PI-PLCbeta is involved in the modulation of the proximal tubule Na+-ATPase by angiotensin II

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.

Role of protein phosphatase 2A in kidney disease (Review)

Kidney disease affects millions of people worldwide and is a financial burden on the healthcare system. Protein phosphatase 2A (PP2A), which is involved in renal development and the function of ion-transport proteins, aquaporin-2 and podocytes, is likely to serve an important role in renal processes. PP2A is associated with the pathogenesis of a variety of different kidney diseases including podocyte injury, inflammation, tumors and chronic kidney disease. The current review aimed to discuss the structure and function of PP2A subunits in the context of kidney diseases. How dysregulation of PP2A in the kidneys causes podocyte death and the inactivation of PP2A in renal carcinoma tissues is discussed. Inhibition of PP2A activity prevents epithelial-mesenchymal transition and attenuates renal fibrosis, creating a favorable inflammatory microenvironment and promoting the initiation and progression of tumor pathogenesis. The current review also indicates that PP2A serves an important role in protection against renal inflammation. Understanding the detailed mechanisms of PP2A provides information that can be utilized in the design and application of novel therapeutics for the treatment and prevention of renal diseases.

Calcium receptor signaling and citrate transport

The calcium sensing receptor (CaSR) in the distal nephron decreases the propensity for calcium stones. Here we investigate if the apical CaSR in the proximal tubule also prevents stone formation acting via regulation of apical dicarboxylate and citrate transport. Urinary citrate, partially reabsorbed as a dicarboxylate in the proximal tubule lumen, inhibits stone formation by complexing calcium. We previously demonstrated a novel apical calcium-sensitive dicarboxylate transport system in OK proximal tubule cells. This calcium-sensitive process has the potential to modulate the amount of citrate available to complex increased urinary calcium. Using isotope labeled succinate uptake in OK cells along with various pharmacologic tools we examined whether the CaSR alters apical dicarboxylate transport and through which signal transduction pathways this occurs. Our results indicate that in the proximal tubule CaSR adjusts apical dicarboxylate transport, and does so via a CaSR → G_q → PKC signaling pathway. Thus, the CaSR may decrease the propensity for stone formation via actions in both proximal and distal nephron segments.

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.

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.

Renin-angiotensin system in the kidney: What is new?

The renin-angiotensin system (RAS) has been known for more than a century as a cascade that regulates body fluid balance and blood pressure. Angiotensin II(Ang II) has many functions in different tissues; however it is on the kidney that this peptide exerts its main functions. New enzymes, alternative routes for Ang IIformation or even active Ang II-derived peptides have now been described acting on Ang II AT₁ or AT₂ receptors, or in receptors which have recently been cloned, such as Mas and AT₄. Another interesting observation was that old members of the RAS, such as angiotensin converting enzyme (ACE), renin and prorenin, well known by its enzymatic activity, can also activate intracellular signaling pathways, acting as an outside-in signal transduction molecule or on the renin/(Pro)renin receptor. Moreover, the endocrine RAS, now is also known to have paracrine, autocrine and intracrine action on different tissues, expressing necessary components for local Ang II formation. This in situ formation, especially in the kidney, increases Ang II levels to regulate blood pressure and renal functions. These discoveries, such as the ACE2/Ang-(1-7)/Mas axis and its antangonistic effect rather than classical deleterious Ang II effects, improves the development of new drugs for treating hypertension and cardiovascular diseases.

ATP7B activity is stimulated by PKCɛ in porcine liver

Copper is necessary for all organisms since it acts as a cofactor in different enzymes, although toxic at high concentrations. ATP7B is one of two copper-transporting ATPases in humans, its vital role being manifested in Wilson disease due to a mutation in the gene that encodes this pump. Our objective has been to determine whether pathways involving protein kinase C (PKC) modulate ATP7B activity. Different isoforms of PKC (α, ɛ, ζ) were found in Golgi-enriched membrane fractions obtained from porcine liver. Cu(I)-ATPase activity was assessed in the presence of different activators and inhibitors of PKC signaling pathways. PMA (10(-8) M), a PKC activator, increased Cu(I)-ATPase activity by 60%, whereas calphostin C and U73122 (PKC and PLC inhibitors, respectively) decreased the activity by 40%. Addition of phosphatase λ decreased activity by 60%, irrespective of pre-incubation with PMA. No changes were detected with 2 μM Ca(2+), whereas PMA plus EGTA increased activity. This enhanced activity elicited by PMA decreased with a specific inhibitor of PKCɛ to levels comparable with those found after phosphatase λ treatment, showing that the ɛ isoform is essential for activation of the enzyme. This regulatory phosphorylation enhanced Vmax without modifying affinities for ATP and copper. It can be concluded that signaling pathways leading to DAG formation and PKCɛ activation stimulate the active transport of copper by ATP7B, thus evidencing a central role for this specific kinase-mediated mechanism in hepatic copper handling.

