(Circulation. 2000;101:1500.)
© 2000 American Heart Association, Inc.
Brief Rapid Communications |
From the Department of Medical Physics, University of Amsterdam, Amsterdam, The Netherlands.
Correspondence to Hans Vink, PhD, Department of Medical Physics, Academic Medical Center, University of Amsterdam, PO Box 22700, 1100 DE Amsterdam, The Netherlands. E-mail h.vink{at}amc.uva.nl
| Abstract |
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Methods and ResultsThe 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 CuSO4 for 6 hours. The dimension of the EC surface layer averaged 0.6±0.1 µm 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.
ConclusionsOxygen-derived free radicals mediate the disruption of the EC surface layer and increase vascular wall adhesiveness by Ox-LDL.
Key Words: atherosclerosis lipoproteins endothelium platelets
| Introduction |
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| Methods |
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Intravital Microscopy
The cremaster muscle was observed with an intravital microscope
(Olympus BHM) and a cooled intensified charge-coupled device
video camera (GenIV ICCD, Princeton Instruments). The
tissue was transilluminated with a mercury lamp (100 W) equipped with a
435-nm bandpass interference filter (blue light) using an applanate,
achromatic condenser set at numerical aperture 1.2 (Universal Applanate
Achromatic Condensor, Olympus). All preparations were examined
with a x60, water-immersion, objective lens (Olympus, UPlanApo with
aperture of 1.2 W or LUMPlanFL with aperture of 0.9 W) and a telescopic
tube to give a final object-to-camera magnification of x250. Images
were displayed on a Philips CM 8833-II video monitor and recorded
using a SVHS videotape recorder (JVC BR-S611E) and a time-coding
interface unit (JVC SA-F911E) for further image analysis.
Data Analysis
Video images were digitized using a frame grabber (DT3152, PCI
Local Bus) and Image-Pro Plus software (Image-Pro Plus version 3.0,
Media Cybernetics). An onscreen caliper using a 1
mm/0.01 mm stage micrometer was used for all
calibrated dimensional measurements. The anatomical capillary diameter
was estimated by positioning the digital calipers at the inside of the
capillary wall. Observed microvessel diameters ranged between 3 and
10 µm, indicating that in addition to true capillary blood
vessels, the observed population of microvessels probably included
terminal arterioles and/or capillary venules. Platelets that
remained stuck to the endothelium for
2 video frames
in the presence of continuously flowing RBCs were counted off-line by
slow motion video play back.
Experimental Protocols
All experimental protocols started 45 to 60 minutes after
completion of the hamster cremaster preparation. Measurements of
capillary dimensions were made starting 10 to 15 minutes before the
injection of either native LDL (n=4) or Ox-LDL (n=7) at 0.4 mg/100 g
body weight. Human LDL (Sigma, L2139) was dialyzed against PBS for 24
hours at 4°C at pH 7.4 without EDTA. LDL was oxidized by the addition
of CuSO4 at a concentration of 7.5 µmol/L
for 6 hours at 37°C. This reaction was stopped by the addition of
0.01 mmol/L EDTA; normal LDL was stored for 6 hours at 4°C.
Finally, both normal LDL and Ox-LDL were dialyzed for 48 hours in PBS
and 0.01% EDTA at 4°C. Protein concentrations were determined using
the method of Lowry et al.3A LDL samples were
stored at 4°C. Using a measurement of
5 mL of plasma volume in a
100-g hamster, the initial systemic Ox-LDL concentration was estimated
as
0.4 mg/5 mL, or
8 mg/dL. This measurement seems to be clinically
relevant on the basis of recent measurements in atherosclerotic
patients; these measurements show Ox-LDL concentrations between 1 and 6
mg/dL.4
To test for the involvement of oxygen-derived free radicals, additional Ox-LDL experiments (n=8) were performed in the presence of superoxide dismutase (SOD) and catalase, as described previously.3
Statistics
Data on the dimension of the spacing between RBCs and the
luminal EC membrane (RBC-EC gap) are presented as means±SE.
RBC-EC gap values after the injection of either normal LDL or Ox-LDL
were compared with their respective controls (preinjection values)
using a paired t test (2-way) to test for significance
at P<0.05.
| Results |
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1 platelet was seen sticking to the
endothelium, giving a total of 15 platelet-EC
adhesions in 7 experiments with Ox-LDL. No adhering platelets were
observed in the presence of SOD and catalase. Figure 2
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| Discussion |
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Treating the RBC surface charge with polycations6 or enzymatically treating the endothelial surface layer7 8 modulates RBC flow through capillary blood vessels and stimulates platelet-EC adhesion. Studies on transvascular exchange demonstrated that the adsorption of albumin and other plasma proteins to the EC glycocalyx confers filter-like properties to the endothelial surface layer and reduces microvascular hydraulic conductivity and solute permeability.9 10 11 Enzymatic disruption of the EC glycocalyx increased the adhesion of blood cells to the vascular wall8 and abolished flow-dependent dilation due to impaired endothelial production of nitric oxide.12 13 These studies demonstrate that the endothelial surface layer is essential for several aspects of endothelial function, including the control of transvascular exchange, providing vessels with an anti-adhesive inner lining, and flow-dependent dilation.
In the present study, we used a recently developed light microscopic technique3 to demonstrate for the first time that a clinically relevant dose of Ox-LDL4 reduces the in vivo dimension of the endothelial surface layer and simultaneously increases platelet-EC adhesion. These effects were completely inhibited by the administration of SOD and catalase, indicating that increased amounts of oxygen-derived free radicals mediate Ox-LDLinduced degradation of the endothelial surface layer and the consequent loss of endothelial anti-adhesive properties. Similarly, Lehr et al14 demonstrated that the increased adhesion of leukocytes to the endothelium of small arterioles and venules after the systemic bolus injection of Ox-LDL could be prevented by vitamin C or SOD. Furthermore, Liao and Granger15 showed that SOD prevented Ox-LDLinduced albumin leakage and leukocyte adhesion to the vascular endothelium.
Oxygen-derived free radicals such as the superoxide anion may degrade the endothelial surface layer and induce the adhesion of platelets by inactivating paracrine anti-platelet substances, such as nitric oxide. Alternatively, oxygen radicals may have unmasked constitutive endothelial adhesion molecules, such as platelet-endothelial cell adhesion molecule, by removing glycocalyx-associated plasma proteins from the EC surface. Readsorption of plasma substances to the EC glycocalyx might then explain the rapid recovery of the endothelial surface layer after its disruption by a bolus injection of Ox-LDL. In agreement with this possibility, our reported layer recovery time (20 to 30 minutes) agrees well with published data on plasma protein reconstitution of the EC surface layer after vascular perfusion with a protein-poor medium.16 However, reconstitution of the EC surface layer by newly synthesized or preformed proteoglycans and/or glycosaminoglycans must also be considered, and additional studies must be designed to distinguish between these mechanisms of surface layer repair.
| Acknowledgments |
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Received October 12, 1999; revision received April 4, 2000; accepted April 8, 2000.
| References |
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