Hemodynamic, renal, and endocrine responses to acute ET(A) blockade at different ANG II plasma levels

Angiotensin (ANG) II effects may be partly mediated by endothelin (ET)-1. This study analyses the hemodynamic, renal, and hormonal responses of acute ET(A) receptor antagonism (LU-135252) at two ANG II plasma levels in eight conscious dogs. Protocol 1 involved a 60-min baseline, followed by two doses of ANG II for 60 min each (4 and 20 ng. kg(-1). min(-1)), termed ANG II 4 (slightly increased) and ANG II 20 (pathophysiologically increased ANG II plasma concentration). Protocol 2 was the same as protocol 1 but included 15 mg/kg iv LU-135252 after the baseline period. Protocol 3 was a 3-h time control. ANG II without LU-135252 did not increase plasma big ET-1 and ET-1, whereas LU-135252 increased ET-1 transiently after injection. This transient ET-1 increase was not reflected in urinary ET-1 excretion. The ANG II induced decreases in sodium, water, and potassium excretion, glomerular filtration rate, and fractional sodium excretion were not different with and without LU-135252. Mean arterial pressure increased during ANG II and was not lower with LU-135252 (-6 mmHg, not significant). Most importantly, during ANG II 20 LU-135252 prevented the decrease in cardiac output. Simultaneously, systemic vascular resistance increased 40% less, pulmonary vascular resistance was maintained at baseline levels, and central venous and wedge pressure were lower. Because ANG II stimulated endothelin de novo synthesis should just have started after 2 h of ANG II infusion, there must be mechanisms other than blocking the coupling of de novo synthesized endothelins to the ET(A) receptors to explain the effects of acute ET(A) receptor inhibition in our setting.     

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.     

Circumventricular organs and ANG II-induced salt appetite: blood pressure and connectivity

A lesion of the subfornical organ (SFO) may reduce sodium depletion-induced salt appetite, which is largely dependent on ANG II, and yet ANG II infusions directly into SFO do not provoke salt appetite. Two experiments were designed to address this apparent contradiction. In experiment 1 sustained infusions of ANG II into SFO did not produce a sustained elevation of blood pressure, and neither a reduction of blood pressure alone with minoxidil and captopril nor a reduction of both blood pressure and volume with furosemide and captopril enhanced salt appetite. Infusions of ANG II in the organum vasculosum laminae terminalis (OVLT) did evoke salt appetite without raising blood pressure. In experiment 2 knife cuts of the afferent and efferent fibers of the rostroventral pole of the SFO abolished water intake during an infusion of ANG II into the femoral vein but failed to reduce salt appetite during an infusion of ANG II into the OVLT. We conclude that 1) hypertension does not account for the failure of infusions of ANG II in the SFO to generate salt appetite and 2) the OVLT does not depend on its connectivity with the SFO to generate salt appetite during ANG II infusions.     

Role of ET(A) receptors in experimental ANG II-induced hypertension in rats

The objectives were to determine if ANG II-induced hypertension is maintained by activation of endothelin type A (ET(A)) receptors by endogenous ET-1 and if this effect is influenced by salt intake. Male rats were maintained on high sodium intake (HS; 6 meq/day) or on normal sodium intake (NS; 2 meq/day). Hypertension was produced by intravenous infusion of ANG II (5 ng/min) for 15 days. Five-day oral dosing with the selective ET(A)-receptor antagonist ABT-627 (~2 mg. kg(-1). day(-1)) reduced mean arterial pressure (MAP) to baseline levels in rats on HS receiving ANG II infusion, but it did not affect MAP in normotensive HS controls. In rats on NS, ABT-627 only transiently decreased MAP in rats receiving ANG II and slightly reduced MAP in normotensive controls. ABT-627 produced mild retention of sodium and water in NS rats receiving ANG II, but not in any other group. These results indicate that ET-1 plays a role in ANG II-induced hypertension via activation of ET(A) receptors and that this role is more prominent in rats on HS.     

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.     

