The effects of ATP on the interactions between monovalent cations and the sodium pump in dialysed squid axons

ATP dependence of Na+-driven Cl-HCO3 exchange in squid axons

Squid giant axons recover from acid loads by activating a Na(+)-driven Cl-HCO(3) exchanger. We internally dialyzed axons to an intracellular pH (pH( i )) of 6.7, halted dialysis and monitored the pH(i) recovery (increase) in the presence of ATP or other nucleotides, using cyanide to block oxidative phosphorylation. We computed the equivalent acid-extrusion rate (J(H)) from the rate of pH(i) increase and intracellular buffering power. In experimental series 1, we used dialysis to vary [ATP](i), finding that Michaelis-Menten kinetics describes J (H) vs. [ATP](i), with an apparent V(max) of 15.6 pmole cm(-2 )s(-1) and K (m) of 124 microM. In series 2, we examined ATP gamma S, AMP-PNP, AMP-PCP, AMP-CPP, GMP-PNP, ADP, ADP beta S and GDP beta S to determine if any, by themselves, could support transport. Only ATP gamma S (8 mM) supported acid extrusion; ATP gamma S also supported the HCO (3)(-) -dependent (36)Cl efflux expected of a Na(+)-driven Cl-HCO(3) exchanger. Finally, in series 3, we asked whether any nucleotide could alter J (H) in the presence of a background [ATP](i) of approximately 230 microM (control J (H) = 11.7 pmol cm(-2 )s(-1)). We found J (H) was decreased modestly by 8 mM AMP-PNP (J (H) = 8.0 pmol cm(-2 )s(-1)) but increased modestly by 1 mM ADP beta S (J (H) = 16.0 pmol cm(-2 )s(-1)). We suggest that ATP gamma S leads to stable phosphorylation of the transporter or an essential activator.

In the squid axon Na+/Ca2+ exchanger the state of the Ca i-regulatory site influences the affinities of the intra- and extracellular transport sites for Na+ and Ca2+

In squid axons, intracellular Mg2+ reduces the activity of the Na+/Ca2+ exchanger by competing with Ca2+ i for its regulatory site. The state of the Ca i-regulatory site (active-inactive) also alters the apparent affinity of intra- and extracellular transport sites. Conditions that hinder the binding of Ca2+ i (low pH i, low [Ca2+]i, high [Mg2+]i) diminish the apparent affinity of intracellular transport sites, in particular for Na i due to its synergism with H+ inhibition, but less noticeably for Ca2+ i because of its antagonism towards (Ha i + Na+ i) and Mg2+ i inhibitions. These are kinetic effects unrelated to the true affinity of the sites. With the Ca i-regulatory site saturated, the intracellular transporting sites are insensitive to [H+]i and to ATP. Likewise, the state of the Ca i-regulatory site (activated or inactivated) influences the affinity of the extracellular Ca o and Na o-transport sites (trans effects). In this case, the effects are opposite to those predicted by any of the transport schemes proposed for the Na+/Ca2+exchanger; i.e. its mechanism remains unexplained. In addition to their intrinsic importance for a full understanding of the properties of the Na+/Ca2+ exchanger, these findings show a new way by which the state of the Ca i-regulatory site may determine net movements of Ca2+ through this system.

Phosphoarginine stimulation of Na(+)-Ca2+ exchange in squid axons--a new pathway for metabolic regulation?

1. [Na+]o-dependent Ca2+ efflux (forward Na(+)-Ca2+ exchange), [32P]ATP wash-out curves and [ATP] were measured in internally dialysed squid giant axons at 17-18 degrees C. 2. We found that dialysing squid axons without ATP and with [Ca2+]i around 1 microM the basal levels of the [Na+]o-dependent Ca2+ efflux were significantly higher in the presence of N omega-phosphoarginine (PA). Phosphocreatine, a related phosphagen, is without effect. 3. PA stimulation of the Na(+)-Ca2+ exchange occurs in the complete absence of ATP (< 1 microM), being independent of, and additive to, the ATP-stimulated [Na+]o-dependent Ca2+ efflux. PA stimulation of [Na+]o-dependent Ca2+ efflux is fully and rapidly reversible with a Km around 7.7 mM. Activation by saturating [PA] is equivalent in magnitude to that of ATP. 4. PA stimulation of Na(+)-Ca2+ exchange is markedly dependent on intracellular Ca2+ and Mg2+ ions. Below 0.5 microM Ca2+i PA effect is negligible, becoming noticeable between 0.8 and 2 microM. In addition, Ca2+i considerably increases the rate at which PA activates the Na(+)-Ca2+ exchange. Although there is no absolute requirement of the PA effect for Mg2+ ions, this divalent cation largely stimulates the PA effect. 5. This work demonstrates, for the first time, the presence in squid axons of a new form of metabolic regulation of the Na(+)-Ca2+ exchange directly and solely related to PA and different from that of MgATP. This novel mechanism is likely to play a physiological role in Ca2+ extrusion through the Na(+)-Ca2+ exchanger, particularly at micromolar [Ca2+]i.

