High External Concentrations Research Articles

Apical Membrane Permeability of Mytilus Gill: Influence of Ultrastructure, Salinity and Competitive Inhibitors on Amino Acid Fluxes

ABSTRACT The apical membrane of gill integumental cells from the mussels Mytilus edulis and M. californianus serves as a permeability barrier separating sea water from a cytoplasm rich in amino acids and other small organic molecules. Morphometric analysis of transmission electronmicrographs indicates that the membrane area of these cells is increased between 10-and 18-fold by the presence of a microvillous brush border. The microvilli do not appear to influence the kinetics of solute transport across the cell apex, as determined using a mathematical model of the relationship between membrane structure and the kinetics of transport. Rates of amino acid loss from the integument were low, and estimates of the upper limit of the passive permeability of the apical membrane to amino acids ranged from 0·5 to 10 × 10−10 cm s−1. Abrupt exposure of intact mussels or isolated gill tissue to 60 % sea water (19%o salinity) resulted in a transient, 40-to 80-fold increase in the rate of loss of all amino acids from integumental tissues. Upon exposure to full-strength sea water, efflux rates returned to near control values. Exposure to 60 % sea water also inhibited the carrier-mediated accumulation of amino acid: uptake of 0·5 μmol l−1 [l4C]alanine and [14C]taurine was reduced by 80% compared to control uptake in 100% sea water. This inhibition was not adequate to account for the increase in net efflux of taurine from gill tissue into 60% artificial sea water (ASW), though the inhibition of alanine uptake may have contributed significantly to the increased loss of this amino acid. Efflux of discrete structural classes of amino acid occurred when integumental tissues were exposed to 50 μmol l−1 concentrations of structurally related analogues. It is concluded that the apical membrane of gill cells has a very low passive permeability to amino acids, and that the overall permeability of the gill can be increased in a reversible fashion by exposure to reduced salinity or to high external concentrations of amino acid.

QUNTITATIVE ION DISTRIBUTION WITHIN ROOT CELLS OF SALT‐SENSITIVE AND SALT‐TOLERANT MAIZE VARIETIES

Significant varietal differences were apparent in the survival of seedlings of maize in saline conditions but only at relatively high external concentrations (200 mol m-3 NaCl), where there was a range from 0 to 66% survival, 25 d after salinization. For the varieties examined there was a strong negative correlation between Na concentrations in the third leaf and survival. Two resistant varieties (Across 8024 and Protador) and one salt-sensitive variety (LG11 ) were identified. The characteristics of ion accumulation were clearly different in salt-tolerant and salt-sensitive types, the difference becoming more pronounced with plant age. The distribution of ions, particular those of Na, K and Cl, was determined within subcellular compartments of roots cells using X-ray microanalysis of freeze-substituted tissue. Salinity induced a greater increase (about 1.7 times) in cytoplasmic Na concentration in the salt-sensitive variety (LG11 ) than in resistant varieties (Across 8024 or Protador). The mean K:Na ratio in the cytoplasm of the root cortical cells in the salt-resistant varieties grown for 15 d in saline conditions (100 mol m-3 NaCl) was twice that found for LG11 . Sodium and Cl concentrations in the vacuoles decreased radially inwards from the epidermal cells in salt-treated roots of LG11 .

Osmotic and ionic regulation in marine littoral Collembola

A comparative study was made on the osmotic and ionic regulation of three species of marine littoral Collembola: the grassland species Isotoma viridis, the salt-marsh Hypogastrura viatica and the intertidal Anurida maritima. Both I. viridis and H. viatica hyper-regulate their osmotic and ionic concentration of the haemolymph at low salt concentrations of the soil water (<50% seawater). At higher external concentrations they behave like osmoconformers. A. maritima is an osmoconformer. It has lost the ability of hyper-regulation, resulting in a high mortality at low salinities. This species is a unique example of a terrestrial arthropod, which has become physiologically tied to a saline environment. Comparison of an inland and a salt-marsh population of I. viridis showed that at freshwater conditions the former population maintains a lower osmotic and ionic concentation than the latter population. The haemolymph of the Collembola consists not only of electrolytes, but also of neutral organic osmotically active solutes, which results in a compromise between the conflicting requirements of ionic concentration and water balance at high external salinities. The degree of volume regulation can be described by a single parameter. It is argued that most water is lost from the haemolymph during osmotic stress. Free amino acid analysis in A. maritima showed an increase of particularly glutamine, alanine and proline at rising salinities.

