The anti-angiogenic VEGF isoform VEGF165b transiently increases hydraulic conductivity, probably through VEGF receptor 1 in vivo

Vascular endothelial growth factor (VEGF) is the principal agent that increases microvascular permeability during physiological and pathological angiogenesis. VEGF is differentially spliced to form two families of isoforms: VEGF(xxx), and VEGF(xxx)b. Whereas VEGF(165) stimulates angiogenesis, VEGF(165)b is anti-angiogenic. To determine the effect of VEGF(165)b on permeability, hydraulic conductivity (L(p)) was measured in individually perfused microvessels in the mesentery of frogs and rats. As with VEGF(165), VEGF(165)b increased L(p) in amphibian (2.4 +/- 0.3-fold) and mammalian (1.9 +/- 0.2-fold) mesenteric microvessels. A dose-response relationship showed that VEGF(165)b (EC(50), 0.65 pm) was approximately 25 times more potent than VEGF(165) (EC(50), 16 pm) in amphibian microvessels. VEGF(165) has been shown to increase permeability through VEGF receptor 2 (VEGF-R2) signalling. However, VEGF(165)b increased L(p) of frog vessels to the same extent in the presence of the VEGF-R2 inhibitor ZM323881, indicating that it does not increase permeability via VEGF-R2 signalling, and was inhibited by the VEGF receptor inhibitor SU5416 at doses that are specific for VEGF receptor 1 (VEGF-R1). VEGF(165)b, in contrast to VEGF(165), did not result in a sustained chronic increase in L(p). These results show that although VEGF(165)b is anti-angiogenic in the mesentery, it does signal in endothelial cells in vivo resulting in a transient, but not sustained, increase in microvascular L(p), probably through VEGF-R1.

Diabetic retinopathy is associated with a switch in splicing from anti- to pro-angiogenic isoforms of vascular endothelial growth factor

AIMS/HYPOTHESIS

Proliferative diabetic retinopathy results from excess blood vessel growth into the vitreous fluid of the eye. Retinal angiogenesis is regulated by expression of vascular endothelial growth factor (VEGF), and many studies have shown that VEGF is critically involved in proliferative diabetic retinopathy. VEGF is alternatively spliced to form the angiogenic (VEGF(xxx)) and potentially anti-angiogenic (VEGF(xxx)b) family of isoforms. The VEGF(xxx)b family is found in normal tissues, but down-regulated in renal and prostate cancer. Previous studies on endogenous expression of VEGF in the eye have not distinguished between the two families of isoforms.

METHODS

We measured VEGF(xxx)b isoform expression in normal human eye tissue (lens, sclera, retina and iris) and vitreous fluid using enzyme-linked immunosorbent assay and Western blotting with a VEGF(xxx)b-specific antibody.

RESULTS

VEGF(xxx)b protein was expressed in lens, sclera, retina, iris and vitreous fluid. Multiple isoforms were seen, including VEGF(165)b, VEGF(121)b, VEGF(145)b, VEGF(183)b and VEGF(189)b. In non-diabetic patients, 64+/-7% of the VEGF in the vitreous was VEGF(xxx)b (n=18), whereas in diabetic patients only 12.5+/-3.6% of total VEGF was VEGF(xxx)b.

CONCLUSIONS/INTERPRETATION

Since VEGF(xxx)b inhibits VEGF(xxx)-induced angiogenesis in a one-to-one stoichiometric manner, these results show that in the eye of diabetic patients VEGF splicing was switched from an anti-angiogenic to a pro-angiogenic environment. This occurred through changes to the ratio of VEGF(xxx):VEGF(xxx)b. Alterations to splicing, and through that to the balance of VEGF isoforms, could therefore be a potential therapeutic strategy for diabetic retinopathy.

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+)].

