Stimulation of the Na+,K(+)-ATPase activity of K562 human erythroleukemia cells by triiodothyronine.

The effect of triiodothyronine (T3) on Na+,K(+)-ATPase activity of K562 human erythroleukemic cell was studied to understand why the erythrocyte sodium pump activity is decreased in hyperthyroidism. Na+,K(+)-ATPase activity of K562 cell lysates was assayed by measuring the release of inorganic phosphate (Pi) from ATP. Na+,K(+)-ATPase activity of K562 cell grown in the presence of T3 for 48 hours was significantly higher than that of control (0.98 +/- 0.05 mumol Pi h-1 mg protein-1 vs 0.82 +/- 0.10 mumol Pi h-1 mg protein-1, p < 0.05). The Na+,K(+)-ATPase activity could be stimulated in a time- and concentration-dependent manner; maximum stimulatory effect of T3 was seen at a concentration of 10(-7) mol/L. When an inducer [cytosine-beta-D-arabino-furanoside (ARA-C)] was added to the culture medium, the K562 cells showed signs of differentiation and synthesised haemoglobin. At the same time, the Na+,K(+)-ATPase activity remained high. We conclude that T3 stimulates Na+,K(+)-ATPase activity of K562 cells and in the presence of T3 during differentiation, the enzyme activity remains high.     

Theoretical considerations on the role of membrane potential in the regulation of endosomal pH.

Na+,K(+)-ATPase has been observed to partially inhibit acidification of early endosomes by increasing membrane potential, whereas chloride channels have been observed to enhance acidification in endosomes and lysosomes. However, little theoretical analysis of the ways in which different pumps and channels may interact has been carried out. We therefore developed quantitative models of endosomal pH regulation based on thermodynamic considerations. We conclude that 1) both size and shape of endosomes will influence steady-state endosomal pH whenever membrane potential due to the pH gradient limits proton pumping, 2) steady-state pH values similar to those observed in early endosomes of living cells can occur in endosomes containing just H(+)-ATPases and Na+,K(+)-ATPases when low endosomal buffering capacities are present, and 3) inclusion of active chloride channels results in predicted pH values well below those observed in vivo. The results support the separation of endocytic compartments into two classes, those (such as early endosomes) whose acidification is limited by attainment of a certain membrane potential, and those (such as lysosomes) whose acidification is limited by the attainment of a certain pH. The theoretical framework and conclusions described are potentially applicable to other membrane-enclosed compartments that are acidified, such as elements of the Golgi apparatus.     

The yeast endosomal Na+K+/H+ exchanger Nhx1 regulates cellular pH to control vesicle trafficking

The relationship between endosomal pH and function is well documented in viral entry, endosomal maturation, receptor recycling, and vesicle targeting within the endocytic pathway. However, specific molecular mechanisms that either sense or regulate luminal pH to mediate these processes have not been identified. Herein we describe the use of novel, compartment-specific pH indicators to demonstrate that yeast Nhx1, an endosomal member of the ubiquitous NHE family of Na+/H+ exchangers, regulates luminal and cytoplasmic pH to control vesicle trafficking out of the endosome. Loss of Nhx1 confers growth sensitivity to low pH stress, and concomitant acidification and trafficking defects, which can be alleviated by weak bases. Conversely, weak acids cause wild-type yeast to present nhx1Delta trafficking phenotypes. Finally, we report that Nhx1 transports K+ in addition to Na+, suggesting that a single mechanism may responsible for both pH and K+-dependent endosomal processes. This presents the newly defined family of eukaryotic endosomal NHE as novel targets for pharmacological inhibition to alleviate pathological states associated with organellar alkalinization.     

Na+,K(+)-ATPase pump currents in giant excised patches activated by an ATP concentration jump.

