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

Inflammatory mediators act at renal pericytes to elicit contraction of vasa recta and reduce pericyte density along the kidney medullary vascular network

Introduction: Regardless of initiating cause, renal injury promotes a potent pro-inflammatory environment in the outer medulla and a concomitant sustained decrease in medullary blood flow (MBF). This decline in MBF is believed to be one of the critical events in the pathogenesis of acute kidney injury (AKI), yet the precise cellular mechanism underlying this are still to be fully elucidated. MBF is regulated by contractile pericyte cells that reside on the descending vasa recta (DVR) capillaries, which are the primary source of blood flow to the medulla. Methods: Using the rat and murine live kidney slice models, we investigated the acute effects of key medullary inflammatory mediators TNF-α, IL-1β, IL-33, IL-18, C3a and C5a on vasa recta pericytes, the effect of AT1-R blocker Losartan on pro-inflammatory mediator activity at vasa recta pericytes, and the effect of 4-hour sustained exposure on immunolabelled NG2+ pericytes. Results and discussion: Exposure of rat and mouse kidney slices to TNF-α, IL-18, IL-33, and C5a demonstrated a real-time pericyte-mediated constriction of DVR. When pro-inflammatory mediators were applied in the presence of Losartan the inflammatory mediator-mediated constriction that had previously been observed was significantly attenuated. When live kidney slices were exposed to inflammatory mediators for 4-h, we noted a significant reduction in the number of NG2+ positive pericytes along vasa recta capillaries in both rat and murine kidney slices. Data collected in this study demonstrate that inflammatory mediators can dysregulate pericytes to constrict DVR diameter and reduce the density of pericytes along vasa recta vessels, further diminishing the regulatory capacity of the capillary network. We postulate that preliminary findings here suggest pericytes play a role in AKI.

Renin-Angiotensin System Induced Secondary Hypertension: The Alteration of Kidney Function and Structure

Long-term hypertension is known as a major risk factor for cardiovascular and chronic kidney disease (CKD). The Renin-angiotensin system (RAS) plays a key role in hypertension pathogenesis. Angiotensin II (Ang II) enhancement in Ang II-dependent hypertension leads to progressive CKD and kidney fibrosis. In the two-kidney one-clip model (2K1C), more renin is synthesized in the principal cells of the collecting duct than juxtaglomerular cells (JGCs). An increase of renal Ang I and Ang II levels and a decrease of renal cortical and medullary Ang 1–7 occur in both kidneys of the 2K1C hypertensive rat model. In addition, the activity of the angiotensin-converting enzyme (ACE) increases, while ACE2's activity decreases in the medullary region of both kidneys in the 2K1C hypertensive model. Also, the renal prolyl carboxypeptidase (PrCP) expression and its activity reduce in the clipped kidneys. The imbalance in the production of renal ACE, ACE2, and PrCP expression causes the progression of renal injury. Intrarenal angiotensinogen (AGT) expression and urine AGT (uAGT) excretion rates in the unclipped kidney are greater than the clipped kidney in the 2K1C hypertensive rat model. The enhancement of Ang II in the clipped kidney is related to renin secretion, while the elevation of intrarenal Ang II in the unclipped kidney is related to stimulation of AGT mRNA and protein in proximal tubule cells by a direct effect of systemic Ang II level. Ang II-dependent hypertension enhances macrophages and T-cell infiltration into the kidney which increases cytokines, and AGT synthesis in proximal tubules is stimulated via cytokines. Accumulation of inflammatory cells in the kidney aggravates hypertension and renal damage. Moreover, Ang II-dependent hypertension alters renal Ang II type 1 & 2 receptors (AT₁R & AT₂R) and Mas receptor (MasR) expression, and the renal interstitial fluid bradykinin, nitric oxide, and cGMP response to AT₁R, AT₂R, or BK B₂-receptor antagonists. Based on a variety of sources including PubMed, Google Scholar, Scopus, and Science-Direct, in the current review, we will discuss the role of RAS-induced secondary hypertension on the alteration of renal function.

Effect of High-Salt Diet on Age-Related High Blood Pressure and Hypothalamic Redox Signaling

Aim: The aim of this study was to investigate the effect of a high salt (HS) diet on age-related changes in blood pressure (BP) and the possible role played by regulatory central mechanisms. Methods: Young (5 months) and old (27 months) male Fischer 344 × Brown Norway (F344/BN) rats were fed standard chow or 8% HS diet for 12 days. BP and heart rate (HR) were measured by telemetry. Results: Mean arterial BP (MAP) was significantly elevated in old rats during the day and night when compared with young animals. The HS diet further elevated MAP in both age groups, and the increase was more pronounced in the old animals, while HR was not altered by age or HS diet. In addition, cardiovascular responses to restraint stress were diminished in the old when compared with the young and were unchanged with HS diet in either age group. Both age and the HS diet elevated the adrenomedullary mRNA levels of tyrosine hydroxylase, an indicator for sympathoexcitation. HS diet enhanced intracerebroventricular angiotensin II (AngII)-induced BP and HR elevations in both age groups. AngII type 1 receptor mRNA increased significantly in the hypothalamus with age and HS diet. Furthermore, hypothalamic p22phox mRNA and gp91phox protein, subunits of NADPH oxidase, as well as NADPH oxidase activity increased with the HS diet in the old animals, whereas antioxidant enzymes that decreased with age yet remained unaltered with the HS diet. Conclusion: Our findings indicate that sensitivity of BP to HS diet increases with age, and that central AngII-induced pressor responses are diminished in old rats compared with the young both under control conditions and during HS diet treatment. These changes are paralleled by increases in the expression and NADPH oxidase activity in the hypothalamus, possibly leading to central oxidative stress-mediated sympathoexcitation and high BP.

Dietary sodium restriction specifically potentiates left ventricular ACE inhibition by zofenopril, and is associated with attenuated hypertrophic response in rats with myocardial infarction

Introduction In patients with myocardial infarction (MI)-induced heart failure, angiotensin-converting enzyme (ACE) inhibitors are proven effective therapy in inhibiting the progression towards overt heart failure. However, the prognosis in these patients is still very poor, and optimisation of therapy is warranted. The antihypertensive and renoprotective effects of ACE inhibitors (ACE-Is) can be substantially enhanced by dietary sodium restriction. In line with the latter, the aim of the present study was to explore whether dietary sodium restriction enhances the efficacy of ACE-I after MI.

Methods Rats with MI-induced left ventricular (LV) dysfunction received ACE-I therapy with zofenopril (5.5 mg/kg/day orally), with or without dietary sodium restriction. ACE activity was measured in non-infarcted LV tissue, kidneys and plasma. Effects on cardiac hypertrophy were examined by means of organ weight/body weight ratios. After blood pressure (BP) measurements, functional consequences of therapy were evaluated as LV pressure development in isolated perfused hearts.

Results Dietary sodium restriction alone had no effect on any of the measured parameters, whereas zofenopril alone significantly reduced plasma and kidney ACE activity, but not LV ACE activity, nor LV weight/body weight ratio. However, only when ACE-I therapy was combined with dietary sodium restriction was LV ACE activity significantly reduced. This effect was paralleled by inhibition of LV hypertrophy. BP was reduced after infarction, and further reduced by zofenopril, but not affected by dietary sodium. Neither treatment was associated with effects on the MI-induced reduction of LV function in vitro.

