Quantitation of hydrogen ion and potential gradients in gastric plasma membrane vesicles

Soraprazan: setting new standards in inhibition of gastric acid secretion

After treatment of millions of patients suffering from gastroesophageal reflux disease (GERD) and other acid-related ailments with proton pump inhibitors, there are still unmet medical needs such as rapid and reliable pain relief, especially for nocturnal acid breakthrough. In this work, we introduce and characterize the biochemistry and pharmacology of the potassium-competitive acid blocker (P-CAB) soraprazan, a novel, reversible, and fast-acting inhibitor of gastric H,K-ATPase. Inhibitory and binding properties of soraprazan were analyzed together with its mode of action, its selectivity, and its in vivo potency. This P-CAB has an IC(50) of 0.1 microM if measured with ion leaky vesicles and of 0.19 microM in isolated gastric glands. With a K(i) of 6.4 nM, a K(d) of 26.4 nM, and a B(max) of 2.89 nmol/mg, this compound is a highly potent and reversible inhibitor of the H,K-ATPase. Soraprazan shows immediate inhibition of acid secretion in various in vitro models and in vivo and was found to be more than 2000-fold selective for H,K-ATPase over Na,K- and Ca-ATPases. Soraprazan is superior to esomeprazole in terms of onset of action and the extent and duration of pH elevation in vivo in the dog. Rapid and consistent inhibition of acid secretion by soraprazan renders the P-CABs a promising group of compounds for therapy of GERD.

Mechanisms of secretion-associated shrinkage and volume recovery in cultured rabbit parietal cells

We have previously shown that stimulation of acid secretion in parietal cells causes rapid initial cell shrinkage, followed by Na(+)/H(+) exchange-mediated regulatory volume increase (RVI). The factors leading to the initial cell shrinkage are unknown. We therefore monitored volume changes in cultured rabbit parietal cells by confocal measurement of the cytoplasmic calcein concentration. Although blocking the presumably apically located K(+) channel KCNQ1 with chromanol 293b reduced both the forskolin- and carbachol-induced cell shrinkage, inhibition of Ca(2+)-sensitive K(+) channels with charybdotoxin strongly inhibited the cell volume decrease after carbachol, but not after forskolin stimulation. The cell shrinkage induced by both secretagogues was partially inhibited by blocking H(+)-K(+)-ATPase with SCH28080 and completely absent after incubation with NPPB, which inhibits parietal cell anion conductances involved in acid secretion. The subsequent RVI was strongly inhibited with the Na(+)/H(+) exchanger 1 (NHE1)-specific concentration of HOE642 and completely by 500 muM dimethyl-amiloride (DMA), which also inhibits NHE4. None of the above substances induced volume changes under baseline conditions. Our results indicate that cell volume decrease associated with acid secretion is dependent on the activation of K(+) and Cl(-) channels by the respective secretagogues. K(+), Cl(-), and water secretion into the secretory canaliculi is thus one likely mechanism of stimulation-associated cell shrinkage in cultured parietal cells. The observed RVI is predominantly mediated by NHE1.

Characterization of the inhibitory activity of tenatoprazole on the gastric H+,K+ -ATPase in vitro and in vivo

Tenatoprazole is a prodrug of the proton pump inhibitor (PPI) class, which is converted to the active sulfenamide or sulfenic acid by acid in the secretory canaliculus of the stimulated parietal cell of the stomach. This active species binds to luminally accessible cysteines of the gastric H+,K+ -ATPase resulting in disulfide formation and acid secretion inhibition. Tenatoprazole binds at the catalytic subunit of the gastric acid pump with a stoichiometry of 2.6 nmol mg(-1) of the enzyme in vitro. In vivo, maximum binding of tenatoprazole was 2.9 nmol mg(-1) of the enzyme at 2 h after IV administration. The binding sites of tenatoprazole were in the TM5/6 region at Cys813 and Cys822 as shown by tryptic and thermolysin digestion of the ATPase labeled by tenatoprazole. Decay of tenatoprazole binding on the gastric H+,K+ -ATPase consisted of two components. One was relatively fast, with a half-life 3.9 h due to reversal of binding at cysteine 813, and the other was a plateau phase corresponding to ATPase turnover reflecting binding at cysteine 822 that also results in sustained inhibition in the presence of reducing agents in vitro. The stability of inhibition and the long plasma half-life of tenatoprazole should result in prolonged inhibition of acid secretion as compared to omeprazole. Further, the bioavailability of tenatoprazole was two-fold greater in the (S)-tenatoprazole sodium salt hydrate form as compared to the free form in dogs which is due to differences in the crystal structure and hydrophobic nature of the two forms.

