High urinary dopa and low urinary dopamine-to-dopa ratio in salt-sensitive hypertension

Decreased urinary excretion of norepinephrine and dopamine in autonomic synucleinopathies

BACKGROUND

Autonomic synucleinopathies feature autonomic failure and intracellular deposition of the protein alpha-synuclein. Three such conditions are the Lewy body diseases (LBDs) Parkinson's disease (PD) and pure autonomic failure (PAF) and the non-LBD synucleinopathy multiple system atrophy (MSA). These diseases all entail catecholaminergic abnormalities in the brain, sympathetically innervated organs, or both; however, little is known about renal catecholaminergic functions in autonomic synucleinopathies. We measured urinary excretion rates of the sympathetic neurotransmitter norepinephrine, the hormone epinephrine, the autocrine-paracrine substance dopamine, the catecholamine precursor 3,4-dihydroxyphenylalanine (DOPA), 3,4-dihydroxyphenylglycol (DHPG, the main neuronal metabolite of norepinephrine), and 3,4-dihydroxyphenylacetic acid (DOPAC, a major dopamine metabolite), in PD, PAF, and MSA groups and controls.

METHODS

Data were reviewed from all research participants who had urine collections (usually 3.5 h) at the National Institutes of Health (NIH) Clinical Center from 1995 to 2024. The control cohort had neither autonomic failure nor a movement disorder.

RESULTS

Norepinephrine excretion rates were decreased compared with controls in PD (p = 0.0001), PAF (p < 0.0001), and MSA (p < 0.0001). Dopamine excretion was also decreased in the three groups (PD: p = 0.0136, PAF: p = 0.0027, MSA: p = 0.0344). DHPG excretion was decreased in PD (p = 0.0004) and PAF (p = 0.0004) but not in MSA. DOPA and epinephrine excretion did not differ among the study groups.

CONCLUSIONS

Autonomic synucleinopathies involve decreased urinary excretion rates of norepinephrine and dopamine. Since virtually all of urinary dopamine in humans is derived from circulating DOPA, the low rates of urinary norepinephrine and dopamine excretion may reflect dysfunctions in the renal sympathetic noradrenergic system, the DOPA-dopamine autocrine-paracrine system, or both systems.

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

Ex Vivo and In Vivo Characterization of Interpolymeric Blend/Nanoenabled Gastroretentive Levodopa Delivery Systems

One approach for delivery of narrow absorption window drugs is to formulate gastroretentive drug delivery systems. This study was undertaken to provide insight into in vivo performances of two gastroretentive systems (PXLNET and IPB matrices) in comparison to Madopar® HBS capsules. The pig model was used to assess gastric residence time and pharmacokinetic parameters using blood, cerebrospinal fluid (CSF), and urine samples. Histopathology and cytotoxicity testing were also undertaken. The pharmacokinetic parameters indicated that levodopa was liberated from the drug delivery systems, absorbed, widely distributed, metabolized, and excreted. C_max were 372.37, 257.02, and 461.28 ng/mL and MRT were 15.36, 14.98, and 13.30 for Madopar HBS capsules, PXLNET, and IPB, respectively. In addition, X-ray imaging indicated that the gastroretentive systems have the potential to reside in the stomach for 7 hours. There was strong in vitro-in vivo correlation for all formulations with r² values of 0.906, 0.935, and 0.945 for Madopar HBS capsules, PXLNET, and IPB, respectively. Consequently, PXLNET and IPB matrices have pertinent potential as gastroretentive systems for narrow absorption window drugs (e.g., L-dopa) and, in this application specifically, enhanced the central nervous system and/or systemic bioavailability of such drugs.

Gastrin stimulates renal dopamine production by increasing the renal tubular uptake of l-DOPA

Gastrin is a peptide hormone that is involved in the regulation of sodium balance and blood pressure. Dopamine, which is also involved in the regulation of sodium balance and blood pressure, directly or indirectly interacts with other blood pressure-regulating hormones, including gastrin. This study aimed to determine the mechanisms of the interaction between gastrin and dopamine and tested the hypothesis that gastrin produced in the kidney increases renal dopamine production to keep blood pressure within the normal range. We show that in human and mouse renal proximal tubule cells (hRPTCs and mRPTCs, respectively), gastrin stimulates renal dopamine production by increasing the cellular uptake of l-DOPA via the l-type amino acid transporter (LAT) at the plasma membrane. The uptake of l-DOPA in RPTCs from C57Bl/6J mice is lower than in RPTCs from normotensive humans. l-DOPA uptake in renal cortical slices is also lower in salt-sensitive C57Bl/6J than in salt-resistant BALB/c mice. The deficient renal cortical uptake of l-DOPA in C57Bl/6J mice may be due to decreased LAT-1 activity that is related to its decreased expression at the plasma membrane, relative to BALB/c mice. We also show that renal-selective silencing of Gast by the renal subcapsular injection of Gast siRNA in BALB/c mice decreases renal dopamine production and increases blood pressure. These results highlight the importance of renal gastrin in stimulating renal dopamine production, which may give a new perspective in the prevention and treatment of hypertension.

African Americans, hypertension and the renin angiotensin system

African Americans have exceptionally high rates of hypertension and hypertension related complications. It is commonly reported that the blood pressure lowering efficacy of renin angiotensin system (RAS) inhibitors is attenuated in African Americans due to a greater likelihood of having a low renin profile. Therefore these agents are often not recommended as initial therapy in African Americans with hypertension. However, the high prevalence of comorbid conditions, such as diabetes, cardiovascular and chronic kidney disease makes treatment with RAS inhibitors more compelling. Despite lower circulating renin levels and a less significant fall in blood pressure in response to RAS inhibitors in African Americans, numerous clinical trials support the efficacy of RAS inhibitors to improve clinical outcomes in this population, especially in those with hypertension and risk factors for cardiovascular and related diseases. Here, we discuss the rationale of RAS blockade as part of a comprehensive approach to attenuate the high rates of premature morbidity and mortality associated with hypertension among African Americans.

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.

