Renin-angiotensin system components in the interstitial fluid of the isolated perfused rat heart. Local production of angiotensin I

Vasodilator reactive oxygen species ameliorate perturbed myocardial oxygen delivery in exercising swine with multiple comorbidities

Multiple common cardiovascular comorbidities produce coronary microvascular dysfunction. We previously observed in swine that a combination of diabetes mellitus (DM), high fat diet (HFD) and chronic kidney disease (CKD) induced systemic inflammation, increased oxidative stress and produced coronary endothelial dysfunction, altering control of coronary microvascular tone via loss of NO bioavailability, which was associated with an increase in circulating endothelin (ET). In the present study, we tested the hypotheses that (1) ROS scavenging and (2) ETA+B-receptor blockade improve myocardial oxygen delivery in the same female swine model. Healthy female swine on normal pig chow served as controls (Normal). Five months after induction of DM (streptozotocin, 3 × 50 mg kg-1 i.v.), hypercholesterolemia (HFD) and CKD (renal embolization), swine were chronically instrumented and studied at rest and during exercise. Sustained hyperglycemia, hypercholesterolemia and renal dysfunction were accompanied by systemic inflammation and oxidative stress. In vivo ROS scavenging (TEMPOL + MPG) reduced myocardial oxygen delivery in DM + HFD + CKD swine, suggestive of a vasodilator influence of endogenous ROS, while it had no effect in Normal swine. In vitro wire myography revealed a vasodilator role for hydrogen peroxide (H2O2) in isolated small coronary artery segments from DM + HFD + CKD, but not Normal swine. Increased catalase activity and ceramide production in left ventricular myocardial tissue of DM + HFD + CKD swine further suggest that increased H2O2 acts as vasodilator ROS in the coronary microvasculature. Despite elevated ET-1 plasma levels in DM + HFD + CKD swine, ETA+B blockade did not affect myocardial oxygen delivery in Normal or DM + HFD + CKD swine. In conclusion, loss of NO bioavailability due to 5 months exposure to multiple comorbidities is partially compensated by increased H2O2-mediated coronary vasodilation.

Combining renin-angiotensin system blockade and sodium-glucose cotransporter-2 inhibition in experimental diabetes results in synergistic beneficial effects

BACKGROUND

Sodium-glucose cotransporter-2 (SGLT2) inhibition exerts cardioprotective and renoprotective effects, often on top of renin-angiotensin system (RAS) blockade. We investigated this in diabetic hypertensive (mREN2)27 rats.

METHODS

Rats were made diabetic with streptozotocin and treated with vehicle, the angiotensin receptor blocker valsartan, the SGLT2 inhibitor empagliflozin, or their combination. Blood pressure (BP) was measured by telemetry.

RESULTS

Diabetes resulted in albuminuria, accompanied by glomerulosclerosis, without a change in glomerular filtration rate. Empagliflozin did not lower BP, while valsartan did, and when combined the BP drop was largest. Only dual blockade reduced cardiac hypertrophy and prevented left ventricular dilatation. Valsartan, but not empagliflozin, increased renin, and the largest renin rise occurred during dual blockade, resulting in plasma angiotensin II [but not angiotensin-(1-7)] upregulation. In contrast, in the kidney, valsartan lowered angiotensin II and angiotensin-(1-7), and empagliflozin did not alter this. Although both valsartan and empagliflozin alone tended to diminish albuminuria, the reduction was significant only when both drugs were combined. This was accompanied by reduced glomerulosclerosis, no change in glomerular filtration rate, and a favorable expression pattern of fibrosis and inflammatory markers (including SGLT2) in the kidney.

CONCLUSION

RAS blockade and SGLT2 inhibition display synergistic beneficial effects on BP, kidney injury and cardiac hypertrophy in a rat with hypertension and diabetes. The synergy does not involve upregulation of angiotensin-(1-7), but may relate to direct RAS-independent effects of empagliflozin in the heart and kidney.

New Approaches Targeting the Renin-Angiotensin System: Inhibition of Brain Aminopeptidase A, ACE2 Ubiquitination, and Angiotensinogen

Despite the availability of various therapeutic classes of antihypertensive drugs, hypertension remains poorly controlled, in part because of poor adherence. Hence, there is a need for the development of antihypertensive drugs acting on new targets to improve control of blood pressure. This review discusses novel insights (including the data of recent clinical trials) with regard to interference with the renin-angiotensin system, focusing on the enzymes aminopeptidase A and angiotensin-converting enzyme 2 (ACE2) in the brain, as well as the substrate of renin- angiotensinogen-in the liver. It raises the possibility that centrally acting amino peptidase A inhibitors (eg, firibastat), preventing the conversion of angiotensin II to angiotensin III in the brain, might be particularly useful in African Americans and patients with obesity. Firibastat additionally upregulates brain ACE2, allowing the conversion of angiotensin II to its protective metabolite angiotensin-(1-7). Furthermore, antisense oligonucleotides or small interfering ribonucleic acids suppress hepatic angiotensinogen for weeks to months after 1 injection and thus could potentially overcome adherence issues. Finally, interference with ACE2 ubiquitination is emerging as a future option for the treatment of neurogenic hypertension, given that ubiquitination resistance might upregulate ACE2 activity.

Beyond Angiotensin-Converting Enzyme Inhibitors: Modulation of the Renin-Angiotensin-Aldosterone System to Delay or Manage Congestive Heart Failure

The renin-angiotensin-aldosterone system (RAAS) consists of bioactive angiotensin peptides, enzymatic pathways, receptors, and the steroid hormone aldosterone. The RAAS regulates blood pressure, sodium, and electrolyte homeostasis and mediates pathologic disease processes. Within this system is an alternative arm that counterbalances the vasoconstrictive, sodium and water retentive, and pro-fibrotic and inflammatory effects of the classical arm. Improved biochemical methodologies in RAAS quantification are elucidating how this complex system changes in health and disease. Future treatments for cardiovascular and kidney disease will likely involve a more nuanced manipulation of this system rather than simple blockade.

Lymphatic vessels and the renin-angiotensin-system

The lymphatic system is an integral part of the circulatory system and plays an important role in the fluid homeostasis of the human body. Accumulating evidence has recently suggested the involvement of lymphatic dysfunction in the pathogenesis of cardio-reno-vascular (CRV) disease. However, how the sophisticated contractile machinery of lymphatic vessels is modulated and, possibly impaired in CRV disease, remains largely unknown. In particular, little attention has been paid to the effect of the renin-angiotensin-system (RAS) on lymphatics, despite the high concentration of RAS mediators that these tissue-draining vessels are exposed to and the established role of the RAS in the development of classic microvascular dysfunction and overt CRV disease. We herein review recent studies linking RAS to lymphatic function and/or plasticity and further highlight RAS-specific signaling pathways, previously shown to drive adverse arterial remodeling and CRV organ damage that have potential for direct modulation of the lymphatic system.

Blood pressure-independent renoprotective effects of small interference RNA targeting liver angiotensinogen in experimental diabetes

BACKGROUND AND PURPOSE

Small interfering RNA (siRNA) targeting liver angiotensinogen lowers blood pressure, but its effects in hypertensive diabetes are unknown.

EXPERIMENTAL APPROACH

To address this, TGR (mRen2)27 rats (angiotensin II-dependent hypertension model) were made diabetic with streptozotocin over 18 weeks and treated with either vehicle, angiotensinogen siRNA, the AT₁ antagonist valsartan, the ACE inhibitor captopril, valsartan + siRNA or valsartan + captopril for the final 3 weeks. Mean arterial pressure (MAP) was measured via radiotelemetry.

KEY RESULTS

MAP before treatment was 153 ± 2 mmHg. Diabetes resulted in albuminuria, accompanied by glomerulosclerosis and podocyte effacement, without a change in glomerular filtration rate. All treatments lowered MAP and cardiac hypertrophy, and the largest drop in MAP was observed with siRNA + valsartan. Treatment with siRNA lowered circulating angiotensinogen by >99%, and the lowest circulating angiotensin II and aldosterone levels occurred in the dual treatment groups. Angiotensinogen siRNA did not affect renal angiotensinogen mRNA expression, confirming its liver-specificity. Furthermore, only siRNA with or without valsartan lowered renal angiotensin I. All treatments lowered renal angiotensin II and the reduction was largest (>95%) in the siRNA + valsartan group. All treatments identically lowered albuminuria, whereas only siRNA with or without valsartan restored podocyte foot processes and reduced glomerulosclerosis.

CONCLUSION AND IMPLICATIONS

Angiotensinogen siRNA exerts renoprotection in diabetic TGR (mRen2)27 rats and this relies, at least in part, on the suppression of renal angiotensin II formation from liver-derived angiotensinogen. Clinical trials should now address whether this is also beneficial in human diabetic kidney disease.

