Impairment of Na+,K+-ATPase activity following enterotoxigenic Campylobacter jejuni infection: changes in Na+, Cl− and 3-O-methyl-D-glucose transport in vitro, in rat ileum

Abstract Unidirectional fluxes of Na⁺, Cl⁻ and 3-O-methyl-D-glucose (3-MG) were measured in vitro across Campylobacter jejuni live culture-infected and control rat ileal short-circuited tissues by the Using Chamber technique. Net secretion of Na⁺ and enhanced secretion of Cl⁻ ions was observed in the infected animals ( P < 0.001, n =6) as compared to the net absorption of Na⁺ and marginal secretion of Cl⁻ ions in the control animals. There was a significant decrease in the mucosal-to-serosal fluxes of 3-MG in C. jejuni -infected rat ileum. The specific Na⁺,K⁺-ATPase activity when measured biochemically in the membrane-rich fraction of enterocytes was found to be significantly lower (58%) in the infected group as compared to the control group ( P < 0.001). Our results therefore suggest that infection with an enterotoxigenic C. jejuni inhibits the Na⁺,K⁺-ATPase activity in rat enterocytes. The impairment of Na⁺,K⁺-ATPase activity thus appears to induce a secondary change in Na⁺,Cl⁻ and 3-MG transport in vitro in rat ileum.     

Effects of chloride and sodium on gas exchange parameters and water relations of Citrus plants

Gas exchange parameters, water relations and Na⁺/Cl^- content were measured on leaves of one-year-old sweet orange ( Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO₃) were used to separate the effects of Cl⁻ and Na⁺ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl⁻ in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na⁺ concentrations of up to 478 m M in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca²⁺ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl⁻ and Na⁺ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.     

Catecholamine Secretion from Bovine Adrenal Chromaffin Cells: The Role of the Na+/Ca2+ Exchanger and the Intracellular Ca2+ Pool

Abstract: The role of the Na⁺/Ca²⁺ exchanger and intracellular nonmitochondrial Ca²⁺ pool in the regulation of cytosolic free calcium concentration ([Ca²⁺]_i) during catecholamine secretion was investigated. Catecholamine secretion and [Ca²⁺]_i were simultaneously monitored in a single chromaffin cell. After high-K⁺ stimulation, control cells and cells in which the Na⁺/Ca²⁺ exchange activity was inhibited showed similar rates of [Ca²⁺]_i elevation. However, the recovery of [Ca²⁺]_i to resting levels was slower in the inhibited cells. Inhibition of the exchanger increased the total catecholamine secretion by prolonging the secretion. Inhibition of the Ca²⁺ pump of the intracellular Ca²⁺ pool with thapsigargin caused a significant delay in the recovery of [Ca²⁺]_i and greatly enhanced the secretory events. These data suggest that both the Na⁺/Ca²⁺ exchanger and the thapsigargin-sensitive Ca²⁺ pool are important in the regulation of [Ca²⁺]_i and, by modulating the time course of secretion, are important in determining the extent of secretion.     

Renal excretion in channel catfish following injection of quinaldine sulphate or 3- trifluoromethyl-4-nitrophenol

Channel catfish, Ictalurus punctatus Rafinesque, injected intraperitoneally with 2-methyl-quinoline sulphate (QdSO₄) or 3-trifluoromethyl-4-nitrophenol (TFM) eliminate most of the dose of these compounds by extra-renal routes. Patterns of renal excretion of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Cl⁻ (ρEq kg⁻¹ h⁻¹) appeared to be associated with the 'stress' of the urine collection technique rather than with the elimination of either compound. Concentrations of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Cl⁻ (mEq/1) were determined in urine, plasma and gall bladder bile.     

Comparison of ion transportation before and after egg hatching in Amphinemura sp. (Plecoptera)

Abstract.  An increase in egg size with embryonic development in stoneflies is believed to result from the uptake of water by osmosis. The present study aims to investigate whether a selective ion transport through egg membranes exists before hatching, and whether ions are released after hatching. Viable and nonviable egg masses are incubated in Petri dishes filled with water, and the concentrations of the ions F⁻, Cl⁻, SO₄²⁻, NO₃⁻, Na⁺, K⁻, Mg²⁺ and Ca²⁺ in the water are determined. The ion transport of an egg mass before and after hatching and a nonviable egg mass is then calculated. Before hatching, Cl⁻, SO₄²⁻, NO₃⁻, Na⁺, Mg²⁺ and Ca²⁺ are taken up from the surrounding water into the inner egg. These ions are selectively taken into the egg. After hatching, Cl⁻, SO₄²⁻, Na⁺, Mg²⁺ and Ca²⁺ are released into the surrounding water. The amount of these ions released after hatching is lower than the amount taken up before hatching. Ions that are not released after hatching are considered to be used in embryonic development.     

