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At least 433 records · Page 24Linked to original sources
PubMed2006

Pulsed high-intensity focused ultrasound enhances thrombolysis in an in vitro model.

PURPOSE: To evaluate the use of pulsed high-intensity focused ultrasound exposures to improve tissue plasminogen activator (tPA)-mediated thrombolysis in an in vitro model. MATERIALS AND METHODS: All experimental work was compliant with institutional guidelines and HIPAA. Clots were formed by placing 1 mL of human blood in closed-off sections of pediatric Penrose tubes. Four experimental groups were evaluated: control (nontreated) clots, clots treated with pulsed high-intensity focused ultrasound only, clots treated with tPA only, and clots treated with pulsed high-intensity focused ultrasound plus tPA. The focused ultrasound exposures (real or sham) were followed by incubations of the clots in tPA with saline or in saline only. Thrombolysis was measured as the relative reduction in the mass of the clot. D-Dimer assays also were performed. Two additional experiments were performed and yielded dose-response curves for two exposure parameters: number of pulses per raster point and total acoustic power. Radiation force-induced displacements caused by focused ultrasound exposures were simulated in the clots. A Tukey-Kramer honestly significant difference test was performed for comparisons between all pairs of experimental groups. RESULTS: The clots treated with focused ultrasound alone did not show significant increases in thrombolysis compared with the control clots. The clots treated with focused ultrasound plus tPA showed a 50% ([30.2/20.1]/20.1) increase in the degree of thrombolysis compared with the clots treated with tPA only (P < .001), further corroborating the d-dimer assay results (P < .001). Additional experiments revealed how increasing both the number of pulses per raster point and the total acoustic power yielded corresponding increases in the thrombolysis rate. In the latter experiment, simulations performed at a range of power settings revealed a direct correlation between increased displacement and observed thrombolysis rate. CONCLUSION: The rate of tPA-mediated thrombolysis can be enhanced by using pulsed high-intensity focused ultrasound exposure in vitro.

Humans↗
PubMed1994

In vitro model to test the thrombogenicity of coronary stents.

Thrombotic occlusion is a major complication limiting the application of stents in coronary arteries. In an in vitro model we investigated the thrombogenicity of different stent materials and several medical regimens to prevent thrombotic occlusion. Experiments were conducted in a closed system of silicon tubing with circulating citrated platelet rich plasma of healthy volunteers (n = 7) and of patients (n = 7 for each condition). Patients were either treated with phenprocoumon or with high or low dose heparin in combination with aspirin alone (100 mg) or aspirin (990 mg) plus dipyridamole (225 mg). After placement of tantalum wire stents into the system platelet aggregates were visible after 13.5 +/- 3.0 min, and occlusion occurred after 15.0 +/- 3.5 min. Similarly, with implanted stainless steel stents aggregation was seen after 13.0 +/- 3.5 min and thrombosis occurred after 14.5 +/- 3.5 min (p < 0.001 vs control without stent). Microscopic examination revealed combined platelet fibrin thrombi occluding the lumen. Platelet components predominately covered stent wires, particularly at crossing points. In all experiments high-dose heparin prevented platelet aggregate formation and stent occlusion independently of additional aspirin or aspirin plus dipyridamole; perfusion time > 60 min (p < 0.001 vs no heparin). Low-dose heparin could not prevent clotting. With aspirin alone aggregates were visible after 16.0 +/- 4.0 min and clotting occurred after 23.0 +/- 5.0 min. In combination with dipyridamole aggregates were visible after 15.5 +/- 5.0 min and clotting after 21.0 +/- 4.0 min (NS vs aspirin alone). Phenprocoumon prevented platelet aggregate formation and stent occlusion; perfusion time > 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2000

Heat transfer to the periodontal ligament during root obturation procedures using an in vitro model.

It appears to be important to avoid thermal injury to the periodontal ligament when using heated guttapercha techniques such as "System B." An in vitro model was developed, consisting of an extracted human tooth rooted in an artificial periodontal ligament (PDL) and alveolar socket, which allowed us to measure the temperature transferred to the root surface. The teeth were instrumented and subsequently embedded in alginate to simulate the PDL. Medium gutta-percha points were fit, sealer was applied, and a fine Buchanan plugger was used for condensation. Temperature measurements were taken simultaneously at the apex and 5 mm from the apex during obturation with two fine gauge thermocouples connected to a digital thermometer. The average temperature increase was approximately 1 degree C at the apex and approximately 2 degrees C at the 5 mm mark. The resulting temperature increases appear to be lower than previously reported by other investigators (Hardie, 1986, 1987; Barkhordar et al., 1990; Weller et al., 1991; Lee et al., 1998), who did not allow for the heat disseminating effect of the PDL.

