Renal accumulation of circulating angiotensin II in angiotensin II-infused rats

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.

Urinary angiotensinogen as an indicator of intrarenal Angiotensin status in hypertension

Angiotensin II (AngII) infusions augment renal angiotensinogen mRNA and protein and urinary angiotensinogen excretion (U(AGT)). Further experiments were performed in 4 groups of rats: normal salt diet with sham operation, NS+Sham, n=6; NS with AngII infusion at 40 ng/min via osmotic minipump, NS+AngII(40), n=9; NS with AngII infusion at 80 ng/min, NS+AngII(80), n=9; high-salt diet with deoxycorticosterone acetate salt pellet (100 mg), HS+DOCA, n=4. These experiments sought to determine whether enhanced U(AGT) is specifically associated with increased kidney AngII levels or is a nonspecific consequence of the hypertension. Systolic BP (SBP) was significantly increased to 131+/-2 and 162+/-2 mm Hg at day 11 in NS+AngII(40) and NS+AngII(80), respectively, compared with NS+Sham (110+/-1). Regression analysis demonstrated a positive relationship (R=0.49) between SBP and U(AGT) for NS+Sham (1.1+/-0.3 nmol AngI/d), NS+AngII(40) (2.5+/-0.9), and NS+AngII(80) (5.5+/-1.5). U(AGT) was also highly correlated (R=0.70) with kidney AngII content for NS+Sham (49+/-6 fmol/g), NS+AngII(40) (215+/-49), and NS+AngII(80) (347+/-47); but not with plasma AngII (R=0.12). HS+DOCA rats also exhibited increased SBP to 134+/-1 mm Hg, but U(AGT) (1.4+/-0.4 nmol AngI/d) and intrarenal AngII content (13+/-2 fmol/g) were not increased despite the hypertension. Infused human angiotensinogen could not be detected in urine of sham-operated or AngII-infused rats (n=4 each). These data demonstrate that U(AGT) increases in AngII-dependent hypertension in a dose- and time-dependent manner, but not in hypertension elicited by HS+DOCA. The results support the hypothesis that AngII-dependent hypertension results in elevated intrarenal AngII and angiotensinogen levels, reflected by increased U(AGT), which does not occur in an AngII-independent hypertensive model.

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.

The mechanism of improved sodium homeostasis of low-dose losartan in preascitic cirrhosis

Renal sodium retention on standing is one aspect of the abnormal renal sodium handling in preascitic, well-compensated patients with cirrhosis. Recently, it has been shown that low doses (7.5 mg) of the angiotensin II (Ang II) receptor antagonist, losartan, can reverse renal sodium retention on high, 200-mmol sodium/d diet in these patients and restore them to sodium balance. Therefore, the effect of 7.5 mg of losartan on sodium excretion, when changing from supine to erect posture for 2 hours, was examined in 10 well-compensated patients with cirrhosis and 9 age- and sex-matched controls on the same sodium diet, under strictly controlled metabolic conditions. In contrast to control subjects, in whom sodium excretion was unaffected, single 7.5-mg doses of losartan again restored the preascitic patients with cirrhosis to sodium balance. In addition, it blunted the fall in erect posture- induced renal sodium excretion by a reduction in proximal and distal tubular reabsorption of sodium. These changes occurred without any significant changes in blood volumes, systemic and renal hemodynamics, or glomerular filtration rate (GFR) and filtered sodium load compared with controls, and despite activation of the systemic renin-angiotensin-aldosterone system, which was still within normal levels. In conclusion, the beneficial natriuretic effects of low-dose losartan on erect posture - induced sodium retention in preascitic cirrhosis supports the suggestion that the pathophysiology of sodium retention in preascites is in part caused by an intrarenal tubular effect of Ang II in that posture.

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.

