ET(B) receptor activation causes exocytic insertion of NHE3 in OKP cells

Acid-base regulation in the renal proximal tubules: using novel pH sensors to maintain homeostasis

The kidneys play a critical role in precisely regulating the composition of the plasma to maintain homeostasis. To achieve this, the kidneys must be able to accurately determine or "sense" the concentration of a wide variety of substances and to make adjustments accordingly. Kidneys face a key challenge in the arena of pH balance, as there is a particularly narrow range over which plasma pH varies in a healthy subject (7.35-7.45) and this pH must constantly be protected against a variety of onslaughts (changes in diet, activity, and even elevation). The proximal tubule, the first segment to come into contact with the forming urine, plays an important role in helping the kidneys to maintain pH homeostasis. Recent studies have identified a number of novel proximal tubule proteins and signaling pathways that work to sense changes in pH and subsequently modulate renal pH regulation. In this review, we will highlight the role of novel players in acid-base homeostasis in the proximal tubule.

Acid Stimulation of the Citrate Transporter NaDC-1 Requires Pyk2 and ERK1/2 Signaling Pathways

Background Urine citrate is reabsorbed exclusively along the renal proximal tubule via the apical Na⁺-dicarboxylate cotransporter NaDC-1. We previously showed that an acid load in vivo and media acidification in vitro increase NaDC-1 activity through endothelin-1 (ET-1)/endothelin B (ET_B) signaling. Here, we further examined the signaling pathway mediating acid-induced NaDC-1 activity.Methods We transiently transfected cultured opossum kidney cells, a model of the proximal tubule, with NaDC-1 and ET_B and measured [¹⁴C]-citrate uptake after media acidification under various experimental conditions, including inactivation of Pyk2 and c-Src, which were previously shown to be activated by media acidification. Wild-type (Pyk2^+/+) and Pyk2-null (Pyk2^-/-) mice were exposed to NH₄Cl loading and euthanized after various end points, at which time we harvested the kidneys for immunoblotting and brush border membrane NaDC-1 activity studies.Results Inhibition of Pyk2 or c-Src prevented acid stimulation but not ET-1 stimulation of NaDC-1 in vitro Consistent with these results, NH₄Cl loading stimulated NaDC-1 activity in kidneys of wild-type but not Pyk2^-/- mice. In cultured cells and in mice, ERK1/2 was rapidly phosphorylated by acid loading, even after Pyk2 knockdown, and it was required for acid but not ET-1/ET_B stimulation of NaDC-1 in vitro Media acidification also induced the phosphorylation of Raf1 and p90RSK, components of the ERK1/2 pathway, and inhibition of these proteins blocked acid stimulation of NaDC-1 activity.Conclusions Acid stimulation of NaDC-1 activity involves Pyk2/c-Src and Raf1-ERK1/2-p90RSK signaling pathways, but these pathways are not downstream of ET-1/ET_B in this process.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Adaptation by the collecting duct to an exogenous acid load is blunted by deletion of the proton-sensing receptor GPR4

We previously reported that the deletion of the pH sensor GPR4 causes a non-gap metabolic acidosis and defective net acid excretion (NAE) in the GPR4 knockout mouse (GPR4−/−) (Sun X, Yang LV, Tiegs BC, Arend LJ, McGraw DW, Penn RB, and Petrovic S. J Am Soc Nephrol 21: 1745–1755, 2010). Since the major regulatory site of NAE in the kidney is the collecting duct (CD), we examined acid-base transport proteins in intercalated cells (ICs) of the CD and found comparable mRNA expression of kidney anion exchanger 1 (kAE1), pendrin, and the a4 subunit of H+-ATPase in GPR4−/− vs. +/+. However, NH4Cl loading elicited adaptive doubling of AE1 mRNA in GPR4+/+, but a 50% less pronounced response in GPR4−/−. In GPR4+/+, NH4Cl loading evoked a cellular response characterized by an increase in AE1-labeled and a decrease in pendrin-labeled ICs similar to what was reported in rabbits and rats. This response did not occur in GPR4−/−. Microperfusion experiments demonstrated that the activity of the basolateral Cl−/HCO3− exchanger, kAE1, in CDs isolated from GPR4−/− failed to increase with NH4Cl loading, in contrast to the increase observed in GPR4+/+. Therefore, the deficiency of GPR4 blunted, but did not eliminate the adaptive response to an acid load, suggesting a compensatory response from other pH/CO2/bicarbonate sensors. Indeed, the expression of the calcium-sensing receptor (CaSR) was nearly doubled in GPR4−/− kidneys, in the absence of apparent disturbances of Ca2+ homeostasis. In summary, the expression and activity of the key transport proteins in GPR4−/− mice are consistent with spontaneous metabolic acidosis, but the adaptive response to a superimposed exogenous acid load is blunted and might be partially compensated for by CaSR.

Akt-mediated regulation of antidepressant-sensitive serotonin transporter function, cell-surface expression and phosphorylation

The present study is focused on the cellular basis for Akt-mediated SERT regulation. SERT has been implicated in mood disorders. SERT is a primary target for antidepressants used in the therapeutic intervention of psychiatric disorders.

Sorting nexin 27 regulates basal and stimulated brush border trafficking of NHE3

In polarized epithelial cells, SNX27 regulates PDZ domain–directed trafficking of NHE3 from endosomes to the plasma membrane and increases the stability of brush border NHE3. This establishes SNX27 as an important regulator of polarized sorting in epithelial cells.

Sorting nexin 27 (SNX27) contains a PDZ domain that is phylogenetically related to the PDZ domains of the NHERF proteins. Studies on nonepithelial cells have shown that this protein is located in endosomes, where it regulates trafficking of cargo proteins in a PDZ domain–dependent manner. However, the role of SNX27 in trafficking of cargo proteins in epithelial cells has not been adequately explored. Here we show that SNX27 directly interacts with NHE3 (C-terminus) primarily through the SNX27 PDZ domain. A combination of knockdown and reconstitution experiments with wild type and a PDZ domain mutant (GYGF → GAGA) of SNX27 demonstrate that the PDZ domain of SNX27 is required to maintain basal NHE3 activity and surface expression of NHE3 in polarized epithelial cells. Biotinylation-based recycling and degradation studies in intestinal epithelial cells show that SNX27 is required for the exocytosis (not endocytosis) of NHE3 from early endosome to plasma membrane. SNX27 is also required to regulate the retention of NHE3 on the plasma membrane. The findings of the present study extend our understanding of PDZ-mediated recycling of cargo proteins from endosome to plasma membrane in epithelial cells.

