Specific inactivation of alpha (+) molecular form of (Na+ + K+)-ATPase by pyrithiamin

Regulation of cough by neuronal Na(+)-K(+) ATPases

The Na(+)-K(+) ATPases play an essential role in establishing the sodium gradients in excitable cells. Multiple isoforms of the sodium pumps have been identified, with tissue and cell specific expression patterns. Because the vagal afferent nerves regulating cough must be activated at sustained high frequencies of action potential patterning to achieve cough initiation thresholds, it is a certainty that sodium pump function is essential to maintaining cough reflex sensitivities in health and in disease. The mechanisms by which Na(+)-K(+) ATPases regulate bronchopulmonary vagal afferent nerve excitability are reviewed as are potential therapeutic strategies targeting the sodium pumps in cough.

Specific expression of an A-kinase anchoring protein subtype, AKAP-150, and specific regulatory mechanism for Na(+),K(+)-ATPase via protein kinase A in the parotid gland among the three major salivary glands of the rat

We have examined the expression of A-kinase anchoring protein (AKAP) in the three major salivary glands, i.e. the parotid gland (PG), submandibular gland (SMG), and sublingual gland (SLG), of the rat to elucidate the functional relevance between saliva secretion and Na(+),K(+)-ATPase regulation by protein kinase A (PKA)-dependent phosphorylation, since an AKAP subtype, AKAP-150, is known to be involved in the regulation of the ATPase in PG. Although AKAP-150 and its mRNA were clearly detected in the PG, they were hardly detectable in either the SMG or SLG. The membrane-bound form of the RII regulatory subunit of PKA, an index for the total amount of AKAP subtypes and therefore of the anchored PKA holoenzyme, was also undetectable in membranes from the SMG and SLG but was found in the PG; though a substantial and comparable amount of Na(+),K(+)-ATPase was present in all of these membrane preparations. Incubation with [gamma-32P]ATP revealed that Na(+),K(+)-ATPase in the PG membranes was quickly phosphorylated upon the addition of cAMP, whereas the ATPases in the membranes from SMG and SLG were not; though they were readily and equally phosphorylated by the exogenously added PKA catalytic subunit. AKAP-150 in the basolateral membranes of PG acinar cells was co-immunoprecipitated with RII by an anti-RII antiserum; and AKAP-150 and Na(+),K(+)-ATPase were immunohistochemically co-localized predominantly on the basolateral membranes, suggesting a possibility that the ATPase might directly interact with the AKAP to form an ATPase/AKAP/PKA complex or associate with the AKAP, such association being mediated via some scaffolding molecule. Expression of AKAP-150 and quick down-regulation of Na(+),K(+)-ATPase by AKAP-anchored PKA in response to cAMP elevation are characteristics specific to PG among the three major salivary glands, suggesting the presence of PG-specific regulatory mechanisms for saliva production/secretion.

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.

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.

Changes in ouabain affinity of Na+, K+-ATPase during focal cerebral ischaemia in the mouse

We investigated the effect of focal cerebral ischaemia on the activity and the affinity of the ouabain sites of Na+,K+-ATPase in the mouse. The Na+,K+-ATPase activity was decreased by 38% as early as 30 min following ischaemia. In the sham group, the dose-response curves for ouabain disclosed three inhibitory states which contribute, respectively, 24.9 +/- 6.7%, 39.1 +/- 7.5% and 36.0% of the total activity (low affinity, LA; high affinity, HA and very high affinity, VHA, respectively). Their computed IC50 values are, respectively: 1.3 X 10(-3) M, 4.5 X 10(-6) M and 2.9 X 10(-9) M. Surprisingly, in ischaemic cortices, only two sites for ouabain were detected. The first site exhibits a LA (IC50 = 2.0 X 10[-4] M) but its relative contribution to the total activity (46.1 +/- 5.2%) is twice that noted for the LA site in non-ischaemic tissues. The second site presents an affinity intermediate between those of HA and VHA sites of the sham group (IC50 = 1.7 X 10[-7] M) and contributes 53.9% to the total activity. Loss in the specific activity of the second site explains that of the total activity. The most likely explanation in the presence of only two ouabain sites of Na+,K+-ATPase following ischaemia may be a change in ouabain affinity of alpha2 and/or alpha3 isoforms, as the presence of all three alpha isoforms has been observed by Western blotting. These results suggest that ischaemia induces intrinsic modifications in Na+,K+-ATPase which result from perturbations in membrane integrity and/or association of the alpha isoforms of this enzyme.