Alginates↗
PubMed1998

An in vitro model fails to demonstrate aerosolization of tumor cells.

BACKGROUND: We investigated the ability of pressurized CO2 gas to aerosolize B16 melanoma (B16) tumor cells in an in vitro model. METHODS: The experimental apparatus consisted of an 18.9-L plastic cylindrical vessel and a compliant latex pouch was attached to the top. Two 5-mm ports penetrated the vessel; insufflation and desufflation were carried out through them. A culture dish containing 20 million B16 cells in liquid culture media was placed at the base within the container. In the first experiment, the vessel was insufflated with CO2 gas to a static pressure of 15 or 30 mm Hg with the outflow port closed. After 10 min, the outflow port was opened and the gas was desufflated through a collecting device containing sterile culture medium. In a second experiment, a continuous flow of CO2 through the vessel was maintained after a pressure of 15 or 30 mm Hg was established. A total of 10 L CO2 was cycled through the vessel. In both experiments, 24 determinations were carried out at each pressure. Each experimental culture dish was microscopically scanned for 2 weeks for the presence of tumor cells. The third and fourth experiments tested for the presence of aerosolized nonviable tumor cells in the expelled gas. Using the model described above, after 10 mins of 30 mm Hg static pressure, the CO2 gas was expelled directly onto a glass slide and cytofixed. Alternately, after 10 mins at 30 mm Hg static pressure, the gas was expelled through a saline-filled Soluset (Abbott Laboratories), centrifuged, and the residue cytofixed onto a glass slide. Each of the five slides per experiment were examined microscopically for the presence of cells. RESULTS: In the first and second experiments, no cells or growth were observed in any of the 96 experimental dishes. In experiments three and four, no cells were detected on any of the slides. CONCLUSIONS: It was not possible with this model to aerosolize tumor cells in a pressurized CO2 environment. Our results suggest that aerosolization of tumor cells is not the mechanism of port site recurrences after laparoscopic surgery for malignant disease.

Aerosols↗
PubMed2002

Anchorage-independent multi-cellular spheroids as an in vitro model of growth signaling in Ewing tumors.

Little is known about the growth-signaling pathways that govern the proliferation of Ewing tumor (ET) cells either in vitro or in vivo. We have studied signal transduction pathways in ET cell lines and compared kinase expression levels and proliferation rates with primary tumors. Cell lines were studied both as conventional adherent monolayers and as anchorage-independent multi-cellular spheroids. Importantly, we observed significant differences between these in vitro models and found that ET spheroids were more closely related to primary tumors with respect to cell morphology, cell-cell junctions, proliferative index and kinase activation. Monolayer ET cells demonstrated serum-dependent phosphorylation of ERK1/2 and AKT and constitutively high serum-independent cyclin D1 protein expression. However, when ET cells were placed in suspension culture, there was immediate serum-independent activation of ERK1/2 and AKT. In addition, cyclin D1 protein expression was completely blocked until stable multi-cellular spheroids had formed, indicating that cell-cell adhesion is necessary for the proliferation of anchorage independent ET cells. This reduction in cyclin D1 expression was post-transcriptional and could be mimicked in monolayer cells by treatment with phosphatidyl inositol-3 kinase (PI3K) inhibitors. Moreover, PI3K inhibition significantly reduced ET cell proliferation and, in primary ET samples, cyclin D1 expression correlated with expression of activated AKT. Thus, the PI3K-AKT pathway appears to be critical for the proliferation of ET cells both in vitro and in vivo and tumor cell growth in vivo may be better represented by the study of anchorage-independent multi-cellular spheroids.

Artificial Gene Fusion↗
PubMed1991

Fluid percussion barotrauma chamber: a new in vitro model for traumatic brain injury.

