Vanadate inhibits (Na+ + K+)ATPase by blocking a conformational change of the unphosphorylated form

The effect of vanadate on Na+,K+-ATPase activity of mouse cerebral cortex during bicuculline-induced seizures

The effect of bicuculline-induced seizures on Na+,K+-ATPase activity of mouse cerebral cortex homogenates, using two different procedures of sample preparation (freezing in situ or decapitation of animals without freezing) is described. Regardless of tissue treatment Na+,K+-ATPase activities during bicuculline-induced seizures did not differ significantly from the appropriate controls when vanadate-free ATP was used as substrate. The response of Na+,K+-ATPase to K+ activation was also similar; the increase in potassium concentration from 2 to 20 mM caused a 33.0 and 32.3% increase of enzyme activity in cortical homogenates from control and convulsing mice, respectively. Vanadate added to the assay medium inhibited Na+,K+-ATPase activity in a dose-dependent manner; with both types of tissue treatment there was, however, a tendency towards lesser inhibition of the enzyme from convulsing mice and at 1 X 10(-7) M vanadate this difference, though slight, was statistically significant: -22.59 vs -27.55% (freezing) and -28.73 vs -38.42% (decapitation) for seizures vs controls, respectively. The reduced sensitivity of Na+,K+-ATPase towards vanadate inhibition in cortical homogenates prepared from mice with convulsions suggests that vanadate might play a role in the modulation of enzyme activity during seizures in vivo.

Homology of ATP binding sites from Ca2+ and (Na,K)-ATPases: comparison of the amino acid sequences of fluorescein isothiocyanate labeled peptides

Ca2+ and (Na,K)-stimulated ATPases from various species and tissues were labeled with fluorescein isothiocyanate (FITC). Labeled peptides were solubilized by tryptic digestion and purified by reverse phase high pressure liquid chromatography. The amino acid sequences of the labeled peptides reveal considerable homology between sarcoplasmic reticulum Ca2+-ATPases from various sources. These Ca2+-ATPases also contain a region of homology with all other ATPases thus far sequenced. A difference was demonstrated between dog skeletal and cardiac Ca2+-ATPases. These results demonstrate homology of the putative ATP binding site of ATPases, which extends over tissue, species, and cation specificity, including the completely conserved amino acid sequence: lys-gly-ala-pro-glu.

The facilitating effect of gangliosides on the electrogenic (Na+/K+) pump and on the resistance of the membrane potential to hypoxia in neuromuscular preparation

The effects have been investigated of a mixture of gangliosides from beef brain cortex (GM1, GD1a, GD1b and GT1) either added to the bathing medium or injected intraperitoneally on muscle fibres and nerve terminals in mouse diaphragm. The electrogenic (Na+/K+) pump activity of muscle fibres enriched with sodium was increased by 38% after 2-h pretreatment with gangliosides (5 X 10(-8) mol X 1(-1]. Muscles from animals treated with gangliosides did not show the substantial depolarization of the resting membrane potential (RMP) in K+-free solution (6 h) shown by control muscles. Further, treatment with gangliosides slowed the changes in muscle fibre RMP and frequency of the miniature end-plate potentials in oxygen deprived muscles.

The amino acid sequence of the fluorescein isothiocyanate reactive site of lamb and rat kidney Na+- and K+-dependent ATPase

Fluorescein 5'-isothiocyanate has been used to label ouabain sensitive and insensitive (Na,K)-ATPases from lamb and rat kidney, respectively. The labeled enzymes were digested with trypsin to generate soluble peptides, which were purified by high performance liquid chromatography and sequenced on a gas phase sequenator. The sequence of the labeled peptide from both species is His-Leu-Leu-Val-Met-Lys-Gly-Ala-Pro-Glu-Arg. Thus, it appears that the primary structure of the fluorescein 5'-isothiocyanate reactive site, and therefore presumably the ATP binding site, is completely conserved in ouabain sensitive and ouabain insensitive (Na,K)-ATPases.