Optimum AT1 receptor-neprilysin inhibition has superior cardioprotective effects compared with AT1 receptor blockade alone in hypertensive rats

Neprilysin inhibitors prevent the breakdown of bradykinin and natriuretic peptides, promoting vasodilation and natriuresis. However, they also increase angiotensin II and endothelin-1. Here we studied the effects of a low and a high dose of the neprilysin inhibitor thiorphan on top of AT1 receptor blockade with irbesartan versus vehicle in TGR(mREN2)27 rats with high renin hypertension. Mean arterial blood pressure was unaffected by vehicle or thiorphan alone. Irbesartan lowered blood pressure, but after 7 days pressure started to increase again. Low- but not high-dose thiorphan prevented this rise. Only during exposure to low-dose thiorphan plus irbesartan did heart weight/body weight ratio, cardiac atrial natriuretic peptide expression, and myocyte size decrease significantly. Circulating endothelin-1 was not affected by low-dose thiorphan with or without irbesartan, but increased after treatment with high-dose thiorphan plus irbesartan. This endothelin-1 rise was accompanied by an increase in renal sodium-hydrogen exchanger 3 protein abundance, and an upregulation of constrictor vascular endothelin type B receptors. Consequently, the endothelin type B receptor antagonist BQ788 no longer enhanced endothelin-1-induced vasoconstriction (indicative of endothelin type B receptor-mediated vasodilation), but prevented it. Thus, optimal neprilysin inhibitor dosing reveals additional cardioprotective effects on top of AT1 receptor blockade in renin-dependent hypertension.

Endothelin-1 regulates H⁺-ATPase-dependent transepithelial H⁺ secretion in zebrafish

Endothelin-1 (EDN1) is an important regulator of H⁺ secretion in the mammalian kidney. EDN1 enhances renal tubule H⁺-ATPase activity, but the underlying mechanism remains unclear. To further elucidate the role of EDN1 in vertebrates' acid-base regulation, the present study used zebrafish as the model to examine the effects of EDN1 and its receptors on transepithelial H⁺ secretion. Expression of EDN1 and one of its receptors, EDNRAa, was stimulated in zebrafish acclimated to acidic water. A noninvasive scanning ion-selective electrode technique was used to show that edn1 overexpression enhances H⁺ secretion in embryonic skin at 3 days post fertilization. EDNRAa loss of function significantly decreased EDN1- and acid-induced H⁺ secretion. Abrogation of EDN1-enhanced H⁺ secretion by a vacuolar H⁺-ATPase inhibitor (bafilomycin A1) suggests that EDN1 exerts its action by regulating the H⁺-ATPase-mediated H⁺ secretion. EDN1 does not appear to affect H⁺ secretion through either altering the abundance of H⁺-ATPase or affecting the cell differentiation of H⁺-ATPase-rich ionocytes, because the reduction in secretion upon ednraa knockdown was not accompanied by decreased expression of H⁺-ATPase or reduced H⁺-ATPase-rich cell density. These findings provide evidence that EDN1 signaling is involved in acid-base regulation in zebrafish and enhance our understanding of EDN1 regulation of transepithelial H⁺ secretion in vertebrates.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Acid increases NHE8 surface expression and activity in NRK cells

We previously demonstrated that there is a paucity of brush-border membrane NHE3 in neonates, the predominant Na(+)/H(+) exchanger in the adult proximal tubule, while NHE8 is relatively highly expressed in neonates compared with adults. We recently showed that metabolic acidosis in neonatal rodents can increase brush-border membrane NHE8 protein expression and Na(+)/H(+) exchange activity. To further examine the regulation of NHE8 by acid, we incubated NRK cells, which express NHE8 but not NHE3, with either acid or control media (6.6 vs. 7.4). There was an increase in Na(+)/H(+) exchanger activity within 6 h of incubation with acid media assessed as the rate of sodium-dependent recovery of pH from an acid load (dpH(i)/dt). The acid stimulation persisted for at least 24 h. The increase in Na(+)/H(+) exchange activity was paralleled by an increase in surface expression of NHE8, assessed by surface biotinylation and streptavidin precipitation. The increase in both apical membrane NHE8 protein expression and Na(+)/H(+) exchange activity with pH 6.6 media compared with 7.4 media was not affected by actinomycin D or cycloheximide consistent with an increase in surface expression independent of mRNA or protein synthesis. Furthermore, there was no increase in total cellular NHE8 protein abundance or mRNA abundance with acid media. Finally, we demonstrate that the increase in surface expression of NHE8 with acid media was blocked by colchicine and cytochalasin D and mediated by acid increasing the rate of exocytosis. In conclusion, NHE8 surface expression and activity are regulated by acid media by increasing the rate of trafficking to the apical membrane.

Contractile effects of endothelins on isolated human ureter

The aim of our study was to investigate mechanism of action of endothelins 1, 2 and 3 on spontaneous activity, tone and intraluminal pressure of human ureter. Both longitudinal tension and intraluminal pressure were recorded from the isolated segments of proximal human ureter. Endothelins 1, 2 and 3 (5.35x10(-11) M - 5.05x10(-8) M) produced concentration-dependent tonic contraction and sustained increase in intraluminal pressure of isolated preparations of human ureter. Endothelins 1 and 3 produced also concentration-dependent inhibition of spontaneous, phasic contractions of the isolated preparations. Selective antagonist of ET(A) receptors BQ123 and selective antagonist of ET(B) receptors BQ788 produced significant inhibition of endothelin-1-induced tonic contraction (pA(2)=8.80 and 6.55, respectively) and increase in intraluminal pressure (pA(2)=8.68 and 7.02, respectively), while they did not affect endothelin-1-induced inhibition of spontaneous activity. Endothelin 1 produces increase in tone and intraluminal pressure of isolated human ureter acting on both ET(A) and ET(B) receptors, the first one being functionally more important. Only endothelins 1 and 3 inhibit spontaneous, phasic activity of human ureter, but this effect was not blocked by selective antagonists of ET(A) and ET(B) receptors.

