Augmentation of intrarenal angiotensin II levels by chronic angiotensin II infusion

Intratubular, Intracellular, and Mitochondrial Angiotensin II/AT1 (AT1a) Receptor/NHE3 Signaling Plays a Critical Role in Angiotensin II-Induced Hypertension and Kidney Injury

Hypertension is well recognized to be the most important risk factor for cardiovascular diseases, stroke, and end-stage kidney failure. A quarter of the world’s adult populations and 46% of the US adults develop hypertension and currently require antihypertensive treatments. Only 50% of hypertensive patients are responsive to current antihypertensive drugs, whereas remaining patients may continue to develop cardiovascular, stroke, and kidney diseases. The mechanisms underlying the poorly controlled hypertension remain incompletely understood. Recently, we have focused our efforts to uncover additional renal mechanisms, pathways, and therapeutic targets of poorly controlled hypertension and target organ injury using novel animal models or innovative experimental approaches. Specifically, we studied and elucidated the important roles of intratubular, intracellular, and mitochondrial angiotensin II (Ang II) system in the development of Ang II-dependent hypertension. The objectives of this invited article are to review and discuss our recent findings that (a) circulating and intratubular Ang II is taken up by the proximal tubules via the (AT₁) AT_1a receptor-dependent mechanism, (b) intracellular administration of Ang II in proximal tubule cells or adenovirus-mediated overexpression of an intracellular Ang II fusion protein selectively in the mitochonria of the proximal tubules induces blood pressure responses, and (c) genetic deletion of AT₁ (AT_1a) receptors or the Na⁺/H⁺ exchanger 3 selectively in the proximal tubules decreases basal blood pressure and attenuates Ang II-induced hypertension. These studies provide a new perspective into the important roles of the intratubular, intracellular, and mitochondrial angiotensin II/AT₁ (AT_1a) receptor signaling in Ang II-dependent hypertensive kidney diseases.

Urinary excretion of renin and angiotensinogen in hypertensive children and adolescents

INTRODUCTION

In recent years hypertension has become an emerging condition in the young population. It has been proposed that the renin-angiotensin system plays an important role in regulation of blood pressure. We assessed whether activation of the intrarenal renin-angiotensin system occurs in hypertensive children and adolescents and what better reflects its activity: urine angiotensinogen (AGT) or urine renin (REN).

MATERIAL AND METHODS

The study was conducted on a sample of 58 subjects with primary hypertension (HT) and 29 normotensive children and adolescents. We measured urine REN and AGT excretion and assessed the values in relation to blood pressure (BP) and other clinical parameters. Both REN and AGT values were calculated by urine creatinine: REN/cr. and AGT/cr., respectively.

RESULTS

We observed higher urine REN/cr. values in hypertensive subjects in comparison to the reference group (6.99 vs. 2.93, p = 0.003). Hypertensive participants showed positive correlations between urine REN/cr. and diastolic 24-hour BP (r = 0.42, p = 0.002) as well as between urine REN/cr. and urine AGT/cr. (r = 0.266, p = 0.044, respectively).

CONCLUSIONS

Increased urine REN/cr. in hypertensive children and adolescents and its positive correlation with BP may indicate its important role in the pathogenesis of HT. Perhaps urine REN/cr. could be a marker of intrarenal renin-angiotensin system activity. Nevertheless, further research should be undertaken to confirm this observation.

Potassium Intake Prevents the Induction of the Renin-Angiotensin System and Increases Medullary ACE2 and COX-2 in the Kidneys of Angiotensin II-Dependent Hypertensive Rats

In angiotensin II (Ang II)-dependent hypertensive rats there is an increased expression of proximal tubule angiotensinogen (AGT), collecting duct renin and angiotensin converting enzyme (ACE), which contributes to intratubular Ang II formation. Ang II acts on Ang II type 1 receptors promoting sodium retention and vasoconstriction. However concurrently, the ACE2-Ang-(1–7) axis and the expression of kallikrein and medullary prostaglandins counteract the effects of Ang II, promoting natriuresis and vasodilation. Human studies demonstrate that dietary potassium (K⁺) intake lowers blood pressure. In this report we evaluate the expression of AGT, ACE, medullary prorenin/renin, ACE2, kallikrein and cyclooxygenase-2 (COX-2) in Ang II-infused rats fed with high K⁺ diet (2%) for 14 days. Dietary K⁺ enhances diuresis in non-infused and in Ang II-infused rats. The rise in systolic blood pressure in Ang II-infused rats was attenuated by dietary K⁺. Ang II-infused rats showed increased renal protein levels of AGT, ACE and medullary prorenin and renin. This effect was attenuated in the Ang II + K⁺ group. Ang II infusion decreased ACE2 compared to the control group; however, K⁺ diet prevented this effect in the renal medulla. Furthermore, medullary COX-2 was dramatically induced by K⁺ diet in non-infused and in Ang II infused rats. Dietary K⁺ greatly increased kallikrein immunostaining in normotensive rats and in Ang II-hypertensive rats. These results indicate that a high K⁺ diet attenuates Ang II-dependent hypertension by preventing the induction of ACE, AGT and collecting duct renin and by enhancing medullary COX-2 and ACE2 protein expression in the kidney.

Bradykinin/B2 receptor activation regulates renin in M-1 cells via protein kinase C and nitric oxide

In the collecting duct (CD), the interactions of renin angiotensin system (RAS) and kallikrein-kinin system (KKS) modulate Na⁺ reabsorption, volume homeostasis, and blood pressure. In this study, we used a mouse kidney cortical CD cell line (M-1 cells) to test the hypothesis that in the CD, the activation of bradykinin B₂ receptor (B₂R) increases renin synthesis and release. Physiological concentrations of bradykinin (BK) treatment of M-1 cells increased renin mRNA and prorenin and renin protein contents in a dose-dependent manner and increased threefold renin content in the cell culture media. These effects were mediated by protein kinase C (PKC) independently of protein kinase A (PKA) because B₂R antagonism with Icatibant and PKC inhibition with calphostin C, prevented these responses, but PKA inhibition with H89 did not modify the effects elicited by the B₂R activation. BK-dependent stimulation of renin gene expression in CD cells also involved nitric oxide (NO) pathway because increased cGMP levels and inhibition of NO synthase with L-NAME prevented it. Complementary renin immunohistochemical studies performed in kidneys from mice with conventional B₂R knockout and conditional B₂R knockout in the CD, showed marked decreased renin immunoreactivity in CD, regardless of the renin presence in juxtaglomerular cells in the knockout mice. These results indicate that the activation of B₂R increases renin synthesis and release by the CD cells through PKC stimulation and NO release, which support further the interactions between the RAS and KKS.

Recent Updates on the Proximal Tubule Renin-Angiotensin System in Angiotensin II-Dependent Hypertension

It is well recognized that the renin-angiotensin system (RAS) exists not only as circulating, paracrine (cell to cell), but also intracrine (intracellular) system. In the kidney, however, it is difficult to dissect the respective contributions of circulating RAS versus intrarenal RAS to the physiological regulation of proximal tubular Na(+) reabsorption and hypertension. Here, we review recent studies to provide an update in this research field with a focus on the proximal tubular RAS in angiotensin II (ANG II)-induced hypertension. Careful analysis of available evidence supports the hypothesis that both local synthesis or formation and AT1 (AT1a) receptor- and/or megalin-mediated uptake of angiotensinogen (AGT), ANG I and ANG II contribute to high levels of ANG II in the proximal tubules of the kidney. Under physiological conditions, nearly all major components of the RAS including AGT, prorenin, renin, ANG I, and ANG II would be filtered by the glomerulus and taken up by the proximal tubules. In ANG II-dependent hypertension, the expression of AGT, prorenin, and (pro)renin receptors, and angiotensin-converting enzyme (ACE) is upregulated rather than downregulated in the kidney. Furthermore, hypertension damages the glomerular filtration barrier, which augments the filtration of circulating AGT, prorenin, renin, ANG I, and ANG II and their uptake in the proximal tubules. Together, increased local ANG II formation and augmented uptake of circulating ANG II in the proximal tubules, via activation of AT1 (AT1a) receptors and Na(+)/H(+) exchanger 3, may provide a powerful feedforward mechanism for promoting Na(+) retention and the development of ANG II-induced hypertension.

The vasoprotective axes of the renin-angiotensin system: Physiological relevance and therapeutic implications in cardiovascular, hypertensive and kidney diseases

The renin-angiotensin system (RAS) is undisputedly one of the most prominent endocrine (tissue-to-tissue), paracrine (cell-to-cell) and intracrine (intracellular/nuclear) vasoactive systems in the physiological regulation of neural, cardiovascular, blood pressure, and kidney function. The importance of the RAS in the development and pathogenesis of cardiovascular, hypertensive and kidney diseases has now been firmly established in clinical trials and practice using renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, type 1 (AT₁) angiotensin II (ANG II) receptor blockers (ARBs), or aldosterone receptor antagonists as major therapeutic drugs. The major mechanisms of actions for these RAS inhibitors or receptor blockers are mediated primarily by blocking the detrimental effects of the classic angiotensinogen/renin/ACE/ANG II/AT₁/aldosterone axis. However, the RAS has expanded from this classic axis to include several other complex biochemical and physiological axes, which are derived from the metabolism of this classic axis. Currently, at least five axes of the RAS have been described, with each having its key substrate, enzyme, effector peptide, receptor, and/or downstream signaling pathways. These include the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor, the ANG II/APA/ANG III/AT₂/NO/cGMP, the ANG I/ANG II/ACE2/ANG (1-7)/Mas receptor, the prorenin/renin/prorenin receptor (PRR or Atp6ap2)/MAP kinases ERK1/2/V-ATPase, and the ANG III/APN/ANG IV/IRAP/AT₄ receptor axes. Since the roles and therapeutic implications of the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor axis have been extensively reviewed, this article will focus primarily on reviewing the roles and therapeutic implications of the vasoprotective axes of the RAS in cardiovascular, hypertensive and kidney diseases.

