Mechanisms of pressure natriuresis: how blood pressure regulates renal sodium transport

The incretin pathway as a therapeutic target in diabetic kidney disease: a clinical focus on GLP-1 receptor agonists

Diabetic kidney disease (DKD) remains the main cause for chronic kidney disease (CKD) and end-stage kidney disease (ESKD) worldwide. Both CKD and ESKD lead to major increases in risk of cardiovascular disease and death in people with diabetes. Despite optimal management of lifestyle, glucose levels and hypertension, residual risk remains high, indicating that additional therapies to mitigate the burden of the disease are desired. In past decades, new treatment options for the management of diabetes have emerged, of which some have showed promising renoprotective potential. This review discusses current understanding of the renal effects of glucagon-like peptide receptor agonists and their potential use in prevention and treatment of DKD.

Renal tubular effects of prolonged therapy with the GLP-1 receptor agonist lixisenatide in patients with type 2 diabetes mellitus

Glucagon-like peptide-1 (GLP-1) receptor agonists (RAs) are well-established glucose-lowering drugs for type 2 diabetes mellitus (T2DM) management. Acute GLP-1RA administration increases urinary excretion of sodium and other electrolytes. However, the renal tubular effects of prolonged GLP-1RA treatment are largely unknown. In this secondary analysis of a randomized trial, we determined the renal tubular effects of 8-wk treatment with 20 μg lixisenatide, a short-acting (prandial) GLP-1RA, versus titrated once-daily insulin glulisine in 35 overweight T2DM-patients on stable insulin glargine background therapy (age: 62 ± 7 yr, glycated hemoglobin: 8.0 ± 0.9%, estimated glomerular filtration rate: >60 ml·min^-1·1.73 m^-2). After a standardized breakfast, lixisenatide increased absolute and fractional excretions of sodium, chloride, and potassium and increased urinary pH. In contrast, lixisenatide reduced absolute and fractional excretions of magnesium, calcium, and phosphate. At week 8, patients treated with lixisenatide had significantly more phosphorylated sodium-hydrogen exchanger isoform 3 (NHE3) in urinary extracellular vesicles than those on insulin glulisine treatment, which suggested decreased NHE3 activity in the proximal tubule. A rise in postprandial blood pressure with lixisenatide partly explained the changes in the urinary excretion of sodium, potassium, magnesium, and phosphate and the changes in urinary pH. In conclusion, lixisenatide affects postprandial urinary excretion of several electrolytes and increases urinary pH compared with insulin glulisine in T2DM patients after 8 wk of treatment. This is most likely explained by a drug-induced rise in blood pressure or direct inhibitory effects on NHE3 in the proximal tubule.

Dietary Fructose Increases the Sensitivity of Proximal Tubules to Angiotensin II in Rats Fed High-Salt Diets

Dietary fructose causes salt-sensitive hypertension. Proximal tubules (PTs) reabsorb 70% of the filtered NaCl. Angiotensin II (Ang II), atrial natriuretic peptide (ANP) and norepinephrine (NE) regulate this process. Although Ang II signaling blockade ameliorates fructose-induced salt-sensitive hypertension, basal PT Na⁺ reabsorption and its sensitivity to the aforementioned factors have not been studied in this model. We hypothesized consuming fructose with a high-salt diet selectively enhances the sensitivity of PT transport to Ang II. We investigated the effects of Ang II, ANP and NE on PT Na reabsorption in rats fed a high-salt diet drinking tap water (HS) or 20% fructose (HS-FRU). Oxygen consumption (QO₂) was used as a measure of all ATP-dependent transport processes. Na⁺/K⁺-ATPase and Na⁺/H⁺-exchange (NHE) activities were studied because they represent primary apical and basolateral transporters in this segment. The effect of 10^-12 mol/L Ang II in QO₂ by PTs from HS-FRU was larger than HS (p < 0.02; n = 7). In PTs from HS-FRU 10^-12 mol/L Ang II stimulated NHE activity by 2.6 ± 0.7 arbitrary fluorescence units/s (p < 0.01; n = 5) but not in those from HS. The stimulatory effect of Ang II on PT Na⁺/K⁺-ATPase activity was not affected by HS-FRU. Responses of QO₂ and NHE activity to ANP did not differ between groups. The response of QO₂ to NE was unaltered by HS-FRU. We concluded that the sensitivity of PT Na⁺ reabsorption specifically to Ang II is enhanced by HS-FRU. This maintains high rates of transport even in the presence of low concentrations of the peptide, and likely contributes to the hypertension.

The Na/K-ATPase Signaling: From Specific Ligands to General Reactive Oxygen Species

The signaling function of the Na/K-ATPase has been established for 20 years and is widely accepted in the field, with many excellent reports and reviews not cited here. Even though there is debate about the underlying mechanism, the signaling function is unquestioned. This short review looks back at the evolution of Na/K-ATPase signaling, from stimulation by cardiotonic steroids (also known as digitalis-like substances) as specific ligands to stimulation by reactive oxygen species (ROS) in general. The interplay of cardiotonic steroids and ROS in Na/K-ATPase signaling forms a positive-feedback oxidant amplification loop that has been implicated in some pathophysiological conditions.

Alteration of Tight Junction Protein Expression in Dahl Salt-Sensitive Rat Kidney

BACKGROUND/AIMS

Altered pressure natriuresis is an important mechanism of hypertension, but it remains elusive at the molecular level. We hypothesized that in the kidney, tight junctions (TJs) may have a role in pressure natriuresis because paracellular NaCl transport affects interstitial hydrostatic pressure.

METHODS

To assess the association of salt-sensitive hypertension with altered renal TJ protein expression, Dahl salt-sensitive (SS) and salt-resistant (SR) rats were put on an 8% NaCl-containing rodent diet for 4 weeks. Systolic blood pressure (SBP) and urine NaCl excretion were measured weekly, and kidneys were harvested for immunoblotting and quantitative PCR analysis at the end of the animal experiments.

RESULTS

SBP was significantly higher in SS rats than in SR rats during the first to fourth weeks of the animal experiments. During the first and second week, urinary NaCl excretion was significantly lower in SS rats as compared with SR rats. However, the difference between the two groups vanished at the third and fourth weeks. In the kidney, claudin-4 protein and mRNA were significantly increased in SS rats as compared with SR rats. On the other hand, occludin protein and mRNA were significantly decreased in SS rats as compared with SR rats. The expression of claudin-2, claudin-7, and claudin-8 did not vary significantly between the two groups.

CONCLUSIONS

In SS rats, SS hypertension was associated with differential changes in renal TJ protein expression. Both upregulation of claudin-4 and downregulation of occludin might increase paracellular NaCl transport in the kidney, resulting in impaired pressure natriuresis in SS rats.

