Capillary permeability in the pancreas and colon: restriction of exogenous and endogenous molecules by fenestrated endothelia

The permeability properties of fenestrated capillaries in the colon and exocrine and endocrine pancreas to exogenous and endogenous molecules were examined. The exogenous tracers horseradish peroxidase (Einstein-Stokes radius [ESR], 3.0 nm), hemoglobin (ESR, 3.2 nm), and ferritin (ESR, 6.1 nm) were injected intravenously and allowed to circulate for 5-90 min. Tissues were removed and processed for cytochemical or standard electron microscopic examination. The endogenous plasma proteins albumin (ESR, 3.5 nm) and IgG (ESR, 5.5 nm) were localized by immunocytochemistry using the protein A-gold technique. All vessels examined were permeable to HRP in less than 5 min. In contrast, these vessels were restrictive to the slightly larger hemoglobin molecule (60-min circulation) and to ferritin (90-min circulation). Capillaries in the exocrine and endocrine pancreas were restrictive to albumin and IgG. These results demonstrate the presence of fenestrated capillary beds, in addition to the choriocapillaris, that are restrictive to molecules with ESR greater than or equal to 3.2 nm. Capillaries in the mucosa of the colon were restrictive to hemoglobin and ferritin but did not restrict albumin or IgG. This indicates that these vessels are of the permeable type. However, the rate of transendothelial movement of molecules is slower than that of other permeable vessels, such as in the ileo-jejunum. This study has provided further evidence for the existence of fenestrated endothelia that are restrictive to exogenous and/or endogenous molecules.     

Octopus microvasculature: permeability to ferritin and carbon.

The permeability of Octopus microvasculature was investigated by intravascular injection of carbon and ferritin. Vessels were tight to carbon while ferritin penetrated the pericyte junction, and was found extravascularly 1-2 min after its introduction. Vesicles occurred rarely in pericytes; fenestrae were absent. The discontinuous endothelial layer did not consitute a permeability barrier. The basement membrane, although retarding the movement of ferritin, was permeable to it; carbon did not penetrate the basement membrane. Evidence indicated that ferritin, and thus similarly sized and smaller water soluble materials, traverse the pericyte junction as a result of bulk fluid flow. Comparisons are made with the convective (or junctional) and slower, diffusive (or vesicular) passage of materials known to occur across the endothelium of continuous capillaries in mammals. Previous macrophysiological determinations concerning the permeability of Octopus vessels are questioned in view of these findings. Possible reasons for some major structural differences in the microcirculatory systems of cephalopods and vertebrates are briefly discussed.     

Immunocytochemical identification of neural elements in the central nervous systems of a snail,some insects,a fish,and a mammal with an antiserum to the molluscan cardio-excitatory tetrapeptide FMRF-amide

Summary The choriocapillaris is the fenestrated capillary network that supplies a large portion of the nutrients required by the retinal pigment epithelium, photoreceptor cells, and other cells of the outer neural retina. The permeability of these capillaries was investigated in the rat by the use of ferritin (mol. wt. approx. 480,000; mol. diam. 110Å) as a tracer. Ninety minutes after intravascular ferritin administration, a high concentration of tracer particles was distributed uniformly in the capillary lumina but few particles were present in Bruch's membrane, the multilayered basement membrane that separates the choriocapillary endothelium from the retinal pigment epithelium. The bulk of the tracer remained in the capillary lumina with a definite blockage seen at fenestral, channel, and vesicle diaphragms. These results indicate that the rat choriocapillary endothelium, unlike the fenestrated endothelia lining other capillary beds, constitutes an important barrier to the passage of ferritin and presumably of circulating native molecules of similar size.Supported by NIH grants EY 01889 and EY 07034 from the National Eye Institute and a grant-in-aid from Fight for Sight, Inc., of New York City     

Surface charges associated with fenestrated brain capillaries. II. In vivo studies on the role of molecular charge in endothelial permeability

We report on the effect of the net charge of a tracer (ferritin) on its permeability in fenestrated capillaries of the brain. Our experiments show that the charge of this tracer actually influences its interaction with the endothelium. Three phases of tracer-endothelial interaction could be discriminated. Anionic and slightly cationic derivatives (pH 4.5-7.8) do not show any affinity to the luminal endothelial membrane. Ferritin derivatives with a pI value between 7.8 and 9.3 result in the labeling of the fenestrae without coating additional luminal plasmalemmal structures (i.e., coated pits and plasmalemmal vesicles). Tracers with a high positive net charge (pI greater than 9.3) led to their endocytotic uptake and extravasation by some transcytotic mechanism. Extravasated cationic ferritin accumulates in the endothelial basement membrane and binds to striated collagen fibrils. It is suggested that the pericapillary collagen fibrils of fenestrated brain capillaries act as a charge filter with respect to macromolecules.     