2-Methoxyestradiol Reduces Angiotensin II-Induced Hypertension and Renal Dysfunction in Ovariectomized Female and Intact Male Mice

Cytochrome P450 1B1 protects against angiotensin II (Ang II)-induced hypertension and associated cardiovascular changes in female mice, most likely via production of 2-methoxyestradiol. This study was conducted to determine whether 2-methoxyestradiol ameliorates Ang II-induced hypertension, renal dysfunction, and end-organ damage in intact Cyp1b1-/-, ovariectomized female, and Cyp1b1+/+ male mice. Ang II or vehicle was infused for 2 weeks and administered concurrently with 2-methoxyestradiol. Mice were placed in metabolic cages on day 12 of Ang II infusion for urine collection for 24 hours. 2-Methoxyestradiol reduced Ang II-induced increases in systolic blood pressure, water consumption, urine output, and proteinuria in intact female Cyp1b1-/- and ovariectomized mice. 2-Methoxyestradiol also reduced Ang II-induced increase in blood pressure, water intake, urine output, and proteinuria in Cyp1b1+/+ male mice. Treatment with 2-methoxyestradiol attenuated Ang II-induced end-organ damage in intact Cyp1b1-/- and ovariectomized Cyp1b1+/+ and Cyp1b1-/- female mice and Cyp1b1+/+ male mice. 2-Methoxyestradiol mitigated Ang II-induced increase in urinary excretion of angiotensinogen in intact Cyp1b1-/- and ovariectomized Cyp1b1+/+ and Cyp1b1-/- female mice but not in Cyp1b1+/+ male mice. The G protein-coupled estrogen receptor 1 antagonist G-15 failed to alter Ang II-induced increases in blood pressure and renal function in Cyp1b1+/+ female mice. These data suggest that 2-methoxyestradiol reduces Ang II-induced hypertension and associated end-organ damage in intact Cyp1b1-/-, ovariectomized Cyp1b1+/+ and Cyp1b1-/- female mice, and Cyp1b1+/+ male mice independent of G protein-coupled estrogen receptor 1. Therefore, 2-methoxyestradiol could serve as a therapeutic agent for treating hypertension and associated pathogenesis in postmenopausal females, and in males.

Prevention and Reversal by Enalapril of Target Organ Damage in Angiotensin II Hypertension

Angiotensin-converting enzyme inhibitors (ACE-Is) prevent target organ damage in several models of hypertension. The aim of this study was to assess the influence of the ACE-I enalapril (10 mg/kg-1per day, gavage) on the cardiovascular alterations and production of free radicals induced by chronic infusion of angiotensin II (Ang II, 200 ng/kg-1per minute, SC) in Sprague-Dawley rats. Enalapril was given concomitantly for the 10 days of Ang II infusion (prevention) or from day 10 to 17 of Ang II infusion (intervention). The influence of the NADPH oxidase inhibitor apocynin (600 mg/L-1in drinking water) was evaluated. mg/L-lin drinking water) was evaluated. Enalapril and apocynin had no effect on hypertension in the prevention and intervention studies. Enalapril prevented the increase in heart weight index (HWI), carotid cross-sectional area (CSA) and albuminuria induced by Ang II. Enalapril reduced HWI and albuminuria whereas CSA I was not affected in the intervention study. Apocynin had effects comparable to enalapril. Both enalapril and apocynin reduced the overproduction of superoxide anion by the left ventricle and rise in advanced oxidation protein products induced by C Ang II. Therefore, the antioxidant but not the antihypertensive effect of enalapril may participate in the prevention and treatment of the Ang II-induced cardiovascular and renal alterations.

Increased angiotensinogen expression, urinary angiotensinogen excretion, and tissue injury in nonclipped kidneys of two-kidney, one-clip hypertensive rats

In angiotensin II (ANG II)-dependent hypertension, there is an angiotensin type 1 receptor-dependent amplification mechanism enhancing intrarenal angiotensinogen (AGT) formation and secretion in the tubular fluid. To evaluate the role of increased arterial pressure, AGT mRNA, protein expression, and urinary AGT (uAGT) excretion and tissue injury were assessed in both kidneys of two-kidney, one-clip Sprague-Dawley hypertensive rats subjected to left renal arterial clipping (0.25-mm gap). By 18-21 days, systolic arterial pressure increased to 180 ± 3 mmHg, and uAGT increased. Water intake, body weights, 24-h urine volumes, and sodium excretion were similar. In separate measurements of renal function in anesthetized rats, renal plasma flow and glomerular filtration rate were similar in clipped and nonclipped kidneys and not different from those in sham rats, indicating that the perfusion pressure to the clipped kidneys remained within the autoregulatory range. The nonclipped kidneys exhibited increased urine flow and sodium excretion. The uAGT excretion was significantly greater in nonclipped kidneys compared with clipped and sham kidneys. AGT mRNA was 2.15-fold greater in the nonclipped kidneys compared with sham (1.0 ± 0.1) or clipped (0.98 ± 0.15) kidneys. AGT protein levels were also greater in the nonclipped kidneys. The nonclipped kidneys exhibited greater glomerular expansion and immune cell infiltration, medullary fibrosis, and cellular proliferation than the clipped kidneys. Because both kidneys have elevated ANG II levels, the greater tissue injury in the nonclipped kidneys indicates that an increased arterial pressure synergizes with increased intrarenal ANG II to stimulate AGT production and exert greater renal injury.

6β-Hydroxytestosterone, a Cytochrome P450 1B1-Testosterone-Metabolite, Mediates Angiotensin II-Induced Renal Dysfunction in Male Mice

6β-Hydroxytestosterone, a cytochrome P450 1B1-derived metabolite of testosterone, contributes to the development of angiotensin II-induced hypertension and associated cardiovascular pathophysiology. In view of the critical role of angiotensin II in the maintenance of renal homeostasis, development of hypertension, and end-organ damage, this study was conducted to determine the contribution of 6β-hydroxytestosterone to angiotensin II actions on water consumption and renal function in male Cyp1b1(+/+) and Cyp1b1(-/-) mice. Castration of Cyp1b1(+/+) mice or Cyp1b1(-/-) gene disruption minimized the angiotensin II-induced increase in water consumption, urine output, proteinuria, and sodium excretion and decreases in urine osmolality. 6β-Hydroxytestosterone did not alter angiotensin II-induced increases in water intake, urine output, proteinuria, and sodium excretion or decreases in osmolality in Cyp1b1(+/+) mice, but restored these effects of angiotensin II in Cyp1b1(-/-) or castrated Cyp1b1(+/+) mice. Cyp1b1 gene disruption or castration prevented angiotensin II-induced renal fibrosis, oxidative stress, inflammation, urinary excretion of angiotensinogen, expression of angiotensin II type 1 receptor, and angiotensin-converting enzyme. 6β-Hydroxytestosterone did not alter angiotensin II-induced renal fibrosis, inflammation, oxidative stress, urinary excretion of angiotensinogen, expression of angiotensin II type 1 receptor, or angiotensin-converting enzyme in Cyp1b1(+/+)mice. However, in Cyp1b1(-/-) or castrated Cyp1b1(+/+) mice, it restored these effects of angiotensin II. These data indicate that 6β-hydroxytestosterone contributes to increased thirst, impairment of renal function, and end-organ injury associated with angiotensin II-induced hypertension in male mice and that cytochrome P450 1B1 could serve as a novel target for treating renal disease and hypertension in male mice.

Augmentation of angiotensinogen expression in the proximal tubule by intracellular angiotensin II via AT1a/MAPK/NF-кB signaling pathways

Long-term angiotensin II (ANG II) infusion significantly increases ANG II levels in the kidney through two major mechanisms: AT1 receptor-mediated augmentation of angiotensinogen (AGT) expression and uptake of circulating ANG II by the proximal tubules. However, it is not known whether intracellular ANG II stimulates AGT expression in the proximal tubule. In the present study, we overexpressed an intracellular cyan fluorescent ANG II fusion protein (Ad-sglt2-ECFP/ANG II) selectively in the proximal tubule of rats and mice using the sodium and glucose cotransporter 2 (sglt2) promoter. AGT mRNA and protein expression in the renal cortex and 24-h urinary AGT excretion were determined 4 wk following overexpression of ECFP/ANG II in the proximal tubule. Systolic blood pressure was significantly increased with a small antinatriuretic effect in rats and mice with proximal tubule-selective expression of ECFP/ANG II (P < 0.01). AGT mRNA and protein expression in the cortex were increased by >1.5-fold and 61 ± 16% (P < 0.05), whereas urinary AGT excretion was increased from 48.7 ± 5.7 (n = 13) to 102 ± 13.5 (n = 13) ng/24 h (P < 0.05). However, plasma AGT, renin activity, and ANG II levels remained unaltered by ECFP/ANG II. The increased AGT mRNA and protein expressions in the cortex by ECFP/ANG II were blocked in AT1a-knockout (KO) mice. Studies in cultured mouse proximal tubule cells demonstrated involvement of AT1a receptor/MAP kinases/NF-кB signaling pathways. These results indicate that intracellular ANG II stimulates AGT expression in the proximal tubules, leading to increased AGT formation and secretion into the tubular fluid, which contributes to ANG II-dependent hypertension.

Everything we always wanted to know about furosemide but were afraid to ask

Furosemide is a widely used, potent natriuretic drug, which inhibits the Na(+)-K(+)-2Cl(-) cotransporter (NKCC)-2 in the ascending limb of the loop of Henle applied to reduce extracellular fluid volume expansion in heart and kidney disease. Undesirable consequences of furosemide, such as worsening of kidney function and unpredictable effects on sodium balance, led to this critical evaluation of how inhibition of NKCC affects renal and cardiovascular physiology. This evaluation reveals important knowledge gaps, involving furosemide as a drug, the function of NKCC2 (and NKCC1), and renal and systemic indirect effects of NKCC inhibition. Regarding renal effects, renal blood flow and glomerular filtration rate could become compromised by activation of tubuloglomerular feedback or by renin release, particularly if renal function is already compromised. Modulation of the intrarenal renin angiotensin system, however, is ill-defined. Regarding systemic effects, vasodilation followed by nonspecific NKCC inhibition and changes in venous compliance are not well understood. Repetitive administration of furosemide induces short-term (braking phenomenon, acute diuretic resistance) and long-term (chronic diuretic resistance) adaptations, of which the mechanisms are not well known. Modulation of NKCC2 expression and activity in kidney and heart failure is ill-defined. Lastly, furosemide's effects on cutaneous sodium stores and on uric acid levels could be beneficial or detrimental. Concluding, a considerable knowledge gap is identified regarding a potent drug with a relatively specific renal target, NKCC2, and renal and systemic actions. Resolving these questions would increase the understanding of NKCCs and their actions and improve rational use of furosemide in pathophysiology of fluid volume expansion.

