Search

Stages of CKD are currently defined by eGFR and require measurement of serum creatinine concentrations. Previous studies have shown a good correlation between salivary and serum urea levels and the stage of CKD. However, quantitative salivary urea assays in current clinical use require costly and labor-intensive commercial kits, which restricts the advantage of using saliva and limits wider applicability as a quick and easy means of assessing renal function. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy has been shown to provide a potentially straightforward, reagent-free method for the identification of a range of disease-related biomarkers and is in current clinical use for analyses of the chemical composition of kidney stones. We assessed the feasibility of ATR-FTIR spectroscopy as an alternative method to measure salivary urea in patients with different stages of CKD. The ATR-FTIR spectra of dried saliva samples from six healthy controls and 20 patients with CKD (stages 1–5) were analyzed to provide their urea concentrations. The lower limit of detection of salivary urea by the ATR-FTIR spectroscopy method was 1–2 mM, at the lower end of the clinically relevant range. Statistically significant differences in salivary urea concentrations were demonstrated between healthy subjects (4.1±0.5 mM) and patients with CKD stages 3–5 (CKD stage 3, 6.8±0.7 mM; CKD stage 4, 9.1±1 mM; CKD stage 5, 14.8±1.6 mM). These salivary urea concentrations correlated well with serum urea levels in the same patients measured by an automated analyzer (Spearman rank correlation coefficient of 0.71; P

Metabolic acidosis, defined as a plasma or serum bicarbonate concentration, Graphical Abstract Graphical Abstract

Phosphate homeostasis is dependent on the interaction and coordination of four main organ systems: thyroid/parathyroids, gastrointestinal tract, bone and kidneys, and three key hormonal regulators, 1,25-hydroxyvitamin D3, parathyroid hormone and FGF23 with its co- factor klotho. Phosphorus is a critical nutritional element for normal cellular function, but in excess can be toxic to tissues, particularly the vasculature. As phosphate, it also has an important interaction and inter-dependence with calcium and calcium homeostasis sharing some of the same controlling hormones, although this is not covered in our article. We have chosen to provide a current overview of phosphate homeostasis only, focusing on the role of two major organ systems, the gastrointestinal tract and kidneys, and their contribution to the control of phosphate balance. We describe in some detail the mechanisms of intestinal and renal phosphate transport, and compare and contrast their regulation. We also consider a significant example of phosphate imbalance, with phosphate retention, which is chronic kidney disease; why consequent hyperphosphatemia is important, and some of the newer means of managing it.

Objective Kidney stone disease seems to be associated with an increased risk of incident cardiovascular outcomes; the aim of this study is to identify differences in 24-h urine excretory profiles and stone composition among stone formers with and without cardiovascular disease (CVD). Methods Data from patients attending the Department of Renal Medicine’s metabolic stone clinic from 1995 to 2012 were reviewed. The sample was divided according to the presence or absence of CVD (myocardial infarction, angina, coronary revascularization, or surgery for calcified heart valves). Univariable and multivariable regression models, adjusted for age, sex, BMI, hypertension, diabetes, eGFR, plasma bicarbonate and potential renal acid load of foods were used to investigate differences across groups. Results 1826 patients had available data for 24-h urine analysis. Among these, 108 (5.9%) had a history of CVD. Those with CVD were older, have higher prevalence of hypertension and diabetes and lower eGFR. Univariable analysis showed that patients with CVD had significantly lower 24-h urinary excretions for citrate (2.4 vs 2.6 mmol/24 h, p = 0.04), magnesium (3.9 vs 4.2 mmol/24 h, p = 0.03) and urinary pH (6.1 vs 6.2, p = 0.02). After adjustment for confounders, differences in urinary citrate and magnesium excretions remained significant. No differences in the probability of stone formation or stone compositions were found. Conclusions Stone formers with CVD have lower renal alkali excretion, possibly suggesting higher acid retention in stone formers with cardiovascular comorbidities. Randomized clinical trials including medications and a controlled diet design are needed to confirm the results presented here. Graphic abstract

Progress in the medical treatment and management of nephrolithiasis has been limited to date and continues to depend on urinary metabolic screening to assess excretion of the main stone constituents, factors determining stone solubility and precipitation, and on dietary and lifestyle recommendations.In this review, we try to highlight some of the broader aspects of kidney stone disease in relation to recent epidemiological and pathophysiological findings, and emerging new treatments. Specifically, this review will cover recent evidence on the association between metabolic risk factors and kidney stone disease, dietary risk factors, and dietary interventions to prevent kidney stones, and how genomics, metabolomics, and proteomics may improve diagnosis and treatment of this troublesome, if rarely fatal, condition. PubMed was used to identify the most suitable references according to our search strategy; only full manuscripts were included.What is emerging is that kidney stone disease is not an isolated disorder but is systemic in nature with links to important and common comorbidities such as diabetes, hypertension, cardiovascular disease, and chronic kidney disease. These associations support the need to take nephrolithiasis seriously as a medical condition and to adopt a more holistic approach to its investigation and treatment.

Excessive activation of the mineralocorticoid receptor (MR) underlies the pathophysiology of heart failure and chronic kidney disease. Hyperkalemia risk limits the therapeutic use of conventional MR antagonists. AZD9977 is a nonsteroidal, selective MR modulator that may protect nonepithelial tissues without disturbing electrolyte balance. This phase I study investigated the safety, tolerability, pharmacokinetics, and pharmacodynamics of multiple oral doses of AZD9977 in healthy volunteers. Twenty-seven male participants aged 23-45 years were randomized 3:1 to receive oral AZD9977 or placebo for 8 days (with twice-daily dosing on days 2-7), in dose cohorts of 50, 150, and 300 mg (AZD9977, n = 6 per cohort; placebo, n = 3 per cohort). Adverse events occurred in 4 of 18 participants receiving AZD9977 (22.2%) and 6 of 9 receiving placebo (66.7%), all of mild or moderate severity; none were serious or led to withdrawal. AZD9977 was rapidly absorbed, with median time of maximum concentration of 0.50-0.84 hours across dose groups. Area under the curve and maximum concentration were approximately dose proportional but elimination and accumulation terminal half-life increased with dose. Steady-state was reached after 3-4 days, with dose-dependent accumulation of 1.2-1.7-fold. Renal clearance was 5.9-6.5 L/hour and 24-37% of AZD9977 was excreted in the urine. Serum aldosterone levels increased dose dependently from days -1 to 7 in participants receiving AZD9977, but serum potassium levels and urinary electrolyte excretion were unchanged. AZD9977 was generally well-tolerated with no safety concerns. Exploratory outcomes suggested reduced hyperkalemia risk compared with MR antagonists. These findings support further clinical development of AZD9977.

