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

Changes in [3H]-ouabain and [3H]-neurotensin binding to rat cerebral cortex membranes after administration of antipsychotic drugs haloperidol and clozapine

Evidences indicate the relationship between neurotensinergic and dopaminergic systems. Neurotensin inhibits synaptosomal membrane Na⁺, K⁺-ATPase activity, an effect blocked by SR 48692, antagonist for high affinity neurotensin receptor (NTS1) type. Assays of high affinity [³H]-ouabain binding (to analyze K⁺ site of Na⁺, K⁺-ATPase) show that in vitro addition of neurotensin decreases binding. Herein potential interaction between NTS1 receptor, dopaminergic D2 receptor and Na⁺, K⁺-ATPase was studied. To test the involvement of dopaminergic D2 receptors in [³H]-ouabain binding inhibition by neurotensin, Wistar rats were administered i.p.with antipsychotic drugs haloperidol (2mg/kg) and clozapine (3, 10 and 30mg/kg). Animals were sacrificed 18h later, cerebral cortices harvested, membrane fractions prepared and high affinity [³H]-ouabain binding assayed in the absence or presence of neurotensin at a 10 micromolar concentration. No differences versus controls for basal binding or for binding inhibition by neurotensin were recorded, except after 10mg/kg clozapine. Rats were administered with neurotensin (3, 10y 30μg, i.c.v.) and 60min later, animals were sacrificed, cerebral cortices harvested and processed to obtain membrane fractions for high affinity [³H]-ouabain binding assays. Results showed a slight but statistically significant decrease in binding with the 30μg neurotensin dose. To analyze the interaction between dopaminergic D2 and NTS1 receptors, [³H]-neurotensin binding to cortical membranes from rats injected with haloperidol (2mg/kg, i.p.) or clozapine (10mg/kg) was assayed. Saturation curves and Scatchard transformation showed that the only statistically significant change occurred in Bmax after haloperidol administration. Hill number was close to the unit in all cases. Results indicated that typical and atypical antipsychotic drugs differentially modulate the interaction between neurotensin and Na⁺, K⁺-ATPase. At the same time, support the notion of an interaction among dopaminergic and neurotensinergic systems and Na⁺, K⁺-ATPase at central synapses.

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.

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.

Norepinephrine induced alpha-adrenoceptor mediated increase in rat brain Na-K ATPase activity is dependent on calcium ion

It has been reported that norepinephrine increases Na-K ATPase activity by acting on alpha-1 adrenoceptors. The mechanism of such an increase was investigated. The norepinephrine induced increase in synaptosomal Na-K ATPase activity was prevented by pretreating the rat brain homogenate with either EDTA, a divalent cation chelator or prazosin, an alpha-1 adrenoceptor blocker. The norepinephrine and EGTA increased the Na-K ATPase activity in the synaptosome prepared from rat brain homogenate untreated with EDTA. The EGTA was ineffective in stimulating the enzyme activity if the synaptosome was prepared from homogenate treated with norepinephrine. However, the EGTA was effective in increasing the enzyme activity if the synaptosome was prepared from the homogenate treated with norepinephrine in the presence of prazosin. Thus, norepinephrine did not increase the Na-K ATPase activity in the presence of EDTA or alpha-1 adrenoceptor blocker. Similarly, the Ca++ chelator, EGTA, could not increase the enzyme activity if the homogenate was pretreated with norepinephrine alone. However, if norepinephrine action was blocked by alpha-1 antagonist prazosin, EGTA increased the enzyme activity possibly by chelation of Ca++. Further, chlorotetracycline fluorescence study showed that norepinephrine removes membrane bound Ca++. Thus, it is likely that norepinephrine acts on adrenoceptors and removes membrane bound Ca++ and thereby increases the Na-K ATPase activity in the synaptosome.

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.

