Transcellular gaps in microvascular walls of frog and rat when permeability is increased by perfusion with the ionophore A23187

Block Copolymer Micelles in Nanomedicine Applications

Polymeric micelles are demonstrating high potential as nanomedicines capable of controlling the distribution and function of loaded bioactive agents in the body, effectively overcoming biological barriers, and various formulations are engaged in intensive preclinical and clinical testing. This Review focuses on polymeric micelles assembled through multimolecular interactions between block copolymers and the loaded drugs, proteins, or nucleic acids as translationable nanomedicines. The aspects involved in the design of successful micellar carriers are described in detail on the basis of the type of polymer/payload interaction, as well as the interplay of micelles with the biological interface, emphasizing on the chemistry and engineering of the block copolymers. By shaping these features, polymeric micelles have been propitious for delivering a wide range of therapeutics through effective sensing of targets in the body and adjustment of their properties in response to particular stimuli, modulating the activity of the loaded drugs at the targeted sites, even at the subcellular level. Finally, the future perspectives and imminent challenges for polymeric micelles as nanomedicines are discussed, anticipating to spur further innovations.

Capillary tone: cyclooxygenase, shear stress, luminal glycocalyx, and hydraulic conductivity (Lp)

Control of capillary hydraulic conductivity (L_p) is the physiological mechanism that underpins systemic hydration. Capillaries form the largest surface of endothelial cells in any species with a cardiovascular system and all capillaries are exposed to the flow-induced force, shear stress (τ). Vasoactive molecules such as prostacyclin (cyclooxygenase product, COX) are released from endothelial cells in response to τ. Little is known about how COX activity impacts capillary L_p. The purpose here was to assess L_p in situ following an acute Δτ stimulus and during COX1/COX2 inhibition. Mesenteric true capillaries (TC) of Rana pipiens (pithed) were cannulated for L_p assessment using the modified Landis technique. Rana were randomized into Control and Test groups. Two capillaries per animal were used (perfusate, 10 mg·mL⁻¹ BSA/frog Ringer's; superfusate, frog Ringer's or indomethacin (10⁻⁵ mol·L⁻¹) mixed in frog Ringer's solution). Three distinct responses of L_p to indomethacin (TC2) were demonstrated (TC1 and TC2 medians: Test Subgroup 1, 3.0 vs. 1.8; Test Subgroup 2, 18.2 vs. 2.2; Test Subgroup 3, 4.2 vs. 10.2 × 10⁻⁷ cm·sec⁻¹·cm H₂O⁻¹). Multiple regression analysis revealed a relationship between capillary L_p and systemic red blood cell concentration or hematocrit, plasma protein concentration, and Δτ (Test Subgroup 1, R² = 0.59, P < 0.0001; Test Subgroup 2, R² = 0.96, P = 0.002), but only during COX inhibition. Maintaining red blood cell and plasma protein levels within a normal range may control barrier function in a healthy state. Recovering barrier function may be an unrecognized benefit of transfusions during blood loss or edema formation.

Biomechanics of Schlemm's canal endothelium and intraocular pressure reduction

Ocular hypertension in glaucoma develops due to age-related cellular dysfunction in the conventional outflow tract, resulting in increased resistance to aqueous humor outflow. Two cell types, trabecular meshwork (TM) and Schlemm's canal (SC) endothelia, interact in the juxtacanalicular tissue (JCT) region of the conventional outflow tract to regulate outflow resistance. Unlike endothelial cells lining the systemic vasculature, endothelial cells lining the inner wall of SC support a transcellular pressure gradient in the basal to apical direction, thus acting to push the cells off their basal lamina. The resulting biomechanical strain in SC cells is quite large and is likely to be an important determinant of endothelial barrier function, outflow resistance and intraocular pressure. This review summarizes recent work demonstrating how biomechanical properties of SC cells impact glaucoma. SC cells are highly contractile, and such contraction greatly increases cell stiffness. Elevated cell stiffness in glaucoma may reduce the strain experienced by SC cells, decrease the propensity of SC cells to form pores, and thus impair the egress of aqueous humor from the eye. Furthermore, SC cells are sensitive to the stiffness of their local mechanical microenvironment, altering their own cell stiffness and modulating gene expression in response. Significantly, glaucomatous SC cells appear to be hyper-responsive to substrate stiffness. Thus, evidence suggests that targeting the material properties of SC cells will have therapeutic benefits for lowering intraocular pressure in glaucoma.

Vascular hyperpermeability, angiogenesis, and stroma generation

It has been known for more than half a century that the tumor microvasculature is hyperpermeable to plasma proteins. However, the identity of the leaky vessels and the consequences of vascular hyperpermeability have received little attention. This article places tumor vascular hyperpermeability in a broader context, relating it to (1) the low-level "basal" permeability of the normal vasculature; (2) the "acute," short-term hyperpermeability induced by vascular permeability factor/vascular endothelial growth factor (VPF/VEGF-A) and other vascular permeabilizing agents; and (3) the "chronic" hyperpermeability associated with longer-term exposure to agents such as VPF/VEGF-A that accompanies many types of pathological angiogenesis. Leakage of plasma protein-rich fluids is important because it activates the clotting system, depositing an extravascular fibrin gel provisional matrix that serves as the first step in stroma generation.

Electron tomography of vesicles

In this issue of Microcirculation, Wagner, Modla, Hossler and Czmmek [25] describe the use of electron tomography to visualize the three-dimensional arrangement of small endothelial vesicles and caveolae of muscle capillaries. Their images show the well-known clusters of fused vesicles communicating with caveolae at the luminal and abluminal surfaces. The advantages of electron tomography are shown by well resolved images of single cytoplasmic vesicles separate from fused vesicle clusters and also by occasional chains of fused vesicles forming trans-endothelial channels. Twenty five to thirty years ago the existence of both trans-endothelial channels and single unattached vesicles was disputed. Also, since some single vesicles and all of the trans-endothelial channels are labeled with a lanthanide tracer present in the perfusate at the time of fixation, this evidence once again raises the question of whether vesicles have a role in vascular permeability to macromolecules. This brief review describes the origin of the vesicle controversy, some of the more recent evidence for and against the participation of vesicles in macromolecular transport and considers some criticisms of ultra-structural evidence for vesicular transport that still require answers.

