Study on nutrient stress tolerance of nine Taraxacum microspecies with a contrasting mineral ecology by cultivation in a low nutrient flowing solution

Growth and mineral status of 9 Taraxacum microspecies were studied under mineral stress conditions, using a flowing solution of low nutrient concentration. Relative growth rate of (whole) plant dry weight, leaf area, and (whole) plant tissue water were used to describe growth. For 4 microspecies, specific uptake rates of NO⁻₃, H₂PO⁻₄, K⁺, Mg²⁺ and Ca²⁺ were investigated.
The applied nutrient condition clearly discriminated between the studied Taraxacum microspecies. With respect to relative growth rate, 3 groups of microspecies could be distinguished: T. nordstedtii > T. lancidens, T. adamii, T. hollandicum, T. taeniatum > T. sellandii, T. eudontum, T. ekmanii, T. ancistrolobum . These categories coincided well with the mineral ecology of the microspecies, going from infertile to fertile sites.
T. nordstedtii , a microspecies of infertile sites, was most efficient in absorbing NO⁻₃, H₂PO⁻₄ and K⁺. T. sellandii and T. eudontum , both occurring in fertile grasslands, showed poor uptake performances for all studied ions. In all Taraxacum microspecies studied, except T. eudontum , internal N concentration appeared to limit growth. Efficiencies in N use, at sub-optimal internal N concentrations, varied with the mineral habitat of the microspecies studied. T. nordstedtii , from infertile sites, and T. sellandii , from fertile sites, were established as high and low extremes, respectively.     

Responses to internal nitrogen and phosphorus concentrations of two Taraxacum microspecies of contrasting mineral ecology: critical N and P concentrations of whole plants

Of the two Taraxacum microspecies used. Taraxacum sellandii Dahlst. usually occurs in grasslands with a high nutrient level; Taraxacum nordstedtii Dahlst. is generally restricted to undisturbed and mineral-poor habitats. Growth response curves for internal N and P were established, based on relative yield of (whole) plant tissue water and (whole plant) internal mineral concentration on a tissue water basis. Critical nutrient concentrations of N and P were determined from the response curves derived. For both macroelements, T. nordstedtii showed lower critical nutrient concentrations. The difference in critical N concentrations coincided with differences in internal NO₃^-₃ concentrations between the microspecies. Finally, we discuss the use of tissue water as a (whole) plant growth parameter and internal mineral concentration on tissue water basis as a parameter describing the mineral status.     

Luxury consumption and specific utilization rates of three macroelements in two Taraxacum microspecies of contrasting mineral ecology

Plants of Taraxacum sellandii Dahlst., a microspecies adapted to fertile, and Taraxacum nordstedtii Dahlst., adapted to infertile soils, were cultured hydroponically, either on a complete nutrient solution or on one deprived of nitrogen, phosphorus, or potassium ions. For all four treatments, the growth and internal mineral concentration of the plants was monitored. For plants cultured on a complete nutrient solution, the uptake rates of nitrate, phosphate, and potassium ions were determined. Luxury consumption of the three macronutrients was computed as the excess of ion absorption over the ion uptake rates minimally required to sustain maximum growth. In these calculations the critical N, P, or K⁺ concentrations, earlier derived, were used as parameters describing the mineral status minimally required to allow maximum growth. Efficiency in use of the three macroelements at various levels of mineral accumulation was also computed. Finally, the response to phosphate starvation as related to phosphate uptake capacity and the accumulation of P was investigated.
The physiological properies investigated provide a causal background for the superior adaptation of T. nordstedtii as compared to T. sellandii to infertile sites. Taraxacum nordstedtii had a higher relative luxury consumption of NO₃^–, H₂PO^-₄, and K⁺, a higher efficiency in N and P use at N– and (severe) P-deficiency, respectively; and, after phosphate starvation, a relatively high preservation of phosphate uptake capacity and an enlargement of P storage. In combination with the low potential growth, luxury consumption will be particularly effective in T. nordstedtii in preventing or minimizing mineral deficiency. The distribution of minerals between cytoplasm and vacuoles as a factor in mineral use efficiency is discussed.     

