Potassium: potassium exchange catalysed by the sodium pump in human red cells

The Na+,K+-ATPase and its stoichiometric ratio: some thermodynamic speculations

 Almost seventy years after its discovery, the sodium-potassium adenosine triphosphatase (the sodium pump) located in the cell plasma membrane remains a source of novel mechanistic and physiologic findings. A noteworthy feature of this enzyme/transporter is its robust stoichiometric ratio under physiological conditions: it sequentially counter-transports three sodium ions and two potassium ions against their electrochemical potential gradients per each hydrolyzed ATP molecule. Here we summarize some present knowledge about the sodium pump and its physiological roles, and speculate whether energetic constraints may have played a role in the evolutionary selection of its characteristic stoichiometric ratio.

Distinct pH dependencies of Na+/K+ selectivity at the two faces of Na,K-ATPase

The sodium pump (Na,K-ATPase) in animal cells is vital for actively maintaining ATP hydrolysis-powered Na⁺ and K⁺ electrochemical gradients across the cell membrane. These ion gradients drive co- and countertransport and are critical for establishing the membrane potential. It has been an enigma how Na,K-ATPase discriminates between Na⁺ and K⁺, despite the pumped ion on each side being at a lower concentration than the other ion. Recent crystal structures of analogs of the intermediate conformations E2·Pi·2K⁺ and Na⁺-bound E1∼P·ADP suggest that the dimensions of the respective binding sites in Na,K-ATPase are crucial in determining its selectivity. Here, we found that the selectivity at each membrane face is pH-dependent and that this dependence is unique for each face. Most notable was a strong increase in the specific affinity for K⁺ at the extracellular face (i.e. E2 conformation) as the pH is lowered from 7.5 to 5. We also observed a smaller increase in affinity for K⁺ on the cytoplasmic side (E1 conformation), which reduced the selectivity for Na⁺ Theoretical analysis of the pK_a values of ion-coordinating acidic amino acid residues suggested that the face-specific pH dependences and Na⁺/K⁺ selectivities may arise from the protonation or ionization of key residues. The increase in K⁺ selectivity at low pH on the cytoplasmic face, for instance, appeared to be associated with Asp⁸⁰⁸ protonation. We conclude that changes in the ionization state of coordinating residues in Na,K-ATPase could contribute to altering face-specific ion selectivity.

Different approaches to the mechanism of the sodium pump

The way in which the sodium pump uses energy from the hydrolysis of ATP to perform osmotic and electrical work is not yet understood. We attempt to bring together the results of a number of different approaches to this problem. One approach has been to correlate biochemical changes and ionic fluxes, both when the pump operates normally and when it operates in various abnormal 'modes' in particular unphysiological conditions. A second approach has been to expose fragments of cell membrane to (gamma-32P)ATP and to study the properties of components of the membrane that become labelled. It is now clear that 32P can be transferred to the beta-carboxy group of an aspartyl residue in a pump polypeptide, but there is controversy about the interrelations of different forms of this polypeptide and its role, if any, in the normal functioning of the pump. A third approach has been to attempt to purify the pump and to determine the properties of the pure enzyme. It seems that the pump contains a polypeptide (molecular weight about 100,000), which bears the phosphorylation site, and a smaller glycopeptide, but there is disagreement about the molecular ratios. The results of these and other approaches cannot yet be fitted into a satisfactory model for the sodium pump, but we shall consider some of the problems involved in this task.

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.

Large diameter of palytoxin-induced Na/K pump channels and modulation of palytoxin interaction by Na/K pump ligands

Palytoxin binds to Na/K pumps to generate nonselective cation channels whose pore likely comprises at least part of the pump's ion translocation pathway. We systematically analyzed palytoxin's interactions with native human Na/K pumps in outside-out patches from HEK293 cells over a broad range of ionic and nucleotide conditions, and with or without cardiotonic steroids. With 5 mM internal (pipette) [MgATP], palytoxin activated the conductance with an apparent affinity that was highest for Na⁺-containing (K⁺-free) external and internal solutions, lowest for K⁺-containing (Na⁺-free) external and internal solutions, and intermediate for the mixed external Na⁺/internal K⁺, and external K⁺/internal Na⁺ conditions; with Na⁺ solutions and MgATP, the mean dwell time of palytoxin on the Na/K pump was about one day. With Na⁺ solutions, the apparent affinity for palytoxin action was low after equilibration of patches with nucleotide-free pipette solution. That apparent affinity was increased in two phases as the equilibrating [MgATP] was raised over the submicromolar, and submillimolar, ranges, but was increased by pipette MgAMPPNP in a single phase, over the submillimolar range; the apparent affinity at saturating [MgAMPPNP] remained ∼30-fold lower than at saturating [MgATP]. After palytoxin washout, the conductance decay that reflects palytoxin unbinding was accelerated by cardiotonic steroid. When Na/K pumps were preincubated with cardiotonic steroid, subsequent activation of palytoxin-induced conductance was greatly slowed, even after washout of the cardiotonic steroid, but activation could still be accelerated by increasing palytoxin concentration. These results indicate that palytoxin and a cardiotonic steroid can simultaneously occupy the same Na/K pump, each destabilizing the other. The palytoxin-induced channels were permeable to several large organic cations, including N-methyl-d-glucamine⁺, suggesting that the narrowest section of the pore must be ∼7.5 Å wide. Enhanced understanding of palytoxin action now allows its use for examining the structures and mechanisms of the gates that occlude/deocclude transported ions during the normal Na/K pump cycle.

A hundred years of sodium pumping

This article gives a history of the evidence (a) that animal cell membranes contain pumps that expel sodium ions in exchange for potassium ions; (b) that the pump derives energy from the hydrolysis of ATP; (c) that it is thermodynamically reversible-artificially steep transmembrane ion gradients make it run backward synthesizing ATP from ADP and orthophosphate; (d) that its mechanism is a ping-pong one, in which phosphorylation of the pump by ATP is associated with an efflux of three sodium ions, and hydrolysis of the phosphoenzyme is associated with an influx of two potassium ions; (e) that each half of the working cycle involves both the transfer of a phosphate group and a conformational change-the phosphate transfer being associated with the occlusion of ions bound at one surface and the conformational change releasing the occluded ions at the opposite surface.

