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

Mechanisms of the ifosfamide-induced inhibition of endocytosis in the rat proximal kidney tubule

The Fanconi syndrome is a common side effect of the chemotherapeutic agent ifosfamide. Current evidences suggest that chloroacetaldehyde (CAA), one of the main metabolites of ifosfamide activation, contributes to its nephrotoxicity. However, the pathophysiology of CAA-induced Fanconi syndrome is not fully understood. The present work examined the adverse effects of CAA on precision-cut rat renal cortical slices, which allowed studying the toxic effect of CAA on proximal endocytosis. We demonstrated that clinically relevant concentrations of CAA (< or =200 microM) are able to inhibit the uptake of horseradish peroxidase, a marker of proximal tubular cell endocytosis in renal tubular proximal cells. CAA > or =75 microM has adverse effects, both on viability parameters and on energy metabolism, as shown by the great decrease in total glutathione and ATP levels. In addition, the V-ATPase, which plays a crucial role in intracellular vesicle trafficking, was inhibited by 100 microM of CAA. By contrast, the slight decrease in Na-K-ATPase activity observed for CAA> or = 125 microM (maximum inhibition: 33%) could not totally explain the inhibition of the reabsorption processes. In conclusion, the addition of the two main adverse effects of CAA (decrease in ATP levels and inhibition of the V-ATPase) could explain the inhibition of endocytosis and the Fanconi syndrome observed during ifosfamide treatments.

A pharmacological examination of Na+ and Cl- transport in two species of freshwater fish

We examined branchial Na(+) and Cl(-) uptake in two species of stenohaline, freshwater fish (goldfish and the Amazonian neon tetra). Kinetic analysis revealed that the two species had similar uptake capacities and affinities for Na(+) and Cl(-). However, while uptakes of Na(+) and Cl(-) (JNain and JClin, respectively) by goldfish were completely inhibited at pH 4.5 and below, uptake in tetras was unaffected by pH down to 3.25. Examination of Cl(-) transport with blockers indicated that goldfish and neon tetras utilize Cl(-)/HCO-3 exchange; SITS and SCN(-) inhibited Cl(-) uptake in both species. In contrast, large differences in Na(+) transport were indicated between the species. In goldfish, exposure to four Na(+)/H(+) exchange blockers, as well as the Na(+) channel blocker phenamil, strongly inhibited JNain. Further, Na(+) and Cl(-) uptake were strongly inhibited by the Na(+)/K(+)/Cl(-) cotransport inhibitor furosemide, as was JNain in "Cl(-)-free" water and JClin in "Na(+)-free" water. This suggests the presence of multiple transporters and possibly even a direct linkage between the transport of Na(+) and Cl(-) in goldfish. In contrast, none of these drugs strongly reduced Na(+) transport in neon tetras, which raises the possibility of a significantly different Na(+) transport mechanism in this acid-tolerant species.

Selective antagonism to the cadherin BT-R1 interferes with calcium-induced adhesion of epithelial membrane vesicles

BT-R(1) is a member of the cadherin superfamily of proteins and is expressed in the midgut epithelium of Manduca sexta during larval development. Previously, we showed that calcium ions influence the structure and stability of BT-R(1) on brush border membrane vesicles (BBMVs) prepared from M. sexta midgut epithelium. In the present study, the effects of calcium and Cry1Ab toxin, produced by Bacillus thuringiensis, on the adhesive properties of BBMVs were investigated. Addition of calcium to a suspension of BBMVs promoted adhesion and aggregation of the vesicles. Treatment of BBMVs with trypsin or lowering the pH (pH 4.0) of the BBMV suspension abolished calcium-induced vesicle aggregation, whereas treatment with deglycosylating enzymes did not affect the aggregation of vesicles, indicating that adhesion and clustering of BBMVs involves protein-protein interactions. Preincubation of BBMVs with Cry1Ab toxin, which specifically binds to BT-R(1) with high affinity and disrupts the midgut epithelium of M. sexta, caused a 50% decrease in calcium-induced vesicle aggregation. The inhibitory effects of the Cry1Ab toxin on BBMV aggregation was blocked completely when the toxin was preincubated with a peptide containing the toxin-binding site of BT-R(1). Cry3A toxin, which is similar in molecular structure to Cry1Ab but does not bind to BT-R(1) and is not toxic to M. sexta larvae, did not affect BBMV aggregation. The results of this study demonstrate that the adhesive function of BT-R(1) is compromised by the Cry1Ab toxin, which acts as a selective antagonist, and supports the notion that BT-R(1) is critical in preserving the integrity of larval midgut epithelium in M. sexta.

