Regulation Of Renal Sodium Transport Research Articles

The Role of Dopamine D2 receptors and Oxidative Stress in the Pathogenesis of Hypertension

Globally, hypertension is the number one risk factor for death, affecting more than 1 billion people. Hypertension is the result of the interactions among genetics, epigenetics, environment, and lifestyle. The long-term regulation of blood pressure rests on renal and non-renal mechanisms. The impaired renal sodium handling in hypertension is caused by aberrant counter-regulatory natriuretic/anti-natriuretic pathways. The sympathetic nervous and renin-angiotensin systems are anti-natriuretic pathways. A counter-regulatory natriuretic pathway is the renal dopaminergic system. Aberrant dopaminergic regulation of renal sodium transport in hypertension is caused by a decrease in renal dopamine synthesis and/or dysfunction of any of the 5 dopamine receptors (D1R, D2R, D3R, D4R, & D5R). Normally, an increase in sodium intake increases while a decrease in sodium intake decreases blood pressure, albeit transiently until sodium balance is achieved. However, ~50 % of hypertensive and ~26% of normotensive subjects have increased blood pressure on high sodium intake, a case of salt sensitivity, while ~20 % have increased blood pressure on a low sodium intake, a case of inverse salt sensitivity. Low and high sodium intakes are associated with increased incidence of cardiovascular events/mortality. In humans with inverse salt sensitivity, there is a linear relationship between the number of single nucleotide polymorphisms in DRD2 (rs6276 and 6277) and decreased renal D2R expression. The increase in blood pressure on a low sodium diet may be due to increased activities of the renin-angiotensin and sympathetic nervous systems that cannot be counteracted by D2R. Hypertension may be a cause or consequence of inflammation or oxidative stress. Deficient D2R function causes renal inflammation independently of the increase in blood pressure. Subjects carrying DRD2 single nucleotide polymorphisms have increased inflammation, mediated by decreased regulation of the miR-217-Wnt5a-Ror2 pathway. The D2R, via paraoxonase2 and sestrin2, maintains normal redox balance and blood pressure. In summary, the D2R is important in the maintenance of normal blood pressure by regulating renal sodium transport, vascular reactivity, inflammation, and redox balance.

Abstract P247: Altered Renal Sodium Transporters Expression Is Associated With Cholinergic-mediated Salt Sensitive Hypertension In Young Spontaneously Hypertensive Rats

High sodium diet is a major contributing factor and has been associated with inflammation in patients suffering from hypertension. Systemic infusion of nicotine induces renal inflammation and the development of premature hypertension in young spontaneously hypertensive rats (SHR) and leads to enhanced expression of the renal Na-K-Cl cotransporter (NKCC2). Sodium sensitivity also plays a role in the development of cholinergic hypertension. We hypothesized that these findings were related to alterations in renal sodium transporters expression. We continuously infused SHR and Wistar Kyoto (WKY, normotensive control) rats with saline or nicotine (15mg/kg/day) and fed either a low-salt (LSD, 0.01% NaCl) or high-salt (HSD, 4% NaCl) diet for 2 weeks. Renal homogenates were assessed by western blot for the presence of the epithelial sodium channel (ENaC), NKCC2, Na-Cl co-transporter (NCC) and Serum and glucocorticoid regulated kinase 1 (SGK-1). Nicotine infusion in WKY induced 1.9-fold increase (p=0.003) in renal expression of SGK-1 under LSD, compared to saline-infused controls. The sharp increase in SGK-1 in nicotine-infused WKY on LSD was accompanied by a 1.9-fold increase in NCC (p=0.004), a 1.3-fold increase in the cleaved alpha ENaC, a 1.2-fold increase in the uncleaved alpha ENaC, and a 1.5-fold increase in the beta-ENaC (p=0.02). Both saline and nicotine infused WKY showed very little expression of renal NKCC2 in either LSD or HSD. There was no effect of either diet or nicotine infusion on expression of SGK-1, NCC, or ENaC subunits in SHR. Nicotine had no effect on NKCC2 expression in SHR on LSD, but HSD alone induced a 2.25-fold increase in renal NKCC2 expression compared to LSD in saline infused SHR (p=0.006). Nicotine infusion further induced a 58% increase in renal NKCC2 expression for SHR fed HSD (p=0.004). Here we show an effect of nicotine to appropriately increase the expression of NCC and ENaC in WKY under LSD and a trend towards decreased expression under HSD. In contrast, nicotine not only has no effect on NCC and ENaC under LSD in SHR but leads to a paradoxical increase in NKCC2 under HSD. We conclude that inappropriate regulation of renal sodium transporters likely plays a role in cholinergic-mediated renal inflammation and hypertension.

