Effects of angiotensin-(1-7) and other bioactive components of the renin-angiotensin system on vascular resistance and noradrenaline release in rat kidney

Modelling hemodynamics regulation in rats and dogs to facilitate drugs safety risk assessment

Pharmaceutical companies routinely screen compounds for hemodynamics related safety risk. In vitro secondary pharmacology is initially used to prioritize compounds while in vivo studies are later used to quantify and translate risk to humans. This strategy has shown limitations but could be improved via the incorporation of molecular findings in the animal-based toxicological risk assessment. The aim of this study is to develop a mathematical model for rat and dog species that can integrate secondary pharmacology modulation and therefore facilitate the overall pre-clinical safety translation assessment. Following an extensive literature review, we built two separate models recapitulating known regulation processes in dogs and rats. We describe the resulting models and show that they can reproduce a variety of interventions in both species. We also show that the models can incorporate the mechanisms of action of a pre-defined list of 50 pharmacological mechanisms whose modulation predict results consistent with known pharmacology. In conclusion, a mechanistic model of hemodynamics regulations in rat and dog species has been developed to support mechanism-based safety translation in drug discovery and development.

α2A-Adrenoceptors Modulate Renal Sympathetic Neurotransmission and Protect against Hypertensive Kidney Disease

BACKGROUND

Increased nerve activity causes hypertension and kidney disease. Recent studies suggest that renal denervation reduces BP in patients with hypertension. Renal NE release is regulated by prejunctional α2A-adrenoceptors on sympathetic nerves, and α2A-adrenoceptors act as autoreceptors by binding endogenous NE to inhibit its own release. However, the role of α2A-adrenoceptors in the pathogenesis of hypertensive kidney disease is unknown.

METHODS

We investigated effects of α2A-adrenoceptor-regulated renal NE release on the development of angiotensin II-dependent hypertension and kidney disease. In uninephrectomized wild-type and α2A-adrenoceptor-knockout mice, we induced hypertensive kidney disease by infusing AngII for 28 days.

RESULTS

Urinary NE excretion and BP did not differ between normotensive α2A-adrenoceptor-knockout mice and wild-type mice at baseline. However, NE excretion increased during AngII treatment, with the knockout mice displaying NE levels that were significantly higher than those of wild-type mice. Accordingly, the α2A-adrenoceptor-knockout mice exhibited a systolic BP increase, which was about 40 mm Hg higher than that found in wild-type mice, and more extensive kidney damage. In isolated kidneys, AngII-enhanced renal nerve stimulation induced NE release and pressor responses to a greater extent in kidneys from α2A-adrenoceptor-knockout mice. Activation of specific sodium transporters accompanied the exaggerated hypertensive BP response in α2A-adrenoceptor-deficient kidneys. These effects depend on renal nerves, as demonstrated by reduced severity of AngII-mediated hypertension and improved kidney function observed in α2A-adrenoceptor-knockout mice after renal denervation.

CONCLUSIONS

Our findings reveal a protective role of prejunctional inhibitory α2A-adrenoceptors in pathophysiologic conditions with an activated renin-angiotensin system, such as hypertensive kidney disease, and support the concept of sympatholytic therapy as a treatment.

Modulating Role of Ang1-7 in Control of Blood Pressure and Renal Function in AngII-infused Hypertensive Rats

BACKGROUND

Indirect evidence suggests that angiotensin 1-7 (Ang1-7) may counterbalance prohypertensive actions of angiotensin II (AngII), via activation of vascular and/or renal tubular receptors to cause vasodilation and natriuresis/diuresis. We examined if Ang1-7 would attenuate the development of hypertension, renal vasoconstriction, and decreased natriuresis in AngII-infused rats and evaluated the mechanisms involved.

METHODS

AngII, alone or with Ang1-7, was infused to conscious Sprague-Dawley rats for 13 days and systolic blood pressure (SBP) and renal excretion were repeatedly determined. In anesthetized rats, acute actions of Ang1-7 and effects of blockade of angiotensin AT1 or Mas receptors (candesartan or A-779) were studied.

RESULTS

Chronic AngII infusion increased SBP from 143 ± 4 to 195 ± 6 mm Hg. With Ang1-7 co-infused, SBP increased from 133 ± 5 to 161 ± 5 mm Hg (increase reduced, P < 0.002); concurrent increases in urine flow (V) and sodium excretion (UNaV) were greater. In anesthetized normotensive or AngII-induced hypertensive rats, Ang1-7 infusion transiently increased mean arterial pressure (MABP), transiently decreased renal blood flow (RBF), and caused increases in UNaV and V. In normotensive rats, candesartan prevented the Ang1-7-induced increases in MABP and UNaV and the decrease in RBF. In anesthetized normotensive, rats intravenous A-779 increased MABP (114 ± 5 to 120 ± 5 mm Hg, P < 0.03) and urine flow. Surprisingly, these changes were not observed with A-779 applied during background Ang1-7 infusion.

CONCLUSIONS

The results suggest that in AngII-dependent hypertension, Ang1-7 deficit contributes to sodium and fluid retention and thereby to BP elevation; a correction by Ang1-7 infusion seems mediated by AT1 and not Mas receptors.

The Role of Mas Receptor on Renal Hemodynamic Responses to Angiotensin 1-7 in Both Irreversible and Reversible Unilateral Ureteral Obstruction Rats

Background:

Unilateral ureteral obstruction (UUO) alters the expression of renin-angiotensin system (RAS) components and angiotensin 1-7 (Ang 1-7) as a main arm of RAS is affected by UUO. The role of Mas receptor antagonist (A779) was examined in renal hemodynamic responses to Ang 1-7 in 3-day UUO and UUO removal (RUUO) in rats.

Materials and Methods:

Forty-five male Wistar rats were randomly divided into three groups of sham operated, UUO, and RUUO, while each group was divided into two subgroups treated with vehicle or A779. Renal blood flow (RBF) and renal vascular resistance (RVR) responses to graded Ang 1-7 infusion were measured at controlled renal perfusion pressure.

Results:

Mean arterial pressure response to Ang 1-7 was increased in vehicle-treated subgroup significantly (P < 0.05) when compared with A779-treated subgroup. However, such observation was not seen in UUO and RUUO rats. The graded Ang 1-7 infusion increased RBF and decreased RVR significantly in vehicle-treated rats (P < 0.005). Furthermore, a significant difference was found between vehicle and A779-treated subgroups in sham, UUO, and RUUO groups (P < 0.005).

Conclusion:

Ang 1-7 could alter the kidney hemodynamics responses in ureteral obstruction models.

