Sodium Transport Mechanisms Research Articles

Deep insights into the sodium transport mechanism and phase evolution in Na4VP2O9 cathode/anode material

For the entire intercalation/deintercalation range of Na4VP2O9, quantum mechanics is applied to investigate sodium transport mechanism and a machine learning force field is used instead of ab initio method to offers an applicable method for the full understanding of the complicated Na + movements dynamically. It shows chain of '8′ shaped units horizontally and repeated alternating round aggregate and '8′ shaped long chain vertically. For primitive Na4VP2O9, facile Na+ transportation along a-axis is observed with an energy barrier of 0.16 V. As a cathode material, Na3VP2O9 also shows fast Na + diffusion with a minimal diffusion energy barrier 0.27 eV along a-axis. However, for the fully sodiated state of Na4VP2O9, Na+ diffusion in the lattice is significantly restricted with an energy barrier of 0.92 eV along b-axis. Accordingly, the sodium ion diffusion coefficients at room temperature for Na3VP2O9, Na4VP2O9 and Na5VP2O9 are 1.2 × 10−9 cm2 s−1, 5.9 × 10−9 cm2 s−1 and 8.2 × 10−11 cm2 s−1, respectively. The formation energy convex hull originated from the removal of Na1 sites and the occupation of E8cO8 cavities reveals an intermediate phase, Na3.5VP2O9, for the NaxVP2O9 (3 ≤ x ≤ 4) and three intermediate phases, Na4.5VP2O9, Na4.625VP2O9 and Na4.75VP2O9, for the NaxVP2O9 (4 ≤ x ≤ 5).

Time course of renal sodium transport in the pregnant rat.

Progressive sodium retention and cumulative plasma volume expansion occur to support the developing fetus during pregnancy. Sodium retention is regulated by individual tubular transporters and channels. An increase or decrease in any single transporter could cause a change in sodium balance. Understanding the time-course for changes in each sodium transporter during pregnancy will enable us to understand progressive sodium retention seen in pregnancy. Here, we examined the activity of the major apical sodium transporters found in the nephron using natriuretic response tests in virgin, early pregnant, mid-pregnant, and late pregnant rats. We also measured renal and serum aldosterone levels. We found that furosemide sensitive sodium transport (NKCC2) is only increased during late pregnancy, thiazide sensitive sodium transport (NDCBE/pendrin) is increased in all stages of pregnancy, and that benzamil sensitive sodium transport (ENaC) is increased beginning in mid-pregnancy. We also found that serum aldosterone levels progressively increased throughout gestation and kidney tissue aldosterone levels increased only during late pregnancy. Here we have shown progressive turning on of specific sodium transport mechanisms to help support progressive sodium retention through the course of gestation. These mechanisms contribute to the renal sodium retention and plasma volume expansion required for an optimal pregnancy.

The crystal structures of a chloride-pumping microbial rhodopsin and its proton-pumping mutant illuminate proton transfer determinants

Microbial rhodopsins are versatile and ubiquitous retinal-binding proteins that function as light-driven ion pumps, light-gated ion channels, and photosensors, with potential utility as optogenetic tools for altering membrane potential in target cells. Insights from crystal structures have been central for understanding proton, sodium, and chloride transport mechanisms of microbial rhodopsins. Two of three known groups of anion pumps, the archaeal halorhodopsins (HRs) and bacterial chloride-pumping rhodopsins, have been structurally characterized. Here we report the structure of a representative of a recently discovered third group consisting of cyanobacterial chloride and sulfate ion-pumping rhodopsins, the Mastigocladopsis repens rhodopsin (MastR). Chloride-pumping MastR contains in its ion transport pathway a unique Thr-Ser-Asp (TSD) motif, which is involved in the binding of a chloride ion. The structure reveals that the chloride-binding mode is more similar to HRs than chloride-pumping rhodopsins, but the overall structure most closely resembles bacteriorhodopsin (BR), an archaeal proton pump. The MastR structure shows a trimer arrangement reminiscent of BR-like proton pumps and shows features at the extracellular side more similar to BR than the other chloride pumps. We further solved the structure of the MastR-T74D mutant, which contains a single amino acid replacement in the TSD motif. We provide insights into why this point mutation can convert the MastR chloride pump into a proton pump but cannot in HRs. Our study points at the importance of precise coordination and exact location of the water molecule in the active center of proton pumps, which serves as a bridge for the key proton transfer.

