Specific measurement of angiotensin metabolites and in vitro generated angiotensin II in plasma

Quantification of endogenous Angiotensin 1-10, 1-9, 1-8, 1-7, and 1-5 in human plasma using micro-UHPLC-MS/MS: Outlining the importance of the pre-analytics for reliable results

Angiotensin peptides (ANGs) play a central role in the renin-angiotensin-aldosterone system, rendering them interesting biomarkers associated with hypertension. Precise quantification of circulating ANGs holds the potential to assess the activity of angiotensin-converting enzyme (ACE), a key protease targeted by widely prescribed drugs, namely ACE inhibitors. This ability could pave the way for personalised medicine, offering insights into the prescription of inhibitors targeting either the proteases or the receptors within the system. Despite recent developments in liquid chromatography-mass spectrometry (LC-MS) methods for measuring circulating ANG concentrations, comprehensive stability studies of ANGs in human plasma are absent in the literature, raising concerns about the reliability of measured concentrations and their link to clinical conditions. To address this critical gap, we conducted an exhaustive evaluation of the pre-analytical stability of ANG1-10, ANG1-9, ANG1-8, ANG1-7, and ANG1-5. By employing surfactants to mitigate non-specific adsorption and a dedicated mix of protease inhibitors to limit protease activity, we established an MS-based assay for these five peptides. We used this method to quantify circulating concentrations of ANGs in the plasma of 11 healthy donors and 3 patients under kidney dialysis. Our findings revealed that ANG1-10 and ANG1-8 circulate at concentrations ranging from 1 to 10 pM in healthy subjects and exhibit a high degree of correlation. Notably, ANG1-9, ANG1-7, and ANG1-5 were undetectable in any of the 14 patients, despite a sub-picomolar limit of detection. This strikingly contrasts with the reference concentrations reported in the literature, which typically fall within the picomolar range. In light of these discrepancies, we strongly advocate for rigorous pre-analytical considerations and comprehensive stability studies to ensure reliable results. We emphasise the pivotal role of heightened pre-analytical awareness within the clinical chemistry community, and we hope for continued growth in this critical area.

Mathematical Modeling of Impacts of Patient Differences on COVID-19 Lung Fibrosis Outcomes

Patient-specific premorbidity, age, and sex are significant heterogeneous factors that influence the severe manifestation of lung diseases, including COVID-19 fibrosis. The renin-angiotensin system (RAS) plays a prominent role in regulating effects of these factors. Recent evidence suggests that patient-specific alteration of RAS homeostasis with premorbidity and the expression level of angiotensin converting enzyme 2 (ACE2), depending on age and sex, is correlated with lung fibrosis. However, conflicting evidence suggests decreases, increases, or no changes in RAS after SARS-CoV-2 infection. In addition, detailed mechanisms connecting the patient-specific conditions before infection to infection-induced fibrosis are still unknown. Here, a mathematical model is developed to quantify the systemic contribution of heterogeneous factors of RAS in the progression of lung fibrosis. Three submodels are connected-a RAS model, an agent-based COVID-19 in-host immune response model, and a fibrosis model-to investigate the effects of patient-group-specific factors in the systemic alteration of RAS and collagen deposition in the lung. The model results indicate cell death due to inflammatory response as a major contributor to the reduction of ACE and ACE2, whereas there are no significant changes in ACE2 dynamics due to viral-bound internalization of ACE2. Reduction of ACE reduces the homeostasis of RAS including angiotensin II (ANGII), while the decrease in ACE2 increases ANGII and results in severe lung injury and fibrosis. The model explains possible mechanisms for conflicting evidence of RAS alterations in previously published studies. Also, the results show that ACE2 variations with age and sex significantly alter RAS peptides and lead to fibrosis with around 20% additional collagen deposition from systemic RAS with slight variations depending on age and sex. This model may find further applications in patient-specific calibrations of tissue models for acute and chronic lung diseases to develop personalized treatments.

Effect of diuretics on plasma aldosterone and potassium in primary hypertension: A systematic review and meta-analysis

AIM

By contrast with drugs inhibiting the renin-angiotensin-aldosterone system (RAAS), diuretics stimulate renin release by the kidneys. Although plasma aldosterone (PA) is thought to be mainly regulated by RAAS activity, serum potassium has been shown to be an important factor in animal models and humans. Here we perform a systematic review and meta-analysis of randomised controlled trials (RCT) in hypertension investigating the effects of diuretic therapy on PA and the correlation of change in PA with that of potassium and blood pressure (BP).

METHODS

Three databases were searched: MEDLINE, EMBASE and the Cochrane Central Register of Controlled Trials (CENTRAL). Titles were first screened by title and abstract for relevance before full-text articles were assessed for eligibility according to a predefined inclusion/exclusion criteria.

RESULTS

A total of 1139 articles were retrieved, of which 42 met the prespecified inclusion/exclusion criteria. The average standardised difference in mean PA was similar for all classes of diuretic: thiazide/thiazide-like 0.299 (95% confidence interval [CI] 0.150, 0.447), loop 0.927 (0.37, 1.49), MRA/potassium-sparing 0.265 (0.173, 0.357) and combination 0.466 (0.137, 0.796), Q = 6.33, P = .097. In subjects untreated with another antihypertensive, there was a significant relationship between change in PA and change in systolic BP but no relationship with the change in potassium.

CONCLUSION

In RCTs of diuretic therapy in hypertension, there is an increase in PA with all classes of diuretic and no significant between-class heterogeneity. Change in PA is not related with potassium but correlates with the change in BP in subjects untreated with another antihypertensive medication.

Robust analysis of angiotensin peptides in human plasma: Column switching-parallel LC/ESI-SRM/MS without adsorption or enzymatic decomposition

Angiotensin (Ang) peptides are the main effectors of the renin-angiotensin system (RAS) regulating diverse physiological conditions and are involved in renal and vascular diseases. Currently, quantitative analyses of Ang peptides in human plasma mainly rely on radioimmunoassay-based methods whose reported levels are quite divergent. Analyses are further complicated by the potential of Ang peptides to bind to solid surfaces, to be enzymatically decomposed during sample preparation, and to undergo post-translational modifications. A column switching-parallel LC/ESI-SRM/MS method has been developed for seven Ang peptides (Ang I, Ang II, Ang III, Ang IV, Ang 1-9, Ang 1-7, and Ang A) in human plasma. Aqueous acetonitrile (5%) containing 50 mM arginine (Arg) as a dissolving solution and a combination of protease inhibitors with formic acid were used to prevent adsorption and enzymatic degradation, respectively. Plasma samples were simply deproteinized with acetonitrile followed by clean-up with an on-line trap column via column-switching. Stable isotope dilution with [¹³C₅,¹⁵N₁-Val]-Ang peptides as internal standards was employed for quantitative analysis. The current methodology has been successfully applied to determine the plasma levels of Ang peptides in healthy participants, suggesting future applicability to studies of various diseases related to RAS.

Modeling the Molecular Impact of SARS-CoV-2 Infection on the Renin-Angiotensin System

SARS-CoV-2 infection is mediated by the binding of its spike protein to the angiotensin-converting enzyme 2 (ACE2), which plays a pivotal role in the renin-angiotensin system (RAS). The study of RAS dysregulation due to SARS-CoV-2 infection is fundamentally important for a better understanding of the pathogenic mechanisms and risk factors associated with COVID-19 coronavirus disease and to design effective therapeutic strategies. In this context, we developed a mathematical model of RAS based on data regarding protein and peptide concentrations; the model was tested on clinical data from healthy normotensive and hypertensive individuals. We used our model to analyze the impact of SARS-CoV-2 infection on RAS, which we modeled through a downregulation of ACE2 as a function of viral load. We also used it to predict the effect of RAS-targeting drugs, such as RAS-blockers, human recombinant ACE2, and angiotensin 1-7 peptide, on COVID-19 patients; the model predicted an improvement of the clinical outcome for some drugs and a worsening for others. Our model and its predictions constitute a valuable framework for in silico testing of hypotheses about the COVID-19 pathogenic mechanisms and the effect of drugs aiming to restore RAS functionality.

Neurocardiac regulation: from cardiac mechanisms to novel therapeutic approaches

Cardiac sympathetic overactivity is a well-established contributor to the progression of neurogenic hypertension and heart failure, yet the underlying pathophysiology remains unclear. Recent studies have highlighted the importance of acutely regulated cyclic nucleotides and their effectors in the control of intracellular calcium and exocytosis. Emerging evidence now suggests that a significant component of sympathetic overactivity and enhanced transmission may arise from impaired cyclic nucleotide signalling, resulting from compromised phosphodiesterase activity, as well as alterations in receptor-coupled G-protein activation. In this review, we address some of the key cellular and molecular pathways that contribute to sympathetic overactivity in hypertension and discuss their potential for therapeutic targeting.