Hepatic and proximal tubule angiotensinogen play distinct roles in kidney dysfunction, glomerular and tubular injury, and fibrosis progression

Components of the renin-angiotensin system, including angiotensinogen (AGT), are critical contributors to chronic kidney disease (CKD) development and progression. However, the specific role of tissue-derived AGTs in CKD has not been fully understood. To define the contribution of liver versus kidney AGT in the CKD development, we performed 5/6 nephrectomy (Nx), an established CKD model, in wild-type (WT), proximal tubule (PT)- or liver-specific AGT knockout (KO) mice. Nx significantly elevated intrarenal AGT expression and elevated blood pressure (BP) in WT mice. The increase of intrarenal AGT protein was completely blocked in liver-specific AGT KO mice with BP reduction, suggesting a crucial role for liver AGT in BP regulation during CKD. Nx-induced glomerular and kidney injury and dysfunction, as well as fibrosis, were all attenuated to a greater extent in liver-specific AGT KO mice compared with PT-specific AGT KO and WT mice. However, the suppression of interstitial fibrosis in PT- and liver-specific AGT KO mouse kidneys was comparable. Our findings demonstrate that liver AGT acts as a critical contributor in driving glomerular and tubular injury, renal dysfunction, and fibrosis progression, whereas the role of PT AGT was limited to interstitial fibrosis progression in chronic renal insufficiency. Our results provide new insights for the development of tissue-targeted renin-angiotensin system intervention in the treatment of CKD.NEW & NOTEWORTHY Chronic kidney disease (CKD) is a major unmet medical need with no effective treatment. Current findings demonstrate that hepatic and proximal tubule angiotensinogen have distinct roles in tubular and glomerular injury, fibrogenesis, and renal dysfunction during CKD development. As renin-angiotensin system components, including angiotensinogen, are important targets for treating CKD in the clinic, the results from our study may be applied to developing better tissue-targeted treatment strategies for CKD and other fibroproliferative diseases.

Angiotensinogen Suppression: A New Tool to Treat Cardiovascular and Renal Disease

Multiple types of renin-angiotensin system (RAS) blockers exist, allowing interference with the system at the level of renin, angiotensin-converting enzyme, or the angiotensin II receptor. Yet, in particular, for the treatment of hypertension, the number of patients with uncontrolled hypertension continues to rise, either due to patient noncompliance or because of the significant renin rises that may, at least partially, overcome the effect of RAS blockade (RAS escape). New approaches to target the RAS are either direct antisense oligonucleotides that inhibit angiotensinogen RNA translation, or small interfering RNA (siRNA) that function via the RNA interference pathway. Since all angiotensins stem from angiotensinogen, lowering angiotensinogen has the potential to circumvent the RAS escape phenomenon. Moreover, antisense oligonucleotides and small interfering RNA require injections only every few weeks to months, which might reduce noncompliance. Of course, angiotensinogen suppression also poses a threat in situations where the RAS is acutely needed, for instance in women becoming pregnant during treatment, or in cases of emergency, when severe hypotension occurs. This review discusses all preclinical data on angiotensinogen suppression, as well as the limited clinical data that are currently available. It concludes that it is an exciting new tool to target the RAS with high specificity and a low side effect profile. Its long-term action might revolutionize pharmacotherapy, as it could overcome compliance problems. Preclinical and clinical programs are now carefully investigating its efficacy and safety profile, allowing an optimal introduction as a novel drug to treat cardiovascular and renal diseases in due time.

Conventional Vasopressor and Vasopressor-Sparing Strategies to Counteract the Blood Pressure-Lowering Effect of Small Interfering RNA Targeting Angiotensinogen

Background

A single dose of small interfering RNA (siRNA) targeting liver angiotensinogen eliminates hepatic angiotensinogen and lowers blood pressure. Angiotensinogen elimination raises concerns for clinical application because an angiotensin rise is needed to maintain perfusion pressure during hypovolemia. Here, we investigated whether conventional vasopressors can raise arterial pressure after angiotensinogen depletion.

Methods and Results

Spontaneously hypertensive rats on a low‐salt diet were treated with siRNA (10 mg/kg fortnightly) for 4 weeks, supplemented during the final 2 weeks with fludrocortisone (6 mg/kg per day), the α‐adrenergic agonist midodrine (4 mg/kg per day), or a high‐salt diet (all groups n=6–7). Pressor responsiveness to angiotensin II and norepinephrine was assessed before and after siRNA administration. Blood pressure was measured via radiotelemetry. Depletion of liver angiotensinogen by siRNA lowered plasma angiotensinogen concentrations by 99.2±0.1% and mean arterial pressure by 19 mm Hg. siRNA‐mediated blood pressure lowering was rapidly reversed by intravenous angiotensin II or norepinephrine, or gradually reversed by fludrocortisone or high salt intake. Midodrine had no effect. Unexpectedly, fludrocortisone partially restored plasma angiotensinogen concentrations in siRNA‐treated rats, and nearly abolished plasma renin concentrations. To investigate whether this angiotensinogen originated from nonhepatic sources, fludrocortisone was administered to mice lacking hepatic angiotensinogen. Fludrocortisone did not increase angiotensinogen in these mice, implying that the rise in angiotensinogen in the siRNA‐treated rats must have depended on the liver, most likely reflecting diminished cleavage by renin.

Conclusions

Intact pressor responsiveness to conventional vasopressors provides pharmacological means to regulate the blood pressure–lowering effect of angiotensinogen siRNA and may support future therapeutic implementation of siRNA.

Kidney Angiotensin in Cardiovascular Disease: Formation and Drug Targeting

The concept of local formation of angiotensin II in the kidney has changed over the last 10-15 years. Local synthesis of angiotensinogen in the proximal tubule has been proposed, combined with prorenin synthesis in the collecting duct. Binding of prorenin via the so-called (pro)renin receptor has been introduced, as well as megalin-mediated uptake of filtered plasma-derived renin-angiotensin system (RAS) components. Moreover, angiotensin metabolites other than angiotensin II [notably angiotensin-(1-7)] exist, and angiotensins exert their effects via three different receptors, of which angiotensin II type 2 and Mas receptors are considered renoprotective, possibly in a sex-specific manner, whereas angiotensin II type 1 (AT1) receptors are believed to be deleterious. Additionally, internalized angiotensin II may stimulate intracellular receptors. Angiotensin-converting enzyme 2 (ACE2) not only generates angiotensin-(1-7) but also acts as coronavirus receptor. Multiple, if not all, cardiovascular diseases involve the kidney RAS, with renal AT1 receptors often being claimed to exert a crucial role. Urinary RAS component levels, depending on filtration, reabsorption, and local release, are believed to reflect renal RAS activity. Finally, both existing drugs (RAS inhibitors, cyclooxygenase inhibitors) and novel drugs (angiotensin receptor/neprilysin inhibitors, sodium-glucose cotransporter-2 inhibitors, soluble ACE2) affect renal angiotensin formation, thereby displaying cardiovascular efficacy. Particular in the case of the latter three, an important question is to what degree they induce renoprotection (e.g., in a renal RAS-dependent manner). This review provides a unifying view, explaining not only how kidney angiotensin formation occurs and how it is affected by drugs but also why drugs are renoprotective when altering the renal RAS. SIGNIFICANCE STATEMENT: Angiotensin formation in the kidney is widely accepted but little understood, and multiple, often contrasting concepts have been put forward over the last two decades. This paper offers a unifying view, simultaneously explaining how existing and novel drugs exert renoprotection by interfering with kidney angiotensin formation.

The adaptive renal response for volume homeostasis during two weeks of dapagliflozin treatment in people with type 2 diabetes and preserved renal function on a sodium-controlled diet

Introduction

Proximal tubule sodium uptake is diminished following sodium glucose cotransporter 2 (SGLT2) inhibition. We previously showed that during SGLT2 inhibition, the kidneys adapt by increasing sodium uptake at distal tubular segments, thereby maintaining body sodium balance. Despite continuous glycosuria, we detected no increased urine volumes. We therefore assessed the adaptive renal responses to prevent excessive fluid loss.

Methods

We conducted a mechanistic open-label study in people with type 2 diabetes mellitus with preserved kidney function, who received a standardized sodium intake (150 mmol/d) to evaluate the effects of dapagliflozin on renin-angiotensin-aldosterone system (RAAS) hormones, volume-related biomarkers, urinary albumin-to-creatinine ratio (UACR), and estimated glomerular filtration rate (eGFR), at start of treatment (day 4), end of treatment (day 14), and follow-up (day 18).

Results

A total of 14 people were enrolled. Plasma renin and angiotensin II and urinary aldosterone and angiotensinogen were acutely and persistently increased during treatment with dapagliflozin. Plasma copeptin level was numerically increased after 4 days (21%). Similarly, fractional urea excretion was significantly decreased at start of treatment (−17%). Free water clearance was significantly decreased after 4 days (−74%) and 14 days (−41%). All changes reversed after dapagliflozin discontinuation.

Conclusion

Dapagliflozin-induced osmotic diuresis triggers kidney adaptive mechanisms to maintain volume and sodium balance in people with type 2 diabetes and preserved kidney function. ClinicalTrials.gov (identification: NCT03152084).