Regulation of renal NaCl transport by nitric oxide, endothelin, and ATP: clinical implications

NaCl absorption along the nephron is regulated not just by humoral factors but also by factors that do not circulate or act on the cells where they are produced. Generally, nitric oxide (NO) inhibits NaCl absorption along the nephron. However, the effects of NO in the proximal tubule are controversial and may be biphasic. Similarly, the effects of endothelin on proximal tubule transport are biphasic. In more distal segments, endothelin inhibits NaCl absorption and may be mediated by NO. Adenosine triphosphate (ATP) inhibits sodium bicarbonate absorption in the proximal tubule, NaCl absorption in thick ascending limbs via NO, and water reabsorption in collecting ducts. Defects in the effects of NO, endothelin, and ATP increase blood pressure, especially in a NaCl-sensitive manner. In diabetes, disruption of NO-induced inhibition of transport may contribute to increased blood pressure and renal damage. However, our understanding of how NO, endothelin, and ATP work, and of their role in pathology, is rudimentary at best.

RasGRP1 stimulation enhances ubiquitination and endocytosis of the sodium-chloride cotransporter

The sodium-chloride cotransporter (NCC) is the principal salt-absorptive pathway in the distal convoluted tubule. Recently, we described a novel pathway of NCC regulation in which phorbol esters (PE) stimulate Ras guanyl-releasing protein 1 (RasGRP1), triggering a cascade ultimately activating ERK1/2 MAPK and decreasing NCC cell surface expression (Ko B, Joshi LM, Cooke LL, Vazquez N, Musch MW, Hebert SC, Gamba G, Hoover RS. Proc Natl Acad Sci USA 104: 20120-20125, 2007). Little is known about the mechanisms which underlie these effects on NCC activity. Regulation of NCC via changes in NCC surface expression has been reported, but endocytosis of NCC has not been demonstrated. In this study, utilizing biotinylation, internalization assays, and a dynamin dominant-negative construct, we demonstrate that the regulation of NCC by PE occurs via an enhancement in internalization of NCC and is dynamin dependent. In addition, immunoprecipitation of NCC and subsequent immunoblotting for ubiquitin showed increased ubiquitination of NCC with phorbol ester treatment. MEK1/2 inhibitors and gene silencing of RasGRP1 indicated that this effect was dependent on RasGRP1 and ERK1/2 activation. Inhibition of ubiquitination prevents any PE-mediated decrease in NCC surface expression as measured by biotinylation or NCC activity as measured by radiotracer uptake. These findings confirmed that the PE effect on NCC is mediated by endocytosis of NCC. Furthermore, ubiquitination of NCC is essential for this process and this ubiquitination is dependent upon RasGRP1-mediated ERK1/2 activation.

cAMP stimulates apical exocytosis of the renal Na(+)-K(+)-2Cl(-) cotransporter NKCC2 in the thick ascending limb: role of protein kinase A

The apical renal Na(+)-K(+)-2Cl(-) cotransporter NKCC2 mediates NaCl absorption by the thick ascending limb (TAL) of Henle's loop. cAMP stimulates NKCC2 by enhancing steady-state apical membrane levels of this protein; however, the trafficking and signaling mechanisms by which this occurs have not been studied. Here, we report that stimulation of endogenous cAMP levels with either forskolin/3-isobutyl-1-methylxanthine (IBMX) or the V2 receptor agonist [deamino-Cys(1),d-Arg(8)]vasopressin increases steady-state surface NKCC2 and that the protein kinase A (PKA) inhibitor H-89 blocks this effect. Confocal imaging of apical surface NKCC2 in isolated perfused TALs confirmed a stimulatory effect of cAMP on apical trafficking that was blocked by PKA inhibition. Selective stimulation of PKA with the agonist N(6)-benzoyl-cAMP (500 microm) stimulated steady-state surface NKCC2, whereas the Epac-selective agonist 8-p-chlorophenylthio-2'-O-methyl-cAMP (100 and 250 microm) had no effect. To explore the trafficking mechanism by which cAMP increases apical NKCC2, we measured cumulative apical membrane exocytosis and NKCC2 exocytic insertion in TALs. By monitoring apical FM1-43 fluorescence, we observed rapid stimulation of apical exocytosis (2 min) by forskolin/IBMX. We also found constitutive exocytic insertion of NKCC2 in TALs over time, which was increased by 3-fold in the presence of forskolin/IBMX. PKA inhibition blunted cAMP-stimulated exocytic insertion but did not affect the rate of constitutive exocytosis. We conclude that cAMP stimulates steady-state apical surface NKCC2 by stimulating exocytic insertion and that this process is highly dependent on PKA but not Epac.

Luminal Na(+)/H (+) exchange in the proximal tubule

The proximal tubule is critical for whole-organism volume and acid-base homeostasis by reabsorbing filtered water, NaCl, bicarbonate, and citrate, as well as by excreting acid in the form of hydrogen and ammonium ions and producing new bicarbonate in the process. Filtered organic solutes such as amino acids, oligopeptides, and proteins are also retrieved by the proximal tubule. Luminal membrane Na(+)/H(+) exchangers either directly mediate or indirectly contribute to each of these processes. Na(+)/H(+) exchangers are a family of secondary active transporters with diverse tissue and subcellular distributions. Two isoforms, NHE3 and NHE8, are expressed at the luminal membrane of the proximal tubule. NHE3 is the prevalent isoform in adults, is the most extensively studied, and is tightly regulated by a large number of agonists and physiological conditions acting via partially defined molecular mechanisms. Comparatively little is known about NHE8, which is highly expressed at the lumen of the neonatal proximal tubule and is mostly intracellular in adults. This article discusses the physiology of proximal Na(+)/H(+) exchange, the multiple mechanisms of NHE3 regulation, and the reciprocal relationship between NHE3 and NHE8 at the lumen of the proximal tubule.