Effect of combining an ACE inhibitor and an angiotensin II receptor blocker on plasma and kidney tissue angiotensin II levels

Increased angiotensin II (AII) activity has been recognized as a risk factor for progression of kidney disease. There is increasing clinical evidence that combining an angiotensin-converting enzyme (ACE) inhibitor with an AII receptor blocker (ARB) reduces proteinuria and blood pressure in patients with renal disease, although the mechanism of this synergistic effect remains poorly defined. This study tested whether the combination of an ACE inhibitor and an ARB reduces plasma AII (AIIp) and kidney tissue AII (AIIk) beyond what is observed with either of these two agents alone. Mean arterial pressure, glomerular filtration rate, AIIp, and AIIk were measured in four groups of Wistar rats after 2 weeks of a low-salt diet and 1 week of treatment with captopril (2.4 mg/d), losartan (1.7 mg/d), combination captopril+losartan (1.7 mg/d of captopril, 0.7 mg/d of losartan), or no treatment (control). Administration of captopril, losartan, and captopril+losartan produced statistically significant reductions in mean arterial pressure (control, 130 +/- 4 mm Hg; captopril, 92 +/- 5 mm Hg; losartan, 88 +/- 4 mm Hg; captopril+losartan, 104 +/- 5 mm Hg) and mild reductions in glomerular filtration rate (control, 3.1 +/- 0.1 mL/min; captopril, 2.2 +/- 0.3 mL/min; losartan, 1.7 +/- 0.3 mL/min; captopril+losartan, 2.3 +/- 0.3 mL/min) when compared with control rats, but no significant differences were observed among the treated groups. Captopril and captopril +losartan reduced AIIp significantly when compared with control (captopril, 43 +/- 8 pg/mL; captopril+losartan, 47 +/- 5 pg/mL; control, 134 pg/mL) and with losartan (99 +/- 2 pg/mL). AIIk values were reduced in captopril (254 +/- 18 pg/g kidney weight) and losartan (292 +/- 33 pg/g kidney weight) when compared with control (1,235 +/- 79 pg/g kidney weight). Captopril+losartan (136 +/- 17 pg/g kidney weight) reduced AIIk to values significantly lower than captopril or losartan alone. Higher doses of captopril (5 mg/d and 7.5 mg/d) or losartan (4 mg/d and 6 mg/d) alone did not reduce AIIk to the levels observed with combination low doses of captopril+losartan. Combining low doses of ACE inhibitor plus ARB reduces AIIk more than higher doses of either agent alone. This reduction in AIIk with ACE inhibitor plus ARB provides a mechanism to understand the synergism of this combination in reducing proteinuria and blood pressure. The reduction in AIIk with ACE inhibitor plus ARB may have important implications in long-term organ protection in hypertension and renal disease.

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.

Intrarenal angiotensin II: interstitial and cellular levels and site of production

BACKGROUND

Both local production and angiotensin II subtype 1 (AT1) receptor-mediated uptake from the circulation contribute to the high levels of angiotensin (Ang) II in the kidney. It is largely unknown where Ang II is produced in the kidney and how much of it originates from the circulation.

METHODS

The concentrations of endogenous and 125I-labeled Ang I and II were measured in renal tissue and in blood from pigs receiving systemic infusions of 125I-Ang I. Pigs were either untreated or treated with the angiotensin converting enzyme (ACE) inhibitor captopril or the AT1 receptor antagonist eprosartan.

RESULTS

125I-Ang I was undetectable in renal tissue but the steady-state concentrations of 125I-Ang II in cortical and medullary tissue were four and two times the concentration in arterial blood plasma, respectively. The tissue concentrations of endogenous Ang II were 100 and 60 times higher than in arterial plasma. Eprosartan reduced 125I-Ang II accumulation by 90%, but did not lower tissue Ang II. Captopril did not alter either 125I-Ang II accumulation or tissue Ang II.

CONCLUSIONS

The bulk of Ang II in the kidney is cell-associated. The high tissue/blood concentration ratio of endogenous Ang II may depend on the same mechanism as demonstrated for 125I-Ang II, that is, AT1 receptor-mediated binding to cells and endocytosis. If so, the results indicate that most renal AT1 receptors are exposed to locally generated Ang II rather than Ang II from the circulation. We propose the existence of a low-Ang II vascular system-related interstitial compartment that is separate from tubular fluid, where, according to micropuncture studies, Ang II levels might be high.

