Endogenous angiotensin concentrations in specific intrarenal fluid compartments of the rat

Renin-angiotensin system revisited

New components and functions of the renin-angiotensin system (RAS) are still being unravelled. The classical RAS as it looked in the middle 1970s consisted of circulating renin, acting on angiotensinogen to produce angiotensin I, which in turn was converted into angiotensin II (Ang II) by angiotensin-converting enzyme (ACE). Ang II, still considered the main effector of RAS was believed to act only as a circulating hormone via angiotensin receptors, AT1 and AT2. Since then, an expanded view of RAS has gradually emerged. Local tissue RAS systems have been identified in most organs. Recently, evidence for an intracellular RAS has been reported. The new expanded view of RAS therefore covers both endocrine, paracrine and intracrine functions. Other peptides of RAS have been shown to have biological actions; angiotensin 2-8 heptapeptide (Ang III) has actions similar to those of Ang II. Further, the angiotensin 3-8 hexapeptide (Ang IV) exerts its actions via insulin-regulated amino peptidase receptors. Finally, angiotensin 1-7 (Ang 1-7) acts via mas receptors. The discovery of another ACE2 was an important complement to this picture. The recent discovery of renin receptors has made our view of RAS unexpectedly complex and multilayered. The importance of RAS in cardiovascular disease has been demonstrated by the clinical benefits of ACE inhibitors and AT1 receptor blockers. Great expectations are now generated by the introduction of renin inhibitors. Indeed, RAS regulates much more and diverse physiological functions than previously believed.

Androgens stimulate proximal tubule transport

BACKGROUND

Disrupting the enzyme cytochrome P4a14 in mice leads to hypertension, which is more severe in male than in female mice and appears to be due to androgen excess. Androgens are known to increase expression of angiotensinogen,but the effect of androgens on proximal tubule transport is unknown.

OBJECTIVE

These studies aimed to determine the effect of androgens on proximal tubule transport.

METHODS

Proximal tubules from knockout (KKO) and wild-ttype (WWT) (SSV/1129) mice were perfused in vitro. Volume resorption (JJ v ) was measured using 3 H-methoxy inulin as a volume marker. In separate experiments, male Sprague-Dawley rats were given dihydrotestosterone (DDHT) injections IP for 10 days. Proximal tubule transport was measured in this model using in vivo microperfusion. The renal expression of angiotensinogen was measured by Northern analysis, and brush border membrane protein abundance of the sodium-hhydrogen exchanger isoform 3 (NNHE3) was measured by Western blotting in the control and DHT-ttreated rats.

RESULTS

Mean (SSE) Jv was significantly elevated in proximal tubules from KO mice compared with WT mice (11.11 [0.006] vs 0.77 [0.112] nL/mm . mm, respectively; P<0.05). The mean proximal tubule Jv rate was significantly higher in DHT-ttreated rats than in control rats given vehicle injections (44.57 [0.331] vs 3.31 [0.223] nL/mm . min, respectively; P<0.01). Luminal perfusion with either enalaprilat or losartan decreased the proximal tubule J v rate in DHT-ttreated rats to a greater degree than in control rats. The DHT-treated rats had higher blood pressures and lower serum angiotensin II concentrations than did the control rats.

CONCLUSION

Results suggest that androgens may directly upregulate the proximal tubule reninangiotensin system, increase the expression of NHE3, and increase the Jv rate, thereby increasing extracel-lular volume and blood pressure and secondarily decreasing serum angiotensin II concentrations.

Role of angiotensin II in the enhancement of ammonia production and secretion by the proximal tubule in metabolic acidosis

Acidosis and angiotensin II stimulate ammonia production and transport by the proximal tubule. We examined the modulatory effect of the type 1 angiotensin II receptor blocker losartan on the ability of metabolic acidosis to stimulate ammonia production and secretion by mouse S2 proximal tubule segments. Mice given NH4Cl for 7 days developed metabolic acidosis (low serum bicarbonate concentration) and increased urinary excretion of ammonia. S2 tubule segments from acidotic mice displayed higher rates of ammonia production and secretion compared with those from control mice. However, when losartan was coadministered in vivo with NH4Cl, both the acidosis-induced increase in urinary ammonia excretion and the adaptive increase in ammonia production and secretion of microperfused S2 segments were largely blocked. In renal cortical tissue, losartan blocked the acid-induced increase in brush-border membrane NHE3 expression but had no effect on the acid-induced upregulation of phosphate-dependent glutaminase or phosphoenolpyruvate carboxykinase 1 in cortical homogenates. Addition of angiotensin II to the microperfusion solution enhanced ammonia secretion and production rates in tubules from NH4Cl-treated and control mice in a losartan-inhibitable manner. These results demonstrate that a 7-day acid challenge induces an adaptive increase in ammonia production and secretion by the proximal tubule and suggest that during metabolic acidosis, angiotensin II signaling is necessary for adaptive enhancements of ammonia excretion by the kidney and ammonia production and secretion by S2 proximal tubule segments, as mediated, in part, by angiotensin receptor-dependent enhancement of NHE3 expression.

Role of the renin angiotensin system in TNF-alpha and Shiga-toxin-induced tissue factor expression

Current evidence implicates a prothrombotic state in the development of Shiga-toxin (Stx)-mediated hemolytic uremic syndrome (HUS). We recently reported that Stx modulates procoagulant activity by enhancing functional tissue factor (TF) activity on cytokine-activated human glomerular endothelial cells (HGECs). Since angiotensin II (Ang II), the key effector of the renin angiotensin system (RAS), has been shown to increase TF expression in vascular tissue, we examined the possible involvement of Ang II in TF expression in HGECs. HGECs were exposed to tumor necrosis factor (TNF)-alpha +/- Stx-1 +/- Ang II. Exogenous Ang II significantly increased TF activity and TF mRNA in TNF-alpha- +/- Stx-1-activated HGECs. This increase was mediated via Ang II type I receptor (AT(1)R), as losartan, an AT(1)R inhibitor, attenuated Ang-II-induced TF activity. To study the effect of endogenous Ang II in TF expression by TNF-alpha +/- Stx-1, HGECs were incubated with losartan or an AT(2)R inhibitor (PD 123319) or an angiotensin-converting enzyme inhibitor (enalapril). Losartan but not PD 123319 decreased TF activity induced by TNF-alpha +/- Stx-1 (P < 0.05). Enalapril, also, dose dependently, downregulated TF expression in HGECs exposed to TNF-alpha +/- Stx-1 (P < 0.05). AT(1)R mRNA was upregulated in TNF-alpha- +/- Stx-1-activated HGECs (P < 0.05). These data indicate that TF expression in TNF-alpha- and Stx-1-activated HGECs is enhanced by exogenous Ang II and that endogenous Ang II production may be upregulated by TNF-alpha +/- Stx-1. Hence, local RAS activation may be important in the development of the thrombotic microangiopathy observed in HUS.

Activation of a local renin angiotensin system in podocytes by glucose

ANG II is a critical mediator of diabetic nephropathy. Pharmacologic inhibition of ANG II slows disease progression beyond what could be predicted by the blood pressure lowering effects alone, suggesting the importance of nonhemodynamic pathways of ANG II in mediating disease. Podocyte injury and loss are cardinal features of diabetic nephropathy. Mounting evidence suggests that the podocyte is a direct target of ANG II-mediated signaling in diabetic renal disease. We have tested the hypothesis that high glucose leads to the activation of a local angiotensin system in podocytes and delineated the underlying pathways involved. Cultured podocytes were exposed to standard glucose (5 mM), high glucose (40 mM), or mannitol as an osmotic control. ANG II levels in cell lysates were measured in the presence or absence of inhibitors of angiotensin-converting enzyme (captopril), chymase (chymostatin), and renin (aliskiren) activity. The effects of glucose on renin and angiotensin subtype 1 receptor expression and protein levels were determined. Exposure to high glucose resulted in a 2.1-fold increase ANG II levels mediated through increased renin activity, as exposure to high glucose increased renin levels and preincubation with Aliskiren abrogated glucose-induced ANG II production. Relevance to the in vivo setting was demonstrated by showing glomerular upregulation of the prorenin receptor in a podocyte distribution early in the course of experimental diabetic nephropathy. Furthermore, high glucose increased angiotensin subtype 1 receptor levels by immunofluorescence and Western blot. Taken together, the resultant activation of a local renin angiotensin system by high glucose may promote progressive podocyte injury and loss in diabetic nephropathy.