Physical activity reduces salt sensitivity of blood pressure: the Genetic Epidemiology Network of Salt Sensitivity Study

Salt sensitivity of blood pressure (BP) is influenced by genetic and environmental factors. A dietary feeding study was conducted from October 2003 to July 2005 that included a 7-day low-sodium intervention (51.3 mmol sodium/day) followed by a 7-day high-sodium intervention (307.8 mmol sodium/day) among 1,906 individuals who were 16 years of age or older and living in rural northern China. Salt sensitivity of BP was defined as mean BP change from the low-sodium intervention to the high-sodium intervention. Usual physical activity during the past 12 months was assessed at baseline using a standard questionnaire. The multivariable-adjusted means of systolic BP responses to high-sodium intervention were 5.21 mm Hg (95% confidence interval (CI): 4.55, 5.88), 4.97 mm Hg (95% CI: 4.35, 5.59), 5.02 mm Hg (95% CI: 4.38, 5.67), and 3.96 mm Hg (95% CI: 3.29, 4.63) among participants from the lowest to the highest quartiles of physical activity, respectively (P = 0.003 for linear trend). The multivariable-adjusted odds ratio of high salt sensitivity of systolic BP was 0.66 (95% CI: 0.49, 0.88) for persons in the highest quartile of physical activity compared with those in the lowest quartile. Physical activity is significantly, independently, and inversely related to salt sensitivity of BP and may be particularly effective in lowering BP among salt-sensitive individuals.

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 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.

Renal dopaminergic defect in C57Bl/6J mice

The C57Bl/6J mouse strain, the genetic background of many transgenic and gene knockout models, is salt sensitive and resistant to renal injury. We tested the hypothesis that renal dopaminergic function is defective in C57Bl/6J mice. On normal NaCl (0.8%, 1 wk) diet, anesthetized and conscious (telemetry) blood pressures were similar in C57Bl/6J and SJL/J mice. High NaCl (6%, 1 wk) increased blood pressure (approximately 30%) in C57Bl/6J but not in SJL/J mice and urinary dopamine to greater extent in SJL/J than in C57Bl/6J mice. Absolute and fractional sodium excretions were lower in SJL/J than in C57Bl/6J mice. The blood pressure-natriuresis plot was shifted to the right in C57Bl/6J mice. Renal expressions of D(1)-like (D(1)R and D(5)R) and angiotensin II AT(1) receptors were similar on normal salt, but high salt increased D(5)R only in C57Bl/6J. GRK4 expression was lower on normal but higher on high salt in C57Bl/6J than in SJL/J mice. Salt increased the excretion of microalbumin and 8-isoprostane (oxidative stress marker) and the degree of renal injury to a greater extent in SJL/J than in C57Bl/6J mice. A D(1)-like receptor agonist increased sodium excretion whereas a D(1)-like receptor antagonist decreased sodium excretion in SJL/J but not in C57Bl/6J mice. In contrast, parathyroid hormone had a similar natriuretic effect in both strains. These results show that defective D(1)-like receptor function is a major cause of salt sensitivity in C57Bl/6J mice, decreased renal dopamine production might also contribute. The relative resistance to renal injury of C57Bl/6J may be a consequence of decreased production of reactive oxygen species.

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.

Mechanisms of disease: the role of GRK4 in the etiology of essential hypertension and salt sensitivity

Hypertension and salt sensitivity of blood pressure are two conditions the etiologies of which are still elusive because of the complex influences of genes, environment, and behavior. Recent understanding of the molecular mechanisms that govern sodium homeostasis is shedding new light on how genes, their protein products, and interacting metabolic pathways contribute to disease. Sodium transport is increased in the proximal tubule and thick ascending limb of Henle of the kidney in human essential hypertension. This Review focuses on the counter-regulation between the dopaminergic and renin-angiotensin systems in the renal proximal tubule, which is the site of about 70% of total renal sodium reabsorption. The inhibitory effect of dopamine is most evident under conditions of moderate sodium excess, whereas the stimulatory effect of angiotensin II is most evident under conditions of sodium deficit. Dopamine and angiotensin II exert their actions via G protein-coupled receptors, which are in turn regulated by G protein-coupled receptor kinases (GRKs). Polymorphisms that lead to aberrant action of GRKs cause a number of conditions, including hypertension and salt sensitivity. Polymorphisms in one particular member of this family-GRK4-have been shown to cause hyperphosphorylation, desensitization and internalization of a member of the dopamine receptor family, the dopamine 1 receptor, while increasing the expression of a key receptor of the renin-angiotensin system, the angiotensin II type 1 receptor. Novel diagnostic and therapeutic approaches for identifying at-risk subjects, followed by selective treatment of hypertension and salt sensitivity, might center on restoring normal receptor function through blocking the effects of GRK4 polymorphisms.

Proximal tubular function and salt sensitivity

Blood pressure response to changes in dietary salt intake is highly variable among individuals. This heterogeneity results from the combined effects of genetic and environmental determinants. In recent years, considerable progress has been made in our understanding of the pathogenic mechanisms leading to the development of salt-sensitive hypertension. Much information has come from the investigation of rare monogenic forms of salt-sensitive hypertension, which has focused attention on alterations of renal sodium handling occurring essentially in the distal nephron. In this paper, we review the experimental, clinical, genetic, and epidemiologic evidence suggesting that proximal tubular function is also an important determinant of the blood pressure response to salt, which deserves greater attention.

Does dietary recall adequately assess sodium, potassium, and calcium intake in hypertensive patients?

OBJECTIVE

A diet low in sodium, high in potassium, and high in calcium is recommended to lower blood pressure. However, compliance with this diet is poor, probably because of dietary intake underestimation. Therefore, we compared electrolyte intake as estimated from dietary recall with a 24-h urinary excretion.

METHODS

Thirty-six patients (26 men and 10 women) with a mean age of 46 +/- 8 y participated in the study. All participants had essential hypertension and were on no drug therapy (n = 20) or non-diuretic monotherapy (n = 16). Patients were instructed to consume a low-sodium (50 mmol/d), high-potassium (supplementation with 30 to 60 mmol/d), and high-calcium (1000 mg/d) diet. Compliance with the diet was assessed at baseline and then 1, 2, and 3 mo after starting the diet. Sodium, potassium, and calcium intakes were carefully estimated from patients' dietary recall and 24-h urinary collection.