GLP-1 and the kidney: from physiology to pharmacology and outcomes in diabetes

The gastrointestinal tract - the largest endocrine network in human physiology - orchestrates signals from the external environment to maintain neural and hormonal control of homeostasis. Advances in understanding entero-endocrine cell biology in health and disease have important translational relevance. The gut-derived incretin hormone glucagon-like peptide 1 (GLP-1) is secreted upon meal ingestion and controls glucose metabolism by modulating pancreatic islet cell function, food intake and gastrointestinal motility, amongst other effects. The observation that the insulinotropic actions of GLP-1 are reduced in type 2 diabetes mellitus (T2DM) led to the development of incretin-based therapies - GLP-1 receptor agonists and dipeptidyl peptidase 4 (DPP-4) inhibitors - for the treatment of hyperglycaemia in these patients. Considerable interest exists in identifying effects of these drugs beyond glucose-lowering, possibly resulting in improved macrovascular and microvascular outcomes, including in diabetic kidney disease. As GLP-1 has been implicated as a mediator in the putative gut-renal axis (a rapid-acting feed-forward loop that regulates postprandial fluid and electrolyte homeostasis), direct actions on the kidney have been proposed. Here, we review the role of GLP-1 and the actions of associated therapies on glucose metabolism, the gut-renal axis, classical renal risk factors, and renal end points in randomized controlled trials of GLP-1 receptor agonists and DPP-4 inhibitors in patients with T2DM.

Renal tubular NHE3 is required in the maintenance of water and sodium chloride homeostasis

The sodium/proton exchanger isoform 3 (NHE3) is expressed in the intestine and the kidney, where it facilitates sodium (re)absorption and proton secretion. The importance of NHE3 in the kidney for sodium chloride homeostasis, relative to the intestine, is unknown. Constitutive tubule-specific NHE3 knockout mice (NHE3^loxloxCre) did not show significant differences compared to control mice in body weight, blood pH or bicarbonate and plasma sodium, potassium, or aldosterone levels. Fluid intake, urinary flow rate, urinary sodium/creatinine, and pH were significantly elevated in NHE3^loxloxCre mice, while urine osmolality and GFR were significantly lower. Water deprivation revealed a small urinary concentrating defect in NHE3^loxloxCre mice on a control diet, exaggerated on low sodium chloride. Ten days of low or high sodium chloride diet did not affect plasma sodium in control mice; however, NHE3^loxloxCre mice were susceptible to low sodium chloride (about -4 mM) or high sodium chloride intake (about +2 mM) versus baseline, effects without differences in plasma aldosterone between groups. Blood pressure was significantly lower in NHE3^loxloxCre mice and was sodium chloride sensitive. In control mice, the expression of the sodium/phosphate co-transporter Npt2c was sodium chloride sensitive. However, lack of tubular NHE3 blunted Npt2c expression. Alterations in the abundances of sodium/chloride cotransporter and its phosphorylation at threonine 58 as well as the abundances of the α-subunit of the epithelial sodium channel, and its cleaved form, were also apparent in NHE3^loxloxCre mice. Thus, renal NHE3 is required to maintain blood pressure and steady-state plasma sodium levels when dietary sodium chloride intake is modified.

Acute renal effects of the GLP-1 receptor agonist exenatide in overweight type 2 diabetes patients: a randomised, double-blind, placebo-controlled trial

AIMS/HYPOTHESIS

This study aimed to investigate the acute renal effects of the glucagon-like peptide-1 receptor agonist (GLP-1RA) exenatide in type 2 diabetes patients.

METHODS

We included overweight (BMI 25-40 kg/m(2)) men and postmenopausal women, aged 35-75 years with type 2 diabetes (HbA1c 48-75 mmol/mol; 6.5-9.0%) and estimated GFR ≥ 60 ml min(-1) 1.73 m(-2). Exenatide or placebo (NaCl solution, 154 mmol/l) was administrated intravenously in an acute, randomised, double-blind, placebo-controlled trial conducted at the Diabetes Center VU University Medical Center (VUMC). GFR (primary endpoint) and effective renal plasma flow (ERPF) were determined by inulin and para-aminohippurate clearance, respectively, based on timed urine sampling. Filtration fraction (FF) and effective renal vascular resistance (ERVR) were calculated, and glomerular hydrostatic pressure (PGLO) and vascular resistance of the afferent (RA) and efferent (RE) renal arteriole were estimated. Tubular function was assessed by absolute and fractional excretion of sodium (FENa), potassium (FEK) and urea (FEU), in addition to urine osmolality, pH and free water clearance. Renal damage markers, BP and plasma glucose were also determined.

RESULTS

Of the 57 patients randomised by computer, 52 were included in the final analyses. Exenatide (n = 24) did not affect GFR (mean difference +2 ± 3 ml min(-1) 1.73 m(-2), p = 0.489), ERPF, FF, ERVR or PGLO, compared with placebo (n = 28). Exenatide increased RA (p < 0.05), but did not change RE. Exenatide increased FENa, FEK, urine osmolality and pH, while FEU, urinary flow and free water clearance were decreased (all p < 0.05). Osmolar clearance and renal damage makers were not affected. Diastolic BP and mean arterial pressure increased by 3 ± 1 and 6 ± 2 mmHg, respectively, whereas plasma glucose decreased by 1.4 ± 0.1 mmol/l (all p < 0.05).

CONCLUSIONS/INTERPRETATION

Exenatide infusion does not acutely affect renal haemodynamics in overweight type 2 diabetes patients at normal filtration levels. Furthermore, acute GLP-1RA administration increases proximal sodium excretion in these patients.

TRIAL REGISTRATION

ClincialTrials.gov NCT01744236 FUNDING : The research leading to these results has been funded from: (1) the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement number 282521 - the SAFEGUARD project; and (2) the Dutch Kidney Foundation, under grant agreement IP12.87.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Role of caveolin 1 in AT1a receptor-mediated uptake of angiotensin II in the proximal tubule of the kidney

Caveolin 1 (CAV-1) functions not only as a constitutive scaffolding protein of caveolae but also as a vesicular transporter and signaling regulator. In the present study, we tested the hypothesis that CAV-1 knockout (CAV-1 KO) inhibits ANG II type 1 [AT1 (AT1a)] receptor-mediated uptake of ANG II in the proximal tubule and attenuates blood pressure responses in ANG II-induced hypertension. To determine the role of CAV-1 in mediating the uptake of FITC-labeled ANG II, wild-type (WT) mouse proximal convoluted tubule cells were transfected with CAV-1 small interfering (si)RNA for 48 h before AT1 receptor-mediated uptake of FITC-labeled ANG II was studied. CAV-1 siRNA knocked down CAV-1 expression by >90% (P < 0.01) and inhibited FITC-labeled ANG II uptake by >50% (P < 0.01). Moreover, CAV-1 siRNA attenuated ANG II-induced activation of MAPK ERK1/2 and Na(+)/H(+) exchanger 3 expression, respectively (P < 0.01). To determine whether CAV-1 regulates ANG II uptake in the proximal tubule, Alexa 488-labeled ANG II was infused into anesthetized WT and CAV-1 KO mice for 60 min (20 ng/min iv). Imaging analysis revealed that Alexa 488-labeled ANG II uptake was decreased by >50% in CAV-1 KO mice (P < 0.01). Furthermore, Val(5)-ANG II was infused into WT and CAV-1 KO mice for 2 wk (1.5 mg·kg(-1)·day(-1) ip). Basal systolic pressure was higher, whereas blood pressure and renal excretory and signaling responses to ANG II were attenuated, in CAV-1 KO mice (P < 0.01). We concluded that CAV-1 plays an important role in AT1 receptor-mediated uptake of ANG II in the proximal tubule and modulates blood pressure and renal responses to ANG II.