The ACE2/Angiotensin-(1-7)/MAS Axis of the Renin-Angiotensin System: Focus on Angiotensin-(1-7)

The renin-angiotensin system (RAS) is a key player in the control of the cardiovascular system and hydroelectrolyte balance, with an influence on organs and functions throughout the body. The classical view of this system saw it as a sequence of many enzymatic steps that culminate in the production of a single biologically active metabolite, the octapeptide angiotensin (ANG) II, by the angiotensin converting enzyme (ACE). The past two decades have revealed new functions for some of the intermediate products, beyond their roles as substrates along the classical route. They may be processed in alternative ways by enzymes such as the ACE homolog ACE2. One effect is to establish a second axis through ACE2/ANG-(1-7)/MAS, whose end point is the metabolite ANG-(1-7). ACE2 and other enzymes can form ANG-(1-7) directly or indirectly from either the decapeptide ANG I or from ANG II. In many cases, this second axis appears to counteract or modulate the effects of the classical axis. ANG-(1-7) itself acts on the receptor MAS to influence a range of mechanisms in the heart, kidney, brain, and other tissues. This review highlights the current knowledge about the roles of ANG-(1-7) in physiology and disease, with particular emphasis on the brain.

Macrophages in neuroinflammation: role of the renin-angiotensin-system

Macrophages are essential players of the innate immune system which are involved in the initiation and progression of various inflammatory and autoimmune diseases including neuroinflammation. In the past few years, it has become increasingly clear that the regulation of macrophage responses by the local tissue milieu is also influenced by mediators which were first discovered as regulators in the nervous or also cardiovascular system. Here, the renin-angiotensin system (RAS) is a major focus of current research. Besides its classical role in blood pressure control, body fluid, and electrolyte homeostasis, the RAS may influence (auto)immune responses, modulate T cells, and particularly act on macrophages via different signaling pathways. Activation of classical RAS pathways including angiotensin (Ang) II and AngII type 1 (AT₁R) receptors may drive pro-inflammatory macrophage responses in neuroinflammation via regulation of chemokines. More recently, alternative RAS pathways were described, such as binding of Ang-(1-7) to its receptor Mas. Signaling via Mas pathways may counteract some of the AngII/AT₁R-mediated effects. In macrophages, the Ang-(1-7)/Mas exerts beneficial effects on neuroinflammation via modulating macrophage polarization, migration, and T cell activation in vitro and in vivo. These data delineate a pivotal role of the RAS in inflammation of the nervous system and identify RAS modulation as a potential new target for immunotherapy with a special focus on macrophages.

Angiotensin-(1–7) Modulates Renal Vascular Resistance Through Inhibition of p38 Mitogen-Activated Protein Kinase in Apolipoprotein E–Deficient Mice

Apolipoprotein E–deficient (apoE(−/−)) mice fed on Western diet are characterized by increased vascular resistance and atherosclerosis. Previously, we have shown that chronic angiotensin (Ang)-(1–7) treatment ameliorates endothelial dysfunction in apoE(−/−) mice. However, the mechanism of Ang-(1–7) on vasoconstrictor response to Ang II is unknown. To examine Ang-(1–7) function, we used apoE(−/−) and wild-type mice fed on Western diet that were treated via osmotic minipumps either with Ang-(1–7) (82 μg/kg per hour) or saline for 6 weeks. We show that Ang II–induced renal pressor response was significantly increased in apoE(−/−) compared with wild-type mice. This apoE(−/−)-specific response is attributed to reactive oxygen species–mediated p38 mitogen–activated protein kinase activation and subsequent phosphorylation of myosin light chain (MLC 20 ), causing renal vasoconstriction. Here, we provide evidence that chronic Ang-(1–7) treatment attenuated the renal pressor response to Ang II in apoE(−/−) mice to wild-type levels. Ang-(1–7) treatment significantly decreased renal inducible nicotinamide adenine dinucleotide phosphate subunit p47phox levels and, thus, reactive oxygen species production that in turn causes decreased p38 mitogen-activated protein kinase activity. The latter has been confirmed by administration of a specific p38 mitogen-activated protein kinase inhibitor SB203580 (5 μmol/L), causing a reduced renal pressor response to Ang II in apoE(−/−) but not in apoE(−/−) mice treated with Ang-(1–7). Moreover, Ang-(1–7) treatment had no effect in Mas(−/−)/apoE(−/−) double-knockout mice confirming the specificity of Ang-(1–7) action through the Mas-receptor. In summary, Ang-(1–7) modulates vascular function via Mas-receptor activation that attenuates pressor response to Ang II in apoE(−/−) mice by reducing reactive oxygen species–mediated p38 mitogen-activated protein kinase activity.

Antenatal betamethasone exposure alters renal responses to angiotensin-(1-7) in uninephrectomized adult male sheep

Antenatal corticosteroid exposure reduces renal function and alters the intrarenal renin-angiotensin system to favor angiotensin activation of angiotensin type 1 receptor (AT1R) mediated responses in ovine offspring. This study aimed to assess whether antenatal steroid exposure would affect renal responses to the direct intrarenal infusion of angiotensin-(1-7) in rams and the angiotensin receptors involved in mediating responses to the peptide. Adult, uninephrectomized rams exposed to either betamethasone or vehicle before birth received intrarenal angiotensin-(1-7) infusions (1 ng/kg/min) alone or in combination with antagonists to angiotensin receptors for 3 h. Basal sodium excretion (UNa) was significantly lower and mean arterial pressure was significantly higher in betamethasone- compared to the vehicle-treated sheep. Angiotensin-(1-7) decreased UNa more in betamethasone- than in vehicle-treated sheep. Candesartan reversed the response to angiotensin-(1-7) but D-Ala(7)-angiotensin-(1-7) did not. Angiotensin-(1-7) infusion decreased effective renal plasma flow in both groups to a similar extent and the response was reversed by candesartan, but was not blocked by D-Ala(7)-angiotensin-(1-7). Glomerular filtration rate increased significantly in both groups after 3 h infusion of angiotensin-(1-7) plus candesartan. These results suggest that antenatal exposure to a clinically relevant dose of betamethasone impairs renal function in rams. Moreover, angiotensin-(1-7) appears capable of activating the AT1R in uninephrectomized rams.