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

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

Immunologic identification of Na+,K(+)-ATPase isoforms in myocardium. Isoform change in deoxycorticosterone acetate-salt hypertension

There are three isoforms of the catalytic (alpha) subunit of the Na+,K(+)-ATPase, each derived from a different gene, that differ in their sensitivity to inhibition by cardiac glycosides. Antibodies specific for the three isoforms were used to study Na+,K(+)-ATPase isoform expression in ventricular myocardium, where an understanding of digitalis receptor diversity is most important. In the rat heart, there is simultaneous expression of two isoforms in adult ventricle, and immunofluorescence studies demonstrated that both isoforms are expressed uniformly in cardiomyocytes. Hypertension and hypertrophy have been reported to selectively depress alpha 2 isoform mRNA levels, and we show in the present study that alpha 2 protein levels were correspondingly depressed in rats made hypertensive by uninephrectomy and treatment with deoxycorticosterone acetate and a high-salt diet. In the human heart, where mRNA for all three alpha isoforms has been reported, we detected all three isoform proteins (alpha 1, alpha 2, and alpha 3). Two isoforms (alpha 1 and alpha 3) predominated in the macaque heart; dissection of the heart showed uniformity of isoform expression in different ventricular regions but markedly less alpha 3 in the atrium. Finally, isoform-specific antibodies were used to detect which alpha isoforms were expressed in the ventricles of several commonly used experimental animals to test the correlation of isoform expression with cardiac glycoside-response heterogeneity. Two isoforms (alpha 1 and alpha 3) were found in canine myocardium, whereas only one (alpha 1) was found in sheep and guinea pig. Expression of Na+,K(+)-ATPase isoforms can thus be readily followed and related to the physiology of the digitalis receptor.

Turnover rates of the canine cardiac Na,K-ATPases

Two functional isoforms alpha (alpha 1) and alpha+ (alpha 3) of the Na,K-ATPase catalytic subunit coexist in canine cardiac myocytes [J. Biol. Chem. (1987) 262, 8941-8943]. The in vitro turnover rates of ATP hydrolysis have been determined in sarcolemma preparations by comparing [3H]ouabain-binding and Na,K-ATPase activity at various doses of ouabain (0.3-300 nM). The correlation between the occupancy of the ouabain-binding sites and the degree of Na,K-ATPase inhibition was not linear. The results showed that the form of low-affinity for ouabain (Kd = 300-700 nM) exhibited a lower turnover rate (88 +/- 10 vs. 147 +/- 15 molecules of ATP hydrolyzed per second per ouabain-binding site) than the high affinity form (Kd = 1-8 nM). Thus our results indicate this specific isoform kinetic difference could contribute to differences in the cardiac cellular function.

Pretreatment of rat brain slices with ouabain decreases chloride-dependent L-glutamate transport in synaptic membrane

Possible roles of (Na(+)-K+)-ATPase on regulation of Cl(-)-dependent L-glutamate (L-Glu) transport in rat synaptic membrane were examined. Pretreatment of rat cerebral slices with 20 micrograms/ml veratrine, 56 mM K+ and 20 microM monensin increased Cl(-)-dependent L-[3H]-Glu uptake into membrane vesicles prepared from the slices. Pretreatment with (Na(+)-K+)-ATPase inhibitors, 100 microM ouabain, 100 microM strophanthidin, 100 microM vanadate and K(+)-free medium, decreased the uptake by 25-30%, while 5 microM A23187 and 10 microM ruthenium red had no effect. Ouabain (100 microM) caused the maximal effect in 10-20 min of incubation. The effect of (Na(+)-K+)-ATPase inhibitors was reversible and characterized by a decrease in Vmax of the uptake. Addition of 5 microM ouabain abolished the increases in the uptake induced by either veratrine, 56 mM K+ or monensin. Dose-inhibition curve of ouabain on the increased Cl(-)-dependent L-[3H]-Glu uptake was bi-phasic; ouabain at 0.5-5 microM selectively diminished the stimulatory effect of veratrine and inhibited the basal uptake at higher concentrations. Veratrine had no effect on L-[3H]-Glu uptake in 5 microM strophanthidin containing or K(+)-free medium. These results suggest the involvement of (Na(+)-K+)-ATPase in the veratrine-induced increase in Cl(-)-dependent L-Glu transport in rat brain synaptic membranes.