Deletion of the angiotensin II type 1 receptor-associated protein enhances renal sodium reabsorption and exacerbates angiotensin II-mediated hypertension

Angiotensin II type 1 receptor (AT1R)–associated protein (ATRAP) promotes AT1R internalization along with suppression of pathological activation of tissue AT1R signaling. However, the functional significance of ATRAP in renal sodium handling and blood pressure regulation under pathological stimuli is not fully resolved. Here we show the blood pressure of mice with a gene-targeted disruption of ATRAP was comparable to that of wild-type mice at baseline. However, in ATRAP-knockout mice, angiotensin II–induced hypertension was exacerbated and the extent of positive sodium balance was increased by angiotensin II. Renal expression of the sodium-proton antiporter 3, a major sodium transporter in the proximal tubules, urinary pH, renal angiotensinogen production, and angiotensin II content was unaffected. Stimulation of the renal expression and activity of the epithelial sodium channel (ENaC), a major sodium transporter in the distal tubules, was significantly enhanced by chronic angiotensin II infusion. The circulating and urinary aldosterone levels were comparable. The blood pressure response and renal ENaC expression by aldosterone were not affected. Thus, ATRAP deficiency exacerbated angiotensin II–mediated hypertension by pathological activation of renal tubular AT1R by angiotensin II. This directly stimulates ENaC in the distal tubules and enhances sodium retention in an aldosterone-independent manner.

Dominant factors that govern pressure natriuresis in diuresis and antidiuresis: a mathematical model

We have developed a whole kidney model of the urine concentrating mechanism and renal autoregulation. The model represents the tubuloglomerular feedback (TGF) and myogenic mechanisms, which together affect the resistance of the afferent arteriole and thus glomerular filtration rate. TGF is activated by fluctuations in macula densa [Cl(-)] and the myogefnic mechanism by changes in hydrostatic pressure. The model was used to investigate the relative contributions of medullary blood flow autoregulation and inhibition of transport in the proximal convoluted tubule to pressure natriuresis in both diuresis and antidiuresis. The model predicts that medullary blood flow autoregulation, which only affects the interstitial solute composition in the model, has negligible influence on the rate of NaCl excretion. However, it exerts a significant effect on urine flow, particularly in the antidiuretic kidney. This suggests that interstitial washout has significant implications for the maintenance of hydration status but little direct bearing on salt excretion, and that medullary blood flow may only play a signaling role for stimulating a pressure-natriuresis response. Inhibited reabsorption in the model proximal convoluted tubule is capable of driving pressure natriuresis when the known actions of vasopressin on the collecting duct epithelium are taken into account.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Impaired pressure natriuresis is associated with interstitial inflammation in salt-sensitive hypertension

PURPOSE OF REVIEW

Impairment of the pressure natriuresis relationship is a central event in the pathogenesis of hypertension. Renal tubulointerstitial inflammation results in salt-sensitive hypertension and, until recently, the changes in pressure natriuresis induced by renal inflammation received little attention.

RECENT FINDINGS

Oxidative stress and increased intrarenal angiotensin II activity, in association with rarefaction and loss of peritubular vascular network, may be involved in the inflammation-induced blunting of the natriuresis resulting from increments in renal perfusion pressure.

SUMMARY

Here, we review the mechanisms for the impairment in pressure natriuresis resulting from renal tubulointerstitial inflammation in reference to the normal physiologic mechanisms involved in this response.

Enhanced angiotensin receptor-associated protein in renal tubule suppresses angiotensin-dependent hypertension

We have previously shown that angiotensin II type 1 receptor-associated protein (ATRAP/Agtrap) interacts with the angiotensin II type 1 receptor and promotes constitutive internalization of the receptor so as to inhibit the pathological activation of its downstream signaling but preserve baseline physiological signaling activity. The present study was designed to investigate the role of renal ATRAP in angiotensin II-dependent hypertension. We generated transgenic mice dominantly expressing ATRAP in the renal tubules, including renal distal tubules. The renal ATRAP transgenic mice exhibited no significant change in blood pressure at baseline on normal salt diet. However, in the renal ATRAP transgenic mice compared with wild-type mice, the following took place: (1) the development of high blood pressure in response to angiotensin II infusion was significantly suppressed based on radiotelemetry, (2) the extent of daily positive sodium balance was significantly reduced during angiotensin II infusion in metabolic cage analysis, and (3) the renal Na+ -Cl- cotransporter activation and α-subunit of the epithelial sodium channel induction by angiotensin II infusion were inhibited. Furthermore, adenoviral overexpression of ATRAP suppressed the angiotensin II-mediated increase in the expression of α-subunit of the epithelial sodium channel in mouse distal convoluted tubule cells. These results indicate that renal tubule-dominant ATRAP activation provokes no evident effects on blood pressure at baseline but exerts an inhibitory effect on the pathological elevation of blood pressure in response to angiotensin II stimulation, thereby suggesting that ATRAP is a potential target of interest in blood pressure modulation under pathological conditions.

Autonomic regulation of kidney function

The kidneys play a central role in cardiovascular homeostasis by ensuring a balance between the fluid taken in and that lost and excreted during everyday activities. This ensures stability of extracellular fluid volume and maintenance of normal levels of blood pressure. Renal fluid handling is controlled via neural and humoral influences, with the former determining a rapid dynamic response to changing intake of sodium whereas the latter cause a slower longer-term modulation of sodium and water handling. Activity in the renal sympathetic nerves arises from an integration of information from the high and low pressure cardiovascular baroreceptors, the somatosensory and visceral systems as well as the higher cortical centers. Each sensory system provides varying input to the autonomic centers of the hypothalamic and medullary areas of the brain at a level appropriate to the activity being performed. In pathophysiological states, such as hypertension, heart failure and chronic renal disease, there may be an inappropriate sympathoexcitation causing sodium retention which exacerbates the disease process. The contribution of the renal sympathetic nerves to these cardiovascular diseases is beginning to be appreciated with the demonstration that renal denervation of resistant hypertensive patients results in a long-term normalization of blood pressure.

