Quantitation of Na+/K+-ATpase and glucose transporter isoforms in rat adipocyte plasma membrane by immunogold labeling

We have quantitated and studied the topology of isoforms of the Na⁺/K⁺-ATPase and of the glucose transporter in rat adipocyte plasma membranes.Adipocytes were incubated with or without insulin for 15 min. Sheets of native plasma membrane, with the cytoplasmic face exposed, were prepared by adsorption to EM grids. Grids were incubated in parallel with monoclonal antibodies against the Na⁺/ K⁺-ATPase isoforms 1 and 2, and the glucose transporter isoforms GLUT1 and GLUT4, followed by immunogold labeling, negative staining and quantitation by counting of the gold particles in electron micrographs. In addition, the distribution of glucose transporters and Na⁺/K⁺-ATPase isoforms in subcellular membrane fractions prepared by an established fractionation procedure was monitored by Western blotting.We found that the Na⁺/K⁺-ATPases and the glucose transporters were confined to the planar part of the plasma membrane, without association to caveolar invaginations.The vast majority of the Na⁺/K⁺-ATPase molecules in the adipocyte plasma membrane were of the 2 isoform; GLUT4 was the dominating glucose transporter isoform.The total number of Na⁺/K⁺-ATPase molecules labeled in the plasma membrane was 3.5×10⁵ per cell, independent of insulin stimulation. Concomitantly, insulin increased GLUT4 labeling sevenfold to a value of 3.5×10⁵ per cell.The authors wish to thank Ulla Blankensteiner, Jonna Harpøth and Lisette Hansen for their skillful technical assistance and Birgit Risto for excellent work with the photographic prints. The 1 and 2 antibodies were kindly donated by K.W. Sweadner, Boston, and the F18 and F27 antibodies were granted by Peer N. Jørgensen, Novo-Nordisk, Bagsværd, Denmark. This work was supported by The NOVO Foundation, The Nordisk Insulin Foundation, The Danish Diabetes Foundation, The P. Carl Petersen Foundation and the Foundation of 17-12-81.     

Role of the Na+,K+-ATPase α2 isoform in the positive inotropic effect of ouabain and marinobufagenin in the rat diaphragm

It was found that ouabain and marinobufagenin, specific inhibitors of Na⁺,K⁺-ATPase, increased the contraction of the isolated rat diaphragm by ～15% (positive inotropic effect) at EC₅₀ = 1.2 ± 0.3 nM and 0.3 ± 0.1 nM, respectively, which was indicative of the participation of the ouabain-sensitive Na⁺,K⁺-ATPase α2 isoform. Analysis of the dose-response curves for the effect of ouabain on the resting membrane potential of muscle fibers in the absence and in the presence of 100 nM acetylcholine (hyperpolarizing the membrane) showed the presence of two pools of Na⁺,K⁺-ATPase α2 that differed in affinity for ouabain. Only the high-affinity pool (IC₅₀ ～ 9 nM) mediates the hyperpolarizing effect of nanomolar concentrations of acetylcholine. Most likely, it is this pool of that is involved in the positive inotropic effect of ouabain, which can be a mechanism of regulation of the muscle function by circulating endogenous inhibitors of Na⁺,K⁺-ATPase.     

Regulation of the Na+/K+-ATPase by insulin: Why and how?

The sodium-potassium ATPase (Na⁺/K⁺-ATPase or Na⁺/K⁺-pump) is an enzyme present at the surface of all eukaryotic cells, which actively extrudes Na⁺ from cells in exchange for K⁺ at a ratio of 3:2, respectively. Its activity also provides the driving force for secondary active transport of solutes such as amino acids, phosphate, vitamins and, in epithelial cells, glucose. The enzyme consists of two subunits ( and ) each expressed in several isoforms. Many hormones regulate Na⁺/K⁺ -ATPase activity and in this review we will focus on the effects of insulin. The possible mechanisms whereby insulin controls Na⁺/K⁺-ATPase activity are discussed. These are tissue- and isoform-specific, and include reversible covalent modification of catalytic subunits, activation by a rise in intracellular Na⁺ concentration, altered Na⁺ sensitivity and changes in subunit gene or protein expression. Given the recent escalation in knowledge of insulin-stimulated signal transduction systems, it is pertinent to ask which intracellular signalling pathways are utilized by insulin in controlling Na⁺/K⁺-ATPase activity. Evidence for and against a role for the phosphatidylinositol-3-kinase and mitogen activated protein kinase arms of the insulin-stimulated intracellular signalling networks is suggested. Finally, the clinical relevance of Na⁺/K⁺-ATPase control by insulin in diabetes and related disorders is addressed.     