WNK4 diverts the thiazide-sensitive NaCl cotransporter to the lysosome and stimulates AP-3 interaction

With-no-lysine kinase 4 (WNK4) inhibits electroneutral sodium chloride reabsorption by attenuating the cell surface expression of the thiazide-sensitive NaCl cotransporter (NCC). The underlying mechanism for this effect remains poorly understood. Here, we explore how WNK4 affects the trafficking of NCC through its interactions with intracellular sorting machinery. An analysis of NCC cell surface lifetime showed that WNK4 did not alter the net rate of cotransporter internalization. In contrast, direct measurements of forward trafficking revealed that WNK4 attenuated the rate of NCC surface delivery, inhibiting the anterograde movement of cotransporters traveling to the plasma membrane from the trans-Golgi network. The response was paralleled by a dramatic reduction in NCC protein abundance, an effect that was sensitive to the lysosomal protease inhibitor leupeptin, insensitive to proteasome inhibition, and attenuated by endogenous WNK4 knockdown. Subcellular localization studies performed in the presence of leupeptin revealed that WNK4 enhanced the accumulation of NCC in lysosomes. Moreover, NCC immunoprecipitated with endogenous AP-3 complexes, and WNK4 increased the fraction of cotransporters that associate with this adaptor, which facilitates cargo transport to lysosomes. WNK4 expression also increased LAMP-2-positive lysosomal content, indicating that the kinase may act by a general AP-3-dependent mechanism to promote cargo delivery into the lysosomal pathway. Taken together, these findings indicate that WNK4 inhibits NCC activity by diverting the cotransporter to the lysosome for degradation by way of an AP-3 transport carrier.

Rho, Rho-kinase, and the actin cytoskeleton regulate the Na+–H+ exchanger in sea urchin eggs

At fertilization, the sea urchin egg undergoes an internal pH (pHi) increase mediated by a Na+ -H+ exchanger. We used antibodies against the mammalian antiporters NHE1 and NHE3 to characterize this exchanger. In unfertilized eggs, only anti-NHE3 cross-reacted specifically with a protein of 81-kDa, which localized to the plasma membrane and cortical granules. Cytochalasin D, C3 exotoxin (blocker of RhoGTPase function), and Y-27632 (inhibitor of Rho-kinase) prevented the pHi change in fertilized eggs. These inhibitors blocked the first cleavage division of the embryo, but not the cortical granule exocytosis. Thus, the sea urchin egg has an epithelial NHE3-like Na+ -H+ exchanger which can be responsible for the pHi change at fertilization. Determinants of this pHi change can be: (i) the increase of exchangers in the plasma membrane (via cortical granule exocytosis) and (ii) Rho, Rho-kinase, and optimal organization of the actin cytoskeleton as regulators, among others, of the intrinsic activity of the exchanger.

Na+/H+ exchangers in renal regulation of acid-base balance

The kidney plays key roles in extracellular fluid pH homeostasis by reclaiming bicarbonate (HCO(3)(-)) filtered at the glomerulus and generating the consumed HCO(3)(-) by secreting protons (H(+)) into the urine (renal acidification). Sodium-proton exchangers (NHEs) are ubiquitous transmembrane proteins mediating the countertransport of Na(+) and H(+) across lipid bilayers. In mammals, NHEs participate in the regulation of cell pH, volume, and intracellular sodium concentration, as well as in transepithelial ion transport. Five of the 10 isoforms (NHE1-4 and NHE8) are expressed at the plasma membrane of renal epithelial cells. The best-studied isoform for acid-base homeostasis is NHE3, which mediates both HCO(3)(-) absorption and H(+) excretion in the renal tubule. This article reviews some important aspects of NHEs in the kidney, with special emphasis on the role of renal NHE3 in the maintenance of acid-base balance.

Na+/H+ exchangers: physiology and link to hypertension and organ ischemia

PURPOSE OF REVIEW

Na/H exchangers (NHEs) are ubiquitous proteins with a very wide array of physiological functions, and they are summarized in this paper in view of the most recent advances. Hypertension and organ ischemia are two disease states of paramount importance in which NHEs have been implicated. The involvement of NHEs in the pathophysiology of these disorders is incompletely understood. This paper reviews the principal findings and current hypotheses linking NHE dysfunction to hypertension and ischemia.

RECENT FINDINGS

With the advent of large-scale sequencing projects and powerful in-silico analyses, we have come to know what is most likely the entire mammalian NHE gene family. Recent advances have detailed the roles of NHE proteins, exploring new functions such as anchoring, scaffolding and pH regulation of intracellular compartments. Studies of NHEs in disease models, even though not conclusive to date, have contributed new evidence on the interplay of ion transporters and the delicate ion balances that may become disrupted.

SUMMARY

This paper provides the interested reader with a concise overview of NHE physiology, and aims to address the implication of NHEs in the pathophysiology of hypertension and organ ischemia in light of the most recent literature.

Glucocorticoids acutely increase cell surface Na+/H+ exchanger-3 (NHE3) by activation of NHE3 exocytosis

Glucocorticoids have important effects on renal function, including the modulation of renal acidification by the major proximal tubular Na(+)/H(+) exchanger, NHE3. While the chronic effect of glucocorticoids is considered to be primarily at the transcriptional level, with increases in NHE3 mRNA and protein expression driving increased transport activity, the mechanisms by which glucocorticoids activate NHE3 in an acute setting have not been investigated. Previous studies have shown that a glucocorticoid-stimulated increase in NHE3 activity can occur before any detectable change in NHE3 mRNA. The present study examines the acute effects of glucocorticoids on NHE3 using opossum kidney (OKP) cells as a cell model. In OKP cells, total NHE3 protein abundance was not changed by 3 h of treatment with dexamethasone (10(-6) M). However, the biotin-accessible fraction representing NHE3 at the apical membrane as well as Na(+)/H(+) exchange activity measured fluorimetrically using the pH-sensitive dye BCECF-AM were significantly increased. These effects were not prevented by the protein synthesis inhibitor cycloheximide. NHE3 insertion (biotinylatable NHE3 after sulfo-NHS-acetate blockade) was stimulated by dexamethasone incubation, with or without cycloheximide. The rate of NHE3 endocytic retrieval, assessed either by the avidin protection assay (early endocytosis) or by the sodium 2-mercaptoethane sulfonate (MesNa) cleavage assay (early and late endocytosis), was not affected by dexamethasone. These findings suggest that trafficking plays a key role in the acute stimulation of NHE3 by glucocorticoids, with exocytosis being the major contributor to the glucocorticoid-induced rapid increase in cell surface NHE3 protein abundance and Na(+)/H(+) exchange activity.