Conclusions Effects of ACE inhibition with zofenopril can be potentiated by additional dietary sodium restriction. However, these effects were tissue-specific, since LV, but not kidney or plasma, ACE activity was affected by the additional dietary sodium restriction. Effects on LV ACE activity were paralleled by reduced LV hypertrophy. Since the measured parameters did not indicate any adverse side-effects, dietary sodium restriction may provide a safe strategy to improve ACE-I efficacy in patients with infarction-induced LV dysfunction.

Acute eprosartan-induced intrarenal vasodilation in hypertensive humans is not influenced by dietary sodium intake or angiotensin II co-infusion

BACKGROUND

Angiotensin II (Ang II) is thought to play an important role in the development of hypertension. Nevertheless, knowledge on the angiotensin II type-1-receptors (AT1Rs) in the hypertensive kidney and the influence of sodium intake and renin-angiotensin system activity on intrarenal AT1R blockade is scarce. To improve our understanding of renal AT1Rs in hypertensive patients, we studied the effects of acute, local administration of AT1R-blocker eprosartan in kidneys of patients with essential hypertension (off medication).

METHOD

In 73 hypertensive patients who were scheduled for diagnostic renal angiography, we measured renal blood flow (Xenon washout method) before and during intrarenal infusion of two incremental doses of eprosartan (3 and 10 μg/kg/min for 15 min per dose). We hypothesized that the vasodilatory effects of eprosartan would be enhanced by low sodium intake and would be reduced during Ang II co-infusion. Therefore, we allocated the patients to either a high or a low sodium diet and coinfused Ang II (1 ng/kg/min) in a subgroup.

RESULTS

Eprosartan infusion resulted in intrarenal vasodilation in all groups. No differences in the magnitude of this effect were found between the groups. No correlation was found between 24-h urinary sodium excretion (a proxy for dietary sodium intake) and the effect of eprosartan.

CONCLUSION

Eprosartan-induced vasodilation is not influenced by sodium intake and/or co-infusion of Ang II. These rather unexpected findings could be explained by differences between circulating and tissue Ang II levels, variations in AT1R expression, and/or stimulation of other vasodilatory pathways.

Gender Difference in Renal Blood Flow Response to Angiotensin II Administration after Ischemia/Reperfusion in Rats: The Role of AT2 Receptor

Background. Renal ischemia/reperfusion (I/R) is one of the major causes of kidney failure, and it may interact with renin angiotensin system while angiotensin II (Ang II) type 2 receptor (AT2R) expression is gender dependent. We examined the role of AT2R blockade on vascular response to Ang II after I/R in rats. Methods. Male and female rats were subjected to 30 min renal ischemia followed by reperfusion. Two groups of rats received either vehicle or AT2R antagonist, PD123319. Mean arterial pressure (MAP), and renal blood flow (RBF) responses were assessed during graded Ang II (100, 300, and 1000 ng/kg/min, i.v.) infusion at controlled renal perfusion pressure (RPP). Results. Vehicle or antagonist did not alter MAP, RPP, and RBF levels significantly; however, 30 min after reperfusion, RBF decreased insignificantly in female treated with PD123319 (P = 0.07). Ang II reduced RBF and increased renal vascular resistance (RVR) in a dose-related fashion (P dose < 0.0001), and PD123319 intensified the reduction of RBF response in female (P group < 0.005), but not in male rats. Conclusion. The impact of the AT2R on vascular responses to Ang II in renal I/R injury appears to be sexually dimorphic. PD123319 infusion promotes these hemodynamic responses in female more than in male rats.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Bosentan normalizes the GFR response to renal nerve stimulation following reversible unilateral ureteric obstruction in the rat

We investigated the renal response to direct renal nerve stimulation, 2 weeks following reversal of 24-h unilateral (left) ureteric obstruction. Renal nerve stimulation caused a 13-15 % fall in renal blood flow, in 4 groups of anesthetized rats following ureteric obstruction (n=9) or a sham operation (n=7) both with (n=9) and without (n=7) treatment with the mixed ET(A/B) receptor antagonist, bosentan. In the sham-operated rats, renal nerve stimulation did not change glomerular filtration rate but reduced urine flow rate (37+/-3 %, P<0.001), and absolute (38+/-4 %, P<0.001) and fractional (35+/-5 %, P<0.01) sodium excretion. Following unilateral ureteric obstruction, renal nerve stimulation increased glomerular filtration rate by 22+/-3 % (P<0.01), but reduced urine flow rate (14+/-2 %, P<0.001) and fractional sodium excretion (23+/-5 %, P<0.01). Bosentan treatment had no effect on baseline or renal responses to renal nerve stimulation in the sham group but normalized the renal response to renal nerve stimulation in the unilateral ureteric obstruction group. We conclude that 14 days after a 24-h period of unilateral ureteric obstruction there is an increase in GFR in response to direct renal nerve stimulation, which is due, in part, to the actions of endothelin at the time of obstruction.

Vascular effects of a tripeptide fragment of novokinine in hypertensive rats: Mechanism of the hypotensive action

BACKGROUND

Activation of angiotensin AT2 receptors (AT2R) counteracts vasoconstrictor effects of AT1R stimulation and contributes to blood pressure control. We examined effects on mean arterial pressure (MAP) and renal hemodynamics of LKP, a tripeptide fragment of novokinine, an established AT2R agonist.

METHODS

Effects of intravenous LKP infusion and then superimposed losartan (AT1R antagonist) on MAP, total renal (RBF, Transonic probe) and renal medullary blood flows (laser-Doppler), and on renal excretion, were examined in anesthetized (1) Wistar rats with acute norepinephrine-induced hypertension, untreated or pretreated with AT2R antagonist (PD 123319) and (2) spontaneously hypertensive rats (SHR) maintained on standard or high-sodium (HS) diet.

RESULTS

In Wistar rats LKP decreased MAP (-4%, p<0.01) and increased renal medullary perfusion, these effects were abolished in rats pretreated with PD 123319 in which a post-LKP increase in MAP (+6%, p<0.02) occurred. LKP did not alter MAP in SHR; in those on HS diet RBF decreased (-14%, p<0.02), the response that was reverted by losartan. Addition of losartan always decreased or tended to decrease MAP.

CONCLUSIONS

LKP lowered MAP in norepinephrine-induced hypertension, probably via activation of AT2R. At reduced availability of AT2R, as in SHR, LKP appeared to bind to different receptors, possibly AT1, and induced systemic or renal vasoconstriction. Compared to the parent novokinine, a smaller LKP molecule might be easier absorbed after oral application and more useful in therapy.

Novel role of mechanosensitive AT1B receptors in myogenic vasoconstriction

Myogenic vasoconstriction is an inherent property of vascular smooth muscle cells (VSMCs) of resistance arteries harboring ill-defined mechanosensing and mechanotransducing elements. G protein-coupled receptors (GPCRs) are discussed as mechanosensors in VSMCs. In this study, we sought to identify and characterize the role and impact of GPCRs on myogenic vasoconstriction. Thus, we analyzed mRNA expression levels of GPCRs in resistance versus preceding conduit arteries revealing a significant enrichment of several GPCRs in resistance vessels. Selective pharmacological blockade of the highly expressed GPCRs in isolated murine mesenteric arteries ex vivo decreased myogenic vasoconstriction. In particular, candesartan and losartan most prominently suppressed myogenic tone, suggesting that AT1 receptors play a central role in myogenic vasoconstriction. Analyzing angiotensinogen(-/-) mice, a relevant contribution of locally produced angiotensin II to myogenic tone could be excluded. Investigation of AT1A (-/-) and AT1B (-/-) murine mesenteric arteries revealed that AT1B, but not AT1A, receptors are key components of myogenic regulation. This notion was supported by examining fura-2-loaded isolated AT1A (-/-) and AT1B (-/-) VSMCs. Our results indicate that in VSMCs, AT1B receptors are more mechanosensitive than AT1A receptors even at comparable receptor expression levels. Furthermore, we demonstrate that the mechanosensitivity of GPCRs is agonist-independent and positively correlates with receptor expression levels.

