Renal uptake of circulating angiotensin II in Val5-angiotensin II infused rats is mediated by AT1 receptor

Why are angiotensin concentrations so high in the kidney?

PURPOSE OF REVIEW

Recent studies have reported that intrarenal angiotensin II content and angiotensin II concentrations in the proximal tubular fluid and renal interstitial fluid are much greater than the circulating angiotensin II levels. These high intrarenal angiotensin II levels are responsible for regulating renal hemodynamics and tubular transport.

RECENT FINDINGS

Intrarenal angiotensin II levels have been assessed from total tissue contents as well as renal interstitial fluid and proximal tubular fluid concentrations. Total tissue contents expressed per gram of tissue weight are greater than plasma angiotensin II concentrations; tubular fluid concentrations and renal interstitial fluid concentrations are even greater in the range of 3-10 pmoles/ml. In hypertensive states, there is also an increased intracellular accumulation of angiotensin II mediated by angiotensin type 1 receptor-dependent endocytosis. The high intrarenal angiotensin II levels are also caused by the presence of angiotensinogen messenger RNA and protein in the proximal tubule cells. Furthermore, there is positive amplification by which increases in circulating angiotensin II stimulate increased production and secretion of angiotensinogen, which is also manifested as an increased urinary excretion rate.

SUMMARY

The ability of the kidney to generate high intratubular and interstitial concentrations allows the kidney to regulate intrarenal levels in accord with the homeostatic needs for the regulation of renal hemodynamics and tubular reabsorption and the regulation of sodium balance. When inappropriately stimulated, high intrarenal angiotensin II levels contribute to excessive salt and water retention, the development of hypertension, and long-term proliferative effects leading to renal injury.

Angiotensin II type 1 receptor-mediated augmentation of renal interstitial fluid angiotensin II in angiotensin II-induced hypertension

BACKGROUND

Angiotensin II (Ang II)-dependent hypertension is associated with augmented intrarenal concentrations of Ang II; however, the distribution of the increased intrarenal Ang II has not been fully established.

OBJECTIVE

To determine the changes in renal interstitial fluid Ang II concentrations in Ang II-induced hypertension and the consequences of treatment with an angiotensin II type 1 (AT1) receptor blocker.

DESIGN AND METHODS

Rats were selected to receive vehicle (5% acetic acid subcutaneously; n = 6), Ang II (80 ng/min subcutaneously, via osmotic minipump; n = 7) or Ang II plus an AT1 receptor antagonist, candesartan cilexetil (10 mg/kg per day, in drinking water; n = 6) for 13-14 days, at which time, experiments were performed on anesthetized rats. Microdialysis probes were implanted in the renal cortex and were perfused at 2 microl/min. The effluent dialysate concentrations of Ang I and Ang II were measured by radioimmunoassay and reported values were corrected for the equilibrium rates at this perfusion rate.

RESULTS

Ang II-infused rats developed greater mean arterial pressures (155 +/- 7 mmHg) than vehicle-infused rats (108 +/- 3 mmHg). Ang II-infused rats showed greater plasma (181 +/- 30 fmol/ml) and kidney (330 +/- 38 fmol/g) Ang II concentrations than vehicle-infused rats (98 +/- 14 fmol/ml and 157 +/- 22 fmol/g, respectively). Renal interstitial fluid Ang II concentrations were much greater than plasma concentrations, averaging 5.74 +/- 0.26 pmol/ml in Ang II-infused rats - significantly greater than those in vehicle-infused rats (2.86 +/- 0.23 pmol/ml). Candesartan treatment prevented the hypertension (87 +/- 3 mmHg) and led to increased plasma Ang II concentrations (441 +/- 27 fmol/ml), but prevented increases in kidney (120 +/- 15 fmol/g) and renal interstitial fluid (2.15 +/- 0.12 pmol/ml) Ang II concentrations.

CONCLUSIONS

These data indicate that Ang II-infused rats develop increased renal interstitial fluid concentrations of Ang II, which may contribute to the increased vascular resistance and reduced sodium excretion. Furthermore, the augmentation of renal interstitial fluid Ang II is the result of an AT1 receptor-mediated process and can be dissociated from the plasma concentrations.

AT1 receptor mediated augmentation of intrarenal angiotensinogen in angiotensin II-dependent hypertension

Angiotensin (Ang) II-infused hypertensive rats exhibit increases in renal angiotensinogen mRNA and protein, as well as urinary angiotensinogen excretion in association with increased intrarenal Ang II content. The present study was performed to determine if the augmentation of intrarenal angiotensinogen requires activation of Ang II type 1 (AT1) receptors. Male Sprague-Dawley rats (200 to 220 g) were divided into 3 groups: sham surgery (n=10), subcutaneous infusion of Ang II (80 ng/min, n=11), and Ang II infusion plus AT1 blocker (ARB), olmesartan (5 mg/d, n=12). Ang II infusion progressively increased systolic blood pressure (SBP) compared with sham (178+/-8 mm Hg versus 119+/-4 at day 11). ARB treatment prevented hypertension (113+/-6 at day 11). Twenty-four-hour urine collections were taken at day 12, and plasma and tissue samples were harvested at day 13. The Ang II+ARB group had a significant increase in plasma Ang II compared with Ang II and sham groups (365+/-46 fmol/mL versus 76+/-9 and 45+/-14, respectively). Nevertheless, ARB treatment markedly limited the enhancement of kidney Ang II by Ang II infusion (65+/-17 fmol/g in sham, 606+/-147 in Ang II group, and 288+/-28 in Ang II+ARB group). Ang II infusion significantly increased kidney angiotensinogen compared with sham (1.69+/-0.21 densitometric units versus 1.00+/-0.17). This change was reflected by increased angiotensinogen immunostaining in proximal tubules. ARB treatment prevented this increase (1.14+/-0.12). Urinary angiotensinogen excretion rates were enhanced 4.7x in Ang II group (4.67+/-0.41 densitometric units versus 1.00+/-0.21) but ARB treatment prevented the augmentation of urinary angiotensinogen (0.96+/-0.23). These data demonstrate that augmentation of intrarenal angiotensinogen in Ang II-infused rats is AT1-dependent and provide further evidence that urinary angiotensinogen is closely linked to intrarenal Ang II in Ang II-dependent hypertension.

