Establishing a novel assay system for measuring renin concentration using cost effective recombinant ovine angiotensinogen

BACKGROUND

Plasma renin can predict future cardiovascular events as well as the prevalence of chronic renal disease in hypertensive subjects. Ovine angiotensinogen (oANG) is a better substrate for measuring renin concentration through activity assay. Recombinant oANG expressed in Escherichia coli cells can be utilized as the substrate while measuring plasma renin. We aim to establish an immunoassay for measuring renin concentration at picomolar level using recombinant oANG.

MATERIAL AND METHODS

Recombinant oANG was expressed in E. coli cells and purified to homogeneity. Various concentrations (0-1.5 pM) of recombinant human renin standard were prepared and incubated with recombinant oANG. Renin activity was determined by angiotensin-I specific enzyme-linked immunosorbent assay.

RESULTS

About 4.5 mg of purified recombinant oANG was obtained from 0.5 L of E. coli culture. The Michaelis constant and turnover number of human renin with recombinant oANG were 0.16 μM and 0.51 s-1, respectively. A linear relationship was obtained when renin activity was plotted as a function of renin concentration using recombinant oANG as the renin substrate. Picomolar amounts of renin can be measured from known renin activity using this method.

CONCLUSION

This study established a novel assay system for measuring renin at picomolar level using cost effective recombinant oANG.

Current laboratory requirements for adrenocorticotropic hormone and renin/aldosterone sample handling are unnecessarily restrictive

Samples for adrenocorticotropic hormone (ACTH) and aldosterone/renin analysis usually require rapid transport to the receiving laboratory for immediate separation and freezing. In practice, this means assessment is limited to hospital settings and many samples are rejected. We examined whether these requirements are necessary by assessing the stability of ACTH, aldosterone and renin over 48 hours in whole blood collected in serum gel and EDTA plasma from 31 participants. Our results show that ACTH collected into EDTA plasma is stable at room temperature for at least 6 hours, mean change at 6 hours -2.6% (95% CI -9.7 to 4.5). Both aldosterone and renin were stable collected on serum gel at room temperature for at least 6 hours: mean change aldosterone +0.2% (95% CI -3.6 to 4.0), renin -1.9% (95% CI -7.0 to3.2). Therefore, by using appropriate preservatives, ACTH and aldosterone/renin can be measured on samples collected at room temperature and processed within 6 hours. This would facilitate outpatient and emergency room assessment of these analytes.

Acute eprosartan-induced intrarenal vasodilation in hypertensive humans is not influenced by dietary sodium intake or angiotensin II co-infusion

BACKGROUND

Angiotensin II (Ang II) is thought to play an important role in the development of hypertension. Nevertheless, knowledge on the angiotensin II type-1-receptors (AT1Rs) in the hypertensive kidney and the influence of sodium intake and renin-angiotensin system activity on intrarenal AT1R blockade is scarce. To improve our understanding of renal AT1Rs in hypertensive patients, we studied the effects of acute, local administration of AT1R-blocker eprosartan in kidneys of patients with essential hypertension (off medication).

METHOD

In 73 hypertensive patients who were scheduled for diagnostic renal angiography, we measured renal blood flow (Xenon washout method) before and during intrarenal infusion of two incremental doses of eprosartan (3 and 10 μg/kg/min for 15 min per dose). We hypothesized that the vasodilatory effects of eprosartan would be enhanced by low sodium intake and would be reduced during Ang II co-infusion. Therefore, we allocated the patients to either a high or a low sodium diet and coinfused Ang II (1 ng/kg/min) in a subgroup.

RESULTS

Eprosartan infusion resulted in intrarenal vasodilation in all groups. No differences in the magnitude of this effect were found between the groups. No correlation was found between 24-h urinary sodium excretion (a proxy for dietary sodium intake) and the effect of eprosartan.

CONCLUSION

Eprosartan-induced vasodilation is not influenced by sodium intake and/or co-infusion of Ang II. These rather unexpected findings could be explained by differences between circulating and tissue Ang II levels, variations in AT1R expression, and/or stimulation of other vasodilatory pathways.

