Flow modulates coronary venular permeability by a nitric oxide-related mechanism

Interactions among Endothelial Nitric Oxide Synthase, Cardiovascular System, and Nociception during Physiological and Pathophysiological States

Nitric oxide synthase (NOS) plays important roles within the cardiovascular system in physiological states as well as in pathophysiologic and specific cardiovascular (CV) disease states, such as hypertension (HTN), arteriosclerosis, and cerebrovascular accidents. This review discusses the roles of the endothelial NOS (eNOS) and its effect on cardiovascular responses that are induced by nociceptive stimuli. The roles of eNOS enzyme in modulating CV functions while experiencing pain will be discussed. Nociception, otherwise known as the subjective experience of pain through sensory receptors, termed “nociceptors”, can be stimulated by various external or internal stimuli. In turn, events of various cascade pathways implicating eNOS contribute to a plethora of pathophysiological responses to the noxious pain stimuli. Nociception pathways involve various regions of the brain and spinal cord, including the dorsolateral periaqueductal gray matter (PAG), rostral ventrolateral medulla (RVLM), caudal ventrolateral medulla, and intermediolateral column of the spinal cord. These pathways can interrelate in nociceptive responses to pain stimuli. The alterations in CV responses that affect GABAergic and glutamatergic pathways will be discussed in relation to mechanical and thermal (heat and cold) stimuli. Overall, this paper will discuss the aggregate recent and past data regarding pain pathways and the CV system.

Lymphatic Collecting Vessel: New Perspectives on Mechanisms of Contractile Regulation and Potential Lymphatic Contractile Pathways to Target in Obesity and Metabolic Diseases

Obesity and metabolic syndrome pose a significant risk for developing cardiovascular disease and remain a critical healthcare challenge. Given the lymphatic system's role as a nexus for lipid absorption, immune cell trafficking, interstitial fluid and macromolecule homeostasis maintenance, the impact of obesity and metabolic disease on lymphatic function is a burgeoning field in lymphatic research. Work over the past decade has progressed from the association of an obese phenotype with Prox1 haploinsufficiency and the identification of obesity as a risk factor for lymphedema to consistent findings of lymphatic collecting vessel dysfunction across multiple metabolic disease models and organisms and characterization of obesity-induced lymphedema in the morbidly obese. Critically, recent findings have suggested that restoration of lymphatic function can also ameliorate obesity and insulin resistance, positing lymphatic targeted therapies as relevant pharmacological interventions. There remain, however, significant gaps in our understanding of lymphatic collecting vessel function, particularly the mechanisms that regulate the spontaneous contractile activity required for active lymph propulsion and lymph return in humans. In this article, we will review the current findings on lymphatic architecture and collecting vessel function, including recent advances in the ionic basis of lymphatic muscle contractile activity. We will then discuss lymphatic dysfunction observed with metabolic disruption and potential pathways to target with pharmacological approaches to improve lymphatic collecting vessel function.

Haemodynamic Wall Shear Stress, Endothelial Permeability and Atherosclerosis-A Triad of Controversy

A striking feature of atherosclerosis is its patchy distribution within the vascular system; certain arteries and certain locations within each artery are preferentially affected. Identifying the local risk factors underlying this phenomenon may lead to new therapeutic strategies. The large variation in lesion prevalence in areas of curvature and branching has motivated a search for haemodynamic triggers, particular those related to wall shear stress (WSS). The fact that lesions are rich in blood-derived lipids has motivated studies of local endothelial permeability. However, the location of lesions, the underlying haemodynamic triggers, the role of permeability, the routes by which lipids cross the endothelium, and the mechanisms by which WSS affects permeability have all been areas of controversy. This review presents evidence for and against the current consensus that lesions are triggered by low and/or oscillatory WSS and that this type of shear profile leads to elevated entry of low density lipoprotein (LDL) into the wall via widened intercellular junctions; it also evaluates more recent evidence that lesion location changes with age, that multidirectional shear stress plays a key role, that LDL dominantly crosses the endothelium by transcytosis, and that the link between flow and permeability results from hitherto unrecognised shear-sensitive mediators.

Dissecting VEGF-induced acute versus chronic vascular hyperpermeability: Essential roles of dimethylarginine dimethylaminohydrolase-1

Vascular endothelial cell growth factor (VEGF) is a key regulator of vascular permeability. Herein we aim to understand how acute and chronic exposures of VEGF induce different levels of vascular permeability. We demonstrate that chronic VEGF exposure leads to decreased phosphorylation of VEGFR2 and c-Src as well as steady increases of nitric oxide (NO) as compared to that of acute exposure. Utilizing heat-inducible VEGF transgenic zebrafish (Danio rerio) and establishing an algorithm incorporating segmentation techniques for quantification, we monitored acute and chronic VEGF-induced vascular hyperpermeability in real time. Importantly, dimethylarginine dimethylaminohydrolase-1 (DDAH1), an enzyme essential for NO generation, was shown to play essential roles in both acute and chronic vascular permeability in cultured human cells, zebrafish model, and Miles assay. Taken together, our data reveal acute and chronic VEGF exposures induce divergent signaling pathways and identify DDAH1 as a critical player and potentially a therapeutic target of vascular hyperpermeability-mediated pathogenesis.

Insulin resistance disrupts cell integrity, mitochondrial function, and inflammatory signaling in lymphatic endothelium

OBJECTIVE

Lymphatic vessel dysfunction and increased lymph leakage have been directly associated with several metabolic diseases. However, the underlying cellular mechanisms causing lymphatic dysfunction have not been determined. Aberrant insulin signaling affects the metabolic function of cells and consequently impairs tissue function. We hypothesized that insulin resistance in LECs decreases eNOS activity, disrupts barrier integrity increases permeability, and activates mitochondrial dysfunction and pro-inflammatory signaling pathways.

METHODS

LECs were treated with insulin and/or glucose to determine the mechanisms leading to insulin resistance.

RESULTS

Acute insulin treatment increased eNOS phosphorylation and NO production in LECs via activation of the PI3K/Akt signaling pathway. Prolonged hyperglycemia and hyperinsulinemia induced insulin resistance in LECs. Insulin-resistant LECs produced less NO due to a decrease in eNOS phosphorylation and showed a significant decrease in impedance across an LEC monolayer that was associated with disruption in the adherence junctional proteins. Additionally, insulin resistance in LECs impaired mitochondrial function by decreasing basal-, maximal-, and ATP-linked OCRs and activated NF-κB nuclear translocation coupled with increased pro-inflammatory signaling.

CONCLUSION

Our data provide the first evidence that insulin resistance disrupts endothelial barrier integrity, decreases eNOS phosphorylation and mitochondrial function, and activates inflammation in LECs.

New insights into shear stress-induced endothelial signalling and barrier function: cell-free fluid versus blood flow

Aims

Fluid shear stress (SS) is known to regulate endothelial cell (EC) function. Most of the studies, however, focused on the effects of cell-free fluid-generated wall SS on ECs. The objective of this study was to investigate how changes in blood flow altered EC signalling and endothelial function directly through wall SS and indirectly through SS effects on red blood cells (RBCs).

