Effect of angiotensin AT2 receptor stimulation on vascular cyclic GMP production in normotensive Wistar Kyoto rats

In the present study in normotensive Wistar Kyoto rats (WKY), we investigated whether any angiotensin II (ANG II) increases in vascular cyclic GMP production were via stimulation of AT(2) receptors. Adult WKY were infused for 4h with ANG II (30 ng/kg per min, i.v.) or vehicle (0.9% NaCl, i.v.) after pretreatment with (1) vehicle, (2) losartan (100 mg/kg p.o.), (3) PD 123319 (30 mg/kg i.v.), (4) losartan+PD 123319, (5) icatibant (500 microg/kg i.v.), (6) L-NAME (1 mg/kg i.v.), (7) minoxidil (3 mg/kg i.v.). Mean arterial blood pressure (MAP) was continuously monitored, and plasma ANG II and aortic cyclic GMP were measured at the end of the study. ANG II infusion over 4h raised MAP by a mean of 13 mmHg. This effect was completely prevented by AT(1) receptor blockade. PD 123319 slightly attenuated the pressor effect induced by ANG II alone (123.4+/-0.8 versus 130.6+/-0.6) but did not alter MAP in rats treated simultaneously with ANG II + losartan (113+/-0.6 versus 114.3+/-0.8). Plasma levels of ANG II were increased 2.2-3.7-fold by ANG II infusion alone or ANG II in combination with the various drugs. The increase in plasma ANG II levels was most pronounced after ANG II+losartan treatment but absent in rats treated with losartan alone. Aortic cyclic GMP levels were not significantly changed by either treatment. Our results demonstrate that the AT(2) receptor did not contribute to the cyclic GMP production in the vascular wall of normotensive WKY.     

Sar1Ile7]angiotensin III, a new selective antagonist of the pressor effect of angiotensin III in conscious rats

Two analogues of angiotensin III were compared as antagonists of the pressor response to angiotensin II (ANG II) and angiotensin III (ANG III) in conscious, unrestrained rats. Dose-mean arterial pressure (MAP) response curves were obtained for ANG II and ANG III in the absence or presence of [Ile7]ANG III (1.3 x 10(-7) mol/kg) or [Sar1 Ile7]ANG III (1.2 x 10(-7) mol/kg). In the presence of [Ile7]ANG III, the dose-MAP response curves for ANG II and ANG III were significantly displaced to the right. [Ile7]ANG III behaved as a partial agonist on ANG II but not ANG III receptors. In the presence of [Sar1 Ile7]ANG III, the dose-MAP response curve for ANG III but not ANG II was significantly displaced to the right. This suggests that [Sar1 Ile7]ANG III is a selective antagonist of ANG III in the vasculature. [Ile7]ANG III, on the other hand, antagonizes both ANG II and ANG III receptors. Our results support the hypothesis of the existence of a sub-class of angiotensin receptors activated by ANG III in the vascular smooth muscle.     

Intradermal angiotensin II administration attenuates the local cutaneous vasodilator heating response

The vasodilation response to local cutaneous heating is nitric oxide (NO) dependent and blunted in postural tachycardia but reversed by angiotensin II (ANG II) type 1 receptor (AT(1)R) blockade. We tested the hypothesis that a localized infusion of ANG II attenuates vasodilation to local heating in healthy volunteers. We heated the skin of a calf to 42 degrees C and measured local blood flow to assess the percentage of maximum cutaneous vascular conductance (%CVC(max)) in eight healthy volunteers aged 19.5-25.5 years. Initially, two experiments were performed; in one, Ringer solution was perfused in three catheters, the response to heating was measured, 2 microg/l losartan, 10 mM nitro-l-arginine (NLA), or NLA + losartan was added to perfusate, and the heat response was remeasured; in another, 10 microM ANG II was given, the heat response was measured, losartan, NLA, or NLA + losartan was added to ANG II, and the heat response was reassessed. The heat response decreased with ANG II, particularly the plateau phase (47 +/- 5 vs. 84 +/- 3 %CVC(max)). Losartan increased baseline conductance in both experiments (from 8 +/- 1 to 20 +/- 2 and 12 +/- 1 to 24 +/- 3). Losartan increased the ANG II response (83 +/- 4 vs. 91 +/- 6 in Ringer). NLA decreased both angiotensin and Ringer responses (31 +/- 4 vs. 43 +/- 3). NLA + losartan blunted the Ringer response (48 +/- 2), but the ANG II response (74 +/- 5) increased. In a second set of experiments, we used dose responses to ANG II (0.1 nM to 10 microM) with and without NLA + losartan to confirm graded responses. Sodium ascorbate (10 mM) restored the ANG II-blunted heating plateau. NO synthase and AT(1)R inhibition cause an NO-independent angiotensin-mediated vasodilation with local heating. ANG II mediates the AT(1)R blunting of local heating, which is not exclusively NO dependent, and is improved by antioxidant supplementation.     

