Renin–Angiotensin–Aldosterone System and the Renal Regulation of Sodium, Potassium, and Blood Pressure Homeostasis

Postprandial sodium sensing by enteric neurons in Drosophila

Sodium is essential for all living organisms1. Animals including insects and mammals detect sodium primarily through peripheral taste cells2-7. It is not known, however, whether animals can detect this essential micronutrient independently of the taste system. Here, we report that Drosophila Ir76b mutants that were unable to detect sodium2 became capable of responding to sodium following a period of salt deprivation. From a screen for cells required for the deprivation-induced sodium preference, we identified a population of anterior enteric neurons, which we named internal sodium-sensing (INSO) neurons, that are essential for directing a behavioural preference for sodium. Enteric INSO neurons innervate the gut epithelia mainly through their dendritic processes and send their axonal projections along the oesophagus to the brain and to the crop duct. Through calcium imaging and CaLexA experiments, we found that INSO neurons respond immediately and specifically to sodium ions. Notably, the sodium-evoked responses were observed only after a period of sodium deprivation. Taken together, we have identified a taste-independent sodium sensor that is essential for the maintenance of sodium homeostasis.

Expression of Components of the Renin-Angiotensin System in Pyogenic Granuloma

Background: There is a growing body of research demonstrating expression of the renin-angiotensin system (RAS) by a putative embryonic stem cell (ESC)-like population within vascular anomalies. This study investigated the expression of components of the RAS in relation to the putative ESC-like population within pyogenic granuloma (PG) that we have recently reported.

Methods: PG samples from 14 patients were analyzed for the expression of components of the RAS: pro-renin receptor (PRR), angiotensin converting enzyme (ACE), angiotensin II receptor 1 (ATIIR1) and angiotensin II receptor 2 (ATIIR2), using 3,3-diaminobenzidine (DAB) immunohistochemical (IHC) staining. Immunofluorescence (IF) IHC staining was performed to localize these proteins on four of the PG samples. RT-qPCR was performed on two snap-frozen PG samples. Western blotting (WB) was performed on one snap-frozen PG sample and two PG-derived primary cell lines.

Results: DAB IHC staining demonstrated the expression of ACE, PRR, ATIIR1, and ATIIR2 in all 14 PG tissue samples. RT-qPCR analysis confirmed abundant mRNA transcripts for PRR, ACE, AIITR1 and ATIIR2, relative to the housekeeping gene. WB confirmed the presence of PRR, ATIIR1, and ACE in the PG tissue sample, and PRR and ATIIR1, in the PG-derived primary cell lines. IF IHC staining demonstrated the expression of PRR, ACE, ATIIR1 on the primitive population that expressed NANOG and SOX2 on the ERG⁺ endothelium of the microvessels within PG.

Conclusion: We have demonstrated the expression of PRR, ACE, and ATIIR1 by the putative the ESC-like population within PG.

A subpressor dose of angiotensin II elevates blood pressure in a normotensive rat model by oxidative stress

Oxidative stress is an imbalance between free radicals and antioxidants, and is an important etiological factor in the development of hypertension. Recent experimental evidence suggests that subpressor doses of angiotensin II elevate oxidative stress and blood pressure. We aimed to investigate the oxidative stress related mechanism by which a subpressor dose of angiotensin II induces hypertension in a normotensive rat model. Normotensive male Wistar rats were infused with a subpressor dose of angiotensin II for 28 days. The control group was sham operated and infused with saline only. Plasma angiotensin II and H2O2 levels, whole-blood glutathione peroxidase, and AT-1a, Cu/Zn SOD, and p22phox mRNA expression in the aorta was assessed. Systolic and diastolic blood pressures were elevated in the experimental group. There was no change in angiotensin II levels, but a significant increase in AT-1a mRNA expression was found in the experimental group. mRNA expression of p22phox was increased significantly and Cu/Zn SOD decreased significantly in the experimental group. There was no significant change to the H2O2 and GPx levels. Angiotensin II manipulates the free radical-antioxidant balance in the vasculature by selectively increasing O2(-) production and decreasing SOD activity and causes an oxidative stress induced elevation in blood pressure in the Wistar rat.

Pathophysiology of hypertensive renal damage: implications for therapy

Unlike the majority of patients with uncomplicated hypertension in whom minimal renal damage develops in the absence of severe blood pressure (BP) elevations, patients with diabetic and nondiabetic chronic kidney disease (CKD) exhibit an increased vulnerability to even moderate BP elevations. Investigations in experimental animal models have revealed that this enhanced susceptibility is a consequence of an impairment of the renal autoregulatory mechanisms that normally attenuate the transmission of elevated systemic pressures to the glomeruli in uncomplicated hypertension. The markedly lower BP threshold for renal damage and the steeper slope of relationship between BP and renal damage in such states necessitates that BP be lowered into the normotensive range to prevent progressive renal damage. When BP is accurately measured using radiotelemetry in animal models, the renal protection provided by renin-angiotensin system (RAS) blockade is proportional to the BP reduction with little evidence of BP-independent protection. A critical evaluation of the clinical data also suggests that the BP-independent renoprotection by RAS blockade has been overemphasized and that achieving lower BP targets is more important than the selection of antihypertensive regimens. However, achievement of such BP goals is difficult in CKD patients without aggressive diuresis, because of their proclivity for salt retention. The effectiveness of RAS blockers in lowering BP in patients who have been adequately treated with diuretics, along with their potassium-sparing and magnesium-sparing effects, provides a more compelling rationale for the use of RAS blockade in the treatment of CKD patients than any putative BP-independent renoprotective superiority.

