Identification of the renal Na+/H+ exchanger with N,N'-dicyclohexylcarbodiimide (DCCD) and amiloride analogues

Effect of monovalent cations on the kinetics of hypoxic conformational change of mitochondrial complex I

Mitochondrial complex I is a large, membrane-bound enzyme central to energy metabolism, and its dysfunction is implicated in cardiovascular and neurodegenerative diseases. An interesting feature of mammalian complex I is the so-called A/D transition, when the idle enzyme spontaneously converts from the active (A) to the de-active, dormant (D) form. The A/D transition plays an important role in tissue response to ischemia and rate of the conversion can be a crucial factor determining outcome of ischemia/reperfusion. Here, we describe the effects of alkali cations on the rate of the D-to-A transition to define whether A/D conversion may be regulated by sodium.

At neutral pH (7–7.5) sodium resulted in a clear increase of rates of activation (D-to-A conversion) while other cations had minor effects. The stimulating effect of sodium in this pH range was not caused by an increase in ionic strength. EIPA, an inhibitor of Na⁺/H⁺ antiporters, decreased the rate of D-to-A conversion and sodium partially eliminated this effect of EIPA. At higher pH (> 8.0), acceleration of the D-to-A conversion by sodium was abolished, and all tested cations decreased the rate of activation, probably due to the effect of ionic strength.

The implications of this finding for the mechanism of complex I energy transduction and possible physiological importance of sodium stimulation of the D-to-A conversion at pathophysiological conditions in vivo are discussed.

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The active/dormant (A/D) transition of complex I is affected by monovalent cations.

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Na⁺ increases the rate of the D/A conversion at neutral pH.

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Lithium and caesium decrease D/A transition at all tested pH

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Matrix ion balance may influence the rate of the activation of the enzyme in situ.

Negative regulation of the platelet Na+/H+ exchanger by trimeric G-proteins

Human platelets contain a Na+/H+ exchanger (NHE) that regulates the cytosolic pH. The role of trimeric G-proteins in NHE control was investigated in plasma membrane vesicles by measuring exchange of intravesicular protons for extravesicular Na+. Exchange was saturable, independent of membrane potential and inhibited by ethylisopropyl amiloride (Ki 0.05 micromol.L-1), demonstrating the involvement of NHE-1. The G-protein activators AlF4- and GMP-P(NH)P reduced exchange by increasing the Km for Na+ from 11.3 +/- 2.1 mM to 21.6 +/- 1.4 mM (AlF4-) and 19.8 +/- 1.1 mM (GMP-P(NH)P), leaving Vmax and the Hill coefficient unchanged. This effect was abolished by inhibitors of Gi-proteins (N-ethylmaleimide, holoenzyme- and A-protomer of pertussis toxin) and by an anti-Galpha Ig and GDP(beta)S. Activation of Gi-proteins by mastoparan and its synthetic analogue Mas7 also strongly reduced NHE activity. These data show that in platelets NHE-1 is under negative control of the Gi-family of trimeric G-proteins.

Characterization of plasma membrane Na+/H+ exchange in eel (Anguilla anguilla) intestinal epithelial cells

The ability of eel intestinal epithelial cells to recover from an acute acid load was analysed using the fluorescent dye 2',7'-bis-carboxy-ethyl-5,6-carboxyfluorescein (BCECF) and cell suspensions. Under these experimental conditions (bicarbonate-free solutions) the resting pHi in cells prepared from sea-water (7.52 +/- 0.031) and fresh-water (7.50 +/- 0.094) adapted animals proved to be similar. The recovery rate (following an acid load) increases by increasing the Na ion concentration in the extracellular medium. This pHi recovery is competitively inhibited by the specific inhibitor dimethylamiloride (DMA) with a low Ki in sea- (1.2 microM) as well as in fresh-water (1.3 microM) adapted animals, indicating the presence of a specific Na/H exchange activity in these cells. Using basolateral membrane vesicles it could be demonstrated that this activity is located on the basolateral side of the enterocyte membrane. The kinetic parameters (Kapp and Jmax) of this exchanger are similar in fresh-water and sea-water adapted animals suggesting that no salinity adaptation occurs, thus excluding the involvement of the antiporter in the osmoregulatory processes. These results are in agreement with the presence in the plasma membrane of the eel enterocytes of a Na/H-1 (housekeeper) form of the antiporter.