Conversion of renal angiotensin II to angiotensin III is critical for AT2 receptor-mediated natriuresis in rats

In the kidney, angiotensin II (Ang II) is metabolized to angiotensin III (Ang III) by aminopeptidase A (APA). In turn, Ang III is metabolized to angiotensin IV by aminopeptidase N (APN). Renal interstitial (RI) infusion of Ang III, but not Ang II, results in angiotensin type-2 receptor (AT(2)R)-mediated natriuresis. This response is augmented by coinfusion of PC-18, a specific inhibitor of APN. The present study addresses the hypotheses that Ang II conversion to Ang III is critical for the natriuretic response. Sprague-Dawley rats received systemic angiotensin type-1 receptor (AT(1)R) blockade with candesartan (CAND; 0.01 mg/kg/min) for 24 hours before and during the experiment. After a control period, rats received either RI infusion of Ang II or Ang II+PC-18. The contralateral kidney received a RI infusion of vehicle in all rats. Mean arterial pressure (MAP) was monitored, and urinary sodium excretion rate (U(Na)V) was calculated separately from experimental and control kidneys for each period. In contrast to Ang II-infused kidneys, U(Na)V from Ang II+PC-18-infused kidneys increased from a baseline of 0.03+/-0.01 to 0.09+/-0.02 micromol/min (P<0.05). MAP was unchanged by either infusion. RI addition of PD-123319, an AT(2)R antagonist, inhibited the natriuretic response. Furthermore, RI addition of EC-33, a selective APA inhibitor, abolished the natriuretic response to Ang II+PC-18. These data demonstrate that RI addition of PC-18 to Ang II enables natriuresis mediated by the AT(2)R, and that conversion of Ang II to Ang III is critical for this response.

Renal disease in recipients of nonrenal solid organ transplantation

Worldwide, more than 250,000 individuals who have received a liver, heart, lung, or intestinal transplant are living longer. Twenty percent to 25% of these recipients experience perioperative acute renal failure, with 10% to 15% requiring renal replacement therapy. Chronic kidney disease (CKD) is also highly prevalent, affecting 30% to 50% of the nonrenal organ transplant population with an annual end-stage renal disease risk of 1.5% to 2.0%. Both acute renal failure and CKD contribute to increased morbidity and premature mortality. The dominant causative factor for renal disorders seen in nonrenal transplant recipients are the calcineurin inhibitors (CNI) and rapamycin analogues, which singly or in combination lead to a variety of nephrotoxic injury. However, 25% to 30% of nonrenal transplant recipients with CKD have other conditions such as hypertension, focal segmental glomerulosclerosis, diabetes mellitus, and hepatitis C infection as the principal underlying cause. Management strategies for renal disease in the nonrenal transplant recipients include the following: (1) delayed introduction of CNI after graft implantation, (2) withdrawal or minimization of long-term CNI therapy, (3) timely use of an appropriate dialysis modality, and (4) expeditious introduction of supportive measures such as anemia management, phosphate binding therapy, and dietary modification. Compared with maintenance dialysis, kidney transplantation reduces long-term mortality by 60% to 70% in nonrenal transplant recipients with end-stage renal disease.

Thiamine attenuates the hypertension and metabolic abnormalities in CD36-defective SHR: uncoupling of glucose oxidation from cellular entry accompanied with enhanced protein O-GlcNAcylation in CD36 deficiency

BACKGROUND AND OBJECTIVES

The spontaneous hypertensive rat (SHR) is a widely studied model of hypertension that exhibits metabolic abnormalities, which share features with the human metabolic syndrome. Genetic linkage studies have revealed a defective CD36 gene, encoding a membrane fatty acid (FA) transporter, in hyperinsulinemia of the SHR. However, there is no unifying mechanism that can explain these phenotypes as a consequence of a defective CD36 gene. Impaired fatty acid uptake is compensated by increased glucose uptake. We hypothesized that (1) the abundant intracellular glucose is not oxidized proportionally and (2) the correction of the uncoupling of glucose oxidation to its cellular entry might be effective against the pathophysiology of CD36-defective SHR. Therefore, we attempted to activate glucose oxidation with the repletion of thiamine, a coenzyme for multiple steps of glucose metabolism.

METHODS AND RESULTS

In one series of experiments, intracellular glucose fate was assessed by the ratio of [(14)C]glucose/[(3)H]deoxyglucose radioactivity, which suggested that glucose oxidation was uncoupled from its cellular entry in SHR. Protein O-GlcNAcylation was intense in the hearts of CD36-defective SHR compared with that of wild-type CD36 rats [Wister Kyoto rats (WKY)], indicating the shunt of glucose through the hexosamine biosynthetic pathway (HBP). In another series of studies, 4-week-old SHR were maintained with water containing 0.2% thiamine for 10 weeks. Systolic blood pressure, plasma insulin and norepinephrine levels were significantly lower in the thiamine-group, as compared with the untreated-group. In epididymal adipose tissue, thiamine repletion down-regulated the expression levels of mRNA transcripts for UDP-N-acetylglucosamine:peptide glycosyltransferase, angiotensinogen, angiotensin type 1 receptor, transforming growth factor-beta1 and plasminogen activator inhibitor-1.

CONCLUSIONS

The hearts of CD36-defective SHR exhibited uncoupling of glucose oxidation from its cellular entry, accompanied with the enhanced protein O-GlcNAcylation, suggesting increased glucose shunt through the HBP. Thiamine repletion in CD36-defective SHR resulted in (1) the correction of the uncoupling of glucose oxidation to its cellular entry, concomitant with reduced protein O-GlcNAcylation, (2) the down-regulation of the expression of mRNAs involved in HBP, the renin-angiotensin system and adipokines in epididymal adipose tissue, and (3) the attenuation of the hypertension and hyperinsulinemia. We propose that interventions targeting glucose oxidation with thiamine repletion may provide a novel adjunctive approach to attenuate metabolic abnormalities and related hypertension.

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.

Effect of ACE inhibitors on angiotensin II in rat mesangial cells cultured in high glucose

This study investigates whether angiotensin converting enzyme (ACE) inhibitors can block high glucose-induced increase of angiotensin II (Ang II) levels in cultured rat mesangial cells. Incubation of cells with high glucose for 5 days increased Ang II in the cell media (extracellular) and cell lysates (intracellular) by approximately 1.5-fold and approximately 2.2-fold, respectively. Captopril blocked high glucose-induced increase in extracellular Ang II levels but not in intracellular Ang II levels. Since the inability of captopril to inhibit intracellular Ang II in intact cells could be explained by failure of the drug to enter the cells, further studies were performed using lysates from cells pre-treated with high glucose for 5 days. Addition of captopril to cell lysates was indeed able to inhibit Ang I conversion to Ang II but only partially. These findings suggest the involvement of non-ACE mechanisms in Ang II formation from Ang I in mesangial cells.

The emerging role of angiotensin-converting enzyme-2 in the kidney

PURPOSE OF REVIEW

The intrarenal renin-angiotensin system contributes to the progression of chronic kidney disease. Angiotensin-converting enzyme-2 is a recently identified protein that is highly expressed in the kidney and results in formation of angiotensin-(1-7). This manuscript highlights current information on the localization and function of angiotensin-converting enzyme-2 in the kidney, along with recent studies on the role of the enzyme in hypertension, glomerulosclerosis, and diabetic nephropathy.

RECENT FINDINGS

Angiotensin-converting enzyme-2 is expressed in relative abundance in the proximal tubule and metabolizes angiotensin II to angiotensin-(1-7). Angiotensin-converting enzyme-2 also catalyzes conversion of angiotensin I to angiotensin-(1-9), which is cleaved to angiotensin-(1-7) by angiotensin-converting enzyme. In mice, gene deletion of angiotensin-converting enzyme-2 elevates blood pressure responses to angiotensin II, and increases renal angiotensin II levels. Male angiotensin-converting enzyme-2-deficient mice exhibit accelerated glomerulosclerosis. In early diabetic nephropathy, proximal tubular angiotensin-converting enzyme-2 expression and activity are increased, suggesting that the enzyme may counterregulate the effects of local angiotensin II.

SUMMARY

Angiotensin-converting enzyme-2 appears to be critical in determining the balance between the intrarenal effects of angiotensin II and angiotensin-(1-7). Angiotensin-converting enzyme-2 could thereby represent a target for novel therapeutic approaches in a variety of kidney disorders.

Toward better renoprotection: Lessons from angiotensin receptor blockers

The rising tide of chronic kidney disease (CKD), especially diabetic nephropathy, has become a worldwide catastrophe. However, therapeutic options to prevent or retard the progression of CKD still remain very limited. The understanding of its molecular mechanisms and the delineation of tools able to modify them are thus of critical importance. The discovery that some antihypertensive agents inhibiting the renin-angiotensin system, such as angiotensin II type 1 receptor blockers, protect the kidney opens new therapeutic perspectives. In this article, we focus on their renoprotective actions beyond blood pressure lowering.

