Development of (Na+-K+)-ATPase in rat cerebrum: correlation with Na+-dependent phosphorylation and K+-paranitrophenylphosphatase

Gestational doxorubicin alters fetal thyroid-brain axis

Administration of chemotherapy during pregnancy may represent a big risk factor for the developing brain, therefore we studied whether the transplacental transport of doxorubicin (DOX) may affect the development of neuroendocrine system. DOX (25 mg/kg; 3 times interaperitoneally/week) was given to pregnant rats during whole gestation period. The disturbances in neuroendocrine functions were investigated at gestation day (GD) 15 and 20 by following the maternal and fetal thyroid hormone levels, fetal nucleotides (ATP, ADP, AMP) levels and adenosine triphosphatase (Na(+), K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase) activities in two brain regions, cerebrum and cerebellum. In control group, the levels of maternal and fetal serum thyroxine (T4), triiodothyronine (T3), thyrotropin (TSH), and fetal serum growth hormone (GH) increased from days 15 to 20, whereas in the DOX group, a decrease in maternal and fetal T4, T3 and increase in TSH levels (hypothyroid status) were observed. Also, the levels of fetal GH decreased continuously from GD 15 to 20 with respect to control group. In cerebrum and cerebellum, the levels of fetal nucleotides and the activities of fetal ATPases in control group followed a synchronized course of development. The fetal hypothyroidism due to maternal administration of DOX decreased the levels of nucleotides, ATPases activities, and total adenylate, instead, the adenylate energy charge showed a trend to an increase in both brain regions at all ages tested. These alterations were dose- and age-dependent and this, in turn, may impair the nerve transmission. Finally, DOX may act as neuroendocrine disruptor causing hypothyroidism and fetal brain energetic dysfunction.

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 Adhesion Molecule on Glia (AMOG) Is Widely Expressed by Astrocytes in Developing and Adult Mouse Brain

Adhesion molecule on glia (AMOG) is a 45 - 50 kD cell surface glycoprotein structurally similar to the Na, K-ATPase beta-subunit and associated with the catalytic subunit of this enzyme. Previous immunofluorescence results had suggested that AMOG is transiently expressed on Bergmann glia during mouse cerebellar development, and antibody-inhibition results have implicated it in the migration of granule neurons. We report that, while AMOG mRNA is detected in Bergmann glia during the migratory period, this astrocyte derivative continues to express AMOG mRNA at similar levels in adult mice suggesting a functional role for AMOG in adulthood. Evidence from RNA and protein blot analyses that AMOG is present before birth, increasing about ten fold in adult mouse brain and cerebellum is also provided. RNA blot analysis of astrocyte-enriched cell populations and in situ hybridization results show that astrocytes synthesize AMOG mRNA in all regions of the developing and adult brain. In the adult, AMOG mRNA is more abundant in grey than white matter and, among grey matter regions, highest in cerebellar cortex. These results indicate a relationship between density of neuronal elements and AMOG expression. It is further speculated that AMOG is part of a Na,K-ATPase complex expressed preferentially by astrocytes in mouse brain.

Influence of development on Na(+)/K(+)-ATPase expression: isoform- and tissue-dependency

The four isoforms of the catalytic subunit of Na(+)/K(+)-ATPase identified in rats differ in their affinities for ions and ouabain. Moreover, its expression is tissue-specific, developmentally and hormonally regulated. The aim of the present work was to evaluate the influence of age on the ratio and density of these isoforms in crude membrane preparations from rat brain hemispheres, brainstem, heart ventricles and kidneys. In all tissues investigated, Na(+)/K(+)-ATPase activity was higher in adults than in neonates but brain tissues presented the most remarkable differences. In these tissues, ouabain inhibition curves for Na(+)/K(+)-ATPase activity revealed the presence of two processes with different sensitivities to ouabain. An increase of approximately sixfold in the expression of the high affinity isoforms was observed between newborn and adult rats. In contrast, the low affinity isoform increased only approximately twofold in brainstem whereas it increased ninefold in brain hemispheres. Unlike brain tissues, a decrease (almost fourfold) in the number of high affinity ouabain binding sites was observed during ontogenesis of the heart. Although limited by the inability to resolve alpha(2) and alpha(3) isoforms, present data indicate that the influence of development on the expression of Na(+)/K(+)-ATPase depends not only on the isoform, but also on the tissue where the enzyme is expressed.

