Microvascular permeability and number of tight junctions are modulated by cAMP

Role of endothelin-1 and cyclic nucleotides in ischemia/reperfusion-mediated microvascular leak

BACKGROUND

A consequence of ischemia/reperfusion (IR) is endothelial barrier dysfunction and intravascular volume loss. The purposes of our study are to explore the impact of: 1) cyclic guanosine monophosphate (cGMP) synthesis inhibition, 2) cyclic adenosine monophosphate (cAMP) synthesis inhibition, 3) treatment with endothelin-1, and 4) endothelin-1 (ET-1)-mediated cAMP changes on IR-induced fluid leak. We hypothesize that IR-mediated microvascular fluid leak results from increased cGMP activity and ET-1 decreases IR-induced fluid leak via cAMP.

METHODS

A micro-cannulation technique was used to determine fluid leak or hydraulic permeability (Lp) in rat mesenteric venules. Lp was measured during IR and after treatment with 1) cGMP synthesis inhibitor (LY83583,10 micromol/L) 2) cAMP synthesis inhibitor (2',5'dideoxyadenosine,10 micromol/L), 3) ET-1 (80 pM), and 4) cAMP synthesis inhibitor plus ET-1 (n=6 in each group; Lp represented as mean+/-standard error of the mean; units 10-cm/sec/cmH2O).

RESULTS

IR resulted in an increase in Lp (Lp=7.07+/-0.20) sevenfold above baseline (1.05+/-0.31) (p<or=0.001). Compared with IR alone, 1) pretreatment with cGMP synthesis inhibitor completely blocked IR-induced fluid leak (Lp=1.08+/-0.18) (p<or=0.001), 2) pretreatment with cAMP synthesis inhibitor attenuated fluid leak (Lp=3.92+/-0.20) (p<or=0.005), 3) treatment with ET-1 decreased fluid leak (Lp=5.38+/-0.28) (p<or=0.005), and 4) pretreatment with a cAMP inhibitor plus treatment with ET-1 reduced fluid leak nearly 50% compared with ET-1 alone (Lp=2.95+/-0.12) (p<or=0.005).

CONCLUSION

cGMP inhibition completely blocks fluid leak, pointing toward a central role as a mediator of IR-induced postcapillary venular leak. ET-1 mildly decreased leak. Furthermore, ET-1 may not exert its effects on microvascular fluid leak during IR via cAMP.

Cyclic nucleotide second messengers (cAMP and cGMP) play a central role in signal transduction and regulation of mesenteric postcapillary fluid leak

BACKGROUND

Endothelial cell receptors involved in post-injury/sepsis fluid extravasation are coupled to G-proteins that stimulate production of cGMP and cAMP. We hypothesize that cGMP and cAMP are endothelial second messengers that control microvascular permeability. The purposes of this series of experiments are to determine microvascular permeability under the following conditions: 1) reduced cGMP levels, 2) elevated cGMP levels, 3) reduced cAMP levels, and 4) elevated cAMP levels.

METHODS

Rat mesenteric venules were cannulated and hydraulic permeability (Lp) was measured at 3 to 5 minute intervals during 1) cGMP synthesis inhibition, 2) inhibition of cGMP degradation, 3) cAMP synthesis inhibition, and 4) inhibition of cAMP degradation (n = 6 in each study group). Lp units are x10 cm(-7)/sec/cmH2O and represented as mean +/- SEM.

RESULTS

Compared with baseline Lp (1.10 +/- 0.06), reduced cGMP levels by inhibiting its synthesis decreased Lp by over 50% (0.50 +/- 0.02, p < 0.001), while elevated cGMP levels by preventing its degradation increased Lp by more than 2-fold (0.91 +/- 0.10 to 2.26 +/- 0.15, p < 0.001). The reduction of cAMP levels by synthesis inhibition elevated Lp over 400% from 0.92 +/- 0.04 to 4.11 +/- 0.54 (p < 0.001), and elevation of cAMP level by blocking its degradation reduced Lp almost 50% from 1.11 +/- 0.04 to 0.59 +/- 0.06 (p < 0.001).