RESULTS

Estimated sodium intake significantly correlated with 24-h urinary excretion (R = 0.43 P < 0.001). However, estimated sodium intake was lower than urinary sodium excretion by 34% at baseline and by 47% after 3 mo of dieting (P < 0.05). Estimated potassium intake correlated with 24-h urinary excretion. Estimated calcium intake significantly increased from 933 +/- 83 mg/d to 1029 +/- 171 mg/d (P < 0.05). Calcium intake derived from patients' recall far exceeded and only slightly correlated with 24-h urinary excretion (R = 0.23, P < 0.01).

CONCLUSIONS

Patients tend to underestimate their sodium intake by 30% to 50%; therefore, urinary sodium excretion is more accurate to assess sodium intake. Thus, 24-h urinary sodium excretion should be used in clinical practice and in clinical trials, especially when dietary non-compliance is suspected.

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.

Links between dietary salt intake, renal salt handling, blood pressure, and cardiovascular diseases

Epidemiological, migration, intervention, and genetic studies in humans and animals provide very strong evidence of a causal link between high salt intake and high blood pressure. The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt. From an evolutionary viewpoint, the human species is adapted to ingest and excrete <1 g of salt per day, at least 10 times less than the average values currently observed in industrialized and urbanized countries. Independent of the rise in blood pressure, dietary salt also increases cardiac left ventricular mass, arterial thickness and stiffness, the incidence of strokes, and the severity of cardiac failure. Thus chronic exposure to a high-salt diet appears to be a major factor involved in the frequent occurrence of hypertension and cardiovascular diseases in human populations.

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.

Dopamine D2 Receptor Modulates Sodium Handling via Local Production of Dopamine in the Kidney

We have recently demonstrated that a deletion of the dopamine D2 receptor gene caused suppression of urinary sodium excretion and salt-sensitive elevation of blood pressure in mice. In order to understand the mechanisms underlying this impaired sodium excretion, we studied renal dopamine production and dopamine-induced sodium excretion in 20- to 30-week-old male D2-receptor knockout (D2KO) mice and age- and sex-matched wildtype (WT) mice. Renal local dopamine synthesis, examined by 24-h urine free dopamine excretion (UDAV), was significantly (p < 0.05) reduced in D2KO mice compared to that in WT mice (D2KO versus WT: 1.06 +/- 0.2 versus 1.5 +/- 0.3 ng/mg creatinine). Such a difference between D2KO and WT mice was also observed after oral administration of 3,4-dihydroxyphenylalanine (L-DOPA), a precursor of dopamine, at 5 mg/kg per day for 24 h. Furthermore, activity of aromatic 1-amino acid decarboxylase, a dopamine synthetase, was significantly suppressed in D2KO mice. Next, we examined changes in 24-h urine flow (UV) and 24-h sodium excretion (UNaV) during chronic infusion of dopamine at sub-pressor doses (3-4 microg/kg per min, sq.) or a vehicle via an osmotic pump. Urine flow in 24 h and UNaV were significantly (p < 0.05) smaller in D2KO mice infused with vehicle than in WT mice infused with vehicle (UV: 210 +/- 43 versus 650 +/- 163 microl/day; UNaV: 20.6 +/- 13.2 versus 44.4 +/- 21.6 microEq/day). After administration of dopamine, UV and UNaV in D2KO mice were restored to a level similar to that in WT mice. These results indicate that D2-dopamine receptors play a significant role in renal local dopamine synthesis and that a shortage of dopamine was, at least in part, responsible for the suppression of UV and UNaV in D2KO mice. However, we could not conclude from the present study whether renal tubular sodium reabsorption is intact in D2KO mice because the baseline dopamine contents in kidneys of D2KO mice and WT mice may be different.

Dopamine and the kidney: a role in hypertension?

PURPOSE OF REVIEW

Defective transduction of the dopamine receptor signal in the kidney has been shown to be important in the pathogenesis of hypertension This review will discuss the genetic mechanism for the defective renal dopaminergic function and the interaction with other gene variant products in the pathogenesis of salt sensitivity and essential hypertension.

RECENT FINDINGS

Single nucleotide polymorphisms of G protein-coupled receptor kinase type 4 (GRK4) phosphorylate, desensitize, and diminish the inhibitory action of D receptors on sodium transport in the kidney. Inhibition of GRK4 expression normalizes renal proximal tubule D receptor function in humans and rodents and ameliorates the hypertension in genetically hypertensive rats. Expression of the GRK4 variant, GRK4gammaA142V, produces hypertension and impairs the natriuretic effect of D receptor stimulation in mice. In humans, GRK4 single nucleotide polymorphisms are associated with essential hypertension, particularly salt sensitive hypertension. The prediction of the hypertensive phenotype is most accurate when elements of the renin-angiotensin system and GRK4 are included in the analysis.

SUMMARY

GRK4 single nucleotide polymorphisms, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic function of angiotensin II type 1 receptors to predominate, may be responsible for salt sensitivity. Hypertension develops with additional perturbations caused by the variants of other genes (e.g., alpha-adducin, angiotensin converting enzyme, angiotensinogen, angiotensin II type 1 receptor, aldosterone synthase, 11beta-hydroxysteroid dehydrogenase type 2), the quantitative interaction of which may vary depending upon the genetic background.

Renal dopamine receptors and hypertension

Dopamine has been recognized as an important modulator of central as well as peripheral physiologic functions in both humans and animals. Dopamine receptors have been identified in a number of organs and tissues, which include several regions within the central nervous system, sympathetic ganglia and postganglionic nerve terminals, various vascular beds, the heart, the gastrointestinal tract, and the kidney. The peripheral dopamine receptors influence cardiovascular and renal function by decreasing afterload and vascular resistance and promoting sodium excretion. Within the kidney, dopamine receptors are present along the nephron, with highest density on proximal tubule epithelial cells. It has been reported that there is a defective dopamine receptor, especially D(1) receptor function, in the proximal tubule of various animal models of hypertension as well as in humans with essential hypertension. Recent reports have revealed the site of and the molecular mechanisms responsible for the defect in D(1) receptors in hypertension. Moreover, recent studies have also demonstrated that the disruption of various dopamine receptor subtypes and their function produces hypertension in rodents. In this review, we present evidence that dopamine and dopamine receptors play an important role in regulating renal sodium excretion and that defective renal dopamine production and/or dopamine receptor function may contribute to the development of various forms of hypertension.

