Rosiglitazone restores renal D1A receptor-Gs protein coupling by reducing receptor hyperphosphorylation in obese rats

Associations of SUCNR1, GRK4, CAMK1D gene polymorphisms and the susceptibility of type 2 diabetes mellitus and essential hypertension in a northern Chinese Han population

AIMS

Diabetes mellitus and hypertension are both complex diseases that are caused by interactions among multiple genetic and physiological factors. To investigate the association of common single-nucleotide polymorphisms (SNPs) of SUCNR1, GRK4 and CAMK1D genes with the susceptibility of the two diseases in a northern Chinese Han population.

METHODS

36 SNPs were genotyped in 2304 clinical patients (1152 type 2 diabetes mellitus, 1152 essential hypertension) and 1152 health controls by Sequenom Mass-ARRAY RS1000.

RESULTS

In this study, we found that BMI, blood press, pulse pressure, FBG, total cholesterol and triglycerides were associated with an increased risk of type 2 diabetes mellitus (T2DM) and essential hypertension (EH). Three SNPs (SUCNR1: rs73168929; GRK4: rs1557213; CAMK1D: rs17151584) significantly associated with the susceptibility of T2DM and EH at the same time. Also, the susceptibility genotypes of 3 SNPs were significantly correlated with liver and renal function parameters.

CONCLUSION

To the best of our knowledge, the present study is the first to report that three SNPs (SUCNR1: rs73168929; GRK4: rs1557213; CAMK1D: rs17151584) contributed to the risk of T2DM and EH in a northern Chinese Han population. These results provide a favourable evidence for better understand of the underlying common mechanism of these two diseases.

Rosiglitazone treatment restores renal responsiveness to atrial natriuretic peptide in rats with congestive heart failure

The thiazolidinedione (TZD) class of Peroxisome proliferator‐activated receptor gamma agonists has restricted clinical use for diabetes mellitus due to fluid retention and potential cardiovascular risks. These side effects are attributed in part to direct salt‐retaining effect of TZDs at the renal collecting duct. A recent study from our group revealed that prolonged rosiglitazone (RGZ) treatment caused no Na+/H₂O retention or up‐regulation of Na⁺ transport‐linked channels/transporters in experimental congestive heart failure (CHF) induced by surgical aorto‐caval fistula (ACF). The present study examines the effects of RGZ on renal and cardiac responses to atrial natriuretic peptide (ANP), Acetylcholine (Ach) and S‐Nitroso‐N‐acetylpenicillamine (SNAP‐NO donor). Furthermore, we assessed the impact of RGZ on gene expression related to the ANP signalling pathway in animals with ACF. Rats subjected to ACF (or sham) were treated with either RGZ (30 mg/kg/day) or vehicle for 4 weeks. Cardiac chambers pressures and volumes were assessed invasively via Miller catheter. Kidney excretory and renal hemodynamic in response to ANP, Ach and SNAP were examined. Renal clearance along with cyclic guanosine monophosphate (cGMP), gene expression of renal CHF‐related genes and ANP signalling in the kidney were determined. RGZ‐treated CHF rats exhibited significant improvement in the natriuretic responses to ANP infusion. This ‘sensitization’ to ANP was not associated with increases in neither urinary cGMP nor in vitro cGMP production. However, RGZ caused down‐regulation of several genes in the renal cortex (Ace, Nos3 and Npr1) and up‐regulation of ACE2, Agtrla, Mme and Cftr along down‐regulation of Avpr2, Npr1,2, Nos3 and Pde3 in the medulla. In conclusion, CHF+RGZ rats exhibited significant enhancement in the natriuretic responses to ANP infusion, which are known to be blunted in CHF. This ‘sensitization’ to ANP is independent of cGMP signalling, yet may involve post‐cGMP signalling target genes such as ACE2, CFTR and V2 receptor. The possibility that TZD treatment in uncomplicated CHF may be less detrimental than thought before deserves additional investigations.

Exposure to Maternal Diabetes Mellitus Causes Renal Dopamine D1 Receptor Dysfunction and Hypertension in Adult Rat Offspring

Epidemiological and experimental studies suggest that maternal diabetes mellitus programs hypertension that is associated with impaired sodium excretion in the adult offspring. However, the underlying mechanisms are not clear. Because dopamine receptor function is involved in the pathogenesis of hypertension, we hypothesized that impaired renal dopamine D1 receptor function is also involved in the hypertension in offspring of maternal diabetes mellitus. Maternal diabetes mellitus was induced by a single intraperitoneal injection of streptozotocin (35 mg/kg) to pregnant Sprague-Dawley rats at day 0 of gestation. Compared with the offspring of mothers injected with citrate buffer (control mother offspring), the diabetic mother offspring (DMO) had increased systolic blood pressure and impaired D1 receptor-mediated diuresis and natriuresis, accompanied by increased renal PKC (protein kinase C) expression and activity, GRK-2 (G protein-coupled receptor kinase-2) expression, D1 receptor phosphorylation, D1 receptor/Gαs uncoupling, and loss of D1 receptor-mediated inhibition of Na+-K+-ATPase activity in renal proximal tubule cells from DMO. Inhibition of PKC reduced the increased GRK-2 expression and normalized D1 receptor function in primary cultures of renal proximal tubule cells from DMO. In addition, DMO, relative to control mother offspring, in vivo, had increased oxidative stress, indicated by decreased renal glutathione and increased renal malondialdehyde and urine 8-isoprostane. Normalization of oxidative stress with tempol also normalized the renal D1 receptor phosphorylation, D1 receptor-mediated diuresis and natriuresis, and blood pressure in DMO. Our present study indicates that maternal diabetes mellitus-programed hypertension in the offspring is caused by impaired renal D1 receptor function because of oxidative stress that is mediated by increased PKC-GRK-2 activity.