Quantification of the alpha(3) subunit of the Na(+)/K(+)-ATPase in developing rat cerebellum

Cerebellar Purkinje neurons of rats have been shown to exhibit a progressive increase in resting membrane potential as the animals develop postnatally. The magnitude of this increase was equivalent in magnitude to the increase in the depolarizing action of ouabain, consistent with a role for the Na(+)/K(+)-pump in the hyperpolarization. Ouabain binding sites in whole cerebellum also increased with age. The present study was undertaken to confirm that the increases in ouabain binding and the electrophysiological responses to ouabain were a consequence of increases in the sodium pump and to determine whether the changes seen at the whole organ level were reflective of changes taking place at the cellular level. Using antibodies directed against the alpha(1), alpha(2), and alpha(3) subunits of the Na(+)/K(+)-ATPase, rats between 13 and 19 days of age exhibited a statistically significant increase in the relative amount of the alpha(3) subunit at the level of the whole organ, as determined by Western and slot blot analyses, with no change in the levels of either the alpha(1) or the alpha(2) subunit. Using immunohistochemistry, the alpha(3) subunit was shown to increase in both the Purkinje cell layer and the white matter during this postnatal time period, while the alpha(1) subunit increased in the granular layer. These results support and extend previous work, which pointed to a role for the electrogenic sodium pump in the developmental increase in Purkinje cell membrane potential. Furthermore, the data provide a cellular mechanism underlying the increase in resting membrane potential, that is, by the specific modulation of the alpha(3) subunit isoform.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

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.

The Na,K-ATPase Hypothesis for Bipolar Disorder: Implications of Normal Development

ABSTRACT The Na,K pump is the indispensable component of excitable tissue that maintains transmembrane ion-based potentials. Adequate human data regarding the developmental trends in Na,K pump activity in childhood are unavailable, but human nonneuronal and animal studies demonstrate a developmental shift toward declining Na,K pump activity with increasing age. The decline in pump activity is paralleled by an age-related increase in intracellular sodium ions and a concomitant decrease in intracellular potassium ions. These changes appear to decrease the potential difference across neuronal membranes with advancing age, suggesting an increase in neuronal excitability. The Na,K-ATPase hypothesis of bipolar disorder proposes that manic symptoms are related to a modest decrease in Na,K pump activity and a consequent increase in neuronal excitability, whereas bipolar depression is consequent to a greater decrease in pump activity and a resultant decrease in neurotransmitter release. Changes in ion transport and distribution with age may offer an explanation for clinical observations of developmental variations in lithium response, including the apparent agerelated decline in maximal pretoxic lithium level, decline in therapeutic blood level range, and perhaps an increase in the therapeutic efficacy of lithium. The clinical implications of the hypothesis regarding the course of bipolar disorder would include an explanation of the increasing frequency and severity of episodes with advancing age. The hypothesis makes numerous predictions which are testable and suggests new avenues of pharmacological intervention.

Regulatory role of a neurotransmitter (5-HT) on glial Na+/K(+)-ATPase in the rat brain

In the present work we studied the effect of serotonin (5-HT) on the kinetics of Na+/K(+)-ATPase in subcellular preparations of the cerebral cortex from male Wistar rats using various concentrations of ATP and K+ with and without added 5-HT. Also we studied the effect of 5-HT on the enzyme in glial or neuronal preparations. The results indicated that there was a significant increase (P < 0.05) of the Vmax in the presence of 5-HT in the whole tissue preparation (homogenate) but not in the subcellular fractions, suggesting that the interaction could be preferentially with the glial pump. Further results supported that this was the case since activation by 5-HT was mainly in the glial preparations. Kinetic data and the binding of [3H]ouabain supported that the enzyme is activated by 5-HT through the exposure of more enzymatic active sites.