Methods and results

Experiments were conducted in individually perfused rat venules. We experimentally induced changes in SS that were quantified by measured flow velocity and fluid viscosity. The concomitant changes in EC [Ca2+]i and nitric oxide (NO) were measured with fluorescent markers, and EC barrier function was assessed by fluorescent microsphere accumulation at EC junctions using confocal imaging. EC eNOS activation was evaluated by immunostaining. In response to changes in SS, increases in EC [Ca2+]i and gap formation occurred only in blood or RBC solution perfused vessels, whereas SS-dependent NO production and eNOS-Ser1177 phosphorylation occurred in both plasma and blood perfused vessels. A bioluminescent assay detected SS-dependent ATP release from RBCs. Pharmacological inhibition and genetic modification of pannexin-1 channels on RBCs abolished SS-dependent ATP release and SS-induced increases in EC [Ca2+]i and gap formation.

Conclusions

SS-induced EC NO production occurs in both cell free fluid and blood perfused vessels, whereas SS-induced increases in EC [Ca2+]i and EC gap formation require the presence of RBCs, attributing to SS-induced pannexin-1 channel dependent release of ATP from RBCs. Thus, changes in blood flow alter vascular EC function through both wall SS and SS exerted on RBCs, and RBC released ATP contributes to SS-induced changes in EC barrier function.

Regulation of pulmonary endothelial barrier function by kinases

The pulmonary endothelium is the target of continuous physiological and pathological stimuli that affect its crucial barrier function. The regulation, defense, and repair of endothelial barrier function require complex biochemical processes. This review examines the role of endothelial phosphorylating enzymes, kinases, a class with profound, interdigitating influences on endothelial permeability and lung function.

Real-time imaging and quantitative analysis of doxorubicin transport in a perfusable microvessel platform

Here we report on real-time imaging and quantitative analysis of solute transport in perfusable cylindrical microvessels formed from Madin-Darby canine kidney (MDCK) cells embedded in a collagen matrix. Fluorescence microscopy was used to image the kinetics of doxorubicin transport following injection. To assess the role of efflux pumps on transport, experiments were performed in microvessels formed from MDCK.2, MDCKII-w/t, and MDCKII-MDR1 cells. MDCKII-w/t and MDCKII-MDR1 showed significant doxorubicin accumulation in the cells, characteristic of the pharmacokinetics of doxorubicin. We present a model for doxorubicin transport that takes into account transport across the cell layer. These results demonstrate how real-time imaging of cell microvessels can be used to analyze the mechanisms of transport and distribution following systemic delivery.

Inhibition of endothelial nitric oxide synthase decreases breast cancer cell MDA-MB-231 adhesion to intact microvessels under physiological flows

Nitric oxide (NO) at different concentrations may promote or inhibit tumor growth and metastasis under various conditions. To test the hypothesis that tumor cells prefer to adhere to the locations with a higher endothelial NO production in intact microvessels under physiological flows and to further test that inhibiting NO production decreases tumor cell adhesion, we used intravital fluorescence microscopy to measure NO production and tumor cell adhesion in postcapillary venules of rat mesentery under normal and reduced flow conditions, and in the presence of an endothelial nitric oxide synthase (eNOS) inhibitor, N(G)-monomethyl-l-arginine (l-NMMA). Rats (SD, 250-300 g) were anesthetized. A midline incision (∼2 inch) was made in the abdominal wall, and the mesentery was taken out from the abdominal cavity and spread over a coverslip for the measurement. An individual postcapillary venule (35-50 μm) was first loaded with 4,5-diaminofluorescein diacetate (DAF-2 DA), a fluorescent indictor for NO. Then the DAF-2 intensity was measured for 30 min under a normal or reduced flow velocity, with and without perfusion with MDA-MB-231 breast cancer cells, and in the presence of l-NMMA. We found that tumor cells prefer to adhere to the microvessel locations with a higher NO production such as curved portions. Inhibition of eNOS by l-NMMA attenuated the flow-induced NO production and reduced tumor cell adhesion. We also found that l-NMMA treatment for ∼40 min reduced microvessel permeability to albumin. Our results suggest that inhibition of eNOS is a good approach to preventing tumor cell adhesion to intact microvessels under physiological flows.

5-HT2a receptor antagonism reduces burn-induced macromolecular efflux in rats

BACKGROUND

Major thermal injuries lead to a systemic inflammatory response with systemic capillary leakage and multiple organ dysfunction. This systemic inflammatory response is induced by a variety of immunmodulative molecules including TNFα and serotonin. Unspecific serotonin antagonism leads to reduced macromolecular efflux in rat mesenteries after burn plasma transfer. The aim of the present study was to evaluate the effect of specific 5-HT2a antagonism on early burn edema.

METHODS

Donor rats (DR) underwent thermal injury (100 °C water, 30% BSA, 12 s) for positive controls. For negative controls, DR underwent sham burn (37 °C water, 30% BSA, 12 s). DR plasma (harvested 4 h post-trauma) was transferred to healthy individuals for positive controls. Study rats received burn plasma (BP) and a Bolus injection of Ketanserin (Ket) (1 mg kg(-1) body weight). Negative controls underwent sham burn plasma infusion. Intravital microscopy was performed in mesenteric venules (0/60/120 min). Edema was assessed by FITC-albumin extravasation. Additionally, leukocyte rolling and sticking (cells mm(-2)) as well as microhemodynamic parameters were assessed.

RESULTS

Significant systemic capillary leakage was observed after BP transfer at 120 min and additional administration of Ket attenuated the postburn edema to sham burn levels. Ket also leads to significantly decreased leukocyte-endothelial interactions when compared to positive controls.

CONCLUSION

5-HT2a antagonism reduces plasma extravasation after burn plasma transfer in healthy individuals. The influence of leukocyte-endothelial interactions on postburn edema remains unclear.

Lymphatic vascular integrity is disrupted in type 2 diabetes due to impaired nitric oxide signalling

AIMS

Lymphatic vessel dysfunction is an emerging component of metabolic diseases and can lead to tissue lipid accumulation, dyslipidaemia, and oedema. While lymph leakage has been implicated in obesity and hypercholesterolaemia, whether similar lymphatic dysfunction exists in diabetes has not been investigated.

METHODS AND RESULTS

To measure the lymphatic integrity of transgenic mice, we developed a new assay that quantifies the solute permeability of murine collecting lymphatic vessels. Compared with age-matched wild-type (WT) controls, the permeability of collecting lymphatics from diabetic, leptin receptor-deficient (db/db) mice was elevated >130-fold. Augmenting nitric oxide (NO) production by suffusion of l-arginine rescued this defect. Using pharmacological tools and eNOS(-/-) mice, we found that NO increased WT lymphatic permeability, but reduced db/db lymphatic permeability. These conflicting actions of NO were reconciled by the finding that phosphodiesterase 3 (PDE3), normally inhibited by NO signalling, was active in db/db lymphatics and inhibition of this enzyme restored barrier function.

CONCLUSION

In conclusion, we identified the first lymphatic vascular defect in type 2 diabetes, an enhanced permeability caused by low NO bioavailability. Further, this demonstrates that PDE3 inhibition is required to maintain lymphatic vessel integrity and represents a viable therapeutic target for lymphatic endothelial dysfunction in metabolic disease.