Role of the AT1A receptor in the CO2-induced stimulation of HCO3- reabsorption by renal proximal tubules

The proximal tubule (PT) is major site for the reabsorption of filtered HCO(3)(-). Previous work on the rabbit PT showed that 1) increases in basolateral (BL) CO(2) concentration ([CO(2)](BL)) raise the HCO(3)(-) reabsorption rate (J(HCO(3))), and 2) the increase that luminal angiotensin II (ANG II) produces in J(HCO(3)) is greatest at 0% [CO(2)](BL) and falls to nearly zero at 20%. Here, we investigate the role of angiotensin receptors in the [CO(2)](BL) dependence of J(HCO(3)) in isolated perfused PTs. We found that, in rabbit S2 PT segments, luminal 10(-8) M saralasin (peptide antagonist of ANG II receptors), lowers baseline J(HCO(3)) (5% CO(2)) to the value normally seen at 0% in the absence of inhibitors and eliminates the J(HCO(3)) response to changes in [CO(2)](BL). However, basolateral 10(-8) M saralasin has no effect. As with saralasin, luminal 10(-8) M candesartan (AT(1) antagonist) reduces baseline J(HCO(3)) and eliminates the [CO(2)](BL) dependence of J(HCO(3)). Luminal 10(-7) M PD 123319 (AT(2) antagonist) has no effect. Finally, we compared PTs from wild-type and AT(1A)-null mice of the same genetic background. Knocking out AT(1A) modestly lowers baseline J(HCO(3)) and, like luminal saralasin or candesartan in rabbits, eliminates the J(HCO(3)) response to changes in [CO(2)](BL). Our accumulated evidence suggests that ANG II endogenous to the PT binds to the apical AT(1A) receptor and that this interaction is critical for both baseline J(HCO(3)) and its response to changes in [CO(2)](BL). Neither apical AT(2) receptors nor basolateral ANG II receptors are involved in these processes.

Hydronephrosis causes salt-sensitive hypertension and impaired renal concentrating ability in mice

AIM

Hypertension is a common disease in the industrialized world and approximately 5% of all cases are secondary to kidney malfunction. We have recently shown that hydronephrosis due to partial unilateral ureteral obstruction (PUUO) causes salt-sensitive hypertension in rats. The mechanisms are still unclear, but appear to be intrarenal and primarily located to the diseased kidney. In the present study, we have developed a model for PUUO to study if hydronephrotic mice develop salt-sensitive hypertension.

METHODS

PUUO was created in 3-week-old mice (C57bl/6J). Blood pressure and heart rate were measured telemetrically in adult animals on normal and high salt diets. Metabolism cages were used to study the renal excretion of electrolytes and water. Plasma samples for renin analysis were collected and renal histological changes were evaluated.

RESULTS

All hydronephrotic animals developed salt-sensitive hypertension that correlated to the degree of hydronephrosis. In hydronephrotic animals, blood pressure increased from 114 +/- 1 mmHg on normal salt diet to 120 +/- 2 mmHg on high salt diet, compared with 103 +/- 1 to 104 +/- 1 in controls. Hydronephrotic animals showed increased diuresis and reduced ability to regulate electrolyte concentration. No differences in plasma renin concentration were found between the groups. The parenchymal weight and glomerular area of contralateral kidneys were significantly increased in the hydronephrotic animals. Histopathology of the hydronephrotic kidneys displayed areas with fibrosis, inflammation and glomerular changes.

CONCLUSION

This study provides a model for PUUO in mice and demonstrates the presence of salt-sensitive hypertension and an impaired renal concentrating ability in mice which has not been described before.

Genetic susceptibility to essential hypertension: insight from angiotensinogen

Although progress in the genetics of essential hypertension may seem disappointing, it has considerable potential in defining research directions that will ultimately translate into clinical practice. The hypothesis that genetic variation at the angiotensinogen locus impacts on individual susceptibility to develop essential hypertension has motivated a substantial body of research by us and many others. We examine how analyses of the mechanisms by which variation in angiotensinogen expression may contribute to disease susceptibility and may have arisen in human populations have progressed in recent years. Although the objective of personalized medicine is still in the future, a genetic hypothesis based on human variation can uniquely empower functional genomics approaches to reach such an ultimate goal.

A local pre-receptor mechanism of hormone stimulus amplification: focus on angiotensin II in resistance blood vessels

BACKGROUND

The in-vivo correlation between vascular tone and the concentration of free angiotensin (Ang) II at the level of the arterioles, under (patho)physiological conditions, is not known.

OBJECTIVE

To examine the in-vivo kinetics of binding of Ang II to Ang II type 1 (AT1) receptors in vascular tissue.

METHODS AND RESULTS

A plane vascular smooth muscle (VSM) sheet containing a single layer of cells, at one side exposed to Ang II, was the starting point for designing a mathematical model based on local receptor density and geometric considerations and on kinetic parameters of Ang II diffusion and Ang II-AT1 receptor complex formation and internalization. Calculations demonstrate that a diffusing Ang II molecule at short distance from the receptor has an almost 100% chance to be actually bound, so that the apparent binding rate constant (per unit of receptor concentration) is greatly augmented. This pre-receptor stimulus amplification (PRESTAMP) mechanism is sustained by AT1 receptor-mediated endocytosis and receptor recycling. On the other hand, PRESTAMP also enhances endocytotic receptor downregulation, and calculations predict that steady-state levels of Ang II above threshold have relatively little additional effect.

CONCLUSION

The results explain why physiological concentrations of free Ang II far below the equilibrium dissociation constant of its reaction with AT1 receptors are sufficient to increase vascular resistance, and why a correlation between blood pressure and the concentration of free Ang II is often difficult to demonstrate.

Angiotensin II inhibits the ROMK-like small conductance K channel in renal cortical collecting duct during dietary potassium restriction

Base-line urinary potassium secretion in the distal nephron is mediated by small conductance rat outer medullary K (ROMK)-like channels. We used the patch clamp technique applied to split-open cortical collecting ducts (CCDs) isolated from rats fed a normal potassium (NK) or low potassium (LK) diet to test the hypothesis that AngII directly inhibits ROMK channel activity. We found that AngII inhibited ROMK channel activity in LK but not NK rats in a dose-dependent manner. The AngII-induced reduction in channel activity was mediated by AT1 receptor (AT1R) binding, because pretreatment of CCDs with losartan but not PD123319 AT1 and AT2 receptor antagonists, respectively, blocked the response. Pretreatment of CCDs with U73122 and calphostin C, inhibitors of phospholipase C (PLC) and protein kinase C (PKC), respectively, abolished the AngII-induced decrease in ROMK channel activity, confirming a role of the PLC-PKC pathway in this response. Studies by others suggest that AngII stimulates an Src family protein-tyrosine kinase (PTK) via PKC-NADPH oxidase. PTK has been shown to regulate the ROMK channel. Inhibition of NADPH oxidase with diphenyliodonium abolished the inhibitory effect of AngII or the PKC activator phorbol 12-myristate 13-acetate on ROMK channels. Suppression of PTK by herbimycin A significantly attenuated the inhibitory effect of AngII on ROMK channel activity. We conclude that AngII inhibits ROMK channel activity through PKC-, NADPH oxidase-, and PTK-dependent pathways under conditions of dietary potassium restriction.

Intrarenal aminopeptidase N inhibition augments natriuretic responses to angiotensin III in angiotensin type 1 receptor-blocked rats

The renal angiotensin angiotensin type 2 receptor has been shown to mediate natriuresis, and angiotensin III, not angiotensin II, may be the preferential angiotensin type 2 receptor activator of this response. Angiotensin III is metabolized to angiotensin IV by aminopeptidase N. The present study hypothesizes that inhibition of aminopeptidase N will augment natriuretic responses to intrarenal angiotensin III in angiotension type 1 receptor-blocked rats. Rats received systemic candesartan for 24 hours before the experiment. After a 1-hour control, cumulative renal interstitial infusion of angiotensin III at 3.5, 7, 14, and 28 nmol/kg per minute (each dose for 30 minutes) or angiotensin III combined with aminopeptidase N inhibitor PC-18 was administered into 1 kidney. The contralateral control kidney received renal interstitial infusion of vehicle. In kidneys infused with angiotensin III alone, renal sodium excretion rate increased from 0.05+/-0.01 micromol/min in stepwise fashion to 0.11+/-0.01 micromol/min at 28 nmol/kg per minute of angiotensin III (overall ANOVA F=3.68; P<0.01). In angiotensin III combined with PC-18, the renal sodium excretion rate increased from 0.05+/-0.01 to 0.32+/-0.08 mumol/min at 28 nmol/kg per minute of angiotensin III (overall ANOVA F=6.2; P<0.001). The addition of intrarenal PD-123319, an angiotensin type 2 receptor antagonist, to renal interstitial angiotensin III plus PC-18 inhibited the natriuretic response. Mean arterial blood pressure and renal sodium excretion rate from control kidneys were unchanged by angiotensin III +/- PC-18 + PD-123319. Angiotensin III plus PC-18 induced a greater natriuretic response than Ang III alone (overall ANOVA F=16.9; P=0.0001). Aminopeptidase N inhibition augmented the natriuretic response to angiotensin III, suggesting that angiotensin III is a major agonist of angiotensin type 2 receptor-induced natriuresis.

C1q nephropathy in association with Gitelman syndrome: a case report

There have been rare reports of glomerulopathies developing in patients with Bartter syndrome (BS) and its milder variant, Gitelman syndrome (GS). We present the first case of C1q nephropathy (C1qN) in an African American child with GS. This child was diagnosed with GS at 9 years of age and subsequently developed nephrotic range proteinuria 3 years later. Renal biopsy revealed mesangial hypercellularity and focal segmental glomerulosclerosis (FSGS). The segmental lesions were generally located at the vascular pole. Dominant C1q (2+) staining along with IgG (1-2+) was demonstrated in the mesangium, which correlated with scattered electron dense mesangial deposits demonstrated by electron microscopy. Treatment with an angiotensin-converting enzyme inhibitor led to an improvement in proteinuria to near-normal values (urine protein/creatinine ratio down to 0.5), but the creatinine clearance declined to approximately 58 ml/min/1.73 m(2). This case highlights the possible association between the milder hypokalemic tubulopathy, GS, and glomerular disease, including C1qN. Prompt evaluation of proteinuria with renal biopsy in these patients is recommended to detect significant glomerular pathology. Further research is needed to define risk factors for this complication.

