Enhanced angiotensin II-induced activation of Na+, K+-ATPase in the proximal tubules of obese Zucker rats

Childhood Obesity: Insight into Kidney Involvement

This review examines the impact of childhood obesity on the kidney from an epidemiological, pathogenetic, clinical, and pathological perspective, with the aim of providing pediatricians and nephrologists with the most current data on this topic. The prevalence of childhood obesity and chronic kidney disease (CKD) is steadily increasing worldwide, reaching epidemic proportions. While the impact of obesity in children with CKD is less pronounced than in adults, recent studies suggest a similar trend in the child population. This is likely due to the significant association between obesity and the two leading causes of end-stage renal disease (ESRD): diabetes mellitus (DM) and hypertension. Obesity is a complex, systemic disease that reflects interactions between environmental and genetic factors. A key mechanism of kidney damage is related to metabolic syndrome and insulin resistance. Therefore, we can speculate about an adipose tissue-kidney axis in which neurohormonal and immunological mechanisms exacerbate complications resulting from obesity. Adipose tissue, now recognized as an endocrine organ, secretes cytokines called adipokines that may induce adaptive or maladaptive responses in renal cells, leading to kidney fibrosis. The impact of obesity on kidney transplant-related outcomes for both donors and recipients is also significant, making stringent preventive measures critical in the pre- and post-transplant phases. The challenge lies in identifying renal involvement as early as possible, as it is often completely asymptomatic and not detectable through common markers of kidney function. Ongoing research into innovative technologies, such as proteomics and metabolomics, aims to identify new biomarkers and is constantly evolving. Many aspects of pediatric disease progression in the population of children with obesity still require clarification. However, the latest scientific evidence in the field of nephrology offers glimpses into various new perspectives, such as genetic factors, comorbidities, and novel biomarkers. Investigating these aspects early could potentially improve the prognosis of these young patients through new diagnostic and therapeutic strategies. Hence, the aim of this review is to provide a comprehensive exploration of the pathogenetic mechanisms and prevalent pathological patterns of kidney damage observed in children with obesity.

The Effect of Treatment With Aminoguanidine, an Advanced Glycation End Product Inhibitor, on Streptozotocin-Induced Diabetic Rats and Its Effects on Physiological and Renal Functions

BACKGROUND/AIM

Diabetes is a multifactorial syndrome that affects the functioning of the renin-angiotensin system (RAS). The role of advanced glycation end products (AGEs) in diabetes is well known. In the present study, we hypothesized that the prevention of AGE accumulation or abrogation of AGE synthesis using an AGE inhibitor, aminoguanidine (AG), in streptozotocin (STZ)-induced diabetic animal models would affect the progression of diabetes and its related complications. We determined the effects of aminoguanidine (AG), an AGE inhibitor, in STZ-induced diabetic rats by determining various indices of RAS and renal functions. Additionally, we also investigated the effect of the drug, AG, on various hemodynamic and physiological functions in the body of the animals.

METHODS

Male Sprague Dawley rats weighing 200-250 g were assigned to four groups (n = 4-6): Vehicle, Vehicle+AG, STZ-induced, and STZ-induced+AG rats. Type 1 diabetes was induced by a single intraperitoneal (IP) injection of streptozotocin (55 mg/kg) dissolved in sodium citrate buffer. The control groups (Vehicle) were injected with buffer. The blood glucose levels were measured after 48 hours, and animals with blood glucose levels > 300 mg/dL were included in the study. Blood glucose levels in the vehicle rats were also determined to ensure non-diabetic conditions. After confirmation, AG was administrated at a dose of 1 g/L in drinking water for two weeks. Urine was collected to measure the glomerular filtration rate (GFR), and the immune reactivity for AT1 and AT2 proteins was analyzed by immunoblotting. Data were expressed as mean ± standard error of the mean (SEM), and a p-value < 0.05 was considered statistically significant.

RESULTS

Diabetic rats had a significant drop in body weight, accompanied by increased food and water consumption. The diabetic rats exhibited significantly increased urine flow and GFR. These phenotypes were significantly or considerately reversed by AG treatment in the STZ+AG-treated diabetic rats. Aminoguanidine prevented the increase in blood sugar levels compared to STZ-induced diabetic rats alone (295.9 ± 50.69 versus 462.3 ± 18.6 mg/dL (p < 0.05)). However, it did not affect the glomerular filtration rate (GFR) and glomerular damage, as assessed by the renal histopathological studies. The STZ-induced diabetic rats had an increased sodium excretion (3.24 ± 0.40 mmol) and significantly increased expression of the AT2 receptor and that of the AT1 receptor, which was slightly reversed by the treatment with AG. Treatment with AG decreased sodium excretion (2.12 ± 0.63, as compared to the diabetic rats). These rats also had modestly decreased expression of the AT2 receptor (0.99 ± 0.07 versus 1.12 ± 0.08, as compared to the STZ-induced diabetic rats), while the AT1 receptors showed a slight increase in the STZ+AG-treated rats compared to the STZ-induced diabetic rats (1.1 ± 0.19 versus 1.08 ± 0.12).

CONCLUSION

This study highlights the action of the drug AG in not exacerbating any damage in diabetic rats. Employing AG as a pharmacological intervention to prevent an increase in blood sugar adds a new dimension to controlling increased blood sugar and preventing diabetic complications. The employability and pharmacological intervention of the drug AG, in diabetes, therefore, need a renewed and further investigation.

Irisin lowers blood pressure in Zucker diabetic rats by regulating the functions of renal angiotensin II type 1 receptor via the inhibition of the NF-κB signaling pathway

BACKGROUND

Irisin, a novel myokine, has been identified to exert a series of favorable effects on metabolic diseases, including diabetes and obesity. This study aimed to explore the effects of chronic irisin administration on blood pressure and the related underlying mechanisms in Zucker diabetic fatty (ZDF) rats.

METHODS AND RESULTS

Male ZDF rats and Zucker lean (ZL) rats received a continuous subcutaneous infusion of irisin or saline for 4 weeks. Compared with ZL counterparts, ZDF rats reported higher systolic blood pressure (SBP), severer renal inflammation, increased oxidative stress, and impaired natriuresis and diuresis; they also had an elevated AT1R expression in renal cortex and augmented candesartan-induced natriuresis and diuresis. The irisin administration lowered SBP, improved diuretic and natriuretic effects, and reduced renal inflammation and oxidative stress in ZDF rats, along with decreased renal expression of AT1R and restored candesartan-mediated natriuresis and diuresis. Further experiments showed that irisin inhibited the translocation of NF-κB from the cytosol to the nucleus and the activation of NF-κB signaling pathway, which may contribute to the reduced AT1R expression and function.

CONCLUSIONS

Irisin administration serves an anti-hypertensive role in ZDF rats by alleviating renal inflammation and oxidative stress, reducing the expression and impact of AT1R, and restoring natriuresis and diuresis. The underlying mechanism may involve the irisin-induced inhibition of the NF-κB signaling pathway.

Obesity and chronic kidney disease: prevalence, mechanism, and management

The prevalence of childhood obesity is increasing worldwide at an alarming rate. While obesity is known to increase a variety of cardiovascular and metabolic diseases, it also acts as a risk factor for the development and progression of chronic kidney disease (CKD). During childhood and adolescence, severe obesity is associated with an increased prevalence and incidence of the early stages of kidney disease. Importantly, children born to obese mothers are also at increased risk of developing obesity and CKD later in life. The potential mechanisms underlying the association between obesity and CKD include hemodynamic factors, metabolic effects, and lipid nephrotoxicity. Weight reduction via increased physical activity, caloric restriction, treatment with angiotensin-converting enzyme inhibitors, and judicious bariatric surgery can be used to control obesity and obesity-related kidney disease. Preventive strategies to halt the obesity epidemic in the healthcare community are needed to reduce the widespread deleterious consequences of obesity including CKD development and progression.

