Partial characterization of an endogenous factor which modulates the effect of catecholamines on synaptosomal Na+, K+-ATPase

High-affinity neurotensin receptor is involved in phosphoinositide turnover increase by inhibition of sodium pump in neonatal rat brain

Phosphoinositide (PI) metabolism is enhanced in neonatal brain by activation of neurotransmitter receptors and by inhibition of the sodium pump with ouabain or endogenous inhibitor termed endobain E. Peptide neurotensin inhibits synaptosomal membrane Na(+), K(+)-ATPase activity, an effect blocked by SR 48692, a selective antagonist for high-affinity neurotensin receptor (NTS1). The purpose of this study was to evaluate potential participation of NTS1 receptor on PI hydrolysis enhancement by sodium pump inhibition. Cerebral cortex miniprisms from neonatal Wistar rats were preloaded with [(3)H]myoinositol in buffer during 60 min and further preincubated for 0 min or 30 min in the absence or presence of SR 48692. Then, ouabain or endobain E were added and incubation proceeded during 20 or 60 min. Reaction was stopped with chloroform/methanol and [(3)H]inositol-phosphates (IPs) accumulation was quantified in the water phase. After 60-min incubation with ouabain, IPs accumulation values reached roughly 500% or 860% in comparison with basal values (100%), if the preincubation was omitted or lasted 30 min, respectively. Values were reduced 50% in the presence of SR 48692. In 20-min incubation experiments, IPs accumulation by ouabain versus basal was 300% or 410% if preincubation was 0 min or 30 min, respectively, an effect blocked 23% or 32% with SR 48692. PI hydrolysis enhancement by endobain E was similarly blocked by SR 48692, being this effect higher when sample incubation with the endogenous inhibitor lasted 60 min versus 20 min. Present results indicate that PI hydrolysis increase by sodium pump inhibition with ouabain or endobain E is partially diminished by SR 48692. It is therefore suggested that NTS1 receptor may be involved in cell signaling system mediated by PI turnover.

The expression of NMDA receptor subunits in cerebral cortex and hippocampus is differentially increased by administration of endobain E, a Na+, K+-ATPase inhibitor

Previous studies showed that endobain E, an endogenous Na+, K+-ATPase inhibitor, decreases dizocilpine binding to NMDA receptor in isolated membranes. The effect of endobain E on expression of NMDA receptor subunits in membranes of rat cerebral cortex and hippocampus was analyzed by Western blot. Two days after administration of 10 mul endobain E (1 microl = 29 mg fresh tissue) NR1 subunit expression enhanced 5-fold and 2.5-fold in cerebral cortex and hippocampus, respectively. NR2A subunit expression increased 2-fold in cerebral cortex and 1.5-fold in hippocampus. The level of NR2B subunit raised 3-fold in cerebral cortex but remained unaltered in hippocampus. NR2C subunit expression was unaffected in either area. NR2D subunit enhanced 1.6 and 2.1-fold for cerebral cortex and hippocampus, respectively. Results indicate that endogenous Na+, K+-ATPase inhibitor endobain E differentially modifies the expression of NMDA receptor subunits.

Endobain E, a brain endogenous factor, is present and modulates NMDA receptor in ischemic conditions

We have isolated from rat cerebral cortex an endogenous Na(+), K(+)-ATPase inhibitor, termed endobain E, which modulates glutamatergic N-methyl-d-aspartate (NMDA) receptor. This endogenous factor allosterically decreases [(3)H]dizocilpine binding to NMDA receptor, most likely acting as a weak channel blocker. In the present study we investigated whether endobain E is present in the cerebral cortex of rats subjected to ischemia and modulates NMDA receptor exposed to the same conditions. Ischemia-reperfusion was carried out by bilateral occlusion of common carotid arteries followed by a 15-min reperfusion period. Elution profile of brain soluble fraction showed that endobain E is present in cerebral cortex of ischemia-reperfusion rats. On assaying its effect on synaptosomal membrane Na(+), K(+)-ATPase activity and [(3)H]dizocilpine binding to cerebral cortex membranes prepared from animals without treatment, it was found that the endogenous modulator isolated from ischemia-reperfusion rats was able to inhibit both enzyme activity and ligand binding. On the other hand, endobain E prepared from rats without treatment also decreased binding to cerebral cortex or hippocampal membranes obtained from animals exposed to ischemia-reperfusion. Since ischemia decreases tissue pH and NMDA receptor activity varies according to proton concentration, pH influence on endobain E effect was tested. Endobain E ( approximately 80 mg original tissue) decreased [(3)H]dizocilpine binding 25% at pH 7.4 or 8.0 but 90% at pH 6.5. These results demonstrate that endobain E is present and also able to modulate NMDA receptor in the short-term period that follows cerebral ischemia and that its effect depends on proton concentration, suggesting greater NMDA receptor modulation by endobain E at low pH, typical of ischemic tissues.