The role of endothelial cell Ca2+store release in the regulation of microvascular permeabilityin vivo

Microvascular permeability is regulated by changes in intracellular calcium concentration. The mechanism by which this increase in calcium determines permeability under normal conditions and during stimulation with agonists remains to be elucidated. In order to determine whether calcium release from intracellular stores could contribute towards the regulation of vascular permeability, hydraulic conductivity (Lp) was measured in frog mesenteric microvessels during stimulation of the endothelial cells of these vessels with agonists that release calcium from the intracellular stores. ATP (which acts through activation of inositol 1,4,5-trisphosphate (IP3) receptors) increased Lp in the absence of calcium influx across the plasma membrane 2.3 +/- 0.3 fold (mean +/- s.e.m., P < 0.01, n = 8), which was less than the increase in the presence of calcium influx (3.1 +/- 1.1 fold). Caffeine (which acts through activation of ryanodine receptors) also increased Lp in the absence of calcium influx across the plasma membrane 3.8 +/- 1.0 fold (P < 0.01, n = 9), but by at least as much as it does in the presence of calcium influx (2.8 +/- 0.5 fold). It is surprising that there was a strong positive correlation between the size of the response during store release and the baseline permeability (r = 0.91 for ATP, r = 0.75 for caffeine). This suggests that the filling state of the stores may regulate the baseline permeability of the microvessels.

Lymphoedema in urological cancer

OBJECTIVES

To review the evidence underlying the diagnosis, pathology and treatment of lymphoedema of the lower extremities and genitalia from or following the treatment for urological cancer, and to suggest possible underlying pathophysiological mechanisms that may explain its development.

METHODS

Reviews of the epidemiological, surgical, and scientific literature and personal experience of treatment of patients are used to build a picture of clinical setting and the physiological principles underlying lymphoedema of the leg.

RESULTS

Lymphoedema of the leg and genitals results in serious morbidity for the patient. The incidence is largely unknown, but varies according to the type and location of tumours and may be up to 50% in advanced stages of penile carcinoma, or following its treatment. Although the aetiology of the condition is either iatrogenic, or associated with malignancy, the underlying pathophysiology is not well understood.

CONCLUSIONS

Recent studies in breast cancer related lymphoedema point to underlying vascular as well as lymphatic problems, but the parallels with lower limb lymphoedema are not known.

Lymphatic density and metastatic spread in human malignant melanoma

Malignant melanoma (MM), the most common cause of skin cancer deaths, metastasises to regional lymph nodes. In animal models of other cancers, lymphatic growth is associated with metastasis. To assess if lymphatic density (LD) was increased in human MM, and its association with metastasis, we measured LD inside and around archival MM samples (MM, n=21), and compared them with normal dermis (n=11), basal cell carcinoma (BCC, n=6) and Merkel cell carcinoma (MCC), a skin tumour thought to metastasise through a vascular route (MCC, n=6). Lymphatic capillary density (mm⁻²), as determined by immunohistochemical staining with the lymphatic specific marker LYVE-1, was significantly increased around MM (10.0±2.5 mm⁻²) compared with normal dermis (2.4±0.9 mm⁻²), BCC (3.0±0.9 mm⁻²) and MCC (2.4±1.4 mm⁻²) (P<0.0001). There was a small decrease in LD inside MM (1.1±0.7 mm⁻²) compared with normal dermis, but a highly significant decrease in BCC (0.14±0.13) and MCC (0.12±2.4) (P<0.01 Kruskal–Wallis). Astonishingly, LD discriminated between melanomas that subsequently metastasised (12.8±1.6 mm⁻²) and those that did not (5.4±1.1 mm⁻², P<0.01, Mann–Whitney). Lymphatic invasion by tumour cells was seen mainly in MM that metastasised (70% compared with 12% not metastasising, P<0.05 Fisher's Exact test). The results show that LD was increased around MMs, and that LD and tumour cell invasion of lymphatics may help to predict metastasis. To this end, a prognostic index was calculated using LD, lymphatic invasion and thickness that clearly discriminated metastatic from nonmetastatic tumours.

Evidence of a role for TRPC channels in VEGF-mediated increased vascular permeability in vivo

