Uncoupled Na+-efflux on reconstituted shark Na,K-ATPase is electrogenic

Na/K Pump Mutations Associated with Primary Hyperaldosteronism Cause Loss of Function

Primary hyperaldosteronism (Conn's syndrome), a common cause of secondary hypertension, is frequently produced by unilateral aldosterone-producing adenomas that carry mutations in ion-transporting genes, including ATP1A1, encoding the Na/K pump's α1 subunit. Whether Na/K pump mutant-mediated inward currents are required to depolarize the cell and increase aldosterone production remains unclear, as such currents were observed in four out of five mutants described so far. Here, we use electrophysiology and uptake of the K⁺ congener ⁸⁶Rb⁺, to characterize the effects of eight additional Na/K pump mutations in transmembrane segments TM1 (delM102-L103, delL103-L104, and delM102-I106), TM4 (delI322-I325 and I327S), and TM9 (delF956-E961, delF959-E961, and delE960-L964), expressed in Xenopus oocytes. All deletion mutants induced abnormal inward currents of different amplitudes at physiological voltages, while I327S lacked such currents. A detailed functional characterization revealed that I327S significantly reduces intracellular Na⁺ affinity without altering affinity for external K⁺. ⁸⁶Rb⁺-uptake experiments show that I327S dramatically impairs function under physiological concentrations of Na⁺ and K⁺. Since Na/K pumps in the adrenal cortex may be formed by association of α1 with β3 instead of β1 subunits, we evaluated whether G99R (another mutant without inward currents when associated with β1) would show inward currents when associated with β3. We found that the kinetic characteristics of either mutant or wild-type α1β3 pumps expressed in Xenopus oocytes to be indistinguishable from those of α1β1 pumps. The observed functional consequences of each hyperaldosteronism mutant point to the loss of Na/K pump function as the common feature of all mutants, which is sufficient to induce hyperaldosteronism.

REM sleep loss increases brain excitability: role of noradrenaline and its mechanism of action

Ever since the discovery of rapid eye movement sleep (REMS), studies have been undertaken to understand its necessity, function and mechanism of action on normal physiological processes as well as in pathological conditions. In this review, first, we briefly surveyed the literature which led us to hypothesise REMS maintains brain excitability. Thereafter, we present evidence from in vivo and in vitro studies tracing behavioural to cellular to molecular pathways showing REMS deprivation (REMSD) increases noradrenaline level in the brain, which stimulates neuronal Na-K ATPase, the key factor for maintaining neuronal excitability, the fundamental property of a neuron for executing brain functions; we also show for the first time the role of glia in maintaining ionic homeostasis in the brain. As REMSD exerts a global effect on most of the physiological processes regulated by the brain, we propose that REMS possibly serves a housekeeping function in the brain. Finally, subject to confirmation from clinical studies, based on the results reviewed here, it is being proposed that the subjects suffering from REMS loss may be effectively treated by reducing either noradrenaline level or Na-K ATPase activity in the brain.

Decoupling the Na+–K+–ATPase in vivo: A possible new role in the gills of freshwater fishes

The literature suggests that when Na(+)-K(+)-ATPase has reduced access to its glycosphingolipid cofactor sulfogalactosyl ceramide (SGC), it is converted to a Na(+) uniporter. We recently showed that such segregation can occur within a single membrane when Na(+)-K(+)-ATPase is excluded from membrane microdomains or 'lipid rafts' enriched in SGC (D. Lingwood, G. Harauz, J.S. Ballantyne, J. Biol. Chem. 280, 36545-36550). Specifically we demonstrated that Na(+)-K(+)-ATPase localizes to SGC-enriched rafts in the gill basolateral membrane (BLM) of rainbow trout exposed to seawater (SW) but not freshwater (FW). We therefore proposed that since the freshwater gill Na(+)-K(+)-ATPase was separated from BLM SGC it should also transport Na(+) only, suggesting a new role for the pump in this epithelium. In this paper we discuss the biochemical evidence for SGC-based modulation of transport stoichiometry and highlight how a unique asparagine-lysine substitution in the FW pump isoform and FW gill transport energetics gear the Na(+)-K(+)-ATPase to perform Na(+) uniport.

