Cardiogenic Pulmonary Edema in Emergency Medicine

Cardiogenic pulmonary edema (CPE) is characterized by the development of acute respiratory failure associated with the accumulation of fluid in the lung's alveolar spaces due to an elevated cardiac filling pressure. All cardiac diseases, characterized by an increasing pressure in the left side of the heart, can cause CPE. High capillary pressure for an extended period can also cause barrier disruption, which implies increased permeability and fluid transfer into the alveoli, leading to edema and atelectasis. The breakdown of the alveolar-epithelial barrier is a consequence of multiple factors that include dysregulated inflammation, intense leukocyte infiltration, activation of procoagulant processes, cell death, and mechanical stretch. Reactive oxygen and nitrogen species (RONS) can modify or damage ion channels, such as epithelial sodium channels, which alters fluid balance. Some studies claim that these patients may have higher levels of surfactant protein B in the bloodstream. The correct approach to patients with CPE should include a detailed medical history and a physical examination to evaluate signs and symptoms of CPE as well as potential causes. Second-level diagnostic tests, such as pulmonary ultrasound, natriuretic peptide level, chest radiograph, and echocardiogram, should occur in the meantime. The identification of the specific CPE phenotype is essential to set the most appropriate therapy for these patients. Non-invasive ventilation (NIV) should be considered early in the treatment of this disease. Diuretics and vasodilators are used for pulmonary congestion. Hypoperfusion requires treatment with inotropes and occasionally vasopressors. Patients with persistent symptoms and diuretic resistance might benefit from additional approaches (i.e., beta-agonists and pentoxifylline). This paper reviews the pathophysiology, clinical presentation, and management of CPE.

Fluid and protein exchange in microvascular networks: Importance of modelling heterogeneity in geometrical and biophysical properties

KEY POINTS

Microvascular network architecture defines coupling of fluid and protein exchange. Network arrangements markedly reduce capillary hydrostatic pressures and resting fluid movement at the same time as increasing the capacity for change The presence of vascular remodelling or angiogenesis puts constraints of network behaviour The sites of fluid and protein exchange can be segregated to different portions of the network Although there is a net filtration of fluid from a network of exchange vessels, there are specific areas where fluid moves into the circulation (reabsorption) and, when protein is moving into tissue, the amount is insufficient under basal conditions to result in changes in oncotic pressure.

ABSTRACT

Integration of functional results obtained across scales, from chemical signalling to the whole organism, is a daunting task requiring the marriage of experimental data with mathematical modelling. In the present study, a novel coupled computational fluid dynamics model is developed incorporating fluid and protein transport using measurements in an in vivo frog (Rana pipiens) mesenteric microvascular network. The influences of network architecture and exchange are explored systematically under the common assumptions of structurally and functionally identical microvessels (Homogeneous Scenario) or microvessels classified by position in flow (Class Uniform Scenario), which are compared with realistic microvascular network components (Heterogeneous Scenario). The model incorporates ten quantities that vary within a microvessel; pressure boundary conditions are calibrated against experimental measurements. The Homogeneous Scenario standard model showed that assuming a single 'typical' capillary hides the influence of vessels arranged into a network architecture, where capillary hydrostatic pressures (pT ) are reduced, resulting in both a nonuniform distribution of blood flow and reduced volume flow rate (Jf,T ). In the Class Uniform Scenario pT was further attenuated to produce a ∼60% reduction in Jf,T . Finally, the Heterogeneous Scenario, incorporating measures of individual vessel surface area, demonstrates additional lowering of pT from inlet values favouring a >70% reduction of Jf,T in the face of a ∼120% increase in protein movement into the tissues relative to the Homogeneous Scenario. Beyond the impacts of network architecture, an unanticipated finding was the influence of a blind-end microvessel on model convergence, indicating a profound influence of the largely unexplored dynamics of vascular remodelling on tissue perfusion.

Atrial natriuretic peptide does not degrade the endothelial glycocalyx: A secondary analysis of a randomized porcine model

BACKGROUND

The atrial natriuretic peptide (ANP) released from the heart regulates intravascular volume and is suspected to increase capillary permeability. Contradictory results regarding ANP and glycocalyx degradation have been reported. The aim of this study was to investigate if an infusion of ANP causes degradation of the endothelial glycocalyx.

METHODS

Twenty pigs, pretreated with 250 mg methylprednisolone, were randomized to receive an infusion of either ANP (50 ng/kg/min) (n = 10) or 0.9% NaCl (n = 10) during 60 min. Endothelial glycocalyx components (heparan sulphate proteoglycan and hyaluronic acid), Hct, calculated plasma volume and colloid osmotic pressure were measured from baseline to 60 min.

RESULTS

There was no difference between the control and intervention groups for heparan sulphate proteoglycan and hyaluronic acid corrected for the change in plasma volume (P = .333 and 0.197). Hct increased with 1.8 ± 2.2% in the intervention group (P = .029) with no change -0.5 ± 2.3% in the control group (P = .504). The plasma volume decreased in the intervention group with -8.4 ± 10% (P = .034) with no change in the control group 3.1 ± 12% (P = .427). Median changes in colloid osmotic pressures in the control and intervention group were -0.39 [95% CI, -1.88-0.13] and 0.9 [95% CI, 0.00-1.58], respectively (P = .012).

CONCLUSIONS

In this randomized porcine study, an ANP infusion did not cause endothelial glycocalyx degradation but decreased the plasma volume most probably due to precapillary vasodilation and increased filtration.

The Colloid Osmotic Pressure Across the Glycocalyx: Role of Interstitial Fluid Sub-Compartments in Trans-Vascular Fluid Exchange in Skeletal Muscle

The primary purpose of these investigations is to integrate our growing knowledge about the endothelial glycocalyx as a permeability and osmotic barrier into models of trans-vascular fluid exchange in whole organs. We describe changes in the colloid osmotic pressure (COP) difference for plasma proteins across the glycocalyx after an increase or decrease in capillary pressure. The composition of the fluid under the glycocalyx changes in step with capillary pressure whereas the composition of the interstitial fluid takes many hours to adjust to a change in vascular pressure. We use models where the fluid under the glycocalyx mixes with sub-compartments of the interstitial fluid (ISF) whose volumes are defined from the ultrastructure of the inter-endothelial cleft and the histology of the tissue surrounding the capillaries. The initial protein composition in the sub-compartments is that during steady state filtration in the presence of a large pore pathway in parallel with the "small pore" glycocalyx pathway. Changes in the composition depend on the volume of the sub-compartment and the balance of convective and diffusive transport into and out of each sub-compartment. In skeletal muscle the simplest model assumes that the fluid under the glycocalyx mixes directly with a tissue sub-compartment with a volume less than 20% of the total skeletal muscle interstitial fluid volume. The model places limits on trans-vascular flows during transient filtration and reabsorption over periods of 30-60 min. The key assumption in this model is compromised when the resistance to diffusion between the base of the glycocalyx and the tissue sub-compartment accounts for more than 1% of the total resistance to diffusion across the endothelial barrier. It is well established that, in the steady state, there can be no reabsorption in tissue such as skeletal muscle. Our approach extends this idea to demonstrate that transient changes in vascular pressure favoring initial reabsorption from the interstitial fluid of skeletal muscle result in much less fluid exchange than is commonly assumed. Our approach should enable critical evaluations of the empirical models of trans-vascular fluid exchange being used in the clinic that do not account for the hydrostatic and COPs across the glycocalyx.

Hypoalbuminemia in COVID-19: assessing the hypothesis for underlying pulmonary capillary leakage

BACKGROUND

Since the first observations of patients with COVID-19, significant hypoalbuminaemia was detected. Its causes have not been investigated yet.

OBJECTIVE

We hypothesized that pulmonary capillary leakage affects the severity of respiratory failure, causing a shift of fluids and proteins through the epithelial-endothelial barrier.

METHODS

One hundred seventy-four COVID-19 patients with respiratory symptoms, 92 admitted to the intermediate medicine ward (IMW) and 82 to the intensive care unit (ICU) at Luigi Sacco Hospital in Milan, were studied.