Hemopexin activity is associated with angiotensin II responsiveness in humans

BACKGROUND

Hemopexin, an acute phase protein, can downregulate the angiotensin (ang) II type 1 receptor (AT1-R) in vitro. Whether hemopexin is involved in the responsiveness to ang II in vivo is unknown. Therefore, we tested whether variations in endogenous hemopexin activity are associated with the responsiveness of blood pressure to ang II in healthy volunteers.

METHOD

Healthy men (n = 33, age 26 ± 9) were studied in balance on low sodium (50 mmol Na per 24 h) and high sodium (200 mmol Na per 24 h) diet, respectively. After baseline measurements of blood pressure, ang II was infused at 0.3, 1 and 3 ng/kg per min for 1 h per dose. Hemopexin activity was measured at baseline in EDTA-plasma samples by an amidolytic assay with a chromogenic substrate suitable for hemopexin activity evaluation.

RESULTS

During high sodium the hemopexin activity was lower; 1.6 × 10 (0.6 × 10 - 4.7 × 10) versus 2.8 × 10 (1.1 × 10 - 5.1 × 10) arbitrary units (P < 0.01) and the pressor response to 3 ng ang II/kg per minute larger than during low sodium (17.6 ± 6.5 versus 14.6 ± 6.9 mmHg, P < 0.01). Hemopexin activity negatively correlated with the pressor response to ang II during either type of sodium intake (high sodium: r = 0.42, P < 0.05; low sodium: r = 0.35, P < 0.05).

CONCLUSION

These in-vivo data obtained in healthy individuals support recent in-vitro data showing that active hemopexin downregulates the availability of the AT1-R. Therefore, activated hemopexin might be considered as a factor mediating ang II effects upon blood pressure by modulating AT1-R availability.

High-salt diets during pregnancy affected fetal and offspring renal renin-angiotensin system

Intrauterine environments are related to fetal renal development and postnatal health. Influence of salty diets during pregnancy on renal functions and renin-angiotensin system (RAS) was determined in the ovine fetuses and offspring. Pregnant ewes were fed high-salt diet (HSD) or normal-salt diet (NSD) for 2 months during middle-to-late gestation. Fetal renal functions, plasma hormones, and mRNA and protein expressions of the key elements of renal RAS were measured in the fetuses and offspring. Fetal renal excretion of sodium was increased while urine volume decreased in the HSD group. Fetal blood urea nitrogen was increased, while kidney weight:body weight ratio decreased in the HSD group. The altered ratio was also observed in the offspring aged 15 and 90 days. Maternal and fetal plasma antidiuretic hormone was elevated without changes in plasma renin activity and Ang I levels, while plasma Ang II was decreased. The key elements of local renal RAS, including angiotensinogen, angiotensin converting enzyme (ACE), ACE2, AT1, and AT2 receptor expression in both mRNA and protein, except renin, were altered following maternal high salt intake. The results suggest that high intake of salt during pregnancy affected fetal renal development associated with an altered expression of the renal key elements of RAS, some alterations of fetal origins remained after birth as possible risks in developing renal or cardiovascular diseases.

High-salt diet increases hormonal sensitivity in skin pre-capillary resistance vessels

AIMS

Recent data indicate that the skin of rats on a high-salt diet is able to accumulate Na(+) without commensurate water. This extrarenal mechanism of Na(+) homoeostasis could affect skin vasoregulation. We hypothesized that the major resistance vessel of rat skin, the pre-capillary arterioles, has increased vasoreactivity within the physiological range of circulating ANG II, a hormone relevant to salt-sensitive hypertension.

METHODS AND RESULTS

Skin arterioles from skin and muscle were isolated using the agar-infusion technique. Vessels from rats fed high-salt and low-salt diet had similar lumen diameter and media area/lumen area ratio. Contractile sensitivity to ANG II was increased in skin vessels from high-salt vessels at all doses tested starting at 10(-10) m (P < 0.01). Pre-capillary arterioles from muscle displayed similar contractions to ANG II, independent of the diet. As ANG II and the renin-angiotensin system are strongly involved in salt conservation, we explored whether vasoreactivity for noradrenaline was increased as well, because this is a functionally unrelated hormone. At low doses, contractions were similar, but at 10(-5) and 10(-4) m, noradrenaline produced stronger contractions in skin vessels from high-salt compared with low-salt rats (P < 0.01).

CONCLUSIONS

Our data demonstrate significantly increased hormonal vasoreactivity of skin vessels from rats on a high-salt diet, which could increase peripheral resistance in many situations and contribute to higher pressure in salt-sensitive hypertension. As vessels from adjacent muscle were unaffected, we raise the interesting possibility that increased vasoreactivity in the skin could be linked to osmotically inactive Na(+) accumulation.

Mechanisms of cyclosporine-induced renal cell apoptosis: a systematic review

BACKGROUND/AIMS

Chronic cyclosporine A (CsA) nephrotoxicity (CCN) is an important cause of chronic renal dysfunction with no effective clinical intervention. To further elucidate the mechanisms of renal cell apoptosis in CCN, all relevant in vivo studies on this subject were analyzed.

METHODS

We searched for in vivo studies on the mechanisms of CsA-induced renal cell apoptosis in Medline (1966-July 2010), Embase (1980-July 2010) and ISI (1986-July 2010). The studies were evaluated for their quality according to a set of in vivo standards, data extracted according to PICOS, and then synthesized.

RESULTS

Renal cell apoptosis was an important feature of CCN and an important factor of renal dysfunction. First, CsA could upregulate Fas/Fas ligand, downregulate Bcl-2/Bcl-XL, and increase caspase-1 and caspase-3. Second, it could induce oxidative stress and damage the antioxidant defense system. Third, it could increase endoplasmic reticulum stress protein in a dose- and time-dependent manner. Fourth, CsA could impair the urine concentration and decrease the expression of hypertonicity-induced genes. Fifth, CsA-induced renal cell apoptosis was significantly decreased by blocking the angiotensin II type 1 receptor using losartan.

CONCLUSIONS

The in vivo mechanisms for CCN are more complex than those found in vitro. CsA can induce renal cell apoptosis using five pathways in vivo and activated caspases might be the ultimate intersection of these pathways and the common intracellular pathway mediating apoptosis. These data provide new potential points for intervention and need to be confirmed by further studies.

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.

Real-time visualization of heterotrimeric G protein Gq activation in living cells

Background

Gq is a heterotrimeric G protein that plays an important role in numerous physiological processes. To delineate the molecular mechanisms and kinetics of signalling through this protein, its activation should be measurable in single living cells. Recently, fluorescence resonance energy transfer (FRET) sensors have been developed for this purpose.

Results

In this paper, we describe the development of an improved FRET-based Gq activity sensor that consists of a yellow fluorescent protein (YFP)-tagged Gγ2 subunit and a Gαq subunit with an inserted monomeric Turquoise (mTurquoise), the best cyan fluorescent protein variant currently available. This sensor enabled us to determine, for the first time, the k_on (2/s) of Gq activation. In addition, we found that the guanine nucleotide exchange factor p63RhoGEF has a profound effect on the number of Gq proteins that become active upon stimulation of endogenous histamine H1 receptors. The sensor was also used to measure ligand-independent activation of the histamine H1 receptor (H1R) upon addition of a hypotonic stimulus.