Metal ion release from titanium-based prosthetic segmental replacements of long bones in baboons: a long-term study

Forty-five baboons that had received titanium-based fiber metal composite segmental bone replacements were studied along with 13 controls without implants. Thirty-eight baboons with implants were sacrificed, and titanium, aluminum, and vanadium levels were assayed in homogenized lung, kidney, spleen, liver, adjacent muscle, and regional lymph nodes. In seven living baboons with implants, blood and urine samples were obtained for trace metal analysis as well as for biochemical and hematological profiles. In the 38 sacrificed baboons with implants, increased titanium levels were noted in the lungs, spleen, adjacent muscle (quadriceps, soleus, and triceps), and regional lymph nodes (inguinal, axillary, and popliteal) in comparison to those of six sacrificed controls without implants. In addition, vanadium was significantly elevated in the lungs of some animals, while aluminum increases were noted in adjacent muscle (quadriceps, soleus, and triceps), lung, and regional lymph nodes (inguinal, axillary, and popliteal). In the seven living baboons with implants, a sixfold increase (p less than 0.0005) in the urine titanium level was noted in comparison to that of seven living controls without implants. Additionally, elevated aluminum levels were found in the serum (p less than 0.0005) of the group with implants. Biochemical and hematological studies did not indicate statistically significant differences in serum electrolytes, in liver and renal function tests, or in complete blood counts between the seven living baboons with implants and their controls.

Bleomycin stimulates both membrane (Na+-K+) ATPase and electrogenic (Na+-K+) pump and partially removes the inhibition by vanadium ions

Bleomycin 2 X 10(-6) and 6 X 10(-6) mol.1(-1) increased the activity of specific (Na+-K+) ATPase of the rat brain microsomes. It also stimulated the electrogenic (Na+-K+) pump in intact skeletal muscle cells. The blocking effect of vanadyl (+4V) on membrane (Na+-K+) ATPase was eliminated completely by the drug, but the action of vanadate (+5V) was counteracted only partially. Electron paramagnetic resonance spectra revealed the formation of a +4V - bleomycin complex which is still able to activate the (Na+-K+) ATPase.

Minireview: physiological and pharmacological properties of vanadium

Vanadium is distributed extensively in nature. It is a trace element and is present in almost all living organisms including man. Even though vanadium was originally recognized for its ability to inhibit membrane Na+-K+-ATPase, various laboratory studies now document that this element has the capacity to affect the activity of various intracellular enzyme systems and may modify their physiological functions. Vanadium may be an essential element for normal development and may play an important role in various homeostatic mechanisms, and thus vanadium deficiency may prove to be an important concern. Abnormalities in biological disposition of vanadium may be involved in the pathogenesis of certain neurological disorders or cardiovascular diseases. While the essentiality of this element for living organisms is yet to be established with certainty, vanadium has become an increasingly important element and is used extensively in various heavy industries such as steel, oil, etc.; thus, the incidence of exposure to toxic levels of vanadium to industrial workers has been an increasing concern for toxicologists. To date, little information is available on the physiological or pharmacological actions of vanadium; hence, it is difficult to reach any definitive conclusion concerning its biological significance, essentiality and its role in pathological states. An attempt has been made in this review to broadly document what is known of various biological actions of vanadium.

Vanadate effect on the Na,K-ATPase and the Na-K pump in in vitro-grown rat vascular smooth muscle cells

