Isolation and cloning of the K+-independent, ouabain-insensitive Na+-ATPase

Reactive oxygen species impair Na+ transport and renal components of the renin-angiotensin-aldosterone system after paraquat poisoning

Paraquat (1,1'-dimethyl-4,4'-bipyridyl dichloride) is an herbicide widely used worldwide and officially banned in Brazil in 2020. Kidney lesions frequently occur, leading to acute kidney injury (AKI) due to exacerbated reactive O2 species (ROS) production. However, the consequences of ROS exposure on ionic transport and the regulator local renin-angiotensin-aldosterone system (RAAS) still need to be elucidated at a molecular level. This study evaluated how ROS acutely influences Na+-transporting ATPases and the renal RAAS. Adult male Wistar rats received paraquat (20 mg/kg; ip). After 24 h, we observed body weight loss and elevation of urinary flow and serum creatinine. In the renal cortex, paraquat increased ROS levels, NADPH oxidase and (Na++K+)ATPase activities, angiotensin II-type 1 receptors, tumor necrosis factor-α (TNF-α), and interleukin-6. In the medulla, paraquat increased ROS levels and NADPH oxidase activity but inhibited (Na++K+)ATPase. Paraquat induced opposite effects on the ouabain-resistant Na+-ATPase in the cortex (decrease) and medulla (increase). These alterations, except for increased serum creatinine and renal levels of TNF-α and interleukin-6, were prevented by 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (tempol; 1 mmol/L in drinking water), a stable antioxidant. In summary, after paraquat poisoning, ROS production culminated with impaired medullary function, urinary fluid loss, and disruption of Na+-transporting ATPases and angiotensin II signaling.

Angiotensin-(3–4) normalizes the elevated arterial blood pressure and abnormal Na+/energy handling associated with chronic undernutrition by counteracting the effects mediated by type 1 angiotensin II receptors

We investigated the mechanisms by which chronic administration of a multideficient diet after weaning alters bodily Na⁺ handling, and culminates in high systolic blood pressure (SBP) at a juvenile age. From 28 to 92 days of age, weaned male Wistar rats were given a diet with low content and poor-quality protein, and low lipid, without vitamin supplementation, which mimics the diets consumed in impoverished regions worldwide. We measured food, energy and Na⁺ ingestion, together with urinary Na⁺ excretion, Na⁺ density (Na⁺ intake/energy intake), plasma Na⁺ concentration, SBP, and renal proximal tubule Na⁺-transporting ATPases. Undernourished rats aged 92 days had only one-third of the control body mass, lower plasma albumin, higher SBP, higher energy intake, and higher positive Na⁺ balance accompanied by decreased plasma Na⁺ concentration. Losartan or Ang-(3–4) normalized SBP, and the combination of the 2 substances induced an accentuated negative Na⁺ balance as a result of strong inhibition of Na⁺ ingestion. Na⁺ density in undernourished rats was higher than in control, irrespective of the treatment, and they had downregulated (Na⁺+K⁺)ATPase and upregulated Na⁺-ATPase in proximal tubule cells, which returned to control levels after Losartan or Ang-(3–4). We conclude that Na⁺ density, not only Na⁺ ingestion, plays a central role in the pathophysiology of elevated SBP in chronically undernourished rats. The observations that Losartan and Ang-(3–4) normalized SBP together with negative Na⁺ balance give support to the proposal that Ang II⇒AT₁R and Ang II⇒AT₂R axes have opposite roles within the renin-angiotensin-aldosterone system of undernourished juvenile rats.

Ceramide-1-Phosphate as a Potential Regulator of the Second Sodium Pump from Kidney Proximal Tubules by Triggering Distinct Protein Kinase Pathways in a Hierarchic Way

Kidney proximal tubules are a key segment in the reabsorption of solutes and water from the glomerular ultrafiltrate, an essential process for maintaining homeostasis in body fluid compartments. The abundant content of Na⁺ in the extracellular fluid determines its importance in the regulation of extracellular fluid volume, which is particularly important for different physiological processes including blood pressure control. Basolateral membranes of proximal tubule cells have the classic Na⁺ + K⁺-ATPase and the ouabain-insensitive, K⁺-insensitive, and furosemide-sensitive Na⁺-ATPase, which participate in the active Na⁺ reabsorption. Here, we show that nanomolar concentrations of ceramide-1 phosphate (C1P), a bioactive sphingolipid derived in biological membranes from different metabolic pathways, promotes a strong inhibitory effect on the Na⁺-ATPase activity (C1P₅₀ ≈ 10 nM), leading to a 72% inhibition of the second sodium pump in the basolateral membranes. Ceramide-1-phosphate directly modulates protein kinase A and protein kinase C, which are known to be involved in the modulation of ion transporters including the renal Na⁺-ATPase. Conversely, we did not observe any effect on the Na⁺ + K⁺-ATPase even at a broad C1P concentration range. The significant effect of ceramide-1-phosphate revealed a new potent physiological and pathophysiological modulator for the Na⁺-ATPase, participating in the regulatory network involving glycero- and sphingolipids present in the basolateral membranes of kidney tubule cells.

