The Na, K-ATPase generates an asymmetric ion gradient that supports multiple cellular functions, including the control of cellular volume, neuronal excitability, secondary ionic transport, and the movement of molecules like amino acids and glucose. The intracellular and extracellular levels of Na+ and K+ ions are the classical local regulators of the enzyme's activity. Additionally, the regulation of Na, K-ATPase is a complex process that occurs at multiple levels, encompassing its total cellular content, subcellular distribution, and intrinsic activity. In this context, the enzyme serves as a regulatory target for hormones, either through direct actions or via signaling cascades triggered by hormone receptors. Notably, FXYDs small transmembrane proteins regulators of Na, K-ATPase serve as intermediaries linking hormonal signaling to enzymatic regulation at various levels. Specifically, members of the FXYD family, particularly FXYD1 and FXYD2, are that undergo phosphorylation by kinases activated through hormone receptor signaling, which subsequently influences their modulation of Na, K-ATPase activity. This review describes the effects of FXYD2, cardiotonic steroid signaling, and hormones such as angiotensin II, dopamine, insulin, and catecholamines on the regulation of Na, K-ATPase. Furthermore, this review highlights the implications of Na, K-ATPase in diseases such as hypertension, renal hypomagnesemia, and cancer.

Aging is associated with a combination of several lower urinary tract (LUT) signs and symptoms, including residual urine, overactive bladder and nocturia. One of the mechanisms of this LUT dysfunction that has not been discussed in dept so far is the role of dopamine (DA).

In this narrative review, we explore the dopaminergic hypothesis in the development of this combination of LUT signs and symptoms in older adults.

DA is one of the neurotransmitters whose regulation and production is disrupted in aging. In synucleinopathies, altered DAergic activity is associated with the occurrence of LUTS and sleep disorders. Projections of DAergic neurons are involved in the regulation of sleep, diuresis, and bladder activity. The low dopamine hypothesis could explain the genesis of a set of LUT signs and symptoms commonly seen in this population, including elevated residual urine, Overactive bladder syndrome and Nocturia (discussed as the RON syndrome). This presentation is however also common in older patients without synucleinopathies or neurological disorders and therefore we hypothesise that altered DAergic activity because of pathological aging, and selective destruction of DAergic neurons, could underpin the presentation of this triad of LUT dysfunction in the older population.

The concept of RON syndrome helps to better understand this common phenotypic presentation in clinical practice, and therefore serves as a useful platform to diagnose and treat LUTS in older adults. Besides recognizing the synucleinopathy "red flag" symptoms, this set of multi-causal LUT signs and symptoms highlights the inevitable need for combination therapy, a challenge in older people with their comorbidities and concomitant medications.

For a better insight into relations between type 2 diabetes mellitus (T2DM) and Na,K-ATPase properties in kidneys, we aimed to characterize two subgroups of ZDF obese (fa/fa) rats, with more and less developed T2DM, and compare them with two controls: lean (fa/+) and Wistar. Na,K-ATPase enzyme kinetics were estimated by measuring the ATP hydrolysis in the range of NaCl and ATP levels. As Na,K-ATPase is sensitive to oxidative stress, we evaluated selected oxidative stress parameters in kidney homogenates. Our results suggest that thiol-disulfide redox balance in the renal medulla and Na,K-ATPase properties in the renal cortex differ between both controls, while observed measurements in lean (fa/+) rats showed deviation towards the values observed in ZDF (fa/fa) rats. In comparison with both controls, Na,K-ATPase enzyme activity was higher in the renal cortex of ZDF rats independent of diabetes severity. This might be a consequence of increased glucose load in tubular fluid. The increase in lipid peroxidation observed in the renal cortex of ZDF rats was not associated with Na,K-ATPase activity impairment. Regarding the differences between subgroups of ZDF animals, well-developed T2DM (glycemia higher than 10 mmol/L) was associated with a higher ability of Na,K-ATPase to utilize the ATP energy substrate.

The dopaminergic system can adapt to the different physiological or pathological situations to which the kidneys are subjected throughout life, maintaining homeostasis of natriuresis, extracellular volume, and blood pressure levels. The role of renal dopamine receptor dysfunction is clearly established in the pathogenesis of essential hypertension. Its associations with other pathological states such as insulin resistance and redox balance have also been associated with dysfunction of the dopaminergic system. The different dopamine receptors (D1-D5) show a protective effect against hypertension and kidney disorders. It is essential to take into account the various interactions of the dopaminergic system with other elements, such as adrenergic receptors. The approach to therapeutic strategies for essential hypertension must go through the blocking of those elements that lead to renal vasoconstriction or the restoration of the normal functioning of dopamine receptors. D1-like receptors are fundamental in this role, and new therapeutic efforts should be directed to the restoration of their functioning in many patients. More studies will be needed to allow the development of drugs that can be targeted to renal dopamine receptors in the treatment of hypertension.

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.

