Effects of acute AT1 receptor blockade by candesartan on arterial pressure and renal function in rats

Dietary fructose enhances angiotensin II-stimulated Na+ transport via activation of PKC-α in renal proximal tubules

Angiotensin II (ANG II) stimulates proximal nephron transport via activation of classical protein kinase C (PKC) isoforms. Acute fructose treatment stimulates PKC and dietary fructose enhances ANG II's ability to stimulate Na⁺ transport, but the mechanisms are unclear. We hypothesized that dietary fructose enhances ANG II's ability to stimulate renal proximal tubule Na⁺ reabsorption by augmenting PKC-α activation and increases in intracellular Ca²⁺. We measured total and isoform-specific PKC activity, basal and ANG II-stimulated oxygen consumption, a surrogate of Na⁺ reabsorption, and intracellular Ca²⁺ in proximal tubules from rats given either 20% fructose in their drinking water (fructose group) or tap water (control group). Total PKC activity was measured by ELISA. PKC-α, PKC-β, and PKC-γ activities were assessed by measuring particulate-to-soluble ratios. Intracelluar Ca²⁺ was measured using fura 2. ANG II stimulated total PKC activity by 53 ± 15% in the fructose group but not in the control group (-15 ± 11%, P < 0.002). ANG II stimulated PKC-α by 0.134 ± 0.026 but not in the control group (-0.002 ± 0.020, P < 0.002). ANG II increased PKC-γ activity by 0.008 ± 0.003 in the fructose group but not in the control group (P < 0.046). ANG II did not stimulate PKC-β in either group. ANG II increased Na⁺ transport by 454 ± 87 nmol·min^-1·mg protein^-1 in fructose group, and the PKC-α/β inhibitor Gö6976 blocked this increase (-96 ± 205 nmol·min^-1·mg protein^-1, P < 0.045). ANG II increased intracellular Ca²⁺ by 148 ± 53 nM in the fructose group but only by 43 ± 10 nM in the control group (P < 0.035). The intracellular Ca²⁺ chelator BAPTA blocked the ANG II-induced increase in Na⁺ transport in the fructose group. We concluded that dietary fructose enhances ANG II's ability to stimulate renal proximal tubule Na⁺ reabsorption by augmenting PKC-α activation via elevated increases in intacellular Ca²⁺.

Purinergic P2X1 receptor, purinergic P2X7 receptor, and angiotensin II type 1 receptor interactions in the regulation of renal afferent arterioles in angiotensin II-dependent hypertension

In ANG II-dependent hypertension, ANG II activates ANG II type 1 receptors (AT₁Rs), elevating blood pressure and increasing renal afferent arteriolar resistance (AAR). The increased arterial pressure augments interstitial ATP concentrations activating purinergic P2X receptors (P2XRs) also increasing AAR. Interestingly, P2X₁R and P2X₇R inhibition reduces AAR to the normal range, raising the conundrum regarding the apparent disappearance of AT₁R influence. To evaluate the interactions between P2XRs and AT₁Rs in mediating the increased AAR elicited by chronic ANG II infusions, experiments using the isolated blood perfused juxtamedullary nephron preparation allowed visualization of afferent arteriolar diameters (AAD). Normotensive and ANG II-infused hypertensive rats showed AAD responses to increases in renal perfusion pressure from 100 to 140 mmHg by decreasing AAD by 26 ± 10% and 19 ± 4%. Superfusion with the inhibitor P2X₁Ri (NF4490; 1 μM) increased AAD. In normotensive kidneys, superfusion with ANG II (1 nM) decreased AAD by 16 ± 4% and decreased further by 19 ± 5% with an increase in renal perfusion pressure. Treatment with P2X₁Ri increased AAD by 30 ± 6% to values higher than those at 100 mmHg plus ANG II. In hypertensive kidneys, the inhibitor AT₁Ri (SML1394; 1 μM) increased AAD by 10 ± 7%. In contrast, treatment with P2X₁Ri increased AAD by 21 ± 14%; combination with P2X₁Ri plus P2X₇Ri (A438079; 1 μM) increased AAD further by 25 ± 8%. The results indicate that P2X₁R, P2X₇R, and AT₁R actions converge at receptor or postreceptor signaling pathways, but P2XR exerts a dominant influence abrogating the actions of AT₁Rs on AAR in ANG II-dependent hypertension.

Role of the afferent renal nerves in sodium homeostasis and blood pressure regulation in rats

New Findings

What is the central question of this study?

What are the differential roles of the mechanosensitive and chemosensitive afferent renal nerves in the reno‐renal reflex that promotes natriuresis, sympathoinhibition and normotension during acute and chronic challenges to sodium homeostasis?

What is the main finding and its importance?

The mechanosensitive afferent renal nerves contribute to an acute natriuretic sympathoinhibitory reno‐renal reflex that may be integrated within the paraventricular nucleus of the hypothalamus. Critically, the afferent renal nerves are required for the maintenance of salt resistance in Sprague–Dawley and Dahl salt‐resistant rats and attenuate the development of Dahl salt‐sensitive hypertension.