Amelioration of renal injury and oxidative stress by the nNOS inhibitor L-VNIO in the salt-sensitive mRen2.Lewis congenic rat

Salt sensitivity is a key risk factor for cardiovascular disease and renal injury. Alterations in renal nitric oxide may contribute to salt-dependent increases in blood pressure and tissue damage. Therefore, we assessed the expression of nitric oxide synthase (NOS) isoforms in the kidney and the effects of nNOS inhibition on renal injury, inflammation, and oxidative stress in the female mRen2.Lewis rat (mRen), a model of salt-sensitive hypertension. We find that a high-salt diet (4% sodium) significantly reduced endothelial NOS mRNA (2.6-fold) and protein (1.5-fold) but increased nNOS mRNA (2.4-fold) and protein (1.9-fold) in the renal cortex of these animals. Immunostaining for nNOS also seemed higher in macula densa and cortical tubules of the rats fed a high-salt diet. Circulating nitrate and nitrite levels were reduced, including the tissue levels of the NOS cofactor tetrahydrobiopterin. Cortical markers of oxidative stress (4HNE, 8-OH-deoxyguanosine) and fibrosis were increased; however, mRNA levels of the NAD(P)H oxidase components NOX4, p22phox, and p47phox were reduced. Chronic treatment with the nNOS inhibitor N-(1-Imino-3-butenyl)-L-ornithine did not influence systolic blood pressure after 4 weeks but significantly attenuated albuminuria, renal fibrosis, inflammation, and indices of oxidative stress. We conclude that an increase in nNOS expression in conjunction with reduced levels of cortical tetrahydrobiopterin may stimulate oxidative stress and renal injury in the salt-sensitive female mRen2.Lewis rat.

Chronic treatment with angiotensin-(1-7) improves renal endothelial dysfunction in apolipoproteinE-deficient mice

BACKGROUND AND PURPOSE

ApolipoproteinE-deficient [apoE (-/-)] mice, a model of human atherosclerosis, develop endothelial dysfunction caused by decreased levels of nitric oxide (NO). The endogenous peptide, angiotensin-(1-7) [Ang-(1-7)], acting through its specific GPCR, the Mas receptor, has endothelium-dependent vasodilator properties. Here we have investigated if chronic treatment with Ang-(1-7) improved endothelial dysfunction in apoE (-/-) mice.

EXPERIMENTAL APPROACH

ApoE (-/-) mice fed on a lipid-rich Western diet were divided into three groups and treated via osmotic minipumps with either saline, Ang-(1-7) (82 µg·kg(-1) ·h(-1) ) or the same dose of Ang-(1-7) together with D-Ala-Ang-(1-7) (125 µg·kg(-1) ·h(-1) ) for 6 weeks. Renal vascular function was assessed in isolated perfused kidneys.

KEY RESULTS

Ang-(1-7)-treated apoE (-/-) mice showed improved renal endothelium-dependent vasorelaxation induced by carbachol and increased renal basal cGMP production, compared with untreated apoE (-/-) mice. Tempol, a reactive oxygen species (ROS) scavenger, improved endothelium-dependent vasorelaxation in kidneys of saline-treated apoE (-/-) mice whereas no effect was observed in Ang-(1-7)-treated mice. Chronic treatment with D-Ala-Ang-(1-7), a specific Mas receptor antagonist, abolished the beneficial effects of Ang-(1-7) on endothelium-dependent vasorelaxation. Renal endothelium-independent vasorelaxation showed no differences between treated and untreated mice. ROS production and expression levels of the NAD(P)H oxidase subunits gp91phox and p47phox were reduced in isolated preglomerular arterioles of Ang-(1-7)-treated mice, compared with untreated mice, whereas eNOS expression was increased.

CONCLUSION AND IMPLICATIONS

Chronic infusion of Ang-(1-7) improved renal endothelial function via Mas receptors, in an experimental model of human cardiovascular disease, by increasing levels of endogenous NO.

Regulation of renal sympathetic neurotransmission by renal α(2A)-adrenoceptors is impaired in chronic renal failure

BACKGROUND AND PURPOSE

The mechanisms underlying increased renal noradrenaline in renal failure are still unclear. In this study, the role of α(2A)-adrenoceptors in controlling sympathetic neurotransmission in chronic renal failure was evaluated in a subtotal nephrectomy model. Also, the influence of this receptor subtype on angiotensin II (Ang II)-mediated noradrenaline release was evaluated.

EXPERIMENTAL APPROACH

α(2A)-adrenoceptor-knockout (KO) and wild-type (WT) mice underwent subtotal (5/6) nephrectomy (SNx) or SHAM-operation (SHAM). Kidneys of WT and KO mice were isolated and perfused. Renal nerves were stimulated with platinum electrodes and noradrenaline release was measured by HPLC.

KEY RESULTS

Noradrenaline release induced by renal nerve stimulation (RNS) was significantly increased in WT mice after SNx. RNS-induced noradrenaline release was significantly higher in SHAM-KO compared with SHAM-WT, but no further increase in noradrenaline release could be observed in SNx-KO. α-adrenoceptor antagonists increased RNS-induced noradrenaline release in SHAM-WT but not in SHAM-KO. After SNx, the effect of α₂-adrenoceptor blockade on renal noradrenaline release was attenuated in WT mice. The mRNA expression of α(2A)-adrenoceptors was not altered, but the inhibitory effect of α₂-adrenoceptor agonists on cAMP formation was abolished after SNx. Ang II facilitated RNS-induced noradrenaline release in SHAM-WT but not in SHAM-KO and SNx-WT.

CONCLUSION AND IMPLICATIONS

In our model of renal failure autoregulation of renal sympathetic neurotransmission was impaired. Presynaptic inhibition of noradrenaline release was diminished and the facilitatory effect of presynaptic angiotensin AT₁ receptors on noradrenaline release was markedly decreased in renal failure and depended on functioning α(2A)-adrenoceptors.

Association of interleukin-6-572G/C gene polymorphisms in the Cantonese population with intracranial aneurysms

Recent studies have demonstrated that cytokines play an important role in the pathogenesis of intracranial aneurysms (IAs). Interleukin-6 (IL-6) is an important proinflammatory cytokine. In our study, we investigated the association of genetic variants within the gene encoding interleukin-6-572G/C (IL-6-572G/C) with IAs in the Cantonese population. The IL-6-572G/C gene polymorphisms in 182 IA cases and 182 controls were analyzed using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Differences in genotype and allele frequencies between patients and controls were tested. There is significant difference of IL-6-572G/C genotype frequencies in IA group compared with control group (P=0.01), and significant difference of IL-6-572G/C allele frequency in IA group (11.54%) compared with control group (4.95%) (P=0.001). Polymorphisms within IL-6-572G/C gene are associated with IAs in the Cantonese population suggesting that the IL-6-572G/C gene is an important candidate gene for IAs.