Effect of bafilomycin A1, a specific inhibitor of vacuolar (V-type) proton ATPases, on the capacitation of rabbit spermatozoa

Mammalian spermatozoa maintain precisely regulated ionic gradients that must be modified during capacitation and the acrosome reaction. In other cell types, ionic gradients are mainly regulated by the presence in plasma membranes of three metabolically different types of ATPases. The modifications induced during in vitro capacitation of rabbit spermatozoa by the specific inhibition of V-type H+-ATPases with bafilomycin A were studied. We used chlortetracycline binding to rabbit spermatozoa to monitor capacitation, and the coomassie brilliant blue method to identify acrosome-reacted sperm cells. There was a significant difference between the percentage of epididymal (66 +/- 7%) and ejaculated (43 +/- 11%) spermatozoa capacitated in vitro, after a 6-h incubation period in the presence of Ca2+ without ATPase inhibitor. The presence of bafilomycin significantly reduced these numbers (25 +/- 11 and 16+/- 8%, epididymal and ejaculated spermatozoa, respectively) and eliminated the difference. Ejaculated spermatozoa capacitated in the absence of bafilomycin showed a linear increase in the percentage of acrosome reactions induced by the addition of A23187 (12 +/- 5, 23+/- 6 and 31 +/- 5 after 15, 30 and 45 min). The presence of 0.2 micromol l-1 bafilomycin during the capacitation incubation induced a significant decrease in the acrosome reaction percentages (4 +/- 2, 8 +/- 3 and 14 +/- 4 after 15, 30 and 45 min). The addition of bafilomycin after the capacitating period had no effect upon the induction of the acrosome reaction by A23187. These results indicate that vacuolar ATPases play an important role during rabbit sperm capacitation. However, once the spermatozoa have been capacitated, V-type ATPases do not have a significant participation during the acrosome reaction.

Functional expression of a vacuolar-type H+-ATPase in the plasma membrane and intracellular vacuoles of Trypanosoma cruzi

Acid-loaded Trypanosoma cruzi amastigotes and trypomastigotes regained normal cytoplasmic pH (pHi), as measured in cells loaded with 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), by a process that was sensitive to bafilomycin A1 at concentrations comparable to those that inhibited vacuolar (V) H+-ATPases from different sources. Steady-state pHi was also decreased by similar concentrations of bafilomycin A1 in a concentration-dependent manner. The efflux of H+ equivalents from amastigotes and trypomastigotes was measured by following changes in the fluorescence of extracellular BCECF. Basal H+ extrusion in the presence of glucose was 15.4+/-2.8 (S.D.) nmol of H+/min per 10(8) amastigotes and 6. 37+/-0.8 nmol of H+/min per 10(8) trypomastigotes. Bafilomycin A1 treatment significantly decreased the efflux of H+ equivalents by amastigotes (8.9+/-2.2 nmol of H+/min per 10(8) cells), but not by trypomastigotes (5.1+/-1.7 nmol of H+/min per 10(8) cells). The localization of the V-H+-ATPase of T. cruzi was investigated by immunocytochemistry. Confocal and electron microscopy indicated that, in addition to being located in cytoplasmic vacuoles, the V-H+-ATPase of different stages of T. cruzi is also located in the plasma membrane. However, no labelling was detected in the plasma membrane lining the flagellar pocket of the different developmental stages. Surface localization of the V-H+-ATPase was confirmed by experiments involving the biotinylation of cell surface proteins and immunoprecipitation with antibodies against the V-H+-ATPase. Taken together, the results are consistent with the presence of a functional V-H+-ATPase in the plasma membrane of amastigotes and with an important role for intracellular acidic compartments in the maintenance of pHi in different stages of T. cruzi.