Participation of renal and circulating endothelin in salt-sensitive essential hypertension

Salt sensitivity of blood pressure is a cardiovascular risk factor, independent of and in addition to hypertension. In essential hypertension, a conglomerate of clinical and biochemical characteristics defines a salt-sensitive phenotype. Despite extensive research on multiple natriuretic and antinatriuretic systems, there is no definitive answer yet about the major causes of salt-sensitivity, probably reflecting the complexity of salt-balance regulation. The endothelins, ubiquitous peptides first described as potent vasoconstrictors, also have vasodilator, natriuretic and antinatriuretic actions, depending on their site of generation and binding to different receptors. We review the available data on endothelin in salt-sensitive essential hypertension and conclude that abnormalities of renal endothelin may play a primary role. More importantly, the salt-sensitive patient may have blood pressure-dependency on endothelin in all states of salt balance, thus predicting that endothelin receptor blockers will have a major therapeutic role in salt-sensitive essential hypertension.

Theodore Cooper Lecture: Renal dopamine system: paracrine regulator of sodium homeostasis and blood pressure

All of the components of a complete dopamine system are present within the kidney, where dopamine acts as a paracrine substance in the control of sodium excretion. Dopamine receptors can be divided into D(1)-like (D(1) and D(5)) receptors that stimulate adenylyl cyclase and D(2)-like (D(2), D(3), and D(4)) receptors that inhibit adenylyl cyclase. All 5 receptor subtypes are expressed in the kidney, albeit in low copy. Dopamine is synthesized extraneuronally in proximal tubule cells, exported from these cells largely into the tubule lumen, and interacts with D(1)-like receptors to inhibit the Na(+)-H(+) exchanger and Na(+),K(+)-ATPase, decreasing tubule sodium reabsorption. During moderate sodium surfeit, dopamine tone at D(1)-like receptors accounts for approximately 50% of sodium excretion. In experimental and human hypertension, 2 renal dopaminergic defects have been described: (1) decreased renal generation of dopamine and (2) a D(1) receptor-G protein coupling defect. Both defects lead to renal sodium retention, and each may play an important role in the pathophysiology of essential hypertension.

Salt sensitivity and hypertension in African Americans: implications for cardiovascular nurses

Hypertension is a major public health problem in the U.S. Salt sensitivity is an important factor associated with hypertension and its complications, yet it has not been addressed in the nursing literature. Salt sensitivity is a directionally appropriate rise or fall in blood pressure when salt is added or removed, respectively. The change in blood pressure in salt-sensitive subjects occurs to a degree exceeding random blood pressure fluctuations. Salt sensitivity is present in 30% of normotensive and over 50% of hypertensive persons. It is more prevalent among African Americans, older persons, and individuals with renal insufficiency or diabetes. This paper provides nurses with an overview of salt sensitivity and its significance in hypertension. It presents conceptual and operational definitions of salt sensitivity, identifies factors contributing to its development, and describes implications for nursing practice.

Renal kallikrein-kinin system, but not renal dopamine system, mediates the natriuretic response to intravenous saline infusion in healthy Chinese subjects

1. To assess the role of renal dopamine (DA), sympathetic nervous system (SNS) activity and the renal kallikrein-kinin system in sodium excretion in Chinese subjects, we studied the effects of intravenous saline infusion on the urinary excretions of sodium, free DA, free noradrenaline (NA) and kallikrein in eight healthy males aged 23-25 years. 2. After a baseline period of 1 h (hour 0), these subjects received 11 of 0.9% saline over 2 h (hours 1 and 2), followed by a 4-h recovery period (hours 3-6). From hours 0-4, subjects remained in the supine position, except to void urine. Distilled water was given orally throughout the study to ensure an adequate diuresis. 3. A 31-39% increase in sodium excretion (P < 0.05) was seen during hours 2 and 3. Urinary DA did not change throughout the study period. Urinary free NA showed no changes while the subjects remained supine, but an increase of 91-105% (P < 0.02) was seen after the subjects became ambulatory. However, there was a 103-140% increase in urinary kallikrein excretion (P < 0.05) during the saline infusion. Urinary kallikrein was still much higher (by 74%) than the basal level 1 h after the completion of the saline infusion. 4. There is no evidence from the present study that renal DA or SNS play any role in the natriuretic response to saline infusion in Chinese subjects. The brisk urinary kallikrein response, despite a relatively small salt load, suggests that the renal kallikrein-kinin system may play an important role in extracellular fluid volume and sodium homeostasis in Chinese subjects.

Deficiency of renal dopaminergic-dependent natriuretic response to acute sodium load in black salt-sensitive subjects in contrast to salt-resistant subjects

OBJECTIVE

To evaluate the involvement of the renal dopaminergic system in the natriuretic responses to acute saline load in salt-resistant (SR) and salt-sensitive (SS) black normotensive (NT) and hypertensive (HT) subjects.

DESIGN AND METHODS

We studied the relationship between the urinary excretion of dopa, dopamine (DA) and its metabolite DOPAC and the natriuretic responses to acute volume expansion (2 l NaCl 0.9% over 2 h) in 20 black NT subjects (12 SR and 8 SS) and 19 black HT subjects (10 SS and 9 SR). Subjects received a low salt (LS) diet (40 mmol sodium/day) for 1 week and a high salt (HS) diet (300 mmol sodium/day) for 1 week; the sequence of the dietary regimens was randomized. Comparisons were made between the results before the saline infusion (baseline) and the results 2 h after the infusion.

