Why are angiotensin concentrations so high in the kidney?

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

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

Effects of dietary salt on intrarenal angiotensin system, NAD(P)H oxidase, COX-2, MCP-1 and PAI-1 expressions and NF-kappaB activity in salt-sensitive and -resistant rat kidneys

BACKGROUND

Chronic consumption of a high-salt diet causes hypertension (HTN) and renal injury in Dahl salt-sensitive (SSR) but not salt-resistant rats (SRR). These events are, in part, mediated by oxidative stress and inflammation in the kidney and vascular tissues. Activation of the angiotensin II type 1 (AT(1)) receptor plays an important role in the pathogenesis of oxidative stress and inflammation in many hypertensive disorders. However, the systemic renin-angiotensin system (RAS) is typically suppressed in salt-sensitive HTN. This study was designed to test the hypothesis that differential response to a high-salt diet in SSR versus SRR may be related to upregulation of tissue RAS and pathways involved in inflammation and reactive oxygen species (ROS) production.

METHODS AND RESULTS

SSR and SRR were studied 3 weeks after consumption of high- (8%) or low-salt (0.07%) diets. The SSR consuming a low-salt diet exhibited significant increases in AT(1) receptor, cyclooxygenase (COX) 2, plasminogen activator inhibitor (PAI) and phospho-I kappaB in the kidney as compared to those found in SRR. The high-salt diet resulted in severe HTN and proteinuria (in SSR but not SRR) and marked elevations of renal tissue monocyte chemoattractant protein 1, p22(phox), NADPH oxidase subunit 4, angiotensin-II-positive cell count, infiltrating T cells and macrophages and further increases in AT(1) receptor, COX-2, PAI-1 and phospho-I kappaB in the SSR group. The high-salt diet significantly lowered plasma renin activity (PRA) in SRR but not in the SSR. COX-1 abundance was similar on the low-salt diet and rose equally with the high-salt diet in both groups. Among subgroups of animals fed the low-salt diet, kidney glutathione peroxidase (GPX) abundance was significantly lower in the SSR than SRR. The high-salt diet raised GPX and mitochondrial superoxide dismutase (SOD) abundance in the SRR kidneys but failed to do so in SSR. Cu/Zn-SOD abundance was similar in the subgroups of SSR and SRR fed the low-salt diet. The high-salt diet resulted in downregulation of Cu/Zn-SOD in SSR but not SRR.

CONCLUSIONS

Salt sensitivity in the SSR is associated with upregulations of the intrarenal angiotensin system, ROS-generating and proinflammatory/profibrotic proteins and an inability to raise antioxidant enzymes and maximally suppress PRA in response to high salt intake. These events can contribute to renal injury with high salt intake in SSR.

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

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

Diabetic nephropathy and proximal tubule ROS: challenging our glomerulocentricity

Diabetic nephropathy is currently viewed as a predominantly glomerular process with glomerular injury driving secondary tubular loss. Brezniceanu and colleagues apply transgenic methods to support a prominent role for reactive oxygen species as mediators and for the proximal tubule as a major site of early disease activity in diabetes. Results support evidence for early tubular apoptosis and atrophy in human diabetic nephropathy.

Angiotensin-converting enzyme I/D and alpha-adducin Gly460Trp polymorphisms: from angiotensin-converting enzyme activity to cardiovascular outcome

The angiotensin-converting enzyme (ACE) I/D and the alpha-adducin (ADD1) Gly460Trp polymorphisms are associated with cardiovascular risk factors. In a prospective population study and in cell models, we investigated the combined effects of these 2 polymorphisms. We randomly recruited 1287 white subjects (women: 50.0%; mean age: 55.9 years). We obtained outcomes from registries and repeat examinations (median 3). Over 9.0 years (median), 178 fatal or nonfatal cardiovascular events occurred. In ADD1 Trp allele carriers, the multivariate-adjusted hazard ratios associated with ACE DD versus I were 1.72 (P=0.007) for total mortality, 2.35 (P=0.02) for cardiovascular mortality, 2.02 (P=0.005) for all cardiovascular events, and 2.59 (P=0.03) for heart failure. In contrast, these hazard ratios did not reach significance in ADD1 GlyGly homozygotes (0.08<or=P<or=0.90). The positive predictive value and attributable risk associated with ACE DD homozygosity combined with mutated ADD1 were 36.2% and 10.3%, respectively. To clarify our epidemiological observations, we investigated the effects of mutated human ADD1 on the membrane-bound ACE activity in fibroblasts from 51 volunteers and in transfected human embryonic kidney cells (31 experiments). In fibroblasts (5.10 versus 3.63 nanomoles of generated hippuric acid per milligram of protein per minute; P=0.0021) and human embryonic kidney cells (1.086 versus 0.081 nmol/mg per minute; P=0.017), the membrane-bound ACE activity increased in the presence but not absence of the ADD1 Trp allele. In conclusion, the combination of ACE DD homozygosity and mutated ADD1 worsened cardiovascular prognosis to a similar extent as classic risk factors, possibly because of increased membrane-bound ACE activity in subjects carrying the ADD1 Trp allele.

