Structure/function studies of mammalian Na-H exchangers--an update

Acidosis defense mechanisms in the preimplantation stages of embryos in BALB/c strain mice

Regulation of intracellular pH (pHi) is an important homeostatic function of cells. There are three major pHi regulatory mechanisms: the HCO3-/Cl- exchanger (AE), which alleviates alkalosis, and the Na+/H+ exchanger (NHE) and Na+,HCO3-/Cl- exchanger (NDBCE), both of which counteract acidosis. NHE activity, which is high at the germinal vesicle stage of oocyte, is inhibited during meiotic maturation, while this inhibition is abolished when the oocyte reaches the pronuclear (PN) stage of the zygote. On the other hand, we have previously found that NDBCE performs complementary regulation against acidosis during meiotic maturation. Additionally, we found that AE activity, which is a defense mechanism against alkalosis, gradually decreases during preimplantation period of embryonic development. Considering that NHE activity is inhibited during meiotic maturation and AE activity gradually decreases during embryonic development stages, we investigated whether NHE and NDBCE activities, both of which act against acidosis, functionally change from the PN zygote to the blastocyst stage of the embryo and identified these pH-regulating proteins at the molecular level in mice of the Balb/c strain. PN zygotes, two-cell (2-c), four-cell (4-c), morula and blastocyst stage embryos were obtained from 5-8-week-old, sexually mature female Balb/c mice by using the classical superovulation procedure. pHi was recorded by using the microspectrofluorometric technique on zygotes and embryos simultaneously loaded with the pH-sensitive fluorophore, 2',7'-Bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The activities of NHE and NDBCE were determined from the recovery curve of induced-acidosis in bicarbonate-free and bicarbonate-containing media, respectively. Specific inhibitors such as cariporide (1 μM), S3226 (1 and 10 μM), EIPA (1, 5, and 25 μM), and amiloride (1 mM) were used to functionally identify NHE isoforms, and the nonspecific inhibitor 4,4'-diisocyanatostilbene-2,2' disulphonic acid, disodium salt (DIDS) was used to confirm NDBCE activity. The isoforms of the pHi-regulatory proteins were also identified by molecular biology using real-time PCR. We found that NHE activity was high at all embryonic stages, and differences between stages were not significant. Functional and molecular findings indicated that isoforms of NHE 1 and 5 are present in the blastocyst, whereas isoforms of NHE 1, 3, and 4 are functional at earlier embryonic stages. Although the contribution of NDBCE activity to recovery from induced-acidosis was detected at all embryonic stages, it was significant only in the PN zygote and the 2-c embryo. This finding was confirmed by molecular analysis, which detected the expression of SLC4A8 encoding NDBCE at all embryonic stages. In conclusion, NHE is an active and important defense mechanism against acidosis and is encoded by at least two protein isoforms in all stages of the Balb/c strain of mice. NDBCE has a supportive function in all embryonic stages, especially in the PN zygote and the 2-c embryo. Preimplantation stage embryos have effective mechanisms to defend against acidosis in response to their metabolic end products (increased acid load) and the acidic environment in utero.

Regulation of Electrolyte Permeability by Herbal Monomers in Edematous Disorders

Edema is a gradual accumulation of fluid in the interstitial tissues or luminal cavities, which is regulated by ion transport pathways and reflects dysfunction of fluid and salt homeostasis. Increasing evidence suggests that some herbal monomers significantly reduce organ/tissue edema. In this review, we briefly summarized the electrolyte permeability involved in pathomechanisms of organ edema, and the benefits of herbal monomers on ionic transport machinery, including Na+-K+-ATPase, Na+ and Cl- channels, Na+-K+-2Cl- co-transporter, etc. Pharmaceutical relevance is implicated in developing advanced strategies to mitigate edematous disorders. In conclusion, the natural herbal monomers regulate electrolyte permeability in many edematous disorders, and further basic and clinical studies are needed.

pH and male fertility: making sense on pH homeodynamics throughout the male reproductive tract

In the male reproductive tract, ionic equilibrium is essential to maintain normal spermatozoa production and, hence, the reproductive potential. Among the several ions, HCO3- and H+ have a central role, mainly due to their role on pH homeostasis. In the male reproductive tract, the major players in pH regulation and homeodynamics are carbonic anhydrases (CAs), HCO3- membrane transporters (solute carrier 4-SLC4 and solute carrier 26-SLC26 family transporters), Na+-H+ exchangers (NHEs), monocarboxylate transporters (MCTs) and voltage-gated proton channels (Hv1). CAs and these membrane transporters are widely distributed throughout the male reproductive tract, where they play essential roles in the ionic balance of tubular fluids. CAs are the enzymes responsible for the production of HCO3- which is then transported by membrane transporters to ensure the maturation, storage, and capacitation of the spermatozoa. The transport of H+ is carried out by NHEs, Hv1, and MCTs and is essential for the electrochemical balance and for the maintenance of the pH within the physiological limits along the male reproductive tract. Alterations in HCO3- production and transport of ions have been associated with some male reproductive dysfunctions. Herein, we present an up-to-date review on the distribution and role of the main intervenient on pH homeodynamics in the fluids throughout the male reproductive tract. In addition, we discuss their relevance for the establishment of the male reproductive potential.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Linaclotide improves gastrointestinal transit in cystic fibrosis mice by inhibiting sodium/hydrogen exchanger 3

Gastrointestinal dysfunction in cystic fibrosis (CF) is a prominent source of pain among patients with CF. Linaclotide, a guanylate cyclase C (GCC) receptor agonist, is a US Food and Drug Administration-approved drug prescribed for chronic constipation but has not been widely used in CF, as the cystic fibrosis transmembrane conductance regulator (CFTR) is the main mechanism of action. However, anecdotal clinical evidence suggests that linaclotide may be effective for treating some gastrointestinal symptoms in CF. The goal of this study was to determine the effectiveness and mechanism of linaclotide in treating CF gastrointestinal disorders using CF mouse models. Intestinal transit, chloride secretion, and intestinal lumen fluidity were assessed in wild-type and CF mouse models in response to linaclotide. CFTR and sodium/hydrogen exchanger 3 (NHE3) response to linaclotide was also evaluated. Linaclotide treatment improved intestinal transit in mice carrying either F508del or null Cftr mutations but did not induce detectable Cl^- secretion. Linaclotide increased fluid retention and fluidity of CF intestinal contents, suggesting inhibition of fluid absorption. Targeted inhibition of sodium absorption by the NHE3 inhibitor tenapanor produced improvements in gastrointestinal transit similar to those produced by linaclotide treatment, suggesting that inhibition of fluid absorption by linaclotide contributes to improved gastrointestinal transit in CF. Our results demonstrate that linaclotide improves gastrointestinal transit in CF mouse models by increasing luminal fluidity through inhibiting NHE3-mediated sodium absorption. Further studies are necessary to assess whether linaclotide could improve CF intestinal pathologies in patients. GCC signaling and NHE3 inhibition may be therapeutic targets for CF intestinal manifestations. NEW & NOTEWORTHY Linaclotide's primary mechanism of action in alleviating chronic constipation is through cystic fibrosis transmembrane conductance regulator (CFTR), negating its use in patients with cystic fibrosis (CF). For the first time, our findings suggest that in the absence of CFTR, linaclotide can improve fluidity of the intestinal lumen through the inhibition of sodium/hydrogen exchanger 3. These findings suggest that linaclotide could improve CF intestinal pathologies in patients.

Recent Updates on the Proximal Tubule Renin-Angiotensin System in Angiotensin II-Dependent Hypertension

It is well recognized that the renin-angiotensin system (RAS) exists not only as circulating, paracrine (cell to cell), but also intracrine (intracellular) system. In the kidney, however, it is difficult to dissect the respective contributions of circulating RAS versus intrarenal RAS to the physiological regulation of proximal tubular Na(+) reabsorption and hypertension. Here, we review recent studies to provide an update in this research field with a focus on the proximal tubular RAS in angiotensin II (ANG II)-induced hypertension. Careful analysis of available evidence supports the hypothesis that both local synthesis or formation and AT1 (AT1a) receptor- and/or megalin-mediated uptake of angiotensinogen (AGT), ANG I and ANG II contribute to high levels of ANG II in the proximal tubules of the kidney. Under physiological conditions, nearly all major components of the RAS including AGT, prorenin, renin, ANG I, and ANG II would be filtered by the glomerulus and taken up by the proximal tubules. In ANG II-dependent hypertension, the expression of AGT, prorenin, and (pro)renin receptors, and angiotensin-converting enzyme (ACE) is upregulated rather than downregulated in the kidney. Furthermore, hypertension damages the glomerular filtration barrier, which augments the filtration of circulating AGT, prorenin, renin, ANG I, and ANG II and their uptake in the proximal tubules. Together, increased local ANG II formation and augmented uptake of circulating ANG II in the proximal tubules, via activation of AT1 (AT1a) receptors and Na(+)/H(+) exchanger 3, may provide a powerful feedforward mechanism for promoting Na(+) retention and the development of ANG II-induced hypertension.