Vascular endothelial growth factor (VEGF) increases vascular permeability by stimulating endothelial Ca(2+) influx. Here we provide evidence that links VEGF-mediated increased permeability and endothelial intracellular Ca(2+) concentration ([Ca(2+)](i)) with diacylglycerol (DAG)-mediated activation of the transient receptor potential channels (TRPCs). We used the Landis-Michel technique to measure changes in hydraulic conductivity (L(p)) and fluorescence photometry to quantify changes in endothelial [Ca(2+)](i) in individually perfused Rana mesenteric microvessels in vivo and transfected nonendothelial cells in vitro. The membrane-permeant DAG analog 1-oleoyl-2-acetyl-sn-glycerol (OAG, 100 microM), which is known to increase Ca(2+) influx through TRPCs, transiently increased L(p) 3.8 +/- 1.2-fold (from 1.6 +/- 0.8 to 9.8 +/- 2.7 x 10(-7) cm.s(-1).cmH(2)O(-1); P < 0.0001; n = 18). Protein kinase C inhibition by bisindolylmaleimide (1 microM) did not affect the OAG-induced increases in L(p). OAG also significantly increased microvascular endothelial [Ca(2+)](i) in vivo (n = 13; P < 0.0001), which again was not sensitive to protein kinase C inhibition. VEGF induced a transient increase in endothelial [Ca(2+)](i) in human embryonic kidney cells (HEK-293) that were cotransfected with VEGF receptor 2 and TRPC-6 but not with control, VEGF receptor 2, or TRPC-6 expression vector alone (P < 0.01; n = 9). Flufenamic acid, which has been shown to enhance activity of TRPC-6 but inhibit TRPC-3 and -7, enhanced the VEGF-mediated increase in L(p) in approximately half of the vessels tested but inhibited the response in the other half of the vessels. These data provide evidence consistent with the hypothesis that VEGF increases vascular permeability via DAG-mediated Ca(2+) entry through TRPCs. Although the exact identities of the TRPCs remain to be confirmed, TRPC-6 appears to be a likely candidate in approximately half of the vessels.

Role of endothelial Ca2+ stores in the regulation of hydraulic conductivity of Rana microvessels in vivo

Vascular permeability is regulated by endothelial cytosolic Ca(2+) concentration ([Ca(2+)](i)). To determine whether vascular permeability is dependent on extracellular Ca(2+) influx or release of Ca(2+) from stores, hydraulic conductivity (L(p)) was measured in single perfused frog mesenteric microvessels in the presence and absence of Ca(2+) influx and store depletion. Prevention of Ca(2+) reuptake into stores by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) inhibition increased L(p) in the absence of extracellular Ca(2+) influx. L(p) was further increased when Ca(2+) influx was restored. Depletion of the Ca(2+) stores with ionomycin and SERCA inhibition increased L(p) in the presence and the absence of extracellular Ca(2+) influx. However, store depletion in itself did not significantly increase L(p) in the absence of active Ca(2+) release from stores into the cytoplasm. There was a significant positive correlation between baseline permeability and the magnitude of the responses to both Ca(2+) store release and Ca(2+) influx, indicating that the Ca(2+) regulating properties of the endothelial cells may regulate the baseline L(p). To investigate the role of Ca(2+) stores in regulation of L(p), the relationship between SERCA inhibition and store release was studied. The magnitude of the L(p) increase during SERCA inhibition significantly and inversely correlated with that during store release by Ca(2+) ionophore, implying that the degree of store depletion regulates the size of the increase on L(p). These data show that microvascular permeability in vivo can be increased by agents that release Ca(2+) from stores in the absence of Ca(2+) influx. They also show that capacitative Ca(2+) entry results in increased L(p) and that the size of the permeability increase can be regulated by the degree of Ca(2+) release.

ZM323881, a novel inhibitor of vascular endothelial growth factor-receptor-2 tyrosine kinase activity

OBJECTIVE

Vascular endothelial growth factor (VEGF) increases vascular permeability and angiogenesis in many pathological conditions including cancer, arthritis, and diabetes. VEGF activates VEGF-Receptor 1(VEGF-R1) and VEGF-Receptor 2 (VEGF-R2), which autophosphorylate to initiate a signaling cascade resulting in angiogenesis and increased microvascular permeability. Here we describe a novel VEGF-R2 selective inhibitor, ZM323881 (5-[[7-(benzyloxy) quinazolin-4-yl]amino]-4-fluoro-2-methylphenol), that is a potent and selective inhibitor of VEGF-R2 tyrosine kinase in vitro (IC(50) < 2 nM), compared with other receptor tyrosine kinases, including VEGF-R1 (IC(50) > 50 microM).

METHODS

Endothelial cell proliferation was assayed by (3)H-thymidine incorporation in response to VEGF-A +/- ZM323881. The effect of ZM323881 on VEGF-mediated permeability was measured in frog microvessels using the Landis Michel technique. To ensure that ZM323881 was effective in frogs, western analysis was performed on protein extracted from frog lungs incubated in the presence or absence of VEGF-A or VEGF-A with ZM323881.