In search of synaptosomal Na+, K+-ATPase regulators

The arrival of the nerve impulse to the nerve endings leads to a series of events involving the entry of sodium and the exit of potassium. Restoration of ionic equilibria of sodium and potassium through the membrane is carried out by the sodium/potassium pump, that is the enzyme Na⁺,K⁺-ATPase. This is a particle-bound enzyme that concentrates in the nerve ending or synaptosomal membranes. The activity of Na⁺,K⁺-ATPase is essential for the maintenance of numerous reactions, as demonstrated in the isolated synaptosomes. This lends interest to the knowledge of the possible regulatory mechanisms of Na⁺,K⁺-ATPase activity in the synaptic region. The aim of this review is to summarize the results obtained in the author's laboratory, that refer to the effect of neurotransmitters and endogenous substances on Na⁺,K⁺-ATPase activity. Mention is also made of results in the field obtained in other laboratories. Evidence showing that brain Na⁺,K⁺-ATPase activity may be modified by certain neurotransmitters and insulin have been presented. The type of change produced by noradrenaline, dopamine, and serotonin on synaptosomal membrane Na⁺,K⁺-ATPase was found to depend on the presence or absence of a soluble brain fraction. The soluble brain fraction itself was able to stimulate or inhibit the enzyme, an effect that was dependent in turn on the time elapsed between preparation and use of the fraction. The filtration of soluble brain fraction through Sephadex G-50 allowed the separation of two active subfractions: peaks I and II. Peak I increased Na⁺,K⁺- and Mg²⁺-ATPases, and peak II inhibited Na⁺,K⁺-ATPase. Other membrane enzymes such as acetylcholinesterase and 5′-nucleotidase were unchanged by peaks I or II. In normotensive anesthetized rats, water and sodium excretion were not modified by peak I but were increased by peak II, thus resembling ouabain effects.³H-ouabain binding was unchanged by peak I but decreased by peak II in some areas of the CNS assayed by quantitative autoradiography and in synaptosomal membranes assayed by a filtration technique. The effects of peak I and II on Na⁺,K⁺-ATPase were reversed by catecholamines. The extent of Na⁺,K⁺-ATPase inhibition by peak II was dependent on K⁺ concentration, thus suggesting an interference with the K⁺ site of the enzyme. Peak II was able to induce the release of neurotransmitter stored in the synaptic vesicles in a way similar to ouabain. Taking into account that peak II inhibits only Na⁺,K⁺-ATPase, increases diuresis and natriuresis, blocks high affinity³H-ouabain binding, and induces neurotransmitter release, it is suggested that it contains an ouabain-like substance.     

Na+-transport modulation induces isoform-specific expression of Na+,K+-ATPase α-subunit isoforms in C2C12 skeletal muscle cell

Changes in demands for Na⁺ transport alter expression of the Na⁺,K⁺-ATPase subunit isoforms. In skeletal muscle, the effects of these changes on expression the 2 isoform, the major isoform expressed in differentiated muscle cell, is not known. Therefore, this study examines regulation of the -subunit isoforms by Na⁺ in the C₂C₁₂ skeletal muscle cell that expresses the 1 and 2 isoforms. Western blot analysis showed that in differentiating C₂C₁₂ muscle cell, but not in undifferentiated myoblast, veratridine, a Na⁺ channel activator, greatly increased expression of the 2 isoform; expression of 1 was unaltered. Because the level of -actinin was unaltered, the data suggest that veratridine treatment did not significantly alter the progression of cell differentiation. Furthermore, a reduction in Na⁺ transport by tetrodotoxin again failed to alter expression of a1. Thus, in C₂C₁₂ skeletal muscle cell, changes in Na⁺ transport alters expression of the 2, but not the 1 isoform. These results differ from those observed previously in muscle cells that express only the 1 isoform. Because mammalian skeletal muscle expresses both the 1- and 2-subunit isoforms, the differential regulation that was observed may be physiologically relevant in these muscle cells in vivo.     