The short-term rinsing of airways by N-acetylcysteine helps expectoration: The mechanism of sodium and chloride transport

N-acetyl-L-cysteine (NAC) mucolytic and antioxidant role is well known, but the effect on epithelial ion transport has not been yet described. The aim of the study was to evaluate the short-term and prolonged influence of NAC on ion transport in the epithelium. The experiment was performed on 108 fragments of rabbit tracheae. Fragments were divided into four groups: inhibited sodium (I) and chloride (II) transport, NAC with inhibited sodium (III) and NAC with inhibited chloride (IV) transport. The changes in electrophysiological parameters were measured in stationary conditions and during mechanical-chemical stimulation after immediate (15 s) and prolonged (60 min) N-acetylcysteine administration on the tissue. Each 15-second stimulation caused repeatable changes in the electric potential of the tissue. In trachea fragments with blocked chloride ion transport, significantly lower (P <0.0001) values of electric potential following prolonged NAC effect were observed when compared to short-term NAC-stimulation. The values of resistance were constant during experiments, which reflects the vitality of the tissue. Short-term NAC administration influences sodium ion transport, which is not observed in a prolonged stimulation. The use of the NAC solution to rinse the airways is of great clinical importance due to the short and intense contact with the epithelium.

Structure of the sodium-dependent phosphate transporter reveals insights into human solute carrier SLC20.

Inorganic phosphate (Pi) is a fundamental and essential element for nucleotide biosynthesis, energy supply, and cellular signaling in living organisms. Human phosphate transporter (hPiT) dysfunction causes numerous diseases, but the molecular mechanism underlying transporters remains elusive. We report the structure of the sodium-dependent phosphate transporter from Thermotoga maritima (TmPiT) in complex with sodium and phosphate (TmPiT-Na/Pi) at 2.3-angstrom resolution. We reveal that one phosphate and two sodium ions (Pi-2Na) are located at the core of TmPiT and that the third sodium ion (Nafore) is located near the inner membrane boundary. We propose an elevator-like mechanism for sodium and phosphate transport by TmPiT, with the TmPiT-Na/Pi complex adopting an inward occluded conformation. We found that disease-related hPiT variants carry mutations in the corresponding sodium- and phosphate-binding residues identified in TmPiT. Our three-dimensional structure of TmPiT provides a framework for understanding PiT dysfunction and for future structure-based drug design.

Role of Deranged Energy Deprivation Signaling in the Pathogenesis of Cardiac and Renal Disease in States of Perceived Nutrient Overabundance.

Sodium-glucose cotransporter 2 inhibitors reduce the risk of serious heart failure and adverse renal events, but the mechanisms that underlie this benefit are not understood. Treatment with SGLT2 inhibitors is distinguished by 2 intriguing features: ketogenesis and erythrocytosis. Both reflect the induction of a fasting-like and hypoxia-like transcriptional paradigm that is capable of restoring and maintaining cellular homeostasis and survival. In the face of perceived nutrient and oxygen deprivation, cells activate low-energy sensors, which include sirtuin-1 (SIRT1), AMP-activated protein kinase (AMPK), and hypoxia inducible factors (HIFs; especially HIF-2α); these enzymes and transcription factors are master regulators of hundreds of genes and proteins that maintain cellular homeostasis. The activation of SIRT1 (through its effects to promote gluconeogenesis and fatty acid oxidation) drives ketogenesis, and working in concert with AMPK, it can directly inhibit inflammasome activation and maintain mitochondrial capacity and stability. HIFs act to promote oxygen delivery (by stimulating erythropoietin and erythrocytosis) and decrease oxygen consumption. The activation of SIRT1, AMPK, and HIF-2α enhances autophagy, a lysosome-dependent degradative pathway that removes dangerous constituents, particularly damaged mitochondria and peroxisomes, which are major sources of oxidative stress and triggers of cellular dysfunction and death. SIRT1 and AMPK also act on sodium transport mechanisms to reduce intracellular sodium concentrations. It is interesting that type 2 diabetes mellitus, obesity, chronic heart failure, and chronic kidney failure are characterized by the accumulation of intracellular glucose and lipid intermediates that are perceived by cells as indicators of energy overabundance. The cells respond by downregulating SIRT1, AMPK, and HIF-2α, thus leading to an impairment of autophagic flux and acceleration of cardiomyopathy and nephropathy. SGLT2 inhibitors reverse this maladaptive signaling by triggering a state of fasting and hypoxia mimicry, which includes activation of SIRT1, AMPK, and HIF-2α, enhanced autophagic flux, reduced cellular stress, decreased sodium influx into cells, and restoration of mitochondrial homeostasis. This mechanistic framework clarifies the findings of large-scale randomized trials and the close association of ketogenesis and erythrocytosis with the cardioprotective and renoprotective benefits of these drugs.