Role of potassium in acid secretion

Potassium (K⁺) ions are critical for the activation and catalytic cycle of the gastric H⁺,K⁺-ATPase, resulting in the secretion of hydrochloric acid into the parietal cell canaliculus. As both symptom, severity and esophageal mucosal damage in gastro-esophageal reflux disease (GERD) are related to the degree of acid exposure, K⁺ is a logical target for approaches to inhibit acid production. The probable K⁺ binding site on the gastric H⁺,K⁺-ATPase has recently been described and studies are elucidating how K⁺ activates the enzyme. K⁺ channels in the apical membrane of the parietal cell are implicated in the recycling of K⁺ and, to date, three potential K⁺ channels (KCNQ1, Kir2.1 and Kir4.1) have been identified. The channels represent theoretical sites for agents to control acid secretion but it will be difficult to develop selective blockers. An alternative strategy is to prevent K⁺ from activating gastric H⁺,K⁺-ATPase; the potassium-competitive acid blocker (P-CAB) class inhibits acid secretion by binding at or near the K⁺ binding site. Ongoing research is further defining the role of K⁺ in the functioning of the gastric H⁺,K⁺-ATPase, as well as determining the clinical utility of agents directed toward this important cation.

A quantitative model for using acridine orange as a transmembrane pH gradient probe

Monitoring the acidification of the internal space of membrane vesicles by proton pumps can be achieved easily with optical probes. Transmembrane pH gradients cause a blue-shift in the absorbance spectrum and the quenching of the fluorescence of the cationic dye acridine orange. It has been postulated that these changes are caused by accumulation and aggregation of the dye inside the vesicles. We tested this hypothesis using liposomes with transmembrane concentration gradients of ammonium sulfate as model system. Fluorescence intensity of acridine orange solutions incubated with liposomes was affected by magnitude of the gradient, volume trapped by vesicles, and temperature. These experimental data were compared to a theoretical model describing the accumulation of acridine orange monomers in the vesicles according to the inside-to-outside ratio of proton concentrations, and the intravesicular formation of sandwich-like piles of acridine orange cations. This theoretical model predicted quantitatively the relationship between the transmembrane pH gradients and spectral changes of acridine orange. Therefore, adequate characterization of aggregation of dye in the lumen of biological vesicles provides the theoretical basis for using acridine orange as an optical probe to quantify transmembrane pH gradients.

Sites of reaction of the gastric H,K-ATPase with extracytoplasmic thiol reagents

The vesicular gastric H,K-ATPase catalyzes an electroneutral H for K exchange allowing acidification of the intravesicular space. There is a total of 28 cysteines present in the alpha subunit of the gastric H,K-ATPase, of which 10 are found in the predicted transmembrane segments and their connecting loop, and 9 are present in the beta subunit, of which 6 are disulfide-linked. To determine which of these was accessible to extracytoplasmic attack, the enzyme was inhibited by four different substituted 2-pyridylmethylsulfinyl benzimidazoles, 5-methoxy-2-[(4-methoxy-3, 5-dimethyl-2-pyridyl)methylsulfinyl]-1H-benzimidazole (omeprazole), 2-[(4-trifluoroethoxy-3-methyl-2-pyridyl)methylsulfinyl]-1H-ben zimida zole (lansoprazole), 5-difluoromethoxy-2-[3, 4-methoxy-2-pyridyl)methylsulfinyl]-1H-benzimidazole (pantoprazole), and 2-[(4-(3-methoxypropoxy)-3-methyl)-2-pyridyl)methylsulfinyl]-1H-++ +benzi midazole (rabeprazole), under acid transporting conditions. All of these compounds are weak bases that accumulate in the acidic space generated by the pump and undergo an acid catalyzed rearrangement to a cationic sulfenamide, which forms disulfides with accessible cysteines. The relative rates of acid activation of these compounds corresponded to the relative rates of inhibition of ATPase activity and acid transport. Fragmentation of the enzyme by trypsin followed by SDS-polyacrylamide gel electrophoresis showed that omeprazole bound covalently to one of the two cysteines in the domains containing the fifth and sixth transmembrane segments and their extracytoplasmic loop and to cysteine 892 in the loop between the seventh and eighth transmembrane segments, but inhibition correlated with the reaction with cysteines in the fifth and sixth domain. Lansoprazole bound to the cysteines in these two domains as well as to cysteine 321 toward the extracytoplasmic end of the third transmembrane segments. Pantoprazole bound only to either cysteine 813 or 822 in the fifth and sixth transmembrane region. The inhibition of Rabeprazole correlated also with its binding to this part of the protein, but this compound continued to bind after full inhibition, eventually binding also to cysteines 321 and 892. No binding was found to any of the cysteines in the seventh to tenth transmembrane segments. Thermolysin digestion of the isolated omeprazole-labeled fifth and sixth transmembrane pair showed that cysteine 813 was the site of labeling. It is concluded that binding of these sided reagents to cysteine 813 in the loop between transmembrane (TM)5 and TM6 is sufficient for inhibition of ATPase activity and acid transport by the gastric acid pump. Of the 10 cysteines present in the membrane and extracytoplasmic domain, only three are exposed sufficiently to allow reactivity with these cationic thiol reagents. The binding to cysteine 813 defines the location of the extracytoplasmic loop between TM5 and TM6 and places the carboxylic acids 820 and 824 conserved between the gastric H,K- and the Na,K-ATPases in TM6, consistent with their assumed role in cation binding.