Differentiation of rat hippocampal neurons induced by estrogen in vitro: effects on neuritogenesis and Na, K-ATPase activity

To gain insight into the mechanisms responsible for differentiation of hippocampal neurons growing in vitro, the effects of estrogen on neuritic development and on activity and distribution of isoforms of the Na, K-ATPase, were evaluated. Dissociated cells from hippocampi of 19-day-old rat fetuses were raised for 5 days in the presence or absence of 100 nM estradiol-17 beta (E2) in minimum essential medium supplemented either with 10% untreated fetal calf serum (MEM-10) or with 10% fetal calf serum previously adsorbed with dextran-activated charcoal (MEM-10-Cha). Cultures in MEM-10 showed larger neuritic length and increased levels of Na, K-ATPase activity than cultures in MEM-10-Cha. In cells cultured in MEM-10 medium, the addition of E2 resulted in selective enhancement of axonal length with a concomitant increase in the alpha-2 isoform of the Na, K-ATPase, whereas a decrease was found in the form most sensitive to ouabain; the total enzymatic activity remained unchanged. Conversely, in cultures raised in MEM-10-Cha, E2 did not affect Na, K-ATPase activity or neuritogenesis. These results show that two presumably independent probes of cellular differentiation of hippocampal neurons (i.e., neuritogenesis and patterns of Na, K-ATPase activity) were concurrently regulated by E2 and that such regulation depended on interaction with factor(s) present in calf serum. The well-known neuritogenic effect of E2 is hereby extended to hippocampal neurons, although for these cells it seems to be restricted to axons.

Na(+)-K(+)-ATPase gene expression during in vitro development of rat fetal forebrain

The existence of at least three isoforms of Na(+)-K(+)-ATPase in adult brain tissues [alpha 1, kidney type; alpha 2 [or alpha(+)]; alpha 3] suggests that these genes might be regulated in a cell-specific and time-dependent manner during development. We have studied this question in serum-free aggregating cell cultures of mechanically dissociated rat fetal telencephalon. At the protein level, the relative rate of synthesis of the pool of alpha 1-, alpha 2-, and alpha 3-subunits increased approximately twofold over 15 days of culture, leading to a marked increase in the immunochemical pool of alpha-subunits as measured by a panspecific polyclonal antibody. Concomitantly, Na(+)-K(+)-ATPase enzyme-specific activity increased three- (lower forebrain) to sixfold (upper forebrain). The transcripts of all three alpha-isoforms and beta-subunit were detected in vitro in similar proportion to the level observed in vivo. alpha 3-mRNA (3.7 kb) was more abundant than alpha 1 (3.7 kb) or alpha 2 (5.3 and 3.4 kb). Cytosine arabinoside (0.4 microM) and cholera toxin (0.1 microM) were used to selectively eliminate glial cells or neurons, respectively. It was found that alpha 2-mRNA is predominantly transcribed in glial cell cultures, whereas alpha 3- and beta 1-mRNA (2.7, 2.3, and 1.8 kb) are predominant in neuronal cultures.

Acid homeostasis following partial ischemia in neonatal brain measured in vivo by 31P and 1H nuclear magnetic resonance spectroscopy

The purpose of this study was to investigate neonatal brain energy metabolism, acid, and lactate homeostasis in the period immediately following partial ischemia. Changes in brain buffering capacity were quantified by measuring mean intracellular brain pH, calculated from the chemical shift of Pi, in response to identical episodes of hypercarbia before and after ischemia. In addition, the relationship between brain buffer base deficit and intracellular pH was compared during and following ischemia. Thus, in vivo 31P and 1H nuclear magnetic resonance spectra were obtained from the brains of seven newborn piglets exposed to sequential episodes of hypercarbia, partial ischemia, and a second episode of hypercarbia in the postischemic recovery period. For the first episode of hypercarbia, brain buffering was similar to values reported for adult animals of other species (percentage pH regulation = 54 +/- 16%). During ischemia, the brain base deficit per unit change in pH was -19 +/- 5 mM/pH unit, which is similar to values reported for adult rats. By 20-35 min postischemia, brain acidosis partly resolved in spite of a net increase in lactate concentration. Therefore, the consumption of lactate could not explain acid homeostasis in the first 35 min following ischemia. We conclude that H+/HCO3- or other proton equivalent translocation mechanisms must be sufficiently developed in piglet brain to support acid regulation. This is surprising, because a substantial body of evidence implies these processes would be less active in immature brain. The second episode of hypercarbia, from 35 to 65 min postischemia, resulted in a smaller decrease in brain pH compared with the first episode, a result indicating an increase in brain buffering capacity (percentage pH regulation = 79 +/- 29%). This was associated with a parallel decrease in brain lactate content, and therefore acid regulation could be attributed to either continued ion translocation or the consumption of lactate. A mild decrease in brain pH and content of energy metabolites was observed, a finding suggesting that the metabolic consequences of severe postischemic hypercarbia are neither particularly dangerous or beneficial.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Development of Na, K-ATPase in neocortical grafts

