Sodium-dependent silicate transport in the apochlorotic marine diatom Nitzschia alba

Silicate uptake by Nitzschia alba cells is higher in medium containing Na⁺ than in media lacking Na⁺ but containing K⁺, Rb⁺, NH₄⁺, Li⁺, or choline⁺. The initial rate is inhibited by monensin and gramicidin but not by valinomycin or nigericin and is less sensitive to inhibition by carbonyl cyanide m-chlorophenylhydrazone (CCCP). In isolated membrane vesicles, silicate is taken up when a Na⁺ gradient is imposed across the membrane or is generated by cytoplasmic Na⁺,K⁺-ATPase. H⁺ or K⁺ gradients in either direction do not stimulate uptake. Na⁺-gradient-dependent uptake is inhibited by monensin but not by CCCP, valinomycin, or vanadate, which inhibits the cytoplasmic Na⁺,K⁺-ATPase. Uptake increases if an internally negative potential is imposed across the membrane. The vesicular uptake shows saturation kinetics with a K_m of 62 μM and a V_max of 4.1 nmol/mg of protein per min. In intact cells, the initial rate of silicate uptake increases with pH up to 9.5. Thus, in N. alba, silicate is symported with Na⁺, and the transport system is driven by the Na⁺ gradient that is generated and maintained across the membrane by the activity of Na⁺,K⁺-ATPase.     

Relative binding affinities of monovalent cations for double-stranded DNA

The competition between sodium and various other monovalent cations that bind to helical DNA in aqueous solution has been studied by ²³Na NMR. Variations in the sodium linewidth with the concentration of the other ion have been analyzed with an equation that describes the competitive binding in terms of two parameters: r, the total extent of counterion binding, and D, a measure of the binding affinity of a cation relative to sodium. The concentration dependence of these parameters was found to be minimal. In the absence of a competing cation the constancy of r has been demonstrated over a range of DNA phosphate concentrations (0.0025-0.015 M) and NaCl concentrations (0.003-1.3 M). For the cations investigated the range in D values is small (0.5-0.9), and the relative binding affinities follow the order: NH₄⁺ > Cs⁺ > K⁺ > Li⁺ > Na⁺.     

Crystal structure of the high-affinity Na+,K+-ATPase–ouabain complex with Mg2+ bound in the cation binding site

The Na⁺,K⁺-ATPase maintains electrochemical gradients for Na⁺ and K⁺ that are critical for animal cells. Cardiotonic steroids (CTSs), widely used in the clinic and recently assigned a role as endogenous regulators of intracellular processes, are highly specific inhibitors of the Na⁺,K⁺-ATPase. Here we describe a crystal structure of the phosphorylated pig kidney Na⁺,K⁺-ATPase in complex with the CTS representative ouabain, extending to 3.4 Å resolution. The structure provides key details on CTS binding, revealing an extensive hydrogen bonding network formed by the β-surface of the steroid core of ouabain and the side chains of αM1, αM2, and αM6. Furthermore, the structure reveals that cation transport site II is occupied by Mg²⁺, and crystallographic studies indicate that Rb⁺ and Mn²⁺, but not Na⁺, bind to this site. Comparison with the low-affinity [K₂]E2–MgF_x–ouabain structure [Ogawa et al. (2009) Proc Natl Acad Sci USA 106(33):13742–13747) shows that the CTS binding pocket of [Mg]E2P allows deep ouabain binding with possible long-range interactions between its polarized five-membered lactone ring and the Mg²⁺. K⁺ binding at the same site unwinds a turn of αM4, dragging residues Ile318–Val325 toward the cation site and thereby hindering deep ouabain binding. Thus, the structural data establish a basis for the interpretation of the biochemical evidence pointing at direct K⁺–Mg²⁺ competition and explain the well-known antagonistic effect of K⁺ on CTS binding.     