ANG II-induced hypertension and the role of the area postrema during normal and increased dietary salt

It has been shown that the area postrema (AP) plays a role in the development of certain types of chronic angiotensin II (ANG II)-induced hypertension in the rat but is not of great importance in the salt sensitivity of arterial pressure. It has recently been proposed, however, that elevated sodium levels may exacerbate the hypertensive effects of ANG II, which by itself dramatically affects salt sensitivity, by acting at sodium-sensing neurons in certain circumventricular organs of the brain. Thus the interactions of ANG II, sodium, and the central nervous system remain to be fully understood. The purpose of this study was to examine the role of the AP in ANG II-induced hypertension during periods of normal and elevated dietary salt. We hypothesized that an intact AP was necessary for the full development of hypertension under chronic ANG II infusion and that its role would be pronounced during periods of increased dietary sodium. To test this, male Sprague-Dawley rats underwent ablation of the area postrema (APx, n = 6) or sham operation (sham, n = 6). After 3 wk of recovery, rats were instrumented with radiotelemetry transducers for constant blood pressure and heart rate monitoring and venous catheters for vehicle infusion. After a 3-day control period of 0.9% saline infusion (7 ml/day) and 0.4% dietary sodium, a 10-day period of ANG II infusion (10 ng.kg(-1).min(-1)) was begun, immediately followed by a second 10-day period during which rats were fed a 4.0% sodium diet. By day 6 of ANG II infusion, mean arterial pressure (MAP) in APx rats had increased to 139 +/- 4 mmHg, whereas MAP in sham rats had increased to 126 +/- 3 mmHg. This difference was found to be significant and continued through day 1 of the high-salt period, after which MAP of the two groups had risen to similar levels. On day 9 of high salt, MAP was again observed to be significantly higher (162 +/- 1 mmHg) in APx rats when compared with sham rats (147 +/- 4 mmHg.) These results do not support the hypothesis that the AP is necessary for the full development of ANG II-induced hypertension at normal or elevated levels of dietary sodium.     

Effects of angiotensin II on autonomic components of nasopharyngeal stimulation in male conscious rabbits.

Angiotensin II (ANG II) is known to activate central sympathetic neurons. In this study we determined the effects of ANG II on the autonomic components of the cardiovascular responses to stimulation of nasopharyngeal receptors with cigarette smoke. Experiments were carried out in conscious New Zealand White rabbits instrumented to record arterial pressure and heart rate. Rabbits were exposed to 50 ml of cigarette smoke before and after subcutaneous osmotic minipump delivery of ANG II at a dose of 50 ng.kg(-1).min(-1) for 1 wk in one group and intracerebroventricular (icv) infusion at a dose of 100 pmol/min for 1 h in a second group. The responses were compared before and after heart rate was controlled by pacing. Autonomic components were evaluated by intravenous administration of atropine methyl bromide (0.2 mg/kg) and prazosin (0.5 mg/kg). ANG II given either systemically or icv significantly blunted the pressor response to smoke (P < 0.05) when the bradycardic response was prevented. This blunted response was not due to an absolute increase in baseline blood pressure after ANG II infusion (71.64 +/- 11.6 vs. 92.1 +/- 19.8 mmHg; P < 0.05) because normalization of blood pressure with sodium nitroprusside to pre-ANG II levels also resulted in a significantly blunted pressor response to smoke. The effect of smoke was alpha(1)-adrenergic receptor-mediated because it was essentially abolished by prazosin in both the pre- and the post-ANG II states (P < 0.05). These results suggest that elevations in central ANG II reduce the sympathetic response to smoke in conscious rabbits. This effect may be due to an augmentation of baseline sympathetic outflow and a reduction in reflex sensitivity similar to the effect of ANG II on baroreflex function.     

Role of endothelin ETB receptor activation in angiotensin II-induced hypertension: effects of salt intake

We showed recently that endothelin (ET)A receptors are involved in the salt sensitivity of ANG II-induced hypertension. The objective of this current study was to characterize the role of endothelin ETB receptor activation in the same model. Male rats on fixed normal (2 meq/day) or high (6 meq/day) salt intake received a continuous intravenous infusion of ANG II or salt only for 15 days. During the middle 5 days of the infusion period, rats were given either the selective ETB receptor antagonist A-192621 or the nonselective endothelin receptor antagonist A-182086 (both at 24 mg x kg(-1) x day(-1) intra-arterially). Infusion of ANG II caused a greater rise in arterial pressure in rats on high-salt intake. The administration of A-192621 increased arterial pressure further in all rats. The chronic hypertensive effect of A-192621 was not significantly affected by salt intake or ANG II. The administration of A-182086 lowered arterial pressure chronically only in rats on normal salt intake receiving ANG II. Thus the salt sensitivity of ANG II-induced hypertension is not caused by changes in ETB receptor function.     