Regulation of Cardiovascular Control Mechanisms by Angiotensin-(1–7) and Angiotensin-Converting Enzyme 2

Among the molecular forms of angiotensin peptides generated by the action of renin on angiotensinogen (Aogen), both angiotensin II (Ang II) and the amino terminal heptapeptide angiotensin-(1–7) [Ang-(1–7)] are critically involved in the long-term control of tissue perfusion, cell-cell communication, development, and growth. Whereas an impressive body of literature continues to uncover pleiotropic effects of Ang II in the regulation of cell function, research on Ang-(1–7) has a shorter history as it was only 16 yr ago that a biological function for this heptapeptide was first demonstrated in the isolated rat neuro-hypophysial explant preparation (1). On the contrary, the synthesis of angiotonin/ hypertensin (now Ang II) was first obtained in 1957 (2), three decades ahead of the discovery of Ang-(1–7) biological properties.

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.

The Effects of Angiotensin-Converting Enzyme Inhibition on the Urinary Excretion of NTproBNP in Male Volunteers

AIMS

This study was designed to test if the renal excretion of the N-terminal prohormone of the B-type natriuretic peptide (NTproBNP) is modulated by angiotensin-converting enzyme inhibition (ACE-I).

METHODS

Following 7 days on a sodium-enriched diet and an induction period of 4 days with incremental dosages of enalapril (2.5, 5, 7.5, 10 mg) or placebo, 10 healthy subjects underwent crossover and double-blind treatment with 20 mg enalapril sodium or placebo at 8:00 h. After 4 h (at 12:00 h), 20 ml.kg(-1) NaCl 0.9% was infused over 60 min. Hemodynamics were determined and blood and urine were sampled at 8:00, 12:00, 13:00, 14:00, 16:00, and 18:00 h. Angiotensin II (AII), NTproANP, and NTproBNP were determined by radio- and electrochemiluminescence immunoassays.

RESULTS

In the whole group, ACE-I led to a lower arterial blood pressure during the fourth day of induction and during the time from 8:00 to 16:00 h, a decrease in AII levels from 8:00 to 14:00 h (p < 0.05), and to a higher cumulative urine output (p < 0.05) in comparison with control. Neither cumulative sodium nor urinary NTproBNP/creatinine excretion were significantly increased after ACE-I. However, a subgroup of 6 volunteers - showing an increase in sodium excretion after ACE-I - also demonstrated lower AII levels at 13:00 h, a higher cumulative urine flow, and a higher urinary NTproBNP/creatinine excretion in comparison with control (all: p < 0.05).

CONCLUSIONS

This suggests that the renal excretion of NTproBNP is modified by enalapril. However, it remains to be determined if this is a direct effect of ACE-I, the decrease in arterial blood pressure, or other potentially confounding variables like bradykinin or endopeptidase activity.

Mechanisms of disease: oxidative stress and inflammation in the pathogenesis of hypertension

Animal studies have shown that oxidative stress and renal tubulointerstitial inflammation are associated with, and have major roles in, the pathogenesis of hypertension. This view is supported by the observations that alleviation of oxidative stress and renal tubulointerstitial inflammation reduce arterial pressure in animal models. Conversely, hypertension has been shown to cause oxidative stress and inflammation in renal and cardiovascular tissues in experimental animals. Taken together, these observations indicate that oxidative stress, inflammation and arterial hypertension participate in a self-perpetuating cycle which, if not interrupted, can lead to progressive cardiovascular disease and renal complications. These events usually occur in an insidious and asymptomatic manner over an extended period following the onset of hypertension. Severe target organ injury can, however, occasionally occur precipitously in the course of malignant or accelerated hypertension. Given the high degree of heterogeneity of hypertensive disorders, the factor(s) initiating the vicious cycle described vary considerably in different forms of hypertension. For instance, oxidative stress in the kidney and vascular tissue is the primary mediator in the pathogenesis of angiotensin-induced, and perhaps lead-induced, hypertension. By contrast, increased arterial pressure is probably the initiating trigger in salt-sensitive hypertension. Although the initiating factor might vary between hypertensive disorders, according to the proposed model, the three components of the cycle eventually coalesce in all forms of hypertension.