CONCLUSIONS

The second messengers, cGMP and cAMP, contribute to the control mechanisms that govern fluid leak across the endothelial barrier: cGMP increases microvascular permeability, while cAMP decreases microvascular permeability. Endothelial cell cyclic nucleotide second messengers are pharmacologically accessible and may be targeted during post-injury/sepsis-associated microvascular fluid leak.

Differential coronary microvascular exchange responses to adenosine: roles of receptor and microvessel subtypes

OBJECTIVE

To assess the role of adenosine receptors in the regulation of coronary microvascular permeability to porcine serum albumin (P(s)(PSA)).

METHODS

Solute flux was measured in single perfused arterioles and venules isolated from pig hearts using fluorescent dye-labeled probes by microspectro-fluorometry. Messenger RNA, protein, and cellular distribution of adenosine receptors in arterioles and venules were analyzed by RT-PCR, immunoblot, and immunofluorescence.

RESULTS

Control venule P(s)(PSA) (10.7 +/- 4.8 x 10(- 7) cm x s(- 1)) was greater than that of arterioles (6.4+/- 2.8 x 10(-7) cm . s(-1); p < .05). Arteriolar P(s)(PSA) decreased (p < .05) with adenosine suffusion over the range from 10(- 8) to 10(-5) M, while venular P(s)(PSA) did not change. The nonselective A(1) and A(2) receptor antagonist, 8-(p-sulfophenyl) theophylline, blocked the adenosine-induced decrease in arteriolar P(s)(PSA). Messenger RNA for adenosine A(1), A(2A), A(2B), and A(3) receptors was expressed in arterioles and venules. Protein for A(1), A(2A), and A(2B), but not A(3), was detected in both microvessel types and was further demonstrated on vascular endothelial cells.

CONCLUSION

Arteriolar P(s)(PSA) decreases with adenosine suffusion but not venular P(s)(PSA). Adenosine A(1), A(2A), and A(2B) receptors are expressed in both arterioles and venules. Selective receptor-linked cellular signaling mechanisms underlying the regulation of permeability remain to be determined.

Lung transplantation: opportunities for research and clinical advancement

Lung transplantation is the only definitive therapy for many forms of end-stage lung diseases. However, the success of lung transplantation is limited by many factors: (1) Too few lungs available for transplantation due to limited donors or injury to the donor lung; (2) current methods of preservation of excised lungs do not allow extended periods of time between procurement and implantation; (3) acute graft failure is more common with lungs than other solid organs, thus contributing to poorer short-term survival after lung transplant compared with that for recipients of other organs; (4) lung transplant recipients are particularly vulnerable to pulmonary infections; and (5) chronic allograft dysfunction, manifest by bronchiolitis obliterans syndrome, is frequent and limits long-term survival. Scientific advances may provide significant improvements in the outcome of lung transplantation. The National Heart, Lung, and Blood Institute convened a working group of investigators on June 14-15, 2004, in Bethesda, Maryland, to identify opportunities for scientific advancement in lung transplantation, including basic and clinical research. This workshop provides a framework to identify critical issues related to clinical lung transplantation, and to delineate important areas for productive scientific investigation.

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.

Physiological aspects of high-altitude pulmonary edema

High-altitude pulmonary edema (HAPE) develops in rapidly ascending nonacclimatized healthy individuals at altitudes above 3,000 m. An excessive rise in pulmonary artery pressure (PAP) preceding edema formation is the crucial pathophysiological factor because drugs that lower PAP prevent HAPE. Measurements of nitric oxide (NO) in exhaled air, of nitrites and nitrates in bronchoalveolar lavage (BAL) fluid, and forearm NO-dependent endothelial function all point to a reduced NO availability in hypoxia as a major cause of the excessive hypoxic PAP rise in HAPE-susceptible individuals. Studies using right heart catheterization or BAL in incipient HAPE have demonstrated that edema is caused by an increased microvascular hydrostatic pressure in the presence of normal left atrial pressure, resulting in leakage of large-molecular-weight proteins and erythrocytes across the alveolarcapillary barrier in the absence of any evidence of inflammation. These studies confirm in humans that high capillary pressure induces a high-permeability-type lung edema in the absence of inflammation, a concept first introduced under the term “stress failure.” Recent studies using microspheres in swine and magnetic resonance imaging in humans strongly support the concept and primacy of nonuniform hypoxic arteriolar vasoconstriction to explain how hypoxic pulmonary vasoconstriction occurring predominantly at the arteriolar level can cause leakage. This compelling but as yet unproven mechanism predicts that edema occurs in areas of high blood flow due to lesser vasoconstriction. The combination of high flow at higher pressure results in pressures, which exceed the structural and dynamic capacity of the alveolar capillary barrier to maintain normal alveolar fluid balance.