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)

Anomalous influence of reduced internal ATP levels on sodium efflux in Myxicola giant axons

Giant axons from the marine annelid, Myxicola infundibulum, were internally dialyzed with ATP-free media and with media with lower than normal ATP levels in an attempt to determine quantitatively the ATP requirement of the Na pump in these cells. This was accomplished by using 22Na ions to measure Na efflux. When [ATP]i in dialysis fluid fell to values within the range of 20-40 microM, a marked stimulation of Na efflux was observed even though an essentially normal ouabain sensitivity of Na efflux persisted; when axons were dialyzed with ATP-free solutions with ouabain present in the external medium throughout the dialysis period, the stimulation of Na efflux still occurred. The stimulation of Na efflux produced by low [ATP]i levels could be reversed by reintroducing normal ATP levels into the dialysis medium. Reversibility was complete provided axons were not depleted of ATP for periods longer than about 1 hr. Longer periods of ATP depletion led to larger and ultimately irreversible increases in Na efflux. The increases in Na efflux occasioned by ATP depletion either prevented or obscured the decrease in Na efflux expected to occur from unfueling the Na pump. Since [ATP]i levels required to significantly unfuel the Na pump lie below the levels at which the Na efflux stimulation occurred, it is problematic to quantitatively assess the influence of [ATP]i levels on Na pump rate by measurements of Na efflux in this preparation. Substitutes for ATP failed to prevent increases in Na efflux. The large increases in Na efflux observed at low [ATP]i occurred with no important changes in the resting membrane potential, and also occurred in Na-free and Ca-free external media. At least part of the increased Na efflux under these conditions may be due to a Na/Na exchange component, as a significant dependence of Na efflux on [Na]o appropriate for this kind of exchange was observed in the ATP-depleted axons. Whether the highly reproducible anomalous effect on Na efflux in Myxicola axons has some fundamental significance in its own right is a matter for future investigation. A few possible explanations of the anomalous effect of reduced ATP levels are discussed.

Vanadate and fluoride effects on Na+-K+-Cl- cotransport in squid giant axon

The effects of vanadate and fluoride on the Na+-K+-Cl- cotransporter of the squid giant axon were assessed. In axons not treated with these agents, intracellular dialysis with ATP-depleting fluids caused bumetanide-inhibitable 36Cl influx to fall with a half time of approximately 16 min. In the presence of either 40 microM vanadate or 5 mM fluoride, the decay of bumetanide-inhibitable 36Cl influx was significantly slowed; half time for vanadate-treated axons is 45 min and for fluoride-treated axons is 37 min. These agents are not exerting their effects on Na+-K+-Cl- cotransport by influencing the rate of ATP depletion of the axon, since they had no effect on the ATP hydrolysis rate of an optic ganglia homogenate. We therefore suggest that these data support the hypothesis that Na+-K+-Cl- cotransport in squid axons is regulated by a phosphorylation-dephosphorylation mechanism and that vanadate and fluoride reduce the rate of dephosphorylation by inhibiting a protein phosphatase.

Effects of pH changes on sodium pump fluxes in squid giant axon

The effects of independently varying intracellular and extracellular pH on sodium pump fluxes were studied in the squid giant axon. By means of intracellular dialysis, we found that changes of intracellular pH (pHi), but not of extracellular pH, affected ouabain-sensitive Na+ efflux and K+ influx over the pH range of 6.0-8.6. Both fluxes were maximum at a pHi of 7.2-7.4. Variations away from this optimal pHi in either the acidic or alkaline direction resulted in a graded inhibition of both ouabain-sensitive fluxes. The kinetic basis for the inhibitory effect of acidic pHi was examined by comparing the kinetic parameters of activation of ouabain-sensitive sodium efflux by intracellular Na+ (Na+i) and extracellular K+ (K+o) at normal pHi with those at acidic pHi. We found that the inhibitory effect of intracellular acidity results from a reversible decrease in maximum velocity (Vmax), without an effect on the activation parameters for Na+i (K1/2 Na+i) or K+o (K1/2 K+o).

