1A-779 attenuates angiotensin-(1-7) depressor response in salt-induced hypertensive rats

Chronic infusion of angiotensin-(1-7) [Ang-(1-7)] lowers blood pressure in salt-induced and spontaneously hypertensive (SHR) rats. In the present study, we have examined the acute effect of Ang-(1-7) in salt-induced hypertension using Dahl salt-sensitive rats placed on low (0.3%) or high (8.0% NaCl) salt diets for 2 weeks. Rats fed a high salt diet showed a greater rise in BP than those fed a low salt diet. Ang-(1-7) (24 microg/kg) reduced mean arterial pressure (MAP), enhanced the release of prostacyclin and nitric oxide, and suppressed thromboxane A(2) levels. A-779 (48 microg/kg, i.v), a selective Ang-(1-7) antagonist, partially blocked these effects of Ang-(1-7). The Ang-(1-7)-induced depressor response observed in these animals was related to an increase in vasodilatory prostanoids, a decrease in the constrictor prostanoid thromboxane A(2), and an increase in nitric oxide levels in both plasma and isolated aortic rings.     

Prevention of angiotensin II-induced cardiac remodeling by angiotensin-(1-7)

Cardiac remodeling, which typically results from chronic hypertension or following an acute myocardial infarction, is a major risk factor for the development of heart failure and, ultimately, death. The renin-angiotensin system (RAS) has previously been established to play an important role in the progression of cardiac remodeling, and inhibition of a hyperactive RAS provides protection from cardiac remodeling and subsequent heart failure. Our previous studies have demonstrated that overexpression of angiotensin-converting enzyme 2 (ACE2) prevents cardiac remodeling and hypertrophy during chronic infusion of angiotensin II (ANG II). This, coupled with the knowledge that ACE2 is a key enzyme in the formation of ANG-(1-7), led us to hypothesize that chronic infusion of ANG-(1-7) would prevent cardiac remodeling induced by chronic infusion of ANG II. Infusion of ANG II into adult Sprague-Dawley rats resulted in significantly increased blood pressure, myocyte hypertrophy, and midmyocardial interstitial fibrosis. Coinfusion of ANG-(1-7) resulted in significant attenuations of myocyte hypertrophy and interstitial fibrosis, without significant effects on blood pressure. In a subgroup of animals also administered [d-Ala(7)]-ANG-(1-7) (A779), an antagonist to the reported receptor for ANG-(1-7), there was a tendency to attenuate the antiremodeling effects of ANG-(1-7). Chronic infusion of ANG II, with or without coinfusion of ANG-(1-7), had no effect on ANG II type 1 or type 2 receptor binding in cardiac tissue. Together, these findings indicate an antiremodeling role for ANG-(1-7) in cardiac tissue, which is not mediated through modulation of blood pressure or altered cardiac angiotensin receptor populations and may be at least partially mediated through an ANG-(1-7) receptor.     

Biological activities of angiotensin II-(1-6)-hexapeptide and angiotensin II-(1-7)-heptapeptide in man

