Endothelin, kidney disease, and hypertension

Chronic exercise improves renal AT1 and ETB receptor functions via modulating GRK4 expression in obese Zucker rats

BACKGROUND

Physical activity has profound benefits on health, especially in patients with cardiovascular and metabolic disease. Exercise training can reduce oxidative stress, improve renal function, and thus lower blood pressure. However, the effect of exercise training on angiotensin II type 1 receptors (AT1R) and endothelin subtype B receptors (ETBR)-mediated diuresis and natriuresis in obese Zucker rats is unclear.

METHODS

Lean and obese Zucker rats were exercised or placed on a nonmoving treadmill for 8 weeks. Blood pressure was measured by tail-cuff plethysmography, and functions of AT1R and ETBR in the kidney were measured by natriuresis, respectively.

RESULTS

Our data showed that exercise training improved glucose and lipid metabolism, renal function and sodium excretion in obese Zucker rats, accompanied by decreased oxidative stress and GRK4 expression in obese Zucker rats. Moreover, exercise training reduced the Candesartan-induced an increase in diuresis and natriuresis and increased ETBR agonists (BQ3020)-mediated diuresis and natriuresis in obese Zucker rats, which were associated with decreased renal AT1R expression and ETBR phosphorylation levels.

CONCLUSIONS

The results demonstrate that exercise training lowers blood pressure via improving renal AT1R and ETBR function through modulating GRK4 expression in Obese Zucker Rats and provides potentially effective targets for obesity-related hypertension.

The circadian clock protein PER1 is important in maintaining endothelin axis regulation in Dahl salt-sensitive rats

Endothelin-1 (ET-1) is a peptide hormone that acts on its receptors to regulate sodium handling in the kidney's collecting duct. Dysregulation of the endothelin axis is associated with various diseases, including salt-sensitive hypertension and chronic kidney disease. Previously, our lab has shown that the circadian clock gene PER1 regulates ET-1 levels in mice. However, the regulation of ET-1 by PER1 has never been investigated in rats. Therefore, we used a novel model where knockout of Per1 was performed in Dahl salt-sensitive rat background (SS Per1 -/-) to test a hypothesis that PER1 regulates the ET-1 axis in this model. Here, we show increased renal ET-1 peptide levels and altered endothelin axis gene expression in several tissues, including the kidney, adrenal glands, and liver in SS Per1 -/- compared with control SS rats. Edn1 antisense lncRNA Edn1-AS, which has previously been suggested to be regulated by PER1, was also altered in SS Per1 -/- rats compared with control SS rats. These data further support the hypothesis that PER1 is a negative regulator of Edn1 and is important in the regulation of the endothelin axis in a tissue-specific manner.

Endothelin A receptor antagonist attenuated renal iron accumulation in iron overload heme oxygenase-1 knockout mice

Progressive iron accumulation and renal impairment are prominent in both patients and mouse models of sickle cell disease (SCD). Endothelin A receptor (ETA) antagonism prevents this iron accumulation phenotype and reduces renal iron deposition in proximal tubules of SCD mice. To better understand the mechanisms of iron metabolism in the kidney and the role of ETA receptor in iron chelation and transport, we studied renal iron handling in a non-sickle cell iron overload model, heme oxygenase-1 (Hmox-1-/-) knockout mice. We found that Hmox-1-/- mice had elevated plasma endothelin-1 (ET-1), cortical ET-1 mRNA expression, and renal iron content compared to Hmox-1+/+ controls. The ETA receptor antagonist, ambrisentan, attenuated renal iron deposition, without any changes to anemia status in Hmox-1-/- mice. This was accompanied by reduced urinary iron excretion. Finally, ambrisentan had an important iron recycling effect by increasing expression of cellular iron exporter, ferroportin-1 (FPN-1) and circulating total iron levels in Hmox-1-/- mice. These findings suggest the ET-1/ETA signaling pathway contributes to in renal iron trafficking in a murine model of iron overload.

Plasma Levels of Big Endothelin-1 Are Associated with Renal Insufficiency and In-Hospital Mortality of Immune Thrombotic Thrombocytopenic Purpura

Immune thrombotic thrombocytopenic purpura (iTTP) is caused by severe deficiency of plasma ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) activity. Despite advances in early diagnosis and management, the mortality rate of acute iTTP remains high in a large part of world where access to some of the most novel therapies is limited. To determine the role of plasma big endothelin-1 (bigET-1) or its bioactive product ET-1 as a biomarker and/or a pathogenic factor in acute iTTP, plasma levels of bigET-1 were determined using an immunoassay in patients with iTTP on admission and during remission, as well as in healthy controls; moreover, the biological effect of ET-1 in thrombus formation was determined by a microfluidic assay. We show that plasma levels of bigET-1 were dramatically increased in patients with acute iTTP on admission, which was significantly decreased during clinical response/remission; elevated admission levels of plasma bigET-1 were associated with low estimated glomerular filtration rate, the need for intensive care unit admission or intubation, and in-hospital mortality. Moreover, an addition of a bioactive product ET-1 to cultured endothelial cells in a microfluidic channel significantly accelerated the rate of thrombus formation under arterial flow. Our results demonstrate for the first time a potential role of measuring plasma bigET-1 in patients with acute iTTP in assessing the disease severity and risk of in-hospital mortality, which may help stratify patients for a more aggressive monitoring and therapeutic strategy; also, the bioactive ET-1, derived from bigET-1, may result in acute renal injury in TTP patient, likely through its vasoconstriction and prothrombotic properties.

A comparison effects of l-citrulline and l-arginine against cyclosporine-induced blood pressure and biochemical changes in the rats

OBJECTIVE

The use of cyclosporine A (CsA) is associated with different adverse effects including hypertension and nephrotoxicity. The present study aimed to compare the inhibitory effects of l-arginine &l-citrulline on CsA-induced blood pressure and biochemical changes in the serum of rats.

METHODS

Thirty-six rats were divided into 6 groups received daily: (1) 1ml distilled water, (2) 200mg/kg l-citrulline IP, (3) 25mg/kg CsA SC, (4) CsA+l-citrulline with the same dose of the former groups, (5) 200mg/kg l-arginine IP and (6) l-arginie+CsA with the same doses of group 4 for 7 days.