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure.

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

G protein‐coupled receptor 37L1 participates in epigenetic gene regulation in human renal proximal tubule cells

G protein‐coupled receptors (GPCRs) in the kidney regulate the reabsorption of essential nutrients, ions, and water from the glomerular filtrate. Abnormality in the renal luminal sodium transport plays an important role in the pathogenesis of essential hypertension. There are a growing number of GPCRs and their interacting protein components that are localized on the nuclear membrane or within the nucleus, where they retain their signaling functions. In most cases, nuclear GPCRs are constitutively expressed on the nuclear membrane. The functions of these nuclear GPCRs are not well understood. Recently we showed that G protein‐coupled receptor 37L1 (GPR37L1), expressed on the apical membrane of renal proximal tubules (RPT), regulates luminal sodium transport and blood pressure via modulating NHE3 function. However, the GPR37L1 signaling pathway in the RPT has not been studied. Co‐immunoprecipitation (Co‐IP), combined with mass spectrometry analyses, revealed the association of GPR37L1 with nuclear import proteins: IPO7, IPO8, KPNA2, KPNB1; nuclear export proteins: XPO1 and XPO7; nuclear pore complex proteins: NUP210; kinases involved in signaling: mTOR, PRKDC, IRS4; and a DNA (Cytosine‐5‐)‐Methyltransferase 1 (DNMT1) in the GPR37L1 protein complex. Immunoblot analyses of nuclear preparations from the human RPT cells (hRPTCs) confirmed the presence of GPR37L1 in the nucleus. Confocal fluorescence imaging of hRPTCs expressing GPR37L1 tagged with green fluorescence protein (GPR37L1‐GFP) showed distinct nuclear membrane expression of GPR37L1. Association of GPR37L1 with mTOR, PRKDC, and DNMT1 proteins was confirmed by Co‐IP followed by immunoblot analyses. Overexpression of GPR37L1 in hRPTCs increased the phosphorylation of PI3, and AKT and decreased the expression of DNMT1, and NHE3 promoter methylation. Our data show that GPR37L1 signals via the PI3K/AKT/mTOR pathway to decrease the expression of DNMT1. Decreased DNMT1 function reduces CpG methylation on the NHE3 promoter that in turn increases NHE3 transcription. This is the first report showing the direct role of a nuclear GPCR in the regulation of renal sodium transport through epigenetic gene regulation.Support or Funding InformationR56DK116828: Prasad Konkalmatt

In Utero Exposure to Fine Particulate Matter Causes Hypertension Due to Impaired Renal Dopamine D1 Receptor in Offspring

