Dopamine is metabolised by different enzymes along the rat nephron

The Role of the Renal Dopaminergic System and Oxidative Stress in the Pathogenesis of Hypertension

The kidney is critical in the long-term regulation of blood pressure. Oxidative stress is one of the many factors that is accountable for the development of hypertension. The five dopamine receptor subtypes (D₁R–D₅R) have important roles in the regulation of blood pressure through several mechanisms, such as inhibition of oxidative stress. Dopamine receptors, including those expressed in the kidney, reduce oxidative stress by inhibiting the expression or action of receptors that increase oxidative stress. In addition, dopamine receptors stimulate the expression or action of receptors that decrease oxidative stress. This article examines the importance and relationship between the renal dopaminergic system and oxidative stress in the regulation of renal sodium handling and blood pressure. It discusses the current information on renal dopamine receptor-mediated antioxidative network, which includes the production of reactive oxygen species and abnormalities of renal dopamine receptors. Recognizing the mechanisms by which renal dopamine receptors regulate oxidative stress and their degree of influence on the pathogenesis of hypertension would further advance the understanding of the pathophysiology of hypertension.

Dopamine D2 receptor modulates Wnt expression and control of cell proliferation

The Wnt/β-catenin pathway is one of the most conserved signaling pathways across species with essential roles in development, cell proliferation, and disease. Wnt signaling occurs at the protein level and via β-catenin-mediated transcription of target genes. However, little is known about the underlying mechanisms regulating the expression of the key Wnt ligand Wnt3a or the modulation of its activity. Here, we provide evidence that there is significant cross-talk between the dopamine D2 receptor (D2R) and Wnt/β-catenin signaling pathways. Our data suggest that D2R-dependent cross-talk modulates Wnt3a expression via an evolutionarily-conserved TCF/LEF site within the WNT3A promoter. Moreover, D2R signaling also modulates cell proliferation and modifies the pathology in a renal ischemia/reperfusion-injury disease model, via its effects on Wnt/β-catenin signaling. Together, our results suggest that D2R is a transcriptional modulator of Wnt/β-catenin signal transduction with broad implications for health and development of new therapeutics.

Dopamine reduces cell surface Na+/H+ exchanger-3 protein by decreasing NHE3 exocytosis and cell membrane recycling

The intrarenal autocrine-paracrine dopamine (DA) system mediates a significant fraction of the natriuresis in response to a salt load. DA inhibits a number of Na⁺ transporters to effect sodium excretion, including the proximal tubule Na⁺/H⁺ exchanger-3 (NHE3). DA represent a single hormone that regulates NHE3 at multiple levels, including translation, degradation, endocytosis, and protein phosphorylation. Because cell surface NHE3 protein is determined by the balance between exocytotic insertion and endocytotic retrieval, we examined whether DA acutely affects the rate of NHE3 exocytosis in a cell culture model. DA inhibited NHE3 exocytosis at a dose-dependent manner with a half maximal around 10^-6 M. The DA effect on NHE3 exocytosis was blocked by inhibition of protein kinase A and by brefeldin A, which inhibits endoplasmic reticulum-to-Golgi transport. NHE3 directly interacts with the ε-subunit of coatomer protein based on yeast-two-hybrid and coimmunoprecipitation. Because NHE3 has been shown to be recycled back to the cell membrane after endocytosis, we measured NHE3 recycling using a biochemical reinsertion assay and showed that reinsertion of NHE3 back to the membrane is also inhibited by DA. In conclusion, among the many mechanisms by which DA reduces apical membrane NHE3 and induces proximal tubule natriuresis, one additional mechanism is inhibition of exocytotic insertion and reinsertion of NHE3 in the apical cell surface.

Concerted regulation of renal plasma flow and glomerular filtration rate by renal dopamine and NOS I in rats on high salt intake

Under high sodium intake renal dopamine (DA) increases while NOS I expression in macula densa cells (MD) decreases. To explore whether renal DA and NOS I, linked to natriuresis and to the stability of the tubuloglomerular feedback, respectively, act in concert to regulate renal plasma flow (RPF) and glomerular filtration rate (GFR). Male Wistar rats were studied under a normal sodium intake (NS, NaCl 0.24%) or a high sodium intake (HS, NaCl 1% in drinking water) during the 5 days of the study. For the last two days, the specific D1-like receptor antagonist SCH 23390 (1 mg kg bwt-1 day-1, sc) or a vehicle was administered. HS intake increased natriuresis, diuresis, and urinary DA while it decreased cortical NOS I expression (P < 0.05 vs. NS), Nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) activity in MD (P < 0.001 vs. NS) and cortical nitrates+nitrites (NOx) production (NS 2.04 ± 0.22 vs. HS 1.28 ± 0.10 nmol mg protein-1, P < 0.01). Treatment with SCH 23390 to rats on HS sharply decreased hydroelectrolyte excretion (P < 0.001 vs. HS) while NOS I expression, NADPH-d activity and NOx production increased (P < 0.05 vs. HS for NOS I and P < 0.001 vs. HS for NADPH-d and NOx). SCH 23390 increased RPF and GFR in HS rats (P < 0.01 HS+SCH vs. HS). It did not cause variations in NS rats. Results indicate that when NS intake is shifted to a prolonged high sodium intake, renal DA through the D1R, and NOS I in MD cells act in concert to regulate RPF and GFR to stabilize the delivery of NaCl to the distal nephron.