Abstract

These studies tested the hypothesis that in normotensive salt‐resistant rat phenotypes the mechanosensitive afferent renal nerve (ARN) reno‐renal reflex promotes natriuresis, sympathoinhibition and normotension during acute and chronic challenges to fluid and electrolyte homeostasis. Selective ARN ablation was conducted prior to (1) an acute isotonic volume expansion (VE) or 1 m NaCl infusion in Sprague–Dawley (SD) rats and (2) chronic high salt intake in SD, Dahl salt‐resistant (DSR), and Dahl salt‐sensitive (DSS) rats. ARN responsiveness following high salt intake was assessed ex vivo in response to noradrenaline and sodium concentration (SD, DSR and DSS) and via in vivo manipulation of renal pelvic pressure and sodium concentration (SD and DSS). ARN ablation attenuated the natriuretic and sympathoinhibitory responses to an acute VE [peak natriuresis (µeq min⁻¹) sham 52 ± 5 vs. ARN ablation 28 ± 3, P < 0.05], but not a hypertonic saline infusion in SD rats. High salt (HS) intake enhanced ARN reno‐renal reflex‐mediated natriuresis in response to direct increases in renal pelvic pressure (mechanoreceptor stimulus) in vivo and ARN responsiveness to noradrenaline ex vivo in SD, but not DSS, rats. In vivo and ex vivo ARN responsiveness to increased renal pelvic sodium concentration (chemoreceptor stimulus) was unaltered during HS intake. ARN ablation evoked sympathetically mediated salt‐sensitive hypertension in SD rats [MAP (mmHg): sham normal salt 102 ± 2 vs. sham HS 104 ± 2 vs. ARN ablation normal salt 103 ± 2 vs. ARN ablation HS 121 ± 2, P < 0.05] and DSR rats and exacerbated DSS hypertension. The mechanosensitive ARNs mediate an acute sympathoinhibitory natriuretic reflex and counter the development of salt‐sensitive hypertension.

Activation of RAAS in a rat model of liver cirrhosis: no effect of losartan on renal sodium excretion

Background

Liver cirrhosis is characterized by avid sodium retention where the activation of the renin angiotensin aldosterone system (RAAS) is considered to be the hallmark of the sodium retaining mechanisms. The direct effect of angiotensin II (ANGII) on the AT-1 receptor in the proximal tubules is partly responsible for the sodium retention. The aim was to estimate the natriuretic and neurohumoral effects of an ANGII receptor antagonist (losartan) in the late phase of the disease in a rat model of liver cirrhosis.

Methods

Bile duct ligated (BDL) and sham operated rats received 2 weeks of treatment with losartan 4 mg/kg/day or placebo, given by gastric gavage 5 weeks after surgery. Daily sodium and potassium intakes and renal excretions were measured.

Results

The renal sodium excretion decreased in the BDL animals and this was not affected by losartan treatment. At baseline the plasma renin concentration (PRC) was similar in sham and BDL animals, but increased urinary excretion of ANGII and an increase P-Aldosterone was observed in the placebo treated BDL animals. The PRC was more than 150 times higher in the losartan treated BDL animals (p < 0.001) which indicated hemodynamic impairment.

Conclusions

Losartan 4 mg/kg/day did not increase renal sodium excretion in this model of liver cirrhosis, although the urinary ANGII excretion was increased. The BDL animals tolerated Losartan poorly, and the treatment induced a 150 times higher PRC.

Modulating Role of Ang1-7 in Control of Blood Pressure and Renal Function in AngII-infused Hypertensive Rats

BACKGROUND

Indirect evidence suggests that angiotensin 1-7 (Ang1-7) may counterbalance prohypertensive actions of angiotensin II (AngII), via activation of vascular and/or renal tubular receptors to cause vasodilation and natriuresis/diuresis. We examined if Ang1-7 would attenuate the development of hypertension, renal vasoconstriction, and decreased natriuresis in AngII-infused rats and evaluated the mechanisms involved.

METHODS

AngII, alone or with Ang1-7, was infused to conscious Sprague-Dawley rats for 13 days and systolic blood pressure (SBP) and renal excretion were repeatedly determined. In anesthetized rats, acute actions of Ang1-7 and effects of blockade of angiotensin AT1 or Mas receptors (candesartan or A-779) were studied.

RESULTS

Chronic AngII infusion increased SBP from 143 ± 4 to 195 ± 6 mm Hg. With Ang1-7 co-infused, SBP increased from 133 ± 5 to 161 ± 5 mm Hg (increase reduced, P < 0.002); concurrent increases in urine flow (V) and sodium excretion (UNaV) were greater. In anesthetized normotensive or AngII-induced hypertensive rats, Ang1-7 infusion transiently increased mean arterial pressure (MABP), transiently decreased renal blood flow (RBF), and caused increases in UNaV and V. In normotensive rats, candesartan prevented the Ang1-7-induced increases in MABP and UNaV and the decrease in RBF. In anesthetized normotensive, rats intravenous A-779 increased MABP (114 ± 5 to 120 ± 5 mm Hg, P < 0.03) and urine flow. Surprisingly, these changes were not observed with A-779 applied during background Ang1-7 infusion.