Cadmium inhibits vacuolar H(+)-ATPase and endocytosis in rat kidney cortex

The mechanism of cadmium (Cd)-induced damage in the mammalian proximal tubule that is manifested by defects in reabsorption of various compounds, is poorly understood. A vacuolar H(+)-ATPase (V-ATPase) in proximal tubule (PT) brush border and intracellular vesicles may be affected by Cd, and this may influence intracellular vesicle trafficking and reabsorption of the filtered proteins. We studied the effects of Cd on V-ATPase and endocytosis in rat renal PT in vivo and on acidification mechanisms in isolated renal cortical organelles in vitro. The V-ATPase activity in brush border membrane (BBM) from Cd-intoxicated rats was 40% lower compared to that in control animals. Immunofluorescence studies in cortical tissue sections and Western blot studies in BBM from Cd-treated rats showed a strongly decreased abundance of the 31 kDa and 70 kDa V-ATPase subunits. Functional studies in vivo showed a dramatically diminished endocytosis of fluorescein-labeled dextran in PT cells from Cd-treated animals, whereas morphological studies revealed a loss of endocytic invaginations and subapical vesicles in the same cells. In studies in vitro, Cd inhibited V-ATPase activity in a concentration- and time-dependent manner in both BBM and endocytic vesicles, whereas in endocytic vesicles, Cd inhibited ATP-driven intravesicular acidification and accelerated the dissipation of transmembrane pH gradients. We conclude that Cd may impair acidification in cell organelles by (a) causing a loss of V-ATPase protein in their limiting membranes, (b) inhibiting the intrinsic V-ATPase activity, and (c) dissipating the transmembrane pH gradient. This may inhibit endocytosis of filtered proteins and impair vesicle-mediated recycling of some membrane transporters, thus contributing to the loss of reabsorptive capacity of the PT.

Vacuolar-type H+-ATPase regulates cytoplasmic pH in Toxoplasma gondii tachyzoites

Cytoplasmic pH (pHi) regulation was studied in Toxoplasma gondii tachyzoites by using the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein. Their mean baseline pHi (7.07+/-0.06; n=5) was not significantly affected in the absence of extracellular Na+, K+ or HCO3(-) but was significantly decreased in a dose-dependent manner by low concentrations of N, N'-dicyclohexylcarbodi-imide (DCCD), N-ethylmaleimide (NEM) or bafilomycin A1. Bafilomycin A1 also inhibited the recovery of tachyzoite pHi after an acid load with sodium propionate. Similar concentrations of DCCD, NEM and bafilomycin A1 produced depolarization of the plasma membrane potential as measured with bis-(1,3-diethylthiobarbituric)trimethineoxonol (bisoxonol), and DCCD prevented the hyperpolarization that accompanies acid extrusion after the addition of propionate, in agreement with the electrogenic nature of this pump. Confocal laser scanning microscopy indicated that, in addition to being located in cytoplasmic vacuoles, the vacuolar (V)-H+-ATPase of T. gondii tachyzoites is also located in the plasma membrane. Surface localization of the V-H+-ATPase was confirmed by experiments using biotinylation of cell surface proteins and immunoprecipitation with antibodies against V-H+-ATPases. Taken together, the results are consistent with the presence of a functional V-H+-ATPase in the plasma membrane of these intracellular parasites and with an important role of this enzyme in the regulation of pHi homoeostasis in these cells.

The role of a H(+)-ATPase in the regulation of cytoplasmic pH in Trypanosoma cruzi epimastigotes

Cytoplasmic pH (pHi) regulation was studied in Trypanosoma cruzi epimastigotes using fluorescent probes. Steady-state pHi was maintained even in the absence of extracellular Na+ or K+, but was significantly decreased in the absence of Cl-. Acid-loaded epimastigotes regained normal pHi by a process that was ATP-dependent and sensitive to N-ethylmaleimide, dicyclohexyl-carbodi-imide and diethylstiboestrol, suggesting involvement of a H(+)-pumping ATPase. Recovery from an acid load was independent of extracellular Na+ or K+ and insensitive to omeprazole, vanadate and low concentrations of bafilomycin A1. Using the fluorescent probe bisoxonol to measure the membrane potential of intact cells, acid loading of epimastigotes was shown to result in a dicyclohexylcarbodi-imide-sensitive hyperpolarization, which suggests electrogenic pumping of protons across the plasma membrane. Addition of glucose, but not of 6-deoxyglucose, produced a transient cellular acidification of possible metabolic origin, and increased the rate of recovery from an acid load. Taken together, these results are consistent with an important role of a H(+)-ATPase in the regulation of pHi homoeostasis in T. cruzi.