Dopamine D2 receptors' effects on renal inflammation are mediated by regulation of PP2A function

Lack or downregulation of the dopamine D2 receptor (D2R) results in increased renal expression of injury markers and proinflammatory factors that is independent of a blood pressure increase. This study aimed to determine the mechanisms involved in the regulation of renal inflammation by D2Rs. Silencing D2Rs in mouse renal proximal tubule cells increased the expression of the proinflammatory TNF-α, monocyte chemoattractant protein-1 (MCP-1), and IL-6. D2R downregulation also increased Akt phosphorylation and activity, and glycogen synthase kinase-3β (GSK3β) phosphorylation and cyclin D1 expression, downstream targets of Akt; however. phosphatidylinositol 3-kinase (PI3K) activity was not affected. Conversely, D2R stimulation decreased Akt and GSK3β phosphorylation and cyclin D1 expression. Increased phospho-Akt, in the absence of increased PI3K activity, may result from decreased Akt dephosphorylation. Inhibition of protein phosphatase 2A (PP2A) with okadaic acid reproduced the effects of D2R downregulation on Akt, GSK3β, and cyclin D1. The PP2A catalytic subunit and regulatory subunit PPP2R2C coimmunoprecipitated with the D2R. Basal phosphatase activity and the expression of PPP2R2C were decreased by D2R silencing that also blunted the increase in phosphatase activity induced by D2R stimulation. Similarly, silencing PPP2R2C also increased the phosphorylation of Akt and GSK3β. Moreover, downregulation of PPP2R2C resulted in increased expression of TNF-α, MCP-1, and IL-6, indicating that decreased phosphatase activity may be responsible for the D2R effect on inflammatory factors. Indeed, the increase in NF-κB reporter activity induced by D2R silencing was blunted by increasing PP2A activity with protamine. Our results show that D2R controls renal inflammation, at least in part, by modulation of the Akt pathway through effects on PP2A activity/expression.

MiR-217 mediates the protective effects of the dopamine D2 receptor on fibrosis in human renal proximal tubule cells

Lack or downregulation of the dopamine D2 receptor (D2R) increases the vulnerability to renal inflammation independent of blood pressure in mice. Common single nucleotide polymorphisms (SNPs) rs6276, 6277, and 1800497 in the human D2R gene are associated with decreased receptor expression/function and hypertension. Human renal proximal tubule cells from subjects carrying these SNPs have decreased D2R expression and increased expression of profibrotic factors and production of extracellular matrix proteins. We tested the hypothesis that the D2R mediates these effects by regulating micro-RNA expression. In cells carrying D2R SNPs, micro-RNAs (miRs)-217, miR-224, miR-335, and miR-1265 were downregulated, whereas miR-1290 was upregulated >4-fold compared with those carrying D2R wild-type alleles. However, only miR-217 was directly regulated by D2R expression. In cells carrying D2R wild-type, miR-217 inhibitor increased the expression of transforming growth factor (TGF)-β1, matrix metalloproteinase 3, fibronectin 1, and collagen 1a, whereas miR-217 mimic had the opposite effect. In cells carrying D2R SNPs, miR-217 mimic also decreased the expression of TGFβ1 and its targets. Wnt5a, a miR-217 target, was increased in cells carrying D2R SNPs and decreased by miR-217 mimic but increased by miR-217 inhibitor in both cell types. In cells carrying D2R wild-type, Wnt5a treatment increased TGFβ1 while silencing Ror2, a Wnt5a receptor, decreased TGFβ1 and blunted the Wnt5a-induced increase in cells carrying D2R wild-type. Our results show that renal proximal tubule cells from subjects carrying D2R SNPs resulting in D2R downregulation have increased TGFβ1 that is mediated by decreased regulation of the miR-217-Wnt5a-Ror2 pathway.

