Cortical cell fluxes of cobalt in roots and transport to the shoots of ryegrass seedlings

Total uptake and transport of ⁵⁸Co as a function of time were measured in seedlings of Lolium perenne L. cv. Premo, using nutrient solutions containing either 0.1 or 1.0 μ M Co²⁺. After an initial shoulder, uptake was linear and about 15% of the Co absorbed was transported to the shoot after 72 h. Log total uptake and transport as a function of log Co concentration (0.01 to 1.0 μ M ) were also linear. Co uptake and transport markedly increased with increasing pH but were unaffected by water flux. Compartmental analysis of ⁵⁸Co efflux data was used to estimate unidirectional fluxes and compartment al concentrations of Co in root cortex, cells. At both levels of external Co, influx to the cytoplasm was passive and cytoplasmic concentrations were comparable. In the 0.1 μ M treatment, cytoplasm concentration was controlled by an efflux pump; fluxes across the tonoplast were passive and concentration in the vacuole was small. In the 0.1 μ M treatment, the concentration of Co in the cytoplasm was regulated by both an efflux pump at the plasmalemma and an influx pump at the tonoplast. Stored Co in the vacuole was largely unavailable for transport. Factors limiting transport, and the significance of Co depletion in nutrient solutions due to uptake, were discussed. We also established that 0.1 μ M Co was sufficient to provide adequate levels of ryegrass shoot Co for ruminant diets.     

Proadrenomedullin N-Terminal 20 Peptide (PAMP), an Endogenous Anticholinergic Peptide: Its Exocytotic Secretion and Inhibition of Catecholamine Secretion in Adrenal Medulla

Abstract: In cultured bovine adrenal medullary cells, stimulation of nicotinic receptors by carbachol evoked the Ca²⁺-dependent exocytotic cosecretion of proadrenomedullin N-terminal 20 peptide (PAMP) (EC₅₀ = 50.1 µ M ) and catecholamines (EC₅₀ = 63.0 µ M ), with the molar ratio of PAMP/catecholamines secreted being equal to the ratio in the cells. Addition of PAMP[1–20]NH₂ inhibited carbachol-induced ²²Na⁺ influx via nicotinic receptors (IC₅₀ = 2.5 µ M ) in a noncompetitive manner and thereby reduced carbachol-induced ⁴⁵Ca²⁺ influx via voltage-dependent Ca²⁺ channels (IC₅₀ = 1.0 µ M ) and catecholamine secretion (IC₅₀ = 1.6 µ M ). It did not alter high K⁺-induced ⁴⁵Ca²⁺ influx via voltage-dependent Ca²⁺ channels or veratridine-induced ²²Na⁺ influx via voltage-dependent Na⁺ channels. PAMP seems to be a novel antinicotinic peptide cosecreted with catecholamines by a Ca²⁺-dependent exocytosis in response to nicotinic receptor stimulation.     

Cations Affect [3H]Mazindol and [3H]WIN 35,428 Binding to the Human Dopamine Transporter in a Similar Fashion

Abstract: The present study addresses the possibility that there are different cocaine-related and mazindol-related binding domains on the dopamine transporter (DAT) that show differential sensitivity to cations. The effects of Zn²⁺, Mg²⁺, Hg²⁺, Li⁺, K⁺, and Na⁺ were assessed on the binding of [³H]mazindol and [³H]WIN 35,428 to the human (h) DAT expressed in C6 glioma cells under identical conditions for intact cell and membrane assays. The latter were performed at both 0 and 21°C. Zn²⁺ (30–100 µ M ) stimulated binding of both radioligands to membranes, with a relatively smaller effect for [³H]mazindol; Mg²⁺ (0.1–100 µ M ) had no effect; Hg²⁺ at ∼3 µ M stimulated binding to membranes, with a relatively smaller effect for [³H]mazindol than [³H]WIN 35,428 at 0°C, and at 30–100 µ M inhibited both intact cell and membrane binding; Li⁺ and K⁺ substitution (30–100 m M ) inhibited binding to membranes more severely than to intact cells; and Na⁺ substitution was strongly stimulatory. With only a few exceptions, the patterns of ion effects were remarkably similar for both radioligands at both 0 and 21°C, suggesting the involvement of common binding domains on the hDAT impacted similarly by cations. Therefore, if there are different binding domains for WIN 35,428 and mazindol, these are not affected differentially by the cations studied in the present experiments, except for the stimulatory effect of Zn²⁺ at 0 and 21°C and Hg²⁺ at 0°C.     