Expression of alpha isoforms of the Na,K-ATPase in human heart

We studied expression of isoforms of Na,K-ATPase in normal and diseased human hearts. Na,K-ATPase alpha-isoform mRNA in samples from normal human left ventricle (LV) was composed of 62.5%, alpha 1, 15% alpha 2 and 22.5% alpha 3 on average. There was an increase in expression of the alpha 3 isoform in samples from failing hearts, but expression of all three isoforms decreased in pressure-overloaded right ventricle (RV).

Glucose and oxygen deprivation induces a Ca(2+)-mediated decrease in (Na(+)+K+)-ATPase activity in rat brain slices

Exposure of rat brain cortical slices to a medium lacking in glucose, oxygen or both glucose and oxygen, resulted in a decrease of the tissue ATP content and a reduction of (Na(+)+K+)-ATPase activity in membranes prepared from the slices. These treatments also inhibited partial reactions of (Na(+)+K+)-ATPase such as Na(+)-dependent phosphorylation and K(+)-stimulated phosphatase, as well as specific binding of [3H]ouabain in membranes prepared from the slices. Glucose deprivation and hypoxia decreased (Na(+)+K+)-ATPase activity in the absence of extracellular Ca2+, but the effects were blocked by 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-acetomethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. Metabolic inhibitors mimicked the effects of glucose deprivation and hypoxia. The effect of glucose-free hypoxia was dependent on extracellular Ca2+. It was blocked by Mg2+ at high concentration, bepridil or amiloride, but not by voltage-sensitive Ca2+ channel antagonists and glutamate receptor antagonists. None of the drugs tested here, except for dithiothreitol, affected the inhibitory effect of glucose-free hypoxia on the enzyme activity. In contrast to brain (Na(+)+K+)-ATPase, the kidney enzyme was insensitive to glucose and oxygen deprivation and metabolic inhibitors which depleted the tissue ATP.

Postnatal change in a Ca(2+)-mediated decrease in (Na+ + K+)-ATPase activity in rat brain slices

The treatment of brain slices from immature rats with veratrine and monensin did not cause any change in (Na+ + K+)-adenosine triphosphatase (ATPase) activity or [3H]ouabain binding in membranes prepared from the slices, though these reagents remarkably stimulated Ca2+ uptake in the slices. Exposure of the slices from adult rats to a glucose-free, hypoxic or both glucose-free and hypoxic medium resulted in a decrease in the enzyme activity, but the enzyme from immature rats was resistant to the conditions.

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)

Expression of Na,K-ATPase isoforms in human heart

The expression pattern of the multiple isoforms of Na,K-ATPase was examined in the human heart. Isoform specific oligonucleotide probes for the alpha 1, alpha 2, alpha 3 and beta 1 subunits were used to probe Northern blots. The adult human ventricle expresses mRNAs for all three alpha subunit isoforms in addition to beta 1 subunit mRNA.

The effect of micromolar Ca2+ on the activities of the different Na+/K(+)-ATPase isozymes in the rat myometrium

In the present work we show the existence of two Na+/K(+)-ATPase isozymes in rat myometrial microsomes and suggest that they have different Ca2+ sensitivities. The catalytic subunits (alpha 1, alpha 2) of Na+/K(+)-ATPase were labelled by fluorescein-isothiocyanate and separated by SDS gel electrophoresis. The two isozyme Ca2(+)-sensitivities were studied by comparing the kinetics of Ca2+, strophantidin, ouabain and N-ethylmaleimide inhibitions. Our results indicate that the activity of the high ouabain-sensitive part (alpha 2 type) of Na+/K(+)-ATPase enzyme could only be inhibited by micromolar Ca2+. Furthermore, treatment of the microsomal preparation with 1mM N-ethylmaleimide selectively inactivated the high Ca2+ sensitive isoform of myometrial Na+/K(+)-ATPase.

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.