AT1 receptor blockade is superior to conventional triple therapy in protecting against end-organ damage in Cyp1a1-Ren-2 transgenic rats with inducible hypertension

OBJECTIVE

In the present study we compared the effects of treatment with the AT1 receptor antagonist candesartan and of 'triple therapy' (hydralazine, hydrochlorothiazide, reserpine) on the course of blood pressure, cardiac hypertrophy and angiotensin II concentrations after induction of hypertension in transgenic rats with inducible expression of the mouse renin gene (Cyp1a1-Ren-2 rats).

METHODS

Hypertension was induced in Cyp1a1-Ren-2 rats through dietary administration of the natural xenobiotic indole-3-carbinol (I3C, 0.3%) for 4 days. Starting on the day before administration of I3C, rats were treated either with candesartan or received triple therapy for 9 days. Systolic blood pressure was measured in conscious animals. Rats were decapitated and angiotensin II levels in plasma and in whole kidney and left ventricular tissues were determined by radioimmunoassay.

RESULTS

Administration of I3C resulted in the development of severe hypertension and cardiac hypertrophy that was accompanied by marked elevations of plasma and tissue angiotensin II concentrations. Candesartan treatment prevented the development of hypertension and cardiac hypertrophy and was associated with a reduction of tissue angiotensin II concentrations. In contrast, triple therapy, despite maintaining systolic blood pressure in the normotensive range, did not prevent the development of cardiac hypertrophy and tissue angiotensin II augmentations.

CONCLUSIONS

Our findings indicate that hypertension in Cyp1a1-Ren-2 rats is a clearly angiotensin II-dependent model of hypertension with elevated circulating and tissue angiotensin II concentrations, and that antihypertensive treatment with AT1 receptor blockade is superior to conventional triple therapy in effective protection against hypertension-induced end-organ damage in this rat model.

Angiotensin II increases chloride absorption in the cortical collecting duct in mice through a pendrin-dependent mechanism

Pendrin (Slc26a4) localizes to type B and non-A, non-B intercalated cells in the distal convoluted tubule, the connecting tubule, and the cortical collecting duct (CCD), where it mediates apical Cl(-)/HCO(3)(-) exchange. The purpose of this study was to determine whether angiotensin II increases transepithelial net chloride transport, J(Cl), in mouse CCD through a pendrin-dependent mechanism. J(Cl) and transepithelial voltage, V(T), were measured in CCDs perfused in vitro from wild-type and Slc26a4 null mice ingesting a NaCl-replete diet or a NaCl-replete diet and furosemide. In CCDs from wild-type mice ingesting a NaCl-replete diet, V(T) and J(Cl) were not different from zero either in the presence or absence of angiotensin II (10(-8) M) in the bath. Thus further experiments employed mice given the high-NaCl diet and furosemide to upregulate renal pendrin expression. CCDs from furosemide-treated wild-type mice had a lumen-negative V(T) and absorbed Cl(-). With angiotensin II in the bath, Cl(-) absorption doubled although V(T) did not become more lumen negative. In contrast, in CCDs from furosemide-treated Slc26a4 null mice, Cl(-) secretion and a V(T) of approximately 0 were observed, neither of which changed with angiotensin II application. Inhibiting ENaC with benzamil abolished V(T) although J(Cl) fell only approximately 50%. Thus substantial Cl(-) absorption is observed in the absence of an electromotive force. Attenuating apical anion exchange with the peritubular application of the H(+)-ATPase inhibitor bafilomycin abolished benzamil-insensitive Cl(-) absorption. In conclusion, angiotensin II increases transcellular Cl(-) absorption in the CCD through a pendrin- and H(+)-ATPase-dependent process.

Effects of Anesthesia on Plasma and Kidney ANG II Levels in Normotensive and ANG II-Dependent Hypertensive Rats

BACKGROUND

Previous studies have implicated that normotensive rats with normal renal renin activity respond to anesthesia and surgery with greater increases in plasma and kidney angiotensin II (ANG II) concentrations than ANG II-dependent hypertensive rats with intrarenal renin depletion. In the present study, we therefore compared plasma and kidney ANG II levels in anesthetized and conscious normotensive and ANG II-dependent hypertensive rats.

METHODS

Salt-replete Hannover-Sprague-Dawley rats (HanSD) served as controls. As models of ANG II-dependent hypertension we used: 1st, transgenic rats harboring the Ren-2 renin gene (TGR); 2nd, two-kidney, one-clip (2K1C) Goldblatt hypertensive rats, and, 3rd, ANG II-infused hypertensive rats. As additional model with enhanced renin-angiotensin system (RAS) activity, salt-depleted HanSD and TGR were employed.

