Elements of a paracrine tubular renin-angiotensin system along the entire nephron

Collecting Duct Renin Does Not Mediate DOCA-Salt Hypertension or Renal Injury

Collecting duct (CD)-derived renin is involved in the hypertensive response to chronic angiotensin-II (Ang-II) administration. However, whether CD renin is involved in Ang-II independent hypertension is currently unknown. To begin to examine this, 12 week old male and female CD-specific renin knock out (KO) mice and their littermate controls were subjected to uni-nephrectomy followed by 2 weeks of deoxycorticosterone acetate (DOCA) infusion combined with a high salt diet. Radiotelemetric blood pressure (BP) was similar between KO and control mice at baseline; BP increased in both groups to a similar degree throughout the 2 weeks of DOCA-salt treatment. Urinary albumin excretion and plasma blood urea nitrogen were comparable between the two groups after DOCA-salt treatment. Fibrosis as assessed by Masson’s Trichrome stain/Sirius Red stain and collagen-1 mRNA expression was similar between control and KO mice. Compared to baseline, DOCA-salt treatment decreased plasma renin concentration (PRC), urinary renin excretion and medullary renin mRNA expression in both floxed and CD renin KO mice with no detectable differences between the two groups. Further, in primary culture of rat inner medullary CD, aldosterone treatment did not change renin activity or total renin content. Taken together, these data suggest that CD derived renin does not play a role in DOCA-salt hypertension.

Hyperaldosteronism and cardiovascular risk in patients with autosomal dominant polycystic kidney disease

Hypertension is commonly associated with autosomal dominant polycystic kidney disease (ADPKD), often discovered before the onset of renal failure, albeit the pathogenetic mechanisms are not well elucidated. Hyperaldosteronism in ADPKD may contribute to the development of insulin resistance and endothelial dysfunction, and progression of cardiorenal disease. The aim of study was to evaluate the prevalence of primary aldosteronism (PA) in ADPKD patients and identify some surrogate biomarkers of cardiovascular risk.We have enrolled 27 hypertensive ADPKD patients with estimated glomerular filtration rate (eGFR) ≥ 60 mL/min, evaluating the renin-angiotensin-aldosterone system (RAAS), inflammatory indexes, nutritional status, homocysteine (Hcy), homeostasis model assessment-insulin resistance (HOMA-IR), mineral metabolism, microalbuminuria, and surrogate markers of atherosclerosis [carotid intima media thickness (cIMT), ankle/brachial index (ABI), flow mediated dilation (FMD), renal resistive index (RRI) and left ventricular mass index (LVMI)]. Furthermore, we have carried out the morpho-functional magnetic resonance imaging (MRI) with high-field 3 T Magnetom Avanto.We have divided patients into group A, with normal plasma aldosterone concentration (PAC) and group B with PA, present in 9 (33%) of overall ADPKD patients. Respect to group A, group B showed a significant higher mean value of LVMI, HOMA-IR and Hcy (P = 0.001, P = 0.004, P = 0.018; respectively), and a lower value of FMD and 25-hydroxyvitamin D (25-OH-VitD) (P = 0.037, P = 0.019; respectively) with a higher prevalence of non-dipper pattern at Ambulatory Blood Pressure Monitoring (ABPM) (65% vs 40%, P < 0.05) at an early stage of the disease.In this study, we showed a high prevalence of PA in ADPKD patients, associated to higher LVMI, HOMA-IR, Hcy, lower FMD, and 25-OH-VitD, considered as surrogate markers of atherosclerosis, compared to ADPKD patients with normal PAC values. Our results indicate a higher overall cardiovascular risk in ADPKD patients with inappropriate aldosterone secretion, and a screening for PA in all patients with ADPKD is recommended.

Calcineurin-inhibition Results in Upregulation of Local Renin and Subsequent Vascular Endothelial Growth Factor Production in Renal Collecting Ducts

BACKGROUND

Tacrolimus (Tac) and Cyclosporine A (CyA) calcineurin inhibitors (CNIs) are 2 effective immunosuppressants which are essential to prevent allograft rejection. Calcineurin inhibitors are known to be nephrotoxic. However, the precise mechanism of nephrotoxicity is not fully understood. In this study, we investigated the in vivo effects of CNIs on the local renal renin-angiotensin system in the collecting duct (CD).

METHODS

Three-week-old mice were treated with either vehicle, CyA (2 mg/kg per day), Tac (0.075 mg/kg per day), CyA + Aliskiren (25 mg/kg per day), or Tac + Aliskiren for 3 weeks. Serum creatinine was measured. Renin and vascular endothelial growth factor (VEGF) contents in CD were evaluated with flow cytometry and multiphoton microscopy. The diameter of vessels was assessed with multiphoton microscopy, and the amount of renal collagen was determined by real-time polymerase chain reaction and Masson staining.

RESULTS

The elevated level of serum creatinine in CNI groups was abolished by Aliskiren. Flow cytometric analysis found elevated renin content in principal cells, which was prevented by Aliskiren. This result was further confirmed with multiphoton microscopy. The VEGF content in CD correlated with reduced capillary diameter and with the formation of fibrotic islands.

CONCLUSIONS

Calcineurin inhibitors induce production of renin in the CD that may contribute to decreased renal blood flow. In turn, CD responds with increased VEGF production, resulting in disproportional vessel growth, further worsening the local hypoxia and striped fibrosis surrounding the CDs. Aliskiren, a direct renin inhibitor blocks these effects and improves CNI-induced nephropathy by decreasing renin production in the CDs. Our data suggest that Aliskiren may be used for the prevention of CNI nephrotoxicity.

20-Hydroxyeicosatetraenoic Acid (HETE)-dependent Hypertension in Human Cytochrome P450 (CYP) 4A11 Transgenic Mice: NORMALIZATION OF BLOOD PRESSURE BY SODIUM RESTRICTION, HYDROCHLOROTHIAZIDE, OR BLOCKADE OF THE TYPE 1 ANGIOTENSIN II RECEPTOR

Male and female homozygous 129/Sv mice carrying four copies of the human cytochrome P450 4A11 gene (CYP4A11) under control of its native promoter (B-129/Sv-4A11(+/+)) develop hypertension (142 ± 8 versus 113 ± 7 mm Hg systolic blood pressure (BP)), and exhibit increased 20-hydroxyeicosatetraenoic acid (20-HETE) in kidney and urine. The hypertension is reversible by a low-sodium diet and by the CYP4A inhibitor HET0016. B-129/Sv-4A11(+/+) mice display an 18% increase of plasma potassium (p < 0.02), but plasma aldosterone, angiotensin II (ANGII), and renin activities are unchanged. This phenotype resembles human genetic disorders with elevated activity of the sodium chloride co-transporter (NCC) and, accordingly, NCC abundance is increased by 50% in transgenic mice, and NCC levels are normalized by HET0016. ANGII is known to increase NCC abundance, and renal mRNA levels of its precursor angiotensinogen are increased 2-fold in B-129/Sv-4A11(+/+), and blockade of the ANGII receptor type 1 with losartan normalizes BP. A pro-hypertensive role for 20-HETE was implicated by normalization of BP and reversal of renal angiotensin mRNA increases by administration of the 20-HETE antagonists 2-((6Z,15Z)-20-hydroxyicosa-6,15-dienamido)acetate or (S)-2-((6Z,15Z)-20-hydroxyicosa-6,15-dienamido)succinate. SGK1 expression is also increased in B-129/Sv-4A11(+/+) mice and paralleled increases seen for NCC. Losartan, HET0016, and 20-HETE antagonists each normalized SGK1 mRNA expression. These results point to a potential 20-HETE dependence of intrarenal angiotensinogen production and ANGII receptor type 1 activation that are associated with increases in NCC and SGK1 and identify elevated P450 4A11 activity and 20-HETE as potential risk factors for salt-sensitive human hypertension by perturbation of the renal renin-angiotensin axis.