Morphologic and transcriptomic assessment of bovine embryos exposed to dietary long-chain fatty acids

The main objectives of this study were to determine the influence of diets enriched in α-linolenic, linoleic or oleic acid on the development and transcriptomic profile of embryos collected from dairy cattle. Non-lactating Holstein cows received one of the three diets supplemented with 8% rolled oilseeds: flax (FLX, n = 8), sunflower (SUN, n = 7) or canola (CAN, n = 8). After a minimum 35-day diet adaptation, cows were superovulated, artificially inseminated and ova/embryos recovered non-surgically after 7.5 days. Cows fed FLX had less degenerated embryos and more viable embryos than those fed CAN or SUN. In total, 175 genes were differentially expressed in blastocysts from cows fed FLX than in cows fed CAN or SUN. These differentially expressed genes were mainly involved in cellular growth and proliferation, cellular development, and cell survival and viability. In conclusion, dietary n-3 polyunsaturated fatty acids reduced early embryonic degeneration possibly through improving embryonic cell survival and viability.

The Physiology and Pathophysiology of Pancreatic Ductal Secretion: The Background for Clinicians

The human exocrine pancreas consists of 2 main cell types: acinar and ductal cells. These exocrine cells interact closely to contribute to the secretion of pancreatic juice. The most important ion in terms of the pancreatic ductal secretion is HCO3. In fact, duct cells produce an alkaline fluid that may contain up to 140 mM NaHCO3, which is essential for normal digestion. This article provides an overview of the basics of pancreatic ductal physiology and pathophysiology. In the first part of the article, we discuss the ductal electrolyte and fluid transporters and their regulation. The central role of cystic fibrosis transmembrane conductance regulator (CFTR) is highlighted, which is much more than just a Cl channel. We also review the role of pancreatic ducts in severe debilitating diseases such as cystic fibrosis (caused by various genetic defects of cftr), pancreatitis, and diabetes mellitus. Stimulation of ductal secretion in cystic fibrosis and pancreatitis may have beneficial effects in their treatment.

Rapamycin Inhibition of mTOR Reduces Levels of the Na+/H+ Exchanger 3 in Intestines of Mice and Humans, Leading to Diarrhea

BACKGROUND & AIMS

The immunosuppressant rapamycin frequently causes noninfectious diarrhea in organ transplant recipients. We investigated the mechanisms of this process.

METHODS

We performed a retrospective analysis of renal transplant recipients treated with rapamycin from 2003 through 2010 at Albany Medical College, collecting data on serum levels of rapamycin. Levels of the Na+/H+ exchanger 3 (NHE3) were measured in human ileal biopsy specimens from patients who did and did not receive rapamycin (controls), in ileum tissues from rats or mice given rapamycin, and in mice with intestine-specific disruption of mammalian target of rapamycin (Mtor) (mTOR(f/f):Villin-cre mice) or Atg7 (Atg7(flox/flox); Villin-Cre). Exchange activity and intestinal water absorption were measured using a pH-sensitive dye and small intestine perfusion, respectively.

RESULTS

Episodes of noninfectious diarrhea occurred in organ recipients after increases in serum levels of rapamycin. The expression of NHE3 was reduced in the ileal brush border of patients with diarrhea. In rats and mice, continuous administration of low doses of rapamycin reduced levels of NHE3 in intestinal tissues; this effect was not observed in mice with intestinal deletion of ATG7, indicating that autophagy is required for the reduction. Administration of single high doses of rapamycin to mice, to model the spikes in rapamycin levels that occur in patients with severe diarrheal episodes, resulted in reduced phosphorylation of S6 and AKT in ileal tissues, indicating inhibition of the mTOR complex (mTORC1 and mTORC2). The intestines of mice with intestine-specific deletion of mTOR were dilated and contained large amounts of liquid stools; they also had reduced levels of total NHE3 and NHERF1 compared with control mice. We observed a significant reduction in Na(+)/H(+) exchange activity in ileum tissues from these mice.

CONCLUSIONS

Rapamycin inhibition of mTOR reduces levels of NHE3 and Na(+)/H(+) exchange activity in intestinal tissues of patients and rodents. This process appears to require the autophagic activity mediated by ATG7. Loss of mTOR regulation of NHE3 could mediate the development of diarrhea in patients undergoing rapamycin therapy.

Methotrexate influx via folate transporters into alveolar epithelial cell line A549

Methotrexate (MTX), a drug used for the treatment of certain cancers as well as rheumatoid arthritis, sometimes induces serious interstitial lung injury. Although lung toxicity of MTX is related to its accumulation, the information concerning MTX transport in the lungs is lacking. In this study, we investigated the mechanisms underlying MTX influx into human alveolar epithelial cell line A549. MTX influx into A549 cells was time-, pH-, and temperature-dependent and showed saturation kinetics. The influx was inhibited by folic acid with IC50 values of 256.1 μM at pH 7.4 and 1.6 μM at pH 5.5, indicating that the mechanisms underlying MTX influx would be different at these pHs. We then examined the role of two folate transporters in MTX influx, reduced folate carrier (RFC) and proton-coupled folate transporter (PCFT). The expression of RFC and PCFT mRNAs in A549 cells was confirmed by reverse transcription polymerase chain reaction. In addition, MTX influx was inhibited by thiamine monophosphate, an RFC inhibitor, at pH 7.4, and by sulfasalazine, a PCFT inhibitor, at pH 5.5. These results indicated that RFC and PCFT are predominantly involved in MTX influx into A549 cells at pH 7.4 and pH 5.5, respectively.

Reduction of proteinuria through podocyte alkalinization

Podocytes are highly differentiated cells and critical elements for the filtration barrier of the kidney. Loss of their foot process (FP) architecture (FP effacement) results in urinary protein loss. Here we show a novel role for the neutral amino acid glutamine in structural and functional regulation of the kidney filtration barrier. Metabolic flux analysis of cultured podocytes using genetic, toxic, and immunologic injury models identified increased glutamine utilization pathways. We show that glutamine uptake is increased in diseased podocytes to couple nutrient support to increased demand during the disease state of FP effacement. This feature can be utilized to transport increased amounts of glutamine into damaged podocytes. The availability of glutamine determines the regulation of podocyte intracellular pH (pHi). Podocyte alkalinization reduces cytosolic cathepsin L protease activity and protects the podocyte cytoskeleton. Podocyte glutamine supplementation reduces proteinuria in LPS-treated mice, whereas acidification increases glomerular injury. In summary, our data provide a metabolic opportunity to combat urinary protein loss through modulation of podocyte amino acid utilization and pHi.

The major facilitative folate transporters solute carrier 19A1 and solute carrier 46A1: biology and role in antifolate chemotherapy of cancer

This review summarizes the biology of the major facilitative membrane transporters, the reduced folate carrier (RFC) (Solute Carrier 19A1) and the proton-coupled folate transporter (PCFT) (Solute Carrier 46A1). Folates are essential vitamins, and folate deficiency contributes to a variety of health disorders. RFC is ubiquitously expressed and is the major folate transporter in mammalian cells and tissues. PCFT mediates the intestinal absorption of dietary folates and appears to be important for transport of folates into the central nervous system. Clinically relevant antifolates for cancer, such as methotrexate and pralatrexate, are transported by RFC, and loss of RFC transport is an important mechanism of methotrexate resistance in cancer cell lines and in patients. PCFT is expressed in human tumors, and is active at pH conditions associated with the tumor microenvironment. Pemetrexed is an excellent substrate for both RFC and PCFT. Novel tumor-targeted antifolates related to pemetrexed with selective membrane transport by PCFT over RFC are being developed. In recent years, there have been major advances in understanding the structural and functional properties and the regulation of RFC and PCFT. The molecular bases for methotrexate resistance associated with loss of RFC transport and for hereditary folate malabsorption, attributable to mutant PCFT, were determined. Future studies should continue to translate molecular insights from basic studies of RFC and PCFT biology into new therapeutic strategies for cancer and other diseases.