Intrarenal angiotensin II and hypertension

Elevations in intrarenal angiotensin II (Ang II) cause reductions in renal function and sodium excretion that contribute to progressive hypertension and lead to renal and vascular injury. Augmentation of intrarenal Ang II occurs by several processes, leading to levels much greater than can be explained from the circulating levels. In Ang II-dependent hypertension, Ang II is internalized via an AT1 receptor mechanism, but there is also sustained intrarenal production of Ang II. Ang II exerts a positive feedback action on intrarenal angiotensinogen (AGT) mRNA and protein. The increased intrarenal AGT production is associated with increased intrarenal and intracellular Ang II contents and urinary AGT excretion rates. The increased urinary AGT indicates spillover of AGT into distal nephron segments supporting enhanced distal Ang II formation and sodium reabsorption. The augmentation of intrarenal Ang II provides the basis for sustained actions on renal function, sodium excretion, and maintenance of hypertension.

Proximal tubular fluid angiotensin II levels in angiotensin II-induced hypertensive rats

BACKGROUND

It has been shown that infusions of low-dose angiotensin II (Ang II) for 2 weeks lead to impaired pressure natriuresis and autoregulatory capability. Although intrarenal renin content and renin mRNA levels are markedly reduced, whole-kidney Ang II content has been shown to be increased. However, the intrarenal distribution of the increased intrarenal Ang II has not been established.

OBJECTIVE

To determine the concentrations of Ang II in the proximal tubule fluid achieved in hypertensive rats (n = 16) infused with Ang II, previously prepared by infusion with Ang II at 60 ng/min via osmotic minipump for 13 days.

METHODS

Rats were anesthetized with pentobarbital sodium and prepared for micropuncture, and then several free-flow proximal tubular fluid collections were obtained and pooled for each rat. At the end of each experiment, a blood sample was collected and the micropunctured kidney was excised and homogenized in chilled methanol. All samples were extracted immediately after collection and stored at 20 degrees C until the day of Ang II radioimmunoassay.

RESULTS

Mean arterial blood pressure averaged 179 +/- 3 mmHg, renal plasma flow was 1.89 +/- 0.15 ml/min per g, and glomerular filtration rate averaged 0.58 +/- 0.04 ml/min per g. The Ang II concentration in proximal tubular fluid averaged 4.5 +/- 1.1 pmol/ml, a value substantially greater than the Ang II concentrations in plasma (0.17 +/- 0.03 pmol/ml), urine (0.06 +/- 0.01 pmol/ml), or total kidney tissue (0.40 +/- 0.10 pmol/g). Plasma renin activity (1.0 +/- 0.21 ng Ang I/ml per h) was markedly suppressed, as observed previously. CONCLUSIONS These findings indicate that Ang II concentrations in proximal tubular fluid collected from kidneys of anesthetized hypertensive rats infused with Ang II are in the nanomolar range, similar to those observed in normotensive rats. The inappropriate maintenance of nanomolar concentrations of Ang II in proximal tubular fluid of Ang II-infused hypertensive rats, even at markedly increased arterial pressures, may contribute to the impaired pressure natriuresis capability previously reported and, thereby, to the development and maintenance of hypertension in this model.

Brain-specific restoration of angiotensin II corrects renal defects seen in angiotensinogen-deficient mice

Mice deficient for angiotensinogen (AGT), or other components of the renin-angiotensin system, show a high rate of neonatal mortality correlated with severe renal abnormalities including hydronephrosis, hypertrophy of renal arteries, and an impaired ability to concentrate urine. Although transgenic replacement of systemic or adipose, but not renal, AGT in AGT-deficient mice has previously been reported to correct some of these renal abnormalities, the tissue target for this complementation has not been defined. In the current study, we have used a novel transgenic strategy to restore the peptide product of the renin-angiotensin system, angiotensin II, exclusively in the brain of AGT-deficient mice and demonstrate that brain-specific angiotensin II can correct the hydronephrosis and partially correct renal dysfunction seen in AGT-deficient mice. Taken together, these results suggest that the renin-angiotensin system affects renal development and function through systemically accessible targets in the brain.

Effects of dietary salt changes on renal renin-angiotensin system in rats

Renin (RA) and angiotensin-converting enzyme (ACE) activities and angiotensinogen, ANG I, and ANG II levels were measured in the kidney (cortex and medulla) and plasma of Wistar-Kyoto rats on a low-sodium (LS; 0.025% NaCl; n = 8), normal-sodium (NS; 1% NaCl; n = 7), or high-sodium (HS; 8% NaCl; n = 7) diet for 21 days. RA, ANG I, and ANG II levels increased in a manner inversely related to sodium content of the diet in both plasma and renal tissues. The LS diet resulted in a 16-, 2.8-, and 1.8-fold increase in plasma RA, ANG I, and ANG II levels, respectively, compared with those in HS rats. In the renal cortex and medulla, RA, ANG I, and ANG II levels were also increased by diminution of dietary salt content but, in contrast to plasma, ANG II levels increased much more than RA or ANG I levels [5.4 (cortex)- and 4.7 (medulla)-fold compared with HS rats]. In summary, we demonstrated variations of ANG II levels in the kidney during dietary salt modifications. Our results confirm that RA and ACE activity are not the steps limiting intrarenal ANG II levels. Nevertheless, despite RA and ACE activity differences between renal cortex and medulla, ANG I and ANG II levels are equivalent in these two tissues; these results argue against a compartmentalization of RAS in these two intrarenal areas.

