Methodological approaches for the study of the aortic endothelium of the rat

The source of heme for vascular heme oxygenase I: heme uptake in rat aorta

During the last decade, heme oxygenase (HO) and carbon monoxide (CO) have garnered substantial research interest in terms of cell and organ regulation, especially as they bear on the central nervous system, organ transplantation, and the cardiovascular system. While the enzymatic mechanism, substrates, and products of HO are well known, it is not clear whether the cardiovascular system derives its supply of the heme substrate through de novo synthesis or uptake from the extracellular milieu. The objective of the present study was to test the latter possibility in rat aorta and to determine the influence of plasma proteins that bind heme in vivo, viz. hemopexin and albumin. Aortic tissue was exposed to [14C]heme in vitro, and the concentration and time dependence of heme uptake was assessed. The presence of hemopexin or albumin in the incubation medium dramatically decreased heme uptake by the aorta. Heme uptake by aortic tissue was not altered after induction of HO-1, which would be expected to increase tissue heme demand. In summary, the rat, isolated aorta was capable of obtaining heme from its external milieu, but this was obtunded in the presence of the plasma proteins hemopexin or albumin. For normal physiological situations, heme uptake may not be a usual source of substrate for vascular HO and hemoenzymes such as nitric oxide synthase, soluble guanylyl cyclase, and cyclooxygenase.Key words: heme uptake, vasculature, heme oxygenase, hemopexin.

A new En face method is useful to quantitate endothelial damage in vivo

Endothelial damage is considered to be an initial change in the atherosclerotic process. However, it has been difficult to detect this initial change in vivo. We established a modified En face immunostaining method that enabled us to obtain clear images of the entire endothelial surface, including at arterial bifurcations, and to quantitate the number of cells of interest in the endothelium. Using this method, we found that treatment with an atherogenic factor, albumin-derived advanced glycosylation end products, for only 2 weeks caused a significant increase in the number of proliferating cell nuclear antigen-positive endothelial cells and the number of macrophages adhering to the endothelium, suggesting that these changes might be relevant to the early events of endothelial dysfunction. In conclusion, the present modified En face immunostaining method may be a promising tool for understanding the pathophysiology of atherosclerosis.

Neurophysiological effects of recurrent laryngeal and thoracic vagus nerves on mediating the neurogenic inflammation of the trachea, bronchi, and esophagus of rats

The present study aims to investigate the neurophysiological effects of recurrent laryngeal nerve and thoracic vagus nerve on the non-cholinergic regulation of neurogenic plasma extravasation of the rat trachea, bronchi, and esophagus. Through thoracotomy, three nerve components, the right thoracic vagal trunk, thoracic vagus nerve, and recurrent laryngeal nerve, were identified. The experiment was sequentially conducted in four steps. First, the individual nerve component was electrically stimulated and the induced inflammatory responses, as quantified by the area density of India ink-labelled blood vessels in the trachea, bronchial trees and esophagus, were compared. Second, we assessed the relative importance of medial and lateral side of the right thoracic vagus nerve in inducing the inflammatory responses by alternative stimulation of one side with simultaneous severance of the other side of this nerve. Third, we examined the effects of transection of the lateral half of the right thoracic vagus nerve on the degeneration of axon fibers located at the following three sites: the nerve segment proximal to cutting site, bronchial and esophageal nerve branches. Finally, we directly observed the inflammatory histopathology of the right lower trachea after stimulation of the medial half of the right thoracic vagus nerve with transection of its lateral half. In this study, we found that the right recurrent laryngeal nerve was predominant in mediating the neurogenic inflammatory responses of upper and dorsal portions of trachea, whereas the right thoracic vagus nerve was predominant in mediating those of the right lower ventral wall of trachea, right main bronchus, and right lobar bronchial trees. The axon fibers of the right thoracic vagus nerve responsible for mediating the neurogenic inflammatory responses of the right lower ventral trachea were mainly accumulated in the medial half, whereas those innervating the right main bronchus, right lobar bronchial trees, and lower esophagus were largely in the lateral half of this nerve. Transection of the lateral half of the right thoracic vagus nerve resulted in significant degeneration of myelinated fibers in its bronchial and esophageal nerve branches. Histopathological examination of the right lower trachea after electrical stimulation of the medial half of thoracic vagus nerve demonstrated the silver-stained leaky venules with accumulations of inflammatory cells. We thus concluded that afferent C-fibers to upper and dorsal portions of trachea were mainly from recurrent laryngeal nerve. In contrast, the neurogenic inflammatory responses of the right lower trachea were predominantly mediated by the medial half of the right thoracic vagus nerve, and those of the right main bronchus, bronchial trees and lower esophagus were largely by the lateral half of this nerve.

