Newly developed renin and prorenin assays and the clinical evaluation of renin inhibitors

An explorative epigenome-wide association study of plasma renin and aldosterone concentration in a Ghanaian population: the RODAM study

BACKGROUND

The epigenetic regulation of the renin-angiotensin-aldosterone system (RAAS) potentially plays a role in the pathophysiology underlying the high burden of hypertension in sub-Saharan Africans (SSA). Here we report the first epigenome-wide association study (EWAS) of plasma renin and aldosterone concentrations and the aldosterone-to-renin ratio (ARR).

METHODS

Epigenome-wide DNA methylation was measured using the Illumina 450K array on whole blood samples of 68 Ghanaians. Differentially methylated positions (DMPs) were assessed for plasma renin concentration, aldosterone, and ARR using linear regression models adjusted for age, sex, body mass index, diabetes mellitus, hypertension, and technical covariates. Additionally, we extracted methylation loci previously associated with hypertension, kidney function, or that were annotated to RAAS-related genes and associated these with renin and aldosterone concentration.

RESULTS

We identified one DMP for renin, ten DMPs for aldosterone, and one DMP associated with ARR. Top DMPs were annotated to the PTPRN2, SKIL, and KCNT1 genes, which have been reported in relation to cardiometabolic risk factors, atherosclerosis, and sodium-potassium handling. Moreover, EWAS loci previously associated with hypertension, kidney function, or RAAS-related genes were also associated with renin, aldosterone, and ARR.

CONCLUSION

In this first EWAS on RAAS hormones, we identified DMPs associated with renin, aldosterone, and ARR in a SSA population. These findings are a first step in understanding the role of DNA methylation in regulation of the RAAS in general and in a SSA population specifically. Replication and translational studies are needed to establish the role of these DMPs in the hypertension burden in SSA populations.

Corpus luteum number and the maternal renin-angiotensin-aldosterone system as determinants of utero-placental (vascular) development: the Rotterdam Periconceptional Cohort

BACKGROUND

Pregnancies with > 1 corpus luteum (CL) display a hyperdynamic circulation and an increased risk of small-for-gestational age deliveries. Among the factors released by the CL is prorenin, the inactive precursor of renin. Since the renin-angiotensin-aldosterone system (RAAS) is involved in early hemodynamic pregnancy adaptation, we linked both CL number and first-trimester concentrations of prorenin (as an indicator of RAAS activity) and the aldosterone/renin ratio (as an indicator of angiotensin-independent aldosterone effectiveness) to non-invasive markers of utero-placental (vascular) development, measured longitudinally from the first trimester onwards.

METHODS

A total of 201 women, who conceived naturally or after in-vitro fertilization treatment (with 0 (n = 8), 1 (n = 143), or > 1 (n = 51) CL), were selected from the Rotterdam Periconceptional Cohort. Maternal RAAS components were determined at 11 weeks gestation. Placental volume and utero-placental vascular volume were measured from transvaginal 3D ultrasound scans at 7, 9 and 11 weeks gestation, pulsatility and resistance indices of the uterine arteries were assessed by pulsed wave Doppler ultrasounds at 7, 9, 11, 13, 22 and 32 weeks gestation. At birth placental weight was obtained using standardized procedures.

RESULTS

Pregnancies without a CL show lower uterine artery indices throughout gestation than 1 CL and > 1 CL pregnancies, while parameters of placental development are comparable among the CL groups. After adjustment for patient- and treatment-related factors, first-trimester prorenin concentrations are positively associated with uterine artery pulsatility and resistance indices (β 0.06, 95% CI 0.01;0.12, p = 0.04 and β 0.10, 95% CI 0.01;0.20, p = 0.04, respectively), while high prorenin concentrations are negatively associated with first-trimester utero-placental vascular volume (β -0.23, 95% CI -0.44;-0.02, p = 0.04) and placental weight (β -93.8, 95%CI -160.3;-27.4, p = 0.006). In contrast, the aldosterone/renin ratio is positively associated with first-trimester placental volume (β 0.12, 95% CI 0.01;0.24, p = 0.04).

CONCLUSIONS

The absence of a CL, resulting in low prorenin concentrations, associates with low uterine artery pulsatility and resistance, while high prorenin concentrations associate with a low utero-placental vascular volume and weight. These data support a scenario in which excess prorenin, by upregulating angiotensin II, increases uterine resistance, thereby preventing normal placental (vascular) development, and increasing the risk of small-for-gestational age deliveries. Simultaneously, high aldosterone concentrations, by ensuring volume expansion, exert the opposite.

Prorenin periconceptionally and in pregnancy: Does it have a physiological role?

Pregnancy demands major cardiovascular, renal and endocrine changes to provide an adequate blood supply for the growing fetus. The renin-angiotensin-aldosterone system plays a key role in this adaptation process. One of its components, prorenin, is released in significant amounts from the ovary and uteroplacental unit. This review describes the sources of prorenin in the periconception period and in pregnancy, including its modulation by in-vitro fertilization protocols, and discusses its potential effects, among others focusing on preeclampsia. It ends with discussing the long-term consequences, even in later life, of inappropriate renin-angiotensin-aldosterone system activity in pregnancy and offers directions for future research. Ultimately, a full understanding of the role of prorenin periconceptionally and during pregnancy will help to develop tools to diagnose and/or prevent reproductive complications.

Prorenin periconceptionally and in pregnancy: Does it have a physiological role?

Pregnancy demands major cardiovascular, renal and endocrine changes to provide an adequate blood supply for the growing fetus. The renin-angiotensin-aldosterone system plays a key role in this adaptation process. One of its components, prorenin, is released in significant amounts from the ovary and uteroplacental unit. This review describes the sources of prorenin in the periconception period and in pregnancy, including its modulation by in-vitro fertilization protocols, and discusses its potential effects, among others focusing on preeclampsia. It ends with discussing the long-term consequences, even in later life, of inappropriate renin-angiotensin-aldosterone system activity in pregnancy and offers directions for future research. Ultimately, a full understanding of the role of prorenin periconceptionally and during pregnancy will help to develop tools to diagnose and/or prevent reproductive complications.