CONCLUSIONS

The results suggest that in AngII-dependent hypertension, Ang1-7 deficit contributes to sodium and fluid retention and thereby to BP elevation; a correction by Ang1-7 infusion seems mediated by AT1 and not Mas receptors.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Antihypertensive action of soluble epoxide hydrolase inhibition in Ren-2 transgenic rats is mediated by suppression of the intrarenal renin-angiotensin system

The aim of the present study was to evaluate the hypothesis that the antihypertensive effects of inhibition of soluble epoxide hydrolase (sEH) are mediated by increased intrarenal availability of epoxyeicosatrienoic acids (EETs), with consequent improvement in renal haemodynamic autoregulatory efficiency and the pressure-natriuresis relationship. Ren-2 transgenic rats (TGR), a model of angiotensin (Ang) II-dependent hypertension, and normotensive transgene-negative Hannover Sprague-Dawley (HanSD) rats were treated with the sEH inhibitor cis-4-(4-(3-adamantan-1-yl-ureido)cyclohexyloxy)benzoic acid (c-AUCB; 26 mg/L) for 48 h. Then, the effects on blood pressure (BP), autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR), and on the pressure-natriuresis relationship in response to stepwise reductions in renal arterial pressure (RAP) were determined. Treatment with c-AUCB did not significantly change BP, renal autoregulation or pressure-natriuresis in normotensive HanSD rats. In contrast, c-AUCB treatment significantly reduced BP, increased intrarenal bioavailability of EETs and significantly suppressed AngII levels in TGR. However, treatment with c-AUCB did not significantly improve the autoregulatory efficiency of RBF and GFR in response to reductions of RAP and to restore the blunted pressure-natriuresis relationship in TGR. Together, the data indicate that the antihypertensive actions of sEH inhibition in TGR are predominantly mediated via significant suppression of intrarenal renin-angiotensin system activity.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Impaired EphA4 signaling leads to congenital hydronephrosis, renal injury, and hypertension

Experimental hydronephrosis induced by partial ureteral obstruction at 3 wk of age causes hypertension and renal impairment in adult rats and mice. Signaling by Ephrin receptors (Eph) and their ligands (ephrins) importantly regulates embryonic development. Genetically modified mice, where the cytoplasmic domain of the EphA4 receptor has been substituted by enhanced green fluorescent protein (EphA4gf/gf), develop spontaneous hydronephrosis and provide a model for further studies of the disorder. The present study aimed to determine if animals with congenital hydronephrosis develop hypertension and renal injuries, similar to that of experimental hydronephrosis. Ultrasound and Doppler techniques were used to visualize renal impairment in the adult mice. Telemetric blood pressure measurements were performed in EphA4gf/gf mice and littermate controls (EphA4+/+) during normal (0.7% NaCl)- and high (4% NaCl)-sodium conditions. Renal excretion, renal plasma flow, and glomerular filtration were studied, and histology and morphology of the kidneys and ureters were performed. EphA4gf/gf mice developed variable degrees of hydronephrosis that correlated with their blood pressure level. In contrast to EphA4+/+, the EphA4gf/gf mice displayed salt-sensitive hypertension, reduced urine concentrating ability, reduced renal plasma flow, and lower glomerular filtration rate. Kidneys from EphA4gf/gf mice showed increased renal injuries, as evidenced by fibrosis, inflammation, and glomerular and tubular changes. In conclusion, congenital hydronephrosis causes hypertension and renal damage, similar to that observed in experimentally induced hydronephrosis. This study further reinforces the supposed causal link between hydronephrosis and later development of hypertension in humans.

Role of cytochrome P-450 metabolites in the regulation of renal function and blood pressure in 2-kidney 1-clip hypertensive rats