Relative roles of principal and intercalated cells in the regulation of sodium balance and blood pressure

The kidney continuously adapts daily renal excretion of NaCl to match dietary intakes in order to maintain the NaCl content of the body, and keep vascular volume constant. Any situation that leads to NaCl retention favors a rise in blood pressure. The aldosterone-sensitive distal nephron, which contains two main types of cells, principal (PC) and intercalated (IC) cells, is an important site for the final regulation of urinary Na(+) excretion. Research over the past 20 years established a paradigm in which PCs are the exclusive site of Na(+) absorption while ICs are solely dedicated to acid-base transport. Recent studies have revealed the unexpected importance of ICs for NaCl reabsorption. Here, we review the mechanisms of Na(+) and Cl(-) transport in the aldosterone-sensitive distal nephron, with emphasis on the role of ICs in maintaining NaCl balance and normal blood pressure.

A local renal renin-angiotensin system activation via renal uptake of prorenin and angiotensinogen in diabetic rats

The mechanism of activation of local renal renin-angiotensin system (RAS) has not been clarified in diabetes mellitus (DM). We hypothesized that the local renal RAS will be activated via increased glomerular filtration and tubular uptake of prorenin and angiotensinogen in diabetic kidney with microalbuminuria. Streptozotocin (STZ)-induced DM and control rats were injected with human prorenin and subsequently with human angiotensinogen. Human prorenin uptake was increased in podocytes, proximal tubules, macula densa, and cortical collecting ducts of DM rats where prorenin receptor (PRR) was expressed. Co-immunoprecipitation of kidney homogenates in DM rats revealed binding of human prorenin to the PRR and to megalin. The renal uptake of human angiotensinogen was increased in DM rats at the same nephron sites as prorenin. Angiotensin-converting enzyme was increased in podocytes, but decreased in the proximal tubules in DM rats, which may have contributed to unchanged renal levels of angiotensin despite increased angiotensinogen. The systolic blood pressure increased more after the injection of 20 μg of angiotensinogen in DM rats than in controls, accompanied by an increased uptake of human angiotensinogen in the vascular endothelium. In conclusion, endocytic uptake of prorenin and angiotensinogen in the kidney and vasculature in DM rats was contributed to increased tissue RAS and their pressor response to angiotensinogen.

Activation of ENaC in collecting duct cells by prorenin and its receptor PRR: involvement of Nox4-derived hydrogen peroxide

The collecting duct (CD) has been recognized as an important source of prorenin/renin, and it also expresses (pro)renin receptor (PRR). The goal of this study was to examine the hypothesis that prorenin or renin via PRR regulates epithelial Na(+) channel (ENaC) activity in mpkCCD cells. Transepithelial Na(+) transport was measured by using a conventional epithelial volt-ohmmeter and was expressed as the calculated equivalent current (Ieq). Amiloride-inhibitable Ieq was used as a reflection of ENaC activity. Administration of prorenin in the nanomolar range induced a significant increase in Ieq that was detectable as early as 1 min, peaked at 5 min, and gradually returned to baseline within 15 min. These changes in Ieq were completely prevented by a newly developed PRR decoy inhibitor, PRO20. Prorenin-induced Ieq was inhibitable by amiloride. Compared with prorenin, renin was less effective in stimulating Ieq Prorenin-induced Ieq was attenuated by apocynin but enhanced by tempol, the latter effect being prevented by catalase. In response to prorenin treatment, the levels of total reactive oxygen species and H2O2 were both increased, as detected by spin-trap analysis and reactive oxygen species (ROS)-Glo H2O2 assay, respectively. Both siRNA-mediated Nox4 knockdown and the dual Nox1/4 inhibitor GKT137892 attenuated prorenin-induced Ieq Overall, our results demonstrate that activation of PRR by prorenin stimulates ENaC activity in CD cells via Nox4-derived H2O2.

Vasopressin/V2 receptor stimulates renin synthesis in the collecting duct

Renin is synthesized in the principal cells of the collecting duct (CD), and its production is increased via cAMP in angiotensin (ANG) II-dependent hypertension, despite suppression of juxtaglomerular (JG) renin. Vasopressin, one of the effector hormones of the renin-angiotensin system (RAS) via the type 2-receptor (V2R), activates the cAMP/PKA/cAMP response element-binding protein (CREB) pathway and aquaporin-2 expression in principal cells of the CD. Accordingly, we hypothesized that activation of V2R increases renin synthesis via PKA/CREB, independently of ANG II type 1 (AT1) receptor activation in CD cells. Desmopressin (DDAVP; 10(-6) M), a selective V2R agonist, increased renin mRNA (∼3-fold), prorenin (∼1.5-fold), and renin (∼2-fold) in cell lysates and cell culture media in the M-1 CD cell line. Cotreatment with DDAVP+H89 (PKA inhibitor) or CREB short hairpin (sh) RNA prevented this response. H89 also blunted DDAVP-induced CREB phosphorylation and nuclear localization. In 48-h water-deprived (WD) mice, prorenin-renin protein levels were increased in the renal inner medulla (∼1.4- and 1.8-fold). In WD mice treated with an ACE inhibitor plus AT1 receptor blockade, renin mRNA and prorenin protein levels were still higher than controls, while renin protein content was not changed. In M-1 cells, ANG II or DDAVP increased prorenin-renin protein levels; however, there were no further increases by combined treatment. These results indicate that in the CD the activation of the V2R stimulates renin synthesis via the PKA/CREB pathway independently of RAS, suggesting a critical role for vasopressin in the regulation of renin in the CD.

Circulating and intrarenal renin-angiotensin systems in healthy men and nonpregnant women

The urinary excretion of renin-angiotensin system (RAS) proteins could reflect the activity of the intrarenal RAS. We hypothesized that the rates of excretion of RAS components into human urine are independent of circulating levels of these proteins and reflect the intrarenal RAS. There are no reports of the simultaneous measurement of prorenin, active renin, angiotensinogen (AGT), and angiotensin-converting enzyme (ACE) excretion in healthy individuals. Therefore, we measured plasma prorenin, ACE, and AGT and urinary renin (uRenin), prorenin (uProrenin), ACE (uACE), and AGT (uAGT) in men and nonpregnant women. Plasma (p) AGT was higher in women then men. Women who were taking estrogen had significantly higher pAGT. In women, pProrenin was negatively correlated with pAGT. There were no correlations between pProrenin, pAGT, and pACE and their urinary counterparts in either men or women. In men, uProrenin/creatinine ratios were lower than in women. There was no effect of estrogen use on urinary excretion of pProrenin, renin, AGT, and ACE. In men, there were significant correlations between uACE/creat and uRen/creat and uAGT/creat; uProrenin/creat and plasma cystatin C levels; and uRenin/creat and uNa/K were also positively correlated. No associations were found in women. In conclusion, urinary excretion of prorenin is sexually dimorphic and is not affected by estrogen use in women. Our data also suggest that the relationship between renal handling of sodium and urinary renin is sexually dimorphic. Since we found no associations between plasma RAS proteins and their urinary counterparts, and the ratio of uProrenin:pProrenin was strikingly different between men and women, levels of urinary RAS proteins in individuals with normal kidney function are most likely the result of tubular secretion, rather than ultrafiltration.

