Regulation of ion permeability in frog brain venules. Significance of calcium, cyclic nucleotides and protein kinase C

Transendothelial Electrical Resistance Measurement across the Blood-Brain Barrier: A Critical Review of Methods

The blood–brain barrier (BBB) represents the tightest endothelial barrier within the cardiovascular system characterized by very low ionic permeability. Our aim was to describe the setups, electrodes, and instruments to measure electrical resistance across brain microvessels and culture models of the BBB, as well as critically assess the influence of often neglected physical and technical parameters such as temperature, viscosity, current density generated by different electrode types, surface size, circumference, and porosity of the culture insert membrane. We demonstrate that these physical and technical parameters greatly influence the measurement of transendothelial electrical resistance/resistivity (TEER) across BBB culture models resulting in severalfold differences in TEER values of the same biological model, especially in the low-TEER range. We show that elevated culture medium viscosity significantly increases, while higher membrane porosity decreases TEER values. TEER data measured by chopstick electrodes can be threefold higher than values measured by chamber electrodes due to different electrode size and geometry, resulting in current distribution inhomogeneity. An additional shunt resistance at the circumference of culture inserts results in lower TEER values. A detailed description of setups and technical parameters is crucial for the correct interpretation and comparison of TEER values of BBB models.

Reversible blood-brain barrier opening utilizing the membrane active peptide melittin in vitro and in vivo

The blood-brain barrier (BBB) tightly controls entry of molecules and cells into the brain, restricting the delivery of therapeutics. Blood-brain barrier opening (BBBO) utilizes reversible disruption of cell-cell junctions between brain microvascular endothelial cells to enable transient entry into the brain. Here, we demonstrate that melittin, a membrane active peptide present in bee venom, supports transient BBBO. From endothelial and neuronal viability studies, we first identify the accessible concentration range for BBBO. We then use a tissue-engineered model of the human BBB to optimize dosing and elucidate the mechanism of opening. Melittin and other membrane active variants transiently increase paracellular permeability via disruption of cell-cell junctions that result in transient focal leaks. To validate the results from the tissue-engineered model, we then demonstrate that transient BBBO can be reproduced in a mouse model. We identify a minimum clinically effective intra-arterial dose of 3 μM min melittin, which is reversible within one day and neurologically safe. Melittin-induced BBBO represents a novel technology for delivery of therapeutics into the brain.

Blood-brain barrier disruption in atrial fibrillation: a potential contributor to the increased risk of dementia and worsening of stroke outcomes?

Atrial fibrillation (AF) has become one of the most significant health problems worldwide, warranting urgent answers to currently pending questions on the effects of AF on brain function. Recent evidence has emerged to show an association between AF and an increased risk of developing dementia and worsening of stroke outcomes. A healthy brain is protected by the blood–brain barrier (BBB), which is formed by the endothelial cells that line cerebral capillaries. These endothelial cells are continuously exposed to shear stress (the frictional force generated by blood flow), which affects endothelial cell structure and function. Flow disturbances as experienced during AF can disrupt the BBB and leave the brain vulnerable to damage. Investigating the plausible mechanisms in detail, linking AF to cerebrovascular damage is difficult in humans, leading to paucity of available clinical data. Here, we discuss the available evidence for BBB disruption during AF due to altered cerebral blood flow, and how this may contribute to an increased risk of dementia and worsening of stroke outcomes.

Ca2+ homeostasis in brain microvascular endothelial cells

Blood brain barrier (BBB) is formed by the brain microvascular endothelial cells (BMVECs) lining the wall of brain capillaries. Its integrity is regulated by multiple mechanisms, including up/downregulation of tight junction proteins or adhesion molecules, altered Ca²⁺ homeostasis, remodeling of cytoskeleton, that are confined at the level of BMVECs. Beside the contribution of BMVECs to BBB permeability changes, other cells, such as pericytes, astrocytes, microglia, leukocytes or neurons, etc. are also exerting direct or indirect modulatory effects on BBB. Alterations in BBB integrity play a key role in multiple brain pathologies, including neurological (e.g. epilepsy) and neurodegenerative disorders (e.g. Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis etc.). In this review, the principal Ca²⁺ signaling pathways in brain microvascular endothelial cells are discussed and their contribution to BBB integrity is emphasized. Improving the knowledge of Ca²⁺ homeostasis alterations in BMVECa is fundamental to identify new possible drug targets that diminish/prevent BBB permeabilization in neurological and neurodegenerative disorders.

