Influence of Na+ intake on dopamine-induced inhibition of renal cortical Na(+)-K(+)-ATPase

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.

Fluoride Exposure Induces Inhibition of Sodium-and Potassium-Activated Adenosine Triphosphatase (Na+, K+-ATPase) Enzyme Activity: Molecular Mechanisms and Implications for Public Health

In this study, several lines of evidence are provided to show that Na + , K + -ATPase activity exerts vital roles in normal brain development and function and that loss of enzyme activity is implicated in neurodevelopmental, neuropsychiatric and neurodegenerative disorders, as well as increased risk of cancer, metabolic, pulmonary and cardiovascular disease. Evidence is presented to show that fluoride (F) inhibits Na + , K + -ATPase activity by altering biological pathways through modifying the expression of genes and the activity of glycolytic enzymes, metalloenzymes, hormones, proteins, neuropeptides and cytokines, as well as biological interface interactions that rely on the bioavailability of chemical elements magnesium and manganese to modulate ATP and Na + , K + -ATPase enzyme activity. Taken together, the findings of this study provide unprecedented insights into the molecular mechanisms and biological pathways by which F inhibits Na + , K + -ATPase activity and contributes to the etiology and pathophysiology of diseases associated with impairment of this essential enzyme. Moreover, the findings of this study further suggest that there are windows of susceptibility over the life course where chronic F exposure in pregnancy and early infancy may impair Na + , K + -ATPase activity with both short- and long-term implications for disease and inequalities in health. These findings would warrant considerable attention and potential intervention, not to mention additional research on the potential effects of F intake in contributing to chronic disease.

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.

Sodium balance, circadian BP rhythm, heart rate variability, and intrarenal renin-angiotensin-aldosterone and dopaminergic systems in acute phase of ARB therapy

We have revealed that even in humans, activated intrarenal renin–angiotensin–aldosterone system (RAAS) enhances tubular sodium reabsorption to facilitate salt sensitivity and nondipper rhythm of blood pressure (BP), and that angiotensin receptor blocker (ARB) could increase daytime urinary sodium excretion rate (U_NaV) to produce lower sodium balance and restore nondipper rhythm. However, the sympathetic nervous system and intrarenal dopaminergic system can also contribute to renal sodium handling. A total of 20 patients with chronic kidney disease (61 ± 15 years) underwent 24‐h ambulatory BP monitoring before and during two‐day treatment with ARB, azilsartan. Urinary angiotensinogen excretion rate (U_AGTV, μg/gCre) was measured as intrarenal RAAS; urinary dopamine excretion rate (U_DAV, pg/gCre) as intrarenal dopaminergic system; heart rate variabilities (HRV, calculated from 24‐h Holter‐ECG) of non‐Gaussianity index λ_25s as sympathetic nerve activity; and power of high‐frequency (HF) component or deceleration capacity (DC) as parasympathetic nerve activity. At baseline, glomerular filtration rate correlated inversely with U_AGTV (r = −0.47, P = 0.04) and positively with U_DAV (r = 0.58, P = 0.009). HF was a determinant of night/day BP ratio (β = −0.50, F = 5.8), rather than DC or λ_25s. During the acute phase of ARB treatment, a lower steady sodium balance was not achieved. Increase in daytime U_NaV preceded restoration of BP rhythm, accompanied by decreased U_AGTV (r = −0.88, P = 0.05) and increased U_DAV (r = 0.87, P = 0.05), but with no changes in HRVs. Diminished sodium excretion can cause nondipper BP rhythm. This was attributable to intrarenal RAAS and dopaminergic system and impaired parasympathetic nerve activity. During the acute phase of ARB treatment, cooperative effects of ARB and intrarenal dopaminergic system exert natriuresis to restore circadian BP rhythm.