Effects of Nitric Oxide on Renal Proximal Tubular Na+ Transport

Nitric oxide (NO) has a wide variety of physiological functions in the kidney. Besides the regulatory effects in intrarenal haemodynamics and glomerular microcirculation, in vivo studies reported the diuretic and natriuretic effects of NO. However, opposite results showing the stimulatory effect of NO on Na⁺ reabsorption in the proximal tubule led to an intense debate on its physiological roles. Animal studies have showed the biphasic effect of angiotensin II (Ang II) and the overall inhibitory effect of NO on the activity of proximal tubular Na⁺ transporters, the apical Na⁺/H⁺ exchanger isoform 3, basolateral Na⁺/K⁺ ATPase, and the Na⁺/HCO₃⁻ cotransporter. However, whether these effects could be reproduced in humans remained unclear. Notably, our recent functional analysis of isolated proximal tubules demonstrated that Ang II dose-dependently stimulated human proximal tubular Na⁺ transport through the NO/guanosine 3′,5′-cyclic monophosphate (cGMP) pathway, confirming the human-specific regulation of proximal tubular transport via NO and Ang II. Of particular importance for this newly identified pathway is its possibility of being a human-specific therapeutic target for hypertension. In this review, we focus on NO-mediated regulation of proximal tubular Na⁺ transport, with emphasis on the interaction with individual Na⁺ transporters and the crosstalk with Ang II signalling.

Superoxide increases angiotensin II AT1 receptor function in human kidney-2 cells

The redox-sensitive Sp family transcription factor has been linked to the regulation of angiotensin II type 1 receptor (AT1R). However, the exact mechanism of AT1R regulation in renal cells is poorly understood. We tested the specificity of reactive oxygen species (ROS), superoxide vs. hydrogen peroxide (H₂O₂), and the specific role of Sp3 transcription factor, if any, in the regulation of AT1R in human kidney cells (HK2 cells). Superoxide dismutase (SOD) inhibitor diethyldithiocarbamate (DETC), but not H₂O₂ treatment, increased fluorescence levels of superoxide probe dihydroethidium (DHE). H₂O_2, but not DETC, treatment increased the fluorescence of the H₂O₂-sensitive probe dichloro-dihydro-fluorescein (DCFH). These data suggest that SOD inhibition by DETC increases the superoxide but not H₂O₂ and exogenously added H₂O₂ is not converted to superoxide in renal cells. Furthermore, DETC, but not H₂O₂, treatment increased nuclear accumulation of Sp3, which was attenuated with the superoxide dismutase (SOD)-mimetic tempol. DETC treatment also increased AT1R mRNA and protein levels that were attenuated with tempol, whereas H₂O₂ did not have any effects on AT1R mRNA. Moreover, Sp3 overexpression increased, while Sp3 depletion by siRNA decreased, protein levels of AT1R. In addition, Sp3 siRNA in the presence of DETC decreased AT1R protein expression. Furthermore, DETC treatment increased the levels of cell surface AT1R as measured by biotinylation and immunofluorescence studies. Angiotensin II increased PKC activity in vehicle-treated cells that further increased in DETC-treated cells, which was attenuated by AT1R blocker candesartan and SOD-mimetic tempol. Taken together, our results suggest that superoxide, but not H₂O₂, via Sp3 up-regulates AT1R expression and function in the renal cells.

Obesity-related glomerulopathy: clinical and pathologic characteristics and pathogenesis

The prevalence of obesity-related glomerulopathy is increasing in parallel with the worldwide obesity epidemic. Glomerular hypertrophy and adaptive focal segmental glomerulosclerosis define the condition pathologically. The glomerulus enlarges in response to obesity-induced increases in glomerular filtration rate, renal plasma flow, filtration fraction and tubular sodium reabsorption. Normal insulin/phosphatidylinositol 3-kinase/Akt and mTOR signalling are critical for podocyte hypertrophy and adaptation. Adipokines and ectopic lipid accumulation in the kidney promote insulin resistance of podocytes and maladaptive responses to cope with the mechanical forces of renal hyperfiltration. Although most patients have stable or slowly progressive proteinuria, up to one-third develop progressive renal failure and end-stage renal disease. Renin-angiotensin-aldosterone blockade is effective in the short-term but weight loss by hypocaloric diet or bariatric surgery has induced more consistent and dramatic antiproteinuric effects and reversal of hyperfiltration. Altered fatty acid and cholesterol metabolism are increasingly recognized as key mediators of renal lipid accumulation, inflammation, oxidative stress and fibrosis. Newer therapies directed to lipid metabolism, including SREBP antagonists, PPARα agonists, FXR and TGR5 agonists, and LXR agonists, hold therapeutic promise.

Mechanisms mediating renal sympathetic nerve activation in obesity-related hypertension

Excessive renal sympathetic nerve activation may be one of the mechanisms underlying obesity-related hypertension. Impaired baroreflex sensitivity, adipokine disorders-such as leptin, adiponectin, and resistin-activation of the renin-angiotensin system, hyperinsulinemia, insulin resistance, and renal sodium retention present in obesity increase renal sympathetic nerve activity, thus contributing to the development of hypertension. Renal sympathetic denervation reduces both renal sympathetic activity and blood pressure in patients with obesity-related hypertension.

Antioxidant resveratrol restores renal sodium transport regulation in SHR

Previously we have shown that in spontaneously hypertensive rats (SHR) renal angiotensin (Ang) II receptor (AT1R) upregulation leads to overstimulation of Na/K-ATPase by Ang II. There are reports that antioxidants can reduce oxidative stress and blood pressure (BP) in SHR, however the effect of these compounds on AT1R function remains to be determined. Therefore, we hypothesized that polyphenol antioxidant resveratrol would mitigate oxidative stress, normalize renal AT1R signaling, and reduce BP in SHR. SHR and wistar-kyoto (WKY) rats were treated with resveratrol for 8 weeks. Untreated SHR exhibited oxidative stress and enhanced renal proximal tubular Ang II-induced G-protein activation and Na/K-ATPase stimulation. Treatment of SHR with resveratrol mitigated oxidative stress, reduced BP, and normalized renal AT1R signaling. In SHR, nuclear expression of transcription factor NF-κB was increased while expression of Nrf2 was reduced. SHR also exhibited a significant decrease in renal antioxidant capacity and activities of phase II antioxidant enzymes. Resveratrol treatment of SHR abolished renal NF-κB activation, restored Nrf2-phase II antioxidant signaling and Ang II-mediated Na/K-ATPase regulation. These data show that in SHR, oxidative stress via activation of NF-κB upregulates AT1R–G-protein signaling resulting in overstimulation Na/K-ATPase which contributes to hypertension. Resveratrol, via Nrf2, activates phase II antioxidant enzymes, mitigates oxidative stress, normalizes AT1R–G-protein signaling and Na/K-ATPase regulation, and decreases BP in SHR.

Chronic NF-κB blockade improves renal angiotensin II type 1 receptor functions and reduces blood pressure in Zucker diabetic rats

Background

Both angiotensin II type 1 receptor (AT₁R) and nuclear factor-kappa B (NF-κB) play significant roles in the pathogenesis of hypertension and type 2 diabetes. However, the role of NF-κB in perpetuating renal AT₁ receptors dysfunction remains unclear. The aim of the present study to determine whether blockade of NF-κB, could reverse the exaggerated renal AT₁R function, reduce inflammatory state and oxidative stress, lower blood pressure in Zucker diabetic fatty (ZDF) rats.