Effect of extracellular pH on presteady-state and steady-state current mediated by the Na+/K+ pump

A ouabain sensitive inward current occurs in Xenopus oocytes in Na+ and K(+)-free solutions. Several laboratories have investigated the properties of this current and suggested that acidic extracellular pH (pHo) produces a conducting pathway through the Na+/K+ pump that is permeable to H+ and blocked by [Na+]o. An alternative suggestion is that the current is mediated by an electrogenic H(+)-ATPase. Here we investigate the effect of pHo and [Na+]o on both transient and steady-state ouabain-sensitive current. At alkaline or neutral pHo the relaxation rate of pre-steady-state current is an exponential function of voltage. Its U-shaped voltage dependence becomes apparent at acidic pHo, as predicted by a model in which protonation of the Na+/K+ pump reduces the energy barrier between the internal solution and the Na+ occluded state. The model also predicts that acidic pHo increases steady-state current leak through the pump. The apparent pK of the titratable group(s) is approximately 6, suggesting that histidine is involved in induction of the conductance pathway. 22Na efflux experiments in squid giant axon and current measurements in oocytes at acidic pHo suggest that both Na+ and H+ are permeant. The acid-induced inward current is reduced by high [Na+]o, consistent with block by Na+. A least squares analysis predicts that H+ is four orders of magnitude more permeant than Na+, and that block occurs when 3 Na+ ions occupy a low affinity binding site (K(0.5) = 130 +/- 30 m M) with a dielectric coefficient of 0.23 +/- 0.03. These data support the conclusion that the ouabain-sensitive conducting pathway is a result of passive leak of both Na+ and H+ through the Na+/K+ pump.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

Investigation of Na(+),K(+)-ATPase on a solid supported membrane: the role of acylphosphatase on the ion transport mechanism

Charge translocation by Na(+),K(+)-ATPase was investigated by adsorbing membrane fragments containing Na(+),K(+)-ATPase from pig kidney on a solid supported membrane (SSM). Upon adsorption, the ion pumps were activated by performing ATP concentration jumps at the surface of the SSM, and the capacitive current transients generated by Na(+),K(+)-ATPase were measured under potentiostatic conditions. To study the behavior of the ion pump under multiple turnover conditions, ATP concentration jump experiments were carried out in the presence of Na(+) and K(+) ions. Current transients induced by ATP concentration jumps were also recorded in the presence of the enzyme alpha-chymotrypsin. The effect of acylphosphatase (AcP), a cytosolic enzyme that may affect the functioning of Na(+),K(+)-ATPase by hydrolyzing its acylphosphorylated intermediate, was investigated by performing ATP concentration jumps both in the presence and in the absence of AcP. In the presence of Na(+) but not of K(+), the addition of AcP causes the charge translocated as a consequence of ATP concentration jumps to decrease by about 50% over the pH range from 6 to 7, and to increase by about 20% at pH 8. Conversely, no appreciable effect of pH upon the translocated charge is observed in the absence of AcP. The above behavior suggests that protons are involved in the AcP-catalyzed dephosphorylation of the acylphosphorylated intermediate of Na(+),K(+)-ATPase.

K+-dependence of electrogenic transport by the NaK-ATPase

Charge translocation by the NaK-ATPase from shark rectal gland was measured by adsorption of proteoliposomes to a planar lipid membrane. The proteoliposomes were prepared by reconstitution of purified NaK-ATPase into liposomes consisting of E. coli lipids. The protein was activated by applying an ATP concentration jump produced by photolysis of a protected derivative of ATP, caged ATP. K+ titrations were used to study the effect of K+ on the charge translocation kinetics of the protein. The time-dependent currents obtained after activation of the enzyme with caged ATP were analyzed with a simplified Albers-Post model (E1 (k1)-->E1ATP (k2)-->E2P (k3)-->E1) taking into account the capacitive coupling of the protein to the measuring system. The results of the K+ titrations show a strong dependence of the rate constant k3 on the K+ concentration at the extracellular side of the protein, indicating the K+ activated dephosphorylation reaction. In contrast, k1 and k2 remained constant. The K+ dependence of the rate k3 could be well described with a K+ binding model with two equivalent binding sites (E2P + 2K+ <==> E2P(K) + K+ <==> E2 P(2K)) followed by a rate limiting reaction (E2P(2K) --> E1(2K)). The half saturating K+ concentration K3,0.5 and the microscopic dissociation constant K3 for the K+ dependence of k3 were 4.5mM and 1.9mM respectively. At saturating K+ concentration the rate constant k3 was approximately 100 s(-1). The relative amount of net charge transported during the Na+ and the K+ dependent reactions could be determined from the experiments. Our results suggest electroneutral K+ translocation and do not support electrogenic K+ binding in an extracellular access channel. This is compatible with a model where 2 negative charges are cotransported with 3Na+ and 2K+ ions. Error analysis gives an upper limit of 20% charge transported during K+ translocation or during electrogenic K+ binding in a presumptive access channel compared to Na+ translocation.

