Angiotensin II regulates H(+)-ATPase activity in rat cortical collecting duct

Activation of the Renin–Angiotensin System Disrupts the Cytoskeletal Architecture of Human Urine-Derived Podocytes

High blood pressure is one of the major public health problems that causes severe disorders in several tissues including the human kidney. One of the most important signaling pathways associated with the regulation of blood pressure is the renin–angiotensin system (RAS), with its main mediator angiotensin II (ANGII). Elevated levels of circulating and intracellular ANGII and aldosterone lead to pro-fibrotic, -inflammatory, and -hypertrophic milieu that causes remodeling and dysfunction in cardiovascular and renal tissues. Furthermore, ANGII has been recognized as a major risk factor for the induction of apoptosis in podocytes, ultimately leading to chronic kidney disease (CKD). In the past, disease modeling of kidney-associated diseases was extremely difficult, as the derivation of kidney originated cells is very challenging. Here we describe a differentiation protocol for reproducible differentiation of sine oculis homeobox homolog 2 (SIX2)-positive urine-derived renal progenitor cells (UdRPCs) into podocytes bearing typical cellular processes. The UdRPCs-derived podocytes show the activation of the renin–angiotensin system by being responsive to ANGII stimulation. Our data reveal the ANGII-dependent downregulation of nephrin (NPHS1) and synaptopodin (SYNPO), resulting in the disruption of the podocyte cytoskeletal architecture, as shown by immunofluorescence-based detection of α-Actinin. Furthermore, we show that the cytoskeletal disruption is mainly mediated through angiotensin II receptor type 1 (AGTR1) signaling and can be rescued by AGTR1 inhibition with the selective, competitive angiotensin II receptor type 1 antagonist, losartan. In the present manuscript we confirm and propose UdRPCs differentiated to podocytes as a unique cell type useful for studying nephrogenesis and associated diseases. Furthermore, the responsiveness of UdRPCs-derived podocytes to ANGII implies potential applications in nephrotoxicity studies and drug screening.

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

H+-ATPase blockade reduced renal gluconeogenesis and plasma glucose in a diabetic rat model

Vacuolar H⁺-adenosine triphosphatase (ATPase) plays important roles in urinary acid excretion, vesicular acidification to activate enzymes, and the membrane recycling of transporters in the kidney. As acidosis stimulates renal gluconeogenesis, we investigated the effect of blockade of H⁺-ATPase on renal gluconeogenesis in diabetic rats. Diabetes mellitus was induced by a single injection of streptozotocin, and a group of DM rats was treated with bafilomycin B1 intraperitoneally for 8 days. In diabetic rats, the renal expression and activity of H⁺-ATPase were increased with elevated urinary ammonium excretion. The blockade of H⁺-ATPase by bafilomycin B1 reduced the renal H⁺-ATPase activity and urinary ammonium excretion in diabetic rats. Treatment with bafilomycin suppressed the enhancement of the renal gluconeogenesis enzymes phosphoenol pyruvate carboxykinase and glucose-6-phosphatase in diabetic rats and reduced the renal cytoplasmic glucose levels, whereas hepatic gluconeogenesis did not change significantly. After a 24-h starvation period, bafilomycin decreased the plasma glucose level to a normal level in diabetic rats. The suppression of renal gluconeogenesis by an H⁺-ATPase inhibitor may therefore be a new therapeutic target for the treatment of diabetes mellitus.

Polymorphisms in Renal Ammonia Metabolism Genes Correlate With 24-Hour Urine pH

Introduction

Urine pH is critical for net acid and solute excretion, but the genetic factors that contribute to its regulation are incompletely understood.

Methods

We tested the association of single nucleotide polymorphisms (SNPs) from 16 genes related to ammonia (NH₃) metabolism (15 biological candidates selected a priori, 1 selected from a previous genome-wide association study analysis) to that of 24-hour urine pH in 2493 individuals of European descent across 2 different cohorts using linear regression, adjusting for age, sex, and body mass index.

Results

Of 2871 total SNPs in these genes, 13 SNPs in ATP6V0A4 (a4 subunit of hydrogen− adenosine triphosphatase), SLC9A3 (sodium/hydrogen exchanger, isoform 3), and RHCG (Rhesus C glycoprotein), and 12 SNPs from insulin-like growth factor binding protein 7 (IGFBP7) had a meta-analysis P value <0.01 in the joint analysis plus a consistent direction of effect and at a least suggestive association (P < 0.1) in both cohorts. The maximal effect size (in pH units) for each additional minor allele of the identified SNPs was −0.13 for IGFBP7, −0.08 for ATP6V0A4, 0.06 for RHCG, and −0.06 for SLC9A3; SNP rs34447434 in IGFBP7 had the lowest meta-analysis P value (P = 7.1 × 10⁻⁸). After adjusting for net alkali absorption, urine pH remained suggestively associated with multiple SNPs in IGFBP, 1 SNP in ATP6V0A4, and a new SNP in GLS (phosphate-dependent glutaminase).

Discussion

Overall, these findings suggest that variants in common genes involved in ammonia metabolism may substantively contribute to basal urine pH regulation. These variations might influence the likelihood of developing disease conditions associated with altered urine pH, such as uric acid or calcium phosphate kidney stones.