Prenatal programming of rat proximal tubule Na+/H+ exchanger by dexamethasone

Prenatal administration of dexamethasone causes hypertension in rats when they are studied as adults. Although an increase in tubular sodium reabsorption has been postulated to be a factor programming hypertension, this has never been directly demonstrated. The purpose of this study was to examine whether prenatal programming by dexamethasone affected postnatal proximal tubular transport. Pregnant Sprague-Dawley rats were injected with intraperitoneal dexamethasone (0.2 mg/kg) daily for 4 days between the 15th and 18th days of gestation. Prenatal dexamethasone resulted in an elevation in systolic blood pressure when the rats were studied at 7-8 wk of age compared with vehicle-treated controls: 131 +/- 3 vs. 115 +/- 3 mmHg (P < 0.001). The rate of proximal convoluted tubule volume absorption, measured using in vitro microperfusion, was 0.61 + 0.07 nl.mm(-1).min(-1) in control rats and 0.93+ 0.07 nl.mm(-1).min(-1) in rats that received prenatal dexamethasone (P < 0.05). Na(+)/H(+) exchanger activity measured in perfused tubules in vitro using the pH-sensitive dye BCECF showed a similar 50% increase in activity in proximal convoluted tubules from rats treated with prenatal dexamethasone. Although there was no change in abundance of NHE3 mRNA, the predominant luminal proximal tubule Na(+)/H(+) exchanger, there was an increase in NHE3 protein abundance on brush-border membrane vesicles in 7- to 8-wk-old rats receiving prenatal dexamethasone. In conclusion, prenatal administration of dexamethasone in rats increases proximal tubule transport when rats are studied at 7-8 wk old, in part by stimulating Na(+)/H(+) exchanger activity. The increase in proximal tubule transport may be a factor mediating the hypertension by prenatal programming with dexamethasone.

Metabolic Alkalosis, Bedside and Bench

Although significant contributions to the understanding of metabolic alkalosis have been made recently, much of our knowledge rests on data from clearance studies performed in humans and animals many years ago. This article reviews the contributions of these studies, as well as more recent work relating to the control of renal acid-base transport by mineralocorticoid hormones, angiotensin, endothelin, nitric oxide, and potassium balance. Finally, clinical aspects of metabolic alkalosis are considered.

Glomerular localization and expression of Angiotensin-converting enzyme 2 and Angiotensin-converting enzyme: implications for albuminuria in diabetes

Angiotensin-converting enzyme 2 (ACE2) expression has been shown to be altered in renal tubules from diabetic mice. This study examined the localization of ACE and ACE2 within the glomerulus of kidneys from control (db/m) and diabetic (db/db) mice and the effect of chronic pharmacologic ACE2 inhibition. ACE2 co-localized with glomerular epithelial cell (podocyte) markers, and its localization within the podocyte was confirmed by immunogold labeling. ACE, by contrast, was seen only in glomerular endothelial cells. By immunohistochemistry, in glomeruli from db/db mice, strong ACE staining was found more frequently than in control mice (db/db 64.6 +/- 6.3 versus db/m 17.8 +/- 3.4%; P < 0.005). By contrast, strong ACE2 staining in glomeruli from diabetic mice was less frequently seen than in controls (db/db 4.3 +/- 2.4 versus db/m 30.6 +/- 13.6%; P < 0.05). For investigation of the significance of reduced glomerular ACE2 expression, db/db mice were treated for 16 wk with a specific ACE2 inhibitor (MLN-4760) alone or combined with telmisartan, a specific angiotensin II type 1 receptor blocker. At the end of the study, glomerular staining for fibronectin, an extracellular matrix protein, was increased in both db/db and db/m mice that were treated with MLN-4760. Urinary albumin excretion (UAE) increased significantly in MLN-4760-treated as compared with vehicle-treated db/db mice (743 +/- 200 versus 247 +/- 53.9 microg albumin/mg creatinine, respectively; P < 0.05), and the concomitant administration of telmisartan completely prevented the increase in UAE associated with the ACE2 inhibitor (161 +/- 56; P < 0.05). It is concluded that ACE2 is localized in the podocyte and that in db/db mice glomerular expression of ACE2 is reduced whereas glomerular ACE expression is increased. The finding that chronic ACE2 inhibition increases UAE suggests that ACE2, likely by modulating the levels of glomerular angiotensin II via its degradation, may be a target for therapeutic interventions that aim to reduce albuminuria and glomerular injury.

The effect of single oral low-dose losartan on posture-related sodium handling in post-TIPS ascites-free cirrhosis

Post-TIPS ascites-free patients with cirrhosis and previous refractory ascites demonstrate subtle sodium retention when challenged with a high sodium load. This is also observed in pre-ascitic patients with cirrhosis. This phenomenon is dependent on an intrarenal angiotensin II (ANG II) mechanism related to the assumption of erect posture. We investigated whether similar mechanisms were involved in post-TIPS ascites-free patients, by studying 10 patients with functioning TIPS and no ascites. We measured the effect of changing from supine to erect posture on sodium excretion at baseline and after single oral low dose losartan (7.5 mg) which has been shown to blunt proximal and distal tubular sodium reabsorption in pre-ascites. At baseline, the assumption of erect posture produced a reduction in sodium excretion (from 0.30+/-0.06 to 0.13+/-0.02 mmol/min, P=.05), which was mainly due to an increase in proximal tubular reabsorption of sodium (PTRNa) (69.7+/-3.1% to 81.1+/-1.8%, P=.003). The administration of losartan resulted in a blunting of PTRNa (supine 69.7+/-3.1% to 63.9+/-3.9%, P=.01 and erect 81.1+/-1.8% to 73.8+/-2.4%, P=.01), accompanied by an increased distal tubular reabsorption of sodium in both postures, with no overall improvement in sodium excretion on standing. In conclusion, post-TIPS ascites-free patients with cirrhosis exhibit erect posture-induced sodium retention. We speculate that (1) this effect is partly mediated by the effect of ANG II on PTRNa and (2) that the inability of low dose losartan to block the erect posture-induced sodium retention may be related to the erect posture-induced rise in aldosterone which is unmodified by losartan.

Hydronephrosis causes salt-sensitive hypertension in rats

BACKGROUND

Hypertension is a common disease in the Western world and approximately 5% of all cases are secondary to kidney malfunction. It is not clear whether unilateral hydronephrosis due to partial obstruction affects blood pressure.

AIM

The aim of this study was to determine whether hypertension develops and to investigate the effects of different salt diets on the blood pressure in hydronephrotic animals.

METHODS

Unilateral partial ureteral obstruction was created in 3-week-old Sprague-Dawley rats. A telemetric device was implanted 4-6 weeks later and blood pressure was measured on normal, low- and high-salt diets. Plasma samples were collected on all diets for renin analysis.

RESULTS

All hydronephrotic animals developed hypertension that correlated to the degree of hydronephrosis. The blood pressure increased slowly with time and was salt sensitive. In severe hydronephrosis, blood pressure increased from 118 +/- 5 mmHg on low salt to 140 +/- 6 mmHg on high salt intake, compared to control levels of 82 +/- 2 and 84 +/- 2 mmHg, respectively. Plasma renin concentration was increased in the hydronephrotic group of animals compared to controls on all diets, but the difference was only significant on a normal salt diet, 165 +/- 15 versus 86 +/- 12 microGU/ml respectively. In animals with severe hydronephrosis the plasma renin levels were lower, and the changes less, than in those with mild and moderate hydronephrosis.

CONCLUSION

This study demonstrates the presence of a salt-sensitive hypertension in hydronephrosis. A systemic effect of the renin-angiotensin system alone cannot be responsible for the hypertension.

Specific receptor for angiotensinogen on human renal cells

BACKGROUND

We recently demonstrated the existence of an angiotensinogen (AGT) receptor on placental cells. It has been established that there is a tissue-specific renin-angiotensin system (RAS) in the human kidney. This study focused on whether human renal proximal tubule epithelial cells possessed an AGT receptor.

METHODS

Human renal proximal tubule epithelial cells were cultured in plastic wells. Binding assays were carried out by adding iodinated angiotensinogen ((125)I-AGT) to each culture well, with or without unlabeled AGT. The cells were washed, lysed, and the radioactivity was measured.

RESULTS

Human renal proximal tubule epithelial cells bound (125)I-AGT in a time-dependent manner. This binding was competitively and specifically inhibited by unlabeled AGT. Bound (125)I-AGT was competitively displaced by AGT. Acid washing removed 30% at 8 h, indicating that 70% bound AGT had been internalized. Scatchard plot binding analysis showed that the identified AGT receptor possessed a single class of high-affinity binding sites (K(d)=1.73 nmol, B(max)=23.39 pmol/10(6) cells).

