Protocol for intraoperative assessment of the human cerebrovascular glycocalyx

INTRODUCTION

Adequate functioning of the blood-brain barrier (BBB) is important for brain homoeostasis and normal neuronal function. Disruption of the BBB has been described in several neurological diseases. Recent reports suggest that an increased permeability of the BBB also contributes to increased seizure susceptibility in patients with epilepsy. The endothelial glycocalyx is coating the luminal side of the endothelium and can be considered as the first barrier of the BBB. We hypothesise that an altered glycocalyx thickness plays a role in the aetiology of temporal lobe epilepsy (TLE), the most common type of epilepsy. Here, we propose a protocol that allows intraoperative assessment of the cerebrovascular glycocalyx thickness in patients with TLE and assess whether its thickness is decreased in patients with TLE when compared with controls.

METHODS AND ANALYSIS

This protocol is designed as a prospective observational case-control study in patients who undergo resective brain surgery as treatment for TLE. Control subjects are patients without a history of epileptic seizures, who undergo a craniotomy or burr hole surgery for other indications. Intraoperative glycocalyx thickness measurements of sublingual, cortical and hippocampal microcirculation are performed by video microscopy using sidestream dark-field imaging. Demographic details, seizure characteristics, epilepsy risk factors, intraoperative haemodynamic parameters and histopathological evaluation are additionally recorded.

ETHICS AND DISSEMINATION

This protocol has been ethically approved by the local medical ethical committee (ID: NL51594.068.14) and complies with the Declaration of Helsinki and principles of Good Clinical Practice. Informed consent is obtained before study enrolment and only coded data will be stored in a secured database, enabling an audit trail. Results will be submitted to international peer-reviewed journals and presented at international conferences.

TRIAL REGISTRATION NUMBER

NTR5568.

Effect of sulodexide on endothelial glycocalyx and vascular permeability in patients with type 2 diabetes mellitus

AIMS/HYPOTHESIS

Endothelial glycocalyx perturbation contributes to increased vascular permeability. In the present study we set out to evaluate whether: (1) glycocalyx is perturbed in individuals with type 2 diabetes mellitus, and (2) oral glycocalyx precursor treatment improves glycocalyx properties.

METHODS

Male participants with type 2 diabetes (n = 10) and controls (n = 10) were evaluated before and after 2 months of sulodexide administration (200 mg/day). The glycocalyx dimension was estimated in two different vascular beds using sidestream dark field imaging and combined fluorescein/indocyanine green angiography for sublingual and retinal vessels, respectively. Transcapillary escape rate of albumin (TER(alb)) and hyaluronan catabolism were assessed as measures of vascular permeability.

RESULTS

Both sublingual dimensions (0.64 [0.57-0.75] μm vs 0.78 [0.71-0.85] μm, p < 0.05, medians [interquartile range]) and retinal glycocalyx dimensions (5.38 [4.88-6.59] μm vs 8.89 [4.74-11.84] μm, p < 0.05) were reduced in the type 2 diabetes group compared with the controls whereas TER(alb) was increased (5.6 ± 2.3% vs 3.7 ± 1.7% in the controls, p < 0.05). In line with these findings, markers of hyaluronan catabolism were increased with diabetes (hyaluronan 137 ± 29 vs 81 ± 8 ng/ml and hyaluronidase 78 ± 4 vs 67 ± 2 U/ml, both p < 0.05). Sulodexide increased both the sublingual and retinal glycocalyx dimensions in participants with diabetes (to 0.93 [0.83-0.99] μm and to 5.88 [5.33-6.26] μm, respectively, p < 0.05). In line, a trend towards TER(alb) normalisation (to 4.0 ± 2.3%) and decreases in plasma hyaluronidase (to 72 ± 2 U/ml, p < 0.05) were observed in the diabetes group.

CONCLUSION/INTERPRETATION

Type 2 diabetes is associated with glycocalyx perturbation and increased vascular permeability, which are partially restored following sulodexide administration. Further studies are warranted to determine whether long-term treatment with sulodexide has a beneficial effect on cardiovascular risk.