The giant-patch technique was used to study the Na+,K(+)-ATPase in excised patches from rat or guinea pig ventricular myocytes. Na+,K(+)-pump currents showed a saturable ATP dependence with aK(m) of approximately 150 microM at 24 degrees C. The pump current can be completely abolished by ortho-vanadate. Dissociation of vanadate from the enzyme in the absence of extracellular Na+ was slow, with a Koff of 3.10(-4) S-1 (K1 approximately 0.5 microM, at 24 degrees C). Stationary currents were markedly dependent on intracellular pH, with a maximum at pH 7.9. Temperature-dependence measurements of the stationary pump current yielded an activation energy of approximately 100 kJ mol-1. Partial reactions in the transport cycle were investigated by generating ATP concentration jumps through photolytic release of ATP from caged ATP at pH 7.4 and 6.3. Transient outward currents were obtained at pH 6.3 with a fast rising phase followed by a slower decay to a stationary current. It was concluded that the fast rate constant of approximately 200 s-1 at 24 degrees C (pH 6.3) reflects a step rate-limiting the electrogenic Na+ release. Simulating the data with a simple three-state model enabled us to estimate the turnover rate under saturating substrate concentrations, yielding rates (at pH 7.4) of approximately 60 s-1 and 200 s-1 at 24 degrees C and 36 degrees C, respectively.     

Uptake and transport of mannosylated ligands by alveolar macrophages. Studies on ATP-dependent receptor-ligand dissociation

During endocytosis, mannosylated ligands enter vesicles which have a density intermediate between that of the plasma membrane and secondary lysosomes. Mannosylated ligands are transferred from these vesicles to lysosomes. A solubilization-precipitation assay was used to study the dissociation of mannosylated ligands from their receptor. In whole cells dissociation was rapid (t 1/2 (37 degrees C) = 8 min) and took place before delivery of the ligand to lysosomes. Receptor-ligand dissociation within membrane vesicles, washed free of cytosol, could be induced by addition of ATP and GTP but not ADP. Receptor-ligand dissociation caused by manipulating the pH of the vesicles suggested that the pH within endosomes was lowered to 5.5 by addition of ATP. Dissociation was blocked by proton ionophores and Zn2+, but was unaffected by inhibitors of the F1, Fo-ATPase or the Na+,K+-ATPase. Dissociation did not require Na+ or K+ and was blocked by anion transport inhibitors. Dissociation was slowed in the absence of permeant anions (Cl-). Receptor-ligand complexes within vesicles isolated as early as 2 min following ligand internalization responded to addition of ATP. The results suggest that receptor-ligand dissociation in endosomes requires ATP, possibly to power endosomal acidification via an ATP-dependent proton pump. Dissociation is enhanced in the presence of permeant anions, suggesting the involvement of an anion channel or carrier.     

Expression of ouabain-sensitive H+,K+-ATPase catalytic subunit gene in the rat epidermis

A comparative localization of Na+,K(+)-ATPase and ouabain-sensitive H+,K(+)-ATPase in rat skin was performed using in situ RNA hybridization and immunohistochemistry. Na+,K(+)-ATPase was predominantly detected in the basal layer of epithelium, whereas the ouabain-sensitive H+,K(+)-ATPase, in the granular and prickle cell layers. The genes of these ATPases are thus expressed in epithelial cells at different stages of their development. The hypothesis was advanced that the ouabain-sensitive H+,K(+)-ATPase is involved in maintaining the skin pH value. The probes specific to the mRNAs of the full-size alpha-subunit of the ouabain-sensitive H+,K(+)-ATPase and its truncated form were used to establish a similar distribution of both mRNA variants in skin. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2002, vol. 28, no. 4; see also http://www.maik.ru.     

Characteristics of (Na+ + K+)-ATPase in the gills of bass

A partial characterization of bass gill (Na+ + K+-ATPase is reported in the present paper. Microsomal preparation from gill homogenate showed optimal (Na+ + K+)-ATPase activity at pH 6,5 in the presence of 100 mM Na+, 20mM K+ and 5mM Mg2+. Under these conditions maximal activity was shown at 45 degrees C, even if an increased lability of the enzyme was shown at temperature greater than 30 degrees C. A complete inhibition of the enzyme occurred in the presence of 1 mM ouabain. The break in the Arrhenius plot occurred approximatively at the temperature of adaptation of these fish (18 degrees C). The energies of activation above and below the break were scarcely different from each other and lower than those reported in other Poikilotherms. Furthermore similar values of Km for Na+ were evidenced at 18 degrees C and 30 degrees C. The whole of data are discussed in comparison with other teleost gill (Na+ + K+)-ATPase reports and related to the physiological role of the enzyme in osmoregulation.     