Methods

Pyrrolidine dithiocarbamate (PDTC), a NF-κB inhibitor (150 mg/kg in drinking water)or vehicle was administered orally to 12-weeks-old ZDF rats and their respective control lean Zucker (LZ) rats for 4 weeks. Blood pressure was measured weekly by tail-cuff method. AT₁R functions were determined by measuring diuretic and natriuretic responses to AT₁R antagonist (candesartan; 10 μg/kg/min iv). The mRNA and protein levels of NF-κB, oxidative stress maker and AT₁R were determined using quantitative real-time PCR and Western blotting, respectively. The NF-κB-DNA binding activity in renal cortex was measured by Electrophoretic mobility shift assay (EMSA).

Results

As compared with LZ rats, ZDF rats had higher blood pressure, impaired natriuresis and diuresis, accompanied with higher levels of oxidative stress and inflammation. Furthermore, AT₁R expression was higher in renal cortex from ZDF rats; candesartan induced natriresis and diuresis, which was augmented in ZDF rats. Treatment with PDTC lowered blood pressure and improved diuretic and natriuretic effects in ZDF rats; meanwhile, the increased oxidative stress and inflammation were reduced; the increased AT₁R expression and augmented candesartan-mediated natriuresis and diuresis were recoverd in ZDF rats. Our further study investigated the mechanisms of PDTC on AT₁R receptor expression. It resulted that PDTC inhibited NF-κB translocation from cytosol to nucleus, inhibited binding of NF-κB with AT₁R promoter, therefore, reduced AT₁R expression and function.

Conclusions

Our present study indicates blockade of NF-κB, via inhibition of binding of NF-κB with AT₁R promoter, reduces renal AT₁R expression and function, improves oxidative stress and inflammatory/anti-inflammatory balance, therefore, lowers blood pressure and recovers renal function in ZDF rats.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-015-0239-7) contains supplementary material, which is available to authorized users.

Oxidative stress causes imbalance of renal renin angiotensin system (RAS) components and hypertension in obese Zucker rats

Background

Oxidative stress plays an important role in the pathogenesis of hypertension, especially in obesity‐related hypertension. The natriuretic and antinatriuretic components of the renal renin angiotensin system (RAS) maintain sodium homeostasis and blood pressure. Here, we test the hypothesis that increased oxidative stress leads to the imbalance of RAS components and hypertension in obese Zucker rats.

Methods and Results

Lean and obese rats received vehicle or tempol, a superoxide dismutase mimetic in the drinking water for 4 weeks. Compared with vehicle‐treated lean rats, vehicle‐treated obese rats exhibited higher blood pressure and increased renal oxidative stress, accompanied by increased diuretic and natriuretic responses to AT₁R antagonist (Candesartan) and AT₂R agonist (CGP‐42112A) and reduced diuretic and natriuretic response to MasR agonist (Ang‐[1 to 7]). Moreover, obese rats had higher ACE, AT₁R and AT₂R, lower ACE2 and MasR expressions in the kidney. All of the above‐mentioned abnormalities were reversed to some degree by tempol treatment. In primary cultures of renal proximal tubular (RPT) cells from lean and obese rats, tempol treatment also increased AT₂R, ACE2, and MasR expressions but decreased AT₁R and ACE expressions in obese rats.

Conclusions

Taken together, our study indicated that the imbalance of renal RAS components was associated with increased oxidative stress in obese rats. Furthermore, antioxidant treatment with tempol reversed the imbalance of renal RAS components and led to diuresis and natriuresis, which, at least in part, explains the blood pressure‐lowering effect of antioxidant supplementation in obesity‐related hypertension.

Modulation of angiotensin II-induced inflammatory cytokines by the Epac1-Rap1A-NHE3 pathway: implications in renal tubular pathobiology

Besides the glomerulus, the tubulointerstitium is often concomitantly affected in certain diseases, e.g., diabetic nephropathy, and activation of the renin-angiotensin system, to a certain extent, worsens its outcome because of perturbations in hemodynamics and possibly tubuloglomerular feedback. Certain studies suggest that pathobiology of the tubulointerstitium is influenced by small GTPases, e.g., Rap1. We investigated the effect of ANG II on inflammatory cytokines, while at the same time focusing on upstream effector of Rap1, i.e., Epac1, and some of the downstream tubular transport molecules, i.e., Na/H exchanger 3 (NHE3). ANG II treatment of LLC-PK1 cells decreased Rap1a GTPase activity in a time- and dose-dependent manner. ANG II treatment led to an increased membrane translocation of NHE3, which was reduced with Epac1 and PKA activators. ANG II-induced NHE3 translocation was notably reduced with the transfection of Rap1a dominant positive mutants, i.e., Rap1a-G12V or Rap1a-T35A. Transfection of cells with dominant negative Rap1a mutants, i.e., Rap1a-S17A, or Epac1 mutant, i.e., EPAC-ΔcAMP, normalized ANG II-induced translocation of NHE3. In addition, ANG II treatment led to an increased expression of inflammatory cytokines, i.e., IL-1β, IL-6, IL-8, and TNF-α, which was reduced with Rap1a-G12V or Rap1a-T35A transfection, while it reverted to previous comparable levels following transfection of Rap1a-S17A or EPAC-ΔcAMP. ANG II-induced expression of cytokines was reduced with the treatment with NHE3 inhibitor S3226 or with Epac1 and PKA activators. These data suggest that this novel Epac1-Rap1a-NHE3 pathway conceivably modulates ANG II-induced expression of inflammatory cytokines, and this information may yield the impetus for developing strategies to reduce tubulointertstitial inflammation in various renal diseases.

Chronic AT2 receptor activation increases renal ACE2 activity, attenuates AT1 receptor function and blood pressure in obese Zucker rats

Abnormal regulation of the renin angiotensin system such as enhanced renal AT1R function and reduced ACE2 activity contributes to obesity-related hypertension. Here we tested whether long-term AT2R activation affects renal function in obesity using lean and obese Zucker rats treated with the AT2R agonist CGP42112A for 2-weeks. This caused blood pressure to decrease by 13 mmHg which was associated with increased urinary sodium excretion in the obese rats. Cortical ACE2 expression and activity, the Mas receptor (MasR), and its ligand angiotensin-(1-7) were all increased in CGP-treated obese compared with control rats. Candesartan-induced natriuresis, a measure of AT1R function, was reduced but cortical AT1R expression and angiotensin II levels were similar in CGP-treated obese compared to control rats. Renin and AT2R expression in obese rats was not affected by CGP-treatment. In HK-2 cells in-vitro, CGP-treatment caused increased ACE2 activity and MasR levels but decreased AT1R levels and renin activity. Thus, long-term AT2R activation shifts the opposing arms of renin angiotensin system and contributes to natriuresis and blood pressure reduction in obese animals. Our study highlights the importance of AT2R as a target for treating obesity related hypertension.

Blood pressure response to angiotensin II is enhanced in obese Zucker rats and is attributed to an aldosterone-dependent mechanism

BACKGROUND AND PURPOSE

Plasma aldosterone levels correlate positively with obesity, suggesting a link between the hypertension associated with obesity and increased mineralocorticoid levels. We tested the hypothesis that aldosterone is involved in the BP response to angiotensin II (AngII) in obese rats.

EXPERIMENTAL APPROACH

Lean (LZR) and obese (OZR) Zucker rats were treated with AngII (9 µg·h(-1) ; 4 weeks), and BP and plasma AngII and aldosterone were determined.

