Transport of insulin and albumin by the microvascular endothelium of the rete mirabile

Wearing the Lab: Advances and Challenges in Skin-Interfaced Systems for Continuous Biochemical Sensing

Continuous, on-demand, and, most importantly, contextual data regarding individual biomarker concentrations exemplify the holy grail for personalized health and performance monitoring. This is well-illustrated for continuous glucose monitoring, which has drastically improved outcomes and quality of life for diabetic patients over the past 2 decades. Recent advances in wearable biosensing technologies (biorecognition elements, transduction mechanisms, materials, and integration schemes) have begun to make monitoring of other clinically relevant analytes a reality via minimally invasive skin-interfaced devices. However, several challenges concerning sensitivity, specificity, calibration, sensor longevity, and overall device lifetime must be addressed before these systems can be made commercially viable. In this chapter, a logical framework for developing a wearable skin-interfaced device for a desired application is proposed with careful consideration of the feasibility of monitoring certain analytes in sweat and interstitial fluid and the current development of the tools available to do so. Specifically, we focus on recent advancements in the engineering of biorecognition elements, the development of more robust signal transduction mechanisms, and novel integration schemes that allow for continuous quantitative analysis. Furthermore, we highlight the most compelling and promising prospects in the field of wearable biosensing and the challenges that remain in translating these technologies into useful products for disease management and for optimizing human performance.

Albumin-based nanoparticles as contrast medium for MRI: vascular imaging, tissue and cell interactions, and pharmacokinetics of second-generation nanoparticles

This multidisciplinary study examined the pharmacokinetics of nanoparticles based on albumin-DTPA-gadolinium chelates, testing the hypothesis that these nanoparticles create a stronger vessel signal than conventional gadolinium-based contrast agents and exploring if they are safe for clinical use. Nanoparticles based on human serum albumin, bearing gadolinium and designed for use in magnetic resonance imaging, were used to generate magnet resonance images (MRI) of the vascular system in rats ("blood pool imaging"). At the low nanoparticle doses used for radionuclide imaging, nanoparticle-associated metals were cleared from the blood into the liver during the first 4 h after nanoparticle application. At the higher doses required for MRI, the liver became saturated and kidney and spleen acted as additional sinks for the metals, and accounted for most processing of the nanoparticles. The multiple components of the nanoparticles were cleared independently of one another. Albumin was detected in liver, spleen, and kidneys for up to 2 days after intravenous injection. Gadolinium was retained in the liver, kidneys, and spleen in significant concentrations for much longer. Gadolinium was present as significant fractions of initial dose for longer than 2 weeks after application, and gadolinium clearance was only complete after 6 weeks. Our analysis could not account quantitatively for the full dose of gadolinium that was applied, but numerous organs were found to contain gadolinium in the collagen of their connective tissues. Multiple lines of evidence indicated intracellular processing opening the DTPA chelates and leading to gadolinium long-term storage, in particular inside lysosomes. Turnover of the stored gadolinium was found to occur in soluble form in the kidneys, the liver, and the colon for up to 3 weeks after application. Gadolinium overload poses a significant hazard due to the high toxicity of free gadolinium ions. We discuss the relevance of our findings to gadolinium-deposition diseases.

Insulin exits skeletal muscle capillaries by fluid-phase transport

Before insulin can stimulate myocytes to take up glucose, it must first move from the circulation to the interstitial space. The continuous endothelium of skeletal muscle (SkM) capillaries restricts insulin's access to myocytes. The mechanism by which insulin crosses this continuous endothelium is critical to understand insulin action and insulin resistance; however, methodological obstacles have limited understanding of endothelial insulin transport in vivo. Here, we present an intravital microscopy technique to measure the rate of insulin efflux across the endothelium of SkM capillaries. This method involves development of a fully bioactive, fluorescent insulin probe, a gastrocnemius preparation for intravital microscopy, an automated vascular segmentation algorithm, and the use of mathematical models to estimate endothelial transport parameters. We combined direct visualization of insulin efflux from SkM capillaries with modeling of insulin efflux kinetics to identify fluid-phase transport as the major mode of transendothelial insulin efflux in mice. Model-independent experiments demonstrating that insulin movement is neither saturable nor affected by insulin receptor antagonism supported this result. Our finding that insulin enters the SkM interstitium by fluid-phase transport may have implications in the pathophysiology of SkM insulin resistance as well as in the treatment of diabetes with various insulin analogs.