Polymorphisms at the F12 and KLKB1 loci have significant trait association with activation of the renin-angiotensin system

Background

Plasma coagulation Factor XIIa (Hageman factor; encoded by F12) and kallikrein (KAL or Fletcher factor; encoded by KLKB1) are proteases of the kallikerin-kinin system involved in converting the inactive circulating prorenin to renin. Renin is a key enzyme in the formation of angiotensin II, which regulates blood pressure, fluid and electrolyte balance and is a biomarker for cardiovascular, metabolic and renal function. The renin-angiotensin system is implicated in extinction learning in posttraumatic stress disorder.

Methods & Results

Active plasma renin was measured from two independent cohorts- civilian twins and siblings, as well as U.S. Marines, for a total of 1,180 subjects. Genotyping these subjects revealed that the carriers of the minor alleles at the two loci- F12 and KLKB1 had a significant association with reduced levels of active plasma renin. Meta-analyses confirmed the association across cohorts. In vitro studies verified digestion of human recombinant pro-renin by kallikrein (KAL) to generate active renin. Subsequently, the active renin was able to digest the synthetic substrate angiotensinogen to angiotensin-I. Examination of mouse juxtaglomerular cell line and mouse kidney sections showed co-localization of KAL with renin. Expression of either REN or KLKB1 was regulated in cell line and rodent models of hypertension in response to oxidative stress, interleukin or arterial blood pressure changes.

Conclusions

The functional variants of KLKB1 (rs3733402) and F12 (rs1801020) disrupted the cascade of enzymatic events, resulting in diminished formation of active renin. Using genetic, cellular and molecular approaches we found that conversion of zymogen prorenin to renin was influenced by these polymorphisms. The study suggests that the variant version of protease factor XIIa due to the amino acid substitution had reduced ability to activate prekallikrein to KAL. As a result KAL has reduced efficacy in converting prorenin to renin and this step of the pathway leading to activation of renin affords a potential therapeutic target.

Angiotensin-(1-7)-induced renal vasodilation in hypertensive humans is attenuated by low sodium intake and angiotensin II co-infusion

Current evidence suggests that angiotensin-(1-7) plays an important role in the regulation of tissue blood flow. This evidence, however, is restricted to studies in animals and human forearm. Therefore, we studied the effects of intrarenal angiotensin-(1-7) infusion on renal blood flow in hypertensive humans. To assess the influence of renin-angiotensin system activity, sodium intake was varied and co-infusion with angiotensin II was performed in a subgroup. In 57 hypertensive patients who were scheduled for renal angiography, renal blood flow was measured ((133)Xenon washout method) before and during intrarenal infusion of angiotensin-(1-7) (3 incremental doses: 0.27, 0.9, and 2.7 ng/kg per minute). Patients were randomized into low or high sodium intake. These 2 groups of patients received angiotensin-(1-7), with or without intrarenal co-infusion of angiotensin II (0.3 ng/kg per minute). Angiotensin-(1-7) infusion resulted in intrarenal vasodilation in patients adhering to a sodium-rich diet. This vasodilatory effect of angiotensin-(1-7) was clearly attenuated by low sodium intake, angiotensin II co-infusion, or both. Regression analyses showed that the prevailing renin concentration was the only independent predictor of angiotensin-(1-7)-induced renal vasodilation. In conclusion, angiotensin-(1-7) induces renal vasodilation in hypertensive humans, but the effect of angiotensin-(1-7) is clearly attenuated by low sodium intake and co-infusion of angiotensin II. This supports the hypothesis that angiotensin-(1-7) induced renal vasodilation depends on the degree of renin-angiotensin-system activation.

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.

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.

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.

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.

Clinical Use of Laboratory Tests for the Identification of Secondary Forms of Arterial Hypertension

The prevalence of secondary hypertension can be underestimated if appropriate tests are not performed. The importance of selecting patients with a high pre-test probability of secondary forms of hypertension is first discussed. The laboratory tests currently used for seeking a cause of hypertension are critically reviewed, with emphasis on their operative features and limitations. Strategies to identify primary aldosteronism, the most frequent form of secondary hypertension, and to determine its unilateral or bilateral causes are described. Treatment entails adrenalectomy in unilateral forms, and mineralocorticoid receptor blockade in bilateral forms. Renovascular hypertension is also a common, curable form of hypertension, that should be identified as early as possible to avoid the onset of cardiovascular target organ damage. The tests for its confirmation or exclusion are discussed. The various tests available for the diagnosis of pheochromocytoma, which is much rarer than the above but extremely important to identify, are also described, with emphasis on recent developments in genetic testing. Finally, the tests for diagnosing some rarer monogenic forms and other renal and endocrine causes of arterial hypertension are explored.