Vasoprotective and atheroprotective effects of angiotensin (1-7) in apolipoprotein E-deficient mice

OBJECTIVE

To evaluate the effectiveness of long-term angiotensin (Ang) (1-7) treatment to inhibit the progression of atherosclerosis in apolipoprotein E-deficient (ApoE(-/-)) mice.

METHODS AND RESULTS

Ang (1-7) is a heptapeptide fragment that has been proposed to counterregulate the Ang II proatherogenic effects. The effect of long-term 4-week Ang (1-7) treatment on both inhibition of atherosclerotic lesion development and improvement of endothelial function was examined in apolipoprotein E(-/-) mice that had been fed an atherogenic high-fat (21%) diet for 16 weeks. Chronic Ang (1-7) treatment significantly improved endothelial function, an effect reversed with either angiotensin type 2 (AT(2)) or Mas receptor blockade. In these vessels, Ang (1-7) treatment significantly decreased superoxide production and increased endothelial nitric oxide synthase immunoreactivity when compared with vehicle treatment. These effects were blocked by both AT(2) and Mas receptor antagonists. Lesion development, assessed as both fatty deposits (oil red O) and intima to media ratio, was also significantly decreased with Ang (1-7) treatment compared with respective controls. Cotreatment with either AT(2) or Mas receptor antagonists reversed Ang (1-7)-mediated reduction in lesion development.

CONCLUSIONS

Long-term Ang (1-7) treatment caused both vasoprotection, via improvement in endothelial function, and atheroprotection, with a reduction in lesion progression in a model of atherosclerosis. These effects appear to be mediated by the restoration of nitric oxide bioavailability and involve a complex interaction of both Mas and AT(2) receptors.

Small but powerful: short peptide hormones and their role in autoimmune inflammation

In the recent years, it has become increasingly clear that the immune response is also influenced by mediators which were first discovered as regulators in the nervous or also cardiovascular system. Here, small peptide hormones may play an important role. Kinins like bradykinins act on the endothelium and play a role for trafficking of lymphocytes over the blood-brain barrier. Neuropeptides like vasoactive intestinal peptide or neuropeptide Y also directly act on T cells and favour the differentiation of Th2 cells or regulatory T cell populations. Recently, the renin-angiotensin system (RAS) came into the focus of interest. Inhibition of the RAS at different levels may influence autoimmune responses and involve T cells as well as antigen-presenting cells, probably via different signalling pathways. Inhibitors of angiotensin converting enzyme and antagonists of the angiotensin 1 receptors are used in the treatment of hypertension, kidney disease or stroke by millions of people worldwide. These inexpensive and safe pharmaceuticals may also represent an interesting and innovative approach for the (combination) treatment of autoimmune diseases like multiple sclerosis.

The importance of the intrarenal renin-angiotensin system

Evidence suggests that virtually every organ system in the human body possesses a local renin-angiotensin system (RAS). These local systems seem to be independently regulated and compartmentalized from the plasma circulation, perhaps with the exception of the vascular endothelial system, which is responsible for maintaining physiological plasma levels of RAS components. Among these local RASs, the kidney RAS--the focus of this Review--seems to be of critical importance for the regulation of blood pressure and salt balance. Indeed, overactivation of the intrarenal RAS in certain disease states constitutes a pathogenic mechanism that leads to tissue injury, proliferation, fibrosis and ultimately, end-organ damage. Intrarenal levels of angiotensin peptides are considerably higher than those in plasma or any other organ tissue. Moreover, the kidney has a unique capacity to degrade angiotensin peptides, perhaps to maintain its intrinsic homeostasis. Interestingly, each local RAS has a distinct enzymatic profile resulting in different patterns of angiotensin fragment generation in different tissues. A better understanding of the autocrine and paracrine mechanisms involved in the renal RAS and other local RASs might direct future organ-specific therapy.

Chronic angiotensin (1-7) injection accelerates STZ-induced diabetic renal injury

AIM

The renin-angiotensin system (RAS) plays a critical role in blood pressure control and body fluid and electrolyte homeostasis. In the past few years, angiotensin (Ang) (1-7) has been reported to counteract the effects of Ang II and was even considered as a new therapeutical target in RAS. The present study aimed to investigate the effect of Ang (1-7) administration on a diabetic animal model and the modulation on local RAS.

METHODS

Streptozotocin (STZ) injection-induced diabetic rats were used in the experiment. The animals were divided into 3 groups: (1) control; (2) STZ-induced diabetes; and (3) STZ-induced diabetes with chronic Ang (1-7) treatment [D+Ang(1-7)]. In the D+Ang(1-7) group, a dose of 25 microg x kg(-1) x h(-1) of Ang (1-7) was continually injected through the jugular vein by embedding miniosmotic pump for 6 weeks. Plasma glucose, ratio of kidney to body weight, and 24 h urine protein and serum creatinine were monitored by conventional measurement. Plasma and renal Ang II levels were measured by radioimmunoassay. Ang-converting enzyme (ACE), ACE2, Ang II type 1 (AT1) receptor, Ang II type 2 (AT2) receptor, Ang (1-7) Mas receptor, and TGF- beta1 mRNA levels were measured by real time PCR; ACE, ACE2, and TGF- beta1 protein levels were analyzed by Western blotting.

RESULTS

The renal function of diabetic rats was significantly retrogressed when compared with that of control rats. After the treatment by constant Ang (1-7) vein injection for 6 weeks, renal function was found to be even worse than diabetic rats, and both TGF-beta1 mRNA and protein levels were elevated in the D+Ang(1-7) group compared with the diabetic rats. The real-time PCR result also showed an increase in ACE mRNA expression and decrease in ACE2 mRNA level in the D+Ang(1-7) group when compared with diabetic rats. The number of AT1 receptors increased in the Ang (1-7)-injected group, while the number of AT2 and Mas receptors decreased.

CONCLUSION

Exogenous Ang (1-7) injection did not ameliorate STZinduced diabetic rat renal injury; on the contrary, it accelerated the progressive diabetic nephropathies.

Effects of angiotensin II and its metabolites in the rat coronary vascular bed: is angiotensin III the preferred ligand of the angiotensin AT2 receptor?