Renal endothelin in chronic angiotensin II hypertension

To determine the influence of chronic ANG II infusion on urinary, plasma, and renal tissue levels of immunoreactive endothelin (ET), ANG II (65 ng/min) or saline vehicle was delivered via osmotic minipump in male Sprague-Dawley rats given either a high-salt diet (10% NaCl) or normal-salt diet (0.8% NaCl). High-salt diet alone caused a slight but not statistically significant increase (7 +/- 1%) in mean arterial pressure (MAP). MAP was significantly increased in ANG II-infused rats (41 +/- 10%), and the increase in MAP was significantly greater in ANG II rats given a high-salt diet (59 +/- 1%) compared with the increase observed in rats given a high-salt diet alone or ANG II infusion and normal-salt diet. After a 2-wk treatment, urinary excretion of immunoreactive ET was significantly increased by approximately 50% in ANG II-infused animals and by over 250% in rats on high-salt diet, with or without ANG II infusion. ANG II infusion combined with high-salt diet significantly increased immunoreactive ET content in the cortex and outer medulla, but this effect was not observed in other groups. In contrast, high-salt diet, with or without ANG II infusion, significantly decreased immunoreactive ET content within the inner medulla. These data indicate that chronic elevations in ANG II levels and sodium intake differentially affect ET levels within the kidney and provide further support for the hypothesis that the hypertensive effects of ANG II may be due to interaction with the renal ET system.     

Renal functional responses to selective intrarenal renin inhibition in Cyp1a1-Ren2 transgenic rats with ANG II-dependent malignant hypertension

Angiotensin (ANG) II-dependent hypertension is characterized by increases in intrarenal ANG II levels, derangement in renal hemodynamics, and augmented tubular sodium reabsorptive capability. Increased nephron expression of renin-angiotensin system components, such as angiotensinogen by proximal tubule cells and renin by collecting duct principal cells, has been associated with an augmented ability of the kidney to form ANG II in hypertensive states. However, the contribution of de novo intrarenal ANG II production to the development and maintenance of ANG II-dependent hypertension remains unclear. The present study was performed to determine the effects of selective intrarenal renin inhibition on whole kidney hemodynamics and renal excretory function in Cyp1a1-Ren2 rats with ANG II-dependent malignant hypertension in the absence of the confounding influence of associated reductions in mean arterial pressure (MAP). Male Cyp1a1-Ren2 transgenic rats were induced to develop malignant hypertension, anesthetized, and surgically prepared for intrarenal administration of the direct renin inhibitor aliskiren (0.01 mg/kg). Following acute aliskiren treatment, urine flow and sodium excretion increased (10.5 ± 1.1 to 15.9 ± 1.9 μl/min, P < 0.001; 550 ± 160 to 1,370 ± 320 neq/min, P < 0.001, respectively) and ANG II excretion decreased (120 ± 30 to 63 ± 17 fmol/h, P < 0.05). There were no significant changes in MAP, glomerular filtration rate, estimated renal plasma flow, plasma ANG II levels, or protein excretion. The present findings demonstrate that selective renal renin inhibition elicits diuretic and natriuretic responses in Cyp1a1-Ren2 rats with ANG II-dependent malignant hypertension. Elevated intraluminal ANG II levels likely act to augment tubular reabsorptive function and, thereby, contribute to the elevated blood pressure in Cyp1a1-Ren2 rats with ANG II-dependent malignant hypertension.     

Effect of chronic IL-6 infusion on acute pressor responses to vasoconstrictors in mice

Interleukin (IL)-6 has been implicated as a contributing factor in the pathogenesis of hypertension, although the mechanisms involved are unclear. Studies conducted in vitro suggest that IL-6 may have a direct effect on vascular tone and may modulate constrictor responses to agonists. Whether this effect can be observed in vivo is unknown. Therefore, mice were treated with either IL-6 (16 ng/h sc) or vehicle for 14 days, and the acute blood pressure and heart rate responses to endothelin (ET)-1, angiotensin II (ANG II), and phenylephrine (PE) were assessed under isoflurane anesthesia. Blood pressure responses to ET-1 were identical in vehicle- and IL-6-infused mice, both in the presence and the absence of ganglion blockade with chlorisondamine. The fall in heart rate during ET-1 responses was significantly attenuated in IL-6-infused mice with autonomic reflexes intact (vehicle vs. IL-6, P < 0.05 at 1 and 3 nmol/kg of ET-1), but this difference was not observed after ganglionic blockade. Both blood pressure and heart rate responses to ANG II were indistinguishable between IL-6- and vehicle-infused mice, as were responses to PE except for a significant increase in the blood pressure response and decrease in the heart rate response in IL-6-infused mice observed only at the highest dose of PE (300 microg/kg; P < 0.05). These findings show that, despite what might be predicted from studies conducted in vitro, chronic exposure to elevated plasma IL-6 concentrations in itself does not predispose the mouse to enhanced responsiveness to vasoconstrictors in vivo.     