Novel Drugs Targeting Hypertension: Renin Inhibitors

The first renin inhibitor, aliskiren, will soon enter the clinical arena. This review summarizes the potential differences between renin inhibitors and the currently existing blockers of the renin-angiotensin system (RAS) [ie, the ACE inhibitors and the angiotensin II type 1 (AT(1)) receptor antagonists], taking also into consideration the recently discovered (pro)renin receptor. This receptor not only activates the inactive precursor of renin, prorenin, but it also exerts direct renin/prorenin-induced effects, independently of angiotensin. The review ends with a brief overview of the available (pre)clinical aliskiren data and a description of its safety profile.

Conformational changes in prorenin during renin inhibition in vitro and in vivo

BACKGROUND

Some renin inhibitors induce changes in the conformation of prorenin in vitro and influence the quantification of active renin by immunoradiometric assays. Whether such changes in renin recognition by monoclonal antibodies exist after oral administration of aliskiren, the first orally available renin inhibitor, is not known.

METHODS

Two commercially available immunoradiometric assays (Cisbio and Nichols) were compared to determine immunoreactive active renin concentrations in plasma samples collected in a single oral dose crossover study comparing the renin inhibitor, aliskiren (300 mg), with the angiotensin II antagonist, valsartan (160 mg), in healthy male subjects.

RESULTS

The addition of aliskiren to plasma samples in vitro, at concentrations of 1-100 micromol/l, increased active renin immunoreactivity in both the Cisbio and Nichols assays. In the crossover study, the two assays gave similar values for the plasma immunoreactive active renin concentration before treatment and following valsartan administration (intraclass coefficient for agreement between the two assays = 0.92). However, a Bland-Altman plot showed a systematic bias towards higher values (1.75-fold higher; 95% confidence interval = 1.02-3.01) in the Nichols than in the Cisbio assay following aliskiren administration. The difference between the results obtained with the two assays depended on incubation time.

CONCLUSIONS

Depending on incubation conditions, circulating renin inhibitors interfere with the recognition of active renin molecules by the monoclonal antibodies used in commercially available assays. Careful consideration must therefore be given to the methodology used for quantifying immunoreactive plasma active renin when patients are treated with renin inhibitors, to avoid an overestimation of the magnitude of active renin release attributable to conformational changes in plasma prorenin.

Mechanisms of adenosine-induced renal vasodilatation in hypertensive patients

BACKGROUND

Adenosine is an endogenous nucleoside with potent vasodilatory capacities, released under ischaemic conditions in particular. Its mechanisms of action, however, remain elusive.

OBJECTIVE

To evaluate the role of adenosine, using a non-selective purinergic receptor antagonist, and the possible involvement of nitric oxide in this mechanism. In addition, the production of renin and catecholamines was studied during infusion of adenosine, caffeine, or both.

METHODS

Thirty-three hypertensive patients who underwent diagnostic renal angiography received intrarenal infusions of adenosine either alone or in combination with caffeine or the nitric oxide synthase inhibitor, N-monomethyl-L-arginine (L-NMMA). The effects on renal blood flow (RBF) were assessed by the xenon-133 washout technique and both arterial and renal venous blood samples were taken for measurement of renin and catecholamine concentrations. Intra-arterial blood pressure and heart rate were monitored continuously.

RESULTS

Adenosine induced a dose-dependent vasodilatation. Caffeine alone did not change RBF, but shifted the dose-response curve of adenosine to the right during concomitant infusion of caffeine. RBF during combined infusion of L-NMMA and adenosine was not different from that during adenosine alone, but the decrease in renal vascular resistance was less pronounced during this combination. Renin secretion did not change during the infusion of either adenosine alone or adenosine in combination with caffeine. Catecholamine concentrations also did not change during any of the experiments.

CONCLUSIONS

Adenosine induces vasodilatation in the human hypertensive kidney and this effect is mediated by the adenosine receptor. Nitric oxide plays, at most, a minor part in the adenosine-induced vasodilatation. Furthermore, renin secretion is not affected by adenosine and caffeine.