RESULTS

The changes in the blood pressure, heart rate, creatinine, BUN, glucose and C-reactive protein (CRP) of the serum were determined in the treated animals. Significant (p<0.001) increase was shown in the blood pressure and heart rate of CsA treated rats compared to the control group. There were also a significant (p<0.05) increase in the creatinine, BUN and glucose, but a decrease in the CRP value in the CsA-treated group. However, l-citrulline significantly (p<0.001) inhibited the changes in the blood pressure and heart rate in CsA-treated as well as it was able to reduce blood pressure in non-treated group significantly (p<0.01). l-citrulline also inhibited the increased levels of BUN and creatinine induced by CsA, while, l-arginine was able to prevent the increased blood pressure and creatinine occurs after administration of CsA.

CONCLUSIONS

These findings suggest that the l-citrulline is more efficient than l-arginine against the adverse effects induced by cyclosporine.

Therapeutic Insights in Chronic Kidney Disease Progression

Chronic kidney disease (CKD) has been recognized as a leading public health problem worldwide. Through its effect on cardiovascular risk and end-stage kidney disease, CKD directly affects the global burden of morbidity and mortality. Classical optimal management of CKD includes blood pressure control, treatment of albuminuria with angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers, avoidance of potential nephrotoxins and obesity, drug dosing adjustments, and cardiovascular risk reduction. Diabetes might account for more than half of CKD burden, and obesity is the most important prompted factor for this disease. New antihyperglycemic drugs, such as sodium-glucose-cotransporter 2 inhibitors have shown to slow the decline of GFR, bringing additional benefit in weight reduction, cardiovascular, and other kidney outcomes. On the other hand, a new generation of non-steroidal mineralocorticoid receptor antagonist has recently been developed to obtain a selective receptor inhibition reducing side effects like hyperkalemia and thereby making the drugs suitable for administration to CKD patients. Moreover, two new potassium-lowering therapies have shown to improve tolerance, allowing for higher dosage of renin-angiotensin system inhibitors and therefore enhancing their nephroprotective effect. Regardless of its cause, CKD is characterized by reduced renal regeneration capacity, microvascular damage, oxidative stress and inflammation, resulting in fibrosis and progressive, and irreversible nephron loss. Therefore, a holistic approach should be taken targeting the diverse processes and biological contexts that are associated with CKD progression. To date, therapeutic interventions when tubulointerstitial fibrosis is already established have proved to be insufficient, thus research effort should focus on unraveling early disease mechanisms. An array of novel therapeutic approaches targeting epigenetic regulators are now undergoing phase II or phase III trials and might provide a simultaneous regulatory activity that coordinately regulate different aspects of CKD progression.

Role of GRK4 in the regulation of the renal ETB receptor in hypertension

The endothelin receptor type B (ETBR) regulates water and electrolyte balance and blood pressure, in part, by inhibiting renal sodium transport. Our preliminary study found that the ETBR-mediated diuresis and natriuresis are impaired in hypertension with unknown mechanism. Persistently increased activity of G protein-coupled receptor kinase 4 (GRK4), caused by increased expression or genetic variants (eg, GRKγ142V), impairs the ability of the kidney to excrete a sodium load, in part, by impairing renal dopamine D₁ receptor function through persistent phosphorylation. Our present study found that although renal ETBR expression was not different between Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHRs), renal ETBR phosphorylation was higher in SHRs. The role of hyper-phosphorylation in impaired ETBR-function was supported by results in human (h) GRK4γ transgenic mice. Stimulation of ETBR by BQ3020-induced natriuresis in human (h) GRK4γ wild-type (WT) mice. However, in hGRK4γ 142V transgenic mice, the renal ETBR was hyperphosphorylated and ETBR-mediated natriuresis and diuresis were not evident. There were co-localization and co-immunoprecipitation of ETBR and GRK4 in renal proximal tubule (RPT) cells from both WKY and SHRs but was greater in the latter than the former group. SiRNA-mediated downregulation of GRK4 expression, recovered the impaired inhibitory effect of ETBR on Na⁺ -K⁺ -ATPase activity in RPT cells from SHR. In vivo downregulation of renal GRK4 expression, via ultrasound-targeted microbubble destruction, decreased ETBR phosphorylation and restored ETBR-mediated natriuresis and diuresis in SHRs. This study provides a mechanism by which GRK4, via regulation of renal ETBR function, participates in the pathogenesis of hypertension.

EDN1-AS, A Novel Long Non-coding RNA Regulating Endothelin-1 in Human Proximal Tubule Cells

Endothelin-1 (ET-1) is a peptide hormone that functions as a vasoconstrictor in the vasculature, whereas in the collecting duct of the kidney it exerts blood pressure-lowering effects via natriuretic actions. Aberrant ET-1 signaling is associated with several pathological states including hypertension and chronic kidney disease. ET-1 expression is regulated largely through transcriptional control of the gene that encodes ET-1, EDN1. Here we report a long, non-coding RNA (lncRNA) that appears to be antisense to the EDN1 gene, called EDN1-AS. Because EDN1-AS represents a potential novel mechanism to regulate ET-1 expression, we examined the regulation of EDN1-AS expression and action. A putative glucocorticoid receptor response (GR) element upstream of the predicted EDN1-AS transcription start site was identified using the ENCODE database and the UCSC genome browser. Two homozygous deletion clones of the element were generated using CRISPR/Cas9. This deletion resulted in a significant increase in the expression of EDN1-AS, which was associated with increased secretion of ET-1 peptide from HK-2 cells (two-fold increase in KO cells vs. CNTL, n = 7, P < 0.05). Phenotypic characterization of these CRISPR clones revealed a difference in cell growth rates. Using a standard growth assay, we determined that the KO1 clone exhibited a three-fold increase in growth over 8 days compared to control cells (n = 4, P < 0.01) and the KO2 clone exhibited a two-fold increase (n = 4, P < 0.01). These results support a role for EDN1-AS as a novel regulatory mechanism of ET-1 expression and cellular proliferation.