RESULTS

Baseline characteristics at admission were considered. Proteins, interleukin 8 (IL-8) and interleukin 10 (IL-10) in bronchoalveolar lavage fluid (BALF) were analysed in 26 ICU patients. In addition, ten autopsy ultrastructural lung studies were performed in patients with COVID-19 and compared with postmortem findings in a control group (bacterial pneumonia-ARDS and H1N1-ARDS). ICU patients had lower serum albumin than IMW patients [20 (18-23) vs 28 (24-33) g L^-1 , P < 0.001]. Serum albumin was lower in more compromised groups (lower PaO₂ -to-FiO₂ ratio and worst chest X-ray findings) and was associated with 30 days of probability of survival. Protein concentration was correlated with IL-8 and IL-10 levels in BALF. Electron microscopy examinations of eight out of ten COVID-19 lung tissues showed loosening of junctional complexes, quantitatively more pronounced than in controls, and direct viral infection of type 2 pneumocytes and endothelial cells.

CONCLUSION

Hypoalbuminaemia may serve as severity marker of epithelial-endothelial damage in patients with COVID-19. There are clues that pulmonary capillary leak syndrome plays a key role in the pathogenesis of COVID-19 and might be a potential therapeutic target.

Drug-induced peripheral oedema: An aetiology-based review

Many drugs are responsible, through different mechanisms, for peripheral oedema. Severity is highly variable, ranging from slight oedema of the lower limbs to anasarca pictures as in the capillary leak syndrome. Although most often noninflammatory and bilateral, some drugs are associated with peripheral oedema that is readily erythematous (eg, pemetrexed) or unilateral (eg, sirolimus). Thus, drug-induced peripheral oedema is underrecognized and misdiagnosed, frequently leading to a prescribing cascade. Four main mechanisms are involved, namely precapillary arteriolar vasodilation (vasodilatory oedema), sodium/water retention (renal oedema), lymphatic insufficiency (lymphedema) and increased capillary permeability (permeability oedema). The underlying mechanism has significant impact on treatment efficacy. The purpose of this review is to provide a comprehensive analysis of the main causative drugs by illustrating each pathophysiological mechanism and their management through an example of a drug.

The endothelial glycocalyx: Barrier functions versus red cell hemodynamics: A model of steady state ultrafiltration through a bi-layer formed by a porous outer layer and more selective membrane-associated inner layer

BACKGROUND

Ultrastructural investigations of the endothelial glycocalyx reveal a layer adjacent to the cell surface with a structure consistent with the primary  ultrafilter of vascular walls. Theory predicts this layer can be no greater than 200-300 nm thick, a result  to be reconciled with observations that red cells and large macromolecules are excluded  from a region 1 micrometer or more from the cell membrane.

OBJECTIVE

To determine whether this apparent inconsistency might be accounted for by a model of steady state water and protein transport through a glycocalyx bi-layer formed by a porous outer layer in series with a more selective inner layer.

METHODS

Expressions for coupled water and albumin fluxes through the two layers were used to describe steady state ultra-filtration though the bi-layer model.

RESULTS

Albumin accumulates at the interface between the porous layer and the selective inner layer. The osmotic pressure of accumulated albumin significantly modifies the observed permeability properties of the microvessel wall by an effective unstirred layer effect.

CONCLUSIONS

The model places significant constraints on the outer layer permeability properties . The only outer layer properties that are consistent with measured steady state filtration rates and models of red cell flux through microvessels are an albumin permeability coefficient and hydraulic conductivity more than an order of magnitude larger than the those of the inner layer.

BMX Represses Thrombin-PAR1-Mediated Endothelial Permeability and Vascular Leakage During Early Sepsis

RATIONALE

BMX (bone marrow kinase on the X chromosome) is highly expressed in the arterial endothelium from the embryonic stage to the adult stage in mice. It is also expressed in microvessels and the lymphatics in response to pathological stimuli. However, its role in endothelial permeability and sepsis remains unknown.

OBJECTIVE

We aimed to delineate the function of BMX in thrombin-mediated endothelial permeability and the vascular leakage that occurs with sepsis in cecal ligation and puncture models.

METHODS AND RESULTS

The cecal ligation and puncture model was applied to WT (wild type) and BMX-KO (BMX global knockout) mice to induce sepsis. Meanwhile, the electric cell-substrate impedance sensing assay was used to detect transendothelial electrical resistance in vitro and, the modified Miles assay was used to evaluate vascular leakage in vivo. We showed that BMX loss caused lung injury and inflammation in early cecal ligation and puncture-induced sepsis. Disruption of BMX increased thrombin-mediated permeability in mice and cultured endothelial cells by 2- to 3-fold. The expression of BMX in macrophages, neutrophils, platelets, and lung epithelial cells was undetectable compared with that in endothelial cells, indicating that endothelium dysfunction, rather than leukocyte and platelet dysfunction, was involved in vascular permeability and sepsis. Mechanistically, biochemical and cellular analyses demonstrated that BMX specifically repressed thrombin-PAR1 (protease-activated receptor-1) signaling in endothelial cells by directly phosphorylating PAR1 and promoting its internalization and deactivation. Importantly, pretreatment with the selective PAR1 antagonist SCH79797 rescued BMX loss-mediated endothelial permeability and pulmonary leakage in early cecal ligation and puncture-induced sepsis.

CONCLUSIONS

Acting as a negative regulator of PAR1, BMX promotes PAR1 internalization and signal inactivation through PAR1 phosphorylation. Moreover, BMX-mediated PAR1 internalization attenuates endothelial permeability to protect vascular leakage during early sepsis.

BMP9 (Bone Morphogenetic Protein-9)/Alk1 (Activin-Like Kinase Receptor Type I) Signaling Prevents Hyperglycemia-Induced Vascular Permeability

Objective- Diabetic macular edema is a major cause of visual impairment. It is caused by blood-retinal barrier breakdown that leads to vascular hyperpermeability. Current therapeutic approaches consist of retinal photocoagulation or targeting VEGF (vascular endothelial growth factor) to limit vascular leakage. However, long-term intravitreal use of anti-VEGFs is associated with potential safety issues, and the identification of alternative regulators of vascular permeability may provide safer therapeutic options. The vascular specific BMP (bone morphogenetic protein) receptor ALK1 (activin-like kinase receptor type I) and its circulating ligand BMP9 have been shown to be potent vascular quiescence factors, but their role in the context of microvascular permeability associated with hyperglycemia has not been evaluated. Approach and Results- We investigated Alk1 signaling in hyperglycemic endothelial cells and assessed whether BMP9/Alk1 signaling could modulate vascular permeability. We show that high glucose concentrations impair Alk1 signaling, both in cultured endothelial cells and in a streptozotocin model of mouse diabetes mellitus. We observed that Alk1 signaling participates in the maintenance of vascular barrier function, as Alk1 haploinsufficiency worsens the vascular leakage observed in diabetic mice. Conversely, sustained delivery of BMP9 by adenoviral vectors significantly decreased the loss of retinal barrier function in diabetic mice. Mechanistically, we demonstrate that Alk1 signaling prevents VEGF-induced phosphorylation of VE-cadherin and induces the expression of occludin, thus strengthening vascular barrier functions. Conclusions- From these data, we suggest that by preventing retinal vascular permeability, BMP9 could serve as a novel therapeutic agent for diabetic macular edema.

Epac1-/- mice have elevated baseline permeability and do not respond to histamine as measured with dynamic contrast-enhanced magnetic resonance imaging with contrast agents of different molecular weights

Aim

Epac1^−/− mice, but not Epac2^−/− mice have elevated baseline permeability to albumin. This study extends the investigations of how Epac‐dependent pathways modulate transvascular exchange in response to the classical inflammatory agent histamine. It also evaluates the limitations of models of blood‐to‐tissue exchange in transgenic mice in DCE‐MRI measurements.

Methods

We measured DCE‐MRI signal intensity in masseter muscle of wt and Epac1^−/− mice with established approaches from capillary physiology to determine how changes in blood flow and vascular permeability contribute to overall changes of microvascular flux. We used two tracers, the high molecular weight tracer (Gadomer‐17, MW 17 kDa, apparent MW 30‐35 kDa) is expected to be primarily limited by diffusion and therefore less dependent on changes in blood flow and the low molecular weight tracer (Dotarem (MW 0.56 kDa) whose transvascular exchange is determined by both blood flow and permeability. Paired experiments in each animal combined with analytical methods provided an internally consistent description of microvascular transport.

Results

Epac1^−/− mice had elevated baseline permeability relative to wt control mice for Dotarem and Gadomer‐17. In contrast to wt mice, Epac1^−/− mice failed to increase transvascular permeability in response to histamine. Dotarem underestimated blood flow and vascular volume and Gadomer‐17 has limited sensitivity in extravascular accumulation.