Polarized distribution of Na, K-ATPase α-subunit isoforms in electrocyte membranes

We have previously demonstrated that Na⁺, K⁺-ATPase activity is present in both differentiated plasma membranes from Electrophorus electricus (L.) electrocyte. Considering that the α subunit is responsible for the catalytic properties of the enzyme, the aim of this work was to study the presence and localization of α isoforms (α1 and α2) in the electrocyte. Dose-response curves showed that non-innervated membranes present a Na⁺, K⁺-ATPase activity 2.6-fold more sensitive to ouabain (I₅₀=1.0±0.1 μM) than the activity of innervated membranes (I₅₀=2.6±0.2 μM). As depicted in [³H]ouabain binding experiments, when the [³H]ouabain-enzyme complex was incubated in a medium containing unlabeled ouabain, reversal of binding occurred differently: the bound inhibitor dissociated 32% from Na⁺, K⁺-ATPase in non-innervated membrane fractions within 1 h, while about 50% of the ouabain bound to the enzyme in innervated membrane fractions was released in the same time. These data are consistent with the distribution of α1 and α2 isoforms, restricted to the innervated and non-innervated membrane faces, respectively, as demonstrated by Western blotting from membrane fractions and immunohistochemical analysis of the main electric organ. The results provide direct evidence for a distinct distribution of Na⁺, K⁺-ATPase α-subunit isoforms in the differentiated membrane faces of the electrocyte, a characteristic not yet described for any polarized cell.     

Na+,K+-ATPase activity in cultured C6 glioma cells

Rat C₆ glioma cells were cultured for 4 days in MEM medium supplemented with 10% bovine serum and Na⁺,K⁺-ATPase activity was determined in homogenates of harvested cells. Approximately 50% of enzyme activity was attained at 1.5 mM K⁺ and the maximum (2.76±0.13 mol Pi/h/mg protein) at 5 mM K⁺. The specific activity of Na⁺,K⁺-ATPase was not influenced by freezing the homogenates or cell suspensions before the enzyme assay. Ten minutes' exposure of glioma cells to 10^–4 or 10^–5 M noradrenaline (NA) remained without any effect on NA⁺,K⁺-ATPase activity. Neither did the presence of NA in the incubation medium, during the enzyme assay, influence the enzyme activity. The nonresponsiveness of Na⁺,K⁺-ATPase of C₆ glioma cells to NA is consistent with the assumption that (+) form of the enzyme may be preferentially sensitive to noradrenaline. Na⁺,K⁺-ATPase was inhibited in a dose-dependent manner by vanadate and 50% inhibition was achieved at 2×10^–7 M concentration. In spite of the fact that Na⁺,K⁺-ATPase of glioma cells was not responsive to NA, the latter could at least partially reverse vanadate-induced inhibition of the enzyme. Although the present results concern transformed glial cells, they suggest the possibility that inhibition of glial Na⁺,K⁺-ATPase may contribute to the previously reported inhibition by vanadate of Na⁺,K⁺-ATPase of the whole brain tissue.     

Structural Peculiarities of Na+,K+-ATPase Isozymes from the Calf Brain

Functionally active preparations of Na⁺,K⁺-ATPase isozymes from calf brain that contain catalytic subunits of three types (1, 2, and 3) were obtained using two approaches: a selective removal of contaminating proteins by the Jorgensen method and a selective solubilization of the enzyme with subsequent reconstitution of their membrane structure by the Esmann method. The ouabain inhibition constants were determined for the isozymes. The real isozyme composition of the Na⁺ pump from the grey matter containing glial cells and the brain stem containing neurons was determined. The plasma membranes of glial cells were shown to contain mainly Na⁺,K⁺-ATPase of the 11 type and minor amounts of isozymes of the 22(1) and the 31(2) type. The axolemma contains 21 and 31 isozymes. A carbohydrate analysis indicated that 11 enzyme preparations from the brain grey matter substantially differ from the renal enzymes of the same composition in the glycosylation of the 1 isoform. An enhanced sensitivity of the 3 catalytic subunit of Na⁺,K⁺-ATPase from neurons to endogenous proteolysis was found. A point of specific proteolysis in the amino acid sequence PNDNR⁴⁹² Y⁴⁹³ was localized (residue numbering is that of the human 3 subunit). This sequence corresponds to one of the regions of the greatest variability in 1-, 2-, 3-, and 4-subunits, but at the same time, it is characteristic of the 3 isoforms of various species. The presence of the 3 isoform of tubulin (cytoskeletal protein) was found for the first time in the high-molecular-mass Na⁺,K⁺-ATPase 31 isozyme complex isolated from the axolemma of brain stem neurons, and its binding to the 3 catalytic subunit was shown.     