Alterations in renal function contribute to Goldblatt two-kidney, one-clip (2K1C) hypertension. A previous study indicated that bioavailability of cytochrome P-450 metabolites epoxyeicosatrienoic acids (EETs) is decreased while that of 20-hydroxyeicosatetraenoic acids (20-HETE) is increased in this model. We utilized the inhibitor of soluble epoxide hydrolase cis-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB) and HET-0016, the inhibitor of 20-HETE production, to study the role of EETs and 20-HETE in the regulation of renal function. Chronic c-AUCB treatment significantly decreased systolic blood pressure (SBP) (133 ± 1 vs. 163 ± 3 mmHg) and increased sodium excretion (1.23 ± 0.10 vs. 0.59 ± 0.03 mmol/day) in 2K1C rats. HET-0016 did not affect SBP and sodium excretion. In acute experiments, renal blood flow (RBF) was decreased in 2K1C rats (5.0 ± 0.2 vs. 6.9 ± 0.2 ml·min(-1)·g(-1)). c-AUCB normalized RBF in 2K1C rats (6.5 ± 0.6 ml·min(-1)·g(-1)). HET-0016 also increased RBF in 2K1C rats (5.8 ± 0.2 ml·min(-1)·g(-1)). Although RBF and glomerular filtration rate (GFR) remained stable in normotensive rats during renal arterial pressure (RAP) reductions, both were significantly reduced at 100 mmHg RAP in 2K1C rats. c-AUCB did not improve autoregulation but increased RBF at all RAPs and shifted the pressure-natriuresis curve to the left. HET-0016-treated 2K1C rats exhibited impaired autoregulation of RBF and GFR. Our data indicate that c-AUCB displays antihypertensive properties in 2K1C hypertension that are mediated by an improvement of RBF and pressure natriuresis. While HET-0016 enhanced RBF, its anti-natriuretic effect likely prevented it from producing a blood pressure-lowering effect in the 2K1C model.

Effect of celecoxib on the antihypertensive effect of losartan in a rat model of renovascular hypertension

Certain nonsteroidal anti-inflammatory drugs have been reported to elevate blood pressure in some hypertensive patients, who are either untreated or treated with antihypertensive agents. This study was undertaken to determine the effect of a selective cyclooxygenase-2 (COX-2) inhibitor, celecoxib, on the antihypertensive effects of the angiotensin II type 1 receptor (AT1) antagonist, losartan potassium. We studied the effect of oral treatment with losartan (30 mg/kg), celecoxib (3 mg/kg), and their combination on the mean arterial blood pressure (MAP), plasma renin activity (PRA), and plasma prostaglandin E2(PGE2) in male Sprague–Dawley rats with renovascular hypertension (RVH) induced by partial subdiaphragmatic aortic constriction. Treatment was continued for 7 days after aortic coarctation. Aortic coarctation led to significant increases in the MAP, PRA, and plasma PGE2. In RVH rats, losartan treatment caused a significant decrease of MAP with a significant increase in both plasma PGE2and PRA. Celecoxib caused a nonsignificant change in MAP with a significant decrease in the raised levels of plasma PGE2and PRA. Concomitant administration of celecoxib and losartan did not significantly affect the lowering effect of losartan on MAP with a subsequent significant decrease in the plasma PGE2and PRA in RVH rats. Therefore, celecoxib could be used in renin-dependent hypertensive patients who receive losartan, without fear of a rise in their blood pressure.

Renal mechanisms contributing to the antihypertensive action of soluble epoxide hydrolase inhibition in Ren-2 transgenic rats with inducible hypertension

In the present study, we examined the effects of soluble epoxide hydrolase (sEH) inhibition on the development of angiotensin II-dependent hypertension and on renal function in transgenic rats with inducible expression of the mouse renin gene (strain name Cyp1a1-Ren-2). Hypertension was induced in these rats by indole-3-carbinol (I3C; 0.3% in the diet) for 12 days. The sEH inhibitor cis-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB) was given in two doses (13 or 26 mg l-1) in drinking water. Blood pressure (BP), body weight (BW) and renal excretory parameters were monitored in conscious animals during the experiment. Renal haemodynamics was assessed at the end of treatment in anaesthetized rats. I3C administration resulted in severe hypertension with a rise in systolic BP from 118 ± 2 to 202 ± 3 mmHg, a loss of BW from 266 ± 5 to 228 ± 4 g and a rise in proteinuria from 14 ± 2 to 34 ± 3 mg day-1. Both doses of c-AUCB significantly attenuated the development of hypertension (systolic BP of 181 ± 4 and 176 ± 4 mmHg, respectively), the loss in BW (256 ± 4 and 259 ± 3 g, respectively) and the degree of proteinuria (27 ± 2 and 25 ± 3 mg day-1, respectively) to a similar extent. Moreover, c-AUCB prevented the reduction in renal plasma flow (5.4 ± 0.4 vs. 4.6 ± 0.3 ml min-1 g-1) and significantly increased sodium excretion (0.84 ± 0.16 vs. 0.38 ± 0.08 μmol min-1 g-1) during I3C administration. These data suggest that the oral administration of c-AUCB displays antihypertensive effects in Ren-2 transgenic rats with inducible malignant hypertension via an improvement of renal function.