Vascular permeability to proteins and peptides in the mouse pineal gland

Summary The permeability of fenestrated capillaries in the mouse pineal gland to proteins and peptides was demonstrated by means of ultrastructural tracers. Horseradish peroxidase (HRP) and microperoxidase (MP) were injected intravenously and allowed to circulate for approximately 30 s, 1 min, 5 min, 1 or 2h. The tissue was then fixed by vascular perfusion or by immersion with aldehydes. In all experiments a pronounced extravasation of HRP and MP occurred. Transendothelial vesicular transport seemed to have occurred across the fenestrated capillaries. The most pronounced tracer labeling of vesicles was found after 1 min of MP- or HRP-circulation. The vesicles were uncoated and more than 70 % of the HRP-and MP-containing vesicles exhibited diameters between 50 and 110 nm. Furthermore, three other transcapillary pathways taken by the tracers are suggested: 1) via intercellular junctions, 2) through fenestrae and 3) via channels formed by fusion of vesicles with the luminal and abluminal cell membranes. Based on these results, it is assumed that the capillaries in the mouse pineal gland are also permeable to peptides synthesized and secreted by the pineal gland.Part of this study was presented at the EMCELL-76 meeting, Copenhagen, 1976     

The ultrastructural basis of transcapillary exchanges

A brief survey is given of current views correlating the ultrastructural and permeability characteristics of capillaries. Observations based on the use of peroxidase (mol wt 40,000), as an in vivo, and colloidal lanthanum, as an in vitro, ultrastructural tracer, are presented. In capillaries with "continuous" endothelium, the endothelial intercellular junctions are thought to be permeable to the tracers, and are regarded as maculae occludentes rather than zonulae occludentes, with a gap of about 40 A in width between the maculae. Some evidence for vesicular transport is also presented. It is inferred that the cell junctions are the morphological equivalent of the small-pore system, and the vesicles the equivalent of the large-pore system. Peroxidase does not apparently cross brain capillaries: the endothelial cell junctions are regarded as zonulae occludentes, and vesicles do not appear to transport across the endothelium. This is regarded as the morphological equivalent of the blood-brain barrier for relatively large molecules. The tracers appear to permeate the fenestrae of fenestrated capillaries, and the high permeability of these capillaries to large molecules is attributed to the fenestrae. Capillaries with discontinuous endothelium readily allow passage of the tracers through the intercellular gaps. A continuous basement membrane may act as a relatively coarse filter for large molecules. In general, the morphology of capillaries correlates well with physiological observations.     

INTESTINAL CAPILLARIES : I. Permeability to Peroxidase and Ferritin

Horseradish peroxidase (mol. diam. 50 A) and ferritin (mol. diam. 110 A) were used as probe molecules for the small and large pore system, respectively, in blood capillaries of the intestinal mucosa of the mouse. Peroxidase distribution was followed in time, after intravenous injection, by applying the Graham-Karnovsky histochemical procedure to aldehyde-fixed specimens. The tracer was found to leave the plasma rapidly and to reach the pericapillary spaces 1 min post injection. Between 1 min and 1 min 30 sec, gradients of peroxidase reaction product could be demonstrated regularly around the capillaries; their highs were located opposite the fenestrated parts of the endothelium. These gradients were replaced by even distribution past 1 min 30 sec. Ferritin, followed directly by electron microscopy, appeared in the pericapillary spaces 3–4 min after i.v. injection. Like peroxidase, it initially produced transient gradients with highs opposite the fenestrated parts of the endothelium. For both tracers, there was no evidence of movement through intercellular junctions, and transport by plasmalemmal vesicles appeared less efficient than outflow through fenestrae. It is concluded that, in the blood capillaries of the inintestinal mucosa, the diaphragms of the endothelial fenestrae contain the structural equivalents of the small pore system. The large pore system seems to be restricted to a fraction of the fenestral population which presumably consists of diaphragm-free or diaphragm-deficient units.     

