C-peptide,Na+,K+-ATPase,and Diabetes

Na⁺,K⁺-ATPase is an ubiquitous membrane enzyme that allows the extrusion of three sodium ions from the cell and two potassium ions from the extracellular fluid. Its activity is decreased in many tissues of streptozotocin-induced diabetic animals. This impairment could be at least partly responsible for the development of diabetic complications. Na⁺,K⁺-ATPase activity is decreased in the red blood cell membranes of type 1 diabetic individuals, irrespective of the degree of diabetic control. It is less impaired or even normal in those of type 2 diabetic patients. The authors have shown that in the red blood cells of type 2 diabetic patients, Na⁺,K⁺-ATPase activity was strongly related to blood C-peptide levels in non–insulin-treated patients (in whom C-peptide concentration reflects that of insulin) as well as in insulin-treated patients. Furthermore, a gene-environment relationship has been observed. The alpha-1 isoform of the enzyme predominant in red blood cells and nerve tissue is encoded by the ATP1A1 gene.Apolymorphism in the intron 1 of this gene is associated with lower enzyme activity in patients with C-peptide deficiency either with type 1 or type 2 diabetes, but not in normal individuals. There are several lines of evidence for a low C-peptide level being responsible for low Na⁺,K⁺-ATPase activity in the red blood cells. Short-term C-peptide infusion to type 1 diabetic patients restores normal Na⁺,K⁺-ATPase activity. Islet transplantation, which restores endogenous C-peptide secretion, enhances Na⁺,K⁺-ATPase activity proportionally to the rise in C-peptide. This C-peptide effect is not indirect. In fact, incubation of diabetic red blood cells with C-peptide at physiological concentration leads to an increase of Na⁺,K⁺-ATPase activity. In isolated proximal tubules of rats or in the medullary thick ascending limb of the kidney, C-peptide stimulates in a dose-dependent manner Na⁺,K⁺-ATPase activity. This impairment in Na⁺,K⁺-ATPase activity, mainly secondary to the lack of C-peptide, plays probably a role in the development of diabetic complications. Arguments have been developed showing that the diabetesinduced decrease in Na⁺,K⁺-ATPase activity compromises microvascular blood flow by two mechanisms: by affecting microvascular regulation and by decreasing red blood cell deformability, which leads to an increase in blood viscosity. C-peptide infusion restores red blood cell deformability and microvascular blood flow concomitantly with Na⁺,K⁺-ATPase activity. The defect in ATPase is strongly related to diabetic neuropathy. Patients with neuropathy have lower ATPase activity than those without. The diabetes-induced impairment in Na⁺,K⁺-ATPase activity is identical in red blood cells and neural tissue. Red blood cell ATPase activity is related to nerve conduction velocity in the peroneal and the tibial nerve of diabetic patients. C-peptide infusion to diabetic rats increases endoneural ATPase activity in rat. Because the defect in Na⁺,K⁺-ATPase activity is also probably involved in the development of diabetic nephropathy and cardiomyopathy, physiological C-peptide infusion could be beneficial for the prevention of diabetic complications.     

The Polarized Distribution of Na+,K+-ATPase: Role of the Interaction between β Subunits

The very existence of higher metazoans depends on the vectorial transport of substances across epithelia. A crucial element of this transport is the membrane enzyme Na⁺,K⁺-ATPase. Not only is this enzyme distributed in a polarized manner in a restricted domain of the plasma membrane but also it creates the ionic gradients that drive the net movement of glucose, amino acids, and ions across the entire epithelium. In a previous work, we have shown that Na⁺,K⁺-ATPase polarity depends on interactions between the β subunits of Na⁺,K⁺-ATPases located on neighboring cells and that these interactions anchor the entire enzyme at the borders of the intercellular space. In the present study, we used fluorescence resonance energy transfer and coprecipitation methods to demonstrate that these β subunits have sufficient proximity and affinity to permit a direct interaction, without requiring any additional extracellular molecules to span the distance.     

Localization of recently characterized membrane transport adenosine triphosphatases

Summary Na⁺, K⁺-ATPase is the best-known member of a family of membrane transport ATPases sharing a number of structural features. Methods recently developed for Na⁺, K⁺-ATPase localization may, therefore, be modifiable for use in studies of the distributions of related enzymes. Our experiences in the use of monophosphate and triphosphate substrate methods for localization of the gastric proton pump ATPase (H⁺, K⁺-ATPase) are described and the prospects for localizing other related enzymes by similar techniques are discussed.     