TRIAL REGISTRATION

www.trialregister.nl NTR780/ http://isrctn.org ISRCTN82695186

FUNDING

An unrestricted Novartis Foundation for Cardiovascular Excellence grant (2006) to M. Nieuwdorp/E. S. G. Stroes, Dutch Heart Foundation (grant number 2005T037).

Hypothesis: arterial glycocalyx dysfunction is the first step in the atherothrombotic process

We present evidence that the 0.5 microm thick gel layer, lining the inner wall of healthy blood vessels, the glycocalyx, is the first line of defence against atherothrombotic disease. All blood vessel linings are coated with this gel, a highly negatively charged structure, rich in anionic sites mostly represented by the sialic acid moieties of glycoproteins and the sulphate and carboxyl groups of heparan-sulphate proteoglycans. Blood flow in arteries is associated with a shear stress at the glycocalyx, which signals the underlying endothelial cells to release nitric oxide (NO), an anti-atherogenic factor. Sites of low shear stress in the arterial tree are more susceptible to atheroma due to lack of NO generation through this mechanism, whereas exercise, by increasing blood flow and shear stress, is protective. We postulate that risk factors for atherothrombosis act by impairing glycocalyx function. That luminal hyperglycaemia causes glycocalyx dysfunction has already been shown; we postulate this to be the first step in the atherothrombotic process in patients with diabetes mellitus and metabolic syndrome (insulin resistance). There is also evidence of glycocalyx defects from exposure to oxidized low-density lipoprotein. We postulate that other risk factors will have a similar action on the glycocalyx as the initiating factor in the disease process, e.g. smoking, hyperlipidaemias and hyperhomocystenaemia. These predictions can now be tested in a large animal model of shear-stress-mediated arterial dilatation.

Perturbation of hyaluronan metabolism predisposes patients with type 1 diabetes mellitus to atherosclerosis

AIMS/HYPOTHESIS

Cardiovascular disease contributes to mortality in type 1 diabetes mellitus, but the specific pathophysiological mechanisms remain to be established. We recently showed that the endothelial glycocalyx, a protective layer of proteoglycans covering the endothelium, is severely perturbed in type 1 diabetes, with concomitantly increased plasma levels of hyaluronan and hyaluronidase. In the present study, we evaluated the relationship between hyaluronan and hyaluronidase with carotid intima-media thickness (cIMT), an established surrogate marker for cardiovascular disease.

SUBJECTS AND METHODS

Non-smoking type 1 diabetes patients without micro- or macrovascular complications and matched controls were recruited and cIMT of both carotid arteries was measured. To evaluate the relationship between cIMT and hyaluronan and hyaluronidase as well as other parameters, uni- or multivariate regression analyses were performed.

RESULTS

We included 99 type 1 diabetes patients (age 10-72 years) and 99 age- and sex-matched controls. Mean cIMT, HbA(1c), high sensitivity C-reactive protein, hyaluronan and hyaluronidase were significantly increased in type 1 diabetes vs controls. Plasma hyaluronan and hyaluronidase were correlated in type 1 diabetes. In univariate regression analyses, mean IMT was associated with plasma hyaluronan, age and male sex, whereas after multivariate analysis only age and sex remained statistically significant.

CONCLUSIONS/INTERPRETATION

We conclude that type 1 diabetes patients show structural changes of the arterial wall associated with increased hyaluronan metabolism. These data may lend further support to altered glycosaminoglycan metabolism in type 1 diabetes as a potential mechanism involved in accelerated atherogenesis.

Vasculoprotective properties of the endothelial glycocalyx: effects of fluid shear stress

The endothelial glycocalyx exerts a wide array of vasculoprotective effects via inhibition of coagulation and leucocyte adhesion, by contributing to the vascular permeability barrier and by mediating shear stress-induced NO release. In this review, we will focus on the relationship between fluid shear stress and the endothelial glycocalyx. We will address the hypothesis that modulation of glycocalyx synthesis by fluid shear stress may contribute to thinner glycocalyces, and therefore more vulnerable endothelium, at lesion-prone sites of arterial bifurcations. Finally, we will discuss the effects of known atherogenic stimuli such as hyperglycaemia on whole body glycocalyx volume in humans and its effect on endothelial function.