Regulation of glomerulotubular balance: flow-activated proximal tubule function

The purpose of this review is to summarize our knowledge and understanding of the physiological importance and the mechanisms underlying flow-activated proximal tubule transport. Since the earliest micropuncture studies of mammalian proximal tubule, it has been recognized that tubular flow is an important regulator of sodium, potassium, and acid-base transport in the kidney. Increased fluid flow stimulates Na⁺ and HCO₃^- absorption in the proximal tubule via stimulation of Na/H-exchanger isoform 3 (NHE3) and H⁺-ATPase. In the proximal tubule, brush border microvilli are the major flow sensors, which experience changes in hydrodynamic drag and bending moment as luminal flow velocity changes and which transmit the force of altered flow to cytoskeletal structures within the cell. The signal to NHE3 depends upon the integrity of the actin cytoskeleton; the signal to the H⁺-ATPase depends upon microtubules. We have demonstrated that alterations in fluid drag impact tubule function by modulating ion transporter availability within the brush border membrane of the proximal tubule. Beyond that, there is evidence that transporter activity within the peritubular membrane is also modulated by luminal flow. Secondary messengers that regulate the flow-mediated tubule function have also been delineated. Dopamine blunts the responsiveness of proximal tubule transporters to changes in luminal flow velocity, while a DA1 antagonist increases flow sensitivity of solute reabsorption. IP3 receptor-mediated intracellular Ca²⁺ signaling is critical to transduction of microvillus drag. In this review, we summarize our findings of the regulatory mechanism of flow-mediated Na⁺ and HCO₃^- transport in the proximal tubule and review available information about flow sensing and regulatory mechanism of glomerulotubular balance.

Atrial natriuretic peptide and renal dopaminergic system: a positive friendly relationship?

Sodium metabolism by the kidney is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between antinatriuretic and natriuretic factors. Renal dopamine plays a central role in this interactive network. The natriuretic hormones, such as the atrial natriuretic peptide, mediate some of their effects by affecting the renal dopaminergic system. Renal dopaminergic tonus can be modulated at different steps of dopamine metabolism (synthesis, uptake, release, catabolism, and receptor sensitization) which can be regulated by the atrial natriuretic peptide. At tubular level, dopamine and atrial natriuretic peptide act together in a concerted manner to promote sodium excretion, especially through the overinhibition of Na+, K+-ATPase activity. In this way, different pathological scenarios where renal sodium excretion is dysregulated, as in nephrotic syndrome or hypertension, are associated with impaired action of renal dopamine and/or atrial natriuretic peptide, or as a result of impaired interaction between these two natriuretic systems. The aim of this review is to update and comment on the most recent evidences demonstrating how the renal dopaminergic system interacts with atrial natriuretic peptide to control renal physiology and blood pressure through different regulatory pathways.

A prospective, randomized study to evaluate the efficacy of various diuretic strategies in acute decompensated heart failure

AIM

To evaluate the safety and efficacy of various initial strategies of loop diuretic administration in patients with acute decompensated heart failure (ADHF) on diuresis, renal function, electrolyte balance and clinical outcomes.

METHODS

Consecutive patients admitted with ADHF were randomized into three groups - intravenous furosemide infusion + intravenous dopamine, intravenous furosemide bolus in two divided doses and intravenous furosemide continuous infusion alone. At 48 h, the treating physician could adjust the diuretic strategy. Primary endpoint was negative fluid balance at 24 h after admission. Secondary end points were duration of hospital stay, negative fluid balance at 48, 72, 96 h, the trend of serum electrolytes, and renal function and 30 day clinical outcome (death and emergency department visits).

RESULTS

Overall ninety patients (thirty in each group) were included in the study. There was a greater diuresis in first 24 h (p = 0.002) and a shorter hospital stay (p = 0.023) with the bolus group. There was no significant difference in renal function and serum sodium and serum potassium levels. There was no difference in the number of emergency department visits among the three groups.

CONCLUSION

All three modes of diuretic therapies can be practiced with no difference in worsening of renal function and electrolyte levels. Bolus dose administration with its rapid volume loss and shorter hospital stay might be a more effective diuretic strategy.

Current strategies for preventing renal dysfunction in patients with heart failure: a heart failure stage approach

Renal dysfunction is common during episodes of acute decompensated heart failure, and historical data indicate that the mean creatinine level at admission has risen in recent decades. Different mechanisms underlying this change over time have been proposed, such as demographic changes, hemodynamic and neurohumoral derangements and medical interventions. In this setting, various strategies have been proposed for the prevention of renal dysfunction with heterogeneous results. In the present article, we review and discuss the main aspects of renal dysfunction prevention according to the different stages of heart failure.