Biological activities of angiotensin II-(1-6)-hexapeptide [ANG-(1-6)] and angiotensin II-(1-7)-heptapeptide [ANG-(1-7)] were studied in 5 normal men and 3 patients with Bartter's syndrome. The angiotensins were infused iv in each subject from 0900 h to 0915 h at a rate of 21 nmol(16.8 micrograms)/kg X min and 18 nmol(16.2 micrograms)/kg X min for ANG-(1-6) and ANG-(1-7), respectively. In the normal men a significant rise in blood pressure was observed by the infusions of both peptides. Average increments of blood pressure for ANG-(1-6) were 17/14, 23/18, 22/15 and 17/14 mmHg at 2, 5, 10 and 15 min, respectively, and those for ANG-(1-7) were 19/15, 20/17, 13/13 and 15/13 mmHg at 2, 5, 10 and 15 min, respectively. The duration of pressor actions after the cessation of the infusions (T) was 10 min for ANG-(1-6) and 20 (for systolic) and 30 (for diastolic) min for ANG-(1-7). T for ANG-(1-6) was shorter than and T for ANG-(1-7) was similar to T for Ile5-angiotensin II (Ile5-ANG II) reported previously in 7 normal men 5 of whom were the same as examined in the present study. On the other hand, both peptides did not cause a rise in blood pressure in the 3 patients with Bartter's syndrome. Both angiotensins did not cause an increase in plasma aldosterone but did cause a significant decrease in plasma renin activity both in the normal men and in the patients. From these results and our previous observations of inactivity of angiotensin II-(5-8)-tetrapeptide, a pressor action of angiotensin II-(4-8)-pentapeptide, and pressor, renin-suppressing and steroidogenic actions of angiotensin II-(3-8)-hexapeptide in normal men, it is thought that ANG-(1-6) and ANG-(1-7) are bound to angiotensin II (ANG II) receptor in the peripheral arterioles and show pressor actions (less than 0.024% and less than 0.028% of Ile5-ANG II, respectively) and suppress renin mainly via short loop feedback and that the shortest biologically active ANG II molecules for pressor, renin-suppressing and steroidogenic actions are Tyr-Ile-His, Val-Tyr-Ile-His and Val-Tyr-Ile-His-Pro-Phe, respectively, in man. It is also evident that ANG-(1-6) is more rapidly metabolized than ANG-(1-7) or Ile5-ANG II in man.     

Formation of angiotensin-(1-7) from angiotensin II by the venom of Conus geographus

The binding of [3H]angiotensin II to AT(1) receptors on Chinese Hamster Ovary cells expressing the human AT(1) receptor (CHO-AT(1) cells) is potently inhibited by venoms of the marine snails Conus geographus and C. betulinus. On the other hand, the binding of the nonpeptide AT(1) receptor-selective antagonist [3H]candesartan is not affected but competition binding curves of angiotensin II and the peptide antagonist [Sar(1),Ile(8)]angiotensin II (sarile) are shifted to the right. These effects resulted from the breakdown of angiotensin II into smaller fragments that do not bind to the AT(1) receptor. In this context, angiotensin-(1-7) is the most prominent fragment and angiotensin-(1-4) and angiotensin-(1-5) are also formed but to a lesser extent. The molecular weight of the involved peptidases exceeds 50 kDa, as determined by gel chromatography and ultrafitration.     

Angiotensin-(1-7) inhibits the angiotensin II-enhanced norepinephrine release in coarcted hypertensive rats

Since it has been suggested that angiotensin (Ang) (1-7) functions as an antihypertensive peptide, we studied its effect on the Ang II-enhanced norepinephrine (NE) release evoked by K+ in hypothalami isolated from aortic coarcted hypertensive (CH) rats. The endogenous NE stores were labeled by incubation of the tissues with 3H-NE during 30 min, and after 90 min of washing, they were incubated in Krebs solution containing 25 mM KCl in the absence or presence of the peptides. Ang-(1-7) not only diminished the K+-evoked NE release from hypothalami of CH rats, but also blocked the Ang II-enhanced NE release induced by K+. Ang-(1-7) blocking action on the Ang II response was prevented by [D-Ala7]Ang-(1-7), an Ang-(1-7) specific antagonist, by PD 123319, an AT2-receptor antagonist, and by Hoe 140, a B2 receptor antagonist. Ang-(1-7) inhibitory effect on the Ang II facilitatory effect on K+-stimulated NE release disappeared in the presence of Nomega-nitro-L-arginine methylester and was restored by L-arginine. Our present results suggest that Ang-(1-7) may contribute to blood pressure regulation by blocking Ang II actions on NE release at the central level. This inhibitory effect is a nitric oxide-mediated mechanism involving AT2 receptors and/or Ang-(1-7) specific receptors and local bradykinin generation.     