Endothelial calcium dynamics, connexin channels and blood-brain barrier function

Situated between the circulation and the brain, the blood-brain barrier (BBB) protects the brain from circulating toxins while securing a specialized environment for neuro-glial signaling. BBB capillary endothelial cells exhibit low transcytotic activity and a tight, junctional network that, aided by the cytoskeleton, restricts paracellular permeability. The latter is subject of extensive research as it relates to neuropathology, edema and inflammation. A key determinant in regulating paracellular permeability is the endothelial cytoplasmic Ca(2+) concentration ([Ca(2+)]i) that affects junctional and cytoskeletal proteins. Ca(2+) signals are not one-time events restricted to a single cell but often appear as oscillatory [Ca(2+)]i changes that may propagate between cells as intercellular Ca(2+) waves. The effect of Ca(2+) oscillations/waves on BBB function is largely unknown and we here review current evidence on how [Ca(2+)]i dynamics influence BBB permeability.

Regulation of blood-brain barrier permeability

The blood-brain barrier minimizes the entry of molecules into brain tissue. This restriction arises by the presence of tight junctions (zonulae occludens) between adjacent endothelial cells and a relative paucity of pinocytotic vesicles within endothelium of cerebral arterioles, capillaries, and venules. Many types of stimuli can alter the permeability characteristics of the blood-brain barrier. Acute increases in arterial blood pressure beyond the autoregulatory capacity of cerebral blood vessels, application of hyperosmolar solutions, application of various inflammatory mediators known to be elevated during brain injury, and/or activation of blood-borne elements such as leukocytes can produce changes in permeability of the blood-brain barrier. The second messenger systems that account for increases in permeability of the blood-brain barrier during pathophysiologic conditions, however, remain poorly defined. This review will summarize studies that have examined factors that influence disruption of the blood-brain barrier, and will discuss the contribution of various cellular second messenger pathways in disruption of the blood-brain barrier during pathophysiologic conditions.

Osmotic opening of the blood-brain barrier: principles, mechanism, and therapeutic applications

1. Osmotic opening of the blood-brain barrier by intracarotid infusion of a hypertonic arabinose or mannitol solution is mediated by vasodilatation and shrinkage of cerebrovascular endothelial cells, with widening of the interendothelial tight junctions to an estimated radius of 200 A. The effect may be facilitated by calcium-mediated contraction of the endothelial cytoskeleton. 2. The marked increase in apparent blood-brain barrier permeability to intravascular substances (10-fold for small molecules) following the osmotic procedure is due to both increased diffusion and bulk fluid flow across the tight junctions. The permeability effect is largely reversed within 10 min. 3. In experimental animals, the osmotic method has been used to grant wide access to the brain of water-soluble drugs, peptides, antibodies, boron compounds for neutron capture therapy, and viral vectors for gene therapy. The method also has been used together with anticancer drugs to treat patients with metastatic or primary brain tumors, with some success and minimal morbidity.

Regulation of cerebral microvascular permeability by histamine in the anaesthetized rat

1. The permeability response of slightly leaky pial venular capillaries to histamine was investigated using the single microvessel occlusion technique. 2. Histamine dose-response curves showed that concentrations between 5 nm and 5 microM increased permeability, while concentrations from 50 microM to 5 mM reduced it. 3. The H2 receptor antagonist cimetidine (2 microM) blocked the effects of lower concentrations of histamine, while the H1 receptor antagonist mepyramine (3 nM) blocked those of higher concentrations of histamine. 4. The effects of lower doses of histamine were mimicked by the H2 receptor agonist dimaprit, and the effects of higher doses of histamine were mimicked by the H1 receptor agonist alpha-2-(2-aminoethyl)pyridine (AEP). 5. Low concentrations of histamine, which normally increase the permeability of Lucifer Yellow (PLY), reduced it when co-applied with the phosphodiesterase 4 (PDE4) inhibitor rolipram. Rolipram also potentiated the response to AEP, but had no effect on that to dimaprit. 6. The effects of dimaprit were blocked by reducing extracellular Ca2+ from 2.5 mM to nominally Ca2+ free, or by applying the calcium entry blocker SKF 96365.

