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

Exercise and fatigue: integrating the role of K+, Na+ and Cl- in the regulation of sarcolemmal excitability of skeletal muscle

Perturbations in K+ have long been considered a key factor in skeletal muscle fatigue. However, the exercise-induced changes in K+ intra-to-extracellular gradient is by itself insufficiently large to be a major cause for the force decrease during fatigue unless combined to other ion gradient changes such as for Na+. Whilst several studies described K+-induced force depression at high extracellular [K+] ([K+]e), others reported that small increases in [K+]e induced potentiation during submaximal activation frequencies, a finding that has mostly been ignored. There is evidence for decreased Cl- ClC-1 channel activity at muscle activity onset, which may limit K+-induced force depression, and large increases in ClC-1 channel activity during metabolic stress that may enhance K+ induced force depression. The ATP-sensitive K+ channel (KATP channel) is also activated during metabolic stress to lower sarcolemmal excitability. Taking into account all these findings, we propose a revised concept in which K+ has two physiological roles: (1) K+-induced potentiation and (2) K+-induced force depression. During low-moderate intensity muscle contractions, the K+-induced force depression associated with increased [K+]e is prevented by concomitant decreased ClC-1 channel activity, allowing K+-induced potentiation of sub-maximal tetanic contractions to dominate, thereby optimizing muscle performance. When ATP demand exceeds supply, creating metabolic stress, both KATP and ClC-1 channels are activated. KATP channels contribute to force reductions by lowering sarcolemmal generation of action potentials, whilst ClC-1 channel enhances the force-depressing effects of K+, thereby triggering fatigue. The ultimate function of these changes is to preserve the remaining ATP to prevent damaging ATP depletion.

KdpFABC reconstituted in Escherichia coli lipid vesicles: substrate dependence of the transport rate

KdpFABC complexes were reconstituted in Escherichia coli lipid vesicles, and ion pumping was activated by addition of ATP to the external medium which corresponds to the cytoplasm under physiological conditions. ATP-driven potassium extrusion was studied in the presence of various substrates potentially influencing transport rate. The pump current was detected as a decrease of the membrane potential by the voltage-sensitive dye DiSC3(5). The results indicate that high cytoplasmic K(+) concentrations have an inhibitory effect on the KdpFABC complex. The pump current decreased to ∼25% of the maximal value at 140 mM K(+) and minimal Mg(2+)concentrations. This effect could be counteracted with increased Mg(2+) concentrations on the cytoplasmic side. This observation may be explained by the Gouy-Chapman effect of two Mg(2+) ions probably bound with a K1/2 of 0.8 mM close to the entrance of the access channel to the binding sites. This factor ensures that under physiological conditions the rate-limiting effect of K(+) release is significantly reduced. Also both ADP and inorganic phosphate are able to reduce the turnover rate of the pump by reversing the phosphorylation step (Ki of 151 μM) and the dephosphorylation step (Ki of 268 μM), respectively. In the case of the DDM-solubilized KdpFABC complex, activation energy under turnover conditions was previously found to be 55 kJ/mol, and the o-vanadate inhibition constant is shown here to be ∼1 μM, which is in agreement with values reported for other P-type ATPases. In the case of the reconstituted enzyme, however, significant differences were observed that have to be assigned to effects of the lipid bilayer environment. The activation energy was increased by a factor of 2, whereas the inhibition by o-vanadate became reduced in a way that only ∼66% of the enzyme could be inhibited and the inhibition constant was increased to a value of ∼60 μM.

My Passion and Passages with Red Blood Cells

This article mainly presents, in sequential panels of time, an overview of my professional involvements and laboratory experiences. I became smitten with red blood cells early on, and this passion remains with me to this day. I highlight certain studies, together with those who performed the work, recognizing that it was necessary to limit the details and the topics chosen for discussion. I am uncertain of the interest a personal account has for others, but at least it's here for the record.

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.