Roles of K+ and Na+ in pH homeostasis and growth of the marine bacterium Vibrio alginolyticus.

The marine bacterium Vibrio alginolyticus, containing 470 mM-K+ and 70 mM-Na+ inside its cells, was able to regulate the cytoplasmic pH (pH(in)) in the narrow range 7.6-7.8 over the external pH (pH(out)) range 6.0-9.0 in the presence of 400 mM-Na+ and 10 mM-K+. In the absence of external K+, however, pHin was regulated only at alkaline pH(out) values above 7.6. When the cells were incubated in the presence of unusually high K+ (400 mM) and 4 mM Na+, the pH(in) was regulated only at acidic pH(out) values below 7.6. These results could be explained by postulating a K+/H+ antiporter as the regulator of pH(in) over the pH(out) range 6.0-9.0. When Na(+)-loaded/K(+)-depleted cells were incubated in 400 mM-Na+ in the absence of K+, an inside acidic delta pH was generated at pH(out) values above 7.0. After addition of diethanolamine the inside acidic delta pH collapsed transiently and then returned to the original value concomitant with the extrusion of Na+, suggesting the participation of a Na+/H+ antiporter for the generation of an inside acidic delta pH. In the presence of 400 mM-K+, at least 5 mM-Na+ was required to support cell growth at pH(out) below 7.5. An increase in Na+ concentration allowed the cells to grow at a more alkaline pH(out). Furthermore, cells containing more Na+ inside could more easily adapt to grow at alkaline pH(out). These results indicated the importance of Na+ in acidification of the cell interior via a Na+/H+ antiporter in order to support cell growth at alkaline pH(out) under conditions where the activity of a K+/H+ antiporter is marginal.     

Relationship of hepatic cholate transport to regulation of intracellular pH and potassium.

Modulation of hepatic cholate transport by transmembrane pH-gradients and during interferences with the homeostatic regulation of intracellular pH and K+ was studied in the isolated perfused rat liver. Within the concentration range studied uptake into the liver was saturable and appeared to be associated with release of OH- and uptake of K+. Perfusate acidification ineffectually stimulated uptake. Application of NH4Cl caused intracellular alkalinization, release of K+ and stimulation of cholate uptake, withdrawal of NH4Cl resulted in intracellular acidification, regain of K+ and inhibition of cholate uptake. Inhibition of Na+/H(+)-exchange with amiloride reduced basal release of acid equivalents into the perfusate, initiated K(+)-release, and inhibited both, control cholate uptake and its recovery following intracellular acidification. K(+)-free perfusion caused K(+)-release and inhibited cholate uptake. K(+)-readmission resulted in brisk K(+)-uptake and recovery of cholate transport. Both effects were inhibited by amiloride. Interference with cholate transport through modulation of pH homeostasis by diisothiocyanostilbenedisulfonate (DIDS) could not be demonstrated because DIDS affected bile acid transport directly. Biliary bile acid secretion was stimulated by intracellular alkalinization and by activation of K(+)-transport. Uncoupling of the mutual interference between pH-dependent cholate uptake and K(+)-transport by amiloride indicates tertiary active transport of cholate. In this, Na+/K(+)-ATPase provides the transmembrane Na(+)-gradient to sustain Na+/H(+)-exchange which maintains the transmembrane pH-gradient and thus supports cholate uptake. Effects of canalicular bile acid secretion are consistent with a saturable, electrogenic transport.     

Na+,K(+)-ATPase phosphorylation in the choroid plexus: synergistic regulation by serotonin/protein kinase C and isoproterenol/cAMP-PK/PP-1 pathways.

