Interactions of chloride and amiloride with the renal Na+/H/ antiporter

Na+/H+ exchangers

Tightly coupled exchange of Na(+) for H(+) occurs across the surface membrane of virtually all living cells. For years, the underlying molecular entity was unknown and the full physiological significance of the exchange process was not appreciated, but much knowledge has been gained in the last two decades. We now realize that, unlike most of the other transporters that specialize in supporting one specific function, Na(+)/H(+) exchangers (NHE) participate in a remarkable assortment of physiological processes, ranging from pH homeostasis and epithelial salt transport, to systemic and cellular volume regulation. In parallel, we have learned a great deal about the biochemistry and molecular biology of Na(+)/H(+) exchange. Indeed, it has now become apparent that exchange is mediated not by one, but by a diverse family of related yet distinct carriers (antiporters) sometimes present in different cell types and located in various intracellular compartments. Each one of these has unique structural features that dictate its functional role and mode of regulation. The biological relevance of Na(+)/H(+) exchange is emphasized by its evolutionary conservation; analogous exchangers are present from bacteria to man. Because of its wide distribution and versatile function, Na(+)/H(+) exchange has attracted an enormous amount of interest and therefore generated a vast literature. The vastness and complexity of the field has been compounded by the multiplicity of NHE isoforms. For reasons of space and in the spirit of this series, this overview is restricted to the family of mammalian NHEs.

HCO 3--dependent volume regulation in alpha-cells of the rat endocrine pancreas

Ion transport activity in pancreatic α-cells was assessed by studying cell volume regulation in response to anisotonic solutions. Cell volume was measured by a video imaging method, and cells were superfused with either 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid-buffered or HCO₃⁻-buffered solutions. α-Cells did not exhibit a regulatory volume increase (RVI) in response to cell shrinkage caused by hypertonic solutions. A RVI was observed, however, in cells that had first undergone a regulatory volume decrease (RVD), but only in HCO₃⁻-buffered solutions. RVI was also observed in response to a HCO₃⁻-buffered hypertonic solution in which the glucose concentration was increased from 4 to 20 mM. The post-RVD RVI and the glucose-induced RVI were both inhibited by 10 μM 5-(N-methyl-N-isobutyl) amiloride or 100 μM 2,2′-(1,2-ethenediyl) bis (5-isothio-cyanatobenzenesulfonic acid), but not by 10 μM benzamil nor 10 μM bumetanide. These data suggest that Na⁺–H⁺ exchangers and Cl⁻–HCO₃⁻ exchangers contribute to volume regulation in α-cells.

Structure and function of the NHE1 isoform of the Na+/H+ exchanger

The Na+/H+ exchanger is a ubiquitous, integral membrane protein involved in pH regulation. It removes intracellular acid, exchanging a proton for an extracellular sodium ion. There are seven known isoforms of this protein that are the products of distinct genes. The first isoform discovered (NHE1) is ubiquitously distributed throughout the plasma membrane of virtually all tissues. It plays many different physiological roles in mammals, including important functions in regulation of intracellular pH, in heart disease, and in cytoskeletal organization. The first 500 amino acids of the protein are believed to consist of 12 transmembrane helices, a membrane-associated segment, and two reentrant loops. A C-terminal regulatory domain of approximately 315 amino acids regulates the protein and mediates cytoskeletal interactions. Studies are underway to determine the amino acid residues important in NHE1 function. At present, it is clear that transmembrane segment IV is important in NHE1 function and that transmembrane segments VII and IX are also involved in transport. Further experiments are required to elucidate the mechanism of transport and regulation of this multifunctional protein.

The changing face of the Na+/H+ exchanger, NHE1: structure, regulation, and cellular actions

The NHE family of ion exchangers includes six isoforms (NHE1-NHE6) that function in an electroneutral exchange of intracellular H(+) for extracellular Na(+). This review focuses on the only ubiquitously expressed isoform, NHE1, which is localized at the plasma membrane where it plays a critical role in intracellular pH (pHi) and cell volume homeostasis. All NHE isoforms share a similar topology: an N-terminus of 12 transmembrane (TM) alpha-helices that collectively function in ion exchange, and a C-terminal cytoplasmic regulatory domain that modulates transport activity by the TM domain. Extracellular signals, mediated by diverse classes of cell-surface receptors, regulate NHE1 activity through distinct signaling networks that converge to directly modify the C-terminal regulatory domain. Modifications in the C-terminus, including phosphorylation and the binding of regulatory proteins, control transport activity by altering the affinity of the TM domain for intracellular H(+). Recently, it was determined that NHE1 also functions as a membrane anchor for the actin-based cytoskeleton, independently of its role in ion translocation. Through its effects on pHi homeostasis, cell volume, and the actin cortical network, NHE1 regulates a number of cell behaviors, including adhesion, shape determination, migration, and proliferation.

