Increased expression of ENaC subunits and increased apical targeting of AQP2 in the kidneys of spontaneously hypertensive rats

Hypertensive rats show increased renal excretion and decreased tissue concentrations of glycine betaine, a protective osmolyte with diuretic properties

Hypertension leads to water-electrolyte disturbances and end-organ damage. Betaine is an osmolyte protecting cells against electrolyte imbalance and osmotic stress, particularly in the kidneys. This study aimed to evaluate tissue levels and hemodynamic and renal effects of betaine in normotensive and hypertensive rats. Betaine levels were assessed using high-performance liquid chromatography-mass spectrometry (HPLC-MS) in normotensive rats (Wistar-Kyoto, WKYs) and Spontaneously Hypertensive rats (SHRs), a model of genetic hypertension. Acute effects of IV betaine on blood pressure, heart rate, and minute diuresis were evaluated. Gene and protein expression of chosen kidney betaine transporters (SLC6a12 and SLC6a20) were assessed using real-time PCR and Western blot. Compared to normotensive rats, SHRs showed significantly lower concentration of betaine in blood serum, the lungs, liver, and renal medulla. These changes were associated with higher urinary excretion of betaine in SHRs (0.20 ± 0.04 vs. 0.09 ± 0.02 mg/ 24h/ 100g b.w., p = 0.036). In acute experiments, betaine increased diuresis without significantly affecting arterial blood pressure. The diuretic response was greater in SHRs than in WKYs. There were no significant differences in renal expression of betaine transporters between WKYs and SHRs. Increased renal excretion of betaine contributes to decreased concentration of the protective osmolyte in tissues of hypertensive rats. These findings pave the way for studies evaluating a causal relation between depleted betaine and hypertensive organ damage, including kidney injury.

Effects of a high-salt diet on MAP and expression levels of renal ENaCs and aquaporins in SHR

An increase in blood pressure by a high-salt (HS) diet may change the expression levels of renal epithelial sodium channels (ENaCs) and aquaporins (AQPs). Spontaneously hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats were exposed to HS and regular-salt (RS) diets for 6 weeks. Mean arterial pressure (MAP) and plasma atrial natriuretic peptide (ANP), angiotensin II (Ang II), aldosterone, and arginine vasopressin (AVP) levels were determined. Expression of mRNA levels of ENaCs and AQPs were quantified by real-time PCR. The MAP was higher in SHRs on the HS diet. Plasma Ang II and aldosterone levels were low while plasma ANP level was high in both strains of rats. Renal expression of mRNA levels of α-, β-, and γ-ENaCs was lowered in SHRs on the HS diet. Meanwhile, renal AQP1, AQP2, and AQP7 mRNA expression levels were lowered in both strains of rats on the HS diet. Suppression of mRNA expression levels of ENaC and AQP subunits suggests that the high-salt-induced increase in the MAP of SHR may not be solely due to renal sodium and water retention.

Lipid Profiles of Urinary Extracellular Vesicles Released during the Inactive and Active Phases of Aged Male Mice with Spontaneous Hypertension

Hypertension remains a major problem, especially in the elderly, as it increases the risk for cardiovascular, coronary artery, cerebrovascular, and kidney diseases. Extracellular vesicles (EVs) play a role in the aging process and contribute to pathophysiology. Our goal was to examine differences in lipid profiles of urinary EVs (uEVs) collected during the inactive and active phases of aged mice and investigate whether these EVs regulate the density of lipid rafts in mouse cortical collecting duct (mpkCCD) principal cells. Here, we demonstrate the epithelial sodium channel (ENaC) inhibitor benzyl amiloride reduced systolic blood pressure in aged male mice during the inactive and active phases. Lipidomics data demonstrate differential enrichment of lipids between the two groups. For example, there are more phosphatidylethanolamine plasmalogens, particularly in the form of alkyl phosphatidylethanolamines, that are enriched in active phase uEVs compared to inactive phase uEVs from the same mice. Amiloride-sensitive transepithelial current increased more in mpkCCD cells challenged with uEVs from the active phase group. Moreover, more ENaC alpha protein was distributed to lipid raft fractions of mpkCCD cells challenged with active phase uEVs. Taken together, the identification of bioactive lipids associated with lipid rafts that are enriched in EVs released during the active phase of aged mice may offer clues to help understand lipid raft organization in recipient principal cells after EV uptake and increased renal ENaC activity, leading to a time-of-day dependent regulation of blood pressure in an aging model.

Apocynin and catalase prevent hypertension and kidney injury in Cyclosporine A-induced nephrotoxicity in rats

Oxidative stress is involved in the pathogenesis of a number of diseases including hypertension and renal failure. There is enhanced expression of nicotinamide adenine dinucleotide (NADPH oxidase) and therefore production of hydrogen peroxide (H2O2) during renal disease progression. This study investigated the effect of apocynin, an NADPH oxidase inhibitor and catalase, an H2O2 scavenger on Cyclosporine A (CsA) nephrotoxicity in Wistar-Kyoto rats. Rats received CsA (25mg/kg/day via gavage) and were assigned to vehicle, apocynin (2.5mmol/L p.o.), catalase (10,000U/kg/day i.p.) or apocynin plus catalase for 14 days. Renal functional and hemodynamic parameters were measured every week, and kidneys were harvested at the end of the study for histological and NADPH oxidase 4 (NOX4) assessment. Oxidative stress markers and blood urea nitrogen (BUN) were measured. CsA rats had higher plasma malondialdehyde (by 340%) and BUN (by 125%), but lower superoxide dismutase and total antioxidant capacity (by 40%, all P<0.05) compared to control. CsA increased blood pressure (by 46mmHg) and decreased creatinine clearance (by 49%, all P<0.05). Treatment of CsA rats with apocynin, catalase, and their combination decreased blood pressure to near control values (all P<0.05). NOX4 mRNA activity was higher in the renal tissue of CsA rats by approximately 63% (P<0.05) compared to controls but was reduced in apocynin (by 64%), catalase (by 33%) and combined treatment with apocynin and catalase (by 84%) compared to untreated CsA rats. Treatment of CsA rats with apocynin, catalase, and their combination prevented hypertension and restored renal functional parameters and tissue Nox4 expression in this model. NADPH inhibition and H2O2 scavenging is an important therapeutic strategy during CsA nephrotoxicity and hypertension.