Effects of i.c.v. losartan on the angiotensin II-mediated vasopressin release and hypothalamic fos expression in near-term ovine fetuses

Our previous studies have shown that central administration of angiotensin (ANG II) causes arginine vasopressin (AVP) release in the fetus at 70-90% gestation. This is evidence that the hypothalamic-neurohypophysial system is relatively mature before birth. However, few data exist regarding central ANG receptor mechanisms-mediated AVP response during fetal life. To determine roles of brain ANG receptor subtypes in this response, AT1 and AT2 receptor antagonists, losartan and PD123319, were investigated in the brain in chronically prepared ovine fetuses at the last third of gestation. Application of losartan intracerebroventricularly (i.c.v.) at 0.5 mg/kg suppressed central ANG II-stimulated plasma AVP release. Losartan at 5 mg/kg (i.c.v.) demonstrated a significant enhancement of AVP increase to i.c.v. ANG II. Associated with the increase of plasma vasopressin levels, c-fos expression in the hypothalamic neurons was significantly different between the low and high doses of losartan. The low dose losartan markedly reduced the dual immunoreactivity for FOS and AVP in the supraoptic nuclei and paraventricular nuclei after i.c.v. ANG II, whereas the high dose losartan together with ANG II, significantly increased the co-localization of positive FOS in the AVP-containing neurons than that induced by i.c.v. ANG II alone. Central ANG II induced fetal plasma vasopressin increase was not altered by PD123319. The data suggest that losartan in the fetal brain has remarkably different effects based on the doses administrated on central ANG II-related neuroendocrine effects at the late gestation, and that the AT1 mechanism is critical in the regulation of fetal body fluid homeostasis related to plasma AVP levels.     

Role of 20-hydroxyeicosatetraenoic acid in the renal and vasoconstrictor actions of angiotensin II

The present study examined the effects of ANG II on the renal synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE) and its contribution to the renal vasoconstrictor and the acute and chronic pressor effects of ANG II in rats. ANG II (10(-11) to 10(-7) mol/l) reduced the diameter of renal interlobular arteries treated with inhibitors of nitric oxide synthase and cyclooxygenase, lipoxygenase, and epoxygenase by 81 +/- 8%. Subsequent blockade of the synthesis of 20-HETE with 17-octadecynoic acid (1 micromol/l) increased the ED(50) for ANG II-induced constriction by a factor of 15 and diminished the maximal response by 61%. Graded intravenous infusion of ANG II (5-200 ng/min) dose dependently increased mean arterial pressure (MAP) in thiobutylbarbitol-anesthetized rats by 35 mmHg. Acute blockade of the formation of 20-HETE with dibromododecenyl methylsulfimide (DDMS; 10 mg/kg) attenuated the pressor response to ANG II by 40%. An intravenous infusion of ANG II (50 ng. kg(-1). min(-1)) in rats for 5 days increased the formation of 20-HETE and epoxyeicosatrienoic acids (EETs) in renal cortical microsomes by 60 and 400%, respectively, and increased MAP by 78 mmHg. Chronic blockade of the synthesis of 20-HETE with intravenous infusion of DDMS (1 mg. kg(-1). h(-1)) or EETs and 20-HETE with 1-aminobenzotriazole (ABT; 2.2 mg. kg(-1). h(-1)) attenuated the ANG II-induced rise in MAP by 40%. Control urinary excretion of 20-HETE averaged 350 +/- 23 ng/day and increased to 1,020 +/- 105 ng/day in rats infused with ANG II (50 ng. kg(-1). min(-1)) for 5 days. In contrast, urinary excretion of 20-HETE only rose to 400 +/- 40 and 600 +/- 25 ng/day in rats chronically treated with ANG II and ABT or DDMS respectively. These results suggest that acute and chronic elevations in circulating ANG II levels increase the formation of 20-HETE in the kidney and peripheral vasculature and that 20-HETE contributes to the acute and chronic pressor effects of ANG II.     