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.     

Angiotensin II utilizes Janus kinase 2 in hypertension, but not in the physiological control of blood pressure, during low-salt intake

Janus kinase (JAK) 2 is activated by ANG II in vitro and in vivo, and chronic blockade of JAK2 by the JAK2 inhibitor AG-490 has been shown recently to attenuate ANG II hypertension in mice. In this study, AG-490 was infused intravenously in chronically instrumented rats to determine if the blunted hypertension was linked to attenuation of the renal actions of ANG II. In male Sprague-Dawley rats, after a control period, ANG II at 10 ng·kg(-1)·min(-1) was infused intravenously with or without AG-490 at 10 ng·kg(-1)·min(-1) iv for 11 days. ANG II infusion (18 h/day) increased mean arterial pressure from 91 ± 3 to 168 ± 7 mmHg by day 11. That response was attenuated significantly in the ANG II + AG-490 group, with mean arterial pressure increasing only from 92 ± 5 to 127 ± 3 mmHg. ANG II infusion markedly decreased urinary sodium excretion, caused a rapid and sustained decrease in glomerular filtration rate to ～60% of control, and increased renal JAK2 phosphorylation; all these responses were blocked by AG-490. However, chronic AG-490 treatment had no effect on the ability of a separate group of normal rats to maintain normal blood pressure when they were switched rapidly to a low-sodium diet, whereas blood pressure fell dramatically in losartan-treated rats on a low-sodium diet. These data suggest that activation of the JAK/STAT pathway is critical for the development of ANG II-induced hypertension by mediating its effects on renal sodium excretory capability, but the physiological control of blood pressure by ANG II with a low-salt diet does not require JAK2 activation.     

Angiotensin II injures the arterial wall causing increased aortic stiffening in apolipoprotein E-deficient mice

Cardiovascular diseases, such as atherosclerosis and hypertension, are associated with arterial stiffening. Previous studies showed that ANG II exacerbated atherosclerosis and induced hypertension and aneurysm formation in apolipoprotein E-deficient (apoE-KO) mice. The aim of the present study was to examine the effects of chronic treatment of ANG II on the arterial elastic properties in apoE-KO mice. We hypothesized that ANG II will injure the arterial wall resulting in increased arterial stiffening. Male apoE-KO mice were infused with either ANG II (1.44 mg. kg(-1). day(-1)) or vehicle (PBS) for 30 days. ANG II treatment accelerated atherosclerosis in the carotid artery by sixfold (P < 0.001) and increased blood pressure by 30% (P < 0.05). Additionally, our data demonstrated that ANG II increased arterial stiffening using both in vivo and in vitro methods. ANG II significantly increased pulse wave velocity by 36% (P < 0.01) and decreased arterial elasticity as demonstrated by a more than 900% increase in maximal stiffening (high strain Young's modulus) compared with vehicle (P < 0.05). These functional changes were correlated with morphological and biochemical changes as demonstrated by an increase in collagen content (60%), a decrease in elastin content (74%), and breaks in the internal elastic lamina in the aortic wall. In addition, endothelium-independent vasorelaxation to sodium nitroprusside was impaired in the aortic rings of ANG II-treated mice compared with vehicle. Thus, the present data indicate that ANG II injures the artery wall in multiple ways and arterial stiffening may be a common outcome of ANG II-induced arterial damage.     

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.     

