Regulation of the renal proximal tubule second sodium pump by angiotensins

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 Handling of Albumin-From Early Findings to Current Concepts

Albumin is the main protein of blood plasma, lymph, cerebrospinal and interstitial fluid. The protein participates in a variety of important biological functions, such as maintenance of proper colloidal osmotic pressure, transport of important metabolites and antioxidant action. Synthesis of albumin takes place mainly in the liver, and its catabolism occurs mostly in vascular endothelium of muscle, skin and liver, as well as in the kidney tubular epithelium. Long-lasting investigation in this area has delineated the principal route of its catabolism involving glomerular filtration, tubular endocytic uptake via the multiligand scavenger receptor tandem—megalin and cubilin-amnionless complex, as well as lysosomal degradation to amino acids. However, the research of the last few decades indicates that also additional mechanisms may operate in this process to some extent. Direct uptake of albumin in glomerular podocytes via receptor for crystallizable region of immunoglobulins (neonatal FC receptor) was demonstrated. Additionally, luminal recycling of short peptides into the bloodstream and/or back into tubular lumen or transcytosis of whole molecules was suggested. The article discusses the molecular aspects of these processes and presents the major findings and controversies arising in the light of the research concerning the last decade. Their better characterization is essential for further research into pathophysiology of proteinuric renal failure and development of effective therapeutic strategies.

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.

The nucleobase adenine as a signalling molecule in the kidney

In 2002, the first receptor activated by the nucleobase adenine was discovered in rats. In the past years, two adenine receptors (AdeRs) in mice and one in Chinese hamsters, all of which belong to the family of G protein-coupled receptors (GPCRs), were cloned and pharmacologically characterized. Based on the nomenclature for other purinergic receptor families (P1 for adenosine receptors and P2 for nucleotide, e.g. ATP, receptors), AdeRs were designated P0 receptors. Pharmacological data indicate the existence of G protein-coupled AdeRs in pigs and humans as well; however, those have not been cloned so far. Current data suggest a role for adenine and AdeRs in renal proximal tubules. Furthermore, AdeRs are suggested to be functional counterplayers of vasopressin in the collecting duct system, thus exerting diuretic effects. We are only at the beginning of understanding the significance of this new class of purinergic receptors, which might become future drug targets.

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.

Cellular localization of adenine receptors in the rat kidney and their functional significance in the inner medullary collecting duct

The Gi-coupled adenine receptor (AdeR) binds adenine with high affinity and potentially reduces cellular cAMP levels. Since cAMP is an important second messenger in the renal transport of water and solutes, we localized AdeR in the rat kidney. Real-time RT-PCR showed higher relative expression of AdeR mRNA in the cortex and outer medulla compared with the inner medulla. Immunoblots using a peptide-derived and affinity-purified rabbit polyclonal antibody specific for an 18-amino acid COOH-terminal sequence of rat AdeR, which we generated, detected two bands between ∼30 and 40 kDa (molecular mass of native protein: 37 kDa) in the cortex, outer medulla, and inner medulla. These bands were ablated by preadsorption of the antibody with the immunizing peptide. Immunofluorescence labeling showed expression of AdeR protein in all regions of the kidney. Immunoperoxidase revealed strong labeling of AdeR protein in the cortical vasculature, including the glomerular arterioles, and less intense labeling in the cells of the collecting duct system. Confocal immunofluorescence imaging colocalized AdeR with aquaporin-2 protein to the apical plasma membrane in the collecting duct. Functionally, adenine (10 μM) significantly decreased (P < 0.01) 1-deamino-8-d-arginine vasopressin (10 nM)-induced cAMP production in ex vivo preparations of inner medullary collecting ducts, which was reversed by PSB-08162 (20 μM, P < 0.01), a selective antagonist of AdeR. Thus, we demonstrated the expression of AdeR in the renal vasculature and collecting ducts and its functional relevance. This study may open a new avenue for the exploration of autocrine/paracrine regulation of renal vascular and tubular functions by the nucleobase adenine in health and disease.

Guanine-induced inhibition of renal Na(+)-ATPase activity: evidence for the involvement of the Gi protein-coupled receptor

There is some evidence to show a possible role of guanosine in the modulation of cellular function, in particular, in the neuronal system. However, nothing is known about the role of guanine in renal function. The aim of the present work was to investigate the role of guanine on modulation of Na+-ATPase activity in isolated basolateral membrane (BLM) of the renal cortex. Guanine inhibited the enzyme activity in a dose-dependent manner with maximal effect (56%) obtained at 10⁻⁶ M. This effect was reversed by DPCPX (8-cyclopentyl-1,3-dipropylxanthine), an antagonist of A₁ receptors, but it was not changed by 10⁻⁸ M DMPX (3,7-dimethyl-1-propargylxanthine) or 10⁻⁸ M MRS (2,3-diethyl-4,5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5-carboxylate), antagonists of A₂ and A₃ receptors, respectively. Furthermore, it was observed that guanine increased [γ-³⁵S]GTP-specific binding with the maximal effect observed at 10⁻⁶ M and this effect was abolished by 10⁻⁶ M GDPβS. The inhibitory effect of 10⁻⁶ M guanine on Na+-ATPase activity was reversed by 10⁻⁶ M GDPβS, 10⁻⁶ M forskolin, 10⁻⁶ M pertussis toxin and 10⁻⁸ M cholera toxin. These results indicate that guanine binds to a DPCPX-sensitive receptor promoting the activation of Gi protein and leading to a decrease in cAMP level and, consequently, inhibition of BLM Na+-ATPase activity.