Mycophenolate mofetil and curcumin provide comparable therapeutic benefit in experimental chronic kidney disease: role of Nrf2-Keap1 and renal dopamine pathways

Increased oxidative stress and inflammation have an important role in the pathophysiology of chronic kidney disease (CKD). On the other hand, more affordable therapeutic alternatives for treating this disease are urgently needed. Therefore, we compared the therapeutic efficacy of curcumin and mycophenolate mofetil (MMF) in 5/6 nephrectomy (5/6 Nx) model of CKD. Also, we evaluated whether both compounds provide benefit through the preservation of similar antioxidant mechanisms. Four groups of male Wistar were studied over a period of 4 wk. Control sham group (n= 12), 5/6 Nx (n = 12), 5/6 Nx + MMF (30 mg/k BW/day, n = 11) and 5/6 Nx + Curcumin (120 mg/k BW/day, n = 12). Renal function and markers of oxidative stress and inflammation were evaluated. Also Nrf2-Keap1 and renal dopamine, antioxidant pathways were assessed. 5/6 Nx induced an altered renal autoregulation response, proteinuria, and hypertension; these effects were in association with increased oxidative stress, endothelial dysfunction and renal inflammation. The mechanisms associated with these alterations included a reduced nuclear translocation of Nrf2 and hyperphosphorylation of dopamine D1 receptor with a concurrent overactivation of renal NADPH oxidase. Treatments with MMF and curcumin provided equivalent therapeutic efficacy as both prevented functional renal alterations as well as preserved antioxidant capacity and avoided renal inflammatory infiltration. Moreover, both treatments preserved Nrf2-Keap1 and renal dopamine antioxidant pathways. In summary, therapeutic strategies aimed to preserve renal antioxidant pathways can help to retard the progression of CKD.

The Role of Aldosterone in Obesity-Related Hypertension

Obese subjects often have hypertension and related cardiovascular and renal diseases, and this has become a serious worldwide health problem. In obese subjects, impaired renal-pressure natriuresis causes sodium retention, leading to the development of salt-sensitive hypertension. Physical compression of the kidneys by visceral fat and activation of the sympathetic nervous system, renin-angiotensin systems (RAS), and aldosterone/mineralocorticoid receptor (MR) system are involved in this mechanism. Obese subjects often exhibit hyperaldosteronism, with increased salt sensitivity of blood pressure (BP). Adipose tissue excretes aldosterone-releasing factors, thereby stimulating aldosterone secretion independently of the systemic RAS, and aldosterone/MR activation plays a key role in the development of hypertension and organ damage in obesity. In obese subjects, both salt sensitivity of BP, enhanced by obesity-related metabolic disorders including aldosterone excess, and increased dietary sodium intake are closely related to the incidence of hypertension. Some salt sensitivity-related gene variants affect the risk of obesity, and together with salt intake, its combination is possibly associated with the development of hypertension in obese subjects. With high salt levels common in modern diets, salt restriction and weight control are undoubtedly important. However, not only MR blockade but also new diagnostic modalities and therapies targeting and modifying genes that are related to salt sensitivity, obesity, or RAS regulation are expected to prevent obesity and obesity-related hypertension.

Human GRK4γ142V Variant Promotes Angiotensin II Type I Receptor-Mediated Hypertension via Renal Histone Deacetylase Type 1 Inhibition

The influence of a single gene on the pathogenesis of essential hypertension may be difficult to ascertain, unless the gene interacts with other genes that are germane to blood pressure regulation. G-protein-coupled receptor kinase type 4 (GRK4) is one such gene. We have reported that the expression of its variant hGRK4γ(142V) in mice results in hypertension because of impaired dopamine D1 receptor. Signaling through dopamine D1 receptor and angiotensin II type I receptor (AT1R) reciprocally modulates renal sodium excretion and blood pressure. Here, we demonstrate the ability of the hGRK4γ(142V) to increase the expression and activity of the AT1R. We show that hGRK4γ(142V) phosphorylates histone deacetylase type 1 and promotes its nuclear export to the cytoplasm, resulting in increased AT1R expression and greater pressor response to angiotensin II. AT1R blockade and the deletion of the Agtr1a gene normalize the hypertension in hGRK4γ(142V) mice. These findings illustrate the unique role of GRK4 by targeting receptors with opposite physiological activity for the same goal of maintaining blood pressure homeostasis, and thus making the GRK4 a relevant therapeutic target to control blood pressure.

Accessory proteins for heterotrimeric G-proteins in the kidney

Heterotrimeric G-proteins play a fundamentally important role in regulating signal transduction pathways in the kidney. Accessory proteins are being identified as direct binding partners for heterotrimeric G-protein α or βγ subunits to promote more diverse mechanisms by which G-protein signaling is controlled. In some instances, accessory proteins can modulate the signaling magnitude, localization, and duration following the activation of cell membrane-associated receptors. Alternatively, accessory proteins complexed with their G-protein α or βγ subunits can promote non-canonical models of signaling activity within the cell. In this review, we will highlight the expression profile, localization and functional importance of these newly identified accessory proteins to control the function of select G-protein subunits under normal and various disease conditions observed in the kidney.

Renal dopaminergic system: Pathophysiological implications and clinical perspectives

Fluid homeostasis, blood pressure and redox balance in the kidney are regulated by an intricate interaction between local and systemic anti-natriuretic and natriuretic systems. Intrarenal dopamine plays a central role on this interactive network. By activating specific receptors, dopamine promotes sodium excretion and stimulates anti-oxidant and anti-inflammatory pathways. Different pathological scenarios where renal sodium excretion is dysregulated, as in nephrotic syndrome, hypertension and renal inflammation, can be associated with impaired action of renal dopamine including alteration in biosynthesis, dopamine receptor expression and signal transduction. Given its properties on the regulation of renal blood flow and sodium excretion, exogenous dopamine has been postulated as a potential therapeutic strategy to prevent renal failure in critically ill patients. The aim of this review is to update and discuss on the most recent findings about renal dopaminergic system and its role in several diseases involving the kidneys and the potential use of dopamine as a nephroprotective agent.

Gender-based differences on the association between salt-sensitive genes and obesity in Korean children aged between 8 and 9 years

BACKGROUND

High sodium intake is associated with the development of chronic diseases such as obesity. Although its role in obesity remains controversial, there may be a correlation between salt sensitivity and the early onset of chronic diseases in obese children.