Effect of secretin and inhibitors of HCO3-/H+ transport on the membrane voltage of rat pancreatic duct cells

The aim of the present study was to study the effect of secretin on the electrophysiological response of pancreatic ducts. Furthermore, we investigated the effects of lipid-soluble buffers and inhibitors of HCO3-/H+ transport. Ducts obtained from fresh rat pancreas were perfused in vitro. Secretin depolarized the basolateral membrane voltage, Vbl, by up to 35 mV (n = 37); a half-maximal response was obtained at 3 x 10(-11) mol/l. In unstimulated ducts a decrease in the luminal Cl- concentration (120 to 37 mmol/l) had a marginal effect on Vbl, but after maximal secretin stimulation it evoked a 14 +/- 2 mV depolarization (n = 6), showing that a luminal Cl- conductance (GCl-) was activated. The depolarizing effect of secretin on Vbl was often preceded by about a 6 mV hyperpolarization, most likely due to an increase in the basolateral GK+. Perfusion of ducts with DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid, 0.01 mmol/l) or addition of ethoxzolamide (0.1 mmol/l) to the bath medium diminished the effect of secretin. Acetate or pre-treatment of ducts with NH4+/NH3 (10 mmol/l in the bath) depolarized the resting Vbl of -65 +/- 2 mV by 16 +/- 4 mV (n = 7) and 19 +/- 3 mV (n = 10), respectively. The fractional resistance of the basolateral membrane (FRbl) doubled, and the depolarizing responses to changes in bath K+ concentrations (5 to 20 mmol/l) decreased from 22 +/- 1 to 11 +/- 2 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the placental brush border membrane Na+/H+ exchanger: identification of thiol-dependent transitions in apparent molecular size

We examined the protein and mRNA encoding the amiloride-sensitive Na+/H+ exchanger from human placenta. Reverse transcriptase PCR of human placental RNA and a human choriocarcinoma cell line showed that the message for the amiloride-sensitive Na+/H+ exchanger from human placenta. Reverse transcriptase PCR of human placental RNA and a human choriocarcinoma cell line showed that the message for the amiloride-sensitive Na+/H+ exchanger is present in the placenta and its derived cell line. Northern blot analysis showed only one species of Na+/H+ exchanger mRNA, of about 5 kb in size. To examine the Na+/H+ exchanger protein two different affinity-purified antibodies were produced against the C-terminal cytoplasmic region of the Na+/H+ exchanger. The antibodies both identified a 105 kDa protein in human placental brush border membrane vesicles. Under non-reducing conditions the amount of 105 kDa protein was greatly decreased, while a 205 kDa protein became apparent. This is probably a dimer of the 105 kDa protein. The monomer-to-dimer transition was dependent on the concentration of beta-mercaptoethanol. The results show that the amiloride-sensitive Na+/H+ exchanger is relatively abundant in human placenta and that it can exist as a larger 205 kDa protein linked by disulphide bonds.

Apical and basolateral Na/H exchange in cultured murine proximal tubule cells (MCT): effect of parathyroid hormone (PTH)

Kidney proximal tubule Na/H exchange is inhibited by PTH. To analyze further the cellular mechanisms involved in this regulation we have used MCT cells (a culture of SV-40 immortalized mouse cortical tubule cells) grown on permeant filter supports. Na/H exchange was measured using single cell fluorescence microscopy (BCECF) and phosphate transport (measured for comparisons) by tracer techniques. MCT cells express apical and basolateral Na/H exchangers which respond differently to inhibition by ethylisopropylamiloride and by dimethylamiloride, the basolateral membrane transporter being more sensitive. Apical membrane Na/H exchange was inhibited by PTH (10(-8) M; by an average of 25%); similar degrees of inhibition were observed when cells were exposed either to forskolin, 8-bromo-cAMP or phorbol ester. Basolateral membrane Na/H exchange was stimulated either by incubation with PTH (to 129% above control levels) or by addition of phorbol ester (to 120% above control levels); it was inhibited after exposure to either forskolin or 8-bromo-cAMP. The above effects of PTH and phorbol ester (apical and basolateral) were prevented by preincubation of cells with protein kinase C antagonists, staurosporine and calphostin C; both compounds did not affect forskolin or 8-bromo-cAMP induced effects. PTH also inhibited apical Na-dependent phosphate influx (29% inhibition at 10(-8) M); it had no effect on basolateral phosphate fluxes (Na-dependent and Na-independent). Incubation with PTH (10(-8) M) resulted in a rapid and transient increase in [Ca2+]i (measured with the fluorescent indicator, fura-2), due to stimulation of a Ca2+ release from intracellular stores. Exposure of MCT cells to PTH did not elevate cellular levels of cAMP. Taken together, these results suggest that PTH utilizes in MCT cells the phospholipase C/protein kinase C pathway to differently control Na/H exchangers (apical vs. basolateral) and to inhibit apical Na/Pi cotransport.

Identification of a small Na+/H+ exchanger-like message in the rabbit myocardium

We examined the Na+/H+ exchanger message in isolated perfused rabbit hearts using Northern blot analysis with cDNA encoding for the rabbit cardiac Na+/H+ exchanger. A cDNA probe from the coding region of the rabbit myocardial Na+/H+ exchanger hybridized to mRNA of 5 kb under high stringency, and to a second 3.8 kb mRNA species under low stringency. When Northern blots were re-probed with a section of the 3'-untranslated region of the cDNA, the 5 kb message was apparent while the smaller 3.8 kb message was not. If isolated working rabbit hearts were subjected to ischemia we observed increases in the 3.8 kb message. Overall, the results show that a 3.8 kb mRNA product, which is homologous to the amiloride sensitive Na+/H+ exchanger, exists in the myocardium and increases during ischemia in the myocardium.