Angiotensin II stimulates thick ascending limb NO production via AT(2) receptors and Akt1-dependent nitric-oxide synthase 3 (NOS3) activation

Angiotensin II (Ang II) acutely stimulates thick ascending limb (TAL) NO via an unknown mechanism. In endothelial cells, activation of Ang II type 2 receptor (AT(2)) stimulates NO. Akt1 activates NOS3 by direct phosphorylation. We hypothesized that Ang II stimulates TAL NO production via AT(2)-mediated Akt1 activation, which phosphorylates NOS3 at serine 1177. We measured NO production by fluorescence microscopy. In isolated TALs, Ang II (100 nm) increased NO production by 1.1 +/- 0.2 fluorescence units/min (p < 0.01). Ang II increased cGMP accumulation by 4.9 +/- 1.3 fmol/microg (p < 0.01). Upon adding the AT(2) antagonist PD123319 (1 microm), Ang II failed to stimulate NO (0.1 +/- 0.1 fluorescence units/min; p < 0.001 versus Ang II); adding the AT(1) antagonist losartan (1 microm) resulted in Ang II stimulating NO by 0.9 +/- 0.1 fluorescence units/min. Akt inhibitor (5 microm) blocked Ang II-stimulated NO (-0.1 +/- 0.2 fluorescence units/min versus inhibitor alone). Phospho-Akt1 increased by 72% after 5 min (p < 0.006), returning to basal after 10 min. Phospho-Akt2 did not change after 5 min but increased by 115 and 163% after 10 and 15 min (p < 0.02). Phospho-Akt3 did not change. An AT(2) agonist increased pAkt1 by 78% (p < 0.02), PI3K inhibition blocked this effect. In TALs transduced with dominant negative Akt1, Ang II failed to stimulate NO (0.1 +/- 0.2 fluorescence units/min versus 1.2 +/- 0.2 for controls; p < 0.001). Ang II increased phospho-NOS3 at serine 1177 by 130% (p < 0.01) and 150% after 5 and 10 min (p < 0.02). Ang II increased phosphoNOS3 at serine 633 by 50% after 5 min (p < 0.01). Akt inhibition prevented NOS3 phosphorylation. We concluded that Ang II enhances TAL NO production via activation of AT(2) and Akt1-dependent phosphorylation of NOS3 at serines 1177 and 633.

Plasmin in nephrotic urine activates the epithelial sodium channel

Proteinuria and increased renal reabsorption of NaCl characterize the nephrotic syndrome. Here, we show that protein-rich urine from nephrotic rats and from patients with nephrotic syndrome activate the epithelial sodium channel (ENaC) in cultured M-1 mouse collecting duct cells and in Xenopus laevis oocytes heterologously expressing ENaC. The activation depended on urinary serine protease activity. We identified plasmin as a urinary serine protease by matrix-assisted laser desorption/ionization time of-flight mass spectrometry. Purified plasmin activated ENaC currents, and inhibitors of plasmin abolished urinary protease activity and the ability to activate ENaC. In nephrotic syndrome, tubular urokinase-type plasminogen activator likely converts filtered plasminogen to plasmin. Consistent with this, the combined application of urokinase-type plasminogen activator and plasminogen stimulated amiloride-sensitive transepithelial sodium transport in M-1 cells and increased amiloride-sensitive whole-cell currents in Xenopus laevis oocytes heterologously expressing ENaC. Activation of ENaC by plasmin involved cleavage and release of an inhibitory peptide from the ENaC gamma subunit ectodomain. These data suggest that a defective glomerular filtration barrier allows passage of proteolytic enzymes that have the ability to activate ENaC.

Plasmin in nephrotic urine activates the epithelial sodium channel

Proteinuria and increased renal reabsorption of NaCl characterize the nephrotic syndrome. Here, we show that protein-rich urine from nephrotic rats and from patients with nephrotic syndrome activate the epithelial sodium channel (ENaC) in cultured M-1 mouse collecting duct cells and in Xenopus laevis oocytes heterologously expressing ENaC. The activation depended on urinary serine protease activity. We identified plasmin as a urinary serine protease by matrix-assisted laser desorption/ionization time of-flight mass spectrometry. Purified plasmin activated ENaC currents, and inhibitors of plasmin abolished urinary protease activity and the ability to activate ENaC. In nephrotic syndrome, tubular urokinase-type plasminogen activator likely converts filtered plasminogen to plasmin. Consistent with this, the combined application of urokinase-type plasminogen activator and plasminogen stimulated amiloride-sensitive transepithelial sodium transport in M-1 cells and increased amiloride-sensitive whole-cell currents in Xenopus laevis oocytes heterologously expressing ENaC. Activation of ENaC by plasmin involved cleavage and release of an inhibitory peptide from the ENaC gamma subunit ectodomain. These data suggest that a defective glomerular filtration barrier allows passage of proteolytic enzymes that have the ability to activate ENaC.

