Differential regulation of angiotensin II receptor subtypes in rat kidney by low dietary sodium

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.

In vivo regulation of renal expression of (pro)renin receptor by a low-sodium diet

Effects of low salt (LS) on (pro)renin receptor (PRR) expression are not well established. We hypothesized that LS enhances renal PRR expression via the cGMP-protein kinase G (PKG) signaling pathway. Sprague-Dawley rats were fed a normal-salt (NS) or LS diet associated with intrarenal cortical administration of vehicle (V), the nitric oxide (NO) synthase inhibitor nitro-l-arginine methyl ester (l-NAME), the NO donor S-nitroso-N-acetyl-dl-penicillamine (SNAP), the cGMP analog 8-bromoguanosine (8-Br)-cGMP, the guanylyl cyclase inhibitor 1H-[1, 2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), or a PKG inhibitor (PKGi) for 6 days via osmotic minipump. We evaluated the effects of each treatment on renal interstitial fluid (RIF) levels of nitrate/nitrite and cGMP and renal PRR expression. There were no significant changes in blood pressure with any of the treatments. Urinary sodium excretion was significantly lower in rats given a LS diet. Compared with NS + V, RIF nitrate/nitrite and cGMP levels increased in LS + V rats. In NS groups, RIF nitrate/nitrite and cGMP levels did not change with l-NAME, ODQ, or PKGi and increased in response to SNAP. 8-Br-cGMP increased RIF cGMP but not RIF nitrate/nitrite. In LS groups, RIF nitrate/nitrite decreased with l-NAME and did not change with ODQ or PKGi whereas RIF cGMP decreased with l-NAME, ODQ, and PKGi. PRR mRNA and protein increased in LS + V. In NS rats, PRR mRNA and protein increased in response to 8-Br-GMP and were not affected by any of other treatments. In LS rats, PRR mRNA and protein decreased significantly in response to l-NAME, ODQ, and PKGi. We conclude that LS intake enhances renal expression of PRR via cGMP-PKG signaling pathway.

Angiotensin AT1 receptor-associated protein Arap1 in the kidney vasculature is suppressed by angiotensin II

Arap1 is a protein that interacts with angiotensin II type 1 (AT(1)) receptors and facilitates increased AT(1) receptor surface expression in vitro. In the present study, we assessed the tissue localization and regulation of Arap1 in vivo. Arap1 was found in various mouse organs, with the highest expression in the heart, kidney, aorta, and adrenal gland. Renal Arap1 protein was restricted to the vasculature and to glomerular mesangial cells and was absent from tubular epithelia. A similar localization was found in human kidneys. To test the hypothesis that angiotensin II may control renal Arap1 expression, mice were subjected to various conditions to alter the activity of the renin-angiotensin system. A high-salt diet (4% NaCl, 7 days) upregulated Arap1 expression in mice by 47% compared with controls (0.6% NaCl, P = 0.03). Renal artery stenosis (7 days) or water restriction (48 h) suppressed Arap1 levels compared with controls (-64 and -62% in the clipped and contralateral kidney, respectively; and -50% after water restriction, P < 0.01). Angiotensin II infusion (2 μg·kg(-1)·min(-1), 7 days) reduced Arap1 mRNA levels compared with vehicle by 29% (P < 0.01), whereas AT(1) antagonism (losartan, 30 mg·kg(-1)·day(-1), 7 days) enhanced Arap1 mRNA expression by 52% (P < 0.01); changes in mRNA were paralleled by Arap1 protein abundance. Experiments with hydralazine and epithelial nitric oxide synthase-/- mice further suggested that Arap1 expression changed in parallel with angiotensin II, rather than with blood pressure per se. Similar to in vivo, Arap1 mRNA and protein were suppressed by angiotensin II in a time- and dose-dependent manner in cultured mesangial cells. In summary, Arap1 is highly expressed in the renal vasculature, and its expression is suppressed by angiotensin II. Thus Arap1 may serve as a local modulator of vascular AT(1) receptor function in vivo.

Atrap deficiency increases arterial blood pressure and plasma volume

The angiotensin receptor-associated protein (Atrap) interacts with angiotensin II (AngII) type 1 (AT1) receptors and facilitates their internalization in vitro, but little is known about the function of Atrap in vivo. Here, we detected Atrap expression in several organs of wild-type mice; the highest expression was in the kidney where it localized to the proximal tubule, particularly the brush border. There was no Atrap expression in the renal vasculature or juxtaglomerular cells. We generated Atrap-deficient (Atrap-/-) mice, which were viable and seemed grossly normal. Mean systolic BP was significantly higher in Atrap-/- mice compared with wild-type mice. Dose-response relationships of arterial BP after acute AngII infusion were similar in both genotypes. Plasma volume was significantly higher and plasma renin concentration was markedly lower in Atrap-/- mice compared with wild-type mice. (125)I-AngII binding showed enhanced surface expression of AT1 receptors in the renal cortex of Atrap-/- mice, accompanied by increased carboanhydrase-sensitive proximal tubular function. In summary, Atrap-/- mice have increased arterial pressure and plasma volume. Atrap seems to modulate volume status by acting as a negative regulator of AT1 receptors in the renal tubules.

Regulation of angiotensin II type I receptor (AT1R) protein levels in the obese Zucker rat kidney and urine

Increased renal expression of the angiotensin II, type-1 receptor (AT1R) has been associated with increased blood pressure (BP) and progression of renal disease. We tested whether common medications used to treat hypertension and the metabolic syndrome alter renal AT1R; and whether urine AT1R can be used as a reasonable noninvasive marker of renal levels in the obese Zucker rat, a model for human metabolic syndrome. Immunoblotting revealed that renal and urinary levels of AT1R were significantly higher in obese versus lean rats and correlated (R = 0.62, p < 0.05). Chronic treatment with BP lowering, candesartan, an AT1R antagonist, increased renal levels of AT1R in both lean (282% of lean controls) and obese (178% of obese controls) rats, but decreased urine AT1R levels in obese rats (72% of obese controls). Similarly, chronic treatment with rosiglitazone (RGZ), a peroxisome proliferator activated receptor (subtype gamma) agonist, significantly decreased urine (43% of obese controls) but not renal AT1R (105%) in obese rats. Blood pressure, measured by radiotelemetry, was significantly correlated in untreated and RGZ-treated rats to renal AT1R (R = 0.57, p = 0.0035). Finally, high- (4%) and medium- (0.4%) NaCl diets increased excretion of AT1R in obese rats to approximately 400% of low- (0.04%) NaCl diet. This effect was markedly blunted in lean rats. Overall, we demonstrate increased renal AT1R levels in obese rats. Urine AT1R correlated with renal levels only in the untreated state. Relative salt-sensitivity of AT1R excretion in obese, relative to lean rats, may have implications for both BP and renal disease in the metabolic syndrome.

Roles of ERK and cPLA2 in the angiotensin II-mediated biphasic regulation of Na+-HCO3(-) transport

Regulation of renal proximal transport by angiotensin II (Ang II) is biphasic: low concentrations (picomolar to nanomolar) stimulate reabsorption, but higher concentrations (nanomolar to micromolar) inhibit reabsorption. Traditionally, the stimulatory effect has been attributed to activation of protein kinase C and/or a decrease in intracellular cAMP, whereas the inhibitory action has been attributed to the activation of phospholipase A2 (PLA2) and the subsequent release of arachidonic acid. The Ang II receptor subtype responsible for these effects and the intracellular signaling pathways involved are not completely understood. We isolated proximal tubules from wild-type, Ang II type 1A receptor (AT1A)-deficient, and group IVA cytosolic phospholipase A2 (cPLA2alpha)-deficient mice, and compared their responses to Ang II. In wild-type mice, we found that the stimulatory and inhibitory effects of Ang II on Na+-HCO3(-) cotransporter activity are both AT1-mediated but that ERK activation only plays a role in the former. The stimulatory effect of Ang II was also observed in AT1A-deficient mice, suggesting that this occurs through AT1B. In contrast, the inhibitory effects of Ang II appeared to be mediated by cPLA2alpha activation because high-concentration Ang II stimulated Na+-HCO3(-) cotransporter activity when cPLA2alpha activity was abrogated by pharmacological means or genetic knockout. Consistent with this observation, we found that activation of the cPLA2alpha/P450 pathway suppressed ERK activation. We conclude that Ang II activates ERK and cPLA2alpha in a concentration-dependent manner via AT1, and that the balance between ERK and cPLA2alpha activities determines the ultimate response to Ang II in intact proximal tubules.

