Proton ATPase in rat renal cortical endocytotic vesicles

Structural properties of the proton translocating complex of the clathrin-coated vesicle

The clathrin-coated vesicle proton pump is a representative member of the new class of endomembrane proton ATPases that share an inhibitor profile which distinguishes them from classic F1F0 and E1E2-type proton pumps. The coated vesicle proton pump is a large (530 kDa) heteroligomer composed of eight polypeptides with molecular masses of 116, 70, 58, 40, 38, 34, 33 and 17 kDa. The 200-fold purified enzyme catalyses ATP-generated proton pumping when reconstituted in liposomes composed of pure lipids. Subunit function has been determined by partial reaction analysis of subunit and subcomplex activities. The isolated 17 kDa subunit, when co-reconstituted with bacteriorhodopsin, forms a dicyclohexylcarbodiimide-inhibitable proton channel. Selective removal of the 116 kDa subunit transforms the proton ATPase from a Mg2+-activatable to a Ca2+-activatable ATPase. Subsequent dissociation and reconstitution of subunits reveals that the 70, 58, 40 and 33 kDa components are required, in composite, to form a functional ATP-hydrolytic core, and that no single subunit or subcomplex deficient in these subunits can catalyse ATP hydrolysis.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Physiological importance of endosomal acidification: potential role in proximal tubulopathies

PURPOSE OF REVIEW

In recent years, there have been significant advances in our understanding of the molecular mechanisms relating proximal tubule abnormalities to the pathogenesis of renal Fanconi syndrome. This review focuses on the role of intra-endosomal acidification-machinery proteins (V-ATPase, CLC-5, NHE-3), as well as apical receptors (megalin and cubilin), in the receptor-mediated endocytosis pathway and in the pathogenesis of proximal tubulopathies.

RECENT FINDINGS

Animal models, including CLC-5 and megalin knockout mice, cubilin-deficient dogs and cadmium-toxicity studies in rats, have shed light on defects leading to low-molecular-weight proteinuria. In particular, the important contribution of defective endosomal acidification and membrane-protein recycling to the pathogenesis of the Fanconi syndrome has emerged from these studies. These observations, together with recent findings in patients with Dent's disease, Lowe's syndrome, autosomal-dominant idiopathic Fanconi syndrome and Imerslund-Grasbeck disease, show that the proteinuria of the Fanconi syndrome is more generalized than previously suspected. High concentrations of polypeptides, including hormones, vitamin-binding proteins and chemokines in urine from these patients and animals may play an important role in the progressive renal failure that is associated with the syndrome.

SUMMARY

The molecular mechanism of proximal tubule protein reabsorption, which is defective in renal Fanconi syndrome, includes a crucial role for endosomal acidification-machinery proteins, in particular the V-ATPase and CLC-5 chloride channels, in the trafficking and acidification-dependent recycling of apical membrane proteins, including the endocytotic receptors megalin and cubilin. An increased understanding of the roles of V-ATPase and CLC-5 in proximal tubule endosomal acidification, in the regulation of the megalin/cubilin-mediated endocytosis pathway and finally in the pathogenesis of human Fanconi syndrome will help in the devising of appropriate strategies for therapeutic intervention for this disorder.

