Microvessel Permeability Research Articles

156 POSTER Antitumor activity of IHL-305, a novel PEGylated liposome containing irinotecan, in huan xenograft models

Cyclophosphamide (CPA) and doxorubicin (DXR)-containing sterically stabilized liposomes (DXR-SL) have a proven clinical activity. We propose that a metronomic CPA dosing schedule enhances accumulation of DXR-SL in solid tumors, because it causes apoptosis in the endothelial cells of the growing tumor vasculature and thereby may increase the permeability of the tumor microvessels. To establish the validity of this hypothesis we investigated the therapeutic benefits of metronomic CPA dosing (p.o.) combined with DXR-SL (i.v.) in a Lewis lung carcinoma, subcutaneously growing in C57BL/6 mouse. The metronomic CPA dosing clearly promoted accumulation and subsequent deep diffusion of SL in the solid tumor as a result of rather a transient increase in the density of CD31+-microvessels, which shows high permeability to SL. It appears that the enhancing effect of metronomic CPA dosing is strongly dependent on the dose of CPA as well as on the time at which the treatment was initiated. Our study indicates that the use of metronomic chemotherapy combined with nanocarriers may be of significant clinical and practical importance in treating intractable solid tumors.

ANALYSIS OF INFLAMMATION

In the past, inflammation has been associated with infections and with the immune system. But more recent evidence suggests that a much broader range of diseases have telltale markers for inflammation. Inflammation is the basic mechanism available for repair of tissue after an injury and consists of a cascade of cellular and microvascular reactions that serve to remove damaged and generate new tissue. The cascade includes elevated permeability in microvessels, attachment of circulating cells to the vessels in the vicinity of the injury site, migration of several cell types, cell apoptosis, and growth of new tissue and blood vessels. This review provides a summary of the major microvascular, cellular, and molecular mechanisms that regulate elements of the inflammatory cascade. The analysis is largely focused on the identification of the major participants, notably signaling and adhesion molecules, and their mode of action in the inflammatory cascade. We present a new hypothesis for the generation of inflammatory mediators in plasma that are derived from the digestive pancreatic enzymes responsible for digestion. The inflammatory cascade offers a large number of opportunities for development of quantitative models that describe various aspects of human diseases.

Adenosine A2A receptor modulation of juvenile female rat skeletal muscle microvessel permeability

Little is known of the regulation of skeletal muscle microvascular exchange under resting or stimulating conditions. Adenosine (ADO) levels in skeletal muscle increase during physiological (exercise) and pathological (hypoxia, inflammation, and ischemia) conditions. Later stages of these pathologies are characterized by the loss of vascular barrier integrity. This study focused on determining which ADO receptor mediates the robust reduction in microvessel permeability to rat serum albumin (P(s)(RSA)) observed in juvenile female rats. In microvessels isolated from abdominal skeletal muscle, ADO suffusion induced a concentration-dependent reduction in arteriolar [log(IC(50)) = -9.8 +/- 0.2 M] and venular [log(IC(50)) = -8.4 +/- 0.2 M] P(s)(RSA). RT-PCR and immunoblot analysis demonstrated mRNA and protein expression of ADO A(1), A(2A), A(2B), and A(3) receptors in both vessel types, and immunofluorescence assay revealed expression of the four subtype receptors in the microvascular walls (endothelium and smooth muscle). P(s)(RSA) responses of arterioles and venules to ADO were blocked by 8-(p-sulphophenyl)theophylline, a nonselective A(1) and A(2) antagonist. An A(2A) agonist, CGS21680, was more potent than the A(1) agonist, cyclopentyladenosine, or the most-selective A(2B) agonist, 5'-(N-ethylcarboxamido)adenosine. The ability of CGS21680 or ADO to reduce P(s)(RSA) was abolished by the A(2A) antagonist, ZM241385. An adenylyl cyclase inhibitor, SQ22536, blocked the permeability response to ADO. In aggregate, these results demonstrate that, in juvenile females (before the production of the reproductive hormones), ADO enhances skeletal muscle arteriole and venule barrier function predominantly via A(2A) receptors using activation of adenylyl cyclase-signaling mechanisms.