Effects of vanadate on Na+,K+-ATPase and on the force of contraction in guinea-pig hearts.

Vanadate has been shown to be a potent inhibitor of isolated Na⁺,K⁺-ATPase. Since the inhibition of this enzyme system has been implicated in a mechanism for the positive inotropic action of cardiac glycosides, the cardiac actions of vanadate were examined in connection with its action on Na⁺,K⁺-ATPase. Vanadate inhibited isolated Na⁺,K⁺-ATPase obtained from various tissues. The differences in the vanadate sensitivity due to enzyme source were relatively small. K⁺-stimulated phosphatase activity was more sensitive than Na⁺,K⁺-stimulated ATP hydrolysis. The compounds was more potent than phosphate in supporting [³H] oubain binding in the presence of Mg²⁺, indicating a higher affinity of the enzyme for vanadate. It, however, failed to inhibit oubain sensitive ⁸⁶Rb uptake in electrically stimulated atrial muscle of guinea-pig hearts in concentrations which would inhibit isolated Na⁺,K⁺-ATPase. These latter concentrations of vanadate also failed to produce positive inotropic effects in electrically stimulated left atrial preparations of guinea-pig hearts. Higher concentrations produced marked negative inotropic effects associated with a shortening of the action potential duration. These results indicate that vanadate is a potent inhibitor of isolated Na⁺,K⁺-ATPase, but cannot inhibit the enzyme in intact myocardial cells or produce positive inotropic effects when applied extracellularly. Inhibitory sites on the enzyme are probably located at the internal surface of the cell membrane which are normally inaccessible to vanadate in intact tissue.     

Glucagon stimulation of hepatic Na+, K+-ATPase

Summary In the perfused rat liver administration of glucagon was shown to result in a transiently increased uptake of K⁺, indicating the possible involvement of the Na⁺, K⁺-ATPase. Direct measurement of the activity of Na⁺, K⁺-ATPase revealed a two-fold stimulation of the enzyme by glucagon. The effect of glucagon on the activity of the enzyme was immediate. Simultaneously with the increase in the activity of the Na⁺, K⁺-ATPase, the activity of Mg²⁺-ATPase decreased. In order to evaluate whether the activation of the Na⁺, K⁺-ATPase by glucagon is related to the metabolic effects of the hormone, experimental conditions known to interfere with the activity of the enzyme were employed and glucagon stimulation of Ca²⁺-efflux, mitochondrial metabolism and gluconeogenesis were measured. K⁺-free perfusate, high K⁺ perfusate or ouabain interfered to varying degrees with the glucagon stimulation of these responses. The combination of K⁺-free perfusate and ouabain almost completely abolished the glucagon stimulation of all three parameters. These results demonstrate the glucagon stimulation of Na⁺, K⁺-ATPase and raise the possibility that the activation of the enzyme by glucagon might be a necessary link for the manifestation of its metabolic effects.     

Effects of prostaglandin A2 on Na+, K+-ATPase activity in basolateral plasma membrane of rat intestine in vitro

Prostagladin A₂, which prevents intestinal ulcers produced by administration of nonsteroidal antiinflammatory compounds such as indomethacin, inhibited the Na⁺,K⁺-ATPase activity in basolateral plasma membrane of rat intestine significantly. Prostaglandin A₂ inhibited mainly the Na⁺-dependent phosphorylation step in the overall reaction of Na⁺,K⁺-ATPase. This decrease of the Na⁺,K⁺-ATPase activity by prostaglandin A₂ was due to the decrease of Vmax of the enzyme and of the affinity of the enzyme for Na⁺. It was also suggested that the presence of both Δ^5,6 and Δ^10,11 structure of prostaglandin A₂ may be necessary for the inhibition of the Na⁺,K⁺-ATPase activity.     