Nigericin-stimulated ATPase activity in microsomal vesicles of tobacco callus

The effect of ionophores on K⁺-stimulated adenosinetriphosphatase (ATPase; ATP phosphohydrolase, EC 3.6.1.3) activity of microsomal vesicles from tobacco callus was investigated. A nigericin-stimulated K⁺-ATPase activity was enriched in a purified microsomal fraction, which was obtained from the interphase of a dextran density gradient between 1.03 and 1.06 g/ml. The purified microsomal fraction was free of mitochondrial membranes and was composed partly of tightly sealed vesicles as indicated by the low K⁺ permeability coefficient. The K⁺-dependent ATPase of this fraction was stimulated slightly by either carbonyl cyanide m-chlorophenylhydrazone (CCCP) (29%) or valinomycin (31%) alone; this ATPase was significantly stimulated by a combination of CCCP and valinomycin (73%) or by nigericin alone (80%). The K⁺-ATPase activity was stimulated by nigericin at pH 6.5 but not at pH 8.5. At pH 6.5, the K⁺-ATPase was inhibited by N,N′-dicyclohexylcarbodiimide but not by oligomycin. Nigericin stimulated the ATPase activity in the absence of initial KCl or pH gradients across the vesicle membrane. These results suggest that nigericin stimulates the ATPase activity by dissipating the H⁺ or K⁺ gradient or both, and support the hypothesis that the K⁺-ATPase mediates a H⁺/K⁺ transport.     

Characterization of the Cation‐Binding Capacity of a Potassium‐Adsorption Filter Used in Red Blood Cell Transfusion

A K⁺‐adsorption filter was developed to exchange K⁺ in the supernatant of stored irradiated red blood cells with Na⁺. To date, however, the filter's adsorption capacity for K⁺ has not been fully evaluated. Therefore, we characterized the cation‐binding capacity of this filter. Artificial solutions containing various cations were continuously passed through the filter in 30 mL of sodium polystyrene sulfonate at 10 mL/min using an infusion pump at room temperature. The cation concentrations were measured before and during filtration. When a single solution containing K⁺, Li⁺, H⁺, Mg²⁺, Ca²⁺, or Al³⁺ was continuously passed through the filter, the filter adsorbed K⁺ and the other cations in exchange for Na⁺ in direct proportion to the valence number. The order of affinity for cation adsorption to the filter was Ca²⁺>Mg²⁺>K⁺>H⁺>Li⁺. In K⁺‐saturated conditions, the filter also adsorbed Na⁺. After complete adsorption of these cations on the filter, their concentration in the effluent increased in a sigmoidal manner over time. Cations that were bound to the filter were released if a second cation was passed through the filter, despite the different affinities of the two cations. The ability of the filter to bind cations, especially K⁺, should be helpful when it is used for red blood cell transfusion at the bedside. The filter may also be useful to gain a better understanding of the pharmacological properties of sodium polystyrene sulfonate.     

Kinetics of Red Cell Na+ and K+ Transport in Prague Hypertensive Rats

Kinetics of ouabain-sensitive, furosemide-sensitive (FS), bumetanide-sensitive (BS) and -resistant Na⁺ and K⁺ transport were studied in erythrocytes of Prague hypertensive rats (PHR) and Prague normotensive rats (PNR). Maximal transport rates (V_max) and apparent affinities for either intracellular Na⁺ or extracellular K⁺ (replaced by Rb⁺) were determined in red cells in which Na⁺ content varied around the physiological range and that were incubated in Na⁺ media. No major differences between PHR and PNR were disclosed in the kinetics of ion transport mediated by the Na⁺-K⁺ pump or BS inward Na⁺-K⁺ cotransport. FS Rb⁺ uptake was higher (due to a greater V_max) in red cells of PHR as compared to PNR. In cells with a lowered Na⁺ content this elevation of FS Rb⁺ uptake was largely due to an augmented K⁺-Cl^? cotransport which exhibits a low affinity for Rb⁺_o and is blocked by 1 mM furosemide but not by 10 μM bumetanide. Red cells of PHR and PNR strains did not differ in either Na⁺ or Rb⁺ leaks. A slight increase of red cell Na⁺ content in PHR was evaluated in terms of the pump-leak concept. The present study did not reveal any obvious kinetic abnormalities of red cell cation transport the presence of which in tissues involved in blood pressure regulation would favor the development or the maintenance of genetic hypertension in PHR.     