The Membrane Potential in a Cytochrome-deficient Species of Bacteroides: Its Magnitude and Mode of Generation

The lipophilic cations butyltriphenylphosphonium (BTPP+), tetraphenylphosphonium (TPP+) and triphenylmethylphosphonium (TPMP+) were taken up into cells of Bacteroides amylophilus H18 under anaerobic conditions. Uptake was dependent on the presence of maltose together with both HCO− 3 and Na+; it was at a maximum at concentrations of ≥ 20 mm-HCO− 3 and ≥2 mm-Na+. The addition of 2-(n-heptyl)-hydroxyquinoline-N-oxide (HpHOQnO) or of an uncoupler of oxidative phosphorylation, or air, resulted in efflux of the lipophilic cations. From the binding behaviour and the physiological effects of the lipophilic cations, a membrane potential (Δψ) of 140 mV was estimated. There was no detectable ΔpH at an external pH of 6·7. The cytoplasmic Na+concentration was estimated to be 0·2 mm, indicating that B. amylophilus can maintain a Na+concentration gradient equivalent to at least 150 mV. Variation in the external Na+concentration (2 to 180 mm) had little influence on Δψ. High external concentrations of the fermentation products acetate, formate and succinate had little effect on the growth rate and the Δψ. The cytoplasmic ATP concentration decreased rapidly on addition of HpHOQnO oxide or of an uncoupler. The maximum internal ATP concentration was only maintained at an external concentration≥2mm-Na+. The NADH: fumarate reductase activity of vesicles of B. amylophilus was associated with alkalization of the suspension medium. The amount of H+taken up was in excess of the expected amount of scalar H+and was partially sensitive to uncoupler. It is concluded that the Δψ was generated via H+translocation driven primarily by the cytochrome-deficient NADH: fumarate reductase system. The transmembrane Na+gradient could be supported via the action of a Na+/2 H+antiporter.

Mercury exchanges and toxicity in the crystalline lens in vitro.

Mercury, (Hg2+, using 203Hg2+ as a tracer) was accumulated by the rabbit lens in vitro in a concentration-dependent manner. Saturation levels for the metal ion were greater at higher external concentrations of Hg2+ and exceeded those in the bathing media. Autoradiographic examination showed that the 203Hg was concentrated in the anterior epithelium and lens bow. Release of accumulated 203Hg was enhanced by external Hg2+ and dithiothreitol but was reduced by lanthanum (La3+). Lanthanum also increased net accumulation of Hg2+. The Hg2+ (10(-4) M for 24 h) appeared to be toxic as shown by excessive accumulation of sodium and loss of potassium by the lens in vitro. Mercuric ions appear to leave and enter the lens by diffusion, a process which may involve anionic pathways on its anterior surface where it is principally bound in the anterior epithelial cells.

Sodium Fluxes, Xylem Transport of Sodium, and K/Na Selectivity in Roots of Seedlings of Hordeum vulgare, cv California Mariout and H.distichon, cv.Villa.

Summary Sodium fluxes were measured by compartmental analysis in roots of intact seedlings of Hordeum vulgare , cv.California Mariout and H distichon , cv.Villa in the concentration range 0.1 to 50 mol m -3 Na + .Apart from xylem transport all sodium fluxes and the cytoplasmic and vacuolar sodium content were higher in Villa than in Mariout, which is known for its high salt tolerance.This was also true for K + -dependent Na + extrusion (KINa exchange) across the plasmalemma, indicating that a higher degree of KINa selectivity at the plasmalemma is not per se decisive for salt tolerance in barley.At high external concentrations xylem transport of Na + was substantially higher in Mariout than in Villa.For both barley species root to shoot sodium transport was almost independent of external concentration between 10 and 50 mol m -3 NaCl and thus appears to be tightly controlled.

