Na<Superscript>+</Superscript>,K<Superscript>+</Superscript>-ATPase activity in an animal model of mania

We evaluated Na⁺,K⁺-ATPase activity in hippocampus of rats submitted to an animal model of mania which included the use of lithium and valproate. In the acute treatment, amphetamine or saline was administered to rats for 14 days, between day 8 and 14, rats were treated with lithium, valproate or saline. In the maintenance treatment, rats were treated with lithium, valproate or saline, between day 8 and 14, amphetamine or saline were administered. Locomotor activity was assessed by open field test and Na⁺,K⁺-ATPase activity was measured. Our results showed that mood stabilizers reversed and prevented amphetamine-induced behavioral effects. Moreover, amphetamine (acute treatment) increased Na⁺,K⁺-ATPase activity, and administration of lithium or valproate reversed this effect. In the maintenance treatment, amphetamine increased Na⁺,K⁺-ATPase activity in saline-pretreated rats. Amphetamine administration in lithium- or valproate-pretreated animals did not alter Na⁺,K⁺-ATPase activity. The findings suggest that amphetamine-induced hyperactivity may be associated with an increase in Na⁺,K⁺-ATPase.     

Alterations of cerebromicrovascular Na, K-ATPase activity due to fatty acids and acute hypertension

Acute hypertension, induced in rats by intravenous injection of angiotensin II, previously has been shown to increase cerebrovascular permeability to macromolecules. The purpose of this study was to examine the effect of acute hypertension on Na⁺, K⁺-ATPase, the enzyme responsible for controlling ionic permeability of the cerebromicrovascular endothelium. The K⁺-dependent p-nitrophenylphosphatase activity of the cerebromicrovascular Na⁺, K⁺-ATPase was determined using microvessels prepared from hypertensive and normotensive rats. When compared to controls, a 70% decrease (P < 0.02) in the maximum rate (V_max) of the Na⁺, K⁺-ATPase from hypertensive rats was evident with no change in the Michaelis constant (K_M). In contrast, γ-glutamyltranspeptidase, a marker enzyme for cerebral endothelic cells, was not significantly affected. Sodium arachidonate (1–100 μM) inhibited the phosphatase activity of the Na⁺, K⁺-ATPase in microvessels isolated from both normotensive and hypertensive rats in a dose-dependent manner. Furthermore, poly-unsaturated fatty acids (sodium linoleate and arachidonate) evoked the greatest inhibition of the enzyme, while sodium oleate and sodium palmitate inhibited the Na⁺, K⁺-ATPase to lesser extents. This regulation of enzyme activity by fatty acids was comparable in control and hypertensive groups. In summary, the data indicate that the cerebromicrovascular Na⁺, K⁺-ATPase was altered as a consequence of acute hypertension and that poly-unsaturated fatty acids can modulate this enzyme in microvessels derived from hypertensive or control rats     

Acute enhancement of spinal cord synaptosomal (Na + K)-ATPase activity in cats following intravenous methylprednisolone

The acute intravenous administration of high doses of methylprednisolone sodium succinate (15–90 mg/kg) to cats enhanced (Na⁺ + K⁺)-ATPase activity as much as 2-fold in synaptosomes prepared from lumbar spinal cord removed following drug administration. The effect was apparent within 5 min and was statistically significant at 1 h following a single injection of methylprednisolone 90 mg/kg. (Na⁺ + K⁺)-ATPase activity had returned to control levels by 24 h post-injection. The implication of these findings to the treatment of spinal cord trauma and the reported neurophysiological effects of glucocorticoids are discussed.     

Effect of repeated restraint stress and clomipramine on Na/K-ATPase activity and behavior in rats

Activation of the limbic-hypothalamic-pituitary-adrenal axis (LHPA) and the release of glucocorticoids are fundamental for the adaptive response and immediate survival of an organism in reaction to acute stimuli. However, high levels of glucocorticoids in the brain may produce neuronal injury and a decrease of Na⁺/K⁺-ATPase activity, with effects on neurotransmitter signaling, neural activity, as well as the whole animal behavior. Clomipramine is a tricyclic antidepressant that inhibits the reuptake of serotonin and norepinephrine by indirect actions on the dopaminergic system and LHPA axis. Its chronic use increases the body's ability to cope with stress; however, high doses can potentiate its side effects on memory, learning, and sensory motor function. The purpose of the present study was to compare the effect of repeated restraint stress and clomipramine treatment on Na⁺/K⁺-ATPase activity and on the behavior of male rats. Changes in the behavioral response were evaluated by measuring the memory, learning, anxiety, and exploratory responses. Our results showed that exposure to repeated restraint stress reduced levels of Na⁺/K⁺-ATPase in brain structures and changed short and long-term memory, learning, and exploratory response when compared to the control group. Exposure to clomipramine treatment increased anxiety levels and reduced Na⁺/K⁺-ATPase activity in the cerebral cortex as well as short term memory, learning, and exploratory response. In conclusion, the present results provide additional evidence concerning how repeated restraint stress and clomipramine chronically administered at higher dose levels affect the neural activity and behavior of male rats.     

