Coordinate regulation of canine glomeruli and adrenal angiotensin receptors by dietary sodium manipulation

Sex-Specific Modulation of Blood Pressure and the Renin-Angiotensin System by ACE (Angiotensin-Converting Enzyme) 2

We showed ACE (angiotensin-converting enzyme) 2 is higher in the kidney of male compared with female mice. To further investigate this sex difference, we examined the role of ACE2 in Ang-[1-8] (angiotensin [1-8])-induced hypertension and regulation of the renin-angiotensin system in the kidney of WT (wild type) and Ace2 KO (knockout) mice. Mean arterial pressure rose faster in WT male than WT female mice after Ang-[1-8] infusion. This sex difference was attenuated in ACE2 KO mice. Ang-[1-8] infusion reduced glomerular AT1R (angiotensin type 1 receptor) binding in WT female mice by 30%, and deletion of Ace2 abolished this effect. In contrast, Ang-[1-8] infusion increased glomerular AT1R binding in WT male mice by 1.2-fold, and this effect of Ang-[1-8] persisted in Ace2 KO male mice (1.3-fold). ACE2 also had an effect on renal protein expression of the neutral endopeptidase NEP (neprilysin), the enzyme that catabolizes Ang-[1-10] (angiotensin [1-10]), the precursor of Ang-[1-8]. Ang-[1-8] infusion downregulated NEP protein expression by 20% in WT male, whereas there was a slight increase in NEP expression in WT female mice. Deletion of Ace2 resulted in lowered NEP expression after Ang-[1-8] infusion in both sexes. These findings suggest sex-specific ACE2 regulation of the renin-angiotensin system contributes to female protection from Ang-[1-8]-induced hypertension. These findings have ramifications for the current coronavirus disease 2019 (COVID-19) pandemic, especially in hypertension since ACE2 is the SARS-CoV-2 receptor and hypertension is a major risk factor for poor outcomes.

Understanding the Two Faces of Low-Salt Intake

Fierce debate has developed whether low-sodium intake, like high-sodium intake, could be associated with adverse outcome. The debate originates in earlier epidemiological studies associating high-sodium intake with high blood pressure and more recent studies demonstrating a higher cardiovascular event rate with both low- and high-sodium intake. This brings into question whether we entirely understand the consequences of high- and (very) low-sodium intake for the systemic hemodynamics, the kidney function, the vascular wall, the immune system, and the brain. Evolutionarily, sodium retention mechanisms in the context of low dietary sodium provided a survival advantage and are highly conserved, exemplified by the renin-angiotensin system. What is the potential for this sodium-retaining mechanism to cause harm? In this paper, we will consider current views on how a sodium load is handled, visiting aspects including the effect of sodium on the vessel wall, the sympathetic nervous system, the brain renin-angiotensin system, the skin as "third compartment" coupling to vascular endothelial growth factor C, and the kidneys. From these perspectives, several mechanisms can be envisioned whereby a low-sodium diet could potentially cause harm, including the renin-angiotensin system and the sympathetic nervous system. Altogether, the uncertainties preclude a unifying model or practical clinical guidance regarding the effects of a low-sodium diet for an individual. There is a very strong need for fundamental and translational studies to enhance the understanding of the potential adverse consequences of low-salt intake as an initial step to facilitate better clinical guidance.

Loss of Resistance to Angiotensin II-Induced Hypertension in the Jackson Laboratory Recombination-Activating Gene Null Mouse on the C57BL/6J Background

Resistance to angiotensin II (Ang II)-induced hypertension in T-cell-deficient male mice with a targeted mutation in the recombination-activating gene-1 (Rag1) on the C57BL/6J background (B6.Rag1^-/- -M), which was reported by 5 independent laboratories including ours before 2015, has been lost. In mice purchased from Jackson Laboratory in 2015 and 2016, the time course and magnitude increase in mean arterial pressure induced by 2 weeks of Ang II infusion at 490 ng/kg per minute was identical between B6.Rag1^-/- -M and male wild-type littermates. Moreover, there were no differences in the time course or magnitude increase in mean arterial pressure at the lowest dose of Ang II (200 ng/kg per minute) that increased mean arterial pressure. This loss in Ang II resistance is independent of T cells. Angiotensin type 1-receptor binding was 1.4-fold higher in glomeruli isolated from recently purchased B6.Rag1^-/- -M suggesting an increase in renal angiotensin type 1-receptor activity masks the blood pressure protection afforded by the lack of T cells. The phenotypic change in B6.Rag1^-/- -M has implications for investigators using this strain to study mechanisms of T-cell modulation of Ang II-dependent blood pressure control. These findings also serve as a reminder that the universal drive for genetic variation occurs in all animals including inbred mouse strains and that spontaneous mutations leading to phenotypic change can compromise experimental reproducibility over time and place. Finally, these observations illustrate the importance of including experimental details about the location and time period over which animals are bred in publications involving animal studies to promote rigor and reproducibility in the scientific literature.