Charge translocation by the Na+/K+ pump under Na+/Na+ exchange conditions: intracellular Na+ dependence

The effect of intracellular (i) and extracellular (o) Na+ on pre-steady-state transient current associated with Na+/Na+ exchange by the Na+/K+ pump was investigated in the vegetal pole of Xenopus oocytes. Current records in response to 40-ms voltage pulses from -180 to +100 mV in the absence of external Na+ were subtracted from current records obtained under Na+/Na+ exchange conditions. Na+-sensitive transient current and dihydroouabain-sensitive current were equivalent. The quantity of charge moved (Q) and the relaxation rate coefficient (ktot) of the slow component of the Nao+-sensitive transient current were measured for steps to various voltages (V). The data were analyzed using a four-state kinetic model describing the Na+ binding, occlusion, conformational change, and release steps of the transport cycle. The apparent valence of the Q vs. V relationship was near 1.0 for all experimental conditions. When extracellular Na+ was halved, the midpoint voltage of the charge distribution (Vq) shifted -25.3+/-0.4 mV, which can be accounted for by the presence of an extracellular ion-well having a dielectric distance delta=0.69+/-0.01. The effect of changes of Nai+ on Nao+-sensitive transient current was investigated. The midpoint voltage (Vq) of the charge distribution curve was not affected over the Nao+ concentration range 3.13-50 mM. As Nai+ was decreased, the amount of charge measured and its relaxation rate coefficient decreased with an apparent Km of 3.2+/-0.2 mM. The effects of lowering Nai+ on pre-steady-state transient current can be accounted for by decreasing the charge available to participate in the fast extracellular Na+ release steps, by a slowly equilibrating (phosphorylation/occlusion) step intervening between intracellular Na+ binding and extracellular Na+ release.

Effect of ADP on Na(+)-Na(+) exchange reaction kinetics of Na,K-ATPase

The whole-cell voltage-clamp technique was used in rat cardiac myocytes to investigate the kinetics of ADP binding to phosphorylated states of Na,K-ATPase and its effects on presteady-state Na(+)-dependent charge movements by this enzyme. Ouabain-sensitive transient currents generated by Na,K-ATPase functioning in electroneutral Na(+)-Na(+) exchange mode were measured at 23 degrees C with pipette ADP concentrations ([ADP]) of up to 4.3 mM and extracellular Na(+) concentrations ([Na](o)) between 36 and 145 mM at membrane potentials (V(M)) from -160 to +80 mV. Analysis of charge-V(M) curves showed that the midpoint potential of charge distribution was shifted toward more positive V(M) both by increasing [ADP] at constant Na(+)(o) and by increasing [Na](o) at constant ADP. The total quantity of mobile charge, on the other hand, was found to be independent of changes in [ADP] or [Na](o). The presence of ADP increased the apparent rate constant for current relaxation at hyperpolarizing V(M) but decreased it at depolarizing V(M) as compared to control (no added ADP), an indication that ADP binding facilitates backward reaction steps during Na(+)-Na(+) exchange while slowing forward reactions. Data analysis using a pseudo three-state model yielded an apparent K(d) of approximately 6 mM for ADP binding to and release from the Na,K-ATPase phosphoenzyme; a value of 130 s(-1) for k(2), a rate constant that groups Na(+) deocclusion/release and the enzyme conformational transition E(1) approximately P --> E(2)-P; a value of 162 s(-1)M(-1) for k(-2), a lumped second-order V(M)-independent rate constant describing the reverse reactions; and a Hill coefficient of approximately 1 for Na(+)(o) binding to E(2)-P. The results are consistent with electroneutral release of ADP before Na(+) is deoccluded and released through an ion well. The same approach can be used to study additional charge-moving reactions and associated electrically silent steps of the Na,K-pump and other transporters.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Volume stabilization in human erythrocytes: combined effects of Ca2+-dependent potassium channels and adenylate metabolism

A mathematical model describing the possible role of Ca(2+)-dependent K+ channels and adenylate metabolism in volume stabilization of human erythrocytes was developed. The model predicts that the red blood cell volume can be stabilized either dynamically or stationary over a broad range of cell membrane permeabilities to cations. The dynamic stabilization results from the operation of Ca(2+)-dependent potassium channels. The erythrocyte volume changes less than 10% if the membrane permeability changes abruptly to a value in the range from half to sevenfold higher than the normal one. The stationary stabilization is achieved via controlling the adenylate metabolism. The stationary value of cell volume changes less than 10% when the membrane permeability varies from half the normal value to 15-fold higher than the normal value.