Structure-activity relations of amiloride and its analogues in blocking the mechanosensitive channel in Xenopus oocytes

1. Patch clamp recording techniques have been used to compare the block caused by amiloride and some of its structural analogues of the mechanosensitive (MS) cation selective channel in frog (Xenopus laevis) oocytes. 2. Like amiloride, the amiloride analogues dimethylamiloride (DMA), benzamil and bromohexamethyleneamiloride (BrHMA) block the MS channel in a highly voltage-dependent manner. 3. All analogues tested were more potent blockers than amiloride with IC50's of 500 microM (amiloride), 370 microM (DMA), 95 microM (benzamil) and 34 microM (BrHMA). 4. Hill plots gave Hill coefficients of 2 (amiloride), 1.8 (DMA), 1 (benzamil) and 1.2 (BrHMA) indicating that the binding of two ligand molecules may be necessary for the block caused by amiloride, DMA and possibly BrHMA whereas only a single ligand molecule may be required for the block by benzamil. 5. The potential use of BrHMA as a light-activated, covalent label of the MS channel protein is discussed. 6. The amiloride analogue 'fingerprinting' of the blocking site on the MS channel indicates it is structurally different from previously described amiloride-sensitive ion transport pathways but may be related to the amiloride binding site on outer hair cells of the ear.

ATP-sensitive Na(+)-H+ antiport in type II alveolar epithelial cells

Type II alveolar epithelial cells in suspension have been previously shown to possess a Na(+)-H+ antiporter that modulates recovery from an intracellular acid load in the nominal absence of HCO-3 [E. Nord, S. Brown, and E. Crandall. Am. J. Physiol. 252 (Cell Physiol. 21): C490-C498, 1987]. Such a Na(+)-dependent mechanism has also been demonstrated in cultured type II cell monolayers (K. Sano et al. Biochim. Biophys. Acta 939: 449-458, 1988). It has recently been suggested that cultured type II cells possess a H(+)-ATPase that contributes to recovery from an intracellular acid load [R. Lubman, S. Danto, and E. Crandall. Am. J. Physiol. 257 (Lung Cell. Mol. Physiol. 1): L438-L445, 1989]. The present study was undertaken to investigate and characterize the mechanisms by which cultured type II cells recover from an intracellular acid load in the nominal absence of HCO-3. Cultured type II cell monolayers were loaded with the pH-sensitive probe 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein, and the characteristics of recovery from an imposed intracellular acid load were studied. Recovery of intracellular pH (pHi) was found to be strictly Na(+)-dependent and inhibited greater than or equal to 95% by 1 mM amiloride. Initial rate of recovery was highly sensitive to pHi, with recovery rates varying inversely with increasing pHi. An acidic extracellular pH (6.5) abolished pHi recovery. Treatment of type II cells with either the sulfhydryl reagent N-ethylmaleimide, a nonspecific sulfhydryl reagent, or 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, a specific vacuolar H(+)-ATPase inhibitor at the concentration tested, resulted in marginal but not statistically significant decrements in pHi recovery. Intracellular ATP depletion, using KCN or replacement of glucose by a nonmetabolizable glucose analogue, reduced pHi recovery by 70-75% relative to control values. Sensitivity to ATP was apparent even under conditions that preserved the transmembrane Na+ gradient. Taken together, these data are most consistent with a single mechanism for pHi recovery in the absence of HCO3-. We interpret this mechanism to be an ATP-sensitive Na(+)-H+ antiporter that acts to reestablish pHi in type II alveolar epithelial cells.

Relative sensitivity to inhibition by cimetidine and clonidine differentiates between the two types of Na(+)-H+ exchangers in cultured cells

Available evidence indicates that there are two types of Na(+)-H+ exchangers, type A (housekeeping type) and type B (epithelial or apical type), in mammalian cells. We have recently reported, using isolated membrane vesicles, that these two types can be differentiated by their relative sensitivities to inhibition by clonidine and cimetidine [Kulanthaivel, Leibach, Mahesh, Cragoe & Ganapathy (1990) J. Biol. Chem. 264, 1249-1252]. The present study was undertaken to determine whether this approach is also effective in cultured cells. The JAR human placental choriocarcinoma cell line and the opossum kidney (OK) cell line, when grown as confluent monolayer cultures on an impermeable plastic support, express Na(+)-H+ exchanger activity which is measurable by determining Na+ uptake into the cells from the culture medium. The JAR cell Na(+)-H+ exchanger was found to be about 100 times more sensitive to inhibition by dimethylamiloride than the OK cell Na(+)-H+ exchanger. Inhibition studies with clonidine and cimetidine were able to differentiate between these two exchangers very clearly. Cimetidine was 18 times more potent than clonidine in inhibiting the JAR cell Na(+)-H+ exchanger. In contrast, clonidine was at least 8 times more potent than cimetidine in inhibiting the Na(+)-H+ exchanger of the OK cell. The results show that the JAR cell expresses the type A Na(+)-H+ exchanger, whereas the OK cell expresses the type B Na(+)-H+ exchanger. This approach also proved to be very effective in correctly identifying the type of Na(+)-H+ exchanger in a third cell line (HeLa). It is concluded that the relative susceptibility to inhibition by clonidine and cimetidine offers an easy and efficient means of differentiating between the two types of Na(+)-H+ exchangers in cultured cells.