Different mechanisms of acute versus long-term antihypertensive effects of soluble epoxide hydrolase inhibition: studies in Cyp1a1-Ren-2 transgenic rats

Recent studies have shown that the long-term antihypertensive action of soluble epoxide hydrolase inhibition (sEH) in angiotensin-II (AngII)-dependent hypertension might be mediated by the suppression of intrarenal AngII levels. To test this hypothesis, we examined the effects of acute (2 days) and chronic (14 days) sEH inhibition on blood pressure (BP) in transgenic rats with inducible AngII-dependent hypertension. AngII-dependent malignant hypertension was induced by 10 days' dietary administration of indole-3-carbinol (I3C), a natural xenobiotic that activates the mouse renin gene in Cyp1a1-Ren-2 transgenic rats. BP was monitored by radiotelemetry. Acute and chronic sEH inhibition was achieved using cis-4-(4-(3-adamantan-1-yl-ureido)cyclohexyloxy) benzoic acid, given at doses of 0.3, 3, 13, 26, 60 and 130 mg/L in drinking water. At the end of experiments, renal concentrations of epoxyeicosatrienoic acids, their inactive metabolites dihydroxyeicosatrienoic acids and AngII were measured. Acute BP-lowering effects of sEH inhibition in I3C-induced rats was associated with a marked increase in renal epoxyeicosatrienoic acids to dihydroxyeicosatrienoic acids ratio and acute natriuresis. Chronic treatment with cis-4-(4-(3-adamantan-1-yl-ureido)cyclohexyloxy) benzoic acid in I3C-induced rats elicited dose-dependent persistent BP lowering associated with a significant reduction of plasma and kidney AngII levels. Our findings show that the acute BP-lowering effect of sEH inhibition in I3C-induced Cyp1a1-Ren-2 transgenic rats is mediated by a substantial increase in intrarenal epoxyeicosatrienoic acids and their natriuretic action without altering intrarenal renin-angiotensin system activity. Long-term antihypertensive action of cis-4-(4-(3-adamantan-1-yl-ureido)cyclohexyloxy) benzoic acid in I3C-induced Cyp1a1-Ren-2 transgenic rats is mediated mostly by suppression of intrarenal AngII concentration.

Effects of chymostatin, a chymase inhibitor, on blood pressure, plasma and tissue angiotensin II, renal haemodynamics and renal excretion in two models of hypertension in the rat

NEW FINDINGS

What is the central question of this study? We examined, in hypertensive rats, whether the angiotensin-converting enzyme-independent enzymes generating angiotensin II in the tissues modulate blood pressure, peripheral circulation and renal function. What is the main finding and its importance? The results suggest that chymostatin-sensitive enzymes diminish vascular tone in renal and extrarenal vascular beds. Chymase or similar chymostatin-sensitive enzymes have a significant role in the synthesis of angiotensin II in different tissues but do not control blood pressure in the short term, similarly in salt-dependent or Goldblatt-type rat hypertension. In salt-dependent hypertension, chymase blockade protected renal outer medullary perfusion, probably by reducing the angiotensin II content in the kidney. Chymase is presumed to be a crucial enzyme of the non-angiotensin-converting enzyme pathway of angiotensin II (Ang II) generation in tissues, a process involved in vascular remodelling and development of hypertension. We examined the role of chymase in hypertension induced by exposure of uninephrectomized rats to high dietary salt intake (UNX HS) and in the Goldblatt renal artery stenosis (two-kidney, one-clip) model. In acute experiments with anaesthetized rats of either model, chymostatin at 2 mg kg(-1) h(-1) or 0.05% DMSO solvent was infused i.v. Mean arterial blood pressure, heart rate, iliac blood flow (a measure of hindlimb perfusion), total renal blood flow and intrarenal regional perfusion (laser-Doppler technique) were measured continuously, along with the glomerular filtration rate and renal excretion. In both models, chymase blockade distinctly decreased plasma and tissue Ang II without lowering mean blood pressure or consistently altering the other functional parameters measured. Unexpectedly, in Goldblatt hypertensive rats the blockade increased the renal and hindlimb vascular resistances by 51 and 33%, respectively (P < 0.05). In UNX HS hypertensive rats, chymase blockade abolished the solvent-induced decrease in outer medullary blood flow. We conclude that chymase or similar chymostatin-sensitive enzyme(s) has a significant role in the synthesis of Ang II in different tissues but does not participate in short-term control of blood pressure in salt-dependent or Goldblatt-type rat hypertension. In the Goldblatt model, chymase appeared to reduce the renal and hindlimb vascular resistances by an unknown mechanism. In salt-dependent hypertension, chymase blockade protected renal outer medullary perfusion, probably by reducing Ang II content in the kidney.

Incomplete Restoration of Angiotensin II-Induced Renal Extracellular Matrix Deposition and Inflammation Despite Complete Functional Recovery in Rats

Some diseases associated with a temporary deterioration in kidney function and/or development of proteinuria show an apparently complete functional remission once the initiating trigger is removed. While it was earlier thought that a transient impairment of kidney function is harmless, accumulating evidence now suggests that these patients are more prone to developing renal failure later in life. We therefore sought to investigate to what extent renal functional changes, inflammation and collagen deposition are reversible after cessation of disease induction, potentially explaining residual sensitivity to damage. Using a rat model of Angiotensin II (Ang II)-induced hypertensive renal disease we show the development of severe hypertension (212 ± 10.43 vs. 146 ± 1.4 mmHg, p<0.001) and proteinuria (51.4 ± 6.3 vs. 14.7 ± 2.0 mg/24h, p<0.01) with declined creatinine clearance (2.0 ± 0.5 vs. 4.9 ± 0.6 mL/min, p<0.001) to occur after 3 weeks of Ang II infusion. At the structural level, Ang II infusion resulted in interstitial inflammation (18.8 ± 4.8 vs. 3.6 ± 0.5 number of macrophages, p<0.001), renal interstitial collagen deposition and lymphangiogenesis (4.1 ± 0.4 vs. 2.2 ± 0.4 number of lymph vessels, p<0.01). Eight weeks after cessation of Ang II, all clinical parameters, pre-fibrotic changes such as myofibroblast transformation and increase in lymph vessel number (lymphangiogenesis) returned to control values. However, glomerular desmin expression, glomerular and periglomerular macrophages and interstitial collagens remained elevated. These dormant abnormalities indicate that after transient renal function decline, inflammation and collagen deposition may persist despite normalization of the initiating pathophysiological stimulus perhaps rendering the kidney more vulnerable to further damage.

Role of stimulated intrarenal angiotensinogen in hypertension

Experimental models of hypertension and patients with inappropriately increased renin formation due to a stenotic kidney, arteriosclerotic narrowing of the renal arterioles or a rare juxtaglomerular cell tumor have shown a progressive augmentation of the intrarenal/intratubular renin-angiotensin system (RAS). The increased intrarenal angiotensin II (Ang II) elicits renal vasoconstriction and enhanced tubular sodium reabsorption in proximal and distal nephron segments. The enhanced intrarenal Ang II levels are due to both increased Ang II type 1 (AT1) receptor mediated Ang II uptake and AT1 receptor dependent stimulation of renal angiotensinogen (AGT) mRNA and augmented AGT production. The increased AGT formation and secretion into the proximal tubular lumen leads to local formation of Ang II, which stimulates proximal transporters such as the sodium/hydrogen exchanger. Enhanced AGT production also leads to spillover of AGT into the distal nephron segments as reflected by AGT in the urine, which provides an index of intrarenal RAS activity. There is also increased Ang II concentration in distal nephron with stimulation of distal sodium transport. Increased urinary excretion of AGT has been demonstrated in patients with hypertension, type 1 and type 2 diabetes mellitus, and several types of chronic kidney diseases indicating an upregulation of intrarenal RAS activity.

Renal hemodynamic and morphological changes after 7 and 28 days of leptin treatment: the participation of angiotensin II via the AT1 receptor

The role of hyperleptinemia in cardiovascular diseases is well known; however, in the renal tissue, the exact site of leptin’s action has not been established. This study was conducted to assess the effect of leptin treatment for 7 and 28 days on renal function and morphology and the participation of angiotensin II (Ang II), through its AT₁ receptor. Rats were divided into four groups: sham, losartan (10 mg/kg/day, s.c.), leptin (0.5 mg/kg/day for the 7 days group and 0.25 mg/kg/day for the 28 days group) and leptin plus losartan. Plasma leptin, Ang II and endothelin 1 (ET-1) levels were measured using an enzymatic immuno assay. The systolic blood pressure (SBP) was evaluated using the tail-cuff method. The renal plasma flow (RPF) and the glomerular filtration rate (GFR) were determined by p-aminohippuric acid and inulin clearance, respectively. Urinary Na⁺ and K⁺ levels were also analyzed. Renal morphological analyses, desmin and ED-1 immunostaining were performed. Proteinuria was analyzed by silver staining. mRNA expression of renin-angiotensin system (RAS) components, TNF-α and collagen type III was analyzed by quantitative PCR. Our results showed that leptin treatment increased Ang II plasma levels and progressively increased the SBP, achieving a pre-hypertension state. Rats treated with leptin 7 days showed a normal RPF and GFR, but increased filtration fraction (FF) and natriuresis. However, rats treated with leptin for 28 showed a decrease in the RPF, an increase in the FF and no changes in the GFR or tubular function. Leptin treatment-induced renal injury was demonstrated by: glomerular hypertrophy, increased desmin staining, macrophage infiltration in the renal tissue, TNF-α and collagen type III mRNA expression and proteinuria. In conclusion, our study demonstrated the progressive renal morphological changes in experimental hyperleptinemia and the interaction between leptin and the RAS on these effects.

