Regulation of Na+,K+-ATPase by persistent sodium accumulation in adult rat thalamic neurones

Deficiency of Thyroid Hormone Reduces Voltage-Gated Na+ Currents as Well as Expression of Na+/K+-ATPase in the Mouse Hippocampus

Mice lacking functional thyroid follicular cells, Pax8^−/− mice, die early postnatally, making them suitable models for extreme hypothyroidism. We have previously obtained evidence in postnatal rat neurons, that a down-regulation of Na⁺-current density could explain the reduced excitability of the nervous system in hypothyroidism. If such a mechanism underlies the development of coma and death in severe hypothyroidism, Pax8^−/− mice should show deficits in the expression of Na⁺ currents and potentially also in the expression of Na⁺/K⁺-ATPases, which are necessary to maintain low intracellular Na⁺ levels. We thus compared Na⁺ current densities in postnatal mice using the patch-clamp technique in the whole-cell configuration as well as the expression of three alpha and two beta-subunits of the Na⁺/K⁺-ATPase in wild type versus Pax8^−/− mice. Whereas the Na⁺ current density in hippocampal neurons from wild type mice was upregulated within the first postnatal week, the Na⁺ current density remained at a very low level in hippocampal neurons from Pax8^−/− mice. Pax8^−/− mice also showed significantly decreased protein expression levels of the catalytic α1 and α3 subunits of the Na⁺/K⁺-ATPase as well as decreased levels of the β2 isoform, with no changes in the α2 and β1 subunits.

Measuring enzyme activities in crude homogenates: Na+/K+-ATPase as a case study in optimizing assays

In this review of assays of Na⁺/K⁺-ATPase (NKA), we explore the choices made by researchers assaying the enzyme to investigate its role in physiological regulation. We survey NKA structure and function in the context of how it is typically assayed, and how technical choices influence what can be said about the enzyme. In comparing different methods for extraction and assay of NKA, we identified a series of common pitfalls that compromise the veracity of results. We include experimental work to directly demonstrate how choices in detergents, salts and substrates influence NKA activities measured in crude homogenates. Our review of assay approaches integrates what is known from enzymology, biomedical physiology, cell biology and evolutionary biology, offering a more robust method for assaying the enzyme in meaningful ways, identifying caveats and future directions to explore its structure and function. The goal is to provide the sort of background on the enzyme that should be considered in exploring the function of the enzyme in comparative physiology.

Synchronization Modulation of Na/K Pumps Induced Membrane Potential Hyperpolarization in Both Physiological and Hyperkalemic Conditions

The capability of the synchronization modulation (SM) technique in enhancing the function of Na/K pumps has been demonstrated in various cells and tissues, including cardiomyocytes, a monolayer of cultured MDCK kidney cells, peripheral blood vessels, and frog skeletal muscles. This study characterized the membrane potential hyperpolarization induced by SM in both physiological and high [K⁺]_o conditions on single skeletal muscle fibers. The results showed that SM could consistently induce membrane potential hyperpolarization by a few millivolts, and this hyperpolarization was not possible in the presence of ouabain. In contrast, the same electrical pulses but with random frequencies, constant frequencies, or synchronization with backward-modulation could not hyperpolarize the membrane potential. Prolonged field application and higher field intensity enhanced the effects of SM-induced hyperpolarization. Finally, the effect of SM was tested on skeletal muscle fibers incubated in a solution with high external potassium. Results showed that the SM electric field could hyperpolarize the membrane potential even if the external K⁺ concentration was higher than the normal, which implied the therapeutic effects of the SM electric field on the hyperkalemic situation.

Two-photon probes for in vivo multicolor microscopy of the structure and signals of brain cells

Imaging the brain of living laboratory animals at a microscopic scale can be achieved by two-photon microscopy thanks to the high penetrability and low phototoxicity of the excitation wavelengths used. However, knowledge of the two-photon spectral properties of the myriad fluorescent probes is generally scarce and, for many, non-existent. In addition, the use of different measurement units in published reports further hinders the design of a comprehensive imaging experiment. In this review, we compile and homogenize the two-photon spectral properties of 280 fluorescent probes. We provide practical data, including the wavelengths for optimal two-photon excitation, the peak values of two-photon action cross section or molecular brightness, and the emission ranges. Beyond the spectroscopic description of these fluorophores, we discuss their binding to biological targets. This specificity allows in vivo imaging of cells, their processes, and even organelles and other subcellular structures in the brain. In addition to probes that monitor endogenous cell metabolism, studies of healthy and diseased brain benefit from the specific binding of certain probes to pathology-specific features, ranging from amyloid-β plaques to the autofluorescence of certain antibiotics. A special focus is placed on functional in vivo imaging using two-photon probes that sense specific ions or membrane potential, and that may be combined with optogenetic actuators. Being closely linked to their use, we examine the different routes of intravital delivery of these fluorescent probes according to the target. Finally, we discuss different approaches, strategies, and prerequisites for two-photon multicolor experiments in the brains of living laboratory animals.

