(Hypertension. 1999;34:212-216.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the Departments of Molecular and Cellular Physiology (I.K., W.C., D.N.G.), and Medicine (R.W.), Center of Excellence in Arthritis and Rheumatism, Louisiana State University Medical Center, Shreveport, La.
Correspondence to D. Neil Granger, PhD, Department of Molecular and Cellular Physiology, Louisiana State University Medical Center, 1501 Kings Hwy, PO Box 33932, Shreveport, LA 71130-3932. E-mail dgrang{at}lsumc.edu
| Abstract |
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Key Words: leukocytes integrins oxidative stress mast cells
| Introduction |
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Although the effects of long-term arterial hypertension on the microvascular responses to I/R have not been characterized, evidence already in the literature supports the possibility that arterial hypertension may alter the vulnerability of the microvasculature to the deleterious actions of I/R. Indeed, published reports suggest that arterial hypertension could either enhance6 7 or attenuate8 9 the inflammatory responses elicited by inflammatory stimuli, such as I/R. The possibility of an enhanced I/R-induced inflammatory response in spontaneously hypertensive rats (SHR) is supported by reports that describe (1) an elevated blood leukocyte count and a greater proportion of basally activated granulocytes circulating in SHR versus Wistar-Kyoto rats (WKY)6 and (2) an increased spontaneous production of reactive oxygen metabolites in the vasculature of SHR10 and hypertensive Dahl salt-sensitive rats.11 Conversely, there are reports that describe an impaired leukocyte-endothelial cell adhesion in mesenteric venules of SHR (versus WKY) exposed to platelet activating factor (PAF), leukotriene B4 (LTB4), or histamine,8 9 all of which have been implicated in the recruitment of leukocytes elicited by I/R.12 Furthermore, it has been shown recently that endotoxin-induced expression of ICAM-1, an endothelial cell adhesion molecule that has been implicated in I/R-induced leukocyte-endothelial cell adhesion,13 is significantly blunted in different vascular beds of SHR versus those of WKY.14
Given the divergent inflammatory responses that have been observed in genetically hypertensive animals, it is difficult to predict whether and in what way this risk factor for ischemic vascular disease alters the microvascular responses to I/R. Hence, the major objective of this study was to determine whether long-term arterial hypertension renders the microvasculature more or less vulnerable to the deleterious inflammatory responses elicited by I/R. Because several leukocyte (CD11/CD18) and endothelial cell (ICAM-1, P-selectin) adhesion glycoproteins have been implicated in the altered immune responses of hypertension,7 8 9 14 we also examined the potential role of these adhesion receptors in mediating the microvascular responses to I/R in both normotensive and hypertensive animals. These objectives were addressed by applying the technique of intravital fluorescence microscopy to venules of SHR and their normotensive WKY controls. The studies were performed with a rat mesentery model of I/R, which allows for simultaneous measurement of leukocyte-endothelial cell adhesion, platelet-leukocyte aggregation, and albumin extravasation. The findings of the present study indicate that long-term arterial hypertension does not significantly modify the leukocyte and platelet recruitment normally elicited in venules by I/R. However, this risk factor does result in an exaggerated albumin leakage response, which is mediated by an interaction between ß2 (CD18) integrins on leukocytes and ICAM-1 on endothelial cells.
| Methods |
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Intravital Microscopy
The methods and techniques used to monitor microvascular and
inflammatory responses in mesenteric postcapillary venules of WKY and
SHR were identical to those previously described.4 13 15
In brief, intravital microscopy was used to monitor leukocyte adherence
and emigration, platelet-leukocyte aggregation, leakage of
FITC-labeled bovine albumin (Sigma Chemical Co), red blood cell
(RBC) velocity, and vessel diameter in mesenteric venules. Wall shear
rate was calculated on the basis of the newtonian definition:
=8(Vmean/D).