Conclusion

The study suggests that the normal barrier loosening effect of histamine in venular microvessels do not function when the normal barrier tightening effect of Epac1 is already compromised. The study also demonstrated that the numerical analysis of DCE‐MRI data with tracers of different molecular weight has significant limitations.

Hypoalbuminemia: Pathogenesis and Clinical Significance

Hypoalbuminemia is associated with inflammation. Despite being addressed repeatedly in the literature, there is still confusion regarding its pathogenesis and clinical significance. Inflammation increases capillary permeability and escape of serum albumin, leading to expansion of interstitial space and increasing the distribution volume of albumin. The half‐life of albumin has been shown to shorten, decreasing total albumin mass. These 2 factors lead to hypoalbuminemia despite increased fractional synthesis rates in plasma. Hypoalbuminemia, therefore, results from and reflects the inflammatory state, which interferes with adequate responses to events like surgery or chemotherapy, and is associated with poor quality of life and reduced longevity. Increasing or decreasing serum albumin levels are adequate indicators, respectively, of improvement or deterioration of the clinical state. In the interstitium, albumin acts as the main extracellular scavenger, antioxidative agent, and as supplier of amino acids for cell and matrix synthesis. Albumin infusion has not been shown to diminish fluid requirements, infection rates, and mortality in the intensive care unit, which may imply that there is no body deficit or that the quality of albumin “from the shelf” is unsuitable to play scavenging and antioxidative roles. Management of hypoalbuminaemia should be based on correcting the causes of ongoing inflammation rather than infusion of albumin. After the age of 30 years, muscle mass and function slowly decrease, but this loss is accelerated by comorbidity and associated with decreasing serum albumin levels. Nutrition support cannot fully prevent, but slows down, this chain of events, especially when combined with physical exercise.

Shutter-speed dynamic contrast-enhanced MRI: Is it fit for purpose?

PURPOSE

To test the ability of shutter-speed dynamic contrast-enhanced (DCE) MRI to estimate water exchange (WX) using simulations and assess its performance in clinical case studies of malignant and benign breast tumors.

METHODS

Data were simulated using a 1-compartment tracer kinetic (TK) model combined with a 2-pool WX model (2PX) and with a 2-compartment TK model. Typical DCE-MRI acquisition parameters were used with both WX-sensitive (8°) and -insensitive (25°) flip angles. Clinical data were obtained from patients with malignant and benign breast tumors. Data were fitted using a 2-compartment TK model and a 1-compartment TK model combined with 4 WX models: fast exchange limit (FXL), no exchange, 2PX, and shutter-speed.

RESULTS

Fits to the 1-compartment simulated data were excellent, but estimates of WX obtained using the 2PX and shutter-speed models were poor. One-compartment TK model fits to the clinical malignant tumor data were bad, except for the shutter-speed model. However, that overestimated TK parameters compared to the best-fit 2-compartment TK model, which predicted a significant blood volume and leaky capillaries (1 tracer compartment is insufficient, 2 are necessary). All models produced excellent fits to the clinical benign tumor data with little variation between parameter estimates (1 tracer compartment is sufficient).

CONCLUSION

The 2PX and shutter-speed models were unable to estimate WX from the DCE-MRI data. A good fit to malignant tumor data using the shutter-speed model was not explained by WX, but the choice of an inappropriate TK model leading to distorted parameter estimates.

Endothelial cAMP deactivates ischemia-reperfusion-induced microvascular hyperpermeability via Rap1-mediated mechanisms

Approaches to reduce excessive edema due to the microvascular hyperpermeability that occurs during ischemia-reperfusion (I/R) are needed to prevent muscle compartment syndrome. We tested the hypothesis that cAMP-activated mechanisms actively restore barrier integrity in postischemic striated muscle. We found, using I/R in intact muscles and hypoxia-reoxygenation (H/R, an I/R mimic) in human microvascular endothelial cells (HMVECs), that hyperpermeability can be deactivated by increasing cAMP levels through application of forskolin. This effect was seen whether or not the hyperpermeability was accompanied by increased mRNA expression of VEGF, which occurred only after 4 h of ischemia. We found that cAMP increases in HMVECs after H/R, suggesting that cAMP-mediated restoration of barrier function is a physiological mechanism. We explored the mechanisms underlying this effect of cAMP. We found that exchange protein activated by cAMP 1 (Epac1), a downstream effector of cAMP that stimulates Rap1 to enhance cell adhesion, was activated only at or after reoxygenation. Thus, when Rap1 was depleted by small interfering RNA, H/R-induced hyperpermeability persisted even when forskolin was applied. We demonstrate that 1) VEGF mRNA expression is not involved in hyperpermeability after brief ischemia, 2) elevation of cAMP concentration at reperfusion deactivates hyperpermeability, and 3) cAMP activates the Epac1-Rap1 pathway to restore normal microvascular permeability. Our data support the novel concepts that 1) different hyperpermeability mechanisms operate after brief and prolonged ischemia and 2) cAMP concentration elevation during reperfusion contributes to deactivation of I/R-induced hyperpermeability through the Epac-Rap1 pathway. Endothelial cAMP management at reperfusion may be therapeutic in I/R injury.NEW & NOTEWORTHY Here, we demonstrate that 1) stimulation of cAMP production deactivates ischemia-reperfusion-induced hyperpermeability in muscle and endothelial cells; 2) VEGF mRNA expression is not enhanced by brief ischemia, suggesting that VEGF mechanisms do not activate immediate postischemic hyperpermeability; and 3) deactivation mechanisms operate via cAMP-exchange protein activated by cAMP 1-Rap1 to restore integrity of the endothelial barrier.

Capillary leak syndrome: etiologies, pathophysiology, and management

In various human diseases, an increase in capillary permeability to proteins leads to the loss of protein-rich fluid from the intravascular to the interstitial space. Although sepsis is the disease most commonly associated with this phenomenon, many other diseases can lead to a "sepsis-like" syndrome with manifestations of diffuse pitting edema, exudative serous cavity effusions, noncardiogenic pulmonary edema, hypotension, and, in some cases, hypovolemic shock with multiple-organ failure. The term capillary leak syndrome has been used to describe this constellation of disease manifestations associated with an increased capillary permeability to proteins. Diseases other than sepsis that can result in capillary leak syndrome include the idiopathic systemic capillary leak syndrome or Clarkson's disease, engraftment syndrome, differentiation syndrome, the ovarian hyperstimulation syndrome, hemophagocytic lymphohistiocytosis, viral hemorrhagic fevers, autoimmune diseases, snakebite envenomation, and ricin poisoning. Drugs including some interleukins, some monoclonal antibodies, and gemcitabine can also cause capillary leak syndrome. Acute kidney injury is commonly seen in all of these diseases. In addition to hypotension, cytokines are likely to be important in the pathophysiology of acute kidney injury in capillary leak syndrome. Fluid management is a critical part of the treatment of capillary leak syndrome; hypovolemia and hypotension can cause organ injury, whereas capillary leakage of administered fluid can worsen organ edema leading to progressive organ injury. The purpose of this article is to discuss the diseases other than sepsis that produce capillary leak and review their collective pathophysiology and treatment.

Increased Vascular Permeability Measured With an Albumin-Binding Magnetic Resonance Contrast Agent Is a Surrogate Marker of Rupture-Prone Atherosclerotic Plaque

BACKGROUND

Compromised structural integrity of the endothelium and higher microvessel density increase vascular permeability. We investigated whether vascular permeability measured in vivo by magnetic resonance imaging using the albumin-binding contrast agent, gadofosveset, is a surrogate marker of rupture-prone atherosclerotic plaque in a rabbit model.