MST3 is involved in ENaC-mediated hypertension

Liddle syndrome is an inherited form of human hypertension caused by increasing epithelial Na⁺ channel (ENaC) expression. Increased Na⁺ retention through ENaC with subsequent volume expansion causes hypertension. In addition to ENaC, the Na⁺-K⁺-Cl^- cotransporter (NKCC) and Na⁺-Cl^- symporter (NCC) are responsible for Na⁺ reabsorption in the kidneys. Several Na⁺ transporters are evolutionarily regulated by the Ste20 kinase family. Ste20-related proline/alanine-rich kinase and oxidative stress-responsive kinase-1 phosphorylate downstream NKCC2 and NCC to maintain Na⁺ and blood pressure (BP) homeostasis. Mammalian Ste20 kinase 3 (MST3) is another member of the Ste20 family. We previously reported that reduced MST3 levels were found in the kidneys in spontaneously hypertensive rats and that MST3 was involved in Na⁺ regulation. To determine whether MST3 is involved in BP stability through Na⁺ regulation, we generated a MST3 hypomorphic mutation and designated MST3^+/- and MST3^-/- mice to examine BP and serum Na⁺ and K⁺ concentrations. MST3^-/- mice exhibited hypernatremia, hypokalemia, and hypertension. The increased ENaC in the kidney played roles in hypernatremia. The reabsorption of more Na⁺ promoted more K⁺ secretion in the kidney and caused hypokalemia. The hypernatremia and hypokalemia in MST3^-/- mice were significantly reversed by the ENaC inhibitor amiloride, indicating that MST3^-/- mice reabsorbed more Na⁺ through ENaC. Furthermore, Madin-Darby canine kidney cells stably expressing kinase-dead MST3 displayed elevated ENaC currents. Both the in vivo and in vitro results indicated that MST3 maintained Na⁺ homeostasis through ENaC regulation. We are the first to report that MST3 maintains BP stability through ENaC regulation.

MST3 (mammalian Ste20-like protein kinase 3), a novel gene involved in ion homeostasis and renal regulation of blood pressure in spontaneous hypertensive rats

Defective renal salt and water excretion, together with increased salt intake, frequently contributes to hypertension. Recent studies indicate that Ste20 family kinases, such as proline-alanine-rich Ste20-related kinase (SPAK) and oxidative stress-response protein 1 (OSR1), are regulators of cell volume, ion transport, and hypertension. The aim of this study was to investigate whether mammalian sterile 20-like protein kinase 3 (MST3), which is also a stress-regulated kinase, is involved in the development of hypertension. MST3 expression was compared in Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) kidneys. MST3 expression was markedly reduced in principal cells of the collecting ducts from the renal inner medulla of SHR. The downregulation of MST3 expression was observed before and after the onset of hypertension in SHR. Mice fed high-salt diets (HS) exhibited a significant increase in MST3 protein level. This is the first study reporting that MST3, a Ste20-like kinase, exerts a conserved regulatory role in sodium homeostasis after high-salt diet and in the development of hypertension.

Altered expression of Aquaporin-2 in one-kidney, one-clip hypertension

AIMS

The aim of the present study was to evaluate the regulation of Aquaporin-2 (AQP2) water channel in the kidney of one-kidney, one-clip rats (Goldblatt-1 model). In addition, some mechanisms that underlie the role of AQP2 in the Goldblatt-1 model were evaluated.

MAIN METHODS

Sprague-Dawley rats were divided in three groups: control two-kidney, no clip (C, 2 K-NC); nephrectomized one-kidney, no clip (N, 1 K-NC) and Goldblatt one-kidney, one-clip (G, 1 K-1C). AQP2 expression (by westernblot, real time PCR, immunohistochemistry and immunofluorescence), vasopressin V2 receptor expression (by real time PCR), cAMP concentration, NFkB and TonEBP (cytosol to nucleus ratio) were evaluated in the renal medulla.

KEY FINDINGS

AQP2 expression, V2 receptor expression and cAMP concentration were decreased in the renal medulla of 1 K-1C rats, NFkB translocation was favoured towards the nucleus suggesting its activation while TonEBP translocation was not altered in this model of hypertension.

SIGNIFICANCE

In this model of hypertension the decrease of AQP2 expression could be a mechanism that counteracts the high blood pressure promoting water excretion and this may be consequence of decreased vasopressin sensitivity and/or the increased activity of NFkB at renomedullary collecting duct level. Given that renovascular hypertension is among the most common causes of secondary hypertension, it is important to elucidate all the relevant mechanisms involved in the generation or in the compensation of the hypertensive state in order to improve the diagnoses and treatment of the patients.

Aquaporins in Cardiovascular System

Recent studies have shown that some aquaporins (AQPs ), including AQP1, AQP4, AQP7 and AQP9, are expressed in endothelial cells, vascular smooth muscle cells and heart of cardiovascular system. These AQPs are involved in the cardiovascular function and in pathological process of related diseases, such as cerebral ischemia , congestion heart failure , hypertension and angiogenesis. Therefore, it is important to understand the accurate association between AQPs and cardiovascular system, which may provide novel approaches to prevent and treat related diseases. Here we will discuss the expression and physiological function of AQPs in cardiovascular system and summarize recent researches on AQPs related cardiovascular diseases.