Angiotensin peptide synthesis and cyclic nucleotide modulation in sympathetic stellate ganglia

Chronically elevated angiotensin II is a widely-established contributor to hypertension and heart failure via its action on the kidneys and vasculature. It also augments the activity of peripheral sympathetic nerves through activation of presynaptic angiotensin II receptors, thus contributing to sympathetic over-activity. Although some cells can synthesise angiotensin II locally, it is not known if this machinery is present in neurons closely coupled to the heart. Using a combination of RNA sequencing and quantitative real-time polymerase chain reaction, we demonstrate evidence for a renin-angiotensin synthesis pathway within human and rat sympathetic stellate ganglia, where significant alterations were observed in the spontaneously hypertensive rat stellate ganglia compared with Wistar stellates. We also used Förster Resonance Energy Transfer to demonstrate that administration of angiotensin II and angiotensin 1-7 peptides significantly elevate cyclic guanosine monophosphate in the rat stellate ganglia. Whether the release of angiotensin peptides from the sympathetic stellate ganglia alters neurotransmission and/or exacerbates cardiac dysfunction in states associated with sympathetic over activity remains to be established.

Tonin Overexpression in Mice Diminishes Sympathetic Autonomic Modulation and Alters Angiotensin Type 1 Receptor Response

Background: Tonin, a serine-protease that forms Angiotensin II (AngII) from angiotensinogen, is increased in failing human heart samples. Increased blood pressure (BP) and decreased heart rate (HR) variabilities are associated with higher risk of cardiovascular morbidity. Losartan has been used to reduce hypertension and, therefore, lowers the risk of fatal and non-fatal cardiovascular events. Determination of tonin's impact on BP and HR variabilities as well as the impact of losartan remain questions to be elucidated.

Aim: Evaluation of cardiovascular autonomic profile in transgenic mice overexpressing the rat tonin enzyme TGM'(rton) and the impact of AT1 receptor blocker, losartan.

Methods: Male C57BL/6 (WT) and TGM'(rTon) mice were cannulated for recording BP (Windaq, 4 MHz) for 30 min at baseline and 30 min after losartan injection (20 mg/kg). BP and HR variabilities were analyzed in time and frequency domain method. Low-frequency (LF) and high-frequency (HF) components were identified for sympathetic and parasympathetic modulations analysis. Ang I, AngII, and Ang1-7 were quantified by high performance liquid chromatography method. The total enzymatic activity for AngI, AngII, and Ang1-7 formation was evaluated in the heart and plasma by Liquid chromatography mass spectrometry (LC-MS/MS).

Results: At the baseline TGM'(rTon) exhibited higher BP, lower cardiac LF, higher cardiac HF, lower LF/HF, and lower alpha index than wild type (WT). After losartan injection, TGM'(rTon) mice presented an additional decrease in cardiac LF and increase in HF in relation to baseline and WT. In the vasculature, losartan caused decreased in BP and LF of systolic BP in WT mice in relation to its baseline. A similar effect was observed in the BP of TGM'(rTon) mice; however, LF of systolic BP increased compared to baseline. Our data also indicates that AT1R receptor signaling has been altered in TGM’(rTon)mice. Interestingly, the dynamics of the renin-angiotensin system kinetics change, favoring production of Ang1-7.

Conclusion: Autonomic evaluation of TGM’(rTon) mice indicates an unclear prognosis for diseases that affect the heart. HR variability in TGM’(rTon) mice indicates high risk of morbidity, and sympathetic and parasympathetic modulation indicate low risk of morbidity. The low risk of morbidity could be the biased production of Ang1-7 in the heart and circulation; however, the altered response of AT1R in the TGM’(rTon) remains to be elucidated, as well aswhether that signaling is pro-protection or pro-pathology.

Angiotensinergic Innervation of the Human Right Atrium: Implications for Cardiac Reflexes

BACKGROUND

The right atrium is densely innervated and provides sensory input to important cardiocirculatory reflexes controlling cardiac output and blood pressure. Its angiotensin (Ang) II-expressing innervation may release Ang II as a neuropeptide cotransmitter to modulate reflexes but has not yet been characterized.

METHODS

Intraoperative surgical biopsies from human right atria (n = 7) were immunocytologically stained for Ang II, tyrosine hydroxylase (TH), and synaptophysin (SYN). Tissue angiotensins were extracted and quantified by radioimmunoassay.

RESULTS

Angiotensinergic fibers were frequent in epicardial nerves and around vessels with variable TH co-localization (none to >50%/bundle). Fibers were also widely distributed between cardiomyocytes and in the endocardium where they were typically nonvaricose, TH/SYN-negative and usually accompanied by varicose catecholaminergic fibers. In the endocardium, some showed large varicosities and were partially TH or SYN-positive. A few endocardial regions showed scattered nonvaricose Ang fibers ending directly between endothelial cells. Occasional clusters of thin varicose terminals co-localizing SYN or TH were located underneath, or protruded into, the endothelium. Endocardial density of Ang and TH-positive fibers was 30-300 vs. 200-450/mm2. Atrial Ang II, III, and I concentrations were 67, 16, and 5 fmol/g (median) while Ang IV and V were mostly undetectable.

CONCLUSIONS

The human right atrium harbors an abundant angiotensinergic innervation and a novel potential source of atrial Ang II. Most peripheral fibers were noncatecholaminergic afferents or preterminal vagal efferents and a minority was presumably sympathetic. Neuronal Ang II release from these fibers may modulate cardiac and circulatory reflexes independently from plasma and tissue Ang II sources.

Resetting of renal tissular renin-angiotensin and bradykinin-kallikrein systems after unilateral kidney denervation in rats

The renal tissular renin-angiotensin and bradykinin-kallikrein systems control kidney function together with the renal sympathetic innervation but their interaction is still unclear. To further elucidate this relationship, we investigated these systems in rats 6 days after left kidney denervation (DNX, n = 8) compared to sham-operated controls (CTR, n = 8). Plasma renin concentration was unchanged in DNX vs. CTR (p = NS). Kidney bradykinin (BK) and angiotensin (Ang) I and II concentrations decreased bilaterally in DNX vs. CTR rats (~20 to 40%, p < 0.05) together with Ang IV and V concentrations that were extremely low (p = NS). Renin, Ang III and dopamine concentrations decreased by ~25 to 50% and norepinephrine concentrations by 99% in DNX kidneys (p < 0.05) but were unaltered in opposite kidneys. Ang II/I and KA were comparable in DNX, contralateral and CTR kidneys. Ang III/II increased in right vs. DNX or CTR kidneys (40-50%, p < 0.05). Ang II was mainly located in tubular epithelium by immunocytological staining and its cellular distribution was unaffected by DNX. Moreover, the angiotensinergic and catecholaminergic innervation of right kidneys was unchanged vs. CTR. We found an important dependency of tissular Ang and BK levels on the renal innervation that may contribute to the resetting of kidney function after DNX. The DNX-induced peptide changes were not readily explained by kidney KA, renin or plasma Ang I generation. However, tissular peptide metabolism and compartmentalization may have played a central role. The mechanisms behind the concentration changes remain unclear and deserve further clarification.

Meta-analysis of effects of obstructive sleep apnea on the renin-angiotensin-aldosterone system

Background

Obstructive sleep apnea (OSA) is the most common cause of resistant hypertension, which has been proposed to result from activation of the renin–angiotensin–aldosterone system (RAAS). We meta-analyzed the effects of OSA on plasma levels of RAAS components.

Methods

Full-text studies published on MEDLINE and EMBASE analyzing fasting plasma levels of at least one RAAS component in adults with OSA with or without hypertension. OSA was diagnosed as an apnea-hypopnea index or respiratory disturbance index ≥ 5. Study quality was evaluated using the Newcastle-Ottawa Scale, and heterogeneity was assessed using the I² statistic. Results from individual studies were synthesized using inverse variance and pooled using a random-effects model. Subgroup analysis, sensitivity analysis, and meta-regression were performed, and risk of publication bias was assessed.

Results

The meta-analysis included 13 studies, of which 10 reported results on renin (n = 470 cases and controls), 7 on angiotensin II (AngII, n = 384), and 9 on aldosterone (n = 439). AngII levels were significantly higher in OSA than in controls [mean differences = 3.39 ng/L, 95% CI: 2.00–4.79, P < 0.00001], while aldosterone levels were significantly higher in OSA with hypertension than OSA but not with hypertension (mean differences = 1.32 ng/dL, 95% CI: 0.58–2.07, P = 0.0005). Meta-analysis of all studies suggested no significant differences in aldosterone between OSA and controls, but a significant pooled mean difference of 1.35 ng/mL (95% CI: 0.88–1.82, P < 0.00001) emerged after excluding one small-sample study. No significant risk of publication bias was detected among all included studies.

Conclusions

OSA is associated with higher AngII and aldosterone levels, especially in hypertensive patients. OSA may cause hypertension, at least in part, by stimulating RAAS activity.