Agents that Promote Protein Phosphorylation Inhibit the Activity of the Na+/Ca2+ Exchanger and Prolong Ca2+ Transients in Bovine Chromaffin Cells

Abstract: The Na⁺/Ca²⁺ exchanger is an important element in the maintenance of calcium homeostasis in bovine chromaffin cells. The Na⁺/Ca²⁺ exchanger from other cell types has been extensively studied, but little is known about its regulation in the cell. We have investigated the role of reversible protein phosphorylation in the activity of the Na⁺/Ca²⁺ exchanger of these cells. Cells treated with 1 m M dibutyryl cyclic AMP (dbcAMP), 1 µ M phorbol 12,13-dibutyrate, 1 µ M okadaic acid, or 100 n M calyculin A showed lowered Na⁺/Ca²⁺ exchange activity and prolonged cytosolic Ca²⁺ transients caused by depolarization. A combination of 10 n M okadaic acid and 1 µ M dbcAMP synergistically inhibited Na⁺/Ca²⁺ exchange activity. Conversely, 50 µ M 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, a protein kinase inhibitor, enhanced Na⁺/Ca²⁺ exchange activity. Moreover, we used cyclic AMP-dependent protein kinase and calcium phospholipid-dependent protein kinase catalytic subunits to phosphorylate isolated membrane vesicles and found that the Na⁺/Ca²⁺ exchange activity was inhibited by this treatment. These results indicate that reversible protein phosphorylation modulates the activity of the Na⁺/Ca²⁺ exchanger and suggest that modulation of the exchanger may play a role in the regulation of secretion.     

Intracellular water volume and internal pH of Streptomyces antibioticus spores

Abstract In the present study we compared the intracellular level of free calcium ([Ca²⁺]_i) and monomeric (G)/total (G + F) actin ratio in HeLa cells infected with diffuse (DAEC) and localised adherent Escherichia coli (LAEC). The level of [Ca²⁺]_i was increased in both DAEC- and LAEC-infected HeLa cells. However, studies with EGTA- and dantrolene-treated cells and also suspension of cells in Ca²⁺-free buffer suggested that the rise of [Ca²⁺]_i in DAEC-infected cells was due to the influx of Ca²⁺ from extracellular medium, whereas Ca²⁺ mobilisation from the intracellular stores was responsible for the enhancement of [Ca²⁺]_i in LAEC-infected cells. It was also evident that the infection of HeLa cells with DAEC and LAEC caused alteration of G / G + F actin ratio as compared to that of control cells. The ratio was much lower in LAEC-infected cells than that of DAEC-infected ones. Moreover, cytochalasin B inhibited both DAEC and LAEC invasion to HeLa cells, suggesting further the role of microfilaments in the invasion process.     

Internalization of Voltage-Dependent Sodium Channels in Fetal Rat Brain Neurons: A Study of the Regulation of Endocytosis

Abstract: In fetal rat brain neurons, activation of voltage-dependent Na⁺ channels induced their own internalization, probably triggered by an increase in intracellular Na⁺ level. To investigate the role of phosphorylation in internalization, neurons were exposed to either activators or inhibitors of cyclic AMP- and cyclic GMP-dependent protein kinases, protein kinase C, and tyrosine kinase. None of the tested compounds mimicked or inhibited the effect of Na⁺ channel activation. An increase in intracellular Ca²⁺ concentration induced either by thapsigargin, a Ca²⁺-ATPase blocker, or by A23187, a Ca²⁺ ionophore, was unable to provoke Na⁺ channel internalization. However, Ca²⁺ seems to be necessary because both neurotoxin- and amphotericin B-induced Na⁺ channel internalizations were partially inhibited by BAPTA-AM. The selective inhibitor of Ca²⁺/calmodulin-dependent protein kinase II, KN-62, caused a dose-dependent inhibition of neurotoxin-induced internalization due to a blockade of channel activity but did not prevent amphotericin B-induced internalization. The rate of increase in Na⁺ channel density at the neuronal cell surface was similar before and after channel internalization, suggesting that recycling of internalized Na⁺ channels back to the cell surface was almost negligible. Pretreatment of the cells with an acidotropic agent such as chloroquine prevented Na⁺ channel internalization, indicating that an acidic endosomal/lysosomal compartment is involved in Na⁺ channel internalization in neurons.     