RESULTS

In anesthetized salt-repleted HanSD, plasma and kidney ANG II levels were higher than in salt-repleted TGR, ANG II-infused and 2K1C rats. Salt depletion caused marked increases in ANG II levels in HanSD but did not alter them in TGR. In contrast, in conscious animals immediately after decapitation plasma and kidney ANG II levels were similar in salt-repleted and salt-depleted TGR, in ANG II-infused rats, in the clipped kidney of 2K1C rats and in salt-depleted HanSD and in all these groups they were significantly higher than in salt-repleted HanSD.

CONCLUSIONS

These findings indicate that anesthesia increases plasma and kidney ANG II levels in HanSD to a greater degree than in ANG II-dependent models of hypertension. Therefore, the results from studies employing anesthetized animals must be interpreted with caution.

ANG II type 2 receptors and neural control of intrarenal blood flow

We tested the hypothesis that activation of angiotensin type 2 (AT(2)) receptors, by both exogenous and endogenous ANG II, modulates neurally mediated vasoconstriction in the renal cortical and medullary circulations. Under control conditions in pentobarbital-anesthetized rabbits, electrical stimulation of the renal nerves (RNS; 0.5-8 Hz) reduced renal blood flow (RBF; -88 +/- 3% at 8 Hz) and cortical perfusion (CBF; -92 +/- 2% at 8 Hz) more than medullary perfusion (MBF; -67 +/- 6% at 8 Hz). Renal arterial infusion of ANG II, at a dose titrated to reduce RBF by approximately 40-50% (5-50 ng.kg(-1).min(-1)) blunted responses of MBF to RNS, without significantly affecting responses of RBF or CBF. Subsequent administration of PD123319 (1 mg/kg plus 1 mg.kg(-1).h(-1)) during continued renal arterial infusion of ANG II did not significantly affect responses of RBF or CBF to RNS but enhanced responses of MBF, so that they were similar to those observed under control conditions. In contrast, administration of PD123319 alone blunted responses of CBF and MBF to RNS. Subsequent renal arterial infusion of ANG II in PD123319-pretreated rabbits restored CBF responses to RNS back to control levels. In contrast, ANG II infusion in PD123319-pretreated rabbits did not alter MBF responses to RNS. These data indicate that exogenous ANG II can blunt neurally mediated vasoconstriction in the medullary circulation through activation of AT(2) receptors. However, AT(2)-receptor activation by endogenous ANG II appears to enhance neurally mediated vasoconstriction in both the cortical and medullary circulations.

Renovascular hypertension: a unique cause of unilateral focal segmental glomerulosclerosis

A 48-year-old man presented with malignant hypertension and massive proteinuria. Renal angiography showed complete obstruction of the left renal artery and 99mTc-mercaptoacetylglycine (MAG3) renography showed a nonfunctioning left kidney. Percutaneous transluminal renal angioplasty of the left renal artery was unsuccessful; hence, the patient underwent left nephrectomy because of uncontrolled hypertension and proteinuria. Histological examination of a right kidney specimen revealed lesions of focal segmental glomerulosclerosis with benign nephrosclerosis. In contrast, histology of the left kidney showed typical ischemic kidney with hypertrophy of arteriolar smooth muscle cells. The patient responded favorably to the nephrectomy, as his blood pressure and urinary protein dramatically decreased with no antihypertensive medication. This case illustrates the heterogeneous effect of the renin-angiotensin system on either kidney in patients with renovascular hypertension due to unilateral renal artery stenosis.

Angiotensin II-induced calcium signaling in the afferent arteriole from rats with two-kidney, one-clip hypertension

The aim of this study was to investigate ANG II-induced Ca2+signaling in freshly isolated afferent arterioles (AA) from two-kidney, one-clip hypertensive (2K1C) rats, which have an elevated plasma and renal ANG II level, and different perfusion pressure and vascular tone in the clipped and nonclipped kidney. The Ca2+responses in vessels from 2K1C and control rats were similar in all groups ( P > 0.1). The intracellular Ca2+(Cai2+) response in the afferent arteriole after 10−8M ANG II stimulation was 0.57 ± 0.10, 0.50 ± 0.07, 0.48 ± 0.04, and 0.36 ± 0.05 in the control, sham, nonclipped, and clipped kidney, respectively. These data were consistent with the finding of unchanged AT1aR mRNA levels in AAs from all groups. Although the absolute values were similar, the dose-response curves to ANG II were different. In the control, sham, and nonclipped kidney from 2K1C, the dose-response curve leveled off between 10−8and 10−6M ANG II. In the clipped kidney, the dose-response curve was linear, with a significantly increased response at 10−6M compared with 10−8M ANG II ( P < 0.05). Inhibition of cyclooxygenase-1 (COX-1) with indomethacin enhanced the ANG II response in the nonclipped (Δ0.30 ± 0.09) and clipped (Δ0.30 ± 0.09) kidneys from 2K1C ( P < 0.005), but not in control rats (Δ−0.02 ± 0.11, P > 0.8). Conclusively, the ANG II-induced Cai2+response was reduced by COX-1-derived prostaglandins in 2K1C, in contrast to control animals, where the COX-1 inhibition had no effect. COX-2 inhibition with NS-398 did not increase the ANG II-mediated Cai2+response in any of the groups.