BACKGROUND: The ion pump Na+,K(+)-ATPase is responsible for the secretion of cerebrospinal fluid from the choroid plexus. In this tissue, the activity of Na+,K(+)-ATPase is inhibited by serotonin via stimulation of protein kinase C-catalyzed phosphorylation. The choroid plexus is highly enriched in two phosphoproteins which act as regulators of protein phosphatase-1 activity, DARPP-32 and inhibitor-1. Phosphorylation catalyzed by cAMP-dependent protein kinase on a single threonyl residue converts DARPP-32 and inhibitor-1 into potent inhibitors of protein phosphatase-1. Previous work has shown that in the choroid plexus, phosphorylation of DARPP-32 and I-1 is enhanced by isoproterenol and other agents that activate cAMP-PK. We have now examined the possible involvement of the cAMP-PK/protein phosphatase-1 pathway in the regulation of Na+,K(+)-ATPase. MATERIALS AND METHODS: The state of phosphorylation of Na+,K(+)-ATPase was measured by determining the amount of radioactivity incorporated into the ion pump following immunoprecipitation from 32P-prelabeled choroid plexuses incubated with various drugs (see below). Two-dimensional phosphopeptide mapping was employed to identify the protein kinase involved in the phosphorylation of Na+,K(+)-ATPase. RESULTS: The serotonin-mediated increase in Na+,K(+)-ATPase phosphorylation is potentiated by okadaic acid, an inhibitor of protein phosphatases-1 and -2A, as well as by forskolin or the beta-adrenergic agonist, isoproterenol, activators of cAMP-dependent protein kinase. Two-dimensional phosphopeptide maps suggest that this potentiating action occurs at the level of a protein kinase C phosphorylation site. Forskolin and isoproterenol also stimulate the phosphorylation of DARPP-32 and protein phosphatase inhibitor-1, which in their phosphorylated form are potent inhibitors of protein phosphatase-1. CONCLUSIONS: The results presented here support a model in which okadaic acid, forskolin, and isoproterenol achieve their synergistic effects with serotonin through phosphorylation of DARPP-32 and inhibitor-1, inhibition of protein phosphatase-1, and a reduction of dephosphorylation of Na+,K(+)-ATPase at a protein kinase C phosphorylation site.     

Monoclonal antibody HK4001 completely inhibits K(+)-dependent ATP hydrolysis and H+ transport of hog gastric H+,K(+)-ATPase

A monoclonal antibody (designated as HK4001) was prepared against hog gastric H+,K(+)-ATPase. It dose-dependently inhibited the H+,K(+)-ATPase activity, formation of the K(+)-sensitive phosphoenzyme, and proton uptake into gastric vesicles. The H+,K(+)-ATPase activity was completely inhibited by addition of the antibody at a molar ratio of 1:2 (antibody/catalytic subunit) at pH 7.8. The maximal inhibition decreased with decrease in pH of the medium (7.8 greater than 7.4 greater than 6.2). The Fab fragment obtained by digestion of the antibody with papain was also inhibitory. The antibody did not inhibit the K(+)-dependent p-nitrophenylphosphatase or the labeling of the enzyme with fluorescein isothiocyanate. It inhibited gastric H+,K(+)-ATPase from rabbits and rats, but did not cross-react with related cation-transport ATPases (Na+,K(+)-ATPase or Ca2(+)-ATPase) or H(+)-ATPase in the multivesicular body. From these and related findings, the antibody was suggested to recognize a highly specific site on the cytosolic surface of H+,K(+)-ATPase. The conformation of the epitope was conserved after treatment with Triton X-100, but not sodium dodecyl sulfate. In addition, judging from the stoichiometry of inactivation of H+,K(+)-ATPase by this antibody, the functional unit of H+,K(+)-ATPase was suggested to be a dimer or a tetramer (not a trimer) of the catalytic unit.     

Distinguishing roles of the membrane-cytoskeleton and cadherin mediated cell-cell adhesion in generating different Na+,K(+)-ATPase distributions in polarized epithelia

