The receptor function of the Na+, K+-activated adenosine triphosphatase system

Decreased hepatocyte membrane potential differences and GABAA-beta3 expression in human hepatocellular carcinoma

To determine whether hepatocyte membrane potential differences (PDs) are depolarized in human HCC and whether depolarization is associated with changes in GABAA receptor expression, hepatocyte PDs and gamma-aminobutyric acid (GABA)A receptor messenger RNA (mRNA) and protein expression were documented in HCC tissues via microelectrode impalement, real-time reverse-transcriptase polymerase chain reaction, and Western blot analysis, respectively. HCC tissues were significantly depolarized (-19.8+/-1.3 versus -25.9+/-3.2 mV, respectively [P<0.05]), and GABAA-beta3 expression was down-regulated (GABAA-beta3 mRNA and protein expression in HCC; 5,693+/-1,385 and 0.29+/-0.11 versus 11,046+/-4,979 copies/100 mg RNA and 0.62+/-0.16 optical density in adjacent tumor tissues, respectively [P=0.002 and P<0.0001, respectively]) when compared with adjacent nontumor tissues. To determine the physiological relevance of the down-regulation, human malignant hepatocytes deficient in GABAA-beta3 receptor expression (Huh-7 cells) were transfected with GABAA-beta3 complementary DNA (cDNA) or vector alone and injected into nu/nu nude mice (n=16-17 group). Tumors developed after a mean (+/-SD) of 51+/-6 days (range: 41-60 days) in 7/16 (44%) mice injected with vector-transfected cells and 70+/-12 days (range: 59-86 days) in 4/17 (24%) mice injected with GABAA-beta3 cDNA-transfected cells (P<0.005).

CONCLUSION

The results of this study indicate that (1) human HCC tissues are depolarized compared with adjacent nontumor tissues, (2) hepatic GABAA-beta3 receptor expression is down-regulated in human HCC, and (3) restoration of GABAA-beta3 receptor expression results in attenuated in vivo tumor growth in nude mice.

Discrimination of transport systems for methylmercury uptake in rat erythrocytes using methylmercury-mercaptalbumin by inhibitors and other factors

This is a continuation of studying the transport system for the uptake of methylmercury (MeHg). The aim of the current study was to study transport systems in rat erythrocyte for the uptake of MeHg while using MeHg-mercaptalbumin (MeHgMASH) complex. The uptake of methylmercury was studied in isolated erythrocytes from rats at 5 degrees C. Different reagents were used to study different transport systems in rat erythrocytes: adenosine 5'-triphosphate (ATP), ouabain and sodium fluoride for the active transport systems; probenecid for the organic anion transport system; 4',4-diisothiocyano-2',2-stilbenedisulphonic acid (DIDS), maleimide and N-ethylmaleimide for Cl- transport system; verapamil for Ca2+ ion transport system; colchicine and vinblastine for the microtubule system; verapamil for Ca2+ ion transport system; colchicine and vinblastine for the microtubule system; valinomycin for the effect of membrane potential; hexanol for the protein-mediated transport system and nonelectrolyte diffusion. The results showed that the uptake of MeHg might be involved in several transport systems: the active transport systems, an organic anion transport system, Cl- ion transport system, and Ca2+ ion transport system. The transport systems were slightly sensitive to the membrane potential. These transport systems seem to share similarities with the transport systems for the uptake of MeHg when using MeHg-cysteine and MeHg-glutathione complexes.

Screening of potential transport systems for methyl mercury uptake in rat erythrocytes at 5 degrees by use of inhibitors and substrates

The current study was designed to screen the potential transport systems for methyl mercury (MeHg) uptake by isolated erythrocytes from rats at 5 degrees. Several inhibitors and substrates were used to test which potential transport system might be involved in MeHg uptake. Probenecid was used to test the organic anion transport system, valinomycin was used to test the effect of the membrane potential, D-glucose and cytochalasin B were used to test the facilitated diffusive D-glucose transport system and colchicine and vinblastine were used to test the microtubule system. The effects of Ca++, Mg++ and Na+ on MeHg uptake have been examined. Ouabain, ATP and glucose were used to test the active transport system, cysteine for the cysteine-facilitated transport system, glycine for system Gly, DL-methionine for system L, and MeHgCl and 4',4-diisothiocyano-2',2-stilbenedisulfonic acid (DIDS) for the Cl- ion transport system. The results showed that MeHg uptake might be involved in the following transport systems at 5 degrees: 1) organic anion transport system; 2) facilitated diffusive D-glucose transport system; 3) cysteine-facilitated transport system; 4) Cl- ion transport system. Moreover, the transport systems for MeHg uptake were sensitive to the membrane potential. Although the mechanisms of interaction of transport systems have not been fully clarified, evidence has been presented which support the existence of several simultaneous transport systems for MeHg uptake.

Effect of inhibitors and substrates on methyl mercury uptake by rat erythrocytes

Methyl mercury (MeHg) uptake by isolated erythrocytes from rats was studied at 20 degrees C. Inhibitors and substrates were used to test which transport system was involved in MeHg uptake. Ouabain and ATP were used to test the active transport system. Glycine was used to test system Gly. DL-Methionine was used to test system L. Cysteine was used to test the cysteine-facilitated transport system. The effects of Ca2+, Mg2+ and Na+ on MeHg uptake have been examined. MeHgCl and 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) were used to test C1- ion transport system. D-Glucose and cytochalasin B were used to test the facilitated diffusive D-Glucose transport system. Colchicine and vinblastine were used to test the microtubule system. Probenecid was used to test the organic acid transport system. Valinomycin was used to test the effect of the membrane potential on MeHg uptake. The results showed that MeHg uptake at 20 degrees C might be involved in the following transport systems: 1) an active transport system; 2) a cysteine-facilitated transport system; 3) a C1- ion transport system; 4) a facilitated diffusive D-glucose transport system; 5) an organic acid transport system. The transport systems for MeHg uptake were sensitive to the membrane potential.