A new electrophoretic variant of alpha subunit of Na+/K(+)-ATPase from the submandibular gland of rats

The alpha catalytic subunits of Na+/K(+)-ATPase were isolated from the kidney and brain of rats (alpha 1 and alpha 2, respectively). The antisera raised against these subunits were used as probes to analyze the isoform of catalytic subunits of Na+/K(+)-ATPase in various tissues of rats. Of 27 rat tissues examined, most had a catalytic subunit identical to alpha 1 but some, such as the nervous and muscle tissues, had both alpha 1 and alpha 2 isoforms as judged by their reactivities to antisera and their electrophoretic mobility. We found that the submandibular gland contained a new electrophoretic variant of immunoreactive alpha subunit (designated alpha(S) in this report) in addition to alpha 1 identical to those found in kidney and brain. The new variant, alpha(S), strongly cross-reacted with anti-alpha 1 antiserum, but to a lesser extent with anti-alpha 2 antiserum. The alpha(S) had a molecular mass which was found to be slightly less (approx. 90 kDa) than brain and kidney alpha 1. We examined whether or not the alpha(S) is formed by proteolytic cleavage of alpha subunits during preparation and concluded that this is not the case. The alpha(S) reacted with [gamma-32P]ATP, resulting in the formation of radioactive alpha subunit which was stabilized by 2 mM ouabain but which was labile in the presence of 70 mM potassium chloride. Since N-terminal amino acid sequence of alpha(S) protein [G()DKY()PAAVS] corresponds exactly and uniquely with the sequence of the alpha 1 chain between residues 1 and 11, it is very probable that alpha(S) protein originated from alpha 1 protein following the post-translational processing.

Sodium affinity of brain Na(+)-K(+)-ATPase is dependent on isozyme and environment of the pump

The sodium affinities for the two forms of the Na(+)-K(+)-ATPase in brain were characterized. To mimic physiological conditions, synaptosomes, which are pinched off presynaptic nerve termini, were used. Examination of the pump in vitro was performed by preparing synaptic plasma membranes (SPMs). It was first shown that synaptosomes contain the two forms of the Na(+)-K(+)-ATPase, alpha 1 and alpha 2, and that these forms have markedly different affinities for the inhibitory cardiac glycoside ouabain. The apparent dissociation constant (K0.5) of alpha 1 for sodium changed from 12 to 9 mM when going from synaptosomes to membranes. For alpha 2, however, a shift from 36 to 12.5 mM was evident. The conclusion is that in vivo alpha 2 exists as a low sodium affinity species but can be altered to a high-affinity form simply by vesicle disruption. By comparison, the Na(+)-K(+)-ATPase from the mouse fibroblast cell line, 3T3-F442A cells, expressed only the alpha 1-isozyme, as shown by immunoblotting and by measurement of its ouabain and sodium affinities. The physiological relevance of these observations is also presented.

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.

Two isoenzymes of Na+,K+-ATPase have different kinetics of K+ dephosphorylation in normal cat and human brain cortex

Analysis of purified Na+,K+-ATPase from cat and human cortex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals two large catalytic subunits called alpha (-) (lower molecular weight) and alpha (+) (higher molecular weight). Differences in K+ dephosphorylation of these two molecular forms have been investigated by measuring the phosphorylation level of each protein after their separation on sodium dodecyl sulfate gels. In the presence of Na+, Mg2+, and ATP, both subunits are phosphorylated. Increasing concentrations (from 0 to 3 mM) of K+ induce progressive dephosphorylation of both alpha-subunits, although the phosphoprotein content of alpha (-) is decreased significantly less than that of alpha (+). Ka values of alpha (-) for K+ are 40% and 50% greater in cat and human cortex, respectively, than values of alpha (+). alpha (-) and alpha (+) are thought to be localized in specific cell types of the brain: alpha (-) is the exclusive form of nonneuronal cells (astrocytes), whereas alpha (+) is the only form of axolemma. Our results support the hypothesis that glial and neuronal Na+,K+-ATPases are different molecular entities differing at least by their K+ sensitivity. Results are discussed in relation to the role of glial cells in the regulation of extracellular K+ in brain.

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.