Angiotensin-(1-7) attenuates hypertension in exercise-trained renal hypertensive rats

Angiotensin-(1-7) [ANG-(1-7)] plays a counterregulatory role to angiotensin II in the renin-angiotensin system. In trained spontaneous hypertensive rats, Mas expression and protein are upregulated in ventricular tissue. Therefore, we examined the role of ANG-(1-7) on cardiac hemodynamics, cardiac functions, and cardiac remodeling in trained two-kidney one-clip hypertensive (2K1C) rats. For this purpose, rats were divided into sedentary and trained groups. Each group consists of sham and 2K1C rats with and without ANG-(1-7) infusion. Swimming training was performed for 1 h/day, 5 days/wk for 4 wk following 1 wk of swimming training for acclimatization. 2K1C rats showed moderate hypertension and left ventricular hypertrophy without changing left ventricular function. Chronic infusion of ANG-(1-7) attenuated hypertension and cardiac hypertrophy only in trained 2K1C rats but not in sedentary 2K1C rats. Chronic ANG-(1-7) treatment significantly attenuated increases in myocyte diameter and cardiac fibrosis induced by hypertension in only trained 2K1C rats. The Mas receptor, ANG II type 2 receptor protein, and endothelial nitric oxide synthase phosphorylation in ventricles were upregulated in trained 2K1C rats. In conclusion, chronic infusion of ANG-(1-7) attenuates hypertension in trained 2K1C rats.     

Chronic infusion of angiotensin-(1-7) reduces heart angiotensin II levels in rats

We tested the hypothesis that the actions of Angiotensin (Ang)-(1-7) in the heart could involve changes in tissue levels of Ang II. This possibility was addressed by determining the effect of chronic infusion of Ang-(1-7) on plasma and tissue angiotensins. Ang-(1-7) was infused subcutaneously (osmotic minipumps) in Wistar rats. Angiotensins were determined by radioimmunoassay (RIA) in plasma, heart, and kidney. Tissue and plasma angiotensin-converting enzyme (ACE) activity and plasma renin activity (PRA) were also measured. Cardiac and renal ACE2 mRNA levels and cardiac angiotensinogen mRNA levels were assessed by semi-quantitative polymerase chain reaction (PCR). AT1 receptor number was evaluated by autoradiograph. Chronic infusion of Ang-(1-7) (2 microg/h, 6 days) produced a marked decrease of Ang II levels in the heart. A less pronounced but significant decrease of Ang-(1-7) was also observed. No significant changes were observed for Ang I. Ang II was not altered in the kidney. In this tissue, a significant increase of Ang-(1-7) and Ang I concentration was observed. A significant increase of plasma Ang-(1-7) and Ang II was also observed. Ang-(1-7) infusion did not change ACE activity or PRA. A selective slight significant increase in ACE2 expression in the heart was observed. Heart angiotensinogen mRNA as well as the number of Ang II binding sites did not change. These results suggest that AT1 receptors-independent changes in heart Ang II concentration might contribute for the beneficial effects of Ang-(1-7) in the heart. Moreover, these results reinforce the hypothesis that this angiotensin plays an important site-specific role within the renin-angiotensin system.     

Angiotensin-(1-7) binds at the type 1 angiotensin II receptors in rat renal cortex.

Significant angiotensin (Ang) (1-7) production occurs in kidney and effects on renal function have been observed. The present study was undertaken to investigate binding characteristics of the heptapeptide to Ang II receptors present in rat renal cortex. [125I]-Ang II binding to rat glomeruli membranes was analyzed in the presence of increasing concentrations of Ang II, Ang-(1-7), DUP 753 and PD 123319. Linearity of the Scatchard plot of the [125I]-Ang II specific binding to rat glomeruli membranes indicated a single population of receptors, with a Kd value of 0.7 +/- 0.1 nM and a Bmax of 198 +/- 0.04 fmol/mg protein. DUP 753, an specific AT1 receptor antagonist, totally displaced the specific binding of [125I]-radiolabelled hormone with a Ki of 15.8 +/- 0.9 nM, while no changes were observed in the presence of the selective AT2 receptor antagonist, PD 123319. The specific [125I]-Ang II binding to rat glomerular membranes was displaced by Ang-(1-7) with high affinity (Ki = 8.0 +/- 3.2 nM). We conclude that radioligand binding assays in the presence of selective Ang II antagonists DUP 753 and PD 123319 suggest the unique presence of AT1, receptors in rat glomeruli and a possible role in the control of the biological renal effects of Ang-(1-7).     