Pieces of cerebral cortex from 14-day rat embryos were transplanted into freshly prepared cavities in the cerebral cortex of adult rats. At various time intervals after implantation. Na,K-ATPase, Mg-ATPase as well as the ratio of two molecular forms of Na,K-ATPase were determined in the grafts and compared with the values obtained from intact cortex at various stages of postnatal development. Up to the 57th postimplantation day (PID), Na.K-ATPase develops very slowly, reaching on PID 57, i.e. on postconceptional day (PCD) 71, only 40% of the enzyme activity of intact brain cortex (PND 49, PCD 71). At PID 90 (PCD 104) Na,K-ATPase activity attained that of the intact adult tissue (PND 82, PCD 104). Mg-ATPase activity in the grafts developed similarly to that of intact brain but was much higher in the early postimplantation phase. The maximum ratio of the two molecular forms of Na,K-ATPase [alfa(+)/alfa] was shifted toward later developmental periods in the grafts as compared with the intact cerebral cortex.

Taurine and beta-alanine uptake in primary astrocytes differentiating in culture: effects of ions

The effects of ions on taurine and beta-alanine uptake were studied in astrocytes during cellular differentiation in primary cultures. The uptakes were strictly Na+-dependent and also inhibited by the omission of K+ and in the presence of ouabain suggesting that their transport is fuelled mainly by these cation gradients. Two sodium ions were associated in the transport of one taurine and beta-alanine molecule across cell membranes. A reduction in Cl- concentration also markedly inhibited the uptake of both amino acids, indicating that this anion is of importance in the transport processes. The similar ion dependency profiles of taurine and beta-alanine uptake corroborate the assumption that the uptake of these amino acids in astrocytes is mediated by the same carrier. In Na+- and K+-free media both taurine and beta-alanine uptakes were reduced significantly more in 14-day-old or older than in 7-day-old cultures. No significant changes occurred in the coupling ratio between Na+ and taurine or beta-alanine as a function of spontaneous cellular differentiation or upon dBcAMP treatment. These results suggest that the uptake systems of these structurally related amino acids in astrocytes have reached a relatively high degree of functional maturity by two weeks in culture.

Tissue specificity, localization in brain, and cell-free translation of mRNA encoding the A3 isoform of Na+,K+-ATPase

The isolation of multiple Na+,K+-ATPase cDNAs from rat brain has led to the discovery of a family of alpha-isoform genes. Using A1 (alpha), A2 (alpha+), and A3 (alpha III) Na+,K+-ATPase gene probes, we have analyzed the distribution of Na+,K+-ATPase mRNAs in adult and fetal rat tissues by RNA blot and hybridization histochemistry. A1 Na+,K+-ATPase mRNA was found ubiquitously among various tissues, with highest levels in transport epithelial and neural tissues. A2 mRNA was found in adult neural and muscle tissues, and A3 mRNA was found only in neural tissues and fetal heart muscle. Both A1 and A2 mRNAs were less abundant in fetal brain than in adult brain; in contrast, A3 mRNA was abundant at both stages. In situ mapping of brain areas that contain A3 mRNA suggests that this Na+,K+-ATPase isoenzyme is expressed predominantly by neural cells. Analysis of Na+,K+-ATPase proteins generated by cell-free translation of synthetic mRNAs suggests that the A3 protein has properties similar to A2 (alpha+).

Inhibitors of cation pump enzyme equally present in normal and ischemic gerbil brain

This was a search for endogenous inhibitors of the cation pump enzyme in normal and ischemic gerbil brain. The first model of ischemia was bilateral common carotid clamping for 30 minutes followed by reperfusion periods of 0, 1, 2.5, and 4 hours. After bilateral clamping, K-paranitrophenylphosphatase (K-pNPPase) activity fell significantly after 0, 1, and 2.5 hours of reflow but only slightly by 4 hours. The second model was decapitation with timed delays of 0, 15, 60 and 240 minutes. There was no decline in K-pNPPase activity after total ischemia produced by decapitation. Thus, the model of partial ischemia was more deleterious to the cation pump enzyme than was the model of total ischemia. All brains contained endogenous K-pNPPase inhibitors. Boiled supernatant fractions inhibited K-pNPPase activity by about 50-60% while Amicon filtrates of brain homogenate inhibited about 12%. Factors in normal and ischemic brain may modify the activity of the cation pump enzyme, but no differences in inhibitory effects were found between normal and ischemic brain extracts.