Single-nucleotide polymorphisms of the dopamine D2 receptor increase inflammation and fibrosis in human renal proximal tubule cells

The dopamine D2 receptor (D2R) negatively regulates inflammation in mouse renal proximal tubule cells (RPTCs), and lack or downregulation of the receptor in mice increases the vulnerability to renal inflammation independent of blood pressure. Some common single-nucleotide polymorphisms (SNPs; rs6276, rs6277, and rs1800497) in the human DRD2 gene are associated with decreased D2R expression and function, as well as high blood pressure. We tested the hypothesis that human RPTCs (hRPTCs) expressing these SNPs have increased expression of inflammatory and injury markers. We studied immortalized hRPTCs carrying D2R SNPs and compared them with cells carrying no D2R SNPs. RPTCs with D2R SNPs had decreased D2R expression and function. The expressions of the proinflammatory tumor necrosis factor-α and the profibrotic transforming growth factor-β1 and its signaling targets Smad3 and Snail1 were increased in hRPTC with D2R SNPs. These cells also showed induction of epithelial mesenchymal transition and production of extracellular matrix proteins, assessed by increased vimentin, fibronectin 1, and collagen I a1. To test the specificity of these D2R SNP effects, hRPTC with D2R SNPs were transfected with a plasmid encoding wild-type DRD2. The expression of D2R was increased and that of transforming growth factor-β1, Smad3, Snail1, vimentin, fibronectin 1, and collagen I a1 was decreased in hRPTC with D2R SNPs transfected with wild-type DRD2 compared with hRPTC-D2R SNP transfected with empty vector. These data support the hypothesis that D2R function has protective effects in hRPTCs and suggest that carriers of these SNPs may be prone to chronic renal disease and high blood pressure.

Proteomic analysis of podocyte exosome-enriched fraction from normal human urine

Urine results from a coordinated activity of glomerular and tubular compartments of the kidney. As a footprint of these cellular functional processes, urinary exosomes, and 40-80 nm membrane vesicles released after fusion with the plasma membrane into the extracellular environment by renal epithelial cells, are a source for identification of proteins and investigation of their role in the kidney. The aim of the present study was the identification of podocyte exosome proteins based on urine immunoabsorption using podocyte-specific CR1-immunocoated beads followed by proteomic analysis using LC MS/MS techniques. This methodology allowed the identification of 1195 proteins. By using a bioinformatic approach, 27 brain-expressed proteins were identified, in which 14 out of them were newly demonstrated to be expressed in the kidney at a mRNA level, and, one of them, the COMT protein, was demonstrated to be expressed in podocytes at a protein level. These results, attesting the reliability of the methodology to identify podocyte proteins, need now to be completed by further experiments to analyze more precisely their biological function(s) in the podocytes.

Intrarenal dopamine inhibits progression of diabetic nephropathy

The kidney has a local intrarenal dopaminergic system, and in the kidney, dopamine modulates renal hemodynamics, inhibits salt and fluid reabsorption, antagonizes the renin-angiotensin system, and inhibits oxidative stress. The current study examined the effects of alterations in the intrarenal dopaminergic system on kidney structure and function in models of type 1 diabetes. We studied catechol-O-methyl-transferase (COMT)(-/-) mice, which have increased renal dopamine production due to decreased dopamine metabolism, and renal transplantation was used to determine whether the effects seen with COMT deficiency were kidney-specific. To determine the effects of selective inhibition of intrarenal dopamine production, we used mice with proximal tubule deletion of aromatic amino acid decarboxylase (ptAADC(-/-)). Compared with wild-type diabetic mice, COMT(-/-) mice had decreased hyperfiltration, decreased macula densa cyclooxygenase-2 expression, decreased albuminuria, decreased glomerulopathy, and inhibition of expression of markers of inflammation, oxidative stress, and fibrosis. These differences were also seen in diabetic mice with a transplanted kidney from COMT(-/-) mice. In contrast, diabetic ptAADC(-/-) mice had increased nephropathy. Our study demonstrates an important role of the intrarenal dopaminergic system to modulate the development and progression of diabetic kidney injury and indicate that the decreased renal dopamine production may have important consequences in the underlying pathogenesis of diabetic nephropathy.

