Enhanced angiotensin II type 2 receptor mechanisms mediate decreases in arterial pressure attributable to chronic low-dose angiotensin II in female rats

The renin-angiotensin system is a far more complex enzymatic cascade than realized previously. Mounting evidence suggests sex-specific differences in the regulation of the renin-angiotensin system and arterial pressure. We examined the hemodynamic responses, angiotensin II receptor subtypes, and angiotensin-converting enzyme 2 gene expression levels after graded doses of angiotensin II in males and females. Mean arterial pressure was measured via telemetry in male and female rats in response to a 2-week infusion of vehicle, low-dose (50 ng/kg per minute SC) or high-dose (400 ng/kg per minute SC) angiotensin II. The effect of concurrent infusion of the angiotensin II type 2 receptor (AT(2)R) blocker (PD123319) was also examined. The arterial pressure response to high-dose angiotensin II was attenuated in females compared with males (24+/-8 mm Hg versus 42+/-5 mm Hg; P for the interaction between sex and treatment <0.002). Remarkably, low-dose angiotensin II decreased arterial pressure (11+/-4 mm Hg; P for the interaction between sex and treatment <0.02) at a dose that did not have an effect in males. This decrease in arterial pressure in females was abolished by AT(2)R blockade. Renal AT(2)R, angiotensin-converting enzyme 2, and left ventricular AT(2)R mRNA gene expressions were markedly greater in females than in males with a renal angiotensin II type 1a receptor:AT(2)R ratio of approximately 1 in females. Angiotensin II infusion did not affect renal AT(2)R mRNA expression but resulted in significantly less left ventricular mRNA expression. Renal angiotensin-converting enzyme 2 mRNA expression levels were greater in females than in males treated with high-dose angiotensin II (approximately 2.5 fold; P for the interaction between sex and treatment <0.05). In females, enhancement of the vasodilatory arm of the renin-angiotensin system, in particular, AT(2)R and angiotensin-converting enzyme 2 mRNA expression, may contribute to the sex-specific differences in response to renin-angiotensin system activation.     

Gender differences in pressure-natriuresis and renal autoregulation: role of the Angiotensin type 2 receptor

Sexual dimorphism in arterial pressure regulation has been observed in humans and animal models. The mechanisms underlying this gender difference are not fully known. Previous studies in rats have shown that females excrete more salt than males at a similar arterial pressure. The renin-angiotensin system is a powerful regulator of arterial pressure and body fluid volume. This study examined the role of the angiotensin type 2 receptor (AT?R) in pressure-natriuresis in male and female rats because AT?R expression has been reported to be enhanced in females. Renal function was examined at renal perfusion pressures of 120, 100, and 80 mm Hg in vehicle-treated and AT?R antagonist-treated (PD123319; 1 mg/kg/h) groups. The pressure-natriuresis relationship was gender-dependent such that it was shifted upward in female vs male rats (P < 0.001). AT?R blockade modulated the pressure-natriuresis relationship, shifting the curve downward in male (P < 0.01) and female (P < 0.01) rats to a similar extent. In females, AT?R blockade also reduced the lower end of the autoregulatory range of renal blood flow (P < 0.05) and glomerular filtration rate (P < 0.01). Subsequently, the renal blood flow response to graded angiotensin II infusion was also measured with and without AT?R blockade. We found that AT?R blockade enhanced the renal vasoconstrictor response to angiotensin II in females but not in males (P < 0.05). In conclusion, the AT?R modulates pressure-natriuresis, allowing the same level of sodium to be excreted at a lower pressure in both genders. However, a gender-specific role for the AT?R in renal autoregulation was evident in females, which may be a direct vascular AT?R effect.     

