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.

Oral NaCl produces a greater natriuresis and diuresis than the intravenous infusion of the same amount of NaCl. Gastrin is the major gastrointestinal hormone taken up by renal proximal tubule (RPT) cells. We hypothesized that renal gastrin and dopamine receptors interact to synergistically increase sodium excretion, an impaired interaction of which may be involved in the pathogenesis of hypertension. In Wistar-Kyoto (WKY) rats, infusion of gastrin induced natriuresis and diuresis, which was abrogated in the presence of a gastrin (CCKBR; CI-988) or D1-like receptor antagonist (SCH23390). Similarly, the natriuretic and diuretic effects of fenoldopam, a D1-like receptor agonist, were blocked by SCH23390, as well as by CI-988. However, the natriuretic effects of gastrin and fenoldopam were not observed in spontaneously hypertensive rats (SHRs). The gastrin/D1-like receptor interaction was also confirmed in RPT cells. In RPT cells from WKY but not SHRs, stimulation of either D1-like or gastrin receptor inhibited Na+-K+-ATPase activity, an effect that was blocked in the presence of SCH23390 or CI-988. In RPT cells from WKY and SHRs, CCKBR and D1 receptor (D1R) co-immunoprecipitated, which was increased after stimulation of either D1R or CCKBR in RPT cells from WKY rats; stimulation of one receptor increased RPT cell membrane expression of the other receptor, effects that were not observed in SHRs. These data suggest that there is a synergism between CCKBR and D1-like receptors to increase sodium excretion. An aberrant interaction between the renal CCKBR and D1-like receptors (e.g., D1R) may play a role in the pathogenesis of hypertension.

Dopamine is important in the pathogenesis of hypertension because of abnormalities in receptor-mediated regulation of renal sodium transport. Dopamine receptors are classified into D1-like (D1, D5) and D2-like (D2, D3, D4) subtypes, all of which are expressed in the kidney. Mice deficient in specific dopamine receptors have been generated to provide holistic assessment on the varying physiological roles of each receptor subtype. This review examines recent studies on these mutant mouse models and evaluates the impact of individual dopamine receptor subtypes on blood pressure regulation.

Abstract-Recent studies have indicated the importance of cholesterol-rich membrane lipid rafts (LRs) in oxidative stress-induced signal transduction. Reduced nicotinamide-adenine dinucleotide phosphate (NADPH) oxidases, the major sources of reactive oxygen species, are implicated in cardiovascular diseases, including hypertension. We tested the hypothesis that NADPH oxidase subunits and activity are regulated by LRs in human renal proximal tubule cells. We report that a high proportion of p22 phox and the small GTPase Rac1 are expressed in LRs in human renal proximal tubule cells.

During conditions of moderate sodium excess, the dopaminergic system sits at the fulcrum of homeostatic control of water and electrolyte balance and blood pressure (1, 2). Dopamine promotes natriuresis by inhibiting sodium chloride reabsorption in specific segments of the nephron. Dopamine exerts its action on dopamine receptors, which belong to the family of G protein-coupled receptors (GPCRs). The signal transduction that follows ligand occupation of a GPCR is tightly regulated to limit the specificity and extent of cellular response. GPCR-mediated signal transduction is rapidly dampened via receptor desensitization or the waning of the responsiveness of the receptor to agonist with time. Desensitization involves receptor phosphorylation and is carried out by either GPCR kinases (GRKs) or second messenger-activated kinases such as protein kinase A and protein kinase C. Homologous desensitization involves GRKs that selectively phosphorylate only agonist-activated receptors, whereas heterologous desensitization is carried out by second messenger-dependent kinases that indiscriminately phosphorylate agonist-activated receptors and those that have not been exposed to the agonist (7).The GRKs are serine/threonine protein kinases comprising seven isoforms that are grouped into three subfamilies. GRK1 and GRK7 belong to the rhodopsin kinase subfamily and are expressed exclusively in the retina (8 -10). GRK2 and GRK3 phosphorylate the ␤-adrenergic receptor and belong to the ␤-adrenergic receptor kinase subfamily (11), and GRK4, GRK5, and GRK6 belong to the GRK4 subfamily. GRK4 is highly enriched in the testis and, to a lesser degree, in the kidneys (12, 13). Four splice variants of human GRK4 result from the alternative splicing of exons 2 and 15 (11). GRK4-␣ is considered the full-length version, whereas GRK4-␤, -␥, and -␦ are shortened versions of . The coding region of the GRK4 gene, whose 4p16.3 locus has been linked to essential hypertension (15, 16), contains several single nucleotide polymorphisms, including R65L, A142V, and A486V, which have been linked to * This work was supported, in whole or in part, by National Institutes of Health Grants HL023081, HL074940, DK039308, HL092196, and HL068686.

