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1 From the Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine; 2 Sumitomo Pharmaceuticals Research Center, Osaka; 3 Department of Ophthalmology, Nagoya City University Medical School, Japan.
| Abstract |
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METHODS. Isolated platelet samples were labeled with carboxyfluorescein diacetate succinimidyl ester. After intravenous administration, platelet behavior in the retinal microcirculation was evaluated with a scanning laser ophthalmoscope. The images were recorded on S-VHS videotape and analyzed with a computer-assisted image analysis system. The plateletendothelial interactions in the retinal microcirculation were also investigated with the use of lipopolysaccharide-stimulated endothelium or platelets activated with thrombin.
RESULTS. Fluorescent platelets were recognized as distinct dots in the retinal microcirculation and could be traced frame by frame. The velocity of platelets in the retinal arteries, capillaries, and veins was 26.1 ± 6.4, 1.6 ± 0.4, and 19.9 ± 8.2 mm/sec, respectively. In control rats, even the activated platelets showed minimal interaction with retinal endothelial cells. In contrast, stimulated retinal endothelium showed active plateletendothelial interactions; many platelets were observed rolling and adhering along the major retinal veins. The interactions between platelets and stimulated endothelial cells were substantially inhibited with the injection of P-selectin monoclonal antibody.
CONCLUSIONS. The present study demonstrated a new method to visualize platelet behavior in the retinal microcirculation in vivo. This method will allow quantitative evaluation of platelet dynamics and plateletendothelial interactions in retinal pathologic conditions.
| Introduction |
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In addition, it is suggested that platelets are involved in the pathogenesis of many inflammatory conditions5 such as ischemiareperfusion injury.6 On activation, accumulated platelets produce free radicals and proinflammatory mediators such as serotonin, leukotrienes, thromboxane A2, and platelet-derived growth factor.7 Moreover, platelets have a potential to modulate leukocyte functional response.8 To recruit flowing platelets to the inflammatory region, it is necessary for platelets to interact with the endothelial cells through distinct adhesion molecules expressed on the surface of platelets and endothelial cells.9 10 Recently, an intravital microscopic study reported that platelets can roll on postischemic endothelium through P-selectin, in the course of accumulation during ischemiareperfusion injury.11
During the past decade, large advances have been made in the research of interactions between leukocytes and retinal endothelial cells.12 13 14 15 This research is supported by biochemical and histologic investigations about adhesion molecules on the surface of leukocytes and endothelial cells.16 17 In spite of the progress in experiments in vitro of plateletendothelial interactions,7 we believe that no information is available about interactions between platelets and retinal endothelium in vivo.18 19 In this study, we developed a new method to visualize and evaluate the dynamics of platelets in the retinal microcirculation. With the use of this system, we investigated plateletendothelial interactions in rat retina.
| Materials and Methods |
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Platelet Preparation
Carboxyfluorescein diacetate succinimidyl ester (CFDASE; Molecular
Probes, Eugene, OR) is a nonfluorescent precursor that diffuses into
cells and forms a stable fluorochrome carboxyfluorescein succinimidyl
ester (CFSE; peak absorbance, 492 nm; peak emission, 518 nm) after
catalysis by esterase. This enzymatic reaction occurs predominantly in
leukocytes and platelets and partially in serum. Intracellular
fluorophores react with lysine residues of protein and remain within
the cell as long as the membrane is intact.20
CFDASE was dissolved in dimethyl sulfoxide (Wako Pure Chemicals, Osaka, Japan) to a concentration of 15.6 mM, and a small aliquot (200 µl) was stored at -70°C until use.
Blood samples from donor rats of either strain were harvested from the abdominal artery and collected in polypropylene tubes containing 2 ml volume of 38 mOsM citric acid, 75 mM trisodium citrate, 100 mOsM dextrose. The blood was centrifuged at 250g for 10 minutes.11 Plateletrich plasma was gently transferred to a fresh tube and centrifuged at 2000g for 10 minutes. The platelet pellet was resuspended in 20 ml Hanks balanced salt solution (Gibco, Grand Island, NY) and incubated with 100 µl CFDASE solution for 30 minutes at 37°C.21 After incubation, the platelet suspension was centrifuged again at 2000g for 10 minutes.11 The platelet pellet was resuspended in Hanks balanced salt solution at a concentration of 1 x 106 platelets/0.3 ml or 2 x 108 platelets/0.3 ml.
