Coronavirus Infection – ACE2, UV, Tryptophan, And Hemoglobin Oxygen Binding

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Diokine

Diokine

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Does tryptophan rich residue "S" spike protein on coronavirus glycoenvelope interface with either tryptophan dioxygenase or indoleamine dioxygenase directly, or does viral infection interfere with coding of hydroxyindole O-methyltransferase?

There is no evidence for this hypothesis, but studies of the structures of ACE2 and "S" spike protein indicate some degree of interaction may be possible. This could have significant implications for virus origin, efficiency, and clinical outcomes.
 
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Terma

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From your article:
ACE2 links amino acid malnutrition to microbial ecology and intestinal inflammation
To test whether the severe colitis phenotype is due to the catalytic activity of ACE2, we treated mice with recombinant soluble ACE2 (rsACE2). We and others have shown that rsACE2 rescues virtually all previously reported in vivo ACE2 functions within the RAS system3,14.
Recombinant ACE2 seems ideal for viral infections, except here:
As it lacks the transmembrane domain, soluble ACE2 cannot associate with the neutral amino acid transporter B0AT1 and thus cannot stabilize its cell-surface localization8. Treatment of mice with rsACE2 did not rescue the severe DSS-induced colitis phenotype of Ace2 mutant mice (Supplementary Fig. 3a–h).
To rule out the possibility that locally increased AngII might cause severe colitis, we generated Agtr1a−/−Ace2−/y mice. Deletion of Agtr1 did not rescue the severe DSS-induced colitis (Supplementary Fig. 4), indicating that these effects are independent of the classical RAS system.
As reported previously8, protein expression of the neutral amino acid transporter B0AT1 was absent in the small intestine of Ace2 mutant mice, whereas mRNA expression was not affected (Supplementary Figs 9e, f and 11a). Owing to the lack of intestinal B0AT1 protein expression, serum levels of the neutral amino acids valine (Val), threonine (Thr) and tyrosine (Tyr), and the essential amino acid tryptophan (Trp) were markedly reduced in Ace2−/y mice
We, therefore, speculated that the function of ACE2 in intestinal uptake of dietary amino acids might underlie the observed phenotype.

[Removed a part here I misread, rest is same...]

Okay, this is all nice, except they say themselves:
The general lack of dietary protein might induce broad effects on several organ systems. We, therefore, set out to define whether a specific amino acid might be responsible for the severe inflammatory phenotype. Because serum tryptophan was markedly decreased in Ace2 mutant mice, we focused on this essential amino acid. Tryptophan is required for the in vivo generation of nicotinamide (also known as vitamin B3 or niacin)17, and insufficient niacin or tryptophan in the diet is the cause of pellagra, a disease still endemic in many countries with protein malnutrition20. More than 90% of pellagra patients develop colitis11 and for nearly 80 years nicotinamide has been used as treatment for pellagra. In Ace2 mutant mice, nicotinamide almost completely alleviated the severe colitis and diarrhoea (Fig. 2a-c; Supplementary Fig. 15).

So that part seems like a non-issue. Supplementing B3 should easily compensate for de-novo synthesis from Trp presumably for most tissues. If it stopped the rest of these symptoms it might also be backing up the kynurenine pathway and increasing those metabolites, at least locally to organs but overall kynurenine levels in the body. It can also itself become rate-limiting if NAD were to become limiting for NADPH synthesis for KMO which you'd have to prevent in the first place. As far as Trp's other issues like heme, supplementing B3 should at least spare some Trp in theory and iirc kynurenine itself or its precursor are effectively IDO or TDO product inhibitors allowing the pathway to back up (don't quote me, memory problems). Even if amino acid uptake is compromised, since for awhile you can catabolize Trp from tissues.

A Trp− diet for wild-type mice also resulted in a significant downregulation of antimicrobial peptides (Fig. 3b). Similar downregulation of antimicrobial peptides was observed on feeding wild-type mice a PFD (Supplementary Fig. 20a). By contrast, dietary nicotinamide (Supplementary Fig. 20b–g) and a Trp+ diet (Fig. 3c and Supplementary Fig. 20h, i) triggered induction of antimicrobial peptides in intestinal epithelial cells from Ace2 mutant and wild-type mice. Thus, dietary tryptophan controls expression of small intestinal antimicrobial peptides.