Dual activity of the H1-H2 domain of the (Na(+)+K+)-ATPase

(Na⁺+K⁺)-ATPase is a target receptor of digitalis (cardiac glycoside) drugs. It has been demonstrated that the H1-H2 domain of the α-subunit of the (Na⁺+K⁺)-ATPase is one of the digitalis drug interaction sites of the enzyme. Despite the extensive studies of the inhibitory effect of digitalis on the (Na⁺+K⁺)-ATPase, the functional property of the H1-H2 domain of the enzyme and its role in regulating enzyme activity is not completely understood. Here we report a surprise finding: instead of inhibiting the enzyme, binding of a specific monoclonal antibody SSA78 to the H1-H2 domain of the (Na⁺+K⁺)-ATPase elevates the catalytic activity of the enzyme. In the presence of low concentration of ouabain, monoclonal antibody SSA78 significantly protects enzyme function against ouabain-induced inhibition. However, higher concentration of ouabain completely inactivates the (Na⁺+K⁺)-ATPase even in the presence of SSA78. These results suggest that the H1-H2 domain of the (Na⁺+K⁺)-ATPase is capable of regulating enzyme function in two distinct ways for both ouabain-sensitive and -resistant forms of the enzyme: it increases the activity of the (Na⁺+K⁺)-ATPase during its interaction with an activator; it also participates in the mechanism of digitalis or ouabain-induced inhibition of the enzyme. Understanding the dual activity of the H1-H2 domain will help better understand the structure-function relationships of the (Na⁺+K⁺)-ATPase and the biological processes mediated by the enzyme.     

Digitalis receptor sugar binding site characteristics: A model based upon studies of Na+, K+-ATPase preparations with differing digitalis sensitivities

The structure-activity relationships of the genin moieties of digitalis glycosides are commonly elucidated by determining the inhibitory potency of a variety of genins toward the plasma membrane Na⁺, K⁺-ATPase; qualitatively these relationships appear to be fairly independent of the specific Na⁺, K⁺-ATPase preparation utilized for the analysis. To determine whether this is the case with regard to the sugar moieties of glycosides, the inhibitory effects of 12 monoglycosides of digitoxigenin toward four Na⁺, K⁺-ATPase preparations of different origin were measured. It was found that while recognition of the major structural determinants of sugar activity appeared to be independent of enzyme source, recognition of the minor structural determinants of activity showed some source dependence. It was also observed that the intrinsic sensitivity to sugar potentiation may be source dependent and unrelated to intrinsic sensitivity to inhibition by digitoxigenin. These observations are compatible with a model of the Na⁺, K⁺-ATPase sugar binding site(s) in which intrinsic sensitivity to sugar attachment as well as recognition characteristics (for sugar structural features) both determine the extent to which a sugar moiety may contribute to the activity of monoglycosides. Further, in these studies one of the Na⁺, K⁺-ATPase preparations employed was obtained from rat brain, a tissue known to contain a mixture of ouabain sensitive and insensitive isoforms. We have observed that the rigorous purification techniques employed appear to have selectively removed from or denatured the less ouabain sensitive al isoform found in this enzyme preparation.     