Modulation of 45Ca2+ Influx into Cells Stably Expressing Recombinant Human NMDA Receptors by Ligands Acting at Distinct Recognition Sites

Abstract: A ⁴⁵Ca²⁺ influx assay has been used to investigate the pharmacology of stably expressed recombinant human NR1a/NR2A and NR1a/NR2B N -methyl- d -aspartate (NMDA) receptors. Inhibition of glutamate-stimulated ⁴⁵Ca²⁺ influx by six glycine-site antagonists and inhibition of glycine-stimulated ⁴⁵Ca²⁺ influx by five glutamate-site antagonists revealed no significant differences between affinity values obtained for NR1a/NR2A and NR1a/NR2B receptors. The polyamine site agonist spermine showed differential modulation of glutamate- and glycine-stimulated ⁴⁵Ca²⁺ influx for recombinant NMDA receptors, inhibiting and stimulating ⁴⁵Ca²⁺ influx into cells expressing NR1a/NR2A receptors (IC₅₀ = 408 µ M ) and NR1a/NR2B receptors (EC₅₀ = 37.3 µ M ), respectively. The antagonist ifenprodil was selective for NR1a/NR2B receptors (IC₅₀ = 0.099 µ M ) compared with NR1a/NR2A receptors (IC₅₀ = 164 µ M ). The effects of putative polyamine site antagonists, redox agents, ethanol, and Mg²⁺ and Zn²⁺ ions were also compared between NR1a/NR2A and NR1a/NR2B receptors. This study demonstrates the use of ⁴⁵Ca²⁺ influx as a method for investigating the pharmacology of the numerous modulatory sites that regulate the function of recombinant human NMDA receptors stably expressed in L(tk-) cells.     

Transport interactions between cadmium and zinc in roots of bread and durum wheat seedlings

Field studies have shown that the addition of Zn to Cd-containing soils can help reduce accumulation of Cd in crop plants. To understand the mechanisms involved, this study used ¹⁰⁹Cd and ⁶⁵Zn to examine the transport interactions of Zn and Cd at the root cell plasma membrane of bread wheat ( Triticum aestivum L.) and durum wheat ( Triticum turgidum L. var. durum ). Results showed that Cd²⁺ uptake was inhibited by Zn²⁺ and Zn²⁺ uptake was inhibited by Cd²⁺. Concentration-dependent uptake of both Cd²⁺ and Zn²⁺ consisted of a combination of linear binding by cell walls and saturable, Michaelis-Menten influx across the plasma membrane. Saturable influx data from experiments with and without 10 µm concentrations of the corresponding inhibiting ion were converted to double reciprocal plots. The results revealed a competitive interaction between Cd²⁺ and Zn²⁺, confirming that Cd²⁺ and Zn²⁺ share a common transport system at the root cell plasma membrane in both bread and durum wheat. The study suggests that breeding or agronomic strategies that aim to decrease Cd uptake or increase Zn uptake must take into account the potential accompanying change in transport of the competing ion.     

Effects of copper on active and passive Rb+ influx in roots of winter wheat

The effects of copper (CuCl₂) on active and passive Rb⁺(⁸⁶Rb⁺) influx in roots of winter wheat grown in water culture for 1 week were studied. External copper concentrations in the range of 10–500 μ M in the uptake nutrient solution reduced active Rb⁺ influx by 20–70%, while passive influx was unaffected (ca 10% of the Rb⁺ influx in the Cu-free solution). At external Rb⁺ concentrations of up to 1 m M , Cu exposure (50 μ M decreased V_max to less than half and increased K_m to twice the value of the control. Short Cu exposure reduced the K⁺ concentration in roots of low K⁺ status. Pretreatment for 5 min in 50 μ M CuCl₂ prior to uptake experiments reduced Rb⁺ influx by 26%. After 60 min pretreatment with Cu, the corresponding reduction was 63%. Cu in the cultivation solution impeded growth, especially of the roots. The Cu concentration in the roots increased linearly with external Cu concentration (0–100 μ M ) while Cu concentration in the shoots was relatively unchanged. The K⁺ concentration in both roots and shoots decreased significantly with increased Cu in the cultivation solutions. Possible effects of Cu on membranes and ion transport mechanisms are discussed.     