Regulator of G protein signalling 2 ameliorates angiotensin II-induced hypertension in mice

Angiotensin II (Ang II) activates signalling pathways predominantly through the G-protein-coupled Ang II type 1 receptor (AT(1)R). The regulator of G protein signalling 2 (RGS2) is a negative G protein regulator. We hypothesized that RGS2 deletion changes blood pressure regulation by increasing the response to Ang II. To address this issue, we infused Ang II (0.5 mg kg(-1) day(-1)) chronically into conscious RGS2-deleted (RGS2(-/-)) and wild-type (RGS2(+/+)) mice, measured mean arterial blood pressure and heart rate (HR) with telemetry and assessed vasoreactivity and gene expression of AT(1A), AT(1B) and AT(2) receptors. Angiotensin II infusion increased blood pressure more in RGS2(-/-) than in RGS2(+/+) mice, while HR was not different between the groups, indicating a resetting of the baroreceptor reflex. Urinary catecholamine excretion was similar in Ang II-infused RGS2(-/-) and RGS2(+/+) mice, indicating a minor role of sympathetic tone for blood pressure differences. Myogenic tone and vasoreactivity in response to Ang II, endothelin-1 and phenylephrine were increased in isolated renal interlobar arterioles of RGS2(-/-) mice compared with RGS2(+/+) mice. The AT(1A), AT(1B) and AT(2) receptor gene expression was not different between RGS2(-/-) and RGS2(+/+) mice. Our findings suggest that RGS2 deletion promotes Ang II-dependent hypertension primarily through an increase of myogenic tone and vasoreactivity, probably by sensitization of AT(1) receptors.

Low sodium intake induces an increase in renal monoamine oxidase activity in the rat. Involvement of an angiotensin II dependent mechanism

AIMS

The interplay between natriuretic dopamine and antinatriuretic angiotensin II represents an important mechanism for the regulation of renal sodium and water excretion. Monoamine oxidase is the main metabolizing pathway for dopamine in the renal cortex. In this study, we have analysed the effect of low sodium feeding and AT1 receptor blockade on renal dopamine metabolism by monoamine oxidase.

METHODS

Four groups of rats were studied: 1, normal salt diet (NS); 2, low salt diet (LS); 3, NS receiving Losartan (Los, specific AT1 receptor antagonist, 20 mg kg(-1) bwt day(-1), NS + Los); 4, LS receiving Los (LS + Los).

RESULTS

Urinary dopamine excretion was lower in LS than in NS rats (543 +/- 32 vs. 680 +/- 34 ng day(-1) 100 g(-1) bwt, P < 0.05). When treated with Los, DOPAC excretion and urinary DOPAC/dopamine ratio fell significantly in the LS + Los group as compared with the LS group (1199 +/- 328 vs. 3081 +/- 681 ng day(-1) 100 g(-1) bwt and 1.90 +/- 0.5 vs. 5.7 +/- 1.2, respectively, both P < 0.02). Losartan increased hydroelectrolyte excretion in the LS group. No changes were found in the NS + Los group. Aromatic L-amino acid decarboxylase activity in cortex was similar in NS and LS rats. Instead, monoamine oxidase activity was higher in cortical homogenates from LS rats (in nmol mg tissue(-1) h(-1): NS 7.66 +/- 0.52; LS 9.82 +/- 0.59, P < 0.05) and this difference was abolished in LS + Los rats (7.34 +/- 0.49 nmol mg tissue(-1) h(-1), P < 0.01, vs. LS).

CONCLUSIONS

We have concluded that low levels of dopamine in the urine of LS rats are because of an increase in the activity of renal monoamine oxidase and that angiotensin II mediates this increase through stimulation of AT1 receptors.

The renal vascular response to ANG II injection is reduced in the nonclipped kidney of two-kidney, one-clip hypertension

The ANG II receptor 1 (AT(1)R) level in the nonclipped kidney of two-kidney, one-clip hypertension (2K1C) has shown to be unchanged despite a high circulating angiotensin (ANG) II level. To examine the vasoreactive response to ANG II in this kidney, injections of ANG II into renal artery were performed 6 wk after clipping of the kidney and compared with normotensive controls. The renal blood flow (RBF) response to 2.5 ng ANG II was measured by a Transonic transit-time flowmeter, before and after indomethacin and candesartan treatment, and analyzed by a computer program. The RBF response to 5 ng arginine-vasopressin (AVP) was examined for comparison with ANG II. The mRNA for AT(1A) and AT(1B) as well as Western blotting for AT(1)R in renal resistance vessels were determined, and plasma renin activity (PRA) was measured. Systolic blood pressure was 183 +/- 4 mmHg in 2K1C rats compared with 113 +/- 1 mmHg in controls (P < 0.001). PRA was significantly increased in 2K1C animals (P < 0.05). Injection of ANG II reduced RBF with 10 +/- 2% in the nonclipped kidney and 24 +/- 3% in controls (P < 0.001). After indomethacin, the RBF response increased from 10 +/- 2 to 20 +/- 3% (P < 0.02) in 2K1C rats and from 24 +/- 3 to 34 +/- 6% in controls (P < 0.01). The doses of candesartan needed to completely inhibit RBF response to ANG II were 30 microg/kg in the nonclipped kidney and 100 microg/kg in controls (P < 0.001). Western blotting and mRNA for AT(1A) and AT(1B) in the nonclipped kidney were similar to the controls. The results indicate that despite no difference in total AT(1)R levels, functional AT(1)R is downregulated in the nonclipped kidney of 2K1C rats.

Postnatal renal development of rats from mothers that received increased sodium intake

The newborn rat kidney is not fully developed until approximately 12 days after birth. Several lines of evidence suggest that angiotensin II (AII) participates in the postnatal development of the kidney. The aim of the present study was to analyze proliferating cell nuclear antigen (PCNA), fibronectin, alpha-smooth muscle-actin (alpha-SM-actin), and AII expression in renal cortex during development in rats born to mothers that received a normal (control) or increased (experimental) sodium intake during pregnancy. Ninety Wistar rats aged 1, 7, 15, and 30 days from the control and experimental groups were killed and the kidneys removed for histological and immunohistochemical studies. The results showed higher fibronectin, alpha-SM-actin, PCNA, and AII expression in the glomerular and tubulointerstitial areas of the renal cortex of 1- and 7-day-old animals, which decreased with renal development. The animals from the experimental group showed at 1 day of age a decrease in alpha-SM-actin, fibronectin, PCNA, and AII expression compared with controls of the same age ( P<0.05). In conclusion, our data show that increased sodium intake during pregnancy induces a reduction of alpha-SM-actin, fibronectin, and PCNA expression in the renal cortex tubulointerstitium and glomeruli of neonatal rats. These alterations may be related to the decrease of AII expression also observed in the renal cortex from these animals.

High Dietary Sodium Blunts Effects of Angiotensin-converting Enzyme Inhibition on Vascular Angiotensin I–to–Angiotensin II Conversion in Rats

High sodium intake blunts the efficacy of angiotensin (Ang)-converting enzyme (ACE) inhibition (ACEi), but the underlying mechanism is incompletely characterized. High sodium has been reported to increase vascular expression and vascular activity of ACE. To investigate whether high-dietary sodium-induced effects on vascular conversion of Ang I might be involved in the sodium-induced blunting of the response to ACEi, the authors studied the vasoconstrictor responses to Ang I and Ang II of isolated aortic rings from healthy rats on low dietary sodium (LS: 0.05% NaCl) and high dietary sodium (HS: 2.0% NaCl) after 3 weeks of ACEi (lisinopril 75 mg/L) or vehicle (CON). Blood pressure was similar in LS and HS in CON, but HS blunted the blood pressure response to ACEi. Functional conversion of Ang I was assessed as the difference in dose-response curves to Ang I and Ang II in parallel aortic rings. Sodium intake did not affect the dose-response curves to Ang I and Ang II in CON. In the ACEi groups, a significant difference was present between the curves for Ang I and Ang II on LS (deltaEC50, 6.7 nM; range, 2.2-13 nM; P < 0.01) but not on HS (deltaEC50: 1.3 nM; range, 0.0-4.1 nM, median [interquartile range], NS). Thus, HS blunts the ACEi-induced reduction of functional vascular Ang I conversion compared with LS. Whether the blunted functional vascular conversion is causally related to the blunted blood pressure response remains to be elucidated.