CONCLUSION

The results of this study provide evidence for the presence of an AGT receptor on human renal proximal tubule epithelial cells. Our finding suggests that the AGT receptor may be an integral component of the renal RAS.

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

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

Angiotensin-(1-7) inhibits angiotensin II-stimulated phosphorylation of MAP kinases in proximal tubular cells

Angiotensin-converting enzyme 2 (ACE2) is a homolog of ACE, which is not blocked by ACE inhibitors. High amounts of ACE2 are present in the proximal tubule, and ACE2 catalyzes generation of angiotensin 1-7 (Ang-(1-7)) by this segment. Ang-(1-7) binds to a receptor distinct from the AT1 or AT2 Ang II receptor, identified as the mas receptor. We studied the effects of Ang-(1-7) on Ang II-mediated cell signaling pathways in proximal tubule. In primary cultures of rat proximal tubular cells, activation of mitogen-activated protein kinases (MAPK) was detected by immunoblotting, in the presence or absence of agonists/antagonists. Transforming growth factor-beta1 (TGF-beta1) was measured by enzyme-linked immunosorbent assay. Ang II (5 min, 10(-7) M) stimulated phosphorylation of the three MAPK (p38, extracellular signal-related kinase (ERK 1/2), and c-Jun N-terminal kinase (JNK)). While incubation of proximal tubular cells with Ang-(1-7) alone did not significantly affect MAPK phosphorylation, Ang-(1-7) (10(-7) M) completely inhibited Ang II-stimulated phosphorylation of p38, ERK 1/2, and JNK. This inhibitory effect was reversed by the Ang-(1-7) receptor antagonist, D-Ala7-Ang-(1-7). Ang II significantly increased production of TGF-beta1 in proximal tubular cells, an effect that was partly inhibited by Ang-(1-7). Ang-(1-7) had no significant effect on cyclic 3',5'-adenosine monophosphate production in these cells. In summary, Ang-(1-7) inhibits Ang II-stimulated MAPK phosphorylation in proximal tubular cells. Generation of Ang-(1-7) by proximal tubular ACE2 could thereby serve a protective role by counteracting the effects of locally generated Ang II.

Angiotensin II production and distribution in the kidney: I. A kinetic model

Information on the levels of angiotensin II (Ang II) and its receptors in the various renal tissue compartments is still incomplete. A model is presented describing the kinetics of Ang II production, distribution, and disposal in the renal cortex. Basic features are: (1) the model is designed to derive, from Ang II measurements in blood and in whole tissue, estimates of the local densities of the Ang II type 1 (AT(1)) and type 2 (AT(2)) receptors, and to calculate the concentrations of endocrine and paracrine Ang II they actually 'see'; (2) glomerular and peritubular tissue are conceived as separate regions (glomerular region (Glom), peritubular region (Pt)); (3) in Glom and in Pt, Ang II is homogeneously distributed in capillary blood and in interstitial fluid; (4) the model allows for local Ang II concentration gradients between interstitium and blood; (5) Ang II from the circulation diffuses into the interstitium of Glom after convective transcapillary transport; (6) Ang II produced in tubules or Pt enters the microcirculation through diffusive overflow from interstitium; (7) the presence of cell-surface-bound Ang II depends on the reaction with AT(1) and AT(2) receptors, and the presence of intracellular Ang II depends on the internalization of Ang II - AT(1) receptor complex; and (8) the model provides for glomerular filtration, vasopeptidase-mediated degradation, and intracellular degradation as mechanisms of elimination. This model can serve as a framework for detailed quantitative studies of the renin-angiotensin system in the kidney.

Angiotensin II formation in the kidney and nephrosclerosis in Ren-2 hypertensive rats

BACKGROUND

Ren-2 transgenic hypertensive rats develop malignant hypertensive nephrosclerosis despite low to normal plasma angiotensin II and suppressed renal renin. We tested the hypothesis that local angiotensin II formation occurs at sites of renal vascular and interstitial injury in this model.

METHODS

Heterozygous Ren-2 transgenic rats were compared with normotensive Sprague-Dawley-Hannover control rats and Ren-2 transgenic rats treated with a very low dose of an angiotensin II type 1 (AT1) receptor antagonist, 1 mg/kg/day losartan, for 4 weeks. Blood pressure measurements, quantifications of urinary albumin, plasma and tissue angiotensin II as well as immunohistochemical analyses were performed.

RESULTS

Systolic blood pressure was not affected by losartan during the study but intra-arterial recordings revealed a decrease of blood pressure. Losartan reduced albumin excretion, cell proliferation, macrophage influx, collagen I and collagen IV deposition. Plasma angiotensin II was decreased, while kidney tissue angiotensin II content was increased in Ren-2 transgenic rats compared with control rats. In Ren-2 transgenic rats, juxtaglomerular renin and angiotensin II staining were reduced, but there was a marked angiotensin II staining at foci of tubulo-interstitial fibrosis and at proliferative malignant vascular lesions.

CONCLUSION

We conclude that local angiotensin II formation is increased in proliferative or fibrotic kidney lesions in the Ren-2 transgenic rat. Local angiotensin II formation may help to explain why the AT1 receptor antagonist prevents or ameliorates this transgenic model of malignant nephrosclerosis despite low to normal plasma angiotensin II and suppressed renal renin.

Renin increases mesangial cell transforming growth factor-beta1 and matrix proteins through receptor-mediated, angiotensin II-independent mechanisms

Recent evidence suggesting a strong interplay between components of the renin-angiotensin system and key mediators of fibrosis led us to hypothesize that renin, independent of its enzymatic action to enhance angiotensin (Ang) II synthesis, directly increases production of the fibrogenic cytokine transforming growth factor (TGF)-beta. Human or rat mesangial cells (MCs) were treated with human recombinant renin (HrRenin) or rat recombinant renin (RrRenin) and the effects on TGF-beta1, plasminogen activator inhibitor-type 1 (PAI-1), fibronectin (FN) and collagen 1 mRNA and protein were investigated. Blockade of the rat MC renin receptor was achieved using siRNA. HrRenin or RrRenin, at doses shown to be physiologically relevant, induced marked dose- and time-dependent increases in TGF-beta1. These effects were not altered by adding an inhibitor of renin's enzymatic action (RO 42-5892), the Ang II receptor antagonist losartan or the Ang-converting enzyme inhibitor enalapril. RrRenin also induced PAI-1, FN and collagen 1 mRNA and PAI-1 and FN protein in a dose-dependent manner. Neutralizing antibodies to TGF-beta partially blocked these effects. Supernatant and cell lysate Ang I and Ang II levels were extremely low. MC angiotensinogen mRNA was undetectable both with and without added renin. Targeting of the rat renin receptor mRNA with siRNA blocked induction of TGF-beta1. We conclude that renin upregulates MC TGF-beta1 through a receptor-mediated mechanism, independent of Ang II generation or action. Renin-induced increases in TGF-beta1 in turn stimulate increases in PAI-1, FN and collagen I. Thus, renin may contribute to renal fibrotic disease, particularly when therapeutic Ang II blockade elevates plasma renin.

The renin-angiotensin system in glomerular podocytes: Mediator of glomerulosclerosis and link to hypertensive nephropathy

The renoprotective effects of pharmacologic inhibition of angiotensin II extend beyond the blood pressure-lowering effects alone, consistent with the observation that angiotensin II is produced locally within the kidney and mediates tissue injury through a series of nonhemodynamic effects. Podocytes are terminally differentiated epithelial cells that contribute to the filtration barrier of the kidney, but also safeguard against the development of glomerulosclerosis. Mounting evidence demonstrates that podocytes are not only a local source of angiotensin II production, but are also vulnerable to its deleterious effects, thus fueling the future development of glomerular scarring. In this review article, we explore the role of a local angiotensin system as a mediator of podocyte injury and discuss its potential link to hypertensive renal disease.

AT1 receptor-mediated accumulation of extracellular angiotensin II in proximal tubule cells: role of cytoskeleton microtubules and tyrosine phosphatases

Long-term angiotensin II (ANG II) administration is associated with increased ANG II accumulation in the kidney, but intrarenal compartment(s) involved in this response remains to be determined. We tested the hypothesis that 1) extracellular ANG II is taken up by proximal tubule cells (PTCs) through AT(1) receptor-mediated endocytosis, 2) this process is regulated by cytoskeleton microtubule- and tyrosine phosphatase-dependent mechanisms, and 3) AT(1) receptor-mediated endocytosis of ANG II has a functional relevance by modulating intracellular cAMP signaling. In cultured PTCs, [(125)I]Tyr-labeled ANG II and fluorescein labeled-ANG II were internalized in a time-dependent manner and colocalized with the endosome marker Alexa Fluor 594-transferrin. Endocytosis of extracellular ANG II was inhibited by the AT(1) receptor blocker losartan (16.5 +/- 4.6%, P < 0.01 vs. ANG II, 78.3 +/- 6.2%) and by the tyrosine phosphatase inhibitor phenylarsine oxide (PAO; 30.0 +/- 3.5%, P < 0.05 vs. ANG II). Intracellular ANG II levels were increased by approximately 58% (basal, 229.8 +/- 11.4 vs. ANG II, 361.3 +/- 11.8 pg ANG II/mg protein, P < 0.01), and the responses were blocked by losartan (P < 0.01), the cytoskeleton microtubule inhibitor colchicine (P < 0.05), and PAO (P < 0.01), whereas depletion of clathrin-coated pits with hyperosmotic sucrose had no effect (356.1 +/- 25.5 pg ANG II/mg protein, not significant). ANG II accumulation was associated with significant inhibition of both basal (control, 15.5 +/- 2.8 vs. ANG II, 9.1 +/- 2.4 pmol/mg protein, P < 0.05) and forskolin-stimulated cAMP signaling (forskolin, 68.7 +/- 8.6 vs. forskolin + ANG II, 42.8 +/- 13.8 pmol/mg protein, P < 0.01). These effects were blocked by losartan and PAO. We conclude that extracellular ANG II is internalized in PTCs through AT(1) receptor-mediated endocytosis and that internalized ANG II may play a functional role in proximal tubule cells by inhibiting intracellular cAMP signaling.

