The Role of Intercalated Cell Nedd4-2 in BP Regulation, Ion Transport, and Transporter Expression

BackgroundNedd4-2 is an E3 ubiquitin-protein ligase that associates with transport proteins, causing their ubiquitylation, and then internalization and degradation. Previous research has suggested a correlation between Nedd4-2 and BP. In this study, we explored the effect of intercalated cell (IC) Nedd4-2 gene ablation on IC transporter abundance and function and on BP.Methods We generated IC Nedd4-2 knockout mice using Cre-lox technology and produced global pendrin/Nedd4-2 null mice by breeding global Nedd4-2 null (Nedd4-2^-/- ) mice with global pendrin null (Slc26a4^-/- ) mice. Mice ate a diet with 1%-4% NaCl; BP was measured by tail cuff and radiotelemetry. We measured transepithelial transport of Cl^- and total CO₂ and transepithelial voltage in cortical collecting ducts perfused in vitro Transporter abundance was detected with immunoblots, immunohistochemistry, and immunogold cytochemistry.Results IC Nedd4-2 gene ablation markedly increased electroneutral Cl^-/HCO₃^- exchange in the cortical collecting duct, although benzamil-, thiazide-, and bafilomycin-sensitive ion flux changed very little. IC Nedd4-2 gene ablation did not increase the abundance of type B IC transporters, such as AE4 (Slc4a9), H⁺-ATPase, barttin, or the Na⁺-dependent Cl^-/HCO₃^- exchanger (Slc4a8). However, IC Nedd4-2 gene ablation increased CIC-5 total protein abundance, apical plasma membrane pendrin abundance, and the ratio of pendrin expression on the apical membrane to the cytoplasm. IC Nedd4-2 gene ablation increased BP by approximately 10 mm Hg. Moreover, pendrin gene ablation eliminated the increase in BP observed in global Nedd4-2 knockout mice.Conclusions IC Nedd4-2 regulates Cl^-/HCO₃^- exchange in ICs., Nedd4-2 gene ablation increases BP in part through its action in these cells.

Acute genetic ablation of pendrin lowers blood pressure in mice

Background

Pendrin, the chloride/bicarbonate exchanger of β-intercalated cells of the renal connecting tubule and the collecting duct, plays a key role in NaCl reabsorption by the distal nephron. Therefore, pendrin may be important for the control of extracellular fluid volume and blood pressure.

Methods

Here, we have used a genetic mouse model in which the expression of pendrin can be switched-on in vivo by the administration of doxycycline. Pendrin can also be rapidly removed when doxycycline administration is discontinued. Therefore, our genetic strategy allows us to test selectively the acute effects of loss of pendrin function.

Results

We show that acute loss of pendrin leads to a significant decrease of blood pressure. In addition, acute ablation of pendrin did not alter significantly the acid-base status or blood K +  concentration.

Conclusion

By using a transgenic mouse model, avoiding off-target effects related to pharmacological compounds, this study suggests that pendrin could be a novel target to treat hypertension.

New Findings on the Pathogenesis of Distal Renal Tubular Acidosis

Background

Distal renal tubular acidosis (dRTA) is characterized by an impairment of the urinary acidification process in the distal nephron. Complete or incomplete metabolic acidosis coupled with inappropriately alkaline urine are the hallmarks of this condition. Genetic forms of dRTA are caused by loss of function mutations of either SLC4A1, encoding the AE1 anion exchanger, or ATP6V1B1 and ATP6V0A4, encoding for the B1 and a4 subunits of the vH⁺ATPase, respectively. These genes are crucial for the function of A-type intercalated cells (A-IC) of the distal nephron.

Summary

Alterations of acid-base homeostasis are variably associated with hypokalemia, hypercalciuria, nephrocalcinosis or nephrolithiasis, and a salt-losing phenotype. Here we report the diagnostic test and the underlying physiopathological mechanisms. The molecular mechanisms identified so far can explain the defect in acid secretion, but do not explain all clinical features. We review the latest experimental findings on the pathogenesis of dRTA, reporting mechanisms that are instrumental for the clinician and potentially inspiring a novel therapeutic strategy.