High sodium intake is known to regulate the renal renin-angiotensin system (RAS) and is a risk factor for the pathogenesis of obesity-related hypertension. The complex nature of the RAS reveals that its various components may have opposing effects on natriuresis and blood pressure regulation. We hypothesized that high sodium intake differentially regulates and shifts a balance between opposing components of the renal RAS, namely, angiotensin-converting enzyme (ACE)-ANG II-type 1 ANG II receptor (AT(1)R) vs. AT(2)-ACE2-angiotensinogen (Ang) (1-7)-Mas receptor (MasR), in obesity. In the present study, we evaluated protein and/or mRNA expression of angiotensinogen, renin, AT(1A/B)R, ACE, AT(2)R, ACE2, and MasR in the kidney cortex following 2 wk of a 8% high-sodium (HS) diet in lean and obese Zucker rats. The expression data showed that the relative expression pattern of ACE and AT(1B)R increased, renin decreased, and ACE2, AT(2)R, and MasR remained unaltered in HS-fed lean rats. On the other hand, HS intake in obese rats caused an increase in the cortical expression of ACE, a decrease in ACE2, AT(2)R, and MasR, and no changes in renin and AT(1)R. The cortical levels of ANG II increased by threefold in obese rats on HS compared with obese rats on normal salt (NS), which was not different than in lean rats. The HS intake elevated mean arterial pressure in obese rats (27 mmHg) more than in lean rats (16 mmHg). This study suggests that HS intake causes a pronounced increase in ANG II levels and a reduction in the expression of the ACE2-AT(2)R-MasR axis in the kidney cortex of obese rats. We conclude that such changes may lead to the potentially unopposed function of AT(1)R, with its various cellular and physiological roles, including the contribution to the pathogenesis of obesity-related hypertension.

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.

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.

Activation of D1 dopamine receptors expressed in the kidneys promotes the excretion of sodium and regulates sodium levels during increases in dietary sodium intake. A decrease in the expression or function of D1 receptors results in increased sodium retention which can potentially lead to the development of hypertension. Studies have shown that in the absence of functional D1 receptors, in null mice, the systolic, diastolic, and mean arterial pressures are higher. Previous studies have shown that the expression and function of D1 receptors in the kidneys are decreased in animal models of diabetes. The mechanisms that down-regulate the expression of renal D1 receptor gene in diabetes are not well understood. Using primary renal cells and acutely isolated kidneys from the streptozotocin-induced rat diabetic model, we demonstrate that the renal D1 receptor expression is down-regulated by the extracellular cAMP-adenosine pathway in vitro and in vivo. In cultures of primary renal cells, a 3 mm, 60-h cAMP treatment down-regulated the expression of D1 receptors. In vivo, we determined that the plasma and urine cAMP levels as well as the expression of 5'-ectonucleotidase, tissue-nonspecific alkaline phosphatase, and adenosine A2a receptors are significantly increased in diabetic rats. Inhibitors of 5'-ectonucleotidase and tissue-nonspecific alkaline phosphatase, α,β-methyleneadenosine 5'-diphosphate, and levamisole, respectively, blocked the down-regulation of D1 receptors in the primary renal cells and in the kidney of diabetic animals. The results suggest that inhibitors of the extracellular cAMP-adenosine pathway reverse the down-regulation of renal D1 receptor in diabetes.

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.

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.

In this study, we investigated the effect of a high n-3 fatty acid diet (eicosapentaenoic and docosahexaenoic acids) in Zucker obese and lean rats on blood pressure in association with physiological parameters, serum biochemistry and oxidative stress analysis. After 150 days of treatment, dietary fish oil supplementation in Zucker obese rats (9 months of age) reduces bodyweight gain and serum triglyceridemia and nitrite levels, increases serum glucose and angiotensin converting enzyme activity, but does not alter blood pressure, cholesterol levels and serum markers of oxidative stress (malondialdehyde, glutathione), compared to the Zucker rats fed control diet. According to these results, we can consider that after 150 days of treatment, fish oil is not enough to regulate parameters involved in the metabolic syndrome, such as cholesterolemia and blood pressure, in a 9 month-old genetically type-2 diabetes rat.

Dopamine produced by renal proximal tubules increases sodium excretion via a decrease in renal sodium reabsorption. Dopamine natriuresis is impaired in obese Zucker rats; however, the mechanism is not fully understood. To test the hypothesis that renal expression of one or more of the subtypes are altered in these rats, we measured whole kidney protein levels by immunoblotting of D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) dopamine receptors in both male and female obese and lean Zucker rats. In obese males on 1% NaCl diet, D1R, D2R, D4R, and D5R were decreased, while D3R was increased, relative to lean rats. Under a 4% NaCl diet, D2R and D3R levels in obese rats were restored to lean levels. 4% NaCl diet reduced D5R in both body types, relative to 1% NaCl diet. Female rats had higher expression of D1R and D3R than did male; however, the sex difference for D1R was markedly blunted in obese rats. In obese rats, dietary candesartan (angiotensin II type 1 receptor blocker) normalized downregulated D1R and D2R, but either decreased (D3R), did not affect (D4R), or further downregulated (D5R) the other subtypes. Candesartan also decreased D4R in lean rats. In summary, reduced renal protein levels of D1R, D2R, D4R, and D5R in obese Zucker rats could induce salt sensitivity and elevate blood pressure. Increased angiotensin II type 1 receptor activity may be mechanistically involved in the decreased expression of D1R and D2R in obese rats. Finally, reduced D1R and D3R in male rats may contribute to sex differences in blood pressure.

It was demonstrated in streptozotocin (STZ)-induced diabetic rats that the D(1) receptor agonist failed to promote sodium excretion as a result of reduced renal D(1) receptor expression and decreased receptor G protein coupling. The present study examined the influence of glycaemic control with insulin on the renal D(1) receptor dysfunction in STZ-induced type 1 diabetes.