Experimental Protocols
After all parameters measured online were in a
steady state, images from the mesenteric preparation were recorded
on videotape for 10 minutes. Immediately thereafter, the superior
mesenteric artery was ligated with a snare created from polyethylene
tubing. On the basis of findings from a previous study,13
an ischemic period of 10 minutes (or 0 minutes for
sham-operated controls) was used to elicit the acute
inflammatory responses in both WKY and SHR. Reperfusion was induced by
gently removing the arterial ligature. All measured
variables were recorded at 10-minute intervals for 30 minutes
after the onset of reperfusion. In some experiments, the SHR and WKY
(n=5 per group) were pretreated (15 minutes before control
measurements) with a monoclonal antibody (mAb) directed against either
CD18 (CL26, 100 µg/rat),16 ICAM-1 (1A29, 2
mg/kg),17 P-selectin (PB1.3, 2 mg/kg),18 or a
nonbinding antibody (P6H6, 2 mg/kg).18 The same protocol
was used with a 10-minute ischemic period. CL26 and 1A29 were
provided by Pharmacia-UpJohn Laboratories, whereas PB1.3 and P6H6 were
provided by Cytel Corporation. The blocking dose used for each mAb was
determined from previously published studies. At the doses used, none
of the mAbs caused leukopenia. The microvascular and inflammatory
responses to these treatment regimens in WKY and SHR were compared with
the responses of animals that did not receive any treatment.
Statistics
The data were analyzed by use of standard statistical
analysis; 1-way ANOVA and the Fisher exact (post hoc)
test. All values are reported as mean±SE from 5 to 7 rats, and
statistical significance was set at P<0.05.
| Results |
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Figure 1 illustrates the effects of I/R on the number of adherent leukocytes in mesenteric venules of WKY and SHR. It shows how the different adhesion glycoprotein-directed mAbs affect the I/R-induced leukocyte adherence responses. In both WKY and SHR groups, I/R elicited a significant and comparable increase in the number of firmly adherent leukocytes. In WKY, mAbs directed against CD18, ICAM-1, and P-selectin resulted in an attenuation of leukocyte adherence by 57%, 49%, and 41%, respectively, whereas the nonbinding mAbs had no effect. A similar pattern of effectiveness of the mAbs in reducing I/R-induced leukocyte adherence was noted in SHR. As shown in Figure 2, the responses of leukocyte emigration to I/R in WKY and SHR largely paralleled the changes noted for leukocyte adherence. A difference between the leukocyte adherence and emigration responses was the inability of the P-selectin specific mAb to reduce I/R-induced leukocyte emigration in both WKY and SHR.
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Figure 3 summarizes the responses of albumin leakage in mesenteric venules of WKY and SHR to I/R and illustrates how the different adhesion glycoprotein-directed mAbs affect the I/R-induced changes in albumin leakage. A difference between WKY and SHR was the inability of I/R to elicit a significant increase in albumin leakage in WKY, although a profound increase was observed in mesenteric venules of SHR. The enhanced I/R-induced albumin leakage noted in SHR was largely prevented by mAbs directed against CD18 or ICAM-1, but not by P-selectin or a nonbinding mAb.
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Exposure of the rat mesentery to I/R is associated with the formation of large platelet-leukocyte aggregates that fill the venule lumen and rapidly course through the vessel with flowing blood.4 Although such aggregates are never observed during control conditions, 37.4±8.4% (in SHR) and 28.5±9.4% (in WKY) of the luminal area of postcapillary venules was occupied by platelet-leukocyte aggregates after exposure to 10 minutes of ischemia and 30 minutes of reperfusion. Table 2 summarizes the effects of the different mAbs on I/R-induced formation of platelet-leukocyte aggregates. Although the most profound reduction in aggregate formation was observed in WKY (84%) and SHR (67%) that received the P-selectin mAb, significant reductions in aggregate formation were also noted in animals treated with an mAb directed against either CD18 (SHR, 73%; WKY, 61%) or ICAM-1 (SHR, 71%; WKY, 60%), but not in those treated with the nonbinding mAb.
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| Discussion |
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The major objective of this study was to determine whether long-term arterial hypertension renders the microvasculature more or less vulnerable to the deleterious inflammatory responses elicited by I/R. By use of SHR and their normotensive counterparts WKY, we compared the ability of postcapillary venules in these animals to mount an inflammatory response after challenge with I/R. Our findings indicate that the microvascular alterations normally elicited by I/R do not differ significantly between SHR and WKY with 1 notable exception, albumin extravasation. The number of firmly adherent and emigrated leukocytes and the amount of platelet-leukocyte aggregation in postischemic venules of SHR were quite similar to those noted in WKY. However, whereas albumin leakage from postcapillary venules was unaffected by 10 minutes of ischemia and 30 minutes of reperfusion in WKY, a dramatic increase in albumin extravasation was noted in SHR. Our experiments with adhesion moleculespecific mAbs also revealed that the I/R-induced albumin leakage across venules in SHR is mediated by an adhesive interaction between CD11/CD18 on leukocytes and ICAM-1 on endothelial cells.