Characteristics of Na+/K(+)-ATPase mediated proton current in Na(+)- and K(+)-free extracellular solutions. Indications for kinetic similarities between H+/K(+)-ATPase and Na+/K(+)-ATPase

The Na+/K(+)-ATPase of an ouabain-resistant mutant of Torpedo californica and of rat was expressed in Xenopus oocytes by microinjection of mRNA coding for the alpha- and the beta-subunit of the protein. Electrophysiological measurements were performed by means of the extracellular giant-patch clamp technique. The pump currents were analyzed in nominal absence of extracellular Na+ and K+ ions. Under these conditions strongly inward rectifying, ouabain-sensitive current was detected with reversal potentials depending on extracellular pH. Presence or absence of intracellular Na+ or K+ ions had nearly no effect on the inward currents, removal of intracellular ATP caused their inhibition. The reversal potentials of the currents generated by the rat pump was shifted by 82.7 +/- 5.4 mV per 10-fold increase of extracellular proton concentration. This refers to an effective valency of 0.71 +/- 0.05. In the absence of a transmembrane proton gradient the pump current reversed at -64.2 +/- 4.4 mV. These findings are not compatible with a proton conducting channel formed by the pump molecule (Wang and Horisberger (1995) Am. J. Physiol. CP 37, C590-595). Therefore, a kinetic model based on the Post-Albers scheme with a modification derived from the reaction scheme of the gastric H+/K(+)-ATPase is proposed. Together with voltage-dependent binding of extracellular protons, this model is compatible with the observe pump currents.

Hydrophobic ion interaction on Na+ activation and dephosphorylation of reconstituted Na+,K(+)-ATPase

In liposomes with reconstituted shark Na+,K(+)-ATPase an uncoupled Na(+)-efflux and a Na+/Na+ exchange can be induced on inside-out oriented pumps by the addition of external (cytoplasmic) Na+ and MgATP to liposomes that either do not contain Na+ (and other alkali cations), or include 130 mM Na+ internally (extracellular). Both modes of exchange are electrogenic and accompanied by a net hydrolysis of ATP. The coupling ratio of positive net charges translocated per ATP split is found to be close to 3:1 and 1:1, respectively, for the two modes of exchange reactions at pH 7.0. By addition of the hydrophobic anion tetraphenylboron (TPB-), which imposes a negative electrostatic membrane potential inside the lipid bilayer, the ATP hydrolysis accompanying uncoupled Na+ efflux is increased with increasing TPB- concentrations. Cholesterol which increases the inner positive dipole potential of the bilayer counteracted this activation by TPB- of uncoupled Na+ efflux. Using the structural analog tetraphenylphosphonium (TPP+), which elicits an inside positive membrane potential, ATP hydrolysis accompanying uncoupled Na(+)-efflux is decreased. The rate of dephosphorylation in the absence of extracellular alkali cations was affected in a similar manner, whereas the dephosphorylation in the presence of extracellular Na+ inducing Na+/Na+ exchange was unaffected by the hydrophobic ions. In both modes of exchange the phosphorylation reaction was independent of the presence of hydrophobic ions. The hydrophobic ions affected the apparent affinity for cytoplasmic Na+, indicating that binding of cytoplasmic Na+ may involve the migration of cations to binding sites through a shallow cytoplasmic access channel. The results are in accordance with the simple electrostatic model for charge translocation in which two negative charges in the cytoplasmic binding domain of the Na+,K(+)-ATPase co-migrate during cation transport.