Contribution of angiotensin II internalization to intrarenal angiotensin II levels in rats

This study was designed to determine the involvement of AT(1) receptors in the uptake of ANG II in the kidney of rats exposed to differing salt intake. Male Wistar-Kyoto rats were treated with a normal-salt (NS; 1% NaCl, n = 7) or a low-salt (LS; 0.025% NaCl, n = 7) diet combined with (LS+Los, n = 7; NS+Los, n = 7) or without losartan (30 mg. kg(-1). day(-1)), an AT(1) receptor antagonist. Renin (RA) and angiotensin-converting enzyme (ACE) activities and angiotensinogen, ANG I, and ANG II levels were measured in plasma, renal cortex, and medulla. In LS rats, in both plasma and renal cortex, the increase in RA was associated with an increase in ANG I and ANG II levels compared with NS rats, but intrarenal ANG II levels increased more than ANG I levels. In NS+Los rats, the increase in RA in plasma was followed by a marked increase in plasma ANG I and ANG II levels compared with NS rats whereas in the kidney the increase of renal RA was followed by a decrease of the levels of these peptides. The same pattern was observed in LS+Los rats, but the decrease in renal ANG II levels was much more pronounced in LS+Los rats than in NS+Los rats. Our results suggest that the increase in renal ANG II levels after salt restriction results mainly from an uptake of ANG II, via AT(1) receptors. Such elevated intrarenal ANG II levels could contribute to maintain sodium and fluid balance and arterial blood pressure during salt-deficiency states.

Regulation of intrarenal angiotensin II in hypertension

Intrarenal angiotensin II (Ang II) is regulated by several complex processes involving formation from both systemically delivered and intrarenally formed substrate, as well as receptor-mediated internalization. There is substantial compartmentalization of intrarenal Ang II, with levels in the renal interstitial fluid and in proximal tubule fluid being much greater than can be explained from the circulating levels. In Ang II--dependent hypertension, elevated intrarenal Ang II levels occur even when intrarenal renin expression and content are suppressed. Studies in Ang II--infused rats have demonstrated that augmentation of intrarenal Ang II is due, in part, to uptake of circulating Ang II via an Ang II type 1 (AT(1)) receptor mechanism and also to sustained endogenous production of Ang II. Some of the internalized Ang II accumulates in the light and heavy endosomes and is therefore potentially available for intracellular actions. The enhanced intrarenal Ang II also exerts a positive feedback action to augment intrarenal levels of angiotensinogen (AGT) mRNA and protein, which contribute further to the increased intrarenal Ang II in hypertensive states. In addition, renal AT(1) receptor protein and mRNA levels are maintained, allowing increased Ang II levels to elicit progressive effects. The increased intrarenal Ang II activity and AGT production are associated with increased urinary AGT excretion rates. The urinary AGT excretion rates show a clear relationship to kidney Ang II content, suggesting that urinary AGT may serve as an index of Ang II--dependent hypertension. Collectively, the data support a powerful role for intrarenal Ang II in the pathogenesis of hypertension.

Effects of antihypertensive therapy on intrarenal angiotensin and bradykinin levels in experimental renal insufficiency

BACKGROUND

Whereas angiotensin converting enzyme inhibitors and angiotensin type 1 receptor antagonists have beneficial effects in the remnant model of renal failure, calcium channel blockers do not consistently improve renal disease in this model. This study examined whether these different means of blood pressure reduction have different effects on renal levels of angiotensin (Ang) and bradykinin peptides.

METHODS

Rats subjected to five-sixths nephrectomy were divided into groups with similar hypertension and proteinuria at 4 to 5 weeks. They then received either no treatment, or enalapril, losartan or nifedipine for 2 weeks. Following repeat measurements of proteinuria and blood pressure, Ang II and bradykinin peptides were measured in the remnant kidney and renin, Ang II, and aldosterone were measured in the plasma.

RESULTS

All three drugs had equivalent blood pressure-lowering effects. Enalapril and losartan reduced proteinuria but nifedipine did not. Reduction of proteinuria in rats treated with enalapril and losartan was associated with a reduction in Ang II levels in both the peri-infarct and intact portions of the remnant kidney. By contrast, nifedipine increased Ang II levels in the intact portion of the remnant kidney. Losartan reduced bradykinin levels in the peri-infarct portion of the remnant kidney while enalapril reduced bradykinin levels in the intact portion of the remnant kidney. Nifedipine had no effect on intrarenal bradykinin levels.

CONCLUSIONS

The differential effects of enalapril, losartan and nifedipine on proteinuria and intrarenal Ang II and bradykinin levels suggest that the ability of an antihypertensive to decrease proteinuria may depend on its ability to decrease kidney Ang II and bradykinin levels.