Sex differences in the pressor and tubuloglomerular feedback response to angiotensin II

Awareness of sex differences in the pathology of cardiovascular disease is increasing. Previously, we have shown a role for the angiotensin type 2 receptor (AT(2)R) in the sex differences in the arterial pressure response to Ang II. Tubuloglomerular feedback (TGF) contributes in setting pressure-natriuresis properties, and its responsiveness is closely coupled to renal Ang II levels. We hypothesize that, in females, the attenuated pressor response to Ang II is mediated via an enhanced AT(2)R mechanism that, in part, offsets Ang II-induced sensitization of the TGF mechanism. Mean arterial pressure was measured via telemetry in male and female wild-type (WT) and AT(2)R knockout (AT(2)R-KO) mice receiving Ang II (600 ng/kg per minute SC). Basal 24-hour mean arterial pressure did not differ among the 4 groups. After 10 days of Ang II infusion, mean arterial pressure increased in the male WT (28±6 mm Hg), male AT(2)R-KO (26±2 mm Hg), and female AT(2)R-KO (26±4 mm Hg) mice, however, the response was attenuated in female WT mice (12±4 mm Hg; P between sex and genotype=0.016). TGF characteristics were determined before and during acute subpressor Ang II infusion (100 ng/kg per minute IV). Basal TGF responses did not differ between groups. The expected increase in maximal change in stop-flow pressure and enhancement of TGF sensitivity in response to Ang II was observed in the male WT, male AT(2)R-KO, and female AT(2)R-KO but not in the female WT mice (P between sex and genotype <0.05; both). In conclusion, these data indicate that an enhanced AT(2)R-mediated pathway counterbalances the hypertensive effects of Ang II and attenuates the Ang II-dependent resetting of TGF activity in females. Thus, the enhancement of the AT(2)R may, in part, underlie the protection that premenopausal women demonstrate against cardiovascular disease.     

Intrarenal aminopeptidase N inhibition augments natriuretic responses to angiotensin III in angiotensin type 1 receptor-blocked rats

The renal angiotensin angiotensin type 2 receptor has been shown to mediate natriuresis, and angiotensin III, not angiotensin II, may be the preferential angiotensin type 2 receptor activator of this response. Angiotensin III is metabolized to angiotensin IV by aminopeptidase N. The present study hypothesizes that inhibition of aminopeptidase N will augment natriuretic responses to intrarenal angiotensin III in angiotension type 1 receptor-blocked rats. Rats received systemic candesartan for 24 hours before the experiment. After a 1-hour control, cumulative renal interstitial infusion of angiotensin III at 3.5, 7, 14, and 28 nmol/kg per minute (each dose for 30 minutes) or angiotensin III combined with aminopeptidase N inhibitor PC-18 was administered into 1 kidney. The contralateral control kidney received renal interstitial infusion of vehicle. In kidneys infused with angiotensin III alone, renal sodium excretion rate increased from 0.05+/-0.01 micromol/min in stepwise fashion to 0.11+/-0.01 micromol/min at 28 nmol/kg per minute of angiotensin III (overall ANOVA F=3.68; P<0.01). In angiotensin III combined with PC-18, the renal sodium excretion rate increased from 0.05+/-0.01 to 0.32+/-0.08 mumol/min at 28 nmol/kg per minute of angiotensin III (overall ANOVA F=6.2; P<0.001). The addition of intrarenal PD-123319, an angiotensin type 2 receptor antagonist, to renal interstitial angiotensin III plus PC-18 inhibited the natriuretic response. Mean arterial blood pressure and renal sodium excretion rate from control kidneys were unchanged by angiotensin III +/- PC-18 + PD-123319. Angiotensin III plus PC-18 induced a greater natriuretic response than Ang III alone (overall ANOVA F=16.9; P=0.0001). Aminopeptidase N inhibition augmented the natriuretic response to angiotensin III, suggesting that angiotensin III is a major agonist of angiotensin type 2 receptor-induced natriuresis.     

Renal and systemic hemodynamic effects of synthetic atrial natriuretic peptide in the anesthetized rat

