Regulation Of Sodium Reabsorption Research Articles

Associations of E-proteinoid 3 receptor genetic polymorphisms with salt sensitivity, longitudinal blood pressure changes, and hypertension incidence in Chinese adults.

The E-proteinoid 3 receptor (PTGER3), a member of the prostaglandin E2 (PGE2) subtype receptor, belongs to the G-protein-coupled superfamily of receptors. Animal studies have demonstrated its involvement in salt sensitivity by regulating sodium reabsorption. This study aimed to investigate the association between genetic variants of PTGER3 and salt sensitivity, longitudinal blood pressure (BP) changes, and the incidence of hypertension in Chinese adults. A chronic salt intake intervention was conducted involving 514 adults from 124 families in the 2004 Baoji Salt-Sensitivity Study Cohort in northern China. These participants followed a 3-day regular baseline diet, followed by a 7-day low-salt diet (3.0g/d) and a 7-day high-salt diet (18g/d), and were subsequently followed for 14years. The findings revealed a significant relationship between the single nucleotide polymorphism (SNP) rs17482751 of PTGER3 and diastolic blood pressure (DBP) response to high salt intervention. Additionally, SNPs rs11209733, rs3765894, and rs2268062 were significantly associated with longitudinal changes in systolic blood pressure (SBP), DBP, and mean arterial pressure (MAP) during the 14-year follow-up period. SNP rs6424414 was significantly associated with longitudinal changes in DBP over 14years. Finally, SNP rs17482751 showed a significant correlation with the incidence of hypertension over 14years. These results emphasize the significant role of PTGER3 gene polymorphism in salt sensitivity, longitudinal BP changes, and the development of hypertension in the Chinese population.

Dietary Cinnamaldehyde Activation of TRPA1 Antagonizes High-Salt-Induced Hypertension Through Restoring Renal Tubular Mitochondrial Dysfunction.

The renal proximal tubule (RPT) plays a pivotal role in regulating sodium reabsorption and thus blood pressure (BP). Transient receptor potential ankyrin 1 (TRPA1) has been reported to protect against renal injury by modulating mitochondrial function. We hypothesize that the activation of TRPA1 by its agonist cinnamaldehyde may mitigate high-salt intake-induced hypertension by inhibiting urinary sodium reabsorption through restoration of renal tubular epithelial mitochondrial function. Trpa1-deficient (Trpa1-/-) mice and wild-type (WT) mice were fed standard laboratory chow [normal diet (ND) group, 0.4% salt], standard laboratory chow with 8% salt [high-salt diet (HS) group], or standard laboratory chow with 8% salt plus 0.015% cinnamaldehyde [high-salt plus cinnamaldehyde diet (HSC) group] for 6 months. Urinary sodium excretion, reactive oxygen species (ROS) production, mitochondrial function, and the expression of sodium hydrogen exchanger isoform 3 (NHE3) and Na+/K+-ATPase of RPTs were determined. Chronic dietary cinnamaldehyde supplementation reduced tail systolic BP and 24-hour ambulatory arterial pressure in HS-fed WT mice. Compared with the mice fed HS, cinnamaldehyde supplementation significantly increased urinary sodium excretion, inhibited excess ROS production, and alleviated mitochondrial dysfunction of RPTs in WT mice. However, these effects of cinnamaldehyde were absent in Trpa1-/- mice. Furthermore, chronic dietary cinnamaldehyde supplementation blunted HS-induced upregulation of NHE3 and Na+/K+-ATPase in WT mice but not Trpa1-/- mice. The present study demonstrated that chronic activation of Trpa1 attenuates HS-induced hypertension by inhibiting urinary sodium reabsorption through restoring renal tubular epithelial mitochondrial function. Renal TRPA1 may be a potential target for the management of excessive dietary salt intake-associated hypertension.