Oestrogen-related receptor α is required for transepithelial H+ secretion in zebrafish

Oestrogen-related receptor α (ERRα) is an orphan nuclear receptor which is important for adaptive metabolic responses under conditions of increased energy demand, such as cold, exercise and fasting. Importantly, metabolism under these conditions is usually accompanied by elevated production of organic acids, which may threaten the body acid-base status. Although ERRα is known to help regulate ion transport by the renal epithelia, its role in the transport of acid-base equivalents remains unknown. Here, we tested the hypothesis that ERRα is involved in acid-base regulation mechanisms by using zebrafish as the model to examine the effects of ERRα on transepithelial H(+) secretion. ERRα is abundantly expressed in H(+)-pump-rich cells (HR cells), a group of ionocytes responsible for H(+) secretion in the skin of developing embryos, and its expression is stimulated by acidic (pH 4) environments. Knockdown of ERRα impairs both basal and low pH-induced H(+) secretion in the yolk-sac skin, which is accompanied by decreased expression of H(+)-secreting-related transporters. The effect of ERRα on H(+) secretion is achieved through regulating both the total number of HR cells and the function of individual HR cells. These results demonstrate, for the first time, that ERRα is required for transepithelial H(+) secretion for systemic acid-base homeostasis.

Angiotensin receptor blocker telmisartan suppresses renal gluconeogenesis during starvation

The kidney plays an important role in gluconeogenesis during starvation. To clarify the anti-diabetic action of angiotensin receptor blockers, we examined the effects of telmisartan on the sodium-glucose co-transporters (SGLT) and the pathways of renal gluconeogenesis in streptozotocin-induced diabetes mellitus (DM) rats. At 4 weeks, the DM rats treated with/without telmisartan for 2 weeks and normal control rats were used for the study after a 24-hour fast. SGLT2 expressed on the brush border membrane of the proximal convoluted tubules increased in the DM rats, but decreased in the rats treated with telmisartan. The expression of restriction enzymes of gluconeogenesis, glucose-6-phosphatase, and phosphoenolpyruvate carboxykinase increased in the proximal tubules in the DM rats, whereas these enzymes decreased in the kidneys of the rats treated with telmisartan. The elevated cytoplasmic glucose-6-phosphate and glucose levels in the kidney of DM rats significantly decreased in those treated with telmisartan, whereas those levels in the liver did not show significant change. Meanwhile, the high plasma glucose levels in the DM rats during the intravenous insulin tolerance tests were ameliorated by telmisartan. The increased fasting plasma glucose levels after 24 hours of starvation in the DM rats thus returned to the control levels by telmisartan treatment. In conclusion, the increased renal SGLT2 expression, elevated renal gluconeogenesis enzymes and extent of insulin-resistance in the DM rats were ameliorated by telmisartan therapy, thus resulting in decreased plasma glucose levels after 24 hours of fasting.

Renin-angiotensin system in ureteric bud branching morphogenesis: implications for kidney disease

Failure of normal branching morphogenesis of the ureteric bud (UB), a key ontogenic process that controls organogenesis of the metanephric kidney, leads to congenital anomalies of the kidney and urinary tract (CAKUT), the leading cause of end-stage kidney disease in children. Recent studies have revealed a central role of the renin-angiotensin system (RAS), the cardinal regulator of blood pressure and fluid/electrolyte homeostasis, in the control of normal kidney development. Mice or humans with mutations in the RAS genes exhibit a spectrum of CAKUT which includes renal medullary hypoplasia, hydronephrosis, renal hypodysplasia, duplicated renal collecting system and renal tubular dysgenesis. Emerging evidence indicates that severe hypoplasia of the inner medulla and papilla observed in angiotensinogen (Agt)- or angiotensin (Ang) II AT 1 receptor (AT 1 R)-deficient mice is due to aberrant UB branching morphogenesis resulting from disrupted RAS signaling. Lack of the prorenin receptor (PRR) in the UB in mice causes reduced UB branching, resulting in decreased nephron endowment, marked kidney hypoplasia, urinary concentrating and acidification defects. This review provides a mechanistic rational supporting the hypothesis that aberrant signaling of the intrarenal RAS during distinct stages of metanephric kidney development contributes to the pathogenesis of the broad phenotypic spectrum of CAKUT. As aberrant RAS signaling impairs normal renal development, these findings advocate caution for the use of RAS inhibitors in early infancy and further underscore a need to avoid their use during pregnancy and to identify the types of molecular processes that can be targeted for clinical intervention.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

The intercalated cells of the mouse kidney OMCD(is) are the target of the vasopressin V1a receptor axis for urinary acidification

BACKGROUND

Vasopressin V1a receptor (V1aR) null mice have insufficient acid-base balance, but the target cell for V1aR signaling which results in the urinary acidification has not been identified.

METHODS

By using a quantitative in situ hybridization technique and a double-staining technique with an anti-AQP3 antibody in mice, we investigated the axial distribution and acidosis-induced expression of V1aR mRNA along the nephron. We also investigated the acidosis-induced morphological change in the tubule cells from wild-type and V1aR-null (V1aR(-/-)) mice.