Renal cortical vasoconstriction contributes to development of salt-sensitive hypertension after angiotensin II exposure

Rats that are administered angiotensin II (AngII) for 2 wk develop persistent salt-sensitive hypertension, which can be prevented by the immunosuppressor mycophenolate mofetil (MMF) given during the AngII infusion. This study examined the contribution of glomerular hemodynamics (GFR dynamics) in the post-AngII hypertensive response to a high-salt diet (HSD) and the effect of MMF treatment. During AngII administration, rats developed severe hypertension (systolic BP [SBP], 185 +/- 3.9 mmHg), proteinuria, afferent and efferent vasoconstriction, and glomerular hypertension. Rats that received AngII+MMF showed similar responses to AngII; however, they developed lower proteinuria (P < 0.05). At 2 wk, AngII was withdrawn and SBP returned toward normal. Rats were then placed on an HSD (4% NaCl), resulting in a progressive increase in SBP (155 +/- 8.2 mmHg at week 1 and 163 +/- 4.5 mmHg at week 5). GFR dynamic alterations persisted after AngII was stopped, i.e., afferent and efferent vasoconstriction, decreased glomerular plasma flow and single-nephron GFR, and lower ultrafiltration coefficient. These changes correlated with the thickening of the afferent arteriole and with focal tubulointerstitial injury. In the AngII+MMF group, SBP remained unchanged throughout the HSD period (146 +/- 2.3 mmHg at week 1 and 148 +/- 4.4 mmHg at week 5) in association with less afferent arteriolar thickening and tubulointerstitial injury. Single-nephron GFR, glomerular plasma flow, efferent resistance, and ultrafiltration coefficient returned to normal with a significant reduction in afferent resistance. These results suggest a critical role of cortical vasoconstriction in salt-sensitive hypertension. The MMF-induced prevention of these changes suggests that immune mechanisms are involved in the vasoconstrictive response.

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:

Mycophenolate mofetil prevents salt-sensitive hypertension resulting from angiotensin II exposure

BACKGROUND

Interstitial mononuclear cell infiltration is a feature of experimental models of salt-sensitive hypertension (SSHTN). Since several products of these cells are capable of modifying local vascular reactivity and sodium reabsorption, we investigated whether mycophenolate mofetil (MMF), a drug known to inhibit infiltration and proliferation of immune cells, would modify the SSHTN induced by angiotensin II (Ang II) infusion.

METHODS

Sprague-Dawley rats received Ang II for two weeks using subcutaneous minipumps. A high-sodium (4% NaCl) diet was started on the third week and was maintained until the eighth week. MMF (30 mg/kg, N = 15), an immunosuppressive drug, or vehicle (N = 15) was given daily by gastric gavage during the initial three weeks. Sham-operated rats (N = 9) were used as controls. Body weight, blood pressure (tail-cuff plethysmography), and serum creatinine were determined weekly. Urinary malondialdehyde (MDA) excretion, renal histology, and immunohistology, including the presence of Ang II and superoxide-producing cells, were analyzed at the end of Ang II infusion and at eight weeks.

RESULTS

MMF treatment did not modify hypertension induced during exogenous Ang II infusion, but prevented the subsequent SSHTN. Tubulointerstitial injury resulting from Ang II infusion was significantly reduced by MMF treatment, as were proliferative activity, T-cell infiltration and activation (interleukin-2 receptor expression), superoxide-producing cells, and urinary MDA excretion. Ang II-producing cells were present in the renal tubulointerstitium of rats with SSHTN (60 +/- 30 Ang II-positive cells/mm(2) at 8 weeks) and were reduced by two thirds in the MMF-treated group. Forty percent of lymphocytes infiltrating the tubulointerstitium stained positive for Ang II. The expression of Ang II receptors in the kidney was unmodified.

CONCLUSIONS

SSHTN resulting from Ang II infusion is associated with infiltration and activation of immune cells that produce Ang II. MMF treatment reduces these features and prevents the development of SSHTN.

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.