Major organ-specific glycoproteins in isolated brain and kidney membranes identified as Na,K-ATPase subunits by combined glycan-, lectin-, and immunoblotting

In the present work combined glycan-, lectin-, and immunoblotting of isolated brain and kidney membranes shows that the alpha and beta subunits of Na,K-ATPase are the most abundant glycoproteins. Further, Datura stramonium and Galanthus nivalis agglutinins recognize the Na,K-ATPase subunits in a mutually exclusive manner in membranes from human, rabbit and rat brain or human, rabbit, rat, pig and dog kidney indicating the presence of species-independent organ-typical glycoforms. The glycosylation status is not related to the ouabain-sensitivity. Taken together, the data reveals organ-specific glycoforms of Na,K-ATPase which might have roles for organ identification and recognition.

Normal sensitivity of Na+/K(+)-ATPase isolated from brain and kidney of spontaneously hypertensive rats to sodium, ouabain or mercury

Genetically hypertensive rats are excellent animal models for investigating putative Na+/K(+)-ATPase alterations associated with the disease. Highly purified Na+/K(+)-ATPase preparations from these animals have not yet been examined. Na+/K(+)-ATPases of two strains of spontaneously hypertensive rats, the Milan hypertensive strain (MHS) and the spontaneously hypertensive rat (SHR) were characterized in comparison with enzymes isolated from their matched normotensive controls; the sensitivity to Na ions as well as the shape and span of the inhibition curves for ouabain and mercury of the isolated Na+/K(+)-ATPases were compared. No functional changes between the purified 'normotensive' and 'hypertensive' Na+/K(+)-ATPases from brain and kidney were detected ruling out drastic structural alterations of the transport system in these two organs of diseased animals.

Na,K-ATPase characterized in artificial membranes. 1. Predominant conformations and ion-fluxes associated with active and inhibited states

The Na,K-ATPase (NKA) system is the receptor for the cardioactive steroids of plant or animal origin. It is not yet known whether passive ion fluxes traverse the inactivated receptor and thereby contribute to the hormonal, pharmacological or toxic actions of these compounds. To look for putative passive ion-fluxes across the ouabain-NKA complex, we incorporated it into the artificial membrane of liposomes. Since this synthetic membrane is virtually impermeable to Na and K ions, the hypothetical ion-fluxes mediated by the NKA can be determined. E2-forms and E2-ouabain-forms of purified NKA were incorporated, in parallel, into separate liposome preparations and the permeability of the resulting E2-liposomes and E2-ouabain-liposomes to K, Na and Ca ions was compared. The E2-liposomes expressed a typical K-permeability which was not observed in the E2-ouabain-liposomes; the latter showed a slightly higher Na-permeability and a similar Ca-permeability as compared to the former. Thus, ouabain does not induce leaks for K or Ca ions in the NKA molecule.

In vitro and in vivo effects of HgCl2 on synaptosomal ATP diphosphohydrolase (EC 3.6.1.5) from cerebral cortex of developing rats

The aim of the present investigation was to evaluate the in vitro (10-500 microM) and in vivo (1-21 subcutaneous injections of 2.5 mg/kg each) effects of HgCl2 on the ATP diphosphohydrolase activity (EC 3.6.1.5; apyrase) of synaptosomes from cerebral cortex of rats at different ages (5, 11, 18 and 25 days of life). The in vitro results showed that HgCl2 (from 10 to 500 microM) inhibited the hydrolysis of both substrates by the synaptosomal enzyme at all ages studied. In contrast, HgCl2 injected in vivo did no affected the normal ontogeny of ATP and ADP hydrolysis. The hydrolysis of both nucleotides increased at the same rate as a function of age in control and HgCl2-treated rats (the specific activity of enzyme increased about 5-fold from the first week of postnatal life of weaning). The results of the present study demonstrated that in vitro HgCl2 inhibited the enzyme, but was ineffective when tested in vivo. Probably the absence of an in vivo effect is due to the low permeability of blood-brain barrier to inorganic forms of mercury.

Role of cell membrane Na,K-ATPase for survival of human lymphocytes in vitro

Lymphocytes are primordial immune cells with variable life times. Besides genetic programming, extracellular factors interacting with cell surface receptors might alter cell survival. We investigated whether the activity of the membrane-embedded Na,K-ATPase (EC3.6.1.37) or sodium pump (NKA) plays a role for cell survival since this ubiquitous system establishes the vital transmembrane Na and K gradients as well as the resulting high intracellular K/Na ratio required for macromolecule synthesis; furthermore, the system exposes an extracellular inhibitory receptors for cardioactive steroids and palytoxin. Isolated human lymphocytes were incubated in vitro and their viability assessed by exclusion of trypan blue. Various incubation conditions were compared; in RPMI-1640 medium cell viability was preserved for 30 h at 37 degrees C. Externally added ouabain, a hydrophilic cardioactive steroid, blocked the [86Rb]potassium uptake at nanomolar concentrations. Despite pump inhibition ouabain did not alter lymphocyte survival, even at 10 mM for 30 h. By contrast, the hydrophilic toxin palytoxin, the most potent animal poison described so far, killed all cells within 2 h at 10 nM; this toxin is known to act via the sodium pump and to provoke deadly cation-leaks by unmasking a channel component. Intracellular Na increased and K decreased as measured by atomic absorption spectrometry in presence of palytoxin; cell swelling was seen by electron microscopy. Ouabain protected the cells from the toxic effect of palytoxin. The results reveal a pivotal role of NKA integrity for lymphocyte survival.

Identification of organ-specific glycosylation of a membrane protein in two tissues using lectins

Since glycosylation of proteins is performed by the host cell, and variable sugar groupings can confer heterogeneity on the same polypeptide, we wished to see whether membrane proteins, in particular the ubiquitous transmembrane Na,K-ATPase, could be glycosylated differently in different organs. Using a highly sensitive enzyme-linked antibody detection system of bound digoxigenin-labelled lectins on nitrocellulose sheets containing electroblotted alpha and beta subunits of kidney and brain Na,K-ATPase, isolated from various rat strains, in combination with isoform-specific immunoblots, we discovered that brain Na,K-ATPase was highly mannosylated in contrast to renal Na,K-ATPase. Thus, we describe the existence of organ-related glycoforms of an integral ubiquitous membrane protein, i.e. diversification of the same polypeptide by organ-typical sugars. At the same time, the presence of the same glycosylation pattern can make distinct protein isoforms occurring in a same organ more homogeneous. Such organ-related glycoforms may serve for tissue identification and as tissue-specific receptors.

