Dopamine receptor signaling defects in spontaneous hypertension

Sorting nexins: role in the regulation of blood pressure

Sorting nexins (SNXs) are a family of proteins that regulate cellular cargo sorting and trafficking, maintain intracellular protein homeostasis, and participate in intracellular signaling. SNXs are also important in the regulation of blood pressure via several mechanisms. Aberrant expression and dysfunction of SNXs participate in the dysregulation of blood pressure. Genetic studies show a correlation between SNX gene variants and the response to antihypertensive drugs. In this review, we summarize the progress in SNX-mediated regulation of blood pressure, discuss the potential role of SNXs in the pathophysiology and treatment of hypertension, and propose novel strategies for the medical therapy of hypertension.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Comprehensive insights in GRK4 and hypertension: From mechanisms to potential therapeutics

G protein-coupled receptors (GPCRs) mediate cellular responses to diverse extracellular stimuli that play vital roles in the regulation of biology, including behavior. Abnormal G protein-coupled receptor kinase (GRK)-mediated regulation of GPCR function is involved in the pathogenesis of hypertension. Among the seven GRK subtypes, GRK4 has attracted attention because of its constitutive activity and tissue-specific expression. Increasing number of studies show that GRK4 affects blood pressure by GPCR-mediated regulation of renal and arterial function. The target receptor of GRK4 is confined not only to GPCRs, but also to other blood pressure-regulating receptors, such as the adiponectin receptor. Genetic studies in humans show that in several ethnic groups, GRK4 gene variants (R65L, A142V, and A486V) are associated with salt-sensitive or salt-resistant essential hypertension and blood pressure responses to antihypertensive medicines. In this article, we present a comprehensive overview of GRK-mediated regulation of blood pressure, focusing on the latest research progress on GRK4 and hypertension and highlighting potential and novel strategies for the prevention and treatment of hypertension.

Angiotensin II inhibition increases diuresis during acute sympathetic activation in intact and denervated kidneys in rats with chronic myocardial infarction

We examined urine excretion during primary acute sympathetic activation (PASA) in Wistar-Kyoto rats with myocardial infarction (MI). The rats underwent unilateral renal denervation (RDN) 7 weeks after coronary artery ligation. 4-10 days later, an acute experiment was performed under anesthetized conditions (n = 8 rats). Isolated carotid sinus pressure was changed stepwise from 60 to 180 mmHg, and the relationship between the arterial pressure (AP) and the normalized urine flow (nUF, urine flow normalized by the body weight) was examined. After obtaining the control data, an angiotensin II type 1 receptor blocker telmisartan (2.5 mg/kg) was intravenously administered. The effects of RDN, telmisartan, and heart weight (biventricular weight) on the relationship between AP and nUF were examined using multiple regression analyses. Regarding the slope of nUF versus AP (nUF_slope), the constant term of the regression was positive (0.315 ± 0.069 μL·min^-1·kg^-1·mmHg^-1), indicating that nUF increased with AP. The heart weight had a negative effect on nUF_slope (P < 0.05), suggesting that the severity of MI was associated with the impairment of urine excretion. Telmisartan increased nUF_slope by 0.358 ± 0.080 μL·min^-1·kg^-1·mmHg^-1 (P < 0.001), whereas RDN had no significant effect on this parameter. The results indicate that unilateral RDN was unable to abolish the effect of the renin-angiotensin system on urine excretion during PASA. Circulating or locally produced angiotensin II, rather than ongoing renal sympathetic nerve activity, played a dominant role in the impairment of urine excretion during PASA in rats with chronic MI.

Reinvestigation of the tonic natriuretic action of intrarenal dopamine: comparison of two variants of salt-dependent hypertension and spontaneously hypertensive rats

The intrarenal dopamine system has been thoroughly investigated at all levels, especially its role in salt-dependent and other forms of hypertension. However, the evidence regarding dopamine's tonic influence on renal tubular transport of sodium remains equivocal. We reinvestigated its tonic influence on sodium excretion and systemic and renal haemodynamics. Early effects of dopamine D1 receptor blockade using 90-min Schering 23390 (SCH) infusion were examined in anaesthetized rats on 7 days' high salt diet (HS), early uninephrectomized rats on 14 days' HS diet, drinking 1% saline (HS/UNX), and in spontaneously hypertensive rats (SHR). In the HS group (baseline BP ~133 mm Hg) renal intracortical SCH promptly decreased sodium, water and total solute excretion (UNa V, V, Uosm V), with significant difference from the solvent-infused group. BP and renal artery blood flow (RBF, Transonic probe) did not change. In HS/UNX model (baseline BP ~150 mm Hg), characterized by hypertrophy of the remaining kidney, the excretion parameters only tended to decrease whereas SCH induced an ~20% fall in RBF. In SHR (BP ~180 mm Hg), UNa V and V tended to increase in solvent-infused rats; this increasing tendency was abolished by SCH infusion. During experiments the renal vascular resistance increased significantly in SCH- and solvent-infused SHR. Despite some contradictory findings regarding the genuine tonic control of renal excretion by intrarenal dopamine, our results clearly support such role in rats on HS diet and in SHR, the model resembling human essential hypertension. The observations strengthen the experimental basis and the rationale for targeting the intrarenal dopamine system in attempts to combat arterial hypertension.

Dopamine D5 receptor-mediated decreases in mitochondrial reactive oxygen species production are cAMP and autophagy dependent

Overproduction of reactive oxygen species (ROS) plays an important role in the pathogenesis of hypertension. The dopamine D₅ receptor (D₅R) is known to decrease ROS production, but the mechanism is not completely understood. In HEK293 cells overexpressing D₅R, fenoldopam, an agonist of the two D₁-like receptors, D₁R and D₅R, decreased the production of mitochondria-derived ROS (mito-ROS). The fenoldopam-mediated decrease in mito-ROS production was mimicked by Sp-cAMPS but blocked by Rp-cAMPS. In human renal proximal tubule cells with DRD1 gene silencing to eliminate the confounding effect of D₁R, fenoldopam still decreased mito-ROS production. By contrast, Sch23390, a D₁R and D₅R antagonist, increased mito-ROS production in the absence of D₁R, D₅R is constitutively active. The fenoldopam-mediated inhibition of mito-ROS production may have been related to autophagy because fenoldopam increased the expression of the autophagy hallmark proteins, autophagy protein 5 (ATG5), and the microtubule-associated protein 1 light chain (LC)3-II. In the presence of chloroquine or spautin-1, inhibitors of autophagy, fenoldopam further increased ATG5 and LC3-II expression, indicating an important role of D₅R in the positive regulation of autophagy. However, when autophagy was inhibited, fenoldopam was unable to inhibit ROS production. Indeed, the levels of these autophagy hallmark proteins were decreased in the kidney cortices of Drd5^-/- mice. Moreover, ROS production was increased in mitochondria isolated from the kidney cortices of Drd5^-/- mice, relative to Drd5^+/+ littermates. In conclusion, D₅R-mediated activation of autophagy plays a role in the D₅R-mediated inhibition of mito-ROS production in the kidneys.

