Direct measurement of immunoreactive renin during changes of posture in man

The effect of 30 min of supine rest on plasma renin concentrations in paediatric patients

AIM

The aim of this study is to interpret plasma renin concentrations (PRCs) correctly in a clinical setting, it is essential to understand to what extent physical activity influences the concentration. Reference values for different postural positions exist for adults, but data for children are sparse. In this study, the effect of 30 min of rest on plasma renin concentrations in paediatric patients was examined.

METHODS

In total, 41 paediatric patients, undergoing either arginine or clonidine provocation tests for growth hormone deficiency, were included. For each stimulation test, two blood samples were drawn. One at time 0 min and one after 30 min of rest. In total, renin concentrations were determined in 86 serum samples.

RESULTS

Renin concentrations decreased from 0 to 30 min with a median value of -20.5% (inter-quartile range: -33.0% to -12.8%). This decrease was not dependent on sex, age, medication or comorbidity, and the variance of renin did not differ between 0 and 30 min.

CONCLUSION

Renin concentrations in children differ according to postural position, which is comparable to adults. Thus, the clinical interpretation of renin results needs to consider the preanalytical conditions applied in the local reference population.

Measurement of Plasma Renin Activity with Use of HPLC-Electrospray-Tandem Mass Spectrometry

Background: The measurement of renin activity is complicated by difficulties in the quantification of angiotensin 1 (Ang1), the product of the renin-catalyzed reaction. We report an HPLC-electrospray-tandem mass spectrometry (HPLC-ESI-MS/MS) method for the quantification of Ang1 as a measure of plasma renin activity (PRA).

Methods: After incubation (37 °C for 3 or 18 h), samples were prepared using C18 solid-phase extraction. [Val]5Ang1 was used as the internal standard (IS). Chromatography was performed on a C18 column, using 200 mL/L ammonium acetate buffer–800 mL/L methanol as the mobile phase. The flow rate was 150 μL/min, with a chromatographic run time of 5 min/sample. Mass spectrometric detection was in the positive ionization mode with selected reaction monitoring (Ang1 m/z 649.0→784.0; IS m/z 641.9→770.4).

Results: The assay was linear over the range 2.5–500 ng Ang1/mL, which corresponded to a limit of detection (signal-to-noise ratio of 3:1) of PRA of 0.14 ng Ang1 · mL−1 · h−1. The imprecision (CV) of the assay at PRA values of 26.1, 13.5, 3.2, and 0.78 ng Ang1 · mL−1 · h−1 was 7.0%, 7.0%, 15%, and 11%, respectively. Absolute recoveries were 92.3% (Ang1) and 87.4% (IS). Incubation times of 3 h vs 18 h in the PRA assay gave good agreement at PRA <2 ng Ang1 · mL−1 · h−1, but samples with a PRA of 2–5 ng Ang1 · mL−1 · h−1 gave lower PRA results after incubation for 18 h than after 3 h. We compared the HPLC-ESI-MS/MS assay and an RIA for the determination of PRA, with PRA incubation times of 3 h and 1.5 h, respectively. The mean PRA based on RIA of Ang1 was higher than that obtained using HPLC-ESI-MS/MS.

Conclusion: The HPLC-ESI-MS/MS method allows sensitive and specific measurement of PRA. The higher activities measured with the RIA method highlight its potential for overestimation of PRA.

Immunoreactive renin concentrations in healthy children from birth to adolescence

We establish normative data for immunoreactive renin concentration in serum of healthy children. In a retrospective study, surplus sera of 281 healthy children, aged 0-18 years, were collected from the laboratory. The determinations were performed with a commercially available two-site immunoradiometric assay. Functional sensitivity was 4.0 mU/l, inter-assay and intra-assay variance were 7.0-18.3% and 3.8-7.5%, respectively. In umbilical cord and during the first 4 days of life, renin concentrations (geometric mean) were significantly higher (P < 0.05) than in older infants and children [umbilical cord: 155.2 mU/l; newborn infants (2-4 days of life): 90.9; newborn infants (5-7 days of life): 32.5; 2 weeks-3 months: 40.8; 4 months-1 year: 54.5; 1-3 years: 46.3; 3-5 years: 48.5; 5-7 years: 51.6; 7-11 years: 38.5; 11-15 years: 37.7; 15-18 years: 31.9]. Newborn infants delivered by Caesarian section had significantly lower renin concentrations in umbilical cord than those delivered vaginally (P < 0.02). Considering the methodological advantages and disadvantages of plasma renin activity and renin concentration assays, renin measurement was at least as valuable and accurate as plasma renin activity determination.

Nongenomic effects of aldosterone on intracellular Ca2+ in vascular smooth muscle cells

Genomic mechanisms of steroid action have been increasingly elucidated over the past four decades. In contrast, rapid steroid actions have been widely recognized only recently, and detailed analysis of the mechanisms involved are still lacking. The present article describes rapid effects of mineralocorticoid hormones on free intracellular calcium in vascular smooth muscle cells as determined by fura 2 spectrofluorometry in single cultured cells from rat aorta. These effects are almost immediate and reach a plateau after only 3 to 5 minutes and are characterized by high specificity for mineralocorticoids versus glucocorticoids. The potent mineralocorticoids aldosterone and fludrocortisone are agonists with estimated apparent EC50 values of approximately 0.1 to 0.5 nmol/L; deoxycorticosterone acetate is an agonist with an EC50 of approximately 5 nmol/L; and progesterone, cortisol, corticosterone, and estradiol have much lower potency (EC50 values of approximately 0.5 to 5 mumol/L). The effect of aldosterone is blocked by neomycin and short-term treatment with phorbol esters but augmented by staurosporine, indicating an involvement of phospholipase C and protein kinase C. The Ca2+ effect appears to involve the release of intracellular Ca2+, as shown by the inhibitory effect of thapsigargin; intriguingly, a relatively small maximum effect (approximately 40 nmol/L increase) is consistently seen. This mechanism operates at physiological subnanomolar aldosterone concentrations and appears to be a likely candidate for rapid fine tuning of cardiovascular responsivity. It may also contribute to known clinical features of mineralocorticoid action that are difficult to explain by the traditional genomic mechanism alone.

Non-classical receptors for aldosterone in plasma membranes from pig kidneys

Rapid, nongenomic in vitro effects of aldosterone on intracellular electrolytes, cell volume and the sodium-proton antiporter have been found in human mononuclear leukocytes (HML), as have related membrane receptors. In the present study, binding of 125I-labeled aldosterone to plasma membrane preparations from pig kidneys was studied, since nongenomic in vitro effects of aldosterone have also been described in cultured kidney cells. In this preparation, binding of aldosterone shares important features with both functional and binding data in HML. These include a very low apparent Ki of approximately 0.1 nM for aldosterone, a high turnover rate and binding selectivity for aldosterone and fludrocortisone. Desoxycorticosterone acetate and corticosterone show intermediate affinity, with apparent Ki values of approximately 1 and 100 nM, with hydrocortisone even less active. Thus binding of aldosterone to kidney plasma membranes is compatible with the major features of its nongenomic renal effects.