Deficiency of angiotensinogen in hepatocytes markedly decreases blood pressure in lean and obese male mice

We recently demonstrated that adipocyte deficiency of angiotensinogen (AGT) ablated high-fat diet-induced elevations in plasma angiotensin II (Ang II) concentrations and obesity-hypertension in male mice. Hepatocytes are the predominant source of systemic AGT. Therefore, in this study, we defined the contribution of hepatocyte-derived AGT to obesity-induced elevations in plasma AGT concentrations and hypertension. Male Agt(fl/fl) mice expressing albumin-driven Cre recombinase were bred to female Agt(fl/fl) mice to generate Agt(fl/fl) or hepatocyte AGT-deficient male mice (Agt(Alb)). Mice were fed a low-fat or high-fat diet for 16 weeks. Hepatocyte AGT deficiency had no significant effect on body weight. Plasma AGT concentrations were increased in obese Agt(fl/fl) mice. Hepatocyte AGT deficiency markedly reduced plasma AGT and Ang II concentrations in lean and obese mice. Moreover, hepatocyte AGT deficiency reduced the content and release of AGT from adipose explants. Systolic blood pressure was markedly decreased in lean (by 18 mm Hg) and obese Agt(Alb) mice (by 54 mm Hg) compared with Agt(fl/fl) controls. To define mechanisms, we quantified effects of Ang II on mRNA abundance of megalin, an AGT uptake transporter, in 3T3-L1 adipocytes. Ang II stimulated adipocyte megalin mRNA abundance and decreased media AGT concentrations. These results demonstrate that hepatocytes are the predominant source of systemic AGT in both lean and obese mice. Moreover, reductions in plasma angiotensin concentrations in obese hepatocyte AGT-deficient mice may have limited megalin-dependent uptake of AGT into adipocytes for the production of Ang II in the development of obesity-hypertension.

Renin and the (pro)renin receptor in the renal collecting duct: Role in the pathogenesis of hypertension

The intrarenal renin-angiotensin system (RAS) plays a critical role in the pathogenesis and progression of hypertension and kidney disease. In angiotensin (Ang) II-dependent hypertension, collecting duct renin synthesis and secretion are stimulated despite suppression of juxtaglomerular (JG) renin. This effect is mediated by the AngII type I receptor (AT1 R), independent of blood pressure. Although the regulation of JG renin has been extensively studied, the mechanisms by which renin is regulated in the collecting duct remain unclear. The augmentation of renin synthesis and activity in the collecting duct may provide a pathway for additional generation of intrarenal and intratubular AngII formation due to the presence of angiotensinogen substrate and angiotensin-converting enzyme in the nephron. The recently described (pro)renin receptor ((P)RR) binds renin or prorenin, enhancing renin activity and fully activating the biologically inactive prorenin peptide. Stimulation of (P)RR also activates intracellular pathways related to fibrosis. Renin and the (P)RR are augmented in renal tissues of AngII-dependent hypertensive rats. However, the functional contribution of the (P)RR to enhanced renin activity in the collecting duct and its contribution to the development of hypertension and kidney disease have not been well elucidated. This review focuses on recent evidence demonstrating the mechanism of renin regulation in the collecting ducts and its interaction with the (P)RR. The data suggest that renin-(P)RR interactions may induce stimulation of intracellular pathways associated with the development of hypertension and kidney disease.

PKC-α-dependent augmentation of cAMP and CREB phosphorylation mediates the angiotensin II stimulation of renin in the collecting duct

In contrast to the negative feedback of angiotensin II (ANG II) on juxtaglomerular renin, ANG II stimulates renin in the principal cells of the collecting duct (CD) in rats and mice via ANG II type 1 (AT1R) receptor, independently of blood pressure. In vitro data indicate that CD renin is augmented by AT1R activation through protein kinase C (PKC), but the exact mechanisms are unknown. We hypothesize that ANG II stimulates CD renin synthesis through AT1R via PKC and the subsequent activation of cAMP/PKA/CREB pathway. In M-1 cells, ANG II increased cAMP, renin mRNA (3.5-fold), prorenin, and renin proteins, as well as renin activity in culture media (2-fold). These effects were prevented by PKC inhibition with calphostin C, PKC-α dominant negative, and by PKA inhibition. Forskolin-induced increases in cAMP and renin expression were prevented by calphostin C. PKC inhibition and Ca2+ depletion impaired ANG II-mediated CREB phosphorylation and upregulation of renin. Adenylate cyclase 6 (AC) siRNA remarkably attenuated the ANG II-dependent upregulation of renin mRNA. Physiological activation of AC with vasopressin increased renin expression in M-1 cells. The results suggest that the ANG II-dependent upregulation of renin in the CD depends on PKC-α, which allows the augmentation of cAMP production and activation of PKA/CREB pathway via AC6. This study defines the intracellular signaling pathway involved in the ANG II-mediated stimulation of renin in the CD. This is a novel mechanism responsible for the regulation of local renin-angiotensin system in the distal nephron.

Roles of collecting duct renin and (pro)renin receptor in hypertension: mini review

In angiotensin (Ang)-II-dependent hypertension, collecting duct renin synthesis and secretion are stimulated despite suppression of juxtaglomerular (JG) renin. This effect is mediated by Ang II type 1 (AT1) receptor independent of blood pressure. Although the regulation of JG renin is known, the mechanisms by which renin is regulated in the collecting duct are not completely understood. The presence of renin activity in the collecting duct may provide a pathway for intratubular Ang II formation since angiotensinogen substrate and angiotensin converting enzyme are present in the distal nephron. The recently named new member of the renin-angiotensin system (RAS), the (pro)renin receptor [(P)RR], is able to bind renin and the inactive prorenin, thus enhancing renin activity and fully activating prorenin. We have demonstrated that renin and (P)RR are augmented in renal tissues from rats infused with Ang II and during sodium depletion, suggesting a physiological role in intrarenal RAS activation. Importantly, (P)RR activation also causes activation of intracellular pathways associated with increased cyclooxygenase 2 expression and induction of profibrotic genes. In addition, renin and (P)RR are upregulated by Ang II in collecting duct cells. Although the mechanisms involved in their regulation are still under study, they seem to be dependent on the intrarenal RAS activation. The complexities of the mechanisms of stimulation also depend on cyclooxygenase 2 and sodium depletion. Our data suggest that renin and (P)RR can interact to increase intratubular Ang II formation and the activation of profibrotic genes in renal collecting duct cells. Both pathways may have a critical role in the development of hypertension and renal disease.

Early urinary angiotensinogen excretion in critically ill neonates

INTRODUCTION

Urinary angiotensinogen is considered a reliable biomarker for intrarenal renin-angiotensin system activity. The aims of this study were to assess the urinary angiotensinogen level during the first day of life and to evaluate its correlation with renal function in critically ill neonates.

METHODS

Urinary angiotensinogen concentration during the first 24 hours of life was measured in 98 critically ill neonates. Neonatal renal function was assessed by urinary levels of cystatin-C, albumin and α1-microglobulin and urinary electrolyte excretion.

RESULTS

Urinary angiotensinogen level decreased with increasing gestational age and body weight in critically ill neonates (P<0.001). After adjustment for gestational age, urinary angiotensinogen level correlated with urinary fractional excretion of sodium and urinary levels of cystatin-C and α1-microglobulin. Multivariate linear regression identified a significant impact of urinary cystatin-C on urinary angiotensinogen level (P<0.001). Furthermore, urinary angiotensinogen was significantly increased in neonates with a urinary cystatin-C-to-creatinine ratio ⩾2500 ng/mg, which was the optimal cut-off value to predict acute kidney injury in our previous study.