Microarray-based identification of differentially expressed genes in families of turbot (Scophthalmus maximus) after infection with viral haemorrhagic septicaemia virus (VHSV)

Viral haemorrhagic septicaemia virus (VHSV) is one of the major threats to the development of the aquaculture industry worldwide. The present study was aimed to identify genes differentially expressed in several turbot (Scophthalmus maximus) families showing different mortality rates after VHSV. The expression analysis was conducted through genome-wide expression profiling with an oligo-microarray in the head kidney. A significant proportion of the variation in the gene expression profiles seemed to be explained by the genetic background, indicating that the mechanisms by which particular species and/or populations can resist a pathogen(s) are complex and multifactorial. Before the experimental infections, fish from resistant families (low mortality rates after VHSV infection) showed high expression of different antimicrobial peptides, suggesting that their pre-immune state may be stronger than fish of susceptible families (high mortality rates after VHSV infection). After infection, fish from both high- and low-mortality families showed an up-modulation of the interferon-induced Mx2 gene, the IL-8 gene and the VHSV-induced protein 5 gene compared with control groups. Low levels of several molecules secreted in the mucus were observed in high-mortality families, but different genes involved in viral entrance into target cells were down-regulated in low-mortality families. Moreover, these families also showed a strong down-modulation of marker genes related to VHSV target organs, including biochemical markers of renal dysfunction and myocardial injury. In general, the expression of different genes involved in the metabolism of sugars, lipids and proteins were decreased in both low- and high-mortality families after infection. The present study serves as an initial screen for genes of interest and provides an extensive overview of the genetic basis underlying the differences between families that are resistant or susceptible to VHSV infection.

Mechanisms of pressure-diuresis and pressure-natriuresis in Dahl salt-resistant and Dahl salt-sensitive rats

Background

Data on blood flow regulation, renal filtration, and urine output in salt-sensitive Dahl S rats fed on high-salt (hypertensive) and low-salt (prehypertensive) diets and salt-resistant Dahl R rats fed on high-salt diets were analyzed using a mathematical model of renal blood flow regulation, glomerular filtration, and solute transport in a nephron.

Results

The mechanism of pressure-diuresis and pressure-natriuresis that emerges from simulation of the integrated systems is that relatively small increases in glomerular filtration that follow from increases in renal arterial pressure cause relatively large increases in urine and sodium output. Furthermore, analysis reveals the minimal differences between the experimental cases necessary to explain the observed data. It is determined that differences in renal afferent and efferent arterial resistances are able to explain all of the qualitative differences in observed flows, filtration rates, and glomerular pressure as well as the differences in the pressure-natriuresis and pressure-diuresis relationships in the three groups. The model is able to satisfactorily explain data from all three groups without varying parameters associated with glomerular filtration or solute transport in the nephron component of the model.

Conclusions

Thus the differences between the experimental groups are explained solely in terms of difference in blood flow regulation. This finding is consistent with the hypothesis that, if a shift in the pressure-natriuresis relationship is the primary cause of elevated arterial pressure in the Dahl S rat, then alternation in how renal afferent and efferent arterial resistances are regulated represents the primary cause of chronic hypertension in the Dahl S rat.

Regulation of renal NaCl and water transport by the ATP/UTP/P2Y2 receptor system

Extracellular nucleotides (e.g., ATP) activate ionotropic P2X and metabotropic P2Y receptors in the plasma membrane to regulate and maintain cell function and integrity. This includes the renal tubular and collecting duct system, where the locally released nucleotides act in a paracrine and autocrine way to regulate transport of electrolytes and water and maintain cell volume. A prominent role has been assigned to Gq-coupled P2Y(2) receptors, which are typically activated by both ATP and UTP. Studies in gene knockout mice revealed an antihypertensive activity of P2Y(2) receptors that is linked to vasodilation and an inhibitory influence on renal salt reabsorption. Flow induces apical ATP release in the thick ascending limb, and first evidence indicates an inhibitory influence of P2Y(2) receptor tone on the expression and activity of the Na-K-2Cl cotransporter NKCC2 in this segment. The apical ATP/UTP/P2Y(2) receptor system in the connecting tubule/cortical collecting duct mediates the inhibitory effect of dietary salt on the open probability of the epithelial sodium channel ENaC and inhibits ENaC activity during aldosterone escape. Connexin 30 has been implicated in the luminal release of the ATP involved in the regulation of ENaC. An increase in collecting duct cell volume in response to manipulating water homeostasis increases ATP release. The subsequent activation of P2Y(2) receptors inhibits vasopressin-induced cAMP formation and water reabsorption, which facilitates water excretion and stabilizes cell volume. Thus recent studies have established the ATP/UTP/P2Y(2) receptor system as a relevant regulator of renal salt and water homeostasis and blood pressure regulation. The pathophysiological relevance and therapeutic potential remains to be determined, but dual effects of P2Y(2) receptor activation on both the vasculature and renal salt reabsorption implicate these receptors as potential therapeutic targets in hypertension.

Sodium intake is associated with carotid artery structure alterations and plasma matrix metalloproteinase-9 upregulation in hypertensive adults

The mechanisms by which dietary sodium modulates cardiovascular risk are not fully understood. This study investigated whether sodium intake is related to carotid structure and hemodynamics and to plasma matrix metalloproteinase (MMP) activity in hypertensive adults. One hundred thirty-four participants were cross-sectionally evaluated by clinical history, anthropometry, carotid ultrasound, and analysis of hemodynamic, inflammatory, and metabolic variables. Daily sodium intake (DSI) was estimated by 24-h recall, discretionary sodium, and a FFQ. In 42 patients, plasma MMP-2 and MMP-9 activities were also analyzed. The mean DSI was 5.52 ± 0.29 g/d. Univariate analysis showed that DSI correlated with common carotid artery systolic and diastolic diameter (r = 0.36 and 0.34; both P < 0.001), peak and mean circumferential tension (r = 0.44 and 0.39; both P < 0.001), Young's Elastic Modulus (r = 0.40; P < 0.001), intima-media thickness (r = 0.19; P < 0.05), and internal carotid artery resistive index (r = 0.20; P < 0.05). Multivariate analyses revealed that only artery diameter, circumferential wall tension, and Young's Elastic Modulus were independently associated with DSI. Conversely, plasma MMP-9 activity was associated with DSI (r = 0.53; P < 0.001) as well as with common carotid systolic diameter (r = 0.33; P < 0.05) and Young's Elastic Modulus (r = 0.38; P < 0.01). In conclusion, sodium intake is associated with carotid alterations in hypertensive adults independently of systemic hemodynamic variables. The present findings also suggest that increased MMP-9 activity might play a role in sodium-induced vascular remodeling.

AT1A angiotensin receptors in the renal proximal tubule regulate blood pressure

Hypertension affects more than 1.5 billion people worldwide but the precise cause of elevated blood pressure (BP) cannot be determined in most affected individuals. Nonetheless, blockade of the renin-angiotensin system (RAS) lowers BP in the majority of patients with hypertension. Despite its apparent role in hypertension pathogenesis, the key cellular targets of the RAS that control BP have not been clearly identified. Here we demonstrate that RAS actions in the epithelium of the proximal tubule have a critical and nonredundant role in determining the level of BP. Abrogation of AT(1) angiotensin receptor signaling in the proximal tubule alone is sufficient to lower BP, despite intact vascular responses. Elimination of this pathway reduces proximal fluid reabsorption and alters expression of key sodium transporters, modifying pressure-natriuresis and providing substantial protection against hypertension. Thus, effectively targeting epithelial functions of the proximal tubule of the kidney should be a useful therapeutic strategy in hypertension.