Identification of sites in the second exomembrane loop and ninth transmembrane helix of the mammalian Na+/H+ exchanger important for drug recognition and cation translocation

Mammalian Na(+)/H(+) exchanger (NHE) isoforms are differentially sensitive to inhibition by several distinct classes of pharmacological agents, including amiloride- and benzoyl guanidinium-based derivatives. The determinants of drug sensitivity, however, are only partially understood. Earlier studies of the drug-sensitive NHE1 isoform have shown that residues within the fourth membrane-spanning helix (M4) (Phe(165), Phe(166), Leu(167), and Gly(178)) and a 66-amino acid segment encompassing M9 contribute significantly to drug recognition. In this report, we have identified two residues within M9, one highly conserved (Glu(350)) and the other non-conserved (Gly(356)), that are major determinants of drug sensitivity. In addition, residues in the second exomembrane loop between M3 and M4 (Gly(152), Phe(157), and Pro(158)) were also found to modestly influence drug sensitivity. A double substitution of crucial sites within M4 and M9 of NHE1 with the corresponding residues present in the drug-resistant NHE3 isoform (i.e. L167F/G356A) greatly reduced drug sensitivity in a cooperative manner to levels nearing that of wild type NHE3. The above mutations did not appreciably affect Na(o)(+) affinity but did markedly decrease the catalytic turnover of the transporter. These data suggest that specific sites encompassing M4 and M9 are critical determinants of both drug recognition and cation translocation.

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

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

Characterization of Mg(2+) efflux from rat erythrocytes non-loaded with Mg(2+)

Non-Mg(2+)-loaded rat erythrocytes with a physiological level of Mg(2+)(i) exhibited Mg(2+) efflux when incubated in nominally Mg(2+)-free media. Two types of Mg(2+) efflux were shown: (1) An Na(+)-dependent Mg(2+) efflux in NaCl and Na gluconate medium, which was inhibited by amiloride and quinidine, as was Na(2+)/Mg(2+) antiport in Mg(2+)-loaded rat erythrocytes; and (2) an Na(+)-independent Mg(2+) efflux in sucrose medium and choline Cl medium, which may be differentiated into SITS-sensitive Mg(2+) efflux at low Cl(-)(o) (in sucrose) and into SITS-insensitive Mg(2+) efflux at high Cl(-)(o) (in 150 mmol/l choline Cl).

Na+/H+ exchangers. Molecular diversity and relevance to heart

During the last several years, significant advances have been made in our understanding of the molecular, cellular, and physiological diversity of mammalian Na+/H+ exchangers. This transporter forms a multigene family of at least six members (NHE1-NHE6) that share approximately 20-60% amino acid identity. NHE1 is the most predominant isoform expressed in heart and it contributes significantly to myocardial pHi homeostasis, which is important for maintaining contractility. However, hyperactivation of NHE1 during episodes of cardiac ischemia and reperfusion disrupts the intracellular ion balance, leading to cardiac dysfunction and damage in several animal models, but which can be prevented by pharmacological antagonists of NHE1. Molecular studies have indicated that the predicted transmembrane segments M4 and M9 contain several residues involved in drug sensitivity. Molecular dissection of the drug binding region should facilitate the rational design of more potent and isoform-specific drugs that may provide therapeutic benefit in the prevention of cardiac ischemia and reperfusion injuries.

Delineation of transmembrane domains of the Na+/H+ exchanger that confer sensitivity to pharmacological antagonists

Plasma membrane Na+/H+ exchanger (NHE) isoforms NHE1 and NHE3 exhibit very different sensitivities to amiloride and its 5-amino-substituted analogues, benzoyl guanidinium derivatives (e.g. (3-methylsulfonyl-4-piperidinobenzoyl)guanidine methanesulfonate (HOE694)), and cimetidine. To define structural domains that confer differential sensitivity to these antagonists, unique restriction endonuclease sites were engineered into cDNAs for each isoform near the regions that encode the putative membrane-spanning domains. These new sites did not modify their pharmacological properties and allowed several chimeric Na+/H+ exchangers to be constructed by exchanging homologous segments. The modified parental (E1' and E3') and chimeric molecules were stably expressed in exchanger-deficient Chinese hamster ovary AP-1 cells and assayed for their sensitivities to amiloride, ethylisopropylamiloride, HOE694, and cimetidine. Most chimeras showed drug sensitivities corresponding to the dominant parental segment. However, interchanging a 66-amino acid segment containing the putative ninth transmembrane (M9) domain and its adjacent loops caused reciprocal alterations in the sensitivities of E1' and E3' to all antagonists. In addition, substituting the first five putative membrane-spanning domains of E3' with the corresponding region of E1' modestly reduced the transporter's sensitivity to cimetidine but not the other compounds. These data indicate that the protein segment between M8 and M10 may be a major site of interaction with these antagonists, although other regions modestly influence sensitivity to certain drugs.