METHODS

In all, 2,163 Korean children (1,106 boys and 1,057 girls) aged 8-9 years were recruited from seven elementary schools in Seoul. To evaluate whether obesity risk was modulated by the salt sensitivity, 11 SNPs related to salt sensitive genes (SSG) became the target of sodium intakes in obese children.

RESULTS

BP, HOMA-IR, LDLc, TG, and the girls' sodium intake significantly increased, but HDLc significantly decreased with increase in BMI. Regardless of sex, the obesity risk was 5.27-fold (CI; 1.320-27.560) higher in the Q2 to Q5 of sodium intake adjusted by energy (4044.9-5058.9 mg/day) than in the lowest Q1 level (2287.6 mg/day) in obese children. BP was sensitively dependent on insulin resistance and lipid accumulation in all subjects; however, sodium intake may be an independent risk factor of obesity without increasing BP in girls. GRK4 A486V mutant homozygote was highly distributed in the obese group, but other SNPs had no impact. The obesity risk increased 7.06, 16.8, and 46.09-fold more in boys with GRK4 A486V, ACE, and SLC12A3 mutants as sodium intake increased. Among girls, the obesity risk increased in GRK4 A486V heterozygote and CYP11β-2 mutant homozygote although sodium intake was relatively lower, implying that ACE, SLC12A, CYP11β-2, and GRK4 A486V polymorphisms showed gender-based differences with regard to interaction between sodium intake and obesity.

CONCLUSION

A high sodium intake markedly increased the obesity risk in variants of GRK4 A486V regardless of sex. The obesity risk increased with GRK4 A486V, ACE, and SLC12A3 variants in boys, whereas it increased with GRK4 A486V and CYP11B2 variants in girls as sodium intake increased. Obese children with the specific gene variants are recommended to reduce their sodium intake.

Common variants of the G protein-coupled receptor type 4 are associated with human essential hypertension and predict the blood pressure response to angiotensin receptor blockade

Non-synonymous GRK4 variants, R65L, A142V and A486V, are associated with essential hypertension in diverse populations. This study replicated the association of GRK4 variants, including GRK4(142V), with human essential hypertension in a Japanese population (n=588; hypertensive, n=486 normotensive controls) and determined whether the presence of GRK4 variants predicted the blood pressure (BP) response to angiotensin receptor blockers (ARBs) in patients with essential hypertension. We analyzed 829 patients and compared the response to ARBs between individuals with no GRK4 variants (n=136) and those with variants at one or any of the three loci (n=693). Carriers of hGRK4(142V) had a greater decrease in systolic BP in response to ARBs than non-carrier hypertensive patients. By contrast, those with variants only at GRK4(486V) were less likely to achieve the BP goal in response to an ARB than those with no variants. These studies showed for the first time the association between GRK4(142V) and a larger decrease in BP with ARBs in hypertensive patients.

Prenatal lipopolysaccharide exposure results in dysfunction of the renal dopamine D1 receptor in offspring

Adverse environment in early life can modulate the adult phenotype, including blood pressure. Lipopolysaccharide (LPS) exposure in utero results in increased blood pressure in the offspring, but the exact mechanisms are not clear. Studies have shown that the renal dopamine D1 receptor (D1R) plays an important role in maintaining sodium homeostasis and normal blood pressure; dysfunction of D1R is associated with oxidative stress and hypertension. In this study, we determined if dysfunction of the renal D1R is involved in fetal-programmed hypertension, and if oxidative stress contributes to this process. Pregnant Sprague-Dawley (SD) rats were intraperitoneally injected with LPS (0.79 mg/kg) or saline at gestation days 8, 10, and 12. As compared with saline-injected (control) dams, offspring of LPS-treated dams had increased blood pressure, decreased renal sodium excretion, and increased markers of oxidative stress. In addition, offspring of LPS-treated dams had decreased renal D1R expression, increased D1R phosphorylation, and G protein-coupled receptor kinase type 2 (GRK2) and type 4 (GRK4) protein expression, and impaired D1R-mediated natriuresis and diuresis. All of the findings in the offspring of LPS-treated dams were normalized after treatment with TEMPOL, an oxygen free radical scavenger. In conclusion, prenatal LPS exposure, via an increase in oxidative stress, impairs renal D1R function and leads to hypertension in the offspring. Normalization of renal D1R function by amelioration of oxidative stress may be a therapeutic target of fetal programming of hypertension.

Unique role of NADPH oxidase 5 in oxidative stress in human renal proximal tubule cells

NADPH oxidases are the major sources of reactive oxygen species in cardiovascular, neural, and kidney cells. The NADPH oxidase 5 (NOX5) gene is present in humans but not rodents. Because Nox isoforms in renal proximal tubules (RPTs) are involved in the pathogenesis of hypertension, we tested the hypothesis that NOX5 is differentially expressed in RPT cells from normotensive (NT) and hypertensive subjects (HT). We found that NOX5 mRNA, total NOX5 protein, and apical membrane NOX5 protein were 4.2±0.7-fold, 5.2±0.7-fold, and 2.8±0.5-fold greater in HT than NT. Basal total NADPH oxidase activity was 4.5±0.2-fold and basal NOX5 activity in NOX5 immunoprecipitates was 6.2±0.2-fold greater in HT than NT (P=<0.001, n=6-14/group). Ionomycin increased total NOX and NOX5 activities in RPT cells from HT (P<0.01, n=4, ANOVA), effects that were abrogated by pre-treatment of the RPT cells with diphenylene-iodonium or superoxide dismutase. Silencing NOX5 using NOX5-siRNA decreased NADPH oxidase activity (-45.1±3.2% vs. mock-siRNA, n=6-8) in HT. D1-like receptor stimulation decreased NADPH oxidase activity to a greater extent in NT (-32.5±1.8%) than HT (-14.8±1.8). In contrast to the marked increase in expression and activity of NOX5 in HT, NOX1 mRNA and protein were minimally increased in HT, relative to NT; total NOX2 and NOX4 proteins were not different between HT and NT, while the increase in apical RPT cell membrane NOX1, NOX2, and NOX4 proteins in HT, relative to NT, was much less than those observed with NOX5. Thus, we demonstrate, for the first time, that NOX5 is expressed in human RPT cells and to greater extent than the other Nox isoforms in HT than NT. We suggest that the increased expression of NOX5, which may be responsible for the increased oxidative stress in RPT cells in human essential hypertension, is caused, in part, by a defective renal dopaminergic system.