Gender differences in vascular reactivity to endothelin-1 (1-31) in mesenteric arteries from diabetic mice

Endothelin-1 (1-31) [ET-1 (1-31)], a novel member of the ET family, comprises 31 amino acids and is derived from the selective hydrolysis of big ET-1 by chymase. Although ET-1 (1-31) reportedly exerts biological effects by direct or indirect [via its conversion to ET-1 (1-21)] mechanisms, it is unclear whether in diabetes the vascular effects of ET-1 (1-31) display gender differences. We investigated this question by exposing mesenteric artery rings to ET-1 (1-31), using arteries from mice in the early or chronic phase of diabetes. In the early stage of diabetes, the ET-1 (1-31)-induced contraction was similar between age- and sex-matched control and streptozotocin (STZ)-induced diabetic mice. In the chronic stage of diabetes, the ET-1 (1-31)-induced contraction was enhanced in diabetic female mice, but not in diabetic male mice (vs. both age-matched control and early-stage diabetic mice). This enhancement was largely prevented by Y27632 (Rho kinase inhibitor), PD98059 [inhibitor of extracellular signal related kinases 1 and 2 (ERK1/2)], or SP600125 [C-jun terminal kinase (JNK) inhibitor]. These data indicate that the ET-1 (1-31)-induced vasoconstriction in the mesenteric artery may be specifically enhanced in established diabetic female mice, and that this enhancement may be due to alterations in the activities of Rho/Rho kinase or mitogen-activated protein kinase.     

Chronic angiotensin-(1-7) prevents cardiac fibrosis in DOCA-salt model of hypertension

Cardiac remodeling is a hallmark hypertension-induced pathophysiology. In the current study, the role of the angiotensin-(1-7) fragment in modulating cardiac remodeling was examined. Sprague-Dawley rats underwent uninephrectomy surgery and were implanted with a deoxycorticosterone acetate (DOCA) pellet. DOCA animals had their drinking water replaced with 0.9% saline solution. A subgroup of DOCA-salt animals was implanted with osmotic minipumps, which delivered angiotensin-(1-7) chronically (100 ng.kg(-1).min(-1)). Control animals underwent sham surgery and were maintained on normal drinking water. Blood pressure was measured weekly with the use of the tail-cuff method, and after 4 wk of treatment, blood pressure responses to graded doses of angiotensin II were determined by direct carotid artery cannulation. Ventricle size was measured, and cross sections of the heart ventricles were paraffin embedded and stained using Masson's Trichrome to measure interstitial and perivascular collagen deposition and myocyte diameter. DOCA-salt treatment caused significant increases in blood pressure, cardiac hypertrophy, and myocardial and perivascular fibrosis. Angiotensin-(1-7) infusion prevented the collagen deposition effects without any effect on blood pressure or cardiac hypertrophy. These results indicate that angiotensin-(1-7) selectively prevents cardiac fibrosis independent of blood pressure or cardiac hypertrophy in the DOCA-salt model of hypertension.     

Effects of angiotensin (1-7) upon right ventricular function in experimental rat pulmonary embolism

Right ventricular (RV) dysfunction contributes to poor clinical prognosis after pulmonary embolism (PE). The present studies evaluate the effects of angiotensin (1-7) (ANG (1-7)) upon RV function during experimental PE in rats. Circulating ANG II increased 8-fold 6 hr after PE (47±13 PE vs. 6±3 pg/mL, control, p<0.05). ACE2 protein was uniformly localized in the RV myocardium of control rats, but showed a patchy distribution with some cells devoid of stain after 6 or 18 hr of PE. RV function decreased 18 hr after PE compared with control treated animals (19±4 vs. 41±1 mmHg, respectively, p<0.05; 669±98 vs. 1354±77 mmHg/sec, respectively, p<0.05), while left ventricular function (LV) was not significantly changed. Animals treated with ANG (1-7) during PE showed improved RV +dP/dt and peak systolic pressure development to values not significantly different from control animals. Protection of RV function by ANG (1-7) was associated with improved arterial blood sO2, base excess and pH. Supplemental delivery of ANG (1-7) reduced the development of RV dysfunction, suggesting a novel approach to protecting RV function in the setting of acute experimental PE.     

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.     