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.

Opposite effect of cAMP signaling in endothelial barriers of different origin

cAMP-mediated signaling mechanisms may destabilize or stabilize the endothelial barrier, depending on the origin of endothelial cells. Here, microvascular coronary [coronary endothelial cells (CEC)] and macrovascular aortic endothelial cell (AEC) monolayers with opposite responses to cAMP were analyzed. Macromolecule permeability, isometric force, activation state of contractile machinery [indicated by phosphorylation of regulatory myosin light chains (MLC), activity of MLC kinase, and MLC phosphatase], and dynamic changes of adhesion complex proteins (translocation of VE-cadherin and paxillin) were determined. cAMP signaling was stimulated by the adenosine receptor agonist 5'-N-(ethylcarboxamido)-adenosine (NECA), the beta-adrenoceptor agonist isoproterenol (Iso), or by the adenylyl cyclase activator forskolin (FSK). Permeability was increased in CEC and decreased in AEC on stimulation with NECA, Iso, or FSK. The effects could be inhibited by the PKA inhibitor Rp-8-CPT-cAMPS and imitated by the PKA activator Sp-cAMPS. Under cAMP/PKA-dependent stimulation, isometric force and MLC phosphorylation were reduced in monolayers of either cell type, due to an activation of MLC phosphatase. In CEC but not in AEC, FSK induced delocalization of VE-cadherin and paxillin from cellular adhesion complexes as indicated by cell fractionation and immunofluorescence microscopy. In conclusion, decline in contractile activation and isometric force contribute to cAMP/PKA-mediated stabilization of barrier function in AEC. In CEC, this stabilizing effect is overruled by cAMP-induced disintegration of cell adhesion structures.

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.

Activation and induction of cyclic AMP phosphodiesterase (PDE4) in rat pulmonary microvascular endothelial cells

Regulation of the rolipram-sensitive cAMP-specific phosphodiesterase 4 (PDE4) gene family was studied in rat pulmonary microvascular endothelial cells (RPMVECs). Total PDE4 hydrolysis was increased within 10 min after addition of forskolin (10 microM), reached a maximum at 20-40 min, and then gradually declined in the cells. A similar activation of PDE4 activity was observed using a protein kinase A (PKA) activator, N(6)-monobutyryl cAMP. Both the forskolin and the N(6)-monobutyryl cAMP activated PDE4 activities were blocked by the PKA-specific inhibitor, H89. This forskolin-stimulated and PKA-mediated short-term activation of PDE4 activity was further confirmed by in vitro phosphorylation of 87kDa PDE4A6 and 83kDa PDE4B3 polypeptides using exogenous PKA Calpha. Increased immunoreactivity of phosphorylated PDE4A6 in situ was detected in Western blots by a PDE4A-phospho antibody specific to the putative PKA phosphorylation sites. Following long-term treatment of RPMVECs with rolipram and forskolin medium (RFM) for more than 60 days, PDE4 activity reached ten-fold higher values than control RPMVECS with twenty-fold increases detected in intracellular cAMP content. The RFM cells showed increased immunoreactivities of the constitutive 4A6 and 4B3 isoforms plus two novel splice variants at 101kDa (4B1) and 71kDa (4B2). Treatment with H89 did not inhibit the PDE4 elevation in RFM cells. In addition to the increased levels of PDE4 in RFM cells, immunofluorescence showed a translocation of PDE4A and 4B to a nuclear region, which was normally not observed in RPMVECs. The PDE4 activity in RFM cells decayed rapidly with an even faster decline of intracellular cAMP content when forskolin/rolipram were removed from the medium. These results suggest that both the activation (short-term) and induction (long-term) of PDE4A/4B isoforms in RPMVECs are closely modulated by the intracellular cAMP content via both post-translational and synthetic mechanisms.