Abstract-Dysfunction of D 2 -like receptors has been reported in essential hypertension. Disruption of D 2 R in mice (D 2 Ϫ/Ϫ ) results in high blood pressure, and several D 2 R polymorphisms are associated with decreased D 2 R expression. Because D 2 R agonists have antioxidant activity, we hypothesized that increased blood pressure in D 2 Ϫ/Ϫ is related to increased oxidative stress. D 2 Ϫ/Ϫ mice had increased urinary excretion of 8-isoprostane, a parameter of oxidative stress; increased activity of reduced nicotinamide-adenine dinucleotide phosphate oxidase in renal cortex; increased expression of the reduced nicotinamide-adenine dinucleotide phosphate oxidase subunits Nox1, Nox2, and Nox4; and decreased expression of the antioxidant enzyme heme-oxygenase-2 in the kidneys, suggesting that regulation of reactive oxygen species (ROS) production by D 2 R involves both pro-oxidant and antioxidant systems. Apocynin, a reduced nicotinamide-adenine dinucleotide phosphate oxidase inhibitor, or hemin, an inducer of heme oxigenase-1, normalized the blood pressure in D 2 Ϫ/Ϫ mice. Because D 2 Rs in the adrenal gland are implicated in aldosterone regulation, we evaluated whether alterations in aldosterone secretion contribute to ROS production in this model. Urinary aldosterone was increased in D 2 Ϫ/Ϫ mice and its response to a high-sodium diet was impaired. Spirolactone normalized the blood pressure in D 2 Ϫ/Ϫ mice and the renal expression of Nox1 and Nox4, indicating that the increased blood pressure and ROS production are, in part, mediated by impaired aldosterone regulation. However, spironolactone did not normalize the excretion of 8-isoprostane and had no effect on expression of Nox2 or heme-oxygenase-2. Our results show that the D 2 R is involved in the regulation of ROS production and that, by direct and indirect mechanisms, altered D 2 R function may result in ROS-dependent hypertension. [1][2][3] There is abundant evidence that an intact dopaminergic system is necessary to maintain normal blood pressure and that genetic hypertension is associated with alterations in dopamine production and receptor function. [1][2][3][4] In humans and rodents, some dopamine receptor genes and their regulators are in loci linked to hypertension. 3,5 The natriuretic effect of D 1 -like agonists is impaired in genetically hypertensive rats 3,4 and in human essential hypertension. 3,4 Alterations in D 2 -like receptor function have also been reported in hypertension. 1,2 Loci in chromosome 11, where the D 2 R gene is located, are linked to hypertension. 5,6 A polymorphism in exon 6 of the D 2 R gene is associated with elevated blood pressure, 7 and a TaqI polymorphism is associated with human essential hypertension. 8 Several D 2 R polymorphisms are associated with decreased D 2 R expression 9,10 and affect D 2 R mRNA stability and synthesis of the receptor. 11 The disruption of any of the dopamine receptor genes in mice produces dopamine receptor subtype-specific hypertension. 3,[12][13][14] Specifically, disruption of the D 2 R...

The dopamine D2 receptor (D2R) regulates renal reactive oxygen species (ROS) production and impaired D2R function results in ROS-dependent hypertension. Paraoxonase 2 (PON2), which belongs to the paraoxonase gene family, is expressed in various tissues, acting to protect against cellular oxidative stress. We hypothesized that PON2 may be involved in preventing excessive renal ROS production and thus may contribute to maintenance of normal blood pressure. Moreover, the D2R may decrease ROS production, in part, through regulation of PON2. D2R co-localized with PON2 in the brush border of mouse renal proximal tubules. Renal PON2 protein was decreased (-33%±6%) in D2-/- relative to D2+/+ mice. The renal subcapsular infusion of PON2 siRNA decreased PON2 protein expression (-55%), increased renal oxidative stress (2.2-fold), associated with increased renal NADPH oxidase expression (Nox1: 1.9-fold; Nox2: 2.9-fold; and Nox4: 1.6-fold) and activity (1.9-fold), and elevated arterial blood pressure (systolic: 134±5 vs. 93±6 mmHg; diastolic: 97±4 vs. 65±7 mmHg; mean: 113±4 vs. 75±7 mmHg). To determine the relevance of the PON2 and D2R interaction in humans, we studied human renal proximal tubule cells. Both D2R and PON2 were found in non-lipid and lipid rafts and physically interacted with each other. Treatment of these cells with the D2R/D3R agonist quinpirole (1μM, 24h) decreased ROS production (-35%±6%), associated with decreased NADPH oxidase activity (-32%±3%) and expression of Nox2 (-41%±7%) and Nox4 (-47%±8%) protein, and increased expression of PON2 mRNA (2.1-fold) and protein (1.6-fold) at 24h. Silencing PON2 (siRNA, 10nM, 48 h) not only partially prevented the quinpiroleinduced decrease in ROS production by 36%, but also increased basal ROS production (1.3-fold) which was associated with an increase in NADPH oxidase activity (1.4-fold) and expression of Nox2 (2.1-fold) and Nox4 (1.8-fold) protein. Inhibition of NADPH oxidase with diphenylene iodonium (10 μM/30 min) inhibited the increase in ROS production caused by PON2 silencing. Our results suggest that renal PON2 is involved in the inhibition of renal NADPH oxidase activity and ROS production and contributes to the maintenance of normal blood pressure. PON2 is positively regulated by D2R and may, in part, mediate the inhibitory effect of renal D2R on NADPH oxidase activity and ROS production.

Hypertension is a multigenic disorder in which abnormal counterregulation between dopamine and (1-4). The D 5 R is widely expressed in the rodent kidney, specifically in the proximal and distal tubules, cortical collecting ducts, medullary ascending limbs of Henle, and arterioles, but not in the glomeruli, juxtaglomerular cells, or macula densa (5). The AT 1 R is also widely expressed in the kidney, specifically in the proximal and distal tubules, cortical