To characterize the hemodynamic events responsible for alterations in renal function during administration of atrial natriuretic peptide, we studied the systemic, renal, and glomerular circulatory effects of intravenous rANP[126-149], administered as a 4 micrograms/kg prime and 0.5 microgram/kg per minute continuous infusion in anesthetized, euvolemic rats. With this protocol, a small decline in mean systemic arterial blood pressure occurred in the context of markedly enhanced urinary sodium excretion, hemoconcentration, and reduced left ventricular end-diastolic pressure and +dP/dt. However, despite a significant decrement in renal vascular resistance, total peripheral resistance remained constant, thereby denoting a preferential renal vasodilatory effect of this peptide in vivo. Whole kidney and single nephron GFR increased by approximately 20%, while effective renal and glomerular plasma flow rates remained stable, resulting in a substantial rise in filtration fraction. Of all the parameters potentially capable of augmenting single nephron GFR, only glomerular capillary hydraulic pressure increased significantly and therefore accounted entirely for the hyperfiltration observed during ANP infusion. This rise in glomerular capillary pressure, in turn, resulted from afferent arteriolar vasodilatation and concurrent efferent arteriolar vasoconstriction, findings that proved independent of both endogenous angiotensin II activity and ANP-induced reductions in renal perfusion pressure. These renal hemodynamic effects are unique when compared with actions of previously studied renal vasodilatory agents.     

Sex differences in renal medullary endothelin receptor function in angiotensin II hypertensive rats

We hypothesized that angiotensin (Ang) II hypertensive rats have impaired natriuresis after renal medullary endothelin (ET) B receptor stimulation that would be more evident in male versus female rats. Acute intramedullary infusion of the ET(B) agonist sarafotoxin 6c in normotensive male rats increased sodium excretion from 0.51±0.11 μmol/min during baseline to 1.64±0.19 μmol/min (P<0.05) after S6c. After 2 weeks of Ang II infusion (260 ng/kg per minute SC), male rats had an attenuated natriuretic response to S6c of 0.62±0.16 μmol/min during baseline versus 0.95±0.07 μmol/min after S6c. In contrast, ET(B)-dependent natriuresis was similar in female hypertensive rats (0.48±0.07 versus 1.5±0.18 μmol/min; P<0.05) compared with normotensive controls (1.05±0.07 versus 2.14±0.24 μmol/min; P<0.05). Because ET(A) receptors also mediate natriuresis in normotensive female rats, we examined ET(A) receptor function in female Ang II hypertensive rats. Intramedullary infusion of ET-1 increased sodium excretion in both hypertensive and normotensive female rats, which was partially blocked by the ET(A) antagonist BQ-123. Maximum ET(B) receptor binding in inner medullary membrane preparations was comparable between vehicle and Ang II hypertensive females; however, maximum ET(B) binding was significantly lower in male hypertensive rats (1952±251 versus 985±176 fmol/mg; P<0.05). These results indicate that renal ET(B) function is impaired in male Ang II hypertension attributed, at least in part, to a reduced number of ET(B) binding sites. Furthermore, renal ET receptor function is preserved in female rats during chronic Ang II infusion, suggesting that renal ET receptor function could serve to limit hypertension in females compared with males.     

Renal angiotensin II type-2 receptors are upregulated and mediate the candesartan-induced natriuresis/diuresis in obese Zucker rats

Recently, there has been a growing interest in studying the role of angiotensin II type-2 (AT(2)) receptor in renal/cardiovascular function in pathological conditions. The present study was designed to determine the functional role of the AT(2) receptors on natriuresis/diuresis and compare the level of the tubular AT(2) receptor expression in obese and lean Zucker rats (12 weeks old). Under anesthesia, candesartan (angiotensin II type 1 [AT(1)]-specific antagonist; 100 microg/kg bolus) produced natriuresis/diuresis to a greater degree in obese than in lean rats. The specific AT(2) antagonist PD123319 (50 microg/kg per minute) after candesartan administration abolished the natriuretic/diuretic effects of candesartan in obese rats but not in lean rats. Infusion of AT(2) receptor agonist, CGP-42112A (1 microg/kg per minute), produced greater increase in sodium and urine excretion over basal in obese than in lean rats. The presence of the AT(2) receptor expression in the brush-border and basolateral membranes was confirmed by Western blotting using specific antibody and antigen-blocking peptide. Densitometric analysis of the bands revealed approximately 1.5- to 2.0-fold increase in the AT(2) receptor proteins in both membranes of obese compared with lean rats. Our results suggest upregulation of the AT(2) receptors, which play a role in mediating the natriuretic/diuretic effects of AT(1) receptor blockers in obese Zucker rats. We speculate that AT(2) receptors, by promoting sodium excretion, may protect obese Zucker rats against blood pressure increase associated with sodium and water retention.     