An H(+)-ATPase regulates cytoplasmic pH in Pneumocystis carinii trophozoites

Pneumocystis carinii is an opportunistic fungus which causes interstitial pneumonia in patients with acquired immunodeficiency syndrome (AIDS). Cytoplasmic pH (pHi) regulation in short-term-cultured P. carinii trophozoites was studied using the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(-6)-carboxyfluorescein. With an extracellular pH of 7.4, the mean baseline pHi of P. carinii trophozoites was 7.40 +/- 0.10 (n = 8). This steady-state pHi was not significantly affected in the absence of extracellular Na+ or K+. Moreover, steady-state pHi was maintained in the nominal absence of HCO3- and was not affected by the Cl-/HCO(3-)-exchanger inhibitor 4, 4'-di-isothiocyanato-dihydrostilbene-2, 2'-disulphonic acid (100 microM), or the Na+/H(+)-exchanger inhibitor N-ethyl-N-isopropylamiloride (100 microM). In contrast, the general inhibitors of ATPases, N-ethylmaleimide (1 mM), and dicyclohexylcarbodi-imide (100 microM), and the inhibitor of yeast H(+)-ATPase, diethylstilbestrol (12.5-100 microM), decreased pHi, while the K+/H(+)-ATPase inhibitor omeprazole (50-400 microM), and the vacuolar-type H(+)-ATPase inhibitor bafilomycin A1 (1-5 microM) only produced a dose-dependent acidification of the cells when used at high concentrations. In addition, steady-state pHi depended on the availability of cellular ATP, since it was decreased by the ATP synthase inhibitors oligomycin (1 microgram/ml) and sodium azide (1 mM), and by the uncoupler of oxidative phosphorylation carbonyl cyanide p-trifluorophenylhydrazone (1 microM), agents that were able to deplete significantly the intracellular ATP levels. Taken together, these results are consistent with an important role of an H(+)-ATPase similar to those found in other fungi in the regulation of pHi homoeostasis in P. carinii trophozoites.

Identification and partial purification of (Ca2+ or Mg2+)-ATPase in renal brush-border membranes

The protein responsible for the (Ca2+ or Mg2+)-ATPase activity in brush-border membranes from pig kidney tubular cells was characterized to distinguish this enzyme from the N-ethylmaleimide-sensitive Mg(2+)-ATPase, also present in renal brush borders. Both enzymes are clearly different in their pH optimum and their sensitivity to divalent cations, nucleoside 5'-triphosphates and inhibitors. Solubilization of the (Ca2+ or Mg2+)-ATPase from brush-border membrane vesicles was accomplished with Nonidet P-40 or dodecylmaltoside. However, simultaneous inactivation of the enzyme was inevitable. A tenfold enrichment of the ATPase activity was obtained by chromatofocusing of Nonidet-P-40-solubilized brush borders. A similar degree of purification was achieved by ion-exchange chromatography of dodecylmaltoside-solubilized preparations. From the SDS/polyacrylamide gels of partially purified (Ca2+ or Mg2+)-ATPase, a few protein bands could still be tentatively identified as responsible for the enzyme activity. Labeling of solubilized brush-border preparations with several radioactive ATP analogues also revealed that a protein band of molecular mass 90 kDa is the most probable candidate for the catalytic peptide of the (Ca2+ or Mg2+)-ATPase. Finally, immunoprecipitation as well as semi-dry blotting with antibodies generated against partially purified enzyme preparations, confirmed that a 90-kDa component is a reasonable candidate for the (Ca2+ or Mg2+)-ATPase in renal brush-border membranes.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Sequence analysis of the catalytic subunit of H(+)-ATPase from porcine renal brush-border membranes