Effect of dietary sodium on estrogen regulation of blood pressure in Dahl salt-sensitive rats

The effects of high-sodium (HS) and normal-sodium (NS) diets on ovarian hormone modulation of mean arterial pressure (MAP) were examined in Dahl salt-resistant (DR) and salt-sensitive (DS) rats. Ovariectomy increased MAP (OVX-Sham) to a greater extent in DS rats maintained for 2 wk on a HS (22 mmHg) compared with a NS (6 mmHg) diet. Ovariectomy had no effect on MAP in DR rats on NS but did increase MAP in rats on HS (10 mmHg) diets. On HS diets, glomerular filtration rate (GFR) was 36% less in the DS-Sham than DR-Sham animals; ovariectomy increased GFR in both strains by 1.4-1.5-fold; glomerular angiotensin II type 1 receptor (AT(1)R) densities were 1.6-fold higher in the DS-Sham than in the DR-Sham group; ovariectomy increased glomerular AT(1)R densities by 1.3-fold in DR rats but had no effect in DS rats; 17beta-estradiol (E(2)) downregulated adrenal AT(1)R densities in both strains on either diet; ovariectomy reduced estrogen receptor-alpha (ER-alpha) protein expression in the renal cortex by 40-50% although renal ER-alpha expression was 34% lower in DS than in DR rats. These observed effects of gonadectomy were prevented by E(2) treatment, suggesting that E(2) deficiency mediates the effects of ovariectomy on MAP, GFR, AT(1)R densities, and renal ER-alpha protein expression. In conclusion, ovariectomy-induced increases in MAP are augmented by HS diet in both strains, and this effect is not mediated by a reduction in GFR. Aberrant renal AT(1)R regulation and reduced renal ER-alpha expression are potential contributors to the hypertensive effects of E(2) deficiency in DS rats. These findings have implications for women with salt-sensitive hypertension and women who are E(2) deficient, such as postmenopausal women.

17β-Estradiol deficiency reduces potassium excretion in an angiotensin type 1 receptor-dependent manner

This study examined the effects of ovariectomy (OVX) and 17β-estradiol (E2) replacement (OVX + E2) on renal function in Sprague-Dawley rats. OVX caused a 40% decrease in the fractional excretion of potassium (FEK+) that was prevented by E2 replacement [Sham, 24.2 ± 2.9%; OVX, 14.5 ± 2.1% ( P < 0.05 vs. OVX + E2); and OVX + E2, 26.2 ± 2.7%; n = 7–11] and that corresponded to significant increases in plasma potassium [(in mmol/l): Sham, 3.15 ± 0.087; OVX, 3.42 ± 0.048 ( P < 0.05 vs. OVX + E2); and OVX + E2, 3.19 ± 0.11; n = 7–11]. No effects of OVX were detected on plasma levels of sodium and aldosterone. Angiotensin II type 1 receptor (AT1R) densities in ovariectomized rats were 1.4-fold and 1.3-fold higher in glomerular [maximum binding capacity (Bmax; in fmol/mg protein): Sham, 482 ± 21; OVX, 666 ± 20 ( P < 0.05 vs. OVX + E2); and OVX + E2, 504 ± 26; n = 7–11] and proximal tubular [Bmax (in fmol/mg protein): Sham, 721 ± 16; OVX, 741 ± 24 ( P < 0.05 vs. OVX + E2); and OVX + E2, 569 ± 23; n = 7–11] membranes compared with E2 replete animals, respectively. Both the angiotensin-converting enzyme inhibitor captopril and the AT1R antagonist losartan prevented the OVX-induced decrease in the FEK+ and the increase in renal AT1R densities, suggesting that E2 deficiency reduces potassium excretion in an ANG II/AT1R-dependent manner. These findings may have implications for renal function in postmenopausal women as well as contribute to the reasons underlying the age-induced increase in susceptibility to hypertension-associated disease in women.