Determinants of Maternal Renin-Angiotensin-Aldosterone-System Activation in Early Pregnancy: Insights From 2 Cohorts

CONTEXT

The corpus luteum (CL) secretes prorenin, renin's inactive precursor. It may thus contribute to the renin-angiotensin-aldosterone-system (RAAS) activation that is required for maternal adaptation in pregnancy. Whether this activation is disturbed in pregnancies lacking a CL is unknown.

OBJECTIVE

The objective of this work is to investigate maternal RAAS determinants in early pregnancy.

DESIGN AND SETTING

Two observational prospective cohort studies.

TOOK PLACE AT

2 tertiary referral hospitals.

PATIENTS AND INTERVENTION(S)

Pregnancies (n = 277) were stratified by CL number and in vitro fertilization (IVF) protocol: 0 CL (programmed cycle frozen embryo transfer [FET], n = 28), 1 CL (natural cycle FET, n = 41 and spontaneous conceptions, n = 139), and more than 1 CL (ovarian stimulation and fresh embryo transfer, n = 69).

METHODS

Quantification was performed for maternal prorenin, renin, and aldosterone blood levels at 5, 9, and 11 weeks of gestation.

RESULTS

Prorenin and renin were lower in the absence of a CL at all time points when compared to 1 CL, whereas prorenin, renin, and aldosterone were higher in the presence of more than 1 CL vs 1 CL (P < .05). Ovarian stimulation with menopausal gonadotropin resulted in higher prorenin, renin, and aldosterone concentrations during the late first trimester than recombinant follicle-stimulating hormone (P < .05). Prorenin, and to a lesser degree renin, correlated positively with serum progesterone and relaxin, but not serum estradiol. Total follicle diameter, body mass index (BMI), polycystic ovary syndrome (PCOS), and antimüllerian hormone (AMH) were additional determinants of circulating prorenin. Finally, pregnancies conceived in the absence of a CL were more disposed to develop preeclampsia.

CONCLUSIONS

CL number, IVF protocol, BMI, PCOS, and AMH affect maternal RAAS activation in early pregnancy, and may thus contribute to pregnancy complications.

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.

Diagnosis and treatment of primary aldosteronism: practical clinical perspectives

Primary aldosteronism (PA), the most common form of secondary hypertension, can be either surgically cured or treated with targeted pharmacotherapy. PA is frequently undiagnosed and untreated, leading to aldosterone-specific cardiovascular morbidity and nephrotoxicity. Thus, clinicians should perform case detection testing for PA at least once in all patients with hypertension. Confirmatory testing is indicated in most patients with positive case detection testing results. The next step is to determine whether patients with confirmed PA have a disease that can be cured with surgery or whether it should be treated medically; this step is guided by computed tomography scan of the adrenal glands and adrenal venous sampling. With appropriate surgical expertise, laparoscopic unilateral adrenalectomy is safe, efficient and curative in patients with unilateral adrenal disease. In patients who have bilateral aldosterone hypersecretion, the optimal management is a low-sodium diet and lifelong treatment with a mineralocorticoid receptor antagonist administered at a dosage to maintain a high-normal serum potassium concentration without the aid of oral potassium supplements.

Do prorenin-synthesizing cells release active, 'open' prorenin?

BACKGROUND

The function of prorenin, the inactive precursor of renin, remains unclear after many decades of research. The discovery of a (pro)renin receptor suggested that prorenin, by binding to this receptor, would become active, that is, obtain an 'open' conformation. However, the receptor only interacted with prorenin at levels that were many orders of magnitude above its normal levels, making such interaction in-vivo unlikely. Prorenin occurs in two conformations, an open, active form, and a closed, inactive form. Under physiological conditions (pH 7.4, 37 °C), virtually all prorenin occurs in the closed conformation. This study investigated to what degree prorenin-synthesizing cells release prorenin in an open conformation.

METHODS AND RESULTS

Renin plus prorenin-synthesizing human mast cells, and prorenin-synthesizing HEK293 cells (transfected with the mammalian expression vector pRhR1100, containing human prorenin) and human decidua cells were incubated with the renin inhibitor VTP-27999. This inhibitor will trap open prorenin, as after VTP-27999 binding, prorenin can no longer return to its closed conformation, thus allowing its detection in a renin immunoradiometric assay. No evidence for the release of open prorenin was found. Moreover, incubating decidua cells with angiotensinogen yielded low angiotensin levels, corresponding with the activity of ≈1% of prorenin in the medium, that is, the amount of open prorenin expected based upon the equilibrium between open and closed prorenin under physiological conditions.

CONCLUSION

Our study does not reveal evidence for the release of open, active prorenin by prorenin-synthesizing cells, at least under cell culture conditions. This argues against prorenin activity at the site of its release.

Biomarkers of activation of renin-angiotensin-aldosterone system in heart failure: how useful, how feasible?

Renin-angiotensin-aldosterone system (RAAS), participated by kidney, liver, vascular endothelium, and adrenal cortex, and counter-regulated by cardiac endocrine function, is a complex endocrine system regulating systemic functions, such as body salt and water homeostasis and vasomotion, in order to allow the accomplishment of physiological tasks, such as orthostasis, physical and emotional stimuli, and to react towards the hemorrhagic insult, in tight conjunction with other neurohormonal axes, namely the sympathetic nervous system, the endothelin and vasopressin systems. The systemic as well as the tissue RAAS are also dedicated to promote tissue remodeling, particularly relevant after damage, when chronic activation may configure as a maladaptive response, leading to fibrosis, hypertrophy and apoptosis, and organ dysfunction. RAAS activation is a fingerprint of systemic arterial hypertension, kidney dysfunction, vascular atherosclerotic disease, and is definitely an hallmark of heart failure, which rapidly shifts from organ disease to a disorder of neurohormonal regulatory systems. Chronic RAAS activation is an indirect or direct target of most effective pharmacological treatments in heart failure, such as beta-blockers, inhibitors of angiotensin converting enzyme, angiotensin receptor blockers, direct renin inhibitors, and mineralocorticoid receptor blockers. Biomarkers of RAAS activation are available, with different feasibility and accuracy, such as plasma renin activity, renin, angiotensin II, and aldosterone, which all accompany the increasing clinical severity of heart failure disease, and are well recognized prognostic factors, even in patients with optimal therapy. Polymorphisms influencing the expression and activity of RAAS pathways have been recognized as clinically relevant biomarkers, likely influencing either the individual clinical phenotype, or the response to drugs. This solid, growing evidence strongly suggests the rationale for the use of biomarkers of the RAAS activation, as a guide to tailor individual therapy in the current practice, and their implementation as a rule-in marker for future trials on novel drugs in the heart failure setting.