They're talking about different peptides, but I've been wondering for awhile the extent of antimicrobial effects of the compounds of the kynurenine pathway, because anthranilic acid is cousin of salicylic acid, so while we view some KP metabolites as immunosuppressive or anti-inflammatory part of it might be through antimicrobial effects, just a thought.

Amino acids and nicotinamide can activate mTOR, which is involved in cell proliferation, survival, protein synthesis and transcription23. In epithelial intestinal cells from unchallenged Ace2 mutant mice, we observed markedly impaired p70S6 kinase activity (Supplementary Fig. 21a) as well as reduced S6 phosphorylation (Fig. 3d and Supplementary Fig. 21b), indicative of reduced mTOR activity. A similar reduction of mTOR activity was recently reported in mice lacking B0AT1 (ref. 24). Administration of a Trp+ diet resulted in increased mTOR activity in the small intestine of Ace2 mutant mice (Fig. 3e). In vivo inhibition of mTOR with rapamycin in wild-type mice resulted in a significant downregulation of antimicrobial peptide expression, an effect that could not be rescued by nicotinamide (Supplementary Fig. 22a). Moreover, administration of rapamycin before the first challenge with DSS increased the severity of colitis (Fig. 3f; Supplementary Fig. 22b–h). Notably, although acute mTOR inhibition may have beneficial effects in murine colitis models25, mTOR blockade has not been proven successful in human clinical trials in inflammatory bowel disease26. Thus, at the molecular level, nicotinamide and dietary tryptophan appear to exert their effects on intestinal antimicrobial peptides and colitis via the mTOR pathway.

This is where you could compare to Lupus: lupus has deranged mTOR activity in immune cells due to unprocessed kynurenine, apparently. You get high IDO and low KMO so Trp--/Kyn++ and Kyn drives mTOR in immune cells, while other cell types become Trp-starved and can't build lean tissue. Trp has to go the right places - because it's a volatile amino acid in biological context.

Importantly, both Trp+ diet and nicotinamide treatment reverted the composition of the intestinal microbiota of Ace2 mutant mice to be more similar to that of untreated wild-type littermates (Fig. 4a–c, Supplementary Fig. 24, Supplementary Tables 1, 2, 5 and 6). Thus, de-regulation of tryptophan amino acid homeostasis in Ace2 mutant mice alters the intestinal microbiome.

We propose the following scenario for how malnutrition and tryptophan deficiency can cause diarrhoea and colitis. Dietary tryptophan is primarily absorbed via the B0AT1/ACE2 transport pathway on the luminal surface of small intestinal epithelial cells. This results in the activation of mTOR, either directly through nutrient sensing and/or through the tryptophan-nicotinamide pathway. mTOR then regulates expression of antimicrobial peptides, which affect the intestinal composition of the gut microbiota.

You could clearly try a combination Trp-starved + high B3 diet in conditions where intestinal Trp usage is defective (whether indoles/serotonin/this), this has probably been discussed, but I find this hard to do for long.

This article is mostly an argument for B3, and since niacinamide is easily absorbed topically in water you can bypass the gut. Obviously lower tyrosine absorption is undesirable unless you're trying to limit adrenaline. At least a small amount of Trp should be spared. Plus its other pharmacological effects are fairly desirable in this case. You could potentially enter a situation where the pentose phosphate pathway can't keep up and not enough PRPP/ribose is produced to assemble NAD, so there's probably an upper bound dose beyond which niacinamide puts pressure on other pathways, and in this situation the sickness may dysregulate glucose metabolism to begin with so you can't assume it's working correctly.

[In actuality this article is an argument for trying exogenous kynurenine, but this is more realistic]
 
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Terma

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The potential use of albumin in COVID-19 patients
Albumin downregulates the expression of the ACE2 receptors (3) and has been shown to improve the ratio of arterial partial pressure of oxygen/fraction of inspired oxygen in patients with acute respiratory distress syndrome as soon as 24 hours after treatment and with an effect that persisted for at least seven days (4). Moreover, researchers who have studied the clinical characteristics of Covid-19 patients have reported again and again that lower serum albumin levels were associated with an increased risk of death, even to suggest that “albumin therapy might be a potential remedy”(5).