Phenytoin dephosphorylates the alpha(-) catalytic subunit of (Na+, K+)-ATPase. A study in mouse, cat and human brain

Phenytoin, a potent antiepileptic drug, has been thought to stimulate Na+, K+ transport across cell membranes, but its influence on (Na+, K+)-ATPase activity remains highly controversial. We have investigated the effects of the drug on the phosphorylation level of (Na+, K+)-ATPase partially purified from mouse, cat and human brain. (Na+, K+)-ATPase catalytic subunits [alpha(+) and alpha(-)] were resolved by sodium dodecylsulfate polyacrylamide gel electrophoresis. Previous experiments had shown that phenytoin dephosphorylates the (Na+, K+)-ATPase catalytic subunit by +/- 50% in C57/BL mice. In the present study, we showed that phenytoin (10(-4) M) decreases the phosphorylation level of (Na+, K+)-ATPase catalytic subunit by the same value in cat and human cortex. Moreover, that effect is predominant on the alpha(-) subunit, thought to be the predominant enzymatic form in non-neuronal or glial cells. The results are thus favoring the hypothesis that phenytoin stimulates the brain (Na+, K+)-ATPase. They further suggest that phenytoin mainly activates the glial enzymatic form, providing central nervous system with an enhanced ability to regulate extracellular K+.

Isoforms of Na,K-ATPase in Artemia salina: II. Tissue distribution and kinetic characterization

To characterize the molecular properties conveyed by the isoforms of the alpha subunit of Na,K-ATPase, the two major transepithelial transporting organs in the brine shrimp (Artemia salina), the salt glands and intestines, were isolated in pure form. The alpha isoforms were quantified by ATP-sensitive fluorescein isothiocyanate (FITC) labeling. The salt gland enzyme exhibits only the alpha 1 isoform, whereas the intestinal enzyme exhibits both the alpha 1 and the alpha 2 isoforms. After 32 hours of development, Na,K-ATPase activity [in mumol Pi/mg protein/hr (1 mu)] in whole homogenates was 32 +/- 6 in the salt glands and 12 +/- 3 in the intestinal preparations (mean +/- SEM). The apparent half-maximal activation constants (K1/2) of the salt gland enzyme as compared to the intestinal enzyme were 3.7 +/- 0.6 mM vs. 23.5 +/- 4 mM (P less than 0.01) for Na+, 16.6 +/- 2.2 mM vs. 8.29 +/- 1.5 mM for K+ (P less than 0.01), and 0.87 +/- 0.8 mM vs. 0.79 +/- 1.1 mM for ATP (NS). The apparent Ki's for ouabain inhibition were 1.1 x 10(-4) M vs. 2 x 10(-5) M, respectively. Treatment of whole homogenates with deoxycholic acid (DOC) produced a maximal Na,K-ATPase activation of 46% in the salt gland as compared to 23% in the intestinal enzyme. Similar differences were found with sodium dodecyl sulfate (SDS). The two distinct forms of Na,K-ATPase isolated from the brine shrimp differed markedly in three kinetic parameters as well as in detergent sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Na,K-ATPase: comparison of the cellular localization of alpha-subunit mRNA and polypeptide in mouse cerebellum, retina, and kidney

A clone encoding mouse brain Na,K-ATPase alpha-subunit was isolated from a mouse brain lambda gt11 cDNA library by using antisera to mouse and bovine brain alpha-subunit. A comparison of the nucleotide sequence of this clone with published sequences of rat brain alpha-subunit isoform clones showed it to be most similar to rat brain alpha 1. An RNA antisense probe prepared from the cDNA insert of the mouse clone detected a single mRNA of approximately 4.5 kb in Northern blots of mouse brain and kidney RNAs. This probe hybridized only to an alpha 1-cDNA insert from rat brain under high stringency conditions on Northern blots. The RNA antisense probe was used for in situ hybridization to sections of mouse kidney, cerebellum, and retina, and the cellular distribution of alpha-subunit mRNA (alpha-mRNA) was compared with that of alpha-subunit polypeptide (alpha-subunit) detected by immunofluorescence in similar sections. In kidney, alpha-mRNA distribution closely paralleled that of the polypeptide with abundant expression in ascending thick limbs and cortical distal tubules and weaker labeling in cortical proximal tubules. The co-distribution of alpha-mRNA and polypeptide in kidney where Na,K-ATPase localization is well established is consistent with the specificity of these probes. In the retina, prominent labeling with both probes was seen in photoreceptor inner segments, inner nuclear layer, and ganglion cell bodies. Plexiform layers and optic fibers expressed abundant alpha-subunit but little mRNA. Light labeling for both was seen in the outer nuclear layer. In cerebellum, alpha-mRNA and alpha-subunit were associated with soma of granule cells, basket cells, and stellate cells. Glomeruli and basket terminals contained abundant alpha-subunit but exhibited little reactivity with the riboprobe. In Purkinje cell bodies, in contrast, the antibody used to identify the cDNA clone did not resolve significant polypeptide in the somal plasmalemma despite abundant somal mRNA expression. Comparison of distribution of the two probes in cerebellum and retina indicates that message accumulation is primarily in cell bodies, while alpha-subunit epitopes may be co-expressed in cell bodies and/or transported to distant sites in cell-specific patterns.