(Pro)renin Receptor Inhibition Reduces Plasma Cholesterol and Triglycerides but Does Not Attenuate Atherosclerosis in Atherosclerotic Mice

Objective: Elevated plasma cholesterol concentrations contributes to ischemic cardiovascular diseases. Recently, we showed that inhibiting hepatic (pro)renin receptor [(P)RR] attenuated diet-induced hypercholesterolemia and hypertriglyceridemia in low-density lipoprotein receptor (LDLR) deficient mice. The purpose of this study was to determine whether inhibiting hepatic (P)RR could attenuate atherosclerosis.

Approach and Results: Eight-week-old male LDLR^−/− mice were injected with either saline or N-acetylgalactosamine-modified antisense oligonucleotides (G-ASOs) primarily targeting hepatic (P)RR and were fed a western-type diet (WTD) for 16 weeks. (P)RR G-ASOs markedly reduced plasma cholesterol concentrations from 2,211 ± 146 to 1,128 ± 121 mg/dL. Fast protein liquid chromatography (FPLC) analyses revealed that cholesterol in very low-density lipoprotein (VLDL) and intermediate density lipoprotein (IDL)/LDL fraction were potently reduced by (P)RR G-ASOs. Moreover, (P)RR G-ASOs reduced plasma triglyceride concentrations by more than 80%. Strikingly, despite marked reduction in plasma lipid concentrations, atherosclerosis was not reduced but rather increased in these mice. Further testing in ApoE^−/− mice confirmed that (P)RR G-ASOs reduced plasma lipid concentrations but not atherosclerosis. Transcriptomic analysis of the aortas revealed that (P)RR G-ASOs induced the expression of the genes involved in immune responses and inflammation. Further investigation revealed that (P)RR G-ASOs also inhibited (P)RR in macrophages and in enhanced inflammatory responses to exogenous stimuli. Moreover, deleting the (P)RR in macrophages resulted in accelerated atherosclerosis in WTD fed ApoE^−/− mice.

Conclusion: (P)RR G-ASOs reduced the plasma lipids in atherosclerotic mice due to hepatic (P)RR deficiency. However, augmented pro-inflammatory responses in macrophages due to (P)RR downregulation counteracted the beneficial effects of lowered plasma lipid concentrations on atherosclerosis. Our study demonstrated that hepatic (P)RR and macrophage (P)RR played a counteracting role in atherosclerosis.

Exclusion of left atrial appendage: effects beyond thromboembolic prevention

PURPOSE OF REVIEW

This review aims to summarize the nonthromboembolic prevention effects of left atrial appendage exclusion (LAAE).

RECENT FINDINGS

Left atrial appendage (LAA) secretes multiple hormones; regulates blood volume and pressure; and generates trigger activities. Exclusion of the LAA by different techniques may lead to downstream effects including changes in blood pressure and cardiac performance, improvement of outcome of atrial fibrillation (AF) ablation, and alteration of metabolism.

SUMMARY

LAAE procedures not only prevent thromboembolic events in patients with AF, but rather may bring additional benefits or side-effect to patients undergoing LAAE.

Perivascular Adipose Tissue in Vascular Function: Does Locally Synthesized Angiotensinogen Play a Role?

ABSTRACT

In recent years, perivascular adipose tissue (PVAT) research has gained special attention in an effort to understand its involvement in vascular function. PVAT is recognized as an important endocrine organ that secretes procontractile and anticontractile factors, including components of the renin-angiotensin-aldosterone system, particularly angiotensinogen (AGT). This review critically addresses the occurrence of AGT in PVAT, its release into the blood stream, and its contribution to the generation and effects of angiotensins (notably angiotensin-(1-7) and angiotensin II) in the vascular wall. It describes that the introduction of transgenic animals, expressing AGT at 0, 1, or more specific location(s), combined with the careful measurement of angiotensins, has revealed that the assumption that PVAT independently generates angiotensins from locally synthesized AGT is incorrect. Indeed, selective deletion of AGT from adipocytes did not lower circulating AGT, neither under a control diet nor under a high-fat diet, and only liver-specific AGT deletion resulted in the disappearance of AGT from blood plasma and adipose tissue. An entirely novel scenario therefore develops, supporting local angiotensin generation in PVAT that depends on the uptake of both AGT and renin from blood, in addition to the possibility that circulating angiotensins exert vascular effects. The review ends with a summary of where we stand now and recommendations for future research.

No evidence for brain renin-angiotensin system activation during DOCA-salt hypertension

Brain renin-angiotensin system (RAS) activation is thought to mediate deoxycorticosterone acetate (DOCA)-salt hypertension, an animal model for human primary hyperaldosteronism. Here, we determined whether brainstem angiotensin II is generated from locally synthesized angiotensinogen and mediates DOCA-salt hypertension. To this end, chronic DOCA-salt-hypertensive rats were treated with liver-directed siRNA targeted to angiotensinogen, the angiotensin II type 1 receptor antagonist valsartan, or the mineralocorticoid receptor antagonist spironolactone (n = 6-8/group). We quantified circulating angiotensinogen and renin by enzyme-kinetic assay, tissue angiotensinogen by Western blotting, and angiotensin metabolites by LC-MS/MS. In rats without DOCA-salt, circulating angiotensin II was detected in all rats, whereas brainstem angiotensin II was detected in 5 out of 7 rats. DOCA-salt increased mean arterial pressure by 19 ± 1 mmHg and suppressed circulating renin and angiotensin II by >90%, while brainstem angiotensin II became undetectable in 5 out of 7 rats (<6 fmol/g). Gene silencing of liver angiotensinogen using siRNA lowered circulating angiotensinogen by 97 ± 0.3%, and made brainstem angiotensin II undetectable in all rats (P<0.05 vs. non-DOCA-salt), although brainstem angiotensinogen remained intact. As expected for this model, neither siRNA nor valsartan attenuated the hypertensive response to DOCA-salt, whereas spironolactone normalized blood pressure and restored brain angiotensin II together with circulating renin and angiotensin II. In conclusion, despite local synthesis of angiotensinogen in the brain, brain angiotensin II depended on circulating angiotensinogen. That DOCA-salt suppressed circulating and brain angiotensin II in parallel, while spironolactone simultaneously increased brain angiotensin II and lowered blood pressure, indicates that DOCA-salt hypertension is not mediated by brain RAS activation.

The emerging role of angiotensinogen in cardiovascular diseases

Angiotensinogen (AGT) is the unique precursor of all angiotensin peptides. Many of the basic understandings of AGT in cardiovascular diseases have come from research efforts to define its effects on blood pressure regulation. The development of novel techniques targeting AGT manipulation such as genetic animal models, adeno-associated viral approaches, and antisense oligonucleotides made it possible to deeply investigate the relationship between AGT and cardiovascular diseases. In this brief review, we provide contemporary insights into the emerging role of AGT in cardiovascular diseases. In light of the recent progress, we emphasize some newly recognized features and mechanisms of AGT in heart failure, hypertension, atherosclerosis, and cardiovascular risk factors.

Megalin: a Novel Determinant of Renin-Angiotensin System Activity in the Kidney?

PURPOSE OF REVIEW

Megalin is well known for its role in the reabsorption of proteins from the ultrafiltrate. Recent studies suggest that megalin also reabsorbs renin and angiotensinogen. Indeed, without megalin urinary renin and angiotensinogen levels massively increase, and even prorenin becomes detectable in urine.

RECENT FINDINGS

Intriguingly, megalin might also contribute to renal angiotensin production, as evidenced from studies in megalin knockout mice. This review discusses these topics critically, concluding that urinary renin-angiotensin system components reflect diminished reabsorption rather than release from renal tissue sites and that alterations in renal renin levels or megalin-dependent signaling need to be ruled out before concluding that angiotensin production at renal tissue sites is truly megalin dependent. Future studies should evaluate megalin-mediated renin/angiotensinogen transcytosis (allowing interstitial angiotensin generation), and determine whether megalin prefers prorenin over renin, thus explaining why urine normally contains no prorenin.

A Randomized Trial of Distal Diuretics versus Dietary Sodium Restriction for Hypertension in Chronic Kidney Disease

BACKGROUND

Distal diuretics are considered less effective than loop diuretics in CKD. However, data to support this perception are limited.

METHODS

To investigate whether distal diuretics are noninferior to dietary sodium restriction in reducing BP in patients with CKD stage G3 or G4 and hypertension, we conducted a 6-week, randomized, open-label crossover trial comparing amiloride/hydrochlorothiazide (5 mg/50 mg daily) with dietary sodium restriction (60 mmol per day). Antihypertension medication was discontinued for a 2-week period before randomization. We analyzed effects on BP, kidney function, and fluid balance and related this to renal clearance of diuretics.

RESULTS

A total of 26 patients (with a mean eGFR of 39 ml/min per 1.73 m²) completed both treatments. Dietary sodium restriction reduced sodium excretion from 160 to 64 mmol per day. Diuretics produced a greater reduction in 24-hour systolic BP (SBP; from 138 to 124 mm Hg) compared with sodium restriction (from 134 to 129 mm Hg), as well as a significantly greater effect on extracellular water, eGFR, plasma renin, and aldosterone. Both interventions resulted in a similar decrease in body weight and NT-proBNP. Neither approaches decreased albuminuria significantly, whereas diuretics did significantly reduce urinary angiotensinogen and β2-microglobulin excretion. Although lower eGFR and higher plasma indoxyl sulfate correlated with lower diuretic clearance, the diuretic effects on body weight and BP at lower eGFR were maintained. During diuretic treatment, higher PGE2 excretion correlated with lower free water clearance, and four patients developed mild hyponatremia.