Short- and long-term regulation of intestinal Na+/H+ exchange activity associated with TLR2 receptor activation is independent of nuclear factor-κB signaling

Type 2 Toll-like receptors (TLR2s) are expressed in cell membranes and recognize a wide range of pathogen-associated molecular patterns derived from bacteria, such as lipoteichoic acid (LTA). The aim of this study was to evaluate the effect of TLR2 activation by LTA on the activity of type 1 Na(+)/H(+) exchanger (NHE) in T84 intestinal epithelial cells. Short-term (0.5 hour) and long-term (18 hours) TLR2 activation significantly inhibited NHE1 activity in a concentration-dependent manner (0.01-100 µg/ml; -7 ± 3 to -21 ± 3% and 3 ± 3 to -21 ± 3% of control values, respectively). S3226 [3-[2-(3-guanidino-2-methyl-3-oxopropenyl)-5-methyl-phenyl]-N-isopropylidene-2-methyl-acrylamide dihydrochloride], an NHE3-selective inhibitor, did not affect the inhibitory effect on NHE activity. LTA-induced NHE inhibition did not occur in the presence ofethylisopropylamiloride (an NHE1 inhibitor). Long-term TLR2 activation decreased NHE1 affinity for Na(+) (Km= 64.98 ± 1.67 mM) compared with control (Km= 20.44 ± 0.54 mM) without changes in Vmax values. After TLR2 activation, we observed tyrosine-protein kinase (SRC) activation, phosphatidylinositol 3-kinase (PI3K) recruitment, and adenylyl cyclase (AC3) phosphorylation. The total amount of AC3 increased (23 ± 8% of control) after long-term treatment with LTA. Anti-AC3 small interfering RNA prevented LTA-induced NHE1 inhibition, similar to that observed with the AC3 inhibitor KH7 [(±)-2-(1H-benzimidazol-2-ylthio)propanoic acid 2-[(5-bromo-2-hydroxyphenyl)methylene]hydrazide]. A significant increase in cAMP levels (32 ± 3% and 14 ± 2% after short- and long-term stimulation, respectively) was detected, and inhibition of protein kinase A (PKA), phospholipase C (PLC), and downregulation of protein kinase C (PKC) prevented NHE1 inhibition. Inhibition of nuclear factor-κΒ (NF-κB) failed to revert NHE1 inhibition. We concluded that activation of TLR2 reduces NHE1 activity in epithelial cells through an alternative pathway that is unrelated to NF-κB, which involves SCR, PI3K, AC3, PKA, PLC, and PKC.

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.

Identification of functionally distinct Na-HCO3 co-transporters in colon

Na-HCO₃ cotransport (NBC) regulates intracellular pH (pHi) and HCO₃ secretion in rat colon. NBC has been characterized as a 5,5′-diisothiocyanato-2-2′-stilbene (DIDS)-sensitive transporter in several tissues, while the colonic NBC is sensitive to both amiloride and DIDS. In addition, the colonic NBC has been identified as critical for pHi regulation as it is activated by intravesicular acid pH. Molecular studies have identified several characteristically distinct NBC isoforms [i.e. electrogenic (NBCe) and electroneutral (NBCn)] that exhibit tissue specific expression. This study was initiated to establish the molecular identity and specific function of NBC isoforms in rat colon. Northern blot and reverse transcriptase PCR (RT-PCR) analyses revealed that electrogenic NBCe1B or NBCe1C (NBCe1B/C) isoform is predominantly expressed in proximal colon, while electroneutral NBCn1C or NBCn1D (NBCn1C/D) is expressed in both proximal and distal colon. Functional analyses revealed that amiloride-insensitive, electrogenic, pH gradient-dependent NBC activity is present only in basolateral membranes of proximal colon. In contrast, amiloride-sensitive, electroneutral, [H⁺]-dependent NBC activity is present in both proximal and distal colon. Both electrogenic and electroneutral NBC activities are saturable processes with an apparent Km for Na of 7.3 and 4.3 mM, respectively; and are DIDS-sensitive with apparent Ki of 8.9 and 263.8 µM, respectively. In addition to Na-H exchanger isoform-1 (NHE1), pHi acidification is regulated by a HCO₃-dependent mechanism that is HOE694-insensitive in colonic crypt glands. We conclude from these data that electroneutral, amiloride-sensitive NBC is encoded by NBCn1C/D and is present in both proximal and distal colon, while NBCe1B/C encodes electrogenic, amiloride-insensitive Na-HCO₃ cotransport in proximal colon. We also conclude that NBCn1C/D regulates HCO₃-dependent HOE694-insensitive Na-HCO₃ cotransport and plays a critical role in pHi regulation in colonic epithelial cells.

Failure to downregulate the epithelial sodium channel causes salt sensitivity in Hsd11b2 heterozygote mice

In vivo, the enzyme 11β-hydroxysteroid dehydrogenase type 2 influences ligand access to the mineralocorticoid receptor. Ablation of the encoding gene, HSD11B2, causes the hypertensive syndrome of apparent mineralocorticoid excess. Studies in humans and experimental animals have linked reduced 11β-hydroxysteroid dehydrogenase type 2 activity and salt sensitivity of blood pressure. In the present study, renal mechanisms underpinning salt sensitivity were investigated in Hsd11b2(+/-) mice fed low-, standard-, and high-sodium diets. In wild-type mice, there was a strong correlation between dietary sodium content and fractional sodium excretion but not blood pressure. High sodium feeding abolished amiloride-sensitive sodium reabsorption, consistent with downregulation of the epithelial sodium channel. In Hsd11b2(+/-) mice, the natriuretic response to increased dietary sodium content was blunted, and epithelial sodium channel activity persisted. High-sodium diet also reduced renal blood flow and increased blood pressure in Hsd11b2(+/-) mice. Aldosterone was modulated by dietary sodium in both genotypes, and salt sensitivity in Hsd11b2(+/-) mice was associated with increased plasma corticosterone levels. Chronic administration of an epithelial sodium channel blocker or a glucocorticoid receptor antagonist prevented salt sensitivity in Hsd11b2(+/-) mice, whereas mineralocorticoid receptor blockade with spironolactone did not. This study shows that reduced 11β-hydroxysteroid dehydrogenase type 2 causes salt sensitivity of blood pressure because of impaired renal natriuretic capacity. This reflects deregulation of epithelial sodium channels and increased renal vascular resistance. The phenotype is not caused by illicit activation of mineralocorticoid receptors by glucocorticoids but by direct activation of glucocorticoid receptors.

Role of Na+ /H+ exchanger 3 in the acidification of the male reproductive tract and male fertility

1. Male fertility is a complex process that is dependent on sex hormones and the normal function of the reproductive organs. Defects of these organs result in abnormal sperm production and function, which, in turn, lead to infertility. 2. Spermatozoa released from the testis are unable to move and fertilize with eggs. These features, known as sperm maturation, are acquired during their transit through the epididymis. 3. Among several processes that take place in the epididymis, absorption and acidification of the luminal fluid are essential for sperm maturation, sperm storage and fertility. Currently, the mechanism by which acidification occurs in the epididymis is still not fully understood. 4. The epididymis is fully equipped with the proteins required for acid/base transport, such as Na(+) /H(+) exchanger 3 (NHE3, SLC9A3), vacuolar-type adenosine triphosphatase (V-ATPase) and various isoforms of enzyme carbonic anhydrase (CA). 5. Most studies, so far, have focused on the role of V-ATPase on H(+) secretion and acidification of the epididymis. The involvement of NHE3 in creating the acidic environment of the epididymal spermatozoa receives little attention. 6. This review presents evidence for and discusses the role of NHE3 in the acidification of the male reproductive tract and its requirement for male fertility.