Glial contribution to seizure: K activation of (Na, K)-ATPase in bulk isolated glial cells and synaptosomes of epileptogenic cortex

Glial and neuronal (Na⁺, K⁺)-ATPase have dissimilar apparent affinities for K⁺ ions. Glial (Na⁺, K⁺)-ATPase is maximally activated by 20 mM K⁺ while neuronal (Na⁺, K⁺)-ATPase is maximally stimulated by 3–5 mM K⁺. Because this glial property may play an important role in the clearance of [K⁺]₀ during seizures, we investigated the K⁺ activation of (Na⁺, K⁺)-ATPase within bulk isolated glial cells and synaptosomes isolated from epileptogenic brains. The primary focus (F), the homolateral brain area around the focus (PF) and the mirror (M) or secondary focus induced by freezing lesions were studied.Results show that both glial and synaptosomal enzyme activities in the epileptogenic state change in comparison with controls, i.e. sham-operated cats. In F and M., glial enzyme decreased reaction velocities between 3 and 18 mM K⁺. In PF, maximum velocities increased in glial (Na⁺, K⁺)-ATPase. These results indicate that in actively firing epileptogenic tissue (F, M), glial (Na⁺, K⁺)-ATPase decreased rate reactions while the catalytic activity was increased in cortical tissues surrounding the focus. These phenomena appeared early, i.e. 1–3 days after production of the freezing lesion, and was associated with a sharp decrease in absolute levels of enzyme activity.Synaptosomal (Na⁺, K⁺)-ATPase from controls always exhibited a saturation curve at 3–6 mM K⁺. Synaptosomal enzyme activities in the primary (F) lesion increased slightly 24 h after lesion production, then progressively decreased 3 days after lesion production. No significant changes were seen in M and PF.     

Could dietary trans fatty acids induce movement disorders? Effects of exercise and its influence on Na⁺K⁺-ATPase and catalase activity in rat striatum

The influence of trans fatty acids (FA) on development of orofacial dyskinesia (OD) and locomotor activity was evaluated. Rats were fed with diets enriched with 20% soybean oil (SO; n − 6 FA), lard (L; saturated FA) or hydrogenated vegetable fat (HVF; trans FA) for 60 weeks. In the last 12 weeks each group was subdivided into sedentary and exercised (swimming). Brains of HVF and L-fed rats incorporated 0.33% and 0.20% of trans FA, respectively, while SO-fed group showed no incorporation of trans FA. HVF increased OD, while exercise exacerbated this in L and HVF-fed rats. HVF and L reduced locomotor activity, and exercise did not modify. Striatal catalase activity was reduced by L and HVF, but exercise increased its activity in the HVF-fed group. Na⁺K⁺-ATPase activity was not modified by dietary FA, however it was increased by exercise in striatum of SO and L-fed rats. We hypothesized that movement disorders elicited by HVF and less by L could be related to increased dopamine levels in striatum, which have been related to chronic trans FA intake. Exercise increased OD possibly by increase of brain dopamine levels, which generates pro-oxidant metabolites. Thus, a long-term intake of trans FA caused a small but significant brain incorporation of trans FA, which favored development of movement disorders. Exercise worsened behavioral outcomes of HVF and L-fed rats and increased Na⁺K⁺-ATPase activity of L and SO-fed rats, indicating its benefits. HVF blunted beneficial effects of exercise, indicating a critical role of trans FA in brain neurochemistry.     