Aminopeptidases metabolize angiotensin II to angiotensin-(2-8) (=angiotensin III) and angiotensin-(3-8) (=angiotensin IV), and carboxypeptidases generate angiotensin-(1-7) from angiotensin I and II. Angiotensin-converting enzyme (ACE) inhibitors and/or angiotensin II type 1 (AT1) receptor blockers affect the concentrations of these metabolites, and they may thus contribute to the beneficial effects of these drugs, possibly through stimulation of non-classical angiotensin AT receptors. Here, we investigated the effects of angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) in the rat coronary vascular bed, with or without angiotensin AT1 - or angiotensin II type 2 (AT2) receptor blockade. Results were compared to those in rat iliac arteries and abdominal aortas. Angiotensin II, angiotensin III and angiotensin IV constricted coronary arteries via angiotensin AT1 receptor stimulation, angiotensin III and angiotensin IV being approximately 20- and approximately 8000-fold less potent than angiotensin II. The angiotensin AT2 receptor antagonist PD123319 greatly enhanced the constrictor effects of angiotensin III, starting at angiotensin III concentrations in the low nanomolar range. PD123319 enhanced the angiotensin II-induced constriction at submicromolar angiotensin II concentrations only. Angiotensin-(1-7) exerted no effects in the coronary circulation, although, at micromolar concentrations, it blocked angiotensin AT1 receptor-induced constriction. Angiotensin AT2 receptor-mediated relaxation did not occur in iliac arteries and abdominal aortas, and the constrictor effects of the angiotensin metabolites in these vessels were identical to those in the coronary vascular bed. In conclusion, angiotensin AT2 receptor activation in the rat coronary vascular bed results in vasodilation, and angiotensin III rather than angiotensin II is the preferred endogenous agonist of these receptors. Angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) do not exert effects through non-classical angiotensin AT receptors in the rat coronary vascular bed, iliac artery or aorta.

Sex and the renin-angiotensin system: inequality between the sexes in response to RAS stimulation and inhibition

The purpose of this review is to examine sex differences in response to stimulation and inhibition of the renin-angiotensin system (RAS). The RAS plays a prominent role in the development of chronic renal disease, and there are known sex differences not only in the expression level of components of the RAS but also in how males and females respond to perturbations of the RAS. In men, renal injury increases in parallel with increased activation of the RAS, while in women, increases in ANG II do not necessarily translate into increases in renal injury. Moreover, both epidemiological and experimental studies have noted sex differences in the therapeutic benefits following angiotensin-converting enzyme inhibitor and angiotensin receptor blocker treatment. Despite these differences, RAS inhibitors are the most commonly prescribed drugs for the treatment of chronic renal disease, irrespective of sex. This review will examine how males and females respond to stimulation and inhibition of the RAS, with a focus on renal disease.

Endothelial nitric oxide synthase is predominantly involved in angiotensin II modulation of renal vascular resistance and norepinephrine release

Nitric oxide (NO) is mainly generated by endothelial NO synthase (eNOS) or neuronal NOS (nNOS). Recent studies indicate that angiotensin II generates NO release, which modulates renal vascular resistance and sympathetic neurotransmission. Experiments in wild-type [eNOS(+/+) and nNOS(+/+)], eNOS-deficient [eNOS(-/-)], and nNOS-deficient [nNOS(-/-)] mice were performed to determine which NOS isoform is involved. Isolated mice kidneys were perfused with Krebs-Henseleit solution. Endogenous norepinephrine release was measured by HPLC. Angiotensin II dose dependently increased renal vascular resistance in all mice species. EC(50) and maximal pressor responses to angiotensin II were greater in eNOS(-/-) than in nNOS(-/-) and smaller in wild-type mice. The nonselective NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 0.3 mM) enhanced angiotensin II-induced pressor responses in nNOS(-/-) and wild-type mice but not in eNOS(-/-) mice. In nNOS(+/+) mice, 7-nitroindazole monosodium salt (7-NINA; 0.3 mM), a selective nNOS inhibitor, enhanced angiotensin II-induced pressor responses slightly. Angiotensin II-enhanced renal nerve stimulation induced norepinephrine release in all species. L-NAME (0.3 mM) reduced angiotensin II-mediated facilitation of norepinephrine release in nNOS(-/-) and wild-type mice but not in eNOS(-/-) mice. 7-NINA failed to modulate norepinephrine release in nNOS(+/+) mice. (4-Chlorophrnylthio)guanosine-3', 5'-cyclic monophosphate (0.1 nM) increased norepinephrine release. mRNA expression of eNOS, nNOS, and inducible NOS did not differ between mice strains. In conclusion, angiotensin II-mediated effects on renal vascular resistance and sympathetic neurotransmission are modulated by NO in mice. These effects are mediated by eNOS and nNOS, but NO derived from eNOS dominates. Only NO derived from eNOS seems to modulate angiotensin II-mediated renal norepinephrine release.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Angiotensin-(1-7) prevents activation of NADPH oxidase and renal vascular dysfunction in diabetic hypertensive rats

BACKGROUND/AIM

We examined the influence of chronic treatment with angiotensin-(1-7) [Ang-(1-7)] on renox (renal NADPH oxidase, NOX-4) and the development of renal dysfunction in streptozotocin-treated spontaneously hypertensive rats (diabetic SHR).

METHODS

Mean arterial pressure, urinary protein and vascular responsiveness of the isolated renal artery to vasoactive agonists were studied in vehicle- or Ang-(1-7)-treated SHR and diabetic SHR.

RESULTS

Ang-(1-7) decreased the elevated levels of renal NADPH oxidase (NOX) activity and attenuated the activation of NOX-4 gene expression in the diabetic SHR kidney. Ang-(1-7) treatment increased sodium excretion but did not affect mean arterial pressure in diabetic SHR. There was a significant increase in urinary protein (266 +/- 22 mg/24 h) in the diabetic compared to control SHR (112 +/- 13 mg/24 h) and treatment of diabetic SHR with Ang-(1-7) reduced the degree of proteinuria (185 +/- 23 mg/24 h, p < 0.05). Ang-(1-7) treatment also attenuated the diabetes-induced increase in renal vascular responsiveness to endothelin-1, norepinephrine, and angiotensin II in SHR, but significantly increased the vasodilation of the renal artery of SHR and diabetic SHR to the vasodilator agonists.

CONCLUSION

These results suggest that treatment with Ang-(1-7) constitutes a potential therapeutic strategy to alleviate NOX-mediated oxidative stress and to reduce renal dysfunction in diabetic hypertensive rats.