Permeability of regenerating and atrophic choriocapillaris in the rabbit

When rabbits receive intravenous injections of sodium iodate, large expanses of the retinal pigment epithelium are destroyed. The adjacent capillary bed, the choriocapillaris, atrophies in response to the loss of the pigment epithelium and then regenerates. This provides a model of the permeability of regenerating and atrophic choriocapillaris, which we studied using intravenously injected horseradish peroxidase and catalase. Regenerating capillaries were permeable to peroxidase but not catalase. The permeability to peroxidase was probably due to endothelial fenestrations, since catalase (which is larger than peroxidase and does not penetrate endothelial fenestrae) was retarded at interendothelial junctional complexes, indicating that they were intact. Atrophic choriocapillaries were impermeable to catalase but displayed a heterogeneous permeability to peroxidase. This was correlated with the presence or absence of fenestrae; capillary profiles lacking fenestrae retained peroxidase in their lumina, whereas if fenestrae were present the tracer penetrated into the pericapillary space. The observations indicate that: (1) the permeability of the regenerating choriocapillaris is qualitatively similar to the mature choriocapillaris, and (2) the atrophic choriocapillaris undergoes changes in permeability that are primarily correlated with the loss of endothelial fenestrae. The observations provide a functional correlate - change in permeability - for structural changes in choriocapillaris endothelium (thickening, loss of fenestrae) in response to destruction of the retinal pigment epithelium, which has been postulated to exert a trophic effect on these capillaries.     

Observations on the permeability of the choriocapillaris of the eye

Summary The choriocapillaris is a fenestrated capillary bed located posterior to the retinal pigment epithelium. It serves as the main source of supply to the photoreceptors, retinal pigment epithelium, and other cells of the outer retina. The permeability of these capillaries to intravenously injected ferritin (MW — approx. 480,000; mol. diam. 11 nm) was examined in the mouse, rabbit, and guinea pig, each of which is characterized by a different type of retinal vascularization. In all three species, the bulk of the ferritin remained in the capillary lumina, where it appeared to be blocked at the level of the diaphragmed fenestrae. Some ferritin was present in endothelial cell vacuoles. The results confirm previous work on the rat choriocapillaris and indicate that the barrier function of the choriocapillary endothelium is present even among species in which the retinal circulation differs significantly.Supported by NIH grant EY03418     

The endothelial pocket

Summary A new endothelial cell structure, named the endothelial pocket, has been found by combined transmission and scanning electron microscopic studies of renal peritubular capillaries. Transmission EM observations made on these and other fenestrated capillaries demonstrated that each pocket consists of an attenuated fold of fenestrated endothelium that projects 200 nm into the lumen above the rest of the endothelial surface. Beneath this luminal fold, there is a space and then another layer of fenestrated endothelium which abuts the basal lamina. The linear density of endothelial pockets was measured in the capillaries of the kidney cortex, intestinal mucosa and exocrine pancreas in mice and determined to be 0.067, 0.017 and 0.007 pockets·m^-1 respectively. Cationic ferritin decoration of the anionic sites on the luminal surface of the endothelium in these capillary beds revealed that both unlabelled and labelled diaphragms are clustered. In such specimens, the majority of the luminal diaphragms on endothelial pockets did not have cationic ferritin binding sites detectable by either scanning or transmission EM. On this account as well as on account of their general morphology, endothelial pockets appear to be multifold versions of the simple transendothelial channel.     

Vascular permeability to lanthanum in the rat incisor pulp

Summary Experiments were performed to compare the permeability of capillaries supplying the endoneurial environment, which is invested by perineurium, with vascular permeability in the pulp where perineurium is absent. Anaesthetised rats were perfused through the aorta with physiological solutions containing lanthanum nitrate at 37° C. Pieces of inferior alveolar nerve and segments of mandibular incisors were immersion-fixed and transverse sections were examined electron microscopically for the distribution of lanthanum. In the pulp the nerve fibres pass between lanthanum-impermeable arterioles and venules en route to the incisal end. In the peripheral pulp a few capillaries were permeable but the most permeable capillaries lay between the odontoblasts. Pulpal capillary permeability was attributed to the fenestrated endothelium and contrasted with the unfenestrated endoneurial capillaries which were impermeable to lanthanum. Whereas the tight junctions of endoneurial capillaries are known to prevent certain blood-borne substances from entering the endoneurium, it was not clear whether the permeability of the pulpal capillaries, which are distant from the nerve fibres, could affect the nerve fibre environment. No extravasated lanthanum reached the pulpal nerve fibres suggesting that they are not affected. Technically it was not possible to examine the incisal third of the tooth where the situation could be different because the volume of the pulp decreases and capillaries lie closer to the nerve fibres.     