RESULTS

In the normal condition, V1aR mRNA was moderately expressed in the medullary thick ascending limb (MTAL) and highly expressed in the intercalated cell (IC) throughout the collecting duct (CD). However, no expression was observed in the proximal tubule, thin limbs of Henle's loop, and the principal cell of the CD. Importantly, V1aR mRNA was upregulated significantly both in the TAL and the IC of the CD in the inner stripe of the outer medulla (MTALis and IC of OMCDis, respectively) when mice were treated with NH4Cl (0.28 mol/L) for 6 days. Acidosis-induced hypertrophy, which was completely attenuated in V1aR(-/-) mice, was observed only in the IC of OMCDis (P < 0.005). In addition, urinary excretion of ammonia (NH3/NH4 (+)) was significantly decreased on day 3 (P < 0.05) and day 6 (P < 0.005) in the V1aR(-/-) mice treated with NH4Cl.

CONCLUSION

In conclusion, the IC of OMCDis may be the target cell stimulated by the vasopressin V1aR axis and contribute to urinary acidification, at least during metabolic acidosis.

Hypothesis: a new role for the Renin-Angiotensin system in ureteric bud branching

Branching morphogenesis in the developing mammalian kidney involves growth and branching of the ureteric bud (UB), leading to formation of its daughter collecting ducts, calyces, pelvis and ureters. Even subtle defects in the efficiency and/or accuracy of this process have profound effects on the ultimate development of the kidney and result in congenital abnormalities of the kidney and urinary tract. This review summarizes current knowledge regarding a number of genes known to regulate UB development and emphasizes an emerging role for the renin-angiotensin system (RAS) in renal branching morphogenesis. Mutations in the genes encoding components of the RAS in mice cause renal papillary hypoplasia, hydronephrosis, and urinary concentrating defect. These findings imply that UB-derived epithelia are targets for angiotensin (ANG) II actions during metanephric kidney development. Here, it is proposed that papillary hypoplasia in RAS-deficient mice is secondary to an intrinsic defect in the development of the renal medulla. This hypothesis is based on the following observations: (a) UB and surrounding stroma express angiotensinogen (AGT) and ANG II AT(1) receptors in vivo; (b) ANG II stimulates UB cell process extension, branching and cord formation in collagen gel cultures in vitro; and (c) AT(1) blockade inhibits ANG II-induced UB cell branching. It is further postulated that ANG II is a novel stroma-derived factor involved in stroma/UB cross-talk which regulates UB branching morphogenesis.

Mechanisms of glomerular albumin filtration and tubular reabsorption

Albumin is filtered through the glomerulus with a sieving coefficient of 0.00062, which results in approximately 3.3 g of albumin filtered daily in human kidneys. The proximal convoluted tubule reabsorbs 71%, the loop of Henle and distal tubule 23%, and collecting duct 3% of the glomerular filtered albumin, thus indicating that the kidney plays an important role in protein metabolism. Dysfunction of albumin reabsorption in the proximal tubules, due to reduced megalin expression, may explain the microalbuminuria in early-stage diabetes. Meanwhile, massive nonselective proteinuria is ascribed to various disorders of the glomerular filtration barrier, including podocyte detachment, glomerular basement membrane rupture, and slit diaphragm dysfunction in focal segmental glomerulosclerosis, membranous nephropathy, and other glomerulonephritis. Selective albuminuria associated with foot process effacement and tight junction-like slit alteration is observed in the patients with minimal-change nephrotic syndrome, and the albumin uptake is enhanced in the podocyte cell body, possibly mediated by albumin receptors in the low-dose puromycin model. The role of enhanced podocyte albumin transport needs to be investigated to elucidate the mechanism of the selective albuminuria in minimal-change disease.

Angiotensin II regulates growth of the developing papillas ex vivo

We tested the hypothesis that lack of angiotensin (ANG) II production in angiotensinogen (AGT)-deficient mice or pharmacologic antagonism of ANG II AT(1) receptor (AT(1)R) impairs growth of the developing papillas ex vivo, thus contributing to the hypoplastic renal medulla phenotype observed in AGT- or AT(1)R-null mice. Papillas were dissected from Hoxb7(GFP+) or AGT(+/+), (+/-), (-/-) mouse metanephroi on postnatal day P3 and grown in three-dimentional collagen matrix gels in the presence of media (control), ANG II (10(-5) M), or the specific AT(1)R antagonist candesartan (10(-6) M) for 24 h. Percent reduction in papillary length was attenuated in AGT(+/+) and in AGT(+/-) compared with AGT(-/-) (-18.4 ± 1.3 vs. -32.2 ± 1.6%, P < 0.05, -22.8 ± 1.3 vs. -32.2 ± 1.6%, P < 0.05, respectively). ANG II blunted the decrease in papilla length observed in respective media-treated controls in Hoxb7(GFP+) (-1.5 ± 0.3 vs. -10.0 ± 1.4%, P < 0.05) or AGT(+/+), (+/-), and (-/-) papillas (-12.8 ± 0.7 vs. -18.4 ± 1.3%, P < 0.05, -16.8 ± 1.1 vs. -23 ± 1.2%, P < 0.05; -26.2 ± 1.6 vs. -32.2 ± 1.6%, P < 0.05, respectively). In contrast, percent decrease in the length of Hoxb7(GFP+) papillas in the presence of the AT(1)R antagonist candesartan was higher compared with control (-24.3 ± 2.1 vs. -10.5 ± 1.8%, P < 0.05). The number of proliferating phospho-histone H3 (pH3)-positive collecting duct cells was lower, whereas the number of caspase 3-positive cells undergoing apoptosis was higher in candesartan- vs. media-treated papillas (pH3: 12 ± 1.4 vs. 21 ± 2.1, P < 0.01; caspase 3: 3.8 ± 0.5 vs. 1.7 ± 0.2, P < 0.01). Using quantitative RT-PCR, we demonstrate that AT(1)R signaling regulates the expression of genes implicated in morphogenesis of the renal medulla. We conclude that AT(1)R prevents shrinkage of the developing papillas observed ex vivo via control of Wnt7b, FGF7, β-catenin, calcineurin B1, and α3 integrin gene expression, collecting duct cell proliferation, and survival.