Angiotensin II hypertension is attenuated in interleukin-6 knockout mice

Plasma levels of IL-6 correlate with high blood pressure under many circumstances, and ANG II has been shown to stimulate IL-6 production from various cell types. This study tested the role of IL-6 in mediating the hypertension caused by high-dose ANG II and a high-salt diet. Male C57BL6 and IL-6 knockout (IL-6 KO) mice were implanted with biotelemetry devices and placed in metabolic cages to measure mean arterial pressure (MAP), heart rate (HR), sodium balance, and urinary albumin excretion. Baseline MAP during the control period averaged 114 +/- 1 and 109 +/- 1 mmHg for wild-type (WT) and IL-6 KO mice, respectively, and did not change significantly when the mice were placed on a high-salt diet (HS; 4% NaCl). ANG II (90 ng/min sc) caused a rapid increase in MAP in both groups, to 141 +/- 9 and 141 +/- 4 in WT and KO mice, respectively, on day 2. MAP plateaued at this level in KO mice (134 +/- 2 mmHg on day 14 of ANG II) but began to increase further in WT mice by day 4, reaching an average of 160 +/- 4 mmHg from days 10 to 14 of ANG II. Urinary albumin excretion on day 4 of ANG II was not different between groups (9.18 +/- 4.34 and 8.53 +/- 2.85 microg/2 days for WT and KO mice). By day 14, albumin excretion was nearly fourfold greater in WT mice, but MAP dropped rapidly back to control levels in both groups when the ANG II was stopped after 14 days. Thus the approximately 30 mmHg greater ANG II hypertension in the WT mice suggests that IL-6 contributes significantly to ANG II-salt hypertension. In addition, the early separation in MAP, the albumin excretion data, and the rapid, post-ANG II recovery of MAP suggest an IL-6-dependent mechanism that is independent of renal injury.     

Adenosine A1-receptor knockout mice have a decreased blood pressure response to low-dose ANG II infusion

Adenosine, acting on A(1)-receptors (A(1)-AR) in the nephron, increases sodium reabsorption, and also increases renal vascular resistance (RVR), via A(1)-ARs in the afferent arteriole. ANG II increases blood pressure and RVR, and it stimulates adenosine release in the kidney. We tested the hypothesis that ANG II-infused hypertension is potentiated by A(1)-ARs' influence on Na(+) reabsorption. Mean arterial pressure (MAP) was measured by radiotelemetry in A(1)-AR knockout mice (KO) and their wild-type (WT) controls, before and during ANG II (400 ng·kg(-1)·min(-1)) infusion. Baseline MAP was not different between groups. ANG II increased MAP in both groups, but on day 12, MAP was lower in A(1)-AR KO mice (KO: 128 ± 3 vs. 139 ± 3 mmHg, P < 0.01). Heart rates were significantly different during days 11-14 of ANG II. Basal sodium excretion was not different (KO: 0.15 ± 0.03 vs. WT: 0.13 ± 0.04 mmol/day, not significant) but was higher in KO mice 12 days after ANG II despite a lower MAP (KO: 0.22 ± 0.03 vs. WT: 0.11 ± 0.02 mmol/day, P < 0.05). Phosphate excretion was also higher in A(1)-AR KO mice on day 12. Renal expression of the sodium-dependent phosphate transporter and the Na(+)/glucose cotransporter were lower in the KO mice during ANG II treatment, but the expression of the sodium hydrogen exchanger isoform 3 was not different. These results indicate that the increase in blood pressure seen in A(1)-AR KO mice is lower than that seen in WT mice but was increased by ANG II nonetheless. The presence of A(1)-ARs during a low dose of ANG II-infusion limits Na(+) and phosphate excretion. This study suggests that A(1)-AR antagonists might be an effective antihypertensive agent during ANG II and volume-dependent hypertension.     

Angiotensin-(1-7) attenuates angiotensin II-induced cardiac remodeling associated with upregulation of dual-specificity phosphatase 1

Chronic hypertension induces cardiac remodeling, including left ventricular hypertrophy and fibrosis, through a combination of both hemodynamic and humoral factors. In previous studies, we showed that the heptapeptide ANG-(1-7) prevented mitogen-stimulated growth of cardiac myocytes in vitro, through a reduction in the activity of the MAPKs ERK1 and ERK2. In this study, saline- or ANG II-infused rats were treated with ANG-(1-7) to determine whether the heptapeptide reduces myocyte hypertrophy in vivo and to identify the signaling pathways involved in the process. ANG II infusion into normotensive rats elevated systolic blood pressure >50 mmHg, in association with increased myocyte cross-sectional area, ventricular atrial natriuretic peptide mRNA, and ventricular brain natriuretric peptide mRNA. Although infusion with ANG-(1-7) had no effect on the ANG II-stimulated elevation in blood pressure, the heptapeptide hormone significantly reduced the ANG II-mediated increase in myocyte cross-sectional area, interstitial fibrosis, and natriuretic peptide mRNAs. ANG II increased phospho-ERK1 and phospho-ERK2, whereas cotreatment with ANG-(1-7) reduced the phosphorylation of both MAPKs. Neither ANG II nor ANG-(1-7) altered the ERK1/2 MAPK kinase MEK1/2. However, ANG-(1-7) infusion, with or without ANG II, increased the MAPK phosphatase dual-specificity phosphatase (DUSP)-1; in contrast, treatment with ANG II had no effect on DUSP-1, suggesting that ANG-(1-7) upregulates DUSP-1 to reduce ANG II-stimulated ERK activation. These results indicate that ANG-(1-7) attenuates cardiac remodeling associated with a chronic elevation in blood pressure and upregulation of a MAPK phosphatase and may be cardioprotective in patients with hypertension.     