Unfavorable Reduction in the Ratio of Endothelin B to A Receptors in Experimental 5/6 Nephrectomy and Adenine Models of Chronic Renal Insufficiency

Chronic renal insufficiency (CRI) is characterized by increased endothelin 1 (ET-1) synthesis. We studied rat kidney endothelin receptor A (ETA) and receptor B (ETB) expressions after 12 and 27 weeks of 5/6 nephrectomy, and after 12 weeks of 0.3% adenine diet, representing proteinuric and interstitial inflammation models of CRI, respectively. Uric acid and calcium-phosphate metabolism were modulated after 5/6 nephrectomy, while ETA blocker and calcimimetic were given with adenine. Endothelin receptor mRNA levels were measured using RT-qPCR and protein levels using autoradiography (5/6 nephrectomy) or ELISA (adenine model). Both 12 and 27 weeks after 5/6 nephrectomy, kidney cortex ETA protein was increased by ~60% without changes in ETB protein, and the ETB:ETA ratio was reduced. However, the ETB:ETA mRNA ratio did not change. In the adenine model, kidney ETA protein was reduced by ~70%, while ETB protein was suppressed by ~95%, and the ETB:ETA ratio was reduced by ~85%, both at the protein and mRNA levels. The additional interventions did not influence the observed reductions in the ETB:ETA ratio. To conclude, unfavorable reduction in the ETB:ETA protein ratio was observed in two different models of CRI. Therefore, ETA blockade may be beneficial in a range of diseases that cause impaired kidney function.

The Role of Endothelin and Endothelin Antagonists in Chronic Kidney Disease

Background

Endothelins (ET) are a family of peptides that act as potent vasoconstrictors and pro-fibrotic growth factors. ET-1 is integral to renal and cardiovascular pathophysiology and exerts effects via autocrine, paracrine and endocrine signaling pathways tied to regulation of aldosterone, catecholamines, and angiotensin. In the kidney, ET-1 is critical to maintaining renal perfusion and controls glomerular arteriole tone and hemodynamics. It is hypothesized that ET-1 influences the progression of chronic kidney disease (CKD), and the objective of this review is to discuss the pathophysiology, and role of ET and endothelin receptor antagonists (ERAs) in CKD.

Summary

The use of ERAs in hypertensive nephropathy has the potential to decrease proteinuria, and in diabetic nephropathy has the potential to restore glycocalyx thickness, also decreasing proteinuria. Focal segmental glomerular sclerosis has no specific Food and Drug Administration-approved therapy currently, however, ERAs show promise in decreasing proteinuria and slowing tissue damage. ET-1 is a potential biomarker for autosomal dominant polycystic kidney disease progression and so it is thought that ERAs may be of some therapeutic benefit.

Key Messages

Multiple studies have shown the utility of ERAs in CKD. These agents have shown to reduce blood pressure, proteinuria, and arterial stiffness. However, more clinical trials are needed, and the results of active or recently concluded studies are eagerly awaited.

Protective Effects of Dexmedetomidine and Oxycodone in Patients Undergoing Limb Ischemia-Reperfusion

Background

Tourniquet-related complications are a common clinical problem. In the present study, we compared the effects of dexmedetomidine vs. oxycodone in patients undergoing limb ischemia-reperfusion.

Material/Methods

Fifty-four patients undergoing unilateral lower-extremity surgery under combined spinal and epidural anesthesia were randomly assigned to a control (ischemia-reperfusion, I/R) group, a dexmedetomidine (Dex) group, and an oxycodone (Oxy) group. Tourniquet-induced hemodynamic parameters changes among groups were compared. The serum concentration of malondialdehyde (MDA), superoxide dismutase (SOD), tumor necrosis factor-a (TNF-α), interleukin-6 (IL-6), fatty acid binding protein 3 (FABP3), endothelin-1 (ET-1), and brain-derived neurotrophic factor (BDNF) were measured using ELISA before anesthesia and at 30 min and at 6 h after tourniquet release.

Results

In the control group, tourniquet use caused significant increases in systolic arterial pressure (SAP), mean arterial pressure (MAP), diastolic arterial pressure (DAP), and rate-pressure product. Compared with Oxy, Dex significantly decreased heart rate (HR). Both Dex and Oxy lowered SAP compared with the control group. No significant difference was observed in DAP between Dex and Oxy. The levels of MDA, TNF-α, IL-6, FABP3, and ET-1 were significantly higher, while the SOD and BDNF were significantly lower compared to baseline in the I/R group, but the variation range of those agents was significantly smaller in the Dex and Oxy groups, and the measured values were comparable between the 2 groups.

Conclusions

Compared with Dex, Oxy was not inferior in mitigating tourniquet-induced hyperdynamic response, ameliorating the inflammatory reaction, and protecting remote multiple organs in lower-extremity surgery patients.

Circadian Variation in Vasoconstriction and Vasodilation Mediators and Baroreflex Sensitivity in Hypertensive Rats

The purpose of this study was to evaluate the relationship between the circadian profile of the vasorelaxing substances calcitonin gene-related peptide (CGRP) and epoxyeicosatrienoic acids (EETs) and the vasconstrictive agent endothelin-1 (ET1) and the daily rhythms of cardiac hemodynamic indices (CHI) and baroreflex (BRS) in Wistar rats with 1 kidney-1 clip model of arterial hypertension (1K-1C AH). The animals were divided into 3 groups: I- sham-operated (SO), II- 4-week and III- 8-week 1K-1C AH rats. Plasma concentration of ET1, CGRP and EET’s were investigated every 4 h. In conscious freely moving 1K-1C AH rats unlike SO animals blood pressure (BP), heart period (HP) and BRS underwent significant circadian fluctuations, with more marked increase in mean values of BP in 8-week hypertensive rats in comparison to 4-week hypertensive rats (179 ± 5 vs. 162 ± 4 mm Hg, p < 0.05). These alterations correlated with more significant reduction in HP (138 ± 5 vs. 150 ± 6 ms, p < 0,05) and BRS (0.44 ± 0.04 vs. 0.58 ± 0.04 ms mm Hg^–1, p < 0.05) in 8-week 1K-1C AH rats. The acrophases of BP in 8-week 1K-1C AH rats in comparison with 4-week were shifted to more late night hours (1:58 a.m. vs. 11:32 p.m.) and in both groups of animals corresponded to lowest circadian plasma levels of CGRP and EETs and to greatest level of ET1. SO rats were characterized by lower values of BP (121 ± 3 mm Hg, p < 0,05) and higher indices of HP (158 ± 2 ms, p < 0,05) and BRS (0.86 ± 0.02 ms mmHg^–1, p < 0,001) in comparison with 1K-1C AH rats 4-week duration. The acrophases of BP, HP and BRS in hypertensive animals were revealed at 14.8 ± 0.5 h, 13.6 ± 0.4 h and 13.1 ± 0.2 h, which correlated with maximal circadian contents of ET1 and CGRP at 24:00 h and EETs at 12:00 h and were shifted in comparison to sham-operated group. In rats with 1K-1C AH, plasma levels of ET1, CGRP and EETs undergo circadian fluctuation with corresponding alterations in CHI and BRS which are more markedly expressed on the late stage of diseases and could be used in future for predictive, preventive, and personalized treatment of arterial hypertension.