Central hypertensive actions of angiotensin I, II and III in conscious rats

The effects of intracerebroventricular administrations of three natural angiotensins, angiotensin I (ANG I 3.8 X 10-11-9.4 X10-10 mol/kg body weight), II (9.6 X 10-12-2.4 X 10-10 mol/kg body weight) and III (2.7 X 10-10 2.5 X 10-9 mol/kg body weight) on systemic blood pressure were investigated in conscious rats. Angiotensin II (ANG II), ANG I and angiotensin III (ANG III), increased blood pressure in a dose-related manner. The order of potency of angiotensins was ANG II greater than ANG I greater than ANG III. The intraventricular administration of a converting enzyme inhibitor (SQ 14225, 6.9 X10-8 mol/kg) abolished the central effect of ANG I, while an angiotensin II analogue ([Sar1-Ala8]ANG II, 1.1 X 10-8 mol/kg) administered intraventricularly inhibited the central pressor effects of these three angiotensins. These results suggest that ANG II is a main mediator of the renin-angiotensin system in the central nervous system.     

HO-1 induction lowers blood pressure and superoxide production in the renal medulla of angiotensin II hypertensive mice

Heme oxygenase-1 (HO-1) induction can attenuate the development of angiotensin II (ANG II)-dependent hypertension. However, the mechanism by which HO-1 lowers blood pressure in this model is not clear. The goal of this study was to test the hypothesis that induction of HO-1 in the kidney can attenuate the increase in reactive oxygen species (ROS) generation in the kidney that occurs during ANG II-dependent hypertension. Mice were divided into four groups, control (Con), cobalt protoporphyrin (CoPP), ANG II, and ANG II + CoPP. CoPP treatment (50 mg/kg) was administered in a single subcutaneous injection 2 days prior to implantation of an osmotic minipump that infused ANG II at a rate of 1 microg x kg(-1) x min(-1). At the end of this period, mean arterial blood pressure (MAP) averaged 93 +/- 5, 90 +/- 5, 146 +/- 8, and 105 +/- 6 mmHg in Con, CoPP-, ANG II-, and ANG II + CoPP-treated mice. To determine whether HO-1 induction resulted in a decrease in ANG II-stimulated ROS generation in the renal medulla, superoxide production was measured. Medullary superoxide production was increased by ANG II infusion and normalized in mice pretreated with CoPP. The reduction in ANG II-mediated superoxide production in the medulla with CoPP was associated with a decrease in extracellular superoxide dismutase protein but an increase in catalase protein and activity. These results suggest that reduction in superoxide and possibly hydrogen peroxide production in the renal medulla may be a potential mechanism by which induction of HO-1 with CoPP lowers blood pressure in ANG-II dependent hypertension.     

Angiotensin II attenuates myocardial interstitial acetylcholine release in response to vagal stimulation

Although ANG II exerts a variety of effects on the cardiovascular system, its effects on the peripheral parasympathetic neurotransmission have only been evaluated by changes in heart rate (an effect on the sinus node). To elucidate the effect of ANG II on the parasympathetic neurotransmission in the left ventricle, we measured myocardial interstitial ACh release in response to vagal stimulation (1 ms, 10 V, 20 Hz) using cardiac microdialysis in anesthetized cats. In a control group (n = 6), vagal stimulation increased the ACh level from 0.85 +/- 0.03 to 10.7 +/- 1.0 (SE) nM. Intravenous administration of ANG II at 10 microg x kg(-1) x h(-1) suppressed the stimulation-induced ACh release to 7.5 +/- 0.6 nM (P < 0.01). In a group with pretreatment of intravenous ANG II receptor subtype 1 (AT(1) receptor) blocker losartan (10 mg/kg, n = 6), ANG II was unable to inhibit the stimulation-induced ACh release (8.6 +/- 1.5 vs. 8.4 +/- 1.7 nM). In contrast, in a group with local administration of losartan (10 mM, n = 6) through the dialysis probe, ANG II inhibited the stimulation-induced ACh release (8.0 +/- 0.8 vs. 5.8 +/- 1.0 nM, P < 0.05). In conclusion, intravenous ANG II significantly inhibited the parasympathetic neurotransmission through AT(1) receptors. The failure of local losartan administration to nullify the inhibitory effect of ANG II on the stimulation-induced ACh release indicates that the site of this inhibitory action is likely at parasympathetic ganglia rather than at postganglionic vagal nerve terminals.     