Hypertension: renin-angiotensin-aldosterone system alterations

Blockers of the renin-angiotensin-aldosterone system (RAAS), that is, renin inhibitors, angiotensin (Ang)-converting enzyme (ACE) inhibitors, Ang II type 1 receptor antagonists, and mineralocorticoid receptor antagonists, are a cornerstone in the treatment of hypertension. How exactly they exert their effect, in particular in patients with low circulating RAAS activity, also taking into consideration the so-called Ang II/aldosterone escape that often occurs after initial blockade, is still incompletely understood. Multiple studies have tried to find parameters that predict the response to RAAS blockade, allowing a personalized treatment approach. Consequently, the question should now be answered on what basis (eg, sex, ethnicity, age, salt intake, baseline renin, ACE or aldosterone, and genetic variance) a RAAS blocker can be chosen to treat an individual patient. Are all blockers equal? Does optimal blockade imply maximum RAAS blockade, for example, by combining ≥2 RAAS blockers or by simply increasing the dose of 1 blocker? Exciting recent investigations reveal a range of unanticipated extrarenal effects of aldosterone, as well as a detailed insight in the genetic causes of primary aldosteronism, and mineralocorticoid receptor blockers have now become an important treatment option for resistant hypertension. Finally, apart from the deleterious ACE-Ang II-Ang II type 1 receptor arm, animal studies support the existence of protective aminopeptidase A-Ang III-Ang II type 2 receptor and ACE2-Ang-(1 to 7)-Mas receptor arms, paving the way for multiple new treatment options. This review provides an update about all these aspects, critically discussing the many controversies and allowing the reader to obtain a full understanding of what we currently know about RAAS alterations in hypertension.

Exercise intensity modulates capillary perfusion in correspondence with ACE I/D modulated serum angiotensin II levels

During exercise the renin-angiotensin system is stimulated. We hypothesized that the increase in serum angiotensin II (AngII) levels after exercise is dependent on exercise intensity and duration and secondly that people with the ACE-II genotype will show a higher increase in AngII serum levels. We also assumed that perfusion of upper limbs is transiently reduced with maximal cycling exercise and that subjects with the ACE-II compared to the ACE-ID/DD genotype will have a higher capillary perfusion due to lower AngII levels. Ten healthy subjects completed a maximal exercise test, a 12-min exercise test at ventilatory threshold and a 3-min test at the respiratory compensation point. AngII serum levels and capillary recruitment of the skin in the third finger were measured before and after exercise and breath-by-breath gas exchange during exercise was assessed. Baseline levels of AngII levels were lower prior to the 3-min test which took place on average 5 days after the last exercise. A two-fold increase compared to baseline levels was found for AngII only immediately after the 3-min test and not after the maximal exercise test and 12-min of exercise. Subjects without the I allele showed a decrease in AngII values after the maximal test in contrast to subjects with the ACE-II/ID genotype. Subjects with the ACE-II genotype had a 1.8 times significant higher capillary perfusion in the finger after exercise. A trend was observed for a 34.3% decreased capillary recruitment in the ACE-ID/DD genotype after exercise. We conclude that the rise in AngII after exercise is intensity dependent and that variability in serum AngII and capillary perfusion is related to the ACE I/D polymorphism.

Blood pressure and renin-angiotensin system resetting in transgenic rats with elevated plasma Val5-angiotensinogen

OBJECTIVE

Increases in plasma angiotensinogen (Ang-N) due to genetic polymorphisms or pharmacological stimuli like estrogen have been associated with a blood pressure (BP) rise, increased salt sensitivity and cardiovascular risk. The relationship between Ang-N, the resetting of the renin-angiotensin system, and BP still remains unclear. Angiotensin (Ang) II-induced genetic hypertension should respond to lisinopril treatment.

METHODS

A new transgenic rat line (TGR) with hepatic overexpression of native (rat) Ang-N was established to study high plasma Ang-N. The transgene contained a mutation producing Val(5)-Ang-II, which was measured separately from nontransgenic Ile-Ang-II in plasma and renal tissue.

RESULTS

Male homozygous TGR had increased plasma Ang-N (~20-fold), systolic BP (ΔBP+26 mmHg), renin activity (~2-fold), renin activity/concentration (5-fold), total Ang-II (~2-fold, kidney 1.7-fold) but decreased plasma renin concentrations (-46%, kidney -85%) and Ile(5)-Ang-I and II (-93%, -94%) vs. controls. Heterozygous TGR exhibited ~10-fold higher plasma Ang-N and 17 mmHg ΔBP. Lisinopril decreased their SBP (-23 vs. -13 mmHg in controls), kidney Ang-II/I (~3-fold vs. ~2-fold) and Ile(5)-Ang-II (-70 vs. -40%), and increased kidney renin and Ile(5)-Ang-I (>2.5-fold vs. <2.5-fold). Kidney Ang-II remained higher and renin lower in TGR compared with controls.

CONCLUSION

High plasma Ang-N increases plasma and kidney Ang-II levels, and amplifies the plasma and renal Ang-II response to a given change in renal renin secretion. This enzyme-kinetic amplification dominates over the Ang-II mediated feedback reduction of renin secretion. High Ang-N levels thus facilitate hypertension via small increases of Ang II and may influence the effectiveness of renin-angiotensin system inhibitors.

Plasma Renin Activity by LC-MS/MS: Development of a Prototypical Clinical Assay Reveals a Subpopulation of Human Plasma Samples with Substantial Peptidase Activity

BACKGROUND

For management and treatment of secondary hypertension, plasma renin activity (PRA) assay is considered an essential diagnostic tool. We developed a liquid chromatography–tandem mass spectrometry (LC-MS/MS)-based approach to PRA offering improvements in laboratory workflow and throughput. During development, we observed a substantial number of clinical samples that have strong degradation activity toward angiotensin (Ang) I during generation. A preliminary characterization of this degradation activity was performed, and we provide here a method by which this degradation can be monitored via the addition of an isotope-labeled degradation standard.

METHODS

Automated online sample extraction coupled with HPLC was used to isolate Ang I and internal standard from plasma. The effluent from the analytical column was directed to a triple quadrupole MS operated in selected reaction monitoring mode, monitoring the a5 and b5 product ions from the [M+3H]+3 precursors. Routine analysis could be achieved with as little as 150 μL plasma.

RESULTS

We identified both C-terminal and N-terminal degradation products of Ang I using isotope-labeled peptides as controls and substrates. In 2%–5% of patient samples, the degradation essentially eliminated any Ang I produced during generation.

CONCLUSIONS

Our method requires reduced sample handling when compared with an RIA and eliminates the need for extended generation times for samples with low renin activity. Degradation of Ang I during generation appears to be a confounding variable in the interpretation of results from some clinical samples. Samples with profound degradation activity can be identified using a degradation standard that is added at the start of generation.

Intraneuronal angiotensinergic system in rat and human dorsal root ganglia

To elucidate the local formation of angiotensin II (Ang II) in the neurons of sensory dorsal root ganglia (DRG), we studied the expression of angiotensinogen (Ang-N)-, renin-, angiotensin converting enzyme (ACE)- and cathepsin D-mRNA, and the presence of protein renin, Ang II, Substance P and calcitonin gene-related peptide (CGRP) in the rat and human thoracic DRG. Quantitative real time PCR (qRT-PCR) studies revealed that rat DRG expressed substantial amounts of Ang-N- and ACE mRNA, while renin mRNA as well as the protein renin were untraceable. Cathepsin D-mRNA and cathepsin D-protein were detected in the rat DRG indicating the possibility of existence of pathways alternative to renin for Ang I formation. Angiotensin peptides were successfully detected with high performance liquid chromatography and radioimmunoassay in human DRG extracts. In situ hybridization in rat DRG confirmed additionally expression of Ang-N mRNA in the cytoplasm of numerous neurons. Intracellular Ang II staining could be shown in number of neurons and their processes in both the rat and human DRG. Interestingly we observed neuronal processes with angiotensinergic synapses en passant, colocalized with synaptophysin, within the DRG. In the DRG, we also identified by qRT-PCR, expression of Ang II receptor AT(1A) and AT(2)-mRNA while AT(1B)-mRNA was not traceable. In some neurons Substance P and CGRP were found colocalized with Ang II. The intracellular localization and colocalization of Ang II with Substance P and CGRP in the DRG neurons may indicate a participation and function of Ang II in the regulation of nociception. In conclusion, these results suggest that Ang II may be produced locally in the neurons of rat and human DRG and act as a neurotransmitter.