Different effects of calcium antagonists on fluid filtration of large arteries and albumin permeability in spontaneously hypertensive rats

OBJECTIVE

To compare the effects of chronic administration of two dihydropyridines, nifedipine and amlodipine, and the non-dihydropyridine Ca2+ antagonist mibefradil on fluid filtration of large arteries and extravasation of albumin in spontaneously hypertensive rats.

METHODS

Spontaneously hypertensive rats aged 2 months were randomly allocated to oral treatment once a day with 30 mg/kg mibefradil (n=12), 100 mg/kg nifedipine (n=12), 20 mg/kg amlodipine (n=12) or placebo (n=12) for 1 month. Instantaneous blood pressure of rats under pentobarbital anaesthesia was recorded at the end of the treatment Fluid filtration across the carotid arterial wall was determined in situ in the isolated carotid artery. Extravasation of 25 mg/kg Evans Blue dye that had been injected intravenously was used to assess whole vascular permeability to albumin after chronic treatment with mibefradil.

RESULTS

Similar reductions in mean arterial pressure were obtained in all Ca2+ antagonist-treated rats. Heart rate was similar in rats in control, nifedipine and amlodipine groups but was significantly lower in mibefradil-treated rats (by 19%, P< 0.001). Fluid filtration across the carotid wall was greater in all Ca2+ antagonist-treated animals. However, fluid filtration was significantly less in mibefradil-treated rats than it was in nifedipine-treated, and amlodipine-treated rats. Furthermore, administration of mibefradil did not significantly modify extravasation of albumin in all tested tissues (pancreas, testis, spleen, lung, kidney, intestine, liver, skeletal muscle) except for cardiac and brain tissues, in which the permeability of albumin was increased by 24 and 33%, respectively, compared with values for the control group (P < 0.05).

CONCLUSION

These results indicate that Ca2+ antagonists increase fluid filtration through large arteries from spontaneously hypertensive rats. That the lower fluid filtration in mibefradil-treated rats was associated with no change in extravasation of albumin in most tissues and especially in skeletal muscle suggests that vascular permeability in hypertensive rats was impaired less by mibefradil treatment than it was by dihydropyridine Ca2+ antagonist treatments.

Alterations in transendothelial electrical resistance by vasoactive agonists and cyclic AMP in a blood-brain barrier model system

We have previously reported that the co-culture of endothelial and glioma cell lines provides an in vitro model for investigating properties of the blood-brain barrier (BBB). To characterise the model system further we have investigated the effects of vasoactive substances implicated in increases in BBB permeability. Additionally, we have also examined whether activation of cyclic AMP signalling pathways, which elevate cerebral endothelial cell barrier function, similarly modulate our model system. ATP, histamine, bradykinin, and serotonin significantly decreased model BBB transendothelial electrical resistance and manipulations which elevate cyclic AMP enhanced culture resistance. These data indicate that our model BBB system responds in a manner characteristic of cerebral microvascular endothelial cells and the BBB in vivo. These data further emphasize the usefulness of our model system.

Two components of blood-brain barrier disruption in the rat

1. Permeability of pial venular capillaries to Lucifer Yellow (PLY) was measured using the single microvessel occlusion technique. 2. PLY was extremely low, when measured shortly after the removal of the meninges, consistent with an intact blood-brain barrier, but rose spontaneously to (1.65 +/- 0.60) x 10(-6) cm s-1 (mean +/- S.D.) within 20-60 min. This first phase of spontaneous disruption lasted 44-164 min. A second phase started when PLY rose sharply, and was characterized by rapid permeability fluctuations with a mean of (12.31 +/- 15.14) x 10(-6) cm s-1. 3. The first phase could be mimicked by applying the divalent cation ionophore A23187 in the presence of Ca2+, when PLY rose by (1.47 +/- 0.25) x 10(-6) cm s-1 (mean +/- S.E.M.). Application of histamine (10 microM) to tight vessels increased PLY by (2.41 +/- 0.22) x 10(-6) cm s-1. 4. Substances that raised intraendothelial cAMP of vessels during the first phase of disruption reduced PLY to the initial blood-brain barrier level. 5. The second phase could be prevented by applying catalase. Similar high and fluctuating PLY values could be produced reversibly by applying arachidonic acid or NH4Cl. 6. This is the first report of two distinct types of permeability increase in the cerebral microvasculature, and reasons for this are discussed.