Advances in the understanding of the pathophysiology of traumatic brain injury have implicated a number of cellular events as fundamental to the evolution of neurologic dysfunction in this process. Following the primary biomechanical insult, a highly complex series of biochemical changes occur, some of which are reversible. The development of fluid percussion injury as an in vivo model for traumatic brain injury has greatly improved our ability to study this disease. However, a comparable in vitro model of biomechanical injury which would enable investigators to study the response to injury in isolated cell types has not been described. We have developed a model of transient barotrauma in cell culture to examine the effects of this form of injury on cell metabolism. This model employs the same fluid percussion device commonly used in in vivo brain injury studies. The effect of this injury was evaluated in monolayers of human glial cells. Cell viability by trypan blue exclusion and the production of leukotrienes following increasing barotrauma was investigated. This model provided a reproducible method of subjecting cells in culture to forces similar to those currently used in animal experimental head injury.

Barotrauma↗
PubMed1989

Comparative activity of cefixime and cefaclor in an in vitro model simulating human pharmacokinetics.

The dependence of the antibacterial activity of the two oral cephalosporins cefixime and cefaclor on pharmacokinetic properties was investigated in an in vitro model using strains of enterobacteria and a streptococcal strain. In the cultures the course of serum concentrations of the respective antibiotic was simulated. The more rapidly attained (1 h) high peak levels (17.5 micrograms/ml) of cefaclor (500 mg dose) in no case showed an advantage over the more slowly reached (3 h) low peak levels (2.5 micrograms/ml) of cefixime (200 mg dose). Cefixime was comparable to cefaclor with respect to its initial killing velocity, whereas it was generally superior with respect to maximum values for reduction of bacterial counts. Due to its long elimination half-life (2.5 h) cefixime prevented regrowth for at least twice as long as cefaclor, which has a short half-life (0.7 h). As a result of its antibacterial activity and pharmacokinetic properties cefixime can be administered less frequently than cefaclor.

Cefaclor↗
PubMed2002

Neuroprotective effect of GMP in hippocampal slices submitted to an in vitro model of ischemia.

1. Guanosine-5'-monophosphate (GMP) was evaluated as a neuroprotective agent against the damage observed in rat hippocampal slices submitted to an in vitro model of ischemia with or without the presence of the ionotropic glutamate receptor agonist, Kainic acid (KA). 2. Cellular injury was evaluated by MTT reduction, lactate dehydrogenase(LDH) release assay, and measurement of intracellular ATP levels. 3. In slices submitted to ischemic conditions, 1 mM GMP partially prevented the decrease in cell viability induced by glucose and oxygen deprivation and the addition of KA. 4. KA or N-methyl-D-aspartate (NMDA) receptor antagonists, gamma-D-glutamylamino-methylsulfonate (GAMS) or (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801, 20 microM) also prevented toxicity in hippocampal slices under ischemic conditions, respectively. 5. The association of GMP with GAMS or MK-801 did not induce additional protection than that observed with GMP or that classical glutamate receptor antagonists alone. 6. GMP, probably by interacting with ionotropic glutamate receptors, attenuated the damage caused by glucose and oxygen deprivation in hippocampal slices. This neuroprotective action of GMP in this model of excitotoxicity is of outstanding interest in the search for effective therapies against ischemic injury.

Adenosine Triphosphate↗
PubMed2001

A novel in vitro model system to grow films of oral bacteria for the study of human tooth root surface caries.

AIMS: To develop a simple and flexible novel in vitro model system to grow films of oral bacteria that could be used to study aspects of dental caries. METHODS AND RESULTS: Standardized suspensions of bacteria were inoculated into Ultrafree-CL (Millipore) ultrafiltration units at various densities. These were incubated for varying time intervals with a range of carbon sources. The bacterial films reproducibly achieved between 107 and 108 cfu cm-2, irrespective of the number inoculated and with no significant changes for 14 d. However, Streptococcus mutans grew through membranes with pores of diameter greater than 0.1 microm after 6 d. Culture of films in sucrose or water for 6 d led to a decreased number of colony-forming units, but returning them to broth reversed this. CONCLUSION: Reproducible films of oral bacteria can be cultured in Ultrafree-CL units. SIGNIFICANCE AND IMPACT OF THE STUDY: This study has shown that Ultrafree-CL units can be used as a simple model system to grow biofilms that could be used for dental caries research.

Biofilms↗
PubMed2001

Dental erosion: in vitro model of wine assessor's erosion.

