Mineralocorticoid receptor-mediated signaling regulates the ion gated sodium channel in vascular endothelial cells and requires an intact cytoskeleton

The release of osteoclast-stimulating factors on supraphysiological loading by osteoprogenitors coincides with expression of genes associated with inflammation and cytoskeletal arrangement

Supraphysiological loading induced by unstable orthopedic implants initiates osteoclast formation, which results in bone degradation. We aimed to investigate which mechanosensitive cells in the peri-implant environment produce osteoclast-stimulating factors and how the production of these factors is stimulated by supraphysiological loading. The release of osteoclast-stimulating factors by different types of isolated bone marrow-derived hematopoietic and mesenchymal stem cells from six osteoarthritic patients was analyzed after one hour of supraphysiological loading (3.0 ± 0.2 Pa, 1 Hz) by adding their conditioned medium to osteoclast precursors. Monocytes produced factors that enhanced osteoclastogenesis by 1.6 ± 0.07-fold and mesenchymal stem cells by 1.4 ± 0.07-fold. Medium from osteoprogenitors and pre-osteoblasts enhanced osteoclastogenesis by 1.3 ± 0.09-fold and 1.4 ± 0.03-fold, respectively, where medium from four patients elicited a response and two did not. Next generation sequencing analysis of osteoprogenitors revealed that genes encoding for inflammation-related pathways and cytoskeletal rearrangements were regulated differently between responders and non-responders. Our data suggest that released osteoclast-stimulating soluble factors by progenitor cells in the bone marrow after supraphysiological loading may be related to cytoskeletal arrangement in an inflammatory environment. This connection could be relevant to better understand the aseptic loosening process of orthopedic implants.

Relationship between plasma aldosterone levels and arterial stiffness parameters in hypertensive patients with subclinical vascular damage

Purpose

Aldosterone plays important role in cardiovascular damage. Aim was to evaluate arterial subclinical damage through arterial stiffness parameters in patients with Essential Hypertension (EH) and Primary Aldosteronism (PA).

Methods

From 2018 to 2019 we consecutively enrolled 82 subjects (37 males and 45 women), distinguished in two groups: 60 EH [systolic blood pressure (SBP) 143.4 ± 16.7 mmHg, diastolic blood pressure (DBP) 89.5 ± 12.1 mmHg] and 22 PA (SBP 149 ± 19.5 mmHg, DBP 92.7 ± 12.4 mmHg) [5 with aldosterone-secreting adrenal adenoma(APA), 17 with idiopathic aldosteronism(IHA)]; 40 matched normotensive subjects (NS) were enrolled (SBP 109.7 ± 6.2 mmHg, DBP 71.3 ± 9.7 mmHg). We used non-invasive applanation tonometer to acquire pressure waveform.

Results

PA patients showed higher μ-Albuminuria (UAE) (65.7 ± 11.0mg/24 h) than EH and NS (21.5 ± 7.0 mg/24 h and 21.5 ± 7.0 mg/24 h, respectively); APA group showed increased levels of arterial stiffness index (11.7 ± 4.8 m/s; p < 0.02) compared to EH subjects (8.3 ± 3 m/s) and NS subjects (7.2 ± 1.7 m/s) as well as higher carotid intima-media thickness (c-IMT); APA patients showed significant reduction of subendocardial viability ratio (SEVR) and travel time of the reflected waves (TI) respect EH and NS. PA groups showed high percentage of augmented “worsening age” (60%), compared to EH (38%) and NS (37%). PAC was positively correlated with Arterial Stiffness Index. Performing multiple linear regression analysis (evaluating anthropometric and biochemical parameters), we found UAE as predictor of Augmentation Index, Arterial Stiffness Index and Travel Time of reflected waves in the enrolled population.

Conclusion

PA patients showed higher cardiovascular subclinical damage respect to EH; UAE excretion had significant correlation with aldosterone, resulting best marker of subclinical vascular remodeling.

Epithelial Sodium Channel δ Subunit Is Expressed in Human Arteries and Has Potential Association With Hypertension

BACKGROUND

Elevated expression and increased activity of vascular epithelial sodium channel (ENaC) can result in vascular dysfunction in small animal models. However, there is limited or no knowledge on expression and function of ENaC channels in human vasculature. Hence, this study explored the expression and function of ENaC in human arteries and their association with hypertension.

METHODS

Human internal mammary artery (IMA) and aorta were obtained from cardiovascular patients undergoing coronary artery bypass graft surgery. Expression of the ENaC subunit was analyzed by polymerase chain reaction, Western blot, and immunohistochemistry. ENaC function was observed by patch-clamp electrophysiology in endothelial cells isolated from IMA. Levels of ENaC subunit expression levels were compared between arteries from normotensive, uncontrolled hypertensive, and controlled hypertensive patients.

RESULTS

For the first time, expression of α, β, γ, and δ was detected at mRNA and protein levels in human IMA and aorta. Single-channel patch-clamp recordings identified both αβγ- and δβγ-like channel conductance in primary endothelial cells isolated and cultured from IMA. Reduced expression of the δ subunit was observed in controlled hypertensive IMA, whereas reduced expression of γ-ENaC was observed in controlled hypertensive aorta.

CONCLUSIONS

These data suggest that functional ENaC channels are expressed in human arteries and their expression levels are associated with hypertension.

Endothelial sodium channel activation mediates DOCA-salt-induced endothelial cell and arterial stiffening

INTRODUCTION

High salt intake and aldosterone are both associated with vascular stiffening in humans. However, our preliminary work showed that high dietary salt alone did not increase endothelial cell (EC) or vascular stiffness or endothelial sodium channel (EnNaC) activation in mice, presumably because aldosterone production was significantly suppressed as a result of the high salt diet. We thus hypothesized that high salt consumption along with an exogenous mineralocorticoid would substantially increase EC and vascular stiffness via activation of the EnNaC.

METHODS AND RESULTS

Mice were implanted with slow-release DOCA pellets and given salt in their drinking water for 21 days. Mice with either specific deletion of the alpha subunit of EnNaC or treated with a pharmacological inhibitor of mTOR, a downstream signaling molecule involved in mineralocorticoid receptor activation of EnNaC, were studied. DOCA-salt treated control mice had increased blood pressure, EC Na⁺ transport activity, EC and arterial stiffness, which were attenuated in both the αEnNaC^-/- and mTOR inhibitor treated groups. Further, depletion of αEnNaC prevented DOCA-salt-induced impairment in EC-dependent vascular relaxation.

CONCLUSION

While high salt consumption alone does not cause EC or vascular stiffening, the combination of EC MR activation and high salt causes activation of EnNaC which increases EC and arterial stiffness and impairs vascular relaxation. Underlying mechanisms appear to include mTOR signaling.

Epithelial Na+ channel and the glycocalyx: a sweet and salty relationship for arterial shear stress sensing

PURPOSE OF REVIEW

The ability of endothelial cells to sense mechanical force, and shear stress in particular, is crucial for normal vascular function. This relies on an intact endothelial glycocalyx that facilitates the production of nitric oxide (NO). An emerging arterial shear stress sensor is the epithelial Na+ channel (ENaC). This review highlights existing and new evidence for the interdependent activity of the glycocalyx and ENaC and its implications for vascular function.

RECENT FINDINGS

New evidence suggests that the glycocalyx and ENaC are physically connected and that this is important for shear stress sensing. The connection relies on N-glycans attached to glycosylated asparagines of α-ENaC. Removal of specific N-glycans reduced ENaC's shear stress response. Similar effects were observed following degradation of the glycocalyx. Endothelial specific viral transduction of α-ENaC increased blood pressure (∼40 mmHg). This increase was attenuated in animals transduced with an α-ENaC version lacking N-glycans.

SUMMARY

These observations indicate that ENaC is connected to the glycocalyx and their activity is interdependent to facilitate arterial shear stress sensation. Future research focusing on how N-glycans mediate this interaction can provide new insights for the understanding of vascular function in health and disease.

Endothelial mineralocorticoid receptor ablation confers protection towards endothelial dysfunction in experimental diabetes in mice

AIM

With diabetes comes a significant risk of macrovascular and microvascular complications. Circulating aldosterone levels increase in patients with diabetes. Aldosterone can directly affect vascular function via activation of the mineralocorticoid receptor (MR). We hypothesized that aldosterone via endothelial MR impairs endothelial function in a murine model of experimental diabetes.