Background/Aims: Adverse environment in utero can modulate adult phenotypes including blood pressure. Fine particulate matter (PM_2.5) exposure in utero causes hypertension in the offspring, but the exact mechanisms are not clear. Renal dopamine D₁ receptor (D₁R), regulated by G protein-coupled receptor kinase type 4 (GRK4), plays an important role in the regulation of renal sodium transport and blood pressure. In this present study, we determined if renal D₁R dysfunction is involved in PM_2.5–induced hypertension in the offspring. Methods: Pregnant Sprague–Dawley rats were given an oropharyngeal drip of PM_2.5 (1.0 mg/kg) at gestation day 8, 10, and 12. The blood pressure, 24-hour sodium excretion, and urine volume were measured in the offspring. The expression levels of GRK4 and D₁R were determined by immunoblotting. The phosphorylation of D₁R was investigated using immunoprecipitation. Plasma malondialdehyde and superoxide dismutase levels were also measured in the offspring. Results: As compared with saline-treated dams, offspring of PM_2.5-treated dams had increased blood pressure, impaired sodium excretion, and reduced D₁R-mediated natriuresis and diuresis, accompanied by decreased renal D₁R expression and GRK4 expression. The impaired renal D₁R function and increased GRK4 expression could be caused by increased reactive oxidative stress (ROS) induced by PM_2.5 exposure. Administration of tempol, a redox-cycling nitroxide, for 4 weeks in the offspring of PM_2.5-treated dam normalized the decreased renal D₁R expression and increased renal D₁R phosphorylation and GRK4 expression. Furthermore, tempol normalized the increased renal expression of c-Myc, a transcription factor that regulates GRK4 expression. Conclusions: In utero exposure to PM_2.5 increases ROS and GRK4 expression, impairs D₁R-mediated sodium excretion, and increases blood pressure in the offspring. These studies suggest that normalization of D₁R function may be a target for the prevention and treatment of the hypertension in offspring of mothers exposed to PM_2.5 during pregnancy.

Abstract P189: Disruption of Cyp2c44 Increases Sodium Chloride Cotransporter Expression and Activity in the Distal Convoluted Tubule

Cytochrome-P450 (CYP) epoxygenase plays an important role in the regulation of renal sodium transport. Previous studies have demonstrated that global disruption of Cyp2c44 , a major epoxygenase in the renal tubules, increased ENaC activity in the collecting duct. In addition, we have demonstrated that an increase in dietary Na intake significantly increased the mRNA expression of CYP2c44 in the DCT. DCT is responsible for the reabsorption of 5-9% of filter Na load and is the target for thiazide diuretic. Na absorption in the DCT is mediated by NaCl cotransporter (NCC) and also by ENaC in the late part of DCT. However, the role of epoxyeicosatrienoic acid (EET) in the regulation Na transport in the DCT is not clear. We now examine the role of CYP2c44 and EET in the regulation of NCC expression. The disruption of Cyp2c44 increases the expression of NCC and ENaC activity in the DCT. The disruption of CYP2c44 also suppressed the renal expression of GPR75, a receptor which is activated by 20-hydroxyeicosatetraenoic acid (20-HETE). Renal Na clearance study also demonstrates that the effect of hydrochlorothiazide on Na excretion in Cyp2c44 knockout (KO) mice was significantly higher than those of WT mice. In addition, the disruption of Cyp2c44 significantly increased the expression of ste20-proline and alanine-rich kinase (SPAK) which is known to stimulate NCC activity by phosphorylation. Increased dietary Na intake decreases NCC expression and this effect is blunted in Cyp2c44 KO mice. The role of EET in the regulation of NCC expression is further suggested by the finding that high fat-diet induced increase in NCC expression is inhibited by application of EET. We conclude that CYP2c44-derived EET plays an important role in inhibiting NCC and ENaC in the DCT.

The cooperative roles of the dopamine receptors, D1R and D5R, on the regulation of renal sodium transport

Determining the individual roles of the two dopamine D1-like receptors (D1R and D5R) on sodium transport in the human renal proximal tubule has been complicated by their structural and functional similarity. Here we used a novel D5R-selective antagonist (LE-PM436) and D1R or D5R-specific gene silencing to determine second messenger coupling pathways and heterologous receptor interaction between the two receptors. D1R and D5R co-localized in renal proximal tubule cells and physically interact, as determined by co-immunoprecipitation and FRET microscopy. Stimulation of renal proximal tubule cells with fenoldopam (D1R/D5R agonist) led to both adenylyl cyclase and phospholipase C (PLC) activation using real-time FRET biosensors ICUE3 and CYPHR, respectively. Fenoldopam increased cAMP accumulation and PLC activity and inhibited both NHE3 and NaKATPase activities. LE-PM436 and D5R siRNA blocked the fenoldopam-stimulated PLC pathway but not cAMP accumulation, while D1R siRNA blocked both fenoldopam-stimulated cAMP accumulation and PLC signaling. Either D1R or D5R siRNA, or LE-PM436 blocked the fenoldopam dependent inhibition of sodium transport. Further studies using the cAMP-selective D1R/D5R agonist SKF83822 and PLC-selective D1R/D5R agonist SKF83959 confirmed the cooperative influence of the two pathways on sodium transport. Thus, D1R and D5R interact in the inhibition of NHE3 and NaKATPase activity, the D1R primarily by cAMP, while the D1R/D5R heteromer modulates the D1R effect through a PLC pathway.