Renin-angiotensin system in ureteric bud branching morphogenesis: insights into the mechanisms

Branching morphogenesis of the ureteric bud (UB) is a key developmental process that controls organogenesis of the entire metanephros. Notably, aberrant UB branching may result in a spectrum of congenital anomalies of the kidney and urinary tract (CAKUT). Genetic, biochemical and physiological studies have demonstrated that the renin-angiotensin system (RAS), a key regulator of the blood pressure and fluid/electrolyte homeostasis, also plays a critical role in kidney development. All the components of the RAS are expressed in the metanephros. Moreover, mutations in the genes encoding components of the RAS in mice or humans cause diverse types of CAKUT which include renal papillary hypoplasia, hydronephrosis, duplicated collecting system, renal tubular dysgenesis, renal vascular abnormalities, abnormal glomerulogenesis and urinary concentrating defect. Despite widely accepted role of the RAS in metanephric kidney and renal collecting system (ureter, pelvis, calyces and collecting ducts) development, the mechanisms by which an intact RAS exerts its morphogenetic actions are incompletely defined. Emerging evidence indicates that defects in UB branching morphogenesis may be causally linked to the pathogenesis of renal collecting system anomalies observed under conditions of aberrant RAS signaling. This review describes the role of the RAS in UB branching morphogenesis and highlights emerging insights into the cellular and molecular mechanisms whereby RAS regulates this critical morphogenetic process.

Renal proximal tubular reabsorption is reduced in adult spontaneously hypertensive rats: roles of superoxide and Na+/H+ exchanger 3

Proximal tubule reabsorption is regulated by systemic and intrinsic mechanisms, including locally produced autocoids. Superoxide, produced by NADPH oxidase enhances NaCl transport in the loop of Henle and the collecting duct, but its role in the proximal tubule is unclear. We measured proximal tubule fluid reabsorption (Jv) in WKY rats and compared that with Jv in the spontaneously hypertensive rat (SHR), a model of enhanced renal superoxide generation. Rats were treated with the NADPH oxidase inhibitor apocynin (Apo) or with small interfering RNA for p22(phox), which is the critical subunit of NADPH oxidase. Jv was lower in SHR compared with Wistar-Kyoto rats (WKY; WKY: 2.3+/-0.3 vs SHR: 1.1+/-0.2 nL/min per millimeter; n=9 to 11; P<0.001). Apo and small interfering RNA to p22(phox) normalized Jv in SHRs but had no effect in WKY rats. Jv was reduced in proximal tubules perfused with S-1611, a highly selective inhibitor of the Na(+)/H(+) exchanger 3, the major Na(+) uptake pathway in the proximal tubule, in WKY rats but not in SHRs. Pretreatment with Apo restored an effect of S-1611 to reduce Jv in the SHRs (SHR+Apo: 2.9+/-0.4 vs SHR+Apo+S-1611: 1.0+/-0.3 nL/min per millimeter; P<0.001). However, because expression of the Na(+)/H(+) exchanger 3 was similar between SHR and WKY rats, this suggests that superoxide affects Na(+)/H(+) exchanger 3 activity. Direct microperfusion of Tempol or Apo into the proximal tubule also restored Jv in SHRs. In conclusion, superoxide generated by NADPH oxidase inhibits proximal tubule fluid reabsorption in SHRs. This finding implies that proximal tubule fluid reabsorption is regulated by redox balance, which may have profound effects on ion and fluid homeostasis in the hypertensive kidney.