METHOD

Endothelial cell-specific mineralocorticoid receptor knockout MR^flox/flox ; Tie2-Cre mice (ECMR-KO) and wild-type FVB littermates were subjected to an experimental type-1 diabetic model by low dose streptozotocin injections (55mg/kg/day) for five consecutive days. After 10 weeks of diabetes, second-order mesenteric resistance arteries were perfused ex vivo to evaluate vessel contractility and endothelial function. The effect of ex vivo incubation with aldosterone with and without the antagonist, spironolactone was determined.

RESULTS

Diabetic ECMR-KO and wild-type mice had similar, elevated, plasma aldosterone concentration while only diabetic wild-type mice displayed elevated urine albumin excretion and cardiac and kidney hypertrophy at 10 weeks. There were no differences in contraction (Emax and EC₅₀ ) to thromboxane receptor agonist (U46619) and elevated K⁺ between groups. Wild-type diabetic mice showed impaired acetylcholine (ACh)-dependent relaxation, while diabetic ECMR-KO mice had intact ACh-mediated relaxation. Aldosterone incubation ex vivo impaired ACh mediated relaxation and rendered responses similar to diabetic WT arteries. Direct, ex vivo aldosterone effects were absent in ECMR-KO animals. Ex vivo inhibitory effects of aldosterone on endothelial relaxation in arteries from WT were abolished by spironolactone.

CONCLUSION

These findings show that endothelial cell mineralocorticoid receptor activation accounts for diabetes-induced systemic endothelial dysfunction in experimental diabetes and may explain the cardiovascular protection by MR antagonists in diabetes.

Increased endothelial sodium channel activity by extracellular vesicles in human aortic endothelial cells: Putative role of MLP1 and bioactive lipids

Extracellular vesicles (EVs) contain biological molecules and are secreted by cells into the extracellular milieu. The endothelial sodium channel (EnNaC) plays an important role in modulating endothelial cell stiffness. We hypothesized EVs secreted from human aortic endothelial cells (hAoEC) positively regulate EnNaC in an autocrine dependent manner. A comprehensive lipidomic analysis using targeted mass spectrometry was performed on multiple preparations of EVs isolated from the conditioned media of hAoEC or complete growth media of these cells. Cultured hAoEC challenged with EVs isolated from the conditioned media of these cells resulted in an increase in EnNaC activity when compared to the same concentration of media derived EVs or vehicle alone. EVs isolated from the conditioned media of hAoEC but not human fibroblast cells were enriched in MARCKS Like Protein 1 (MLP1). The pharmacological inhibition of the negative regulator of MLP1, protein kinase C, in cultured hAoEC resulted in an increase in EV size and release compared to vehicle or pharmacological inhibition of protein kinase D. The MLP1 enriched EVs increased the density of actin filaments in cultured hAoEC compared to EVs isolated from human fibroblast cells lacking MLP1. We quantified 141 lipids from glycerolipids, glycerophospholipids, and sphingolipids in conditioned media EVs that represented twice the number found in control media EVs. The concentrations of sphingomyelin, lysophosphatidylcholine and phosphatidylethanolamine were higher in conditioned media EVs. These results provide the first evidence for EnNaC regulation in hAoEC by EVs and provide insight into a possible mechanism involving MLP1, unsaturated lipids, and bioactive lipids.

Detrimental or beneficial: Role of endothelial ENaC in vascular function

In the past, it was believed that the expression of the epithelial sodium channel (ENaC) was restricted to epithelial tissues, such as the distal nephron, airway, sweat glands, and colon, where it is critical for sodium homeostasis. Over the past two decades, this paradigm has shifted due to the finding that ENaC is also expressed in various nonepithelial tissues, notably in vascular endothelial cells. In this review, the recent findings of the expression, regulation, and function of the endothelial ENaC (EnNaC) are discussed. The expression of EnNaC subunits is reported in a variety of endothelial cell lines and vasculatures, but this is controversial across different species and vessels and is not a universal finding in all vascular beds. The expression density of EnNaC is very faint compared to ENaC in the epithelium. To date, little is known about the regulatory mechanism of EnNaC. Through it can be regulated by aldosterone, the detailed downstream signaling remains elusive. EnNaC responds to increased extracellular sodium with the feedforward activation mechanism, which is quite different from the Na⁺ self-inhibition mechanism of ENaC. Functionally, EnNaC was shown to be a determinant of cellular mechanics and vascular tone as it can sense shear stress, and its activation or insertion into plasma membrane causes endothelial stiffness and reduced nitric oxide production. However, in some blood vessels, EnNaC is essential for maintaining the integrity of endothelial barrier function. In this context, we discuss the possible reasons for the distinct role of EnNaC in vasculatures.

Role of the vascular endothelial sodium channel activation in the genesis of pathologically increased cardiovascular stiffness

Cardiovascular (CV) stiffening represents a complex series of events evolving from pathological changes in individual cells of the vasculature and heart which leads to overt tissue fibrosis. While vascular stiffening occurs naturally with ageing it is accelerated in states of insulin (INS) resistance, such as obesity and type 2 diabetes. CV stiffening is clinically manifested as increased arterial pulse wave velocity and myocardial fibrosis-induced diastolic dysfunction. A key question that remains is how are these events mechanistically linked. In this regard, heightened activation of vascular mineralocorticoid receptors (MR) and hyperinsulinaemia occur in obesity and INS resistance states. Further, a downstream mediator of MR and INS receptor activation, the endothelial cell Na+ channel (EnNaC), has recently been identified as a key molecular determinant of endothelial dysfunction and CV fibrosis and stiffening. Increased activity of the EnNaC results in a number of negative consequences including stiffening of the cortical actin cytoskeleton in endothelial cells, impaired endothelial NO release, increased oxidative stress-meditated NO destruction, increased vascular permeability, and stimulation of an inflammatory environment. Such endothelial alterations impact vascular function and stiffening through regulation of vascular tone and stimulation of tissue remodelling including fibrosis. In the case of the heart, obesity and INS resistance are associated with coronary vascular endothelial stiffening and associated reductions in bioavailable NO leading to heart failure with preserved systolic function (HFpEF). After a brief discussion on mechanisms leading to vascular stiffness per se, this review then focuses on recent findings regarding the role of INS and aldosterone to enhance EnNaC activity and associated CV stiffness in obesity/INS resistance states. Finally, we discuss how coronary artery-mediated EnNaC activation may lead to cardiac fibrosis and HFpEF, a condition that is especially pronounced in obese and diabetic females.

Mechanical Strain-Mediated Tenogenic Differentiation of Mesenchymal Stromal Cells Is Regulated through Epithelial Sodium Channels

It has been suggested that mechanical strain may elicit cell differentiation in adult somatic cells through activation of epithelial sodium channels (ENaC). However, such phenomenon has not been previously demonstrated in mesenchymal stromal cells (MSCs). The present study was thus conducted to investigate the role of ENaC in human bone marrow-derived MSCs (hMSCs) tenogenic differentiation during uniaxial tensile loading. Passaged-2 hMSCs were seeded onto silicone chambers coated with collagen I and subjected to stretching at 1 Hz frequency and 8% strain for 6, 24, 48, and 72 hours. Analyses at these time points included cell morphology and alignment observation, immunocytochemistry and immunofluorescence staining (collagen I, collagen III, fibronectin, and N-cadherin), and gene expression (ENaC subunits, and tenogenic markers). Unstrained cells at similar time points served as the control group. To demonstrate the involvement of ENaC in the differentiation process, an ENaC blocker (benzamil) was used and the results were compared to the noninhibited hMSCs. ENaC subunits' (α, β, γ, and δ) expression was observed in hMSCs, although only α subunit was significantly increased during stretching. An increase in tenogenic genes' (collagen1, collagen3, decorin, tenascin-c, scleraxis, and tenomodulin) and proteins' (collagen I, collagen III, fibronectin, and N-cadherin) expression suggests that hMSCs underwent tenogenic differentiation when subjected to uniaxial loading. Inhibition of ENaC function resulted in decreased expression of these markers, thereby suggesting that ENaC plays a vital role in tenogenic differentiation of hMSCs during mechanical loading.

ENaC in Salt-Sensitive Hypertension: Kidney and Beyond

PURPOSE OF REVIEW

The main goal of this article is to discuss the role of the epithelial sodium channel (ENaC) in extracellular fluid and blood pressure regulation.