RESULTS

In all the groups saline infusion induced significant increases in urinary volume (ml/4 h) of two- to three-fold and in urinary sodium excretion (mmol/4 h) of three- to ten-fold; these increases were significantly greater during the HS diet than during the LS diet. Saline infusion significantly increased the mean arterial pressure (MAP) by 5 mmHg in HT-SS subjects and by 4-5 mmHg in NT-SS subjects, but the MAP did not changed in the NT-SR and HT-SR groups. Under the LS diet, saline infusion changed the DA excretion (in nmol/4 h) by -49+/-89 in HT-SS subjects, by 17+/-52 in NT-SS subjects, by 235+/-72 in HT-SR subjects and by 220+/-86 in NT-SR subjects (P < 0.05 between SR and SS subjects). The saline infusion-induced changes in DA excretion correlated significantly with the increases in urinary sodium excretion (r = 0.71, P < 0.01) in the NT-SR and HT-SR subjects under the LS diet, but not in the SR groups on the HS diet nor in the SS groups (HT and NT) on either diet. Saline infusion significantly reduced the DA/dopa ratio in SS (NT and HT) but not SR (NT and HT) subjects, whereas the DA/DOPAC (dihydroxyphenylacetic acid) ratios were similar in all the groups.

CONCLUSIONS

The urinary dopaminergic system may participate in the natriuretic responses to acute sodium load only in SR subjects (NT and HT) and only under LS diets, but not in SS subjects (NT and HT). This strongly suggests that black NT- and HT-SS subjects have an underlying impairment in the activity of the renal dopaminergic system which may be associated with a reduced decarboxylation of dopa into DA.

Acute hypotensive, natriuretic, and hormonal effects of nifedipine in salt-sensitive and salt-resistant black normotensive and hypertensive subjects

In a randomized double-blind study, we compared the short-term effects of nifedipine (10 mg 3x daily for 1 day) versus placebo on 24-h blood pressure, diuresis, natriuresis, urinary excretion of dopamine and metabolites, and on plasma renin activity (PRA) and plasma aldosterone levels in 18 black hypertensive (HT) patients [eight salt-resistant (HT-SR) and 10 salt-sensitive (HT-SS)], and in 20 black normotensive (NT) subjects (12 NT-SR and eight NT-SS) who were studied randomly with both a high- (HS) and a low-salt (LS) diet. In comparison to placebo, nifedipine significantly decreased 24-h mean BP in all groups either with HS or LS diets (all p<0.05). With HS, greater hypotensive effects were achieved in NT-SS (-10+/-2 mm Hg) versus NT-SR (-3+/-1 mm Hg; p<0.05) and in HT-SS (-18+/-2 mm Hg) versus HT-SR (-12+/-2 mm Hg; p<0.05). In NT-SS and HT-SS, nifedipine induced greater (p<0.05) BP decrease with HS (-10+/-2 and -18+/-2 mm Hg) than with LS (-4+/-1 and -9+/-1 mm Hg, respectively), whereas in NT-SR and HT-SR, the hypotensive effect did not differ between HS and LS. Nifedipine versus placebo significantly increased natriuresis and fractional excretion of sodium in all groups only with HS (p<0.05) but not with LS diets. Only in HT-SS were the hypotensive and natriuretic effects of nifedipine significantly correlated (r = -0.77; p<0.01). Nifedipine produced a similar increase of the urinary excretion of dopamine, L-DOPA, and of DOPAC in all subjects, which did not correlate with hypotensive and natriuretic effects. Nifedipine did not modify plasma levels of renin and of aldosterone except in NT-SS with HS, in whom nifedipine increased PRA levels (p <0.05). We conclude that although nifedipine reduces BP in all groups of NT and HT with LS and HS diets, the effect is greater in salt-sensitive subjects with HS. Although in HT-SS with HS, the short-term natriuretic response to nifedipine may contribute to its hypotensive effects, the diuretic-natriuretic effect of nifedipine is not necessary for the expression of its hypotensive effect. Moreover, it is unlikely that any short-term effects of nifedipine either on the renal dopaminergic system or on the secretion of aldosterone explain nifedipine short-term hypotensive and diuretic-natriuretic effects.

A novel fully automated method for the measurement of sulphoconjugated catecholamines in urine using the Gilson ASTED-XL sample preparation system and high-performance liquid chromatography

Dopamine is produced in the kidney where it causes sodium excretion. Dopamine sulphate is deconjugated in vivo, and may be a physiological reservoir for this active renal dopamine. To investigate the role of dopamine and dopamine sulphate in sodium homeostasis we have developed a fully automated HPLC assay for free, total and sulphoconjugated dopamine. Using the Gilson ASTED-XL sample preparation unit, with temperature controlled racks, urinary free and total dopamine are measured pre-and post-incubation with arylsulphatase. Dopamine sulphate is calculated from the difference between free and total measurements. Acidified 24 h urines are processed automatically. Free dopamine assay follows buffering to pH 7.0, addition of internal standard, addition of EDTA to stabilize free catecholamines at neutral pH, and incubation at 37 degrees C for 30 min. This mixture is trace enriched on a HEMA-SB TEC prior to ion-paired HPLC with amperometric detection. To measure total dopamine the entire process is automatically repeated with addition of arylsulphatase (400 mU/mL urine) at the beginning of the 37 degrees C incubation. The working range of the assay is up to 7 micromol/L total dopamine. Within-and between-run imprecision for dopamine sulphate is less than 3 and 7% respectively. Median dopamine sulphate excretion in 12 normotensive subjects was 4.3 micromol/24 h.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Differential catecholamine responses to protein intake in healthy and hypertensive subjects

Protein intake-induced natriuresis previously related to increased urinary dopamine excretion was reexamined in an extensive controlled study comparing healthy and hypertensive subjects. In healthy subjects, ingestion of 1 g/kg wt tuna induced natriuresis that was associated, between postprandial hours 1 and 2, with increased plasma tyrosine [191 +/- 13% (mean +/- SE); P < 0.01], 3, 4-dihydroxyphenylalanine (104 +/- 12%, P < 0.05 in plasma; 162 +/- 20%, P < 0.05 in urine), plasma free dopamine (156 +/- 32%; P < 0. 05), and dopamine sulfate (191 +/- 11%, P < 0.001 in plasma; 199 +/- 15%, P < 0.01 in urine) but affected urinary free dopamine excretion only at limits of significance. Hypertensive subjects had less (P < 0.02) natriuresis and, despite comparable plasma tyrosine and dopamine sulfate increases, no increase in plasma and urinary 3, 4-dihydroxyphenylalanine and plasma free dopamine. Their plasma and urinary free epinephrine responses were less (P < 0.05) than the borderline increases in control subjects. Compared with control subjects, they significantly increased plasma 3, 4-dihydroxyphenylalanine sulfate (P < 0.05), epinephrine sulfate (P < 0.05), and the dopamine sulfate-to-free dopamine ratio (P < 0.02). Postprotein natriuresis is thus associated with nutritional priming-induced plasma but not urinary free dopamine increase. Hypertensive subjects have attenuated natriuretic and plasma free dopamine responses and less free epinephrine increase. This may partly result from higher circulating 3,4-dihydroxyphenylalanine, dopamine, and epinephrine sulfoconjugates leaving fewer free amines for biological actions.