Methodologic issues in the measurement of urinary renin

BACKGROUND AND OBJECTIVES

Alge et al. recently reported that urinary renin may be a prognostic biomarker for AKI after cardiac surgery. However, their urinary renin levels far exceeded published plasma renin levels, whereas normally, urinary renin is <10% of plasma renin. This result raises questions about the specificity of the new Quantikine Renin ELISA Kit used in the work by Alge et al., which is claimed to detect total renin (i.e., renin and prorenin). Therefore, this study tested this assay.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Plasma and urine from 30 patients with hypertension, diabetes, or preeclampsia and 10 healthy pregnant women (randomly selected from sample sets obtained earlier to investigate urinary renin-angiotensin system components) were used to compare the ELISA with a validated renin immunoradiometric assay and an in-house enzyme kinetic assay. Measurements were performed before and after in vitro prorenin activation, representing renin and total renin, respectively.

RESULTS

Total renin measurements by ELISA, immunoradiometric assay, and enzyme kinetic assay were highly correlated. However, ELISA results were consistently ≥10-fold higher. The ELISA standard yielded low to undetectable levels in the immunoradiometric assay and enzyme kinetic assay, except after prorenin activation, when the results were ≥10-fold lower than the ELISA results. In plasma, prorenin activation increased ELISA results by 10%-15%. Urine contained no detectable prorenin.

CONCLUSIONS

The ELISA renin kit standard is prorenin, and its immunoreactivity and enzymatic activity after conversion to renin do not match the International Reference Preparation of human renin that has been used to validate previous immunoradiometric assays and enzyme kinetic assays; in fact, they are at least 10-fold lower, and thus, any measurements obtained with this ELISA kit yield levels that are at least 10-fold too high. The ELISA antibodies detect both renin and prorenin, with a preference for the former. Given these inconsistencies, urinary renin levels should be measured by established renin assays.

Multiple ascending dose study with the new renin inhibitor VTP-27999: nephrocentric consequences of too much renin inhibition

This study compared the pharmacodynamic/pharmacokinetic profile of the new renin inhibitor VTP-27999 in salt-depleted healthy volunteers, administered once daily (75, 150, 300, and 600 mg) for 10 days, versus placebo and 300 mg aliskiren. VTP-27999 was well tolerated with no significant safety issues. It was rapidly absorbed, attaining maximum plasma concentrations at 1 to 4 hours after dosing, with a terminal half-life of 24 to 30 hours. Plasma renin activity remained suppressed during the 24-hour dosing interval at all doses. VTP-27999 administration resulted in a dose-dependent induction of renin, increasing the concentration of plasma renin maximally 350-fold. This induction was greater than with aliskiren, indicating greater intrarenal renin inhibition. VTP-27999 decreased plasma angiotensin II and aldosterone. At 24 hours and later time points after dosing on day 10 in the 600-mg group, angiotensin II and aldosterone levels were increased, and plasma renin activity was also increased at 48 and 72 hours, compared with baseline. VTP-27999 decreased urinary aldosterone excretion versus placebo on day 1. On day 10, urinary aldosterone excretion was higher in the 300- and 600-mg VTP-27999 dose groups compared with baseline. VTP-27999 decreased blood pressure to the same degree as aliskiren. In conclusion, excessive intrarenal renin inhibition, obtained at VTP-27999 doses of 300 mg and higher, is accompanied by plasma renin rises, that after stopping drug intake, exceed the capacity of extrarenal VTP-27999 to block fully the enzymatic reaction. This results in significant rises of angiotensin II and aldosterone. Therefore, renin inhibition has an upper limit.

Recruited renin-containing renal microvascular cells demonstrate the calcium paradox regulatory phenotype

Renin is the critical regulatory enzyme for production of angiotensin (Ang)-II, a potent vasoconstrictor involved in regulating blood pressure and in the pathogenesis of hypertension. Chronic sodium deprivation enhances renin secretion from the kidney, due to recruitment of additional cells from the afferent renal microvasculature to become renin-producing rather than just increasing release from existing juxtaglomerular (JG) cells. JG cells secrete renin inversely proportional to extra- and intracellular calcium, a unique phenomenon characteristic of the JG regulatory phenotype known as the "calcium paradox." It is not known if renin secreted from recruited renin-containing cells is regulated similarly to native JG cells, and therefore acquires this JG cell phenotype. We hypothesized that non-JG cells in renal microvessels recruited to produce renin in response to chronic dietary sodium restriction would demonstrate the calcium paradox, characteristic of the JG cell phenotype. Histology showed recruitment of upstream arteriolar renin in response to sodium restriction compared to normal-diet rats. Renin fluorescence intensity increased 53% in cortices of sodium-restricted rats (P<0.001). We measured renin release from rat afferent microvessels, isolated using iron oxide nanopowder and incubated in either normal or low-calcium media. Basal renin release from normal sodium-diet rat microvessels in normal calcium media was 298.1±44.6 ng AngI/mL/hour/mg protein, and in low-calcium media increased 39% to 415.9±71.4 ng AngI/mL/hour/mg protein (P<0.025). Renin released from sodium-restricted rat microvessels increased 50% compared to samples from normal-diet rats (P<0.04). Renin release in normal calcium media was 447.0±54.3 ng AngI/mL/hour/mg protein, and in low-calcium media increased 36% to 607.6±96.1 ng AngI/mL/hour/mg protein (P<0.05). Thus, renin-containing cells recruited in the afferent microvasculature not only express and secrete renin but demonstrate the calcium paradox, suggesting renin secretion from recruited renin-containing cells share the JG phenotype for regulating renin secretion.

