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

Expression and activity of the na-k ATPase in ischemic injury of primary cultured astrocytes

Astrocytes are reported to have critical functions in ischemic brain injury including protective effects against ischemia-induced neuronal dysfunction. Na-K ATPase maintains ionic gradients in astrocytes and is suggested as an indicator of ischemic injury in glial cells. Here, we examined the role of the Na-K ATPase in the pathologic process of ischemic injury of primary cultured astrocytes. Chemical ischemia was induced by sodium azide and glucose deprivation. Lactate dehydrogenase assays showed that the cytotoxic effect of chemical ischemia on astrocytes began to appear at 2 h of ischemia. The expression of Na-K ATPase α1 subunit protein was increased at 2 h of chemical ischemia and was decreased at 6 h of ischemia, whereas the expression of α1 subunit mRNA was not changed by chemical ischemia. Na-K ATPase activity was time-dependently decreased at 1, 3, and 6 h of chemical ischemia, whereas the enzyme activity was temporarily recovered to the control value at 2 h of chemical ischemia. Cytotoxicity at 2 h of chemical ischemia was significantly blocked by reoxygenation for 24 h following ischemia. Reoxygenation following chemical ischemia for 1 h significantly increased the activity of the Na-K ATPase, while reoxygenation following ischemia for 2 h slightly decreased the enzyme activity. These results suggest that the critical time for ischemia-induced cytotoxicity of astrocytes might be 2 h after the initiation of ischemic insult and that the increase in the expression and activity of the Na-K ATPase might play a protective role during ischemic injury of astrocytes.

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.

Postnatal development of electrogenic sodium pump activity in rat hippocampal pyramidal neurons

We assessed the development of electrogenic sodium pump (Na+ pump) activity in CA1 pyramidal neurons of rat hippocampal slices by studying the prolonged hyperpolarization which follows glutamate-induced depolarization (postglutamate hyperpolarization or PGH) at different postnatal ages. We also examined the development of membrane-bound enzyme in the hippocampal CA1 subfield with light microscopic immunocytochemistry and an antiserum against Na+,K(+)-ATPase. The PGH, which has previously been shown to be due to activation of an electrogenic Na+ pump in adult hippocampal CA1 neurons, was eliminated by strophanthidin, a Na+,K(+)-ATPase inhibitor, at all ages. It was unaffected by several potassium channel blockers, an intracellular calcium chelator, intracellular Cl- injection or tetrodotoxin (TTX) perfusion. The PGH thus appeared to be independent of K+ and Cl- conductances and produced by an electrogenic Na+ pump in adult and immature animals activated in large part by entry of Na+ through the glutamate receptor-channel complex. The size (integrated area) of the PGH was directly proportional to the area of preceding glutamate-induced depolarization (GD) and relatively voltage independent. Similar GDs could be elicited from postnatal day (P) 7 to P greater than or equal to 35, however, only very small PGHs were produced in neurons from P7-11 animals. A ratio of PGH area to GD area (PGH ratio) was calculated for each neuron and used to compare Na+ pump activity at different ages. There was a significant increase in the mean PGH ratio with age when P7-11, P21-25 and P35-39 groups were compared. Na+ pump activity estimated from the PGH ratio is very low in the first postnatal week but develops gradually over the first 5 weeks of life. Immunostaining for Na+,K(+)-ATPase in adult rat hippocampi revealed a punctate reaction product surrounding pyramidal cell bodies, whereas the staining was uniform along plasmalemma of dendrites in stratum radiatum and stratum oriens. By contrast, only minimum staining was present surrounding cell bodies and dendrites of P7 hippocampi and staining in stratum pyramidale was not punctate at this age. Na+,K(+)-ATPase activity estimated grossly from immunocytochemical staining is very low in the first postnatal week, increases during the first 5 weeks and develops a characteristic focal localization.(ABSTRACT TRUNCATED AT 400 WORDS)