Impaired dopamine D1 receptor-mediated vasorelaxation of mesenteric arteries in obese Zucker rats

Background

Obesity plays an important role in the pathogenesis of hypertension. Renal dopamine D₁-like receptor-mediated diuresis and natriuresis are impaired in the obese Zucker rat, an obesity-related hypertensive rat model. The role of arterial D₁ receptors in the hypertension of obese Zucker rats is not clear.

Methods

Plasma glucose and insulin concentrations and blood pressure were measured. The vasodilatory response of isolated mesenteric arteries was evaluated using a small vessel myograph. The expression and phosphorylation of D₁ receptors were quantified by co-immunoprecipitation and immunoblotting To determine the effect of hyperinsulinemia and hyperglycemia on the function of the arterial D₁ receptor, we studied obese Zucker rats (six to eight-weeks old) fed (6 weeks) vehicle or rosiglitazone, an insulin sensitizer (10 mg/kg per day) and lean Zucker rats (eight to ten-weeks old), fed high-fat diet to induce hyperinsulinemia or injected intraperitoneally with streptomycin (STZ) to induce hyperglycemia.

Results

In obese Zucker rats, the vasorelaxant effect of D₁-like receptors was impaired that could be ascribed to decreased arterial D₁ receptor expression and increased D₁ receptor phosphorylation. In these obese rats, rosiglitazone normalized the arterial D₁ receptor expression and phosphorylation and improved the D₁-like receptor-mediated vasorelaxation. We also found that D₁ receptor-dependent vasorelaxation was decreased in lean Zucker rats with hyperinsulinemia or hyperglycemia but the D₁ receptor dysfunction was greater in the former than in the latter group. The ability of insulin and glucose to decrease D₁ receptor expression and increase its phosphorylation were confirmed in studies of rat aortic smooth muscle cells.

Conclusions

Both hyperinsulinemia and hyperglycemia caused D₁ receptor dysfunction by decreasing arterial D₁ receptor expression and increasing D₁ receptor phosphorylation. Impaired D₁ receptor-mediated vasorelaxation is involved in the pathogenesis of obesity-related hypertension.

Renal dopamine receptors, oxidative stress, and hypertension

Dopamine, which is synthesized in the kidney, independent of renal nerves, plays an important role in the regulation of fluid and electrolyte balance and systemic blood pressure. Lack of any of the five dopamine receptor subtypes (D1R, D2R, D3R, D4R, and D5R) results in hypertension. D1R, D2R, and D5R have been reported to be important in the maintenance of a normal redox balance. In the kidney, the antioxidant effects of these receptors are caused by direct and indirect inhibition of pro-oxidant enzymes, specifically, nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase, and stimulation of anti-oxidant enzymes, which can also indirectly inhibit NADPH oxidase activity. Thus, stimulation of the D2R increases the expression of endogenous anti-oxidants, such as Parkinson protein 7 (PARK7 or DJ-1), paraoxonase 2 (PON2), and heme oxygenase 2 (HO-2), all of which can inhibit NADPH oxidase activity. The D5R decreases NADPH oxidase activity, via the inhibition of phospholipase D2, and increases the expression of HO-1, another antioxidant. D1R inhibits NADPH oxidase activity via protein kinase A and protein kinase C cross-talk. In this review, we provide an overview of the protective roles of a specific dopamine receptor subtype on renal oxidative stress, the different mechanisms involved in this effect, and the role of oxidative stress and impairment of dopamine receptor function in the hypertension that arises from the genetic ablation of a specific dopamine receptor gene in mice.

GRK2-mediated inhibition of adrenergic and dopaminergic signaling in right ventricular hypertrophy: therapeutic implications in pulmonary hypertension

BACKGROUND

The cause and consequences of impaired adrenergic signaling in right ventricular failure/hypertrophy (RVH) are poorly understood. We hypothesized that G protein-coupled receptor kinase-2 (GRK2)-mediated uncoupling of β-adrenergic receptor signaling impairs inotropic reserve. The implications of right ventricular (RV) adrenergic remodeling for inotrope selection and the therapeutic benefit of interrupting Gβγ-GRK2 interaction, using gallein, were tested.

METHODS AND RESULTS

Chamber-specificity and cellular localization of adrenergic remodeling were compared in rodent RVH associated with pulmonary arterial hypertension (PAH-RVH; SU5416+chronic-hypoxia or Monocrotaline) versus pulmonary artery banding-induced RVH (PAB-RVH). Results were corroborated in RV arrays from 10 PAH patients versus controls. Inotropic reserve was assessed in RV- and left ventricular-Langendorff models and in vivo. Gallein therapy (1.8 mg/kg/day ×2-weeks) was assessed. Despite similar RVH, cardiac output (58.3±4.9 versus 82.9±4.8 mL/min; P<0.001) and treadmill distance (41.5±11.6 versus 244.1±12.4 m; P<0.001) were lower in PAH-RVH versus PAB-RVH. In PAH-RVH versus PAB-RVH there was greater downregulation of β1-, α1- and dopamine-1 receptors, more left ventricular involvement, and greater impairment of RV contractile reserve. RV GRK2 activity increased in parallel with a reduction in both adrenergic receptor expression and inotrope-stimulated cAMP levels (P<0.01). β1-receptor downregulation also occurred in human PAH-RVH. Dobutamine was superior to dopamine as an RV inotrope, both ex vivo and in vivo.

CONCLUSIONS

GRK2-mediated desensitization-downregulation of adrenergic and dopaminergic receptors impairs inotropic reserve in PAH-RVH. Acute inotropic support in RVH is best accomplished by dobutamine, reflecting its better coupling to adenylyl cyclase and the reliance of dopamine on dopamine-1-receptor signaling, which is impaired in RVH. Inhibiting Gβγ-GRK2 interactions has therapeutic benefit in RVH.