Key Message

Primary dRTA is usually intended as a single-cell disease because the A-IC are mainly affected. However, novel evidence shows that different cell types of the nephron may contribute to the signs and symptoms, moving the focus from a single-cell towards a renal disease.

Renal Atp6ap2/(Pro)renin Receptor Is Required for Normal Vacuolar H+-ATPase Function but Not for the Renin-Angiotensin System

ATPase H⁺-transporting lysosomal accessory protein 2 (Atp6ap2), also known as the (pro)renin receptor, is a type 1 transmembrane protein and an accessory subunit of the vacuolar H⁺-ATPase (V-ATPase) that may also function within the renin-angiotensin system. However, the contribution of Atp6ap2 to renin-angiotensin-dependent functions remains unconfirmed. Using mice with an inducible conditional deletion of Atp6ap2 in mouse renal epithelial cells, we found that decreased V-ATPase expression and activity in the intercalated cells of the collecting duct impaired acid-base regulation by the kidney. In addition, these mice suffered from marked polyuria resistant to desmopressin administration. Immunoblotting revealed downregulation of the medullary Na⁺-K⁺-2Cl^- cotransporter NKCC2 in these mice compared with wild-type mice, an effect accompanied by a hypotonic medullary interstitium and impaired countercurrent multiplication. This phenotype correlated with strong autophagic defects in epithelial cells of medullary tubules. Notably, cells with high accumulation of the autophagosomal substrate p62 displayed the strongest reduction of NKCC2 expression. Finally, nephron-specific Atp6ap2 depletion did not affect angiotensin II production, angiotensin II-dependent BP regulation, or sodium handling in the kidney. Taken together, our results show that nephron-specific deletion of Atp6ap2 does not affect the renin-angiotensin system but causes a combination of renal concentration defects and distal renal tubular acidosis as a result of impaired V-ATPase activity.

Rh glycoproteins in epithelial cells: lessons from rat and mice studies

Rhesus glycoproteins are a recently discovered family of ammonium transporters and a new branch of the Mep/AMT proteins superfamily that was identified more than 15 years ago in lower organisms and plants. Despite many ex vivo studies showing evidences that Rh glycoproteins can accelerate transmembrane NH3 or NH4+ transfer, their role in normal and disease physiology remains unknown. This review focuses on some of the different studies carried out in animal models to gain insight into Rh glycoprotein function. Immunolocalization studies have added new evidence that this protein family is related to ammonium transport or metabolism in epithelial cells. However, the absence of distal tubular acidosis or hyperammonemia in Rhbg KO mice have raised new questions about the physiological significance of these proteins.

Localization and functional characterization of Na+/H+ exchanger isoform NHE4 in rat thick ascending limbs

The Na+/H+ exchanger NHE4 was cloned from a rat stomach cDNA library and shown to be expressed predominantly in the stomach and less dramatically in the kidney. The role and precise localization of NHE4 in the kidney are still unknown. A polyclonal antibody against a unique NHE4 decapeptide was used for immunohistochemistry in rat kidney. Simultaneous use of antibodies to Tamm-Horsfall glycoprotein and aquaporin-2 or -3 permitted identification of thick ascending limbs and collecting ducts, respectively. The results indicate that NHE4 is highly expressed in basolateral membranes of thick ascending limb and distal convoluted tubule, whereas collecting ducts from cortex to inner medulla and proximal tubules showed weaker basolateral NHE4 expression. Western blot analysis of NHE4 in membrane fractions prepared from the inner stripe of the outer medulla revealed the presence of a 95-kDa protein that was enriched in basolateral membrane vesicles isolated from medullary thick ascending limbs. The inhibition curve of H+-activated (22)Na uptake by 5-(N-ethyl-N-isopropyl)amiloride (EIPA) was consistent with the presence, beyond the EIPA high-affinity NHE1 isoform, of an EIPA low-affinity NHE with apparent half-maximal inhibition of 2.5 microM. Kinetic analyses showed that the extracellular Na+ dependence of NHE4 activity followed a simple hyperbolic relationship, with an apparent affinity constant of 12 mM. Intravesicular H+ activated NHE4 by a positive cooperative mechanism. NHE4 had an unusual low affinity for intravesicular H+ with a half-maximal activation value of pK 6.21. We conclude that NHE4, like NHE1, is expressed on the basolateral membrane of multiple nephron segments. Nevertheless, these two proteins exhibited dramatically different affinities for intracellular H+, suggesting that they may play distinct physiological roles in the kidney.