CONCLUSIONS

Distal diuretics are noninferior to dietary sodium restriction in reducing BP and extracellular volume in CKD. Diuretic sensitivity in CKD is maintained despite lower diuretic clearance.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

DD-study: Diet or Diuretics for Salt-sensitivity in Chronic Kidney Disease (DD), NCT02875886.

Novel Therapeutic Approaches Targeting the Renin-Angiotensin System and Associated Peptides in Hypertension and Heart Failure

Despite the success of renin-angiotensin system (RAS) blockade by angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 receptor (AT1R) blockers, current therapies for hypertension and related cardiovascular diseases are still inadequate. Identification of additional components of the RAS and associated vasoactive pathways, as well as new structural and functional insights into established targets, have led to novel therapeutic approaches with the potential to provide improved cardiovascular protection and better blood pressure control and/or reduced adverse side effects. The simultaneous modulation of several neurohumoral mediators in key interconnected blood pressure-regulating pathways has been an attractive approach to improve treatment efficacy, and several novel approaches involve combination therapy or dual-acting agents. In addition, increased understanding of the complexity of the RAS has led to novel approaches aimed at upregulating the ACE2/angiotensin-(1-7)/Mas axis to counter-regulate the harmful effects of the ACE/angiotensin II/angiotensin III/AT1R axis. These advances have opened new avenues for the development of novel drugs targeting the RAS to better treat hypertension and heart failure. Here we focus on new therapies in preclinical and early clinical stages of development, including novel small molecule inhibitors and receptor agonists/antagonists, less conventional strategies such as gene therapy to suppress angiotensinogen at the RNA level, recombinant ACE2 protein, and novel bispecific designer peptides.

Angiotensin II type 2 receptor blocker PD123319 has more beneficial effects than losartan on ischemia-reperfusion injury and oxidative damage in isolated rat heart

Our study aimed to determine the effects of losartan and PD123319 in ischemia-reperfusion (IR) injury in isolated perfused rat heart. The study used 40 male Wistar albino rats that were grouped as Control, IR, and IR treatment groups that received losartan (20 mg/kg), PD123319 (20 mg/kg), and losartan+PD123319. The hearts were attached to Langendorff isolated heart system by employing in situ cannulation method, and cardiodynamic parameters were recorded during the experiment. At the end of experiment, hearts were retained for biochemical analysis and all data were statistically evaluated. A partial recovery of cardiodynamic parameters was observed in all treatment groups. A significant increase in oxidative stress parameters were seen in the IR group, whereas all treatment groups exhibited lower increase. Furthermore, levels of all antioxidant parameters were significantly lower in the IR group, but higher in the treatment groups. Effects on all parameters were much more remarkable in the PD123319 group. Levels of angiotensin II and renin were increased (P < 0.001) with IR application and decreased (P < 0.001) with the treatment of both antagonists. In conclusion, treatment of losartan and PD123319 played a cardioprotective role against IR injury, PD123319 being more effective in this protection.

Food-derived bioactive peptides and their role in ameliorating hypertension and associated cardiovascular diseases

Non-communicable diseases including cardiovascular diseases (CVDs) and associated metabolic disorders are responsible for nearly 40 million deaths globally per year. Hypertension or high blood pressure (BP) is one of the primary reasons for the development of CVDs. A healthy nutritional strategy complementing with physical activity can substantially reduce high BP and prevent the occurrence of CVD-associated morbidity and mortality. Bioactive peptides currently are the next wave of the promising bench to clinic options for potential targeting chronic and acute health issues including hypertension. Peptides demonstrating anti-inflammatory, anti-oxidant, and angiotensin-converting enzyme-I inhibitory activity are widely studied for the amelioration of hypertension and associated CVDs. Isolating these potent bioactive peptides from different food sources is a promising endeavor toward nutraceutical based dietary management and prevention of hypertension. Understanding the pathophysiology of hypertension and the action mechanisms of the bioactive peptides would complement in designing and characterizing more potent peptides and suitable comprehensive dietary plans for the prevention of hypertension and associated CVDs.

Angiotensin generation in the brain: a re-evaluation

The existence of a so-called brain renin-angiotensin system (RAS) is controversial. Given the presence of the blood-brain barrier, angiotensin generation in the brain, if occurring, should depend on local synthesis of renin and angiotensinogen. Yet, although initially brain-selective expression of intracellular renin was reported, data in intracellular renin knockout animals argue against a role for this renin in angiotensin generation. Moreover, renin levels in brain tissue at most represented renin in trapped blood. Additionally, in neurogenic hypertension brain prorenin up-regulation has been claimed, which would generate angiotensin following its binding to the (pro)renin receptor. However, recent studies reported no evidence for prorenin expression in the brain, nor for its selective up-regulation in neurogenic hypertension, and the (pro)renin receptor rather displays RAS-unrelated functions. Finally, although angiotensinogen mRNA is detectable in the brain, brain angiotensinogen protein levels are low, and even these low levels might be an overestimation due to assay artefacts. Taken together, independent angiotensin generation in the brain is unlikely. Indeed, brain angiotensin levels are extremely low, with angiotensin (Ang) I levels corresponding to the small amounts of Ang I in trapped blood plasma, and Ang II levels at most representing Ang II bound to (vascular) brain Ang II type 1 receptors. This review concludes with a unifying concept proposing the blood origin of angiotensin in the brain, possibly resulting in increased levels following blood-brain barrier disruption (e.g. due to hypertension), and suggesting that interfering with either intracellular renin or the (pro)renin receptor has consequences in an RAS-independent manner.

The macula densa prorenin receptor is essential in renin release and blood pressure control

The prorenin receptor (PRR) was originally proposed to be a member of the renin-angiotensin system (RAS); however, recent work questioned their association. The present paper describes a functional link between the PRR and RAS in the renal juxtaglomerular apparatus (JGA), a classic anatomical site of the RAS. PRR expression was found in the sensory cells of the JGA, the macula densa (MD), and immunohistochemistry-localized PRR to the MD basolateral cell membrane in mouse, rat, and human kidneys. MD cell PRR activation led to MAP kinase ERK1/2 signaling and stimulation of PGE₂ release, the classic pathway of MD-mediated renin release. Exogenous renin or prorenin added to the in vitro microperfused JGA-induced acute renin release, which was inhibited by removing the MD or by the administration of a PRR decoy peptide. To test the function of MD PRR in vivo, we established a new mouse model with inducible conditional knockout (cKO) of the PRR in MD cells based on neural nitric oxide synthase-driven Cre-lox recombination. Deletion of the MD PRR significantly reduced blood pressure and plasma renin. Challenging the RAS by low-salt diet + captopril treatment caused further significant reductions in blood pressure, renal renin, cyclooxygenase-2, and microsomal PGE synthase expression in cKO vs. wild-type mice. These results suggest that the MD PRR is essential in a novel JGA short-loop feedback mechanism, which is integrated within the classic MD mechanism to control renin synthesis and release and to maintain blood pressure.

AT1-receptor blockade, but not renin inhibition, reduces aneurysm growth and cardiac failure in fibulin-4 mice

AIMS

Increasing evidence supports a role for the angiotensin II-AT1-receptor axis in aneurysm development. Here, we studied whether counteracting this axis via stimulation of AT2 receptors is beneficial. Such stimulation occurs naturally during AT1-receptor blockade with losartan, but not during renin inhibition with aliskiren.

METHODS AND RESULTS

Aneurysmal homozygous fibulin-4 mice, displaying a four-fold reduced fibulin-4 expression, were treated with placebo, losartan, aliskiren, or the β-blocker propranolol from day 35 to 100. Their phenotype includes cystic media degeneration, aortic regurgitation, left ventricular dilation, reduced ejection fraction, and fractional shortening. Although losartan and aliskiren reduced hemodynamic stress and increased renin similarly, only losartan increased survival. Propranolol had no effect. No drug rescued elastic fiber fragmentation in established aneurysms, although losartan did reduce aneurysm size. Losartan also increased ejection fraction, decreased LV diameter, and reduced cardiac pSmad2 signaling. None of these effects were seen with aliskiren or propranolol. Longitudinal micro-CT measurements, a novel method in which each mouse serves as its own control, revealed that losartan reduced LV growth more than aneurysm growth, presumably because the heart profits both from the local (cardiac) effects of losartan and its effects on aortic root remodeling.

CONCLUSION

Losartan, but not aliskiren or propranolol, improved survival in fibulin-4 mice. This most likely relates to its capacity to improve structure and function of both aorta and heart. The absence of this effect during aliskiren treatment, despite a similar degree of blood pressure reduction and renin-angiotensin system blockade, suggests that it might be because of AT2-receptor stimulation.