Adult renal function is modified by perinatal taurine status in conscious male rats

Perinatal taurine exposure influences renal function in adult female offspring. This study tests the hypothesis that prenatal rather than postnatal taurine exposure alters renal function in adult conscious male rats. Female Sprague Dawley rats were fed normal rat chow and tap water alone (Control), tap water containing 3% β-alanine (taurine depletion, TD) or tap water containing 3% taurine (taurine supplementation, TS) either from conception until delivery (fetal period; TDF or TSF) or from delivery until weaning (lactation period; TDL or TSL). After weaning, male offspring were fed with the normal rat chow and tap water ad libitum. At 7-8 weeks of age, renal function was studied in conscious, restrained rats. Mean arterial pressures were slightly higher in rats receiving taurine supplementation during either the fetal or lactation periods (compared to Control and TD groups), but heart rates were not significantly different among groups. Effective renal blood flows were lower in TDF, TDL, and TSF rats (TDF 4.6±0.8 ml/min/g kidney weight (KW), TDL 3.0±0.9 ml/min/g KW, and TSF 2.8±0.7 ml/min/g KW) than in TSL (7.7±0.9 ml/min/g KW) or Control rats (7.3±1.6 ml/min/g KW). These differences were correlated with significant increases in renal vascular resistance in TDF, TDL, and TSF groups compared to TSL and Control rats. In contrast, glomerular filtration rates were not significantly different among groups. Although basal water and sodium excretion were slightly lower in TDL and TSF rats compared to other groups, their diuretic and natriuretic responses to an acute saline load were not different from Control. The present data indicate that in adult male rats, both perinatal supplementation and depletion of taurine can alter renal hemodynamics, and these effects are differentially time-dependent.

The sodium pump and cardiotonic steroids-induced signal transduction protein kinases and calcium-signaling microdomain in regulation of transporter trafficking

The Na/K-ATPase was discovered as an energy transducing ion pump. A major difference between the Na/K-ATPase and other P-type ATPases is its ability to bind a group of chemicals called cardiotonic steroids (CTS). The plant-derived CTS such as digoxin are valuable drugs for the management of cardiac diseases, whereas ouabain and marinobufagenin (MBG) have been identified as a new class of endogenous hormones. Recent studies have demonstrated that the endogenous CTS are important regulators of renal Na(+) excretion and blood pressure. The Na/K-ATPase is not only an ion pump, but also an important receptor that can transduce the ligand-like effect of CTS on intracellular protein kinases and Ca(2+) signaling. Significantly, these CTS-provoked signaling events are capable of reducing the surface expression of apical NHE3 (Na/H exchanger isoform 3) and basolateral Na/K-ATPase in renal proximal tubular cells. These findings suggest that endogenous CTS may play an important role in regulation of tubular Na(+) excretion under physiological conditions; conversely, a defect at either the receptor level (Na/K-ATPase) or receptor-effector coupling would reduce the ability of renal proximal tubular cells to excrete Na(+), thus culminating/resulting in salt-sensitive hypertension.

Placental oxidative stress in malnourished rats and changes in kidney proximal tubule sodium ATPases in offspring

1. Intrauterine malnutrition has been linked to the development of adult cardiovascular and renal diseases, which are related to altered Na(+) balance. Here we investigated whether maternal malnutrition increases placental oxidative stress with subsequent impact on renal ATP-dependent Na(+) transporters in the offspring. 2. Maternal malnutrition was induced in rats during pregnancy by using a basic regional diet available in north-eastern Brazil. Placental oxidative stress was evaluated by measuring thiobarbituric acid-reactive substances, which were 35-40% higher in malnourished dams (MalN). Na(+) pumps were evaluated in control and prenatally malnourished rats (at 25 and 90 days of age). 3. Identical Na(+)/K(+)-ATPase activity was found in both groups at 25 days (approximately 150 nmol P(i)/mg per min). However, although Na(+)/K(+)-ATPase increased by 40% with growth in control rats, it remained constant in pups from MalN. 4. In juvenile rats, the activity of the ouabain-insensitive Na(+)-ATPase was higher in MalN than in controls (70 vs 25 nmol P(i)/mg per min). Nevertheless, activity did not increase with kidney and body growth: at 90 days, it was 50% lower in MalN than in controls. The maximal stimulation of the Na(+)-ATPase by angiotensin (Ang) II was 35% lower in MalN than in control rats and was attained only with a much higher concentration of the peptide (10(-10) mol/L) than in controls (10(-14) mol/L). 5. Protein kinase C activity, which mediates the effects of AngII on Na(+)-ATPase was only one-third of normal values in the MalN group. 6. These results indicate that placental oxidative stress may contribute to fetal undernutrition, which leads to later disturbances in Na(+) pumps from proximal tubule cells.

An update on the relationship between the kidney, salt and hypertension

The relationship between salt intake and blood pressure has been noteworthy for a long time. Nevertheless, the complex hemodynamic alterations associated with the salt-induced blood pressure changes have become clear only quite recently. Despite this knowledge, the concept of Guyton, which postulates that any increase in blood pressure should lead to a pressure natriuresis normalising blood pressure over the long term is still valid. As a conclusion, we have to remember that an elevation of arterial pressure can only be maintained if renal function as indicated by pressure natriuresis is impaired.

Acute angiotensin II infusions elicit pressure natriuresis in mice and reduce distal fractional sodium reabsorption

Acute angiotensin II (Ang II) infusions into mice increase arterial pressure (AP) and elicit pressure natriuresis. We used this model of pressure natriuresis to delineate the distal nephron responses to AP-mediated increases in distal sodium delivery. In the first group, we measured changes in urinary sodium excretion (U(Na)V) in male C57/BL6 anesthetized mice (n=9) before and during acute Ang II infusions (5 ng/g of body weight per minute). Acute Ang II infusions increased AP (98+/-3 to 126+/-5 mm Hg; P<0.001), urine flow (2.7+/-0.5 to 6.0+/-0.8 microL/min; P<0.01), and U(Na)V (0.6+/-0.2 to 1.3+/-0.2 microEq/min; P<0.05). There were significant relationships between U(Na)V and urine flow (y=0.207x+0.030; P<0.0001) and between U(Na)V and AP (y=0.027x-2.100). In a separate series, distal sodium delivery and fractional reabsorption of distal sodium delivery were determined in control (n=12) and Ang II-infused mice (n=8) by comparing U(Na)V before and after blockade of the 2 major distal nephron sodium transporters with amiloride (5 mg/kg of body weight) plus bendroflumethiazide (12 mg/kg of body weight). A positive relationship was found between U(Na)V (y=0.015x-1.100; P<0.0001) or distal sodium delivery (y=0.027x-0.900; P<0.0001) and AP. An inverse relationship was found between fractional reabsorption of distal sodium delivery and AP (y=-0.511x+128.300; P<0.01). These data indicate that Ang II-mediated pressure natriuresis involves an increase in distal sodium delivery combined with a reduced distal nephron fractional sodium reabsorption, suggesting that increased AP prevents the distal nephron transport mechanisms from accommodating the increased distal delivery.