The catalytic subunit of the H(+)-ATPase from brush-border membranes of porcine renal proximal tubules was labeled with the hydrophobic SH-group reagent 10-N-(bromoacetyl)amino-1-decyl-beta-glucopyranoside (BADG) which irreversibly inhibits proton pump activity in the absence but not in the presence of ATP. The labeled protein was purified and digested with proteinases. After isolation and sequencing of proteolytic peptides two BADG-labeled cysteines were identified. The amino acid sequences of the obtained proteolytic peptides were homologous to the catalytic subunit of V-ATPases. From mRNA of porcine kidney cortex a catalytic H(+)-ATPase subunit was cloned. 181 of the 183 amino acids which overlap in the sequence derived from the cDNA and the proteolytic peptides were identical, and the two deviations are due to single base exchanges. A comparison of the amino acid sequence derived from the cloned cDNA with sequences of catalytic H(+)-ATPase subunits communicated by other laboratories revealed 98%, 96% and 94% identity with sequences from bovine adrenal medulla, from bovine kidney medulla and from clathrin-coated vesicles of bovine brain. Between 64% and 69% identity was obtained with sequences from fungi and plants. The data show that the catalytic subunit of V-ATPases is highly conserved during evolution. They indicate organ and species specificity in mammalians.

Characterization of an ATP-driven H+ pump in human placental brush-border membrane vesicles

The presence of an ATP-driven H+ pump as measured by H+ uptake upon addition of ATP was not demonstrable in human placental brush-border membrane vesicles when used in their native form, owing to their right-side-out orientation. However, the presence of the H+ pump in these membranes became evident when the membrane vesicles were transiently exposed to 1% cholate, with subsequent removal of the detergent to re-form the vesicles. Apparently, cholate pretreatment reoriented the H+ pump from an inward-facing configuration to outward-facing. Consequently, H+ uptake in response to externally added ATP was easily demonstrable in these cholate-pretreated vesicles by using the delta pH indicator Acridine Orange. In addition, bafilomycin A1-sensitive ATPase activity was measurable in cholate-pretreated vesicles, but not in native intact vesicles, indicating reorientation of the H+ pump. The reoriented H+ pump was electrogenic because H+ uptake was stimulated by an inside-negative anion-diffusion potential or when the vesicles were voltage-clamped. ATP supported H+ uptake with an apparent Km of 260 microM. ITP and GTP supported the pump activity partially, whereas CTP and UTP did not. Mg2+ and Mn2+ were the most preferred bivalent cations. Co2+ and Zn2+ showed partial activity, whereas Ca2+ and Ba2+ showed little or no activity. The pump was inhibited by nanomolar concentrations of bafilomycin A1 and micromolar concentrations of N-ethylmaleimide, p-chloromercuribenzenesulphonate, NN-dicyclohexylcarbodi-imide and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, but was relatively insensitive to oligomycin, vanadate and NaN3. The inhibition by N-ethylmaleimide was protectable by ATP. It is concluded that human placental brush-border membranes possess an ATP-driven H+ pump and that, on the basis of its characteristics, it belongs to the class of vacuolar (V-type) H+ pumps.

The structure and biochemistry of the vacuolar H+ ATPase in proximal and distal urinary acidification

Vacuolar H+ ATPases participate in renal hydrogen ion secretion in both the proximal and distal nephron. These plasma membrane forms of the vacuolar H+ ATPase are regulated physiologically to maintain the acid-base balance of the organism. Proton transporting renal cells have requirements for constitutive acidification of intracellular compartments for normal endocytic and secretory functions. Recent experiments have begun to reveal how the kidney regulates these proton pumps independently. Vacuolar H+ ATPases are a family of structurally similar enzyme which differ in the composition of specific subunits. Cytosolic regulatory enzymes are present in renal cells which may affect vacuolar H+ ATPases in certain membrane compartments selectively. The vacuolar H+ ATPase in the plasma membrane of intercalated cells resides in a specialized proton-transporting apparatus that translocates the enzyme between an intracellular membrane pool and the plasma membrane in response to physiologic stimuli.

H+ extrusion by an apical vacuolar-type H(+)-ATPase in rat renal proximal tubules