The impact of vanadate on the Na,K-ATPase system in the vascular smooth muscle cell is poorly understood. The present study describes the kinetics of the effect of vanadate on Na,K-ATPase and the Na-K pump in in vitro grown rat VSMC's. Vanadate interaction with the Na,K-ATPase system in vascular smooth muscle cells was examined by observing its influence on ouabain-sensitive adenosine triphosphate hydrolysis in disrupted cells rendered permeable by osmotic shock, and the uptake of rubidium by intact cells. The I50 for vanadate inhibition of ouabain-sensitive hydrolysis of adenosine triphosphate occurred at vanadate concentrations of 10(-6) to 10(-7) M. This inhibition was potassium dependent. The maximal inhibitory effect of vanadate occurred at potassium concentrations of 10-20 mEq/liter. Sodium exerted a moderate antagonistic influence on vanadate inhibition of ouabain-sensitive adenosine triphosphate hydrolysis. Rubidium uptake by vascular smooth muscle cells was not altered within 120 minutes when 10(-5) M vanadate was added to the medium containing intact vascular smooth muscle cells. Yet, vanadium concentrations in the vascular smooth muscle cells within this incubation period reached levels 1.48-fold higher than the extracellular vanadate concentrations of 10(-5) M. These observations indicate that vanadate is a potent inhibitor of the VSMC Na,K-ATPase in disrupted vascular smooth muscle cells. However, in intact vascular smooth muscle cells vanadium gaining access into the vascular smooth muscle cell's interior does not inhibit the Na-K pump, probably because of its binding to intracellular proteins and/or conversion from the vanadate to the vanadyl ion.

The effects of several ligands on the potassium-vanadate interaction in the inhibition of the (Na+ + K+)-ATPase and the Na+, K+ pump

Inhibition by vanadate of the K+-dependent p-nitrophenylphosphatase activity catalyzed by the (Na+ + K+)-ATPase partially purified from pig kidney showed competitive behavior with the substrate, K+ and Mg2+ acted as cofactors in promoting that inhibition. Ligands which inhibited the K+-dependent p-nitrophenyl phosphate hydrolysis (Na+, nucleotide polyphosphates, inorganic phosphate) protected against inhibition by vanadate. The magnitude of that protection was proportional to the inhibition produced in the absence of vanadate. In the presence of only p-nitrophenyl phosphate and Mg2+, or when the protective ligands were tested alone, the activation of p-nitrophenyl phosphate hydrolysis by K+ followed a sigmoid curve in the presence as well in the absence of vanadate. However, the combination of 100 mM NaCl and 3 mM ATP resulted in a biphasic effect of K+ on the p-nitrophenyl phosphate hydrolysis in the presence of vanadate. After an initial rise at low K+ concentration, the p-nitrophenylphosphatase activity declined at high K+ concentrations; this decline became more pronounced as the vanadate concentration was increased. This biphasic response was not seen when a nonphosphorylating ATP analog was combined with Na+ (which favors the nucleotide binding) or with inorganic phosphate (a requirement for K+ - K+ exchange). Experiments with inside-out resealed vesicles from human red cells showed that in the absence of Na+ plus ATP, K+ promoted vanadate inhibition of p-nitrophenylphosphatase activity in a nonbiphasic manner, acting at cytoplasmic sites. On the other hand, in the presence of Na+ plus ATP, the biphasic response of p-nitrophenyl phosphate hydrolysis is due to K+ acting on extracellular sites. In vanadate-poisoned intact red blood cells, the biphasic response of the ouabain-sensitive Rb+ influx as a function of the external Rb+ concentration failed to develop when there was no Na+ in the extracellular media. In addition, in the absence of extracellular Na+, external Rb+ did not influence the magnitude of inhibition. The present findings indicate that external K+ favors vanadate inhibition by displacing Na+ from unspecified extracellular membrane sites.

Glucocorticoid receptor stabilization: relative effects of molybdate ion on inactivation by alkaline phosphatase and phospholipase A2

Glucocorticoid receptors in cytosol preparations from rat liver or mouse L cells are inactivated by phospholipase A2 or calf intestine alkaline phosphatase. Molybdate ion, an inhibitor of a variety of phosphatase enzymes, does not prevent inactivation of glucocorticoid binding capacity by alkaline phosphatase but it blocks inactivation by phospholipase A2. In neither case is the enzyme itself inhibited, and the effect of molybdate on phospholipase-mediated inactivation appears to reflect the ability of molybdate to prevent receptor inactivation by the detergent action of lysophosphatides.