Renal dopamine receptor function in hypertension

Dopamine plays an important role in the regulation of renal sodium excretion. The synthesis of dopamine and the presence of dopamine receptor subtypes (D1A, D1B, as D1-like and D2, and D3 as D2-like) have been shown within the kidney. The activation of D1-like receptors located on the proximal tubules causes inhibition of tubular sodium reabsorption by inhibiting Na,H-exchanger and Na,K-ATPase activity. The D1-like receptors are linked to the multiple cellular signaling systems (namely, adenylyl cyclase, phospholipase C, and phospholipase A2) in the different regions of the nephron. Defective renal dopamine production and/or dopamine receptor function have been reported in human primary hypertension as well as in genetic models of animal hypertension. There may be a primary defect in D1-like receptors and an altered signaling system in the proximal tubules that lead to reduced dopamine-mediated effects on renal sodium excretion in hypertension. Recently, it has been shown in animal models that the disruption of either D1A or D3 receptors at the gene level causes hypertension in mice. Dopamine and dopamine receptor agonists also provide therapeutic potential in treatment of various cardiovascular pathological conditions, including hypertension. However, because of the poor bioavailability of the currently available compounds, the use of D1-like agonists is limited to the management of patients with severe hypertension when a rapid reduction of blood pressure is clinically indicated and in acute management of patients with heart failure. In conclusion, there is convincing evidence that dopamine and dopamine receptors play an important role in regulation of renal function, suggesting that a defective dopamine receptor/signaling system may contribute to the development and maintenance of hypertension. Further studies need to be directed toward establishing a direct correlation between defective dopamine receptor gene in the kidney and development of hypertension. Subsequently, it may be possible to use a therapeutic approach to correct the defect in dopamine receptor gene causing the hypertension.

Salt sensitivity: concept and pathogenesis

Almost two decades ago, the existence of a subset of essential hypertensive patients, who were sensitive (according to the increase in blood pressure levels) to the intake of a diet with a high salt content, was described. These patients are characterized by an increase in blood pressure and in body weight when switched from a low to a high sodium intake. The increase in body weight is due to the incapacity of the kidneys to excrete the whole intake of sodium until renal perfusion pressure (mean blood pressure) attains a level that is able to restore pressure-natriuresis relationship to values that enable the kidney to excrete the salt ingested or administered intravenously. Salt sensitivity does not seem to depend on the existence of an intrinsic renal defect to handle sodium, but on the existence of subtle abnormalities in the regulation of the sympathetic nervous system, the renin-angiotensin system or endothelial function. It is also relevant that organ damage secondary to arterial hypertension, has been shown in animal models and in hypertensive humans sensitive to a high salt intake to be significantly higher when compared with that of salt-resistant animals or humans. Interestingly, in humans, salt sensitivity has been shown to correlate with microalbuminuria, an important predictor of cardiovascular morbidity and mortality, which correlates with most of the cardiovascular risk factors commonly associated with arterial hypertension. One of these factors is insulin resistance, that usually accompanies high blood pressure in overweight and obese hypertensives. Insulin resistance and hyperinsulinism are present in a significant percentage of hypertensive patients developing cardiovascular symptoms or death. For these reasons, therapy of arterial hypertension must be directed, not only to facilitate the lowering of BP level, but also, to halt the mechanisms underlying the increase in BP, when salt intake is increased. Furthermore, therapy must preferably improve the diminished insulin sensitivity present in salt-sensitive subjects that contribute independently to increased cardiovascular risk.

Apical and basolateral uptake and intracellular fate of dopamine precursor L-dopa in LLC-PK1 cells

The present study was aimed at the uptake of L-3,4-dihydroxyphenylalanine (L-dopa) and its intracellular decarboxylation to dopamine. The accumulation of L-dopa from the apical side in cells cultured in collagen-treated plastic was found to be a saturable process with a Michaelis constant (Km) of 123 +/- 17 microM and a maximal velocity (Vmax) of 6.0 +/- 0.2 nmol.mg protein-1.6 min-1. The uptake of L-dopa applied from either the apical or basal cell borders in cells cultured in polycarbonate filters was also found to be saturable; nonlinear analysis of saturation curves for apical and basal application revealed Km values of 63.8 +/- 17.0 and 42.5 +/- 9.6 microM and Vmax values of 32.0 +/- 5.8 and 26.2 +/- 3.4 nmol.mg protein-1.6 min-1, respectively. Cell monolayers incubated with L-dopa, applied from either the apical or the basal side, in the absence of benserazide, led to the accumulation of newly formed dopamine. The intracellular accumulation of newly formed dopamine was a saturable process with apparent Km values of 20.5 +/- 8.2 and 247.3 +/- 76.8 microM when the substrate was applied from the apical and basal side, respectively. Some of the newly formed dopamine escaped to the extracellular milieu. The basal outward transfer of dopamine was five- to sevenfold of that occurring at the apical side and was uniform over a wide range of concentrations of intracellular dopamine; the apical outward transfer of the amine depended on the intracellular concentration of dopamine and was a nonsaturable process. The apical and basal outward transfers of dopamine were insensitive to cocaine (10 and 30 microM) and GBR-12909 (1 and 3 microM). The accumulation of exogenous dopamine in LLC-PK1 cells was found to be saturable; nonlinear analysis of the saturation curves revealed for the apical and basal application of dopamine a Km of 17.7 +/- 4.3 and 96.0 +/- 28.1 microM and a Vmax of 2.0 +/- 0.1 and 2.2 +/- 0.3 nmol.mg protein-1.6 min-1, respectively. However, both cocaine (10, 30, or 100 microM) and GBR-12909 (1 or 3 microM) were found not to affect the uptake of 100 microM dopamine applied from either the apical or the basal cell border. In conclusion, the data presented here show that LLC-PK1 cells are endowed with considerable aromatic L-amino acid decarboxylase (AADC) activity and transport L-dopa quite efficiently through both the apical and basal cell borders. On the other hand, our observations support the possibility of a basal-to-apical gradient of AADC activity and the possibility that LLC-PK1 cells might constitute an interesting in vitro model for the study of the renal dopaminergic physiology.