Two active Na+/K+-ATPases of high affinity for ouabain in adult rat brain membranes

The degree of heterogeneity of active Na+/K(+)-ATPases has been investigated in terms of ouabain sensitivity. A mathematical analysis of the dose-response curves (inhibition of Na+/K(+)-ATPase) at equilibrium is consistent with the putative existence of three inhibitory states for ouabain two of high (very high plus high) and one of low affinity. The computed IC50 values are: 23.0 +/- 0.15 nM, 460 +/- 4.0 nM and 320 +/- 4.6 microM, respectively. The relative abundance of the three inhibitory states was estimated as: 39%, 36% and 20%, respectively. Direct measurements of [3H]ouabain-binding at equilibrium carried out on membrane preparations with ATP, Mg2+ and Na+ also revealed two distinct high affinity-binding sites, the apparent Kd values of which were 17.0 +/- 0.2 nM (very high) and 80 +/- 1 nM (high), respectively. Dissociation processes were studied at different ouabain concentrations according to both reversal of enzyme inhibition and [3H]ouabain release. The reversal of enzyme inhibition occurred at three different rates, depending upon the ouabain doses used (10 nM, 2 and 100 microM). When the high-affinity sites were involved (ouabain doses lower than 2 microM) the dissociation process was biphasic. A similar biphasic pattern was also detected by [3H]ouabain-release. The time-course of [3H]ouabain dissociation (0.1 microM) was also biphasic. These data indicate that the three catalytic subunits of rat brain Na+/K(+)-ATPase alpha 1, alpha 2 and alpha 3 (Hsu, Y.-M. and Guidotti, G. (1989) Biochemistry 28, 569-573) are able to hydrolyse ATP and exhibit different affinities for cardiac glycosides.

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.

Na+,K+-ATPase in developing fetal guinea pig brain and the effect of maternal hypoxia

Na+,K+-ATPase activity was determined in fetal guinea pig brain at 35, 40, 45, 50, 55, and 60 days of gestation. The activity remained at a constant level during the early periods (35-45 days) of gestation and increased significantly during 45-60 days. Following maternal hypoxia, the activity of Na+,K+-ATPase in the term (60 days) fetal brain was reduced by 50% whereas the preterm (50 days) brain activity was unaffected. Under identical hypoxic conditions, the enzymatic activity of adult brain was significantly reduced by 20%. Na+,K+-ATPase obtained from fetal brain (50 days of gestation) has both a low and a high affinity for ATP (Km values = 0.50 and 0.053 mM and corresponding Vmax values = 10.77 and 2.82 mumoles Pi/mg protein/hr), whereas the enzyme in the adult brain has only a low affinity (Km = 1.67 mM and Vmax = 20.32 mumoles Pi/mg protein/hr). The high and low affinity sites for ATP in the fetal brain suggests a mechanism essential for the maintenance of cellular ionic gradients at low concentrations of ATP and which would provide the fetal brain with a greater tolerance to hypoxia. The high sensitivity of Na+,K+-ATPase activity to hypoxia in guinea pig brain at term suggests that the cell membrane functions of the fetal brain may be more susceptible to hypoxia at term than it is earlier in gestation.

Difference between neuronal and nonneuronal (Na+ + K+)-ATPases in their conformational equilibrium