Open-loop analysis on sympathetically mediated arterial pressure and urine output responses in spontaneously hypertensive rats: effect of renal denervation

Primary acute sympathetic activation (PASA) causes a subsequent arterial pressure (AP) elevation. In this case, an antidiuretic effect via the renal innervation and pressure diuresis can act antagonistically on the kidneys. We examined the effect of PASA on urine output in spontaneously hypertensive rats (SHR) 4-7 days after unilateral renal denervation (RDN) (n = 9). The slope of the plot of urine flow versus AP was positive (0.120 ± 0.031 μL min-1 kg-1 mmHg-1) on the intact side, but it was less than 1/3 of the slope observed previously in normotensive Wistar-Kyoto rats (WKY). RDN did not normalize the slope of urine flow versus AP (0.179 ± 0.025 μL min-1 kg-1 mmHg-1, P = 0.098 versus the intact side). The urine flow at the operating point of the AP tended to be greater on the denervated than the intact side (29.0 ± 1.8 vs. 25.3 ± 1.9 μL min-1 kg-1, P = 0.055). The percent increase (17.2 ± 7.2%) was not different from that observed previously in WKY. Although high-resting sympathetic nerve activity is prerequisite for maintaining hypertension in SHR, the effect of sympathetic innervation on the urine output function was not greater than that in WKY.

Renal Dopamine Receptors and Oxidative Stress: Role in Hypertension

Significance: The kidney plays an important role in the long-term control of blood pressure. Oxidative stress is one of the fundamental mechanisms responsible for the development of hypertension. Dopamine, via five subtypes of receptors, plays an important role in the control of blood pressure by various mechanisms, including the inhibition of oxidative stress. Recent Advances: Dopamine receptors exert their regulatory function to decrease the oxidative stress in the kidney and ultimately maintain normal sodium balance and blood pressure homeostasis. An aberration of this regulation may be involved in the pathogenesis of hypertension. Critical Issues: Our present article reviews the important role of oxidative stress and intrarenal dopaminergic system in the regulation of blood pressure, summarizes the current knowledge on renal dopamine receptor-mediated antioxidation, including decreasing reactive oxygen species production, inhibiting pro-oxidant enzyme nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and stimulating antioxidative enzymes, and also discusses its underlying mechanisms, including the increased activity of G protein-coupled receptor kinase 4 (GRK4) and abnormal trafficking of renal dopamine receptors in hypertensive status. Future Directions: Identifying the mechanisms of renal dopamine receptors in the regulation of oxidative stress and their contribution to the pathogenesis of hypertension remains an important research focus. Increased understanding of the role of reciprocal regulation between renal dopamine receptors and oxidative stress in the regulation of blood pressure may give us novel insights into the pathogenesis of hypertension and provide a new treatment strategy for hypertension.

Dopamine Receptors and the Kidney: An Overview of Health- and Pharmacological-Targeted Implications

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.

Is renal ß-adrenergic-WNK4-NCC pathway important in salt hypertension of Dahl rats?

In 2011 Fujita and coworkers proposed that ß-adrenergic stimulation causes decreased serine/threonine-protein kinase WNK4 transcription leading to the activation of Na-Cl cotransporter (NCC) which participates in salt sensitivity and salt hypertension development in rodents. The aim of our study was to investigate whether the above hypothesis is also valid for salt hypertension of Dahl rats, which are characterized by high sympathetic tone and abnormal renal sodium handling. Male 8-week-old salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) Dahl rats were fed either low-salt diet (LS, 0.4 % NaCl) or high-salt diet (HS, 4 % NaCl) for 6 weeks. Half of the animals on either diet were chronically treated with non-selective ß-blocker propranolol (100 mg/kg/day). At the end of the experiment diuresis and sodium excretion were measured prior and after hydrochlorothiazide injection (HCTZ, 10 mg/kg i.p.). Furthermore, blood pressure (BP), heart rate (HR), sympathetic (pentolinium 5 mg/kg i.v.) and NO-dependent (L-NAME 30 mg/kg i.v.) BP components were determined. Chronic HS diet feeding increased BP through sympathoexcitation in SS/Jr but not in SR/Jr rats. Concomitant propranolol treatment did not lower BP in either experimental group. Under the conditions of low salt intake HCTZ increased diuresis, natriuresis and fractional sodium excretion in SR/Jr but not in SS/Jr rats. HS diet feeding attenuated renal response to HCT in SR/Jr rats, whereas no HCTZ effect was observed in SS/Jr rats fed HS diet. Propranolol treatment did not modify diuresis or natriuresis in any experimental group. In conclusions, our present data do not support the idea on the essential importance of renal ß-adrenergic-WNK4-NCC pathway in pathogenesis and/or maintenance of salt hypertension in Dahl rats.

A new common functional coding variant at the DDC gene change renal enzyme activity and modify renal dopamine function

The intra-renal dopamine (DA) system is highly expressed in the proximal tubule and contributes to Na+ and blood pressure homeostasis, as well as to the development of nephropathy. In the kidney, the enzyme DOPA Decarboxylase (DDC) originating from the circulation. We used a twin/family study design, followed by polymorphism association analysis at DDC locus to elucidate heritable influences on renal DA production. Dense single nucleotide polymorphism (SNP) genotyping across the DDC locus on chromosome 7p12 was analyzed by re-sequencing guided by trait-associated genetic markers to discover the responsible genetic variation. We also characterized kinetics of the expressed DDC mutant enzyme. Systematic polymorphism screening across the 15-Exon DDC locus revealed a single coding variant in Exon-14 that was associated with DA excretion and multiple other renal traits indicating pleiotropy. When expressed and characterized in eukaryotic cells, the 462Gln variant displayed lower Vmax (maximal rate of product formation by an enzyme) (21.3 versus 44.9 nmol/min/mg) and lower Km (substrate concentration at which half-maximal product formation is achieved by an enzyme.)(36.2 versus 46.8 μM) than the wild-type (Arg462) allele. The highly heritable DA excretion trait is substantially influenced by a previously uncharacterized common coding variant (Arg462Gln) at the DDC gene that affects multiple renal tubular and glomerular traits, and predicts accelerated functional decline in chronic kidney disease.

Downregulation of Renal G Protein-Coupled Receptor Kinase Type 4 Expression via Ultrasound-Targeted Microbubble Destruction Lowers Blood Pressure in Spontaneously Hypertensive Rats

BACKGROUND

G protein-coupled receptor kinase type 4 (GRK4) plays a vital role in the long-term control of blood pressure (BP) and sodium excretion by regulating renal G protein-coupled receptor phosphorylation, including dopamine type 1 receptor (D1R). Ultrasound-targeted microbubble destruction (UTMD) is a promising method for gene delivery. Whether this method can deliver GRK4 small interfering RNA (siRNA) and lower BP is not known.

METHODS AND RESULTS

BP, 24-hour sodium excretion, and urine volume were measured after UTMD-targeted GRK4 siRNA delivery to the kidney in spontaneously hypertensive rats. The expression levels of GRK4 and D1R were determined by immunoblotting. The phosphorylation of D1R was investigated using immunoprecipitation. The present study revealed that UTMD-mediated renal GRK4 siRNA delivery efficiently reduced GRK4 expression and lowered BP in spontaneously hypertensive rats, accompanied by increased sodium excretion. The increased sodium excretion might be accounted for by the UTMD regulation of D1R phosphorylation and function in spontaneously hypertensive rats. Further analysis showed that, although UTMD had no effect on D1R expression, it reduced D1R phosphorylation in spontaneously hypertensive rats kidneys and consequently increased D1R-mediated natriuresis and diuresis.