Macula densa Na(+)/H(+) exchange activities mediated by apical NHE2 and basolateral NHE4 isoforms

Functional and immunohistochemical studies were performed to localize and identify Na(+)/H(+) exchanger (NHE) isoforms in macula densa cells. By using the isolated perfused thick ascending limb with attached glomerulus preparation dissected from rabbit kidney, intracellular pH (pH(i)) was measured with fluorescence microscopy by using 2',7'-bis-(2-carboxyethyl)-5-(and -6) carboxyfluorescein. NHE activity was assayed by measuring the initial rate of Na(+)-dependent pH(i) recovery from an acid load imposed by prior lumen and bath Na(+) removal. Removal of Na(+) from the bath resulted in a significant, DIDS-insensitive, ethylisopropyl amiloride (EIPA)-inhibitable decrease in pH(i). This basolateral transporter showed very low affinity for EIPA and Hoechst 694 (IC(50) = 9.0 and 247 microM, respectively, consistent with NHE4). The recently reported apical NHE was more sensitive to inhibition by these drugs (IC(50) = 0.86 and 7.6 microM, respectively, consistent with NHE2). Increasing osmolality, a known activator of NHE4, greatly stimulated basolateral NHE. Immunohistochemical studies using antibodies against NHE1-4 peptides demonstrated expression of NHE2 along the apical and NHE4 along the basolateral, membrane, whereas NHE1 and NHE3 were not detected. These results suggest that macula densa cells functionally and immunologically express NHE2 at the apical membrane and NHE4 at the basolateral membrane. These two isoforms likely participate in Na(+) transport, pH(i), and cell volume regulation and may be involved in tubuloglomerular feedback signaling by these cells.

Regulation by PKC isoforms of Na(+)/H(+) exchanger in luminal membrane vesicles isolated from cortical tubules

The present study was designed to determine the Na/H exchanger isoforms present in luminal membrane vesicles (LMV) isolated from rat kidney cortical tubule suspensions, as well as the effects of acute phorbol ester (phorbol myristate acetate, PMA) and angiotensin II (ANG II) pretreatment of suspensions on NHE activity and protein kinase C (PKC) isoform abundance. In LMV, both NHE3 and NHE2 proteins were found by Western blot analysis, but only ethylisopropylamiloride-sensitive and almost completely Hoe-694-resistant Na/H exchange activity was observed from (22)Na uptake and thus attributed to NHE3. PMA pretreatment increased Na/H exchange activity and PKC isoforms alpha, delta, and epsilon abundance in LMV, and these effects were prevented by PKC inhibition. Low-dose ANG II (10(-11) M) pretreatment increased Na/H exchange activity and only PKC-zeta abundance in LMV, and these effects were also prevented by PKC inhibition. After high-dose ANG II (10(-7) M), Na/H exchange activity was decreased in LMV. PKC inhibition did not prevent this effect. In conclusion, the stimulating effects of PMA and low-dose ANG II are explained by the translocation of different isoforms of PKC in LMV, whereas the inhibitory effect of high-dose ANG II is not PKC dependent.