Renal ischemia-reperfusion leads to hypertension and changes in proximal tubule Na+ transport and renin-angiotensin-aldosterone system: Role of NADPH oxidase

Acute renal injury (AKI) is a risk factor for the development of hypertension, which involves oxidative stress, changes in Na⁺ handling, and the intrarenal renin-angiotensin-aldosterone system (RAAS) as underlying mechanisms. We investigated in rats whether renal ischemia-reperfusion (IR) leads to changes in the proximal tubule ATP-dependent Na⁺ transport and the intrarenal content of RAAS components, as well as the role of NADPH oxidase. Rats weighing 300-350 g were submitted to AKI by bilateral IR (n = 25). After IR injury, the animals were followed up for 4 weeks. One part (n = 7) received daily treatment with the NADPH oxidase inhibitor apocynin (100 mg/kg, drinking water), while another part (n = 9) received apocynin 24 h before and after IR. One group was submitted to sham surgery (n = 8). Four weeks after IR, the rats presented elevated systolic blood pressure, as well as increased lipid peroxidation, NADPH oxidase activity, (Na⁺+K⁺)ATPase activity, and upregulation of type 1 angiotensin II receptor in the renal cortex. On the other hand, there was a decrease in Na⁺-ATPase activity and downregulation of the isoforms 1 and 2 of the angiotensin-converting enzyme, type 2 angiotensin II receptor, and of the α and ε isoforms of protein kinase C. Most of these alterations was prevented by both apocynin treatment protocols. Thus, we conclude that AKI-induced by IR may induce changes in proximal tubule ATPases and RAAS components compatible with renal Na⁺ retention and hypertension. These data also indicate that the NADPH oxidase represents a key factor in the origin of these alterations.

The Functioning of Na+-ATPases from Protozoan Parasites: Are These Pumps Targets for Antiparasitic Drugs?

The ENA ATPases (from exitus natru: the exit of sodium) belonging to the P-type ATPases are structurally very similar to the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA); they exchange Na+ for H+ and, therefore, are also known as Na+-ATPases. ENA ATPases are required in alkaline milieu, as in the case for Aspergillus, where other transporters cannot mediate an uphill Na+ efflux. They are also important for salt tolerance, as described for Arabidopsis. During their life cycles, protozoan parasites might encounter a high pH environment, thus allowing consideration of ENA ATPases as possible targets for controlling certain severe parasitic diseases, such as Chagas' Disease. Phylogenetic analysis has now shown that, besides the types IIA, IIB, IIC, and IID P-type ATPases, there exists a 5th subgroup of ATPases classified as ATP4-type ATPases, found in Plasmodium falciparum and Toxoplasma gondii. In malaria, for example, some drugs targeting PfATP4 destroy Na+ homeostasis; these drugs, which include spiroindolones, are now in clinical trials. The ENA P-type (IID P-type ATPase) and ATP4-type ATPases have no structural homologue in mammalian cells, appearing only in fungi, plants, and protozoan parasites, e.g., Trypanosoma cruzi, Leishmania sp., Toxoplasma gondii, and Plasmodium falciparum. This exclusivity makes Na+-ATPase a potential candidate for the biologically-based design of new therapeutic interventions; for this reason, Na+-ATPases deserves more attention.

Two sodium pumps in the hepatopancreas of the intertidal euryhaline crab Neohelice granulata: biochemical characteristics and differential modulation after feeding

