Effects of pulmonary ischemia-reperfusion on platelet adhesion in subpleural arterioles in rabbits

Transient heat stress protects from severe endothelial damage and dysfunction during prolonged experimental ex-vivo lung perfusion

Introduction

The pulmonary endothelium is the primary target of lung ischemia-reperfusion injury leading to primary graft dysfunction after lung transplantation. We hypothesized that treating damaged rat lungs by a transient heat stress during ex-vivo lung perfusion (EVLP) to elicit a pulmonary heat shock response could protect the endothelium from severe reperfusion injury.

Methods

Rat lungs damaged by 1h warm ischemia were reperfused on an EVLP platform for up to 6h at a constant temperature (T°) of 37°C (EVLP37°C group), or following a transient heat stress (HS) at 41.5°C from 1 to 1.5h of EVLP (EVLPHS group). A group of lungs exposed to 1h EVLP only (pre-heating conditions) was added as control (Baseline group). In a first protocol, we measured lung heat sock protein expression (HSP70, HSP27 and Hsc70) at selected time-points (n=5/group at each time). In a second protocol, we determined (n=5/group) lung weight gain (edema), pulmonary compliance, oxygenation capacity, pulmonary artery pressure (PAP) and vascular resistance (PVR), the expression of PECAM-1 (CD31) and phosphorylation status of Src-kinase and VE-cadherin in lung tissue, as well as the release in perfusate of cytokines (TNFα, IL-1β) and endothelial biomarkers (sPECAM, von Willebrand Factor -vWF-, sE-selectin and sICAM-1). Histological and immunofluorescent studies assessed perivascular edema and formation of 3-nitrotyrosine (a marker of peroxinitrite) in CD31 lung endothelium.

Results

HS induced an early (3h) and persisting expression of HSP70 and HSP27, without influencing Hsc70. Lungs from the EVLP37°C group developed massive edema, low compliance and oxygenation, elevated PAP and PVR, substantial release of TNFα, IL-1β, s-PECAM, vWF, E-selectin and s-ICAM, as well as significant Src-kinase activation, VE-cadherin phosphorylation, endothelial 3-NT formation and reduced CD31 expression. In marked contrast, all these alterations were either abrogated or significantly attenuated by HS treatment.

Conclusion

The therapeutic application of a transient heat stress during EVLP of damaged rat lungs reduces endothelial permeability, attenuates pulmonary vasoconstriction, prevents src-kinase activation and VE-cadherin phosphorylation, while reducing endothelial peroxinitrite generation and the release of cytokines and endothelial biomarkers. Collectively, these data demonstrate that therapeutic heat stress may represent a promising strategy to protect the lung endothelium from severe reperfusion injury.

Intravital assessment of precapillary pulmonary arterioles of type 1 diabetic mice shows oxidative damage and increased tone in response to NOS inhibition

Microvascular dilation, important for peripheral tissue glucose distribution, also modulates alveolar perfusion and is inhibited by loss of bioavailable nitric oxide (NO) in diabetes mellitus (DM). We hypothesized that DM-induced oxidative stress decreases bioavailable NO and pulmonary precapillary arteriolar diameter, causing endothelial injury. We examined subpleural pulmonary arterioles after acute NO synthase (NOS) inhibition with NG-nitro-l-arginine methyl ester (l-NAME) in streptozotocin (STZ)- and saline (CTRL)-treated C57BL/6J mice. Microvascular changes were assessed by intravital microscopy in the right lung of anesthetized mice with open chest and ventilated lungs. Arteriolar tone in pulmonary arterioles (27.2-48.7 µm diameter) increased in CTRL mice (18.0 ± 11% constriction, P = 0.034, n = 5) but decreased in STZ mice (13.6 ± 7.5% dilation, P = 0.009, n = 5) after l-NAME. Lung tissue dihydroethidium (DHE) fluorescence (superoxide), inducible NOS expression, and protein nitrosylation (3-nitrotyrosine) increased in STZ mice and correlated with increased glucose levels (103.8 ± 8.8 mg/dL). Fluorescently labeled fibrinogen administration and fibrinogen immunostaining showed fibrinogen adhesion, indicating endothelial injury in STZ mice. In CTRL mice, vasoconstriction to l-NAME was likely due to the loss of bioavailable NO. Vasodilation in STZ mice may be due to decreased formation of a vasoconstrictor or emergence of a vasodilator. These findings provide novel evidence that DM targets the pulmonary microcirculation and that decreased NO bioavailability and increased precapillary arteriolar tone could potentially lead to ventilation-perfusion abnormalities, exacerbating systemic DM complications.NEW & NOTEWORTHY Diabetes pulmonary and microvascular consequences are well recognized but have not been characterized. We assessed lung microvascular changes in a live anesthetized mouse model of type 1 diabetes, using a novel intravital microscopy technique. Our results show new evidence that a diabetes-induced decrease in lung nitric oxide bioavailability underlies oxidative damage, enhanced platelet activation, and endothelial injury causing pulmonary microvascular dysfunction and altered vasoreactivity. These findings could provide novel strategies to prevent or reverse diabetes systemic consequences.