Polarized expression of different monocarboxylate transporters in rat medullary thick limbs of Henle

Extracellular lactic acid is a major fuel for the mammalian medullary thick ascending limb (MTAL), whereas under anoxic conditions, this nephron segment generates a large amount of lactic acid, which needs to be excreted. We therefore evaluated, at both the functional and molecular levels, the possible presence of monocarboxylate transporters in basolateral (BLMVs) and luminal (LMVs) membrane vesicles isolated from rat MTALs. Imposing an inward H(+) gradient induced the transient uphill accumulation of L-[(14)C]lactate in both types of vesicles. However, whereas the pH gradient-stimulated uptake of L-[(14)C]lactate in BLMVs was inhibited by anion transport blockers such as alpha-cyano-4-hydroxycinnamate, 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS), and furosemide, it was unaffected by these agents in LMVs, indicating the presence of a L-lactate/H(+) cotransporter in BLMVs, but not in LMVs. Under non-pH gradient conditions, however, the uptake of L-[(14)C]lactate in LMVs was transstimulated 100% by L-lactate, but by only 30% by D-lactate. Furthermore, this L-lactate self-exchange was markedly inhibited by alpha-cyano-4-hydroxycinnamate and DIDS and almost completely by 1 mM furosemide, findings consistent with the existence of a stereospecific carrier-mediated lactate transport system in LMVs. Using immunofluorescence confocal microscopy and immunoblotting, the monocarboxylate transporter (MCT)-2 isoform was shown to be specifically expressed on the basolateral domain of the rat MTAL, whereas the MCT1 isoform could not be detected in this nephron segment. This study thus demonstrates the presence of different monocarboxylate transporters in rat MTALs; the basolateral H(+)/L-lactate cotransporter (MCT2) and the luminal H(+)-independent organic anion exchanger are adapted to play distinct roles in the transport of monocarboxylates in MTALs.

Kidney cortex cells derived from SV40 transgenic mice retain intrinsic properties of polarized proximal tubule cells

BACKGROUND

We have developed a nontransformed immortalized mice kidney cortex epithelial cell (MKCC) culture from a mouse transgenic for a recombinant plasmid adeno-SV40 (PK4). Methods and Results. After 12 months in culture, the immortalized cells had a stable homogeneous epithelial-like phenotype, expressed simian virus 40 (SV40) T-antigen, but failed to induce tumors after injection in nude mice. Epithelium exhibited polarity with an apical domain bearing many microvilli separated from lateral domains by junctional complexes with ZO1 protein. The transepithelial resistance was low. A Na-dependent glucose uptake sensitive to phlorizin and a Na-dependent phosphate uptake sensitive to arsenate were present. Western blot analysis of membrane fractions showed that anti-Na-Pi antiserum reacted with a 87 kD protein. The Na/H antiporters NHE-1, NHE-2, and NHE-3 mRNAs were detected by reverse transcription-polymerase chain reaction (RT-PCR). The corresponding proteins with molecular weights of 111, 81, and 75 kD, respectively, could be detected by Western blot and were shown to be functional. Parathyroid hormone (PTH) induced a tenfold increase in cAMP and reduced the Na-dependent phosphate uptake and NHE-3 activity, as observed in proximal tubule cells. Isoforms alpha, delta, epsilon, and zeta of protein kinase C (PKC) were present in the cells. Angiotensin II (Ang II) elicited a translocation of the PKC-alpha toward the basolateral and apical domains.

CONCLUSION

Thus, the MKCC culture retains the structural and functional properties of proximal tubular cells. To our knowledge, it is the first cell culture obtained from transgenic mice that exhibits the NHE-3 antiporter and type II Na-Pi cotransporter. MKCCs also display functional receptors for PTH and Ang II. Thus, MKCCs offer a powerful in vitro system to study the cellular mechanisms of ion transport regulation in proximal epithelium.