No study has been done on the existence, biochemical characteristics, and modulation of K+-independent ouabain-insensitive Na+ ATPase activity (the second sodium pump) in the digestive tract of intertidal euryhaline crabs and moreover on the coexistence and modulation under distinct physiological and (or) environmental conditions of different sodium pumps. We determined the occurrence, characteristics, and responses at different times (0, 1, 24, 48, and 120 h) after feeding upon distinct salinities of Na+ ATPase activity and Na+/K+ ATPase in the hepatopancreas of Neohelice granulata (Dana, 1851), which is a model species. The stimulation by Na+ under total inhibition of Na+/K+ ATPase activity revealed the occurrence of Na+ ATPase activity that was totally inhibited by 2 mmol·L–1 furosemide, exhibits Michaelis–Menten kinetics for ATP (apparent Km = 0.52 ± 0.16 mmol·L–1), and highest activity at around pH 7.4. In crabs acclimated to 35 psu (osmoconforming conditions), Na+ ATPase activity was highly increased (about 15-fold) (532 ± 58 nmol Pi·mg protein−1·min−1) in the hepatopancreas 48 h after feeding. In 10 psu (hyper-regulating conditions), Na+ ATPase activity decreased in the hepatopancreas 24 h after feeding (7 ± 9 nmol Pi·mg protein−1·min−1) and recovered initial values after 48 h (24 ± 35 nmol Pi·mg protein−1·min−1). Unlike Na+ ATPase, Na+/K+ ATPase activity did not change after feeding at any salinity, suggesting the specific modulation of the second sodium pump and its role in postprandial adjustments in the hepatopancreas.

Protective outcomes of low-dose doxycycline on renal function of Wistar rats subjected to acute ischemia/reperfusion injury

Renal ischemia-reperfusion injury (IRI) is a major cause of acute renal failure. Doxycycline (Dc) belongs to the tetracycline-class of antibiotics with demonstrated beneficial molecular effects in the brain and heart, mainly through matrix metalloproteinases inhibition (MMP). However, Dc protection of renal function has not been demonstrated. We determined whether low doses of Dc would prevent decreases in glomerular filtration rate (GFR) and maintain tubular Na⁺ handling in Wistar rats subjected to kidney I/R. Male Wistar rats underwent bilateral kidney ischemia for 30min followed by 24h reperfusion (I/R). Doxycycline (1, 3, and 10mg/kg, i.p.) was administered 2h before surgery. Untreated I/R rats showed a 250% increase in urine volume and proteinuria, a 60% reduction in GFR, accumulation of urea-nitrogen in the blood, and a 60% decrease in the fractional Na⁺ excretion due to unbalanced Na⁺ transporter activity. Treatment with Dc 3mg/kg maintained control levels of urine volume, proteinuria, GFR, blood urea-nitrogen, fractional Na⁺ excretion, and equilibrated Na⁺ transporter activities. The Dc protection effects on renal function were associated with kidney structure preservation and prevention of TGFβ and fibronectin deposition. In vitro, total MMP activity was augmented in I/R and inhibited by 25 and 50μM Dc. In vivo, I/R augmented MMP-2 and -9 protein content without changing their activities. Doxycycline treatment downregulated total MMP activity and MMP-2 and -9 protein content. Our results suggest that treatment with low dose Dc protects from IRI, thereby preserving kidney function.

Vasorelaxation of goat mesenteric artery is mediated by endothelial Na(+)-K(+)-ATPase

OBJECTIVE

To examine the role of Na(+)-K(+)-ATPase and K(+) channels in mediating vasorelaxation in the superior mesenteric artery of Capra hircus.

MATERIALS AND METHODS

Goat superior mesenteric artery (GSMA) was cut into 1.5-2 mm circular rings and mounted in a thermostatically controlled (37°C ± 0.5°C) organ bath containing 20 ml of modified Krebs-Henseleit saline (MKHS) (pH 7.4), with continuous aeration under 1.5 g tension for 90 min. Endothelium-intact (ED+) or endothelium-denuded (ED-) GSMA ring was contracted with phenylephrine (PE) or 5-hydroxytryptamine (5-HT) (1 μM-0.1 mM) in the absence or presence of ouabain (0.1 μM). KCl (1 μM-10 mM) was added cumulatively to K(+)-free MKHS-pre-contracted (ED+/-) rings in the absence or presence of ouabain (0.1 μM) or barium (1 μM) or 4-aminopyridine (1 μM).

RESULTS

Ouabain did not alter the basal tone of the arterial ring. The contractile response induced by PE (Emax: 50.46 ± 2.68, pD2: 5.53 ± 0.04) and 5-HT (Emax: 30.86 ± 1.33, pD2: 6.17 ± 0.03) in ED+ ring was significantly (P < 0.001) augmented in ED- rings (PE: Emax: 93.30 ± 2.11, pD2: 6.41 ± 0.04; 5-HT: Emax: 95.07 ± 0.99, pD2: 6.27 ± 0.03). The contractile response induced by PE and 5-HT in ED+ or ED- rings in the presence of ouabain was almost identical with that of ED- rings. Vasorelaxation of KCl (Emax: 2.90 ± 1.14, pD2: 3.9 ± 0.03) was significantly attenuated in the presence of ouabain (Emax: 73.8 ± 5.16, pD2: 4.3 ± 0.04), Ba(2+) (Emax: 16.34 ± 4.7, pD2: 3.22 ± 0.02), 4-AP (Emax: 18.16 ± 2.4, pD2: 3.68 ± 0.03), ouabain and Ba(2+) (Emax: 70.09 ± 3.66, pD2: 4.41 ± 0.04), and ouabain and 4-AP (Emax: 66.98 ± 4.61, pD2: 4.13 ± 0.06).