Investigation of the mechanisms by which chronic infusion of an acutely subpressor dose of angiotensin II induces hypertension

The mechanisms by which chronic infusion of an initially subpressor low dose of angiotensin II (ANG II) causes a progressive and sustained hypertension remain unclear. In conscious sheep (n = 6), intravenous infusion of ANG II (2 microg/h) gradually increased mean arterial pressure (MAP) from 82 +/- 3 to 96 +/- 5 mmHg over 7 days (P < 0.001). This was accompanied by peripheral vasoconstriction; total peripheral conductance decreased from 44.6 +/- 6.4 to 38.2 +/- 6.7 ml.min(-1).mmHg(-1) (P < 0.001). Cardiac output and heart rate were unchanged. In the regional circulation, mesenteric, renal, and iliac conductances decreased but blood flows were unchanged. There was no coronary vasoconstriction, and coronary blood flow increased. Ganglion blockade (125 mg/h hexamethonium for 4 h) reduced MAP by 13 +/- 1 mmHg in the control period and by 7 +/- 2 mmHg on day 8 of ANG II treatment. Inhibition of central AT(1) receptors by intracerebroventricular infusion of losartan (1 mg/h for 3 h) had no effect on MAP in the control period or after 7 days of ANG II infusion. Pressor responsiveness to incremental doses of intravenous ANG II (5, 10, 20 microg/h, each for 15 min) was unchanged after 7 days of ANG II infusion. ANG II caused no sodium or water retention. In summary, hypertension due to infusion of a low dose of ANG II was accompanied by generalized peripheral vasoconstriction. Indirect evidence suggested that the hypertension was not neurogenic, but measurement of sympathetic nerve activity is required to confirm this conclusion. There was no evidence for a role for central angiotensinergic mechanisms, increased pressor responsiveness to ANG II, or sodium and fluid retention.     

Renal medullary nitric oxide deficit of Dahl S rats enhances hypertensive actions of angiotensin II

Studies were designed to examine the hypothesis that the renal medulla of Dahl salt-sensitive (Dahl S) rats has a reduced capacity to generate nitric oxide (NO), which diminishes the ability to buffer against the chronic hypertensive effects of small elevations of circulating ANG II. NO synthase (NOS) activity in the outer medulla of Dahl S rats (arginine-citrulline conversion assay) was significantly reduced. This decrease in NOS activity was associated with the downregulation of protein expression of NOS I, NOS II, and NOS III isoforms in this region as determined by Western blot analysis. In anesthetized Dahl S rats, we observed that a low subpressor intravenous infusion of ANG II (5 ng. kg(-1). min(-1)) did not increase the concentration of NO in the renal medulla as measured by a microdialysis with oxyhemoglobin trapping technique. In contrast, ANG II produced a 38% increase in the concentration of NO (87 +/- 8 to 117 +/- 8 nmol/l) in the outer medulla of Brown-Norway (BN) rats. The same intravenous dose of ANG II reduced renal medullary blood flow as determined by laser-Doppler flowmetry in Dahl S, but not in BN rats. A 7-day intravenous ANG II infusion at a dose of 3 ng. kg(-1). min(-1) did not change mean arterial pressure (MAP) in the BN rats but increased MAP in Dahl S rats from 120 +/- 2 to 138 +/- 2 mmHg (P < 0.05). ANG II failed to increase MAP after NO substrate was provided by infusion of L-arginine (300 microg. kg(-1). min(-1)) into the renal medulla of Dahl S rats. Intravenous infusion of L-arginine at the same dose had no effect on the ANG II-induced hypertension. These results indicate that an impaired NO counterregulatory system in the outer medulla of Dahl S rats makes them more susceptible to the hypertensive actions of small elevations of ANG II.     