Cholesterol modulation of molecular activity of reconstituted shark Na+,K(+)-ATPase

The cholesterol content of liposome bilayers has been varied between 0-40 mol% to study the effects on reconstituted Na+,K(+)-ATPase. The maximum hydrolytic activity of reconstituted Na+,K(+)-ATPase was increased by cholesterol at concentrations above 10 mol% for both the physiological Na+/K(+)-exchange reactions, as well as for the partial reactions Na+/Na(+)-exchange and uncoupled Na+ efflux. Omission of cholesterol from the liposome bilayer modified the activation by cytoplasmic Na+, indicating effects on both Vmax and on the Na(+)-affinity. Several other kinetic parameters were found to be strongly influenced as well, most notable the steady-state phosphorylation level, and the characteristics of the phosphorylation/dephosphorylation reactions. These results indicate that cholesterol interacts directly with the Na+,K(+)-ATPase as an essential effector perhaps by affecting its conformational mobility or monomer interaction.

Comparative aspects of Na+/K+ pump-mediated uncoupled Na+ efflux in red blood cells and kidney proteoliposomes

Ouabain-sensitive uncoupled Na+ efflux has been studied in human, pig, and rat red cells and in vesicles containing reconstituted kidney Na+/K+ pumps obtained from these same species. The red cells from the different species gave qualitatively similar results; the uncoupled Na+ efflux was 15-30% of the Na+/K+ exchange rate, and this flux was inhibited at 5 mM extracellular Na+ (Na+o). At higher levels of Na+o there was a monotonic increase in the Na+ efflux. As has previously been observed in human red cells, the uncoupled efflux from pig red cells consists of Na+ and anion cotransport, suggesting that anion cotransport may be a general characteristic of uncoupled Na+ efflux in red cells. The uncoupled Na+ efflux carried out by pig and rat kidney Na+/K+ pumps differs from the red cell activity in that it represents no more than 2-4% of the Na+/K+ exchange rate and that 5 mM Na+o does not inhibit this efflux. Furthermore, the efflux does not appear to be dependent on anion cotransport. Vesicles containing human kidney Na+/K+ pumps differ from vesicles derived from pig or rat kidneys in that the Na+ efflux is not inhibited or stimulated by Na+ present on the opposite side; it thus appears that the Na+,K(+)-ATPase in these vesicles may be incapable of Na+/Na+ exchange. These results indicate that the ligand and kinetic properties of the uncoupled Na+ efflux mode of red cells are markedly different from kidney-derived Na+/K+ pumps reconstituted into proteoliposomes. The basis for these differences may be inherent in the Na+/K+ pumps themselves or represent differences between the two types of preparations studied.

Pre-steady-state transient currents mediated by the Na/K pump in internally perfused Xenopus oocytes

Pre-steady-state transient currents have been investigated in the vegetal pole of Xenopus oocytes using the open-oocyte vaseline-gap technique of Taglialatela, Toro, and Stefani (Biophysical Journal. 61:78-82, 1992). Voltage pulses 40 ms in duration were made from a holding potential of -40 mV to command potentials over the range -160 to +60 mV in increments of 20 mV. Current records (averaged 20X; sampled every 200 microseconds) in the presence of dihydroouabain (DHO) or absence of external Na+ (Nao) were subtracted from current records obtained under Na/Na exchange conditions, i.e. internally perfused with 50 mM Na+, 5 mM ATP, and 5 mM ADP (K(+)-free) and externally superfused with 100 mM Na+,K(+)-free solution. Transient currents were dependent on intracellular Na+ and nucleotides, and diminished by activation of forward pumping; they were also reduced by 10 micrograms ml-1 of oligomycin B applied to the external solution. These properties of the pre-steady state currents are consistent with the Na/K pump operating in its electroneutral Na/Na exchange mode. The voltage dependence of the DHO- and Nao-sensitive transient currents was analyzed using a pseudo two-state model in which only the rate coefficient for Nao-binding/reocclusion is voltage-dependent (Rakowski, R. F. 1993. J. Gen. Physiol. 101:117-144). The apparent valence of the charge moved during the on (zq-on) and off (zq-off) of the pulse were 0.96 +/- 0.05 and 0.95 +/- 0.05 for Nao-sensitive, and 1.10 +/- 0.07 and 0.85 +/- 0.06 for DHO-sensitive transient currents, respectively. The total amount of charge moved (Qtot) and the mid-point voltage of the charge distribution (Vq) were 230 +/- 15 pC and -56.2 +/- 5.1 mV, and 268 +/- 34 pC and -67.0 +/- 7.6 mV for Nao- and DHO-sensitive transient currents, respectively. The apparent valence (zk) and the voltage at which the forward and backward rates are equal (Vk) obtained from the relaxation rates were 0.80 +/- 0.05 and -129.3 +/- 10.0 mV, and 0.86 +/- 0.10 and -135.1 +/- 9.0 mV for the Nao- and DHO-sensitive pre-steady state currents, respectively. The values of the parameters were not statistically significantly different between the Nao- and DHO-sensitive transient currents. Excluding the first 600 microseconds after the onset of a voltage step which was not temporally resolved, transient currents showed no indication of a rising phase. These results support the idea that charge translocation occurs within an external access channel at a rate that is governed by a voltage-dependent binding/reocclusion process and a voltage-independent deocclusion/unbinding process.