Novel Strategies for Endothelial Preservation in Lung Transplant Ischemia-Reperfusion Injury

Lung ischemia reperfusion (IR) injury inevitably occurs during lung transplantation. The pulmonary endothelium is the primary target of IR injury that potentially results in severe pulmonary dysfunction. Over the last decades, various molecules, receptors, and signaling pathways were identified in order to develop treatment strategies for the preservation of the pulmonary endothelium against IR injury. We here review the latest and most promising therapeutic strategies for the protection of the endothelium against IR injury. These include the stabilization of the endothelial glycocalyx, inhibition of endothelial autophagy, inhibition of adhesion molecules, targeting of angiotensin-converting enzyme, and traditional viral and novel non-viral gene transfer approaches. Though some of these strategies proved to be promising in experimental studies, very few of these treatment concepts made the transfer into clinical application. This dilemma underscores the need for more experimental evidence for the translation into clinical studies to invent therapeutic concepts against IR injury-mediated endothelial damage.

Fucoidan attenuates hyperoxia-induced lung injury in newborn rats by mediating lung fibroblasts differentiate into myofibroblasts

Background

Hyperoxia-induced lung injury is one of the most common and frequent diseases in premature infants and may develop into bronchopulmonary dysplasia (BPD). Fucoidan, extracted from brown seaweed and brown algae, has anti-apoptosis, antioxidative and anti-fibrosis effects. This study aimed to explore whether fucoidan could alleviate hyperoxia-induced lung injury in newborn rats.

Methods

Lung wet-weight/dry-weight (W/D) ratio, total protein (TP) content, total cell counts, and lactate dehydrogenase (LDH) levels are used to evaluate lung injury. Masson staining is used to evaluate lung fibrotic. Tunnel assay and Hoechst 33258 assay were used to evaluate apoptosis. The levels of serum superoxide dismutase (SOD), malondialdehyde (MDA), and glutathione (GSH) were measured using ELISA to assess oxidative stress. Western blot assay was used to detect apoptosis-related proteins Bcl-1, Bax, and myofibroblast proteins α-SMA.

Results

The data indicating fucoidan treatment remarkably reduces the lung W/D ratio and TP content, total cell counts, and LDH levels in bronchoalveolar lavage fluid (BALF). Also, fucoidan treatment significantly inhibited cell apoptosis with the elevated expression of Bcl-2/Bax in cultured lung fibroblasts. Moreover, treatment with fucoidan suppressed the levels of MDA significantly and elevated the level of SOD and GSH, showing that oxidative stress was restrained by fucoidan. Furthermore, the decreased expression levels of α-SMA and collagen I was detected in fibroblast treated with fucoidan.

Conclusions

These data suggest fucoidan may protect the lung from hyperoxia via suppressing cell apoptosis, mitigating oxidative stress, and inhibiting lung fibroblasts from differentiating into myofibroblasts.

Expression of Claudin-4 in Lung Ischemia-Reperfusion Injury in Experimental Lung Transplantation

OBJECTIVE

To assess the presence of CLDN4 in bronchoalveolar lavage fluid (BALF) and pulmonary tissue as an early indicator of LIRI and its relationship with changes in pulmonary physiology, edema formation and histology in an experimental porcine model of LTx with CIT of 50 min or 6 h.

METHODS

In 12 pigs, LIRI was produced by: group I (n = 6) LTx with 50 min of CIT (LTx-50 min-CIT); and group II (n = 6) LTx with 6 h of CIT (LTx-6h-CIT). The lung function, edema formation, macroscopic and microscopic changes were assessed. CLDN4 expression in BALF and pulmonary tissue were determined.