Conversion of brain angiotensin II to angiotensin III is critical for pressor response in rats

The present investigation measured the relative pressor potencies of intracerebroventricularly infused ANG II, ANG III, and the metabolically resistant analogs d-Asp(1)ANG II and d-Arg(1)ANG III in alert freely moving rats. The stability of these analogs was further facilitated by pretreatment with the specific aminopeptidase A inhibitor EC33 or the aminopeptidase N inhibitor PC18. The results indicate that the maximum elevations in mean arterial pressure (MAP) were very similar for each of these compounds across the dose range 1, 10, and 100 pmol/min during a 5-min infusion period. However, d-Asp(1)ANG II revealed significantly extended durations of pressor effects before return to base level MAP. Pretreatment intracerebroventricular infusion with EC33 blocked the pressor activity induced by the subsequent infusion of d-Asp(1)ANG II, whereas EC33 had no effect on the pressor response to subsequent infusion of d-Arg(1)ANG III. In contrast, pretreatment infusion with PC18 extended the duration of the d-Asp(1)ANG II pressor effect by about two to three times and the duration of d-Arg(1)ANG III's effect by approximately 10 to 15 times. Pretreatment with the specific AT(1) receptor antagonist losartan blocked the pressor responses induced by the subsequent infusion of both analogs indicating that they act via the AT(1) receptor subtype. These results suggest that the brain AT(1) receptor may be designed to preferentially respond to ANG III, and ANG III's importance as a centrally active ligand has been underestimated.     

Effects of losartan on the blood-brain barrier permeability in long-term nitric oxide blockade-induced hypertensive rats

Hypertension is closely associated with vascular endothelial dysfunction. The aim of this study was to investigate the effects of Angiotensin II (ANG II) receptor antagonist losartan on the blood-brain barrier (BBB) permeability in L-NAME-induced hypertension and/or in ANG II-induced acute hypertension in normotensive and hypertensive rats. Systolic blood pressure was measured by tail cuff method before, during and following L-NAME treatment (1 g/L). Losartan (3 mg/kg) was given to the animal for five days. Acute hypertension was induced by ANG II (60 microg/kg). Arterial blood pressure was directly measured on the day of the experiment. BBB disruption was quantified according to the extravasation of the albumin-bound Evans blue dye. Losartan significantly reduced the mean arterial blood pressure from 169 +/- 3.9 mmHg to 82 +/- 2.9 mmHg in L-NAME and from 171 +/- 2.9 mmHg to 84 +/- 2.9 in L-NAME plus losartan plus ANG II groups (p < 0.05). The content of Evans blue dye in the cerebral cortex significantly increased in L-NAME (p < 0.01). Moreover, the content of Evans blue dye markedly increased in the cerebellum (p < 0.001) and slightly increased in diencephalon region (p < 0.05) in L-NAME plus ANG II. Losartan reduced the increased BBB permeability to Evans blue dye in L-NAME (p < 0.01) and L-NAME plus ANG II (p < 0.001). These results indicate that L-NAME and L-NAME plus ANG II both lead to an increase in microvascular Evans blue dye efflux to brain, and losartan treatment attenuates this protein-bound dye transport into brain tissue presumably due to its protective effect on endothelial cells of brain vessels.     

Effect of AT(1) receptor blockade on cardiac apoptosis in angiotensin II-induced hypertension