Dendrobium huoshanense polysaccharides ameliorate ulcerative colitis by improving intestinal mucosal barrier and regulating gut microbiota

• Long-term administration of DHP improved symptoms of UC mice. • DHP improved intestinal mucosal barrier function and reduced inflammation. • DHP regulated intestinal microecological disorders. • DHP effectually restored abnormal metabolism in the intestinal flora of UC mice. The gut microbiota is inextricably linked to ulcerative colitis (UC). The therapeutic effect of Dendrobium huoshanense polysaccharides (DHP) on ulcerative colitis has been confirmed, but whether it acts through the intestinal flora is still unknown. The study aimed to investigate the potential mechanisms of DHP intervention in UC mice by combining metabolomics of intestinal contents and gut flora analysis. The results showed that DHP alleviated UC symptoms while improving colonic mucosal barrier function and inflammatory manifestations. A total of 12 potential biomarkers were significantly altered in metabolomics concerning tryptophan metabolism, vitamin B6 metabolism, regulation of sodium reabsorption by aldosterone, and steroid hormone biosynthesis. DHP reversed the abundance of Fusobacteriota , Deferribacterota and Bacteroidetes , and significantly increased Desulfobacteriota . DHP significantly decreased the relative abundance of Clostridioides , while increasing Alistipes , and Rikenella. In conclusion, through metabolomics and 16s RNA sequencing methods, we explored the mechanism of preventive effects of Dendrobium huoshanense on UC.

Associations of genetic variations in NEDD4L with salt sensitivity, blood pressure changes and hypertension incidence in Chinese adults.

Neural precursor cell expressed developmentally downregulated 4‐like (NEDD4L), a member of the E3 ubiquitin‐protein ligases, encoded by NEDD4L gene, was found to be involved in in salt sensitivity by regulating sodium reabsorption in salt‐sensitive rats. The authors aimed to explore the associations of NEDD4L genetic variants with salt sensitivity, blood pressure (BP) changes and hypertension incidence in Chinese adults. Participants from 124 families in Northern China in the Baoji Salt‐Sensitive Study Cohort in 2004, who received the chronic salt intake intervention, including a 7‐day low‐salt diet (3.0 g/day) and a 7‐day high‐salt diet (18 g/day), were analyzed. Besides, the development of hypertension over 14 years was evaluated. NEDD4L single nucleotide polymorphism (SNP) rs74408486 was shown to be significantly associated with systolic BP (SBP), diastolic BP (DBP) and mean arterial pressure (MAP) responses to low‐salt diet, while SNPs rs292449 and rs2288775 were significantly associated with pulse pressure (PP) response to high‐salt diet. In addition, SNP rs4149605, rs73450471, and rs482805 were significantly associated with the longitudinal changes in SBP, DBP, MAP, or PP at 14 years of follow‐up. SNP rs292449 was significantly associated with hypertension incidence over the 14‐year follow‐up. Finally, this gene‐based analysis found that NEDD4L was significantly associated with longitudinal BP changes and the incidence of hypertension over the 14‐year follow‐up. This study indicated that gene polymorphism in NEDD4L serve an important function in salt sensitivity, longitudinal BP change and development of hypertension in the Chinese population.

MO068: Paraoxonase 2 Protects the Kidney Tissue from Aldosterone-Promoted Oxidative Stress in Milan Normotensive Rats Strain

Abstract BACKGROUND AND AIMS Kidney plays a pivotal role on the maintenance of blood pressure by the regulation of sodium reabsorption. Aldosterone sensitive distal nephron finely tunes salt reabsorption through several transporters, including the epithelial sodium channel (ENaC). Alteration in ENaC function leads to clinical relevant conditions associated with high or low blood pressure when gain or loss of function mutations occur, respectively. Recently, accumulating evidence suggests the putative role of renal Paraoxonase 2 (PON2) as regulator of ENaC function. PON2 is a membrane-associated enzyme showing hydrolytic activity against 3-oxo-C12 homoserine lactone, a signaling molecule produced by bacteria and mainly Pseudomonas aeruginosa. PON2 also exhibits antioxidant property. In the distal nephron, PON2 is expressed in principal cells, and in vitro it physically interacts with ENaC subunits and inhibits ENaC channel activity in Xenopus oocytes likely by modulating channel assembly in the endoplasmic reticulum. However, the role of PON2 in the kidney is still not known. In the present study, we investigate whether PON2 is regulated by changes in aldosterone state and its role in aldosterone-induced oxidative stress in the kidneys. METHOD We addressed these issues utilizing both in vivo and in vitro approaches, such as RESULTS We first demonstrated that in Milan normotensive rats strain, PON2 is upregulated by low-salt state according with α-ENaC activation. This regulation is aldosterone sensitive since pretreatment with mineralocorticoid receptor antagonist spironolactone prevented the stimulatory effects of sodium restricted diet. Furthermore, low-sodium diet inducing PON2 upregulation was associated with reduced ROS levels, more specifically superoxide levels, but treatment with spironolactone prevented this beneficial effect, by increasing ROS level in kidney cortex. Since antihypertensive drugs spironolactone and canrenone contain a lactone residue in their molecular structure, we tested whether they could directly interact with PON2. We demonstrated in vitro that spironolactone and canrenone were able to inhibit PON2 lactonase activity and so to directly interfere with PON2 function. Finally, Milan hypertensive rats, carrying a mutation of α-adducin as determinant of their hypertensive phenotype, presented with higher level of ROS while PON2 was upregulated. CONCLUSION These results demonstrate PON2 as a novel target of steroid hormone aldosterone. PON2 may serve as a potential ROS scavenger mechanism protecting the kidneys from aldosterone-promoted oxidative stress.