Na(+)-H+ exchanger subtypes: a predictive review

In recent years, many reports have appeared describing distinct heterogeneity of proteins that heretofore were considered to be a single species or type. The division of proteins into different classes or subtypes is aided by pharmacological tools such as selective ligands, functional measurements such as those examining kinetic or regulatory differences, and molecular biological approaches that have identified distinct genes coding for similar yet distinguishable gene products. Currently, much effort is directed toward understanding the significance of these sometimes subtle differences in terms of functional consequences for the cells in which they exist. Although most reports to date involve hormone and neurotransmitter receptor subtypes, it is also possible that other cell surface molecules such as ion transporters exist as multiple subtypes. In this paper we review the current evidence that Na(+)-H+ exchange activity is mediated by different Na(+)-H+ exchanger subtypes. Although subtypes have not been identified with certainty, we can predict certain distinguishing characteristics that these putative subtypes may have that may be of value in correlating predicted gene products obtained from cDNA cloning with previously characterized Na(+)-H+ exchangers.

Inhibition of the rat renal Na+/H+ exchanger by beta-adrenergic antagonists

The beta-adrenergic antagonists, alprenolol and propranolol, inhibit the Na+/H+ exchanger in rat renal brush-border membrane vesicles. Half-maximal inhibition occurs at 86 microM alprenolol and 36 microM propranolol. Similar to amiloride and Na+, propranolol protects the Na+/H+ exchanger from irreversible inhibition by the carboxyl group reagent, N,N'-dicyclohexyl-carbodiimide (DCCD). Protection is incomplete, depends on propranolol concentration, and reaches a maximum at 0.4 mM propranolol. With a comparable dose dependence, propranolol protects a 65 kDa band from labeling with [14C]DCCD. The data indicate that beta-adrenergic antagonists specifically interact with the proximal tubular Na+/H+ exchanger.

Identification of the protein and cDNA of the cardiac Na+/H+ exchanger

We examined the myocardial form of the Na+/H+ exchanger. A partial length cDNA clone was isolated from a rabbit cardiac library and it encoded for a Na+/H+ exchange protein. In comparison with the human Na+/H+ exchanger, the sequence of the 5' end of the cDNA was highly conserved, much more than the 3' region, while the deduced amino acid sequence was also highly conserved. To further characterize the myocardial Na+/H+ exchange protein, we examined Western blots of isolated sarcolemma with antibody produced against a fusion protein of the Na+/H+ exchanger. The antibodies reacted with a sarcolemma protein of 50 kDa and with a protein of 70 kDa. The results show that the rabbit myocardium does possess a Na+/H+ exchanger protein homologous to the known human Na+/H+ exchanger.

Uptake of glutamate in Corynebacterium glutamicum. 1. Kinetic properties and regulation by internal pH and potassium

The active uptake system for glutamate in Corynebacterium glutamicum is inducible by growth on glutamate as sole energy and carbon source and is also susceptible to catabolite repression by glucose. The basic level of uptake activity is low in glucose-grown cells (1.5 nmol.mg dry mass-1.min-1), it is intermediate when acetate is the carbon source (3.8 nmol.mg dry mass-1.min-1) and becomes fully induced by glutamate (15 nmol.mg dry mass-1.min-1). In all cases the uptake has, except for different Vmax values, identical kinetic and energetic properties, and is characterized by a low apparent Km value of 0.5-1.3 microM and by high substrate specificity. The transported substrate species is the deprotonated form which can also be concluded from the extremely high pH optimum of transport above pH 9. Glutamate uptake in cells grown in media with low K+ concentration is not influenced by external Na+ but is drastically stimulated by addition of K+. Stimulation by K+ could be separated into two different mechanisms. (a) Addition of K+ increases the internal pH, thereby stimulating glutamate uptake which is regulated by the internal pH in C. glutamicum. The apparent pK of the internal 'pH switch' is 6.6; below this value, uptake of glutamate is inhibited. (b) Internal K+ also directly promotes glutamate uptake. Effective uptake of glutamate can be observed only when the cytosolic K+ concentration exceeds a threshold value of about 200 mM. Stimulation of glutamate uptake by external K+ is not due to functional coupling of K+ and glutamate transport but reveals the necessity to replenish the internal K+ pool.