Phosphoinositide hydrolysis increase by angiotensin-(1–7) in neonatal rat brain

Angiotensin (Ang)-(1-7) is an endogenous peptide hormone of the renin-angiotensin system which exerts diverse biological actions, some of them counterregulate Ang II effects. In the present study potential effect of Ang-(1-7) on phosphoinositide (PI) turnover was evaluated in neonatal rat brain. Cerebral cortex prisms of seven-day-old rats were preloaded with [(3)H]myoinositol, incubated with additions during 30 min and later [(3)H]inositol-phosphates (IPs) accumulation quantified. It was observed that PI hydrolysis enhanced 30% to 60% in the presence of 0.01 nM to 100 nM Ang-(1-7). Neither 10 nM [D-Ala(7)]Ang-(1-7), an Ang-(1-7) specific antagonist, nor 10 nM losartan, an angiotensin II type 1 (AT(1)) receptor antagonist, blocked the effect of 0.1 nM Ang-(1-7) on PI metabolism. The effect of 0.1 nM Ang-(1-7) on PI hydrolysis was not reduced but it was even significantly increased in the simultaneous presence of [D-Ala(7)]Ang-(1-7) or losartan. PI turnover enhancement achieved with 0.1 nM Ang-(1-7) decreased roughly 30% in the presence of 10 nM PD 123319, an angiotensin II type 2 (AT(2)) receptor antagonist. The antagonists alone also enhanced PI turnover. Present findings showing an increase in PI turnover by Ang-(1-7) represent a novel action for this peptide and suggest that it exerts a function in this signaling system in neonatal rat brain, an effect involving, at least partially, angiotensin AT(2) receptors.

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.

The role of angiotensin(1-7) in renal vasculature of the rat

OBJECTIVE

Angiotensin(1-7) is an active component of the renin-angiotensin-aldosterone system. Its exact role in renal vascular function is unclear. We therefore studied the effects of angiotensin(1-7) on the renal vasculature in vitro and in vivo.

METHODS

Isolated small renal arteries were studied in an arteriograph system by constructing concentration-response curves to angiotensin II, without and with angiotensin(1-7). In isolated perfused kidneys, the response of angiotensin II on renal vascular resistance was measured without and with angiotensin(1-7). The influence of angiotensin(1-7) on angiotensin II-induced glomerular afferent and efferent constriction was assessed with intravital microscopy in vivo under anaesthesia. In freely moving rats, we studied the effect of angiotensin(1-7) on angiotensin II-induced reduction of renal blood flow with an electromagnetic flow probe.

RESULTS

Angiotensin(1-7) alone had no effect on the renal vasculature in any of the experiments. In vitro, angiotensin(1-7) antagonized angiotensin-II-induced constriction of isolated renal arteries (9.71 +/- 1.21 and 3.20 +/- 0.57%, for control and angiotensin(1-7) pre-treated arteries, respectively; P < 0.0005). In isolated perfused kidneys, angiotensin(1-7) reduced the angiotensin II response (100 +/- 16.6 versus 72.6 +/- 15.6%, P < 0.05) and shifted the angiotensin II dose-response curve rightward (pEC50, 6.69 +/- 0.19 and 6.26 +/- 0.12 for control and angiotensin(1-7) pre-treated kidneys, respectively; P < 0.05). Angiotensin(1-7), however, was devoid of effects on angiotensin-II-induced constriction of glomerular afferent and efferent arterioles and on angiotensin-II-induced renal blood flow reduction in freely moving rats in vivo.

CONCLUSION

Angiotensin(1-7) antagonizes angiotensin II in renal vessels in vitro, but does not appear to have a major function in normal physiological regulation of renal vascular function in vivo.

Are multiple angiotensin receptor types involved in angiotensin (1-7) actions on isolated rat portal vein

Angiotensin (1-7) [Ang (1-7)] is a bioactive component of the renin angiotensin system. Ang (1-7) may interact with angiotensin type 1 (AT1) or type 2 (AT2) receptors and with Ang (1-7) - specific receptors. We examined the interactions between different doses of Ang (1-7) (1 nM-1 microM) and angiotensin II (Ang II) (10 and 100 nM) on isolated rat portal vein. In endothelium-denuded portal vein rings, Ang (1-7) inhibited contractile effects induced by Ang II. The effects of Ang (1-7) were modified by indomethacin, N(G)-nitro-L-arginine methyl ester (L-NAME), (D-Ala7)-Angiotensin (1-7) (H-2888) and losartan. Our results suggest that on rat isolated portal vein rings without endothelium, Ang (1-7) reduces Ang II-induced contractions by acting mostly on Ang (1-7) specific receptors, and this effect is mediated by vasodilatatory prostaglandins. At high concentrations, Ang (1-7) effects are mediated by AT1-receptors, though to a lesser extent than by Ang (1-7) specific receptors.

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.

Angiotensin I-converting enzyme-dependent and neutral endopeptidase-dependent generation and degradation of angiotensin II contrarily modulate noradrenaline release: implications for vasopeptidase-inhibitor therapy?

OBJECTIVES

Vasopeptidase inhibitors inhibit neutral endopeptidase (NEP) and angiotensin I-converting enzyme (ACE). Since angiotensin (ANG) II availability is decreased by ACE inhibition but is increased by NEP inhibition, we evaluated the influence of the vasopeptidase inhibitor omapatrilat on ANG II-dependent noradrenaline (NA) release.

DESIGN

The functional relevance of ACE-dependent and NEP-dependent generation and degradation of ANG II on NA overflow was determined in pithed rats by applications of ANG I (0.1-100 microg/kg) or ANG II (0.01-10 microg/kg) after single injections of ramipril (1 mg/kg), the NEP inhibitor candoxatril (100 mg/kg), or the vasopeptidase inhibitor omapatrilat (30 mg/kg).

RESULTS

Blood pressure was equipotently decreased by ramipril and omapatrilat, but not by candoxatril. NA overflow was increased after ANG I infusions in controls (EC50 = 9.0 microg/kgANG I, Emax = 5680 pg/ml), but almost completely suppressed by ramipril and omapatrilat. Candoxatril decreased EC50 (4.1 microg/kg) and increased Emax (7259 pg/ml). NA overflow after ANG II infusions was enhanced by candoxatril or omapatrilat. Ex vivo ACE activity was extensively inhibited by ramipril or omapatrilat, whereas ex vivo NEP activity was reduced by omapatrilat and candoxatril only. In vitro, omapatrilat inhibited NEP and ACE with similar potencies (IC50 NEP/IC50 ACE = 0.4).

CONCLUSIONS

Vasopeptidase inhibitors influence ANG II-related NA release depending on their ability to modulate the availability of ANG II via ACE or NEP. After acute application, the vasopeptidase inhibitor suppresses NA release in response to ANG I due to a predominant reduction of ANG II formation. These results indicate that the ratio of ACE-inhibitory and NEP-inhibitory potencies of vasopeptidase inhibitors may be relevant for sympathetic activation in chronic therapy.

