Lanthanum and some other cation-induced changes in fluidity of synaptosomal membrane studied with nitroxide stearate spin labels.

Using nitroxide fatty acid spin labels, the effects of some cations such as La3+, Cd2+ and Hg2+ on synaptosomal membranes were studied by observing changes in their ESR spectra. The labels were incorporated almost instantaneously into synaptosomes isolated from rat brain cortex. ESR spectra of the spin-labeled synaptosomes were significantly braodened immediately upon adding La3+, Ce3+, Cd2+ or Hg2+ but hardly affected by Ca2+, Sr2+ and Ba2+. The magnitude of the change in the separation of the outer two peaks in ESR spectra (2T') depends on the number (n) of methylene units between the polar head group and the spin-label (nitroxide) group; that is, it increases with decreasing n. Among these ions, the effect of La3+ was the greatest and appeared to be in parallel with the amount of La3+ bound with the synaptosomes. On the other hand, K+, Rb+ or Li+ causes hardly any significant changes.     

Inhibition of the biosynthesis ofN-acetylneuraminic acid by metal ions and seleniumin vitro

In liver homogenate the biosynthesis ofN-acetylneuraminic acid usingN-acetylglucosamine as precursor can be followed stepwise by applying different chromatographic procedures. In this cell-free system 16 metal ions (Zn²⁺, Mn²⁺, La³⁺, Co²⁺, Cu²⁺, Hg²⁺, VO ₃ ^– , Pb²⁺, Ce³⁺, Cd²⁺, Fe²⁺, Fe³⁺, Al³⁺, Sn²⁺, Cs⁺ and Li⁺) and the selenium compounds, selenium(IV) oxide and sodium selenite, have been checked with respect to their ability to influence a single or possible several steps of the biosynthesis ofN-acetylneuraminic acid. It could be shown that the following enzymes are sensitive to these metal ions (usually applied at a concentration of 1 mmoll^–1):N-acetylglucosamine kinase (inhibited by Zn²⁺ and vandate), UDP-N-acetylglucosamine-2-epimerase (inhibited by zn²⁺, Co²⁺, Cu²⁺, Hg²⁺, VO ₃ ^– , Pb²⁺, Cd²⁺, Fe³⁺, Cs⁺, Li⁺, selenium(IV) oxide and selenite), andN-acetylmannosamine kinase (inhibited by Zn²⁺, Cu²⁺, Cd²⁺, and Co²⁺). Dose dependent measurements have shown that Zn²⁺, Cu²⁺ and selenite are more efficient inhibitors of UDP-N-acetylglucosamine-2-epimerase than vanadate. As for theN-acetylmannosamine kinase inhibition, a decreasing inhibitory effect exists in the following order Zn²⁺, Cd²⁺, Co²⁺ and Cu²⁺. In contrast, La³⁺, Al³⁺ and Mn²⁺ (1 mmoll^–1) did not interfere with the biosynthesis ofN-acetylneuraminic acid. Thus, the conclusion that the inhibitory effect of the metal ions investigated cannot be regarded as simply unspecific is justified.Dedicated to Professor Theodor Günther on the occasion of his 60th birthday     

Effects of Cadmium, Lead, Mercury and Zinc on °-Aminolevulinic Acid Dehydratase Activity from Radish Leaves

The purpose of the present study was to investigate the in vitro and the in vivo effects of cadmium, zinc, mercury and lead on -aminolevulinic acid dehydratase (ALA-D) activity from radish leaves. The in vivo effect of these metals on growth, DNA and protein content was also evaluated. The results demonstrated that among the elements studied Cd²⁺ presented the highest toxicity for radish. 50% inhibition of ALA-D activity (IC₅₀) in vitro was at 0.39, 2.39, 2.29, and 1.38 mM Cd²⁺, Zn²⁺, Hg²⁺ and Pb²⁺, respectively. After in vivo exposure Cd²⁺, Zn²⁺, Hg²⁺ and Pb²⁺ inhibited ALA-D by about 40, 26, 34 and 15%, respectively. Growth was inhibited by about 40, 10, 25, and 5% by Cd²⁺, Zn²⁺, Hg²⁺, and Pb²⁺, respectively. DNA content was reduced about 35, 30, 20, and 10% for Cd²⁺, Zn²⁺, Hg²⁺, and Pb²⁺, respectively. The metal concentration in radish leaves exposed to Cd²⁺, Zn²⁺, Hg²⁺, and Pb²⁺ was 18, 13, 6, and 7 mol g^–1, respectively. The marked ability of radish to accumulate Cd²⁺ and Zn²⁺ raises the possibility of using this vegetable as a biomonitor of environmental contamination by these metals.     