RESULTS

Both groups presented similar clinical, edema, and histological damage, as well as similar expression of CLDN4 in BALF and tissue (p > 0.05, RM-ANOVA).

CONCLUSION

CLDN4 expressed in BALF and the pulmonary tissue during the first 5 h within 72 h of the PGD window are not associated by the deterioration of lung function, edema and lung histological injury, in LTx with CIT 50 min or 6 h, CLDN4 does not seem to be a valuable indicator of LIRI.

Diabetic Microvascular Disease and Pulmonary Fibrosis: The Contribution of Platelets and Systemic Inflammation

Diabetes is strongly associated with systemic inflammation and oxidative stress, but its effect on pulmonary vascular disease and lung function has often been disregarded. Several studies identified restrictive lung disease and fibrotic changes in diabetic patients and in animal models of diabetes. While microvascular dysfunction is a well-known complication of diabetes, the mechanisms leading to diabetes-induced lung injury have largely been disregarded. We described the potential involvement of diabetes-induced platelet-endothelial interactions in perpetuating vascular inflammation and oxidative injury leading to fibrotic changes in the lung. Changes in nitric oxide synthase (NOS) activation and decreased NO bioavailability in the diabetic lung increase platelet activation and vascular injury and may account for platelet hyperreactivity reported in diabetic patients. Additionally, the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway has been reported to mediate pancreatic islet damage, and is implicated in the onset of diabetes, inflammation and vascular injury. Many growth factors and diabetes-induced agonists act via the JAK/STAT pathway. Other studies reported the contribution of the JAK/STAT pathway to the regulation of the pulmonary fibrotic process but the role of this pathway in the development of diabetic lung fibrosis has not been considered. These observations may open new therapeutic perspectives for modulating multiple pathways to mitigate diabetes onset or its pulmonary consequences.

Temporal determination of lung NO system and COX-2 upregulation following ischemia-reperfusion injury

BACKGROUND

Pulmonary ischemia-reperfusion (IR) is a biopathological event detectable in several clinical conditions, including lung transplantation, cardiopulmonary bypass, resuscitation, and pulmonary embolism. The understanding behind the activation of various inflammatory mediators regulating the apoptotic pathways remains largely unknown. We investigated the temporal expression of endothelial nitric oxide (eNOS), inducible (iNOS), and cyclooxygenase-2 (COX-2) proteins following lung-IR injury.

METHODS

Lung IR was induced in anesthetized rats. One hour ischemia was performed by clamping the left hilum. eNOS, iNOS, and COX-2 levels in the bronchoalveolar lavage (BAL) were measured at different time points after restoring lung perfusion in conjunction with histological changes and cellular apoptosis.

RESULTS

BAL-eNOS levels were increased as early as 3 hours post IR, attaining the highest values (5.5 U/mL) at 3 hours, compared to non-IR values (2.8 U/mL). BAL-iNOS increased at 3-hour post-IR (3 U/mL). iNOS reached the highest levels at 24 hours (4.5 U/mL) as compared to nonischemic lungs (1.8 U/mL). COX-2 peaked at 12 hours (.025 U/mL) compared to 3, 24, and 48 hours. Highest apoptotic rates were detected at 12 and 48 hours following IR.

CONCLUSIONS

The time-associated involvement of eNOS, iNOS, and COX-2 enzymes during the evolution of IR injury may point to an early reaction of the NOSs system versus the COX-2. Similar patterns of enzymatic activity were previously shown in the context of lung IR injury. This temporal activation may indicate an involvement of eNOS in an early reparative response, and possibly the late-pathological response, mediated by the coinduction of iNOS-COX-2.