Modulation of Aspartate Release by Ascorbic Acid and Endobain E, an Endogenous Na+, K+-ATPase Inhibitor

The isolation of a soluble brain fraction which behaves as an endogenous ouabain-like substance, termed endobain E, has been described. Endobain E contains two Na+, K+ -ATPase inhibitors, one of them identical to ascorbic acid. Neurotransmitter release in the presence of endobain E and ascorbic acid was studied in non-depolarizing (0 mM KCl) and depolarizing (40 mM KCl) conditions. Synaptosomes were isolated from cerebral cortex of male Wistar rats by differential centrifugation and Percoll gradient. Synaptosomes were preincubated in HEPES-saline buffer with 1 mM D-[3H]aspartate (15 min at 37 degrees C), centrifuged, washed, incubated in the presence of additions (60 s at 37 degrees C) and spun down; radioactivity in the supernatants was quantified. In the presence of 0.5-5.0 mM ascorbic acid, D-[3H]aspartate release was roughly 135-215% or 110-150%, with or without 40 mM KCI, respectively. The endogenous Na+, K+ -ATPase inhibitor endobain E dose-dependently increased neurotransmitter release, with values even higher in the presence of KCl, reaching 11-times control values. In the absence of KCl, addition of 0.5-10.0 mM commercial ouabain enhanced roughly 100% D-[3H]aspartate release; with 40 mM KCl a trend to increase was recorded with the lowest ouabain concentrations to achieve statistically significant difference vs. KCl above 4 mM ouabain. Experiments were performed in the presence of glutamate receptor antagonists. It was observed that MPEP (selective for mGluR5 subtype), failed to decrease endobain E response but reduced 50-60% ouabain effect; LY-367385 (selective for mGluR1 subtype) and dizocilpine (for ionotropic NMDA glutamate receptor) did not reduce endobain E or ouabain effects. These findings lead to suggest that endobain E effect on release is independent of metabotropic or ionotropic glutamate receptors, whereas that of ouabain involves mGluR5 but not mGluR1 receptor subtype. Assays performed at different temperatures indicated that in endobain E effect both exocytosis and transporter reversion are involved. It is concluded that endobain E and ascorbic acid, one of its components, due to their ability to inhibit Na+, K+ -ATPase, may well modulate neurotransmitter release at synapses.

Endobain E, a brain Na+, K+-ATPase inhibitor, decreases norepinephrine uptake in rat hypothalamus

The ability of an endogenous brain Na+, K+ -ATPase inhibitor, termed endobain E, to increase [3H]norepinephrine release in rat hypothalamus was previously reported. Endobain E effect on neurotransmitter uptake was studied by assaying [3H]norepinephrine uptake in rat hypothalamus preparations, to observe uptake inhibition, which reached 60% with endobain E equivalent to 100 mg fresh cerebral cortex, an effect achieved with 40 or 400 microM ouabain. Results support the proposal that endobain E behaves as an ouabain-like substance. Taken jointly results obtained on neurotransmitter release and uptake, the suggestion that endobain E may enhance norepinephrine availability in the synaptic gap and thus lead to an increase in noradrenergic activity is advanced.

Allosteric modulation of [3H]dizocilpine binding to N-methyl-d-aspartate receptor by an endogenous Na+, K+-ATPase inhibitor: dependence on receptor activation

An endogenous Na(+), K(+)-ATPase inhibitor, termed endobain E, has been isolated from rat brain and proved to decrease [3H]dizocilpine binding to cerebral cortex N-methyl-D-aspartate (NMDA) receptor, an effect independent of sodium pump activity. The purpose of this study was to disclose the mechanism of [3H]dizocilpine binding reduction by endobain E by performing saturation, kinetic and competitive assays. In saturation binding assays, endobain E increased K(d) without modifying B(max) value. To determine whether competitive or allosteric interaction was involved, kinetics of [3H]dizocilpine binding to cerebral cortex membranes was studied. Endobain E increased [3H]dizocilpine dissociation rate constant and induced an initial fast phase, without modifying association rate constant, indicating an allosteric interaction. In competitive [3H]dizocilpine binding assays, no additive effect was observed with endobain E plus competitive antagonists for glutamate or glycine sites (2-amino-5-phosphonopentanoic acid (AP-5) and 7-chlorokynurenic acid, respectively), indicating that coagonist site blockade interferes with endobain E effect. However, the higher glutamate and glycine concentration, the greater its effect. Endobain E binding reduction was partially additive with that induced by ketamine or Mg(2+) (receptor-associated channel blockers). Results suggest that the greater the channel activation by glutamate and glycine, the greater endobain E allosteric effect. Furthermore, as ketamine and Mg(2+) interfere with endobain E effect, this factor most likely binds to the inner surface of the NMDA associated channel.