Aquaporin 9 expression along the male reproductive tract

Fluid movement across epithelia lining portions of the male reproductive tract is important for modulating the luminal environment in which sperm mature and reside, and for increasing sperm concentration. Some regions of the male reproductive tract express aquaporin (AQP) 1 and/or AQP2, but these transmembrane water channels are not detectable in the epididymis. Therefore, we used a specific antibody to map the cellular distribution of another AQP, AQP9 (which is permeable to water and to some solutes), in the male reproductive tract. AQP9 is enriched on the apical (but not basolateral) membrane of nonciliated cells in the efferent duct and principal cells of the epididymis (rat and human) and vas deferens, where it could play a role in fluid reabsorption. Western blotting revealed a strong 30-kDa band in brush-border membrane vesicles isolated from the epididymis. AQP9 is also expressed in epithelial cells of the prostate and coagulating gland where fluid transport across the epithelium is important for secretory activity. However, it was undetectable in the seminal vesicle, suggesting that an alternative fluid transport pathway may be present in this tissue. Intracellular vesicles in epithelial cells along the reproductive tract were generally poorly stained for AQP9. Furthermore, the apical membrane distribution of AQP9 was unaffected by microtubule disruption. These data suggest that AQP9 is a constitutively inserted apical membrane protein and that its cell-surface expression is not acutely regulated by vesicular trafficking. AQP9 was detectable in the epididymis and vas deferens of 1-wk postnatal rats, but its expression was comparable with adult rats only after 3--4 wk. AQP9 could provide a route via which apical fluid and solute transport occurs in several regions of the male reproductive tract. The heterogeneous and segment-specific expression of AQP9 and other aquaporins along the male reproductive tract shown in this and in our previous studies suggests that fluid reabsorption and secretion in these tissues could be locally modulated by physiological regulation of AQP expression and/or function.

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.

Use of inhibitors of ion transport to differentiate iron transporters in erythroid cells

Iron uptake by rabbit reticulocytes and mature erythrocytes was investigated using 4 incubation systems: 1. Fe-transferrin in NaCl at pH 7.4, 2. Fe-transferrin in sucrose at pH 5.9, 3. Fe(II)-sucrose in sucrose at pH 6.5, and 4.Fe(II)-sucrose in KCl at pH 7.0. These systems were compared with respect to their magnitude and response to many membrane transport inhibitors and modifying agents. Iron uptake via the first 3 systems had many similar features that were quite distinct from those of iron uptake in the fourth system. On the basis of these results, it is concluded that erythroid cells contain two iron transport mechanisms, one with high affinity and relatively low capacity for iron transport, which can be studied using incubation systems 1-3, and the other of low affinity but high capacity (incubation system 4). High-affinity transport is present only in immature erythroid cells, is relatively sensitive to inhibition by N-ethylmaleimide (NEM), N,N1- dicyclohexylcarbodiimide (DCCD), and 7-chloro-4-nitrobenz-2-oxa-1,3 diazole (NBD), and is probably the mechanism by which iron, released from transferrin within endosomes, is transported across the endosomal membrane into the cytosol. DCCD and NBD are also inhibitors of the endosomal H(+)-ATPase, which is in keeping with the hypothesis that this ATPase functions as the iron transporter in endosomal membranes. However, the more-specific inhibitor of this enzyme, bafilomycin A1, inhibited iron uptake only in incubation system 1, where its action can be attributed to inhibition of endosomal acidification. Hence, it is unlikely that the ATPase also functions as the iron transporter. The low-affinity uptake mechanism is sensitive to inhibition by amiloride, valinomycin, quinidine, imipramine, quercetin, and diethylstilbestrol (to all of which high-affinity transport is relatively resistant), and is present in mature erythrocytes as well as reticulocytes.

Diseases of renal adenosine triphosphatase

Most renal transport is a primary or secondary result of the action of one of three membrane bound ion translocating ATPase pumps. The proximal tubule mechanisms for the reabsorption of salt, volume, organic compounds, phosphate, and most bicarbonate reabsorption depend upon the generation and maintenance of a low intracellular sodium concentration by the basolateral membrane Na-K-ATPase pump. The reabsorption of fluid and salt in the loop of Henle is similarly dependent on the energy provided by Na-K-ATPase activity. Some proximal tubule bicarbonate reabsorption and all distal nephron proton excretion is a product of one of two proton translocating ATPase pumps, either an electrogenic H-ATPase or an electroneutral H-K-ATPase. In this article, the authors review the biochemistry and physiology of pump activity and consider the pathophysiology of proximal and distal renal tubular acidosis, the Fanconi syndrome, and Bartter's syndrome as disorders of ATPase pump function.