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.

Renal molecular mechanisms underlying altered Na+ handling and genesis of hypertension during adulthood in prenatally undernourished rats

In the present study, we investigated the development of hypertension in prenatally undernourished adult rats, including the mechanisms that culminate in dysfunctions of molecular signalling in the kidney. Dams were fed a low-protein multideficient diet throughout gestation with or without α-tocopherol during lactation. The time course of hypertension development followed in male offspring was correlated with alterations in proximal tubule Na+-ATPase activity, expression of angiotensin II (Ang II) receptors, and activity of protein kinases C and A. After the establishment of hypertension, Ang II levels, cyclo-oxygenase 2 (COX-2) and NADPH oxidase subunit expression, lipid peroxidation and macrophage infiltration were examined in renal tissue. Lipid peroxidation in undernourished rats, which was very intense at 60 d, decreased at 90 d and returned to control values by 150 d. During the prehypertensive phase, prenatally undernourished rats exhibited elevated renal Na+-ATPase activity, type 2 Ang II receptor down-regulation and altered protein kinase A:protein kinase C ratio. Stable late hypertension coexisted with highly elevated levels of Ang II-positive cells in the cortical tubulointerstitium, enhanced increase in the expression of p47phox (NADPH oxidase regulatory subunit), marked down-regulation of COX-2 expression, expanded plasma volume and decreased creatinine clearance. These alterations were reduced when the dams were given α-tocopherol during lactation. The offspring of well-nourished dams treated with α-tocopherol exhibited most of the alterations encountered in the offspring of undernourished dams not treated with α-tocopherol. Thus, alterations in proximal tubule Na+ transport, subcellular signalling pathways and reactive oxygen species handling in renal tissue underpin the development of hypertension.

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.

How the kidney is impacted by the perinatal maternal environment to develop hypertension

Environmental conditions during perinatal development such as maternal undernutrition, maternal glucocorticoids, placental insufficiency, and maternal sodium overload can program changes in renal Na(+) excretion leading to hypertension. Experimental studies indicate that fetal exposure to an adverse maternal environment may reduce glomerular filtration rate by decreasing the surface area of the glomerular capillaries. Moreover, fetal responses to environmental insults during early life that contribute to the development of hypertension may include increased expression of tubular apical or basolateral membrane Na(+) transporters and increased production of renal superoxide leading to enhanced Na(+) reabsorption. This review will address the role of these potential renal mechanisms in the fetal programming of hypertension in experimental models induced by maternal undernutrition, fetal exposure to glucocorticoids, placental insufficiency, and maternal sodium overload in the rat.

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.

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.

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.

Atrial natriuretic peptides and urodilatin modulate proximal tubule Na(+)-ATPase activity through activation of the NPR-A/cGMP/PKG pathway

The signaling pathway mediating modulation of Na(+)-ATPase of proximal tubule cells by atrial natriuretic peptides (ANP) and urodilatin through receptors located in luminal and basolateral membranes (BLM) is investigated. In isolated BLM, 10(-11)M ANP or 10(-11)M urodilatin inhibited the enzyme activity (50%). Immunodetection revealed the presence of NPR-A in BLM and LLC-PK1 cells. Both compounds increased protein kinase G (PKG) activity (80%) and this effect did not occur with 10(-6)M LY83583, a specific inhibitor of guanylyl cyclase. The inhibitory effect of these peptides on Na(+)-ATPase activity did not occur after addition of 10(-6)M KT5823, a specific inhibitor of PKG. LLC-PK1 cells were used to investigate if ANP and urodilatin change the activity of sodium pumps by luminal receptor interaction. ANP and urodilatin inhibited Na(+)-ATPase activity (50%), with maximal effect at 10(-10)M, similar to 10(-7)M db-cGMP, and did not occur with 10(-7)M LY83583, a guanylyl cyclase inhibitor. ANP and urodilatin specifically inhibit Na(+)-ATPase activity by activation of the cGMP/PKG pathway through NPR-A located in luminal membrane and BLM, increasing understanding of the mechanism of natriuretic peptides on renal sodium excretion, with proximal tubule Na(+)-ATPase one possible target.

