Role of the circulating renin-angiotensin system in the pathogenesis of hypertension in transgenic rats. TGR(mREN2)27

Effects of Renal Denervation on the Enhanced Renal Vascular Responsiveness to Angiotensin II in High-Output Heart Failure: Angiotensin II Receptor Binding Assessment and Functional Studies in Ren-2 Transgenic Hypertensive Rats

Detailed mechanism(s) of the beneficial effects of renal denervation (RDN) on the course of heart failure (HF) remain unclear. The study aimed to evaluate renal vascular responsiveness to angiotensin II (ANG II) and to characterize ANG II type 1 (AT₁) and type 2 (AT₂) receptors in the kidney of Ren-2 transgenic rats (TGR), a model of ANG II-dependent hypertension. HF was induced by volume overload using aorto-caval fistula (ACF). The studies were performed two weeks after RDN (three weeks after the creation of ACF), i.e., when non-denervated ACF TGR enter the decompensation phase of HF whereas those after RDN are still in the compensation phase. We found that ACF TGR showed lower renal blood flow (RBF) and its exaggerated response to intrarenal ANG II (8 ng); RDN further augmented this responsiveness. We found that all ANG II receptors in the kidney cortex were of the AT₁ subtype. ANG II receptor binding characteristics in the renal cortex did not significantly differ between experimental groups, hence AT₁ alterations are not responsible for renal vascular hyperresponsiveness to ANG II in ACF TGR, denervated or not. In conclusion, maintained renal AT₁ receptor binding combined with elevated ANG II levels and renal vascular hyperresponsiveness to ANG II in ACF TGR influence renal hemodynamics and tubular reabsorption and lead to renal dysfunction in the high-output HF model. Since RDN did not attenuate the RBF decrease and enhanced renal vascular responsiveness to ANG II, the beneficial actions of RDN on HF-related mortality are probably not dominantly mediated by renal mechanism(s).

Intrapulmonary Activation of the Angiotensin-Converting Enzyme Type 2/Angiotensin 1-7/G-Protein-Coupled Mas Receptor Axis Attenuates Pulmonary Hypertension in Ren-2 Transgenic Rats Exposed to Chronic Hypoxia

The present study was performed to evaluate the role of intrapulmonary activity of the two axes of the renin-angiotensin system (RAS): vasoconstrictor angiotensin-converting enzyme (ACE)/angiotensin II (ANG II)/ANG II type 1 receptor (AT1) axis, and vasodilator ACE type 2 (ACE2)/angiotensin 1-7 (ANG 1-7)/Mas receptor axis, in the development of hypoxic pulmonary hypertension in Ren-2 transgenic rats (TGR). Transgene-negative Hannover Sprague-Dawley (HanSD) rats served as controls. Both TGR and HanSD rats responded to two weeks´ exposure to hypoxia with a significant increase in mean pulmonary arterial pressure (MPAP), however, the increase was much less pronounced in the former. The attenuation of hypoxic pulmonary hypertension in TGR as compared to HanSD rats was associated with inhibition of ACE gene expression and activity, inhibition of AT1 receptor gene expression and suppression of ANG II levels in lung tissue. Simultaneously, there was an increase in lung ACE2 gene expression and activity and, in particular, ANG 1-7 concentrations and Mas receptor gene expression. We propose that a combination of suppression of ACE/ANG II/AT1 receptor axis and activation of ACE2/ANG 1-7/Mas receptor axis of the RAS in the lung tissue is the main mechanism explaining attenuation of hypoxic pulmonary hypertension in TGR as compared with HanSD rats.

Association between renin-angiotensin-aldosterone system-related genes and blood pressure in a Korean population

AIMS. Blood pressure control is influenced by various genetic and environmental factors, and genetic susceptibility is important in the development of essential hypertension. Because the renin-angiotensin-aldosterone system (RAAS) has a key role in vasoconstriction, vasodilation, and sodium and electrolyte balance, it is central in blood pressure control and so is an appropriate target in hypertension treatments. The present study assessed the association of RAAS-related genes with blood pressure and hypertension in a Korean population. Single nucleotide polymorphisms (SNPs, n = 114) in nine RAAS-related genes (AGT, REN, ACE, ACE2, AGTR1, CYP11B2, NR3C2, MAS1, and CMA1) were assessed for their correlation with blood pressure and hypertension using genotype data of 8842 individuals from the Korea Association Resource subject pool. MAJOR FINDINGS. Linear regression analysis revealed a statistically significant association with blood pressure of 10 SNPs in six genes (ACE, ACE2, CYP11B2, NR3C2, MAS1, and CMA1). An additional hypertension case-control study identified 10 SNPs in NR3C2 and ACE that were linked to hypertension. PRINCIPAL CONCLUSION. Three SNPs (rs11737660, rs6810951, and rs10519963) in NR3C2 correlate with both blood pressure and hypertension. Genetic polymorphisms in RAAS-related genes appear to be associated with hypertension in a Korean population.