Conclusions

Our observations reveal that the application of a truncated mTurquoise as donor and a YFP-tagged Gγ2 as acceptor in FRET-based Gq activity sensors substantially improves their dynamic range. This optimization enables the real-time single cell quantification of Gq signalling dynamics, the influence of accessory proteins and allows future drug screening applications by virtue of its sensitivity.

Sex differences in acute ANG II-mediated hemodynamic responses in mice

Male sex is associated with higher blood pressure and greater renal injury, perhaps related to greater sensitivity to ANG II. In anesthetized male and female C57BLK/6 mice, we assessed responses of mean arterial pressure (MAP) and renal vascular resistance (RVR; Transonic flow probe) to acute bolus injections of ANG II (0.3-3.0 microg/kg iv) and phenylephrine (PE; 30-300 microg/kg) during low-, normal-, and high-sodium diets. The role of reactive oxygen species was determined by coadministration of tempol. ANG II type 1 and type 2 (AT1 and AT2) receptor and endothelial nitric oxide synthase (NOS3) expression were determined in dissected kidney vessels. While no difference was found on the low-sodium (LS) diet, MAP and RVR responses to ANG II were greater in males during the normal-sodium (NS) and high-sodium (HS) diets (e.g., RVR response at ANG II 3.0 microg/kg during NS: +329 +/- 22 vs. +271 +/- 28 mmHg.ml(-1).min, P = 0.029, effect size = 0.75). Tempol had no effect on the sex-dependent responses on any of the diets. On the LS diet, AT1 and AT2 receptor expression was higher in males. No sex differences were found on the NS diet. On the HS diet, AT1 was higher, and NOS3 expression was lower in males. Acute responses to ANG II are greater in male mice during NS and HS diets, which is, in part, related to differences in AT1, AT2, and NOS3 expression in kidney vessels. Mouse models will be useful to study the role of sex differences in ANG II sensitivity for cardiovascular and renal disease.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Detection of fetal cells in the maternal kidney during gestation in the mouse

It has been reported that fetal cells migrate into maternal blood and organs. Since these fetal chimeric cells could be involved in maternal allogeneic tolerance to the fetus, the fetal chimeric cells might be implicated in maternal-fetal immunology and development of maternal autoimmune diseases. However, the mechanism and role of fetal microchimerism remains unclear. We aimed to describe the mechanism by which fetal cells become associated with maternal organs during pregnancy, using a mouse fetal microchimerism model. Non-obese diabetic/severe combined immunodeficiency (NOD/SCID) female mice, which are useful for tracking the behavior of fetal cells in the maternal body, were mated with transgenic males expressing enhanced green fluorescent protein (GFP), and the presence of GFP-positive cells were examined in peripheral blood and organs of pregnant mothers. By flow cytometry, we showed that 0.95 +/- 0.48% of mononuclear cells detected in the maternal peripheral blood were GFP-positive, and thus of fetal origin, during the first gestational week. This value decreased to 0.10 +/- 0.13% during the third gestational week (p < 0.05). GFP-positive cells were detected in the extraglomerular mesangial region and among the epithelial cells of the proximal renal tubule of the maternal kidney. These GFP-positive cells also expressed angiotensin II receptor subtype 2 (AT2), which is known to participate in regulating organogenesis and vasoreactivity. Fetal cells expressing AT2 may therefore be involved in the regulation of vascular tone in the maternal kidney. These observations suggest that fetal cells could influence maternal renal function through activation of the AT2 signaling.

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

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

Gq-coupled receptors as mechanosensors mediating myogenic vasoconstriction

Despite the central physiological function of the myogenic response, the underlying signalling pathways and the identity of mechanosensors in vascular smooth muscle (VSM) are still elusive. In contrast to present thinking, we show that membrane stretch does not primarily gate mechanosensitive transient receptor potential (TRP) ion channels, but leads to agonist-independent activation of G(q/11)-coupled receptors, which subsequently signal to TRPC channels in a G protein- and phospholipase C-dependent manner. Mechanically activated receptors adopt an active conformation, allowing for productive G protein coupling and recruitment of beta-arrestin. Agonist-independent receptor activation by mechanical stimuli is blocked by specific antagonists and inverse agonists. Increasing the AT(1) angiotensin II receptor density in mechanically unresponsive rat aortic A7r5 cells resulted in mechanosensitivity. Myogenic tone of cerebral and renal arteries is profoundly diminished by the inverse angiotensin II AT(1) receptor agonist losartan independently of angiotensin II (AII) secretion. This inhibitory effect is enhanced in blood vessels of mice deficient in the regulator of G-protein signalling-2. These findings suggest that G(q/11)-coupled receptors function as sensors of membrane stretch in VSM cells.

Obesity augments vasoconstrictor reactivity to angiotensin II in the renal circulation of the Zucker rat

Obesity is an emerging risk factor for renal dysfunction, but the mechanisms are poorly understood. Obese patients show heightened renal vasodilation to blockade of the renin-angiotensin system, suggesting deficits in vascular responses to angiotensin II (ANG II). This study tested the hypothesis that obesity augments renal vasoconstriction to ANG II. Lean (LZR), prediabetic obese (OZR), and nonobese fructose-fed Zucker rats (FF-LZR) were studied to determine the effects of obesity and insulin resistance on reactivity of blood pressure and renal blood flow to vasoconstrictors. OZR showed enlargement of the kidneys, elevated urine output, increased sodium intake, and decreased plasma renin activity (PRA) vs. LZR, and renal vasoconstriction to ANG II was augmented in OZR. Renal reactivity to norepinephrine and mesenteric vascular reactivity to ANG II were similar between LZR and OZR. Insulin-resistant FF-LZR had normal reactivity to ANG II, indicating the insulin resistance was an unlikely explanation for the changes observed in OZR. Four weeks on a low-sodium diet (0.08%) to raise PRA reduced reactivity to ANG II in OZR back to normal levels without effect on LZR. From these data, we conclude that in the prediabetic stages of obesity, a decrease in PRA is observed in Zucker rats that may lead to increased renal vascular reactivity to ANG II. This increased reactivity to ANG II may explain the elevated renal vasodilator effects observed in obese humans and provide insight into early changes in renal function that predispose to nephropathy in later stages of the disease.

Pathophysiological Mechanisms of Salt-Dependent Hypertension

Changes in salt intake are associated in general with corresponding changes in arterial blood pressure. An exaggerated increment in blood pressure driven by a salt load is characteristic of salt-sensitive hypertension, a condition affecting more than two thirds of individuals with essential hypertension who are older than 60 years. In the last decade, significant insight was gained about the role of the kidney in the increment in blood pressure induced by sodium retention. The present review focuses on the pathophysiological characteristics of the blood pressure increase driven by expansion of extracellular fluid and the increment in plasma sodium concentration. In addition, we discuss systemic and renal conditions that result in decreased urinary sodium excretion and were implicated in the development of salt-sensitive hypertension.