A bacterial view of the periodic table: genes and proteins for toxic inorganic ions

Essentially all bacteria have genes for toxic metal ion resistances and these include those for Ag⁺, AsO ⁻₂ , AsO ³⁻₄ , Cd²⁺, Co²⁺, CrO ²⁻₄ , Cu²⁺, Hg²⁺, Ni²⁺, Pb²⁺, TeO ²⁻₃ , Tl⁺ and Zn²⁺. The largest group of resistance systems functions by energy-dependent efflux of toxic ions. Fewer involve enzymatic transformations (oxidation, reduction, methylation, and demethylation) or metal-binding proteins (for example, metallothionein SmtA, chaperone CopZ and periplasmic silver binding protein SilE). Some of the efflux resistance systems are ATPases and others are chemiosmotic ion/proton exchangers. For example, Cd²⁺-efflux pumps of bacteria are either inner membrane P-type ATPases or three polypeptide RND chemiosmotic complexes consisting of an inner membrane pump, a periplasmic-bridging protein and an outer membrane channel. In addition to the best studied three-polypeptide chemiosmotic system, Czc (Cd²⁺, Zn²⁺, and Co²), others are known that efflux Ag⁺, Cu⁺, Ni²⁺, and Zn²⁺. Resistance to inorganic mercury, Hg²⁺ (and to organomercurials, such as CH₃Hg⁺ and phenylmercury) involve a series of metal-binding and membrane transport proteins as well as the enzymes mercuric reductase and organomercurial lyase, which overall convert more toxic to less toxic forms. Arsenic resistance and metabolizing systems occur in three patterns, the widely-found ars operon that is present in most bacterial genomes and many plasmids, the more recently recognized arr genes for the periplasmic arsenate reductase that functions in anaerobic respiration as a terminal electron acceptor, and the aso genes for the periplasmic arsenite oxidase that functions as an initial electron donor in aerobic resistance to arsenite.

     

Interaction Between Metals and Chelating Agents Affects Glutamate Binding on Brain Synaptic Membranes

The present study investigates the possible effects of Hg²⁺, Pb²⁺, and Cd²⁺ on [³H]-glutamate binding. To better understand the role of the thiol-disulfide status on the toxicity of such metals toward glutamatergic neurotransmission, we used three thiol chelating agents, 2,3-dimercaptopropanol (BAL), 2,3-dimercaptopropane 1-sulfonate (DMPS), and meso-2,3-dimercaptosuccinic acid (DMSA). Dithiotreitol (DTT) was tested for its ability to prevent metals-induced inhibition on [³H]-glutamate binding. Hg²⁺, Pb²⁺, and Cd²⁺ showed a concentration-dependent inhibition on [³H]-glutamate binding, and mercury was the most effective inhibitor. BAL did not prevent [³H]-glutamate binding inhibition by Hg²⁺, Cd²⁺, and Pb²⁺. However, DMPS and DMSA prevented the inhibition caused by Cd²⁺ and Pb²⁺, but not by Hg²⁺. DTT did not prevent the inhibition on [³H]-glutamate binding caused by 10 M Hg²⁺. In contrast, it was able to partially prevent [³H]-glutamate binding inhibition caused by 40 M Pb²⁺ and Cd²⁺. These results demonstrated that the heavy metals present an inhibitory effect on [³H]-glutamate binding. In addition, BAL was less effective to protect [³H]-glutamate binding inhibition caused by these metals than other chelating agents studied.     