Differential Inhibition of Secretagogue-Stimulated Sodium Uptake in Adrenal Chromaffin Cells by Activation of D4 and D5 Dopamine Receptors

Abstract: Recent studies have demonstrated that D1-selective and D2-selective dopamine receptor agonists inhibit catecholamine secretion and Ca²⁺ uptake into bovine adrenal chromaffin cells by receptor subtypes that we have identified by PCR as D5, a member of the D1-like dopamine receptor subfamily, and D4, a member of the D2-like dopamine receptor subfamily. The purpose of this study was to determine whether activation of D5 or D4 receptors inhibits influx of Na⁺, which could explain inhibition of secretion and Ca²⁺ uptake by dopamine agonists. D1-selective agonists preferentially inhibited both dimethylphenylpiperazinium- (DMPP) and veratridine-stimulated ²²Na⁺ influx into chromaffin cells. The D1-selective agonists chloro-APB hydrobromide (CI-APB; 100 µ M ) and SKF-38393 (100 µ M ) inhibited DMPP-stimulated Na⁺ uptake by 87.5 ± 2.3 and 59.7 ± 4.5%, respectively, whereas the D2-selective agonist bromocriptine (100 µ M ) inhibited Na⁺ uptake by only 22.9 ± 5.0%. Veratridine-stimulated Na⁺ uptake was inhibited 95.1 ± 3.2 and 25.7 ± 4.7% by 100 µ M CI-APB or bromocriptine, respectively. The effect of CI-APB was concentration dependent. A similar IC₅₀ (∼18 µ M ) for inhibition of both DMPP- and veratridine-stimulated Na⁺ uptake was obtained. The addition of 8-bromo-cyclic AMP (1 m M ) had no effect on either DMPP- or veratridine-stimulated Na⁺ uptake. These observations suggest that D1-selective agonists are inhibiting secretagogue-stimulated Na⁺ uptake in a cyclic AMP-independent manner.     

Effects of Protein Kinase Inhibitors on Morphology and Function of Cultured Bovine Adrenal Chromaffin Cells: KN-62 Inhibits Secretory Function by Blocking Stimulated Ca2+ Entry

Abstract: In cultured bovine adrenal chromaffin cells, a nonselective protein kinase inhibitor, staurosporine, inhibits secretory function and induces neurite outgrowth. In the present study, effects of other nonselective protein kinase inhibitors (K-252a, H-7, and H-8) and reportedly selective protein kinase inhibitors (KN-62 and chelerythrine chloride) were examined on bovine adrenal chromaffin cell morphology, secretory function, and ⁴⁵Ca²⁺ uptake. Treatment of chromaffin cells with 10 µ M K-252a, 50 µ M H-7, or 50 µ M H-8 induced changes in cell morphology within 3 h; these compounds also induced a time-dependent inhibition of stimulated catecholamine release. Chelerythrine chloride, a selective inhibitor of Ca²⁺/phospholipid-dependent protein kinase, did not induce outgrowth or inhibit secretory function under our treatment conditions. KN-62, a selective inhibitor of Ca²⁺/calmodulin-dependent protein kinase II (CaMK II), significantly inhibited stimulated catecholamine release (IC₅₀ value of 0.32 µ M ), but had no effect on cell morphology. The reduction of secretory function induced by 1 µ M KN-62 was significant within 5 min and rapidly reversible. Unlike H-7, H-8, and staurosporine, KN-62 significantly inhibited stimulated ⁴⁵Ca²⁺ uptake. KN-04, a structural analogue of KN-62 that does not inhibit CaMK II, inhibited stimulated ⁴⁵Ca²⁺ uptake and catecholamine release like KN-62. These studies indicate that KN-62 inhibits secretory function via the direct blockade of activated Ca²⁺ influx. The nonselective inhibitors, K-252a, H-7, H-8, and staurosporine, inhibit secretory function by another mechanism, perhaps one involving alterations in the cytoskeleton.     