A comparative study between a brain Na+,K(+)-ATPase inhibitor (endobain E) and ascorbic acid

In the search of Na+,K(+)-ATPase modulators, we have reported the isolation by gel filtration and HPLC of a brain fraction, termed endobain E, which highly inhibits Na+,K(+)-ATPase activity. In the present study we compared some properties of endobain E with those of ascorbic acid. Kinetic experiments assaying synaptosomal membrane K(+)-p-nitrophenylphosphatase (K(+)-p-NPPase) activity in the presence of endobain E or ascorbic acid showed that in neither case did enzyme inhibition prove competitive in nature versus K+ or p-NPP concentration. At pH 5.0, endobain E and ascorbic acid maximal UV absorbance was 266 and 258 nm, respectively; alkalinization to pH 14.0 led to absorption drop and shift for endobain E but to absorbance disappearance for ascorbic acid. After cysteine treatment, endobain E absorbance decreased, whereas that of ascorbic acid remained unaltered; iodine treatment led to absorbance drop and shift for endobain E but to absorbance disappearance for ascorbic acid. HPLC analysis of endobain E disclosed the presence of two components: one eluting with retention time and UV spectrum indistinguishable from those of ascorbic acid and a second, as yet unidentified, both exerting Na+,K(+)-ATPase inhibition.

The effect of an endogenous Na+, K+-ATPase inhibitor on rat lens transparency and ultrastructure

1. The purpose of the present study was to analyze the possible effect of ouabain and an endogenous ouabain-like substance (endobain E), on lenses of 100- and 400-g body weight rats. 2. Lenses were incubated with ouabain or endobain E for 120 min, either at room temperature or in the cold; opalescence was checked by gross examination and ultrastructure by electron microscopy. 3. Lenses from 400-g rats invariably remained translucent whereas those from 100-g rats presented variable opalescence. 4. As disclosed with the electron microscope, lenses of 100-g rats incubated at room temperature, with or without ouabain or endobain E, presented variable degrees of ultrastructural changes: with ouabain, there was fiber separation and vacuole formation but with endobain E, no vacuoles were found and fibers, though disorganized, appeared attached. After incubation in an ice bath, lenses were markedly altered in all conditions assayed. 5. It is concluded that ouabain and endobain E effect on lens transparency depends on the rat age and that in young animals, it is crucial incubation temperature during experimental procedure.

Metabotropic glutamate receptor involvement in phosphoinositide hydrolysis stimulation by an endogenous Na(+), K(+)-ATPase inhibitor and ouabain in neonatal rat brain

The mechanism of action of an endogenous Na(+), K(+)-ATPase inhibitor, termed endobain E, on phosphoinositide hydrolysis was studied in neonatal rat brain cortex and compared with that of ouabain. Lack of additivity for endobain E and glutamate paired stimulation on inositol phosphates accumulation suggested that they share at least a common step on inositol phosphate metabolism, as previously advanced for ouabain. In addition, Cd(2+) sensitivity of endobain E and ouabain effects strengthened the involvement of glutamate receptors. The participation of ionotropic glutamate receptors on endobain E- and ouabain-induced phosphoinositide hydrolysis seems untenable, since antagonists dizocilpine and CNQX proved unable to inhibit these effects. However, the endobain E effect was blocked by 2 x 10 (-4) M L-AP3 (an antagonist for group I mGluRs) when at least a 15-min preincubation protocol was employed. Maximal inhibition of endobain E effect (42%) occurred when L-AP3 preincubation was extended to 60 min, as already shown with glutamate, but only a trend to decrease was recorded with ouabain. At variance, the ouabain effect was reduced to 50% employing 5 x 10 (-4) M MCPG (a competitive antagonist for group I mGluRs), whereas no blockade was observed with endobain E or glutamate. In addition, MPEP (a selective mGluR5 antagonist) partially reduced ouabain, endobain E and glutamate responses and the selective mGluR1 antagonist LY367385 showed no activity at all. To sum up, the present findings support the involvement of mGluR5 in both endobain E and ouabain phosphoinositide hydrolysis stimulation in neonatal rat brain, in spite of dissimilar response to tested antagonists.

An endogenous Na+, K+-ATPase inhibitor enhances phosphoinositide hydrolysis in neonatal but not in adult rat brain cortex

The effect of an endogenous Na+, K+-ATPase inhibitor, termed endobain E, on phosphoinositide hydrolysis was studied in rat brain cortical prisms and compared with that of ouabain. As already shown for ouabain, a transient effect was obtained with endobain E; maximal accumulation of inositol phosphates induced by endobain E was 604 +/- 138% and 186 +/- 48% of basal values in neonatal and adult rats, respectively. The concentration-response plot for the interaction between endobain E and phosphoinositide turnover differed from that of ouabain, thus suggesting the involvement of distinct mechanisms. In the presence of endobain E plus ouabain at saturating concentrations, no additive effect was recorded, suggesting that both substances share at least a common step in their activation mechanism of inositol phosphates metabolism or that they enhance phosphatidylinositol 4,5-biphosphate breakdown from the same membrane precursor pool, until its exhaustion. Experiments with benzamil, a potent blocker of Na+/Ca2+ exchanger, showed that it partially and dose-dependently inhibited endobain E effect. These results indicate that the endogenous Na+, K+-ATPase inhibitor endobain E, like ouabain, is able to stimulate phosphoinositide turnover transiently during postnatal brain development.