Intrarenal AT(1) receptor and ACE binding in ANG II-induced hypertensive rats

The intrarenal expression of angiotensin II (ANG II) type 1 (AT(1)) receptors and angiotensin-converting enzyme (ACE) was determined in ANG II-induced hypertensive rats (80 ng/min; 2 wk). Systolic blood pressure averaged 184 +/- 3 and 125 +/- 1 mmHg in ANG II-infused compared with Sham rats on day 12. Total kidney AT(1) receptor protein levels were not altered significantly. AT(1) receptor binding mapped by quantitative in vitro autoradiography was significantly decreased in glomeruli (172 +/- 25 vs. 275 +/- 34 disintegrations. min(-1). mm(-2)) and the inner stripe of the outer medulla (121 +/- 17 vs. 178 +/- 19 disintegrations. min(-1). mm(-2)), but not proximal convoluted tubules (48 +/- 9 vs. 58 +/- 6 disintegrations. min(-1). mm(-2)) of ANG II-infused compared with Sham rats. Proximal tubule ACE binding was significantly augmented (132 +/- 4 vs. 97 +/- 3 disintegrations. min(-1). mm(-2)) in ANG II-infused rats. In summary, during ANG II-induced hypertension, glomeruli and inner stripe of the outer medulla have reduced AT(1) receptor binding. Proximal convoluted tubules exhibit maintained AT(1) receptor density and increased ACE binding, which together with the elevated ANG II levels suggest that ANG II exerts a sustained influence on tubular reabsorption and consequently contributes to the development and maintenance of ANG II-dependent hypertension.

Ang II accumulation in rat renal endosomes during Ang II-induced hypertension: role of AT(1) receptor

Hypertension induced by long-term infusion of angiotensin II (Ang II) is associated with augmented intrarenal Ang II levels to a greater extent than can be explained on the basis of the circulating Ang II levels. Although part of this augmentation is due to AT(1) receptor-dependent internalization, the intracellular compartments involved in this Ang II accumulation remain unknown. In the present study, we sought to determine whether Ang II trafficking into renal cortical endosomes is increased during Ang II hypertension, and if so, whether the AT(1) receptor antagonist, candesartan, prevents this accumulation. Compared with controls (n=12; 114+/-2 mm Hg), Ang II-infused rats (n=12; 80 ng/kg/min, SC, for 13 days) developed hypertension with systolic blood pressure rising to 185+/-4 mm Hg by Day 12. In Ang II hypertensive rats, plasma renin activity was suppressed, whereas plasma and kidney Ang II levels were increased by 3-fold (348+/-58 versus 119+/-16 fmol/mL) and 2-fold (399+/-39 versus 186+/-26 fmol/g). Intracellular endosomal Ang II levels were increased by more than 10-fold (1100+/-283 versus 71+/-12 fmol/mg protein), whereas intermicrovillar cleft Ang II levels were increased by more than 2-fold (88+/-22 versus 37+/-7 fmol/mg protein). Flow cytometric analysis detected significant increases in AT(1A) receptor antibody binding in endosomal and intermicrovillar clefts of Ang II-infused rats. The hypertension induced by Ang II was prevented in rats treated concurrently with candesartan (2 mg/kg/d, 119+/-3 mm Hg). Candesartan treatment (n=8) also prevented increases in kidney (215+/-19 fmol/g), endosomal (96+/-29 fmol/mg protein), and intermicrovillar cleft Ang II levels (11+/-2 fmol/mg protein). These results indicate that there is substantial intracellular accumulation of angiotensin peptides in renal cortical endosomes during Ang II-dependent hypertension via an AT(1) receptor-mediated process.

Urinary vasodilator and vasoconstrictor angiotensins during menstrual cycle, pregnancy, and lactation

Since normal human pregnancy is characterized by normotension in the face of an increased renin-angiotensin-aldosterone system (RAAS), we evaluated the temporal pattern of urinary excretion of a novel vasodilator within this system, angiotensin-(1-7) (Ang-[1-7]), during the menstrual cycle, pregnancy, and lactation. The urinary profiles of Ang I, Ang II, human chorionic gonadotropin, 17beta-estradiol, and progesterone were also determined. During the menstrual cycle, urinary Ang-(1-7) and Ang II remained stable (mean cycle value: 94.6 +/- 11.3 and 11.4 +/- 1.1 pmol/g of creatinine, respectively) in nine females. In 10 normal pregnant women, urinary Ang-(1-7) and Ang II increased throughout gestation, averaging 1499.8 +/- 310 and 224.4 +/- 58 pmol/g of creatinine, respectively (p < 0.05) at wk 35 and falling during lactation to 394.0 +/- 95 and 65.7 +/- 20 pmol/ g of creatinine (p < 0.05), respectively. The Ang-(1-7)/Ang II ratio was unchanged in the different reproductive periods. During the menstrual cycle, Ang II and Ang-(1-7) correlated with 17beta-estradiol and progesterone using multivariate analysis (r = 0.31, p < 0.001) and r = 0.28, p < 0.02, respectively). During gestation, 17beta-estradiol and progesterone correlated with urinary Ang-(1-7) (r = 0.48, p < 0.001 and r = 0.47, p < 0.001, respectively) and Ang II (r = 0.24, p < 0.03 and r = 0.25, p < 0.03, respectively); by multiple regression, only Ang-(1-7) correlated with both steroids (r = 0.49,p < 0.001). The progressive rise of Ang-(1-7) throughout gestation, probably modulated by estrogen and progesterone, suggests a physiologic counterregulation within the RAAS.