Biphasic regulation of renal proximal bicarbonate absorption by luminal AT(1A) receptor

Angiotensin II (AngII) regulates renal proximal transport in a biphasic way. It has been recently shown that the basolateral type 1A receptor (AT(1A)) mediates the biphasic regulation of Na(+)-HCO(3)(-) cotransporter (NBC) by AngII. However, the receptor subtype(s) responsible for the luminal AngII actions remained to be established. To clarify this issue, the luminal AngII effects in isolated proximal tubules from wild-type (WT) and AT(1A)-deficient mice (AT(1A) KO) were compared. In WT, the rate of bicarbonate absorption (JHCO(3)(-)), analyzed with a stop-flow microspectrofluorometric method, was stimulated by 10(-10) mol/L luminal AngII but was inhibited by 10(-6) mol/L luminal AngII. Both stimulatory and inhibitory effects of AngII were completely blocked by valsartan (AT(1) antagonist) but unaffected by PD 123,319 (AT(2) antagonist). In AT(1A) KO, in contrast, luminal AngII (10(-10) - 10(-6) mol/L) did not change JHCO(3)(-). In WT, 10(-6) mol/L luminal AngII increased cell Ca(2+) concentrations ([Ca(2+)](i)), which was again blocked by valsartan but not by PD 123,319. However, luminal AngII did not increase [Ca(2+)](i) in AT(1A) KO. On the other hand, the addition of arachidonic acid similarly inhibited JHCO(3)(-) in WT and AT(1A) KO. Furthermore, the acute activation of protein kinase C by phorbol 12-myristate 13-acetate similarly stimulated JHCO(3)(-) in WT and AT1A KO, indicating that the inhibitory and stimulatory pathways necessary for the AngII actions were preserved in AT(1A) KO. These results indicate that the luminal AT(1A) mediates the biphasic regulation of bicarbonate absorption by luminal AngII, while no evidence was obtained for a role of AT(2).

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.

Blood pressure response to chronic episodic hypoxia: the renin-angiotensin system

By using an inspired oxygen fraction that produces oxyhemoglobin desaturation equivalent to that seen in human sleep apnea, we have demonstrated that 35 days of recurrent episodic hypoxia (every 30 s for 7 h/day) results in an 8-13 mmHg persistent increase in diurnal systemic mean arterial blood pressure (MAP) in rats. Blockade of angiotensin II receptors (AT(1a)) eliminates this response. Separate groups of male Sprague-Dawley rats were fed high-salt (8%), ad libitum-salt, or low-salt (0.1%) diets for 7 wk: 2 wk of wash-in for baseline blood pressure measurement and 5 wk of experimental conditions. Rats in each salt group were subjected to episodic hypoxia whereas controls remained unhandled under normoxic conditions. MAP remained at basal levels in all nonepisodic hypoxia controls as well as high-salt-diet episodic hypoxia-exposed rats. Ad lib and low-salt episodic hypoxia rats showed an increase in MAP from 106 and 104 mmHg at baseline to 112 and 113 mmHg, respectively (P < 0.05). Whole kidney renin mRNA was suppressed in high-salt controls and episodic hypoxia rats, whereas kidney AT(1a) mRNA showed opposite changes. Suppression of the renin-angiotensin system with a high-salt diet blocks the increase in MAP in episodic hypoxia-challenged rats, in part by suppressing local tissue renin levels. Upregulation of the tissue angiotensin II system appears to be necessary for the chronic blood pressure changes that occur from episodic hypoxia.

The angiotensin II type 1 receptor and receptor-associated proteins

The mechanisms of regulation, activation and signal transduction of the angiotensin II (Ang II) type 1 (AT1) receptor have been studied extensively in the decade after its cloning. The AT1 receptor is a major component of the renin-angiotensin system (RAS). It mediates the classical biological actions of Ang II. Among the structures required for regulation and activation of the receptor, its carboxyl-terminal region plays crucial roles in receptor internalization, desensitization and phosphorylation. The mechanisms involved in heterotrimeric G-protein coupling to the receptor, activation of the downstream signaling pathway by G proteins and the Ang II signal transduction pathways leading to specific cellular responses are discussed. In addition, recent work on the identification and characterization of novel proteins associated with carboxyl-terminus of the AT1 receptor is presented. These novel proteins will advance our understanding of how the receptor is internalized and recycled as they provide molecular mechanisms for the activation and regulation of G-protein-coupled receptors.

Hyperplastic vascular smooth muscle cells of the intrarenal arteries in angiotensin II type 1a receptor null mutant mice

BACKGROUND

Angiotensin II (Ang II), which contracts vascular smooth muscle cells (VSMCs), has been reported to regulate VSMC growth. Recently formed transgenic mice without angiotensinogen or Ang II receptors showed vascular alterations. However, it is still unclear how their VSMCs alter. We explored the role of Ang II via the Ang II type 1a receptor (AT1a) in VSMCs in vivo using AT1a null mutant mice.

METHODS

We analyzed the ultrastructure of the intrarenal arteries in AT1a null mutant mice that were homozygous for a targeted disruption of AT1a receptor gene using light and electron microscopy.

RESULTS

The structural changes of the intrarenal arteries in AT1a null mutant mice showed the wall thickening, which in the interlobar, arcuate, and proximal interlobular arteries consisted of two additional populations of VSMCs, on the luminal and abluminal sides of the media. The luminal overpopulation of smooth muscle cells (SMCs) was arranged in a longitudinal direction separated by increased interposed elastic laminae. The abluminal overpopulation of SMCs ran in circumferential directions separated from the main population. The cytological structure of VSMCs in AT1a null mutant mice was smaller in size, contained more organelles for protein synthesis and secretion than in control mice, and had poorly developed contractile apparatus.

CONCLUSIONS

The lack of AT1a signaling causes structural abnormalities in the renal vascular system and transforms the phenotype of VSMCs into cell proliferation, induces the escape of VSMCs from the circular mechanical integrity, and results in increased synthesis of extracellular matrices.

Coordinate regulation of canine glomeruli and adrenal angiotensin receptors by dietary sodium manipulation

BACKGROUND

This study evaluated the effects of dietary sodium manipulation in dogs on the regulation of canine angiotensin receptors (cAT1 and cAT2) in the kidney and adrenal.

METHODS

Isolated glomeruli and membranes from renal medulla and the adrenal gland were used in radioligand binding assays from two groups of dogs: dogs maintained on low-sodium diet for two weeks followed by a high-sodium diet for two weeks (H), and dogs were maintained on the reverse schedule (L).

RESULTS

Analysis of the binding data showed that dietary sodium manipulation had no significant effects on cAT1 and cAT2 receptor binding affinities in glomeruli, renal medulla, and adrenal tissues. In contrast, dietary sodium loading induced a marked increase in cAT1 receptor expression in both the glomeruli and adrenal compared with receptor expression in salt-restricted animals [H/L ratio: glomeruli (1.5), renal medulla (1.1), adrenal (1.6)] that inversely correlated with the activity of the plasma renin angiotensin system. Conversely, adrenal cAT2 receptor expression was regulated in an inverse manner in the H and L animal groups [H/L ratio: 0.7].

CONCLUSIONS

This study demonstrates that renal glomerular and adrenal AT1 receptors in the dog are coordinately down-regulated by dietary sodium restriction compared with sodium loading, which is distinctly different from the reciprocal regulation observed for rat AT1 receptors in these tissues. Collectively, these data suggest that postreceptor events in dogs are determinants of the aldosterone response observed during sodium restriction. These findings have important implications for the regulation of the renin-angiotensin system in humans, and suggest that coordinate regulation of AT1 receptors in the adrenal and glomeruli represent a negative feedback mechanism that when functioning normally prevents fluctuations of arterial blood pressure and development of arterial hypertension in response to changes in dietary sodium.

Different types of antagonism by losartan and irbesartan on the effects of angiotensin II and its degradation products in rabbit arteries

A previous study by our group has demonstrated that the selective AT1-receptor antagonist losartan behaves as a noncompetitive antagonist in rabbit isolated renal artery (RA). In the present investigation, the influence of losartan and irbesartan on the contractile effects of angiotensin II (AII) and its degradation products angiotensin III (AIII) and angiotensin IV (AIV) was determined in the rabbit isolated RA and femoral artery (FA). The arteries were set up in organ chambers and changes in isometric force were recorded. In both rabbit isolated RA and FA preparations, AII, AIII and AIV elicited significant contractile responses with a similar efficacy. These effects were impaired by the presence of functional endothelium in RA preparations but not in FA preparations. In both preparations studied, the effects of AII, AIII and AIV were influenced neither by the aminopeptidase-A and -M inhibitor amastatin (10 microM), nor by the aminopeptidase-B and -M inhibitor bestatin (10 microM). In endothelium-denuded FA preparations, preincubation with losartan (3-300 nM) antagonized AII-, AIII- and AIV-induced contractions in a competitive manner. However, in endothelium-denuded RA preparations, losartan depressed the maximal contractile responses induced by AII but not those induced by AIII and AIV. In the same preparations, preincubation of another selective AT1-receptor antagonist irbesartan (3-30 nM) concentration-dependently shifted AII and AIII curves to the right in an insurmountable manner. The reduction of the maximal response of AII is more potent when compared to that of AIII (47.7 +/- 1.51% vs. 66.7 +/- 1.88%, percentage of the initial maximal response; P < 0.05; n=5). The selective AT2-receptor antagonist PD123177 (1 microM) did not influence the responses to all three peptides in both RA and FA preparations. These heterogeneous antagonistic effects of the two AT1-receptor antagonists studied with respect to the contractile actions of AII, AIII and AIV suggest the possible existence of multiple, functionally relevant AT1-receptor subtypes in rabbit RA preparations.

