Side-dependent effects of internal versus external Na and K on ouabain binding to reconstituted human red blood cell ghosts

On the functional use of the membrane compartmentalized pool of ATP by the Na+ and Ca++ pumps in human red blood cell ghosts

Previous evidence established that a sequestered form of adenosine triphosphate (ATP pools) resides in the membrane/cytoskeletal complex of red cell porous ghosts. Here, we further characterize the roles these ATP pools can perform in the operation of the membrane's Na(+) and Ca(2+) pumps. The formation of the Na(+)- and Ca(2+)-dependent phosphointermediates of both types of pumps (E(Na)-P and E(Ca)-P) that conventionally can be labeled with trace amounts of [gamma-(3)P]ATP cannot occur when the pools contain unlabeled ATP, presumably because of dilution of the [gamma-(3)P]ATP in the pool. Running the pumps forward with either Na(+) or Ca(2+) removes pool ATP and allows the normal formation of labeled E(Na)-P or E(Ca)-P, indicating that both types of pumps can share the same pools of ATP. We also show that the halftime for loading the pools with bulk ATP is 10-15 minutes. We observed that when unlabeled "caged ATP" is entrapped in the membrane pools, it is inactive until nascent ATP is photoreleased, thereby blocking the labeled formation of E(Na)-P. We also demonstrate that ATP generated by the membrane-bound pyruvate kinase fills the membrane pools. Other results show that pool ATP alone, like bulk ATP, can promote the binding of ouabain to the membrane. In addition, we found that pool ATP alone functions together with bulk Na(+) (without Mg(2+)) to release prebound ouabain. Curiously, ouabain was found to block bulk ATP from entering the pools. Finally, we show, with red cell inside-outside vesicles, that pool ATP alone supports the uptake of (45)Ca by the Ca(2+) pump, analogous to the Na(+) pump uptake of (22)Na in this circumstance. Although the membrane locus of the ATP pools within the membrane/cytoskeletal complex is unknown, it appears that pool ATP functions as the proximate energy source for the Na(+) and Ca(2+) pumps.

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.

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.

Ouabain binding, ATP hydrolysis, and Na+,K(+)-pump activity during chemical modification of brain and muscle Na+,K(+)-ATPase

The effects of 16 group-specific, amino acid-modifying agents were tested on ouabain binding, catalytical activity of membrane-bound (rat brain microsomal), sodium dodecyl sulfate-treated Na+,K(+)-ATPase, and Na+,K(+)-pump activity in intact muscle cells. With few exceptions, the potency of various tryptophan, tyrosine, histidine, amino, and carboxy group-oriented drugs to suppress ouabain binding and Na+,K(+)-ATPase activity correlated with inhibition of the Na+,K(+)-pump electrogenic effect. ATP hydrolysis was more sensitive to inhibition elicited by chemical modification than ouabain binding (membrane-bound or isolated enzyme) and than Na+,K(+)-pump activity. The efficiency of various drugs belonging to the same "specificity" group differed markedly. Tyrosine-oriented tetranitromethane was the only reagent that interfered directly with the cardiac receptor binding site as its inhibition of ouabain binding was completely protected by ouabagenin preincubation. The inhibition elicited by all other reagents was not, or only partially, protected by ouabagenin. It is surprising that agents like diethyl pyrocarbonate (histidine groups) or butanedione (arginine groups), whose action should be oriented to amino acids not involved in the putative ouabain binding site (represented by the -Glu-Tyr-Thr-Trp-Leu-Glu- sequence), are equally effective as agents acting on amino acids present directly in the ouabain binding site. These results support the proposal of long-distance regulation of Na+,K(+)-ATPase active sites.

Digitalis-like properties of an inhibitor of the Na+/K+ pump in human cerebrospinal fluid

Work originally reported by my laboratory has established that a constituent of human cerebrospinal fluid (CSF) specifically inhibits the Na+/K+ pump in human red cells and the activity of the enzyme (Na+/K+)-ATPase. Furthermore, we have shown that the inhibitory compound has a molecular weight of approx. 600 and is sensitive to proteolytic digestion, indicating that it is a small peptide. I describe here that the inhibitor of the Na+/K+ pump in human CSF mimics the effects the digitalis glycosides in 3 different assay systems: the Na+/K+ pump in human red cell, the (Na+/K+)-ATPase activity of a purified enzyme and the specific binding of [3H]ouabain to its receptor in the red cell membrane. Moreover, the inhibitor in human CSF is a competitive inhibitor of the stimulation of the Na+/K+ pump by extracellular K+. Based on these findings, I propose that a small peptide with digitalis-like properties present in human CSF is an endogenous regulator of the Na+/K+ pump in cells of the central nervous system. This digitalis-like substance could be one factor regulating the K+ concentration of the CSF and controlling the secretion of CSF by the choroid plexus.

