypothesis of capillary permeability and suggest that the endothelial glycocalyx contributes as much as 60% of the hydraulic resistance of the capillary wall.

Effects of hydroxyethyl rutosides upon the permeability of single capillaries in the frog mesentery

1. We have investigated the effects of a standardised mixture of hydroxyethyl rutosides (HR) upon the permeability of the walls of single capillaries and venules of the frog mesentery. 2. In each experiment a single vessel was perfused via a micropipette with frog Ringer solutions containing bovine serum albumin (10 mg ml-1) and Ficoll 70 (40 mg ml-1) first in the absence of HR and then with HR added to the perfusate. The permeability of the vessel walls was assessed during each perfusion by using a development of the Landis micro-occlusion technique to estimate their hydraulic permeability (Lp) and the effective osmotic pressure (sigma delta pi) exerted across them by the perfusate macromolecules. 3. Measurements were made both in vessels which appeared to be healthy and in vessels showing signs of stasis or inflammation before perfusion. 4. HR at concentrations of 1.0, 0.1 and 0.01 mg ml-1 reduced hydraulic permeability to approximately half of its value in the absence of HR. It increased sigma delta pi to macromolecules at concentrations of 10, 1.0, 0.1, 0.01 and 0.001 mg ml-1. The effects of HR upon permeability were not reversed within 10 min of perfusion with an HR-free solution. 5. Ultrastructural examination of a number of vessels in which initial high values of permeability were reduced to values within the normal range of permeabilities by HR, showed clear signs of damage to the endothelium, with large gaps between adjacent endothelial cells. 6. These observations suggest that HR does reduce microvascular permeability both in healthy vessels and vessels showing of inflammation. The reduction in permeability of inflamed vessels does not appear to be the result of closure of the gaps between adjacent endothelial cells.

Capillary permeability and how it may change

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The effect of varying albumin concentration of the hydraulic conductivity of the rabbit common carotid artery

Rabbit common carotid arteries were cannulated in situ, after ligation of their branches, and transferred to a perfusion apparatus in such a way that they were maintained at their physiological dimensions and the endothelium remained intact. The vessels were pressurized to 150 cm H2O with Krebs solution and the wall smooth muscle was relaxed with 10(-4) M NaNO2. The rate of inflow of perfusate into the vessels was measured by following the movement of a bubble in a calibrated capillary which, when steady, was taken to indicate the transmural filtration rate. The filtration rate was 1.48 +/- 0.26 X 10(-6) cm sec-1 (11) (mean, SD, n) with 1 g/dl bovine serum albumin in Krebs solution. The values with 0, 4, 7, and 10 g/dl, normalized by the 1 g/dl value were 1.38 +/- 0.16 (7), 0.80 +/- 0.05 (9), 0.65 +/- 0.03 (8), and 0.47 +/- 0.06 (9), respectively. The hydraulic conductivity of the wall was also found to depend on perfusate albumin concentration. The 1 g/dl value was 0.92 +/- 0.17 X 10(-8) cm sec-1 (cm H2O)-1 (11) and the values with 0, 4, 7 and 10 g/dl normalized by the 1 g/dl value were 1.35 +/- 0.16 (7), 0.87 +/- 0.06 (9), 0.81 +/- 0.03 (8), and 0.72 +/- 0.06 (9), respectively. The findings were analyzed in relation to models involving interaction of albumin with the endothelial glycocalyx, concentration polarization at the blood/wall interface, dependence of flux on solvent viscosity and dependence of the porosity of the wall interstitium on solvent flux. Both concentration polarization and variation of the porosity of the wall interstitium provide reasonable quantitative explanations for the findings.