A gene-based analysis of variants in the serum/glucocorticoid regulated kinase (SGK) genes with blood pressure responses to sodium intake: the GenSalt Study.

BackgroundSerum and glucocorticoid regulated kinase (SGK) plays a critical role in the regulation of renal sodium transport. We examined the association between SGK genes and salt sensitivity of blood pressure (BP) using single-marker and gene-based association analysis.MethodsA 7-day low-sodium (51.3 mmol sodium/day) followed by a 7-day high-sodium intervention (307.8 mmol sodium/day) was conducted among 1,906 Chinese participants. BP measurements were obtained at baseline and each intervention using a random-zero sphygmomanometer. Additive associations between each SNP and salt-sensitivity phenotypes were assessed using a mixed linear regression model to account for family dependencies. Gene-based analyses were conducted using the truncated p-value method. The Bonferroni-method was used to adjust for multiple testing in all analyses.ResultsIn single-marker association analyses, SGK1 marker rs2758151 was significantly associated with diastolic BP (DBP) response to high-sodium intervention (P = 0.0010). DBP responses (95% confidence interval) to high-sodium intervention for genotypes C/C, C/T, and T/T were 2.04 (1.57 to 2.52), 1.79 (1.42 to 2.16), and 0.85 (0.30 to 1.41) mmHg, respectively. Similar trends were observed for SBP and MAP responses although not significant (P = 0.15 and 0.0026, respectively). In addition, gene-based analyses demonstrated significant associations between SGK1 and SBP, DBP and MAP responses to high sodium intervention (P = 0.0002, 0.0076, and 0.00001, respectively). Neither SGK2 nor SGK3 were associated with the salt-sensitivity phenotypes in single-maker or gene-based analyses.ConclusionsThe current study identified association of the SGK1 gene and BP salt-sensitivity in the Han Chinese population. Further studies are warranted to identify causal SGK1 gene variants.

Abstract MP15: Single And Joint Associations Of Genetic Variants In The Serum/glucocorticoid Regulated Kinase (sgk) Genes With Blood Pressure Responses To Sodium Intake: The Gensalt Study

Background: Serum and glucocorticoid regulated kinase (SGK) plays a critical role in the regulation of renal sodium transport. We examined the association between SGK genes and salt sensitivity of blood pressure (BP) using single-marker and gene-based association analyses. Methods: A 7-day low-sodium (51.3 mmol sodium/day) followed by a 7-day high-sodium intervention (307.8 mmol sodium/day) was conducted among 1,906 Chinese participants. BP measurements were obtained at baseline and each intervention using a random-zero sphygmomanometer. Additive associations between each SNP and salt-sensitivity phenotypes were assessed using a mixed linear regression model to account for family dependencies. Gene-based analyses were conducted using the truncated p-value method. The Bonferroni-method was used to adjust for multiple testing in all analyses. Results: In single-marker association analyses, SGK1 marker rs2758151 was significantly associated with diastolic BP (DBP) response to high-sodium intervention (P=0.0010). DBP responses (95% confidence interval) to high-sodium intervention for genotypes C/C, C/T, and T/T were 2.04 (1.57 to 2.52), 1.79 (1.42 to 2.16), and 0.85 (0.30 to 1.41) mmHg, respectively. Similar non-significant trends were observed for SBP and MAP responses (P=0.15 and 0.0026, respectively). In addition, gene-based analyses demonstrated significant associations between SGK1 and SBP, DBP and MAP responses to high sodium intervention (P=0.0002, 0.0076, and 0.00001, respectively). Neither SGK2 nor SGK3 were associated with the salt-sensitivity phenotypes in single-maker and gene-based analyses. Conclusions: The current study identified single-marker and gene-based association of the SGK1 gene and BP salt-sensitivity in the Han Chinese population. Further studies are warranted to identify causal SGK1 gene variants.