BACKGROUND: Myeloperoxidase released after neutrophil and monocyte activation can generate reactive oxygen species, leading to host tissue damage. Extracellular glomerular myeloperoxidase deposition, seen in ANCA-associated vasculitis, may enhance crescentic GN through antigen-specific T and B cell activation. Myeloperoxidase-deficient animals have attenuated GN early on, but augmented T cell responses. We investigated the effect of myeloperoxidase inhibition, using the myeloperoxidase inhibitor AZM198, to understand its potential role in treating crescentic GN. METHODS: We evaluated renal biopsy samples from patients with various forms of crescentic GN for myeloperoxidase and neutrophils, measured serum myeloperoxidase concentration in patients with ANCA-associated vasculitis and controls, and assessed neutrophil extracellular trap formation, reactive oxygen species production, and neutrophil degranulation in ANCA-stimulated neutrophils in the absence and presence of AZM198. We also tested the effect of AZM198 on ANCA-stimulated neutrophil-mediated endothelial cell damage in vitro, as well as on crescentic GN severity and antigen-specific T cell reactivity in the murine model of nephrotoxic nephritis. RESULTS: All biopsy specimens with crescentic GN had extracellular glomerular myeloperoxidase deposition that correlated significantly with eGFR and crescent formation. In vitro, AZM198 led to a significant reduction in neutrophil extracellular trap formation, reactive oxygen species production, and released human neutrophil peptide levels, and attenuated neutrophil-mediated endothelial cell damage. In vivo, delayed AZM198 treatment significantly reduced proteinuria, glomerular thrombosis, serum creatinine, and glomerular macrophage infiltration, without increasing adaptive T cell responses. CONCLUSIONS: Myeloperoxidase inhibition reduced neutrophil degranulation and neutrophil-mediated endothelial cell damage in patients with ANCA-associated vasculitis. In preclinical crescentic GN, delayed myeloperoxidase inhibition suppressed kidney damage without augmenting adaptive immune responses, suggesting it might offer a novel adjunctive therapeutic approach in crescentic GN.

The proximal tubule is responsible for reclaiming water, phosphates and amino acids from the tubular filtrate. There are genetic defects in both phosphate and amino acid transporters leading to nephrolithiasis. This review also explores genetic defects in regulators of phosphate and calcium transport in this nephron segment that lead to stone formation.

Aims:\ud \ud Recent reports suggest that iron deficiency impacts both intestinal calcium and phosphate absorption, although the exact transport pathways and intestinal segment responsible have not been determined. Therefore, we aimed to systematically investigate the impact of iron deficiency on the cellular mechanisms of transcellular and paracellular calcium and phosphate transport in different regions of the rat small intestine.\ud \ud Methods:\ud \ud Adult, male Sprague‐Dawley rats were maintained on a control or iron deficient diet for two weeks and changes in intestinal calcium and phosphate uptake were determined using the in situ intestinal loop technique. The circulating levels of the hormonal regulators of calcium and phosphate were determined by ELISA, while the expression of transcellular calcium and phosphate transporters, and intestinal claudins were determined using qPCR and western blotting.\ud \ud Results:\ud \ud Diet‐induced iron deficiency significantly increased calcium absorption in the duodenum but had no impact in the jejunum and ileum. In contrast, phosphate absorption was significantly inhibited in the duodenum and to a lesser extent the jejunum, but remained unchanged in the ileum. The changes in duodenal calcium and phosphate absorption in the iron deficient animals were associated with increased claudin 2 and 3 mRNA and protein levels, while levels of parathyroid hormone, fibroblast growth factor‐23 and 1,25‐dihydroxy vitamin D3 were unchanged.\ud \ud Conclusion:\ud \ud We propose that iron deficiency alters calcium and phosphate transport in the duodenum. This occurs via changes to the paracellular pathway, whereby upregulation of claudin 2 increases calcium absorption and upregulation of claudin 3 inhibits phosphate absorption.

Aquaporin‐2 (AQP2) is a homotetrameric water channel responsible for the final water reuptake in the kidney. Disease‐causing AQP2 mutations induce nephrogenic diabetes insipidus (NDI), a condition that challenges the bodily water balance by producing large urinary volumes. In this study, we characterize three new AQP2 mutations identified in our lab from NDI patients (A120D, A130V, T179N) along the previously reported A47V variant. Using Xenopus oocytes, we compared the key functional and biochemical features of these mutations against classical recessive (R187C) and dominant (R254Q) forms, and once again found clear functional recovery features (increased protein stability and function) for all mutations under study. This behaviour, attributed to heteromerization to wt‐AQP2, challenge the classical model to NDI which often depicts recessive mutations as ill‐structured proteins unable to oligomerize. Consequently, we propose a revised model to the cell pathophysiology of AQP2‐related NDI which accounts for the functional recovery of recessive AQP2 mutations., Mutations to AQP2 induce nephrogenic diabetes insipidus (NDI), where low/inactive water channels cause large urinary waste of possible fatal outcome. Our past studies have shown that several AQP2 mutations could be functionally recovered by associating to wild‐type AQP2 (wt‐AQP2) into the protein complex (tetramer). This study presents the characterization of three new AQP2 mutations from actual case‐studies in regard to recovery capacities, and conclude by proposing a revised pathophysiological model to NDI, one where AQP2 mutations are now presented as mostly recoverable through wt‐AQP2 association into functional water channels.