Low-dose candesartan improves renal blood flow and kidney oxygen tension in rats with endotoxin-induced acute kidney dysfunction

Sepsis is associated with an activation of the renin-angiotensin system and causes acute kidney injury. The aim was to examine the effects of a low, nondepressor dose of the selective angiotensin II type 1 receptor antagonist candesartan on renal hemodynamics and function in endotoxemic rats. Endotoxemia was induced in Sprague-Dawley rats by a dose of LPS (Escherichia coli O127:B8; 7.5 mg kg(-1), i.p.). At 16 h after endotoxin administration, renal clearance experiments were performed in thiobutabarbital anesthetized rats. Study groups (1) sham-saline, (2) LPS-saline, and (3) LPS-candesartan received isotonic saline or candesartan (10 microg kg(-1), i.v.) after baseline measurements. Kidney function, renal blood flow (RBF), and cortical and outer medullary perfusion (laser-Doppler flowmetry) and oxygen tension (P(O2); Clark-type microelectrodes) were analyzed during 2 h after drug administration. At baseline, endotoxemic rats showed an approximately 50% reduction in glomerular filtration rate and RBF (P < 0.05), a decline in cortical and outer medullary perfusion, and Po2 (P < 0.05), but no significant alterations in MAP compared with saline-injected controls. Candesartan treatment significantly improved RBF (+40% +/- 6% vs. baseline), cortical perfusion (+18% +/- 3% vs. baseline), and cortical (+19% +/- 7% vs. baseline) and outer medullary (+22% +/- 10% vs. baseline) P(O2) in endotoxemic rats (P < 0.05 vs. LPS-saline). Candesartan did not significantly influence MAP or glomerular filtration rate, whereas filtration fraction was reduced by 27% +/- 5% vs. baseline (P < 0.05 vs. LPS-saline). In conclusion, candesartan, in a dose that did not significantly decrease MAP, caused renal vasodilation and markedly improved RBF and intrarenal P(O2) in endotoxemic rats. These findings suggest renoprotective effects of candesartan in sepsis.

AT1 receptor blockade prevents interstitial and glomerular apoptosis but not fibrosis in pigs with neonatal induced partial unilateral ureteral obstruction

Obstruction-induced fibrosis is a leading cause of end-stage renal failure in children. The pathophysiological mechanisms may involve apoptosis and the renin-angiotensin system. We studied apoptosis and fibrosis in a well-established neonatal pig model with unilateral partial ureteral obstruction (PUUO) induced during ongoing nephrogenesis in 2-day-old piglets. The role of angiotensin II (ANG II) was studied using the AT(1) receptor blocker CV-11974 (0.12 mg/h candesartan from age 23 to 30 days). At day 30 the kidneys were perfusion fixed and fibrosis, apoptosis, and tubular lengths were quantitated using stereological methods, picro Sirius red staining, and immunohistochemical techniques identifying activated caspase 3, aquaporin-2 (AQP2), and von Willebrand factor. The collagen content was assessed by hydroxyproline density. Neonatal induced PUUO increased interstitial and glomerular cell apoptosis and fibrosis. At this stage, PUUO did not increase tubular cell apoptosis or decrease tubular length and cell number. AT(1) receptor blockade prevented the PUUO-induced interstitial and glomerular cell apoptosis but did not attenuate fibrosis. In conclusion, AT(1) receptor blockade after the end of nephrogenesis may prevent interstitial and glomerular cell apoptosis but not fibrosis, suggesting that pathways not involving AT(1) receptor stimulation contribute to neonatal obstruction-induced fibrosis or that prevention of interstitial cell apoptosis counteracts a potential antifibrotic effect of AT(1) receptor blockade in this pig model of congenital obstructive nephropathy. Our results demonstrate that ANG II plays a role in PUUO-induced glomerular cell apoptosis.

Angiotensin II, interstitial inflammation, and the pathogenesis of salt-sensitive hypertension

Transient administration of ANG II causes persistent salt-sensitive hypertension associated with arteriolopathy, interstitial inflammation, and cortical vasoconstriction; blocking the vascular and inflammatory changes with mycophenolate mofetil (MMF) prevents vasoconstriction. While infiltrating leukocytes during the salt-sensitive hypertension phase express ANG II, the functional role of ANG II during this phase is not known. We examined the acute effect of candesartan on renal hemodynamics during the established salt-sensitive hypertensive phase and related these findings to direct measurement of intrarenal ANG II and inflammatory cells in rats previously exposed to ANG II with or without MMF treatment. Sham controls were also examined. The administration of ANG II, followed by exposure to high-salt diet, resulted in hypertension, cortical vasoconstriction, an increase in interstitial inflammatory cells (44.8 +/- 1.3 lymphocytes/mm2, and 30.8 +/- 1.2 macrophages/mm2 ANG II vs. 19.6 +/- 2 lymphocytes/mm2, and 22 +/- 0.7 macrophages/mm2 Sham), and increase in renal ANG II levels (1,358 +/- 74.6 pg/ml ANG II vs. 194 +/- 9.28 pg/ml Sham). Treatment with MMF during the administration of exogenous ANG II resulted in reduction in renal interstitial inflammation (19.7 +/- 0.9 lymphocytes/mm2 and 15.9 +/- 0.8 machophages/mm2), ANG II levels (436.9 +/- 52.29 pg/ml), cortical vasoconstriction, and stable blood pressure levels during the subsequent challenge with a high-salt diet. Acute administration of candesartan similarly reduced renal vasoconstriction and blood pressure. We conclude that the cortical vasoconstriction occurring with salt-sensitive hypertension following exposure to ANG II is mediated by intrarenal ANG II, related, at least in part, to the interstitial inflammation.