Angiotensin II modulates renal sympathetic neurotransmission through nitric oxide in AT2 receptor knockout mice

OBJECTIVE

Angiotensin (Ang) II enhances renal sympathetic neurotransmission and stimulates nitric oxide (NO) release. The present study investigates whether Ang II-mediated modulation of sympathetic neurotransmission is dependent on NO production in the kidney. AT2 -/y receptor-deficient mice are used to identify the Ang II receptor subtype involved.

METHODS

Mice kidneys were isolated and perfused with Krebs-Henseleit solution. Drugs were added to the perfusion solution in a cumulative manner. Release of endogenous noradrenaline (NA) was measured by high-performance liquid chromatography (HPLC). AT1 receptor expression was analysed by real-time polymerase chain reaction (PCR).

RESULTS

Ang II (0.01-30 nmol/l) dose dependently increased pressor responses in kidneys of AT2 -/y mice and wild-type (AT2 +/y) mice. Maximal pressor responses and EC50 values for Ang II was greater in AT2 -/y than in AT2 +/y mice. L-NAME (N(omega)-nitro-L-arginine methyl ester; 0.3 mmol/l) enhanced Ang II-induced pressor responses in both strains. In AT2 -/y mice, Ang II-induced facilitation of NA release was more pronounced than in AT2 +/y mice. L-NAME reduced Ang II-mediated facilitation of NA release in both strains. This reduction was more potent in AT2 -/y mice. In kidneys of AT2 -/y mice the AT1 receptor expression was significantly upregulated.

CONCLUSION

These results suggest that activation of AT1 receptors by Ang II releases NO in mouse kidney to modulate sympathetic neurotransmission. Since AT1 receptors are upregulated in AT2 -/y mice kidneys, NO-dependent effects were greater in these mice. Thus, NO seems to play an important modulatory role for renal sympathetic neurotransmission.

Angiotensin‐converting enzyme 2 as a novel target for gene therapy for hypertension

Less than one-third of patients with hypertension have their blood pressures (BP) controlled with current traditional therapeutic approaches for the treatment and control of hypertension. Pharmacological approaches may have reached a plateau in their effectiveness and thus newer innovative strategies need to be studied not only to increase the number of patients that can achieve BP control, but also to find a way to cure, not just manage, the disease. Continuous advances in gene delivery systems coupled with the completion of the Human Genome Project, now make it possible to investigate genetic means for the treatment and possible cure for hypertension. The renin-angiotensin system (RAS) has long been known to regulate BP, and salt and water metabolism. This system is unique in having both a peripheral circulating system and a tissue-based system. Each of these components have been ascribed a variety of physiological effects that have been associated with not only an increase in BP, but also in a variety of the pathophysiological manifestations associated with hypertension, such as cardiac hypertrophy and kidney dysfunction. We and others have used an antisense gene therapy approach, targeting the classical components of the RAS, to effectively attenuate the development of hypertension and related cardiovascular pathophysiologies in numerous experimental models of hypertension. Recently other components of the RAS have been elucidated and some of these components may be potential targets in a gene therapy approach. This article will focus on angiotensin-converting enzyme 2 (ACE2) as a new, potential target of gene therapy for hypertensive disorders.

Effects of angiotensin-(1–7) blockade on renal function in rats with enhanced intrarenal Ang II activity

BACKGROUND

Increasing evidence suggests that angiotensin-(1-7) [Ang-(1-7)] acts as an endogenous antagonist of Ang II when the renin-angiotensin system (RAS) is activated. In the present study, we therefore compared the effects of acute intrarenal (i.r.) Ang-(1-7) receptor blockade on renal function under conditions of normal and increased intrarenal Ang II concentration.

METHODS

Salt-replete Hannover-Sprague Dawley rats (HanSD) served as control animals. As models with enhanced action of Ang II we first used transgenic rats harboring the Ren-2 renin gene (TGR), second, Ang II-infused rats, third, 2-kidney, 1-clip (2K1C) hypertensive rats on normal salt intake, and fourth, salt-depleted TGR and HanSD.

RESULTS

I.r. Ang-(1-7) receptor blockade elicited significant decreases in glomerular filtration rate (GFR), renal plasma flow (RPF), and sodium excretion in 2K1C rats, and in salt-depleted TGR and HanSD. In contrast, i.r. Ang-(1-7) receptor blockade did not significantly change GFR, RPF, and sodium excretion in salt-replete TGR and HanSD, or in Ang II-infused rats.

CONCLUSION

These findings suggest that under conditions of normal intrarenal RAS activity and increased intrarenal Ang II action by infusion of Ang II or by insertion of a renin gene in salt-replete conditions, Ang-(1-7) is not an important factor in the regulation of renal function. In contrast, under conditions of endogenous RAS activation due to clipping of the renal artery or to sodium restriction, Ang-(1-7) serves as opponent of the vasoconstrictor actions of Ang II.

Angiotensin II Receptor Modulation of Renal Vascular Resistance and Neurotransmission in Young and Adult Spontaneously Hypertensive Rats

BACKGROUND/AIMS

Angiotensin (Ang) II modulates vascular resistance and sympathetic neurotransmission through Ang II type 1 (AT1) receptors. Recent studies reported an involvement of AT2 receptors. We investigated whether AT2 receptors participate in modulation of vascular resistance and sympathetic neurotransmission in spontaneously hypertensive rats (SHR).

METHODS

Kidneys of 6- and 16-week-old normotensive (WKY) and SHR were isolated and perfused.

RESULTS

Noradrenaline release induced by renal nerve stimulation (RNS) was increased in SHR (WKY: 1,837 +/- 128, SHR: 2,310 +/- 192 pg/g). Ang I- and II-induced pressor responses and enhancement of noradrenaline release were greater in SHR than in WKY. Pressor responses to Ang I and II were greater in adult compared with young SHR. The AT1 receptor antagonist EXP3174 (0.1 microM) blocked Ang I- and II-induced renal vasoconstriction and noradrenaline release to RNS in both strains. In contrast, the selective AT2 receptor antagonist PD 123319 (1 microM) had no influence in young and adult WKY and SHR.

CONCLUSION

Ang I and II had a greater impact on renal vascular resistance and neurotransmission in SHR, which was more pronounced in adult SHR. All effects are mediated by the AT1 receptor and no modulatory influence of the AT2 receptor could be found.