KEY POINTS: The presence of plasma proteins in urine is difficult to interpret quantitatively. It may be a result of impaired glomerular filtration or impaired proximal tubule (PT) reabsorption, or both. Dent1 disease (CLCN5 mutation) abolishes PT protein reabsorption leaving glomerular function intact. Using urine protein measurements from patients with Dent1 disease and normal individuals, we devised a mathematical model that incorporates two PT transport processes with distinct kinetics. This model predicts albumin, α1 -microglobulin (α1 -m), β2 -microglobulin (β2 -m) and retinol-binding protein 4 (RBP4) urine concentrations. Our results indicate that the urinary excretion of β2 -m and RBP4 differs from that of albumin and α1 -m in their sensitivity to changes in the glomerular filtration rate, glomerular protein leak, tubular protein uptake via endocytosis and PT water reabsorption. The model predicts quantitatively how hyperfiltration and glomerular leak interact to promote albuminuria. Our model should contribute to improved understanding and interpretation of urine protein measurements in renal disease.ABSTRACT: To clarify the relative contributions of glomerular filtration and tubular uptake to urinary protein excretion, we developed a mathematical model of protein reabsorption in the human proximal tubule (PT) using Michaelis-Menten kinetics and molar urinary protein measurements taken from human Dent1 disease (CLCN5 loss-of-function mutation). β2 -Microglobulin (β2 -m) and retinol-binding protein 4 (RBP4) are normally reabsorbed with 'very high' efficiency uptake kinetics and fractional urinary excretion of 0.025%, whereas albumin and α1 -microglobulin (α1 -m) are reabsorbed by 'high' efficiency uptake kinetics and 50-fold higher fractional urinary excretion of 1.15%. Our model correctly predicts the urinary β2 -m, RBP4 and α1 -m content in aristolochic acid nephropathy, and elevated β2 -m excretion with increased single nephron glomerular filtration rate (SNGFR) following unilateral-nephrectomy. We explored how altered endocytic uptake, water reabsorption, SNGFR and glomerular protein filtration affect excretion. Our results help to explain why β2 -m and RBP4 are more sensitive markers of PT dysfunction than albumin or α1 -m, and suggest that reduced PT sodium and water reabsorption in Fanconi syndrome may contribute to proteinuria. Transition of albumin excretion from normal to microalbuminuria, a 5-fold increase, corresponds to a 3.5-fold elevation in albumin glomerular filtration, supporting the use of microalbuminuria screening to detect glomerular leak in diabetes. In macroalbuminuria, small albumin permeability changes produce large changes in excretion. However, changes in SNGFR can alter protein excretion, and hyperfiltration with glomerular leak can combine to increase albuminuria. Our model provides a validated quantitative description of the transport processes underlying the protein composition of human urine in normal and pathophysiological states.

The proximal tubule (PT) reabsorbs most of the glomerular filtrate and plays an important role in the uptake, metabolism and excretion of xenobiotics. Some therapeutic drugs are harmful to the PT, and resulting nephrotoxicity is thought to be responsible for approximately 1 in 6 of cases of children hospitalized with acute kidney injury (AKI). Clinically, PT dysfunction leads to urinary wasting of important solutes normally reabsorbed by this nephron segment, leading to systemic complications such as bone demineralization and a clinical scenario known as the renal Fanconi syndrome (RFS). While PT defects can be diagnosed using a combination of blood and urine markers, including urinary excretion of low molecular weight proteins (LMWP), standardized definitions of what constitutes clinically significant toxicity are lacking, and identifying which patients will go on to develop progressive loss of kidney function remains a major challenge. In addition, much of our understanding of cellular mechanisms of drug toxicity is still limited, partly due to the constraints of available cell and animal models. However, advances in new and more sophisticated in vitro models of the PT, along with the application of high-content analytical methods that can provide readouts more relevant to the clinical manifestations of nephrotoxicity, are beginning to extend our knowledge. Such technical progress should help in discovering new biomarkers that can better detect nephrotoxicity earlier and predict its long-term consequences, and herald a new era of more personalized medicine.

IntroductionDiabetic kidney disease (DKD) is the leading cause of end-stage kidney disease worldwide and a major cause of premature mortality in diabetes mellitus (DM). While improvements in care have reduced the incidence of kidney disease among those with DM, the increasing prevalence of DM means that the number of patients worldwide with DKD is increasing. Improved understanding of the biology of DKD and identification of novel therapeutic targets may lead to new treatments. A major challenge to progress has been the heterogeneity of the DKD phenotype and renal progression. To investigate the heterogeneity of DKD we have set up The East and North London Diabetes Cohort (HEROIC) Study, a secondary care-based, multiethnic observational study of patients with biopsy-proven DKD. Our primary objective is to identify histological features of DKD associated with kidney endpoints in a cohort of patients diagnosed with type 1 and type 2 DM, proteinuria and kidney impairment.Methods and analysisHEROIC is a longitudinal observational study that aims to recruit 500 patients with DKD at high-risk of renal and cardiovascular events. Demographic, clinical and laboratory data will be collected and assessed annually for 5 years. Renal biopsy tissue will be collected and archived at recruitment. Blood and urine samples will be collected at baseline and during annual follow-up visits. Measured glomerular filtration rate (GFR), echocardiography, retinal optical coherence tomography angiography and kidney and cardiac MRI will be performed at baseline and twice more during follow-up. The study is 90% powered to detect an association between key histological and imaging parameters and a composite of death, renal replacement therapy or a 30% decline in estimated GFR.Ethics and disseminationEthical approval has been obtained from the Bloomsbury Research Ethics Committee (REC 18-LO-1921). Any patient identifiable data will be stored on a password-protected National Health Services N3 network with full audit trail. Anonymised imaging data will be stored in a ISO27001-certificated data warehouse.Results will be reported through peer-reviewed manuscripts and conferences and disseminated to participants, patients and the public using web-based and social media engagement tools as well as through public events.