Interplay of adenosine monophosphate-activated protein kinase/sirtuin-1 activation and sodium influx inhibition mediates the renal benefits of sodium-glucose co-transporter-2 inhibitors in type 2 diabetes: A novel conceptual framework.

Long-term treatment with sodium-glucose co-transporter-2 (SGLT2) inhibitors slows the deterioration of renal function in patients with diabetes. This benefit cannot be ascribed to an action on blood glucose, ketone utilization, uric acid or systolic blood pressure. SGLT2 inhibitors produce a striking amelioration of glomerular hyperfiltration. Although initially ascribed to an action of these drugs to inhibit proximal tubular glucose reabsorption, SGLT2 inhibitors exert renoprotective effects, even in patients with meaningfully impaired levels of glomerular function that are sufficient to abolish their glycosuric actions. Instead, the reduction in intraglomerular pressures may be related to an action of SGLT2 inhibitors to interfere with the activity of sodium-hydrogen exchanger isoform 3, thereby inhibiting proximal tubular sodium reabsorption and promoting tubuloglomerular feedback. Yet, experimentally, such an effect may not be sufficient to prevent renal injury. It is therefore noteworthy that the diabetic kidney exhibits an important defect in adenosine monophosphate-activated protein kinase (AMPK) and sirtuin-1 (SIRT1) signalling, which may contribute to the development of nephropathy. These transcription factors exert direct effects to mute oxidative stress and inflammation, and they also stimulate autophagy, a lysosomally mediated degradative pathway that maintains cellular homeostasis in the kidney. SGLT2 inhibitors induce both AMPK and SIRT1, and they have been shown to stimulate autophagy, thereby ameliorating cellular stress and glomerular and tubular injury. Enhanced AMPK/SIRT1 signalling may also contribute to the action of SGLT2 inhibitors to interfere with sodium transport mechanisms. The dual effects of SGLT2 inhibitors on AMPK/SIRT1 activation and renal tubular sodium transport may explain the protective effects of these drugs on the kidney in type 2 diabetes.

Regulation of CSF and Brain Tissue Sodium Levels by the Blood-CSF and Blood-Brain Barriers During Migraine

Cerebrospinal fluid (CSF) and brain tissue sodium levels increase during migraine. However, little is known regarding the underlying mechanisms of sodium homeostasis disturbance in the brain during the onset and propagation of migraine. Exploring the cause of sodium dysregulation in the brain is important, since correction of the altered sodium homeostasis could potentially treat migraine. Under the hypothesis that disturbances in sodium transport mechanisms at the blood-CSF barrier (BCSFB) and/or the blood-brain barrier (BBB) are the underlying cause of the elevated CSF and brain tissue sodium levels during migraines, we developed a mechanistic, differential equation model of a rat's brain to compare the significance of the BCSFB and the BBB in controlling CSF and brain tissue sodium levels. The model includes the ventricular system, subarachnoid space, brain tissue and blood. Sodium transport from blood to CSF across the BCSFB, and from blood to brain tissue across the BBB were modeled by influx permeability coefficients PBCSFB and PBBB, respectively, while sodium movement from CSF into blood across the BCSFB, and from brain tissue to blood across the BBB were modeled by efflux permeability coefficients and , respectively. We then performed a global sensitivity analysis to investigate the sensitivity of the ventricular CSF, subarachnoid CSF and brain tissue sodium concentrations to pathophysiological variations in PBCSFB, PBBB, and . Our results show that the ventricular CSF sodium concentration is highly influenced by perturbations of PBCSFB, and to a much lesser extent by perturbations of . Brain tissue and subarachnoid CSF sodium concentrations are more sensitive to pathophysiological variations of PBBB and than variations of PBCSFB and within 30 min of the onset of the perturbations. However, PBCSFB is the most sensitive model parameter, followed by PBBB and , in controlling brain tissue and subarachnoid CSF sodium levels within 3 h of the perturbation onset.

