Cyclooxygenase-2 Modulates Glycosaminoglycan Production in the Skin During Salt Overload

Tangram of Sodium and Fluid Balance

Homeostasis of fluid and electrolytes is a tightly controlled physiological process. Failure of this process is a hallmark of hypertension, chronic kidney disease, heart failure, and other acute and chronic diseases. While the kidney remains the major player in the control of whole-body fluid and electrolyte homeostasis, recent discoveries point toward more peripheral mechanisms leading to sodium storage in tissues, such as skin and muscle, and a link between this sodium and a range of diseases, including the conditions above. In this review, we describe multiple facets of sodium and fluid balance from traditional concepts to novel discoveries. We examine the differences between acute disruption of sodium balance and the longer term adaptation in chronic disease, highlighting areas that cannot be explained by a kidney-centric model alone. The theoretical and methodological challenges of more recently proposed models are discussed. We acknowledge the different roles of extracellular and intracellular spaces and propose an integrated model that maintains fluid and electrolyte homeostasis and can be distilled into a few elemental players: the microvasculature, the interstitium, and tissue cells. Understanding their interplay will guide a more precise treatment of conditions characterized by sodium excess, for which primary aldosteronism is presented as a prototype.

The role of glycosaminoglycans in blood pressure regulation

Essential hypertension (HT) is the global health problem and is a major risk factor for the development of cardiovascular and kidney disease. High salt intake has been associated with HT and impaired kidney sodium excretion is considered to be a major mechanism for the development of HT. Although kidney has a very important role in regulation of BP, this traditional view of BP regulation was challenged by recent findings suggesting that nonosmotic tissue sodium deposition is very important for BP regulation. This new paradigm indicates that sodium can be stored and deposited nonosmotically in the interstitium without water retention and without increased BP. One of the major determinants of this deposition is glycosaminoglycans (GAGs). By binding to GAGs found in the endothelial surface layer (ESL) which contains glycocalyx, sodium is osmotically inactivated and not induce concurrent water retention. Thus, GAGs has important function for homeostatic BP and sodium regulation. In the current review, we summarized the role of GAGs in ESL and BP regulation.

Sodium in the skin: a summary of the physiology and a scoping review of disease associations

A large and growing body of research suggests that the skin plays an important role in regulating total body sodium, challenging traditional models of sodium homeostasis that focused exclusively on blood pressure and the kidney. In addition, skin sodium may help to prevent water loss and facilitate macrophage-driven antimicrobial host defence, but may also trigger immune dysregulation via upregulation of proinflammatory markers and downregulation of anti-inflammatory processes. We performed a systematic search of PubMed for published literature on skin sodium and disease outcomes and found that skin sodium concentration is increased in patients with cardiometabolic conditions including hypertension, diabetes and end-stage renal disease; autoimmune conditions including multiple sclerosis and systemic sclerosis; and dermatological conditions including atopic dermatitis, psoriasis and lipoedema. Several patient characteristics are associated with increased skin sodium concentration including older age and male sex. Animal evidence suggests that increased salt intake results in higher skin sodium levels; however, there are conflicting results from small trials in humans. Additionally, limited data suggest that pharmaceuticals such as diuretics and sodium-glucose co-transporter-2 inhibitors approved for diabetes, as well as haemodialysis may reduce skin sodium levels. In summary, emerging research supports an important role for skin sodium in physiological processes related to osmoregulation and immunity. With the advent of new noninvasive magnetic resonance imaging measurement techniques and continued research on skin sodium, it may emerge as a marker of immune-mediated disease activity or a potential therapeutic target.

Na+ is shifted from the extracellular to the intracellular compartment and is not inactivated by glycosaminoglycans during high salt conditions in rats

Recently, studies have emerged suggesting that the skin plays a role as major Na⁺ reservoir via regulation of the content of glycosaminoglycans and osmotic gradients. We investigated whether there were electrolyte gradients in skin and where Na⁺ could be stored to be inactivated from a fluid balance viewpoint. Na⁺ accumulation was induced in rats by a high salt diet (HSD) (8% NaCl and 1% saline to drink) or by implantation of a deoxycorticosterone acetate (DOCA) tablet (1% saline to drink) using rats on a low salt diet (LSD) (0.1% NaCl) on tap water as control. Na⁺ and K⁺ were assessed by ion chromatography in tissue eluates, and the extracellular volume by equilibration of ⁵¹ Cr-EDTA. By tangential sectioning of the skin, we found a low Na⁺ content and extracellular volume in epidermis, both parameters rising by ∼30% and 100%, respectively, in LSD and even more in HSD and DOCA when entering dermis. We found evidence for an extracellular Na⁺ gradient from epidermis to dermis shown by an estimated concentration in epidermis ∼2 and 4-5 times that of dermis in HSD and DOCA-salt. There was intracellular storage of Na⁺ in skin, muscle, and myocardium without a concomitant increase in hydration. Our data suggest that there is a hydration-dependent high interstitial fluid Na⁺ concentration that will contribute to the skin barrier and thus be a mechanism for limiting water loss. Salt stress results in intracellular storage of Na⁺ in exchange with K⁺ in skeletal muscle and myocardium that may have electromechanical consequences. KEY POINTS: Studies have suggested that Na⁺ can be retained or removed without commensurate water retention or loss, and that the skin plays a role as major Na⁺ reservoir via regulation of the content of glycosaminoglycans and osmotic gradients. In the present study, we investigated whether there were electrolyte gradients in skin and where Na⁺ could be stored to be inactivated from a fluid balance viewpoint. We used two common models for salt-sensitive hypertension: high salt and a deoxycorticosterone salt diet. We found a hydration-dependent high interstitial fluid Na⁺ concentration that will contribute to the skin barrier and thus be a mechanism for limiting water loss. There was intracellular Na⁺ storage in muscle and myocardium without a concomitant increase in hydration, comprising storage that may have electromechanical consequences in salt stress.

High salt diet impairs dermal tissue remodeling in a mouse model of IMQ induced dermatitis

Recent animal studies, as well as quantitative sodium MRI observations on humans demonstrated that remarkable amounts of sodium can be stored in the skin. It is also known that excess sodium in the tissues leads to inflammation in various organs, but its role in dermal pathophysiology has not been elucidated. Therefore, our aim was to study the effect of dietary salt loading on inflammatory process and related extracellular matrix (ECM) remodeling in the skin. To investigate the effect of high salt consumption on inflammation and ECM production in the skin mice were kept on normal (NSD) or high salt (HSD) diet and then dermatitis was induced with imiquimod (IMQ) treatment. The effect of high salt concentration on dermal fibroblasts (DF) and peripheral blood mononuclear cells (PBMC) was also investigated in vitro. The HSD resulted in increased sodium content in the skin of mice. Inflammatory cytokine Il17 expression was elevated in the skin of HSD mice. Expression of anti-inflammatory Il10 and Il13 decreased in the skin of HSD or HSD IMQ mice. The fibroblast marker Acta2 and ECM component Fn and Col1a1 decreased in HSD IMQ mice. Expression of ECM remodeling related Pdgfb and activation phosphorylated (p)-SMAD2/3 was lower in HSD IMQ mice. In PBMCs, production of IL10, IL13 and PDGFB was reduced due to high salt loading. In cultured DFs high salt concentration resulted in decreased cell motility and ECM production, as well. Our results demonstrate that high dietary salt intake is associated with increased dermal pro-inflammatory status. Interestingly, although inflammation induces the synthesis of ECM in most organs, the expression of ECM decreased in the inflamed skin of mice on high salt diet. Our data suggest that salt intake may alter the process of skin remodeling.