Duodenal ulcer. Calcium status in isolated parietal cells

Although the etiology of duodenal ulcer is not known, its treatment with drugs that reduce acid secretion is well accepted. The central role of calcium in stimulus-secretion coupling resulting in acid secretion by gastric parietal cells is documented. However, the status of intracellular calcium in gastric parietal cells in the basal state in patients with duodenal ulcer is not known. Multiple endoscopic gastric mucosal biopsies from the corpus of the stomach of 52 patients were processed and isolated parietal cells were studied. Intracellular calcium was estimated using fura-2-acetoxymethyl ester. Influx and efflux were determined by using radioactive calcium. Acridine orange retention was used to assess acid production. Only calcium influx at 20 min was significantly (P < 0.01) more in patients with duodenal ulcer as compared to the control group. There was no difference between the groups in calcium influx at 0 and 60 min; calcium efflux at 0, 20, and 60 min; intracellular free calcium and acid secretion. We conclude that in the unstimulated state calcium homeostasis in isolated parietal cells of patients with duodenal ulcer shows only a minimal difference as compared to controls.

Site-directed antibodies as topographical probes of the gastric H,K-ATPase alpha-subunit

Gastric acid is secreted by an ATP-driven H+ and K+ exchanger (H,K-ATPase), an integral apical membrane protein of parietal cells. Although the primary structure of the enzyme is known, its higher order structure is uncertain. In order to acquire topographical probes of native, microsomal H,K-ATPase, synthetic peptides corresponding to the 17 amino-terminal (N-peptide) and 16 carboxyl-terminal (C-peptide) residues of pig gastric H,K-ATPase alpha-subunit were coupled to keyhole limpet hemocyanin (KLH). Rabbits were immunized with peptide-KLH conjugates and their sera were tested for specificity by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and immunocytochemistry. All sera showed high ELISA reactivities with synthetic peptides, peptide-BSA conjugates, and microsomal H,K-ATPase adsorbed to microtiter wells (some titers greater than 1:10(4)). Immunoblots of H,K-ATPase resolved by SDS-PAGE showed both N-peptide and C-peptide antibodies reacting with a single 94 kDa band. All sera selectively stained parietal cells in pig gastric mucosal sections. Preimmune sera gave negative or weak signals in all assays. In competition ELISAs, N-peptide antibodies, but not C-peptide antibodies, were displaced from the corresponding bound synthetic peptides by added microsomal H,K-ATPase. One of the N-peptide antibodies inhibited H,K-ATPase activity by more than 50%; binding of this antibody was decreased when ATP or K+ were bound to the enzyme. These results indicate a cytoplasmically-oriented alpha-subunit N-terminus which may participate conformationally in the H,K-ATPase catalytic cycle, and suggest that antibodies against synthetic H,K-ATPase peptides are potentially useful probes of native microsomal H,K-ATPase topography.

Angiotensin II directly increases rabbit renal brush-border membrane sodium transport: presence of local signal transduction system

In the present study, we have examined the direct actions of angiotensin II (AII) in rabbit renal brush border membrane (BBM) where binding sites for AII exist. Addition of AII (10(-11)-10(-7) M) was found to stimulate 22Na+ uptake by the isolated BBM vesicles directly. All did not affect the Na(+)-dependent BBM glucose uptake, and the effect of AII on BBM 22Na+ uptake was inhibited by amiloride, suggesting the involvement of Na+/H+ exchange mechanism. BBM proton permeability as assessed by acridine orange quenching was not affected by AII, indicating the direct effect of AII on Na+/H+ antiport system. In search of the signal transduction mechanism, it was found that AII activated BBM phospholipase A2 (PLA) and that BBM contains a 42-kDa guanine nucleotide-binding regulatory protein (G-protein) that underwent pertussis toxin (PTX)-catalyzed ADP-ribosylation. Addition of GTP potentiated, while GDP-beta S or PTX abolished, the effects of AII on BBM PLA and 22Na+ uptake, suggesting the involvement of G-protein in AII's actions. On the other hand, inhibition of PLA by mepacrine prevented AII's effect on BBM 22Na+ uptake, and activation of PLA by mellitin or addition of arachidonic acid similarly enhanced BBM 22Na+ uptake, suggesting the role of PLA activation in mediating AII's effect on BBM 22Na+ uptake. In summary, results of the present study show a direct stimulatory effect of AII on BBM Na+/H+ antiport system, and suggest the presence of a local signal transduction system involving G-protein mediated PLA activation.