Metabolic disturbances and inflammatory dysfunction predict severity of coronavirus disease 2019 (COVID-19): a retrospective study
Decrease in lymphocyte count, serum total protein, serum albumin, high-density lipoprotein cholesterol (HDL-C), ApoA1, CD3+T%, and CD8+T% were found to be valuable in predicting the transition of COVID-19 from mild to severe illness.

This really makes it sound as if ACE2 is a secondary issue to albumin, but since nobody is trying it like he says, who can tell? His reference was on kidney:

Sci-Hub | Albumin caused the increasing production of angiotensin II due to the dysregulation of ACE/ACE2 expression in HK2 cells. Clinica Chimica Acta, 403(1-2), 23–30 | 10.1016/j.cca.2008.12.015
Human serum albumin (HSA) up regulated the production of bothTGF-βand endothelin in a time and dose dependent manner, whichwas inhibited by heparin under basal and stimulatory conditions[20].Furthermore, albumin upregulated the expression of TGF-βtype II receptor in proximal tubular cells, which could amplify the matrix-stimulatory actions of TGF-β, thereby contributing to the development of tubulointerstitial fibrosis[21]

So fibrosis is good? Maybe something else:
Because albumin is known to be the predominant proteinexcreted in the urine of patients with diabetic nephropathy, thecurrent study focused on the influence of albumin on RAS. Firstly, weinvestigated the expression of ACE and ACE2 in the HK-2 cells. Wefound that the relative abundance of ACE mRNA determined by realtime RT- PCR and ACE protein detected by western blot wasimpressively increased induced by BSA. In contrast, ACE2 wasprofoundly decreased compared to the control. Evidence of bothupregulation of ACE and downregulation of ACE2 would suggest thatalbumin activates proximal tubular cells by interrupting the balanceof ACE and ACE2.

So with bovine albumin you increase risk of kidney fibrosis by increasing proportion of ACE. Then you want to use that to lower ACE2, to lower viral entry? So if despite that you have association of low albumin with corona fatalities then it suggests either albumin is critical in some other respect like Trp binding - or higher ACE breaking down bradykinin. For reference ACE inhibitors do the opposite: https://www.ahajournals.org/doi/pdf/10.1161/01.HYP.0000118054.86193.ce
So maybe albumin is important but these are strange things to gloss over. (You could say it'd be irresponsible to treat with albumin without considering the need for an ARB or trialing without)
 
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Diokine

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It's kind of a goofy reference.

So fibrosis is good? Maybe something else:

This may be somewhat out of context. Albumin would be expected to increase TGF-β through changes in oncotic pressure on the endothelial network, encouraging remodeling. TGF-β responds to mechanical stimulus in a variety of tissue types to encourage remodeling. So I would expect it it to increase in proportion to the presence of albumin, indicating pressure. In any case, levels of TGF-β beyond accommodation wold probably encourage fibrosis, but I don't think albumin is the primary factor in this.

Regulation of endothelial cell plasticity by TGF-β
Endothelial TGF-β signalling drives vascular inflammation and atherosclerosis
Stretch-induced Activation of Transforming Growth Factor-β1 in Pulmonary Fibrosis


Human serum albumin (HSA) up regulated the production of both TGF-β and endothelin in a time and dose dependent manner, which was inhibited by heparin under basal and stimulatory conditions[20]
The references used here were studying mouse models of proteinuria and used greatly exaggerated doses of BSA in mice.


I guess the question is do normal levels of albumin contribute to kidney pathology? I could't find much evidence for hyperalbuminemia beyond high-protein diets and other pathologies sometimes associated with the liver. I wouldn't suspect albumin to cause kidney injuries or fibrosis via ACE/ACE2 balance in vivo like he's stating, in fact most nephropathies exhibit reduced levels of albumin. In any case, the hypothetical downregulation of ACE2 in live kidneys by albumin would not be on my mind for possible efficacy of albumin in treating coronavirus disease. The evidence does show that in the case of ARDS anyway, albumin infusion helps with PaO2/FiO2 ratio, though it doesn't necessarily reduce mortality. The literature also supports several mechanisms and evidence for the use of albumin in correcting several factors considered in critical illness like sepsis, though again it doesn't seem to improve mortality according to several studies. I can say that if I were a doctor treating someone with a critical illness, I would make it a point to correct hypoalbuminemia.
 