Mechanisms and functions of AT(1) angiotensin receptor internalization

The type 1 (AT(1)) angiotensin receptor, which mediates the known physiological and pharmacological actions of angiotensin II, activates numerous intracellular signaling pathways and undergoes rapid internalization upon agonist binding. Morphological and biochemical studies have shown that agonist-induced endocytosis of the AT(1) receptor occurs via clathrin-coated pits, and is dependent on two regions in the cytoplasmic tail of the receptor. However, it is independent of G protein activation and signaling, and does not require the conserved NPXXY motif in the seventh transmembrane helix. The dependence of internalization of the AT(1) receptor on a cytoplasmic serine-threonine-rich region that is phosphorylated during agonist stimulation suggests that endocytosis is regulated by phosphorylation of the AT(1) receptor tail. beta-Arrestins have been implicated in the desensitization and endocytosis of several G protein-coupled receptors, but the exact nature of the adaptor protein required for association of the AT(1) receptor with clathrin-coated pits, and the role of dynamin in the internalization process, are still controversial. There is increasing evidence for a role of internalization in sustained signal generation from the AT(1) receptor. Several aspects of the mechanisms and specific function of AT(1) receptor internalization, including its precise mode and route of endocytosis, and the potential roles of cytoplasmic and nuclear receptors, remain to be elucidated.

Angiotensin II infusion exacerbates radiation nephropathy

We hypothesized that angiotensin II will exacerbate radiation nephropathy in a time-specific manner. Experimental radiation nephropathy is treatable with angiotensin-converting enzyme inhibition or angiotensin II (AII) receptor blockers. These interventions are particularly important between 3 and 10 weeks after irradiation. We therefore undertook studies in which AII infusions were given at particular intervals after irradiation. Rats received total body irradiation (TBI) plus syngeneic bone marrow transplantation followed (or not) by AII infusion at 200 or 400 ng/kg/min. Infusions were given from 0 to 4 or 4 to 8 weeks after irradiation. An additional group was unirradiated but infused at 800 ng/kg/min for 8 weeks. Kidney function was assessed over 26 weeks, and histology was evaluated after the animals were killed. AII infusion alone did not cause azotemia. There was transient hypertension during AII infusion at 800 ng/kg/min but only minor histologic injury. Irradiation caused azotemia and hypertension, which were not exacerbated by AII infusion at 200 ng/kg/min. Irradiation plus AII infusion at 400 ng/kg/min from 4 to 8 weeks after TBI caused significantly greater azotemia than irradiation alone or irradiation with AII infusion from 0 to 4 weeks. The blood pressure was higher in irradiated rats infused with AII from 4 to 8 weeks. Arteriolar fibrinoid necrosis was a prominent feature in kidneys of rats infused with AII from 4 to 8 weeks after TBI. The worsening of radiation nephropathy by AII infusion from 4 to 8 weeks after irradiation strongly supports the idea of specific and sequential events in the pathogenesis of kidney failure in this model. Hypertension may play a role in these events in addition to the effect of AII alone. The occurrence of arteriolar fibrinoid necrosis in the irradiated, 4-to-8-week-infused animals suggests that vascular injury during that interval determines later outcome.

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.

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.

Unraveling the Mystery of Goldblatt Hypertension

In hypertension caused by unilateral renal artery stenosis, the nonstenotic kidney becomes renin depleted but fails to prevent hypertension. The nonstenotic kidney mysteriously develops elevated intrarenal angiotensin II (ANG II) content. Rats chronically infused with ANG II exhibit a similar hypertensive process. The augmentation of intrarenal ANG II is due to receptor-mediated internalization and continued ANG II formation, which provide a hypertensinogenic stimulus.

Renal tissue angiotensins during converting enzyme inhibition in the spontaneously hypertensive rat