Na+/H+ exchange modulates acidification of early rat liver endocytic vesicles

Endocytic vesicles are acidified by an electrogenic vacuolar H(+)-ATPase. These studies examined whether rat liver endosomes also exhibit Na+/H+ exchange and whether this transporter alters acidification. Extravesicular Na+ caused saturable proton efflux from acidified endosomes with a Michaelis constant for Na+ of 7.6 mM, whereas an in-to-out Na+ gradient caused endosome acidification without MgATP and accelerated acidification with MgATP. Na(+)-driven proton fluxes were little altered by valinomycin or carbonyl cyanide m-chlorophenylhydrazone. Na+/H+ exchange was inhibited by Li+ but was not affected by K+, Cl-, amiloride (1 mM), or 5-(N,N-dimethyl) amiloride (0.1 mM). Na+/H+ exchange was detected in "early" but not in "late" liver endosomes or in lysosomes. These data suggest that early rat liver endosomes exhibit Na+/H+ exchange that, immediately after endosome formation, may accelerate vesicular acidification. Because of its insensitivity to amiloride, this exchanger may be a pharmacologically altered form of Na+/H+ exchanger-1 or a new isoform.

Mutational analysis of transmembrane histidines in the amiloride-sensitive Na+/H+ exchanger

The histidine-reactive reagent, diethyl pyrocarbonate (DEPC) inhibits the human amiloride-sensitive Na+/H+ exchanger (NHE1) in stably transfected fibroblasts. NHE1 was protected by cimetidine and amiloride from DEPC, and DEPC inhibition was reversed with hydroxylamine, suggesting a role for critical histidine groups in NHE activity. We replaced the histidines (H) in putative transmembrane domains (H35, H120, H349) with glycine (G) using site-directed mutagenesis. There was no significant change in NHE activity of the H120G; H349G; H120,349G; and H35,120,349G mutants compared with wild type. The 50% inhibition concentration values for amiloride, ethyl isopropyl amiloride (EIPA), and cimetidine of the H349G mutant were significantly increased compared with the wild-type NHE1. We also examined the DEPC effect on the transport activity of the triple histidine mutant (H35,120,349G) and found that NHE1 activity was still inhibited by DEPC with reversal by hydroxylamine and protected by amiloride and cimetidine. Kinetic analysis of DEPC inhibition indicated that two "critical" histidine residues are required for NHE transport activity. Substitutions of H349 with asparagine (N), glutamine (Q), serine (S), tyrosine (Y), valine (V), leucine (L), and phenylalanine (F) were also examined. There were no changes in NHE activity of these mutants compared with wild type. The H349G and H349L mutants became more resistant to amiloride, whereas the H349Y and H349F mutants became more sensitive to amiloride. The H349S (mimics NHE3) and H349Y (mimics NHE4) mutations had only modest effects on amiloride sensitivity. These results indicate that H349 affects the interaction of NHE1 with its inhibitors, even though substitutions at this site, per se, do not appear to explain the differences in amiloride sensitivity between different NHE isoforms. Despite clear-cut effects of the H349G mutation on the competitive interaction of NHE1 with cimetidine and EIPA, this mutation did not affect the affinity of NHE1 for its cationic substrates (Na+, Li+).

Renal apical membrane sodium-hydrogen exchange in genetic salt-sensitive hypertension