Salt-Sensitive Hypertension of the Renal Tubular Cell-Specific NFAT5 (Nuclear Factor of Activated T-Cells 5) Knockout Mice.

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URINARY AMMONIUM EXCRETION BY ALDOSTERONE THROUGH THE REGULATION OF RH C GLYCOPROTEIN IN THE INTERCALATED CELLS OF THE RENAL COLLECTING DUCT

Objective: Aldosterone is a key hormone that regulates sodium reabsorption in the principal cells (PCs) of the renal collecting duct, maintaining blood pressure. Alternatively, aldosterone is involved in urinary acid excretion mainly via ammonia as ammonium ions in the intercalated cells (ICs) of the collecting duct, although its mechanisms are obscure. Rh C glycoprotein (Rhcg), an ammonia transporter, is expressed mainly in the intercalated cells. Direct regulation of ammonium excretion by aldosterone through the Rhcg in metabolic acidosis was investigated. Design and method: C57BL/6J mice were divided into four groups: 1) sham-operation, 2) sham and NH4Cl drinking (Sham-NH4Cl), 3) adrenalectomy and NH4Cl drinking (ADX-NH4Cl), and 4) adrenalectomy with continuous administration of aldosterone and NH4Cl drinking (ADX-Aldo-NH4Cl). Mice were sacrificed three days after NH4Cl drinking. Urinary ammonium excretion and localizations of Rhcg and ubiquitin were examined. For in vitro experiments, IN-IC cells, an intercalated cell-derived cell line, that stably express flag-tagged Rhcg (Rhcg-flag) were generated. The effects of aldosterone, spironolactone, Go6983 (a PKC inhibitor) and MG132 (a proteasome inhibitor) on the expression of Rhcg-flag were examined. Results: NH4Cl-induced urinary ammonium excretion was inhibited by adrenalectomy in ADX-NH4Cl mice. Aldosterone replacement restored ammonium excretion in ADX-Aldo-NH4Cl mice. Immunohistochemical study revealed that NH4Cl drinking increased the accumulation of Rhcg at apical and basolateral membranes and subapical sites of the ICs in sham-NH4Cl mice. Adrenalectomy and aldosterone replacement decreased and increased the accumulation of Rhcg at apical membrane of the ICs in ADX-NH4Cl and ADX-Aldo-NH4Cl mice, respectively. Ubiquitin expression was not changed between in PCs and ICs of sham-NH4Cl mice, while it was less in ICs than in PCs in ADX-NH4Cl mice. Aldosterone restored the ubiquitin expression in ICs. In IN-IC cells, Western blotting showed that aldosterone increased the expression of Rhcg-flag in the membrane fraction. Spironolactone and Go6983 each inhibited the expression of Rhcg-flag. Treatment with MG132 increased the expression of Rhcg-flag in whole cell lysate. Immunoprecipitation revealed the association of Rhcg with the ubiquitin. Conclusions: Aldosterone could directly regulate Rhcg expression through the mineralocorticoid receptor-PKC pathway. Furthermore, aldosterone could modulate Rhcg expression through the ubiquitin-proteasome pathway.

Aldosterone Decreases Vasopressin-Stimulated Water Reabsorption in Rat Inner Medullary Collecting Ducts.