Malic Acid Production in Callus Cultures of Zinc and Lead Tolerant and Non-tolerant Anthoxanthum odoratum

Summary Callus cultures derived from two zinc and lead tolerant clones and one non-tolerant clone of Anthoxanthum odoratum were grown in medium containing high concentrations of zinc (20 mg· 1 -1 ), lead (25 mg · 1 -1 ), or copper (1 mg · 1 -1 ), then the malic acid levels measured after three weeks. High concentrations of zinc caused an increase to twice the control level of malic acid in both tolerant and non-tolerant clones. Lead also caused an increase in malic acid levels in all the three clones whereas in the presence of copper there was a small increase in malic acid in the non-tolerant clone and a reduction in the two zinc and lead tolerant clones. It was concluded that the accumulation of malic acid in all clones occurred as a response to high external concentrations of metal. The accumulation of malic acid was not seen to have a primary role in the mechanism of metal tolerance.

Polyamine Uptake, Kinetics, and Competition among Polyamines and between Polyamines and Inorganic Cations

Polyamine uptake, the kinetics of this uptake, and the competition among polyamines and between polyamines and inorganic cations were studied in petals of Saintpaulia ionantha Wendl. Uptake experiments using (14)C-labeled polyamines were carried out on single petals, at room temperaure (20 degrees C) and in the light. The results show that putrescine, spermidine, and spermine uptake was dependent on the external pH and occurred up to high external polyamine concentrations with K(m) values of 8.6, 1.2, and 2.1 millimolar, respectively, with spermidine being the most absorbed at low concentration (17 micromolar). Putrescine and spermidine did not seem to compete for the same site of absorption. Furthermore, putrescine and spermidine uptake was not inhibited by Ca(2+), Mg(2+), and K(+) at the same concentrations (17 micromolar), whereas 1.7 millimolar Ca(2+) inhibited and K(+) enhanced spermidine uptake. The intracellular localization of the absorbed putrescine was determined using two different methods. Very little label was found in the apoplast, while most of it was localized in the 98,500g supernatant. According to our data the vacuole, which represents a substantial part of Saintpaulia parenchyma cells, could be a site of putrescine accumulation. 2,4-Dinitrophenol and diethylstilbestrol did not inhibit uptake; however, at 0 degrees C there was a 35% inhibition of spermidine uptake, compared with the controls kept at 20 degrees C as well as a 68% inhibition with 20 millimolar NaSCN.

Inhibition of Ca 2+-activated K + channels in pig pancreatic acinar cells by Ba 2+, Ca 2+, quinine and quinidine

Patch-clamp whole-cell and single-channel current recordings were made from pig pancreatic acinar cells to test the effects of quinine, quinidine, Ba 2+ and Ca 2+. Voltage-clamp current recordings from single isolated cells showed that high external concentrations of Ba 2+ or Ca 2+ (88 mM) abolished the outwad K + currents normally associated with depolarizing voltage steps. Lower concentrations of Ca 2+ only had small inhibitory effects whereas 11 mM Ba 2+ almost blocked the K + current. 5.5 mM Ba 2+ reduced the outward K + current to less than 30% of the control value. Both external quinine and quinidine (200–500 μM) markedly reduced whole-cell outward K + currents. In single-channel current studies it was shown that external Ba 2+ (1–5 mM) markedly reduced the probability of opening of high-conductance Ca 2+ and voltage-activated K + channels whereas internal Ba 2+ (6 · 10 −6 to 3 · 10 −5 M) caused activation at negative membrane potentials and inhibition at positive potentials. Quinidine (200–400 μM) evoked rapid chopping of single K + channel openings acting both from the outside and inside of the membrane and in this way markedly reduced the total current passing through the channels.

Role of calcium mobilization in mediation of acetylcholine‐evoked chloride currents in Xenopus laevis oocytes.