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

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

Mechanism responsible for oligomycin-induced occlusion of Na+ within Na/K-ATPase

The mechanism whereby oligomycin occludes Na+ within Na/K-ATPase was investigated to study Na+ and K+ transport mechanisms. Oligomycin stimulated Na+ binding to Na/K-ATPase but inhibited Na-K and Na-Na exchange. The oligomycin concentration required to stimulate Na+ binding to half-maximal was 4.5 microM, which was close to the concentration that reduced Na-Na and Na-K exchange and ATPase activity to half-maximal, suggesting that Na/K-ATPase possesses an oligomycin binding site responsible for stimulating Na+ binding and reducing ion exchange and ATPase activity. In contrast, neither K+ binding nor K+ transport was affected by oligomycin. Limited tryptic digestion of Na/K-ATPase showed that, unlike Na+, K+, and ouabain, oligomycin treatment did not result in a specific digestion pattern. Oligomycin appeared to inhibit ouabain binding in a noncompetitive manner, whereas it did not affect ATP binding. Na/K-ATPase isoforms with low and high sensitivities to ouabain were equally sensitive to oligomycin. These results suggest that the oligomycin binding site is located on the extracellular side of Na/K-ATPase, at a different position from the ouabain binding site, and this antibiotic did not induce a conformational change of Na/K-ATPase. We propose that oligomycin interacts with the Na+ occlusion site from the extracellular side of Na/K-ATPase, which delays Na+ release to the extracellular side without inducing a conformational change, suggesting that the pathways responsible for Na+ and K+ transport differ.

K+ self-exchange by the Na+ pump: regulation by P(i) and metabolic perturbations

We have previously demonstrated that the Na(+)-K+ pump on the basolateral membrane of the rabbit cortical collecting duct can function in the K+/K+ exchange mode. Increasing intracellular phosphate in red blood cells inhibits the Na+ pump and increases K+/K+ exchange. We found that maneuvers designed to increase intracellular phosphate in collecting duct cells caused an increase in K+/K+ exchange. Subjecting the cells to a metabolic insult (cyanide) increased K+/K+ exchange by the pump as judged by its ouabain sensitivity and lack of electrogenic or conductive characteristics. The results demonstrate that the rate of K+/K+ exchange by the Na(+)-K+ pump can be altered by changes in intracellular phosphate over a range that is physiologically or pathologically achievable. The results also suggest a mechanism for inhibition of vectorial Na+ transport during metabolic stress.

Investigation of ion binding to the cytoplasmic binding sites of the Na,K-pump

A dual-wavelength fluorimeter was constructed, which used two light emitting diodes (LEDs) to excite the fluorescence dye RH 421 alternately with two different wavelengths. The ratio of the emissions at the two excitation wavelengths provided a drift-insensitive signal, which allowed detection of very small changes of the fluorescence intensity. Those small changes were induced by ion binding and release in conformation E1 of the Na,K-ATPase. Titration experiments were performed to determine equilibrium dissociation constants (+/- standard deviation) for each step in the complete binding and release sequence: 0.12 +/- 0.01 mM (E2(K2)<==>KE1), 0.08 +/- 0.01 mM (KE1<==>E1A), 3.0 +/- 0.2 mM (NaE1<==>E1), 5.2 +/- 0.4 mM (Na2E1<==>NaE1) and 6.5 +/- 0.4 mM (Na3E1<==>Na2E1) at pH 7.2 and T = 16 degrees C. These numbers show that the affinities of the binding sites exposed to the cytoplasm, are higher for K+ than for Na+ ions, similar to what was found on the extracellular side. The physiological requirement for extrusion of Na+ from the cytoplasm, and for import of K+ from the extracellular medium seems to be facilitated not by favorable binding affinities in state E1 but by the two ATP-driven reaction steps of the cycle, E2(K2) + ATP-->K2E1.ATP and Na3E1.ATP<==>(Na3) E1-P, which border the ion exchange reactions at the binding sites in conformation E1.

The alpha 1 Na(+)-K+ pump of the Dahl salt-sensitive rat exhibits altered Na+ modulation of K+ transport in red blood cells

The properties of the alpha 1 Na(+)-K+ pump were compared in Dahl salt-sensitive (DS) and salt-resistant (DR) strains by measuring ouabain-sensitive fluxes (mmol/liter cell x hr = FU, Mean +/- SE) in red blood cells (RBCs) and varying internal (i) and external (o) Na+ and K+ concentrations. Kinetic parameters of several modes of operation, i.e., Na+/K+, K+/K+, Na+/Na+ exchanges, were characterized and analyzed for curve-fitting using the Enzfitter computer program. In unidirectional flux studies (n = 12 rats of each strain) into fresh cells incubated in 140 mM Na(+) + 5 mM K+, ouabain-sensitive K+ influx was substantially lower in the DS than in DR RBCs, while ouabain-sensitive Na+ efflux and Nai were similar in both strains. Thus, the coupling ratio between unidirectional Na+:K+ fluxes was significantly higher in DS than in DR cells at similar RBC Na+ content. In the presence of 140 mM Nao, activation of ouabain-sensitive K+ influx by Ko had a lower Km and Vmax in DS as estimated by the Garay equation (N = 2.70 +/- 0.33, Km 0.74 +/- 0.09 mM; Vmax 2.87 +/- 0.09 FU) than in DR rats (N = 1.23 +/- 0.36, Km 2.31 +/- 0.16 mM; Vmax 5.70 +/- 0.52 FU). However, the two kinetic parameters were similar following Nao removal. The activation of ouabain-sensitive K+ influx by Nai had significantly lower Vmax in DS (9.3 +/- 0.4 FU) than in DR (14.5 +/- 0.6 FU) RBCs but similar Km. These data suggest that the low K+ influx in DS cells is caused by a defect in modulation by Nao and Nai. Na+ efflux showed no differences in Nai activation or trans effects by Nao and Ko, thus accounting for the different Na+:K+ coupling ratio in the Dahl strains. Further evidence for the differences in the coupling of ouabain-sensitive fluxes was found in studies of net Na+ and K+ fluxes, where the net ouabain-sensitive Na+ losses showed similar magnitudes in the two Dahl strains while the net ouabain-sensitive K+ gains were significantly greater in the DR than the DS RBCs. Ouabain-sensitive Na+ influx and K+ efflux were also measured in these rat RBCs. The inhibition of ouabain-sensitive Na+ influx by Ko was fully competitive for the DS but not for the DR pumps. Thus, for DR pumps, Ko could activate higher K+ influx in DR pumps without a complete inhibition of ouabain-sensitive Na+ influx.(ABSTRACT TRUNCATED AT 400 WORDS)

Annual review prize lecture. 'All hands to the sodium pump'

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Role of protein conformation changes and transphosphorylations in the function of Na+/K(+)-transporting adenosine triphosphatase: an attempt at an integration into the Na+/K+ pump mechanism