Inflammatory changes in permeability and ultrastructure of single vessels in the frog mesenteric microcirculation

1. In fifteen experiments, single microvessels in the exposed mesenteries of pithed frogs were perfused with Ringer solutions containing bovine serum albumin (40 mg ml-1). For each vessel, the hydraulic permeability of its walls (Lp) and effective osmotic pressure exerted across them (sigma delta pi) were determined from measurements of fluid filtration rates at two capillary pressures (Michel, Mason, Curry, Tooke & Hunter, 1974) before and at predetermined times after the tissue temperature had been raised abruptly from approximately 15 degrees C to 30-35 degrees C. Temperatures greater than 30 degrees C appear to damage the tissues of frogs acclimatized to temperatures of 5-10 degrees C. 2. In fourteen out of fifteen experiments Lp rose when the temperature was raised to 30-35 degrees C. In twelve of these experiments the increase in Lp was greater than expected from the fall in water viscosity with temperature, and was progressive. In five vessels where measurements were made 1-2 min after tissue temperature was raised, Lp increased from an initial mean value (+/- S.E.M.) of 3.76 (+/- 0.54) x 10(-3) micron s-1 cmH2O-1 to one of 8.72 (+/- 1.68) x 10(-3) micron s-1 cmH2O-1. In nine vessels where measurements were made at 10 min after tissue temperature was raised, Lp increased from an initial mean value of 4.03 (+/- 0.72) x 10(-3) micron s-1 cmH2O-1 to one of 16.9 (+/- 3.5) x 10(-3) micron s-1 cmH2O-1. Further increases in Lp were seen at 15 and 20 min. 3. The changes in the effective osmotic pressure opposing filtration, sigma delta pi, were very variable. Out of the twelve vessels which showed large changes in Lp with tissue heating, four showed no reduction in sigma delta pi after 10 min, though sigma delta pi fell in three of these vessels at 15 min. In the other eight vessels, sigma delta pi fell as Lp rose. A quantitative theory developed in this paper allowed the changes in sigma delta pi to be analysed in terms of a component across the regions of vessel wall of increased permeability (sigma H delta pi H) and a component across regions where Lp was unchanged. In six vessels sigma H delta pi H approximated to zero; in two vessels sigma H delta pi H was 6.2 and 4.5 cmH2O.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of chemical fixation on the permeability of frog mesenteric capillaries

1. We have investigated the effects on microvascular permeability of two chemical fixatives, glutaraldehyde/formaldehyde mixture and osmium tetroxide, both of which are used widely in electron microscopy. 2. The permeability of single perfused frog mesenteric capillaries was assessed using the technique of Michel (1980) to estimate the hydraulic conductance (Lp) of the vessel walls and the effective osmotic pressure (sigma delta pi) which could be exerted across them by the neutral macromolecule, Ficoll 70. 3. In each experiment, Lp and sigma delta pi were estimated in the same capillary before and after chemical fixation with either an aldehyde mixture (n = 6) or 1% osmium tetroxide (n = 6). In the absence of serum proteins in the perfusate, the values of Lp recorded prior to fixation were high. The effect of both fixatives was to reduce Lp. With aldehydes the mean value (+/- S.E.M.) of Lp fell from 22.1 (+/- 6.3) x 10(-3) to 5.7 (+/- 2.0) x 10(-3) microns s-1 cmH2O-1. With osmium tetroxide the mean Lp fell from 29.1 (+/- 8.5) x 10(-3) to 8.6 (+/- 3.6) x 10(-3) microns s-1 cmH2O-1. 4. The perfusates contained the neutral macromolecule, Ficoll 70, at a concentration of 60 mg ml-1 which exerted an osmotic pressure of 35 cmH2O in a membrane osmometer. In the absence of plasma proteins in the perfusate, sigma delta pi was only a fraction of this perfusate oncotic pressure prior to fixation. After aldehyde treatment there was a small but insignificant decrease in sigma delta pi. Osmium tetroxide, however, increased sigma delta pi from a mean (+/- S.E.M.) of 6.3 (+/- 1.6) cmH2O before fixation to 16.4 (+/- 2.8) cmH2O after fixation, P less than 0.01 using a paired t test. 5. In three experiments, the reflection coefficient of the capillary wall to NaCl sigma NaCl, was measured using the technique of Curry, Mason & Michel (1976) before and after osmium fixation. Fixation raised sigma NaCl from a mean value (+/- S.E.M.) of 0.009 (+/- 0.001) to one of 0.017 (+/- 0.004). Although these changes may be interpreted as an increase in the contribution to the total Lp of the capillary wall of channels which are available only to water, the changes are too small for such a mechanism to account for the increase in reflection coefficient to Ficoll 70. 6. The reduction in Lp following chemical fixation could represent an increase in the hydraulic resistance of either the capillary wall itself, or in the tissues surrounding the vessel.(ABSTRACT TRUNCATED AT 400 WORDS)

