Targeted proteomic profiling of renal Na(+) transporter and channel abundances in angiotensin II type 1a receptor knockout mice

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Angiotensin II-Type-1a Receptor and Renal K + Wasting during Overnight Low-Na + Intake

Key Points

Background

Chronic angiotensin II perfusion stimulates Kir4.1/Kir5.1 of the distal convoluted tubule (DCT) via angiotensin II–type-1a-receptor (AT1aR), and low‐sodium intake also stimulates Kir4.1/Kir5.1. However, the role of AT1aR in mediating the effect of low salt on Kir4.1/Kir5.1 is not explored.

Methods

We used the patch-clamp technique to examine Kir4.1/Kir5.1 activity of the DCT, employed immunoblotting to examine Na-Cl cotransporter (NCC) expression/activity, and used the in vivo perfusion technique to measure renal Na+ and renal K+ excretion in control, kidney tubule–specific–AT1aR-knockout mice (Ks-AT1aR-KO) and DCT-specific–AT1aR-knockout mice (DCT-AT1aR-KO).

Results

Angiotensin II acutely stimulated the 40-pS-K+ channel (Kir4.1/Kir5.1-heterotetramer) and increased whole-cell Kir4.1/Kir5.1-mediated K+ currents and the negativity of DCT membrane potential only in late DCT2 but not in early DCT. Acute angiotensin II increased thiazide-induced renal Na+ excretion (ENa). The effect of angiotensin II on Kir4.1/Kir5.1 and hydrochlorothiazide-induced ENa was absent in Ks-AT1aR-KO mice. Overnight low-salt intake stimulated the expression of Agtr1a mRNA in DCT, increased whole-cell Kir4.1/Kir5.1-mediated K+ currents in late DCT, hyperpolarized late DCT membrane, augmented the expression of phosphor-Na-Cl-cotransporter, and enhanced thiazide-induced renal-ENa in the control mice. However, the effect of overnight low-salt intake on Kir4.1/Kir5.1 activity, DCT membrane potential, and NCC activity/expression was abolished in DCT-AT1aR-KO or Ks-AT1aR-KO mice. Overnight low-salt intake had no effect on baseline renal K+ excretion (EK) and plasma K+ concentrations in the control mice, but it increased baseline renal-EK and decreased plasma K+ concentrations in DCT-AT1aR-KO or in Ks-AT1aR-KO mice.

Conclusions

Acute angiotensin II or overnight low-salt intake stimulated Kir4.1/Kir5.1 in late DCT, and AT1aR was responsible for acute angiotensin II or overnight low-salt intake–induced stimulation of Kir4.1/Kir5.1 and NCC. AT1aR of the DCT plays a role in maintaining adequate baseline renal-EK and plasma K+ concentrations during overnight low-salt intake.

Juxtaglomerular apparatus-mediated homeostatic mechanisms: therapeutic implication for chronic kidney disease

INTRODUCTION

Juxtaglomerular apparatus (JGA)-mediated homeostatic mechanism links to how sodium-glucose cotransporter 2 inhibitors (SGLT2is) slow progression of chronic kidney disease (CKD) and may link to how tolvaptan slows renal function decline in autosomal dominant polycystic kidney disease (ADPKD).

AREA COVERED

JGA-mediated homeostatic mechanism has been hypothesized based on investigations of tubuloglomerular feedback and renin-angiotensin system. We reviewed clinical trials of SGLT2is and tolvaptan to assess the relationship between this mechanism and these drugs.

EXPERT OPINION

When sodium load to macula densa (MD) increases, MD increases adenosine production, constricting afferent arteriole (Af-art) and protecting glomeruli. Concurrently, MD signaling suppresses renin secretion, increases urinary sodium excretion, and counterbalances reduced sodium filtration. However, when there is marked increase in sodium load per-nephron, as in advanced CKD, MD adenosine production increases, relaxing Af-art and maintaining sodium homeostasis at the expense of glomeruli. The beneficial effects of tolvaptan on renal function in ADPKD may also depend on the JGA-mediated homeostatic mechanisms since tolvaptan inhibits sodium reabsorption in the thick ascending limb.The JGA-mediated homeostatic mechanism regulates Af-arts, constricting to relaxing according to homeostatic needs. Understanding this mechanism may contribute to the development of pharmacotherapeutic compounds and better care for patients with CKD.

Role of Angiotensin II Type 1a Receptor (AT1aR) of Renal Tubules in Regulating Inwardly Rectifying Potassium Channels 4.2 (Kir4.2), Kir4.1, and Epithelial Na+ Channel (ENaC)

BACKGROUND

Kir4.2 and Kir4.1 play a role in regulating membrane transport in the proximal tubule (PT) and in the distal-convoluted-tubule (DCT), respectively.

METHODS

We generated kidney-tubule-specific-AT1aR-knockout (Ks-AT1aR-KO) mice to examine whether renal AT1aR regulates Kir4.2 and Kir4.1.

RESULTS

Ks-AT1aR-KO mice had a lower systolic blood pressure than Agtr1aflox/flox (control) mice. Ks-AT1aR-KO mice had a lower expression of NHE3 (Na+/H+-exchanger 3) and Kir4.2, a major Kir-channel in PT, than Agtr1aflox/flox mice. Whole-cell recording also demonstrated that the membrane potential in PT of Ks-AT1aR-KO mice was lesser negative than Agtr1aflox/flox mice. The expression of Kir4.1 and Kir5.1, Kir4.1/Kir5.1-mediated K+ currents of DCT and DCT membrane potential in Ks-AT1aR-KO mice, were similar to Agtr1aflox/flox mice. However, angiotensin II perfusion for 7 days hyperpolarized the membrane potential in PT and DCT of the control mice but not in Ks-AT1aR-KO mice, while angiotensin II perfusion did not change the expression of Kir4.1, Kir4.2, and Kir5.1. Deletion of AT1aR did not significantly affect the expression of αENaC (epithelial Na+ channel) and βENaC but increased cleaved γENaC expression. Patch-clamp experiments demonstrated that deletion of AT1aR increased amiloride-sensitive Na+-currents in the cortical-collecting duct but not in late-DCT. However, tertiapin-Q sensitive renal outer medullary potassium channel currents were similar in both genotypes.