Renal function, blood pressure, the natriuretic response to 5% volume expansion (VE) and the effects of the D(1) receptor agonist fenoldopam on natriuresis and on Na(+)/K(+)-ATPase activity in renal tubules were evaluated in uninephrectomized and sham-operated Wistar rats treated with STZ and compared with controls and STZ-treated rats made euglycaemic with insulin. D(1) receptor immunohistochemistry and protein abundance by western blot were also determined in all groups.

Treatment of sham and uninephrectomized rats with STZ caused a 4-fold increase in glucose plasma levels compared to controls and euglycaemic diabetic rats. A blunted natriuretic response to VE was observed in both sham and uninephrectomized hyperglycaemic diabetic rats, and this was accompanied by failure of fenoldopam to increase natriuresis and to inhibit renal Na(+)/K(+)-ATPase activity. In contrast, in both sham and uninephrectomized euglycaemic diabetic rats, the natriuretic response to VE, the fenoldopam-induced natriuresis and the accompanied inhibition of Na(+)/K(+)-ATPase activity were similar to those of the corresponding controls. D(1) receptor immunodetection and protein abundance were reduced in hyperglycaemic diabetic rats, but not in euglycaemic diabetic animals.

We conclude that the renal expression and natriuretic response to D(1) receptor activation is compromised in both sham and uninephrectomized rats with STZ-induced diabetes. These abnormalities were prevented by lowering glucose blood levels with insulin, thus providing evidence for the involvement of hyperglycaemia in the disturbances that underlie the compromised dopamine-sensitive natriuresis and increase of blood pressure in type 1 diabetes.

We tested the effects of inflammation on renal dopamine D1 receptor signaling cascade, a key pathway that maintains sodium homeostasis and blood pressure during increased salt intake. Inflammation was produced by administering lipopolysaccharide (LPS; 4 mg/kg ip) to rats provided without (normal salt) and with 1% NaCl in drinking water for 2 wk (high salt). Control rats had saline injection and received tap water. We found that LPS increased the levels of inflammatory cytokines, interleukin-6, and tumor necrosis factor-alpha in the rats given either normal- or high-salt intake. Also, these rats had higher levels of oxidative stress markers, malondialdehyde and nitrotyrosine, and lower levels of antioxidant enzyme superoxide dismutase in the renal proximal tubules (RPTs). The nuclear levels of transcription factors NF-kappaB increased and Nrf2 decreased in the RPTs in response to LPS in rats given normal and high salt. Furthermore, D1 receptor numbers, D1 receptor proteins, and D1 receptor agonist (SKF38393)-mediated (35)S-GTPgammaS binding decreased in the RPTs in these rats. The basal activities of Na-K-ATPase in the RPTs were similar in control and LPS-treated rats given normal and high salt. SKF38393 caused inhibition of Na-K-ATPase activity in the primary cultures of RPTs treated with vehicle but not in the cultures treated with LPS. Furthermore, LPS caused an increase in blood pressure in the rats given high salt but not in the rats given normal salt. These results suggest that LPS differentially regulates NF-kappaB and Nrf2, produces inflammation, decreases antioxidant enzyme, increases oxidative stress, and causes D1 receptor dysfunction in the RPTs. The LPS-induced dysfunction of renal D1 receptors alters salt handling and causes hypertension in rats during salt overload.

Insulin resistance is associated with hypertension by mechanisms likely involving the kidney. To determine how the major apical sodium transporter of the thick ascending limb, the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) is regulated by high-fat feeding, we treated young male, Fischer 344 X Brown Norway (F344BN) rats for 8 wk with diets containing either normal (NF, 4%) or high (HF, 36%) fat, by weight, primarily as lard. HF-fed rats had impaired glucose tolerance, increased urine excretion of 8-isoprostane (a marker of oxidative stress), increased protein levels for NKCC2 (50-125%) and the renal outer medullary potassium channel (106%), as well as increased natriuretic response to furosemide (20-40%). To test the role of oxidative stress in this response, in study 2, rats were fed the NF or HF diet plus plain drinking water, or water containing N(G)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor (100 mg/l), or tempol, a superoxide dismutase mimetic (1 mmol/l). The combination of tempol with HF nullified the increase in medullary NKCC2, while l-NAME with HF led to the highest expression of medullary NKCC2 (to 498% of NF mean). However, neither of these drugs dramatically affected the elevated natriuretic response to furosemide with HF. Finally, l-NAME led to a marked increase in blood pressure (measured by radiotelemetry), which was significantly enhanced with HF. Mean arterial blood pressure at 7 wk was as follows (mmHg): NF, 100 +/- 2; NF plus l-NAME, 122 +/- 3; and HF plus l-NAME, 131 +/- 2. Overall, HF feeding increased the abundance of NKCC2. Inappropriately high sodium reabsorption in the thick ascending limb via NKCC2 may contribute to hypertension with insulin resistance.

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.

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.