Lack of cardiac fibrosis in a new model of high prorenin hyperaldosteronism

The aim of the present study was to test the hypothesis that elevation of prorenin in plasma is sufficient to induce cardiac fibrosis. Normotensive cyp1a1ren-2 transgenic rats with normal plasma prorenin and aldosterone levels were given 0.125% indole-3-carbinol (I3C) orally for a period of 12 wk. Plasma prorenin and aldosterone levels were determined in 4-wk intervals, and cardiac marker enzymes for hypertrophy, fibrosis, and oxidative stress as well as cardiac pathology were investigated. In I3C-treated cyp1a1 ren-2 transgenic rats, plasma prorenin concentrations were >100-fold elevated (> or = 7.1 + or - 2.6 microg ANG I.ml(-1).h(-1) vs. < or = 0.07 + or - 0.1; P < 0.001), whereas active renin levels were suppressed (0.09 + or - 0.02 vs. 0.2 + or - 0.1; P < 0.05). Aldosterone concentrations were elevated three- to fourfold for a period of >4 wk (574 + or - 51 vs. 160 + or - 68 pg/ml; P < 0.01). After 12 wk of I3C, rats exhibited moderate cardiac hypertrophy (heart weight/body weight 2.5 + or - 0.04 vs. 3.1 + or - 0.1 mg/g; P < 0.01). There was a slight increase in mRNA contents of endothelin 1 (1.21 + or - 0.08 vs. 0.75 + or - 0.007; P < 0.001), NADP oxidase-2 (1.03 + or - 0.006 vs. 0.76 + or - 0.04; P < 0.001), transforming growth factor-beta (0.99 + or - 0.06 vs. 0.84 + or - 0.04; P < 0.05), collagen type I (1.32 + or - 0.32 vs. 0.94 + or - 0.18; P < 0.05), and intercellular adhesion molecule-1 (1.12 + or - 0.12 vs. 0.84 + or - 0.08; P < 0.05). These genes are known to be stimulated by the renin-angiotensin system. There were no histological signs of fibrosis in the heart. We found that prorenin and aldosterone alone are not sufficient to induce considerable cardiac fibrosis in the absence of sodium load.

Secretory and cytosolic (pro)renin in kidney, heart, and adrenal gland

Renin is commonly known as a secretory glycoprotein, which is expressed, stored, and secreted in a regulated manner by the kidney. The rat kidney exclusively expresses secretory renin. In this organ, renin regulates glomerular filtration rate, vascular resistance, and sodium reabsorbtion. In the adult rat heart, secretory preprorenin is not expressed. Instead, an alternative renin transcript is expressed that encodes for a previously unrecognized cytosolic renin. The expression of cytosolic but not of secretory renin increases markedly after myocardial infarction, indicating a role specifically for cytosolic renin in postischemic repair processes. In the adrenal gland, secretory renin is expressed and provides the basis for an intra-adrenal angiotensin (ANG) II amplification system. This amplification system reduces the demand for circulating ANGII to stimulate aldosterone production and thus minimizes any detrimental effects of circulating ANGII in other tissues. The adrenal gland additionally expresses cytosolic renin, which is targeted to mitochondria. Adrenal cytosolic renin increases aldosterone production plasma renin independently.

Effects of Dietary Salt Load and Salt Depletion on the Course of Hypertension and Angiotensin II Levels in Male and Female Heterozygous Ren-2 Transgenic Rats

BACKGROUND

In the present study we evaluated plasma and kidney angiotensin II (ANG II) levels in female and male Ren-2 transgenic rats (TGR) in comparison to age-matched female and male normotensive Hannover Sprague-Dawley rats.

METHODS

The rats were maintained on a normal sodium (NS) diet (0.6% NaCl) or fed a high sodium (HS) diet (2% NaCl) for 4 days or were sodium depleted by administration of 40 mg furosemide per liter drinking water overnight followed by 3 days of low sodium diet (0.01% NaCl) (LS + F). ANG II levels were determined by radioimmunoassay.