CONCLUSIONS

AT1aR determines the baseline membrane potential of PT by controlling Kir4.2 expression/activity but AT1aR is not required for determining the baseline membrane potential of the DCT and Kir4.1/Kir5.1 activity/expression. However, AT1aR is required for angiotensin II-induced hyperpolarization of basolateral membrane of PT and DCT. Deletion of AT1aR had no effect on baseline renal outer medullary potassium channel activity but increased ENaC activity in the CCD.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Local and downstream actions of proximal tubule angiotensin II signaling on Na+ transporters in the mouse nephron

The renal nephron consists of a series of distinct cell types that function in concert to maintain fluid and electrolyte balance and blood pressure. The renin-angiotensin system (RAS) is central to Na⁺ and volume balance. We aimed to determine how loss of angiotensin II signaling in the proximal tubule (PT), which reabsorbs the bulk of filtered Na⁺ and volume, impacts solute transport throughout the nephron. We hypothesized that PT renin-angiotensin system disruption would not only depress PT Na⁺ transporters but also impact downstream Na⁺ transporters. Using a mouse model in which the angiotensin type 1a receptor (AT_1aR) is deleted specifically within the PT (AT_1aR PTKO), we profiled the abundance of Na⁺ transporters, channels, and claudins along the nephron. Absence of PT AT_1aR signaling was associated with lower abundance of PT transporters (Na⁺/H⁺ exchanger isoform 3, electrogenic Na⁺-bicarbonate cotransporter 1, and claudin 2) as well as lower abundance of downstream transporters (total and phosphorylated Na⁺-K⁺-2Cl^- cotransporter, medullary Na⁺-K⁺-ATPase, phosphorylated NaCl cotransporter, and claudin 7) versus controls. However, transport activities of Na⁺-K⁺-2Cl^- cotransporter and NaCl cotransporter (assessed with diuretics) were similar between groups in order to maintain electrolyte balance. Together, these results demonstrate the primary impact of angiotensin II regulation on Na⁺ reabsorption in the PT at baseline and the associated influence on downstream Na⁺ transporters, highlighting the ability of the nephron to integrate Na⁺ transport along the nephron to maintain homeostasis.NEW & NOTEWORTHY Our study defines a novel role for proximal tubule angiotensin receptors in regulating the abundance of Na⁺ transporters throughout the nephron, thereby contributing to the integrated control of fluid balance in vivo.

Renin-angiotensin system in mammalian kidney development

Mutations in the genes of the renin-angiotensin system result in congenital anomalies of the kidney and urinary tract (CAKUT), the main cause of end-stage renal disease in children. The molecular mechanisms that cause CAKUT are unclear in most cases. To improve the care of children with CAKUT, it is critical to determine the underlying mechanisms of CAKUT. In this review, we discuss recent advances that have helped to better understand how disruption of the renin-angiotensin system during kidney development contributes to CAKUT.

The Renal Physiology of Pendrin-Positive Intercalated Cells

Intercalated cells (ICs) are found in the connecting tubule and the collecting duct. Of the three IC subtypes identified, type B intercalated cells are one of the best characterized and known to mediate Cl- absorption and HCO3- secretion, largely through the anion exchanger pendrin. This exchanger is thought to act in tandem with the Na+-dependent Cl-/HCO3- exchanger, NDCBE, to mediate net NaCl absorption. Pendrin is stimulated by angiotensin II and aldosterone administration via the angiotensin type 1a and the mineralocorticoid receptors, respectively. It is also stimulated in models of metabolic alkalosis, such as with NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, of probably more physiological or clinical significance is the role of these pendrin-positive ICs in blood pressure regulation, which occurs, at least in part, through pendrin-mediated renal Cl- absorption, as well as their effect on the epithelial Na+ channel, ENaC. Aldosterone stimulates ENaC directly through principal cell mineralocorticoid hormone receptor (ligand) binding and also indirectly through its effect on pendrin expression and function. In so doing, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. In addition to its role in Na+ and Cl- balance, pendrin affects the balance of other ions, such as K+ and I-. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contribution of pendrin-positive ICs in the kidney to distal nephron function and blood pressure.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Role of sodium/glucose cotransporter inhibition on a rat model of angiotensin II-dependent kidney damage

Background

Renal proximal tubular sodium and glucose reabsorption are regulated by the sodium-glucose cotransporter (SGLT2). Changes in this transporter can play a role in hyperglycaemia and reactive oxygen species (ROS) production. We demonstrated increased glucose absorption in proximal tubule membrane vesicles and increased expression of SGLT2 in hypertensive rats. Here we investigated Angiotensin II (Ang II) -dependent SGLT2 expression induction and the role of SGLT2 induction in the development of Ang II-dependent kidney damage. The aim of this study was to determine whether SGLT2 induction by Ang II is associated with Ang II-dependent kidney damage. We propose the following objectives a) to demonstrate that Ang II induces SGLT2 expression and b) to demonstrate that prevention of SGLT2 expression and activity prevent Ang II-induced kidney damage.

Methods

We used chronic Ang II infusion as a model of kidney damage in male Wistar rats and evaluated systolic blood pressure by telemetric methods. SGLT2 mRNA and protein expression were evaluated by PCR and immunoblotting. SGLT2 activity was evaluated in brush border membrane vesicles by measuring glucose uptake. ROS production was measured by confocal microscopy. The glomerular filtration rate (GFR) was evaluated by the inulin excretion method, and urinary protein excretion was evaluated by the Bradford method. Biological parameter evaluations were performed, after two weeks of infusion of Ang II. We compared the effects of Angiotensin II (AT1) receptor blockade by Losartan and SGLT2 inhibition by Empagliflozin both as monotherapy treatments and in combination on the development of kidney damage.

Results

Chronic Ang II infusion led to a blood pressure elevation and increased SGLT2 mRNA expression and activity as well as kidney damage, as reflected by increased ROS production, decreased GFR and increased urinary protein excretion. AT1 receptor blockade prevented all these changes. By contrast, SGLT2 inhibition did not affect blood pressure and had a small effect on kidney damage. However, the combination of both drugs resulted in the potentiation of the effects observed by AT1 receptor blockade alone.

Conclusions

We suggest that Ang II-dependent increased SGLT2 induction is one mechanism by which Ang II induces kidney damage.

Electronic supplementary material

The online version of this article (10.1186/s12882-019-1490-z) contains supplementary material, which is available to authorized users.

Podocyte Injury Augments Intrarenal Angiotensin II Generation and Sodium Retention in a Megalin-Dependent Manner

We have previously shown that podocyte injury increases the glomerular filtration of liver-derived Agt (angiotensinogen) and the generation of intrarenal Ang II (angiotensin II) and that the filtered Agt is reabsorbed by proximal tubules in a manner dependent on megalin. In the present study, we aimed to study the role of megalin in the generation of renal Ang II and sodium handling during nephrotic syndrome. We generated proximal tubule-specific megalin KO (knockout) mice and crossed these animals with NEP25 mice, in which podocyte-specific injury can be induced by injection of the immunotoxin LMB2. Without podocyte injury, renal Agt staining was markedly diminished and urinary Agt increased in KO mice. However, renal Ang II was similar between KO and control mice on average: 117 (95% CI, 101-134) versus 101 (95% CI, 68-133) fmol/g tissue. We next tested the effect of megalin KO on intrarenal Ang II generation with podocyte injury. Control NEP25 mice showed markedly increased renal Agt staining and renal Ang II levels: 450 (336-565) fmol/g tissue. Megalin KO/NEP25 mice showed markedly diminished Agt reabsorption and attenuated renal Ang II: 199 (156-242) fmol/g tissue (P<0.001). Compared with control NEP25 mice, megalin KO/NEP25 mice excreted 5-fold more sodium in the urine. Western blot analysis showed that megalin KO decreased NHE3 and the cleaved α and γ forms of Epithelial Na Channel. These data indicate that Agt reabsorbed by proximal tubules via megalin in nephrotic syndrome is converted to Ang II, which may contribute to sodium retention and edema formation by activating NHE3 and Epithelial Na Channel.