The expression of D1 dopamine (DA) receptor gene is regulated during development, aging, and pathophysiology. The extracellular factors and signaling mechanisms that modulate the expression of D1 DA receptor have not been well characterized. Here, we present novel evidence that endogenous D1 DA receptor expression is inhibited by extracellular cAMP in the Cath.A Derived (CAD) catecholaminergic neuronal cell line. CAD cells express the multi-drug resistance protein 5 transporters and secrete cAMP. Addition of exogenous cAMP decreases D1 receptor mRNA and protein greater than fourfold in 24 h. The cAMP-induced decrease of D1 receptor mRNA levels is blocked by cGMP and by 1,3-dipropyl-8-(p-sulfo-phenyl)xanthine, an inhibitor of ecto-phosphodiestrase. Extracellular AMP, a metabolite of cAMP, also independently decreased D1 receptor mRNA levels. Inhibitors of ecto-nucleotidases, alpha,beta-methyleneadenosine 5'-di-phosphate and GMP, completely blocked the decrease of D1 receptor mRNA by extracellular cAMP, but only partially blocked the decrease induced by extracellular AMP. Levamisole, an inhibitor of tissue non-specific alkaline phosphatase, completely blocked the AMP-induced decrease of D1 receptor mRNA. The extracellular cAMP, AMP, and adenosine (ADO)-induced decrease in D1 receptor mRNA expression are mediated by A2a ADO receptor subtype. The results suggest a novel molecular mechanism linking activation of A2a ADO receptors with inhibition of D1 DA receptor expression.

Abnormal neurohormonal regulation of renal sodium handling plays an important role in obesity-associated hypertension. We investigated the effect of experimental obesity on renal response to atrial natriuretic peptide (ANP).

The effect of ANP was studied in three groups of rats: (1) lean controls, (2) animals made obese by a highly palatable diet, (3) rats treated with adipose tissue hormone, leptin, for 7 days to reproduce hyperleptinemia observed in obesity.

ANP administered at a dose of 50 pmol/kg min(-1) induced about a 3-fold lower increase in Na+ and cGMP excretion in obese and leptin-treated rats than in the control group. ANP decreased Na+,K+-ATPase activity in the renal medulla only in the control group. Natriuretic effect of exogenous cGMP was also impaired in obese and leptin-treated rats. In contrast, hydrolysis-resistant cGMP derivative, 8-bromo-cGMP exerted comparable natriuretic effects in all groups. Neutral endopeptidase inhibitor, phosphoramidon, and ANP clearance receptor antagonist, C-ANP, increased urinary ANP excretion in all groups to a similar level, but their natriuretic effect was impaired in obese and leptin-treated groups. A specific inhibitor of cGMP-degrading phosphodiesterase, zaprinast, had comparable natriuretic and Na+,K+-ATPase-lowering effects in all groups and restored normal sensitivity to ANP.

(1) Dietary-induced obesity is accompanied by impaired natriuretic effect of ANP, (2) ANP resistance in obesity may be accounted for by increased leptin level, (3) accelerated degradation of cGMP may contribute to ANP resistance associated with obesity and hyperleptinemia, suggesting that inhibiting cGMP-specific phosphodiesterases may be useful in the treatment of obesity-associated hypertension.

Renal angiotensin II (AII) is suggested to play a role in the enhanced sodium reabsorption that causes a shift in pressure natriuresis in obesity related hypertension; however, the mechanism is not known. Therefore, to assess the influence of AII on tubular sodium transport, we determined the effect of AII on the Na+, K+-ATPase activity (NKA), an active transporter regulated by the AT1 receptor activity, in the isolated proximal tubules of lean and obese Zucker rats. Also, we determined the levels of the tubular AT1 receptor and associated signal transducing G proteins, as the initial signaling components that mediate the effects of AII on Na+, K+-ATPase activity. In the isolated proximal tubules, AII produced greater stimulation of the NKA activity in obese compared with lean rats. Determination of the AT1 receptors by Scatchard analysis of the [125I] Sar-Ang II binding and Western blot analysis in the basolateral (BLM) and brush border membrane (BBM) revealed a modest but significant increase (23%) in the AT1 receptor number mainly in the BLM of obese compared with lean rats. The AII affinity for AT1 receptors, as determined by IC50 values of AII to displace [125I] Sar-Ang II binding in BLM and BBM were similar in lean and obese rats. Western blot analysis revealed significant increases in Gialpha1, Gialpha2, Gialpha3, and Gq/11alpha in BLM and Gialpha1, Gialpha3, and Gq/11alpha in BBM of obese as compared with lean rats. The increase in the levels of the AT1 receptor and G proteins, mainly in the BLM, may be contributing to the enhanced AII-induced activation of NKA in the proximal tubules of obese rats. This phenomenon, in part, may be responsible for the increased sodium reabsorption and the development of hypertension in obese Zucker 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.