Regulation of blood vascular permeability in the skin

Regulation of blood vessel permeability is essential for the homeostasis of peripheral tissues. This regulation controls the trafficking of plasma contents, including water, vitamins, ions, hormones, cytokines, amyloids, lipoproteins, carrier proteins, and immunoglobulins. The properties of blood vessels vary among tissues based on their structural differences: continuous, fenestrated, or sinusoidal. These three types of blood vessels have different charge and size barrier properties. The anionic luminal glycocalyx layer on endothelial cells establishes the "charge barrier" that repels the attachment of negatively charged blood cells and plasma molecules. In contrast, the "size barrier" of blood vessels largely relies on the interendothelial junctions (IEJs) between endothelial cells, which define the paracellular permeability. As in most peripheral tissues, blood capillaries in the skin are composed of continuous and/or fenestrated blood vessels that have relatively tighter IEJs compared to those in the internal organs. Small vesicles in the capillary endothelium were discovered in the 1950s, and studies have since confirmed that blood endothelial cells transport the plasma contents by endocytosis and subsequent transcytosis and exocytosis-this process is called transcellular permeability. The permeability of blood vessels is highly variable as a result of intrinsic and extrinsic factors. It is significantly elevated upon tissue inflammations as a result of disabled IEJs and increased paracellular permeability due to inflammatory mediators. An increase in transcellular permeability during inflammation has also been postulated. Here, we provide an overview of the general properties of vascular permeability based on our recent observations of murine skin inflammation models, and we discuss its physiological significance in peripheral homeostasis.

Endothelial Transcytosis of Insulin: Does It Contribute to Insulin Resistance?

Most research on insulin resistance has focused on impaired signaling at the level of target tissues like skeletal muscle. Insulin delivery is also important and includes recruitment and perfusion of capillaries bearing insulin, but also the transit of insulin across the capillary endothelium. The mechanisms of this second stage (insulin transcytosis) and whether it contributes to insulin resistance remain uncertain.

Skeletal Muscle Vascular Function: A Counterbalance of Insulin Action

Insulin is a vasoactive hormone that regulates vascular homeostasis by maintaining balance of endothelial-derived NO and ET-1. Although there is general agreement that insulin resistance and the associated hyperinsulinemia disturb this balance, the vascular consequences for hyperinsulinemia in isolation from insulin resistance are still unclear. Presently, there is no simple answer for this question, especially in a background of mixed reports examining the effects of experimental hyperinsulinemia on endothelial-mediated vasodilation. Understanding the mechanisms by which hyperinsulinemia induces vascular dysfunction is essential in advancing treatment and prevention of insulin resistance-related vascular complications. Thus, we review literature addressing the effects of hyperinsulinemia on vascular function. Furthermore, we give special attention to the vasoregulatory effects of hyperinsulinemia on skeletal muscle, the largest insulin-dependent organ in the body. This review also characterizes the differential vascular effects of hyperinsulinemia on large conduit vessels versus small resistance microvessels and the effects of metabolic variables in an effort to unravel potential sources of discrepancies in the literature. At the cellular level, we provide an overview of insulin signaling events governing vascular tone. Finally, we hypothesize a role for hyperinsulinemia and insulin resistance in the development of CVD.

Caveolae: One Function or Many?

Caveolae are small, bulb-shaped plasma membrane invaginations. Mutations that ablate caveolae lead to diverse phenotypes in mice and humans, making it challenging to uncover their molecular mechanisms. Caveolae have been described to function in endocytosis and transcytosis (a specialized form of endocytosis) and in maintaining membrane lipid composition, as well as acting as signaling platforms. New data also support a model in which the central function of caveolae could be related to the protection of cells from mechanical stress within the plasma membrane. We present evidence for these diverse roles and consider in vitro and in vivo experiments confirming a mechanoprotective role. We conclude by highlighting current gaps in our knowledge of how mechanical signals may be transduced by caveolae.

Endothelial cells actively concentrate insulin during its transendothelial transport

OBJECTIVE

We examined insulins uptake and transendothelial transport by endothelial cells in order to: (i) ascertain whether insulin accumulates within the cells to concentrations greater than in the media; (ii) compare trans endothelial insulin transport to that of inulin (using the latter as a tracer for passive transport or leaked); and; (iii) determine whether insulins transported depended on insulin action.

METHODS

Using 125I-insulin at physiologic concentrations we measured both the uptake and trans endothelial transport of insulin by bovine aortic endothelial cells and measured cell volume using tritiated 3-O-methylglucose.

RESULTS

Bovine aortic endothelial cells accumulate insulin to > five-fold above the media concentrations and the trans endothelial transport of insulin, but not inulin, is saturable and requires intact PI-3-kinase and MEK signaling.

CONCLUSION

The insulin receptor and downstream signaling from the receptor regulates endothelial insulin transport. Insulin is accumulated against a concentration gradient by the endothelial cell. We suggest that insulin uptake is rate limiting for insulin trans endothelial transport.

Nitrosation-dependent caveolin 1 phosphorylation, ubiquitination, and degradation and its association with idiopathic pulmonary arterial hypertension