Aluminum-induced alterations in [3H]ouabain binding and ATP hydrolysis catalyzed by the rat brain synaptosomal (Na(+)+K+)-ATPase

The (Na(+)+K+)-ATPase is responsible for maintenance of the ionic milieu of cells. The objective of this study is to investigate the effect of aluminum, an ion implicated in several neurological disorders, on ATP hydrolysis catalyzed by the rat brain synaptosomal (Na(+)+K+)-ATPase and on the binding of [3H]ouabain to this enzyme. AlCl3 (25-100 microM) inhibits the phosphatase activity of the (Na(+)+K+)-ATPase in a dose-dependent manner. AlCl3 appears to act as a reversible, noncompetitive inhibitor of (Na(+)+K+)-ATPase activity by decreasing the maximum velocity of the enzyme without significantly affecting the apparent dissociation constant with respect to ATP. AlCl3 may affect Mg2+ sites on the (Na(+)+K+)-ATPase but does not appear to interact with Na+ or K+ sites on the enzyme. In contrast to this inhibitory effect on the phosphatase function of the enzyme, AlCl3 (1-100 microM) stimulates the binding of [3H]ouabain to the (Na(+)+K+)-ATPase. This effect is due to an increase in the maximum [3H]ouabain binding capacity of the enzyme with no change in the [3H]ouabain binding affinity. These data support the hypothesis that AlCl3 may stabilize the phosphorylated form of the synaptosomal (Na(+)+K+)-ATPase which increases [3H]ouabain binding while inhibiting the phosphatase activity of the enzyme.

Na,K-ATPase and carboxyfluorescein distinctly alter vesicle formation in vitro

The mechanism of vesicle formation as well as the precise reasons for their stability are not known. Thus, it is necessary to simulate the process in vitro for studying its mechanism. If phospholipids are suspended in physiological solution by means of cholate and the detergent is then removed by dialysis, the phospholipids self-assemble to form unilamellar vesicles. We report here that the addition of Na,K-ATPase (an integral membrane protein) to the phospholipids changes the vesicle structure, they become larger and a multilamellar population appears. By contrast, carboxyfluorescein, a compound commonly used for labelling the aqueous vesicle compartment, produces an unexpected effect on vesicle structure by inducing complex, tore-like intravesicular multilayer formations associated with a 5-fold increase in diameter. Thus, the presence of a protein in the membrane phase or of a compound in the water phase can influence and direct vesicle formation in vitro; these model systems might give some clues to possible physicochemical or biological factors governing the formation of natural membrane structures.

Modulation of embryonic Na(+)-K(+)-ATPase activity and mouse preimplantation development in vitro in media containing high concentrations of potassium

The effect of various potassium concentrations (ranging from 1.4 mM to 30 mM K+) in modified Tyrode's medium on the culture of mouse zygotes obtained after in vitro fertilization to the blastocyst stage was examined. A clear dose-dependent negative effect of increasing K+ concentrations on the preimplantation embryonic development in vitro was found. We have previously shown that significantly more two-cell embryos reach the blastocyst stage when cultured during the second day postinsemination in medium supplemented with taurine. Because taurine, an amino acid that abounds in the reproductive tract, has been reported to inhibit the enzyme Na(+)-K(+)-adenosine triphosphatase (Na(+)-K(+)-ATPase), we used two other conditions known to inhibit the Na(+)-K(+)-ATPase to study their effect on mouse embryo development. Culturing embryos during a short period (the second day postinsemination) in low extracellular K+ concentrations (1.4 mM) or in medium supplemented with ouabain (50 microM) showed positive effects similar to those of culturing in medium with taurine (10 mM). This beneficial effect of ouabain was found in various K+ concentrations tested, including the high concentrations present in the oviduct. Although the effects of low K+ and taurine can possibly be ascribed to their other cellular effects, the effect of ouabain shows that inhibition of the Na(+)-K(+)-ATPase during the two-cell stage in the mouse is beneficial for further embryonic development to the blastocyst stage.

Control of the Na+,K(+)-ATPase under normal and pathological conditions

The Na+,K(+)-ATPase is an important enzyme in determining the ionic milieu of the cerebromicrovasculature and neurons. The effect of hypertension or aging on this enzyme, as well as its susceptibility to regulation by fatty acids or aluminum, is the focus of this study. A significant increase (34%) in the apparent affinity constant (KD) but no change in the maximum binding capacity (Bmax) for [3H]ouabain binding to the cerebromicrovascular Na+,K(+)-ATPase occurs after induction of acute hypertension. In addition, long chain unsaturated fatty acids stimulate the binding of [3H]ouabain to the enzyme in microvessels from normotensive and hypertensive rats. The synaptosomal Na+,K(+)-ATPase is sensitive to aluminum. AlCl3 (1-100 microM) inhibits the K(+)-dependent-p-nitrophenylphosphatase (K(+)-NPPase) activity of the Na+,K(+)-ATPase in a dose-dependent manner. AlCl3 (100 microM) decreases the Vmax by 14% but does not alter the KM, suggestive of non-competitive inhibition. The enzyme from aged brain displays a greater Vmax, but shows the same susceptibility to AlCl3 as the enzyme from younger brain. In summary, disruption of the Na+,K(+)-ATPase may underlie, at least in part, abnormalities of nerve and vascular cell function in disorders where elevated concentrations of fatty acids or metal ions are involved.

Ouabain--a link in the genesis of high blood pressure?