METHODS AND RESULTS

New Zealand white rabbits (n=10) were rendered atherosclerotic by cholesterol-diet and endothelial denudation. Plaque rupture was triggered with Russell's viper venom and histamine. Animals were imaged pre-triggering, at 3 and 12 weeks, to quantify plaque area, vascular permeability, vasodilation, and stiffness and post-triggering to identify thrombus. Plaques identified on the pretrigger scans were classified as stable or rupture-prone based on the absence or presence of thrombus on the corresponding post-trigger magnetic resonance imaging, respectively. All rabbits had developed atherosclerosis, and 60% had ruptured plaques. Rupture-prone plaques had higher vessel wall relaxation rate (R1; 2.30±0.5 versus 1.86±0.3 s-1; P<0.001), measured 30 minutes after gadofosveset administration, and higher R1/plaque area ratio (0.70±0.06 versus 0.47±0.02, P= 0.01) compared with stable plaque at 12 weeks. Rupture-prone plaques had higher percent change in R1 between the 3 and 12 weeks compared with stable plaque (50.80±7.2% versus 14.22±2.2%; P<0.001). Immunohistochemistry revealed increased vessel wall albumin and microvessel density in diseased aortas and especially in ruptured plaque. Electron microscopy showed lack of structural integrity in both luminal and microvascular endothelium in diseased vessels. Functionally, the intrinsic vasodilation of the vessel wall decreased at 12 weeks compared with 3 weeks (18.60±1.0% versus 23.43±0.8%; P<0.001) and in rupture-prone compared with stable lesions (16.40±2.0% versus 21.63±1.2%; P<0.001). Arterial stiffness increased at 12 weeks compared with 3 weeks (5.00±0.1 versus 2.53±0.2 m/s; P<0.001) both in animals with stable and rupture-prone lesions.

CONCLUSIONS

T1 mapping using an albumin-binding contrast agent (gadofosveset) could quantify the changes in vascular permeability associated with atherosclerosis progression and rupture-prone plaques.

Nitric oxide synthase inhibition causes acute increases in glomerular permeability in vivo, dependent upon reactive oxygen species

There is increasing evidence that the permeability of the glomerular filtration barrier (GFB) is partly regulated by a balance between the bioavailability of nitric oxide (NO) and that of reactive oxygen species (ROS). It has been postulated that normal or moderately elevated NO levels protect the GFB from permeability increases, whereas ROS, through reducing the bioavailability of NO, have the opposite effect. We tested the tentative antagonism between NO and ROS on glomerular permeability in anaesthetized Wistar rats, in which the left ureter was cannulated for urine collection while simultaneously blood access was achieved. Rats were systemically infused with either l-NAME or l-NAME together with the superoxide scavenger Tempol, or together with l-arginine or the NO-donor DEA-NONOate, or the cGMP agonist 8-bromo-cGMP. To measure glomerular sieving coefficients (theta, θ) to Ficoll, rats were infused with FITC-Ficoll 70/400 (mol/radius 10-80 Å). Plasma and urine samples were analyzed by high-performance size-exclusion chromatography (HPSEC) for determination of θ for Ficoll repeatedly during up to 2 h. l-NAME increased θ for Ficoll_70Å from 2.27 ± 1.30 × 10^-5 to 8.46 ± 2.06 × 10^-5 (n = 6, P < 0.001) in 15 min. Tempol abrogated these increases in glomerular permeability and an inhibition was also observed with l-arginine and with 8-bromo-cGMP. In conclusion, acute NO synthase inhibition in vivo by l-NAME caused rapid increases in glomerular permeability, which could be reversed by either an ROS antagonist or by activating the guanylyl cyclase-cGMP pathway. The data strongly suggest a protective effect of NO in maintaining normal glomerular permeability in vivo.

Perivascular Macrophages Limit Permeability

OBJECTIVE

Perivascular cells, including pericytes, macrophages, smooth muscle cells, and other specialized cell types, like podocytes, participate in various aspects of vascular function. However, aside from the well-established roles of smooth muscle cells and pericytes, the contributions of other vascular-associated cells are poorly understood. Our goal was to ascertain the function of perivascular macrophages in adult tissues under nonpathological conditions.

APPROACH AND RESULTS

We combined confocal microscopy, in vivo cell depletion, and in vitro assays to investigate the contribution of perivascular macrophages to vascular function. We found that resident perivascular macrophages are associated with capillaries at a frequency similar to that of pericytes. Macrophage depletion using either clodronate liposomes or antibodies unexpectedly resulted in hyperpermeability. This effect could be rescued when M2-like macrophages, but not M1-like macrophages or dendritic cells, were reconstituted in vivo, suggesting subtype-specific roles for macrophages in the regulation of vascular permeability. Furthermore, we found that permeability-promoting agents elicit motility and eventual dissociation of macrophages from the vasculature. Finally, in vitro assays showed that M2-like macrophages attenuate the phosphorylation of VE-cadherin upon exposure to permeability-promoting agents.

CONCLUSIONS

This study points to a direct contribution of macrophages to vessel barrier integrity and provides evidence that heterotypic cell interactions with the endothelium, in addition to those of pericytes, control vascular permeability.

Regulators of endothelial integrity as severity predictors in dengue

INTRODUCTION

Dengue is currently among the mosquito-borne diseases of greatest global impact. The clinical course of the disease can be unpredictable, so proper handling in its early stages is critical to ensure optimal outcomes. 

OBJECTIVE

To evaluate serum regulators of endothelial integrity (VEGF, sICAM-1, sEndoglina, Ang-1, and Ang-2) as predictive markers of dengue severity. 

MATERIALS AND METHODS

We conducted a case-control study nested in an appropriate cohort. Endothelial regulator levels were first measured by ELISA, after which analysis was performed using logistic regression of clinical and regulatory variables, with severity as an output variable. A possible severity prediction model, based on the variables of interest and output, was defined using the best area under the ROC curve. 

RESULTS

The median subject age was 24 years. Severe cases were associated with Ang-2 serum levels of ≥1,490 ng/ml (OR=3.1; p=0.015). Serum levels of Ang-2 (≥1,490 ng/ml) contributed to the severity prediction model, as did a 0.73 area under the ROC curve, together with the variables rash, impaired consciousness and abdominal pain, with an OR of 3.2 (CI 95%: 1.16 to 8.9; p=0.024). 

CONCLUSION

The endothelial regulator Ang-2 could be a predictor of severity in dengue.

Evidence That Cingulin Regulates Endothelial Barrier Function In Vitro and In Vivo

OBJECTIVE

Cingulin is a cytoplasmic component of tight junctions. Although modulation of cingulin levels in cultured epithelial model systems has no significant effect on barrier function, evidence from cingulin knockout mice suggests that cingulin may be involved in the regulation of the behavior of epithelial or endothelial cells. Here, we investigate the role of cingulin in the barrier function of endothelial cells.

APPROACH AND RESULTS

We show that cingulin is expressed in human endothelial cells of the skin, brain, and lung in vivo and in vitro. Endothelial cingulin colocalizes and coimmunoprecipitates with the tight junction proteins zonula occludens-1 and guanine nucleotide exchange factor-H1. Cingulin overexpression in human umbilical vein endothelial cell induces tight junction formation, increases transendothelial electric resistance, and strengthens barrier function for low and high molecular weight tracers. In contrast, cultured endothelial cells lacking cingulin are more permeable for low molecular weight tracers. In cingulin knockout mice, neurons of the area postrema and Purkinje cells show an increased uptake of small molecular weight tracers indicating decreased barrier function at these sites.

CONCLUSIONS

We demonstrate that cingulin participates in the modulation of endothelial barrier function both in human cultured cells in vitro and in mouse brains in vivo. Understanding the role of cingulin in maintaining tight barriers in endothelia may allow developing new strategies for the treatment of vascular leak syndromes.

Ultrastructural changes in the pulmonary mechanical barriers in a rat model of severe acute pancreatitis-associated acute lung injury

This study examined the ultrastructural changes in the pulmonary mechanical barriers in a rat model of severe acute pancreatitis (SAP)-associated acute lung injury (ALI). Animals were randomized into the SAP group (n = 60) and the control group (n = 60). SAP was induced by retrograde injection of 5% taurocholic acid into the biliopancreatic duct. The morphological abnormalities assessed by histology and the lung wet/dry weight ratio and the ultrastructural abnormalities assessed by transmission electron microscope and scanning electron microscope examinations plus lanthanum nitrate tracing were compared between the two groups at 6, 12, and 24 h post-SAP induction (n = 10/group/time point). The SAP group had significantly greater extravascular effusion than the control group at each time point as assessed by the lung wet/dry weight ratio (p < .001). The severity of the tissue damage increased in the lung and pancreas over time in the SAP group (all p < .001). In the SAP group, ultrastructural damages to the endothelial, epithelial, and pleural barriers were apparent and the damages to the endothelial barrier were detected earlier than the other two barriers, suggesting its fundamental role in preventing the further development of SAP-associated ALI. Moreover, the ultrastructural abnormalities were detected earlier than symptoms and morphological changes. The ultrastructural damages in the endothelial, epithelial, and pleural barriers occurred in the early stage of SAP. The endothelial barrier is likely to be the first line to prevent the further development in this rat model of SAP-associated ALI.