Increased hypothalamic angiotensin-(1-7) levels in rats with aortic coarctation-induced hypertension

Since angiotensin (Ang) (1-7) injected into the brain blocked Ang II pressor actions in rats made hypertensive by aortic coarctation (CH), we examined systemic and tissue angiotensin peptide levels, specifically concentrating on the hypothalamic Ang-(1-7) levels. Plasma, heart and kidney isolated from CH rats showed increased levels of Ang I, Ang II and Ang-(1-7) compared with the normotensive group, with Ang II being the predominant peptide in heart and kidney. In the hypothalamus, equimolar amounts of Ang II and Ang-(1-7) were found in the sham group, whereas only Ang-(1-7) levels increased in CH rats. We conclude that aortic coarctation activates systemic and tissue renin-angiotensin system. The increased central levels of Ang-(1-7) in the CH rats suggest a potential mitigating role of this peptide in central control of the hypertensive process.     

Angiotensin-(1-7) regulates the levels of angiotensin II receptor subtype AT1 mRNA differentially in a strain-specific fashion

Ang-(1-7) is an effector peptide of the renin-angiotensin system with several distinct actions that are likely mediated by a specific receptor. Regulatory effects of angiotensin (Ang) peptides, Ang-(1-7) and Ang II, on Ang receptor subtype 1 (AT1) mRNA expression were investigated in vascular smooth muscle cells (VSMC) from four University of Akron (Akr) rat strains (WKY, SHR and two backcross consomic lines SHR/y and SHR/a), and in SHR and WKY cells from Charles River Laboratories (Crl). In WKY/Akr and SHR/Akr, Ang-(1-7) treatment increased the levels of AT1 mRNA. This effect was inhibited by the specific Ang-(1-7) antagonist, A-779, in WKY/Akr but not SHR/Akr. Ang II had no effect in Akr cells, but it down-regulated AT1 mRNA in WKY/Crl and SHR/Crl VSMC. Ang-(1-7) did not affect AT1 mRNA levels in Crl lines. In conclusion, Ang-(1-7) regulates the AT1 receptor either directly or indirectly in a strain-specific fashion. The Ang-(1-7) antagonist, A-779, blocks the actions of Ang-(1-7) only in VSMC from WKY/Akr rats, suggesting either that the binding sites for Ang-(1-7) have different properties in SHR/Akr and WKY/Akr cell lines, or that some of the effects of Ang-(1-7) are not receptor mediated. Further, we found differences between Akr cells and Crl cells that are consistent with their genetic heterogeneity.     

Identification of angiotensin-(1-7) in rat brain. Evidence for differential processing of angiotensin peptides

Tissue and plasma forms of angiotensin (Ang) peptides were characterized by reverse-phase high performance liquid chromatography and three specific radioimmunoassays. This method allowed resolution of 10 Ang peptides and revealed distinctive distributions for the three principal Ang peptides in the brain, adrenal gland, and plasma. In extracts from the rat hypothalamus, approximately equimolar amounts of Ang-(1-7), Ang-II, and Ang-I were detected (1.10, 1.18, and 1.45 pmol/g of tissue, respectively). A similar profile was observed in the medulla oblongata and amygdala, although the content of these three peptides was 40-70% less than that seen in the hypothalamus. In the adrenal gland, the predominant peptide was Ang-II (1.07 pmol/g); levels of Ang-(1-7) (0.19 pmol/g) and Ang-I (0.14 pmol/g) were approximately 20% that of Ang-II. In plasma, the major angiotensin was Ang-I (0.13 pmol/ml), with lower levels of Ang-(1-7) and Ang-II (0.01-0.02 pmol/ml). This study is the first demonstration of the endogenous presence of Ang-(1-7) in central and peripheral tissues of the rat. Moreover, the data suggest tissue-specific processing of angiotensins, with Ang-(1-7) being a predominant Ang peptide in the central nervous system. In light of the recent biological properties described for this peptide, Ang-(1-7) may represent an active member of Ang peptides in the brain.     