α2C-Adrenoceptors modulate L-DOPA uptake in opossum kidney cells and in the mouse kidney

Targeted deletion or selective pharmacological inhibition of α(2C)-adrenoceptors in mice results in increased brain tissue levels of dopamine and its precursor l-3,4-dihydroxyphenylalanine (l-DOPA), without significant changes in l-DOPA synthesis. l-DOPA uptake is considered the rate-limiting step in dopamine synthesis in the kidney. Since α(2C)-adrenoceptors may influence the transport of l-DOPA, we investigated the effect of α(2C)-adrenoceptor activation on l-DOPA uptake in a kidney cell line (opossum kidney cells). l-DOPA and dopamine kidney tissue levels in α(2C)-adrenoceptor knockout (α(2C)KO) mice and in mice treated with the selective α(2C)-adrenoceptor antagonist JP-1302 were also evaluated. The α(2)-adrenoceptor agonist medetomidine (0.1-1,000 nM) produced a concentration-dependent decrease in l-DOPA uptake in opossum kidney cells (IC(50): 2.5 ± 0.5 nM and maximal effect: 28 ± 5% of inhibition). This effect was abolished by a preincubation with JP-1302 (300 nM). Furthermore, the effect of medetomidine (100 nM) was abolished by a preincubation with U-0126 (10 μM), a MEK1/2 inhibitor. Kidney tissue levels of l-DOPA were significantly higher in α(2C)KO mice compared with wild-type mice (wild-type mice: 58 ± 2 pmol/g tissue and α(2C)KO mice: 81 ± 15 pmol/g tissue, P < 0.05) and in mice treated with JP-1302 (3 μmol/kg body wt) compared with control mice (control mice: 62 ± 2 pmol/g tissue and JP-1302-treated mice: 75 ± 1 pmol/g tissue, P < 0.05), both without significant changes in dopamine kidney tissue levels. However, mice treated with JP-1302 on a high-salt diet presented significantly higher dopamine levels in the kidney and urine compared with control animals on a high-salt diet. In conclusion, in a kidney cell line, α(2C)-adrenoceptor activation inhibits l-DOPA uptake, and in mice, deletion or blockade of α(2C)-adrenoceptors increases l-DOPA kidney tissue levels.

Regulation of glomerulotubular balance. I. Impact of dopamine on flow-dependent transport

In response to volume expansion, locally generated dopamine decreases proximal tubule reabsorption by reducing both Na/H-exchanger 3 (NHE3) and Na-K-ATPase activity. We have previously demonstrated that mouse proximal tubules in vitro respond to changes in luminal flow with proportional changes in Na(+) and HCO(3)(-) reabsorption and have suggested that this observation underlies glomerulotubular balance. In the present work, we investigate the impact of dopamine on the sensitivity of reabsorptive fluxes to changes in luminal flow. Mouse proximal tubules were microperfused in vitro at low and high flow rates, and volume and HCO(3)(-) reabsorption (J(v) and J(HCO3)) were measured, while Na(+) and Cl(-) reabsorption (J(Na) and J(Cl)) were estimated. Raising luminal flow increased J(v), J(Na), and J(HCO3) but did not change J(Cl). Luminal dopamine did not change J(v), J(Na), and J(HCO3) at low flow rates but completely abolished the increments of Na(+) absorption by flow and partially inhibited the flow-stimulated HCO(3)(-) absorption. The remaining flow-stimulated HCO(3)(-) absorption was completely abolished by bafilomycin. The DA1 receptor blocker SCH23390 and the PKA inhibitor H89 blocked the effect of exogenous dopamine and produced a two to threefold increase in the sensitivity of proximal Na(+) reabsorption to luminal flow rate. Under the variety of perfusion conditions, changes in cell volume were small and did not always parallel changes in Na(+) transport. We conclude that 1) dopamine inhibits flow-stimulated NHE3 activity by activation of the DA1 receptor via a PKA-mediated mechanism; 2) dopamine has no effect on flow-stimulated H-ATPase activity; 3) there is no evidence of flow stimulation of Cl(-) reabsorption; and 4) the impact of dopamine is a coordinated modulation of both luminal and peritubular Na(+) transporters.

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.

Dopamine and angiotensin as renal counterregulatory systems controlling sodium balance

PURPOSE OF REVIEW

To review the recent evidence demonstrating how the renal dopaminergic and angiotensin systems control renal electrolyte balance through various receptor-mediated pathways with counterregulatory interactions.