Effects of Blocking the Renin-Angiotensin System on Expression and Translocation of Protein Kinase C Isoforms in the Kidney of Diabetic Rats

BACKGROUND

High glucose and angiotensin II (Ang II) can activate protein kinase C (PKC) in diabetes mellitus. However, it is not clear which isoform of PKC is activated by glucose or Ang II. Our study focused on the effects of angiotensin blockade, using the angiotensin-converting enzyme inhibitor fosinopril, the Ang II receptor blocker irbesartan and their combination, on the expression and translocation of PKC isoforms alpha and betaII in the renal cortex and medulla in diabetes.

METHODS

Hyperglycemia was induced with streptozotocin and diabetic rats were randomized to 4 groups: diabetic control, irbesartan group (40 mg/kg daily), fosinopril group (40 mg/kg daily) and combination group (irbesartan plus fosinopril, 20 mg/kg daily, respectively); age-matched normal rats served as normal control. After 4 weeks, expression and translocation of PKC-alpha and -betaII in the renal cortex and medulla were assessed by immunohistochemistry and Western immunoblotting.

RESULTS

The expression of PKC-alpha in the membrane and cytosol fractions from the renal cortex was significantly higher in diabetic rats (276.83 +/- 32.44% in membrane, 149.04 +/- 23.42% in cytosol) than that in normal ones. The expression of PKC-betaII in the renal cortex of diabetic rats decreased significantly in the membrane (50.00 +/- 11.68%, p < 0.05) and remained unchanged in the cytosol (94.51 +/- 11.69%, p > 0.05) compared with normal controls. Treatment with irbesartan, fosinopril and their combination partially corrected the abnormalities mentioned above. For the expression of PKC-alpha and -betaII in the medulla, no difference was detected among the 5 groups.

CONCLUSION

The renin-angiotensin system was implicated in the pathogenesis of diabetic nephropathy by regulating the activation of PKC isoforms.

Renal angiotensin type 2 receptors mediate natriuresis via angiotensin III in the angiotensin II type 1 receptor-blocked rat

Whereas angiotensin (Ang) II is the major effector peptide of the renin-angiotensin system, its metabolite, des-aspartyl1-Ang II (Ang III), may also have biologic activity. We investigated the effects of renal interstitial (RI) administration of candesartan (CAND), a specific Ang II type 1 receptor (AT1) blocker, with and without coinfusion of PD-123319 (PD), a specific Ang II type 2 receptor (AT2) blocker, on Na+ excretion (UNaV) in uninephrectomized rats. We also studied the effects of unilateral RI infusion of Ang II or Ang III on UNaV with and without systemic infusion of CAND with the noninfused kidney as control. In rats receiving normal Na+ intake, RI CAND increased UNaV from 0.07+/-0.08 to 0.82+/-0.17 micromol/min (P<0.01); this response was abolished by PD. During Na+ restriction, CAND increased UNaV from 0.06+/-0.02 to 0.1+/-0.02 micromol/min (P<0.05); this response also was blocked by PD. In rats with both kidneys intact, in the absence of CAND, unilateral RI infusion of Ang III did not significantly alter UNaV. However, with systemic CAND infusion, RI Ang III increased U(Na)V from 0.08+/-0.01 micromol/min to 0.18+/-0.04 micromol/min (P<0.01) at 3.5 nmol/kg per minute, and UNaV remained elevated throughout the infusion; this response was abolished by PD. However, RI infusion of Ang II did not significantly alter UNaV at any infusion rate (3.5 to 80 nmol/kg per minute) with or without systemic CAND infusion. These results suggest that intrarenal AT1 receptor blockade engenders natriuresis by activation of AT2 receptors. AT2 receptor activation via Ang III, but not via Ang II, mediates the natriuretic response in the presence of systemic AT1 receptor blockade.

Macrophages and progressive tubulointerstitial disease

Macrophages and progressive tubulointerstitial disease. In chronic renal disease, tubulointerstitial inflammation and injury is associated with infiltrating macrophages. As a consequence of primary injury, proteinuria, chronic hypoxia, and glomerular-derived cytokines may all differentially modulate the expression of factors that promote macrophage recruitment. In addition to adhesion molecules and chemokines, products of complement system and renin-angiotensin system activation may direct this process. Once present at interstitial sites, macrophages interact with resident cells and extracellular matrix to generate a proinflammatory microenvironment that amplifies tissues injury and promotes scarring. There is now increasing evidence for the efficacy of interventions directed against factors that recruit, activate, or are produced by macrophages. A detailed understanding of the biology of this area may lead to the further development of therapies that will improve the outcome of renal disease.

Nephron function in transgenic mice with selective vascular or tubular expression of Angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) null mice display aberrant renal pathology. Inadequate formation of angiotensin II (Ang II) results in hypotension, loss of fluid homeostasis, lack of urine concentration, and failure to regulate GFR through the tubuloglomerular feedback (TGF) mechanism. For examining the tissue-specific role of ACE in renal structure and regulation of renal filtrate formation, single-nephron GFR, proximal tubular fluid reabsorption, and TGF responsiveness were determined in mice that expressed ACE in only one tissue. Maximum TGF responses in mice that expressed somatic ACE (sACE) in proximal tubule cells (Gs strain) or germinal ACE in the serum (Pg strain) were reduced significantly compared with wild-type (WT) mice. In contrast, TGF responses in mice that expressed sACE in vascular endothelial cells (Ts strain) were not different from control. Single-nephron GFR was reduced in Ts compared with WT mice, but fractional reabsorption and therefore glomerulotubular balance were not distinguishable. BP responses to exogenous Ang I were diminished in Ts, Gs, and Pg mice, whereas those to Ang II were the same in the different strains. Plasma and renal tissue Ang I of all transgenic mouse strains was significantly higher than WT, whereas Ang II levels were generally lower; aldosterone levels were significantly lower than WT in Ts mice but not in the two other transgenic strains. Our results demonstrate that vascular expression of sACE can largely but not completely restore TGF regulation of GFR. Proximal fluid reabsorption in the chronic absence of proximal tubule ACE is normal.

Functional expression of the renin-angiotensin system in human podocytes

Experimental and clinical studies impressively demonstrate that angiotensin-converting enzyme inhibitors (ACEI) and angiotensin receptor blockers (ARB) significantly reduce proteinuria and retard progression of glomerular disease. The underlying intraglomerular mechanisms are not yet fully elucidated. As podocyte injury constitutes a critical step in the pathogenesis of glomerular proteinuria, beneficial effects of ACEI and ARB may partially result from interference with a local renin-angiotensin system (RAS) in podocytes. The knowledge of expression and function of a local RAS in podocytes is limited. In this study, we demonstrate functional expression of key components of the RAS in differentiated human podocytes: podocytes express mRNA for angiotensinogen, renin, ACE type 1, and the AT(1) and AT(2) angiotensin receptor subtypes. In Western blot experiments and immunostainings, expression of the AT(1) and AT(2) receptor was demonstrated both in differentiated human podocytes and in human kidney cortex. ANG II induced a concentration-dependent increase in cytosolic Ca(2+) concentration via AT(1) receptors in differentiated human podocytes, whereas it did not increase cAMP. Furthermore, ANG II secretion was detected, which was blocked by neither the ACEI captopril nor the renin inhibitor remikiren nor the chymase inhibitor chymostatin. ANG II secretion of podocytes was not increased by mechanical stress. Finally, ANG II was found to increase staurosporine-induced apoptosis in podocytes. We speculate that ACEI and ARB exert their beneficial effects, in part, by interfering with a local RAS in podocytes. Further experiments are required to identify the underlying molecular mechanism(s) of podocyte protection.