Stimulation of K-C1 cotransport in rat red cells by a hemolytic anemia-producing metabolite of dapsone

Dapsone, a sulfone compound used in the treatment of leprosy and, more recently, Pneumocystis carinii pneumonia, produces as a major side effect a hemolytic anemia. This anemia is characterized by oxidation of hemoglobin to methemoglobin and increased splenic uptake of red blood cells. Using a rat model, Grossman and Jollow (J. Pharmacol. Exp. Ther. 244: 118-125, 1988) found that dapsone hydroxylamine (DDS-NOH), a dapsone metabolite, is responsible for its hemolytic effect in vivo. DDS-NOH also promotes hemoglobin binding to SH groups on rat red cell membrane proteins (Budinsky et al., FASEB J. 2: A801, 1988). Since the binding of hemoglobin and other reagents (e.g., N-ethylmaleimide) to membrane SH groups has been associated with increased K transport in red blood cells, we examined the effect of DDS-NOH on K efflux from rat red blood cells in vitro. Cells shrink when exposed to DDS-NOH (100 microM) in media with plasma-like ionic composition. This shrinkage is prevented if extracellular K is raised to 110 mM or if intra- and extracellular Cl are replaced by methylsulfate (MeSO4), suggesting involvement of a K-Cl cotransport pathway. Indeed, 100 microM DDS-NOH produces a 4- to 5-fold increase in K efflux in cells containing Cl but less than a 2-fold increase in cells containing MeSO4. This stimulatory effect is specific for K; Na efflux is slightly inhibited by 100 microM DDS-NOH. The concentrations of DDS-NOH required for half-maximal stimulation of Cl-dependent K efflux (53 microM) is similar to its half-maximal hemolytic concentration in rats (approximately 100 microM). Furthermore, the stimulation of Cl-dependent K efflux by DDS-NOH is greater than 80% reversed by subsequent treatment of the cells with dithiothreitol, suggesting involvement of SH groups. Our results indicate that DDS-NOH exposure stimulates an apparent K-Cl cotransport in rat red blood cells, resulting in cell shrinkage under physiological ionic conditions. Since shrinkage of red blood cells renders them less deformable (Mohandas et al., J. Clin. Invest. 66: 563-573, 1980), this suggests a pathophysiological mechanism whereby DDS-NOH exposure in vivo could promote increased splenic uptake of red blood cells and hemolytic anemia.

Different sensitivity of ATP + Mg + Na (I) and Pi + Mg (II) dependent types of ouabain binding to phospholipase A2

The effect of phospholipase A2 and of related agents on ouabain binding and Na, K-ATPase activity were studied in intact and detergent-treated membrane preparations of rat brain cortex and pig kidney medulla. It was found that phospholipase A2 (PLA2) may distinguish or dissociate ouabain binding complexes I (ATP + Mg + Na) and II (Pi + Mg), stimulating the former and inhibiting the latter. Procedures which break the permeability barriers of vesicular membrane preparations, such as repeated freezing-thawing, sonication or hypoosmotic shock failed to mimic the effect of PLA2, indicating that it was not acting primarily by opening the inside-out oriented vesicles. The detergent digitonin exhibited similar effects on ouabain binding in both ATP + Mg + Na and Pi + Mg media. Other detergents were ineffective. The ability of PLA2 to distinguish between ouabain binding type I and II can be manifested even in SDS-treated, purified preparations of Na, K-ATPase. The number of ATP + Mg + Na-dependent sites is unchanged, while the Pi + Mg-dependent sites are decreased in number in a manner similar to that seen in original membranes. This inhibition is completely lost in the reconstituted Na, K-ATPase system, where the ATP- as well as Pi-oriented ouabain sites are inhibited by PLA2.