CONCLUSIONS

The urinary angiotensinogen level correlates with the overall maturity of renal function during the early postnatal period in critically ill neonates and an increased urinary angiotensinogen level might reflect renal injury in immature neonates.

Changes in urinary angiotensinogen posttreatment in pediatric IgA nephropathy patients

BACKGROUND

Recently, we demonstrated that urinary angiotensinogen (AGT) levels are increased and reflect intrarenal renin-angiotensin system (RAS) status in pediatric patients with chronic glomerulonephritis. Therefore, this study was performed to test the hypothesis that urinary AGT (UAGT) levels provide a specific index of intrarenal RAS status associated with RAS blockade treatment in pediatric IgA nephropathy (IgAN) patients.

METHODS

We measured plasma and UAGT levels and urinary transforming growth factor beta (TGF-β) levels, after which we performed immunohistochemical analysis of AGT, angiotensin II (Ang II), and TGF-β in 24 pediatric IgAN patients treated with RAS blockades for 2 years. Paired tests were used to analyze the changes from baseline to study end.

RESULTS

Although there was no change in plasma AGT levels, UAGT and TGF-β levels were significantly decreased after RAS blockade, which was accompanied by the expression levels of AGT, Ang II, and TGF-β, as well as the magnitude of glomerular injury. Baseline UAGT levels positively correlated with diastolic blood pressure, urinary protein levels, scores for mesangial hypercellularity, and the expression levels of AGT, Ang II, and TGF-β in renal tissues.

CONCLUSIONS

These data indicate that UAGT is a useful biomarker of intrarenal RAS activation, which is associated with glomerular injury during RAS blockade in pediatric IgAN patients.

Nephron-specific deletion of the prorenin receptor causes a urine concentration defect

The prorenin receptor (PRR), a recently discovered component of the renin-angiotensin system, is expressed in the nephron in general and the collecting duct in particular. However, the physiological significance of nephron PRR remains unclear, partly due to developmental abnormalities associated with global or renal-specific PRR gene knockout (KO). Therefore, we developed mice with inducible nephron-wide PRR deletion using Pax8-reverse tetracycline transactivator and LC-1 transgenes and loxP flanked PRR alleles such that ablation of PRR occurs in adulthood, after induction with doxycycline. Nephron-specific PRR KO mice have normal survival to ∼1 yr of age and no renal histological defects. Compared with control mice, PRR KO mice had 65% lower medullary PRR mRNA and protein levels and markedly diminished renal PRR immunofluorescence. During both normal water intake and mild water restriction, PRR KO mice had significantly lower urine osmolality, higher water intake, and higher urine volume compared with control mice. No differences were seen in urine vasopressin excretion, urine Na(+) and K(+) excretion, plasma Na(+), or plasma osmolality between the two groups. However, PRR KO mice had reduced medullary aquaporin-2 levels and arginine vasopressin-stimulated cAMP accumulation in the isolated renal medulla compared with control mice. Taken together, these results suggest nephron PRR can potentially modulate renal water excretion.

Urinary Angiotensinogen as a Potential Biomarker of Intrarenal Renin-angiotensin System Activity in Chinese Patients with Chronic Kidney Disease

Urinary angiotensinogen (AGT) mainly derives from the AGT produced in the proximal tubular cells. Evidence exists that supports the correlation between urinary AGT and circulating AGT. Previous studies measured urinary AGT by radioimmunoassay which is not convenient for clinical practice. In this study, we utilized an enzyme-linked immunosorbent assay (ELISA) based method to quantify urinary AGT. We analysed the relationship between urinary AGT and intrarenal angiotensin II (Ang II) activity in patients with chronic kidney disease (CKD). Urinary and plasma renin activity, AGT, Ang II and aldosterone were measured by radioimmunoassay or ELISA in 128 CKD patients. Furthermore, expression levels of intrarenal renin, AGT, Ang II and Ang II receptor were examined by immunohistochemistry staining (IHCS) in 72 CKD patients undergoing renal biopsy. Average urinary AGT was 2.02 ± 0.55 ng/(mg Cr). Hypertension, urinary protein, urinary Ang II and urinary Type IV collagen (Col IV) positively correlated with urinary AGT. Estimated glomerular filtration rate (eGFR), urinary sodium and serum AGT negatively correlated with urinary AGT. Multiple regression analysis indicated that low serum AGT, high urinary protein, urinary Ang II and urinary Col IV correlated significantly with high urinary AGT. Moreover, we observed positive correlation between urinary AGT and positive IHCS area of AGT, Ang II and Ang II Type 1 receptor inrenal tissue. These data suggest that urinary AGT might be a potential biomarker of intrarenal angiotensin II activity in CKD patients.

Crosstalk between (Pro)renin receptor and COX-2 in the renal medulla during angiotensin II-induced hypertension

Angiotensin II (AngII) is an octapeptide hormone that plays a central role in regulation of sodium balance, plasma volume, and blood pressure. Its role in the pathogenesis of hypertension is highlighted by the wide use of inhibitors of the renin-angiotensin system (RAS) as the first-line antihypertensive therapy. However, despite intensive investigation, the mechanism of AngII-induced hypertension is still incompletely understood. Although diverse pathways are likely involved, increasing evidence suggests that the activation of intrarenal RAS may represent a dominant mechanism of AngII-induced hypertension. (Pro)renin receptor (PRR), a potential regulator of intrarenal RAS, is expressed in the intercalated cells of the collecting duct (CD) and induced by AngII, in parallel with increased renin in the principal cells of the CD. Activation of PRR elevated PGE2 release and COX-2 expression in renal inner medullary cells whereas COX-2-derived PGE2via the EP4 receptor mediates the upregulation of PRR during AngII infusion, thus forming a vicious cycle. The mutually stimulatory relationship between PRR and COX-2 in the distal nephron may play an important role in mediating AngII-induced hypertension.

Special deLIVERy: podocyte injury promotes renal angiotensin II generation from liver-derived angiotensinogen

The role of the circulating renin-angiotensin system (RAS) in regulating systemic blood pressure and sodium balance is well established. More recently, researchers have turned their focus to the local generation of angiotensin II (Ang II) in specific tissues. Matsusaka et al. revisit the renal RAS and provide evidence that liver-derived angiotensinogen (AGT) is a major determinant of renal Ang II levels in a model of podocyte injury.

Classical Renin-Angiotensin system in kidney physiology

The renin-angiotensin system has powerful effects in control of the blood pressure and sodium homeostasis. These actions are coordinated through integrated actions in the kidney, cardiovascular system and the central nervous system. Along with its impact on blood pressure, the renin-angiotensin system also influences a range of processes from inflammation and immune responses to longevity. Here, we review the actions of the "classical" renin-angiotensin system, whereby the substrate protein angiotensinogen is processed in a two-step reaction by renin and angiotensin converting enzyme, resulting in the sequential generation of angiotensin I and angiotensin II, the major biologically active renin-angiotensin system peptide, which exerts its actions via type 1 and type 2 angiotensin receptors. In recent years, several new enzymes, peptides, and receptors related to the renin-angiotensin system have been identified, manifesting a complexity that was previously unappreciated. While the functions of these alternative pathways will be reviewed elsewhere in this journal, our focus here is on the physiological role of components of the "classical" renin-angiotensin system, with an emphasis on new developments and modern concepts.