RESULTS

ZM323881 inhibits VEGF-A-induced endothelial cell proliferation (IC(50) = 8 nM) and VEGF-R2 tyrosine phosphorylation in vitro. VEGF-A-mediated increases in vascular permeability in perfused mesenteric microvessels in vivo were reversibly abolished by both ZM323881 and the class III receptor tyrosine kinase inhibitor PTK787/ZK222584.

CONCLUSIONS

These data suggest that VEGF-R2 phosphorylation is necessary for VEGF-A-mediated increases in microvascular permeability in vivo.

Regulation of vascular permeability by vascular endothelial growth factors

Increased vascular permeability is one of the first stages in both physiological and pathological angiogenesis-the generation of new blood vessels from preexisting vasculature. Although this has been hypothesised to be true in physiological angiogenesis, it is clearly a mark of blood vessel growth in disease. Normal, healthy blood vessel growth (physiological angiogenesis) occurs throughout development as well as during tissue repair and growth in adult tissues. Angiogenesis is also seen in a wide variety of diseases, which include all the major causes of mortality in the West-heart disease, cancer, stroke, vascular disease, and diabetes. Much of this angiogenesis is significantly different from normal blood vessel growth and is termed pathological angiogenesis. Angiogenesis is regulated by vascular growth factors, the most notable being the vascular endothelial growth factor family of proteins (VEGF). These act on specific receptors in the vascular system to stimulate new vessel growth by a number of mechanisms. VEGFs also directly stimulate increased vascular permeability to water and large molecular weight proteins and vasodilatation. These two effects result in a large flux of water and macromolecules from the vasculature to the interstitium, often resulting in oedema. This review will outline the mechanisms by which VEGFs do this and discuss some of the difficulties in interpreting data from VEGF studies due to the conflicting and synergistic effects of these actions.

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.

Regulation of microvascular permeability by vascular endothelial growth factors

Generation of new blood vessels from pre-existing vasculature (angiogenesis) is accompanied in almost all states by increased vascular permeability. This is true in physiological as well as pathological angiogenesis, but is more marked during disease states. Physiological angiogenesis occurs during tissue growth and repair in adult tissues, as well as during development. Pathological angiogenesis is seen in a wide variety of diseases, which include all the major causes of mortality in the west: heart disease, cancer, stroke, vascular disease and diabetes. Angiogenesis is regulated by vascular growth factors, particularly the vascular endothelial growth factor family of proteins (VEGF). These act on two specific receptors in the vascular system (VEGF-R1 and 2) to stimulate new vessel growth. VEGFs also directly stimulate increased vascular permeability to water and large-molecular-weight proteins. We have shown that VEGFs increase vascular permeability in mesenteric microvessels by stimulation of tyrosine auto-phosphorylation of VEGF-R2 on endothelial cells, and subsequent activation of phospholipase C (PLC). This in turn causes increased production of diacylglycerol (DAG) that results in influx of calcium across the plasma membrane through store-independent cation channels. We have proposed that this influx is through DAG-mediated TRP channels. It is not known how this results in increased vascular permeability in endothelial cells in vivo. It has been shown, however, that VEGF can stimulate formation of a variety of pathways through the endothelial cell, including transcellular gaps, vesiculovacuolar organelle formation, and fenestrations. A hypothesis is outlined that suggests that these all may be part of the same process.

Vascular endothelial growth factor increases Rana vascular permeability and compliance by different signalling pathways