RESULTS

Female TGR at the age of 38 days were already hypertensive and had developed cardiac hypertrophy, whereas male TGR at this age still exhibited a normotensive phenotype. HS diet increased the blood pressure (BP) but did not alter the ANG II levels in TGR at any age. LS + F decreased the BP without significant change in ANG II concentrations in TGR. Female TGR responded to salt loading and salt depletion by more pronounced changes in BP than male TGR.

CONCLUSIONS

Female TGR develop hypertension more rapidly and the salt-sensitive component of hypertension is more pronounced in female than in male TGR.

Angiotensin II formation in the kidney and nephrosclerosis in Ren-2 hypertensive rats

BACKGROUND

Ren-2 transgenic hypertensive rats develop malignant hypertensive nephrosclerosis despite low to normal plasma angiotensin II and suppressed renal renin. We tested the hypothesis that local angiotensin II formation occurs at sites of renal vascular and interstitial injury in this model.

METHODS

Heterozygous Ren-2 transgenic rats were compared with normotensive Sprague-Dawley-Hannover control rats and Ren-2 transgenic rats treated with a very low dose of an angiotensin II type 1 (AT1) receptor antagonist, 1 mg/kg/day losartan, for 4 weeks. Blood pressure measurements, quantifications of urinary albumin, plasma and tissue angiotensin II as well as immunohistochemical analyses were performed.

RESULTS

Systolic blood pressure was not affected by losartan during the study but intra-arterial recordings revealed a decrease of blood pressure. Losartan reduced albumin excretion, cell proliferation, macrophage influx, collagen I and collagen IV deposition. Plasma angiotensin II was decreased, while kidney tissue angiotensin II content was increased in Ren-2 transgenic rats compared with control rats. In Ren-2 transgenic rats, juxtaglomerular renin and angiotensin II staining were reduced, but there was a marked angiotensin II staining at foci of tubulo-interstitial fibrosis and at proliferative malignant vascular lesions.

CONCLUSION

We conclude that local angiotensin II formation is increased in proliferative or fibrotic kidney lesions in the Ren-2 transgenic rat. Local angiotensin II formation may help to explain why the AT1 receptor antagonist prevents or ameliorates this transgenic model of malignant nephrosclerosis despite low to normal plasma angiotensin II and suppressed renal renin.

Effects of changes in sodium balance on plasma and kidney angiotensin II levels in anesthetized and conscious Ren-2 transgenic rats

OBJECTIVE

Since there is as yet no general agreement regarding the role of plasma and kidney angiotensin II (ANG II) in the development of hypertension in Ren-2 transgenic rats (TGR), in the present study we evaluated plasma and kidney ANG II levels in anesthetized and conscious TGR and in normotensive Hannover-Sprague-Dawley rats (HanSD) fed a normal salt diet (NS). Given the importance of ANG II in the development of salt-sensitive hypertension, and the fact that hypertensinogenic actions of ANG II are mediated via ANG II type 1 (AT1) receptors, the effects of high salt (HS) intake and of sodium depletion on blood pressure (BP), ANG II levels and kidney AT1 receptor protein expression in TGR and HanSD were also examined.

METHODS

Rats were maintained on a NS diet (0.6% NaCl) or fed a HS diet (2% NaCl) for 4 days or were sodium depleted (40 mg/l furosemide for 1 day followed by 3 days of 0.01% NaCl diet). They were sacrificed either by an overdose of anesthetic (thiopental sodium) or by decapitation (without anesthetic) and plasma and kidney ANG II levels were determined by radioimmunoassay during the prehypertensive (32 days old), the early (52 days) and the maintenance (90 days) phases of hypertension. Total kidney AT1 receptor protein levels were assessed by Western blot analysis.

RESULTS

In anesthetized animals fed the NS diet, plasma ANG II levels were lower in 32-day-old TGR than in HanSD, but at 52 and 90 days of age no significant differences were noted. ANG II concentrations in kidney tissue were similar in 32- and 90-day-old TGR and HanSD, but were higher in 52-day-old TGR than in HanSD. In contrast, in conscious animals immediately after decapitation, plasma and kidney ANG II levels were higher in TGR than in HanSD at all ages. HS diet did not change BP but suppressed ANG II levels in HanSD at all ages. In contrast, HS diet increased BP but did not decrease plasma and kidney ANG II levels in TGR at all ages. Sodium restriction did not alter BP and resulted in a marked increase in ANG II levels in HanSD, but caused a significant decrease in BP in TGR without altering plasma or tissue ANG II concentrations. There were no significant differences in renal AT1 receptor protein expression between HanSD and TGR at any age of any of the experimental groups.