Sex differences in the development of angiotensin II-induced hypertension in conscious mice

Sex has an important influence on blood pressure (BP) regulation. There is increasing evidence that sex hormones interfere with the renin-angiotensin system. Thus the purpose of this study was to determine whether there are sex differences in the development of ANG II-induced hypertension in conscious male and female mice. We used telemetry implants to measure aortic BP and heart rate (HR) in conscious, freely moving animals. ANG II (800 ng.kg(-1).min(-1)) was delivered via an osmotic pump implanted subcutaneously. Our results showed baseline BP in male and female mice to be similar. Chronic systemic infusion of ANG II induced a greater increase in BP in male (35.1 +/- 5.7 mmHg) than in female mice (7.2 +/- 2.0 mmHg). Gonadectomy attenuated ANG II-induced hypertension in male mice (15.2 +/- 2.4 mmHg) and augmented it in female mice (23.1 +/- 1.0 mmHg). Baseline HR was significantly higher in females relative to males (630.1 +/- 7.9 vs. 544.8 +/- 16.2 beats/min). In females, ANG II infusion significantly decreased HR. However, the increase in BP with ANG II did not result in the expected decrease in HR in either intact male or gonadectomized mice. Moreover, the slope of the baroreflex bradycardia to phenylephrine was blunted in males (-5.6 +/- 0.3 to -2.9 +/- 0.5) but not in females (-6.5 +/- 0.5 to -5.6 +/- 0.3) during infusion of ANG II, suggesting that, in male mice, infusion of ANG II results in a resetting of the baroreflex control of HR. Ganglionic blockade resulted in greater reduction in BP on day 7 after ANG II infusion in males compared with females (-61.0 +/- 8.9 vs. -36.6 +/- 6.6 mmHg), suggesting an increased contribution of sympathetic nerve activity in arterial BP maintenance in male mice. Together, these data indicate that there are sex differences in the development of chronic ANG II-induced hypertension in conscious mice and that females may be protected from the increases in BP induced by ANG II.     

Stimulation of ANP by angiotensin-(1-9) via the angiotensin type 2 receptor

Aims

Angiotensin-(1-9) [Ang-(1-9)] and Ang-(1-7) are cleaved by Ang converting enzyme 2 forming Ang I and Ang II, respectively, and the truncated Angs play a role in regulating atrial natriuretic peptide (ANP) secretion. Previously, we found that Ang-(1-7) stimulates ANP secretion via the Mas receptor. However, the effect of Ang-(1-9) on ANP secretion is still unknown. The aim of the present study is to determine whether Ang-(1-9) stimulates ANP secretion and to characterize the signaling pathway involved in stimulating secretion.

Main methods

We examined the effects of Ang-(1-9) on ANP secretion and atrial contractility with and without inhibitors in isolated perfused atria.

Key findings

Ang-(1-9) stimulated ANP secretion and concentration without change in atrial contractility. Ang-(1-9)-induced-ANP secretion was increased from 5% to 60% by 3 μM Ang-(1-9) during the low-stretch state of the atrium. This stimulatory effect of Ang-(1-9) on ANP secretion was attenuated by pretreatment with an Ang II type 2 receptor (AT₂R) antagonist but not by AT₁R or Mas receptor antagonist. In addition, pretreatment with inhibitors of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) blocked Ang-(1-9)-induced ANP secretion. In the high-stretch atrial state, Ang-(1-9)-induced ANP secretion was increased more than in the low-stretch state following addition of 1 μM Ang-(1-9) (from 108% to 170%). In an in vivo experiment, acute infusion of Ang-(1-9) increased plasma ANP level without altering arterial blood pressure. This effect was attenuated by pretreatment with AT₂R antagonist but not by Mas receptor antagonist.

Significance

These results suggest that Ang-(1-9) stimulates ANP secretion via the AT₂R-PI3K-Akt-NO-cGMP pathway.     

Prevention of Dahl salt-induced hypertension by chronic dietary tryptophan

Four-week-old inbred Dahl salt-sensitive (DS/JR) and Dahl salt-resistant (DR/JR) rats were placed on an 8% salt diet with or without a supplemental 2.5% tryptophan (Trp). Blood pressures were monitored for the next 5 weeks. Urine volumes and ion concentrations were measured during the 6th week. Blood pressures of DS/JR rats on control diets elevated rapidly and markedly, whereas pressures of DS/JR rats on the Trp-supplemented diet were not significantly elevated over those of DR/JR rats. Pressures of DR/JR rats were unaffected by Trp supplementation. Urinary sodium was significantly greater in DR/JR rats compared with DS/JR rats and was unaffected by Trp supplementation. This suggests that the antihypertensive effect of Trp was not at the level of the kidney. We conclude that dietary Trp blocks the development of hypertension in DS/JR rats maintained on a high salt diet.     