Obesity, cardiometabolic syndrome, and chronic kidney disease: the weight of the evidence

The epidemic of obesity experienced in both industrialized and nonindustrialized countries largely accounts for the increase in the prevalence of the cardiometabolic syndrome (CMS). Obesity and the CMS significantly increase the risk for cardiovascular disease (CVD) and chronic kidney disease (CKD). Multiple abnormalities that can lead to kidney injury have been identified in overweight and obese people, including insulin resistance, compensatory hyperinsulinemia, inappropriate activation of the renin-angiotensin-aldosterone system and increased oxidative stress, endoplasmic reticulum stress, coagulability, and impaired fibrinolysis. The combined effects of these conditions induce in the kidneys impaired pressure natriuresis, glomerular hypertension, endothelial dysfunction, and vasoconstriction, as well as matrix proliferation and expansion. Among the consequences are microalbuminuria, now known to be a surrogate of diffuse endothelial dysfunction as well as a predictor of CVD, and CKD. Diet and regular physical activity are the cornerstones of weight management, and they add to currently available pharmacologic agents and bariatric surgery. The understanding of the pathophysiology of obesity/CMS helps to explain the benefits of agents that improve insulin sensitivity, control inflammation, and block the renin-angiotensin-aldosterone system. The increasing prevalence of obesity and CMS contribute to the growing frequency of CKD and demands the development of multifactorial strategies directed at identifying people at risk, as well as preventing excessive weight gain and its deleterious consequences.

Angiotensin II, interstitial inflammation, and the pathogenesis of salt-sensitive hypertension

Transient administration of ANG II causes persistent salt-sensitive hypertension associated with arteriolopathy, interstitial inflammation, and cortical vasoconstriction; blocking the vascular and inflammatory changes with mycophenolate mofetil (MMF) prevents vasoconstriction. While infiltrating leukocytes during the salt-sensitive hypertension phase express ANG II, the functional role of ANG II during this phase is not known. We examined the acute effect of candesartan on renal hemodynamics during the established salt-sensitive hypertensive phase and related these findings to direct measurement of intrarenal ANG II and inflammatory cells in rats previously exposed to ANG II with or without MMF treatment. Sham controls were also examined. The administration of ANG II, followed by exposure to high-salt diet, resulted in hypertension, cortical vasoconstriction, an increase in interstitial inflammatory cells (44.8 +/- 1.3 lymphocytes/mm2, and 30.8 +/- 1.2 macrophages/mm2 ANG II vs. 19.6 +/- 2 lymphocytes/mm2, and 22 +/- 0.7 macrophages/mm2 Sham), and increase in renal ANG II levels (1,358 +/- 74.6 pg/ml ANG II vs. 194 +/- 9.28 pg/ml Sham). Treatment with MMF during the administration of exogenous ANG II resulted in reduction in renal interstitial inflammation (19.7 +/- 0.9 lymphocytes/mm2 and 15.9 +/- 0.8 machophages/mm2), ANG II levels (436.9 +/- 52.29 pg/ml), cortical vasoconstriction, and stable blood pressure levels during the subsequent challenge with a high-salt diet. Acute administration of candesartan similarly reduced renal vasoconstriction and blood pressure. We conclude that the cortical vasoconstriction occurring with salt-sensitive hypertension following exposure to ANG II is mediated by intrarenal ANG II, related, at least in part, to the interstitial inflammation.

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

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

Cardiometabolic syndrome and chronic kidney disease

Chronic kidney disease (CKD) is increasingly recognized as a major risk factor for end-stage renal disease (ESRD), cardiovascular (CV) disease, and CV-related premature death. More than 8 million people in the United States have CKD; therefore, preventive stratiegies should be directed at identifying risk factors for this condition. There is growing evidence implicating the cardiometabolic syndrome, a clustering of CV risk factors that include obesity, insulin resistance, compensatory hyperinsulinemia, dysglycemia, atherogenic dyslipidemia, and hypertension. Factors mediating this relationship include increased glomerular filtration, increased vascular permeability, oxidative and endoplasmic reticulum stress, activation of the renin-angiotensin system, and inappropriate secretion of growth factors. The consequences are microalbuminuria, a marker of inflammation and endothelial dysfunction, renal vascular proliferation, extracellular matrix expansion, and CKD. Prevention of CKD should be directed at controlling all components of the cardiometabolic syndrome, with the ultimate goal of reducing the burden imposed by ESRD.

Salt consumption increases blood pressure and abolishes the light/dark rhythm in angiotensin AT1a receptor deficient mice

Experiments were performed to study the role of angiotensin (Ang) AT1a receptors in dietary sodium-induced changes in blood pressure (BP). We measured light/dark rhythms in BP, heart rate (HR) and drinking behavior in Ang AT1a deficient (AT1a -/-) and wild type (AT1a +/+) mice with arterial telemetric catheters. Mice were given ad libitum access to a high salt diet (8% NaCl, HSD for 8 days) and tap water. The major finding was that the Ang AT1a -/- mice showed enhanced sodium sensitivity. This was seen by a greater percentage increase in BP (+21% vs. +12%) and an earlier onset of BP change (increase on day 5 vs. day 8) in AT1a -/- vs. AT1a +/+. The normal light/dark BP rhythm was abolished in AT1a -/- after 5 days of HSD. HSD produced an increase in water intake (drinking activity and volume consumed) in both groups with no difference in the percentage increase or the light/dark drinking rhythm. HSD produced no changes in plasma osmolality, hematocrit or body weight in either group. Evidence shows that a deficiency of Ang AT1a receptors results in an enhancement in sodium sensitivity along with a disruption of the normal light/dark BP rhythm. The data combined with previous findings suggests that activation of other components of the renin angiotensin system and/or sympathetic pathways may be responsible for the cardiovascular changes in AT1a deficient mice.

Increased renal vascular reactivity to ANG II after unilateral nephrectomy in the rat involves 20-HETE

This study examined the role of intrarenal ANG II in the renal vascular reactivity changes occurring in the remaining kidney undergoing adaptation following contralateral nephrectomy. Renal blood flow responses to intrarenal injections of ANG II (0.25 to 5 ng) were measured in anesthetized euvolemic male Wistar rats 1, 4, 12, and 24 wk after uninephrectomy (UNX) or sham procedure (SHAM). At week 4, renal vasoconstriction induced by 2 ng ANG II was greater in UNX (69 +/- 5%) than in SHAM rats (50 +/- 3%; P < 0.01). This response was inhibited, by 50 and 66%, and by 20 and 25%, in SHAM and UNX rats, after combined injections of ANG II and losartan, or PD-123319 (P < 0.05), respectively. Characteristics of ANG II receptor binding in isolated preglomerular resistance vessels were similar in the two groups. After prostanoid inhibition with indomethacin, renal vasoconstriction was enhanced by 42 +/- 8% (P < 0.05), only in SHAM rats, whereas after 20-HETE inhibition with HET0016, it was reduced by 53 +/- 16% (P < 0.05), only in UNX rats. These differences vanished after concomitant prostanoid and 20-HETE inhibition in the two groups. After UNX, renal cortical protein expression of cytochrome P-450 2c23 isoform (CYP2c23) and cyclooxygenase-1 (COX-1) was unaltered, but it was decreased for CYP4a and increased for COX-2. In conclusion, renal vascular reactivity to ANG II was significantly increased in the postuninephrectomy adapted kidney, independently of protein expression, but presumably involving interactions between 20-HETE and COX in the renal microvasculature and changes in the paracrine activity of ANG II and 20-HETE.