Kinin receptors in human vascular tissue: their role in atheromatous disease

Using samples of many human blood vessels, obtained at autopsy and specific antibodies directed to peptide sequences of the kinin B1 and B2 receptors, we demonstrate the localisation of these receptors within the human vascular system using standard immunolabelling techniques. In large elastic arteries and veins, kinin receptors are present only in the endothelial cells whereas in all muscular arteries and arterioles, these receptors are present in both the endothelial and smooth muscle cells. The identification of kinin receptors in human blood vessels confirms that kinins may modulate both vascular permeability and contractility. The incidental finding at histology, of patchy atheromatous disease in the coronary, femoral, vertebral and pericallosal arteries, assisted in elucidating the role of these receptors in the commonest disease affecting human blood vessels. Intense labelling for B1 receptors was observed in the endothelial cells, foamy macrophages, inflammatory cells and fibroblasts within the thickened intima of the plaque as well as in smooth muscle cells of the underlying tunica media. Immunoreactive B2 receptors were also observed in these cells but with reduced intensity. The intense immunolabelling of B1 receptors in these regions suggest that these may be induced by atheromatous disease and may have therapeutic importance for the B1 receptor antagonists.

Mechanisms of alpha-thrombin, histamine, and bradykinin induced endothelial permeability

alpha-Thrombin, bradykinin, and histamine are endogenous mediators that increase endothelial permeability. We examined the mechanism by which these three vasoactive mediators could alter permeability to albumin of human umbilical vein endothelial cells (HUVEC). HUVEC were grown to confluence on Transwell membranes and we monitored the flux of fluorescein isothiocyanate-labeled human serum albumin across the membrane from the upper to lower chamber of the Transwell. Addition of alpha-thrombin, bradykinin, or histamine increased the permeability coefficient of the HUVEC monolayer. At 30 min the permeability coefficient for alpha-thrombin was 4.92 x 10(-6) cm/sec while histamine was 4.47 x 10(-6) cm/sec. Maximum changes in the permeability coefficient were about three-fold control baseline values (1.59 x 10(-6) cm/sec). There was also a temporal difference in the magnitude of the permeability coefficient. alpha-Thrombin and bradykinin induced HUVEC permeability was increased for the first 90 min after which it returned to control levels. In contrast, histamine increased the permeability of the HUVEC monolayer throughout the 2 h experiment. To determine a possible intracellular mechanism of the altered permeability coefficients, HUVEC were labeled with FURA-2 and intracellular calcium was monitored by digital fluorescence ratio imaging. Maximum intracellular calcium in HUVEC was increased by alpha-thrombin (245 +/- 20 nM) and histamine (210 +/- 22 nM), but not by bradykinin (70 +/- 7 nM) as compared to control (69 +/- 10). Fluorescent photomicrographs of HUVEC stimulated with the three agonists indicated that alpha-thrombin and histamine substantially altered HUVEC f-actin arrangement, while bradykinin had no effect on HUVEC f-actin distribution. These data support previous in vitro and in vivo studies demonstrating increased permeability by all three agonists. These data also show, for the first time, that histamine and alpha-thrombin increased permeability by calcium-dependent intracellular pathways, but bradykinin operates through a calcium-independent mechanism.

Astroglial cells inhibit the increasing permeability of brain endothelial cell monolayer following hypoxia/reoxygenation

Blood-brain barrier (BBB) is known to be structured with astroglial cells (AGs) and brain endothelial cells (BECs), and it has been proposed that these cells play different roles in the BBB. We cultivated AGs and BECs from infant rats (2-week-old), and these cells were cultured on the opposing side of collagen membrane to produce a co-culture model of the BBB in vitro. Permeability of the cell layer was evaluated by the electrical resistance through the membrane. To clarify the role of AGs in the BBB disruption following ischemia/reperfusion, electrical resistance of the co-culture model was compared to that of BEC monolayer following hypoxia/reoxygenation. The electrical resistance through BEC monolayer showed 55.5 +/- 15.1 percent reduction at 4 h of hypoxia, and 93.3 +/- 5.4 percent reduction at 8 h of hypoxia (n = 8). However, the co-culture model showed attenuation of the reduction (24.8 +/- 14.2 percent) at 4 h of hypoxia (n = 8, P < 0.01), but not at 8 h of hypoxia (95.3 +/- 5.0 percent). These results indicate that AGs reduce the increasing permeability of the BEC monolayer following short duration of hypoxia/reoxygenation. It is suggested that AGs may have a protective effect to the BBB disruption following ischemia/reperfusion.