Cytotoxic Swelling of Sick Excitable Cells - Impaired Ion Homeostasis and Membrane Tension Homeostasis in Muscle and Neuron

When they become simultaneously leaky to both Na⁺ and Cl^-, excitable cells are vulnerable to potentially lethal cytotoxic swelling. Swelling ensues in spite of an isosmotic milieu because the entering ions add osmolytes to the cytoplasm's high concentration of impermeant anionic osmolytes. An influx of osmotically-obliged water is unavoidable. A cell that cannot stanch at least one the leaks will succumb to death by Donnan effect. "Sick excitable cells" are those injured through ischemia, trauma, inflammation, hyperactivity, genetically-impaired membrane skeletons and other insults, all of which foster bleb-damage to regions of the plasma membrane. Nav channels resident in damaged membrane exhibit left-shifted kinetics; the corresponding Nav window conductance constitutes a Na⁺-leak. In cortical neurons, sustained depolarization to ∼-20mV elicits a sustained lethal gCl. Underlying V_rest in skeletal muscle is a constitutively active gCl; not surprisingly therefore, dystrophic muscle fibers, which are prone to bleb damage and which exhibit Nav-leak and Na⁺-overload, are prone to cytotoxic swelling. To restore viability in cytotoxically swelling neurons and muscle, the imperative of fully functional ion homeostasis is well-recognized. However, as emphasized here, in a healthy excitable cell, fully functional membrane tension homeostasis is also imperative. ATPase-pumps keep plasma membrane batteries charged, and ATPase-motor proteins maintain membrane tone. In sick excitable cells, neither condition prevails.

Chloride Dysregulation, Seizures, and Cerebral Edema: A Relationship with Therapeutic Potential

Pharmacoresistant seizures and cytotoxic cerebral edema are serious complications of ischemic and traumatic brain injury. Intraneuronal Cl^- concentration ([Cl^-]_i) regulation impacts on both cell volume homeostasis and Cl^--permeable GABA_A receptor-dependent membrane excitability. Understanding the pleiotropic molecular determinants of neuronal [Cl^-]_i - cytoplasmic impermeant anions, polyanionic extracellular matrix (ECM) glycoproteins, and plasmalemmal Cl^- transporters - could help the identification of novel anticonvulsive and neuroprotective targets. The cation/Cl^- cotransporters and ECM metalloproteinases may be particularly druggable targets for intervention. We establish here a paradigm that accounts for recent data regarding the complex regulatory mechanisms of neuronal [Cl^-]_i and how these mechanisms impact on neuronal volume and excitability. We propose approaches to modulate [Cl^-]_i that are relevant for two common clinical sequela of brain injury: edema and seizures.

Transepithelial glucose transport and Na+/K+ homeostasis in enterocytes: an integrative model

The uptake of glucose and the nutrient coupled transcellular sodium traffic across epithelial cells in the small intestine has been an ongoing topic in physiological research for over half a century. Driving the uptake of nutrients like glucose, enterocytes must have regulatory mechanisms that respond to the considerable changes in the inflow of sodium during absorption. The Na-K-ATPase membrane protein plays a major role in this regulation. We propose the hypothesis that the amount of active Na-K-ATPase in enterocytes is directly regulated by the concentration of intracellular Na(+) and that this regulation together with a regulation of basolateral K permeability by intracellular ATP gives the enterocyte the ability to maintain ionic Na(+)/K(+) homeostasis. To explore these regulatory mechanisms, we present a mathematical model of the sodium coupled uptake of glucose in epithelial enterocytes. Our model integrates knowledge about individual transporter proteins including apical SGLT1, basolateral Na-K-ATPase, and GLUT2, together with diffusion and membrane potentials. The intracellular concentrations of glucose, sodium, potassium, and chloride are modeled by nonlinear differential equations, and molecular flows are calculated based on experimental kinetic data from the literature, including substrate saturation, product inhibition, and modulation by membrane potential. Simulation results of the model without the addition of regulatory mechanisms fit well with published short-term observations, including cell depolarization and increased concentration of intracellular glucose and sodium during increased concentration of luminal glucose/sodium. Adding regulatory mechanisms for regulation of Na-K-ATPase and K permeability to the model show that our hypothesis predicts observed long-term ionic homeostasis.