The activity of Na+/H(+)-exchange and H(+)-ATPase was measured in the absence of CO2/HCO3 by microfluorometry at the single cell level in rat proximal tubules (superficial S1/S2 segments) loaded with BCECF [2'7'-bis(carboxyethyl)5-6-carboxyfluorescein- acetoxymethylester]. Intracellular pH (pHi) was lowered by a NH4Cl-prepulse technique. In the absence of Na+ in the superfusion solutions, pHi recovered from the acid load by a mechanism inhibited by 0.1 microM bafilomycin A1, a specific inhibitor of a vacuolar-type H(+)-ATPase. Readdition of Na+ in the presence of bafilomycin A1 produced an immediate recovery of pHi by a mechanism sensitive to the addition of 10 microM EIPA (ethylisopropylamiloride), a specific inhibitor of Na+/H+ exchange. The transport rate of the H(+)-ATPase is about 40% of Na+/H(+)-exchange activity at a similar pHi (0.218 +/- 0.028 vs. 0.507 +/- 0.056 pH unit/min. Pre-exposure of the tubules to 30 mM fructose, 0.5 mM iodoacetate and 1 mM KCN (to deplete intracellular ATP) prevented a pHi recovery in Na(+)-free media; readdition of Na+ led to an immediate pHi recovery. Tubules pre-exposed to Cl(-)-free media for 2 hr also reduced the rate of Na(+)-independent pHi recovery. In free-flow electrophoretic separations of brush border membranes and basolateral membranes, a bafilomycin A1-sensitive ATPase activity was found to be associated with the brush border membrane fraction; half maximal inhibition is at 6 x 10(-10) M bafilomycin A1.(ABSTRACT TRUNCATED AT 250 WORDS)

Proteinases inhibit H(+)-ATPase and Na+/H+ exchange but not water transport in apical and endosomal membranes from rat proximal tubule

A marked increase in water permeability can be induced in Xenopus oocytes by injection of mRNA from tissues that express water channels, suggesting that the water channel is a protein. In view of this and previous reports which showed that proteinases may interfere with mercurial inhibition of water transport in red blood cells (RBC), we examined the influence of trypsin, chymotrypsin, papain, pronase, subtilisin and thermolysin on water permeability as well as on ATPase activity, H(+)-pump, passive H+ conductance, and Na+/H+ exchange in apical brush-border vesicles (BBMV) and endosomal (EV) vesicles from rat renal cortex. H+ transport was measured by Acridine orange fluorescence quenching and water transport by stopped-flow light scattering. As measured by potential-driven H+ accumulation in BBMV and EV, proteinase treatment had little effect on vesicle integrity. In BBMV, ecto-ATPase activity was inhibited by 15-30%, Na+/H+ exchange by 20-55%, and H+ conductance was unchanged. Osmotic water permeability (Pf) was 570 microns/s and was inhibited 85-90% by 0.6 mM HgCl2; proteinase treatment did not affect Pf or the HgCl2 inhibition. In EV, NEM-sensitive H+ accumulation and ATPase activity were inhibited by greater than 95%. Pf (140 microns/s) and HgCl2 inhibition (75-85%) were not influenced by proteinase treatment. SDS-PAGE showed selective digestion of multiple polypeptides by proteinases. These results confirm the presence of water channels in BBMV and EV and demonstrate selective inhibition of ATPase function and Na+/H+ exchange by proteinase digestion. The lack of effect of proteinases on water transport by mercurials. We conclude that the water channel may be a small integral membrane protein which, unlike the H(+)-ATPase and Na+/H+ exchanger, has no functionally important membrane domains that are sensitive to proteolysis.

Expression and distribution of renal vacuolar proton-translocating adenosine triphosphatase in response to chronic acid and alkali loads in the rat

Renal hydrogen ion excretion increases with chronic acid loads and decreases with alkali loads. We examined the mechanism of adaptation by analyzing vacuolar proton-translocating adenosine triphosphatase (H+ ATPase) 31-kD subunit protein and mRNA levels, and immunocytochemical distribution in kidneys from rats subjected to acid or alkali loads for 1, 3, 5, 7, and 14 d. Acid- and alkali-loaded rats exhibited adaptive responses in acid excretion, but showed no significant changes in H+ ATPase protein or mRNA levels in either cortex or medulla. In contrast, there were profound adaptive changes in the immunocytochemical distribution of H+ ATPase in collecting duct intercalated cells. In the medulla, H+ ATPase staining in acid-loaded rats shifted from cytoplasmic vesicles to plasma membrane, whereas in alkali-loaded rats, cytoplasmic vesicle staining was enhanced, and staining of plasma membrane disappeared. In the cortical collecting tubule, acid loading increased the number of intercalated cells showing enhanced apical H+ ATPase staining and decreased the number of cells with basolateral or poorly polarized apical staining. The results indicate that both medulla and cortex participate in the adaptive response to acid and alkali loading by changing the steady-state distribution of H+ ATPase, employing mechanisms that do not necessitate postulating interconversion of intercalated cells with opposing polarities.