Activation of sodium transport in human erythrocytes by beta-adrenoceptor stimulation in vivo

Beta-adrenoceptor stimulation in vivo shifts potassium into the cells. To examine whether human erythrocytes participate in this process, we measured, along with serum or plasma potassium, the concentrations of potassium and sodium in erythrocytes. Beta-adrenoceptor stimulation was obtained by infusion of either fenoterol or hexoprenaline into 6 volunteers at rest or by endogenous amines provoked in 14 volunteers during ergometric exercise. Metabolic effects were followed at rest on serum insulin, C-peptide, and growth hormone levels, and during exercise on pH on lactate concentration in blood. The potassium concentration (mean +/- S.E.M.) dropped (p less than 0.01) in serum from 4.64 +/- 0.37 to 3.19 +/- 0.43 mmol x l-1 in the first hour at rest and in plasma from 5.70 +/- 0.93 to 4.63 +/- 0.45 in 90 sec directly after exercise. The concentration of erythrocyte sodium dropped (p less than 0.001) from 9.68 +/- 0.73 to 8.81 +/- 0.62 mmol x l-1 in cells and from 9.62 +/- 1.16 to 8.55 +/- 1.24 during exercise for 90 s, respectively. Changes in the concentration ratio of cellular sodium to potassium confirmed this sodium shift. An increased sodium transport in erythrocytes due to beta-adrenoceptor stimulation in vivo appears to complement a shift of serum potassium into the cells and may be mediated by the membrane-bound sodium, potassium ATPase.

(Na+,K+)-ATPase and noradrenergic function: effects of chronic ethanol

The experiments in this paper examined interactions between ethanol and repeated noradrenergic stimulation in vivo on regulation of (Na+,K+)-ATPase. The increase in ouabain binding and K+-phosphatase activity associated with (Na+,K+)-ATPase in rats treated with repeated yohimbine injections was prevented by chronic ethanol. Ethanol did not affect the yohimbine-induced alterations in noradrenergic receptor binding or in content of the norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol, showing that prevention of noradrenergic stimulation of (Na+,K+)-ATPase was not caused by a decrease in availability of norepinephrine. In addition, norepinephrine depletion with the neurotoxin DSP4 did not prevent the increases in (Na+,K+)-ATPase indices during chronic ethanol treatment, showing that they did not result from increased norepinephrine exposure. These results suggest that chronic ethanol reduces sensitivity of (Na+,K+)-ATPase to norepinephrine in vivo, possibly as a consequence of membrane effects of ethanol tolerance.

Potassium-chloride cotransport in resealed human red cell ghosts

Furosemide-inhibitable K influx is threefold higher in resealed ghosts of human erythrocytes than in intact cells. The enhancement is specific for K in that furosemide-inhibitable Na influx is the same in resealed ghosts and intact cells. The enhanced K influx resembles K-Cl cotransport in intact cells in that it requires Cl but not Na. N-ethylmaleimide (NEM), which stimulates furosemide-inhibitable K influx in intact cells, is without effect (or slightly inhibitory) in resealed ghosts. The failure of NEM to enhance the flux was not due to low ATP in the ghosts. These findings suggest that enhancement of the K flux in ghosts occurs by oxidation of membrane protein sulfhydryl groups, known to occur with lysis, the same sulfhydryl groups at which NEM acts by alkylation. This conclusion is supported by two observations: dithiothreitol completely prevents the increase in K influx in ghosts; this agent inhibits both oxidation of sulfhydryl groups and alkylation of them by NEM; and K influx in resealed ghosts is sensitive to changes in cell volume, just as it is in NEM-treated intact cells.

Interaction of cardiac glycosides and Na,K-ATPase is regulated by effector-controlled equilibrium between two limit enzyme conformers

The paper describes the dissociation parameters of the complexes between [3H]-digitoxin and Na,K-ATPase (Na+ + K+-activated, Mg2+-dependent ATP phosphohydrolase, E.C. 3.6.1.3) from pig cardiac muscle and brain cortex formed and dissociated in the presence of different combinations and concentrations of the enzyme effectors ATP, Mg2+, Na+ and K+. Systematic variation of effector-ligation of Na,K-ATPase allowed production of glycoside complexes with two enzyme conformers only, which showed either rapid or slow dissociation kinetics. Appropriate changes of enzyme ligation allowed the interconversion of the two conformer types. Biphasic, rapid and slow glycoside release was not bound with the presence of two Na,K-ATPase isozymes, but caused by the enzyme ligation-determined coexistence of the two conformers of Na,K-ATPase. The rate constants for the rapid and slow glycoside release were within the complexes of each dissociation type much alike indicating uniform isomerization kinetics of the two conformers even when differently liganded. Taken together, the observations indicated the effector-controlled isomerizations of two conformers of Na,K-ATPase possessing different geometries of the glycoside binding domain. Present findings and relevant literature data were integrated in a circular, consecutive and simultaneous model for induced conformation changes that accounted for the regulation of the interaction of cardiac glycosides and Na,K-ATPase through an effector-controlled equilibrium between two limit enzyme conformers.