Enhancement of angiotensinogen expression in angiotensin II-dependent hypertension

Chronic infusion of angiotensin (Ang) II leads to the development of hypertension and enhances intrarenal Ang II content to levels greater than can be explained from the circulating concentrations of the peptide. We previously reported that renal angiotensinogen (Ao) mRNA is enhanced in Ang II-dependent hypertension and may contribute to augmented intrarenal Ang II levels, but the Ao protein levels were not significantly increased. Because a high-salt diet (H/S) has been shown to suppress renal expression of Ao mRNA, we examined the effects of chronic Ang II infusion on kidney and liver Ao mRNA and protein levels in male Sprague-Dawley rats (n=12) maintained on an 8% salt diet. Ang II was administered via osmotic minipumps (40 ng/min) to 1 group (n=6) while the remaining rats were sham-operated. A H/S diet alone did not alter systolic blood pressure in sham animals (109+/-6 mm Hg at day 12); however, Ang II infusions to the H/S rats significantly increased systolic blood pressure (167+/-7 at day 12) and intrarenal Ang II content (459+/-107 fmol/g versus 270+/-42) despite a marked suppression of plasma renin activity (0.9+/-0.2 ng Ang I. mL(-1). h(-1) versus 2.8+/-1.3). Ang II infusions significantly increased kidney Ao mRNA compared with the H/S diet alone by 1.9+/-0.1-fold. Western blot analysis of kidney protein extracts showed that the Ang II-infused rats had increased kidney Ao protein levels compared with the H/S diet alone (1.9+/-0.1-fold). Liver Ao mRNA and protein and plasma Ao protein were also significantly increased by Ang II infusions. These data demonstrate the effects of Ang II infusion to stimulate Ao mRNA and protein. Thus, the augmented intrarenal Ang II in Ang II-dependent hypertension may result, in part, by a positive amplification mechanism to activate renal expression of AO:

Intrarenal angiotensin and bradykinin peptide levels in the remnant kidney model of renal insufficiency

BACKGROUND

The remnant kidney model of renal failure is associated with normal or suppressed plasma renin and angiotensin (Ang) II levels when hypertension is established. However, the hypertension responds to angiotensin-converting enzyme (ACE) inhibition and Ang II receptor antagonism, suggesting a role for Ang II in the hypertensive process. Bradykinin (BK) is a potent vasoactive peptide that may also participate in this model.

METHODS

Ang II and BK peptides were measured in the ischemic peri-infarct portion and the intact portion of the remnant kidney at two, five, and seven weeks after surgery. Plasma Ang II, renin, angiotensinogen, and aldosterone levels were also measured.

RESULTS

Ang II levels in the peri-infarct portion were higher than in the intact portion at all time points and were higher than in sham-operated kidney at two weeks. Ang II levels in the intact portion were similar to the levels in kidneys of sham-operated rats at two and five weeks and were suppressed at seven weeks. BK levels were increased in the peri-infarct portion at all time points and in the intact portion at two and five weeks. Plasma Ang II and aldosterone levels were also elevated at two weeks.

CONCLUSIONS

Peri-infarct renal tissue Ang II levels and plasma Ang II and aldosterone levels increase transiently during the evolution of hypertension in the remnant kidney model. Sustained hypertension is associated with an increase in intrarenal BK levels but not with persistent increases in intrarenal or circulating Ang II levels.

Expression of angiotensinogen mRNA and protein in angiotensin II-dependent hypertension

Chronic elevations in circulating angiotensin II (AngII) levels produce sustained hypertension and increased intrarenal AngII contents through multiple mechanisms, which may include sustained or increased local production of AngII. This study was designed to test the hypothesis that chronic AngII infusion increases renal angiotensinogen mRNA and protein levels, thus contributing to the increase in intrarenal AngII levels. AngII (80 ng/min) was infused subcutaneously for 13 d into Sprague-Dawley rats, using osmotic minipumps. Control rats underwent sham operations. By day 12, systolic arterial BP increased to 184 +/- 3 mmHg in AngII-treated rats, whereas values for sham-treated rats remained at control levels (125 +/- 1 mmHg). Plasma renin activity was markedly suppressed (0.2 +/- 0.1 versus 5.3 +/- 1.2 ng AngI/ml per h); however, renal AngII contents were significantly increased in AngII-treated rats (273 +/- 29 versus 99 +/- 18 fmol/g). Western blot analyses of plasma and liver protein using a polyclonal anti-angiotensinogen antibody demonstrated two specific immunoreactive bands, at 52 and 64 kD, whereas kidney tissue exhibited one band, at 52 kD. Densitometric analyses demonstrated that AngII infusion did not alter plasma (52- or 64-kD), renal (52-kD), or hepatic (52-kD) angiotensinogen protein levels; however, there was a significant increase in hepatic expression of the highly glycosylated 64-kD angiotensinogen protein, of almost fourfold (densitometric value/control value ratios of 3.79 +/- 1.16 versus 1.00 +/- 0.35). Renal and hepatic expression of angiotensinogen mRNA, which was examined by semiquantitative reverse transcription-PCR, was significantly increased in AngII-treated rats, compared with shamtreated rats (kidney, densitometric value/glyceraldehyde-3-phosphate dehydrogenase mRNA value ratios of 0.82 +/- 0.11 versus 0.58 +/- 0.04; liver, densitometric value/glyceraldehyde-3-phosphate dehydrogenase mRNA value ratios of 2.34 +/- 0.07 versus 1.32 +/- 0.15). These results indicate that increases in circulating AngII levels increase intrarenal angiotensinogen mRNA levels, which may contribute to the sustained renal AngII-generating capacity that paradoxically occurs in AngII-treated hypertensive rats.