The potency of substituted benzimidazoles such as E3810, omeprazole, Ro 18-5364 to inhibit gastric H+, K(+)-ATPase is correlatedwith the rate of acid-activation of the inhibitor

The half maximal inhibitory concentrations (IC50) of substituted benzimidazoles for the H+, K(+)-ATPase in hog gastric vesicles were measured by using the pyruvate kinase-lactate dehydrogenase-linked system in which hydrolysis of ATP was coupled with the oxidation of NADH. The vesicles were incubated in a solution containing a high concentration of KCl, valinomycin and Mg-ATP, and the intravesicular medium was acidified. The inhibitor was activated in the acidic medium and reacted with SH groups on the luminal (intravesicular) side of the ATPase. The active compound formed in the extravesicular medium (pH 6.11) was quenched by GSH. Under these conditions, IC50 of new compound E3810, 2[(4-(3-methoxypropoxy)-3-methylpyridine-2-yl)methyl-sulfinyl]-1H- benzimidazole sodium salt, was 0.072 microM and that of omeprazole was 0.47 microM at 25 degrees. On the other hand, the rates of formation of active compounds, tetracyclic sulfenamide derivatives, from original substituted benzimidazoles in 0.1 N HCl (k) were determined by measuring optical density at the characteristic wavelengths of the active compounds. There was a good correlation between IC50 and k for various substituted benzimidazoles including E3810, methoxy derivative of E3810, omeprazole, Ro 18-5364, H compound, picoprazole and timoprazole. This fact suggest that the rate of the formation of the acid-activated compound is a main factor determining the potency of the inhibitor.

Increased Na+/H+ antiport activity in the renal brush border membrane of SHR

Defect in renal salt excretion may play an important role in the pathogenesis of hypertension. We examined sodium (Na+) uptake by brush border membrane (BBM) vesicles of young (6 week old) spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) of the same age. SHR had lower urinary Na+ excretion (223.1 +/- 9.3 vs. 266.3 +/- 3.7 microEq/day/100 g, N = 8, P less than 0.01) and higher systolic blood pressure (98.9 +/- 1.2 vs. 82.9 +/- 1.8 mm Hg, N = 8, P less than 0.01) than WKY. BBM vesicle Na+ uptake, measured by rapid filtration technique, was higher in SHR when compared to WKY (1.44 +/- 0.03 vs. 1.01 +/- 0.06 nmol/mg/5 sec, N = 4, P less than 0.01). This increase in Na+ influx was apparent only in the present of an outward-directed proton (H+) gradient and was abolished by 1 mM amiloride. BBM permeability to H+ as assessed by acridine orange quenching was not different between SHR and WKY. Kinetic analyses of the amiloride-sensitive BBM Na+ uptake revealed a higher Vmax (2.13 +/- 0.27 vs. 0.70 +/- 0.30 nmol/mg/5 sec, N = 4, P less than 0.01) and a higher km for Na+ (3.55 +/- 0.32 vs. 1.23 +/- 0.14 mM, N = 4, P less than 0.05) in SHR. These findings thus demonstrate an intrinsic derangement in BBM Na+ transport in young SHR which is characterized by increased Na+/H+ antiport activity. This alteration in antiport activity is not attributable to changes in membrane permeability to H+, and is characterized by higher Vmax and km.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP-dependent H+ transport by the turtle bladder: NBD-C1 preferentially inhibits the vanadate-insensitive component in isolated membranes

We investigated the inhibitory effects of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) on ATP-dependent H+ accumulation by membrane vesicles prepared from the turtle urinary bladder epithelium. NBD-Cl at 30 microM was found to completely inhibit the vanadate-insensitive component of H+ transport, with half-maximal inhibition occurring at 4.2 to 5.4 microM. In contrast, the vanadate-inhibitable component was unaffected by 30 microM NBD-Cl. At high concentrations (300 microM), both components were fully inhibited. The results confirm the presence of two distinct H+ transport processes in turtle bladder membranes and identify selective inhibitors, NBD-Cl and vanadate, for each process.

Inhibition of gastric H+/K+-ATPase by substituted imidazo[1,2-a]pyridines

A hydrophobic imidazopyridine, SCH 28080 (3-cyanomethyl-2-methyl-8-phenylmethoxy)imidazo[1,2-a]pyridine) has previously been shown to inhibit gastric acid secretion in vivo and in vitro. Studies of isolated gastric H+/K+-ATPase have demonstrated that SCH 28080 reversibly inhibited the enzyme and competitively interacted with the K+-stimulated ATPase and p-nitrophenylphosphatase activities of the H+/K+-ATPase. To elucidate the mechanism of inhibition further, for example to establish whether the inhibitor interaction occurs on the luminal or the cytosolic side of the enzyme or if compound pKa influences inhibition, SCH 28080 and three analogues have been studied. We have examined the effects on K+-stimulated ATPase activity in isolated ion-permeable membrane vesicles at different pH values and KCl concentrations. In ion-tight membrane fractions the effect on acid formation was estimated. The results are in agreement with the hypothesis that the protonated, and thus positively charged, form of SCH 28080 is the active species, and that the inhibitory effect is exerted by binding of the compound to the luminal side of the H+/K+-ATPase.