Plasma renin and aldosterone measurements in low renin hypertensive states

Plasma renin concentrations are extremely low, requiring high sensitivity methods to detect low renin hypertensive states. Moreover, plasma prorenin must not cryoactivate to renin to avoid falsely high values. The enzyme kinetic plasma renin activity (PRA) test has the required sensitivity, whereas direct renin assays and PRA tests with short incubation times are usually not accurate enough. Test specificity is essential for plasma aldosterone. The Nichols Advantage aldosterone assay is fast and automated but requires great attention to quality control. Here, the impact of renin on the aldosterone:renin ratio as a screening test for primary aldosteronism is reviewed. A sensitive plasma renin test is essential for the diagnosis of low renin hypertensive states and, currently, can be consistently achieved only with the PRA radioimmunoassay.

Validation of a New Automated Renin Assay

Background: Measurement of plasma renin is important for the treatment of patients with congenital adrenal hyperplasia (CAH) and in the evaluation of patients with suspected hyperaldosteronism. Immunologic assays for plasma renin offer easier implementation and standardization than enzyme-kinetic assays for plasma renin activity, but their sensitivity and specificity have been questioned. We studied a renin immunochemiluminescence assay on an automated platform.

Methods: Renin was measured by an enzymatic assay, by IRMA, and by the new Nichols Advantage Specialty System immunochemiluminometric assay (ICMA), in plasmas from unselected individuals from our outpatient departments and in samples from patients with selected diagnoses.

Results: The detection limit in the ICMA was 0.1 mU/L. The recovery was &gt;90%, and the imprecision (CV) was generally &lt;9%. Mean (SD) concentrations measured by ICMA were 32 (21)% lower than those measured by IRMA. Renin concentrations as measured by ICMA were identical in serum and EDTA-, heparin-, and citrate-anticoagulated plasmas. Prolonged incubation of whole blood at room temperature before centrifugation did not affect renin concentrations. The central 95% interval for 80 healthy adults was 6–85.5 mU/L. Plasma renin as assessed by ICMA in patients with primary hyperaldosteronism was &lt;0.2 mU/L.

Conclusions: The performance characteristics of the new renin ICMA allow its use for patients with CAH and for the diagnosis of mineralocorticoid hypertension. In view of the variability of renin concentrations, use for other forms of hypertension or physiologic research calls for the development of uniform sampling protocols.

Long-term renoprotective effects of losartan in diabetic nephropathy: interaction with ACE insertion/deletion genotype?

OBJECTIVE

Several observational follow-up studies have found that the D allele of the insertion (I)/deletion (D) polymorphism of the ACE gene (ACE/ID) is associated with an increased risk of renal function loss, even during ACE inhibition. Therefore, we investigated the long-term effect of the angiotensin II subtype-1 (AT1) receptor antagonist losartan (100 mg o.d.) on kidney function in II and DD type 1 diabetic patients with diabetic nephropathy.

RESEARCH DESIGN AND METHODS

A total of 54 hypertensive type 1 diabetic patients with diabetic nephropathy homozygous for the insertion (n = 26) or the deletion (n = 28) allele were included in the study. After a 4-week washout, the patients received losartan (tablet, 100 mg o.d.) and were followed prospectively with a mean follow-up period of 36 months. Patients and investigators were blinded to ACE genotypes. At baseline, after 2 and 4 months and every 6 months thereafter, glomerular filtration rate (GFR), albuminuria, and 24-h blood pressure were determined.

RESULTS

At baseline, GFR, albuminuria, and blood pressure were similar in the two genotype groups, II versus DD: mean (SD), 86 (22) vs. 88 (24) ml. min(-1). 1.73 m(-2); median (interquartile range), 1,134 (598-2,023) vs. 1,451 (893-1,766) mg/24 h; and mean (SD), 156/82 (17/9) vs. 153/80 (17/11) mmHg, respectively. GFR decreased similarly in both genotype groups, versus DD, respectively (P = 0.4): geometric mean (95% CI), 2.9 (2.0-4.2) vs. 3.4 (2.3-5.1) ml. min(-1). year(-1). Albuminuria and arterial blood pressure were significantly reduced during the study; no differences were noted between groups. During follow-up, albuminuria was decreased by 75% (95% CI 59-85) and 73% (56-83) in the II and DD groups, respectively (P < 0.01 vs. baseline). Mean systolic and diastolic blood pressures were 139/74 mmHg (14/8) in both genotype groups during the study (P < 0.01 vs. baseline).