Aldosterone indirectly regulates water reabsorption in the distal tubule by regulating sodium reabsorption. However, the direct effect of aldosterone on vasopressin-regulated water and urea permeability in the rat inner medullary collecting duct (IMCD) has not been tested. We investigated whether aldosterone regulates osmotic water permeability in isolated perfused rat IMCDs. Adding aldosterone (500 nM) to the bath significantly decreased osmotic water permeability in the presence of vasopressin (50 pM) in both male and female rat IMCDs. Aldosterone significantly decreased aquaporin-2 (AQP2) phosphorylation at S256 but did not change it at S261. Previous studies show that aldosterone can act both genomically and non-genomically. We tested the mechanism by which aldosterone attenuates osmotic water permeability. Blockade of gene transcription with actinomycin D did not reverse aldosterone-attenuated osmotic water permeability. In addition to AQP2, the urea transporter UT-A1 contributes to vasopressin-regulated urine concentrating ability. We tested aldosterone-regulated urea permeability in vasopressin-treated IMCDs. Blockade of gene transcription did not reverse aldosterone-attenuated urea permeability. In conclusion, aldosterone directly regulates water reabsorption through a non-genomic mechanism. Aldosterone-attenuated water reabsorption may be related to decreased trafficking of AQP2 to the plasma membrane. There may be a sex difference apparent in the inhibitory effect of aldosterone on water reabsorption in the inner medullary collecting duct. This study is the first to show a direct effect of aldosterone to inhibit vasopressin-stimulated osmotic water permeability and urea permeability in perfused rat IMCDs.

Salt-sensitive transcriptome of isolated kidney distal tubule cells.

In the distal kidney tubule, the steroid hormone aldosterone regulates sodium reabsorption via the epithelial sodium channel (ENaC). Most studies seeking to identify ENaC-regulating aldosterone-induced proteins have used transcriptional profiling of cultured cells. To identify salt-sensitive transcripts in an in vivo model, we used low-NaCl or high-NaCl diet to stimulate or suppress endogenous aldosterone, in combination with magnetic- and fluorescence-activated cell sorting to isolate distal tubule cells from mouse kidney for transcriptional profiling. Of the differentially expressed transcripts, 162 were more abundant in distal tubule cells isolated from mice fed low-NaCl diet, and 161 were more abundant in distal tubule cells isolated from mice fed high-NaCl diet. Enrichment analysis of Gene Ontology biological process terms identified multiple statistically overrepresented pathways among the differentially expressed transcripts that were more abundant in distal tubule cells isolated from mice fed low-NaCl diet, including ion transmembrane transport, regulation of growth, and negative regulation of apoptosis. Analysis of Gene Ontology molecular function terms identified differentially expressed transcription factors, transmembrane transporters, kinases, and G protein-coupled receptors. Finally, comparison with a recently published study of gene expression changes in distal tubule cells in response to administration of aldosterone identified 18 differentially expressed genes in common between the two experiments. When expression of these genes was measured in cortical collecting ducts microdissected from mice fed low-NaCl or high-NaCl diet, eight were differentially expressed. These genes are likely to be regulated directly by aldosterone and may provide insight into aldosterone signaling to ENaC in the distal tubule.

Aldosterone‐Induced Transcripts Identified from Rapidly Isolated Collecting Duct Cells