RECENT FINDINGS

Besides its role in sodium handling in the kidney, recent studies have found that ENaC expressed in other cells including immune cells can influence blood pressure via extra-renal mechanisms. Dendritic cells (DCs) are activated and contribute to salt-sensitive hypertension in an ENaC-dependent manner. We discuss recent studies on how ENaC is regulated in both the kidney and other sites including the vascular smooth muscles, endothelial cells, and immune cells. We also discuss how this extra-renal ENaC can play a role in salt-sensitive hypertension and its promise as a novel therapeutic target. The role of ENaC in blood pressure regulation in the kidney has been well studied. Recent human gene sequencing efforts have identified thousands of variants among the genes encoding ENaC, and research efforts to determine if these variants and their expression in extra-renal tissue play a role in hypertension will advance our understanding of the pathogenesis of ENaC-mediated cardiovascular disease and lead to novel therapeutic targets.

Cardiomyopathies and Adrenal Diseases

Cardiomyopathies are myocardial disorders in which heart muscle is structurally and/or functionally abnormal. Previously, structural cardiomyocyte disorders due to adrenal diseases, such as hyperaldosteronism, hypercortisolism, and hypercatecholaminism, were misunderstood, and endomyocardial biopsy (EMB) was not performed because was considered dangerous and too invasive. Recent data confirm that, if performed in experienced centers, EMB is a safe technique and gives precious information about physiopathological processes implied in clinical abnormalities in patients with different systemic disturbances. In this review, we illustrate the most important features in patients affected by primary aldosteronism (PA), Cushing's syndrome (CS), and pheochromocytoma (PHEO). Then, we critically describe microscopic and ultrastructural aspects that have emerged from the newest EMB studies. In PA, the autonomous hypersecretion of aldosterone induces the alteration of ion and water homeostasis, intracellular vacuolization, and swelling; interstitial oedema could be a peculiar feature of myocardial toxicity. In CS, cardiomyocyte hypertrophy and myofibrillolysis could be related to higher expression of atrogin-1. Finally, in PHEO, the hypercontraction of myofilaments with the formation of contraction bands and occasional cellular necrosis has been observed. We expect to clear the role of EMB in patients with cardiomyopathies and adrenal disease, and we believe EMB is a valid tool to implement new management and therapies.

Subclinical atherosclerosis due to increase of plasma aldosterone concentrations in essential hypertensive individuals

BACKGROUND AND AIMS

The adrenal mineralocorticoid system plays a key role in cardiovascular, metabolic and renal damage. This study aimed to assess the relationship between plasma aldosterone concentration (PAC) and some surrogate markers of subclinical atherosclerosis, such as carotid intima-media thickness (cIMT), ankle-brachial index (ABI) and biochemical parameters in patients with essential hypertension.

METHODS AND RESULTS

From January 2014 to December 2017, we consecutively enrolled 804 essential hypertensive patients (407 men and 397 women, mean age 50 ± 14 years) without cardiovascular complications, distinguishing patients in quartiles according to PAC. Compared with the first quartile, the highest PAC quartile was associated with the highest levels of serum uric acid (SUA) (5.3 ± 1.3 vs. 5.0 ± 1.0 mg/dl; P = 0.01), triglycerides (117.5 ± 15.7 vs. 106.8 ± 10.5 mg/dl; P < 0.05), 24-h urinary albumin excretion (UAE) (38.8 ±  vs. 7.6 ± mg/24 h; P < 0.05), cIMT (0.87 ± 0.22 vs. 0.80 ± 0.21 mm; P = 0.001) and increased prevalence of carotid plaques (26 vs. 16%; P < 0.005). Moreover, we found that in patients with PAC more than 150 pg/ml, the ABI was significantly lower than those with PAC < 150 pg/ml (1.01 ± 0.09 vs. 1.10 ± 0.09; P < 0.022). PAC was also found to be an independent predictor of the presence of carotid plaques and pathological ABI (<0.9) in essential hypertensive individuals.

CONCLUSION

Our results revealed that higher PAC values are strongly associated with some metabolic variables, as triglycerides, UAE, cIMT, worse ABI and major prevalence of carotid plaques that, together with elevated blood pressure values, are strictly correlated with higher risk of atherosclerosis and cardiovascular complications.

Epithelial Na+ Channel (ENaC) Formed by One or Two Subunits Forms Functional Channels That Respond to Shear Force

Canonical epithelial sodium channels (ENaCs) are heterotrimers formed by α, β, and γ ENaC subunits in vertebrates and belong to the Degenerin/ENaC family of proteins. Proteins from this family form mechanosensitive channels throughout the animal kingdom. Activity of canonical ENaC is regulated by shear force (SF) mediating Na⁺ absorption in the kidney and vascular tone of arteries. Expression analysis suggests that non-canonical ENaC, formed by single or only two subunits, exist in certain tissues, but it is unknown if these channels respond to SF. α, β, γ, and δ ENaC subunits were expressed either alone or in combinations of two subunits in Xenopus oocytes. Amiloride-sensitive currents and the responses to SF were assessed using two-electrode voltage clamp recordings. With the exception of γ ENaC, all homomeric channels provided amiloride-sensitive currents and responded to SF applied via a fluid stream directed onto the oocytes. Channels containing two subunits were also activated by SF. Here, the presence of the γ ENaC subunit when co-expressed with α or δ augmented the SF response in comparison to the αβγ/δβγ ENaC. Overall, we provide evidence that non-canonical ENaC can form channels that respond to SF. This supports a potential function of non-canonical ENaC as mechanosensors in epithelial, vascular, and sensory cells.

Regulation of ion channels in the microcirculation by mineralocorticoid receptor activation

The mineralocorticoid receptor (MR) has classically been studied in the renal epithelium for its role in regulating sodium and water balance and, subsequently, blood pressure. However, the MR also plays a critical role in the microvasculature by regulating ion channel expression and function. Activation of the MR by its endogenous agonist aldosterone results in translocation of the MR into the nucleus, where it can act as a transcription factor. Although most of the actions of the aldosterone can be attributed to its genomic activity though MR activation, it can also act by nongenomic mechanisms. Activation of this ubiquitous receptor increases the expression of epithelial sodium channels (ENaC) in both the endothelium and smooth muscle cells of peripheral and cerebral vessels. MR activation also regulates activity of calcium channels, calcium-activated potassium channels, and various transient receptor potential (TRP) channels. Modification of these ion channels results in a myriad of negative consequences, including impaired endothelium-dependent vasodilation, alterations in generation of myogenic tone, and increased inflammation and oxidative stress. Taken together, these studies demonstrate the importance of studying the impact of the MR on ion channel function in the vasculature. While research in this area has made advances in recent years, there are still many large gaps in knowledge that need to be filled. Crucial future directions of study include defining the molecular mechanisms involved in this interaction, as well as elucidating the potential sex differences that may exist, as these areas of understanding are currently lacking.

Shear force sensing of epithelial Na+ channel (ENaC) relies on N-glycosylated asparagines in the palm and knuckle domains of αENaC

The ability to sense mechanical forces is essential for all living organisms. Extracellular tethers have been proposed to mediate mechanical activation of channels belonging to the epithelial Na⁺ channel (ENaC)/degenerin protein family. The nature and architecture of the tethers that link the channel protein with the extracellular matrix are unknown. Our study provides experimental evidence that glycosylated asparagines and their N-glycans are part of tethers for mechanical activation of ENaC by shear force. The identified asparagines are also important for arterial blood pressure regulation in vivo. These findings provide insights into how mechanical forces are sensed by mechanosensitive ENaC channels to regulate blood pressure.

Mechanosensitive ion channels are crucial for normal cell function and facilitate physiological function, such as blood pressure regulation. So far little is known about the molecular mechanisms of how channels sense mechanical force. Canonical vertebrate epithelial Na⁺ channel (ENaC) formed by α-, β-, and γ-subunits is a shear force (SF) sensor and a member of the ENaC/degenerin protein family. ENaC activity in epithelial cells contributes to electrolyte/fluid-homeostasis and blood pressure regulation. Furthermore, ENaC in endothelial cells mediates vascular responsiveness to regulate blood pressure. Here, we provide evidence that ENaC’s ability to mediate SF responsiveness relies on the “force-from-filament” principle involving extracellular tethers and the extracellular matrix (ECM). Two glycosylated asparagines, respectively their N-glycans localized in the palm and knuckle domains of αENaC, were identified as potential tethers. Decreased SF-induced ENaC currents were observed following removal of the ECM/glycocalyx, replacement of these glycosylated asparagines, or removal of N-glycans. Endothelial-specific overexpression of αENaC in mice induced hypertension. In contrast, expression of αENaC lacking these glycosylated asparagines blunted this effect. In summary, glycosylated asparagines in the palm and knuckle domains of αENaC are important for SF sensing. In accordance with the force-from-filament principle, they may provide a connection to the ECM that facilitates vascular responsiveness contributing to blood pressure regulation.