Brain and peripheral angiotensin II type 1 receptors mediate renal vasoconstrictor and blood pressure responses to angiotensin IV in the rat

OBJECTIVES: Angiotensin (Ang) IV was reported to increase renal cortical blood flow (CBF) via putative angiotensin IV receptor (AT4) stimulation but reduce total renal blood flow (RBF) via angiotensin II type 1 (AT1) receptors. We investigated the effect of Ang IV on simultaneously measured mean arterial pressure (MAP), RBF, and CBF. The possible involvement of AT1 or AT4 receptors, the possible natriuretic effect, and responses to central administration were also explored. METHODS AND RESULTS: Intravenous injections of Ang IV dose dependently increased MAP and decreased CBF and RBF; these effects were abolished by AT1 receptor blockade. These reductions in CBF and RBF highly correlated as did renal vascular responses to Ang II and fenoldopam. Ang IV did not induce renal vasodilation even following AT1 receptor blockade. Intrarenal Ang IV infusion reduced CBF and RBF but had no natriuretic effect. Central Ang IV administration induced an AT1-mediated immediate increase in MAP and renal vascular resistance and a secondary increase in RBF. AT4 selective ligands, LVV-hemorphin-7 and AT4-16 (intravenous, intrarenal or intracerebroventricular), had no effects on MAP, RBF or urinary sodium excretion. Additional in-vitro experiments indicated that the majority of the Ang IV-sensitive aminopeptidase activity in kidney membranes is attributed to aminopeptidase-N. CONCLUSION: Insulin-regulated aminopeptidase (IRAP)/AT4 receptors are involved in neither the regulation of RBF or CBF nor in the handling of renal sodium. Ang IV increases MAP and induces renal vasoconstriction via stimulation of brain and peripheral AT1 receptors and may be involved in the regulation of renal blood flow and blood pressure.     

Exogenous angiotensin II does not facilitate norepinephrine release in the heart

Studies on the effect of angiotensin II on norepinephrine release from sympathetic nerve terminals through stimulation of presynaptic angiotensin II type 1 receptors are equivocal. Furthermore, evidence that angiotensin II activates the cardiac sympathetic nervous system in vivo is scarce or indirect. In the intact porcine heart, we investigated whether angiotensin II increases norepinephrine concentrations in the myocardial interstitial fluid (NE(MIF)) under basal conditions and during sympathetic activation and whether it enhances exocytotic and nonexocytotic ischemia-induced norepinephrine release. In 27 anesthetized pigs, NE(MIF) was measured in the left ventricular myocardium using the microdialysis technique. Local infusion of angiotensin II into the left anterior descending coronary artery (LAD) at consecutive rates of 0.05, 0.5, and 5 ng/kg per minute did not affect NE(MIF), LAD flow, left ventricular dP/dt(max), and arterial pressure despite large increments in coronary arterial and venous angiotensin II concentrations. In the presence of neuronal reuptake inhibition and alpha-adrenergic receptor blockade, left stellate ganglion stimulation increased NE(MIF) from 2.7+/-0.3 to 7.3+/-1.2 before, and from 2.3+/-0.4 to 6.9+/-1.3 nmol/L during, infusion of 0.5 ng/kg per minute angiotensin II. Sixty minutes of 70% LAD flow reduction caused a progressive increase in NE(MIF) from 0.9+/-0.1 to 16+/-6 nmol/L, which was not enhanced by concomitant infusion of 0.5 ng/kg per minute angiotensin II. In conclusion, we did not observe any facilitation of cardiac norepinephrine release by angiotensin II under basal conditions and during either physiological (ganglion stimulation) or pathophysiological (acute ischemia) sympathetic activation. Hence, angiotensin II is not a local mediator of cardiac sympathetic activity in the in vivo porcine heart.     