RECENT FINDINGS

Stimulation of the renal rennin-angiotensin system results in increased sodium reabsorption, whereas the opposite is true for stimulation of the renal dopaminergic system. An underactive renal dopaminergic system has been associated with increased sodium reabsorption and hypertension. Recent findings indicate novel cell surface receptor-mediated mechanisms by which these two renal endocrine systems directly counterregulate each other. Each of the dopamine receptors (D1R through D5R) have been implicated in dopamine-mediated natriuresis, in addition to counterregulating the angiotensin type 1 R. Dopamine D1-like (D1R and D5R) stimulation has also been found to induce an AT2 receptor- dependent natriuresis. Recently, it has also been discovered that reactive oxygen species can play a role in inactivating the D1 receptor and activating the angiotensin type 1 R.

SUMMARY

Current therapeutic interventions for hypertension predominantly involve correction of an overactive rennin-angiotensin aldosterone system. Recent evidence suggests that stimulation of the renal dopaminergic system and possibly activation of AT2 receptors, as well as decreasing reactive oxygen species, may provide additional therapeutic approaches.

Trafficking of Na-K-ATPase and dopamine receptor molecules induced by changes in intracellular sodium concentration of renal epithelial cells

Most of the transepithelial transport of sodium in proximal tubules occurs through the coordinated action of the apical sodium/proton exchanger and the basolateral Na-K-ATPase. Hormones that regulate proximal tubule sodium excretion regulate the activities of these proteins. We have previously demonstrated that the level of intracellular sodium concentration modulates the regulation of Na-K-ATPase activity by angiotensin II and dopamine. An increase of a few millimolars in intracellular sodium concentration leads to increased Na-K-ATPase activity without a statistically significant increase in the number of plasma membrane Na-K-ATPase molecules, as determined by cell surface protein biotinylation. Using total internal reflection fluorescence, we detected an increased number of Na-K-ATPase molecules in cytosolic compartments adjacent to the plasma membrane, suggesting that the increased intracellular sodium concentration induces a movement of Na-K-ATPase molecules toward the plasma membrane. While intracellular compartments containing Na-K-ATPase molecules are very close to the plasma membrane, compartments containing type 1 dopamine receptors (D1Rs) are distributed in different parts of the cell cytosol. Fluorescence determinations indicate that an increased intracellular sodium concentration induces the increased colocalization of dopamine receptors with Na-K-ATPase molecules in the region of the plasma membrane. We propose that under in vivo conditions, in response to a sodium load in the lumen of proximal tubules, an increased level of intracellular sodium in epithelial cells is an early event that triggers the cellular response that leads to dopamine inhibition of proximal tubule sodium reabsorption.

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

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

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.

Ouabain-insensitive acidification by dopamine in renal OK cells: primary control of the Na(+)/H(+) exchanger

The present study was aimed at evaluating the role of D(1)- and D(2)-like receptors and investigating whether inhibition of Na(+) transepithelial flux by dopamine is primarily dependent on inhibition of the apical Na(+)/H(+) exchanger, inhibition of the basolateral Na(+)-K(+)-ATPase, or both. The data presented here show that opossum kidney cells are endowed with D(1)- and D(2)-like receptors, the activation of the former, but not the latter, accompanied by stimulation of adenylyl cyclase (EC(50) = 220 +/- 2 nM), marked intracellular acidification (IC(50) = 58 +/- 2 nM), and attenuation of amphotericin B-induced decreases in short-circuit current (28.6 +/- 4.5% reduction) without affecting intracellular pH recovery after CO(2) removal. These results agree with the view that dopamine, through the activation of D(1)- but not D(2)-like receptors, inhibits both the Na(+)/H(+) exchanger (0.001933 +/- 0.000121 vs. 0.000887 +/- 0.000073 pH unit/s) and Na(+)-K(+)-ATPase without interfering with the Na(+)-independent HCO transporter. It is concluded that dopamine, through the action of D(1)-like receptors, inhibits both the Na(+)/H(+) exchanger and Na(+)-K(+)-ATPase, but its marked acidifying effects result from inhibition of the Na(+)/H(+) exchanger only, without interfering with the Na(+)-independent HCO transporter and Na(+)-K(+)-ATPase.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Concerted action of dopamine on renal and intestinal Na(+)-K(+)-ATPase in the rat remnant kidney