CONCLUSION

The vasorelaxation in GSMA is mediated by the endothelium-derived hyperpolarizing factor (EDHFs) such as ouabain-sensitive Na(+)-K(+)-ATPase, KIR and Kv channels.

Altered signaling pathways linked to angiotensin II underpin the upregulation of renal Na(+)-ATPase in chronically undernourished rats

This study has investigated the participation of altered signaling linked to angiotensin II (Ang II) that could be associated with increased Na(+) reabsorption in renal proximal tubules during chronic undernutrition. A multideficient chow for rats (basic regional diet, BRD) was used, which mimics several human diets widely taken in developing countries. The Vmax of the ouabain-resistant Na(+)-ATPase resident in the basolateral membranes increased >3-fold (P<0.001) accompanied by an increase in Na(+) affinity from 4.0 to 0.2mM (P<0.001). BRD rats had a >3-fold acceleration of the formation of phosphorylated intermediates in the early stage of the catalytic cycle (in the E1 conformation) (P<0.001). Immunostaining showed a huge increase in Ang II-positive cells in the cortical tubulointerstitium neighboring the basolateral membranes (>6-fold, P<0.001). PKC isoforms (α, ε, λ, ζ), Ang II type 1 receptors and PP2A were upregulated in BRD rats (in %): 55 (P<0.001); 35 (P<0.01); 125, 55, 11 and 30 (P<0.001). PKA was downregulated by 55% (P<0.001). With NetPhosK 1.0 and NetPhos 2.0, we detected 4 high-score (>0.70) regulatory phosphorylation sites for PKC and 1 for PKA in the primary sequence of the Na(+)-ATPase α-subunit, which are located in domains that are key for Na(+) binding and catalysis. Therefore, chronic undernutrition stimulates tubulointerstitial activity of Ang II and impairs PKC- and PKA-mediated regulatory phosphorylation, which culminates in an exaggerated Na(+) reabsorption across the proximal tubular epithelium.

Mechanisms involving Ang II and MAPK/ERK1/2 signaling pathways underlie cardiac and renal alterations during chronic undernutrition

Background

Several studies have correlated protein restriction associated with other nutritional deficiencies with the development of cardiovascular and renal diseases. The driving hypothesis for this study was that Ang II signaling pathways in the heart and kidney are affected by chronic protein, mineral and vitamin restriction.

Methodology/Principal Findings

Wistar rats aged 90 days were fed from weaning with either a control or a deficient diet that mimics those used in impoverished regions worldwide. Such restriction simultaneously increased ouabain-insensitive Na⁺-ATPase and decreased (Na⁺+K⁺)ATPase activity in the same proportion in cardiomyocytes and proximal tubule cells. Type 1 angiotensin II receptor (AT₁R) was downregulated by that restriction in both organs, whereas AT₂R decreased only in the kidney. The PKC/PKA ratio increased in both tissues and returned to normal values in rats receiving Losartan daily from weaning. Inhibition of the MAPK pathway restored Na⁺-ATPase activity in both organs. The undernourished rats presented expanded plasma volume, increased heart rate, cardiac hypertrophy, and elevated systolic pressure, which also returned to control levels with Losartan. Such restriction led to electrical cardiac remodeling represented by prolonged ventricular repolarization parameters, induced triggered activity, early after-depolarization and delayed after-depolarization, which were also prevented by Losartan.

Conclusion/Significance

The mechanisms responsible for these alterations are underpinned by an imbalance in the PKC- and PKA-mediated pathways, with participation of angiotensin receptors and by activation of the MAPK/ERK1/2 pathway. These cellular and molecular alterations culminate in cardiac electric remodeling and in the onset of hypertension in adulthood.