Renoprotection by ACE inhibition or aldosterone blockade is blood pressure-dependent

Renin-angiotensin-aldosterone system blockade has been shown to protect against renal damage in salt-supplemented, stroke-prone spontaneously hypertensive rats (SHRsp). Based on intermittent tail-cuff blood pressure (BP) measurements, it has been claimed that such protection is BP-independent and mediated by a blockade of the direct tissue-damaging effects of angiotensin and/or aldosterone. BP radiotelemetry was performed for 8 weeks in approximately 10-week-old male SHRsp who received a standard diet and either tap water (n=10) or 1% NaCl to drink. Saline-drinking SHRsp were either left untreated (n=12), received enalapril (50 mg/L) in drinking fluid (n=9), or had subcutaneous implantation of time-release 200-mg pellets of aldactone (n=10). The average systolic BP (mean+/-SEM) during the final 3 weeks was significantly higher (P<0.05) in untreated saline-drinking (215+/-6 mm Hg) SHRsp but not aldactone-treated (198+/-4 mm Hg) or enalapril-treated treated SHRsp (173+/-1 mm Hg), as compared with tap water-drinking SHRsp (197+/-3 mm Hg). Histological renal damage scores at 8 weeks paralleled the BP in all groups, with an excellent correlation (r=0.8, P<0.001, n=41). Moreover, a renal damage score of >5 was only observed in SHRsp whose average systolic BP during the final 3 weeks exceeded 200 mm Hg, indicating a threshold relation with BP. These data show that protection by renin-angiotensin-aldosterone system blockade in this model is BP-dependent and mediated by preventing the severe increases in BP seen in untreated salt-supplemented SHRsp and further underscore the limitations of interpretations based on conventional tail-cuff BP measurements.

Enhanced natriuretic response to neutral endopeptidase inhibition in heart-transplant recipients

Heart-transplant recipients (Htx) generally present with body fluid and sodium handling abnormalities and hypertension. To investigate whether neutral endopeptidase inhibition (NEP-I) increases endogenous atrial natriuretic peptide (ANP) and enhances natriuresis and diuresis after heart transplantation, ecadotril was given orally to 8 control subjects and 8 matched Htx, and levels of volume-regulating hormones and renal water, electrolyte, and cyclic guanosine monophosphate (cGMP) excretions were monitored for 210 minutes. Baseline plasma ANP, brain natriuretic peptide (BNP), and cGMP were elevated in Htx, but renin and aldosterone, like urinary parameters, did not differ between groups. NEP-I increased plasma ANP (Htx, 20.6+/-2.3 to 33.2+/-5.9 pmol/L, P<0.01; controls, 7.7+/-1. 2 to 10.6+/-2.6 pmol/L) and cGMP, but not BNP. Renin decreased similarly in both groups, whereas aldosterone decreased significantly only in Htx. Enhanced urinary sodium (1650+/-370% versus 450+/-150%, P=0.01), cGMP, and water excretions were observed in Htx and urinary cGMP positively correlated with natriuresis in 6 of the Htx subjects. Consistent with a normal circadian rhythm of blood pressure, without excluding a possible effect of NEP-I, mean systemic blood pressure increased similarly in both groups at the end of the study (6.9+/-2.0% versus 7.4+/-2.8% in controls and Htx). Thus, systemic hypertension, mild renal impairment, and raised plasma ANP levels are possible contributory factors in the enhanced natriuresis and diuresis with NEP-I in Htx. These results support a physiological role for the cardiac hormone after heart transplantation and suggest that long-term studies may be useful to determine the potential of NEP-I in the treatment of sodium retention and water retention after heart transplantation.

Renin is not synthesized by cardiac and extrarenal vascular tissues. A review of experimental evidence

A comprehensive review of physiological and molecular biological evidence refutes claims for synthesis of renin by cardiac and vascular tissues. Cardiovascular tissue renin completely disappears after binephrectomy. Residual putative reninlike activity, where investigated, has had the characteristics of lysosomal acid proteases. Occasional reports of renin or renin mRNA in vascular and cardiac tissues can be ascribed to failure to remove the kidneys 24 hours beforehand, overloading of detection systems, problems with stringency in identification, and illegitimate transcripts after more than 25 cycles of polymerase chain reaction. Others, using more stringent criteria, have failed to detect cardiac and vascular renin mRNA. Accordingly, a growing number of investigators have concluded that the kidneys are the only source of cardiovascular tissue renin. Although prorenin is secreted from extrarenal tissues as well as from the kidneys, there is no evidence that it is ever converted to renin in the circulation. The kidney is the only tissue with known capacity to convert prorenin to renin and to secrete active renin into the circulation. Accordingly, renin of renal origin determines plasma and hence, extracellular fluid renin levels. In these loci, angiotensin (Ang) I, formed by renin cleavage of circulating and interstitial fluid angiotensinogen, is in turn cleaved by angiotensin converting enzyme, located in plasma and extracellular fluids and on the luminal surface of pulmonary and systemic vascular endothelial cells, to Ang II, which perfuses and bathes the heart and vasculature. Consistent with this model, plasma renin and angiotensin and the antihypertensive action of renin inhibitors, converting enzyme inhibitor, or Ang II antagonists all disappear after binephrectomy. Thus, the plasma renin level, via Ang II formation, determines renin system vasoconstrictor activity, the antihypertensive potential of anti-renin system drugs, and the risk of heart attack in hypertensive patients. This analysis redirects renin research to renal mechanisms that create the plasma renin level, to renal prorenin biosynthesis and its processing to renin, and to their regulated secretion, extracellular distribution, and possible binding to by target tissues. In this context, it is still possible that changes in circulating and interstitial renin substrate or available converting enzyme might exert subtle modulating influences on Ang II formation. However, this analysis redefines the importance of plasma renin measurements to assess clinical situations, because plasma renin is the only known initiator driving the cardiovascular renin-angiotensin system, and its strength can be measured.