Aplysiallene,a New Bromoallene as an Na,K-ATPase Inhibitor from the Sea Hare,Aplysia kurodai

A new bromoallene metabolite, named aplysiallene, was isolated from the Japanese sea hare, Aplysia kurodai, as an Na,K-ATPase inhibitor. Its structure was elucidated by spectroscopic methods. The known metabolites, laurinterol and debromolaurinterol, isolated from this animal were also evaluated for their Na,K-ATPase inhibitory activity.     

Insulin Sensitivity of Rat Muscle Sodium Pump

The aim of the present study was to examine the possible physiological responsiveness of the sodium pump to insulin in rat muscle, an effect that has never been convincingly demonstrated. The insulin stimulation of the sodium pump was estimated by two well-established parameters: ouabain binding to pieces of soleus muscle, and NalK-ATPase activity of purified plasma membranes. For both parameters the dose dependence of the insulin effect on the sodium pump shows the characteristic bell-shaped stimulation pattern, with a maximum in the physiological hormone concentration range. This result has not been observed in previous studies where insulin concentrations two to three orders of magnitude higher were used. It can be concluded that an effect of insulin on the regulation of the Na pump in muscle might well be operating in vivo     

Dopamine Oxidation Products Inhibit Na+, K+-ATPase Activity in Crude Synaptosomal–mitochondrial Fraction from Rat Brain

The diverse damaging effects of dopamine (DA) oxidation products on brain subcellular components including mitochondrial electron transport chain have been implicated in dopaminergic neuronal death in Parkinson's disease. It has been shown in this study that DA (50–200?μM) causes dose-dependent inhibition of Na⁺, K⁺-ATPase activity of rat brain crude synaptosomal–mitochondrial fraction during in vitro incubation up to 2?h. The enzyme inactivation is prevented by catalase and the metal-chelator (diethylenetriamine penta-acetic acid) but not by superoxide dismutase or hydroxyl-radical scavengers like mannitol and dimethylsulphoxide (DMSO). Further, reduced glutathione and cysteine, markedly prevent DA-mediated inactivation of Na⁺, K⁺-ATPase. Under similar conditions of incubation, DA (200?μM) leads to the formation of quinoprotein adducts (protein-cysteinyl catechol) with synaptosomal–mitochondrial proteins and the phenomenon is also prevented by glutathione (5?mM) or cysteine (5?mM).

The available data imply that the inactivation of Na⁺, K⁺-ATPase in this system involves both H²O² and metal ions. The reactive quinones by forming adducts with protein thiols also probably contribute to the process, since reduced glutathione and cysteine which scavenge quinones from the system protect Na⁺, K⁺-ATPase from DA-mediated damage. The inactivation of neuronal Na⁺, K⁺-ATPase by DA may give rise to various toxic sequelae with potential implications for dopaminergic cell death in Parkinson's disease.     

Selectivity analyses of γ-benzylidene digoxin derivatives to different Na,K-ATPase α isoforms: a molecular docking approach

Digoxin and other cardiotonic steroids (CTS) exert their effect by inhibiting Na,K-ATPase (NKA) activity. CTS bind to the various NKA isoforms that are expressed in different cell types, which gives CTS their narrow therapeutic index. We have synthesised a series of digoxin derivatives (γ-Benzylidene digoxin derivatives) with substitutions in the lactone ring (including non-oxygen and ether groups), to obtain CTS with better NKA isoform specificity. Some of these derivatives show some NKA isoform selective effects, with BD-3, BD-8, and BD-13 increasing NKA α2 activity, BD-5 inhibiting NKA α1 and NKA α3, BD-10 reducing NKA α1, but stimulating NKA α2 and α3; and BD-14, BD-15, and BD-16 enhancing NKA α3 activity. A molecular-docking approach favoured NKA isoform specific interactions for the compounds that supported their observed activity. These results show that BD compounds are a new type of CTS with the capacity to target NKA activity in an isoform-specific manner.     

Role of Apical H-K Exchange and Basolateral K Channel in the Regulation of Intracellular pH in Rat Distal Colon Crypt Cells