In the present study, we tested the hypothesis that chronic inflammation and oxidative/nitrosative stress induce caveolin 1 (Cav-1) degradation, providing an underlying mechanism of endothelial cell activation/dysfunction and pulmonary vascular remodeling in patients with idiopathic pulmonary arterial hypertension (IPAH). We observed reduced Cav-1 protein despite increased Cav-1 messenger RNA expression and also endothelial nitric oxide synthase (eNOS) hyperphosphorylation in human pulmonary artery endothelial cells (PAECs) from patients with IPAH. In control human lung endothelial cell cultures, tumor necrosis factor α-induced nitric oxide (NO) production and S-nitrosation (SNO) of Cav-1 Cys-156 were associated with Src displacement and activation, Cav-1 Tyr-14 phosphorylation, and destabilization of Cav-1 oligomers within 5 minutes that could be blocked by eNOS or Src inhibition. Prolonged stimulation (72 hours) with NO donor DETANONOate reduced oligomerized and total Cav-1 levels by 40%-80%, similar to that observed in IPAH patient-derived PAECs. NO donor stimulation of endothelial cells for >72 hours, which was associated with sustained Src activation and Cav-1 phosphorylation, ubiquitination, and degradation, was blocked by NOS inhibitor L-NAME, Src inhibitor PP2, and proteosomal inhibitor MG132. Thus, chronic inflammation, sustained eNOS and Src signaling, and Cav-1 degradation may be important causal factors in the development of IPAH by promoting PAEC dysfunction/activation via sustained oxidative/nitrosative stress.

Remodelling of membrane rafts expression in lung cells as an early sign of mechanotransduction-signalling in pulmonary edema

Membrane rafts (MRs) are clusters of lipids, organized in a "quasicrystalline" liquid-order phase, organized on the cell surface and whose pattern of molecules and physicochemical properties are distinct from those of the surrounding plasma membrane. MRs may be considered an efficient and fairly rapid cell-activated mechanism to express or mask surface receptors aimed at triggering specific response pathways. This paper reports observations concerning the role of MRs in the control of lung extravascular water that ought to be kept at minimum to assure gas diffusion, supporting the hypothesis that MRs expression is a potential mechanism of sensing minor changes in the volume of extravascular water. We present the evidence that MRs expression specifically relates to signal-transduction processes evoked by mechanical stimuli arising in the interstitial lung compartment when a small increase in extravascular volume occurs. We further hypothesize that a differential expression of MRs might also reflect the damage to precise components of the extracellular matrix caused by the perturbation in water balance and thus can trigger a molecule-oriented specific matrix remodelling.

The vascular actions of insulin control its delivery to muscle and regulate the rate-limiting step in skeletal muscle insulin action

Evidence suggests that insulin delivery to skeletal muscle interstitium is the rate-limiting step in insulin-stimulated muscle glucose uptake and that this process is impaired by insulin resistance. In this review we examine the basis for the hypothesis that insulin acts on the vasculature at three discrete steps to enhance its own delivery to muscle: (1) relaxation of resistance vessels to increase total blood flow; (2) relaxation of pre-capillary arterioles to increase the microvascular exchange surface perfused within skeletal muscle (microvascular recruitment); and (3) the trans-endothelial transport (TET) of insulin. Insulin can relax resistance vessels and increase blood flow to skeletal muscle. However, there is controversy as to whether this occurs at physiological concentrations of, and exposure times to, insulin. The microvasculature is recruited more quickly and at lower insulin concentrations than are needed to increase total blood flow, a finding consistent with a physiological role for insulin in muscle insulin delivery. Microvascular recruitment is impaired by obesity, diabetes and nitric oxide synthase inhibitors. Insulin TET is a third potential site for regulating insulin delivery. This is underscored by the consistent finding that steady-state insulin concentrations in plasma are approximately twice those in muscle interstitium. Recent in vivo and in vitro findings suggest that insulin traverses the vascular endothelium via a trans-cellular, receptor-mediated pathway, and emerging data indicate that insulin acts on the endothelium to facilitate its own TET. Thus, muscle insulin delivery, which is rate-limiting for its metabolic action, is itself regulated by insulin at multiple steps. These findings highlight the need to further understand the role of the vascular actions of insulin in metabolic regulation.

Endothelial transcytosis in health and disease

The visionaries predicted the existence of transcytosis in endothelial cells; the cell biologists deciphered its mechanisms and (in part) the molecules involved in the process; the cell pathologists unravelled the presence of defective transcytosis in some diseases. The optimistic perspective is that transcytosis, in general, and receptor-mediated transcytosis, in particular, will be greatly exploited in order to target drugs and genes to exclusive sites in and on endothelial cells (EC) or underlying cells. The current recognition that plasmalemmal vesicles (caveolae) are the vehicles involved in EC transcytosis has moved through various phases from initial considerations of caveolae as unmovable sessile non-functional plasmalemma invaginations to the present identification of a multitude of molecules and a crowd of functions associated with these ubiquitous structures of endothelial and epithelial cells. Further understanding of the molecular machinery that precisely guides caveolae through the cells so as to reach the target membrane (fission, docking, and fusion), to avoid lysosomes, or on the contrary, to reach the lysosomes, and discharge the cargo molecules will assist in the design of pathways that, by manipulating the physiological route of caveolae, will carry molecules of choice (drugs, genes) at controlled concentrations to precise destinations.