Regulation of active cation flux by vanadate in beating rat heart muscle cells in culture

Summary Looking for a supposed digitalis-like action of compounds of the trace element vanadium, we have investigated the influence of vanadate (Na₃VO₄) on beating and on active cation flux of [⁴²K⁺] and [⁸⁶Rb⁺] in cultured rat heart muscle cells:Na₃VO₄ (10^–6–10^–3M) exerts a positive chronotropic effect and increases contraction velocity and beating automaticity of the cells. Vanadate-induced alteration of beating is paralleled by stimulated uptake of [⁴²K⁺] and [⁸⁶Rb⁺] up to 75%. This stimulation has to be attributed to increased activity of (Na⁺ +K⁺)-ATPase and cannot solely be explained by the enhanced beating frequency. In contrast to ouabain, vanadate raises intracellular potassium content up to 15% and prevents cell contractures of ouabain-intoxicated heart muscle cells.The experimental data speak against a possible digitalis-like action of vanadate in cultured rat heart muscle cells.

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Regulation des aktiven Kationenflusses durch Vanadat in kontrahierenden Rattenherzmuskelzellen in Kultur

Zusammenfassung Im Hinblick auf eine vermutete digitalisähnliche Wirkung von Verbindungen des Spurenelementes Vanadium haben wir den Einfluß von Natriumvanadat (Na₃VO₄) auf Kontraktionsverhalten und aktiven Kationenfluß von [⁴²K⁺] und [⁸⁶Rb⁺] in kultivierten Rattenherzmuskelzellen untersucht:Na₃VO₄ (10^–6–10^–3 M) wirkt positiv chronotrop und erhöht die Kontraktionsge-schwindigkeit sowie die Automatieneigung der Zellen. Den Veränderungen des Kontraktionsverhaltens läuft eine gesteigerte Aufnahme ins Zellinnere von [⁴²K⁺] und [⁸⁶Rb⁺] (bis zu 75%) parallel. Diese Stimulation ist auf eine erhöhte Aktivität der (Na⁺+K⁺)-ATPase zurückzuführen und läßt sich nicht allein durch die erhöhte Kontraktionsfrequenz erklären. Im Gegensatz zu Herzglykosiden bewirkt Vanadat eine Steigerung des intrazellulären Kaliumgehaltes bis zu 15% und verhindert in g-Strophanthin-intoxikierten Zellen die Ausbildung von Kontrakturen.Die experimentellen Befunde sprechen gegen eine digitalisähnliche Wirkung von Vanadat in kultivierten Rattenherzmuskelzellen.

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The data have been presented at the Symposium Cardiac Effects of Vanadate, Munich, October 26–27, 1979.

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With 1 figure

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Supported by Wilhelm-Sander-Stiftung 14/1978-We 1 and Deutsche Forschungsgemeinschaft (ER 65/2).     

Potassium secretion in rat distal colon during dietary potassium loading: role of pH regulated apical potassium channels

Background—Chronicdietary K⁺ loading increases the abundance of largeconductance (210 pS) apical K⁺ channels in surface cells ofrat distal colon, resulting in enhanced K⁺ secretion inthis epithelium. However, the factors involved in the regulation ofthese K⁺ channels are at present unclear.
Aims—To evaluate theeffect of dietary K⁺ loading on intracellular pH and itsrelation to large conductance apical K⁺ channel activity insurface cells of rat distal colon.
Methods/Results—Asassessed by fluorescent imaging, intracellular pH was higher inK⁺ loaded animals (7.48 (0.09)) than in controls (7.07 (0.04); p<0.01) when surface cells were bathed in NaCl solution, and asimilar difference in intracellular pH was observed when cells werebathed in Na₂SO₄ solution (7.67 (0.09) and 6.92 (0.05) respectively; p<0.001). Ethylisopropylamiloride (EIPA; aninhibitor of Na⁺-H⁺ exchange; 1 µM) decreasedintracellular pH when surface cells from K⁺ loaded animalswere bathed in either solution, although the decrease was greater whenthe solution contained NaCl (ΔpH 0.50 (0.03)) rather thanNa₂SO₄ (ΔpH 0.18 (0.02); p<0.05). Incontrast, EIPA had no effect in cells from control animals. As assessedby patch clamp recording techniques, the activity of large conductance K⁺ channels in excised inside-out membrane patches fromdistal colonic surface cells of K⁺ loaded animals increasedtwofold when the bath pH was raised from 7.40 to 7.60. As assessed bycell attached patches in distal colonic surface cells fromK⁺ loaded animals, the addition of 1 µM EIPA decreasedK⁺ channel activity by 50%, consistent with reversal ofNa⁺-H⁺ exchange mediated intracellular alkalinisation.
Conclusion—Intracellularalkalinisation stimulates pH sensitive large conductance apicalK⁺ channels in rat distal colonic surface cells as part ofthe K⁺ secretory response to chronic dietary K⁺loading. Intracellular alkalinisation seems to reflect an increase inEIPA sensitive Na⁺-H⁺ exchange, which may be amanifestation of the secondary hyperaldosteronism associated with thismodel of colonic K⁺ adaptation.