Plasma-accessible carbonic anhydrase at the tissue of a teleost fish may greatly enhance oxygen delivery: in vitro evidence in rainbow trout, Oncorhynchus mykiss

During a generalized acidosis in rainbow trout, catecholamines are released into the blood, activating red blood cell (RBC) Na+/H+ exchange (βNHE), thus protecting RBC intracellular pH (pHi) and subsequent O2 binding at the gill. Because of the presence of a Root effect (a reduction in oxygen carrying capacity of the blood with a reduction in pH), the latter could otherwise be impaired. However, plasma-accessible carbonic anhydrase (CA) at the tissues (and absence at the gills) may result in selective short-circuiting of RBC βNHE pH regulation. This would acidify the RBCs and greatly enhance O2 delivery by exploitation of the combined Bohr-Root effect, a mechanism not previously proposed. As proof-of-principle, an in vitro closed system was developed to continuously monitor extracellular pH (pHe) and O2 tension (PO2) of rainbow trout blood. In this closed system, adding CA to acidified, adrenergically stimulated RBCs short-circuited βNHE pH regulation, resulting in an increase in PO2 by >30 mmHg, depending on the starting Hb-O2 saturation and degree of initial acidification. Interestingly, in the absence of adrenergic stimulation, addition of CA still elevated PO2, albeit to a lesser extent, a response that was absent during general NHE inhibition. If plasma-accessible CA-mediated short-circuiting is operational in vivo, the combined Bohr-Root effect system unique to teleost fishes could markedly enhance tissue O2 delivery far in excess of that in vertebrates possessing a Bohr effect alone and may lead to insights about the early evolution of the Root effect.

Use it or lose it? Sablefish, Anoplopoma fimbria, a species representing a fifth teleostean group where the βNHE associated with the red blood cell adrenergic stress response has been secondarily lost

Like most teleosts, sablefish (Anoplopoma fimbria Pallas 1814) blood exhibits a moderate Root effect (~35% maximal desaturation), where a reduction in blood pH dramatically reduces O2 carrying capacity, a mechanism important for oxygenating the eye and filling the swim bladder (SB) in teleosts. Although sablefish lack a SB, we observed a well-defined choroid rete at the eye. The adrenergically mediated cell swelling typically associated with a functional red blood cell (RBC) β-adrenergic Na+/H+ exchanger (βNHE), which would normally protect RBC pH, and thus O2 transport, during a generalized acidosis, was not observed in sablefish blood. Neither isoproterenol (a β-agonist) nor 8-bromo cAMP could elicit this response. Furthermore, RBC osmotic shrinkage, known to stimulate NHEs in general and βNHE in other teleosts such as trout and flounder, resulted in no significant regulatory volume increase (RVI), further supporting the absence of a functional RBC βNHE. The onset of the Root effect occurs at a much lower RBC pH (6.83–6.92) than in other teleosts, and thus RBC βNHE may not be required to protect O2 transport during a generalized acidosis in vivo. Phylogenetically, sablefish may represent a fifth group of teleosts exhibiting a secondary reduction or loss of βNHE activity. However, sablefish have not lost the choroid rete at the eye (unlike in the other four groups), which may still function with the Root effect to oxygenate the retina, but the low pH onset of the Root effect may ensure haemoglobin (Hb)-O2 binding is not compromised at the respiratory surface during a general acidosis in the absence of RBC βNHE. The sablefish may represent an anomaly within the framework of Root effect evolution, in that they possess a moderate Root effect and a choroid rete at the eye, but lack the RBC βNHE and the SB system.

Ouabain stimulates Na-K-ATPase through a sodium/hydrogen exchanger-1 (NHE-1)-dependent mechanism in human kidney proximal tubule cells

Recent investigations demonstrate increased Na/H exchanger-1 (NHE-1) activity and plasma levels of ouabain-like factor in spontaneously hypertensive rats. At nanomolar concentrations, ouabain increases Na-K-ATPase activity, induces cell proliferation, and activates complex signaling cascades. We hypothesize that the activity of NHE-1 and Na-K-ATPase are interdependent. To test whether treatment with picomolar ouabain regulates Na-K-ATPase through an NHE-1-dependent mechanism, we examined the role of NHE-1 in ouabain-mediated stimulation of Na-K-ATPase in kidney proximal tubule cell lines [opossum kidney (OK), HK-2, HKC-5, and HKC-11] and rat kidney basolateral membranes. Ouabain stimulated Na-K-ATPase activity and tyrosine phosphorylation in cells that express NHE-1 (OK, HKC-5, and HKC-11) but not in HK-2 cells that express very low levels of NHE-1. Inhibition of NHE-1 with 5 microM EIPA, a NHE-1-specific inhibitor, prevented ouabain-mediated stimulation of (86)Rb uptake and Na-K-ATPase phosphorylation in OK, HKC-5, and HKC-11 cells. Expression of wild-type NHE-1 in HK2 cells restored regulation of Na-K-ATPase by picomolar ouabain. Treatment with picomolar ouabain increased membrane expression of Na-K-ATPase and enhanced NHE-1-Na-K-ATPase alpha1-subunit association. Treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) increased Na-K-ATPase activity, expression, phosphorylation, and association with NHE-1 increased in rat kidney cortical basolateral membranes. Eight days' treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) resulted in increased blood pressure in these rats. These results suggest that the association of NHE-1 with Na-K-ATPase is critical for ouabain-mediated regulation of Na-K-ATPase and that these effects may play a role in cardioglycoside-stimulated hypertension.

Molecular identification of Na(+)-H(+) exchanger isoforms (NHE2) in the gills of the euryhaline teleost Fundulus heteroclitus

In the current study, reverse-transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) PCR were used to clone full-length putative Na(+)-H(+) exchanger isoforms (NHE2a) cDNA from the gills of Fundulus heteroclitus. The 2480 bp cDNA includes a coding region for a protein that shows a 57% amino acid homology to rabbit NHE2. These sequences allowed data mining of available fish genome data, which revealed at least three NHE2 subtypes in some teleost species.

Angiotensin II directly regulates intestinal epithelial NHE3 in Caco2BBE cells

BACKGROUND

Angiotensin II (AII) effects on intestinal Na+ transport may be multifactorial. To determine if AII might have a direct effect on intestinal epithelial Na+ transport, we investigated its actions on Na+ transport in human intestinal epithelial Caco2BBE cells.

RESULTS

AII increased apical (brush border) sodium-hydrogen exchanger (NHE)-3, but not NHE2, activity within one hour. Similarly, only apical membrane NHE3 abundance increased at 1-2 hours without any change in total NHE3 protein abundance. From 4-48 hours, AII stimulated progressively larger increases in apical NHE3 activity and surface abundance, which was associated with increases in NHE3 protein expression. At 4-24 hours, NHE3 mRNA increases over baseline expression, suggesting increased gene transcription. This was supported by AII induced increases in rat NHE3 gene promoter-reporter activity. AII induction of NHE3 was blocked by the AII type I receptor antagonist losartan. Acute changes in AII-induced increases in NHE3 exocytosis were blocked by a phospholipase C inhibitor, an arachidonic acid cytochrome P450 epoxygenase inhibitor, as well as phosphatidylinositol 3 kinase (PI3K) inhibitors and Akt inhibitor, partially blocked by a metalloproteinase inhibitor and an EGF (epidermal growth factor) receptor kinase inhibitor, but not affected by an inhibitor of MEK-1 (MAPKK-1, mitogen activated protein kinase kinase-1).

CONCLUSION

We conclude that angiotensin II has a direct role in regulating intestinal fluid and electrolyte absorption which may contribute to its overall effects in regulation systemic volume and blood pressure. AII activates several key signaling pathways that induce acute and chronic changes in NHE3 membrane trafficking and gene transcription.