Collecting duct intercalated cell function and regulation

Intercalated cells are kidney tubule epithelial cells with important roles in the regulation of acid-base homeostasis. However, in recent years the understanding of the function of the intercalated cell has become greatly enhanced and has shaped a new model for how the distal segments of the kidney tubule integrate salt and water reabsorption, potassium homeostasis, and acid-base status. These cells appear in the late distal convoluted tubule or in the connecting segment, depending on the species. They are most abundant in the collecting duct, where they can be detected all the way from the cortex to the initial part of the inner medulla. Intercalated cells are interspersed among the more numerous segment-specific principal cells. There are three types of intercalated cells, each having distinct structures and expressing different ensembles of transport proteins that translate into very different functions in the processing of the urine. This review includes recent findings on how intercalated cells regulate their intracellular milieu and contribute to acid-base regulation and sodium, chloride, and potassium homeostasis, thus highlighting their potential role as targets for the treatment of hypertension. Their novel regulation by paracrine signals in the collecting duct is also discussed. Finally, this article addresses their role as part of the innate immune system of the kidney tubule.

The intrarenal generation of angiotensin II is required for experimental hypertension

Hypertension is a major risk factor for cardiovascular disease. While the cause of hypertension is multifactorial, renal dysregulation of salt and water excretion is a major factor. All components of the renin-angiotensin system are produced locally in the kidney, suggesting that intrarenal generation of angiotensin II plays a key role in blood pressure regulation. Here, we show that two mouse models lacking renal angiotensin converting enzyme (ACE) are protected against angiotensin II and l-NAME induced hypertension. In response to hypertensive stimuli, mice lacking renal ACE do not produce renal angiotensin II. These studies indicate that the intrarenal renin-angiotensin system works as an entity separate from systemic angiotensin II generation. Renal ACE appears necessary for experimental hypertension.

Clinical relevance of urinary angiotensinogen and renin as potential biomarkers in patients with overt proteinuria

Urinary angiotensinogen (AGT) and renin have been reported to reflect the intrarenal renin-angiotensin system (RAS) activity. However, the adequacy and clinical significance of these markers have not been evaluated in overtly proteinuric patients. In patients with biopsy-proven glomerulonephritis, plasma and urinary AGT and renin were analyzed. A cohort of 75 patients treated with RAS inhibitors was followed for 1 year. Among the 207 patients, 105 had subnephrotic and 102 had nephrotic-range proteinuria. Mean age, estimated glomerular filtration rate (eGFR), and urinary protein-to-creatinine ratio (P/Cr) of all patients were 48 years, 79.7 mL/min/1.73 m(2), and 5.66 mg/mg, respectively. Both natural logarithm of urinary AGT/creatinine (ln [urinary AGT/Cr]) and ln (urinary renin/Cr) showed positive correlations with urinary P/Cr. There was a positive correlation between ln (urinary AGT/Cr) and ln (urinary renin/Cr). Ln (urinary renin/Cr) was not affected by ln (plasma renin) regardless of the degree of proteinuria. The treatment response to RAS inhibitors was greatest in patients with high urinary AGT and renin. However, the predictive value of those parameters was no longer present when the values were adjusted by the degree of proteinuria. Ln (urinary renin/Cr) and initial eGFR were independently associated with the changes in renal function for 1 year. Ln (urinary AGT/Cr) was associated with persistent overt proteinuria after 1 year. Our study suggests that urinary renin may be a better marker in heavy proteinuria, and the treatment response to RAS inhibitors may be enhanced in patients with high urinary renin and AGT. Further studies will be necessary to explore the value of urinary AGT and renin.

Direct regulation of ENaC by bradykinin in the distal nephron. Implications for renal sodium handling

PURPOSE OF REVIEW

Locally produced peptide hormones kinins, such as bradykinin, are thought to oppose many of the prohypertensive actions of the renin-angiotensin-aldosterone system. In the kidney, bradykinin, via stimulation of B2 receptors (B2R), favors natriuresis mostly due to the inhibition of tubular Na reabsorption. Recent experimental evidence identifies the epithelial Na channel (ENaC) as a key end effector of bradykinin actions in the distal tubular segments. The focus of this review is the physiological relevance and molecular details of the bradykinin signal to ENaC.

RECENT FINDINGS

The recent epidemiological GenSalt study demonstrated that genetic variants of the gene encoding B2R show significant associations with the salt sensitivity of blood pressure. Bradykinin was shown to have an inhibitory effect on the distal nephron sodium transport via stimulation of B2 receptor-phospholipase C (B2R-PLC) cascade to decrease ENaC open probability. Genetic ablation of bradykinin receptors in mice led to an augmented ENaC function, particularly during elevated sodium intake, likely contributing to the salt-sensitive hypertensive phenotype. Furthermore, augmentation of bradykinin signaling in the distal nephron was demonstrated to be an important component of the natriuretic and antihypertensive effects of angiotensin converting enzyme inhibition.

SUMMARY

Salt-sensitive inhibition of ENaC activity by bradykinin greatly advances our understanding of the molecular mechanisms that are responsible for shutting down distal tubule sodium reabsorption during volume expanded conditions to avoid salt-sensitive hypertension.

Renal generation of angiotensin II and the pathogenesis of hypertension

The existence of a complete and functional renin-angiotensin system along the nephron is widely recognized. However, its precise role in blood pressure control and, by extension, hypertension is still uncertain. While most investigators agree that overexpressing RAS components along the nephron results in hypertension, two important issues remain: whether the local RAS works as a separate entity or represents an extension of the systemic RAS and whether locally generated angiotensin II has specific renal effects on blood pressure that are distinct from systemic angiotensin II. This review addresses these issues while emphasizing the unique role of local angiotensin II in the response of the kidney to hypertensive stimuli and the induction of hypertension.

Collecting duct-specific knockout of renin attenuates angiotensin II-induced hypertension

The physiological and pathophysiological significance of collecting duct (CD)-derived renin, particularly as it relates to blood pressure (BP) regulation, is unknown. To address this question, we generated CD-specific renin knockout (KO) mice and examined BP and renal salt and water excretion. Mice containing loxP-flanked exon 1 of the renin gene were crossed with mice transgenic for aquaporin-2-Cre recombinase to achieve CD-specific renin KO. Compared with controls, CD renin KO mice had 70% lower medullary renin mRNA and 90% lower renin mRNA in microdissected cortical CD. Urinary renin levels were significantly lower in KO mice (45% of control levels) while plasma renin concentration was significantly higher in KO mice (63% higher than controls) during normal-Na intake. While no observable differences were noted in BP between the two groups with varying Na intake, infusion of angiotensin II at 400 ng·kg(-1)·min(-1) resulted in an attenuated hypertensive response in the KO mice (mean arterial pressure 111 ± 4 mmHg in KO vs. 128 ± 3 mmHg in controls). Urinary renin excretion and epithelial Na(+) channel (ENaC) remained significantly lower in the KO mice following ANG II infusion compared with controls. Furthermore, membrane-associated ENaC protein levels were significantly lower in KO mice following ANG II infusion. These findings suggest that CD renin modulates BP in ANG II-infused hypertension and these effects are associated with changes in ENaC expression.

Nuclear expression of renin-angiotensin system components in NRK-52E renal epithelial cells

INTRODUCTION

Isolated nuclei of sheep proximal tubules express angiotensin (Ang) receptors as well as angiotensinogen (AGT) and renin. The present study characterized the NRK-52E tubular epithelial cell line for the intracellular expression of renin-angiotensin system (RAS) components.

METHODS

RAS components were visualized by immunofluorescent staining in intact cells and protein expression in isolated nuclei.