Effect of inflammatory agents on electrical resistance across the blood-brain barrier in pial microvessels of anaesthetized rats

The effect of histamine, bradykinin and serotonin on blood-brain barrier permeability was investigated using in situ measurement of transendothelial electrical resistance in pial microvessels of anaesthetized rats. Mean resistance of vessels superfused with artificial cerebrospinal fluid was 1800 omega cm2, indicating a tight barrier with extremely low ion permeability. In paired experiments from continuous measurements in single vessels, addition of 10(-3) M serotonin to the solution bathing the brain had no marked effect on resistance; whereas both histamine and bradykinin, applied at a concentration of 10(-4) M, caused a rapid and reversible decrease in resistance. Mean resistance was 408 and 505 omega cm2 in 10(-4) M histamine and bradykinin, respectively, and approximately 50% of vessels had a resistance less than 250 omega cm2, compared to 12% in controls, indicating a leaky blood-brain barrier that is not capable of normal brain ion homeostasis. Histamine and bradykinin had similar dose-response relations, and a maximal effect was observed between 20 and 50 microM. Thus, histamine and bradykinin act at the abluminal (brain-facing) membranes of the cerebral endothelium to mediate blood-brain barrier opening. These results support a role for histamine and bradykinin in brain oedema formation.

Effects of second messengers on the permeability and morphology of eel rete capillaries

The effects of second-messenger concentration changes on capillary diffusion capacity (permeability-surface area product [PS]) to cellular and paracellular tracers and on capillary ultrastructure were studied during countercurrent perfusion of the rete of the eel swim bladder. Cyclic nucleotide effects were investigated with isoproterenol, forskolin, and dibutyryl cAMP. Isoproterenol (5 x 10(-6) mol/L) did not modify water and solute permeability or capillary structure. Forskolin (10(-4) mol/L) immediately raised the concentrations of cAMP in the rete and produced interstitial edema but did not change permeability. The addition of dibutyryl cAMP (10(-6) mol/L) to the perfusate had rapid effects: it reduced the PS of [3H]water and oxygen and increased the PS of [125I]albumin, [14C]sucrose, and 22Na. No structural changes were observed. Phosphoinositide effects were studied with 1,2-dioctanoyl-sn-glycerol (DG) and phorbol 12-myristate 13-acetate (PMA). DG (10(-5) mol/L) had no effect on the permeability of the rete to water and solutes, while inducing cell membrane vacuolization. PMA (10(-5) mol/L) progressively reduced the PS of [3H]water. In contrast, PS values of [125I]albumin, [14C]sucrose, and 22Na rose gradually. Membrane vacuoles bulging into the lumen and in the cytoplasm were a common feature. The Ca2+ effect was investigated with the Ca2+ ionophore A23187. At 5 x 10(-6) mol/L, unsteady permeability changes and extensive cytolysis were observed. At 5 x 10(-7) mol/L, the PS of [125I]albumin, [14C]sucrose, and 22Na rapidly increased. The PS values for water were not modified. No structural changes were identified.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of endothelin receptor antagonists on bradykinin-induced increases in macromolecular efflux

The goal of this study was to determine the effects of endothelin receptor antagonists on agonist-induced increases in macromolecular extravasation in the hamster cheek pouch in vivo. We used intravital fluorescent microscopy and fluorescein isothiocyanate dextran (FITC-dextran; mol wt = 70 K) to examine extravasation from postcapillary venules in response to bradykinin and endothelin before and following application of inhibitors of endothelin receptors (ETAB and ETA). Increases in extravasation of macromolecules were quantitated by counting the number of venular leaky sites. Bradykinin (0.5 and 1.0 microM) and endothelin-1 (0.01 and 0.1 nM) produced a dose-related increase in the number of venular leaky sites and superfusion of PD 142893 (ETAB antagonist), and PD 147953 and BQ-123 (ETA antagonists) significantly decreased bradykinin- and endothelin-induced responses. Addition of calcium to the superfusate restored bradykinin-induced increases in venular leaky sites in the presence of endothelin receptor antagonism. Thus, the findings of the present study suggest that endothelin receptor antagonists abrogate bradykinin- and endothelin-induced increases in macromolecular efflux from postcapillary venules. The mechanism for the effects of endothelin receptor antagonists appears to be related to inhibition of the ETA receptor which, in turn, alters the mobilization of calcium across venular endothelium.