Isolation of basolateral and brush-border membranes from the rabbit kidney cortex. Vesicle integrity and membrane sidedness of the basolateral fraction

A rapid and reproducible method has been developed for the simultaneous isolation of basolateral and brush-border membranes from the rabbit renal cortex. The basolateral membrane preparation was enriched 25-fold in (Na+ + K+)-ATPase and the brush-border membrane fraction was enriched 12-fold in alkaline phosphatase, whereas the amount of cross-contamination was low. Contamination of these preparations by mitochondria and lysosomes was minimal as indicated by the low specific activities of enzyme markers, i.e., succinate dehydrogenase and acid phosphatase. The basolateral fraction consisted of 35-50% sealed vesicles, as demonstrated by detergent (sodium dodecyl sulfate) activation of (Na+ + K+)-ATPase activity and [3H]ouabain binding. The sidedness of the basolateral membranes was estimated from the latency of ouabain-sensitive (Na+ + K+)-ATPase activity assayed in the presence of gramicidin, which renders the vesicles permeable to Na+ and K+. These studies suggest that nearly 90% of the vesicles are in a right-side-out orientation.

Inhibition of the sodium pump in guinea-pig ventricular muscle by dihydro-ouabain: effects of external potassium and sodium

The inhibition of the electrogenic pump current in quiescent guinea-pig ventricular muscle by dihydro-ouabain (DHO) was studied with the three-micro-electrode voltage-clamp technique described previously (Daut, 1982c). From dose-response curves of the drug-induced current change (ID) the equilibrium dissociation constant of the binding of DHO to the Na-K pump (KD) and the electrogenic pump current flowing in the steady state (Ip) were inferred (Daut & Rüdel, 1982b). The external K concentration ([K]o) was varied between 2 and 4.5 mM (substituted by Na). KD was found to increase with increasing [K]o. A plot of log KD versus log [K]o gave a straight line with a slope of about 1.5. The time constants of the onset (tau on) and decay (tau off) of ID are supposed to represent the chemical kinetics of binding and unbinding of the drug (Daut & Rüdel, 1981, 1982b). Tau on was found to be inversely related to [K]o whereas tau off was found to be independent of [K]o. Ip was found to be independent of [K]o. This was interpreted to indicate that in the steady state Ip is mainly determined by the passive influx of Na into the cell, which may be relatively insensitive to small changes in [K]o. The effects of [K]o on the drug-induced current change are consistent with competitive inhibition of the binding of DHO to the Na-K pump. It is suggested that K ions and cardiac glycosides compete for extracellular binding sites on the same conformation of the Na-K pump. The external Na concentration ([Na]o) was varied between 147 and 49 mM (substituted by choline or Tris). Reduction of [Na]o produced a proportional decrease of Ip. This may be a consequence of the accompanying reduction of passive Na influx and the resulting decrease in intracellular Na activity (alpha iNa). Reduction of [Na]o markedly increased KD. This effect may be mediated by competition between Na and K at the K-loading sites of the pump and/or by separate modulatory Na-binding sites. It is concluded that the well known effects of external Na and K on the positive inotropic action of cardiac glycosides can be fully accounted for by the marked changes in the apparent binding affinity of the drug reported here.

[3H]Ouabain binding and Na+, K+-ATPase in resealed human red cell ghosts

The interaction of the cardiac glycoside [3H]ouabain with the Na+, K+ pump of resealed human erythrocyte ghosts was investigated. Binding of [3H]ouabain to high intracellular Na+ ghosts was studied in high extracellular Na+ media, a condition determined to produce maximal ouabain binding rates. Simultaneous examination of both the number of ouabain molecules bound per ghost and the corresponding inhibition of the Na+, K+-ATPase revealed that one molecule of [3H]ouabain inhibited one Na+, K+-ATPase complex. Intracellular magnesium or magnesium plus inorganic phosphate produced the lowest ouabain binding rate. Support of ouabain binding by adenosine diphosphate (ADP) was negligible, provided synthesis of adenosine triphosphate (ATP) through the residual adenylate kinase activity was prevented by the adenylate kinase inhibitor Ap5A. Uridine 5'-triphosphate (UTP) alone did not support ouabain binding after inhibition of the endogenous nucleoside diphosphokinase by trypan blue and depletion of residual ATP by the incorporation of hexokinase and glucose. ATP acting solely at the high-affinity binding site of the Na+, K+ pump (Km approximately 1 microM) promoted maximal [3H]ouabain binding rates. Failure of 5'-adenylyl-beta-gamma-imidophosphate (AMP-PNP) to stimulate significantly the rate of ouabain binding suggests that phosphorylation of the pump was required to expose the ouabain receptor.