Studies on ouabain-complexed (Na+ +K+)-ATPase carried out with vanadate

Vanadate is able to promote the binding of ouabain to (Na+ +K+)-ATPase and it is shown that vanadate is trapped in the enzyme-ouabain complex. Also ouabain-bound enzyme, the formation of which was facilitated by (Mg2+ +Na+ +ATP) or (Mg2+ +Pi), is accessible to vanadate when washed free of competing ligands used for the promotion of ouabain binding. For vanadate binding to (Na+ +K+)-ATPase and to enzyme-ouabain complexes a divalent cation (Mg2+ or Mn2+) is indispensable, indicating that the cation does not remain attached to the ouabain-bound enzyme. K+ further increases vanadate binding in the absence of ouabain, but seems to have no additional role in case of vanadate binding to enzyme-ouabain complexes. Mn2+ is more efficient than Mg2+ in promoting binding of vanadate and ouabain to (Na+ +K+)-ATPase. That K+ in combination with Mn2+, in analogy with the effect in combination with Mg2+, increases the equilibrium binding level of vanadate and decreases that of ouabain does not seem to favour the hypothesis of selection of a special E2-subconformation by Mn2+. The vanadate-trapped enzyme-ouabain complex was examined for simultaneous nucleotide binding which could demonstrate a two-substrate mechanism per functional unit of the enzyme. The acceleration by (Na+ +ATP) of ouabain release from the (Mg2+ +Pi)-facilitated enzyme-ouabain complex does not, as anticipated, support such a mechanism. On the other hand, the deceleration of vanadate release as well as of ouabain release from a (Mg2+ +vanadate)-promoted complex could be consistent with a two-substrate mechanism working out-of-phase.

Occlusion of rubidium ions by the sodium-potassium pump: its implications for the mechanism of potassium transport

1. The occlusion of rubidium ions by Na, K-ATPase has been investigated by suspending enzyme prepared from pig kidney outer medulla in media containing low concentrations of (86)Rb, forcing the suspensions rapidly through small columns of cation-exchange resin, and measuring the amounts of radioactivity emerging from the columns.2. When the suspension media contained 2 mM-ATP or ADP, or 15 mM-NaCl, the amounts of radioactivity emerging from the columns were greatly (and similarly) reduced, presumably because both nucleotides and sodium ions stabilized the enzyme in the E(1) form. (See p. 19 for definition of E(1) and E(2)). The extra radioactivity carried through the columns when nucleotides and sodium were absent was taken as a measure of the amount of rubidium occluded within the enzyme (in the E(2) form) when it emerged from the resin.3. By varying the flow rate, and therefore the time spent by the enzyme on the resin, and relating this to the amount of radioactivity emerging from the columns, we have been able to estimate the rate constant for the conformational change (E(2) --> E(1)) that allows the occluded rubidium ions to escape. At 20 degrees C, and in the absence of nucleotides, it is about 0.1 S(-1).4. The rate constant for rubidium release was the same in a sodium-containing as in a potassium-containing medium. The opposite effects of sodium and potassium ions on the poise of the equilibrium between the E(1) and the E(2) forms of the enzyme must, therefore, be due solely to opposite effects of these ions on the rate of conversion of E(1) to E(2).5. The rate constant for rubidium release was greatly increased by ATP and by ADP. Both nucleotides appeared to act at low-affinity sites and without phosphorylating the enzyme.6. Orthovanadate, in the presence of magnesium ions, stabilized the enzyme in the occluded-rubidium (E(2)Rb) form.7. Ouabain, in the presence of magnesium ions, prevented the occlusion of rubidium ions.8. We have measured the amount of rubidium occluded by the enzyme as a function of rubidium concentration, and estimate that at saturating rubidium concentrations about three rubidium ions can be occluded per phosphorylation site (or per ouabain-binding site).9. We have found that the occluded-rubidium form of the enzyme can also be formed by allowing rubidium ions to catalyse the hydrolysis of phosphoenzyme generated by the addition of ATP to enzyme suspended in a high-sodium medium.10. The properties of the occluded-rubidium form of the enzyme, and of the two routes that can lead to its formation, suggest that an analagous occluded-potassium form plays a central role in the transport of potassium ions through the sodium-potassium pump. This hypothesis is supported by a detailed consideration of the probable magnitudes of the rate constants of the individual reactions making up the two routes.