Correlation between vascular endothelial growth factor and c-met expressions in breast carcinoma

10621 Background: Vascular endothelial growth factor (VEGF) is a specific mitogen and survival factor for endothelial cells and a key promoter of angiogenesis. This cytokine selectively induces cells proliferation and migration, increases permeability of microvessels and activates proteolitic enzymes involved in tumor invasiveness. C-met is tyrosine kinase receptor; its activation can increase tumorigenicity, induce cell motility and enhance invasiveness and metastasis. This protein can increase production of protease and urokinase which are associated with membrane degradation crucial for metastasis. The relationships between both cytokines are not clearly identified. In this study, we evaluated VEGF and c-met expression in homogenous group of 99 patients with stage II ductal breast carcinomas and analyzed correlations between both markers. Methods: Microscopic studies were performed on formalin-fixed, paraffin-embedded cancer tissues, obtained during surgery and stained routinely with haematoxylin and eosin. Expression of VEGF and c-met was evaluated using a standard immunoperoxidase technique and area with molecule expression, intensity of color reaction and product of both findings were counted. Association between markers expressions was tested by Spearman Correlation Coefficient. Results: Expression of c-met was found in 37 (37.37%) breast tumors, while expression of VEGF was observed in 67 (67.67%) cases. There were strong correlations between expressions of both cytokines. The relationships were found between area with molecules’ expression (p = 0.0438) and intensity of reaction (p = 0.0461), also in the product of both findings (p = 0.0229). Conclusions: The results of our study confirms hypothesis that c-met may act as an indirect angiogenic factor by inducing expression of VEGF. No significant financial relationships to disclose.

Glucocorticoid effects on mouse microvascular endothelial barrier permeability are brain specific

Endothelial cells (ECs) from different vascular beds display certain common qualities, but each subtype is uniquely adapted to meet the demands of the underlying tissues. The structural peculiarities of intercellular junctions are, for instance, considered to account for the differences in permeability displayed by various vascular beds: strong occludin expression is unique to cerebral ECs and considered to account for the high electrical resistance and low paracellular permeability of brain microvessels which constitute the blood-brain barrier (BBB). The integrity of the BBB is compromised in many disorders of the human CNS; therapeutic strategies include treatment with glucocorticoids (GCs), which improve barrier properties of the BBB. In contrast, positive effects of GCs on peripheral vascular permeability could not be demonstrated clearly, while side-effects of prolonged GC treatment are considerable. In an effort to elucidate this difference, we analysed the expression of occludin and the glucocorticoid receptor (GR) in BBB and non-BBB (myocardium) endothelial cells. Our results demonstrate complete GR downregulation by GCs in murine non-BBB endothelial cells in vivo, whereas GC administration led to nuclear concentration of GRs in BBB endothelium. In correlation with these in vivo data, the use of cerebral and myocardial endothelial cell lines proved GR downregulation in non-BBB cells in vitro in response to GC treatment. Divergent transactivating activity of GRs in the BBB and non-BBB endothelial cellular context could be demonstrated after transfection of endothelial cells with a model GC-responsive test promoter plasmid in the presence and absence of dexamethasone. Our results thus suggest differential signalling mechanisms involved in endothelial barrier regulation, arguing for the development of tissue-specific drugs for therapeutic applications.

Dynamic contrast-enhanced magnetic resonance imaging is a poor measure of rectal cancer angiogenesis

The aim of this study was to investigate the use of magnetic resonance imaging (MRI) for non-invasive measurement of rectal cancer angiogenesis and hypoxia. Fifteen patients with rectal adenocarcinoma underwent preoperative dynamic contrast-enhanced (DCE) and blood oxygenation level-dependent (BOLD) MRI. Microvessel density (CD31 level), and expression of vascular endothelial growth factor (VEGF) and carbonic anhydrase (CA) 9 were measured immunohistochemically in histological tumour sections from 12 patients. Serum VEGF levels were also measured in 14 patients. Correlations between quantitative imaging indices and immunohistochemical variables were examined. There was good correlation between circulating VEGF and CD31 expression (r(S) = 0.88, P < 0.001). CD31 expression did not correlate with any dynamic MRI parameter, except transfer constant, with which it correlated inversely (r(S) = -0.65, P = 0.022). Tissue and circulating VEGF levels did not correlate, and neither correlated with any tumour DCE MRI parameter. No relationship was seen between BOLD MRI and CA-9 expression. The negative correlation between transfer constant (reflecting tumour blood flow and microvessel permeability) with CD31 expression is paradoxical. DCE MRI methods for assessing tissue vascularity correlate poorly with histological markers of angiogenesis and hypoxia, suggesting that DCE MRI does not simply reflect static histological vascular properties in patients with rectal cancer.