(Na+ + K+)-adenosinetriphosphatase in the brain of Shiverer (Shi/Shi) mice

The myelin-deficient Shiverer (Shi/Shi) mutant mouse may be a useful model in assessing the dependence of brain (Na+ + K+)-ATPase concentration and composition on myelin membrane formation. Brain microsomal membranes from age-matched control (+/+) and Shiverer (Shi/Shi) mice were fractionated by differential centrifugation and sucrose gradient sedimentation. No reduction in (Na+ + K+)-ATPase specific activity was measured in whole homogenates, high- and low-speed fractions or gradient fractions from brains of Shi/Shi mice as compared to those of +/+ mice. In addition, sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting with antisera specific for mouse brain (Na+ + K+)-ATPase revealed no significant difference in catalytic subunit composition between fractions of +/+ and Shi/Shi brains. The similar results obtained for both +/+ and myelin-deficient Shi/Shi mice suggest that myelin contributes little to total brain (Na+ + K+)-ATPase.

Vanadate inhibition of rat cerebral cortex Na+,K+-ATPase during postnatal development

The Na+,K+-ATPase activity and its response to vanadate inhibition was investigated in cerebral cortex homogenates of 7-, 12- and 18-day-old rats. The enzyme was inhibited by vanadate in a dose-dependent manner in all these age groups. Furthermore, there was a different sensitivity towards vanadate during postnatal development; the concentration of V+5 needed for 50% inhibition of Na+, K+-ATPase was 1.1 X 10(-6)M, 2 X 10(-7)M and 4.4 X 10(-7)M for 7-, 12- and 18-day-old rats, respectively. It is suggested that the different sensitivity of Na+, K+-ATPase towards vanadate inhibition during postnatal development might be due to age-dependent changes in the ratio of various cell types.

(Na+ + K+)-ATPase in C6 glioma and rat cerebrum

The content and distribution of the membrane-bound enzyme (Na+ + K+)-ATPase in a rat cerebral C6 glioma was determined by immunocytochemistry, immunoblots and enzyme assay. In the C6 glioma cell culture (Na+ + K+)-ATPase activity was about 20% of (Mg2+ + Na+ + K+)-ATPase activity. However, (Mg2+ + Na+ + K+)-ATPase activity in the cerebral C6 gliomas was very close to Mg2+ baseline and not significantly increased by Na+ and K+. As shown by immunoblotting, (Na+ + K+)-ATPase catalytic subunit was detected in excised samples of control cerebrum and as a trace in the intracerebral portions of C6 glioma but not at all in the extracranial portions of C6 glioma or in C6 glioma cell culture. (Na+ + K+)-ATPase was not detected immunocytochemically in paraffin sections of the extracranial or intracerebral portions of rat cerebral C6 glioma. The absence of staining for (Na+ + K+)-ATPase clearly demarcated projections of glioma within normal brain. These results suggest that C6 glioma has little if any expression of (Na+ + K+)-ATPase in vitro or in vivo. The small amount of enzyme epitope in the intracerebral portions represents contamination by normal cerebrum in the extracts.

Diurnal variation of cation pump enzyme activity in pineal and seven other rat brain regions

Adult female Long-Evans rats were maintained on an automatically regulated artificial lighting schedule of light:dark (L:D) 14.5:9.5 for 12 wk. After sacrifice at 0630, 1130, 1600, 1800, 2000, 2200, 0230, or 0400, the pineals were removed, weighed, and assayed for N-acetyltransferase (NAT), melatonin, Mg++-paranitrophenylphosphatase (pNPPase), and K-pNPPase activity. The brains were quickly dissected into the following areas: cerebellum, superior colliculi, inferior colliculi, visual cortex, auditory cortex, sensorimotor cortex, and the hypothalamic area around the suprachiasmatic nucleus. These regions were weighed and 10% sucrose homogenates were prepared for determinations of protein, Mg++-pNPPase, and K+-pNPPase activity. Pineal melatonin rose over six-fold from 144 +/- 70 pg/gland at 1130 to 981 +/- 173 pg/gland at 0230. Similarly, pineal NAT activity rose over 11-fold, from 119 +/- 12 pmol/gland/h to 1315 +/- 232 pmol/gland/h at the same times. K+-pNPPase activity rose by about two-thirds, from 133 +/- 12.8 nmol/gland/h to 224 +/- 22.3 nmol/gland/h from 1600 to 0230. However, when expressed per mg protein, these differences in pNPPase activity were not significant. There were no significant daily rhythms discernible in any of the seven other brain regions across these times. We conclude that cation pump enzyme activity varies only slightly with time in the rat brain and pineal gland, in spite of definite daily rhythms of pineal melatonin and NAT activity.