Regulation by sodium intake of type 1 angiotensin II receptor mRNAs in the kidney of Sabra rats

OBJECTIVE

To study the relationship between the sensitivity to sodium content of the diet in terms of development of hypertension and the regulation of the expression of type 1 angiotensin II receptor subtypes by such a diet.

METHODS

The expression of angiotensin II receptor subtype (AT1A and AT1B) mRNAs was studied by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in the four zones of the kidneys of Sabra rats, sensitive or resistant to DOCA salt-induced hypertension (SBH/y and SBN/y, respectively). Rats were fed a high (8%) or normal (0.4%) NaCl diet. As vasopressin is known to be elevated in SBH/y rats and to be involved in DOCA-salt hypertension, we studied an additional group of SBH/y rats, fed a high sodium diet, enriched in water.

RESULTS

With the absence of DOCA, SBH/y rats did not develop hypertension. The high sodium diet induced a greater fall in the plasma renin activity in the SBH/y (-95%) than in the SBN/y (-63%). In the cortex (C) and inner stripe (IS), the high sodium diet decreased AT1A and AT1B mRNAs in SBH/y and SBN/y, with a higher magnitude for SBH/y, than for SBN/y (C, -28 versus -20%; IS, -42 versus -20%). The addition of water to the high sodium diet lessened the effect of sodium in the C and IS, although the plasma renin activity (PRA) was not altered.

CONCLUSION

A high sodium diet significantly decreases both AT1A and AT1B gene expression in two specific zones of the rat kidney containing the target cells of angiotensin II (C and IS). This down-regulation is organ-specific since it was observed in the kidney and adrenals, but not in the liver. Finally, SBH/y and SBN/y rats differ in the basal level of AT1 mRNA expression in the IS, and in the ability to modulate AT1 mRNA level under sodium intake.

Angiotensin II type 1 receptor gene polymorphism predicts response to losartan and angiotensin II

Angiotensin II type 1 receptor gene polymorphism predicts response to losartan and angiotensin II.

BACKGROUND

Most of the known actions of angiotensin II (Ang II) are mediated by the Ang II type 1 receptor (AGT1R). A noncoding polymorphism of the AGT1R gene has been described in which there is either an adenine (A) or cytosine (C) base at position 1166. The functional significance of this polymorphism is unknown, prompting us to examine the relationship between this polymorphism and the systemic and renal responses to AGT1R blockade and subpressor Ang II infusion.

METHODS

Sixty-six healthy Caucasian men and women, genotyped for the AGT1R polymorphism by polymerase chain reaction, were chosen to form two homogeneous groups: AA and AC/CC. Renal hemodynamic function was assessed with inulin and para-aminohippurate clearance before and after AGT1R receptor blockade with losartan and Ang II infusion.

RESULTS

The mean values at baseline for glomerular filtration rate (GFR), renal plasma flow (ERPF), and renal blood flow (RBF) were significantly lower in the AC/CC group compared with the AA group. Losartan increased the GFR and decreased the mean arterial pressure (MAP) in the AC/CC group, but did not influence these parameters in the AA group. The aldosterone responses to losartan were blunted in the AA subgroup. During Ang II infusion, AC/CC subjects maintained GFR despite equivalent declines in RBF, suggesting an enhanced efferent arteriolar constrictive response.

CONCLUSIONS

Taken together, these results suggest that there is a relationship between the AGT1R A1166-->C polymorphism and the humoral and renal hemodynamic responses to AGT1R blockade and to Ang II infusion in the sodium-replete state, and that the C allele is associated with enhanced intrarenal and peripheral Ang II activity. Further studies are required to determine the genetic locus for this effect.

Effects of angiotensin II and antagonists on AT(1) receptor expression in mesangial cells

Rat mesangial cells were exposed to angiotensin II, angiotensin AT(1) receptor antagonists such as losartan, EXP 3174 and candesartan, or dexamethasone for increasing periods (1-24 h). Angiotensin AT(1A) and AT(1B) receptor mRNA were measured by reverse transcription-polymerase chain reaction (RT-PCR). Angiotensin II, losartan and EXP 3174 did not modify significantly angiotensin AT(1A) and AT(1B) receptor mRNA. Candesartan increased angiotensin AT(1B) receptor mRNA and, to a lesser extent, angiotensin AT(1A) receptor mRNA. In contrast, dexamethasone decreased mainly angiotensin AT(1B) receptor mRNA. As shown by Western blot analysis, exposure of mesangial cells to angiotensin II, losartan or EXP 3174 did not produce any change in angiotensin AT(1) receptor protein, whereas dexamethasone and candesartan exerted inhibitory effects. In conclusion, the angiotensin AT(1B) receptor subtype, the most abundantly distributed in rat mesangial cells, is inhibited by glucocorticoids. The effect of candesartan is more complex with a slight stimulation of angiotensin AT(1B) mRNA and a marked inhibition of angiotensin AT(1) receptor protein. In contrast, angiotensin II and the other angiotensin AT(1) receptor antagonists studied are inactive on angiotensin AT(1) mRNA and protein.

Distribution of angiotensin AT1 and AT2 receptor subtypes in the rat kidney

ANG II contributes importantly to the regulation of renal vascular resistance, glomerular filtration, and tubular epithelial transport, yet there remains a paucity of information regarding the localization of the ANG II type 1 and 2 (AT1 and AT2) receptors within the rat kidney particularly within the vasculature. The present study was designed to localize the transcriptional and translational site(s) of AT1 and AT2 receptor (AT1R and AT2R, respectively) expression within the rat kidney. Using immunohistochemistry, we detected the AT(1)R translational sites throughout the kidney, with the strongest labeling found in the vasculature of the renal cortex and the proximal tubules of the outer medulla. The AT2R protein expression was found throughout the rat kidney, although there was little to no expression found in the glomerulus and medullary thick ascending limbs of Henle (TAL). Gene-specific primers were then designed to distinguish between the receptor subtypes within microdissected renal tubular and vascular segments using RT-PCR. AT1AR, AT1BR, and AT2R mRNA were found within the renal vasculature (afferent arterioles, arcuate artery, and outer medullary descending vasa recta). The mRNA for both the AT1R isoforms was also detected in the glomeruli and the renal tubules (proximal tubules, TAL, and collecting ducts); however, no AT2R mRNA was detected within the glomerulus and was inconsistently found within the medullary TAL (MTAL). Taken together, these data show that mRNA for the AT1R subtypes was located in all of the renal tubular and vascular segments. Evidence for AT2R mRNA was also found in all but two of the vascular and tubular segments, the MTAL, and the glomeruli. These results are consistent with the whole tissue immunohistochemically localized receptors.

Angiotensin receptors: molecular biology and signalling

1. The active peptide hormone angiotensin II (AngII) is formed from its prohormone angiotensinogen by way of inactive angiotensin I. The highly specific protease, renin, responsible for the initiation of this system was elusive and considered unstable. We isolated it in a pure and stable form from the kidney of the pig, human, rat, and land submandibular glands of the mouse. It was shown that there is only one type of renin with highly stringent substrate specificity, except certain strains of the mouse which have two gene products. 2. The well-known diversity of action of AngII can be attributed to the presence of more than two subtypes, AT1 and AT2, as well as multiple signalling pathways for both of them. 3. The first subtype AT1 was shown to mediate most of the traditionally recognized AngII functions such as vasoconstriction, electrolyte homeostasis etc. 4. Although the identification of the signalling modes of the second subtype AT2 still remains elusive, we and others have shown evidence that its action is generally antagonistic to that of AT1. AT2 inhibits AT1 (growth factor-stimulated cell growth), AT2 attenuates the vasoconstriction induced by AT1. Since AT2 seems to mediate nitric oxide formation in the renal cells, it may initiate a natriuretic pathway in contrast to the sodium-retaining action of AT1-mediated AngII action. 5. Newer mechanisms and functions of these and other receptors will be clarified by the combination of molecular, cellular and integrated physiological studies.