New insights regarding epithelial Na+ channel regulation and its role in the kidney, immune system and vasculature

PURPOSE OF REVIEW

This review describes recent findings regarding the epithelial Na channel (ENaC) and its roles in physiologic and pathophysiologic states. We discuss new insights regarding ENaC's structure, its regulation by various factors, its potential role in hypertension and nephrotic syndrome, and its roles in the immune system and vasculature.

RECENT FINDINGS

A recently resolved structure of ENaC provides clues regarding mechanisms of ENaC activation by proteases. The use of amiloride in nephrotic syndrome, and associated complications are discussed. ENaC is expressed in dendritic cells and contributes to immune system activation and increases in blood pressure in response to NaCl. ENaC is expressed in endothelial ENaC and has a role in regulating vascular tone.

SUMMARY

New findings have emerged regarding ENaC and its role in the kidney, immune system, and vasculature.

The acute blood pressure-lowering effect of amiloride is independent of endothelial ENaC and eNOS in humans and mice

AIMS

The epithelial sodium channel (ENaC) is expressed in cultured endothelial cells and inhibitory coupling to eNOS activity has been proposed. The present study tested the hypothesis that ENaC blockers increase systemic NO-products and lower blood pressure in patients and mice, depending on eNOS.

METHODS

NO-products and cGMP were measured in diabetes patient urine and plasma samples before and after amiloride treatment (20-40 mg for two days, plasma n = 22, urine n = 12 and 5-10 mg for eight weeks, plasma n = 52, urine n = 55). Indwelling catheters were implanted in the femoral artery and vein in mice for continuous arterial blood pressure and heart rate recordings and infusion.

RESULTS

Treatment with amiloride for two days increased plasma and urine NO-products, while plasma cGMP decreased and urinary cGMP was unchanged in patient samples. Eight weeks of treatment with amiloride did not alter NO-products and cGMP. In mice, amiloride boli of 5, 50, and 500 µg/kg lowered heart rate and arterial blood pressure significantly and acutely. Benzamil had no effect on pressure and raised heart rate. In hypertensive eNOS^-/- and L-NAME-treated mice, amiloride lowered blood pressure significantly. L-NAME infusion significantly decreased NO-products in plasma; amiloride and eNOS-deletion had no effect. An acetylcholine bolus resulted in acute blood pressure drop that was attenuated in eNOS^-/- and L-NAME mice. ENaC subunit expressions were not detected consistently in human and mouse arteries and endothelial cells.

CONCLUSION

Amiloride has an acute hypotensive action not dependent on ENaC and eNOS and likely related to the heart.

The Deletion of Endothelial Sodium Channel α (αENaC) Impairs Endothelium-Dependent Vasodilation and Endothelial Barrier Integrity in Endotoxemia in Vivo

The role of epithelial sodium channel (ENaC) activity in the regulation of endothelial function is not clear. Here, we analyze the role of ENaC in the regulation of endothelium-dependent vasodilation and endothelial permeability in vivo in mice with conditional αENaC subunit gene inactivation in the endothelium (endo-αENaC^KO mice) using unique MRI-based analysis of acetylcholine-, flow-mediated dilation and vascular permeability. Mice were challenged or not with lipopolysaccharide (LPS, from Salmonella typhosa, 10 mg/kg, i.p.). In addition, changes in vascular permeability in ex vivo organs were analyzed by Evans Blue assay, while changes in vascular permeability in perfused mesenteric artery were determined by a FITC-dextran-based assay. In basal conditions, Ach-induced response was completely lost, flow-induced vasodilation was inhibited approximately by half but endothelial permeability was not changed in endo-αENaC^KO vs. control mice. In LPS-treated mice, both Ach- and flow-induced vasodilation was more severely impaired in endo-αENaC^KO vs. control mice. There was also a dramatic increase in permeability in lungs, brain and isolated vessels as evidenced by in vivo and ex vivo analysis in endotoxemic endo-αENaC^KO vs. control mice. The impaired endothelial function in endotoxemia in endo-αENaC^KO was associated with a decrease of lectin and CD31 endothelial staining in the lung as compared with control mice. In conclusion, the activity of endothelial ENaC in vivo contributes to endothelial-dependent vasodilation in the physiological conditions and the preservation of endothelial barrier integrity in endotoxemia.

Epithelial Na+ channel differentially contributes to shear stress-mediated vascular responsiveness in carotid and mesenteric arteries from mice

A potential "new player" in arteries for mediating shear stress responses is the epithelial Na⁺ channel (ENaC). The contribution of ENaC as shear sensor in intact arteries, and particularly different types of arteries (conduit and resistance), is unknown. We investigated the role of ENaC in both conduit (carotid) and resistance (third-order mesenteric) arteries isolated from C57Bl/6J mice. Vessel characteristics were determined at baseline (60 mmHg, no flow) and in response to increased intraluminal pressure and shear stress using a pressure myograph. These protocols were performed in the absence and presence of the ENaC inhibitor amiloride (10 µM) and after inhibition of endothelial nitric oxide synthase (eNOS) by N^ω-nitro-l-arginine methyl ester (l-NAME; 100 µM). Under no-flow conditions, amiloride increased internal and external diameters of carotid (13 ± 2%, P < 0.05) but not mesenteric (0.5 ± 0.9%, P > 0.05) arteries. In response to increased intraluminal pressure, amiloride had no effect on the internal diameter of either type of artery. However, amiloride affected the stress-strain curves of mesenteric arteries. With increased shear stress, ENaC-dependent effects were observed in both arteries. In carotid arteries, amiloride augmented flow-mediated dilation (9.2 ± 5.3%) compared with control (no amiloride, 6.2 ± 3.3%, P < 0.05). In mesenteric arteries, amiloride induced a flow-mediated constriction (-11.5 ± 6.6%) compared with control (-2.2 ± 4.5%, P < 0.05). l-NAME mimicked the effect of ENaC inhibition and prevented further amiloride effects in both types of arteries. These observations indicate that ENaC contributes to shear sensing in conduit and resistance arteries. ENaC-mediated effects were associated with NO production but may involve different (artery-dependent) downstream signaling pathways. NEW & NOTEWORTHY The epithelial Na⁺ channel (ENaC) contributes to shear sensing in conduit and resistance arteries. In conduit arteries ENaC has a role as a vasoconstrictor, whereas in resistance arteries ENaC contributes to vasodilation. Interaction of ENaC with endothelial nitric oxide synthase/nitric oxide signaling to mediate the effects is supported; however, cross talk with other shear stress-dependent signaling pathways cannot be excluded.

Does Aldosterone Play a Significant Role for Regulation of Vascular Tone?

Besides the well-known renal effects of aldosterone, the hormone is now known to have direct vascular effects. Clinical observations underline substantial adverse effects of aldosterone on cardiovascular function. The source of systemic circulating aldosterone is the adrenal gland zona glomerulosa cells through stimulus-secretion coupling involving depolarization, opening of L- and T-type calcium channels and aldosterone synthase activation. Local formation and release in peripheral tissues such as perivascular fat is recognized. Where does aldosterone affect the vasculature? Mineralocorticoid receptors (MRs) are present in endothelial and vascular smooth muscle cells, and MR-independent pathways are also involved. The vascular effects of aldosterone are complex, both concentration and temporal and spatial aspects are relevant. The acute response includes vasodilation through endothelial nitric oxide formation and vasoconstrictor effects through endothelial-contracting cyclooxygenase-derived factors and a changed calcium handling. The response to aldosterone can change within the same blood vessels depending on the exposure time and status of the endothelium. Chronic responses involve changed levels of reactive oxygen radicals, endothelial Na-influx and smooth muscle calcium channel expression. Furthermore, perivascular cells for example mast cells have also been suggested to participate in the chronic response. Moreover, the vascular effect of aldosterone depends on the status of the endothelium which is likely the cause of the very different responses to aldosterone and MR treatment observed in human studies going from increased to decreased flow depending on whether the patient had prior cardiovascular disease with endothelial dysfunction or not. A preponderance of constrictor versus dilator responses to aldosterone could therefore be involved in the detrimental vascular actions of the hormone in the setting of endothelial dysfunction and contribute to explain the beneficial action of MR blockers on blood pressure and target organ injury.