Fragment Screening of Human Aquaporin 1

Aquaporins (AQPs) are membrane proteins that enable water transport across cellular plasma membranes in response to osmotic gradients. Phenotypic analyses have revealed important physiological roles for AQPs, and the potential for AQP water channel modulators in various disease states has been proposed. For example, AQP1 is overexpressed in tumor microvessels, and this correlates with higher metastatic potential and aggressiveness of the malignancy. Chemical modulators would help in identifying the precise contribution of water channel activity in these disease states. These inhibitors would also be important therapeutically, e.g., in anti-cancer treatment. This perceived importance contrasts with the lack of success of high-throughput screens (HTS) to identify effective and specific inhibitors of aquaporins. In this paper, we have screened a library of 1500 "fragments", i.e., smaller than molecules used in HTS, against human aquaporin (hAQP1) using a thermal shift assay and surface plasmon resonance. Although these fragments may not inhibit their protein target, they bound to and stabilized hAQP1 (sub mM binding affinities (KD), with an temperature of aggregation shift ΔTagg of +4 to +50 °C) in a concentration-dependent fashion. Chemically expanded versions of these fragments should follow the determination of their binding site on the aquaporin surface.

Angiotensin II and Anti Diuretic Hormone Exert Synergistic Effects on Thick Ascending Limb Transport in Spontaneously Hypertensive Rats

BACKGROUND

Sodium reabsorption is increased in the thick ascending limb (TAL) of Henle in several hypertensive models. In this segment, while transport is increased by ADH via cAMP, sodium reabsorption results from Ang II-induced superoxide (O2(-)) production. Surprisingly, it is unknown whether these mechanisms overlap in hypertension. We hypothesized that Ang II and ADH have accumulative effects on TAL's transport during hypertension.

METHODS

The effect of ADH/Ang II in TALs from spontaneously hypertensive rats (SHR) on oxygen consumption (QO2), cAMP and O2(-) was measured.

RESULTS

Basal QO2 was 113.3 ± 14.2 nmol O2/min/mg protein. Addition of ADH (1 nM) increased QO2 by 198%. In the presence of ADH, Ang II (1 nM) elicited a QO2 transient response and then rose to 321.5 ± 28.3 (p = 0.003 vs. ADH). These accumulative effects could be due to nitric oxide synthase (NOS) uncoupling, lower Ang II ability to decrease cAMP or increased O2(-). We first measured QO2 using a NOS inhibitor. Pretreatment with L-NAME did not block the observed interaction (p = 0.001 Ang II vs. ADH). Also, Ang II blocked the ADH-stimulated cAMP accumulation in TAL of SHRs. In the presence of ADH, Ang II increased O2(-) production in TALs from SHR by 309% (p = 0.015 vs. basal). The O2(-) scavenger tempol blocked the Ang II effects on QO2. In the presence of the NADPH oxidase inhibitor apocynin, the accumulative effects of ADH and Ang II were abolished. We conclude that (1) in SHR, Ang II has accumulative effects on ADH-stimulated transport; (2) this effect is mediated by AT1 receptors, and increased O2(-) production.

Dysregulation of ENaC in Animal Models of Nephrotic Syndrome and Liver Cirrhosis

Nephrotic syndrome and liver cirrhosis are common clinical manifestations, and are associated with avid sodium retention leading to the development of edema and ascites. However, the mechanism for the sodium retention is still incompletely understood and the molecular basis remains undefined. We examined the changes of sodium (co)transporters and epithelial sodium channels (ENaCs) in the kidneys of experimental nephrotic syndrome and liver cirrhosis. The results demonstrated that puromycin- or HgCl2-induced nephrotic syndrome was associated with 1) sodium retention, decreased urinary sodium excretion, development of ascites, and increased plasma aldosterone level; 2) increased apical targeting of ENaC subunits in connecting tubule and collecting duct segments; and 3) decreased protein abundance of type 2 11β-hydroxysteroid dehydrogenase (11βHSD2). Experimental liver cirrhosis was induced in rats by CCl₄ treatment or common bile duct ligation. An increased apical targeting of alpha-, beta-, and gamma-ENaC subunits in connecting tubule, and cortical and medullary collecting duct segments in sodium retaining phase of liver cirhosis but not in escape phase of sodium retention. Immunolabeling intensity of 11βHSD2 in the connecting tubule and cortical collecting duct was significantly reduced in sodium retaining phase of liver cirrhosis, and this was confirmed by immunoblotting. These observations therefore strongly support the view that the renal sodium retention associated with nephrotic syndrome and liver cirrhosis is caused by increased sodium reabsorption in the aldosterone sensitive distal nephron including the connecting tubule and collecting duct, and increased apical targeting of ENaC subunits plays a role in the development of sodium retention in nephrotic syndrome and liver cirrhosis.

Hydrogen sulphide and tempol treatments improve the blood pressure and renal excretory responses in spontaneously hypertensive rats

Oxidative stress and suppressed H2S production lead to increased renal vascular resistance, disturbed glomerular hemodynamics, and abnormal renal sodium and water handling, contribute to the pathogenesis and maintenance of essential hypertension in man and the spontaneously hypertensive rat. This study investigated the impact of H2S and tempol alone and in combination on blood pressure and renal hemodynamics and excretory functions in the SHR. Groups of WKY rats or SHR (n=6) were treated for 4 weeks either as controls or received NaHS (SHR+NaHS), tempol (SHR+Tempol), or NaHS plus tempol (SHR+NaHS +Tempol). Metabolic studies were performed on days 0, 14, and 28, thereafter animals were anaesthetized to measure renal hemodynamics and plasma oxidative and antioxidant markers. SHR control rats had higher mean arterial blood pressure (140.0 ± 2 vs. 100.0 ± 3 mmHg), lower plasma and urinary H2S, creatinine clearance, urine flow rate and urinary sodium excretion, and oxidative stress compared to WKY (all p<0.05). Treatment either with NaHS or with tempol alone decreased blood pressure and oxidative stress and improved renal hemodynamic and excretory function compared to untreated SHR. Combined NaHS and tempol therapy in SHRs caused larger decreases in blood pressure (∼20-22% vs. ∼11-15% and ∼10-14%), increases in creatinine clearance, urinary sodium excretion and fractional sodium excretion and up-regulated the antioxidant status compared to each agent alone (all p<0.05). These findings demonstrated that H2S and tempol together resulted in greater reductions in blood pressure and normalization of kidney function compared with either compound alone.