CONCLUSIONS

In contrast to previous observational studies with ACE inhibitors, long-term treatment with losartan has similar beneficial renoprotective effects on progression of diabetic nephropathy in hypertensive type 1 diabetic patients with ACE II and DD genotypes.

Additive effect of ACE inhibition and angiotensin II receptor blockade in type I diabetic patients with diabetic nephropathy

Albuminuria and hypertension are predictors of poor renal and cardiovascular outcome in diabetic patients. This study tested whether dual blockade of the renin-angiotensin system (RAS) with both an angiotensin-converting enzyme (ACE) inhibitor (ACE-I) and an Angiotensin-II receptor blocker (ARB) is superior to either drug alone in type I diabetic patients with diabetic nephropathy (DN). A randomized double-blind crossover trial was performed with 8-wk treatment with placebo, 20 mg of benazepril once daily, 80 mg of valsartan once daily, and the combination of 20 mg of benazepril and 80 mg of valsartan. Twenty type I diabetic patients with DN were included. At the end of each treatment period, albuminuria, 24-h BP, and GFR were measured. Eighteen patients completed the study. Placebo values were: albuminuria [mean (95% CI)], 701 (490 to 1002) mg/24 h; BP [mean (SEM)], 144 (4)/79 (2) mmHg, and GFR [mean (SEM)], 82 (7) ml/min per 1.73 m(2). Treatment with benazepril, valsartan, or dual blockade significantly reduced albuminuria and BP compared with placebo. Benazepril and valsartan were equally effective. Dual blockade induced an additional reduction in albuminuria of 43 % (29 to 54 %) compared with any type of monotherapy, and a reduction in systolic BP of 6 (0 to 13) mmHg and 7 (1 to 14) mmHg (versus benazepril and valsartan, respectively) and a reduction of 7 (4 to 10) mmHg diastolic compared with both monotherapies. GFR was reversibly reduced on dual blockade compared with monotherapy and placebo. All treatments were safe and well tolerated. In conclusion, dual blockade of the RAS may offer additional renal and cardiovascular protection in type I diabetic patients with DN.

Acute intrarenal administration of cortisol has no effect on renal blood flow in hypertensive individuals

BACKGROUND

Cortisol is known to increase blood pressure. One possible mechanism is the reported increase in renal vascular resistance (RVR). It is unknown whether this is due to a direct effect of cortisol on the kidneys.

OBJECTIVE

To study the effect of infusion of cortisol directly into the renal artery on renal blood flow (RBF) and on renal 11beta-hydroxysteroid dehydrogenase (11beta-HSD)-mediated conversion of cortisol to cortisone in patients with primary hypertension.

DESIGN AND METHODS

Twenty-seven patients with primary hypertension participated in this study. Fifteen received placebo and 12 received glycyrrhetinic acid (GRA; 500 mg) orally 2.5 h before the study. After a 10 min infusion of 5% glucose, cortisol was infused in stepwise increasing doses (0.625, 1.25 and 2.5 microg/kg per min), for 10 min each dose. At the end of each infusion step, RBF was measured using the xenon-133 washout technique. Plasma samples from the femoral artery and renal vein were taken for measurement of cortisol and cortisone. Urine was collected for measurement of steroid concentrations for 6 h on the day before the infusion and for 6 h after the infusion.

RESULTS

After placebo or GRA, cortisol infusion did not change RVR, RBF or blood pressure. RVR values were 0.72 (0.45-0.89) mmHg/ml per min per 100 ml tissue [median (first and third quartiles)] and 0.71 (0.64-0.91) mmHg/ml per min per 100 ml tissue during infusion of 5% glucose and infusion of the highest dose of cortisol, respectively ( P= NS). Cortisol infusion increased the venous-arterial difference in plasma cortisone concentration across the kidney from 76 (40-115) nmol/l to 138 (100-186) nmol/l (P< 0.05) and increased the cortisol : cortisone ratios in the renal vein and in urine (both P< 0.05). As compared with placebo, administration of GRA increased the cortisol : cortisone ratios in peripheral and renal veins and in the urine.