Differential permeability of pineal capillaries to lanthanum ion in the rat (Rattus norvegicus), gerbil (Meriones unguiculatus) and golden hamster (Mesocricetus auratus)

Summary Capillaries in the pineal gland of the rat (Rattus norvegicus), gerbil (Meriones unguiculatus) and golden hamster (Mesocricetus auratus) were investigated by means of electron-microscopical tracer studies using lanthanum. The tracer was administered together with the fixative solution by perfusion via the left cardiac ventricle. In the rat, endothelial junctions of fenestrated capillaries are permeable to lanthanum. In the gerbil the tracer does not leave the capillaries, which are of the non-fenestrated type throughout the organ. In the golden hamster the two portions of the pineal system have different types of capillaries. While in the superficial pineal fenestrated capillaries permeable to lanthanum predominate, the deep pineal possesses capillaries that are, without exception, devoid of fenestrations and impermeable to the tracer. It is suggested that differences in the structure of the capillaries are related to differences in the extent of a specialized contact of the pineal gland to the third ventricle.The authors dedicate this paper to Professor H. Rollhäuser, Münster, on the occasion of his 65th birthday     

The penetration of exogenous tracers through the enameloid organ of developing teleost fish teeth

In order to determine whether exogenous materials permeate to the forming tooth enameloid matrix, teleost species were injected intramuscularly with horseradish peroxidase (HRP) or myoglobin, or; intracardially with lanthanum nitrate or HRP, then killed a predetermined intervals post-injection. Tooth bearing bones were processed for transmission electron microscopy. At the enameloid matrix formation stage, capillaries associated with the enameloid organ were few in number and rarely fenestrated. Both organic tracers reached the matrix at cervical but not coronal, regions of the teeth in all species examined. Lanthanum was rarely observed extravascularly and never extended to the enameloid matrix at the secretion stage. At the enameloid mineralization stage, fenestrated capillaries were closely associated with the outer dental epithelial cells (ODE). All tracers were observed in the plasma membrane invaginations of the ODE. Only intracardially injected HRP compromised the apical intercellular junctions of the inner dental epithelial cells (IDE) to reach the mineralizing enameloid Lanthanum did not extend past the ODE-IDE cell junctions. It is concluded that the close association of mineralization stage fenestrated capillaries with the highly invaginated ODE cells result in increased tracer penetration compared to the secretory stage. The deeper penetration of the organic tracers, compared with lanthanum, between mineralization stage IDE cells may be due to longer in vivo circulation of the former material. The apical junctions of mineralization stage IDE cells, however, remained impermeable to the organic tracers. The absence of mineral in secretory stage enameloid mineral could not be due to specialized cell junctions preventing access of molecules to the matrix. It is suggested that controlling factors other than cellular permeability initiate enameloid mineralization.     

Cytochemical studies of the vascular endothelium

Cytochemical methods have been used to examine the vascular endothelium. With hemeproteins and immunocytochemistry, investigators have demonstrated the pathways that blood-borne molecules can take to gain access to the extravascular space (Ghitescu et al. 1986; Milici et al. 1987; Schneeberger and Karnovsky 1971; Simionescu et al. 1975). These same cytochemical methods have also provided evidence that morphologically similar endothelia may have different permeability properties (Hart and Pino 1985b, 1986; Pino 1985; Pino and Essner 1980, 1981). Differences in the location and chemical composition of cell surface moieties have been ascertained with enzyme digestion methods, lectins, and cationic ferritin (De Bruyn and Michelson 1978; Pino 1984c, 1986a, b; Simionescu et al. 1981a). The author hopes that he has provided the reader with representative examples of how investigators have used these cytochemical methods for their studies. As new methods are developed and applications are found for existing techniques such as ultracryomicrotomy (Milici et al. 1987) and colloidal gold markers (Pino 1987b), cytochemistry will remain a fundamental tool for the study of the structure and function of the vascular endothelium.     