Angiotensin II-induced hypertension blunts thick ascending limb NO production by reducing NO synthase 3 expression and enhancing threonine 495 phosphorylation

Thick ascending limbs reabsorb 30% of the filtered NaCl load. Nitric oxide (NO) produced by NO synthase 3 (NOS3) inhibits NaCl transport by this segment. In contrast, chronic angiotensin II (ANG II) infusion increases net thick ascending limb transport. NOS3 activity is regulated by changes in expression and phosphorylation at threonine 495 (T495) and serine 1177 (S1177), inhibitory and stimulatory sites, respectively. We hypothesized that NO production by thick ascending limbs is impaired by chronic ANG II infusion, due to reduced NOS3 expression, increased phosphorylation of T495, and decreased phosphorylation of S1177. Rats were infused with 200 ng·kg(-1)·min(-1) ANG II or vehicle for 1 and 5 days. ANG II infusion for 5 days decreased NOS3 expression by 40 ± 12% (P < 0.007; n = 6) and increased T495 phosphorylation by 147 ± 26% (P < 0.008; n = 6). One-day ANG II infusion had no significant effect. NO production in response to endothelin-1 was blunted in thick ascending limbs from ANG II-infused animals [ANG II -0.01 ± 0.06 arbitrary fluorescence units (AFU)/min vs. 0.17 ± 0.02 AFU/min in controls; P < 0.01]. This was not due to reduced endothelin-1 receptor expression. Phosphatidylinositol 3,4,5-triphosphate (PIP3)-induced NO production was also reduced in ANG II-infused rats (ANG II -0.07 ± 0.06 vs. 0.13 ± 0.04 AFU/min in controls; P < 0.03), and this correlated with an impaired ability of PIP3 to increase S1177 phosphorylation. We conclude that in ANG II-induced hypertension NO production by thick ascending limbs is impaired due to decreased NOS3 expression and altered phosphorylation.

Mechanisms of AT1a receptor-mediated uptake of angiotensin II by proximal tubule cells: a novel role of the multiligand endocytic receptor megalin

The present study tested the hypothesis that the multiligand endocytic receptor megalin is partially involved in the uptake of ANG II and downstream signaling responses in mouse proximal tubule cells (mPCT) by interacting with AT1a receptors. mPCT cells of wild-type (WT) and AT1a receptor-deficient (AT1a-KO) mice were treated with vehicle, the AT1 receptor blocker losartan (10 μM), or a selective megalin small interfering (si) RNA for 48 h. The uptake of fluorescein (FITC)-labeled ANG II (10 nM, 37°C) and downstream signaling responses were analyzed by fluorescence imaging and Western blotting. AT1a receptors and megalin were abundantly expressed in mPCT cells, whereas AT1a receptors were absent in AT1a-KO mPCT cells (P < 0.01). In WT mPCT cells, FITC-ANG II uptake was visualized at 30 min in the cytoplasm and in the nuclei 1 h after exposure. Losartan alone completely blocked the uptake of FITC-ANG II, whereas megalin siRNA inhibited only 30% of the response (P < 0.01). The remaining FITC-ANG II uptake in the presence of megalin siRNA was completely abolished by losartan. ANG II induced threefold increases in phosphorylated MAP kinases ERK1/2 and a onefold increase in phosphorylated sodium and hydrogen exchanger 3 (NHE3) proteins, which were also blocked by losartan and megalin-siRNA. By contrast, losartan and megalin siRNA had no effects on these signaling proteins in AT1a-KO mPCT cells. We conclude that the uptake of ANG II and downstream MAP kinases ERK1/2 and NHE3 signaling responses in mPCT cells are mediated primarily by AT1a receptors. However, megalin may also play a partial role in these responses to ANG II.

Do intravenous and subcutaneous angiotensin II increase blood pressure by different mechanisms?

Angiotensin (Ang) II plays a key role in blood pressure regulation. Mechanisms of the pressor effect of chronic intravenous AngII administration include vasoconstriction, stimulation of the sympathetic nervous system and aldosterone production, as well as direct effects on renal excretion of sodium and water.  Chronic AngII administration by subcutaneous minipump at doses higher than required to increase blood pressure by the intravenous route has identified additional pressor mechanisms, including the immune system, cytokines and matrix metalloproteinases. However, pressor doses of subcutaneous AngII may exceed the angiotensinogen synthesis rate and produce inflammation, fibrosis and necrosis of skin overlying the minipump.  Evidence that chronic subcutaneous and intravenous AngII increase blood pressure by different mechanisms includes the prevention of the pressor effects of subcutaneous, but not intravenous, AngII by angiotensin-converting enzyme inhibition. Furthermore, low doses of subcutaneous AngII reduce blood pressure of female, but not male, rodents and higher doses are less pressor in females than in males, whereas intravenous AngII is equally pressor in males and females.  Pressor doses of chronic subcutaneous AngII produce greater weight loss, anorexia and reduced kidney weight and cause greater vascular, cardiac and renal pathology than equally pressor doses of chronic intravenous AngII.  The different effects of chronic intravenous and subcutaneous AngII suggest that these two models of hypertension give different information and may differ in their relevance to blood pressure regulation in physiological and pathological states such as hypertension in humans.

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

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

Activation of the renin-angiotensin system by a low-salt diet does not augment intratubular angiotensinogen and angiotensin II in rats

In angiotensin II (ANG II) infusion hypertension, there is an augmentation of intratubular angiotensinogen (AGT) and ANG II leading to increased urinary AGT and ANG II excretion rates associated with tissue injury. However, the changes in urinary AGT and ANG II excretion rates and markers of renal injury during physiologically induced stimulation of the renin-angiotensin system (RAS) by a low-salt diet remain unclear. Male Sprague-Dawley rats received a low-salt diet (0.03% NaCl; n = 6) and normal-salt diet (0.3% NaCl, n = 6) for 13 days. Low-salt diet rats had markedly higher plasma renin activity and plasma ANG II levels. Kidney cortex renin mRNA, kidney AGT mRNA, and AGT immunoreactivity were not different; however, medullary renin mRNA, kidney renin content, and kidney ANG II levels were significantly elevated by the low-salt diet. Kidney renin immunoreactivity was also markedly increased in juxtaglomerular apparati and in cortical and medullary collecting ducts. Urinary AGT excretion rates and urinary ANG II excretion rates were not augmented by the low-salt diet. The low-salt diet caused mild renal fibrosis in glomeruli and the tubulointerstitium, but no other signs of kidney injury were evident. These results indicate that, in contrast to the response in ANG II infusion hypertension, the elevated plasma and intrarenal ANG II levels caused by physiological stimulation of RAS are not reflected by increased urinary AGT or ANG II excretion rates or the development of renal injury.

Collecting Duct Renin: A major player in Angiotensin II-dependent Hypertension

Recently, the focus of interest on the role of the renin angiotensin system in the pathophysiology of hypertension has shifted towards greater emphasis on new developments in local renin angiotensin systems in specific tissues. We have focused our recent investigations on the role of the intrarenal-intratubular RAS in hypertension. All of the components needed for angiotensin II generation are present within the various compartments in the kidney. This brief review is focused on recent evidence that inappropriate activation of renin in distal nephron segments, by acting on angiotensinogen generated in the proximal tubule cells and delivered to the distal nephron may contribute to increased distal intrarenal angiotensin II formation, sodium retention and development and progression of hypertension.

Evolving concepts on regulation and function of renin in distal nephron

Sustained stimulation of the intrarenal/intratubular renin-angiotensin system in a setting of elevated arterial pressure elicits renal vasoconstriction, increased sodium reabsorption, proliferation, fibrosis, and eventual renal injury. Activation of luminal AT(1) receptors in proximal and distal nephron segments by local Ang II formation stimulates various transport systems. Augmented angiotensinogen (AGT) production by proximal tubule cells increases AGT secretion contributing to increased proximal Ang II levels and leading to spillover of AGT into the distal nephron segments, as reflected by increased urinary AGT excretion. The increased distal delivery of AGT provides substrate for renin, which is expressed in principal cells of the collecting tubule and collecting ducts, and is also stimulated by AT(1) receptor activation. Renin and prorenin are secreted into the tubular lumen and act on the AGT delivered from the proximal tubule to form more Ang I. The catalytic actions of renin and or prorenin may be enhanced by binding to prorenin receptors on the intercalated cells or soluble prorenin receptor secreted into the tubular fluid. There is also increased luminal angiotensin converting enzyme in collecting ducts facilitating Ang II formation leading to stimulation of sodium reabsorption via sodium channel and sodium/chloride co-transporter. Thus, increased collecting duct renin contributes to Ang II-dependent hypertension by augmenting distal nephron intratubular Ang II formation leading to sustained stimulation of sodium reabsorption and progression of hypertension.

Sexual dimorphism in urinary angiotensinogen excretion during chronic angiotensin II-salt hypertension

BACKGROUND

The intrarenal renin-angiotensin system contributes to hypertension by regulating sodium and water reabsorption throughout the nephron. Sex differences in the intrarenal components of the renin-angiotensin system have been involved in the greater incidence of high blood pressure and progression to kidney damage in males than females.

OBJECTIVE

This study investigated whether there is a sex difference in the intrarenal gene expression and urinary excretion of angiotensinogen (AGT) during angiotensin II (Ang II)-dependent hypertension and high-salt (HS) diet.

METHODS

Male and female Sprague-Dawley rats were divided into 5 groups for each sex: Normal-salt control, HS diet (8% NaCl), Ang II-infused (80 ng/min), Ang II-infused plus HS diet, and Ang II-infused plus HS diet and treatment with the Ang II receptor blocker, candesartan (25 mg/L in the drinking water). Rats were evaluated for systolic blood pressure (SBP), kidney AGT mRNA expression, urinary AGT excretion, and proteinuria at different time points during a 14-day protocol.

RESULTS

Both male and female rats exhibited similar increases in urinary AGT, with increases in SBP during chronic Ang II infusion. HS diet greatly exacerbated the urinary AGT excretion in Ang II-infused rats; males had a 9-fold increase over Ang II alone and females had a 2.5-fold increase. Male rats displayed salt-sensitive SBP increases during Ang II infusion and HS diet, and female rats did not. In the kidney cortex, males displayed greater AGT gene expression than females during all treatments. During Ang II infusion, both sexes exhibited increases in AGT gene message compared with same-sex controls. In addition, HS diet combined with Ang II infusion exacerbated the proteinuria in both sexes. Concomitant Ang II receptor blocker treatment during Ang II infusion and HS diet decreased SBP and urinary AGT similarly in both sexes; however, the decrease in proteinuria was greater in the females.

CONCLUSION

During Ang II-dependent hypertension and HS diet, higher intrarenal renin-angiotensin system activation in males, as reflected by higher AGT gene expression and urinary excretion, indicates a mechanism for greater progression of high blood pressure and might explain the sex disparity in development of salt-sensitive hypertension.