The particular aim of the review on some basic facets of the mechanism of Na+/K(+)-transporting ATPase (Na/K-ATPase) has been to integrate the experimental findings concerning the Na(+)- and K(+)-elicited protein conformation changes and transphosphorylations into the perspective of an allosterically regulated, phosphoryl energy transferring enzyme. This has led the authors to the following summarizing evaluations. 1. The currently dominating hypothesis on a link between protein conformation changes ('E1 in equilibrium with E2') and Na+/K+ transport (the 'Albers-Post scheme') has been constructed from a variety of partial reactions and elementary steps, which, however, do not all unequivocally support the hypothesis. 2. The Na(+)- and K(+)-elicited protein conformation changes are inducible by a variety of other ligands and modulatory factors and therefore cannot be accepted as evidence for their direct participation in effecting cation translocation. 3. There is no evidence that the 'E1 in equilibrium with E2' protein conformation changes are moving Na+ and K+ across the plasma membrane. 4. The allosterically caused ER in equilibrium with ET ('E1 in equilibrium with E2') conformer transitions and the associated cation 'occlusion' in equilibrium with 'de-occlusion' processes regulate the actual catalytic power of an enzyme ensemble. 5. A host of experimental variables determines the proportion of functionally competent ER enzyme conformers and incompetent ET conformers so that any enzyme population, even at the start of a reaction, consists of an unknown mixture of these conformers. These circumstances account for the occurrence of contradictory observations and apparent failures in their comparability. 6. The modelling of the mechanism of the Na/K-ATPase and Na+/K+ pump from the results of reductionistically designed experiments requires the careful consideration of the physiological boundary conditions. 7. Na+ and K+ ligandation of Na/K-ATPase controls the geometry and chemical reactivity of the catalytic centre in the cycle of E1 in equilibrium with E2 state conversions. This is possibly effected by hinge-bending, concerted motions of three adjacent, intracellularly exposed peptide sequences, which shape open and closed forms of the catalytic centre in lock-and-key responses. 8. The Na(+)-dependent enzyme phosphorylation with ATP and the K(+)-dependent hydrolysis of the phosphoenzyme formed are integral steps in the transport mechanism of Na/K-ATPase, but the translocations of Na+ and K+ do not occur via a phosphate-cation symport mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphatase activity and potassium transport in liposomes with Na+,K(+)-ATPase incorporated

We have used liposomes with incorporated pig kidney Na+,K(+)-ATPase to study vanadate sensitive K(+)-K+ exchange and net K+ uptake under conditions of acetyl- and p-nitrophenyl phosphatase activities. The experiments were performed at 20 degrees C. Cytoplasmic phosphate contamination was minimized with a phosphate trapping system based on glycogen, phosphorylase a and glucose-6-phosphate dehydrogenase. In the absence of Mg2+ (no phosphatase activity) 5-10 mM p-nitrophenyl phosphate slightly stimulated K(+)-K+ exchange whereas 5-10 mM acetyl phosphate did not. In the presence of 3 mM MgCl2 (high rate of phosphatase activity) acetyl phosphate did not affect K(+)-K+ exchange whereas p-nitrophenyl phosphate induced a greater stimulation than in the absence of Mg2+; a further addition of 1 mM ADP resulted in a 35-65% inhibition of phosphatase activity with an increase in K(+)-K+ exchange, which sometimes reached the levels seen with 5 mM phosphate and 1 mM ADP. The net K+ uptake in the presence of 3 mM MgCl2 was not affected by acetyl phosphate or p-nitrophenyl phosphate, whereas it was inhibited by 5 mM phosphate (with and without 1 mM ADP). The results of this work suggest that the phosphatase reaction is not by itself associated to K+ translocation. The ADP-dependent stimulation of K(+)-K+ exchange in the presence of phosphatase activity could be explained by the overlapping of one or more step/s of the reversible phosphorylation from phosphate with the phosphatase cycle.

Stoichiometry and voltage dependence of the sodium pump in voltage-clamped, internally dialyzed squid giant axon

The stoichiometry and voltage dependence of the Na/K pump were studied in internally dialyzed, voltage-clamped squid giant axons by simultaneously measuring, at various membrane potentials, the changes in Na efflux (delta phi Na) and holding current (delta I) induced by dihydrodigitoxigenin (H2DTG). H2DTG stops the Na/K pump without directly affecting other current pathways: (a) it causes no delta I when the pump lacks Na, K, Mg, or ATP, and (b) ouabain causes no delta I or delta phi Na in the presence of saturating H2DTG. External K (Ko) activates Na efflux with Michaelis-Menten kinetics (Km = 0.45 +/- 0.06 mM [SEM]) in Na-free seawater (SW), but with sigmoid kinetics in approximately 400 mM Na SW (Hill coefficient = 1.53 +/- 0.08, K1/2 = 3.92 +/- 0.29 mM). H2DTG inhibits less strongly (Ki = 6.1 +/- 0.3 microM) in 1 or 10 mM K Na-free SW than in 10 mM K, 390 mM Na SW (1.8 +/- 0.2 microM). Dialysis with 5 mM each ATP, phosphoenolpyruvate, and phosphoarginine reduced Na/Na exchange to at most 2% of the H2DTG-sensitive Na efflux. H2DTG sensitive but nonpump current caused by periaxonal K accumulation upon stopping the pump, was minimized by the K channel blockers 3,4-diaminopyridine (1 mM), tetraethylammonium (approximately 200 mM), and phenylpropyltriethylammonium (20-25 mM) whose adequacy was tested by varying [K]o (0-10 mM) with H2DTG present. Two ancillary clamp circuits suppressed stray current from the axon ends. Current and flux measured from the center pool derive from the same membrane area since, over the voltage range -60 to +20 mV, tetrodotoxin-sensitive current and Na efflux into Na-free SW, under K-free conditions, were equal. The stoichiometry and voltage dependence of pump Na/K exchange were examined at near-saturating [ATP], [K]o and [Na]i in both Na-free and 390 mM Na SW. The H2DTG-sensitive F delta phi Na/delta I ratio (F is Faraday's constant) of paired measurements corrected for membrane area match, was 2.86 +/- 0.09 (n = 8) at 0 mV and 3.05 +/- 0.13 (n = 6) at -60 to -90 mV in Na-free SW, and 2.72 +/- 0.09 (n = 7) at 0 mV and 2.91 +/- 0.21 (n = 4) at -60 mV in 390 mM Na SW. Its overall mean value was 2.87 +/- 0.07 (n = 25), which was not significantly different from the 3.0 expected of a 3 Na/2 K pump.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage dependence of partial reactions of the Na+/K+ pump: predictions from microscopic models

A theoretical treatment of the voltage dependence of electroneutral Na+-Na+ and K+-K+ exchange mediated by the Na+/K+ pump is given. The analysis is based on the Post-Albers reaction scheme in which the overall transport process is described as a sequence of conformational transitions and ion-binding and ion-release steps. The voltage dependence of the exchange rate is determined by a set of 'dielectric coefficients' reflecting the magnitude of charge translocations associated with individual reaction steps. Charge movement may result from conformational changes of the transport protein and/or from migration of ions in an access channel connecting the binding sites with the aqueous medium. It is shown that valuable mechanistic information may be obtained by studying the voltage dependence of transport rates at different (saturating and nonsaturating) ion concentrations.