Connexin channels provide a target to manipulate brain endothelial calcium dynamics and blood-brain barrier permeability

The cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) is an important factor determining the functional state of blood-brain barrier (BBB) endothelial cells but little is known on the effect of dynamic [Ca(2+)](i) changes on BBB function. We applied different agonists that trigger [Ca(2+)](i) oscillations and determined the involvement of connexin channels and subsequent effects on endothelial permeability in immortalized and primary brain endothelial cells. The inflammatory peptide bradykinin (BK) triggered [Ca(2+)](i) oscillations and increased endothelial permeability. The latter was prevented by buffering [Ca(2+)](i) with BAPTA, indicating that [Ca(2+)](i) oscillations are crucial in the permeability changes. Bradykinin-triggered [Ca(2+)](i) oscillations were inhibited by interfering with connexin channels, making use of carbenoxolone, Gap27, a peptide blocker of connexin channels, and Cx37/43 knockdown. Gap27 inhibition of the oscillations was rapid (within minutes) and work with connexin hemichannel-permeable dyes indicated hemichannel opening and purinergic signaling in response to stimulation with BK. Moreover, Gap27 inhibited the BK-triggered endothelial permeability increase in in vitro and in vivo experiments. By contrast, [Ca(2+)](i) oscillations provoked by exposure to adenosine 5' triphosphate (ATP) were not affected by carbenoxolone or Gap27 and ATP did not disturb endothelial permeability. We conclude that interfering with endothelial connexin hemichannels is a novel approach to limiting BBB-permeability alterations.

Extravasation of polymeric nanomedicines across tumor vasculature

Tumor microvasculature is fraught with numerous physiological barriers which hinder the efficacy of anticancer agents. These barriers include chaotic blood supply, poor tumor vasculature permeability, limited transport across the interstitium due to high interstitial pressure and absence of lymphatic network. Abnormal microvasculature also leads to hypoxia and acidosis which limits effectiveness of chemotherapy. These barriers restrict drug or drug carrier extravasation which hampers tumor regression. Targeting key features of the tumor microenvironment such as tumor microvessels, interstitial hypertension and tumor pH is a promising approach to improving the efficacy of anticancer drugs. This review highlights the current knowledge on the distinct tumor microenvironment generated barriers which limit extravasation of drugs and focuses on modalities for overcoming these barriers using multi-functional polymeric carriers. Special attention is given to utilizing polymeric nanomedicines to facilitate extravasation of anticancer drugs for future cancer therapy.

Ramp acceleration and hydraulic conductivity (L(p)) of living capillaries

Living mesenteric capillaries with either an intact or disrupted glycocalyx were challenged with ramp change in shear stress (Deltatau). Animals (Rana pipiens) were divided randomly into two experimental groups, and two true capillaries (TC) per animal were investigated. The modified Landis technique was combined with intravital microscopy to view individual TC and assess hydraulic conductivity (L(p)), an index of capillary function. Median L(p) was 3.2 x 10(-7) for control and 11.8 x 10(-7) cm s(-1) cm H(2)O(-1) after mild, brief (1 min) pronase treatment (P<0.0001). Analysis by stimulus component showed that L(p) for untreated capillaries was related negatively to ramp acceleration (R(2)=0.46, P<0.0001, n=38) and positively to Deltatau magnitude (R(2)=0.28, P=0.0006, n=38). Disrupting the capillary glycocalyx revealed a positive and previously unknown relationship between ramp acceleration and L(p) (R(2)=0.44, P=0.002, n=19) plus an upward shift (increased intercept) of the magnitude Deltatau-L(p) relationship compared to abrupt stimulation. These data suggest that bloodstream hemodynamics may impact capillary function. Further, an intact glycocalyx may protect capillaries when blood flow changes.

VEGF-A and the induction of pathological angiogenesis

Tumors, wounds, and chronic inflammatory disorders generate a new vascular supply by a process known as pathological angiogenesis. Whereas formation of the normal blood vasculature requires the interaction of many different agonists and inhibitors, including vascular endothelial growth factor-A (VEGF-A) and other members of the vascular permeability factor/VEGF family, pathological angiogenesis is a cruder, simpler process that can be replicated by a single VEGF-A isoform, VEGF-A(164/5). VEGF-A(164/5) induces the formation of several distinctly different types of new blood vessels that differ from normal blood vessels with respect to organization, structure, and function. Elucidating the properties of these new vessels has led to a better understanding of angiogenesis and will hopefully lead to new approaches to antiangiogenic therapy.

Vascular permeability, vascular hyperpermeability and angiogenesis

The vascular system has the critical function of supplying tissues with nutrients and clearing waste products. To accomplish these goals, the vasculature must be sufficiently permeable to allow the free, bidirectional passage of small molecules and gases and, to a lesser extent, of plasma proteins. Physiologists and many vascular biologists differ as to the definition of vascular permeability and the proper methodology for its measurement. We review these conflicting views, finding that both provide useful but complementary information. Vascular permeability by any measure is dramatically increased in acute and chronic inflammation, cancer, and wound healing. This hyperpermeability is mediated by acute or chronic exposure to vascular permeabilizing agents, particularly vascular permeability factor/vascular endothelial growth factor (VPF/VEGF, VEGF-A). We demonstrate that three distinctly different types of vascular permeability can be distinguished, based on the different types of microvessels involved, the composition of the extravasate, and the anatomic pathways by which molecules of different size cross-vascular endothelium. These are the basal vascular permeability (BVP) of normal tissues, the acute vascular hyperpermeability (AVH) that occurs in response to a single, brief exposure to VEGF-A or other vascular permeabilizing agents, and the chronic vascular hyperpermeability (CVH) that characterizes pathological angiogenesis. Finally, we list the numerous (at least 25) gene products that different authors have found to affect vascular permeability in variously engineered mice and classify them with respect to their participation, as far as possible, in BVP, AVH and CVH. Further work will be required to elucidate the signaling pathways by which each of these molecules, and others likely to be discovered, mediate the different types of vascular permeability.

Vacuolar structures can be identified by AFM elasticity mapping

Fluid-filled organelles like vesicles, endosomes and pinosomes are inevitable parts of cellular signalling and transport. Endothelial cells, building a barrier between blood and tissue, can form vacuolar organelles. These structures are implicated in upregulated fluid transport across the endothelium under inflammatory conditions. Vacuolar organelles have been described by transmission electron microscopy so far. Here, we present a method that images and mechanically characterizes intracellular structures in whole cells by atomic force microscopy (AFM). After crosslinking the cellular proteins with the fixative glutaraldehyde, plasma membrane depressions become observable, which are scattered around the cell nucleus. Nanomechanical analysis identifies them as spots of reduced stiffness. Scanning electron microscopy confirms their pit-like appearance. In addition, fluorescence microscopy detects an analogous pattern of protein-poor spots, thereby confirming mechanical rigidity to arise from crosslinked proteins. This AFM application opens up a mechanical dimension for the investigation of intracellular organelles.