(Na+,K+)-ATPase of mammalian brain: effects of temperatures on cation and ATP interactions regulating phosphatase activity

The effects of temperature on interactions between univalent cations or ATP and the p-nitrophenylphosphatase activity associated with brain (Na+,K+)-ATPase were examined. The apparent affinity for K+ activation under conditions favoring the moderate affinity site was temperature dependent, increasing with decreasing temperature. A comparison of univalent cations showed that the negative apparent delta H and delta S for cation binding increased with increasing apparent cation affinity. In contrast to the case with the moderate affinity sites, apparent affinity for the high affinity K+ site was independent of temperature. As temperature decreased, properties of moderate affinity site binding approached those of the high affinity site. The temperature dependence of ATP inhibition was opposite to that for K+ activation, with positive apparent delta H and delta S. The apparent delta H and delta S for cation binding approached those for the overall conformational change to K+-sensitive enzyme as cation affinity increased. These data suggest that E2, the K+-sensitive form of (Na+,K+)-ATPase, is stabilized by forces that require a decrease in entropy, explaining the predominant existence of E1 at physiologic temperatures. A conformational change leading to stabilization of E2 at higher temperatures can be produced by binding of univalent cations to a moderate affinity, presumably intracellular, site. This effect is counteracted by ATP. ATP also appears to alter the selectivity of this site to favor Na+ over K+ binding.

The concentration dependence of active K+ transport in the turkey erythrocyte. Hill analysis and evidence for positive cooperativity between ion binding sites

A mathematical model is presented which describes the theoretical relationship between ligand concentration and physiological response for systems in which the response is dependent upon simultaneous occupancy of two receptor ligand-binding sites. The treatment considers both the possibility of intrinsic differences between the binding sites with regard to ligand affinity, as well as the possibility of mutually induced changes in affinity resulting from allosteric interactions. Unlike the Monod-Wyman-Changeux formulation for allosteric enzymes, the general model put forward here makes double occupancy an absolute requirement for enzymatic function. It is shown that such a model leads to the prediction of a curvilinear Hill plot from which one can obtain an explicit estimate of the degree of allosteric interaction between the two ligand binding sites as well as the Gibbs standard free energy change for the overall binding reaction. It is then shown that, in the specific instance of Na, K-ATPase-mediated K+ transport by the turkey erythrocyte, the configuration of the Hill curve describing the rate of ouabain-sensitive K+ transport as a function of external K+ concentration conforms closely to that predicted by the model described above. The results are of particular interest because they indicate a strongly cooperative interaction between the two K+ binding sites on the transport protein such that occupancy of one site results in an enhancement of the affinity of the other site for K+ by a minimum of 15- to 20-fold. Finally, we consider in detail a model of the Monod-Wyman-Changeux type in which, by contrast, both singly and doubly occupied forms of the enzyme are assumed to be catalytically active, and which we analogously extend to allow for the possibility of asymmetry between the two ligand binding sites. Although it is shown that the two models can not be differentiated from each other in the present experimental system, they yield virtually identical estimates for the degree of positive cooperativity between the two K+ binding sites.

Comparative effects of external monovalent cations on sodium pump activity and ouabain inhibition rates in squid giant axon

1. A number of external monovalent cations were compared with regard to their effects on Na pump rate and the rate of ouabain inhibition of the pump in squid giant axon. 2. External ions which stimulate active Na efflux (K, Rb, and Cs) were found to decrease the rate at which low concentrations of ouabain inhibit the pump, and those ions which inhibit the pump externally (Na and Li) to increase the rate of inhibition. 3. In Na- and Li-containing solutions, pump rate appeared to be the major factor in determining the rate of ouabain inhibition regardless of whether K, Cs, or Rb was used to stimulate active Na efflux. 4. When choline was substituted for external Na, ouabain inhibition rates were more than twice as rapid when Cs was used as the pump-stimulating cation than when K was activating the pump to a similar level. 5. These results suggest that external monovalent cations modulate ouabain inhibition in squid axon at two classes of sites: pump activation sites, and also separate regulatory sites, whose occupation can significantly increase the rate of ouabain inhibition independent of pump turnover rate.