Interaction of magnesium with the sodium pump of the human red cell

1. In broken red cell membranes, Mg2+ inhibition of Na+,K+-adenosine-5'-triphosphatase (Na+,K+-ATPase) activity was partially competitive with MgATP. Mg2+ inhibition of Na+,K+-ATPase activity was uncompetitive with K+ in broken red cell membranes, and Mg2+ inhibition of ouabain-sensitive K+ influx in K+-free resealed ghosts was uncompetitive with external K+. 2. When Na+,K+-ATPase activity was measured at relatively high K+ concentration, Mg2+ inhibition was partially competitive with Na+. Mg2+ inhibition of ouabain-sensitive K+ influx in K+-free resealed ghosts was competitive with cell Na+. Magnesium was a more effective inhibitor of the uncoupled Na+ efflux in low-Na+ ghosts than in high-Na+ ghosts. 3. These findings indicate that Mg2+ inhibition results from combination of the ion with the enzyme form E2K at high intracellular Na+ and K+, and from combination with the form E1 at low intracellular Na+ and K+. 4. In ghosts containing high concentrations of MgPO4, inhibition of the K+-K+ exchange by Mg2+ was more effective at high than at low nucleotide concentrations. At high MgPO4 and low Mg2+ concentration the activity of the exchange increased monotonically with nucleotide concentration, but at a higher Mg2+ concentration, nucleotide activation of the exchange was biphasic: the K+-K+ exchange rate increased, reached a maximum, and then decreased with increasing nucleotide concentration.

Phosphate inhibition of the human red cell sodium pump: simultaneous binding of adenosine triphosphate and phosphate

1. The Na+-K+ exchange carried out by the Na+ pump of human red cell ghosts and the Na+ + K+-dependent adenosine triphosphatase (Na+,K+-ATPase) activity of human red cell membranes are inhibited by MgPO4 rather than by free phosphate; similarly, the substrate for the K+-K+ exchange carried out by the pump is MgPO4 rather than free phosphate. 2. Inhibition of the Na+, K+-ATPase activity by MgPO4 is only partially competitive (mixed type) with ATP, and MgPO4 inhibition of the Na+-K+ exchange measured in Na+-free solutions and in K+-free ghosts which contain ATP at relatively high concentration is partially uncompetitive (mixed type) with external K+. 3. When measurements were made in K+-free ghosts and Na+-free solutions, or when Na+,K+-ATPase activity was measured at high ATP concentrations, inhibition by MgPO4 was non-competitive with cell Na+. This observation is not consistent with the Albers-Post reaction mechanism of the Na+ pump, and suggests the presence of an alternative reaction pathway in which ATP combines with the enzyme before phosphate is released. 4. MgPO4 monotonically inhibited the uncoupled Na+ efflux which occurs in solutions free of both Na+ and K+. The uncoupled efflux seemed to be more sensitive to MgPO4 inhibition than the Na+-K+ exchange. 5. Trinitrophenyladenosine-5'-tetraphosphate stimulated the K+-K+ exchange in the presence of MgPO4, and the characteristics of stimulation by TNP adenosine tetraphosphate were little different from the characteristics of stimulation by trinitrophenyladenosine-5'-triphosphate or -5'-diphosphate. The nucleotide binding site at which K+-K+ exchange is stimulated must be able to accommodate a nucleotide with a linear array of four phosphate groups.

Phosphatase activity of Na+/K+-ATPase. Enzyme conformations from ligands interactions and Rb occlusion experiments

The present work compares the effects of several ligands (phosphatase substrates, MgCl2, RbCl and inorganic phosphate) and temperature on the phosphatase activity and the E2(Rb) occluded conformation of Na+/K+-ATPase. Cooling from 37 degrees C to 20 degrees C and 0 degrees C (hydrolysis experiments) or from 20 degrees C to 0 degrees C (occlusion experiments) had the following consequences: (i) dramatically reduced the Vmax for p-nitrophenyl phosphate and acetyl phosphate hydrolysis but it produced little or no changes in the Km for the substrates; (ii) led to a 5-fold drop in the Km for the inorganic phosphate-induced di-occlusion of E2(Rb); (iii) reduced the K0.5 and curve sigmoidicity of the Rb-stimulated hydrolysis of p-nitrophenyl phosphate and acetyl phosphate and the Rb-promoted E2(Rb) formation. At 20 degrees C, in the presence of 1 mM RbCl and no Mg2+, acetyl phosphate did not affect E2(Rb); with 3 mM MgCl2, acetyl phosphate stimulated a release of Rb from E2(Rb) both in the presence and absence of RbCl in the incubation mixture. As a function of acetyl phosphate concentration the Km for iRb release was indistinguishable from the Km found for stimulation of hydrolysis and enzyme phosphorylation under identical experimental conditions; in addition, the extrapolated di-occluded fraction corresponding to maximal hydrolysis was not different from 100%. These results indicate that although E2(K) might be an intermediary in the phosphatase reaction, the most abundant enzyme conformation during phosphatase turnover is E2 which has no K+ occluded in it. The ligand interactions associated to phosphatase activity do not support an equivalence of this reaction with the dephosphorylation step in the Na+ + K+-dependent ATP hydrolysis; on the other hand, there are similarities with the reversible binding of inorganic phosphate in the presence of Mg2+ and K+ ions.

Properties of the Na+-K+ pump in human red cells with increased number of pump sites

We studied the Na+/K+ pump in red cells from an obese human subject (MAJ) in which the number of pumps/cell was 10-20 times higher than normal. Through measurements of the kinetic properties of several modes of operation of the Na+/K+ pump we determined that the pumps in MAJ cells are kinetically normal. In the presence of adequate metabolic substrate the maximum rates of Na+ pumping and lactate production saturated at 60 and 12 nmol/1 cell per h, respectively. Under physiological conditions pump and "leak" Na+ fluxes were similar in MAJ and normal cells. Since internal Na+ was lower in MAJ than in normal cells (Nai+ approximately 2 and 8 mmol/1 cell, respectively), we conclude that the reduction in cell Na+ allows the Na+/K+ pump in MAJ cells to operate at lower fraction of maximum capacity and to compensate for the increased number of pumps.