Change in shear stress (Deltatau)/hydraulic conductivity (Lp) relationship after pronase treatment of individual capillaries in situ

A complex glycoprotein meshwork covers the inner wall of blood vessels and is implicated in mechanotransduction of fluid shear stress (tau). A relationship between Deltatau and capillary Lp has been established. The purpose of this study was to evaluate Lp in response to Deltatau after exposing the capillary lumen to a mild, non-specific protease selected to disrupt its inner matrix. We hypothesized that Lp would not correlate with Deltatau after enzyme treatment. Frogs (Rana pipiens, n=69) were pithed and the mesentery was exteriorized. Lp was assessed at 30 cm H2O using the modified Landis technique after an abrupt, square wave Deltatau produced by a physiologically relevant increase in pressure. Perfusate solutions were 10 mg ml-1 BSA/frog Ringer's (Control) or 0.1 mg ml-1 pronase in BSA/Ringer's (1 min) then BSA/Ringer's alone (Test). Mean (+/-SE) control Lp following Deltatau was 2.2+/-0.2 x 10(-7) cm s-1 cm H2O-1 and individual values correlated positively with Deltatau (r=0.85, P<0.0001, n=41). After pronase, mean Test Lp (17.6+/-2.5 x 10(-7) cm s-1 cm H2O-1) was higher compared to control and Deltatau/Lp plots revealed two subsets of capillaries. Lp correlated strongly with Deltatau in capillaries with diameters<or=15 microm (r=0.91, P=0.0006, n=14) and also in a second subset of capillaries with diameters >15 microm (r=0.96, P=0.0001, n=8). Slopes were 3.9- and 8.7-fold higher, respectively, compared to control. These data suggest a protective role for luminal constituents of intact capillaries. Mechanisms involved in capillary responses to flow-induced, mechanical stimuli may be located in the cellular structures that form capillaries.

Bradykinin shifts endothelial fluid passage from para- to transcellular routes

The signalling peptide bradykinin (BK) is implicated in inflammation and angiogenesis. It promotes fluid transport from blood vessels to interstitial space, and thus facilitates oedema formation. To clarify whether paracellular or transcellular pathways mediate this effect, we investigated the BK-stimulated fluid transport across endothelial monolayers in vitro by comparison of electrical and fluorescence methods. Electrical cell impedance sensing monitored a biphasic response of cell layers to BK with high time resolution: a short decrease (18%, 1 min) was followed by a more sustained increase in paracellular resistance (30%, 10 min). The two phases can be assigned to second messengers of the BK-signalling pathway: Ca(2+) for the decrease and cyclic adenosine monophosphate for the rise of resistance, respectively. Despite tightening of the intercellular clefts, BK increased the fluid permeability by 39%, indicating transcellular fluid transport. Additionally, BK stimulated both in- and outwardly directed membrane trafficking as assessed by vesicular fluid uptake (by 49%) and secretion of von Willebrandt factor (by 85%). In conclusion, the combination of electrical and fluorescence data suggests that BK induces a shift from para- to transcellular fluid transport across endothelium.

Permeability properties of tumor surrogate blood vessels induced by VEGF-A

Malignant tumors generate new blood vessels by secreting growth factors, particularly members of the vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) family. Overall, the new blood vessels that form are hyperpermeable to plasma proteins, a property that is thought to be important for generating new stroma. However, tumor blood vessels are structurally heterogeneous and include microvessels of at least the following distinct types: mother vessels (MV), glomeruloid microvascular proliferations (GMP), arterio-venous-like vascular malformations and capillaries. Our goal was to determine whether macromolecular tracers leaked from all or from only a subset of these vessel types and to elucidate the extravasation pathways. As blood vessels are only a minor component of tumors, and therefore, difficult to study in situ, we used an adenoviral vector to express VEGF-A164, the most important member of the VPF/VEGF family, in mouse tissues. So expressed, VEGF-A164 induces large numbers of surrogate vessels of each type found in tumors in a highly reproducible manner. Overall permeability to plasma proteins was assessed qualitatively with Evan's blue dye and quantitatively with a dual tracer method employing radioactive albumin. Leaky vessels were identified by confocal microscopy (FITC-dextran) and by electron microscopy (ferritin). MV, and to a lesser extent GMP, were found to be hyperpermeable but capillaries and vascular malformations were not. Ferritin extravasated primarily by two trans-cellular routes, vesiculo-vacuolar organelles (VVOs) and fenestrae. This occurred despite a considerable reduction in VVO frequency as VVO membranes translocated to the plasma membrane during MV formation. However, reduction in the number and complexity of VVOs was offset by extensive endothelial cell thinning and a greatly shortened extravasation pathway. Extrapolating these findings to tumors predicts that only a subset of tumor vessels, MV and GMP, is hyperpermeable, and that measures of overall vessel permeability greatly underestimate the permeability of individual MV and GMP.

Current state, achievements, and future prospects of polymeric micelles as nanocarriers for drug and gene delivery

Polymeric micelles, self-assemblies of block copolymers, are promising nanocarrier systems for drug and gene delivery. Until now, several micellar formulations of antitumor drugs have been intensively studied in preclinical and clinical trials, and their utility has been demonstrated. Even compared with long-circulating liposomes, polymeric micelles might have several advantages, such as controlled drug release, tissue-penetrating ability and reduced toxicity such as hand-foot syndrome and hypersensitivity reaction. Importantly, critical features of the polymeric micelles as drug carriers, including particle size, stability, and loading capacity and release kinetics of drugs, can be modulated by the structures and physicochemical properties of the constituent block copolymers. Also, nano-engineering of block copolymers might allow the preparation of polymeric micelles with integrated smart functions, such as specific-tissue targetability, as well as chemical or physical stimuli-sensitivity. Thus, polymeric micelles are nanotechnology-based carrier systems that might exert the activity of potent bioactive compounds in a site-directed manner, ensuring their effectiveness and safety in the clinical use.

'What controls aqueous humour outflow resistance?'