Roles of the circulating renin-angiotensin-aldosterone system in human pregnancy

This review describes the changes that occur in circulating renin-angiotensin-aldosterone system (RAAS) components in human pregnancy. These changes depend on endocrine secretions from the ovary and possibly the placenta and decidua. Not only do these hormonal secretions directly contribute to the increase in RAAS levels, they also cause physiological changes within the cardiovascular system and the kidney, which, in turn, induce reflex release of renal renin. High levels of ANG II play a critical role in maintaining circulating blood volume, blood pressure, and uteroplacental blood flow through interactions with the ANG II type I receptor and through increased production of downstream peptides acting on a changing ANG receptor phenotype. The increase in ANG II early in gestation is driven by estrogen-induced increments in angiotensinogen (AGT) levels, so there cannot be negative feedback leading to reduced ANG II production. AGT can exist in various forms in terms of redox state or complexed with other proteins as polymers; these affect the ability of renin to cleave ANG I from AGT. Thus, during pregnancy the rate of ANG I production varies not only because levels of renin change in response to homeostatic demand but also because AGT changes not only in concentration but in form. Activation of the circulating and intrarenal RAASs is essential for normal pregnancy outcome subserving the increased demand for salt and, hence, water during pregnancy. Thus, the complex integration of the secretions and actions of the circulating maternal renin-angiotensin system in pregnancy plays a key role in pregnancy outcome.

Serum level of soluble (pro)renin receptor is modulated in chronic kidney disease

BACKGROUND

Prorenin, the precursor of renin, binds to the (pro)renin receptor [(P)RR] and triggers intracellular signaling. The ligand binding sites of (P)RR are disconnected and are present in the soluble form of the receptor in serum. Given that the clinical significance of serum prorenin and soluble (P)RR in chronic kidney disease (CKD) is unclear, we investigated the relationship between serum prorenin, soluble (P)RR, and various clinical parameters in patients with CKD.

METHODS

A total of 374 patients with CKD were enrolled. Serum samples were collected, and the levels of soluble (P)RR and prorenin were measured using ELISA kits. Serum creatinine (Cr), blood urea nitrogen (BUN), uric acid (UA), hemoglobin (Hb), soluble secreted α-Klotho, and the urine protein/Cr ratio were also measured. Similarly, clinical parameters were also evaluated using serum and urine sample collected after 1 year (n = 204).

RESULTS

Soluble (P)RR levels were positively associated with serum Cr (P < 0.0001, r = 0.263), BUN (P < 0.0001, r = 0.267), UA (P < 0.005, r = 0.168) levels, CKD stage (P < 0.0001, r = 0.311) and urine protein/Cr ratio (P < 0.01, r = 0.157), and inversely with estimated glomerular infiltration rate (eGFR) (P < 0.0001, r = -0.275) and Hb (P < 0.005, r = -0.156). Soluble (P)RR levels were inversely associated with α-Klotho levels (P < 0.001, r = -0.174) but did not correlate with prorenin levels. With respect to antihypertensive drugs, soluble (P)RR levels were significantly lower in patients treated with an angiotensin II receptor blocker (ARB) than in those without ARB therapy (P < 0.005). Soluble (P)RR levels were significantly lower in CKD patients with diabetes mellitus or primary hypertension than in those without these conditions (P < 0.05). In contrast, serum levels of prorenin did not correlate with parameters related to renal function. Serum prorenin levels were significantly higher in CKD patients with diabetes mellitus than in nondiabetic patients (P < 0.05), but not in CKD patients with hypertension (P = 0.09). Finally, with respect to the relationship between basal soluble (P)RR levels and the progression rates of renal function, soluble (P)RR levels were positively associated with ΔCr (P < 0.05, r = 0.159) and inversely associated with ΔeGFR (P < 0.05, r = -0.148).

CONCLUSION

Serum levels of soluble (P)RR correlated with the stage of CKD. Our findings suggest that soluble (P)RR may be involved in renal injury and influence the progression of CKD.

New roles for renin and prorenin in heart failure and cardiorenal crosstalk

The renin-angiotensin-aldosterone-system (RAAS) plays a central role in the pathophysiology of heart failure and cardiorenal interaction. Drugs interfering in the RAAS form the pillars in treatment of heart failure and cardiorenal syndrome. Although RAAS inhibitors improve prognosis, heart failure–associated morbidity and mortality remain high, especially in the presence of kidney disease. The effect of RAAS blockade may be limited due to the loss of an inhibitory feedback of angiotensin II on renin production. The subsequent increase in prorenin and renin may activate several alternative pathways. These include the recently discovered (pro-) renin receptor, angiotensin II escape via chymase and cathepsin, and the formation of various angiotensin subforms upstream from the blockade, including angiotensin 1–7, angiotensin III, and angiotensin IV. Recently, the direct renin inhibitor aliskiren has been proven effective in reducing plasma renin activity (PRA) and appears to provide additional (tissue) RAAS blockade on top of angiotensin-converting enzyme and angiotensin receptor blockers, underscoring the important role of renin, even (or more so) under adequate RAAS blockade. Reducing PRA however occurs at the expense of an increase plasma renin concentration (PRC). PRC may exert direct effects independent of PRA through the recently discovered (pro-) renin receptor. Additional novel possibilities to interfere in the RAAS, for instance using vitamin D receptor activation, as well as the increased knowledge on alternative pathways, have revived the question on how ideal RAAS-guided therapy should be implemented. Renin and prorenin are pivotal since these are at the base of all of these pathways.