Direct Observation of Serotonin 5-HT3 Receptor-Induced Increases in Calcium Levels in Individual Brain Nerve Terminals

Abstract: Confocal microscopy was used to assess internal calcium level changes in response to presynaptic receptor activation in individual, isolated nerve terminals (synaptosomes) from rat corpus striatum, focusing, in particular, on the serotonin 5-HT₃ receptor, a ligand-gated ion channel. The 5-HT₃ receptor agonist-induced calcium level changes in individual synaptosomes were compared with responses evoked by K⁺ depolarization. Using the fluorescent dye fluo-3 to measure relative changes in internal free Ca²⁺ concentration ([Ca²⁺]_i), K⁺-induced depolarization resulted in variable but rapid increases in apparent [Ca²⁺]_i among the individual terminals, with some synaptosomes displaying large transient [Ca²⁺]_i peaks of varying size (two- to 12-fold over basal levels) followed by an apparent plateau phase, whereas others displayed only a rise to a sustained plateau level of [Ca²⁺]_i (two- to 2.5-fold over basal levels). Agonist activation of 5-HT₃ receptors induced slow increases in [Ca²⁺]_i (rise time, 15–20 s) in a subset (∼5%) of corpus striatal synaptosomes, with the increases (averaging 2.2-fold over basal) being dependent on Ca²⁺ entry and inhibited by millimolar external Mg²⁺. We conclude that significant increases in brain nerve terminal Ca²⁺, rivaling that found in response to excitation by depolarization but having distinct kinetic properties, can therefore result from the activation of presynaptic ligand-gated ion channels.     

Hypoxia-Induced Catecholamine Release and Intracellular Ca2+ Increase via Suppression of K+ Channels in Cultured Rat Adrenal Chromaffin Cells

Abstract: Hypoxia (5% O₂) enhanced catecholamine release in cultured rat adrenal chromaffin cells. Also, the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) increased within 3 min in ∼50% of the chromaffin cells under hypoxic stimulation. The increase depended on the presence of extracellular Ca²⁺. Nifedipine and ω-conotoxin decreased the population of the cells that showed the hypoxia-induced [Ca²⁺]_i increase, showing that the Ca²⁺ influx was attributable to L- and N-type voltage-dependent Ca²⁺ channels. The membrane potential was depolarized during the perfusion with the hypoxic solution and returned to the basal level following the change to the normoxic solution (20% O₂). Membrane resistance increased twofold under the hypoxic condition. The current-voltage relationship showed a hypoxia-induced decrease in the outward K⁺ current. Among the K⁺ channel openers tested, cromakalim and levcromakalim, both of which interact with ATP-sensitive K⁺ channels, inhibited the hypoxia-induced [Ca²⁺]_i increase and catecholamine release. The inhibitory effects of cromakalim and levcromakalim were reversed by glibenclamide and tolbutamide, potent blockers of ATP-sensitive K⁺ channels. These results suggest that some fractions of adrenal chromaffin cells are reactive to hypoxia and that K⁺ channels sensitive to cromakalim and glibenclamide might have a crucial role in hypoxia-induced responses. Adrenal chromaffin cells could thus be a useful model for the study of oxygen-sensing mechanisms.     