The effects of salt-induced hypertension on alpha1-adrenoreceptor expression and cardiovascular physiology in the rainbow trout (Oncorhynchus mykiss)

Experiments were conducted on rainbow trout to determine the impact of dietary salt on arterial blood pressure. After 4-6 wk, fish fed a salt-enriched diet exhibited a 37% elevation of dorsal aortic pressure (from 23.8 +/- 1.2 to 32.6 +/- 1.4 mmHg) and an 18% increase in ventral aortic pressure (from 33.0 +/- 1.5 to 38.9 +/- 1.3 mmHg). The hypertension presumably reflected the increase in cardiac output (from 31.0 +/- 0.8 to 36.4 +/- 2.2 ml.min(-1).kg(-1)) because systemic and branchial resistances were statistically unaltered by salt feeding. The chronic hypertension was associated with a decrease in the pressor responses of the systemic vasculature to catecholamines and hypercapnia in the salt-fed fish. The reduction in alpha-adrenergic responsiveness of the systemic vasculature is consistent with desensitization or loss of functional alpha-adrenoceptors (alpha-ARs). In support of this idea, the salt-fed fish exhibited significantly decreased levels of alpha(1D)-AR mRNA in the dorsal aorta and the afferent (ABA) and efferent branchial arteries (EBA). In contrast, however, the results obtained from norepinephrine dose-response curves for EBA and ABA vascular rings in vitro did not provide evidence for loss of function of branchial artery alpha(1)-ARs in the salt-fed fish. Indeed, the EC(50) for the EBA norepinephrine dose-response curve was significantly reduced (from 3.75 x 10(-7) to 2.12 x 10(-7) M) in the salt-fed fish, indicating an increase in the binding affinity of the alpha(1)-ARs.     

Angiotensin-(1-7) potentiates responses to bradykinin but does not change responses to angiotensin I

Angiotensin-(1-7) (Ang-(1-7)), a bioactive peptide in the renin-angiotensin system, has counterregulatory actions to angiotensin II (Ang II). However, the mechanism by which Ang-(1-7) enhances vasodepressor responses to bradykinin (BK) is not well understood. In the present study, the effects of Ang-(1-7) on responses to BK, BK analogs, angiotensin I (Ang I), and Ang II were investigated in the anesthetized rat. The infusion of Ang-(1-7) (55 pmol/min i.v.) enhanced decreases in systemic arterial pressure in response to i.v. injections of BK and the BK analogs [Hyp3, Tyr(Me)8]-bradykinin (HT-BK) and [Phe8psi (CH2-NH) Arg9]-bradykinin (PA-BK) without altering pressor responses to Ang I or II, or depressor responses to acetylcholine and sodium nitroprusside. The angiotensin-converting enzyme (ACE) inhibitor enalaprilat enhanced responses to BK and the BK analog HT-BK without altering responses to PA-BK and inhibited responses to Ang I. The potentiating effects of Ang-(1-7) and enalaprilat on responses to BK were not attenuated by the Ang-(1-7) receptor antagonist A-779. Ang-(1-7)- and ACE inhibitor-potentiated responses to BK were attenuated by the BK B2 receptor antagonist Hoe 140. The cyclooxygenase inhibitor sodium meclofenamate had no significant effect on responses to BK or Ang-(1-7)-potentiated BK responses. These results suggest that Ang-(1-7) potentiates responses to BK by a selective B2 receptor mechanism that is independent of an effect on Ang-(1-7) receptors, ACE, or cyclooxygenase product formation. These data suggest that ACE inhibitor-potentiated responses to BK are not mediated by an A-779-sensitive mechanism and are consistent with the hypothesis that enalaprilat-induced BK potentiation is due to decreased BK inactivation.