Change of Na+ pump current reversal potential in sheep cardiac Purkinje cells with varying free energy of ATP hydrolysis

1. The Na(+)-K+ pump current, Ip, of cardioballs from isolated sheep cardiac Purkinje cells was measured at 30-34 degrees C by means of whole-cell recording. 2. Under physiological conditions Ip is an outward current. Experimental conditions which cause a less negative free energy of intracellular ATP hydrolysis (delta GATP) and steeper sarcolemmal gradients for the pumped Na+ and Cs+ ions evoked an Ip in the inward direction over a wide range of membrane potentials. The reversal of the Ip direction was reversible. 3. The inwardly directed Ip increased with increasingly negative membrane potentials and amounted to -0.13 +/- 0.03 microA cm-2 (mean +/- S.E.M.; n = 6) at -95 mV. 4. The reversal potential (Erev) of Ip was studied as a function of delta GATP at constant sarcolemmal gradients of the pumped cations. 5. In order to vary delta GATP the cell interior was dialysed with patch pipette solutions containing 10 mM ATP and different concentrations of ADP and inorganic phosphate. The media were composed to produce delta GATP levels of about -58, -49 and -39 kJ mol-1. 6. A less negative delta GATP shifted Erev to more positive membrane potentials. From measurements of Ip as a function of membrane potential Erev was estimated to be -195, -115 and -60 mV at delta GATP levels of approximately -58, -49 and -39 kJ mol-1, respectively. The calculated Erev amounted to -224 mV at delta GATP approximately -58 kJ mol-1, -126 mV at delta GATP approximately 49 kJ mol-1 and -24 mV at delta GATP approximately -39 kJ mol-1. 7. Possible reasons for the discrepancy between estimated and calculated Erev values are discussed. 8. Shifting delta GATP to less negative values not only altered Erev but also diminished Ip at each membrane potential tested. The maximal Ip (Ip,max), which can be activated by external Cs+ (Cs+o), decreased under these conditions, whereas [Cs+]o causing half-maximal Ip activation remained unchanged. Similarly, the voltage dependence of Ip activation by Cs+o was unaffected. 9. It is concluded that Erev of Ip varies with delta GATP at constant sarcolemmal gradients of the pumped cations. This agrees with thermodynamic considerations.

Functional coupling of phosphorylation and nucleotide binding sites in the proteolytic fragments of Na+/K(+)-ATPase

Cleavage of the alpha-subunit of Na+/K(+)-ATPase by trypsin at Arg438-Ala439 causes enzyme inhibition which has been suggested to be due to altered alignment of phosphorylation site on the 48-kDa N-terminal fragment with nucleotide binding site on the 64-kDa C-terminal fragment. Our aims were to test this hypothesis and to assess the effect of the cleavage on the enzyme's two ATP sites. Na(+)-dependent phosphorylation of the partially cleaved enzyme by ATP showed that K0.5 values of ATP for phosphorylations of intact alpha and 48-kDa peptide were the same (0.4 microM). Unchanged interactions among the residues across the cleavage site were also indicated by data showing that reaction of fluorescein isothiocyanate with the 64-kDa peptide blocked phosphorylation of the 48-kDa peptide by ATP. ATP is known to block the reaction of fluorescein isothiocyanate with the enzyme. Experiments on the partially cleaved enzyme showed that K0.5 of ATP for protection of alpha was 30-60 microM, and the value for the protection of interacting 48-kDa and 64-kDa peptides was 1-3 mM. Evidently, while the cleavage does not affect the high affinity catalytic site, it disrupts the allosteric low affinity ATP site. Experiments on reconstituted preparations showed that the cleavage abolished ATP-dependent Na+/K+ exchange, Pi+ATP-dependent Rb+/Rb+ exchange, ATP-dependent Na+/Na+ exchange, and ADP+ATP-dependent Na+/Na+ exchange activities. Selective disruption of the low affinity ATP site accounts for the inhibitions of all functions involving K+(Rb+), based on the established role of this site in the control of K+ access channels. Cleavage-induced inhibitions of other activities, however, suggest additional roles of the low affinity ATP site in the reaction cycle.