Regulation of NCX1 by palmitoylation

Palmitoylation (S-acylation) is the reversible conjugation of a fatty acid (usually C16 palmitate) to intracellular cysteine residues of proteins via a thioester linkage. Palmitoylation anchors intracellular regions of proteins to membranes because the palmitoylated cysteine is recruited to the lipid bilayer. NCX1 is palmitoylated at a single cysteine in its large regulatory intracellular loop. The presence of an amphipathic α-helix immediately adjacent to the NCX1 palmitoylation site is required for NCX1 palmitoylation. The NCX1 palmitoylation site is conserved through most metazoan phlya. Although palmitoylation does not regulate the normal forward or reverse ion transport modes of NCX1, NCX1 palmitoylation is required for its inactivation: sodium-dependent inactivation and inactivation by PIP2 depletion are significantly impaired for unpalmitoylatable NCX1. Here we review the role of palmitoylation in regulating NCX1 activity, and highlight future questions that must be addressed to fully understand the importance of this regulatory mechanism for sodium and calcium transport in cardiac muscle.

Inner ear pathologies impair sodium-regulated ion transport in Meniere\u2019s disease

Meniere’s disease (MD), a syndromal inner ear disease, is commonly associated with a pathological accumulation of endolymphatic fluid in the inner ear, termed “idiopathic” endolymphatic hydrops (iEH). Although numerous precipitating/exacerbating factors have been proposed for MD, its etiology remains elusive. Here, using immunohistochemistry and in situ protein–protein interaction detection assays, we demonstrate mineralocorticoid-controlled sodium transport mechanisms in the epithelium of the extraosseous portion of the endolymphatic sac (eES) in the murine and human inner ears. Histological analysis of the eES in an extensive series of human temporal bones consistently revealed pathological changes in the eES in cases with iEH and a clinical history of MD, but no such changes were found in cases with “secondary” EH due to other otological diseases or in healthy controls. Notably, two etiologically different pathologies—degeneration and developmental hypoplasia—that selectively affect the eES in MD were distinguished. Clinical records from MD cases with degenerative and hypoplastic eES pathology revealed distinct intergroup differences in clinical disease presentation. Overall, we have identified for the first time two inner ear pathologies that are consistently present in MD and can be directly linked to the pathogenesis of EH, and which potentially affect the phenotypical presentation of MD.

Mechanisms of Sodium Transport in Plants-Progresses and Challenges.

Understanding the mechanisms of sodium (Na+) influx, effective compartmentalization, and efflux in higher plants is crucial to manipulate Na+ accumulation and assure the maintenance of low Na+ concentration in the cytosol and, hence, plant tolerance to salt stress. Na+ influx across the plasma membrane in the roots occur mainly via nonselective cation channels (NSCCs). Na+ is compartmentalized into vacuoles by Na+/H+ exchangers (NHXs). Na+ efflux from the plant roots is mediated by the activity of Na+/H+ antiporters catalyzed by the salt overly sensitive 1 (SOS1) protein. In animals, ouabain (OU)-sensitive Na+, K+-ATPase (a P-type ATPase) mediates sodium efflux. The evolution of P-type ATPases in higher plants does not exclude the possibility of sodium efflux mechanisms similar to the Na+, K+-ATPase-dependent mechanisms characteristic of animal cells. Using novel fluorescence imaging and spectrofluorometric methodologies, an OU-sensitive sodium efflux system has recently been reported to be physiologically active in roots. This review summarizes and analyzes the current knowledge on Na+ influx, compartmentalization, and efflux in higher plants in response to salt stress.

Endothelin-1 mediates natriuresis but not polyuria during vitamin D-induced acute hypercalcaemia.