Lack of association between polymorphisms of angiotensin II receptor genes and response to short-term angiotensin II infusion

OBJECTIVES: The physiological effects of polymorphisms of the renin-angiotensin-aldosterone system (RAAS) are poorly understood. Long-term effects of genetic variants can be studied in cross-sectional linkage studies. In this study, we examined the short-term effects of genetic polymorphisms of the angiotensin II AT1 - and AT2-receptor subtypes in humans by means of angiotensin II infusion. METHODS: In 120 male, white, young (26 +/- 3 years) subjects with normal or mildly elevated blood pressure, changes in mean arterial blood pressure, aldosterone levels, glomerular filtration rate (GFR), and renal plasma flow (RPF) were measured in response to angiotensin II infusion (0.5 ng/kg per min and 3.0 ng/kg per min, each over 30 min). The -2228 G/A polymorphism of the AT1-receptor gene, and the +1675 G/A polymorphism of the AT2-receptor gene were determined by restriction digestion and single strand conformation polymorphism analysis, respectively. RESULTS: Infusion of angiotensin II resulted in an increase in mean arterial pressure, serum aldosterone levels and GFR, and in a decrease in RPF (all P< 0.001). However, at similar baseline mean arterial pressure, aldosterone levels, and renal haemodynamics, the response to angiotensin II did not significantly differ across the AT1 - and AT2-receptor genotypes with the sample size of our study being adequate to detect relevant differences across the genotypes with a power of > 90% for all parameters. CONCLUSIONS: The response to angiotensin II infusion does not differ across the the AT1- and AT2-receptor genotypes examined in our study. However, long-term effects of variants of angiotensin II receptor genes cannot be ruled out with this approach.     

Disparate roles of AT2 receptors in the renal cortical and medullary circulations of anesthetized rabbits

The contributions of angiotensin II type 1 (AT1) and type 2 (AT2) receptors to the control of regional kidney blood flow were determined in pentobarbital-anesthetized rabbits. Intravenous candesartan (AT1 antagonist; 10 microg/kg plus 10 microg x kg(-1) x h(-1)) reduced mean arterial pressure (12%) and increased total renal blood flow (29%) and cortical laser-Doppler flux (18%) but not medullary laser-Doppler flux. Neither intravenous PD123319 (AT2 antagonist; 1 mg/kg plus 1 mg x kg(-1) x h(-1)) nor saline vehicle significantly affected these variables, and the responses to candesartan plus PD123319 were indistinguishable from those of candesartan alone. In vehicle-treated rabbits, renal-arterial infusions of angiotensin II (1 to 25 ng x kg(-1) x min(-1)) and angiotensin III (5 to 125 ng x kg(-1) x min(-1)) dose-dependently reduced renal blood flow (up to 51%) and cortical laser-Doppler flux (up to 50%) but did not significantly affect medullary laser-Doppler flux or arterial pressure. Angiotensin(1-7) (20 to 500 ng x kg(-1) x min(-1)) had similar effects but of lesser magnitude. CGP42112A (20 to 500 ng x kg(-1) x min(-1)) did not significantly affect these variables. After PD123319 administration, angiotensin II and angiotensin III dose-dependently increased medullary laser-Doppler flux (up to 84%), and reductions in renal blood flow in response to angiotensin II were enhanced. Candesartan abolished renal hemodynamic responses to the angiotensin peptides, even when given in combination with PD123319. We conclude that AT2 receptor activation counteracts AT1-mediated vasoconstriction in the renal cortex but also counteracts AT1-mediated vasodilatation in vascular elements controlling medullary perfusion. These mechanisms might have an important effect on the control of medullary perfusion under conditions of activation of the renin-angiotensin system.     

Effects of exogenous heme on renal function: role of heme oxygenase and cyclooxygenase

We examined the effects of heme administration (15 mg/kg IV) on indexes of renal carbon monoxide production and contrasted the renal functional response to heme in anesthetized rats pretreated and not pretreated with stannous mesoporphyrin (40 micromol/kg IV) to inhibit heme oxygenase or sodium meclofenamate (5 mg/kg IV plus infusion at 10 microg/kg per minute) to inhibit cyclooxygenase. In rats without drug pretreatment, heme administration decreased renal vascular resistance and increased renal blood flow, urine volume, and sodium excretion associated with augmented urinary excretion of 6-keto-PGF1alpha and enhanced concentration of carbon monoxide in the renal cortical microdialysate. Pretreatment with stannous mesoporphyrin did not prevent heme from producing renal vasodilation and increasing renal blood flow but abolished the diuretic and natriuretic responses. Conversely, pretreatment with sodium meclofenamate blunted the renal vasodilatory effect of heme but affected neither the diuretic nor the natriuretic effect. We conclude that heme-induced renal vasodilation is a cyclooxygenase-dependent response involving increased synthesis of PGI2, whereas heme-induced diuresis and natriuresis are heme oxygenase-dependent responses involving inhibition of tubular reabsorption of sodium and water through undefined mechanisms.     