Permeability of muscle capillaries to microperoxidase

In this study we attempted to identify a morphologic counterpart of the small pore of muscle capillaries. The existence of such a pore has been postulated by physiologists to explain the permeability of muscle capillaries to small macromolecules. We injected mice intravenously with microperoxidase (MP) and fixed specimens of diaphragm at intervals of 0-250 s after the injection to localize the tracer by electron microscopy. The small size of MP (1,900 mol wt and 20 A molecular diameter [MD]) ensures its ready passage through the small pore since the latter is thought to be either a cylindrical channel 90 A in diameter or a slit 55 A wide. MP appears in the pericapillary interstitium within 30 s of initiation of its intravenous injection. The patterns of localization of MP observed within clefts between adjacent capillary endothelial cells indicate that some endothelial junctions are permeable to this tracer. Although small vesicles transfer MP across the endothelium, we do not believe that the vesicles transfer substantial amounts of MP into the pericapillary interstitium. We did not obtain evidence that MP crosses the endothelium of capillaries through channels formed either by a single vesicle or by a series of linked vesicles opening simultaneously at both surfaces of the endothelial cell. From our observations we conclude that some endothelial junctions of capillaries are permeable to MP, and that these permeable junctions are a plausible morphologic counterpart of the small pore.     

The permeability of capillaries among the small granule-containing cells in rat superior cervical ganglia: an ultrastructural lanthanum tracer study.

The permeability of blood capillaries associated with small granule-containing (SGC) cells in rat superior cervical ganglia was investigated at ultrastructural level by employing ionic lanthanum as an electron dense tracer. In rat superior cervical ganglia, the majority of blood capillaries were nonfenestrated. Both fenestrated and nonfenestrated capillaries were observed in the area associated with SGC cells. Lanthanum tracer was observed in the luminal surface, the interendothelial cleft and the subendothelial perivascular spaces of both fenestrated and nonfenestrated capillaries associated with SGC cells. The external lamina of the Schwann cell which surrounded the neurons, nerve fibres and SGC cells were clearly delineated by the lanthanum tracer. Furthermore, the perineuronal space, the periaxonal space, and the pericellular space of the SGC cells were readily accessible to the lanthanum ion. The results demonstrated an absence of blood-nerve barrier, blood-ganglionic and blood-SGC cell barrier to the lanthanum ion in the parenchymal area of the SGC cells in rat superior cervical ganglia. It is proposed that lanthanum may pass through the endothelial cells via 1) the fenestrae of fenestrated capillaries, 2) the intercellular junctions of both fenestrated and nonfenestrated capillaries, i.e., a paracellular pathway; and 3) the process of endocytosis/exocytosis, i.e., a transcellular pathway, to reach the subendothelial space and be distributed in the parenchyma of SGC cells in rat superior cervical ganglia.     

THE INFLUENCE OF INTRAVASCULAR FLUID VOLUME ON THE PERMEABILITY OF NEWBORN AND ADULT MOUSE LUNGS TO ULTRASTRUCTURAL PROTEIN TRACERS

The permeability of the alveolar-capillary membrane of newborn and adult mice to horseradish peroxidase (HRP) and catalase was studied by means of ultrastructural cytochemistry, and the permeability to ferritin was studied by electron microscopy. The influence of varying volumes of intravenously injected fluid on the rate of leakage of the tracers from pulmonary capillaries was examined. The tracers were injected intravenously and the mice were sacrificed at timed intervals. Experiments on newborn mice with intranasally instilled HRP were also done. The tissues were fixed in formaldehyde-glutaraldehyde fixative. Chopped sections were incubated in Graham and Karnovsky's medium for peroxidase and in a modification of this medium for catalase. Tissues were postfixed in OsO₄ and processed for electron microscopy. In both newborn and adult mice, the ready passage of peroxidase through endothelial clefts was dependent on the injection of the tracer in large volumes of saline. When the tracer was injected in small volumes of saline, its passage through endothelial clefts was greatly reduced. Endothelial junctions of newborn mice were somewhat more permeable to HRP than those of adult mice. In all animals, alveolar epithelial junctions were impermeable to HRP. Catalase and ferritin did not pass through endothelial junctions. Intranasally instilled HRP in newborn mice was taken up by pinocytotic vesicles and tubules of flat alveolar cells.     