CONCLUSIONS

Taken together, these study results indicate that UTMD-targeted GRK4 siRNA delivery to the kidney effectively reduces D1R phosphorylation by inhibiting renal GRK4 expression, improving D1R-mediated natriuresis and diuresis, and lowering BP, which may provide a promising novel strategy for gene therapy for hypertension.

ISN Forefronts Symposium 2015: Maintaining Balance Under Pressure-Hypertension and the Proximal Tubule

Renal control of effective circulating volume (ECV) is key for circulatory performance. When renal sodium excretion is inadequate, blood pressure rises and serves as a homeostatic signal to drive natriuresis to re-establish ECV. Recognizing that hypertension involves both renal and vascular dysfunction, this report concerns proximal tubule sodium hydrogen exchanger 3 (NHE3) regulation during acute and chronic hypertension. NHE3 is distributed in tall microvilli (MV) in the proximal tubule, where it reabsorbs a significant fraction of the filtered sodium. NHE3 redistributes, in the plane of the MV membrane, between the MV body, where NHE3 is active, and the MV base, where NHE3 is less active. A high-salt diet and acute hypertension both retract NHE3 to the base and reduce proximal tubule sodium reabsorption independent of a change in abundance. The renin angiotensin system provokes NHE3 redistribution independent of blood pressure: The angiotensin-converting enzyme (ACE) inhibitor captopril redistributes NHE3 to the base and subsequent angiotensin II (AngII) infusion returns NHE3 to the body of the MV and restores reabsorption. Chronic AngII infusion presents simultaneous AngII stimulation and hypertension; that is, NHE3 remains in the body of the MV, due to the high local AngII level and inflammation, and exhibits a compensatory decrease in abundance driven by the hypertension. Genetically modified mice with blunted hypertensive responses to chronic AngII infusion (due to lack of the proximal tubule AngII receptors interleukin-17A or interferon-γ expression) exhibit reduced local AngII accumulation and inflammation and larger decreases in NHE3 abundance, which improves the pressure natriuresis response and reduces the need for elevated blood pressure to facilitate circulating volume balance.

Blood pressure decrease in spontaneously hypertensive rats folowing renal denervation or dopamine β-hydroxylase inhibition with etamicastat

Overactivity of the sympathetic nervous system has an important role in the development and progression of arterial hypertension. Catheter-based renal nerve ablation for the treatment of drug-resistant hypertension has recently been developed. An alternative strategy for the modulation of sympathetic nerve function is to reduce the biosynthesis of noradrenaline (NA) by inhibiting dopamine β-hydroxylase (DβH), the enzyme that catalyzes the conversion of dopamine (DA) to NA in the sympathetic nerves. Renal denervation (RDN) surgery was performed in spontaneously hypertensive rats (SHR) to evaluate the effect of RDN on the DA and NA levels and on blood pressure over a 28-day period. The selective peripheral DβH inhibitor etamicastat (30 mg kg (-1)day(-1)) was administered to another cohort of SHR. RDN and etamicastat treatment had no effect on the renal function, as assessed by measuring the water balance response, renal function and urinary electrolyte levels. RDN significantly decreased the systolic blood pressure (SBP) and the diastolic blood pressure (DBP). A gradual return of the SBP and the DBP to the high baseline levels was observed over time. Conversely, treatment with etamicastat resulted in a significant decrease in the SBP and the DBP at all time points. On the last day of the assessment, NA levels in renal tissue were significantly decreased in both RDN and etamicastat-treated groups. In contrast, the NA levels in the left ventricle were decreased only in the etamicastat-treated group. Thus, RDN produces transitory decreases in blood pressure, whereas prolonged downregulation of sympathetic drive with the DβH inhibitor etamicastat results in a sustained decrease in the SBP and the DBP.

Transcriptional regulation of renal dopamine D1 receptor function during oxidative stress

There exists a strong link between oxidative stress, renal dopaminergic system, and hypertension. It is reported that reactive oxygen species attenuate renal proximal tubular dopamine receptor (D1R) function, which disrupts sodium regulation and leads to hypertension. However, the mechanisms for renal D1R dysfunction are not clear. We investigated the role of redox-sensitive transcription factors AP1 and SP3 in transcriptional suppression of D1R gene and subsequent D1R signaling. Human kidney proximal tubular cells were treated with a pro-oxidant l-buthionine sulfoximine (BSO) with and without an antioxidant tempol. In human kidney cells, BSO caused oxidative stress and reduced D1R mRNA and membrane receptor expression. Incubation of human kidney cells with SKF38393, a D1R agonist, caused a concentration-dependent inhibition of Na/K-ATPase. However, SKF38393 failed to inhibit Na/K-ATPase in BSO-treated cells. BSO increased AP1 and SP3 nuclear expression. Transfection with AP1- or SP3-specific siRNA abolished BSO-induced D1R downregulation. Treatment of rats with BSO for 4 weeks increased oxidative stress and SP3-AP1 expression and reduced D1R numbers in renal proximal tubules. These rats exhibited high blood pressure, and SKF38393 failed to inhibit proximal tubular Na/K-ATPase activity. Control rats were kept on tap water. Tempol per se had no effect on D1R expression or other signaling molecules but prevented BSO-induced oxidative stress, SP3-AP1 upregulation, and D1R dysfunction in both human kidney cells and rats. These data show that oxidative stress via AP1-SP3 activation suppresses D1R transcription and function. Tempol mitigates oxidative stress, blocks AP1-SP3 activation, and prevents D1R dysfunction and hypertension.

First D1-like receptor PET imaging of the rat and primate kidney: implications for human disease monitoring

The intrarenal dopamine system is important for signaling and natriuresis, and significant dysfunction is associated with hypertension and kidney disease in ex vivo studies. Dopamine receptors also modulate and are modulated by the renin-angiotensin-aldosterone system. Here, we show the first in vivo measurement of D1-like receptors in the renal cortex of Sprague-Dawley rat and Papio anubis baboon using [(11)C]NNC 112, a positron emission tomography radioligand for D1-like receptors. In addition, we show a D1-like binding potential response to angiotensin II blockade in rats using losartan. Demonstration of self-saturable binding in the rat as well as specific and saturable binding in Papio anubis validate the use of [(11)C]NNC 112 in the first in vivo measurement of renal dopamine D1-like receptors. Furthermore, [(11)C]NNC 112 is a radioligand tool already validated for use in probing human central nervous system (CNS) D1-like receptors. Our work demonstrates specific and saturable non-CNS binding in higher animals and the ability to quantify physiological response to drug treatment and provides a clear path to extend use of [(11)C]NNC 112 to study renal dopamine in humans.