The electrogenic sodium pump in guinea-pig ventricular muscle: inhibition of pump current by cardiac glycosides

1. The inhibition of the electrogenic sodium pump in guinea-pig ventricular muscle by cardiac glycosides was studied with a voltage-clamp technique.2. Superfusion of the preparation with dihydro-ouabain (DHO) produced a reversible depolarization of up to 7 mV. When the membrane potential was clamped to a constant value near the resting potential application of DHO produced a corresponding current change in the inward direction which reached a steady state in less than 1 min.3. The drug-induced current change (I(D)) was found to be the result of a parallel shift of the current-voltage relation. The contributions of a change in extracellular K or intracellular Na to the measured I(D) were shown to be very small. From these findings and the results summarized below it was concluded that I(D) represents the blockage of the electrogenic pump current by DHO and that it is proportional to the number of drug molecules bound to the Na-K-ATPase in the intact cell.4. The dependence of I(D) on the concentration of DHO applied (5 x 10(-6)-8 x 10(-4) M) was found to be consistent with the predictions of the law of mass action for reversible one-to-one binding of the drug to the Na-K pump under equilibrium conditions. From a Scatchard-type plot the equilibrium dissociation constant (K(D)) of DHO was determined to be 4.6 (+/-2.3) x 10(-5) M.5. The steady-state pump current in the resting preparation was calculated to be 0.81+/-0.26 muA/cm(2). It contributed 6.4+/-0.9 mV to the resting potential in Tyrode solution containing 3 mM-K.6. In the smallest preparations used the measured time course of the onset and decay of I(D) agreed with the chemical kinetics of binding and unbinding calculated for various DHO concentrations. The rate constant of unbinding (k(2)) was found to be 3.4 (+/-0.7) x 10(-2) S(-1) and the average rate constant of binding (k(1)) was 7.4 x 10(2) M(-1) S(-1).7. By comparing the effects of ouabain and DHO in the same preparation the following estimates of the chemical constants of ouabain binding to the Na-K pump were obtained: K(D) approximately 1.5 x 10(-6) M; k(1) approximately 4 x 10(3) M(-1) S(-1); k(2) approximately 6 x 10(-3) S(-1).8. An analysis of the transmembrane movements of Na and K in the steady state showed that the measured pump current density is consistent with a counter-transport of 3 Na and 2 K ions.

[The problem of the cellular receptor for cardiac glycosides (author's transl)]

This review concerns the Na+, K+ -ATPase as well as the Na+, K+ -pump in the intact membrane and the highly specific inhibition of this transport system by cardiac glycosides. The interaction between glycoside and enzyme and the regulation of the kinetics of glycoside binding by ATP, K+, Na+, Mg2+ and Ca2+ are described. Emphasis is placed on the significance of the Na+, K+ -pump as the pharmacological receptor for cardiac glycosides. The problem encountered and progress made in attempting to correlate the inotropic action of cardiac glycosides with the binding of these drugs to the heart muscle and with the inhibition of the Na+, K+ -pump are reported. Recent results concerning increases of the intracellular Na+ concentration which are obtained by a partial inhibition of the Na+, K+ -pump and which are followed by an elevation of the intracellular Ca2+ -activity are reviewed. The discovery of a digitalis-like endogenous activity corresponds to the high specificity of the receptor for cardiac glycosides.

The current concept for the cardiac glycoside receptor

This brief review emphasizes the significance of the Na+,K+-ATPase or the Na+,K+ pump of the intact membrane as the pharmacological receptor for cardiac glycosides. The properties of transport enzyme and the regulation of glycoside binding are described. An outline is given of the problems encountered and of the progress made in attempting to correlate the inotropic action of cardiac glycosides with the binding of these drugs to the heart muscle and with the inhibition of the Na+,K+ pump. Furthermore, the correlation of intracellular Ca2+ activity an Na+ concentration with the inhibition of the Na+,K+ pump is discussed. The existence of a digitalis-like endogenous activity may also indicate an important role of the Na+,K+ pump as a receptor for a physiological regulatory control of cardiac contractility.