The pathogenesis of hypertension in autosomal dominant polycystic kidney disease

BACKGROUND

Hypertension is a common and serious complication of autosomal dominant polycystic kidney disease (ADPKD), often occurring early in the disease before the renal function starts to decrease. The pathogenesis of this early hypertension is controversial.

OBJECTIVE

To review studies on the pathogenesis of early and late hypertension in ADPKD.

STUDY SELECTION

Studies on ADPKD and hypertension were retrieved from Medline from the last 20 years, with an emphasis on the last 10 years. These studies, together with selected published abstracts from recent hypertension and nephrology meetings, were reviewed critically.

RESULTS

Cyst growth, renal handling of sodium, activation of the renin-angiotensin-aldosterone system, volume expansion, an elevated plasma volume, and increased plasma atrial natriuretic peptide and plasma endothelin levels have all been found to be associated with hypertension in ADPKD. In some studies an inappropriate activity of the renin-angiotensin-aldosterone system that could be related to cyst growth and intrarenal ischemia was found. An increase in renal vascular resistance has been demonstrated and might be caused by intrarenal release of angiotensin II. Interestingly, the protective effect of angiotensin converting enzyme inhibitors on the renal function could not be demonstrated in ADPKD patients with a moderately decreased renal function. The importance, if any, of endothelial vasodilatory factors is not known. Sympathetic nervous activity seems to be increased in ADPKD, but the importance of this for the blood pressure level is not known.

CONCLUSION

The pathogenesis of hypertension in ADPKD is complex and likely to be dependent on the interaction of hemodynamic, endocrine and neurogenic factors.

Differential human renal tubular responses to dopamine type 1 receptor stimulation are determined by blood pressure status

We performed the present studies to determine whether a proximal renal tubular dopamine D1-like receptor defect exists in human essential hypertension. Twenty-four subjects were studied (13 normotensive and 11 hypertensive) in a randomized, double-blind, vehicle-controlled study using fenoldopam, a selective D1-like receptor agonist. Subjects were studied in sodium metabolic balance at 300 mEq/d, after which the salt sensitivity of their blood pressure was determined. Fenoldopam at peak doses of 0.1 to 0.2 microgram/kg per minute decreased mean arterial pressure in hypertensive subjects but did not change mean pressure in normotensive subjects. Fenoldopam increased renal plasma flow to a greater extent in hypertensive than normotensive subjects. Fenoldopam increased both urinary and fractional sodium excretions in the hypertensive and normotensive groups. In normotensive but not hypertensive subjects, fenoldopam increased the fractional excretion of lithium and distal sodium delivery. In contrast, both distal fractional sodium reabsorption and sodium-potassium exchange fell significantly in hypertensive subjects. We conclude that human essential hypertension is associated with a reduction in the proximal tubular response to D1-like receptor stimulation compared with normotensive subjects. Hypertensive subjects appear to have a compensatory upregulation of renal vascular and distal tubular D1-like receptor function that offsets the proximal tubular defect, resulting in an enhanced natriuretic response to D1-like receptor stimulation.

Salt sensitivity of blood pressure in humans

A variety of different techniques have been used for the assessment of the blood pressure response to changes in salt and water balance in humans. These have generally been found to be reproducible and to yield congruent results. This review surveys the characteristics of subjects identified as salt sensitive and salt resistant by different investigators from demographic and physiological perspectives.

Abnormal atrial natriuretic peptide and renal responses to saline infusion in nonmodulating essential hypertensive patients

BACKGROUND

Nonmodulation seems to represent an inheritable trait characterized by abnormal angiotensin-mediated control of aldosterone release and renal blood supply and salt-sensitive hypertension. Recently, we demonstrated that atrial natriuretic peptide (ANP) response to angiotensin II also is altered in nonmodulators. Moreover, an abnormal ANP response to acute volume expansion has been shown by others in hypertensive patients displaying some features of nonmodulators. These data induced us to hypothesize that nonmodulators. These data induced us to hypothesize that nonmodulation could be characterized by an abnormal ANP response to saline load.

METHODS AND RESULTS

Forty-three essential hypertensive men were subdivided into low-renin patients (n = 12), nonmodulators (n = 15), and modulators (n = 16) according to their renin profile and ability to modulate aldosterone and p-aminohippurate clearance responses to a graded angiotensin II infusion (1.0 ng.kg-1.min-1 and 3.0 ng.kg-1.min-1 for 30 minutes each) on both a low- (10 mmol Na+ per day) and a high- (210 mmol Na+ per day) Na+ intake. The intravenous saline load (0.25 mL.kg-1.min-1 for 2 hours) performed on a low-Na+ diet increased plasma ANP levels in low-renin (from 14.30 +/- 4.68 to 23.30 +/- 7.52 fmol/mL at 120 minutes, P < .05) and modulating patients (from 10.95 +/- 3.55 to 18.21 +/- 5.42 fmol/mL at 120 minutes, P < .05), whereas it did not change the hormone levels in nonmodulators (from 10.77 +/- 3.25 to 13.83 +/- 5.70 fmol/mL at 120 minutes, P = NS). When patients switched from a low- to a high-NaCl diet, plasma ANP levels increased significantly in all groups. However, when the saline load was repeated on a high-NaCl intake, ANP levels increased in both low-renin and modulating patients (P < .05), whereas it failed to increase in nonmodulators.

CONCLUSIONS

Nonmodulating hypertensive patients showed a reduced ANP response to saline infusion in the presence of a normal increase of plasma ANP with dietary NaCl load. The impaired ANP response to saline infusion could be due to a different distribution of volume load and contribute to determining the reduced ability to excrete sodium that is commonly described in nonmodulators.