The bulk of aqueous humour outflow resistance is generated in or near the inner wall endothelium of Schlemm's canal in normal eyes, and probably also in glaucomatous eyes. Fluid flow through this region is controlled by the location of the giant vacuoles and pores found in cells of the endothelium of Schlemm's canal, but the flow resistance itself is more likely generated either in the extracellular matrix of the juxtacanalicular connective tissue or the basement membrane of Schlemm's canal. Future studies utilizing in vitro perfusion studies of inner wall endothelial cells may give insights into the process by which vacuoles and pores form in this unique endothelium and why inner wall pore density is greatly reduced in glaucoma.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Cytosolic Ca2+ concentration and rate of increase of the cytosolic Ca2+ concentration in the regulation of vascular permeability in Rana in vivo

Vascular permeability is assumed to be regulated by the cytosolic Ca(2+) concentration ([Ca(2+)](c)) of the endothelial cells. When permeability is increased, however, the maximum [Ca(2+)](c) appears to occur after the maximum permeability increase, suggesting that Ca(2+)-dependent mechanisms other than the absolute Ca(2+) concentration may regulate permeability. Here we investigate whether the rate of increase of the [Ca(2+)](c) (d[Ca(2+)](c)/dt) may more closely approximate the time course of the permeability increase. Hydraulic conductivity (L(p)) and endothelial [Ca(2+)](c) were measured in single perfused frog mesenteric microvessels in vivo. The relationships between the time courses of the increased L(p), [Ca(2+)](c) and d[Ca(2+)](c)/dt were examined. L(p) peaked significantly earlier than [Ca(2+)](c) in all drug treatments examined (Ca(2+) store release, store-mediated Ca(2+) influx, and store-independent Ca(2+) influx). When L(p) was increased in a store-dependent manner the time taken for L(p) to peak (3.6 +/- 0.9 min during store release, 1.2 +/- 0.3 min during store-mediated Ca(2+) influx) was significantly less than the time taken for [Ca(2+)](c) to peak (9.2 +/- 2.8 min during store release, 2.1 +/- 0.7 min during store-mediated influx), but very similar to that for the peak d[Ca(2+)](c)/dt to occur (4.3 +/- 2.0 min during store release, 1.1 +/- 0.5 min during Ca(2+) influx). Additionally, when the increase was independent of intracellular Ca(2+) stores, L(p) (0.38 +/- 0.03 min) and d[Ca(2+)](c)/dt (0.30 +/- 0.1 min) both peaked significantly before the [Ca(2+)](c) (1.05 +/- 0.31 min). These data suggest that the regulation of vascular permeability by endothelial cell Ca(2+) may be regulated by the rate of change of the [Ca(2+)](c) rather than the global [Ca(2+)].

Regulation of actin dynamics is critical for endothelial barrier functions

We tested the hypothesis that the equilibrium between F- and G-actin in endothelial cells modulates the integrity of the actin cytoskeleton and is important for the maintenance of endothelial barrier functions in vivo and in vitro. We used the actin-depolymerizing agent cytochalasin D and jasplakinolide, an actin filament (F-actin) stabilizing and promoting substance, to modulate the actin cytoskeleton. Low doses of jasplakinolide (0.1 microM), which we have previously shown to reduce the permeability-increasing effect of cytochalasin D, had no influence on resting permeability of single-perfused mesenteric microvessels in vivo as well as on monolayer integrity. The F-actin content of cultured endothelial cells remained unchanged. In contrast, higher doses (10 microM) of jasplakinolide increased permeability (hydraulic conductivity) to the same extent as cytochalasin D and induced formation of intercellular gaps in cultured myocardial endothelial (MyEnd) cell monolayers. This was accompanied by a 34% increase of F-actin and pronounced disorganization of the actin cytoskeleton in MyEnd cells. Furthermore, we tested whether an increase of cAMP by forskolin and rolipram would prevent the cytochalasin D-induced barrier breakdown. Conditions that increase intracellular cAMP failed to block the cytochalasin D-induced permeability increase in vivo and the reduction of vascular endothelial cadherin-mediated adhesion in vitro. Taken together, these data support the hypothesis that the state of polymerization of the actin cytoskeleton is critical for maintenance of endothelial barrier functions and that both depolymerization by cytochalasin D and hyperpolymerization of actin by jasplakinolide resulted in an increase of microvessel permeability in vivo. However, cAMP, which is known to support endothelial barrier functions, seems to work by mechanisms other than stabilizing F-actin.

Intact capillaries sensitive to rate, magnitude, and pattern of shear stress stimuli as assessed by hydraulic conductivity (Lp)

Magnitude of abrupt changes in fluid shear stress (Delta tau) correlates with capillary hydraulic conductivity (Lp). Cultured endothelial cells differentiate between rate of change in and pattern of fluid stimulation; however, neither rate nor pattern sensitivity has been evaluated in individual capillaries. We hypothesized that Lp would be greater following abrupt compared to slow Delta tau and that magnitude of Delta tau would correlate with Lp regardless of rate. Mesenteric venular capillaries (VC, n=41) located in pithed North American leopard frogs (Rana pipiens) were cannulated either above or at in situ pressure to induce abrupt (<0.1 s) or slow (1-5 min) Delta tau. Lp was assessed always at the same pressure (30 cm H2O) using the modified Landis technique. Mean +/- SE Lp was sixfold higher (P=0.005) following abrupt (19.0 +/- 3.9 x 10(-7)) compared to slow (2.7 +/- 0.9 x 10(-7) cm/s/cm H2O) Delta tau after accounting for stimulus pattern variability. Linear relationships between Lp and Delta tau existed for both abrupt [Lp = 0.026(Delta tau)-1.6, R2=0.90, P=0.0001] and slow [Lp = 0.005(Delta tau)-0.3, R2=0.82, P=0.03] stimuli. These results suggest that frog mesenteric VC sense unique characteristics of Delta tau and respond by altering Lp.

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.

Ultrastructural studies define soluble macromolecular, particulate, and cellular transendothelial cell pathways in venules, lymphatic vessels, and tumor-associated microvessels in man and animals

We present de novo studies and review published efforts from our laboratory, spanning 12 years (from 1988 to 2000), where we have used ultrastructural approaches to study the functional anatomy of the microvasculature in man and animals in health and disease. These efforts have defined a new endothelial cell organelle, termed the vesiculo-vacuolar organelle (VVO), which participates in the regulated transendothelial cell passage of soluble macromolecules. The studies defining this organelle utilized ultrathin serial sections, three-dimensional computer-assisted reconstructions, and ultrastructural electron-dense tracers to establish luminal to abluminal transendothelial cell continuity of VVOs. Commonality of VVOs and caveolae is suggested by the ultrastructural anatomy of individual units of VVOs and caveolae, the presence of caveolin in both structures, and a mathematical analysis of morphometric data, all of which suggest that VVOs form from fusions of individual size units equivalent to vesicles of caveolar size. Ultrastructural studies have localized potent permeability factors and their specific receptors to VVOs in in vivo tumor and allergic inflammation models. Regulation of permeability through VVOs has been quantified and shown to be increased in tumor microvessels and in control vessels exposed to potent permeability-inducing mediators. The transendothelial cell passage of particulate macromolecules occurs by vacuolar transport in tumor vessels; in permeability factor-exposed control vessels, colloidal carbon traversed endothelial cells via the development of pores that did not communicate with or disrupt intercellular junctions by gap formation. Serial section and computer-assisted reconstructions established these findings and suggested the possible development of transendothelial cell pores from VVOs. Serial sectioning and computer-assisted three-dimensional reconstructions of ultrastructural samples of an acute inflammation model revealed a transendothelial cell traffic route for motile neutrophils and platelets in the absence of classical ultrastructural criteria for regulated secretion from either cell.