Intracellular renin increases the inward calcium current in smooth muscle cells of mesenteric artery of SHR. Implications for hypertension and vascular remodeling

The influence of intracellular renin on the inward calcium current in isolated smooth muscle cells from SHR mesenteric arteries was investigated. Measurements of calcium current were performed using the whole cell configuration of pCLAMP. The results indicated that: 1) renin (100nM) dialyzed into smooth muscle cells, increased the inward calcium current; 2) verapamil (10-9M) administered to the bath inhibited the effect of renin on the inward calcium current; 3) concurrently with the increase of calcium current a depolarization of 6.8+/-2.1mV (n=16)(P<0.05) was found in cells dialyzed with renin; 4) intracellular dialysis of renin (100nM) into smooth muscle cells isolated from mesenteric arteries of normal Wystar Kyoto rats showed no significant change on calcium current; 5) aliskiren (10-9M) dialyzed into the cell together with renin (100nM) abolished the effect of the enzyme on the calcium current in SHR; 6) Ang II (100nM) dialyzed into the smooth muscle cell from mesenteric artery of SHR in absence of renin, decreased the calcium current-an effect greatly reduced by valsartan (10-9M) added to the cytosol; 7) administration of renin (100nM) plus angiotensinogen (100nM) into the cytosol of muscles cells from SHR rats reduced the inward calcium current; 8) extracellular administration of Ang II (100nM) increased the inward calcium current in mesenteric arteries of SHR.

CONCLUSIONS

intracellular renin in vascular resistance vessels from SHR due to internalization or expression, contributes to the regulation of vascular tone and control of peripheral resistance-an effect independently of Ang II. Implications for hypertension and vascular remodeling are discussed.

Pathological Ace2-to-Ace enzyme switch in the stressed heart is transcriptionally controlled by the endothelial Brg1-FoxM1 complex

Genes encoding angiotensin-converting enzymes (Ace and Ace2) are essential for heart function regulation. Cardiac stress enhances Ace, but suppresses Ace2, expression in the heart, leading to a net production of angiotensin II that promotes cardiac hypertrophy and fibrosis. The regulatory mechanism that underlies the Ace2-to-Ace pathological switch, however, is unknown. Here we report that the Brahma-related gene-1 (Brg1) chromatin remodeler and forkhead box M1 (FoxM1) transcription factor cooperate within cardiac (coronary) endothelial cells of pathologically stressed hearts to trigger the Ace2-to-Ace enzyme switch, angiotensin I-to-II conversion, and cardiac hypertrophy. In mice, cardiac stress activates the expression of Brg1 and FoxM1 in endothelial cells. Once activated, Brg1 and FoxM1 form a protein complex on Ace and Ace2 promoters to concurrently activate Ace and repress Ace2, tipping the balance to Ace2 expression with enhanced angiotensin II production, leading to cardiac hypertrophy and fibrosis. Disruption of endothelial Brg1 or FoxM1 or chemical inhibition of FoxM1 abolishes the stress-induced Ace2-to-Ace switch and protects the heart from pathological hypertrophy. In human hypertrophic hearts, BRG1 and FOXM1 expression is also activated in endothelial cells; their expression levels correlate strongly with the ACE/ACE2 ratio, suggesting a conserved mechanism. Our studies demonstrate a molecular interaction of Brg1 and FoxM1 and an endothelial mechanism of modulating Ace/Ace2 ratio for heart failure therapy.

Blood pressure-independent renoprotection in diabetic rats treated with AT1 receptor-neprilysin inhibition compared with AT1 receptor blockade alone

ARNI [dual AT1 (angiotensin II type 1) receptor-neprilysin inhibition] exerts beneficial effects on blood pressure and kidney function in heart failure, compared with ARB (AT1 receptor blockade) alone. We hypothesized that ARNI improves cardiac and kidney parameters in diabetic TGR(mREN2)27 rats, an angiotensin II-dependent hypertension model. Rats were made diabetic with streptozotocin for 5 or 12 weeks. In the final 3 weeks, rats were treated with vehicle, irbesartan (ARB) or irbesartan+thiorphan (ARNI). Blood pressure, measured by telemetry in the 5-week group, was lowered identically by ARB and ARNI. The heart weight/tibia length ratio in 12-week diabetic animals was lower after ARNI compared with after ARB. Proteinuria and albuminuria were observed from 8 weeks of diabetes onwards. ARNI reduced proteinuria more strongly than ARB, and a similar trend was seen for albuminuria. Kidneys of ARNI-treated animals showed less severe segmental glomerulosclerosis than those of ARB-treated animals. After 12 weeks, no differences between ARNI- and ARB-treated animals were found regarding diuresis, natriuresis, plasma endothelin-1, vascular reactivity (acetylcholine response) or kidney sodium transporters. Only ARNI-treated rats displayed endothelin type B receptor-mediated vasodilation. In conclusion, ARNI reduces proteinuria, glomerulosclerosis and heart weight in diabetic TGR(mREN2)27 rats more strongly than does ARB, and this occurs independently of blood pressure.

The Role of the (Pro)renin Receptor in Hypertensive Disease

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin (together denoted as (pro)renin). Since tissue renin levels are usually higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. The (pro)renin receptor ((P)RR), discovered in 2002, appeared a promising candidate, although its nanomolar affinity for renin/prorenin is many orders of magnitude above their levels in blood. This review discusses (P)RR-related research since its discovery. First, encouraging in vitro findings supported detrimental effects of (pro)renin-(P)RR interaction, even resulting in angiotensin-independent signaling. Moreover, the putative (P)RR blocker "handle region peptide" (HRP) yielded beneficial effects in various cardiovascular animal models. Then doubt arose whether such interaction truly occurs in vivo, and (P)RR deletion unexpectedly turned out to be lethal. Moreover, HRP results could not be confirmed. Finally, it was discovered that the (P)RR actually is a component of vacuolar-type H(+)-ATPase, a multisubunit protein found in virtually every cell type which is essential for vesicle trafficking, protein degradation, and coupled transport. Nevertheless, selective (P)RR blockade in the brain with the putative antagonist PRO20 (corresponding with the first 20 amino acids of prorenin's prosegment) reduced blood pressure in the deoxycorticosteroneacetate (DOCA)-salt model, and (P)RR gene single nucleotide polymorphisms associate with hypertension. To what degree this relates to (pro)renin remains uncertain. The concept of (P)RR blockade in hypertension, if pursued, requires rigorous testing of any newly designed antagonist, and may not hold promise given the early death of tissue-specific (P)RR knockout animals.

Optimum AT1 receptor-neprilysin inhibition has superior cardioprotective effects compared with AT1 receptor blockade alone in hypertensive rats

Neprilysin inhibitors prevent the breakdown of bradykinin and natriuretic peptides, promoting vasodilation and natriuresis. However, they also increase angiotensin II and endothelin-1. Here we studied the effects of a low and a high dose of the neprilysin inhibitor thiorphan on top of AT1 receptor blockade with irbesartan versus vehicle in TGR(mREN2)27 rats with high renin hypertension. Mean arterial blood pressure was unaffected by vehicle or thiorphan alone. Irbesartan lowered blood pressure, but after 7 days pressure started to increase again. Low- but not high-dose thiorphan prevented this rise. Only during exposure to low-dose thiorphan plus irbesartan did heart weight/body weight ratio, cardiac atrial natriuretic peptide expression, and myocyte size decrease significantly. Circulating endothelin-1 was not affected by low-dose thiorphan with or without irbesartan, but increased after treatment with high-dose thiorphan plus irbesartan. This endothelin-1 rise was accompanied by an increase in renal sodium-hydrogen exchanger 3 protein abundance, and an upregulation of constrictor vascular endothelin type B receptors. Consequently, the endothelin type B receptor antagonist BQ788 no longer enhanced endothelin-1-induced vasoconstriction (indicative of endothelin type B receptor-mediated vasodilation), but prevented it. Thus, optimal neprilysin inhibitor dosing reveals additional cardioprotective effects on top of AT1 receptor blockade in renin-dependent hypertension.

Deterioration of kidney function by the (pro)renin receptor blocker handle region peptide in aliskiren-treated diabetic transgenic (mRen2)27 rats

Dual renin-angiotensin system (RAS) blockade in diabetic nephropathy is no longer feasible because of the profit/side effect imbalance. (Pro)renin receptor [(P)RR] blockade with handle region peptide (HRP) has been reported to exert beneficial effects in various diabetic models in a RAS-independent manner. To what degree (P)RR blockade adds benefits on top of RAS blockade is still unknown. In the present study, we treated diabetic TGR(mREN2)27 rats, a well-established nephropathy model with high prorenin levels [allowing continuous (P)RR stimulation in vivo], with HRP on top of renin inhibition with aliskiren. Aliskiren alone lowered blood pressure and exerted renoprotective effects, as evidenced by reduced glomerulosclerosis, diuresis, proteinuria, albuminuria, and urinary aldosterone levels as well as diminished renal (P)RR and ANG II type 1 receptor expression. It also suppressed plasma and tissue RAS activity and suppressed cardiac atrial natriuretic peptide and brain natriuretic peptide expression. HRP, when given on top of aliskiren, did not alter the effects of renin inhibition on blood pressure, RAS activity, or aldosterone. However, it counteracted the beneficial effects of aliskiren in the kidney, induced hyperkalemia, and increased plasma plasminogen activator-inhibitor 1, renal cyclooxygenase-2, and cardiac collagen content. All these effects have been linked to (P)RR stimulation, suggesting that HRP might, in fact, act as a partial agonist. Therefore, the use of HRP on top of RAS blockade in diabetic nephropathy is not advisable.