Renal plasma flow: glomerular filtration rate relationships in man during direct renin inhibition with aliskiren

We examined the relation between change in renal plasma flow (RPF) and change in glomerular filtration rate (GFR) in healthy humans on a low-salt diet during direct renin inhibition with aliskiren. We measured the renal hemodynamic response to acute dosing of 300mg aliskiren by mouth to 19 healthy normotensive subjects (age, 33+/-3 years; baseline RPF, 575+/-23; GFR, 138+/-14mL/min/1.73m(2)) on a low-sodium diet (10mmol/day). GFR and RPF were measured by the clearance of inulin and para-aminohippurate. There was a marked increase in average RPF (169+/-24mL/min/1.73m(2)) and a small rise in average GFR (1.4+/-5mL/min/1.73m(2)) from baseline in response to aliskiren. There was a clear correlation between the change in RPF and the change in GFR between subjects (r=0.65; P < .003). A substantial increase in RPF was accompanied by a rise in GFR. Dependence of GFR on RPF was identified in healthy humans after RPF rose significantly with aliskiren. The responsible mechanism likely involves intravascular oncotic pressure along the glomerular capillary resulting in greater surface area available for filtration.

Endogenous angiotensinergic system in neurons of rat and human trigeminal ganglia

To clarify the role of Angiotensin II (Ang II) in the sensory system and especially in the trigeminal ganglia, we studied the expression of angiotensinogen (Ang-N)-, renin-, angiotensin converting enzyme (ACE)- and cathepsin D-mRNA, and the presence of Ang II and substance P in the rat and human trigeminal ganglia. The rat trigeminal ganglia expressed substantial amounts of Ang-N- and ACE mRNA as determined by quantitative real time PCR. Renin mRNA was untraceable in rat samples. Cathepsin D was detected in the rat trigeminal ganglia indicating the possibility of existence of pathways alternative to renin for Ang I formation. In situ hybridization in rat trigeminal ganglia revealed expression of Ang-N mRNA in the cytoplasm of numerous neurons. By using immunocytochemistry, a number of neurons and their processes in both the rat and human trigeminal ganglia were stained for Ang II. Post in situ hybridization immunocytochemistry reveals that in the rat trigeminal ganglia some, but not all Ang-N mRNA-positive neurons marked for Ang II. In some neurons Substance P was found colocalized with Ang II. Angiotensins from rat trigeminal ganglia were quantitated by radioimmunoassay with and without prior separation by high performance liquid chromatography. Immunoreactive angiotensin II (ir-Ang II) was consistently present and the sum of true Ang II (1-8) octapeptide and its specifically measured metabolites were found to account for it. Radioimmunological and immunocytochemical evidence of ir-Ang II in neuronal tissue is compatible with Ang II as a neurotransmitter. In conclusion, these results suggest that Ang II could be produced locally in the neurons of rat trigeminal ganglia. The localization and colocalization of neuronal Ang II with Substance P in the trigeminal ganglia neurons may be the basis for a participation and function of Ang II in the regulation of nociception and migraine pathology.

Renal and hormonal responses to direct renin inhibition with aliskiren in healthy humans

BACKGROUND

Pharmacological interruption of the renin-angiotensin system focuses on optimization of blockade. As a measure of intrarenal renin activity, we have examined renal plasma flow (RPF) responses in a standardized protocol. Compared with responses with angiotensin-converting enzyme inhibition (rise in RPF approximately 95 mL x min(-1) x 1.73 m(-2)), greater renal vasodilation with angiotensin receptor blockers (approximately 145 mL x min(-1) x 1.73 m(-2)) suggested more effective blockade. We predicted that blockade with the direct oral renin inhibitor aliskiren would produce renal vascular responses exceeding those induced by angiotensin-converting enzyme inhibitors and angiotensin receptor blockers.

METHODS AND RESULTS

Twenty healthy normotensive subjects were studied on a low-sodium (10 mmol/d) diet, receiving separate escalating doses of aliskiren. Six additional subjects received captopril 25 mg as a low-sodium comparison and also received aliskiren on a high-sodium (200 mmol/d) diet. RPF was measured by clearance of para-aminohippurate. Aliskiren induced a remarkable dose-related renal vasodilation in low-sodium balance. The RPF response was maximal at the 600-mg dose (197+/-27 mL x min(-1) x 1.73 m(-2)) and exceeded responses to captopril (92+/-20 mL x min(-1) x 1.73 m(-2); P<0.01). Furthermore, significant residual vasodilation was observed 48 hours after each dose (P<0.01). The RPF response on a high-sodium diet was also higher than expected (47+/-17 mL x min(-1) x 1.73 m(-2)). Plasma renin activity and angiotensin levels were reduced in a dose-related manner. As another functional index of the effect of aliskiren, we found significant natriuresis on both diets.

CONCLUSIONS

Renal vasodilation in healthy people with the potent renin inhibitor aliskiren exceeded responses seen previously with angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. The effects were longer lasting and were associated with significant natriuresis. These results indicate that aliskiren may provide more complete and thus more effective blockade of the renin-angiotensin system.

Pharmacokinetic/pharmacodynamic modeling of renin biomarkers in subjects treated with the renin inhibitor aliskiren

A semimechanistic pharmacokinetic/pharmacodynamic (PK/PD) model was developed to evaluate the effects of aliskiren on the renin-angiotensin system (RAS) in humans. Mean data were extracted from a three-way crossover, placebo-controlled study. Outcome measures included the time-course of plasma renin activity (PRA) and plasma concentrations of aliskiren, active renin (AR), angiotensin I (ANG I), and angiotensin II (ANG II). The disposition of aliskiren may be best described as a two-compartment model with nonlinear elimination and distribution. The four biomarkers of RAS inhibition were co-modeled, and the AR showed a dose-dependent increase after the administration of aliskiren. This effect was described in terms of an indirect stimulatory response model in conjunction with an empirical submodel of functional adaptation. The estimated concentration of aliskiren necessary for producing 50% inhibition of PRA is 0.66 ng/ml, which is similar to in vitro estimates (0.33 ng/ml) after correction for plasma protein binding. The final and reduced models test the current hypothesis that RAS is inhibited by direct renin antagonism, and also provide suitable platforms for future clinical study design and analysis.

Renin inhibition with aliskiren: where are we now, and where are we going?

With the development of aliskiren, blockade of the renin-angiotensin-aldosterone system (RAAS) at the level of the interaction of renin with a substrate has become a clinical reality. This review covers the specific features of the first agent likely to achieve widespread clinical exposure, aliskiren. The potential of renin inhibition must be viewed in the context of the remarkable efficacy of both angiotensin-converting enzyme (ACE) inhibition and angiotensin receptor blockers (ARBs). The implications of blockade of the renin system at its rate-limiting step are reviewed, with the therapeutic implications for both the renin inhibitor employed alone or the renin inhibitor combined with an ACE inhibitor or ARB. The relevant and necessary studies are ongoing.

Effect of hypertension on angiotensin-(1–7) levels in rats with different angiotensin-I converting enzyme polymorphism

To determine circulating angiotensin-(1-7) [Ang-(1,7)] levels in rats with different angiotensin converting enzyme (ACE) genotypes and to evaluate the effect of hypertension on levels of this heptapeptide, plasma levels of angiotensin II (Ang II) and Ang-(1-7) were determined by HPLC and radioimmunoassay in (a) normotensive F0 and F2 homozygous Brown Norway (BN; with high ACE) or Lewis (with low ACE) rats and (b) in hypertensive F2 homozygous male rats (Goldblatt model). Genotypes were characterized by PCR and plasma ACE activity measured by fluorimetry. Plasma ACE activity was 2-fold higher (p < 0.05) in homozygous BN compared to homozygous Lewis groups. In the Goldblatt groups, a similar degree of hypertension and left ventricular hypertrophy was observed in rats with both genotypes. Plasma Ang II levels were between 300-400% higher (p < 0.05) in the BN than in the Lewis rats, without increment in the hypertensive animals. Plasma Ang-(1-7) levels were 75-87% lower in the BN rats (p < 0.05) and they were significantly higher (p < 0.05) in the hypertensive rats from both genotypes. Plasma levels of Ang II and Ang-(1-7) levels were inversely correlated in the normotensive rats (r = -0.64; p < 0.001), but not in the hypertensive animals. We conclude that there is an inverse relationship between circulating levels of Ang II and Ang-(1-7) in rats determined by the ACE gene polymorphism. This inverse relation is due to genetically determined higher ACE activity. Besides, plasma levels of Ang-(1-7) increase in renovascular hypertension.

Increased Aortic NADPH Oxidase Activity in Rats With Genetically High Angiotensin-Converting Enzyme Levels

In humans and rats, angiotensin I–converting enzyme activity is significantly determined by a gene polymorphism. Homozygous Brown Norway rats have higher plasma angiotensin I–converting enzyme activity and circulating angiotensin II (Ang II) levels than Lewis rats. Because Ang II induces NAD(P)H oxidase activation, we hypothesized here that Brown Norway rats have higher vascular NAD(P)H oxidase activity and superoxide anion production than Lewis rats. Homozygous Brown Norway (n=15) and Lewis (n=13) male rats were used. Plasma angiotensin I–converting enzyme activity (by fluorimetry), Ang II levels (by high-performance liquid chromatography and radioimmunoassay), and aortic NAD(P)H oxidase activity, as well as superoxide anion production (by chemiluminescence with lucigenin) were measured. Plasma angiotensin I–converting enzyme activity and Ang II levels were 100% higher in Brown Norway rats than in Lewis rats ( P <0.05). Aortic angiotensin I– converting enzyme, but not Ang II, was elevated ( P <0.05). Aortic superoxide anion production and NAD(P)H oxidase activity were 300% and 260% higher in Brown Norway than in Lewis rats, respectively ( P <0.05), which was not observed in Brown Norway rats treated with candesartan (10 mg/kg per day for 7 days). Endothelial NO synthase activity in the aorta from Brown Norway rats was significantly lower than in Lewis rats. However, inducible NO synthase activity and both endothelial NO synthase and inducible NO synthase mRNA and protein levels were similar in both genotypes. In summary, Brown Norway rats have higher vascular NAD(P)H oxidase activity and superoxide anion production than Lewis rats, suggesting the presence of a higher level of vascular oxidative stress in rats with genetically higher angiotensin I–converting enzyme levels. This effect is mediated through the angiotensin I receptor.