Regulation of blood-brain barrier endothelial cells by nitric oxide

Nitric oxide (NO) synthesized by vascular endothelial cells is a potent vasodilator substance. The actions of NO extend well beyond its vasodilatory properties, and increasingly, NO has been recognized as an important signal for intercellular and intracellular communication. Recently, NO has been implicated in the regulation of vascular and blood-brain barrier permeability. NO has also been shown to modulate ion channels in excitable cells, thus affecting neuronal firing. We report the results of patch-clamp experiments that show a modulatory action of NO as well as cGMP and cAMP on a hyperpolarization-activated current (Iha) carried by both Na+ and K+ ions in blood-brain barrier endothelial cells. Iha was recorded in cells dialyzed with 0.2 mmol/L GTP-gamma-S to inhibit a large inwardly rectifying potassium current. This ionic current and its modulation by NO may play a role in the regulation of the transport of ions, nutrients, and other molecules to the brain and serve as an integral part of the blood-brain barrier. The modulation of Iha by a cyclic guanosine nucleotide may also explain previous reports suggesting a role for NO in the regulation of blood-brain barrier function.

Permeability of disrupted cerebral microvessels in the frog

1. This study reports the results of varying the hydrostatic pressure on measurements of permeability coefficient to the low molecular weight impermeant dye carboxyfluorescein (MW = 376) in single leaky cerebral microvessels. A mathematical model, that solved the convective diffusion equations used to analyse the measurements, showed that the measurements were consistent with leakiness being due to 22 nm wide parallel-sided slits between endothelial cells. 2. Microvessels on the surface of the frog's brain were cannulated with a micropipette and perfused with an artificial cerebrospinal fluid containing the dye. Vessels were occluded with a glass microneedle and the rate of change in dye concentration in a 12 microns length section was measured using video-intensified microscopy. 3. It was found that the rate of dye loss at all points along the occluded microvessel segment could be accounted for by a model for convection and diffusion, and that changes in dye concentration at a point remote from the segment entrance can give a good measure of diffusive permeability. 4. When series of measurements were carried out on a single vessel, permeability rose over the course of 20 min. Mean permeability for all measurements was 3.01 x 10(-5) cm sec-1, n = 64 (mode, 2.0; range, 0.48-9.6). The hydrostatic pressure applied during the perfusion had no effect on the measured permeability. 5. The dye concentration along the vessel axis was measured at the steady state and was shown to respond to changes in hydrostatic perfusion pressure in a way predicted by the model. This indicates that hydrostatically driven bulk flow can be important, and thus convection may account for effects previously ascribed to vesicular transcytosis. 6. The possible anatomical basis for the porous pathway is discussed in the light of recent observations on the presence of 0.5 microns perijunctional gaps, the possibility of transendothelial channels, and the unzipping of tight junctions to leave a 22 nm wide slit.

Effect of histamine and antagonists on electrical resistance across the blood-brain barrier in rat brain-surface microvessels

The effect of histamine on blood-brain barrier permeability was investigated using in situ measurement of transendothelial electrical resistance in brain-surface microvessels of anaesthetized rats. Mean resistance of vessels superfused with artificial cerebrospinal fluid was 1500 omega.cm2, indicating a tight barrier with low ion permeability. The addition of 10(-4) M histamine resulted in a 75% decrease in resistance, in both arterial and venous vessels, indicating a marked increase in barrier permeability. To determine the nature of the response to histamine, rats were given presurgical intraperitoneal injections of promethazine (H1 receptor antagonist), cimetidine (H2 receptor antagonist) or indomethacin (cyclo-oxygenase inhibitor), singularly and in combinations. Cimetidine completely blocked the histamine-mediated increase in barrier permeability whereas promethazine only had a small effect and indomethacin was ineffective. In addition, cimetidine treatment resulted in a 100% increase in basal resistance in both arterial and venous vessels, suggesting endogenous histamine was acting to increase blood-brain barrier permeability. It is concluded that histamine causes an increase in blood-brain barrier permeability which is mediated via endothelial H2 receptors, and that the electrical resistance in cimetidine-treated rats most closely represents the true permeability of the blood-brain barrier.