The involvement of Ca ions in the mediation of muscarinic Cl- current responses in Xenopus oocytes was studied using the voltage-clamp technique and direct measurements of 45Ca efflux. The injection of Ca into the oocytes produced a dose-dependent transient inward (depolarizing) current carried by Cl. This current was occasionally followed by a second, long-lasting inward current. The muscarinic response was evoked by the application of acetylcholine (ACh). It consisted of a transient inward current response, and a long-lasting inward current response, both inward currents carried by Cl ions. Both responses were inhibited by intracellular injection of ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetraacetic acid (EGTA), the long-lasting response being inhibited faster than the transient response. The calmodulin inhibitor, trifluoperazine, inhibited both the Cl-current responses to ACh and to Ca injection. ACh (10 microM) evoked a release of 45Ca from pre-loaded oocytes. This effect was inhibited by atropine (1 microM). In the absence of external Ca, the muscarinic transient and long-lasting responses were partially inhibited. The long-lasting response was more sensitive to the external Ca depletion than the transient response. Repetitive applications of ACh in the absence of external Ca resulted in a progressive decrease in the response amplitudes. Under these conditions, a temporary exposure to normal Ca solution ('Ca window') resulted in a partial recovery of the response amplitudes. The muscarinic inward current responses were not inhibited by nifedipine (20 microM). In the presence of a high external concentration of Mn ions ([Mn]o = 18 mM), the transient response was potentiated. Subsequent applications of ACh in high [Mn]o resulted in progressively decreasing responses. It is concluded that the muscarinic Cl responses in Xenopus oocytes are mediated by an increase in the intracellular free Ca activity, aiCa. Ca ions involved in the mediation of the muscarinic Cl current responses are released from cellular Ca stores. It is also proposed that the transient and long-lasting responses result from the release of Ca from two different stores.

The neurotoxicity of excitatory amino acids is produced by passive chloride influx

In the 15 years since the neurotoxic properties of glutamate and related amino acids were first described, there has been no thoroughly convincing explanation of the pathophysiology of excitatory amino acid-induced neuronal death. These substances depolarize central neurons, increase the frequency of neuronal discharge, and augment synaptic activity, leading to the suggestion that one or more of these properties may in some way be responsible for toxicity. More recently, an excessive calcium influx triggered by amino acids has been implicated in this process. As isolation of the different factors potentially involved in amino acid neurotoxicity is virtually impossible in vivo, dispersed hippocampal cultures were used to define the pathophysiology of this process in vitro. The toxicity of glutamate, N-methyl-D-aspartate, and kainate was unaffected when calcium was deleted and tetrodotoxin added to the balanced salt solution bathing the cultures. In parallel experiments, the calcium ionophore A23187 was not toxic in the presence of calcium. These experiments failed to confirm a role for neuronal activity or calcium influx in this process. However, when depolarization was blocked by deleting sodium from the control salt solution, neither glutamate, N-methyl-D-aspartate, nor kainate produced obvious changes. Alternately, when passive chloride influx was prevented by largely deleting chloride from the bath, the cells were also unchanged by the amino acids. Further experiments showed that depolarization produced by high external potassium concentrations or veratridine was also toxic, but only in the presence of external chloride. These experiments suggest that the pathophysiology of amino acid neurotoxicity may be rather straightforward.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of naphthalene and aqueous crude-oil extracts on the green flagellate Chlamydomonas angulosa. VI. Phosphate uptake and retention

Chlamydomonas angulosa when grown in normal Bold's basal medium (BBM) (1.7 mM Pi) has a very high cell P content (120 fg-at. P∙cell−1) and low C: P atomic ratio (35: 1) and is only able to show net Pi uptake in media with Pi concentrations higher than 0.1 mM when grown in the light. The presence of light enhances net phosphate uptake or decreases net phosphate loss under all external Pi concentrations. Cells transferred to low-Pi media will, however, grow rapidly while reducing their cell P to almost 1/10 of the initial level, indicating that most of the cell P in cells grown in high-P media represents reserve P, probably in the form of polyphosphates. The presence of naphthalene and crude-oil components in the culture media decreases net uptake of Pi at high external Pi concentrations and increases the rate of Pi loss at low external concentrations. Naphthalene present at 100% saturation level in BBM causes rapid loss of a large fraction of cell P under all conditions tested. The phosphorus lost appears as reactive phosphorus in the medium, suggesting that cell polyphosphates are rapidly converted to Pi, which then leaks out of the cells in response to the presence of saturating naphthalene concentrations in the medium. BBM with 50% naphthalene saturation causes much less loss of cell P under most of the incubation conditions, and net uptake can take place at the highest external Pi concentrations tested in the light. Media saturated with crude oil are even less effective than 50% naphthalene saturated media in causing P efflux, or in reducing net Pi uptake, although at low external P concentrations, uptake of P is decreased compared with the control.