Ceramide-activated protein kinases A and C zeta inhibit kidney proximal tubule cell Na(+)-ATPase

The basolateral membranes of kidney proximal tubule cells have (Na(+)+K(+))-ATPase and Na(+)-ATPase activities, involved in Na(+) reabsorption. We showed that ceramide (Cer) modulates protein kinase A (PKA) and protein kinase C (PKC), which are involved in regulating ion transporters. Here we show that ceramide, promotes 60% inhibition of Na(+)-ATPase activity (I(50) approximately 100nM). This effect was completely reversed by inhibiting PKA but did not involve the classic PKC signaling pathway. In these membranes we found the Cer-activated atypical PKC zeta (PKCzeta) isoform. When PKCzeta is inhibited, Cer ceases to inhibit the Na(+)-ATPase, allowing the cAMP/PKA signaling pathway to recover its stimulatory effect on the pump. There were no effects on the (Na(+)+K(+))-ATPase. These results reveal Cer as a potent physiological modulator of the Na(+)-ATPase, participating in a regulatory network in kidney cells and counteracting the stimulatory effect of PKA via PKCzeta.

PKA-mediated effect of MAS receptor in counteracting angiotensin II-stimulated renal Na+-ATPase

We showed previously that angiotensin-(1-7) [Ang-(1-7)] reversed stimulation of proximal tubule Na+-ATPase promoted by angiotensin II (Ang II) through a D-ala(7)-Ang-(1-7) (A779)-sensitive receptor. Here we investigated the signaling pathway coupled to this receptor. According to our data, Ang-(1-7) produces a MAS-mediated reversal of Ang II-stimulated Na+-ATPase by a Gs/PKA pathway because: (1) the Ang-(1-7) effect is reversed by GDPbetaS, an inhibitor of trimeric G protein and Gs polyclonal antibody. Cholera toxin, an activator of Gs protein, mimicked it; (2) in the presence of Ang II, Ang-(1-7) increased the PKA activity 10-fold; (3) the peptide inhibitor of PKA blocked the Ang-(1-7) effect on Ang II-stimulated Na+-ATPase; (4) Ang-(1-7) reverses the Ang II-stimulated PKC activity; (5) cAMP mimicked the Ang-(1-7) effect on the Ang II-stimulated Na+-ATPase. Our results provide new understanding about the signaling mechanisms coupled to MAS receptor-mediated renal Ang-(1-7) effects.

The stimulatory effect of angiotensin II on Na(+)-ATPase activity involves sequential activation of phospholipases and sustained PKC activity

Angiotensin II (Ang II) stimulates the proximal tubule Na(+)-ATPase through the AT(1) receptor/phosphoinositide phospholipase Cbeta (PI-PLCbeta)/protein kinase C (PKC) pathway. However, this pathway alone does not explain the sustained effect of Ang II on Na(+)-ATPase activity for 30 min. The aim of the present work was to elucidate the molecular mechanisms involved in the sustained effect of Ang II on Na(+)-ATPase activity. Ang II induced fast and correlated activation of Na(+)-ATPase and PKC activities with the maximal effect (115%) observed at 1 min and sustained for 30 min, indicating a pivotal role of PKC in the modulation of Na(+)-ATPase by Ang II. We observed that the sustained activation of PKC by Ang II depended on the sequential activation of phospholipase D and Ca(2+)-insensitive phospholipase A(2), forming phosphatidic acid and lysophosphatidic acid, respectively. The results indicate that PKC could be the final target and an integrator molecule of different signaling pathways triggered by Ang II, which could explain the sustained activation of Na(+)-ATPase by Ang II.