Ultracytochemical localisation of Na+, K+-ATPase in cerebral endothelium in acute hypertension

Summary This ultracytochemical study was undertaken to determine whether increased arteriolar permeability in acute hypertension is accompanied by altered localisation of the ouabain-sensitive, K⁺-dependent p-nitrophenylphosphatase (K⁺-NPPase), a component of the Na⁺, K⁺-ATPase system. Rats were injected with horseradish peroxidase (HRP) intravenously and acute hypertension was induced by a 2-min infusion of angiotensin amide. Rats were killed at 3 and 15 min, following which brains were sliced and reacted for demonstration of K⁺-NPPase and HRP reaction product. Vessels of normotensive and hypertensive rats that were nonpermeable to HRP showed discontinuous distribution of K⁺-NPPase on the outer plasma membranes of endothelial and adventitial cells of arterioles and endothelial cells and pericytes of capillaries. Arterioles of the hypertensive rats which were permcable to HRP showed marked reduction of K⁺-NPPase localisation in their walls at 3 min while at 15 min when the blood pressure had returned to resting levels the enzyme localisation was similar to controls. This study demonstrates transient alteration of the NA⁺, K⁺-ATPase system during increased endothelial permeability in acute hypertension. The implication of this finding and our previous observation of reduced Ca²⁺-ATPase localisation in endothelial plasma membranes in acute hypertension has been discussed.Supported by the Heart and Stroke Foundation of Ontario     

Effects of mammalian brain extracts and chlormadinone acetate on neuronal Na, K-ATPase and electrogenic Na, K-pump activity in vitro

Acid-acetone extracts of brain (from beef and guinea pig) and chlormadinone acetate (CMA) were compared with ouabain for their ability to inhibit the electrogenic Na⁺,K⁺-pump and the Na⁺,K⁺-ATPase of neuronal tissues. The membrane potential of neurones in the paravertebral sympathetic ganglion of the bullfrog was recorded in K⁺-free Ringer's solution by means of the sucrose gap technique. The potassium activated hyperpolarization (K_H⁺), induced by the re-introduction of potassium, was used as an index of electrogenic Na⁺,K⁺-pumping. The K_H⁺ was blocked by 1 μM ouabain. Na⁺,K⁺-ATPase activity was measured in microsomal membrane preparations of frog and beef brain using a continuous spectrophotometric assay. Although ouabain consistently inhibited beef brain Na⁺,K⁺-ATPase (IC₅₀ = 2.2 μM), acid-acetone extracts prepared from guinea pig and beef brain produced only partial inhibition. Neither of the extracts significantly reduced the K_H⁺ of the frog ganglion. CMA inhibited Na⁺,K⁺-ATPase prepared from bullfrog brain and spinal cord with slightly greater potency (IC₅₀ = 4.5 μM) than did ouabain (IC₅₀ = 10 μM). In contrast, electrogenic Na⁺,K⁺-pumping (i.e. the K_H⁺) in the frog ganglion was not affected by this steroid. It is concluded that although both the extracts and CMA inhibited Na⁺,K⁺-ATPase, neither can be considered ouabain-like due to their failure to affect the electrogenic Na⁺,K⁺-pump in situ.     

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.     

Methylphenidate treatment increases Na<Superscript>+</Superscript>, K<Superscript>+</Superscript>-ATPase activity in the cerebrum of young and adult rats

Methylphenidate is a central nervous system stimulant used for the treatment of attention-deficit hyperactivity disorder. Na⁺, K⁺-ATPase is a membrane-bound enzyme necessary to maintain neuronal excitability. Considering that methylphenidate effects on central nervous system metabolism are poorly known and that Na⁺, K⁺-ATPase is essential to normal brain function, the purpose of this study was to evaluate the effect of this drug on Na⁺, K⁺-ATPase activity in the cerebrum of young and adult rats. For acute administration, a single injection of methylphenidate (1.0, 2.0, or 10.0 mg/Kg) or saline was given to rats on postnatal day 25 or postnatal day 60, in the young and adult groups, respectively. For chronic administration, methylphenidate (1.0, 2.0, or 10.0 mg/Kg) or saline injections were given to young rats starting at postnatal day 25 once daily for 28 days. In adult rats, the same regimen was performed starting at postnatal day 60. Our results showed that acute methylphenidate administration increased Na⁺, K⁺-ATPase activity in hippocampus, prefrontal cortex, and striatum of young and adult rats. In young rats, chronic administration of methylphenidate also enhanced Na⁺, K⁺-ATPase activity in hippocampus and prefrontal cortex, but not in striatum. When tested in adult rats, Na⁺, K⁺-ATPase activity was increased in all cerebral structures studied. The present findings suggest that increased Na⁺, K⁺-ATPase activity may be associated with neuronal excitability caused by methylphenidate.     