Vascular arrangement and ultrastructure of the European eelpout Zoarces viviparus ovary: implications for maternal-embryonic exchange

The structural basis for exchange between maternal serum and ovarian fluid in the viviparous teleost Zoarces viviparus was investigated. Casts of the ovarian vasculature showed that blood supply to the ovary is initially directed to the follicular appendages lining the ovarian wall through thick-walled muscular arteries running along the ovary wall and within the follicular appendages. The follicles had a rich capillary network with diffusion distances between maternal blood and ovarian fluid comparable to those found for gill epithelia, suggesting this is the primary site of gas exchange between maternal plasma and ovarian fluid. Follicular capillary beds were continuous with those in the ovary wall and were eventually drained by the ovarian and intestinal venous systems. The barrier between ovarian fluid and maternal blood consisted of the endothelial cells of the maternal blood vessels and a layer of epithelial cells lining the ovarian lumen, with an intermittent layer of loose connective fibers. Junctional complexes between cells were predominantly anchoring junctions with the occurrence of occasional occluding junctions, supporting the possibility of paracellular transport from maternal serum to ovarian fluid of small molecular weight compounds. Heavy investment in keratin filaments suggests that follicles are tissues of high structural integrity. Evidence for protein synthesis in the ovarian lining was found in the form of Golgi apparatus and rough endoplasmic reticulum. Although numerous cytoplasmic vacuoles and secretory granules were present in both epithelial and endothelial cells, the fate of synthesized protein remains to be determined.

Molecular determinants of endothelial transcytosis and their role in endothelial permeability

Caveolae transcytosis with its diverse mechanisms-fluid phase, adsorptive, and receptor-mediated-plays an important role in the continuous exchange of molecules across the endothelium. We will discuss key features of endothelial transcytosis and caveolae that have been studied recently and have increased our understanding of caveolae function in transcytosis at the molecular level. During transcytosis, caveolae "pinch off" from the plasma membrane to form discrete vesicular carriers that shuttle to the opposite front of endothelial cells, fuse with the plasma membrane, and discharge their cargo into the perivascular space. Endothelial transcytosis exhibits distinct properties, the most important being rapid and efficient coupling of endocytosis to exocytosis on opposite plasma membrane. We address herein the membrane fusion-fission reactions that underlie transcytosis. Caveolae move across the endothelial cells with their cargo predominantly in the fluid phase through an active process that bypasses the lysosomes. Endothelial transcytosis is a constitutive process of vesicular transport. Recent studies show that transcytosis can be upregulated in response to pathological stimuli. Transcytosis via caveolae is an important route for the regulation of endothelial barrier function and may participate in different vascular diseases.

In vitro study of LDL transport under pressurized (convective) conditions

It is difficult to assess the transport pathways that carry low-density lipoprotein (LDL) into the artery wall in vivo, and there has been no previous in vitro study that has examined transendothelial transport under physiologically relevant pressurized (convective) conditions. Therefore, we measured water, albumin, and LDL fluxes across bovine aortic endothelial cell (BAEC) monolayers in vitro and determined the relative contributions of vesicles, paracellular transport through "breaks" in the tight junction, and "leaky" junctions associated with dying or dividing cells. Our results show that leaky junctions are the dominant pathway for LDL transport (>90%) under convective conditions and that albumin also has a significant component of transport through leaky junctions (44%). Transcellular transport of LDL by receptor-mediated processes makes a minor contribution (<10%) to overall transport under convective conditions.

The vascular endothelial cell mediates insulin transport into skeletal muscle

The pathways by which insulin exits the vasculature to muscle interstitium have not been characterized. In the present study, we infused FITC-labeled insulin to trace morphologically (using confocal immunohistochemical methods) insulin transport into rat skeletal muscle. We biopsied rectus muscle at 0, 10, 30, and 60 min after beginning a continuous (10 mU x min(-1) x kg(-1)), intravenous FITC-insulin infusion (with euglycemia maintained). The FITC-insulin distribution was compared with that of insulin receptors (IR), IGF-I receptors (IGF-IR), and caveolin-1 (a protein marker for caveolae) in skeletal muscle vasculature. We observed that muscle endothelium stained strongly for FITC-insulin within 10 min, and this persisted to 60 min. Endothelium stained more strongly for FITC-insulin than any other cellular elements in muscle. IR, IGF-IR, and caveolin-1 were also detected immunohistochemically in muscle endothelial cells. We further compared their intracellular distribution with that of FITC-insulin in cultured bovine aortic endothelial cells (bAECs). Considerable colocalization of IR or IGF-IR with FITC-insulin was noted. There was some but less overlap of IR or IGF-IR or FITC-insulin with caveolin-1. Immunoprecipitation of IR coprecipitated caveolin-1, and conversely the precipitation of caveolin-1 brought down IR. Furthermore, insulin increased the tyrosine phosphorylation of caveolin-1, and filipin (which inhibits caveolae formation) blocked insulin uptake. Finally, the ability of insulin, IGF-I, and IGF-I-blocking antibody to diminish insulin transport across bAECs grown on transwell plates suggested that IGF-IR, in addition to IR, can also mediate transendothelial insulin transit. We conclude that in vivo endothelial cells rapidly take up and concentrate insulin relative to plasma and muscle interstitium and that IGF-IR, like IR, may mediate insulin transit through endothelial cells in a process involving caveolae.