Parathyroid hormone regulation of Na+/H+ exchange in opossum kidney cells: polarity and mechanisms

In previous work we have shown that parathyroid hormone (PTH) inhibits Na+/H+ exchange in cellular suspensions of OK (opossum kidney) cells (an established renal epithelial cell line) in a dose-dependent manner. PTH effects could be mimicked by pharmacological activation of both protein kinase A and protein kinase C (Helmle-Kolb et al. 1990). In the present paper we extend these observations and analyze the PTH-dependent control of Na+/H+ exchange in OK cells kept in epithelial configuration (monolayer). Na+/H+ exchange activity is examined by microfluorometry using the intracellularly trapped pH-sensitive dye 2'7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein. Cells recovered from an acid load (NH4Cl prepulse) after addition of apical Na+. Ethylisopropylamiloride inhibits Na(+)-dependent pHi recovery at micromolar concentrations. PTH leads to an inhibition of apical Na+/H+ exchange activity; inhibition is observed even at a concentration of 5 pM PTH. PTH given at maximally effective concentrations (24 nM) reduces the total Na+/H+ exchange capacity by 60%-70%. Apical as well as basolateral hormone additions elicit an inhibitory response at low (5 pM) or high (24 nM) concentrations. Forskolin (activation of protein kinase A) and phorbol esters (activation of protein kinase C) lead to an inhibition of Na+/H+ exchange activity (60%-70% inhibition). These observations suggest that Na+/H+ exchange activity is preferentially located in the apical membranes of OK cells kept in monolayer configuration.(ABSTRACT TRUNCATED AT 250 WORDS)

Polarity and kinetics of Na(+)-H+ exchange in cultured opossum kidney cells

Opossum kidney (OK) cells (an established cell line) were loaded with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF; a fluorescent dye with a pH-sensitive spectrum), and intracellular pH (pHi) was examined by microfluorometry. Single cells, within a confluent monolayer and grown on a permeant support, were examined for the mechanism of recovery from an acid load as imposed by exposure to ammonium chloride (NH4 prepulse). The Na(+)-dependent recovery of pHi from an acid load (Na(+)-H+ exchange) is examined in terms of the Na+ activation kinetics of the recovery and the polarity of the response. In 80% of the cells examined (33/41), both apical and basolateral Na+ cause recovery from an acid load. The response of cells to apical Na+ is well fit by Michaelis-Menten kinetics [Kt(Na) = 35 mM], but the response to basolateral Na+ is not. The response to basolateral Na+ addition is modeled in terms of variable transepithelial leak of Na+ and variable amounts of basolateral Na(+)-H+ exchange. Despite an average response to basolateral (145 mM) Na+ that is 34% of the response to apical Na+, modeling suggests that basolateral Na(+)-H+ exchange must be less than 10% of the cellular total to fit the basolateral Na+ activation kinetics. The model, and experiments using ordered addition of Na+ from the apical vs. basolateral medium, also suggest that transepithelial leak (of basolateral Na+ to the apical compartment) is required to explain the pHi recovery observed due to addition of basolateral Na+. Direct estimation of (basolateral to apical) transepithelial leak demonstrates that the response due to basolateral Na+ addition is explained by transepithelial leak and a Na(+)-H+ exchange that is expressed solely in the apical membrane.

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.

Cation transport probes: the amiloride series

The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, and of effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. We have reviewed the pharmacology of inhibition of these processes by amiloride an its analogs, as well as the use of amiloride analogs as potential probes for the characterization of ion transport systems.

Regulation of Na+/H+ exchange in opossum kidney cells by parathyroid hormone, cyclic AMP and phorbol esters

Parathyroid hormone (PTH) controls two proximal tubular brush border membrane transport systems, Na+/phosphate co-transport and Na+/H+ exchange. In OK cells, a cell line with proximal tubular transport characteristics, PTH acts via kinase C and kinase A activation to inhibit Na+/phosphate co-transport [6, 8, 9, 19, 22]. In the present study, we show that PTH inhibits Na+/H+ exchange and that this effect can be mimicked by pharmacological activation of kinase A and kinase C. Ionomycin-dependent increases in cytoplasmic Ca2+ concentration do not induce inhibition of Na+/H+ exchange; PTH-dependent inhibition of Na+/H+ exchange is not prevented by ionomycin or by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (Ca2+ clamping). Detailed dose-response curves for the different agonists, given either alone or in combination, suggest that the two regulatory cascades (kinase A and kinase C) are operating independent of each other and reach a common final target, resulting in 40-50% inhibition of Na+/H+ exchange. An analysis of intracellular pH sensitivity of Na+/H+ exchange suggests that inhibition is not related to a shift in set point, but is rather explained by a reduced Vmax of Na+/H+ exchange and/or reduced affinity for protons at the internal membrane surface. It is suggested that kinase A as well as kinase C can mediate PTH inhibition of renal proximal tubular Na+/H+ exchange and that the relative importance of a particular regulatory cascade is determined by the PTH-concentration-dependent rates in the liberation of diacylglycerol (phospholipase C/kinase C) and cAMP (adenylate cyclase/kinase A).