BACKGROUND: Wine makers and assessors frequently experience severe dental erosion. The objectives of this study were to develop an in vitro model of dental erosion caused by frequent wine contact with teeth, and to use this model to assess the effectiveness of a variety of methods which might protect against this form of erosion. METHODS: An initial pilot study found that riesling style wine was more erosive than champagne style, and both more than claret. Wine tasting was simulated by subjecting exposed windows of enamel and root surfaces on 50 intact, extracted human teeth to 1400 one minute exposures to white wine (ph 3.2). A variety of dental materials were applied to the exposed windows on groups of teeth prior to erosive challenge, to assess their protective ability. RESULTS: Protective resin coatings and fluoride varnishes protected both enamel and roots against wine induced erosion. A high degree of protection was provided by apf gel, with less by naf gel. CONCLUSIONS: It was concluded that dentists may be able to help minimise erosion resulting from frequent wine-tasting in their patients by the clinical application of one or a combination of these agents at times prior to prolonged assessment periods.

Acidulated Phosphate Fluoride↗
PubMed2006

Development of an in vitro model to study clot lysis activity of thrombolytic drugs.

BACKGROUND: Thrombolytic drugs are widely used for the management of cerebral venous sinus thrombosis patients. Several in vitro models have been developed to study clot lytic activity of thrombolytic drugs, but all of these have certain limitations. There is need of an appropriate model to check the clot lytic efficacy of thrombolytic drugs. In the present study, an attempt has been made to design and develop a new model system to study clot lysis in a simplified and easy way using a thrombolytic drug, streptokinase. METHODS: Whole blood from healthy individuals (n = 20) was allowed to form clots in a pre-weighed sterile microcentrifuge tubes; serum was removed and clot was weighed. After lysis by streptokinase fluid was removed and remnants of clot were again weighed along with the tube. Percentage of Clot lysis was calculated on the basis of the weight difference of microcentrifuge tubes obtained before and after clot lysis. RESULTS: There was a significant percentage of clot lysis observed when streptokinase was used. On the other hand with water (negative control), minimal (2.5%) clot lysis was observed. There was a significant difference between clot lysis done by streptokinase and water. CONCLUSION: Our study could be a rapid and effective methodology to study clot-lytic effect of newly developed drugs as well as known drugs.

Journal Article↗
PubMedDate not supplied

An in vitro model study of BSp73 rat tumour cell invasion into endothelial monolayer.

In order to study the process of invasion in more detail we developed an in vitro model of the vessel wall. Rat tumour cells derived from an adenocarcinoma of the pancreas, BSp73 AS--of high invasive but low metastatic capacity--and BSp73 ASML--not invasive but highly metastatic--were compared for their mode of invasion into confluent monolayers of endothelial cells. Corneal as well as vascular endothelial cells were plated alternatively onto the basal lamina-like bovine lens capsule that was mounted in a combi-ring dish or reconstituted extracellular matrix (Basement Membrane Matrigel) as substrata. The endothelial monolayers were confronted with AS- and ASML-tumour cells. The interaction of the various cell types was followed by scanning and transmission electron microscopy. The invasive cell type AS was able to force the endothelial cells to retract and subsequently undermined the endothelial cell layer. In the noninvasive cell population ASML most cells remained in the typical roundish morphology and did not interact with the endothelial cell layers. Only a very minor fraction of ASML populations was able to attach to and also invade into the endothelial cell monolayer. It could be shown that AS-cells individually and as small groups penetrated the endothelial cell layer. The results of transmission and scanning electron microscopy suggest that endothelial cell retraction and underlapping of adjacent endothelial cells by tumour cells play an important role in invasion and extravasation through blood vessels. Against all expectations, the nonmetastasizing tumour cell variant (AS-cells) exhibited a dramatic invasive behaviour whereas the highly metastatic ASML-variant mostly retained its spherical shape and showed invasive activity only in exceptional cases.

Adenocarcinoma↗
PubMed1997

Distribution of Marcaine in an in vitro model of the subarachnoid space conforming to actual spinal column geometries.

To study factors influencing the distribution of local anaesthetics in the subarachnoid space, an in vitro model is constructed which takes into account the natural curvature of the spinal column and the volume occupation of spinal cord and nerve fibres to resemble the in vivo situation. Three Marcaine solutions of different baricity (1003, 1008, 1030 kg/m3) are injected with a 22 G, a 27 G Quincke point needle and a 18 G multiport catheter into three models of non-pathological spinal columns with injection flow speeds of 0.6, 0.2 or 0.1 ml/s. Methylene blue is added for visual and qualitative assessment of drug distribution. Baricity is the main actor in the spreading of the drug solution. For all other variables, no significant difference is found after ten minutes, though the initial distribution may differ according to the geometry used. A hypobaric solution yields a remarked difference between fast and slower injections. The position of the catheter should be controlled.