Effect of inflammatory stimuli on the silver staining pattern of the rat carotid endothelium

Silver nitrate stains the intercellular junctions of the endothelium and other cytoplasmic or membrane components. Two protocols are described for the silver staining of rat carotid endothelium that exclude the use of pressurized fixatives and simplify the technique previously described for rat aorta. The entire surface of the carotid endothelium was examined and several parameters (stigmata, granularity, clustering of anionic sites, transversal lines, weakening of silver lines and leukocyte adhesion) were evaluated. We studied the pattern of silver staining in two situations: (1) endothelial activation and (2) neurogenic inflammation. Endothelial activation was produced by the intravenous administration of a proinflammatory albumin or polyinosinic acid. Both products cause a marked increase in leukocyte adhesion concomitant with a decrease in argyrophilia and a weakness or loss of silver lines. Neurogenic inflammation, which is mediated by substances released from sensory nerves, was induced by the intravenous administration of substance P or capsaicin. Both stimuli produced an increase in argyrophilia and weakness or loss of silver lines. Substance P caused a clustering of anionic sites, whereas this phenomenon was more discrete with capsaicin. Nearly 80% of all examined rats (controls and inflammatory stimuli treated) showed endothelial membrane disruptions formed by clusters of cells often in the shape of streaks aligned with the long axis of the vessel. The detection of these discontinuities is important, as loss of endothelial integrity is central in the initiation of pathological events.

Induction of kinin B1 receptor-dependent vasoconstriction following balloon catheter injury to the rabbit carotid artery

1. Balloon catheter injury to the rabbit carotid artery damaged the endothelium and induced neointima formation over 7 days. The area of intima, expressed as a percentage of the media, was 16.2 +/- 4.2% and 8.2 +/- 0.1% in balloon catheter-injured and sham-operated arteries. 2. Seven days after arterial injury, carotid arteries were isolated and set up as ring preparations in organ baths for isometric tension measurements. Balloon catheter-injured arteries first contracted with noradrenaline (0.01-0.1 microM), contracted further in a concentration-dependent manner to bradykinin (BK; pD2, 5.98 +/- 0.22; Emax, 41.3 +/- 5.2% of KCl) and to des-Arg9-BK (pD2, 7.12 +/- 0.36; Emax, 46.0 +/- 9.9% of KCl). In contrast, vessel segments with endothelium either intact or acutely removed were unresponsive to both BK receptor agonists. 3. The concentration-contraction curves for BK and for des-Arg9-BK were shifted to the right by the B1 receptor antagonist, [Leu8]des-Arg9-BK (3 microM), but not by the selective B2 receptor antagonist, Hoe 140 (1 microM). 4. Thus, BK and its metabolite, des-Arg9-BK act as vasoconstrictor agents following balloon catheter injury. These effects appear to be mediated by activation of B1 receptors.

Phagocytosis of intravenously administered particles by leukocytes adhered to the aortic endothelium of the rat

Phagocytosis has been used to characterize on a functional basis leukocytes adhered to the aortic endothelium of the rat. After intravenous administration of particles, phagocytosis was observed microscopically in esterase-positive leukocytes adhered to the endothelium in whole mounts of aorta. Polybead blue and red, 0.5 and 1 microns particle size, were inadequate because they were insufficiently colored to be identified individually at 400 x. Fluoresbrite YG 0.25 and 0.50 micron at doses of 0.2 and 2 x 0.3 ml/100 g, respectively, produced endothelial lesions. The same occurred with Monastral blue B (MbB) at 0.3 ml/100 g, red iron at 2 x 16 mg/100 g and India ink at different concentrations depending on the supplier. At lower particle doses, lesions were not found. Deferoxamine mesylate 1.5 mg/100 g intravenous and allopurinol 5 mg/100 g intraperitoneal administered before the particles diminished the number and intensity of lesions. In none of the cases studied was the percentage of phagocytic cells greater than 50%. Clearance curves of MbB and Fluoresbrite indicated rapid disappearance of particles from the blood. Results indicate that administration of particulate suspensions is not a good method for characterizing the phagocytic leukocytes adhering to the aortic endothelium because low doses produce rapid clearance of particles, thus impeding sufficient leukocyte loading, and higher doses produce endothelial lesions that often impair reliable counting of the adhering leukocytes.