Paradoxical role of angiotensin II type 2 receptors in resistance arteries of old rats

The role of angiotensin II type 2 receptors (AT2Rs) remains a matter of controversy. Its vasodilatory and antitrophic properties are well accepted. Nevertheless, in hypertensive rats, AT2R stimulation induces a vasoconstriction counteracting flow-mediated dilation (FMD). This contraction is reversed by hydralazine. Because FMD is also decreased in aging, another risk factor for cardiovascular diseases, we hypothesized that AT2R function might be altered in old-rat resistance arteries. Mesenteric resistance arteries (250 mum in diameter) were isolated from old (24 months) and control (4 months) rats receiving hydralazine (16 mg/kg per day; 2 weeks) or water. FMD, NO-mediated dilation, and endothelial NO synthase expression were lower in old versus control rats. AT2R blockade improved FMD in old rats, suggesting that AT2R stimulation produced vasoconstriction. AT2R expression was higher in old rats and mainly located in the smooth muscle layer. In old rats, AT2R stimulation induced endothelium-independent contraction, which was suppressed by the antioxidant Tempol. Reactive oxygen species level was higher in old-rat arteries than in controls. Hydralazine improved FMD and NO-dependent dilation in old rats without change in AT2R expression and location. In old rats treated with hydralazine, reactive oxygen species level was reduced in endothelial and smooth muscle cells, and AT2R-dependent contraction was abolished. Thus, AT2R stimulation induced vasoconstriction through activation of reactive oxygen species production, contributing to decrease FMD in old-rat resistance arteries. Hydralazine suppressed AT2R-dependent reactive oxygen species production and AT2R-dependent contraction, improving FMD. Importantly, endothelial alterations in aging were reversible. These findings are important to consider in the choice of vasoactive drugs in aging.     

Dopamine and angiotensin type 2 receptors cooperatively inhibit sodium transport in human renal proximal tubule cells

Little is known regarding how the kidney shifts from a sodium and water reclaiming state (antinatriuresis) to a state where sodium and water are eliminated (natriuresis). In human renal proximal tubule cells, sodium reabsorption is decreased by the dopamine D(1)-like receptors (D(1)R/D(5)R) and the angiotensin type 2 receptor (AT(2)R), whereas the angiotensin type 1 receptor increases sodium reabsorption. Aberrant control of these opposing systems is thought to lead to sodium retention and, subsequently, hypertension. We show that D(1)R/D(5)R stimulation increased plasma membrane AT(2)R 4-fold via a D(1)R-mediated, cAMP-coupled, and protein phosphatase 2A-dependent specific signaling pathway. D(1)R/D(5)R stimulation also reduced the ability of angiotensin II to stimulate phospho-extracellular signal-regulated kinase, an effect that was partially reversed by an AT(2)R antagonist. Fenoldopam did not increase AT(2)R recruitment in renal proximal tubule cells with D(1)Rs uncoupled from adenylyl cyclase, suggesting a role of cAMP in mediating these events. D(1)Rs and AT(2)Rs heterodimerized and cooperatively increased cAMP and cGMP production, protein phosphatase 2A activation, sodium-potassium-ATPase internalization, and sodium transport inhibition. These studies shed new light on the regulation of renal sodium transport by the dopaminergic and angiotensin systems and potential new therapeutic targets for selectively treating hypertension.     

Angiotensin AT2 receptors: control of renal sodium excretion and blood pressure.