New renin inhibitor VTP-27999 alters renin immunoreactivity and does not unfold prorenin

Renin inhibitors like aliskiren not only block renin but also bind prorenin, thereby inducing a conformational change (like the change induced by acid) allowing its recognition in a renin-specific assay. Consequently, aliskiren can be used to measure prorenin. VTP-27999 is a new renin inhibitor with an aliskiren-like IC50 and t1/2, and a much higher bioavailability. This study addressed (pro)renin changes during treatment of volunteers with VTP-27999 or aliskiren. Both drugs increased renin immunoreactivity. Treatment of plasma samples from aliskiren-treated subjects with excess aliskiren yielded higher renin immunoreactivity levels, confirming the presence of prorenin. Unexpectedly, this approach did not work in VTP-27999-treated subjects, although an assay detecting the prosegment revealed that their blood still contained prorenin. Subsequent in vitro analysis showed that VTP-27999 increased renin immunoreactivity for a given amount of renin by ≥ 30% but did not unfold prorenin. Yet, it did bind to acid-activated, intact prorenin and then again increased immunoreactivity in a renin assay. However, no such increase in immunoreactivity was seen when measuring acid-activated prorenin bound to VTP-27999 with a prosegment-directed assay. The VTP-27999-induced rises in renin immunoreactivity could be competitively prevented by aliskiren, and antibody displacement studies revealed a higher affinity of the active site-directed antibodies in the presence of VTP-27999. In conclusion, VTP-27999 increases renin immunoreactivity in renin immunoassays because it affects the affinity of the active site-directed antibody. Combined with its lack of effect on prorenin, these data show that VTP-27999 differs from aliskiren. The clinical relevance of these results needs to be established.

Renin and prorenin as biomarkers in hypertension

PURPOSE OF REVIEW

This review examines the evidence that plasma renin and/or prorenin level may be used to guide therapy in hypertension and as an independent risk factor for future cardiovascular events.

RECENT FINDINGS

A large number of retrospective analyses of patient populations in clinical trials, in whom 'baseline' renin measurements were available, supports that high renin, but not high prorenin levels, are indicative of future cardiovascular disease and death, particularly in patients with kidney dysfunction and/or hypertension. The relationship is not affected by the use of renin-angiotensin system (RAS) blockers. High renin levels also tend to support the use of RAS inhibitors as first-choice antihypertensive agents. However, the added value of a renin measurement on top of traditional risk factors is modest, and the pressure response to RAS blockade, even in high-renin patients, varies widely.

SUMMARY

Measuring 'baseline' renin as a marker of future cardiovascular events or to determine the choice of drug is of limited value in an individual patient.

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.

The (pro)renin receptor. A decade of research: what have we learned?

The discovery of a (pro)renin receptor ((P)RR) in 2002 provided a long-sought explanation for tissue renin–angiotensin system (RAS) activity and a function for circulating prorenin, the inactive precursor of renin, in end-organ damage. Binding of renin and prorenin (referred to as (pro)renin) to the (P)RR increases angiotensin I formation and induces intracellular signalling, resulting in the production of profibrotic factors. However, the (pro)renin concentrations required for intracellular signalling in vitro are several orders of magnitude above (patho)physiological plasma levels. Moreover, the phenotype of prorenin-overexpressing animals could be completely attributed to angiotensin generation, possibly even without the need for a receptor. The efficacy of the only available putative (pro)renin receptor blocker handle region peptide remains doubtful, leading to inconclusive results. The fact that, in contrast to other RAS components, (P)RR knock-outs, even tissue-specific, are lethal, points to an important, (pro)renin-independent, function of the (P)RR. Indeed, recent research has highlighted ancillary functions of the (P)RR as an essential accessory protein of the vacuolar-type H⁺-ATPase (V-ATPase), and in this role, it acts as an intermediate in Wnt signalling independent of (pro)renin. In conclusion, (pro)renin-dependent signalling is unlikely in non-(pro)renin synthesizing organs, and the (P)RR role in V-ATPase integrity and Wnt signalling may explain some, if not all of the phenotypes previously associated with (pro)renin-(P)RR interaction.

(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.

Evaluation of a direct prorenin assay making use of a monoclonal antibody directed against residues 32-39 of the prosegment

BACKGROUND

Prorenin is an early marker of microvascular complications in diabetes. However, it can only be measured indirectly (following its conversion to renin), with a renin immunoradiometric assay (IRMA). Unfortunately, treatment with a renin inhibitor interferes with this assay, because renin inhibitors induce a conformational change in prorenin, thereby allowing its detection as renin.

METHODS

We evaluated Molecular Innovation's new direct prorenin ELISA, which makes use of an antibody that recognizes an epitope near prorenin's putative cleavage site (R 43 L 44), thus no longer requiring prorenin activation. Plasma samples of 41 diabetic individuals treated with aliskiren (renin inhibitor) or irbesartan were tested. Semi-purified recombinant prorenin was used as standard, because the ELISA standard yielded approximately 10-fold lower values in the renin IRMA following its conversion to renin.

RESULTS

The ELISA detected prorenin levels that were identical to those determined by the IRMA in untreated and irbesartan-treated individuals. Yet, it yielded higher prorenin levels in aliskiren-treated individuals. Aliskiren, at levels reached in plasma during treatment, did not interfere with the ELISA, but allowed the detection of up to 20-30% of prorenin as renin in the IRMA, thereby resulting in a significant overestimation of renin and an underestimation of prorenin. The ELISA rendered results within 2 h and did not require a pretreatment period of several days to convert prorenin to renin.

CONCLUSION

The new direct assay allows rapid prorenin detection, is not hampered by aliskiren when used at clinically relevant doses, and might be used to identify diabetic patients developing retinopathy and/or nephropathy.

Renal responses to three types of renin-angiotensin system blockers in patients with diabetes mellitus on a high-salt diet: a need for higher doses in diabetic patients?