The role of sodium ions in methanogenesis. Formaldehyde oxidation to CO2 and 2H2 in methanogenic bacteria is coupled with primary electrogenic Na+ translocation at a stoichiometry of 2-3 Na+/CO2

Cell suspensions of Methanosarcina barkeri were found to oxidize formaldehyde to CO2 and 2H2 (delta G0' = -27 kJ/mol CO2), when methanogenesis was inhibited by 2-bromoethanesulfonate. We report here that this reaction is coupled with (a) primary electrogenic Na+ translocation at a stoichiometry of 2-3 Na+/CO2, (b) with secondary H+ translocation via a Na+/H+ antiporter and (c) with ATP synthesis driven by an electrochemical proton potential. This is concluded from the following findings. Formaldehyde oxidation to CO2 and 2H2 was dependent on Na+ ions, 2-3 mol Na+/mol formaldehyde oxidized were extruded. Na+ translocation was inhibited by Na+ ionophores, but not affected by protonophores of Na+/H+ antiport inhibitors. Formaldehyde oxidation was associated with the build up of a membrane potential in the order of 100 mV (inside negative), which could be dissipated by sodium ionophores rather than by protonophores. Formaldehyde oxidation was coupled with ATP synthesis, which could be inhibited by Na+ ionophores, Na+/H+ antiport inhibitors, by protonophores and by the H+-translocating-ATP-synthase inhibitor, dicyclohexylcarbodiimide. With cell suspensions of Methanobacterium thermoautotrophicum similar results were obtained.

Characterization and reconstitution of the Na+/H+ antiporter from the plasma membrane of the halotolerant alga Dunaliella

Na+/H+ exchange activity in plasma membrane preparations isolated from the unicellular halotolerant alga, Dunaliella salina, is shown to be competitively inhibited by amiloride or Li+, with Ki values of 25 and 30 microM, respectively. The activity can be followed by either the sodium-dependent change in transvesicular delta pH, as monitored by absorbance changes of Acridine orange, or by the delta pH-dependent uptake of 22Na+ into the intravesicular space. The activity was solubilized, by extraction with Triton, and reconstituted into active proteoliposomes. The activity of the reconstituted proteoliposomes was strongly stimulated by the presence of valinomycin and KCl, suggesting that the exchanger is electrogenic, presumably exchanging more than one proton for each Na+ ion. Partial purification of the Triton-extracted exchanger was obtained by fractionation on a DEAE-cellulose column.

Inhibition of Na+-dependent phosphate transport by group-specific covalent reagents in rat kidney brush border membrane vesicles. Evidence for the involvement of tyrosine and sulfhydryl groups on the interior of the membrane

The effects of tyrosine- and sulfhydryl-specific reagents on the Na+-dependent transport of phosphate in brush border membrane vesicles prepared from rat renal cortex were investigated. This study is the first to show that the tyrosine-specific reagents 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole and tetranitromethane inactivate the transporter in a concentration- and time-dependent fashion while the membrane impermeant tyrosine reagent, N-acetylimidazole, has no effect on phosphate uptake. The membrane permeant sulfhydryl reagent N-ethylmaleimide also caused a time- and concentration-dependent inactivation of this transport process but the membrane impermeant reagents 7-chloro-4-sulfobenzo-2-oxa-1,3-diazole and eosin-5-maleimide had little effect on phosphate uptake. The inhibitory effects of both tyrosine- and sulfhydryl-specific reagents were additive, but no protection from inactivation by tyrosine-specific reagents could be achieved by preincubation of the vesicles with the substrates of the transporter or with competitive inhibitors of the transport process. These results suggest that the amino acids modified by these agents are located either within the membrane or on the cytosolic surface of the transporter. These residues may not participate in substrate binding, but may be important for the conformational change of the transporter necessary for the translocation of phosphate across these membranes. This study also shows that Na+-dependent phosphate transport can be inactivated by other reagents which covalently modify histidine, carboxyl, and amino groups on proteins.

Relation of ATPases in rat renal brush-border membranes to ATP-driven H+ secretion

In the presence of inhibitors for mitochondrial H+-ATPase, (Na+ + K+)- and Ca2+-ATPases, and alkaline phosphatase, sealed brush-border membrane vesicles hydrolyse externally added ATP demonstrating the existence of ATPases at the outside of the membrane ("ecto-ATPases"). These ATPases accept several nucleotides, are stimulated by Ca2+ and Mg2+, and are inhibited by N.N'-dicyclohexylcarbodiimide (DCCD), but not by N-ethylmaleimide (NEM). They occur in both brush-border and basolateral membranes. Opening of brush-border membrane vesicles with Triton X-100 exposes ATPases located at the inside (cytosolic side) of the membrane. These detergent-exposed ATPases prefer ATP, are activated by Mg2+ and Mn2+, but not by Ca2+, and are inhibited by DCCD as well as by NEM. They are present in brush-border, but not in basolateral membranes. As measured by an intravesicularly trapped pH indicator. ATP-loaded brush-border membrane vesicles extrude protons by a DCCD- and NEM-sensitive pump. ATP-driven H+ secretion is electrogenic and requires either exit of a permeant anion (Cl-) or entry of a cation, e.g., Na+ via electrogenic Na+/D-glucose and Na+/L-phenylalanine uptake. In the presence of Na+, ATP-driven H+ efflux is stimulated by blocking the Na+/H+ exchanger with amiloride. These data prove the coexistence of Na+-coupled substrate transporters, Na+/H+ exchanger, and an ATP-driven H+ pump in brush-border membrane vesicles. Similar location and inhibitor sensitivity reveal the identity of ATP-driven H+ pumps with (a part of) the DCCD- and NEM- sensitive ATPases at the cytosolic side of the brush-border membrane.

