Blockade of angiotensin II type-1 receptor increases salt sensitivity in Sprague–Dawley rats

Effect of Angiotensin receptor blockade on Plasma Osmolality and Neurohumoral Responses to High Environmental Temperature in Rats Fed a High Salt Diet

Plasma osmolality (pOsmol) and neurohumoral signals play important roles in the pathophysiology of cardiovascular diseases. Our study investigated the effect of high environmental temperature (HET) on neurohumoral responses and pOsmol in rats fed a high salt diet (HSD), with and without angiotensin II receptor blockade (ARB), using telmisartan.  Fifty-six male 8-week old Sprague-Dawley rats (95-110g) were randomly assigned into seven groups of 8 rats. These included control rats (I) fed with 0.3% NaCl diet (normal diet, ND); salt-loaded rats (II) fed with 8% NaCl (high salt) diet; ND rats (III) exposed to HET (38.5±0.5oC ) 4 hours daily per week; rats (IV) fed with 8% NaCl diet and exposed to HET daily. Others included rats (V) fed with 8% NaCl diet and treated with telmisartan (30mg/kg); ND rats (VI) exposed to HET and treated with telmisartan; rats (VI) fed with 8% NaCl diet, exposed to HET and treated with telmisartan. Plasma angiotensin II, aldosterone, vasopressin and norepinephrine (NE) concentrations were determined by ELISA technique; pOsmol from plasma K+, Na+ and Urea. HSD combined with HET in rats synergistically increased pOsmol (P<0.001) with an associated non-synergistic rise in fluid intake (P<0.001), fluid balance (P<0.001), plasma angiotensin II (P<0.01) and aldosterone (P<0.05), NE (P<0.001) and vasopressin (P<0.05) concentrations compared to control. Telmisartan did not alter pOsmol in all the treated-rats, but normalized fluid intake levels and plasma vasopressin in the rats exposed to either HSD or HEt alone. Prolonged exposure of rats to hot environment exacerbated the effect of excess dietary salt on pOsmol, with no effect on angiotensin II-mediated neurohumoral responses.

Salt sensitivity and its implication in clinical practice

Hypertension (HTN) is a complex multi-factorial disease and is considered one of the foremost modifiable risk factors for stroke, heart failure, ischemic heart disease and renal dysfunction. Over the past century, salt and its linkage to HTN and cardiovascular (CV) mortality has been the subject of intense scientific scrutiny. There is now consensus that different individuals have different susceptibilities to blood pressure (BP)-raising effects of salt and this susceptiveness is called as salt sensitivity. Several renal and extra-renal mechanisms are believed to play a role. Blunted activity of the renin–angiotensin–aldosterone system (RAAS), adrenal Rac1-MR-Sgk1-NCC/ENaC pathway, renal SNS-GR-WNK4-NCC pathway, defect of membrane ion transportation, inflammation and abnormalities of Na⁺/Ca²⁺ exchange have all been implicated as pathophysiological basis for salt sensitive HTN. While salt restriction is definitely beneficial recent observation suggests that treatment with Azilsartan may improve salt sensitivity by selectively reducing renal proximal tubule Na⁺/H⁺ exchange. This encourages the future potential benefits of recognizing and therapeutically addressing the salt sensitive phenotype in humans.

Brain natriuretic peptide as a potential novel marker of salt-sensitivity in chronic kidney disease patients without cardiac dysfunction

Although the renin-angiotensin system (RAS) is counter-balanced by a salt-sensitive mechanism in the hypertensive state, both are reported to be up-regulated in chronic kidney disease (CKD) patients. We conducted this study to evaluate the associations among the RAS, renal function, hypertension, and atherosclerosis, as well as to identify markers for salt-sensitivity. A total of 213 pre-dialysis CKD patients with preserved cardiac function (EF >50 %) were enrolled. Their renal and cardiac biochemical markers and plasma renin activity (PRA) were measured, and echocardiography and carotid artery ultrasound were performed. Their salt intake was estimated by the NaCl excretion from a 24-h collected urine sample. The PRA was higher in patients with hypertension (p = 0.018), and had a significant negative correlation with the eGFR (r = -0.23, p = 0.0067). Importantly, the PRA had a strong negative correlation with the brain natriuretic peptide (BNP) level (r = -0.28, p = 0.017) regardless of whether the patients were being treated with RAS inhibitors. The BNP level was related to the renal functions (eGFR: p = 0.001, ACR: p = 0.009). There was a significant positive correlation between the BNP level and carotid intima-media thickness (p < 0.001). A multivariate analysis revealed that older age and an excess of NaCl excretion were independent predictors of BNP elevation (p = 0.02 and 0.003, respectively). Our analysis revealed details of the counterbalance between BNP and PRA, as well as identifying that excess salt intake is a predictor of BNP elevation. These results indicate that the BNP could be a possible valuable marker for salt sensitivity, and that high salt sensitivity could facilitate atherosclerosis in CKD patients.