Angiotensin II induces apoptosis in human and rat alveolar epithelial cells

Recent work from this laboratory demonstrated potent inhibition of apoptosis in human alveolar epithelial cells (AECs) by the angiotensin-converting enzyme inhibitor captopril [B. D. Uhal, C. Gidea, R. Bargout, A. Bifero, O. Ibarra-Sunga, M. Papp, K. Flynn, and G. Filippatos. Am. J. Physiol. 275 (Lung Cell. Mol. Physiol. 19): L1013-L1017, 1998]. On this basis, we hypothesized that apoptosis in this cell type might be induced by angiotensin II (ANG II) through its interaction with the ANG II receptor. Purified ANG II induced dose-dependent apoptosis in both the human AEC-derived A549 cell line and in primary type II pneumocytes isolated from adult Wistar rats as detected by nuclear and chromatin morphology, caspase-3 activity, and increased binding of annexin V. Apoptosis also was induced in primary rat AECs by purified angiotensinogen. The nonselective ANG II-receptor antagonist saralasin completely abrogated both ANG II- and angiotensinogen-induced apoptosis at a concentration of 50 microgram/ml. With RT-PCR, both cell types expressed the ANG II-receptor subtypes 1 and 2 and angiotensin-converting enzyme (ACE). The nonthiol ACE inhibitor lisinopril blocked apoptosis induced by angiotensinogen, but not apoptosis induced by purified ANG II. These data demonstrate the presence of a functional ANG II-dependent pathway for apoptosis in human and rat AECs and suggest a role for the ANG II receptor and ACE in the induction of AEC apoptosis in vivo.

Angiotensin II plays a pathogenic role in immune-mediated renal injury in mice

Several lines of evidence show the importance of angiotensin II (AII) in renal injuries, especially when hemodynamic abnormalities are involved. To elucidate the role of AII in immune-mediated renal injury, we studied anti-glomerular basement membrane (GBM) nephritis in AII type 1a receptor (AT1a)-deficient homozygous (AT1a-/-) and wild-type (AT1a+/+) mice. A transient activation of the renin-angiotensin system (RAS) was observed in both groups of mice at around day 1. A renal expression of monocyte chemoattractant protein-1 (MCP-1) was transiently induced at six hours in both groups, which was then downregulated at day 1. In the AT1a+/+ mice, after RAS activation, the glomerular expression of MCP-1 was exacerbated at days 7 and 14. Thereafter, severe proteinuria developed, and the renal expressions of transforming growth factor-beta1 (TGF-beta1) and collagen type I increased, resulting in severe glomerulosclerosis and interstitial fibrosis. In contrast, glomerular expression of MCP-1, proteinuria, and tissue damage were markedly ameliorated in the AT1a-/- mice. Because this amelioration is likely due to the lack of AT1a, we can conclude that AII action, mediated by AT1a, plays a pathogenic role in anti-GBM nephritis, in which AII may contribute to the exacerbation of glomerular MCP-1 expression. These results suggest the involvement of AII in immune-mediated renal injuries.

Effects of ACTH on adrenal angiotensin II receptor subtype expression in vivo

Control of adrenal aldosterone secretion is an important endocrine mechanism mediated by angiotensin II (Ang. II). Recently three subtypes of angiotensin receptors have been demonstrated in the rat adrenal. Our aim was to examine if these receptors are affected by hormone treatment in vivo. Treatment of rats with ACTH resulted in a decrease in AT2 receptor binding from 766 +/- 95 to 310 +/- 51 fmol/mg protein (P < 0.05), without significant changes in AT1 receptors. AT2 receptor mRNA was also decreased (18 +/- 2%, of control, P < 0.05; 25+/-7% of control, P < 0.05) after both 2 and 7 day treatment with ACTH. Changes in AT1a receptor mRNA or total AT1 mRNA did not reach statistical significance. Moreover, treatment with dexamethasone or aldosterone did not affect AT1a, AT1b, or AT2 mRNA. These results demonstrate that ACTH treatment results in subtype-specific changes in adrenal AT receptor gene expression in vivo.

Effects of long-term intrarenal angiotensin II infusion on renal vascular responsiveness to vasoactive agents

1. We tested whether chronic intrarenal angiotensin II (AngII) infusion altered renal vascular responsiveness to vasoactive agents, which would provide evidence of vascular structural changes. 2. The renal blood flow (RBF) responses to renal arterial administration of bolus doses of acetylcholine, glyceryl trinitrate, AngII and noradrenaline were measured before commencement of and 1 day after cessation of 28 days intrarenal AngII infusion (0.5 ng/kg per min) in chronically instrumented conscious dogs. 3. The RBF responses to these vasoactive agents were unaltered by chronic intrarenal AngII infusion in conscious dogs. 4. These functional studies provide no evidence for renal vascular hypertrophy in response to chronic intrarenal AngII infusion in conscious dogs.

Inhibition of the expression of the gene for the angiotensin AT1 receptor by angiotensin II in the rat adrenal gland

The expression of angiotensin AT1A and AT1B receptor mRNA after continuous angiotensin II administration was investigated in the rat adrenal gland. Angiotensin AT1 receptor mRNA detected by Northern blot analysis decreased to 52.7+/-16.1% of control after the administration of angiotensin II (20 microg/h) for 24 h, and to 70.8+/-8.0% after 1 week. A low dose of angiotensin II (0.2 microg/h) also decreased angiotensin AT1 receptor mRNA to 73.0+/-5.5% after 1 week. Competitive reverse transcription and polymerase chain reaction (RT-PCR) experiments revealed that both angiotensin AT1A and AT1B receptor mRNAs decreased after administration of angiotensin II (20 or 0.2 microg/h) for 1 week. Analysis of the angiotensin AT1A promoter by using luciferase-reporter system showed that angiotensin II (up to 1 microM) did not have any effects on the promoter activity (106+/-5.7% after 0.1 microM angiotensin II stimulation) in Y1 cells and cultured vascular smooth muscle cells, although phorbol myristate acetate (PMA) decreased the promoter activity by about 40% compared with control. These results suggest that angiotensin AT1 receptor gene expression in the rat adrenal gland is inhibited by angiotensin II and it may not be due to suppression of promoter activity. Other mechanisms such as destabilization of angiotensin AT1 receptor mRNA or angiotensin II-induced increased blood pressure may be involved in the inhibition.

Regulation of vascular type 1 angiotensin II receptor in hypertension and sodium loading: role of angiotensin II

OBJECTIVE

To test the hypotheses that a high sodium intake increases steady state messenger RNA levels of the type 1 angiotensin II receptor in the aorta and mesenteric resistance arteries, and that this increase is mediated by suppression of production of angiotensin II induced by a high sodium intake; and to test the hypotheses that angiotensin II administered at a pressor dose increases steady state messenger RNA levels of the angiotensin II type 1 receptor in the aorta and mesenteric resistance arteries, and that this increase is mediated by activation of angiotensin II type 1 receptors in these vessels.

METHODS

In experiment 1, male Wistar rats were divided into four groups and treated for 2 weeks with a (0.5%) normal sodium diet, a normal-sodium diet plus angiotensin II, a high (4%) sodium diet, or a high-sodium plus angiotensin II. We infused 25 ng/kg per min angiotensin II subcutaneously by using osmotic pumps. In experiment 2, male Wistar rats were divided into four groups and treated for 2 weeks with vehicle, 1 mg/kg per day losartan by oral gavage, 250 ng/kg per min angiotensin II by using an osmotic pump), and losartan plus angiotensin II. Angiotensin II type 1 messenger mRNA was measured with the use of quantitative reverse transcriptase-polymerase chain reaction in the presence of an angiotensin II type 1 receptor mutant complementary RNA as internal standard.

RESULTS

Results from experiment 1 show that body weight and systolic tail-cuff blood pressures did not differ among our four groups (P > 0.05). Angiotensin II type 1 messenger RNA levels of rats in high-salt diet group were 73% (aorta) and 171% (mesenteric resistance arteries) greater than those of rats in normal-salt diet group (P < 0.05). In contrast, angiotensin II type 1 messenger RNA levels both in aorta and in mesenteric resistance arteries of rats in normal-salt diet plus angiotensin II and high-salt diet plus angiotensin II groups did not differ from those of rats in normal-salt diet group. Results from experiment 2 show that systolic blood pressures in rats treated with angiotensin II and with losartan plus angiotensin II were higher than those in rats administered vehicle (P < 0.05). Mean response of arterial pressure to bolus injection of angiotensin II was suppressed in losartan-treated rats compared with that in rats administered vehicle and in rats treated with losartan plus angiotensin II compared with that in rats treated with angiotensin II (P < 0.05). Angiotensin II type 1 messenger RNA levels were higher by 73% (in aorta) and 63% (in mesenteric resistance arteries) in rats treated with angiotensin II than they were in rats administered vehicle (P < 0.05), but not in both aorta and mesenteric resistance arteries in rats treated with losartan and losartan plus angiotensin II versus rats administered vehicle.