Inhibition and labeling of the rat renal Na+/H+-exchanger by an antagonist of muscarinic acetylcholine receptors

A covalently binding label for muscarinic acetylcholine receptors, propylbenzilylcholine mustard (PrBCM), irreversibly inhibits the Na+/H+ exchanger in rat renal brush-border membrane vesicles. Substrates of the antiporter, Na+ and Li+, as well as inhibitors, amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA) and propranolol, protect the antiporter from inactivation by PrBCM. With [3H]PrBCM a band with an app. Mr of 65 kDa is predominantly labeled. Amiloride protects this band from labeling with [3H]PrBCM and [14C]-N,N'-dicyclohexylcarbodiimide (DCCD) proving its identity with the renal Na+/H+ exchanger. Our data reveal a specific interaction of PrBCM with the Na+/H+ exchanger and suggest structural relations between antiporter and receptors.

Solubilization and reconstitution of renal brush border Na+-H+ exchanger

In order to permit future characterization and possible isolation of the Na+-H+ exchanger from the apical membrane of proximal tubular cells, studies were performed to solubilize and reconstitute this transporter. Rabbit brush border membranes were prepared by a magnesium aggregation method, solubilized with the detergent octyl glucoside, and reconstituted into artificial phospholipid vesicles. In the presence of a pH gradient (pHin 6.0, pHout 8.0), the uptake of 1 mM 22Na+ into the proteoliposomes was five- to sevenfold higher than into liposomes. Amiloride (2 mM) inhibited proton gradient-stimulated uptake of sodium by 50%. As compared to proton gradient conditions, the uptake of sodium was lower in the absence of a pH gradient but was significantly higher when the outside and inside pH was 6.0 than 8.0. The Ka for sodium in reconstituted proteoliposomes studied under pH gradient conditions was 4 mM. The uptake of sodium in proteoliposomes prepared from heat-denatured membrane proteins was significantly decreased. These studies demonstrate that proteoliposomes prepared from octyl glucoside-solubilized brush border membrane proteins and asolectin exhibit proton gradient-stimulated, amiloride-inhibitable, electroneutral uptake of sodium. The ability to solubilize and reconstitute the Na+-H+ exchanger from the apical membrane of the proximal tubule will be of value in isolating and characterizing this transporter.

Amiloride and its analogs as tools in the study of ion transport

Amiloride inhibits most plasma membrane Na+ transport systems. We have reviewed the pharmacology of inhibition of these transporters by amiloride and its analogs. Thorough studies of the Na+ channel, the Na+/H+ exchanger, and the Na+/Ca2+ exchanger, clearly show that appropriate modification of the structure of amiloride will generate analogs with increased affinity and specificity for a particular transport system. Introduction of hydrophobic substituents on the terminal nitrogen of the guanidino moiety enhances activity against the Na+ channel; whereas addition of hydrophobic (or hydrophilic) groups on the 5-amino moiety enhances activity against the Na+/H+ exchanger. Activity against the Na+/Ca2+ exchanger and Ca2+ channel is increased with hydrophobic substituents at either of these sites. Appropriate modification of amiloride has produced analogs that are several hundred-fold more active than amiloride against specific transporters. The availability of radioactive and photoactive amiloride analogs, anti-amiloride antibodies, and analogs coupled to support matrices should prove useful in future studies of amiloride-sensitive transport systems. The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, as well as effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. One must consider whether the effects seen on various cellular processes are direct or due to a cascade of events triggered by an effect on an ion transport system.

LLC-PK1 mutant with increased Na+-H+ exchange and decreased sensitivity to amiloride

LLC-PK1 cells contain a well-characterized Na+-H+ antiporter that is sensitive to ethylisopropylamiloride (EIPA) in the submicromolar range. Using a modification of the method of Franchi et al. (J. Biol. Chem. 261: 14614-14620, 1986), we have selected mutants that can recover from an acid load in the presence of 100 microM EIPA. One such mutant, designated PKE20, has been studied in detail. The maximal velocity (Vmax) for the Na+-H+ antiporter, assayed as EIPA-sensitive 22Na+ uptake, has increased from 44 nmol.min-1.10(6) cells-1 in the parent cells to 106 nmol.min-1.10(6) cells-1 in PKE20. No detectable change has occurred in the Km for Na+ (118 mM in the parent, 111 mM in the mutant) or in the dependence of Na+ uptake on intracellular pH. However, the PKE20 antiporter exhibits a greatly decreased sensitivity to amiloride and its derivatives, with drops in inhibitory potency ranging from 25-fold (amiloride) to 100-fold (EIPA). The mutation is specific for the antiporter; measurements of Na+-K+ pump and Na+-dependent amino acid uptake show only small changes, which appear to result from minor antiporter-induced alterations in internal Na+ concentration. PKE20 cells should prove useful in experiments to identify and isolate the antiporter protein.