The Na+/K+ pump of cardiac Purkinje cells is preferentially fuelled by glycolytic ATP production

The role of glycolysis and oxidative phosphorylation in providing the ATP for the cardiac Na+/K+ pump was studied in cardioballs from sheep Purkinje fibres. As an indicator of the pump activity, the pump current Ip was measured at -20 mV and 30-33 degrees C by means of whole-cell recording. During intracellular perfusion with a pipette solution containing 5 mM ATP and 15 mM glucose Ip reached a maximum within 8 min and declined to 50% of this value within 27 min after gaining access to the cell interior. Perfusion with an ATP- and glucose-free medium barely enhanced the Ip decline. Inhibition of the oxidative phosphorylation by carbonylcyanide m-chlorophenylhydrazone (CCCP, 2 microM or 20 microM) moderately accelerated the effect of the ATP- and glucose-free pipette solution. Addition of 2 mM iodoacetic acid (an inhibitor of glycolysis) to the latter medium further enhanced the Ip decrease with time. Inhibition of the glycolytic ATP synthesis by 2-deoxy-D-glucose (5 mM) caused a dramatic decline of Ip to half of its maximum within 7.3 min. Pyruvate (5 mM) and inorganic phosphate (2 mM) did not affect the fast Ip decline evoked by the ATP- and glucose-free, 2-deoxyglucose-containing medium, whereas 2 microM CCCP still hastened the fast Ip decrease slightly. This effect of complete metabolic inhibition was reversed by switching to an inhibitor-free pipette solution containing 15 mM ATP. It is concluded that the Na+/K+ pump of cardiac Purkinje cells is preferentially fuelled by glycolytic ATP synthesis.

Na+/K+/Cl- cotransport in resealed ghosts from erythrocytes of the Milan hypertensive rats

The erythrocytes (RBC) of the Milan hypertensive rats (MHS) have a smaller volume and faster Na+/K+/Cl- cotransport than RBC from normotensive controls (MNS). The difference in Na+/K+/Cl- cotransport is no longer present in inside-out Vesicles (IOV) of RBC membrane. To differentiate between cytoplasmic or membrane skeleton abnormalities as possible causes of these differences. Resealed ghosts (RG) were used to measure ion transport systems. The following results have been obtained: (1) RG from MHS have a smaller volume than MNS (mean +/- S.E. 20.7 +/- 0.45 vs. 22.09 +/- 0.42 fl, P < 0.05). (2) RG showed a bumetanide-sensitive Na efflux that retains the characteristics of the Na+/K+/Cl- cotransport of the original RBC: it is K(+)- and Cl(-)-sensitive and dependent on the intracellular Na+ concentration. (3) The Na+/K+/Cl- cotransport was faster in RG from MHS than in those from MNS (mean +/- S.E. 0.095 +/- 0.01 vs. 0.066 +/- 0.01 rate constant h-1, P < 0.01). These results, together with those of IOV, support the hypothesis that an abnormality in the membrane skeletal proteins may play a role in the different Na+/K+/Cl- cotransport modulation between MHS and MNS erythrocytes.