Effects of chronic dietary salt loading on the renin angiotensin and adrenergic systems of rainbow trout (Oncorhynchus mykiss)

Previous studies have demonstrated that chronic dietary salt loading causes hypertension and a decreased sensitivity of the systemic vasculature to α-adrenergic stimulation and other hypertensive stimuli (e.g. hypercapnia) in rainbow trout (Oncorhynchus mykiss). This reduced sensitivity to hypertensive stimuli is consistent with a possible blunting of homeostatic responses normally aimed at raising blood pressure. To test this idea, we examined the consequences of long-term salt feeding and the associated hypertension on the interactive capacities of the renin angiotensin system (RAS) and adrenergic systems to elevate blood pressure in trout. Secretion of catecholamines in response to a range of doses of homologous ANG II in vivo and in situ (using a perfused posterior cardinal vein preparation) was reduced in the salt-fed fish. The reduced sensitivity to ANG II could not be explained by alterations in stored catecholamine (adrenaline or noradrenaline) levels or the general responsiveness of the chromaffin cells to depolarizing stimuli (60 mmol/l KCl). Despite the decreased responsiveness of the chromaffin cells to ANG II, plasma catecholamines were increased to a greater extent in the salt-fed fish during acute hypoxia (a condition that activates the RAS). Interestingly, the pressor effects of ANG II in vivo were actually heightened in the salt-fed fish. The increased pressor response to exogenous ANG II was likely attributable to its direct interaction with vascular ANG II receptors because the effect persisted even after blockade of α-adrenergic receptors. Treating fish with the vascular smooth muscle relaxant papaverine caused similar reductions in blood pressure and increases in plasma ANG II levels regardless of diet. Similarly, inhibition of angiotensin converting enzyme with lisinopril reduced blood pressure equally in control and salt-fed fish. These results indicate that, while long-term dietary salt loading blunts the response of trout chromaffin cells to ANG II, the RAS itself appears to be unaffected. Indeed, the capacity of ANG II to elevate blood pressure is not compromised nor do fish exhibit a reduced capacity to mount an acute humoral adrenergic stress response during acute hypoxia.     

Sex differences in angiotensin signaling in bulbospinal neurons in the rat rostral ventrolateral medulla

Sex differences may play a significant role in determining the risk of hypertension. Bulbospinal neurons in the rostral ventrolateral medulla (RVLM) are involved in the tonic regulation of arterial pressure and participate in the central mechanisms of hypertension. Angiotensin II (ANG II) acting on angiotensin type 1 (AT(1)) receptors in RVLM neurons is implicated in the development of hypertension by activating NADPH oxidase and producing reactive oxygen species (ROS). Therefore, we analyzed RVLM bulbospinal neurons to determine whether there are sex differences in: 1) immunolabeling for AT(1) receptors and the key NADPH oxidase subunit p47 using dual-label immunoelectron microscopy, and 2) the effects of ANG II on ROS production and Ca(2+) currents using, respectively, hydroethidine fluoromicrography and patch-clamping. In tyrosine hydroxylase-positive RVLM neurons, female rats displayed significantly more AT(1) receptor immunoreactivity and less p47 immunoreactivity than male rats (P < 0.05). Although ANG II (100 nM) induced comparable ROS production in dissociated RVLM bulbospinal neurons of female and male rats (P > 0.05), an effect mediated by AT(1) receptors and NADPH oxidase, it triggered significantly larger dihydropyridine-sensitive long-lasting (L-type) Ca(2+) currents in female RVLM neurons (P < 0.05). These observations suggest that an increase in AT(1) receptors in female RVLM neurons is counterbalanced by a reduction in p47 levels, such that ANG II-induced ROS production does not differ between females and males. Since the Ca(2+) current activator Bay K 8644 induced larger Ca(2+) currents in females than in male RVLM neurons, increased ANG II-induced L-type Ca(2+) currents in females may result from sex differences in calcium channel densities or dynamics.