OBJECTIVE

Activation of the renal renin-angiotensin system in patients with diabetes mellitus appears to contribute to the risk of nephropathy. Recently, it has been recognized than an elevation of prorenin in plasma also provides a strong indication of risk of nephropathy. This study was designed to examine renin-angiotensin system control mechanisms in the patient with diabetes mellitus.

METHODS

We enrolled 43 individuals with type 2 diabetes mellitus. All individuals were on a high-salt diet to minimize the contribution of the systemic renin-angiotensin system. After an acute exposure to captopril (25 mg), they were randomized to treatment with either irbesartan (300 mg) or aliskiren (300 mg) for 2 weeks.

RESULTS

All agents acutely lowered blood pressure and plasma aldosterone, and increased renal plasma flow and glomerular filtration rate. Yet, only captopril and aliskiren acutely increased plasma renin and decreased plasma angiotensin II, whereas irbesartan acutely affected neither renin nor angiotensin II. Plasma renin and angiotensin II subsequently did increase upon chronic irbesartan treatment. When given on day 14, irbesartan and aliskiren again induced the above hemodynamic, renal and adrenal effects, yet without significantly changing plasma renin. Irbesartan at that time did not affect plasma angiotensin II, whereas aliskiren lowered it to almost zero.

CONCLUSION

The relative resistance of the renal renin response to acute (irbesartan) and chronic (irbesartan and aliskiren) renin-angiotensin system blockade supports the concept of an activated renal renin-angiotensin system in diabetes, particularly at the level of the juxtaglomerular cell, and implies that diabetic patients might require higher doses of renin-angiotensin system blockers to fully suppress the renal renin-angiotensin system.

Measuring and targeting aldosterone and renin in atherosclerosis-a review of clinical data

Our understanding of the development and progression of atherosclerosis has increased substantially over the past decades. A significant role for the renin-angiotensin-aldosterone system (RAAS) in this process has gained appreciation in recent years. Preclinical and clinical studies have associated components of the RAAS with various cardiovascular disease conditions. Classically known for its contribution to hypertension, dysregulation of the system is now also believed to promote vascular inflammation, fibrosis, remodeling, and endothelial dysfunction, all intimately related to atherosclerosis. The reduction in cardiovascular mortality and morbidity, as seen with the use of angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers, supports the concept that RAAS is involved in the pathogenesis of atherosclerotic disease. However, the underlying molecular mechanisms of the pathophysiology remain to be completely understood. Evidence points toward additional benefit from therapeutic approaches aiming at more complete inhibition of the system and the possible utility of renin or aldosterone in the prediction of cardiovascular outcome. This review will summarize the current knowledge from clinical studies regarding the presumptive role of renin and aldosterone in the prediction and management of patients with atherosclerosis. For this purpose, a literature search was performed, focusing on available clinical data regarding renin or aldosterone and cardiovascular outcome.

A comparative study on inter and intralaboratory reproducibility of renin measurement with a conventional enzymatic method and a new chemiluminescent assay of immunoreactive renin

BACKGROUND

The activity of the renin-angiotensin system is usually evaluated as plasma renin activity (PRA, ngAI/ml per h) but the reproducibility of this enzymatic assay is notoriously scarce. We compared the inter and intralaboratory reproducibilities of PRA with those of a new automated chemiluminescent assay, which allows the direct quantification of immunoreactive renin [chemiluminescent immunoreactive renin (CLIR), microU/ml].

METHODS

Aliquots from six pool plasmas of patients with very low to very high PRA levels were measured in 12 centres with both the enzymatic and the direct assays. The same methods were applied to three control plasma preparations with known renin content.

RESULTS

In pool plasmas, mean PRA values ranged from 0.14 +/- 0.08 to 18.9 +/- 4.1 ngAI/ml per h, whereas those of CLIR ranged from 4.2 +/- 1.7 to 436 +/- 47 microU/ml. In control plasmas, mean values of PRA and of CLIR were always within the expected range. Overall, there was a significant correlation between the two methods (r = 0.73, P < 0.01). Similar correlations were found in plasmas subdivided in those with low, intermediate and high PRA. However, the coefficients of variation among laboratories found for PRA were always higher than those of CLIR, ranging from 59.4 to 17.1% for PRA, and from 41.0 to 10.7% for CLIR (P < 0.01). Also, the mean intralaboratory variability was higher for PRA than for CLIR, being respectively, 8.5 and 4.5% (P < 0.01).

CONCLUSION

The measurement of renin with the chemiluminescent method is a reliable alternative to PRA, having the advantage of a superior inter and intralaboratory reproducibility.

Comparative efficacy and safety of aliskiren and irbesartan in patients with hypertension and metabolic syndrome

Metabolic syndrome, a cluster of risk factors that increase the risk of cardiovascular morbidity and mortality, is common in patients with hypertension. Chronic renin-angiotensin-aldosterone system (RAAS) activation, shown by elevated plasma renin activity (PRA), is implicated in many of the features of metabolic syndrome. The direct renin inhibitor aliskiren may be of benefit in this patient group as aliskiren targets the RAAS at the rate-limiting step. In this double-blind study, 141 patients with hypertension (mean baseline BP 155/93 mm Hg) and metabolic syndrome (modified National Cholesterol Education Program ATP III criteria) were randomized to aliskiren 300 mg or irbesartan 300 mg once daily. Patients treated with aliskiren 300 mg had their mean sitting blood pressure (BP) lowered by 13.8/7.1 mm Hg after 12 weeks, significantly greater (P≤0.001) than the 5.8/2.8 mm Hg reduction observed in patients treated with irbesartan 300 mg. A significantly greater proportion of patients treated with aliskiren achieved BP control to <135/85 mm Hg (29.2 vs 16.7% with irbesartan; P=0.019). Aliskiren treatment led to a 60% decrease in PRA from baseline, whereas irbesartan increased PRA by 99% (both P<0.001). Aliskiren and irbesartan had similar effects on glucose and lipid profiles and on a panel of biomarkers of inflammation and cardiovascular risk. Both aliskiren and irbesartan were well tolerated. Collectively, these results suggest that aliskiren 300 mg may offer treatment benefits compared with irbesartan 300 mg for BP reduction in patients with hypertension and metabolic syndrome.