Vascular endothelial growth factor (VEGF) chronically increases microvascular permeability, compliance and vessel diameter. To determine the signalling pathways by which VEGF exerts these effects, we investigated the role of Ca2+ influx and mitogen-activated protein kinase (MAPK) phosphorylation on the increase in hydraulic conductivity (Lp), diameter and compliance in mesenteric microvessels in the anaesthetised frog (Rana species). The VEGF-mediated chronically increased permeability was attenuated by co-perfusion of VEGF with 5 mM NiCl2, previously shown to inhibit Ca2+ influx. MAPK phosphorylation inhibition by PD98059 did not affect the chronic increase in Lp. To determine whether other agonists which increased Ca2+ influx also chronically increased Lp, the effect of ATP perfusion on chronic Lp was measured. ATP perfusion also chronically increased Lp. The chronic increase in Lp was therefore dependent on an initial transient Ca2+ influx, and not MAPK activation, and was not unique to VEGF stimulation. Inhibition of Ca2+ influx did not inhibit the increase in microvascular diameter or compliance brought about by VEGF. Both these increases were inhibited by PD98059. The VEGF-mediated increase in compliance and diameter was therefore dependent on MAPK activation, not on Ca2+ influx. The chronic increase in Lp stimulated by VEGF perfusion 24 h previously was reduced when the vessel was perfused with 5 mM NiCl2. The sustained, high Lp was therefore dependent on Ca2+ influx. The endothelial cell calcium concentration ([Ca2+]i) of vessels previously perfused with VEGF or ATP, and with a chronically increased Lp, was not significantly increased compared to [Ca2+]i of endothelial cells in vessels before agonist perfusion These experiments show that VEGF acts through different pathways to stimulate increased permeability and compliance. The data are consistent with the hypothesis that VEGF chronically increases Lp through an acute stimulation of Ca2+ influx, but increases compliance and diameter by acute stimulation of the MAPK signalling pathway. They also suggest that the increase in Lp is dependent on a sustained Ca2+ influx, even though the endothelial [Ca2+]i is not raised.

In vivo mechanisms of vascular endothelial growth factor-mediated increased hydraulic conductivity of Rana capillaries

1. Vascular endothelial growth factor (VEGF) increases hydraulic conductivity (L(p)) in vivo. To determine the signal transduction cascade through which this is mediated, we measured the effect of inhibition of various signalling pathways on VEGF-mediated acute increases in L(p) in individually perfused frog mesenteric microvessels. 2. VEGF receptors have previously been shown to activate phospholipase C-gamma (PLCgamma), protein kinase C (PKC) and MEK, the mitogen-activated and extracellular signal-related kinase (ERK) kinase. To determine the role of these signalling pathways we measured the effects of inhibitors of each on the VEGF-mediated increase in L(p). 3. VEGF-mediated increases in L(p) were attenuated by pre-treatment with the PLC inhibitor U73122, but not affected by treatment with the inactive enantiomer U73343. The PLC inhibitor was also able to attenuate the increase in L(p) mediated by the inflammatory mediator ATP. 4. Inhibition of either PKC or MEK activation using the selective inhibitors bisindolylmaleimide (BIM, 1 microM) and PD98059 (30 microM), respectively, did not change the VEGF-mediated increase in L(p). However, PD98059, BIM and U73122 all reduced phosphorylation of ERK1/2 determined by Western blot analysis with anti-phospho-ERK1/2 antibodies. 5. Furthermore, inhibition of the conversion of diacyl glycerol (DAG) to arachidonic acid, by perfusion with the DAG lipase inhibitor RHC80267 (50 microM), did not attenuate the increase in L(p) brought about by VEGF. 6. These data suggest that VEGF acutely increases microvascular permeability in vivo through a mechanism that is dependent on PLC stimulation, but is independent of PKC or MEK activation or production of arachidonic acid from DAG. We therefore propose that VEGF acutely acts to increase L(p) through the direct actions of DAG, independently of PKC or arachidonic acid.

Differential effects of vascular endothelial growth factor-C and placental growth factor-1 on the hydraulic conductivity of frog mesenteric capillaries

Vascular endothelial growth factors (VEGFs) are known to increase vascular permeability. VEGF-A acts on two receptor tyrosine kinases, VEGF receptor-1 (VEGF-R1 or flt-1) and VEGF receptor-2 (VEGF-R2, flk-1 or KDR). VEGF-C acts only on VEGF-R2 on vascular endothelial cells, whereas placental growth factor-1 (PlGF-1) acts only on VEGF-R1. The effects of perfusion of these receptor-specific proteins on hydraulic conductivity (L(p)) was measured in frog mesenteric capillaries. The effect of PlGF on L(p) was not conclusive, and overall fluid flux did not increase during that time. VEGF-C acutely and transiently increased L(p) (4.5 +/- 0.9-fold), which was more obvious in a subset of vessels, in a similar manner to that reported for VEGF-A. In the subset of vessels in which VEGF-C significantly increased L(p) acutely, there was a sustained 12-fold increase in L(p) 20 min after perfusion, but this was not seen in those vessels which did not respond acutely to VEGF-C, or in vessels exposed to PlGF-1. L(p) was also increased 24 h after perfusion with VEGF-C, but not with PlGF-1. Western blot analysis showed that VEGF-R1 and VEGF-R2 are both present in frog tissue. These data show that the VEGFs that stimulate VEGF-R2 chronically increase L(p), but not those that stimulate VEGF-R1 only. This supports the hypothesis that chronic increases in microvascular permeability induced by VEGF are mediated via activation of VEGF-R2 rather than VEGF-R1.