The stepwise hydrolysis of adenine nucleotides by ectoenzymes of rat renal brush-border membranes

Evidence is presented for the existence of ectoenzymes in rat renal cortical brush-border membrane vesicles that produce adenosine as a final product using either ATP, ADP or AMP as substrate. The enzymes are insensitive to levamisole, ouabain, oligomycin and N-ethylmaleimide, and have absolute requirement for divalent cations with following order of activation Mg2+ greater than Ca2+ greater than Mn2+ greater than Ba2+ greater than Zn2+. At least two separate enzymes can be distinguished. One is capable of hydrolyzing ATP, other nucleoside triphosphates and ADP, but not AMP. The enzyme is insensitive to concanavalin A. The other enzyme hydrolyzes AMP and is strongly inhibited by this lectin. Mg2(+)-stimulated ATP hydrolysis displays saturation kinetics which is not of the simple Michaelis-Menten type, but is biphasic with a high-affinity (K'm = 0.16 mM) and low-affinity site (K'm = 9.0 mM), respectively. The low-affinity site hydrolyzes ATP, ITP and GTP to a similar extent, whereas CTP and UTP with about 40% lower rate. The high-affinity site splits ATP much better than other nucleoside triphosphates. Hydrolysis of ADP follows simple Michaelis-Menten saturation kinetic with apparent Km = 0.38 +/- 0.06 mM. Inhibition, activation and substrate specificity studies indicate that nucleoside triphosphatase and nucleoside diphosphatase may reside on the same protein. Kinetics of the AMP hydrolysis is hyperbolic with apparent Km = 76 +/- 9 microM. The cascade of ectonucleotidases in the brush-border membrane of the proximal tubule may catalyze the degradation of filtered nucleotides into adenosine and phosphate, the compounds which are thereafter probably reabsorbed by separate transport systems.

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 inside-out oriented H(+)-ATPases in cholate-pretreated renal brush-border membrane vesicles

Exposure of porcine renal brush-border membrane vesicles to 1.2% cholate and subsequent detergent removal by dialysis reorients almost all N-ethylmaleimide (NEM)-sensitive ATPases from the vesicle inside to the outside. ATP addition to cholate-pretreated, but not to intact, vesicles causes H+ uptake as visualized by the delta pH indicator, acridine orange. The reoriented H(+)-pump is electrogenic because permeant extravesicular anions or intravesicular K+ plus valinomycin enhance H+ transport. ATP stimulates H+ uptake with an apparent Km of 93 microM. Support of H+ uptake and Pi liberation by ATP greater than GTP approximately ITP greater than UTP indicates a preference for ATP and utilization of other nucleotides at lower efficiency. ADP is a potent, competitive inhibitor of ATP-driven H+ uptake (Ki, 24 microM), Mg2+ and Mn2+ support ATP-driven H+ uptake, but Ca2+, Ba2+, and Zn2+ do not, 1 mM Zn2+ inhibits MgATP-driven H+ transport completely. NEM-sensitive Pi liberation is stimulated by Mg2+ and Mg2+ and, unlike H+ uptake, also by Ca2+ suggesting Ca2(+)-dependent ATP hydrolysis unrelated to H+ transport. The inside-out oriented H(+)-pump is relatively insensitive toward oligomycin, azide, N,N'-dicyclohexylcarbodiimide (DCCD) and vanadate, but efficiently inhibited by NEM (apparent Ki, 0.77 microM), and 4-chloro-7-nitro-benzoxa-1,3-diazole (NBD-Cl; apparent Ki, 0.39 microM). Taken together, the H(+)-ATPase of proximal tubular brush-border membranes exhibits characteristics very similar to those of "vacuolar type" (V-type) H(+)-ATPases. Hence, V-type H(+)-ATPases occur not only in intracellular organelles but also in specialized plasma membrane areas.