Low-affinity Na+ sites on (Na+ +K+)-ATPase modulate inhibition of Na+-ATPase activity by vanadate

Na+-ATPase activity is extremely sensitive to inhibition by vanadate at low Na+ concentrations where Na+ occupies only high-affinity activation sites. Na+ occupies low-affinity activation sites to reverse inhibition of Na+-ATPase and (Na+, K+)-ATPase activities by vanadate. This effect of Na+ is competitive with respect to both vanadate and Mg2+. The apparent affinity of the enzyme for vanadate is markedly increased by K+. The principal effect of K+ may be to displace Na+ from the low-affinity sites at which it activates Na+-ATPase activity.

In vivo electrical stimulation alters sensitivity of the brain (Na+ + K+)ATPase toward inhibition by vanadate

It has recently been shown that electrical stimulation of the brain cortex in vivo blocks invasion of cortical spreading depression (SD) into the stimulated area. The effect has been interpreted as a result of activating a K+ pumping mechanism that prevents the accumulation of this ion in the extracellular space to the high levels required for SD propagation. In the present experiments (Na+ + K+)ATPase activity was determined in the electrically stimulated region of the rat brain cortex. When ATP preparations containing vanadate were used as substrate, elevation of K concentration in the assay medium from 2 to 20 mM inhibited enzyme activity in homogenates from the normal cortex but not that from homogenates of the electrically stimulated cortical region. With vanadate-free ATP (Boehringer) as a substrate, slight stimulation by 20 mM K+ has been observed in both cases. Vanadate (0.25 microM) added to the assay medium containing Boehringer ATP and 20 mM K+ inhibited ATPase activity from the normal cortex but not that from the stimulated cortical area. Electrical stimulation may activate (Na+ + K+)ATPase at least partly by diminution of its susceptibility toward the inhibitory action of vanadate.

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

1. The efflux of Na in dialysed axons of the squid has been used to monitor the sidedness of the interactions of the Na pump with Na(+) ions, K(+) ions and ATP. The axons were under conditions such that most of the Na efflux went through the Na pump by means of a complete cycle of ATP hydrolysis.2. With 310 mm-K(i) (+), 70 mm-Na(i) (+) and 10 mm-K(+) artificial sea water (ASW) more than 97% of the Na efflux was abolished by removal of ATP. The efflux of Na was stimulated by ATP with a K((1/2)) of about 200 mum. This is similar to the K((1/2)) of 150 mum found for the ATP dependence of a ouabain-sensitive Na,K-ATPase activity in membrane fragments isolated from squid optical nerves.3. A 100-fold reduction in the ATP concentration (from 3-5 mm to 30-50 mum) increased the apparent affinity of the Na pump for K(o) (+) about 8-fold. In addition, the maximal rate of K(o) (+)-stimulated Na efflux was reduced by a similar factor. Analogous results were seen in axons dialysed with 310 mm-K(i) (+) or without K(i) (+).4. The relative effectiveness of external monovalent cations as activators of the Na efflux was a function of the ATP concentration inside the axon. With 3-5 mm-ATP the order of effectiveness was K(+) > NH(4) (+) > Rb(+). With 30-50 mum-ATP the sequence was NH(4) (+) >> K(+) >> Rb(+). These results were not affected by the removal of K(i) (+).5. When the ATP concentration was 3 mm and the Na(i) (+) concentration 70 mm, the removal of K(i) (+) produced a slight and reversible increase in the total efflux of Na (15%) and no change in the ATP-dependent Na efflux. When the ATP concentration was reduced to 30-50 mum, or the Na(i) (+) concentration lowered to 5-10 mm, the removal of K(i) (+) reversibly increased the total and the ATP-dependent efflux of Na. The largest increase in Na efflux was seen when both ATP and Na(i) (+) were simultaneously reduced. The ATP-dependent extra Na efflux resulting from the exclusion of K(i) (+) was abolished by 10(-4)m-ouabain in the sea waters.6. The increase in the ATP-dependent Na efflux observed in axons dialysed with 0 K(i) (+) + 10 mm-K(+) ASW was not seen in axons perfused with 310 mm-K(i) (+) + 450 mm-K(+) ASW. However, both experimental conditions gave rise to a similar (and small) ATP-independent and ouabain-insensitive efflux of Na. This indicates that the effects on the Na pump of removing K(i) (+) are not due to the simultaneous membrane depolarization. In addition, it suggests that K(i) (+) has an inhibitory effect on the Na pump, and that that effect is antagonized by Na(i) (+) and ATP.7. The present results are consistent with the idea that the same conformation of the Na pump (and Na,K-ATPase) can be reached by interaction with external K(+) after phosphorylation and with internal K(+) before rephosphorylation. This enzyme conformation produces an enzyme-K complex from which K(+) ions are not easily released unless high concentrations of ATP are present. This also stresses a non-phosphorylating regulatory role of ATP.