ANG-(3-4) inhibits renal Na+-ATPase in hypertensive rats through a mechanism that involves dissociation of ANG II receptors, heterodimers, and PKA

The physiological roles of ANG-(3-4) (Val-Tyr), a potent ANG II-derived peptide, remain largely unknown. The present study 1)investigates whether ANG-(3-4) modulates ouabain-resistant Na(+)-ATPase resident in proximal tubule cells and 2) verifies whether its possible action on pumping activity, considered the fine tuner of Na(+) reabsorption in this nephron segment, depends on blood pressure. ANG-(3-4) inhibited Na(+)-ATPase activity in membranes of spontaneously hypertensive rats (SHR) at nanomolar concentrations, with no effect in Wistar-Kyoto (WKY) rats or on Na(+)-K(+)-ATPase. PD123319 (10(-7) M) and PKA(5-24) (10(-6) M), an AT2 receptor (AT2R) antagonist and a specific PKA inhibitor, respectively, abrogated this inhibition, indicating that AT2R and PKA are central in this pathway. Despite the lack of effect of ANG-(3-4) when assayed alone in WKY rats, the peptide (10(-8) M) completely blocked stimulation of Na(+)-ATPase induced by 10(-10) M ANG II in normotensive rats through a mechanism that also involves AT2R and PKA. Tubular membranes from WKY rats had higher levels of AT2R/AT1R heterodimers, which remain associated in 10(-10) M ANG II and dissociate to a very low dimerization state upon addition of 10(-8) M ANG-(3-4). This lower level of heterodimers was that found in SHR, and heterodimers did not dissociate when the same concentration of ANG-(3-4) was present. Oral administration of ANG-(3-4) (50 mg/kg body mass) increased urinary Na(+) concentration and urinary Na(+) excretion with a simultaneous decrease in systolic arterial pressure in SHR, but not in WKY rats. Thus the influence of ANG-(3-4) on Na(+) transport and its hypotensive action depend on receptor association and on blood pressure.

Inorganic phosphate uptake in Trypanosoma cruzi is coupled to K(+) cycling and to active Na(+) extrusion

BACKGROUND

Orthophosphate (Pi) is a central compound in the metabolism of all organisms, including parasites. There are no reports regarding the mechanisms of Pi acquisition by Trypanosoma cruzi.

METHODS

(32)Pi influx was measured in T. cruzi epimastigotes. The expression of Pi transporter genes and the coupling of the uptake to Na(+), H(+) and K(+) fluxes were also investigated. The transport capacities of different evolutive forms were compared.

RESULTS

Epimastigotes grew significantly more slowly in 2mM than in 50mM Pi. Influx of Pi into parasites grown under low Pi conditions took place in the absence and presence of Na(+). We found that the parasites express TcPho84, a H(+):Pi-symporter, and TcPho89, a Na(+):Pi-symporter. Both Pi influx mechanisms showed Michaelis-Menten kinetics, with a one-order of magnitude higher affinity for the Na(+)-dependent system. Collapsing the membrane potential with carbonylcyanide-p-trifluoromethoxyphenylhydrazone strongly impaired the influx of Pi. Valinomycin (K(+) ionophore) or SCH28028 (inhibitor of (H(+)+K(+))ATPase) significantly inhibited Pi uptake, indicating that an inwardly-directed H(+) gradient energizes uphill Pi entry and that K(+) recycling plays a key role in Pi influx. Furosemide, an inhibitor of the ouabain-insensitive Na(+)-ATPase, decreased only the Na(+)-dependent Pi uptake, indicating that this Na(+) pump generates the Na(+) gradient utilized by the symporter. Trypomastigote forms take up Pi inefficiently.

CONCLUSIONS

Pi starvation stimulates membrane potential-sensitive Pi uptake through different pathways coupled to Na(+) or H(+)/K(+) fluxes.

GENERAL SIGNIFICANCE

This study unravels the mechanisms of Pi acquisition by T. cruzi, a key process in epimastigote development and differentiation to trypomastigote forms.

Is the second sodium pump electrogenic?

Transepithelial sodium transport is a process that involves active Na⁺ transport at the basolateral membrane of the epithelial cell. This process is mediated by the Na⁺/K⁺ pump, which exchanges 3 internal Na⁺ by 2 external K⁺ inducing a net charge movement and the second Na⁺ pump, which transports Na⁺ accompanied by Cl⁻ and water. It has been suggested that this pump could also be electrogenic. Herein, we evaluated, in MDCK cells, the short-circuit current (I_sc) generated by these Na⁺ pumps at the basolateral membrane of the epithelial cells, using amphotericin B as an apical permeabilizing agent. In Cl⁻-containing media, I_sc induced by amphotericin B is totally inhibited by ouabain, indicating that only the electrogenic Na⁺/K⁺ pump is detectable in the presence of Cl⁻. Electrogenicity of the second Na⁺ pump can be demonstrated in Cl⁻-free media. The existence of a furosemide-sensitive component of I_sc, in addition to an ouabain-sensitive one, was identified in absence of chloride. Passive Cl⁻ movement associated with the function of the second Na⁺ pump seems to be regulated by the pump itself. These results demonstrate that the second Na⁺ pump is an electroneutral mechanism result from the stoichiometric movement of Na⁺ and Cl⁻ across the basolateral plasma membrane of the epithelial cell.