SCH28080 prevents omeprazole inhibition of the gastric H+/K+-ATPase

The interaction between SCH28080 and omeprazole, two specific inhibitors of gastric H+/K+-ATPase, was investigated using gastric glands and isolated gastric membranes. For gastric glands, inhibition of acid formation by SCH28080 was not reversed by washing whereas inhibition by omeprazole was partially reversed after washing. These features are opposite to what is found with isolated membranes. However, if gastric glands were permeabilized with digitonin after exposure to the inhibitors and recovery measured as ATP-dependent acid formation or H+/K+-ATPase activity, inhibition by SCH28080 was completely reversed while inhibition by omeprazole was non-reversible. Using a procedure of pretreatment with inhibitors followed by permeabilization and assay of recovered activity, it was found that a combined treatment with SCH28080 plus omeprazole prevented the irreversible inhibition by omeprazole, i.e. acid forming capability and ATPase activity were fully recovered. In order to test the possibility that SCH28080 prevented activation of omeprazole by dissipating an acid environment, control experiments were performed with SCN, which gave equivalent dissipation of the acid gradient but did not prevent the irreversible inhibition by omeprazole. These results were confirmed in isolated gastric membranes where residual p-nitrophenylphosphatase activity was assayed following exposure of acid transporting vesicles to omeprazole. Compared to control conditions, omeprazole inhibited 48% of the phosphatase activity whereas simultaneous addition of SCH28080 reduced the inhibition to 14%. The results therefore suggest that SCH28080 selectively blocks irreversible inhibition by omeprazole and thus that these two agents interact at a common region of the luminal aspect of the gastric H+/K+-ATPase.

Transport ratios of reconstituted (H+ + K+)-ATPase

Gastric (H+ + K+)-ATPase was reconstituted into artificial phosphatidylcholine/cholesterol vesicles by means of a freeze-thaw-sonication procedure. The passive and active transport mediated by these vesicles were measured (Skrabanja, A.T.P., Asty, P., Soumarmon, A., De Pont, J.J.H.H.M. and Lewin, M.J.M. (1986) Biochim. Biophys. Acta 860, 131-136). To determine real initial velocities, the proteoliposomes were separated from non-incorporated enzyme, by means of centrifugation on a sucrose gradient. The purified proteoliposomes were used to measure active H+ and Rb+ transport, giving at room-temperature velocities of 46.3 and 42.5 mumol per mg per h, respectively. A transport ratio of two cations per ATP hydrolyzed was also measured. These figures indicate that the enzyme catalyzes an electroneutral H+-Rb+ exchange.

Inhibition of (H+ + K+)-ATPase by omeprazole in isolated gastric vesicles requires proton transport

Omeprazole was found to inhibit the (H+ + K+)-ATPase activity in isolated gastric vesicles only when acid was accumulated in the vesicle lumen. The ATPase activity was time- and dose-dependently inhibited in the presence of K+ and valinomycin. Under conditions in which no pH-gradient was generated, i.e., in the presence of K+ alone or NH4+, no effect of omeprazole was found. The degree of inhibition was directly correlated to the amount of inhibitor bound to the preparation. A stoichiometry of 2 mol radiolabelled inhibitor bound per mol phosphoenzyme was found on total inhibition of the K+ plus valinomycin-stimulated activity. This inhibitory action of omeprazole on the ATPase activity could be fully reversed by addition of beta-mercaptoethanol. The inhibition of the proton transport in the (H+ + K+)-ATPase-containing vesicles by omeprazole was also strictly correlated to the amount of bound inhibitor. The stoichiometry of binding at total inhibition of this reaction was found to be 1.4 mol per mol phosphoenzyme. The K+-stimulated p-nitrophenylphosphatase activity was inhibited in parallel with the ATPase activity, whereas the phosphoenzyme levels were affected to a lesser extent by omeprazole. Gel electrophoresis of an omeprazole-inhibited vesicle preparation showed that the radiolabel was mainly found at 94 kDa, the molecular weight of the (H+ + K+)-ATPase catalytic subunit(s).

Gastric (H+,K+)-ATPase

Gastric acid secretion results from the activity of a specific ATPase, the (H+,K+)-ATPase. This enzyme, discovered in 1973, exchanges H+ for K+. It has two ATP binding sites, both involved in enzyme activity, whose affinities vary as a function of the H+ and K+ concentrations. Hydrolysis of ATP at the highest affinity site leads to the synthesis of a covalent aspartyl phosphate which accumulates in the absence of K+. The presence of this cation accelerates dephosphorylation resulting in the stimulation of ATPase (and PNPPase) activity. The structure of membranous (H+,K+)-ATPase is poorly defined. n-Octylglucoside solubilizes an active enzyme of 390-420 kDa which can be partly depolymerized using cholate. The monomer, characterized in SDS has a 95 kDa molecular mass and is inactive. In the presence of magnesium, (H+,K+)-ATPase catalyzes the active and neutral exchange of H+ for K+ at the expense of ATP. In the absence of ATP, (H+,K+)-ATPase acts as a passive transporter exchanging K+ for K+ at maximal rate and H+ for K+ at a 20 times slower rate.