KEY RESULTS

Chronic AngII increased the BP in OZR markedly more so than in LZR. Plasma AngII levels in LZR and OZR were similar after AngII treatment. The AngII stimulated a rise in plasma aldosterone that was sixfold more in OZR than in LZR. The thickness of the zona glomerulosa of the adrenal glands was selectively increased by AngII in OZR. Adrenal mRNA levels of CYP11B2 aldosterone synthase and the AT(1B) receptor were selectively increased in AngII-treated OZR. The BP response to chronic AngII stimulation was diminished in OZR after adrenalectomy when plasma aldosterone was absent. Acute bolus injections of AngII did not increase the BP response or aldosterone release in OZR.

CONCLUSIONS AND IMPLICATIONS

The AngII-induced BP response is enhanced in obesity and this is associated with a specific increase in circulating aldosterone. Due to the AngII-induced growth of the zona glomerulosa in OZR, the AT(1B) receptors and aldosterone synthase may be selectively enhanced in obesity under concomitant AngII stimulation, increasing the adrenal synthesis of aldosterone. Our results confirm functionally that aldosterone plays a major role in obesity-related hypertension.

Altered functioning of both renal dopamine D1 and angiotensin II type 1 receptors causes hypertension in old rats

Activation of renal dopamine D1 (D1R) and angiotensin II type 1 receptors (AT(1)Rs) influences the activity of proximal tubular sodium transporter Na,K-ATPase and maintains sodium homeostasis and blood pressure. We reported recently that diminished D1R and exaggerated AT(1)R functions are associated with hypertension in old Fischer 344 × Brown Norway F1 (FBN) rats, and oxidative stress plays a central role in this phenomenon. Here we studied the mechanisms of age-associated increase in oxidative stress on diminished D1R and exaggerated AT(1)R functions in the renal proximal tubules of control and antioxidant Tempol-treated adult and old FBN rats. Although D1R numbers and D1R agonist SKF38393-mediated stimulation of [(35)S]-GTPγS binding (index of D1R activation) were lower, G protein-coupled receptor kinase 4 (kinase that uncouples D1R) levels were higher in old FBN rats. Tempol treatment restored D1R numbers and G protein coupling and reduced G protein-coupled receptor kinase 4 levels in old FBN rats. Angiotensin II-mediated stimulation of [(35)S]-GTPγS binding and Na,K-ATPase activity were higher in old FBN rats, which were also restored with Tempol treatment. We also measured renal AT(1)R function in adult and old Fischer 344 (F344) rats, which, despite exhibiting an age-related increase in oxidative stress and diminished renal D1R function, are normotensive. We found that diuretic and natriuretic responses to candesartan (indices of AT(1)R function) were similar in F344 rats, a likely explanation for the absence of age-associated hypertension in these rats. Perhaps, alterations in both D1R (diminished) and AT(1)R (exaggerated) functions are necessary for the development of age-associated hypertension, as seen in old FBN rats.

AT2 receptor non-peptide agonist C21 promotes natriuresis in obese Zucker rats

Previously, we demonstrated that angiotensin II type 2 (AT(2)) receptors have a role in natriuresis in obese Zucker rats (OZR). In the present study, we investigated the role of a novel, non-peptide agonist, C21, in natriuresis via AT(2) receptor activation in OZR. Infusion of C21 (1 and 5 μg kg(-1) min(-1)) into rats under anesthesia caused a dose-dependent increase in urine flow (UF) and urinary Na volume (U(Na)V). These effects of C21 were blocked by pre-infusion of the AT(2) receptor antagonist, PD123319, (50 μg kg(-1) min(-1)), suggesting involvement of the AT(2) receptor. Infusion of C21 (5 μg kg(-1) min(-1)) significantly increased the fractional excretion of sodium without changing the glomerular filtration rate or blood pressure, suggesting a tubular effect. Similarly, C21 infusion increased the fractional excretion of lithium, suggesting a proximal tubular effect. Furthermore, we tested the effect of C21 on natriuresis after blocking two main, distal-nephron Na transporters, the epithelial Na channels (ENaC), with amiloride (AM, 3 mg kg(-1) body wt), and the NaCl cotransporters (NCC), with bendroflumethiazide (BFTZ, 7 mg kg(-1) body wt). Infusion of AM + BFTZ caused significant increases in both diuresis and natriuresis, which were further increased by infusion of C21 (5 μg kg(-1) min(-1)). Natriuresis in response to C21 was associated with increases in urinary NO and cGMP levels. The data indicate that the AT(2) receptor agonist, C21, promotes natriuresis via AT(2) receptor activation and that this effect is potentially based in the proximal tubules and linked to the nitric oxide/cyclic guanosine monophosphate pathway. The natriuretic response to C21 may have therapeutic significance by improving kidney function in obesity.

D3 dopamine receptor regulation of D5 receptor expression and function in renal proximal tubule cells

Dopamine receptor, via D(1)-like and D(2)-like receptors, increases sodium excretion in kidney. We have reported positive interactions between D(3) and D(1) receptors in renal proximal tubule (RPT) cells. These reports, however do not preclude that there may be also interaction between D(3) and D(5) receptors, because of the lack of selective D(1) and D(5) receptor agonists or antagonists. We hypothesize that D(3) receptors can regulate D(5) receptors, and that D(3) receptor regulation of D(5) receptors in RPTs is impaired in spontaneously hypertensive rats (SHRs). It showed that a D(3) receptor agonist, PD128907, by the activation of protein kinase C activity, increased the expression of D(5) receptors in a concentration- and time-dependent manner in RPT cells from Wistar-Kyoto (WKY) rats. The stimulatory effect of the D(3) receptor on D(5) receptor expression was impaired in RPT cells from SHRs. The effect of D(3) receptor on D(5) receptor is functionally relevant; stimulation of D(5) receptor decreases Na(+)-K(+) adenosine triphosphatase (ATPase) activity in WKY cells. Pretreatment with D(3) receptor agonist for 24 h enhances the D(5) receptor expression and D(5) receptor-mediated inhibitory effect on Na(+)-K(+) ATPase activity in WKY cells, but decreases them in SHR cells. The effect of D(3) receptor on D(5) receptor expression and function was also confirmed in the D(5) receptor-transfected HEK293 cells. It indicates that activation of D(3) receptor increases D(5) receptor expression and function. Altered regulation of D(3) receptor on D(5) receptors may have a role in the pathogenesis of hypertension.

Leptin and the Regulation of Renal Sodium Handling and Renal Na-Transporting ATPases: Role in the Pathogenesis of Arterial Hypertension

Leptin, an adipose tissue hormone which regulates food intake, is also involved in the pathogenesis of arterial hypertension. Plasma leptin concentration is increased in obese individuals. Chronic leptin administration or transgenic overexpression increases blood pressure in experimental animals, and some studies indicate that plasma leptin is elevated in hypertensive subjects independently of body weight. Leptin has a dose- and time-dependent effect on urinary sodium excretion. High doses of leptin increase Na(+) excretion in the short run; partially by decreasing renal Na(+),K(+)-ATPase (sodium pump) activity. This effect is mediated by phosphatidylinositol 3-kinase (PI3K) and is impaired in animals with dietary-induced obesity. In contrast to acute, chronic elevation of plasma leptin to the level observed in patients with the metabolic syndrome impairs renal Na(+) excretion, which is associated with the increase in renal Na(+),K(+)-ATPase activity. This effect results from oxidative stress-induced deficiency of nitric oxide and/or transactivation of epidermal growth factor receptor and subsequent stimulation of extracellular signal-regulated kinases. Ameliorating "renal leptin resistance" or reducing leptin level and/or leptin signaling in states of chronic hyperleptinemia may be a novel strategy for the treatment of arterial hypertension associated with the metabolic syndrome.