Urinary kallikrein and salt sensitivity in essential hypertensive males

A strong influence of urinary kallikrein excretion on the salt sensitivity of blood pressure has been recently suggested in normotensive patients. To evaluate the relationship between kallikrein and salt sensitivity in essential hypertension, active kallikrein excretion, plasma renin activity, atrial natriuretic peptide and aldosterone levels were evaluated in 37 male hypertensives (mean age 43.3 +/- 4.7 years) after two weeks on a normal NaCl diet (120 mmol NaCl per day). After kallikrein determination, salt sensitivity was assessed in a randomized cross-over double-blind fashion by evaluating the blood pressure response to a high (240 mmol NaCl per day for two weeks) and a low (40 mmol NaCl per day for 2 weeks) NaCl intake. Blood pressure changes were evaluated considering as baseline blood pressure the measurement taken at the end of the 2 weeks under normal NaCl intake. Patients were classified as salt sensitive when a diastolic blood pressure change of 10 mm Hg or more occurred after both periods of low and high NaCl intake. At the end of the assessment of salt sensitivity, 19 hypertensive patients (mean age 43.0 +/- 4.6 years) were resistant. The urinary excretion of active kallikrein was significantly lower (P < 0.0001) in salt sensitive (0.51 +/- 0.36 U/24 hr) than in salt resistant patients (1.28 +/- 0.48 U/24 hr). Also, plasma atrial natriuretic peptide levels were higher in salt sensitive than in salt resistant hypertensives (P < 0.02), and a significant correlation between urinary kallikrein and plasma atrial natriuretic peptide was demonstrated in salt sensitive hypertensives (r = -0.691, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Salt sensitivity in hypertension. Renal and cardiovascular implications

The mechanisms responsible for the increase in blood pressure response to high salt intake in salt-sensitive patients with essential hypertension are complex and only partially understood. A complex interaction between neuroendocrine factors and the kidney may underlie the propensity for such patients to retain salt and develop salt-dependent hypertension. The possible role of vasodilator and natriuretic agents, such as the prostaglandins, endothelium-derived relaxing factor, atrial natriuretic factor, and kinin-kallikrein system, requires further investigation. An association between salt sensitivity and a greater propensity to develop renal failure has been described in certain groups of hypertensive patients, such as blacks, the elderly, and those with diabetes mellitus. Salt-sensitive patients with essential hypertension manifest a deranged renal hemodynamic adaptation to a high dietary salt intake. During a low salt diet, salt-sensitive and salt-resistant patients have similar mean arterial pressure, glomerular filtration rate, effective renal plasma flow, and filtration fraction. On the other hand, during a high salt intake glomerular filtration rate does not change in either group, and effective renal blood flow increases in salt-resistant but decreases in salt-sensitive patients; filtration fraction and glomerular capillary pressure decrease in salt-resistant but increase in salt-sensitive patients. Salt-sensitive patients are also more likely than salt-resistant patients to manifest left ventricular hypertrophy, microalbuminuria, and metabolic abnormalities that may predispose them to cardiovascular diseases. In conclusion, salt sensitivity in hypertension is associated with substantial renal, hemodynamic, and metabolic abnormalities that may enhance the risk of cardiovascular and renal morbidity.

Lack of renal dopamine production during acute volume expansion in Dahl salt-sensitive rats

Endogenous kidney dopamine (DA) is reported to contribute to the natriuretic response to acute volume expansion (VE). Several studies suggest that a defect in renal DA-ergic system may play a role in genetic hypertension in humans and rats. The present study was performed to determine the role of renal DA and tubular DA-1 receptors in the natriuretic response to VE in age-matched inbred Dahl salt sensitive (SS/Jr) and salt resistant (SR/Jr) rats of 9-11 weeks of age. In pentobarbital anesthetized rats, VE was carried out by intravenous infusion of isotonic sodium chloride (5% body weight) over a period of 60 min. This maneuver evoked marked increases in urine output and urinary sodium excretion in both SR/Jr and SS/Jr species. However, the natriuretic and diuretic response to VE was significantly reduced in SS/Jr as compared to SR/Jr rats. It was also observed that the urinary excretion of DA was significantly increased during VE only in SR/Jr, but not in SS/Jr rats. In separate group of animals, infusion of DA (1 microgram/kg/min) produced similar increases in urine output and urinary sodium excretion without causing any alterations in blood pressure or heart rate in either SS/Jr or SR/Jr rats. These results suggest that SS/Jr rats are not able to eliminate an acute increase in sodium load as efficiently as SR/Jr, which may be partly due to an impaired endogenous kidney DA production.

Plasma and cerebrospinal fluid neurochemical pattern in Menkes disease

Menkes disease is a neurodegenerative disorder of copper metabolism. Because the enzyme dopamine-beta-hydroxylase requires copper to catalyze the conversion of dopamine to norepinephrine, we reasoned that patients with Menkes disease would have a neurochemical pattern similar to that seen in patients with congenital absence of dopamine-beta-hydroxylase, i.e., high levels of dopamine, the dopamine metabolite dihydroxyphenylacetic acid (DOPAC), and the catecholamine precursor dihydroxyphenylalanine (DOPA), and low levels of norepinephrine and its neuronal metabolite dihydroxyphenylglycol (DHPG). We measured plasma and cerebrospinal fluid (CSF) levels of catechols in 10 patients ranging in age from 9 days to 27 months. In contrast to patients with congenital absence of dopamine-beta-hydroxylase, norepinephrine levels were normal in plasma of 4 Menkes patients and in CSF of all 10 patients. However, the ratios of DOPA:DHPG and DOPAC:DHPG in plasma and CSF of Menkes patients were invariably increased beyond the ranges of control values. These neurochemical findings indicate partial deficiency of dopamine-beta-hydroxylase in Menkes patients, with compensatory increases in catecholamine biosynthesis in sympathetic nerves and in the brain. Increased tyrosine hydroxylation and increased exocytotic release of norepinephrine may be responsible for preservation of plasma and CSF norepinephrine levels in Menkes patients. The abnormal neurochemical pattern, including high ratios of DOPA:DHPG and DOPAC:DHPG, may serve as a biochemical marker for Menkes disease and provide a baseline against which the influence of proposed therapies can be judged.