Evidence for a functional intracellular angiotensin system in the proximal tubule of the kidney

ANG II is the most potent and important member of the classical renin-angiotensin system (RAS). ANG II, once considered to be an endocrine hormone, is now increasingly recognized to also play novel and important paracrine (cell-to-cell) and intracrine (intracellular) roles in cardiovascular and renal physiology and blood pressure regulation. Although an intracrine role of ANG II remains an issue of continuous debates and requires further confirmation, a great deal of research has recently been devoted to uncover the novel actions and elucidate underlying signaling mechanisms of the so-called intracellular ANG II in cardiovascular, neural, and renal systems. The purpose of this article is to provide a comprehensive review of the intracellular actions of ANG II, either administered directly into the cells or expressed as an intracellularly functional fusion protein, and its effects throughout a variety of target tissues susceptible to the impacts of an overactive ANG II, with a particular focus on the proximal tubules of the kidney. While continuously reaffirming the roles of extracellular or circulating ANG II in the proximal tubules, our review will focus on recent evidence obtained for the novel biological roles of intracellular ANG II in cultured proximal tubule cells in vitro and the potential physiological roles of intracellular ANG II in the regulation of proximal tubular reabsorption and blood pressure in rats and mice. It is our hope that the new knowledge on the roles of intracellular ANG II in proximal tubules will serve as a catalyst to stimulate further studies and debates in the field and to help us better understand how extracellular and intracellular ANG II acts independently or interacts with each other, to regulate proximal tubular transport and blood pressure in both physiological and diseased states.

Involvement of cytochrome P-450 1B1 in renal dysfunction, injury, and inflammation associated with angiotensin II-induced hypertension in rats

We investigated the contribution of cytochrome P-450 1B1 (CYP1B1) to renal dysfunction and organ damage associated with ANG II-induced hypertension in rats. ANG II (300 ng·kg(-1)·min(-1)) or vehicle were infused for 2 wk, with daily injections of a selective CYP1B1 inhibitor, 2,4,3',5'-tetramethoxystilbene (TMS; 300 μg/kg ip), or its vehicle. ANG II increased blood pressure and renal CYP1B1 activity that were prevented by TMS. ANG II also increased water intake and urine output, decreased glomerular filtration rate, increased urinary Na(+) and K(+) excretion, and caused proteinuria, all of which were prevented by TMS. ANG II infusion caused hypertrophy, endothelial dysfunction, and increased reactivity of renal and interlobar arteries to vasoconstrictor agents and renal vascular resistance and interstitial fibrosis as indicated by accumulation of α-smooth muscle actin, fibronectin, and collagen, and inflammation as indicated by increased infiltration of CD-3(+) cells; these effects were inhibited by TMS. ANG II infusion also increased production of reactive oxygen species (ROS) and activities of NADPH oxidase, ERK1/2, p38 MAPK, and c-Src that were prevented by TMS. TMS alone had no effect on any of the above parameters. These data suggest that CYP1B1 contributes to the renal pathophysiological changes associated with ANG II-induced hypertension, most likely via increased ROS production and activation of ERK1/2, p38 MAPK, and c-Src and that CYP1B1 could serve as a novel target for treating renal disease associated with hypertension.

Increased renin excretion is associated with augmented urinary angiotensin II levels in chronic angiotensin II-infused hypertensive rats

Renin expression in principal cells of collecting ducts (CD) is upregulated in angiotensin II (ANG II)-dependent hypertensive rats; however, it remains unclear whether increased CD-derived renin undergoes tubular secretion. Accordingly, urinary levels of renin (uRen), angiotensinogen (uAGT), and ANG II (uANG II) were measured in chronic ANG II-infused Sprague-Dawley rats (80 ng/min for 14 days, n = 10) and sham-operated rats (n = 10). Systolic blood pressure increased in the ANG II rats by day 5 and continued to increase throughout the study (day 13; ANG II: 175 ± 10 vs. sham: 116 ± 2 mmHg; P < 0.05). ANG II infusion increased renal cortical and medullary ANG II levels (cortical ANG II: 606 ± 72 vs. 247 ± 43 fmol/g; P < 0.05; medullary ANG II: 2,066 ± 116 vs. 646 ± 36 fmol/g; P < 0.05). Although plasma renin activity (PRA) was suppressed in the ANG II-infused rats (0.3 ± 0.2 vs. 5.5 ± 1.8 ng ANG I·ml(-1)·h(-1); P < 0.05), renin content in renal medulla was increased (12,605 ± 1,343 vs. 7,956 ± 765 ng ANG I·h(-1)·mg(-1); P < 0.05). Excretion of uAGT and uANG II increased in the ANG II rats [uAGT: 1,107 ± 106 vs. 60 ± 26 ng/day; P < 0.0001; uANG II: 3,813 ± 431 vs. 2,080 ± 361 fmol/day; P < 0.05]. By day 13, despite suppression of PRA, urinary prorenin content increased in ANG II rats [15.7 ± 3 vs. 2.6 ± 1 × 10(-3) enzyme units excreted (EUE)/day, P < 0.01] as was the excretion rate of renin (8.6 ± 2 × 10(-6) EUE/day) compared with sham (2.8 ± 1 × 10(-6) EUE/day; P < 0.05). Urinary renin and prorenin protein levels examined by Western blot were augmented ∼10-fold in the ANG II-infused rats. Concomitant AT(1) receptor blockade with candesartan prevented the increase. Thus, in ANG II-dependent hypertensive rats with marked PRA suppression, increased urinary levels of renin and prorenin reflect their augmented secretion by CD cells into the luminal fluid. The greater availability of renin and AGT in the urine reflects the capability for intratubular ANG II formation which stimulates sodium reabsorption in distal nephron segments.

New insights and perspectives on intrarenal renin-angiotensin system: focus on intracrine/intracellular angiotensin II

Although renin, the rate-limiting enzyme of the renin-angiotensin system (RAS), was first discovered by Robert Tigerstedt and Bergman more than a century ago, the research on the RAS still remains stronger than ever. The RAS, once considered to be an endocrine system, is now widely recognized as dual (circulating and local/tissue) or multiple hormonal systems (endocrine, paracrine and intracrine). In addition to the classical renin/angiotensin I-converting enzyme (ACE)/angiotensin II (Ang II)/Ang II receptor (AT₁/AT₂) axis, the prorenin/(Pro)renin receptor (PRR)/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, and the Ang IV/AT₄/insulin-regulated aminopeptidase (IRAP) axis have recently been discovered. Furthermore, the roles of the evolving RAS have been extended far beyond blood pressure control, aldosterone synthesis, and body fluid and electrolyte homeostasis. Indeed, novel actions and underlying signaling mechanisms for each member of the RAS in physiology and diseases are continuously uncovered. However, many challenges still remain in the RAS research field despite of more than one century's research effort. It is expected that the research on the expanded RAS will continue to play a prominent role in cardiovascular, renal and hypertension research. The purpose of this article is to review the progress recently being made in the RAS research, with special emphasis on the local RAS in the kidney and the newly discovered prorenin/PRR/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, the Ang IV/AT₄/IRAP axis, and intracrine/intracellular Ang II. The improved knowledge of the expanded RAS will help us better understand how the classical renin/ACE/Ang II/AT₁ receptor axis, extracellular and/or intracellular origin, interacts with other novel RAS axes to regulate blood pressure and cardiovascular and kidney function in both physiological and diseased states.

Salt-induced renal injury in SHRs is mediated by AT1 receptor activation

OBJECTIVE

This study aimed to examine the effects of salt loading, with or without simultaneous angiotensin receptor blocker (ARB) treatment, on the systemic and tissue renin-angiotensin system (RAS) in spontaneously hypertensive rats (SHRs).

METHOD

Evaluation was performed early (4 weeks) in the course of salt loading in order to examine initial mediating events of cardiovascular and renal damage produced by salt excess. Four groups of rats were studied. Group 1 received regular rat chow (normal-salt diet); group 2 received normal-salt diet and an ARB (losartan, 30 mg/kg per day); group 3 received high-salt (8%) chow; and group 4 received high-salt diet and losartan.

RESULTS

High-salt diet increased systolic pressure to 193±1 mmHg compared to 180±2 in normal-salt diet group. Losartan reduced SBP in SHRs fed normal-salt diet but did not reduce SBP in the SHRs fed high-salt diet (192±2 mmHg). High-salt diet markedly increased urinary protein excretion from 27±4 to 64±13 mg/day and this increase was ameliorated by losartan (40±9 mg/day). In SHRs on high-salt diet, plasma angiotensin II concentration increased three to four-fold, whereas urinary angiotensinogen excretion increased 10-fold; and these changes were significantly reduced by losartan. High-salt diet accelerated glomerular injury and interstitial fibrosis in SHRs which were reduced by losartan.

CONCLUSION

These results demonstrate that the activity of RAS was either not suppressed or, even augmented, after 4 weeks of salt loading despite high salt intake and increased SBP. The data suggest that an augmented intrarenal RAS during high-salt diet may contribute to the development of renal injury in this experimental model.

Role of cytochrome P-450 metabolites in the regulation of renal function and blood pressure in 2-kidney 1-clip hypertensive rats

Alterations in renal function contribute to Goldblatt two-kidney, one-clip (2K1C) hypertension. A previous study indicated that bioavailability of cytochrome P-450 metabolites epoxyeicosatrienoic acids (EETs) is decreased while that of 20-hydroxyeicosatetraenoic acids (20-HETE) is increased in this model. We utilized the inhibitor of soluble epoxide hydrolase cis-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB) and HET-0016, the inhibitor of 20-HETE production, to study the role of EETs and 20-HETE in the regulation of renal function. Chronic c-AUCB treatment significantly decreased systolic blood pressure (SBP) (133 ± 1 vs. 163 ± 3 mmHg) and increased sodium excretion (1.23 ± 0.10 vs. 0.59 ± 0.03 mmol/day) in 2K1C rats. HET-0016 did not affect SBP and sodium excretion. In acute experiments, renal blood flow (RBF) was decreased in 2K1C rats (5.0 ± 0.2 vs. 6.9 ± 0.2 ml·min(-1)·g(-1)). c-AUCB normalized RBF in 2K1C rats (6.5 ± 0.6 ml·min(-1)·g(-1)). HET-0016 also increased RBF in 2K1C rats (5.8 ± 0.2 ml·min(-1)·g(-1)). Although RBF and glomerular filtration rate (GFR) remained stable in normotensive rats during renal arterial pressure (RAP) reductions, both were significantly reduced at 100 mmHg RAP in 2K1C rats. c-AUCB did not improve autoregulation but increased RBF at all RAPs and shifted the pressure-natriuresis curve to the left. HET-0016-treated 2K1C rats exhibited impaired autoregulation of RBF and GFR. Our data indicate that c-AUCB displays antihypertensive properties in 2K1C hypertension that are mediated by an improvement of RBF and pressure natriuresis. While HET-0016 enhanced RBF, its anti-natriuretic effect likely prevented it from producing a blood pressure-lowering effect in the 2K1C model.