Role of the T cell in the genesis of angiotensin II induced hypertension and vascular dysfunction

Hypertension promotes atherosclerosis and is a major source of morbidity and mortality. We show that mice lacking T and B cells (RAG-1^−/− mice) have blunted hypertension and do not develop abnormalities of vascular function during angiotensin II infusion or desoxycorticosterone acetate (DOCA)–salt. Adoptive transfer of T, but not B, cells restored these abnormalities. Angiotensin II is known to stimulate reactive oxygen species production via the nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase in several cells, including some immune cells. Accordingly, adoptive transfer of T cells lacking the angiotensin type I receptor or a functional NADPH oxidase resulted in blunted angiotensin II–dependent hypertension and decreased aortic superoxide production. Angiotensin II increased T cell markers of activation and tissue homing in wild-type, but not NADPH oxidase–deficient, mice. Angiotensin II markedly increased T cells in the perivascular adipose tissue (periadventitial fat) and, to a lesser extent the adventitia. These cells expressed high levels of CC chemokine receptor 5 and were commonly double negative (CD3⁺CD4⁻CD8⁻). This infiltration was associated with an increase in intercellular adhesion molecule-1 and RANTES in the aorta. Hypertension also increased T lymphocyte production of tumor necrosis factor (TNF) α, and treatment with the TNFα antagonist etanercept prevented the hypertension and increase in vascular superoxide caused by angiotensin II. These studies identify a previously undefined role for T cells in the genesis of hypertension and support a role of inflammation in the basis of this prevalent disease. T cells might represent a novel therapeutic target for the treatment of high blood pressure.

A novel mechanism of renal blood flow autoregulation and the autoregulatory role of A1 adenosine receptors in mice

Autoregulation of renal blood flow (RBF) is mediated by a fast myogenic response (MR; approximately 5 s), a slower tubuloglomerular feedback (TGF; approximately 25 s), and potentially additional mechanisms. A1 adenosine receptors (A1AR) mediate TGF in superficial nephrons and contribute to overall autoregulation, but the impact on the other autoregulatory mechanisms is unknown. We studied dynamic autoregulatory responses of RBF to rapid step increases of renal artery pressure in mice. MR was estimated from autoregulation within the first 5 s, TGF from that at 5-25 s, and a third mechanism from 25-100 s. Genetic deficiency of A1AR (A1AR-/-) reduced autoregulation at 5-25 s by 50%, indicating a residual fourth mechanism resembling TGF kinetics but independent of A1AR. MR and third mechanism were unaltered in A1AR-/-. Autoregulation in A1AR-/- was faster at 5-25 than at 25-100 s suggesting two separate mechanisms. Furosemide in wild-type mice (WT) eliminated the third mechanism and enhanced MR, indicating TGF-MR interaction. In A1AR-/-, furosemide did not further impair autoregulation at 5-25 s, but eliminated the third mechanism and enhanced MR. The resulting time course was the same as during furosemide in WT, indicating that A1AR do not affect autoregulation during furosemide inhibition of TGF. We conclude that at least one novel mechanism complements MR and TGF in RBF autoregulation, that is slower than MR and TGF and sensitive to furosemide, but not mediated by A1AR. A fourth mechanism with kinetics similar to TGF but independent of A1AR and furosemide might also contribute. A1AR mediate classical TGF but not TGF-MR interaction.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Mice lacking P2Y2 receptors have salt-resistant hypertension and facilitated renal Na+ and water reabsorption

Extracellular nucleotides (e.g., ATP) regulate many physiological and pathophysiological processes through activation of nucleotide (P2) receptors in the plasma membrane. Here we report that gene-targeted (knockout) mice that lack P2Y2 receptors have salt-resistant arterial hypertension in association with an inverse relationship between salt intake and heart rate, indicating intact baroreceptor function. Knockout mice have multiple alterations in their handling of salt and water: these include suppressed plasma renin and aldosterone concentrations, lower renal expression of the aldosterone-induced epithelial sodium channel alpha-ENaC, greater medullary expression of the Na-K-2Cl-cotransporter NKCC2, and greater furosemide-sensitive Na+ reabsorption in association with greater renal medullary expression of aquaporin-2 and vasopressin-dependent renal cAMP formation and water reabsorption despite similar vasopressin levels compared with wild type. Of note, smaller increases in plasma aldosterone were required to adapt renal Na+ excretion to restricted intake in knockout mice, suggesting a facilitation in renal Na+ retention. The results thus identify a previously unrecognized role for P2Y2 receptors in blood pressure regulation that is linked to an inhibitory influence on renal Na+ and water reabsorption. Based on these findings in knockout mice, we propose that a blunting in P2Y2 receptor expression or activity is a new mechanism for salt-resistant arterial hypertension.

Mechanisms of renal blood flow autoregulation: dynamics and contributions

Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, ANG II, or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of ANG II and nitric oxide (NO). MR requires < 10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes approximately 50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism < 15%. NO attenuates the strength, speed, and contribution of MR, whereas ANG II does not modify the balance of the autoregulatory mechanisms.

Na+, K+-ATPase: An Indispensable Ion Pumping-Signaling Mechanism Across Mammalian Cell Membranes

Na(+), K(+)-adenosine triphosphatase is a ubiquitous enzyme present in higher eukaryotes responsible for the maintenance of ionic gradients across the plasma membrane. It creates appropriate conditions for critical cellular processes such as secondary transport of solutes and water, for pH regulation, and also for creating an electrical potential that gives singular qualities to excitable cells. It also served as a platform for a higher level of cellular complexity because many important signaling networks appear to be downstream events of the pump's function. Renal physiology and pathology are affected significantly by its presence, and it seems that both molecular and pharmacologic manipulations of its action can create different venues to deal with diverse disease states.

Renal autoregulation: new perspectives regarding the protective and regulatory roles of the underlying mechanisms

When the kidney is subjected to acute increases in blood pressure (BP), renal blood flow (RBF) and glomerular filtration rate (GFR) are observed to remain relatively constant. Two mechanisms, tubuloglomerular feedback (TGF) and the myogenic response, are thought to act in concert to achieve a precise moment-by-moment regulation of GFR and distal salt delivery. The current view is that this mechanism insulates renal excretory function from fluctuations in BP. Indeed, the concept that renal autoregulation is necessary for normal renal function and volume homeostasis has long been a cornerstone of renal physiology. This article presents a very different view, at least regarding the myogenic component of this response. We suggest that its primary purpose is to protect the kidney against the damaging effects of hypertension. The arguments advanced take into consideration the unique properties of the afferent arteriolar myogenic response that allow it to protect against the oscillating systolic pressure and the accruing evidence that when this response is impaired, the primary consequence is not a disturbed volume homeostasis but rather an increased susceptibility to hypertensive injury. It is suggested that redundant and compensatory mechanisms achieve volume regulation, despite considerable fluctuations in distal delivery, and the assumed moment-by-moment regulation of renal hemodynamics is questioned. Evidence is presented suggesting that additional mechanisms exist to maintain ambient levels of RBF and GFR within normal range, despite chronic alterations in BP and severely impaired acute responses to pressure. Finally, the implications of this new perspective on the divergent roles of the myogenic response to pressure vs. the TGF response to changes in distal delivery are considered, and it is proposed that in addition to TGF-induced vasoconstriction, vasodepressor responses to reduced distal delivery may play a critical role in modulating afferent arteriolar reactivity to integrate the regulatory and protective functions of the renal microvasculature.