Urinary renin and angiotensinogen in type 2 diabetes: added value beyond urinary albumin?

OBJECTIVE

Urinary levels of renin-angiotensin-aldosterone system (RAAS) components may reflect renal RAAS activity and/or the renal efficacy of RAAS inhibition. Our aim was to determine whether urinary angiotensinogen and renin are circulating RAAS-independent markers during RAAS blockade.

METHODS

Urinary and plasma levels of angiotensinogen, renin, and albumin were measured in 22 patients with type 2 diabetes, hypertension, and albuminuria, during 2-month treatment periods with placebo, aliskiren, irbesartan, or their combination in random order in a crossover study.

RESULTS

Aliskiren and irbesartan both increased plasma renin 3-4-fold, and above 10-fold when combined. Irbesartan decreased plasma angiotensinogen by approximately 25%, and no changes in plasma angiotensinogen were observed during the combination. Urine contained aliskiren at micromolar levels, blocking urinary renin by above 90%. Both blockers reduced urinary angiotensinogen, significant for irbesartan only. Combination blockade reduced urinary angiotensinogen even further. Reductions in urinary angiotensinogen paralleled albuminuria changes, and the urine/plasma concentration ratio of angiotensinogen was identical to that of albumin under all conditions. In contrast, urinary renin did not follow albumin, and remained unaltered after all treatments. Yet, the urine/plasma concentration ratio of renin was more than 100-fold higher than that of angiotensinogen and albumin, and approximately 4-fold reduced by single RAAS blockade, and more than 10-fold by dual RAAS blockade.

CONCLUSIONS

Aliskiren filters into urine and influences urinary renin measurements. The urine/plasma renin ratio, but not urinary renin alone, may reflect the renal efficacy of RAAS blockade. Urinary angiotensinogen is a marker of filtration barrier damage rather than intrarenal RAAS activity.

Urinary markers of intrarenal renin-angiotensin system activity in vivo

Recent interest focuses on urinary renin and angiotensinogen as markers of renal renin-angiotensin system activity. Before concluding that these components are independent markers, we need to exclude that their presence in urine, like that of albumin (a protein of comparable size), is due to (disturbed) glomerular filtration. This review critically discusses their filtration, reabsorption and local release. Given the close correlation between urinary angiotensinogen and albumin in human studies, it concludes that, in humans, urinary angiotensinogen is a filtration barrier damage marker with the same predictive power as urinary albumin. In contrast, in animals, tubular angiotensinogen release may occur, although tubulus-specific knockout studies do not support a functional role for such angiotensinogen. Urinary renin levels, relative to albumin, are >200-fold higher and unrelated to albumin. This may reflect release of renin from the urinary tract, but could also be attributed to activation of filtered, plasma-derived prorenin and/or incomplete tubular reabsorption.

The intrarenal renin-angiotensin system: does it exist? Implications from a recent study in renal angiotensin-converting enzyme knockout mice

A large body of evidence supports the presence of local production of angiotensins in the kidney. It is widely believed that renin-angiotensin system (RAS) blockers, through interference with such production and/or the local effects of angiotensin (Ang) II, exert protective renal effects. Yet, whether such production affects blood pressure independently from the circulating RAS is still a matter of debate. To investigate this, a recent study by Gonzalez-Villalobos et al. (J Clin Invest 2013; 123: 2011-2023) has studied the consequences of infusing Ang II or the nitric oxide synthase inhibitor l-NAME in mice lacking renal angiotensin-converting enzyme (ACE). They observed blunted blood pressure and renal responses in the renal ACE knockout mice versus wild-type controls. This review discusses to what degree these findings can be considered as unequivocal evidence for ACE-mediated Ang II formation in the kidney as an independent determinant of hypertension.

The (pro)renin receptor blocker handle region peptide upregulates endothelium-derived contractile factors in aliskiren-treated diabetic transgenic (mREN2)27 rats

BACKGROUND

Elevated prorenin levels associate with microvascular complications in patients with diabetes mellitus, possibly because prorenin affects vascular function in diabetes mellitus, for example by generating angiotensins following its binding to the (pro)renin receptor [(P)RR]. Here we evaluated whether the renin inhibitor aliskiren, with or without the putative (P)RR antagonist handle region peptide (HRP) improved the disturbed vascular function in diabetic TGR(mREN2)27 rats, a high-prorenin, high-(P)RR hypertensive model.

METHODS

Telemetry transmitters were implanted to monitor blood pressure. After 3 weeks of treatment, rats were sacrificed, and iliac and mesenteric arteries were removed to evaluate vascular reactivity.

RESULTS

Diabetes mellitus enhanced the contractile response to nitric oxide synthase (NOS) blockade, potentiated the response to phenylephrine, diminished the effectiveness of endothelin type A (ETA) receptor blockade and allowed acetylcholine to display constrictor, cyclo-oxygenase-2 mediated, endothelium-dependent responses in the presence of NOS inhibition and blockers of endothelium-derived hyperpolarizing factors. Aliskiren normalized blood pressure, suppressed renin activity, and reversed the above vascular effects, with the exception of the altered effectiveness of ETA receptor blockade. Remarkably, when adding HRP on top of aliskiren, its beneficial vascular effects either disappeared or were greatly diminished, although HRP did not alter the effect of aliskiren on blood pressure and renin activity.

CONCLUSIONS

Renin inhibition improves vascular dysfunction in diabetic hypertensive rats, and HRP counteracts this effect independently of blood pressure and angiotensin. (P)RR blockade therefore is unlikely to be a new tool to further suppress the renin-angiotensin system (RAS) on top of existing RAS blockers.

Intracellular renin alters the electrical properties of the intact heart ventricle of adult Sprague Dawley rats

The influence of intracellular renin injection on the electrical properties of the intact left ventricle from adult Sprague Dawley rat heart was investigated. Intracellular renin injection was performed using intracellular microelectrodes filled with solution containing renin (120pM). Pressure pulses (40-70psi) for short periods of time (20ms), were applied to the micropipette while recording the action potential simultaneously from the same fiber. The results indicated that intracellular renin caused a depolarization of ventricular fibers of 7.3±2±mV (n=38) (4 animals) (P<0.05) and a decrease of the action potential duration at 50% and at 90% repolarization, respectively. Moreover, the refractoriness was significantly decreased with consequent generation of triggered activity. The effect of intracellular renin was seen within 3min of enzyme injection. The shortening of the action potential was related to an increase of potassium current which was measured in isolated ventricular myocytes before and after intracellular dialysis of renin (10(-9)M) using a voltage whole cell clamp configuration. Valsartan (10(-8)M) dialyzed together with renin (120pM) into the cell decreased drastically the effect of renin on potassium current. An increment of potassium current was also found when intracellular renin was dialyzed into cardiomyocytes exposed to Krebs solution containing valsartan (10(-8)M) for 10min prior to renin administration. Bis-1 which is a specific inhibitor of PKC, abolished the effect of intracellular renin on potassium current.

IN CONCLUSION

intracellular renin decreases the action potential duration and cardiac refractoriness in the intact left ventricle of adult Sprague Dawley rats. The shortening of the action potential was related to an increase in total potassium current. The effect of renin on total potassium currents was inhibited by valsartan and by Bis-1. Implication for cardiac arrhythmias was discussed.

Plasma levels of prorenin and renin in blacks and whites: their relative abundance and associations with plasma aldosterone concentration

BACKGROUND

All renin arises from prorenin. The proportion of renin relative to prorenin could influence overall renin-angiotensin-aldosterone activity. We sought to determine whether prorenin levels were related to extracellular volume, as reflected by the levels of plasma renin activity (PRA), and to aldosterone.

METHODS

We analyzed plasma levels of prorenin, renin, and aldosterone, as well as their interactions, in 129 young blacks and whites.

RESULTS

Blacks had lower plasma renin concentration (PRC) and PRA, but had prorenin levels similar to whites (69 pg/ml in blacks vs. 62 pg/ml in whites, P = 0.41). As a result, the renin-to-total renin ratio was significantly lower in blacks (11.5% in blacks as compared to 19.8% in whites; P = 0.0001). Because prorenin also resides in tissues including the adrenal where it can bind to a specific receptor to generate angiotensin II, we examined the relationship of prorenin levels to plasma aldosterone concentrations (PAC). While a positive association between PRC and PAC was found in both blacks and whites, PAC was positively related to prorenin in whites (P = 0.04) but negatively in blacks, an observation that we hypothesize was due to reduced prorenin-to-renin conversion in blacks.