Deciphering the mechanisms of the Na+/H+ exchanger-3 regulation in organ dysfunction

The Na+/H+ exchanger-3 (NHE3) belongs to the mammalian NHE protein family and catalyzes the electro-neutral exchange of extracellular sodium for intracellular proton across cellular membranes. Its transport function is of essential importance for the maintenance of the body's salt and water homeostasis as well as acid-base balance. Indeed, NHE3 activity is finely regulated by a variety of stimuli, both acutely and chronically, and its transport function is fundamental for a multiplicity of severe and world-wide infection-pathological conditions. This review aims to provide a concise overview of NHE3 physiology and discusses the role of NHE3 in clinical conditions of prominent importance, specifically in hypertension, diabetic nephropathy, heart failure, acute kidney injury, and diarrhea. Study of NHE3 function in models of these diseases has contributed to the deciphering of mechanisms that control the delicate ion balance disrupted in these disorders. The majority of the findings indicate that NHE3 transport function is activated before the onset of hypertension and inhibited thereafter; NHE3 transport function is also upregulated in diabetic nephropathy and heart failure, while it is reported to be downregulated in acute kidney injury and in diarrhea. The molecular mechanisms activated during these pathological conditions to regulate NHE3 transport function are examined with the aim of linking NHE3 dysfunction to the analyzed clinical disorders.

Abnormal increase in urinary aquaporin-2 excretion in response to hypertonic saline in essential hypertension

Background

Dysregulation of the expression/shuttling of the aquaporin-2 water channel (AQP2) and the epithelial sodium channel (ENaC) in renal collecting duct principal cells has been found in animal models of hypertension. We tested whether a similar dysregulation exists in essential hypertension.

Methods

We measured urinary excretion of AQP2 and ENaC β-subunit corrected for creatinine (u-AQP2_CR, u-ENaC_β-CR), prostaglandin E2 (u-PGE₂) and cyclic AMP (u-cAMP), fractional sodium excretion (FE_Na), free water clearance (C_H2O), as well as plasma concentrations of vasopressin (AVP), renin (PRC), angiotensin II (Ang II), aldosterone (Aldo), and atrial and brain natriuretic peptide (ANP, BNP) in 21 patients with essential hypertension and 20 normotensive controls during 24-h urine collection (baseline), and after hypertonic saline infusion on a 4-day high sodium (HS) diet (300 mmol sodium/day) and a 4-day low sodium (LS) diet (30 mmol sodium/day).

Results

At baseline, no differences in u-AQP2_CR or u-ENaC_β-CR were measured between patients and controls. U-AQP2_CR increased significantly more after saline in patients than controls, whereas u-ENaC_β-CR increased similarly. The saline caused exaggerated natriuretic increases in patients during HS intake. Neither baseline levels of u-PGE₂, u-cAMP, AVP, PRC, Ang II, Aldo, ANP, and BNP nor changes after saline could explain the abnormal u-AQP2_CR response.

Conclusions

No differences were found in u-AQP2_CR and u-ENaC_β-CR between patients and controls at baseline. However, in response to saline, u-AQP2_CR was abnormally increased in patients, whereas the u-ENaC_β-CR response was normal. The mechanism behind the abnormal AQP2 regulation is not clarified, but it does not seem to be AVP-dependent.

Clinicaltrial.gov identifier

NCT00345124.

Altered expression of renal aquaporins and α-adducin polymorphisms may contribute to the establishment of salt-sensitive hypertension

BACKGROUND

Sodium-sensitive hypertension is caused by renal tubular dysfunction, leading to increased retention of sodium and water. Previous findings have suggested that single-nucleotide polymorphisms of the cytoskeletal protein, α-adducin, are associated with increased membrane expression of the Na/K pump and abnormal renal sodium transport in Milan hypertensive strain (MHS) rats and in humans. However, the possible contribution of renal aquaporins (AQPs) to water retention remains undefined in MHS rats.

METHODS

Kidneys from MHS rats were analyzed and compared with those from age-matched Milan normotensive strain (MNS) animals by quantitative-PCR, immunoblotting, and immunoperoxidase. Endocytosis assay was performed on renal cells stably expressing AQP4 and co-transfected either with wild-type normotensive (NT) or with mutated hypertensive (HT) α-adducin.

RESULTS

Semiquantitative immunoblotting revealed that AQP1 abundance was significantly decreased only in HT MHS whereas AQP2 was reduced in both young pre-HT and adult-HT animals. On the other hand, AQP4 was dramatically upregulated in MHS regardless of the age. These results were confirmed by immunoperoxidase microscopy. Endocytosis assays clearly showed that the expression of mutated adducin strongly reduced the rate of constitutive AQP4 endocytosis, thereby increasing its abundance at the plasma membrane.

CONCLUSIONS

We provide here the first evidence that AQP1, AQP2, and AQP4 are dysregulated in the kidneys of MHS animals. In particular, we provide evidence that α-adducin mutations may be responsible for AQP4 upregulation. The downregulation of AQP1 and AQP2 and the upregulation of AQP4 may be relevant for the onset and maintenance of salt-sensitive hypertension.

Dipeptidyl peptidase IV inhibition attenuates blood pressure rising in young spontaneously hypertensive rats

OBJECTIVES

The present study aimed to assess the effect of the specific dipeptidyl peptidase IV (DPPIV) inhibitor sitagliptin on blood pressure and renal function in young prehypertensive (5-week-old) and adult spontaneously hypertensive rats (SHRs; 14-week-old).

METHODS

Sitagliptin (40 mg/kg twice daily) was given by oral gavage to young (Y-SHR + IDPPIV) and adult (A-SHR + IDPPIV) SHRs for 8 days. Kidney function was assessed daily and compared with age-matched vehicle-treated SHR (Y-SHR and A-SHR) and with normotensive Wistar-Kyoto rats (Y-WKY and A-WKY). Arterial blood pressure was measured in these animals at the end of the experimental protocol. Additionally, Na/H exchanger isoform 3 (NHE3) function and expression in microvilli membrane vesicles were assessed in young animals.