In silico identification and characterization of the ion transport specificity for P-type ATPases in the Mycobacterium tuberculosis complex

Background

P-type ATPases hydrolyze ATP and release energy that is used in the transport of ions against electrochemical gradients across plasma membranes, making these proteins essential for cell viability. Currently, the distribution and function of these ion transporters in mycobacteria are poorly understood.

Results

In this study, probabilistic profiles were constructed based on hidden Markov models to identify and classify P-type ATPases in the Mycobacterium tuberculosis complex (MTBC) according to the type of ion transported across the plasma membrane. Topology, hydrophobicity profiles and conserved motifs were analyzed to correlate amino acid sequences of P-type ATPases and ion transport specificity. Twelve candidate P-type ATPases annotated in the M. tuberculosis H37Rv proteome were identified in all members of the MTBC, and probabilistic profiles classified them into one of the following three groups: heavy metal cation transporters, alkaline and alkaline earth metal cation transporters, and the beta subunit of a prokaryotic potassium pump. Interestingly, counterparts of the non-catalytic beta subunits of Hydrogen/Potassium and Sodium/Potassium P-type ATPases were not found.

Conclusions

The high content of heavy metal transporters found in the MTBC suggests that they could play an important role in the ability of M. tuberculosis to survive inside macrophages, where tubercle bacilli face high levels of toxic metals. Finally, the results obtained in this work provide a starting point for experimental studies that may elucidate the ion specificity of the MTBC P-type ATPases and their role in mycobacterial infections.

The Drosophila NKCC Ncc69 is required for normal renal tubule function

Epithelial ion transport is essential to renal homeostatic function, and it is dysregulated in several diseases, such as hypertension. An understanding of the insect renal (Malpighian) tubule yields insights into conserved epithelial ion transport processes in higher organisms and also has implications for the control of insect infectious disease vectors. Here, we examine the role of the Na(+)-K(+)-2Cl(-) (NKCC) cotransporter Ncc69 in Drosophila tubule function. Ncc69 mutant tubules have decreased rates of fluid secretion and K(+) flux, and these phenotypes were rescued by expression of wild-type Ncc69 in the principal cells of the tubule. Na(+) flux was unaltered in Ncc69 mutants, suggesting Na(+) recycling across the basolateral membrane. In unstimulated tubules, the principal role of the Na(+)-K(+)-ATPase is to generate a favorable electrochemical gradient for Ncc69 activity: while the Na(+)-K(+)-ATPase inhibitor ouabain decreased K(+) flux in wild-type tubules, it had no effect in Ncc69 mutant tubules. However, in the presence of cAMP, which stimulates diuresis, additional Na(+)-K(+)-ATPase-dependent K(+) transport pathways are recruited. In studying the effects of capa-1 on wild-type and Ncc69 mutant tubules, we found a novel antidiuretic role for this hormone that is dependent on intact Ncc69, as it was abolished in Ncc69 mutant tubules. Thus, Ncc69 plays an important role in transepithelial ion and fluid transport in the fly renal tubule and is a target for regulation in antidiuretic states.

Perinatal Na+ overload programs raised renal proximal Na+ transport and enalapril-sensitive alterations of Ang II signaling pathways during adulthood

BACKGROUND

High Na(+) intake is a reality in nowadays and is frequently accompanied by renal and cardiovascular alterations. In this study, renal mechanisms underlying perinatal Na(+) overload-programmed alterations in Na(+) transporters and the renin/angiotensin system (RAS) were investigated, together with effects of short-term treatment with enalapril in terms of reprogramming molecular alterations in kidney.