Therapeutic potential of fucoidan in myocardial ischemia

Fucoidan, a sulfated polysaccharide extracted from brown seaweed, is a candidate for the treatment of ischemic diseases. The aim of this study was to measure the therapeutic potential of fucoidan in a rat model of myocardial ischemia-reperfusion injury. Forty rats were submitted to myocardial ischemia-reperfusion injury by transient occlusion of the left coronary artery. Rats were then randomized into 2 groups: fucoidan (5 mg/kg, intramuscularly; n = 20) or control (saline intramuscularly; n = 20) was administered 1 hour before injury and daily thereafter for 1 month. At 1 month, plasma levels of stromal cell-derived factor-1α (SDF-1α) were assessed by enzyme-linked immunosorbent assay kit. Hearts were evaluated by histoimmunochemistry. Fucoidan induced significant antifibrotic effects, reducing the infarct scar size by almost 30% on Sirius red-stained sections (9.45% ± 4.27% vs. 13% ± 5.67% in controls; P = 0.03). Vascular density in the fucoidan group (α-actin, RECA-1, or lectin BS1 stained) was increased by 40% (2.18 ± 0.79 mm vs. 1.49 ± 0.42 mm in controls ×200; P = 0.001). Plasma SDF-1α at 1 month was not significantly different between the 2 groups. However, increased immunostaining density of SDF-1α and vascular endothelial growth factor in fibrotic ischemic tissues was observed in fucoidan-treated animals versus controls. In conclusion, fucoidan enhanced tissue repair in myocardial ischemia-reperfusion by promoting revascularization (in situ vascular endothelial growth factor and SDF-1α overexpression) and limiting fibrosis. Consequently, fucoidan may be useful for myocardial ischemic patients.

Interplay of endothelial and inducible nitric oxide synthases modulates the vascular response to ischaemia-reperfusion in the rabbit lung

AIM

Lung ischaemia-reperfusion induces nitric oxide synthesis and reactive nitrogen species, decreasing nitric oxide bioavailability. We hypothesized that in the ventilated lung, this process begins during ischaemia and intensifies with reperfusion, contributing to ischaemia-reperfusion-induced pulmonary vasoconstriction. The aim was to determine whether ischaemia-reperfusion alters inducible and endothelial nitric oxide synthase expression/activity, reactive nitrogen species generation, and nitric oxide bioavailability, potentially affecting pulmonary perfusion.

METHODS

Ischaemia-reperfusion was induced for various times in anesthetized rabbits with ventilated lungs by reversibly occluding the right pulmonary artery and initiating reperfusion. Nitric oxide synthase activity/expression and phosphorylation, reactive nitrogen species generation and total nitrate/nitrite were determined in lung tissue.

RESULTS

Inducible nitric oxide synthase expression and activity, and reactive nitrogen species formation coincided with increased pulmonary vascular resistance during reperfusion and increased with ischaemia duration, further increasing after 2-h reperfusion. Total nitrate/nitrite also increased with ischaemia but decreased after 2-h reperfusion. Pre-treatment with an inducible nitric oxide synthase inhibitor (1400W; Cayman Chemical Company, Ann Arbor, MI, USA) attenuated inducible nitric oxide synthase activity, reactive nitrogen species generation and pulmonary vascular resistance, but did not affect total nitrate/nitrite. Endothelial nitric oxide synthase expression was unchanged by ischaemia-reperfusion; however, its phosphorylation on serine 1177 and dephosphorylation on threonine 495 was uncoupled, suggesting decreased endothelial nitric oxide synthase activity. 1400W prevented uncoupling of endothelial nitric oxide synthase phosphorylation, maintaining its activity during reperfusion.

CONCLUSION

Ischaemia-reperfusion up-regulates inducible nitric oxide synthesis and/activity, which coincides with reduced endothelial nitric oxide synthase activity as suggested by its uncoupling and may contribute to ischaemia-reperfusion-induced pulmonary vasoconstriction.

Soluble p-selectin and the risk of primary graft dysfunction after lung transplantation

BACKGROUND

Platelet activation with subsequent neutrophilic adherence to the vasculature initiates ischemia-reperfusion injury. We hypothesized that higher plasma P-selectin levels reflecting platelet activation would therefore be associated with primary graft dysfunction (PGD) after lung transplantation.

METHODS

In a prospective, multicenter cohort study of 376 patients who had undergone lung transplantation between 2002 and 2007, we measured soluble P-selectin levels before lung transplantation and at 6 and 24 h after lung reperfusion in 20 patients with grade III PGD (Pao(2)/fraction of inspired oxygen, < 200 mm Hg [with alveolar infiltrates seen on chest radiographs]) at 72 h after transplantation and 61 control subjects without PGD.

RESULTS

Higher postoperative soluble P-selectin levels were associated with an increased risk of PGD at 72 h after transplantation (odds ratio [OR] per 1 natural log increase in soluble P-selectin at 6 h after lung allograft reperfusion, 3.5; 95% confidence interval [CI], 1.01 to 11.8; p = 0.048) and at 24 h after lung allograft reperfusion (OR, 4.8; 95% CI, 1.4 to 16.1; p = 0.01). Higher preoperative mean pulmonary artery pressure and the use of cardiopulmonary bypass were also associated with an increased risk of PGD.