cAMP protects endothelial barrier functions by preventing Rac-1 inhibition

cAMP enhances endothelial barrier properties and is protective against various inflammatory mediators both in vivo and in vitro. However, the mechanisms whereby cAMP stabilizes the endothelial barrier are largely unknown. Recently we demonstrated that the Rho family GTPase Rac-1 is required for maintenance of endothelial barrier functions in vivo and in vitro. Therefore, in the present study we investigated the effect of forskolin (5 microM)- and rolipram (10 microM)-induced cAMP increase on reduction of barrier functions in response to Rac-1 inhibition by Clostridium sordellii lethal toxin (LT). Forskolin and rolipram treatment blocked LT (200 ng/ml)-induced hydraulic conductivity (Lp) increase in mesenteric microvessels in vivo. Likewise, LT-induced intercellular gap formation in monolayers of cultured microvascular myocardial endothelial (MyEnd) cells and LT-induced loss of adhesion of vascular endothelial cadherin-coated microbeads were abolished. Inhibition of PKA by myristoylated inhibitor peptide (14-22) of PKA (100 microM) reduced the protective effect of cAMP on LT-induced Lp increase in vivo and gap formation in vitro, indicating that the effect of cAMP on Rac-1 inhibition was PKA dependent. Glucosylation assays demonstrated that cAMP prevents inhibitory Rac-1 glucosylation by LT, indicating that one way that cAMP enhances endothelial barrier functions may be by regulating Rac-1 signaling. Our study suggests that cAMP may provide its well-established protective effects at least in part by regulation of Rho proteins.

A model for the modulation of microvessel permeability by junction strands

To investigate the effect of junction strands on microvessel permeability, we extend the previous analytical model developed by Fu et al. (1994, J. Biomech. Eng., 116, pp. 502-513), for the interendothelial cleft to include multiple junction strands in the cleft and an interface between the surface glycocalyx layer and the cleft entrance. Based on the electron microscopic observations by Adamson et al. (1998, Am. J. Physiol., 274(43), pp. H1885-H1894), that elevation of intracellular cAMP levels would increase number of tight junction strands, this two-junction-strand and two-pore model can successfully account for the experimental data for the decreased permeability to water, small and intermediate-sized solutes by cAMP.

Airway epithelial integrity is protected by a long-acting beta2-adrenergic receptor agonist

Airway epithelial integrity may be impaired by bacterial exoproducts, which are able to degrade tight junction-associated proteins such as zonula occludens 1 (ZO-1). We have investigated the protective effect of salmeterol, a long-acting beta(2)-adrenergic agonist, on Pseudomonas aeruginosa-induced alteration of the epithelial junctional barrier. We demonstrate in human airway epithelial cells (HAEC) that salmeterol induces a time-dependent increase in ZO-1 protein, although no significant change in ZO-1 transcripts was observed. When HAEC cultures were exposed to P. aeruginosa (PAO1) supernatants, apical expression of ZO-1 protein was maintained in salmeterol-pretreated HAEC cultures, whereas it disappeared after PAO1 exposure in cultures not pretreated with salmeterol. Western blot experiments showed that the 220-kD ZO-1 protein was decreased after PAO1 incubation but was still present in salmeterol-pretreated HAEC extracts. The functional activity of ZO-1 protein was monitored by measuring transepithelial resistance and analyzing the diffusion of a low molecular weight tracer through the intercellular spaces. After PAO1 incubation, the epithelial integrity of HAEC was impaired, as shown by a decrease in transepithelial resistance and increased paracellular permeability, but was not significantly altered after salmeterol preincubation. These results demonstrate that salmeterol may contribute to the protection of the airway epithelium barrier against bacterial virulence factors.

A dual-pathway ultrastructural model for the tight junction of rat proximal tubule epithelium