[3H]dizocilpine binding to N-methyl-D-aspartate (NMDA) receptor is modulated by an endogenous Na+, K+-ATPase inhibitor. Comparison with ouabain

An endogenous Na+, K+-ATPase inhibitor termed endobain E has been isolated from rat brain which shares several biological properties with ouabain. This cardiac glycoside possesses neurotoxic properties attributable to Na+, K+-ATPase inhibition, which leads to NMDA receptor activation, thus supporting the concept that Na+/K+ gradient impairment has a critical impact on such receptor function. To evaluate potential direct effects of endobain E and ouabain on NMDA receptors, we assayed [3H]dizocilpine binding employing a system which excludes ionic gradient participation. Brain membranes thoroughly washed and stored as pellets ('non-resuspended' membranes) or after resuspension in sucrose ('resuspended' membranes) were employed. Membrane samples were incubated with 4 or 10 nM ligand with or without added endobain E or ouabain, in the presence of different glutamate plus glycine combinations, with or without spermidine. [3H]dizocilpine basal binding and Na+, K+- and Mg2+-ATPase activities proved very similar in 'non-resuspended' or 'resuspended' membranes. Endobain E decreased [3H]dizocilpine binding to 'resuspended' membranes in a concentration-dependent manner, attaining roughly 50% binding inhibition with the highest endobain E concentration assayed. Among tested conditions, only in 'resuspended' membranes, with 4 nM ligand and with 1x10(-8) M glutamate plus 1x10(-5) M glycine, was [3H]dizocilpine binding enhanced roughly +24% by ouabain (1 mM). After Triton X-100 membrane treatment, which drastically reduces Na+, K+-ATPase activity, the effect of ouabain on binding was lost whereas that of endobain E remained unaltered. Results indicate that not only membrane preparation but also treatment and storage are crucial to observe direct endobain E and ouabain effects on NMDA receptor, which are not attributable to changes in Na+, K+-ATPase activity or to Na+/K+ equilibrium alteration.

The effect of EGTA and Ca++ in regulation of the brain Na/K-ATPase by noradrenaline

BACKGROUND

The Na/K-ATPase activity of the brain synaptic plasma membranes (SPM) is regulated by noradrenaline (NA) and the synaptosomal factor SF (soluble protein obtained from the synaptosome cytosol). In the absence of SF, NA inhibits Na/K-ATPase, while, on addition of SF to the reaction medium, there is a NA-dependent activation of Na/K-ATPase. On the other hand, EGTA augments the Na/K-ATPase activity and attenuates the ability of NA to inhibit Na/K-ATPase.

RESULTS

Considering that Ca2+ ion is a Na/K-ATPase modifier, it can be assumed that the effect of NA and SF is a Ca2+-dependent process. However, in the presence of 0.3 mM EGTA and 0.1 mM NA, the apparent inhibition constant for Ca2+ (at [Ca2+] > 0.3 mM) is not SF dependent, while the apparent activation constant for SF does not change at increasing Ca2+ concentration ([Ca2+] < 0.3 mM). At various Ca2+ concentrations (0.06, 0.35 and 0.6 mM), no significant changes occur in the mode of action of NA on the Na/K-ATPase activity in the presence of 5 microg/ml SF. EGTA also has no effect on the NA-independent activation of Na/K-ATPase evoked by high SF concentrations.

CONCLUSIONS

Taking into account that in the absence of EGTA similar results have been obtained, it can be concluded that the effect of NA and SF on brain Na/K-ATPase is a Ca2+-independent process.

How many endobains are there?

Oxidative metabolism is very active in brain, where large amounts of chemical energy as ATP molecules are consumed, mostly required to maintain cellular Na+/K+ gradients through the participation of the sodium pump (Na+,K+-ATPase), whose activity is selectively and potently inhibited by the alkaloid ouabain. Na+/K+ gradients are involved in nerve impulse propagation, in neurotransmitter release and cation homeostasis in the nervous system. Likewise, enzyme activity modulation is crucial for maintaining normal blood pressure and cardiovascular contractility as well as renal sodium excretion. The present article reviews the progress in disclosing putative ouabain-like substances, examines their denomination according to different research teams, tissue or biological fluid sources, extraction and purification, assays, biological properties and chemical and biophysical features. When data is available, comparison with ouabain itself is mentioned. Likewise, their potential action in normal physiology as well as in experimental and human pathology is summarized.

Kinetics of Na+, K+-ATPase inhibition by an endogenous modulator (II-A)

We have previously reported the isolation by gel filtration and anionic exchange HPLC of two brain Na+, K+-ATPase inhibitors, II-A and II-E, and kinetics of enzyme interaction with the latter. In the present study we evaluated the kinetics of synaptosomal membrane Na+, K+-ATPase with II-A and found that inhibitory activity was independent of ATP (2-8 mM), Na+ (3.1-100 mM), or K+ (2.5-40 mM) concentration. Hanes-Woolf plots showed that II-A decreases Vmax in all cases; KM value decreased for ATP but remained unaltered for Na+ and K+, indicating respectively uncompetitive and noncompetitive interaction. However, II-A became a stimulator at 0.3 mM K+ concentration. It is postulated that brain endogenous factor II-A may behave as a sodium pump modulator at the synaptic region, an action which depends on K+ concentration.