Changes in the Distribution of the α 3 Na(+)/K(+) ATPase Subunit in Heterozygous Lurcher Purkinje Cells as a Genetic Model of Chronic Depolarization during Development

A common assumption of excitotoxic mechanisms in the nervous system is that the ionic imbalance resulting from overstimulation of glutamate receptors and increased Na⁺ and Ca⁺⁺ influx overwhelms cellular energy metabolic systems leading to cell death. The goal of this study was to examine how a chronic Na⁺ channel leak current in developing Purkinje cells in the heterozygous Lurcher mutant (+/Lc) affects the expression and distribution of the α3 subunit of the Na⁺/K⁺ ATPase pump, a key component of the homeostasis system that maintains ionic equilibrium in neurons. The expression pattern of the catalytic α3 Na⁺/K⁺ ATPase subunit was analyzed by immunohistochemistry, histochemistry, and Western Blots in wild type (WT) and +/Lc cerebella at postnatal days P10, P15, and P25 to determine if there are changes in the distribution of active Na⁺/K⁺ ATPase subunits in degenerating Purkinje cells. The results suggest that the expression of the catalytic α3 subunit is altered in chronically depolarized +/Lc Purkinje cells, although the density of active Na⁺/K⁺ ATPase pumps is not significantly altered compared with WT in the cerebellar cortex at P15, and then declines from P15 to P25 in the +/Lc cerebellum as the +/Lc Purkinje cells degenerate.

A novel fluorescence imaging approach to monitor salt stress-induced modulation of ouabain-sensitive ATPase activity in sunflower seedling roots

Seedlings exposed to salt stress are expected to show modulation of intracellular accumulation of sodium ions through a variety of mechanisms. Using a new methodology, this work demonstrates ouabain (OU)-sensitive ATPase activity in the roots of sunflower seedlings subjected to salt stress (120 mM NaCl). 9-Anthroylouabain (a derivative of ouabain known to inhibit Na(+), K(+) -ATPase activity in animal systems, EC 3.6.3.9) has been used as a probe to analyze OU-sensitive ATPase activity in sunflower (Helianthus annuus) seedling roots by spectrofluorometric estimation and localization of its spatial distribution using confocal laser scanning microscopy. Salt stress for 48 h leads to a significant induction of OU-sensitive ATPase activity in the meristematic region of the seedling roots. Calcium ions (10 mM) significantly inhibit enzyme activity and a parallel accumulation of sodium ions in the cytosol of the columella cells, epidermis and in the cells of the meristematic region of the roots is evident. As a rapid response to NaCl stress, the activity of OU-sensitive ATPase gets localized in the nuclear membrane of root protoplasts and it gets inhibited after treatment with calcium ions. Nuclear membrane localization of the OU-sensitive ATPase activity highlights a possible mechanism to efflux sodium ions from the nucleus. Thus, a correlation between OU-sensitive ATPase activity, its modulation by calcium ions and accumulation of sodium ions in various regions of the seedling roots, has been demonstrated using a novel approach in a plant system.

The Na+ transport in gram-positive bacteria defect in the Mrp antiporter complex measured with 23Na nuclear magnetic resonance

(23)Na nuclear magnetic resonance (NMR) has previously been used to monitor Na(+) translocation across membranes in gram-negative bacteria and in various other organelles and liposomes using a membrane-impermeable shift reagent to resolve the signals resulting from internal and external Na(+). In this work, the (23)Na NMR method was adapted for measurements of internal Na(+) concentration in the gram-positive bacterium Bacillus subtilis, with the aim of assessing the Na(+) translocation activity of the Mrp (multiple resistance and pH) antiporter complex, a member of the cation proton antiporter-3 (CPA-3) family. The sodium-sensitive growth phenotype observed in a B. subtilis strain with the gene encoding MrpA deleted could indeed be correlated to the inability of this strain to maintain a lower internal Na(+) concentration than an external one.