Sidedness of the effects of sodium and potassium ions on the conformational state of the sodium-potassium pump

1. (Na,K)ATPase from kidney membranes has been reconstituted into proteoliposomes following solubilization in cholate, by the freeze-thaw sonication procedure described by Kasahara & Hinkle (1977). The method is rapid and convenient.2. Upon addition of ATP to the exterior medium the reconstituted vesicles sustain high rates of active (22)Na uptake and (86)Rb efflux with many properties similar to those of the Na/K pump in well characterized cells such as erythrocytes.3. Observations on both active and passive transport of (22) Na and (86)Rb indicate that the vesicle population is heterogeneous; about 40 per cent contain Na/K pumps and the remainder seem to be plain lipid vesicles.4. The major Na(+)- or K(+)-stabilized non-phosphorylated conformational forms of the (Na, K)ATPase, E(1). Na and E(2). (K) respectively, have been investigated in the proteoliposomes, with particular regard to sidedness of the actions of Na(+) and K(+).5. Tryptic digestion of the vesicles reveals the Na(+)- and K(+)-stabilized conformations E(1). Na and E(2). (K) as characterized originally for purified (Na, K)ATPase (Jørgensen, 1975). Controlled trypsinolysis of Tris(+)-loaded vesicles in a Na(+)-or K(+)-containing medium leads to typical biphasic (Na(+)) or simple exponential (K(+)) time courses respectively, for loss of ATP-dependent (22)Na uptake (assayed subsequent to the tryptic digestion in the presence of inophores valinomycin plus FCCP). Tryptic digestion of K(+)- or Rb(+)- or Tris(+)-loaded vesicles suspended in a Na(+) medium results only in the biphasic (E(1). Na) pattern of loss of ATP-dependent (22)Na uptake.6. ATP-dependent (22)Na uptake and (86)Rb efflux are reduced by about the same extent following a short tryptic digestion in a Na(+)-containing medium.7. Vanadate ions inhibit ATP-dependent (22)Na uptake into the vesicles, at low concentrations (K(0.5) approximately 2 x 10(-7)m), following pre-incubation together with Mg(2+) and K(+) ions. K(+) ions in the medium are effective, K(+) ions within the vesicle are not. Na(+) ions in the medium prevent inhibition by vanadate+Mg(2+) but do not reverse inhibition in vesicles pre-incubated with vanadate, Mg(2+) and K(+) ions.8. The results show that the conformational forms E(1). Na and E(2). (K) are stabilized by Na(+) or K(+) ions respectively, bound to sites on the Na/K pump normally facing the cytoplasm. The significance of this finding is discussed in relation to the cation transport function of the pump.