Ca2+ and Reactive Oxygen Species in Staurosporine-Induced Neuronal Apoptosis

Abstract: Staurosporine (0.03–0.5 µ M ) induced a dose-dependent, apoptotic degeneration in cultured rat hippocampal neurons that was sensitive to 24-h pretreatments with the protein synthesis inhibitor cycloheximide (1 µ M ) or the cell cycle inhibitor mimosine (100 µ M ). To investigate the role of Ca²⁺ and reactive oxygen species in staurosporine-induced neuronal apoptosis, we overexpressed calbindin D_28K, a Ca²⁺ binding protein, and Cu/Zn superoxide dismutase, an antioxidative enzyme, in the hippocampal neurons using adenovirus-mediated gene transfer. Infection of the cultures with the recombinant adenoviruses (100 multiplicity of infection) resulted in a stable expression of the respective proteins assessed 48 h later. Overexpression of both calbindin D_28K and Cu/Zn superoxide dismutase significantly reduced staurosporine neurotoxicity compared with control cultures infected with a β-galactosidase overexpressing adenovirus. Staurosporine-induced neuronal apoptosis was also significantly reduced when the culture medium was supplemented with 10 or 30 m M K⁺, suggesting that Ca²⁺ influx via voltage-sensitive Ca²⁺ channels reduces this apoptotic cell death. In contrast, neither the glutamate receptor agonist NMDA (1–10 µ M ) nor the NMDA receptor antagonist dizocilpine (MK-801; 1 µ M ) was able to reduce staurosporine neurotoxicity. Cultures treated with the antioxidants U-74500A (1–10 µ M ) and N -acetylcysteine (100 µ M ) also demonstrated reduced staurosporine neurotoxicity. These results suggest a fundamental role for both Ca²⁺ and reactive oxygen species in staurosporine-induced neuronal apoptosis.     

Nitroprusside and Cyclic GMP Stimulate Na+-Ca2+ Exchange Activity in Neuronal Preparations and Cultured Rat Astrocytes

Abstract: The effects of nitric oxide (NO)-generating agents on ⁴⁵Ca²⁺ uptake in rat brain slices and cultured rat astrocytes were studied in the presence of monensin, which is considered to drive the Na⁺-Ca²⁺ exchanger in the reverse mode. Sodium nitroprusside (SNP) at >10 µ M increased monensin-stimulated Ca²⁺ uptake in the slices, although it did not affect high K⁺-stimulated Ca²⁺ uptake. Another NO donor, 3-morpholinosydnonimine, was effective. The effect of SNP was antagonized by hemoglobin (50 µ M ), a NO scavenger, and mimicked by 8-bromo-cyclic GMP (100 µ M ). In rat brain synaptosomes, SNP increased monensin-stimulated Ca²⁺ uptake, but it did not affect high K⁺-stimulated Ca²⁺ uptake. 8-Bromocyclic GMP, but not SNP, increased Na⁺-dependent Ca²⁺ uptake significantly in synaptic membrane vesicles in the absence of monensin. In cultured rat astrocytes, SNP and 8-bromo-cyclic GMP increased Ca²⁺ uptake in the presence of ouabain and monensin, which were required for the Ca²⁺ uptake in the cells. These findings suggest that NO stimulates the Na⁺-Ca²⁺ exchanger in neuronal preparations and astrocytes in a cyclic GMP-dependent mechanism.     