Angiotensin II AT2 receptors act as a functional antagonist for the AT1 receptors in various tissues. We previously reported that activation of the renal AT2 receptors promotes natriuresis and diuresis; however, the mechanism is not known. The present study was designed to investigate whether activation of AT2 receptors affects the activity of Na+-K+-ATPase (NKA), an active tubular sodium transporter, in the proximal tubules isolated from Sprague-Dawley rats. The AT2 receptor agonist CGP-42112 (10(-10)-10(-7) M) produced a dose-dependent inhibition of NKA activity (9-38%); the inhibition was attenuated by the presence of the AT2 receptor antagonist PD-123319 (1 microM), suggesting the involvement of the AT2 receptors. The AT1 receptor antagonist losartan (1 microM) did not affect the CGP-42112 (100 nM)-induced inhibition of NKA activity. The presence of guanylyl cyclase inhibitor ODQ (10 microM) and the nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 100 microM) abolished the CGP-42112 (100 nM)-induced NKA inhibition. ANG II (100 nM), in the presence of losartan, significantly inhibited NKA activity; the inhibition was attenuated by PD-123319. CGP-42112 also, in a dose-dependent manner, stimulated NO production (approximately 0-230%) and cGMP accumulation (approximately 25-100%). The CGP-42112 (100 nM)-induced NO and cGMP increases were abolished by the AT2 receptor antagonist PD-123319, ODQ, and L-NAME. The data suggest that the activation of the AT2 receptor via stimulation of the NO/cGMP pathway causes inhibition of NKA activity in the proximal tubules. This phenomenon provides a plausible mechanism responsible for the AT2 receptor-mediated natriuresis-diuresis in rodents.

Desensitization of G-protein-coupled receptors (GPCR) includes receptor endocytosis. This phenomenon is suggested, at least for some receptors, to be associated with receptor resensitization. Here, we examined the role of receptor endocytosis for two different GPCR, the dopamine-1 (D1) receptor and the beta1-adrenoceptor (beta(1)-AR) in renal tissue. The functional role of receptor endocytosis was examined on Na+, K+ -ATPase activity in microdissected proximal tubules from rat kidney. The spatial regulation of endogenous D1 receptors and beta(1)-AR was examined by confocal microscopy techniques in LLCPK cells. Phenylarsine oxide (PAO) an endocytosis inhibitor, attenuated isoproterenol-induced decrease in Na+, K+ -ATPase activity but had no such effect on dopamine-induced decrease in Na+, K+ -ATPase activity. We have previously shown that isoproterenol sensitizes the renal dopamine system, by recruiting silent D1 receptors from the interior of the cell towards the plasma membrane. This effect was attenuated by PAO as well as by cytochalasin D while these substances had no effect on dopamine-induced D1 receptor recruitment. The beta(1)-AR was localized to the plasma membrane in control cells. Isoproterenol induced a rapid internalization of the beta(1)-AR; which was prevented by PAO. The results suggest that endocytosis of beta(1)-AR in renal proximal tubular cells is an important step in signal generation, while endocytosis of proximal tubular D1 receptor is not.

Dopamine D(1A) receptor function is impaired in obesity-induced insulin resistance, contributing to sodium retention. We showed previously that uncoupling of D(1A) receptors from G proteins is responsible for diminished natriuretic response to dopamine in obese Zucker rats (OZRs). We hypothesized that overexpression of G protein-coupled receptor kinases (GRKs) leads to increased phosphorylation of D(1A) receptors, which in turn causes uncoupling of the receptors from G(s) proteins in proximal tubules of OZRs. We also examined effects of an insulin sensitizer, rosiglitazone, in correcting these defects. We found that basal and agonist (fenoldopam)-induced coupling of D(1A) receptors to G(s) proteins was impaired in proximal tubules of OZRs compared with lean Zucker rats (LZRs). Moreover, basal serine phosphorylation of D(1A) receptors was elevated two- to threefold in proximal tubules of OZRs compared with LZRs. Fenoldopam increased D(1A) receptor phosphorylation in proximal tubules of LZRs but not OZRs. Compared with that in LZRs, GRK4 expression in OZRs was elevated 200-300% in proximal tubule cell lysates and GRK2 expression was approximately 30% higher in plasma membranes isolated from proximal tubules of OZRs. Rosiglitazone treatment restored basal and agonist-induced coupling of D(1A) receptors to G(s) proteins and reduced basal serine phosphorylation of D(1A) receptors, GRK4 expression, and translocation of GRK2 to the plasma membrane in proximal tubules of OZRs. Furthermore, rosiglitazone significantly reduced fasting blood glucose and plasma insulin in OZRs. Collectively, these results suggest that insulin resistance is responsible for GRK4 overexpression and GRK2 translocation leading to hyperphosphorylation of D(1A) receptors and their uncoupling from G(s) proteins as rosiglitazone treatment corrects these defects in OZRs.

In essential hypertension, the defect in renal dopamine (DA) D(1) receptor function is intrinsic to proximal tubules as this phenomenon is also seen in primary proximal tubule cultures from spontaneously hypertensive rats (SHR) and essential hypertensive patients. Previously, a defect was reported in renal D(1) receptor function in obese Zucker rats. In the present study, we sought to determine whether this D(1) receptor dysfunction is intrinsic in these animals. In primary proximal tubular epithelial cells (PTECs) from lean and obese rats, DA inhibited Na-K-ATPase (NKA) activity in PTECs from both groups of rats. Basal NKA activity, D(1) receptor protein expression, and their coupling to G proteins were similar in cells from both groups. However, when PTECs from lean and obese rats were cultured in 20% serum from obese rats, DA failed to inhibit NKA activity, which was accompanied by a reduction in D(1) receptor expression and a defect in D(1) receptor-G protein coupling. No such defects in the inhibitory effect of DA on NKA activity, D(1) receptor numbers, or coupling were seen when PTECs from both lean and obese rats were grown in 20% serum from lean or rosiglitazone-treated obese (RTO) rats. RTO rat serum had normal blood glucose and reduced plasma levels of insulin compared with serum from obese rats. Furthermore, chronic insulin treatment of PTECs from lean and obese rats caused an attenuation in DA-induced NKA inhibition, a decrease in D(1) receptor expression, and D(1) receptor-G protein uncoupling. These results suggest that defective D(1) receptor function in obese Zucker rats is not inherited but contributed to by hyperinsulinemia and/or other circulating factors associated with obesity.