Hypertension or high blood pressure is a risk factor that increases risk of myocardial infarction, renal failure or cerebral stroke. The pathogenesis of hypertension is due to a variety of causes, including inherited predisposition, dietary habits, especially salt intake, smoking, and also 'general lifestyle'. But for the scientist interested in the complex interplay of physiological and molecular factors, the actual causes of high blood pressure remain uninvestigated. The following article is concerned with new reports that ouabain, a plant derivative, occurs in human beings, in whom it appears to have a hormonal function; ouabain may even play a key role in the pathogenesis of hypertension. We are thus brought a step closer to the background of cardiovascular disease; we may also be afforded a lead to a new therapeutic principle.

Measurement of plasma membrane potential in isolated rat hepatocytes using the lipophilic cation, tetraphenylphosphonium: correction of probe intracellular binding and mitochondrial accumulation

The lipophilic cation tetraphenylphosphonium (TPP+) has been extensively utilized as the probe for the membrane potential (Vm) in various cells. For application to mammalian cells, however, two serious problems require resolution: (1), correction of TPP+ binding to intracellular constituents and (2), estimation of the considerable TPP+ accumulation in mitochondria. We propose here a simple corrective method for the TPP+ binding and its accumulation. TPP+ distribution is assumed as: (1), two compartments (a cytosolic and a mitochondrial space); (2), a proportional relationship between TPP+ bound amount and its unbound concentration in each compartment. We theoretically derived the simple equation: Vm = - RT/F ln(C/Mphys ratio/C/Mabol ratio) where R, T and F have their usual thermodynamic significance. Here, the C/M ratio is defined as the ratio of TPP+ concentration of apparent intracellular to extracellular space. The suffixes phys and abol, respectively, mean the physiological and solely Vm-abolished conditions. This equation was checked with hepatocytes, because estimating hepatocytes Vm with TPP+ distribution is not considered possible because of the relatively high mitochondrial content. The selective Vm abolition was achieved by permeabilization with 20 microM of amphotericin B. The Vm value was, thus, estimated to be -38.6 +/- 0.3 mV, compatible with those obtained with microelectrodes in other laboratories. Vm in hepatocytes is composed of transmembrane K+ diffusion potential (-20.6 +/- 0.3 mV) and electrogenic Na+/K(+)-ATPase (-19.6 +/- 0.4 mV). Addition of rheogenic L-alanine caused a transient but significant depolarization (from control to -34 +/- 0.3 mV). These results taken together indicate that hepatocyte Vm can be accurately determined with the present simple method, so that it may possibly be applicable to the evaluation of Vm in other mammalian cells.

Na+/K(+)-ATPase: modes of inhibition by Mg2+

Adding 15 mM free Mg2+ decreased Vmax of the Na+/K(+)-ATPase reaction. Mg2+ also decreased the K0.5 for K+ activation, as a mixed inhibitor, but the increased inhibition at higher K+ concentrations diminished as the Na+ concentration was raised. Inhibition was greater with Rb+ but less with Li+ when these cations substituted for K+ at pH 7.5, while at pH 8.5 inhibition was generally less and essentially the same with all three cations: implying an association between inhibition and ion occlusion. On the other hand, Mg2+ increased the K0.5 for Na(+)-activation of the Na+/K(+)-ATPase and Na(+)-ATPase reactions, as a mixed inhibitor. Changing incubation pH or temperature, or adding dimethylsulfoxide affected inhibition by Mg2+ and K0.5 for Na+ diversely. Presteady-state kinetic studies on enzyme phosphorylation, however, showed competition between Mg2+ and Na+. In the K(+)-phosphatase reaction catalyzed by this enzyme Mg2+ was a (near) competitor toward K+. Adding Na+ with K+ inhibited phosphatase activity, but under these conditions 15 mM Mg2+ stimulated rather than inhibited; still higher Mg2+ concentrations then inhibited with K+ plus Na+. Similar stimulation and inhibition occurred when Mn2+ was substituted for Mg2+, although the concentrations required were an order of magnitude less. In all these experiments no ionic substitutions were made to maintain ionic strength, since alternative cations, such as choline, produced various specific effects themselves. Kinetic analyses, in terms of product inhibition by Mg2+, require Mg2+ release at multiple steps. The data are accommodated by a scheme for the Na+/K(+)-ATPase with three alternative points for release: before MgATP binding, before K+ release and before Na+ binding. The latter alternatives necessitate two Mg2+ ions bound simultaneously to the enzyme, presumably to divalent cation-sites associated with the phosphate and the nucleotide domains of the active site.

Partial purification and properties of (Na+ + K+)-ATPase from Potamon potamios

1. The tissue distribution of the (Na+ + K+)-ATPase in the freshwater/land crab Potamon Potamios was studied. 2. Gills were found to display the highest total activity in the whole animal (47%) but the highest specific activity was detected in the heart (15.15 mumol Pi/mg protein/min). 3. All other organs tested were found to have low enzyme activity. 4. The freshwater/land crab ATPase enzyme was inhibited by ouabain with a Ki of 0.5 mM.Km values for ATP, Mg2+ and K+ were 1.4, 4.0 and 1.2 mM respectively. The enzyme also showed a break in the Arrhenius plot at 23 degrees C. 5. A purification method of microsomal ATPase is described involving ultracentrifugation and electrofocusing.

Characterization of ouabain-induced phosphoinositide hydrolysis in brain slices of the neonatal rat