Nucleotide specificity of the E2K----E1K transition in (Na+ + K+)-ATPase as probed with tryptic inactivation and fragmentation

The nucleotide specificity for the E2K----E1K conformational transition in (Na+ + K+)-ATPase as the key step for overall hydrolytic activity and coupled cation transport has been investigated. Use has been made of tryptic inactivation, which is biexponential in time for the enzyme in the presence of Na+ with or without nucleotides (E1 conformation) and monoexponential in the presence of K+ (E2 conformation). ATP, AdoPP[NH]P and CTP in order of decreasing effectivity induce the biphasic tryptic inactivation pattern in the presence of K+. Their order of effectivity is inversely related to the rate constant of the second (slow) phase of inactivation. In the presence of K+ and either ITP or GTP tryptic inactivation remains monoexponential, indicating that these nucleotides cannot drive the E2K----E1K transition. Tryptic inactivation has been compared with tryptic fragmentation of the alpha-subunit (apparent mol. wt. 94 kDa) of (Na+ + K+)-ATPase. In the E1 conformation (Na+ present) a 71 kDa fragment is formed during the second phase of inactivation. In the E2 conformation (K+ present) the alpha-subunit is split to fragments of 41 and 52 kDa. In the presence of K+ and ATP, ADP, AdoPP[NH]P or CTP the 71 kDa fragment is formed in amounts which decrease in the order ATP approximately equal to ADP greater than AdoPP[NH]P greater than CTP. In the presence of K+ and AMP, ITP or GTP the 71 kDa fragment is absent and only the E2 fragments are formed. From these and literature data we arrive at a specificity order for the E2K----E1K transition of ATP greater than ADP greater than AdoPP[NH]P greater than CTP greater than ITP = GTP = AMP. The same order holds for K+ transport in the K+-K+ exchange and for overall hydrolytic activity (Na+ + K+ present) with the natural nucleoside triphosphates as substrates. This marks the E2K----E1K transition as the step in the reaction mechanism that determines nucleotide specificity for (Na+ + K+)-activated hydrolysis and coupled cation transport.

H+ transport by reconstituted gastric (H+ + K+)-ATPase

Gastric (H+ + K+)-ATPase was reconstituted into artificial phosphatidylcholine/cholesterol liposomes by means of a freeze-thaw-sonication technique. Upon addition of MgATP, active H+ transport was observed, with a maximal rate of 2.1 mumol X mg-1 X min-1, requiring the presence of 100 mM K+ at the intravesicular site. However, in the absence of ATP an H+-K+ exchange with a maximal rate of 0.12 mumol X mg-1 X min-1 was measured, which could be inhibited by the well-known ATPase inhibitors vanadate and omeprazole, giving the first evidence of a passive K+-H+ exchange function of gastric (H+ + K+)-ATPase. An Na+-H+ exchange activity was also measured, which was fully inhibited by 1 mM amiloride. Simultaneous reconstitution of Na+/H+ antiport and (H+ + K+)-ATPase could explain why reconstituted ATPase appeared less cation-specific than the native enzyme (Rabon, E.C., Gunther, R.B., Soumarmon, A., Bassilian, B., Lewin, M.J.M. and Sachs, G. (1985) J. Biol. Chem. 260, 10200-10212).

Potassium-potassium exchange as part of the over-all reaction mechanism of the sodium pump of the human red blood cell

When the efflux components of the Na-K exchange and K-K exchange are measured under identical conditions, the apparent K 1/2 (the concentration of K at which the velocity is half-maximal) for external K of the two processes differ. The discrepancy diminishes when the measurements are made in solutions containing low concentrations of Na, and in these solutions the uncoupled Na efflux is also partially suppressed. It is possible to demonstrate an uncoupled K efflux into solutions free of Na and K. This uncoupled efflux is also partially inhibited by low concentrations of external Na. At high enough concentrations, intracellular Na completely inhibits the K-K exchange. Inhibition of the K-K exchange by cell Na is competitive with cell K, and inhibition of the Na-K exchange by cell K is competitive with cell Na. In each case the characteristics of the competition suggest that both ions competitively interact with sites on the same enzyme form. The Albers-Post model of the Na-K pump reaction mechanism, modified to account for the uncoupled Na and K efflux, accounts in detail for these observations. If the K-K exchange is part of the over-all Na-K exchange, as indicated by the findings, pump models in which Na must add to the pump at the inside before K is released are excluded.

Effects of altering the ATP/ADP ratio on pump-mediated Na/K and Na/Na exchanges in resealed human red blood cell ghosts

Resealed human red blood cell ghosts were prepared to contain a range of ADP concentrations at fixed ATP concentrations and vice versa. ATP/ADP ratios ranging from approximately 0.2 to 50 were set and maintained (for up to 45 min) in this system. ATP and ADP concentrations were controlled by the addition of either a phosphoarginine- or phosphocreatine-based regenerating system. Ouabain-sensitive unidirectional Na efflux was determined in the presence and absence of 15 mM external K as a function of the nucleotide composition. Na/K exchange was found to increase to saturation with ATP (K 1/2 approximately equal to 250 microM), whereas Na/Na exchange (measured in K-free solutions) was a saturating function of ADP (K 1/2 approximately equal to 350 microM). The elevation of ATP from approximately 100 to 1,800 microM did not appreciably affect Na/Na exchange. In the presence of external Na and a saturating concentration of external K, increasing the ADP concentration at constant ATP was found to decrease ouabain-sensitive Na/K exchange. The decreased Na/K exchange that still remained when the ADP/ATP ratio was high was stimulated by removal of external Na. Assuming that under normal substrate conditions the reaction cycle of the Na/K pump is rate-limited by the conformational change associated with the release of occluded K [E2 X (K) X ATP----E1 X ATP + K], increasing ADP inhibits the rate of these transformations by competition with ATP for the E2(K) form. A less likely alternative is that inhibition is due to competition with ATP at the high-affinity site (E1). The acceleration of the Na/K pump that occurs upon removing external Na at high levels of ADP evidently results from a shift in the forward direction of the transformation of the intermediates involved with the release of occluded Na from E1P X (Na). Thus, the nucleotide composition and the Na gradient can modulate the rate at which the Na/K pump operates.

The locus of nucleotide specificity in the reaction mechanism of (Na+ + K+)-ATPase determined with ATP and GTP as substrates

ATP and GTP have been compared as substrates for (Na+ + K+)-ATPase in Na+-activated hydrolysis, Na+-activated phosphorylation, and the E2K----E1K transition. Without added K+ the optimal Na+-activated hydrolysis rates in imidazole-HCl (pH 7.2) are equal, but are reached at different Na+ concentrations: 80 mM Na+ for GTP, 300 mM Na+ for ATP. The affinities of the substrates for the enzyme are widely different: Km for ATP 0.6 microM, for GTP 147 microM. The Mg-complexed nucleotides antagonize activation as well as inhibition by Na+, depending on the affinity and concentration of the substrate. The optimal 3-s phosphorylation levels in imidazole-HCl (pH 7.0) are equally high for the two substrates (3.6 nmol/mg protein). The Km value for ATP is 0.1-0.2 microM and for GTP it ranges from 50 to 170 microM, depending on the Na+ concentration. The affinity of Na+ for the enzyme in phosphorylation is lower with the lower affinity substrate: Km (Na+) is 1.1 mM with ATP and 3.6 mM with GTP. The GTP-phosphorylated intermediate exists, like the ATP-phosphorylated intermediate, in the E2P conformation. Addition of K+ increases the optimal hydrolytic activity 30-fold for ATP (at 100 mM Na+ + 10 mM K+) and 2-fold for GTP (at 100 mM Na+ + 0.16 mM K+). K+ greatly increases the Km values for both substrates (to 430 microM for ATP and 320 microM for GTP). Above 0.16 mM K+ inhibits GTP hydrolysis. GTP does not reverse the quenching effect of K+ on the fluorescence of the 5-iodoacetamidofluorescein-labeled enzyme. ATP fully reverses this effect, which represents the transition from E1K to E2K. Hence GTP is unable to drive the E2K----E1K transition.