Thyroxine regulates the activity and the concentration of synaptic plasma membrane Na,K-ATPase in the developing rat brain cortex

The administration of thyroxine to neonatal rats stimulates the activity of synaptic membrane Na,K-ATPase in the brain cortex of euthyroid and hypothyroid animals. Thyroxine also increases NA,K-ATPase activity of isolated neuronal perikarya in neonatal rats. Binding studies employing [3H]ouabain indicate that the hormone increases the amount of Na,K-ATPase in the synaptic membranes of two-week-old rats. Thyroxine treatment however did not affect synaptic membrane Na,K-ATPase in rats aged 30 days. The results suggest that thyroid hormones are important for the maturation of the synaptic plasma membrane during the critical period of rat brain development.

Development and regional distribution of two molecular forms of the catalytic subunit of the Na,K-ATPase in rat brain

Two molecular forms of the catalytic subunit of the Na,K-ATPase can be isolated from brain (1). Only the alpha form was detected in rat embryo brain at 13 days of gestation (E13). The alpha (+) form, which is characteristic of myelinated axons, appeared at E15 before myelination begins. Hence its expression is not dependent on prior myelination. Axonal transport of the alpha (+) form was demonstrated in 4 day-old rats. The ratio of alpha (+): alpha was 1:1 in adult retina, cortex and cerebellum and 10:1 in brain stem. Although alpha (+) is characteristic of myelinated axons, this regional difference was present not only in enzyme extracted from crude microsomes, that contain myelinated axon fragments, but also in enzyme from isolated synaptosomes. Hence, the alpha (+): alpha ratio is an inherent characteristic of the neuron and does not depend on regional differences in myelination.

Ionic behaviors and nerve growth factor dependence in developing chick ganglia. II. Studies with neurons of dorsal root ganglia

Using intact dorsal root ganglia (DRG) from embryonic (E) chick and measuring 22Na+ accumulation, the authors have recently shown that (i) ionic control by the ganglia has a complete requirement for exogenous NGF between E6 and E10, and (ii) control of ion pump mechanisms independent of exogenous NGF is progressively acquired by these ganglia from E10 to E16. Similar experiments have now been carried out using enriched suspensions of ganglionic neurons to test whether the acquisition of endogenous control by older ganglia was (1) due to the close association between neurons and nonneurons, and (2) correlated with a decreasing need by these neurons for exogenous NGF for survival in culture. In this enriched neuronal population, Na+ accumulation in the absence of NGF increases from E7 to E10, paralleling the increase in Na+ accessible space under ouabain, but then decreases conspicuously between E10 and E16, despite little change in the ouabain-sensitive Na+ space. NGF prevents Na+ accumulation during the early period, and becomes increasingly irrelevant for this behavior in later (after E10) development. K+ movements (traced with 86Rb+) behaved similarly. Active K+ influx (Na+, K+-pump mediated) also increases severalfold between E7 and E10. This K+ influx is sensitive to NGF at E7 and E10 but not at E14, paralleling the observed Na+ and K+ behaviors. These data suggest that the control of Na+, K+-pump performances acquired by these neurons between E10 and E16 represents the development of a neuronal self-sufficiency. This increase in ionic control is not due to an increase in pump molecules or pumping efficiency. No increases in the binding of [3H]ouabain or active K+ influx occur between E10 and E16, when ionic control is developing. The ionic dependence on NGF by the DRG neurons changes with their developmental age along the same temporal pattern displayed by their survival response to NGF in culture.