Differential Effects of the Egg Jelly Molecules FSG and SAP-I on Elevation of Intracellular Ca2+ and pH in Sea Urchin Spermatozoa

We examined the effects of two egg jelly components, a fucose sulfate glycoconjugate (FSG) and sperm-activating peptide I (SAP-I: Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly), on the intracellular pH (pH_i) and Ca²⁺ ([Ca²⁺]_i) of spermatozoa of the sea urchin Hemicentrotus pulcherrimus . FSG and/or SAP-I induced elevations of [Ca²⁺]; and pH_i in the spermatozoa at pH 8.0. At pH 8.0, a second addition of FSG did not induced further elevation of the [Ca²⁺]_i or pH_i of spermatozoa treated with FSG, but addition or FSG after SAP-I or of SAP-I after FSG induced further increases of [Ca²⁺]_i and pH_i, At pH 6.6, FSG and/or SAP-I did not induce significant elevation of the [Ca²⁺]_i, although SAP-I elevated the pH_i, its half-maximal effective concentration being 10 to 100 pM. At pH 8.0, tetraethyl-ammonium, a voltage-sensitive K⁺-channel blocker, inhibited induction of the acrosome reaction and elevations of [Ca²⁺]_i and pH_i by FSG, but did not affect those by SAP-I. These results suggest that FSG and SAP-I activate different Ca²⁺ and H⁺ transport systems.     

Influence of light and darkness and the experimental design on kinetics of K+ (86Rb) influx in roots of intact spring wheat

Seedlings of spring wheat ( Triticum aestivum L. cv. Svenno) were cultivated at 20°C in continuous light or darkness with the roots in nutrient solutions for six days. The plants were starved for K⁺ during different periods of time to produce plants with various K⁺ status. In one cultivation light-grown plants were pretreated in darkness, and vice versa, before the uptake experiment. In all experiments, roots were put in a complete nutrient medium containing 2.0 m M K⁺ radiolabelled with ⁸⁶Rb. The uptake time was varied (5, 60 or 120 min).
The K⁺ concentration in the roots, [K⁺]_root, increased during the course of the uptake experiments, especially in light and at initially low [K⁺]_root, At the same time K⁺ (⁸⁶Rb) influx in the roots decreased. The simoidal relationship obtained between K⁺ (⁸⁶Rb) influx and [K⁺]_root was affected by these changes, and Hill plots gave various Hill coefficients, n_H, depending on the duration of the uptake experiments. n_H from three apparently straight line segments of the same plot, in different [K⁺]_root - intervals, indicated a falling degree of interaction between the binding sites as [K⁺]_root increased. For the dark-grown plants negative cooperativity could not be demonstrated.     

Presynaptic Mechanism of Action of 4-Aminopyridine: Changes in Intracellular Free Ca2+ Concentration and Its Relationship to B-50 (GAP-43) Phosphorylation

Abstract: Recently we have shown that 4-aminopyridine (4-AP), a drug known to enhance transmitter release, stimulates the phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat brain synaptosomes and that this effect is dependent on the presence of extracellular Ca²⁺. Hence, we were interested in the relationship between changes induced by 4-AP in the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) and B-50 phosphorylation in synaptosomes. 4-AP (100 μ M ) elevates the [Ca²⁺]_i (as determined with fura-2) to approximately the same extent as depolarization with 30 m M K⁺ (from an initial resting level of 240 n M to ∼480 n M after treatment). However, the underlying mechanisms appear to be different: In the presence of 4-AP, depolarization with K⁺ still evoked an increase in [Ca²⁺]_i, which was additive to the elevation caused by 4-AP. Several Ca²⁺ channel antagonists (CdCl₂, LaCl₃, and diphenylhydantoin) inhibited the increase in B-50 phosphorylation by 4-AP. It is interesting that the increase in [Ca²⁺]_i and the increase in B-50 phosphorylation by 4-AP were attenuated by tetrodotoxin, a finding pointing to a possible involvement of Na⁺ channels in this action. These results suggest that 4-AP (indirectly) stimulates both Ca²⁺ influx and B-50 phosphorylation through voltage-dependent channels by a mechanism dependent on Na⁺ channel activity.     