Optical recording of membrane potential responses from early embryonic chick ganglia using voltage-sensitive dyes

Changes in absorbance of voltage-sensitive merocyanine-rhodanine dyes were used to monitor electrical responses in the semilunar ganglion of 4–10-day-old developing chick embryos. The electrical responses were simultaneously recorded from many positions in the ganglion. Stimulation of the afferent nerve fibers (the ophthalmic division of cranial nerve V) with a suction electrode led to changes in light absorption of the stained ganglia. With both the depolarizing and hyperpolarizing pulses, the change was largest at 700 nm and was eliminated at a wavelength of 620 nm where the voltage-dependent absorption change of the dyes disappears. In the 4–10-day-old embryonic ganglia, two types of optical membrane potential responses, ‘non-conducted’ and ‘conducted’ responses, were identified. The non-conducted response varied with the intensity of the stimulus and had the nature of an electrotonic spread. Furthermore, this non-conducted response exhibited an ‘initial upstroke-response’ followed by the steady-state plateau evoked by larger depolarizing pulses. The conducted responses were blocked by tetrodotoxin (TTX) or by high external potassium concentration. The incidence of the conducted responses increased as development proceeded from the 5th to the 10th day of age. Thus, the TTX-sensitive action potential activity is probably generated initially in the semilunar ganglion during the 5–10-day-stage of development. These data represent the first demonstration of membrane potential responses in early embryonic intact nervous system. Furthermore, these studies demonstrate the usefulness of voltage-sensitive dyes in the analysis of the organizing process of embryonic neuronal functions during these early stages of development.

Accumulation of pollutants in fish

1. 1. The amount of radioactivity which derived from 14C-labeled pollutants was determined in liver, kidney, intestine, blood, muscle and gills of carp, exposed for 6, 24 and 72 hr to high external concentrations of urea, methanol, atrazine and PCP. 2. 2. The results allowed one to calculate roughly the uptake rate for these compounds. It was low for urea (0.055 μg/g per hr), higher for methanol (0.12) and atrazine (0.16) and highest for PCP (1.5). 3. 3. The bioaccumulation factors (BFs) were determined for the different substances and organs. They correlated with the hydrophilic-lipophilic nature of the chemicals. The more lipophilic the substances the more accumulation occurred in the liver. 4. 4. PCP accumulated the most. BF was 300–400 in most tissues except muscle where it was quite low. The BF was 3–4 for atrazine in liver, kidney and intestine, but just 1 in blood, muscle and gills. 5. 5. There is some evidence that the BF for methanol equals 1 in liver, kidney, gills and intestine. It is less than 1 in blood and muscle. Urea was equally distributed in all organs and in the external medium.

Amine Transport in Riccia fluitans

The cytoplasmic and vacuolar pH and changes thereof in the presence of ammonia (NH(4)Cl) and methylamine (CH(3)NH(3)Cl) have been measured in rhizoid cells of Riccia fluitans by means of a pH-sensitive microelectrode.On addition of 1 micromolar NH(4)Cl, the cytoplasmic pH of 7.2 to 7.4 drops by 0.1 to 0.2 pH units, but shifts to pH 7.8 in the presence of 50 micromolar NH(4)Cl or 500 micromolar CH(3)NH(3)Cl. The pH of the vacuole increases drastically from 4.5 to 5.7 with these latter concentrations. Since a NH(4) (+)/CH(3)NH(3) (+) uniporter has been demonstrated in the plasmalemma of R. fluitans previously (Felle 1983 Biochim Biophys Acta 602:181-195), the concentration-dependent shifts of cytoplasmic pH are interpreted as results of two processes: first, acidification through deprotonation of the actively transported NH(4) (+); and second, alkalinization through protonation of NH(3) which is taken up to a significant extent from high external concentrations. Furthermore, it is concluded that the determination of intracellular pH by means of methylamine distribution is not a reliable method for eucaryotic systems.