The endothelial αENaC contributes to vascular endothelial function in vivo

The Epithelial Sodium Channel (ENaC) is a key player in renal sodium homeostasis. The expression of α β γ ENaC subunits has also been described in the endothelium and vascular smooth muscle, suggesting a role in vascular function. We recently demonstrated that endothelial ENaC is involved in aldosterone-modulated endothelial stiffness. Here we explore the functional role of the endothelial αENaC subunit in vascular function in vivo. Compared to littermates, mice with conditional αENaC subunit gene inactivation in the endothelium only (endo-αENaC Knock Out mice) had no difference in their physiological parameters such as systolic blood pressure or heart rate. Acute and long-term renal Na⁺ handlings were not affected, indicating that endothelial αENaC subunit is not involved in renal sodium balance. Pharmacological inhibition of ENaC with benzamil blunted acetylcholine-induced nitric oxide production in mesenteric arteries from wild type mice but not in endo-αENaC ^KO mice, suggesting a critical role of endothelial ENaC in agonist-induced nitric oxide production. In endo-αENaC ^KO mice, compensatory mechanisms occurred and steady state vascular function was not altered except for flow-mediated dilation. Our data suggest that endothelial αENaC contributes to vascular endothelial function in vivo.

Juvenile growth reduces the influence of epithelial sodium channels on myogenic tone in skeletal muscle arterioles

Previous studies have documented that rapid juvenile growth is accompanied by functional changes in the arteriolar endothelium, but much less is known about functional changes in arteriolar smooth muscle over this period. In this study, we investigate the possible contribution of epithelial sodium channels (ENaC) to the myogenic behaviour of arterioles at two stages of juvenile growth. The effects of the ENaC inhibitor benzamil on different levels of myogenic tone were studied in isolated gracilis muscle arterioles from rats aged 21-28 days ("weanlings") and 42-49 days ("juveniles"). ENaC subunit expression in the arteriolar wall was also determined, and the interaction between ENaC and nitric oxide (NO) in regulating vascular tone was explored by combined use of benzamil and N^G -monomethyl-l-arginine (l-NMMA). At physiological pressures, both steady-state myogenic tone and the dynamic adjustments in this tone triggered by acute pressure changes were less in juvenile arterioles than in weanling arterioles. α, β and γ ENaC protein was present in arterioles at both ages, but benzamil only had an effect on myogenic tone in weanling arterioles. In these vessels, benzamil increased, rather than decreased, myogenic tone, and this effect was prevented by l-NMMA or endothelial removal. These findings suggest that although ENaC is present in gracilis muscle arterioles of both weanling and juvenile rats, it is not obligatory for the genesis of myogenic activity in these vessels at either age. However, ENaC activity can significantly modulate the level of myogenic tone through stimulation of endothelial NO release at an early stage of growth.

Oxidized low-density lipoprotein stimulates epithelial sodium channels in endothelial cells of mouse thoracic aorta

BACKGROUND AND PURPOSE

The epithelial sodium channel (ENaC) is expressed in endothelial cells and acts as a negative modulator of vasodilatation. Oxidized LDL (ox-LDL) is a key pathological factor in endothelial dysfunction. In the present study we examined the role of ENaC in ox-LDL-induced endothelial dysfunction and its associated signal transduction pathway.

EXPERIMENTAL APPROACH

Patch clamp techniques combined with pharmacological approaches were used to examine ENaC activity in the endothelial cells of a split-open mouse thoracic aorta. Western blot analysis was used to determine ENaC expression in the aorta. The aorta relaxation was measured using a wire myograph assay.

KEY RESULTS

Ox-LDL, but not LDL, significantly increased ENaC activity in the endothelial cells attached to split-open thoracic aortas, and the increase was inhibited by a lectin-like ox-LDL receptor-1 (LOX-1) antagonist (κ-carrageenan), an NADPH oxidase inhibitor (apocynin), and a scavenger of ROS (TEMPOL). Sodium nitroprusside, an NO donor, diminished the ox-LDL-mediated activation of ENaC, and this effect was abolished by inhibiting soluble guanylate cyclase (sGC) and PKG. Ox-LDL reduced the endothelium-dependent vasodilatation of the aorta pectoralis induced by ACh, and this reduction was partially restored by blocking ENaC.

CONCLUSION AND IMPLICATIONS

Ox-LDL stimulates ENaC in endothelial cells through LOX-1 receptor-mediated activation of NADPH oxidase and accumulation of intracellular ROS. Since the stimulation of ENaC can be reversed by elevating NO, we suggest that both inhibition of ENaC and an elevation of NO may protect the endothelium from ox-LDL-induced dysfunction.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Dietary salt blunts vasodilation by stimulating epithelial sodium channels in endothelial cells from salt-sensitive Dahl rats

BACKGROUND AND PURPOSE

Our recent studies show that the reduced activity of epithelial sodium channels (ENaC) in endothelial cells accounts for the adaptation of vasculature to salt in Sprague-Dawley rats. The present study examines a hypothesis that enhanced ENaC activity mediates the loss of vasorelaxation in Dahl salt-sensitive (SS) rats.

EXPERIMENTAL APPROACH

We used the cell-attached patch-clamp technique to record ENaC activity in split-open mesenteric arteries. Western blot and immunofluorescence staining were used to evaluate the levels of aldosterone, ENaC, eNOS and NO. Blood pressure was measured with the tail-cuff method and the artery relaxation was measured with the wire myograph assay.

KEY RESULTS

High-salt (HS) diet significantly increased plasma aldosterone and ENaC activity in the endothelial cells of Dahl SS rats. The endothelium-dependent artery relaxation was blunted by HS challenge in these rats. Amiloride, a potent blocker of ENaC, increased both phosphorylated eNOS and NO and therefore prevented the HS-induced loss of vasorelaxation. As, in SS rats, endogenous aldosterone was already elevated by HS challenge, exogenous aldosterone did not further elevate ENaC activity in the rats fed with HS. Eplerenone, a mineralocorticoid receptor antagonist, attenuated the effects of HS on both ENaC activity and artery relaxation.

CONCLUSIONS AND IMPLICATIONS

These data suggest that HS diet blunts artery relaxation and causes hypertension via a pathway associated with aldosterone-dependent activation of ENaC in endothelial cells. This pathway provides one of the mechanisms by which HS causes hypertension in Dahl SS rats.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Gadolinium released by the linear gadolinium-based contrast-agent Gd-DTPA decreases the activity of human epithelial Na+ channels (ENaCs)

BACKGROUND

Gadolinium-based-contrast-agents (GBCAs) are used for magnetic-resonance-imaging and associated with renal and cardiovascular adverse reactions caused by released Gd³⁺ ions. Gd³⁺ is also a modulator of mechano-gated ion channels, including the epithelial Na⁺ channel (ENaC) that is expressed in kidney epithelium and the vasculature. ENaC is important for salt-/water homeostasis and blood pressure regulation and a likely target of released Gd³⁺ from GBCAs causing the above-mentioned adverse reactions. Therefore this study examined the effect of Gd³⁺ and GBCAs on ENaC's activity.

METHODS

Human αβγENaC was expressed in Xenopus laevis oocytes and exposed to Gd³⁺, linear (Gd-DTPA, Magnevist) or cyclic (Dotarem) GBCAs. Transmembrane ion-currents (I_M) were recorded by the two-electrode-voltage-clamp technique and Gd³⁺-release by Gd-DTPA was confirmed by inductively coupled plasma-mass spectrometry.

RESULTS

Gd³⁺ exerts biphasic effects on ENaC's activity: ≤0.3mmol/l decreased I_M which was preventable by DEPC (modifies histidines). Strikingly Gd³⁺≥0.4mmol/l increased I_M and this effect was prevented by cysteine-modifying MTSEA. Linear Gd-DTPA and Magnevist mimicked the effect of ≤0.3mmol/l Gd³⁺, whereas the chelator DTPA showed no effect. Gd³⁺ and Gd-DTPA increased the IC₅₀ for amiloride, but did not affect ENaC's self-inhibition. Interestingly, cyclic Gd-DOTA (Dotarem) increased I_M to a similar extent as its chelator DOTA, suggesting that the chelator rather than released Gd³⁺ is responsible for this effect.