Renal dopamine and angiotensin II receptor signaling in age-related hypertension

Kidneys play a vital role in long-term regulation of blood pressure. This is achieved by actions of many renal and nonrenal factors acting on the kidney that help maintain the body's water and electrolyte balance and thus control blood pressure. Several endogenously formed or circulating hormones/peptides, by acting within the kidney, regulate fluid and water homeostasis and blood pressure. Dopamine and angiotensin II are the two key renal factors that, via acting on their receptors and counterregulating each other's function, maintain water and sodium balance. In this review, we provide recent advances in the signaling cascades of these renal receptors, especially at the level of their cross talk, and discuss their roles in blood pressure regulation in the aging process.

G protein-coupled receptor kinases in cardiovascular disease: why "where" matters

Cardiac function is mainly controlled by β-adrenergic receptors (β-ARs), members of the G protein-coupled receptor (GPCR) family. GPCR signaling and expression are tightly controlled by G protein-coupled receptor kinases (GRKs), which induce GPCR internalization and signal termination through phosphorylation. Reduced β-AR density and activity associated with elevated cardiac GRK expression and activity have been described in various cardiovascular diseases. Moreover, alterations in extracardiac GRKs have been observed in blood vessels, adrenal glands, kidneys, and fat cells. The broad tissue distribution of GPCRs and GRKs suggests that a keen appreciation of integrative physiology may drive future therapeutic development. In this review, we provide a brief summary of GRK isoforms, subcellular localization, and interacting partners that impinge directly or indirectly on the cardiovascular system. We also discuss GRK/GPCR interactions and their implications in cardiovascular pathophysiology.

The connection between GRKs and various signaling pathways involved in diabetic nephropathy

Diabetic nephropathy (DN) is a known microvascular complication in patients with diabetes mellitus. DN has become one of the main causes of death in diabetic patients. The occurrence and development of DN results from the comprehensive action of multi-factors, though the exact mechanism is not very clear. Recently, a study found that numerous pathways are activated during the course of the disease, including the PGE2-EP-G protein system, the renin-angiotensin system, protein kinase C, MAPK and oxidative stress, and transforming growth factor-β. G protein-coupled receptor kinases (GRKs), specifically recognize and phosphorylate agonist-activated G protein-coupled receptors, which play a major role in the above-mentioned pathways. The purpose of this paper is to review current information concerning the connection between GRKs and various signaling pathways involved in DN.

Potential dopamine-1 receptor stimulation in hypertension management

The role of dopamine receptors in blood pressure regulation is well established. Genetic ablation of both dopamine D1-like receptor subtypes (D1, D5) and D2-like receptor subtypes (D2, D3, D4) results in a hypertensive phenotype in mice. This review focuses on the dopamine D1-like receptor subtypes D1 and D5 (especially D1 receptors), as they play a major role in regulating sodium homeostasis and blood pressure. Studies mostly describing the role of renal dopamine D1-like receptors are included, as the kidneys play a pivotal role in the maintenance of sodium homeostasis and the long-term regulation of blood pressure. We also attempt to describe the interaction between D1-like receptors and other proteins, especially angiotensin II type 1 and type 2 receptors, which are involved in the maintenance of sodium homeostasis and blood pressure. Finally, we discuss a new concept of renal D1 receptor regulation in hypertension that involves oxidative stress mechanisms.

G protein-coupled receptor kinases: more than just kinases and not only for GPCRs

G protein-coupled receptor (GPCR) kinases (GRKs) are best known for their role in homologous desensitization of GPCRs. GRKs phosphorylate activated receptors and promote high affinity binding of arrestins, which precludes G protein coupling. GRKs have a multidomain structure, with the kinase domain inserted into a loop of a regulator of G protein signaling homology domain. Unlike many other kinases, GRKs do not need to be phosphorylated in their activation loop to achieve an activated state. Instead, they are directly activated by docking with active GPCRs. In this manner they are able to selectively phosphorylate Ser/Thr residues on only the activated form of the receptor, unlike related kinases such as protein kinase A. GRKs also phosphorylate a variety of non-GPCR substrates and regulate several signaling pathways via direct interactions with other proteins in a phosphorylation-independent manner. Multiple GRK subtypes are present in virtually every animal cell, with the highest expression levels found in neurons, with their extensive and complex signal regulation. Insufficient or excessive GRK activity was implicated in a variety of human disorders, ranging from heart failure to depression to Parkinson's disease. As key regulators of GPCR-dependent and -independent signaling pathways, GRKs are emerging drug targets and promising molecular tools for therapy. Targeted modulation of expression and/or of activity of several GRK isoforms for therapeutic purposes was recently validated in cardiac disorders and Parkinson's disease.

Dysfunction of endothelium-dependent relaxation to insulin via PKC-mediated GRK2/Akt activation in aortas of ob/ob mice

In diabetic states, hyperinsulinemia may negatively regulate Akt/endothelial nitric oxide synthase (eNOS) activation. Our main aim was to investigate whether and how insulin might negatively regulate Akt/eNOS activities via G protein-coupled receptor kinase 2 (GRK2) in aortas from ob/ob mice. Endothelium-dependent relaxation was measured in aortic rings from ob/ob mice (a type 2 diabetes model). GRK2, β-arrestin2, and Akt/eNOS signaling-pathway protein levels and activities were mainly assayed by Western blotting. Plasma insulin was significantly elevated in ob/ob mice. Insulin-induced relaxation was significantly decreased in the ob/ob aortas [vs. age-matched control (lean) ones]. The response in ob/ob aortas was enhanced by PKC inhibitor or GRK2 inhibitor. Akt (at Thr308) phosphorylation and eNOS (at Ser1177) phosphorylation, and also the β-arrestin2 protein level, were markedly decreased in the membrane fraction of insulin-stimulated ob/ob aortas (vs. insulin-stimulated lean ones). These membrane-fraction expressions were enhanced by GRK2 inhibitor and by PKC inhibitor in the ob/ob group but not in the lean group. PKC activity was much greater in ob/ob than in lean aortas. GRK2 protein and activity levels were increased in ob/ob and were greatly reduced by GRK2 inhibitor or PKC inhibitor pretreatment. These results suggest that in the aorta in diabetic mice with hyperinsulinemia an upregulation of GRK2 and a decrease in β-arrestin2 inhibit insulin-induced stimulation of the Akt/eNOS pathway and that GRK2 overactivation may result from an increase in PKC activity.