CONCLUSIONS

On the basis of our present results we conclude that TGR exhibit a disrupted interaction between sodium homeostasis and the regulation of the renin-angiotensin system (RAS) activity which results in the loss of BP regulation in this model.

The role of intrarenal angiotensin II in the development of hypertension in Ren-2 transgenic rats

OBJECTIVE

We investigated the responses of mean arterial pressure and renal blood flow to intravenous and intrarenal angiotensin II, plasma and kidney angiotensin II concentrations and renal angiotensin receptor subtype 1 protein expression, and renal functional responses to intravenous and intrarenal angiotensin receptor 1 blockade with candesartan.

METHODS

In male anaesthetized transgenic rats and Hannover Sprague-Dawley rats aged 36-38 days mean arterial pressure and renal blood flow were determined after intravenous and intrarenal boluses of angiotensin II. Mean arterial pressure, renal blood flow and sodium excretion after intravenous or intrarenal candesartan were studied. Plasma and kidney angiotensin II concentrations were determined by radioimmunoassay. Renal angiotensin receptor subtype 1 protein levels were analysed by immunoblotting.

RESULTS

The responses of mean arterial pressure and renal blood flow to angiotensin II were significantly greater in transgenic than in Hannover Sprague-Dawley rats. The administration of candesartan resulted in comparable decreases in mean arterial pressure and increases in renal blood flow and sodium excretion in both groups of rats. Renal angiotensin receptor subtype 1 protein levels were no different between Hannover Sprague-Dawley and transgenic rats.

CONCLUSIONS

Plasma and kidney angiotensin II levels were lower in anaesthetized transgenic rats but, in contrast, were higher in decapitated transgenic rats when compared with Hannover Sprague-Dawley rats, suggesting that the kidney function of prehypertensive transgenic rats is under inappropriately high angiotensin II-dependent influence.

Hypertension in the (mRen-2)27 Rat Is Not Explained by Enhanced Kinetics of Transgenic Ren-2 Renin

Enhanced efficiency of the reaction between transgenic Ren-2 mouse renin and endogenous rat angiotensinogen has been suggested as 1 mechanism that contributes to the accelerated hypertension and increased tissue angiotensin of the (mRen-2)27 transgenic rat. This was tested in a study conducted at pH 7.4 in vitro that compared the kinetic constants of purified mouse Ren-2 and rat renin (each at 100, 75, 50, and 25 pmol/L) reacting with physiologic concentrations of rat angiotensinogen (0 to 4 μmol/L). Under these conditions, the kinetic constants for Ren-2 ( K m , 1.8 μmol/L; K cat , 0.07/s; and K cat / K m , 0.04 L · μmol −1 · s −1 ) were not different from rat renin. However, Ren-2 renin acting on its homologous mouse angiotensinogen was confirmed as being much slower. We conclude that hypertension in the Ren-2 rat is not related to renin kinetics. Other mechanisms are considered, with reference to human essential hypertension.

Functional significance of prorenin internalization in the rat heart

Intracardiac renin is considered to be involved in the pathogenesis of cardiac hypertrophy, fibrosis, and myocardial infarction. Cardiac renin is predominantly derived from the circulation, because preprorenin is not expressed locally and uptake of renin has been demonstrated. One mechanism of internalization recently described involves the mannose-6-phosphate receptor and requires glycosylation of renin. Based on previous observations, we considered the existence of another pathway of uptake, not requiring glycosylation and predominantly involving prorenin. This hypothesis and its functional consequences were investigated in vitro and in vivo. We demonstrate that isolated adult cardiomyocytes internalize unglycosylated prorenin, which is followed by the generation of angiotensins. We further show that transgenic rats, expressing the ren-2(d) renin gene in an inducible manner, exhibit markedly enhanced levels of unglycosylated renin within intracellular compartments in the heart as a consequence of the induction of hepatic transgene expression and the rise of circulating unglycosylated prorenin levels. Because in this model severe cardiac damage occurs as a consequence of the rise of circulating prorenin levels, internalization of prorenin into cardiac cells is likely to play a key role in this process.