Dopamine causes natriuresis and diuresis via activation of D1 receptors located on the renal proximal tubules and subsequent inhibition of the sodium transporters, Na-H exchanger and Na+/K+ ATPase. We have reported that dopamine fails to inhibit the activities of these two transporters in the obese Zucker rats (OZR). The present study was designed to examine the functional consequence of this phenomenon by determining the natriuretic and diuretic response to D1 receptor activation in lean Zucker rats (LZR) and OZR. In 11-12 week-old OZR and LZR, natriuretic and diuretic responses to intravenously administered D1 receptor agonist, SKF 38393 (3 microg/kg/min for 30 min) were measured under Inactin anesthesia. Plasma insulin and glucose levels were significantly higher in the obese rats as compared to the lean rats. Intravenous infusion of SKF 38393 caused significant increases in urine flow, urinary sodium excretion (U(Na)V), fractional excretion of sodium (FE(Na)), and glomerular filtration rate (GFR) in the lean rats. However, the natriuretic and diuretic response to SKF 38393 was markedly blunted in OZR. Infusion of SKF 38393 did not cause significant changes in the mean blood pressure and heart rate in either of the two groups. We suggest that the diminished natriuretic response to D1 receptor activation in OZR is the consequence of the previously reported defect in the D1 receptor-G-protein coupling and the failure of dopamine to inhibit the sodium transporters in these animals.

Obese Zucker rats (OZR) are hyperinsulenemic, hyperglycemic and dyslipidemic and develop salt dependent hypertension. Since salt sensitivity is considered to be due to impaired handling of renal sodium excretion, these studies were conducted in the obese and lean Zucker rats (LZR) anesthetized with Inactin to evaluate renal function under basal conditions and during acute isotonic fluid volume expansion (VE). Mean Arterial blood pressure (MBP), heart rate (HR), renal blood flow(RBF) and glomerular filtration rate (GFR) were not significantly different between the lean Zucker rats fed normal diet or that fed salt rich diet(8% NaCI). However, basal UV and UNaV were significantly greater in the LZR fed high salt. During VE essentially identical increases occurred in GFR, UV and UNaV in both the lean groups. In the OZR fed salt rich diet also, there were no significant changes in the heart rate, RBF and GFR. However, arterial blood pressure of the OZR fed salt rich diet was significantly greater than that of the OZR on the normal diet as well as that of both the lean groups. Also, as in the LZR, basal UV and UNaV were significantly greater in the salt fed obese rats. During volume expansion there were no impairments in the ability of the obese groups fed normal or salt rich diet to eliminate sodium and water during volume load. In fact, the net sodium and water excretions during and 60 min after VE in both the obese groups were significantly greater than that of corresponding lean groups. Furthermore, these values in the OZR fed salt rich diet were significantly greater than that of the obese rats on normal salt diet perhaps due to the contribution of pressure natriuretic mechanisms'. These data demonstrate that although OZR are salt sensitive, the renal mechanisms that would collectively respond to acute isotonic VE were fully functional. An unexpected and a novel finding in these studies is that the salt rich diet, in addition to increasing arterial blood pressure also significantly lowered plasma of insulin levels and enhanced glucose and cholesterol levels in the obese Zucker rats.

Dopamine, via activation of renal D(1) 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 type I diabetes. However, it is not known whether the response to D(1) receptor activation is altered in type I diabetes. The present study was designed to examine the effect of streptozotocin-induced type I diabetes on renal D(1) receptor expression and function. Streptozotocin treatment of Sprague-Dawley rats caused a fourfold increase in plasma levels of glucose along with a significant decrease in insulin levels compared with control rats. Intravenous administration of SKF-38393, a D(1) receptor agonist, caused a threefold increase in sodium excretion in control rats. However, SKF-38393 failed to produce natriuresis in diabetic rats. SKF-38393 caused a concentration-dependent inhibition of Na-K-ATPase activity in renal proximal tubules of control rats. However, the ability of SKF-38393 to inhibit Na-K-ATPase activity was markedly diminished in diabetic rats. D(1) receptor numbers and protein abundance as determined by [(3)H]SCH-23390 ligand binding and Western blot analysis were markedly reduced in diabetic rats compared with control rats. Moreover, SKF-38393 failed to stimulate GTP gamma S binding in proximal tubular membranes from diabetic rats compared with control rats. We conclude that the natriuretic response to D(1) receptor activation is reduced in type I diabetes as a result of a decrease in D(1) receptor expression and defective receptor G protein coupling. These abnormalities may contribute to the sodium retention associated with type I diabetes.