Electron microscope study of blood-brain barrier opening induced by immunological targeting of the endothelial barrier antigen

Barrier vessels in the central nervous system are lined with endothelial cells which constitute the blood-brain barrier (BBB) and show selective expression of certain biochemical markers. One of these, the endothelial barrier antigen (EBA), is specific to the rat. The exact role of EBA in the BBB is not known, although several studies have shown a correlation between the reduction in EBA expression in endothelial cells and the opening of the BBB. However, in these studies it was not possible to determine if EBA reduction was a primary event or was secondary to opening of the BBB. A recent light microscope study demonstrated that immunological targeting of EBA in vivo, by intravenous injection of a monoclonal antibody (anti-EBA), leads to acute and widespread opening of the BBB. In the current study we have employed this model together with tracer application and immunoperoxidase electron microscopy to determine the site of binding of the injected antibody and the route of opening of the BBB. The results showed that (a) the anti-EBA injected in vivo became bound to brain endothelial cells, principally to luminal membranes. (b) Endothelial cells showed widened intercellular junctions and increased cytoplasmic vesicles and vacuoles. (c) Many perivascular astrocytic processes were swollen. (d) The macromolecular tracer HRP was present in vesicles, vacuoles, widened paracellular clefts, the perivascular space and brain parenchyma. In conclusion, the in vivo targeting of EBA leads to opening of the BBB apparently via paracellular and transcellular routes. This model is useful for the study of vascular permeability in the CNS and experimental manipulation of the BBB. It may have a potential application in experimental studies on drug delivery throughout the CNS.

"Bronchial artery delivery of viral vectors for gene delivery in cystic fibrosis; superior to airway delivery?"

BACKGROUND

Attempts at gene therapy for the pulmonary manifestations of Cystic Fibrosis have relied mainly on airway delivery. However the efficiency of gene transfer and expression in the airway epithelia has not reached therapeutic levels. Access to epithelial cells is not homogenous for a number of reasons and the submucosal glands cannot be reached via the airways.

PRESENTATION

We propose to inject gene delivery vectors directly into bronchial arteries combined with pre-delivery of vascular endothelial growth factor to increase vascular endothelial permeability and post-delivery flow reduction by balloon occlusion. Thus it may be possible to reach mucous secreting cells of the bronchial luminal epithelium and the submucosal glands in an increased and homogenous fashion.

TESTING

This combination of techniques to the best of our knowledge has not previously been investigated, and may enable us to overcome some of the current limitations to gene therapy for Cystic Fibrosis.

Openings in frog microvascular endothelium at different rates of increase in pressure and at different temperatures

Experiments were carried out on single mesenteric capillaries and venules of pithed frogs to determine whether the rate of increase in intravascular pressure (dP/dt) influenced the critical pressure (P(B)) which increases wall permeability. Vessels, microperfused with frog Ringer solutions containing 0.1% bovine serum albumin and red cells, were occluded downstream before pressure was raised either as a ramp or in a series of 13.6 cmH2O steps. By varying step duration, the mean dP/dt could be matched to dP/dt applied as a steady ramp. P(B) was recorded as the pressure at which there was an abrupt increase in filtration with red cells passing to and through one or more sites in the vessel wall. In all vessels, increasing dP/dt raised P(B), with no differences between steps and ramps. The relation between P(B) and dP/dt was linear, consistent with a latent period, T (the slope), between a critical pressure being reached and the abrupt increase in permeability being observed. Direct observation confirmed this latent period. Between 12 and 20 (o)C, T was 8.5 +/- 0.47 s; between 0 and 5 degrees C, T was 11.5 +/- 0.97 s. Tissue cooling did not influence the time constant, tau, describing the rate of stretch of wall following a step increase in pressure and used to measure wall visco-elastic properties. Nor was the value of tau (1.15 +/- 0.06 s, n = 42) consistent with T being accounted for by visco-elasticity. It is suggested that the latent period may indicate an active response of the endothelium.

Inhibitors of myosin light chain kinase and phosphodiesterase reduce ventilator-induced lung injury

Alveolar overdistension due to high peak inflation pressures (PIP) is associated with an increased capillary filtration coefficient (K(fc)). To determine which signal pathways contribute to this injury, we perfused isolated rat lungs with 5% bovine albumin in Krebs solution and measured K(fc) after successive 30-min periods of ventilation with peak inflation pressures (PIP) of 7, 20, 30, and 35 cmH(2)O. In a high-PIP control group, K(fc) increased significantly after ventilation with 30 and 35 cmH(2)O PIP, but significant increases were prevented by treatment with 100 microM trifluoperazine, an inhibitor of Ca(2+)/calmodulin, 500 nM ML-7, an inhibitor of myosin light chain kinase (MLCK), a combination of isoproterenol (20 microM) and rolipram (10 microM) to enhance intracellular cAMP levels, and a dose of KT-5720 (2 microM), which inhibits MLCK and protein kinase C. These studies suggest that the Ca(2+)/calmodulin-MLCK pathway augments capillary fluid leak after a modest high-PIP injury and that this is attenuated by kinase inhibition and increased intracellular cAMP.

The cutaneous microcirculation

The cutaneous microcirculation is organized as two horizontal plexuses. One is situated 1-1.5 mm below the skin surface and the other is at the dermal-subcutaneous junction. Ascending arterioles and descending venules are paired as they connect the two plexuses. From the upper layer, arterial capillaries rise to form the dermal papillary loops that represent the nutritive component of the skin circulation. There are sphincter-like smooth muscle cells at the point where the ascending arterioles divide to form the arteriolar component of the upper horizontal plexus. At the dermal-subcutaneous junction, there are collecting veins with two cusped valves that are oriented to prevent the retrograde flow of blood. Laser Doppler flowmetry has demonstrated vasomotion of red cell flux localized to the sites of ascending arterioles. The simultaneous recording by laser Doppler flowmetry of red cell flux and the concentration of moving red blood cells from individual sites allows one to construct topographic maps of these two values. These two maps, based on initial studies using correlative skin biopsies, can define 1 mm3 volumes of skin that are predominantly arteriolar in composition, venular in composition, or essentially devoid of all microvascular elements. The electron and light microscopic features that define the microvascular segments, when coupled with that ability of laser Doppler flowmetry to define the predominant microvascular segments under the probe, allow one to study both the mechanisms of normal physiologic states and the pathogenetic mechanisms underlying pathologic skin disorders in which the microvasculature plays a predominant role.