In the distal nephron, the steroid hormone aldosterone regulates sodium reabsorption via the epithelial sodium channel ENaC. Previous attempts to identify the aldosterone‐induced proteins that mediate the action of aldosterone on ENaC have primarily used transcriptional profiling of cultured cells. Here, we present a method to rapidly isolate collecting duct cells from the mouse kidney for transcriptional profiling. Our objective is to use this method, in combination with dietary sodium manipulation, to identify transcripts that are differentially expressed in the collecting ducts of mouse kidney under conditions of high versus low plasma aldosterone.An antibody against the cell adhesion molecule L1‐CAM was previously used for fluorescence activated cell sorting (FACS) of collecting duct cells from human kidney. We verified L1‐CAM is also expressed in the collecting ducts of mouse kidney using immunofluorescence. We next developed a protocol for physical and enzymatic separation of mouse kidney into a single cell suspension. The kidney cells were labeled with L1‐CAM magnetic microbeads as well as antibodies against markers for endothelial cells (CD31) and immune cells (CD45). This allowed isolation of an L1‐CAM+ cell population using magnetic activated cell sorting (MACS), followed by separation of collecting duct cells from L1‐CAM+ endothelial and immune cells using FACS. RNA was purified from the FACS‐isolated collecting duct cells and, following poly(A) selection, used to generate Illumina TruSeq libraries. The libraries were sequenced on the Illumina HiSeq platform and the reads were aligned to the mouse genome using the Subread aligner. The aligned reads were mapped to transcripts using feature Counts and differentially expressed transcripts are identified using EdgeR.To identify transcripts that are differentially expressed in the collecting ducts of mouse kidney under conditions of high versus low plasma aldosterone, C57BL/6J mice were fed sodium‐deficient diet (to stimulate endogenous aldosterone) or high‐sodium diet (to suppress endogenous aldosterone). After 5 days of dietary sodium manipulation, the kidneys were harvested, collecting duct cells were isolated, and RNA was sequenced and analyzed according to the above protocol. Of the differentially expressed transcripts, 234 were up‐regulated by aldosterone (more highly expressed in the sodium‐deficient diet samples) while 276 were down‐regulated by aldosterone (more highly expressed in the high‐sodium diet samples); false discovery rate < 0.05, no log2FC or log2CPM threshold. The transcriptome data were then compared with recently published single‐cell RNA‐seq data for the major collecting duct cell types, specifically intercalated cells and principal cells. It was found that 16 of the up‐regulated transcripts and 32 of the down‐regulated transcripts were highly expressed in principal cells, thereby limiting the number of candidate transcripts for subsequent validation. Of the transcripts that were highly expressed in principal cells, Sgk1 was the most differentially expressed transcript induced by aldosterone.Support or Funding InformationU.S. Department of Veterans Affairs: I01BX002228This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Involvement of ENaC in the development of salt-sensitive hypertension.

Salt-sensitive hypertension is associated with renal and vascular dysfunctions, which lead to impaired fluid excretion, increased cardiac output, and total peripheral resistance. It is commonly accepted that increased renal sodium handling and plasma volume expansion are necessary factors for the development of salt-induced hypertension. The epithelial sodium channel (ENaC) is a trimeric ion channel expressed in the distal nephron that plays a critical role in the regulation of sodium reabsorption in both normal and pathological conditions. In this mini-review, we summarize recent studies investigating the role of ENaC in the development of salt-sensitive hypertension. On the basis of experimental data obtained from the Dahl salt-sensitive rats, we and others have demonstrated that abnormal ENaC activation in response to a dietary NaCl load contributes to the development of high blood pressure in this model. The role of different humoral factors, such as the components of the renin-angiotensin-aldosterone system, members of the epidermal growth factors family, arginine vasopressin, and oxidative stress mediating the effects of dietary salt on ENaC are discussed in this review to highlight future research directions and to determine potential molecular targets for drug development.

A low-salt diet increases the expression of renal sirtuin 1 through activation of the ghrelin receptor in rats.

Previous studies have shown that sirtuin 1 (Sirt1) is renoprotective; however, details regarding its distribution and functions in the kidney remain unknown. Here, we demonstrated that Sirt1 was mainly expressed in the tubulointerstitial cells of normal rat kidneys and was co-localized with aquaporin 2, indicating it may be involved in water/salt regulation. Renal Sirt1 expression increased in the non-glomerular cytoplasmic portion of the kidney after a 24-h fast, but no significant changes in Sirt1 expression occurred after water loading (50 mL/kg) or 24-h water deprivation. After consuming a low-salt (0.075%) or 60% calorie restriction diet for 7 days, Sirt1 expression in the rat kidney was significantly increased, whereas a high-salt (8%) diet did not change the level of Sirt1 expression. The low-salt diet also increased Sirt1 expression in the heart, muscle, brain, and fat tissues. The increased Sirt1 that was observed in rats on a low-salt diet was associated with increased ghrelin expression in the distal nephron, with both molecules exhibiting similar distribution patterns. An in vitro experiment suggested that ghrelin increases Sirt1 expression in cortical collecting duct cells by activating ghrelin receptors. Our study indicates that this ‘ghrelin-Sirt1 system’ may participate in regulating sodium reabsorption in the distal nephron.