Blood cells and endothelial barrier function

The barrier properties of endothelial cells are critical for the maintenance of water and protein balance between the intravascular and extravascular compartments. An impairment of endothelial barrier function has been implicated in the genesis and/or progression of a variety of pathological conditions, including pulmonary edema, ischemic stroke, neurodegenerative disorders, angioedema, sepsis and cancer. The altered barrier function in these conditions is often linked to the release of soluble mediators from resident cells (e.g., mast cells, macrophages) and/or recruited blood cells. The interaction of the mediators with receptors expressed on the surface of endothelial cells diminishes barrier function either by altering the expression of adhesive proteins in the inter-endothelial junctions, by altering the organization of the cytoskeleton, or both. Reactive oxygen species (ROS), proteolytic enzymes (e.g., matrix metalloproteinase, elastase), oncostatin M, and VEGF are part of a long list of mediators that have been implicated in endothelial barrier failure. In this review, we address the role of blood borne cells, including, neutrophils, lymphocytes, monocytes, and platelets, in the regulation of endothelial barrier function in health and disease. Attention is also devoted to new targets for therapeutic intervention in disease states with morbidity and mortality related to endothelial barrier dysfunction.

Mechanosensing at the vascular interface

Mammals are endowed with a complex set of mechanisms that sense mechanical forces imparted by blood flow to endothelial cells (ECs), smooth muscle cells, and circulating blood cells to elicit biochemical responses through a process referred to as mechanotransduction. These biochemical responses are critical for a host of other responses, including regulation of blood pressure, control of vascular permeability for maintaining adequate perfusion of tissues, and control of leukocyte recruitment during immunosurveillance and inflammation. This review focuses on the role of the endothelial surface proteoglycan/glycoprotein layer-the glycocalyx (GCX)-that lines all blood vessel walls and is an agent in mechanotransduction and the modulation of blood cell interactions with the EC surface. We first discuss the biochemical composition and ultrastructure of the GCX, highlighting recent developments that reveal gaps in our understanding of the relationship between composition and spatial organization. We then consider the roles of the GCX in mechanotransduction and in vascular permeability control and review the prominent interaction of plasma-borne sphingosine-1 phosphate (S1P), which has been shown to regulate both the composition of the GCX and the endothelial junctions. Finally, we consider the association of GCX degradation with inflammation and vascular disease and end with a final section on future research directions.

Nitric oxide, S-nitrosation, and endothelial permeability

S-Nitrosation is rapidly emerging as a regulatory mechanism in vascular biology, with particular importance in the onset of hyperpermeability induced by pro-inflammatory agents. This review focuses on the role of endothelial nitric oxide synthase (eNOS)-derived nitric oxide (NO) in regulating S-Nitrosation of adherens junction proteins. We discuss evidence for translocation of eNOS, via caveolae, to the cytosol and analyze the significance of eNOS location for S-Nitrosation and onset of endothelial hyperpermeability to macromolecules.

Microvascular permeability to water is independent of shear stress, but dependent on flow direction

Endothelial cells in a cultured monolayer change from a "cobblestone" configuration when grown under static conditions to a more elongated shape, aligned with the direction of flow, after exposure to sustained uniform shear stress. Sustained blood flow acts to protect regions of large arteries from injury. We tested the hypothesis that the stable permeability state of individually perfused microvessels is also characteristic of flow conditioning. In individually perfused rat mesenteric venular microvessels, microvascular permeability, measured as hydraulic conductivity (Lp), was stable [mean 1.0 × 10(-7) cm/(s × cmH2O)] and independent of shear stress (3-14 dyn/cm(2)) for up to 3 h. Vessels perfused opposite to the direction of normal blood flow exhibited a delayed Lp increase [ΔLp was 7.6 × 10(-7) cm/(s × cmH2O)], but the increase was independent of wall shear stress. Addition of chondroitin sulfate and hyaluronic acid to perfusates increased the shear stress range, but did not modify the asymmetry in response to flow direction. Increased Lp in reverse-perfused vessels was associated with numerous discontinuities of VE-cadherin and occludin, while both proteins were continuous around the periphery of forward-perfused vessels. The results are not consistent with a general mechanism for graded shear-dependent permeability increase, but they are consistent with the idea that a stable Lp under normal flow contributes to prevention of edema formation and also enables physiological regulation of shear-dependent small solute permeabilities (e.g., glucose). The responses during reverse flow are consistent with reports that disturbed flows result in a less stable endothelial barrier in venular microvessels.

Erythrocyte-derived sphingosine-1-phosphate stabilizes basal hydraulic conductivity and solute permeability in rat microvessels

Exogenous sphingosine-1-phosphate (S1P), a lipid mediator in blood, attenuates acute microvascular permeability increases via receptor S1P1 to stabilize the endothelium. To evaluate the contribution of erythrocytes as an endogenous source of S1P to the regulation of basal permeability, we measured permeability coefficients in intact individually perfused venular microvessels of rat mesentery. This strategy also enabled the contributions of other endogenous S1P sources to be evaluated. Apparent permeability coefficients (P(S)) to albumin and α-lactalbumin and the hydraulic conductivity of mesenteric microvessels were measured in the presence or absence of rat erythrocytes or rat erythrocyte-conditioned perfusate. Rat erythrocytes added to the perfusate were the principal source of S1P in these microvessels. Basal P(S) to albumin was stable and typical of blood-perfused microvessels (mean 0.5 × 10(-6) cm/s) when erythrocytes or erythrocyte-conditioned perfusates were present. When they were absent, P(S) to albumin or α-lactalbumin increased up to 40-fold (over 10 min). When exogenous S1P was added to perfusates, permeability returned to levels comparable with those seen in the presence of erythrocytes. Addition of SEW 2871, an agonist specific for S1P1, in the absence of red blood cells reduced P(S)(BSA) (40-fold reduction) toward basal. The specific S1P1 receptor antagonist (W-146) reversed the stabilizing action of erythrocytes and increased permeability (27-fold increase) in a manner similar to that seen in the absence of erythrocytes. Erythrocytes are a primary source of S1P that maintains normal venular microvessel permeability. Absence of erythrocytes or conditioned perfusate in in vivo and in vitro models of endothelial barriers elevates basal permeability.

Myosin light chain kinase signaling in endothelial barrier dysfunction

Microvascular barrier dysfunction is a serious problem that occurs in many inflammatory conditions, including sepsis, trauma, ischemia-reperfusion injury, cardiovascular disease, and diabetes. Barrier dysfunction permits extravasation of serum components into the surrounding tissue, leading to edema formation and organ failure. The basis for microvascular barrier dysfunction is hyperpermeability at endothelial cell-cell junctions. Endothelial hyperpermeability is increased by actomyosin contractile activity in response to phosphorylation of myosin light chain by myosin light chain kinase (MLCK). MLCK-dependent endothelial hyperpermeability occurs in response to inflammatory mediators (e.g., activated neutrophils, thrombin, histamine, tumor necrosis factor alpha, etc.), through multiple cell signaling pathways and signaling molecules (e.g., Ca(++) , protein kinase C, Src kinase, nitric oxide synthase, etc.). Other signaling molecules protect against MLCK-dependent hyperpermeability (e.g., sphingosine-1-phosphate or cAMP). In addition, individual MLCK isoforms play specific roles in endothelial barrier dysfunction, suggesting that isoform-specific inhibitors could be useful for treating inflammatory disorders and preventing multiple organ failure. Because endothelial barrier dysfunction depends upon signaling through MLCK in many instances, MLCK-dependent signaling comprises multiple potential therapeutic targets for preventing edema formation and multiple organ failure. The following review is a discussion of MLCK-dependent mechanisms and cell signaling events that mediate endothelial hyperpermeability.