Hypercalcaemia can occur under various pathological conditions, such as primary hyperparathyroidism, malignancy or granulomatosis, and it induces natriuresis and polyuria in various species via an unknown mechanism. A previous study demonstrated that hypercalcaemia induced by vitamin D in rats increased endothelin (ET)-1 expression in the distal nephron, which suggests the involvement of the ET system in hypercalcaemia-induced effects. In the present study, we demonstrate that, during vitamin D-induced hypercalcaemia, the activation of ET system by increased ET-1 is responsible for natriuresis but not for polyuria. Vitamin D-treated hypercalcaemic mice showed a blunted response to amiloride, suggesting that epithelial sodium channel function is inhibited. We have identified an original pathway that specifically mediates the effects of vitamin D-induced hypercalcaemia on sodium handling in the distal nephron without affecting water handling. Acute hypercalcaemia increases urinary sodium and water excretion; however, the underlying molecular mechanism remains unclear. Because vitamin D-induced hypercalcaemia increases the renal expression of endothelin (ET)-1, we hypothesized that ET-1 mediates the effects of hypercalcaemia on renal sodium and water handling. Hypercalcaemia was induced in 8-week-old, parathyroid hormone-supplemented, male mice by oral administration of dihydrotachysterol (DHT) for 3days. DHT-treated mice became hypercalcaemic and displayed increased urinary water and sodium excretion compared to controls. mRNA levels of ET-1 and the transcription factors CCAAT-enhancer binding protein β and δ were specifically increased in the distal convoluted tubule and downstream segments in DHT-treated mice. To examine the role of the ET system in hypercalcaemia-induced natriuresis and polyuria, mice were treated with the ET-1 receptor antagonist macitentan, with or without DHT. Mice treated with both macitentan and DHT displayed hypercalcaemia and polyuria similar to that in mice treated with DHT alone; however, no increase in urinary sodium excretion was observed. To identify the affected sodium transport mechanism, we assessed the response to various diuretics in control and DHT-treated hypercalcaemic mice. Amiloride, an inhibitor of the epithelial sodium channel (ENaC), increased sodium excretion to a lesser extent in DHT-treated mice compared to control mice. Mice treated with either macitentan+DHT or macitentan alone had a similar response to amiloride. In summary, vitamin D-induced hypercalcaemia increases the renal production of ET-1 and decreases ENaC activity, which is probably responsible for the rise in urinary sodium excretion but not for polyuria.

Intraspecies variation in sodium partitioning, potassium and proline accumulation under salt stress in Casuarina equisetifolia Forst

Casuarina equisetifolia Forst., a member of the Casuarinaceae family, is widely planted in coastal areas due to its ability to function as potential barrier against wind and waves. Significant variation has been reported in the ability of C. equisetifolia to grow under salinity stress. In the present study, 82 clones of C. equisetifolia were assessed for their response to 50 mM incremental NaCl concentrations ranging from 50 mM to 550 mM in Hoagland’s solution and clones with contrasted salt tolerance were identified. Several earlier reports attribute salt sensitivity in Casuarina species to the toxic effect of sodium. Intraclonal variation in the levels of sodium accumulation was therefore analysed. However, sodium content in the shoots and roots, showed little correlation (0.351 and −0.171) with salt tolerance in C. equisetifolia. Similarly, sodium to potassium ratio in the shoots and roots of NaCl treated and untreated clones also did not show correlation with mortality although certain tolerant clones exhibited selectivity of potassium over sodium under salt stress. Analysis of the shoot to root ratio of sodium however, showed better correlation (0.448) with salt tolerance, suggesting that restricted translocation of sodium to shoots and its relative retention in roots might play a crucial role in the salt tolerant clones of C. equisetifolia, and that shoot to root ratio of sodium could be a better parameter for salt tolerance in C. equisetifolia clones. The higher salt tolerance observed in certain clones despite higher sodium accumulation or shoot to root ratio of sodium suggests the presence of different multiple adaptive mechanisms that may be operating in different clones to help protect the cells from the toxic effects of sodium. The tolerant clone, TNIPT 4, which accumulated high concentrations of Na+, had low shoot to root ratio of Na+, and also a higher constitutive as well as NaCl induced accumulation of the compatible osmolyte, proline. The study thus emphasizes the need for characterising the genetic components involved in sodium transport, proline metabolism and other mechanisms contributing to salinity tolerance. The identified clones with contrasted stress tolerance mechanisms would thus be a valuable resource for transcriptomic, proteomic and metabolomic exploration in addition to their utility for field evaluation in flooded and coastal saline tracts.