Several experiments were carried out to study the difference between two isozymes (alpha(+) and alpha) of (Na+ + K+)-ATPase in the conformational equilibrium. Rat brain (Na+ + K+)-ATPase was much more thermolabile than the kidney enzyme. Both enzymes were protected from heat inactivation not only by Na+ and K+, but also by choline in varying degrees, though there was a difference between the two enzymes in the protection by the ligands. The brain enzyme was partially protected from N-ethylmaleimide (NEM) inactivation by both Na+ and K+, but the effects of the ligands on NEM inactivation of the kidney enzyme were more complex. Though ligands differentially affected the thermostability and NEM sensitivity of the two enzymes, the effects were not simply related to the conformational states. The sensitivity of phosphoenzyme (EP) formed in the presence of ATP, Na+, and Mg2+ to ADP or K+ and K+-p-nitrophenyl phosphatase (pNPPase) was then studied as a probe of the differences in the conformational equilibrium between the two isozymes. The EP of the brain enzyme was partially sensitive to ADP, while those of the heart and kidney enzymes were not. At physiological Na+ concentrations the percentages of E1P formed by the brain and kidney enzymes were determined to be about 40-50 and 10-20% of the total EP, respectively. The hydrolytic activity of pNPP in the presence of Li+, a selective activator at catalytic sites of the reaction, was much higher in the kidney enzyme than in the brain enzyme. The inhibition of K+-stimulated pNPPase by ATP and Na+ was greater in the latter enzyme than in the former. These results suggest that neuronal and nonneuronal (Na+ + K+)-ATPases differ in their conformational equilibrium: the E1 or E1P may be more stable in the alpha(+) than in the alpha during the turnover, and conversely the E2 or E2P may be more stable in the latter than in the former.

(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.

Phosphorylation of two isozymes of (Na+ + K+)-ATPase by inorganic phosphate

The phosphorylation of two isozymes (alpha(+) and alpha) of (Na+ + K+)-ATPase by 32Pi was studied under equilibrium conditions in various enzyme preparations from rat medulla oblongata, rat cerebral cortex, rat cerebellum, rat kidney, guinea pig kidney, and rabbit kidney. In ouabain-sensitive (Na+ + K+)-ATPases such as the brain, guinea pig kidney, and rabbit kidney enzymes, ouabain stimulated the Mg2+-dependent phosphorylation at lower concentrations, while a higher concentration was required for the stimulation of rat kidney (Na+ + K+)-ATPase, an ouabain-insensitive enzyme. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that two isozymes of the brain (Na+ + K+)-ATPase were also phosphorylated by 32Pi in the presence of ouabain. The properties of the phosphorylation were compared between the medullar oblongata (referred to as alpha(+] and the kidney (referred to as alpha) (Na+ + K+)-ATPases. The steady-state level of phosphorylation was achieved faster in the kidney enzymes than in the medulla oblongata enzyme. Phosphorylation without ouabain was greater in the kidney enzymes than in the brain enzymes. Furthermore, the former enzymes were inhibited by K+ much more than the latter. These findings suggest that the two isozymes of (Na+ + K+)-ATPase differ in their conformational changes during enzyme turnover.

Effects of ethanol on ouabain inhibition of mouse brain (Na+,K+)ATPase activity

Plots of ouabain inhibition of mouse cerebral cortical (Na+,K+)ATPase activity fitted a two-site model significantly better than a one-site model, consistent with the presence of two forms of the enzyme with different affinities for ouabain. The fraction of enzyme activity with high affinity for ouabain (HAO: Ki = 500 nM), suggested to be localized neuronally, constituted the major portion (60-70%) of activity. Ouabain inhibition of both components of enzyme activity was reduced as KCl concentrations were increased. In vitro, only high concentrations of ethanol affected (Na+,K+)ATPase activity and ouabain inhibition of activity. Ethanol (500 mM) selectively reduced the activity, and increased the sensitivity to ouabain inhibition, of the HAO component, with no significant effect on the low-affinity (LAO) component. On the other hand, following chronic treatment of mice with ethanol in vivo, in a paradigm that produced tolerance and physical dependence, the sensitivity to ouabain of the HAO form of the enzyme was selectively increased. The relative proportions, and the activities of the HAO and LAO components, were not altered. The effects of ethanol, added in vitro, on the HAO component were decreased in ethanol-tolerant animals. The selective effect of chronic ethanol ingestion on (Na+,K+)ATPase activity indicates the specificity of action of ethanol in the CNS.

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.