The present study evaluated renal and intestinal adaptations in sodium handling in uninephrectomized (Unx) rats and the role of dopamine. Two weeks after uninephrectomy, the remnant kidney in Unx rats weighed 33 +/- 2% more than the corresponding kidney in sham-operated (Sham) animals. This was accompanied by increases in urinary levels of dopamine and major metabolites [3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid] and increases in maximal velocity values (169 vs. 115 nmol. mg protein(-1). 15 min(-1)) for renal aromatic L-amino acid decarboxylase, the enzyme responsible for the synthesis of renal dopamine. High salt (HS) intake increased (P < 0.05) the urinary excretion of dopamine and DOPAC in Unx and Sham rats. However, the urinary levels of L-3,4-dihydroxyphenylalanine, dopamine, and DOPAC in Sham rats during HS intake were lower than in Unx rats. Blockade of dopamine D(1) receptors (Sch-23390, 2 x 30 microg/kg) reduced the urinary excretion of sodium in Unx (31% decrease) more pronouncedly than in Sham (19% decrease) rats. However, inhibition of renal Na(+)-K(+)-ATPase activity by dopamine was of similar magnitude in Unx and Sham rats. In parallel, it was observed that uninephrectomy resulted in a significant reduction in jejunal sodium absorption and Na(+)-K(+)-ATPase activity in jejunal epithelial cells. In jejunal epithelial cells from Sham rats, dopamine (1 microM) failed to inhibit Na(+)-K(+)-ATPase activity, whereas in Unx rats it produced a significant reduction. It is concluded that uninephrectomy results in increased renal dopaminergic activity and dopamine-sensitive enhanced natriuresis. Furthermore, it is suggested that decreased jejunal absorption of sodium may take place in response to partial renal ablation, as an example of renal-intestinal cross talk.

Molecular modulation of inward and outward apical transporters of L-dopa in LLC-PK(1) cells

The present study examined the nature of the apical inward and outward L-3,4-dihydroxyphenylalanine (L-dopa) transporters in LLC-PK(1) cells and whether protein kinases differentially modulate the activities of these transporters. The apical inward transfer of L-dopa was promoted through an energy-dependent and sodium-insensitive transporter (Michaelis constant = 38 microM; maximum velocity = 2608 pmol. mg protein(-1). 6 min(-1)). This transporter was insensitive to N-(methylamino)-isobutyric acid but competitively inhibited by 2-aminobicyclo(2,2, 1)-heptane-2-carboxylic acid (BHC; IC(50) = 251 microM). Modulators of protein kinase A (cAMP, forskolin, IBMX, and cholera toxin), protein kinase G (cGMP, zaprinast, LY-83583 and sodium nitroprusside), and protein kinase C (phorbol 12,13-dibutirate and chelerythrine) failed to affect the accumulation of L-dopa. The Ca(2+)/calmodulin inhibitors calmidazolium and trifluoperazine inhibited L-dopa uptake (IC(50) of 72 and 55 microM, respectively). The inhibitory effect of calmidazolium on the accumulation of L-dopa was of the noncompetitive type. The organic anion inhibitor DIDS, but not p-aminohippurate, and the protein tyrosine kinase (PTK) inhibitor genistein significantly increased L-dopa accumulation, which was mainly due to inhibition of apical outward transfer of L-dopa. It is concluded that LLC-PK(1) cells take up L-dopa over the apical cell border through the L-type amino acid transporter, which appears to be under the control of Ca(2+)-calmodulin-mediated pathways. The apical outward transfer of L-dopa may be promoted through a DIDS-sensitive transport mechanism and appears to be under the tonic control of PTK.

Inhibition of proximal convoluted tubule transport by dopamine

BACKGROUND

Dopamine can produce a natriuresis and diuresis independent of changes in renal hemodynamics. However, previous studies have failed to demonstrate an inhibition of transport by dopamine in intact proximal convoluted tubules.

METHODS

Rabbit proximal convoluted tubules were perfused in vitro with an ultrafiltrate-like solution and bathed in a serum-like albumin solution.

RESULTS

In the present study, the addition of 10-5 M dopamine to the lumen or bath of proximal convoluted tubules perfused in vitro had no effect on transport. In proximal convoluted tubules, addition of 10-6 M bath norepinephrine increased the rate of volume absorption from 0.65 +/- 0.08 to 0.93 +/- 0.08 nl/mm. min (P < 0.01). Addition of 10-5 M luminal dopamine in the presence of bath norepinephrine inhibited the rate of volume absorption to 0.72 +/- 0.10 nl/mm. min (P = 0.01). The inhibition in the rate of volume absorption by luminal dopamine in the presence of bath norepinephrine was completely blocked by the DA1 antagonist, SCH 23390. The DA1 agonist luminal 10-5 M fenoldopam also inhibited volume absorption in the presence of bath norepinephrine, but the DA2 agonist luminal 10-5 M quinpirole was without effect. Bath 10-5 M dopamine had no effect on volume absorption in the presence of bath norepinephrine.