Humoral and ionic regulation of osteoclast acidity

Regulation of the acidity of osteoclasts was determined in situ on the endocranial surfaces of mouse calvaria using acridine orange, a fluorescent weak base. Osteoclasts could be identified by large size, multiple nuclei, relatively small numbers of cells, and the way and the extent to which they took up the dye. Nonosteoclastic cells were stained mainly in their nuclei and occasionally in a few lysosomes surrounding their nuclei, which were uniformly single in nonosteoclasts. Nuclei in osteoclasts were also stained, but the staining of the nuclei was partially masked by the intensity and completeness of the staining of the cytoplasm. In some cells the cytoplasmic staining appeared to be in discrete granules, giving the cytoplasm a bright, frothy appearance. This fluorescence was present in both treated and untreated cells and aided in identifying the osteoclasts. Acridine orange fluorescence at 624 nm intensity, and hence, osteoclast acidity, was increased by parathyroid hormone and prostaglandin E2. Parathyroid hormone-induced increases in acidity were inhibited by calcitonin, cortisol, sodium fluoride, and prostaglandin E2. Furthermore, osteoclast acidity was dependent largely or partially on maintenance of K+ and Na+ gradients, patent Na+ channels, chloride-bicarbonate exchange, and H+, K+-ATPase. These findings demonstrate that osteoclasts become acidified by mechanisms similar to those occurring in gastric parietal cells.

H+ transport by reconstituted gastric (H+ + K+)-ATPase

Gastric (H+ + K+)-ATPase was reconstituted into artificial phosphatidylcholine/cholesterol liposomes by means of a freeze-thaw-sonication technique. Upon addition of MgATP, active H+ transport was observed, with a maximal rate of 2.1 mumol X mg-1 X min-1, requiring the presence of 100 mM K+ at the intravesicular site. However, in the absence of ATP an H+-K+ exchange with a maximal rate of 0.12 mumol X mg-1 X min-1 was measured, which could be inhibited by the well-known ATPase inhibitors vanadate and omeprazole, giving the first evidence of a passive K+-H+ exchange function of gastric (H+ + K+)-ATPase. An Na+-H+ exchange activity was also measured, which was fully inhibited by 1 mM amiloride. Simultaneous reconstitution of Na+/H+ antiport and (H+ + K+)-ATPase could explain why reconstituted ATPase appeared less cation-specific than the native enzyme (Rabon, E.C., Gunther, R.B., Soumarmon, A., Bassilian, B., Lewin, M.J.M. and Sachs, G. (1985) J. Biol. Chem. 260, 10200-10212).

Gastric K+-stimulated p-nitrophenylphosphatase cytochemistry

A cytochemical study of gastric K+-stimulated p-nitrophenylphosphatase (K-NPPase) activity, corresponding to a K+-stimulated phosphoprotein phosphatase of H-K-ATPase system, has been made by a new cytochemical method. Sections of fixed guinea pig gastric mucosa in a mixture of 2% paraformaldehyde and 0.25% glutaraldehyde, were incubated with the incubation medium (1.0 M glycine-0.1 M KOH buffer, pH 9.0, 2.5 ml; 1.1 M KCl, 0.5 ml; 10 mM lead citrate dissolved in 50 mM KOH, 4 ml; levamisole, 6.0 mg; dimethyl sulfoxide, 2.0 ml; 0.1 M p-nitrophenylphosphate (Mg-salt), 1.0 ml; ouabain, 73.0 mg) for 30 min at room temperature. Under a light microscope the specific gastric K-NPPase reaction was distributed only in the parietal cells of the fundic glands. The electron microscopic cytochemistry showed that the gastric K-NPPase activity was localized on the membrane lining the apical surfaces, secretory canaliculi and tubulovesicles. On the other hand, ouabain-sensitive K-NPPase activity (Na-K-ATPase) was demonstrated to localize only in the basolateral membrane of parietal cells with Mayahara's method. These findings support the interrelationships between the apical surface membrane, secretory canalicular membrane and tubulovesicles, and the functional differentiation of the membrane between the secretory membrane and basolateral membrane.

Stimulation of carbonic anhydrase in osteoclasts by parathyroid hormone

Changes in the acidity of osteoclasts were evaluated by direct measurement of the fluorescent intensity of osteoclasts exposed to acridine orange, a fluorescent weak base which becomes concentrated in acid-containing subcellular compartments. Parathyroid hormone (PTH) produced dose-dependent increases in fluorescent intensity; maximal increases in intensity occurred at doses between 3 and 10 micrograms PTH/ml of culture medium. Acetazolamide, a potent inhibitor of carbonic anhydrase, inhibited the increase in fluorescence induced by PTH, but this drug was less effective in reducing fluorescence in maximally than in submaximally stimulated osteoclasts, indicating that either more enzyme or more resistant enzyme was present in the PTH-stimulated cells. Because increased fluorescence of acridine orange is a sign of greater acidity, these results suggest that (1) PTH stimulates the acidity of osteoclasts, (2) carbonic anhydrase activity is necessary for maximum acidity, and (3) carbonic anhydrase is activated by PTH.