The renin-angiotensin system is a coordinated hormonal cascade of crucial importance in cardiovascular and renal function. The primary effector peptide angiotensin II functions at two major receptors, the AT1 and AT2 receptors. AT2 receptors mediate vasodilation and natriuresis. Regarding vasodilator actions, AT2 receptors oppose the AT1 receptor-mediated vasoconstrictor action of angiotensin II. Regarding the natriuretic actions of AT2 receptors, des-aspartyl 1-angiotensin II, rather than angiotensin II, is the preferred agonist. Regarding both the vasodilator and natriuretic properties of AT2 receptors, the beneficial blood pressure reduction and natriuretic responses to AT1 receptor blockade are mediated, at least in part, by AT2 receptor activation. In addition, AT2 receptor activation suppresses renin biosynthesis and release at renal juxtaglomerular cells. Therefore, AT2 receptors are potential therapeutic targets in hypertension.     

Angiotensin II sensitivity is associated with the angiotensin II type 1 receptor A(1166)C polymorphism in essential hypertensives on a high sodium diet

Several investigations have shown heterogeneity in the functional responses to angiotensin II (Ang II) in patients with essential hypertension. The present study was initiated to evaluate whether the A(1166)C polymorphism of the Ang II type 1 receptor (AT(1)R) gene contributes to this variability in Ang II responses. After 7 days of a high-sodium diet (220 mmol Na(+) per day), we measured in 42 essential hypertensive patients blood pressure, heart rate, effective renal plasma flow (ERPF), glomerular filtration rate (GFR), active plasma renin concentration, aldosterone, and atrial natriuretic peptide (ANP) before and during Ang II infusion (increasing doses of 0.3, 1.0, and 3.0 ng/kg per minute). Calculated variables were filtration fraction and renal vascular resistance (RVR). Patients in the 3 genotype groups (AA: n=14; AC: n=17; CC: n=11) were matched for gender, age, and body mass index. At baseline, CC patients had decreased GFR (P:=0.06) and aldosterone (P:<0.05) and increased ANP (P:<0.05) compared with AA patients. Moreover, responses of ERPF, GFR, and RVR to the lowest concentration of Ang II (0.3 ng/kg per minute) were more pronounced in CC patients than in AA patients (ERPF/GFR: P:<0.05; RVR: P:=0.07), whereas maximal responses were all comparable between the groups. Heart rate was decreased at all levels of Ang II infusion in CC patients, while it did not change in AA or AC patients. There were no differences in responses of active plasma renin concentration, aldosterone, and ANP to Ang II between the 3 groups. From these data, we conclude that the C allele of the AT(1)R A(1166)C polymorphism is associated with increased sensitivity but not reactivity to Ang II. An augmented response to Ang II may well be responsible for the increased incidence of cardiovascular abnormalities found in patients with 1 or 2 C alleles.     

Conversion of renal angiotensin II to angiotensin III is critical for AT2 receptor-mediated natriuresis in rats

In the kidney, angiotensin II (Ang II) is metabolized to angiotensin III (Ang III) by aminopeptidase A (APA). In turn, Ang III is metabolized to angiotensin IV by aminopeptidase N (APN). Renal interstitial (RI) infusion of Ang III, but not Ang II, results in angiotensin type-2 receptor (AT(2)R)-mediated natriuresis. This response is augmented by coinfusion of PC-18, a specific inhibitor of APN. The present study addresses the hypotheses that Ang II conversion to Ang III is critical for the natriuretic response. Sprague-Dawley rats received systemic angiotensin type-1 receptor (AT(1)R) blockade with candesartan (CAND; 0.01 mg/kg/min) for 24 hours before and during the experiment. After a control period, rats received either RI infusion of Ang II or Ang II+PC-18. The contralateral kidney received a RI infusion of vehicle in all rats. Mean arterial pressure (MAP) was monitored, and urinary sodium excretion rate (U(Na)V) was calculated separately from experimental and control kidneys for each period. In contrast to Ang II-infused kidneys, U(Na)V from Ang II+PC-18-infused kidneys increased from a baseline of 0.03+/-0.01 to 0.09+/-0.02 micromol/min (P<0.05). MAP was unchanged by either infusion. RI addition of PD-123319, an AT(2)R antagonist, inhibited the natriuretic response. Furthermore, RI addition of EC-33, a selective APA inhibitor, abolished the natriuretic response to Ang II+PC-18. These data demonstrate that RI addition of PC-18 to Ang II enables natriuresis mediated by the AT(2)R, and that conversion of Ang II to Ang III is critical for this response.     