A dual-pathway model is proposed for transport across the tight junction (TJ) in rat proximal tubule: large slit breaks formed by infrequent discontinuities in the TJ complex and numerous small circular pores, with spacing similar to that of claudin-2. This dual-pathway model is developed in the context of a proximal tubule model (Weinstein AM. Am J Physiol Renal Fluid Electrolyte Physiol 247: F848-F862, 1984) to provide an ultrastructural view of solute and water fluxes. Tubule model paramters (TJ reflection coefficient and water permeability), plus the measured epithelial NaCl and sucrose permeabilities, provide constraints for the dual-pathway model, which yields the small-pore radius and spacing and large slit height and area. For a small-pore spacing of 20.2 nm, comparable to the distance between adjacent particle pairs in apposing TJ strands, the small-pore radius is 0.668 nm and the large slit breaks have a height of 19.6 nm, occupying 0.04% of the total TJ length. This pore/slit geometry also satisfies the measured permeability for mannitol. The numerous small circular pores account for 91.25% of TJ NaCl permeability but only 5.0% of TJ water permeability. The infrequent large slit breaks in the TJ account for 95.0% of TJ water permeability but only 8.7% of TJ NaCl permeability. Sucrose and mannitol (4.6- and 3.6-A radius) can pass through both the large slit breaks and the small pores. For sucrose, 78.3% of the flux is via the slits and 21.7% via the pores; for mannitol, the flux is split nearly evenly between the two pathways, 50.8 and 49.2%. In this ultrastructural model, the TJ water permeability is 21.2% of the entire transepithelial water permeability and thus an order of magnitude greater than that predicted by the single-pore/slit theory (Preisig PA and Berry CA. Am J Physiol Renal Fluid Electrolyte Physiol 249: F124-F131, 1985).

Phosphodiesterase 3 activity is reduced in dog lung following pacing-induced heart failure

We hypothesized that decreases in expression and/or activity of cAMP-specific phosphodiesterases (PDE) contribute to protective adaptations observed in lung after heart failure. In this study, we compared PDE activity in lung parenchyma isolated from control dogs and those paced to heart failure by assaying cyclic nucleotide hydrolysis in fractions of homogenate supernatant eluted from DEAE-Trisacryl columns. Cyclic nucleotide hydrolysis due to PDE3, PDE4, and PDE5 isoforms was predominant in both control and paced groups. The ratio of PDE3 activity to total cAMP PDE activity was decreased in the paced group compared with control (P < 0.05), whereas PDE4 or PDE5 activity ratios were not different between the two groups. With the use of RT-PCR, message expression for PDE3A or PDE3B did not differ between the two groups. Cilostamide, a selective PDE3 inhibitor, and forskolin, a nonspecific agonist for adenylyl cyclase, both inhibited thapsigargin-induced increases in endothelial permeability in control lung. We conclude that PDE3 activity, but not mRNA expression, is reduced in lung from dogs paced to heart failure, a change that could contribute to heart failure-induced attenuation of the lung endothelial permeability response to injury.

Intracellular signalling involved in modulating human endothelial barrier function

The endothelium dynamically regulates the extravasation of hormones, macromolecules and other solutes. In pathological conditions, endothelial hyperpermeability can be induced by vasoactive agents, which induce tiny leakage sites between the cells, and by cytokines, in particular vascular endothelial growth factor, which increase the exchange of plasma proteins by vesicles and intracellular pores. It is generally believed that the interaction of actin and non-muscle myosin in the periphery of the endothelial cell, and the destabilization of endothelial junctions, are required for endothelial hyperpermeability induced by vasoactive agents. Transient short-term hyperpermeability induced by histamine involves Ca2+/calmodulin-dependent activation of the myosin light chain (MLC) kinase. Prolonged elevated permeability induced by thrombin in addition involves activation of the small GTPase RhoA and Rho kinase, which inhibits dephosphorylation of MLC. It also involves the action of other protein kinases. Several mechanisms can increase endothelial barrier function, depending on the tissue affected and the cause of hyperpermeability. They include blockage of specific receptors, and elevation of cyclic AMP by agents such as beta2-adrenergic agents. Depending on the vascular bed, nitric oxide and cyclic GMP can counteract or aggravate endothelial hyperpermeability. Finally, inhibitors of RhoA activation and Rho kinase represent a potentially valuable group of agents with endothelial hyperpermeability-reducing properties.