RESULTS

An antibody to the angiotensin I (Ang I) sequence of AGT (AI-AGT) revealed only cytosolic staining, while an antibody to an internal sequence of AGT (Int-AGT) revealed primarily nuclear staining. Immunoblots of nuclear and cytosolic fractions confirmed the differential cell staining of AGT. Immunostaining for renin was present on nuclei of intact cells. Nuclear renin activity averaged 0.77±0.05 nmol/mg protein/h that was reduced by aliskiren (0.13±0.01 nmol/mg/h, n=3, p<0.01); trypsin activation increased activity three-fold. Peptide staining localized angiotensin II (Ang II) and Ang-(1-7) to the nucleus and peptide content averaged 59±2 and 57±22 fmol/mg (n=4), respectively. Peptide metabolism in isolated nuclei revealed the processing of Ang I to Ang-(1-7) by thimet oligopeptidase.

CONCLUSION

We conclude that the NRK-52E cells express an intracellular RAS localized to the nucleus and may be an appropriate cell model to elucidate the functional relevance of this system.

Prostaglandin E-prostanoid4 receptor mediates angiotensin II-induced (pro)renin receptor expression in the rat renal medulla

Angiotensin II (Ang II) stimulates (pro)renin receptor (PRR) expression in the renal collecting duct, triggering the local renin response in the distal nephron. Our recent study provided evidence for involvement of cyclooxygenase-2-prostaglandin E2 pathway in Ang II-dependent stimulation of PRR expression in the collecting duct. Here, we tested the role of E-prostanoid (EP) subtypes acting downstream of cyclooxygenase-2 in this phenomenon. In primary rat inner medullary collecting duct cells, Ang II treatment for 12 hours induced a 1.8-fold increase in the full-length PRR protein expression. To assess the contribution of EP receptor, the cell was pretreated with specific EP receptor antagonists: SC-51382 (for EP1), L-798106 (for EP3), L-161982 (for EP4), and ONO-AE3-208 (ONO, a structurally distinct EP4 antagonist). The upregulation of PRR expression by Ang II was consistently abolished by L-161982 and ONO and partially suppressed by SC-51382 but was unaffected by L-798106. The PRR expression was also significantly elevated by the EP4 agonist CAY10598 in the absence of Ang II. Sprague-Dawley rats were subsequently infused for 1 or 2 weeks with vehicle, Ang II alone, or in combination with ONO. Ang II infusion induced parallel increases in renal medullary PRR protein and renal medullary and urinary renin activity and total renin content, all of which were blunted by ONO. Both tail cuff plethysmography and telemetry demonstrated attenuation of Ang II hypertension by ONO. Overall, these results have established a crucial role of the EP4 receptor in mediating the upregulation of renal medullary PRR expression and renin activity during Ang II hypertension.

COX-2 mediates angiotensin II-induced (pro)renin receptor expression in the rat renal medulla

(Pro)renin receptor (PRR) is predominantly expressed in the distal nephron where it is activated by angiotensin II (ANG II), resulting in increased renin activity in the renal medulla thereby amplifying the de novo generation and action of local ANG II. The goal of the present study was to test the role of cycloxygenase-2 (COX-2) in meditating ANG II-induced PRR expression in the renal medulla in vitro and in vivo. Exposure of primary rat inner medullary collecting duct cells to ANG II induced sequential increases in COX-2 and PRR protein expression. When the cells were pretreated with a COX-2 inhibitor NS-398, ANG II-induced upregulation of PRR protein expression was almost completely abolished, in parallel with the changes in medium active renin content. The inhibitory effect of NS-398 on the PRR expression was reversed by adding exogenous PGE2. A 14-day ANG II infusion elevated renal medullary PRR expression and active and total renin content in parallel with increased urinary renin, all of which were remarkably suppressed by the COX-2 inhibitor celecoxib. In contrast, plasma and renal cortical active and total renin content were suppressed by ANG II treatment, an effect that was unaffected by COX-2 inhibition. Systolic blood pressure was elevated with ANG II infusion, which was attenuated by the COX-2 inhibition. Overall, the results obtained from in vitro and in vivo studies established a crucial role of COX-2 in mediating upregulation of renal medullary PRR expression and renin content during ANG II hypertension.

Urinary angiotensinogen as a potential biomarker of intrarenal renin-angiotensin system activity in Chinese chronic kidney disease patients

BACKGROUND

Urinary angiotensinogen (AGT) mainly derives from the AGT produced in proximal tubular cells. Evidence exists that supports the correlation between urinary AGT and circulating AGT.

AIM

To investigate the role of urinary AGT as a potential biomarker of intrarenal renin-angiotensin system activity in Chinese chronic kidney disease (CKD) patients.

METHODS

ELISA-based method used to quantify urinary AGT. Analyzed the relationship between urinary AGT and intrarenal angiotensin II (Ang II) activity in 128 CKD patients. ELISA was applied to measure the urinary and plasma renin activity, AGT, Ang II and aldosterone. Furthermore expression levels of intrarenal renin, AGT, Ang II and Ang II receptor were examined by immunohistochemistry staining (IHCS) in 72 CKD patients undergoing renal biopsy.

RESULTS

The logarithmic transformation Log(urinary AGT/UCre) levels showed a normal distribution. Therefore, Log(urinary AGT/UCre) levels were used for the analyses. Average urinary AGT was 2.02 ± 0.55 ng/(mg Cr). Hypertension, urinary protein, urinary Ang II and urinary type IV collagen (Col IV) positively correlated with urinary AGT. Estimated glomerular filtration rate (eGFR), urinary sodium and serum AGT negatively correlated with urinary AGT. Multiple regression analysis indicated that low serum AGT, high urinary protein, urinary Ang II and urinary Col IV correlated significantly with high urinary AGT.

CONCLUSIONS

We observed positive correlation between urinary AGT and positive IHCS area of AGT, Ang II and Ang II type 1 receptor in renal tissue. These data suggest that urinary AGT might be a potential biomarker of intrarenal Ang II activity in CKD patients.

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.

Regulation of luminal acidification by the V-ATPase

Specialized cells in the body express high levels of V-ATPase in their plasma membrane and respond to hormonal and nonhormonal cues to regulate extracellular acidification. Mutations in or loss of some V-ATPase subunits cause several disorders, including renal distal tubular acidosis and male infertility. This review focuses on the regulation of V-ATPase-dependent luminal acidification in renal intercalated cells and epididymal clear cells, which are key players in these physiological processes.

Blood pressure and amiloride-sensitive sodium channels in vascular and renal cells

Sodium transport in the distal nephron is mediated by epithelial sodium channel activity. Proteolytic processing of external domains and inhibition with increased sodium concentrations are important regulatory features of epithelial sodium channel complexes expressed in the distal nephron. By contrast, sodium channels expressed in the vascular system are activated by increased external sodium concentrations, which results in changes in the mechanical properties and function of endothelial cells. Mechanosensitivity and shear stress affect both epithelial and vascular sodium channel activity. Guyton's hypothesis stated that blood pressure control is critically dependent on vascular tone and fluid handling by the kidney. The synergistic effects, and complementary regulation, of the epithelial and vascular systems are consistent with the Guytonian model of volume and blood pressure regulation, and probably reflect sequential evolution of the two systems. The integration of vascular tone, renal perfusion and regulation of renal sodium reabsorption is the central underpinning of the Guytonian model. In this Review, we focus on the expression and regulation of sodium channels, and we outline the emerging evidence that describes the central role of amiloride-sensitive sodium channels in the efferent (vascular) and afferent (epithelial) arms of this homeostatic system.