Internal potassium stimulates the sodium-potassium pump by increasing cell ATP concentration

1. Intracellular K increases the ouabain-sensitive Na-K exchange in human red blood cells. Pump rate increases hyperbolically with internal K with a K12 for K of 2.58 m-mole/l. red blood cells. Li also stimulates the pump rate, but with a much higher K12. The stimulation does not result from a change in the affinity of the pump for its substrates Na, K or Mg or for the product, phosphate. 2. The effect of cell K on the Na-Na exchange is biphasic. At low concentrations it decreases the exchange rate but then the exchange increases linearly with cell k concentration. 3. Stimulation of the pump rate by internal K can be demonstrated in reconstituted ghosts but only if the ratio of the volume of cells to that of solution at the time of haemolysis is high. Stimulation is not observed if the ghosts contain an efficient system for rephosphorylating ADP to ATP, such as creatine phosphate and creatine kinase, or if the measurements are made with iodoacetamide which inhibits rephosphorylation of ADP by inhibiting the enzyme glyceraldehyde-3-phosphate dehydrogenase. 4. Cells with low internal K and Li have low ATP concentrations, and ATP increases hyperbolically with internal K or Li with the same K12 as does the pump rate. In cells depleted of substrate intracellular K does not stimulate the pump rate. 5. The effect of K and Li on the pump rate probably does not result from enhanced activity of any of the enzymes below phosphofructokinase in the glycolytic pathway.

Effect of intracellular potassium upon the electrogenic pump of frog retinal pigment epithelium

We have studied the hyperpolarizing, electrogenic pump located on the apical membrane of the retinal pigment epithelium (RPE) in an vitro preparation of bullfrog RPE-choroid. Changes in RPE [K+]i alter the current produced by this pump. Increasing [K+]o in the solution perfusing the basal membrane increases RPE [K+]i (measured with a K+-specific microelectrode), and also depolarizes the apical membrane. The depolarization is due to a decrease in electrogenic pump current flowing across the apical membrane resistance, since it is abolished when the pump is inhibited by apical ouabain, by cooling the tissue, or by 0 mM [K+]o outside the apical membrane. Removal of Cl- from the solution perfusing the basal membrane abolishes the K+-evoked apical depolarization by preventing the entry of K+ (as KCl) into the cell. We conclude that the increase in [K+]i causes the decrease in pump current. This result is consistent with the finding that [K+]i is a competitive inhibitor of the Na+ - K+ pump in red blood cells. It is possible that the light-evoked changes in [K+]o in the distal retina could alter RPE [K+]i, and thus could affect the pump from both sides of the apical membrane. Any change in pump current is likely to influence retinal function, since this pump helps to determine the composition of the photoreceptor extracellular space.

The correlation between ouabain binding and potassium pump inhibition in human and sheep erythrocytes

1. [3H]Ouabain binding to human and sheep red blood cells was shown to be specific for receptors associated with Na/K transport. Virtually all tritium binding was abolished by dilution with unlabelled drug. Saturation levels of binding were independent of glycoside concentration and were identical to those associated with 100% inhibition of K pumping. 2. [3H]Ouabain binding and 42K influx were measured simultaneously in order to correlate the degree of K pump inhibition with the amount of glycoside bound. Results by this method agreed exactly with those obtained by pre-exposing cells to drug, followed by washing and then measuring K influx. 3. Plots of [3H]oubain binding vs. K pump inhibition were rectilinear for human and low K (LK) sheep red cells, indicating one glycoside receptor per K pump site and functional homogeneity of pump sites. High K (HK) sheep red cells exhibited curved plots of binding versus inhibition, which were best explained in terms of one receptor per pump, but a heterogeneous population of pump sites. 4. External K reduced the rate of glycoside binding, but did not alter the relationship between binding and inhibition. 5. The number of K pump sites was estimated as 450--500 per human cell and 30--50 per LK sheep cell. HK sheep cells had 90--130 sites per cell, of which eighty to ninety were functionally dominant. The number of K pump sites on LK sheep cells was not changed by anti-L, although the maximum velocity of pump turnover was increased.

Modulation of ouabain binding and potassium pump fluxes by cellular sodium and potassium in human and sheep erythrocytes

1. Erythrocytes were treated with nystatin to alter internal Na (Nai) and K (Ki) composition. Although the rates of K pumping and [3H]ouabain binding were altered dramatically, the relationship between glycoside binding and K pump inhibition was unaffected. 2. Human cells with high Nai and low Ki exhibited an increased rate of ouabain binding as compared to high Ki, low Nai cells; this paralleled the stimulated K pump activity of high Nai cells. 3. At constant Ki, increasing internal Na stimulated K pump and ouabain binding rates concomitantly. 4. At low Nai, increasing Ki inhibited both K pumping and ouabain binding. However, at high Nai, increasing Ki from 4 to 44 mM stimulated the rate of glycoside binding, parallel to its effect of increasing the rate of active K influx. 5. Anti-L, an isoantibody to low K (LK) sheep red cells, increased the rate of ouabain binding via its stimulation of K pump turnover. Since the latter effect is the result of affinity changes at the internal cation activation site(s) of the pump (Lauf, Rasmusen, Hoffman, Dunham, Cook, Parmelee & Tosteson, 1970), the antibody's effect on ouabain binding reflected the positive correlation between the rates of K pump turnover and glycoside binding. 6. These data provide the first evidence in intact cells for the occurrence of a Nai-induced conformational change in the Na/K pump during its normal operational cycle.

