Andreas Rohrwasser

Effects of Dietary Sodium and Genetic Background on Angiotensinogen and Renin in Mouse

Elements of a renin-angiotensin system expressed along the entire nephron, including angiotensinogen secreted by proximal tubule and renin expressed in connecting tubule, may participate in the regulation of sodium reabsorption at multiple sites of the nephron. The response of this tubular renin-angiotensin system to stepwise changes in dietary sodium was investigated in 2 mouse strains, the sodium-sensitive inbred C57BL/6 and the sodium-resistant CD1 outbred. Plasma angiotensinogen was not affected by sodium regimen, whereas plasma renin increased 2-fold under low sodium. In both strains, the variation in urinary parameters did not parallel the changes observed in plasma. Angiotensinogen and renin excretion were significantly higher under high sodium than under low sodium. Water deprivation, by contrast, induced significant activation in the tubular expression of angiotensinogen and renin. C57BL/6 exhibited significantly higher urinary excretion of angiotensinogen than did CD1 animals under both conditions of sodium intake. The extent to which these urinary parameters reflect systemic or tubular responses to challenges of sodium homeostasis may depend on the relative contribution of sodium restriction and volume depletion.

Development of genetic hypotheses in essential hypertension

AbstractEssential hypertension illustrates the formidable task presented by the identification of genetic determinants of common disease. Making an initial genetic inference may prove difficult enough; the subsequent demonstration of functional significance at various levels of biological integration may be even more challenging. We review three instances in which an initial genetic inference has led to the development of testable hypotheses pursued at increasingly higher levels of biological organization. These include the adducin, the G protein β3 subunit, and the angiotensinogen hypotheses.

Significance of urinary angiotensinogen in essential hypertension as a function of plasma renin and aldosterone status.

Objective This study was performed to test the significance of urinary angiotensinogen (UAGT) in essential hypertensive patients stratified as a function of plasma renin and aldosterone. Methods and results A sample of 248 essential hypertensives, investigated under their usual sodium diet and either off-medication or under a standardized treatment, was separated into two groups on the basis of upright plasma active renin and aldosterone medians. Patients with plasma active renin and aldosterone below medians are referred to as the low renin–aldosterone essential hypertensive group (LRA-EH). Others subjects are defined as other essential hypertensives (O-EH). Blood pressure (BP) was recorded by 24-h ambulatory monitoring. UAGT was measured by a specific enzyme-linked immunosorbent assay for total angiotensinogen. Because UAGT was markedly increased in the presence of overt proteinuria (≥ 300 mg/24 h), proteinuric patients (n = 29) were excluded from subsequent analyses. UAGT was a significant predictor of systolic and diastolic BP in LRA-EH females (P < 0.01 and P = 0.05, respectively) but not in males. By contrast, urinary sodium excretion (P < 0.001) and maintenance of treatment (P = 0.002) were significant predictors of systolic BP in males. These correlations were not observed in O-EH, whether males or females. Conclusions In the present study, UAGT stands as a strong predictor of BP in women with low plasma renin/aldosterone, suggesting an involvement of the tubular renin–angiotensin system in these subjects. Higher sodium intake or the need to maintain treatment may account in part for the lack of a similar relationship in males.

Functional Analysis of a Mutation Occurring between the Two In-frame AUG Codons of Human Angiotensinogen

Angiotensinogen (ANG) is the specific substrate of the renin-angiotensin system, a major participant in blood pressure control. We have identified a natural mutation at the −30 amino acid position of the angiotensinogen signal peptide, in which an arginine is replaced by a proline (R−30P). Heterozygous individuals with R−30P showed a tendency to lowered plasma angiotensinogen level (1563 ng of ANG I/ml (range 1129–1941)) compared with normal individuals in the family (1892 ng of ANG I/ml (range 1603–2072)). Human angiotensinogen mRNA has two in-phase translation initiation codons (AUG) starting upstream 39 and 66 nucleotides from the cap site. R−30P occurs in a cluster of basic residues adjacent to the first AUG codon that may affect intracellular sorting of the nascent protein. Pulse-chase experiments in transiently transfected cultured cells revealed that the R−30P mutation was associated with reduced amounts of both intra- and extracellular protein. In a cell-free system, we found that two forms of native angiotensinogen were generated by alternative initiation of translation at either AUG codon. Alteration of either the first or second AUG codons abolished the synthesis of the longer and the shorter form of native angiotensinogen, respectively. Furthermore, the rate of secretion of the shorter form was lower than that of the longer form. By transplanting angiotensinogen signal peptide onto green fluorescence protein, however, we found that both forms of the signal peptide could target green fluorescence protein, normally localized in the cytoplasm, to the secretory pathway. Although the R−30P mutation may not affect intracellular sorting of angiotensinogen in a qualitative manner, it leads to a quantitative reduction in the net secretion of mature angiotensinogen through decreased translocation or increased residence time in the endoplasmic reticulum.