Utilizing proteomics to understand and define hypertension: where are we and where do we go?

INTRODUCTION

Hypertension is a complex and multifactorial cardiovascular disorder. With different mechanisms contributing to a different extent to an individual's blood pressure, the discovery of novel pathogenetic principles of hypertension is challenging. However, there is an urgent and unmet clinical need to improve prevention, detection, and therapy of hypertension in order to reduce the global burden associated with hypertension-related cardiovascular diseases. Areas covered: Proteomic techniques have been applied in reductionist experimental models including angiotensin II infusion models in rodents and the spontaneously hypertensive rat in order to unravel mechanisms involved in blood pressure control and end organ damage. In humans proteomic studies mainly focus on prediction and detection of organ damage, particularly of heart failure and renal disease. While there are only few proteomic studies specifically addressing human primary hypertension, there are more data available in hypertensive disorders in pregnancy, such as preeclampsia. We will review these studies and discuss implications of proteomics on precision medicine approaches. Expert commentary: Despite the potential of proteomic studies in hypertension there has been moderate progress in this area of research. Standardized large-scale studies are required in order to make best use of the potential that proteomics offers in hypertension and other cardiovascular diseases.

Compromised regulation of the collecting duct ENaC activity in mice lacking AT1a receptor

ENaC-mediated sodium reabsorption in the collecting duct (CD) is a critical determinant of urinary sodium excretion. Existing evidence suggest direct stimulatory actions of Angiotensin II (Ang II) on ENaC in the CD, independently of the aldosterone-mineralocorticoid receptor (MR) signaling. Deletion of the major renal AT₁ receptor isoform, AT_1a R, decreases blood pressure and reduces ENaC abundance despite elevated aldosterone levels. The mechanism of this insufficient compensation is not known. Here, we used patch clamp electrophysiology in freshly isolated split-opened CDs to investigate how AT_1a R dysfunction compromises functional ENaC activity and its regulation by dietary salt intake. Ang II had no effect on ENaC activity in CDs from AT_1a R -/- mice suggesting no complementary contribution of AT₂ receptors. We next found that AT_1a R deficient mice had lower ENaC activity when fed with low (<0.01% Na⁺ ) and regular (0.32% Na⁺ ) but not with high (∼2% Na⁺ ) salt diet, when compared to the respective values obtained in Wild type (WT) animals. Inhibition of AT₁ R with losartan in wild-type animals reproduces the effects of genetic ablation of AT_1a R on ENaC activity arguing against contribution of developmental factors. Interestingly, manipulation with aldosterone-MR signaling via deoxycosterone acetate (DOCA) and spironolactone had much reduced influence on ENaC activity upon AT_1a R deletion. Consistently, AT_1a R -/- mice have a markedly diminished MR abundance in cytosol. Overall, we conclude that AT_1a R deficiency elicits a complex inhibitory effect on ENaC activity by attenuating ENaC P_o and precluding adequate compensation via aldosterone cascade due to decreased MR availability.

Renal intercalated cells and blood pressure regulation

Type B and non-A, non-B intercalated cells are found within the connecting tubule and the cortical collecting duct. Of these cell types, type B intercalated cells are known to mediate Cl- absorption and HCO3- secretion largely through pendrin-dependent Cl-/HCO3- exchange. This exchange is stimulated by angiotensin II administration and is also stimulated in models of metabolic alkalosis, for instance after aldosterone or NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, the role of pendrin in blood pressure regulation is likely of more physiological or clinical significance. Pendrin regulates blood pressure not only by mediating aldosterone-sensitive Cl- absorption, but also by modulating the aldosterone response for epithelial Na+ channel (ENaC)-mediated Na+ absorption. Pendrin regulates ENaC through changes in open channel of probability, channel surface density, and channels subunit total protein abundance. Thus, aldosterone stimulates ENaC activity through both direct and indirect effects, the latter occurring through its stimulation of pendrin expression and function. Therefore, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contributory role of pendrin in distal nephron function and blood pressure.

Increased renal oxidative stress in salt-sensitive human GRK4γ486V transgenic mice

We tested the hypothesis that salt-sensitive hypertension is caused by renal oxidative stress by measuring the blood pressure and reactive oxygen species-related proteins in the kidneys of human G protein-coupled receptor kinase 4γ (hGRK4γ) 486V transgenic mice and non-transgenic (Non-T) littermates on normal and high salt diets. High salt diet increased the blood pressure, associated with impaired sodium excretion, in hGRK4γ486V mice. Renal expressions of NOX isoforms were similar in both strains on normal salt diet but NOX2 was decreased by high salt diet to a greater extent in Non-T than hGRK4γ486V mice. Renal HO-2, but not HO-1, protein was greater in hGRK4γ486V than Non-T mice on normal salt diet and normalized by high salt diet. On normal salt diet, renal CuZnSOD and ECSOD proteins were similar but renal MnSOD was lower in hGRK4γ486V than Non-T mice and remained low on high salt diet. High salt diet decreased renal CuZnSOD in hGRK4γ486V but not Non-T mice and decreased renal ECSOD to a greater extent in hGRK4γ486V than Non-T mice. Renal SOD activity, superoxide production, and NOS3 protein were similar in two strains on normal salt diet. However, high salt diet decreased SOD activity and NOS3 protein and increased superoxide production in hGRK4γ486V mice but not in Non-T mice. High salt diet also increased urinary 8-isoprostane and 8-hydroxydeoxyguanosine to a greater extent in hGRK4γ486V than Non-T mice. hGRK4γwild-type mice were normotensive and hGRK4γ142V mice were hypertensive but both were salt-resistant and in normal redox balance. Chronic tempol treatment partially prevented the salt-sensitivity of hGRK4γ486V mice. Thus, hGRK4γ486V causes salt-sensitive hypertension due, in part, to defective renal antioxidant mechanisms.

Prenatal Testosterone Exposure Decreases Aldosterone Production but Maintains Normal Plasma Volume and Increases Blood Pressure in Adult Female Rats

Plasma testosterone levels are elevated in pregnant women with preeclampsia and polycystic ovaries; their offspring are at increased risk for hypertension during adult life. We tested the hypothesis that prenatal testosterone exposure induces dysregulation of the renin-angiotensin-aldosterone system, which is known to play an important role in water and electrolyte balance and blood pressure regulation. Female rats (6 mo old) prenatally exposed to testosterone were examined for adrenal expression of steroidogenic genes, telemetric blood pressure, blood volume and Na⁺ and K⁺ levels, plasma aldosterone, angiotensin II and vasopressin levels, and vascular responses to angiotensin II and arg⁸-vasopressin. The levels of Cyp11b2 (aldosterone synthase), but not the other adrenal steroidogenic genes, were decreased in testosterone females. Accordingly, plasma aldosterone levels were lower in testosterone females. Plasma volume and serum and urine Na⁺ and K⁺ levels were not significantly different between control and testosterone females; however, prenatal testosterone exposure significantly increased plasma vasopressin and angiotensin II levels and arterial pressure in adult females. In testosterone females, mesenteric artery contractile responses to angiotensin II were significantly greater, while contractile responses to vasopressin were unaffected. Angiotensin II type-1 receptor expression was increased, while angiotensin II type-2 receptor was decreased in testosterone arteries. These results suggest that prenatal testosterone exposure downregulates adrenal Cyp11b2 expression, leading to decreased plasma aldosterone levels. Elevated angiotensin II and vasopressin levels along with enhanced vascular responsiveness to angiotensin II may serve as an underlying mechanism to maintain plasma volume and Na⁺ and K⁺ levels and mediate hypertension in adult testosterone females.