H(+)-ATPase and transport of DOPAC, HVA, and 5-HIAA in monoamine neurons

The effects of N-methylmaleimide (N-MtM), a vacuolar H(+)-ATPase inhibitor, were evaluated in the putamen of the cat to study the in vivo transport mechanisms of dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindolacetic acid (5-HIAA), using the brain focal microdialysis technique combined with HPLC. The addition of N-MtM to the perfusate altered invariably the flux of the DOPAC, HVA, and 5-HIAA in a similar pattern, resulting in a decrease of the extracellular levels of such metabolites, its extent being N-MtM concentration dependent, thus indicating that the mechanism(s) of such a decrease is (are) related most likely to decreased transport from the intracellular to the extracellular space as the consequence of the inhibition of the vacuolar H(+)-ATPase of DA and 5-HT neurons by the N-MtM. Furthermore, N-MtM masked the release of DA and 5-HT produced by KCl 120 mmol/l. Indeed, N-MtM increased the extracellular levels of such transmitters to values exceeding 4 to 6 times of those produced by KCl 120 mmol/l alone, which suggests that vacuolar H(+)-ATPase is probably involved also in the retention and/or reuptake process of DA and 5-HT.

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)

Dicyclohexylcarbodiimide and vanadate sensitive ATPase of lung lamellar bodies

Lung surfactant is synthesized in lung epithelial type II cells and stored in the lamellar bodies prior to its secretion onto the alveolar surface. The lamellar bodies, like other secretory organelles, maintain an ATP-dependent pH gradient that is sensitive to inhibitors of H(+)-ATPase. This report shows that the ATPase activity of lamellar bodies is enriched in a fraction prepared from lamellar bodies that were disrupted after isolation. The apparent Vmax for this enzyme was 150 nmol ATP hydrolyzed per min per mg protein and apparent Km for ATP was approximately 50 microM. The enzyme activity was sensitive to N-ethylmaleimide (NEM), dicyclohexylcarbodiimide (DCCD) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) (all inhibitors of vacuolar-type H(+)-ATPase) and vanadate (inhibitor of phosphoenzyme-type ATPase). Besides, the activity could also be inhibited with diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and Ca2+. Two proteins (of approximately 45 kDa and 17 kDa) of this fraction showed acid-stable phosphorylation with ATP. The labeling of proteins with ATP (-gamma-32P) could be chased with unlabelled ATP, suggesting that phosphorylation and dephosphorylation of these proteins is associated with the ATPase activity. Our results on inhibition characteristics of the enzyme activity suggest that besides a vacuolar type H(+)-ATPase, the lamellar bodies also contain a phosphoenzyme type ATPase that is sensitive to inhibitors of vacuolar type H(+)-ATPase.

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.

Heterogeneity in ATP-dependent acidification in endocytic vesicles from kidney proximal tubule. Measurement of pH in individual endocytic vesicles in a cell-free system

Measurement of membrane transport in suspensions of isolated membrane vesicles provides averaged information over a potentially very heterogeneous vesicle population. To examine the regulatory mechanisms for ATP-dependent acidification, methodology was developed to measure pH in individual endocytic vesicles. Endocytic vesicles from proximal tubule apical membrane of rat kidney were labeled in vivo by intravenous infusion of FITC-dextran (9 kD); a microsomal fraction was obtained from dissected renal cortex by homogenization and differential centrifugation. Vesicles were immobilized on a polylysine coated coverglass and imaged at high magnification by a silicon intensified target camera. ATP-dependent acidification was not influenced by endosome immobilization. Endosome pH was determined from the integrated fluorescence intensity of individual labeled vesicles after background subtraction. Calibration studies with high K and nigericin showed nearly identical fluorescence vs. pH curves for different endosomes with a standard deviation for a single pH measurement in a single endosome of approximately 0.2 pH units. In response to addition of 1 mM MgATP in the presence of K and valinomycin, endosome pH decreased from 7.2 to a mean of 6.4 with a unimodal distribution with width at half-maximum of approximately 1 pH unit. The drop in endosome pH increased and the shape of the distribution changed when the time between FITC-dextran infusion and kidney removal was increased from 5 to 20 min. Differences in ATP-dependent acidification could not be attributed to heterogeneity in passive proton conductance. These results establish a direct method to measure pH in single endocytic vesicles and demonstrate remarkable heterogeneity in ATP-dependent acidification which was interpreted in terms of heterogeneity in the number and/or activity of proton pumps at serial stages of endocytosis.