Protein Carbonylation of an Amino Acid Residue of the Na/K-ATPase α1 Subunit Determines Na/K-ATPase Signaling and Sodium Transport in Renal Proximal Tubular Cells.

BackgroundWe have demonstrated that cardiotonic steroids, such as ouabain, signaling through the Na/K‐ATPase, regulate sodium reabsorption in the renal proximal tubule. By direct carbonylation modification of the Pro222 residue in the actuator (A) domain of pig Na/K‐ATPase α1 subunit, reactive oxygen species are required for ouabain‐stimulated Na/K‐ATPase/c‐Src signaling and subsequent regulation of active transepithelial 22Na+ transport. In the present study we sought to determine the functional role of Pro222 carbonylation in Na/K‐ATPase signaling and sodium handling.Methods and ResultsStable pig α1 knockdown LLC‐PK1‐originated PY‐17 cells were rescued by expressing wild‐type rat α1 and rat α1 with a single mutation of Pro224 (corresponding to pig Pro222) to Ala. This mutation does not affect ouabain‐induced inhibition of Na/K‐ATPase activity, but abolishes the effects of ouabain on Na/K‐ATPase/c‐Src signaling, protein carbonylation, Na/K‐ATPase endocytosis, and active transepithelial 22Na+ transport.ConclusionsDirect carbonylation modification of Pro224 in the rat α1 subunit determines ouabain‐mediated Na/K‐ATPase signal transduction and subsequent regulation of renal proximal tubule sodium transport.

Coordinated Control of ENaC and Na+,K+-ATPase in Renal Collecting Duct.

Tubular reabsorption of filtered sodium is tightly controlled to maintain body volume homeostasis. The rate of sodium transport by collecting duct (CD) cells varies widely in response to dietary sodium intake, GFR, circulating hormones, neural signals, and local regulatory factors. Reabsorption of filtered sodium by CD cells occurs via a two-step process. First, luminal sodium crosses the apical plasma membrane along its electrochemical gradient through epithelial sodium channels (ENaC). Intracellular sodium is then actively extruded into the interstitial space by the Na(+),K(+)-ATPase located along the basolateral membrane. Mismatch between sodium entry and exit induces variations in sodium intracellular concentration and cell volume that must be maintained within narrow ranges for control of vital cell functions. Therefore, renal epithelial cells display highly coordinated apical and basolateral sodium transport rates. We review evidence from experiments conducted in vivo and in cultured cells that indicates aldosterone and vasopressin, the two major hormones regulating sodium reabsorption by CD, generate a coordinated stimulation of apical ENaC and basolateral Na(+),K(+)-ATPase. Moreover, we discuss evidence suggesting that variations in sodium entry per se induce a coordinated change in Na(+),K(+)-ATPase activity through the signaling of protein kinases such as protein kinase A and p38 mitogen-activated protein kinase.

Adult nephron-specific MR-deficient mice develop a severe renal PHA-1 phenotype.

Aldosterone is the main mineralocorticoid hormone controlling sodium balance, fluid homeostasis, and blood pressure by regulating sodium reabsorption in the aldosterone-sensitive distal nephron (ASDN). Germline loss-of-function mutations of the mineralocorticoid receptor (MR) in humans and in mice lead to the "renal" form of type 1 pseudohypoaldosteronism (PHA-1), a case of aldosterone resistance characterized by salt wasting, dehydration, failure to thrive, hyperkalemia, and metabolic acidosis. To investigate the importance of MR in adult epithelial cells, we generated nephron-specific MR knockout mice (MR(Pax8/LC1)) using a doxycycline-inducible system. Under standard diet, MR(Pax8/LC1) mice exhibit inability to gain weight and significant weight loss compared to control mice. Interestingly, despite failure to thrive, MR(Pax8/LC1) mice survive but develop a severe PHA-1 phenotype with higher urinary Na(+) levels, decreased plasma Na(+), hyperkalemia, and higher levels of plasma aldosterone. This phenotype further worsens and becomes lethal under a sodium-deficient diet. Na(+)/Cl(-) co-transporter (NCC) protein expression and its phosphorylated form are downregulated in the MR(Pax8/LC1) knockouts, as well as the αENaC protein expression level, whereas the expression of glucocorticoid receptor (GR) is increased. A diet rich in Na(+) and low in K(+) does not restore plasma aldosterone to control levels but is sufficient to restore body weight, plasma, and urinary electrolytes. In conclusion, MR deletion along the nephron fully recapitulates the features of severe human PHA-1. ENaC protein expression is dependent on MR activity. Suppression of NCC under hyperkalemia predominates in a hypovolemic state.