Effects of ACE inhibition on proximal tubule sodium transport

Angiotensin-converting enzyme (ACE) inhibitors such as captopril, which block ANG II formation, are commonly used for treatment of hypertension. There is substantial evidence that the proximal tubule (PT) is a primary target site for captopril but the molecular mechanisms for its action in PT are not well defined. The aim of this study was to determine the physiological and molecular changes in PT provoked by acute captopril treatment in the absence of changes in blood pressure or glomerular filtration rate (GFR). Captopril (infused at 12 μg/min for 20 min) did not change blood pressure or GFR but induced an immediate (<10 min) increase in PT flow measured with a nonobstructive optical method (to 117 ± 14% of baseline) along with a rapid diuresis from 2.1 ± 0.6 mg/min (baseline) to 3.7 ± 0.9 mg/min (captopril). Captopril also provoked a significant retraction of PT Na+/H+exchanger isoform 3 (NHE3), NHE regulatory factor (NHERF)-1, myosin-VI, and Na+-Picotransporter type 2 (NaPi2), but not ACE, out of apical microvillus-enriched membranes. Proteomic analysis with MALDI-TOF MS revealed an additional eight abundant membrane-associated proteins that redistributed out of the microvillus-enriched membrane during captopril treatment: megalin, myosin II-A, clathrin, aminopeptidase N, DPPIV, ezrin, moesin, and vacuolar H+-ATPase subunit β2. In summary, captopril can rapidly depress PT reabsorption in the absence of a change in GFR or BP and provokes the redistribution of a set of transporters and transporter-associated proteins that likely participate in the decrease in PT reabsorption and may also contribute to the blood pressure-lowering effect of ACE inhibitors.

Aldosterone stimulates activity and surface expression of NHE3 in human primary proximal tubule epithelial cells (RPTEC)

The steroid hormone aldosterone is a major regulator of extracellular volume and blood pressure. Aldosterone effectors are for example the epithelial Na(+) channel (ENaC), the Na(+)-K(+)-ATPase and the proximal tubule Na(+)/H(+) exchanger isoform 3 (NHE3). The aim of this study was to investigate whether aldosterone acts directly on proximal tubule cells to stimulate NHE3 and if so whether the EGF-receptor (EGFR) is involved. For this purpose, primary human renal proximal tubule cells were exposed to aldosterone. NHE3 activity was determined from Na(+)- dependent pH-recovery, NHE3 surface expression was determined by biotinylation and immunoblotting. EGFR-expression was assessed by ELISA. pH(i)- measurements revealed an aldosterone-induced increase in NHE3 activity, which was inhibited by the mineralocorticoid receptor blocker spironolactone and by the EGFR-kinase inhibitor AG1478. Immunoprecipitation and immunoblot analysis showed an aldosterone-induced increase in NHE3 surface expression, which was also inhibited by spironolactone and AG1478. Furthermore, aldosterone enhanced EGFR-expression. In conclusion, aldosterone stimulates NHE3 in human proximal tubule cells. The underlying mechanisms include AG1478 inhibitable kinase and are paralleled by enhanced EGFR expression, which could be compatible with EGF-receptor-pathway-dependent surface expression and activity of NHE3 in human primary renal proximal tubule epithelial cells.

Mechanisms mediating pressure natriuresis: what we know and what we need to find out

1. It is well established that pressure natriuresis plays a key role in long-term blood pressure regulation, but our understanding of the mechanisms underlying this process is incomplete. 2. Pressure natriuresis is chiefly mediated by inhibition of tubular sodium reabsorption, because both total renal blood flow and glomerular filtration rate are efficiently autoregulated. Inhibition of active sodium transport within both the proximal and distal tubules likely makes a contribution. Increased renal interstitial hydrostatic pressure (RIHP) likely inhibits sodium reabsorption by altering passive diffusion through paracellular pathways in 'leaky' tubular elements. 3. Nitric oxide and products of cytochrome P450-dependent arachidonic acid metabolism are key signalling mechanisms in pressure natriuresis, although their precise roles remain to be determined. 4. The key unresolved question is, how is increased renal artery pressure 'sensed' by the kidney? One proposal rests on the notion that blood flow in the renal medulla is poorly autoregulated, so that increased renal artery pressure leads to increased renal medullary blood flow (MBF), which, in turn, leads to increased RIHP. An alternative proposal is that the process of autoregulation of renal blood flow leads to increased shear stress in the preglomerular vasculature and, so, release of nitric oxide and perhaps products of cytochrome P450-dependent arachidonic acid metabolism, which, in turn, drive the cascade of events that inhibit sodium reabsorption. 5. Central to the arguments underlying these opposing hypotheses is the extent to which MBF is autoregulated. This remains highly controversial, largely because of the limitations of presently available methods for measurement of MBF.

Blood pressure maintenance in NHE3-deficient mice with transgenic expression of NHE3 in small intestine

NHE3 Na+/H+ exchanger knockout ( Nhe3−/−) mice have severe absorptive deficits in the kidney proximal tubule and intestinal tract. The resulting hypovolemia has confounded efforts to carefully evaluate the specific effects of NHE3 deficiency on kidney function. Development of mice with transgenic expression of NHE3 in the small intestine (tg Nhe3−/−) has allowed us to analyze the role of renal NHE3 in overall maintenance of blood pressure, pressure natriuresis, and autoregulation of both glomerular filtration rate (GFR) and renal blood flow (RBF). Ambulatory blood pressure, measured by telemetry, was lower in tg Nhe3−/− mice than in wild-type controls (tg Nhe3+/+) when the mice were maintained on a normal NaCl diet but was normalized when they were provided with a high NaCl intake. Furthermore, administration of the AT1-receptor blocker losartan showed that circulating ANG II plays a major role in maintaining blood pressure in tg Nhe3−/− mice fed normal NaCl but not in those receiving high NaCl. Clearance studies revealed a blunted pressure-natriuresis response in tg Nhe3−/− mice at lower blood pressures but a robust response at higher blood pressures. Autoregulation of GFR and RBF was normal in tg Nhe3−/− mice. These results show that dietary NaCl loading normalizes blood pressure in awake tg Nhe3−/− mice and that alterations in NHE3 activity are not essential for normal autoregulation of GFR and RBF. Furthermore, the data strongly support the hypothesis that NHE3 plays an important role in the diuretic and natriuretic responses to increases in blood pressure but also show that mechanisms not involving NHE3 mediate pressure natriuresis in the higher range of blood pressures studied.