Endothelin-1 Increases Arachidonic Acid Release in C6 Glioma Cells Through a Potassium-Modulated Influx of Calcium

Abstract: Endothelin-1 (Et-1) but not a range of other receptor agonists stimulated the release of arachidonic acid (AA) in C6 glioma. Et-1 activation was concentration dependent and was inhibited by chelation of extracellular calcium. The calcium ionophores A23187 and ionomycin could also stimulate release of AA. Et-1 caused an early increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) followed by a sustained but lower plateau level. The sensitivity of the response to quinacrine, its dependence on Ca²⁺, and the demonstration of an increase in phospholipase A₂ (PLA₂) activity that was insensitive to dithiothreitol suggested that the release of AA was due to activation of cytosolic PLA₂ in the cells. Staurosporine, a protein kinase C (PKC) inhibitor, had no effect on Et-1-induced AA release but abolished that by phorbol 12-myristate 13-acetate, demonstrating that the Et-1 response was PKC independent. Raised levels of extracellular KCI inhibited both AA release and the increase in [Ca²⁺]_i triggered by Et-1, whereas valinomycin, which causes K⁺ efflux, not only caused a rapid rise in [Ca²⁺]_i but also caused AA mobilisation. The results therefore suggest that Et-1 activation of PLA₂ in this cell type requires calcium influx dependent on K⁺ efflux.     

Tachykinins Potentiate N-Methyl-D-Aspartate Responses in Acutely Isolated Neurons from the Dorsal Horn

Abstract: Substance P and neurokinin A both potentiated N -methyl- d -aspartate (NMDA)-induced currents recorded in acutely isolated neurons from the dorsal horn of the rat. To elucidate the mechanism underlying this phenomenon, we measured the effects of tachykinins and glutamate receptor agonists on [Ca²⁺]_i in these cells. Substance P, but not neurokinin A, increased [Ca²⁺]_i in a subpopulation of neurons. The increase in [Ca²⁺]_i was found to be due to Ca²⁺ influx through voltage-sensitive Ca²⁺ channels. Substance P and neurokinin A also potentiated the increase in [Ca²⁺]_i produced by NMDA, but not by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, or 50 m M K⁺. Phorbol esters enhanced the effects of NMDA and staurosporine inhibited the potentiation of NMDA effects by tachykinins. It is concluded that activation of protein kinase C may mediate the enhancement of NMDA effects by tachykinins in these cells. However, the effects of tachykinins on [Ca²⁺]_i can be dissociated from their effects on NMDA receptors.     

Nitric Oxide Disrupts Ca2+ Homeostasis in Hippocampal Neurons

Abstract: Nitric oxide has been recognized in recent years as an important mediator of neuronal toxicity, which in many cases involves alterations of the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i). In [Ca²⁺]_i fluorimetric experiments on cultured hippocampal neurons, the nitric oxide-releasing agent S -nitrosocysteine produced a delayed rise in [Ca²⁺]_i over a 20-min exposure, which was accompanied by a progressive slowing of the kinetics of recovery from depolarization-induced [Ca²⁺]_i transients. These effects were blocked by oxyhemoglobin and by superoxide dismutase, confirming nitric oxide as the responsible agent, and suggesting that they involved peroxynitrite formation. Similar alterations of [Ca²⁺]_i homeostasis were produced by the mitochondrial ATP synthase inhibitor oligomycin, and when an ATP-regenerating system was supplied via the patch pipette in combined whole-cell patch-clamp-[Ca²⁺]_i fluorimetry experiments, S -nitrosocysteine had no effect on the resting [Ca²⁺]_i or on the recovery kinetics of [Ca²⁺]_i transients induced by direct depolarization. We conclude that prolonged exposure to nitric oxide disrupts [Ca²⁺]_i homeostasis in hippocampal neurons by impairing Ca²⁺ removal from the cytoplasm, possibly as a result of ATP depletion. The resulting persistent alterations in [Ca²⁺]_i may contribute to the delayed neurotoxicity of nitric oxide.     