Polarity, diversity, and plasticity in proximal tubule transport systems

This contribution first reviews the distribution of transport systems within the plasma membrane of the proximal tubule cell (polarity), with particular emphasis on transport systems located in the basal-lateral plasma membranes and on the role of cascade coupling in tubular transport. Then, the differences between transport systems in the pars convoluta and the pars recta of the proximal tubule are discussed (diversity). Finally, evidence is presented that changes in the microenvironment of sodium cotransport systems can alter the mode of operation of the transporter (plasticity). The two examples mainly addressed are the sodium-D-glucose and the sodium-glutamate cotransport system.

Molecular size of the Na+-H+ antiport in renal brush border membranes, as estimated by radiation inactivation

The radiation inactivation method was applied to brush border membrane vesicles from rat kidney, in order to estimate the molecular size of the Na+-H+ antiporter. Sodium influx (1mM) driven by an acid intravesicular pH was unaffected by the high osmolarity of the cryoprotective solution. Initial rate of influx was estimated by linear regression performed on the first 10 seconds of transport: 0.512 pmol/micrograms protein/s. There was no binding component involved. Incubation performed in the presence of 1 mM amiloride, an inhibitor of the Na+-H+ antiport gave an initial rate of only 0.071 pmol/microgram/s, an 82% inhibition. Membrane vesicles were irradiated at -78 degrees C in a Gammacel Model 220. Sodium influx was reduced, as the dose of radiation increased, but the influx remained linear for the period of time (10s) during which the initial rate was estimated, indicating no alteration of the proton driving force during this time period. Amiloride-insensitive flux remained totally unaffected by the radiation dose, indicating that the passive permeability of the membrane towards sodium was unaffected. The amiloride-sensitive pathway presented a monoexponential profile of inactivation, allowing the molecular size to be estimated at 321 kDa. Based on DCCD-binding studies suggesting the molecular size of the monomer to be around 65 kDa for rat kidney, our results suggest that the functional transporter in the membrane to be a multimer.

Inhibition of sodium-dependent transport systems in rat renal brush-border membranes with N,N'-dicyclohexylcarbodiimide

Treatment of rat renal brush-border membrane vesicles with N,N'-dicyclohexylcarbodiimide (DCCD) causes irreversible inhibition of the Na+-coupled transport systems for D-glucose, L-phenylalanine, L-glutamate, and sulfate. The DCCD-reactive side groups of these transport systems differ in their sensitivity towards DCCD and protection by substrates. The D-glucose and L-glutamate transporters cannot be protected by their substrates. In contrast, Na+ protects the transport systems for L-phenylalanine and sulfate from inactivation by DCCD. The data suggest covalent modification by DCCD of D-glucose and L-glutamate transporters apart from their substrate binding sites and of L-phenylalanine and sulfate transporters within their Na+-binding regions.

Evidence for histidyl and carboxy groups at the active site of the human placental Na+-H+ exchanger

The Na+-H+ exchanger of the human placental brush-border membrane was inhibited by pretreatment of the membrane vesicles with a histidyl-group-specific reagent, diethyl pyrocarbonate and with a carboxy-group-specific reagent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline. In both cases the inhibition was irreversible and non-competitive in nature. But, if the membrane vesicles were treated with these reagents in the presence of amiloride, cimetidine or clonidine, there was no inhibition. Since amiloride, cimetidine and clonidine all interact with the active site of the exchanger in a mutually exclusive manner, the findings provide evidence for the presence of essential histidyl and carboxy groups at or near the active site of the human placental Na+-H+ exchanger. This conclusion was further substantiated by the findings that Rose Bengal-catalysed photo-oxidation of histidine residues as well as covalent modification of carboxy residues with NN'-dicyclohexylcarbodi-imide irreversibly inhibited the Na+-H+ exchanger and that amiloride protected the exchanger from inhibition caused by NN'-dicyclohexylcarbodi-imide.

Species differences between rat and rabbit renal Na+/H+ exchangers

Amiloride, 5-ethylisopropylamiloride (EIPA), and 5-ethylisopropyl-6-bromo-amiloride (Br-EIPA) inhibit rabbit and rat renal Na+/H+ exchangers with comparable potency. Irreversible inhibitions by Br-EIPA after irradiation of rat and rabbit renal brush-border vesicles are also similar. In contrast, irreversible inhibition by N,N'-dicyclohexylcarbodiimide (DCCD) of the rabbit renal antiporter requires higher DCCD concentrations as compared to the rat. Rabbit renal brush-border membranes show highest [14C]DCCD incorporation at MW 80,000, 51,000 and 36,000 and lack the amiloride-protectable MW 65,000 protein previously identified as Na+/H+ exchanger in rat kidney cortex. The data indicate species differences with respect to renal Na+/H+ exchangers.