G Protein-coupled receptor kinase 2 (GRK2): A novel modulator of insulin resistance

G protein-coupled receptor kinase 2 (GRK2) is emerging as a key, integrative node in many signalling pathways. Besides its canonical role in the modulation of the signalling mediated by many G protein-coupled receptors (GPCR), this protein can display a very complex network of functional interactions with a variety of signal transduction partners, in a stimulus, cell type, or context-specific way. We review herein recent data showing that GRK2 can regulate insulin-triggered transduction cascades at different levels and that this protein plays a relevant role in insulin resistance and obesity in vivo, what uncovers GRK2 as a potential therapeutic target in the treatment of these disorders.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Inhibitory effect of the D(3) dopamine receptor on insulin receptor expression and function in vascular smooth muscle cells

BACKGROUND

Vascular smooth muscle cell (VSMC) proliferation is regulated by numerous hormones and humoral factors. Our previous study found that stimulation of D(1)-like dopamine receptors inhibited insulin receptor expression and function in VSMCs. We hypothesize that there is also an interaction between D(3) dopamine and insulin receptors, i.e., stimulation of the D(3) receptor inhibits insulin receptor expression and function.

METHODS

Receptor expression was determined by immunoblotting, immunohistochemisty, and reverse transcriptase-PCR; VSMC proliferation was determined by 3-(4,5-dimethylthiazol-2-yl)-diphenyl-tetrazolium bromide (MTT) assay and cell number.

RESULTS

Insulin receptor protein is increased in the aorta of D(3) receptor deficient mice. Stimulation of the D(3) receptor inhibited insulin receptor mRNA and protein expression and insulin-mediated VSMC proliferation, and increased protein kinase A (PKA) activity, insulin receptor phosphorylation, and degradation in immortalized aortic VSMCs (A10 cells). These effects were blocked by a PKA inhibitor, indicating that the D(3) receptor-mediated decrease in insulin receptor expression was related to a decrease in transcription/post-transcription and increased degradation, involving PKA signaling.

CONCLUSIONS

D(3) receptor stimulation may be a target to reduce the adverse effect of insulin in hypertension by inhibition of insulin receptor expression and function in arterial VSMCs.

Exercise reduces oxidative stress but does not alleviate hyperinsulinemia or renal dopamine D1 receptor dysfunction in obese rats

Impairment of renal dopamine D1 receptor (D1R)-mediated natriuresis is associated with hypertension in humans and animal models, including obese Zucker rats. We have previously reported that treatment of these rats with antioxidants or insulin sensitizers reduced insulin levels and oxidative stress, restored D1R-mediated natriuresis, and reduced blood pressure. Furthermore, the redox-sensitive transcription factor, nuclear factor-κB (NF-κB), has been implicated in impairment of D1R-mediated natriuresis during oxidative stress. In this study, we investigated the effect of exercise on insulin levels, oxidative stress, nuclear translocation of NF-κB, blood pressure, albuminuria, and D1R-mediated natriuresis. The exercise protocol involved treadmill exercise from 3 wk of age for 8 wk. Exercise reduced oxidative stress, nuclear translocation of NF-κB, and albuminuria. However, exercise did not reduce plasma insulin levels or blood pressure. Also, selective D1R agonist (SKF-38393)-mediated increases in sodium excretion and guanosine 5'-O-(3-thiotriphosphate) binding were impaired in obese rats compared with lean rats, and exercise did not restore this defect. We conclude that, while exercise is beneficial in reducing oxidative stress and renal injury, reducing insulin levels may be required to restore D1R-mediated natriuresis in this model of obesity and metabolic syndrome. Furthermore, this study supports previous observations that restoring D1R function contributes to blood pressure reduction in this model.

G protein-coupled receptor kinase 2 plays a relevant role in insulin resistance and obesity

OBJECTIVE

Insulin resistance is associated with the pathogenesis of metabolic disorders as type 2 diabetes and obesity. Given the emerging role of signal transduction in these syndromes, we set out to explore the possible role that G protein-coupled receptor kinase 2 (GRK2), first identified as a G protein-coupled receptor regulator, could have as a modulator of insulin responses.

RESEARCH DESIGN AND METHODS

We analyzed the influence of GRK2 levels in insulin signaling in myoblasts and adipocytes with experimentally increased or silenced levels of GRK2, as well as in GRK2 hemizygous animals expressing 50% lower levels of this kinase in three different models of insulin resistance: tumor necrosis factor-α (TNF-α) infusion, aging, and high-fat diet (HFD). Glucose transport, whole-body glucose and insulin tolerance, the activation status of insulin pathway components, and the circulating levels of important mediators were measured. The development of obesity and adipocyte size with age and HFD was analyzed.

RESULTS

Altering GRK2 levels markedly modifies insulin-mediated signaling in cultured adipocytes and myocytes. GRK2 levels are increased by ∼2-fold in muscle and adipose tissue in the animal models tested, as well as in lymphocytes from metabolic syndrome patients. In contrast, hemizygous GRK2 mice show enhanced insulin sensitivity and do not develop insulin resistance by TNF-α, aging, or HFD. Furthermore, reduced GRK2 levels induce a lean phenotype and decrease age-related adiposity.

CONCLUSIONS

Overall, our data identify GRK2 as an important negative regulator of insulin effects, key to the etiopathogenesis of insulin resistance and obesity, which uncovers this protein as a potential therapeutic target in the treatment of these disorders.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

G-protein-coupled receptor kinases in cardiovascular conditions: focus on G-protein-coupled receptor kinase 2, a gain in translational medicine

With increasing knowledge of the regulatory mechanisms of G-protein-coupled receptor signaling in heart physiology, many studies have focused on the role of this system in cardiovascular disease. In recent years, scientists have moved their attention from the receptors to their regulatory proteins: the G-protein-coupled receptor kinases. This class of protein is indispensable for terminating signaling of G-protein-coupled receptors through receptor desensitization and downregulation. This article attempts to assemble the currently available information regarding G-protein-coupled receptor kinases and their role in cardiovascular disease and, in particular, the potential employment of G-protein-coupled receptor kinase 2 as biomarker of cardiac dysfunction.