VEGF and ATP act by different mechanisms to increase microvascular permeability and endothelial [Ca(2+)](i)

Vascular endothelial growth factor (VEGF) increases hydraulic conductivity (L(p)) by stimulating Ca(2+) influx into endothelial cells. To determine whether VEGF-mediated Ca(2+) influx is stimulated by release of Ca(2+) from intracellular stores, we measured the effect of Ca(2+) store depletion on VEGF-mediated increased L(p) and endothelial intracellular Ca(2+) concentration ([Ca(2+)](i)) of frog mesenteric microvessels. Inhibition of Ca(2+) influx by perfusion with NiCl(2) significantly attenuated VEGF-mediated increased [Ca(2+)](i). Depletion of Ca(2+) stores by perfusion of vessels with thapsigargin did not affect the VEGF-mediated increased [Ca(2+)](i) or the increase in L(p). In contrast, ATP-mediated increases in both [Ca(2+)](i) and L(p) were inhibited by thapsigargin perfusion, demonstrating that ATP stimulated store-mediated Ca(2+) influx. VEGF also increased Mn(2+) influx after perfusion with thapsigargin, whereas ATP did not. These data showed that VEGF increased [Ca(2+)](i) and L(p) even when Ca(2+) stores were depleted and under conditions that prevented ATP-mediated increases in [Ca(2+)](i) and L(p). This suggests that VEGF acts through a Ca(2+) store-independent mechanism, whereas ATP acts through Ca(2+) store-mediated Ca(2+) influx.

Vascular endothelial growth factor and microvascular permeability

Vascular Endothelial Growth Factors (VEGFs) are endogenously produced vascular cytokines which result in angiogenesis, vasodilatation, and increased microvascular permeability in vivo. They are endothelial specific and result in mitosis, migration, stress fiber formation and increased permeability of endothelial cells in culture. They have been critically implicated in a host of pathological conditions including solid tumor growth and diabetes, and been proposed as a therapy for coronary and peripheral ischemic disease. However, the potent permeability-enhancing properties of VEGFs are very poorly understood. The pharmacology, signal transduction pathways, intracellular signaling mechanisms, and ultrastructural changes which result in increased permeability are still not clear. This review discusses the available evidence for how VEGFs increase permeability in vivo, and some of the pitfalls in interpreting experiments which do not take into account the vasoactive properties of VEGFs. It also discusses the clinical implications of the permeability enhancing effect of VEGFs, and the relevance of these studies to development of new therapies.

The chronic effect of vascular endothelial growth factor on individually perfused frog mesenteric microvessels

1. Hydraulic conductivity (Lp) of the wall of perfused microvessels has previously been shown to be chronically increased 24 h after a 10 min perfusion with vascular endothelial growth factor (VEGF). In order to investigate this further, Lp and the effective oncotic pressure difference (f3DeltaPi) acting across the vessel walls was measured before exposure to VEGF and 24 h later after the mesentery had been replaced in the abdominal cavity. 2. Acute 10 min perfusion with VEGF did not chronically change f3DeltaPi despite chronically increasing Lp 6.8 +/- 1.2-fold. This suggests that pathways formed 24 h after perfusion with VEGF which increase hydraulic conductivity of the capillary walls have the same reflection coefficient as those present before VEGF. 3. Acute 10 min perfusion with VEGF significantly increased the diameter of vessels after 24 h by 48 +/- 13%. To determine whether this was due to changes in the compliance of the vessel wall, the distensibility of microvessels was measured before and 24 h after perfusion with VEGF. The distensibility was increased 45 +/- 15% by VEGF but this was not great enough to account for the increase in diameter. 4. The chronic increase in Lp could be attenuated by inhibition of nitric oxide synthase with L-NAME. In addition, the chronic increase in permeability was correlated with the acute response to VEGF (r = 0.71, P < 0.01) suggesting that the acute and chronic changes may be related. 5. These results show that VEGF chronically increases Lp without affecting the oncotic reflection coefficient. This may be due to reduced pore path length, or increased small pore numbers, which are properties of fenestrated capillaries. They also show that VEGF increases microvascular distensibility and diameter.