Impact of Angiotensin Type 1A Receptors in Principal Cells of the Collecting Duct on Blood Pressure and Hypertension

The main actions of the renin-angiotensin system to control blood pressure (BP) are mediated by the angiotensin type 1 receptors (AT1Rs). The major murine AT1R isoform, AT1AR, is expressed throughout the nephron, including the collecting duct in both principal and intercalated cells. Principal cells play the major role in sodium and water reabsorption. Although aldosterone is considered to be the dominant regulator of sodium reabsorption by principal cells, recent studies suggest a role for direct actions of AT1R. To specifically examine the contributions of AT1AR in principal cells to BP regulation and the development of hypertension in vivo, we generated inbred 129/SvEv mice with deletion of AT1AR from principal cells (PCKO). At baseline, we found that BPs measured by radiotelemetry were similar between PCKOs and controls. During 1-week of low-salt diet (<0.02% NaCl), BPs fell significantly (P<0.05) and to a similar extent in both groups. On a high-salt (6% NaCl) diet, BP increased but was not different between groups. During the initial phase of angiotensin II-dependent hypertension, there was a modest but significant attenuation of hypertension in PCKOs (163±6 mm Hg) compared with controls (178±2 mm Hg; P<0.05) that was associated with enhanced natriuresis and decreased alpha epithelial sodium channel activation in the medulla of PCKOs. However, from day 9 onward, BPs were indistinguishable between groups. Although effects of AT1AR on baseline BP and adaptation to changes in dietary salt are negligible, our studies suggest that direct actions of AT1AR contribute to the initiation of hypertension and epithelial sodium channel activation.

The role of pendrin in blood pressure regulation

Pendrin is a Na(+)-independent Cl(-)/HCO3(-) exchanger found in the apical regions of type B and non-A, non-B intercalated cells within the aldosterone-sensitive region of the nephron, i.e., the distal convoluted tubule (DCT), the connecting tubule (CNT), and the cortical collecting duct (CCD). Type B intercalated cells mediate Cl(-) absorption and HCO3(-) secretion primarily through pendrin-mediated Cl(-)/HCO3(-) exchange. This exchanger is upregulated with angiotensin II administration and in models of metabolic alkalosis, such as following administration of aldosterone or NaHCO3. In the absence of pendrin-mediated HCO3(-) secretion, an enhanced alkalosis is observed following aldosterone or NaHCO3 administration. However, probably of more significance is the role of pendrin in the pressor response to aldosterone. Pendrin mediates Cl(-) absorption and modulates aldosterone-induced Na(+) absorption mediated by the epithelial Na channel (ENaC). Pendrin changes ENaC activity by changing both channel open probability (Po) and surface density (N), at least partly by altering luminal HCO3(-) and ATP concentration. Thus aldosterone and angiotensin II stimulate pendrin expression and function, which stimulates ENaC activity, thereby contributing to the pressor response of these hormones. However, pendrin may modulate blood pressure partly through its extrarenal effects. For example, pendrin is expressed in the adrenal medulla, where it modulates catecholamine release. The increase in catecholamine release observed with pendrin gene ablation likely contributes to the increment in vascular contractile force observed in the pendrin null mouse. This review summarizes the signaling mechanisms that regulate pendrin abundance and function as well as the contribution of pendrin to distal nephron function.

The role of pendrin in renal physiology

Pendrin is a Na(+)-independent Cl(-)/HCO3(-) exchanger that localizes to type B and non-A, non-B intercalated cells, which are expressed within the aldosterone-sensitive region of the nephron, i.e., the distal convoluted tubule, the connecting tubule, and the cortical collecting duct. Type B cells mediate Cl(-) absorption and HCO3(-) secretion primarily through pendrin-mediated Cl(-)/HCO3(-) exchange. At least in some treatment models, pendrin acts in tandem with the Na(+)-dependent Cl(-)/HCO3(-) exchanger (NDCBE) encoded by Slc4a8 to mediate NaCl absorption. The pendrin-mediated Cl(-)/HCO3(-) exchange process is greatly upregulated in models of metabolic alkalosis, such as following aldosterone administration or dietary NaHCO3 loading. It is also upregulated by angiotensin II. In the absence of pendrin [Slc26a4 (-/-) or pendrin null mice], aldosterone-stimulated NaCl absorption is reduced, which lowers the blood pressure response to aldosterone and enhances the alkalosis that follows the administration of this steroid hormone. Pendrin modulates aldosterone-induced Na(+) absorption by changing ENaC abundance and function through a kidney-specific mechanism that does not involve changes in the concentration of a circulating hormone. Instead, pendrin changes ENaC abundance and function at least in part by altering luminal HCO3(-) and ATP concentrations. Thus, aldosterone and angiotensin II also stimulate pendrin expression and function, which likely contributes to the pressor response of these hormones. This review summarizes the contribution of the Cl(-)/HCO3(-) exchanger pendrin in distal nephron function.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Electroneutral absorption of NaCl by the aldosterone-sensitive distal nephron: implication for normal electrolytes homeostasis and blood pressure regulation

Sodium absorption by the distal part of the nephron, i.e., the distal convoluted tubule, the connecting tubule, and the collecting duct, plays a major role in the control of homeostasis by the kidney. In this part of the nephron, sodium transport can either be electroneutral or electrogenic. The study of electrogenic Na(+) absorption, which is mediated by the epithelial sodium channel (ENaC), has been the focus of considerable interest because of its implication in sodium, potassium, and acid-base homeostasis. However, recent studies have highlighted the crucial role played by electroneutral NaCl absorption in the regulation of the body content of sodium chloride, which in turn controls extracellular fluid volume and blood pressure. Here, we review the identification and characterization of the NaCl cotransporter (NCC), the molecule accounting for the main part of electroneutral NaCl absorption in the distal nephron, and its regulators. We also discuss recent work describing the identification of a novel "NCC-like" transport system mediated by pendrin and the sodium-driven chloride/bicarbonate exchanger (NDCBE) in the β-intercalated cells of the collecting system.

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.

Renal angiotensin-converting enzyme and blood pressure control

PURPOSE OF REVIEW

This review presents novel findings regarding the renal angiotensin-converting enzyme (ACE) and its role in blood pressure (BP) control.