Ischemia is a major cause of kidney damage. Proximal tubular epithelial cells (PTECs) are highly susceptible to ischemic insults that frequently cause acute kidney injury (AKI), a potentially life-threatening condition with high mortality. Accumulating evidence has identified altered mitochondrial function as a central pathologic feature of AKI. The mitochondrial NAD+-dependent enzyme sirtuin 5 (SIRT5) is a key regulator of mitochondrial form and function, but its role in ischemic renal injury (IRI) is unknown. SIRT5 expression was increased in murine PTECs after IRI in vivo and in human PTECs (hPTECs) exposed to an oxygen/nutrient deprivation (OND) model of IRI in vitro. SIRT5-depletion impaired ATP production, reduced mitochondrial membrane potential, and provoked mitochondrial fragmentation in hPTECs. Moreover, SIRT5 RNAi exacerbated OND-induced mitochondrial bioenergetic dysfunction and swelling, and increased degradation by mitophagy. These findings suggest SIRT5 is required for normal mitochondrial function in hPTECs and indicate a potentially important role for the enzyme in the regulation of mitochondrial biology in ischemia.

Cognitive impairment is an increasingly recognized major cause of chronic disability and is commonly found in patients with chronic kidney disease (CKD). Knowledge of the relationship between kidney dysfunction and impaired cognition may improve our understanding of other forms of cognitive dysfunction. Patients with CKD are at an increased risk (compared with the general population) of both dementia and its prodrome, mild cognitive impairment (MCI), which are characterized by deficits in executive functions, memory and attention. Brain imaging in patients with CKD has revealed damage to white matter in the prefrontal cortex and, in animal models, in the subcortical monoaminergic and cholinergic systems, accompanied by widespread macrovascular and microvascular damage. Unfortunately, current interventions that target cardiovascular risk factors (such as anti-hypertensive drugs, anti-platelet agents and statins) seem to have little or no effect on CKD-associated MCI, suggesting that the accumulation of uraemic neurotoxins may be more important than disturbed haemodynamic factors or lipid metabolism in MCI pathogenesis. Experimental models show that the brain monoaminergic system is susceptible to uraemic neurotoxins and that this system is responsible for the altered sleep pattern commonly observed in patients with CKD. Neural progenitor cells and the glymphatic system, which are important in Alzheimer disease pathogenesis, may also be involved in CKD-associated MCI. More detailed study of CKD-associated MCI is needed to fully understand its clinical relevance, underlying pathophysiology, possible means of early diagnosis and prevention, and whether there may be novel approaches and potential therapies with wider application to this and other forms of cognitive decline.

In proteinuric renal diseases, excessive plasma nonesterified free fatty acids bound to albumin can leak across damaged glomeruli to be reabsorbed by renal proximal tubular cells and cause inflammatory tubular cells damage by as yet unknown mechanisms. The present study was designed to investigate these mechanisms induced by palmitic acid (PA; one of the nonesterified free fatty acids) overload. Our results show that excess PA stimulates ATP release through the pannexin 1 channel in human renal tubule epithelial cells (HK-2), increasing extracellular ATP concentration approximately threefold compared with control. The ATP release is dependent on caspase-3/7 activation induced by mitochondrial reactive oxygen species. Furthermore, extracellular ATP aggravates PA-induced monocyte chemoattractant protein-1 secretion and monocyte infiltration of tubular cells, enlarging the inflammatory response in both macrophages and HK-2 cells via the purinergic P2X7 receptor-mammalian target of rapamycin-forkhead box O1-thioredoxin-interacting protein/NOD-like receptor protein 3 inflammasome pathway. Hence, PA increases mitochondrial reactive oxygen species-induced ATP release and inflammatory stress, which cause a “first hit,” while ATP itself is a “second hit” in amplifying the renal tubular inflammatory response. Thus, inhibition of ATP release or the purinergic P2X7 receptor may be an approach to reduce renal inflammation and improve renal function.

Rationale and objective Kidney stones are a common condition in the general population, however, high-quality evidence for its management is scarce. We propose the creation of an international network with the aim of sharing practice patterns and patient data towards an improvement of our knowledge of the disease. Study design Cross-sectional survey. Setting and participants An online survey was circulated through several scientific societies. Items were grouped into six domains. Each center’s overall score (OS) was also calculated. Analytical approach Chi square and Mann–Whitney tests were performed for differences across centers. Results The countries that contributed most were Italy (8.6%), Turkey (6.6%), France and Spain (6.1%). Some type of nutritional work-up was implemented in 62% of centers. A DEXA scan was performed by 46% of centers, whereas some kind of acidification test was performed by 25% of centers. Most centers (80%) implemented blood investigations at least at baseline. With regard to 24-h urine exams, 7 out of 16 were performed by at least 50% of centers. Information on stone composition was collected by 58% of centers. The OS was significantly higher among higher-volume centers compared with lower-volume centers (p = 0.002). Significant differences between EU and non-EU centers were found. Limitations Cross-sectional design; no validation on information. Conclusions Our survey highlights the potential for the creation of a network of centers that could share information in a common database for observational research and for enrollment of patients in interventional trials.

Alpha intercalated cells (αICs) in the kidney collecting duct (CD) belong to a family of mitochondria rich cells (MRCs) and have a crucial role in acidifying the urine via apical V-ATPase pumps. The nature of metabolism in αICs and its relationship to transport was not well-understood. Here, using multiphoton live cell imaging in mouse kidney tissue, FIB-SEM, and other complementary techniques, we provide new insights into mitochondrial structure and function in αICs. We show that αIC mitochondria have a rounded structure and are not located in close proximity to V-ATPase containing vesicles. They display a bright NAD(P)H fluorescence signal and low uptake of voltage-dependent dyes, but are energized by a pH gradient. However, expression of complex V (ATP synthase) is relatively low in αICs, even when stimulated by metabolic acidosis. In contrast, anaerobic glycolytic capacity is surprisingly high, and sufficient to maintain intracellular calcium homeostasis in the presence of complete aerobic inhibition. Moreover, glycolysis is essential for V-ATPase-mediated proton pumping. Key findings were replicated in narrow/clear cells in the epididymis, also part of the MRC family. In summary, using a range of cutting-edge techniques to investigate αIC metabolism in situ, we have discovered that these mitochondria dense cells have a high glycolytic capacity.