The Metabotropic Glutamate Receptor mGluR5 Induces Calcium Oscillations in Cultured Astrocytes via Protein Kinase C Phosphorylation

Abstract: The metabotropic glutamate receptor mGluR5, but not the closely related mGluR1, is expressed in cultured astrocytes, and this expression is up-regulated by specific growth factors. We investigated the capability and underlying mechanisms of mGluR5 to induce oscillatory responses of intracellular calcium concentration ([Ca²⁺]_i) in cultured rat astrocytes. Single-cell [Ca²⁺]_i recordings indicated that an mGluR-selective agonist, (1 S ,3 R )-1-aminocyclopentane-1,3-dicarboxylate (1 S ,3 R -ACPD), elicits [Ca²⁺]_i oscillations in good agreement with the growth factor-induced up-regulation of mGluR5 in cultured astrocytes. A protein kinase C (PKC) inhibitor, bisindolylmaleimide I, converted a 1 S ,3 R -ACPD-mediated oscillatory response into a nonoscillatory response. In addition, the PKC activator phorbol 12-myristate 13-acetate completely abolished the [Ca²⁺]_i increase. These and other pharmacological properties of 1 S ,3 R -ACPD-induced [Ca²⁺]_i oscillations correlate well with those of the cloned mGluR5 characterized in heterologous expression systems. Furthermore, the potential involvement of protein phosphatases in [Ca²⁺]_i oscillations is suggested. The present study demonstrates that mGluR5 is capable of inducing [Ca²⁺]_i oscillations in cultured astrocytes and that phosphorylation/dephosphorylation of mGluR5 is critical in [Ca²⁺]_i oscillations, analogous to the cloned mGluR5 expressed in heterologous cell lines.     

The self-incompatibility response in Papaver rhoeas is mediated by cytosolic free calcium

The role of Ca²⁺ signalling during the self-incompatibility (SI) response in Papaver rhoeas L. has been investigated using Ca²⁺-sensitive dyes. Pollen tubes were micro-injected with Calcium Green-1 and cytosolic free calcium ([Ca²⁺]_i) imaged using laser scanning confocal microscopy (LSCM). Addition of incompatible stigmatic S-glycoproteins induced a transient increase in the level of [Ca²⁺]_i in pollen tubes. In contrast, no rise in [Ca²⁺]_i was detectable after addition of either compatible or heat-denatured incompatible stigmatic S-glycoproteins. The elevation of [Ca²⁺]_i was followed by the specific inhibition of pollen tube growth in incompatible reactions. It has been shown previously that gene expression in pollen tubes is switched on during an incompatible reaction. Since the [Ca²⁺]_i transient appeared to originate from the region where the nuclei are located, Ca²⁺ may be involved in locally regulating the expression of these genes. The photoactivation of caged Ca²⁺ to artificially elevate [Ca²⁺]_i resulted in the inhibition of pollen tube growth and thus mimicked the SI response. Taken together, the results provide an important link between a transient rise in [Ca²⁺]_i and the biological phenomenon of inhibition of pollen tube growth and demonstrate, for the first time, direct evidence that the SI response in P. rhoeas is mediated by [Ca²⁺]_i.     

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.     

Nerve Growth Factor, Epidermal Growth Factor, and Insulin Differentially Potentiate ATP-Induced [Ca2+]i Rise and Dopamine Secretion in PC12 Cells

Abstract: To study how growth factors affect stimulus-secretion coupling pathways, we examined the effects of nerve growth factor (NGF), epidermal growth factor (EGF), and insulin on ATP-induced [Ca²⁺]_i rise and dopamine secretion in PC12 cells. After a 4-day incubation of cells, all three factors increased ATP-induced dopamine secretion significantly. We then examined which step of ATP-induced secretion was affected by the growth factors. Cellular levels of dopamine-β-hydroxylase and catecholamines were increased by NGF treatment but were not affected by EGF or insulin. The ATP-induced [Ca²⁺]_i rise was also enhanced after growth factor treatment. The EC₅₀ of ATP for inducing [Ca²⁺]_i rise and dopamine secretion was increased by NGF treatment but not by treatment with EGF or insulin. Accordingly, the dependence on [Ca²⁺]_i of dopamine secretion was increased significantly only in NGF-treated cells. Our results suggest that for EGF- and insulin-treated PC12 cells, the increase in secretion is mainly due to increased potency of ATP in inducing [Ca²⁺]_i rise. NGF treatment not only increased the potency of ATP but also decreased the Ca²⁺ sensitivity of the secretory pathway, which as a result becomes more tightly regulated by changes in [Ca²⁺]_i.     