Immune complexes alter cerebral microvessel permeability: roles of complement and leukocyte adhesion

Immune complexes (ICs) are potent inflammatory mediators in peripheral tissues. However, very few studies have examined the ability of ICs to induce inflammatory responses in the brain. Therefore, using preformed ICs or the reverse passive Arthus (RPA) model to localize ICs to the pial microvasculature of mice, we aimed to investigate the ability of ICs to induce an inflammatory response in the cerebral (pial) microvasculature. Application of preformed ICs immediately increased pial microvascular permeability, with a minimal change in leukocyte adhesion in pial postcapillary venules. In contrast, initiation of the RPA response in the pial microvasculature induced changes in cerebral microvascular permeability and increased leukocyte adhesion in pial postcapillary venules. The RPA response induced deposition of C3 in perivascular regions adjacent to sites of IC formation. Depletion of C3 abrogated RPA-induced microvascular permeability and leukocyte adhesion, indicating that the complement pathway was critical for this response. Inhibition of leukocyte adhesion via CD18 blockade also reduced IC-induced microvascular permeability. However, this did not require intercellular adhesion molecule-1, inasmuch as blockade of intercellular adhesion molecule-1 did not alter RPA-induced microvascular permeability and adhesion. These findings demonstrate that ICs are capable of rapidly inducing inflammatory responses in the cerebral microvasculature, with the complement pathway and leukocyte recruitment playing critical roles in microvascular dysfunction.

Leukocyte-platelet aggregate adhesion and vascular permeability in intact microvessels: role of activated endothelial cells

Leukocyte-platelet aggregation and aggregate adhesion have been indicated as biomarkers of the severity of tissue injury during inflammation or ischemic reperfusion. The objective of this study is to investigate the mechanisms of the aggregate adhesion and quantitatively evaluate its relationship with microvessel permeability. A combined autologous blood perfusion with single microvessel perfusion technique was employed in rat mesenteric venular microvessels. The aggregate adhesion was induced by systemic application of TNF-alpha plus local application of platelet-activating factor (PAF). Changes in permeability were determined by measurements of hydraulic conductivity (Lp) before and after aggregate adhesion in the same individually perfused microvessels. The compositions of the adherent aggregates were identified with fluorescent labeling and confocal imaging. In contrast to leukocyte adhesion as single cells resulting in no increase in microvessel permeability, aggregate adhesion induced prolonged increases in microvessel Lp (6.1 +/- 0.9 times the control, n = 9) indicated by the initial Lp measurements after 3 h of blood perfusion, which is distinct from the transient Lp increase caused by PAF-induced endothelial activation in the absence of blood. Isoproteronol (Iso) attenuated aggregate adhesion-mediated Lp increases if applied after autologous blood perfusion and prevented the aggregate adhesion if the initial endothelial activation is inhibited by applying Iso before PAF administration but showed less effect on single leukocyte adhesion. This study demonstrated that leukocyte-platelet aggregate adhesion via a mechanism different from that of single leukocyte adhesion caused a prolonged increase in microvessel permeability. Our results also indicate that the initial activation of endothelial cells by PAF plays a crucial role in the initiation of leukocyte-platelet aggregate adhesion.

Activation of Epac/Rap1 attenuates venular permeability response to platelet activating factor (PAF) in vivo

Stimulation of intracellular cAMP strongly inhibits acute permeability response induced by many inflammatory mediators including PAF. We have reported that both PKA-dependent and PKA-independent pathways are involved. A novel PKA-independent pathway is the activation of Epac, a cAMP specific guanine nucleotide exchange factor for the small GTPase Rap1. In cultured endothelial monolayers Rap1 increases cortical actin and promotes redistribution of tight junction and adherens junction components to cell-cell junctions (Blood, 2005; 105:–1955). The cAMP analog 8-pCPT-2′-O-Methyl-cAMP (O-Me-cAMP) is specific for the Epac/Rap1 pathway. We tested the efficacy in vivo of O-Me-cAMP to block PAF stimulated hyperpermeability in single rat mesentery post-capillary venules. We found that O-Me-cAMP significantly attenuates the PAF response as measured by hydraulic permeability (Lp) (p<0.05, n = 6, Wilcoxon). PAF alone increased Lp 25 ± 7-fold over baseline (0.47 ± 0.05 × 10-7 cm/(s×cmH2O); with O-Me-cAMP (100 μM, 30 min pretreatment) the PAF peak Lp was 8 ± 2-fold over baseline (mean ± SEM). Similar results were found with 500 μM O-Me-cAMP. We conclude that activation of the Epac/Rap1 pathway is a significant component of the activity of cAMP to block increased permeability in normal microvessels. This approach will enable further differentiation of PKA dependent and PKA independent mechanisms to regulate microvessel permeability. NIH HL28607

