A negative slope in the current-voltage relationship of the Na+/K+ pump in Xenopus oocytes produced by reduction of external [K+]

Charge movement by the Na/K pump in Xenopus oocytes

Pre-steady-state transient currents (1986. Nakao, M., and D. C. Gadsby. Nature [Lond.]. 323:628-630) mediated by the Na/K pump were measured under conditions for Na/Na exchange (K-free solution) in voltage-clamped Xenopus oocytes. Signal-averaged (eight times) current records obtained in response to voltage clamp steps over the range -160 to +60 mV after the addition of 100 microM dihydroouabain (DHO) or removal of external Na (control) were subtracted from test records obtained before the solution change. A slow component of DHO- or Na-sensitive difference current was consistently observed and its properties were analyzed. The quantity of charge moved was well described as a Boltzmann function of membrane potential with an apparent valence of 1.0. The relaxation rate of the current was fit by the sum of an exponentially voltage-dependent reverse rate coefficient plus a voltage-independent forward rate constant. The quantity of charge moved at the on and off of each voltage pulse was approximately equal except at extreme negative values of membrane potential where the on charge tended to be less than the off. The midpoint voltage of the charge distribution function (Vq) was shifted by -24.8 +/- 1.7 mV by changing the external [Na] in the test condition from 90 to 45 mM and by +14.7 +/- 1.7 mV by changing the test [Na] from 90 to 120 mM. A pseudo three-state model of charge translocation is discussed in which Na+ is bound and occluded at the internal face of the enzyme and is released into an external-facing high field access channel (ion well). The model predicts a shift of the charge distribution function to more hyperpolarized potentials as extracellular [Na] is lowered; however, several features of the data are not predicted by the model.

Molecular recognition and processing of periodic signals in cells: study of activation of membrane ATPases by alternating electric fields

A molecule which is immobilized, oriented or tumbling more slowly than the frequency of a periodic field, may interact with the field to produce chemical effects that are uncommon in a homogeneous solution. Among these effects are the alteration of the rate of a chemical reaction and the exchange of energy between the oscillating field and the conformation of the molecule. When certain conditions are satisfied, this exchange allows the molecule to absorb and couple the energy of the field to drive an endergonic reaction. The efficiency of energy coupling depends on field strength and frequency and on the ligand concentration. There are windows of these parameters to achieve efficient coupling. These windows can be expressed in terms of the rate constants and equilibrium constants of the catalytic reactions, and the amplitude and frequency of the periodic field. This mechanism allows cells to receive, process and transmit energy of high and medium level periodic potentials by means of membrane enzymes or receptors. A theory for the transduction of electric energy, electroconformational coupling (ECC) will be discussed. The electric field induced cation pumping activities of Na,K-ATPase and Ca-ATPase of human erythrocytes and the ATP synthetic activity of beef heart mitochondrial ATPase will then be used to test an ECC membrane transport model. For the processing of low level periodic signals, a theory of an oscillatory activation barrier (OAB), which considers resonance transduction between an oscillating field and the activation barrier of the rate limiting step in an enzymic reaction, will be discussed. The OAB mechanism successfully interprets the AC stimulated ATP hydrolysis activity of Ecto-ATPase from chicken oviduct and F0F1-ATPase from beef heart. We propose that mechanisms similar to an OAB model are adopted by cells to sense weak electric, acoustic, mechanical, concentration (i.e., chemical potential) and other types of signals, and to communicate with other cells by these signals. The experimental data and mechanistic information presented in this communication give us a glimpse of the molecular electronic designs in living cells. This information is also relevant with respect to environmental issues. Environmental electromagnetic fields and sonic pollutants may interfere with normal communications of cells and organisms. Their benefit, if any, and detrimental effects can be assessed and dealt with only if we fully understand mechanisms of cellular interactions with these fields and pollutants, at the molecular level.