Endothelial vesiculo-vacuolar organelles, pockets and multi-layered fenestrated lamellae in the capillaries of the mouse carotid body

Fenestrated capillaries represent the basic structural unit in the carotid body. They mediate a characteristic hyperpermeability state in this organ. Endothelial fenestrae and plasmalemmal vesicles are of particular importance in this respect. The present electron microscopic study of the capillaries of the mouse carotid body demonstrates prominent endothelial cell structures that are suggested to be closely related to endothelial fenestrae and plasmalemmal vesicles. These structures include: (1) Vesiculo-vacuolar organelles formed by fusion and intercommunication of vesicles and vacuoles of variable dimensions. (2) Pockets in the form of fenestrated membrane-bound vacuoles that communicate either with the capillary lumen, pericapillary space or both via multiple apertures or fenestrae. (3) Multi-layered fenestrated Lamellae where the endothelial cytoplasm is divided into multiple attenuated sheets provided with several fenestrae. The latter two structures are preferentially located in the thick perinuclear region of the endothelial cell. Their fenestrae are always distributed in linear series and show close similarity to the usual chains of fenestrae in the attenuated periphery of the endothelial cells. The individual apertures of the fenestrated vacuoles and multi-layered fenestrated lamellae are closely similar to the stomata of fully opened plasmalemmal vesicles suggesting a relationship between them. Morphological and morphometrical analysis of a series of fenestrae belonging to these structures revealed that they are identical to the usual chains of fenestrae in the attenuated periphery of the endothelial cells.

Genetic Ablation of Caveolin-1 Confers Protection Against Atherosclerosis

Objective— The development of atherosclerosis is a process characterized by the accumulation of lipids in the form of modified lipoproteins in the subendothelial space. This initiating step is followed by the subsequent recruitment and proliferation of other cell types, including monocytes/macrophages and smooth muscle cells. Here, we evaluate the potential role of caveolae membrane domains in the pathogenesis of atherosclerosis by using apolipoprotein E-deficient (ApoE−/−) mice as a model system.

Methods and Results— Caveolin-1 (Cav-1) is a principal structural protein component of caveolae membrane domains. To directly assess the in vivo role of caveolae and Cav-1 in atherosclerosis, we interbred Cav-1−/− mice with ApoE−/− mice. Interestingly, loss of Cav-1 resulted in a dramatic >2-fold increase in non-HDL plasma cholesterol levels in the ApoE−/− background. However, despite this hypercholesterolemia, we found that loss of Cav-1 gene expression was clearly protective against the development of aortic atheromas, with up to an ≈70% reduction in atherosclerotic lesion area. Mechanistically, we demonstrated that loss of Cav-1 resulted in the dramatic downregulation of certain proatherogenic molecules, namely, CD36 and vascular cell adhesion molecule-1.

Conclusions— Taken together, our results indicate that loss of Cav-1 can counteract the detrimental effects of atherogenic lipoproteins. Thus, Cav-1 is a novel target for drug development in the pharmacologic prevention of atheroma formation. Our current data also provide the first molecular genetic evidence to support the hypothesis that caveolar transcytosis of modified lipoproteins (from the blood to the sub-endothelial space) is a critical initiating step in atherosclerosis.

Caveolin, caveolae, and endothelial cell function

Caveolae are 50- to 100-nm cell-surface plasma membrane invaginations observed in terminally differentiated cells. They are particularly abundant in endothelial cells, where they are believed to play a major role in the regulation of endothelial vesicular trafficking and signal transduction. The use of caveolin-1-deficient mice has provided many new insights into the roles of caveolae and caveolin-1 in the regulation of endothelial cell function. These novel findings suggest an important role for caveolin-1 in the pathogenesis of cancer, atherosclerosis, and vascular disease.

Caveolae/raft-dependent endocytosis

Although caveolae are well-characterized subdomains of glycolipid rafts, their distinctive morphology and association with caveolins has led to their internalization being considered different from that of rafts. In this review, we propose that caveolae and rafts are internalized via a common pathway, caveolae/raft-dependent endocytosis, defined by its clathrin independence, dynamin dependence, and sensitivity to cholesterol depletion. The regulatory role of caveolin-1 and ligand sorting in this complex endocytic pathway are specifically addressed.

Distinct caveolae-mediated endocytic pathways target the Golgi apparatus and the endoplasmic reticulum

Internalization of autocrine motility factor (AMF) into the endoplasmic reticulum is sensitive to the cholesterol-extracting reagent methyl-beta-cyclodextrin, inhibited by the dynamin-1 K44A mutant and negatively regulated by caveolin-1. Thus, AMF internalization requires a caveolae-mediated endocytic pathway. Similarly, we show here that endocytosis of cholera toxin (CTX) in NIH-3T3 fibroblasts is inhibited by adenoviral expression of the dynamin-1 K44A mutant but only partially by expression of the clathrin hub. Treatment with methyl-beta-cyclodextrin and overexpression of caveolin-1, but not the clathrin hub, selectively diminishes CTX endocytosis to the Golgi apparatus but not to endosomes. CTX is therefore targeted via a caveolin-1-regulated caveolae-mediated pathway to the Golgi. Disruption of Golgi-, caveosome- or endosome-mediated trafficking with brefeldin A, nocodazole or a 20 degrees C temperature block, respectively, inhibit CTX endocytosis to the Golgi but do not affect AMF delivery to the endoplasmic reticulum. Following an incubation of only five minutes in the presence of the clathrin hub, AMF and CTX are not cointernalized, and AMF is delivered to the AMF-R-positive smooth ER. The internalization of both ligands is nevertheless sensitive to the tyrosine kinase inhibitor genistein, confirming that they are both internalized via caveolae/raft pathways. Two distinct caveolae-mediated endocytic pathways therefore exist, including a novel direct pathway to the ER from the plasma membrane.