Elevated plasma endothelin-1 is associated with reduced weight loss post vertical sleeve gastrectomy

BACKGROUND

Obesity and insulin resistance are positively correlated with plasma endothelin-1 (ET-1) levels; however, the mechanisms leading to increased ET-1 are not understood. Similarly, the full physiological complexity of ET-1 has yet to be described, especially in obesity. To date, one of the best treatments available for morbid obesity is bariatric surgery to quickly reduce body fat and the factors associated with obesity-related disease; however, the effects of vertical sleeve gastrectomy (SG) on plasma ET-1 have not been described.

OBJECTIVES

To determine if SG will reduce plasma ET-1 levels and to determine if plasma ET-1 concentration is associated with weight loss after surgery.

SETTING

The studies were undertaken at a University Hospital.

METHODS

This was tested by measuring plasma ET-1 levels from 12 obese patients before and after SG. All data were collected from clinic visits before SG, 6 weeks after SG, and 6 months after surgery.

RESULTS

At 6 weeks after SG, plasma ET-1 levels increased by 24%; however, after 6 months, there was a 27% decrease compared with presurgery. Average weight loss in this cohort was 11.3% ± 2.4% body weight after 6 weeks and 21.4% ± 5.7% body weight after 6 months. Interestingly, we observed an inverse relationship between baseline plasma ET-1 and percent body weight loss (R² = .49, P = .01) and change in body mass index 6 months (R² = .45, P = .011) post bariatric surgery.

CONCLUSIONS

Our results indicate that SG reduces plasma ET-1 levels, a possible mechanism for improved metabolic risk in these patients. These data also suggest that ET-1 may serve as a predictor of weight loss after bariatric surgery.

High dietary sodium causes dyssynchrony of the renal molecular clock in rats

Speed JS, Hyndman KA, Roth K, Heimlich JB, Kasztan M, Fox BM, Johnston JG, Becker BK, Jin C, Gamble KL, Young ME, Pollock JS, Pollock DM. High dietary sodium causes dyssynchrony of the renal molecular clock in rats. Am J Physiol Renal Physiol 314: F89-F98, 2018. First published September 27, 2017; doi:10.1152/ajprenal.00028.2017.-Dyssynchrony of circadian rhythms is associated with various disorders, including cardiovascular and metabolic diseases. The cell autonomous molecular clock maintains circadian control; however, environmental factors that may cause circadian dyssynchrony either within or between organ systems are poorly understood. Our laboratory recently reported that the endothelin (ET-1) B (ETB) receptor functions to facilitate Na+ excretion in a time of day-dependent manner. Therefore, the present study was designed to determine whether high salt (HS) intake leads to circadian dyssynchrony within the kidney and whether the renal endothelin system contributes to control of the renal molecular clock. We observed that HS feeding led to region-specific alterations in circadian clock components within the kidney. For instance, HS caused a significant 5.5-h phase delay in the peak expression of Bmal1 and suppressed Cry1 and Per2 expression in the renal inner medulla, but not the renal cortex, of control rats. The phase delay in Bmal1 expression appears to be mediated by ET-1 because this phenomenon was not observed in the ETB-deficient rat. In cultured inner medullary collecting duct cells, ET-1 suppressed Bmal1 mRNA expression. Furthermore, Bmal1 knockdown in these cells reduced epithelial Na+ channel expression. These data reveal that HS feeding leads to intrarenal circadian dyssynchrony mediated, in part, through activation of ETB receptors within the renal inner medulla.

Diurnal pattern in skin Na+ and water content is associated with salt-sensitive hypertension in ETB receptor-deficient rats

Impairment in the ability of the skin to properly store Na⁺ nonosmotically (without water) has recently been hypothesized as contributing to salt-sensitive hypertension. Our laboratory has shown that endothelial production of endothelin-1 (ET-1) is crucial to skin Na⁺ handling. Furthermore, it is well established that loss of endothelin type B receptor (ET_B) receptor function impairs Na⁺ excretion by the kidney. Thus we hypothesized that rats lacking functional ET_B receptors (ET_B-def) will have a reduced capacity of the skin to store Na⁺ during chronic high-salt (HS) intake. We observed that ET_B-def rats exhibited salt-sensitive hypertension with an approximate doubling in the diurnal amplitude of mean arterial pressure compared with genetic control rats on a HS diet. Two weeks of HS diet significantly increased skin Na⁺ content relative to water; however, there was no significant difference between control and ET_B-def rats. Interestingly, HS intake led to a 19% increase in skin Na⁺ and 16% increase in water content (relative to dry wt.) during the active phase (zeitgeber time 16) versus inactive phase (zeitgeber time 4, P < 0.05) in ET_B-def rats. There was no significant circadian variation in total skin Na⁺ or water content of control rats fed normal or HS. These data indicate that ET_B receptors have little influence on the ability to store Na⁺ nonosmotically in the skin during long-term HS intake but, rather, appear to regulate diurnal rhythms in skin Na⁺ content and circadian blood pressure rhythms associated with a HS diet.