Biochemical characterization of an electrogenic vacuolar proton pump in purified chicken osteoclast plasma membrane vesicles

A well-characterized chicken osteoclast plasma membrane vesicle preparation manifested Mg2(+)-dependent ATP hydrolyzing activity of 0.213 mumol inorganic phosphate released per mg protein per minute (n = 7). The Mg2+ dependence showed a high-affinity component with a KMg of 1.293 microM and Vmax of 0.063 mumol Pi per mg protein per minute, and a low-affinity component with a KMg of 297.6 microM and a Vmax of 0.232 mumol Pi per mg protein per minute. The Mg2(+)-ATPase activity was inhibited by N,N'-dicyclohexylcarbodiimide (DCCD, 0.2 mM, 50.7%), N-ethylmaleimide (0.5 mM, 34.6%), nolinium bromide (1 mM, 29.9%), 4,4'-diisothiocyano-2,2'-stilbene sulfonic acid (DIDS, 1 mM, 45.1%), and p-chloromercuribenzoic acid (PCMB, 0.1 mM, 33.8%). Sodium orthovanadate (Na3 VO4) at 1 microM had no effect but caused 29.5% inhibition at 1 mM. Na+ could substitute for K+ without loss of activity, NO3- caused 19.5% inhibition when substituted for Cl-, and acetate replacement of Cl- resulted in 36.4% stimulation of Mg2(+)-ATPase. ATP, GTP, ITP, CTP, and ADP were all hydrolyzed effectively. DCCD (0.2 mM), NEM (0.5 mM), nolinium bromide (1 mM), and DIDS (50 microM) almost completely abolished proton transport as measured spectrofluorometrically by acridine orange quenching. Na3 VO4 (1 mM) had no effect, and duramycin (80 micrograms/ml) inhibited transport 52.7%. K+ replacement of Na+ caused a 79.2% increase in initial proton transport rate. NO3- and acetate substitution of Cl- resulted in a 46.1 and 55.7% decrease in transport, respectively. ATP supports transport far more effectively than the other nucleotides tested. ADP was ineffective. Experiments using the potassium ionophore, valinomycin, indicated that the proton pump functions electrogenically, with Cl- most likely cotransported by an anion transporter. The proton pump also seems to have at least one anion-sensitive site, elucidated by experiments in the presence of NO3- and Cl-.

Evidence of chloride/bicarbonate exchange mediating bicarbonate efflux from S3 segments of rabbit renal proximal tubule

The mechanism of HCO3- exit from rabbit renal proximal tubule S3 segments was investigated. Isolated tubules were perfused luminally and peritubularly with test solutions and cell pH (pHi), cell Cl- activity ([Cl-]i) and cell Na+ activity ([Na+]i) were measured with ion-selective microelectrodes. From the response of pHi and [Cl-]i to changes in bath Cl- or HCO3- concentrations a Cl-/HCO3- exchanger was identified in the basolateral cell membrane. It was reversibly inhibited by millimolar concentrations of the disulfonic stilbene SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid). Cell potential measurements and preliminary determinations of initial ion flux rates suggested a stoichiometry of Cl- to HCO3- flux near 1.0. The transport rate appeared to saturate already at low bath Cl- concentrations (approximately 30 mmol/l), but it was independent of bath pH in the range of 7.4-6.4. Cl-/HCO3- exchange was not directly coupled to Na+ flux although in approximately half of the experiments long-term incubation in Na(+)-free solutions indirectly inhibited the exchanger. Sudden application of SITS under control conditions revealed that the exchanger normally facilitates the exit of HCO3- from cell to interstitium at the expense of Cl- uptake into the cell. How Cl- ions recirculate towards the peritubular surface is presently not known.

Stimulation of canine kidney BBMV ATPase activity by acidic pH in the presence of Zn2+: an ATPase activity distinct from transport ATPases and alkaline phosphatase that may be an ecto-ATPase

Renal brush border membrane vesicles (BBMV) of the dog possess at least two ATPase activities. In the present study, we have examined the effect of pH, ions, and inhibitors on the activity of ATPase in BBMV. Two different sets of conditions were identified that produced stimulation of ATPase activity. A unique stimulation of BBMV ATPase activity occurred at acidic pH in the presence of 1 mM ZnCl2. In the absence of Zn2+, a second ATPase activity was stimulated by alkaline pH values with peak stimulation occurring between pH 8.5 and 9.0. The results suggest that the alkaline pH-stimulated hydrolysis of ATP probably represents the activity of BBMV alkaline phosphatase. The unique acidic pH + Zn2(+)-stimulated ATPase activity must represent the activity of a second protein other than the alkaline phosphatase, since purified alkaline phosphatase did not show this activity. The biochemical identity and physiological function of this renal BBMV ATPase activity remain to be determined, but it may be an ecto-ATPase.