Cation activation of the pig kidney sodium pump: transmembrane allosteric effects of sodium

We have studied activation by Na or Rb ions of different transport modes of the Na-K pump, using phospholipid vesicles reconstituted with pig kidney Na-K-ATPase. The shape of the activation curves, sigmoid or quasi-hyperbolic, depends on the nature of the cation at the opposite surface and not on the specific mode of transport. ATP-dependent Na uptake into K-containing vesicles (Na-K exchange) is activated by cytoplasmic Na along a highly sigmoid curve in the absence of extracellular Na (Hill number, nH = 1.9). Activation displays progressively less-sigmoid curves as extracellular Na is raised to 150 mM (nH = 1.2). The maximal rate of the Na-K exchange is not affected. Na is not transported from the extracellular face by the pump in the presence of excess extracellular K, and the transmembrane effects of the extracellular Na are therefore 'allosteric' in nature. ATP-dependent Na-Na exchange (Lee & Blostein, 1980) and classical ATP-plus-ADP-dependent Na-Na exchange are activated by cytoplasmic Na along hyperbolic curves. ATP-dependent Na uptake into Tris-containing vesicles is activated by cytoplasmic Na along a somewhat sigmoidal curve. (ATP + Pi)-dependent Rb-Rb exchange is activated by cytoplasmic and extracellular Rb along strictly hyperbolic curves. The same applies for Rb-Rb exchange in the presence or absence of ATP or Pi alone. The presence of a high concentration of extracellular Na together with extracellular Rb induces a sigmoidal activation by cytoplasmic Rb of (ATP + Pi)-dependent Rb-Rb exchange (nH = 1.45) but does not affect the maximal rate of exchange. Slow passive Rb fluxes through the pump observed in the absence of other pump ligands (see Karlish & Stein, 1982 alpha) are activated by cytoplasmic Rb along a strictly hyperbolic curve with extracellular Rb, nH = 1.0 (Rb-Rb exchange), along a strongly sigmoid curve with extracellular Na, nH = 1.5 (Rb-Na exchange), and along less-sigmoid curves with extracellular Tris, nH = 1.24 (net Rb flux) or extracellular Li, nH = 1.2 (Rb-Li exchange). Activation of the passive Rb fluxes by extracellular Rb is hyperbolic in the presence of cytoplasmic Rb, Li or Tris but is sigmoid in the presence of cytoplasmic Na (nH = 1.36). Inhibition by cytoplasmic Na of passive Rb fluxes from the cytoplasmic to the extracellular face of the pump depends on the nature of the cation at the extracellular surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Current generated by backward-running electrogenic Na pump in squid giant axons

The sodium pump of animal cells is electrogenic, that is, it normally exports more sodium ions than it imports potassium ions. In the squid giant axon, the resulting net outward electric current has a density of a few microA cm-2, and contributes 1-2 mV to the resting membrane potential. The pump is driven by the free energy of hydrolysis of ATP, and in some instances it has been possible to run the pump backwards and synthesize ATP by lowering the [ATP]/[ADP] X [Pi] ratio and steepening the transmembrane Na+ and K+ gradients. Here we have examined the question of whether a backward-running sodium pump conserves its Na+/K+ greater than 1 stoichiometry. We demonstrate reversal of the sodium pump of squid giant axon, and find that backward pumping indeed produces a net inward electric current. This current is voltage-sensitive. Our observations have mechanistic implications for models of the sodium pump.

Na+-K+ transport and volume of rat erythrocytes under dietary K+ deficiency

Red cell Na+ and K+ content and transport were studied in Sprague-Dawley rats in the course of a dietary K+ depletion ranging 1-6 wk. Plasma K+ fell to below 2 mM, and red cell K+ decreased. Cellular Na+ rose due to an increase of the Na+ leak. Inward Rb+ and outward Na+ transport by the Na+-K+ pump (determined at 2 mM external Rb+) were accelerated by the rise in cell Na+ concentration. K+ depletion caused a cation deficit of up to 30% of total red cell Na+ plus K+ and a consecutive cell shrinkage with an increase in mean cellular hemoglobin content (MCHC). The cell shrinkage, in turn, was paralleled by up to a 10-fold increase in the maximum capacity of the furosemide-sensitive, chloride-dependent Na+-K+ cotransport system. This system participated with up to 50% of the total K+ movements across the red cell membrane in severe K+ deficiency. In normal cells shrunken by osmotic means, Na+-K+ cotransport was similarly accelerated severalfold, indicating that the cell shrinkage occurring during K+ depletion is a major factor inducing the changes in Na+-K+ cotransport. However, a second unknown factor is also involved. It is concluded that in the rat, not only genetic but also environmental parameters contribute in determining the actual activity of the red cell Na+-K+ cotransport system. The cell volume and MCHC must be considered when judging Na+ and K+ transport changes observed in rat erythrocytes under various pathophysiological conditions.

Comparison between fluxes of potassium and of chloride in astrocytes in primary cultures

Transport processes operating in astrocytes were examined by measuring unidirectional fluxes of 42K and 36Cl in primary cultures of mouse astrocytes, at steady-state with respect to ion composition. The total K+ uptake rate was 2025 nmol X mg-1 protein X min-1. This rate was not influenced by furosemide (2 mM), an inhibitor of Cl- uptake and K+-K+ exchange, or acetazolamide (0.1 mM), a carbonic anhydrase inhibitor. Ouabain (1 mM) inhibited the uptake rate by 541 nmol X mg-1 X min-1. The equilibrated K+ content was determined to be 696 nmol X mg-1. The rate constant for efflux was 2.76 min-1. This equals an efflux rate of 1921 nmol X mg-1 X min-1, i.e. a similar value as the influx. Furosemide and ouabain did not inhibit the efflux. The equilibrated Cl- content was found to be 167 nmol X mg-1 and it decreased in furosemide-treated cells to 68.1 nmol X mg-1. The total Cl- uptake was 35 nmol X mg-1 X min-1 and it was inhibited by furosemide or bumetanide by 27 nmol X mg-1 X min-1. The mean resting membrane potential was found to be -77.4 mV. From these data we conclude: (1) that the K+ uptake rates are high, as can be expected from estimates based on literature data for K+ conductance in mammalian glial cells in situ; and (2) that the cells possess a very low relative Cl- permeability.