Abstract 141: G-Protein Coupled Receptors GPR37 and GPR37L1 Regulate Sodium Reabsorption in Renal Proximal Tubule Cells

GPR37 and GPR37L1 are closely related G-protein coupled receptors that are expressed mainly in brain glial cells and muscle-myenteric nerve layers in the GI tract. GPR37L1 transgenic mice have decreased systolic blood pressure (SBP) whereas GPR37L1 KO mice have increased SBP. However, there are no studies reporting kidney expression of GPR37 or GPR37L1 or their role in renal blood pressure regulation. Immunostaining and immunoblotting showed that GPR37 and GPR37L1 are expressed in the apical membrane of proximal tubules of the mouse kidney; RT-PCR on proximal tubule and collecting duct cells obtained by laser capture micro-dissection of mouse kidney sections, confirmed these findings. In addition, chronic high salt diet increased the renal expression of prosaposin, a precursor for saposin C, a natural ligand for GPR37 and GPR37L1. Infusion of prosaptide, a synthetic ligand for GPR37 and GPR37L1, decreased SBP in mice by 10 mm Hg. To determine the roles of GPR37 and GPR37L1 in renal Na+ transport, we over-expressed separately the two proteins in human renal proximal tubule (RPT) cells (n=3). Intracellular Na+ was increased in GPR37- or GPR37L1-transfected RPT cells (3.1 ± 0.8 and 3.2 ± 0.6 fold respectively, P<0.001) compared with mock-transfected cells. Immunoblot analyses showed increased phosphorylation of Erk1/2 (1.33 ± 0.03 and 1.52 ± 0.06 fold, P<0.05), and ribosomal S6 protein (1.29 ± 0.03.5 and 1.39 ± 0.08 fold, P<0.01) in RPT cells over-expressing GPR37 or GPR37L1, respectively. Na+,K+-ATPase expression was decreased by 29% ± 3.5 (P<0.05) in GPR37L1 transfected RPT cells, but was unaltered in GPR37 transfected RPT cells. Taken together, these results show that both GPR37 and GPR37L1 are expressed in RPT cells and signal via the Erk1/2 pathway. GPR37L1 increases intracellular Na+ in RPT cells by decreasing the exit of Na+ due to a decrease in Na+,K+-ATPase expression and activity at the basolateral membrane while GPR37 increases intracellular Na+, by a mechanism independent of Na+,K+-ATPase. These results indicate that GPR37 and GPR37L1 may play a role in Na+ reabsorption in RPT cells and may be novel targets to designing drugs to treat patients with hypertension.

STE20/SPS1-related proline/alanine-rich kinase (SPAK) is critical for sodium reabsorption in isolated, perfused thick ascending limb.

SPAK [STE20 (sterile 20)/SPS1-related proline/alanine-rich kinase] kinase consists of a full-length (FL-) and an alternatively spliced kidney-specific (KS-) isoform. SPAK regulates the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). The relative abundance and role of FL- vs. KS-SPAK in regulating Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) in thick ascending limb (TAL) are not completely understood. Here, we report that FL-SPAK mRNA was the most abundant in medullary TAL (mTAL), followed by cortical TAL (cTAL) and DCT. KS-SPAK mRNA abundance was relatively lower than FL-SPAK. The ratios of FL-SPAK to KS-SPAK in mTAL, cTAL, and DCT were 12.3, 12.5, and 10.2, respectively. To examine the role of SPAK in the regulation of sodium transport in TAL, we used in vitro microperfusion of mTAL and cTAL isolated from wild-type (WT) and SPAK knockout mice (SPAK-KO) that lack both FL- and KS-SPAK. The rates of sodium absorption in cTAL and mTAL of SPAK-KO mice were 34.5 and 12.5% of WT tubules, respectively. The mRNA levels of related OSR1 kinase and SPAK protease Dnpep in SPAK-KO tubules were not significantly different from WT tubules. We next examined the role of SPAK in the regulation of sodium reabsorption by vasopressin in TAL. Vasopressin increased sodium reabsorption by ∼80% in both mTAL and cTAL from WT mice. While baseline sodium reabsorption was lower in SPAK-KO tubules, vasopressin increased sodium reabsorption over twofold. In conclusion, the combined net effect of SPAK isoforms on sodium reabsorption in TAL is stimulatory. SPAK is not essential for vasopressin stimulation of sodium reabsorption in TAL.