Rat strain effects of AT1 receptor activation on D1 dopamine receptors in immortalized renal proximal tubule cells

The dopaminergic and renin-angiotensin systems regulate blood pressure, in part, by affecting sodium transport in renal proximal tubules (RPTs). We have reported that activation of a D1-like receptor decreases AT1 receptor expression in the mouse kidney and in immortalized RPT cells from Wistar-Kyoto (WKY) rats. The current studies were designed to test the hypothesis that activation of the AT1 receptor can also regulate the D1 receptor in RPT cells, and this regulation is aberrant in spontaneously hypertensive rats (SHRs). Long-term (24 hours) stimulation of RPT cells with angiotensin II, via AT1 receptors increased total cellular D1 receptor protein in a time- and concentration-dependent manner in WKY but not in SHR cells. Short-term stimulation (15 minutes) with angiotensin II did not affect total cellular D1 receptor protein in either rat strain. However, in the short-term experiments, angiotensin II decreased cell surface membrane D1 receptor protein in WKY but not in SHR cells. D1 and AT1 receptors colocalized (confocal microscopy) and their coimmunoprecipitation was greater in WKY than in SHRs. However, AT1/D1 receptor coimmunoprecipitation was decreased by angiotensin II (10(-8) M/24 hours) to a similar extent in WKY (-22+/-8%) and SHRs (-22+/-12%). In summary, these studies show that AT1 and D1 receptors interact differently in RPT cells from WKY and SHRs. It is possible that an angiotensin II-mediated increase in D1 receptors and dissociation of AT1 from D1 receptors serve to counter regulate the long-term action of angiotensin II in WKY rats; different effects are seen in SHRs.

Effects of suppressing intrarenal angiotensinogen on renal transforming growth factor-beta1 expression in acute ureteral obstruction

BACKGROUND

Angiotensin II (Ang II) mediates the up-regulation of fibrogenic factors such as transforming growth factor-beta1 (TGF-beta1) in chronic renal diseases. In addition, it has been proposed that the intrarenal renin-angiotensin system (RAS) is as important as the systemic RAS in kidney disease progression.

METHODS

We suppressed angiotensinogen (AGT) gene expression in the kidney by transferring recombinant adenoviral vectors carrying a transgene expressing AGT antisense mRNA, and determined the effect of the local inhibition of the RAS on TGF-beta1 synthesis in the kidneys of rats with unilateral ureteral obstruction (UUO). Immediately after UUO, recombinant adenovirus vectors were injected intraparenchymally into the cortex of obstructed kidneys.

RESULTS

beta-galactosidase (beta-gal)-stained kidney sections revealed the efficient transduction of the recombinant adenoviral vectors into tubular epithelial cells. Kidney cortex injected with AGT antisense showed significantly lower native AGT mRNA and protein expressions than control UUO kidneys at 24 hours and 5 days post-UUO. TGF-beta1 was significantly up-regulated in the renal cortex 24 hours and 5 days post-UUO, whereas AGT antisense-injected UUO rats showed significantly reduced TGF-beta1 expression compared to control UUO rats. Both fibronectin and collagen type I expressions were increased 24 hours and 5 days post-UUO, and these augmentations were considerably reduced by AGT antisense RNA treatment.

CONCLUSION

This study demonstrates that the suppression of intrarenal RAS prevents the formation of renal cortical TGF-beta1, and of related fibrogenic factors, in early UUO.

Angiotensin II stimulates alpha3(IV) collagen production in mouse podocytes via TGF-beta and VEGF signalling: implications for diabetic glomerulopathy

BACKGROUND

The podocyte is bathed in an angiotensin II (AngII)-rich ultrafiltrate, but the impact of AngII on podocyte pathobiology is not well known. Because podocytes play a direct role in the glomerular basement membrane (GBM) thickening of diabetes, the alpha3(IV) collagen chain was examined. Podocyte expression of alpha3(IV) collagen may involve the transforming growth factor-beta (TGF-beta) and vascular endothelial growth factor (VEGF) systems.

METHODS

Cultured mouse podocytes were treated with various doses of AngII for selected periods of time, with or without inhibitors of TGF-beta and VEGF signalling, SB-431542 and SU5416, respectively. TGF-beta1 and VEGF were assayed by enzyme-linked immunosorbent assay (ELISA); alpha3(IV) collagen, TGF-beta type II receptor and phospho-Smad2 were assayed by immunoblotting.

RESULTS

AngII >or=10(-10) M was found to stimulate the production of alpha3(IV) collagen significantly in as short a time as 3 h. The expression of alpha3(IV) collagen was influenced by the TGF-beta system, but AngII did not increase the podocyte's production of TGF-beta1 ligand; rather, it increased the expression of the TGF-beta type II receptor and activated the TGF-beta signalling system through Smad2. Despite the TGF-beta receptor upregulation, synergy between AngII and TGF-beta1 to boost alpha3(IV) collagen production was not observed. However, blockade of TGF-beta signalling with SB-431542 prevented AngII from stimulating alpha3(IV) collagen production. Podocyte expression of alpha3(IV) collagen was also increased by the autocrine activity of VEGF. Podocytes were stimulated to secrete VEGF by 10(-10) M or higher AngII after 48 h. Blockade of the endogenous VEGF activity by SU5416 prevented AngII-stimulated alpha3(IV) collagen production.

CONCLUSIONS

AngII stimulates the podocyte to produce alpha3(IV) collagen protein via mechanisms involving TGF-beta and VEGF signalling. Alterations in alpha3(IV) collagen production may contribute to GBM thickening and perhaps proteinuria in diabetes.

Micropuncture determination of nephron function in mice without tissue angiotensin-converting enzyme

To determine the role of the local renin-angiotensin system in renal function, micropuncture was performed on two lines of mice in which genetic changes to the angiotensin-converting enzyme (ACE) gene markedly reduced or eliminated the expression of renal tissue ACE. Whereas blood pressure is low in one line (ACE 2/2), it is normal in the other (ACE 1/3) due to ectopic hepatic ACE expression. When normalized for renal size, levels of glomerular filtration rate [GFR; μl·min−1·g kidney wt−1(KW)] and single-nephron GFR (SNGFR; nl·min−1·g KW−1) were similar between wild-type (WT) and ACE 1/3 mice, while both measures were significantly reduced in ACE 2/2 mice (WT: 500 ± 63 and 41.7 ± 3.5; ACE 1/3: 515.8 ± 71 and 44.3 ± 3.3; ACE 2/2: 131.4 ± 23 and 30.3 ± 3.5). Proximal fractional reabsorption was not significantly different between WT and ACE 1/3 mice (51 ± 3.5 and 49 ± 2.3%), and it was increased significantly in ACE 2/2 mice (74 ± 3.5%). Infusion of ANG II (50 ng·kg−1·min−1) increased mean arterial pressure by ∼7 mmHg in all groups of mice and reduced SNGFR in WT and ACE 1/3 mice (to 30.9 ± 2.8 and 31.9 ± 2.5 nl·min−1·g KW−1) while increasing it in ACE 2/2 mice (to 55.3 ± 5.3 nl·min−1·g KW−1) despite an increase in total renal vascular resistance. The tubuloglomerular feedback (TGF) response was markedly reduced in ACE 1/3 mice (stop-flow pressure change −2.5 ± 0.9 mmHg) compared with WT despite similar blood pressures (−8.3 ± 0.6 mmHg). In ACE 2/2 mice, TGF was absent (−0.7 ± 0.2 mmHg). We conclude that the chronic lack of ACE, and presumably ANG II generation, in the proximal tubule was not associated with sustained proximal fluid transport defects. However, renal tissue ACE is an important contributor to TGF.

Combination therapy with ACE inhibitors and angiotensin II receptor blockers to halt progression of chronic renal disease: Pathophysiology and indications

It is no a secret that we are confronted by an alarmingly increasing number of patients with progressive renal disease. There is ample evidence for the notion that angiotensin II (Ang II) is a major culprit in progression. The vasopeptide Ang II turned out to have also multiple nonhemodynamic pathophysiologic actions on the kidney, including proinflammatory and profibrogenic effects. Diverse complex Ang II generating systems have been identified, including specifically local tissue-specific renin-angiotensin systems (RAS). For example, proximal tubular cells have all components required for a functional RAS capable of synthesizing Ang II. On the other hand, Ang II is not the only effector of the RAS and other peptides generated by the RAS influence renal function and structure as well. Moreover, the discoveries that Ang II can be generated by enzymes other than angiotensin-converting enzyme (ACE) and that Ang II and other RAS derived peptides bind to various receptors with different functional consequences have further added to the complexity of this system. Several major clinical trials have clearly shown that ACE inhibitor treatment slows the progression of renal diseases, including in diabetic nephropathy. Well-controlled studies demonstrated that this effect is in part independent of blood pressure control. More recently, with Ang II type 1 receptor (AT(1)) receptor antagonists a similarly protective effect on renal function was seen in patients with type 2 diabetes. Neither ACE inhibitor treatment nor AT(1) receptor blockade completely abrogate progression of renal disease. A recently introduced novel therapeutic approach is combination treatment comprising both ACE inhibitor and AT(1) receptor antagonists. The rationale for this approach is based on several considerations. Small-scale clinical studies, mainly of crossover design, documented that combination therapy is more potent in reducing proteinuria in patients with different chronic renal diseases. Blood pressure as an important confounder was, however, significantly lower in the majority of this studies in the combination treatment arms compared to the respective monotherapies. In a recent prospective study Japanese authors avoided this confounder and demonstrated that combination therapy reduced hard end-points (end stage renal failure or doubling of serum creatinine concentration) by 50% compared to the respective monotherapies. This effect could not be explained by a more pronounced reduction of blood pressure in the combination therapy group. Although these results are encouraging, administration of combination therapy should be reserved currently to special high risk groups. Further studies are necessary to confirm these promising results. It is possible that combination therapy may increase the risk of hyperkalemia, particularly when with coadministered with medications such as nonsteroidal anti-inflammatory drugs (NSAIDs) or spironolactone. In our opinion patients with proteinuria >1 g/day despite optimal blood pressure control under RAS-blocking monotherapy are a high-risk group which will presumably benefit from combination therapy.