Properties of the Mg2+-induced low-affinity nucleotide binding site of (Na+ + K+)-activated ATPase

The Mg2+-induced low-affinity nucleotide binding by (Na+ + K+)-ATPase has been further investigated. Both heat treatment (50-65 degrees C) and treatment with N-ethylmaleimide reduce the binding capacity irreversibly without altering the Kd value. The rate constant of inactivation is about one-third of that for the high-affinity site and for the (Na+ + K+)-ATPase activity. Thermodynamic parameters (delta H degree and delta S degree) for the apparent affinity in the ATPase reaction (Km ATP) and for the true affinity in the binding of AdoPP[NH]P (Kd and Ki) differ greatly in sign and magnitude, indicating that one or more reaction steps following binding significantly contribute to the Km value, which thus is smaller than the Kd value. Ouabain does not affect the capacity of low-affinity nucleotide binding, but only increases the Kd value to an extent depending on the nucleotide used. GTP and CTP appear to be most sensitive, ATP and ADP intermediately sensitive and AdoPP[NH]P and AMP least sensitive to ouabain. Ouabain reduces the high-affinity nucleotide binding capacity without affecting the Kd value. The nucleotide specificity of the low-affinity binding site is the same for binding (competition with AdoPP[NH]P) and for the ATPase activity (competition with ATP): AdoPP[NH]P greater than ATP greater than ADP greater than AMP. The low-affinity nucleotide binding capacity is preserved in the ouabain-stabilized phosphorylated state, and the Kd value is not increased more than by ouabain alone. It is inferred that the low-affinity site is located on the enzyme, more specifically its alpha-subunit, and not on the surrounding phospholipids. It is situated outside the phosphorylation centre. The possible functional role of the low-affinity binding is discussed.

Stimulation and inhibition by ATP and orthophosphate of the potassium-potassium exchange in resealed red cell ghosts

The potassium:potassium (K-K) exchange through the sodium pump has been measured as the ouabain-sensitive 86Rb uptake by Na-free ghosts resealed to contain various concentrations of ATP, orthophosphate and K. The exchange is activated by increasing either internal or external K+ (Rb+) ion concentration. The activation curves can be described by simple Michaelis kinetics as: exchange = Vmax [K]/(Kapp + [K]). Increasing ATP concentration increases the apparent affinity for external K ions but decreases the apparent affinity for internal K (Ki+). Increasing [ATP] from 1 microM to 1 mM typically increases the Kapp for Ki+ from less than 1 mM to about 30 mM. Increasing ATP first activates the exchange but, after an optimal concentration is reached, further increase of ATP inhibits. The value of ATP concentration which gives the maximum flux depends on the internal and external K+ concentrations. The higher [Ki], the greater the optimal ATP concentration. Increasing external K (Rb) decreases the optimal ATP concentration. Increasing the concentration of orthophosphate (Pi) activates the exchange at high ATP but inhibits at low ATP concentration. A concentration of Pi which stimulates the exchange at high external K (Rb) can inhibit at low external K (Rb). These findings are in agreement with a consecutive or ping-pong model of the K-K exchange. We suggest that previous experiments have not shown the inhibitory effects of ATP and Pi because of the particular range of concentrations investigated.

Effects of maintained illumination upon [K+]0 in the subretinal space of the isolated retina of the toad

Illumination of the vertebrate retina evokes a transient decrease in extracellular potassium concentration, [K+]0, in the subretinal space. During maintained illumination, [K+]0 recovers toward its dark-adapted level. The mechanisms most likely to contribute to this recovery process were examined by using K+-selective microelectrodes to measure [K+]0 in the isolated retina of the toad. Bufo marinus. Although both diffusion of K+ and changes in the rod membrane voltage contribute to the recovery of [K+]0 during maintained illumination, other factors are likely to be involved as well. It is suggested that this recovery process could be due in part to inhibition of the Na+/K+ pump in the rod photo-receptors during maintained illumination.

Effect of pH and different substrates on the electrokinetic properties of (Na+, K+)-ATPase vesicles

Some biophysical properties of a (Na+, K+)-ATPase preparation from guinea-pig kidney have been analysed. The recently developed technique of laser Doppler spectroscopy was applied to measure particle mobility under electrophoretic conditions. The following results were obtained: 1. magnesium ions at pH 7.3 decrease the mobility of the ATPase containing vesicles by binding to negatively charged surface groups. At pH 3.3 the competitive binding of protons causes a shift of the mobility vs. [Mg2+] curve to higher values of [Mg2+], 2. binding of ATP at pH 7.3 (Kd = 0.9 X 10(-4) M for (mM 1 NaCl, 0.2 KCl, 0.1 MgCl2, 0.1 Tris) was measured as an increase in particle mobility depending also on [Mg2+]. At pH 3.3 also unspecific ATP-binding occurred, 3. ITP and GTP had the same Kd value as ATP; ADP a slightly lower one (Kd = 1.2 X 10(-4) M). Tris-H3PO4 (Kd = 2.6 X 10(-4) M) was also able to increase particle mobility, but only at higher concentrations and not to the same extent as ATP; AMP induced only very small changes, 4. from the mobility-pH curve an isoelectric point of 4.1 is derived (buffer: 1 mM NaCl, 0.2 mM KCl, 0.1 mM MgCl2, 0.1 mM Tris). In the presence of 0.9 mM ATP the isoelectric point is shifted to 3.2. As the electrophoretic mobility is directly proportional to the net charge of the vesicles, the results may be interpreted as changes in surface charge density, originating from both a conformational change of the ATPase polypeptide and a decrease in vesicle size.

Potassium transport across the frog retinal pigment epithelium

Previous experiments indicate that the apical membrane of the frog retinal pigment epithelium contains electrogenic Na:K pumps. In the present experiments net potassium and rubidium transport across the epithelium was measured as a function of extracellular potassium (rubidium) concentration, [K]0 ( [Rb]0). The net rate of retina-to-choroid 42K(86Rb) transport increased monotonically as [K]0 ( [Rb]0) increased from approximately 0.2 to 5 mM on both sides of the tissue or on the apical (neural retinal) side of the tissue. No further increase was observed when [K]0 ( [Rb]0) was elevated to 10 mM. Net sodium transport was also stimulated by elevating [K]0. The net K transport was completely inhibited by 10-4 M ouabain in the solution bathing the apical membrane. Ouabain inhibited the unidirectional K flux in the direction of net flux but had no effect on the back-flux in the choroid-to-retina direction. The magnitude of the ouabain-inhibitable 42K(86Rb) flux increased with [K]0 ( [Rb]0). These results show that the apical membrane Na:K pumps play an important role in the net active transport of potassium (rubidium) across the epithelium. The [K]0 changes that modulate potassium transport coincide with the light-induced [K]0 changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium.