Active Na-K transport and the rate of ouabain binding. The effect of insulin and other stimuli on skeletal muscle and adipocytes

1. The effect of stimulation or inhibition of active Na-K transport on [(3)H]ouabain binding has been investigated in isolated soleus muscles and adipocytes.2. In rat soleus muscle, the ouabain-sensitive component of (42)K influx was stimulated by insulin (100 m-u/ml.), adrenaline (6 x 10(-6)M), and by pre-incubation with veratrine (10(-5)M) or in a K-free buffer. In all of these instances, the rate of ouabain binding was increased by 41-113%. Conversely, pre-treatment with tetracaine (0.2 mM) decreased the (42)K-influx and diminished the rate of [(3)H]ouabain binding by 36%.3. Neither insulin, adrenaline or tetracaine produced any detectable change in the total number of ouabain-binding sites (as measured under equilibrium conditions) in rat soleus muscle.4. In mouse and guinea-pig soleus muscle and in fat cells isolated from rats, insulin also increased the rate of [(3)H]ouabain binding without producing any significant change in the total number of ouabain-binding sites.5. Both in soleus muscle and the epididymal fat pad of the rat, there was a linear correlation between (42)K influx and the initial rate of [(3)H]ouabain binding.6. It is concluded that the rate of ouabain binding is determined significantly by the rate of active Na-K transport, but within the time intervals studied (4-6 hr) stimulation or inhibition of the Na pump does not lead to any appreciable change in the total number of Na pumps. It seems unlikely that the stimulation of active Na-K transport by insulin or adrenaline is due to unmasking or de novo synthesis of Na pumps.

Comparison of the side-dependent effects of Na and K on orthophosphate-, UTP-, and ATP-promoted ouabain binding to reconstituted human red blood cell ghosts

This paper is concerned with analyzing the sidedness of action of various determinants which alter the rate of ouabain binding to human red blood cell ghosts. Thus, ouabain binding promoted by orthophosphate (Pi) and its inhibition by Na are shown to be due to inside Pi and inside Na. External K inhibits Pi-promoted ouabain binding and Nao acts to decrease the effectiveness of Ko. Similarly, inside uridine triphosphate (UTPi) stimulates the rate of ouabain binding which can be antagonized by either Nai or Ko acting alone. The actions of Nai and Ko are different when ouabain binding is promoted by Pi and UTPi compared to inside adenosine triphosphate (ATPi). With ATPi, the ouabain binding rate is only affected when Nai and Ko are both present. Possible differences in the mechanism of action of K and Na on Pi-and UTP-promoted binding are discussed in the light of their sidedness of action.

Effects of Mg and Ca on the side dependencies of Na and K on ouabain binding to red blood cell ghosts and the control of Na transport by internal Mg

The effect of alteration in the concentration of internal Mg on the rate of ouabain binding to reconstituted human red blood cell ghosts has been evaluated as well as the effect of Mgi on Na:Na compared to Na:K exchange. It was found that the dependence of the rate of ATP-promoted ouabain binding on the combined presence of Nai and Ko which occurs at high [Mg]i is lost when the concentration of Mgi is lowered. The sensitivity of the external surface for Ko is also changed since Ko can now inhibit the ouabain binding rate in the absence of Nai; on the other hand Nao at low [Mg]i can stimulate ouabain binding indicating that the relative affinity of the outside surface for Nao has either increased or that for Ko has decreased or both. Thus the effects of changes in [Mg]i result in a change in the side-dependent actions of Na and K and emphasize the possible difficulties of interpreting results obtained on systems lacking sidedness. Mgi was found to be required for Pi-promoted ouabain binding and that the inhibitory action of Nai increased as [Mg]i was increased. In addition, Ca was found to be most effective in inhibiting the rate of ATP-promoted ouabain binding when Na and K were present together than when either was present alone. Na:K exchange was found to be more sensitive to the concentration of Mgi than Na:Na exchange; at low [Mg]i Na:K exchange could be stimulated without changing the extent of Na:Na exchange. These results are consistent with the idea that conformational states of the pump complex are directly influenced by [Mg]i.