The blood volumes of the primary and secondary circulatory system in the Atlantic cod Gadus morhua L, using plasma bound Evans Blue and compartmental analysis

The volume of the primary (PCS) and secondary (SCS) circulatory system in the Atlantic cod Gadus morhua was determined using a modified dye dilution technique. Cod (N=10) were chronically cannulated in the second afferent branchial artery with PE-50 tubing. Evans Blue dye was bound to harvested fish plasma at a concentration of 1 mg dye ml(-1) plasma, and injected at a concentration of 1 mg kg(-1) body mass. Serial sampling from the cannula produced a dye dilution curve, which could be described by a double exponential decay equation. Curve analysis enabled the calculation of the primary circulatory and total distribution volume. The difference between these volumes is assumed to be the volume of the SCS. From the dilution curve, it was also possible to calculate flow rates between and within the systems. The results of these experiments suggest a plasma volume in the PCS of 3.42+/-0.89 ml 100 g(-1) body mass, and in the SCS of 1.68+/-0.35 ml 100 g(-1) body mass (mean +/- S.D.) or approximately 50% that of the PCS. Flow rates to the SCS were calculated as 2.7% of the resting cardiac output. There was an allometric relationship between body mass and blood volumes. Increasing condition factor showed a tendency towards smaller blood volumes of the PCS, expressed as percentage body mass, but this was not evident for the volume of the SCS.

Increased endothelial vesicular transport correlates with increased blood-tumor barrier permeability induced by bradykinin and leukotriene C4

Bradykinin and leukotriene C4 (LTC4) have been shown to increase molecular transport across the blood-tumor barrier (BTB). The aim of this study was to quantitatively assess whether an increase in vesicular transport or opening of tight junctions was responsible for this increase in permeability. Wistar rats bearing RG2 or C6 gliomas were infused with bradykinin or LTC4 through the right carotid artery for 15 min and then perfused to achieve fixation. Prepared specimens were observed using transmission electron microscopy. Infusion of either bradykinin or LTC4 resulted in significantly increased vesicular density in capillary endothelial cells of the BTB but not in normal brain capillaries. The opening of tight junctions, assessed by determining a cleft index, was found to be greater in tumor capillaries compared to normal controls. However, neither bradykinin nor LTC4 produce variations in the cleft index. A significant accumulation of horseradish peroxidase was seen in the intercellular peri-capillary spaces and in endothelial transport vesicles after infusion of bradykinin, demonstrating that the formation of vesicles was associated with macromolecular transcytosis. These findings suggest that pinocytotic vesicular transport is the primary means by which luminal to abluminal transport occurs in response to vasomodulation with bradykinin or LTC4.

Sequential morphological and permeability changes in the rete capillaries during hyperglycaemia

With the rete model of the eel swimbladder, we have studied the appearance and development of a microangiopathy during a 2-year period of hyperglycaemia. Hyperglycaemia was induced in the eel by chronic exposure to cold water. At 3-5 months, basement membrane thickness was twice the normal value and increased only slightly thereafter. Diffusion coefficients of permeability were measured in counter-current perfusion experiments for a variety of tracers that are believed to use different pathways of transcapillary transport. The permeability to sucrose was the first to significantly increase, at 6-8 months, followed by that of albumin, insulin, and inulin, at 9-11 months and that of sodium, at 18-24 months. The permeability to water and antipyrine remained stable throughout the study. The results indicate that in the rete model, chronic hyperglycaemia induces a rapid thickening of the capillary basement membrane and selective permeability increments in the various paths of transcapillary transport.

Transcytosis of plasma macromolecules in endothelial cells: a cell biological survey