Anesthesia, Spinal↗
PubMed1993

[An in vitro model of tumour cell invasion]

Tumorigenicity and malignancy of transformed cells have thus far been demonstrated in animal experiments only. Therefore it seems desirable to develop an in vitro model to study aspects of malignancy such as invasive growth and metastasis in order to provide information about the in vivo behavior of transformed cells. In this study a tissue culture system has been developed which was used to investigate two pivotal characteristics of malignant cells: migration and production of proteases. Proteases are enzymes that degrade extracellular matrix and also may be capable of destroying cells directly. In order to study behavior of malignant cells in vitro, rat liver epithelial cells were transfected with the ras oncogene. Expression of several genes was induced, including a protease and a protease inhibitor. The cells were found to be morphologically and neoplastically transformed and to migrate constitutively. Furthermore the cells grew in semi-solid medium, which is another property of transformed cells. These in vitro characteristics correlated with in vivo behavior (invasive growth in athymic mice). It is hoped that this correlation will enable further studies to be carried out with tissue culture systems, so limiting the use of live animals. This study has been published in detail (Zoller, J., 1991. Biochemische und molekularbiologische Analyse der Invasion von Tumorzellen. Konstanzer Dissertationen 301. Konstanz: Hartung-Gorre Verlag).

Journal Article↗
PubMed2001

Osteoclast-like cells in an in vitro model of bone destruction by rheumatoid synovium.

OBJECTIVE: Osteoclasts may be involved in the process of rheumatoid bone destruction. To test this hypothesis, we developed an in vitro model of bone destruction by osteoclast-like cells derived from cultured rheumatoid synovial tissue without using any inducers. METHODS: Synovial tissues were obtained from rheumatoid arthritis and osteoarthritis patients and tissue pieces of about 2 mm(3) that contained synovial lining were cultured. Multinucleated cells derived from cultured synovial tissues were studied cytochemically and morphologically for osteoclast-specific markers. RESULTS: Fibroblast-like and macrophage-like cells from the tissue pieces proliferated in the coexistence of lymphocytes. After 14 days of culture, multinucleated cells with tartrate-resistant acid phosphatase activity appeared. These cells expressed vacuolar H(+)-ATPase, the vitronectin receptor and cathepsin K. Although binding of (125)I-labelled salmon calcitonin was very low, the cells contained ringed structures of F-actin and showed strong bone-resorbing activity on ivory slices. Proliferation of macrophage-like cells and formation of multinucleated cells continued during 6 months of culture in the presence of fibroblast-like cells. The bone-resorbing activity of multinucleated cells derived from rheumatoid synovial tissue was much higher than that of cells from osteoarthritis synovial tissue, and was related to the disease activity of rheumatoid arthritis. CONCLUSION: Our culture system reproduced in vitro the process of bone destruction by rheumatoid synovium, including the proliferation and fusion of precursor cells, polarization, activation and bone tissue resorption. This system may provide a tool for understanding the mechanisms of bone destruction in rheumatoid arthritis and for the development of new therapies to prevent bone destruction.

Actins↗
PubMed1999

Immunolocalization of the fodrin, E-cadherin, and beta-catenin adhesion complex in infiltrating ductal carcinoma of the breast-comparison with an in vitro model.

Fodrin, E-cadherin, and beta-catenin immunolocalization was studied in 54 cases of infiltrating ductal carcinoma of the breast and compared with an in vitro model in order to study the dynamic relationship between these components of an adhesion complex. In low-grade tumours, the staining patterns were similar for both fodrin and E-cadherin, with localization of these proteins to the cell membranes. beta-Catenin showed reduced membrane staining compared with non-neoplastic epithelium. High-grade tumours displayed strong membranous as well as cytoplasmic immunolocalization of fodrin, while E-cadherin staining was fragmented or lost from the membranes, with only occasional weak intracellular staining. beta-Catenin showed fragmented membrane staining and cytoplasmic accumulation. In addition, nuclear staining of beta-catenin was occasionally observed. In a v-src-transformed MDCK cell line, following 15min of src activation, beta-catenin began to detach from the cell membrane and localize to the cytoplasm, while fodrin and E-cadherin remained unchanged. After 30-45min of src activation, the cells lost their cuboidal shape and began to lose cell-to-cell contact. Fodrin staining remained mostly membranous while that of E-cadherin and beta-catenin was fragmented and spiky. After 60min of src activation, fodrin localized completely in the cell cytoplasm, while E-cadherin and beta-catenin were partly cytoplasmic with fragmented and spiky membranous staining. Occasionally, beta-catenin was seen in the nucleus. Both in vivo and in vitro findings clearly demonstrated a disruption of the E-cadherin/beta-catenin/fodrin/cytoskeleton linkage concomitant with the loss of cell-to-cell adhesion and change in cell shape, from epithelioid to a fibroblastoid phenotype. Membranous localization of E-cadherin showed a positive correlation with oestrogen and progesterone expression, whereas loss of membranous E-cadherin and cytoplasmic accumulation of fodrin was more often observed in high-grade carcinomas and showed a positive correlation with p53 expression.