Translational studies on augmentation of intratubular renin-angiotensin system in hypertension

Various models of experimental hypertension and clinical examples of increased renin formation from a stenotic kidney or a juxtaglomerular cell tumor have shown that increased circulating angiotensin II (Ang II) stimulates the intrarenal/intratubular renin–angiotensin system (RAS) that elicits renal vasoconstriction, enhanced tubular sodium reabsorption, and progressive development of hypertension and renal injury. The enhanced intrarenal Ang II activity is due to both receptor-mediated Ang II uptake and Ang II type 1 (AT1) receptor–mediated stimulation of renal angiotensinogen (AGT) mRNA and protein by proximal tubule cells. The increased AGT secretion leads to local formation of Ang II and spillover of AGT into the distal nephron segments as reflected by increased AGT excretion in the urine, which provides an index of intrarenal RAS activity. In clinical studies, increased urinary excretion of AGT has been demonstrated in hypertension, type 1 and type 2 diabetes mellitus, and several types of chronic kidney diseases. In addition, renin secretion from principal cells of the collecting ducts is increased by AT1 receptor activation and acts on AGT from the proximal tubule to form more Ang I. Renin and/or (pro)renin activity is enhanced by binding to the (pro)renin receptor (PRR) on intercalated cells or secreted as soluble PRR contributing further to AGT cleavage, thus making more substrate available for Ang II conversion by local angiotensin-converting enzyme. The augmented intratubular Ang II concentrations together with elevated renal interstitial Ang II concentrations contribute to sustained stimulation of sodium reabsorption, vasoconstriction, development of hypertension, and progressive renal injury and fibrosis.

Angiotensin II blockade and renal protection

Current national guidelines have recommended the use of renin-angiotensin system inhibitors, including angiotensin II type 1 receptor blockers (ARBs), in preference to other antihypertensive agents for treating hypertensive patients with chronic kidney disease. However, the mechanisms underlying the renoprotective effects of ARBs are multiple and complex. Blood pressure reduction by systemic vasodilation with an ARB contributes to its beneficial effects in treating kidney disease. Furthermore, ARB-induced renal vasodilation results in an increase in renal blood flow, leading to improvement of renal ischemia and hypoxia. ARBs are also effective in reducing urinary albumin excretion through a reduction in intraglomerular pressure and the protection of glomerular endothelium and/or podocyte injuries. In addition to blocking angiotensin II-induced renal cell and tissue injuries, ARBs can decrease intrarenal angiotensin II levels by reducing proximal tubular angiotensinogen and production of collecting duct renin, as well as angiotensin II accumulation in the kidney. In this review, we will briefly summarize our current understanding of the pharmacological effects of an ARB in the kidney. We will also discuss the possible mechanisms responsible for the renoprotective effects of ARBs on type 2 diabetic nephropathy.

Urinary angiotensinogen level is correlated with proteinuria in renal transplant recipients

OBJECTIVE

Along with immunologic mechanisms, intrarenal renin-angiotensin system (RAS) activation has been suggested to play a role in the development and progression of chronic allograft injury. In various glomerular diseases, urinary angiotensinogen (AGT) level is a good indicator for the activation of intrarenal RAS. In this study, we aimed to investigate the parameters associated with urinary AGT level in patients with kidney transplantation.

METHODS

Seventy renal transplant patients with stable graft function (≥ 6 months after transplantation, serum creatinine level <2 mg/dL) and 21 healthy volunteers were included in the study. Patients were taking standard triple immunosuppressive treatment. Demographic characteristics of patients and healthy volunteers, drug use, and 24-hour ambulatory blood pressure measurements were recorded. Morning second urine and fasting blood samples were taken from all participants. Serum biochemical markers and urine Na, K, uric acid, creatinine, and protein levels were measured. Urinary AGT levels were determined by enzyme-linked immunosorbent assay.

RESULTS

Mean systolic and diastolic blood pressures in patients with renal transplantation were higher than in healthy volunteers. Both urinary AGT-urinary creatinine ratio (UAGT/UCr) and urinary protein-urinary creatinine ratio (UPro/UCr) were higher in kidney transplant patients than in healthy volunteers (P < .01; P < .0001; respectively). In patients with renal transplantation, UAGT/UCr was positively correlated with UPro/UCr and negatively correlated with estimated glomerular filtration rate (eGFR) (r = 0.738; P = .01; and r = -0.397; P = .01; respectively). There was no correlation between UAGT/UCr and other study parameters, including bood pressure levels.

CONCLUSIONS

Our findings indicate that high urinary excretion of AGT is associated with proteinuria and lower eGFR in kidney transplant recipients without overt chronic allograft injury. These preliminary results encourage us to design a long-term longitudinal analysis using urinary AGT along with multiple markers to obtain early diagnosis and to predict the prognosis of chronic allograft dysfunction.

Redox signaling is an early event in the pathogenesis of renovascular hypertension

Activation of the renin-angiotensin-aldosterone system plays a critical role in the development of chronic renal damage in patients with renovascular hypertension. Although angiotensin II (Ang II) promotes oxidative stress, inflammation, and fibrosis, it is not known how these pathways intersect to produce chronic renal damage. We tested the hypothesis that renal parenchymal cells are subjected to oxidant stress early in the development of RVH and produce signals that promote influx of inflammatory cells, which may then propagate chronic renal injury. We established a reproducible murine model of RVH by placing a tetrafluoroethylene cuff on the right renal artery. Three days after cuff placement, renal tissue demonstrates no histologic abnormalities despite up regulation of both pro- and anti-oxidant genes. Mild renal atrophy was observed after seven days and was associated with induction of Tnfα and influx of CD3⁺ T cells and F4/80⁺ macrophages. By 28 days, kidneys developed severe renal atrophy with interstitial inflammation and fibrosis, despite normalization of plasma renin activity. Based on these considerations, we propose that renal parenchymal cells initiate a progressive cascade of events leading to oxidative stress, interstitial inflammation, renal fibrosis, and atrophy.

A link between the intrarenal renin angiotensin system and hypertension in autosomal dominant polycystic kidney disease

BACKGROUND/AIMS

Early onset of hypertension and its consequences account for the great majority of deaths in patients with autosomal dominant polycystic kidney disease (ADPKD). Renin-angiotensin system (RAS) components have been shown in ADPKD kidneys independent of systemic RAS. Thus, we examined the urinary angiotensinogen (UAGT) levels as a biomarker of intrarenal RAS status in ADPKD patients with/without hypertension and healthy subjects.

METHODS

Eighty-four ADPKD patients (43 with hypertension and 41 without hypertension) and 40 healthy controls were studied cross-sectionally. Patients with glomerular filtration rate <60 ml/min were excluded from the study. Hypertension was diagnosed with ambulatory blood pressure monitoring. Urinary and plasma concentration of angiotensinogen, spot urine microprotein and creatinine (UCre) levels were recorded for each participant.

RESULTS

UAGT/UCre levels were higher in hypertensive ADPKD patients (23.7 ± 8.4) compared with normotensive ADPKD patients (16.6 ± 5.2) and healthy controls (6.9 ± 3.3; p < 0.001). In univariate analysis, UAGT correlated with systolic blood pressure, diastolic blood pressure (DBP) and proteinuria. The independence of these correlations was analyzed in a regression model, and UAGT was shown to be significantly predicted by proteinuria and DBP.

CONCLUSION

Intrarenal RAS activation which is monitored by UAGT levels clinically may be a harbinger of hypertension and kidney disease in ADPKD patients.

Urinary markers of intrarenal renin-angiotensin system activity in vivo

Recent interest focuses on urinary renin and angiotensinogen as markers of renal renin-angiotensin system activity. Before concluding that these components are independent markers, we need to exclude that their presence in urine, like that of albumin (a protein of comparable size), is due to (disturbed) glomerular filtration. This review critically discusses their filtration, reabsorption and local release. Given the close correlation between urinary angiotensinogen and albumin in human studies, it concludes that, in humans, urinary angiotensinogen is a filtration barrier damage marker with the same predictive power as urinary albumin. In contrast, in animals, tubular angiotensinogen release may occur, although tubulus-specific knockout studies do not support a functional role for such angiotensinogen. Urinary renin levels, relative to albumin, are >200-fold higher and unrelated to albumin. This may reflect release of renin from the urinary tract, but could also be attributed to activation of filtered, plasma-derived prorenin and/or incomplete tubular reabsorption.