Intracellular Na+ and metabolic modulation of Na/K pump and excitability in the rat suprachiasmatic nucleus neurons

Na/K pump activity and metabolic rate are both higher during the day in the suprachiasmatic nucleus (SCN) that houses the circadian clock. Here we investigated the role of intracellular Na+ and energy metabolism in regulating Na/K pump activity and neuronal excitability. Removal of extracellular K+ to block the Na/K pump excited SCN neurons to fire at higher rates and return to normal K+ to reactivate the pump produced rebound hyperpolarization to inhibit firing. In the presence of tetrodotoxin to block the action potentials, both zero K+-induced depolarization and rebound hyperpolarization were blocked by the cardiac glycoside strophanthidin. Ratiometric Na+ imaging with a Na+-sensitive fluorescent dye indicated saturating accumulation of intracellular Na+ in response to pump blockade with zero K+. The Na+ ionophore monensin also induced Na+ loading and hyperpolarized the membrane potential, with the hyperpolarizing effect of monensin abolished in zero Na+ or by pump blockade. Conversely, Na+ depletion with Na+-free pipette solution depolarized membrane potential but retained residual Na/K pump activity. Cyanide inhibition of oxidative phosphorylation blocked the Na/K pump to depolarize resting potential and increase spontaneous firing in most cells, and to raise intracellular Na+ levels in all cells. Nonetheless, the Na/K pump was incompletely blocked by cyanide but completely blocked by iodoacetate to inhibit glycolysis, indicating the involvement of both oxidative phosphorylation and glycolysis in fueling the Na/K pump. Together, the results indicate the importance of intracellular Na+ and energy metabolism in regulating Na/K pump activity as well as neuronal excitability in the SCN neurons.

Changes in Na+, K+-ATPase activity and alpha 3 subunit expression in CNS after administration of Na+, K+-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.

Capillary endothelial Na(+), K(+), ATPase transporter homeostasis and a new theory for migraine pathophysiology

BACKGROUND

Cerebrospinal fluid sodium concentration ([Na(+)](csf)) increases during migraine, but the cause of the increase is not known.

OBJECTIVE

Analyze biochemical pathways that influence [Na(+)](csf) to identify mechanisms that are consistent with migraine.

METHOD

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

RESULTS

Increased capillary endothelial cell (CEC) Na(+), K(+), -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na(+)](csf). Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na(+)](e)) and potassium ([K(+)](e)).

CONCLUSIONS

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K(+)](e), facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na(+)](e), increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

K+-dependent paradoxical membrane depolarization and Na+ overload, major and reversible contributors to weakness by ion channel leaks

Normal resting potential (P1) of myofibers follows the Nernst equation, exhibiting about -85 mV at a normal extracellular K(+) concentration ([K(+)](o)) of 4 mM. Hyperpolarization occurs with decreased [K(+)](o), although at [K(+)](o) < 1.0 mM, myofibers paradoxically depolarize to a second stable potential of -60 mV (P2). In rat myofiber bundles, P2 also was found at more physiological [K(+)](o) and was associated with inexcitability. To increase the relative frequency of P2 to 50%, [K(+)](o) needed to be lowered to 1.5 mM. In the presence of the ionophore gramicidin, [K(+)](o) reduction to only 2.5 mM yielded the same effect. Acetazolamide normalized this increased frequency of P2 fibers. The findings mimic hypokalemic periodic paralysis (HypoPP), a channelopathy characterized by hypokalemia-induced weakness. Of myofibers from 7 HypoPP patients, up to 25% were in P2 at a [K(+)](o) of 4 mM, in accordance with their permanent weakness, and up to 99% were in P2 at a [K(+)](o) of 1.5 mM, in accordance with their paralytic attacks. Of 36 HypoPP patients, 25 had permanent weakness and myoplasmic intracellular Na(+) ([Na(+)](i)) overload (up to 24 mM) as shown by in vivo (23)Na-MRI. Acetazolamide normalized [Na(+)](i) and increased muscle strength. HypoPP myofibers showed a nonselective cation leak of 12-19.5 microS/cm(2), which may explain the Na(+) overload. The leak sensitizes myofibers to reduced serum K(+), and the resulting membrane depolarization causes the weakness. We postulate that the principle of paradoxical depolarization and loss of function upon [K(+)](o) reduction may apply to other tissues, such as heart or brain, when they become leaky (e.g., because of ischemia).