CONCLUSION

These results confirm Gd³⁺-release from linear Gd-DTPA and indicate that the released Gd³⁺ amount is sufficient to interfere with ENaC's activity to provide putative explanations for GBCA-related adverse effects.

Biosensor cell assay for measuring real-time aldosterone-induced release of histamine from mesenteric arteries

AIMS

The aims were to develop a method for real-time detection of histamine release and to test whether incubation with aldosterone induces histamine release from isolated, perfused mice mesenteric arteries.

METHODS

Fura-2-loaded HEK-293 cells transfected with the histamine H1 receptor was used as a sensitive biosensor assay for histamine release from isolated mouse mesenteric arteries. Activation of the H1 receptor by histamine was measured as an increased number of intracellular Ca²⁺ transient peaks using fluorescence imaging.

RESULTS

The developed biosensor was sensitive to histamine in physiological relevant concentrations and responded to substances released by the artery preparation. Aldosterone treatment of mesenteric arteries from wild-type mice for 50 min resulted in an increased number of intracellular Ca²⁺ transient peaks in the biosensor cells, which was significantly inhibited by the histamine H1 blocker pyrilamine. Mesenteric arteries from mast cell-deficient SASH mice induced similar pyrilamine-sensitive Ca²⁺ transient response in the biosensor cells. Mesenteric arteries from wild-type and SASH mice expressed histamine decarboxylase mRNA, indicating that mast cells are not the only source of histamine release.

CONCLUSION

The developed biosensor assay can measure release of substances from vascular preparations. Histamine is released from the vessel preparation in response to aldosterone treatment independently of mast cells. The assay enables us to study a new signaling mechanism for vascular responses induced by aldosterone.

AMP-Activated Protein Kinase Attenuates High Salt-Induced Activation of Epithelial Sodium Channels (ENaC) in Human Umbilical Vein Endothelial Cells

Recent studies suggest that the epithelial sodium channel (ENaC) is expressed in the endothelial cells. To test whether high salt affects the NO production via regulation of endothelial ENaC, human umbilical vein endothelial cells (HUVECs) were incubated in solutions containing either normal or high sodium (additional 20 mM NaCl). Our data showed that high sodium treatment significantly increased α-, β-, and γ-ENaC expression levels in HUVECs. Using the cell-attached patch-clamp technique, we demonstrated that high sodium treatment significantly increased ENaC open probability (P_O). Moreover, nitric oxide synthase (eNOS) phosphorylation (Ser 1177) levels and NO production were significantly decreased by high sodium in HUVECs; the effects of high sodium on eNOS phosphorylation and NO production were inhibited by a specific ENaC blocker, amiloride. Our results showed that high sodium decreased AMP-activated kinase (AMPK) phosphorylation in endothelial cells. On the other hand, metformin, an AMPK activator, prevented high sodium-induced upregulation of ENaC expression and P_O. Moreover, metformin prevented high salt-induced decrease in NO production and eNOS phosphorylation. These results suggest that high sodium stimulates ENaC activation by negatively modulating AMPK activity, thereby leading to reduction in eNOS activity and NO production in endothelial cells.

Contribution of concentration-sensitive sodium channels to the absorption of alveolar fluid in mice

The concentration-sensitive sodium channel (Nac) is activated by an increase in the extracellular sodium concentration. Although the expression of Nac in alveolar type II epithelial cells (AEC II) has been reported previously, the physiological role of Nac in the lung has not been established. We characterized Nac expression and examined amiloride-insensitive sodium transport mediated by Nac in mouse lung. Immunofluorescence studies revealed that Nac did not colocalize with either aquaporin 5 or cystic fibrosis transmembrane conductance regulator, but partially colocalized with the epithelial sodium channel γ-subunit. Immunoelectron microscopy studies showed that Nac localized at the basolateral membrane of pulmonary microvascular endothelial cells (PMVECs). Nac mRNA and protein were expressed in PMVECs isolated from the lungs of mice. Image analysis indicated that sodium influx into the alveolar wall was dependent on increases in extracellular sodium concentration. We conclude that Nac expressed in PMVECs and AEC II contributes to the reabsorption of sodium via an amiloride-insensitive pathway during alveolar fluid clearance.

Modulation of Immunity and Inflammation by the Mineralocorticoid Receptor and Aldosterone

The mineralocorticoid receptor (MR) is a ligand dependent transcription factor. MR has been traditionally associated with the control of water and electrolyte homeostasis in order to keep blood pressure through aldosterone activation. However, there is growing evidence indicating that MR expression is not restricted to vascular and renal tissues, as it can be also expressed by cells of the immune system, where it responds to stimulation or antagonism, controlling immune cell function. On the other hand, aldosterone also has been associated with proinflammatory immune effects, such as the release of proinflammatory cytokines, generating oxidative stress and inducing fibrosis. The inflammatory participation of MR and aldosterone in the cardiovascular disease suggests an association with alterations in the immune system. Hypertensive patients show higher levels of proinflammatory mediators that can be modulated by MR antagonism. Although these proinflammatory properties have been observed in other autoimmune and chronic inflammatory diseases, the cellular and molecular mechanisms that mediate these effects remain unknown. Here we review and discuss the scientific work aimed at determining the immunological role of MR and aldosterone in humans, as well as animal models.

Cooperative Role of Mineralocorticoid Receptor and Caveolin-1 in Regulating the Vascular Response to Low Nitric Oxide-High Angiotensin II-Induced Cardiovascular Injury

Aldosterone interacts with mineralocorticoid receptor (MR) to stimulate sodium reabsorption in renal tubules and may also affect the vasculature. Caveolin-1 (cav-1), an anchoring protein in plasmalemmal caveolae, binds steroid receptors and also endothelial nitric oxide synthase, thus limiting its translocation and activation. To test for potential MR/cav-1 interaction in the vasculature, we investigated if MR blockade in cav-1-replete or -deficient states would alter vascular function in a mouse model of low nitric oxide (NO)-high angiotensin II (AngII)-induced cardiovascular injury. Wild-type (WT) and cav-1 knockout mice (cav-1(-/-)) consuming a high salt diet (4% NaCl) received Nω-nitro-l-arginine methyl ester (L-NAME) (0.1-0.2 mg/ml in drinking water at days 1-11) plus AngII (0.7-2.8 mg/kg per day via an osmotic minipump at days 8-11) ± MR antagonist eplerenone (EPL) 100 mg/kg per day in food. In both genotypes, blood pressure increased with L-NAME + AngII. EPL minimally changed blood pressure, although its dose was sufficient to block MR and reverse cardiac expression of the injury markers cluster of differentiation 68 and plasminogen activator inhibitor-1 in L-NAME+AngII treated mice. In aortic rings, phenylephrine and KCl contraction was enhanced with EPL in L-NAME+AngII treated WT mice, but not cav-1(-/-) mice. AngII-induced contraction was not different, and angiotensin type 1 receptor expression was reduced in L-NAME + AngII treated WT and cav-1(-/-) mice. In WT mice, acetylcholine-induced relaxation was enhanced with L-NAME + AngII treatment and reversed with EPL. Acetylcholine relaxation in cav-1(-/-) mice was greater than in WT mice, not modified by L-NAME + AngII or EPL, and blocked by ex vivo L-NAME, 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), or endothelium removal, suggesting the role of NO-cGMP. Cardiac endothelial NO synthase was increased in cav-1(-/-) versus WT mice, further increased with L-NAME + AngII, and not affected by EPL. Vascular relaxation to the NO donor sodium nitroprusside was increased with L-NAME + AngII in WT mice but not in cav-1(-/-) mice. Plasma aldosterone levels increased and cardiac MR expression decreased in L-NAME + AngII treated WT and cav-1(-/-) mice and did not change with EPL. Thus, during L-NAME + AngII induced hypertension, MR blockade increases contraction and alters vascular relaxation via NO-cGMP, and these changes are absent in cav-1 deficiency states. The data suggest a cooperative role of MR and cav-1 in regulating vascular contraction and NO-cGMP-mediated relaxation during low NO-high AngII-dependent cardiovascular injury.

Dietary salt regulates epithelial sodium channels in rat endothelial cells: adaptation of vasculature to salt

BACKGROUND AND PURPOSE

The epithelial sodium channel (ENaC) is expressed in vascular endothelial cells and is a negative modulator of vasodilation. However, the role of endothelial ENaCs in salt-sensitive hypertension remains unclear. Here, we have investigated how endothelial ENaCs in Sprague-Dawley (SD) rats respond to high-salt (HS) challenge.