Dopamine and diabetes mellitus are reported to have close link between them. We have studied the effect of six-week treatment with D1 receptor agonist fenoldopam (1 mg/kg, i.p., daily) on glucose, lipid, and renal profile in streptozotocin (STZ)-induced (non-insulin dependent) type 2 diabetic rats. Streptozotocin (90 mg/kg, i.p.) was injected to two day old Sprague-Dawley pups. Streptozotocin produced hyperglycemia, hyperinsulinemia, hyperlipidemia, hypertension, increase in serum urea and creatinine by the time animals were 10 week old. Treatment with fenoldopam significantly decreased serum glucose, insulin, cholesterol, triglyceride, urea, creatinine, and blood pressure. During oral glucose tolerance test (OGTT), diabetic rats showed increase in AUC(glucose) and AUC(insulin). Fenoldopam significantly decreased AUC(glucose) in diabetic rats. Diabetic rats showed lower insulin sensitivity index (K(TTT)) that was significantly increased by treatment with fenoldopam in diabetic rats. Diabetic rats showed decrease in urinary sodium. Fenoldopam treatment significantly increased urine output as well as urinary sodium indicating reduced sodium retention. Our data indicates fenoldopam treatment improves peripheral insulin sensitivity and renal function in STZ-induced type 2 diabetic rats.

Angiotensin II (Ang II) via the activation of AT1 receptors and subsequent stimulation of the tubular sodium transporters increases sodium and water reabsorption in the proximal tubule. An enhanced tubular action of Ang II is implicated in obesity related hypertension; however, the mechanism of such a phenomenon is unknown. Present study was designed to determine the AT1 receptor numbers and function in the proximal tubule of obese and lean Zucker rats. Obese Zucker rats were hypertensive and hyperinsulinemic. The plasma renin activity was similar in the lean and obese rats. Angiotensin II stimulated the Na,H-exchanger (NHE) activity in the proximal tubule, but the stimulatory response was markedly greater in obese than in lean rats. Similarly, Ang II caused greater inhibition in cAMP accumulation in the proximal tubule of obese compared to lean rats. The (125I]sar-Ang II binding revealed a 100% increase in the AT1 receptor number in the brush border membrane (BBM) of obese compared to lean rats. The Western blot analysis revealed a 36-51% increase in the Gi(alpha)1 and Gi(alpha)3 in the BBM of obese compared to lean rats. We conclude that increases in the AT1 receptor number and abundance of the Gi(alpha) on BBM may be responsible for the enhanced signaling and subsequent greater stimulation of NHE by Ang II in proximal tubules of obese rats. The greater stimulation of NHE by Ang II may contribute to the increased tubular sodium reabsorption and to the hypertension in obese Zucker rats.

The present study examined intestinal dopaminergic activity and its response to high salt (HS, 1% NaCl over a period of 24 hours) intake in obese (OZR) and lean Zucker rats (LZR). The basal Na+,K+-ATPase activity (nmol Pi/mg protein/min) in the jejunum of OZR was higher than in LZR on normal salt (NS) (OZR-NS = 111.3 +/- 6.0 vs. LZR-NS = 88.0 +/- 8.3). With the increase in salt intake, the basal Na+,K+-ATPase activity significantly increased in both animals (OZR-HS = 145.9 +/- 11.8; LZR-HS = 108.8 +/- 6.7). SKF 38393 (10 nM), a specific D1-like dopamine receptor agonist, inhibited the jejunal Na+,K+-ATPase activity in OZR on HS intake, but failed to inhibit enzyme activity in OZR on NS intake and LZR on NS and HS intakes. The aromatic L-amino acid decarboxylase (AADC) activity in OZR was lower than in LZR on NS intake. The HS intake increased AADC activity in OZR, but not in LZR. During the NS intake the jejunal monoamine oxidase (MAO) activity in OZR was similar to that in LZR. The HS intake significantly decreased MAO activity in both OZR and LZR. The jejunal COMT activity in OZR was higher than in LZR on NS intake. The HS intake reduced COMT activity in OZR but not LZR. It is concluded that inhibition of jejunal Na+,K+-ATPase activity through D1 dopamine receptors is dependent on salt intake in OZR, whereas in LZR, the enzyme failed to respond to the activation of D1 dopamine receptors irrespective of their salt intake.

We have studied the effect of chronic treatment with dopamine D1 receptor agonist fenoldopam (1 mg/kg, i.p. daily for 6 weeks) on renal function and metabolic parameters in streptozotocin (STZ)-diabetic rats. Diabetes was induced by a single tail vein injection of STZ (45 mg/kg). STZ produced severe hyperglycemia, hypoinsulinemia, hypercholesterolemia, hypertriglyceridemia, hypertension and bradycardia. Fenoldopam treatment significantly reduced fasting but not fed blood glucose levels and lowered the blood pressure in diabetic animals. Significant change was not observed in insulin, cholesterol, triglyceride levels. Diabetic animals showed increase in AUCglucose and decrease in AUCinsulin during oral glucose tolerance test. Fenoldopam treatment did not significantly change these values in diabetic animals. STZ produced increase in serum urea, creatinine and blood urea nitrogen. Diuresis and urinary sodium retention was observed in diabetic animals. Renal hypertrophy was observed as seen from increased kidney weight/body weight ratio and increased total RNA content as well as decreased total DNA content. Fenoldopam treatment significantly lowered serum urea, creatinine and blood urea nitrogen. Urinary sodium retention was significantly reduced and renal hypertrophy was prevented with fenoldopam treatment as seen from the improved kidney weight/body weight ratio. Fenoldopam treatment significantly prevented reduction in total DNA content and increase in total RNA content further substantiating reduced renal hypertrophy. Our data suggest that STZ induced diabetes is associated with renal dysfunctions and fenoldopam treatment could be beneficial in a condition where diabetes mellitus co-exists with hypertension and compromised renal function.