The effect of the Na/K-ATPase inhibitor ouabain on phosphoinositide (Ptdlns) hydrolysis was studied in rat brain cortical slices. Ouabain induced a dose-dependent accumulation of inositol phosphates (InsPs) which was much higher in neonatal rats (1570 +/- 40% of basal) than in adult animals (287 +/- 18% of basal). For this reason, all experiments were conducted with 7 day-old rats. Strophantidin caused a similar stimulation of Ptdlns hydrolysis, although it was less potent than ouabain. The order of potency for ouabain-stimulated InsPs accumulation in brain areas was hippocampus greater than cortex greater than brainstem greater than cerebellum. The effect of ouabain was not blocked by antagonists for the muscarinic, alpha1 -adrenergic and glutamate receptors. Also ineffective were the K+ channel blockers 4-aminopyridine and tetraethylammonium, the sodium channel blocker tetrodotoxin, and the calcium channel blocker verapamil, whereas the Na/Ca exchanger blocker amiloride partially antagonized the effect of ouabain. The accumulation of InsPs induced by ouabain was additive to that of carbachol and norepinephrine, as well as to that induced by high K+ and veratrine, but not to that of glutamate. Removal of Na+ ions from the incubation buffer completely prevented the accumulation of InsPs induced by ouabain. The effect of ouabain was also dependent upon extracellular calcium and was under negative feedback control of protein kinase C. Despite the higher effect of ouabain on Ptdlns hydrolysis of immature rats, the density of [3H]ouabain binding sites, as well as the activity of Na/K-ATPase were higher in adult animals. Furthermore, a poor correlation was found between ouabain-stimulated Ptdlns hydrolysis and [3H]ouabain binding in brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of [3H]ouabain binding to cerebromicrovascular (Na+ + K+)-ATPase by metal ions and proteins

The (Na+ + K+)-ATPase is localized to the cerebral endothelium, i.e. the blood-brain barrier, and is important for the maintenance of the brain electrolyte environment. Data from the present study indicate that Pb2+ inhibits the binding of [3H]ouabain to the cerebral microvascular (Na+ + K+)-ATPase in a time- and dose-dependent manner. Pb2(+)-induced inhibition developed slowly with a maximum obtained after 40 min. Inhibition of [3H]ouabain binding to the enzyme was 48% at 10 microM Pb2+ and appeared maximal (89%) at 100 microM Pb2+ when compared to [3H]ouabain binding in untreated microvessels at 40 min. In contrast, 100 microM Al3+ caused a 55% increase in [3H]ouabain binding to the (Na+ + K+)-ATPase, relative to untreated microvessels at 40 min. Insulin or bovine serum albumin stimulated [3H]ouabain binding to the enzyme when added at similar concentrations. However, the addition of both insulin and bovine serum albumin did not result in an additive effect. These results show that insulin exerts a nonspecific effect on [3H]ouabain binding to the (Na+ + K+)-ATPase similar to that evoked by bovine serum albumin. However, the metal ions Pb2+ and Al3+ provoke selective alterations in the cerebromicrovascular (Na+ + K+)-ATPase with Pb2+ inhibiting and Al3+ stimulating [3H]ouabain binding.

Chelation of mercury by ouabain-sensitive and ouabain-resistant renal Na,K-ATPase

The SH-reactive HgCl2 inhibits the Na,K-ATPase activity potently in a manner antagonized only partially by EDTA or cysteine; solely dimercaprol, a dithiol antidote for mercury, blocks the HgCl2 effects entirely as confirmed also by 203Hg-binding experiments. The results reveal the presence of a chelating component in pure Na,K-ATPase with an affinity for mercury superior to EDTA. The mercury-sensitivity of the Na,K-ATPase is not related to the ouabain-sensitivity. This criterion will be useful for the distinction between ouabain-like and mercury-like inhibitors from body fluids and tissues.

Elevation of bovine endothelial cell angiotensin converting enzyme by cationophores and inhibition by ouabain

We recently reported that calcium ionophore A23187 causes a several-fold elevation of angiotensin converting enzyme (ACE) activity of bovine pulmonary artery endothelial cells in culture and that this elevation is dependent upon extracellular calcium. Now we have observed that monensin, a sodium ionophore, also elevates the ACE activity of these cells. This elevation in ACE was not inhibited by 0.2 mM EGTA or the calcium channel inhibitor nifedipine, and monensin did not alter intracellular calcium as measured by fluorimetric assessment of fura-2/AM-loaded cells. When confluent endothelial cells were incubated with monensin or A23187 in the presence of 10-20 nM ouabain, a specific inhibitor of Na+/K(+)-ATPase, the elevation in ACE produced by both of the ionophores was totally eliminated. Concentrations of ouabain greater than 10 nM also inhibited baseline levels of ACE activity. Ca2+ measurements of fura-2/AM-loaded cells showed that ouabain had no effect on the influx of Ca2+ produced by A23187. The elevation of ACE seemed to require new protein synthesis, since 0.1 micrograms/ml cycloheximide inhibited the elevation produced by monensin and A23187. Other sodium transport inhibitors such as amiloride or bumetanide had no effect on ACE elevation caused by monensin. These results suggest that ACE levels of bovine endothelial cells in culture are under cation regulation and may be modulated by ouabain-sensitive Na+/K(+)-ATPase.

Sodium transport defect of ouabain-resistant renal Na,K-ATPase

The murine renal Na,K-ATPase is resistant to cardiac glycosides. It is not yet known however whether altered active transport is associated with the drug-resistance. To investigate this problem Na,K-ATPases were purified from the outer medulla of both rat and rabbit kidneys and reconstituted identically into liposomes. The Na-stimulation of the Na,K-ATPase activity before reconstitution and of the Na-transport after reconstitution was measured. A Na-defect inherent in the ouabain-resistant rat Na,K-ATPase was discovered indicating a link between the cardiac glycoside sensitivity and the Na-transport.

Characterization of fatty acid interaction with ouabain and vanadate binding to (Na+ + K+)-activated ATPase

The candidateship of unsaturated fatty acids as endogenous ouabain-like factors was studied. Binding of the artificial ligand vanadate at the intracellular phosphorylation epitope of membrane-bound Na+/K+-ATPase was unaffected by linoleic and arachidonic acid. In the (Mg2+ + Pi)-facilitated system for ouabain binding they were characterized as noncompetitive inhibitors of cardiac glycoside binding, however. The ouabain binding capacity as well as the affinity decreased and the ouabain dissociation rate was accelerated by fatty acids. In the presence of vanadate for facilitation of ouabain binding an increase in ouabain affinity was seen. It is concluded that elementary criteria for the characterization of unsaturated fatty acids as ouabain-like factors are not fulfilled. The ratio between E2-subconformations of Na+/K+-ATPase with different ouabain affinities may be changed by incorporation of fatty acids in the lipid membrane.