RESULTS

Mean arterial blood pressure of Y-SHR + IDPPIV was significantly lower than that of Y-SHR (104 ± 3 vs. 123 ± 5 mmHg, P < 0.01) and was similar to Y-WKY (94 ± 4 mmHg, P > 0.05). Compared to Y-SHR, Y-SHR + IDPPIV exhibited enhanced cumulative urinary flow and sodium excretion and decreased NHE3 activity and expression in proximal tubule microvilli. In the A-SHR, sitagliptin treatment had no significant effect on either renal function or arterial blood pressure.

CONCLUSION

Our data suggest that DPPIV inhibition attenuates blood pressure rising in young prehypertensive SHRs, partially by inhibiting NHE3 activity in renal proximal tubule.

Programmed hypertension in rats treated with a NF-κB inhibitor during nephrogenesis: renal mechanisms

Suppression of the renin-angiotensin system (RAS) during murine lactation causes progressive renal injury, indicating a physiological action of angiotensin II on nephrogenesis. The nuclear factor NF-κB system is one of the main intracellular mediators of angiotensin II. We investigated whether inhibition of this system with pyrrolidine dithiocarbamate (PDTC) during rat nephrogenesis would lead to similar hypertension and renal injury as observed with RAS suppressors. Immediately after delivery, 32 Munich-Wistar dams, each nursing 6 male pups, were divided into 2 groups: C, untreated, and PDTC, receiving PDTC, 280 mg kg(-1) day(-1) orally, during 21 days. After weaning, the offspring were followed until 10 months of age without treatment. Adult rats that received neonatal PDTC exhibited stable hypertension and myocardial injury, without albuminuria. To gain additional insight into this process, the renal expression of RAS components and sodium transporters were determined by quantitative real-time PCR (qRT-PCR) at 3 and 10 months of life. Renal renin and angiotensinogen were upregulated at 3 and downregulated at 10 months of age, suggesting a role for early local RAS activation. Likewise, there was early upregulation of the proximal sodium/glucose and sodium/bicarbonate transporters, which abated later in life, suggesting that additional factors sustained hypertension in the long run. The conclusions drawn from the findings were as follows: (1) an intact NF-κB system during nephrogenesis may be essential to normal renal and cardiovascular function in adult life; (2) neonatal PDTC represents a new model of hypertension, lacking overt structural injury or functional impairment of the kidneys; and (3) hypertension in this model seems associated with early temporary activation of renal RAS and sodium transporters.

Posttranslational mechanisms associated with reduced NHE3 activity in adult vs. young prehypertensive SHR

Abnormalities in renal proximal tubular (PT) sodium transport play an important role in the pathophysiology of essential hypertension. The Na+/H+ exchanger isoform 3 (NHE3) represents the major route for sodium entry across the apical membrane of renal PT cells. We therefore aimed to assess in vivo NHE3 transport activity and to define the molecular mechanisms underlying NHE3 regulation before and after development of hypertension in the spontaneously hypertensive rat (SHR). NHE3 function was measured as the rate of bicarbonate reabsorption by means of in vivo stationary microperfusion in PT from young prehypertensive SHR (Y-SHR; 5-wk-old), adult SHR (A-SHR; 14-wk-old), and age-matched Wistar Kyoto (WKY) rats. We found that NHE3-mediated PT bicarbonate reabsorption was reduced with age in the SHR (1.08 ± 0.10 vs. 0.41 ± 0.04 nmol/cm2×s), while it was increased in the transition from youth to adulthood in the WKY rat (0.59 ± 0.05 vs. 1.26 ± 0.11 nmol/cm2×s). Higher NHE3 activity in the Y-SHR compared with A-SHR was associated with a predominant microvilli confinement and a lower ratio of phosphorylated NHE3 at serine-552 to total NHE3 (P-NHE3/total). After development of hypertension, P-NHE3/total increased and NHE3 was retracted out of the microvillar microdomain along with the regulator dipeptidyl peptidase IV (DPPIV). Collectively, our data suggest that the PT is playing a role in adapting to the hypertension in the SHR. The molecular mechanisms of this adaptation possibly include an increase of P-NHE3/total and a redistribution of the NHE3-DPPIV complex from the body to the base of the PT microvilli, both predicted to decrease sodium reabsorption.

Exposure to maternal diabetes induces salt-sensitive hypertension and impairs renal function in adult rat offspring

OBJECTIVE

Epidemiological and experimental studies have led to the hypothesis of fetal origin of adult diseases, suggesting that some adult diseases might be determined before birth by altered fetal development. We have previously demonstrated in the rat that in utero exposure to maternal diabetes impairs renal development leading to a reduction in nephron number. Little is known on the long-term consequences of in utero exposure to maternal diabetes. The aim of the study was to assess, in the rat, long-term effects of in utero exposure to maternal diabetes on blood pressure and renal function in adulthood.

RESEARCH DESIGN AND METHODS

Diabetes was induced in Sprague-Dawley pregnant rats by streptozotocin on day 0 of gestation. Systolic blood pressure, plasma renin activity, and renal function were measured in the offspring from 1 to 18 months of age. High-salt diet experiments were performed at the prehypertensive stage, and the abundance of tubular sodium transporters was evaluated by Western blot analysis. Kidney tissues were processed for histopathology and glomerular computer-assisted histomorphometry.

RESULTS AND CONCLUSIONS

We demonstrated that in utero exposure to maternal diabetes induces a salt-sensitive hypertension in the offspring associated with a decrease in renal function in adulthood. High-salt diet experiments show an alteration of renal sodium handling that may be explained by a fetal reprogramming of tubular functions in association or as a result of the inborn nephron deficit induced by in utero exposure to maternal diabetes.