Effects of vanadate on the functional properties of the isolated toad bladder

1. Vanadate, considered by some as a candidate for physiological modulation of Na pumping activity, was studied in the isolated toad urinary bladder. It produced a decrease in Na transport activity that was reversible and could be partially antagonized by pretreatment with a disulphonic distilbene derivative, SITS. This suggests an intracytoplasmic site of action for vanadate. Other transition metal salts, prepared from Ta and Nb, produced instead a transient rise in Na transport and this was unaffected by SITS. 2. Comparison between ouabain and vanadate showed clear-cut differences: inhibition of Na transport by the former, not the latter, was partially overcome by increasing of Na transport by the former, not the latter, was partially overcome by increasing cell Na or serosal K. Unexpectedly intracellular K did not decrease appreciably following vanadate treatment, in contrast to what occurs when ouabain was used instead. 3. Vasopressin-induced hydro-osmotic flow was irreversibly inhibited by vanadate but not by ouabain; pretreatment with SITS attenuated this effect. Moreover, vanadate blocked urinary acidification in this epithelium, thus supporting the hypothesis that proton flow occurs through an enzyme distinct from the mitochondrial ATPase, insensitive to vanadate.

Possible mechanisms for inotropic actions of vanadate in isolated guinea pig and rat heart preparations

The mechanism of inotropic actions of vanadate studied in isolated, electrically stimulated atrial and ventricular muscle preparations of rat or guinea-pig heart. Vanadate produced a negative inotropic effect in guinea-pig left atrial preparations associated with a marked shortening of the action potential plateau. In guinea-pig papillary muscle, or rat atrial or ventricular muscle preparations, vanadate produced a positive inotropic effect, which was not affected by either propranolol, phentolamine or metiamide. The positive inotropic effect was observed when action potential duration was either increased or decreased. Inotropic concentrations of vanadate failed to significantly alter the ouabain-sensitive 86Rb+-uptake, an estimate of sodium pump activity, or tissue concentration of cyclic AMP in electrically stimulated preparations. In partially depolarized rat atrial preparations in which fast sodium channels were inactivated in the presence of a high concentration of K+ (22 mmol/l), vanadate restored electrical activity (calcium-dependent action potentials) and the contraction, similar to isoproterenol. This action of vanadate was abolished by Mn2+, a slow channel inhibitor, but not by tetrodotoxin. The characteristic of vanadate- and isoproterenol-restored preparations, however, were substantially different. Moreover, vanadate failed to restore the contraction or action potential in partially depolarized guinea pig atrial preparations unlike isoproterenol. These results indicate that vanadate may either enhance or inhibit slow channels in cardiac muscle.

Vanadate inhibition of the Ca2+-ATPase from human red cell membranes

(1) VO3(-) combines with high affinity to the Ca2+-ATPase and fully inhibits Ca2+-ATPase and Ca2+-phosphatase activities. Inhibition is associated with a parallel decrease in the steady-state of the Ca2+-dependent phosphoenzyme. (2) VO3(-) blocks hydrolysis of ATP at the catalytic site. The sites for VO3(-) also exhibit negative interactions in affinity with the regulatory sites for ATP of the Ca2+-ATPase. (3) The sites for VO39-) show positive interaactions in affinity with sites for Mg2+ and K+. This accounts for the dependence on Mg2+ and K+ of the inhibition by VO3(-). Although, with less effectiveness, Na2+ and K+ substitutes for K+ whereas Li+ does not. The apparent affinites for Mg24 and K+ for inhibiton by VO3(-) seem to be less than those for activation of the Ca2+-ATPase. (4) Inhibition by VO3(-) is independent of Ca2+ at concentrations up to 50 microM. Higher concentrations of Ca2+ lead to a progressive release of the inhibitiory effect of VO3(-).

A specific enzyme is not necessary for vanadate-induced oxidation of NADH

It has recently been found that ortho- or metavanadate can effectively block (Na+ + K+)ATPase and that it loses its blocking potency when reduced to the vanadyl (VO2+) ion. The question arose whether vanadate could be involved (reduced) in an NAD-linked enzymatic redox system of the cell. Here we have studied the effect of vanadate on malate dehydrogenase (MDH, EC1.1.1.37) catalysed oxidation of NADH during the formation of malate from oxalacetate in vitro. The MDH reaction was accelerated by vanadate, but we found thatr vanadate does not require the presence of any specific enzyme or substrate to mediate NADH oxidation.