Significance of the β-Subunit in the Biogenesis of Na+/K+-ATPase

This review summarizes our experiments on the significance of the -subunit in the functional expression of Na⁺/K⁺-ATPase. The -subunit acts like a receptor for the -subunit in the biogenesis of Na⁺/K⁺-ATPase and facilitates the correct folding of the -subunit in the membrane. The -subunit synthesized in the absence of the -subunit is subjected to rapid degradation in the endoplasmic reticulum. Several assembly sites are assigned in the sequence of the -subunit from the cytoplasmic NH₂-terminal domain to the extracellular COOH-terminus: the NH₂-terminal region of the extracellular domain, the conservative proline in the third disulfide loop, the hydrophobic amino acid residues near the COOH-terminus and the cysteine residues forming the second and the third disulfide bridges. Upon assembly, the -subunit confers a resistance to trypsin on the -subunit. The conformations induced in the -subunit of Na⁺/K⁺-ATPase by Na⁺/K⁺- and H⁺/K⁺-ATPase -subunits are somehow different from each other and are named the NK-type and KH-type, respectively. The extracellular domain of the -subunit is involved in the folding of the -subunit leading to trypsin-resistant conformations. The sequences from Cys150 to the COOH-terminus of the Na⁺/K⁺-ATPase -subunit and from Ile89 to the COOH–terminus of the H⁺/K⁺-ATPase -subunit are necessary to form trypsin-resistant conformations of the NK- and HK-type. respectively. The first disulfide loop of the extracellular domain of the -subunits is critical in the expression of functional Na⁺/K⁺-ATPase.     

Evidence That Antioxidants Prevent the Inhibition of Na+,K+-ATPase Activity Induced by Octanoic Acid in Rat Cerebral Cortex in Vitro

The objective of the present study was to investigate the in vitro effects of octanoic acid, which accumulates in medium-chain acyl-CoA dehydrogenase (MCAD) deficiency and in Reye syndrome, on key enzyme activities of energy metabolism in the cerebral cortex of young rats. The activities of the respiratory chain complexes I–IV, creatine kinase, and Na⁺, K⁺-ATPase were evaluated. Octanoic acid did not alter the electron transport chain and creatine kinase activities, but, in contrast, significantly inhibited Na⁺, K⁺-ATPase activity both in synaptic plasma membranes and in homogenates prepared from cerebral cortex. Furthermore, decanoic acid, which is also increased in MCAD deficiency, and oleic acid strongly reduced Na⁺, K⁺-ATPase activity, whereas palmitic acid had no effect. We also examined the effects of incubating glutathione and trolox (-tocopherol) alone or with octanoic acid on Na⁺, K⁺-ATPase activity. Tested compounds did not affect Na⁺, K⁺-ATPase activity by itself, but prevented the inhibitory effect of octanoic acid. These results suggest that inhibition of Na⁺, K⁺-ATPase activity by octanoic acid is possibly mediated by oxidation of essential groups of the enzyme. Considering that Na⁺, K⁺-ATPase is critical for normal brain function, it is feasible that the significant inhibition of this enzyme activity by octanoate and also by decanoate may be related to the neurological dysfunction found in patients affected by MCAD deficiency and Reye syndrome.     

Na+, K+-ATPase Isozyme Diversity; Comparative Biochemistry and Physiological Implications of Novel Functional Interactions

Na⁺, K⁺-ATPase is ubiquitously expressed in the plasma membrane ofall animal cells where it serves as the principal regulator of intracellularion homeostasis. Na⁺, K⁺-ATPase is responsible for generating andmaintaining transmembrane ionic gradients that are of vital importance forcellular function and subservient activities such as volume regulation, pHmaintenance, and generation of action potentials and secondary activetransport. The diversity of Na⁺, K⁺-ATPase subunit isoforms andtheir complex spatial and temporal patterns of cellular expression suggestthat Na⁺, K⁺-ATPase isozymes perform specialized physiologicalfunctions. Recent studies have shown that the subunit isoformspossess considerably different kinetic properties and modes of regulationand the subunit isoforms modulate the activity, expression and plasmamembrane targeting of Na⁺, K⁺-ATPase isozymes. This review focuseson recent developments in Na⁺, K⁺-ATPase research, and in particular reportsof expression of isoforms in various tissues and experiments aimed atelucidating the intrinsic structural features of isoforms important forNa⁺, K⁺-ATPase function.     

Expression of the Catalytic Subunit of Ouabain-Sensitive H+,K+-ATPase in Rat Skin Epidermis

A comparative localization of Na⁺,K⁺-ATPase and ouabain-sensitive H⁺,K⁺-ATPase in rat skin was performed using in situ RNA hybridization and immunohistochemistry. Na⁺,K⁺-ATPase was predominantly detected in the basal layer of the epithelium, whereas the ouabain-sensitive H⁺,K⁺-ATPase, in the granular and prickle cell layers. The genes of these ATPases are thus expressed in epithelial cells at different stages of their development. The hypothesis was advanced that the ouabain-sensitive H⁺,K⁺-ATPase is involved in maintaining the skin pH value. The probes specific to the mRNAs of the full-size -subunit of the ouabain-sensitive H⁺,K⁺-ATPase and its truncated form were used to establish a similar distribution of both mRNA variants in skin.     