Inhibition of renal Na+-ATPase activity by inosine is mediated by A1 receptor-induced inhibition of the cAMP signaling pathway

We have previously demonstrated that adenosine is deaminated to inosine in the isolated basolateral membrane (BLM) of kidney proximal tubules. This work investigates the possible effect of inosine on proximal tubule Na(+)-ATPase activity. Inosine reduced Na(+)-ATPase activity by 70%. This effect of inosine was completely attenuated by 10(-8) M DPCPX, an A(1) receptor-selective antagonist, but it was not affected by either 10(-8) M DMPX or 10(-7) M MRS1523, A(2) and A(3) receptor-selective antagonists, respectively. The inhibitory effect of inosine was blocked by: (1) 10(-6) M GDPbetaS, a trimeric G protein inhibitor; (2) 1microg/ml pertussis toxin, a Gi protein inhibitor; (3) 10(-6) M forskolin, an adenylyl cyclase activator; (4) 10(-9) M cholera toxin, a Gs protein activator; (5) 10(-6)M cAMP. Our results demonstrate that the inhibitory effect of inosine on the sodium pump is mediated by the A(1) receptor/Gi/cAMP pathway.

Na(+)-ATPase in spontaneous hypertensive rats: possible AT(1) receptor target in the development of hypertension

Clinical and experimental data show an increase in sodium reabsorption on the proximal tubule (PT) in essential hypertension. It is well known that there is a link between essential hypertension and renal angiotensin II (Ang II). The present study was designed to examine ouabain-insensitive Na(+)-ATPase activity and its regulation by Ang II in spontaneously hypertensive rats (SHR). We observed that Na(+)-ATPase activity was enhanced in 14-week-old but not in 6-week-old SHR. The addition of Ang II from 10(-12) to 10(-6) mol/L decreased the enzyme activity in SHR to a level similar to that obtained in WKY. The Ang II inhibitory effect was completely reversed by a specific antagonist of AT(2) receptor, PD123319 (10(-8) mol/L) indicating that a system leading to activation of the enzyme in SHR is inhibited by AT(2)-mediated Ang II. Treatment of SHR with losartan for 10 weeks (weeks 4-14) prevents the increase in Na(+)-ATPase activity observed in 14-week-old SHR. These results indicate a correlation between AT(1) receptor activation in SHR and increased ouabain-insensitive Na(+)-ATPase activity. Our results open new possibilities towards our understanding of the pathophysiological mechanisms involved in the increased sodium reabsorption in PT found in essential hypertension.

Placental oxidative stress in malnourished rats and changes in kidney proximal tubule sodium ATPases in offspring

1. Intrauterine malnutrition has been linked to the development of adult cardiovascular and renal diseases, which are related to altered Na(+) balance. Here we investigated whether maternal malnutrition increases placental oxidative stress with subsequent impact on renal ATP-dependent Na(+) transporters in the offspring. 2. Maternal malnutrition was induced in rats during pregnancy by using a basic regional diet available in north-eastern Brazil. Placental oxidative stress was evaluated by measuring thiobarbituric acid-reactive substances, which were 35-40% higher in malnourished dams (MalN). Na(+) pumps were evaluated in control and prenatally malnourished rats (at 25 and 90 days of age). 3. Identical Na(+)/K(+)-ATPase activity was found in both groups at 25 days (approximately 150 nmol P(i)/mg per min). However, although Na(+)/K(+)-ATPase increased by 40% with growth in control rats, it remained constant in pups from MalN. 4. In juvenile rats, the activity of the ouabain-insensitive Na(+)-ATPase was higher in MalN than in controls (70 vs 25 nmol P(i)/mg per min). Nevertheless, activity did not increase with kidney and body growth: at 90 days, it was 50% lower in MalN than in controls. The maximal stimulation of the Na(+)-ATPase by angiotensin (Ang) II was 35% lower in MalN than in control rats and was attained only with a much higher concentration of the peptide (10(-10) mol/L) than in controls (10(-14) mol/L). 5. Protein kinase C activity, which mediates the effects of AngII on Na(+)-ATPase was only one-third of normal values in the MalN group. 6. These results indicate that placental oxidative stress may contribute to fetal undernutrition, which leads to later disturbances in Na(+) pumps from proximal tubule cells.