Inorganic polyphosphate elicits pro-inflammatory responses through activation of the mammalian target of rapamycin complexes 1 and 2 in vascular endothelial cells

BACKGROUND

Inorganic polyphosphate (polyP) elicits pro-inflammatory signaling responses in endothelial cells through interaction with two receptors, RAGE and P2Y1 . It is known that polyP activates mTOR signaling in breast cancer cells.

OBJECTIVES

The objective of this study is to understand the mechanism of the polyP-mediated signaling pathway in endothelial cells and to determine whether polyP exerts its pro-inflammatory effect through activation of mTOR.

METHODS

mTOR activation by polyP or platelet releasates in cellular and animal models was monitored in the absence and presence of pharmacological inhibitors and/or siRNA knockdown of specific signaling molecules.

RESULTS

PolyP effectively induced phosphorylation of mTOR complex 1 (mTORC1) substrate, p70S6K, in endothelial cells by an AKT-dependent but ERK-independent mechanism. The siRNA knockdown of both RAGE and P2Y1 or specific inhibitors of the PI3K/PLC/PKC/Ca(2+) signaling axis inhibited polyP-mediated p70S6K phosphorylation. Moreover, either rapamycin or siRNA knockdown of raptor (mTORC1-specific component) abrogated polyP-mediated phosphorylation of p70S6K. By contrast, the siRNA knockdown of rictor (mTOR complex 2-specific component) but not raptor eliminated the barrier-disruptive effect of polyP. Specific NF-κB inhibitors abrogated polyP-mediated phosphorylation of p70S6K and rapamycin suppressed polyP-induced activation of NF-κB. Finally, specific inhibitors of the mTOR signaling network eliminated polyP-mediated vascular leakage and leukocyte recruitment in animal models.

CONCLUSIONS

PolyP, through interaction with RAGE and P2Y1 , activates both the mTORC1 and mTORC2 signaling network. Both the pro-inflammatory and mTOR signaling functions of polyP are linked.

Factor VIIa binding to endothelial cell protein C receptor protects vascular barrier integrity in vivo

BACKGROUND

Recent studies have shown that factor VIIa binds to endothelial cell protein C receptor(EPCR), a cellular receptor for protein C and activated protein C. At present, the physiologic significance of FVIIa interaction with EPCR in vivo remains unclear.

OBJECTIVE

To investigate whether exogenously administered FVIIa, by binding to EPCR, induces a barrier protective effect in vivo.

METHODS

Lipopolysaccharide(LPS)-induced vascular leakage in the lung and kidney,and vascular endothelial growth factor (VEGF)-induced vascular leakage in the skin, were used to evaluate the FVIIa-induced barrier protective effect. Wild-type, EPCR-deficient, EPCR-overexpressing and hemophilia A mice were used in the studies.

RESULTS

Administration ofFVIIa reduced LPS-induced vascular leakage in the lung and kidney; the FVIIa-induced barrier protective effect was attenuated in EPCR-deficient mice. The extent of VEGF-induced vascular leakage in the skin was highly dependent on EPCR expression levels. Therapeutic concentrations of FVIIa attenuated VEGF-induced vascular leakage in control mice but not in EPCR-deficient mice.Blockade of FVIIa binding to EPCR with a blocking mAb completely attenuated the FVIIa-induced barrier protective effect. Similarly, administration of protease activated receptor 1 antagonist blocked the FVIIa induced barrier protective effect. Hemophilic mice showed increased vascular permeability, and administration of therapeutic concentrations of FVIIa improved barrier integrity in these mice.

CONCLUSIONS

This is the first study to demonstrate that FVIIa binding to EPCR leads to a barrier protective effect in vivo. This finding may have clinical relevance, as it indicates additional advantages of using FVIIa in treating hemophilic patients.

Combination treatment with N-acetyl-seryl-aspartyl-lysyl-proline and tissue plasminogen activator provides potent neuroprotection in rats after stroke

BACKGROUND AND PURPOSE

N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP), an endogenously produced circulating peptide in humans and rodents, exerts anti-inflammatory and cardioprotective activities in various cardiovascular diseases.

METHODS

The present study evaluated the neuroprotective effect of AcSDKP alone and in combination with thrombolytic therapy in a rat model of embolic focal cerebral ischemia.

RESULTS

We found that treatment with AcSDKP alone at 1 hour or the combination treatment with AcSDKP and tissue plasminogen activator (tPA) at 4 hours after stroke onset substantially increased AcSDKP levels in plasma and cerebrospinal fluid and robustly reduced infarct volume and neurological deficits, without increasing the incidence of brain hemorrhage compared with ischemic rats treated with saline, AcSDKP alone at 4 hours, and tPA alone at 4 hours. Moreover, the combination treatment considerably reduced the density of nuclear transcription factor-κB (NF-κB), transforming growth factor β (TGF-β), and plasminogen activator inhibitor-1 (PAI-1) positive cerebral blood vessels in the ischemic brain, all of which were associated with reduced microvascular fibrin extravasation and platelet accumulation compared with tPA monotherapy. In vitro, AcSDKP blocked fibrin-elevated TGF-β1, PAI-1, and NF-κB proteins in primary human brain microvascular endothelial cells.

CONCLUSIONS

Our data indicate that AcSDKP passes the blood-brain barrier, and that treatment of acute stroke with AcSDKP either alone at 1 hour or in combination with tPA at 4 hours of the onset of stroke is effective to reduce ischemic cell damage in a rat model of embolic stroke. Inactivation of TGF-β and NF-κB signaling by AcSDKP in the neurovascular unit may underlie the neuroprotective effect of AcSDKP.

Mice with targeted inactivation of ppap2b in endothelial and hematopoietic cells display enhanced vascular inflammation and permeability

OBJECTIVE

Lipid phosphate phosphatase 3 (LPP3), encoded by the PPAP2B gene, is an integral membrane enzyme that dephosphorylates, and thereby terminates, the G-protein-coupled receptor-mediated signaling actions of lysophosphatidic acid (LPA) and sphingosine-1-phosphate. LPP3 is essential for normal vascular development in mice, and a common PPAP2B polymorphism is associated with increased risk of coronary artery disease in humans. Herein, we investigate the function of endothelial LPP3 to understand its role in the development and human disease.

APPROACH AND RESULTS

We developed mouse models with selective LPP3 deficiency in endothelial and hematopoietic cells. Tyrosine kinase Tek promoter-mediated inactivation of Ppap2b resulted in embryonic lethality because of vascular defects. LPP3 deficiency in adult mice, achieved using a tamoxifen-inducible Cre transgene under the control of the Tyrosine kinase Tek promoter, enhanced local and systemic inflammatory responses. Endothelial, but not hematopoietic, cell LPP3 deficiency led to significant increases in vascular permeability at baseline and enhanced sensitivity to inflammation-induced vascular leak. Endothelial barrier function was restored by pharmacological or genetic inhibition of either LPA production by the circulating lysophospholipase D autotaxin or of G-protein-coupled receptor-dependent LPA signaling.

CONCLUSIONS

Our results identify a role for the autotaxin/LPA-signaling nexus as a mediator of endothelial permeability in inflammation and demonstrate that LPP3 limits these effects. These findings have implications for therapeutic targets to maintain vascular barrier function in inflammatory states.

Extracellular fetal hemoglobin induces increases in glomerular permeability: inhibition with α1-microglobulin and tempol

Extracellular fetal hemoglobin (HbF) and adult hemoglobin (HbA) are proinflammatory and generate ROS. Increased plasma levels of extracellular HbF have recently been reported to occur in early preeclampsia. α1-Microglobulin (A1M) is a physiological heme-binding protein and radical scavenger that has been shown to counteract vascular permeability increases induced by HbA in the perfused placenta. The present study was performed to investigate whether HbF and HbA will increase glomerular permeability in vivo and to test whether A1M and tempol, a ROS scavenger, can prevent their effects. Anesthetized Wistar rats were continuously infused intravenously with either HbA, HbF, or cyano-inactivated HbF together with FITC-Ficoll-70/400, inulin, and (51)Cr-labeled EDTA for 2 h. Plasma samples and urine samples (left ureter) were taken repeatedly and analyzed by high-performance size exclusion chromatography to assess glomerular sieving coefficients for Ficoll of radius 10-80 Å. In separate experiments, A1M or tempol was given before and during Hb infusions. Extracellular HbF caused rapid, transient increases in glomerular permeability to large Ficoll molecules (50-80Å), contrary to the effects of HbA and cyano-inactivated HbF. For HbF, glomerular sieving coefficients for Ficoll of radius 60Å increased from 3.85 ± 0.85 × 10(-5) to 2.60 ± 0.96 × 10(-4) at 15 min, changes that were abrogated by tempol and reduced by A1M. In conclusion, our data demonstrate that extracellular HbF, infused systemically, can acutely increase glomerular permeability through inducing oxidative stress.