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Diokine

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So I asked myself how I would imagine the vascular endothelial system, what kind of imaginary mental model would I create to help me understand this information in context.


upload_2020-4-13_21-37-34.png


Continuous capillary located mostly in the skin and muscle – least permeable

upload_2020-4-13_21-37-29.png


Fenistrated capillary primarily located in the endocrine glands, intestines, pancreas, and the glomeruli of the kidney – medium permeability


upload_2020-4-13_21-37-23.png


Sinusoid capillary located in rich vascular environments, liver, bone marrow, spleen – most permeable


So it becomes apparent that there are significant differences in the kinds of capillaries, with degrees of permeability dependent on the role of the tissue involved. We can imagine a fine elastic mesh – the vascular endothelial net – that supports a fine web of various types and sizes of cells. I like to imagine the fine nylon mesh used in pantyhose. We could even use pantyhose to hold water, but we’d have to pump it in very fast because it would all leak out very quickly. Imagine if we made the elastic fibers much finer, or larger, so that we could actually pump juice through the tube effectively. We can imagine how the distribution of stress and strain might look through this net at a pressure. We can imagine the changes in stress and strain induced with mechanical pumping of fluid.

The regulation of this network is complicated. Too much pressure may tear the mesh. So we need some way to control the elasticity. Too much elasticity wouldn’t allow us to pump effectively – farther ends of the network would see very little pressure. So there is a combination of systemic and local control features to help exquisitely tune this instrument. Another trick is that the sensitivity of this system, or ability to experience large changes in response with respect to stimulation, is dependent on the compliance of the tissue. The compliance of the tissue is dependent on finer structure, finer structure requires maximal energy. The regulation of this network also requires quick recovery from fatigue. Damage from excessive stretch must be limited and repairs must be swift otherwise cumulative injury could critically damage vessel integrity.

Albumin provides a large degree of oncotic pressure, or mechanical force distributed across the vascular network. This mechanical force is generated from the electrostatic and osmotic interactions between amino acids and the surrounding plasma, displacing water. This has a very important effect of pushing water rich with nutrients and other small molecules into the interstitial space bathing cells. Modulation of albumin is a very important factor in optimal delivery of nutrients, hormones, vitamins, minerals, and amino acids. Increasing albumin has an important effect on kidney filtration, and kidney injury typically results in loss of albumin from the blood plasma through urine.

This loss of albumin requires intervention to maintain pressure and delivery of nutrients. The body has many ways of increasing pressure and accommodating different conditions. The renin-angiotensin-aldosterone system is one of these ways. Renin increases angiotensin production systemically, which is then acted upon locally by angiotensin converting enzyme to produce angiotensin I and II, which is the active molecule. Angiotensin II increases vascular pressure. It is further locally acted on by angiotensin converting enzyme II (ACE2,) producing angiotensin 1-7, which is a vasodilator. So we can see here a small example of the way the wisdom of the body organizes local response to mechanical stress in response to pressure changes.

These pressure changes and modulation by chemical endocrine systems are not always optimally handled by the body. This requires a degree of mechanical modulation, in the form tissue remodeling. Formation of collagen is an important kind of tissue remodeling, particularly in the endothelial network. Continuous pressure and strain developed beyond limit leads to tears and scarring. This scarring is the basis of atherosclerosis. So many factors influence remodeling that it is impossible to examine any of them here sufficiently, but vitamin C requires special mention. Vitamin C is critically important for the maintenance of flexible and elastic vascular tissue. Insufficient ascorbic acid in the presence of mechanical stress will result in accumulation of damage to the delicate matrix supporting life.