Fluorescence study on cardiac glycoside binding to the Na,K-pump. Ouabain binding is associated with movement of electrical charge

Recently we have presented evidence that the fluorescence probe RH 421 can be used to detect binding and release of ions at the extracellular face of the pump since these processes are associated with translocation of electrical charge. Applying this method to experiments with cardiac glycosides we found that: (1) ouabain induced fluorescence changes of the electrochromic dye, RH 421, were caused by the change of charges bound to the enzyme; (2) independent of the sodium concentration, the final fluorescence amplitude indicated that approximately 2 Na+ ions were bound to the pump; (3) the sodium release to the extracellular side involved two distinct electrogenic steps; (4) the kinetics of inhibition depended on the Na(+)-concentration. Experiments with hydrophobic ions indicated that the kinetics of ouabain binding to the Na-ATPase is voltage dependent; and (5) the applied technique is a convenient tool to characterize binding of cardiac glycosides to the Na,K-pump.

Voltage-dependent stimulation of Na+/K(+)-pump current by external cations: selectivity of different K+ congeners

Currents generated by the endogenous Na+/K+ pump in the oocytes of Xenopus laevis were determined under voltage-clamp as currents activated by different K+ congeners. The voltage dependence of the pump current reflects voltage-dependent steps in the reaction cycle. The decrease of K(+)-activated pump current at positive potentials has been attributed to voltage-dependent stimulation by the external K+ (Rakowski, Vasilets, LaTona and Schwarz (1991) J. Membr. Biol. 121, 177-187). In Na(+)-free solution, activation of the pump by external cations seems to be the dominating voltage-dependent and rate-determining step in the reaction cycle. Under these conditions, the voltage dependence of apparent Km values for pump activation can be analyzed. The dependence suggests voltage-dependent binding of extracellular cations assuming that an effective charge of about 0.4 of an elementary charge is moved in the electrical field during a step associated with the cation binding. The apparent Km values at 0 mV differ for various cations that stimulate pump activity. The values are in mM: 0.10 for Tl+, 0.63 for K+, 0.71 for Rb+, 9.3 for NH4+, and 12.9 for Cs+. The corresponding apparent affinities follow the same sequence as the cation permeability of the K(+)-selective delayed rectifier channel of nerve cells. The results are compatible with the interpretation that the cations have to pass an ion-selective access channel to reach their binding sites in the pump molecule.

Regulation of endogenous and expressed Na+/K+ pumps in Xenopus oocytes by membrane potential and stimulation of protein kinases

Modulation of the current generated by the Na+/K+ pump by membrane potential and protein kinases was investigated in oocytes of Xenopus laevis. In addition to a positive slope region in the current-voltage (I-V) relationship of the Na+/K+ pump, a negative slope region has been described in these cells (Lafaire & Schwarz, 1986) and has been attributed to a voltage-dependent apparent Km value for pump stimulation by external [K+] (Rakowski et al., 1991). To study this feature in more detail, Xenopus oocytes were used for comparative analysis of the negative slope of the I-V relationship of the endogenous Na+/K+ pump and of the Na+/K+ pump of the electric organ of Torpedo californica expressed in the oocytes. The effects of stimulation of protein kinases A and C on the negative slope were also analyzed. To investigate the negative slope over a wide potential range, experiments were performed in Na(+)-free solution and in the presence of high concentrations of Ba2+ and tetraethylammonium, to block all nonpump related K(+)-sensitive currents. Pump currents and pump-mediated fluxes were determined as differences of currents or fluxes in solutions with and without extracellular K+. The voltage dependence of the Km value for stimulation of the Na+/K+ pump by external [K+] shows significant species differences. Over the entire voltage range from -140 to +20 mV, the Km value for the Na+/K+ pump of Torpedo electroplax is substantially higher than for the endogenous pump and exhibits more pronounced voltage dependence. For the Xenopus pump, the voltage dependence can be described by voltage-dependent stimulation by external [K+] and can be interpreted by voltage-dependent K+ binding, assuming that an effective charge between 0.37 and 0.56 of an elementary charge is moved in the electrical field. An analogous evaluation of the voltage dependence of the Torpedo pump requires the assumption of movement of two effective charges of 0.16 and 1.0 of an elementary charge. Application of 1,2-dioctanoyl-sn-glycerol (diC8, 10-50 microM), which is known to stimulate protein kinase C, reduces the maximum activity of the Xenopus pumps in the oocyte membrane by 40% and modulates the voltage dependence of K+ stimulation. For the endogenous Xenopus pump, the apparent effective charge increased from 0.37 to 0.51 of elementary charge and the apparent Km at 0 mV increased from 0.46 to 0.83 mM. For the Torpedo pump, one of the apparent effective charges increased from 1.0 to 2.5 of elementary charge.(ABSTRACT TRUNCATED AT 400 WORDS)