An apical membrane ouabain-sensitive H-K exchange and a barium-sensitive basolateral membrane potassium channel are present in colonic crypt cells and may play a role in both K absorption and intracellular pH (pH_i) regulation. To examine the possible interrelationship between apical membrane H-K exchange and basolateral membrane K movement in rat distal colon in the regulation of pH_i, experiments were designed to assess whether changes in extracellular potassium can alter pH_i. pH_i in isolated rat crypts was determined using microspectrofluorimetric measurements of the pH-sensitive dye BCECF-AM (2′,7′-bis(carboxyethyl-5(6)-carboxy-fluorescein acetoxy methylester). After loading with the dye, crypts were superfused with a Na-free solution which resulted in a rapid and reversible fall in pH_i (7.36 ± 0.02 to 6.98 ± 0.03). Following an increase in extracellular [K] to 20 mm, in the continued absence of Na, there was a further decrease in pH_i (0.20 ± 0.02, P < 0.01). K-induced acidification was blocked both by 2 mm bath barium, a K channel blocker, and by 0.5 mm lumen ouabain. K-induced acidification was also observed when intracellular acidification was induced by a NH₄Cl prepulse. These observations suggest that increased basolateral K movement increases intracellular [K] resulting in a decrease in pH_i that is mediated by a ouabain-sensitive apical membrane H,K-ATPase. Our results demonstrate an interrelationship between basolateral K movement and apical H-K exchange in the regulation of pH_i and apical K entry in rat distal colon. Received: 31 March 1998/Revised: 8 September 1998     

Estradiol modulates the sodium pump in the heart sarcolemma

Cardiovascular effects of estrogens and particularly that of estradiol involve protection of the heart against ischemia. These effects were believed to be mainly indirect, mediated via changes in the blood and blood vessels. In the present paper a direct action of estradiol on the heart is demonstrated. Estradiol stimulates (p < 0.001) the Na,K-ATPase activity of cardiac sarcolemmal membranes by stimulating in an allosteric manner, the activation of the enzyme by potassium. The latter activation involves also an increase in affinity to potassium of the potassium binding sites on the enzyme molecule, but remains without any effect on the capacity and K_Dvalue of specific ouabain binding to the Na,K-ATPase. Estradiol is also antagonizing the depression of Na,K-ATPase activity that may be caused by ischemia and it is stimulating (p < 0.01) the ouabain-sensitive uptake of ⁸⁶Rb into the heart cells.Our results indicate, that in addition to the known indirect effects of estradiol on the heart, the hormone also stimulates the activity and improves the kinetics of interaction of cardiac sarcolemmal Na,K-ATPase with ATP as well as with Na⁺ and K⁺ ions. This direct action may also account for the cardioprotective effects of estradiol.     

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 and subunits of Na,K-ATPase are the most abundant glycoproteins. Further,Datura stramonium andGalanthus 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.Abbreviations NKA Na,K-ATPase (EC 3.6.1.37) - PAGE polyacrylamide gel electrophoresis in dodecylsulfate - Con-A Concanavalin A - DSA Datura stramonium agglutinin - GNA Galanthus nivalis agglutinin - MAA Maackia amurensis agglutinin - PNA Peanut agglutinin - SNA Sambucus nigra agglutinin - WGA Wheat germ agglutinin Abbreviations used in figures K kidney - B brain - Cr Crude - De Detergent-treated - Fe fetuin - Ct creatinase - I-blot immuno-blot - L-blot lectin-blot     

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 (EC 3.6.1.37) 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 °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.Abbreviations BCA bicinchonic acid - D-PBS Dulbecco's Phosphate Buffered Saline - HBSS Hanks' Balanced Salt Solution - PYX palytoxin (used in figures only) - NKA Na,K-ATPase     

Fast charge translocations associated with partial reactions of the Na,K-pump: I. Current and voltage transients after photochemical release of ATP

Summary Nonstationary electric currents are described which are generated by the Na,K-pump. Flat membrane sheets 0.2–1 m in diameter containing a high density of oriented N,K-ATPase molecules are bound to a planar lipid bilayer acting as a capacitive electrode. In the aqueous phase adjacent to the bound membrane sheets, ATP is released within milliseconds from an inactive, photolabile precursor (caged ATP) by an intense flash of light. After the ATP-concentration jump, transient current and voltage signals can be recorded in the external circuit corresponding to a translocation of positive charge across the pump protein from the cytoplasmic to the extracellular side. These electrical signals which can be suppressed by inhibitors of the Na,K-ATPase require the presence of Na⁺ but not of K⁺ in the aqueous medium. The intrinsic pump currentI _p (t) can be evaluated from the recorded current signal, using estimated values of the circuit parameters of the compound membrane system.I _p (t) exhibits a biphasic behavior with a fast rising period, followed by a slower decline towards a small quasistationary current. The time constant of the rising phase ofI _p (t) is found to depend on the rate of photochemical ATP release. Further information on the microscopic orgin of the current transient can be obtained by double-flash experiments and by chymotrypsin modification of the protein. These and other experiments indicate that the observed charge-translocation is associated with early events in the normal transport cycle. After activation by ATP, the pump goes through the first steps of the cycle and then enters a long-lived state from which return to the initial state is slow.