     

Insulin stimulation of K+ uptake in 3T3-L1 fibroblasts involves phosphatidylinositol 3-kinase and protein kinase C-zeta

Summary Despite the important physiological role of insulin in the regulation of ionic homeostasis, primarily mediated by the Na⁺/K⁺-ATPase and Na⁺/K⁺/2Cl^– cotransporter, the intracellular signalling molecules mediating this effect of insulin have not been elucidated. Treatment of 3T3-L1 fibroblasts with insulin increased total ⁸⁶Rb⁺ (K⁺) uptake from 0.8 ± 0.04 to 1.02 ± 0.05 nmol · mg^–1· protein^–1· min^–1 (p < 0.005). These changes in K⁺ flux, though small, can alter the membrane potential. Uptake occurred through both the Na⁺/K⁺-ATPase and Na⁺/K⁺/2Cl^– cotransporter and both were stimulated by insulin. Interestingly, when bumetanide was used to inhibit the Na⁺/K⁺/2Cl^– cotransporter prior to insulin action, no increase in ⁸⁶Rb⁺ uptake via the Na⁺/K⁺-ATPase was observed. The structurally distinct phosphatidylinositol 3-kinase inhibitors wortmannin (50–200 nmol/l) and LY294 002 (50 μmol/l) attenuated both total insulin-stimulated ⁸⁶Rb⁺ uptake as well as uptake via the Na⁺/K⁺-ATPase and Na⁺/K⁺/2Cl^– cotransporter. Neither the inhibitor of p70 S6 kinase activation, rapamycin (30 ng/ml) nor the mitogen activated protein kinase kinase inhibitor, PD098 059 (50 μmol/l), had any effect on insulin's stimulation of K⁺ influx. A 10 μmol/l concentration of the protein kinase C (PKC) inhibitor bisindolylmaleimide attenuated insulin action but at 1 μmol/l it was ineffective, suggesting involvement of the atypical PKC-ζ isoform. We conclude that insulin-stimulated K⁺ uptake in 3T3-L1 fibroblasts appears to involve concerted regulation of both the Na⁺/K⁺-ATPase and Na⁺/K⁺/2Cl^– cotransporter and we show for the first time that this process is signalled via a pathway involving phosphatidylinositol 3-kinase and PKC-ζ. [Diabetologia (1998) 41: 1199–1204] Received: 20 March 1998 and in revised form: 2 June 1998     

Relaxation Study of Complex Formation between Monovalent Cations and Cyclic Polyethers

Equilibrium quotients for the 1:1 complexes of dibenzo-30-crown-10 with Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺, and T1⁺ in methanol solution have been determined spectrophotometrically. The observed selectivity pattern is discussed in terms of the hydration energy of the cations and the cation-ligand binding energy. The rate of complex formation is diffusion-controlled. Relaxation amplitude data are consistent with a mechanism that involves conformational change of the crown compound.     

K+-dependent paradoxical membrane depolarization and Na+ overload, major and reversible contributors to weakness by ion channel leaks

Normal resting potential (P1) of myofibers follows the Nernst equation, exhibiting about −85 mV at a normal extracellular K⁺ concentration ([K⁺]_o) of 4 mM. Hyperpolarization occurs with decreased [K⁺]_o, although at [K⁺]_o < 1.0 mM, myofibers paradoxically depolarize to a second stable potential of −60 mV (P2). In rat myofiber bundles, P2 also was found at more physiological [K⁺]_o and was associated with inexcitability. To increase the relative frequency of P2 to 50%, [K⁺]_o needed to be lowered to 1.5 mM. In the presence of the ionophore gramicidin, [K⁺]_o reduction to only 2.5 mM yielded the same effect. Acetazolamide normalized this increased frequency of P2 fibers. The findings mimic hypokalemic periodic paralysis (HypoPP), a channelopathy characterized by hypokalemia-induced weakness. Of myofibers from 7 HypoPP patients, up to 25% were in P2 at a [K⁺]_o of 4 mM, in accordance with their permanent weakness, and up to 99% were in P2 at a [K⁺]_o of 1.5 mM, in accordance with their paralytic attacks. Of 36 HypoPP patients, 25 had permanent weakness and myoplasmic intracellular Na⁺ ([Na⁺]_i) overload (up to 24 mM) as shown by in vivo ²³Na-MRI. Acetazolamide normalized [Na⁺]_i and increased muscle strength. HypoPP myofibers showed a nonselective cation leak of 12–19.5 μS/cm², which may explain the Na⁺ overload. The leak sensitizes myofibers to reduced serum K⁺, and the resulting membrane depolarization causes the weakness. We postulate that the principle of paradoxical depolarization and loss of function upon [K⁺]_o reduction may apply to other tissues, such as heart or brain, when they become leaky (e.g., because of ischemia).     