Na+, K+-ATPase: relation of conformational transitions to function

Summary In its native environment, Na⁺, K⁺-ATPase of the plasma membrane is an oligomer consisting of two or more of each of two major subunits. Na⁺ and K⁺ move across the membrane through the channels that exist between the catalytic subunits of this oligomer. Two distinct ligand-induced conformational transitions (one due to the binding of K⁺ and ATP to the enzyme, and the other resulting from the phosphorylation of the enzyme in the presence of Na⁺ and ATP) cause changes in the geometries of the intersubunit channels, and provide the necessary energy-linked gating mechanisms for the transmembrane movements of ions against electrochemical gradients.     

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.     

Transport ATPase of erythrocyte membrane: sensitivities of Na plus, K, plus-ATPase and Kplus-phosphatase activities to ouabain.

Cardiac glycosides are inhibitors of Na⁺,K⁺-ATPase, and K⁺-phosphatase activities of the transport enzyme. Previous studies have shown that when the sensitivities of these two activities to ouabain are compared by the addition of varying concentrations of the drug to the assay media, the K⁺-phosphatase is significantly less sensitive than Na⁺,K⁺-ATPase. This work was done to seek an explanation for this phenomenon. 3-O-Methyl-fluorescein phosphate was used as substrate for the continuous fluorimetric assay of K⁺-phosphatase obtained from human red cells. When ouabain was added to the assay medium, a time-dependent inhibition of K⁺-phosphatase was observed. The rate of inhibition was also influenced by the order of additions of K⁺ and ouabain. In view of these results, several enzyme samples exposed to ouabain for varying lengths of time were prepared, and their Na⁺,K⁺-ATPase and K⁺-phosphatase activities were then determined. A good correlation between the extent of inhibition of the two activities was obtained. These results prove that the previously observed discrepancies between the sensitivities of Na⁺,K⁺-ATPase and K⁺-phosphatase to ouabain are due to the different kinetics of drug interaction with the enzyme under the different conditions of the two assays and that once a certain level of ouabain binding to the enzyme is achieved, both activities are equally inhibited.     

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 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.     

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.     

A kinetic comparison of cardiac glycoside interactions with Na+,K+-ATPases from skeletal and cardiac muscle and from kidney

The rates of association of [³H]ouabain to Na⁺,K⁺-ATPase and the rates of dissociation of the enzyme-ouabain complexes were determined for enzymes isolated from dog skeletal muscle, beef heart muscle, and lamb kidney medulla. The rates of association were strongly influenced by the presence of ligands such as magnesium, sodium, potassium, ATP, and inorganic phosphate. For a particular set of binding ligands, the rates of association did not vary much amongst the three enzymes studied, although enzyme from skeletal muscle was the fastest. In contrast, the rates of dissociation were relatively independent of the ligand conditions. The rates of dissociation also varied greatly amongst the enzyme sources, with skeletal muscle Na⁺,K⁺-ATPase being the fastest. Although the major determinant of the affinity of the Na⁺,K⁺-ATPase for ouabain is the rate of dissociation, the rate of association also plays a role. Since the binding of ouabain to the Na⁺,K⁺-ATPase in the presence of magnesium, ATP, sodium, and potassium is very slow, it is difficult to obtain an I₅₀ (equilibrium) value for the inhibition of hydrolytic activity by ouabain. If measurements of activity are made after a long period of time (3 h), the affinity of the enzyme for ouabain, estimated from inhibition of Na⁺,K⁺-ATPase activity, approached the value calculated from [³H]ouabain binding. The ratio of the I₅₀ value for ouabagenin to that for ouabain for the skeletal muscle enzyme was the same as that for cardiac muscle enzyme, indicating that the sugar moiety of ouabain was interacting with the receptor of both enzymes. It is apparent, therefore, that the absence of a sugar binding site in skeletal Na⁺,K⁺-ATPase is not the reason for the faster dissociation rate of this enzyme.     

The relationship between localization of Na+, K+-ATPase and cellular fine structure in the rat parotid gland

Summary A method for the ultrastructural localization of the Na⁺-pump enzyme, Na⁺, K⁺-ATPase has been applied to the rat parotid gland. A ouabain-sensitive final reaction product, dependent on the presence of K⁺ and Mg²⁺, was found to be evenly distributed along the basal and basolateral plasma membranes of acinar and striated duct cells. In both cases, it was localized predominantly in the cytoplasm of the extensive foldings of these membranes.It is concluded that the reaction product meets the criteria for valid localization of this enzyme and that potential Na⁺-pump sites have been demonstrated. This study supports previous findings in salivary glands and contributes to an increasing body of evidence that contraluminal Na⁺-pumps are common to both reabsorbing and secreting epithelial cells.     