Azilsartan Improves Salt Sensitivity by Modulating the Proximal Tubular Na+-H+ Exchanger-3 in Mice

A potent angiotensin II type-1 receptor blocker, azilsartan, has been reported to reduce blood pressure more effectively than candesartan. Interestingly, azilsartan can also restore the circadian rhythm of blood pressure. We hypothesized that azilsartan could also improve salt sensitivity; thus, we examined the effect of azilsartan on sodium handling in renal tubules. Subtotal nephrectomized C57BL/6 mice received azilsartan (1.0 mg/kg/day), candesartan (0.3 mg/kg/day), or vehicle via the oral route in conjunction with a normal- (0.3%) or high-salt (8.0%) diet. Two weeks later, the azilsartan group showed significantly lower blood pressure during the light period than the candesartan and vehicle groups (azilsartan: 103.1 ± 1.0; candesartan: 111.7 ± 2.7; vehicle: 125.5 ± 2.5 mmHg; P < 0.05; azilsartan or candesartan vs. vehicle). The azilsartan group also showed higher urinary fractional excretion of sodium during the dark period than the candesartan and vehicle groups (azilsartan: 21.37 ± 3.69%; candesartan: 14.17 ± 1.42%; vehicle: 13.85 ± 5.30%; P < 0.05 azilsartan vs. candesartan or vehicle). A pressure—natriuresis curve demonstrated that azilsartan treatment restored salt sensitivity. Immunofluorescence and western blotting showed lower levels of Na⁺-H⁺ exchanger-3 (NHE3) protein (the major sodium transporter in renal proximal tubules) in the azilsartan group, but not in the candesartan or vehicle groups. However, azilsartan did not affect NHE3 transcription levels. Interestingly, we did not observe increased expression of downstream sodium transporters, which would have compensated for the increased flow of sodium and water due to non-absorption by NHE3. We also confirmed the mechanism stated above using cultured opossum kidney proximal tubular cells. Results revealed that a proteasomal inhibitor (but not a lysosomal inhibitor) blocked the azilsartan-induced decrease in NHE3 protein expression, suggesting that azilsartan increases NHE3 ubiquitination. In conclusion, azilsartan (but not candesartan) improved salt sensitivity possibly by decreasing NHE3 expression via ubiquitin—proteasomal degradation.

Carbonyl stress induces hypertension and cardio-renal vascular injury in Dahl salt-sensitive rats

One major precursor of carbonyl stress, methylglyoxal (MG), is elevated in the plasma of chronic kidney disease (CKD) patients, and this precursor contributes to the progression of vascular injury, hypertension and renal injury in diabetic nephropathy patients. This molecule induces salt-sensitive hypertension via a reactive oxygen species-mediated pathway. We examined the role of MG in the pathogenesis of hypertension and cardio–renal injury in Dahl salt-sensitive (Dahl S) rats, which is a rat model of CKD. Nine-week-old Dahl S rats were fed a 1% NaCl diet, and 1% MG was added to their drinking water for up to 12 weeks. Blood pressure and cardio–renal injuries were compared with rats treated with tap water alone. The angiotensin II receptor blocker (ARB), candesartan (10 mg kg⁻¹ day⁻¹), was administered to MG Dahl S rats to determine the impact of this drug on the pathogenesis of MG-induced CKD. A progressive increase in systolic blood pressure was observed (123±1–148±5 mm Hg) after 12 weeks of MG administration. MG administration significantly increased urinary albumin excretion, glomerular sclerosis, tubular injury, myocardial collagen content and cardiac perivascular fibrosis. MG also enhanced the renal expression of Nɛ-carboxyethyl-lysine (an advanced glycation end product), 8-hydroxydeoxyguanosine (a marker of oxidative stress), macrophage (ED-1) positive cells (a marker of inflammation) and nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase activity. Candesartan treatment for 4 weeks significantly reduced these parameters. These results suggest that MG-induced hypertension and cardio–renal injury and increased inflammation and carbonyl and oxidative stress, which were partially preventable by an ARB.

Duration until nighttime blood pressure fall indicates excess sodium retention

Impaired renal sodium excretion causes sodium retention, which prevents the nocturnal dip in blood pressure (BP); thus, high BP persists until excess sodium is excreted. The authors defined "dipping time" (DT) as the duration until the nocturnal BP falls below 90% of the daytime average. Diuretic (e.g., hydrochlorothiazide [HCTZ]) and angiotensin receptor blocker (ARB) are able to eliminate sodium retention and restore the non-dipper BP rhythm. Reanalysis of two previous studies demonstrate that HCTZ and ARB shortened the DT. Shortening DT correlated directly with the increase in daytime urinary sodium excretion (Study 2). DT can be used as a preliminary indicator of sodium retention.