A method for estimating free Ca within human red blood cells, with an application to the study of their Ca-dependent K permeability

Murphy, Coll, Rich and Williamson (J. Biol. Chem. 255:6600--6608, 1980) described a null-point method for estimating intracellular free Ca in liver cells. They used digitonin to lyse the cells in solutions of varying Ca concentration. This method has been adapted for use with human red cells. The values found are about 0.4 micron or micrometer Ca in fresh cells, and from 0.4 to 0.7 micron or micrometer Ca in blood-bank cells, at pH 7.2 and 37 degrees C. These are likely to be overestimates, and the errors and limitations of the method are discussed. Red cells may be loaded with Ca by metabolic depletion in Ca-containing solutions. Such cells have an elevated K permeability, and the relationships between free Ca, total Ca and K permeability were investigated, using 86Rb as a tracer for K. 86Rb flux studies show that the affinity of the K channel for Ca is the same in cells as in resealed ghosts where intracellular Ca can be controlled with Ca buffers, but the rate of tracer equilibration is 3-6 times faster in ghosts than in cells.

Kinetics of the sodium pump in red cells of different temperature sensitivity

Ouabain-sensitive K influx into ground squirrel and guinea pig red cells was measured at 5 and 37 degrees C as a function of external K and internal Na. In both species the external K affinity increases on cooling, being three- and fivefold higher in guinea pig and ground squirrel, respectively, at 5 than at 37 degrees C. Internal Na affinity also increased on cooling, by about the same extent. The effect of internal Na on ouabain-sensitive K influx in guinea pig cells fits a cubic Michaelis-Menten-type equation, but in ground squirrel cells this was true only at high [Na]i. There was still significant ouabain-sensitive K influx at low [Na]i. Ouabain-binding experiments indicated around 800 sites/cell for guinea pig and Columbian ground squirrel erythrocytes, and 280 sites/cell for thirteen-lined ground squirrel cells. There was no significant difference in ouabain bound per cell at 37 and 5 degrees C. Calculated turnover numbers for Columbian and thirteen-lined ground squirrel and guinea pig red cell sodium pumps at 37 degrees C were about equal, being 77-100 and 100-129 s-1, respectively. At 5 degrees C red cells from ground squirrels performed significantly better, the turnover numbers being 1.0-2.3 s-1 compared with 0.42-0.47 s-1 for erythrocytes of guinea pig. The results do not accord with a hypothesis that cold-sensitive Na pumps are blocked in one predominant form.

Effects of atp or phosphate on passive rubidium fluxes mediated by Na-K-ATPase reconstituted into phospholipid vesicles

1. The passive Rb fluxes mediated by the Na-K pump in reconstituted vesicles, described by Karlish & Stein (1982), are affected by ATP or by phosphate acting separately.2. Rb-Rb exchange through inside-out pumps is stimulated by ATP at low concentrations and is inhibited at high concentrations. There are mutual effects of Rb at cytoplasmic sites and ATP. The higher is the Rb concentration, the greater is the degree of stimulation and the less is the inhibition of exchange by ATP, and the higher are the concentrations of ATP required to produce effects. ATP stimulates Rb-Rb exchange maximally by about 5-fold.3. There are similar effects of ATP on zero-trans net Rb uptake through inside-out pumps. However, much lower degrees of stimulation and greater inhibition of the net flux by ATP are observed, and much lower concentrations of ATP are required for these effects, by comparison with those on Rb-Rb exchange.4. Rb uptake on inside-out pumps in Na-loaded vesicles shows only inhibition by ATP.5. Phosphate effects require the presence of Mg(0) ions. At low Mg(0) concentrations (up to 100 muM) phosphate moderately stimulates Rb uptake into Rb-free or Rb-loaded vesicles (about 50%), but has no effect on Rb uptake into Na-loaded vesicles. At millimolar concentrations of Mg(0) ions, phosphate strongly inhibits the Rb uptake into Rb-free or Na-loaded vesicles but has no effect on Rb uptake into Rb-loaded vesicles.6. The separate effects of ATP and of phosphate are explained in terms of the model proposed by Karlish & Stein (1982), modified to take into account stimulation of the conformational transition E(2)(Rb)(occ) --> E(1) Rb by ATP, and stimulation of the conformational transition E(2)(Rb)(occ) --> E(2) Rb by phosphate due to phosphorylation of the protein.

Combined effects of ATP and phosphate on rubidium exchange mediated by Na-K-ATPase reconstituted into phospholipid vesicles

1. Phospholipid vesicles reconstituted with Na-K-ATPase show an (ATP+phosphate)-stimulated Rb-Rb exchange, with properties similar to the K-K exchange of human red cells. This includes a rate 15-20% of the rate of active ATP-dependent Na-K exchange.2. We have studied activation of this Rb-Rb exchange by ATP at fixed phosphate concentrations and by phosphate at fixed ATP concentrations. It is found for both ATP and phosphate that with low concentrations of the fixed ligand an increase in concentration of the complementary ligand produces first stimulation and then inhibition of Rb-Rb exchange. At high concentrations of the fixed ligand the complementary ligand shows only saturation behaviour.3. The pattern of activation and of inhibition by ATP and by phosphate is affected by the Rb(0) concentration in the exterior medium, in that higher concentrations of Rb(0) counteract inhibitory effects of high concentrations of ATP and phosphate.4. (ATP+phosphate)-stimulated Rb-Rb exchange is activated by Rb(0) in the exterior medium along a sigmoid curve. An increase of Rb(i) within the vesicles, which raises the maximal velocity of Rb-Rb exchange, is accompanied by a smaller increase in the Rb(0) concentration required for half-maximal stimulation of the Rb-Rb exchange.5. The data are interpreted in terms of a model similar to those proposed by Karlish & Stein (1982a,b), but extended to include simultaneous effects of ATP and phosphate. Inhibitions by high concentration of ATP or phosphate arise as a result of stabilization of E(1) ATP or E(2)-P forms respectively, in the presence of low concentrations of the complementary ligand. With high concentrations of the fixed ligand, saturation behaviour of the varying ligand is observed because the occluded Rb forms become the dominant transport intermediates. The occluded Rb forms bind both ATP and phosphate weakly and independently. The effects of ATP together with phosphate are accounted for by a simple combination of their separate effects on the Rb-Rb exchange.6. We suggest that the functional role of the occluded Rb form E(2) (Rb)(occ) in active transport is to minimize passive cation leaks through the system and allow control of the direction of cation movements by binding of physiological ligands such as ATP or phosphate.