Calcium in plant senescence and fruit ripening

Abstract. Calcium has long been associated with regulation of the ripening process of fruit and post‐harvest storage life. Specifically, maintenance of relatively high calcium concentrations in fruit tissue results in a slower rate of ripening, as seen in lower respiration rates, reduced ethylene production, and slower softening of fruit flesh. There are also some specific fruit disorders such as bitter pit, which can be prevented if sufficient calcium is present. Senescence of other plant tissues such as leaves and flowers has also been shown to be retarded by the application of calcium.Work leading to the above information is reviewed and discussed in the context of what is currently known of cellular regulation of calcium in plants. The major sites for the action of calcium in senescence and ripening are suggested to be in membrane structure and function, and in cell wall structure. Although high external concentrations of calcium are an advantage in reducing the rate of senescence or ripening, it is emphasized that normal cell function requires the maintenance of low concentrations of free calcium in the cell cytosol. It is suggested that one possible feature of senescence is a breakdown in such cellular regulation.

Effects of 1,25- and 24,25-dihydroxycholecalciferol on parathyroid hormone release from human parathyroid cells in vitro.

The effects of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) and 24,25-dihydroxycholecalciferol (24,25-(OH)2D3) on parathyroid hormone (PTH) release from human parathyroid cells were investigated using an in vitro system of dispersed cells. The cells were obtained from 7 patients with primary hyperparathyroidism (HPT) and adenoma, 4 patients with primary HPT due to hyperplasia and 2 patients with parathyroid hyperplasia secondary to chronic renal failure. The dispersed cells were incubated in tissue culture medium at low, normal and high external calcium concentrations for 2-16 h. There was a gradual suppression of PTH release (5-55%) when the calcium concentration in the medium was increased from 0.5 to 3.0 mM, thus indicating retained regulation of hormone release. The addition of 1,25-(OH)2D3 in concentrations of 0.1 and 1 ng/ml and of 24,25-(OH)2D3 in concentrations of 1.0 and 10 ng/ml during the incubations did not further affect the amount of PTH released by the cells. The concentrations of the different vitamin D metabolites tested closely correspond to levels observed under normal physiological conditions and during treatment with high doses of vitamin D in vivo. Thus, the findings contradict the idea of any direct short-term regulatory effect of either 1,25-(OH)2D3 or 24,25-(OH)2D3 on the secretion of PTH from hyperfunctioning human parathyroid tissue.

Resolution of cellular compartments involved in membrane potential changes accompanying IgE-mediated degranulation of rat basophilic leukemia cells.

The overall membrane potential of rat basophilic leukemia cells (RBL-2H3) calculated from the transmembrane distribution of the lipophilic, tritium-labelled cation tetraphenyl-phosphonium [( 3H]TPP+) was resolved into its mitochondrial and plasma membrane potential components. Using the mitochondrial uncoupler carbonylcyanide-p-trifluormethoxyphenyl hydrazone (FCCP) which collapses the mitochondrial potential, it was shown that about one third of the overall potential resulted from the mitochondrial contribution. Degranulation of the RBL cells induced by two different IgE-cross-linking agents (specific antigen and anti-IgE antibodies), was accompanied by, and well correlated with, a decrease in the overall potential. However, evaluation of the source of these observed potential changes revealed that the FCCP-insensitive fraction of the overall potential, delta psi P, (representing the plasma membrane potential), was not affected. In contrast, the FCCP-sensitive component due to the mitochondrial potential decreased when receptor cross-linking increased. Thus, the observed decrease in the overall potential is most probably a secondary event in the sequence leading from stimulus to secretion. Indeed, exposure of the RBL cells either to a high external concentration of K+ ions or to a high amount of external TPP+, both causing depolarization, failed to trigger degranulation. It is suggested that the apparent decrease in the measured overall potential is a reflection of the mitochondrial membrane depolarization. The latter is most probably caused by mitochondrial Ca2+ uptake initiated by the increase in the intracellular concentration of Ca2+ which follows cells activation.