Molecular pathogenetic mechanisms of nephrotic edema: progress in understanding

Molecular and pathogenetic mechanisms in sodium retention and water reabsorption of nephrotic edema are discussed. Are reported and analyzed molecular mechanisms about sodium retention in collecting duct cells regarding activation and surface expression of epithelial sodium channels (ENaC) and sodium-potassium-ATPase (Na,K-ATPase) by aldosterone, vasopressin, natriuretic peptide system (underfill theory): is necessary a better understanding about the dysregulation of ENaC and Na,K-ATPase surface expression and the resistance to natriuretic peptide system. Are also reported and analyzed molecular mechanisms of sodium retention in proximal tubule cells regarding intrinsic albumin toxicity upon type 3 sodium-hydrogen exchanger ionic pump and the activity of sodium-hydrogen exchanger regulatory factor protein (overfill theory): a better knowledge about the link between albumin, sodium-hydrogen exchanger type 3 (NHE3) ionic pump, sodium-hydrogen exchanger regulatory factor protein is necessary. Then molecular mechanisms of vasopressin free water retention through acquaporin water channels in collecting duct cells are discussed: further studies are necessary to understand vasopressin release pathway (osmotic/nonosmotic) and V2 receptor activation with cell surface expression of renal acquaporins water channel.

MOLECULAR PHYSIOLOGY OF INTESTINAL N+/H+EXCHANGE

The sodium/hydrogen exchange (NHE) gene family plays an integral role in neutral sodium absorption in the mammalian intestine. The NHE gene family is comprised of nine members that are categorized by cellular localization (i.e., plasma membrane or intracellular). In the gastrointestinal (GI) tract of multiple species, there are resident plasma membrane isoforms including NHE1 (basolateral) and NHE2 (apical), recycling isoforms (NHE3), as well as intracellular isoforms (NHE6, 7, 9). NHE3 recycles between the endosomal compartment and the apical plasma membrane and functions in both locations. NHE3 regulation occurs during normal digestive processes and is often inhibited in diarrheal diseases. The C terminus of NHE3 binds multiple regulatory proteins to form large protein complexes that are involved in regulation of NHE3 trafficking to and from the plasma membrane, turnover number, and protein phosphorylation. NHE1 and NHE2 are not regulated by trafficking. NHE1 interacts with multiple regulatory proteins that affect phosphorylation; however, whether NHE1 exists in large multi-protein complexes is unknown. Although intestinal and colonic sodium absorption appear to involve at least NHE2 and NHE3, future studies are necessary to more accurately define their relative contributions to sodium absorption during human digestion and in pathophysiological conditions.

Transactivation of Trk neurotrophin receptors by G-protein-coupled receptor ligands occurs on intracellular membranes

Neurotrophins, such as NGF and BDNF, activate Trk receptor tyrosine kinases through receptor dimerization at the cell surface followed by autophosphorylation and intracellular signaling. It has been shown that activation of Trk receptor tyrosine kinases can also occur via a G-protein-coupled receptor (GPCR) mechanism, without involvement of neurotrophins. Two GPCR ligands, adenosine and pituitary adenylate cyclase-activating polypeptide (PACAP), can activate Trk receptor activity to increase the survival of neural cells through stimulation of Akt activity. To investigate the mechanism of Trk receptor transactivation, we have examined the localization of Trk receptors in PC12 cells and primary neurons after treatment with adenosine agonists and PACAP. In contrast to neurotrophin treatment, Trk receptors were sensitive to transcriptional and translational inhibitors, and they were found predominantly in intracellular locations particularly associated with Golgi membranes. Biotinylation and immunostaining experiments confirm that most of the transactivated Trk receptors are found in intracellular membranes. These results indicate that there are alternative modes of activating Trk receptor tyrosine kinases in the absence of neurotrophin binding at the cell surface and that receptor signaling may occur and persist inside of neuronal cells.

Lysophosphatidic acid induces exocytic trafficking of Na(+)/H(+) exchanger 3 by E3KARP-dependent activation of phospholipase C

Lysophosphatidic acid (LPA) stimulates Na(+)/H(+) exchanger 3 (NHE3) activity in opossum kidney proximal tubule (OK) cells by increasing the apical membrane amount of NHE3. This occurs by stimulation of exocytic trafficking of NHE3 to the apical plasma membrane by an E3KARP-dependent mechanism. However, it is still unclear how E3KARP leads to the LPA-induced exocytosis of NHE3. In the current study, we demonstrate that stable expression of exogenous E3KARP increases LPA-induced phospholipase C (PLC) activation and subsequent elevation of intracellular Ca(2+) in opossum kidney proximal tubule (OK) cells. Pretreatment with U73122, a PLC inhibitor, prevented the LPA-induced NHE3 activation and the exocytic trafficking of NHE3. To understand how the elevation of intracellular Ca(2+) leads to the stimulation of NHE3, we pretreated OK cells with BAPTA-AM, an intracellular Ca(2+) chelator. BAPTA-AM completely blocked the LPA-induced increase of NHE3 activity and surface NHE3 amount by decreasing the LPA-induced exocytic trafficking of NHE3. Pretreatment with GF109203X, a PKC inhibitor, did not affect the percent of LPA-induced NHE3 activation and increase of surface NHE3 amount. From these results, we suggest that E3KARP plays a necessary role in LPA-induced PLC activation, and that PLC-dependent elevation of intracellular Ca(2+) but not PKC activation is necessary for the LPA-induced increase of NHE3 exocytosis.

À quoi sert le système endothéline ?

Endothelins are a family of three peptides of 21 amino acids with strong vasoconstrictor effects. The three peptides are encoded by three different genes and derived from precursors (" big endothelins") which are cleaved by metalloproteases, named endothelin-converting enzyme. Two receptors have been cloned, ET-A and ET-B which bind the three endothelins with various affinities. The diverse expression pattern of the endothelin system (ET) components is associated with a complex pharmacology and its counteracting physiological actions. New modulators of the ET system have been described : retinoic acid, leptin, prostaglandins, hypoxia. Endothelins can be considered as regulators working in paracrine and autocrine fashion in a variety of organs in different cellular types. The ET system has beneficial and detrimental roles in mammals. The different components have been shown to be essential for a normal embryonic and neonatal development, for renal homeostasis and maintenance of basal vascular tone. They are involved in physiological and tumoral angiogenesis. They affect the physiology and pathophysiology of the liver, muscle, skin, adipose tissue and reproductive tract. The endothelin system participates in the development of atherosclerosis as well as pulmonary hypertension, and mediates cardiac remodeling in heart failure. Elaboration of new animal models (knock-out, pathophysiological models em leader ) will allow the clear genetic dissection of physiological and pathophysiological roles of the endothelin system.