CONCLUSION

A high-salt diet increases angiotensin II type 1 messenger RNA levels both in aorta and in mesenteric resistance arteries. This increase is completely suppressed by simultaneous nonpressor infusion of angiotensin II, suggesting that angiotensin II negatively regulates vascular angiotensin II type 1 messenger RNA in normotensive rats. Hypertension induced by pressor infusion of angiotensin II increases angiotensin II type 1 messenger RNA levels both in aorta and in mesenteric resistance arteries. This increase can be prevented by administration of losartan at a nondepressor dose, suggesting that angiotensin II positively regulates vascular angiotensin II type 1 messenger RNA via activation of the angiotensin II type 1 receptor during hypertension.

Role of cytoplasmic tail of the type 1A angiotensin II receptor in agonist- and phorbol ester-induced desensitization

To investigate mechanisms underlying the agonist-induced desensitization of the type 1A angiotensin II receptor (AT1A-R), we have stably expressed in Chinese hamster ovary (CHO) cells the wild-type receptor and truncated mutants lacking varying lengths of the cytoplasmic tail. Assay of inositol 1,4,5-trisphosphate (IP3) formation in response to agonist demonstrated that the truncated mutants T318, T328, and T348 lacking the last 42, 32, or 12 amino acid residues, respectively, couple with Gq protein with an efficiency similar to that of full-length receptors, whereas coupling of Gq protein was abolished in the T310 truncated mutant devoid of the carboxyl-terminal 50 amino acids. Exposure of CHO/AT1A-R cells expressing the wild-type AT1A-R to angiotensin II resulted in rapid and dose-dependent homologous desensitization of receptor-mediated IP3 formation, which was independent of the receptor internalization. Mastoparan, an activator of G protein-coupled receptor kinase (GRK), induced desensitization of the AT1A-R. The agonist-induced desensitization of the receptor was largely prevented by heparin, a potent inhibitor of GRK, whereas it was only partially attenuated by a protein kinase C (PKC)-specific inhibitor. The homologous or heterologous desensitization of the receptor was greatly impaired in the truncated mutants T318 and T328, lacking the Ser/Thr-rich (13 or 12 Ser/Thr residues) cytoplasmic tail of the AT1A-R. Deletion of the last two Ser residues, including one PKC consensus site in the receptor tail, prevented only phorbol 12-myristate 13-acetate-induced desensitization by 30%. Moreover, we found an agonist-induced translocation of a heparin-sensitive kinase activity. The angiotensin II-stimulated heparin-sensitive kinase could phosphorylate a thioredoxin fusion protein containing the entire AT1A-R cytoplasmic tail (N295 to E359), which lacks consensus phosphorylation sites for GRK1, GRK2, and GRK3. The heparin-sensitive kinase may not be GRK2, GRK3, or GRK6 expressed in CHO/AT1A-R cells, since angiotensin II did not induce translocation of these receptor kinases. Potential Ser/Thr phosphorylation sites located between S328 and S347 in the cytoplasmic tail of AT1A-R seem to play a critical role in the heterologous and homologous desensitization of the receptor. A heparin-sensitive kinase other than GRK2, GRK3, or GRK6 may be involved in the agonist-induced homologous desensitization of the AT1A-R.

Acute sodium depletion modifies septo-preoptic neuron sensitivities to neurohormones

Sodium (Na+) depletion induces sodium appetite to replenish Na+ loss. It appears to be a consequence of enhanced levels of aldosterone (Aldo) and angiotensin II (AII) in the plasma as well as in the brain. Mineralocorticoid pretreatment modifies the sensitivity of septo-preoptic neurons to locally applied AII and Aldo. Therefore, we investigated septo-preoptic neuronal sensitivities to AII and Aldo, as well as to the specific AII type-1 receptor (AT-1) non-peptide antagonist losartan (Los) and to the specific AII type-2 receptor (AT-2) non-peptide antagonist PD123319 after one Na+ depletion without repletion. We found that one Na+ depletion induced increases in the proportion of neurons inhibited by iontophoretic application of AII (20.5% vs. 7.8%, p=0.004) whereas, the proportion of neurons excited by Aldo was increased, (23.7% vs. 5%, p=0.001). Moreover, the proportion of neurons changing sensitivity to AII after one application of Aldo was increased in the furosemide group (44.2% vs. 20.4%, p=0.0123). The proportion of neurons inhibited by application of losartan was enhanced, (26.4% vs. 9.3%, p=0.03). No significant changes were found in response to PD123319 by itself. Moreover, there were more neurons which co-localized responses to both Los and PD123319 in the furosemide group than in the control group (29.7% vs. 8.6%, p=0.027). It is known that multidepletions induce an increased need-free sodium appetite and our present findings could well form part of the neuronal basis of this behavior.

Renal effects of renin-angiotensin system blockade

Pharmacological interruption of the renin-angiotensin system at different pathway levels has extended our knowledge on the distribution of angiotensin receptors in different nephron segments, its regulation and tubular cell responses. Novel beneficial effects obtained with blockade of this peptide on cellular proliferation and its interaction with other vasoactive systems are particularly important for preventing renal damage.

Expression of angiotensin type-1 (AT1) and type-2 (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review

The discovery that all components of the renin-angiotensin system (RAS) are present in the central nervous system led investigators to postulate the existence of a local brain RAS. Supporting this, angiotensin immunoreactive neurons have been visualized in the brain. Two major pathways were described: a forebrain pathway which connects circumventricular organs to the median preoptic nucleus, paraventricular nucleus, and supraoptic nucleus, and a second pathway connecting the hypothalamus to the medulla oblongata. Blood-brain barrier deficient circumventricular organs are rich in angiotensin II receptors. By activating these receptors, circulating angiotensin II may act on central cardiovascular centers via angiotensinergic neurons, providing a link between peripheral and central angiotensin II systems. Among the effector peptides of the brain RAS, angiotensin II and angiotensin III have the same affinity for the two pharmacologically well-defined receptors: type 1 (AT1) and type 2 (AT2). When injected in the brain, these peptides increase blood pressure, water intake, and anterior and posterior pituitary hormone release and may modify memory and learning. The cloning of AT1 and AT2 receptor cDNAs has revealed that these receptors belong to the seven transmembrane domain receptor family. In rodents, two AT1 receptor subtypes, AT1A and AT1B, have been isolated. Using specific riboprobes for in situ hybridization histochemistry, recent studies mapped the distribution of AT1A, AT1B, and AT2 receptor mRNAs in the adult rat and found a predominant expression of AT1A and AT2 mRNA in the brain and of AT1B in the pituitary. Very limited overlap was found between the brain expression of AT1A and AT2 mRNAs. In several functional entities of the brain, such as the preoptic region, the hypothalamus, the olivocerebellary system, and the brainstem baroreflex arc, the colocalization of receptor mRNA, binding sites, and angiotensin immunoreactive nerve terminals suggests local synthesis and expression of angiotensin II receptors. In other areas, such as the bed nucleus of the stria terminalis, the median eminence, or certain parts of the nucleus of the solitary tract, angiotensin II receptors are likely of extrinsic origin. The neuronal expression of AT1A and AT2 receptors was demonstrated in the subfornical organ, the hypothalamus, and the lateral septum. By using double label in situ hybridization, AT1A receptor expression was localized in corticotropin releasing hormone but not in vasopressin containing neurons in the hypothalamus. The information is discussed together with functional data concerning the role of brain angiotensins, in an attempt to provide a better understanding of the physiological and functional roles of each receptor subtype.

Regulation of type 1 ANG II receptor in vascular tissue: role of alpha1-adrenoreceptor

Angiotensin II (ANG II) and norepinephrine (NE) are important regulators of vascular function and structure. Recent studies showed that there are multiple interactions between these two potent vasoconstrictor agents. The present experiment was designed to investigate the effect of NE on the expression of the type 1 ANG II receptor (AT1) in the aorta and cultured vascular smooth muscle cells (VSMC) of rats. Rats were subcutaneously infused with either NE (0.5 microg x kg(-1) x min(-1), n = 6) or the alpha1-adrenoreceptor antagonist prazosin (3.5 microg x kg(-1) x min(-1), n = 6) for 2 wk. Body weight and tail cuff systolic blood pressure were not modified compared with the vehicle control (P > 0.05). Northern blot analysis showed that AT1 mRNA levels in aorta were decreased by 38% in NE-treated rats and increased 117% in prazosin-treated rats (P < 0.05) compared with control. To determine whether NE directly regulates expression of vascular AT1 mRNA and AT1 receptor density, Northern blot analysis and radioligand binding experiments were performed in cultured VSMC. Incubation of VSMC with NE (10(-7) M) led to 44% decrease in AT1 mRNA levels (P < 0.05) and 39% decrease in AT1 receptor density (P < 0.05). Prazosin, but not the alpha2-adrenoreceptor antagonist yohimbine, prevented NE-induced decrease in AT1 mRNA and AT1 receptor density in these cells. Taken together, our results indicate that vascular AT1 gene expression and receptor protein are regulated by ambient NE levels, and NE-induced downregulation of AT1 mRNA and receptor protein is mediated, at least in part, by activating alpha1-adrenoreceptors.