RECENT FINDINGS

The textbook flow diagram of the renin-angiotensin system (RAS) shows the pulmonary endothelium as the main source of the ACE that converts angiotensin I to angiotensin II. However, ACE is made in large quantities by the kidneys, which raises the important question of what precisely is the function of renal ACE? Recent studies in gene-targeted mice indicates that renal ACE plays a dominant role in regulating the response of the kidney to experimental hypertension. In particular, renal ACE and locally generated angiotensin II affect the activity of several key sodium transporters and the induction of sodium and water retention resulting in the elevation of BP.

SUMMARY

New experimental data link the renal ACE/angiotensin II pathway and the local regulation of sodium transport as key elements in the development of hypertension.

Targeted proteomics using immunoblotting technique for studying dysregulation of ion transporters in renal disorders

Renal salt and water transport physiology has benefited tremendously from the rapid advance of proteomics. Proteomics developed as a fast-throughput means of screening for global changes in proteins in a selected tissue, organ or cell type, as a logical offshoot of similar comprehensive, messenger RNA array-type technology. Targeted proteomics utilizes similar techniques but examines a predetermined set of proteins. One approach that has been rigorously employed over the last 10 years to evaluate differences in renal protein abundances due to a treatment or genotype has been parallel semiquantitative immunoblotting using antibody arrays. This approach, and newer ones on the horizon, provide a rapid global overview of regulation of the individual proteins whose integrated action determines overall renal sodium or water reabsorption.

Chronic angiotensin II infusion drives extensive aldosterone-independent epithelial Na+ channel activation

The inability of mineralocorticoid receptor (MR) blockade to reduce hypertension associated with high angiotensin (Ang) II suggests direct actions of Ang II to regulate tubular sodium reabsorption via the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. We used freshly isolated aldosterone-sensitive distal nephron from mice to delineate the synergism and primacy between aldosterone and Ang II in controlling functional ENaC activity. Inhibition of MR specifically prevented the increased number of functionally active ENaC, but not ENaC open probability elicited by a low sodium diet. In contrast, we found no functional role of glucocorticoid receptors in the regulation of ENaC activity by dietary salt intake. Simultaneous inhibition of MR and Ang II type 1 receptors ameliorated the enhanced ENaC activity caused by low dietary salt intake and produced significantly greater natriuresis than either inhibitor alone. Chronic systemic Ang II infusion induced more than 2 times greater increase in ENaC activity than observed during dietary sodium restriction. Importantly, ENaC activity remained greatly above control levels during maximal MR inhibition. We conclude that during variations in dietary salt intake both aldosterone and Ang II contribute complementarily to the regulation of ENaC activity in the aldosterone-sensitive distal nephron. In contrast, in the setting of Ang II-dependent hypertension, ENaC activity is upregulated well above the physiological range and is not effectively suppressed by inhibition of the aldosterone-MR axis. This provides a mechanistic explanation for the resistance to MR inhibition that occurs in hypertensive subjects having elevated intrarenal Ang II levels.

Angiotensin II-induced hypertension increases plasma membrane Na pump activity by enhancing Na entry in rat thick ascending limbs

Thick ascending limbs (TAL) reabsorb 30% of the filtered NaCl load. Na enters the cells via apical Na-K-2Cl cotransporters and Na/H exchangers and exits via basolateral Na pumps. Chronic angiotensin II (ANG II) infusion increases net TAL Na transport and Na apical entry; however, little is known about its effects on the basolateral Na pump. We hypothesized that in rat TALs Na pump activity is enhanced by ANG II-infusion, a model of ANG II-induced hypertension. Rats were infused with 200 ng·kg(-1)·min(-1) ANG II or vehicle for 7 days, and TAL suspensions were obtained. We studied plasma membrane Na pump activity by measuring changes in 1) intracellular Na (Nai) induced by ouabain; and 2) ouabain-sensitive oxygen consumption (QO2). We found that the ouabain-sensitive rise in Nai in TALs from ANG II-infused rats was 12.8 ± 0.4 arbitrary fluorescent units (AFU)·mg(-1)·min(-1) compared with only 9.9 ± 1.1 AFU·mg(-1)·min(-1) in controls (P < 0.024). Ouabain-sensitive oxygen consumption was 17 ± 5% (P < 0.043) greater in tubules from ANG II-treated than vehicle rats. ANG II infusion did not alter total Na pump expression, the number of Na pumps in the plasma membrane, or the affinity for Na. When furosemide (1.1 mg·kg(-1)·day(-1)) was coinfused with ANG II, no increase in plasma membrane Na pump activity was observed. We concluded that in ANG II-induced hypertension Na pump activity is increased in the plasma membrane of TALs and that this increase is caused by the chronically enhanced Na entry occurring in this model.

Direct activation of ENaC by angiotensin II: recent advances and new insights

Angiotensin II (Ang II) is the principal effector of the renin-angiotensin-aldosterone system (RAAS). It initiates myriad processes in multiple organs integrated to increase circulating volume and elevate systemic blood pressure. In the kidney, Ang II stimulates renal tubular water and salt reabsorption causing antinatriuresis and antidiuresis. Activation of the RAAS is known to enhance activity of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. In addition to its well described stimulatory actions on aldosterone secretion, Ang II is also capable of directly increasing ENaC activity. In this brief review, we discuss recent findings about non-classical Ang II actions on ENaC and speculate about its relevance for renal sodium handling.

The absence of intrarenal ACE protects against hypertension

Activation of the intrarenal renin-angiotensin system (RAS) can elicit hypertension independently from the systemic RAS. However, the precise mechanisms by which intrarenal Ang II increases blood pressure have never been identified. To this end, we studied the responses of mice specifically lacking kidney angiotensin-converting enzyme (ACE) to experimental hypertension. Here, we show that the absence of kidney ACE substantially blunts the hypertension induced by Ang II infusion (a model of high serum Ang II) or by nitric oxide synthesis inhibition (a model of low serum Ang II). Moreover, the renal responses to high serum Ang II observed in wild-type mice, including intrarenal Ang II accumulation, sodium and water retention, and activation of ion transporters in the loop of Henle (NKCC2) and distal nephron (NCC, ENaC, and pendrin) as well as the transporter activating kinases SPAK and OSR1, were effectively prevented in mice that lack kidney ACE. These findings demonstrate that ACE metabolism plays a fundamental role in the responses of the kidney to hypertensive stimuli. In particular, renal ACE activity is required to increase local Ang II, to stimulate sodium transport in loop of Henle and the distal nephron, and to induce hypertension.