Sphingosine‐1‐phosphate (S1P) attenuates acute inflammatory response to mediators platelet activating factor (PAF) and bradykinin (Bk)

Inhibition of Rac1 increases microvessel permeability (AJPHeart 286:H394, 2004). Activation of Rac1 inhibits acute hyperpermeability stimulated by PAF and enhances peripheral actin, and translocation of both actin-binding protein VASP and the Rac1 dependent effector cortactin to cell-cell junctions of endothelial monolayers. Here, we predicted that S1P would inhibit the response to mediators Bk and PAF. We determined hydraulic conductivity (Lp) of single post-capillary venules of rat mesentery. Vessels were perfused with solution containing: 1) serum albumin (BSA, 10 mg/ml) in Ringer’s solution to establish control Lp (25 min), 2) S1P (1 μM, 30 min), 3) S1P and PAF (10 nM, 30 min), 4) BSA (30 min), and 5) PAF without S1P (20 min). A second group of venules were tested for responsiveness to Bk (10 nM) using the same protocol. S1P significantly attenuated the response to PAF. Mean peak Lp response (Lppeak) to PAF was 4.6 ± 0.9 × 10−7 cm/(s×cmH2O) with S1P and was 13 ± 3.6 × 10−7 cm/(s×cmH2O) without (p<0.05, n=6, Wilcoxon). Similarly, the mean Lppeak to Bk was 7.6 ± 3.0 × 10−7 cm/(s×cmH2O) with S1P and 26.4 ± 9.2 × 10-7 cm/(s×cmH2O) without (p<0.05, n=11). A recent publication indicates very similar inhibitory action of S1P on PAF (AJPHeart 289:H840, 2005). Also, Rac1 activation via S1P is protective against lipopolysaccharide-induced lung edema in mice (Cell Signal 17:131, 2005). The results are consistent with the hypothesis that Rac1 activation protects endothelium against acute inflammatory stimuli through stabilization of the peripheral actin-cell adhesion complex. NIH HL28607

Mechanical analysis on the role of the endothelial surface glycocalyx in microvessel permeability

Mechanical analysis on the role of the endothelial surface glycocalyx in microvessel permeability

FMLP-stimulated release of reactive oxygen species from adherent leukocytes increases microvessel permeability

Our previous study (Am J Physiol Heart Circ Physiol 288: H1331-H1338, 2005) demonstrated that TNF-alpha induced significant leukocyte adhesion without causing increases in microvessel permeability, and that formyl-Met-Leu-Phe-OH (fMLP)-stimulated neutrophils in the absence of adhesion increased microvessel permeability via released reactive oxygen species (ROS). The objective of our present study is to investigate the mechanisms that regulate neutrophil respiratory burst and the roles of fMLP-stimulated ROS release from adherent leukocytes in microvessel permeability. A technique that combines single-microvessel perfusion with autologous blood perfusion was employed in venular microvessels of rat mesenteries. Leukocyte adhesion was induced by systemic application of TNF-alpha. Microvessel permeability was assessed by measuring hydraulic conductivity (L(p)). The 2-h autologous blood perfusion after TNF-alpha application increased leukocyte adhesion from 1.2 +/- 0.2 to 13.3 +/- 1.6 per 100 microm of vessel length without causing increases in L(p). When fMLP (10 microM) was applied to either perfusate (n = 5) or superfusate (n = 8) in the presence of adherent leukocytes, L(p) transiently increased to 4.9 +/- 0.9 and 4.4 +/- 0.3 times the control value, respectively. Application of superoxide dismutase or an iron chelator, deferoxamine mesylate, after fMLP application prevented or attenuated the L(p) increase. Chemiluminescence measurements in isolated neutrophils demonstrated that TNF-alpha alone did not induce ROS release but that preexposure of neutrophils to TNF-alpha in vivo or in vitro potentiated fMLP-stimulated ROS release. These results suggest a priming role of TNF-alpha in fMLP-stimulated neutrophil respiratory burst and indicate that the released ROS play a key role in leukocyte-mediated permeability increases during acute inflammation.