Effect of spontaneous pathology and thrombin on leukocyte adhesion to rat aortic endothelium

Leukocyte adhesion and other injury parameters have been studied in the aortic endothelium of Sprague-Dawley rats in two situations: (1) spontaneous pathology in conventional rats with antibodies to Mycoplasma pulmonis and/or Kilham or Sendai viruses, and (2) intravascular coagulation by thrombin administration in SPF rats. Adhesion (esterase (+) leukocytes/mm2) in SPF rats was 8 +/- 5 (n = 12). Adhesion in 38% of the conventional rats was 54 +/- 27 (n = 8), half of them being non-analyzed and the rest having antibodies to M. pulmonis and/or Kilham rat virus. In 19 rats with antibodies to M. pulmonis and/or Kilham or Sendai viruses, AgNO3 and hematoxylin staining of the aortic endothelium showed an increase in leukocyte adhesion, and the presence of argyrophilic cells, stigmata and granularity--severe endothelial lesions being observed in some cases. Adhesion in rats after 0.25, 1, 3 and 6 h of thrombin administration (30 units/100 g) was not different from controls. Adhesion after 24 h was 108 +/- 53 (n = 10) and 60 +/- 59 (n = 10), and 22 +/- 20 (n = 10) in rats treated with thrombin plus heparin or hirudin, respectively. Thrombin produced endothelial lesions at all times studied, and these included membrane blebs, platelet and erythrocyte adhesion and alterations in the pattern of endothelial esterase activity.

Technical considerations in evaluating the endothelial integrity of rat aortic preparations with silver staining

Assessment of endothelial integrity is an obligatory step in many pharmacological studies. Integrity of endothelium is affected by manipulations performed during the removal and cleaning of the vessel and by some of the silver-staining techniques utilized for demonstrating interendothelial junctions. When aortas were cleaned of periadventitial tissue in cold Tris-saline (once separated from the animal) by untrained personnel, only 45% of the endothelium was preserved. When cleaning was performed in situ by trained personnel while flushing with cold Krebs-Ringer-6% albumin, over 95% was left intact. AgNO3-staining performed before fixation produced a 50% loss of endothelium when using NH4Br and (NH4)2S as developers. AgNO3-staining performed after fixation produced over 95% recuperation of endothelium when 2% glutaraldehyde, 150 mM NaCl, 40 mM phosphate buffer, pH 7.4, were utilized as initial fixative, NH4Br and (NH4)2S being equally effective as developers. Chloride ions were necessary to intensify silver lines. Several patterns of deendothelization were produced by mechanical and chemical injury with saponin, NH4Br and (NH4)2S. In all cases, hematoxylin staining was employed as an auxiliary technique to interpret images of injured endothelium. Presence of albumin protected the endothelium from mechanical damage.

Effect of transmural pressure on low density lipoprotein and albumin transport and distribution across the intact arterial wall

To investigate the effect of hyperpressure on the transport of low density lipoprotein (LDL) and albumin in the arterial wall, we measured in vitro the uptake of both iodine-131-labeled LDL and iodine-125-labeled albumin in intact rabbit thoracic aorta, held at in vivo length and pressurized to 70 or 160 mm Hg. Arteries were incubated for 2 hours (n = 8) at 70 mm Hg, and for 5 minutes (n = 4), 30 minutes (n = 4), 1 hour (n = 5), and 2 hours (n = 5) at 160 mm Hg. The transmural distribution of the relative concentrations of LDL (CLDL) and albumin (Calb) across the wall was determined by using a serial frozen sectioning technique. At 70 mm Hg, the mean medial CLDL and Calb values were 0.0018 +/- 0.0007 and 0.0039 +/- 0.0013, respectively. At 160 mm Hg, CLDL and Calb were markedly increased. The distribution of labeled albumin was almost uniform across the media and reached a steady state after 30 minutes, whereas labeled LDL accumulated in the first inner layers, a steady state being achieved after 1 hour. The 1-hour values of CLDL in the first and second luminal sections (0.24 +/- 0.03 and 0.13 +/- 0.05, respectively) were much higher than those of Calb, the CLDL/Calb ratios being 4.12 +/- 0.94 and 2.34 +/- 0.42 (p less than 0.01), respectively. In the subsequent sections, the CLDL decreased markedly and became much lower than the Calb, the CLDL/Calb ratio averaging 0.2 in the two-thirds outer media. To investigate whether LDL was trapped at high pressure in the inner layers, vessels were exposed to a tracer-free intraluminal solution for 30 minutes, after a 30-minute incubation with tracers. After washout, albumin was almost totally removed from the wall, while the CLDL were practically unchanged. Compaction of the media induced by high distending stresses applied to the vessel might have hindered the efflux of LDL, whereas albumin moved freely through the wall. Synergy between increased endothelial permeability and compaction of the media together with enhanced pressure-driven convection might account for the marked increase in LDL concentration observed in the inner wall at high pressure.