Effect of angiotensin II blockade on a new congenic model of hypertension derived from transgenic Ren-2 rats

The generation of the Lew.Tg(mRen2) congenic hypertensive rat strain, developed through a backcross of the hypertensive (mRen2)27 transgenic rat with normotensive Lewis rats, provides a new model by which primary hypertension can be studied without the genetic variability found in the original strain. The purpose of this study was to characterize the Lew.Tg(mRen2) rats by dually investigating the effects of type 1 angiotensin II (ANG II) receptor (AT(1)) blockade and angiotensin-converting enzyme (ACE) activity inhibition on the ANG-(1-7)/ACE2 axis of the renin-angiotensin system in this new hypertensive model. The control of blood pressure elicited by 12-day administration of either lisinopril (mean difference change = 92 +/- 2, P < 0.05) or losartan (mean difference change = 69 +/- 2, P < 0.05) was associated with 54% and 33% increases in cardiac ACE2 mRNA and 54% and 43% increases in cardiac ACE mRNA, respectively. Lisinopril induced a 3.1-fold (P < 0.05) increase in renal cortical expression of ACE2, whereas losartan increased ACE2 mRNA 3.5-fold (P < 0.05). Both treatment regimens increased renal ACE mRNA 2.6-fold (P < 0.05). The two therapies augmented ACE2 protein activity, as well as increased cardiac and renal AT(1) receptor mRNAs. ACE inhibition reduced plasma ANG II levels (81%, P < 0.05) and increased plasma ANG-(1-7) (265%, P < 0.05), whereas losartan had no effect on the peptides. In contrast with what had been shown in normotensive rats, ACE inhibition decreased renal ANG II excretion and transiently decreased ANG-(1-7) excretion, whereas losartan treatment was associated with a consistent decrease in ANG-(1-7) urinary excretion rates. In response to the treatments, the expression of both renal cortical renin and angiotensinogen mRNAs was significantly augmented. The paradoxical effects of blockade of ANG II synthesis and activity on urinary excretion rates of the peptides and plasma angiotensins levels suggest that, in Lew.Tg(mRen2) congenic rats, a failure of compensatory ACE2 and ANG-(1-7)-dependent vasodepressor mechanisms may contribute both to the development and progression of hypertension driven by increased formation of endogenous ANG II.

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.

Effects of changes in sodium balance on plasma and kidney angiotensin II levels in anesthetized and conscious Ren-2 transgenic rats

OBJECTIVE

Since there is as yet no general agreement regarding the role of plasma and kidney angiotensin II (ANG II) in the development of hypertension in Ren-2 transgenic rats (TGR), in the present study we evaluated plasma and kidney ANG II levels in anesthetized and conscious TGR and in normotensive Hannover-Sprague-Dawley rats (HanSD) fed a normal salt diet (NS). Given the importance of ANG II in the development of salt-sensitive hypertension, and the fact that hypertensinogenic actions of ANG II are mediated via ANG II type 1 (AT1) receptors, the effects of high salt (HS) intake and of sodium depletion on blood pressure (BP), ANG II levels and kidney AT1 receptor protein expression in TGR and HanSD were also examined.

METHODS

Rats were maintained on a NS diet (0.6% NaCl) or fed a HS diet (2% NaCl) for 4 days or were sodium depleted (40 mg/l furosemide for 1 day followed by 3 days of 0.01% NaCl diet). They were sacrificed either by an overdose of anesthetic (thiopental sodium) or by decapitation (without anesthetic) and plasma and kidney ANG II levels were determined by radioimmunoassay during the prehypertensive (32 days old), the early (52 days) and the maintenance (90 days) phases of hypertension. Total kidney AT1 receptor protein levels were assessed by Western blot analysis.