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Chronic regulation of the renal Na(+)/H(+) exchanger NHE3 by dopamine: translational and posttranslational mechanisms

The intrarenal autocrine/paracrine dopamine (DA) system contributes to natriuresis in response to both acute and chronic Na(+) loads. While the acute DA effect is well described, how DA induces natriuresis chronically is not known. We used an animal and a cell culture model to study the chronic effect of DA on a principal renal Na(+) transporter, Na(+)/H(+) exchanger-3 (NHE3). Intraperitoneal injection of Gludopa in rats for 2 days elevated DA excretion and decreased total renal cortical and apical brush-border NHE3 antigen. Chronic treatment of an opossum renal proximal cell line with DA decreased NHE3 activity, cell surface and total cellular NHE3 antigen, but not NHE3 transcript. The decrease in NHE3 antigen was dose and time dependent with maximal inhibition at 16-24 h and half maximal effect at 3 × 10(-7) M. This is in contradistinction to the acute effect of DA on NHE3 (half maximal at 2 × 10(-6) M), which was not associated with changes in total cellular NHE3 protein. The DA-induced decrease in total NHE3 protein was associated with decrease in NHE3 translation and mediated by cis-sequences in the NHE3 5'-untranslated region. DA also decreased cell surface and total cellular NHE3 protein half-life. The DA-induced decrease in total cellular NHE3 was partially blocked by proteasome inhibition but not by lysosome inhibition, and DA increased ubiquitylation of total and surface NHE3. In summary, chronic DA inhibits NHE3 with mechanisms distinct from its acute action and involves decreased NHE3 translation and increased NHE3 degradation, which are novel mechanisms for NHE3 regulation.

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

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

Potential dopamine-1 receptor stimulation in hypertension management

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

Dopamine receptors and hypertension

Dopamine plays an important role in regulating renal function and blood pressure. Dopamine synthesis and dopamine receptor subtypes have been shown in the kidney. Dopamine acts via cell surface receptors coupled to G proteins; the receptors are classified via pharmacologic and molecular cloning studies into two families, D1-like and D2-like. Two D1-like receptors cloned in mammals, the D1 and D5 receptors (D1A and D1B in rodents), are linked to adenylyl cyclase stimulation. Three D2-like receptors (D2, D3, and D4) have been cloned and are linked mainly to adenylyl cyclase inhibition. Activation of D1-like receptors on the proximal tubules inhibits tubular sodium reabsorption by inhibiting Na/H-exchanger and Na/K-adenosine triphosphatase activity. Reports exist of defective renal dopamine production and/or dopamine receptor function in human primary hypertension and in genetic models of animal hypertension. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to hypertension. A primary defect in D1-like receptors and an altered signaling system in proximal tubules may reduce dopamine-mediated effects on renal sodium excretion. The molecular basis for dopamine receptor dysfunction in hypertension is being investigated, and may involve an abnormal posttranslational modification of the dopamine receptor.

Dysregulation of dopamine-dependent mechanisms as a determinant of hypertension: studies in dopamine receptor knockout mice

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones/humoral factors, such as aldosterone, angiotensin, catecholamines, endothelin, oxytocin, prolactin pro-opiomelancortin, reactive oxygen species, renin, and vasopressin. Dopamine receptors are classified into D(1)-like (D(1) and D(5)) and D(2)-like (D(2), D(3), and D(4)) subtypes based on their structure and pharmacology. In recent years, mice deficient in one or more of the five dopamine receptor subtypes have been generated, leading to a better understanding of the physiological role of each of the dopamine receptor subtypes. This review summarizes the results from studies of various dopamine receptor mutant mice on the role of individual dopamine receptor subtypes and their interactions with other G protein-coupled receptors in the regulation of blood pressure.

The dopaminergic system in hypertension

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport, vascular smooth muscle contractility and production of reactive oxygen species and by interacting with the renin–angiotensin and sympathetic nervous systems. Dopamine receptors are classified into D1-like (D1 and D5) and D2-like (D2, D3 and D4) subtypes based on their structure and pharmacology. Each of the 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 and/or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. This review summarizes the physiology of the different dopamine receptors in the regulation of blood pressure, and the relationship between dopamine receptor subtypes and hypertension.

Signaling pathways involved in atrial natriuretic factor and dopamine regulation of renal Na+, K+ -ATPase activity

Dopamine (DA) and atrial natriuretic factor (ANF) share a number of physiological effects. We hypothesized that ANF and the renal dopaminergic system could interact and enhance the natriuretic and diuretic effects of the peptide. We have previously reported that the ANF-stimulated DA uptake in renal tubular cells is mediated by the natriuretic peptide type-A receptor (NPR-A). Our aim was to investigate the signaling pathways that mediate ANF effects on renal 3H-DA uptake. Methylene blue (10 microM), an unspecific inhibitor of guanylate cyclase (GC), blunted ANF elicited increase of DA uptake. ODQ (10 microM) a specific inhibitor of soluble GC, did not modify DA uptake and did not reverse ANF-induced increase of DA uptake; then the participation of nitric oxide-dependent pathways must be discarded. The second messenger was the cGMP since the analogous 125 microM 8-Br-cGMP mimicked ANF effects. The specific inhibitor of the protein kinase G (PKG), KT 5823 (1 microM) blocked ANF effects indicating that PKG is involved. We examined if ANF effects on DA uptake were able to modify Na+, K+ -adenosine triphosphatase (Na+, K+ -ATPase) activity. The experiments were designed by means of inhibition of renal DA synthesis by carbidopa and neuronal DA uptake blocked by nomifensine. In these conditions renal Na+, K+ -ATPase activity was increased, in agreement with the decrease of DA availability. When in similar conditions, exogenous DA was added to the incubation medium, the activity of the enzyme tended to decrease, following to the restored availability of DA. The addition of ANF alone had similar effects to the addition of DA on the sodium pump, but when both were added together, the activity of Na(+), K(+)-ATPase was decreased. Moreover, the extraneuronal uptake blocker, hydrocortisone, inhibited the latter effect. In conclusion, ANF stimulates extraneuronal DA uptake in external cortex tissues by activation of NPR-A receptors coupled to GC and it signals through cGMP as second messenger and PKG. Dopamine and ANF may achieve their effects through a common pathway that involves reversible deactivation of renal tubular Na+, K+ -ATPase activity. This mechanism demonstrates a DA-ANF relationship involved in the modulation of both decreased sodium reabsorption and increased natriuresis.

Identification of human dopamine receptors agonists from Chinese herbs

AIM

To find human dopamine receptors, especially D1-like receptor specific agonists from Chinese herbs as potential antihypertension drug leads.

METHODS

Two D1-like receptor cell lines carrying a beta-lactamase reporter gene, and a D2 receptor cell line coexpressing a promiscuous G protein G15 were constructed using HEK293 cells. A natural compound library made from fractionated samples of herbal extracts was used for high-throughput screening (HTS) against one of the cell lines, HEK/D5R/CRE-blax. The interested hits were evaluated for their activities against various dopamine receptors.

RESULTS

Fourteen hits were identified from primary screening, of which 2 of the better hit samples, HD0522 and HD0059, were selected for further material and activity analysis, and to obtain 2 compounds that appeared as 2 single peaks in HPLC, HD0522H01 and HD0059H01. HD0059H01 could activate D1, D2, and D5 receptors, with EC(50 ) values of 2.28 microg/mL, 0.85 microg/mL, and 1.41 microg/mL, respectively. HD0522H01 could only activate D1R and D5R with EC(50 ) values of 2.95 microg/mL and 8.38 microg/mL.