Systemic and Regional Hemodynamic Responses to Tempol in Angiotensin II-Infused Hypertensive Rats

-Recent studies have indicated that angiotensin II (Ang II) can stimulate oxidative stress. The present study was conducted to assess the contribution of oxygen radicals to hypertension and regional circulation during Ang II-induced hypertension. With radioactive microspheres, the responses of systemic and regional hemodynamics to the membrane-permeable, metal-independent superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (tempol) were assessed in conscious Ang II-infused hypertensive rats. Ang II-infused rats (80 ng/min SC for 12 days: n=25) showed higher mean arterial pressure (MAP: 161+/-4 mm Hg) and total peripheral resistance (TPR: 1.59+/-0.08 mm Hg. min(-1). mL(-1)) than vehicle-infused normotensive rats (116+/-3 mm Hg and 0.95+/-0.04 mm Hg. min(-1). mL(-1), respectively; n=23). The blood flow rates in the brain, spleen, large intestine, and skin were significantly reduced in Ang II-infused rats compared with vehicle-infused rats, whereas rates in the lung, heart, liver, kidney, stomach, small intestine, mesenterium, skeletal muscle, and testis were similar. Vascular resistance was significantly increased in every organ studied except the lung, in which the resistance was similar. Tempol (216 µmol/kg IV) significantly reduced MAP by 30+/-4% from 158+/-7 to 114+/-5 mm Hg and TPR by 35+/-6% from 1.57+/-0.17 to 0.95+/-0.04 mm Hg. min(-1). g(-1) in Ang II-infused rats (n=9) but had no effect on these parameters in vehicle-infused rats (n=8). In Ang II-infused rats, tempol did not affect regional blood flow but significantly decreased vascular resistance in the brain (29+/-6%), heart (31+/-6%), liver (37+/-7%), kidney (30+/-7%), small intestine (38+/-6%), and large intestine (47+/-7%). Ang II-infused hypertensive rats showed doubled vascular superoxide production (assessed with lucigenin chemiluminescence), which was normalized by treatment with tempol (3 mmol/L, n=7). Further studies showed that the NO synthase inhibitor, N:(omega)-nitro-L-arginine methyl ester (11 µmol. kg(-1). min(-1) IV, n=11) markedly attenuated the systemic and regional hemodynamic responses of tempol in Ang II-infused rats. These results suggest that in this model of hypertension, oxidative stress may have contributed to the alterations in systemic blood pressure and regional vascular resistance through inactivation of NO.

Metabolism alters the selectivity of angiotensin-(1-7) receptor ligands for angiotensin receptors

The present study examined whether metabolism of the putative angiotensin-(1-7) receptor agonist and antagonist [angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7), respectively] altered their ability to interact with angiotensin AT(1), AT(2), and AT(4) receptor subtypes. Both angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7) competed with low affinity for (125)I-sarcosine(1), isoleucine(8) angiotensin II binding to AT(1) and AT(2) receptors in rat liver and adrenal medulla membranes, respectively, and competed with low affinity for (125)I-angiotensin IV binding to AT(4) receptors in bovine kidney epithelial cell membranes. In vitro renal metabolism of the angiotensin-(1-7) receptor ligands (incubating peptides with rat cortical tissue homogenates) had minimal influence on low-affinity binding to AT(1) and AT(2) receptors, yet caused a significant and dramatic shift toward high-affinity binding for AT(4) receptors. Low-affinity angiotensin II binding to the AT(4) receptor was also shifted toward high-affinity binding following renal metabolism of the peptide. Conversely, angiotensins with high affinity for the AT(4) receptor (e.g., angiotensin IV) were shifted toward low-affinity binding states following peptide metabolism. Incubation of (125)I-angiotensin-(1-7) with rat cortical tissue generated the high-affinity AT(4) receptor ligand (125)I-angiotensin-(3-7), whereas the renal metabolism of (125)I-angiotensin II generated both (125)I-angiotensin-(3-7) and (125)I-angiotensin IV. These results reveal that renal metabolism of angiotensin-(1-7) receptor ligands and angiotensin II yields products that have high affinity for the AT(4) receptor and could potentially contribute to the biologic actions of the parent peptide in the kidney.

Afferent arteriolar reactivity to angiotensin II is enhanced during the early phase of angiotensin II hypertension

Increased renal microvascular reactivity may contribute to the blunted pressure natriuretic response and increase in blood pressure during the development of angiotensin II hypertension. The current studies were performed to determine renal microvascular reactivity during the early phases of angiotensin II-infused hypertension. Male-Sprague Dawley rats received angiotensin II (60 ng/min) or vehicle via an osmotic minipump. Normotensive and angiotensin II hypertensive rats were studied 1 and 2 weeks after implantation of the minipump. Systolic blood pressure averaged 117 +/- 4 mm Hg (n = 31) before pump implantation. Angiotensin II infusion increased systolic blood pressure to 149 +/- 3 and 187 +/- 5 mm Hg on infusion days 6 and 12, respectively. Renal microvascular responses to angiotensin II and norepinephrine at renal perfusion pressures of 100 and 150 mm Hg were observed using the in vitro juxtamedullary nephron preparation. Afferent arteriolar diameters of 1-week normotensive animals averaged 22 +/- 1 microm and after 2 weeks of vehicle infusion averaged 21 +/- 1 microm at a perfusion pressure of 100 mm Hg. In animals infused with angiotensin II for 1 or 2 weeks, diameters of the afferent arterioles perfused at a pressure of 100 mm Hg were 20% and 9% smaller, respectively. Additionally, 1- and 2-week hypertensive animals had an enhanced responsiveness of the renal microvasculature to angiotensin II. At a perfusion pressure of 100 mm Hg, angiotensin II (10 nmol/L) decreased afferent arteriolar diameter by 26 +/- 5% and 22 +/- 3% in the 1- and 2-week angiotensin II hypertensive rats, respectively. In 1- and 2-week normotensive animals, angiotensin II (10 nmol/L) decreased afferent arteriolar diameter by 18 +/- 2% and 15 +/- 2%, respectively, at a perfusion pressure of 100 mm Hg. In contrast, the afferent arteriolar response to norepinephrine was not altered in angiotensin II hypertensive rats. These data demonstrate an elevated renal microvascular resistance and enhanced vascular reactivity that is selective for angiotensin II in the early phases of hypertension development after infusion of angiotensin II. Thus, an alteration in renal microvascular function contributes to the blunted pressure natriuretic response and progressive development of hypertension.