Overexpression of Renin in the collecting duct causes elevated blood pressure

BACKGROUND

Renin is synthesized in the collecting duct and is regulated differently than renin in the juxtaglomerular apparatus. However, the physiological relevance of collecting duct renin remains unknown, particularly with regard to its ability to regulate blood pressure.

METHODS

We used gene targeting to generate mice with overexpression of renin in the collecting duct. A conditional mutant mouse line was created with the mouse renin transcript distal to a "transcriptional stop sequence" such that gene expression only occurred when the stop sequence was excised. These mice were bred with mice transgenic for the aquaporin-2 promoter driving Cre recombinase in order to achieve collecting duct-specific overexpression of renin.

RESULTS

RNA analysis confirmed kidney-specific recombination, and medullary renin mRNA levels were increased 5-fold in collecting duct renin mice. Blood pressure was recorded by telemetry and plasma and urine was collected in 24-hour metabolic cages on normal, high-, and low-Na+ diets. Although no significant differences in 24-hour urinary Na+ excretion between targeted and control mice were detected, renin overexpresser mice had elevated blood pressure compared with controls on a high-Na+ diet. Urinary renin excretion was 2-fold higher in targeted mice as compared with controls on normal and low-Na+ diets. Plasma renin concentration was significantly suppressed in targeted mice as compared with controls on normal and high-Na+ diets.

CONCLUSION

Taken together, these results suggest that collecting duct-derived renin has the potential to modulate blood pressure independent of the systemic renin-angiotensin system.

Urinary angiotensinogen as a potential biomarker of severity of chronic kidney diseases

We previously reported that urinary excretion rates of angiotensinogen (AGT) provide a specific index of the activity of the intrarenal renin-angiotensin system in angiotensin II-dependent hypertensive rats. Meanwhile, we have recently developed direct enzyme-linked immunosorbent assays (ELISAs) to measure plasma and urinary AGT in humans. This study was performed to test a hypothesis that urinary AGT levels are enhanced in chronic kidney disease (CKD) patients and correlated with some clinical parameters. Eighty patients with CKD (37 women and 43 men, from 18 to 94 years old) and seven healthy volunteers (two women and five men, from 27 to 43 years old) were included. Plasma AGT levels showed a normal distribution; however, urinary AGT-creatinine ratios (UAGT/UCre) deviated from the normal distribution. When a logarithmic transformation was executed, Log(UAGT/UCre) levels showed a normal distribution. Therefore, Log(UAGT/UCre) levels were used for further analyses. Log(UAGT/UCre) levels were not correlated with age, gender, height, body weight, body mass index, systolic blood pressure, diastolic blood pressure, serum sodium levels, serum potassium levels, urinary sodium-creatinine ratios, plasma renin activity, or plasma AGT levels. However, Log(UAGT/UCre) levels were significantly correlated positively with urinary albumin-creatinine ratios, fractional excretion of sodium, urinary protein-creatinine ratios, and serum creatinine, and correlated negatively with estimated glomerular filtration rate. Log(UAGT/UCre) levels were significantly increased in CKD patients compared with control subjects (1.8801 +/- 0.0885 vs. 0.9417 +/- 0.1048; P = .0024). These data confirmed our earlier report and showed that a new ELISA assay is a valid approach for measuring urinary AGT.

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 cortical collecting duct plays a pivotal role in the kidney's local renin-angiotensin system]

The renin-angiotensin system is one of the most important hormone systems in the body, and the regulations as well as the role in the juxtaglomerular apparatus are well known. The present review focuses on renin secretion in a recently described localization, the cortical collecting duct. The authors display it in parallel of the copying strategy of an adult and a developing kidney. Furthermore, based on different animal studies it highlights the local role of renin released from the collecting duct. In chronic angiotensin II-infused, 2-kidney, 1-clip hypertensive model as well as in diabetic rats the major source of (pro)renin is indeed the collecting duct. In this localization this hormone can reach both the systemic circulation and the interstitial renin-angiotensin system components including the newly described (pro)renin receptor, by which (pro)renin is able to locally activate pro-fibrotic intracellular signal pathways. Consequently, one can postulate that in the future renin may serve either as a new therapeutic target in nephropathy associated with both hypertension and diabetes or as an early diagnostic marker in chronic diseases leading to nephropathy.

Proximal tubule angiotensinogen modulation of arterial pressure

PURPOSE OF REVIEW

Although the existence of a complete intrarenal renin-angiotensin system is now well established, its role in modulating tubule sodium transport and blood pressure is incompletely understood. Several recent studies have shed light on one component of the system, proximal tubule-derived angiotensinogen (AGT). This review discusses the synthesis, regulation and function of AGT in the proximal tubule.

RECENT FINDINGS

Under normal sodium intake, AGT within the S1 and S2 segments of the proximal tubule may derive from the systemic circulation, whereas the S3 segment synthesizes AGT. Urinary AGT likely primarily reflects proximal tubule-derived AGT. Proximal tubule AGT synthesis is regulated by high Na intake, angiotensin-II and inflammatory cytokines. Transgenic expression of mouse AGT in the proximal tubule causes hypertension. Overexpression of rat AGT in the proximal tubule leads to hypertension, enhanced reactive oxygen species generation via NADPH oxidase, tubular apoptosis and tubulointerstitial fibrosis; these effects can be mitigated by catalase overexpression.

SUMMARY

Proximal tubule-derived AGT has the potential to modulate blood pressure and sodium balance, and promote renal injury. Interactions with the systemic renin-angiotensin system may influence the role of proximal tubule-derived AGT in the kidney.

Role of complement 3a in the growth of mesangial cells from stroke-prone spontaneously hypertensive rats

Vascular smooth muscle cells (VSMCs) derived from spontaneously hypertensive rats (SHR) show exaggerated growth with a synthetic phenotype and angiotensin II (Ang II) production associated with increased production of complement (C3). We hypothesized that C3 is involved in the growth of mesangial cells (MCs) from hypertensive rats. We examined the effects of a C3a receptor inhibitor on proliferation, phenotype and Ang II generation in MCs from stroke prone-spontaneously hypertensive rats (SHR)-SP, SHR and Wistar-Kyoto (WKY) rats. Expression of C3 and C3a receptor were evaluated by immunohistochemical staining of the renal cortex. We examined the effects of the C3a inhibitor, SB290157, on proliferation, the expression of phenotype-marker mRNAs and Ang II production in cells from SHR-SP, SHR and WKY rats. Immunostaining of C3 was stronger in SHR and SHRSP glomeruli. MCs from SHR-SP and SHR abundantly express pre-pro C3 mRNA. SB290157 significantly inhibited basal DNA synthesis and proliferation of MCs from SHR-SP and SHR. Expression of osteopontin mRNA in MCs from SHR-SP and SHR was decreased with SB290157 treatment, whereas MC basal expression of α-SMA mRNA was decreased. SB290157 significantly decreased the production of Ang II in MCs from SHR-SP and SHR. Endogenous C3a promotes exaggerated growth with a synthetic phenotype and the production of Ang II in MCs from SHR-SP and SHR. The C3 and C3a receptor system may primarily be involved in the pathogenesis of renal remodeling in hypertensive rats.