CONCLUSION

We established cellbased assays for 3 different human dopamine receptors and identified specific agonists HD0522H01 and HD0059H01 through HTS. The specific agonist to D1-like receptors, HD0522H01, may become a new natural product-based drug lead for antihypertension treatment.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

Atrial natriuretic factor stimulates renal dopamine uptake mediated by natriuretic peptide-type A receptor

To determine the effects of atrial natriuretic factor (ANF) on renal dopamine (DA) metabolism, 3H-DA and 3H-L-DOPA uptake by renal tubular cells was measured in experiments carried out in vitro in Sprague-Dawley rats. The receptor type involved was also analyzed. The results indicate that ANF increased at 30 min, DA uptake in a concentration-response fashion having 10 pM ANF as the threshold concentration. Conversely, the uptake of the precursor L-DOPA was not modified by the peptide. ANF effects were observed in tissues from external and juxtamedullar cortex and inner medulla. On this basis, 100 nM ANF was used to continue the studies in external cortex tissues. DA uptake was characterized as extraneuronal uptake, since 100 microM hydrocortisone blocked ANF-induced increase of DA uptake. Renal DA uptake was decreased at 0 degrees C and in sodium-free medium. The effects of ANF in these conditions were not present, confirming that renal DA uptake is mediated by temperature- and sodium-dependent transporters and that the peptide requires the presence of the ion to exhibit its actions on DA uptake. The biological natriuretic peptide type A receptor (NPR-A) mediates ANF effects, since 100 nM anantin, a specific blocker, reversed ANF-dependent increase of DA uptake. The natriuretic peptide type C receptor (NPR-C) is not involved, since the specific analogous 100 nM 4-23 ANF amide has no effect on renal DA uptake and does not alter the effects of 100 nM ANF. In conclusion, ANF stimulates DA uptake by kidney tubular cells. ANF effects are mediated by NPR-A receptors coupled to guanylate cyclase and cGMP as second messenger. The process involved was characterized as a typical extraneuronal uptake, and characterized as temperature- and sodium-dependent. This mechanism could be related to DA effects on sodium reabsorption and linked to ANF enhanced natriuresis in the kidney. The increment of endogenous DA into tubular cells, as a consequence of increased DA uptake, would permit D1 receptor recruitment and Na+,K+-ATPase activity inhibition, which results in decreased sodium reabsorption and increased natriuresis.

The Abnormal Renal Vasodilator Response to D1-Like Receptor Stimulation in Conscious SHR Can Be Normalized by AT1 Blockade

BACKGROUND

We previously showed that the renal vasodilator response to a D1-like receptor agonist is blunted in conscious SHR compared with WKY rats. The mechanism of this impaired dopaminergic responsiveness in SHR is unclear. An altered balance between the renin-angiotensin-aldosterone system (RAAS) and the dopaminergic system may be involved. To determine the interaction between the RAAS and the dopaminergic system in the blunted D1-like responsiveness in SHR, we studied the renal vasodilator response to the D1-like receptor agonist fenoldopam before and after 7 days of pretreatment with the AT1-receptor antagonist (AT1-A) L158,809 in conscious SHR and WKY rats.

METHODS

Effective renal plasma flow (ERPF) was measured by the clearance of I-hippuran. Mean arterial pressure (MAP) was measured via an intraarterial catheter.

RESULTS

Without pretreatment, MAP was reduced to comparable degrees by fenoldopam in WKY (-7 +/- 4%, ns) and SHR (-6 +/- 1%, P < 0.05). However, ERPF was significantly more increased (P < 0.006) by fenoldopam in WKY (+26 +/- 2%, P < 0.0001) than in SHR (+2 +/- 2%, ns). AT1-A treatment reduced MAP and increased ERPF and glomerular filtration rate significantly in both strains. Pretreatment with AT1-A significantly potentiated the fenoldopam-induced rise in ERPF in SHR, but not in WKY, without affecting the blood pressure responses in either strain. As a result, during pretreatment with an AT1-A, the rise in ERPF by fenoldopam was similar in both strains (SHR +25 +/- 2%, P < 0.0001; WKY +33 +/- 2%, P < 0.0001).

CONCLUSIONS

These results suggest that the RAAS accounts for the blunted renal vasodilator response to a D1-like receptor agonist in SHR. A dysbalance between the dopaminergic system and the RAAS may be involved in the abnormal renal hemodynamic regulation in SHR.

The dopamine precursorl-dihydroxyphenylalanine is transported by the amino acid transporters rBAT and LAT2 in renal cortex

The intrarenal autocrine-paracrine dopamine (DA) system is critical for Na+homeostasis. l-Dihydroxyphenylalanine (l-DOPA) uptake from the glomerular filtrate and plasma provides the substrate for DA generation by the renal proximal tubule. The transporter(s) responsible for proximal tubule l-DOPA uptake has not been characterized. Renal cortical poly-A+RNA injected into Xenopus laevis oocytes induced l-DOPA uptake in a time- and dose-dependent fashion with biphasic Kms in the millimolar and micromolar range and independent of inward Na+, K+, or H+gradients, suggesting the presence of low- and high-affinity l-DOPA carriers. Complementary RNA from two amino acid transporters yielded l-DOPA uptake significantly above water-injected controls the rBAT/b0,+AT dimer (rBAT) and the LAT2/4F2 dimer (LAT2). In contradistinction to renal cortical poly-A+, l-DOPA kinetics of rBAT and LAT2 showed classic Michaelis-Menton kinetics with Kms in the micromolar and millimolar range, respectively. Sequence-specific antisense oligonucleotides to rBAT or LAT2 (AS) caused inhibition of rBAT and LAT2 cRNA-induced l-DOPA transport and cortical poly-A+-induced arginine and phenylalanine transport. However, the same ASs only partially blocked poly-A+-induced l-DOPA transport. In cultured kidney cells, silencing inhibitory RNA (siRNA) to rBAT significantly inhibited l-DOPA uptake. We conclude that rBAT and LAT2 can mediate apical and basolateral l-DOPA uptake into the proximal tubule, respectively. Additional l-DOPA transport mechanisms exist in the renal cortex that remain to be identified.

Reduced natriuretic response to acute sodium loading in COMT gene deleted mice

BACKGROUND

The intrarenal natriuretic hormone dopamine (DA) is metabolised by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO). Inhibition of COMT, as opposed to MAO, results in a potent natriuretic response in the rat. The present study in anaesthetized homozygous and heterozygous COMT gene deleted mice attempted to further elucidate the importance of COMT in renal DA and sodium handling. After acute intravenous isotonic sodium loading, renal function was followed.

RESULTS

COMT activity in heterozygous mice was about half of that in wild type mice and was zero in the homozygous mice. MAO activity did not differ between the genotypes. Urinary sodium excretion increased 10-fold after sodium loading in wild type mice. In heterozygous and homozygous mice, the natriuretic effects of sodium loading were only 29 % and 39 %, respectively, of that in wild type mice. Arterial pressure and glomerular filtration rate did not differ between genotypes. Baseline norepinephrine and DA excretions in urine were elevated in the homozygous, but not in heterozygous, COMT gene deleted mice. Urinary DA excretion increased after isotonic sodium loading in the wild type mice but not in the COMT gene deleted mice.

CONCLUSIONS

Mice with reduced or absent COMT activity have altered metabolism of catecholamines and are unable to increase renal DA activity and produce normal natriuresis in response to acute sodium loading. The results support the hypothesis that COMT has an important role in the DA-mediated regulation of renal sodium excretion.