Mechanisms of blood-brain barrier breakdown

The functional status of the blood-brain barrier (BBB) must be taken into account when designing and interpreting brain imaging techniques. The integrity of the BBB is affected in many diseases of the brain, with the potential involvement of a number of different but poorly understood cellular mechanisms. Factors known to disrupt the BBB experimentally include arachidonic acid and the eicosanoids, bradykinin, histamine and free radicals. These active compounds, released in pathological tissue, may alter cytosolic calcium levels and induce second messenger systems leading to an alteration in BBB permeability. Extravasation of plasma proteins may occur via disrupted tight junctions, stimulation of fluid-phase vesicular transport or the formation of transcellular pores or channels.

Role of actin and myosin in the control of paracellular permeability in pig, rat and human vascular endothelium

1. We have investigated the endothelial actomyosin system with particular emphasis on its possible role in actively opening a paracellular route for permeability. 2. Actin and myosin comprised 16% of total endothelial protein with a molar actin/myosin ratio of 16.2 which is close to the actin/myosin ratio of muscle (studies on freshly isolated pig pulmonary arterial endothelial cells, PAEC). 3. By immunocytochemistry at the light and electron microscope levels the bulk of actin and myosin was colocalized in close vicinity to the intercellular clefts of both micro- and macrovascular endothelial cells in situ and in vitro. 4. Calcium-ionophore-induced rise in permeability of human umbilical venous endothelial cells (HUVEC) and PAEC monolayers grown on filters in a two-chamber permeability system was caused by opening of intercellular gaps. Gap formation depended on the rise in intracellular Ca2+ and could be blocked by the calmodulin-binding drugs trifluperazine (TFP) and W7. 5. In skinned monolayers of cultured PAEC and in isolated sheets of HUVEC gap formation was shown to require ATP and occurred only when free myosin binding sites were available on endothelial actin filaments (experiments with myosin subfragment 1 modified by N-ethylmaleimide, S1-NEM). 6. These experiments suggest that actin and myosin in endothelial cells play a central role in regulating the width of the intercellular clefts, thereby controlling the paracellular pathway of vascular permeability.

Magnetic resonance imaging increases the blood-brain barrier permeability to 153-gadolinium diethylenetriaminepentaacetic acid in rats

In a qualitative electron microscopy study we initially reported that exposure of rats to a standard clinical magnetic resonance imaging (MRI) procedure temporarily increased the blood-brain barrier (BBB) permeability to horseradish peroxidase. In this study, we quantitatively support our initial finding. Rats were injected intracardially with radio-labelled diethylenetriaminepentaacetic acid [( 153Gd]DTPA) in the middle of two sequential 23.2 min MRI exposures. Exposed rats (n = 21) showed significantly greater (29%, P = 0.006) retention of [153 Gd]DTPA than sham-exposed rats (n = 22) 1 h after the end of the last 23.2 min exposure. These findings suggest that magnetic fields may alter BBB permeability.

Roles of calcium, cyclic nucleotides, and protein kinase C in regulation of endothelial permeability

We studied the effects of calcium, cyclic nucleotides, and protein kinase C on albumin transfer, electrical resistance, and cytoskeletal actin filaments in cultured human umbilical vein endothelial cells. The endothelial monolayer grown on collagen-treated filters markedly restricted the transfer of albumin relative to its transfer across the filter alone. Both Ca++ ionophore A23187 and ethyleneglycol tetraacetic acid disrupted the integrity of the endothelial monolayer, thereby increasing endothelial albumin transfer and decreasing electrical resistance in a concentration-dependent manner. Neither W-7, a calmodulin antagonist, nor TMB-8, an intracellular Ca++ antagonist, influenced endothelial permeability. In contrast, increases in intracellular cyclic adenosine 5'-monophosphate (AMP) and/or cyclic guanosine 5'-monophosphate (GMP) induced by dibutyryl cyclic AMP, forskolin, 3-isobutyl-1-methylxanthine, 8-bromo cyclic GMP, dibutyryl cyclic GMP, or sodium nitroprusside significantly elevated endothelial electrical resistance and inhibited albumin transfer; similar effects resulted from activation of protein kinase C by phorbol-12-myristate-13-acetate or 1-oleoyl-2-acetyl-glycerol. These substances ruffled the dense peripheral bands of F-actin without compromising the integrity of endothelial monolayer. These results suggest that calcium, cyclic nucleotides, and protein kinase C play important roles in the regulation of endothelial permeability and the maintenance of endothelial integrity.