Nitration as a Mechanism of Na+, K+-ATPase Modification During Hypoxia in the Cerebral Cortex of the Guinea Pig Fetus

Previous studies have shown that hypoxia induces nitric oxide synthase-mediated generation of nitric oxide free radicals leading to peroxynitrite production. The present study tests the hypothesis that hypoxia results in NO-mediated modification of Na⁺, K⁺-ATPase in the fetal brain. Studies were conducted in guinea pig fetuses of 58-days gestation. The mothers were exposed to FiO₂ of 0.07% for 1 hour. Brain tissue hypoxia in the fetus was confirmed biochemically by decreased ATP and phosphocreatine levels. P₂ membrane fractions were prepared from normoxic and hypoxic fetuses and divided into untreated and treated groups. The membranes were treated with 0.5 mM peroxynitrite at pH 7.6. The Na⁺, K⁺-ATPase activity was determined at 37°C for five minutes in a medium containing 100 mM NaCl, 20 mM KCl, 6.0 mM MgCl₂, 50 mM Tris HCl buffer pH 7.4, 3.0 mM ATP with or without 10 mM ouabain. Ouabain sensitive activity was referred to as Na⁺, K⁺-ATPase activity. Following peroxynitrite exposure, the activity of Na⁺, K⁺-ATPase in guinea pig brain was reduced by 36% in normoxic membranes and further 29% in hypoxic membranes. Enzyme kinetics was determined at varying concentrations of ATP (0.5 mM-2.0 mM). The results indicate that peroxynitrite treatment alters the affinity of the active site of Na⁺, K⁺-ATPase for ATP and decreases the Vmax by 35% in hypoxic membranes. When compared to untreated normoxic membranes Vmax decreases by 35.6% in treated normoxic membranes and further to 52% in treated hypoxic membranes. The data show that peroxynitrite treatment induces modification of Na⁺, K⁺-ATPase. The results demonstrate that peroxynitrite decreased activity of Na⁺, K⁺-ATPase enzyme by altering the active sites as well as the microenvironment of the enzyme. We propose that nitric oxide synthase-mediated formation of peroxynitrite during hypoxia is a potential mechanism of hypoxia-induced decrease in Na⁺, K⁺-ATPase activity.     

Inhibition of Rat Brain Na+, K+-ATPase Activity Induced by Homocysteine Is Probably Mediated by Oxidative Stress

The objective of the present study was to investigate the effects of preincubation of hippocampus homogenates in the presence of homocysteine or methionine on Na⁺, K⁺-ATPase and Mg²⁺-ATPase activities in synaptic membranes of rats. Homocysteine significantly inhibited Na⁺, K⁺-ATPase activity, whereas methionine had no effect. Mg²⁺-ATPase activity was not altered by the metabolites. We also evaluated the effect of incubating glutathione, cysteine, dithiothreitol, trolox, superoxide dismutase and GM1 ganglioside alone or incubation with homocysteine on Na⁺, K⁺-ATPase activity. Tested compounds did not alter Na⁺, K⁺-ATPase and Mg²⁺-ATPase activities, but except for trolox, prevented the inhibitory effect of homocysteine on Na⁺, K⁺-ATPase activity. These results suggest that inhibition of this enzyme activity by homocysteine is possibly mediated by free radicals and may contribute to the neurological dysfunction found in homocystinuric patients.     