Protective effects of IRFI-042 in monensin induced neurotoxicity in chicks

Monensin, a well known ionophore antibiotic, may cause severe damage in neuronal cells by altering Na+/K+-ATPase and Ca2+-ATPase. We investigated whether IRFI-042, a synthetic analogue of vitamin E, may block lipid peroxidation in neuronal cells and protect against monensin neurotoxicity in chicks. Monensin toxicity was induced in chicks by once-daily administration (150 mg/kg by oral gavages), for 8 days. Sham animals received a saline solution and were used as controls. All animals were randomized to receive either IRFI-042 (20 mg/kg) or its vehicle. Survival rate, brain lipid peroxidation, mRNA for neuronal and inducible nitric oxide synthases (nNOS and iNOS) and brain histological evaluations, including immunohistochemical expression of nNOS and iNOS were performed. Monensin administration decreased survival rate, induced behavioural changes, increased brain lipid peroxidation, reduced brain nNOS mRNA and immunostaining and enhanced iNOS mRNA and immunostaining in the brain in chicks. IRFI-042 significantly improved the survival rate and counteracted monensin-induced changes in chick brains. Our data suggest that monensin is responsible of neurotoxicity in chicks by inducing oxidative stress/lipid peroxidation and that IRFI-042 might represent a useful pharmacological approach to protect against the neuronal damage induced by this monovalent carboxylic ionophorous polyether antibiotic.

Expression of mutated mouse myocilin induces open-angle glaucoma in transgenic mice

We developed a genetic mouse model of open-angle glaucoma by expression of mutated mouse myocilin (Myoc) in transgenic (Tg) mice. The Tyr423His point mutation, corresponding to the severe glaucoma-causing Tyr437His mutation in the human MYOC gene, was introduced into bacterial artificial chromosome DNA encoding the full-length mouse Myoc gene and long flanking regions. Both wild-type (Wt) and Tg animals expressed Myoc in tissues of the irido-corneal angle and the sclera. Expression of mutated Myoc induced the accumulation of Myoc in cell cytoplasm and prevented its secretion into the extracellular space. The levels of ATPase-1 were reduced in the irido-corneal angle of Tg mice compared with Wt animals. Tg mice demonstrated a moderate elevation of intraocular pressure, the loss of approximately 20% of the retinal ganglion cells (RGCs) in the peripheral retina, and axonal degeneration in the optic nerve. RGC depletion was associated with the shrinkage of their nuclei and DNA fragmentation in the peripheral retina. Pathological changes observed in the eyes of Tg mice are similar to those observed in glaucoma patients.

Properties of the new fluorescent Na+ indicator CoroNa Green: comparison with SBFI and confocal Na+ imaging

Neuronal activity causes substantial Na+ transients in fine cellular processes such as dendrites and spines. The physiological consequences of such Na+ transients are still largely unknown. High-resolution Na+ imaging is pivotal to study these questions, and, up to now, two-photon imaging with the fluorescent Na+ indicator sodium-binding benzofuran isophthalate (SBFI) has been the primary method of choice. Recently, a new Na+ indicator dye, CoroNa Green (CoroNa), that has its absorbance maximum at 492 nm, has become available. In the present study, we have compared the properties of SBFI with those of CoroNa by performing Na+ measurements in neurons of hippocampal slices. We show that CoroNa is suitable for measurement of Na+ transients using non-confocal wide-field imaging with a CCD camera. However, substantial transmembrane dye leakage and lower Na+ sensitivity are clearly disadvantages when compared to SBFI. We also tested CoroNa for its suitability for high-resolution imaging of Na+ transients using a confocal laser scanning system. We demonstrate that CoroNa, in contrast to SBFI, can be employed for confocal imaging using a conventional argon laser and report the first Na+ measurements in dendrites using this dye. In conclusion, CoroNa may prove to be a valuable tool for confocal Na+ imaging in fine cellular processes.