Activation of neuronal endothelin B receptors mediates pressor response through alpha-1 adrenergic receptors

Abnormalities in activity of the endothelin (ET) system have been widely reported in a number of cardiovascular disease states such as hypertension and heart failure. Although the vascular responses to ET are well established, the interaction between ET and other important modulators of blood pressure, such as the sympathetic nervous system, are less understood. Previous reports implicate ET signaling through ET type B (ET_B) receptors in increasing neuronal activity. Therefore, we hypothesized that activation of ET_B receptors on sympathetic nerves would increase blood pressure through an adrenergic‐mediated mechanism. Thus, we used anesthetized ET_B‐deficient rats, which only express functional ET_B receptors on adrenergic neurons, and genetic controls, which express functional ET_B receptors in vascular tissue and kidney epithelium. We determined the pressor response to the selective ET_B receptor agonist sarafotoxin c (S6c). Separate groups of rats were treated with the α₁‐adrenergic receptor antagonist prazosin or the β‐adrenergic receptor antagonist propranolol to elucidate the role of adrenergic signaling in mediating the blood pressure response. We observed a dose‐dependent pressor response to S6c in ET_B‐deficient rats that was reversed by prazosin treatment and augmented by propranolol. In genetic control rats, the effects of S6c on sympathetic neurons were mostly masked by the direct activity of ET_B receptor activation on the vasculature. Heart rate was mostly unaffected by S6c across all groups and treatments. These results suggest that ET_B activation on sympathetic neurons causes an increase in blood pressure mediated through α₁‐adrenergic receptor signaling.

The Matrikine Acetylated Proline-Glycine-Proline Couples Vascular Inflammation and Acute Cardiac Rejection

The extracellular matrix (ECM) is a dynamic, bioactive structure critical to organ development, structure and function. Excessive remodeling of the ECM is a hallmark of a variety of inflammatory conditions including vascular disease. Endothelin-1 (ET1) synthesis is understood to promote cardiovascular diseases including acute cardiac transplant rejection; however, the contribution of ECM-derived chemokines (matrikines) to vascular inflammation remains poorly understood. Herein we report that the matrikine acetylated Pro-Gly-Pro (PGP) stimulates vascular inflammation through activation of endothelial CXC Chemokine Receptor 2 (CXCR2) and production of endothelin-1 both in vitro and in vivo. As a proof of hypothesis, we demonstrate that coronary PGP levels associate with both circulating endothelin-1 and acute rejection in cardiac transplant patients (sensitivity of 100% and specificity of 86%). These findings establish PGP as a novel mediator in cardiovascular disease, and implicate bioactive matrix fragments as underappreciated agents potentially active in numerous conditions propagated by progressive vascular inflammation.

Plasma levels of endothelin-1 and renal function among young and healthy adults

Background:

Endothelin-1 (ET-1), a vasoconstrictive and pro-inflammatory peptide, is associated with several cardiovascular risk factors and outcomes. We aimed to investigate the association of plasma ET-1 levels and renal function among young and healthy adults.

Methods:

Individuals aged 25–41 years were enrolled in a population-based cohort study. Main exclusion criteria were established kidney disease, cardiovascular diseases, diabetes mellitus and a body mass index>35 kg/m2. Fasting venous plasma samples were used to measure creatinine, cystatin C and ET-1. The estimated glomerular filtration rate (eGFR) was calculated using the creatinine based chronic kidney disease epidemiology collaboration (CKD-EPI) formula. Multivariable regression models were constructed to assess interrelationships of plasma ET-1 with parameters of renal function.

Results:

Median age of the 2139 participants was 37 years, 47% males. Median creatinine and eGFR were 67 μmol/L and 112 mL/min/1.73 m2, respectively. Using quartile one as the reference group, the β-coefficients (95% confidence intervals [CIs]) for eGFR were 0.06 (− 1.22 to 1.35),−0.66 (− 1.95 to 0.62) and−1.70 (− 3.01 to−0.39) for quartiles 2–4 (p-for-trend=0.0056), respectively and β-coefficients (95% CIs) for cystatin C were 0.002 (− 0.01 to 0.02), 0.02 (0.003–0.03) and 0.03 (0.01–0.04) for quartiles 2–4 (p-for-trend<0.0001), respectively. Using ET-1 as a continuous variable, the β-coefficient (95% CI) for eGFR per 1-unit increase was−1.82 (− 3.19 to−0.44, p=0.0095) and 0.02 (0.01–0.04, p=0.0003) for cystatin C. Similar results were found between creatinine and ET-1 levels.

Conclusions:

ET-1 levels are strongly associated with parameters of renal function among young and healthy adults, suggesting an important role of ET-1 and endothelial function in the regulation of kidney function.

Renal denervation attenuates hypertension but not salt sensitivity in ETB receptor-deficient rats

Hypertension is a prevalent pathology that increases risk for numerous cardiovascular diseases. Because the etiology of hypertension varies across patients, specific and effective therapeutic approaches are needed. The role of renal sympathetic nerves is established in numerous forms of hypertension, but their contribution to salt sensitivity and interaction with factors such as endothelin-1 are poorly understood. Rats deficient of functional ET_B receptors (ET_B-def) on all tissues except sympathetic nerves are hypertensive and exhibit salt-sensitive increases in blood pressure. We hypothesized that renal sympathetic nerves contribute to hypertension and salt sensitivity in ET_B-def rats. The hypothesis was tested through bilateral renal sympathetic nerve denervation and measuring blood pressure during normal salt (0.49% NaCl) and high-salt (4.0% NaCl) diets. Denervation reduced mean arterial pressure in ET_B-def rats compared with sham-operated controls by 12 ± 3 (SE) mmHg; however, denervation did not affect the increase in blood pressure after 2 wk of high-salt diet (+19 ± 3 vs. +16 ± 3 mmHg relative to normal salt diet; denervated vs. sham, respectively). Denervation reduced cardiac sympathetic-to-parasympathetic tone [low frequency-high frequency (LF/HF)] during normal salt diet and vasomotor LF/HF tone during high-salt diet in ET_B-def rats. We conclude that the renal sympathetic nerves contribute to the hypertension but not to salt sensitivity of ET_B-def rats.