CONCLUSION

Acute infusion of cortisol in high doses directly into the renal artery in patients with primary hypertension did not affect RBF or RVR. Infusion of cortisol resulted in increased cortisol-cortisone conversion by renal 11beta-HSD2, but the concurrent increase in renal and urinary cortisol : cortisone ratio suggests a relative insufficiency of renal 11beta-HSD2 activity as a result of enzyme saturation. This may enhance mineralocorticoid receptor stimulation by cortisol.

Renoprotective effects of losartan in diabetic nephropathy: interaction with ACE insertion/deletion genotype?

BACKGROUND

The beneficial short- and long-term renoprotective effects of angiotensin I-converting enzyme (ACE) inhibition are lower in albuminuric diabetic patients homozygous for the deletion compared to the insertion polymorphism of the ACE gene. In an attempt to overcome this interaction, we evaluated the short-term renoprotective effect in diabetic nephropathy of the angiotensin II receptor antagonist losartan in patients homozygous for the insertion or the deletion allele.

METHODS

Fifty-four hypertensive type 1 diabetic patients with diabetic nephropathy homozygous for the insertion (I; N = 26) or the deletion (D; N = 28) allele of the ACE/ID polymorphism were included. After four weeks of washout, the patients received losartan 50 mg daily followed by 100 mg in two treatment periods each lasting two months. Patients and investigators were blinded to ACE genotypes. At baseline and in the end of the treatment periods, 24-hour blood pressure, albuminuria and glomerular filtration rate values were determined.

RESULTS

At baseline, blood pressure, albuminuria and glomerular filtration rate (GFR) values were similar in the two genotype groups [II vs. DD, 1134 (238 to 5302) vs. 1451 (227 to 8129) mg/24 h, median (range); 156/82 (17/9) vs. 153/80 (17/11) mm Hg, mean (SD); and 86 (22) vs. 88 (24) mL/min/1.73 m2, respectively]. Both doses of losartan significantly lowered blood pressure, albuminuria, and GFR (P < 0.05 vs. baseline). Losartan 100 mg was more effective than 50 mg in reducing albuminuria, 51% (95% CI; 40 to 61) versus 33% (23 to 42), respectively (P < 0.01). No differences in the impact of losartan between the II and DD groups were observed: Losartan 100 mg lowered systolic/diastolic blood pressure by 12/6 and 10/4 mm Hg, whereas albuminuria decreased by 55% (35 to 68) and 46% (28 to 61), in the II and DD groups, respectively (P = NS).

CONCLUSION

Our data suggest that losartan offers similar short-term renoprotective and blood pressure lowering effects in albuminuric hypertensive type 1 diabetic patients with the ACE II and DD genotypes. However, the long-term renoprotective effects remain to be evaluated.

Functional tests for primary aldosteronism: value of captopril suppression

With the introduction of more simple screening tests such as the aldosterone/renin ratio, the detection rate of primary aldosteronism has increased considerably. Until now, no reference values have been available for reporting the aldosterone/renin ratio using plasma aldosterone values expressed in SI units (pmol/L) and plasma active renin (ng/L) measured by immunoradiometric assay. We studied 153 subjects who had either normal blood pressure, essential hypertension, or primary aldosteronism. Essential hypertensive patients usually have aldosterone/renin (pmol/L/ng/L) ratios below 100, whereas ratios for patients with primary aldosteronism are above 140. Results that fall between 100 and 140 suggest a need for repeat testing. Patients with elevated aldosterone/renin ratios require confirmatory testing to demonstrate nonsuppressive autonomous aldosterone production. To this end, salt loading is widely used, but this approach may be contraindicated in patients with severe hypertension. The captopril suppression test appears as effective as salt loading in confirming a diagnosis of primary aldosteronism. In addition, the captopril test is safe, well tolerated, and cost-effective.

Point mutation in the stalk of angiotensin-converting enzyme causes a dramatic increase in serum angiotensin-converting enzyme but no cardiovascular disease

BACKGROUND

Angiotensin-converting enzyme (ACE) metabolizes many small peptides and plays a key role in blood pressure regulation. Elevated serum ACE is claimed to be associated with an increased risk for cardiovascular disease. Previously, two families with dramatically increased serum ACE were described, but no systematic survey of affected individuals was performed, and the molecular background of this trait is unknown.