Charge transport of ion pumps on lipid bilayer membranes

Ion pumps create ion gradients across cell membranes while consuming light energy or chemical energy. The ion gradients are used by the corresponding cell types for passive-ion transport via ion channels or carriers or for accumulation of nutrients like sugar or amino acids via cotransport systems or antiporters.

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.

Cis-allosteric effects of cytoplasmic Na+/K+ discrimination at varying pH. Low-affinity multisite inhibition of cytoplasmic K+ in reconstituted Na+/K(+)-ATPase engaged in uncoupled Na(+)-efflux

In liposomes with reconstituted shark Na+/K(+)-ATPase the effect of cytoplasmic K+ was investigated in the absence of extracellular alkali ions. During such conditions the Na+/K(+)-ATPase is engaged in the so called uncoupled Na+ efflux mode in which cytoplasmic Na+ activates and binds to the enzyme and becomes translocated without countertransport of K+ as in the physiological Na+/K+ exchange mode. In this uncoupled flux mode only low-affinity inhibition by K+cyt is found to be present. The inhibition pattern is consistent with a model in which cytoplasmic K+ exhibit mixed inhibition of Na+ activation, probably by binding at the three cytoplasmic loading sites on E1ATP (E1A). With determined intrinsic binding constants for cytoplasmic Na+ to this form of KS1, KS2, KS3 = 40 mM, 2 mM, 2 mM the inhibition pattern can be simulated assuming three K+cyt sites with equal affinity for Ki = 40 mM, similar to KS1 for the first Na+cyt site. The discrimination between cytoplasmic Na+ and K+ is therefore enhanced by allosteric interaction initiated from the cis-side due to binding of the first Na+, as opposed to K+, which induces the positive cooperatively in the successive Na+ bindings. pH is found to influence the pattern of K+cyt inhibition: A lowering of the pH potentiates the K+cyt inhibition, whereas at increased pH the inhibition is decreased and transformed into a pure competitive competition.

A voltage-activated cation transport pathway associated with the sodium pump

In proteoliposomes containing reconstituted shark Na,K-ATPase, inside positive potentials open a cation conductance characterized by a voltage-dependence very similar to that found in mammalian erythrocytes. In both proteoliposomes and erythrocytes, the voltage-activated pathway is inhibited by external oligomycin, which traps the Na,K-ATPase in a Na-occluded E1 form. These results indicate that a cation permeable pathway, activated by inside positive potentials, can be ascribed to the Na-K pump--possibly through interaction with its gating mechanism.

Electrogenic pump current of sarcoplasmic reticulum Ca(2+)-ATPase reconstituted at high lipid/protein ratio

When Ca(2+)-ATPase from sarcoplasmic reticulum was reconstituted with excess phospholipid (at a 1:800 weight ratio) in a monomeric state and activated by Ca2+ and ATP a transmembrane potential developed which could be continuously recorded by the fluorochrome oxonol VI. The results demonstrate the electrogenicity of active Ca2+ transport during continuous turnover. The fluorescence signal can be quantified in terms of net current electrical flow through the vesicular membranes and compared to the ATP hydrolysis to give the number of electrostatic charges transferred during Ca2+ transport. From such measurements a stoichiometry of 1.8 +/- 0.4 Ca2+ per ATP hydrolyzed at pH 7.1 can be obtained. The method is also convenient for determination of the kinetics of Ca(2+)-ATPase activation by ATP and free Ca2+.

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.

Variable stoichiometry in reconstituted shark Na,K-ATPase engaged in uncoupled efflux

In liposomes with reconstituted shark Na,K-ATPase produced to contain no internal K+ or Na+ addition of external Na+ and ATP induce an uncoupled Na+ efflux on inside-out oriented pumps which is electrogenic and accompanied by hydrolysis of ATP (Cornelius, F. (1989) Biochem. Biophys. Res. Commun. 160, 801-807). At saturating cytoplasmic Na+ the net-charge translocated per ATP molecule split is compatible with a coupling ratio of Nacyt transported per ATP split of 3:1 at pH greater than or equal to 7.0. However, this ratio decreases to 1.5:1 below pH 7.0. At non-saturating cytoplasmic Na+ the 3:1 stoichiometry is attained at pH 7.0-7.5, whereas outside this range of pH the net-charge translocated per ATP molecule split decreases.