Cardiotonic Steroids in Adaptation to Dietary Salt Intake

For many years the concept relating salt to blood pressure (BP) changes has been debated and the concept of natriuretic hormone eliminating excessive sodium by direct inhibition of Na/K-ATPase has raised controversy. However, a recently discovered Na/K-ATPase signaling function has been widely confirmed and provided a novel mechanism to explain the relationship between sodium and blood pressure. Recently, we have demonstrated that activation of this Na/KATPase signaling function regulates sodium reabsorption in renal proximal tubules (RPTs) to correct sodium retention related volume expansion and BP increase. This mechanism demonstrates that rather than contributing to development and maintenance of hypertension, a properly regulated RPT Na/K-ATPase signaling has a protective effect by stimulating renal sodium excretion. A clear understanding of molecular mechanisms whereby the Na/K-ATPase signaling axis counterbalance volume expansion would have major pathophysiological and therapeutic implications for volume expansion mediated hypertension. In this review, we will focus on the effect of the newly appreciated cardiotonic steroids (CTS)-Na/K-ATPase signaling on RPT-mediated sodium handling by coordinated regulation of the Na/K-ATPase and sodium/proton exchanger isoform 3 (NHE3).

Pressure natriuresis and the renal control of arterial blood pressure.

The regulation of extracellular fluid volume by renal sodium excretion lies at the centre of blood pressure homeostasis. Renal perfusion pressure can directly regulate sodium reabsorption in the proximal tubule. This acute pressure natriuresis response is a uniquely powerful means of stabilizing long-term blood pressure around a set point. By logical extension, deviation from the set point can only be sustained if the pressure natriuresis mechanism is impaired, suggesting that hypertension is caused or sustained by a defect in the relationship between renal perfusion pressure and sodium excretion. Here we describe the role of pressure natriuresis in blood pressure control and outline the cascade of biophysical and paracrine events in the renal medulla that integrate the vascular and tubular response to altered perfusion pressure. Pressure natriuresis is impaired in hypertension and mechanistic insight into dysfunction comes from genetic analysis of blood pressure disorders. Transplantation studies in rats show that blood pressure is determined by the genotype of the kidney and Mendelian hypertension indicates that the distal nephron influences the overall natriuretic efficiency. These approaches and the outcomes of genome-wide-association studies broaden our view of blood pressure control, suggesting that renal sympathetic nerve activity and local inflammation can impair pressure natriuresis to cause hypertension. Understanding how these systems interact is necessary to tackle the global burden of hypertension.

Involvement of Reactive Oxygen Species in a Feed-forward Mechanism of Na/K-ATPase-mediated Signaling Transduction

Cardiotonic steroids (such as ouabain) signaling through Na/K-ATPase regulate sodium reabsorption in the renal proximal tubule. We report here that reactive oxygen species are required to initiate ouabain-stimulated Na/K-ATPase·c-Src signaling. Pretreatment with the antioxidant N-acetyl-L-cysteine prevented ouabain-stimulated Na/K-ATPase·c-Src signaling, protein carbonylation, redistribution of Na/K-ATPase and sodium/proton exchanger isoform 3, and inhibition of active transepithelial (22)Na(+) transport. Disruption of the Na/K-ATPase·c-Src signaling complex attenuated ouabain-stimulated protein carbonylation. Ouabain-stimulated protein carbonylation is reversed after removal of ouabain, and this reversibility is largely independent of de novo protein synthesis and degradation by either the lysosome or the proteasome pathways. Furthermore, ouabain stimulated direct carbonylation of two amino acid residues in the actuator domain of the Na/K-ATPase α1 subunit. Taken together, the data indicate that carbonylation modification of the Na/K-ATPase α1 subunit is involved in a feed-forward mechanism of regulation of ouabain-mediated renal proximal tubule Na/K-ATPase signal transduction and subsequent sodium transport.