Intratubular Renin-Angiotensin System in Hypertension

It is well recognized that the renin-angiotensin system plays an important role in the regulation of arterial pressure and sodium homeostasis. Recent years, many studies have shown that local tissue angiotensin II levels are differentially regulated and cannot be explained on the basis of circulating concentrations. All of the components needed for angiotensin II generation are present within the various compartments in the kidney including the renal interstitium and the tubular network. The cascade of the renin-angiotensin system demonstrates three major possible sites for the pharmacological interruption of the renin-angiotensin system: the interaction of renin with its substrate, angiotensinogen, the angiotensin converting enzyme, and angiotensin II type 1 receptors. This brief article will focus on the role of the intratubular renin-angiotensin system in the pathophysiology of hypertension and the responses to the renin-angiotensin system blockade by renin inhibitors, angiotensin converting enzyme inhibitors and angiotensin II type 1 receptor blockers.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Posttranscriptional mechanisms contribute to osmotic regulation of ANG type 1 receptors in cultured rat renomedullary interstitial cells

Previously, we showed that ANG II receptors in cultured rat renomedullary interstitial cells (RMICs) are osmotically regulated (19). The current study examined the mechanisms underlying this osmotic regulation in RMICs cultured in isoosmotic (300 mosmol/kgH2O) and hyperosmotic (600 mosmol/kgH2O) conditions. Radioligand competition analysis coupled with RNase protection assays (RPA) and ligand-mediated receptor internalization studies revealed that RMICs primarily express the type 1a angiotensin receptor (AT(1a)R). When cultured under hyperosmotic conditions, the density (B(max)) of AT1R in RMIC membranes decreased by 31% [B(max) (pmol/mg protein): 300 mosmol/kgH2O, 6.44 +/- 0.46 vs. 600 mosmol/kgH2O, 4.42 +/- 0.37, n = 8, P < 0.01], under conditions in which no detectable changes in AT(1a)R mRNA expression or in the kinetics of ligand-mediated AT1R internalization were observed. RNA electromobility shift assays showed that RNA protein complex (RPC) formation between RMIC cytosolic RNA binding proteins and the 5' leader sequence (5'LS) of the AT(1a)R was increased 1.5-fold under hyperosmotic conditions [5'LS RPC (arbitrary units): 300 mosmol/kgH2O, 0.79 +/- 0.08 vs. 600 mosmol/kgH2O, 1.17 +/- 0.07, n = 4, P < 0.01]. These results suggest that the downregulation of AT(1a)R expression in RMICs cultured under hyperosmotic conditions is regulated at the posttranscriptional level by RNA binding proteins that interact within the 5'LS of the AT(1a)R mRNA. The downregulation of AT(1a)R expression under hyperosmotic conditions may be an important mechanism by which the activity of ANG II is regulated in the hyperosmotic renal medulla.

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.

Increased glomerular angiotensin II binding in rats exposed to a maternal low protein diet in utero

In the rat, protein restriction during pregnancy increases offspring blood pressure by 20-30 mmHg. We have shown in an earlier study that this is associated with a reduction in nephron number and increased glomerular sensitivity to angiotensin II (Ang II) in vivo. Hence, we hypothesized that exposure to a maternal low-protein diet increases glomerular Ang II AT1 receptor expression and decreases AT2 receptor expression. To test this hypothesis, pregnant Wistar rats were fed isocalorific diets containing either 18% (control) or 9% (LP) protein from conception until birth. At 4 weeks of age, the kidneys of male offspring were harvested to measure cortical AT1 and AT2 receptor expression, 125I-Ang II glomerular binding, tissue renin activity, tissue Ang II and plasma aldosterone concentrations. AT1 receptor expression was increased (62%) and AT2 expression was decreased (35%) in LP rats. Maximum 125I-Ang II (125I-Ang II) binding (Bmax) was increased in LP rats (control n = 9, 291.6 +/- 27.4 versus LP n = 7, 445.7 +/- 27.4 fmol (mg glomerular protein)(-1), P < 0.01), but affinity (KD) was not statistically different from controls (control 2.87 +/- 0.85 versus LP 0.84 +/- 0.20 pmol 125I-Ang II, P = 0.059). Renal renin activity, tissue Ang II and plasma aldosterone concentrations did not differ between control and LP rats. Increased AT1 receptor expression in LP rat kidneys is consistent with greater haemodynamic sensitivity to Ang II in vivo. This may result in an inappropriate reduction in glomerular filtration rate, salt and water retention, and an increase in blood pressure.

Modulation of body fluids and angiotensin II receptors in a rat model of intra-uterine growth restriction

We previously reported that sodium restriction during pregnancy reduces plasma volume expansion and promotes intra-uterine growth restriction (IUGR) in rats while it activates the renin-angiotensin-aldosterone system (RAAS). In the present study, we proceeded to determine whether expression of the two angiotensin II (ANGII) receptor subtypes (AT(1) and AT(2)) change in relation to maternal water-electrolyte homeostasis and fetal growth. To this end, pregnant (gestation day 15) and non-pregnant Sprague-Dawley rats were randomly assigned to two groups fed either normal, or Na(+)-restricted diets for 7 days. At the end of the treatment period, plasma aldosterone and renin activity as well as plasma and urine electrolytes were measured. Determinations for AT(1) and AT(2) mRNA and protein were made by RNase protection assay and photoaffinity labelling, respectively, using a number of tissues implicated in volume regulation and fetal growth. In non-pregnant rats, Na(+) restriction decreases Na(+) excretion without altering plasma volume, plasma Na(+) concentration or the expression of AT(1) and AT(2) mRNA or protein in the tissues examined. In normally fed pregnant rats when compared to non-pregnant controls, AT(1) mRNA increases in the hypothalamus as well as pituitary and declines in uterine arteries, while AT(1) protein decreases in the kidney and AT(2) mRNA declines in the adrenal cortex. In pregnant rats, Na(+) restriction induces a decrease in plasma Na(+), an increase in plasma urea, as well as a decline in renal urea and creatinine clearance rates. Protein levels for both AT(1) and AT(2) in the pituitary and AT(2) mRNA in the adrenal cortex are lower in the Na(+)-restricted pregnant group when compared to normally fed pregnant animals. Na(+) restriction also induces a decrease in AT(1) protein in the placenta. In conclusion, these results suggest that pregnancy may increase sensitivity to Na(+) depletion by the tissue-specific modulation of ANGII receptors. Finally, these receptors may be implicated in the IUGR response to low Na(+).

AT1 receptors mediate angiotensin II–induced release of nitric oxide in afferent arterioles

BACKGROUND

Recent studies have indicated that angiotensin II (Ang II) possibly activates the nitric oxide (NO) system. We investigated the role of AT receptor subtypes (AT-R) in mediating the Ang II-induced NO release in afferent arterioles (Af) of mice.

METHODS

Isolated Af of mice were perfused, and the isotonic contraction measured. Further, NO release was determined using DAF-FM, a fluorescence indicator for NO. Moreover, we qualitatively assessed the expression of AT-R at the mRNA level using reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Ang II reduced luminal diameters dose dependently (67.3 +/- 6.3% at 10(-6) mol/L). Inhibition of AT2-R with PD123.319 did not change the Ang II contractile response. AT1-R blockade with ZD7155 inhibited contraction. Stimulation of AT2-R during AT1-R inhibition with ZD7155, and preconstriction with norepinephrine (NE) had no influence on the diameter. Drug application via the perfusion pipette changed flow and pressure, and enhanced NO fluorescence by DeltaF = 4.0 +/- 0.4% (N= 14, background). Luminal application of Ang II (10(-7) mol/L) increased the NO fluorescence by DeltaF = 9.9 +/- 1.2% (N= 8). AT1-R blockade blunted the increase to background levels (DeltaF to 4.0 +/- 0.3%, N= 6, P < 0.05), but AT2-R blockade did not (8.1 +/- 0.9%, N= 9). L-NAME nearly abolished the Ang II effect on the NO fluorescence (DeltaF = 1.6 +/- 0.5% (N= 8). NE did not increase NO release beyond the background levels. RT-PCR showed expression of both AT1-R and AT2-R.