Cooperative interaction of trp melastatin channel transient receptor potential (TRPM2) with its splice variant TRPM2 short variant is essential for endothelial cell apoptosis

RATIONALE

Oxidants generated by activated endothelial cells are known to induce apoptosis, a pathogenic feature of vascular injury and inflammation from multiple pathogeneses. The melastatin-family transient receptor potential 2 (TRPM2) channel is an oxidant-sensitive Ca2+ permeable channel implicated in mediating apoptosis; however, the mechanisms of gating of the supranormal Ca2+ influx required for initiating of apoptosis are not understood.

OBJECTIVE

Here, we addressed the role of TRPM2 and its interaction with the short splice variant TRPM2 short variant (TRPM2-S) in mediating the Ca2+ entry burst required for induction of endothelial cell apoptosis.

METHODS AND RESULTS

We observed that TRPM2-S was basally associated with TRPM2 in the endothelial plasmalemma, and this interaction functioned to suppress TRPM2-dependent Ca2+ gating constitutively. Reactive oxygen species production in endothelial cells or directly applying reactive oxygen species induced protein kinase C-α activation and phosphorylation of TRPM2 at Ser 39. This in turn stimulated a large entry of Ca2+ and activated the apoptosis pathway. A similar TRPM2-dependent endothelial apoptosis mechanism was seen in intact vessels. The protein kinase C-α-activated phosphoswitch opened the TRPM2 channel to allow large Ca2+ influx by releasing TRPM2-S inhibition of TRPM2, which in turn activated caspase-3 and cleaved the caspase substrate poly(ADP-ribose) polymerase.

CONCLUSIONS

Here, we describe a fundamental mechanism by which activation of the trp superfamily TRPM2 channel induces apoptosis of endothelial cells. The signaling mechanism involves reactive oxygen species-induced protein kinase C-α activation resulting in phosphorylation of TRPM2-S that allows enhanced TRPM2-mediated gating of Ca2+ and activation of the apoptosis program. Strategies aimed at preventing the uncoupling of TRPM2-S from TRPM2 and subsequent Ca2+ gating during oxidative stress may mitigate endothelial apoptosis and its consequences in mediating vascular injury and inflammation.

Inhibition of vascular response in inflammation by crude aqueous extract of the root bark of zanthoxylum xanthoxyloides

BACKGROUND

The root bark extract of Zanthoxylum xanthozyloides is used in folklore medicine in Ghana and Nigeria to treat inflammation. A previous pharmacological study confirmed the anti-inflammatory activity of the extract.

OBJECTIVE

To study the effect of the extract on vascular response in inflammation.

METHOD

The extract was obtained by Soxhlet extraction and rotatory evaporation, followed by freeze-drying. Groups of rats (with carrageenin-induced paw inflammation) and mice (with xylene-induced pinna inflammation) were, respectively, assigned randomly to treatment groups. The animals were given three different treatments orally: 0.9% saline (control), the extract (400mg/kg and 800mg/kg for mice; 1000mg/kg, 2000 mg/kg, and 4000mg/kg for rats), and indomethacin (5mg/kg and 10mg/kg for mice; 10mg/kg, 20mg/kg, and 40mg/kg for rats). In another set of experiment, each treatment group received phenylephrine subcutaneously (30µg/kg for rats and 20µg/kg for mice) in addition to the specified treatment aforementioned. In both sets of experiments, each group of rats was rotated through the entire treatment groups such that each animal served as control as well as received all the treatments. Analysis of variance was used as the statistical test.

RESULTS

The extract and indomethacin both caused dose-dependent reduction in the carrageenin-induced increase in paw volume in rats and also reduced xylene-induced increase in blood flow in mice pinna arteries. Phenylephrine enhanced the decrease in capillary permeability and vasodilatation caused by low dose extract but not that caused by high dose extract or both low and high dose indomethacin.

CONCLUSION

The extract reduced vasodilatation and decreased capillary permeability in inflammation.

An integrative model of coupled water and solute exchange in the heart

Physiologists have devised many models for interpreting water and solute exchange data in whole organs, but the models have typically neglected key aspects of the underlying physiology to present the simplest possible model for a given experimental situation. We have developed a physiologically realistic model of microcirculatory water and solute exchange and applied it to diverse observations of water and solute exchange in the heart. Model simulations are consistent with the results of osmotic weight transient, tracer indicator dilution, and steady-state lymph sampling experiments. The key model features that permit this unification are the use of an axially distributed blood-tissue exchange region, inclusion of a lymphatic drain in the interstitium, and the independent computation of transcapillary solute and solvent fluxes through three different pathways.

Transient transcapillary exchange of water driven by osmotic forces in the heart

Osmotic transient responses in organ weight after changes in perfusate osmolarity have implied steric hindrance to small-molecule transcapillary exchange, but tracer methods do not. We obtained osmotic weight transient data in isolated, Ringer-perfused rabbit hearts with NaCl, urea, glucose, sucrose, raffinose, inulin, and albumin and analyzed the data with a new anatomically and physicochemically based model accounting for 1) transendothelial water flux, 2) two sizes of porous passages across the capillary wall, 3) axial intracapillary concentration gradients, and 4) water fluxes between myocytes and interstitium. During steady-state conditions approximately 28% of the transcapillary water flux going to form lymph was through the endothelial cell membranes [capillary hydraulic conductivity (Lp) = 1.8 +/- 0.6 x 10-8 cm. s-1. mmHg-1], presumably mainly through aquaporin channels. The interendothelial clefts (with Lp = 4.4 +/- 1.3 x 10-8 cm. s-1. mmHg-1) account for 67% of the water flux; clefts are so wide (equivalent pore radius was 7 +/- 0.2 nm, covering approximately 0.02% of the capillary surface area) that there is no apparent hindrance for molecules as large as raffinose. Infrequent large pores account for the remaining 5% of the flux. During osmotic transients due to 30 mM increases in concentrations of small solutes, the transendothelial water flux was in the opposite direction and almost 800 times as large and was entirely transendothelial because no solute gradient forms across the pores. During albumin transients, gradients persisted for long times because albumin does not permeate small pores; the water fluxes per milliosmolar osmolarity change were 200 times larger than steady-state water flux. The analysis completely reconciles data from osmotic transient, tracer dilution, and lymph sampling techniques.

A rise of the serum level of von Willebrand factor occurs before clinical manifestation of the severe form of ovarian hyperstimulation syndrome

BACKGROUND

Ovarian hyperstimulation syndrome (OHSS) appears to be caused by increased capillary permeability in the vascular endothelial cells. Such cells secrete excess amounts of von Willebrand factor (vWF), a large adhesive glycoprotein.

METHODS

We retrospectively evaluated the circulating levels of vWF and of vascular endothelial growth factor (VEGF) twice, on the days of oocyte retrieval and embryo transfer, in 46 women who developed early-onset OHSS.

RESULTS

Nineteen, 14, and 13 women developed mild, moderate, and severe OHSS, respectively. Inconsistent changes were observed in the VEGF during oocyte retrieval and embryo transfer. However, the net increase in serum vWF during that period showed an increase in absolute value at the time of embryo transfer that paralleled an increase in the severity of OHSS. That is, in mild OHSS, the serum vWF increased from 140 +/- 44 to 164 +/- 28%; in moderate OHSS, it increased from 113 +/- 47 to 186 +/- 22%; and in severe OHSS, it increased from 120 +/- 35 to 274 +/- 63%. All 9 women with a vWF level > 230% at embryo transfer developed severe OHSS, while 9 of 13 women with severe OHSS exhibited a vWF > 230% at embryo transfer.

CONCLUSION

The results suggest that a rise of the serum level of vWF occurs prior to clinical manifestation of OHSS in patients with severe OHSS but not in patients with mild OHSS.