Brain Soluble Fractions Which Modulate Na+, K+-ATPase Activity Likewise Modify Muscarinic Receptor

Two brain soluble fractions, named peaks I and II, which respectively stimulate and inhibit neuronal Na⁺, K⁺-ATPase activity, have been isolated by gel filtration in Sephadex G-50. Since cholinergic transmission seems related to such enzyme activity, in this study we evaluated the effect of brain peak I, peak II, a more purified fraction II-E and commercial ouabain, on specific binding of the muscarinic antagonist [³H]quinuclidinyl benzilate to membranes from rat cerebellum, hippocampus and cerebral cortex. We found that binding was increased by peak I and decreased by peak II, II-E and ouabain, all effects proving concentration-dependent. Since the changes exerted on the muscarinic receptor followed a pattern similar to the one already described for synaptosomal membrane Na⁺, K⁺-ATPase activity, both systems seem to interact at a functional level.     

Reduction of Na(+),K(+)-ATPase activity in hippocampus of rats subjected to chemically induced hyperhomocysteinemia

Hyperhomocysteinemia occurs in homocystinuria, an inherited metabolic disease clinically characterized by thromboembolic episodes and a variable degree of neurological dysfunction whose pathophysiology is poorly known. In this study, we induced elevated levels of homocysteine (Hcy) in blood (500 M), comparable to those of human homocystinuria, and in brain (60 nmol/g wet tissue) of young rats by injecting subcutaneously homocysteine (0.3-0.6 mol/g of body weight) twice a day at 8-hr intervals from the 6th to the 28th postpartum day. Controls received saline in the same volumes. Na⁺,K⁺-ATPase and Mg²⁺-ATPase activities were determined in the hippocampus of treated Hcy- and saline-treated rats. Chronic administration of Hcy significantly decreased (40%) Na⁺,K⁺-ATPase activity but did not alter Mg²⁺-ATPase activity. Considering that Na⁺,K⁺-ATPase plays a crucial role in the central nervous system, our results suggest that the brain dysfunction found in homocystinuria may be related to the reduction of brain Na⁺,K⁺-ATPase activity.     

Modulation of Aspartate Release by Ascorbic Acid and Endobain E,an Endogenous Na<Superscript>+</Superscript>, K<Superscript>+</Superscript>-ATPase Inhibitor

The isolation of a soluble brain fraction which behaves as an endogenous ouabain-like substance, termed endobain E, has been described. Endobain E contains two Na⁺, K⁺-ATPase inhibitors, one of them identical to ascorbic acid. Neurotransmitter release in the presence of endobain E and ascorbic acid was studied in non-depolarizing (0 mM KCl) and depolarizing (40 mM KCl) conditions. Synaptosomes were isolated from cerebral cortex of male Wistar rats by differential centrifugation and Percoll gradient. Synaptosomes were preincubated in HEPES-saline buffer with 1 mM d-[³H]aspartate (15 min at 37°C), centrifuged, washed, incubated in the presence of additions (60 s at 37°C) and spun down; radioactivity in the supernatants was quantified. In the presence of 0.5–5.0 mM ascorbic acid, d-[³H]aspartate release was roughly 135–215% or 110–150%, with or without 40 mM KCl, respectively. The endogenous Na⁺, K⁺-ATPase inhibitor endobain E dose-dependently increased neurotransmitter release, with values even higher in the presence of KCl, reaching 11-times control values. In the absence of KCl, addition of 0.5–10.0 mM commercial ouabain enhanced roughly 100% d-[³H]aspartate release; with 40 mM KCl a trend to increase was recorded with the lowest ouabain concentrations to achieve statistically significant difference vs. KCl above 4 mM ouabain. Experiments were performed in the presence of glutamate receptor antagonists. It was observed that MPEP (selective for mGluR5 subtype), failed to decrease endobain E response but reduced 50–60% ouabain effect; LY-367385 (selective for mGluR1 subtype) and dizocilpine (for ionotropic NMDA glutamate receptor) did not reduce endobain E or ouabain effects. These findings lead to suggest that endobain E effect on release is independent of metabotropic or ionotropic glutamate receptors, whereas that of ouabain involves mGluR5 but not mGluR1 receptor subtype. Assays performed at different temperatures indicated that in endobain E effect both exocytosis and transporter reversion are involved. It is concluded that endobain E and ascorbic acid, one of its components, due to their ability to inhibit Na⁺, K⁺-ATPase, may well modulate neurotransmitter release at synapses.     