The paradox of dopamine and angiotensin II-mediated Na(+), K(+)-ATPase regulation in renal proximal tubules

Accumulated studies reported that the natruretic dopamine (DA) and the anti-natruretic angiotensin II (Ang II) represent an important mechanism to regulate renal Na(+) and water excretion through intracellular secondary messengers to inhibit or activate renal proximal tubule (PT) Na(+), K(+)-ATPase (NKA). The antagonistic actions were mediated by the phosphorylation of different position of NKA α₁-subunit and different Pals-associated tight junction protein (PATJ) PDZ domains, the different protein kinase C (PKC) isoforms (PKC-β, PKC-ζ), the common adenylyl cyclase (AC) pathway, and the crosstalk and balance between DA and Ang II to NKA regulation. Besides, Ang II-mediated NKA modulation has bi-phasic effects.

Impaired stimulatory effect of ETB receptor on D₃ receptor in immortalized renal proximal tubule cells of spontaneously hypertensive rats

BACKGROUND

Activation of renal D₃ receptor induces natriuresis and diuresis in Wistar-Kyoto (WKY) rats; in the presence of ETB receptor antagonist, the natriuretic effect of D₃ receptor in WKY rats is reduced. We hypothesize that ETB receptor activation may regulate D₃ receptor expression in renal proximal tubule (RPT) cells from WKY rats, which is impaired in RPT cells from spontaneously hypertensive rats (SHRs).

METHODS

D₃ receptor expression was determined by immunoblotting; the D₃/ETB receptor linkage was checked by coimmunoprecipitation; Na(+)-K(+)-ATPase activity was determined as the rate of inorganic phosphate released in the presence or absence of ouabain.

RESULTS

In RPT cells from WKY rats, the ETB receptor agonist BQ3020 increased D₃ receptor protein. In contrast, in RPT cells from SHRs, BQ3020 did not increase D₃ receptor. There was coimmunoprecipitation between D₃ and ETB receptors in RPT cells from WKY and SHRs. Activation of ETB receptor increased D₃/ETB coimmunoprecipitation in RPT cells from WKY rats, but not from SHRs. The basal levels of D₃/ETB receptor coimmunoprecipitation were greater in RPT cells from WKY rats than in those from SHRs. Stimulation of D₃ receptor inhibited Na(+)-K(+)-ATPase activity, which was augmented by the pretreatment with the ETB receptor agonist BQ3020 in WKY RPT cells, but not in SHR RPT cells.

CONCLUSION

ETB receptors regulate and physically interact with D₃ receptors differently in WKY rats and SHRs. The impaired natriuretic effect in SHRs may be, in part, related to impaired ETB and D₃ receptor interactions.

Oxidative stress alters renal D1 and AT1 receptor functions and increases blood pressure in old rats

Aging is associated with an increase in oxidative stress and blood pressure (BP). Renal dopamine D1 (D1R) and angiotensin II AT1 (AT1R) receptors maintain sodium homeostasis and BP. We hypothesized that age-associated increase in oxidative stress causes altered D1R and AT1R functions and high BP in aging. To test this, adult (3 mo) and old (21 mo) Fischer 344 × Brown Norway F1 rats were supplemented without/with antioxidant tempol followed by determining oxidative stress markers (urinary antioxidant capacity, proximal tubular NADPH-gp91phox, and plasma 8-isoprostane), D1R and AT1R functions, and BP. The D1R and AT1R functions were determined by measuring diuretic and natriuretic responses to D1R agonist (SKF-38393; 1 μg·kg(-1)·min(-1) iv) and AT1R antagonist (candesartan; 10 μg/kg iv), respectively. We found that the total urinary antioxidant capacity was lower in old rats, which increased with tempol treatment. In addition, tempol decreased the elevated NADPH-gp91phox and 8-isoprostane levels in old rats. Systolic, diastolic, and mean arterial BPs were higher in old rats and were reduced by tempol. Although SKF-38393 produced diuresis in both adult and old rats, urinary sodium excretion (UNaV) increased only in adult rats. While candesartan increased diuresis and UNaV in adult and old rats, the magnitude of response was greater in old rats. Tempol treatment in old rats reduced candesartan-induced increase in diuresis and UNaV. Our results demonstrate that diminished renal D1R and exaggerated AT1R functions are associated with high BP in old rats. Furthermore, oxidative stress may cause altered renal D1R and AT1R functions and high BP in old rats.

Inhibition of NAD(P)H oxidase potentiates AT2 receptor agonist-induced natriuresis in Sprague-Dawley rats

A positive association between renin-angiotensin system, especially AT1 receptor, and oxidative stress in the pathogenesis of hypertension and cardiovascular/renal diseases has been suggested. However, the role of oxidative stress, especially superoxide radicals in renal sodium handling in response to AT1 and AT2 receptors, is not known. Therefore, the present study was designed to investigate the role of NAD(P)H oxidase (NOX), a major superoxide radical producing enzyme, in AT1 and AT2 receptor function on natriuresis/diuresis in Sprague-Dawley rats. The rats under anesthesia were intravenously infused with NOX inhibitor apocynin (3.5 μg·kg(-1)·min(-1)), the AT1 receptor antagonist candesartan (100 μg/kg; bolus), and the AT2 receptor agonist CGP-42112A (1 μg·kg(-1)·min(-1)) alone and in combinations. Candesartan alone significantly increased urinary flow (UF; μl/30 min) by 53 and urinary Na excretion (U(Na)V; μmol/min) by 0.4 over basal. Preinfusion of apocynin had no effect on the net increase in UF or U(Na)V in response to candesartan. On the other hand, apocynin preinfusion caused profound increases in CGP-42112A-induced UF by 72, U(Na)V by 1.14, and fractional excretion of Na by 7.8. Apocynin and CGP-42112A alone did not cause significant increase in UF or U(Na)V over the basal. CGP-42112A infusion in the presence of apocynin increased urinary nitrite/nitrates and cGMP over basal. The infusion of candesartan, apocynin, and CGP-42112A alone or in combinations had no effect on the blood pressure or the glomerular filtration rate, suggesting tubular effects on natriuresis/diuresis. The data suggest that NOX may have an antagonistic role in AT2 receptor-mediated natriuresis/diuresis possibly via neutralizing nitric oxide and thereby influence fluid-Na homeostasis.

Chronic hyperleptinemia induces resistance to acute natriuretic and NO-mimetic effects of leptin

Apart from controlling energy balance, leptin, secreted by adipose tissue, is also involved in the regulation of cardiovascular function. Previous studies have demonstrated that acutely administered leptin stimulates natriuresis and vascular nitric oxide (NO) production and that these effects are impaired in obese animals. However, the mechanism of resistance to leptin is not clear. Because obesity is associated with chronically elevated leptin, we examined if long-term hyperleptinemia impairs acute effects of leptin on sodium excretion and NO production in the absence of obesity. Hyperleptinemia was induced in lean rats by administration of exogenous leptin at a dose of 0.5mg/kg/day for 7 days, and then acute effect of leptin (1mg/kg i.v.) was studied under general anesthesia. Leptin increased fractional sodium excretion and decreased Na(+),K(+)-ATPase activity in the renal medulla. In addition, leptin increased the level of NO metabolites and cyclic GMP in plasma and aortic wall. These acute effects of leptin were impaired in hyperleptinemic animals. In both control and hyperleptinemic groups the effect of leptin on Na(+) excretion and renal Na(+),K(+)-ATPase was abolished by phosphoinositide 3-kinase (PI3K) inhibitor, wortmannin, but not by protein kinase B/Akt inhibitor, triciribine,. In contrast, acute effect of leptin on NO metabolites and cGMP was abolished by triciribine but not by wortmannin. Leptin stimulated Akt phosphorylation at Ser(473) in aortic tissue but not in the kidney, and this effect was comparable in control and hyperleptinemic groups. These results suggest that hyperleptinemia may mediate "renal" and "vascular" leptin resistance observed in obesity.