Angiotensin II (ANG II) via AT(1) receptors induces apoptosis in cardiomyocytes in vitro. We tested the hypothesis that in vivo AT(1) receptor stimulation is accompanied by cardiac apoptosis and attempted to elucidate the molecular mechanisms involved in the death signaling pathway. Male Sprague-Dawley rats received ANG II (120 ng x kg(-1) x min(-1) sc) for 7 days with or without the AT(1) receptor antagonist losartan (10 mg x kg(-1) x day(-1) orally). Cardiac function was assessed by echocardiography. Apoptosis in the heart was detected and quantified by in situ TdT-mediated dUTP nick-end labeling (TUNEL) and radiolabeled DNA laddering. Expression of bax, bcl-2, caspase 3, and AT(1) and AT(2) receptors was examined by Western blot analysis. Activity of caspase 3 was also measured by a fluorometric immunosorbent enzyme assay. Tail cuff systolic blood pressure was elevated (P < 0.01, n = 6) in ANG II-infused rats (173 +/- 3 mmHg) versus controls (111 +/- 2 mmHg) and reduced by losartan (134 +/- 4 mmHg). Cardiac function was essentially unchanged in ANG II-infused rats. Increased internucleosomal DNA cleavage by TUNEL assay and radiolabeled DNA laddering showed results compatible with enhanced cardiomyocyte apoptosis in the hearts of ANG-II infused rats. The bax-to-bcl-2 ratio, expression of the active form of caspase 3 (17 kDa), and activity of caspase 3 in the hearts of the ANG II group increased more than twofold above controls. Protein expression of AT(1) and AT(2) receptors was significantly increased in ANG II-infused rats compared with control rats. Losartan-treated ANG II-infused rats exhibited normalized apoptosis, bax, caspase 3 activity, and AT(1) receptors. ANG II stimulation of AT(1) receptors in the heart in vivo is associated with an increased rate of apoptosis without major hemodynamic consequences. Bax and caspase 3 are involved in the apoptotic signaling pathway in this experimental paradigm.     

Chronic angiotensin II infusion attenuates the renal sympathoinhibitory response to acute volume expansion

In this study the hypothesis was tested that chronic infusion of ANG II attenuates acute volume expansion (VE)-induced inhibition of renal sympathetic nerve activity (SNA). Rats received intravenous infusion of either vehicle or ANG II (12 ng. kg(-1). min(-1)) for 7 days. ANG II-infused animals displayed an increased contribution of SNA to the maintenance of mean arterial pressure (MAP) as indicated by ganglionic blockade, which produced a significantly (P < 0.01) greater decrease in MAP (75 +/- 3 mmHg) than was observed in vehicle-infused (47 +/- 8 mmHg) controls. Rats were then anesthetized, and changes in MAP, mean right atrial pressure (MRAP), heart rate (HR), and renal SNA were recorded in response to right atrial infusion of isotonic saline (20% estimated blood volume in 5 min). Baseline MAP, HR, and hematocrit were not different between groups. Likewise, MAP was unchanged by acute VE in vehicle-infused animals, whereas VE induced a significant bradycardia (P < 0.05) and increase in MRAP (P < 0.05). MAP, MRAP, and HR responses to VE were not statistically different between animals infused with vehicle vs. ANG II. In contrast, VE significantly (P < 0.001) reduced renal SNA by 33.5 +/- 8% in vehicle-infused animals but was without effect on renal SNA in those infused chronically with ANG II. Acutely administered losartan (3 mg/kg iv) restored VE-induced inhibition of renal SNA (P < 0.001) in rats chronically infused with ANG II. In contrast, this treatment had no effect in the vehicle-infused group. Therefore, it appears that chronic infusion of ANG II can attenuate VE-induced renal sympathoinhibition through a mechanism requiring AT(1) receptor activation. The attenuated sympathoinhibitory response to VE in ANG II-infused animals remained after arterial barodenervation and systemic vasopressin V(1) receptor antagonism and appeared to depend on ANG II being chronically increased because ANG II given acutely had no effect on VE-induced renal sympathoinhibition.     

Impaired responsiveness of renal mechanosensory nerves in heart failure: role of endogenous angiotensin

Increasing renal pelvic pressure results in PGE(2)-mediated release of substance P. Substance P increases afferent renal nerve activity (ARNA), which leads to a reflex increase in urinary sodium excretion (U(Na)V). Endogenous ANG II modulates the responsiveness of renal mechanosensory nerves. The ARNA and U(Na)V responses are suppressed by low- and enhanced by high-sodium diet. We examined whether the ARNA responses are altered in rats with congestive heart failure (CHF), a condition characterized by increased ANG II and sodium retention. The ARNA responses to increasing renal pelvic pressure 相似文献   

Enhanced renal expression of preproendothelin mRNA during chronic angiotensin II hypertension