Polymorphism in gene coding for ACE determines different development of myocardial fibrosis in rats

In humans, the effect of angiotensin-converting enzyme (ACE) gene polymorphisms in cardiovascular disease is still controversial. In the rat, a microsatellite marker in the ACE gene allows differentiation of the ACE gene polymorphism among strains with different ACE levels. We tested the hypothesis that this ACE gene polymorphism determines the extent of cardiac fibrosis induced by isoproterenol (Iso) in the rat. We used a male F2generation (homozygous LL and BB ACE genotypes determined by polymerase chain reaction) derived from two rat strains [Brown-Norway (BB) and Lewis (LL)] that differ with respect to their plasma ACE activities. For induction of left ventricular (LV) hypertrophy (LVH) and cardiac fibrosis, rats were infused with Iso (5 mg·kg–1·day–1) or saline (control) for 10 days and euthanized at day 1 after the last injection. The interstitial collagen volumetric fraction (ICVF), collagen I, and fibronectin content, but not collagen III content, were significantly higher in the homozygous BB rats than in homozygous LL rats. Differences in metalloprotease (MMP)-9, but not in MMP-2 activities as well as in cardiac cell proliferation, were also detected between LL and BB rats treated with Iso. LV ACE activity was higher in BB rats than LL rats and correlated with ICVF ( r = 0.61, P < 0.002). No changes were observed in plasma ACE activities, ANG II plasma or LV levels, plasma renin activity, and ACE and ANG II type 1 receptor (AT1R) mRNA levels in the LV of rats with the two different ACE polymorphisms. Iso induced a similar degree of LVH [assessed by an increase in LV weight 100 per body weight, LV-to-right ventricle (RV) ratio, and LV protein content] in LL and BB rats. We concluded that rats in the F2generation with high plasma ACE activity developed more fibrosis but to a similar degree of LVH compared with rats with low plasma ACE activity.

Local renin-angiotensin systems: the unanswered questions

The concept of local renin-angiotensin systems has been introduced almost 20 years ago to explain the beneficial blood pressure-independent effects of ACE inhibitors and AT(1) receptor antagonists in cardiovascular diseases. In the past decade, research has focussed on the local effects of angiotensin II rather than on the mechanism(s) of its local generation. This review addresses several of the unanswered questions with regard to tissue angiotensin II generation, focussing in particular on the heart and vascular wall: (1) what is the origin of the renin that is required to generate angiotensin II locally, (2) where does tissue angiotensin generation occur (intra- versus extracellular), (3) what is the importance of alternative (non-renin, non-ACE) angiotensin-generating enzymes, (4) do ACE inhibitors and AT(1) receptor antagonists exert local effects that are renin-angiotensin system independent (thereby incorrectly leading to the conclusion that they interfere with the local generation or effects of angiotensin II), and (5) to what degree do differences in tissue angiotensin generation underlie the association between cardiovascular diseases and renin-angiotensin system gene polymorphisms?

Tissue-specific response to interstitial angiotensin II in humans

Angiotensin II is synthesized locally in various tissues; however, the role of interstitial angiotensin II in the regulation of regional metabolism and tissue perfusion is not clear. We characterized the effect of interstially applied angiotensin II in skeletal muscle and subcutaneous adipose tissue of young, normal-weight, healthy subjects by using the microdialysis technique. Furthermore, we tested the hypothesis that the effect of interstitial angiotensin II is modulated by nitric oxide. Tissues were perfused with 0.01, 0.1, and 1 micro mol/L angiotensin II in the presence of the L- or D-isomer of N(G)-nitro-arginine-methyl ester (L- or D-NAME), the effective and noneffective isomer, respectively, for blocking nitric oxide synthase. Dialysate ethanol, glycerol, glucose, lactate, and pyruvate concentrations were measured to assess changes in blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively. Baseline blood flow and dialysate concentrations of the metabolites were similar with L- and D-NAME in both tissues. Blood flow and dialysate glucose and lactate did not change significantly in both tissues during perfusion with angiotensin II. Dialysate glycerol dose-dependently increased in adipose tissue (P<0.0438) but decreased in muscle (P<0.007). In muscle, dialysate pyruvate increased (P<0.0002), whereas lactate/pyruvate ratio decreased (P<0.001), both dose-dependently. All effects were similar with L- and D-NAME and could be reversed by nitroprusside. We conclude that in contrast to the profound hemodynamic effect of intravascular angiotensin II, interstitial angiotensin II has a minimal acute effect on blood flow in both tissues. However, interstitial angiotensin II modulates lipid and carbohydrate metabolism in a tissue specific fashion. Thus, the physiology of interstitial angiotensin II cannot be predicted from intravascular studies.

Effects of dietary salt changes on renal renin-angiotensin system in rats

Renin (RA) and angiotensin-converting enzyme (ACE) activities and angiotensinogen, ANG I, and ANG II levels were measured in the kidney (cortex and medulla) and plasma of Wistar-Kyoto rats on a low-sodium (LS; 0.025% NaCl; n = 8), normal-sodium (NS; 1% NaCl; n = 7), or high-sodium (HS; 8% NaCl; n = 7) diet for 21 days. RA, ANG I, and ANG II levels increased in a manner inversely related to sodium content of the diet in both plasma and renal tissues. The LS diet resulted in a 16-, 2.8-, and 1.8-fold increase in plasma RA, ANG I, and ANG II levels, respectively, compared with those in HS rats. In the renal cortex and medulla, RA, ANG I, and ANG II levels were also increased by diminution of dietary salt content but, in contrast to plasma, ANG II levels increased much more than RA or ANG I levels [5.4 (cortex)- and 4.7 (medulla)-fold compared with HS rats]. In summary, we demonstrated variations of ANG II levels in the kidney during dietary salt modifications. Our results confirm that RA and ACE activity are not the steps limiting intrarenal ANG II levels. Nevertheless, despite RA and ACE activity differences between renal cortex and medulla, ANG I and ANG II levels are equivalent in these two tissues; these results argue against a compartmentalization of RAS in these two intrarenal areas.

Contribution of angiotensin II internalization to intrarenal angiotensin II levels in rats

This study was designed to determine the involvement of AT(1) receptors in the uptake of ANG II in the kidney of rats exposed to differing salt intake. Male Wistar-Kyoto rats were treated with a normal-salt (NS; 1% NaCl, n = 7) or a low-salt (LS; 0.025% NaCl, n = 7) diet combined with (LS+Los, n = 7; NS+Los, n = 7) or without losartan (30 mg. kg(-1). day(-1)), an AT(1) receptor antagonist. Renin (RA) and angiotensin-converting enzyme (ACE) activities and angiotensinogen, ANG I, and ANG II levels were measured in plasma, renal cortex, and medulla. In LS rats, in both plasma and renal cortex, the increase in RA was associated with an increase in ANG I and ANG II levels compared with NS rats, but intrarenal ANG II levels increased more than ANG I levels. In NS+Los rats, the increase in RA in plasma was followed by a marked increase in plasma ANG I and ANG II levels compared with NS rats whereas in the kidney the increase of renal RA was followed by a decrease of the levels of these peptides. The same pattern was observed in LS+Los rats, but the decrease in renal ANG II levels was much more pronounced in LS+Los rats than in NS+Los rats. Our results suggest that the increase in renal ANG II levels after salt restriction results mainly from an uptake of ANG II, via AT(1) receptors. Such elevated intrarenal ANG II levels could contribute to maintain sodium and fluid balance and arterial blood pressure during salt-deficiency states.

Angiotensin I-converting enzyme gene polymorphism influences chronic hypertensive response in the rat Goldblatt model

BACKGROUND AND OBJECTIVE

In humans, the insertion/deletion polymorphism in the angiotensin (Ang) I converting enzyme (ACE) gene significantly determines ACE activity. The deletion allele induces higher ACE levels and is associated with hypertension in men. In the rat, a microsatellite marker in the ACE gene allows differentiation of the ACE alleles among strains with different ACE levels. We evaluated the effect of genetically determined ACE expression on the development of renovascular hypertension in the rat.