In simple epithelia, the distribution of ion transporting proteins between the apical or basal-lateral domains of the plasma membrane is important for determining directions of vectorial ion transport across the epithelium. In the choroid plexus, Na+,K(+)-ATPase is localized to the apical plasma membrane domain where it regulates sodium secretion and production of cerebrospinal fluid; in contrast, Na+,K(+)-ATPase is localized to the basal-lateral membrane of cells in the kidney nephron where it regulates ion and solute reabsorption. The mechanisms involved in restricting Na+,K(+)-ATPase distribution to different membrane domains in these simple epithelia are poorly understood. Previous studies have indicated a role for E-cadherin mediated cell-cell adhesion and membrane-cytoskeleton (ankyrin and fodrin) assembly in regulating Na+,K(+)-ATPase distribution in absorptive kidney epithelial cells. Confocal immunofluorescence microscopy reveals that in chicken and rat choroid plexus epithelium, fodrin, and ankyrin colocalize with Na+,K(+)-ATPase at the apical plasma membrane, but fodrin, ankyrin, and adducin also localize at the lateral plasma membrane where Na+,K(+)- ATPase is absent. Biochemical analysis shows that fodrin, ankyrin, and Na+,K(+)-ATPase are relatively resistant to extraction from cells in buffers containing Triton X-100. The fractions of Na+,K(+)-ATPase, fodrin, and ankyrin that are extracted from cells cosediment in sucrose gradients at approximately 10.5 S. Further separation of the 10.5 S peak of proteins by electrophoresis in nondenaturing polyacrylamide gels revealed that fodrin, ankyrin, and Na+,K(+)-ATPase comigrate, indicating that these proteins are in a high molecular weight complex similar to that found previously in kidney epithelial cells. In contrast, the anion exchanger (AE2), a marker protein of the basal- lateral plasma membrane in the choroid plexus, did not cosediment in sucrose gradients or comigrate in nondenaturing polyacrylamide gels with the complex of Na+,K(+)-ATPase, ankyrin, and fodrin. Ca(++)- dependent cell adhesion molecules (cadherins) were detected at lateral membranes of the choroid plexus epithelium and colocalized with a distinct fraction of ankyrin, fodrin, and adducin. Cadherins did not colocalize with Na+,K(+)-ATPase and were absent from the apical membrane. The fraction of cadherins that was extracted with buffers containing Triton X-100 cosedimented with ankyrin and fodrin in sucrose gradients and comigrated in nondenaturing gels with ankyrin and fodrin in a high molecular weight complex. Since a previous study showed that E-cadherin is an instructive inducer of Na+,K(+)-ATPase distribution, we examined protein distributions in fibroblasts transfected with B- cadherin, a prominent cadherin expressed in the choroid plexus epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)     

Response of alkalinization or acidification by phytohemagglutinin is dependent on the activity of protein kinase C in human peripheral T Cells

The increase of intracellular free calcium concentration ([Ca(2+)](i)) and protein kinase C (PKC) activity are two major early mitogenic signals to initiate proliferation of human T cells. However, a rapid change in intracellular pH (pH(i)), acidification or alkalinization during the activation, is also associated after these two signals. The aim of this study was to define whether the change in pH(i) is affected by calcium and protein kinase C (PKC), in phytohemagglutinin (PHA)-stimulated T cells. T cells were isolated from human peripheral blood. The [Ca(2+)](i) and the pH(i) were measured using, respectively, the fluorescent dyes, Fura-2, and BCECF. In addition, down-regulation of PKC activity by PMA (1 microM, 18 h) was confirmed in these cells using a protein kinase assay. The results indicated that, (1) alkalinization was induced by PHA or PMA in T cells; the results of alkalinization was PKC-dependent and Ca(2+)-independent, (2) in PKC down-regulated T cells, PHA induced acidification; this effect was enhanced by pre-treating the cells with the Na(+)/H(+) exchange inhibitor, 5-(N,N-dimethyl)-amiloride, (DMA, 10 microM, 20 min), (3) the acidification was dependent on the Ca(2+) influx and blocked by removal of extracellular calcium or the addition of the inorganic channel blocker, Ni(2+), and (4) Thapsigargin (TG), a Ca(2+)-ATPase inhibitor, confirmed that acidification by the Ca(2+) influx occurred in T cells in which PKC was not down-regulated. These findings indicate two mechanisms, alkalinization by PKC and acidification by Ca(2+) influx, exist in regulating pH(i) in T cells. This is the first report that PHA stimulates the acidification by Ca(2+) influx but not alkalinization in T cells after down-regulation of PKC. In conclusion, the activity of PKC in T cells determines the response in alkalinization or acidification by PHA.     