Endothelial glycocalyx: permeability barrier and mechanosensor

Endothelial cells are covered with a polysaccharide rich layer more than 400 nm thick, mechanical properties of which limit access of circulating plasma components to endothelial cell membranes. The barrier properties of this endothelial surface layer are deduced from the rate of tracer penetration into the layer and the mechanics of red and white cell movement through capillary microvessels. This review compares the mechanosensor and permeability properties of an inner layer (100-150 nm, close to the endothelial membrane) characterized as a quasi-periodic structure which accounts for key aspects of transvascular exchange and vascular permeability with those of the whole endothelial surface layers. We conclude that many of the barrier properties of the whole surface layer are not representative of the primary fiber matrix forming the molecular filter determining transvascular exchange. The differences between the properties of the whole layer and the inner glycocalyx structures likely reflect dynamic aspects of the endothelial surface layer including tracer binding to specific components, synthesis and degradation of key components, activation of signaling pathways in the endothelial cells when components of the surface layer are lost or degraded, and the spatial distribution of adhesion proteins in microdomains of the endothelial cell membrane.

Wall structures of myocardial precapillary arterioles and postcapillary venules reexamined and reconstructed in vitro for studies on barrier functions

The barrier functions of myocardial precapillary arteriolar and postcapillary venular walls (PCA or PCV, respectively) are of considerable scientific and clinical interest (regulation of blood flow and recruitment of immune defense). Using enzyme histochemistry combined with confocal microscopy, we reexamined the cell architecture of human PCA and PVC and reconstructed appropriate in vitro models for studies of their barrier functions. Contrary to current opinion, the PCA endothelial tube is encompassed not by smooth muscle cells but rather by a concentric layer of pericytes cocooned in a thick, microparticle-containing extracellular matrix (ECM) that contributes substantially to the tightness of the arteriolar wall. This core tube extends upstream into the larger arterioles, there additionally enwrapped by smooth muscle. PCV consist of an inner layer of large, contractile endothelial cells encompassed by a fragile, wide-meshed pericyte network with a weakly developed ECM. Pure pericyte and endothelial cell preparations were isolated from PCA and PCV and grown in sandwich cultures. These in vitro models of the PCA and PCV walls exhibited typical histological and functional features. In both plasma-like (PLM) and serum-containing (SCM) media, the PCA model (including ECM) maintained its low hydraulic conductivity (L(P) = 3.24 ± 0.52·10(-8)cm·s(-1)·cmH(2)O(-1)) and a high selectivity index for transmural passage of albumin (SI(Alb) = 0.95 ± 0.02). In contrast, L(P) and SI(Alb) in the PCV model (almost no ECM) were 2.55 ± 0.32·10(-7)cm·s(-1)·cmH(2)O(-1) and 0.88 ± 0.03, respectively, in PLM, and 1.39 ± 0.10·10(-6)cm·s(-1)·cmH(2)O(-1) and 0.49 ± 0.04 in SCM. With the use of these models, systematic, detailed studies on the regulation of microvascular barrier properties now appear to be feasible.

Neutrophil transmigration, focal adhesion kinase and endothelial barrier function

Neutrophil activation is an essential component of innate immune defense against infection and injury. In response to inflammatory stimulation, circulating neutrophils undergo a series of dynamic and metabolic changes characterized by β2-intergrin mediated adhesion to microvascular endothelium and subsequent transendothelial migration. During this process, neutrophils release granular contents containing digestive enzymes and produce cytotoxic agents such as reactive oxygen species and cytokines. These products target endothelial barriers inducing phosphorylation-triggered junction dissociation, actin stress fiber formation, and actomyosin contraction, manifest as paracellular hyperpermeability. Endothelial cell-matrix focal adhesions play an integral role in this process by providing structural support for endothelial conformational changes that facilitate neutrophil transmigration, as well as by recruiting intracellular molecules that constitute the hyperpermeability signaling cascades. As a central connector of the complex signaling network, focal adhesion kinase (FAK) is activated following neutrophil adhesion, and further mediates the reorganization of endothelial integrin-matrix attachments in a pattern coordinating with cytoskeleton contraction and junction opening. In this review, we present recent experimental evidence supporting the importance of FAK in neutrophil-dependent regulation of endothelial permeability. The discussion focuses on the mechanisms by which neutrophils activate FAK and its downstream effects on endothelial barriers.

3,3′-Dioctadecylindocarbocyanine-low-density lipoprotein uptake and flow patterns in the rabbit aorta–iliac bifurcation under three perfusion flow conditions

The aim of this study was to elucidate which of the following two factors plays a more important role in the localization of atherogenesis: the barrier function of the arterial endothelium modulated by wall shear stress or flow-dependent low-density lipoprotein (LDL) concentration at the blood/wall interface. To determine this, the rabbit aorto-iliac bifurcation was perfused with 3,3′-dioctadecylindocarbocyanine (DiI)-LDL solution under three different flow conditions: (i) forward flow (perfused in the in vivo flow direction); (ii) backward flow (perfused in a reversed flow direction); and (iii) static group (no flow). The results showed that there was a peak in the curve of DiI-LDL uptake distribution along the lateral wall of the bifurcation for all three groups, which was located in the branching areas where the endothelial cells were round and polygonal with no preferred orientation. Nevertheless, the peak of the forward flow group was much sharper than those of the other two groups. The overall DiI-LDL uptake was the highest for the static group. The present experimental study supports the concept that both the barrier function of the endothelium modulated by wall shear stress and the mass transport phenomenon of LDL concentration polarization are involved in the infiltration/accumulation of atherogenic lipids within the arterial wall. Nevertheless, the latter might play a larger role in the localization of atherogenesis.