Effects of Ca2+ and polyamines on Na+ and Rb+ influx in excised maize roots

When 1 m M spermidine or spermine was included in an absorption solution which contained 20 m M Na⁺ and 1 m M Rb⁺, Na⁺ influx into excised maize roots ( Zea mays L. cv. Golden Cross Bantam) was reduced. Rb⁺ influx was reduced in the presence of spermidine and uneffected in the presence of spermine when compared with control solutions. When 1 m M Ca²⁺ replaced the polyamines, Na⁺ influx was strongly reduced and Rb⁺ influx was promoted. Rb⁺ influx from 1 m M Rb⁺ solutions which did not contain Na⁺ was also promoted by 1 m M Ca²⁺, but was inhibited by 1 m M spermidine. This Ca²⁺ promotion of Rb⁺ influx could be reversed by 10 times greater concentration of spermidine in the absorption solution. H⁺ efflux from excised roots was inhibited by spermidine when compared with Ca²⁺ or control solutions, however, the plasma membrane ATPase was not inhibited by spermidine. It is concluded that external Ca²⁺ plays two separate roles in membrane function, only one of which can be substituted for by polyamines. The first role, maintenance of membrane integrity, can be substituted for by spermidine or spermine. The second function, maintenance of the Rb⁺ transport mechanism, is Ca²⁺ specific and cannot be substituted for by spermidine or spermine. The results of this study are discussed in terms of electrostatic interactions between the plasma membrane and the Ca²⁺ or polyamines.     

Histamine-Evoked Chromaffin Cell Scinderin Redistribution, F-Actin Disassembly, and Secretion: In the Absence of Cortical F-Actin Disassembly, an Increase in Intracellular Ca2+ Fails to Trigger Exocytosis

Abstract: Histamine is a known chromaffin cell secretagogue that induces Ca²⁺-dependent release of catecholamines. However, conflicting evidence exists as to the source of Ca²⁺ utilized in histamine-evoked secretion. Here we report that histamine-H₁ receptor activation induces redistribution of scinderin, a Ca²⁺-dependent F-actin severing protein, cortical F-actin disassembly, and catecholamine release. Histamine evoked similar patterns of distribution of scinderin and filamentous actin. The rapid responses to histamine occurred in the absence of extracellular Ca²⁺ and were triggered by release of Ca²⁺ from intracellular stores. The trigger for the release of Ca²⁺ was inositol 1,4,5-trisphosphate because U-73122, a phospholipase C inhibitor, but not its inactive isomer (U-73343), inhibited the increases in IP₃ and intracellular Ca²⁺ levels, scinderin redistribution, cortical F-actin disassembly, and catecholamine release in response to histamine. Thapsigargin, an agent known to mobilize intracellular Ca²⁺, blocked the rise in intracellular Ca²⁺ concentration, scinderin redistribution, F-actin disassembly, and catecholamine secretion in response to histamine. Calphostin C and chelerythrine, two inhibitors of protein kinase C, blocked all responses to histamine with the exception of the release of Ca²⁺ from intracellular stores. This suggests that protein kinase C is involved in histamine-induced responses. The results also show that in the absence of F-actin disassembly, rises in intracellular Ca²⁺ concentration are not by themselves capable of triggering catecholamine release.     