Characterization of gill (Na+ + K+)-ATPase in the sea bass (Dicentrarchus labrax L.)

Bass gill microsomal preparations contain both a Na+, K+ and Mg2+-dependent ATPase, which is completely inhibited by 10(-3)M ouabain and 10(-2)M Ca2+, and also a ouabain insensitive ATP-ase activity in the presence of both Mg2+ and Na+. Under the optimal conditions of pH 6.5, 100 mM Na+, 20 mM K+, 5 mM ATP and 5 mM Mg2+, (Na+ + K+)-ATPase activity at 30 degrees C is 15.6 mumole Pi hr/mg protein. Bass gill (Na+ + K+)-ATPase is similar to other (Na+ + K+)-ATPases with respect to the sensitivity to ionic strength, Ca2+ and ouabain and to both Na+/K+ and Mg2+/ATP optimal ratios, while pH optimum is lower than poikilotherm data. The enzyme requires Na+, whereas K+ can be replaced efficiently by NH+4 and poorly by Li+. Both Km and Vm values decrease in the series NH+4 greater than K+ greater than Li+. The break of Arrhenius plot at 17.7 degrees C is close to the adaptation temperature. Activation energies are scarcely different from each other and both lower than those generally reported. The Km for Na+ poorly decreases as the assay temperature lowers. The comparison with literature data aims at distinguishing between distinctive and common features of bass gill (Na+ + K+)-ATPase.     

Sodium-potassium pump inhibitor in the mechanism of one-kidney, one wrap hypertension in dogs.

Using ouabain sensitive 86Rb uptake by the vessel wall, we previously showed that sodium-potassium pump activity is decreased in the arteries and veins, and that the sodium-potassium pump inhibitor (SPI) is increased in the plasma of dogs with one-kidney, one wrap (1-K, 1W) hypertension, a low renin model of hypertension. We also showed in rats with a similar type of hypertension that the membrane potential of vascular smooth muscle cells in arteries is decreased, and that this decrease can be reproduced in arterial cells in arteries from normal rats by applying plasma from the hypertensive animals. One endogenous SPI in human plasma has been reported to be ouabain or its isomer. In this study, we used a newly available Dupont ouabain enzyme immunoassay kit to examine plasma and kidneys for SPI in dogs with 1-K, 1W hypertension. We also examined 1) the inhibiting activity of plasma of Na+, K(+)-ATPase obtained from normal kidneys, and 2) the Na+, K(+)-ATPase activity of the kidneys from these hypertensive animals. 1-K, 1W hypertension was produced in dogs by wrapping the left kidney in a silk bag and removing the right kidney. The removed kidney was kept at -70 degrees C till assayed. After 4 weeks of hypertension, the remaining kidney was removed and stored at -70 degrees C till assayed. Blood samples were drawn before and at weeks 3 and 4 of hypertension. Plasma levels of "ouabain" and Na+, K(+)-ATPase inhibitory activity were increased at weeks 3 and 4 of hypertension, compared to pre-hypertension levels. Renal tissue "ouabain" levels were also increased at week 4 of hypertension. However, renal Na+, K(+)-ATPase activity was unchanged. These findings, using two different assays, confirm our 1980 conclusion that SPI is elevated in the plasma of dogs with 1-K, 1W hypertension. The absence of renal Na+, K(+)-ATPase inhibition, despite increased plasma and renal SPI in these animals, may have important implications for the development of this type of hypertension.     

AS160 associates with the Na+,K+-ATPase and mediates the adenosine monophosphate-stimulated protein kinase-dependent regulation of sodium pump surface expression

The Na(+),K(+)-ATPase is the major active transport protein found in the plasma membranes of most epithelial cell types. The regulation of Na(+),K(+)-ATPase activity involves a variety of mechanisms, including regulated endocytosis and recycling. Our efforts to identify novel Na(+),K(+)-ATPase binding partners revealed a direct association between the Na(+),K(+)-ATPase and AS160, a Rab-GTPase-activating protein. In COS cells, coexpression of AS160 and Na(+),K(+)-ATPase led to the intracellular retention of the sodium pump. We find that AS160 interacts with the large cytoplasmic NP domain of the α-subunit of the Na(+),K(+)-ATPase. Inhibition of the activity of the adenosine monophosphate-stimulated protein kinase (AMPK) in Madin-Darby canine kidney cells through treatment with Compound C induces Na(+),K(+)-ATPase endocytosis. This effect of Compound C is prevented through the short hairpin RNA-mediated knockdown of AS160, demonstrating that AMPK and AS160 participate in a common pathway to modulate the cell surface expression of the Na(+),K(+)-ATPase.     