A new role for the renin-angiotensin system in the development of the ureteric bud and renal collecting system

The renin-angiotensin system (RAS) plays a critical role in kidney development. Mutations in the genes encoding components of the RAS or pharmacological inhibition of RAS in mice or humans cause a spectrum of congenital abnormalities of the kidney and urinary tract (CAKUT). The observed defects include renal vascular abnormalities, abnormal glomerulogenesis, renal papillary hypoplasia, hydronephrosis, aberrant ureteric bud (UB) budding, duplicated collecting system and renal tubular dysgenesis. Little is known about the potential role of Ang II and its receptors in the morphogenesis of the UB and renal collecting system. This review emphasizes a novel role for the RAS in the development of the UB, collecting ducts and renal medulla. We observe that UB and surrounding stroma express angiotensinogen and Ang II AT1 receptors (AT1R) in vivo. Ang II stimulates UB cell branching in collagen gel cultures in vitro and induces UB morphogenesis in intact whole embryonic metanephroi grown ex vivo. In contrast, treatment of metanephroi with the AT1R antagonist candesartan inhibits UB branching. In addition, Ang II induces tyrosine phosphorylation of the epidermal growth factor receptor (EGFR) in UB cells. Furthermore, Ang II-stimulated UB morphogenesis is abrogated by inhibition of EGFR tyrosine kinase activity. In summary: 1) Ang II, acting via the AT1R, stimulates UB branching; 2) This process depends on tyrosine phosphorylation of the EGFR. Together, these data indicate that cooperation of AT1R and EGFR signaling performs essential functions during renal collecting system development via control of UB branching morphogenesis.

Angiotensin II activates H+-ATPase in type A intercalated cells

We reported previously that angiotensin II (AngII) increases net Cl(-) absorption in mouse cortical collecting duct (CCD) by transcellular transport across type B intercalated cells (IC) via an H(+)-ATPase-and pendrin-dependent mechanism. Because intracellular trafficking regulates both pendrin and H(+)-ATPase, we hypothesized that AngII induces the subcellular redistribution of one or both of these exchangers. To answer this question, CCD from furosemide-treated mice were perfused in vitro, and the subcellular distributions of pendrin and the H(+)-ATPase were quantified using immunogold cytochemistry and morphometric analysis. Addition of AngII in vitro did not change the distribution of pendrin or H(+)-ATPase within type B IC but within type A IC increased the ratio of apical plasma membrane to cytoplasmic H(+)-ATPase three-fold. Moreover, CCDs secreted bicarbonate under basal conditions but absorbed bicarbonate in response to AngII. In summary, angiotensin II stimulates H(+) secretion into the lumen, which drives Cl(-) absorption mediated by apical Cl(-)/HCO(3)(-) exchange as well as generates more favorable electrochemical gradient for ENaC-mediated Na(+) absorption.

Expression of NG,NG-dimethylarginine dimethylaminohydrolase and protein arginine N-methyltransferase isoforms in diabetic rat kidney: effects of angiotensin II receptor blockers

OBJECTIVE

The nitric oxide (NO) synthase inhibitor asymmetric dimethylarginine (ADMA) is generated by protein arginine N-methyltransferase (PRMT)-1 and is metabolized by N(G),N(G)-dimethylarginine dimethylaminohydrolase (DDAH). We tested the hypothesis that increased serum ADMA (S(ADMA)) in the streptozotocin (STZ)-induced diabetic rat model of diabetes is mediated by an angiotensin receptor blocker-sensitive change in DDAH or PRMT expression.

RESEARCH DESIGN AND METHODS

Data were compared from four groups of rats: sham-injected controls, untreated STZ-induced diabetic rats at 4 weeks, STZ-induced diabetic rats administered the angiotensin II (Ang II) receptor blocker telmisartan for 2 weeks, and control rats administered telmisartan for 2 weeks.

RESULTS

Immunostaining and Western blotting of microdissected nephron segments localized DDAH I in the proximal tubules and DDAH II in the glomeruli, afferent arterioles, macula densa, and distal nephron. Renal Ang II and S(ADMA) increased with diabetes but were normalized by 2 weeks of telmisartan. DDAH I expression was decreased in diabetic kidneys, while DDAH II expression was increased. These changes were reversed by telmisartan, which also reduced expression of PRMT-1 and -5. Telmisartan increased expressions of DDAH I but decreased DDAH II in Ang II-stimulated kidney slices ex vivo.

CONCLUSIONS

Renal Ang II and S(ADMA) are increased in insulinopenic diabetes. They are normalized by an Ang II receptor blocker, which increases the renal expression of DDAH I, decreases PRMT-1, and increases renal NO metabolites.

Angiotensin II causes hypertension and cardiac hypertrophy through its receptors in the kidney

Essential hypertension is a common disease, yet its pathogenesis is not well understood. Altered control of sodium excretion in the kidney may be a key causative feature, but this has been difficult to test experimentally, and recent studies have challenged this hypothesis. Based on the critical role of the renin-angiotensin system (RAS) and the type I (AT1) angiotensin receptor in essential hypertension, we developed an experimental model to separate AT1 receptor pools in the kidney from those in all other tissues. Although actions of the RAS in a variety of target organs have the potential to promote high blood pressure and end-organ damage, we show here that angiotensin II causes hypertension primarily through effects on AT1 receptors in the kidney. We find that renal AT1 receptors are absolutely required for the development of angiotensin II-dependent hypertension and cardiac hypertrophy. When AT1 receptors are eliminated from the kidney, the residual repertoire of systemic, extrarenal AT1 receptors is not sufficient to induce hypertension or cardiac hypertrophy. Our findings demonstrate the critical role of the kidney in the pathogenesis of hypertension and its cardiovascular complications. Further, they suggest that the major mechanism of action of RAS inhibitors in hypertension is attenuation of angiotensin II effects in the kidney.