RESULTS

In anesthetized animals fed the NS diet, plasma ANG II levels were lower in 32-day-old TGR than in HanSD, but at 52 and 90 days of age no significant differences were noted. ANG II concentrations in kidney tissue were similar in 32- and 90-day-old TGR and HanSD, but were higher in 52-day-old TGR than in HanSD. In contrast, in conscious animals immediately after decapitation, plasma and kidney ANG II levels were higher in TGR than in HanSD at all ages. HS diet did not change BP but suppressed ANG II levels in HanSD at all ages. In contrast, HS diet increased BP but did not decrease plasma and kidney ANG II levels in TGR at all ages. Sodium restriction did not alter BP and resulted in a marked increase in ANG II levels in HanSD, but caused a significant decrease in BP in TGR without altering plasma or tissue ANG II concentrations. There were no significant differences in renal AT1 receptor protein expression between HanSD and TGR at any age of any of the experimental groups.

CONCLUSIONS

On the basis of our present results we conclude that TGR exhibit a disrupted interaction between sodium homeostasis and the regulation of the renin-angiotensin system (RAS) activity which results in the loss of BP regulation in this model.

Augmentation of Intrarenal Angiotensin II Levels in Uninephrectomized Aldosterone/Salt-Treated Hypertensive Rats; Renoprotective Effects of an Ultrahigh Dose of Olmesartan

Recent studies have suggested that aldosterone plays a role in the pathogenesis of renal injury. In this study, we investigated whether local angiotensin II (Ang II) activity contributes to the progression of renal injury in aldosterone/salt-induced hypertensive rats. Uninephrectomized rats were treated with 1% NaCl in a drinking solution and one of the following combinations for 6 weeks: vehicle (2% ethanol, s.c.; n=9), aldosterone (0.75 mug/h, s.c.; n=8), aldosterone+Ang II type 1 receptor blocker olmesartan (10 mg/kg/day, p.o.; n=8), or aldosterone+olmesartan (100 mg/kg/day, p.o.; n=9). Aldosterone/salt-treated hypertensive rats exhibited severe proteinuria and renal injury characterized by glomerular sclerosis and tubulointerstitial fibrosis. Aldosterone/salt-induced renal injury was associated with augmented expression of angiotensin converting enzyme and Ang II levels in the renal cortex and medullary tissues. Renal cortical and medullary mRNA expression of transforming growth factor-beta (TGF-beta) and connective tissue growth factor (CTGF) as well as the collagen contents were increased in aldosterone/salt-treated hypertensive rats. Treatment with olmesartan (10 or 100 mg/kg/day) had no effect on blood pressure but attenuated proteinuria in a dose-dependent manner. Olmesartan at 10 mg/kg/day tended to decrease renal cortical and medullary Ang II levels, TGF-beta and CTGF expression, and collagen contents; however, these changes were not significant. On the other hand, an ultrahigh dose of olmesartan (100 mg/kg/day) significantly decreased these values and ameliorated renal injury. These data suggest that augmented local Ang II activity contributes, at least partially, to the progression of aldosterone/salt-dependent renal injury.

Albumin endocytosis in proximal tubule cells is modulated by angiotensin II through an AT2 receptor-mediated protein kinase B activation

Albumin endocytosis in renal proximal tubule cells is a clathrin- and receptor-mediated mechanism that, in several pathophysiological conditions, is involved in initiating or promoting tubule-interstitial disease. Although much work has been done on this pathway, the regulation of albumin endocytosis in proximal tubule cells is not well understood. Here, we study the modulation by angiotensin II (Ang II) of albumin endocytosis in LLC-PK1, a model of proximal tubule cells. We observed that Ang II increases albumin endocytosis by approximately 100% at 10(-9) M. This effect is completely reversed by 10(-9) M PD123319, a specific AT(2) receptor antagonist, but not by losartan, a specific AT(1) receptor antagonist, at concentrations up to 10(-7) M. The Ang II effect on albumin endocytosis is also reversed by: phosphoinositide 3-kinase inhibitors LY294002 (2.5 x 10(-6) M) or wortmannin (10(-7) M), the protein kinase B inhibitor (2 x 10(-5) M), and staurosporine (2 x 10(-6) M), an inhibitor of 3'-phosphoinositide-dependent kinase 1. Ang II induced the selective phosphorylation of protein kinase B (PKB) at the Thr-308 residue without a change in Ser-473 phosphorylation, a combination that leads to an increase in PKB activity. These effects were completely abolished by 3 x 10(-6) M staurosporine or 10(-8) M PD123319. Our experiments also showed that PKB is present in the membrane fraction in overnight-starved LLC-PK1 cells. Taken together, these data show that Ang II increases albumin endocytosis through an AT(2) receptor mediated by activation of PKB in the plasma membrane, which depends on the basal activity of the phosphatidyl-inositol 3-kinase.