EXPERIMENTAL APPROACH

BP and plasma aldosterone levels were measured. We used patch-clamp technique to record ENaC activity in split-open mesenteric arteries (MAs). Western blot and Griess assay were used to detect expression of α-ENaCs, eNOS and NO. Vasorelaxation in second-order MAs was measured with wire myograph assays.

KEY RESULTS

Functional ENaCs were observed in endothelial cells and their activity was significantly decreased after 1 week of HS diet. After 3 weeks of HS diet, ENaC expression was also reduced. When either ENaC activity or expression was reduced, endothelium-dependent relaxation (EDR) of MAs, in response to ACh, was enhanced. This enhancement of EDR was mimicked by amiloride, a blocker of ENaCs. By contrast, HS diet significantly increased contractility of MAs, accompanied by decreased eNOS activity and NO levels. However, ACh-induced release of NO was much higher in MAs isolated from HS rats than those from NS rats.

CONCLUSIONS AND IMPLICATIONS

HS intake increased the BP of SD rats, but simultaneously enhanced EDR by reducing ENaC activity and expression due to feedback inhibition. Therefore, ENaCs may play an important role in endothelial cells allowing the vasculature to adapt to HS conditions.

Between Rho(k) and a hard place: the relation between vessel wall stiffness, endothelial contractility, and cardiovascular disease

Vascular stiffness is a mechanical property of the vessel wall that affects blood pressure, permeability, and inflammation. As a result, vascular stiffness is a key driver of (chronic) human disorders, including pulmonary arterial hypertension, kidney disease, and atherosclerosis. Responses of the endothelium to stiffening involve integration of mechanical cues from various sources, including the extracellular matrix, smooth muscle cells, and the forces that derive from shear stress of blood. This response in turn affects endothelial cell contractility, which is an important property that regulates endothelial stiffness, permeability, and leukocyte-vessel wall interactions. Moreover, endothelial stiffening reduces nitric oxide production, which promotes smooth muscle cell contraction and vasoconstriction. In fact, vessel wall stiffening, and microcirculatory endothelial dysfunction, precedes hypertension and thus underlies the development of vascular disease. Here, we review the cross talk among vessel wall stiffening, endothelial contractility, and vascular disease, which is controlled by Rho-driven actomyosin contractility and cellular mechanotransduction. In addition to discussing the various inputs and relevant molecular events in the endothelium, we address which actomyosin-regulated changes at cell adhesion complexes are genetically associated with human cardiovascular disease. Finally, we discuss recent findings that broaden therapeutic options for targeting this important mechanical signaling pathway in vascular pathogenesis.

Endothelial sodium channels trigger endothelial salt sensitivity with aging

The epithelial sodium channel is also expressed in vascular endothelium (endothelial sodium channel [EnNaC]). Depending on ambient sodium concentration, EnNaC is associated with mechanical stiffening of the endothelial cell cortex, leading to endothelial dysfunction. Because the incidence of both salt sensitivity and endothelial dysfunction increases with age, we investigated the abundance of EnNaC in aging mice. To assess EnNaC functionality and endothelial salt sensitivity, stiffness was measured while ambient sodium was varied. Aortae of young (3 months) and old (15 months) C57BL/6J wild-type mice were kept ex vivo on a physiological concentration of aldosterone (0.45 nmol/L). Spironolactone (10 nmol/L) and amiloride (1 μmol/L) were applied for aldosterone antagonism and EnNaC blockage, respectively. EnNaC at the endothelial cell surface was quantified by immunofluorescence staining. Cortical stiffness was monitored by atomic force microscopy when ambient sodium was raised from 135 to 150 mmol/L. In ex vivo aortae of older mice, endothelial cells had significantly higher EnNaC numbers than those of younger mice (+23%). In parallel, cortical stiffness was found increased (+8.5%). Acute application of high sodium led to an immediate rise in stiffness in both groups but was pronounced in endothelium of older mice (+18% versus +26%). Spironolactone and amiloride lowered EnNaC abundance and prevented endothelial stiffening under all conditions. We conclude that EnNaC mediates endothelial salt sensitivity in the aging process. This mechanism might contribute to the development of age-related cardiovascular disease and suggests the usage of spironolactone and amiloride specifically in the elderly.

Feedforward activation of endothelial ENaC by high sodium

Kidney epithelial sodium channels (ENaCs) are known to be inactivated by high sodium concentrations (feedback inhibition). Recently, the endothelial sodium channel (EnNaC) was identified to control the nanomechanical properties of the endothelium. EnNaC-dependent endothelial stiffening reduces the release of nitric oxide, the hallmark of endothelial dysfunction. To study the regulatory impact of sodium on EnNaC, endothelial cells (EA.hy926 and ex vivo mouse endothelium) were incubated in aldosterone-free solutions containing either low (130 mM) or high (150 mM) sodium concentrations. By applying atomic force microscopy-based nanoindentation, an unexpected positive correlation between increasing sodium concentrations and cortical endothelial stiffness was observed, which can be attributed to functional EnNaC. In particular, an acute rise in sodium concentration (+20 mM) was sufficient to increase EnNaC membrane abundance by 90% and stiffening of the endothelial cortex by 18%. Despite the absence of exogenous aldosterone, these effects were prevented by the aldosterone synthase inhibitor FAD286 (100 nM) or the mineralocorticoid receptor (MR)-antagonist spironolactone (100 nM), indicating endogenous aldosterone synthesis and MR-dependent signaling. Interestingly, in the presence of high-sodium concentrations, FAD286 increased the transcription of the MR by 69%. Taken together, a novel feedforward activation of EnNaC by sodium is proposed that contrasts ENaC feedback inhibition in kidney.

Nanomechanics of vascular endothelium

The mechanical characteristics of endothelial cells reveal four distinct compartments, namely glycocalyx, cell cortex, cytoplasm and nucleus. There is accumulating evidence that endothelial nanomechanics of these individual compartments control vascular physiology. Depending on protein composition, filament formation and interaction with cross-linker proteins, these four compartments determine endothelial stiffness. Structural organization and mechanical properties directly influence physiological processes such as endothelial barrier function, nitric oxide release and gene expression. This review will focus on endothelial nanomechanics and its impact on vascular function.

Blood pressure and amiloride-sensitive sodium channels in vascular and renal cells

Sodium transport in the distal nephron is mediated by epithelial sodium channel activity. Proteolytic processing of external domains and inhibition with increased sodium concentrations are important regulatory features of epithelial sodium channel complexes expressed in the distal nephron. By contrast, sodium channels expressed in the vascular system are activated by increased external sodium concentrations, which results in changes in the mechanical properties and function of endothelial cells. Mechanosensitivity and shear stress affect both epithelial and vascular sodium channel activity. Guyton's hypothesis stated that blood pressure control is critically dependent on vascular tone and fluid handling by the kidney. The synergistic effects, and complementary regulation, of the epithelial and vascular systems are consistent with the Guytonian model of volume and blood pressure regulation, and probably reflect sequential evolution of the two systems. The integration of vascular tone, renal perfusion and regulation of renal sodium reabsorption is the central underpinning of the Guytonian model. In this Review, we focus on the expression and regulation of sodium channels, and we outline the emerging evidence that describes the central role of amiloride-sensitive sodium channels in the efferent (vascular) and afferent (epithelial) arms of this homeostatic system.

Vascular endothelium: a vulnerable transit zone for merciless sodium

In humans, when plasma sodium concentration rises slightly beyond 140 mM, vascular endothelium sharply stiffens and nitric oxide release declines. In search of a vascular sodium sensor, the endothelial glycocalyx was identified as being a negatively charged biopolymer capable of selectively buffering sodium ions. Sodium excess damages the glycocalyx and renders vascular endothelium increasingly permeable for sodium. In the long term, sodium accumulates in the interstitium and gradually damages the organism. It was discovered that circulating red blood cells (RBC) 'report' surface properties of the vascular endothelium. To some extent, the RBC glycocalyx mirrors the endothelial glycocalyx. A poor (charge-deprived) endothelial glycocalyx causes a poor RBC glycocalyx and vice versa. This observation led to the assumption that the current state of an individual's vascular endothelium in terms of electrical surface charges and sodium-buffering capabilities could be read simply from a blood sample. Recently, a so-called salt blood test was introduced that quantifies the RBC sodium buffer capacity and thus characterizes the endothelial function. The arguments are outlined in this article spanning a bridge from cellular nano-mechanics to clinical application.