Membrane transporters and folate homeostasis: intestinal absorption and transport into systemic compartments and tissues

Members of the family of B9 vitamins are commonly known as folates. They are derived entirely from dietary sources and are key one-carbon donors required for de novo nucleotide and methionine synthesis. These highly hydrophilic molecules use several genetically distinct and functionally diverse transport systems to enter cells: the reduced folate carrier, the proton-coupled folate transporter and the folate receptors. Each plays a unique role in mediating folate transport across epithelia and into systemic tissues. The mechanism of intestinal folate absorption was recently uncovered, revealing the genetic basis for the autosomal recessive disorder hereditary folate malabsorption, which results from loss-of-function mutations in the proton-coupled folate transporter gene. It is therefore now possible to piece together how these folate transporters contribute, both individually and collectively, to folate homeostasis in humans. This review focuses on the physiological roles of the major folate transporters, with a brief consideration of their impact on the pharmacological activities of antifolates.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Down-regulated in adenoma Cl/HCO3 exchanger couples with Na/H exchanger 3 for NaCl absorption in murine small intestine

BACKGROUND & AIMS

Electroneutral NaCl absorption across small intestine contributes importantly to systemic fluid balance. Disturbances in this process occur in both obstructive and diarrheal diseases, eg, cystic fibrosis, secretory diarrhea. NaCl absorption involves coupling of Cl(-)/HCO(3)(-) exchanger(s) primarily with Na(+)/H(+) exchanger 3 (Nhe3) at the apical membrane of intestinal epithelia. Identity of the coupling Cl(-)/HCO(3)(-) exchanger(s) was investigated using mice with gene-targeted knockout (KO) of Cl(-)/HCO(3)(-) exchangers: Slc26a3, down-regulated in adenoma (Dra) or Slc26a6, putative anion transporter-1 (Pat-1).

METHODS

Intracellular pH (pH(i)) of intact jejunal villous epithelium was measured by ratiometric microfluoroscopy. Ussing chambers were used to measure transepithelial (22)Na(36)Cl flux across murine jejunum, a site of electroneutral NaCl absorption. Expression was estimated using immunofluorescence and quantitative polymerase chain reaction.

RESULTS

Basal pH(i) of DraKO epithelium, but not Pat-1KO epithelium, was alkaline, whereas pH(i) in the Nhe3KO was acidic relative to wild-type. Altered pH(i) was associated with robust Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange activity in the DraKO and Nhe3KO villous epithelium, respectively. Contrary to genetic ablation, pharmacologic inhibition of Nhe3 in wild-type did not alter pH(i) but coordinately inhibited Dra. Flux studies revealed that Cl(-) absorption was essentially abolished (>80%) in the DraKO and little changed (<20%) in the Pat-1KO jejunum. Net Na(+) absorption was unaffected. Immunofluorescence demonstrated modest Dra expression in the jejunum relative to large intestine. Functional and expression studies did not indicate compensatory changes in relevant transporters.

CONCLUSIONS

These studies provide functional evidence that Dra is the major Cl(-)/HCO(3)(-) exchanger coupled with Nhe3 for electroneutral NaCl absorption across mammalian small intestine.

A combined treatment with ethanol and 6-dimethylaminopurine is effective for the activation and further embryonic development of oocytes from Sprague-Dawley and Wistar rats

In nuclear-transferred or round spermatid-injected oocytes, artificial activation is required for further development in mammals. Although strontium chloride is widely used as the reagent for inducing oocyte activation in mice, the optimal method for oocyte activation remains controversial in rats because ovulated rat oocytes are spontaneously activated in vitro before artificial activation is applied. In our previous study, we found that cytostatic factor activity, which is indispensable for arrest at the MII stage, is potentially low in rats and that this activity differs greatly between two outbred rats (Slc: Sprague-Dawley (SD) and Crj: Wistar). Therefore, it is necessary to establish an optimal protocol for oocyte activation independent of strains. Given that comparative studies of the in vitro development of oocytes activated by different activation protocols are very limited, we compared four different protocols for oocyte activation (ethanol, ionomycin, strontium and electrical pulses) in two different SD and Wistar rats. Our results show that oocytes derived from SD rats have significantly higher cleavage and blastocyst formation than those from Wistar rats independent of activation regimes. In both types of rat, ethanol treatment provided significantly higher developmental ability at cleavage and blastocyst formation compared to the other activation protocols. However, the initial culture in a fertilization medium (high osmolarity mR1ECM) for 24 h showed a detrimental effect on the further in vitro development of parthenogenetic rat oocytes. Taken together, our results show that ethanol treatment is the optimal protocol for the activation of rat oocytes in SD and Wistar outbred rats. Our data also suggest that high-osmolarity media are inadequate for the in vitro development of parthenogenetically activated oocytes compared with fertilized oocytes.

Gastrointestinal distribution and kinetic characterization of the sodium-hydrogen exchanger isoform 8 (NHE8)

NHE8 is a newly identified NHE isoform expressed in rat intestine. To date, the kinetic characteristics and the intestinal segmental distribution of this NHE isoform have not been studied. This current work was performed to determine the gene expression pattern of the NHE8 transporter along the gastrointestinal tract, as well as its affinity for Na(+), H(+), and sensitivity to known NHE inhibitors HOE694 and S3226. NHE8 was differentially expressed along the GI tract. Higher NHE8 expression was seen in stomach, duodenum, and ascending colon in human, while higher NHE8 expression was seen in jejunum, ileum and colon in adult mouse. Moreover, the expression level of NHE8 is much higher in the stomach and jejunum in young mice compared with adult mice. To evaluate the functional characterictics of NHE8, the pH indicator SNARF-4 was used to monitor the rate of intra-cellular pH (pH(i)) recovery after an NH(4)Cl induced acid load in NHE8 cDNA transfected PS120 cells. The NHE8 cDNA transfected cells exhibited a sodium-dependent proton exchanger activity having a Km for pH(i) of approximately pH 6.5, and a Km for sodium of approximately 23 mM. Low concentration of HOE694 (1 microM) had no effect on NHE8 activity, while high concentration (10 microM) significantly reduced NHE8 activity. In the presence of 80 microM S3226, the NHE8 activity was also inhibited significantly. In conclusion, our work suggests that NHE8 is expressed along the gastrointestinal tract and NHE8 is a functional Na(+)/H(+) exchanger with kinetic characteristics that differ from other apically expressed NHE isoforms.

PGE2 reduces net water and chloride absorption from the rat colon by targeting the Na+/H+ exchanger and the Na+ K+ 2Cl- cotransporter

An effect of PGE2 on water and chloride absorption was already established in a previous work. This study is an attempt to find the mechanism of action of the prostaglandin by investigating the involvement of three major transporters namely the Na+ -K+ ATPase, the Na+/H+ exchanger and the Na+ K+ 2Cl- cotransporter. Rats were injected with PGE2 and 15 min later, the colon was perfused in situ with Krebs Ringer buffer, and net water and chloride absorption were determined. When the involvement of the cotransporter and/or the exchanger was investigated, animals were injected with, respectively, furosemide and amiloride 10 min before PGE2. Superficial and crypt colonocytes were then isolated and the protein expression of the Na+ -K+ ATPase and the Na+ K+ 2Cl- was determined by western blot analysis. The effect of PGE2 on the pump activity in presence or absence of the transporters' inhibitors was also studied. PGE2 decreased net water and chloride absorption from the colon, increased the Na+ -K+ ATPase activity in superficial cells and reduced it in crypt cells. The prostaglandin was found to stimulate secretion in superficial cells by targeting the Na+ K+ 2Cl- symporter, and reduce absorption in crypt cells by targeting the Na+/H+ antiporter. Changes in the activity of the pump are secondary to changes in the activity of the other transporters.

Pharmacodynamic characteristics and cardioprotective effects of new NHE1 inhibitors

Inhibitors of Na(+)/H(+) exchanger (NHE) 1 have been shown to exert protective effects on various myocardial injuries. In this study, we characterized the pharmacodynamic properties of new guanidine NHE1 inhibitors (cariporide, sabiporide, KR-32511, KR-32570, and KR-33028) to analyze their myocardial protective effects. Although NHE1 is the major NHE isoform in cardiomyocytes, IC(50)values of these chemicals tested in rat cardiomyocytes were significantly different from those in PS120/hNHE1 cells where human NHE1 is heterologously expressed. In rat cardiomyocytes, KR-32570 and KR-33028 exhibited the highest potencies and their IC(50)values were 7+/-2 nM and 9+/-3 nM, respectively. The IC(50)values of all the chemicals tested on rat submandibular gland NHE2 were in the micromolar range, and they showed no inhibitory effects on hNHE3 and epithelial Na(+) channels up to 30 microM, suggesting a high selectivity toward NHE1. Sabiporide and KR-32570 exhibited slow dissociation kinetics with NHE1 inhibition persisting even after rinsing-out. When the cytoprotective effects of chemicals against hypoxic damage of rat cardiomyocytes were examined, the order of potency was KR-32570>or=KR-33028>sabiporide>cariporide>KR-32511. This order was exactly the same as that for the NHE1 inhibition in rat cardiomyocytes and did not correlate with any other properties, including the slow dissociation kinetics. Taken together, these results suggest that KR-32570 and KR-33028 are potent candidates for cardioprotective agents, and that the IC(50) in the target organ is the most critical factor governing the cytoprotective effects of NHE1 inhibitors.