Inbred Dahl/Rapp salt-sensitive and salt-resistant rats differ in their blood pressure response to dietary salt. We studied sodium-hydrogen (Na-H) exchanger kinetics in renal brush border membrane vesicles prepared from both strains on either a 1% or 8% NaCl diet. Kinetics measurements were made with the acridine orange fluorescence quenching technique in vesicles prepared at pH 6.0. The initial Na-H exchange rate was measured using preparations with similar initial quench values. The maximal transport rate (Vmax, fluorescence units per second per milligram protein [+/- SEM]) in salt-sensitive rats on a 1% NaCl diet was significantly lower than that in salt-resistant rats (36.9 +/- 4.4 versus 51.8 +/- 5.5, respectively, P < .0005). With the 8% NaCl diet for 1 week, the Vmax of salt-resistant rats decreased and became similar to that of salt-sensitive rats. The affinity for sodium (Km, millimoles per liter [+/- SEM]) was also lower in salt-sensitive rats than in salt-resistant rats while on a 1% NaCl diet (11.8 +/- 1.0 versus 19.6 +/- 2.3, respectively, P < .002). These values converged when both strains were fed an 8% NaCl diet for 1 week. Inhibition by 25 mumol/L amiloride was less in salt-sensitive rats than in salt-resistant rats on the 1% NaCl diet. These results show that salt-sensitive rats have lower renal apical membrane Na-H exchange activity than salt-resistant rats on a 1% NaCl diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Basolateral mechanisms of intracellular pH regulation in the colonic epithelial cell line HT29 clone 19A

The intracellular pH (pHi) of the colonic tumour cell line HT29 cl.19A was studied by microspectrofluorometry using the pH-sensitive dye BCECF. Single cells within a confluent monolayer, grown in a polarized manner on permeable supports, were examined. An amiloride-sensitive Na+/H+ exchange and a stilbene-insensitive Cl-/HCO3- exchange mechanism have been identified in the basolateral membrane. Removal of Na+ from the basolateral solution caused a decrease of pHi by 0.50 +/- 0.09 unit (n = 4). Amiloride or Na(+)-free solution at the apical side had no effect on pHi. Cl- removal at the basolateral side led to an increase of pHi by 0.20 +/- 0.03 unit (n = 4) whereas apical removal had no influence on pHi. This effect was independent of Na+ and was insensitive to 0.2 mM 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulphonic acid. A basolateral Cl-/HCO3- exchanger is the most likely explanation for this observation. The Na+/H+ exchange mechanism in the basolateral membrane is an acid extruder, whereas the Cl-/HCO3- exchanger is an acid loader. Both of these mechanisms are important for the maintenance of intracellular pH in HT29 cl.19A cells.

Endothelin reverses the effects of acidosis on the intracellular Ca2+ transient and contractility in ferret myocardium

Endothelin may play an important role in modulating myocardial contractility under certain pathophysiological conditions. To determine whether endothelin beneficially modulates myocardial contractility in the common clinical condition of acidosis, we compared the effects of endothelin-1 on intracellular Ca2+ transients and isometric contractions under normal (extracellular pH [pH(o)] 7.4) and acidotic (pH(o) 6.4) conditions in ferret papillary muscles (n = 33) loaded with the Ca(2+)-regulated bioluminescent indicator aequorin. A pH(o) of 6.4 was induced by replacing 92% of HCO3- with Cl- in the bathing medium. The effects of endothelin at pH(o) 6.4 differed from the effects at pH(o) 7.4 in that 1) the minimally effective concentration of endothelin was 30-fold lower (1 x 10(-10) M at pH(o) 6.4; 3 x 10(-9) M at pH(o) 7.4) and the concentration-response curve of endothelin was significantly shifted to the left with a decrease in log EC50 from -7.83 +/- 0.13 to -8.92 +/- 0.10 (p less than 0.001), indicating an increased sensitivity of myocardium to endothelin; 2) endothelin produced an increase of approximately 375% in tension development at pH(o) 6.4 (approximately 62% at pH(o) 7.4) (p less than 0.001) without increasing peak [Ca2+]i (approximately 13% increase at pH(o) 7.4, p less than 0.001), indicating an increase in myofilament Ca2+ responsiveness; and 3) endothelin significantly abbreviated (approximately -19%, p less than 0.001) the prolonged intracellular Ca2+ transient induced by acidosis (pH(o) 6.4). In addition, pretreatment with 10 microM of the Na(+)-H+ exchange inhibitor 5-(N-methyl-N-isobutyl)-amiloride significantly attenuated endothelin-induced effects on the intracellular Ca2+ transient and contraction during acidosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyanide-induced alteration of cytosolic pH: involvement of cellular hydrogen ion handling processes