The role of ENaC in vascular endothelium

Once upon a time, the expression of the epithelial sodium channel (ENaC) was mainly assigned to the kidneys, colon and sweat glands where it was considered to be the main determinant of sodium homeostasis. Recent, though indirect, evidence for the possible existence of ENaC in a non-epithelial tissue was derived from the observation that the vascular endothelium is a target for aldosterone. Inhibitory actions of the intracellular aldosterone receptors by spironolactone and, more directly, by ENaC blockers such as amiloride supported this view. Shortly after, direct data on the expression of ENaC in vascular endothelium could be demonstrated. There, endothelial ENaC (EnNaC) could be defined as a major regulator of cellular mechanics which is a critical parameter in differentiating between vascular function and dysfunction. Foremost, the mechanical stiffness of the endothelial cell cortex, a layer 50-200 nm beneath the plasma membrane, has been shown to play a crucial role as it controls the production of the endothelium-derived vasodilator nitric oxide (NO) which directly affects the tone of the vascular smooth muscle cells. In contrast to soft endothelial cells, stiff endothelial cells release reduced amounts of NO, the hallmark of endothelial dysfunction. Thus, the combination of endothelial stiffness and myogenic tone might increase the peripheral vascular resistance. An elevation of arterial blood pressure is supposed to be the consequence of such functional changes. In this review, EnNaC is discussed as an aldosterone-regulated plasma membrane protein of the vascular endothelium that could significantly contribute to maintaining of an appropriate arterial blood pressure but, if overexpressed, could participate in the pathogenesis of arterial hypertension.

Aldosterone affects blood flow and vascular tone regulated by endothelium-derived NO: therapeutic implications

Aldosterone, in doses inappropriate to the salt status, plays an important role in the development of cardiovascular injury, including endothelial dysfunction, independent of its hypertensive effects. Acute non-genomic effects of aldosterone acting on mineralocorticoid receptors are inconsistent in healthy humans: vasoconstriction or forearm blood flow decrease via endothelial dysfunction, vasodilatation mediated by increased NO actions, or no effects. However, in studies with experimental animals, aldosterone mostly enhances vasodilatation mediated by endothelium-derived NO. Chronic exposure to aldosterone, which induces genomic responses, results in impairments of endothelial function through decreased NO synthesis and action in healthy individuals, experimental animals and isolated endothelial cells. Chronic aldosterone reduces NO release from isolated human endothelial cells only when extracellular sodium is raised. Oxidative stress is involved in the impairment of endothelial function by promoting NO degradation. Aldosterone liberates endothelin-1 (ET-1) from endothelial cells, which elicits ET(A) receptor-mediated vasoconstriction by inhibiting endothelial NO synthesis and action and through its own direct vasoconstrictor action. Ca(2+) flux through T-type Ca(2+) channels activates aldosterone synthesis and thus enhances unwanted effects of aldosterone on the endothelium. Mineralocorticoid receptor inhibitors, ET(A) receptor antagonists and T-type Ca(2) + channel blockers appear to diminish the pathophysiological participation of aldosterone in cardiovascular disease and exert beneficial actions on bioavailability of endothelium-derived NO, particularly in resistant hypertension and aldosteronism.

Histamine-dependent prolongation by aldosterone of vasoconstriction in isolated small mesenteric arteries of the mouse

In arterioles, aldosterone counteracts the rapid dilatation (recovery) following depolarization-induced contraction. The hypothesis was tested that this effect of aldosterone depends on cyclooxygenase (COX)-derived products and/or nitric oxide (NO) synthase (NOS) inhibition. Recovery of the response to high K+ was observed in mesenteric arteries of wild-type and COX-2−/− mice but it was significantly diminished in preparations from endothelial NOS (eNOS)−/− mice. Aldosterone pretreatment inhibited recovery from wild-type and COX-2−/− mice. The NO donor sodium nitroprusside (SNP) restored recovery in arteries from eNOS−/− mice, and this was inhibited by aldosterone. Actinomycin-D abolished the effect of aldosterone, indicating a genomic effect. The effect was blocked by indomethacin and by the COX-1 inhibitor valeryl salicylate but not by NS-398 (10−6 mol/l) or the TP-receptor antagonist S18886 (10−7 mol/l). The effect of aldosterone on recovery in arteries from wild-type mice and the SNP-mediated dilatation in arteries from eNOS−/− mice was inhibited by the histamine H2 receptor antagonist cimetidine. RT-PCR showed expression of mast cell markers in mouse mesenteric arteries. The adventitia displayed granular cells positive for toluidine blue vital stain. Confocal microscopy of live mast cells showed loss of quinacrine fluorescence and swelling after aldosterone treatment, indicating degranulation. RT-PCR showed expression of mineralocorticoid receptors in mesenteric arteries and in isolated mast cells. These findings suggest that aldosterone inhibits recovery by stimulation of histamine release from mast cells along mesenteric arteries. The resulting activation of H2 receptors decreases the sensitivity to NO of vascular smooth muscle cells. Aldosterone may chronically affect vascular function through paracrine release of histamine.

Epithelial sodium channel stiffens the vascular endothelium in vitro and in Liddle mice

Liddle syndrome, an inherited form of hypertension, is caused by gain-of-function mutations in the epithelial Na(+) channel (ENaC), the principal mediator of Na(+) reabsorption in the kidney. Accordingly, the disease pathology was ascribed to a primary renal mechanism. Whether this is the sole responsible mechanism, however, remains uncertain as dysregulation of ENaC in other tissues may also be involved. Previous work indicates that ENaC in the vascular endothelium is crucial for the regulation of cellular mechanics and thus vascular function. The hormone aldosterone has been shown to concomitantly increase ENaC surface expression and stiffness of the cell cortex in vascular endothelial cells. The latter entails a reduced release of the vasodilator nitric oxide, which eventually leads to an increase in vascular tone and blood pressure. Using atomic force microscopy, we have found a direct correlation between ENaC surface expression and the formation of cortical stiffness in endothelial cells. Stable knockdown of αENaC in endothelial cells evoked a reduced channel surface density and a lower cortical stiffness compared with the mock control. In turn, an increased αENaC expression induced an elevated cortical stiffness. More importantly, using ex vivo preparations from a mouse model for Liddle syndrome, we show that this disorder evokes enhanced ENaC expression and increased cortical stiffness in vascular endothelial cells in situ. We conclude that ENaC in the vascular endothelium determines cellular mechanics and hence might participate in the control of vascular function.

An emerging concept of vascular salt sensitivity

Excessive amounts of salt in food, as usually consumed worldwide, affect the vascular system, leading to high blood pressure and premature disabilities. Salt entering the vascular bed after a salty meal is transiently bound to the endothelial glycocalyx, a negatively charged biopolymer lining the inner surface of the blood vessels. This barrier protects the endothelium against salt overload. A poorly-developed glycocalyx increases the salt permeability of the vascular system and the amount of salt being deposited in the body, which affects organ function. A simple test system is now available that evaluates vascular salt sensitivity in humans and identifies individuals who are at risk of salt-induced hypertension. This short review aims to discuss how the underlying basic research can be translated into medical practice and, thus, meaningful health outcomes.

A physiological concept unmasking vascular salt sensitivity in man

About one third of the population worldwide is supposed to be salt sensitive which is a major cause for arterial hypertension later in life. For preventive actions it is thus desirable to identify salt-sensitive individuals before the appearance of clinical symptoms. Recent observations suggest that the vascular endothelium consists of two salt-sensitive barriers in series, the glycocalyx that buffers sodium and the endothelial cell membrane that contains sodium channels. Glycocalyx sodium buffer capacity and sodium channel activity are conversely related to each other. For proof of concept, a so-called salt provocation test (SPT) was developed that should unmask vascular salt sensitivity in humans at virtually any age. Nineteen healthy subjects, ranging from 25 to 63 years of age, underwent two series of 1-h blood pressure measurements after acute ingestion of a salt cocktail with or without addition of a sodium channel blocker effective in vascular endothelium. Differential analysis of the changes in diastolic blood pressure (net ∆DP) identified 12 individuals (63 %) as being salt resistant (net ∆DP = −0.05 ± 0.62 mmHg) and seven individuals (37 %) as being salt sensitive (net ∆DP = +6.98 ± 0.75 mmHg). Vascular salt sensitivity was not related to the age of the study participants. It is concluded that the SPT could be useful for identifying vascular salt sensitivity in humans already early in life.