Up-regulation of sodium pump activity in Xenopus laevis oocytes by expression of heterologous beta 1 subunits of the sodium pump

Recent evidence suggests that the beta subunit of the Na+ pump is essential for the alpha subunit to express catalytic activity and for assembly of the holoenzyme in the plasma membrane. We report here that injection into Xenopus laevis oocytes of cRNAs specific for beta 1 subunit isoforms of the Na+ pump of four species (Torpedo californica, chicken, mouse and rat) causes a time-dependent increase in the number of ouabain-binding sites, both in the plasma membrane and in internal membranes. Expression of the beta 1 subunit of the Na+ pump of mouse and rat in the oocytes could be substantiated by immunoprecipitation using a polyclonal antiserum against the mouse beta 1 subunit. Scatchard analysis in permeabilized cells disclosed that the affinity for ouabain is unchanged after expression of each of the beta 1 subunits. A proportional increase in ouabain-sensitive 86Rb+ uptake indicates that the additionally expressed ouabain-binding sites on the cell surface represent functional Na+ pumps. The findings support the concept of Geering. Theulaz, Verrey, Häuptle & Rossier [(1989) Am. J. Physiol. 257, C851-C858] that beta 1 subunits expressed in oocytes associate with an excess of endogenous alpha subunits of the Na+ pump to form a hybrid enzyme. In addition, all of the beta 1 isoforms investigated in the present study were also capable of combining with the co-expressed alpha 1 subunit of the Torpedo Na+ pump to produce a functional enzyme. Injection of cRNA encoding for the Torpedo alpha 1 subunit alone had no effect on the ouabain-binding capacity of the surface and intracellular membranes of the oocyte.

Coexpression of alpha 1 with putative beta 3 subunits results in functional Na+/K+ pumps in Xenopus oocytes

The active Na+/K+ pump is composed of an alpha and a beta subunit. Until now, three putative isoforms of the beta subunit have been identified that share sequence similarity. We have expressed the beta 1 and beta 3 isoforms of Xenopus laevis Na+/K(+)-ATPase in Xenopus oocytes to compare functional properties of the Na+/K+ pump, including either of these two isoforms. Na+/K+ pump current, estimated as K(+)-induced outward current in voltage-clamped oocytes, was doubled by coexpression of alpha 1 subunits with either isoform of the beta subunit compared to expression of alpha 1 subunits alone. The kinetics of activation by external K+ and the voltage dependence of the electrogenic activity of the Na+/K+ pump were similar with both beta isoforms, indicating that both beta 1 and beta 3 isoforms can support expression at the oocyte surface of an active Na+/K+ pump with similar functional properties.

Dependence of Na+ pump current on external monovalent cations and membrane potential in rabbit cardiac Purkinje cells

1. The effect of membrane potential and various extracellular monovalent cations on the Na+ pump current (Ip) was studied on isolated, single Purkinje cells of the rabbit heart by means of whole-cell recording. 2. Ip was identified as current activated by external K+ or its congeners NH4+ and Tl+. The current was blocked by dihydroouabain (1-5 x 10(-4) M) over the whole range of membrane potentials tested. 3. In Na(+)-containing solution half-maximum Ip activation (K0.5) occurred at 0.4 mM-Tl+, 1.9 mM-K+ and 5.7 mM-NH4+ (holding potential, -20 mV). 4. The pump current (Ip)-voltage (V) relationship of the cells in Na(+)-containing media with K+ or its congeners at the tested concentrations greater than K0.5 displayed a steep positive slope at negative membrane potentials between -120 and -20 mV. Little voltage dependence of Ip was observed at more positive potentials up to +40 mV. At even more positive potentials Ip measured at 2 and 5.4 mM-K+ decreased. 5. Lowering the concentration of K+ or its congeners below the K0.5 value in Na(+)-containing solution induced a region of negative slope of the Ip-V curve at membrane potentials positive to -20 mV. 6. The shape of the Ip-V relationship remained unchanged when the K+ concentration (5.4 mM) of the Na(+)-containing medium was replaced by NH4+ or Tl+ concentrations of similar potency to activate Ip (20 mM-NH4+ or 2 mM-Tl+). 7. In Na(+)-free, choline-containing solution half-maximum Ip activation occurred at 0.13 mM-K+ (holding potential, -20 mV). 8. At negative membrane potentials the positive slope of the Ip-V curve was flatter in Na(+)-free than in Na(+)-containing media. A reduced voltage dependence of Ip persisted, regardless of whether choline ions or Li+ were used as a Na+ substitute. 9. Lowering the K+ concentration of the Na(+)-free, choline-containing solution to 0.05 mM evoked an extended region of negative slope in the Ip-V relationship at membrane potentials between -40 and +60 mV. 10. It is concluded that the apparent affinity of the Na(+)-K+ pump towards K+ in cardiac Purkinje cells depends on both the membrane potential and the extracellular Na+ concentration. 11. The region of negative slope of the Ip-V curve observed in cells which were superfused with media containing low concentrations of K+ or its congeners strongly suggests the existence of at least two voltage-sensitive steps in the cardiac Na(+)-K+ pump cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional expression of N-terminal truncated alpha-subunits of Na,K-ATPase in Xenopus laevis oocytes