Lysophosphatidylcholine: an enigmatic lysolipid

increased endothelial permeability of pulmonary microvessels is the major contributor of acute lung injury, sepsis, and acute respiratory distress syndrome ([2][1], [7][2]). Studies have established that change in cell shape resulting in concurrent formation of gaps between endothelial cells is a

Immune Complexes Alter Cerebral Microvessel Permeability: Roles of Complement and Leukocyte Adhesion

Immune complexes (ICs) are potent inflammatory mediators in peripheral tissues. However, very few studies have examined the ability of ICs to induce inflammatory responses in the brain. Therefore, ...

Technical and Practical Considerations for Permeability Modeling of Dynamic Contrast Enhanced MRI

Characterization of the microvasculature by kinetic modeling of dynamic contrast enhanced MRI (dCE-MRI) is emerging as a powerful approach to the study of angiogenesis and the monitoring of therapeutic approaches in pre-clinical models and clinical settings alike. Applications center on oncology but can also be found in a number of other pathologies, including inflammation (e.g. arthritis), toxic insult and myocardial infarction. Using dynamic contrast enhanced MRI to estimate time-varying tissue concentrations of contrast medium, or tracer, permits simple kinetic modeling to yield physiologically relevant parameters relating to the microvasculature and representing such entities as fractional blood volume, microvessel permeability and extravascular, extracellular space.

Inactivation of CD11b in a mouse transgenic model protects against sepsis-induced lung PMN infiltration and vascular injury

To inactivate chronically the beta2-integrin CD11b (Mac-1), we made a transgenic model in mice in which we expressed the CD11b antagonist polypeptide neutrophil inhibitory factor (NIF). Using these mice, we determined the in vivo effects of CD11b inactivation on polymorphonuclear leukocyte (PMN) function and acute lung injury (ALI) induced by Escherichia coli septicemia. In wild-type PMNs, CD11b expression was induced within 1 h after E. coli challenge, whereas this response was significantly reduced in NIF(+/+) PMNs. Coimmunoprecipitation studies showed that NIF associated with CD11b in NIF(+/+) PMNs. To validate the effectiveness of CD11b blockade, we compared PMN function in NIF(+/+) and Mac-1-deficient (Mac-1(-/-)) mice. Adhesion of both Mac-1(-/-) and NIF(+/+) PMNs to endothelial cells in response to LPS was reduced in both types of PMNs and fully blocked only by the addition of anti-CD11a monoclonal antibody. This finding is indicative of intact CD11a function in the NIF(+/+) PMNs but the blockade of CD11b function. CD11b inactivation in NIF(+/+) mice interfered with lung PMN infiltration induced by E. coli and prevented the increase in lung microvessel permeability and edema formation, with most of the protection seen in the 1-h period after the E. coli. Thus our results demonstrate that CD11b plays a crucial role in mediating lung PMN sequestration and vascular injury in the early phase of gram-negative septicemia. The NIF(+/+) mouse model, in which CD11b is inactivated by binding to NIF, is a potentially useful model for in vivo assessment of the role of PMN CD11b in the mechanism of vascular inflammation.

Blood flow and permeability in microvessels

The mechanics of blood flow in microvessels and microvessel permeability are reviewed. In the first part, characteristics of blood flow in vivo and in vitro are described from a fluid-mechanical point of view, and mathematical models for blood flow in microvessels are presented. Possible causes of the increased flow resistance obtained in vivo compared to in vitro are examined, including the effects of irregularities of vessel lumen, the presence of endothelial surface glycocalyx and white blood cells. In the second part, the ultrastructural pathways and mechanisms whereby endothelial cells and the clefts between the cells modulate microvessel permeability to water and solutes are introduced. Previous and current models for microvessel permeability to water and solutes are reviewed. These models examine the role of structural components of interendothelial cleft, such as junction strands and surface glycocalyx, in the determination of water and solute transport across the microvessel walls. Transport models in the tissue space surrounding the microvessel are also described.