Renal function in the AT1A receptor knockout mouse during normal and volume-expanded conditions

BACKGROUND

Genetically altered mice lacking the AT1A angiotensin II (Ang II) receptor were used to examine the role of AT1A receptors in regulating renal hemodynamics, sodium excretion, glomerulotubular balance, and Ang II levels in plasma and kidney during normal and volume-expanded conditions.

METHODS

AT1A receptor-deficient mice and their wild-type controls were anesthetized with inactin and ketamine, and were prepared to allow intravenous infusions of solutions and measurements of aortic pressure and urine collections. Inulin and para-aminohippurate (PAH) solutions were infused intravenously for clearance determinations under conditions of euvolemia (2.5 microliter/min infusion of isotonic saline) or volume-expansion conditions (12.5 microliter/min). After three 30-minute urine collections, blood samples were collected, and kidneys were harvested. Plasma and kidney Ang II measurements were made by radioimmunoassay.

RESULTS

In the euvolemic state, mean arterial pressures (MAPs) were significantly lower in the AT1A receptor-deficient mice (68 +/- 4 mm Hg) compared with wild-type controls (89 +/- 3 mm Hg). Despite the lower MAP, the glomerular filtration rate (GFR), renal plasma flow (RPF), absolute sodium excretion, and fractional sodium excretion were not significantly different between wild-type and AT1A-/- mice. Volume expansion did not alter MAP in wild-type mice, but significantly increased MAP in the AT1A-/- mice (68 +/- 4 to 83 +/- 5 mm Hg). Similar increases in GFR, RPF, absolute sodium excretion, and fractional sodium excretion in AT1A+/+ and AT1A-/- mice were observed. Glomerulotubular balance was not disrupted by the absence of AT1A receptors. During euvolemia, plasma Ang II concentrations were significantly higher in the AT1A-/- mice compared with wild-type mice (536 +/- 172 vs. 198 +/- 36 fmol/ml). Although volume expansion had no effect on plasma Ang II levels in the AT1A+/+ group, plasma Ang II concentrations were markedly suppressed in the AT1A-/- mice to levels that were not different from those in wild-type mice. In contrast, kidney tissue Ang II contents were reduced in the AT1A-/- mice and were not significantly altered during volume expansion in either the AT1A-/- or the AT1A+/+ mice.

CONCLUSIONS

The absence of AT1A receptors does not impair chronic regulation of renal blood flow, GFR, or glomerulotubular balance. The prompt restoration of MAP following volume expansion suggests that low blood pressure in the AT1A receptor-deficient mice is primarily due to reduced effective plasma and extracellular fluid volume. Normalization of plasma Ang II levels with volume expansion demonstrates a dominant effect of extracellular fluid volume and blood pressure over AT1A receptor-mediated short-loop feedback in the regulation of plasma Ang II levels.

Renal endosomes contain angiotensin peptides, converting enzyme, and AT(1A) receptors

Kidney cortex and proximal tubular angiotensin II (ANG II) levels are greater than can be explained on the basis of circulating ANG II, suggesting intrarenal compartmentalization of these peptides. One possible site of intracellular accumulation is the endosomes. In the present study, we tested for endosomal ANG I, ANG II, angiotensin type 1A receptor (AT(1A)), and angiotensin converting enzyme (ACE) activity and determined whether these levels are regulated by salt intake. Male Sprague-Dawley rats were fed chow containing either high or low dietary sodium for 10-14 days. Blood and kidneys were harvested and processed for measurement of plasma, kidney, and renal intermicrovillar cleft and endosomal angiotensin levels. Kidney ANG I averaged 179 +/- 20 fmol/g and ANG II averaged 258 +/- 36 fmol/g in rats fed a high-sodium diet and were significantly higher, averaging 347 +/- 58 fmol/g and 386 +/- 55 fmol/g, respectively, in rats fed a low-salt diet. Renal intermicrovillar clefts and endosomes contained ANG I and ANG II. Intermicrovillar cleft ANG I and ANG II levels averaged 8.4 +/- 2.6 and 74 +/- 26 fmol/mg, respectively, in rats fed a high-salt diet and 7.6 +/- 1.7 and 70 +/- 25 fmol/mg in rats fed a low-salt diet. Endosomal ANG I and ANG II levels averaged 12.3 +/- 4.4 and 43 +/- 19 fmol/mg, respectively, in rats fed a high-salt diet, and these levels were similar to those observed in rats fed a low-salt diet. Renal endosomes from rats fed a low-salt diet demonstrated significantly more AT(1A) receptor binding compared with rats fed a high-salt diet. ACE activity was detectable in renal intermicrovillar clefts and was 2.5-fold higher than the levels observed in renal endosomes. Acute enalaprilat treatment decreased ACE activity in renal intermicrovillar clefts by 90% and in renal endosomes by 84%. Likewise, intermicrovillar cleft and endosomal ANG II levels decreased by 61% and 52%, respectively, in enalaprilat-treated animals. These data demonstrate the presence of intact angiotensin peptides and ACE activity in renal intermicrovillar clefts and endosomes, indicating that intact angiotensin peptides are formed and/or trafficked through intracellular endosomal compartments and are dependent on ACE activity.