Cortical distal nephron Cl(-) transport in volume homeostasis and blood pressure regulation

Renal intercalated cells mediate the secretion or absorption of Cl(-) and OH(-)/H(+) equivalents in the connecting segment (CNT) and cortical collecting duct (CCD). In so doing, they regulate acid-base balance, vascular volume, and blood pressure. Cl(-) absorption is either electrogenic and amiloride-sensitive or electroneutral and thiazide-sensitive. However, which Cl(-) transporter(s) are targeted by these diuretics is debated. While epithelial Na(+) channel (ENaC) does not transport Cl(-), it modulates Cl(-) transport probably by generating a lumen-negative voltage, which drives Cl(-) flux across tight junctions. In addition, recent evidence indicates that ENaC inhibition increases electrogenic Cl(-) secretion via a type A intercalated cells. During ENaC blockade, Cl(-) is taken up across the basolateral membrane through the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) and then secreted across the apical membrane through a conductive pathway (a Cl(-) channel or an electrogenic exchanger). The mechanism of this apical Cl(-) secretion is unresolved. In contrast, thiazide diuretics inhibit electroneutral Cl(-) absorption mediated by a Na(+)-dependent Cl(-)/HCO3(-) exchanger. The relative contribution of the thiazide and the amiloride-sensitive components of Cl(-) absorption varies between studies and probably depends on the treatment model employed. Cl(-) absorption increases markedly with angiotensin and aldosterone administration, largely by upregulating the Na(+)-independent Cl(-)/HCO3(-) exchanger pendrin. In the absence of pendrin [Slc26a4((-/-)) or pendrin null mice], aldosterone-stimulated Cl(-) absorption is significantly reduced, which attenuates the pressor response to this steroid hormone. Pendrin also modulates aldosterone-induced changes in ENaC abundance and function through a kidney-specific mechanism that does not involve changes in the concentration of a circulating hormone. Instead, pendrin changes ENaC abundance and function, at least in part, by altering luminal HCO3(-). This review summarizes mechanisms of Cl(-) transport in CNT and CCD and how these transporters contribute to the regulation of extracellular volume and blood pressure.

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

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

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.

Angiotensin II increases activity of the epithelial Na+ channel (ENaC) in distal nephron additively to aldosterone

Dietary salt intake controls epithelial Na+ channel (ENaC)-mediated Na+ reabsorption in the distal nephron by affecting status of the renin-angiotensin-aldosterone system (RAAS). Whereas regulation of ENaC by aldosterone is generally accepted, little is known about whether other components of RAAS, such as angiotensin II (Ang II), have nonredundant to aldosterone-stimulatory actions on ENaC. We combined patch clamp electrophysiology and immunohistochemistry in freshly isolated split-opened distal nephrons of mice to determine the mechanism and molecular signaling pathway of Ang II regulation of ENaC. We found that Ang II acutely increases ENaC Po, whereas prolonged exposure to Ang II also induces translocation of α-ENaC toward the apical membrane in situ. Ang II actions on ENaC Po persist in the presence of saturated mineralocorticoid status. Moreover, aldosterone fails to stimulate ENaC acutely, suggesting that Ang II and aldosterone have different time frames of ENaC activation. AT1 but not AT2 receptors mediate Ang II actions on ENaC. Unlike its effect in vasculature, Ang II did not increase [Ca2+]i in split-opened distal nephrons as demonstrated using ratiometric Fura-2-based microscopy. However, application of Ang II to mpkCCDc14 cells resulted in generation of reactive oxygen species, as probed with fluorescent methods. Consistently, inhibiting NADPH oxidase with apocynin abolished Ang II-mediated increases in ENaC Po in murine distal nephron. Therefore, we concluded that Ang II directly regulates ENaC activity in the distal nephron, and this effect complements regulation of ENaC by aldosterone. We propose that stimulation of AT1 receptors with subsequent activation of NADPH oxidase signaling pathway mediates Ang II actions on ENaC.

AT1 receptors in the collecting duct directly modulate the concentration of urine

Mice lacking AT(1) angiotensin receptors have an impaired capacity to concentrate the urine, but the underlying mechanism is unknown. To determine whether direct actions of AT(1) receptors in epithelial cells of the collecting duct regulate water reabsorption, we used Cre-Loxp technology to specifically eliminate AT(1A) receptors from the collecting duct in mice (CD-KOs). Although levels of AT(1A) receptor mRNA in the inner medulla of CD-KO mice were significantly reduced, their kidneys appeared structurally normal. Under basal conditions, plasma and urine osmolalities and urine volumes were similar between CD-KO mice and controls. The increase in urine osmolality in response to water deprivation or vasopressin administration, however, was consistently attenuated in CD-KO mice. Similarly, levels of aquaporin-2 protein in inner and outer medulla after water deprivation were significantly lower in CD-KO mice compared with controls, despite its normal localization to the apical membrane. In summary, these results demonstrate that AT(1A) receptors in epithelial cells of the collecting duct directly modulate aquaporin-2 levels and contribute to the concentration of urine.

Angiotensin II acts through the angiotensin 1a receptor to upregulate pendrin

Pendrin is an anion exchanger expressed in the apical regions of B and non-A, non-B intercalated cells. Since angiotensin II increases pendrin-mediated Cl(-) absorption in vitro, we asked whether angiotensin II increases pendrin expression in vivo and whether angiotensin-induced hypertension is pendrin dependent. While blood pressure was similar in pendrin null and wild-type mice under basal conditions, following 2 wk of angiotensin II administration blood pressure was 31 mmHg lower in pendrin null than in wild-type mice. Thus pendrin null mice have a blunted pressor response to angiotensin II. Further experiments explored the effect of angiotensin on pendrin expression. Angiotensin II administration shifted pendrin label from the subapical space to the apical plasma membrane, independent of aldosterone. To explore the role of the angiotensin receptors in this response, pendrin abundance and subcellular distribution were examined in wild-type, angiotensin type 1a (Agtr1a) and type 2 receptor (Agtr2) null mice given 7 days of a NaCl-restricted diet (< 0.02% NaCl). Some mice received an Agtr1 inhibitor (candesartan) or vehicle. Both Agtr1a gene ablation and Agtr1 inhibitors shifted pendrin label from the apical plasma membrane to the subapical space, independent of the Agtr2 or nitric oxide (NO). However, Agtr1 ablation reduced pendrin protein abundance through the Agtr2 and NO. Thus angiotensin II-induced hypertension is pendrin dependent. Angiotensin II acts through the Agtr1a to shift pendrin from the subapical space to the apical plasma membrane. This Agtr1 action may be blunted by the Agtr2, which acts through NO to reduce pendrin protein abundance.

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.

Through a glass darkly: salt transport by the distal tubule

The distal convoluted tubule (DCT) plays a central role in blood pressure and potassium homeostasis, as evidenced by diseases that occur when its function is modified. The paper by van der Lubbe and colleagues makes clear that angiotensin II itself increases the activity and abundance of the thiazide-sensitive Na-Cl cotransporter (NCC), independent of changes in circulating aldosterone. This Commentary provides additional perspective on that work.