The blood-brain barrier in a reptile, Anolis carolinensis

An electron microscopic study was made of the ultrastructure and permeability of the capillaries in the cerebral hemispheres of the lizard, Anolis carolinensis. The brain of Anolis is vascularized by a loop-type pattern consisting exclusively of arteriovenous capillary loops. The ultrastructure of the endothelium and the arrangement of the various layers from the capillary lumen to the central nervous tissue is similar to that of mammals. The endothelial cells form a continuous layer around the lumen and are joined by tight interendothelial junctions. The basal lamina of the endothelium is also continuous and encloses pericyte processes. The cells of the nervous tissue rest directly on the basal lamina of the capillary and are separated from each other by a 200 Å space. Intravenously injected horseradish peroxidase (MW 40,000) and ferritin (MW 500,000) were used to study the permeability of the capillaries. The entry of horseradish peroxidase and ferritin into the intercellular spaces of the brain is restricted by the tightness of the interendothelial junctions. No vesicular transport of either tracer occurs; however, ferritin does enter the endothelial cells in vacuoles. No tracer molecules are present in the basal lamina, pericytes, or nervous tissue. The different responses of the endothelial cell to the tracers used in this study suggest that endocytotic activities of endothelial cells involve different processes. Vacuoles formed by marginal folds, vacuoles formed by endothelial surface projections or deep invaginations of the plasma membrane, 600–800 Å vesicles, and coated vesicles all seem to differ in the nature of the substances which they endocytose.     

Ultrastructural studies on the barrier properties of the paraganglia in the rat recurrent laryngeal nerve.

The permeability of blood capillaries in the paraganglia of the rat recurrent laryngeal nerve (RLN) was investigated by employing the ionic lanthanum tracer at ultrastructural level. Two types of blood capillaries, namely, fenestrated and nonfenestrated types, were observed in the rat RLN and its associated paraganglia (RLN paraganglia). A preferential distribution of fenestrated capillaries in the RLN paraganglia was noted. Nonfenestrated capillaries were distributed in the area of RLN devoid of paraganglia. Minute aberrant ganglia consisting of 4-8 neurons were frequently encountered in the rat RLN near the paraganglia. The capillaries in these neuronal areas were also nonfenestrated. The lanthanum tracer was limited within the vascular lumen, but not in the extravascular space, in the RLN proper and in the area of RLN paraganglia where the neurons were identified. In the RLN paraganglia, the tracer was located in the vascular lumen, extravascular space, periaxonal space of nerve fibers, and the intercellular space of the RLN paraganglionic cells. We concluded that (1) a blood-nerve barrier and a blood-ganglion (or blood-neuron) barrier exist in the area of RLN devoid of paraganglia, and (2) blood-paraganglion barrier and blood-nerve barrier were lacking in the rat RLN paraganglia.     

Leaky intra-acinar arteries in rat lungs perfused with hydrogen peroxide

We have investigated the effects of H2O2 (150 or 300 microM) on the ultrastructure and permeability of the pulmonary endothelium in rat lungs perfused for 60 min with buffered Hanks' bovine serum albumin medium. In one group of experiments, we examined the effect of H2O2 on the uptake and transport of cationized ferritin (CF) by endothelial cells in intra-acinar arteries, alveolar capillaries, and interlobular veins. The influence of the oxidant on endothelial adsorptive endocytic processes was assessed by measuring the density of ferritin particles in luminal vesicles, multivesicular bodies, and basal lamina. In a second group of experiments, we examined the effects of H2O2 on the fine structure and permeability to electron-dense macromolecules of arterial, microvascular, and venous endothelium. For this purpose, at the end of the 60-min perfusion with H2O2, CF was perfused to identify leaky vessels. We found that H2O2 caused a dose-dependent inhibition of transcytosis of CF in all vascular segments. At the lower dose of H2O2, inhibition of transcytotic activity was not associated with structural injury to the vascular endothelium or with elevation of wet-to-dry ratios. At the higher oxidant dose, inhibition of transcytosis was associated with leaky arterial endothelium and elevation of wet-to-dry ratios (6.44 +/- 0.12 vs. 5.64 +/- 0.16, P less than 0.02). The effects of H2)2 were prevented by adding catalase to the perfusate. The selective loss of structural integrity and leakiness of the arterial endothelium were diminished but not completely abolished by perfusing the oxidant retrograde from the venous side.(ABSTRACT TRUNCATED AT 250 WORDS)