Steady-state fluid filtration at different capillary pressures in perfused frog mesenteric capillaries

1. The theory of steady-state filtration through capillary walls (Michel, 1984) has been developed and investigated in experiments on single capillaries of the frog mesentery perfused with Ringer solutions containing bovine serum albumin (BSA) and Ficoll 70. 2. In each experiment, the micro-occlusion technique of Michel, Mason, Curry, Tooke & Hunter (1974) has been used to investigate the relation between fluid movements per unit area of capillary wall (Jv/A) and capillary pressure (Pc) under two sets of conditions in the same vessel. First, the relation has been determined following brief perfusions, where the difference in oncotic pressure across the capillary wall approximated to the perfusate oncotic pressure at all values of Pc. These results are referred to as the transient data. Secondly, the relation was investigated by estimating Jv/A at values of Pc which had been maintained constant during at least 2 min of perfusion prior to the measurement. Under these conditions the concentration of macromolecules in the pericapillary fluid was determined by the steady-state composition of the filtrate passing through the capillary wall, and these results are referred to as steady-state data. 3. In all fifteen capillaries investigated, the relationship between Jv/A and Pc was linear for the transient data and conspicuously non-linear in the steady state. When Pc exceeded the oncotic pressure of the perfusate, steady-state values for Jv/A lay slightly above but parallel to the transient values for the same vessel. At values of Pc less than the perfusate oncotic pressure, the transient data showed reabsorption of fluid from the tissues, but in the steady state either fluid movements were so small as to be undetected or slight filtration was observed. The steady-state data followed the pattern predicted by theory. 4. The transient data were used to estimate the reflection coefficient of the capillary wall (sigma) to the macromolecular solute. In seven vessels, the mean sigma to BSA was 0.76 (S.E. of mean +/- 0.04) and in eight different vessels mean sigma to Ficoll 70 in the presence of BSA (10 mg ml-1) was 0.98 (S.E. of mean +/- 0.05). The steady-state data were consistent with the prediction that the oncotic pressure opposing high filtration rates approximates to sigma 2 pi c in the steady state, where pi c is the perfusate oncotic pressure.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of albumin on the osmotic pressure exerted by myoglobin across capillary walls in frog mesentery

1. Individual capillaries in mesenteries of pithed frogs were perfused sequentially with two frog Ringer solutions containing myoglobin at a concentration of 70 mg/ml. The first perfusate solution contained bovine serum albumin at a concentration of 10 mg/ml. The second perfusate contained no albumin. The modified Landis micro-occlusion technique (Michel, Mason, Curry & Tooke, 1974) was used to measure the capillary hydraulic conductivity and the effective osmotic pressure of the myoglobin solution across the capillary wall under conditions where the transcapillary concentration difference was established by steady-state ultrafiltration. 2. In six capillaries the effective osmotic pressure of the myoglobin solution at 20-22 degrees C was 15.9 +/- 2.2 (S.E. of mean) cmH2O when albumin was in the perfusate and 10.1 +/- 1.1 cmH2O after albumin was removed from the perfusate. 3. The hydraulic conductivity of the capillary wall increased from a mean value of 6.5 +/- 1.4 X 10(-7) cm/(s cmH2O) when albumin was in the perfusate to 15.6 +/- 3.8 X 10(-7) cm/(s cmH2O) during perfusion with Ringer solution containing myoglobin alone. 4. Control experiments to measure steady-state filtration rates at pressures 10-30 cmH2O were carried out to check the assumption in the method that the reduction in the measured osmotic pressure of myoglobin was the result of changes in the properties of the molecular sieve within the capillary wall and was not simply the result of solute accumulation on the tissue side of the capillary wall. 5. The measured reduction in the effective osmotic pressure of myoglobin when albumin was removed from the perfusate does not conform to the hypothesis (Crone, 1984) that the tight segment of the intercellular junction is the principal molecular filter for myoglobin in the wall of frog mesenteric capillaries. 6. Our results do conform to the hypothesis that a network of fibrous molecules, reinforced by adsorbed albumin, forms the principal molecular filter at the capillary wall in frog mesenteric capillaries.