ATP dependence of Na(+)-K+ pump of cold-sensitive and cold-tolerant mammalian red blood cells

1. The ATP concentration of intact, cold-tolerant (ground squirrel) red cells and cold-sensitive (guinea-pig and human) red cells was monitored by use of the firefly tail, luciferin-luciferase assay. ATP kinetics of the pump in intact red blood cells was investigated by altering cell [ATP] by progressive depletion of ATP in the presence of 2-deoxy-D-glucose and then by measurement of ouabain-sensitive K+ influx at each level of [ATP] at various temperatures between 37 and 5 degrees C. Na(+)-K(+)-ATPase activity of broken membranes was also determined in parallel experiments using ouabain-sensitive release of 32P from [gamma-32P]ATP as a measure of activity. 2. Without depletion, there is no immediate decrease in [ATP] of intact cold-sensitive cells at low temperature (5 degrees C) at times when there are marked differences in the activities of the Na(+)-K+ pump of cold-tolerant and cold-sensitive cells. 3. At 37 degrees C Na(+)-K(+)-ATPase of all three species exhibited two components of ATP dependence at 37 degrees C, one with high velocity, low affinity, the other with low velocity, high affinity. Affinities of both components rose with cooling. 4. A similar, two component pattern was observed in intact guinea-pig and human red cells at 37 degrees C, except that the segment corresponding to the high affinity component had an apparent Km (Michaelis-Menten constant) 3- to 4-fold higher than that of the broken membrane preparation. 5. Cooling intact guinea-pig and human red cells decreased the apparent affinity of the high velocity, low affinity component for ATP, so that at 20 degrees C the value of Km approached or exceeded the levels of physiological ATP concentration. Below 20 degrees C only one component with values corresponding to that of the low velocity, high affinity component could be observed. 6. In intact ground squirrel cells only the low affinity, high velocity component was apparent between 37 and 5 degrees C. Its affinity for ATP rose with cooling between 37 and 5 degrees C.

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.

NMR detection of creatine kinase expressed in liver of transgenic mice: determination of free ADP levels

To use the equilibrium established by creatine kinase (CK) to determine hepatic free ADP levels, the transcriptional control elements of the transthyretin gene were used to direct expression of the CK B isozyme to the livers of transgenic mice. Activities of CK ranging from 80-250 mumol per min per g (wet weight) were detected in liver extracts from five founder mice. The CK activity was stably transmitted to subsequent generations. Isozyme gels and immunoblots confirmed that the activity detected in extracts was due to the B isozyme of CK. Immunohistology indicated that the protein was expressed uniformly throughout the liver and was localized primarily to the cytoplasm. 31P NMR spectroscopy was used to detect the metabolic product of the CK reaction, phosphocreatine, demonstrating that the enzyme was active in vivo. The phosphocreatine level fell rapidly during anoxia (t1/2 = 1 min), indicating that the CK reaction was integrated into hepatic energy metabolism. The equilibrium established by CK was used to calculate a hepatic free ADP level of 0.059 +/- 0.004 mumol/g (wet weight). In vivo NMR studies of these mice will be valuable for studying the role of free ADP in regulating liver metabolism.

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)

Effects of the ATP, ADP and inorganic phosphate on the transport rate of the Na+,K+-pump

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. In the reconstituted system the pump can be activated by adding ATP to the external medium. ATP-driven potassium extrusion by the Na+,K+-pump was studied using a voltage-sensitive dye in the presence of valinomycin. ADP strongly reduced the turnover rate of the pump with a concentration for half-maximal inhibition of cD,1/2 = 0.1 mM. cD,1/2 was found to be virtually independent of ATP concentration, indicating that the inhibition is non-competitive with respect to ATP. The non-competitive inhibition by ADP can be explained on the basis of the Post-Albers reaction cycle of the Na+,K+-pump, assuming that the main action of ADP is the reversal of the phosphorylation step. A similar 'product inhibition' was observed with inorganic phosphate, but at much higher concentrations (cP,1/2 = 14 mM).