The purpose of this study was to determine the role of endothelin in mediating the renal hemodynamic and arterial pressure changes observed during chronic ANG II-induced hypertension. ANG II (50 ng x kg(-1) x min(-1)) was chronically infused into the jugular vein by miniosmotic pump for 2 wk in male Sprague-Dawley rats with and without endothelin type A (ET(A))-receptor antagonist ABT-627 (5 mg x kg(-1) x day(-1)) pretreatment. Arterial pressure increased in ANG II rats compared with control rats (149 +/- 5 vs. 121 +/- 6 mmHg, P < 0.05, respectively). Renal expression of preproendothelin mRNA was increased by approximately 50% in both the medulla and cortex of ANG II rats. The hypertensive effect of ANG II was completely abolished in rats pretreated with the ET(A)-receptor antagonist (114 +/- 5 mmHg, P < 0.05). Glomerular filtration rate was decreased by 33% in ANG II rats, and this response was attenuated in rats pretreated with ET(A)-receptor antagonist. These data indicate that activation of the renal endothelin system by ANG II may play an important role in mediating chronic renal and hypertensive actions of ANG II.     

Diuretic response to acute hypertension is blunted during angiotensin II clamp

Acute hypertension inhibits proximal tubule (PT) fluid reabsorption. The resultant increase in end proximal flow rate provides the error signal to mediate tubuloglomerular feedback autoregulation of renal blood flow and glomerular filtration rate and suppresses renal renin secretion. To test whether the suppression of the renin-angiotensin system during acute hypertension affects the magnitude of the inhibition of PT fluid and sodium reabsorption, plasma ANG II levels were clamped by infusion of the angiotensin-converting enzyme (ACE) inhibitor captopril (12 microg/min) and ANG II after pretreatment with the bradykinin B(2) receptor blocker HOE-140 (100 microg/kg bolus). Because ACE also degrades bradykinin, HOE-140 was included to block effect of accumulating vasodilatory bradykinins during captopril infusion. HOE-140 increased the sensitivity of arterial blood pressure to ANG II: after captopril infusion without HOE-140, 20 ng x kg(-1) x min(-1) ANG II had no pressor effect, whereas with HOE-140, 20 ng x kg(-1) x min(-1) ANG II increased blood pressure from 104 +/- 4 to 140 +/- 6 mmHg. ANG II infused at 2 ng x kg(-1) x min(-1) had no pressor effect after captopril and HOE-140 infusion ("ANG II clamp"). When blood pressure was acutely increased 50-60 mmHg by arterial constriction without ANG II clamp, urine output and endogenous lithium clearance increased 4.0- and 6.7-fold, respectively. With ANG II clamp, the effects of acute hypertension were reduced 50%: urine output and endogenous lithium clearance increased two- and threefold, respectively. We conclude that HOE-140, an inhibitor of the B(2) receptor, potentiates the sensitivity of arterial pressure to ANG II and that clamping systemic ANG II levels during acute hypertension blunts the magnitude of the pressure diuretic response.     

Covalent crosslinking analysis of angiotensin receptors on differentiated NG108-15 cells

Neuroblastoma x glioma hybrid cells (NG108-15) were used as a model system to characterize neuronal-glial type angiotensin (ANG) receptors by covalent crosslinking analysis. After differentiation with 1.5% DMSO and 0.5% fetal bovine serum for four to five days, saturation analysis revealed a single high affinity site with a Kd = 1.35 +/- 0.42 nM and a Bmax = 468 +/- 106 fmol/mg protein. Using the homobifunctional crosslinking reagent bis(sulfosuccinimidyl) suberate (BS3), a site with an estimated Mr of 78 kDa was specifically labeled with 125I-ANG II as determined by SDS-polyacrylamide gel electrophoresis. Both ANG II and ANG III (10(-6) M) inhibited specific labeling. The Ki for ANG III binding was similar by both pharmacologic (Ki = 3.33 +/- 0.98 nM) and gel densitometric (Ki = 2.65 +/- 0.32 nM) analyses. We conclude that the 78 kDa protein represents a high affinity ANG binding site with similar affinities for both ANG II and ANG III.     

Interactions between angiotensin II and nitric oxide during exercise in normal and heart failure rats.