CONCLUSIONS

We observed a disproportionately high level of prorenin in blacks. These high circulating prorenin levels however do not result in greater adrenal angiotensin II and aldosterone production in healthy young blacks.

AT1 antagonism and renin inhibition in mice: pivotal role of targeting angiotensin II in chronic kidney disease

The role of the renin-angiotensin system in chronic kidney disease involves multiple peptides and receptors. Exerting antipodal pathophysiological mechanisms, renin inhibition and AT(1) antagonism ameliorate renal damage. However, it is unclear which mechanism exerts better nephroprotection. We compared the renin inhibitor aliskiren with the AT(1) antagonist losartan in mice with chronic kidney disease due to renal ablation. Doses were adjusted to equipotent inhibition of the renin-angiotensin system, determined via a dose-response quantifying plasma and renal renin expression. Six-week treatment with either 500 mg/l drinking water losartan or 50 mg·kg(-1)·day(-1) aliskiren significantly decreased albuminuria, glomerular damage, and transcription rates of renal injury markers to a similar extent. An array analysis comparing renal gene expression of losartan- and aliskiren-treated mice evaluating >34,000 transcripts demonstrated regulation for 14 genes only, with small differences. No superior nephroprotection was found by combining losartan and aliskiren. Compared with plasma concentrations, aliskiren accumulated ∼7- to 29-fold in the heart, liver, lung, and spleen and ∼156-fold in the kidney. After withdrawal, plasma concentrations dropped to zero within 24 h, whereas renal tissue concentrations declined slowly over days. Withdrawal of aliskiren in mice with chronic kidney disease revealed a significantly delayed re-increase in albuminuria compared with withdrawal of losartan. This study demonstrates equieffective nephroprotection of renin inhibition and AT(1) antagonism in mice with chronic kidney disease without additional benefit of combination therapy. These observations underscore the pivotal role of targeting ANG II to reduce renal injury.

(Pro)renin and its receptors: pathophysiological implications

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin [together denoted as (pro)renin]. Since tissue renin levels are usually somewhat higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. Several candidates have been evaluated in the past three decades, including a renin-binding protein, the mannose 6-phosphate/insulin-like growth factor II receptor and the (pro)renin receptor. Although the latter seemed the most promising, its nanomolar affinity for renin and prorenin is several orders of magnitude above their actual (picomolar) levels in blood, raising doubt on whether (pro)renin–(pro)renin receptor interaction will ever occur in vivo. A wide range of in vitro studies have now demonstrated (pro)renin-receptor-induced effects at nanomolar renin and prorenin concentrations, resulting in a profibrotic phenotype. In addition, beneficial in vivo effects of the putative (pro)renin receptor blocker HRP (handle region peptide) have been observed, particularly in diabetic animal models. Despite these encouraging results, many other studies have reported either no or even contrasting effects of HRP, and (pro)renin-receptor-knockout studies revealed lethal consequences that are (pro)renin-independent, most probably due to the fact that the (pro)renin receptor co-localizes with vacuolar H+-ATPase and possibly determines the stability of this vital enzyme. The present review summarizes all of the recent findings on the (pro)renin receptor and its blockade, and critically compares it with the other candidates that have been proposed to mediate (pro)renin uptake from blood. It ends with the conclusion that the (pro)renin–(pro)renin receptor interaction, if it occurs in vivo, is limited to (pro)renin-synthesizing organs such as the kidney.

Angiotensin II type 2 receptor agonists: where should they be applied?

INTRODUCTION

Angiotensin II, the active endproduct of the renin-angiotensin system (RAS), exerts its effects via angiotensin II type 1 and type 2 (AT(1), AT(2)) receptors. AT(1) receptors mediate all well-known effects of angiotensin II, ranging from vasoconstriction to tissue remodeling. Thus, to treat cardiovascular disease, RAS blockade aims at preventing angiotensin II-AT(1) receptor interaction. Yet RAS blockade is often accompanied by rises in angiotensin II, which may exert beneficial effects via AT(2) receptors.

AREAS COVERED

This review summarizes our current knowledge on AT(2) receptors, describing their location, function(s), endogenous agonist(s) and intracellular signaling cascades. It discusses the beneficial effects obtained with C21, a recently developed AT(2) receptor agonist. Important questions that are addressed are do these receptors truly antagonize AT(1) receptor-mediated effects? What about their role in the diseased state and their heterodimerization with other receptors?

EXPERT OPINION

The general view that AT(2) receptors exclusively exert beneficial effects has been challenged, and in pathological models, their function sometimes mimics that of AT(1) receptors, for example, inducing vasoconstriction and cardiac hypertrophy. Yet given its upregulation in various pathological conditions, the AT(2) receptor remains a promising target for treatment, allowing effects beyond blood pressure-lowering, for example, in stroke, aneurysm formation, inflammation and myocardial fibrosis.

The role of the renin-angiotensin system in thoracic aortic aneurysms: clinical implications

Thoracic aortic aneurysms (TAAs) are a potential life-threatening disease with limited pharmacological treatment options. Current treatment options are aimed at lowering aortic hemodynamic stress, predominantly with β-adrenoceptor blockers. Increasing evidence supports a role for the renin-angiotensin system (RAS) in aneurysm development. RAS blockade would not only lower blood pressure, but might also target the molecular pathways involved in aneurysm formation, in particular the transforming growth factor-β and extracellular signal-regulated kinase 1/2 pathways. Indeed, the angiotensin II type 1 (AT₁) receptor blocker losartan was effective in lowering aortic root growth in mice and patients with Marfan's syndrome. RAS inhibition (currently possible at 3 levels, i.e. renin, ACE and the AT₁ receptor) is always accompanied by a rise in renin due to interference with the negative feedback loop between renin and angiotensin II. Only during AT₁ receptor blockade will this result in stimulation of the non-blocked angiotensin II type 2 (AT₂) receptor. This review summarizes the clinical aspects of TAAs, provides an overview of the current mouse models for TAAs, and focuses on the RAS as a new target for TAA treatment, discussing in particular the possibility that AT₂ receptor stimulation might be crucial in this regard. If true, this would imply that AT₁ receptor blockers (and not ACE inhibitors or renin inhibitors) should be the preferred treatment option for TAAs.

Beneficial cardiac effects of the renin inhibitor aliskiren in spontaneously hypertensive rats

BACKGROUND

The blood pressure-lowering effect of the renin inhibitor aliskiren equals that of angiotensin-converting enzyme (ACE) inhibitors and angiotensin (Ang) II type 1 (AT1) receptor blockers. Whether aliskiren offers end-organ protection remains to be investigated. Here, we compared the cardiac effects of aliskiren, the AT1 receptor blocker irbesartan and the ACE inhibitor captopril in spontaneously hypertensive rats (SHR) at equi-hypotensive doses.

METHODS AND RESULTS

SHR were treated for 1-3 weeks with vehicle, aliskiren, captopril or irbesartan (100, 3 and 15 mg/kg per day, respectively) using an osmotic minipump, and compared to vehicle-treated Wistar-Kyoto (WKY) controls. All drugs lowered (but not normalized) mean arterial pressure in SHR equi-effectively, as monitored by radiotelemetry, without altering heart rate. All drugs also reduced the increased cardiomyocyte area in SHR, and tended to normalize the elevated brain natriuretic peptide plasma levels. In the Langendorff set-up, all drugs normalized the diminished endothelium-dependent vasodilator response to bradykinin in SHR. Moreover, aliskiren and irbesartan, but not captopril, decreased the enhanced coronary Ang II response in SHR. Aliskiren reduced plasma renin activity and the plasma and tissue angiotensin levels at 1 week of treatment; yet, after 3 weeks of aliskiren treatment only the cardiac angiotensin levels remained suppressed, whereas no tissue angiotensin reductions were seen with captopril or irbesartan.

CONCLUSION

For a given decrease in blood pressure, aliskiren improves coronary endothelial function and decreases cardiac hypertrophy in SHR to at least the same degree as ACE inhibition and AT1 receptor blockade. In addition, aliskiren diminishes the enhanced Ang II response in the coronary circulation of SHR and offers superior long-term cardiac angiotensin suppression.

Steroidogenesis vs. steroid uptake in the heart: do corticosteroids mediate effects via cardiac mineralocorticoid receptors?

OBJECTIVE

To test whether glucocorticoids act as the endogenous agonist of cardiac mineralocorticoid receptors, we evaluated the cardiac effects of aldosterone and corticosterone and cardiac steroidogenesis vs. steroid uptake from plasma.