Anion-coupled Na efflux mediated by the human red blood cell Na/K pump

The red cell Na/K pump is known to continue to extrude Na when both Na and K are removed from the external medium. Because this ouabain-sensitive flux occurs in the absence of an exchangeable cation, it is referred to as uncoupled Na efflux. This flux is also known to be inhibited by 5 mM Nao but to a lesser extent than that inhibitable by ouabain. Uncoupled Na efflux via the Na/K pump therefore can be divided into a Nao-sensitive and Nao-insensitive component. We used DIDS-treated, SO4-equilibrated human red blood cells suspended in HEPES-buffered (pHo 7.4) MgSO4 or (Tris)2SO4, in which we measured 22Na efflux, 35SO4 efflux, and changes in the membrane potential with the fluorescent dye, diS-C3 (5). A principal finding is that uncoupled Na efflux occurs electroneurally, in contrast to the pump's normal electrogenic operation when exchanging Nai for Ko. This electroneutral uncoupled efflux of Na was found to be balanced by an efflux of cellular anions. (We were unable to detect any ouabain-sensitive uptake of protons, measured in an unbuffered medium at pH 7.4 with a Radiometer pH-STAT.) The Nao-sensitive efflux of Nai was found to be 1.95 +/- 0.10 times the Nao-sensitive efflux of (SO4)i, indicating that the stoichiometry of this cotransport is two Na+ per SO4=, accounting for 60-80% of the electroneutral Na efflux. The remainder portion, that is, the ouabain-sensitive Nao-insensitive component, has been identified as PO4-coupled Na transport and is the subject of a separate paper. That uncoupled Na efflux occurs as a cotransport with anions is supported by the result, obtained with resealed ghosts, that when internal and external SO4 was substituted by the impermeant anion, tartrate i,o, the efflux of Na was inhibited 60-80%. This inhibition could be relieved by the inclusion, before DIDS treatment, of 5 mM Cli,o. Addition of 10 mM Ko to tartrate i,o ghosts, with or without Cli,o, resulted in full activation of Na/K exchange and the pump's electrogenicity. Although it can be concluded that Na efflux in the uncoupled mode occurs by means of a cotransport with cellular anions, the molecular basis for this change in the internal charge structure of the pump and its change in ion selectivity is at present unknown.

Na,K-ATPase in artificial lipid vesicles. Comparison of Na,K and Na-only pumping mode

Na,K-ATPase from rabbit kidney outer medulla was reconstituted in large unilamellar lipid vesicles by detergent dialysis. Vesicles prepared in the presence or absence of potassium allowed to study two different transport modes: the (physiological) Na,K-mode in buffers containing Na+ and K+ and the Na-only mode in buffers containing Na+ but no K+. The ATP hydrolysis activity was obtained by determination of the liberated inorganic phosphate, Pi, and the inward directed Na+ flux was measured by 22Na-tracer flux. Electrogenic transport properties were studied using the membrane potential sensitive fluorescence-dye oxonol VI. The ratio upsilon(Na,K)/upsilon(Na) of the turnover rates in the Na,K-mode and in the Na-only mode is 6.6 +/- 2.0 under otherwise identical conditions and nonlimiting Na+ concentrations. Strong evidence is found that the Na-only mode exhibits a stoichiometry of 3Na+cyt/2Na+ext/1ATP, i.e. the extracellular (= intravesicular) Na+ has a potassium-like effect. In the Na-only mode one high-affinity binding side for ATP (KM congruent to 50 nM) was found, in the Na,K-mode a high- and low-affinity binding side with equilibrium dissociation constants, KM, of 60 nM and 13 microM, respectively. The sensitivity against the noncompetitively inhibiting ADP (KI = 6 microM) is higher by a factor of 20 in the Na-only mode compared to the Na,K-mode. From the temperature dependence of the pumping activity in both transport modes, activation energies of 160 kJ/mol for the Na,K-mode and 110 kJ/mol for the Na-only mode were determined.