Competence of this system is crucial for life, and impairment to a critical degree can cause rapid death. Sepsis is one form of this failure. The presence of severe levels of inflammatory mediators essentially renders the endothelial vasculature unable to maintain pressure, or unable to support levels of nutrients required for life. In some cases resuscitation through hydration or other pressors can restore dynamics back to subcritical levels, but some cases the trauma is so severe or blood so poisoned that nothing can be done. In other cases, some aspect of fine control is so severely disrupted that the capillaries no longer effectively hold pressure, and rapid decline sets in. I believe this is the case with tryptophan. In a sense, the body is losing discrimination of where vascular beds end and tissue begins.



upload_2020-4-13_21-36-37.png


Endothelial progenitor cell tube formation in vitro. Bright-field image of human microvascular endothelial cells (HMECS) forming vascular networks on Matrigel after 12 hours(A) Bright field image of EPC matrix (B). Fluorescent images of CellTracker Red-labeled HMECs at 9 hours on Matrigel (C). A sculpture designed from five snapshots of a computer simulation of branching morphogenesis illustrating the forces lung cells exert as they form capillaries.


upload_2020-4-13_21-36-59.png


HIM imaging of glomerular endothelial cells. (A) Two adjacent endothelial cells from a glomerular capillary (GA-fixed, dehydrated using the extended methanol freeze-substitution protocol), imaged from the luminal side. The most striking features of these cells are the numerous, round fenestrations that are present over the entire cell surface. The raised ridges (arrows) represent the location of the tight junction between the two cells. Bar = 175 nm. (B) Higher magnification showing details of the fenestrations. In some of them, a substructure consisting of faint spokes like a bicycle wheel can be seen (arrows). Bar = 80 nm. doi:10.1371/journal.pone.0057051.g004​
 
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Diokine

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Terma said:
So if despite that you have association of low albumin with corona fatalities then it suggests either albumin is critical in some other respect like Trp binding - or higher ACE breaking down bradykinin. For reference ACE inhibitors do the opposite: https://www.ahajournals.org/doi/pdf/10.1161/01.HYP.0000118054.86193.ce

I think bradykinin is probably significantly involved. I haven't spent any time seeing how it ties in. I'm reading the reference you linked here and came across this endothelium-derived hyperpolarizing factor or EDHF. This would indicate that smooth muscle cell reaction to degrees of electrical polarization would in part be controlled by Ca2+ flux. This heavily implicates transient receptor potential channels. Indeed TRP channels are associated with smooth muscle control. The modes of calcium flux display some interesting characteristics, namely Ca2+ oscillations.

Smooth muscle cell calcium activation mechanisms

Smooth muscle cell (SMC) contraction is controlled by the Ca2+ and Rho kinase signalling pathways. While the SMC Rho kinase system seems to be reasonably constant, there is enormous variation with regard to the mechanisms responsible for generating Ca2+ signals. One way of dealing with this diversity is to consider how this system has been adapted to control different SMC functions. Phasic SMCs (vas deferens, uterus and bladder) rely on membrane depolarization to drive Ca2+ influx across the plasma membrane. This depolarization can be induced by neurotransmitters or through the operation of a membrane oscillator. Many tonic SMCs (vascular, airway and corpus cavernosum) are driven by a cytosolic Ca2+ oscillator that generates periodic pulses of Ca2+. A similar oscillator is present in pacemaker cells such as the interstitial cells of Cajal (ICCs) and atypical SMCs that control other tonic SMCs (gastrointestinal, urethra, ureter). The changes in membrane potential induced by these cytosolic oscillators does not drive contraction directly but it functions to couple together individual oscillators to provide the synchronization that is a characteristic feature of many tonic SMCs.

There is a degree of feedback built into the system. Disruption of any of the feedback mechanisms represents significant risk of injury. There may exist conditions in which the disruption of feedback is sufficiently particular to induce extremely large excursions of range possibly resulting in significant or catastrophic injury.