METHODS AND RESULTS

Systolic BP (SBP), ACE and angiotensin II (Ang II) levels were measured using the Goldblatt (Gb) model (two kidneys, one clip) in homozygous males of two inbred strains (F2) of Lewis x Brown-Norway (BN) rats. SBP was significantly higher in the BN-Gb rats compared to the Lewis-Gb rats throughout the study (F = 239.6, P < 0.001). An interaction was observed between SBP and strain (F = 2.92, P < 0.01). Plasma ACE activity was 100% higher in the BN-Gb than in the Lewis-Gb rats (P < 0.05). Ang II plasma levels were higher in the BN-sham than in the Lewis-sham rats (255 +/- 22 versus 161 +/- 16 pg/ml, P < 0.05), increased in both Gb groups and correlated significantly with SBP (r = 0.58, P < 0.01).

CONCLUSIONS

Genetically determined ACE expression in male rats enhances the chronic hypertensive response after the induction of renovascular hypertension. A relationship between circulating Ang II and the development of hypertension was also observed in this experimental model of genetically modulated hypertension.

Vasoconstriction is determined by interstitial rather than circulating angiotensin II

1. We investigated why angiotensin (Ang) I and II induce vasoconstriction with similar potencies, although Ang I-II conversion is limited. 2. Construction of concentration-response curves to Ang I and II in porcine femoral arteries, in the absence or presence of the AT(1) or AT(2) receptor antagonists irbesartan and PD123319, revealed that the approximately 2 fold difference in potency between Ang I and II was not due to stimulation of different AT receptor populations by exogenous and locally generated Ang II. 3. Measurement of Ang I and II and their metabolites at the time of vasoconstriction confirmed that, at equimolar application of Ang I and II, bath fluid Ang II during Ang I was approximately 18 times lower than during Ang II and that Ang II was by far the most important metabolite of Ang I. Tissue Ang II was 2.9+/-1.5% and 12.2+/-2.4% of the corresponding Ang I and II bath fluid levels, and was not affected by irbesartan or PD123319, suggesting that it was located extracellularly. 4. Since approximately 15% of tissue weight consists of interstitial fluid, it can be calculated that interstitial Ang II levels during Ang II resemble bath fluid Ang II levels, whereas during Ang I they are 8.8 - 27 fold higher. Consequently at equimolar application of Ang I and II, the interstitial Ang II levels differ only 2 - 4 fold. 5. Interstitial, rather than circulating Ang II determines vasoconstriction. Arterial Ang I, resulting in high interstitial Ang II levels via its local conversion by ACE, may be of greater physiological importance than arterial Ang II.

[Early prevention of experimental left ventricular hypertrophy in experimental hypertension and angiotensin II levels]

INTRODUCTION

Angiotensin II levels can be partially inhibited during chronic administration of angiotensin converting enzyme (ACE) inhibitors, limiting from a clinical point of view its efficacy in the treatment of hypertension. There are few studies relating ACE activity directly with early prevention of left ventricular hypertrophy (LVH) in systemic hypertension during the administration of an ACE inhibitor (ACEI).

AIM

To evaluate the effects of early ACE inhibition with perindopril on the development of hypertension, LVH and levels of angiotensin II (Ang II) in plasma as well as in LV in the rat Goldblatt model (Gb; 2 kidneys-1 clip), 2 weeks after surgery.

RESULTS

Systolic blood pressure and relative LV mass increased by 42% and 20% respectively, in the Gb group (p < 0.001). Plasma and LV ACE activities were significantly higher in the Gb rats compared with the control rats. Plasma and LV Ang II levels also increased by 129% and 800%, respectively. Perindorpil prevented hypertension and LVH development by inhibiting plasma ACE (and also LV ACE), and also circulation Ang II in plasma and in the LV.

CONCLUSIONS

In this experimental model of hypertensive LVH, there is an early activation of plasma and cardiac ACE. Early administration of an ACE inhibitor prevents the development of hypertension and LVH by inhibiting the increases of plasma and LV Ang II.

Metabolic and hemodynamic response of adipose tissue to angiotensin II

OBJECTIVE

Recent studies have revealed the presence of a local renin-angiotensin system in adipose tissue. To examine the possible role of this system in adipose tissue, we performed microdialysis studies on the effect of angiotensin II (Ang II) on blood flow and metabolism in abdominal subcutaneous adipose tissue (aSAT) and femoral subcutaneous adipose tissue (fSAT) in young healthy men.

RESEARCH METHODS AND PROCEDURES

Using the microdialysis technique, two different protocols were run perfusion with Ringer's solution + 50 mM ethanol with the subsequent addition of 125, 250, and 500 microg/liter Ang II (n = 8) and Ringers's solution + 50 mM ethanol with the subsequent addition of isoproterenol (1 microM) alone and in combination with 500 microg/liter Ang II (n = 6). Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured for estimating blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively.

RESULTS

Perfusion with Ang II resulted in a dose-dependent decrease in blood flow (fSAT > aSAT), lipolysis (fSAT > aSAT), and glucose uptake (fSAT = aSAT). Isoproterenol increased blood flow and lipolysis at both sites and those effects could be returned to baseline values by the addition of Ang II in aSAT but not fSAT.

DISCUSSION

In conclusion, our data indicate that in addition to its well-known vasoconstricting effect, Ang II inhibits lipolysis in adipose tissue, whereby femoral fat depots seem to be more sensitive to this effect than abdominal depots.

Measurement of Immunoreactive Angiotensin-(1–7) Heptapeptide in Human Blood

Background: The renal enzyme renin cleaves from the hepatic α2-globulin angiotensinogen angiotensin-(1–10) decapeptide [Ang-(1–10)], which is further metabolized to smaller peptides that help maintain cardiovascular homeostasis. The Ang-(1–7) heptapeptide has been reported to have several physiological effects, including natriuresis, diuresis, vasodilation, and release of vasopressin and prostaglandins.

Methods: To investigate Ang-(1–7) in clinical settings, we developed a method to measure immunoreactive (ir-) Ang-(1–7) in 2 mL of human blood and to estimate plasma concentrations by correcting for the hematocrit. A sensitive and specific antiserum against Ang-(1–7) was raised in a rabbit. Human blood was collected in the presence of an inhibitor mixture including a renin inhibitor to prevent peptide generation in vitro. Ang-(1–7) was extracted into ethanol and purified on phenylsilylsilica. The peptide was quantified by radioimmunoassay. Increasing doses of Ang-(1–7) were infused into volunteers, and plasma concentrations of the peptide were measured.

Results: The detection limit for plasma ir-Ang-(1–7) was 1 pmol/L. CVs for high and low blood concentrations were 4% and 20%, respectively, and between-assay CVs were 8% and 13%, respectively. Reference values for human plasma concentrations of ir-Ang-(1–7) were 1.0–9.5 pmol/L (median, 4.7 pmol/L) and increased linearly during infusion of increasing doses of Ang-(1–7).

Conclusions: Reliable measurement of plasma ir-Ang-(1–7) is achieved with efficient inhibition of enzymes that generate or metabolize Ang-(1–7) after blood sampling, extraction in ethanol, and purification on phenylsilylsilica, and by use of a specific antiserum.

Specific and non-specific measurements of tissue angiotensin II cascade members

Although angiotensin II (ANG II) has been the focal regulatory peptide of the renin-angiotensin system, its proteolytic fragments have recently been demonstrated to have biological effects. Conventional measurement of angiotensins involves radioimmunoassay (RIA), which is a sensitive binding technique capable of measuring low physiological concentrations. However, ANG II antibody cross-reacts with ANG II and its fragments (ANG II cascade), rendering RIA measurement alone to be a non-specific measure of immunoreactive ANG II (ir-ANG II). On the other hand, high-performance liquid chromatography (HPLC) is capable of separating immunoreactive ANG II cascade members, but may not be sensitive enough to detect these low peptide concentrations often present in biological samples. Consequently, a reverse-phase HPLC method, with triethylammoniun formate as an ion-pair reagent, was developed to separate ANG II and its fragments, ANG III, ANG IV and ANG V. This HPLC separation was applied to extracts from normal canine hearts and ANG II cascade immunoreactive fractions were collected. Collected fractions were quantified by RIA, with the use of separate standard curves. The isocratic HPLC separation of ANG II, ANG III, ANG IV and ANG V was achieved in less than 5 min with adjacent peaks having baseline resolution. Measured cardiac left ventricle ANG III, ANG IV and ANG V concentrations (mean+/-SD) were 5.3+/-2.2,4.0+/-1.0 and 3.1+/-1.0 fmol/g (n=9), respectively. There was a significant difference (P=0.003, n=9) between left ventricular immunoreactive ANG II and 'true' ANG II, corrected for recovery rates of 86.2+/-22.5 and 53.5+/-16.2 fmol/g, respectively. We conclude that the combination of HPLC with RIA ensures the specific measurement of the ANG II cascade family members while non-chromatographic processing of tissue renders ANG II measurement non-specific. In addition, the use of triethylammonium formate as mobile phase additive is superior in the HPLC separation of the angiotensins.