Evolutionary origins of eukaryotic sodium/proton exchangers

More than 200 genes annotated as Na+/H+ hydrogen exchangers (NHEs) currently reside in bioinformation databases such as GenBank and Pfam. We performed detailed phylogenetic analyses of these NHEs in an effort to better understand their specific functions and physiological roles. This analysis initially required examining the entire monovalent cation proton antiporter (CPA) superfamily that includes the CPA1, CPA2, and NaT-DC families of transporters, each of which has a unique set of bacterial ancestors. We have concluded that there are nine human NHE (or SLC9A) paralogs as well as two previously unknown human CPA2 genes, which we have named HsNHA1 and HsNHA2. The eukaryotic NHE family is composed of five phylogenetically distinct clades that differ in subcellular location, drug sensitivity, cation selectivity, and sequence length. The major subgroups are plasma membrane (recycling and resident) and intracellular (endosomal/TGN, NHE8-like, and plant vacuolar). HsNHE1, the first cloned eukaryotic NHE gene, belongs to the resident plasma membrane clade. The latter is the most recent to emerge, being found exclusively in vertebrates. In contrast, the intracellular clades are ubiquitously distributed and are likely precursors to the plasma membrane NHE. Yeast endosomal ScNHX1 was the first intracellular NHE to be described and is closely related to HsNHE6, HsNHE7, and HsNHE9 in humans. Our results link the appearance of NHE on the plasma membrane of animal cells to the use of the Na+/K(+)-ATPase to generate the membrane potential. These novel observations have allowed us to use comparative biology to predict physiological roles for the nine human NHE paralogs and to propose appropriate model organisms in which to study the unique properties of each NHE subclass.

Gialpha3 protein-coupled dopamine D3 receptor-mediated inhibition of renal NHE3 activity in SHR proximal tubular cells is a PLC-PKC-mediated event

This study evaluated the transduction pathway associated with type 3 Na(+)/H(+) exchanger (NHE3) activity-induced inhibition during dopamine D(3) receptor activation in immortalized renal proximal tubular epithelial cells from the spontaneously hypertensive rat. The dopamine D(3) receptor agonist 7-OH-DPAT decreased NHE3 activity, which was prevented by the D(2)-like receptor antagonist S-sulpiride, pertussis toxin (PTX; overnight treatment), and the PKC inhibitor chelerythrine, but not by cholera toxin (overnight treatment), the MAPK inhibitor PD-098059, or the p38 inhibitor SB-203580. The PKA inhibitor H-89 abolished the inhibitory effects of forskolin on NHE3 activity, but not that of 7-OH-DPAT. The phospholipase C (PLC) inhibitor U-73122 prevented the inhibitory effects of 7-OH-DPAT, whereas PDBu and 7-OH-DPAT increased PLC activity and reduced NHE3 activity; downregulation of PKC abolished the inhibitory effects of both PDBu and 7-OH-DPAT on NHE activity. The inhibition of NHE3 activity by GTPgammaS and the prevention of the effect of 7-OH-DPAT by PTX suggest an involvement of a G(i/o) protein coupled to the dopamine D(3) receptor. Indeed, the 7-OH-DPAT-induced decrease in NHE3 activity was abolished in cells treated overnight with the anti-G(i)alpha3 antibody, but not in cells treated with antibodies against G(q/11), G(s)alpha, G(beta), and G(i)alpha1,2 proteins. The calcium ionophore A-23187 and the Ca(2+)-ATPase inhibitor thapsigargin increased intracellular Ca(2+) but did not affect NHE3 activity. However, the inhibitory effects of PDBu and 7-OH-DPAT on NHE3 activity were completely abolished by A-23287 and thapsigargin. It is concluded that inhibition of NHE3 activity by dopamine D(3) receptors coupled to G(i)alpha3 proteins is a PLC-PKC-mediated event, modulated by intracellular Ca(2+).

Renal interstitial guanosine cyclic 3', 5'-monophosphate mediates pressure-natriuresis via protein kinase G

Pressure-natriuresis is the physiological protective mechanism whereby elevation of blood pressure induces a rapid increase in renal sodium (Na+) excretion. Pressure-natriuresis abnormalities are common to all forms of hypertension. We tested the hypothesis that pressure-natriuresis is mediated by renal interstitial (RI) cGMP and protein kinase G (PKG). We used anesthetized, uninephrectomized Sprague-Dawley rats and a standard pressure-natriuresis model in which bilateral adrenalectomy and renal denervation was done on rats. Renal perfusion pressure (RPP) was adjusted by manipulating clamps above and below the renal artery, and RI cGMP was quantified by microdialysis. RI cGMP increased from 3.1+/-0.5 to 5.5+/-0.4 fmol/min (P<0.05) when RPP was raised from 100 to 140 mm Hg. This increase in RI cGMP was eliminated by RI infusion of soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,2-alpha]quinoxalin-1-one (ODQ). Raising RPP from 100 to 140 mm Hg increased urinary sodium excretion from 0.2+/-0.1 to 0.8+/-0.1 micromol/min, fractional sodium excretion from 0.2+/-0.1% to 0.8+/-0.1%, and fractional lithium excretion from 20.1+/-3.0% to 62.7+/-3.7% (all P<0.05). These responses were eliminated by RI infusion of nitric oxide synthase inhibitor N-nitro-l-arginine methyl ester, ODQ, and PKG inhibitors Rp-8-pCPT-cGMP and Rp-8-Br-cGMP. Increasing RPP from 100 to 140 mm Hg decreased fractional proximal sodium reabsorption without influencing fractional distal Na+ reabsorption or glomerular filtration rate. In conclusion, pressure-natriuresis is mediated by RI cGMP and a PKG signaling pathway in target renal proximal tubule cells.

Neural control of renal function

The renal nerves are the communication link between the central nervous system and the kidney. In response to multiple peripheral and central inputs, efferent renal sympathetic nerve activity is altered so as to convey information to the major structural and functional components of the kidney, the vessels, glomeruli, and tubules, each of which is innervated. At the level of each of these individual components, information transfer occurs via interaction of the neurotransmitter released at the sympathetic nerve terminal-neuroeffector junction with specific postjunctional receptors coupled to defined intracellular signaling and effector systems. In response to normal physiological stimuli, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of renal blood flow, glomerular filtration rate, renal tubular epithelial cell solute and water transport, and hormonal release. Afferent input from sensory receptors located in the kidney participates in this reflex control system via renorenal reflexes that enable total renal function to be self-regulated and balanced between the two kidneys. In pathophysiological conditions, abnormal regulation of efferent renal sympathetic nerve activity contributes significantly to the associated abnormalities of renal function which, in turn, are of importance in the pathogenesis of the disease.