Inhibition of lipid peroxidation by IRFI 042, a vitamin E analogue, decreases monensin cardiotoxicity in chicks

Monensin, a well-known ionophore antibiotic, may cause severe damage in myocardial cells. We investigated whether IRFI 042, a new analogue of vitamin E, may block lipid peroxidation in myocardial cells and in turn protect against monensin toxicity. Monensin toxicity was induced by repeated daily administration of the ionophore antibiotic (150 mg/kg/day for 7 days). Sham animals received by oral gavages only a saline solution and were used as controls. All animals were randomized to receive concomitantly by oral gavages IRFI 042 (20 mg/kg) or its vehicle. The experiment lasted 8 days. Survival rate, heart lipid peroxidation, studied by means of thiobarbituric acid-reactive substances (TBARs) levels, cardiac expression of endothelial nitric oxide (e-NOS) and histological analysis of the heart were performed. Monensin administration caused a decrease in survival rate. Mortality appeared following the second monensin injection and at day 7 caused a survival rate of 20%. Thereafter, no further mortality was observed. IRFI 042 administration improved survival rate. Injection of the ionophore antibiotic resulted in a marked cardiac lipid peroxidation and in a significant reduction in cardiac e-NOS message and protein expression. IRFI 042 decreased heart TBARs levels (Monensin + vehicle = 6.5 +/- 0.8 nmol/mg; Monensin + IRFI 042 = 3.2 +/- 1.1 nmol/mg; P < 0.001) and increased e-NOS message and protein expression. Histological analysis showed that IRFI 042 improved myocardial cells damage and enhanced the depressed e-NOS expression in chick heart samples following monensin administration. Our data suggest that IRFI 042 is a promising drug to reduce monensin cardio-toxicity in chicks.

Normal expression and inflammation-induced changes of Na and Na/K ATPase activity in spinal dorsal horn of the rat

We tested whether that peripheral inflammation induces changes in the spinal dorsal horn ATPase activity. Adult Sprague-Dawley rats were anesthetized (thiobarbital), the left hind paw (inflammation group; n = 15) was immersed in water at 60 degrees C for 60s, which induced a local inflammation. A control group (n = 12) was tested with water at room temperature. After 60 min of peripheral inflammation left (LDH) or right lumbar dorsal horn (RDH) were processed for total, Na/K, Na and remanent ATPase activities (nM P(i) (mgprotein)(-1) min(-1)). In control animals isoenzymatic activities were: Na (31.2%); Na/K (20.6%) and remanent (48.2%) from total ATPase activity. No LDH-RDH asymmetry was found. The inflammation group presented an ipsilateral increase of total ATPase activity in LDH (X+/-S.E.M.; 4798.9+/-601) over the RDH (3982.2+/-451; Delta+817; P<0.05). This is due to an increase in Na ATPase activity (1609.3+/-297) over RDH (1164.2+/-166; Delta+445; P<0.05). ATPase activities were increased in LDH from inflamed over the control group as follows: total (4798.9+/-601; Delta+840; P<0.05), Na/K (1298.1+/-301; Delta+483; P<0.05) and Na (1609.3+/-297; Delta+373; P<0.05). These increased ATPase activities, induced in a short time, can be considered a functional marker of nociceptive neuronal activity.

Calcium localisation by X-ray microanalysis and fluorescence microscopy in larvae of zooxanthellate and azooxanthellate corals

X-ray microanalysis and fluorescence microscopy (Calcium Orangetrade mark) was used to determine the distribution of intracellular calcium (I(Ca)), in the form of total and ionic calcium respectively, in planulae and settled larvae of a zooxanthellate coral. The distribution of total calcium only was determined in larvae of an azooxanthellate coral. In azooxanthellate planulae and settled larvae, total I(Ca) concentration in the oral ectoderm was high and similar to that in seawater (SW). Calcium concentration did not vary (P > 0.05) between planulae and settled larvae. However, settled larvae accumulated large amounts of calcium in gastrodermal lipid-containing cells. In contrast, zooxanthellate planulae possessed significantly (P < 0.01) lower concentrations of total I(Ca) within ectodermal cells in comparison to settled larvae. In addition, in settled zooxanthellate larvae total calcium concentration in the mesogloea and coelenteron was significantly (P < 0.05) higher than in the oral ectodermal and gastrodermal cells, respectively. Total I(Ca) concentrations in the oral ectoderm of settled larvae were also significantly (P < 0.01) lower than that of the calicoblastic ectoderm. In zooxanthellate settled larvae, ionic I(Ca) levels in the aboral epithelium surrounding rapidly growing septa were high. These levels increased significantly (P < 0.05) within the tissue surrounding growing septa after incubation in high-calcium SW.