Advection and diffusion of substances in biological tissues with complex vascular networks

For highly diffusive solutes the kinetics of blood-tissue exchange is only poorly represented by a model consisting of sets of independent parallel capillary-tissue units. We constructed a more realistic multicapillary network model conforming statistically to morphometric data. Flows through the tortuous paths in the network were calculated based on constant resistance per unit length throughout the network and the resulting advective intracapillary velocity field was used as a framework for describing the extravascular diffusion of a substance for which there is no barrier or permeability limitation. Simulated impulse responses from the system, analogous to tracer water outflow dilution curves, showed flow-limited behavior over a range of flows from about 2 to 5 ml min(-1) g(-1), as is observed for water in the heart in vivo. The present model serves as a reference standard against which to evaluate computationally simpler, less physically realistic models. The simulated outflow curves from the network model, like experimental water curves, were matched to outflow curves from the commonly used axially distributed models only by setting the capillary wall permeability-surface area (PS) to a value so artifactually low that it is incompatible with the experimental observations that transport is flow limited. However, simple axially distributed models with appropriately high PSs will fit water outflow dilution curves if axial diffusion coefficients are set at high enough values to account for enhanced dispersion due to the complex geometry of the capillary network. Without incorporating this enhanced dispersion, when applied to experimental curves over a range of flows, the simpler models give a false inference that there is recruitment of capillary surface area with increasing flow. Thus distributed models must account for diffusional as well as permeation processes to provide physiologically appropriate parameter estimates.

Facilitated diffusion and membrane permeation of fatty acid in albumin solutions

Facilitated transport is characteristic of most living systems, and usually involves a series of consecutive adjacent transfer regions, each having different transport properties. As a first step in the analysis of the multiregional problem, we consider in a single unstirred layer the facilitated diffusion of fatty acid (F) in albumin (A) solution under conditions of slow versus rapid association-dissociation, accounting for differing diffusivities of the albumin-fatty acid complex (AF). Diffusion gradients become established in an unstirred layer between a source of constant concentration of A, AF, and F in equilibrium, and a membrane permeable to F. The posited system does not reduce to a thin- or thick-layer approximation. The transient state is prolonged by slower on/off binding rates and by increasing the thickness of the unstirred layer. Solutions to transient and steady state depend upon the choice of boundary conditions, especially for thin regions. When there are two regions (each with its specific binding protein) separated by a permeable membrane, the steady-state fluxes and concentration profiles depend on the rates of association and dissociation reactions, on the diffusion coefficients, local consumption rates, and on the membrane permeability. Sensitivity analysis reveals the relative importance of these mechanisms.

Pathophysiology of fluid imbalance

Fluid imbalance can arise due to hypovolemia, normovolemia with maldistribution of fluid, and hypervolemia. Trauma is among the most frequent causes of hypovolemia, with its often profuse attendant blood loss. Another common cause is dehydration, which primarily entails loss of plasma rather than whole blood. The consequences of hypovolemia include reduction in circulating blood volume, lower venous return and, in profound cases, arterial hypotension. Myocardial failure may result from increased myocardial oxygen demand in conjunction with reduced tissue perfusion. Finally, anaerobic metabolism due to reduced perfusion may produce acidosis and, together with myocardial dysfunction, precipitate multi-organ failure. The splanchnic organs are particularly susceptible to the deleterious effects of hypotension and hypovolemic shock, and these effects, depending upon their duration and severity, may be irreversible despite restoration of normovolemia by fluid administration. Patient monitoring in the intensive care unit typically relies upon central venous pressure devices, whereas the primary focus in the operating theater is blood volume deficit estimated from suction devices. However, estimates of intraoperative blood loss can be inaccurate, potentially leading to inappropriate fluid management. Normovolemia with maldistribution of fluid can be encountered in shock-specific microcirculatory disorders secondary to hypovolemia, as well as pain and stress. Consequent vasoconstriction and reduced tissue driving pressure, as well as leukocyte and platelet adhesion, and liberation of humoral and cellular mediators, may impair or abolish blood flow in certain areas. The localized perfusion deficit may contribute to multi-organ failure. Choice of resuscitation fluid may be important in this context, since some evidence suggests that at least certain colloids might be helpful in diminishing post-ischemic microvascular leukocyte adherence. Excessive volume administration may lead to fluid overload and associated impairment of pulmonary function. However, entry of fluid into the lungs may also be facilitated by increased vascular permeability in certain pathologic conditions, especially sepsis and endotoxemia, even in the absence of substantially rising hydrostatic pressure. Another condition associated with elevated vascular permeability is systemic capillary leak syndrome. The chief goal of fluid management, based upon current understanding of the pathophysiology of fluid imbalance, should be to ensure adequate oxygen delivery by optimizing blood oxygenation, perfusion pressure, and circulating volume.

Potassium and thallium uptake in dog myocardium

We sought to ascertain the rates and mechanisms of uptake of markers for regional myocardial blood flows.

METHODS

The rates of exchange of potassium and thallium across capillary walls and cell membranes in isolated blood-perfused dog hearts were estimated from multiple indicator dilution curves recorded for 131I-albumin, 42K and 201Tl from the coronary sinus outflow following injection into arterial inflow. Analysis involved fitting the observed dilution curves with a model composed of a capillary-interstitial fluid-cell exchange region and nonexchanging larger vessels.

RESULTS

Capillary permeability surface products (PSc) for potassium and thallium were similar, 0.82 +/- 0.33 (mean +/- s.d., n = 19) and 0.87 +/- 0.32 ml min-1 g-1 (n = 24) with a ratio for simultaneous pairs of 1.02 +/- 0.27 (n = 19). For the myocardial cells, PSpc averaged 3.7 +/- 3.1 ml min-1 g-1 (n = 19) for K+ and 9.5 +/- 3.9 (n = 24) for Tl+; the ratio of potassium to thallium averaged 0.40 +/- 0.19 (n = 18), thereby omitting a single high value for potassium. This high cellular influx for thallium is interpreted as due to its passage through ionic channels for both Na+ and K+.

CONCLUSION

The high permeabilities and large volumes of distribution make thallium and potassium among the best ionic deposition markers for regional flow. Their utility for this purpose is compromised by significant capillary barrier limitation retarding uptake; so regional flow is underestimated modestly in high-flow regions particularly.

Efficient numerical methods for nonlinear-facilitated transport and exchange in a blood-tissue exchange unit

The analysis of experimental data obtained by the multiple-indicator method requires complex mathematical models for which capillary blood-tissue exchange (BTEX) units are the building blocks. This study presents a new, nonlinear, two-region, axially distributed, single capillary, BTEX model. A facilitated transporter model is used to describe mass transfer between plasma and intracellular spaces. To provide fast and accurate solutions, numerical techniques suited to nonlinear convection-dominated problems are implemented. These techniques are the random choice method, an explicit Euler-Lagrange scheme, and the MacCormack method with and without flux correction. The accuracy of the numerical techniques is demonstrated, and their efficiencies are compared. The random choice, Euler-Lagrange and plain MacCormack method are the best numerical techniques for BTEX modeling. However, the random choice and Euler-Lagrange methods are preferred over the MacCormack method because they allow for the derivation of a heuristic criterion that makes the numerical methods stable without degrading their efficiency. Numerical solutions are also used to illustrate some nonlinear behaviors of the model and to show how the new BTEX model can be used to estimate parameters from experimental data.

Fractal 15O-labeled water washout from the heart

To characterize the washout of water from the heart, we used a flow-limited (not diffusion- or permeability-limited) marker for blood-tissue exchange, namely, tracer-labeled water. Experiments were performed by injecting 15O-labeled water into the inflow to isolated blood-perfused rabbit hearts and by recording the tracer content in the heart and in the outflow simultaneously for up to 5 minutes. The data exhibit a particular combination of power law forms: (1) The downslopes of the residue and outflow curves were both power law functions, with the residue diminishing as t-alpha and the outflow as t-alpha-1, where alpha is interpreted to be the dimensionless exponent of a fractal power law relation characterizing the self-similarity inherent in each curve. (2) The fractional escape rate, given by the outflow curve divided by the residue curve, diminished almost exactly as t-1. In 18 sets of curves, alpha averaged 2.21 +/- 0.27. These results lead to an improved method for extrapolating the downslopes of indicator dilution curves to estimate their areas and therefore the blood flows. The evidence also points strongly to the conclusions that myocardial water washout is a fractal process and that stirred tank models are inappropriate for the heart.