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4− and HPO42−). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

This not an original publication or a current and up-to-date review of purinergic signalling and kidney function, but rather a tribute to Professor Geoffrey Burnstock, written as a short and personal memoir of our early collaborative work together on this topic: our beginnings and the subsequent journey we took with our many valued collaborators along the way.

To date, the role of the small intestine in regulating post-prandial phosphate homeostasis has remained unclear and controversial. Previous studies have proposed the presence of a gut-derived phosphaturic factor that acts independently of changes in plasma phosphate concentration or parathyroid hormone (PTH) level; however, these early studies used duodenal luminal phosphate concentrations in the molar range and therefore the physiological relevance of this is uncertain. In the present study, we used both in vivo and in vitro approaches to investigate the presence of this putative ‘intestinal phosphatonin’. Instillation of 1.3 M phosphate into the duodenum rapidly induced phosphaturia, but in contrast to previous reports, this was associated with significant hyperphosphataemia and elevated PTH level; however, there was not the expected decrease in abundance of the renal sodium-phosphate cotransporter NaPi-IIa. Instillation of a physiological (10 mM) phosphate load had no effect on plasma phosphate concentration, PTH level or phosphate excretion. Moreover, phosphate uptake by opossum kidney cells was unaffected after incubation with serosal fluid collected from intestinal segments perfused with different phosphate concentrations. Taken together, these findings do not support the concept of a gut-derived phosphaturic factor that can mediate rapid signalling between gut and kidney, leading to increased urinary phosphate excretion, as part of normal phosphate homeostasis. This article is protected by copyright. All rights reserved

New Findings What is the topic of this review? This review summarizes the evidence on the localization, electrophysiological properties, agonist specificity and putative physiological role of sodium–glucose transporter 3 (SGLT3). What advances does it highlight? Published information is reviewed in some detail by comparing human and rodent isoforms, as well as advances in testing hypotheses for the physiological role of SGLT3 as a glucose sensor or incretin release mediator. We provide a critical overview of available published data and discuss a putative functional role for SGLT3 in human and mouse physiology. Sodium–glucose transporter 3 (SGLT3) has attracted interest because of its putative role as a glucose sensor, rather than a sugar transporter, in contrast to its co-family members SGLT1 and SGLT2. Significant progress has been made in characterizing the electrophysiological properties in vitro of the single human SGLT3 isoform and the two mouse isoforms, SGLT3a and SGLT3b. Although early reports indicated SGLT3 expression in the small intestinal myenteric and submucosal neurones, hypothalamic neurones, portal vein and kidney, a lack of reliable antibodies has left unanswered its exact tissue and cellular localization. Several hypotheses for a role of SGLT3 in glucose sensing, gastric emptying, glucagon-like peptide-1 release and post-Roux-en-Y gastric bypass remodelling have been explored, but so far there is only limited and indirect supportive evidence using non-specific agonists/antagonists, with no firm conclusions. There are no published or available data in knockout animals, and translation is difficult because of its different isoforms in human versus rodent, as well as a lack of selective agonists or antagonists, all of which make SGLT3 challenging to study. However, its unique electrophysiological properties, ubiquitous expression at the mRNA level, enrichment in the small intestine and potential, but uncertain, physiological role demand more attention. The purpose of this overview and review of SGLT3 biology is to provide an update, highlight the gaps in our knowledge and try to signpost potential ways forward to define its likely function in vivo.

Previous studies in our laboratory have suggested that P2X7 could contribute to the progression of diabetic nephropathy and modulated klotho expression. The aim of this study was to investigate if P2X7 receptor is related to the expression of klotho in the onset of diabetic nephropathy in rats. Seven-week-old male Wistar rats weighing 210 g were all uninephrectomized; two-third of the animals were induced to diabetes with 60 mg/kg streptozotocin i.v., and one-third received its vehicle (control rats). At 4th day of the fifth week of the protocol, half of the diabetic rats received a small interfering RNA targeting for P2X7 mRNA, and the other half received its vehicle. Euthanasia was made at the eighth week. Diabetic animals reproduced all classic symptoms of the disease; besides, they showed reduced renal function and low NO bioavailability; also, SOD1, SOD2, and catalase were increased, probably due to the oxidative stress which was elevated in this situation. Metabolic data of diabetic rats did not change by silencing P2X7 receptor. For the other hand, silencing P2X7 was able to contribute to balance oxidative and nitrosative profile, ultimately improving the renal function and increasing plasma and membrane forms of klotho. These findings suggest that the management of P2X7 receptor can benefit the kidneys with diabetic nephropathy. Further studies are needed to show the therapeutic potential of this receptor inhibition to provide a better quality of life for the diabetic patient. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s11302-020-09695-1) contains supplementary material, which is available to authorized users.

Oxalate crystal-induced renal inflammation is associated with progressive kidney failure due to activation of the NLRP3/CASP-1 inflammasome. It has been suggested previously that purinergic P2X7 receptor signaling is critical for crystal-induced inflammasome activation and renal injury. Therefore, we investigated the role of the P2X7 receptor in response to crystal-induced cytokine release, inflammation, and kidney failure using in vitro and in vivo models. Dendritic cells and macrophages derived from murine bone marrow and human peripheral blood mononucleated cells stimulated with calcium-oxalate crystals, monosodium urate crystals, or ATP lead to the robust release of interleukin-1beta (IL-1ß). Treatment with the P2X7 inhibitor A740003 or the depletion of ATP by apyrase selectively abrogated ATP-induced, but not oxalate and urate crystal-induced IL-1ß release. In line with this finding, dendritic cells derived from bone marrow (BMDCs) from P2X7−/− mice released reduced amounts of IL-1ß following stimulation with ATP, while oxalate and urate crystal-induced IL-1ß release was unaffected. In sharp contrast, BMDCs from Casp1−/− mice exhibited reduced IL-1ß release following either of the three stimulants. In addition, P2X7−/− mice demonstrated similar degrees of crystal deposition, tubular damage and inflammation when compared with WT mice. In line with these findings, increases in plasma creatinine were no different between WT and P2X7−/− mice. In contrast to previous reports, our results indicate that P2X7 receptor is not required for crystal-induced CKD and it is unlikely to be a suitable therapeutic target for crystal-induced progressive kidney disease.