Adaptation of NHE-3 in the rat thick ascending limb: effects of high sodium intake and metabolic alkalosis

The present studies examined the effects of chronic NaCl administration and metabolic alkalosis on NHE-3, an apical Na+/H+ exchanger of the rat medullary thick ascending limb of Henle (MTAL). NaCl administration had no effect on NHE-3 mRNA abundance as assessed by competitive RT-PCR, as well as on NHE-3 transport activity estimated from the Na+-dependent cell pH recovery of Na+-depleted acidified MTAL cells, in the presence of 50 microM Hoe-694, which specifically blocks NHE-1 and NHE-2. Two models of metabolic alkalosis were studied, one associated with high sodium intake, i.e., NaHCO3 administration, and one not associated with high sodium intake, i.e., chloride depletion alkalosis (CDA). In both cases, the treatment induced a significant metabolic alkalosis that was associated with a decrease in NHE-3 transport activity (-27% and -25%, respectively). Negative linear relationships were observed between NHE-3 activity and plasma pH or bicarbonate concentration. NHE-3 mRNA abundance and NHE-3 protein abundance, assessed by Western blot analysis, also decreased by 35 and 25%, respectively, during NaHCO3-induced alkalosis, and by 47 and 33%, respectively, during CDA. These studies demonstrate that high sodium intake has per se no effect on MTAL NHE-3. In contrast, chronic metabolic alkalosis, regardless of whether it is associated with high sodium intake or not, leads to an appropriate adaptation of NHE-3 activity, which involves a decrease in NHE-3 protein and mRNA abundance.

Immunolocalization of the Na+/H+ exchanger isoform NHE2 in rat kidney

Four Na+/H+ exchangers (NHE1 to NHE4) have been detected in the kidney. Renal NHE2 expression sites have not been fully established. We have raised rabbit antisera against an oligopeptide related to the amino acids 652 to 661 of rat NHE2. Western blot analysis of plasma membrane fractions isolated from rat renal cortex showed that affinity-purified anti-NHE2 antibody detected an 85-kDa protein in apical but not in basolateral membranes. The labeling of this 85-kDa protein was specifically blocked by preincubation of the antibody with its monomeric peptide, indicating specific recognition. Indirect immunolabeling was performed on sections of paraformaldehyde-fixed rat kidney embedded in paraffin. Strong staining was seen in the apical membrane of cortical thick ascending limbs, distal convoluted tubules, and connecting tubules. Much weaker apical staining was found in medullary thick ascending limbs of Henle. In the inner medulla, some thin limbs were intensively labeled by the anti-NHE2 antibody. No staining could be detected in any segments of the proximal tubule and collecting duct.

Evidence for an amiloride-insensitive Na+/H+ exchanger in rat renal cortical tubules

We have characterized the Na+/H+ exchanger (NHE) isoforms expressed in rat renal cortical tubule fragments. Amiloride sensitivity of the Na(+)-dependent intracellular pH (pHi) recovery in suspended tubules that had been acid loaded by an NH4+ prepulse was determined in nominally CO2/HCO3(-)-free solution, using the fluorescent pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. In the presence of 140 mM extracellular Na+, 800 microM amiloride inhibited the rate of Na(+)-dependent pHi recovery by only 65%, demonstrating the presence of a Na(+)-dependent amiloride-insensitive H+ extrusion system. This system was not affected by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid but was activated by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Lowering extracellular Na+ concentration permitted 300 microM amiloride to completely inhibit Na(+)-dependent pHi recovery. These results can be explained by the expression of a Na+/H+ exchange with the pharmacological properties of NHE4. Using reverse transcriptase-polymerase chain reaction, we found specific mRNA for NHE1, NHE2, NHE3, and NHE4 isoforms in the renal cortex. Immunohistochemical studies using polyclonal antibodies against rat NHE4 peptide demonstrated that NHE4 is heterogeneously expressed on basolateral membrane domains of cortical tubules. These results strongly suggest that amiloride-insensitive Na+/H+ exchange expressed in renal cortical tubule suspensions is mediated by NHE4.