Immunocytochemical localization of Na+,K(+)-ATPase in rat retinal pigment epithelial cells

We investigated quantitatively the ultrastructural localization of the alpha-subunit of Na+,K(+)-ATPase in rat retinal pigment epithelial cells by the protein A-gold technique, using an affinity-purified antibody against the alpha-subunit of rat kidney Na+,K(+)-ATPase. Immunoblot analysis showed that the antibody bound specifically to the alpha- and alpha(+)-subunits of Na+,K(+)-ATPase in the whole retina [the sensory retina plus retinal pigment epithelium (RPE)]. Rat eyes were fixed by perfusion with 4% paraformaldehyde containing 1% glutaraldehyde and embedded in Lowicryl K4M. Ultra-thin sections were incubated with affinity-purified antibody against the alpha-subunit of rat kidney Na+,K(+)-ATPase and subsequently with protein A-gold complex. Light microscopy with a silver enhancement procedure revealed Na+,K(+)-ATPase localized to both the apical and the basal plasma membrane domains of the RPE. Quantitative immunocytochemical analysis by electron microscopy showed a higher density of gold particles on the apical surface than on the basolateral one. Microvilli are so well developed on the apical surface of the RPE that the apical surface profile is much longer than the basolateral one. This means that Na+,K(+)-ATPase is mainly located on the apical surface of the RPE cells.     

Aldosterone-mediated regulation of Na+, K(+)-ATPase gene expression in adult and neonatal rat cardiocytes.

By altering the Na+/K+ electrochemical gradient, Na+,K(+)-ATPase activity profoundly influences cardiac cell excitability and contractility. The recent finding of mineralocorticoid hormone receptors in the heart implies that Na+,K(+)-ATPase gene expression, and hence cardiac function, is regulated by aldosterone, a corticosteroid hormone associated with certain forms of hypertension and classically involved in regulating Na+,K(+)-ATPase gene expression and transepithelial Na+ transport in tissues such as the kidney. The regulation by aldosterone of the major cardiac Na+,K(+)-ATPase isoform genes, alpha-1 and beta-1, were studied in adult and neonatal rat ventricular cardiocytes grown in defined serum-free media. In both cell types, aldosterone-induced a rapid and sustained 3-fold induction in alpha-1 mRNA accumulation within 6 h. beta-1 mRNA was similarly induced. alpha-1 mRNA induction occurred over the physiological range with an EC50 of 1-2 nM, consistent with binding of aldosterone to the high affinity mineralocorticoid hormone receptor. In adult cardiocytes, this was associated with a 36% increase in alpha subunit protein accumulation and an increase in Na(+)-K(+)-ATPase transport activity. Aldosterone did not alter the 3-h half-life of alpha-1 mRNA, indicating an induction of alpha-1 mRNA synthesis. Aldosterone-dependent alpha-1 mRNA accumulation was not blocked by the protein synthesis inhibitor cycloheximide, whereas amiloride inhibited both an aldosterone-dependent increase in intracellular Na+ [Na+]i) and alpha-1 mRNA accumulation. This demonstrates that aldosterone directly stimulates Na+,K(+)-ATPase alpha-1 subunit mRNA synthesis and protein accumulation in cardiac cells throughout development and suggests that the heart is a mineralocorticoid-responsive organ. An early increase in [Na+]i may be a proximal event in the mediation of the hormone effect.     

Mutant Phe788 --> Leu of the Na+,K+-ATPase is inhibited by micromolar concentrations of potassium and exhibits high Na+-ATPase activity at low sodium concentrations.