D2 -like receptor stimulation decreases effective renal plasma flow and glomerular filtration rate in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHRs) the dopaminergic D1-like renal vasodilator response is impaired. The renal vascular response to D2-like receptor stimulation in vivo is incompletely known. Therefore, renal hemodynamics were studied in conscious SHRs during continuous infusion of D2-like agonist N,N-Di-n-propyldopamine (DPDA) (10 microg/kg/min) with Wistar-Kyoto (WKY) rats as controls. As sodium status may affect dopaminergic responses, rats were studied during both low- and high-sodium diets. D2-like stimulation reduced mean arterial pressure and effective renal plasma flow and glomerular filtration rate (GFR) similarly in SHR and WKY rats. Renal vascular resistance increased significantly in both strains. The response to DPDA is modified by sodium status, with a more pronounced fall in blood pressure (in WKYs and SHRs) and GFR (in WKYs) during high-sodium conditions. The responses were blocked by co-infusion with D2 antagonist domperidone. Thus, D2-like renal vascular responses are normal in SHRs irrespective of sodium intake. The combination of a preserved D2-like renal vasoconstrictive and an impaired D1-like renal vasodilatory response may contribute to maintenance of hypertension in SHRs.

Dopamine acutely stimulates Na+/H+ exchanger (NHE3) endocytosis via clathrin-coated vesicles: dependence on protein kinase A-mediated NHE3 phosphorylation

Dopamine (DA) is a key hormone in mammalian sodium homeostasis. DA induces natriuresis via acute inhibition of the renal proximal tubule apical membrane Na(+)/H(+) exchanger NHE3. We examined the mechanism by which DA inhibits NHE3 in a renal cell line. DA acutely decreases surface NHE3 antigen in dose- and time-dependent fashion without altering total cellular NHE3. Although DA(1) receptor agonist alone decreases surface NHE3, simultaneous DA(2) agonist synergistically enhances the effect of DA(1). Decreased surface NHE3 antigen, caused by stimulation of NHE3 endocytosis, is dependent on intact functioning of the GTPase dynamin and involves increased binding of NHE3 to the adaptor protein AP2. DA-stimulated NHE3 endocytosis can be blocked by pharmacologic or genetic protein kinase A inhibition or by mutation of two protein kinase A target serines (Ser-560 and Ser-613) on NHE3. We conclude that one mechanism by which DA induces natriuresis is via protein kinase A-mediated phosphorylation of proximal tubule NHE3 leading to endocytosis of NHE3 via clathrin-coated vesicles.

Regulation of sodium/potassium ATPase activity: impact on salt balance and vascular contractility

Na+,K+-ATPase distributes ions between the intracellular and extracellular space and is responsible for total-body sodium homeostasis. The activity of this ion pump is regulated by catecholamines and peptide hormones; by the ligand of Na+,K+-ATPase, ouabain; and by direct interaction with cytoskeleton proteins. This review summarizes recent advances in the field of short-term regulation of Na+,K+-ATPase and the implications of these advances for the regulation of blood pressure. Renal Na+,K+-ATPase activity is bidirectionally regulated by natriuretic and antinatriuretic hormones, and a shift in the balance between these forces may lead to salt retention and hypertension. Dopamine plays a key role in this interactive regulation. By inhibiting vascular Na+,K+-ATPase activity, an excess of circulating ouabain may increase calcium concentration in vascular cells and lead to increased vascular contractility. Finally, mutations in cytoskeleton proteins may stimulate renal Na+,K+-ATPase activity by way of protein/protein interaction and lead to salt retention and hypertension. Abnormalities in the systems regulating Na+,K+-ATPase should be explored further in the search for the multiple causes of essential hypertension.

Iloprost inhibits inositol-1,4,5-trisphosphate-mediated calcium mobilization stimulated by angiotensin II in cultured preglomerular vascular smooth muscle cells

In a previous study of cultured preglomerular vascular smooth muscle cells, it was demonstrated that, although the stable prostacyclin analog iloprost alone had no effect on the intracellular calcium concentration ([Ca2+](i)), it did significantly attenuate the increase in [Ca2+](i) stimulated by angiotensin II (AngII). In this study, the mechanisms by which iloprost interacts with calcium signaling pathways stimulated by AngII were examined. [Ca2+](i) was assessed using the calcium-sensitive fluorescent dye fura-2. Initial studies identified two major components of the [Ca2+](i) response to AngII in this homogeneous preparation of vascular smooth muscle cells from renal resistance vessels. Mobilization of internal stores was evident as an immediate TMB-8-sensitive peak increase in [Ca2+](i) (52 +/- 6 to 297 +/- 26 nM, P: < 0.001) in a calcium-free medium. After [Ca2+](i) had returned to baseline levels during continued AngII stimulation, a nifedipine-sensitive entry pathway was revealed by the sustained stimulatory effect of added external calcium, which increased [Ca2+](i) to 112 +/- 13 nM (P: < 0.001). Coadministration of iloprost with AngII attenuated both the immediate peak (154 +/- 14 nM) and sustained plateau (61 +/- 9 nM) phases. Increases in endogenous levels of cAMP induced by the phosphodiesterase inhibitor milrinone mirrored the actions of iloprost, suggesting that the prostacyclin analog exerted its actions via cAMP activation. Blockade of cAMP-dependent protein kinase with KT 5720 reversed the effects of both iloprost and milrinone. When iloprost or milrinone was introduced after the initial mobilization peak had dissipated, the plateau phase of calcium entry was unchanged (92 +/- 9 nM). The concept that iloprost does not directly modulate calcium entry was further supported by data showing that the activation of L-type calcium channels by BAY-K 8644 was unchanged during iloprost treatment. On the basis of the observation that iloprost did not alter thapsigargin stimulation of Ca(2+)-ATPase activity, it is concluded that the actions of cAMP are distinct from increasing calcium uptake into the sarcoplasmic reticulum. This study provides new information on the ability of iloprost to primarily attenuate inositol-1,4,5-triphosphate-mediated calcium mobilization via cAMP, with secondary inhibition of L-type calcium entry channels. These data clarify the mechanism by which prostaglandins buffer AngII constriction in resistance arterioles.

Characterization of acute inhibition of Na/H exchanger NHE-3 by dopamine in opossum kidney cells

BACKGROUND

Dopamine (DA) is a principal natriuretic hormone that defends extracellular fluid volume from a Na load. Natriuresis is effected partly through inhibiting the proximal tubule Na/H exchanger NHE-3. Changes in NHE-3 phosphorylation is one mechanism by which NHE-3 activity is regulated.

METHODS

We used opossum kidney (OK) cells to characterize the differential and synergistic effects of DA receptor subtype-1 (DA1) and -2 (DA2) agonists and the effect of blockade of protein kinase A (PKA) or protein kinase C (PKC) on NHE-3 activity and phosphorylation.

RESULTS

DA and DA1 agonists inhibited NHE-3 activity, and DA1 antagonist blocked the effect of either DA or DA1 agonist. DA2 agonist alone had no effect, but DA2 antagonist reduced the DA effect on NHE-3 activity. DA1 and DA2 agonists together were more potent than DA1 alone. PKA inhibition eliminated the effect of DA1 agonist and partially blocked the effect of DA on NHE-3 activity. PKC inhibition did not block the DA effect. DA1 agonist and PKA activation phosphorylated NHE-3 on identical sites. Despite lack of effect on NHE-3 activity, DA2 agonists increased NHE-3 phosphorylation. DA-induced NHE-3 phosphorylation was distinct from DA1 and PKA but closely resembled DA2.