Inhibition of Bradykinin-Induced Cytosolic Ca2+ Elevation by Muscarinic Stimulation Without Attenuation of Inositol 1,4,5-Trisphosphate Production in Human Neuroblastoma SK-N-BE(2)C Cells

Abstract: Cross talk between two phospholipase C (PLC)-linked receptor signalings was investigated in SK-N-BE(2)C human neuroblastoma cells. Sequential stimulation with two agonists at 5-min intervals was performed to examine the interaction between muscarinic and bradykinin (BK) receptors. Pretreatment of cells with a maximal effective concentration (5 µ M ) of BK did not affect the subsequent carbachol (CCh)-induced [Ca²⁺]_i rise, but CCh (1 m M ) pretreatment completely abolished the BK-induced [Ca²⁺]_i rise without inhibition of BK-induced inositol 1,4,5-trisphosphate (IP₃) generation. Thapsigargin (1 µ M ) pretreatment abolished the subsequent BK- and CCh-induced [Ca²⁺]_i rise, though it did not affect agonist-induced IP₃ generation. However, the addition of atropine at plateau phases of CCh-induced [Ca²⁺]_i rise and IP₃ production caused a rapid decline to the basal levels and then restored the [Ca²⁺]_i rise by BK. Treatment of cells with both CCh and BK at the same time showed additive effects in IP₃ production. However, the [Ca²⁺]_i rise induced by both agonists in the presence or absence of extracellular Ca²⁺ was the same as the responses triggered by CCh alone. The results suggest that each receptor or receptor-linked PLC activity is not influenced by pretreatment with the other agonist but IP₃-sensitive Ca²⁺ stores are shared by signal pathways from both receptors.     

Effects of Calcium Channel Antagonists on Calcium Entry and Glutamate Release from Cultured Rat Cerebellar Granule Cells

Abstract: Using a range of Ca²⁺ channel blockers we have investigated the Ca²⁺ channel subtypes that mediate the depolarisation-induced elevation of the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) and glutamate release from cultured rat cerebellar granule cells. ω-Conotoxin-GVIA had little effect on either the transient or plateau phase of the depolarisation-induced [Ca²⁺]_i rise or on glutamate release, ruling out a significant role for N-type Ca²⁺ channels. Nifedipine substantially inhibited the initial transient rise in [Ca²⁺]_i and the plateau phase of the [Ca²⁺]_i rise and glutamate release, suggesting the involvement of L-type Ca²⁺ channels. Both ω-agatoxin and ω-conotoxin-MVIIC also inhibited the transient rise in [Ca²⁺]_i and glutamate release but not the plateau phase of the [Ca²⁺]_i rise. The inhibitions by nifedipine were not increased by coaddition of ω-conotoxin-MVIIC, suggesting overlapping sensitivity to these channel blockers. These data show that glutamate release from granule cells in response to depolarisation with a high KCI level involves Ca²⁺ currents that are sensitive to nifedipine, ω-agatoxin-IVA, and also ω-conotoxin-MVIIC. The overlapping sensitivity of the channels to these toxins prevents attribution of any of the phases of the [Ca²⁺]_i rise or glutamate release to distinct P-, Q-, or O-type Ca²⁺ currents.     