Regulation of Na,K-ATPase expression by endothelin-1 in transformed human ciliary non-pigmented epithelial (HNPE) cells

Endothelin-1 (ET-1) (1-100 nM) decreases the activity of Na,K-ATPase, a key enzyme responsible for aqueous humor formation, in transformed human non-pigmented ciliary epithelial (HNPE) cells. The present study sought to determine if ET-1 alters the expression of the catalytically active alpha subunit of Na,K-ATPase in HNPE cells and identify mechanisms underlying these effects. We report that acute (15 and 30 min) treatment with ET-1 results in an increase in mRNA expression of the alpha 1 subunit of Na,K-ATPase. Similar to ET-1's effects, ouabain (100 microM), a selective inhibitor of Na,K-ATPase, and monensin (10 microM), a sodium ionophore, also increased Na,K-ATPase expression in HNPE cells. The increase in Na,K-ATPase expression by short-term treatment with ouabain and monensin was dependent on their ability to elevate intracellular sodium concentrations. However, acute ET-1 treatment mediated increase in Na,K-ATPase expression was independent of changes in intracellular sodium. A prolonged (24 hr) ET-1 treatment results in an increase in both mRNA and protein levels of the alpha 1 subunit of Na,K-ATPase. These observations suggest that ET-1 could play a homeostatic role in modulating aqueous humor formation by having differential effects on the activity and expression of Na,K-ATPase by the ciliary epithelium in the eye.

Production and role of extracellular guanosine cyclic 3', 5' monophosphate in sodium uptake in human proximal tubule cells

The present study was designed to determine the capability of human renal proximal tubule (RPT) to generate and export guanosine cyclic 3', 5' monophosphate (cGMP) in response to direct stimulation of soluble guanylyl cyclase by nitric oxide (NO) donors. In addition, we investigated whether cGMP extrusion from human RPT cells is required for inhibition of cellular sodium uptake. RPT cells were cultured from fresh human kidneys (normotensive subjects, n=4, mean age 65+/-4.7 years, 3 men, 1 woman; hypertensive patients, n=6, mean age 64+/-6.1 years, 4 men, 2 women) after unilateral nephrectomy. The fluorescence dye Sodium Green was employed to determine cytoplasmic Na+ concentration. In the presence of the Na+/K+ ATPase inhibitor ouabain, fluorescence was monitored at the appropriate wavelength (excitation 485 nm, emission 535 nm). Nitric oxide donor, S-nitroso-N-acetylpenicillamine (SNAP, 10(-4) M), increased both intracellular and extracellular cGMP (from 1.26+/-0.21 to 88.7+/-12.6 pmol/mg protein and from 0.58+/-0.10 to 9.24+/-1.9 pmol/mL, respectively, P<0.01) and decreased cellular Na+ uptake by 37.4+/-6.8% (P<0.05) compared with control. The effects of SNAP on cGMP production were similar in normotensive and hypertensive subjects. The increases in intracellular and extracellular cGMP concentration because of SNAP were blocked completely by soluble guanylyl cyclase inhibitor ODQ (1-H-[1,2,4] oxadiazolo [4,2-alpha] quinoxalin-1-one). Probenecid, an organic anion transport inhibitor, augmented the SNAP (10(-6) M)-induced increase in intracellular cGMP accumulation (from 4.9+/-0.9 to 9.8+/-1.5 pmol/mg protein, P<0.05), abrogated the SNAP-induced increase in extracellular cGMP extrusion (from 1.07+/-0.4 to 0.37+/-0.1 pmol/L, P<0.05) and blocked the SNAP-induced reduction in cellular Na+ uptake. Neither intracellular nor extracellular cGMP were influenced by l-arginine, the metabolic precursor of NO, or N(G)-nitro-L-arginine methyl ester, an inhibitor of NO synthase. After exogenous administration of cGMP (10(-5) M) or its membrane-permeable analogue 8-Br-cGMP (10(-5) M), only 8-Br-cGMP crossed the cell membrane to increase intracellular cGMP (from 1.36+/-0.19 to 289.7+/-29.4 pmol/mg protein, P<0.01). However, both cGMP and 8-Br-cGMP were effective in decreasing cellular Na+ uptake. In conclusion, human RPT cells contain soluble guanylyl cyclase and are able to generate and export cGMP in response to NO. Because human RPT cells do not themselves contain constitutive NO synthase, the NO-generating cGMP must be derived from sources outside the human RPT. The cGMP cellular export system is critical in the regulation of RPT cellular Na+ absorption in humans.