Visualisation of intramural coronary vasculature by an imaging cryomicrotome suggests compartmentalisation of myocardial perfusion areas

A technique is presented for the 3D visualisation of the coronary arterial tree using an imaging cryomicrotome. After the coronary circulation of the excised heart was filled with a fluorescent plastic, the heart was frozen and mounted in the cryomicrotome. The heart was then sliced serially, with a slice thickness of 40 microm, and digital images were taken from each cutting plane of the remaining bulk material using appropriate excitation and emission filters. Using maximum intensity projections over a series of images in the cutting plane and perpendicular plane, the structural organisation of intramural vessels was visualised in the present study. The branching end in the smallest visible vessels, which define tissue areas that are well delineated from each other by 1-2 mm wide bands populated only by vessels less than 40 microm in diameter. The technique presented here allows further quantification in the future of the 3D structure of the coronary arterial tree by image analysis techniques.

Elevated capillary tube hematocrit reflects degradation of endothelial cell glycocalyx by oxidized LDL

Proteoglycans and plasma proteins bound to the endothelial cell glycocalyx are essential for vascular function, but at the same time, they lower capillary tube hematocrit by reducing capillary volume available to flowing blood. Because oxidized low-density lipoproteins (oxLDL) reduce the effective thickness of the glycocalyx (Vink H, Constantinescu AA, and Spaan JAE. Circulation 101: 1500-1502, 2000), we designed the present study to determine whether this is caused by pathological degradation of glycocalyx constituents or increased glycocalyx deformation by elevated shear forces of flowing blood. Capillaries from the right cremaster muscle of 24 hamsters were examined by using intravital microscopy after systemic administration of normal LDL (n = 4), moderate oxLDL (6-h oxidation with CuSO(4), n = 7), severe oxLDL (18-h oxidation, n = 5), and moderate oxLDL plus superoxide dismutase (SOD) and catalase (n = 8). Capillary tube hematocrit increased from 0.16 +/- 0.03 to 0.37 +/- 0.05 and from 0.15 +/- 0.01 to 0.31 +/- 0.03 after moderate oxLDL and severe oxLDL, respectively. These changes were paralleled by increases in red blood cell flux from 8.7 +/- 1.9 to 13.8 +/- 3 and from 10.7 +/- 2.1 to 16.3 +/- 3.2 cells/s after moderate oxLDL and severe oxLDL, respectively, in the absence of changes in anatomic capillary diameter. Red blood cell velocity, as a measure for the shear forces on the glycocalyx, was not affected by oxLDL, whereas tissue pretreatment with SOD and catalase completely abolished the effects of oxLDL on glycocalyx thickness, capillary hematocrit, and red blood cell flux. We conclude that elevation of capillary tube hematocrit by oxLDL reflects degradation of the endothelial glycocalyx by oxygen-derived free radicals.

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.

Capillary endothelial surface layer selectively reduces plasma solute distribution volume

We previously reported that a 0.4- to 0.5-microm-thick endothelial surface layer confines Dextran 70 (70 kDa) to the central core of hamster cremaster muscle capillaries. In the present study we used a variety of plasma tracers to probe the barrier properties of the endothelial surface layer using combined fluorescence and brightfield intravital microscopy. No permeation of the endothelial surface layer was observed for either neutral or anionic dextrans >/=70 kDa, but a neutral Dextran 40 (40 kDa) and neutral free dye (rhodamine, 0.4 kDa) equilibrated with the endothelial surface layer within 1 min. In contrast, small anionic tracers of similar size (0. 4-40 kDa) permeated the endothelial surface layer relatively slowly with half-times (tau(50)) between 11 and 60 min, depending on tracer size. Furthermore, two plasma proteins, fibrinogen (340 kDa) and albumin (67 kDa), moved slowly into the endothelial surface layer at the same rates, despite greatly differing sizes (tau(50) approximately 40 min). Dextran 70, which did not enter the glycocalyx over the course of these experiments, entered at the same rate as free albumin when it was conjugated to albumin. These findings demonstrate that for anionic molecules size and charge have a profound effect on the penetration rate into the glycocalyx. The equal rates of penetration of the glycocalyx demonstrated by the different protein molecules suggests that multiple factors may influence the penetration of the barrier, including molecular size, charge, and structure.