METHODOLOGY/PRINCIPAL FINDINGS

Male adult Wistar rats were obtained from dams maintained throughout pregnancy and lactation on a standard diet and drinking water (control) or 0.17 M NaCl (saline group). Enalapril (100 mg/l), an angiotensin converting enzyme inhibitor, was administered for three weeks after weaning. Ninety day old offspring from dams that drank saline presented with proximal tubules exhibiting increased (Na(+)+K(+))ATPase expression and activity. Ouabain-insensitive Na(+)-ATPase activity remained unchanged but its response to angiotensin II (Ang II) was lost. PKC, PKA, renal thiobarbituric acid reactive substances (TBARS), macrophage infiltration and collagen deposition markedly increased, and AT(2) receptor expression decreased while AT(1) expression was unaltered. Early treatment with enalapril reduced expression and activity of (Na(+)+K(+))ATPase, partially recovered the response of Na(+)-ATPase to Ang II, and reduced PKC and PKA activities independently of whether offspring were exposed to high perinatal Na(+) or not. In addition, treatment with enalapril per se reduced AT(2) receptor expression, and increased TBARS, macrophage infiltration and collagen deposition. The perinatally Na(+)-overloaded offspring presented high numbers of Ang II-positive cortical cells, and significantly lower circulating Ang I, indicating that programming/reprogramming impacted systemic and local RAS.

CONCLUSIONS/SIGNIFICANCE

Maternal Na(+) overload programmed alterations in renal Na(+) transporters and in its regulation, as well as severe structural lesions in adult offspring. Enalapril was beneficial predominantly through its influence on Na(+) pumping activities in adult offspring. However, side effects including down-regulation of PKA, PKC and AT(2) receptors and increased TBARS could impair renal function in later life.

Overexpression of HGF transgene attenuates renal inflammatory mediators, Na(+)-ATPase activity and hypertension in spontaneously hypertensive rats

Renal inflammation and oxidative stress are constantly present in experimental hypertension. Since the spontaneously hypertensive rat (SHR) has reduced levels of hepatocyte growth factor (HGF), which suppresses the activation of the proinflammatory nuclear transcription factor kappa B (NF-κB), we speculated that HGF deficiency could play a key role in the pathogenesis of hypertension in the SHR. To test this hypothesis we increased HGF in the SHR by HGF gene delivery. We found that kidneys of 15-week-old SHR had an important reduction in HGF mRNA and protein expression. Adult SHRs were randomly assigned to receive weekly hydrodynamic injection (1mg/kg) of a naked plasmid containing human HGF (hHGF) gene associated with a cytomegalovirus promoter (pCMV-HGF) or empty vector (pcDNA3.1) during 6weeks. WKY rats treated with pcDNA3.1 and pCMV-HGF served as controls. The kidneys in the hypertensive SHR untreated and treated with the empty vector had increased NF-κB activation, elevated mRNA and protein expression of RANTES, MCP-1 and IL-6 and increased oxidative stress. Activity of Na(+)-ATPase was increased while activity of Na(+), K(+)-ATPase was normal. hHGF gene therapy normalized renal NF-κB activity, proinflammatory cytokines, antioxidant status (GSH, SOD and CAT), Na(+)-ATPase activity, reduced renal injury and ameliorated hypertension. Our results suggest that reduction in HGF production plays a major role in the pathogenesis of hypertension in the SHR and increasing HGF is a potential therapeutic target in the treatment of hypertension.

The second sodium pump: from the function to the gene

Transepithelial Na(+) transport is mediated by passive Na(+) entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na(+)/K(+) pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na(+)/K(+)-ATPase and the ouabain-insensitive, furosemide-inhibitable Na(+)-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na(+)-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na(+)- and Na(+)/K(+)-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na(+)-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na(+)-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na(+)-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na(+) transport and cellular Na(+) homeostasis leads us to reconsider its role in health and disease.

Angiotensin-(3-4) counteracts the Angiotensin II inhibitory action on renal Ca2+-ATPase through a cAMP/PKA pathway