Mechanisms of dietary Cu uptake in freshwater rainbow trout: evidence for Na-assisted Cu transport and a specific metal carrier in the intestine

Copper (Cu) is both a vital nutrient and a potent toxicant. The objective of this study was to analyze the mechanistic nature of intestinal Cu transport in rainbow trout using radiolabeled Cu (64Cu) and an in vitro gut sac technique. Reduction of mucosal NaCl levels inhibited Cu transport while increase caused stimulation; Na(2)SO(4) had an identical effect, implicating Na(+) rather than the anion. These responses were unrelated to solvent drag, osmotic pressure or changes in transepithelial potential. The presence of elevated luminal Ag stimulated Cu and Na(+) uptake. Phenamil caused a partial inhibition of both Cu and Na(+) uptake while hypercapnia stimulated Na(+) and Cu transport. Cu uptake was sensitive to luminal pH and inhibited by a tenfold excess of Fe and Zn. These factors had no effect on Na(+ )uptake. On the basis of these results we propose a novel Na(+)-assisted mechanism of Cu uptake wherein the Na(+) gradient stimulates an increase in the H(+) concentration of the brushborder creating a suitable microenvironment for the effective transport of Cu via either DMT1 or Ctr1.

The putative mechanism of Na+ absorption in euryhaline elasmobranchs exists in the gills of a stenohaline marine elasmobranch, Squalus acanthias

We recently cloned an NHE3 orthologue from the gills of the euryhaline Atlantic stingray (Dasyatis sabina), and generated a stingray NHE3 antibody to unequivocally localize the exchanger to the apical side of epithelial cells that are rich with Na(+)/K(+)-ATPase (A MRC). We also demonstrated an increase in NHE3 expression when stingrays are in fresh water, suggesting that NHE3 is responsible for active Na(+) absorption. However, the vast majority of elasmobranchs are only found in marine environments. In the current study, immunohistochemistry with the stingray NHE3 antibody was used to localize the exchanger in the gills of the stenohaline marine spiny dogfish shark (Squalus acanthias). NHE3 immunoreactivity was confined to the apical side of cells with basolateral Na(+)/K(+)-ATPase and was excluded from cells with high levels of vacuolar H(+)-ATPase. Western blots detected a single protein of 88 kDa in dogfish gills, the same size as NHE3 in stingrays and mammals. These immunological data demonstrate that the putative cell type responsible for active Na(+) absorption in euryhaline elasmobranchs is also present in stenohaline marine elasmobranchs, and suggest that the inability of most elasmobranchs to survive in fresh water is not due to a lack of the gill ion transporters for Na(+) absorption.

Different ionic conditions prompt NHE2 and NHE3 translocation to the plasma membrane

We tested whether NHE3 and NHE2 Na(+)/H(+) exchanger isoforms were recruited to the plasma membrane (PM) in response to changes in ion homeostasis. NHE2-CFP or NHE3-CFP fusion proteins were functional Na(+)/H(+) exchangers when transiently expressed in NHE-deficient PS120 fibroblasts. Confocal morphometry of cells whose PM was labeled with FM4-64 measured the fractional amount of fusion protein at the cell surface. In resting cells, 10-20% of CFP fluorescence was at PM and stable over time. A protocol commonly used to activate the Na(+)/H(+) exchange function (NH(4)-prepulse acid load sustained in Na(+)-free medium), increased PM percentages of PM NHE3-CFP and NHE2-CFP. Separation of cellular acidification from Na(+) removal revealed that only NHE3-CFP translocated when medium Na(+) was removed, and only NHE2-CFP translocated when the cell was acidified. NHE2/NHE3 chimeric proteins demonstrate that the Na(+)-removal response element resides predominantly in the NHE3 cytoplasmic tail and is distinct from the acidification response sequence of NHE2.

The role of carbonic anhydrases in renal physiology

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO(2). CA is expressed in most segments of the kidney. CAII and CAIV predominate in human and rabbit kidneys; in rodent kidneys, CAXII, and CAXIV are also present. CAIX is expressed by renal cell carcinoma (RCC). Most of these isoforms, except for rodent CAIV, have high turnover rates. CAII is a cytoplasmic enzyme, whereas the others are membrane-associated; CAIV is anchored by glycosylphosphatidylinositol linkage. Membrane polarity is apical for CAXIV, basolateral for CAXII, and apical and basolateral for CAIV. Luminal membrane CAs facilitate the dehydration of carbonic acid (H(2)CO(3)) that is formed when secreted protons combine with filtered bicarbonate. Basolateral CA enhances the efflux of bicarbonate via dehydration of H(2)CO(3). CAII and CAIV can associate with bicarbonate transporters (e.g., AE1, kNBC1, NBC3, and SCL26A6), and proton antiporter, NHE1 in a membrane protein complex called a transport metabolon. CAXII and CAXIV may also be associated with transporters in normal kidney and CAIX in RCCs. The multiplicity of CAs implicates their importance in acid-base and other solute transport along the nephron. For example, CAII on the cytoplasmic face and CAIV on the extracellular surface provide the 'push' and 'pull' for bicarbonate transport by supplying and dissipating substrate respectively.

The sodium/hydrogen exchanger: a possible mediator of immunity

Immune cells such as macrophages and neutrophils provide the first line of defence of the immune system using phagocytosis, cytokine and chemokine synthesis and release, as well as Reactive Oxygen Species (ROS) generation. Many of these functions are positively coupled with cytoplasmic pH (pHi) and/or phagosomal pH (pHp) modification; an increase in pHi represents an important signal for cytokine and chemokine release, whereas a decrease in pHp can induce an efficient antigen presentation. However, the relationship between pHi and ROS generation is not well understood. In immune cells two main transport systems have been shown to regulate pHi: the Na+/H+ Exchanger (NHE) and the plasmalemmal V-type H+ ATPase. NHE is a family of proteins which exchange Na+ for H+ according to their concentration gradients in an electroneutral manner. The exchanger also plays a key role in several other cellular functions including proliferation, differentiation, apoptosis, migration, and cytoskeletal organization. Since not much is known on the relationship between NHE and immunity, this review outlines the contribution of NHE to different aspects of innate and adaptive immune responses such as phagosomal acidification, NADPH oxidase activation and ROS generation, cytokine and chemokine release as well as T cell apoptosis. The possibility that several pro-inflammatory diseases may be modulated by NHE activity is evaluated.

Amiloride combined with small-volume resuscitation with hypertonic saline is superior in ameliorating trauma-hemorrhagic shock-induced lung injury in rats to the administration of either agent alone

OBJECTIVE

Recognition of the limitations of standard crystalloid resuscitation has led to exploration for alternative resuscitation strategies that might better prevent the development of trauma-hemorrhage-induced organ dysfunction and systemic inflammation. Thus, the goal of this study was to compare the effects of two resuscitation strategies alone and in combination with that of standard resuscitation with Ringer's lactate. These two strategies were intravenous injection of amiloride, an inhibitor of Na/H exchange and epithelial Na channels, and resuscitation with hypertonic saline.

DESIGN

Prospective animal study with concurrent control.

SETTING

Small animal laboratory.

SUBJECTS

Adult male Sprague-Dawley rats.

INTERVENTIONS

Rats injected with amiloride or its vehicle were subjected to trauma-hemorrhagic shock (T/HS) or trauma sham-shock (T/SS) and resuscitated with Ringer's lactate or hypertonic saline. The T/HS model consisted of a laparotomy plus 90 mins of shock (mean arterial pressure 30 mm Hg). Three hours after the end of the shock or sham-shock period, lung permeability, lung histology, pulmonary neutrophil sequestration, neutrophil CD11b expression, gut injury, and red blood cell rigidification were assessed.