Changes in Na<Superscript>+</Superscript>, K<Superscript>+</Superscript>-ATPase Activity and Alpha 3 Subunit Expression in CNS After Administration of Na<Superscript>+</Superscript>, K<Superscript>+</Superscript>-ATPase Inhibitors

The expression of Na⁺, K⁺-ATPase α3 subunit and synaptosomal membrane Na⁺, K⁺-ATPase activity were analyzed after administration of ouabain and endobain E, respectively commercial and endogenous Na⁺, K⁺-ATPase inhibitors. Wistar rats received intracerebroventricularly ouabain or endobain E dissolved in saline solution or Tris–HCl, respectively or the vehicles (controls). Two days later, animals were decapitated, cerebral cortex and hippocampus removed and crude and synaptosomal membrane fractions were isolated. Western blot analysis showed that Na⁺, K⁺-ATPase α3 subunit expression increased roughly 40% after administration of 10 or 100 nmoles ouabain in cerebral cortex but remained unaltered in hippocampus. After administration of 10 μl endobain E (1 μl = 28 mg tissue) Na⁺, K⁺-ATPase α3 subunit enhanced 130% in cerebral cortex and 103% in hippocampus. The activity of Na⁺, K⁺-ATPase in cortical synaptosomal membranes diminished or increased after administration of ouabain or endobain E, respectively. It is concluded that Na⁺, K⁺-ATPase inhibitors modify differentially the expression of Na⁺, K⁺-ATPase α3 subunit and enzyme activity, most likely involving compensatory mechanisms.     

Immunocytochemical localization of Na+, K+-ATPase in primary cultures of rat retina

Conditions have been established which allow growth of embryonic rat retinal cells in dissociated cell culture for up to one month. Na⁺,K⁺-ATPase localization was stuied in both neuronal and mixed neuronal-glial (flat cell) cultures. High Na⁺,K⁺-ATPase-like-immunoreactivity was associated with plasma membranes of neuronal cell bodies and their processes. Markedly lower immunoreactivity was found in the underlying flat cells in mixed cultures. Staining was generally uniform over perikaryal plasma membranes and showed a bead-like appearance in neuronal processes, supporting previous studies in brain tissue which used histocytochemical procedures specific for the Na⁺,K⁺-ATPase. This system should be useful for examining distribution of the enzyme in developing nerve and glial cells and may help to resolve questions regarding Na⁺–K⁺ homeostasis by neurons and glia.Dedicated to Henry McIlwain.     

An endogenous factor which interacts with synaptosomal membrane Na+, K+-ATPase activation by K+

In previous papers, the isolation of brain soluble fractions able to modify neuronal Na⁺, K⁺-ATPase activity has been described. One of those fractions-peak I-stimulates membrane Na⁺, K⁺-ATPase while another-peak II-inhibits this enzyme activity, and has other ouabain-like properties. In the present study, synaptosomal membrane Na⁺, K⁺-ATPase was analyzed under several experimental conditions, using ATP orp-nitrophenylphosphate (p-NPP) as substrate, in the absence and presence of cerebral cortex peak II. Peak II inhibited K⁺-p-NPPase activity in a concentration dependent manner. Double reciprocal plots indicated that peak II uncompetitively inhibits K⁺-p-NPPase activity regarding substrate, Mg²⁺ and K⁺ concentration. Peak II failed to block the known K⁺-p-NPPase stimulation caused by ATP plus Na⁺. At various K⁺ concentrations, percentage K⁺-p-NPPase inhibition by peak II was similar regardless of the ATP plus Na⁺ presence, indicating lack of correlation with enzyme phosphorylation. Na⁺, K⁺-ATPase activity was decreased by peak II depending on K⁺ concentration. It is postulated that the inhibitory factor(s) present in peak II interfere(s) with enzyme activation by K⁺.     

Reduction of hippocampal Na+, K+-ATPase activity in rats subjected to an experimental model of depression

The effect of a model of depression using female rats on Na⁺, K⁺-ATPase activity in hippocampal synaptic plasma membranes was studied. In addition, the effect of further chronic treatment with fluoxetine on this enzyme activity was verified. Sweet food consumption was measured to evaluate the efficacy of this model in inducing a state of reduced response to rewarding stimili. After 40 days of mild stress, a reduction in sweet food ingestion was observed. Reduction of hippocampal Na⁺, K⁺-ATPase activity was also observed. Treatment with fluoxetine increased this enzyme activity and reversed the effect of stress. Chronic fluoxetine decreased the ingestion of sweet food in both groups. This result is in agreement with suggestions that reduction of Na⁺, K⁺-ATPase activity is a caracteristic of depressive disorders. Fluoxetine reversed this effect. Therefore it is possible that altered Na⁺, K⁺-ATPase activity may be involved in the pathophysiology of depression in patients.