Dopamine, a neurotransmitter, precursor of noradrenaline, is responsible for cardiovascular and renal actions, such as increase in myocardial contractility and cardiac output, without changes in heart rate, producing passive and active vasodilatation, diuresis and natriuresis. These cardiovascular and renal actions take place through the interaction with dopamine receptors, D(1), D(2), D(3), D(4), and D(5). Recent findings point to the possibility of D(6) and D(7)receptors. Dopamine is known to influence the control of arterial pressure by influencing the central and peripheral nervous system and target organs such as kidneys and adrenal glands, in some types of hypertension. Although dopamine and its derivatives have been shown to have antihypertensive effects, these are still being studied; therefore it is important to explain some physiological and pharmacological aspects of dopamine, its receptors, and the clinical uses it could have in the treatment of arterial hypertension and more recently in obesity, based on evidence proving a clear association between obesity and the decrease in the expression of D(2) receptors in the brain of obese persons.

Regulation by dopamine of cardiovascular function, renal function and systemic blood pressure regulation is multifaceted. Each of the five dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. The D1, D3, and D4 receptors interact with the renin-angiotensin system, while the D2 and D5 receptors interact with the sympathetic nervous system to regulate blood pressure.

Dopamine via the activation of D1-like receptors inhibits Na,K-ATPase and Na,H-exchanger and subsequently increases sodium excretion. We have previously reported that dopamine failed to inhibit Na,K-ATPase in the proximal tubules (PTs) of obese Zucker rats. The present study was designed to determine the effect of dopamine on Na,H-exchanger in PTs of lean and obese Zucker rats, and examine D1-like receptor-coupled signal transduction pathway mediating the inhibition of Na,H-exchanger. We found that dopamine inhibited Na,H-exchanger in the PTs of lean rats but this response was absent in obese rats. In brush border membranes, [3H]SCH 23390 binding revealed a approximately 45% reduction in D1-like receptor binding sites in obese compared to lean rats. Dopamine stimulated cAMP accumulation in PTs of lean but not in obese rats. Forskolin-mediated stimulation of cAMP was similar in lean and obese rats. Dopamine as well as forskolin and dibutyryl cAMP-mediated stimulation of protein kinase A (PKA) was reduced in PTs of obese compared to lean rats. The data suggest that reduction in D1-like receptor binding sites, defective coupling with signaling pathway and inability of PKA activation may be responsible for the failure of dopamine to inhibit Na,H-exchanger in PTs of obese rats. This phenomenon may contribute to an increase in sodium reabsorption and development of hypertension in obese Zucker rats.

Renal sodium retention, as a result of increased abundance of sodium transporters, may play a role in the development and/or maintenance of the increased blood pressure in obesity. To address this hypothesis, we evaluated the relative abundances of renal sodium transporters in lean and obese Zucker rats at 2 and 4 mo of age by semiquantitative immunoblotting. Mean systolic blood pressure was higher in obese rats relative to lean at 3 mo, P < 0.02. Furthermore, circulating insulin levels were 6- or 13-fold higher in obese rats compared with lean at 2 or 4 mo of age, respectively. The abundances of the alpha(1)-subunit of Na-K-ATPase, the thiazide-sensitive Na-Cl cotransporter (NCC or TSC), and the beta-subunit of the epithelial sodium channel (ENaC) were all significantly increased in the obese rats' kidneys. There were no differences for the sodium hydrogen exchanger (NHE3), the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2 or BSC1), the type II sodium-phosphate cotransporter (NaPi-2), or the alpha-subunit of ENaC. These selective increases could possibly increase sodium retention by the kidney and therefore could play a role in obesity-related hypertension.

Obesity-related non-insulin dependent diabetes mellitus (NIDDM) is frequently accompanied by hypertension. The present study was designed to clarify this mechanism. We first determined the blood pressure in male Wistar fatty rats (WFR), one of the NIDDM model rats, and in Wistar lean rats (WLR) as the control, with a normal (0.7% NaCl) or high (7% NaCl) salt diet. We observed no difference in systolic and mean blood pressures between WFR and WLR. WFR, however, became extremely hypertensive as a result of ingesting the high salt diet. We next investigated the mechanism for sodium sensitivity in WFR. Although the urinary excretion of dopamine (DA), a potent natriuretic factor, which reflects the ability for renal DA production, was preserved in WFR, the sodium balance with the high salt diet was positive. Moreover, Na-K-ATPase activity in isolated proximal convoluted tubules (PCT) from WFR with a normal salt diet was significantly (p<0.05) higher than that from WLR. A high salt load produced a significant (p<0.05) decrease in Na-K-ATPase activity in WLR but not in WFR. Similarly, Na-K-ATPase activity in WLR with a normal salt diet was significantly (p<0.05) inhibited by DA (10(-5) M), but this was not true in WFR. Furthermore, urinary excretion of norepinephrine in WFR with a high salt diet was the highest among all the groups. These results indicate that WFR tend to develop salt-sensitive hypertension that could be caused by the excessive sodium retention occurring as the results of a defective dopaminergic system in the kidney that fails to inhibit Na-K-ATPase activity. Augmentation of the renal sympathetic nervous system may play some role in this setting.