Manganese fluxes and manganese-dependent neurotransmitter release in presynaptic nerve endings isolated from rat brain.

The uptake and efflux of 54Mn and 45Ca, and the release of dopamine (DA) were measured in pinched-off presynaptic nerve endings (synaptosomes) isolated from rat brain. The uptake of Mn and Ca was increased when forebrain or striatal synaptosomes were incubated in a depolarizing, K-rich solution. The time courses of K-stimulated Mn and Ca entry were similar: there was initially a high rate of ion accumulation, lasting 1-3 s, that gradually levelled off. The initial uptake of Mn, like that of Ca, was greatly diminished by a 10 s pre-incubation in K-rich solution prior to the addition of radiotracer. Several Ca channel blockers, including Ni (0.03 mM), Sr (2.0 mM), Co (0.04 mM), Ba (1.5 mM) and La (0.2 mM), suppressed the K-stimulated uptake of Mn and of Ca to a similar extent. The K-stimulated uptake of Mn increased as a function of the external Mn concentration, and saturated at high external concentrations of Mn. These high concentrations of Mn also blocked the K-stimulated uptake of Ca. There was a decreased efflux of Ca, but not of Mn, from the synaptosomes when the external Na concentration was reduced. The Na-dependent efflux of Ca was diminished by external Mn, but was unaffected when the synaptosomes were loaded with Mn. The rate of [3H]DA release from striatal synaptosomes was less than 0.001 s-1 in non-depolarizing, low-K solutions, in the absence or presence of Mn and Ca (1 mM). The rate of release was also unchanged in depolarizing, K-rich solutions in the absence of these divalent cations. The addition of 1 mM-Mn to a K-rich solution increased the rate of DA release by about 40%, and the time course of release was linear for at least 30 s. The addition of 1 mM-Ca increased the rate of release nearly 100-fold during the first second, and thereafter the rate of release rapidly declined. Ni (1 mM) and, to a lesser extent, Mg (10 mM) reduced the rate of K-stimulated DA release that is dependent on either Mn or Ca. The pattern of inhibition of DA release resembled the pattern of inhibition of K-stimulated uptake of Mn and Ca. The addition of Mn to K-rich solutions stimulated the release of the neurotransmitters 5-hydroxytryptamine and gamma-aminobutyric acid, but not acetylcholine, from striatal synaptosomes.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitrate content and nitrate reductase activity in Rumex obtusifolius L. : II. Responses to nitrate starvation and nitrogen fertilization.

The aim of this work was to investigate the effect of nitrogen starvation and subsequent fentilization with nitrate or ammonium on nitrate content and nitrate reductase activity of Rumex obtusifolius L. under natural conditions.When plants were transplanted to nitrate-poor media, endogenous nitrate was reduced within a few days. In parallel, nitrage reductase activities dropped to about 25% of the initial values. As a consequence of nitrate fertilization (1; 10 or 100 mmol KNO3/l substrate), endogenous nitrate content of the plant abruptly increased within one day. In extreme cases, nitrate concentrations of up to 10% of plant dry weight could be observed without being lethal. High external nitrate concentrations caused an inhibition of nitrate reductase within the leaves, while low external concentrations provoked an increase in the enzyme activity of about 450% within one day. Ammonium fertilization (5 mmol (NH4)2SO4/l substrate) also caused an increase in nitrate reductase activity and nitrate content within leaf blades. This observation indicates a rapid nitrification of ammonium in the substrate. When plants were fertilized with ammonium plus nitrate (2.5 mmol (NH4)2SO4+ 5 mmol KNO3/l substrate), an extremely high and long term increase in nitrate reduction could be observed. Due to an intensive enzymatic nitrate turnover, the nitrate content of leaf blades then remained relatively low. Our observations do not point to an inhibition of nitrate reductase activity in leaves of Rumex obtusifolius by ammonium. Despite temporarily high endogenous nitrate concentrations, Rumex obtusifolius may not be termed as a "nitrate storage" plant, since the accumulation of nitrate is a short term process only.