Intrinsic nitric oxide and superoxide production regulates descending vasa recta contraction

Descending vasa recta (DVR) are 12- to 15-μm microvessels that supply the renal medulla with blood flow. We examined the ability of intrinsic nitric oxide (NO) and reactive oxygen species (ROS) generation to regulate their vasoactivity. Nitric oxide synthase (NOS) inhibition with N(ω)-nitro-l-arginine methyl ester (l-NAME; 100 μmol/l), or asymmetric N(G),N(G)-dimethyl-l-arginine (ADMA; 100 μmol/l), constricted isolated microperfused DVR by 48.82 ± 4.34 and 27.91 ± 2.91%, respectively. Restoring NO with sodium nitroprusside (SNP; 1 mmol/l) or application of 8-Br-cGMP (100 μmol/l) reversed DVR vasoconstriction by l-NAME. The superoxide dismutase mimetic Tempol (1 mmol/l) and the NAD(P)H inhibitor apocynin (100, 1,000 μmol/l) also blunted ADMA- or l-NAME-induced vasoconstriction, implicating a role for concomitant generation of ROS. A role for ROS generation was also supported by an l-NAME-associated rise in oxidation of dihydroethidium that was prevented by Tempol or apocynin. To test whether H(2)O(2) might play a role, we examined its direct effects. From 1 to 100 μmol/l, H(2)O(2) contracted DVR whereas at 1 mmol/l it was vasodilatory. The H(2)O(2) scavenger polyethylene glycol-catalase reversed H(2)O(2) (10 μmol/l)-induced vasoconstriction; however, it did not affect l-NAME-induced contraction. Finally, the previously known rise in DVR permeability to (22)Na and [(3)H]raffinose that occurs with luminal perfusion was not prevented by NOS blockade. We conclude that intrinsic production of NO and ROS can modulate DVR vasoactivity and that l-NAME-induced vasoconstriction occurs, in part, by modulating superoxide concentration and not through H(2)O(2) generation. Intrinsic NO production does not affect DVR permeability to hydrophilic solutes.

Shear stress and the endothelial transport barrier

The shear stress of flowing blood on the surfaces of endothelial cells that provide the barrier to transport of solutes and water between blood and the underlying tissue modulates the permeability to solutes and the hydraulic conductivity. This review begins with a discussion of transport pathways across the endothelium and then considers the experimental evidence from both in vivo and in vitro studies that shows an influence of shear stress on endothelial transport properties after both acute (minutes to hours) and chronic (hours to days) changes in shear stress. Next, the effects of shear stress on individual transport pathways (tight junctions, adherens junctions, vesicles and leaky junctions) are described, and this information is integrated with the transport experiments to suggest mechanisms controlling both acute and chronic responses of transport properties to shear stress. The review ends with a summary of future research challenges.

Myosin light chain kinase in microvascular endothelial barrier function

Microvascular barrier dysfunction is implicated in the initiation and progression of inflammation, posttraumatic complications, sepsis, ischaemia-reperfusion injury, atherosclerosis, and diabetes. Under physiological conditions, a precise equilibrium between endothelial cell-cell adhesion and actin-myosin-based centripetal tension tightly controls the semi-permeability of microvascular barriers. Myosin light chain kinase (MLCK) plays an important role in maintaining the equilibrium by phosphorylating myosin light chain (MLC), thereby inducing actomyosin contractility and weakening endothelial cell-cell adhesion. MLCK is activated by numerous physiological factors and inflammatory or angiogenic mediators, causing vascular hyperpermeability. In this review, we discuss experimental evidence supporting the crucial role of MLCK in the hyperpermeability response to key cell signalling events during inflammation. At the cellular level, in vitro studies of cultured endothelial monolayers treated with MLCK inhibitors or transfected with specific inhibiting peptides have demonstrated that induction of endothelial MLCK activity is necessary for hyperpermeability. Ex vivo studies of live microvessels, enabled by development of the isolated, perfused venule method, support the importance of MLCK in endothelial permeability regulation in an environment that more closely resembles in vivo tissues. Finally, the role of MLCK in vascular hyperpermeability has been confirmed with in vivo studies of animal disease models and the use of transgenic MLCK210 knockout mice. These approaches provide a more complete view of the role of MLCK in vascular barrier dysfunction.

The NO cascade, eNOS location, and microvascular permeability

The nitric oxide (NO) cascade and endothelial NO synthase (eNOS) are best known for their role in endothelium-mediated relaxation of vascular smooth muscle. Activation of eNOS by certain inflammatory stimuli and enhanced NO release have also been shown to promote increased microvascular permeability. However, it is not entirely clear why activation of eNOS by certain vasodilatory agents, like acetylcholine, does not affect microvascular permeability, whereas activation of eNOS by other inflammatory agents that increase permeability, like platelet-activating factor, does not cause vasodilation. In this review, we discuss the evidence demonstrating the role of eNOS in the elevation of microvascular permeability. We also examine the relative importance of eNOS phosphorylation and localization in its function to promote elevated microvascular permeability as well as emerging topics with regard to eNOS and microvascular permeability regulation.

Lymphatic endothelial cells adapt their barrier function in response to changes in shear stress

BACKGROUND

Lymphatic endothelial cells form an important barrier necessary for normal lymph formation and propulsion. However, little is known about how physical forces within lymphatic vessels affect endothelial barrier function. The purpose of this study was to characterize how laminar flow affects lymphatic endothelial barrier function and to test whether endothelial cells respond to flow changes by activating the intracellular actin cytoskeleton to enhance barrier function.

METHODS AND RESULTS

Cultured adult human dermal microlymphatic endothelial cells (HMLEC-d) were grown on small gold electrodes arranged within a flow channel, and transendothelial electrical resistance (TER), an index of barrier function, was determined. Laminar flow was applied to the cells at a baseline shear stress of 0.5 dynes/cm(2), and was increased to 2.5, 5.0, or 9.0 dynes/cm(2), causing a magnitude-dependent increase in barrier function that was reversed 30 min later when the shear stress was returned to baseline. This response was abolished by blockade of actin dynamics with 10 microM phalloidin, and significantly inhibited by blockade of Rac1 activity with 50 microM NSC23766. Blockade of protein kinase A (10 microM H-89) did not inhibit the response. Mathematical modeling based on our impedance data showed that the flow-induced changes in TER were primarily due to altered current flow between cells and not beneath cells.

CONCLUSIONS

These results suggest that lymphatic endothelial cells dynamically alter their morphology and barrier function in response to changes in shear stress by a mechanism dependent upon Rac1-mediated actin dynamics.

Regulation of hydraulic conductivity in response to sustained changes in pressure

The present study addresses the effect of a sustained change in pressure on microvascular permeability assessed by hydraulic conductivity ( Lp) measurements from microvessels of the rat mesentery. With a microperfusion technique, transvascular filtration (normalized to surface area; Jv/ S) and Lp were measured in small arterioles (baseline Lp = 0.26 × 10−7 cm·s−1·cmH2O−1) and venules (baseline Lp = 2.88 × 10−7 cm·s−1·cmH2O−1). The main finding of this study is that step increases in microvascular pressure led to time-dependent alterations of Lp. Immediately after a twofold step increase in pressure, Jv/ S increased in proportion to the pressure change. This observation is consistent with Starling's law that predicts filtration proportional to the overall pressure gradient when Lp is constant. However, when Jv/ S measurements continued for 60–90 min past the step in pressure, there was an initial decrease in Jv/ S for 30 min (“sealing effect”) followed by a substantial increase in Jv/ S out to 90 min. The sustained increase in Jv/ S suggests an increase in Lp of 36 ± 7% for small arterioles and 42 ± 5% for small venules ( P < 0.05 for both). In addition, the increase in Lp in response to an increase in pressure was attenuated significantly by nitric oxide synthase inhibition. These results indicate that a pressure-induced mechanical stimulus (possibly Jv) activates a NO-dependent biochemical response that leads to an increase in hydraulic conductivity.