Developmental programming of a reduced nephron endowment: more than just a baby's birth weight

The risk of developing many adult-onset diseases, including hypertension, type 2 diabetes, and renal disease, is increased in low-birth-weight individuals. A potential underlying mechanism contributing to the onset of these diseases is the formation of a low nephron endowment during development. Evidence from the human, as well as many experimental animal models, has shown a strong association between low birth weight and a reduced nephron endowment. However, other animal models, particularly those in which the mother is exposed to elevated glucocorticoids for a short period, have shown a 20-40% reduction in nephron endowment without discernible changes in the birth weight of offspring. Such findings emphasize that a low birth weight is one, but certainly not the only, predictor of nephron endowment and suggests reduced nephron endowment and risk of developing adult-onset disease, even among normal-birth-weight individuals. Recognition of the dissociation between birth weight and nephron endowment is important for future studies aimed at elucidating the role of a reduced nephron endowment in the developmental programming of adult disease.

A new trick for an old dogma: ENaC proteins as mechanotransducers in vascular smooth muscle

Myogenic constriction is a vasoconstriction of blood vessels to increases in perfusion pressure. In renal preglomerular vasculature, it is an established mechanism of renal blood flow autoregulation. Recently, myogenic constriction has been identified as an important protective mechanism, preventing the transmission of systemic pressure to the fragile glomerular vasculature. Although the signal transduction pathways mediating vasoconstriction are well known, how the increases in pressure trigger vasoconstriction is unclear. The response is initiated by pressure-induced stretch of the vessel wall and thus is dependent on mechanical signaling. The identity of the sensor detecting VSMC stretch is unknown. Previous studies have considered the role of extracellular matrix-integrin interactions, ion conduction units (channels and/or transporters), and the cytoskeleton as pressure detectors. Whether, and how, these structures fit together in VSMCs is poorly understood. However, a model of mechanotransduction in the nematode Caenorhadbditis elegans (C. elegans) has been established that ties together extracellular matrix, ion channels, and cytoskeletal proteins into a large mechanosensing complex. In the C. elegans mechanotransducer model, a family of evolutionarily conserved proteins, referred to as the DEG/ENaC/ASIC family, form the ion-conducting pore of the mechanotransducer. Members of this protein family are expressed in VSMC where they may participate in pressure detection. This review will address how the C. elegans mechanotransducer model can be used to model pressure detection in mammalian VSMCs and provide a new perspective to pressure detection in VSMCs.

Pathogenesis of essential hypertension: historical paradigms and modern insights

Since its first identification in the late 1800s, a variety of etiologies for essential hypertension have been proposed. In this paper we review the primary proposed hypotheses in the context of both the time in which they were proposed as well as the subsequent studies performed over the years. From these various insights, we propose a current paradigm to explain the renal mechanisms underlying the hypertension epidemic today. Specifically, we propose that hypertension is initiated by agents that cause systemic and intrarenal vasoconstriction. Over time intrarenal injury develops with microvascular disease, interstitial T cell and macrophage recruitment with the induction of an autoimmune response, with local angiotensin II formation and oxidant generation. These changes maintain intrarenal vasoconstriction and hypoxia with a change in local vasoconstrictor-vasodilator balance favoring sodium retention. Both genetic and congenital (nephron number) mechanisms have profound influence on this pathway. As blood pressure rises, renal ischemia is ameliorated and sodium balance restored completely (in salt-resistant) or partially (in salt-sensitive) hypertension, but at the expense of a rightward shift in the pressure natriuresis curve and persistent hypertension.

Acute rejection modulates gene expression in the collecting duct

Kidney transplantation, especially when associated with acute rejection, leads to changes in the expression of many genes, including those encoding solute transporters and water channels. In a rat model of acute rejection after allogeneic renal transplantation, impaired renal function, increased urine volume, and increased fractional excretion of sodium were observed. Gene array analysis revealed that these findings were associated with significant downregulation of water channels (aquaporin-1, -2, -3, and -4) and transporters of sodium, glucose, urea, and other solutes. In addition, changes in expression of various receptors, kinases, and phosphatases that modulate the expression or activity of renal transport systems were observed. Syngeneic transplantation or treatment with cyclosporine A following allogeneic transplantation did not impair graft function but did lead to the downregulation of aquaporin-1, -3, and -4 and several solute transporters. However, expression of aquaporin-2 and the epithelial sodium channel did not change, suggesting that the downregulation of these transporters following allogeneic transplantation is rejection-dependent. In conclusion, changes in gene expression may explain the impaired handling of solute and water after allogeneic transplantation, especially during acute rejection. Treatment with cyclosporine A improves the regulation of solute and water by preventing the downregulation of aquaporin-2 and epithelial sodium channel, even though many other transporter genes remain downregulated.

Pathogenesis of oedema in nephrotic syndrome: role of epithelial sodium channel

Nephrotic syndrome is associated with avid sodium retention, leading to the development of oedema and ascites. Studies in experimental animals suggest that sodium retention in nephotic syndrome is due to increased sodium re-absorption in the collecting duct, which is also the action site of vasoregulatory hormones. However, the mechanisms underlying sodium retention in nephrotic syndrome are incompletely understood and the molecular basis remains undefined. This review summarizes recent insight into the role of epithelial sodium channels (ENaC) in animal models of nephrotic syndrome induced by puromycin aminonucleoside - or HgCl(2) treatment. The sodium retention associated with nephrotic syndrome is caused by increased sodium re-absorption in the aldosterone-sensitive distal nephron segments including the connecting tubule and collecting duct, in which an increased apical targeting of ENaC subunits also plays an important role in the development of sodium retention in nephrotic syndrome.