Differential effects of L-NAME on rat venular hydraulic conductivity

The role of nitric oxide (NO) in microvascular permeability remains unclear because both increases and decreases in permeability by NO synthase (NOS) inhibitors have been reported. We sought to determine whether blood-borne constituents modify venular permeability responses to the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). We assessed hydraulic conductivity (L(p)) of pipette-perfused rat mesenteric venules before and after exposure to 10(-4) M L-NAME. In the absence of blood-borne constituents, L-NAME reduced L(p) by nearly 50% (from a median of 2.4 x 10(-7) cm x s(-1) x cmH(2)O(-1), n = 17, P < 0.001). The reduction in L(p) by L-NAME was inhibited by a 10-fold molar excess of L-arginine but not D-arginine (n = 6). In a separate group of venules, blood flow was allowed to resume during exposure to L-NAME. In vessels perfused by blood during L-NAME exposure, L(p) increased by 78% (from 1.4 x 10(-7) cm x s(-1) x cmH(2)O(-1), n = 10, P < 0.01). N(G)-nitro-D-arginine methyl ester did not affect L(p) in either of the two groups. These data imply that NO has direct vascular effects on permeability that are opposed by secondary changes in permeability mediated by blood-borne constituents.

Advanced glycation end products induce blood-retinal barrier dysfunction in normoglycemic rats

Advanced glycation end products (AGEs) have been implicated in the progressive vascular dysfunction which occurs during diabetic retinopathy. In the current study we have examined the role of these adducts in blood-retinal barrier (BRB) breakdown and investigated expression of the vasopermeabilizing agent vascular endothelial growth factor (VEGF) in the retina. When normoglycemic rats were injected with AGE-modified albumin daily for up to 10 days there was widespread leakage of FITC-dextran and serum albumin from the retinal vasculature when compared to control animals treated with nonmodified albumin. Ultrastructural examination of the vasculature revealed areas of attenuation of the retinal vascular endothelium and increased vesicular organelles only in the AGE-exposed rats. Quantitative RT-PCR and in situ hybridization demonstrated a significant increase in retinal VEGF mRNA expression (P < 0.05). These results suggest that AGEs can initiate BRB dysfunction in nondiabetic rats and a concomitant increase in retinal VEGF expression. These findings may have implications for the role of AGEs in the pathogenesis of diabetic retinopathy.

Different pathways of macromolecule extravasation from hyperpermeable tumor vessels

Tumor microvessels are hyperpermeable to plasma proteins, a consequence of tumor cell-secreted vascular permeability factor/vascular endothelial growth factor (VPF/VEGF). However, the pathways by which macromolecules extravasate from tumor vessels have been little investigated. To characterize tumor vessels more precisely and to elucidate the pathways by which macromolecules extravasated from them, we studied two well-defined, VPF/VEGF-secreting murine carcinomas, MOT and TA3/St. Whether grown in ascites or solid form, MOT tumors induced large, pericyte-poor "mother" vessels whose lining endothelium developed fenestrae that involved 1.8-5.6% of the surface. Fenestrae developed in parallel with markedly reduced endothelial cell vesiculo-vacuolar organelles (VVOs). TA3/St tumors, which secreted more VPF/VEGF than MOT tumors, elicited mother vessels with unchanged VVOs and without fenestrae. In both tumors, a plasma protein tracer, ferritin, extravasated through VVOs and in MOT tumors ferritin also extravasated through fenestrae. Endothelial gaps were not observed in either tumor. Thus, not all VPF/VEGF-secreting tumors induce fenestrated endothelium. Also, VVOs provide an internal store of membrane that can be transferred to the endothelial cell surface to provide the substantial increase in plasma membrane necessary for mother vessel formation in MOT tumors. Such transfer was apparently unnecessary in TA3/St tumors in which extensive early endothelial cell division provided the increased plasma membrane necessary for forming mother vessels.

Endothelial cell shrinkage increases permeability through a Ca2+-dependent pathway in single frog mesenteric microvessels

1. We tested whether calcium (Ca2+)-dependent mechanisms were essential for our previous observation that a change in the endothelial cell (EC)-extracellular matrix (ECM) attachment caused an increase in microvessel hydraulic permeability (Lp) after exposure to hypertonic solutions in single perfused mesenteric microvessels in pithed frogs (Rana pipiens). 2. In microvessels where integrin-dependent EC-ECM attachments were disrupted by pretreatment with the peptide Gly-Arg-Gly-Asp-Thr-Pro (GRGDTP; 0.3 mmol l-1), we measured microvessel Lp after exposure to hypertonic solutions under experimental conditions that reduced Ca2+ influx into endothelial cells. 3. High K+ solutions (59.7 and 100 mmol l-1 K+) were used to depolarize the endothelial membrane and therefore to reduce the electrochemical driving force for Ca2+ influx through conductive Ca2+ channels. These solutions abolished the increase in Lp caused by hypertonic solutions in the microvessels pretreated with GRGDTP. 4. We previously suggested that the removal of albumin from the perfusate may reduce EC-ECM attachment because hypertonic solutions increased the Lp of microvessels above that due to removal of albumin alone. This additional increase in Lp was attenuated by the 59.7 mmol l-1 K+ solution and was completely abolished by the 100 mmol l-1 K+ solution. 5. Bumetanide, an inhibitor of the Na+-K+-2Cl- co-transporter and one of the mechanisms of regulatory volume increase after exposure to hypertonic solutions in endothelial cells, did not change the response of microvessels to high K+ solutions. 6. Our findings indicate that Ca2+ entry into endothelial cells via passive conductance channels is necessary to increase microvessel Lp after exposure to hypertonic solutions in microvessels where EC-ECM attachments are disrupted.

Microvascular permeability

This review addresses classical questions concerning microvascular permeabiltiy in the light of recent experimental work on intact microvascular beds, single perfused microvessels, and endothelial cell cultures. Analyses, based on ultrastructural data from serial sections of the clefts between the endothelial cells of microvessels with continuous walls, conform to the hypothesis that different permeabilities to water and small hydrophilic solutes in microvessels of different tissues can be accounted for by tortuous three-dimensional pathways that pass through breaks in the junctional strands. A fiber matrix ultrafilter at the luminal entrance to the clefts is essential if microvascular walls are to retain their low permeability to macromolecules. Quantitative estimates of exchange through the channels in the endothelial cell membranes suggest that these contribute little to the permeability of most but not all microvessels. The arguments against the convective transport of macromolecules through porous pathways and for the passage of macromolecules by transcytosis via mechanisms linked to the integrity of endothelial vesicles are evaluated. Finally, intracellular signaling mechanisms implicated in transient increases in venular microvessel permeability such as occur in acute inflammation are reviewed in relation to studies of the molecular mechanisms involved in signal transduction in cultured endothelial cells.