D3 dopamine receptor regulation of ETB receptors in renal proximal tubule cells from WKY and SHRs

BACKGROUND

The dopaminergic and endothelin systems, by regulating sodium transport in the renal proximal tubule (RPT), participate in the control of blood pressure. The D(3) and ETB receptors are expressed in RPTs, and D(3) receptor function in RPTs is impaired in spontaneously hypertensive rats (SHRs). Therefore, we tested the hypothesis that D(3) receptors can regulate ETB receptors, and that D(3) receptor regulation of ETB receptors in RPTs is impaired in SHRs.

METHODS

ETB receptor expression in RPT cells was measured by immunoblotting and reverse transcriptase-PCR and ETB receptor function by measuring Na(+)-K(+) ATPase activity. D(3)/ETB receptor interaction was studied by co-immunoprecipitation.

RESULTS

In Wistar-Kyoto (WKY) RPT cells, the D(3) receptor agonist, PD128907, increased ETB receptor protein expression, effects that were blocked by removal of calcium in the culture medium. The stimulatory effect of D(3) on ETB receptor mRNA and protein expression was also blocked by nicardipine. In contrast, in SHR RPT cells, PD128907 decreased ETB receptor expression. Basal D(3)/ETB receptor co-immunoprecipitation was three times greater in WKY than in SHRs. The absolute amount of D(3)/ETB receptor co-immunoprecipitation induced by a D(3) receptor agonist was also greater in WKY than in SHRs. Stimulation of ETB receptors decreased Na(+)-K(+) ATPase activity in WKY but not in SHR cells. Pretreatment with PD128907 augmented the inhibitory effect of BQ3020 on Na(+)-K(+) ATPase activity in WKY but not in SHR cells.

CONCLUSIONS

D(3) receptors regulate ETB receptors by physical receptor interaction and govern receptor expression and function. D(3) receptor regulation of ETB receptors is aberrant in RPT cells from SHRs.

Protective role of angiotensin II subtype 2 receptor in blood pressure increase in obese Zucker rats

Earlier, we reported that there was an increase in angiotensin II type 2 (AT(2)) receptor expression in the renal proximal tubule, and selective activation of the AT(2) receptor by AT(2) agonist inhibits Na,K-ATPase activity in the proximal tubules and increases urinary Na excretion in obese Zucker rats. We hypothesized that the AT(2) receptor has a protective role against blood pressure increase in obese Zucker rats. To test this hypothesis, we treated obese Zucker rats with the AT(2) receptor antagonist PD123319 (PD; 30 microg/kg per minute) using osmotic pumps. Age-matched lean rats and vehicle-treated obese Zucker rats served as controls. On day 15 of the treatment with PD, arterial blood pressure was measured by cannulation of the left carotid artery under anesthesia. Control obese rats exhibited higher mean arterial pressure (122.0+/-3.4 mm Hg) compared with lean control rats (97.0+/-4.8 mm Hg). The PD treatment of obese rats raised mean arterial pressure further by 13 mm Hg. The plasma renin activity was significantly increased in the PD-treated obese compared with control-obese or lean rats. Western blot analysis revealed that the PD treatment in obese rats caused an approximately 3-fold increase in the renin expression in the kidney cortex but had no effect on the expression of the cortical angiotensin II type 1 and AT(2) receptors. The present study suggests that the renal AT(2) receptors provide a protective role against blood pressure increase in obese Zucker rats, and this protective effect, in part, could be because of the ability of the AT(2) receptors to keep the kidney renin expression low in obese rats.

Loss of biphasic effect on Na/K-ATPase activity by angiotensin II involves defective angiotensin type 1 receptor-nitric oxide signaling

Oxidative stress causes changes in angiotensin (Ang) type 1 receptor (AT1R) function, which contributes to hypertension. Ang II affects blood pressure via maintenance of sodium homeostasis by regulating renal Na(+) absorption through its effects on Na/K-ATPase (NKA). At low concentrations, Ang II stimulates NKA; higher concentrations inhibit the enzyme. We examined the effect of oxidative stress on renal AT1R function involved in biphasic regulation of NKA. Male Sprague-Dawley rats received tap water (control) and 30 mmol/L of L-buthionine sulfoximine (BSO), an oxidant, with and without 1 mmol/L of Tempol (antioxidant) for 2 weeks. BSO-treated rats exhibited increased oxidative stress, AT1R upregulation, and hypertension. In proximal tubules from control rats, Ang II exerted a biphasic effect on NKA activity, causing stimulation of the enzyme at picomolar and inhibition at micromolar concentrations. However, in BSO-treated rats, Ang II caused stimulation of NKA at both of the concentrations. The effect of Ang II was abolished by the AT1R antagonist candesartan and the mitogen-activated protein kinase inhibitor UO126, whereas the Ang type 2 receptor antagonist PD-123319 and NO synthase inhibitor N(G)-nitro-L-arginine methyl ester had no effect. The inhibitory effect of Ang II was sensitive to candesartan and N(G)-nitro-L-arginine methyl ester, whereas PD-123319 and UO126 had no effect. In BSO-treated rats, Ang II showed exaggerated stimulation of NKA, mitogen-activated protein kinase, proline-rich-tyrosine kinase 2, and NADPH oxidase but failed to activate NO signaling. Tempol reduced oxidative stress, normalized AT1R signaling, unmasked the biphasic effect on NKA, and reduced blood pressure in BSO-treated rats. In conclusion, oxidative stress-mediated AT1R upregulation caused a loss of NKA biphasic response and hypertension. Tempol normalized AT1R signaling and blood pressure.

Na+,K+-ATPase is modulated by angiotensin II in diabetic rat kidney--another reason for diabetic nephropathy?

Angiotensin II (ANGII) plays a central role in the enhanced sodium reabsorption in early type 1 diabetes in man and in streptozotocin-induced (STZ) diabetic rats. This study investigates the effect of untreated STZ-diabetes leading to diabetic nephropathy in combination with ANGII treatment, on the abundance and localization of the renal Na(+),K(+)-ATPase (NKA), a major contributor of renal sodium handling. After 7 weeks of STZ-diabetes (i.v. 65 mg kg(-1)) a subgroup of control (C) and diabetic (D7) Wistar rats were treated with ANGII (s.c. minipump 33 microg kg(-1) h(-1) for 24 h; CA and D7A). We measured renal function and mRNA expression, protein level, Serin23 phosphorylation, subcellular distribution, and enzyme activity of NKA alpha-1 subunit in the kidney cortex. Diabetes increased serum creatinine and urea nitrogen levels (C versus D7), as did ANGII (C versus CA, D7 versus D7A). Both diabetes (C versus D7) and ANGII increased NKA alpha-1 protein level and enzyme activity (C versus CA, D7 versus D7A). Furthermore, the combination led to an additive increase (D7 versus D7A, CA versus D7A). NKA alpha-1 Ser23 phosphorylation was higher both in D7 and ANGII-treated rats in the non-cytoskeletal fraction, while no signal was detected in the cytoskeletal fraction. Control kidneys showed NKA alpha-1 immunopositivity on the basolateral membrane of proximal tubular cells, while both D7 and ANGII broadened NKA immunopositivity towards the cytoplasm. Our study demonstrates that diabetes mellitus (DM) increases the mRNA expression, protein level, Ser23 phosphorylation and enzyme activity of renal NKA, which is further elevated by ANGII. Despite an increase in total NKA quantity in diabetic nephropathy, the redistribution to the cystosol suggests the Na(+) pump is no longer functional. ANGII also caused translocation from the basolateral membrane, thus in diabetic states where ANGII level is acutely elevated, the loss of NKA will be exacerbated. This provides another mechanism by which ANGII blockade is likely to be protective.