Vascular endothelial growth factor increases microvascular permeability via a Ca(2+)-dependent pathway

We tested the hypothesis that vascular endothelial growth factor (VEGF) increases microvascular permeability by increasing calcium influx into endothelial cells forming the vessel walls. We measured microvessel hydraulic conductivity (Lp) in isolated perfused MS-222-anesthetized frog mesenteric microvessels during perfusion with VEGF under conditions that attenuate calcium influx. VEGF increased Lp during a second successive perfusion in the same microvessel by 7.8-fold, which was not significantly different from that brought about by an initial application of VEGF (5.0-fold). However, under depolarizing conditions, the increase in Lp was reduced from 11.1- to 5.6-fold when depolarized to -10 mV (58 mM K+) and to 2.8-fold when depolarized to 0 mV (100 mM K+). Attenuating calcium influx by the addition of nickel ions resulted in a similar attenuation of the increase in Lp (from 13- to 2.5-fold). VEGF also increased the intracellular calcium concentration in endothelial cells of perfused microvessels as determined by measurement with fura 2. We therefore conclude that VEGF increases Lp by increasing calcium influx.

Vascular endothelial growth factor increases hydraulic conductivity of isolated perfused microvessels

These experiments report the first direct measurement of microvessel permeability coefficients after exposure to vascular endothelial growth factor (VEGF). The Landis technique was extended to enable measurement of the resistance of the microvessel wall to water flow, hydraulic conductivity (Lp), on the same microvessel in the frog mesentery during the initial exposure to VEGF (acute) and 24 and 72 h after initial exposure (chronic). Control measurements of Lp showed no change either acutely or chronically. Exposure to 1 nM VEGF rapidly and transiently increased microvessel Lp within 30 s (to 7.8-fold greater than baseline values) and returned to control within 2 min. The baseline Lp was fivefold greater after 24 h than the initial baseline as a result of VEGF perfusion and returned to its original value after 72 h. These experiments confirm the hypothesis that VEGF acts both acutely (over a period of a few minutes) and chronically (over a few hours) to increase microvascular permeability.

Quantification of rate and depth of pitting in human edema using an electronic tonometer

An instrument (tonometer) was developed to measure objectively the rate as well as depth of pitting of edematous limbs under a sudden local load. Displacement versus time curves were obtained in vivo in postmastectomy edema arms and also in vitro (compression of sponges) and were analyzed in terms of spring and dashpot constants. There was no significant difference between the quasi-instantaneous indentation of tissue in the edematous and normal arms (median 2.9mm), and the two correlated strongly (r = 0.91, p < 0.0001). An exponentially slowing indentation followed. The mean difference between initial and final deformation (X infinity-X0) was greater in the swollen arms (5.7mm) than in the normal arms (1.3mm, p < 0.01). The time constant of indentation (tau) was significantly greater in the swollen arms (227s) than in the normal arms (71s). There was no correlation between the duration of the edema and any of the pitting characteristics. There was a significant negative correlation between glycosaminoglycan concentration of interstitial fluid and rate constant 1/tau (r = 0.9, p < 0.01). The tonometer thus provides an objective way of quantifying the rate and depth of pitting edema.

Starling pressures in the human arm and their alteration in postmastectomy oedema

1. Surgery and radiotherapy to axillary lymph nodes during breast cancer treatment is often followed, commonly years later, by chronic postmastectomy oedema (PMO). PMO is considered a 'high protein' oedema due to reduced axillary lymph drainage. Since oedema formation also depends on fluid input (capillary filtration), we studied the Starling pressures in the affected and contralateral arm. Colloid osmotic pressure was measured in patient serum (pi p) and interstitial fluid (pi i). Subcutis fluid was collected from PMO arms by both wick and aspiration methods, and from the control arm by the wick method only. Interstitial hydraulic pressure (P(i)) was measured by the wick-in-needle method. 2. Oedema pi i was 19.2 +/- 4.1 cmH2O (n = 13, wick) to 16.3 +/- 4.4 cmH2O (n = 41, aspirate; difference not significant; mean +/- S.D. throughout). This was significantly lower than pi i in the control arm (21.4 +/- 3.8 cmH2O, n = 14, P < 0.01, analysis of variance). Also, there was a negative correlation between oedema pi i and the percentage increase in arm volume (correlation coefficient r = -0.35, P < 0.05) in contrast to conventional expectation. 3. Oedema P(i) (1.9 +/- 2.0 cmH2O, n = 28) exceeded the subatmospheric control P(i) (-2.8 +/- 3.0 cmH2O; P < 0.01). Venous and arterial pressures were normal but pi p was subnormal (31.1 +/- 2.7 cmH2O, n = 47). 4. Net pressure opposing capillary blood pressure, P(o), was calculated as P(i) + sigma (pi p-pi i) for a reflection coefficient, sigma, of 0.90-0.99.(ABSTRACT TRUNCATED AT 250 WORDS)