Signaling pathways involved with the stimulatory effect of angiotensin II on vacuolar H+-ATPase in proximal tubule cells

It has been documented that angiotensin II (ANG II) (10(-9) M) stimulates proton extrusion via H(+)-adenosine triphosphatase (ATPase) in proximal tubule cells. In the present study, we investigated the signaling pathways involved in the effects of ANG II on H(+)-ATPase activity and on the cytosolic free calcium concentration in immortalized rat proximal tubule cells, a permanent cell line derived from rat proximal tubules. The effects of ANG on pH(i) and [Ca(+2)](i) were assessed by the fluorescent probes, 2',7-bis (2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxy-methyl ester and fluo-4-acetoxy-methyl ester, in the absence of Na(+) to block the Na(+)/H(+) exchanger. In the control situation, the pH recovery rate following intracellular acidification with NH(4)Cl was 0.073+/-0.011 pH units/min (n=12). This recovery was significantly increased with ANG II (10(-9 )M), to 0.12+/-0.015 pH units/min, n=10. This last effect was also followed by a significant increase of Ca(+2) (i), from 99.72+/-1.704 nM (n=21) to 401.23+/-33.91 nM (n=39). The stimulatory effect of ANG II was blocked in the presence of losartan, an angiotensin II subtype 1 (AT(1)) receptor antagonist. H89 [protein kinase A (PKA) inhibitor] plus ANG II had no effect on the pH recovery. Staurosporine [protein kinase C (PKC) inhibitor] impaired the effect of ANG II. Phorbol myristate acetate (PKC activator) mimicked in part the stimulatory effect of ANG II, but reduced Ca(+2) (i). 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (intracellular calcium chelator) alone reduced the pH(i) recovery rate below control levels and impaired the effect of ANG II, in a way similar to that of trimethoxy benzoate (a blocker of Ca(+2) (i) mobilization). We conclude that ANG II regulates rat proximal tubule vacuolar H(+)-ATPase by a PKA-independent mechanism and that PKC and intracellular calcium play a critical role in this regulation.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

ANG II reduces net acid secretion in rat outer medullary collecting duct

In rat outer medullary collecting duct (OMCD), the mechanism(s) and regulation of H+ secretion are not understood fully. The effect of changes in acid-base balance and the renin-angiotensin system on net H+ secretion was explored. Rats received NaCl, NaHCO3, NH4Cl, or nothing in their drinking water for 7 days. Total ammonia and total CO2 (JtCO2) fluxes were measured in OMCD tubules perfused in vitro from rats in each treatment group. JtCO2 was reduced in tubules from rats drinking NH4Cl relative to those drinking NaHCO3. Because NH4Cl intake increases plasma renin and aldosterone, we asked if upregulation of the renin-angiotensin system reduces net H+ secretion. Deoxycorticosterone pivalate administered in vivo did not affect JtCO2. However, ANG II given in vivo at 0.1 ng/min reduced JtCO2 by 35%. To determine if ANG II has a direct effect on acid secretion, JtCO2 was measured with ANG II applied in vitro. ANG II (10-8 M) present in the bath solution reduced JtCO2 by 35%. This ANG II effect was not observed in the presence of the AT1 receptor blocker candesartan. In conclusion, in rat OMCD, JtCO2 is paradoxically reduced with NH4Cl ingestion. Increased circulating ANG II, as occurs during metabolic acidosis, reduces JtCO2.

Regulation of sodium transporters in the thick ascending limb of rat kidney: response to angiotensin II

The effect of ANG II treatment of rats for 7 days was examined with respect to the abundance and subcellular localization of key thick ascending limb (TAL) Na+ transporters. Rats were on a fixed intake of Na+ and water and treated with 0, 12.5, 25, 50 (ANG II-50), 100 (ANG II-100), and 200 (ANG II-200) ng x min(-1) x kg(-1) ANG II (sc). Semiquantitative immunoblotting revealed that Na+/H+ exchanger 3 (NHE3) abundance in the inner stripe of the outer medulla (ISOM) of ANG II-treated rats was significantly increased: 179 +/- 28 (ANG II-50, n = 5), 166 +/- 23 (ANG II-100, n = 7), and 167 +/- 19% (ANG II-200, n = 4) of control levels (n = 6, P < 0.05), whereas lower doses of ANG II were ineffective. The abundance of the bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter (BSC-1) in the ISOM was also increased to 187 +/- 28 (ANG II-50), 162 +/- 23 (ANG II-100), and 166 +/- 19% (ANG II-200) of control levels (P < 0.05), but there were no changes in the abundance of Na(+)-K(+)-ATPase and the electroneutral Na(+)-HCO3 cotransporter NBCn1. Immunocytochemistry confirmed the increase in NHE3 and BSC-1 labeling in medullary TAL (mTAL). In the cortex and the outer strip of the outer medulla, NHE3 abundance was unchanged, whereas immunocytochemistry revealed markedly increased NHE3 labeling of the proximal tubule brush border, suggesting subcellular redistribution of NHE3 or differential protein-protein interaction. Despite this, ANG II-treated rats (50 ng x min(-1) x kg(-1) for 5 days, n = 6) had a higher urinary pH compared with controls. NH4Cl loading completely blocked all effects of ANG II infusion on NHE3 and BSC-1, suggesting a potential role of pH as a mediator of these effects. In conclusion, increased abundance of NHE3 and BSC-1 in mTAL cells as well as increased NHE3 in the proximal tubule brush border may contribute to enhanced renal Na+ and HCO3 reabsorption in response to ANG II.