Inhibition of effects of flow on potassium permeability in single perfused frog mesenteric capillaries

1. We have investigated the effects of various potential inhibitors on flow-dependent K+ permeability (PK) of single perfused mesenteric microvessels in pithed frogs. 2. Neither superfusion with a nitric oxide synthase inhibitor, NG-monomethyl-L-arginine (10 or 100 micromol l-1), nor the addition of indomethacin (30 micromol l-1) to both perfusate and superfusate reduced the positive correlation between PK and flow velocity (U). 3. In the presence of agents known to raise intracellular levels of adenosine 3',5'-cyclic monophosphate (noradrenaline, 8-bromo-cAMP and a combination of forskolin and rolipram) the slope of the relation between PK and U was no longer significant, so that PK was no longer flow dependent. 4. These results confirm that the flow dependence of PK is a biological process and not an artefact of measurement and suggest a role for intracellular cAMP rather than nitric oxide or prostacyclin in the flow-dependent modulation of PK in frog mesenteric microvessels.

Vascular permeability factor/vascular endothelial growth factor and the significance of microvascular hyperpermeability in angiogenesis

This Chapter has reviewed the literature concerning VPF/VEGF as a potent vascular permeabilizing cytokine. In accord with this important role, microvessels have been found to be hyperpermeable to plasma proteins and other circulating macromolecules at sites where VPF/VEGF and its receptors are overexpressed, i.e., in tumors, healing wounds, retinopathies, many important inflammatory conditions and in certain physiological processes, such as ovulation and corpus luteum formation. Moreover, microvascular hyperpermeability to plasma proteins was shown to have an important consequence: the laying down of a fibrin-rich extracellular matrix. This provisional matrix, in turn, favors and supports the ingrowth of fibroblasts and endothelial cells which, together, transform the provisional matrix into the mature stroma characteristic of tumors and healed wounds. Finally, we have considered the pathways by which these and other circulating macromolecules cross the endothelium of normal and VPF/VEGF-permeabilized microvessels. These pathways include VVOs and trans-endothelial openings that have been variously interpreted as inter-endothelial cell gaps or trans-endothelial cell pores. At least some trans-endothelial cell pores may arise from VVOs. In conclusion, these data provide new insights into the mechanisms of angiogenesis and stroma formation, insights which are potentially applicable to a wide variety of disease states and which may lead to identification of new targets for therapeutic intervention.

Transient and prolonged increase in endothelial permeability induced by histamine and thrombin: role of protein kinases, calcium, and RhoA

In the present study, we differentiated between short- and long-term effects of vasoactive compounds on human endothelial permeability in an in vitro model. Histamine induced a rapid and transient (<3 minutes) decrease in barrier function, as evidenced by a decreased transendothelial electrical resistance and an increased passage of 22Na ions. This increase in permeability was inhibited completely by chelation of intracellular calcium ions by BAPTA-AM and inhibition of calmodulin activity and myosin light chain (MLC) phosphorylation. The presence of serum factors prolonged the barrier dysfunction induced by histamine. Thrombin by itself induced a prolonged barrier dysfunction (>30 minutes) as evidenced by an increased passage of peroxidase and 40 kDa dextran. It was dependent only partially on calcium ions and calmodulin. The protein tyrosine kinase inhibitors genistein and herbimycin A, but not the inactive analogue daidzein, inhibited to a large extent the increase in permeability induced by thrombin. Genistein and BAPTA-AM inhibited the thrombin-induced permeability in an additive way, causing together an almost complete prevention of the thrombin-induced increase in permeability. Inhibition of protein tyrosine kinase was accompanied by a decrease in MLC phosphorylation and a reduction in the extent of F-actin fiber and focal attachment formation. Inhibition of RhoA by C3 transferase toxin reduced both the thrombin-induced barrier dysfunction and MLC phosphorylation. Genistein and C3 transferase toxin did not elevate the cellular cAMP levels. No evidence was found for a significant role of protein kinase C in the thrombin-induced increase in permeability or in the accompanying MLC phosphorylation. These data indicate that in endothelial cell monolayers that respond to histamine in a physiological way, thrombin induces a prolonged increase in permeability by "calcium sensitization," which involves protein tyrosine phosphorylation and RhoA activation.

Endothelial gaps and adherent leukocytes in allergen-induced early- and late-phase plasma leakage in rat airways

Exposure of sensitized individuals to antigen can induce allergic responses in the respiratory tract, manifested by early and late phases of vasodilatation, plasma leakage, leukocyte influx, and bronchoconstriction. Similar responses can occur in the skin, eye, and gastrointestinal tract. The early-phase response involves mast cell mediators and the late-phase response is leukocyte dependent, but the mechanism of leakage is not understood. We sought to identify the leaky blood vessels, to determine whether these vessels contained endothelial gaps, and to analyze the relationship of the gaps to adherent leukocytes, using biotinylated lectins or silver nitrate to stain the cells in situ and Monastral blue as a tracer to quantify plasma leakage. Most of the leakage occurred in postcapillary venules (< 40-microns diameter), whereas most of the leukocyte migration (predominantly neutrophils) occurred in collecting venules. Capillaries and arterioles did not leak. Endothelial gaps were found in the leaky venules, both by silver nitrate staining and by scanning electron microscopy, and 94% of the gaps were distinct from sites of leukocyte adhesion or migration. We conclude that endothelial gaps contribute to both early and late phases of plasma leakage induced by antigen, but most leakage occurs upstream to sites of leukocyte adhesion.

Regulation of transport pathways in tumor vessels: role of tumor type and microenvironment

Novel anti-neoplastic agents such as gene targeting vectors and encapsulated carriers are quite large (approximately 100-300 nm in diameter). An understanding of the functional size and physiological regulation of transvascular pathways is necessary to optimize delivery of these agents. Here we analyze the functional limits of transvascular transport and its modulation by the microenvironment. One human and five murine tumors including mammary and colorectal carcinomas, hepatoma, glioma, and sarcoma were implanted in the dorsal skin-fold chamber or cranial window, and the pore cutoff size, a functional measure of transvascular gap size, was determined. The microenvironment was modulated: (i) spatially, by growing tumors in subcutaneous or cranial locations and (ii) temporally, by inducing vascular regression in hormone-dependent tumors. Tumors grown subcutaneously exhibited a characteristic pore cutoff size ranging from 200 nm to 1.2 microm. This pore cutoff size was reduced in tumors grown in the cranium or in regressing tumors after hormone withdrawal. Vessels induced in basic fibroblast growth factor-containing gels had a pore cutoff size of 200 nm. Albumin permeability was independent of pore cutoff size. These results have three major implications for the delivery of therapeutic agents: (i) delivery may be less efficient in cranial tumors than in subcutaneous tumors, (ii) delivery may be reduced during tumor regression induced by hormonal ablation, and (iii) permeability to a molecule is independent of pore cutoff size as long as the diameter of the molecule is much less than the pore diameter.