Mechanisms of dopamine D(1) and angiotensin type 2 receptor interaction in natriuresis

Renal dopamine D(1)-like receptors (D(1)Rs) and angiotensin type 2 receptors (AT(2)Rs) are important natriuretic receptors counterbalancing angiotensin type 1 receptor-mediated tubular sodium reabsorption. Here we explore the mechanisms of D(1)R and AT(2)R interactions in natriuresis. In uninephrectomized, sodium-loaded Sprague-Dawley rats, direct renal interstitial infusion of the highly selective D(1)R agonist fenoldopam induced a natriuretic response that was abolished by the AT(2)R-specific antagonist PD-123319 or by microtubule polymerization inhibitor nocodazole but not by actin polymerization inhibitor cytochalasin D. By confocal microscopy and immunoelectron microscopy, fenoldopam translocated AT(2)Rs from intracellular sites to the apical plasma membranes of renal proximal tubule cells, and this translocation was abolished by nocodazole. Because D(1)R activation induces natriuresis via an adenylyl cyclase/cAMP signaling pathway, we explored whether this pathway is responsible for AT(2)R recruitment and AT(2)R-mediated natriuresis. Renal interstitial coinfusion of the adenylyl cyclase activator forskolin and 3-isobutly-1-methylxanthine induced natriuresis that was abolished either by PD-123319 or nocodazole but was unaffected by specific the D(1)R antagonist SCH-23390. Coadministration of forskolin and 3-isobutly-1-methylxanthine also translocated AT(2)Rs to the apical plasma membranes of renal proximal tubule cells; this translocation was abolished by nocodazole but was unaffected by SCH-23390. The results demonstrate that D(1)R-induced natriuresis requires AT(2)R recruitment to the apical plasma membranes of renal proximal tubule cells in a microtubule-dependent manner involving an adenylyl cyclase/cAMP signaling pathway. These studies provide novel insights regarding the mechanisms whereby renal D(1)Rs and AT(2)Rs act in concert to promote sodium excretion in vivo.     

Renal effects of atrial natriuretic factor during control of the renin-angiotensin system in anesthetized dogs

The contribution of the renin-angiotensin system to the natriuretic responses to intrarenal infusions of 1, 5, 25, and 125 pmol/kg/min synthetic rat atrial natriuretic peptide 101-126 was determined in one-kidney anesthetized dogs. In vehicle-treated dogs, atrial natriuretic peptide 101-126 increased fractional sodium excretion from 1.8 +/- 0.6% to a peak response of 5.1 +/- 0.9% during infusion of 25 pmol/kg/min. The peptide progressively decreased mean arterial pressure from 110 +/- 5 to 94 +/- 4 mm Hg, renal vascular resistance from 0.40 +/- 0.02 to 0.30 +/- 0.02 mm Hg/ml/min, and arterial plasma renin activity from 4.3 +/- 1.6 to 3.1 +/- 0.8 ng/ml/hr. When the renin-angiotensin system was blocked by 3 mg/kg i.v. enalaprilat, baseline pressure fell to 86 +/- 4 mm Hg, and subsequent infusions of atrial natriuretic peptide 101-126 did not affect fractional sodium excretion. The decreases in blood pressure (from 86 +/- 4 to 76 +/- 4 mm Hg) and in renal vascular resistance (from 0.27 +/- 0.03 to 0.23 +/- 0.02 mm Hg/ml/min) were also ameliorated compared with the control responses. Intravenous infusion of 2.5 ng/kg/min angiotensin II restored mean arterial pressure and potentiated the natriuretic and renal vascular responses to atrial natriuretic peptide 101-126. In two additional groups of anesthetized dogs, enalaprilat did not produce the profound hypotension and did not affect the natriuretic responses to atrial natriuretic peptide 101-126. When renal vascular resistance was elevated by intrarenal infusion of angiotensin II in enalaprilat-treated dogs, the natriuretic response was improved.(ABSTRACT TRUNCATED AT 250 WORDS)