Defective conformational response in a selectively trypsinized (Na+ + K+)-ATPase studied with tryptophan fluorescence

1. Monitoring protein conformations of purified (Na+ + K+)-ATPase with intrinsic fluorescence we have examined if altered conformational responses accompany the defective catalytic and transport processes in selectively modified 'invalid' (Na+ + K+)-ATPase which is obtained by graded tryptic digestion of the Na+ form of the protein. 2. The protein fluorescence intensity of the K+ form (E2K) of both control and invalid (Na+ + K+)-ATPase is 2--3% higher than that of the Na+ form (E1Na). By varying the NaCl concentration we found evidence for different fluorescence intensities of the two phosphoenzymes; E2P has the same fluorescence intensity as E2K and the intensity of E1P is similar to that of E1Na. The fraction of phosphoenzyme present as E2P can therefore be determined as the amplitude of the fluorescence change accompanying phosphorylation in the absence of K+ divided by the amplitude of the full response to K+. 3. Titration of the fluorescence responses of the invalid (Na+ + K+)-ATPase shows that the tryptic split alters the noise of the equilibria between the cation-bound conformations, E1Na and E2K, and between the phosphoforms, E1P and E2P, in the direction of the E1 forms. 4. Vanadate binds to the Mg2+-bound form of E2K and prevents further changes in fluorescence intensity of the protein. The conformative responses of invalid (Na+ + K+)-ATPase are insensitive to vanadate in agreement with the reduced vanadate binding affinity of this enzyme. 5. The defective conformative response of the invalid (Na+ + K+)-ATPase in relation to its catalytic defects, reduced Na+ transport, and insensitivity to vanadate suggest that the transitions between Na+ forms (E1) and K+ forms (E2) of the protein are coupled to the catalytic and transport reactions of the (Na+ + K+)-pump.

The effects of vanadate on the fluxes of sodium and potassium ions through the sodium pump

1. The effects of sodium orthovanadate on the fluxes of sodium and potassium (or rubidium) ions through the sodium pump have been investigated in intact human red cells and in resealed ghosts prepared from them. Sodium-potassium exchange, potassium-potassium exchange, pump reversal, sodium-sodium exchange and uncoupled sodium efflux have each been studied.2. When intact human red cells were incubated in high-sodium media containing vanadate in low concentrations, inhibition of potassium or rubidium influx was marked only if the potassium or rubidium concentration in the medium was sufficiently high to cause nearly maximal influx in the absence of vanadate. The absence of inhibition at lower potassium or rubidium concentrations cannot be explained by supposing that the onset of inhibition by vanadate is slower in these conditions.3. Lowering the extracellular sodium concentration, or raising the vanadate concentration, decreased the minimum concentration of extracellular potassium or rubidium at which inhibition by vanadate was detected.4. Experiments on potassium influx into intact red cells treated with the ionophore A23187 showed that magnesium ions act at intracellular sites to promote inhibition by vanadate.5. Measurements of potassium efflux from intact red cells incubated in high-sodium media, with or without potassium, showed that potassium-potassium exchange was inhibited by vanadate at low concentrations whereas reversal of the pump was not.6. Measurements of sodium efflux from intact red cells or resealed ghosts incubated in high-sodium media, with or without potassium, showed that vanadate had little or no effect on sodium-sodium exchange at concentrations at which sodium-potassium exchange was markedly reduced. Much higher concentrations of vanadate did cause partial inhibition of sodium-sodium exchange.7. Experiments to determine whether vanadate in low concentrations inhibited uncoupled sodium efflux were inconclusive, but suggested that the flux was inhibited. Measurements of the ATP hydrolysis that is thought to be associated with the uncoupled sodium efflux showed that this hydrolysis was strongly inhibited.8. The different effects of vanadate on the different fluxes are discussed, and related to the way in which vanadate is thought to act on the sodium pump.