Dopamine acutely decreases apical membrane Na/H exchanger NHE3 protein in mouse renal proximal tubule

BACKGROUND

Dopamine is a principal natriuretic hormone in mammalian Na+ homeostasis. Dopamine acutely alters glomerular filtration rate (GFR) and decreases Na+ absorption in both the proximal and distal nephron. Proximal tubule natriuresis is effected through inhibition of the apical membrane Na/H exchanger NHE3.

METHODS

We examined whether dopamine directly and acutely decreases apical membrane NHE3 protein using renal tissue in two in vitro systems: renal cortical slices and in vitro perfused single tubules. After incubation with dopamine, NHE3 activity was measured by 22Na flux and NHE3 antigen was measured by immunoblot in apical membrane and total cellular membranes.

RESULTS

Direct application of dopamine to either cortical slices or microperfused tubules acutely decreases NHE3 activity and antigen at the apical membrane of the proximal tubule. No change in total cellular NHE3 was detected.

CONCLUSION

One mechanism by which dopamine causes natriuresis is via direct and acute reduction of NHE3 protein at the apical membrane via changes in NHE3 protein trafficking.

Albumin Regulates the Na+/H+Exchanger 3 in OKP Cells

ABSTRACT. Albumin filtered by the glomerulus is reabsorbed in the proximal tubule. We have shown previously that proteinuria stimulates the proximal tubular Na+/H+exchanger 3 (NHE3) in rats. Activation of NHE3 may be a pathophysiologically important factor in the development of renal salt and water retention observed in the nephrotic syndrome. For examining whether albumin is a specific inducer of proximal tubular Na+/H+exchange and to determine the molecular mechanisms by which it regulates Na+/H+exchange, the effect of albumin on NHE3 in opossum kidney cells was studied. Albumin activated Na+/H+exchange in a time- and dose-dependent manner up to 100% in 48 h. In the early phase of stimulation (2 to 12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, increased NHE3 activity was accompanied by increase in cell surface NHE3 protein. The increase in surface NHE3 was associated with increased bidirectional trafficking of NHE3 protein between intracellular compartments and the cell surface. At 48 h, total cell NHE3 protein abundance and mRNA were increased as well. Whereas NHE3 translation was increased, NHE3 protein half-life remained unchanged. The effects of albumin on NHE3 protein abundance were modified by hydrocortisone in a complicated pattern. These results indicate that albumin directly regulates proximal tubular NHE3 at multiple levels.

Angiotensin II stimulates NHE3 activity by exocytic insertion of the transporter: role of PI 3-kinase

BACKGROUND

Low-concentration angiotensin II (Ang II) stimulates Na+/H+ exchanger 3 (NHE3) activity in renal proximal tubule mainly via angiotensin II type 1 (AT1) receptors. The mechanisms that mediate the increase in NHE3 activity elicited by Ang II remain incompletely settled.

METHODS

To assess a potential role of NHE3 trafficking in the Ang II effect, NHE3 activity was measured by H+-driven initial rate of 22Na uptake resistant to 50 micromol/L of the Na+/H+ exchange inhibitor cariporide (HOE642), and sensitive to 300 micromol/L ethyl isopropyl amiloride (EIPA), in a model of cultured proximal tubular cells (MKCC), in which functional apical NHE3 and AT receptors are normally present. Apical expression of NHE3 protein was determined by cell surface biotinylation and immunoblotting.

RESULTS

Ang II (10-10 mol/L, 43 minutes) increased NHE3 activity and biotinylated NHE3 protein without any change in total amount of NHE3 protein. Both effects were suppressed by specific AT1 receptor antagonists. When 2-mercaptoethanesulphonic acid (MESNA) was used to cleave biotin from all apical proteins, intracellular biotinylated NHE3 protein remained unchanged after Ang II incubation compared to control. When sulfo-N-hydrosuccinimide (NHS)-acetate was used first to block all apical reactive sites, an increase in biotinylated NHE3 protein was observed following Ang II incubation. To evaluate the role of phosphatidylinositol 3-kinase (PI 3-kinase), the specific inhibitor wortmannin was used. It suppressed Ang II-induced increase in NHE3 activity and trafficking. Furthermore, latrunculin B, inhibitor of actin filament polymerization, prevented both Ang II stimulatory effects.

CONCLUSION

Ang II stimulates NHE3 activity, at least in part, by exocytic insertion of the protein into the apical membrane. This effect is mediated by PI 3-kinase and required integrity of actin cytoskeleton.

Bimodal acute effects of A1 adenosine receptor activation on Na+/H+ exchanger 3 in opossum kidney cells

Regulation of renal apical Na+/H+ exchanger 3 (NHE3) activity by adenosine has been suggested to contribute to acute control of mammalian Na(+) homeostasis. The mechanism by which adenosine controls NHE3 activity in a renal cell line was examined. The adenosine analog, N(6)-cyclopentyladenosine (CPA) exerts a bimodal effect on NHE3: CPA concentrations >10(-8) M inactivate NHE3, whereas concentrations <10(-8) M stimulate NHE3 activity. Acute CPA-induced control of NHE3 was blocked by antagonists of A1 adenosine receptors and inhibition of phospholipase C, pretreatment with BAPTA-AM (chelator of cellular calcium), and exposure to pertussis toxin. Stimulatory and to some extent also inhibitory CPA concentrations attenuated 8-bromo-cAMP and dopamine-mediated inhibition of NHE3. BAPTA eliminated the ability of a stimulatory dose of CPA to attenuate 8-bromo-cAMP-induced suppression of NHE3 activity. Upon inhibition of protein kinase C, CPA at an inhibitory dose provoked activation of NHE3, which is partially reverted by 8-bromo-cAMP and suppressed by pre-incubation with BAPTA-AM. Cytochalasin B, an actin-modifying agent, selectively prevented downregulation but did not affect upregulation of NHE3 activity by CPA. In conclusion, these observations demonstrate that (1) CPA modulates NHE3 activity by elevation of cellular Ca(2+) exerting a negative control on adenylate cyclase activity, (2) protein kinase C is the determining factor leading to CPA-induced downregulation of NHE3 activity, and (3) alterations of surface NHE3 abundance may contribute to A1 adenosine receptor-dependent inhibition of NHE3 activity.