Prolonged diastolic time fraction protects myocardial perfusion when coronary blood flow is reduced

BACKGROUND

Because coronary blood flow is impeded during systole, the duration of diastole is an important determinant of myocardial perfusion. The aim of this study was to show that coronary flow modulates the duration of diastole at constant heart rate.

METHODS AND RESULTS

In anesthetized, open-chest dogs, diastolic time fraction (DTF) increased significantly when coronary flow was reduced by lowering perfusion pressure from 100 to 70, 55, and 40 mm Hg. On average, DTF increased from 0.47+/-0.04 to 0.55+/-0.03 after a pressure step from 100 to 40 mm Hg in control, from 0.42+/-0.04 to 0.47+/-0.04 after administration of adenosine, and from 0.46+/-0.07 to 0.55+/-0.06 after L-NMMA (mean+/-SD, 6 dogs for control and adenosine, 4 dogs for L-NMMA, all P<0.05). Flow normalized to its value at full dilation and pressure of 90 mm Hg (375+/-25 mL/min) increased during the period of reduced pressure at 40 mm Hg; control, from 0.005+/-63 (2 seconds after pressure step) to 0.09+/-0.06 (15 seconds after pressure step); with adenosine, from 0.19+/-0.06 to 0. 22+/-0.06; and with L-NMMA, from 0.013+/-0.007 to 0.12+/-0.02 (all P<0.05). The increase in DTF at low pressure may be explained by a decrease in interstitial volume at low pressure, which either decreases the preload of the myocytes or reduces the buffer capacity for ions determining repolarization, thereby causing an earlier onset of relaxation.

CONCLUSIONS

Because the largest increase in DTF occurs at pressures below the autoregulatory range when blood flow to the subendocardium is closely related to DTF, modulation of DTF by coronary blood flow can provide an important regulatory mechanism to match supply and demand of the myocardium when vasodilatory reserve is exhausted.

Identification of distinct luminal domains for macromolecules, erythrocytes, and leukocytes within mammalian capillaries

A thick endothelial surface coat consisting of the glycocalyx and associated plasma proteins has been hypothesized to reduce functional capillary volume available for flowing plasma macromolecules and blood cells. The purpose of this study was to compare anatomic and functional capillary diameters available for macromolecules, RBCs, and WBCs in hamster cremaster muscle capillaries. Bright-field and fluorescence microscopy provided similar estimates (mean +/- SE) of the anatomic capillary diameter: 5.1 +/- 0.1 microns (bright field, 39 capillaries in 10 animals) and 5.1 +/- 0.2 microns (membrane dye PKH26, 18 capillaries in 2 animals). Estimates of functional diameters were obtained by measuring the width of RBCs and WBCs and the intracapillary distribution of systemically injected fluorescein isothiocyanate (FITC)-dextran 70. WBCs (5.1 +/- 0.2 microns) fully occupied the anatomic capillary cross section. In contrast, the widths of RBCs (3.9 +/- 0.2 microns, 21 capillaries in 8 animals) and FITC-dextran (4.3 +/- 0.2 microns, 21 capillaries in 8 animals) were significantly smaller than the anatomic capillary diameter. Continuous (1- to 5-minute) excitation of fluorochromes in the capillary lumen (light-dye treatment) increased the width of RBCs passing the treated site from 3.6 +/- 0.3 to 4.4 +/- 0.3 microns (6 capillaries in 4 animals) and the width of the FITC-dextran column from 4.1 +/- 0.2 to 4.6 +/- 0.3 microns (10 capillaries in 7 animals). Furthermore, light-dye treatment increased capillary tube hematocrit by 60% in 40-microns-long capillary segments compared with untreated sites in the same capillaries. It is concluded that the wall of skeletal muscle capillaries is decorated with a 0.4- to 0.5-microns-thick endothelial surface coat, which may represent the true active interface between blood and the capillary wall.