Breast Neoplasms↗
PubMed2001

The cerebrovascular response to elevated potassium--role of nitric oxide in the in vitro model of isolated rat middle cerebral arteries.

We investigated the role of nitric oxide (NO) in the vascular response to high extraluminal K(+)-concentrations in the in vitro model of isolated rat middle cerebral arteries (MCA). Under control conditions, rat MCA dilated at 20, 30, 40 and 60 mM K(+). At 80 mM K(+), a slight vasoconstriction occurred. The unspecific NO synthase (NOS)-inhibitor L(omega)-nitro-L-arginine (L-NNA) increased the resting tone at 3 mM K(+) by 31+/-5% (P<0.01). While the vasodilatative effect of 20 mM K(+) was unaffected by L-NNA, NOS-inhibition resulted in vasoconstriction at > or = 40 mM K(+) (P<0.01). In presence of L-NNA, the basal vessel diameter was restored by either the NO-donor S-nitroso-N-acetylpenicillamine (SNAP) or the cell-permeable guanosine-3',5'-cyclic monophosphate (cGMP) analogue 8-Br-cGMP. Co-application of L-NNA with either SNAP or 8-Br-cGMP resulted in partial restitution of the vasodilatative effect of 40 mM K(+), respectively. In presence of the soluble guanylyl cyclase inhibitor 1 H-[l,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), the vascular response to 40 mM K(+) was abolished. Our findings together with findings from the literature indicate a modulator role of NO at K(+) > or = 40 mM K(+), involving a cGMP-dependent mechanism.

Animals↗
PubMedDate not supplied

Passage of low density lipoproteins through monolayers of human arterial endothelial cells. Effects of vasoactive substances in an in vitro model.

The endothelium controls the influx of lipoproteins into the arterial wall, a process that may be disturbed in arteriosclerotic blood vessels. We have used an in vitro model to investigate the characteristics of the passage of low density lipoproteins (LDL) through monolayers of human arterial endothelial cells. Umbilical artery, aorta, or carotid artery endothelial cells were cultured on polycarbonate filters and formed a tight monolayer in which the cells were connected by tight junctions. Passage of 125I-LDL through these monolayers proceeded linearly over a 24-hour period. It was threefold lower through monolayers of aorta or carotid artery cells than through monolayers of umbilical artery cells. The LDL passage process did not show saturation with LDL concentrations up to 800 micrograms/ml LDL-protein (i.e., 1.6 nmol/ml apolipoprotein B) between 2 and 4 hours after addition. However, during the first 30 to 60 minutes after addition of high concentrations of LDL, a reduction of the passage rate of both LDL and peroxidase, resulting in an apparent saturation of the passage process, was observed. The passage rate of the negatively charged acetylated LDL was twofold lower than that of native LDL. Addition of histamine to the endothelial monolayer resulted in a large, but transient, increase in permeability paralleled by a decrease in electrical resistance. The effects of histamine were mediated via an H1 receptor. Thrombin and Ca++ ionophore also induced an increase in permeability of the monolayer, while bradykinin did not. The effects of histamine and thrombin were paralleled by a rapid and marked increase in cytoplasmatic Ca++ concentration of the endothelial cells, while bradykinin induced only a small increase. Although the cyclic adenosine 5'-monophosphate-elevating agent, forskolin, markedly decreased the basal rate of LDL passage through the endothelial cell monolayers, it did not change the relative increase in permeability induced by histamine. Thus, histamine induces small, but significant, increases in the permeability of tight endothelial cell monolayers.

1-Methyl-3-isobutylxanthine↗