Translational regulation of vascular endothelial growth factor expression in renal epithelial cells by angiotensin II

ANG II regulates growth factor expression in the kidney. We investigated whether ANG II regulated vascular endothelial growth factor (VEGF) synthesis in proximal tubular epithelial (MCT) cells. ANG II (1 nM) increased VEGF protein expression within 5 min, the effect lasting for 30 min. There was no change in VEGF mRNA levels or mRNA stability, and transcription inhibitors did not affect ANG II-induced VEGF expression. Regulation of VEGF translation was investigated. Polyribosomal analysis revealed selective enrichment of heavy ribosomes (polysomes) with VEGF mRNA transcripts compared with light ribosomes in ANG II-treated cells, although distribution of GAPDH was unaltered. In vitro translation of total RNA from polysomal fractions showed selective increase in VEGF protein synthesis in ANG II-treated cells. Preincubation with LY-294002, a PI 3-kinase inhibitor, or expression of dominant-negative Akt prevented ANG II-stimulated increase in VEGF translation. ANG II increased phosphorylation of eukaryotic initiation factor 4E and its binding protein 4E-BP1, critical events that regulate the initiation phase of protein translation. ANG II failed to increase VEGF mRNA translation in cells stably expressing the phosphorylation mutant of 4E-BP1. Our data illustrate that a rapid increase in VEGF protein expression by ANG II is regulated at the initiation phase of translation of VEGF mRNA in renal epithelial cells. Regulation of VEGF translation by ANG II represents a novel pathway of renal response to injury.

Activity of angiotensin-converting enzyme after treatment with L-arginine in renovascular hypertension

The renin-angiotensin system plays a role in the pathophysiology of renovascular hypertension. In addition, some studies have demonstrated a beneficial effect of L-arginine (L-Arg), the precursor of nitric oxide (NO), in this model of hypertension. This study was designed to investigate the effects of L-Arg on cardiovascular parameters and on the activity of the angiotensin-converting enzyme (ACE), after 14 days of renovascular hypertension. The experiments were performed on conscious male Wistar rats. Two-kidney, one-clip renovascular hypertension (2KIC) was initiated in rats by clipping the left renal artery during 14 days, while control rats were sham-operated. One group was submitted to a similar procedure and treated with L-Arg (10 mg/ml; average intake of 300mg/day) from the 7th to the 14th day after surgery, whereas the respective control group received water instead. At the end of the treatment period, the mean arterial pressure (MAP) was measured in conscious animals. The rats were sacrificed and the ACE activity was assayed in heart and kidneys, using Hip-His-Leu as substrate. In a separate group, the heart was removed, the left ventricle (LV) was weighed and the LV/body weight ratios (LV/BW) were determined. We observed significant differences in MAP between the L-Arg-treated and untreated groups (129 +/- 7 vs. 168 +/- 6 mmHg; P< 0.01). The cardiac hypertrophy described for this model of hypertension was attenuated in the 2K1C-L-Arg-treated group (14th day, wet LV/BW: 2K1C-L-Arg = 1.88 +/- 0.1; 2K1C = 2.20 +/- 0.1 mg/g; P < 0.05). L-Arg administration caused an important decrease in cardiac ACE activity (2K1C-L-Arg: 118 +/- 15; 2K1C: 266 +/- 34 micromol/min/mg; P < 0.01). L-Arg also decreased the ACE activity in the clipped kidney by 47% (P < 0.01), but not in the nonclipped kidney. These data suggest that increased NO formation and reduced angiotensin II formation are involved in the anthihypertensive effect of orally administered L-arginine.

A novel mechanism for angiotensin II formation in streptozotocin-diabetic rat glomeruli

Recent evidence suggests that the intrarenal renin-angiotensin system (RAS) may play an important role in the development of glomerular changes associated with diabetic nephropathy. In this study, the glomerular RAS was examined in male Sprague-Dawley rats made diabetic with streptozotocin (STZ), and the findings compared with those obtained in control nondiabetic rats. In diabetic rat glomerular extracts, angiotensinogen and angiotensin II (ANG II) levels were increased significantly by 2.2- and 1.9-fold, respectively, compared with nondiabetic controls. No significant differences in ANG I and angiotensin-converting enzyme (ACE) levels were observed between these groups. The HPLC analysis of the glomerular extracts demonstrated that exogenous ANG I was converted into various ANG peptides including ANG II, ANG1-9, and ANG1-7. A significant increase in formation of ANG II from exogenous ANG I was observed in STZ rats compared with control rats. Preincubation of glomerular extracts with captopril resulted in a 20-30% decrease in ANG II conversion from exogenous ANG I in diabetic and control rats. The possible role of ANG1-9 in formation of ANG II was examined by HPLC. Exogenous ANG1-9 in glomerular extracts was converted into ANG II, this conversion being significantly higher in STZ rats than in control rats. These findings provide new information that ANG1-9 is produced in rat glomerular extracts, can be converted to ANG II, and that this conversion is also stimulated in diabetic rat glomeruli. Thus this study demonstrates that in diabetic rats, glomerular ANG II levels are increased due to an increase in angiotensinogen and an increase in the formation of ANG II.

Renoprotective effect of benazepril on diabetic nephropathy mediated by P42/44MAPK

The effects of benazepril on P42/44MAPK, angiotensin II expression in renal tissue and renal pathological change of the experimental diabetic rats were assessed and the possible mechanism of benazepril's renoprotective effect was explored. Adult male Wistar rats, 11-12 weeks age, weighing initially 160 to 200 g were randomly allocated into 2 groups: control group (A, n = 6) and experimental group (n = 12). Diabetic rats in experimental group were rendered diabetic by intraperitoneal injection of Streptozotocin (60 mg/kg body weight), and randomly subdivided into B group (diabetic control) and C group (diabetic rats treated with benazepril, 6 mg/kg every day). Studies were performed 8 weeks after induction of diabetes. Twenty-four h urine of every rat was collected to detect urine creatinine. Serum glucose concentration and serum creatinine were determined by collecting blood samples from the inferior vena cava. Body and kidney weight were recorded. Creatinine clearance (Ccr) and ratio of kidney weight to body weight were calculated. Plasma and renal tissue angiotensin II concentration was assayed by radioimmunoassay (RIA). The phospo-p44/42MAPK protein expression was detected by Western-blot. The results showed that benazepril had no significant effect on the blood glucose level in diabetic rats in two experimental groups. Ccr and ratio of kidney weight to body weight were increased in group B (P < 0.01) as compared with normal rats at the end of the 8th week. At the end of the 8th week, Ccr in group C was lower than that in group B (P < 0.01). The ratio of kidney weight to body weight in group C was lower than that in group B at the 8th week. There were glomeruli hypertrophy and slight or moderate mesangium proliferation in diabetic rats, while there was fragmentally proliferative mesangium in group C at the end of the 8th week. Renal tissue angiotensin II concentration was significantly increased in group B, while benazepril could significantly decrease the concentration of angiotensin II in renal tissue. The expression of the phospo-p44/42MAPK protein in group B was increased as compared with group A, while it was decreased in group C as compared with group B. P42/ 44MAPK pathway participated in the pathogenesis of diabetic nephropathy. Benazepril can eliminate high filtration of glomeruli, decrease proteinuria, and eliminate renal hypertrophy as well as renal destruction. Renoprotective effect of benazepril in diabetic rats may be partly related to the inhibition of angiotensin II -P42/44MAPK pathway.

Physiological Relevance of Renin/Prorenin Binding and Uptake

There is compelling physiological evidence of binding and uptake of renin and prorenin in tissues. A number of molecules with the ability to bind renin and prorenin have been identified and have been characterized to varying degrees. It remains unclear, however, just how many renin/prorenin binding proteins and receptors exist and what their physiological functions may be. The possible functions of renin/prorenin binding and uptake are manifold, and include clearance of renin and prorenin from the circulation, local generation of angiotensins, activation of prorenin on the cell surface, trafficking of prorenin between cellular and extracellular compartments as part of a complex processing machinery, and signal transduction both via direct receptor mediated signaling, and via modulation of O-linkage of N-acetyl-glucosamine to cellular proteins. Some of these functions may involve single renin/prorenin binding sites or receptors, while others may require multiple binding sites and receptors. This review describes the physiological studies that have provided evidence of renin/prorenin uptake from the circulation, summarizes our knowledge of renin/prorenin binding proteins and receptors, and postulates new roles for renin/prorenin binding and uptake in tissues.