Immunohistochemical localization of Na, K-ATPase in the choroid plexus

To determine if canine and rat choroid plexus Na⁺, K⁺-ATPase can be localized by immunoperoxidase staining after fixation and embedding, we prepared rabbit antiserum to purified canine kidney medulla Na⁺, K⁺-ATPase. When sodium dodecylsulfate polyacrylamide electrophoretic gels of purified canine kidney Na⁺, K⁺-ATPase and canine kidney microsomes were treated with antiserum followed by [¹²⁵I]protein A and autoradiography, the canine microsomes and purified Na⁺, K⁺-ATPase showed a prominent radioactive band coincident with the α-, β- and γ-subunits of the purified canine kidney enzyme.When the rabbit immunoglobulin that was purified from the Na⁺, K⁺-ATPase antiserum through DEAE-cellulose ion exchange chromatography was used for immunoperoxidase staining of the choroid plexus fixed with Bouin's fixative, intense immunoreactive staining was present on the epithelial cells of both choroid plexuses but was not found in the tissue around the vessel. The staining was especially confined to apical surfaces of the epithelial cells. The same procedure was performed in the canine kidney, and immunostaining was obtained in the tubules where Baskin and Stahl described the enzyme localization. No staining was seen with pre-immune serum of the normal rabbit. We concluded that both the canine and rat choroid plexus are rich in Na⁺, K⁺-ATPase, which plays an important role in cerebrospinal fluid (CSF) secretion.     

Erythrocyte membrane sodium — potassium adenosine triphosphatase activity in affective disorders

Summary Erythrocyte membrane Na⁺,K⁺-ATPase activity was studied in drug naive patients with bipolar (BP) mania (n=62) and unipolar (UP) depression (n=60) and normal controls (n=66). Compared to controls there was a significantly decreased Na⁺,K⁺-ATPase activity in UP depressives but no change in BP manics. However, lithium treatment caused a significant increase in Na⁺,K⁺-ATPase activity although there was no correlation between plasma lithium levels and enzyme activity. Plasma cortisol correlated inversely with Na⁺,K⁺-ATPase in UP depressives. Interestingly, the lithium responders [<50% Beck Rafaelson's Mania Rating Scale (BRMS) score] showed a significant increase in Na⁺,K⁺-ATPase activity compared to lithium nonresponders (>50% BRMS score). These observations indicate that monitoring of Na⁺,K⁺-ATPase activity during lithium therapy is useful to predict a therapeutic response.     

Role of opioidergic and monoaminergic neurotransmission in the GnRH release mechanism of EBP-primed OVX rats

We examined the effect of intracerebroventricular (i.c.v.) administration of μ-opioid agonist, morphine, and its antagonist naloxone followed by morphine on the activities of monoamine-metabolizing enzymes, namely tyrosine hydroxylase (TH) and monoamine oxidase (MAO) along with adenosinetriphosphatase (Na⁺, K⁺ -ATPase), the enzyme responsible for the maintenance of ionic gradients across the membrane, in seven discrete regions of brain from estrogen- and progesterone-primed ovariectomized rats. TH activity decreased after morphine treatment in some areas such as the median eminence-arcuate region (ME-ARC), the amygdala, and the thalamus, showing statistically significant change. MAO activity increased in all the areas studied, but more appreciable change was observed in medial preoptic area (mPOA), the ME-ARC region, and the cortex. Pronounced increase in Na⁺, K⁺ -ATPase enzyme activity was observed after the drug treatment. Naloxone given prior to morphine injection resulted in recovery of the enzyme activities in most of the areas studied. Our study may provide insights into the precise opioidergic modulation of gonadotropin releasing hormone (GnRH) release mechanisms through the involvement of monoaminergic system, elucidating the basis of various neuronal dysfunctions and their management in opioid addicts.     

Microsphere embolism-induced changes in noradrenaline uptake of the cerebral cortex in rats

The present study was undertaken to elucidate pathophysiological changes in noradrenaline (NA) transporter and Na⁺/K⁺-ATPase, key regulators of cation gradient across the plasma membrane, in nerve terminals of the cerebral cortex after microsphere-induced cerebral embolism in rats. The V_max value of NA uptake, when analyzed by the Eadie–Hofstee plot, tended to decrease on the 1st day and decreased on the 3rd and 7th days after the embolism without any change in the K_m value. The NA content in cerebrocortical synaptosomes did not alter on the 1st day, but decreased on the 3rd and 7th days after the embolism. Ouabain (1 mM) inhibited NA uptake on the 1st day, but did not alter the uptake on the 3rd and 7th days after the embolism. The activity of Na⁺/K⁺-ATPase of cerebrocortical synaptosomes increased on the 1st day and gradually decreased up to the 7th day after the embolim. These results suggest that NA uptake in nerve terminals of the cerebral cortex decreased after microsphere embolism, which may be due to a reduction in function of NA transporters. The changes in Na⁺/K⁺-ATPase following microsphere embolism may represent a compensatory action to maintain ion homeostasis in nerve terminals at an early stage of ischemic injury.     