CONCLUSION

Dopamine has no direct epithelial action on the proximal convoluted tubule. However, luminal dopamine antagonizes the stimulation in transport produced by norepinephrine. These studies suggest that luminal dopamine may play a role to modulate sodium transport in the presence of renal nerve activity.

Phosphatidylinositol 3-kinase-mediated endocytosis of renal Na+, K+-ATPase alpha subunit in response to dopamine

Dopamine (DA) inhibition of Na+,K+-ATPase in proximal tubule cells is associated with increased endocytosis of its alpha and beta subunits into early and late endosomes via a clathrin vesicle-dependent pathway. In this report we evaluated intracellular signals that could trigger this mechanism, specifically the role of phosphatidylinositol 3-kinase (PI 3-K), the activation of which initiates vesicular trafficking and targeting of proteins to specific cell compartments. DA stimulated PI 3-K activity in a time- and dose-dependent manner, and this effect was markedly blunted by wortmannin and LY 294002. Endocytosis of the Na+,K+-ATPase alpha subunit in response to DA was also inhibited in dose-dependent manner by wortmannin and LY 294002. Activation of PI 3-K generally occurs by association with tyrosine kinase receptors. However, in this study immunoprecipitation with a phosphotyrosine antibody did not reveal PI 3-K activity. DA-stimulated endocytosis of Na+, K+-ATPase alpha subunits required protein kinase C, and the ability of DA to stimulate PI 3-K was blocked by specific protein kinase C inhibitors. Activation of PI 3-K is mediated via the D1 receptor subtype and the sequential activation of phospholipase A2, arachidonic acid, and protein kinase C. The results indicate a key role for activation of PI 3-K in the endocytic sequence that leads to internalization of Na+,K+-ATPase alpha subunits in response to DA, and suggest a mechanism for the participation of protein kinase C in this process.

Deamination of newly-formed dopamine in rat renal tissues

1. The present study has examined the formation of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in slices of the rat renal cortex and the renal medulla loaded with exogenous L-beta-3,4-dihydroxyphenylalanine (L-DOPA). The effects of pargyline and of two selective inhibitors of monoamine oxidase (MAO) types A and B, respectively Ro 41-1049 and Ro 19-6327, on the deamination of newly-synthesized dopamine in kidney slices incubated with exogenous L-DOPA were also tested. The assay of L-DOPA, dopamine, noradrenaline and DOPAC was performed by means of h.p.l.c. with electrochemical detection. 2. Incubation of renal slices with exogenous L-DOPA resulted in a concentration-dependent accumulation of dopamine and DOPAC; the tissue levels of newly-formed dopamine and DOPAC in slices of the renal medulla were 6-8% of those in cortical slices. 3. Pargyline (0.1 mM) produced a marked decrease (84% reduction) in the formation of DOPAC in kidney slices loaded with 1.0 mM L-DOPA; this effect was accompanied by a 17% increase in the accumulation of dopamine. Similar effects were obtained at higher concentrations of pargyline (0.5 and 1.0 mM). At 5.0 and 10.0 mM pargyline, a marked decrease (46 and 76% reduction) in the accumulation of newly-formed dopamine was observed. 4. The accumulation of dopamine and DOPAC was found to be time-dependent in experiments in which tissues were incubated with 5 and 10 microM L-DOPA for 5, 10, 20 and 30 min. Pargyline (0.1 mM) produced an increase in the accumulation of dopamine at all incubation periods and decreased the formation of DOPAC. 6. It is concluded that deamination of newly-formed dopamine in kidney slices loaded with L-DOPA constitutes an important mechanism of amine inactivation. The results presented also suggest that most of the MAO, located inside the compartment where the synthesis of dopamine occurs, is of the A type.