Diabetic nephropathy: New insights into established therapeutic paradigms and novel molecular targets

Diabetic nephropathy is one of the most prevalent microvascular complication in patients suffering from diabetes and is reported to be the major cause of renal failure when compared to any other kidney disease. Currently, available therapies provide only symptomatic relief and unable to treat the underlying pathophysiology of diabetic nephropathy. This review will explore new insights into the established therapeutic paradigms targeting oxidative stress, inflammation and endoplasmic reticulum stress with the focus on recent clinical developments. Apart from this, the involvement of novel cellular and molecular mechanisms including the role of endothelin-receptor antagonists, Wnt signaling pathway, epigenetics and micro RNA is also discussed so that key molecular switches involved in the pathogenesis of diabetic nephropathy can be identified. Elucidating new molecular pathways will help in the development of novel therapeutics for the prevention and treatment of diabetic nephropathy.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Cardiac Fibrosis: The Fibroblast Awakens

Myocardial fibrosis is a significant global health problem associated with nearly all forms of heart disease. Cardiac fibroblasts comprise an essential cell type in the heart that is responsible for the homeostasis of the extracellular matrix; however, upon injury, these cells transform to a myofibroblast phenotype and contribute to cardiac fibrosis. This remodeling involves pathological changes that include chamber dilation, cardiomyocyte hypertrophy and apoptosis, and ultimately leads to the progression to heart failure. Despite the critical importance of fibrosis in cardiovascular disease, our limited understanding of the cardiac fibroblast impedes the development of potential therapies that effectively target this cell type and its pathological contribution to disease progression. This review summarizes current knowledge regarding the origins and roles of fibroblasts, mediators and signaling pathways known to influence fibroblast function after myocardial injury, as well as novel therapeutic strategies under investigation to attenuate cardiac fibrosis.

Endothelin, sex, and pregnancy: unique considerations for blood pressure control in females

Endothelin-1 (ET-1) is a potent vasoconstrictor, and dysregulation of the endothelin (ET) system has been implicated in the development of hypertension. Sex differences in the ET system have been identified in ET receptor expression and activation, levels of ET-1, and downstream mediators of the ET system. More specifically, males have greater ET-1/ETA receptor activation, whereas females exhibit greater ETB receptor activation. These differences have been suggested to contribute to the sex differences observed in blood pressure control, with greater ETB receptor activation in females potentially acting as an important pathway contributing to the lower prevalence of hypertension in young females compared with age-matched males. This hypothesis is further supported by studies in pregnancy; the role of the ET system is enhanced during pregnancy, with dysregulation of the ET system resulting in preeclampsia. Further research is necessary to elucidate the relative roles of the ET system in blood pressure control in both sexes and to further explore the potential benefits of pharmacological ET blockade in women.

Endothelin and renal ion and water transport

The renal tubular epithelial cells produce more endothelin-1 (ET-1) than any other cell type in the body. Moving down the nephron, the amount of ET-1 produced appears fairly consistent until reaching the inner medullary collecting duct, which produces at least 10 times more ET-1 than any other segment. ET-1 inhibits Na(+) transport in all parts of the nephron through activation of the ETB receptor, and, to a minor extent, the ETA receptor. These effects are most prominent in the collecting duct where ETB-receptor activation inhibits activity of the epithelial Na(+) channel. Effects in other parts of the nephron include inhibition of Na(+)/H(+) exchange in the proximal tubule and the Na(+), K(+), 2Cl(-) co-transporter in the thick ascending limb. In general, the renal epithelial ET-1 system is an integral part of the body's response to a high salt intake to maintain homeostasis and normal blood pressure. Loss of ETB-receptor function results in salt-sensitive hypertension. The role of renal ET-1 and how it affects Na(+) and water transport throughout the nephron is reviewed.

Sex-Specific Effect of Endothelin in the Blood Pressure Response to Acute Angiotensin II in Growth-Restricted Rats

The renal endothelin system contributes to sex differences in blood pressure with males demonstrating greater endothelin type-A receptor-mediated responses relative to females. Intrauterine growth restriction programs hypertension and enhance renal sensitivity to acute angiotensin II in male growth-restricted rats. Endothelin is reported to work synergistically with angiotensin II. Thus, this study tested the hypothesis that endothelin augments the blood pressure response to acute angiotensin II in male growth-restricted rats. Systemic and renal hemodynamics were determined in response to acute angiotensin II (100 mg/kg per minute for 30 minutes) with and without the endothelin type-A receptor antagonist, Atrasentan (ABT-627; 10 ng/kg per minute for 30 minutes), in rats pretreated with enalapril (250 mg/L for 1 week) to normalize the endogenous renin-angiotensin system. Endothelin type-A receptor blockade reduced angiotensin II-mediated increases in blood pressure in male control and male growth-restricted rats. Endothelin type-A receptor blockade also abolished hyper-responsiveness to acute angiotensin II in male growth-restricted rats. Yet, blood pressure remained significantly elevated above baseline after endothelin type-A receptor blockade, suggesting that factors in addition to endothelin contribute to the basic angiotensin II-induced pressor response in male rats. We also determined sex-specific effects of endothelin on acute angiotensin II-mediated hemodynamic responses. Endothelin type-A receptor blockade did not reduce acute angiotensin II-mediated increases in blood pressure in female control or growth-restricted rats, intact or ovariectomized. Thus, these data suggest that endothelin type-A receptor blockade contributes to hypersensitivity to acute angiotensin II in male growth-restricted rats and further supports the sex-specific effect of endothelin on blood pressure.