Oxidative stress-induced renal angiotensin AT1 receptor upregulation causes increased stimulation of sodium transporters and hypertension

Reactive oxygen species have emerged as important molecules in cardiovascular dysfunction such as diabetes and hypertension. Recent work has shown that oxidative stress and angiotensin II signaling mutually regulate each other by multiple mechanisms and contribute to the development of hypertension. Most of the known biological actions of angiotensin II can be attributed to AT1 receptors. The present study was carried out to investigate the role of renal AT1 receptor signaling in oxidative stress-mediated hypertension. Male Sprague-Dawley rats received tap water (control) or 30 mM L-buthionine sulfoximine (BSO), an oxidant, with and without 1 mM tempol (an antioxidant) for 2 wk. Compared with control rats, BSO-treated rats exhibited increased oxidative stress and reduced antioxidant levels and developed hypertension. BSO treatment also caused increased renal proximal tubular AT1 receptor protein abundance, message levels, and ligand binding. In these rats, angiotensin II caused significantly higher accumulation of inositol trisphosphate (IP3) and phospholipase C (PLC) activation which was sensitive to blockade by AT1 but not to AT2 antagonist. Also, angiotensin II-mediated, AT1-dependent MAP kinase, Na-K-ATPase, and Na/H exchanger 3 activation was higher in BSO-treated rats than in control rats. Tempol supplementation of BSO-treated rats restored redox status, normalized AT1 receptor expression, and decreased blood pressure. Tempol also normalized the angiotensin II-mediated, AT1-dependent IP3 accumulation and PLC, MAP kinase, Na-K-ATPase, and Na/H exchanger 3 stimulation. These data suggest that oxidative stress leads to AT1 receptor upregulation, which in turn causes overstimulation of sodium transporters and subsequently contributes to sodium retention and hypertension. Tempol, while reducing oxidative stress, normalizes AT1 receptor signaling and decreases blood pressure.

Transactivation of epidermal growth factor receptor in vascular and renal systems in rats with experimental hyperleptinemia: role in leptin-induced hypertension

We examined the role of epidermal growth factor (EGF) receptor in the pathogenesis of leptin-induced hypertension in the rat. Leptin, administered in increasing doses (0.1-0.5 mg/kg/day) for 10 days, increased phosphorylation levels of non-receptor tyrosine kinase, c-Src, EGF receptor and extracellular signal-regulated kinases (ERK) in aorta and kidney, which was accompanied by the increase in plasma concentration and urinary excretion of isoprostanes and H2O2. Blood pressure and renal Na+,K+-ATPase activity were higher, whereas urinary sodium excretion was lower in animals receiving leptin. The effects of leptin on renal Na+,K+-ATPase, natriuresis and blood pressure were abolished by NADPH oxidase inhibitor, apocynin, Src kinase inhibitor, PP2, EGF receptor inhibitor, AG1478, protein farnesyltransferase inhibitor, manumycin A, and ERK inhibitor, PD98059. In contrast, inhibitors of insulin-like growth factor-1 and platelet-derived growth factor receptors, AG1024 and AG1295, respectively, only slightly reduced ERK phosphorylation and had no effect on blood pressure in rats receiving leptin. These data indicate that: (1) experimental hyperleptinemia is associated with oxidative stress and c-Src-dependent transactivation of the EGF receptor, which stimulates ERK in vascular wall and the kidney, (2) overactivity of EGF receptor-ERK pathway contributes to leptin-induced hypertension by stimulating renal Na+,K+-ATPase and reducing sodium excretion, (3) inhibitors of c-Src, EGF receptor and ERK may be considered as a novel therapy for hypertension associated with hyperleptinemia, e.g. in patients with obesity and metabolic syndrome.

Impaired angiotensin II AT(1) receptor function and enhanced Na, K-ATPase affinity for sodium in proximal tubule of streptozotocin-treated diabetic rats

We determined angiotensin II (Ang II) AT(1) receptor function in terms of Na-K-ATPAse (NKA) stimulation in the proximal tubule (PTs) of streptozotocin-induced diabetic rats. Ang II (10 pM) stimulated NKA activity in PTs of control rats but not diabetic rats. The AT(1) receptor expression was similar, but the expression of G-proteins (G(i)alpha2 and G(i)alpha3) in the PTs was decreased in diabetic compared with control rats. Kinetic studies revealed an increase in NKA affinity, low K(0.5,) for Na, with no changes in V(max) of the enzyme in diabetic compared with control rats. Basal Ser-phosphorylation of NKA alpha1-subunit was lower in diabetic compared with control rats. This data suggest that the higher basal NKA affinity for Na, possibly due to lower Ser-phosphorylaion of alpha1-subunit and not the AT(1) receptor function, in the PTs may be responsible for increased renal Na reabsorption associated with early stage of streptozotocin-induced diabetes.

Obesity augments vasoconstrictor reactivity to angiotensin II in the renal circulation of the Zucker rat

Obesity is an emerging risk factor for renal dysfunction, but the mechanisms are poorly understood. Obese patients show heightened renal vasodilation to blockade of the renin-angiotensin system, suggesting deficits in vascular responses to angiotensin II (ANG II). This study tested the hypothesis that obesity augments renal vasoconstriction to ANG II. Lean (LZR), prediabetic obese (OZR), and nonobese fructose-fed Zucker rats (FF-LZR) were studied to determine the effects of obesity and insulin resistance on reactivity of blood pressure and renal blood flow to vasoconstrictors. OZR showed enlargement of the kidneys, elevated urine output, increased sodium intake, and decreased plasma renin activity (PRA) vs. LZR, and renal vasoconstriction to ANG II was augmented in OZR. Renal reactivity to norepinephrine and mesenteric vascular reactivity to ANG II were similar between LZR and OZR. Insulin-resistant FF-LZR had normal reactivity to ANG II, indicating the insulin resistance was an unlikely explanation for the changes observed in OZR. Four weeks on a low-sodium diet (0.08%) to raise PRA reduced reactivity to ANG II in OZR back to normal levels without effect on LZR. From these data, we conclude that in the prediabetic stages of obesity, a decrease in PRA is observed in Zucker rats that may lead to increased renal vascular reactivity to ANG II. This increased reactivity to ANG II may explain the elevated renal vasodilator effects observed in obese humans and provide insight into early changes in renal function that predispose to nephropathy in later stages of the disease.

Insulin's impact on renal sodium transport and blood pressure in health, obesity, and diabetes

Insulin has been shown to have antinatriuretic actions in humans and animal models. Moreover, endogenous hyperinsulinemia and insulin infusion have been correlated to increased blood pressure in some models. In this review, we present the current state of understanding with regard to the regulation of the major renal sodium transporters by insulin in the kidney. Several groups, using primarily cell culture, have demonstrated that insulin can directly increase activity of the epithelial sodium channel, the sodium-phosphate cotransporter, the sodium-hydrogen exchanger type III, and Na-K-ATPase. We and others have demonstrated alterations in the expression at the protein level of many of these same proteins with insulin infusion or in hyperinsulinemic models. We also discuss how this regulation is perturbed in type I and type II diabetes mellitus. Finally, we discuss a potential role for regulation of insulin receptor signaling in the kidney in contributing to sodium balance and blood pressure.