CONCLUSION

We postulate the following: (1) DA modifies NHE-3 phosphorylation by activating PKA through DA1 and by other kinases/phosphatases via DA2. (2) DA1 is sufficient to inhibit NHE-3, while DA2 is insufficient but plays a synergistic role by altering NHE-3 phosphorylation.

Intrarenal dopamine: a key signal in the interactive regulation of sodium metabolism

The kidney regulates sodium metabolism with extraordinary precision and sensitivity. This is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between anti-natriuretic and natriuretic factors. Dopamine, produced in renal proximal tubule cells, plays a central role in this interactive network. Natriuretic hormones that are released from extrarenal sources, such as atrial natriuretic peptide, mediate some of their effects via renal dopamine receptors. On the level of the tubules, dopamine acts by opposing the effects of anti-natriuretic factors, such as angiotensin II and alpha-adrenergic receptors. Sodium retention leads to an increase in renal dopamine tonus, and the natriuretic effects of dopamine are more prominent under this condition. Inhibition or down-regulation of dopamine receptors significantly attenuates the natriuretic response to salt loading. Renal dopamine is modulated by the supply of filtered L-DOPA and the metabolism of dopamine via catechol-O-methyldopamine. The importance of dopamine as a natriuretic hormone is reflected by its capacity to inhibit the majority of renal tubule sodium transporters. Notably, the activity of Na+, K+ ATPase is inhibited in most tubule segments by dopamine. Recent studies have elucidated many of the signaling pathways for renal dopamine receptors. Novel principles for homologous and heterologous sensitization of dopamine receptors have been detected that may explain some of the interaction between dopamine and other first messengers that modulate renal tubule sodium transport. A broad understanding of the renal dopamine system has become increasingly important, since there is now strong evidence from both clinical and experimental studies that dysregulation of the renal dopamine system plays a role in many forms of multigenetic hypertension.

Dopamine D1 receptor gene polymorphism is associated with essential hypertension

Dopamine has been shown to influence renal sodium excretion through a direct interaction with the dopamine receptor (DR). The dopamine D1 receptor (DRD1) has been localized to the proximal tubules and is known to increase sodium excretion by inhibiting Na-H exchanger and Na,K-ATPase activity. Defective renal dopamine production and/or DR function have been reported in essential hypertension (EH) as well as in genetic models of animal hypertension. With a restriction fragment length polymorphism of the DRD1 gene, we performed an association study in patients with EH. One hundred thirty-one subjects with EH and 136 age-matched normotensive (NT) controls were studied. Polymerase chain reaction was used to amplify the A-48G polymorphic site in the DRD1 gene, and restriction analysis of the polymerase chain reaction product was used to score the A and G alleles. The allele frequencies in the EH group and NT group were then compared. The G allele was observed more frequently in the EH group than in the NT group, and the allele frequencies in the 2 groups differed significantly (chi(2)=6.5, P=0.01). Multiple logistic linear regression analysis revealed that the genotype frequencies of A/A, A/G, and G/G differed significantly (odds ratio=2.1; 95% CI=1.19 to 3.66) between the EH and NT groups. EH patients who possess the G allele had a higher diastolic blood pressure than those lacking the G allele (P<0.01). Thus, the alleles detected by this restriction fragment length polymorphism in the DRD1 gene are associated with EH, and they appear to influence the diastolic blood pressure of Japanese EH patients.

Type 1A dopamine receptor expression in the heart is not altered in spontaneously hypertensive rats

We have recently demonstrated that type 1A dopamine (D1A) receptor is expressed in the rat heart, but its function still remains unknown. In the present study, we investigated possible changes in the expression level and the distribution of the cardiac D1A receptor in the development of left ventricular hypertrophy in spontaneously hypertensive rats/Izumo strain (SHR/Izm) at the ages of 4, 8, and 20 weeks. We examined D1A receptor protein distribution by immunohistochemistry and gene expression by competitive polymerase chain reaction (competitive PCR). In SHR/Izm, compared with the age-matched Wistar Kyoto rats/Izmo strain (WKY/Izm), blood pressure and heart/body weight ratio were significantly increased at 8 and 20 weeks. By immunohistochemistry, the D1A receptor was localized in cardiomyocytes and vascular smooth muscle cells of coronary arteries, but not in interstitial fibrotic tissue. D1A receptor distribution was not changed either by the strain or the age. Competitive PCR analysis showed that the D1A receptor mRNA level was significantly higher at 4 weeks than at 8 and 20 weeks in both strains of rats and that there was no significant difference in D1A receptor mRNA between SHR/Izm and WKY/Izm at any age (43.2 +/- 10.4 attomol x 10(-3)/L v 43.1 +/- 11.2 attomol x 10(-3)/L at 4 weeks, P = not significant, 3.9 +/- 0.9 attomol x 10(-3)/L v 4.0 +/- 1.3 attomol x 10(-3)/L at 8 weeks, P = not significant, 3.0 +/- 1.2 attomol x 10(-3)/L v 1.9 +/- 1.6 attomol x 10(-3)/L at 20 weeks, P = not significant). These results do not support the hypothesis that changes in D1A receptor expression are associated with the development of left ventricular hypertrophy in SHR.

Renal proximal tubule sodium transport and genetic mechanisms of essential hypertension

Renal sodium re-absorption is a closely regulated process serving to maintain both extracellular fluid volume and arterial blood pressure. Proteins participating in sodium re-absorption and its regulation are therefore important candidate proteins whose genes may contain sequence variation contributing to the inherited tendency for increased arterial blood pressure (essential hypertension). Important insight has come from rare forms of single-gene hypertension in human subjects and from polygenic animal models of genetic hypertension. Both indicate the primacy of altered renal function in the genesis of hypertension, and suggest that genes contributing to the disease are members of the subset of genes expressed in the kidney. This review examines evidence for abnormalities in renal sodium re-absorption in hypertension and focuses on the proximal tubule as a site of relevant dysfunction. Identification of the proteins participating in renal sodium re-absorption and its regulation, particularly those involved in the renal pressure-natriuresis mechanism, will allow gene cloning and sequencing which in turn may lead to the identification of novel gene sequence variation participating in hypertension.

Renal dopaminergic mechanisms and hypertension: a chronology of advances

Dopamine (DA) has been shown to influence kidney function through endogenous synthesis and subsequent interaction with locally expressed dopamine receptor subtypes (D1, D5 as D1-like and D2, D3, and D4 as D2-like). DA, and DA-receptor specific agonists and antagonists can alter renal water and electrolyte excretion along with renin release when infused systemically or intrarenally. Such effects are brought about by a combination of renal hemodynamic and direct tubular effects evoked along the full length of the nephron. The cellular mechanisms that direct these dopamine-mediated renal electrolyte fluxes have recently been clarified and include alterations in adenylyl cyclase, phospholipase C, and phospholipase A1 activity. The dopaminergic system also interacts directly with the renal kallikrein-kinin, prostaglandin and other neurohumoral systems. Aberrant renal dopamine production and/or dopamine receptor function have been reported in salt-dependent and low-renin forms of human primary hypertension as well as in genetic models of animal hypertension, including the SHR and Dahl SS rat. DA D1 or D3 receptor knockout mice have been shown to develop hypertension.