Vascular hypertrophy and increased P70S6 kinase in mice lacking the angiotensin II AT(2) receptor

BACKGROUND

Angiotensin II activates 2 distinct G protein-coupled receptors, the AT(1) and AT(2) receptors. Most of the known cardiovascular effects of angiotensin II are mediated by the AT(1) receptor subtype. The aim of the present study was to test whether deletion of the AT(2) receptor gene in mice (AT(2)-KO mice) leads to long-term functional or structural alterations in the cardiovascular system.

METHODS AND RESULTS

In vivo pressure responses to angiotensin II or the alpha(1)-adrenergic receptor agonist phenylephrine were greatly enhanced in AT(2)-KO mice. Deletion of the angiotensin AT(2) receptor did not lead to a compensatory increase of the activity of the circulating renin-angiotensin system, and arterial blood pressure was identical in wild-type control mice (WT) and AT(2)-KO mice. Cardiac contractility as assessed by LV catheterization and by rapid MRI also did not differ between AT(2)-KO and WT mice. Isolated femoral arteries from AT(2)-KO mice, however, showed enhanced vasoconstriction to angiotensin II, norepinephrine, and K(+) depolarization compared with WT. Morphometric analysis of large and small femoral arteries revealed a significant hypertrophy of media smooth muscle cells. Phospho-P70S6 kinase levels were significantly increased in aortas from AT(2)-KO mice compared with WT mice. Treatment of mice with an ACE inhibitor for 8 weeks abolished the increased pressure responsiveness, vascular hypertrophy, and enhanced P70S6 kinase phosphorylation in AT(2)-KO mice.

CONCLUSIONS

These results indicate that vascular AT(2) receptors inhibit the activity and, hence, hypertrophic signaling by the P70S6 kinase in vivo and thus are important regulators of vascular structure and function.

Circadian rhythms in the renin-angiotensin system and adrenal steroids may contribute to the inverse blood pressure rhythm in hypertensive TGR(mREN-2)27 rats

The transgenic TGR(mREN-2)27 rat is not only characterized by fulminant hypertension, but also by a disturbance in circadian blood pressure regulation, resulting in inverse circadian blood pressure profiles. The reasons for these alterations are not very well understood at present. We therefore investigated the circadian rhythms in several hormones participating in blood pressure regulation. From TGR and Sprague-Dawley (SPRD) control rats synchronized to 12h light and 12h dark (LD 12:12) blood was collected at different circadian times (07, 11, 15, 19, 23, 03, and 07 again, 5 rats per strain and time). The activities of plasma renin and converting enzyme, as well as plasma concentrations of corticosterone and aldosterone, were determined by radioimmunoassay (RIA). SPRD rats showed significant circadian rhythms in all variables except plasma renin activity, with maxima occurring during the day. TGR rats showed significant circadian rhythmicity in plasma renin activity and corticosterone and daily variation in aldosterone; angiotensin-converting enzyme (ACE) activity did not reach statistical significance. In TGR rats, 24h means in plasma renin activity and aldosterone were approximately sevenfold and fourfold higher, respectively, than in SPRD rats. Peak concentrations in corticosterone around 15h were more than two times higher in TGR rats than in SPRD rats, whereas no differences were observed during the night. It is concluded that, in TGR rats, the overall increase in plasma renin activity and aldosterone may contribute to the elevated blood pressure. The comparatively high levels in corticosterone and plasma renin activity during daytime may be involved in the inverse circadian blood pressure profiles in the transgenic animals.

Local renin-angiotensin system is involved in K+-induced aldosterone secretion from human adrenocortical NCI-H295 cells