Regulation of type 1 angiotensin II receptor in adrenal gland: role of alpha1-adrenoreceptor

We have previously shown that sodium restriction upregulates the genes encoding angiotensin II receptor (AT1) subtypes, AT1A and AT1B, in the adrenal gland and that this upregulation is mediated by activation of the AT1 receptor. There are multiple interactions between the renin-angiotensin and the adrenergic nervous systems; thus, we conducted the present experiment to investigate whether low sodium-induced upregulation of adrenal AT1A and AT1B is modulated by the alpha1-adrenoreceptor. Seven-week-old male Wistar rats were divided into four groups and given normal sodium diet (0.5%, NS), NS+prazosin (3.5 microg x kg(-1) x min(-1) by osmotic pump), low sodium diet (0.07%, LS), or LS+prazosin. Body weight and mean arterial pressure were not modified over the 2 weeks of treatment (P>.05). Pressor responses to bolus injection of the alpha1-agonist phenylephrine were inhibited in both prazosin groups, compared with NS and LS rats (P<.05). Adrenal AT1A mRNA, determined by Northern blot analysis, was increased in LS (P<.05) but not in NS+prazosin (P>.05), compared with NS. Prazosin enhanced the LS-induced increase of AT1A mRNA (P<.05). Adrenal AT1B mRNA was increased in both LS and NS+prasozin rats, compared with NS rats (P<.05). Prazosin also enhanced the LS-induced increase in AT1B mRNA (P<.05). Therefore, blockade of alpha1-adrenoreceptor results in an enhancement of LS-induced upregulation of adrenal mRNA for AT1A and AT1B. These data suggest that the sympathetic nervous system exerts an inhibitory action, via activation of the alpha1-adrenoreceptor, on AT1A and AT1B gene expression in the adrenal gland during sodium depletion.

Na depletion-induced Na appetite of sheep is independent of brain angiotensin II

Previous experiments indicated that the Na appetite of Na-deplete sheep is decreased by systemically administered captopril. The assumption that captopril does not readily cross the blood-brain barrier, lead to the conclusion that circulating ANG II acting in brain areas without a blood-brain barrier, i.e., circumventricular organs such as the subfornical organ or organum vasculosum of the lamina terminalis, contributes to Na appetite induced by Na depletion. The present experiments investigated the possibility that systemically administered captopril does, in fact, cross the blood-brain-barrier and thereby influence brain angiotensin II formation and that brain angiotensin II contributes to Na depletion-induced Na appetite of sheep. The results showed that systemically administered captopril blocked water intake caused by intracerebroventricular infusion of angiotensin I, and that Na depletion induced Na appetite was not decreased by intracerebroventricular infusion of various antagonists of the renin-angiotensin system. Thus, the results suggest that although captopril crosses the blood-brain-barrier and can influence the formation of brain angiotensin II, brain angiotensin II is not involved in the Na appetite of Na-deplete sheep.

Reversal of microvascular rarefaction and reduced renal mass hypertension

This study examined the microcirculatory and renin-angiotensin system changes following the reversal of hypertension in reduced renal mass rats. Nine-week-old Sprague-Dawley reduced renal mass rats were placed on a low or high sodium diet for 4 or 8 weeks or a combination of 4 weeks of high sodium followed by 4 weeks of low sodium. Blood pressure was directly measured during the development of hypertension and its reversal. Plasma renin activity, angiotensin-converting enzyme activity, and angiotensin II concentrations were measured throughout the experiment. The cremaster and hindlimb muscles were removed, and microvascular density was determined by quantitative stereology. Four weeks of high sodium increased blood pressure (152+/-7 mm Hg) and reduced microvessel density (13.7%). Reduced renal mass hypertension was rapidly reversed after the rats were returned to a low sodium diet (124+/-7 mm Hg after 3 days), and microvascular density returned to control levels. After 4 weeks of high sodium, circulating plasma renin activity and angiotensin II fell by 94% and 82%, respectively. Plasma angiotensin-converting enzyme activity was increased after 2 weeks of high sodium but returned to control levels after 4 weeks of high sodium. This study demonstrates that microvascular density is reduced in reduced renal mass hypertensive rats following exposure to high sodium diet and this is associated with a fall in circulating plasma renin activity and angiotensin II levels. Microvascular density can return to normal levels after a reactivation of the circulating renin-angiotensin system. This study provides further evidence for the hypothesis that modulation of the renin-angiotensin system is important in the regulation of microvascular structure.

Regulation of angiotensin II receptor AT1 subtypes in renal afferent arterioles during chronic changes in sodium diet

Studies determined the effects of chronic changes in sodium diet on the expression, regulation, and function of different angiotensin II (ANG II) receptor subtypes in renal resistance vessels. Rats were fed low- or high-sodium diets for 3 wk before study. Receptor function was assessed in vivo by measuring transient renal blood flow responses to bolus injections of ANG II (2 ng) into the renal artery. ANG II produced less pronounced renal vasoconstriction in rats fed a low- compared with high-sodium diet (16% vs. 56% decrease in renal blood flow, P < 0.001). After acute blockade of ANG II formation by iv enalaprilat injection in sodium-restricted animals, ANG II produced a 40% decrease in renal blood flow, a level between untreated dietary groups and less than high salt diet. Intrarenal administration of angiotensin II receptor type 1 (AT1) receptor antagonists losartan or EXP-3174 simultaneously with ANG II caused dose-dependent inhibition of ANG II responses. Based on maximum vasoconstriction normalized to 100% ANG II effect in each group, AT1 receptor antagonists produced the same degree of blockade in all groups, with an apparent maximum of 80-90%. In contrast, similar doses of the angiotensin II receptor type 2 (AT2) receptor ligand CGP-42112 had only a weak inhibitory effect. In vitro equilibrium-saturation 125I-ANG II binding studies on freshly isolated afferent arterioles indicated that ANG II receptor density was lower in the low- vs. high-sodium animals (157 vs. 298 fmol/mg, P < 0.04); affinity was similar (0.65 nM). Losartan and EXP-3174 displaced up to 80-90% of the ANG II binding; fractional displacement was similar in both diet groups. In contrast, the AT2 receptor analogues PD-123319 and CGP-42112 at concentrations < 10(-6) M had no effect on ANG II binding. RT-PCR assays revealed the expression of both angiotensin II receptor type 1A (AT(1A)) and angiotensin II receptor type 1B (AT(1B)) subtypes in freshly isolated afferent arterioles, while there was very little AT2 receptor expression. Total AT1 receptor mRNA expression was suppressed by low sodium intake to 66% of control levels, whereas it was increased to 132% of control by high-sodium diet, as indicated by ribonuclease protection assay. Receptor regulation was associated with parallel changes in AT(1A) and AT(1B) expression; the AT(1A)/AT(1B) ratio was stable at 3.7. We conclude that AT1 receptors are the predominant ANG II receptor type in renal resistance vessels of 7-wk-old rats. Chronic changes in sodium intake caused parallel regulation of expression and amount of receptor protein of the two AT1 receptor genes that modulate receptor function and altered reactivity of renal vessels to ANG II.

Modulation of renal glomerular angiotensin II receptors by ace inhibition and AT1 receptor antagonism

Angiotensin-converting enzyme inhibitors (ACE-I) and specific nonpeptide angiotensin II (ANG II) receptor antagonists have been used extensively to treat a variety of cardiovascular disorders in experimental animals and humans. Despite their widespread use, only a limited amount of data has been published regarding the effect that renin-angiotensin system (RAS) blockade may have on ANG II receptors, and very often this information is contradictory. The present study was designed to investigate whether changes in plasma ANG II levels induced by RAS blockade could alter glomerular ANG II receptor characteristics. Captopril was employed as an ACE-I with losartan and TCV-116, two AT1 receptor antagonists of different chemical structure. Two experimental protocols were established. Protocol 1 contained 3 experimental groups: controls (Sprague-Dawley rats, 250-300 g BW), and animals treated with either captopril (0.5 g/l via drinking water) or losartan (10 mg/kg BW p.o.). In protocol 2, the animals were treated as in protocol 1 except that losartan was replaced by TCV-116 (1 mg/kg BW p.o.). At the end of treatment (3 days), all groups were killed by decapitation, blood was collected for plasma renin activity (PRA) measurement, and hearts and kidneys were excised. ANG II receptors were assessed by radioligand binding assays on membrane preparations of purified glomeruli, by displacement of 125I-[Sar1, Ile8]-ANG II with specific nonpeptide antagonists of AT1 (losartan) and AT2 (PD 123319) receptor subtypes. RAS blockade by either ACE-I or AT1 antagonists increased PRA. The binding assays showed that renal glomeruli from treated rats and controls expressed a single population (AT1) of ANG II receptors. The density of glomerular AT1 receptors was not modulated by captopril, but was significantly lower in animals treated with either losartan (Bmax: 854 +/- 169 vs. 379 +/- 79 fmol/mg protein and Kd: 59 +/- 6 vs. 45 +/- 6 nM for controls and losartan, respectively) or TCV-116 (480 +/- 72 vs. 188 +/- 16 fmol/mg protein and Kd: 45 +/- 9 vs. 37 +/- 18 nM for controls and TCV-116, respectively) than in their controls. No changes in receptor affinity (Kd) were detected. Previous membrane "acid-wash" did not modify the results. We conclude that short-term RAS blockade by AT1 antagonists, but not by ACE-I, induces true downregulation of renal glomerular ANG II receptors. No AT2 receptor subtype was detected.