Multiple modes of regulation of Na+/H+ exchangers

Mammalian Na(+)/H(+) exchangers (NHE) mediate electroneutral countertransport of H(+) for Na(+) across the plasmalemmal and organellar membranes. They contribute to cellular and organellar pH and volume regulation and transepithelial Na(+) transport. The aim of this review is to illustrate the complex regulation of these transporters by focusing on the multiple mechanisms controlling the epithelial isoform, NHE3. A variety of agents and conditions (e.g., hormones, growth factors, cellular pH, and medium osmolarity) act in concert to achieve short-term and long-term regulation of this isoform. The underlying mechanism involves changes in the number of transporters on the cell surface and/or altered activity of the individual exchangers due to allosteric activation by intracellular protons, phosphorylation and interaction with accessory proteins and the cytoskeleton. A similar regulatory versatility probably applies to other NHE isoforms, and the lessons learned from studying members of the NHE family could serve as a useful reference when exploring the modes and levels of regulation of other transporters.

Regulation of the proximal tubular sodium/proton exchanger NHE3 in rats with puromycin aminonucleoside (PAN)-induced nephrotic syndrome

Excessive proteinuria due to loss of glomerular permselectivity in nephrotic syndrome can cause disturbances in renal salt and water handling with edema formation. Apart from oncotic and hydrostatic mechanisms associated with hypoalbuminemia, primary derangements in renal tubular sodium transport may contribute to the pathogenesis of nephrotic edema. Whereas there is evidence for an increase of cortical collecting duct sodium reabsorption in nephrotic rats, it remains controversial whether proximal tubule sodium transport may also be activated in this condition. The regulation of the cortical Na/H exchanger NHE3, the main pathway for Na reabsorption in the proximal tubule (PT), was investigated in rats with puromycin aminonucleoside (PAN)-induced nephrotic syndrome. PAN rats developed reduced GFR, severe proteinuria, and sodium retention within 3 d. After 10 d, immunoblots of brush border vesicles revealed a decreased abundance of NHE3 in nephrotic animals. However, the Na/H antiporter activity in the same vesicle preparations was not significantly altered. Antiporter activity normalized for NHE3 protein was increased by 88% in nephrotic animals (P = 0.025). Immunohistochemistry with the same polyclonal antibody as for immunoblots revealed a decrease of NHE3 abundance in PT. In contrast, immunoreactivity for the monoclonal antibody 2B9, which specifically recognizes the non-megalin-associated, transport-competent pool of NHE3, was higher in PAN-treated rats than in controls. In conclusion, increased sodium reabsorption might be associated with a shift of NHE3 from an inactive pool to an active pool, thus contributing to sodium retention in a state of proteinuria.

Na(+)/H(+) exchanger 3 is in large complexes in the center of the apical surface of proximal tubule-derived OK cells

Cell biological approaches were used to examine the location and function of the brush border (BB) Na(+)/H(+) exchanger NHE3 in the opossum kidney (OK) polarized renal proximal tubule cell line. NHE3 epitope tagged with the vesicular stomatitis virus glycoprotein epitope (NHE3V) was stably expressed and called OK-E3V cells. On the basis of cell surface biotinylation studies, these cells had 10-15% of total NHE3 on the BB. Intracellular NHE3V largely colocalized with Rab11 and to a lesser extent with EEA1. The BB location of NHE3V was examined by confocal microscopy relative to the lectins wheat germ aggluttinin (WGA) and phytohemagluttin E (PHA-E), as well as the B subunit of cholera toxin (CTB). The cells were pyramidal, and NHE3 was located in microvilli in the center of the apical surface. In contrast, PHA-E, WGA, and CTB were diffusely distributed on the BB. Detergent extraction showed that total NHE3V was largely soluble in Triton X-100, whereas virtually all surface NHE3V was insoluble. Sucrose density gradient centrifugation demonstrated that total NHE3V migrated at the same size as approximately 400- and approximately 900-kDa standards, whereas surface NHE3V was enriched in the approximately 900-kDa form. Under basal conditions, NHE3 cycled between the cell surface and the recycling pathway through a phosphatidylinositol (PI) 3-kinase-dependent mechanism. Measurements of surface and intracellular pH were obtained by using FITC-WGA. Internalization of FITC-WGA occurred largely into the juxtanuclear compartment that contained Rab11 and NHE3V. pH values on the apical surface and in endosomes in the presence of the NHE3 blocker, S3226, were elevated, showing that NHE3 functioned to acidify both compartments. In conclusion, NHE3V in OK cells exists in distinct domains both in the center of the apical surface and in a juxtanuclear compartment. In the BB fraction, NHE3 is largely in the detergent-insoluble fraction in lipid rafts and/or in large heterogenous complexes ranging from approximately 400 to approximately 900 kDa.

Insulin activates Na(+)/H(+) exchanger 3: biphasic response and glucocorticoid dependence

Insulin is an important regulator of renal salt and water excretion, and hyperinsulinemia has been implicated to play a role in hypertension. One of the target proteins of insulin action in the kidney is Na(+)/H(+) exchanger 3 (NHE3), a principal Na(+) transporter responsible for salt absorption in the mammalian proximal tubule. The molecular mechanisms involved in activation of NHE3 by insulin have not been studied so far. In opossum kidney (OK) cells, insulin increased Na(+)/H(+) exchange activity in a time- and concentration-dependent manner. This effect is due to activation of NHE3 as it persisted after pharmacological inhibition of NHE1 and NHE2. In the early phase of stimulation (2-12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, enhanced NHE3 activity was accompanied by an increase in total and cell surface NHE3 protein and NHE3 mRNA abundance. All the effects of insulin on NHE3 activity, protein, and mRNA were amplified in the presence of hydrocortisone. These results suggest that insulin stimulates renal tubular NHE3 activity via a biphasic mechanism involving posttranslational factors and an increase in NHE3 gene expression and the effects are dependent on the permissive action of hydrocortisone.