Growth, contents of K+ and kinetics of K+(86Rb) uptake in barley cultured at different low supply rates of potassium

Plants of barley ( Hordeum vulgare L. cv. Salve) were grown with 6.5–35% relative increase of K⁺ supply per day (RKR) using a special computer-controlled culture unit. After a few days on the culture solution the plants adapted their relative growth rate (RGR) to the rate of nutrient supply. The roots of the plants remained in a low salt status irrespective of the rate of nutrient supply, whereas the concentration of K⁺ in shoots increased with RKR. Both V_max and K_m for K⁺(⁸⁶Rb) influx increased with RKR. It is concluded that with a continuous and stable K⁺ stress, the K⁺ uptake system is adjusted to provide an effective K⁺ uptake at each given RKR. Allosteric regulation of K⁺ influx does not occur and efflux of K⁺ is very small.     

Electrical signalling and cytokinins mediate effects of light and root cutting on ion uptake in intact plants

Nutrient acquisition in the mature root zone is under systemic control by the shoot and the root tip. In maize, exposure of the shoot to light induces short-term (within 1–2 min) effects on net K⁺ and H⁺ transport at the root surface. H⁺ efflux decreased (from −18 to −12 nmol m⁻² s⁻¹) and K⁺ uptake (∼2 nmol m⁻² s⁻¹) reverted to efflux (∼−3 nmol m⁻² s⁻¹). Xylem probing revealed that the trans-root (electrical) potential drop between xylem vessels and an external electrode responded within seconds to a stepwise increase in light intensity; xylem pressure started to decrease after a ∼3 min delay, favouring electrical as opposed to hydraulic signalling. Cutting of maize and barley roots at the base reduced H⁺ efflux and stopped K⁺ influx in low-salt medium; xylem pressure rapidly increased to atmospheric levels. With 100 m m NaCl added to the bath, the pressure jump upon cutting was more dramatic, but fluxes remained unaffected, providing further evidence against hydraulic regulation of ion uptake. Following excision of the apical part of barley roots, influx changed to large efflux (−50 nmol m⁻² s⁻¹). Kinetin (2–4  µ m ), a synthetic cytokinin, reversed this effect. Regulation of ion transport by root-tip-synthesized cytokinins is discussed.     

Comparison of Voltage-Dependent 45Ca2+ Uptake Rates by Synaptosomes Isolated from Rat Brain Regions

Abstract: ⁴⁵Ca²⁺ uptake by synaptosomes isolated from cerebral cortex, cerebellum, midbrain, and brain stem of male Sprague-Dawley rats was measured at 1-, 3-, 5-, 15-, 30-, and 60-s time periods. The fastest rate of depolarization-dependent calcium uptake occurred in each brain region between 0 and 1 s. Uptake rates dropped off quickly with 3–5-s rates at approximately 15–20% of those observed at 0–1 s in cerebral cortex, cerebellum, and midbrain. Uptake rates at the 1–3-s interval were maintained at a relatively high rate in these three brain regions suggesting mixed fast- and slow-phase processes. The magnitude and rate of ⁴⁵Ca²⁺ uptake were similar in synaptosomes from cerebral cortex, cerebellum, and midbrain but were significantly less in brain stem synaptosomes. These results suggest a fast and a slow component to voltage-dependent ⁴⁵Ca²⁺ uptake by presynaptic nerve terminals from various brain regions.     

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.     

Lack of active K+ uptake in aeroponically grown wheat seedlings

The K⁺ (⁸⁶Rb⁺) uptake and the growth of intact wheat seedlings ( Triticum aestivum L. cv. GK Szeged) grown in 0.5 m M CaCl₂ solution and of seedlings grown on wet filter paper in Petri dishes were compared under different experimental conditions. Aeroponic (AP) and hydroponic (HP) conditions brought about striking differences in the growth of the roots, whereas the shoot growth was not influenced. The dry weight of the roots was higher for the AP plants than for the HP plants. The AP grown seedlings exhibit a low rate of K⁺ uptake, which seems to be a passive process. The effect of 2, 4–dinitrophenol (2, 4–DNP) clearly shows the absence of an active component of the K⁺ uptake in roots grown in air with a high relative humidity. In plants grown under AP conditions the effect of Ca²⁺ on the K⁺ uptake is unfavourable, i.e. there is an inhibition (negative Viets effect). Results relating to the effect of 2,4–DNP suggest that the "negative Viets effect" is a feature of the passive K⁺ uptake. The data suggest that the AP growth conditions play a very important role in the induction and/or development of the ion transport system(s), which becomes impaired under the AP conditions.     