Receptor Protein Tyrosine Phosphatase μ Regulates the Paracellular Pathway in Human Lung Microvascular Endothelia

The pulmonary vascular endothelial paracellular pathway and zonula adherens (ZA) integrity are regulated, in part, through protein tyrosine phosphorylation. ZA-associated protein tyrosine phosphatase (PTP)s are thought to counterregulate tyrosine phosphorylation events within the ZA multiprotein complex. One such receptor PTP, PTPmu, is highly expressed in lung tissue and is almost exclusively restricted to the endothelium. We therefore studied whether PTPmu, in pulmonary vascular endothelia, associates with and/or regulates both the tyrosine phosphorylation state of vascular endothelial (VE)-cadherin and the paracellular pathway. PTPmu was expressed in postconfluent human pulmonary artery and lung microvascular endothelial cells (ECs) where it was almost exclusively restricted to EC-EC boundaries. In human lung microvascular ECs, knockdown of PTPmu through RNA interference dramatically impaired barrier function. In immortalized human microvascular ECs, overexpression of wild-type PTPmu enhanced barrier function. PTPmu-VE-cadherin interactions were demonstrated through reciprocal co-immunoprecipitation assays and co-localization with double-label fluorescence microscopy. When glutathione S-transferase-PTPmu was incubated with purified recombinant VE-cadherin, and when glutathione S-transferase-VE-cadherin was incubated with purified recombinant PTPmu, PTPmu directly bound to VE-cadherin. Overexpression of wild-type PTPmu decreased tyrosine phosphorylation of VE-cadherin. Therefore, PTPmu is expressed in human pulmonary vascular endothelia where it directly binds to VE-cadherin and regulates both the tyrosine phosphorylation state of VE-cadherin and barrier integrity.

Sphingosine 1-phosphate prevents platelet-activating factor-induced increase in hydraulic conductivity in rat mesenteric venules: pertussis toxin sensitive

Sphingosine 1-phosphate (S1P) is a biologically active lipid. In vitro, S1P tightens the endothelial barrier, as assessed by a rapid increase in electrical resistance and a decrease in solute permeability. We hypothesized that this activity of S1P would also occur in vivo. Hydraulic conductivity (Lp), an assessment of endothelial barrier function, was measured in individually perfused venules in rat mesenteries. S1P (1 microM) decreased basal Lp by 63% when basal Lp was between 3.6 and 4.1 x 10(-7) cm x s(-1) x cmH2O(-1) but showed no effect when basal Lp was below 2 x 10(-7) cm x s(-1) x cmH2O(-1). Under either condition, S1P blocked the sixfold increase in Lp induced by platelet-activating factor (PAF, 10 nM). Perfusion of venules with pertussis toxin (0.1 microg/ml), a specific inhibitor of the inhibitory G protein, Gi, for 3 h did not affect basal Lp or the increased Lp induced by PAF. Pertussis toxin, however, significantly attenuated the inhibitory action of S1P on the PAF-induced increase in Lp, indicating the involvement of the Gi protein. Measurement of endothelial cytoplasmic Ca2+ concentration ([Ca2+]i) in venules loaded with fura-2 AM showed that S1P alone transiently increased basal endothelial [Ca2+]i (from 89 nM to 193 nM) but had no effect on the magnitude and time course of the PAF-induced increase in endothelial [Ca2+]i. These results indicate that S1P functions in vivo to prevent the PAF-induced increase in microvessel permeability. The inhibitory action of S1P involves the pertussis toxin-sensitive Gi protein and is not mediated by prevention of the PAF-induced increase in endothelial [Ca2+]i.

Blood tumor permeability of experimental brain tumor: An electron microscopic study using lanthanum

In an attempt to assess the permeability of microvessels in the experimental brain tumor model, lanthanum ion (La3+) was used as a low-molecular weight electron microscopic probe. Rat glioma 9 L and adenocarcinoma ACL15 were transplanted to the brain and subflank of rats. The rats were then anesthetized sequentially perfused with saline, saline plus La3+ followed by a fixative in phosphate buffer. The brain and subcutaneous tumors were removed, further fixed, and processed for electron microscopy. La3 did not pass through the tight junctions of the normal cerebral endothelium. Similarly, La3+ did not penetrate the endothelial cell wall of the microvessels in the transplanted brain tumors. In contrast, extravasation of La3+ from the microvessels in the transplanted subcutaneous tumors was observed. The electron microscopy examination results indicate that the vesicular transport was a predominant mechanism in the penetration of La3+ through the endothelial cell wall. Since most chemotherapeutic agents similar as La3+ are of low molecular weight, we can suggest from the results of our present study that the blood tumor permeability of the anti-cancer agents in the rat model of brain glioma transplantation differs from that in the rat model of subcutaneous glioma transportation. In other words, our results indicate that when the subcutaneous glioma transplantation model is used in sensitivity tests of anti-cancer agents, it will possibly be very difficult to predict the anti-neoplastic effect in vivo.