Pendrin modulates ENaC function by changing luminal HCO3-

The epithelial Na(+) channel, ENaC, and the Cl(-)/HCO(3)(-) exchanger, pendrin, mediate NaCl absorption within the cortical collecting duct and the connecting tubule. Although pendrin and ENaC localize to different cell types, ENaC subunit abundance and activity are lower in aldosterone-treated pendrin-null mice relative to wild-type mice. Because pendrin mediates HCO(3)(-) secretion, we asked if increasing distal delivery of HCO(3)(-) through a pendrin-independent mechanism "rescues" ENaC function in pendrin-null mice. We gave aldosterone and NaHCO(3) to increase pendrin-dependent HCO(3)(-) secretion within the connecting tubule and cortical collecting duct, or gave aldosterone and NaHCO(3) plus acetazolamide to increase luminal HCO(3)(-) concentration, [HCO(3)(-)], independent of pendrin. Following treatment with aldosterone and NaHCO(3), pendrin-null mice had lower urinary pH and [HCO(3)(-)] as well as lower renal ENaC abundance and function than wild-type mice. With the addition of acetazolamide, however, acid-base balance as well as ENaC subunit abundance and function was similar in pendrin-null and wild-type mice. We explored whether [HCO(3)(-)] directly alters ENaC abundance and function in cultured mouse principal cells (mpkCCD). Amiloride-sensitive current and ENaC abundance rose with increased [HCO(3)(-)] on the apical or the basolateral side, independent of the substituting anion. However, ENaC was more sensitive to changes in [HCO(3)(-)] on the basolateral side of the monolayer. Moreover, increasing [HCO(3)(-)] on the apical and basolateral side of Xenopus kidney cells increased both ENaC channel density and channel activity. We conclude that pendrin modulates ENaC abundance and function, at least in part by increasing luminal [HCO(3)(-)] and/or pH.

Decreased ENaC expression compensates the increased NCC activity following inactivation of the kidney-specific isoform of WNK1 and prevents hypertension

Mutations in WNK1 and WNK4 lead to familial hyperkalemic hypertension (FHHt). Because FHHt associates net positive Na(+) balance together with K(+) and H(+) renal retention, the identification of WNK1 and WNK4 led to a new paradigm to explain how aldosterone can promote either Na(+) reabsorption or K(+) secretion in a hypovolemic or hyperkalemic state, respectively. WNK1 gives rise to L-WNK1, an ubiquitous kinase, and KS-WNK1, a kinase-defective isoform expressed in the distal convoluted tubule. By inactivating KS-WNK1 in mice, we show here that this isoform is an important regulator of sodium transport. KS-WNK1(-/-) mice display an increased activity of the Na-Cl cotransporter NCC, expressed specifically in the distal convoluted tubule, where it participates in the fine tuning of sodium reabsorption. Moreover, the expression of the ROMK and BKCa potassium channels was modified in KS-WNK1(-/-) mice, indicating that KS-WNK1 is also a regulator of potassium transport in the distal nephron. Finally, we provide an alternative model for FHHt. Previous studies suggested that the activation of NCC plays a central role in the development of hypertension and hyperkalemia. Even though the increase in NCC activity in KS-WNK1(-/-) mice was less pronounced than in mice overexpressing a mutant form of WNK4, our study suggests that the activation of Na-Cl cotransporter is not sufficient by itself to induce a hyperkalemic hypertension and that the deregulation of other channels, such as the Epithelial Na(+) channel (ENaC), is probably required.

Upregulation of renal sodium transporters in D5 dopamine receptor-deficient mice

D(5) dopamine receptor (D(5)R)-deficient (D(5)(-/-)) mice have hypertension that is aggravated by an increase in sodium intake. The present experiments were designed to test the hypothesis that a dysregulation of renal sodium transporters is related to the salt sensitivity in D(5)(-/-) mice. D(5)R was expressed in the renal proximal tubule, thick ascending limb, distal convoluted tubule, and cortical and outer medullary collecting ducts in D(5)(+/+) mice. On a control Na(+) diet, renal protein expressions of NKCC2 (sodium-potassium-2 chloride cotransporter), sodium chloride cotransporter, and alpha and gamma subunits of the epithelial sodium channel were greater in D(5)(-/-) than in D(5)(+/+) mice. Renal renin abundance and urine aldosterone levels were similar but renal angiotensin II type 1 receptor (AT(1)R) protein expression was increased in D(5)(-/-) mice. An elevated Na(+) diet increased further the elevated blood pressure of D(5)(-/-) mice but did not affect the normal blood pressure of D(5)(+/+) mice. The increased levels of NKCC2, sodium chloride cotransporter, and alpha and gamma subunits of the epithelial sodium channel persisted with the elevated Na(+) diet and unaffected by chronic AT(1)R blockade (losartan) in D(5)(-/-) mice. The expressions of proximal sodium transporters NHE3 (sodium hydrogen exchanger type 3) and NaPi2 (sodium phosphate cotransporter type 2) were increased by the elevated Na(+) diet in D(5)(-/-) mice; the increased expression of NHE3 but not NaPi2 was abolished by AT(1)R blockade. Our findings suggest that the increased protein expression of sodium transporters/channels in distal nephron segments may be the direct consequence of the disruption of D(5)R, independent of the renin-angiotensin aldosterone system.

Candesartan Differentially Regulates Epithelial Sodium Channel in Cortex Versus Medulla of Streptozotocin-Induced Diabetic Rats

Diabetes is associated with an activated renal renin-angiotensin-aldosterone system (RAAS) and it was shown that streptozotocin (STZ)-induced diabetic rats had increased whole kidney protein levels of the epithelial sodium channel subunits (α-, β- and γ-ENaC). However, the role of the RAAS on the regional, i.e., cortical versus medullary, regulation of ENaC is unclear. Male Sprague-Dawley rats were injected with STZ (intravenous, 65 mg/kg·bw, n=12/group). After 14 days, half of them received drinking water with candesartan (2 mg/kg·bw/day), an angiotensin-II type-1 receptor (AT1R) antagonist, for one week. In the medulla, i.e., inner stripe of the outer medulla (ISOM), base and/or tip of the inner medulla, immunoblotting revealed increased protein abundances of α1 Na-K-ATPase and ENaC subunits with diabetes (200-600% of controls), which were not reversed by candesartan. In fact, candesartan increased all ENaC subunits and α1 Na-K-ATPase in the ISOM and/or base in control rats. In contrast, in the cortex, diabetes did not increase these proteins. However, candesartan reduced cortical β- and γ-ENaC regardless of diabetic state. In summary, diabetes-induced increases in ENaC were seen preferentially in the medulla. These changes appeared to be due to a mechanism clearly distinct from AT1R activation, because they were not abolished by candesartan. In fact, candesartan treatment tended to increase some of these medullary proteins, perhaps in compensation for increased NaCl load. In contrast, cortical β- and γ-ENaC were reduced by candesartan regardless of diabetic state suggesting their regulation by AT1R at this site; however this did not appear to be a site of diabetes-induced ENaC up-regulation.

Angiotensin II regulation of renal vascular ENaC proteins

BACKGROUND

The importance of beta and gamma epithelial Na(+) channel (ENaC) proteins in vascular smooth muscle cell (VSMC)-mediated pressure-induced constriction in renal interlobar arteries has been demonstrated recently. In renal epithelial tissue, ENaC expression is regulated by angiotensin II (Ang II). However, whether Ang II regulates vascular ENaC expression has never been determined. Therefore, the goal of the current investigation was to determine whether Ang II affects vascular ENaC expression and its contribution to pressure-induced constriction.

METHODS

To address this goal, Sprague-Dawley rats were infused with Ang II (50 ng/kg/min) via osmotic minipump for 1 week. Mean arterial pressure (MAP) was measured using radiotelemetry. Interlobar arteries were isolated from these animals to assess VSMC ENaC protein expression, pressure-induced constriction, and agonist induced vascular reactivity.