Brain soluble fractions which modulate Na+, K+-ATPase activity likewise modify muscarinic receptor

Two brain soluble fractions, named peaks I and II, which respectively stimulate and inhibit neuronal Na+, K+-ATPase activity, have been isolated by gel filtration in Sephadex G-50. Since cholinergic transmission seems related to such enzyme activity, in this study we evaluated the effect of brain peak I, peak II, a more purified fraction II-E and commercial ouabain, on specific binding of the muscarinic antagonist [3H]quinuclidinyl benzilate to membranes from rat cerebellum, hippocampus and cerebral cortex. We found that binding was increased by peak I and decreased by peak II, II-E and ouabain, all effects proving concentration-dependent. Since the changes exerted on the muscarinic receptor followed a pattern similar to the one already described for synaptosomal membrane Na+, K+-ATPase activity, both systems seem to interact at a functional level.

A brain Na+, K+-ATPase inhibitor (endobain E) enhances norepinephrine release in rat hypothalamus

We have shown that synaptosomal membrane Na+, K+-ATPase activity is stimulated or inhibited by norepinephrine according to the presence or absence of a brain soluble fraction. Gel filtration of such soluble fraction has allowed the separation of two fractions, peaks I and II, able to stimulate and inhibit Na+, K+-ATPase activity, respectively. Peak II behaves much like ouabain, which has suggested the term endobain. From peak II, a subfraction termed II-E (endobain E), which highly inhibits Na+, K+-ATPase, has been separated by anionic exchange chromatography in a Synchropack AX-300 column. We determined the in vitro effect of endobain E obtained from rat cerebral cortex on neuronal norepinephrine release by incubating rat hypothalamic tissue in the presence of [3H]norepinephrine. Neuronal norepinephrine release was quantified as the factor above basal [3H]norepinephrine released to the medium at experimental and three post-experimental periods. Endobain E was found to increase norepinephrine release in a concentration-dependent fashion, reaching 200%, equivalent to the effect achieved with 400 microM ouabain. Ouabain effect persisted along three post-experimental periods whereas that of endobain E remained only during the first post-experimental period. These results led us to conclude that endobain increases norepinephrine release in hypothalamic neurons at the presynaptic nerve ending level, an effect resembling that of ouabain. It is postulated that endobain E may enhance catecholamine availability in the synaptic gap, leading to an increase in noradrenergic activity.

Na+,K+-ATPase interaction with a brain endogenous inhibitor (endobain E)

Na+,K+-ATPase activity of rat brain synaptosomal membranes was evaluated in the presence of an inhibitory fraction II-E (termed endobain E), isolated by gel filtration and anionic exchange HPLC of a rat brain soluble fraction. We studied endobain E aging, analyzed its inhibitory potency in the absence or presence of ouabain as well as its ability to block high affinity [3H]ouabain binding to cerebral cortex membranes. Similar loss of endobain E activity was observed when samples were stored either dried or in solution. Endobain E fraction inhibited synaptosomal membrane Na+,K+-ATPase activity in a concentration-dependent manner and the slope of the corresponding curve strongly resembled that of ouabain. Assays performed in the presence of endobain E and ouabain indicated that the inhibitory effect was additive or less than additive, depending on their respective concentrations during preincubation and/or incubation. High affinity [3H]ouabain binding to cerebral cortex membranes proved concentration-dependent from 0.10 to 0.50 mg protein per ml; binding inhibition by endobain E was independent of protein concentration within the above range. [3H]ouabain binding inhibition by endobain E was concentration-dependent over a 10-fold range, an effect similar to that found for Na+,K+-ATPase inhibition. The extent of endobain E effect on Na+,K+-ATPase inhibition was much higher (90-100%) than that on [3H]ouabain binding blockade (50%). Findings suggest some type of interaction between endobain E and ouabain inhibitory mechanisms and favour the view that the former behaves as an endogenous ouabain.

Endogenous modulators of brain Na+,K(+)-ATPase at early postnatal stages of rat development

The presence of endogenous modulators (peaks I and II) of synaptosomal Na+, K(+)-ATPase activity from adult rat cerebral cortex was previously suggested. In this study, the presence of such modulators at different postnatal stages of rat development was examined and their effect was tested on Na+, K(+)-ATPase activity. Synaptosomal membrane Na+, K(+)-ATPase activity was enhanced 20-30% by peak I and inhibited 70-75% by peak II obtained from 4-, 10-, 20- and 35-40-day-old rats. A fraction purified from peak II by anionic exchange HPLC (termed II-E) highly inhibits enzyme activity and behaves as a ouabain-like factor. Inhibitory activity of a 4-day-old II-E fraction proved higher than the corresponding fraction obtained from adult rats. Since expression of cerebral Na+, K(+)-ATPase has been shown to increase 10-fold during development whereas peak II concentration was observed to remain constant, and given the higher potency of purified neonatal II-E fraction, the effect of the latter may be greater at early postnatal stages of development than during adult life. It is suggested that the II-E fraction, which contains an ouabain-like factor, may play a role in neuronal development.