We recently demonstrated that Angiotensin-(3-4) [Ang-(3-4)], an Ang II-derived dipeptide, overcomes inhibition of plasma membrane Ca(2+)-ATPase promoted by nanomolar concentrations of Ang II in basolateral membranes of renal proximal tubule cells, with involvement of a so far unknown AT(2)R-dependent and NO-independent mechanism. The present study investigates the signaling pathway triggered by Ang-(3-4) that is responsible for counteracting the inhibitory effect of Ang II, and attempts to elucidate the functional interaction of the dipeptide with Ang II at the level of AT(2)R. Stimulation by cholera toxin of G(s)α protein structurally linked to AT(2)R--as revealed by their co-immunoprecipitation--mimicked the effect of Ang-(3-4) on Ca(2+)-ATPase activity. Furthermore, addition of dibutyril-cAMP (db-cAMP) mimicked Ang-(3-4), whereas the specific PKA inhibitor, PKAi(5-24) peptide, suppressed the counter-regulatory effect of Ang-(3-4) and the AT(2)R agonist, CGP42112A. Membrane-associated PKA activity was stimulated by Ang-(3-4) or CGP42112A to comparable levels as db-cAMP, and the Ang-(3-4) effect was abrogated by the AT(2)R antagonist PD123319, whereas the AT(1)R antagonist Losartan had no effect. Ang-(3-4) stimulated PKA-mediated phosphorylation of Ca(2+)-ATPase and activated PKA to comparable levels. Binding assays demonstrated that Ang-(3-4) could not displace (3)H-Ang II from HEK 293T cells expressing AT(2)R, but 10(-10) mol/L Ang-(3-4) resulted in the appearance of a probable higher-affinity site (picomolar range) for Ang II. The results presented herein demonstrate that Ang-(3-4), acting as an allosteric enhancer, suppresses Ang II-mediated inhibition of Ca(2+)-ATPase through an AT(2)R/cAMP/PKA pathway, after inducing conformational changes in AT(2)R that results in generation of higher-affinity sites for Ang II.

Recent experiments towards a model for fluid secretion in Rhodnius Upper Malpighian Tubules (UMT)

Three different methods have been used to improve a model for fluid secretion in Upper Malpighian Tubules (UMT) of the blood sucking insect Rhodnius prolixus. (I) In the first, UMT double perfusions in 5th instar Rhodnius were used to measure their fluid secretion rate. They were stimulated to secrete with 5-HT. Double perfusions allowed access separately to the basolateral and the apical cell membranes with pharmacological agents known to block different ion transport functions, namely ATPases, cotransporters and/or countertransporters and ion and water channels: ouabain, bafilomycin A1, furosemide, bumetanide, SITS, acetazolamide, amiloride, DPC, BaCl(2), pCMBS and DTT. The basic assumption is that changes in water movement reflect changes in ion transport mechanisms. (II) Intracellular Na(+) concentrations were measured with a fluorometric method in dissected R. prolixus UMT, under several experimental conditions. (III) ATPase activities were measured in R. prolixus UMT. A tentative model for the function of the UMT cell is presented. We find that (a) at the basolateral cell membrane, fundamental is a Na(+)-K(+)-2Cl(-) cotransporter; of intermediate importance are the Na(+)-K(+)-ATPase and a ouabain-insensitive Na(+)-ATPase, ion channels and Rp-MIP water channels. (b) At the apical cell membrane, most important are a V-H(+)-ATPase; and a K(+) and/or Na(+)-H(+) exchanger.

Metabolic programming during lactation stimulates renal Na+ transport in the adult offspring due to an early impact on local angiotensin II pathways

Background

Several studies have correlated perinatal malnutrition with diseases in adulthood, giving support to the programming hypothesis. In this study, the effects of maternal undernutrition during lactation on renal Na⁺-transporters and on the local angiotensin II (Ang II) signaling cascade in rats were investigated.

Methodology/Principal Findings

Female rats received a hypoproteic diet (8% protein) throughout lactation. Control and programmed offspring consumed a diet containing 20% protein after weaning. Programming caused a decrease in the number of nephrons (35%), in the area of the Bowman's capsule (30%) and the capillary tuft (30%), and increased collagen deposition in the cortex and medulla (by 175% and 700%, respectively). In programmed rats the expression of (Na⁺+K⁺)ATPase in proximal tubules increased by 40%, but its activity was doubled owing to a threefold increase in affinity for K⁺. Programming doubled the ouabain-insensitive Na⁺-ATPase activity with loss of its physiological response to Ang II, increased the expression of AT₁ and decreased the expression of AT₂ receptors), and caused a pronounced inhibition (90%) of protein kinase C activity with decrease in the expression of the α (24%) and ε (13%) isoforms. Activity and expression of cyclic AMP-dependent protein kinase decreased in the same proportion as the AT₂ receptors (30%). In vivo studies at 60 days revealed an increased glomerular filtration rate (GFR) (70%), increased Na⁺ excretion (80%) and intense proteinuria (increase of 400% in protein excretion). Programmed rats, which had normal arterial pressure at 60 days, became hypertensive by 150 days.

Conclusions/Significance

Maternal protein restriction during lactation results in alterations in GFR, renal Na⁺ handling and in components of the Ang II-linked regulatory pathway of renal Na⁺ reabsorption. At the molecular level, they provide a framework for understanding how metabolic programming of renal mechanisms contributes to the onset of hypertension in adulthood.