The phenotype of the human materno-fetal endothelial barrier: molecular occupancy of paracellular junctions dictate permeability and angiogenic plasticity

In vitro models predict that molecular occupancy of endothelial junctions may regulate both barrier function and angiogenesis. Whether this is true in human vascular beds undergoing physiological angiogenesis has not been shown. This review presents data which demonstrate there are two distinct junctional phenotypes, 'activated' and 'stable', present in the vascular tree of the human placenta taken from two distinct highly angiogenic gestational periods (first and last trimester). Stability is conferred by the presence of occludin in tight junctions and plakoglobin in adherens junctions. Their localization may be influenced by vascular endothelial growth factor and angiopoietins 1 and 2 that have a similar temporal and site-specific differential expression. The junctional phenotypes are reversible, as shown in studies with endothelial cells isolated from placental microvessels and grown in the presence/absence of cAMP-enhancing agents. Reductions in protein levels and loss of junctional localization of adhesion molecules result in increased permeability to macromolecules, whilst up-regulation and re-targeting of these molecules inhibit cell proliferation and increase transendothelial resistance. These studies suggest junctional adhesion molecules can regulate physiological angiogenesis and vascular re-modelling. Moreover, the activated junctional phenotype of placental microvessels allows them to participate in increased growth and proliferation. This junctional immaturity appears to be at the expense of barrier function resulting in sites of maximal materno-fetal solute exchange.

Extracellular matrix, junctional integrity and matrix metalloproteinase interactions in endothelial permeability regulation

Vascular endothelial permeability is maintained by the regulated apposition of adherens and tight junctional proteins whose organization is controlled by several pharmacological and physiological mediators. Endothelial permeability changes are associated with: (1) the spatial redistribution of surface cadherins and occludin, (2) stabilization of focal adhesive bonds and (3) the progressive activation of matrix metalloproteinases (MMPs). In response to peroxide, histamine and EDTA, endothelial cells sequester VE-cadherin and alter its cytoskeletal binding. Simultaneously, these mediators enhance focal adhesion to the substratum. Oxidants, cytokines and pharmacological mediators also trigger the activation of matrix metalloproteinases (MMPs) in a cytoskeleton and tyrosine phosphorylation dependent manner to degrade occludin, a well-characterized tight junction element. These related in vitro phenomena appear to co-operate during inflammation, to increase endothelial permeability, structurally stabilize cells while also remodelling cell junctions and substratum.

Differential phosphodiesterase activity contributes to restrictive endothelial barrier function during angiogenesis

Angiogenic endothelial hyperpermeability is abruptly diminished between days 4.5 and 5.0 of the 18-day lifespan of the chick chorioallantoic membrane. Here, we evaluated phosphodiesterase (PDE) activity during the differentiation of barrier function. At day 4.5, rolipram-mediated inhibition of cAMP-specific PDE IV reduced FITC-dextran extravasation. Moreover, inhibition of PDE III by HL 725, but not PDE I by 8-IBMX, decreased the temporal angiogenic endothelial hyperpermeability. Reduced FITC-dextran was also observed at day 4.5 after application of KT 5823, a selective inhibitor of cGMP-specific protein kinase G (PKG), LY 83583, an inhibitor of soluble guanylate cyclase, or LNMMA, an inhibitor of nitric oxide synthase. At day 5.0, Rp-cAMPS-mediated inhibition of cAMP-specific protein kinase A (PKA) diminished barrier function and interstitial accumulation of FITC-dextran was increased. In all cases, the mean widths of interendothelial separation remained uniform. Together, the results support the concept that differentiation of restrictive angiogenic endothelial barrier function in vivo includes inactivation of PDE III and PDE IV with consequent up-regulation of cAMP/PKA signaling and down-regulation of the cGMP/PKG pathway.

Cyclic AMP and acidic fibroblast growth factor have opposing effects on tight and adherens junctions in microvascular endothelial cells in vitro

Endothelial adherens junctions (AJ) and tight junctions (TJ) are important determinants of vascular permeability and cell morphology. Here, we investigate their regulation, in primary human placental microvascular endothelial cell (HPMEC) cultures, by either aFGF plus heparin (ECGS) or elevated cAMP. The proliferation of HPMEC was weakly stimulated by ECGS, while cAMP was inhibitory. ECGS had little effect on transendothelial resistance (TER), but increased macromolecular permeability, whereas cAMP induced a twofold increase in TER and reduced macromolecular permeability. Ultrastructurally, ECGS-treated HPMEC exhibited an "activated" phenotype typified by proliferating cells, with poorly organized cell-cell junctions, whereas cAMP-treated cells appeared quiescent and markedly flattened with extended paracellular junctions, resembling endothelium in situ. The expression and localization of junctional molecules, F-actin, and junctional phosphotyrosine were examined by confocal microscopy and immunoblotting. Junctional molecules in ECGS-treated cells were less organized at lateral membranes than in control cells, whereas in cAMP-treated cells, they were highly localized at continuous contacts. These differences correlated with the intensity of junctional phosphotyrosine, being lowest with cAMP treatment. In the AJ of ECGS-treated and control cells, beta-catenin predominated but in cAMP-treated cells, gamma-catenin/plakoglobin was enriched. In addition, cAMP upregulated junctional expression of VE-cadherin and PECAM-1 and increased the levels of the TJ molecules occludin and ZO-1. The expression levels of junctional components, and their tyrosine phosphorylation, play an important role in dynamic regulation of endothelial cell-cell junctions.