Altered renal sodium transporter expression in an animal model of type 2 diabetes mellitus

Hemodynamic factors play an important role in the development and/or progression of diabetic nephropathy. We hypothesized that renal sodium transporter dysregulation might contribute to the hemodynamic alterations in diabetic nephropathy. Otsuka Long Evans Tokushima Fatty (OLETF) rats were used as an animal model for type 2 diabetes. Long Evans Tokushima (LETO) rats were used as controls. Renal sodium transporter regulation was investigated by semiquantitative immunoblotting and immunohistochemistry of the kidneys of 40-week-old animals. The mean serum glucose level in OLETF rats was increased to 235+/-25 mg/dL at 25 weeks, and the hyperglycemia continued up to the end of 40 weeks. Urine protein/ creatinine ratios were 10 times higher in OLETF rats than in LETO rats. At 40th week, the abundance of the epithelial sodium channel (ENaC) beta-subunit was increased in OLETF rats, but the abundance of the ENaC gamma-subunit was decreased. No significant differences were observed in the ENaC alpha-subunit or other major sodium transporters. Immunohistochemistry for the ENaC beta-subunit showed increased immunoreactivity in OLETF rats, whereas the ENaC gamma-subunit showed reduced immunoreactivity in these rats. In OLETF rats, ENaC beta-subunit upregulation and ENaC gamma-subunit downregulation after the development of diabetic nephropathy may reflect an abnormal sodium balance.

Renal sodium-glucose cotransporter activity and aquaporin-2 expression in rat kidney during chronic nitric oxide synthase inhibition

BACKGROUND/AIMS

The renal sodium glucose cotransporter (SGLT2) and the water channel aquaporin-2 (AQP2) play a critical role in tubular sodium and water reabsorption and in the regulation of extracellular fluid volume both in physiologic and pathophysiologic conditions. However, there is little information about SGLT2 and AQP2 expression and/or activity in hypertension and there are no reports during hypertension induced by chronic nitric oxide synthase (NOS) inhibition.

METHODS

Hypertension was induced in rats by oral administration of N(G)-nitro-L-arginine methyl ester (L-NAME) (20 mg/kg/24 h) for 6 (H6) or 12 (H12) weeks. SGLT2 activity was measured using alpha-(14)C-methylglucose active uptake. The expression level of transporters was assessed by immunohistochemistry and/or immunoblotting.

RESULTS

SGLT2 activity was reduced in both H6 and H12; this was due neither to a decrease in SGLT2 expression nor to a change in membrane phospholipid composition. In H6, AQP2 expression diminished only in the inner medulla (IM), while in H12 it diminished in both outer (OM) and IM. This reduced expression of AQP2 may partially account for the increased urinary volume and decreased urinary osmolality in H12, since we obtained a strong correlation between AQP2 expression and these urinary parameters in both OM and IM.

CONCLUSION

We propose that in rats in which hypertension is induced by NOS inhibition, SGLT2 activity and AQP2 expression are modified to compensate for the elevated arterial pressure. However, we cannot discount the possibility that the observed changes are due to the decrease in NO production itself.

Effects of antioxidant drugs in rats with acute renal injury

Acute renal failure is mainly caused by ischemia/reperfusion (I/R) injury or nephrotoxic drugs, in which reactive oxygen species (ROS) may play an important role. Therefore, antioxidants are expected to decrease the vulnerability of renal injury associated with oxidative challenges. α-Lipoic acid (α-LA), potent antioxidant, could act as ROS scavengers, iron chelators and enzyme modulators. In rats with acute renal injury, dysregulation of aquaporin (AQP) water channels and sodium transporters has been noted. I/R injury or cisplatin induced marked down-regulation of AQP1, AQP2 and AQP3 water channels, and type-3 Na-H exchanger, Na,K-ATPase, and Na-K-2Cl cotransporters, in association with impairment of urinary concentration and tubular sodium reabsorption. Treatment with α-LA prevented the dysregulation of AQP channels and sodium transporters, along with improved urinary concentrating capability and renal sodium reabsorption.

Function and regulation of epithelial sodium transporters in the kidney of a salt-sensitive hypertensive rat model

OBJECTIVE

To determine the function and regulation of thiazide-sensitive NaCl co-transporters (NCC), NaK2Cl co-transporters (NKCC2), and epithelial sodium channels (ENaC) in the kidneys of a salt-sensitive hypertensive model.

DESIGN AND METHODS

Neonatal Wistar rats were treated with capsaicin or vehicle. Seven-week-old male rats were treated for 2 weeks with: vehicle plus a normal (Con-NS) or high (Con-HS) sodium diet, and capsaicin pretreatment plus a normal (Cap-NS) or high (Cap-HS) sodium diet. Mean arterial pressure (MAP), renal excretory function, and protein expression determined by western blot were performed.

RESULTS

MAP was increased in Cap-HS compared with other groups. Trichlormethiazide increased urine sodium excretion (UNaV) and urine flow rate (UFR) and decreased MAP in Cap-HS rats only. Furosemide increased UNaV and UFR in Cap-NS, Con-HS and Cap-HS, and decreased MAP in Cap-HS rats only. Amiloride had no effect on UNaV, UFR and MAP in any group. Renal NCC contents were increased in Cap-HS compared with Con-NS, Con-HS and Cap-NS rats, and NKCC2 expression was increased in Cap-NS, Con-HS and Cap-HS compared with Con-NS rats. No change was found in ENaC alpha subunit expression. The capsaicin-induced release of calcitonin gene-related peptide from renal tissues was decreased in Cap-HS and Cap-NS compared with Con-HS and Con-NS rats.

CONCLUSION

NCC and possibly NKCC2, but not ENaC, were functionally upregulated in the kidneys of rats subjected to sensory nerve degeneration plus high salt intake, suggesting that sensory neurotransmitters may regulate the expression of the former but not the latter, which may underlie the development of salt-sensitive hypertension in this model.

Increased Renal Expression of Aquaporin-3 in Rats Inhibited Type 2 11β-Hydroxysteroid Dehydrogenase

AIMS

To investigate whether the regulation of aquaporin (AQP) channels is altered by inhibition of type 2 11beta-hydroxysteroid dehydrogenase (11betaHSD2).