MEASUREMENTS AND MAIN RESULTS

Both amiloride and hypertonic saline reduced T/HS-induced pulmonary permeability and neutrophil sequestration, and coadministration of these two agents was more efficacious than administration of the individual agents. In contrast, whereas gut injury was attenuated by both amiloride and hypertonic saline, combined administration of amiloride and hypertonic saline failed to further protect the gut. Additionally, hypertonic saline reduced both neutrophil CD11b expression and red blood cell rigidification, whereas amiloride was without effect.

CONCLUSIONS

Combined administration of amiloride and small-volume resuscitation with hypertonic saline may be a strategy worthy of further evaluation in the therapy of shock-induced distant organ injury.

Luminal hyperosmolarity decreases Na transport and impairs barrier function of sheep rumen epithelium

The effects of luminal hyperosmolarity on Na and Cl transport were studied in rumen epithelium of sheep. An increase of luminal osmotic pressure with mannitol (350 and 450 mosm/l) caused a significant increase of tissue conductance, G (T), which is linearly correlated with flux rates of (51)Cr-EDTA and indicates an increase of passive permeability. Studies with microelectrodes revealed, that an increase of the osmotic pressure caused a significant increase of the conductance of the shunt pathway from 1.23 +/- 0.10 (control) to 1.92 +/- 0.14 mS cm(-2) (450 mosm/l) without a change of fractional resistance. Hyperosmolarity significantly increased J (sm) and reduced J (net) Na. The effect of hyperosmolarity on J (ms) Na is explained by two independent and opposed effects: increase of passive permeability and inhibition of the Na(+)/H(+) exchanger. Hypertonic buffer solution induced a decrease of the intracellular pH (pH(i)) of isolated ruminal cells, which is consistent with an inhibition of Na(+)/H(+) exchange, probably isoform NHE-3, because NHE-3-mRNA was detectable in rumen epithelium. These data are in contrast to previous reports and reveal a disturbed Na transport and an impaired barrier function of the rumen epithelium, which predisposes translocation of rumen endotoxins and penetration of bacteria.

Action of ANG II and ANP on colon epithelial cells

The interaction of angiotensin II (ANG II) and atrial natriuretic peptide (ANP) on intracellular pH (pH(i)) and calcium ([Ca2+](i)) was investigated in T84 cells (a permanent cell line derived from human colon epithelium) using the fluorescent stains BCECF/AM and Fluo 4/AM, respectively. pH(i) recovery rate mediated by the Na(+)/H+ exchanger (NHE) was examined following an NH4Cl pulse. Under control conditions pH(i) recovered at 0.114+/-0.005 pH units/min (n=35). ANG II (10(-12) or 10(-9) M) increased this value, whilst ANG II (10(-7) M) decreased it. These effects of ANG II were impaired by simultaneous addition of 1 microM or 25 microM HOE-694, indicating that the stimulatory and inhibitory effects of ANG II on pH(i) recovery are mediated in part via the NHE1 and NHE2 isoforms. ANG II increased [Ca2+]i concentration-dependently. ANP (10(-6) M) or dimethyl-BAPTA/AM (50 microM) blocked the effects of ANG II on [Ca2+]i and on the rate of pH(i) recovery. Thapsigargin (10(-5) M) enhanced the effect of ANG II on [Ca2+]i and reversed its stimulatory effect on the rate of pH(i) recovery to an inhibitory one. External Ca(2+)-free solution did not affect the effects of ANG II on these parameters. These data suggest that the [Ca2+]i increase induced by ANG II is dependent on intracellular calcium stores. They are compatible with the demonstration of two sites on the C-terminal of the Na(+)/H+ exchanger, one stimulating Na(+)/H+ activity by increases of [Ca2+]i in the lower range (at 10(-12) or 10(-9) M ANG II) and the other inhibiting this activity at high [Ca2+]i levels (at 10(-7) M ANG II). ANP or dimethyl-BAPTA/AM, by impairing the pathway mediating the increase in [Ca2+]i, block both the stimulatory and inhibitory effects of ANG II.

Ligand-dependent complex formation between the Angiotensin II receptor subtype AT2 and Na+/H+ exchanger NHE6 in mammalian cells

Involvement of Angiotensin II (Ang II) in the regulation of sodium levels by modulating the Na+/H+ exchangers is demonstrated in many tissues. Screening of a mouse 17-day fetus cDNA library with the Angiotensin II receptor AT2 as the bait in yeast two-hybrid assay led us to identify an AT2-interacting mouse fetus peptide that shared 98% amino acid identity with the corresponding region of the human NHE6. NCBI Blast search showed that the clone 6430520C02 (GenBank Accession # AK032326) of the mouse genome project carried the complete sequence of this new mouse NHE6 isoform. The human and mouse NHE6 peptides share 97% overall homology. Further analysis showed that the region spanning the third intracellular loop and C-terminal cytoplasmic tail of the AT2 directly interacted with a 182 amino acid region that spans the predicted 5th intracellular loop and the initial part of the C-terminus of the mouse NHE6 in yeast two-hybrid assay. This 182-amino acid region that interacted with the AT2 also shares 98% homology with the corresponding region of rat NHE6 and therefore is highly conserved across species. We detected widespread expression of this NHE6 isoform in several rat tissues including 10-day fetus, 17-day fetus, and 30-day post-natal tissues of heart, brain, kidney and muscle. Moreover, the AT2 co-immunoiprecipitated with a hemagglutinin tagged NHE6 when expressed in human cell line MCF-7, and activated by AngII. This ligand-dependent complex formation between the AT2 and NHE6 suggests that the hormone Ang II may act as a regulator of NHE6, and Ang II-mediated direct protein-protein interaction between AT2 and NHE6 could be a mechanism for modulating the functions of the ubiquitously expressed NHE6 in different tissues.

Functional linkage of H+/peptide transporter PEPT2 and Na+/H+ exchanger in primary cultures of astrocytes from mouse cerebral cortex

In our previous studies, we demonstrated that the high-affinity type peptide transporter PEPT2 is expressed in rat cerebral cortex using synaptosomal membrane study and that the uptake of dipeptide [14C]glycylsarcosine into synaptosomes was stimulated by an inwardly directed H+ gradient (Fujita et al., Brain Res. 972, 52-61, 2004). However, there is no information available for the driving force of PEPT2 function in the nervous system. In the present study, we investigated functional characteristics of PEPT2 mediated transport of Gly-Sar in primary cultured astrocytes from mouse cerebral cortex and examined the effects of Na+/H+ exchanger (NHE) inhibitor on Gly-Sar uptake in mouse astrocytes. In mouse astrocytes, extracellular H+ influenced only the maximal velocity (Vmax) of Gly-Sar uptake without affecting the apparent affinity (Kt). Interestingly, removal of Na+ from uptake buffer significantly reduced Gly-Sar uptake and Gly-Sar uptake was modulated by NHE inhibitors. The treatment of EIPA, an NHE inhibitor, altered the Vmax value of Gly-Sar uptake but had no effect on its Kt value. RT-PCR revealed that NHE1 and NHE2 mRNA are expressed in mouse cerebrocortical astrocytes. These results demonstrated that NHE activity is required to allow optimal uptake of dipeptides mediated by PEPT2 into the astrocytes. This study represents the first description of the functional co-operation of PEPT2 and NHE1 and/or NHE2 in cerebrocortical astrocytes.