Peroxisome proliferator activated receptor – alpha regulation of sodium transport mechanisms in human primary renal proximal tubule epithelial cells during acute Angiotensin II treatment

Angiotensin II (Ang II) is important for Na+ regulation in the kidney. Proteins in the proximal tubule (PT) epithelial cells such as the sodium hydrogen exchanger (NHE3), sodium hydrogen exchanger regulatory factor‐1 (NHERF‐1), Na+/K+ ATPase, NaPi2 serve as Na+ transporters. Studies have demonstrated that NHERF‐1 interacts with NaPi2 and NHE3 to regulate Na+ transport. Peroxisome proliferator activated receptor‐alpha (PPAR‐α) activation reduces Na+/K+ ATPase activity but is important for its expression. This study tests the hypothesis that PPAR‐α activation stimulates NHERF‐1 to interact with NHE3, NaPi2 and Na+/K+ ATPase to reduce Na+ reabsorption in PT cells during Ang II stimulation. Human primary renal proximal tubule epithelial cells (PREC) were used during passage 4 and plated at a density of 2 × 106/dish. PRECs were treated for 20 hours as follows: control plates, Ang II (10−9 M), fenofibrate (a PPAR‐α agonist)( 10−7 M) and Ang II + fenofibrate. Western blots were performed to determine the expression (optical density/area) of NHERF‐1, NHE3, Na+/K+ ATPase and NaPi2 in PRECs. Fenofibrate, Ang II and Ang II + Fenofibrate significantly increased NHERF‐1 expression in PRECs above control. We did not observe differences in NaPi2 or Na+/K+ ATPase expression between the groups. Our results suggest fenofibrate regulates Na+ reabsorption in PRECs by increasing NHERF‐1 expression. Our results also indicate that a low dose of Ang II directly stimulates NHERF‐1 expression in PRECs. Future studies are needed to determine the dose dependent effect of Ang II and fenofibrate on NaPi2, Na+/K+ ATPase, NHE3 expression and activity. Support: 5K01HL092593–05

Small intestinal ion transport

In this review, we focus on the recent (March 2010 to September 2011) advances in small intestinal ion transport, with particular emphasis on sodium, chloride, bicarbonate, and calcium transport mechanisms under physiological and pathophysiological conditions. Knockout of NHERF1 and NHERF2 allowed translation of the data largely derived from the in-vitro models into a living organism. These studies also expand our knowledge about the complexity of intestinal transporter interactomes, define the role for scaffolding proteins in basal and regulated apical transport, and help identify potential targets for pharmacological approaches. We continue to accumulate novel information about the function and regulation of NHE3 (including its role in regulating paracellular Ca2+ flux), NHE8, as well as about the complexity of the intestinal Cl- and HCO3- transport in health and disease. Thanks to the new genetically engineered mouse models, a significant progress has been made in our understanding of the role of NHERF proteins in regulation of intestinal Na+ absorption. Significant novel data on the coordinated function of bicarbonate, chloride, and sodium transporters contributes to our current views of the integrative physiology of the small intestinal electrolyte transport.