Crosstalk between the signaling pathways triggered by angiotensin II and adenosine in the renal proximal tubules: implications for modulation of Na(+)-ATPase activity

We have previously demonstrated that adenosine (Ado) reverses the stimulatory effect of angiotensin II (Ang II) on Na(+)-ATPase activity via the A(2A) receptor. In this work, the molecular mechanism involved in Ado-induced shutdown in the signaling pathway triggered by 10(-8)M Ang II was investigated. It was observed that: (1) both 10(-12)M PMA (a PKC activator) and 5x10(-8)M U73122 (an inhibitor of PI-PLCbeta) prevent the reversion effect induced by 10(-6)M Ado (only observed in the presence of 10(-6)M DPCPX (an A(1) receptor antagonist)) on Ang II-stimulated Na(+)-ATPase and PKC activities; (2) Ang II-stimulated PKC activity was reversed by 10(-6)M forskolin (an adenylyl cyclase activator) or 10(-8)M PKA inhibitory peptide and 10(-8)M DMPX (an A(2) receptor-selective antagonist). Considering that PMA prevents the inhibitory effect of Ado on Ang II-stimulated Na(+)-ATPase and PKC activities, it is likely that the PMA-induced effect, i.e. PKC activation, is downstream of the target for Ado-induced reversion of Ang II stimulation of Na(+)-ATPase activity. We investigated the hypothesis that PI-PLCbeta could be the target for Ado-induced PKA activation. Our data demonstrate that Ang II-stimulated PI-PLCbeta activity was reversed by Ado or 10(-7)M cAMP; the reversibility of the Ado-induced effect was prevented by either DMPX or PKA inhibitory peptide. These data demonstrate that Ado-induced PKA activation reduces Ang II-induced stimulation of PI-PLCbeta.

B2 receptor-mediated dual effect of bradykinin on proximal tubule Na+ -ATPase: sequential activation of the phosphoinositide-specific phospholipase Cbeta/protein kinase C and Ca2+ -independent phospholipase A2 pathways

In a previous paper we showed that bradykinin (BK), interacting with its B2 receptor, inhibits proximal tubule Na+ -ATPase activity but does not change (Na+ +K+)ATPase activity. The aim of this paper was to investigate the molecular mechanisms involved in B2-mediated modulation of proximal tubule Na+ -ATPase by BK. To abolish B1 receptor-mediated effects, all experiments were carried out in the presence of (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Leu), des-Arg9-[Leu8]-BK (DALBK), a specific antagonist of B1 receptor. A dual effect on the Na+ -ATPase activity through the B2 receptor was found: short incubation times (1-10 min) stimulate the enzyme activity; long incubation times (10-60 min) inhibit it. The stimulatory effect of BK is mediated by activation of phosphoinositide-specific phospholipase C beta (PI-PLCbeta)/protein kinase C (PKC); its inhibitory action is mediated by Ca2+ -independent phospholipase A2 (iPLA2). Prior activation of the PI-PLCbeta/PKC pathway is required to activate the iPLA2-mediated inhibitory phase. These results reveal a new mechanism by which BK can modulate renal sodium excretion: coupling between B2 receptor and activation of membrane-associated iPLA2.

The angiotensin receptor type 1-Gq protein-phosphatidyl inositol phospholipase Cbeta-protein kinase C pathway is involved in activation of proximal tubule Na+-ATPase activity by angiotensin(1-7) in pig kidneys

In a previous study, we observed that angiotensin(1-7) (Ang(1-7)) stimulates proximal tubule Na+-ATPase activity through the angiotensin receptor type 1 (AT1R). Here we aimed to study the signalling pathways involved. Our results show that the stimulatory effect of Ang(1-7) on Na+-ATPase activity through AT1R involves a Gq protein-phosphatidyl inositol-phospholipase Cbeta (PI-PLCbeta) pathway because: (1) the effect was reversed by GDPbetaS, a non-hydrolysable GDP analogue, and by a monoclonal Gq protein antibody; (2) the effect was similar and not additive to that of GTPgammaS, a non-hydrolysable GTP analogue; (3) Ang(1-7) induced a rapid decrease (30 s) in phosphatidylinositol 4,5-bisphosphate levels, a PI-PLCbeta substrate; and (4) U73122, a specific inhibitor of PI-PLCbeta, abolished Ang(1-7)-induced stimulation of Na+-ATPase activity. Angiotensin(1-7) increased the protein kinase C (PKC) activity similarly to phorbol-12-myristate-13-acetate (PMA), an activator of PKC. This effect was reversed by losartan, a specific antagonist of AT1R. The stimulatory effects of Ang(1-7) and PMA on Na+-ATPase activity are similar, non-additive and reversed by calphostin C, a specific inhibitor of PKC. A catalytic subunit of PKC (PKC-M) increased the Na+-ATPase activity. These data show that Ang(1-7) stimulates Na+-ATPase activity through the AT1R-Gq protein-PI-PLCbeta-PKC pathway. This effect is similar to that described for angiotensin II, showing for the first time that these compounds could have similar effects in the renal system.