Evidence that cell surface charge reduction modifes capillary red cell velocity-flux relationships in hamster cremaster muscle

1. From capillary red cell velocity (V)-flux (F) relationships of hamster cremaster muscle a yield velocity (VF = 0) can be derived at which red cell flux is zero. Red cell velocity becomes intermittent and/or red blood cells come to a complete standstill for velocities close to this yield velocity, and, at the same time, capillary tube haematocrit becomes very low. 2. We have tested whether the net negative charge of red blood cells (RBCs) contributes to the magnitude of VF = 0. Velocity-flux relationships were measured for normal cells, normal cells labelled with the fluorescent dye calcein (LRBCs), and red cells treated with hexadimethrine to mask negative charge and labelled with calcein as well (HDM-LRBCs). Measurements were done in a hamster cremaster muscle preparation applying video in vivo microscopy. 3. Hexadimethrine treatment reduced the net negative surface charge of red cells to 20% of control as estimated from transmission electron microscopy using a ferritin tagging technique. The values of VF = 0 found for normal red cells and HDM-LRBCs were 86 +/- 15 and 31 +/- 17 microns s-1, +/- S.E.M., n = 12, respectively, which were significantly different (P < 0.05). For normal cells and cells labelled with calcein only, VF = 0 values were 63 +/- 14 and 65 +/- 13 microns s-1, n = 8, respectively, which were not significantly different. The effect of HDM treatment did not alter filterability of the red cells as estimated from transit times through 5 microns pores. 4. The present findings demonstrate that the net negative charge of RBCs contributes significantly to the yield velocity for red blood cells entering capillaries and flowing through them. HDM treatment reduced the net negative charge of red blood cells and may have caused cells to enter capillaries more easily owing to reduced electrostatic repulsion at the capillary entrance. In addition, HDM treatment may have lowered intracapillary flow resistance by a reduction in electrostatic repulsive forces between red blood cells and negatively charged (macromolecules on) capillary endothelial cells at sites of irregular capillary cross-sectional shape, without significantly affecting the lubricating properties of the capillary endothelial glycocalyx and/or associated plasma macromolecules.

Red blood cell velocity in nailfold capillaries during hyperbaric oxygenation

Microcirculatory hemodynamics of the skin during hyperbaric oxygenation were assessed by determination of nailfold capillary red blood cell velocity (Vrbc). Under hyperbaric conditions a continuous increase in Vrbc was found. Control values, 0.43 +/- 0.12 mm. sec-1 (mean +/- sem), were significantly (P < 0.05) lower compared with Vrbc at the end of hyperbaric oxygenation (0.62 +/- 0.16 mm.sec-1).

Heterogeneous NADH fluorescence during post-anoxic reactive hyperemia in saline perfused rat heart

In the present study epicardial NADH fluorescence photographs were taken of rat hearts during dynamic transitions of oxygen content of the myocardium. Hearts were perfused in a Langendorff set-up where it was possible to switch between low and high-pO2 perfusates. NADH fluorescence photographs were taken with a suitable fluorescence set-up and photo negatives digitized and analyzed by use of a computer. Restoration of perfusion with a high-pO2 solution resulted in a reactive hyperemic flow being established. Prior to the occlusion being lifted high NADH fluorescence was observed. Reactive hyperemic flow was associated with heterogenic NADH fluorescence patterns which diminished as control flow was restored. The patterns observed during reactive hyperemia were identical to those observed when tissue oxygen was restored by high-pO2 perfusion following high flow hypoxia achieved by low-pO2 perfusion. This study shows that heterogenic epicardial flow patterns are associated with reactive hyperemia.