CONCLUSION

Higher postoperative soluble P-selectin levels were associated with an increased risk of PGD at 72 h following lung transplantation.

Differential effects of nitric oxide synthesis on pulmonary vascular function during lung ischemia-reperfusion injury

Lung ischemia-reperfusion (IR) injury causes alveolar, epithelial and endothelial cell dysfunction which often results in decreased alveolar perfusion, characteristic of an acute respiratory distress syndrome. Nitric oxide (NO) from endothelium-derived NO synthase (eNOS) helps maintain a low pulmonary vascular resistance. Paradoxically, during acute lung injury, overproduction of NO via inducible NO synthase (iNOS) and oxidative stress lead to reactive oxygen and nitrogen species (ROS and RNS) formation and vascular dysfunction. RNS potentiate vascular and cellular injury by oxidation, by decreasing NO bioavailability, and by regulating NOS isoforms. RNS potentiate their own production by uncoupling NO production through eNOS by oxidation and disruption of Akt-mediated phosphorylation of eNOS. This review focuses on effects of NO which cause vascular dysfunction in the unique environment of the lung and presents a hypothesis for interplay between eNOS and iNOS activation with implications for development of new strategies to treat vascular dysfunction associated with IR.

Lesion-targeted thrombopoietin potentiates vasculogenesis by enhancing motility and enlivenment of transplanted endothelial progenitor cells via activation of Akt/mTOR/p70S6kinase signaling pathway

Thrombopoietin (TPO), a physiological regulator of megakaryocyte and platelet development, is a multifunctional positive regulator in early hematopoiesis by hematopoietic stem cells. In this study, we investigated the effect of TPO on endothelial progenitor cells (EPCs) for therapeutic vasculogenesis in vitro and in vivo, and the intracellular signaling mechanism exerting the activity of EPCs. 7-day culture-expanded EPCs derived from human peripheral blood mononuclear cells were applied to each assay. Flow cytometry demonstrated the expression of c-Mpl, the receptor of TPO, in cultured EPCs. In vitro experiments revealed enhanced migration and survival of cultured EPCs by TPO. In vivo, TPO was intramuscularly administered into the foci of ischemic hindlimbs in athymic nude mice, immediately followed by intravenous injection of cultured EPCs, to assess the booster effect of TPO on vascular regeneration. At day 4 post-transplantation, transplanted EPCs were 1.7-fold higher in TPO-treated animals compared to control. At day 28, blood perfusion was recovered in the TPO-treated group, accompanied by an increase in microvascular density. The signaling transduction pathway underlying TPO-mediated activities of cultured EPCs was assessed by Western blotting. TPO induced sequential phosphorylations of Akt to p70S6kinase through mTOR. Inhibition of the PI3-kinase/Akt/mTOR/p70S6kinase signaling pathway negated the biological functions of cultured EPCs, either migration (by LY294002 for PI3-kinase and Rapamycin for mTOR) or survival and tubulogenesis (by Rapamycin). These findings provide evidence that TPO possesses booster potential for therapeutic vasculogenesis, by activating the PI3-kinase/Akt/mTOR/p70S6kinase pathway crucial to the biological activities of EPCs.

Lung ischemia-reperfusion injury: implications of oxidative stress and platelet-arteriolar wall interactions

Pulmonary ischemia-reperfusion (IR) injury may result from trauma, atherosclerosis, pulmonary embolism, pulmonary thrombosis and surgical procedures such as cardiopulmonary bypass and lung transplantation. IR injury induces oxidative stress characterized by formation of reactive oxygen (ROS) and reactive nitrogen species (RNS). Nitric oxide (NO) overproduction via inducible nitric oxide synthase (iNOS) is an important component in the pathogenesis of IR. Reaction of NO with ROS forms RNS as secondary reactive products, which cause platelet activation and upregulation of adhesion molecules. This mechanism of injury is particularly important during pulmonary IR with increased iNOS activity in the presence of oxidative stress. Platelet-endothelial interactions may play an important role in causing pulmonary arteriolar vasoconstriction and post-ischemic alveolar hypoperfusion. This review discusses the relationship between ROS, RNS, P-selectin, and platelet-arteriolar wall interactions and proposes a hypothesis for their role in microvascular responses during pulmonary IR.