Purification of α-amylase from Bacillus lentus cultures

Acetone fractionation of Bacillus lentus culture filtrate yielded the highest -amylase activity and the 66.6% fraction reached 13-fold that of the crude enzyme preparation. Gel filtration and ion exchange chromatography afforded a pure -amylase (relative molecular mass, 42 000). The pure enzyme was highly active on starch and dextrin. It produced a mixture of oligosaccharides as major products of starch hydrolysis. Maximal activity was reached at 70° C and pH 6.1. Ca²⁺, Na⁺, K⁺ and Sr²⁺ ions stabilized or slightly stimulated the enzyme whereas Ag⁺, Co²⁺, Hg²⁺, Zn²⁺, Cd²⁺ and Fe³⁺ ions strongly inhibited the activity. The enzyme contained 16 amino acids, of which aspartic and glutamic acids were present in the highest proportions. Correspondence to: S. H. Omar     

Effects of heavy metals on pollen tube growth and ultrastructure

Summary The influence of different concentrations of the heavy metals cadmium (Cd²⁺), cobalt (Co²⁺), copper (Cu²⁺), iron (Fe²⁺ and Fe³⁺), mercury (Hg²⁺), manganese (Mn²⁺), and zinc (Zn²⁺), plus aluminium (Al³⁺) (a toxic metal in polluted areas), on pollen germination and tube growth ofLilium longiflorum was investigated using light microscopy. Effects could be observed with 3 M and 100 M of heavy metal, added as chloride salts to the medium. Cd²⁺, Cu²⁺, and Hg²⁺, showed the greatest toxicity, whereas germination and growth rate was less affected by Mn²⁺. Affected tubes showed swelling of the tip region. Tubes treated with Cd²⁺, Co²⁺, Fe²⁺, Fe³⁺, Hg²⁺, and Mn²⁺ were also prepared for ultrastructural studies. In all cases, the main effect was abnormal cell wall organization, mostly at the tip, where round, fibrillar aggregates, the shape and size of secretory Golgi vesicles were formed. They built up a loose network which could be up to 10 m thick compared to untreated tubes where the cell wall was composed of thin layers of long fibrils and about 100 nm thick. Cd²⁺ was the only metal which produced effects at the intracellular level: organelle distribution within the tip region appeared disorganized. A general mechanism of heavy metal action on pollen tube growth is discussed.     

Effects of Heavy Metals on Stomatal Movements in Broad Bean Leaves

How do heavy metals affect stomatal movements and whether water channels are involved in stomatal movements was investigated in broad bean (Vicia faba L.) leaves. Three-week old fully expanded leaves were harvested. Leaf epidermis was peeled off and soaked in the Mes–KOH buffer containing the salts of heavy metals. Stomatal aperture was measured under the microscope. The tested heavy metal ions, such as Hg²⁺, Pb²⁺, Zn²⁺, and La³⁺, partly inhibited stomatal opening in light or closing in darkness at submillimolar concentrations, while K⁺, Na⁺ and Mg²⁺ had no visible effects on stomatal movements. As compared to La³⁺, Hg²⁺ affected stomatal movements more significantly. Stomatal movements were almost completely inhibited under a combined Hg²⁺ and La³⁺ treatment. Apparently, La³⁺, a Ca²⁺ channel blocker, inhibits the changes in the cytosolic Ca²⁺ concentration in guard cells, thus affecting stomatal movements. The inhibitory effect of Hg²⁺ on stomatal movements may be explained by the inhibition of water channels. Like Hg²⁺, Zn²⁺ and Pb²⁺ interfered with stomatal movements. It is concluded that heavy metals at submillimolar concentrations inhibit stomatal movements. They may affect water fluxes through guard cell membranes in different ways, i.e., Hg²⁺, Pb²⁺, and Zn²⁺ inhibit water channels, whereas La³⁺ block ion channels. Water channels may be involved in stomatal movements by regulating water fluxes and play a dominant and primary role in stomatal movements.     