Donnan equilibrium and pH gradient in isolated tracheal apical membrane vesicles

Apical plasma membrane vesicles isolated from bovine tracheal epithelium were found to possess both an electrical potential gradient (delta psi) and a transmembrane pH gradient (delta pH). The delta psi was calculated from the distribution of 86Rb+ in the presence of the ionophore valinomycin, and delta pH was determined from the distribution of [14C]methylamine under conditions of apparent equilibrium. Maximal values for delta psi of -54.3 +/- 2.5 mV and for delta pH of 0.75 +/- 0.07 pH units were obtained under low ionic strength conditions. Increasing the ionic strength by the addition of 50 mM of either permeant or impermeant electrolytes reduced delta psi to near zero values. The delta pH varied in parallel with the delta psi. The results suggest that both 86Rb+ in the presence of valinomycin and H+ are in a Donnan equilibrium with impermeant negative charge in the vesicle interior.

Redistribution of gastric K+-NPPase in vertebrate oxyntic cells in relation to hydrochloric acid secretion: a cytochemical study

Gastric K+-NPPase represents a partial reaction of the (K+-H+)ATPase system, which is considered to be the proton pump in mammalian parietal cells. In the present paper, K+-NPPase activity was cytochemically studied by the method of Mayahara et al. (1980) in gastric glands of birds, amphibia, and mammals, either in the resting state induced by cimetidine or after stimulation of HCl secretion by histamine. The gastric K+-NPPase cytochemical reaction was localized only in oxyntic cells of the gastric mucosa in the three species tested. The subcellular distribution of the K+-NPPase reaction product drastically changes with the secretory state of HCl. In resting cells, the K+-NPPase staining is associated with the membranes of the endocellular tubular system while in HCl-secreting cells, it is associated with the plasma membrane of the elaborate secretory surface characteristic of this functional state. The above results demonstrate that the same enzymatic activity, which is associated with the gastric proton pump, is present in both membranous systems of the oxyntic cell secretory pole. This fact supports the proposal that the tubular system represents a membrane reserve that inserts the proton pump into the luminal plasma membrane in vertebrate oxyntic cells under the action of HCl secretagogues.

The H+/ATP transport ratio of the (K+ + H+)-ATPase of pig gastric membrane vesicles

Various values have been reported for the H+/ATP transport ratio of the (K+ + H+)-ATPase of the gastric parietal cell: 4, 2 and 1. We have, therefore, reinvestigated this matter with a vesicle preparation isolated from pig gastric mucosa. The vesicles are suspended in glycylglycine buffer (pH 6.11) at 22 degrees C, and incubated until equalization of the K+ concentration inside and outside (75 mM). After addition of ATP, the initial rates of H+ uptake and ATP hydrolysis are then measured. Proton uptake is inhibited in the absence of K+ or in the presence of nigericin. The K0.5 value for proton transport is 154 microM and the Km value for ATP hydrolysis is 61 microM. The Lineweaver-Burk plot for ATP hydrolysis vs. ATP concentration is linear with a Vmax of 5.5 nmol/mg protein per s, but that for H+ uptake is not. Thus with increasing ATP concentration (6.7 to 1670 microM) the transport ratio increases from 0.3 to 1.8. Extrapolation to infinite ATP concentration gives a value of 1.89. (S.E. 0.13, N = 5) and a Hill coefficient of n = 1.21 (S.E. 0.06, N = 5) implying that the true transport ratio is 2 H+/ATP with positive cooperativity between the protons.

Effect of carbachol or histamine stimulation on rat gastric membranes enriched in (H+-K+)-ATPase

We have examined histamine- or carbachol-induced changes in rat gastric membranes enriched in K+-stimulated ATPase. Stimulation of secretion by both secretagogues in vivo produced a class of microsomal membranes which exhibited valinomycin-independent, KCl-dependent H+ transport. In contrast, membrane vesicles isolated from cimetidine inhibited resting mucosa exhibited largely the ionophore-dependent H+ transport. In addition, only in the carbachol-stimulated membranes a portion of the ionophore-independent H+ transport was refractory to cimetidine pretreatment. The gastric microsomal membranes were resolved into light and heavy fractions by centrifugation over isotonic 2H2O media. The ionophore-independent H+ transport was almost exclusively associated with the heavy microsomal fraction while the ionophore-dependent H+ transport was detected in the light fraction. Also, these fractions were considerably different from each other in their appearance in electron micrographs and SDS gel electrophoresis patterns. Secretagogue stimulation increased the population of the heavy microsomal membrane vesicles exhibiting the valinomycin-independent, K+-dependent H+ transport and their overall content of K+-stimulated ATPase. Cimetidine treatment, on the other hand, increased the ATPase activity associated with the light microsomes, and produced the heavy microsomal membranes showing only a marginal degree of the ionophore independent H+ accumulation, even though they were very similar to the carbachol-stimulated heavy membranes in the specific activity of K+-stimulated ATPase. SDS gel patterns and appearance in electron micrograph. These observations suggest that activation of secretion involves at least two distinctive events; transformation of the light to the heavy gastric membranes containing a K+-dependent H+ pump and an increased KCl permeability in the latter.