Reciprocal changes in renal ACE/ANG II and ACE2/ANG 1-7 are associated with enhanced collecting duct renin in Goldblatt hypertensive rats

Alterations in the balance between ANG II/ACE and ANG 1-7/ACE2 in ANG II-dependent hypertension could reduce the generation of ANG 1-7 and contribute further to increased intrarenal ANG II. Upregulation of collecting duct (CD) renin may lead to increased ANG II formation during ANG II-dependent hypertension, thus contributing to this imbalance. We measured ANG I, ANG II, and ANG 1-7 contents, angiotensin-converting enzyme (ACE) and ACE2 gene expression, and renin activity in the renal cortex and medulla in the clipped kidneys (CK) and nonclipped kidneys (NCK) of 2K1C rats. After 3 wk of unilateral renal clipping, systolic blood pressure and plasma renin activity increased in 2K1C rats (n = 11) compared with sham rats (n = 9). Renal medullary angiotensin peptide levels were increased in 2K1C rats [ANG I: (CK = 171 ± 4; NCK = 251 ± 8 vs. sham = 55 ± 3 pg/g protein; P < 0.05); ANG II: (CK = 558 ± 79; NCK = 328 ± 18 vs. sham = 94 ± 7 pg/g protein; P < 0.001)]; and ANG 1-7 levels decreased (CK = 18 ± 2; NCK = 19 ± 2 pg/g vs. sham = 63 ± 10 pg/g; P < 0.001). In renal medullas of both kidneys of 2K1C rats, ACE mRNA levels and activity increased but ACE2 decreased. In further studies, we compared renal ACE and ACE2 mRNA levels and their activities from chronic ANG II-infused (n = 6) and sham-operated rats (n = 5). Although the ACE mRNA levels did not differ between ANG II rats and sham rats, the ANG II rats exhibited greater ACE activity and reduced ACE2 mRNA levels and activity. Renal medullary renin activity was similar in the CK and NCK of 2K1C rats but higher compared with sham. Thus, the differential regulation of ACE and ACE2 along with the upregulation of CD renin in both the CK and NCK in 2K1C hypertensive rats indicates that they are independent of perfusion pressure and contribute to the altered content of intrarenal ANG II and ANG 1-7.

Enhanced urinary angiotensinogen excretion in Cyp1a1-Ren2 transgenic rats with inducible ANG II-dependent malignant hypertension

INTRODUCTION

Previous studies have demonstrated that the urinary excretion of angiotensinogen is significantly increased in ANG II-infused hypertensive rats, which is associated with an augmentation of intrarenal ANG II levels. These findings suggest that urinary angiotensinogen excretion rates provide an index of intrarenal ANG II levels in ANG II-dependent hypertensive states. However, little information is available regarding the urinary excretion of angiotensinogen in ANG II-dependent malignant hypertension.

METHODS

This study was performed to determine if urinary angiotensinogen excretion is increased in Cyp1a1-Ren2 transgenic rats [strain name: TGR(Cyp1aRen2)] with inducible ANG II-dependent malignant hypertension. Adult male Cyp1a1-Ren2 rats (n = 6) were fed a normal diet containing 0.3% indole-3-carbinol (I3C) for 10 days to induce ANG II-dependent malignant hypertension.

RESULTS

Rats induced with I3C exhibited pronounced increases in systolic blood pressure (208 ± 7 versus 127 ± 3 mm Hg; P < 0.001), marked proteinuria (29.4 ± 3.6 versus 5.9 ± 0.3 mg/d; P < 0.001) and augmented urinary angiotensinogen excretion (996 ± 186 versus 241 ± 31 ng/d; P < 0.01). Chronic administration of the AT₁ receptor antagonist, candesartan (25 mg/L in drinking water, n = 6), prevented the I3C-induced increases in systolic blood pressure (125 ± 5 mm Hg; P < 0.001), proteinuria (7.3 ± 1.0 mg/d; P < 0.001) and urinary angiotensinogen excretion (488 ± 51 ng/d, P < 0.01).

CONCLUSIONS

These data demonstrate that the urinary excretion of angiotensinogen is markedly augmented in ANG II-dependent malignant hypertension. Such increased urinary angiotensinogen excretion may contribute to augmented intrarenal ANG II levels and, thereby, to the increased blood pressure in Cyp1a1-Ren2 transgenic rats with inducible ANG II-dependent malignant hypertension.

Enhancement of renin and prorenin receptor in collecting duct of Cyp1a1-Ren2 rats may contribute to development and progression of malignant hypertension

To determine whether in the transgenic rat model [TGR(Cyp1a1Ren2)] with inducible ANG II-dependent malignant hypertension changes in the activation of intrarenal renin-angiotensin system may contribute to the pathogenesis of hypertension, we examined the gene expression of angiotensinogen (AGT) in renal cortical tissues and renin and prorenin receptor [(P)RR] in the collecting duct (CD) of the kidneys from Cyp1a1Ren2 rats (n = 6) fed a normal diet containing 0.3% indole-3-carbinol (I3C) for 10 days and noninduced rats maintained on a normal diet (0.6% NaCl diet; n = 6). Rats induced with I3C developed malignant hypertension and exhibited alterations in the expression of renin and (P)RR expressed by the CD cells. In the renal medullary tissues of the Cyp1a1Ren2 transgenic rats with malignant hypertension, renin protein levels in CD cells were associated with maintained renin content and lack of suppression of the endogenous Ren1c gene expression. Furthermore, these tissues exhibited increased levels of (P)RR transcript, as well as of the protein levels of the soluble form of this receptor, the s(P)RR. Intriguingly, although previous findings demonstrated that urinary AGT excretion is augmented in Cyp1a1Ren2 transgenic rats with malignant hypertension, in the present study we did not find changes in the gene expression of AGT in renal cortical tissues of these rats. The data suggest that upregulation of renin and the s(P)RR in the CD, especially in the renal medullary tissues of Cyp1a1Ren2 transgenic rats with malignant hypertension, along with the previously demonstrated increased availability of AGT in the urine of these rats, may constitute a leading mechanism to explain elevated formation of kidney ANG II levels in this model of ANG II-dependent hypertension.

Angiotensin II type 1 receptor-mediated augmentation of urinary excretion of endogenous angiotensin II in Val5-angiotensin II-infused rats

Rats infused chronically with Val(5)-Angiotensin (Ang) II exhibit increased urinary excretion of endogenous Ile(5)-Ang II by the 12th day of infusion, suggesting the stimulation of endogenous Ang II formation by Val(5)-Ang II infusion. The present study determined the time course of increased urinary Ang II excretion and the effects of Ang II type 1 receptor blockade (candesartan, 2 mg/kg per day) on the urinary excretion rates of Ile(5)-Ang II in Val(5)-Ang II-infused (80 ng/min) rats. Ile(5)-Ang II was separated from Val(5)-Ang II by high-performance liquid chromatography and measured by radioimmunoassay. Systolic blood pressure increased progressively (215+/-2 mm Hg) in Val(5)-Ang II-infused rats (n=5), whereas the candesartan-treated group (n=6) remained normotensive (124+/-3 mm Hg). Candesartan treatment significantly increased the level of plasma Ile(5)-Ang II (24.0+/-7.6 versus 156.9+/-24.6 fmol/mL; P<0.01); in contrast, there was a markedly lower intrarenal Ile(5)-Ang II content (357.9+/-76.6 versus 21.1+/-2.8 fmol/g; P<0.01). Urinary Ile(5)-Ang II excretion rates were elevated by day 9 (2185.7+/-283.2 fmol/24 hours) in Val(5)-Ang II-infused rats but not in candesartan-treated rats (740.6+/-110.3 fmol/24 hours). Thus, Ang II type 1 receptor blockade prevents the increase in urinary excretion of endogenous Ang II in rats subjected to chronic Ang II infusion. These data indicate that the increased urinary excretion of endogenous Ang II in Val(5)-Ang II-infused rats is primarily attributed to Ang II type 1 receptor-dependent secretion into and/or de novo formation of Ang II within the tubular lumen.

Intrarenal cytochrome P-450 metabolites of arachidonic acid in the regulation of the nonclipped kidney function in two-kidney, one-clip Goldblatt hypertensive rats

OBJECTIVE

The contribution of epoxyeicosatrienoic acids (EETs) and 20-hydroxyeicosatetraenoic acid (20-HETE) as cytochrome P-450 metabolites of arachidonic acid in the regulation of the nonclipped kidney function in two-kidney, one-clip (2K1C) Goldblatt hypertensive rats was investigated during the phases of initial and stable hypertension, that is, 7 or 27 days after clipping, respectively.

METHODS

Male Hannover Sprague-Dawley rats had the right renal artery clipped or underwent sham operation. Urinary excretion of EETs, their inactive metabolites dihydroxyeicosatrienoic acids and of 20-HETE was measured. Intrarenal cytochrome P-450 protein expression and the activities of epoxygenase, omega-hydroxylase and soluble epoxide hydrolase were also determined. The responses of renal hemodynamics and electrolyte excretion of the nonclipped kidney to left renal artery infusions of inhibitors of EETs or 20-HETE formation (MS-PPOH and DDMS, respectively) were measured.

RESULTS

In 2K1C rats, urinary excretion of EETs was significantly lower and that of 20-HETE was higher than that in sham-operated animals only on day 27 after clipping. Intrarenal inhibition of EETs significantly decreased renal hemodynamics and sodium excretion in sham-operated but not in 2K1C rats. Intrarenal inhibition of 20-HETE decreased sodium excretion in sham-operated rats but elicited increases in renal hemodynamics and sodium excretion in 2K1C rats.

CONCLUSION

Our results indicate that the nonclipped kidney of Goldblatt 2K1C rats in the phase of sustained hypertension exhibits decreased intrarenal EETs and elevated 20-HETE levels as compared with the kidney of sham-operated animals. This suggests that altered production and action of cytochrome P-450-derived metabolites during this stable phase contributes to the mechanism of Goldblatt 2K1C hypertension.