Effects of the direct renin inhibitor aliskiren and atenolol alone or in combination in patients with hypertension

INTRODUCTION

Aliskiren is the first in a new class of direct renin inhibitors to be approved for the treatment of hypertension.

PATIENTS AND METHODS

In this double-blind, multicentre trial, 694 patients with hypertension (mean sitting diastolic blood pressure [BP] > or = 95 and < 110 mmHg) were randomised to once-daily aliskiren 150 mg (n=231), atenolol 50 mg (n=231) or the combination (150/50 mg; n=232) for six weeks, followed by a further six weeks on double the initial doses of aliskiren and atenolol. Efficacy (reduction from baseline in mean sitting systolic and diastolic BP) and tolerability of study treatments were assessed; plasma renin activity (PRA) was measured in a subset of patients.

RESULTS

At Week 12 endpoint, aliskiren, atenolol and aliskiren/atenolol lowered systolic and diastolic BP from baseline by 14.3/11.3, 14.3/13.7 and 17.3/14.1 mmHg, respectively. Systolic BP reductions with aliskiren/atenolol were significantly greater than those with aliskiren (p=0.039) or atenolol (p=0.034) alone, and diastolic BP reductions were greater than with aliskiren alone (p<0.001). Diastolic BP changes were larger with atenolol than with aliskiren (p=0.003, correlating with the large reductions in pulse rate (> 10 bpm) observed with atenolol. Aliskiren, atenolol and aliskiren/atenolol reduced geometric mean PRA from baseline by 65%, 52% and 61%, respectively. In patients with moderate or high baseline PRA (> or = 0.65 ng/ml/hour), PRA was reduced to low levels (< 0.65 ng/ml/hour) at Week 12 endpoint in a greater proportion of patients receiving aliskiren (11/15 patients, 73.3%) or aliskiren/atenolol (18/23, 78.3%) than with atenolol (10/21, 47.6%). Aliskiren treatment was associated with numerically lower rates of adverse events and discontinuations due to adverse events compared with atenolol or combination treatment, and unlike atenolol was not associated with bradycardia.

CONCLUSIONS

Direct renin inhibition with aliskiren may be an appropriate substitute for beta-blocker treatment in patients with uncomplicated hypertension. Aliskiren also represents an attractive option for dual therapy with atenolol to improve systolic BP/pulse pressure reductions and BP control with maintained tolerability compared with atenolol alone.

Measurement of plasma renin concentration in cats by use of a fluorescence resonance energy transfer peptide substrate of renin

OBJECTIVE

To evaluate the use of a commercially available 5-carboxyfluorescein-based, intramolecularly quenched, fluorescence resonance energy transfer (FRET) peptide substrate of renin for measurement of plasma renin concentration in cats.

SAMPLE POPULATION

Plasma samples obtained during a previous study of renal autograft ischemia-reperfusion injury in 10 cats and samples of fetal bovine serum containing recombinant human renin (rh-renin).

PROCEDURES

Experiments involving samples of fetal bovine serum containing rh-renin were conducted to identify a suitable control vehicle, optimal substrate concentration, and appropriate duration of incubation. With the use of the identified assay conditions, a standard curve was constructed to allow conversion of relative fluorescent units into values of renin concentration (ng/mL). Subsequently, plasma samples obtained from cats before and after renal autograft ischemia-reperfusion injury were assayed to determine endogenous renin concentration.

RESULTS

Under conditions of a 1:50 substrate dilution and 4-hour incubation period, the assay detected small amounts of rh-renin in fetal bovine serum. A linear relationship (R(2) = 0.996) between the relative fluorescent units generated and exogenous rh-renin concentration was evident. The assay detected renin in plasma samples obtained from cats after renal autograft ischemia-reperfusion, and renin concentrations on days 1 and 2 after transplant differed significantly.

CONCLUSIONS AND CLINICAL RELEVANCE

The study data indicated that the assay involving the FRET peptide substrate of renin is potentially a rapid and specific method for measurement of plasma renin concentration in cats.

Signal transduction of the (pro)renin receptor as a novel therapeutic target for preventing end-organ damage

The (pro)renin receptor ((P)RR) not only represents a novel component of the renin-angiotensin system but is also a promising novel drug target because of its crucial involvement in the pathogenesis of renal and cardiac end-organ damage. This review discusses the signal transduction of the (P)RR with its adapter protein promyelocytic zinc-finger protein, the impact of this receptor, especially on cardiovascular disease, and its putative interaction with renin inhibitors such as aliskiren. Furthermore, the increasing complexity regarding the cellular function of the (P)RR is addressed, which arises by the intimate link with proton pumps and the phosphatase PRL-1, as well as by the presence of different subcellular localizations and of a soluble isoform of the (P)RR. Finally, the rationale and strategy for the development of small-molecule antagonists of the (P)RR, called renin/prorenin receptor blockers, are presented.

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.

Activity Assays and Immunoassays for Plasma Renin and Prorenin: Information Provided and Precautions Necessary for Accurate Measurement

Background: Measurement of plasma renin is important for the clinical assessment of hypertensive patients. The most common methods for measuring plasma renin are the plasma renin activity (PRA) assay and the renin immunoassay. The clinical application of renin inhibitor therapy has thrown into focus the differences in information provided by activity assays and immunoassays for renin and prorenin measurement and has drawn attention to the need for precautions to ensure their accurate measurement.

Content: Renin activity assays and immunoassays provide related but different information. Whereas activity assays measure only active renin, immunoassays measure both active and inhibited renin. Particular care must be taken in the collection and processing of blood samples and in the performance of these assays to avoid errors in renin measurement. Both activity assays and immunoassays are susceptible to renin overestimation due to prorenin activation. In addition, activity assays performed with peptidase inhibitors may overestimate the degree of inhibition of PRA by renin inhibitor therapy. Moreover, immunoassays may overestimate the reactive increase in plasma renin concentration in response to renin inhibitor therapy, owing to the inhibitor promoting conversion of prorenin to an open conformation that is recognized by renin immunoassays.