Common channels for water and protons at apical and basolateral cell membranes of frog skin and urinary bladder epithelia. Effects of oxytocin, heavy metals, and inhibitors of H(+)-adenosine triphosphatase

We have compared the response of proton and water transport to oxytocin treatment in isolated frog skin and urinary bladder epithelia to provide further insights into the nature of water flow and H+ flux across individual apical and basolateral cell membranes. In isolated spontaneous sodium-transporting frog skin epithelia, lowering the pH of the apical solution from 7.4 to 6.4, 5.5, or 4.5 produced a fall in pHi in principal cells which was completely blocked by amiloride (50 microM), indicating that apical Na+ channels are permeable to protons. When sodium transport was blocked by amiloride, the H+ permeability of the apical membranes of principal cells was negligible but increased dramatically after treatment with antidiuretic hormone (ADH). In the latter condition, lowering the pH of the apical solution caused a voltage-dependent intracellular acidification, accompanied by membrane depolarization, and an increase in membrane conductance and transepithelial current. These effects were inhibited by adding Hg2+ (100 microM) or dicyclohexylcarbodiimide (DCCD, 10(-5) M) to the apical bath. Net titratable H+ flux across frog skin was increased from 30 +/- 8 to 115 +/- 18 neq.h-1.cm-2 (n = 8) after oxytocin treatment (at apical pH 5.5 and serosal pH 7.4) and was completely inhibited by DCCD (10(-5) M). The basolateral membranes of the principal cells in frog skin epithelium were found to be spontaneously permeable to H+ and passive electrogenic H+ transport across this membrane was not affected by oxytocin. Lowering the pH of the basolateral bathing solution (pHb) produced an intracellular acidification and membrane depolarization (and an increase in conductance when the normal dominant K+ conductance of this membrane was abolished by Ba2+ 1 mM). These effects of low pHb were blocked by micromolar concentrations of heavy metals (Zn2+, Ni2+, Co2+, Cd2+, and Hg2+). Lowering pHb in the presence of oxytocin (50 mU/ml) produced a transepithelial current (3 microA.cm-2 at pHb 5.5) which was blocked by 100 microM of Hg2+, Zn2+, or Ni2+ at the basolateral side, and by DCCD (10(-5) M) or Hg2+ (100 microM) from the apical side. The net hydroosmotic water flux (JH2O) induced by oxytocin in frog bladder sacs was blocked by inhibitors of H(+)-adenosine triphosphatase (ATPase). Diethylstilbestrol (DES 10(-5) M), oligomycin (10(-8) M), and DCCD (10(-5) M) prevented JH2O when present in the lumen. These effects cannot be attributed to inhibition of metabolism since cyanide (10(-4) M), or 2-deoxyglucose (10(-3) M) had no effect on JH2O.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Functional colocalization of water channels and proton pumps in endosomes from kidney proximal tubule