Inhibition of purified (Na,K)-ATPase activity from porcine cerebral cortex by NO generating drugs

We tested the effects of several nitric oxide (NO) generating compounds on the activity of sodium-potassium adenosine 5′-triphosphatase [(Na⁺,K⁺)-ATPase] purified from porcine cerebral cortex. Sodium nitroprusside (SNP),S-nitroso-N-acetylpenicillamine (SNAP), 3-morpholinosydnonimine (SIN-1) and (dl)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide (NOR 3) inhibited the (Na⁺,K⁺)-ATPase activity dose dependently. Superoxide dismutase, a NO scavenger, and sulfhydryl (SH) compounds, reduced-form glutathione (rGSH) and dithiothreitol (DTT), prevented the inhibitory action of SNAP, SIN-1 and NOR 3 but not of SNP, when applied simultaneously with NO generating compounds, and this enzyme inhibition could be reactivated by the incubation with these SH compounds but not with SOD. The inhibitory action by SNP was magnified by simultaneous application of DTT. These results suggest that NO generating compounds, SNAP, SIN-1 and NOR 3 but not SNP, may release NO or NO-derived products and may inhibit (Na⁺,K⁺)-ATPase activity by interacting with a SH group at the active site of the enzyme.     

Cholesterol oxidation reduces Ca + Mg-ATPase activity, interdigitation, and increases fluidity of brain synaptic plasma membranes

These experiments examined effects of cholesterol oxidation on Ca²⁺+Mg²⁺-ATPase activity, Na⁺+K⁺-ATPase activity, and membrane structure of brain synaptic plasma membranes (SPM). Cholesterol oxidase [E.C.1.1.3.6 fromBrevibacterium sp.] was used to oxidize cholesterol. Two cholesterol pools were identified in synaptosomal membranes based on their accessibility to cholesterol oxidase. A rapidly oxidized cholesterol pool was observed with a¹t₁₂ of 1.19±0.09 min and a second pool with a²t₁₂ of 38.30±4.16 min. Activity of Ca²⁺+Mg²⁺-ATPase was inhibited by low levels of cholesterol oxidation. Ten percent cholesterol oxidation, for example, resulted in approximately 35% percent inhibition of Ca²⁺+Mg²⁺-ATPase activity. After 13% cholesterol oxidation, further inhibition of Ca²⁺+Mg²⁺-ATPase activity was not observed. Activity of Na⁺+K⁺-ATPase was not affected by different levels of cholesterol oxidation (5%–40%). SPM interdigitation was significantly reduced and fluidity was significantly increased by cholesterol oxidation. The relatiobship observed between SPM interdigitation and Ca²⁺+Mg²⁺-ATPase activity was consistent with studies using model membranes [7]. Brain SPM function and structure were altered by relatively low levels of cholesterol oxidation and is a new approach to understanding cholesterol dynamics and neuronal function. The sensitivity of brain SPM to cholesterol oxidation may be important with respect to the proposed association between oxygen free radicals and certain neurodegenerative diseases.     

Brain Na, K-ATPase activity possibly regulated by a specific serotonin receptor

Na⁺, K⁺-ATPase activity in 6 regions of adult brain was measured after incubation with varying concentrations of serotonin. A concentration-dependent increase in enzyme activity was observed in 4 regions, with cerebral cortex and cerebellum showing the largest response. These results together with previous ones suggest that serotoninmmodulates brain Na⁺, K⁺-ATPase activity through a specific receptor located in target neurons or glial cells.     