Enhancing cardiac protection after myocardial infarction: rationale for newer clinical trials of angiotensin receptor blockers

Treatment of acute myocardial infarction (MI) has been advanced considerably in the past 20 years with the advent of acute reperfusion strategies such as thrombolytic therapy or primary angioplasty and the use of adjunctive medical therapies such as aspirin, beta-blockers, and HMG-CoA reductase inhibitors (statins); each has been proven to reduce morbidity and mortality rates after MI. Angiotensin-converting enzyme (ACE) inhibitors have earned an important place in this list of winners, particularly in patients clinically assessed as being at higher risk for cardiovascular death. The benefits of treating such patients with an ACE inhibitor have clearly shown significant improvements in survival that are both complementary and additive to the other proven therapies. However, a significant subset of patients treated with ACE inhibitors will die or have worsening congestive heart failure despite adequate therapy. Appropriate risk stratification can help guide the clinician in identifying patients at greatest risk for cardiovascular events after MI. Fortunately, investigators are currently exploring the potential benefits of more specific and selective blockade of the renin-angiotensin system with AT(1) receptor blockers (ARBs) in the hope of enhancing survival beyond that evidenced with ACE inhibition alone. These agents pharmacologically inhibit the renin-angiotensin system at the angiotensin (Ang) II type 1 receptor level. Although there are theories postulating why blocking the harmful effects of Ang II at this receptor would be more effective than inhibiting ACE-mediated conversion from inactive Ang I to Ang II, extrapolation to the clinical setting remains highly speculative. These hypotheses can only be tested by direct comparisons of ACE inhibitors and ARBs in the appropriate patient populations. The VALIANT (Valsartan in Acute Myocardial Infarction) trial is testing the hypothesis that interruption of the renin-angiotensin pathway by using the ARB valsartan alone or in combination with an ACE inhibitor will be more effective in saving lives than an ACE inhibitor alone in treating patients at high risk. This trial will enroll patients with symptoms of congestive heart failure or depressed left ventricular ejection fraction and randomly assign them to either captopril, valsartan, or the combination. Other proven conventional post-MI therapies are encouraged. This study will definitively determine whether the use of valsartan offers additional benefits over those achieved with ACE inhibitor monotherapy in patients at high risk after MI.

Does the addition of losartan improve the beneficial effects of ACE inhibitors in patients with anterior myocardial infarction? A pilot study

OBJECTIVE

To verify the efficacy of the combination of captopril (75 mg day) and losartan (25 mg/day) in early postinfarction phases of reperfused anterior acute myocardial infarction.

DESIGN AND PATIENTS

99 patients, hospitalised for suspected anterior acute myocardial infarction within four hours from the onset of symptoms, were randomised into two groups: group A included 50 patients who received captopril 75 mg/day and placebo; group B included 49 patients who received captopril 75 mg/day within three days of admission plus losartan 12.5 mg, as a first dose, and 25 mg/day successively. An additional 23 patients with anterior acute myocardial infarction received losartan 25 mg alone and acted as controls (group C) to check the effects of losartan on plasma angiotensin II (AII) concentrations. Noradrenaline (norepinephrine) (NA) and AII plasma concentrations were measured on the third and 10th day after admission in 93 patients (35 from group A, 35 from group B, and 23 from group C). 90 days after admission patients underwent echocardiography to determine end systolic volume (ESV) and ejection fraction (EF).

RESULTS

Patients in groups A and B were similar with regard to age, sex, creatine kinase peak, EF, ESV, and risk factors. Group B (captopril plus losartan) patients showed a significant reduction in mean (SD) systolic blood pressure within the group (basal 128 (10) mm Hg; 10 days after admission 105 (9) mm Hg, p < 0.001), and in comparison with group A (captopril) patients (basal 127 (11) mm Hg; 10 days after admission 116 (10) mm Hg, p < 0. 001). Diastolic blood pressure was also lower in group B patients versus group A (66 (11) v 77 (11) mm Hg). Group C (losartan) patients also showed a significant reduction in systolic blood pressure (131 (13) mm Hg down to 121 (12) mm Hg, p < 0.001). Neither NA nor AII plasma concentrations in groups A and B differed significantly in basal samples (NA 673 (138) v 675 (141) pg/ml; AII 12.77 (4.79) v 12.65 (4.71) pg/ml) or 10 days after admission (NA 283 (93) v 277 (98) pg/ml; AII 5.31 (2.25) v 6.09 (3.31) pg/ml). However, patients in group C had higher plasma concentrations of AII (14.79 (5.7) pg/ml on the third day and 7.98 (4.92) pg/ml on the 10th day) than patients in either group A or B (p = 0.006). After 90 days following treatment, group B (captopril plus losartan) patients had a smaller ESV than patients in group A (captopril) and group C (losartan).

CONCLUSION

The data suggest that the combination of captopril plus losartan is feasible in the early treatment of acute myocardial infarction patients, and it appears that this combination has more effect on ESV than captopril alone in the short term.

Adenosine causes the release of active renin and angiotensin II in the coronary circulation of patients with essential hypertension

OBJECTIVES

The aim of the study was to evaluate whether adenosine infusion can induce production of active renin and angiotensin II in human coronary circulation.

BACKGROUND

Adenosine can activate angiotensin production in the forearm vessels of essential hypertensive patients.

METHODS

In six normotensive subjects and 12 essential hypertensive patients adenosine was infused into the left anterior descending coronary artery (1, 10, 100 and 1,000 microg/min x 5 min each) while active renin (radioimmunometric assay) and angiotensin II (radioimmunoassay after high performance liquid chromatography purification) were measured in venous (great cardiac vein) and coronary arterial blood samples. In five out of 12 hypertensive patients adenosine infusion and plasma samples were repeated during intracoronary angiotensin-converting enzyme inhibitor benazeprilat (25 microg/min) administration. Finally, in adjunctive hypertensive patients, the same procedure was applied during intracoronary sodium nitroprusside (n = 4) or acetylcholine (n = 4).

RESULTS

In hypertensive patients, but not in control subjects, despite a similar increment in coronary blood flow, a significant (p < 0.05) transient increase of venous active renin (from 10.7 +/- 1.4 [95% confidence interval 9.4 to 11.8] to a maximum of 13.8 +/- 2.1 [12.2 to 15.5] with a consequent drop to 10.9 +/- 1.8 [9.7 to 12.1] pg/ml), and angiotensin II (from 14.6 +/- 2.0 [12.7 to 16.5] to a maximum of 20.4 +/- 2.7 [18.7 to 22.2] with a consequent drop to 16.3 +/- 1.8 [13.9 to 18.7] pg/ml) was observed under adenosine infusion, whereas arterial values did not change. Calculated venous-arterial active renin and angiotensin II release showed a strong correlation (r = 0.78 and r = 0.71, respectively; p < 0.001) with circulating active renin. This adenosine-induced venous angiotensin II increase was significantly blunted by benazeprilat. Finally, both sodium nitroprusside and acetylcholine did not affect arterial and venous values of active renin and angiotensin II.

CONCLUSIONS

These data indicate that exogenous adenosine stimulates the release of active renin and angiotensin II in the coronary arteries of essential hypertensive patients, and suggest that this phenomenon is probably due to renin release from tissue stores of renally derived renin.

Dual pathway for angiotensin II formation in human internal mammary arteries

1. Angiotensin converting enzyme (ACE) is thought to be the main enzyme to convert antiotensin I to the vasoactive angiotensin II. Recently, in the human heart, it was found that the majority of angiotensin II formation was due to another enzyme, identified as human heart chymase. In the human vasculature however, the predominance of either ACE or non-ACE conversion of angiotensin I remains unclear. 2. To study the effects of ACE- and chymase-inhibition on angiotensin II formation in human arteries, segments of internal mammary arteries were obtained from 37 patients who underwent coronary bypass surgery. 3. Organ bath experiments showed that 100 microM captopril inhibited slightly the response to angiotensin I (pD2 from 7.09+/-0.11-6.79+/-0.10, P<0.001), while 100 microM captopril nearly abolished the response to [pro10] angiotensin I, a selective substrate for ACE, and the maximum contraction was reduced from 83+/-19%-23+/-17% of the control response (P=0.01). A significant decrease of the pD2 of angiotensin I similar to captopril was observed in the presence of 50 microM chymostatin (pD2 from 7.36+/-0.13-6.99+/-0.15, P<0.039), without influencing the maximum response. In the presence of both inhibitors, effects were much more pronounced than either inhibitor alone, and a 300 times higher dose was needed to yield a significant contraction response to angiotensin I. 4 These results indicate the presence of an ACE and a non-ACE angiontensin II forming pathway in human internal mammary arteries.