Urinary sodium pump inhibitor raises cytosolic free calcium concentration in rat aorta

We were able to purify two distinct sodium pump inhibitors to homogeneity from human urine based on [3H]ouabain-displacing activity from intact human erythrocytes. The polar and less polar compounds were eluted off the C18 reverse-phase column with 18% and 31% acetonitrile, respectively. The polar compound cross-reacted very weakly with specific antidigoxin antibody and lacked a characteristic ultraviolet absorption peak between 190 and 300 nm. The less polar compound showed a prominent digoxinlike immunoreactivity and had an ultraviolet spectrum similar to that of digoxin. We examined the effects of these compounds on cytosolic free calcium concentration in cultured rat vascular smooth muscle cells (A10 cells) using the fluorescent calcium chelator fura-2. Only the polar ouabain-displacing compound caused a significant increase, from 108 +/- 7 to 162 +/- 8 nM (n = 6, p less than 0.01), in cytosolic free calcium concentration in A10 cells. The rise in cytosolic free calcium concentration induced by the polar ouabain-displacing compound tended to be slower in onset and more sustained than that induced by arginine vasopressin. In contrast, ouabain and bufalin had no appreciable effects on cytosolic free calcium concentration in A10 cells. These results suggest that the polar ouabain-displacing compound we isolated from human urine may possess a vasoactive property and may play an important role in the modulation of vascular tone.

The human glutathione S-transferases. Immunohistochemical studies of the developmental expression of Alpha- and Pi-class isoenzymes in liver

Immunohistochemical studies of the developmental expression of the Alpha- and Pi-class glutathione S-transferases in human liver have shown that the Pi enzyme is expressed in bile-duct epithelium and some hepatocytes but not in haematopoietic cells. This locus is down-regulated during gestation in hepatocytes but not in epithelium. The enzymes of the Alpha set were also found in only some hepatocytes, and it appears that many cells express neither these nor the Pi forms.

Effect of fatty acids on [3H]ouabain binding to cerebromicrovascular (Na+ + K+)-ATPase

The effects of short- and long-chain fatty acids on the cerebromicrovascular (Na+ + K+)-ATPase were investigated using specific [3H]ouabain binding to the enzyme. Specific binding increased linearly with total microvessel protein (37-110 micrograms) and was time-dependent with maximum binding obtained by 10 min. Arachidonic acid, but not palmitic acid, stimulated [3H]ouabain binding in a dose-dependent manner, with a 105% increase over basal levels at 100 microM arachidonic acid. Preincubation of the microvessels with arachidonic acid did not alter the stimulation observed. 4-Pentenoic acid stimulated [3H]ouabain binding only at high concentrations (10 mM). Scatchard analysis of [3H]ouabain binding to untreated microvessels yielded a single class of "high-affinity" binding sites with an apparent binding affinity (KD) of 64.7 +/- 2.0 nM and a binding capacity (Bmax) of 10.1 +/- 1.5 pmol/mg protein. In the presence of 100 microM arachidonic acid, a monophasic Scatchard plot also was obtained, but the KD significantly decreased to 51.9 +/- 2.7 nM (p less than 0.01), whereas the Bmax remained virtually unchanged (12.5 +/- 1.2 pmol/mg protein). The stimulation of [3H]ouabain binding in the presence of arachidonic acid was potentiated by 4-pentenoic acid, but not by indomethacin or eicosatetraynoic acid. These data suggest that long-chain polyunsaturated fatty acids may be involved in the regulation of blood-brain barrier (Na+ + K+)-ATPase and may play a role in the cerebral dysfunction associated with diseases in which plasma levels of nonesterified fatty acids are elevated.

Endogenous cardiac glycosidelike compounds

The possibility that endogenous inhibitors of the sodium pump exist and bind to the cardiac glycoside binding site on Na+,K+-adenosine triphosphatase (ATPase) has been a source of much controversy. Although numerous hormones and inorganic ions that modulate Na+,K+-ATPase activity have been described, most of these affect the sodium pump indirectly by varying the intracellular sodium concentration or by increasing the number of enzyme units. None of these endogenous compounds has been shown conclusively to modulate sodium pump activity by binding to the cardiac glycoside binding site on Na+,K+-ATPase. However, the near-universal presence of three high-affinity binding sites on the alpha-subunit of the enzyme has engendered much speculation that endogenous ligands for these receptors must exist. In addition, none of the hormones known to indirectly affect sodium pump activity in vivo has been shown to modulate Na+,K+-ATPase activity in response to extracellular volume expansion or to play a role in the pathogenesis of hypertension or chronic renal failure, conditions in which a circulating inhibitor of Na+,K+-ATPase has been implicated. This report presents a condensed history of the search for endogenous inhibitors of Na+,K+-ATPase and describes recent advances in the field. Despite progress in identifying and characterizing compounds that could affect Na+,K+-ATPase activity in vivo, definitive proof for the existence of endogenous ligands for the cardiac glycoside binding site remains elusive.

Ouabain-binding site of (Na+ + K+)-ATPase in right-side-out vesicles has not an externally accessible SH group

The fluorescing sulfhydryl reagent N-(7-dimethylamino-4-methylcoumarinyl)maleimide (DACM) inactivates purified (Na+ + K+)-ATPase at 20 microM. This inactivation results in a decrease of the ouabain-binding capacity of the enzyme. Treatment of (Na+ + K+)-ATPase, embedded in right-side-out-oriented vesicles, by DACM does not affect ouabain binding to the enzyme. Incorporation of DACM into the alpha subunit of (Na+ + K+)-ATPase embedded in right-side-out vesicles is also not affected by the presence or absence of 100 microM ouabain. It is therefore concluded that a sulfhydryl group does not reside within the ouabain-binding site of (Na+ + K+)-ATPase.