Structures,Bonding and Sensor Properties of Some Alkaline o-Phthalatocuprates

Comprehensive study of the structure and bonding of disodium, dipotassium and diammonium di-o-phthalatocuprates(II) dihydrates has been undertaken. The crystal structure of ammonium o-phthalatocuprate has been determined. The identity of structures of phthalatocuprate chains in potassium and ammonium salts has been revealed. Vibrational spectra of all three compounds have been recorded, and the assignment of vibrational bands has been made. Force field calculations have shown a minor effect of outer-sphere cations (Na⁺, K⁺, NH₄⁺) on both intraligand (C-O) and metal–ligand bond strengths. Synthesized compounds have been tested as electrochemical sensors on D-glucose, dopamine and paracetamol. Their sensitivity to analytes varied in the order of Na⁺ > K⁺ > NH₄⁺. This effect has been explained by the more pronounced steric hindrance of copper ions in potassium and ammonium salts.     

The Effect of Melittin on Na+ and Rb+ Transport in Cultured Skin Fibroblasts of the Spontaneously Hypertensive Rat

Melittin effect on transport of Na⁺ and Rb⁺(K⁺ analog) was examined in cultured skin fibroblasts originating from the Spontaneously Hypertensive, Wistar Kyoto and Wistar rats. Melittin increased both Na⁺ (²²Na⁺₊) uptake and 86Rb⁺ efflux as well as the activity of the Na⁺-pump (ouabain sensitive ⁸⁶Rb⁺ uptake) in all three preparations. The effect of the toxin was maximal at a dose of 160–240ng/10⁵ cells/ml. At this does, fibroblasts of the Spontaneously Hypertensive rat demonstrated the greatest response to melittin with respect to the increase in Na⁺ and Rb⁺ fluxes and increase in the intracellular Na⁺ concentrations. It is concluded that melittin can be utilized as a probe to delineate subtle differences in the cellular regulation of Na⁺ and K⁺ in the Spontaneously Hypertensive rat as compared with its normotensive controls.     

Alterations of Na/K-ATPase function in caveolin-1 knockout cardiac fibroblasts

Recent studies have demonstrated that the Na⁺/K⁺-ATPase is not only an ion pump, but also a membrane receptor that confers the ligand-like effects of cardiotonic steroids (CTS) such as ouabain on protein kinases and cell growth. Because CTS have been implicated in cardiac fibrosis, this study examined the role of caveolae in the regulation of Na⁺/K⁺-ATPase function and CTS signaling in cardiac fibroblasts. In cardiac fibroblasts prepared from wild-type and caveolin-1 knockout [Cav-1(−/−)] mice, we found that the absence of caveolin-1 did not affect total cellular amount or surface expression of Na⁺/K⁺-ATPase α1 subunit. However, it did increase ouabain-sensitive ⁸⁶Rb⁺ uptake. While knockout of caveolin-1 increased basal activities of Src and ERK1/2, it abolished the activation of these kinases induced by ouabain but not angiotensin II. Finally, ouabain stimulated collagen synthesis and cell proliferation in wild type but not Cav-1(−/−) cardiac fibroblasts. Thus, we conclude that caveolae are important for regulating both pumping and signal transducing functions of Na⁺/K⁺-ATPase. While depletion of caveolae increases the pumping function of Na⁺/K⁺-ATPase, it suppresses CTS-induced signal transduction, growth, and collagen production in cardiac fibroblasts.     