Functional evidence for alternative ANG II-forming pathways in hamster cardiovascular system

Like human chymase, hamster chymase is an ANG II-forming enzyme, but pathophysiological roles of chymase are still unknown. We determined the functional conversion of ANG I and [Pro11, D-Ala12]ANG I, a chymase-selective substrate, to ANG II in the hamster cardiovascular system. ANG I and [Pro11, D-Ala12]ANG I produced similar dose-dependent pressor responses in conscious hamsters. Captopril and CV-11974, an ANG II type 1 (AT1)-receptor antagonist, inhibited the responses to ANG I; in contrast, the pressor responses to [Pro11, D-Ala12]ANG I were suppressed only by CV-11974. In the isolated aorta, captopril suppressed ANG I-induced contraction by 84%; administration of captopril with either chymostatin or aprotinin eliminated the contraction. [Pro11, D-Ala12]ANG I-induced contraction was not affected by captopril but was attenuated by chymostatin (71%) and aprotinin (57%). CV-11974 abolished the responses to both substrates, whereas PD-123319, an AT2-receptor antagonist, had no effect. In homogenates of the aorta and heart, soybean trypsin inhibitor-inhibitable ANG II formation predominated over captopril- or aprotinin-inhibitable ANG II formation. These data suggest that [Pro11,D-Ala12]ANG I and part of ANG I were functionally converted to ANG II by chymase and other serine protease(s) in hamster vessels, inducing AT1-receptor-mediated vasoconstriction. Biochemical data supported a role for chymase in the alternative pathway.

A radioimmunoassay for the determination of angiotensin II in elasmobranch fish

A homologous radioimmunoassay was developed to determine the concentration of angiotensin II (Asn1, Pro3, Ile5)-Ang II) in elasmobranchs. Cross-reactivity with elasmobranch angiotensin I and other heterologous angiotensins was high and therefore all potentially cross-reacting angiotensins were separated by high performance liquid chromatography after prior extraction with Sep-Pak C18 cartridges. The validity of the assay for the determination of elasmobranch Ang II was demonstrated by parallelism with a series of Ang II standards with serially diluted elasmobranch plasma extracts. Overall recovery of elasmobranch Ang II added to a plasma pool was 75.1 +/- 5.2%. Plasma Ang II concentrations measured by our RIA were similar in fish adapted to 70, 100, or 120% SW at 139 +/- 20.1, 109 +/- 15.3, and 119 +/- 16.3 fmol . ml-1, respectively.

Angiotensin II has depressor effects in pregnant and nonpregnant women

Studies in anesthetized animals suggest that angiotensin II evokes a depressor as well as a pressor effect, which becomes evident on cessation of infusion. We have studied 18 nonpregnant and 8, 23, and 22 women in the first, second, and third trimesters of pregnancy to determine whether such an effect is present in conscious women, whether it is dose dependent, and whether it is influenced by pregnancy. Angiotensin II was infused intravenously in doubling concentrations at 10-minute intervals until a pressor effect of approximately 20 mm Hg was observed. The infusion was stopped, and blood pressure was monitored at 2-minute intervals for 30 minutes. There was a significant diastolic depressor effect after stopping angiotensin II in the nonpregnant women and those in the second and third trimesters of pregnancy. Individual women required differing doses of angiotensin II to evoke the standardized pressor response. It was thus possible to examine the depressor response in each group in relation to infused doses of angiotensin II. In nonpregnant women and in those in the second and third trimesters of pregnancy, the depressor response was dose dependent (P<.001). At any given dose, the depressor response deepened as pregnancy progressed (P<.001). Basal plasma prostacyclin concentrations rise in pregnancy, and angiotensin II can stimulate prostacyclin synthesis. This might mediate the depressor effect. In conclusion, the diminished pressor response to angiotensin II in normal pregnancy may be partly due to an increasing depressor effect of the hormone.

Renal tissue angiotensins during converting enzyme inhibition in the spontaneously hypertensive rat

To compare the effects of an angiotensin-converting enzyme inhibitor on circulating and tissue renin-angiotensin system (RAS), we measured different RAS parameters during the first day of treatment (Day1) as well as after two weeks of treatment (Day14). Ramipril was given orally once daily to adult male spontaneously hypertensive rats (SHR). Renin activity (RA), angiotensin converting enzyme (ACE) activity and levels of angiotensin I (ang I) and angiotensin II (ang II) in the plasma, renal cortex and renal medulla were assessed at Day1 and Day14 of the treatment. In the plasma, both RA and ang I increased 10 to 15 fold one to four hours after acute as well as at Day14 of ramipril treatment and then returned to basal values within 24 hours. Plasma ang II levels were not significantly decreased at Day1 or Day14. The decrease in the ang II/ang I ratio suggested a sustained inhibition of plasma ACE at Day14. In the renal cortex and medulla, a clearly different pattern was observed: in ramipril treated rats, RA in the renal cortex and medulla did not change at Day1 but at Day14 we observed a slight and sustained increase in RA. Despite very high basal levels of RA, ang I levels in the renal cortex were comparable to those in the plasma. The ang I level increased only one-fold one hour after ramipril intake at Day1 and Day14. This suggests that angiotensinogen may have a limiting role in the synthesis of ang I in the kidney. Ang II levels were slightly higher in the renal cortex and medulla than in the plasma suggesting local synthesis of the peptide. In the kidney, ang II levels decreased one and four hours after the acute or prolonged ramipril treatment and the ang II/ang I ratio was reduced at the same time. Our results show that the responses of the plasma and kidney components of the RAS to ACE inhibition are different in the plasma and the kidney suggesting that the circulating and tissue RAS are at least in part independent.

Functional and biochemical analysis of angiotensin II-forming pathways in the human heart

Blockade of the renin-angiotensin system by inhibition of angiotensin-converting enzyme (ACE) is beneficial for the treatment of hypertension and congestive heart failure. However, it is unclear how complete the blockade by ACE inhibitors is and if there is continuing angiotensin II (Ang II) formation during chronic treatment with ACE inhibitors. Indeed chymase, a serine protease, which is able to form angiotensin II from angiotensin I (Ang I) and cannot be blocked by ACE inhibitors, has been shown to be present in human heart. The goal of the present study was to evaluate the extent of renin-angiotensin system blockade and the Ang II-forming pathways in cardiac tissue of patients chronically treated with ACE inhibitors or in patients without ACE inhibition therapy. Our studies indicate an incomplete ACE inhibition in human heart tissue after chronic ACE inhibitor therapy. Moreover, ACE contributes only a small portion to the total Ang I conversion, as shown in biochemical studies in ventricular and coronary homogenates or functionally as Ang I contractions in isolated rings of coronary arteries. A serine protease was responsible for the majority of Ang II production in both the membrane preparation and Ang I-induced contractions of isolated coronary arteries. In humans, the serine protease pathway is likely to play an important role in cardiac Ang II formation. Thus, drugs such as renin inhibitors and Ang II receptor blockers might be able to induce a more complete blockade of the renin-angiotensin system, providing a more efficacious therapy.

Tissue renin-angiotensin system: a site of drug action?

In this review, we present background material that provides partial support for a tissue renin-angiotensin system (RAS). Evidence for the existence of this system relied in part on the use of drugs, which has entailed using low doses or concentrations of angiotensin-converting enzyme inhibitors, renin inhibitors, and angiotensin antagonists to block the RAS in vascular beds and in isolated arteries or organs. Other evidence for a tissue RAS has depended upon measurements of the components of the system, i.e. enzymes, substrates, and mRNAs for these proteins. All of these components were first believed to be present in the heart and blood vessels; however, it is now known that renin in the circulating blood derived from the kidney is used for the local synthesis of angiotensins. The main emphasis of the review is on the renal RAS because it is believed that the local RAS is most prominent in this organ. The renal RAS is probably involved in the long-rather than short-term regulation of renal vascular resistance and maintenance of normal blood pressure through the regulation of sodium reabsorption.

Pathologic consequences of increased angiotensin II activity

Advances in molecular medicine and pharmacology have allowed clinicians to critically reassess the renin-angiotensin system. Angiotensin II (AII) participates in the control of cardiovascular function and electrolyte balance, and plays a part in the regulation of cellular oncogenes and the expression of growth factors. The expression of the proteins of the renin-angiotensin system in organs other than the kidneys suggests that these diverse actions are associated with the peptide in the local environment. Tissue renin-angiotensin activity has prompted the investigation of alternate pathways for the production of AII and characterization of novel forms of angiotensin peptides that counteract the vasoconstrictor and proliferative actions of AII. The heptapeptide angiotensin-(1-7) appears to be critically involved in regulating the angiotensinogen activity of AII through stimulation of vasodilator prostaglandins and release of nitric oxide. Study in this area has been accelerated by the identification of receptors that convey the actions of angiotensin peptides at the cellular level and the pharmacologic characterization of agents that inhibit the ability of AII to bind to target receptors. The introduction of a new class of orally active AII-receptor blockers has provided a specific test of the role of AII in the development of essential hypertension and the potential for improved therapy for hypertension and cardiac and vascular sequelae.