Constitutive eNOS-derived nitric oxide is a determinant of endothelial junctional integrity

Basal vascular endothelial permeability is normally kept low in part by the restrictiveness of interendothelial junctions (IEJs). We investigated the possible role of nitric oxide (NO) in controlling IEJ integrity and thereby regulating basal vascular permeability. We determined the permeability of continuous endothelia in multiple murine vascular beds, including lung vasculature, of wild-type mice, endothelial nitric oxide synthase (eNOS) null mice, and mice treated with NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME). Light and electron microscopic studies revealed that L-NAME treatment resulted in IEJs opening within a few minutes with a widespread response within 30 min. We observed a 35% increase in transendothelial transport of albumin, using as tracer dinitrophenylated albumin in mouse lungs and other organs studied. To rule out the involvement of blood cells in the mechanism of increased endothelial permeability, vascular beds were flushed free of blood, treated with L-NAME, and perfused with the tracer. The open IEJs observed in these studies indicated a direct role for NO in preserving the normal structure of endothelial junctions. We also used the electron-opaque tracer lanthanum chloride to assess vascular permeability. Lanthanum chloride was presented by perfusion to various vascular beds of mice lacking NO. Open IEJs were seen only in capillary and venular endothelial segments of mice lacking NO, and there was a concomitant increase in vascular permeability to the tracer. Together, these data demonstrate that constitutive eNOS-derived NO is a crucial determinant of IEJ integrity and thus serves to maintain the low basal permeability of continuous endothelia.

Involvement of ROCK-mediated endothelial tension development in neutrophil-stimulated microvascular leakage

Neutrophil-induced coronary microvascular barrier dysfunction is an important pathophysiological event in heart disease. Currently, the precise cellular and molecular mechanisms of neutrophil-induced microvascular leakage are not clear. The aim of this study was to test the hypothesis that rho kinase (ROCK) increases coronary venular permeability in association with elevated endothelial tension. We assessed permeability to albumin (P(a)) in isolated porcine coronary venules and in coronary venular endothelial cell (CVEC) monolayers. Endothelial barrier function was also evaluated by measuring transendothelial electrical resistance (TER) of CVEC monolayers. In parallel, we measured isometric tension of CVECs grown on collagen gels. Transference of constitutively active (ca)-ROCK protein into isolated coronary venules or CVEC monolayers caused a significant increase in P(a) and decreased TER in CVECs. The ROCK inhibitor Y-27632 blocked the ca-ROCK-induced changes. C5a-activated neutrophils (10(6)/ml) also significantly elevated venular P(a), which was dose-dependently inhibited by Y-27632 and a structurally distinct ROCK inhibitor, H-1152. In CVEC monolayers, activated neutrophils increased permeability with a concomitant elevation in isometric tension, both of which were inhibited by Y-27632 or H-1152. Treatment with ca-ROCK also significantly increased CVEC monolayer permeability and isometric tension, coupled with actin polymerization and elevated phosphorylation of myosin regulatory light chain on Thr18/Ser19. The data suggest that during neutrophil activation, ROCK promotes microvascular leakage in association with actin-myosin-mediated tension development in endothelial cells.

Regulation of capillary hydraulic conductivity in response to an acute change in shear

The effects of mechanical perturbations (shear stress, pressure) on microvascular permeability primarily have been examined in micropipette-cannulated vessels or in endothelial monolayers in vitro. The objective of this study is to determine whether acute changes in blood flow shear stress might influence measurements of hydraulic conductivity (L(p)) in autoperfused microvessels in vivo. Rat mesenteric microvessels were observed via intravital microscopy. Occlusion of a third-order arteriole with a micropipette was used to divert and increase flow through a nonoccluded capillary or fourth-order arteriolar branch. Transvascular fluid filtration rate in the branching vessel was measured with a Landis technique. Flow (shear)-induced increases in L(p) disappeared within 20-30 s of the removal of the shear and could be eliminated with nitric oxide synthase inhibition. The shear-induced increase in L(p) was greater in capillaries compared with terminal arterioles. An acute change in shear may regulate L(p) by a nitric oxide-dependent mechanism that displays heterogeneity within a microvascular network.

Sexual dimorphism in the permeability response of coronary microvessels to adenosine

Gender influences volume regulation via several mechanisms; whether these include microvascular exchange, especially in the heart, is not known. In response to adenosine (Ado), permeability (P(s)) to protein of coronary arterioles of female pigs decreases acutely. Whether Ado induces similar P(s) changes in arterioles from males or whether equivalent responses occur in coronary venules of either sex has not been determined. Hypotheses that 1) basal P(s) properties and 2) P(s) responses to vasoactive stimuli are sex independent were evaluated from measures of P(s) to two hydrophilic proteins, alpha-lactalbumin and porcine serum albumin (PSA), in arterioles and venules isolated from hearts of adult male and female pigs. Consistent with hypothesis 1, basal P(s) values of both microvessel types were independent of sex. Contrary to hypothesis 2, P(s) responses to Ado varied with sex, protein, and vessel type. Confirming earlier studies, Ado induced a approximately 20% decrease in P(s) to both proteins in coronary arterioles from females. In arterioles from males, Ado did not change P(s) for alpha-lactalbumin (P(s)(alpha-lactalb), 3 +/- 13%), whereas P(s) for PSA (P(s)(PSA)) decreased by 27 +/- 8% (P < 0.005). In venules from females, Ado elevated P(s)(PSA) by 44 +/- 20% (P < 0.05), whereas in those from males, Ado reduced P(s)(PSA) by 24 +/- 5% (P < 0.05). The variety of outcomes is consistent with transvascular protein and protein-carried solute flux being regulated by multiple sex-dependent mechanisms in the heart and provides evidence of differences in exchange homeostasis of males and females in health and, likely, disease.

The protein kinase MEK1/2 mediate vascular endothelial growth factor- and histamine-induced hyperpermeability in porcine coronary venules

Mitogen-activated protein kinases (MAPKs) have been implicated in the signal transduction of the endothelial response to growth factors and inflammatory stimuli. The objective of this study was to test the hypothesis that the p42/44 MAPK pathway plays a common role in mediating the microvascular hyperpermeability response to vascular endothelial growth factor (VEGF) and histamine. The apparent permeability coefficient of albumin was measured in isolated and perfused coronary venules. Application of VEGF induced a rapid increase in venular permeability, and the effect was blocked by PD98059 and UO126, selective inhibitors of the mitogen-activated protein kinase kinase MEK1/2, in a dose-dependent pattern. The same MEK1/2 inhibitors dose-dependently attenuated the increase in venular permeability caused by histamine. In addition, the increases in venular permeability caused by agents that are known to activate the nitric oxide pathway, including the calcium ionophore ionomycin, the nitric oxide donor S-nitroso-N-acetylpenicillamine, and the protein kinase G activator 8-bromo-cGMP, were significantly attenuated in venules pretreated with the MEK1/2 inhibitors. Furthermore, transfection of venules with active MEK1 increased baseline permeability. In contrast, transfection of active ERK1, a downstream target of MEK1/2, did not significantly alter the basal permeability of venules. Moreover, inhibition of ERK1/2 with a specific inhibiting peptide did not prevent the hyperpermeability response to VEGF or histamine. The results suggest that activation of MEK1/2 may play a central role in the signal transduction of microvascular hyperpermeability in response to growth factors and inflammatory mediators.