Technical considerations in evaluating the adhesion of leukocytes to aortic endothelium of the rat

Comments on techniques for characterizing leukocytes adhered to the aortic endothelium of the rat are given. Alpha-naphthyl acetate esterase positive leukocytes were studied by optical microscopy of en face intima-media preparations. Results indicate 1) 1% paraformaldehyde-2% glutaraldehyde is a better fixative than formalin-calcium or 4% paraformaldehyde with or without 1.5 mM CaCl2; the latter produces distortion of leukocytes, endothelial desquamation and enzymate inhibition, 2) washing the aorta with phosphate-buffered saline for 90 sec prior to fixation-perfusion produces a notable decrease in the number of leukocytes adhered, 3) diazotized parasaniline is better than fast blue RR salt as coupling agent in the esterase reaction, and 4) counterstaining with 1% methyl green for 1 min, before or after the esterase reaction, is not adequate because of limited contrast and the heavy staining of smooth muscle. Counterstaining with Gill's hematoxylin No. 3 for 90 sec is adequate only when done before the esterase reaction. Inhibition of endothelial esterase activity by hematoxylin decreases background, favors contrast of adhered leukocytes and makes it possible to observe nucleus-cytoplasm relations.

Morphological and functional techniques for assessing endothelial integrity: the use of Evans blue dye, silver stains, and endothelial derived relaxing factor

The endothelium is an important aspect of vascular function and pathology. Simple and reliable methods for assessing the presence or absence of endothelium in vascular specimens are presented. Intravenous Evans blue dye that stains endothelial denuded vessel walls in recommended for the macroscopic visualization of endothelial injury in gross intact arterial specimens. Scanning electron microscopy (SEM) is often used for investigating microscopic endothelial injury, but the differentiation of endothelial cells from underlying vessel wall structures can be difficult. By demarcating endothelial cell borders, silver stains allow for better endothelial identification by SEM. A third technique describes assessing endothelial integrity by how well it functions and involves the selective production of endothelium derived relaxing factor. As cardiovascular surgeons become more involved in basic vascular research, a review of these techniques should be appropriate and helpful.

Adhesion of leukocytes to the aortic endothelium of conventional, specific pathogen free (SPF) and hypercholesterolemic SPF rats

Adhesion of leukocytes to the aortic endothelium was studied in specific pathogen-free (SPF) and conventional rats and in SPF rats with diet-induced hypercholesterolemia. Nonspecific esterase activity with alpha-naphthyl acetate as substrate was used to characterize the adhered cells. Phagocytic activity was determined by injecting i.v. 0.1-0.4 ml/100 g doses of Monastral blue B (MbB). Adhesion in SPF rats was 8 +/- 4 esterase (+) cells/mm2. Adhesion in conventional rats was of the same order except in 2 cases with antibodies to Mycoplasma pulmonis and Kilham rat virus, where adhesion was 44 and 68 esterase (+) cells/mm2, respectively. For all MbB doses studied, phagocytic activity arose in a percentage of the adherent cells, ranging from 5 to 85%. Rats fed the hyperlipidic diet for 15 days developed severe hypercholesterolemia and adhesion was drastically increased to 200-700 esterase (+) cells/mm2. Results indicate that: (1) spontaneous pathology in rats may produce an increased adhesion of leukocytes to the endothelium, and (2) phagocytic activity is only expressed in a fraction of the esterase (+) cells adhered to the endothelium.