Neuronal cells exposed to cyanide rapidly lose the capacity to regulate internal Ca2+ homeostasis, thereby accumulating an excess cytosolic Ca2+ load. The present study was undertaken to examine the effects of KCN on another important ion: hydrogen ion. KCN (1-10 mM) rapidly decreased intracellular pH (pHi) of cultured pheochromocytoma (PC12) cells as indicated by the pH-sensitive fluorescent dye 2',7-bis(carboxyethyl)-5(6)-carboxyfluorescein. Removal of Ca2+ from the media or pretreating the cells with diltiazem (10(-5) M), a calcium channel blocker, delayed the onset and reduced the magnitude of the drop in pHi. Lowering the pH of the incubation medium (pHo) to 6.9 exaggerated the drop in pHi, while raising it to 7.9 attenuated the change in pHi. Removal of Na+ from the media enhanced the cyanide effect. Reintroduction of Na+ or substitution with Li+ reversed the cytosolic acidification, suggesting involvement of the Na+/H+ exchanger in the cyanide action. Pretreatment of cells with amiloride, 0.2 mM, blunted the cytosolic acidification induced by KCN, possibly by decreasing intracellular Na+ accumulation and disrupting H+ efflux. Cyanide thus produces a rapid dysfunction of hydrogen ion handling mechanisms and this may play a role in cyanide neurotoxicity.

Reclamation of filtered bicarbonate

Approximately 85% of the filtered bicarbonate load is reabsorbed in the proximal convoluted tubule. Transport in this segment displays saturation kinetics, and exhibits a higher capacity for reabsorption in the earliest portion. Reclamation of bicarbonate is highly regulated in the proximal tubule: an increase in luminal [HCO3-], flow rate and arterial PCO2 increase, while alkalinization of the peritubular surface inhibits bicarbonate absorption. Angiotensin II also appears to regulate bicarbonate transport, especially in the S1 segment. The majority of the filtered bicarbonate load which escapes reabsorption in the proximal tubule is reabsorbed in the thick ascending limb of Henle's loop. Bicarbonate reclamation in this segment is enhanced by luminal [HCO3-] and furosemide, and by chronic metabolic acidosis and increased dietary sodium intake. Amiloride, AVP and glucagon inhibit absorption in the thick ascending limb.

Characterization of Na(+)-independent Mg2+ efflux from erythrocytes

Na(+)-independent Mg2+ efflux from Mg2(+)-loaded human, rat and chicken erythrocytes was reduced by extracellular Cl-. Na(+)-independent Mg2+ efflux at low extracellular Cl- concentration (sucrose medium) was inhibited by SITS and was nearly insensitive to SITS in 150 mM choline Cl medium. The inhibition of Mg2+ efflux by extracellular Cl- and DIDS could be overcome by the lipophilic permeant tetraphenylphosphonium cation. Na(+)-independent Mg2+ efflux from human and rat erythrocytes in sucrose and choline Cl medium was inhibited by cAMP and by amiloride and amiloride analogues. The results indicate that Na(+)-independent Mg2+ efflux in high Cl- medium is performed by a similar or the same Mg2+ efflux system, operating in sucrose medium in which the efflux of Mg2+ is accompanied by the efflux of Cl- for charge compensation.

An essential role for vicinal dithiol groups in the catalytic activity of the human placental Na(+)-H+ exchanger

We examined the effects of phenylarsine oxide, a reagent specific for vicinal dithiol groups, on the catalytic activities, Na+ influx and H+ efflux, of the human placental Na(+)-H+ exchanger. Treatment of the placental brush-border membrane vesicles with the reagent markedly inhibited both the activities. The inhibition was partially reversible by dithiols. The effect of phenylarsine oxide was to reduce the maximal velocity of the exchanger without influencing its affinity for Na+. The exchanger was partially protected from this inhibition by amiloride but not by cimetidine even though both these compounds interacted with the Na(+)-binding site. The data demonstrate that vicinal dithiol groups are essential for the catalytic function of the placental Na(+)-H+ exchanger and that the critical dithiol groups are located at a site distinct from the Na(+)-binding site.

Methyl isobutyl amiloride: a new probe to assess the number of Na-H antiporters

We measured the binding of [3H]-5-(N-methyl-N-isobutyl) amiloride (MIA) to purified rabbit renal brush border membranes. MIA binding was protein, temperature and time dependent with optimal binding at pH 8.0 or above. At low pH MIA binding was inhibited, suggesting competition between H+ ions and MIA for the MIA binding site. There was 70-80% specific binding which reached a plateau at 30 min and remained stable thereafter for 150 min. Scatchard analysis revealed one family of binding sites with Bmax of 3.4 +/- 0.4 pmoles/mg protein and Kd of 30.5 +/- 2.3 nM. MIA inhibited the Vmax of the Na-H antiporter (assessed by acridine orange quenching) in a dose dependent fashion with 100% inhibition at MIA concentration of 10(-3) M and this inhibition was greater than that of amiloride. We conclude that MIA, a potent inhibitor of the Na-H antiporter, displays a high percentage of specific binding to renal brush border membranes and can be used to assess the number of the Na-H antiporters.