Angiotensin II-induced hypertension regulates AT1 receptor subtypes and extracellular matrix turnover in mouse retinal pigment epithelium

Accumulation of specific deposits and extracellular molecules under the retinal pigment epithelium (RPE) has been previously observed in eyes with age-related macular degeneration (AMD) and may play a role in the pathogenesis of AMD. Even though age is the major determinant for developing AMD, clinical studies have revealed hypertension (HTN) as another systemic risk factor. Angiotensin II (Ang II) is considered the most important hormone associated with HTN. To evaluate the relationship of Ang II to AMD, we studied whether mouse RPE expresses functional Ang II receptor subtypes and whether HTN-induced Ang II regulates expression of these receptors as well as critical ECM molecules (MMP-2 and type IV collagen) involved in ECM turnover in RPE. We used 9-month-old C57BL/6 male mice infused with Ang II alone or Ang II in combination with the AT1 receptor antagonist candesartan or the AT2 receptor antagonist PD123319 for 4 weeks to determine whether HTN-associated Ang II was important for ECM regulation in RPE. We found that mouse RPE expressed both Ang II receptor subtypes at the mRNA and protein levels. Infusion with Ang II induced HTN and elevated plasma and ocular Ang II levels. Ang II also regulated AT1a and AT1b receptor mRNA expression, the intracellular concentration of calcium [Ca(2+)](i), MMP-2 activity, and type IV collagen accumulation. Concurrent administration of Ang II with the AT1 receptor blocker prevented the increase in blood pressure and rise in ocular Ang II levels, as well as the calcium and MMP-2 responses. In contrast, the type IV collagen response to Ang II was prevented by blockade of AT2 receptors, but not AT1 receptors. Plasma Ang II levels were not modified by the AT1 or AT2 receptor blockade. Since the effects of Ang II on MMP-2 and type IV collagen require inhibition of both Ang II receptor subtypes, these receptors may play a role as a potential therapeutic targets to prevent ECM turnover dysregulation in the RPE basement membrane, suggesting a pathogenic mechanism to explain the link between HTN and AMD.

Augmentation of endogenous intrarenal angiotensin II levels in Val5-ANG II-infused rats

In angiotensin II (ANG II)-induced hypertension, intrarenal ANG II levels are increased by AT(1) receptor-mediated ANG II internalization and endogenous ANG II generation. The objective of the present study was to determine the relative contribution of de novo formation of endogenous ANG II. Male Sprague-Dawley rats were divided into three groups: sham operated (n = 6), Val(5)-ANG II infused (n = 16), and Ile(5)-ANG II infused (n = 6). Val(5)-ANG II and Ile(5)-ANG II were infused at 80 ng/min via subcutaneous osmotic minipump for 13 days, followed by harvesting of blood and kidney samples. In six Val(5)-ANG II-infused rats, urine was collected on the day before infusion and on day 12 of infusion. Extracted samples were subjected to HPLC to separate Val(5)-ANG II from Ile(5)-ANG II followed by RIA. Systolic blood pressure increased significantly from 121 +/- 2 to 206 +/- 4 mmHg in the Val(5)-ANG II-infused rats and from 124 +/- 3 to 215 +/- 5 mmHg in the Ile(5)-ANG II-infused rats. In the Val(5)-ANG II-infused rats, the plasma Ile(5)-ANG II levels increased 196.2 +/- 70.1% compared with sham plasma Ile(5)-ANG II concentration. Val(5)-ANG II levels were 150.0 +/- 28.2 fmol/ml which accounted for 53.5 +/- 10.1% of the total ANG II in plasma. The kidney Ile(5)-ANG II levels in the Val(5)-ANG II-infused rats increased 69.9 +/- 30.7% compared with sham kidney Ile(5)-ANG II concentrations. Intrarenal accumulation of Val(5)-ANG II accounted for 52.5 +/- 5.3% of the total kidney ANG II during Val(5)-ANG II infusion while endogenous Ile(5)-ANG II accounted for 47.5 +/- 8.6%. The urinary Ile(5)-ANG II excretion rate on day 12 increased 93.2 +/- 32.1% compared with preinfusion level indicating increased formation of endogenous ANG II. Thus, the increases in intrarenal ANG II levels during chronic ANG II infusions involve substantial stimulation of endogenous ANG II formation which contributes to overall augmentation of intrarenal ANG II.

Collecting duct renin is upregulated in both kidneys of 2-kidney, 1-clip goldblatt hypertensive rats

Renin in collecting duct cells is upregulated in chronic angiotensin II-infused rats via angiotensin II type 1 receptors. To determine whether stimulation of collecting duct renin is a blood pressure-dependent effect; changes in collecting duct renin and associated parameters were assessed in both kidneys of 2-kidney, 1-clip Goldblatt hypertensive (2K1C) rats. Renal medullary tissues were used to avoid the contribution of renin from juxtaglomerular cells. Systolic blood pressure increased to 184+/-9 mm Hg in 2K1C rats (n=19) compared with sham rats (121+/-6 mm Hg; n=12). Although renin immunoreactivity markedly decreased in juxtaglomerular cells of nonclipped kidneys (NCK: 0.2+/-0.0 versus 1.0+/-0.0 relative ratio) and was augmented in clipped kidneys (CK: 1.7+/-1.0 versus 1.0+/-0.0 relative ratio), its immunoreactivity increased in cortical and medullary collecting ducts of both kidneys of 2K1C rats (CK: 2.8+/-1.0 cortex; 2.1+/-1.0 medulla; NCK: 4.6+/-2.0 cortex, 3.2+/-1.0 medulla versus 1.0+/-0.0 in sham kidneys). Renal medullary tissues of 2K1C rats showed greater levels of renin protein (CK: 1.4+/-0.2; NCK: 1.5+/-0.3), renin mRNA (CK: 5.8+/-2.0; NCK: 4.9+/-2.0), angiotensin I (CK: 120+/-18 pg/g; NCK: 129+/-13 pg/g versus sham: 67+/-6 pg/g), angiotensin II (CK: 150+/-32 pg/g; NCK: 123+/-21 pg/g versus sham: 91+/-12 pg/g; P<0.05), and renin activity (CK: 8.6 microg of angiotensin I per microgram of protein; NCK: 8.3 microg of angiotensin I per microgram of protein; sham: 3.4 microg of angiotensin I per microgram of protein) than sham rats. These data indicate that enhanced collecting duct renin in 2K1C rats occurs independently of blood pressure. Upregulation of distal tubular renin helps to explain how sustained intrarenal angiotensin II formation occurs even during juxtaglomerular renin suppression, thus allowing maintained effects on tubular sodium reabsorption that contribute to the hypertension.

Renal interstitial adenosine is increased in angiotensin II-induced hypertensive rats

Since marked renal vasoconstriction is observed in angiotensin II (ANG II)-mediated hypertensive rats, we studied the possible interaction between ANG II and adenosine in this model. ANG II was infused into male Wistar rats through osmotic minipumps (435 ng·kg−1·min−1) for 14 days. In sham and ANG II groups, renal tissue and interstitial adenosine were measured; both increased to a similar twofold extent in the ANG II-treated rats (31.40 ± 4 vs. 62.0 ± 8.4 nM, sham vs. ANG II, interstitial adenosine; P< 0.001). The latter decreased by 47% with the specific blockade of 5′-nucleotidase. Glomerular hemodynamics demonstrated marked renal vasoconstriction in the angiotensin-treated group, which was reverted by an adenosine A1-receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, 10 μg·kg−1·min−1). 5′-Nucleotidase and adenosine deaminase (ADA) activities were measured in the cytosolic and membrane fractions. Only the membrane ADA activity decreased from 1,202 ± 80 to 900 ± 50 mU/mg protein in the ANG II-treated rats ( P< 0.05), as well as in their protein and mRNA expression. Despite the adenosine elevation, A1and A2breceptor protein did not change; in contrast, downregulation was observed in A2areceptor and upregulation in A3receptor. A similar pattern was found in the cortex and in the medulla; mRNA significantly decreased only in the A3receptor in both segments. These results suggest that the elevation of renal tissue and interstitial adenosine contributes to the renal vasoconstriction observed in the ANG II-induced hypertension and that it is mediated by a decrease in the activity and expression of ADA, increased production of adenosine, and an induced imbalance in adenosine receptors.

Splanchnic Circulation Is a Critical Neural Target in Angiotensin II Salt Hypertension in Rats

Chronic angiotensin II (Ang II) infusion, in rats fed high salt, engages the sympathetic nervous system to increase venomotor tone. The splanchnic sympathetic nervous system is the most important regulator of venous tone, indicating that splanchnic sympathetic nervous system activity may be increased in Ang II salt hypertension. We hypothesized that celiac ganglionectomy (CGx), to selectively disrupt sympathetic innervation to the splanchnic circulation, would attenuate arterial pressure (AP), and venous tone increases in Ang II salt hypertension. Rats fed 2% or 0.4% NaCl were instrumented to allow AP measurement by radiotelemetry at the same time as surgical CGx or sham operation. Ang II was delivered by minipump (150 ng/kg per minute) for 14 days. CGx reduced AP independent of salt diet during control. CGx markedly attenuated Ang II hypertension in rats on 2% NaCl but had little effect in rats fed 0.4% NaCl. To test the possibility that CGx exerted its effects via renal denervation, rats were subjected to the same protocol but received selective bilateral renal denervation. Renal denervation decreased AP during control but had no protective effect on Ang II hypertension and actually tended to exacerbate the pressor response. Finally, separate groups of rats underwent CGx or sham operation and were instrumented to allow repeated measures of mean circulatory filling pressure, an index of venous tone. In addition to attenuating Ang II salt hypertension, CGx completely prevented Ang II salt-induced increases in mean circulatory filling pressure and substantially attenuated depressor responses to acute ganglion blockade. We conclude that, in the presence of high salt, Ang II activates the splanchnic sympathetic nervous system to increase venomotor tone and AP.

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.