Characterization of (Na+ + K+)-ATPase-liposomes. III. Controlled activation and inhibition of symmetric pumps by timed asymmetric ATP, RbCl, and cardiac glycoside addition

Inside-out as well as right-side-out oriented (Na+ + K+)-ATPase molecules reconstituted in liposomes are activated successively by timed asymmetric addition of ATP to the internal and external liposome compartment; this presents the first functional confirmation of the symmetric pump-orientation in cholate-dialysed preparations revealed previously by the equal distribution of intramembrane particles on the concave and convex surface of freeze-fractured (Na+ + K+)-ATPase-liposomes. The initial transport rates of the symmetrically oriented pump populations are regulated by varying the bilateral K or Rb ion concentrations; ATP, ouabain, digoxin or vanadate are used to activate or block selectively the right-side-out, inside-out or both (Na+ + K+)-ATPase populations. Finally, these liposomes of the second generation present a new tool to evaluate the membrane-permeability as well as the effects of receptor-ligands or other probes in a single preparation.

Autoradiographic visualization and characterization of [3H]ouabain binding to the Na+,K+-ATPase of rat brain and pineal

Ouabain binds to the catalytic subunit of Na+,K+-ATPase and specific [3H]ouabain binding can be used as a measure of the number of active enzyme molecules present in a given tissue. Specific [3H]ouabain binding can be demonstrated in frozen, cryostat sections from rat brain and pineal and these sites have the characteristics of Na+,K+-ATPase. Incubations carried out in the absence of ATP or the presence of excess unlabeled ouabain reduces specific binding by greater than or equal to 98%. The addition of K+ or omission of Mg2+ also result in a decrease in specific binding. Strophanthidin, digoxin and digoxigenin displace [3H]ouabain binding with IC50 values of 0.73, 0.48 and 1.4 microM, respectively. Scatchard analyses of specific [3H]ouabain binding in brain sections shows a single class of non-interacting binding sites with an apparent affinity (Kd) of 339 nM and a maximal binding capacity (Bmax) of 34.9 pmol/mg protein. [3H]Ouabain binding is unevenly distributed throughout the brain with the olfactory nuclei, superior colliculus, dentate gyrus, pontine nuclei and pineal gland having a relatively high density of binding sites. The outer layers (1-3) of the cerebral cortex show more labeling than the inner layers (4-6) and most other brain areas have intermediate levels of [3H]ouabain binding sites, whereas white matter has virtually no specific binding. Computer-assisted densitometry was used to measure changes in specific [3H]ouabain binding after kainic acid injection into the caudate nucleus. An initial increase in [3H]ouabain binding was observed at 1 and 24 h after lesioning and a decrease in [3H]ouabain binding was evident by 9 days after lesioning.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenovirus-dependent changes in cell membrane permeability: role of Na+, K+-ATPase

Adenovirus-dependent release of choline phosphate from KB cells at pH 6.0 was partially blocked by ouabain. In K+-containing medium, maximum inhibition of release was obtained by 10(-5) M ouabain and half-maximal inhibition was achieved by about 0.5 X 10(-6)M ouabain. Ouabain did not block either the binding or the uptake of adenovirus by KB cells. Without K+, about 25% of cell-associated choline phosphate was released by adenovirus, whereas with 1 mM K+ about 50% was released. This activation by K+ was blocked by 0.1 mM ouabain. HeLa cells behaved like KB cells, but a mutant of HeLa cells resistant to ouabain (D98-OR) released much lower amounts of choline phosphate in response to human adenovirus type 2 (Ad2). Wild-type D98-OR cells bound nearly the same amount of adenovirus as did normal HeLa cells. Ad2 also increased the activity of Na+,K+-ATPase in KB cells, with maximum activation at 50 micrograms of Ad2 per ml. In D98-OR cells, Ad2 failed to activate Na+,K+-ATPase activity. Ad2-dependent lysis of endocytic vesicles (receptosomes) was assayed by measuring Ad2-dependent enhancement of epidermal growth factor-Pseudomonas exotoxin toxicity. This action of adenovirus was increased when K+ was present in the medium. Under the conditions used, K+ had no effect on the amount of Ad2 or epidermal growth factor taken up by the cells. On the basis of these results, it is suggested that Ad2-dependent cellular efflux of choline phosphate and adenovirus-dependent lysis of receptosomes may require Na+,K+-ATPase activity.

A quantitative receptor assay for "digitalis-like" compounds in serum. Demonstration of raised concentrations in essential hypertension and correlation with arterial blood pressure

The influence of serum from patients with essential hypertension on [3H]ouabain binding to isolated (Na+ +K+)-ATPase and on the reactivity with digoxin-specific antibodies was investigated. [3H]Ouabain binding to (Na+ +K+)-ATPase was significantly decreased (P less than 0.001) by sera of 18 hypertensive patients (34.9 +/- 1.5 pmol/U) as compared with 22 normotensive controls (43.8 +/- 1.2 pmol/U). The factor, whose concentration is increased in the serum of patients with essential hypertension, competed with [3H]ouabain at isolated (Na+ +K+)-ATPase. Therefore, it was possible to quantify this "digitalis-like" factor with a receptor assay in ouabain equivalents. Three times higher mean serum levels were found in hypertensive patients (234.8 +/- 48.7 nM) than in normotensive controls (76.3 +/- 9.3 nM). Deproteinization of the sera by ultrafiltration through steroid-adsorbing membranes and by boiling of acidified sera for 10 min led to a significant reduction of the inhibitory activity and to the complete loss of a difference between the sera of normotensives and hypertensives. After deproteinization by boiling for 15 min, sera of normotensives showed levels of "digitalis-like" compounds of 16.53 +/- 2.15 nM and hypertensives of 41.65 +/- 8.41 nM (P less than 0.05). Though significantly elevated concentrations of "digitalis-like" factor were measured with the receptor assay, no significant increase of digoxin-like activity could be detected with digoxin-specific antibodies in untreated serum.