Effect of glycocalyx on shear-dependent albumin uptake in endothelial cells

The glycocalyx layer on the surface of an endothelial cell is an interface barrier for uptake of macromolecules, such as low-density lipoprotein and albumin, in the cell. The shear-dependent uptake of macromolecules thus might govern the function of the glycocalyx layer. We therefore studied the effect of glycocalyx on the shear-dependent uptake of macromolecules into endothelial cells. Bovine aorta endothelial cells were exposed to shear stress stimulus ranging from 0.5 to 3.0 Pa for 48 h. The albumin uptake into the cells was then measured using confocal laser scanning microscopy, and the microstructure of glycocalyx was observed using electron microscopy. Compared with the uptake into endothelial cells under static conditions (no shear stress stimulus), the albumin uptake at a shear stress of 1.0 Pa increased by 16% and at 3.0 Pa decreased by 27%. Compared with static conditions, the thickness of the glycocalyx layer increased by 70% and the glycocalyx charge increased by 80% at a shear stress of 3.0 Pa. The albumin uptake at a shear stress of 3.0 Pa for cells with a neutralized (no charge) glycocalyx layer was almost twice that of cells with charged layer. These findings indicate that glycocalyx influences the albumin uptake at higher shear stress and that glycocalyx properties (thickness and charge level) are involved with the shear-dependent albumin uptake process.

Vascular transfer of adenovirus is augmented by nitric oxide in the rat heart

Reversible opening of the endothelial barrier remains a major obstacle when hearts are transfected via the coronary system. Our aim was to establish an experimental system permitting the continuous analysis of vascular transfer of virus in the intact heart. Isolated saline-perfused rat hearts were inverted and covered with a latex cap to collect interstitial transudate (IT) on the pericardial surface. Adenovirus (10(9) pfu/ml) was stably labeled with rhodamine fluorescent dye. Analysis of IT and coronary perfusate revealed that under baseline conditions, adenovirus in the IT reached 75% of its vascular concentration within 3 min. The nitric oxide-donors S-nitroso-N-acetyl penicillamine (SNAP) and bradykinin (BK) were the most effective substances to increase total IT volume and adenoviral interstitial concentration. Perfusion with 9% serum markedly reduced IT volume flow and delayed the SNAP/BK effect. Our findings demonstrate that SNAP and BK effectively increased coronary transfer of adenovirus suggesting that the inverted isolated heart is a suitable model to optimize vascular transfer of virus under standardized conditions.

A transmural pressure gradient induces mechanical and biological adaptive responses in endothelial cells

A sudden increase in the transmural pressure gradient across endothelial monolayers reduces hydraulic conductivity ( Lp), a phenomenon known as the sealing effect. To further characterize this endothelial adaptive response, we measured bovine aortic endothelial cell (BAEC) permeability to albumin and 70-kDa dextran, Lp, and the solvent-drag reflection coefficients (σ) during the sealing process. The diffusional permeability coefficients for albumin (1.33 ± 0.18 × 10–6cm/s) and dextran (0.60 ± 0.16 × 10–6cm/s) were measured before pressure application. The effective permeabilities (measured when solvent drag contributes to solute transport) of albumin and dextran ( Pealband Pedex) were measured after the application of a 10 cmH2O pressure gradient; during the first 2 h of pressure application, Pealb, Pedex, and Lpwere significantly reduced by 2.0 ± 0.3-, 2.1 ± 0.3-, and 3.7 ± 0.3-fold, respectively. Immunostaining of the tight junction (TJ) protein zonula occludens-1 (ZO-1) was significantly increased at cell-cell contacts after the application of transmural pressure. Cytochalasin D treatment significantly elevated transport but did not inhibit the adaptive response, whereas colchicine treatment had no effect on diffusive permeability but inhibited the adaptive response. Neither cytoskeletal inhibitor altered σ despite significantly elevating both Lpand effective permeability. Our data suggest that BAECs actively adapt to elevated transmural pressure by mobilizing ZO-1 to intercellular junctions via microtubules. A mechanical (passive) component of the sealing effect appears to reduce the size of a small pore system that allows the transport of water but not dextran or albumin. Furthermore, the structures of the TJ determine transport rates but do not define the selectivity of the monolayer to solutes (σ).

Mass transport in arteries and the localization of atherosclerosis

Atherosclerosis is a disease of the large arteries that involves a characteristic accumulation of high-molecular-weight lipoprotein in the arterial wall. This review focuses on the mass transport processes that mediate the focal accumulation of lipid in arteries and places particular emphasis on the role of fluid mechanical forces in modulating mass transport phenomena. In the final analysis, four mass transport mechanisms emerge that may be important in the localization of atherosclerosis: blood phase controlled hypoxia, leaky endothelial junctions, transient intercellular junction remodeling, and convective clearance of the subendothelial intima and media. Further study of these mechanisms may contribute to the development of therapeutic strategies for atherosclerotic diseases.

Effects of perfusion rate on permeability of frog and rat mesenteric microvessels to sodium fluorescein

The permeability, P(S), to sodium fluorescein (Stokes-Einstein radius = 0.45 nm) has been measured in single mesenteric capillaries of pithed frogs and anaesthetised rats as perfusion velocity, U, was varied over a range from 400 up to 2000-10,000 microm s(-1). P(S) increased linearly with U. In 20 frog capillaries, mean (+/- S.E.M.) P(S) (in microm s(-1)) = 9.35 (+/- 1.55)U x 10(-5) + 0.244 (+/- 0.0291). Similarly, in nine rat venules, mean P(S) = 1.62 (+/- 0.385)U x 10(-4) + 0.375 (+/- 0.025). The flow-dependent component of permeability could be reversibly abolished in frog capillaries by superfusing with 100 microM noradrenaline and by superfusing rat venules with the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine (20 microM). It was shown that changes in microvascular pressure accompanying changes in U during free perfusion could account for only 15 % of the changes in P(S), i.e. 85 % of the changes in P(S) were changes in the permeability coefficient itself. A comparison between the changes in P(S) with U and the previously described changes in microvascular permeability to K(+) with U, suggest that if the flow-dependent component of permeability is modelled as a population of pores of constant size, these have radii of 0.8 nm. Such a pathway would limit flow-dependent permeability to small hydrophilic molecules and have minimal effect on net fluid exchange.