Genetic deletion of AT1a receptors attenuates intracellular accumulation of ANG II in the kidney of AT1a receptor-deficient mice

We and others have previously shown that high levels of ANG II are accumulated in the rat kidney via a type 1 (AT(1)) receptor-mediated mechanism, but it is not known which AT(1) receptor is involved in this process in rodents. We tested the hypothesis that AT(1a) receptor-deficient mice (Agtr1a-/-) are unable to accumulate ANG II intracellularly in the kidney because of the absence of AT(1a) receptor-mediated endocytosis. Adult male wild-type (Agtr1a+/+), heterozygous (Agtr1a+/-), and Agtr1a-/- were treated with vehicle, ANG II (40 ng/min ip via osmotic minipump), or ANG II plus the AT(1) antagonist losartan (10 mg.kg(-1).day(-1) po) for 2 wk. In wild-type mice, ANG II induced hypertension (168 +/- 4 vs. 113 +/- 3 mmHg, P < 0.001), increased kidney-to-body weight ratio (P < 0.01), caused pressure natriuresis (P < 0.05), and elevated plasma and whole kidney ANG II levels (P < 0.001). Concurrent administration of ANG II with losartan attenuated these responses to ANG II. In contrast, Agtr1a-/- mice had lower basal systolic pressures (P < 0.001), smaller kidneys with much fewer AT(1b) receptors (P < 0.001), higher basal 24-h urinary sodium excretion (P < 0.01), as well as basal plasma and whole kidney ANG II levels (P < 0.01). However, intracellular ANG II levels in the kidney were lower in Agtr1a-/- mice. In Agtr1a-/- mice, ANG II slightly increased systolic pressure (P < 0.05) but had no effect on the kidney weight, urinary sodium excretion, and whole kidney ANG II levels. Losartan restored systolic pressure to basal levels and decreased whole kidney ANG II levels by approximately 20% (P < 0.05). These results demonstrate a predominant role of AT(1a) receptors in blood pressure regulation and in the renal responses to long-term ANG II administration, that AT(1b) receptors may play a limited role in blood pressure control and mediating intrarenal ANG II accumulation in the absence of AT(1a) receptors.

Intrarenal renin-angiotensin system and counteracting protective mechanisms in angiotensin II-dependent hypertension

It is now well accepted that alterations in kidney function, due either to primary renal disease or to inappropriate hormonal influences on the kidney, are a cardinal characteristic in all forms of hypertension, and lead to a reduced ability of the kidneys to excrete sodium and the consequent development of elevated arterial pressures. However, it is also apparent that many extrarenal factors are important contributors to altered kidney function and hypertension. Central to many hypertensinogenic processes is the inappropriate activation of the renin-angiotensin system (RAS) and its downstream consequences by various pathophysiologic mechanisms. There may also be derangements in arachidonic acid metabolites, endothelium derived factors such as nitric oxide and carbon monoxide, and various paracrine and neural systems that normally interact with or provide a counteracting balance to the actions of the RAS. Thus, when the capacity of the kidneys to maintain sodium balance and extracellular fluid volume within appropriate ranges is compromised, increases in arterial pressure become necessary to re-establish normal balance.

Renal angiotensin II concentration and interstitial infiltration of immune cells are correlated with blood pressure levels in salt-sensitive hypertension

Renal immune cell infiltration and cells expressing angiotensin II (AII) in tubulointerstitial areas of the kidney are features of experimental models of salt-sensitive hypertension (SSHTN). A high-salt intake tends to suppress circulating AII levels, but intrarenal concentrations of AII have not been investigated in SSHTN. This study explored the relationship between these features to gain insight into the pathophysiology of SSHTN. Plasma angiotensin II (AII) and renal interstitial AII (microdialysis technique) and the infiltration of macrophages, lymphocytes, and AII-positive cells were determined in SSHTN induced by 5 wk of a high-salt diet (HSD) after short-term infusion of AII in rats with (n = 10) and without (n = 11) treatment with mycophenolate mofetil (MMF) and in control rats fed a high- (n = 7) and normal (n = 11) salt diet. As in previous studies, MMF did not affect AII-associated hypertension but reduced the interstitial inflammation and the SSHTN in the post-AII-period. During the HSD period, the AII group untreated with MMF had mean +/- SD) low plasma (2.4 +/- 1.4 pg/ml) and high interstitial AII concentration (1,310 +/- 208 pg/ml); MMF treatment resulted in a significantly lower interstitial AII (454 +/- 128 pg/ml). Renal AII concentration and the number of tubulointerstitial AII-positive cells were correlated. Blood pressure correlated positively with interstitial AII and negatively with plasma AII, thus giving compelling evidence of the paramount role of the AII within the kidney in the AII-induced model of salt-driven hypertension.

Genetic clamping of renin gene expression induces hypertension and elevation of intrarenal Ang II levels of graded severity in Cyp1a1-Ren2 transgenic rats

INTRODUCTION

Transgenic rats with inducible angiotensin II (Ang II)-dependent hypertension (strain name: TGR[Cyp1a1-Ren2]) were generated by inserting the mouse Ren2 renin gene, fused to the cytochrome P450 1a1 (Cyp1a1) promoter, into the genome of the rat. The present study was performed to characterise the changes in plasma and kidney tissue Ang II levels and in renal haemodynamic function in Cyp1a1-Ren2 rats following induction of either slowly developing or malignant hypertension in these transgenic rats.

MATERIALS AND METHODS

Arterial blood pressure (BP) and renal haemodynamics and excretory function were measured in pentobarbital sodium-anaesthetised Cyp1a1- Ren2 rats fed a normal diet containing either a low dose (0.15%, w/w for 1415 days) or high dose (0.3%, w/w for 1112 days) of the aryl hydrocarbon indole-3-carbinol (I3C) to induce slowly developing and malignant hypertension, respectively. In parallel experiments, arterial blood samples and kidneys were harvested for measurement of Ang II levels by radioimmunoassay.

RESULTS

Dietary I3C increased plasma renin activity (PRA), plasma Ang II levels, and arterial BP in a dose-dependent manner. Induction of different fixed levels of renin gene expression and PRA produced hypertensive phenotypes of varying severity with rats developing either mild or malignant forms of hypertensive disease. Administration of I3C, at a dose of 0.15% (w/w), induced a slowly developing form of hypertension whereas administration of a higher dose (0.3%) induced a more rapidly developing hypertension and the clinical manifestations of malignant hypertension including severe weight loss. Both hypertensive phenotypes were characterised by reduced renal plasma flow, increased filtration fraction, elevated PRA, and increased plasma and intrarenal Ang II levels. These I3C-induced changes in renal haemodynamics, PRA and kidney Ang II levels were more pronounced in Cyp1a1-Ren2 rats with malignant hypertension. Chronic administration of the AT1-receptor antagonist, hypertension, the associated changes in renal haemodynamics, and the augmentation of intrarenal Ang II levels.

CONCLUSIONS

Activation of AT1-receptors by Ang II generated as a consequence of induction of the Cyp1a1-Ren2 transgene mediates the increased arterial pressure and the associated reduction of renal haemodynamics and enhancement of intrarenal Ang II levels in hypertensive Cyp1a1-Ren2 transgenic rats.

Chronic low-dose angiotensin II infusion increases venomotor tone by neurogenic mechanisms

The purpose of this study was to identify changes in venomotor tone in the chronic low-dose angiotensin II (Ang II) model of hypertension and to establish the contribution of sympathetic nerve activation to these venomotor tone changes. Male Sprague-Dawley rats were acclimatized to a 0.4% or 2.0% NaCl diet for 7 days and then catheterized to allow chronic and repeated measures of arterial pressure, central venous pressure, and mean circulatory filling pressure (MCFP), an index of venous smooth muscle tone, in conscious undisturbed rats. After 4 days of recovery and a 3-day control period, an Ang II or physiological saline-filled osmotic minipump was implanted subcutaneously to deliver Ang II (150 ng/kg per minute) or vehicle control for 14 days. MCFP was measured in duplicate before and after acute ganglionic blockade with hexamethonium (30 mg/kg i.v.) on control day 2 and Ang II infusion on days 1, 3, 7, and 14. Blood volume was also measured on these days and was unchanged for the duration of the study in all of the groups. Arterial pressure was increased for the duration of Ang II infusion in rats on both 0.4% and 2% NaCl diets, but the increase was significantly greater in the 2% NaCl group and completely abolished by hexamethonium. MCFP was significantly increased for the entire Ang II infusion period only in rats fed 2% NaCl, and this increase was completely abolished by hexamethonium. We conclude that the combination of chronic low-dose Ang II infusion and high dietary salt intake engages the sympathetic nervous system to increase venomotor tone.

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.

Differential expression of nuclear AT1 receptors and angiotensin II within the kidney of the male congenic mRen2. Lewis rat

We established a new congenic model of hypertension, the mRen(2). Lewis rat and assessed the intracellular expression of angiotensin peptides and receptors in the kidney. The congenic strain was established from the backcross of the (mRen2)27 transgenic rat that expresses the mouse renin 2 gene onto the Lewis strain. The 20-wk-old male congenic rats were markedly hypertensive compared with the Lewis controls (systolic blood pressure: 195 +/- 2 vs. 107 +/- 2 mmHg, P < 0.01). Although plasma ANG II levels were not different between strains, circulating levels of ANG-(1-7) were 270% higher and ANG I concentrations were 40% lower in the mRen2. Lewis rats. In contrast, both cortical (CORT) and medullary (MED) ANG II concentrations were 60% higher in the mRen2. Lewis rats, whereas tissue ANG I was 66 and 84% lower in CORT and MED. For both strains, MED ANG II, ANG I, and ANG-(1-7) were significantly higher than CORT levels. Intracellular ANG II binding distinguished nuclear (NUC) and plasma membrane (PM) receptor using the ANG II radioligand 125I-sarthran. Isolated CORT nuclei exhibited a high density (Bmax >200 fmol/mg protein) and affinity for the sarthran ligand (KD<0.5 nM); the majority of these sites (>95%) were the AT1 receptor subtype. CORT ANG II receptor Bmax and KD values in nuclei were 75 and 50% lower, respectively, for the mRen2. Lewis vs. the Lewis rats. In the MED, the PM receptor density (Lewis: 50 +/- 4 vs. mRen2. Lewis: 21 +/- 5 fmol/mg protein) and affinity (Lewis: 0.31 +/- 0.1 vs. 0.69 +/- 0.1 nM) were lower in the mRen2. Lewis rats. In summary, the hypertensive mRen2. Lewis rats exhibit higher ANG II in both CORT and MED regions of the kidney. Evaluation of intracellular ANG II receptors revealed lower CORT NUC and MED PM AT1 sites in the mRen2. Lewis. The downregulation of AT1 sites in the mRen2. Lewis rats may reflect a compensatory response to dampen the elevated levels of intrarenal ANG II.