Metal cations as synaptosomal calcium blockers in studies with Fura-2

Metal ions are often used to block calcium channels in various tissues, including synaptosomes. In the present study, Fura-2 was used to determine the effectiveness of various metal ions as calcium channel blockers in rat brain synaptosomes in vitro. In buffer solutions, La³⁺ and Cd²⁺ increased the Fura-2 fluorescence in a manner similar to Ca²⁺. Ni²⁺ and Mn²⁺ appeared to be fluorescence quenching cations, and Sr²⁺ and Co²⁺ had little effect on the fluorescence of Fura-2. In suspensions of synaptosomes under resting conditions, Cd²⁺, Ni²⁺ and Mn²⁺ were found to be not suitable for use in synaptosome studies. On the other hand, La³⁺ and Co²⁺ had little effect on the Fura-2 fluorescence of resting synaptosomes, and under depolarizing conditions, La³⁺ and Co²⁺ decreased the Fura-2 fluorescence. These resuls, therefore, suggest that La³⁺ and Co²⁺ may be suitable as calcium channel blockers in synaptosome studies.     

Binding of mercuric and other heavy metal ions by microbial growth media

Ion-specific electrodes were used to study the binding of Hg²⁺, Pb²⁺, Cu²⁺, and Cd²⁺ ions to widely used bacterial growth media (Nutrient broth, trypticase soy broth, the medium of Foot and Taylor [6] and of Nelsonet al.[12]) and to media components [yeast extract, peptone, tryptone, proteose peptone, and casamino acids (acid hydrolyzed casein)]. Volatilization of Hg²⁺ from aqueous solutions could be prevented by any of the growth media or their components. All media bound large amounts of Hg²⁺, Pb²⁺, and Cu²⁺, but much less Cd²⁺. Of the media components, casamino acids showed the most binding activity for all metal ions; the relative affinity of other media components to different ions varied with the cation studied. In general, the Irving-Williams [8] series for cation affinity to organic ligands was followed: Hg²⁺>Pa²⁺ Cu²⁺ Cd²⁺.After adding 20 ppm of Hg²⁺, Pb²⁺, or Cu²⁺ (concentrations inhibitory to the growth of most microorganisms) to the growth media, 80 ppb or less remained as free cations in the solution. This might suggest that such ions enter bacterial cells as organic complexes, or that bacterial cells can compete successfully with growth media for the bound ions.     

Differential action of Hg2+ and Cd2+ on the phycobilisomes and chlorophyll a fluorescence,and photosystem II dependent electron transport in the cyanobacterium Anabaena flos-aquae

The effect of equimolar concentrations of Hg²⁺ and Cd²⁺ on the whole cell absorption spectra, absorption spectra of the extracted phycocyanin (PC) and fluorescence emission spectra of phycobilisomes (PBS) was investigated in the cells of Anabaena flos-aquae. The PC component of the PBS was found to be extremely sensitive to the Hg²⁺ rather than the Cd²⁺ ions. Further, the results showed that Hg²⁺ and Cd²⁺ induced decrease in the rate of Hill activity (H₂O - DCPIP) was partially restored by the electron donor NH₂OH, not by the diphenyl carbazide. Similarly, chlorophyll a fluorescence emission in the presence of metals showed that addition of NH₂OH could effectively reverse the metal induced alterations in the fluorescence emission intensity. These results, together, suggested that Hg²⁺ and Cd²⁺ caused damage to the photosystems (PS) II reaction center. However, a relatively higher stimulation of the chlorophyll a emission at 695 nm with a red shift of 4.0 nm in the presence of Hg²⁺, and Cd²⁺ induced preferential decrease in the emission intensity at 676 nm as compared with the peak at 695 nm were indicative of the differential action of Hg²⁺ and Cd²⁺ on the PS II.     