Kinetics of Na+, K+-ATPase inhibition by a rat brain endogenous factor (II-E)

Previous work from this laboratory led to the isolation by gel filtration and anionic exchange HPLC of a rat brain fraction named II-E, which highly inhibits synaptosomal membrane Na+, K+-ATPase activity. In this study we evaluated the kinetics of such inhibition and found that inhibitory potency was independent of Na+ (1.56-200 mM), K+ (1.25-40 mM), or ATP (1-8 mM) concentration. Hanes-Woolf plots indicated that II-E decreases Vmax but does not alter KM value, and suggested uncompetitive inhibition for Na+, K+ or ATP. However, II-E became a stimulator at 0.5 mM ATP concentration. It is postulated that this brain factor may modulate ionic transport at synapses, thus participating in central neurotransmission.

Differential properties between an endogenous brain Na+, K(+)-ATPase inhibitor and ouabain

By means of a Sephadex G-50 column and anionic exchange HPLC a cerebral cortex soluble fraction (II-E) which highly inhibits neuronal Na(+)-K(+)-ATPase activity has been previously obtained. Herein, II-E properties are compared with those of the cardenolide ouabain, the selective and specific Na+, K(+)-ATPase inhibitor. It was observed that alkali treatment destroyed II-E but not ouabain inhibitory activity. II-E presented a maximal absorbance at 265 nm both at pH 7 and pH 2 which diminished at pH 10. Ouabain showed a maximum at 220 nm which was not altered by alkalinization. II-E was not retained in a C-18 column, indicating its hydrophilic nature, whereas ouabain presented a 26-min retention time in reverse phase HPLC. Therefore, it is concluded that the inhibitory factor present in II-E is structurally different to ouabain.

Kinetics of K(+)-p-nitrophenyl phosphatase stimulation by a brain soluble fraction

We have already described the separation of two brain soluble fractions by Sephadex G-50, one of which stimulates (peak I) and the other inhibits (peak II) Na+, K(+)-ATPase and K(+)-p-nitrophenylphosphatase (K(+)-p-NPPase) activities. Here we examine the features of synaptosomal membrane p-NPPase activity in the presence and absence of brain peak I. It was observed that stimulation of Mg2+, K(+)-p-NPPase activity by peak I was concentration dependent. The ability of peak I to stimulate p-NPPase activity was lost by heat treatment followed by brief centrifugation. Pure serum albumin also stimulated enzyme activity. K(+)-p-NPPase stimulation by peak I proved dependent on K+ concentration but independent of Mg2+ and substrate p-nitrophenylphosphate concentrations. Since our determinations were performed in a non-phosphorylating condition reflecting the Na+, K(+)-ATPase Na+ site, it is suggested that peak I may stimulate the Na+-dependent enzyme phosphorylation known to take place from the internal cytoplasmic side.

An endogenous factor which interacts with synaptosomal membrane Na+, K(+)-ATPase activation by K+

In previous papers, the isolation of brain soluble fractions able to modify neuronal Na+, K(+)-ATPase activity has been described. One of those fractions-peak I-stimulates membrane Na+, K(+)-ATPase while another-peak II-inhibits this enzyme activity, and has other ouabain-like properties. In the present study, synaptosomal membrane Na+, K(+)-ATPase was analyzed under several experimental conditions, using ATP or p-nitrophenylphosphate (p-NPP) as substrate, in the absence and presence of cerebral cortex peak II. Peak II inhibited K(+)-p-NPPase activity in a concentration dependent manner. Double reciprocal plots indicated that peak II uncompetitively inhibits K(+)-p-NPPase activity regarding substrate, Mg2+ and K+ concentration. Peak II failed to block the known K(+)-p-NPPase stimulation caused by ATP plus Na+. At various K+ concentrations, percentage K(+)-p-NPPase inhibition by peak II was similar regardless of the ATP plus Na+ presence, indicating lack of correlation with enzyme phosphorylation. Na+, K(+)-ATPase activity was decreased by peak II depending on K+ concentration. It is postulated that the inhibitory factor(s) present in peak II interfere(s) with enzyme activation by K+.