The apical membrane of mammalian proximal tubule undergoes rapid membrane cycling by exocytosis and endocytosis. Osmotic water and ATP-driven proton transport were measured in endocytic vesicles from rabbit and rat proximal tubule apical membrane labeled in vivo with the fluid phase marker fluorescein-dextran. Osmotic water permeability (Pf) was determined from the time course of fluorescein-dextran fluorescence after exposure of endosomes to an inward osmotic gradient in a stopped-flow apparatus. Pf was 0.009 (rabbit) and 0.029 cm/s (rat) (23 degrees C) and independent of osmotic gradient size. Pf in rabbit endosomes was inhibited reversibly by HgCl2 (KI = 0.2 mM) and had an activation energy of 6.4 +/- 0.5 kcal/mol (15-35 degrees C). Endosomal proton ATPase activity was measured from the time course of internal pH, measured by fluorescein-dextran fluorescence, after the addition of external ATP. Endosomes contained an ATP-driven proton pump that was sensitive to N-ethylmaleimide and insensitive to vanadate and oligomycin. In response to saturating [ATP] the pump acidified the endosomal compartment at a rate of 0.17 (rat) and 0.029 pH unit/s (rabbit); at an external pH of 7.4, the steady-state pH was 6.4 (rat) and 6.5 (rabbit). To examine whether water channels and the proton ATPase were present in the same endosome, the time course of fluorescein-dextran fluorescence was measured in response to an osmotic gradient in the presence and absence of ATP. ATP did not alter endosome Pf, but decreased the amplitude of the fluorescence signal by 43 +/- 3% (rabbit) and 47 +/- 4% (rat).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Short-term and long-term stimulation of Na+-H+ exchange in cortical brush-border membranes during compensatory growth of the rat kidney

The effect of unilateral nephrectomy on Na+-H+ exchange in rat renal cortical brush-border membrane vesicles (BBMV) was studied by the method of acridine orange fluorescence quenching. The exchanger activity in BBMV from remnant kidney increased rapidly by 70-75% within first 30 min following uninephrectomy. Only a slight further increase was found in later stages of renal growth, i.e. 30 min to 7 days following uninephrectomy. The changes in antiporter activity were restricted to Vmax, whereas the Km for Na+ was similar in control and compensatory growing kidney. The increase of Na+-H+ exchange at 15 min was not affected by actinomycin D in vivo, whereas the increase at 48 h was completely abolished indicating that protein synthesis could be involved in the late, but not in the initial stimulation of renal Na+-H+ exchange. The late, but not the initial stimulations of Na+-H+ exchange were associated with elevated activities of cortical (Na++K+)-ATPase indicating that changes in antiporter activity precede those in the (Na++K+)-pump. The early stimulation of Na+-H+ exchange in BBMV in one kidney was induced also by the occlusion of blood flow through the contralateral kidney for 15 min, without removing it. Thirty min after the occlusion was removed and the reflow established, the Na+-H+ exchange in BBMV from the intact kidney decreased to the control values. The observed modulations in renal Na+-H+ exchanger may be regulated by phosphorylation-dephosphorylation events.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of a proton-pumping ATPase in rat kidney

The distribution of vacuolar H+ATPase in rat kidney was examined by immunocytochemistry using affinity-purified antibodies against the 31-, 56-, and 70-kD subunits of the bovine kidney proton pump. Proximal convoluted tubules were labeled over apical plasma membrane invaginations, and in the initial part of the thin descending limb, apical and basolateral plasma membranes were moderately stained. Thick ascending limbs and distal convoluted tubules were apically stained although the intensity was greater in the distal convoluted tubule. Collecting duct principal cells were virtually unlabeled, but intercalated cells had intense staining with an apical, basolateral or diffuse pattern in the cortex, and exclusively apical staining in the medulla. These results (a) show the presence of an H+ATPase in the apical plasma membrane of the proximal tubule that may contribute to H+ transport in this segment; (b) provide direct evidence that the intercalated cell contains most of the H+ATPase detectable in the collecting duct, supporting its proposed role in H+ transport; (c) demonstrate that subpopulations of cortical intercalated cells have opposite polarities of an H+ATPase, consistent with the presence of both proton- and bicarbonate-secreting cells; and (d) suggest a role for the H+ATPase in acid/base regulation or H+ transport in segments other than the collecting duct and the proximal tubule.