Capillary endothelial transport of uric acid in guinea pig heart

Much of the adenosine formed in the heart is degraded by endothelial enzymes to uric acid, which is exported across the coronary capillary endothelial cell membrane before renal excretion. Because previous experiments suggested that cell permeability for uric acid is either very high (similar to water) or very low, multiple indicator-dilution experiments were carried out to distinguish between the two possibilities. An intravascular reference tracer, 131I-labeled albumin, and an extracellular reference tracer, L-[3H]glucose, were injected together with [14C]uric acid as a bolus into the coronary inflow, while fractionating the venous outflow for 90 s. Recovery of injected uric acid averaged 99.0 +/- 2.9% (mean +/- SD, n = 12) that of L-glucose. Peak capillary extraction of L-glucose and uric acid averaged 0.38 +/- 0.032 and 0.42 +/- 0.035 (P less than 0.005) compared with albumin. Except at the peaks, the dilution curves for [14C]uric acid and L-[3H]glucose coincided closely, indicating that little uric acid was transported into cells. The dilution curves were analyzed using an axially distributed, multipathway, four region mathematical model, to estimate membrane permeability-surface area (PS) products. Since the endothelial cell PS for uric acid was low (0.12 +/- 0.09 ml.g-1.min-1), approximately 3% of the PS reported for adenosine, the possibility of flow-limited exchange for uric acid is ruled out. To estimate steady-state endothelial concentrations of uric acid in vivo, equations were developed describing electrochemical potential gradients for dissociated and undissociated forms of a weak acid. Despite endothelial production, intracellular concentrations that are lower than outside are expected because the negative membrane potential and lower cellular pH assist uric acid efflux.

Blood-tissue exchange via transport and transformation by capillary endothelial cells

The escape of solutes from the blood during passage along capillaries in heart and skeletal muscle occurs via diffusion through clefts between endothelial cells and, for some solutes, via adsorption to or transport across the luminal plasmalemma of the endothelial cell. To quantitate the rates of permeation via these two routes of transport across capillary wall, we have developed a linear model for transendothelial transport and illustrated its suitability for the design and analysis of multiple simultaneous indicator dilution curves from an organ. Data should be obtained for at least three solutes: 1) an intravascular reference, albumin; 2) a solute transported by endothelial cells; and 3) another reference solute, of the same molecular size as solute 2, which neither binds nor traverses cell membranes. The capillary-tissue convection-permeation model is spatially distributed and accounts for axial variation in concentrations, transport through and around endothelial cells, accumulation and consumption within them, exchange with the interstitium and parenchymal cells, and heterogeneity of regional flows. The upslope of the dilution curves is highly sensitive to unidirectional rate of loss at the luminal endothelial surface. There is less sensitivity to transport across the antiluminal surface, except when endothelial retention is low. The model is useful for receptor kinetics using tracers during steady-state conditions and allows distinction between equilibrium binding and reaction rate limitations. Uptake rates at the luminal surface are readily estimated by fitting the model to the experimental dilution curves. For adenosine and fatty acids, endothelial transport accounts for 30-99% of the transcapillary extraction.

Through the microcirculatory maze with machete, molecule, and minicomputer (1986 Alza lecture)

This is a personal historical essay on meanderings through the jungle of the microcirculatory swamp. Because one pretends that the wandering was purposefully exploratory, a few guideposts are placed at positions where one could discern blaze-marks from earlier wanderers, and the path cut a little wider along some of the routes that may be enjoyed by investigators wanting to put their blazes along more distant paths. Naturally, one starts by coming up the broad rivers, then branching into the little streams. Each of us chooses to seek a different "mother lode," up a different stream.

Terminology for mass transport and exchange

Virtually all fields of physiological research now encompass various aspects of solute transport by convection, diffusion, and permeation across membranes. Accordingly, this set of terms, symbols, definitions, and units is proposed as a means of clear communication among workers in the physiological, engineering, and physical sciences. The goal is to provide a setting for quantitative descriptions of physiological transport phenomena.

A comparison of ascorbate and glucose transport in the heart

Multiple indicator-dilution experiments were done to compare the transcapillary exchange of tracer amounts of L-[14C]ascorbate and D-[3H]glucose (against an intravascular reference 131I-albumin) in Ringer-perfused (5 mM glucose) isolated rabbit hearts. The indicator-dilution curves for the two were virtually superimposed over the first 40-80 s. Estimates of the capillary permeability-surface area products, PSc, were the same, 2.3 +/- 0.7 (SD) ml X g-1 X min-1 (n = 18), in accord with the coincidence of their instantaneous extractions. The similarity of glucose and ascorbate permeabilities is explained by the similarity in molecular weights and passive diffusivity, their lipophobic nature, and the paucity of carrier-mediated endothelial transport for either molecule. The data were analyzed via a model composed of aggregates of spatially distributed capillary-tissue units (capillary blood, interstitium, myocytes) accounting for the heterogeneity of regional flows. The interstitial volumes in this preparation are enlarged, 0.30 +/- 0.04 ml/g. There is substantial entry into myocardial cells, the cell permeability-surface area products being approximately 2-3 ml X g-1 X min-1 for ascorbate and glucose. The estimated volumes of interstitial and intracellular space, 0.30 and 0.47 ml X g-1 X min-1, reflect interstitial edema and are very close to measured values, giving reassurance concerning the methods of modeling analysis.

Transcapillary exchange and retention of fluoride, strontium, EDTA, sucrose, and antipyrine in bone

The extractions of 85Sr2+, 18F-, sucrose-14C, EDTA-51Cr, and antipyrine-14C in bone were determined by the multiple indicator-dilution method. Fluoride and strontium extractions were 18 to 70% during a single transcapillary passage, and those of EDTA and sucrose were from 11 to 59%, whereas extraction of antipyrine was 87%. Injections of 85Sr2+ and 18F- made when perfusion was done alternately with blood and plasma resulted in similar fractional extractions. When flow and extraction were measured simultaneously, extraction was related inversely to flow.

Regional distribution of diffusible tracers and carbonized microspheres in the left ventricle of isolated dog hearts

Microspheres of different sizes, ¹²⁵I-labeled antipyrine (I-Ap), and ⁴²KCl or ⁸⁶RbCl were injected into the aortic inflow of isolated, Langendorff, perfused, nonworking dogs hearts at blood flows of 1.3–4.8 ml/min g⁻¹. After 15 seconds to 5 minutes, the left ventricle was sectioned into about 300 ordered pieces, and the amount of each tracer was determined. For all tracers, the relative density of deposition was generally higher in the endocardial region, except in one heart in which the aortic pressure and the total coronary flow were low. The deposition of ⁴²K and that of I-Ap were essentially similar in three hearts over a large range of regional variation. This finding suggests either that both tracers were distributed in proportion to flow or that a small diminution in relative density of deposition of ⁴²K in high-flow regions due to lower transcapillary extraction was quantitatively similar to a decrease in the residual fraction of I-Ap in these same regions due to faster washout in the first 15–30 seconds after injection. Large microspheres were deposited preferentially in regions of high flow, exaggerating the apparent heterogeneity of regional flows. The distribution of the smaller microspheres was closer to that for I-Ap or ⁴²K.

Circulatory transport of iodoantipyrine and water in the isolated dog heart

The exchanges of ¹²⁵I-labeled 4-iodoantipyrine (I-Ap), ¹⁴C-labeled antìpyrine (¹⁴C-Ap), and tritiated water (THO) were studied in isolated blood-perfused, beating, nonworking dog hearts. From a first set of experiments, analysis of externally monitored myocardial clearance curves of I-Ap after its injection into coronary artery blood showed its washout to be flow limited at flows ranging from 0.8 to 3.8 ml · g⁻¹ · min⁻¹. Therefore, these curves can be used for estimating coronary blood flow. In a second set of experiments, coronary sinus dilution curves of simultaneously injected I-Ap and THO were found to be indistinguishable in shape at high coronary flows. At low flows (<1.8 ml · g⁻¹ · min⁻¹), THO curves showed an earlier upslope and higher peak than antipyrine, indicating either a diffusional shunt for water or a larger volume of distribution for antipyrine. ¹⁴C-Ap had a slightly faster washout than I-Ap. The differences are partially attributable either to differences in solubility of I-Ap, ¹⁴C-Ap, and THO in erythrocytes or to differences in their volumes of distribution, and partially to diffusional shunting of water.