METHODS

Male Sprague-Dawley rats were treated with glycyrrhizic acid (GA, 2 g/l drinking water) for 7 days. The expression of AQP2 and AQP3 was determined in the kidney by immunoblotting and immunohistochemistry. The expression of Gsalpha and type VI adenylyl cyclase, and the activity of adenylyl cyclase were also determined.

RESULTS

Following the GA treatment, the expression of 11betaHSD2 was significantly decreased in the kidney. The expression of AQP3 was increased, while that of AQP2 remained unchanged. Plasma renin activity and serum aldosterone levels were decreased. Plasma arginine vasopressin (AVP) levels were comparable between the groups. Neither the forskolin-stimulated cAMP generation nor the expression of Gsalpha and type VI adenylyl cyclase was altered significantly.

CONCLUSION

A decreased expression of 11betaHSD2 may result in an upregulation of AQP3, in which AVP/cAMP-dependent mechanisms are unlikely to be involved.

Na channel expression and activity in the medullary collecting duct of rat kidney

The expression and activity of epithelial Na(+) channels (ENaC) in the medullary collecting duct of the rat kidney were examined using a combination of whole cell patch-clamp measurements of amiloride-sensitive currents (I(Na)) in split-open tubules and Western blot analysis of alpha-, beta-, and gamma-ENaC proteins. In the outer medullary collecting duct, amiloride-sensitive currents were undetectable in principal cells from control animals but were robust when rats were treated with aldosterone (I(Na) = 960 +/- 160 pA/cell) or fed a low-Na diet (I(Na) = 440 +/- 120 pA/cell). In both cases, the currents were similar to those measured in principal cells of the cortical collecting duct from the same animals. In the inner medullary collecting duct, currents were much lower, averaging 120 +/- 20 pA/cell in aldosterone-treated rats. Immunoblots showed that all three ENaC subunits were expressed in the cortex, outer medulla, and inner medulla of the rat kidney. When rats were fed a low-Na diet for 1 wk, similar changes in alpha- and gamma-ENaC occurred in all three regions of the kidney; the amounts of full-length as well as putative cleaved alpha-ENaC protein increased, and the fraction of gamma-ENaC protein in the cleaved state increased at the expense of the full-length protein. The appearance of a presumably fully glycosylated form of beta-ENaC in Na-depleted animals was observed mainly in the outer and inner medulla. These findings suggest that the capability of hormone-regulated, channel-mediated Na reabsorption by the nephron extends at least into the outer medullary collecting duct.

StAR expression and the long-term aldosterone response to high-potassium diet in Wistar-Kyoto and spontaneously hypertensive rats

ANG II and potassium are known to increase steroidogenic acute regulatory protein (StAR) levels. However, a corresponding increase in StAR mRNA levels has so far been observed only in response to ANG II. We therefore studied the regulation of adrenal StAR mRNA expression in the context of dietary potassium-stimulated aldosterone production. Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were fed a diet containing either 1 or 4% KCl for 5 days. The high-potassium diet increased StAR mRNA levels within the zona glomerulosa in both strains, as demonstrated by in situ hybridization. However, aldosterone production increased in WKY but not in SHR (WKY: from 22.8 +/- 4.8 to 137 +/- 25 ng/100 ml, P < 0.001, vs. SHR: from 29 +/- 3.8 to 51 +/- 10.2 ng/100 ml, not significant). This increase was associated with an increase in Cyp11b2 mRNA levels in WKY (3-fold; P < 0.001) but not in SHR. In both strains, the 4% KCl diet was associated with increased plasma renin-independent aldosterone production, as indicated by the marked increase of the aldosterone-to-renin ratios (from 1.4 +/- 0.3 to 9 +/- 3 in WKY and from 3 +/- 1 to 14 +/- 5 in SHR; P < 0.002). We conclude that an increase of StAR mRNA levels within the outer cortex is involved in the long-term adrenal response to potassium. This increase alone is not sufficient to increase aldosterone production in the presence of normal Cyp11b2 mRNA levels.

Increased Expression of Renal Aquaporin Water Channels in Spontaneously Hypertensive Rats

AIMS

The present study was aimed to determine whether there exists an altered regulation of aquaporin (AQP) water channels in hypertension.

METHODS

Male spontaneously hypertensive rats (SHR) aged 10-12 weeks were used. Age-matched Wistar-Kyoto (WKY) rats served as control. The abundance of AQP1-4 proteins in the kidney was determined by Western blot analysis. The protein expression and activity of adenylyl cyclase were also determined.

RESULTS

The medullary expression of AQP2 and AQP3 proteins was increased in SHR compared with that in WKY rats. The expression of AQP1 protein was also significantly increased in the inner medulla, while that of AQP4 was not. Immunohistochemistry of AQP2 revealed that principal cells of the collecting duct have strong immunoreactivity, the degree of which was augmented with prominent apical labeling in SHR. The plasma level of arginine vasopressin (AVP) was higher in SHR; the adenylyl cyclase activity stimulated by AVP was augmented, along with increased expression of type VI adenylyl cyclase. The urine was more concentrated with its volume decreased in SHR.

CONCLUSION

The expression of AQP1-3 channels is increased in the kidney, in association with enhanced activity of the AVP/cAMP pathway, in SHR.

Aquaporins as potential drug targets

The aquaporins (AQP) are a family of integral membrane proteins that selectively transport water and, in some cases, small neutral solutes such as glycerol and urea. Thirteen mammalian AQP have been molecularly identified and localized to various epithelial, endothelial and other tissues. Phenotype studies of transgenic mouse models of AQP knockout, mutation, and in some cases humans with AQP mutations have demonstrated essential roles for AQP in mammalian physiology and pathophysiology, including urinary concentrating function, exocrine glandular fluid secretion, brain edema formation, regulation of intracranial and intraocular pressure, skin hydration, fat metabolism, tumor angiogenesis and cell migration. These studies suggest that AQP may be potential drug targets for not only new diuretic reagents for various forms of pathological water retention, but also targets for novel therapy of brain edema, inflammatory disease, glaucoma, obesity, and cancer. However, potent AQP modulators for in vivo application remain to be discovered.