Inhibition by taurine of Na(+)-Ca2+ exchange in sarcolemmal membrane vesicles from bovine and guinea pig hearts

1. Taurine, but not GABA, beta-alanine and glycine, inhibited Na(+)-dependent Ca2+ uptake in bovine cardiac sarcolemmal membrane vesicles in a dose-dependent manner. 2. The inhibition of Na(+)-dependent Ca2+ uptake was noncompetitive with respect to Ca2+ concentration. 3. The inhibitory effect of taurine on the exchange was also observed in cardiac sarcolemmal vesicles prepared from guinea pig, but not from rat. 4. Taurine did not affect Na(+)-dependent Ca2+ efflux nor ATP-dependent Ca2+ uptake in the bovine cardiac membranes.

Purification of cardiac (Na+,K+)-activated adenosine triphosphatase from rat

A procedure is described for preparation of highly active (Na+,K+)-ATPase from rat heart which has a specific activity of 200-600 mumol Pi/mg/h. The procedure is simple and can be applied to small amounts of heart muscle (approximately 1 g). The ATPase activity was more than 90% sensitive to ouabain (at concentrations up to 1 mM). The ouabain sensitivity is biphasic with about 20% of the ATPase activity being inhibited at approximately 3 X 10(-7) M ouabain.

Cardiac Na+,K+-ATPase isoenzymes: sensitivity to prednisolone bisguanylhydrazone

Prednisolone-3,20-bisguanylhydrazone (PBGH), a steroid derivative, has been shown to inhibit Na+,K+-ATPase isolated from guinea-pig heart or kidney in concentrations significantly lower than those required to inhibit the enzyme obtained from other sources. Because Na+,K+-ATPases obtained from guinea-pig heart or kidney are predominantly of the alpha isoform, the hypothesis that PBGH selectively inhibits the alpha isoform over alpha (+) isoform of the enzyme was tested. Sodium dodecylsulfate polyacrylamide gel electrophoresis of the enzyme preparations revealed the presence of only the higher mobility, alpha isoform in guinea-pig heart and ferret kidney, whereas those from guinea-pig brain, dog brain and ferret heart showed both high and low mobility isoforms corresponding to alpha and alpha (+) isoforms. Na+,K+-ATPase obtained from the guinea-pig heart was most sensitive to PBGH and those isolated from ferret heart or ferret kidney had the lowest sensitivity. Enzyme preparations obtained from dog brain, dog heart or guinea-pig brain had intermediate sensitivity. This spectrum of enzyme sensitivity to PBGH was markedly different from that to ouabain. In ferret heart Na+,K+-ATPase, a low concentration of PBGH preferentially inhibited [3H]ouabain binding to the high affinity ouabain binding sites (alpha(+) isoform). These results indicate that PBGH is not a specific inhibitor of the alpha isoforms of Na+,K+-ATPase. Affinity of the enzyme for PBGH is determined by the species and tissue rather than isoforms of Na+,K+-ATPase.

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.

Nerve growth factor induces Na+,K+-ATPase in a nerve cell line

The effects of nerve growth factor (NGF) on induction of Na+,K+-ATPase were examined in a rat pheochromocytoma cell line, PC12h. Na+,K+-ATPase activity in a crude particulate fraction from the cells increased from 0.37 +/- 0.02 (n = 19) to 0.55 +/- 0.02 (n = 20) (means +/- SEM, mumol Pi/min/mg of protein) when cultured with NGF for 5-11 days. The increase caused by NGF was prevented by addition of specific anti-NGF antibodies. Epidermal growth factor and insulin had only a small effect on induction of Na+,K+-ATPase. A concentration of basic fibroblast growth factor three times higher than that of NGF showed a similar potency to NGF. The molecular form of the enzyme was judged as only the alpha form in both the untreated and the NGF-treated cells by a simple pattern of low-affinity interaction with cardiotonic steroids: inhibition of enzyme activity by strophanthidin (Ki approximately 1 mM) and inhibition of Rb+ uptake by ouabain (Ki approximately 100 microM). As a consequence, during differentiation of PC12h cells to neuron-like cells, NGF increases the alpha form of Na+,K+-ATPase, but does not induce the alpha(+) form of the enzyme, which has a high sensitivity for cardiotonic steroid and is a characteristic form in neurons.