Regulation of renal ouabain-resistant Na+-ATPase by leptin, nitric oxide, reactive oxygen species, and cyclic nucleotides: implications for obesity-associated hypertension

This study examined the effect of leptin on renal ouabain-resistant Na(+)-ATPase, which drives the reabsorption of about 10% of sodium transported in the proximal tubule. Chronic leptin administration (0.25 mg/kg s.c. twice daily for seven days) increased Na(+)-ATPase activity by 62.9%. This effect was prevented by the coadministration of superoxide dismutase mimetic, tempol, or the NADPH oxidase inhibitor, apocynin (2 mM in the drinking water). Acutely administered NO donors decreased Na(+)-ATPase activity. This effect was abolished by soluble guanylate cyclase inhibitor, ODQ, but not by protein kinase G inhibitors. Exogenous cGMP reduced Na(+)-ATPase activity, but its synthetic analogues, 8-bromo-cGMP and 8-pCPT-cGMP, were ineffective. The inhibitory effect of NO donors and cGMP was abolished by EHNA, an inhibitor of cGMP-stimulated phosphodiesterase (PDE2). Exogenous cAMP analogue and dibutyryl-cAMP increased Na(+)-ATPase activity and abolished the inhibitory effect of cGMP. Finally, the administration of superoxide-generating mixture (xanthine oxidase+hypoxanthine) increased Na(+)-ATPase activity. The results suggest that nitric oxide decreases renal Na(+)-ATPase activity by stimulating cGMP, which in turn activates PDE2 and decreases cAMP concentration. Increased production of reactive oxygen species may lead to the elevation of Na(+)-ATPase activity by scavenging NO and limiting its inhibitory effect. Chronic hyperleptinemia is associated with increased Na(+)-ATPase activity due to excessive oxidative stress.

Polyunsaturated fatty acids mobilize intracellular Ca2+in NT2 human teratocarcinoma cells by causing release of Ca2+from mitochondria

In a variety of disorders, overaccumulation of lipid in nonadipose tissues, including the heart, skeletal muscle, kidney, and liver, is associated with deterioration of normal organ function, and is accompanied by excessive plasma and cellular levels of free fatty acids (FA). Increased concentrations of FA may lead to defects in mitochondrial function found in diverse diseases. One of the most important regulators of mitochondrial function is mitochondrial Ca2+([Ca2+]m), which fluctuates in coordination with intracellular Ca2+([Ca2+]i). Polyunsaturated FA (PUFA) have been shown to cause [Ca2+]imobilization albeit by unknown mechanisms. We have found that PUFA but not monounsaturated or saturated FA cause [Ca2+]imobilization in NT2 human teratocarcinoma cells. Unlike the [Ca2+]iresponse to the muscarinic G protein-coupled receptor agonist carbachol, PUFA-mediated [Ca2+]imobilization in NT2 cells is independent of phospholipase C and inositol-1,4,5-trisphospate (IP3) receptor activation, as well as IP3-sensitive internal Ca2+stores. Furthermore, PUFA-mediated [Ca2+]imobilization is inhibited by the mitochondria uncoupler carboxyl cyanide m-chlorophenylhydrozone. Direct measurements of [Ca2+]mwith X-rhod-1 and45Ca2+indicate that PUFA induce Ca2+efflux from mitochondria. Further studies show that ruthenium red, an inhibitor of the mitochondrial Ca2+uniporter, blocks PUFA-induced Ca2+efflux from mitochondria, whereas inhibitors of the mitochondrial permeability transition pore cyclosporin A and bongkrekic acid have no effect. Thus PUFA-gated Ca2+release from mitochondria, possibly via the Ca2+uniporter, appears to be the underlying mechanism for PUFA-induced [Ca2+]imobilization in NT2 cells.