Toward an understanding of hypertension resistance

PHYSIOLOGISTS HAVE LONG APPRECIATED the central role of renal salt excretion in the control of blood pressure (4). Maintenance of a constant intravascular fluid volume and blood pressure depends on the ability of the kidneys to regulate urinary sodium excretion through the concerted action of several parallel sodium transport mechanisms. While abnormal function of any one transport process can usually be compensated for, there is a price. That is, a greater than normal change in arterial blood pressure must occur for natriuresis to be brought into balance with an increased Na intake. This concept has been reinforced though the study of rare Mendelian diseases of hypertension or hypotension, since all of the monogenic blood pressure syndromes characterized to date are caused by mutations in genes that influence sodium homeostasis (8). Individuals with these disorders often develop dramatic phenotypes of hypertension or salt wasting, usually through the inheritance of alleles harboring single nucleotide mutations. In recent years, considerable effort has been directed toward understanding whether these genes or others contribute substantially to blood pressure variation in the population as a whole. The approach for a number of these analyses has involved the unbiased search for associations between common genetic polymorphisms and blood pressure through genomewide association (GWA) studies. In some cases, GWA scans have confirmed the involvement of signaling pathways known to regulate renal salt excretion (10), or uncovered novel potential targets for antihypertensive therapy (6). In other examples, however, they have failed to identify common variants that reach genome-wide statistical significance (7, 11). More recent efforts by large-scale consortia have yielded success at identifying new associations (6). However, questions remain regarding the effect size of individual variants (3), and, in general, GWA studies have only identified a fraction of the total predicted genetic component for a variety of complex disease traits. Thus a shift in attention away from common polymorphisms, and toward the analysis of rare variants has been called for, so that their relative contributions may be assessed (3). In a genetic tour-de-force, Ji et al. (5) recently took such an approach, testing whether rare mutations in kidney salt transport genes might contribute substantially to blood pressure variation in the general population. The authors recognized that the heterozygous carrier state for Bartter’s and Gitelman’s syndromes, two Mendelian salt-wasting disorders, should be prevalent in at least 1% of individuals worldwide. Thus they asked whether rare coding variants in genes mutated in these disorders affect blood pressure in a large study population, the Framingham Offspring Cohort. They chose to sequence the exons of three candidates that affect renal sodium transport: SLC12A1 [Na-K-Cl cotransporter (NKCC2), mutated in type 1 Bartter’s syndrome], KCNJ1 (ROMK, mutated in type 2 Bartter’s syndrome), and SLC12A3 [Na-Cl cotransporter (NCC), mutated in Gitelman’s syndrome]. To select for relevant coding variants, Ji et al. first identified carriers for mutations in NKCC2, ROMK, and NCC already proven previously to cause hereditary salt wasting. As a second strategy, the authors applied stringent criteria to identify additional variants that could be inferred with confidence to lead to loss of function. To make the cutoff, a variant had to change an evolutionarily conserved residue and be present in the study population at a low frequency (to favor alleles under purifying selection). This approach identified 30 mutations across 49 subjects in 3,000 individuals. The authors found that carriers for one of the proven or inferred mutations in NKCC2, NCC, or ROMK had

The Na+-dependent chloride-bicarbonate exchanger SLC4A8 mediates an electroneutral Na+ reabsorption process in the renal cortical collecting ducts of mice

Regulation of sodium balance is a critical factor in the maintenance of euvolemia, and dysregulation of renal sodium excretion results in disorders of altered intravascular volume, such as hypertension. The amiloride-sensitive epithelial sodium channel (ENaC) is thought to be the only mechanism for sodium transport in the cortical collecting duct (CCD) of the kidney. However, it has been found that much of the sodium absorption in the CCD is actually amiloride insensitive and sensitive to thiazide diuretics, which also block the Na-Cl cotransporter (NCC) located in the distal convoluted tubule. In this study, we have demonstrated the presence of electroneutral, amiloride-resistant, thiazide-sensitive, transepithelial NaCl absorption in mouse CCDs, which persists even with genetic disruption of ENaC. Furthermore, hydrochlorothiazide (HCTZ) increased excretion of Na+ and Cl- in mice devoid of the thiazide target NCC, suggesting that an additional mechanism might account for this effect. Studies on isolated CCDs suggested that the parallel action of the Na+-driven Cl-/HCO3- exchanger (NDCBE/SLC4A8) and the Na+-independent Cl-/HCO3- exchanger (pendrin/SLC26A4) accounted for the electroneutral thiazide-sensitive sodium transport. Furthermore, genetic ablation of SLC4A8 abolished thiazide-sensitive NaCl transport in the CCD. These studies establish what we believe to be a novel role for NDCBE in mediating substantial Na+ reabsorption in the CCD and suggest a role for this transporter in the regulation of fluid homeostasis in mice.