Activation of TRPV1-dependent Calcium Oscillation Exacerbates Seawater Inhalation-Induced Acute Lung Injury
Calcium is an important second messenger and it is widely recognized that acute lung injury (ALI) is often caused by oscillations of cytosolic free Ca2+. Previous studies have indicated that the activation of transient receptor potential‑vanilloid (TRPV) channels and subsequent Ca2+ entry initiates an acute calcium‑dependent permeability increase during ALI. However, whether seawater exposure induces such an effect through the activation of TRPV channels remains unknown. In the current study, the effect of calcium, a component of seawater, on the inflammatory reactions that occur during seawater drowning‑induced ALI, was examined. The results demonstrated that a high concentration of calcium ions in seawater increased lung tissue myeloperoxidase activity and the secretion of inflammatory mediators, such as tumor necrosis factor‑α (TNF‑α) and interleukin (IL)‑1β and IL‑6. Further study demonstrated that the seawater challenge elevated cytosolic Ca2+ concentration, indicated by [Ca2+]c, by inducing calcium influx from the extracellular medium via TRPV1 channels. The elevated [Ca2+c] may have resulted in the increased release of TNF‑α and IL‑1β via increased phosphorylation of nuclear factor‑κB (NF‑κB). It was concluded that a high concentration of calcium in seawater exacerbated lung injury, and TRPV1 channels were notable mediators of the calcium increase initiated by the seawater challenge. Calcium influx through TRPV1 may have led to greater phosphorylation of NF‑κB and increased release of TNF‑α and IL‑1β.

What are the particular conditions that could cause these excursions? Nitric oxide, cGMP, and I suspect low vitamin D, are major players. Also:

A Protein Kinase G-sensitive Channel Mediates Flow-Induced Ca(2+) Entry Into Vascular Endothelial Cells

The hemodynamic force generated by blood flow is considered to be the physiologically most important stimulus for the release of nitric oxide (NO) and prostacyclin (PGI(2)) from vascular endothelial cells (1). NO and PGI(2) then act on the underlying smooth muscle cells, causing vasodilation and thus lowering blood pressure (2, 3). One critical early event occurring in this flow-induced regulation of vascular tone is that blood flow induces Ca(2+) entry into vascular endothelial cells, which in turn leads to the formation of NO (4, 5). Here we report a mechanosensitive Ca(2+)-permeable channel in vascular endothelial cells. The activity of the channel was inhibited by 8-Br-cGMP, a membrane-permeant activator of protein kinase G (PKG), in cell-attached membrane patches. The inhibition could be reversed by PKG inhibitor KT5823 or H-8. A direct application of active PKG in inside-out patches blocked the channel activity. Gd(3+), Ni(2+), or SK&F-96365 also inhibited the channel activity. A study of fluorescent Ca(2+) entry revealed a striking pharmacological similarity between the Ca(2+) entry elicited by flow and the mechanosensitive Ca(2+)-permeable channel we identified, suggesting that this channel is the primary pathway mediating flow-induced Ca(2+) entry into vascular endothelial cells.

H2O2-Induced Dilation in Human Coronary Arterioles: Role of Protein Kinase G Dimerization and Large-Conductance Ca2+-Activated K+ Channel Activation
Hydrogen peroxide (H2O2) serves as a key endothelium-derived hyperpolarizing factor mediating flow-induced dilation in human coronary arterioles (HCAs). The precise mechanisms by which H2O2 elicits smooth muscle hyperpolarization are not well understood. An important mode of action of H2O2 involves the oxidation of cysteine residues in its target proteins, including protein kinase G (PKG)-Iα, thereby modulating their activities.

From the earlier article Regulation of Vascular Tone and Blood Pressure by a Tryptophan-Derived Tricyclic Hydroperoxide, we have evidence that oxidized tryptophan residues, intermediaries of the kynurenine pathway, participate in this reaction.

This is good evidence for the existence of several mechanisms, which through their various interactions, influence vascular competence to such a degree that they are capable of causing catastrophic failure of microcirculation. While this evidence supports many factors in these effects, I think a significant role can be assigned to tryptophan metabolism. Differences in tissues account for different expressions, and spontaneous capillary leaking has been documented.

Narrative Review: The Systemic Capillary Leak Syndrome
The systemic capillary leak syndrome (SCLS) is a rare disease of reversible plasma extravasation and vascular collapse accompanied by hemoconcentration and hypoalbuminemia. Its cause is unknown, although it is believed to be a manifestation of transient endothelial dysfunction due to endothelial contraction, apoptosis, injury, or a combination of these
 

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