Na+/H+ exchange activity and NHE-3 expression in renal tubules from the spontaneously hypertensive rat

BACKGROUND

The NHE-3 isoform of the Na+/H+ antiporter, in the apical membrane of renal proximal tubule, is responsible for the bulk transport of Na+ and fluid reabsorption. Studies have reported that apical NHE-3 translocates to internal pools, thereby facilitating natriuresis when blood pressure increases abruptly.

METHODS

The present study examined Na+/H+ exchange activity and NHE-3 expression in renal cortical tubules from the spontaneously hypertensive rat (SHR) and WKY rats before and after the development of hypertension. SHR 4 to 6 weeks of age were pre-hypertensive, 6 to 7 weeks old had mild hypertension, and 8 to 13 weeks old had severe hypertension. Renal proximal tubules (PTs) were isolated and purified by Percoll gradient centrifugation. NHE-3 protein and mRNA levels were determined by Western and Northern blots, respectively. Apical brush border membrane vesicles (BBMV) were prepared using the MgSO4 aggregation method and Na+/H+ exchange activity assessed using the acridine orange method.

RESULTS

Na+/H+ exchange activity, determined as the rate of Na+-dependent intracellular pH (pHi) recovery assessed using BCECF after an acute acid load, was significantly greater in PTs from SHR than in WKY rats at all age groups (4 to 6 weeks, 0.30 +/- 0.04 vs. 0.24 +/- 0.02 pH U/30 sec, P < 0.05; 6 to 7 weeks, 0.42 +/- 0.07 vs. 0.29 +/- 0.05 pH U/30 sec, P < 0.05; and 8 to 13 weeks, 0.48 +/- 0.07 vs. 0.40 +/- 0.07 pH U/30 sec, P < 0.05). The Na+-dependent recovery in BBMV was also greater in SHR than WKY rats (1464 +/- 62 vs. 1042 +/- 79 fluorescence. U/5 sec, P < 0.001) and was unaffected by cariporide, a specific NHE-1 inhibitor. NHE-3 protein levels also were significantly higher in SHR than age-matched WKY rats at all stages during the development of hypertension (pre-hypertensive 1.8-fold; early onset hypertension twofold; established hypertension 1.5-fold; each P < 0.05). By contrast, NHE-3 mRNA levels were not different between SHR and WKY rats at each age group.

CONCLUSIONS

Na+/H+ exchange activity and NHE-3 protein abundance in renal proximal tubules from the SHR are increased while NHE-3 mRNA is not. A post-transcriptional event(s) best explains the increase in NHE-3 protein expression since mRNA levels were not increased. The alterations in the SHR antedate the development of hypertension and fail to decrease as blood pressure increases with age in the SHR, which likely results in inappropriate renal sodium retention in the face of a chronic rise in blood pressure.

Cholangiocytes exhibit dynamic, actin-dependent apical membrane turnover

The present studies of cholangiocytes used complementary histological, biochemical, and electrophysiological methods to identify a dense population of subapical vesicles, quantify the rates of vesicular trafficking, and assess the contribution of the actin cytoskeleton to membrane trafficking. FM 1-43 fluorescence measured significant basal rates of total exocytosis (1.33 +/- 0.16% plasma membrane/min) in isolated cholangiocytes and apical exocytosis in cholangiocyte monolayers. Cell surface area remained unchanged, indicating that there was a concurrent, equivalent rate of endocytosis. FM 1-43 washout studies showed that 36% of the endocytosed membrane was recycled to the plasma membrane. 8-(4-Chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP; cAMP analog) increased exocytosis by 71 +/- 31%, whereas the Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS; protein kinase A inhibitor) diminished basal exocytosis by 53 +/- 11%. A dense population of 140-nm subapical vesicles arose, in part, from apical membrane endocytosis. Phalloidin staining showed that a subpopulation of the endocytosed vesicles was encapsulated by F-actin. Furthermore, membrane trafficking was inhibited by disrupting the actin cytoskeleton with cytochalasin D (51 +/- 13% of control) or jasplakinolide (58 +/- 9% of control). These studies indicate that there is a high rate of vesicular trafficking at the apical membrane of cholangiocytes and suggest that both cAMP and the actin cytoskeleton contribute importantly to these events.

Endothelin-1/endothelin-B receptor-mediated increases in NHE3 activity in chronic metabolic acidosis

Decreases in blood pH activate NHE3, the proximal tubular apical membrane Na/H antiporter. In cultured renal epithelial cells, activation of the endothelin-B (ET(B)) receptor increases NHE3 activity. To examine the role of the ET(B) receptor in the response to acidosis in vivo, the present studies examined ET(B) receptor-deficient mice, rescued from neonatal lethality by expression of a dopamine beta-hydroxylase promoter/ET(B) receptor transgene (Tg/Tg:ET(B)(-/-) mice). In proximal tubule suspensions from Tg/Tg:ET(B)(+/-) mice, 10(-8) M endothelin-1 (ET-1) increased NHE3 activity, but this treatment had no effect on tubules from Tg/Tg:ET(B)(-/-) mice. Acid ingestion for 7 days caused a greater decrease in blood HCO(3)(-) concentration in Tg/Tg:ET(B)(-/-) mice compared with Tg/Tg:ET(B)(+/+) and Tg/Tg:ET(B)(+/-) mice. Whereas acid ingestion increased apical membrane NHE3 by 42-46% in Tg/Tg:ET(B)(+/+) and Tg/Tg:ET(B)(+/-) mice, it had no effect on NHE3 in Tg/Tg:ET(B)(-/-) mice. In C57BL/6 mice, excess acid ingestion increased renal cortical preproET-1 mRNA expression 2.4-fold and decreased preproET-3 mRNA expression by 37%. On a control diet, Tg/Tg:ET(B)(-/-) mice had low rates of ammonium excretion, which could not be attributed to an inability to acidify the urine, as well as hypercitraturia, with increased titratable acid excretion. Acid ingestion increased ammonium excretion, citrate absorption, and titratable acid excretion to the same levels in Tg/Tg:ET(B)(-/-) and Tg/Tg:ET(B)(+/+) mice. In conclusion, metabolic acidosis increases ET-1 expression, which increases NHE3 activity via the ET(B) receptor.