Insulin treatment enhances AT1receptor function in OK cells

Increased renal sodium retention is considered a major risk factor contributing to hypertension associated with chronic hyperinsulinemia and obesity. However, the molecular mechanism involved is not understood. The present study investigates the effect of insulin treatment on AT1receptor expression and ANG II-induced stimulation of Na/H exchanger (NHE) and Na-K-ATPase (NKA) in opossum kidney (OK) cells, a proximal tubule cell line. The presence of the AT1receptors in OK cells was confirmed by the specific binding of125I-sar-ANG II and by detecting ∼43-kDa protein on Western blot analysis with AT1receptor antibody and blocking peptide as well as by expression of AT1receptor mRNA as determined by RT-PCR. Insulin treatment (100 nM for 24 h) caused an increase in125I-sar-ANG II binding, AT1receptor protein content, and mRNA levels. The whole cell lysate and membrane showed similar insulin-induced increase in the AT1receptor protein expression, which was blocked by genistein (100 nM), a tyrosine kinase inhibitor, and cycloheximide (1.5 μg/ml), a protein synthesis inhibitor. Determination of ethyl isopropyl amiloride-sensitive22Na+uptake, a measure of the NHE activity, revealed that ANG II (1–100 pM)-induced stimulation of NHE in insulin-treated cells was significantly greater than in the control cells. Similarly, ANG II (1–100 pM)-induced stimulation of ouabain-sensitive86Rb+uptake, a measure of NKA activity in insulin-treated cells, was significantly greater than in the control cells. ANG II stimulation of both the transporters was blocked by AT1receptor antagonist losartan, suggesting the involvement of AT1receptors. Thus chronic insulin treatment causes upregulation of AT1receptors, which evoked ANG II-induced stimulation of NHE and NKA. We propose that insulin-induced increase in the renal AT1receptor function serves as a mechanism responsible for the increased renal sodium reabsorption and thus may contribute to development of hypertension in conditions associated with hyperinsulinemia.

Angiotensin II AT1receptor blockade decreases vasopressin-induced water reabsorption and AQP2 levels in NaCl-restricted rats

Vasopressin and ANG II, which are known to play a major role in renal water and sodium reabsorption, are mainly coupled to the cAMP/PKA and phosphoinositide pathways, respectively. There is evidence for cross talk between these intracellular signaling pathways. We therefore hypothesized that vasopressin-induced water reabsorption could be attenuated by ANG II AT1receptor blockade in rats. To address this, three protocols were used: 1) DDAVP treatment (20 ng/h sc for 7 days, n = 8); 2) DDAVP (20 ng/h sc for 7 days) and candesartan (1 mg·kg−1·day−1sc for 7 days) cotreatment ( n = 8); and 3) vehicle infusion as the control ( n = 8). All rats were maintained on a NaCl-deficient diet (0.1 meq Na+·200 g body wt−1·day−1) during the experiment. DDAVP treatment alone resulted in a significant decrease in urine output (3.1 ± 0.2 ml/day) compared with controls (11.5 ± 2.2 ml/day, P < 0.05), whereas the urine output was significantly increased in response to DDAVP and candesartan cotreatment (9.8 ± 1.0 ml/day, P < 0.05). Consistent with this, rats cotreated with DDAVP and candesartan demonstrated decreased urine osmolality (1,319 ± 172 mosmol/kgH2O) compared with rats treated with DDAVP alone (3,476 ± 182 mosmol/kgH2O, P < 0.05). Semiquantitative immunoblotting revealed significantly decreased expression of medullary aquaporin-2 (AQP2) and AQP2 phosphorylated in the PKA phosphorylation consensus site Ser-256 (p-AQP2) in response to DDAVP and candesartan cotreatment compared with DDAVP treatment alone. In addition, cortical and medullary AQP1 was also downregulated. Fractional sodium excretion (FENa) and plasma potassium levels were markedly increased, and the expressions of the cortical type 3 Na+/H+exchanger (NHE3), thiazide-sensitive Na-Cl cotransporter (NCC), and Na-K-ATPase were significantly decreased in response to DDAVP and candesartan cotreatment. Moreover, medullary type 1 bumetanide-sensitive Na-K-2Cl cotransporter expression showed a marked gel mobility shift from 160 to ∼180 kDa corresponding to enhanced glycosylation, whereas expression was unchanged. In conclusion, ANG II AT1receptor blockade in DDAVP-treated rats was associated with decreased urine concentration and decreased AQP2 and AQP1 expression. Moreover, FENawas increased in parallel with decreased expression of NHE3, NCC, and Na-K-ATPase. These results suggest that ANG II AT1receptor activation plays a significant role in regulating aquaporin and sodium transporter expression and modulating urine concentration in vivo.