Posttranscriptional mechanisms contribute to osmotic regulation of ANG type 1 receptors in cultured rat renomedullary interstitial cells

Previously, we showed that ANG II receptors in cultured rat renomedullary interstitial cells (RMICs) are osmotically regulated (19). The current study examined the mechanisms underlying this osmotic regulation in RMICs cultured in isoosmotic (300 mosmol/kgH2O) and hyperosmotic (600 mosmol/kgH2O) conditions. Radioligand competition analysis coupled with RNase protection assays (RPA) and ligand-mediated receptor internalization studies revealed that RMICs primarily express the type 1a angiotensin receptor (AT(1a)R). When cultured under hyperosmotic conditions, the density (B(max)) of AT1R in RMIC membranes decreased by 31% [B(max) (pmol/mg protein): 300 mosmol/kgH2O, 6.44 +/- 0.46 vs. 600 mosmol/kgH2O, 4.42 +/- 0.37, n = 8, P < 0.01], under conditions in which no detectable changes in AT(1a)R mRNA expression or in the kinetics of ligand-mediated AT1R internalization were observed. RNA electromobility shift assays showed that RNA protein complex (RPC) formation between RMIC cytosolic RNA binding proteins and the 5' leader sequence (5'LS) of the AT(1a)R was increased 1.5-fold under hyperosmotic conditions [5'LS RPC (arbitrary units): 300 mosmol/kgH2O, 0.79 +/- 0.08 vs. 600 mosmol/kgH2O, 1.17 +/- 0.07, n = 4, P < 0.01]. These results suggest that the downregulation of AT(1a)R expression in RMICs cultured under hyperosmotic conditions is regulated at the posttranscriptional level by RNA binding proteins that interact within the 5'LS of the AT(1a)R mRNA. The downregulation of AT(1a)R expression under hyperosmotic conditions may be an important mechanism by which the activity of ANG II is regulated in the hyperosmotic renal medulla.

Translational control of the rat angiotensin type 1a receptor by alternative splicing

The rat angiotensin type 1a receptor (AT1aR) is comprised of three exons. Two transcripts are possible due to alternative splicing of exon 2 (E1,3 and E1,2,3). Both transcripts code for identical AT1aR proteins since they differ only in the length of their 5' leader sequence (5'LS). We investigated the functional differences of these two transcripts in stably transfected Chinese hamster ovary (CHO) cells and also determined the splice variant composition in rat tissues. E1,3 expressing cells exhibited 1.8-fold higher AT1R densities and five-fold higher levels of Ang II-stimulated inositol phosphate production compared to E1,2,3 expressing cells. No differences in E1,3 and E1,2,3 mRNA levels or mRNA stability were seen. In vitro translation assays revealed 1.8-fold higher AT1aR protein levels from E1,3 compared to E1,2,3 transcripts, suggesting exon 2 reduces functional AT1R expression by inhibiting translation. Deletion of 10 nucleotides in exon 2 increased translation of the mutated E1,2,3 transcript to levels which were indistinguishable from E1,3, suggesting that this loop region of a predicted hairpin contributes to the inhibitory RNA cis element within exon 2. Comparison of AT1aR exonic composition and AT1R densities in rat tissues suggests alternative splicing is regulated in a tissue-specific manner and contributes to tissue-specific differences in AT1R density.

The effect of hypothalamo-pituitary disconnection on the renin-angiotensin system in the late-gestation fetal sheep