Change in macromolecular composition of interstitial fluid from swollen arms after breast cancer treatment, and its implications

1. The pathophysiology of chronic arm oedema after treatment of breast cancer was investigated by collecting serum and subcutaneous interstitial fluid from the affected and contralateral arms by the wick method (both arms) and by aspiration (oedematous arm). The fluids were analysed for total protein, albumin, glycosaminoglycan and viscosity, and arm volume was measured. 2. Total protein concentration in the aspirated oedema fluid was 32.4 +/- 7.5 g/l (mean +/- SD throughout; n = 39). Protein concentration in wick fluid from the oedematous arm (35.8 +/- 7.3 g/l, n = 14) was not significantly different from that in aspirated fluid. The oedema protein concentrations were significantly lower than in wick fluid from the non-swollen arm (41.4 +/- 6.7 cmH2O, n = 13, P < 0.01, analysis of variance). This was surprising in view of the common assumption that, the condition being of lymphatic origin, the oedema protein concentration should be raised. 3. The ratio of aspirate protein concentration to serum protein concentration showed a weak but highly significant negative correlation with the percentage increase in arm volume (r = -0.47, n = 35, P < 0.005), again in contrast to conventional expectation. The demonstration of a reduced protein concentration in the swollen arm did not therefore depend solely on a comparison with the wick control results. The volume increased by on average 33% and the ratio of aspirate protein concentration to serum protein concentration averaged 0.52 +/- 0.11 on the swollen side and 0.64 +/- 0.13 on the unaffected side. 4. Serum protein concentration in the patients with arm swelling (61.2 +/- 4.9 g/l) was significantly lower than that in postmastectomy patients without this complication (65.0 +/- 6.2 g/l). Most of the decrease occurred in the albumin fraction (oedema patients, 38.3 +/- 5.1 g/l; control patients, 42.0 +/- 2.1 g/l). In oedema patients receiving the anti-oestrogen tamoxifen serum albumin concentration was on average 2.3 g/l lower than in oedema patients not under medication (P < 0.05, t-test). 5. Glycosaminoglycan concentration in oedema fluid was 0.8 +/- 0.14 g/l (n = 21) and 75% was sulphated. Along with the plasma protein this raised the relative viscosity of the fluid to 1.34 +/- 0.16 (n = 11). 6. The reduction in interstitial protein concentration in the swollen arm, contrary to expectation in lymphoedema, could be explained in several ways. One possible hypothesis in light of reported haemodynamic abnormalities in such arms is that capillary pressure rises, increasing capillary filtration rate.(ABSTRACT TRUNCATED AT 400 WORDS)

Capillary underperfusion in chronic venous insufficiency: a cause for leg ulceration?

Dermal capillaries in the goiter area of the lower leg were examined by video-microscopy before and after the administration of intravenous fluorescein in 13 patients with chronic venous insufficiency (CVI) who were at risk of developing leg ulceration, and in 13 normal controls. The influence of posture on capillary perfusion was determined by viewing the same area of skin with the leg in both the supine and dependent positions. Capillary density was lower in patients than in controls, irrespective of the position of the leg (P < 0.01). Fluorescence angiography studies in normal controls showed a reduction in capillary density with dependency (P < 0.01), but patients with CVI showed no significant change. Fluorescence angiography revealed a greater number of capillaries than seen during native capillaroscopy (P < 0.05). The decreased capillary density, and the loss of the postural vasoconstrictor reflex in patients with chronic venous incompetence may play a role in the pathogenesis of ulceration.