The paradox of hyperdopaminuria in aromatic L-amino Acid deficiency explained

Aromatic L-amino acid decarboxylase (AADC) decarboxylates 3,4-L-dihydroxylphenylalanine (L-dopa) to dopamine, and 5-hydroxytryptophan to serotonin. In AADC deficiency, dopamine and serotonin deficiency leads to a severe clinical picture with mental retardation, oculogyric crises, hypotonia, dystonia, and autonomic dysregulation. However, despite dopamine deficiency in the central nervous system, urinary dopamine excretion in AADC-deficient patients is normal to high.In human, renal AADC-activity is very high compared to other tissues including brain tissue. Plasma L-dopa levels are increased in AADC deficiency. In this study, the hypothesis that in AADC deficiency relatively high-residual renal AADC-activity combined with high substrate availability of L-dopa leads to normal or elevated levels of urinary dopamine is tested and verified using 24-h urine collection of two AADC-deficient patients.Renal dopamine is a major regulator of natriuresis and plays a crucial role in the maintenance of sodium homeostasis. Therefore, the preservation of sufficient renal AADC-activity in AADC deficiency might be crucial for survival of AADC-deficient patients.In this study, we underpinned an empirical finding with theory, thereby putting a clinical observation into its physiological context. Our study stresses the difference - not qualitatively but quantitatively - between dopamine production in the central nervous system and peripheral organs. Furthermore, this study clarifies the so far unexplained observation that neurotransmitter profiles in urine should be interpreted with extreme caution in the diagnostic work-up of patients suspected to suffer from neurometabolic 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.

Vesicular monoamine transporter 1 mediates dopamine secretion in rat proximal tubular cells

Renal dopamine, synthesized by proximal tubules, plays an important role in the regulation of renal sodium excretion. Although the renal dopaminergic system has been extensively investigated in both physiological and pathological situations, the mechanisms whereby dopamine is stored and secreted by proximal tubule cells remain obscure. In the present study we investigated whether vesicular monoamine transporters (VMAT)-1 and -2, which participate in amine storing and secretion, are expressed in rat renal proximal tubules, and we defined their involvement in dopamine secretion. By combining RT-PCR, Western blot, and immunocytochemistry we showed that VMAT-1 is the predominant isoform expressed in isolated proximal tubule cells. These results were confirmed by immunohistochemistry analysis of rat renal cortex showing that VMAT-1 was found in proximal tubules but not in glomeruli. Functional studies showed that, as previously reported for VMAT-dependent amine transporters, dopamine release by cultured proximal tubule cells was partially inhibited by disruption of intracellular H(+) gradient. In addition, dopamine secretion was prevented by the VMAT-1/VMAT-2 inhibitor reserpine but not by the VMAT-2 inhibitor tetrabenazine. Finally, we demonstrated that tubular VMAT-1 mRNA and protein expression were significantly upregulated during a high-sodium diet. In conclusion, our results show for the first time the expression of a VMAT in the renal proximal tubule and its involvement in regulation of dopamine secretion. These data represent the first step toward the comprehension of the role of this transporter in renal dopamine handling and its involvement in pathological situations.

Low sodium intake induces an increase in renal monoamine oxidase activity in the rat. Involvement of an angiotensin II dependent mechanism

AIMS

The interplay between natriuretic dopamine and antinatriuretic angiotensin II represents an important mechanism for the regulation of renal sodium and water excretion. Monoamine oxidase is the main metabolizing pathway for dopamine in the renal cortex. In this study, we have analysed the effect of low sodium feeding and AT1 receptor blockade on renal dopamine metabolism by monoamine oxidase.

METHODS

Four groups of rats were studied: 1, normal salt diet (NS); 2, low salt diet (LS); 3, NS receiving Losartan (Los, specific AT1 receptor antagonist, 20 mg kg(-1) bwt day(-1), NS + Los); 4, LS receiving Los (LS + Los).

RESULTS

Urinary dopamine excretion was lower in LS than in NS rats (543 +/- 32 vs. 680 +/- 34 ng day(-1) 100 g(-1) bwt, P < 0.05). When treated with Los, DOPAC excretion and urinary DOPAC/dopamine ratio fell significantly in the LS + Los group as compared with the LS group (1199 +/- 328 vs. 3081 +/- 681 ng day(-1) 100 g(-1) bwt and 1.90 +/- 0.5 vs. 5.7 +/- 1.2, respectively, both P < 0.02). Losartan increased hydroelectrolyte excretion in the LS group. No changes were found in the NS + Los group. Aromatic L-amino acid decarboxylase activity in cortex was similar in NS and LS rats. Instead, monoamine oxidase activity was higher in cortical homogenates from LS rats (in nmol mg tissue(-1) h(-1): NS 7.66 +/- 0.52; LS 9.82 +/- 0.59, P < 0.05) and this difference was abolished in LS + Los rats (7.34 +/- 0.49 nmol mg tissue(-1) h(-1), P < 0.01, vs. LS).

CONCLUSIONS

We have concluded that low levels of dopamine in the urine of LS rats are because of an increase in the activity of renal monoamine oxidase and that angiotensin II mediates this increase through stimulation of AT1 receptors.