Gene expression of central and peripheral renin-angiotensin system components upon dietary sodium intake in rats

The effects of dietary sodium intake on the gene expression of the renin-angiotensin system (RAS) were investigated in rat central and peripheral tissues in a single set of experiment. Northern and reverse transcriptase-polymerase chain reaction (RT-PCR) techniques were used to detect mRNA expression in rats fed a low- or a high-sodium diet (5 or 500 mmol Na+/kg diet) for 20 days. Plasma and renal renin levels were elevated in rats maintained on the low-sodium diet. Sodium deprivation enhanced the expression of angiotensinogen, renin, AT1A and AT1B receptor subtypes in the hypothalamus, but suppressed them in the brainstem. Kidney and adrenal levels of those mRNAs were also enhanced in the sodium-restricted rats. Both AT1A and AT1B mRNAs changed in a similar magnitude in each tissue examined upon dietary sodium intake. AT1A was the predominant receptor subtype of AT1 in all the tissues examined in the present study except the adrenal gland. The present study demonstrated that dietary sodium modulated the gene expression of the RAS components in the central and peripheral tissues. It also showed that the RAS components in the brainstem and hypothalamus were differentially expressed upon sodium deprivation. This suggests different roles of the RAS in these tissues in maintaining body fluid homeostasis in response to different sodium intakes.

Losartan blocks aldosterone and renal vascular responses to angiotensin II in humans

In vitro and animal studies have demonstrated that the effect of angiotensin II (Ang II) on aldosterone is mediated through the Ang II type 1 receptor. However, it has been difficult to demonstrate an effect of Ang II type 1 receptor blockade on aldosterone levels in human studies. One possible explanation is that subjects have not been studied under salt-controlled conditions. Therefore, we examined the effects of losartan on the aldosterone and renal plasma flow responses to Ang II infusion in six normotensive subjects under low and high salt conditions. Ang II was infused in graded doses (0.3 to 10 ng/kg per minute) in the presence and absence of losartan (a single 50-mg oral dose). Renal plasma flow was assessed by measurement of para-aminohippurate clearance. Blood pressure, plasma aldosterone levels (low salt conditions only), and para-aminohippurate clearance were measured before and after each Ang II dose. Losartan had no effect on baseline systolic pressure but attenuated the systolic pressure response to exogenous Ang II during both low salt (0.7 +/- 1.9 versus 6.7 +/- 1.4 mm Hg, P = .001) and high salt (2.0 +/- 1.9 versus 12.3 +/- 2.1 mm Hg, P = .006) conditions. Under low salt conditions, losartan reduced the baseline plasma aldosterone level from 1135 +/- 204 to 558 +/- 102 pmol/L (P = .015) and blocked the aldosterone response to Ang II (-49 +/- 110 versus +436 +/- 83 pmol/L, P = .019). During high salt conditions, losartan had no effect on baseline renal plasma flow but attenuated the renal plasma flow response to Ang II (-90.1 +/- 15.1 versus -185.1 +/- 2.6 mL/min per 1.73 m2, P = .013). These data confirm that losartan lowers both basal and exogenous Ang II-stimulated aldosterone levels under low salt conditions. Losartan does not significantly affect baseline renal plasma flow but does attenuate the renal plasma flow response to exogenous Ang II under high salt conditions.

Angiotensin II receptor subtype antagonists can both stimulate and inhibit salt appetite in rats

In urethane-anaesthetised male Wistar rats iontophoretic application of the angiotensin II (Ang II) type 1 (AT-1) receptor specific nonpeptide antagonist losartan in the septo-preoptic continuum can produce neuronal excitation of short- and long-term duration which has been interpreted as removal of tonic Ang II-induced inhibition. Mineralocorticoid pretreatment, which increases neuronal sensitivity to Ang II to enhance salt appetite, also removes this losartan-induced long-term excitation. These results suggested steroid modulation of the AT-1 receptor and a complex involvement of Ang II in salt appetite. To investigate the role of the inhibitory action of central Ang II on salt appetite, we gave intracerebroventicular injections of a single, low dose (10 ng) of losartan in normal euhydrated rats and this produced, paradoxically, a progressive increase in salt intake (1.6 +/- 0.3 ml/day to 3.5 +/- 0.9 ml/day, n = 15, P < 0.05). Treatment of these salt enhanced rats with DOCA (0.5 mg/day, s.c., for 3 days) further increased the salt appetite, but then a second i.c.v. injection of the same dose of losartan significantly inhibited the enhanced salt appetite (4.7 +/- 0.7 to 1.3 +/- 0.4, n = 9, P < 0.05). These results provide evidence for a complex action of Ang II on the At-1 receptor mediating the generation of salt appetite that appears to involve either at least two functional subtypes of this AT-1 receptor, as already suggested by previous electrophysiological experiments, or one AT-1 receptor with several activation states.

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.

Distinct mechanisms of upregulation of type 1A angiotensin II receptor gene expression in kidney and adrenal gland

We previously demonstrated that type 1A angiotensin II (Ang II) receptor (AT1A) is the predominant renal subtype and is upregulated by a low sodium diet. We have now tested the hypothesis that upregulation of AT1A mRNA induced by sodium deficiency is renal specific and is mediated by activation of type 1 Ang II receptor (AT1). Male Wistar rats were divided into four groups (n = 5 each) and treated for 2 weeks with normal sodium diet (0.5%), normal sodium plus 3 mg/kg per day losartan, low sodium diet (0.07%), or low sodium diet plus losartan. At the end of the 2 weeks, body weight and mean arterial pressure were not different among the four groups (P > .05). Plasma renin activity was elevated by losartan treatment, sodium restriction, or the combination of the two versus control (P < .05). Northern blot analysis showed that the ratio of renal AT1A to glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA was increased by losartan treatment, sodium restriction, or the combination of the two versus control (P < .05). In contrast, the ratio of adrenal AT1A to GAPDH mRNA was increased only by sodium restriction versus three other groups (P < .05). Thus, sodium deficiency increases AT1A mRNA in both kidney and adrenal gland, while Ang II receptor blockade by losartan prevents low sodium-induced AT1A mRNA only in adrenal gland.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of adenovirus-mediated gene transfer in cultured thoracic aorta and renal artery of rats

Replication-deficient recombinant adenovirus vectors have been used to transfer foreign genes effectively to a wide variety of cell types in vivo and in vitro. We have now used adenovirus containing either the Escherichia coli beta-galactosidase (beta-gal) gene (AdHCMVsp1LacZ) or the firefly luciferase gene (Ad5-luc3) to test the hypothesis that efficiencies of adenovirus-mediated gene delivery into organ cultures of smooth muscle differ according to the anatomic origin of the muscle. Thoracic aorta and renal artery were isolated from 9-week-old male Sprague-Dawley rats and exposed to adenovirus after 16 hours of incubation with serum-free medium (Dulbecco's modified Eagle's medium). With the use of histochemical methods, beta-gal staining was noted in both endothelial and adventitial cells but not in the muscular media of thoracic aorta and renal artery exposed to AdHCMVsp1LacZ. The efficiency of the transfection, assessed either by counting of beta-gal-stained cells in intact vessels or by measurement of beta-gal activity in tissue extracts, was higher in renal artery than thoracic aorta (P < .05). Consistent with this result, luciferase activity in renal artery exposed to Ad5-luc3 (15.9 +/- 2.1 x 10(6) relative light units per milligram protein) was higher than that in thoracic aorta (8.3 +/- 2.0 x 10(6), P < .05). To determine whether increased efficiency of adenovirus-mediated gene transfer into renal artery is a function of the replication status of vessels, we assessed [3H]thymidine incorporation. [3H]Thymidine uptake by thoracic aorta was only 63% of that in renal artery (P < .05), indicating that more proliferating cells are present in renal artery. We conclude that the efficiency of adenovirus-mediated gene transfer into cultured renal artery is enhanced compared with that into thoracic aorta and propose that the increase in efficiency is related to the higher proliferative activity of renal artery.