Renal tubular dopamine outward transfer during Na(+)-H+ exchange activation by alpha 1- and alpha 2-adrenoceptor agonists

1. The present work describes the effects of inhibitors (amiloride and ethylisopropylamiloride) and activators (quinoxaline and phenylephrine) of the Na(+)-H+ exchanger on the outflow of dopamine in rat kidney slices loaded with L-dihydroxyphenylalanine (L-DOPA). 2. Incubation of kidney slices loaded with 50 microM L-DOPA in the presence of increasing concentrations of amiloride or ethylisopropylamiloride (EIPA) resulted in a concentration-dependent decrease in the outflow of newly-formed dopamine; the IC50 value for EIPA was 5.6 +/- 0.3 microM. Phenylephrine and quinoxaline were found to produce a concentration-dependent increase in the outflow of newly-formed dopamine; the EC50 values for the phenylephrine and quinoxaline were, respectively, 0.9 +/- 0.1 and 0.08 +/- 0.01 microM. 3. The facilitatory effect of phenylephrine on the outflow of dopamine was found not to be affected by yohimbine (100 nM), but was abolished by prazosin (1 microM), whereas that of quinoxaline was found to be selectively antagonized by yohimbine (100 nM), but not by prazosin (1 microM); EIPA (10 microM) was also found to abolish the effect of both phenylephrine and quinoxaline. The facilitatory effect of quinoxaline was also found to be reduced by 42-48% and 56-78% by, dibutyryl adenosine cyclic 3',5'-monophosphate (dibutyryl cyclic AMP; 250 microM) and forskolin (10 microM), respectively, but not by the protein kinase C (PKC) inhibitor, (+)-sphingosine (10 microM). In contrast, (+)-sphingosine (10 microM) was found to antagonize markedly (43- 69% reduction) the facilitatory effect of phenylephrine; dibutyrylcyclic AMP (250 microM) and forskolin (10 microM) were also found to reduce significantly the facilitatory effect of phenylephrine, by 42-53% and 44-59% respectively.4. A synergistic effect between alphal- and alpha2-adrenoceptors was observed for submaximal concentrations of quinoxaline (50 nM) and phenylephrine or submaximal concentrations of phenylephrine (0.5 microM) and quinoxaline, but not for maximal effective concentrations of either agonist. Dibutyryl cyclic AMP(250 microM) or forskolin (10 microM) produced a marked decrease (35-85% reduction) of the synergistic effect between phenylephrine and quinoxaline. The addition of phorbol 12,13-dibutyrate (PDBu; 500 nM) was found not to alter the outflow of newly-formed dopamine, but did potentiate (18-42% increase) the facilitatory effect of quinoxaline on the amine outflow. This effect was found to occur for submaximal concentrations of quinoxaline (10, 50 and 100 nM) and found to be antagonized by (+)-sphingosine(10 microM). In contrast, PDBu (500 nM) was found not to potentiate the facilitatory effect of phenylephrine on dopamine outflow.5. In conclusion, inhibition of the Na+-H+ antiport by amiloride and EIPA results in considerable reduction in the outflow of newly-formed dopamine, whereas the activation of this mechanism by both phenylephrine and quinoxaline results in facilitation of the outflow of dopamine; this effect is selectively reversed by alphal- and alpha2-adrenoceptor antagonists and EIPA. The synergistic effect between quinoxaline and phenylephrine may be related to an amplification of a reaction at a given point in the post-receptor transducing pathway.

Kinetic study of the tubular dopamine outward transporter in the rat and dog kidney

1. The present study has determined the kinetic characteristics of the outflow of dopamine of renal origin in slices of rat and dog renal cortex loaded with exogenous L-dihydroxyphenylalanine (L-DOPA 5 to 5000 microM). 2. In both dog and rat renal tissues the production of dopamine was found to be dependent on the concentration of L-DOPA used and reached its maximum at 2500 microM L-DOPA. The decarboxylation of L-DOPA in rat cortical slices (16.4 +/- 2.6 to 1479.2 +/- 85.2 nmol g-1) was 6 fold that in the dog (2.2 +/- 0.4 to 252.1 +/- 21.2 nmol g-1). In the rat kidney a large amount (approximately 50%) of the dopamine (5.2 +/- 0.6 to 743.4 +/- 58.3 nmol g-1) was found to escape into the incubation medium, whereas in dog renal slices the amount of newly-formed dopamine escaping into the incubation medium (0.7 +/- 0.2 to 46.5 +/- 9.3 nmol g-1) was less than 25% of the total amount of the amine formed. 3. The application of the Michaelis-Menten equation to the net transport of newly-formed dopamine has allowed the identification of a saturable (carrier-mediated transfer) and a non-saturable component (diffusion). The Vmax (nmol g-1 15 min-1) and Km (nM) values for the saturable component were, respectively, 340 +/- 41 and 396 +/- 45 in the rat kidney and 112 +/- 16 and 319 +/- 35 in the dog kidney. In both rat and dog renal tissues, the magnitude of the non-saturable component was found to be of minor importance up to a concentration of 250 nmol g-1 of dopamine to be transported. At high concentrations of the amine (greater then 250 nmol g-1), only attainable in rat kidney slices, most of the dopamine was found to leave the compartment where the synthesis did occur through a non-saturable transport system.4. In conclusion, the results presented here show that the outflow of newly-formed dopamine in both dog and rat kidney slices loaded with exogenous L-DOPA follows Michaelis-Menten kinetics with a saturable component and a non-saturable one, the latter assuming particular importance only at higher concentrations of the amine.