Protective role of the angiotensin AT2 receptor in a renal wrap hypertension model

We evaluated the role of the renal angiotensin II type 2 (AT2) receptor in blood pressure regulation in rats with 2-kidney, 1 figure-8 wrap (Grollman) hypertension. Renal wrapping increased systolic blood pressure (SBP). Renal interstitial fluid (RIF) bradykinin (BK), nitric oxide end-products (NOX), and cGMP were higher in the contralateral intact kidney than in the wrapped kidney. In rats with Grollman hypertension, losartan normalized SBP and increased renal function, RIF BK, NOX, and cGMP only in contralateral kidneys. In contrast, PD 123319, a specific AT2-receptor antagonist, significantly increased SBP and decreased RIF BK, NOX, and cGMP in both kidneys. Combined administration of losartan and PD 123319 prevented the decrease in SBP and the increase in RIF BK, NOX, and cGMP levels observed with losartan alone. BK-receptor blockade caused a significant increase in RIF BK and a decrease in RIF NOX and cGMP in both kidneys similar to that observed during administration of PD 123319. In rats that underwent sham operation, RIF BK increased in response to angiotensin II, an effect that was blocked by PD 123319. These data demonstrate that angiotensin II mediates renal production of BK, which, in turn, releases nitric oxide and cGMP via stimulation of AT2 receptors. The increase in blood pressure and the decrease in renal BK, nitric oxide, and cGMP during AT2-receptor blockade suggests that the AT2 receptor mediates counterregulatory vasodilation in Grollman hypertension and prevents a further increase in blood pressure.

Differential response of angiotensin peptides in the urine of hypertensive animals

Urinary excretion rates of angiotensin I (Ang I), angiotensin II (Ang II), and angiotensin-(1-7) [Ang-(1-7)] were determined in normotensive Sprague Dawley (SD), spontaneously hypertensive (SHR), and mRen-2 transgenic hypertensive animals before and following blockade of Ang II synthesis or activity for two weeks. This study was performed to determine for the first time whether inhibition of Ang II alters the excretion of angiotensin peptides in the urine. Rats were given either tap water or water medicated with lisinopril, losartan or both agents in combination. Blood pressure was monitored at regular intervals during the experiment by the tail-cuff method, and once again at the end of the study with a catheter implant into a carotid artery. Metabolic studies and 24 h urinary excretion variables and angiotensin peptides were determined before and during the procedures. While all three treatments normalized the blood pressure of hypertensive animals, therapy with either lisinopril or the combination of lisinopril and losartan had a greater antihypertensive effect in both SHR and [mRen-2]27 transgenic hypertensive rats. In the urine, the concentration of the angiotensins (normalized by 24-h creatinine excretion) was several-fold higher in the untreated hypertensive animals than in normotensive SD rats. In SD rats, lisinopril or lisinopril and losartan produced a sustained rise in urinary levels of Ang-(1-7) without changes in the excretion of Ang I and Ang II. In contrast, Ang I and Ang-(1-7) were significantly elevated in SHR medicated with lisinopril alone or in combination with losartan. Only losartan, however, augmented urinary levels of Ang II in the SHR. The antihypertensive effects of the three separate regimens had no effect on the urinary excretion of angiotensin peptides in [mRen-2]27 transgenic hypertensive rats. These data show that Ang I and Ang-(1-7) are excreted in large amounts in the urine of SD, SHR and [mRen-2]27 hypertensive rats. The unchanged Ang-(1-7) excretion in transgenic hypertensive (Tg+) rats after inhibition of the renin-angiotensin system agrees with the previous finding of a reduced plasma clearance of the peptide in this model of hypertension. The data suggest that this form of hypertension may be associated with increased activity of an endogenous converting enzyme inhibitor.

Proximal tubular angiotensin II levels and renal functional responses to AT1 receptor blockade in nonclipped kidneys of Goldblatt hypertensive rats

-Previous studies have shown that whereas the nonclipped kidney in two-kidney, one clip (2K1C) rats undergoes marked depletion of renin content and renin mRNA, intrarenal angiotensin II (Ang II) levels are not suppressed; however, the distribution and functional consequences of intrarenal Ang II remain unclear. The present study was performed to assess the plasma, kidney, and proximal tubular fluid levels of Ang II and the renal responses to intrarenal Ang II blockade in the nonclipped kidneys of rats clipped for 3 weeks. The Ang II concentrations in proximal tubular fluid averaged 9.19+/-1.06 pmol/mL, whereas plasma Ang II levels averaged 483+/-55 fmol/mL and kidney Ang II content averaged 650+/-66 fmol/g. Thus, as found in kidneys from normal rats with normal renin levels, proximal tubular fluid concentrations of Ang II are in the nanomolar range. To avoid the confounding effects of decreases in mean arterial pressure (MAP), we administered the nonsurmountable AT1 receptor antagonist candesartan directly into the renal artery of nonclipped kidneys (n=10). The dose of candesartan (0.5 microg) did not significantly decrease MAP in 2K1C rats (152+/-3 versus 148+/-3 mm Hg), but effectively prevented the renal vasoconstriction elicited by an intra-arterial bolus of Ang II (2 ng). Candesartan elicited significant increases in glomerular filtration rate (GFR) (0.65+/-0. 06 to 0.83+/-0.11 mL. min-1. g-1) and renal blood flow (6.3+/-0.7 to 7.3+/-0.9 mL. min-1. g-1), and proportionately greater increases in absolute sodium excretion (0.23+/-0.07 to 1.13+/-0.34 micromol. min-1. g-1) and fractional sodium excretion (0.38+/-0.1% to 1.22+/-0. 35%) in 2K1C hypertensive rats. These results show that proximal tubular fluid concentrations of Ang II are in the nanomolar range and are much higher than can be explained on the basis of plasma levels. Further, the data show that the intratubular levels of Ang II in the nonclipped kidneys of 2K1C rats remain at levels found in kidneys with normal renin content and could be exerting effects to suppress renal hemodynamic and glomerular function and to enhance tubular reabsorption rate.