An analysis of the permeability of a fenestra

In this analysis of published data we evaluate the permeabilities of unit area of fenestral pathway to water and small solutes. These properties are then used to assess the functional significance of the fenestral diaphragm which, along with the glycocalyx and basement membrane, makes up the fenestral pathway. Endothelial hydraulic conductance and permeability to small lipophobic solutes increase with fenestral density in a variety of mammalian tissue. The increase in conductance per unit increment in fenestral area (Kfen) averages 0.38 micron X s-1 X mm Hg-1 (regression analysis). This value means that a single fenestra (diam 0.06 micron, area 0.003 micron2) has a greater conductance than 1 micron2 of continuous (skeletal muscle) endothelium. Similarly the diffusional permeability of unit fenestral area (Pfen) to B12 and inulin is high. But neither Kfen nor Pfen is as great as might be expected from the extreme thinness of the diaphragm (less than or equal to 5 nm). Pfen depends on diffusivity (D) and pathlength (delta chi): Pfen = D'/delta chi. D' depends on the fraction of the fenestral surface available for exchange and on restriction to diffusion. These relations were applied to test the view that the diaphragm, rather than the glycocalyx or basement membrane, is the major barrier to fluid and small solutes in the fenestral pathway. If this were turn, over 98 1/2% of the diaphragm's area would have to be totally impermeable in order to yield the observed values of Pfen--a result consequent upon the extreme thinness of the diaphragm. In this event there would be less than one equivalent pore (radius 5-11 nm) per diaphragm on average-which is incompatible with ultrastructural evidence. The resistance of a 5-nm-thick diaphragm containing one or more pores is not high enough to account for fenestral resistance. It is concluded that much thicker structures, such as the glycocalyx and/or basement membrane, and not the diaphragm, account for fenestral resistance to small-solute and water transport.

The presence of plasma proteins facilitates the uptake of 125I-thrombin by the rabbit thoracic aorta endothelium in vitro

Various purified proteins, protein derivatives and two polysaccharides were added individually to a physiological medium in order to effect uptake of 125I-thrombin by the rabbit aorta endothelium. Over a wide range of concentration (0.004-40 mg/ml), the presence of either purified rabbit or bovine albumin during thrombin uptake encouraged an increase (70-110%) in 125I-thrombin binding by the endothelium and subendothelium compared to uptake by aorta segments in the absence of added protein. Pretreatment of aorta segments with albumin before incubation with 125I-thrombin in the absence of albumin did not encourage thrombin uptake to the same extent as having 125I-thrombin and albumin together. Purified human transferrin, rabbit IgG, chicken ovalbumin or denatured bovine casein could replace albumin to produce a similar enhancement of thrombin uptake. Replacing active concentrations of albumin by either reduced-carboxymethylated albumin, defatted albumin, plasmin-treated or thermolysin-treated albumin also caused an increase (50-130%) in thrombin binding, whereas replacement by acid-hydrolysed albumin or with polyglutamic acid was either ineffective or even inhibitory. Lysine-modified or arginine-modified albumins caused a small enhancement (14-32%) and no enhancement of thrombin uptake, respectively. Dextran, at low concentration (0.04-0.4 mg/ml) did not influence thrombin uptake, and at higher concentration (4-40 mg/ml) caused a decrease in uptake by both the endothelium and subendothelial layers. Low concentration of dextran sulphate inhibited thrombin uptake to 20-30% of control values. These data express the importance of accompanying protein in the response of the vascular endothelium during binding of thrombin. The possibility that other protein-cell interactions may be similarly influenced by macromolecular solutes is also discussed.