Low sodium diet inhibits the local counter-regulator effect of angiotensin-(1-7) on angiotensin II

OBJECTIVE

The heptapeptide angiotensin-(1-7) [Ang-(1-7)] has been identified as a versatile, endogenous inhibitor of the renin-angiotensin system (RAS). As the therapeutic response to exogenous RAS inhibitors, such as AT1 receptor antagonists, is altered by changes in salt intake, we investigated the effect of a low, normal and high sodium diet on the antagonism of Ang II by Ang-(1-7). The role of angiotensin receptor subtypes and the endothelium was assessed.

METHODS

Male Wistar rats received a normal sodium (0.3% NaCl), high sodium (2.0% NaCl) or low sodium (0.05% NaCl) diet for 10 days. Vascular responses were assessed ex vivo in thoracic aortic rings in the presence of the nitric oxide (NO) inhibitor N-monomethyl-l-arginine (l-NMMA) to avoid aspecific vasodilator effects of Ang-(1-7).

RESULTS

After a normal or high salt diet, Ang-(1-7) significantly decreased maximal Ang II-induced vascular constrictions by 40-50%. After a low salt diet this non-competitive antagonism disappeared. The AT2 receptor antagonist PD 123319 and the Ang-(1-7) receptor antagonist A779 attenuated the effect of Ang-(1-7) found in rats fed with a normal or high sodium diet. Further, removal of endothelium and pretreatment with the prostaglandin synthesis inhibitor indomethacin (10 mol/l) abolished the non-competitive antagonism by Ang-(1-7).

CONCLUSION

Ang-(1-7) elicits a specific, endothelium-dependent and non-competitive antagonism of Ang II, which involves AT2 and Ang-(1-7) receptors but is independent of NO production. This non-competitive antagonism of Ang-(1-7) is abolished by a low sodium intake in normotensive rats, suggesting that it serves as a negative feedback towards Ang II in response to an altered sodium intake.

Haemodynamic effects of dual blockade of the renin-angiotensin system in spontaneously hypertensive rats: influence of salt

OBJECTIVE

To elucidate the mechanisms responsible for the adverse renal effects induced by dual blockade of the renin-angiotensin system (RAS) and the role of salt therein.

METHODS

The effects of enalapril, losartan and their combination on blood pressure, renal haemodynamics, renal function and RAS were investigated over a wide range of doses in spontaneously hypertensive rats fed either a low-sodium or a high-sodium diet.

RESULTS

In rats fed the low-sodium diet, the losartan-enalapril combination induced the same dose-dependent haemodynamic and hormonal changes as did three- to 10-fold greater doses of enalapril or losartan alone. When a strong decrease (> 50%) in blood pressure was achieved (with 10 mg/kg enalapril plus 10 mg/kg losartan, 100 mg/kg enalapril or 100 mg/kg losartan), a massive renal vasoplegia occurred and renal insufficiency developed. In addition, because of the huge release of renin, angiotensinogen concentrations were reduced, leading to a decrease in intrarenal angiotensins. In rats fed the high-sodium diet, those treated with the enalapril 30 mg/kg plus losartan 30 mg/kg combination, despite complete functional RAS blockade, exhibited smaller decreases in blood pressure and renal resistance, lesser release of renin and angiotensinogen consumption, and a normal renal function. These effects were similar to those produced by 100 mg/kg of enalapril or losartan in rats fed the high-salt diet, or by 10 mg/kg of enalapril or of losartan in rats fed the low-salt diet.

CONCLUSIONS

Dual RAS blockade could be either beneficial, when sodium intake is unrestricted, or dangerous, when sodium intake is restricted.

Spiral artery associated restricted growth (SPAARG): a computer model of pathophysiology resulting from low intervillous pressure having fetal programming implications

Failure of adequate trophoblastic conversion of maternal spiral arteries is associated with intrauterine growth restriction (IUGR). In addition to poor oxygen delivery, raised spiral artery resistance reduces placental intervillous pressure. An iterative type computer model was formed by linking an existing model of the fetus and a new nine cotyledon placental model. Simulation of compression cuffing of the spiral arteries to progressively restrict uteroplacental flow was performed, while observing various fetal and placental variables. Water moved to the fetus in the cotyledonary core villi, and to the mother in the outer villous layers. While the fetus could match villous capillary pressure to changes in intervillous pressure, net transplacental water movement was minimal, but when spiral artery resistance was increased sufficiently to cause mean intervillous pressure to fall below that which the fetus could match, a net flow to the mother appeared. That continued until the resulting fetal blood hemoconcentration produced a sufficient increase in colloid osmotic pressure to restrict further loss. All cells within the fetal-placental unit are then required to operate in an abnormal ionic environment, which may significantly affect systems such as the renin-angiotensin set-point, with implications for post-natal homeostasis such as control of adult blood pressure. Furthermore, in vivo, cells of the feto-placental unit respond to the increased intravascular osmotic pressure by production of intracellular osmolytes in order to match intracellular and vascular/interstitial osmotic pressures. This may explain the observed effects on postnatal water balance in growth restricted infants and could also provide a possible mechanism for the association of the systemic maternal complications associated with impaired placentation and reduced intervillus flow.

AT1 and AT2-receptor antagonists inhibit Ang II-mediated facilitation of noradrenaline release in human atria

It is generally accepted that regulation of blood pressure and sympathetic neurotransmission by angiotensin (Ang) II is brought about through activation of AT1-receptors. Since recent studies demonstrated a high proportion of AT2-receptors in the human heart, the aim of our study was to investigate whether Ang II modulates noradrenaline release also through activation of AT2-receptors in this tissue. Human atrial appendages were prelabeled with [3H]-noradrenaline and electrically field-stimulated. Stimulation-induced outflow of radioactivity was taken as an index of endogenous noradrenaline release. Ang I and II enhanced noradrenaline release in a dose-dependent manner up to 55 and 72%, respectively. These effects were blocked by the selective AT1-receptor antagonists EXP3174 and irbesartan (10 nmol/L). Moreover, the selective AT2-receptor antagonists PD123319 and CGP42112A (0.1 and 1 micromol/L) also inhibited Ang II-induced facilitation of noradrenaline release. Captopril (5 micromol/L) shifted the dose response curve for Ang I less potent to the right than EXP3174 (10 nmol/L). Ang I and II enhanced the stimulation-induced noradrenaline release significantly more potent in tissues of patients pretreated with ACE inhibitors than without. In conclusion, both AT1- and AT2-receptors seem to play a role in Ang II-mediated facilitation of noradrenaline release in the human heart. Chronic treatment with ACE inhibitors appears to affect cardiac sympathetic neurotransmission possibly by upregulation of presynaptic Ang II receptors.