AMPA Receptor-Mediated Excitotoxicity in Human NT2-N Neurons Results from Loss of Intracellular Ca2+ Homeostasis Following Marked Elevation of Intracellular Na+

Abstract: Human NT2-N neurons express Ca²⁺-permeable α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid glutamate receptors (AMPA-GluRs) and become vulnerable to excitotoxicity when AMPA-GluR desensitization is blocked with cyclothiazide. Although the initial increase in intracellular Ca²⁺ levels ([Ca²⁺]_i) was 1.9-fold greater in the presence than in the absence of cyclothiazide, Ca²⁺ entry via AMPA-GluRs in an early phase of the exposure was not necessary to elicit excitotoxicity in these neurons. Rather, subsequent necrosis was caused by a >40-fold rise in [Na⁺]_i, which induced a delayed [Ca²⁺]_i rise. Transfer of the neurons to a 5 m M Na⁺ medium after AMPA-GluR activation accelerated the delayed [Ca²⁺]_i rise and intensified excitotoxicity. Low-Na⁺ medium-enhanced excitotoxicity was partially blocked by amiloride or dizocilpine (MK-801), and completely blocked by removal of extracellular Ca²⁺, suggesting that Ca²⁺ entry by reverse operation of Na⁺/Ca²⁺ exchangers and via NMDA glutamate receptors was responsible for the neuronal death after excessive Na⁺ loading. Our results serve to emphasize the central role of neuronal Na⁺ loading in AMPA-GluR-mediated excitotoxicity in human neurons.     

Systemin triggers an increase of cytoplasmic calcium in tomato mesophyll cells: Ca2+ mobilization from intra- and extracellular compartments

We show here that, within 1–2 min of application, systemin triggers a transient increase of cytoplasmic free calcium concentration ([Ca²⁺]_c) in cells from Lycopersicon esculentum mesophyll. The systemin-induced Ca²⁺ increase was slightly but not significantly reduced by L-type Ca²⁺ channel blockers (nifedipine, verapamil and diltiazem) and the Ca²⁺ chelator [ethylene glycol tetraacetic acid (EGTA)], whereas inorganic Ca²⁺ channel blockers (LaCl₃, CdCl₂ and GdCl₃) and compounds affecting the release of intracellular Ca²⁺ from the vacuole (ruthenium red, LiCl, neomycin) strongly reduced the systemin-induced [Ca²⁺]_c increase. By contrast, no inhibitory effect was seen with the potassium and chloride channel blockers tested. Unlike systemin, other inducers of proteinase inhibitor (PI) and of wound-induced protein synthesis, such as jasmonic acid (JA) and bestatin, did not trigger an increase of cytoplasmic Ca²⁺. The systemin-induced elevation of cytoplasmic Ca²⁺ which might be an early step in the systemin signalling pathway, appears to involve an influx of extracellular Ca²⁺ simultaneously through several types of Ca²⁺ permeable channels, and a release of Ca²⁺ from intracellular stores sensitive to blockers of inositol 1,4,5-triphosphate (IP₃)- and cyclic adenasine 5'-diphosphoribose (cADPR)-mediated Ca²⁺ release.     

Apoplastic pH and inorganic ion levels in tomato fruit: A potential means for regulation of cell wall metabolism during ripening

Apoplastic pH and ionic conditions exert strong influence on cell wall metabolism of many plant tissues; however, the nature of the apoplastic environment of ripening fruit has been the subject of relatively few studies. In this report, a pressure-bomb technique was used to extract apoplastic fluid from tomato fruit ( Lycopersicon esculentum Mill.) pericarp at several developmental stages. pH and the levels of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻ and P were determined and compared with the values for the bulk pericarp and locule tissues. The pH of the apoplastic fluid from pericarp tissue decreased from 6.7 in immature and mature-green fruits to 4.4 in fully-ripe fruit. During the same period, the K⁺ concentration increased from 13 to 37 m M . The levels of Na⁺ and divalent cations did not change, whereas the anions P and Cl⁻ increased in ripe fruit. Ca²⁺ levels remained relatively constant during ripening at 4–5 m M , concentrations that effectively limit pectin solubilization. The electrical conductivity of the apoplastic liquid increased 3-fold during ripening, whereas osmotically active solutes increased 2-fold. Pressure-treated fruit retained the capacity to ripen. The decline in apoplastic pH and increase in ionic strength during tomato fruit ripening may regulate the activity of cell wall hydrolases. The potential role of apoplastic changes in fruit ripening and softening is discussed.     