CONCLUSION

The results indicate an Ang II-induced NO release in Af of mice, which is mediated by AT1-R. Thus, Ang II balances its own constrictor action in Af. This control mechanism is very important in view of high renin and angiotensin II concentration in the juxtaglomerular apparatus.

Effect of Sodium on Vasoconstriction and Angiotensin II Type 1 Receptor mRNA Expression in Cold‐induced Hypertensive Rats

Angiotensin II and sodium play an important pathogenetic role in several models of hypertension. Now, we investigated the effects of sodium on vasoconstriction and angiotensin II type 1 (AT1) and type 2 (AT2) receptor mRNA expression in aortic vessels from cold-induced hypertensive rats. Wistar rats on low sodium and high sodium diet were exposed to cold-stress for 8 weeks. The effects of angiotensin II infusion on mean arterial blood pressure were investigated in these rats. In addition, angiotensin II induced contraction was measured using aortic rings. Expression of AT1 receptor mRNA and AT2 receptor mRNA was assessed in aortic vessels by reverse transcription polymerase chain reaction. After infusion of angiotensin II mean arterial blood pressure in cold-induced hypertensive rats on high sodium diet was significantly higher compared to cold-induced hypertensive rats on low sodium diet (p < 0.05). Angiotensin II-induced contraction of aortic rings was significantly higher in cold-induced hypertensive rats on high sodium diet compared to cold-induced hypertensive rats on low sodium diet (2.39 +/- 0.03 g vs. 2.21 +/- 0.04 g, n = 12, p < 0.01). Angiotensin AT1 receptor mRNA was significantly higher in cold-induced hypertensive rats on high sodium diet compared to cold-induced hypertensive rats on low sodium diet (p < 0.05). It is concluded that in this nongenetic, nonsurgical animal model of cold-induced hypertension increased vasoconstriction and increased AT1 receptor mRNA expression in aortic vessels are dependent on sodium intake.

Angiotensin II type 2 receptor inhibits prorenin processing in juxtaglomerular cells

Long-term treatment with an angiotensin II type 1 receptor blocker (ARB) has been shown to decrease the plasma renin activity (PRA) of hypertensive patients, whereas PRA remains elevated during angiotensin-converting enzyme inhibitor (ACEI) treatment. In the present study, we used rat juxtaglomerular (JG) cells to elucidate the mechanism(s) involved in the differential regulation of PRA between ARB and ACEI treatment. Addition of 100 nmol/l angiotensinogen (Aogen) to JG cells (n=6 primary cultures) significantly increased the medium angiotensin (Ang) II levels from 14 +/- 2 to 440 +/- 9 pg/ml and suppressed the renin secretion rate (RSR) from 39.6 +/- 5.4% to 6.3 +/- 1.8% without affecting active renin content (ARC) or total renin content (TRC). In the Aogen-treated cells, the ACEI, delapril hydrochloride (CV3317, 10 micromol/l), significantly decreased the medium Ang II levels to 58 +/- 14 pg/ml and increased RSR to 39.8 +/- 4.1% without affecting ARC or TRC. The ARB, an active metabolite of candesartan cilexetil (CV11974, 10 micromol/l), however, significantly increased the medium Ang II levels and RSR to 486 +/- 15 pg/ml and 40.9 +/- 9.8%, respectively, and decreased ARC from 63.2 +/- 6.8 to 21.6 +/- 3.6 ng of Ang l x h(-1) x million cells(-1) without affecting TRC. The decreases in ARC of the Aogen+CV11974-treated cells (n=6 primary cultures) were inhibited by an Ang II type 2 receptor blocker, PD123319 (10 micromol/l). JG cells (n=6 primary cultures) were also treated with an Ang II type 2 receptor agonist, CGP42212A (0.1 micromol/l). CGP42212A significantly increased RSR from 38.2 +/- 1.6% to 49.7 +/- 4.7% and decreased ARC from 60.8 +/- 3.0 to 25.3 +/- 2.8 ng of Ang l x h(-1) million cells(-1) without affecting TRC. Addition of CV11974 did not alter the RSR, ARC, or TRC of the CGP42212A-treated cells; however, PD123319 abolished the effects of CGP42212A. These results indicate that, distinct from ACEIs, ARBs inhibit prorenin processing of JG cells through Ang II type 2 receptors. Long-term treatment with an ARB may decrease PRA in part by diminishing the storage of active renin in JG cells.

A gammaGT-AT1A receptor transgene protects renal cortical structure in AT1 receptor-deficient mice

To understand the physiological role of angiotensin type 1 (AT(1)) receptors in the proximal tubule of the kidney, we generated a transgenic mouse line in which the major murine AT(1) receptor isoform, AT(1A), was expressed under the control of the P1 portion of the gamma-glutamyl transpeptidase (gammaGT) promoter. In transgenic mice, this promoter has been shown to confer cell-specific expression in epithelial cells of the renal proximal tubule. To avoid random integration of multiple copies of the transgene, we used gene targeting to produce mice with a single-copy transgene insertion at the hypoxanthine phosphoribosyl transferase (Hprt) locus on the X chromosome. The physiological effects of the gammaGT-AT(1A) transgene were examined on a wild-type background and in mice with targeted disruption of one or both of the murine AT(1) receptor genes (Agtr1a and Agtr1b). On all three backgrounds, gammaGT-AT(1A) transgenic mice were healthy and viable. On the wild-type background, the presence of the transgene did not affect development, blood pressure, or kidney structure. Despite relatively low levels of expression in the proximal tubule, the transgene blunted the increase in renin expression typically seen in AT(1)-deficient mice and partially rescued the kidney phenotype associated with Agtr1a(-/-)Agtr1b(-/-) mice, significantly reducing cortical cyst formation by more than threefold. However, these low levels of cell-specific expression of AT(1) receptors in the renal proximal tubule did not increase the low blood pressures or abolish sodium sensitivity, which are characteristic of AT(1) receptor-deficient mice. Although our studies do not clearly identify a role for AT(1) receptors in the proximal tubules of the kidney in blood pressure homeostasis, they support a major role for these receptors in modulating renin expression and in maintaining structural integrity of the renal cortex.