Our finding that the I/R-induced leukocyte recruitment responses in SHR are similar to those noted in WKY contrasts with reports describing an attenuated recruitment of adherent leukocytes in mesenteric venules of SHR (relative to WKY) that are exposed to inflammatory mediators such as PAF, LTB4, and histamine.8 9 Antagonists and biosynthesis inhibitors that target these mediators have been shown to blunt I/R-induced leukocyte-endothelial cell adhesion in mesenteric venules of normotensive rats.12 Hence, a comparable attenuation of leukocyte adhesion in SHR would be expected after exposure of venules to I/R.
However, published evidence suggests that I/R should elicit an exacerbation of leukocyte adhesion in venules of SHR versus WKY. For example, it has been shown that a large number of circulating leukocytes in SHR (versus WKY) are in an activated state, as reflected by an enhanced rate of superoxide production.6 Furthermore, there is evidence that the constitutive (basal) level of ICAM-1 expression is significantly higher in the splanchnic microvasculature of SHR than in WKY.14 Finally, a recent report describes an enhanced adhesion of isolated monocytes to aortic endothelial cells from SHR versus WKY.22 An explanation for the inconsistency between these observations and the reports describing a reduced sensitivity of venules in SHR to PAF, LTB4, or histamine8 9 is not readily available. However, it may reflect the involvement of different mechanisms for I/R-induced recruitment of adherent leukocytes in WKY versus SHR.
A notable difference in the responses of the microvasculature of SHR versus WKY to I/R is the significantly greater albumin leakage that occurs in SHR. This observation is interesting in view of published reports that describe either a diminished23 or unchanged24 endothelial barrier function in hypertensive humans or SHR. Consistent with some reports of a diminished endothelial barrier function, previous studies of I/R-induced albumin leakage in normotensive animals have used longer ischemic durations to elicit the level of endothelial barrier dysfunction that we observed in venules of SHR. Nonetheless, previous studies have established a clear cause and effect relationship between I/R-induced albumin leakage and the adhesion and emigration of leukocytes in postcapillary venules.13 Indeed, it has been shown that the magnitude of the albumin extravasation in mesenteric venules elicited by I/R is directly proportional to the number of adherent and emigrated leukocytes.13 Furthermore, mAbs that blunt the recruitment of adherent leukocytes after I/R are similarly effective in reducing the accompanying albumin leakage response. In that regard, it is interesting that the findings of the present study demonstrate that, although the intensity of I/R-induced leukocyte recruitment in venules of SHR is no greater than that observed in WKY, the endothelial barrier in venules of SHR appears to be more vulnerable to the leukocyte-mediated damage induced by I/R. This enhanced vulnerability may reflect an enhanced capacity of leukocytes to mediate tissue injury in SHR, because it has been shown that neutrophils from SHR degranulate more readily and produce more superoxide when stimulated than their counterparts do in WKY.6 7 Alternatively, it may be proposed that vascular endothelial cells in SHR produce less NO than those in WKY, and, as a consequence, SHR venules have a reduced capacity to defend against neutrophil-derived superoxide. However, this possibility appears unlikely in view of a previous report that describes nearly identical leukocyte-endothelial cell adhesion responses after NO synthase inhibition in mesenteric venules of SHR and WKY8 .
In conclusion, our findings suggest that long-term arterial hypertension does not exacerbate the recruitment of leukocytes into postischemic tissues to the same degree as other cardiovascular risk factors, such as diabetes and hypercholesterolemia. However, hypertension does appear to render the endothelial barrier more susceptible to permeability. This increases the effect of I/R, an action that is mediated by an interaction between CD11/CD18 on leukocytes and ICAM-1 on endothelial cells.
| Acknowledgments |
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Received December 22, 1998; first decision February 25, 1999; accepted March 25, 1999.
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