Cyclic AMP-induced depolarization measured by bis-oxonol fluorescence in bovine adrenal medullary chromaffin cells

Summary Effects of cyclic AMP on membrane potentials were examined by measuring the changes of bis-oxonol fluorescence in bovine adrenal medullary chromaffin cells. 8-Bromo cyclic AMP (8Br-cAMP) or forskolin caused a gradual and long lasting increase of the fluorescence intensity. The effects of 8br-cAMP was blocked by cyclic AMP-dependent protein kinase inhibitor, adenosine-3, 5-cyclic monophosphothioate, Rp-diastereomer (Rp-cAMPS) and there was no further increase in the fluorescence by 8br-cAMP in the cells depolarized with 56 mM KCl or gramicidin D. Ouabain or the removal of extracellular K⁺ ([K⁺]₀free) which block Na⁺, K⁺-ATPase also increased the fluorescence. The effect of 8br-cAMP on the fluorescence was counteracted by ouabain or [K⁺]₀ free and was blocked in the absence of extracellular Na⁺ but not by tetrodotoxin or the removal of Ca²⁺ from the medium. These results may suggest that cyclic AMP causes the membrane depolarization by accumulating Na⁺ through the inhibition of Na⁺, K⁺-ATPase in adrenal chromaffin cells.     

Transport pathways for lithium ions in neuroblastoma × glioma hybrid cells at ‘therapeutic’ concentrations of Li

The pathways of Li⁺ transport in neuroblastoma × glioma hybrid cells were studied at 2 mM external Li⁺. Five components of Li⁺ transport were identified. (1) A Na⁺-dependent Li⁺ countertransport system mediating Li⁺ transport in both directions across the plasma membrane. This transport pathway is insensitive to ouabain or external K⁺. It shows trans-stimulation (i.e. acceleration of Li⁺ extrusion by external Na⁺ and stimulation of Li⁺ uptake by internal Na⁺) and cis-inhibition (i.e. reduction of Li⁺ uptake by external Na⁺). (2) The Na⁺K⁺ pump mediates Li⁺ uptake but not Li⁺ release in cells with physiological Na⁺ and K⁺ content. Li⁺ uptake by the pump in choline media is inhibited by both external Na⁺ and K⁺. In Na⁺ media, external K⁺ exhibits a biphasic effect: in concentrations up to about 1 mM, K⁺ accelerates, and at higher concentrations, K⁺ inhibits, Li⁺ uptake by the pump. (3) Li⁺ can enter the voltage-dependent Na⁺ channel. Li⁺ uptake through this pathway is stimulated by veratridine and scorpion toxin, the stimulation being blocked by tetrodotoxin. Residual pathways comprise (4) a saturable component, which is comparable to basal Na⁺ uptake, and (5) a ouabain-resistant component promoting Li⁺ extrusion against an electrochemical gradient in choline media. The mechanisms for Li⁺ extrusion described here possibly explain how neuronal cells maintain the steady-state ratio of internal to external Li⁺ below 1 during chronic exposure to 1–2 mM external Li⁺.     

Control of the Na, K -pump by nerve growth factor is essential to neuronal survival

Recently we have shown that nerve growth factor (NGF) controls the performance of the Na⁺, K⁺ -pump in its target ganglionic neurons in suspension cultures. In the present study, enriched neuronal preparations of embryonic day 8 (E8) chick dorsal root ganglia (DRG) were obtained by means of a differential attachment procedure using tissue culture plastic dishes. Neurons were routinely seeded into polyornithine-coated 16 mm culture wells in the presence of NGF. After 18 h, cultures were switched to media with or without NGF, and containing either⁸⁶Rb⁺ (as a tracer for K⁺) or²²Na⁺ (as a tracer for Na ions). Over the next 12–15 h the cultures were assessed for numbers of surviving neurons and accumulated radioactivity. Cultured E8 chick DRG neurons fail to maintain their intracellular K⁺ concentration when deprived of NGF over 4–6 h. The NGF-deprived and K⁺- depleted neurons reaccumulate K⁺ within minutes of delayed NGF administration. The occurrence of this K⁺ response in culture to added NGF parallels the response occurring in E8 neuronal suspensions, including the time of onset of irreversibility. Similar experiments performed with²²Na⁺ indicate corresponding ionic behaviors for cultured E8 DRG neurons. These NGF-controlled ionic responses in monolayer cultures occur for E7 and E10 neurons, but not E14 neurons and parallel the survival response to NGF of the same neurons. Blocking the pump performance by NGF deprivation leads to neuronal death. Identical results are obtained by addition of oubain or omission of external K⁺ in the presence of NGF. Partial reduction of pump performance by any one of these treatments leads to partial survival of the neuronal population in a precisely predictable manner. Therefore, control of the pump by NGF is an essential component of the NGF action on neuronal survival.