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

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.

A role for benzamil-sensitive proteins of the central nervous system in the pathogenesis of salt-dependent hypertension

1. Although increasing evidence suggests that salt-sensitive hypertension is a disorder of the central nervous system (CNS), little is known about the critical proteins (e.g. ion channels or exchangers) that play a role in the pathogenesis of the disease. 2. Central pathways involved in the regulation of arterial pressure have been investigated. In addition, systems such as the renin-angiotensin-aldosterone axis, initially characterized in the periphery, are present in the CNS and seem to play a role in the regulation of arterial pressure. 3. Central administration of amiloride, or its analogue benzamil hydrochloride, has been shown to attenuate several forms of salt-sensitive hypertension. In addition, intracerebroventricular (i.c.v.) benzamil effectively blocks pressor responses to acute osmotic stimuli, such as i.c.v. hypertonic saline. Amiloride or its analogues have been shown to interact with the brain renin-angiotensin-aldosterone system (RAAS) and to effect the expression of endogenous ouabain-like compounds. Alterations of brain RAAS function and/or endobain expression could play a role in the interaction between amiloride compounds and arterial pressure. Peripheral treatments with benzamil, even at higher doses than those given centrally, have little or no effect on arterial pressure. These data provide strong evidence that benzamil-sensitive proteins (BSPs) of the CNS play a role in cardiovascular responsiveness to sodium. 4. Mineralocorticoids have been linked to human hypertension; many patients with essential hypertension respond well to pharmacological agents antagonizing the mineralocorticoid receptor and certain genetic forms of hypertension are caused by chronically elevated levels of aldosterone. The deoxycorticosterone acetate (DOCA)-salt model of hypertension is a benzamil-sensitive model that incorporates several factors implicated in the aetiology of human disease, including mineralocorticoid action and increased dietary sodium. The DOCA-salt model is ideal for investigating the role of BSPs in the pathogenesis of hypertension, because mineralocorticoid action has been shown to modulate the activity of at least one benzamil-sensitive protein, namely the epithelial sodium channel. 5. Characterizing the BSPs involved in the pathogenesis of hypertension may provide a novel clinical target. Further studies are necessary to determine which BSPs are involved and where, in the nervous system, they are located.

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.

High-affinity neurotensin receptor is involved in phosphoinositide hydrolysis stimulation by carbachol in neonatal rat brain

Ontogenetic studies indicate that inositol phosphate accumulation in rodent brain tissue by cholinergic muscarinic agonists as well as expression of high-affinity neurotensin receptor (NTS1) peak at 7 days after birth. Herein, potential participation of this receptor in such effect was investigated. Cerebral cortex prisms of 7-day-old rats were preloaded with [3H]myoinositol and later incubated during 60 or 20 min in the presence of muscarinic agonist carbachol plus neurotensin and SR 48692, a non-peptide NTS1 antagonist. In 60-min incubation experiments, inositol phosphate accumulation by 10(-3) M carbachol was roughly 320%, an effect which remained unaltered plus 10(-6) M to 10(-4) M neurotensin but partially decreased with equimolar SR 48692 concentration. In 20-min incubation experiments, inositol phosphate accumulation by 10(-3) M carbachol was circa 240%, a value which attained 320-360% plus 10(-7) M neurotensin; this effect was totally blocked by 10(-7) M SR 48692. It was concluded that in inositol phosphate accumulation by carbachol, besides the cholinergic muscarinic receptor, the NTS1 receptor is likewise involved; findings at 60 min are attributable to the effect of endogenous neurotensin whereas those at 20 min most likely involve both endogenous and exogenously added peptide.

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.