Early Events in Free Radical-Mediated Damage of Isolated Nerve Terminals: Effects of Peroxides on Membrane Potential and Intracellular Na+ and Ca2+ Concentrations

Abstract: The effects of peroxides were investigated on the membrane potential, intracellular Na⁺ ([Na⁺]_i) and intracellular Ca²⁺ ([Ca²⁺]_i) concentrations, and basal glutamate release of synaptosomes. Both H₂O₂ and the organic cumene hydroperoxide produced a slow and continuous depolarization, parallel to an increase of [Na⁺]_i over an incubation period of 15 min. A steady rise of the [Ca²⁺]_i due to peroxides was also observed that was external Ca²⁺ dependent and detected only at an inwardly directed Ca²⁺ gradient of the plasma membrane. These changes did not correlate with lipid peroxidation, which was elicited by cumene hydroperoxide but not by H₂O₂. Resting release of glutamate remained unchanged during the first 15 min of incubation in the presence of peroxides. These alterations may indicate early dysfunctions in the sequence of events occurring in the nerve terminals in response to oxidative stress.     

Endogenous abscisic acid levels are linked to decreased growth of bush bean plants treated with NaCl

This paper studies the relative importance of endogenous ABA and ion toxicity in the leaf growth inhibition caused by NaCl in salt-adapted and unadapted bush beans. Adaptation to salt-stress was achieved by germination of seeds in 75 m M NaCl, while unadapted plants were germinated in tap water. The adaptation process caused a transitory increase in leaf ABA, Na⁺ and Cl⁻ concentrations, while leaf expansion was inhibited. However, when grown for 8 or 13 days in 75 m M NaCl-containing nutrient solution, primary and first trifoliolate leaves of salt-adapted plants had greater areas than those of unadapted plants. Concentrations of ABA, Na⁺ and Cl⁻ in these leaves were lower in adapted plants, and a strong negative correlation between leaf expansion growth and either leaf Na⁺, Cl⁻ or ABA concentrations could be established. However, in the second trifoliolate leaves only the ABA, but not the Na⁺ or Cl⁻, concentrations were significantly correlated with leaf expansion. Our results suggest that salt-induced inhibition of leaf expansion growth in bush beans is mediated by ABA rather than Na⁺ or Cl⁻ toxicity. Moreover, the increase of ABA, induced by the salt-pretreatment, seems to play an important role in limiting the accumulation of Na⁺ and Cl⁻ in the leaves, leading to adaptation of bush beans to salt-stress.     

Involvement of Wheat Germ Agglutinin (WGA)-Binding Protein in the Induction of the Acrosome Reaction of the Sea Urchin, Strongylocentrotus intermedius. II. Antibody against WGA-Binding Protein Induces the Acrosome Reaction

We obtained a polyclonal antibody against the WGA-binding protein (WGAbp) of Strongylocentrotus intermedius sperm, which is a membrane glycoprotein of 260 kD under non-reducing condition. Anti-WGAbp antibody induced increases in both intracellular Ca²⁺ ([Ca²⁺]i) and intracellular pH (pHi), resulting in the onset of the AR. The increases in [Ca²⁺]i and pHi required extracellular Ca²⁺ and Na⁺, respectively, and were suppressed by the pretreatment with WGA, resulting in the inhibition of the AR. Anti-WGAbp antibody-induced AR was inhibited also by lowered extracellular pH. elevated K⁺, removal of Na⁺ from seawater and the treatment with verapamil, a Ca²⁺ channel inhibitor. These inhibitory conditions are identical with those of the egg jelly-induced AR. Monovalent Fab fragments from anti-WGAbp antibody also induced the AR at relatively high concentration. These results suggest that the WGAbp on the sperm plasma membrane is involved in the regulation of Ca²⁺ influx and Na⁺/H⁺ exchange associated with the AR of S. intermedius sperm. It is a strong candidate for the receptor of the AR-inducing substance in the egg jelly.     