In search of synaptosomal Na+, K+-ATPase regulators

The arrival of the nerve impulse to the nerve endings leads to a series of events involving the entry of sodium and the exit of potassium. Restoration of ionic equilibria of sodium and potassium through the membrane is carried out by the sodium/potassium pump, that is the enzyme Na+,K(+)-ATPase. This is a particle-bound enzyme that concentrates in the nerve ending or synaptosomal membranes. The activity of Na+,K(+)-ATPase is essential for the maintenance of numerous reactions, as demonstrated in the isolated synaptosomes. This lends interest to the knowledge of the possible regulatory mechanisms of Na+,K(+)-ATPase activity in the synaptic region. The aim of this review is to summarize the results obtained in the author's laboratory, that refer to the effect of neurotransmitters and endogenous substances on Na+,K(+)-ATPase activity. Mention is also made of results in the field obtained in other laboratories. Evidence showing that brain Na+,K(+)-ATPase activity may be modified by certain neurotransmitters and insulin have been presented. The type of change produced by noradrenaline, dopamine, and serotonin on synaptosomal membrane Na+,K(+)-ATPase was found to depend on the presence or absence of a soluble brain fraction. The soluble brain fraction itself was able to stimulate or inhibit the enzyme, an effect that was dependent in turn on the time elapsed between preparation and use of the fraction. The filtration of soluble brain fraction through Sephadex G-50 allowed the separation of two active subfractions: peaks I and II. Peak I increased Na+,K(+)- and Mg(2+)-ATPases, and peak II inhibited Na+,K(+)-ATPase. Other membrane enzymes such as acetylcholinesterase and 5'-nucleotidase were unchanged by peaks I or II. In normotensive anesthetized rats, water and sodium excretion were not modified by peak I but were increased by peak II, thus resembling ouabain effects. 3H-ouabain binding was unchanged by peak I but decreased by peak II in some areas of the CNS assayed by quantitative autoradiography and in synaptosomal membranes assayed by a filtration technique. The effects of peak I and II on Na+,K(+)-ATPase were reversed by catecholamines. The extent of Na+,K(+)-ATPase inhibition by peak II was dependent on K+ concentration, thus suggesting an interference with the K+ site of the enzyme. Peak II was able to induce the release of neurotransmitter stored in the synaptic vesicles in a way similar to ouabain. Taking into account that peak II inhibits only Na+,N(+)-ATPase, increases diuresis and natriuresis, blocks high affinity 3H-ouabain binding, and induces neurotransmitter release, it is suggested that it contains an ouabain-like substance.

Inhibitory effect of insulin and cytoplasmic factor(s) on brain (Na(+) + K+) ATPase

(Na+ + K+)ATPase activity in cerebral cortex was modulated by insulin action depending on the Mg2+ concentration. Thus, in homogenates in the presence of 1-3 mM Mg2+, insulin stimulated the enzyme, whereas in the presence of 4-6 mM Mg2+ inhibition was observed. Exposure of synaptosomal membranes to the soluble fraction resulted in inhibition of ATPase activity in a dose-dependent manner. The inhibitory effect of insulin was regulated by a cytoplasmic factor in a dose-dependent manner. Similar variations to those obtained with a crude synaptosomal fraction were obtained by using a partially purified ATPase. These results indicated the importance of soluble factors in the modulation of ATPase by insulin and add more evidence in support for a role of insulin as a neuromodulator.

Effect of Na+, K(+)-ATPase modifiers on high-affinity ouabain binding determined by quantitative autoradiography

There is growing evidence on the existence of endogenous ouabain-like factors that modulate Na+, K(+)-ATPase activity. In this laboratory, two soluble subfractions (peaks I and II) were previously separated from rat cerebral cortex, which had opposite effects on Na+, K(+)-ATPase activity. Peak I stimulated and peak II inhibited the enzyme (Rodríguez de Lores Arnaiz and Antonelli de Gómez de Lima, Neurochem Res 11:933-947, 1986). The same effects are now reported for K(+)-p-nitrophenyphosphatase activity. Localization of high-affinity ouabain binding in rat brain was done by quantitative autoradiography using a microcomputer digital imaging system. Peak I did not modify, whereas peak II blocked ouabain binding in areas 3-4 of cerebral cortex, dentate gyrus, stria terminalis, thalamic nuclei, and basal ganglia. Similar results were obtained when ouabain binding was determined in rabbit cerebral cortex and by a conventional filtration assay in nerve ending membranes obtained from rat cerebral cortex. These results favour the idea that the factor present in peak II fraction might behave as an ouabain-like substance.

Effect of tissue specificity of brain soluble fractions on Na+, K(+)-ATPase activity

Previous evidence from this laboratory indicated that catecholamines and brain endogenous factors modulate Na+,K(+)-ATPase activity of the synaptosomal membranes. The filtration of a brain total soluble fraction through Sephadex G-50 permitted the separation of two fractions -peaks I and II-which stimulated and inhibited Na+,K(+)-ATPase, respectively (Rodríguez de Lores Arnaiz and Antonelli de Gomez de Lima, Neurochem. Res. 11, 1986, 933). In order to study tissue specificity a rat kidney total soluble was fractionated in Sephadex G-50 and kidney peak I and II fractions were separated; as control, a total soluble fraction prepared from rat cerebral cortex was also processed. The UV absorbance profile of the kidney total soluble showed two zones and was similar to the profile of the brain total soluble. Synaptosomal membranes Na+,K(+)- and Mg2(+)-ATPases were stimulated 60-100% in the presence of kidney and cerebral cortex peak I; Na+,K(+)-ATPase was inhibited 35-65% by kidney peak II and 60-80% by brain peak II. Mg2(+)-ATPase activity was not modified by peak II fractions. ATPases activity of a kidney crude microsomal fraction was not modified by kidney peak I or brain peak II, and was slightly increased by kidney peak II or brain peak I. Kidney purified Na+,K(+)-ATPase was increased 16-20% by brain peak I and II fractions. These findings indicate that modulatory factors of ATPase activity are not exclusive to the brain. On the contrary, there might be tissue specificity with respect to the enzyme source.