Protein Phosphorylation and Calcium Uptake into Rat Forebrain Synaptosomes: Modulation by the σ Ligand, 1,3-Ditolylguanidine

Abstract: The σ ligand 1,3-di- O -tolylguanidine (DTG) increased basal dynamin and decreased depolarization-stimulated phosphorylation of the synaptosomal protein synapsin Ib without having direct effects on protein kinases or protein phosphatases. DTG dose-dependently decreased the basal cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) and blocked the depolarization-dependent increases in [Ca²⁺]_i. These effects were inhibited by the σ antagonists rimcazole and BMY14802. The nitric oxide donors sodium nitroprusside (SNP) and 8-( p -chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate decreased basal [Ca²⁺]_i and the KCl-evoked rise in [Ca²⁺]_i to an extent similar to DTG. SNP, but not DTG, produced a rise in cyclic GMP levels, suggesting that the effect of DTG on [Ca²⁺]_i was not mediated via downstream regulation of cyclic GMP levels. DTG increased ⁴⁵Ca²⁺ uptake and efflux under basal conditions and inhibited the ⁴⁵Ca²⁺ uptake induced by depolarization with KCl. The KCl-evoked rise in [Ca²⁺]_i was inhibited by ω-conotoxin (ω-CgTx)-GVIA and -MVIIC but not nifedipine and ω-agatoxin-IVA. The effect of DTG on decreasing the KCl-evoked rise in [Ca²⁺]_i was additive with ω-CgTx-MVIIC but not with ω-CgTx-GVIA. These data suggest that DTG was producing some of its effects on synapsin I and dynamin phosphorylation and intrasynaptosomal Ca²⁺ levels via inhibition of N-type Ca²⁺ channels.     

Effect of K+ and H+ stress and role of Ca2+ in the regulation of intracellular K+ concentration in mung bean roots

The effects of external K⁺, H⁺ and Ca2⁺ concentrations on the intracellular K+ concentration, [K⁺]i, and the K⁺-ATPase activity in 2-day-old mung bean roots [ Vigna mungo (L.) Hepper] were investigated. [K⁺]i, in mung bean roots was markedly decreased by external K⁺ or H⁺ stress and did not recover the initial value even after the stress was removed. This decrease in [K⁺]i, gradually disappeared with the addition of (Ca2⁺. Ca2⁺ may offset the harmful effects of ion stress. Ca2⁺ seems to have two effects on K+ transport; control of K⁺ permeability and activation of K⁺ uptake, although K⁺-ATPase activity was inhibited by Ca2⁺ concentrations higher than 10–4 M. We suggest that Ca2⁺ activates K⁺ uptake indirectly through the acidification of the cytoplasm.     

K/Na selectivity at the plasmalemma of cortical root cells and preferential release of sodium to the xylem vessels in roots of Atriplex hortensis

Using excised roots of Atriplex hortensis L., cv. Gelbe Gartenmelde, the uptake, accumulation and xylem transport of K⁺ and Na⁺ have been measured. Influx as well as xylem transport proved to discriminate little between K⁺ and Na⁺, when considered in relation to the external solution. Both K⁺ and Na⁺ inhibited the uptake and xylem transport of each other to about the same degree. Measurements of intracel-lular Na⁺ fluxes by means of compartment analysis indicated that the low degree of K/Na discrimination during uptake was due to low influx selectivity. Moreover, K⁺/Na⁺ exchange at the plasmalemma was not very efficient in Atriplex roots. In order to establish the basis of the low K/Na discrimination in xylem transport, the rates of K⁺ and Na⁺ transport were related to the cytoplasmic K⁺ and Na⁺ concentrations to yield the selectivity ratio of transport, S(transport) = (φ_cx(K) × [Na⁺]_c)/(φ_cx(Na) × [K⁺]_c). Under all conditions this ratio was far below one indicating that Na⁺ was favoured during xylem release in excised roots of Atriplex at low external concentrations. The implications of this discrimination in favour of Na+ are discussed with respect to salt tolerance of A. hortensis .