The role of the kidney in cardiovascular medicine

The relationship between the kidney and hypertension is complex. The kidney is both culprit and victim. Renal disease and even renal structural abnormalities (nephron underdosing) lead to hypertension. On the other hand, blood pressure (even blood pressure values in the range of normotension) accelerates a progressive loss of renal function in patients with primary renal disease. This review discusses some recent work in this field, emphasizing that multiple mechanisms are operative in renal hypertension, particularly a shift in the natriuresis-blood pressure relationship (blood pressure natriuresis), inappropriate activation of the renin-angiotensin system, sympathetic overactivity, and impaired endothelial cell-dependent vasodilatation. It also emphasizes the substantial revision of past recommendations concerning blood pressure targets. In renal patients, blood pressure should be reduced to levels below 125/75 mm Hg. In addition to blood pressure, a reduction in proteinuria is widely considered an additional treatment target.

The role of intrarenal angiotensin II in the development of hypertension in Ren-2 transgenic rats

OBJECTIVE

We investigated the responses of mean arterial pressure and renal blood flow to intravenous and intrarenal angiotensin II, plasma and kidney angiotensin II concentrations and renal angiotensin receptor subtype 1 protein expression, and renal functional responses to intravenous and intrarenal angiotensin receptor 1 blockade with candesartan.

METHODS

In male anaesthetized transgenic rats and Hannover Sprague-Dawley rats aged 36-38 days mean arterial pressure and renal blood flow were determined after intravenous and intrarenal boluses of angiotensin II. Mean arterial pressure, renal blood flow and sodium excretion after intravenous or intrarenal candesartan were studied. Plasma and kidney angiotensin II concentrations were determined by radioimmunoassay. Renal angiotensin receptor subtype 1 protein levels were analysed by immunoblotting.

RESULTS

The responses of mean arterial pressure and renal blood flow to angiotensin II were significantly greater in transgenic than in Hannover Sprague-Dawley rats. The administration of candesartan resulted in comparable decreases in mean arterial pressure and increases in renal blood flow and sodium excretion in both groups of rats. Renal angiotensin receptor subtype 1 protein levels were no different between Hannover Sprague-Dawley and transgenic rats.

CONCLUSIONS

Plasma and kidney angiotensin II levels were lower in anaesthetized transgenic rats but, in contrast, were higher in decapitated transgenic rats when compared with Hannover Sprague-Dawley rats, suggesting that the kidney function of prehypertensive transgenic rats is under inappropriately high angiotensin II-dependent influence.

Adenosine reverses the stimulatory effect of angiotensin II on the renal Na+-ATPase activity through the A2 receptor

In the present paper, we report the modulation of the Angiotensin II (Ang II)-stimulated Na+-ATPase activity of the proximal tubule basolateral membrane by adenosine (Ado). Preincubation of isolated basolateral membrane with 10(-8)M Ang II increases the Na+-ATPase activity from 7.5+/-0.3 (control) to 14.6+/-0.9 nmol Pi x mg(-1)x min(-1)nmol Pi x mg(-1) x min(-1) (p<0.05). Incubation of Ang II-stimulated enzyme with 10(-6)M Ado, in the presence of the A1 receptor antagonist DPCPX (10(-6)M), completely reverses the Ang II-induced effect bringing the Na+-ATPase activity to the basal level. The following evidences demonstrate involvement of the A2 receptor/Gs protein/adenylyl cyclase/PKA signaling pathway in the inhibitory effect induced by Ado on the Ang II-stimulated Na+-ATPase activity in the presence of the DPCPX: 1) the inhibitory effect of Ado is abolished by the A2 receptor selective antagonist DMPX (10(-8)M); 2) the effect induced by Ado is blocked by 10(-8)M GDPbetaS and mimicked by 10(-9)M cholera toxin and 10(-8)M GTPgammaS; 3) the stimulatory effect of Ang II is reduced by 10(-6)M forskolin, an activator of adenylyl cyclase, or 10(-6)M cAMP; 4) Ado stimulates PKA activity; 5) the inhibitory effect induced by this nucleoside is reversed by the PKA inhibitor peptide.