Endothelin contributes to blunted renal autoregulation observed with a high-salt diet

Autoregulation of renal blood flow (RBF) is an essential function of the renal microcirculation that has been previously shown to be blunted by excessive dietary salt. Endogenous endothelin 1 (ET-1) is increased following a high-salt (HS) diet and contributes to the control of RBF but the differential effects of ET-1 on renal microvessel autoregulation in response to HS remain to be established. We hypothesized that a HS diet increases endothelin receptor activation in normal Sprague-Dawley rats and blunts autoregulation of RBF. The role of ET-1 in the blunted autoregulation produced by a HS diet was assessed in vitro and in vivo using the blood-perfused juxtamedullary nephron preparation and anesthetized rats, respectively. Using highly selective antagonists, we observed that blockade of either ETA or ETB receptors was sufficient to restore normal autoregulatory behavior in afferent arterioles from HS-fed rats. Additionally, normal autoregulatory behavior was restored in vivo in HS-fed rats by simultaneous ETA and ETB receptor blockade, whereas blockade of ETB receptors alone showed significant improvement of normal autoregulation of RBF. Consistent with this observation, autoregulation of RBF in ETB receptor-deficient rats fed HS was similar to both ETB-deficient rats and transgenic control rats on normal-salt diets. These data support the hypothesis that endogenous ET-1, working through ETB and possibly ETA receptors, contributes to the blunted renal autoregulatory behavior in rats fed a HS diet.

The renal dopaminergic system: novel diagnostic and therapeutic approaches in hypertension and kidney disease

Salt sensitivity of blood pressure, whether in hypertensive or normotensive subjects, is associated with increased cardiovascular risk and overall mortality. Salt sensitivity can be treated by reducing NaCl consumption. However, decreasing salt intake in some may actually increase cardiovascular risk, including an increase in blood pressure, that is, inverse salt sensitivity. Several genes have been associated with salt sensitivity and inverse salt sensitivity. Some of these genes encode proteins expressed in the kidney that are needed to excrete a sodium load, for example, dopamine receptors and their regulators, G protein-coupled receptor kinase 4 (GRK4). We review here research in this field that has provided several translational opportunities, ranging from diagnostic tests to gene therapy, such as (1) a test in renal proximal tubule cells isolated from the urine of humans that may determine the salt-sensitive phenotype by analyzing the recruitment of dopamine D1 receptors to the plasma membrane; (2) the presence of common GRK4 gene variants that are not only associated with hypertension but may also be predictive of the response to antihypertensive therapy; (3) genetic testing for polymorphisms of the dopamine D2 receptor that may be associated with hypertension and inverse salt sensitivity and may increase the susceptibility to chronic kidney disease because of loss of the antioxidant and anti-inflammatory effects of the renal dopamine D2 receptor, and (4) in vivo renal selective amelioration of renal tubular genetic defects by a gene transfer approach, using adeno-associated viral vectors introduced to the kidney by retrograde ureteral infusion.

High salt diet increases the pressor response to stress in female, but not male ETB-receptor-deficient rats

Acute stress in both rodents and humans causes a transient rise in blood pressure associated with an increase in plasma endothelin-1 (ET-1). High salt (HS) intake also increases ET-1 production, and interestingly, blunts the pressor response to acute air jet stress in rats. We previously reported that female rats lacking functional ET_B receptors everywhere except sympathetic nerves (ET_B def) had a greater degree of hypertension in response to a HS diet compared to their male counterparts when measured by the tail cuff method. However, we now report that salt-induced hypertension is not different between sexes when measured by telemetry. Therefore, additional experiments were designed to test the hypothesis that female ET_B def rats are more sensitive to acute stress when on a HS diet. The pressor response, measured by telemetry, to acute air jet stress was similar between male transgenic control (Tg control) and ET_B def rats following chronic HS intake. In contrast, female ET_B def rats had a significantly greater pressor response (about twofold higher) than female or male Tg control or male ET_B def rats maintained on HS, a finding that cannot be explained by increased vascular reactivity to ET-1 in female rats as we observed that male ET_B def rats had a greater pressor response to i.v. infusion of ET-1 compared to females. Furthermore, HS feeding exacerbated the pressor response to ET-1 in both male and female ET_B def rats. Given our previous studies demonstrating that the ET_A receptor functions to reduce the pressor response to acute stress, these findings further support a role for the ET receptor system in the pressor response to acute stress and that female rats have reduced ET_A receptor activity when on a HS diet compared to males.

Inhibition of ENaC by endothelin-1

The amiloride-sensitive epithelial Na(+) channel (ENaC) is a key player in the regulation of Na(+) homeostasis. Its functional activity is under continuous control by a variety of signaling molecules, including bioactive peptides of endothelin family. Since ENaC dysfunction is causative for disturbances in total body Na(+) levels associated with the abnormal regulation of blood volume, blood pressure, and lung fluid balance, uncovering the molecular mechanisms of inhibitory modulation or inappropriate activation of ENaC is crucial for the successful treatment of a variety of human diseases including hypertension. The precise regulation of ENaC is particularly important for normal Na(+) and fluid homeostasis in organs where endothelins are known to act: the kidneys, lung, and colon. Inhibition of ENaC by endothelin-1 (ET-1) has been established in renal cells, and several molecular mechanisms of inhibition of ENaC by ET-1 are proposed and will be reviewed in this chapter.

Cre recombinase-regulated Endothelin1 transgenic mouse lines: novel tools for analysis of embryonic and adult disorders

Endothelin-1 (EDN1) influences both craniofacial and cardiovascular development and a number of adult physiological conditions by binding to one or both of the known endothelin receptors, thus initiating multiple signaling cascades. Animal models containing both conventional and conditional loss of the Edn1 gene have been used to dissect EDN1 function in both embryos and adults. However, while transgenic Edn1 over-expression or targeted genomic insertion of Edn1 has been performed to understand how elevated levels of Edn1 result in or exacerbate disease states, an animal model in which Edn1 over-expression can be achieved in a spatiotemporal-specific manner has not been reported. Here we describe the creation of Edn1 conditional over-expression transgenic mouse lines in which the chicken β-actin promoter and an Edn1 cDNA are separated by a strong stop sequence flanked by loxP sites. In the presence of Cre, the stop cassette is removed, leading to Edn1 expression. Using the Wnt1-Cre strain, in which Cre expression is targeted to the Wnt1-expressing domain of the central nervous system (CNS) from which neural crest cells (NCCs) arise, we show that stable chicken β-actin-Edn1 (CBA-Edn1) transgenic lines with varying EDN1 protein levels develop defects in NCC-derived tissues of the face, though the severity differs between lines. We also show that Edn1 expression can be achieved in other embryonic tissues utilizing other Cre strains, with this expression also resulting in developmental defects. CBA-Edn1 transgenic mice will be useful in investigating diverse aspects of EDN1-mediated-development and disease, including understanding how NCCs achieve and maintain a positional and functional identity and how aberrant EDN1 levels can lead to multiple physiological changes and diseases.