Activation of Sea Urchin Eggs by Halothane and Its Inhibition by Dantrolene

Eggs of the sea urchin, Hemicentrotus pulcherrimus , were stimulated by halothane, known to induce Ca²⁺ release from sarcosome, to cause fertilization membrane formation in normal and Ca²⁺ free artificial sea water. In the absence of external Ca²⁺, halothane-induced formation of fertilization membrane was inhibited by dantrolene, an inhibitor of Ca²⁺ release from sarcosome, but was not blocked by nifedipine, a Ca²⁺ antagonist specific to Ca²⁺ channels in plasma membrane. Ca²⁺ release from sedimentable fraction isolated from eggs was induced by halothane and was inhibited by dantrolene, but was not blocked by nifedipine. In normal artificial sea water, halothane-caused egg activation was not inhibited either by dantrolene or by nifedipine, but was blocked in the presence of both compounds. ⁴⁵Ca²⁺ influx was substantially stimulated by halothane in eggs exposed to ⁴⁵CaCl₂. Halothane-induced ⁴⁵Ca²⁺ influx into eggs was inhibited by nifedipine but was not blocked by dantrolene. When Ca²⁺ release from intracellular organellae is blocked, Ca²⁺ transport through Ca²⁺ channels in plasma membrane probably acts as a "fail-safe" system to induce an increase in cytosolic Ca²⁺ level, resulting in egg activation.     

Excretion of ions (Cd2+, Li+, Na+ and Cl−) by Tamarix aphylla

Excretion of minerals by the NaCl-resistant and comparatively cadmium-resistant tree Tamarix aphylla (L.) Karst, was investigated. Cd²⁺ was excreted by plants exposed for 1–10 days to 9 or 45 μ M Cd²⁺ solutions. Excretion of this toxic ion increased considerably with time but was less than 5% of the quantities that had been accumulated in the shoots. Excretion of Na⁺ and Cl⁻ was positively correlated with NaCl concentration (1.5, 10, 50 m M ) of the medium. The Na⁺/Cl⁻ ratios of the excrete were positively correlated with the concentration of the treatment solution. Ca²⁺ excretion decreased with increasing NaCl concentrations of the solution. Excretion of K⁺ and Mg²⁺ was only little affected by NaCl. Excretion of Li⁺ occurred whenever this element was supplied in the uptake solution; daily excretion rates of Li⁺ increased with time. The ecological significance of excretion is discussed in relation to the low selectivity of the mechanism in T. aphylla .     

Intracellular Ascorbic Acid Inhibits the Na+-Ca2+ Exchanger in Cultured Rat Astrocytes

Abstract: The effect of ascorbic acid on Ca²⁺ uptake in cultured rat astrocytes was examined in the presence of ouabain and monensin, which are considered to drive the Na⁺-Ca²⁺ exchanger in the reverse mode. Ascorbic acid at 0.1–1 m M inhibited Na⁺-dependent Ca²⁺ uptake significantly but not Na⁺-dependent glutamate uptake in the cells, although the inhibition required pretreatment for more than 30 min. The effect of ascorbic acid on the Ca²⁺ uptake was blocked by simultaneous addition of ascorbate oxidase (10 U/ml). Na⁺-dependent Ca²⁺ uptake was also inhibited by isoascorbate at 1 m M but not by ascorbate 2-sulfate, dehydroascorbate, and sulfhydryl-reducing reagents such as glutathione and 2-mercaptoethanol. The inhibitory effect of ascorbic acid was observed even in the presence of an inhibitor of lipid peroxidation, o -phenanthroline, or a radical scavenger, mannitol, and the degrading enzymes such as catalase and superoxide dismutase. On the other hand, the inhibitory effect was not observed under the Na⁺-free conditions that inhibited the uptake of ascorbic acid in astrocytes. When astrocytes were cultured for 2 weeks in a medium containing ascorbic acid, the content of ascorbic acid in the cells was increased and conversely Na⁺-dependent Ca²⁺ uptake was decreased. These results suggest that an increase in intracellular ascorbic acid results in a decrease of Na⁺-Ca²⁺ exchange activity in cultured astrocytes and the mechanism is not related to lipid peroxidation.     

Calcium and plant action potentials

Abstract. Under normal conditions the action potential in Characeae is dependent on the presence of both Cl⁻ and Ca²⁺. Cl⁻ seems to play a straightforward part as a transient depolarizing flow. The role of Ca²⁺, however, is emerging as an increasingly complex one: there are Ca²⁺ concentration changes in the cytoplasm, as well as transient Ca²⁺ currents across the plasmalemma and possibly the tonoplast. In most Characeae Ca²⁺ is necessary for the Cl⁻ channel to function, and it is also involved in the cessation of the cytoplasmic streaming observed at the time of excitation.
The function of Ca²⁺ at the time of the action potential is being revealed by experimental techniques of increasing sophistication. The development of these methods and possible associated artefacts are considered.