We hypothesized that nitric oxide (NO) opposes ANG II-induced increases in arterial pressure and reductions in renal, splanchnic, and skeletal muscle vascular conductance during dynamic exercise in normal and heart failure rats. Regional blood flow and vascular conductance were measured during treadmill running before (unblocked exercise) and after 1) ANG II AT(1)-receptor blockade (losartan, 20 mg/kg ia), 2) NO synthase (NOS) inhibition [N(G)-nitro-L-arginine methyl ester (L-NAME); 10 mg/kg ia], or 3) ANG II AT(1)-receptor blockade + NOS inhibition (combined blockade). Renal conductance during unblocked exercise (4.79 +/- 0.31 ml x 100 g(-1) x min(-1) x mmHg(-1)) was increased after ANG II AT(1)-receptor blockade (6.53 +/- 0.51 ml x 100 g(-1) x min(-1) x mmHg(-1)) and decreased by NOS inhibition (2.12 +/- 0.20 ml x 100 g(-1) x min(-1) x mmHg(-1)) and combined inhibition (3.96 +/- 0.57 ml x 100 g(-1) x min(-1) x mmHg(-1); all P < 0.05 vs. unblocked). In heart failure rats, renal conductance during unblocked exercise (5.50 +/- 0.66 ml x 100 g(-1) x min(-1) x mmHg(-1)) was increased by ANG II AT(1)-receptor blockade (8.48 +/- 0.83 ml x 100 g(-1) x min(-1) x mmHg(-1)) and decreased by NOS inhibition (2.68 +/- 0.22 ml x 100 g(-1) x min(-1) x mmHg(-1); both P < 0.05 vs. unblocked), but it was unaltered during combined inhibition (4.65 +/- 0.51 ml x 100 g(-1) x min(-1) x mmHg(-1)). Because our findings during combined blockade could be predicted from the independent actions of NO and ANG II, no interaction was apparent between these two substances in control or heart failure animals. In skeletal muscle, L-NAME-induced reductions in conductance, compared with unblocked exercise (P < 0.05), were abolished during combined inhibition in heart failure but not in control rats. These observations suggest that ANG II causes vasoconstriction in skeletal muscle that is masked by NO-evoked dilation in animals with heart failure. Because reductions in vascular conductance between unblocked exercise and combined inhibition were less than would be predicted from the independent actions of NO and ANG II, an interaction exists between these two substances in heart failure rats. L-NAME-induced increases in arterial pressure during treadmill running were attenuated (P < 0.05) similarly in both groups by combined inhibition. These findings indicate that NO opposes ANG II-induced increases in arterial pressure and in renal and skeletal muscle resistance during dynamic exercise.     

Involvement of endogenous nitric oxide in angiotensin II-induced activation of vascular mitogen-activated protein kinases

Angiotensin II (ANG II) is a powerful activator of mitogen-activated protein (MAP) kinase cascades in cardiovascular tissues through a redox-sensitive mechanism. Nitric oxide (NO) is considered to antagonize the vasoconstrictive and proarteriosclerotic actions of ANG II. However, the role of endogenous NO in ANG II-induced redox-sensitive signal transduction is not yet clear. In this study using catheterized, conscious rats, we found that acute intravenous administration of N(G)-nitro-L-arginine methyl ester (L-NAME; 5 mg/kg) enhanced phosphorylation of aortic MAP kinases extracellular signal regulated kinase (ERK) 1/2 and p38, which were suppressed only partially by a superoxide dismutase mimetic (Tempol), whereas ANG II-induced MAP kinase phosphorylation was markedly suppressed by Tempol. FK409, a NO donor, had little effect on vascular MAP kinase phosphorylation. On the other hand, acute exposure to a vasoconstrictor dose of ANG II (200 ng x kg(-1) x min(-1) iv) failed to enhance phosphorylation of aortic MAP kinases in the chronically L-NAME-treated rats, whereas the vasoconstrictor effect of ANG II was not affected by L-NAME treatment. Furthermore, three different inhibitors of NO synthase suppressed, in a dose-dependent manner, ANG II-induced MAP kinase phosphorylation in rat vascular smooth muscle cells, which was closely linked to superoxide generation in cells. These results indicate the involvement of endogenous NO synthase in ANG II-induced signaling pathways, leading to activation of MAP kinase, and that NO may have dual effects on the vascular MAP kinase activation associated with redox sensitivity.