The modern exploration of endothelial cell biology is a largely interdisciplinary exercise. Cell biological, physiological, and more recently molecular biology approaches were used to study the pathways and the organelles involved in transcytosis of macromolecules in endothelial cell (EC). Here we discuss mainly the cell biological findings that revealed that EC have the attributes to fulfill the transport function. They are polarized cells, heterogeneous, and, thus, structurally and functionally adapted to the vascular bed in which they reside. The structural heterogeneity involves the number and distribution of plasmalemmal vesicles (caveolae), their generated channels, and the organization of intercellular junctions. The closely related functional heterogeneity comprises the degree of permeability for plasma molecules that vary as a function of organ. The EC are endowed with the cellular machinery to perform (1) endocytosis, that is to take up plasma proteins and the molecules they carry to be used for themselves (cholesterol-carrying low density lipoproteins, fatty acid carrying albumin, iron carrying transferrin, etc.), and (2) transcytosis, which implies to transport plasma proteins to the subjacent cells and tissues. The possible pathways for transport of molecules are transcellular, via caveolae and channels, and paracellular via intercellular junctions. Most of the results obtained, so far, indicate that transcytosis of albumin, low-density lipoproteins, metaloproteases, and insulin, is performed by cargo-vesicles and their generated channels. The paracellular pathway can be used for water and ions; in postcapillary venules, at the level of which approximately 30% of junctions are open to a space of 6 nm, small molecules may take this route. Recent data obtained by molecular biology techniques revealed that caveolae are endowed with the molecular machinery for fusion/fission, docking, and movement across cells. Moreover, the various and numerous molecules that have been detected in the caveolae membrane and the different functions assumed by this differentiated microdomain strengthen the postulate that there are at least two or more types of vesicles molecularly tailored for the local physiological requirements.

Morphological and cytochemical aspects of capillary permeability

Transport of plasma soluble constituents across the capillary wall is of primordial importance in cardiovascular physiology. While physiological experiments have concluded with the existence of two sets of pores, a large one responsible for the transport of proteins and a small one designed for the diffusion of small solutes, the morphological counterparts have yet to get general agreement. In this review, we present the different proposed paths within and between the endothelial cells that do allow passage of plasma constituents and may respond to the definitions established by physiological means. The vesicular system existing in endothelial cells has been the first transendothelial path to be proposed. Several data have demonstrated the involvement of this system in transport, although others have systematically brought controversy. One alternative to the vesicles has been the demonstration of membrane-bound tubules creating, in certain cases, transendothelial channels that would allow diffusion of plasma proteins and other constituents across the capillary wall. Access to this tubulo-vesicular system could be restrained by the stomatal diaphragm and facilitated by specific membrane receptors. Further, we have demonstrated for the first time with morpho-cytochemical tools, that the intercellular clefts are the site of diffusion for small molecules such as peptides having a molecular weight inferior to 3,000. For the fenestrated capillary bed, we have shown that fenestrae are the site through which plasma constituents cross the capillary wall. However, and in spite of the existence of these large open pores, the endothelial cells still display the tubulo-vesicular system involved in transport of large molecules and their intercellular clefts are also the site of diffusion of small molecules. Making consensus on the existence of an intracellular tubulo-vesicular system in non-fenestrated capillaries, responsible for the transport of large molecules by the endothelial cells, and understanding the rational for the fenestrated capillary to have three paths for transport--the fenestrae, the tubulo-vesicular system, and the inter-endothelial clefts--require further investigation.

Structure and function of endothelial caveolae

Caveolae are spherical invaginations of the plasma membrane and associated vesicles that are found at high surface densities in most cells, endothelia included. Their structural framework has been shown to consist of oligomerized caveolin molecules interacting with cholesterol and sphingolipids. Caveolae have been involved in many cellular functions such as endocytosis, signal transduction, mechano-transduction, potocytosis, and cholesterol trafficking. Some confusion still persists in the field with respect to the relationship between caveolae and the lipid rafts, which have been involved in many of the above functions. In addition to all these, endothelial caveolae have been involved in capillary permeability by their participation in the process of transcytosis. This short review will focus on their structure and components, methods used to determine these components, and the role of caveolae in the transendothelial exchanges between blood plasma and the interstitial fluid.

Caveolin-1 is a negative regulator of caveolae-mediated endocytosis to the endoplasmic reticulum

Caveolae are flask-shaped invaginations at the plasma membrane that constitute a subclass of detergent-resistant membrane domains enriched in cholesterol and sphingolipids and that express caveolin, a caveolar coat protein. Autocrine motility factor receptor (AMF-R) is stably localized to caveolae, and the cholesterol extracting reagent, methyl-beta-cyclodextrin, inhibits its internalization to the endoplasmic reticulum implicating caveolae in this distinct receptor-mediated endocytic pathway. Curiously, the rate of methyl-beta-cyclodextrin-sensitive endocytosis of AMF-R to the endoplasmic reticulum is increased in ras- and abl-transformed NIH-3T3 cells that express significantly reduced levels of caveolin and few caveolae. Overexpression of the dynamin K44A dominant negative mutant via an adenovirus expression system induces caveolar invaginations sensitive to methyl-beta-cyclodextrin extraction in the transformed cells without increasing caveolin expression. Dynamin K44A expression further inhibits AMF-R-mediated endocytosis to the endoplasmic reticulum in untransformed and transformed NIH-3T3 cells. Adenoviral expression of caveolin-1 also induces caveolae in the transformed NIH-3T3 cells and reduces AMF-R-mediated endocytosis to the endoplasmic reticulum to levels observed in untransformed NIH-3T3 cells. Cholesterol-rich detergent-resistant membrane domains or glycolipid rafts therefore invaginate independently of caveolin-1 expression to form endocytosis-competent caveolar vesicles via rapid dynamin-dependent detachment from the plasma membrane. Caveolin-1 stabilizes the plasma membrane association of caveolae and thereby acts as a negative regulator of the caveolae-mediated endocytosis of AMF-R to the endoplasmic reticulum.