To compare the effects of an angiotensin-converting enzyme inhibitor on circulating and tissue renin-angiotensin system (RAS), we measured different RAS parameters during the first day of treatment (Day1) as well as after two weeks of treatment (Day14). Ramipril was given orally once daily to adult male spontaneously hypertensive rats (SHR). Renin activity (RA), angiotensin converting enzyme (ACE) activity and levels of angiotensin I (ang I) and angiotensin II (ang II) in the plasma, renal cortex and renal medulla were assessed at Day1 and Day14 of the treatment. In the plasma, both RA and ang I increased 10 to 15 fold one to four hours after acute as well as at Day14 of ramipril treatment and then returned to basal values within 24 hours. Plasma ang II levels were not significantly decreased at Day1 or Day14. The decrease in the ang II/ang I ratio suggested a sustained inhibition of plasma ACE at Day14. In the renal cortex and medulla, a clearly different pattern was observed: in ramipril treated rats, RA in the renal cortex and medulla did not change at Day1 but at Day14 we observed a slight and sustained increase in RA. Despite very high basal levels of RA, ang I levels in the renal cortex were comparable to those in the plasma. The ang I level increased only one-fold one hour after ramipril intake at Day1 and Day14. This suggests that angiotensinogen may have a limiting role in the synthesis of ang I in the kidney. Ang II levels were slightly higher in the renal cortex and medulla than in the plasma suggesting local synthesis of the peptide. In the kidney, ang II levels decreased one and four hours after the acute or prolonged ramipril treatment and the ang II/ang I ratio was reduced at the same time. Our results show that the responses of the plasma and kidney components of the RAS to ACE inhibition are different in the plasma and the kidney suggesting that the circulating and tissue RAS are at least in part independent.

Thyroid hormone stimulates renin synthesis in rats without involving the sympathetic nervous system

The present study was performed to examine whether renal expression of the renin gene is regulated by thyroid hormone. Thirty male Sprague-Dawley rats were divided into hypothyroid, control, and hyperthyroid groups by use of daily intraperitoneal administration of methimazole, saline vehicle, or thyroxine, respectively. Each group was further subdivided into sympathetic innervated and sympathetic denervated subgroups by use of intraperitoneal administration of saline vehicle or 6-hydroxydopamine. Plasma renin activity and renal levels of renin were measured by radioimmunoassays after 8 wk. Renal expression of renin mRNA was evaluated by a semiquantitative reverse transcriptase-polymerase chain reaction. Compared with control animals, plasma renin activity, renal level of renin, and renal expression of renin mRNA were reduced (82, 94, and 71%, respectively) in hypothyroid animals and elevated (155, 1,182, and 152%, respectively) in hyperthyroid animals. Sympathetic denervation had no independent effect on these renin values. Our results indicate that thyroid hormone stimulates renin synthesis without involving the sympathetic nervous system.

Receptor-mediated intrarenal angiotensin II augmentation in angiotensin II-infused rats

Chronic low-dose angiotensin II (Ang II) infusion for 13 days mimics two-kidney, one clip Goldblatt hypertension and increase intrarenal Ang II levels. We performed studies to determine the time course for the enhancement of intrarenal Ang II levels and whether the increased intrarenal Ang II is a tissue-specific event and requires a receptor-mediated step. Male Sprague-Dawley rats were uninephrectomized, and either vehicle or Ang II (40 ng/min) was infused via a subcutaneous osmotic minipump. Plasma and renal Ang II levels were measured 3, 7, 10, and 13 days after minipump implantation. Compared with controls (126 +/- 2 mm Hg), systolic pressure in Ang II-infused rats exhibited a detectable increase by day 6 (146 +/- 2 mm Hg) and continued to increase to 189 +/- 5 mm Hg by day 12. Plasma Ang II levels were elevated by day 3, whereas intrarenal Ang II levels were not significantly elevated until 10 days of Ang II infusion. Renal injury characterized by focal and segmental glomerulosclerosis was evident after 13 days of Ang II infusion. Losartan (30 mg/kg per day) prevented the development of hypertension in the Ang II-infused rats for the duration of the infusion period (125 +/- 1 mm Hg) and reduced the degree of glomerular injury. Plasma renin activity was suppressed in the Ang II-infused group but was elevated markedly in both losartan-treated groups. Plasma Ang II levels were elevated in the Ang II-infused rats and were even higher during losartan treatment. Intrarenal Ang II levels were enhanced significantly (354 +/- 60 versus 164 +/- 23 fmol/g) in the Ang II-infused rats. However, losartan treatment prevented the augmentation of intrarenal Ang II caused by Ang II infusion. Heart and adrenal Ang II levels were not significantly increased in the Ang II-infused rats but were significantly elevated during losartan treatment. These results suggest that the tissue-specific elevations of intrarenal Ang II levels caused by chronic Ang II infusion are mediated by angiotensin type 1 receptor activation, which leads to either receptor-mediated internalization of Ang II, enhancement of intrarenal Ang II formation, or both.