RESULTS

MAP was not different in control (113 +/- 2 mm Hg) and Ang II- (114 +/- 2 mm Hg) infused mice. We found that Ang II infusion decreased renal VSMC beta and gammaENaC immunolabeling by 18%. Consistent with this finding, we also found that ENaC-dependent peak pressure-induced constriction was inhibited from 38 +/- 3% to 25 +/- 1% at 125 mm Hg. Vasoreactivity to KCl, phenylephrine (PE), and acetylcholine (ACh) was unchanged.

CONCLUSIONS

Ang II suppression of pressure-induced constrictor responses in renal interlobar arteries may be mediated, at least in part, by inhibition of beta and gammaENaC protein expression.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Losartan chemistry and its effects via AT1 mechanisms in the kidney

Besides the importance of the renin-angiotensin system (RAS) in the circulation and other organs, the local RAS in the kidney has attracted a great attention in research in last decades. The renal RAS plays an important role in the body fluid homeostasis and long-term cardiovascular regulation. All major components and key enzymes for the establishment of a local RAS as well as two important angiotensin II (Ang II) receptor subtypes, AT1 and AT2 receptors, have been confirmed in the kidney. In additional to renal contribution to the systemic RAS, the intrarenal RAS plays a critical role in the regulation of renal function as well as in the development of kidney disease. Notably, kidney AT1 receptors locating at different cells and compartments inside the kidney are important for normal renal physiological functions and abnormal pathophysiological processes. This mini-review focuses on: 1) the local renal RAS and its receptors, particularly the AT1 receptor and its mechanisms in physiological and pathophysiological processes; and 2) the chemistry of the selective AT1 receptor blocker, losartan, and the potential mechanisms for its actions in the renal RAS-mediated disease.

Chronic candesartan alters expression and activity of NKCC2, NCC, and ENaC in the obese Zucker rat

The obese Zucker rat reportedly has increased activity of the intrarenal renin-angiotensin-aldosterone system, which conceptually could contribute to elevated salt sensitivity and blood pressure (BP). Our aim was to determine whether there was increased angiotensin II type 1 receptor (AT(1)R)-mediated upregulation of expression or activity of the bumetanide-sensitive Na-K-2Cl cotransporter, the thiazide-sensitive Na-Cl cotransporter (NCC), and/or the epithelial sodium channel (ENaC) in obese vs. lean Zucker rats. Male obese and lean Zucker rats (10-wk old) were fed either 1) control chow (1% NaCl) or 2) chow with candesartan (CAN), an AT(1)R antagonist (25 mg/kg.diet) for 14 wk (n = 8/treatment/body type). BP measured by radiotelemetry, was markedly reduced by CAN ( approximately 20-25 mmHg) in both lean and obese rats with no body-type differences. Obese rats had significantly greater net natriuretic response to single injections of hydrochlorothiazide and benzamil, suggesting increased activity of NCC and ENaC, respectively; however, only the response to benzamil was reduced by CAN. CAN led to a significant reduction in whole kidney levels of NCC and gamma-ENaC (70-kDa band) in both lean and obese rats. However, it significantly increased alpha-ENaC and Na-K-2Cl cotransporter levels, and these increases were greater in obese rats. These studies suggest that relatively increased ENaC, but not NCC activity, in obese rats is due to enhanced AT(1)R activity. CAN attenuated the reduction of several renal transporters in the obese rat kidney. Finally, differences in intrarenal AT(1)R activity do not seem directly responsible for BP differences between lean and obese rats.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Reduced ENaC protein abundance contributes to the lower blood pressure observed in pendrin-null mice

Pendrin (encoded by Pds, Slc26a4) is a Cl(-)/HCO(3)(-) exchanger expressed in the apical regions of type B and non-A, non-B intercalated cells of kidney and mediates renal Cl(-) absorption, particularly when upregulated. Aldosterone increases blood pressure by increasing absorption of both Na(+) and Cl(-) through increased protein abundance and function of Na(+) transporters, such as the epithelial Na(+) channel (ENaC) and the Na(+)-Cl(-) cotransporter (NCC), as well as Cl(-) transporters, such as pendrin. Because aldosterone analogs do not increase blood pressure in Slc26a4(-/-) mice, we asked whether Na(+) excretion and Na(+) transporter protein abundance are altered in kidneys from these mutant mice. Thus wild-type and Slc26a4-null mice were given a NaCl-replete, a NaCl-restricted, or NaCl-replete diet and aldosterone or aldosterone analogs. Abundance of the major renal Na(+) transporters was examined with immunoblots and immunohistochemistry. Slc26a4-null mice showed an impaired ability to conserve Na(+) during dietary NaCl restriction. Under treatment conditions in which circulating aldosterone is increased, alpha-, beta-, and 85-kDa gamma-ENaC subunit protein abundances were reduced 15-35%, whereas abundance of the 70-kDa fragment of gamma-ENaC was reduced approximately 70% in Slc26a4-null relative to wild-type mice. Moreover, ENaC-dependent changes in transepithelial voltage were much lower in cortical collecting ducts from Slc26a4-null than from wild-type mice. Thus, in kidney, ENaC protein abundance and function are modulated by pendrin or through a pendrin-dependent downstream event. The reduced ENaC protein abundance and function observed in Slc26a4-null mice contribute to their lower blood pressure and reduced ability to conserve Na(+) during NaCl restriction.

Novel roles of intracrine angiotensin II and signalling mechanisms in kidney cells

Angiotensin II (Ang II) has powerful sodium-retaining, growth-promoting and pro- inflammatory properties in addition to its physiological role in maintaining body salt and fluid balance and blood pressure homeostasis. Increased circulating and local tissue Ang II is one of the most important factors contributing to the development of sodium and fluid retention, hypertension and target organ damage. The importance of Ang II in the pathogenesis of hypertension and target organ injury is best demonstrated by the effectiveness of angiotensin- converting enzyme (ACE) inhibitors and AT1-receptor antagonists in treating hypertension and progressive renal disease including diabetic nephropathy. The detrimental effects of Ang II are mediated primarily by the AT1-receptor, while the AT2-receptor may oppose the AT1-receptor. The classical view of the AT1-receptor-mediated effects of Ang II is that the agonist binds its receptors at the cell surface, and following receptor phosphorylation, activates downstream signal transduction pathways and intracellular responses. However, evidence is emerging that binding of Ang II to its cell surface AT1-receptors also activates endocytotic (or internalisation) processes that promote trafficking of both the effector and the receptor into intracellular compartments. Whether internalised Ang II has important intracrine and signalling actions is not well understood. The purpose of this article is to review recent advances in Ang II research with focus on the mechanisms underlying high levels of intracellular Ang II in proximal tubule cells and the contribution of receptor-mediated endocytosis of extracellular Ang II. Further attention is devoted to the question whether intracellular and/or internalised Ang II plays a physiological role by activating cytoplasmic or nuclear receptors in proximal tubule cells. This information may aid future development of drugs to prevent and treat Ang II-induced target organ injury in cardiovascular and renal diseases by blocking intracellular and/or nuclear actions of Ang II.