Further biochemical characterization of an Na+ pump inhibitor purified from human urine

An increase in endogenous Na+,K+-ATPase inhibitor(s) with digitalis-like properties has been reported in chronic renal insufficiency, in Na+-dependent experimental hypertension and in some essential hypertensive patients. The present study specifies some properties and some biochemical characteristics of a semipurified compound from human urine having digitalis-like properties. The urine-derived inhibitor (endalin) inhibits Na+,K+-ATPase activity and [3H]-ouabain binding, and cross-reacts with anti-digoxin antibodies. The inhibitory effect on ATPases of endalin is higher on Na+,K+-ATPase than on Mg2+-ATPase and Ca2+-ATPase. The mechanism of endalin action on highly purified Na+,K+-ATPase was compared to that of ouabain and was similar in that it reversibly inhibited Na+,K+-ATPase activity; it inhibited Na+,K+-ATPase non-competitively with ATP; its inhibitory effect was facilitated by Na+; K+ decreased its inhibitory effect on Na+,K+-ATPase; it competitively inhibited ouabain binding to the enzyme; its binding was maximal in the presence of Mg2+ and Pi; it decreased the Na+ pump activity in human erythrocytes; it reduced serotonin uptake by human platelets; and it was diuretic and natriuretic in rat bioassay. The endalin differed from ouabain in only three aspects: its inhibitory effect was not really specific for Na+,K+-ATPase; its binding to the enzyme was undetectable in the presence of Mg2+ and ATP; it was not kaliuretic in rat bioassay. Endalin is a reversible and partial specific inhibitor of Na+,K+-ATPase, its Na+,K+-ATPase inhibition closely resembles that of ouabain and it could be considered as one of the natriuretic hormones.

Studies of the development of basic, neutral and acidic isoenzymes of glutathione S-transferase in human liver, adrenal, kidney and spleen

The ontogeny of basic, near-neutral and acidic glutathione S-transferase isoenzymes was studied by using chromatofocusing and ion-exchange chromatography. These isoenzyme sets demonstrated tissue-specific patterns of expression. For example, whereas basic isoenzymes were identified in all liver and adrenal cytosols obtained after 10 weeks gestation, these forms were not detected in kidney until 10 weeks post-natal age and in spleen until about 40 weeks post-natal age. Our data indicate that the basic monomers B1 and B2 are present in liver cytosol at 21 weeks gestation. Expression of the near-neutral isoenzymes was usually weak; for example, they were not generally expressed in liver until 30 weeks gestation, and no developmental patterns in their expression could be identified in adrenal, kidney and spleen. The acidic isoenzymes were usually strongly expressed in adrenal, kidney and spleen, although there was a decline in the level of expression in kidney after birth.

Endogenous digitalis-like factors in hypertension and chronic renal insufficiency

Endogenous digitalis-like factors have been implicated in the adaptations that accompany renal insufficiency and in the pathogenesis of hypertension. We recently described several fractions of normal human plasma that inhibit NaK-ATPase and exhibit apparent digoxin-like immunoreactivity. To determine if hypertension and/or renal insufficiency affect plasma levels of these factors, we examined four patient groups: normotensive controls; hypertensive subjects with normal renal function; hypertensives with moderate renal insufficiency; and chronic dialysis patients. Plasma levels of digoxin-like immunoreactivity and NaK-ATPase inhibitory activity were significantly increased in hypertensive patients with mild renal failure (7.6 +/- 1.1 ouabain equivalents, mean +/- SEM, N = 21 vs 4.1 +/- 1.1 in normotensive controls, N = 20, P less than 0.05). NaK-ATPase inhibitory activity tended to be higher in patients with primary hypertension and normal renal function (5.5 +/- 0.7 ouabain equivalents, P less than 0.07); in dialysis patients, it was not different from controls. There was no correlation between NaK-ATPase inhibitory activity and blood pressure in any group. There was a significant rise in plasma NaK-ATPase inhibitory activity during dialysis (+ 1.8 +/- 0.7 ouabain equivalents, N = 22, P less than 0.03). As we have found that NaK-ATPase inhibitory activity in the plasma of normal humans can be separated into three distinct fractions, EI1, EI2, and EI3, we analyzed the plasma of 10 dialysis patients further. The increase in NaK-ATPase inhibitory activity could be attributed to fractions EI1 and EI3. These results suggest that plasma NaK-ATPase inhibitors increase with chronic renal insufficiency, but not hypertension alone. Although hemodialysis may acutely raise plasma levels, long-term dialysis returns them to the normal range.

Erythrocyte Na(+)-K+ ATPase activity in the genesis of reducing renal mass in hypertensive rats

To study the relationship between natriuretic factor and experimental low renin hypertension, the erythrocyte Na⁺-K⁺ ATPase activities in the genesis of reducing renal mass in hypertensive rats were measured. The results were as follows : 1. The fractional excretion of Na in hypertensive, reduced renal mass rats (saline-drinking) was higher than that of normotensive, reduced renal mass rats (water-drinking) (p<.001). The plasma renin activities in the hypertensive group were markedly lower than those of the normotensive group (p<.001). 2. The total ATPase activities of rat erythrocyte membrane in the hypertensive group were lower than in those of the normal and the normotensive groups (p<.001, p<.02). 3. The Mg⁺⁺-ATPase activities of rat erythrocyte membrane in the normotensive group tended to decrease significantly (p<.001), but the differences between the normotensive and hypertensive group were not significant. 4. The Na⁺-K⁺ ATPase activities of rat erythrocyte membrane in the hypertensive group were lower than those of the normal and the normotensive group (p<.001). However, the difference between the normal and the normotensive groups was not significant. 5. When erythrocyte membrane taken from normal rats was incubated with supernates of boiled plasma from normotensive rats, the total ATPase activities of erythrocyte membrane were not different from those of hypertensive rats. 6. When erythrocyte membrane taken from normal rats was incubated with supernates of boiled plasma from normotensive rats, the Mg⁺⁺ ATPase activities of erythrocyte membrane were lower than those of hypertensive rats (p<.01). 7. When erythrocyte membrane taken from normal rats was incubated with supernates of boiled plasma from hypertensive rats, the Na⁺-K⁺ ATPase activities of erythrocyte membrane were lower than those of normotensive rats (p<.01). Based on the above findings, it is suggested that the pathogenesis of low renin, reduced renal mass hypertension is primarily mediated by a natriuretic factor (or an ouabain-like factor, inhibitor of Na⁺-K⁺ ATPase) produced by extracellular fluid volume expansion.