SGK regulation of renal sodium transport.

The serum and glucocorticoid regulated kinase (SGK) family of protein kinases shares similar biochemical and hormonal signaling properties; however, the SGK kinases also exhibit distinct differences in regulating renal sodium (Na(+)) transport. This review will highlight recent advances in our understanding of the specificity of SGK kinase signaling and regulation of renal Na(+) transport. Differential expression of SGK kinases at the cellular and subcellular levels contributes to signaling specificity. New evidence indicates that SGK1 associates with the apical cell membrane of cortical collecting duct cells to regulate open probability of the epithelial Na(+) channel (ENaC). Scaffold proteins can also recruit SGK1 to multiprotein complexes for regulation of ENaC expression in the apical membrane. Recent SGK1 knockout models have implicated the NaCl co-transporter (NCC) as another target of SGK1 regulation. Less is known about the function of SGK2 or SGK3, but both kinases can regulate Na(+)/H(+) exchanger 3 (NHE3) activity. The SGK kinases assume distinct roles in regulating Na transport in both proximal and distal elements of the kidney tubule. Future examination of the molecular mechanisms by which the SGK kinases regulate specific substrates will inform our understanding of how these kinases contribute to the physiology of renal Na(+) transport.

Dopamine and Angiotensin Type 2 Receptors Cooperatively Inhibit Sodium Transport in Human Renal Proximal Tubule Cells

Little is known regarding how the kidney shifts from a sodium and water reclaiming state (antinatriuresis) to a state where sodium and water are eliminated (natriuresis). In human renal proximal tubule cells, sodium reabsorption is decreased by the dopamine D(1)-like receptors (D(1)R/D(5)R) and the angiotensin type 2 receptor (AT(2)R), whereas the angiotensin type 1 receptor increases sodium reabsorption. Aberrant control of these opposing systems is thought to lead to sodium retention and, subsequently, hypertension. We show that D(1)R/D(5)R stimulation increased plasma membrane AT(2)R 4-fold via a D(1)R-mediated, cAMP-coupled, and protein phosphatase 2A-dependent specific signaling pathway. D(1)R/D(5)R stimulation also reduced the ability of angiotensin II to stimulate phospho-extracellular signal-regulated kinase, an effect that was partially reversed by an AT(2)R antagonist. Fenoldopam did not increase AT(2)R recruitment in renal proximal tubule cells with D(1)Rs uncoupled from adenylyl cyclase, suggesting a role of cAMP in mediating these events. D(1)Rs and AT(2)Rs heterodimerized and cooperatively increased cAMP and cGMP production, protein phosphatase 2A activation, sodium-potassium-ATPase internalization, and sodium transport inhibition. These studies shed new light on the regulation of renal sodium transport by the dopaminergic and angiotensin systems and potential new therapeutic targets for selectively treating hypertension.

Insulin Resistance, Obesity, Hypertension, and Renal Sodium Transport

Sodium transport through various nephron segments is quite important in regulating sodium reabsorption and blood pressure. Among several regulators of this process, insulin acts on almost all the nephron segments and is a strong enhancer of sodium reabsorption. Sodium-proton exchanger type 3 (NHE3) is a main regulator of sodium reabsorption in the luminal side of proximal tubule. In the basolateral side of the proximal tubule, sodium-bicarbonate cotransporter (NBCe1) mediates sodium and bicarbonate exit from tubular cells. In the distal nephron and the connecting tubule, epithelial sodium channel (ENaC) is of great importance to sodium reabsorption. NHE3, NBCe1, and ENaC are all regulated by insulin. Recently with-no-lysine (WNK) kinases, responsible for familial hypertension, stimulating sodium reabsorption in the distal nephron, have been found to be also regulated by insulin. We will discuss the regulation of renal sodium transport by insulin and its roles in the pathogenesis of hypertension in insulin resistance.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets.