Background and purpose Excessive MR activation is implicated in the pathogenesis of cardiovascular disease. MR antagonists (MRA) are effective and key guideline-directed therapy in patients with heart failure (HF) with reduced ejection fraction. However, clinical adverse events due to hyperkalaemia and a decline in renal function significantly restricts the treatment. AZD9977 is a novel, non-steroidal selective MR modulator that, compared with MRA, displays a differentiated MR-binding pattern and cofactor recruitment profile, and exerts similar organ protective effects to eplerenone with minimal effects on urinary electrolyte handling. Methods We performed a randomized, placebo-controlled, single-blind phase 1 study to evaluate the safety, tolerability and pharmacokinetics of oral AZD9977 in healthy men. Three sequential ascending dose cohorts were evaluated, and participants received either AZD9977 (n=6) or placebo (n=3) for 8 days. AZD9977 target doses of 50mg bid, 150mg bid and 300mg bid were explored based on a model predicted dose range comparable to and exceeding the target receptor occupancy levels achieved with approved MRA drugs. The dose of AZD9977 50mg bid is predicted to be equipotent to spironolactone 25mg daily. Results 27 healthy male subjects aged 23–45 years completed the study. All doses of AZD9977 were well tolerated with no safety concerns identified. A total of 7 adverse events occurred in 4 participants (22.2%) receiving AZD9977 and 8 events occurred in 6 participants (66.7%) receiving placebo, none severe or resulting in withdrawal from the study. Rapid absorption of AZD9977 was observed with median Tmax values ranging from 0.5 to 0.8-hours post-dose and approximate dose proportional kinetics between 50 to 300 mg twice daily, as measured by AUCτ and Cmax. Steady-state levels in plasma were generally reached within 3–4 days. Target engagement was confirmed by a robust dose-dependent rise in mean serum aldosterone levels between Day −1 and Day 7 (Placebo = 43±172pmol/L, AZD9977 50mg bid = 132±83pmol/L, 150mg bid = 447±242pmol/L, 300mg bid = 808±330pmol/L; Figure 1). AZD9977 had no effect on blood pressure or heart rate and did not alter serum electrolyte levels, urinary Na+ excretion, log urinary Na+/K+ ratio (Figure 2), estimated GFR, or urine volumes when compared to placebo-treated subjects. Figure 1 and Figure 2 Conclusion These observations in humans are consistent with pre-clinical studies, demonstrating robust MR-receptor engagement without changes in serum K+ and renal electrolyte handling. These characteristics could potentially realise the therapeutic benefit of MR blockade with a low risk of hyperkalaemia. This hypothesis is currently being tested clinically in a head-to-head trial of AZD9977 versus spironolactone in patients with HF and impaired renal function (NCT03682497). Acknowledgement/Funding The study was Sponsored and funded by AstraZeneca

The kidney proximal tubule is the first part of the nephron after the filtering glomerulus and performs the bulk of reabsorption of filtered solutes. Proximal tubular cells are highly adapted to this task and express numerous sodium-coupled transporters. They also express two large receptors called

Chronic kidney disease (CKD) is now seen as a systemic disease involving also the central nervous system [1], but the link between the kidney and different organ systems and disease went unnoticed for a long time. The king of Poland, Stephen Bathory (1533-86), suffered from CKD due to polycystic kidney disease and depression [2]. Similarly, Wolfgang Amadeus Mozart was also thought to have had CKD [3] and depression [4]. A list of 'Famous People Who Have Died from Kidney Disease' [5] includes many who suffered from both CKD and depression or other signs of mental illness. Is this a coincidence or actually evidence of a link between kidney disease and brain dysfunction? This is not merely an academic question because all forms of mental illness can seriously impair an individual's quality of life, and are frequently associated with progression of diseases and premature mortality, so it is worth the effort of trying to answer it. Europe and much of the industrialized countries are experiencing growing numbers of patients with CKD within their ageing populations [6]. CKD is complex and potentially fatal: (i) all organs are affected, sooner or later; (ii) the balance of plasma volume, electrolytes, acid-base and minerals, metabolites, hormones and proteins is disturbed; and (iii) patients often need a multidisciplinary team approach managing complex comorbidities, drug regimens and special diets. Although the prognosis of patients with CKD remains poor, their increasing life expectancy has shifted medical attention from life-threatening emergencies to long-term complications and sequelae, and how to improve quality of life [7]. Indeed, kidney failure has detrimental effects on health-related quality of life (HRQoL), reaching levels similar to those seen in patients with metastatic cancer [8]. This might be due to psychological factors, both kidney disease and cancer being chronic diseases with a bad prognosis. However, although the effect of CKD on quality of life is more evident in advanced stages (stage G4P) and in older patients [9, 10], a large study has shown a significant decrease in HRQoL as early as CKD stage G2 [11]. Notably, neurological and cognitive impairments [12], and depression [13] are among the most debilitating consequences of CKD contributing to the significantly reduced HRQoL [14].