Experimental Procedure
Each rat of either strain was anesthetized with xylazine
hydrochloride (4 mg/kg) and ketamine hydrochloride (10 mg/kg). The
pupils were dilated with 0.5% tropicamide and 2.5% phenylephrine
hydrochloride. A contact lens was used to retain corneal clarity
throughout the experiment. Each rat had a catheter inserted into the
tail vein and was placed on a stereotaxic platform. Arterial blood
pressure and heart rate were monitored with the blood pressure analyzer
(IITC, Woodland Hills, CA). Platelets (1 x
106) were infused to measure the velocity of each
platelet in the retinal microcirculation, and 2 x
108 platelets were used to evaluate the
interactions with the retinal endothelial cells. A low dose of CFSE
solution (Molecular Probes) was administered to delineate the retinal
vasculature. The fundus was observed with a scanning laser
ophthalmoscope (SLO; Rodenstock, Munich, Germany) in the 20° or 40°
field. The argon blue laser (wavelength, 488 nm) was used for the
illumination source, with a regular emission filter for fluorescein
angiography. The obtained images were recorded on an S-VHS videotape at
the video rate of 30 frames/sec for further analysis.
Experimental Design
The endothelial cells of retinal vessels in recipient rats or
platelet samples from donor rats were activated in some experiments to
investigate the plateletendothelial interactions in the retinal
microcirculation. To stimulate the endothelial cells of the retinal
vessels, 200 µg lipopolysaccharide (LPS) from Salmonella
typhimurium (Difco, Detroit, MI) was injected into the footpad of
the recipient rats. In this study, recipient rats were used at 12 hours
after LPS treatment.12
To activate platelets, purified
platelets were incubated with 0.2 U/ml thrombin (Sigma, St. Louis, MO)
for 15 minutes at 37°C after labeling with CFDASE.9
10
As a control, six female Lewis rats were administered nonactivated platelets (group 1). To investigate the effects of activation of endothelial cells on the plateletendothelial interactions, nonactivated platelets were injected into six female Lewis rats treated with LPS (group 3). Activated platelets were injected into the nonsurgical Lewis rats (group 2; n = 6) or Lewis rats treated with LPS (group 4; n = 6). To evaluate the involvement of P-selectin in plateletendothelial interactions, 2 mg/kg anti-rat P-selectin monoclonal antibody (mAb) ARP2-422 was administered intravenously to the female Lewis rats treated with LPS, 5 minutes before injection of nonactivated platelets (group 5; n = 6).
Image Analysis
The video recordings were analyzed with an image analysis system,
consisting of a personal computer (Apple Computer, Cupertino, CA)
equipped with a video digitizer (Radius, San Jose, CA). The latter
digitizes the video image in real time (30 frame/sec) to 640 horizontal
and 480 vertical pixels with an intensity resolution of 256 steps. We
investigated the behavior of platelets in the retinal vessels to
evaluate the plateletendothelial interactions. Rolling platelets were
defined as platelets that moved at a slower velocity than free-flowing
platelets in a given vessel and that made intermittent adhesive
contacts with vascular endothelial cells.9
10
11
12
13
14
15
A platelet
was defined as adherent to vascular endothelium if it remained
stationary for longer than 30 seconds.11
The number of
rolling platelets was calculated as the total number of rolling
platelets along all major veins for 1 minute within a circle with a
radius of 500 µm from the center of the optic disc. Velocity of
rolling platelets was calculated as the time required for a platelet to
travel a given distance along the vessel.9
10
11
12
13
14
15
The number
of platelets adhering to the venous endothelial walls was quantified
within the same circle. All parameters were evaluated after a
stabilization period of 5 minutes after the administration of
platelets.
After these experiments, the rats were killed with an overdose of anesthesia and the eyes enucleated to determine a calibration factor with which to convert values measured on a computer monitor (in pixels) into real values (in micrometers).
Statistical Analysis
All values are presented as means ± SD. The data were
analyzed using a one-way analysis of variance with Scheffés
post hoc test. Differences were considered statistically significant at
P < 005.