Shear stress regulates occludin content and phosphorylation

Previous studies determined that shear stress imposed on bovine aortic endothelial cell (BAEC) monolayers increased the hydraulic conductivity (L(P)); however, the mechanism by which shear stress increases L(P) remains unknown. This study tested the hypothesis that shear stress regulates paracellular transport by altering the expression and phosphorylation state of the tight junction protein occludin. The effect of shear stress on occludin content was examined by Western blot analysis. Ten dyn/cm(2) significantly reduced occludin content in a time-dependent manner such that after a 3 h exposure to shear, occludin content decreased to 44% of control. Twenty dyn/cm(2) decreased occludin content to 50% of control and increased L(P) by 4.7-fold after 3 h. Occludin expression and L(P) depend on tyrosine kinase activity because erbstatin A (10 microM) attenuated both the shear-induced decrease in occludin content and increase in L(P). Shear stress increased occludin phosphorylation after 5 min, 15 min, and 3 h exposures. The shear-induced increase in occludin phosphorylation was attenuated with dibutyryl (DB) cAMP (1 mM), a reagent previously shown to reverse the shear-induced increase in L(P). We conclude that shear stress rapidly (< or = 5 min) increases occludin phosphorylation and significantly decreases the expression of occludin over 1-4 h. Alterations in the occludin phosphorylation state and occludin total content are potential mechanisms by which shear stress increases L(P).

Permeability of endothelial monolayers to albumin is increased by bradykinin and inhibited by prostaglandins

Using monolayers of bovine aortic endothelial cells (BAEC) in modified Boyden chambers, we examined the role of prostaglandins (PGs) in the bradykinin (BK)-induced increase of albumin permeability. BK induced a concentration-dependent increase of the permeability of BAEC, which reached 49.9 +/- 1% at the concentration of 10(-8) M. Two inhibitors of the prostaglandin G/H synthase, indomethacin (2.88 microM) and ibuprofen (10 microM), potentiated BK-induced permeability 1.8- and 3.9-fold, respectively. Exogenously administered PGE2 and iloprost, a stable analog of prostacyclin, attenuated the effect of BK in a concentration-dependent manner. Butaprost equally reduced the effect of BK, suggesting the participation of the EP2 receptor in this phenomenon. However, the EP4-selective antagonist AH-23848 did not significantly inhibit the protective effect of PGE2. The inhibitory effect of PGE2 was reversed by the adenylate cyclase inhibitor MDL-12330A (10 microM). These results suggest that BK-induced increase of permeability of BAEC monolayer to (125)I-labeled albumin is negatively regulated by PGs. This postulated autocrine activity of PGs may involve an increase in the intracellular level of cAMP.