Reinterpretation of endothelial cell gaps induced by vasoactive mediators in guinea-pig, mouse and rat: many are transcellular pores

1. In response to vascular permeabilizing agents, particulates circulating in the blood extravasate from venules through endothelial cell openings. These openings have been thought to be intercellular gaps though recently this view has been challenged. 2. To define the precise location of endothelial cell gaps, serial section electron microscopy and three-dimensional reconstructions were performed in skin and cremaster muscle of guinea-pigs, mice and rats injected locally with agents that enhance microvascular permeability: vascular permeability factor, histamine or serotonin. Ferritin and colloidal carbon were injected intravenously as soluble and particulate macromolecular tracers, respectively. 3. Both tracers extravasated from venules in response to all three permeability enhancing agents. The soluble plasma protein ferritin extravasated primarily by way of vesiculo-vacuolar organelles (VVOs), interconnected clusters of vesicles and vacuoles that traverse venular endothelium. In contrast, exogenous particulates (colloidal carbon) and endogenous particulates (erythrocytes, platelets) extravasated from plasma through transendothelial openings. 4. Serial electron microscopic sections and three-dimensional reconstructions demonstrated that eighty-nine of ninety-two openings were transendothelial pores, not intercellular gaps. Pore frequency increased 3- to 33-fold when carbon was used as tracer. 5. The results demonstrate that soluble and particulate tracers extravasate from venules by apparently different transcellular pathways in response to vasoactive mediators. However, some pores may derive from rearrangements of VVOs.

Effect of cell shrinkage on permeability of cultured bovine aortic endothelia and frog mesenteric capillaries

1. We have tested the hypothesis that a reduction in endothelial cell volume increases microvessel permeability and that the degree of endothelial cell attachment to their basement membranes determines the magnitude of permeability changes caused by a reduction in endothelial cell volume. 2. A decrease in endothelial cell volume was imposed on both intact microvessels and cultured endothelial monolayers by raising osmolarity by 100 mosmol l-1. 3. We found that hypertonic solutions did not increase the hydraulic permeability (Lp) of individually perfused venular microvessels in frog mesentery when the perfusate contained albumin. Hypertonic solutions did increase Lp, however, after we perfused the microvessels with the peptide Gly-Arg-Gly Asp-Thr-Pro (GRGDTP; 0.3 mmol l-1), to disrupt integrin-dependent endothelial cell (EC) attachment to the extracellular matrix (ECM). 4. After albumin was removed from the perfusate, hypertonic solutions increased Lp of microvessels and the permeability of endothelial monolayers to alpha-lactalbumin. 5. Our findings indicate that endothelial cell integrin-ECM binding plays a role in transducing changes in cell volume and/or shape into changes in permeability. We hypothesize that removal of albumin from the vascular perfusate may compromise EC-ECM interactions via an integrin-dependent mechanism.

Differing effects of histamine and serotonin on microvascular permeability in anaesthetized rats

1. We have investigated simultaneous changes in the hydraulic permeability (Lp) and the retention of perfusate macromolecules in single mesenteric venules of anaesthetized rats during perfusion with either histamine or serotonin. 2. The venules were microperfused in situ. Retention of macromolecules was assessed from the effective oncotic pressure (omega delta pi) exerted by the perfusate across the vessel walls. Lp and omega delta pi were estimated by the red cell microperfusion technique. 3. Perfusion with histamine (at concentrations between 16 microM and 3.26 mM) and serotonin (at concentrations between 26 microM and 1.3 mM) transiently increased Lp and reduced omega delta pi. Maximal changes were seen at 6-9 min with histamine and at 3 min with serotonin. 4. Maximal increases in Lp were greater with histamine (approximately 3-fold) than with serotonin (1.5- to 2-fold). Serotonin, however, decreased omega delta pi from a baseline of 14-15 cmH2O to one of 6-7 cmH2O whereas the fall of omega delta pi with histamine was only from 14-15 cmH2O to 10-11 cmH2O. 5. The data are consistent with the hypothesis that serotonin increases permeability by inducing openings in the venular endothelium which do not retain macromolecules. If histamine also increases permeability by gap formation, these gaps are able to retain macromolecules to a significant extent.

Openings in frog microvascular endothelium induced by high intravascular pressures

1. We have investigated the effects of microvascular pressures, high enough to rupture the vessel wall, upon the ultrastructure and permeability of mesenteric capillaries in pithed frogs. 2. The vessels were perfused via micropipettes with Ringer solutions containing bovine serum albumin (1 mg ml-1) and a few human red cells. After a perfused section of vessel had been closed downstream, pressure (applied via the micropipette) was raised in a series of steps of 10 mmHg, each lasting approximately 10 s, until the vessel ruptured. Fluid filtration through the vessel wall prior to rupture was estimated from the movements of the red cells. 3. Seven vessels were fixed in glutaraldehyde immediately after rupture and prepared for electron microscopy. The electron micrographs revealed openings in the vessel walls and thirty-six of these gaps were completely defined in runs of serial sections made on four of the vessels. Twenty-nine of these gaps passed through the endothelial cells (transcellular) and seven were intercellular. 4. The pressure at which a vessel ruptured, Pb, was measured in twenty-nine vessels and had a mean +/- S.E.M. value of 79.6 +/- 5.0 cmH2O. In ten of these vessels, which had a mean +/- S.E.M. Pb of 84.2 +/- 6.5 cmH2O, microvascular pressure was lowered immediately after the initial rupture and the vessel perfused at a pressure of 20 cmH2O. Pb was then remeasured and found to be 69.9 +/- 8.4 cmH2O, which was not significantly different from its initial value. 5. Hydraulic permeability (Lp) was measured in six vessels over the range of 15-30 cmH2O before and 10 min after the vessel wall ruptured at high pressure. Mean values were 5.5 x 10(-7) and 4.0 x 10(-7) cms-1 cmH2O-1 and were not significantly different. 6. At pressures equal to and 10 mmHg below Pb, small short-lived increases in filtration rate were observed. It is suggested that these may correspond to the increased permeability to fluid and macromolecules observed at high microvascular pressures in intact capillary beds.