Currently an evidence-based approach to nephrolithiasis is hampered by a lack of randomized controlled trials. Thus, there is a need for common platforms for data sharing and recruitment of patients to interventional studies. A first step in achieving this objective would be to share practice methods and protocols for subsequent standardization in what is still a heterogeneous clinical field. Here, we present the results of a pilot survey performed across 24 European clinical kidney stone centers. The survey was distributed by a voluntary online questionnaire circulated between June 2017 and January 2018. About 46% of centers reported seeing on average 20 or more patients per month. Only 21% adopted any formal referral criteria. Centers were relatively heterogeneous in respect of the definition of an incident stone event. The majority (71%) adopted a formal follow-up scheme; of these, 65% included a follow-up visit at 3 and 12 months, and 41% more than 12 months. In 79% of centers some kind of imaging was performed systematically. 75% of all centers performed laboratory analyses on blood samples at baseline and during follow-up. All centers performed laboratory analyses on 24-h urine samples, the majority (96%) at baseline and during follow-up. There was good correspondence across centers for analyses performed on 24-h urine samples, although the methods of 24-h urine collection and analysis were relatively heterogeneous. Our survey among 24 European stone centers highlights areas of homogeneity and heterogeneity that will be investigated further. Our aim is the creation of a European network of stone centers sharing practice patterns and hosting a common database for research and guidance in clinical care.

Historically, the control of renal vascular and tubular function has, for the most part, concentrated on neural and endocrine regulation. However, in addition to these extrinsic factors, it is now appreciated that several complex humoral control systems exist within the kidney that can act in an autocrine and/or paracrine fashion. These paracrine systems complement neuroendocrine regulation by dynamically fine-tuning renal vascular and tubular function to buffer rapid changes in nephron perfusion and flow rate of tubular fluid. One of the most pervasive is the extracellular nucleotide/P2 receptor system, which is central to many of the intrinsic regulatory feedback loops within the kidney such as renal haemodynamic autoregulation and tubuloglomerular feedback (TGF). Although physiological actions of extracellular adenine nucleotides were reported almost 100 years ago, the conceptual framework for purinergic regulation of renal function owes much to the work of Geoffrey Burnstock. In this review, we reflect on our >20-year collaboration with Professor Burnstock and highlight the research that is still unlocking the potential of the renal purinergic system to understand and treat kidney disease.

Aim We aimed to determine whether the sodium/glucose cotransporter family member SGLT3, a proposed glucose sensor, is expressed in the intestine and/or kidney, and if its expression is altered in mouse models of obesity and in humans before and after weight-loss surgery. Main methods We used in-situ hybridization and quantitative PCR to determine whether the Sglt3 isoforms 3a and 3b were expressed in the intestine and kidney of C57, leptin-deficient ob/ob, and diabetic BTBR ob/ob mice. Western blotting and immunohistochemistry were also used to assess SGLT3 protein levels in jejunal biopsies from obese patients before and after weight-loss Roux-en-Y gastric bypass surgery (RYGB), and in lean healthy controls. Key findings Sglt3a/3b mRNA was detected in the small intestine (duodenum, jejunum and ileum), but not in the large intestine or kidneys of mice. Both isoforms were detected in epithelial cells (confirmed using intestinal organoids). Expression of Sglt3a/3b mRNA in duodenum and jejunum was significantly lower in ob/ob and BTBR ob/ob mice than in normal-weight littermates. Jejunal SGLT3 protein levels in aged obese patients before RYGB were lower than in lean individuals, but substantially upregulated 6 months post-RYGB. Significance Our study shows that Sglt3a/3b is expressed primarily in epithelial cells of the small intestine in mice. Furthermore, we observed an association between intestinal mRNA Sglt3a/3b expression and obesity in mice, and between jejunal SGLT3 protein levels and obesity in humans. Further studies are required to determine the possible role of SGLT3 in obesity.

The International Society of Nephrology Forefronts Symposium Immunomodulation of Cardio-Renal Function took place October 22 to 25, 2015, in Shenzhen, China. The program covered basic and clinical aspects of cardio-renal pathophysiology and immunity. Leading scientists from different and related disciplines of clinical and basic research described and reviewed recent discoveries, and discussed emerging topics under the headings “Immunity and Renal Pathophysiology”; “Autoimmunity and the Inflammasome”; “Immunity and the Gut Microbiome”; “Immuno-Metabolism”; “Immunogenetics, Transcriptomics and Epigenetics; “Immunity and Hypertension”; and “Immunity, Fibrosis, and Kidney Disease.”

What is the central question of this study? Although SGLT2 inhibitors represent a promising treatment for patients suffering from diabetic nephropathy, the influence of metabolic disruption on the expression and function of glucose transporters is largely unknown. What is the main finding and its importance? In vivo models of metabolic disruption (Goto-Kakizaki type II diabetic rat and junk-food diet) demonstrate increased expression of SGLT1, SGLT2 and GLUT2 in the proximal tubule brush border. In the type II diabetic model, this is accompanied by increased SGLT- and GLUT-mediated glucose uptake. A fasted model of metabolic disruption (high-fat diet) demonstrated increased GLUT2 expression only. The differential alterations of glucose transporters in response to varying metabolic stress offer insight into the therapeutic value of inhibitors. SGLT2 inhibitors are now in clinical use to reduce hyperglycaemia in type II diabetes. However, renal glucose reabsorption across the brush border membrane (BBM) is not completely understood in diabetes. Increased consumption of a Western diet is strongly linked to type II diabetes. This study aimed to investigate the adaptations that occur in renal glucose transporters in response to experimental models of diet-induced insulin resistance. The study used Goto-Kakizaki type II diabetic rats and normal rats rendered insulin resistant using junk-food or high-fat diets. Levels of protein kinase C-βI (PKC-βI), GLUT2, SGLT1 and SGLT2 were determined by Western blotting of purified renal BBM. GLUT- and SGLT-mediated d-[(3) H]glucose uptake by BBM vesicles was measured in the presence and absence of the SGLT inhibitor phlorizin. GLUT- and SGLT-mediated glucose transport was elevated in type II diabetic rats, accompanied by increased expression of GLUT2, its upstream regulator PKC-βI and SGLT1 protein. Junk-food and high-fat diet feeding also caused higher membrane expression of GLUT2 and its upstream regulator PKC-βI. However, the junk-food diet also increased SGLT1 and SGLT2 levels at the proximal tubule BBM. Glucose reabsorption across the proximal tubule BBM, via GLUT2, SGLT1 and SGLT2, is not solely dependent on glycaemic status, but is also influenced by diet-induced changes in glucose metabolism. We conclude that different metabolic disturbances result in complex adaptations in renal glucose transporter protein levels and function.