Conclusions: The successful application of renin assays to patient care requires that the clinician and the clinical chemist understand the information provided by these assays and of the precautions necessary to ensure their accuracy.

Intrarenal renin-angiotensin system

Phylogenetically the renin-angiotensin system (RAS) is an ancient regulatory system which has attracted the attention of researchers for about a century. As a result of their efforts, different types of RAS inhibitors are now widely used as therapeutic medicines. The scientific enthusiasm toward RAS remains undiminished and new findings and discoveries are to be expected. Early investigators described the role of RAS in the local control of renal hemodynamics. This correlated well with the morphology of juxtaglomerular apparatus (JGA). Recently developed imaging techniques has allowed for in vivo visualization of cellular functions and the use of molecular biological tools have shed new light on the morphology and physiology of renal RAS, especially in connection with the tubular system. RAS has gained recognition to be more than just an endocrine regulatory system for regulating hemodynamics and water/salt metabolism. RAS is a local tissue and/or cellular regulator with a wide range of effects exerted via various receptors. Local RAS is crucially involved in basic physiological processes like ontogenesis and cell proliferation as well as pathophysiological conditions such as inflammation and tissue fibrosis. These findings may open new frontiers for novel therapeutic approaches. This review focuses only on some specific - less discussed and recently described or hypothesized - morphological and functional aspects of intrarenal RAS, including in vivo imaging of RAS, its effects on juxtaglomerular apparatus and possible cooperative mechanisms among various local renal RAS systems.

Diabetic complications: a role for the prorenin-(pro)renin receptor-TGF-beta1 axis?

Morbidity and mortality of diabetes mellitus are strongly associated with cardiovascular disease including nephropathy. A discordant tissue renin-angiotensin system (RAS) might be a mediator of the endothelial dysfunction leading to both micro- and macrovascular complications of diabetes. The elevated plasma levels of prorenin in diabetic subjects with microvascular complications might be part of this discordant RAS, especially since the plasma renin levels in diabetes are low. Prorenin, previously thought of as an inactive precursor of renin, is now known to bind to a (pro)renin receptor, thus activating locally angiotensin-dependent and -independent pathways. In particular, the stimulation of the transforming growth factor-beta (TGF-beta) system by prorenin could be an important contributor to diabetic disease complications. This review discusses the concept of the prorenin-(pro)renin receptor-TGF-beta(1) axis, concluding that interference with this pathway might be a next logical step in the search for new therapeutic regimens to reduce diabetes-related morbidity and mortality.

(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.

Aliskiren fails to lower blood pressure in patients who have either low PRA levels or whose PRA falls insufficiently or reactively rises

BACKGROUND

Suppressed baseline plasma renin activity (PRA) levels or large reactive increases in renin secretion are two possible reasons for treatment failure with antirenin system drugs.

METHODS

To investigate their prevalence we reanalyzed data from three published clinical trials of the renin inhibitor aliskiren.

RESULTS

Aliskiren failed to lower systolic blood pressure (SBP) by at least 10 mm Hg in half of all patients. It was very effective in two-thirds of medium- to high-renin patients (-19 mm Hg). But BP did not fall in most low-renin patients, or in 30% of medium- to high-renin patients. BP actually rose by >10 mm Hg in 5% of patients taking aliskiren and in >10% of patients when aliskiren was added to an angiotensin receptor blocker (ARB) or angiotensin converting enzyme inhibitor (ACEI). PRA changed in parallel with BP. Although PRA fell in most patients, it actually rose in 5% and in up to 30% when aliskiren was added to an ARB or ACEI.

CONCLUSIONS

There are two reasons for treatment failure with aliskiren. Many hypertensive patients have large BP falls. But, BP does not fall in most low-renin patients or in medium- to high-renin patients whose PRA levels do not fall sufficiently. If the concept of that treatment resistance is caused by excessive reactive increases in renin secretion is confirmed, then a PRA determination during treatment could be used to guide subsequent addition or subtraction of either natriuretic or antirenin drug types, to thereby correct BP and reduce cardiovascular risk.

Components of the renin-angiotensin-aldosterone system in plasma and ascites in hepatic cirrhosis

BACKGROUND

Decompensated liver cirrhosis is characterized by activation of the renin-angiotensin-aldosterone system (RAAS). We investigated whether compartmentalization of these components occurs in ascitic fluid.

METHODS

In 26 patients with cirrhosis RAAS components and albumin were quantified in simultaneously obtained plasma and ascitic fluid samples. Renin degradation was determined in vitro in plasma and ascites.

RESULTS

Plasma angiotensinogen was below normal reference values in all but two patients and correlated inversely with plasma renin (r = -0.73, P < 0.001). Plasma renin activity was elevated in most subjects. The plasma and ascites concentrations of renin, prorenin, angiotensinogen and aldosterone were closely (P < 0.001) correlated. Expressed as a percentage of plasma levels, the angiotensinogen level (18 +/- 11%) was slightly lower than the albumin level (23 +/- 8%), whereas the aldosterone level (43 +/- 18%) was considerably higher (P < 0.0001). For renin and prorenin these percentages were much lower (P < 0.0001), despite the fact that their molecular weight is lower than that of albumin and angiotensinogen. This was not due to a more rapid degradation of renin in ascites fluid, since the in-vitro degradation rates of renin in plasma and ascitic fluid were identical.

CONCLUSION

In hepatic cirrhosis ascites can be regarded as an ultrafiltrate of plasma RAAS components. Since differences in molecular weight or metabolic rate cannot explain the low ascites-to-plasma ratio of renin and prorenin, either their transcapillary transport is impaired and/or they selectively bind to (pro)renin binding sites.