Influence of ANG II on cytoplasmic sodium in cultured rabbit nonpigmented ciliary epithelium

Angiotensin (ANG) II receptors have been reported in the nonpigmented ciliary epithelium (NPE) of the eye. In cultured NPE, we found ANG II caused a dose-dependent rise of cytoplasmic sodium. The sodium increase was inhibited by the AT(1)-AT(2) receptor antagonist saralasin (IC(50) = 3.7 nM) and the AT(1) antagonist losartan (IC(50) = 0.6 nM) but not by the AT(2) antagonist PD-123319. ANG II also caused a dose-dependent increase in the rate of ouabain-sensitive (86)Rb uptake. The ANG II-induced cell sodium increase and (86)Rb uptake increase were reduced by dimethylamiloride (DMA; 10 microM). On the basis of this finding, we propose that Na(+)/H(+) exchange is stimulated by ANG II. Simultaneously, ANG II appears to inhibit H(+)-ATPase-mediated proton export. Thus Ang II (10 nM) did not alter the baseline cytoplasmic pH (pH(i)) but reduced pH(i) in cells that were also exposed to 10 microM DMA. Consistent with the notion of H(+)-ATPase inhibition in ANG II-treated NPE, bafilomycin A(1) (100 nM) (BAF) and ANG II were both observed to suppress the pH(i) increase that occurs upon exposure to a mixture of epinephrine (1 microM) and acetylcholine (10 microM) and the pH(i) increase elicited by depolarization. In ATP hydrolysis measurements, H(+)-ATPase activity (bafilomycin A(1)-sensitive ATP hydrolysis) was reduced significantly in cells that had been pretreated 10 min with 10 nM ANG II. In summary, these studies suggest that ANG II causes H(+)-ATPase inhibition and an increase of cell sodium due to activation of Na(+)/H(+) exchange.

Ureteric bud derivatives express angiotensinogen and AT1 receptors

Inactivation of the renin-angiotensin system interferes with the morphogenesis of the renal medulla. Thus ureteric bud (UB) derivatives may be a target for angiotensin production and action. To begin to test this hypothesis, we examined the cellular expression of angiotensinogen (Ao) and AT(1) receptor proteins during rat metanephrogenesis by immunohistochemistry. In addition, we tested whether UB-derived cells in culture express the Ao and AT(1) proteins. On embryonic day E15, Ao and AT(1) are expressed in the UB branches and stromal mesenchyme. S-shaped bodies, including the vascular cleft, express AT(1) but not Ao. The metanephric mesenchyme and pretubular aggregates are Ao negative and AT(1) negative. Expression of Ao and AT(1) in UB branches and ampullae is also observed on E16. However, UB expression of Ao is transient and is no longer detectable in the developing distal nephron beyond E17. On E17, both Ao and AT(1) are expressed in capillary loop glomeruli and proximal tubules, whereas UB branches express AT(1) only. By E18, the majority of Ao immunoreactivity is clustered in terminally differentiated proximal tubules, whereas AT(1) receptors are expressed in both proximal and distal nephron segments. The specificity of Ao and AT(1) staining was documented by the elimination/attenuation of immunoreactivity after preadsorption of the primary antibodies with their respective antigens. Consistent with the in vivo findings, the AT(1) protein is abundantly expressed in cellular lysates of mouse UB (E11.5) and IMCD3 (adult) cells. Moreover, AT(1) receptors in UB and IMCD3 cells are functional, since angiotensin II treatment elicits the tyrosine phosphorylation of the mitogen-activated protein kinases, ERK1/2. To our knowledge, this is the first demonstration of Ao and AT(1) protein expression in the developing distal nephron. Angiotensin II may have a paracrine role in the ontogeny of the collecting system.

Angiotensin II increases vasopressin-stimulated facilitated urea permeability in rat terminal IMCDs

Angiotensin II receptors are present along the rat inner medullary collecting duct (IMCD), although their physiological role is unknown. Because urea is one of the major solutes transported across the terminal IMCD, we measured angiotensin II's effect on urea permeability. In the perfused rat terminal IMCD, angiotensin II had no effect on basal urea permeability but significantly increased vasopressin-stimulated urea permeability by 55%. Angiotensin II, both without and with vasopressin, also increased the amount of (32)P incorporated into urea transporter (UT)-A1 in inner medullary tissue exposed to these hormones ex vivo. Because angiotensin II activates protein kinase C, we tested the effect of staurosporine (SSP). In the absence of angiotensin II, SSP had no effect on vasopressin-stimulated urea permeability in the perfused terminal IMCD. However, SSP completely and reversibly blocked the angiotensin II-mediated increase in vasopressin-stimulated urea permeability. SSP and chelerythrine reduced the angiotensin II-stimulated (32)P incorporation into UT-A1 in inner medullary tissue exposed ex vivo. We conclude that angiotensin II increases vasopressin-stimulated facilitated urea permeability and (32)P incorporation into the 97- and 117-kDa UT-A1 proteins via a protein kinase C-mediated signaling pathway. These data suggest that angiotensin II augments vasopressin-stimulated facilitated urea transport in the rat terminal IMCD and may play a physiological role in the urinary concentrating mechanism by augmenting the maximal response to vasopressin.