Role of renal medullary oxidative and/or carbonyl stress in salt-sensitive hypertension and diabetes

1. Salt-sensitive hypertension is commonly associated with diabetes, obesity and chronic kidney disease. The present review focuses on renal mechanisms involved in the development of this type of hypertension. 2. The renal medullary circulation plays an important role in the development of salt-sensitive hypertension. In vivo animal studies have demonstrated that the balance between nitric oxide (NO) and reactive oxygen species (ROS) in the renal medulla is an important element of salt-sensitive hypertension. The medullary thick ascending limb (mTAL) in the outer medulla is an important source of NO and ROS production and we have explored the mechanisms that stimulate their production, as well as the effects of NO superoxide and hydrogen peroxide on mTAL tubular sodium reabsorption and the regulation of medullary blood flow. 3. Angiotensin II-stimulated NO produced in the mTAL is able to diffuse from the renal mTAL to the surrounding vasa recta capillaries, providing a mechanism by which to increase medullary blood flow and counteract the direct vasoconstrictor effects of angiotensin II. Enhanced oxidative stress attenuates NO diffusion in this region. 4. Carbonyl stress, like oxidative stress, can also play an important role in the pathogenesis of chronic kidney disease, such as insulin resistance, salt-sensitive hypertension and renal vascular complications. 5. Despite the large number of studies undertaken in this area, there is as yet no drug available that directly targets renal ROS. Oxidative and/or carbonyl stress may be the next target of drug discovery to protect against salt-sensitive hypertension and associated end-organ damage.

Long-term antihypertensive efficacy of losartan/hydrochlorothiazide combination therapy on home blood pressure control

Angiotensin receptor blocker (ARB)/hydrochlorothiazide (HCTZ) combination therapy has been shown to produce a prompt reduction in clinic blood pressure (BP) without serious adverse effects; however, long-term antihypertensive efficacy on home BP has not been fully investigated. In this open-label multicenter observational study, a total of 151 hypertensive patients uncontrolled with antihypertensive regimens including standard dose of ARBs were switched to the fixed-dose combination of losartan (50 mg)/HCTZ (12.5 mg) (mean age 66.9 ± 9.5 years, 51% male, 19% with diabetes mellitus, and 57% with dyslipidemia). After 3 months, losartan/HCTZ treatment significantly reduced mean home systolic BP/diastolic BP from a baseline level of 153 ± 11/85 ± 9 mm Hg to 136 ± 12/77 ± 10 mm Hg (P < .001) and mean clinic BP from 158 ± 9/87 ± 9 to 136 ± 12/77 ± 10 (P < .001), which were maintained through the study period of 12 months (132 ± 11/75 ± 9 and 136 ± 12/77 ± 10; home and clinic BP at 12 months, respectively, P < .001). Furthermore, younger patients (< 65 years) receiving ARB monotherapy at the start of the study showed a significantly greater reduction in home BP, but not in clinic BP, compared with elderly patients (≥ 65 years). In conclusion, losartan/HCTZ combination therapy exerted a 1-year long-term efficacy on home BP as well as clinic BP. In patients uncontrolled with ARB monotherapy, the antihypertensive efficacy on home BP is more pronounced in younger patients compared with that in elderly patients.

Changes in renal vessels following the long-term administration of an angiotensin II receptor blocker in Zucker fatty rats

INTRODUCTION

The nephro-protective effects of angiotensin II receptor blockers (ARBs) are widely known; however, there are few reports of long-term effects focusing on the renal vessels. We studied afferent arteriolar changes induced by the long-term administration of an ARB.

MATERIALS AND METHODS

Thirty-two 6-week-old male Zucker fatty rats (ZFRs) were divided into following four groups (n = 8 in each): ZFR Group and ZFR+High Group fed a standard or high-salt diet, respectively; ZFR+ARB Group and ZFR+High+ARB Group fed a standard or high-salt diet with ARB (Olmesartan, 5 mg/kg/day), respectively. Blood pressure, proteinuria, morphological examinations and glomerular haemodynamics in vivo were studied.

RESULTS

Marked proliferative changes in the afferent arteriolar smooth muscle cells (SMCs) were frequently observed in the two groups given ARBs; in the ZFR+ARB group (77.3±10.3%) compared with the two groups without ARB (1.7%, p < 0.005; 1.2%, p < 0.0005) and 37.4±15.6% in the ZFR+High+ARB group. Proteinuria markedly decreased in the groups treated with ARBs, but the glomerular erythrocyte velocities showed no differences.

CONCLUSIONS

Our findings indicate that long-term ARB administration induced unusual proliferative changes in SMCs of afferent arterioles of ZFRs. These changes could narrow arteriolar lumens and reduce intraglomerular pressure, but they could cause also irreversible damage to the arterioles.