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.

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.

Adenine-induced inhibition of Na(+)-ATPase activity: Evidence for involvement of the Gi protein-coupled receptor in the cAMP signaling pathway

In the present work, we demonstrate that adenine reduced Na(+)-ATPase activity in isolated basolateral membrane (BLM) of proximal tubule in a dose-dependent manner. Adenine metabolism was ruled out by TLC analysis of the potential [(3)H]adenine derived-metabolites. Specific binding of [(3)H]adenine to isolated BLM was observed in a dose-dependent manner with K(d) and B(max) of 242.6+/-27.6 nM and 2749.9+/-104.9 fmolmg(-1), respectively. Adenine increased the [(35)S]GTPgammaS specific binding and it was completely abolished by 10(-6)M GDPbetaS (G protein inhibitor) but it was not modified by DPCPX, DMPX and MRS1523, selective antagonists for A(1), A(2) and A(3) receptors, respectively. Furthermore, the inhibitory effect of adenine on the Na(+)-ATPase activity was blocked by 10(-6)M GDPbetaS, 1 microg/ml pertussis toxin (Gi protein inhibitor), 10(-6)M foskolin (adenylyl cyclase activator) and 10(-8)M cAMP. These data demonstrate that adenine inhibits the proximal tubule Na(+)-ATPase activity through the Gi protein-coupled receptor.

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

In previous papers we showed that Ang II increases the proximal tubule Na+-ATPase activity through AT1/PKC pathway [L.B. Rangel, C. Caruso-Neves, L.S. Lara, A.G. Lopes, Angiotensin II stimulates renal proximal tubule Na+-ATPase activity through the activation of protein kinase C. Biochim. Biophys. Acta 1564 (2002) 310-316, L.B.A. Rangel, A.G. Lopes, L.S. Lara, C. Caruso-Neves, Angiotensin II stimulates renal proximal tubule Na+)-ATPase activity through the activation of protein kinase C. Biochim. Biophys. Acta 1564 (2002) 310-316]. In the present paper, we study the involvement of PI-PLCbeta on the stimulatory effect of angiotensin II (Ang II) on the proximal tubule Na+-ATPase activity. Western blotting assays, using a polyclonal antibody for PI-PLCbeta, show a single band of about 150 KDa, which correspond to PI-PLCbeta isoforms. Ang II induces a rapid decrease in PIP2 levels, a PI-PLCbeta substrate, being the maximal effect observed after 30 s incubation. This effect of Ang II is completely abolished by 5 x 10(-8) M U73122, a specific inhibitor of PI-PLCbeta. In this way, the effect of 10(-8) M Ang II on the proximal tubule basolateral membrane (BLM) Na+-ATPase activity is completely abolished by 5 x 10(-8) M U73122. The increase in diacylglycerol (DAG) concentration, an product of PI-PLCbeta, from 0.1 to 10 nM raises the Na+-ATPase activity from 6.1+/-0.2 to 13.1+/-1.8 nmol Pi mg(-1) min(-1). This effect is similar and non-additive to that observed with Ang II. Furthermore, the stimulatory effect of 10 nM DAG is completely reversed by 10(-8) M calphostin C (Calph C), an inhibitor of PKC. Taken together these data indicate that Ang II stimulates the Na+-ATPase activity of proximal tubule BLM through a PI-PLCbeta/PKC pathway.

Adenosine reverses the stimulatory effect of angiotensin II on the renal Na+-ATPase activity through the A2 receptor

In the present paper, we report the modulation of the Angiotensin II (Ang II)-stimulated Na+-ATPase activity of the proximal tubule basolateral membrane by adenosine (Ado). Preincubation of isolated basolateral membrane with 10(-8)M Ang II increases the Na+-ATPase activity from 7.5+/-0.3 (control) to 14.6+/-0.9 nmol Pi x mg(-1)x min(-1)nmol Pi x mg(-1) x min(-1) (p<0.05). Incubation of Ang II-stimulated enzyme with 10(-6)M Ado, in the presence of the A1 receptor antagonist DPCPX (10(-6)M), completely reverses the Ang II-induced effect bringing the Na+-ATPase activity to the basal level. The following evidences demonstrate involvement of the A2 receptor/Gs protein/adenylyl cyclase/PKA signaling pathway in the inhibitory effect induced by Ado on the Ang II-stimulated Na+-ATPase activity in the presence of the DPCPX: 1) the inhibitory effect of Ado is abolished by the A2 receptor selective antagonist DMPX (10(-8)M); 2) the effect induced by Ado is blocked by 10(-8)M GDPbetaS and mimicked by 10(-9)M cholera toxin and 10(-8)M GTPgammaS; 3) the stimulatory effect of Ang II is reduced by 10(-6)M forskolin, an activator of adenylyl cyclase, or 10(-6)M cAMP; 4) Ado stimulates PKA activity; 5) the inhibitory effect induced by this nucleoside is reversed by the PKA inhibitor peptide.