Insulin resistance and impaired functional vasodilation in obese Zucker rats

Individuals with metabolic syndrome exhibit insulin resistance and an attenuated functional vasodilatory response to exercise. We have shown that impaired functional vasodilation in obese Zucker rats (OZRs) is associated with enhanced thromboxane receptor (TP)-mediated vasoconstriction. We hypothesized that insulin resistance, hyperglycemia/hyperlipidemia, and the resultant ROS are responsible for the increased TP-mediated vasoconstriction in OZRs, resulting in impaired functional vasodilation. Eleven-week-old male lean Zucker rats (LZRs) and OZRs were fed normal rat chow or chow containing rosiglitazone (5 mg.kg(-1).day(-1)) for 2 wk. In another set of experiment, LZRs and OZRs were treated with 2 mM tempol (drinking water) for 7-10 days. After the treatments, spinotrapezius muscles were prepared, and arcade arteriolar diameters were measured following muscle stimulation and arachidonic acid (AA) application (10 muM) in the absence and presence of the TP antagonist SQ-29548 (1 muM). OZRs exhibited higher insulin, glucose, triglyceride, and superoxide levels and increased NADPH oxidase activity compared with LZRs. Functional and AA-induced vasodilations were impaired in OZRs. Rosiglitazone treatment improved insulin, glucose, triglyceride, and superoxide levels as well as NADHP oxidase activity in OZRs. Both rosiglitazone and tempol treatment improved vasodilatory responses in OZRs with no effect in LZRs. SQ-29548 treatment improved vasodilatory responses in nontreated OZRs with no effect in LZRs or treated OZRs. These results suggest that insulin resistance and the resultant increased ROS impair functional dilation in OZRs by increasing TP-mediated vasoconstriction.

Nutrigenomics: concepts and applications to pharmacogenomics and clinical medicine

The maintenance of health and the prevention and treatment of chronic diseases are influenced by naturally occurring chemicals in foods. In addition to supplying the substrates for producing energy, a large number of dietary chemicals are bioactive--that is, they alter the regulation of biological processes and, either directly or indirectly, the expression of genetic information. Nutrients and bioactives may produce different physiological phenotypes among individuals because of genetic variability and not only alter health, but also disease initiation, progression and severity. The study and application of gene-nutrient interactions is called nutritional genomics or nutrigenomics. Nutrigenomic concepts, research strategies and clinical implementation are similar to and overlap those of pharmacogenomics, and both are fundamental to the treatment of disease and maintenance of optimal health.

Insulin causes renal dopamine D1 receptor desensitization via GRK2-mediated receptor phosphorylation involving phosphatidylinositol 3-kinase and protein kinase C

The renal dopamine system plays an important role in sodium homeostasis and a defect in dopamine D1 receptor (D1R) function is present in hypertension, diabetes, and aging. Our previous studies in hyperinsulinemic animals and in renal cell cultures treated with insulin showed decrease in D1R number and defective coupling to G proteins; however, the exact mechanisms remained unknown. Therefore, we investigated insulin-mediated D1R desensitization and underlying molecular mechanism in opossum kidney (OK) cells. Chronic exposure (24 h) of OK cells to 10 nM insulin caused significant decrease in D1R number and agonist affinity. The D1R was hyperserine phosphorylated, uncoupled from G proteins and SKF38393 , a D1R agonist, failed to stimulate G proteins and inhibit Na-K-ATPase activity. Insulin increased protein kinase C (PKC) activity and caused G protein-coupled receptor kinase 2 (GRK2) translocation to the membranes. Tyrosine kinase inhibitor genistein and phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin blocked insulin-mediated PKC activation and GRK2 membranous translocation. In addition to genistein and wortmannin, GRK2 membranous tranlocation was also blocked by PKC inhibitor chelerythrine chloride and GRK2-specific siRNA. Genistein, wortmannin, chelerythrine chloride, and GRK2 siRNA abrogated D1R serine phosphorylation and normalized D1R expression and affinity in insulin-treated cells. Furthermore, these inhibitors and siRNA restored D1R G protein coupling and ability of SKF38393 to inhibit Na-K-ATPase activity. In conclusion, insulin-induced D1R desensitization involves PI3K, PKC, and GRK2. Insulin activates PI3K-PKC-GRK2 cascade, causing D1R serine phosphorylation, which leads to D1R downregulation and uncoupling from G proteins, and results in the failure of SKF38393 to stimulate G proteins and inhibit Na-K-ATPase activity.

Oxidative stress reduces renal dopamine D1 receptor-Gq/11alpha G protein-phospholipase C signaling involving G protein-coupled receptor kinase 2

The dopamine D1 receptors (D1R), expressed in renal proximal tubules, participate in the regulation of sodium transport. A defect in the coupling of the D1R to its G protein/effector complex in renal tubules has been reported in various conditions associated with oxidative stress. Because G protein-coupled receptor kinases (GRKs) are known to play an important role in D1R desensitization, we tested the hypothesis that increased oxidative stress in obese Zucker rats may cause GRK2 upregulation and, subsequently, D1R dysfunction. Lean and obese rats were given normal diet or diet supplemented with antioxidant lipoic acid for 2 wk. Compared with lean rats, obese rats exhibited oxidative stress, D1R were uncoupled from G(q/11)alpha at basal level, and SKF-38393 failed to elicit D1R-G protein coupling, stimulate phospholipase C (PLC), and inhibit Na-K-ATPase activity. These animals showed increased basal protein kinase C (PKC) activity and membranous translocation of GRK2 and increased GKR2-G(q/11)alpha interaction and D1R serine phosphorylation. Enzymatic dephosphorylation of D1R restored SKF-38393-induced adenylyl cyclase stimulation but not PLC activation. Treatment of obese rats with lipoic acid restored D1R-G protein coupling and SKF-38393-induced PLC stimulation and Na-K-ATPase inhibition. Lipoic acid treatment also normalized PKC activity, GRK2 sequestration, and GKR2-G(q/11)alpha interaction. In conclusion, these data show that oxidative stress increases PKC activity causing GRK2 membranous translocation. GRK2 interacts with G(q/11)alpha and acts, at least in part, as a regulator of G protein signaling leading to the D1R-G(q/11)alpha uncoupling, causing inability of SKF-38393 to stimulate PLC and inhibit Na/K-ATPase. Lipoic acid, while reducing oxidative stress, normalized PKC activity and restored D1R-G(q/11)alpha-PLC signaling and the ability of SKF-38393 to inhibit Na-K-ATPase activity.