Intrarenal dopamine coordinates the effect of antinatriuretic and natriuretic factors

The precision by which sodium balance is regulated suggests an intricate interaction between modulatory factors released from intra- and extrarenal sources. Intrarenally produced dopamine has a central role in this interactive network. Dopamine, produced in renal tubular cells acts as an autocrine and paracrine factor to inhibit the activity of Na+,K+-ATPase as well as of a number of sodium influx pathways. The natriuretic effect of dopamine is most prominent under high salt diet. The antinatriuretic effects of noradrenaline, acting on alpha-adrenoceptors and angiotensin II are opposed by dopamine as well as by atrial natriuretic peptide (ANP). Several lines of evidence have suggested that ANP acts via the renal dopamine system and recent studies from our laboratory have shown that this effect is attributed to recruitment of silent D1 receptors from the interior of the cell towards the plasma membrane. Taken together, the observations suggest that dopamine coordinates the effects of antinatriuretic and natriuretic factors and indicate that an intact renal dopamine system is of major importance for the maintenance of sodium homeostasis and normal blood pressure.

Idiopathic edema

Idiopathic edema is a syndrome of real or perceived excessive weight gain. This article reviews what is known about the possible causes, evaluation, and treatment. Although the cause is unknown but often thought to be due to secondary hyperaldosteronism, primary abnormalities of the hypothalamus, thyroid, dopaminergic release or renal dopaminergic metabolism, vascular basement membrane, or capillary sphincter control could perhaps contribute in some patients. The diagnosis requires careful attention to possible abnormalities of the liver, heart, kidneys, gastrointestinal tract, thyroid, and pancreas. The history must include an evaluation for risks of bulimia and purging; diuretic and laxative screening should be performed. Specific records of daily weights, urinary outputs, and menstral cycle dates are useful. Treatment may include dietary counseling to provide weight control and a constant carbohydrate intake, treatments for depression, compression stockings, spironolactone, amiloride, angiotensin II inhibitors, or sympathomimetic agents, depending on the severity and timing of the patient's symptoms. Unfortunately, idiopathic edema may be a multifactorial disorder that has not been completely delineated. Further research into possible causative mechanisms is required before a more useful algorithm for evaluation and treatment is available.

Altered sodium pump alpha and gamma subunit gene expression in nephron segments from hypertensive rats

OBJECTIVE

To determine the qualitative and quantitative expression of alpha and gamma sodium pump subunits in whole kidney and nephron segment RNA from Sprague Dawley rats, spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats.

DESIGN

A novel reverse transcription polymerase chain reaction technique was devised which provides accurate and precise measurement of the number of molecules of specific transcript abundance, a measurement of gene expression. This allows the quantitative comparison of multiple samples across multiple subjects and, since the estimates are accurate rather than relative, can also be used to make quantitative comparisons across expressed genes, such as isoforms and subunits of the heterotrimeric renal sodium pump.

METHODS

We examined which catalytic isoforms were expressed and then quantified transcript abundance in whole kidney and convoluted and straight segments of the proximal tubule.

RESULTS

Alpha 1 and gamma transcripts, but not alpha 2, alpha 3 or alpha 4 isoforms, were consistently observed in nephron segments. Levels of alpha 1 were lower in kidney RNA from 15-16-week-old SHR than in WKY rats of the same age (P = 0.001), but were not different between SHR and WKY in 4-5-week-old animals. No significant difference was observed in gamma subunit abundance in kidney RNA from 4-5-week-old animals; however, at 15-16 weeks, the expression in SHR was one-third that in WKY rats (P = 0.003). In proximal convoluted tubules from 4-5-week-old animals, the level of alpha 1 RNA expression was lower (P = 0.03) in SHR than in WKY rats. In addition, levels of alpha 1 in proximal straight tubule from the 4-5-week-old SHR were also lower than in WKY rats (P = 0.02). This difference was even greater in 15-16-week-old animals: in SHR, alpha 1 expression was less than 20% of the level of expression in WKY rats (P = 0.0003). Expression of the gamma subunit exhibited a similar pattern of downregulation in SHR. In RNA from proximal convoluted tubules and proximal straight tubules from both 4-5- and 15-16-week-old animals, expression of the gamma subunit was demonstrated to be significantly lower in SHR than in WKY rats.

CONCLUSION

The results indicate a coordinate reduction in the abundance of sodium pump alpha and gamma subunits in the proximal tubules of SHR, which occurs early during the development of hypertension.

Impaired renal vascular response to a D1-like receptor agonist but not to an ACE inhibitor in conscious spontaneously hypertensive rats

The natriuretic response to a dopamine 1-like receptor agonist is blunted in spontaneously hypertensive rats (SHRs). Whether the renal vasodilator response to D1-like receptor stimulation in SHRs is defective also is unclear. To determine whether the renal hemodynamic response to a D1-like receptor is impaired in SHR, we examined the effect of a continuous infusion of the D1-like receptor agonist fenoldopam (2 microg/kg/min) on systemic and renal hemodynamics in conscious SHRs and Wistar-Kyoto (WKY) rats. As an active control, we used an equivalent antihypertensive dosage of captopril (10 mg/kg). Fenoldopam significantly increased effective renal plasma flow (ERPF) in WKY rats (+22 +/- 5%; p < 0.01), whereas this response was absent in SHRs (+7 +/- 3%; NS). Mean arterial pressure (MAP) was significantly reduced in SHRs (-11 +/- 2%; p < 0.001), demonstrating a systemic vasodilator response to fenoldopam in SHRs. The reduction in renal vascular resistance (RVR) was more pronounced in WKY rats (-24 +/- 2%) than in SHRs (-13 +/- 4%; p < 0.05). Captopril significantly increased ERPF in SHRs (+16 +/- 3%; p < 0.001), demonstrating a preserved renal vasodilatory capacity in SHRs. The blunting of the renal vasodilatory response to fenoldopam in SHRs is present during a high as well as a low sodium intake. In conscious SHRs, the renal vasodilatory response to a D1-like receptor agonist is impaired, whereas the blood pressure response is more pronounced. The preserved renal vasodilatory response to captopril indicates that the defective vasodilatory response in SHRs is functional rather than due to altered structural properties of the renal vascular bed.

New insights into dopaminergic receptor function using antisense and genetically altered animals

Dopaminergic receptors are widespread throughout the central and peripheral nervous systems, where they regulate a variety of physiological, behavioral, and endocrine functions. These receptors are also clinically important drug targets for the treatment of a number of disorders, such as Parkinson's disease, schizophrenia, and hyperprolactinemia. To date, five different dopamine receptor subtypes have been cloned and characterized. Many of these subtypes are pharmacologically similar, making it difficult to selectively stimulate or block a specific receptor subtype in vivo. Thus, the assignment of various physiological or behavioral functions to specific dopamine receptor subtypes using pharmacological tools is difficult. In view of this, a number of investigators have--in order to elucidate functional roles--begun to use highly selective genetic approaches to alter the expression of individual dopamine receptor subtypes in vivo. This review discusses recent studies involving the use of genetic approaches for the study of dopaminergic receptor function.