Decrease of high affinity ouabain binding in rat cerebellum and hypothalamus by thiamin deficiency

The effect of dietary thiamin deficiency on high affinity [3H]ouabain binding was examined in different regions of rat brain. The binding activity in the cerebellum and hypothalamus was significantly lower in the thiamin-deficient group than in the pair-fed control and freely fed control groups. The decrease was due to a change in Bmax but not in Kd. The results suggest a possible involvement of (Na+,K+)-ATPase in neurological manifestations of thiamin deficiency.

(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.

Mode of inhibition of brain Na+,K+-ATPase by agelasidines and agelasines from a sea sponge

The Na+,K+-ATPase from brain or kidney and sarcoplasmic reticulum (SR) Ca2+-ATPase were inhibited potently by agelasidine C (Agd-C) and agelasine B (Ags-B), the bioactive principles of sea sponge, while Agd-C and Ags-B exerted less potent inhibition of heart Na+,K+-ATPase. The ionized moiety in Agd-C and the long nonpolar side chains in Ags-B play important roles in their inhibitory action. The inhibition of Na+,K+-ATPase by Agd-C or Ags-B was virtually reversed by diluting with the inhibitor-free solution. Kinetic analysis of the inhibitory effects of Agd-C and Ags-B indicates that the inhibition of pig brain Na+,K+-ATPase is parabolic and noncompetitive with respect to ATP. This may indicate that the inhibition of Na+,K+-ATPase results from the binding of two molecules of Agd-C or Ags-B to one enzyme. The sigmoidal behavior (n = 1.3-1.4) in the K+ activation curve for Na+,K+-ATPase was strikingly intensified (n = 2.1) by Agd-C, whereas it was almost eliminated (n = 1.1) by Ags-B. These results suggest that the cooperative interaction between K+-binding sites on the enzyme was dramatically altered.

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.

Studies on the ontogenesis of the different isoenzymes of Na+, K+-ATPase in rat brain in vivo and in vitro in relation to their regulation and cellular localisation

Na+,K+ ATPase isoenzyme activities (alpha(+)-high ouabain affinity; alpha low ouabain affinity) were investigated in developing rat brain in vivo and in whole rat brain reaggregating cultures in vitro. The perinatal profile of the two isoenzyme forms in vivo revealed that, although alpha activity predominates in immature (P14.5-P16) brain, the activity alpha(+) form increases more increases more rapidly such that it is predominant at birth in both cerebellum and forebrain. No regional variation in the proportional activities of the two isoenzyme forms was seen perinatally to explain the previously reported, differential sensitivity of the cerebellar alpha isoenzyme to neonatally induced hypothyroidism. Whole rat brain reaggregating cultures seeded at P16 show a normal development of Na+,K+ ATPase isoenzyme activity if grown for 14 days in a serum supplemented medium (S+). Cultivation of whole rat brain reaggregates in serum deprived medium (S-) leads to a retarded development of alpha isoenzyme activity possibly due to the absence of T3 from the medium. Hormonally-induced changes in the development of the brain Na+, K+-ATPase isoenzymes are discussed in relation to their possible function and cellular localization.

Acute changes in myo-inositol uptake and 22Na+ flux in murine neuroblastoma cells (N1E-115) following insulin

myo-Inositol uptake was investigated in a murine neuroblastoma clone (N1E-115) to determine the effect of altered Na+,K+-ATPase activity. The Na+ ionophore monensin, and veratridine, an alkaloid affecting voltage-dependent Na+ entry, increased acute 22Na+ uptake and 22Na+ efflux from pre-loaded cells, concomitant with enhanced myo-inositol uptake. This effect was also seen following insulin. Insulin-stimulated myo-inositol uptake was inhibited by amiloride, ouabain and pyrithiamine. Amiloride inhibition suggests that activation of Na+/H+ exchange preceding Na+,K+-ATPase activation is involved in insulin stimulation of myo-inositol uptake. Pyrithiamine inhibition is an indication of prior activation of the Na+,K+-ATPase alpha + catalytic subunit by insulin. The results provide evidence that insulin contributes to the maintenance of Na+,K+-ATPase in neuronal tissue.