METHODS AND RESULTS

Both corticosterone and aldosterone increased left ventricular pressure in the rat heart. Aldosterone decreased coronary flow, whereas corticosterone increased it. All corticosterone effects were blocked by the glucocorticoid receptor antagonist, RU486, and unaltered by the mineralocorticoid receptor antagonist, canrenoate, or the 11beta-hydroxysteroid dehydrogenase (HSD11B)2 inhibitor, carbenoxolone. Unlike mineralocorticoid receptor blockade, RU486 did not ameliorate postischemia infarct size and arrhythmias. Corticosterone, when added to the perfusion buffer, rapidly accumulated at cardiac tissue sites, reaching steady-state levels that were identical to those in coronary effluent, independently of the presence of aldosterone, RU486 or canrenoate. After stopping the perfusion, cardiac corticosterone fully washed away with a half-life of less than 1 min. Measurements of steroid-synthesizing enzyme gene expression levels in human ventricular and atrial tissue pieces from heart-beating organ donors, patients with end-stage heart failure and hypertrophic cardiomyopathy patients revealed that under no condition, the human heart was capable of synthesizing aldosterone or cortisol de novo. Yet, expression of HSD11B1, HSD11B2, mineralocorticoid receptors and glucocorticoid receptors was found, and HSD11B2 and mineralocorticoid receptors were upregulated in pathological conditions. Moreover, aldosterone reduced cardiac inotropy in a Na/K/2Cl cotransporter-dependent manner.

CONCLUSION

Both cortisol/corticosterone and aldosterone accumulate in the cardiac interstitium. The presence of HSD11B2 and mineralocorticoid receptors/glucocorticoid receptors at cardiac tissue sites allows both steroids to exert their effects via separate mechanisms.

Mast cell degranulation mediates bronchoconstriction via serotonin and not via renin release

To verify the recently proposed concept that mast cell-derived renin facilitates angiotensin II-induced bronchoconstriction bronchial rings from male Sprague-Dawley rats were mounted in Mulvany myographs, and exposed to the mast cell degranulator compound 48/80 (300 microg/ml), angiotensin I, angiotensin II, bradykinin or serotonin (5-hydroxytryptamine, 5-HT), in the absence or presence of the renin inhibitor aliskiren (10 micromol/l), the ACE inhibitor captopril (10 micromol/l), the angiotensin II type 1 (AT1) receptor blocker irbesartan (1 micromol/l), the mast cell stabilizer cromolyn (0.3 mmol/l), the 5-HT2A/2C receptor antagonist ketanserin (0.1 micromol/l) or the alpha1-adrenoceptor antagonist phentolamine (1 micromol/l). Bath fluid was collected to verify angiotensin generation. Bronchial tissue was homogenized to determine renin, angiotensinogen and serotonin content. Compound 48/80 contracted bronchi to 24+/-4% of the KCl-induced contraction. Ketanserin fully abolished this effect, while cromolyn reduced the contraction to 16+/-5%. Aliskiren, captopril, irbesartan and phentolamine did not affect this response, and the angiotensin I and II levels in the bath fluid after 48/80 exposure were below the detection limit. Angiotensin I and II equipotently contracted bronchi. Captopril shifted the angiotensin I curve approximately 10-fold to the right, whereas irbesartan fully blocked the effect of angiotensin II. Bradykinin-induced constriction was shifted approximately 100-fold to the left with captopril. Serotonin contracted bronchi, and ketanserin fully blocked this effect. Finally, bronchial tissue contained serotonin at micromolar levels, whereas renin and angiotensinogen were undetectable in this preparation. In conclusion, mast cell degranulation results in serotonin-induced bronchoconstriction, and is unlikely to involve renin-induced angiotensin generation.

The increase in renin during renin inhibition: does it result in harmful effects by the (pro)renin receptor?

Renin inhibitors, similar to all renin-angiotensin system (RAS) blockers, increase the plasma concentration of renin because they attenuate the negative feedback effect of angiotensin (Ang) II on renin release. The increase in renin has been suggested to be higher than that during other types of RAS blockade. This could potentially limit the effectiveness of renin inhibition, either because Ang II generation might occur again ('Ang II escape'), possibly even at the levels above baseline, as has been described before for angiotensin-converting enzyme inhibitors, or because high levels of renin will stimulate the recently discovered (pro)renin receptor, and thus induce effects in an Ang-independent manner. This review shows first that the cause(s) of the renin increase during treatment with the renin inhibitor aliskiren is the consequence of a combination of factors, including an assay artifact, allowing the detection of prorenin as renin, and a change in renin half-life. When correcting for these phenomena the increase is unlikely to be as excessive as originally thought. The review then critically describes the consequence(s) of such a increase, concluding (i) that an Ang II escape is highly unlikely, given the [aliskiren]/[renin] stoichiometry, and (ii) that renin and prorenin downregulate their receptor (similar to many agonists). On the basis of the latter, one could even speculate that this will be more substantial when the renin and prorenin levels are higher. Thus, from this point of view the larger increase in renin during renin inhibition will cause a stronger reduction in (pro)renin receptor expression, and a greater suppression of (pro)renin receptor-mediated effects than other renin-Ang blockers.

(Pro)renin receptors: are they biologically relevant?

PURPOSE OF REVIEW

The recent introduction of a renin inhibitor, aliskiren, into the clinical arena has revived interest in renin and its precursor prorenin. In addition, a renin-binding and prorenin-binding receptor has been found, which not only activates prorenin but also induces angiotensin-independent signaling. This review addresses the question of whether this receptor has any biological relevance.

RECENT FINDINGS

Prorenin is the preferred agonist of the (pro)renin receptor. When bound to the receptor, prorenin undergoes a conformational change allowing it to display full enzymatic activity. Receptor activation by renin/prorenin triggers the mitogen-activated protein kinase-extracellular signal-regulated kinase 1/2 signaling pathway, and human (pro)renin receptor transgenic rats develop glomerulosclerosis and hypertension in the absence of changes in renin or angiotensin. Aliskiren prevents angiotensin I generation by receptor-bound prorenin but does not block signaling. Conflicting results have been obtained with the putative (pro)renin receptor antagonist 'handle region peptide', suggesting that its efficacy depends on experimental conditions.

SUMMARY

Although it is tempting to speculate that the (pro)renin receptor is the missing link providing a role for prorenin in tissue angiotensin generation, the discrepant results with handle region peptide and the lack of clinical studies with (pro)renin receptor blockers do not yet firmly support such a role.

Prolonged exposure of cardiac cells to renin plus angiotensinogen reduces intracellular renin in the failing heart. On the role of angiotensin II-AT1 complex internalization

To investigate the influence of prolonged exposure of cardiac cells to renin plus angiotensinogen (Ao) on intracellular renin levels, myocytes were isolated from the ventricle of cardiomyopathic hamsters(TO-2) and incubated in Krebs solution containing renin(128 pmol Ang ml/min) plus Ao (110 pmol Ang I generated by renin to exhaustion) for a period of 24 h. Membrane-bound and intracellular AT1 receptors levels as well as intracellular renin were studied using immunological methods and quantified by flow cytometry. The results indicated: a) intracellular renin levels were higher in the failing heart at an advanced stage of the disease (8 months) than in age-matched controls; b) the intracellular renin levels were significantly reduced in cells exposed to renin (128 pmol Ang I.ml/min) plus angiotensinogen (Ao)(110 pmol Ang I generated by renin to exhaustion) for a period of 24 h; c) incubation of the cardiomyocytes with renin (128 pmol Ang I.ml/min) alone did not reduced the intracellular renin levels; d) the fall of the intracellular renin level was related to the formation of angiotensin II (Ang II) at the surface cell membrane and internalization of the Ang II-AT1 complex because losartan (10(-7) M) added to the incubation medium containing renin plus Ao, blocked the internalization of AT1 and suppressed the decline of the intracellular renin levels; e) no internalization of renin or renin secretion was found in these experiments.

IN CONCLUSION

prolonged exposure of cardiac cells to renin plus Ao (24 h) reduced intracellular renin levels through the internalization of Ang II-AT1 complex and inhibition of renin expression.

The impact of left ventricular assist device-induced left ventricular unloading on the myocardial renin-angiotensin-aldosterone system: therapeutic consequences?

AIMS

Angiotensin-converting enzyme inhibitors (ACE-Is) prevent the rise in myocardial angiotensin II that occurs after left ventricular assist device (LVAD) implantation, but do not fully normalize cardiac function. Here, we determined the effect of LVAD implantation, with or without ACE-Is, on cardiac renin, aldosterone, and norepinephrine, since these hormones, like angiotensin II, are likely determinants of myocardial recovery during LVAD support.

METHODS AND RESULTS

Biochemical measurements were made in paired LV myocardial samples obtained from 20 patients before and after LVAD support in patients with and without ACE-I therapy. Pre-LVAD renin levels were 100x normal and resulted in almost complete cardiac angiotensinogen depletion. In non-ACE-I users, LVAD support, by normalizing blood pressure, reversed this situation. Cardiac aldosterone decreased in parallel with cardiac renin, in agreement with the concept that cardiac aldosterone is blood-derived. Cardiac norepinephrine increased seven-fold, possibly due to the rise in angiotensin II. Angiotensin-converting enzyme inhibitor therapy prevented these changes: renin and aldosterone remained high, and no increase in norepinephrine occurred.

CONCLUSION

Although LV unloading lowers renin and aldosterone, it allows cardiac angiotensin generation to increase and thus to activate the sympathetic nervous system. Angiotensin-converting enzyme inhibitors prevent the latter, but do not affect aldosterone. Thus, mineralocorticoid receptor antagonist therapy during LVAD support may play a role in further promoting recovery.