Pendrin as a regulator of ECF and blood pressure

Abnormal renal sodium handling plays a pivotal role in the pathophysiology of arterial hypertension. Though sodium is present in the diet as a chloride salt, most studies have focused on the renal mechanisms of sodium transport and their regulation, and very few have investigated the impact of chloride. However, strong evidence indicates that chloride may play a very important role in the 'prohypertensive' effects of sodium. Several observations indicate that sodium has to be administered as NaCl and not as NaHCO3 to raise arterial blood pressure. Recently, genetic studies have pointed out the critical role of the most distal parts of the nephron in regulating sodium balance and blood pressure. Interestingly, in those segments, the transport of Na+ and Cl- proceeds separately, through different molecules. Recent evidence has demonstrated that an apical Cl-/HCO3- exchanger called pendrin is an absolute requirement for chloride reabsorption in the collecting system, indicating that chloride absorption is not passive and paracellular but is mediated by the intercalated cells. Importantly, pendrin disruption protects mice against mineralocorticoid-induced hypertension and impairs renal adaptation to a low NaCl diet. Moreover, recent findings suggest several mechanisms by which chloride transport might affect NaCl balance and blood pressure. Pendrin-mediated chloride transport through intercalated cells appears to be an independent and important determinant of renal NaCl handling and therefore might represent a new target for blood pressure control.

Sodium Ion Transporters as New Therapeutic Targets in Heart Failure

Sodium ion transporters in sarcolemma are involved in numerous vital cell functions, such as excitability, excitation-contraction coupling, energy metabolism, pH and volume regulation, development and growth. In a number of cardiac pathologies, the intracellular sodium concentration ([Na+]i) is elevated. Since [Na+]i and intracellular Ca2+ concentration ([Ca2+]i are coupled through the Na+/Ca(2+)-exchanger, these cardiac pathologies display disturbed calcium handling. For instance, [Na+]i is increased in heart failure (HF) leading to Na+/Ca(2+)-exchanger mediated increase in [Ca2+]i, reduced contractility and increased propensity to arrhythmias. Several studies support the contention that an increase in [Na+]i and [Ca2+]i transduces a signal the nucleus, that triggers development of cardiac remodelling and hypertrophy. Pharmacological intervention, which favourably interferes with [Na+]i and [Ca2+]i homeostasis, might prevent hypertrophy, cardiac remodelling, arrhythmias and HF. The most important sodium transport mechanisms that may underlie increased [Na+]i are: Na+/H(+)-exchanger (NHE-1), Na+-HCO(3)(-) co-transporter (NBC), Na(+)-K(+)-Cl(-) co-transporter (NKCC), Na(+)-channel, Na+/K(+)-ATPase and Na+/Ca(2+)-exchanger (NCX). Preclinical studies showed that pharmacological interventions, targeted against sarcolemmal sodium ion transporters, proved effective in ameliorating heart failure. In this respect: 1) NHE-1 inhibition reduces cardiac remodelling, hypertrophy and HF, although, in the patients following coronary artery bypass graft surgery, it was associated with an increase of stroke. 2) The activity of NBC is up-regulated, during the development of hypertrophy and may be a therapeutic strategy to prevent the development of hypertrophy and HF. 3) NKCC is increased in post-infarction HF, and the inhibition of NKCC attenuated post-infarction remodelling. 4) Inactivation of sodium channels is impaired in HF, which may result, in increased Na+ influx and prolongation of the action potential. 5) Blockade of NCX may be useful as a part of a combined therapeutic approach. Inhibition of reversed mode, or activation of forward mode NCX reduce Ca2+ overload. 6) Inhibition of Na+/K(+)-ATPase (digoxin), is used to increase contractility, however, it enhances progression of HF. Oppositely, new drugs which increase activity of Na+/K(+)-ATPase may prevent the development of cardiac remodelling hypertrophy and HF.

Linking valve closure behavior and sodium transport mechanism in freshwater clam Corbicula fluminea in response to copper

The purpose of this study is to develop a mechanistic model to describe a conceptually new “flux–biological response” approach based on biotic ligand model (BLM) and Michaelis–Menten (M–M) kinetics to allow the linkage between valve closure behavior and sodium (Na) transport mechanism in freshwater clam Corbicula fluminea in response to waterborne copper (Cu). We test the proposed model against published data regarding Na uptake kinetics in rainbow trout and Na uptake profile in C. fluminea, confirming that the predictive model is robust. Here, we show that the predicted M–M maximum Cu internalization flux in C. fluminea is 0.369 μmol g −1 h −1 with a half-saturation affinity constant of 7.87 × 10 −3 μM. Dynamics of Na uptake and valve closure daily rhythm driven by external Cu can also be predicted simultaneously. We suggest that this “Na transport–valve closure behavior” approach might provide the basis of a future design of biomonitoring tool.