Interaction between Tubulin and Na2+,K2+-ATPase in Brain Stem Neurons

Functionally active Na²⁺,K²⁺-ATPase isozymes containing three types of the catalytic subunits (1, 2, and 3) were obtained from calf brain by two methods: selective removal of contaminating proteins according to Jorgensen (1974) and selective solubilization of the enzyme with subsequent reformation of the membrane structure according to Esmann (1988). All preparations were characterized with respect to ouabain-inhibition constants. The presence of the cytoskeleton protein tubulin (3 isoform) in the high-molecular-weight complex of Na²⁺,K²⁺-ATPase 31 isozyme from brain stem axolemma and the junction between Na²⁺,K²⁺-ATPase 3 subunit and tubulin 3 subunit are shown for the first time.     

Role of heme oxygenase in heme-mediated inhibition of rat brain Na+−K+-ATPase: Protection by tin-protoporphyrin

Hemoglobin has been shown to inhibit brain Na⁺–K⁺-ATPase through an iron-dependent mechanism. Both hemoglobin and iron cause spontaneous peroxidation of brain lipids. Release of iron from the heme molecule in animal tissues is dependent on the activity of heme oxygenase. We hypothesized that inhibition of heme catabolism by heme oxygenase prevents the iron-mediated inhibition of Na⁺–K⁺-ATPase and might subsequently reduce the tissue damage. Therefore, we studied the effect of heme and tin-protoporphyrin, an inhibitor of heme oxygenase, on the activity of partially purified Na⁺–K⁺-ATPase from rat brain in the presence and absence of purified hepatic heme oxygenase. Heme alone at a concentration of 30 M did not inhibit Na⁺–K⁺-ATPase. However, in the presence of heme oxygenase, heme inhibited Na⁺–K⁺-ATPase by 75%. Pretreatment of rats with SnCl₂, a known inducer of heme oxygenase, reduced the basal activity of the brain Na⁺–K⁺-ATPase by 50%. Inhibition of heme oxygenase by tin-protoporphyrin (30 M) prevented the inhibition of Na⁺–K⁺-ATPase which occurred in the presence of heme and heme oxygenase. It is concluded that suppression of heme oxygenase by tin-protoporphyrin might be a therapeutic approach to management of hemoglobin-associated brain injury following CNS hemorrhage.     

Purification and characterization of the ouabain-sensitive H/K-ATPase from guinea-pig distal colon

Distal colon absorbs K⁺ through a Na⁺-independent, ouabain-sensitive H⁺/K⁺-exchange, associated to an apical ouabain-sensitive H⁺/K⁺-ATPase. Expression of HKα2, gene associated with this ATPase, induces K⁺-transport mechanisms, whose ouabain susceptibility is inconsistent. Both ouabain-sensitive and ouabain-insensitive K⁺-ATPase activities have been described in colonocytes. However, native H⁺/K⁺-ATPases have not been identified as unique biochemical entities. Herein, a procedure to purify ouabain-sensitive H⁺/K⁺-ATPase from guinea-pig distal colon is described. H⁺/K⁺-ATPase is Mg²⁺-dependent and activated by K⁺, Cs⁺ and NH₄⁺ but not by Na⁺ or Li⁺, independently of K⁺-accompanying anion. H⁺/K⁺-ATPase was inhibited by ouabain and vanadate but insensitive to SCH-28080 and bafilomycin-A. Enzyme was phosphorylated from [³²P]-γ-ATP, forming an acyl-phosphate bond, in an Mg²⁺-dependent, vanadate-sensitive process. K⁺ inhibited phosphorylation, effect blocked by ouabain. H⁺/K⁺-ATPase is an α/β-heterodimer, whose subunits, identified by Tandem-mass spectrometry, seems to correspond to HKα2 and Na⁺/K⁺-ATPase β1-subunit, respectively. Thus, colonic ouabain-sensitive H⁺/K⁺-ATPase is a distinctive P-type ATPase.