The release of calcium from heart mitochondria by sodium

The release of Ca²⁺ from heart mitochondria could be induced by addition of 20 to 50 mm Na⁺ to the incubation medium. Of the other cations tested, K⁺, Rb⁺, Cs⁺, and Mg²⁺ were without effect, whereas some release was induced by Li⁺. In the presence of ruthenium red, which prevented the re-binding of the released Ca²⁺, 2 to 5 mm Na⁺ were sufficient to produce a measurable release of Ca²⁺. The amount of Ca²⁺ freed from the mitochondria was proportional to the amount of Na⁺ added, and to the concentration of Ca²⁺ present in the mitochondria. The release appeared to be a biphasic process, and in the first 15 s up to 3 nmol Ca²⁺ per mg of mitochondrial protein could be dissociated from the mitochondria.The phenomenon was more evident at pH 6.5 than under slightly alkaline conditions. The significance of this release in the process of heart contraction is discussed.     

Altered Na+ transport after an intracellular α-subunit deletion reveals strict external sequential release of Na+ from the Na/K pump

The Na/K pump actively exports 3 Na⁺ in exchange for 2 K⁺ across the plasmalemma of animal cells. As in other P-type ATPases, pump function is more effective when the relative affinity for transported ions is altered as the ion binding sites alternate between opposite sides of the membrane. Deletion of the five C-terminal residues from the α-subunit diminishes internal Na⁺ (Na_i⁺) affinity ≈25-fold [Morth et al. (2007) Nature 450:1043–1049]. Because external Na⁺ (Na_o⁺) binding is voltage-dependent, we studied the reactions involving this process by using two-electrode and inside-out patch voltage clamp in normal and truncated (ΔKESYY) Xenopus-α1 pumps expressed in oocytes. We observed that ΔKESYY (i) decreased both Na_o⁺ and Na_i⁺ apparent affinities in the absence of K_o⁺, and (ii) did not affect apparent Na_o⁺ affinity at high K_o⁺. These results support a model of strict sequential external release of Na⁺ ions, where the Na⁺-exclusive site releases Na⁺ before the sites shared with K⁺ and the ΔKESYY deletion only reduces Na_o⁺ affinity at the shared sites. Moreover, at nonsaturating K_o⁺, ΔKESYY induced an inward flow of Na⁺ through Na/K pumps at negative potentials. Guanidinium⁺ can also permeate truncated pumps, whereas N-methyl-D-glucamine cannot. Because guanidinium_o⁺ can also traverse normal Na/K pumps in the absence of both Na_o⁺ and K_o⁺ and can also inhibit Na/K pump currents in a Na⁺-like voltage-dependent manner, we conclude that the normal pathway transited by the first externally released Na⁺ is large enough to accommodate guanidinium⁺.     

C-peptide stimulates rat renal tubular Na+, K+-ATPase activity in synergism with neuropeptide Y

Summary This study was performed in order to test the hypothesis that the connecting peptide of proinsulin, C-peptide, might in itself possess biological activity. Renal tubular Na⁺, K⁺-ATPase, which is a well-established target for many peptide hormones, was chosen as a model. Rat C-peptide (I) was found to stimulate Na⁺, K⁺-ATPase activity in single, proximal convoluted tubules dissected from rat kidneys. C-peptide increased the Na⁺ affinity of the enzyme and all subsequent studies were performed at non-saturating Na⁺ concentrations. C-peptide stimulation of Na⁺, K⁺-ATPase activity occurred in a concentration-dependent manner in the dose range 10^–8–10^–6 mol/l. The presence of neuropeptide Y, 5×10^–9 mol/l, enhanced this effect and stimulation of Na⁺, K⁺-ATPase activity then occurred in the C-peptide dose range 10^–11–10^–8 mol/l. C-peptide stimulation of Na⁺, K⁺-ATPase activity was abolished in tubules pretreated with pertussis toxin. It was also abolished in the presence of FK 506, a specific inhibitor of the Ca²⁺-calmodulin-dependent protein phosphatase 2B. These results indicate that C-peptide stimulates Na⁺, K⁺-ATPase activity, probably by activating a receptor coupled to a pertussis toxin-sensitive G-protein with subsequent activation of Ca²⁺-dependent intracellular signalling pathways.Abbreviations PTX Pertussis toxin - NPY neuropeptide Y - PCT proximal convoluted tubule - BSA bovine serum albumin - dB cAMP dibutyryl cyclic adenosine monophosphate - PP2B Ca²⁺/calmodulin-dependent protein phosphatase 2B - PKC protein kinase C - [Ca²⁺] intracellular calcium concentration     