Mutant Phe788 --> Leu of the rat kidney Na+,K(+)-ATPase was expressed in COS cells to active-site concentrations between 40 and 60 pmol/mg of membrane protein. Analysis of the functional properties showed that the discrimination between Na+ and K+ on the two sides of the system is severely impaired in the mutant. Micromolar concentrations of K+ inhibited ATP hydrolysis (K(0.5) for inhibition 107 microM for the mutant versus 76 mM for the wild-type at 20 mM Na+), and at 20 mM K+, the molecular turnover number for Na+,K(+)-ATPase activity was reduced to 11% that of the wild-type. This inhibition was counteracted by Na+ in high concentrations, and in the total absence of K+, the mutant catalyzed Na(+)-activated ATP hydrolysis ("Na(+)-ATPase activity") at an extraordinary high rate corresponding to 86% of the maximal Na+,K(+)-ATPase activity. The high Na(+)-ATPase activity was accounted for by an increased rate of K(+)-independent dephosphorylation. Already at 2 mM Na+, the dephosphorylation rate of the mutant was 8-fold higher than that of the wild-type, and the maximal rate of Na(+)-induced dephosphorylation amounted to 61% of the rate of K(+)-induced dephosphorylation. The cause of the inhibitory effect of K+ on ATP hydrolysis in the mutant was an unusual stability of the K(+)-occluded E2(K2) form. Hence, when E2(K2) was formed by K+ binding to unphosphorylated enzyme, the K(0.5) for K+ occlusion was close to 1 microM in the mutant versus 100 microM in the wild-type. In the presence of 100 mM Na+ to compete with K+ binding, the K(0.5) for K+ occlusion was still 100-fold lower in the mutant than in the wild-type. Moreover, relative to the wild-type, the mutant exhibited a 6-7-fold reduced rate of release of occluded K+, a 3-4-fold increased apparent K+ affinity in activation of the pNPPase reaction, a 10-11-fold lower apparent ATP affinity in the Na+,K(+)-ATPase assay with 250 microM K+ present (increased K(+)-ATP antagonism), and an 8-fold reduced apparent ouabain affinity (increased K(+)-ouabain antagonism).     

Purification of Na+,K+-ATPase expressed in Pichia pastoris reveals an essential role of phospholipid-protein interactions

Na+,K+-ATPase (porcine alpha/his10-beta) has been expressed in Pichia Pastoris, solubilized in n-dodecyl-beta-maltoside and purified to 70-80% purity by nickel-nitrilotriacetic acid chromatography combined with size exclusion chromatography. The recombinant protein is inactive if the purification is done without added phospholipids. The neutral phospholipid, dioleoylphosphatidylcholine, preserves Na+,K+-ATPase activity of protein prepared in a Na+-containing medium, but activity is lost in a K+-containing medium. By contrast, the acid phospholipid, dioleoylphosphatidylserine, preserves activity in either Na+- or K+-containing media. In optimal conditions activity is preserved for about 2 weeks at 0 degrees C. Both recombinant Na+,K+-ATPase and native pig kidney Na+,K+-ATPase, dissolved in n-dodecyl-beta-maltoside, appear to be mainly stable monomers (alpha/beta) as judged by size exclusion chromatography and sedimentation velocity. Na+,K+-ATPase activities at 37 degrees C of the size exclusion chromatography-purified recombinant and renal Na+,K+-ATPase are comparable but are lower than that of membrane-bound renal Na+,K+-ATPase. The beta subunit is expressed in Pichia Pastoris as two lightly glycosylated polypeptides and is quantitatively deglycosylated by endoglycosidase-H at 0 degrees C, to a single polypeptide. Deglycosylation inactivates Na+,K+-ATPase prepared with dioleoylphosphatidylcholine, whereas dioleoylphosphatidylserine protects after deglycosylation, and Na+,K+-ATPase activity is preserved. This work demonstrates an essential role of phospholipid interactions with Na+,K+-ATPase, including a direct interaction of dioleoylphosphatidylserine, and possibly another interaction of either the neutral or acid phospholipid. Additional lipid effects are likely. A role for the beta subunit in stabilizing conformations of Na+,K+-ATPase (or H+,K+-ATPase) with occluded K+ ions can also be inferred. Purified recombinant Na+,K+-ATPase could become an important experimental tool for various purposes, including, hopefully, structural work.