NCI-H295, a human adrenocarcinoma cell line, has been proposed as a model system to define the role of the renin-angiotensin system in the regulation of aldosterone production in humans. Because the precise cellular localization of the components of the renin-angiotensin system in human adrenal cortical cells remains unclear, we investigated their localization in this defined cell system. NCI-H295 cells expressed both angiotensinogen and renin as shown by reverse transcriptase polymerase chain reaction and immunohistochemistry. Human angiotensin-converting enzyme (ACE) was not detectable by immunocytochemistry, ACE binding, or reverse transcriptase polymerase chain reaction. However, 3.5 mmol/L K+ stimulated the formation of both angiotensin I and angiotensin II 1. 9- and 2.5-fold, respectively, and increased aldosterone release 3. 0-fold. The K+-induced stimulation of aldosterone release was decreased by captopril and enalaprilat (24% and 26%, respectively) and by the angiotensin type 1 (AT1)-receptor antagonist losartan (28%). Angiotensin II-induced stimulation of aldosterone release was abolished by losartan treatment. Specific [125I]Sar1-angiotensin II binding was detected by receptor autoradiography. The binding of [125I]Sar1-angiotensin II was completely displaced by the AT1 antagonist losartan but not by the AT2 receptor ligand PD 123319, confirming the expression of angiotensin II AT1 receptors in NCI-H295 cells. Our results demonstrate that NCI-H295 cells express most of the components of the renin-angiotensin system. Our failure to detect ACE, however, suggests that the production of angiotensin II in NCI-H295 cells may be ACE independent. NCI-H295 cells are able to produce angiotensin II, and K+ increases aldosterone secretion in part through an angiotensin-mediated pathway. The production of angiotensin II in NCI-H295 cells demonstrates that this human cell line can be useful to characterize the role of locally produced angiotensin II in the regulation of aldosterone release.

Angiotensin II AT1-receptor blockade inhibits monocyte activation and adherence in transgenic (mRen2)27 rats

This study investigated whether angiotensin II AT1-receptor blockade with losartan inhibits endothelium-monocyte interactions originating from long-term activation of the renin-angiotensin system in hypertensive transgenic rats [TGR(mRen2)27]. The number of circulating activated monocytes, monocytes adhered to thoracic aorta endothelium, and the extent of endothelial cell injury were compared in adult male transgenic (mRen2)27 and age-matched Hannover Sprague-Dawley (SD) rats after 12 days of continuous subcutaneous administration of saline (120 microl/24 h), losartan (10 mg/kg/24 h), or the vasodilator hydralazine (3 mg/kg/24 h). At the doses administered in this experiment, both losartan and hydralazine normalized mRen2 rat blood pressures equal to values in similarly treated SD rats. Compared with saline infusion, administration of either antihypertensive in mRen2 rats reduced (p<0.05) endothelial cell injury, but only losartan significantly (p<0.05) decreased the number of activated circulating and endothelium-adherent monocytes. Infusion of antihypertensives in SD rats had no effect on blood pressures, monocyte activity, or endothelial injury compared with saline administration. These findings suggest that the recruitment and infiltration of leukocytes into the subendothelium associated with renin-angiotensin system-induced hypertension is partly mediated by pressure-independent AT1-receptor pathways.

Losartan and angiotensin II inhibit aldosterone production in anephric rats via different actions on the intraadrenal renin-angiotensin system

Angiotensin II (ANG II) is a major stimulator of aldosterone biosynthesis. When investigating the relative contribution of circulating and locally produced ANG II, we were therefore surprised to find that ANG II, given chronically s.c. (200 ng/kg x min), markedly inhibits a nephrectomy (NX)-induced rise of aldosterone concentrations (from 10 +/- 2 to 465 +/- 90 ng/100 ml in vehicle infused, and from 9 +/- 2 to 177 +/- 35 in ANG II infused rats 55 h after NX and hemodialysis). We further observed, by in situ hybridization, that bilateral NX increases the number of adrenocortical cells expressing renin and that this rise was prevented by ANG II. Moreover, the rise of aldosterone levels was also inhibited by the AT1-receptor antagonist, losartan (10 microg/kg x min, chronically i.p. from 8 +/- 2 to 199 +/- 26 ng/100 ml), despite the absence of circulating renin and a reduction of ANG I to less than 10%. These data demonstrate that aldosterone production, after NX, is regulated by an intraadrenal renin-angiotensin system and that this system is physiologically suppressed by circulating angiotensin. Because the effects of losartan or ANG II on aldosterone production involved a latency period of at least 30 h after NX and were associated with a modulation or recruitment of renin-producing cells, we suggest that the intraadrenal renin-angiotensin system operates via regulation of cell differentiation on a long-term scale, rather than or additionally to its short-term effects on aldosterone synthase activity.