Renal dopamine-1 receptors in hypertensive inbred rat strains with and without hyperactivity

Renal dopamine-1 (DA-1) receptors are involved in the regulation of sodium transport in several nephron segments, including the proximal convoluted tubule (PCT). DA-1 receptors in the PCT and cortical collecting duct of normotensive rats are linked to the stimulation of adenylyl cyclase (AC). We have reported a defect in the DA-1 receptor/AC coupling in the PCT of the spontaneously hypertensive rat (SHR) of the Okamoto-Aoki strain. Hyperactivity and hypertension are both expressed in the SHR. To determine if the DA-1 receptor coupling defect is associated with hyperactivity or hypertension, we studied the DA-1 receptor in the PCT of two new inbred rat strains derived from the SHR: the hyperactive WKHA and the hypertensive WKHT rat. Tail-cuff blood pressures taken at 4 weeks indicated that WKHT rats were not hypertensive (86 +/- 3 mm Hg, n = 6), whereas at 12 weeks systolic pressures in both SHR and WKHT rats exceeded 150 mm Hg. Hyperactivity, however, was noted in WKHA rats even at this early age. Basal AC activity was similar in WKHA and WKHT PCT in either age group. In the older rats, the DA-1 agonist fenoldopam (10(-7) mol/L) stimulated AC activity in WKHA (70.6 +/- 16.1 fmol per 3 mm PCT per 20 minutes, n = 3) but not in WKHT PCT (43.3 +/- 5.3 fmol per 3 mm PCT per 20 minutes, n = 4). Gpp(NH)p (10(-5) mol/L), a nonhydrolyzable GTP analogue, stimulated AC activity to a similar extent in WKHA and WKHT PCT.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of alpha-human atrial natriuretic peptide on the synthesis of dopamine in the rat kidney

1. The present study has examined the influence of alpha-human atrial natriuretic peptide (alpha-hANP) on the synthesis of dopamine and its deamination into 3,4-dihydroxyphenylacetic acid (DOPAC) in rat kidney slices loaded with exogenous L-dihydroxyphenylalanine (L-DOPA). 2. alpha-hANP (3.3 and 330 nM) was found to produce a marked reduction (63-78% reduction) in the time-dependent accumulation of newly-formed dopamine and of its deaminated metabolite DOPAC in kidney slices loaded with 10 microM L-DOPA. alpha-hANP (330 nM) was also found to decrease the accumulation of newly-formed dopamine (45-66% reduction) and DOPAC (38-61% reduction) in experiments in which increasing concentrations (1-100 microM) of L-DOPA were used. This inhibitory effect was found to be potentiated by zaprinast (M&B 22,948; 10 microM), a guanosine cyclic 3',5'-monophosphate (cyclic GMP) phosphodiesterase inhibitor. Alone, zaprinast also decreased the accumulation of both dopamine (54-71% reduction) and DOPAC (73-92% reduction). 3. In kidney homogenates, alpha-hANP (330 nM) was found to affect neither the formation of dopamine nor its deamination to DOPAC. 4. Both alpha-hANP (330 nM) and zaprinast (10 microM) were found not to affect the formation of dopamine and DOPAC in kidney slices obtained from rats on a high salt diet during the previous 6 weeks. A similar situation was also found to occur when kidney slices obtained from 24-months old rats were used.5. The results obtained suggest that the inhibitory effect of alpha-hANP on the renal synthesis of dopamine is dependent on the activation of a membrane-operated mechanism, coupled to the enzyme guanylate cyclase, controlling the entry of L-DOPA into the cells.