Regulation of renal ouabain-resistant Na+-ATPase by leptin, nitric oxide, reactive oxygen species, and cyclic nucleotides: implications for obesity-associated hypertension

This study examined the effect of leptin on renal ouabain-resistant Na(+)-ATPase, which drives the reabsorption of about 10% of sodium transported in the proximal tubule. Chronic leptin administration (0.25 mg/kg s.c. twice daily for seven days) increased Na(+)-ATPase activity by 62.9%. This effect was prevented by the coadministration of superoxide dismutase mimetic, tempol, or the NADPH oxidase inhibitor, apocynin (2 mM in the drinking water). Acutely administered NO donors decreased Na(+)-ATPase activity. This effect was abolished by soluble guanylate cyclase inhibitor, ODQ, but not by protein kinase G inhibitors. Exogenous cGMP reduced Na(+)-ATPase activity, but its synthetic analogues, 8-bromo-cGMP and 8-pCPT-cGMP, were ineffective. The inhibitory effect of NO donors and cGMP was abolished by EHNA, an inhibitor of cGMP-stimulated phosphodiesterase (PDE2). Exogenous cAMP analogue and dibutyryl-cAMP increased Na(+)-ATPase activity and abolished the inhibitory effect of cGMP. Finally, the administration of superoxide-generating mixture (xanthine oxidase+hypoxanthine) increased Na(+)-ATPase activity. The results suggest that nitric oxide decreases renal Na(+)-ATPase activity by stimulating cGMP, which in turn activates PDE2 and decreases cAMP concentration. Increased production of reactive oxygen species may lead to the elevation of Na(+)-ATPase activity by scavenging NO and limiting its inhibitory effect. Chronic hyperleptinemia is associated with increased Na(+)-ATPase activity due to excessive oxidative stress.

Enhanced AT1receptor-mediated vasocontractile response to ANG II in endothelium-denuded aorta of obese Zucker rats

In the present study, we tested the hypothesis that ANG II causes a greater vasoconstriction in obese Zucker rats, a model of type 2 diabetes, with mild hypertension. Measurement of isometric tension in isolated aortic rings with intact endothelium revealed a modest but not significantly greater ANG II-induced contraction in obese than lean rats. Removal of endothelium or inhibition of nitric oxide (NO) synthase by NG-nitro-l-arginine methyl ester (l-NAME) enhanced 1) ANG II-induced contraction in both lean and obese rats, being significantly greater in obese rats (Emaxg/g tissue, denuded: lean 572 ± 40 vs. obese 664 ± 16; l-NAME: lean 535 ± 14 vs. obese 818 ± 23) and 2) ANG II sensitivity in obese compared with lean rats, as revealed by the pD2values. Endothelin-1 and KCl elicited similar contractions in the aortic rings of lean and obese rats. ACh, a NO-dependent relaxing hormone, produced greater relaxation in the aortic rings of obese than lean rats, whereas sodium nitroprusside, an NO donor, elicited similar relaxations in both rat strains. The expression of the ANG type 1 (AT1) receptor protein and mRNA in the endothelium-intact aorta was significantly greater in obese than lean rats, whereas the endothelium-denuded rings expressed modest but not significantly greater levels of AT1receptors in obese than lean rats. The endothelial NO synthase protein and mRNA expression levels were higher in the aorta of obese than lean animals. We conclude that, although ANG II produces greater vasoconstriction in obese rat aortic rings, enhanced endothelial AT1receptor-mediated NO production appears to counteract the increased ANG II-induced vasoconstriction, suggesting that arterial AT1receptor may not be a contributing factor to hypertension in this model of obesity.

Time-dependent transition from H(2)O(2)-extracellular signal-regulated kinase- to O(2)-nitric oxide-dependent mechanisms in the stimulatory effect of leptin on renal Na+/K+/-ATPase in the rat

1. Recent studies suggest that leptin, a peptide hormone secreted by white adipose tissue, is involved in the pathogenesis of arterial hypertension, in part by regulating renal sodium handling. Previously, we have demonstrated that in normal rats leptin has a time-dependent effect on renal Na(+)/K(+)-ATPase that drives tubular sodium reabsorption. Short-term leptin infusion results in a transient decrease in Na(+)/K(+)-ATPase activity, whereas prolonged administration stimulates the enzyme. 2. In the present study, we investigated whether these acute effects of leptin are preserved in rats with experimentally induced chronic hyperleptinaemia. 3. Hyperleptinaemia was induced by administration of exogenous leptin (0.25 mg/kg twice daily, s.c., for 7 days). Acute effects of leptin in anaesthetized control (normoleptinaemic) and hyperleptinaemic animals was investigated. Leptin was infused into the abdominal aorta proximally to the renal arteries for 0.5, 1, 2 or 3 h. 4. Leptin (1 microg/min per kg) had a time-dependent effect on renal Na(+)/K(+)-ATPase in both the control and hyperleptinaemic groups. The inhibitory effect observed after 0.5 h infusion was impaired in the hyperleptinaemic group. However, in both groups this effect was abolished by the Janus kinase inhibitor tyrphostin AG490 (100 nmol/min per kg), as well as by the phosphatidylinositol 3-kinase inhibitors wortmannin (10 nmol/min per kg) and LY294002 (1 micromol/min per kg). 5. The stimulatory effect of leptin on Na(+)/K(+)-ATPase activity was observed after 3 h of infusion and was of similar magnitude in control and hyperleptinaemic groups. In the control group, the stimulatory effect of leptin was abolished by the NADPH oxidase inhibitor apocynin (1 micromol/min per kg), the H(2)O(2) scavenger catalase (1 mg/min per kg) and the extracellular signal-regulated kinase (ERK) inhibitor PD98059 (100 nmol/min per kg). In contrast, in the hyperleptinaemic group, the stimulatory effect of leptin was abolished by the cGMP analogue 8-bromo-cGMP (100 nmol/min per kg) and by the superoxide dismutase mimetic tempol (100 micromol/min per kg) but was not affected by catalase or PD98059. 6. Leptin increased urinary H(2)O(2) excretion and ERK phosphorylation in the renal tissue only in the control group. 7. The results suggest that the acute stimulatory effect of leptin on renal Na(+)/K(+)-ATPase is mediated by divergent mechanisms depending on the chronic leptin level (i.e. by H(2)O(2)-dependent stimulation of ERK in normoleptinaemic animals and by superoxide-dependent impairment of the nitric oxide-cGMP pathway in hyperleptinaemic rats).

H2O2 and Src-dependent transactivation of the EGF receptor mediates the stimulatory effect of leptin on renal ERK and Na+, K+-ATPase

We examined the mechanism through which leptin increases Na(+), K(+)-ATPase activity in the rat kidney. Leptin was infused under anaesthesia into the abdominal aorta proximally to the renal arteries and then Na(+), K(+)-ATPase activity was measured in the renal cortex and medulla. Leptin (1mug/kgmin) increased Na(+), K(+)-ATPase activity after 3h of infusion, which was accompanied by the increase in urinary H(2)O(2) excretion and phosphorylation level of extracellular signal regulated kinase (ERK). The effect of leptin on ERK and Na(+), K(+)-ATPase was abolished by catalase, specific inhibitors of epidermal growth factor (EGF) receptor, AG1478 and PD158780, as well as by ERK inhibitor, PD98059, and was mimicked by both exogenous H(2)O(2) and EGF. The effect of leptin was also prevented by the inhibitor of Src tyrosine kinase, PP2. Leptin and H(2)O(2) increased Src phosphorylation at Tyr(418). We conclude that leptin-induced stimulation of renal Na(+), K(+)-ATPase involves H(2)O(2) generation, Src kinase, transactivation of the EGF receptor, and stimulation of ERK.