Characterization of mitochondrial glutamate dehydrogenase from dark‐grown soybean seedlings

Purified preparations of NAD(H)‐glutamate dehydrogenase (GDH, EC 1.4.1.2.) were assayed to determine the effects of mono‐ and divalent cations, nucleotides and select carbon compounds on NAD(H)‐dependent GDH activity. The amination reaction was stimulated 2‐ to 17‐fold by divalent cations (Ca²⁺ > Cd²⁺ > Co²⁺ > Mg²⁺ > Mn²⁺ > Zn²⁺ between 1 and 1000 µ M ), but the reaction was unaffected by monovalent cations (Na ⁺ and K ⁺). The amination reaction was most responsive to changes in Ca²⁺ in a NADH‐dependent manner. The addition of EDTA or EGTA nullified the stimulatory effects of Ca²⁺. Calmodulin alone or in combination with calmodulin antagonists did not affect the amination reaction. Divalent cations (at 1 m M ) inhibited the rate of the deamination reaction by 15 to 25%, while monovalent cations had no effect. ATP inhibited the amination reaction by 10 to 60%, while ADP had little or no effect. ATP or ADP decreased the rate of the deamination reaction 23 to 60 or 20 to 38%, respectively. Many tricarboxylic acid cycle intermediates inhibited the amination reaction, 20 to 50% of the inhibition could be attributed to the chelating capacity of intermediates. Conversely, most of the carbon sources tested did not affect the deamination reaction, the only appreciable differences were increases in activity with sucrose (21%) and glucose (41%) and a decrease in activity with pyruvate (34%). Inhibitors of sulfhydryl groups were used to examine the importance of reduced thiol groups in the amination or deamination reactions. The amination was not dependent on reduced thiol groups, whereas the deamination reaction was dependent on reduced thiol groups.     

Inhibition of Maitotoxin-Induced Ca2+ Influx in Rat Glioma C6 Cells by Brevetoxins and Synthetic Fragments of Maitotoxin

Abstract: ⁴⁵Ca²⁺ influx in rat glioma C6 cells induced by 0.3 n M maitotoxin (MTX) was markedly inhibited by brevetoxin A (PbTx1) and brevetoxin B (PbTx2), with EC₅₀ values of 16 and 13 µ M , respectively. This inhibition was observed immediately after addition of MTX when monitored with fura-2, which suggests that PbTx2 directly blocks the action of MTX. This blockade by PbTx2 was not affected by tetrodotoxin, which excludes the involvement of voltage-sensitive sodium channels. The depolarizing effects of these brevetoxins were also not a likely cause of this inhibition, because melittin, a channel-forming peptide, did not significantly block MTX-induced ⁴⁵Ca²⁺ influx. Instead, this inhibition was thought to be mediated by blockade of an MTX-binding site by the brevetoxins, based on the fact that these toxins, particularly PbTx2, closely mimic the partial structure of MTX. Synthetic fragments of MTX corresponding to the hydrophilic EF-GH rings (200 µ M ) and LM-NO rings (500 µ M ) of MTX significantly reduced MTX-elicited Ca²⁺ influx. The observation that the effects of MTX were inhibited by structural mimics of both its hydrophobic and hydrophilic portions implies that both portions of the MTX molecule recognize its target.     