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

Altered Regulation of Renal Sodium Transporters in Salt-Sensitive Hypertensive Rats Induced by Uninephrectomy

Uninephrectomy (uNx) in young rats causes salt-sensitive hypertension (SSH). Alterations of sodium handling in residual nephrons may play a role in the pathogenesis. Therefore, we evaluated the adaptive alterations of renal sodium transporters according to salt intake in uNx-SSH rats. uNx or sham operations were performed in male Sprague-Dawley rats, and normal-salt diet was fed for 4 weeks. Four experimental groups were used: sham-operated rats raised on a high-salt diet for 2 weeks (CHH) or on a low-salt diet for 1 week after 1 week's high-salt diet (CHL) and uNx rats fed on the same diet (NHH, NHL) as the sham-operated rats were fed. Expression of major renal sodium transporters were determined by semiquantitative immunoblotting. Systolic blood pressure was increased in NHH and NHL groups, compared with CHH and CHL, respectively. Protein abundances of Na+/K+/2Cl- cotransporter (NKCC2) and Na+/Cl- cotransporter (NCC) in the CHH group were lower than the CHL group. Expression of epithelial sodium channel (ENaC)-γ increased in the CHH group. In contrast, expressions of NKCC2 and NCC in the NHH group didn't show any significant alterations, compared to the NHL group. Expressions of ENaC-α and ENaC-β in the NHH group were higher than the CHH group. Adaptive alterations of NKCC2 and NCC to changes of salt intake were different in the uNx group, and changes in ENaC-α and ENaC-β were also different. These altered regulations of sodium transporters may be involved in the pathogenesis of SSH in the uNx rat model.

Dopamine, kidney, and hypertension: studies in dopamine receptor knockout mice.

Dopamine is important in the pathogenesis of hypertension because of abnormalities in receptor-mediated regulation of renal sodium transport. Dopamine receptors are classified into D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), D(4)) subtypes, all of which are expressed in the kidney. Mice deficient in specific dopamine receptors have been generated to provide holistic assessment on the varying physiological roles of each receptor subtype. This review examines recent studies on these mutant mouse models and evaluates the impact of individual dopamine receptor subtypes on blood pressure regulation.

Endogenous digitalis: pathophysiologic roles and therapeutic applications.

Endogenous digitalis-like factors, also called cardiotonic steroids, have been thought for nearly half a century to have important roles in health and disease. The endogenous cardiotonic steroids ouabain and marinobufagenin have been identified in humans, and an effector mechanism has been delineated by which these hormones signal through the sodium/potassium-transporting ATPase. These findings have increased interest in this field substantially. Although cardiotonic steroids were first considered important in the regulation of renal sodium transport and arterial pressure, subsequent work has implicated these hormones in the control of cell growth, apoptosis and fibrosis, among other processes. This Review focuses on the role of endogenous cardiotonic steroids in the pathophysiology of essential hypertension, congestive heart failure, end-stage renal disease and pre-eclampsia. We also discuss potential therapeutic strategies that have emerged as a result of the increased understanding of the regulation and actions of cardiotonic steroids.

Estradiol regulates the expression of renal sodium transporters in ovariectomized rats

Although recent studies suggest that estrogen may alter renal expression of certain sodium transporters, conflicting results make its effects on the regulation of renal sodium transporters complicated. To determine the effect of estrogen on the expression of major sodium transporters, we performed semiquantitative immunoblotting analysis of the kidneys from ovariectomized (OVX) Sprague‐Dawley rats treated with (n=6) or without (n=5) high dose of 17β‐estradiol benzoate (E2) (14mcg/100g BW) for 10 days. Physiologic data including body weight (BW), blood pressure (BP), fractional excretion of sodium (FENa) and plasma aldosterone level were also measured.BP showed no difference between two groups. BW was 278 ± 20 g in E2‐replaced rats and 311 ±15 g in OVX rats(p=0.028). FENa of the E2‐replaced rats increased without significant difference (E2 vs. OVX, 0.57 ± 0.22% vs. 0.36 ± 0.19%; mean ± SD, p=0.14). Plasma aldosterone level was 179 ± 125 pg/ml in E2‐replaced rats and 315 ± 218 pg/ml in OVX rats(p=0.20). Administration of E2 led to significant decrease in the protein abundances for Na‐Cl cotransporter, Na‐K‐2Cl cotransporter, the Na‐K‐ATPase, α‐ and γ‐subunits of ENaC in the OVX rats (66%, 32%, 52%, 59% and 49% of control, respectively; p<0.05).E2 replacement to OVX rats resulted in down‐regulation of major renal sodium transporters, which might be associated with reduced plasma aldosterone level.