Protein kinase C-mediated cell death mode switch induced by high glucose

Cortical neurons rapidly die in necrosis due to poor glucose uptake in the low-density (LD) culture under serum-free condition without any supplements. The scanning and transmission electron microscopical analyses characterized the necrosis by membrane disruption, mitochondrial swelling and loss of cytoplasmic electron density. High-glucose treatment delayed the neuronal death by suppressing necrosis, but induced apoptosis through increase in Bax levels, cytochrome c release, caspase-3 activation and DNA ladder formation. Although pyruvate as well as high glucose inhibited necrotic cell death and rapid decrease in cellular ATP levels, possibly related to decreased [(3)H]-2-deoxy glucose uptake under the serum-free condition, it did not induce apoptosis. Protein kinase C inhibitors blocked these changes related to the cell death mode switch. Several neurotrophic factors did not affect the necrosis, but potentiated high-glucose-induced survival activity, while inhibiting cytochrome c release. All these results suggest that high-glucose treatment causes neuronal cell death mode switch by inhibiting necrosis, while inducing apoptosis, which is prevented by neurotrophic factors.

Coupling of calcium homeostasis to axonal sodium in axons of mouse optic nerve

Axonal populations in neonatal and mature optic nerves were selectively stained with calcium dyes for analysis of calcium homeostasis and its possible coupling to axonal Na. Repetitive nerve stimulation causes a rise in axonal [Ca(2+)](i) the posttetanus recovery of which is impeded by increasing the number of action potentials in the tetanus. This effect is augmented in 4-aminopyridine (4-AP; 1 mM), which dramatically increases the calcium and presumably sodium load during the tetanus. Increasing axonal [Na](i) with the Na-ionophore monensin (4-50 microM) and ouabain (30 microM) retards posttetanus calcium decline, suggesting that efficient calcium clearance depends on a low level of axonal [Na](i). Posttetanus calcium clearance is not affected by K-mediated depolarization. To further examine coupling between axonal [Na](i) and [Ca(2+)](i), the resting axonal [Ca(2+)](i) was monitored as axonal [Na(+)](i) was elevated with ouabain, veratridine, and monensin. In all cases, elevation of axonal [Na(+)](i) evokes a calcium influx into axons. This influx is unrelated to activation of calcium channels but is consistent with calcium influx via reversal of the Na/Ca exchanger expected as a consequence of axonal [Na(+)](i) elevation. In conclusion, this study demonstrates that calcium homeostasis in the axons of the optic nerve is strongly coupled to axonal [Na(+)](i) in a manner consistent with the Na/Ca exchanger playing a major role in extruding calcium following nerve activity.

The lethal expression of the GluR2flip/GluR4flip AMPA receptor in HEK293 cells

alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) -type glutamate receptors play a critical role in excitotoxicity associated with cerebral hypoxia, ischaemia and other acute brain insults. AMPA receptors are composed of GluR1-GluR4 subunits in homomeric and heteromeric assemblies, forming nonselective cation channels. In addition, each subunit has alternative splice variants, flip and flop forms. Heterologous expression studies showed that the AMPA receptor channels exhibit diverse properties depending on subunit/variant composition. For example, the absence of the GluR2 subunit makes AMPA receptor assemblies Ca2+-permeable. Excitotoxicity induced by activating AMPA receptor channels has been linked to excessive Ca2+ influx through the GluR2-lacking channels. Here we demonstrate that coexpression of the AMPA receptor GluR2flip and GluR4flip subunits exerts a lethal effect on HEK293 cells, whereas no lethal activity is observed in other homomeric or heteromeric combinations of AMPA receptor subunits. Patch clamp recordings and Ca2+ imaging analyses have revealed that this GluR2flip/GluR4flip receptor exhibits a low Ca2+ permeability. This subunit combination, however, showed prolonged Na+ influx following AMPA stimulation, even in the absence of cyclothiazide, which attenuates AMPA receptor desensitization. Furthermore, the GluR2flip/GluR4flip-mediated lethality was potentiated by the interruption of cellular Na+ extrusion mechanisms using ouabain or benzamil. These observations suggest that the GluR2flip/GluR4flip receptor-mediated excitotoxicity is attributed to Na+ overload, but not Ca2+ influx.