Difference in phospholipid dependence between two isozymes of brain (Na+ + K+)-ATPase

The effect of phospholipase C on two isozymes (alpha (+) and alpha forms) of rat brain (Na+ + K+)-ATPase and the temperature-dependence of their activities were investigated. Phospholipase C from Clostridium welchii inhibited the activities of the enzymes treated with and without pyrithiamin or N-ethylmaleimide, a preferential inhibitor of the alpha (+) form, but the extent of the inhibition was higher in the control enzyme than in the treated enzymes. The treatment of the (Na+ + K+)-ATPase with phospholipase C altered a ratio between high- and low-affinity components for ouabain inhibition. It also caused the similar change in a ratio between the alpha (+) and alpha forms of Na+-stimulated phosphorylation from [gamma-32P]ATP. These findings indicate that the alpha (+) form of rat brain (Na+ + K+)-ATPase is more sensitive to phospholipase C than the alpha form. Analysis of Arrhenius plots of the activities of the control and pyrithiamin-treated enzymes showed that there was a difference between the two enzymes in a break point. We suggest that two isozymes of rat brain (Na+ + K+)-ATPase differ in the interaction with phospholipids or in the lipid-environment.

Involvement of sulfhydryl groups in the inhibition of brain (Na+ + K+)-ATPase by pyrithiamin

Brain (Na+ + K+)-ATPase was protected by low concentrations of GSH from the inhibitory effect of pyrithiamin. The possible involvement of sulfhydryl groups in the inhibition was then studied by comparing the effect of pyrithiamin with that of N-ethylmaleimide on the enzyme. The treatment of rat brain (Na+ + K+)-ATPase with thesee inhibitors caused a significant decrease in reactivity of the enzyme to N-ethyl[3H]maleimide. N-Ethylmaleimide, like pyrithiamin, inhibited the partial reactions of (Na+ + K+)-ATPase system in parallel with the inhibition of the overall reaction. An SDS-polyacrylamide gel electrophoresis procedure indicated that pyrithiamin and N-ethylmaleimide inhibited Na+-dependent phosphorylation of the alpha(+) form of rat brain (Na+ + K+)-ATPase more than that of alpha, though the selectivity for the alpha(+) seemed to be higher with the former inhibitor than in the latter. The treatment also decreased sensitivity of the enzyme to ouabain inhibition. However, pyrithiamin- and N-ethylmaleimide-induced inactivations of the enzyme differed in the efficacy of GSH for protection and in the effect of the kind of ligands present during the reaction. Furthermore, pyrithiamin did not appear to interact directly with sulfhydryl groups, but caused the formation of disulfide in bovine brain (Na+ + K+)-ATPase. In contrast to N-ethylmaleimide, pyrithiamin did not affect the sulfhydryl-enzymes such as alcohol dehydrogenase and L-alanine dehydrogenase. It is concluded that pyrithiamin modifies the functional sulfhydryl groups of brain (Na+ + K+)-ATPase in a way different from N-ethylmaleimide and causes a structural change and inactivation of the enzyme.

Effect of thyroid status on the development of the different molecular forms of Na+,K+-ATPase in rat brain

The effect of thyroid status on the postnatal development of the two molecular forms of Na+,K+-ATPase, distinguished kinetically on the basis of their ouabain sensitivity, was examined in rat brain. Hypothyroidism induced by PTU from day 1 postnatally significantly reduced the Na+,K+-ATPase activity in cerebellum (22-30 days) but not forebrain, whereas hyperthyroidism (T4 treatment from day 1) had no effect. The hypothyroidism-induced reduction in cerebellum was reflected by a 20-45% reduction in the activity of the alpha(+) form of Na+,K+-ATPase (high ouabain affinity) against control brains compared to a 60-70% reduction in the activity of the alpha form (low ouabain affinity). These results show that neonatally induced hypothyroidism leads to a selectively greater impairment of the ontogenesis of the activity of cerebellar alpha form of Na+,K+-ATPase. This may possibly reflect a retarded development of a selective cerebellar cell population containing predominantly the alpha form of the enzyme.