Dietary sodium regulates angiotensin AT1a and AT1b mRNA expression in mouse brain

Previous results showed that angiotensin (Ang) AT1a and AT1b receptor mRNA are expressed in mouse hypothalamus (HYP), brainstem (BS) and anterior pituitary (PIT). To extend these findings, we developed a real-time polymerase chain reaction (PCR) method to differentiate and quantify Ang AT1a and AT1b mRNA in mouse brain. An experiment was conducted in male C57Bl/6J mice to determine the effects of low and high dietary salt (0.04 or 8% NaCl for 2 weeks) on mRNA expression. Physiological measurements showed that high salt increased water intake (15.1 +/- 0.6 ml/day), whereas low salt decreased water intake (3.2 +/- 0.1 ml/day). There were no significant changes in body weight, hematocrit or plasma osmolality. Real-time PCR was effective in distinguishing AT1a and AT1b receptor mRNA. The PCR efficiencies for AT1a, AT1b and 18S ribosome were tested to be identical, making it possible to quantify mRNA levels. There were differences in angiotensin receptor expression, related to diet and brain region. In hypothalamus, both the high salt and low salt diet decreased AT1a expression (to 63 +/- 4% and 62 +/- 1%), although there were no changes in AT1b. In brainstem, there was a marked increase in AT1a (to 365 +/- 60%) and AT1b (to 372 +/- 23%) after high salt, although there was only a marked decrease for AT1b (to 23 +/- 5%) after low salt. In anterior pituitary, both high salt and low salt diet increased AT1a expression (to 152 +/- 8% and 123 +/- 9%), although there were no changes in AT1b. Results document that both AT1 receptor subtypes are present in mouse hypothalamus, brainstem and anterior pituitary, and that there is differential regulation of expression in response to changes in dietary salt.

Dietary salt intake modulates progression of antithymocyte serum nephritis through alteration of glomerular angiotensin II receptor expression

Dietary salt intake modulates the renin-angiotensin system (RAS); however, little is known about the effect of salt intake on the progression of glomerulonephritis. We investigated the glomerular expression of TGF-β1type I (TβRI) and II (TβRII) TGF-β receptors and RAS components in rats with antithymocyte serum (ATS) nephritis on normal (NSI)-, low (LSI)-, and high-salt intake (HSI) and on HSI rats receiving candesartan cilexetil (CC) and LSI rats receiving PD-123319. Glomerular lesions were less severe in rats on LSI and aggravated in those on HSI compared with those on NSI. Intrarenal renin and glomerular ANG II levels were significantly higher in LSI and lower in HSI rats. In ATS nephritis, HSI increased glomerular TβRI, TβRII, and ANG II type 1 receptor (AT1R), and decreased glomerular ANG II type 2 receptor (AT2R), whereas LSI decreased glomerular TGF-β1and TβRI and increased glomerular AT2R. CC ameliorated glomerular lesions, reduced glomerular TGF-β1and TβRII, and increased glomerular AT2R. PD-123319 aggravated glomerular lesions and increased glomerular TGF-β1and TβRII. Our results suggest that dietary salt intake influences progression of ATS nephritis by modulating glomerular TGF-β1and TβR expression resulting, at least in part, from altered glomerular AT1R and AT2R expression.

Angiotensin receptor subtypes in thin and muscular juxtamedullary efferent arterioles of rat kidney

ANG II controls the vascular tone of pre- and postglomerular arterioles, and thereby glomerular filtration, through binding to either AT1A, AT1B, or AT2 receptors. AT1 receptors, which are coupled to intracellular Ca2+ signaling, have vasoconstricting effects, whereas AT2 receptors, whose signaling mechanism is unknown, induce vasodilatation. The angiotensin receptors have been characterized in afferent arterioles, which express the three types of receptors, but not in efferent arterioles. Two subpopulations of juxtamedullary efferent arterioles, muscular ones which terminate as vasa rectae and thin ones which terminate as peritubular capillaries, have been described. They display functional heterogeneity with regard to the ANG II response. To evaluate whether these differences are associated with differential expression of ANG II receptors, we examined the expression pattern of AT1A, AT1B, and AT2 receptor mRNAs by RT-PCR in these arterioles and studied the effect of valsartan, a specific AT1-receptor antagonist. Results indicate that muscular arterioles express AT1A, AT1B, and AT2 receptors, whereas thin arterioles only express the AT1A and AT2 types, and at a much lower level. Valsartan fully inhibited ANG II-induced increases in intracellular Ca2+ in both arteriolar types, but with different kinetics. In muscular arterioles, inhibition was monoexponential, whereas it displayed a marked positive cooperativity in thin arterioles. Finally, the apparent affinity for valsartan was higher in muscular than in thin arterioles. In conclusion, this study further documents the differences between muscular and thin efferent arterioles with regard to ANG II signalization in the rat kidney.

Sodium load increases renal angiotensin type 1 receptors and decreases bradykinin type 2 receptors

The regulation of both angiotensin receptors and bradykinin receptors during sodium intake is poorly understood. We hypothesized that an altered balance between renal angiotensin type 1 (AT1) receptors and bradykinin type 2 (B2) receptors might contribute to an increase in blood pressure during periods of high-sodium intake. We studied the effects of high-sodium intake on renal AT1 receptors and B2 receptors in 5-6-week-old spontaneously hypertensive rats (SHR) receiving high-sodium chloride (6% NaCl) or mineral salts (10.5%, composition: 57% NaCl, 28% KCl, 12% MgSO4) compared to those receiving a low-sodium (NaCl 0.125%) diet for 10 weeks. Mineral salt intake was included due to its beneficial effects on blood pressure and cardiac hypertrophy. Receptor densities were measured by quantitative autoradiography. AT1 receptors were quantified using incubation with 125I-Sar1-Ile8-angiotensin II and displacement was measured with PD123319 (10 micromol/l), whereas B2 receptors were quantified using 125I-HPP-icatibant and displacement was measured with icatibant (3 micromol/l). Compared to the SHR controls, a further increase in blood pressure occurred after 2 weeks in the 6% NaCl group and after 6 weeks in the mineral salt group. AT1 receptor density increased in the renal cortex by 41% (p<0.01) in the 6% NaCl group and by 26% (p<0.05) in the mineral salt group. B2 receptor density decreased in the renal medulla by 26% (p<0.01) in the 6% NaCl group, and decreased even more i.e., by 45% (p<0.001), in the mineral salt group. It was shown that a 6% NaCl or a 10.5% mineral salt loading was capable of increasing renal AT1 receptor density and decreasing renal B2 receptor density. An altered balance between these receptors might be associated with hypertension under conditions of sodium loading.

Type 1A angiotensin II receptor is regulated differently in proximal and distal nephron segments

Angiotensin II plays important roles in renal vasoconstriction, sodium reabsorption in proximal tubules, and cell proliferation. Angiotensin II receptors are present not only in proximal but also in distal tubules. We investigated the effects of dehydration on the mRNA expression of type 1A angiotensin II receptor (AT1A) in proximal and distal nephron segments and on the expression of type 1 angiotensin II receptor (AT1) protein. Competitive polymerase chain reaction was employed to quantitatively examine mRNA expression, and AT1-specific polyclonal antibody was used for Western blot analysis. AT1A mRNA expression was most abundant in glomeruli. Collecting ducts showed higher expressions than did proximal tubules or thick ascending limbs. Dehydration caused an increase of AT1A mRNA expression in glomeruli, proximal straight tubules (PST), and medullary and cortical thick ascending limbs (MAL and CAL, respectively). In contrast, dehydration decreased AT1A mRNA expression in cortical, outer medullary, and inner medullary collecting ducts (CCD, OMCD, and IMCD, respectively). Incubation of isolated glomeruli, PST, and IMCD in hypertonic solution made by NaCl and mannitol in vitro increased AT1A mRNA expression. Incubation of IMCD with AVP (10(-7) mol/l) also increased AT1A mRNA expression. AT1 was detected at 45 kDa by Western blotting. Dehydration caused a decrease and increase of AT1 expression in the cortex and the medulla, respectively. In summary, these data showed that the mechanisms of the regulation of AT1A differ between proximal and distal tubules. The finding that AT1 was up-regulated in the medulla during dehydration may suggest that this receptor plays an important role in dehydration in the distal tubules.