Isoform-specific alterations in cardiac and erythrocyte Na ,K-ATPase activity induced by norepinephrine

Background: Myocardial Na⁺,K⁺-ATPase activities are decreased in congestive heart failure because of an increase in plasma norepinephrine levels, but it is difficult to monitor the activities in the clinical setting.Methods and Results: This study investigated whether erythrocyte Na⁺,K⁺-ATPase activity can reflect myocardial enzyme activity and whether isoform-specific alterations occur in the presence of catecholamine. Na⁺,K⁺-ATPase activity was measured by the colorimetric method by using the left ventricular myocardium and erythrocytes prepared from eight rabbits given norepinephrine for 7 days and from eight control rabbits that received saline. The protein levels of total catalytic subunit and α₁ - or α₃-isoform of Na⁺,K⁺-ATPase were determined by Western blot analysis. Na⁺,K⁺-ATPase activity was lower in both myocardium and erythrocytes from norepinephrine-treated rabbits than control rabbits (P < .01 and P < .01, respectively). There was a close correlation in Na⁺,K⁺-ATPase activity between myocardium and erythrocytes (r = .963). Total catalytic subunit protein level was lower in myocardium from norepinephrine-treated rabbits than control rabbits, but the α₁-isoform level was similar between the two groups. The α₃-isoform level was lower in norepinephrinetreated rabbits than control rabbits. In erythrocytes, α₁-isoform was lower in norepinephrinetreated rabbits than control rabbits.Conclusions: Na⁺,K⁺-ATPase activity in myocardium could be reflected in erythrocyte membrane, although there was a difference in isoform-specific regulation between the two.     

Mutation I810N in the α3 isoform of Na+,K+-ATPase causes impairments in the sodium pump and hyperexcitability in the CNS

In a mouse mutagenesis screen, we isolated a mutant, Myshkin (Myk), with autosomal dominant complex partial and secondarily generalized seizures, a greatly reduced threshold for hippocampal seizures in vitro, posttetanic hyperexcitability of the CA3-CA1 hippocampal pathway, and neuronal degeneration in the hippocampus. Positional cloning and functional analysis revealed that Myk/+ mice carry a mutation (I810N) which renders the normally expressed Na⁺,K⁺-ATPase α3 isoform inactive. Total Na⁺,K⁺-ATPase activity was reduced by 42% in Myk/+ brain. The epilepsy in Myk/+ mice and in vitro hyperexcitability could be prevented by delivery of additional copies of wild-type Na⁺,K⁺-ATPase α3 by transgenesis, which also rescued Na⁺,K⁺-ATPase activity. Our findings reveal the functional significance of the Na⁺,K⁺-ATPase α3 isoform in the control of epileptiform activity and seizure behavior.     

Molecular and functional characterization of a novel low-affinity cation transporter (LCT1) in higher plants

The transport of cations across membranes in higher plants plays an essential role in many physiological processes including mineral nutrition, cell expansion, and the transduction of environmental signals. In higher plants the coordinated expression of transport mechanisms is essential for specialized cellular processes and for adaptation to variable environmental conditions. To understand the molecular basis of cation transport in plant roots, a Triticum aestivum cDNA library was used to complement a yeast mutant deficient in potassium (K⁺) uptake. Two genes were cloned that complemented the mutant: HKT1 and a novel cDNA described in this report encoding a cation transporter, LCT1 (low-affinity cation transporter). Analysis of the secondary structure of LCT1 suggests that the protein contains 8–10 transmembrane helices and a hydrophilic amino terminus containing sequences enriched in Pro, Ser, Thr, and Glu (PEST). The transporter activity was assayed using radioactive isotopes in yeast cells expressing the cDNA. LCT1 mediated low-affinity uptake of the cations Rb⁺ and Na⁺, and possibly allowed Ca²⁺ but not Zn²⁺ uptake. LCT1 is expressed in low abundance in wheat roots and leaves. The precise functional role of this cation transporter is not known, although the competitive inhibition of cation uptake by Ca²⁺ has parallels to whole plant and molecular studies that have shown the important role of Ca²⁺ in reducing Na⁺ uptake and ameliorating Na⁺ toxicity. The structure of this higher plant ion transport protein is unique and contains PEST sequences.