Inhibition of inducible nitric oxide synthase attenuates platelet adhesion in subpleural arterioles caused by lung ischemia-reperfusion in rabbits

Oxidative stress, induced by lung ischemia-reperfusion, leads to platelet and leukocyte activation and may contribute to decreased alveolar perfusion by platelet adhesion to the arteriolar wall. We investigated the hypothesis that ischemia-reperfusion injury increases inducible nitric oxide synthase (iNOS) activity and subsequent generation of reactive nitrogen species with P-selectin-dependent platelet-endothelial interactions and vasoconstriction during lung reperfusion. Subpleural arterioles, labeled platelets, and leukocytes were examined in anesthetized, open-chest rabbits by intravital fluorescence microscopy. Ischemia was caused by reversible occlusion of the right pulmonary artery for 1 or 2 h (1IR and 2IR groups). During 2 h of reperfusion, postischemic platelet rolling and adhesion were independent from leukocyte-arteriolar wall interactions and correlated with pulmonary arteriolar constriction in proportion to the length of ischemia. In rabbits treated with an iNOS inhibitor (1400W) before occlusion (2IR + 1400W group), platelet-arteriolar wall interactions and vasoconstriction were prevented. iNOS expression and activity in ischemic lung tissue were markedly greater than control and also were proportional to ischemia duration. NOS activity, immunochemically detected P-selectin, and nitrotyrosine expression in ischemic lung tissue from animals subjected to ischemia-reperfusion, as well as the plasma level of soluble P-selectin, were significantly higher than in nonischemic lungs and were inhibited by pretreatment with 1400W. These results show that platelet adhesion and arteriolar constriction during early reperfusion in the ventilated lung can result from increased iNOS activity and is highly correlated with reactive nitrogen species and P-selectin expression.

Thrombopoietin gene transfer-mediated enhancement of angiogenic responses to acute ischemia

The development of new blood vessels is a complex process, likely requiring the synergy of multiple angiogenic mediators. This study focuses on the proximal angiogenic response using the platelet as a complex carrier of critical mediators of angiogenesis. Platelet levels are controlled by circulating levels of thrombopoietin (TPO) functioning to activate megakaryocyte differentiation and platelet release through the c-mpl receptor. We hypothesized that TPO gene transfer should enhance correction of experimental ischemia by providing increased levels of platelets and hence platelet-derived mediators of angiogenesis. To evaluate this hypothesis, we dissected the role of the TPO-c-mpl-megakaryocyte-platelet pathway in the angiogenic response using a model of acute hindlimb ischemia of wild-type, TPO(-/-), and c-mpl(-/-) mice. The data demonstrate that infusion of platelets will enhance the angiogenic response in wild-type mice and that the endogenous angiogenic response is blunted in TPO(-/-) and c-mpl(-/-) mice. Consistent with this observation, adenovirus (Ad)-mediated transfer of TPO (AdTPO) enhanced the correction of ischemia in wild-type and TPO(-/-), but not c-mpl(-/-), mice. Local versus systemic administration of AdTPO showed that the effect of TPO gene transfer was systemic, not local, and it could be replaced by gene transfer of VEGF, one of the many mediators of angiogenesis carried by the platelets, even in the absence of components in the TPO-c-mpl-megakaryocyte-platelet pathway.

Pressure-induced leukocyte margination in lung postcapillary venules

Although pressure elevation in lung postcapillary venules increases endothelial P-selectin expression, the extent to which P-selectin causes lung leukocyte margination remains controversial. To address this issue, we optically viewed postcapillary venules of the isolated blood-perfused rat lung by real-time fluorescence imaging. To determine leukocyte margination in single postcapillary venules, we quantified the fluorescence of leukocytes labeled in situ with rhodamine 6G (R6G). Although baseline fluorescence was sparse, a 10-min pressure elevation by 10 cmH(2)O markedly increased R6G fluorescence. Both stopping blood flow during pressure elevation and eliminating leukocytes from the perfusion blocked the fluorescence increase, affirming that these fluorescence responses were attributable to pressure-induced leukocyte margination. A P-selectin-blocking MAb and the L- and P-selectin blocker fucoidin each inhibited the fluorescence increase, indicating that P-selectin was critical for inducing margination. Time-dependent imaging of blood-borne fluorescent beads revealed reduction of plasma velocity during pressure elevation. After pressure returned to baseline, a similar reduction of plasma velocity, established by manually decreasing the perfusion rate, prolonged margination. Our findings show that in lung postcapillary venules, the decrease in plasma velocity critically determines pressure-induced leukocyte margination.