Desipramine modulates 3H-ouabain binding in rat hypothalamus

We have previously shown that Na+, K(+)-ATPase activity in hypothalamus is increased after administration of an acute dose of desipramine, a noradrenaline uptake inhibitor (Viola et al., Cell Molec Neurobiol 9:263-271, 1989). In this report the same treatment (10 mg per kg) was applied to evaluate 3H-ouabain binding in rat brain sections by quantitative autoradiography. Results disclosed an increase in the number of ouabain binding sites in hypothalamus but not in cerebral cortex. Concomitantly, such acute DMI treatment enhanced K(+)-stimulated-p-nitrophenylphosphatase activity in hypothalamus membranes whereas it failed to modify cerebral cortex membranes. A direct interaction of DMI with the enzyme was ruled out since in vitro DMI is known to inhibit the enzyme. It may be speculated that DMI indirectly stimulates Na+, K(+)-ATPase through the increase in noradrenaline which acts in turn on the external phosphorylated site of the enzyme.

Na+,K(+)-ATPase activity is selectively increased in thalamus in thiamine deficiency prior to the appearance of neurological symptoms

The relationship between progression of neurological status and the activities of both Na+,K(+)- and Mg(2+)-dependent-ATPase (adenosine 5'-triphosphate phosphohydrolase) was investigated in brain regions of pyrithiamine-induced thiamine deficient rats. Thalamic Na+,K(+)-ATPase activity was selectively increased by 200% (P < 0.01) prior to the appearance of symptoms of thiamine deficiency and normalized in symptomatic rats. This selective transitory activation precludes a mediation by brain soluble fraction Na+,K(+)-ATPase modifiers as does the unaltered distribution in regional high-affinity [3H]ouabain binding densities observed throughout the time-course used in these experiments. Na+,K(+)-ATPase maintains cellular ionic gradients and has been implicated in neurotransmitter uptake and release mechanisms. The fact that the increased thalamic Na+,K(+)-ATPase activity coincides with the early alterations in serotonin metabolism observed in similarly treated animals and the concomitantly early increase in glucose utilization previously observed in the thalamus of thiamine-deficient rats is discussed.

Decrease of nerve Na+,K(+)-ATPase activity in the pathogenesis of human diabetic neuropathy

A decrease in Na+,K(+)-ATPase activity is claimed to play a central role in the pathogenesis of electrophysiological and morphological abnormalities that characterize the neuropathic complications in different animal models of diabetes mellitus. The peripheral nerves from 17 patients with either type I or type II diabetes mellitus were studied to assess the importance of changes in Na+,K(+)-ATPase activity in chronic human diabetic neuropathy. Sixteen nerves from age- and sex-matched normal individuals, and 12 nerves from non-diabetic neuropathic subjects undergoing vascular or orthopedic surgery served as negative and positive controls, respectively. All specimens were processed blind. Ouabain-sensitive ATPase activity was measured by a modified spectrophotometric coupled-enzyme assay. Standard histology, fiber teasing and electron microscopy were used to establish the normal or neuropathological patterns of surgical material. Morphometric analysis permitted calculation of fiber density in each nerve specimen and correlation of this figure with the relevant enzymatic activity. Na+,K(+)-ATPase activity was approximately 59% lower in nerves from diabetic patients than in normal controls (P < 0.01) and approximately 38% lower in nerves from non-diabetic patients with neuropathy (P < 0.01). Although nerves from both neuropathic conditions had significantly fewer fibers than those from normal individuals (diabetic -33%, and non-diabetic -22%), the decreases in Na+,K(+)-ATPase activity and fiber density were not correlated only in specimens from diabetic patients (r2 = 0.096; P = 0.22). Taken together with data from experimental animal models, these results suggest that the reduction in Na+,K(+)-ATPase activity in diabetic nerves is not an epiphenomenon secondary to fiber loss; rather, it may be an important factor in the pathogenesis and self-maintenance of human diabetic neuropathy.

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.