Inhibition of rat brain microsomal (Na+ + K+)-ATPase and K+-p-nitrophenylphosphatase by periodic acid

The effects of mild periodate exposure on the kinetics of (Na+ + K+)-ATPase and K+-p-nitrophenylphosphatase were studied using rat cerebral microsome preparations. Fifty percent inhibition of both enzyme activities was attained near 3 microM periodate concentrations. This inhibition was biphasic with time. Mg2+-ATPase and Mg2+-p-nitrophenylphosphatase activities were much less inhibited by periodate. Periodate inhibition was partially reversed by dimercaprol and dithiothreitol but not by diffusion. The possible reaction products formic acid, formaldehyde, glyceraldehyde, and acetaldehyde had no inhibitory effects in similar concentrations. Periodate exposure produced no detectable changes in the activation of (Na+ + K+)-ATPase by Na+, K+, Mg2+, or ATP. Residues shared by both (Na+ + K+)-ATPase and K+-p-nitrophenylphosphatase are both critical to hydrolytic function and sensitive to mild oxidation by periodate.

Asterixis and encephalopathy following metrizamide myelography: investigations into possible mechanisms and review of the literature

Marked asterixis occurred in two patients following metrizamide myelography. One also suffered generalized seizures and the other had severe stuttering speech for seven days. The spectrum of toxic manifestations of metrizamide is reviewed with emphasis on the unusual lethargy and other depressive effects seen with this relatively safe agent. The hypothesis that metrizamide exerts a ouabain-like effect on the cortical surface was tested. Metrizamide in concentrations as high as 20 mM had no inhibitory effect on rat cerebral K+-para-nitrophenylphosphatase, a partial reaction of (Na+K+)-adenosine triphosphatase. Because metrizamide is a 2-deoxyglucose analogue, a competitive inhibition of hexokinase at the first step in glycolysis was also postulated. Metrizamide was found to competitively inhibit commercial (microbial) hexokinase. The Michaelis constant for glucose rises from 0.13 to 0.25 to 0.33 to 0.91 mM in the presence of 0, 0.4, 1.0, and 2.0 mM metrizamide, respectively. Since the concentration of metrizamide over the cerebral cortex after routine myelography may be approximately 50 mM compared with a glucose concentration of only 3.6 mM (65 mg/dl), it is postulated that impaired brain glucose metabolism may be responsible for some of the toxic effects of metrizamide.

Developmental changes in polypeptide composition of, and precursor incorporation into, cellular and subcellular fractions of rat cerebral cortex

Neuronal-enriched and glial-enriched fractions from rat cerebral cortex at 2, 5, 9, 14 and 23 days postnatally, and subcellular fractions from 2, 14 and 46 day old rat were prepared. The polypeptide composition of all fractions was analysed by sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis and quantified by densitometry. Fifty-nine polypeptides (mol. wts., 13,200-251,000) were resolved in the cell fractions of which the majority remained unchanged throughout postnatal development. Three polypeptides (mol. wts., 102,000, 56,000, 53,700) were found to increase in amount developmentally in both cellular fractions, the latter two showing a peak in relative amount on day 14 and a subsequent decline. Three polypeptides (mol. wts., 47,000, 28,200, 17,400) were found to be common to the glial-enriched fraction as well as the myelin fraction, and all showed a developmental increase. The neuronal-enriched fraction was found to be enriched in five polypeptides of which one (mol. wt., 51,900) showed a developmental increase after ten days postnatally, the others (mol. wts., 178,700, 142,000, 109,000, 24,000) showing a decrease. In vitro incorporation of [35S]-methionine into the glial-enriched fraction was carried out, and a developmental decline was observed in the labelling of a polypeptide of 42,000 mol. wt.

Immunocytochemical localization of (Na+, K+)-ATPase in the goldfish optic nerve

Antiserum to the catalytic subunit of goldfish brain (Na+, K+)-ATPase has been employed at the electron microscopic level by means of the peroxidase-antiperoxidase immunohistochemical method. In optic nerve, antigenic sites are restricted to the nodes of Ranvier. No reaction product is detected in underlying internodal neurolemma. Outgrowing neurites for cultured retinal explants devoid of glial ensheathment exhibit a continuous distribution of the enzyme subunit. Antibodies against eel electroplax (Na+, K+)-ATPase cross-react with the goldfish brain enzyme and show a similar immunocytochemical distribution pattern.