Measurement of hydraulic conductivity of single perfused Rana mesenteric microvessels between periods of controlled shear stress

A new method for the determination of hydraulic conductivity in individually perfused microvessels in vivo is described. A vessel is cannulated at both ends with glass micropipettes and the fluid filtration rate across the vessel wall measured from the velocities of red cells when the pressure in the micropipettes is balanced. Hydraulic conductivity measured using this double-cannulation method (2.6 (+/- 0.9) x 10(-7) cm s(-1) cmH(2)O(-1)) was not significantly different from that measured using a previously described technique in the same vessel (2.4 (+/- 0.9) x 10(-7) cm s(-1) cmH(2)O(-1) using the Landis-Michel method). Shear stress on the vessel wall was controlled by changing the difference between the inflow and outflow pressures during periods of perfusion. The volume flow through the vessel, calculated from red cell velocity either in the vessel or in the pipette, was linearly proportional to this pressure difference. Higher flow rates could only be calculated from red cell velocities in the micropipette. There was no relationship between the imposed shear stress and intervening measurements of hydraulic conductivity (r = 0.029). This novel technique has advantages over the Landis-Michel method, which include the control of outflow resistance, the measurement of shear stress under conditions of controlled pressure, the elimination of compression damage to the vessel (since vessel occlusion is not necessary) and assessment of hydraulic conductivity over the same length of vessel throughout the experiment. The measurement of solute concentrations by indwelling micropipette electrodes and the collection of perfusate for analysis are other possibilities.

Similar permeability responses to nitric oxide synthase inhibitors of venules from three animal species

The influence of nitric oxide (NO) on microvascular permeability remains unclear. NO synthase (NOS) inhibitors have been reported to increase as well as to decrease permeability in different experimental models and animal species. We tested the hypothesis that NOS inhibitors influence venular permeability differently in amphibians and mammals. Permeability coefficients to albumin (P(alb)(s)) were measured on in situ mesenteric venules of the frog and rat and excised pig coronary venules before and after exposure to NOS inhibitors. Despite individual variability in magnitude of responses, NOS inhibitors resulted in a reduction in P(alb)(s) in each species. Superfusion with 10(-5) M N(G)-monomethyl-l-arginine (l-NMMA) reduced P(alb)(s) of frog mesenteric venules by 42% (from a median of 11.4 x 10(-7) cm s(-1), n = 12, P < 0.01) and by 67% in porcine coronary venules (from 12.5 x 10(-7) cm s(-1), n = 5, P < 0.05). The response was attenuated in rat mesenteric venules; 10(-4) M N(G)-nitro-l-arginine methyl ester (l-NAME) reduced P(alb)(s) by 23% (from 7.6 x 10(-7) cm s(-1), n = 9, P = 0.01). The inactive d-enantiomers of the NOS inhibitors were without effect on P(alb)(s) in each model. In pig venules, perfusion with blood modified the permeability responses to l-NMMA, suggesting that effects of NO on permeability are modified by one or more elements of blood. These data support a role of nitric oxide release on venular permeability to albumin that is conserved among the three animal species.

Effects of L-NA and sodium nitroprusside on ischemia/reperfusion-induced leukocyte adhesion and macromolecular leakage in hamster cheek pouch venules

Our objective was to study how the topical application of a nitric oxide synthase inhibitor (l-NA, Nomega-nitro-L-arginine) and a nitric oxide donor, sodium nitroprusside (SNP), could modulate leukocyte adhesion (sticking) and microvascular permeability as altered by ischemia/reperfusion (I/R) and topically applied histamine after I/R. Golden hamsters were prepared for intravital microscopy. Ischemia was induced by an inflatable silicon rubber cuff mounted around the neck of the cheek pouch prepared for intravital microscopy. Saline, L-NA, sodium nitroprusside, and histamine were applied in the superfusion solution. FITC-dextran was injected iv 30 min before initiation of ischemia as a marker of microvascular permeability. L-NA 10(-5) M inhibited both the increase in number of sticking leukocytes and the increase in vascular permeability after I/R compared with the untreated control group of hamsters. SNP neutralized this effect of L-NA on leukocytes and vascular permeability and caused arteriolar dilation at the concentration used, 10(-6) M. Both SNP and L-NA + SNP enhanced the I/R-induced macromolecular leakage. The topical application of SNP and SNP + L-NA did not modify the response to histamine after I/R compared with the untreated control group. In hamsters not subjected to I/R, histamine-induced macromolecular leakage was inhibited by L-NA and L-NA + SNP but was unchanged by SNP. It is concluded that inhibition of nitric oxide formation by L-NA reduced both leukocyte adhesion in postcapillary venules and the increase in macromolecular leakage and that a NO donor such as SNP could enhance the macromolecular leakage response to I/R.

Effect of altered flow on the pattern of permeability around rabbit aortic branches

Uptake of circulating macromolecules by the aortic wall is greater downstream than upstream of branch sites in immature rabbits, but the opposite pattern is seen at later ages. The mature pattern is nitric oxide dependent; we tested whether it is also flow dependent. Intercostal arteries of anesthetized rabbits were occluded, sham operated, or left alone. Uptake of rhodamine-labeled albumin was assessed by quantitative fluorescence microscopy of the sections through the aorta. In mature animals, uptake was higher upstream than downstream of the control and sham-operated branches, but the pattern was reversed at occluded branches. In young animals, uptake was not significantly different between regions upstream and downstream of control, sham-operated, or occluded branches. The absence of the normal immature pattern may reflect an influence of anesthesia and will assist in the elucidation of mechanisms underlying this pattern. The data for mature animals provide the first direct evidence that flow determines permeability near arterial branches and may account for the inverse spatial correlation between shear stress and disease prevalence at branches of adult human arteries.

Generation of superoxide anion impairs histamine-induced increases in macromolecular efflux

The first goal of this study was to determine the effect of generation of superoxide anion using pyrogallol on histamine-induced increases in macromolecular efflux. We used intravital microscopy and fluorescein isothiocyanate-dextran (FITC-dextran; MW 70K) to examine macromolecular extravazation from postcapillary venules in the hamster cheek pouch in response to histamine before and following topical application of vehicle or pyrogallol. Extravazation of macromolecules was quantitated by counting venular leaky sites. Histamine elicited reproducible increases in venular leaky sites before and during infusion of vehicle. In contrast, topical application of pyrogallol (0.5 mM) abolished histamine-induced increases in formation of venular leaky sites. Our second goal was to examine whether pyrogallol-induced inhibition of venular leaky site formation could be reversed by superoxide dismutase. Application of superoxide dismutase (300 U/ml) to the cheek pouch in the presence of pyrogallol restored histamine-induced increases in venular leaky sites. Thus, the generation of superoxide anion alters histamine-induced increases in macromolecular efflux. These results support the concept that disease states that produce oxidative stress may impair agonist-induced increases in microvascular permeability via inactivation of nitric oxide.

Regulation of blood-brain barrier permeability

The blood-brain barrier minimizes the entry of molecules into brain tissue. This restriction arises by the presence of tight junctions (zonulae occludens) between adjacent endothelial cells and a relative paucity of pinocytotic vesicles within endothelium of cerebral arterioles, capillaries, and venules. Many types of stimuli can alter the permeability characteristics of the blood-brain barrier. Acute increases in arterial blood pressure beyond the autoregulatory capacity of cerebral blood vessels, application of hyperosmolar solutions, application of various inflammatory mediators known to be elevated during brain injury, and/or activation of blood-borne elements such as leukocytes can produce changes in permeability of the blood-brain barrier. The second messenger systems that account for increases in permeability of the blood-brain barrier during pathophysiologic conditions, however, remain poorly defined. This review will summarize studies that have examined factors that influence disruption of the blood-brain barrier, and will discuss the contribution of various cellular second messenger pathways in disruption of the blood-brain barrier during pathophysiologic conditions.