Several previous studies have reported an increase in Hounsfield unit density of the renal papillae in patients with nephrolithiasis compared with controls. Kidney stone formers (KSF) were found to have higher papillary and cortical density in both kidneys, irrespective of which side had calculi, and it was proposed that this might be related to the presence of underlying hypercalciuria. The current study was designed: (1) to determine whether recurrent KSF do have higher papillary density compared with healthy controls; (2) to test an association between higher renal papillary density and the presence of hypercalciuria in KSF. This retrospective case-matched controlled study was carried out at the Royal Free Hospital, London, UK. We investigated 111 patients, 57 of whom were KSF and 54 healthy controls. The CT attenuation values were measured within a 0.2 cm(2) area of the renal papilla in the upper, middle, and lower segments of each kidney, and were compared between KSF and non-stone formers, and between KSF with and without hypercalciuria. There were no significant differences in age and sex between groups. Papillary density was significantly higher in KSF by both crude and adjusted analyses (p 0.001). However, there was no association between higher papillary density and hypercalciuria in KSF. The papillary density measured by CT is a useful, non-invasive tool to differentiate between KSF and healthy controls. The absence of any correlation between papillary density and hypercalciuria suggests that the presence of clinically significant underlying renal stone disease, rather than urinary metabolic abnormalities, correlates with radiologically detectable increased papillary density.

International audience; Gain-of-function mutations in with no lysine (K) 1 (WNK1) and WNK4 genes are responsible for familial hyperkalemic hypertension (FHHt), a rare, inherited disorder characterized by arterial hypertension and hyperkalemia with metabolic acidosis. More recently, FHHt-causing mutations in the Kelch-like 3-Cullin 3 (KLHL3-CUL3) E3 ubiquitin ligase complex have shed light on the importance of WNK's cellular degradation on renal ion transport. Using full exome sequencing for a 4-generation family and then targeted sequencing in other suspected cases, we have identified new missense variants in the WNK1 gene clustering in the short conserved acidic motif known to interact with the KLHL3-CUL3 ubiquitin complex. Affected subjects had an early onset of a hyperkalemic hyperchloremic phenotype, but normal blood pressure values"Functional experiments in Xenopus laevis oocytes and HEK293T cells demonstrated that these mutations strongly decrease the ubiquitination of the kidney-specific isoform KS-WNK1 by the KLHL3-CUL3 complex rather than the long ubiquitous catalytically active L-WNK1 isoform. A corresponding CRISPR/Cas9 engineered mouse model recapitulated both the clinical and biological phenotypes. Renal investigations showed increased activation of the Ste20 proline alanine-rich kinase-Na+-Cl- cotransporter (SPAK-NCC) phosphorylation cascade, associated with impaired ROMK apical expression in the distal part of the renal tubule. Together, these new WNK1 genetic variants highlight the importance of the KS-WNK1 isoform abundance on potassium homeostasis.

Background For many patients with kidney failure, the cause and underlying defect remain unknown. Here, we describe a novel mechanism of a genetic order characterized by renal Fanconi syndrome and kidney failure. Methods We clinically and genetically characterized members of five families with autosomal dominant renal Fanconi syndrome and kidney failure. We performed genome-wide linkage analysis, sequencing, and expression studies in kidney biopsy specimens and renal cells along with knockout mouse studies and evaluations of mitochondrial morphology and function. Structural studies examined the effects of recognized mutations. Results The renal disease in these patients resulted from monoallelic mutations in the gene encoding glycine amidinotransferase (GATM), a renal proximal tubular enzyme in the creatine biosynthetic pathway that is otherwise associated with a recessive disorder of creatine deficiency. In silico analysis showed that the particular GATM mutations, identified in 28 members of the five families, create an additional interaction interface within the GATM protein and likely cause the linear aggregation of GATM observed in patient biopsy specimens and cultured proximal tubule cells. GATMaggregates-containing mitochondria were elongated and associated with increased ROS production, activation of the NLRP3 inflammasome, enhanced expression of the profibrotic cytokine IL-18, and increased cell death. Conclusions In this novel genetic disorder, fully penetrant heterozygous missense mutations in GATM trigger intramitochondrial fibrillary deposition of GATM and lead to elongated and abnormal mitochondria. We speculate that this renal proximal tubular mitochondrial pathology initiates a response from the inflammasome, with subsequent development of kidney fibrosis.

In the kidney, purinergic (P2) receptor‐mediated ATP signaling has been shown to be an important local regulator of epithelial sodium transport. Appropriate sodium regulation is crucial for blood pressure (BP) control and disturbances in sodium balance can lead to hypo‐ or hypertension. Links have already been established between P2 receptor signaling and the development of hypertension, attributed mainly to vascular and/or inflammatory effects. A transgenic mouse model with deletion of the P2X4 receptor (P2X4−/−) is known to have hypertension, which is thought to reflect endothelial dysfunction and impaired nitric oxide (NO) release. However, renal function in this model has not been characterized; moreover, studies in vitro have shown that the P2X4 receptor can regulate renal epithelial Na+ channel (ENaC) activity. Therefore, in the present study we investigated renal function and sodium handling in P2X4−/− mice, focusing on ENaC‐mediated Na+ reabsorption. We confirmed an elevated BP in P2X4−/− mice compared with wild‐type mice, but found that ENaC‐mediated Na+ reabsorption is no different from wild‐type and does not contribute to the raised BP observed in the knockout. However, when P2X4−/− mice were placed on a low sodium diet, BP normalized. Plasma aldosterone concentration tended to increase according to sodium restriction status in both genotypes; in contrast to wild‐types, P2X4−/− mice did not show an increase in functional ENaC activity. Thus, although the increased BP in P2X4−/− mice has been attributed to endothelial dysfunction and impaired NO release, there is also a sodium‐sensitive component.