Diuretic and natriuretic effect of a brain soluble fraction that inhibits neuronal Na+,K(+)-ATPase

The separation by Sephadex G-50 of two subfractions, peak I and II, from the brain soluble fraction has been previously described. These fractions were able to stimulate and inhibit synaptosomal membrane Na+,K(+)-ATPase, respectively (Rodríguez de Lores Arnaiz and Antonelli de Gómez de Lima, Neurochem. Res. 11, 933-948, 1986). Experimental evidence indicates that the alteration of Na+,K(+)-ATPase activity may result in changes of renal and cardiovascular parameters. In the present study, we have analyzed the effect of peak I and II fractions prepared from rat cerebral cortex on water and sodium excretion and on heart rate and arterial pressure in normotensive anesthetized rats. It was observed that water and sodium excretion were not modified by the administration of peak I fraction but that they were increased by peak II fraction. The cardiovascular parameters were not significantly modified by either of the fractions. The results indicate that brain soluble factor (s) which is (are) present in peak II fraction may modify some aspects of renal physiology after systemic administration.

The inhibitory activity of a brain extract on synaptosomal Na+, K+-ATPase is sensitive to carboxypeptidase A and to chelating agents

In the present study some properties of an inhibitory extract of synaptosomal membrane Na+, K+-ATPase were investigated. This extract (peak II) was prepared by gel filtration in Sephadex G-50 of a soluble fraction of the rat cerebral cortex. Ultrafiltration of peak II through Amicon membranes indicated that the inhibitor has a low MW (less than 1000). The inhibitory activity was not modified by heating in neutral pH at 95 degrees C for 20 min but it was destroyed by charring in acid pH at 200 degrees C for 120 min. The inhibitory activity decreased by incubation of peak II with carboxypeptidase A. These findings suggest that the factor responsible for the inhibition of Na+, K+-ATPase activity is probably a polypeptide. On the other hand, the inhibition was reverted by the chelators EDTA and EGTA, indicating the participation of an ionic compound as well. The increase of Mg2+ concentration during the enzyme assay did not increase the inhibition, indicating that the ion involved might not be vanadate. It is suggested that both a polypeptide and an ionic compound coparticipate in the inhibitory effect of peak II on Na+, K+-ATPase activity.

Different properties of two brain extracts separated in Sephadex G-50 that modify synaptosomal ATPase activities

We have previously reported that Na+,K+-ATPase of nerve ending membranes is stimulated by catecholamines only in the presence of a brain soluble fraction. The filtration of this soluble fraction through Sephadex G-50 permitted the separation of two extracts of maximal UV absorbance (peaks I and II) which showed different effects on ATPases. Peak I stimulated both Na+, K+-ATPase and Mg2+-ATPase activities and peak II inhibited Na+, K+-ATPase activity. We have now studied the activity of ATPases in the presence of the whole eluate obtained from the Sephadex G-50 column. It was observed that maximal effects on ATPases were obtained with peaks I and II. Peak I and peak II fractions were unable to modify the activity of acetylcholinesterase or 5'-nucleotidase present in the synaptosomal membranes. The stimulatory effect of peak I on ATPases was concentration dependent (up to 1:100), it was stable at different pHs and it was reverted by catecholamines. The inhibitory effect of peak II on Na+,K+-ATPase was concentration dependent (up to 1:50,000), it was stable only at acid pH, and it was partially reverted by catecholamines. These findings indicate that the factors responsible for the effects of peaks I and II have different properties and that their actions on ATPases show enzyme specificity.

Stimulation of brain Na,K-ATPase by norepinephrine but not taurine

The effect of taurine on rat and hamster brain Na,K-ATPase was examined and compared to norepinephrine (NE) stimulation of the enzyme. Although NE stimulation of microsomal Na,K-ATPase was observed in the presence of the cell cytosolic fraction, taurine was without effect in the presence and absence of this fraction. Taurine also failed to modulate pubescent and mature hamster brain Na,K-ATPase. Presence or absence of ion chelators did not change taurine's effect. These results are discussed in relation to previous reports of taurine and catecholamine stimulation of Na,K-ATPase.

The aging of a brain soluble fraction modifies its effect on the activity of neuronal Na+, K+-ATPase

In previous work we presented evidence showing that a brain soluble fraction was necessary to observe the stimulation of membrane Na+,K+-ATPase activity by catecholamines. Preliminary experiments suggested to us that the soluble fraction by itself was able to modify this enzyme activity. In the present study we have assayed the activity of synaptosomal Na+,K+-ATPase in the presence of a soluble fraction (aqueous supernatant after 100,000 g 30 min) prepared from rat cerebral cortex. The soluble fraction was used at different times after its preparation and different conditions in the incubation period previous to the enzyme assay were tested. It was observed that the enzyme activity increased 70% in the presence of a "0 min" soluble fraction. This effect was not found: a) in the presence of a "30 min" soluble fraction or b) when the membranes plus a "0 min" soluble fraction were incubated for 30 min (15 min at 37 degrees C + 15 min at 0 degree C) before the ATPase assay. In the presence of a "60 min" or "24 h" soluble fraction Na+,K+-ATPase activity was inhibited 50%. Results obtained indicate that Na+,K+-ATPase activity of synaptosomal membranes can be stimulated, inhibited or unchanged, depending on the aging of the soluble fraction.