Identification of cis-regulatory sequences in the human angiotensinogen gene by transgene coplacement and site-specific recombination

The function of putative regulatory sequences identified in cell transfection experiments can be elucidated only through in vivo experimentation. However, studies of gene regulation in transgenic mice (TgM) are often compromised by the position effects, in which independent transgene insertions differ in expression depending on their location in the genome. In order to overcome such a dilemma, a method called transgene coplacement has been developed in Drosophila melanogaster. In this method, any two sequences can be positioned at exactly the same genomic site by making use of Cre/loxP recombination. Here we applied this method to mouse genetics to characterize the function of direct repeat (DR) sequences in the promoter of the human angiotensinogen (hAGT) gene, the precursor of the vasoactive octapeptide angiotensin II. We modified a hAGT bacterial artificial chromosome to use Cre/loxP recombination in utero to generate TgM lines bearing a wild-type or a mutant promoter-driven hAGT locus integrated at a single chromosomal position. The expression analyses revealed that DR sequences contribute 50 or >95% to hAGT transcription in the liver and kidneys, respectively, whereas same sequences are not required in the heart and brain. This is the first in vivo dissection of DNA cis elements that are demonstrably indispensable for regulating both the level and cell type specificity of hAGT gene transcription.

Antioxidative effect of p38 mitogen-activated protein kinase inhibitor in the kidney of hypertensive rat

OBJECTIVE

Nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase is regulated by angiotensin II, interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha via p38 mitogen-activated protein kinase (MAPK). We hypothesized that p38 MAPK inhibitor, FR167653, may suppress NAD(P)H oxidase and its oxygen radical production and ameliorate renal damage in Dahl salt-sensitive rats with heart failure (DSHF).

METHODS

DSHF rats were fed with 8% NaCl diet from 6 to 18 weeks old. Eleven-week-old DSHF rats received either vehicle or FR167653 (2 mg/kg per day) for 7 weeks and the renal NAD(P)H oxidase p47phox and nitric oxide synthase (NOS), superoxide production and renal damage were evaluated in comparison with the control Dahl salt-resistant rat fed with 8% NaCl diet.

RESULTS

In the kidney of DSHF rat, phosphorylated p38 MAPK was enhanced with an increased IL-1beta and TNF-alpha production compared with control rats. Treatment with FR167653 significantly suppressed p38 MAPK, IL-1beta and TNF-alpha. Renal NAD(P)H oxidase p47phox expression and superoxide production were significantly increased in the DSHF rats and treatment with FR167653 suppressed NAD(P)H oxidase expression and reduced superoxide formation. Renal endothelial and inducible NOS were reduced in DSHF rats compared with control rats, but FR167653 increased NOS and NO production in the kidney. Proteinuria, glomerulosclerosis and interstitial macrophage migration via intercellular adhesion molecule-1 (ICAM-1) were enhanced in DSHF rat and they were ameliorated by FR167653.

CONCLUSION

The inhibition of p38 MAPK by FR167653 reduced renal IL-1beta and TNF-alpha production and ameliorated renal damage in hypertensive rat via suppression of NAD(P)H oxidase and enhanced NO bioavailability.

Angiotensin II induces carbon monoxide production in the perfused kidney: relationship to protein kinase C activation

Heme oxygenase (HO)-derived carbon monoxide (CO) attenuates vascular reactivity to constrictor stimuli. ANG II produces vasoconstriction and induces HO-1 isoform expression. However, direct evidence that ANG II promotes HO product generation is lacking. Therefore, we examined the effects of ANG II on CO release and HO isoform expression in isolated rat kidneys. Kidneys were perfused with oxygenated Krebs buffer. ANG II (1 μmol/l) increased ( P < 0.05) perfusion pressure from 97 ± 9 to 150 ± 14 mmHg; it also increased ( P < 0.05) the concentration of CO in the venous effluent (from 27.1 ± 11.9 to 45.6 ± 11.7, 62.5 ± 16.7, 94.8 ± 20.7, and 101.9 ± 13.1 nmol/l after 30, 60, 90, and 120 min, respectively). The pressor effect of ANG II was blunted ( P < 0.05) in kidneys perfused with buffer containing losartan (10 μmol/l) or PKC inhibitors staurosporine (0.1 μmol/l) or calphostin C (1 μmol/l). Kidneys perfused with buffer containing ANG II for 120 min also displayed increased ( P < 0.05) HO-1 expression. Stannous mesoporphyrin (30 μmol/l) decreased CO release ( P < 0.05) in preparations perfused with and without ANG II; the HO inhibitor also increased ( P < 0.05) perfusion pressure, more so in kidneys perfused with that without ANG II. We conclude that ANG II stimulates CO production and release in isolated, perfused rat kidneys. This action of ANG II is linked to the activation of AT1receptors and involves PKC activation and upregulation of renal HO-1 but not of HO-2 protein expression. The study suggests upregulation of renal HO-1 and CO release are protagonic events in a counterregulatory mechanism that attenuates ANG II-induced renal vasoconstriction.