Role of oxidative stress in defective renal dopamine D1 receptor-G protein coupling and function in old Fischer 344 rats

Aging is associated with an increase in oxidative stress. Previously, we have reported that dopamine failed to inhibit proximal tubular Na-K-ATPase and to promote sodium excretion in old rats (Beheray S, Kansra V, Hussain T, and Lokhandwala MF. Kidney Int 58: 712–720, 2000). This was due to uncoupling of dopamine D1 receptors from G proteins resulting from hyperphosphorylation of D1 receptors. The present study was designed to test the role of oxidative stress in the age-related decline in renal dopamine D1 receptor function. We observed that old animals had increased malondialdehyde (MDA) levels, a biomarker of oxidative stress, and decreased D1 receptor number and protein in the proximal tubules (PT) compared with adult rats. In old rats, there was increased G protein-coupled receptor kinase-2 (GRK-2) abundance, increased basal serine phosphorylation of D1 receptors, and defective D1 receptor-G protein coupling in PT membranes. Interestingly, supplementation with an antioxidant, tempol (1 mmol/l in drinking water for 15 days), lowered MDA levels and normalized D1 receptor number and protein in old rats to the level seen in adult rats. Furthermore, tempol decreased GRK-2 abundance and D1 receptor serine phosphorylation and restored D1 receptor-G protein coupling in PT of old rats. The functional consequence of these changes was the restoration of the natriuretic response to D1 receptor activation in tempol-supplemented old rats. Therefore, in old rats, tempol reduces oxidative stress and prevents GRK-2 membranous abundance and hyperphosphorylation of D1 receptors, resulting in restoration of D1 receptor-G protein coupling and the natriuretic response to SKF-38393. Thus tempol, by lowering oxidative stress, normalizes the age-related decline in dopamine receptor function.

The complex role of PPARgamma in renal dysfunction in obesity: managing a Janus-faced receptor

Obesity is frequently accompanied by insulin resistance, type II diabetes, hypertension and atherosclerosis, a cluster of pathologies that are the major components of the metabolic syndrome. Obesity is a known cause for renal dysfunction that leads to two major renal pathologies: hypertension and glomerular and tubulointerstitial injury. Peroxizome proliferator activated receptors (PPARs) are transcription factors belonging to the nuclear hormone receptor superfamily with important functions in the regulation of metabolism. The role of PPARgamma isoforms in adipogenesis and vascular inflammation associated to obesity has been vastly studied and is well recognized, albeit not completely mechanistically understood. Also, the effect of various PPARgamma agonists on blood pressure reduction in different forms of hypertension, including obesity related hypertension has been reported, but the mechanisms involved are only beginning to be studied. Even less clear is the concurrent beneficial effect of PPARgamma agonists thiazolinendiones (TZD) on blood pressure reduction in different forms of hypertension and, at the same time, in some cases, the significant water retention leading to edema and heart failure. The occurrence of both these apparently opposite effects on the renal water and sodium handling suggests a complex role of PPARgamma in the kidney that is likely related to the metabolic state. Also, PPARgamma activation leads to a reduction in mesangial cell proliferation while stimulating apoptosis. TZD treatment reduces albuminuria in obese and diabetic humans and rodent models suggesting protective effects against renal tubuloglomerular injury. The focus of this review is to present and critically discuss the recent findings on the roles of PPARgamma in the kidney in direct relation to renal function and renal injury in obesity and obesity-initiated diabetes.

Tempol reduces oxidative stress and restores renal dopamine D1-like receptor- G protein coupling and function in hyperglycemic rats

Dopamine via activation of renal D1-like receptors inhibits the activities of Na-K-ATPase and Na/H exchanger and subsequently increases sodium excretion. Decreased renal dopamine production and sodium excretion are associated with hyperglycemic conditions. We have earlier reported D1-like receptor-G protein uncoupling and reduced response to D1-like receptor activation in streptozotocin (STZ)-treated hyperglycemic rats (Marwaha A, Banday AA, and Lokhandwala MF. Am J Physiol Renal Physiol 286: F451-F457, 2004). The present study was designed to test the hypothesis that oxidative stress associated with hyperglycemia increases basal D1-like receptor serine phosphorylation via activation of the PKC-G protein receptor kinase (GRK) pathway, resulting in loss of D1-like receptor-G protein coupling and function. We observed that STZ-treated rats exhibited oxidative stress as evidenced by increased lipid peroxidation. Furthermore, PKC activity and expression of PKC-betaI- and -delta-isoforms were increased in STZ-treated rats. In addition, in STZ-treated rats there was increased GRK2 translocation to proximal tubular membrane and increased basal serine D1-like receptor phosphorylation. Supplementation with the antioxidant tempol lowered oxidative stress in STZ-treated rats, led to normalization of PKC activity, and prevented GRK2 translocation. Furthermore, tempol supplementation in STZ-treated rats restored D1-like receptor-G protein coupling and inhibition of Na-K-ATPase activity on D1-like receptor agonist stimulation. The functional consequence was the restoration of the natriuretic response to D1-like receptor activation. We conclude that oxidative stress associated with hyperglycemia causes an increase in activity and expression of PKC. This leads to translocation of GRK2, subsequent phosphorylation of the D1-like receptor, its uncoupling from G proteins and loss of responsiveness to agonist stimulation.