| Results |
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| Discussion |
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To our knowledge, little information is currently available about the velocity of platelets in retinal microcirculation.18 In this study, the velocity of platelets in the retinal arteries, veins, and capillaries in control rats was 26.1 ± 6.4, 19.9 ± 8.2, and 1.6 ± 0.4 mm/sec, respectively. In a previous study with acridine orange digital fluorography, Nishiwaki et al.27 reported that the velocity of leukocytes was 29.5 ± 7.3, 17.4 ± 5.3, and 1.4 ± 0.4 mm/sec in the major retinal arteries, veins, and capillaries, respectively. Their results were relatively comparable with our current findings. However, as they discussed, digital processing used in the SLO is restricted spatially and temporally.27 In major retinal vessels, especially in arteries, fluorescent particles sometimes disappear when constitutive frames are analyzed. It is possible that the movement of the fastest platelets could not be traced consecutively, because the video rate was 30 frames/sec. Otherwise, because the SLO uses an interlaced scanning system,28 the SLO may not have been able to detect the figure of a platelet between two lines of the scanning laser.29 Moreover, the velocity of platelets depends on the pulsatile nature of the flow in the artery or on the cell location across the vessels in the artery or vein.30 Blood cells flowing closer to the axis flow faster compared with those in the marginal stream.31 Therefore, there is a limitation in measuring the velocity of platelets in the major retinal vessels. In contrast, the velocity of platelets in the capillaries is comparable with that of leukocytes. Leukocytes, which are larger than the caliber of the capillary, have to be squeezed during passage through the narrow capillary, accompanied by platelets and erythrocytes in rouleaux formations.32 This accounts for the similarity of velocity in the capillaries between platelets and leukocytes.
It has been suggested that P-selectin mediates plateletendothelial interactions,9 10 11 and Zachem et al.5 have recently shown that P-selectin mAb attenuates platelet accumulation in glomerulonephritis. In this study, we investigated plateletendothelial interactions in the retinal microcirculation with the use of LPS-stimulated endothelium or platelets activated with thrombin. Thrombin can activate platelets and induce rapid expression of P-selectin on their surface within a few minutes.33 Under basal conditions of the endothelium, even activated platelets showed minimal interactions with retinal endothelial cells. These findings indicate that activation of platelets had a minor influence on plateletendothelial interactions. Perhaps, nitric oxide or prostaglandin I2 derived from endothelial cells contributes partially to the antiplatelet property of the endothelium.34 In contrast, retinal endothelial cells stimulated with LPS showed active interactions with platelets, similar to the findings of Miyamoto et al.,12 who showed leukocytes rolling along retinal veins in LPS-treated rats, with a peak 12 hours after induction. Although injected platelets may also be activated with various kinds of cytokines induced by LPS treatment immediately after intravenous infusion,7 certainly, retinal endothelial cells were activated with the treatment of LPS and expressed P-selectin on their surfaces.15 P-selectin expression on the endothelial cells is essential for platelet rolling on the venous walls.10 11 Our current observation has been supported by an intravital microscopic study of Frenette et al.10 of P-selectindeficient mice. These investigators have shown that platelets from P-selectindeficient and wild-type mice can roll along the stimulated endothelium of wild-type mice but not along the stimulated endothelium of P-selectindeficient mice. P-selectin, which is stored in WeibelPalade bodies of endothelial cells, modulate the recruitment of not only leukocytes but also platelets to the inflammatory regions.35
In another study of P-selectindeficient mice, Massberg et al.11 observed platelet rolling on the arterial and venous walls in the postischemic mesentery. In our study, no platelet rolling was observed along the major retinal arteries. However, we demonstrated thrombus formation and platelet tethering in the retinal arteries. The expression of P-selectin is reported to be limited substantially on the arterial endothelium, compared with that on the veins.35 Moreover, in the major retinal artery 10 to 50 µm in diameter, platelet adhesion is rarely observed because of the high shear stress.4 30 Therefore, this interaction is mediated by other adhesion molecules.2 3 As Savage et al.4 have recently suggested, various kinds of adhesion molecules work together in the thrombus formation under high-flow conditions in the retinal arteries.
In conclusion, we developed a new method to evaluate quantitatively the platelet dynamics and plateletendothelial interactions in the retinal microcirculation in vivo. In platelet adhesion to vascular walls, it is necessary for flowing platelets to interact with the endothelial cells through distinct adhesion molecules expressed on the surface of platelets and endothelial cells.9 10 11 In this study, activation of platelets had a minor influence on plateletendothelial interactions; however, endothelial cells treated with LPS showed active interactions with platelets. P-selectin expressed on the endothelial cells modulated the recruitment of platelets to the inflamed retina.
| Footnotes |
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Submitted for publication March 23, 1999; revised June 10, 1999; accepted July 6, 1999.
Commercial relationships policy: N.
Corresponding author: Junichi Kiryu, MD, Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine, Sakyo-ku, Kyoto 606-8507, Japan. E-mail: kiryu{at}kuhp.kyoto-u.ac.jp
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