METHODS AND RESULTS

Eight families were identified with autosomal dominant inheritance of a dramatic (5-fold) increase of serum ACE activity. Strikingly, no clinical abnormalities were apparent in the affected subjects. Isolated blood cells were used for genetic and biochemical analysis. The level of ACE expression on the blood leukocytes and dendritic cells and total cell-associated ACE of the affected individuals was similar to that in nonaffected relatives; however membrane-bound mutant ACE was much more efficiently clipped from the cell surface compared with its wild-type counterpart. A point mutation causing Pro1199Leu in the stalk region of the ACE molecule cosegregates with the increase in serum ACE (LOD score, 6.63).

CONCLUSIONS

A point mutation in the stalk region of the ACE protein causes increased shedding, leading to increased serum ACE, whereas cell-bound ACE is unaltered, and affected individuals exhibit no clinical abnormalities. These findings qualify the importance of serum ACE and establish a new determinant of ACE solubilization.

Nitric oxide dependence of renal blood flow in patients with renal artery stenosis

In ischemia, nitric oxide (NO) production is increased, possibly to preserve flow. The role of NO was investigated in hypertensive patients with or without renal artery stenosis (RAS). Fifty-five hypertensive patients scheduled to undergo diagnostic renal angiography underwent mean renal blood flow (MRBF) measurements before and after an intrarenal injection of the NO synthase blocker N(g)-monomethyl-L-arginine (L-NMMA) at 0.03 microg/kg, before angiography. A dose-response study indicated that this dose of L-NMMA significantly blocked NO synthesis. MRBF was measured at baseline and 1, 5, 10, and 20 min after L-NMMA treatment. On the basis of the angiographic results, patients were divided into three diagnostic categories, i.e., essential hypertension (n = 26), unilateral RAS (n = 16), or bilateral RAS (n = 8). In essential hypertension, MRBF was decreased by 18 +/- 4% at 20 min. In unilateral RAS, L-NMMA did not affect MRBF in the stenotic kidney but reduced MRBF in the nonstenotic kidney by 40 +/- 9% at 20 min. In bilateral RAS, L-NMMA reduced flow by 32 +/- 14% at 20 min. In the nonstenotic kidney in unilateral RAS, a positive correlation was observed between the effect of NO blockade on MRBF and arterial renin levels (P = 0.009). In the stenotic kidney, in contrast, this correlation was inverse (P = 0.007). In conclusion, MRBF depends on NO in hypertensive patients, except in the stenotic kidney in unilateral RAS. In the nonstenotic kidney in unilateral RAS, NO bioavailability is increased. It is suggested that a compensatory mechanism, regulated by NO and possibly angiotensin II, may preserve renal function.

High-affinity prorenin binding to cardiac man-6-P/IGF-II receptors precedes proteolytic activation to renin

Mannose-6-phosphate (man-6-P)/insulin-like growth factor-II (man-6-P/IgF-II) receptors are involved in the activation of recombinant human prorenin by cardiomyocytes. To investigate the kinetics of this process, the nature of activation, the existence of other prorenin receptors, and binding of native prorenin, neonatal rat cardiomyocytes were incubated with recombinant, renal, or amniotic fluid prorenin with or without man-6-P. Intact and activated prorenin were measured in cell lysates with prosegment- and renin-specific antibodies, respectively. The dissociation constant (K(d)) and maximum number of binding sites (B(max)) for prorenin binding to man-6-P/IGF-II receptors were 0.6 +/- 0.1 nM and 3,840 +/- 510 receptors/myocyte, respectively. The capacity for prorenin internalization was greater than 10 times B(max). Levels of internalized intact prorenin decreased rapidly (half-life = 5 +/- 3 min) indicating proteolytic prosegment removal. Prorenin subdivision into man-6-P-free and man-6-P-containing fractions revealed that only the latter was bound. Cells also bound and activated renal but not amniotic fluid prorenin. We concluded that cardiomyocytes display high-affinity binding of renal but not extrarenal prorenin exclusively via man-6-P/IGF-II receptors. Binding precedes internalization and proteolytic activation to renin thereby supporting the concept of cardiac angiotensin formation by renal prorenin.