A Salt-Induced Reno-Cerebral Reflex Activates Renin-Angiotensin Systems and Promotes CKD Progression

Salt intake promotes progression of CKD by uncertain mechanisms. We hypothesized that a salt-induced reno-cerebral reflex activates a renin-angiotensin axis to promote CKD. Sham-operated and 5/6-nephrectomized rats received a normal-salt (0.4%), low-salt (0.02%), or high-salt (4%) diet for 2 weeks. High salt in 5/6-nephrectomized rats increased renal NADPH oxidase, inflammation, BP, and albuminuria. Furthermore, high salt activated the intrarenal and cerebral, but not the systemic, renin-angiotensin axes and increased the activity of renal sympathetic nerves and neurons in the forebrain of these rats. Renal fibrosis was increased 2.2-fold by high versus low salt, but intracerebroventricular tempol, losartan, or clonidine reduced this fibrosis by 65%, 69%, or 59%, respectively, and renal denervation or deafferentation reduced this fibrosis by 43% or 38%, respectively (all P<0.05). Salt-induced fibrosis persisted after normalization of BP with hydralazine. These data suggest that the renal and cerebral renin-angiotensin axes are interlinked by a reno-cerebral reflex that is activated by salt and promotes oxidative stress, fibrosis, and progression of CKD independent of BP.

Endothelin@25 - new agonists, antagonists, inhibitors and emerging research frontiers: IUPHAR Review 12

Since the discovery of endothelin (ET)-1 in 1988, the main components of the signalling pathway have become established, comprising three structurally similar endogenous 21-amino acid peptides, ET-1, ET-2 and ET-3, that activate two GPCRs, ETA and ETB . Our aim in this review is to highlight the recent progress in ET research. The ET-like domain peptide, corresponding to prepro-ET-193-166 , has been proposed to be co-synthesized and released with ET-1, to modulate the actions of the peptide. ET-1 remains the most potent vasoconstrictor in the human cardiovascular system with a particularly long-lasting action. To date, the major therapeutic strategy to block the unwanted actions of ET in disease, principally in pulmonary arterial hypertension, has been to use antagonists that are selective for the ETA receptor (ambrisentan) or that block both receptor subtypes (bosentan). Macitentan represents the next generation of antagonists, being more potent than bosentan, with longer receptor occupancy and it is converted to an active metabolite; properties contributing to greater pharmacodynamic and pharmacokinetic efficacy. A second strategy is now being more widely tested in clinical trials and uses combined inhibitors of ET-converting enzyme and neutral endopeptidase such as SLV306 (daglutril). A third strategy based on activating the ETB receptor, has led to the renaissance of the modified peptide agonist IRL1620 as a clinical candidate in delivering anti-tumour drugs and as a pharmacological tool to investigate experimental pathophysiological conditions. Finally, we discuss biased signalling, epigenetic regulation and targeting with monoclonal antibodies as prospective new areas for ET research.

Endothelin and endothelin antagonists in chronic kidney disease

The incidence and prevalence of chronic kidney disease (CKD), with diabetes and hypertension accounting for the majority of cases, is on the rise, with up to 160 million individuals worldwide predicted to be affected by 2020. Given that current treatment options, primarily targeted at the renin-angiotensin system, only modestly slow down progression to end-stage renal disease, the urgent need for additional effective therapeutics is evident. Endothelin-1 (ET-1), largely through activation of endothelin A receptors, has been strongly implicated in renal cell injury, proteinuria, inflammation, and fibrosis leading to CKD. Endothelin receptor antagonists (ERAs) have been demonstrated to ameliorate or even reverse renal injury and/or fibrosis in experimental models of CKD, whereas clinical trials indicate a substantial antiproteinuric effect of ERAs in diabetic and nondiabetic CKD patients even on top of maximal renin-angiotensin system blockade. This review summarizes the role of ET in CKD pathogenesis and discusses the potential therapeutic benefit of targeting the ET system in CKD, with attention to the risks and benefits of such an approach.

End o' the line revisited: moving on from nitric oxide to CGRP

When endothelin-1(ET-1) was discovered it was hailed as the prototypical endothelium-derived contracting factor (EDCF). However, over the years little evidence emerged convincingly demonstrating that the peptide actually contributes to moment-to-moment changes in vascular tone elicited by endothelial cells. This has been attributed to the profound inhibitory effect of nitric oxide (NO) on both the production (by the endothelium) and the action (on vascular smooth muscle) of ET-1. Hence, the peptide is likely to initiate acute changes in vascular diameter only under extreme conditions of endothelial dysfunction when the NO bioavailability is considerably reduced if not absent. The present essay discusses whether or not this concept should be revised, in particular in view of the potent inhibitory effect exerted by calcitonin gene related peptide (CGRP) released from sensorimotor nerves on vasoconstrictor responses to ET-1.

Sex differences in the developmental programming of hypertension

Experimental models of developmental programming provide proof of concept and support Barker's original findings that link birthweight and blood pressure. Many experimental models of developmental insult demonstrate a sex difference with male offspring exhibiting a higher blood pressure in young adulthood relative to their age-matched female counterparts. It is well recognized that men exhibit a higher blood pressure relative to age-matched women prior to menopause. Yet, whether this sex difference is noted in individuals born with low birthweight is not clear. Sex differences in the developmental programming of blood pressure may originate from innate sex-specific differences in expression of the renin angiotensin system that occur in response to adverse influences during early life. Sex differences in the developmental programming of blood pressure may also involve the influence of the hormonal milieu on regulatory systems key to the long-term control of blood pressure such as the renin angiotensin system in adulthood. In addition, the sex difference in blood pressure in offspring exposed to a developmental insult may involve innate sex differences in oxidative status or the endothelin system or may be influenced by age-dependent changes in the developmental programming of cardiovascular risk factors such as adiposity. Therefore, this review will highlight findings from different experimental models to provide the current state of knowledge related to the mechanisms that contribute to the aetiology of sex differences in the developmental programming of blood pressure and hypertension.

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.