Regulation of renal sodium transporters during severe inflammation

Regulation of renal sodium transporters during severe inflammation

Regulation of Renal Sodium Transporters during Severe Inflammation

Sepsis-associated acute renal failure is characterized by decreased GFR and tubular dysfunction. The pathogenesis of endotoxemic tubular dysfunction with failure in urine concentration and increased fractional sodium excretion is poorly understood. This study investigated the regulation of renal sodium transporters during severe inflammation in vivo and in vitro. Injection of high-dosage LPS reduced BP and GFR, increased fractional sodium excretion, and strongly decreased the expression of Na(+)/H(+)-exchanger, renal outer medullary potassium channel, Na(+)-K(+)-2Cl(-) co-transporter, epithelial sodium channel, and Na(+)/K(+)-ATPase in mice. Also, injection of TNF-alpha, IL-1beta, or IFN-gamma decreased renal function and expression of renal sodium transporters. LPS-induced downregulation of sodium transporters was not affected in cytokine-knockout mice. However, supplementary glucocorticoid treatment, which inhibited LPS-induced increase of tissue cytokine concentrations, attenuated LPS-induced renal dysfunction and downregulation of tubular sodium transporters. Injection of low-dosage LPS increased renal tissue cytokines and downregulated renal sodium transporters without arterial hypotension. In vitro, in cortical collecting duct cells, cytokines also decreased expression of renal outer medullary potassium channel, epithelial sodium channel, and Na(+)/K(+)-ATPase. Renal hypoperfusion by renal artery clipping did not influence renal sodium transporter expression, in contrast to renal ischemia-reperfusion injury, which depressed transporter expression. These findings demonstrate downregulation of renal sodium transporters that likely accounts for tubular dysfunction during inflammation. These data suggest that alteration of renal sodium transporters during LPS-induced acute renal failure is mediated by cytokines rather than renal ischemia. However, in a complex in vivo model of severe inflammation, the possible presence and influence of renal hypoperfusion and reperfusion on the expression of renal sodium transporters cannot be completely excluded.

Altered expression of renal sodium transporters in the animal model of type 2 diabetes mellitus

Hemodynamic factors including systemic and intraglomerular hypertension play an important role in development and progression of diabetic nephropathy. We hypothesized that dysregulation of renal sodium transporters might be responsible for hemodynamic alteration in diabetic nephropathy. We used Otsuka Long Evans Tokushima Fatty (OLETF) rats, as a model of type 2 diabetes and Long Evans Tokushima (LETO) rats as controls. We investigated regulation of renal sodium transporters by using semiquantitative immunoblotting and immunohistochmistry of kidney. At 12 weeks of age, OLETF rats were heavier and had higher blood glucose level. At 40 weeks of age, OLETF rats had elevated SBP, increased urine volume, glucose level, and urine protein-to-creatinine ratio. At 12 weeks of age, relative band densities of epithelial sodium channel (ENaC) γ-subunits in OLETF rats (% of LETO rats) was decreased (44 ± 4%, p<0.05). At 40 weeks of age, relative band densities of β-subunit was increased (171 ± 20%, p<0.05) and that of γ-subunit was decreased (32 ± 9%, p<0.05) significantly. Consistent with these results, ENaC β-subunit immunohistochemistry showed remarkable increase in immunoreactivity in 40-week-old OLETF rats and γ-subunit immunohistochemistry showed decrease in immunoreactivity in 12 and 40-week-old OLETF rats. In OLETF rats, increase in β-ENaC abundance after development of diabetic nephropathy may reflect hemodynamic changes and be associated with the systemic and intraglomerular hypertension and progression of kidney disease.