Leukocyte adhesion and microvessel permeability

To investigate the direct effect of leukocyte adherence to microvessel walls on microvessel permeability, we developed a method to measure changes in hydraulic conductivity (L(p)) before and after leukocyte adhesion in individually perfused venular microvessels in frog mesentery. In 19 microvessels that were initially free of leukocyte sticking or rolling along the vessel wall, control L(p) was measured first with Ringer-albumin perfusate. Blood flow was then restored in each vessel with a reduced flow rate in the range of 30-116 microm/s to facilitate leukocyte adhesion. Each vessel was recannulated in 45 min. The mean number of leukocytes adhering to the vessel wall was 237 +/- 22 leukocytes/mm(2). At the same time, L(p) increased to 4.7 +/- 0.5 times the control value. Superfusion of isoproterenol (10 microM) after leukocyte adhesion brought the increased L(p) back to 1.1 +/- 0.2 times the control in 5-10 min (n = 9). Superfusing isoproterenol before leukocyte adhesion prevented the increase in L(p) (n = 6). However, the number of leukocytes adhering to the vessel wall was not significantly affected. These results demonstrated that leukocyte adhesion caused an increase in microvessel permeability that could be prevented or restored by increasing cAMP levels in endothelial cells using isoproterenol. Thus cAMP-dependent mechanisms that regulate inflammatory agent-induced increases in permeability also modulate leukocyte adhesion-induced increases in permeability but act independently of mechanisms that regulate leukocyte adhesion to the microvessel wall. Application of ketotifen, a mast cell stabilizer, and desferrioxamine mesylate, an iron-chelating reagent, attenuated the increase in L(p) induced by leukocyte adhesion, suggesting the involvement of oxidants and the activation of mast cells in leukocyte adhesion-induced permeability increase. Furthermore, with the use of an in vivo silver stain technique, the locations of the adherent leukocytes on the microvessel wall were identified quantitatively in intact microvessels.

Orosomucoid has a cAMP-dependent effect on human endothelial cells and inhibits the action of histamine

The plasma protein orosomucoid (alpha(1)-acid glycoprotein) has previously been shown to constitute a critical component of the capillary barrier. The protein has also been suggested to act as an anti-inflammatory mediator in a diversity of experimental situations. Recently we reported that orosomucoid is synthesized by the microvascular endothelial cells per se. In the present study, the effects of orosomucoid on primary cultures of human umbilical vein endothelial cells (HUVEC) were studied using the Cytosensor microphysiometer. We found that 1) orosomucoid (0.01 g/l) increased the metabolic activity of HUVEC as reflected by the increased acidification rate of +14 +/- 1%; 2) pretreatment with 0.5 mM 8-bromo-cAMP for 20 min markedly and reversibly inhibited the effect of orosomucoid, whereas 8-bromo-cGMP did not; 3) histamine elicited a dose-dependent response that was abolished by pretreatment with either cAMP or cGMP; and finally, 4) pretreatment of HUVEC for 6 min with orosomucoid (0.01 g/l) inhibited the action of histamine. In summary, this is the first report demonstrating that orosomucoid affects human endothelial cells and that it does so by using cAMP as a second messenger. This provides an explanation for previous findings of anti-inflammatory effects of the protein and shows that orosomucoid affects the endothelium during both normal and pathophysiological conditions.

Dominant role of cAMP in regulation of microvessel permeability

We reported previously that increasing cAMP levels in endothelial cells attenuated ATP-induced increases in hydraulic conductivity (L(p)), and that the activation of cGMP-dependent pathways was a necessary step to increase L(p) in response to inflammatory mediators. The aim of the present study was to evaluate the role of basal levels of cAMP in microvessel permeability under resting conditions and to evaluate the cross talk between cAMP- and cGMP-dependent signaling mechanisms in regulation of microvessel permeability under stimulated conditions, using individually perfused microvessels from frog and rat mesenteries. We found that reducing cAMP levels by inhibition of adenylate cyclase or inhibiting cAMP-dependent protein kinase through the use of H-89 increased basal L(p) in both frog and rat mesenteric venular microvessels. We also found that 8-bromocAMP (8-BrcAMP, 0.2 and 2 mM) was sufficient to attenuate or abolish the increases in L(p) due to exposure of frog mesenteric venular microvessels to 8-BrcGMP (2 mM) and ATP (10 microM). Similarly, in rat mesenteric venular microvessels, application of 8-BrcAMP (2 mM) abolished the increases in L(p) due to exposure to 8-BrcGMP alone (2 mM) or with the combination of bradykinin (1 nM). In addition, application of erythro-9-(2-hydroxy-3-nonyl)adenine, an inhibitor of cGMP-stimulated phosphodiesterase, significantly attenuated both 8-BrcGMP- and bradykinin-induced increases in L(p). These results demonstrate that basal levels of cAMP are critical to maintaining normal permeability under resting conditions, and that increased levels of cAMP are capable of overcoming the activation of cGMP-dependent pathways, therefore preventing increases in microvessel permeability. The balance between endothelial concentrations of these two opposing cyclic nucleotides controls microvessel permeability, and cAMP levels play a dominant role.

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.