The activity of the renin-angiotensin system (RAS) increases significantly in the late-gestation fetal sheep. Fetal cortisol is also increased during this time, and it is thought that the increase in cortisol may modulate the RAS changes. Previous studies have examined the effects of cortisol infusion on RAS activity, but the effects of blocking the peripartum increase in cortisol concentrations on the developmental changes in the RAS are not known. Therefore, we utilized the technique of hypothalamic-pituitary disconnection (HPD), which prevents the cortisol surge from occurring, to investigate the importance of the late-gestation increase in cortisol on the ontogenic changes in RAS activity. HPD of fetal sheep was performed at 120 days of gestational age (dGA), and fetuses were delivered between 135 and 139 dGA. Control fetuses were sham operated. HPD blocked the late-gestation cortisol increase but did not alter renal renin mRNA, renal renin or prorenin protein content, nor plasma renin levels compared with sham operated. However, HPD fetuses had increased ANG II receptor subtype 1 (AT1) mRNA and protein expression in the kidney and lungs. ANG II receptor subtype 2 (AT2) expression was not altered in these tissues at either mRNA or protein level. HPD did not change AT1 or AT2 mRNA in the left ventricle but did result in decreased protein levels for both receptors. These studies demonstrate that blockade of the naturally occurring increase in fetal cortisol concentration in late gestation is associated with tissue-specific alterations in expression of AT1 and AT2 receptors. These changes may impact on fetal tissue maturation and hence have consequences in postnatal life.

Ovariectomy augments hypertension in aging female Dahl salt-sensitive rats

The ovariectomized (OVX) Dahl salt-sensitive (DS) rat fed a low-salt diet is a model of postmenopausal hypertension. In addition to estrogen loss, aging can also contribute to postmenopausal hypertension. We hypothesized that: (1) female DS rats on a low-salt diet become hypertensive with age; (2) ovariectomy accelerates age-dependent hypertension in the DS rat caused by estrogen depletion; and (3) this hypertension correlates with increased type 1 angiotensin receptor (AT1R) number (Bmax). Blood pressure was monitored by telemetry from 3 to 12 months and AT1R Bmax was determined by Scatchard analysis in glomeruli and adrenal cortex. Three groups of DS rats were studied: intact, OVX, and 17beta-estradiol-replaced OVX (OVX+E). In intact rats, aging to 12 months resulted in hypertension (159+/-6 mm Hg) and an 82% decrease in estrogen. Blood pressure in OVX was significantly higher than OVX+E through 12 months of age (173+/-4 versus 150+/-8 mm Hg). At 4 months, OVX increased AT1R Bmax compared with intact and OVX+E in both glomeruli and adrenal cortex. Aging also increased AT1R Bmax in these tissues in intact rats. In summary, female DS rats fed a low-salt diet have hypertension develop with age, that is accelerated by OVX and attenuated by estrogen replacement. Concurrently, AT1Rs are upregulated by age and OVX, which is prevented by estrogen replacement. This study suggests that an increased activity of the renin angiotensin system contributes to the development of hypertension, and estrogen protects against this process.

Genetic variation in CYP11B2 and AT1R influences heart rate variability conditional on sodium excretion

Sympathetic tone increases with stimulation of the renin-angiotensin system and is under the influence of salt intake. In the European Project On Genes in Hypertension (EPOGH), we investigated whether polymorphisms in the genes encoding aldosterone synthase (CYP11B2 C-344T) and the type-1 angiotensin II receptor (AT1R A1166C) affect the autonomic modulation of heart rate at varying levels of salt intake. We measured the low frequency (LF) and high frequency (HF) components of heart rate variability and their ratio (LF:HF) in the supine and standing positions in 1797 participants (401 families and 320 unrelated subjects) randomly selected from 6 European populations, whose average urinary sodium excretion ranged from 163 to 245 mmol/d. In multivariate analyses with sodium excretion analyzed as a continuous variable, we explored the phenotype-genotype associations using generalized estimating equations and quantitative transmission disequilibrium tests. Across populations, there was no heterogeneity in the phenotype-genotype relations. The genotypic effects differed according to sodium excretion. In subjects with sodium excretion <190 mmol/d (median), supine heart rate, LF, and LF:HF increased and HF decreased with the number of CYP11B2 -344T alleles, and the orthostatic changes in LF, HF, and LF:HF were blunted in carriers of the AT1R 1166C allele. In subjects with sodium excretion >190 mmol/d, these associations with the CYP11B2 and AT1R polymorphisms were nonsignificant or in the opposite direction, respectively. Thus, CYP11B2 C-344T and AT1R A1166C polymorphisms affect the autonomic modulation of heart rate, but these genetic effects depend on sodium excretion.