Nitric oxide regulates renal cortical cyclooxygenase-2 expression

We have previously shown that cyclooxygenase-2 (COX-2) is localized to the cortical thick ascending limb of the loop of Henle (cTALH)/macula densa of the rat kidney, and expression increases in response to low-salt diet and/or angiotensin-converting enzyme (ACE) inhibition. Because of the localization of neuronal nitric oxide synthase (nNOS) to macula densa and surrounding cTALH, the present study investigated the role of nitric oxide (NO) in the regulation of COX-2 expression. For in vivo studies, rats were fed a normal diet, low-salt diet or low-salt diet combined with the ACE inhibitor captopril. In each group, one-half of them were treated with the nNOS inhibitors 7-nitroinidazole (7-NI) or S-methyl-thiocitrulline. Both of these NOS inhibitors inhibited increases in COX-2 mRNA and immunoreactive protein in response to low salt and low salt+captopril. For in vitro studies, COX-2 expression was studied in primary cultures of rabbit cTALH cells immunodisssected with Tamm-Horsfall antibody. Basal COX-2 immunoreactivity expression was stimulated by S-nitroso-N-acetyl-penicillamine (SNAP), an NO donor, and intracellular cGMP concentration. The cultured cells expressed immunoreactive nNOS, and 7-NI inhibited basal COX-2 immunoreactivity expression, which could be partially overcome by cGMP. In summary, these studies indicate that NO is a mediator of increased renal cortical COX-2 expression seen in volume depletion and suggest important interactions between the NO and COX-2 systems in the regulation of arteriolar tone and the renin-angiotensin system by the macula densa.

Angiotensin II stimulates vesicular H+-ATPase in rat proximal tubular cells

Two mechanisms of H+ ion secretion in the proximal tubule that mediate bicarbonate reabsorption have been identified: the brush border Na/H exchanger and electrogenic H+ ion secretion. Angiotensin II (AII) has been shown to be a regulator of the luminal Na+/H+ exchanger and the basolateral Na+/HCO3- cotransporter. In the present study, we examined the effects of AII on H+-ATPase activity in isolated proximal tubule fragments. H+-ATPase activity was assessed by monitoring intracellular pH after Na+ removal from the bath. In addition, we investigated the effects on pH recovery of the proton pump inhibitor bafilomycin A1, removal of Cl-, and of colchicine. pH was continuously measured with the pH-sensitive fluorescent dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Recovery of cell pH was observed in the absence of external Na+ and was significantly accelerated by AII. The AII-stimulated pH recovery was completely abolished by bafilomycin A1, by removal of Cl-, by NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate; a potent Cl- channel blocker], and by colchicine. We conclude from these studies that AII stimulates proton extrusion via H+-ATPase by a Cl--dependent process involving brush border insertion of vesicles. This process may contribute to up-regulation of HCO3- reabsorption along the proximal tubule when tubules are exposed to AII.

Role of angiotensin II in dietary modulation of rat late distal tubule bicarbonate flux in vivo

We have reported that overnight fasting stimulates bicarbonate reabsorption (JtCo2) in rat distal tubules. The present in vivo microperfusion studies evaluated the hypothesis that endogenous angiotensin II (AII) mediates this response. Rat late distal (LD) tubules were perfused at 8 nl/min in vivo with a hypotonic solution containing 28 mM bicarbonate. In overnight-fasted rats, LD JtCO2 was significantly higher than in normally fed rats (50 +/- 4 vs. 16 +/- 6 pmol/min.mm, P < 0.05). When overnight-fasted rats were salt-loaded, JtCO2 fell significantly (38 +/- 3 pmol/min.mm, P < 0.05). Conversely, in fed rats ingesting a zero-salt diet, JtCO2 increased three-fold (45 +/- 5 pmol/min.mm, P < 0.05). Enalaprilat infusion (0.25 micrograms/kg body wt, intravenously), in these zero-salt and overnight-fasted rats, reduced LD JtCO2 values to normal. Further, infusion of losartan (5 mg/kg body wt, intravenously), the specific AII AT1 receptor blocker, reduced JtCO2 in overnight-fasted rats by two-thirds (16 +/- 4 pmol/min.mm, P < 0.05). Finally, we perfused 10(-11) M AII intraluminally with and without 10(-6) M losartan: AII increased JtCO2 to 45 +/- 6 pmol/min.mm, equal to the zero-salt flux. This was completely abrogated by simultaneous losartan perfusion. Therefore, these results suggest that AII is an in vivo stimulator of late distal tubule bicarbonate reabsorption.