Actual escape area and lateral escape from the xylem of the alkali ions Na+, K+, Rb+ and Cs+ in tomato

Approximation of the total escape area of the xylem in an inbred line of tomato (Ly-copersicon escutentum Mill. cv. Tiny Tim) with help of the frequency distribution of xylem vessel radii provides the possibility to calculate realistic escape constant values from uptake experiments of several elements into tomato stem segments. Comparison of the lateral escape rates of ²⁴Na⁺, ⁴²K⁺, ⁸⁶Rb⁺ and ¹³⁴Cs⁺ indicate that Na⁺ escape is rate-limited by its uptake into a rather constant number of surrounding cells, regardless of changes in the total escape area of the xylem vessels. The escape of K⁺, Rb⁺ and Cs⁺ seems to be proportional to the surface area of the xylem vessels and their escape is apparently controlled by their transport across the cell walls of the transport channels. The calculated small values for the escape rate constants (apparent permeability of the xylem cell walls, ca 2–3 · 10−⁹ m s−⁷) are probably due to the presence of lignin in the xylem cell walls, the discrimination between ions as a result of differing affinities and selectivities and the presence of other solutes in the applied solution.     

Ca2+-Dependent Enhancement of [3H]Dopamine Uptake in Rat Striatum: Possible Involvement of Calmodulin-Dependent Kinases

Abstract: We studied effects of Ca²⁺ in the incubation medium on [³H]dopamine ([³H]DA) uptake by rat striatal synaptosomes. Both the duration of the preincubation period with Ca²⁺ (0–30 min) and Ca²⁺ concentration (0–10 m M ) in Krebs-Ringer medium affected [³H]DA uptake by the synaptosomes. The increase was maximal at a concentration of 1 m M Ca²⁺ after a 10-min preincubation (2.4 times larger than the uptake measured without preincubation), which reflected an increase in V _max of the [³H]DA uptake process. On the other hand, [³H]DA uptake decreased rapidly after addition of ionomycin in the presence of 1 m M Ca²⁺. The Ca²⁺-dependent enhancement of the uptake was still maintained after washing synaptosomes with Ca²⁺-free medium following preincubation with 1 m M Ca²⁺. Protein kinase C inhibitors did not affect apparently Ca²⁺-dependent enhancement of the uptake, whereas 1-[ N,O -bis(1,5-isoquinolinesulfonyl)- N -methyl- l -tyrosyl]-4-phenylpiperazine (KN-62; a Ca²⁺/calmodulin-dependent kinase II inhibitor) and wortmannin (a myosin light chain kinase inhibitor) significantly reduced it. Inhibitory effects of KN-62 and wortmannin appeared to be additive. N -(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7; a calmodulin antagonist) also remarkably inhibited the enhancement. These results suggest that Ca²⁺-dependent enhancement of [³H]DA uptake is mediated by activation of calmodulin-dependent protein kinases.     

Effects of ABA on 86Rb+ fluxes at plasmalemma and tonoplast of stomatal guard cells

Abscisic acid (ABA) induces a transient stimulation of ⁸⁶Rb⁺ from isolated guard cells of Commelina communis L. When ABA is added after 30–50 min of wash-out in the absence of ABA, when tracer is almost entirely vacuolar, its effects on vacuolar release are measured. When ABA is added early in the wash-out (at 2–4 min), when both cytoplasm and vacuole are labelled, the resulting efflux includes both vacuolar and cytoplasmic contributions. Detailed comparison of rates of efflux in the absence of ABA, and in the presence of ABA added early and late in the wash-out, allows the effects of ABA on plasmalemma and tonoplast fluxes to be assessed. Three effects of ABA can be distinguished: these are stimulation of the ⁸⁶Rb⁺ flux from vacuole to cytoplasm (by twofold to 6.7-fold); stimulation of the plasmalemma efflux, by up to twofold, a smaller factor than that of the tonoplast effect and variable between experiments; and a doubling of the half-time for cytoplasmic exchange in ABA, taken to reflect an increase in cytoplasmic ion content as ions flood out of the vacuole. Concentrations of ABA of 0.1–0.2 µM and 1–10 µM are equally effective in the stimulation of plasmalemma efflux, but the effects on tonoplast fluxes are both delayed and reduced at low external concentrations of ABA. It is argued that the delay reflects the need for a threshold internal ABA to be reached before the initiation of vacuolar release, and the reduction reflects the sensitivity of the extent of activation of tonoplast ion channels to concentration of internal ABA. It is likely that the plasmalemma change is mediated by external ABA, and could be the result of the modulation of the stretch-activated channel suggested previously.