Inhibition of (H+ + K+)-ATPase and H+ accumulation in hog gastric membranes by trifluoperazine, verapamil and 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate

The mechanism of gastric antisecretory action for trifluoperazine, verapamil and 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) has been studied utilizing isolated hog gastric membranes enriched with (H+ + K+)-ATPase. The drugs inhibited the gastric ATPase due to their apparent competition with K+ for the luminal high-affinity K+-site of the ATPase. The dose to inhibit 50% (ID50) of the ATPase in the membranes rendered freely permeable to K+ (20 mM) was 50 microM for trifluoperazine and 1.5 mM for verapamil and TMB-8. In intact hog gastric membranes which develop a pH gradient in the presence of valinomycin, ATP and KCl, however, trifluoperazine at 4 microM, verapamil and TMB-8 at 15 microM inhibited 40 and 30% of the valinomycin-stimulated ATPase activity, respectively, and also blocked the ionophore-dependent intravesicular acidification as measured by aminopyrine accumulation. The enhanced potency of the drugs to inhibit the ATPase in the intact membrane vesicles may be attributed to the accumulation of the drugs as a weak base within the vesicles, where the luminal K+-site of the ATPase is accessible. Calmodulin and Ca2+ had no effect on the extent of H+-accumulation as measured by aminopyrine accumulation in the membrane vesicles which were prepared in the presence of 1 mM EGTA. Since the drugs showed similar potency in interfering with H+ movements either in the membrane vesicles or isolated rabbit gastric glands stimulated by dibutyryl cAMP, it is reasonable to suggest the inhibitory effect of the drugs on (H+ + K+)-ATPase as a primary cause for such interferences in both cases. A trifluoperazine analog and other lipophilic amine drugs similarly inhibited (H+ + K+)-ATPase and H+ accumulation in the membrane vesicles or in the glands. We have concluded that a tertiary amine, the only common functional group among these drugs, is primarily responsible for their ability to interact with the high-affinity K+ site of the gastric ATPase.

Reconstituted amiloride-inhibited sodium transporter from rabbit kidney medulla is responsible for Na+-H+ exchange

Microsomes formed from rabbit kidney medulla and reconstituted proteoliposomes formed from these microsomes were capable of amiloride-inhibited Na+ transport that was insensitive to valinomycin either with or without K+. This indicated that the Na+ transport process was electroneutral. This Na+ transport process was insensitive to extravesicular Cl- or HCO-3 and not stimulated by high intravesicular gradients of K+, Ca2+ or Mg2+, which indicated that the process did not require NaCl or NaHCO3 co-transport or Na+/K+, Na+/Ca2+ or Na+/Mg2+ counter-transport. Na+ uptake into microsomes or proteoliposomes was inhibited by extravesicular K+, Ca2+, Mg2+ or La3+, which indicated that these ions interacted with the Na+-binding site on the transport protein. Na+ uptake into microsomes was stimulated by intravesicular protons and inhibited by extravesicular protons. This suggested that microsomes were capable of Na+-H+ exchange and this was confirmed when Na+ was shown to stimulate H+ efflux from microsomes. The amiloride-inhibited Na+ transporter from medulla microsomes which has been reconstituted into proteoliposomes is most likely a Na+-H+ exchanger. This Na+ transporter was totally insensitive to the uncoupler 1799, either in the presence or absence of valinomycin plus K+ and less sensitive to NH3 than to amiloride. This indicated that amiloride inhibited Na+ transport not merely by acting as a weak-base uncoupler but by directly interacting with the protein responsible for Na+-H+ exchange.

Anion exchange in oxyntic cell apical membrane: relationship to thiocyanate inhibition of acid secretion

The effects of SCN- on H+-accumulation by inside-out gastric vesicles derived from the apical membrane of secreting oxyntic cells are reported. SCN- inhibited the formation of pH gradients in Cl- and isethionate media. In Cl-, the concentration of SCN- required to achieve a certain degree of inhibition of H+ uptake (or dissipation of performed gradients) was increased with the increase in Cl- concentration, indicating some competitive phenomena between these anions. Comparison of the rates of dissipation of similar pH gradients achieved in Cl- vs. isethionate suggested the existence of a fast Cl-/SCN- exchange. In addition, direct isotopic fluxes confirmed the existence of rapid anion exchange and K-salt transport for both Cl- and SCN-. The rates of anion-exchange and K-salt transport were of similar magnitude, and the rates for SCN- in either countertransport against Cl- or cotransport with K+ were twice as fast as the equivalent values for Cl-. These mediated pathways in the apical membrane provide the possible means for rapid access of SCN- to the acidic canalicular spaces of the oxyntic cell that is implicit in recent proposals to explain SCN- inhibition of gastric HCl secretion.

Amiloride-inhibited Na+ uptake into toad bladder microsomes is Na+-H+ exchange

Amiloride-inhibited Na+ transport into toad urinary bladder microsomes is sensitive to a pH gradient across the vesicular membrane. The magnitude of the gradient was measured directly with acridine orange. Also Na+ could stimulate amiloride-sensitive proton efflux from the microsomes. These results indicated that the transport process was Na+-H+ exchange.