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.

Kinetics of (Na+ + K+)-ATPase: analysis of the influence of Na+ and K+ by steady-state kinetics

The influence of Na+ and K+ on the steady-state kinetics at 37 degrees C of (Na+ + K+)-ATPase was investigated. From an analysis of the dependence of slopes and intercepts (from double-reciprocal plots or from Hanes plots) of the primary data on Na+ and K+ concentrations a detailed model for the interaction of the cations with the individual steps in the mechanism may be inferred and a set of intrinsic (i.e. cation independent) rate constants and cation dissociation constants are obtained. A comparison of the rate constants with those obtained from an analogous analysis of Na+-ATPase kinetics (preceding paper) provides evidence that the ATP hydrolysis proceeds through a series of intermediates, all of which are kinetically different from those responsible for the Na+-ATPase activity. The complete model for the enzyme thus involves two distinct, but doubly connected, hydrolysis cycles. The model derived for (Na+ + K+)-ATPase has the following properties: The empty, substrate free, enzyme form is the K+-bound form E2K. Na+ (Kd = 9 mM) and MgATP (Kd = 0.48 mM), in that order, must be bound to it in order to effect K+ release. Thus Na+ and K+ are simultaneously present on the enzyme in part of the reaction cycle. Each enzyme unit has three equivalent and independent Na+ sites. K+ binding to high-affinity sites (Kd = 1.4 mM) on the presumed phosphorylated intermediate is preceded by release of Na+ from low-affinity sites (Kd = 430 mM). The stoichiometry is variable, and may be Na:K:ATP = 3:2:1. To the extent that the transport properties of the enzyme are reflected in the kinetic ATPase model, these properties are in accord with one of the models shown by Sachs ((1980) J. Physiol. 302, 219-240) to give a quantitative fit of transport data for red blood cells.

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)

In squid axons intracellular Mg2+ is essential for ATP-dependent Na+ efflux in the absence and presence of strophanthidin

The effect on Na+ efflux of removal of intracellular Mg2+ was studied in squid giant axons dialyzed without internal Ca2+. In the absence of Mg2i+, ATP was unable to stimulate any efflux of Na+ above the baseline of about 1 pmol . cm-2 . s-1. This behavior was observed in otherwise normal axons and in axons poisoned with 50 microM strophanthidin in the sea water. Reinstatement of 4 mM MgCl2 in excess to ATP in the dialysis solution brought about the usual response of Na+ efflux to ATP, external K+ and strophanthidin. The present experiments show that, regardless of the mechanism for the ATP-dependent Na+ efflux in strophanthidin-poisoned axons, this type of flux shares with the active Na+ extrusion the need for the simultaneous presence of intracellular ATP and Mg2+.

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.

Characterization of the the (Na + +K+)-ATPase in a membranous preparation from the optic ganglion of the squid (Loligo pealei)

(1) A membrane fraction enriched in (Na+ +K+)-ATPase (EC 3.6.1.3) was obtained from optic ganglia of the squid (Loligo pealei) by density gradient fractionation of membranes followed by treatment with either SDS or Brij-58. The resulting membrane had an (Na+ +K+)-ATPase specific activity of approx. 2 units/mg and was greater than 95% ouabain-sensitive. (2) The (Na+ +K+)-ATPase had a Km for ATP of 0.42 +/- 0.04 mM and a pH optimum of 7.0. It was inhibited by ouabain with a Ki of 0.32 +/- 0.04 microM. (3) Optimum monovalent cation concentrations were: 240 mM NaCl, 60 mM KCl, tested with NaCl + KCl = 300 mM. (4) The Mg2+ dependence of hydrolysis varied with the absolute ATP concentration. At 3 mM ATP, the Km for Mg2+ was 0.86 +/- 0.10 mM, and at 6 mM ATP, the Km was 1.86 +/- 0.44 mM. High levels of Mg2+ caused inhibition of hydrolysis. (5) The interactions of Na+ and K+ were examined over a range of conditions. K+ levels caused modulations in the Na+ dependence in the range of 1-150 mM. (6) The (Na+ +K+)-ATPase prepared from squid optic ganglion displays properties similar to those of the sodium pump in injected nerves.