Heterologous expression of rat NHE4: a highly amiloride-resistant Na+/H+ exchanger isoform

Molecular cloning and expression have previously defined three members of the Na+/H+ exchanger (NHE) gene family NHE1 and NHE2 are sensitive to inhibition by amiloride and its 5'-amino alkyl-substituted analogues, whereas NHE3 is quite resistant to amiloride inhibition. Each of these exchangers has narrowly defined cation specificities for Na+ and Li+. Expression studies with NHE4 have not been as successful, with only a description of modest expression of activity (C. Bookstein, M. W. Musch, A. DePaoli, Y. Xie, M. Villereal, M. C. Rao, and E. B. Chang. J. Biol. Chem. 269: 29704-29709, 1994). We now report that NHE4 activity in stably transfected fibroblasts is activated by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), permitting functional characterization of this NHE isoform. The activating effect of DIDS was unique among the disulfonic stilbenes, and competition studies suggested a cross-linking mechanism. NHE4 is extremely resistant to amiloride and ethylisopropylamiloride inhibition and, unlike other NHE isoforms, affects K+/H+ exchange as well as Na+/H+ and Li+/H+ exchange. These findings demonstrate that NHE4 is a functionally distinct member of the NHE gene family and suggest a unique physiological role for this cation/H+ exchanger.

Signaling pathways in the biphasic effect of angiotensin II on apical Na/H antiport activity in proximal tubule

Low concentrations of angiotensin II (Ang II) increase, whereas high concentrations inhibit the apical Na/H antiporter activity in the proximal tubule, but the respective roles of the different signaling pathways in mediating these effects remains unsettled. We studied the effects of both low and high doses of Ang II in the presence of selective signaling pathway inhibitors, on the apical Na/H antiport activity of rat proximal tubule. Experiments were carried out in intact cells of freshly prepared tubule fragments obtained from the outer third of cortex, that is, devoid of basolateral Na/H antiport activity in the absence of bicarbonate transport and H(+)-ATPase activity. In tubules acid-loaded by an NH4Cl prepulse, Na/H antiport activity was assessed by the initial rate of intracellular pH recovery (dpHi/dt), measured with BCECF. When tubules were preincubated with low dose Ang II (10(-11) M for 3 min), dpHi/dt increased by 25 +/- 8%, whereas incubation with high dose Ang II (10(-7) M for 3 min) decreased dpHi/dt by 30 +/- 4%, compared to control (P < 0.01 in both cases). Both effects were abolished in the presence of 2.10(-3) M amiloride. Low dose Ang II-induced increase in dpHi/dt was not affected by preincubation with a specific PKA inhibitor, Rp-CPT-cAMP 10(-4) M, and was completely abolished by preincubation with PKC inhibitors, staurosporine 10(-7) M, sphingosine 5.10(-6) M, or calphostin 10(-6) M. In addition, pretreatment of rats with pertussis toxin led to a partial inhibition of the effect of low dose Ang II. The high dose-Ang II-induced decrease in dpHi/dt was not affected by pretreatment with a calcium-calmodulin kinase inhibitor W-7 10(-4) M. Conversely, pretreatment with the cytochrome P-450 inhibitor econazole 10(-5) M reversed the inhibitory effect of high dose Ang II to a stimulatory effect (24 +/- 8%, P < 0.01), quantitatively similar to the effect of low dose Ang II. In addition, arachidonate was found to exert an econazole-sensitive dose-dependent inhibitory effect on dpHi/dt, and 5,6-EET 10(-6) M, a cytochrome P-450 derived-arachidonic acid metabolite, induced a 38 +/- 9% inhibition, similar to that observed with high dose Ang II alone. There was no additive effect of 5,6-EET and high dose Ang II. Finally, pretreatment with two PLA2 inhibitors (BromoPhenacylBromide, 6.10(-6) M, and oleyloxyethyl phosphorylcholine, 5.10(-6) M) reversed the inhibitory effect of high dose Ang II to a stimulatory effect (32 +/- 11% and 25 +/- 11%, respectively, P < 0.05 for both inhibitors). We conclude that, in intact rat proximal cells, low dose Ang II stimulates the apical Na/H antiport through a pertussis toxin-sensitive G protein-dependent PKC pathway, whereas high dose Ang II inhibits the Na/H antiport activity through the PLA2- and cytochrome P-450-dependent metabolites of arachidonate.