The rete mirabile of the eel: a useful model for the study of transcapillary passage of MR contrast agents

Our purpose was to study the capillary leakage of MR contrast media using a pure capillary model, the rete mirabile of the eel. The rete is a countercurrent-exchange organ composed of an arterial and a venous capillary system that can be catheterized and perfused. Substances are introduced at the arterial input by a constant infusion, and their steady-state concentrations are measured at the arterial and venous outputs. The capillary leakage of four MR contrast agents--Gd-DOTA(MW = 561 D), carboxymethyldextran-Gd-DTPA (MW = 38,900 D), albumin-Gd-DTPA (MW = 92,000 D), AMI-227 (400,000 D<MW<900,000 D)--was characterized by reference to radioactive tracers (3HHO, 22Na, 14C-sucrose, 125I-albumin) by two parameters. These parameters were the concentration ratio of the venous output over the arterial input [C(VOUT)(%)] and the permeability coefficient (P). The transcapillary pathway mechanisms for carboxymethyldextran-Gd-DTPA and albumin-Gd-DTPA were studied by electron microscopy. P values for Gd-DOTA (9.4+/-3.6 x 10(-7) cm/s) and albumin-Gd-DTPA (11.8+/-5.5 x 10(-7) cm/s) were close to P values for 14C-sucrose, while P values for carboxymethyldextran-Gd-DTPA (6.4+/-4.9 x 10(-7) cm/sec) were similar to P values for 125I-albumin. The lowest permeability was observed with AMI-227 (2.7+/-2 x 10(-7) cm/sec). Vesicular transport was demonstrated for carboxymethyldextran-Gd-DTPA and albumin-Gd-DTPA. The transcapillary passage of several MR contrast agents can be characterized with the rete mirabile model. Molecular weight is the major factor influencing transport.

Localization of autocrine motility factor receptor to caveolae and clathrin-independent internalization of its ligand to smooth endoplasmic reticulum

Autocrine motility factor receptor (AMF-R) is a cell surface receptor that is also localized to a smooth subdomain of the endoplasmic reticulum, the AMF-R tubule. By postembedding immunoelectron microscopy, AMF-R concentrates within smooth plasmalemmal vesicles or caveolae in both NIH-3T3 fibroblasts and HeLa cells. By confocal microscopy, cell surface AMF-R labeled by the addition of anti-AMF-R antibody to viable cells at 4 degreesC exhibits partial colocalization with caveolin, confirming the localization of cell surface AMF-R to caveolae. Labeling of cell surface AMF-R by either anti-AMF-R antibody or biotinylated AMF (bAMF) exhibits extensive colocalization and after a pulse of 1-2 h at 37 degreesC, bAMF accumulates in densely labeled perinuclear structures as well as fainter tubular structures that colocalize with AMF-R tubules. After a subsequent 2- to 4-h chase, bAMF is localized predominantly to AMF-R tubules. Cytoplasmic acidification, blocking clathrin-mediated endocytosis, results in the essentially exclusive distribution of internalized bAMF to AMF-R tubules. By confocal microscopy, the tubular structures labeled by internalized bAMF show complete colocalization with AMF-R tubules. bAMF internalized in the presence of a 10-fold excess of unlabeled AMF labels perinuclear punctate structures, which are therefore the product of fluid phase endocytosis, but does not label AMF-R tubules, demonstrating that bAMF targeting to AMF-R tubules occurs via a receptor-mediated pathway. By electron microscopy, bAMF internalized for 10 min is located to cell surface caveolae and after 30 min is present within smooth and rough endoplasmic reticulum tubules. AMF-R is therefore internalized via a receptor-mediated clathrin-independent pathway to smooth ER. The steady state localization of AMF-R to caveolae implicates these cell surface invaginations in AMF-R endocytosis.

Evidence of a tubular system for transendothelial transport in arterial capillaries of the rete mirabile

The arterial endothelial cells of the rete capillaries of the eel were examined by transmission electron microscopy on thin sections, on freeze-fracture replicas, by scanning electron microscopy, after cytochemical osmium impregnation and perfusion with peroxidase. The study revealed the existence of membrane-bound tubules and vesicles that open at both the luminal and abluminal poles of the cell and at the level of the intercellular space. The tubules are straight or present successive dilations and constrictions. They branch in various directions and intrude deeply into the cell cytoplasm, forming a complex tubular network within the cell. Immunocytochemical techniques were applied on immersion-fixed tissues and on perfusion of the capillaries with albumin and insulin. These demonstrated that the tubular-vesicular system is involved in the transport of circulating proteins. Furthermore, protein A-gold immunocytochemistry has revealed the association of actin with the membranes of this system. On the basis of these results, we suggest that the transendothelial transport of serum proteins takes place by a transcytotic process through a membrane-bound tubular-vesicular system and is equivalent to the large pore system presumed from functional studies.