Dietary salt intake modulates progression of antithymocyte serum nephritis through alteration of glomerular angiotensin II receptor expression

Dietary salt intake modulates the renin-angiotensin system (RAS); however, little is known about the effect of salt intake on the progression of glomerulonephritis. We investigated the glomerular expression of TGF-β1type I (TβRI) and II (TβRII) TGF-β receptors and RAS components in rats with antithymocyte serum (ATS) nephritis on normal (NSI)-, low (LSI)-, and high-salt intake (HSI) and on HSI rats receiving candesartan cilexetil (CC) and LSI rats receiving PD-123319. Glomerular lesions were less severe in rats on LSI and aggravated in those on HSI compared with those on NSI. Intrarenal renin and glomerular ANG II levels were significantly higher in LSI and lower in HSI rats. In ATS nephritis, HSI increased glomerular TβRI, TβRII, and ANG II type 1 receptor (AT1R), and decreased glomerular ANG II type 2 receptor (AT2R), whereas LSI decreased glomerular TGF-β1and TβRI and increased glomerular AT2R. CC ameliorated glomerular lesions, reduced glomerular TGF-β1and TβRII, and increased glomerular AT2R. PD-123319 aggravated glomerular lesions and increased glomerular TGF-β1and TβRII. Our results suggest that dietary salt intake influences progression of ATS nephritis by modulating glomerular TGF-β1and TβR expression resulting, at least in part, from altered glomerular AT1R and AT2R expression.

Estrogen regulates adrenal angiotensin type 1 receptors by modulating adrenal angiotensin levels

Estrogen inhibits adrenal angiotensin type 1 receptor (AT(1)R) binding sites and attenuates the adrenal responsivity to angiotensin II (Ang II). Ang II modulates AT(1)R expression. Here, we determined if estrogen-induced down-regulation of adrenal AT(1)Rs involves modulation of adrenal Ang II. Female rats were ovariectomized (OVX) and injected with 17beta-estradiol benzoate (E(2); 40 micro g/kg) or vehicle for 7 d. Adrenal Ang II was separated from other angiotensin peptides by HPLC and measured by RIA. Scatchard analysis of radioligand binding curves showed that E(2) or captopril (Cap; 0.5 g/liter water) significantly reduced adrenal AT(1)R binding (maximum binding capacity) by 22% and 19%, respectively, compared with OVX (276 +/- 2.09 fmol/mg protein). E(2) and Cap lowered adrenal Ang II levels by 39% and 21%, respectively, compared with OVX (4.10 +/- 0.44 pmol/g). E(2) caused no further reductions in adrenal AT(1)R binding or in Ang II levels in Cap-treated OVX rats. High-dose Ang II infusion (1000 ng/kg.min) increased adrenal Ang II levels by 71% and lowered AT(1)R binding by 18%. Under these infusion conditions, E(2) did not reduce adrenal Ang II or AT(1)R binding. No differences in AT(1)R affinity (dissociation constant) were observed among groups. These data suggest that E(2) regulates the number of adrenal AT(1)R binding sites indirectly by modulating adrenal Ang II.

Angiotensinogen genotype affects renal and adrenal responses to angiotensin II in essential hypertension

BACKGROUND

Renovascular and adrenal responses to infused angiotensin II (Ang II) are intermediate phenotypes that may indirectly reflect tissue renin-angiotensin system activity. We examine herein angiotensinogen (AGT) as a candidate gene to help elucidate potential mechanisms for previously reported AGT linkage and association studies.

METHODS AND RESULTS

Renal plasma flow and plasma aldosterone were measured before and after a 45-minute infusion of Ang II (3 ng x kg(-1) x min(-1)) in 190 hypertensive patients who were on carefully controlled high- and low-salt diets. Reduced renal vascular (P=0.0002) and adrenal (P=0.002) responses to infused exogenous Ang II were associated with the AGT -6A allele. In multiple logistic regression, greater body mass index, lower basal renal plasma flow, and higher diastolic blood pressure together with AGT -6A genotype were associated with lower renal vascular response. In contrast, only male sex and AGT -6A genotype were associated with lower adrenal response. When both the renal and adrenal responses to Ang II were in the lowest tertile, the AGT -6AA genotype was present in 55.6%; in contrast, when both responses were in the upper 2 tertiles, the -6AA genotype was present in only 17.8% (P=0.001).

CONCLUSIONS

A clear association between AGT genotype and response to infused Ang II was demonstrated for both the renal vasculature and the adrenal, consistent with the hypothesis that the AGT -6A genotype results in increased tissue expression of angiotensinogen and Ang II.