Aortic blood flow subtraction: an alternative method for measuring total renal blood flow in conscious dogs

We have measured total renal blood flow (TRBF) as the difference between signals from ultrasound flow probes implanted around the aorta above and below the renal arteries. The repeatability of the method was investigated by repeated, continuous infusions of angiotensin II and endothelin-1 seven times over 8 wk in the same dog. Angiotensin II decreased TRBF (350 +/- 16 to 299 +/- 15 ml/min), an effect completely blocked by candesartan (TRBF 377 +/- 17 ml/min). Subsequent endothelin-1 infusion reduced TRBF to 268 +/- 20 ml/min. Bilateral carotid occlusion (8 sessions in 3 dogs) increased arterial blood pressure by 49% and decreased TRBF by 12%, providing an increase in renal vascular resistance of 69%. Dynamic analysis showed autoregulation of renal blood flow in the frequency range <0.06-0.07 Hz, with a peak in the transfer function at 0.03 Hz. It is concluded that continuous measurement of TRBF by aortic blood flow subtraction is a practical and reliable method that allows direct comparison of excretory function and renal blood flow from two kidneys. The method also allows direct comparison between TRBF and flow in the caudal aorta.

Volume expansion during acute angiotensin II receptor (AT(1)) blockade and NOS inhibition in conscious dogs

The responses to AT(1)-receptor blockade (candesartan 1 mg/kg) and to concomitant volume expansion (saline 35 ml/kg for 90 min) with and without nitric oxide synthase (NOS) inhibition (N(G)-nitro-L-arginine methyl ester 30 microg small middle dot kg(-1) small middle dot min(-1)) were investigated in separate experiments in normal dogs. AT(1) blockade decreased arterial pressure (106 +/- 4 to 96 +/- 5 mmHg) and increased glomerular filtration rate (GFR) by 17% and sodium excretion threefold. NOS inhibition increased arterial pressure (103 +/- 3 to 116 +/- 3 mmHg) and decreased GFR by 21% and reduced sodium excretion by some 80%. Volume expansion increased arterial pressure significantly in all series involving this procedure, most pronounced during combined AT(1) blockade and NOS inhibition (21 +/- 4 mmHg). Volume expansion during AT(1) blockade elicited marked natriuresis (26 +/- 11 to 274 +/- 55 micromol/min) that was severely reduced by concomitant NOS inhibition (10 +/- 3 to 45 +/- 11 micromol/min), but still much larger than that seen with volume expansion during NOS inhibition alone (2 +/- 1 to 23 +/- 7 micromol/min). Volume expansion during AT(1) blockade increased GFR (+30%), less so during combined AT(1) blockade and NOS inhibition (+13%), but it did not increase GFR significantly (P = 0.07) during NOS inhibition alone. Plasma ANG II increased greater than sevenfold with AT(1) blockade and doubled with NOS inhibition (paired t-test, P < 0.05), whereas it decreased by 50-80% during volume expansion irrespective of pretreatment, i.e., during NOS inhibition, volume expansion did not generate subnormal plasma ANG II concentrations. In conclusion, 1) acute AT(1) blockade leads to hyperfiltration, natriuresis, and hyperresponsiveness to volume expansion, 2) these responses are >85% inhibitable by unspecific NOS inhibition, and 3) NOS inhibition alone is followed by increases in plasma ANG II, hypofiltration, and severe antinatriuresis that may be counterbalanced but not overwhelmed by volume expansion. Thus NOS inhibition virtually abolishes the volume expansion natriuresis, at least in part, due to the lack of appropriate inhibition of the renin-angiotensin-aldosterone system.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Roles of ANG II and bradykinin in the renal regional blood flow responses to ACE inhibition in sodium-depleted dogs

The relative contributions of ANG II and bradykinin (BK) to the renal regional blood flow responses during angiotensin-converting enzyme (ACE) inhibition remain unclear. This study was performed to evaluate renal cortical (CBF) and medullary blood flow (MBF) responses to intrarterial administration of enalaprilat (33 microg. kg(-1). min (-1)) after blockade of the ANG II AT(1 )receptors with candesartan (100 microg) in 7 dogs fed a low-salt diet (0.01%) for 5 days. Laser-Doppler flowmetry was used to measure relative changes in CBF and MBF. Candesartan alone increased CBF (+20 +/- 2%) and MBF (+22 +/- 7%). Enalaprilat infusion after candesartan administration resulted in further increases in both CBF (+21 +/- 5%) and MBF (+41 +/- 8%). However, the relative changes in MBF were significantly greater (P < 0.01) than those in CBF. Administration of the BK B(2) receptor blocker icatibant (300 microg) after enalaprilat returned CBF and MBF to values seen with candesartan alone. These data support a substantive role for BK potentiation during ACE inhibitor-induced renal vasodilation in dogs maintained on a low-sodium diet, with a relatively greater effect on MBF compared to CBF.