Specific regulation of StAR expression in the rat adrenal zona glomerulosa. An in situ hybridization study

Steroid acute regulatory protein (StAR) plays an essential role in steroidogenesis because it is responsible for the transfer of cholesterol from cellular stores to the inner mitochondrial membrane. We investigated the distribution and regulation of StAR expression in association with aldosterone production in the rat adrenal gland in vivo. Using nonradioactive in situ hybridization, we demonstrate that the outermost five to seven parenchymal cell layers express the StAR gene only weakly and inhomogeneously. The strongest expression is found in the zona fasciculata and zona reticularis. In addition, some cells in the adrenal medulla also stained positively. To differentiate between functionally active glomerulosa and inactive intermediate cells, we compared the expression pattern of StAR with that of aldosterone synthase. The expression of the latter is localized to two or three cell layers only, located immediately below the capsule. However, the cells of the intermedia are capable of expressing both genes prominently, as shown after stimulation with bilateral nephrectomy for 2 days. All zones of the adrenal cortex by then expressed StAR gene to the same extent. This was accompanied by a 50-fold elevated plasma aldosterone concentration. Our data demonstrate that the width of the aldosterone-producing zone can increase within a short period of time by recruiting hormonally inactive cells to steroidogenesis.

Mineralocorticoid receptor knockout mice: pathophysiology of Na+ metabolism

Mineralocorticoid receptor (MR)-deficient mice were generated by gene targeting. These animals had a normal prenatal development. During the first week of life, MR-deficient (-/-) mice developed symptoms of pseudohypoaldosteronism. They finally lost weight and eventually died at around day 10 after birth from dehydration by renal sodium and water loss. At day 8, -/- mice showed hyperkalemia, hyponatremia, and a strong increase in renin, angiotensin II, and aldosterone plasma concentrations. Methods were established to measure renal clearance and colonic transepithelial Na+ reabsorption in 8-day-old mice in vivo. The fractional renal Na+ excretion was elevated >8-fold. The glomerular filtration rate in -/- mice was not different from controls. The effect of amiloride on renal Na+ excretion and colonic transepithelial voltage reflects the function of amiloide-sensitive epithelial Na+ channels (ENaC). In -/- mice, it was reduced to 24% in the kidney and to 16% in the colon. There was, however, still significant residual ENaC-mediated Na+ reabsorption in both epithelia. RNase protection analysis of the subunits of ENaC and (Na++ K+)-ATPase did not reveal a decrease in -/- mice. The present data indicate that MR-deficient neonates die because they are not able to compensate renal Na+ loss. Regulation of Na+ reabsorption via MR is not achieved by transcriptional control of ENaC and (Na+ + K+)-ATPase in RNA abundance but by transcriptional control of other as yet unidentified genes. MR knockout mice will be a suitable tool for the search of these genes.

Additive effects of enalapril and losartan in (mREN-2)27 transgenic rats

Blockade of angiotensin II AT1 receptors combined with angiotensin I-converting enzyme inhibition might amplify the potency of the renin-angiotensin system blockade. We studied whether chronic and simultaneous administration of enalapril and losartan would result in additive or synergistic effects in the (mREN-2)27 transgenic rat (TGR), the investigated targets being blood pressure, cardiac hypertrophy, renin-angiotensin system blockade achieved, and plasma active renin concentration. In addition, the origin (renal or extrarenal, rat or mouse) of the induced renin release was determined. Adult TGRs were treated orally and daily for 5 to 7 weeks with 1 mg/kg (E1) or 3 mg/kg (E3) enalapril or 1 mg/kg (L1) or 3 mg/kg (L3) losartan, or their combinations (E1L1 and E3L3). At the end of the treatment period, enalapril and losartan exerted dose-dependent and, when combined, additive effects in terms of blood pressure fall and cardiac hypertrophy limitation, and synergistic effects in terms of plasma active renin stimulation and blockade of exogenous angiotensin I pressor effects, with E3L3>E3>L3, E1L1>E1> or =L1, and E1L1=E3>L3). This indicates that in the TGR, (1) the greater the renin-angiotensin system blockade achieved, the greater are the reduction in blood pressure, the limitation of cardiac hypertrophy, and the reactive rise in plasma renin concentration elicited, and (2) the enalapril-losartan combinations are more potent at achieving these goals than any of their constituents individually. In contrast, there was no interaction between the two drugs regarding aldosteronuria reduction. Measurement of plasma renin concentration and renal renin at pH 6.5 and 8.5, ie, the optimal pH values for rat and mouse renin activities, respectively, indicates that in TGRs the counterregulatory process for renin release elicited by enalapril, losartan, or their combination involves primarily rat renin of renal origin, a finding supported further by the observed increase in the rat renal renin hybridization index.