Rapid redistribution and inhibition of renal sodium transporters during acute pressure natriuresis

Pages F1004–F1014: Y. Zhang, A. K. Mircheff, C. B. Hensley, C. E. Magyar, D. G. Warnock, R. Chambrey, K.-P. Yip, D. J. Marsh, N.-H. Holstein-Rathlou, and A. A. McDonough. “Rapid redistribution and inhibition of renal sodium transporters during acute pressure natriuresis.” The immunoblot panels in Figures 2 and 5–7 were inadvertently printed from low-resolution copies of the original artwork; in addition, the panels in Fig. 6 were incorrectly labeled. The correct figures are reproduced on the following pages.

(See PDF)

Rapid redistribution and inhibition of renal sodium transporters during acute pressure natriuresis

Pages F1004'F1014: Y. Zhang, A. K. Mircheff, C. B. Hensley, C. E. Magyar, D. G. Warnock, R. Chambrey, K.-P. Yip, D. J. Marsh, N.-H. Holstein-Rathlou, and A. A. McDonough. “Rapid redistribution and inhibition of renal sodium transporters during acute pressure natriuresis.” The immunoblot panels in Figures 2 and 5–7 were inadvertently printed from low-resolution copies of the original artwork; in addition, the panels in Fig. 6 were incorrectly labeled. The correct figures are reproduced on the following pages.

(See PDF)

Rapid redistribution and inhibition of renal sodium transporters during acute pressure natriuresis

Acute arterial hypertension provokes a rapid decrease in proximal tubule (PT) Na+ reabsorption, increasing flow to the macula densa, the signal for tubuloglomerular feedback. We tested the hypothesis, in rats, that Na+ transport is decreased due to rapid redistribution of apical Na+/H+ exchangers and basolateral Na+ pumps to internal membranes. Arterial pressure was increased 50 mmHg by constricting various arteries. We also tested whether transporter internalization occurred when PT Na+ reabsorption was inhibited with the carbonic anhydrase inhibitor benzolamide. Five minutes after initiating either natriuretic stimuli, cortex was removed, and membranes were fractionated by density gradient centrifugation. Urine output and endogenous lithium clearance increased threefold in response to either stimuli. Acute hypertension provoked a redistribution of apical Na+/H+ exchanger NHE3, alkaline phosphatase, and dipeptidyl peptidase IV to higher density membranes enriched in the intracellular membrane markers. Basolateral membrane Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity decreased 50%, 25-30% of the alpha 1-and beta 1-subunits redistributed to higher density membranes, and the remainder is attributed to decreased activity of the transporters. Benzolamide did not alter Na+ transporter activity or distribution, implying that decreasing apical Na+ uptake does not initiate redistribution or inhibition of basolateral Na(+)-K(+)-ATPase. We conclude that PT natriuresis provoked by acute arterial pressure is mediated by both endocytic removal of apical Na+/H+ exchangers and basolateral Na+ pumps as well as decreased total Na+ pump activity.

Renal Na+/H+ exchanger isoforms and their regulation by thyroid hormone

Na+ crosses the luminal membrane of the proximal tubule primarily via Na+/H+ exchange (NHE), and NHE activity is influenced by thyroid status. Pharmacological, immunological, and kinetic studies indicate multiple isoforms of NHE, and four full-length cDNAs have been cloned to date. The aims of this study were to determine which NHE mRNAs (NHE1, -2, -3, and -4) were expressed in the rat proximal tubule, the relative abundance of each in the renal cortex, and the effect of thyroid status on their expression. By blot hybridization of poly(A)+ RNA, all NHE isoform mRNAs were detected in the rat renal cortex; NHE1, -2, and -3 in the proximal tubule; and NHE1 and -3 in LLC-PK1 cells. NHE3 mRNA abundance was fourfold higher than the other three isoforms in renal cortex. The effect of thyroid status was assessed in renal cortex from euthyroid, hypothyroid, and hyperthyroid rats. Although none of the NHE mRNA levels was altered in the transition from euthyroid to hypothyroid states, both NHE2 and NHE3 mRNA levels increased 1.5-fold in the transition from hypo- to hyperthyroidism. NHE3 protein, measured by immunoblot with the use of an NHE3-specific antibody, was detected at 83-85 kDa in renal cortex and codistributed on sorbitol gradients with the brush-border marker alkaline phosphatase. No significant difference in NHE3 protein abundance was detected between hypothyroid and hyperthyroid rats. In conclusion, in the renal cortex, the NHE3 isoform predominates at the mRNA level, is expressed in apical membranes, and increases at the mRNA but not the protein levels in response to thyroid hormone treatment, suggesting parallel changes in synthesis and turnover of NHE3 by thyroid hormone.