N-terminal deletion mutants of Na,K-ATPase alpha 1 isoforms initiating translation at Met34 (alpha 1T1) or at Met43 (alpha 1T2) were expressed in X. laevis oocytes. Compared to beta 3 cRNA injected controls, the co-expression of alpha 1wt, alpha 1T1, alpha 1T2 with beta 3 subunits results in a 2- to 3-fold increase of ouabain binding sites, parallelled by a concomitant increase in Na,K-pump current. The apparent K1/2 for potassium activation of the alpha 1T2/beta 3 Na,K-pumps is significantly higher than that of the alpha 1wt/beta 3 or alpha 1T1/beta 3 Na,K-pumps expressed at the cell surface. Total deletion of the lysine-rich N-terminal domain thus allows the expression of active Na,K-pump but with distinct cation transport properties.

Conditions for a backward-running Na+/K+ pump in Xenopus oocytes

Current generated by the electrogenic Na+/K+ pump protein was determined in oocytes of Xenopus laevis as strophantidine-sensitive current measured under voltage clamp. Under conditions of reduced intracellular [Na+] and [ATP], both to values below 1 mM, and in extracellularly K(+)-free medium, the Na+/K+ pump seems to operate in a reversed mode pumping Na+ into the cell and K+ out of the cell. This is demonstrated by strophantidine-induced hyperpolarization of the membrane and inward-directed current mediated by the pump protein. In addition, strophantidine-sensitive uptake of 22Na+ can be demonstrated under these conditions. The pump current decreases with membrane depolarization as expected for a pump cycle that involves inward movement of positive charges during Na+ translocation.

Charge translocation by the Na,K-pump: II. Ion binding and release at the extracellular face

In the first part of the paper, evidence has been presented that electrochromic styryl dyes, such as RH 421, incorporate into Na,K-ATPase membranes isolated from mammalian kidney and respond to changes of local electric field strength. In this second part of the paper, fluorescence studies with RH-421-labeled membranes are described, which were carried out to obtain information on the nature of charge-translocating reaction steps in the pumping cycle. Experiments with normal and chymotrypsin-modified membranes show that phosphorylation by ATP and occlusion of Na+ are electroneutral steps, and that release of Na+ from the occluded state to the extracellular side is associated with translocation of charge. Fluorescence signals observed in the presence of K+ indicate that binding and occlusion of K+ at the extracellular face of the pump is another major electrogenic reaction step. The finding that the fluorescence signals are insensitive to changes of ionic strength leads to the conclusion that the binding pocket accommodating Na+ or K+ is buried in the membrane dielectric. This corresponds to the notion that the binding sites are connected with the extracellular medium by a narrow access channel ("ion well"). This notion is further supported by experiments with lipophilic ions, such as tetraphenylphosphonium (TPP+) or tetraphenylborate (TPB-), which are known to bind to lipid bilayers and to change the electrostatic potential inside the membrane. Addition of TPP+ leads to a decrease of binding affinity for Na+ and K+, which is thought to result from the TPP(+)-induced change of electric field strength in the access channel.