The human Na,K-ATPase alpha4 isoform is a ouabain-sensitive alpha isoform that is expressed in sperm

The Na,K-ATPase generates electrochemical gradients across the plasma membrane that are responsible for numerous cellular and physiological processes. The active Na,K-ATPase is minimally composed of an alpha and a beta subunit and families of isoforms for both subunits exist. Recent studies have identified a physiological role for the rat Na,K-ATPase alpha4 isoform in sperm motility. However, very little is known about the human Na,K-ATPase alpha4 isoform other than its genomic sequence and structure and its mRNA expression pattern. Here, the human alpha4 isoform of the Na,K-ATPase is cloned, expressed, and characterized. Full length cDNAs encoding the putative human alpha4 isoform of the Na,K-ATPase were identified from a number of ESTs and a protein product corresponding to this isoform was shown to be expressed from these cDNAs. The human Na,K-ATPase alpha4 isoform protein was found to be expressed in mature sperm in human testes sections and it is localized specifically to the principle piece of human sperm. In addition, the presence of the Na,K-ATPase alpha4 isoform is absent in immature testes however its expression appears coincident with sexual maturity. And finally, the human Na,K-ATPase alpha4 isoform was shown to be as sensitive to cardiac glycoside inhibition as the human Na,K-ATPase alpha1 isoform. Considering the important role of the rat Na,K-ATPase alpha4 isoform in rat sperm motility, the demonstration that the human alpha4 isoform is a sperm-specific protein localized to the flagellum suggests a role for the human Na,K-ATPase alpha4 isoform in human sperm physiology.

pH Microdomains in Oligodendrocytes

Oligodendrocytes (OLs) are cells that produce myelin in the central nervous system. Here we use ratiometric pH indicator dye to analyze intracellular pH in OLs in culture. The results reveal alkaline microdomains, which predominate in the perikaryon and proximal dendrites, and acidic microdomains, which predominate in distal dendrites. Spatial nonuniformity of pH is generated by differential subcellular distribution of Na(+)/H(+) exchanger (NHE), which is localized in a punctate distribution in the perikaryon and proximal processes, Na(+)/HCO(3)(-) cotransporter (NBC), which is localized in a punctate distribution in distal dendrites, and carbonic anhydrase isotype II (CAII), which is colocalized with either NHE or NBC. Inhibition of NHE activity by amiloride inhibits regeneration of alkaline microdomains after cytoplasmic acidification, whereas the inhibition of CAII activity with ethoxyzolamide inhibits acidification of dendrites. Fluorescence correlation spectroscopy analysis of CAII microinjected into OLs reveals freely diffusing protein throughout the cell as well as protein associated predominantly with NHE in the perikaryon and predominantly with NBC in the dendrites. Alkaline and acidic microdomains could be generated by transport metabolons consisting of CAII associated with NHE or NBC, respectively. This study provides the first evidence for pH microdomains in cells and describes a mechanism for how they are generated.

Release of canine parvovirus from endocytic vesicles

Canine parvovirus (CPV) is a small nonenveloped virus with a single-stranded DNA genome. CPV enters cells by clathrin-mediated endocytosis and requires an acidic endosomal step for productive infection. Virion contains a potential nuclear localization signal as well as a phospholipase A(2) like domain in N-terminus of VP1. In this study we characterized the role of PLA(2) activity on CPV entry process. PLA(2) activity of CPV capsids was triggered in vitro by heat or acidic pH. PLA(2) inhibitors inhibited the viral proliferation suggesting that PLA(2) activity is needed for productive infection. The N-terminus of VP1 was exposed during the entry, suggesting that PLA(2) activity might have a role during endocytic entry. The presence of drugs modifying endocytosis (amiloride, bafilomycin A(1), brefeldin A, and monensin) caused viral proteins to remain in endosomal/lysosomal vesicles, even though the drugs were not able to inhibit the exposure of VP1 N-terminal end. These results indicate that the exposure of N-terminus of VP1 alone is not sufficient to allow CPV to proliferate. Some other pH-dependent changes are needed for productive infection. In addition to blocking endocytic entry, amiloride was able to block some postendocytic steps. The ability of CPV to permeabilize endosomal membranes was demonstrated by feeding cells with differently sized rhodamine-conjugated dextrans together with the CPV in the presence or in the absence of amiloride, bafilomycin A(1), brefeldin A, or monensin. Dextran with a molecular weight of 3000 was released from vesicles after 8 h of infection, while dextran with a molecular weight of 10,000 was mainly retained in vesicles. The results suggest that CPV infection does not cause disruption of endosomal vesicles. However, the permeability of endosomal membranes apparently changes during CPV infection, probably due to the PLA(2) activity of the virus. These results suggest that parvoviral PLA(2) activity is essential for productive infection and presumably utilized in membrane penetration process of the virus, but CPV also needs other pH-dependent changes or factors to be released to the cytoplasm from endocytic vesicles.

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.

Molecular cloning and characterization of a sodium-pump ATPase of the moss Physcomitrella patens

Physcomitrella patens grew slowly at 600 mm Na+, pH 6.0, affected by the low water potential but without signs of suffering Na+ toxicity. At pH 8.0, tolerance seemed to be lower but it grew at 200 mm Na+, again without signs of Na+ toxicity. The resistance of Physcomitrella cells to the toxic effects of Na+ can be accounted for by their capacity to keep high K+:Na+ ratios and to extrude Na+ by a system that is not dependent on DeltapH. Physcomitrella expresses two P-type ATPases similar in sequence to fungal ENA-type Na+-ATPases. A functional study in yeast demonstrated that one of these ATPases, PpENA1, is an Na+-pump. We also found that P. patens has a plant-type SOS1 Na+/H+ antiporter. We discuss that Na+-ATPases existed in early land plants but that they were lost during the evolution of bryophytes to flowering plants.

Zoniporide: a potent and selective inhibitor of the human sodium-hydrogen exchanger isoform 1 (NHE-1)

The sodium-hydrogen exchanger isoform-1 (NHE-1) plays an important role in the myocardial response to ischemia-reperfusion; inhibition of this exchanger protects against ischemic injury, including reduction in infarct size. Herein we describe a novel, potent, and highly selective NHE-1 inhibitor, zoniporide (CP-597,396; [1-(quinolin-5-yl)-5-cyclopropyl-1H-pyrazole-4-carbonyl] guanidine). Zoniporide inhibits human NHE-1 with an IC(50) of 14 nM, has >150-fold selectivity vs. other NHE isoforms, and potently inhibits ex vivo NHE-1-dependent swelling of human platelets. This compound is well tolerated in preclinical animal models, exhibits moderate plasma protein binding, has a t(1/2) of 1.5 h in monkeys, and has one major active metabolite. In both in vitro and in vivo rabbit models of myocardial ischemia-reperfusion injury, zoniporide markedly reduced infarct size without adversely affecting hemodynamics or cardiac function. In the isolated heart (Langendorff), zoniporide elicited a concentration-dependent reduction in infarct size (EC(50) = 0.25 nM). At 50 nM it reduced infarct size by 83%. This compound was 2.5-20-fold more potent than either eniporide or cariporide (EC(50)s of 0.69 and 5.11 nM, respectively), and reduced infarct size to a greater extent than eniporide. In open chest, anesthetized rabbits, zoniporide also elicited a dose-dependent reduction in infarct size (ED(50) = 0.45 mg/kg/h) and inhibited NHE-1-mediated platelet swelling (93% inhibition at 4 mg/kg/h). Furthermore, zoniporide attenuated postischemic cardiac contractile dysfunction in conscious primates, and reduced both the incidence and duration of ischemia-reperfusion-induced ventricular fibrillation in rats. Zoniporide represents a novel class of potent and selective human NHE-1 inhibitors with potential utility for providing cardioprotection in a clinical setting.

Acid-base regulation in fishes: cellular and molecular mechanisms

The mechanisms underlying acid-base transfers across the branchial epithelium of fishes have been studied for more than 70 years. These animals are able to compensate for changes to internal pH following a wide range of acid-base challenges, and the gill epithelium is the primary site of acid-base transfers to the water. This paper reviews recent molecular, immunohistochemical, and functional studies that have begun to define the protein transporters involved in the acid-base relevant ion transfers. Both Na(+)/H(+) exchange (NHE) and vacuolar-type H(+)-ATPase transport H(+) from the fish to the environment. While NHEs have been thought to carry out this function mainly in seawater-adapted animals, these proteins have now been localized to mitochondrial-rich cells in the gill epithelium of both fresh and saltwater-adapted fishes. NHEs have been found in the gill epithelium of elasmobranchs, teleosts, and an agnathan. In several species, apical isoforms (NHE2 and NHE3) appear to be up-regulated following acidosis. In freshwater teleosts, H(+)-ATPase drives H(+) excretion and is indirectly coupled to Na(+) uptake (via Na(+) channels). It has been localized to respiratory pavement cells and chloride cells of the gill epithelium. In the marine elasmobranch, both branchial NHE and H(+)-ATPase have been identified, suggesting that a combination of these mechanisms may be utilized by marine elasmobranchs for acid-base regulation. An apically located Cl(-)/HCO(3)(-) anion exchanger in chloride cells may be responsible for base excretion in fresh and seawater-adapted fishes. While only a few species have been examined to date, new molecular approaches applied to a wider range of fishes will continue to improve our understanding of the roles of the various gill membrane transport processes in acid-base balance.