Enzymatic and functional evidence for adaptation of the vacuolar H(+)-ATPase in proximal tubule apical membranes from rats with chronic metabolic acidosis

The present work examined the effects of chronic metabolic acidosis on the vacuolar proton-translocating adenosine triphosphatase (H(+)-ATPase) activity both in rat renal cortical homogenates and in their luminal membranes. Moreover, to assess the effect of acidosis on H+ transport by the apical H(+)-ATPase, we have developed a detergent-dilution procedure, resulting in the formation of sealed vesicles having this enzyme at their external surface. NH4Cl loading for 4 days had no effect on homogenates H(+)-ATPase activity, estimated with either N-ethylmaleimide or bafilomycin A1. In contrast, H(+)-ATPase activities were increased significantly by about 30% in both native apical membranes prepared by Ca2+ aggregation and detergent-treated luminal vesicles from acidotic animal. Kinetic analysis revealed that this stimulation was solely through changes in the Vmax for ATP. In membranes prepared by Mg2+ aggregation, acidosis also caused significant stimulation of the H(+)-ATPase activity. In addition, the initial rate of ATP-induced intravesicular acidification was 25% higher in reoriented H(+)-ATPase vesicles from acidotic rats, whereas passive proton permeability was identical in both groups. Finally, both vesicle enrichments and yields of luminal markers and de-enrichments and yields of intracellular membrane markers were identical in the two groups. These results provide enzymatic and functional evidence suggesting that chronic acidosis induces an adaptative change in the rat brush border H(+)-ATPase.

Heterogeneity in the effects of membrane potentials on pantothenate and glucose uptakes by rabbit renal apical membranes

1. Previous studies using renal brush-border membrane vesicles have established that both the pantothenate and the low Km (Michaelis-Menten constant), low Vmax (maximal rate) D-glucose systems have a stoichiometry of 2 Na+: 1 organic molecule. In this study, we compared the mechanisms by which the membrane potential energizes pantothenate and D-glucose uptakes by brush-border membrane vesicles isolated from the whole cortex of rabbit kidney. 2. In the absence of Na+, varying the membrane potential from +60 to -60 mV decreased pantothenate uptake, whereas D-glucose uptake was increased in a linear manner. These results suggested the existence of a conductive pathway for pantothenate in these membranes. They also suggested that the pantothenate free carrier is electroneutral, while the glucose free carrier is negatively charged. 3. In the presence of an inwardly directed Na+ gradient, varying the membrane potential from +60 to -60 mV increased Na(+)-dependent pantothenate influx linearly. In contrast, a shift from +60 to +40 mV in the membrane potential had no influence on Na(+)-dependent D-glucose influx, whereas influx was a linear function of the membrane potential from +40 to -60 mV, indicating that there is a threshold membrane potential required for membrane potential-dependent D-glucose movement to occur. 4. Kinetic studies revealed that the effect of membrane potential on pantothenate uptake is through changes in the Km, while Vmax was unchanged. On the other hand, the membrane potential exerted its effect on D-glucose transport solely on the Vmax. 5. Finally, binding studies revealed that membrane potential, both in the presence and absence of a Na+ gradient, elicited effects on phlorizin binding qualitatively similar to those observed for D-glucose transport. 6. Implications of these findings for tubular regulation of these electrogenic secondary active transport systems are discussed.