A new rhodamine‐based fluorescent chemodosimeter for mercuric ions in water media

A new rhodamine–ethylenediamine–nitrothiourea conjugate (RT) was synthesized and its sensing property as a fluorescent chemodosimeter toward metal ions was explored in water media. Analytical results from absorption and fluorescence spectra revealed that the addition of Hg²⁺ ions to the aqueous solution of the chemodosimeter RT caused a distinct fluorescence OFF–ON response with a remarkable visual color change from colorless to pink; however, no clear spectral and color changes were observed from other metal ions including: Zn²⁺, Cu²⁺, Cd²⁺, Pb²⁺, Ag⁺, Fe²⁺, Cr³⁺, Co³⁺, Ni²⁺, Ca²⁺, Mg²⁺, K⁺ and Na⁺. The sensing results and the molecular structure suggested that a Hg²⁺‐induced a desulfurization reaction and cyclic guanylation of the thiourea moiety followed by ring‐opening of the rhodamine spirolactam in RT are responsible for a distinct fluorescence turn‐on signal, indicating that RT is a remarkably sensitive and selective chemodosimeter for Hg²⁺ ions in aqueous solution. Hg²⁺ within a concentration range from 0.1 to 25 μM can be determined using RT as a chemodosimeter and a detection limit of 0.04 μM is achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of trace metals on mouse B16 melanoma cells in culture

The effects of fourteen metal ions (As³⁺, As⁵⁺, Cd²⁺, Co²⁺, Cr³⁺, Cr⁶⁺, Hg²⁺, Li⁺, Mg²⁺, Mn²⁺, Ni²⁺, Se⁴⁺, V⁵⁺, VO²⁺) on the proliferation and differentiation in mouse B16 melanoma cells cultivated in vitro were analyzed. Cell number assays, melanin, and protein measurements, a 3(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide reduction test (MTT survival test), and a clonal growth assay were performed. At 10⁻⁴ M, metal ions such as As³⁺, As⁵⁺, Cd²⁺, Cr⁶⁺, Se⁴⁺, V⁵⁺, VO²⁺, and, to a minor extent, Li⁺, Hg²⁺, and Co²⁺ significantly reduced the number of the B16 melanoma cells. For the same molar concentration, the order of the levels of cell toxicity of the metal compounds to B16 cells as measured by the MTT test was as follows: Hg²⁺>Cr⁶⁺=Cd²⁺>As³⁺, As⁵⁺>V⁵⁺, VO²⁺>Se⁴⁺=Ni²⁺=Co²⁺=Li⁺. An increased synthesis of melanin in B16 cells was noted after incubation with Co²⁺, Ni²⁺, Cd²⁺, and Li⁺, whereas Se⁴⁺ had, on the contrary, an inhibiting effect on melanogenesis.     

Isolation of Cadmium- and Mercury-sensitive Mutants of Escherichia coli and Some Factors Influencing Their Sensitivities

Several Cd²⁺ hypersensitive and Hg²⁺ sensitive mutants of Escherichia coli K-12 were isolated after repeated mutagenesis with nitrosoguanidine. The Cd-hypersensitive mutant, CD17P, could not grow in a chemically defined liquid medium containing 0.5 μm Cd²⁺, and the growth of Hg-sensitive mutant, HG17P-52, was inhibited by 0.9 μm Hg²⁺. Thus, CD 17P and HG17P-52 were respectively about 1000 fold and 4 fold more sensitive than the parental strain.

Both mutants were also considerably more sensitive to Zn²⁺ than their parent, but they did not show any appreciable change in sensitivity to other metals tested. Among the various metabolites and chemicals tested, reducing agents such as cysteine, dithiothreitol and vitamin C showed a protective effect against metal toxicity. Reduced glutathione was effective only against Hg²⁺. EDTA specifically and efficiently reversed only Cd²⁺ toxicity.     

Metal ion-induced permeability changes in cell membranes: A minireview

Summary 1. The paper summarizes the effects of the metal ions Cu²⁺, Pb²⁺, Ag⁺, Hg²⁺, Zn²⁺, and Cd²⁺ applied externally or internally to the surface membrane of different excitable cells.2. Conductance changes induced by metal ions, and metal ion-activated current, are compared with respect to their ion and voltage dependence.3. It is suggested that metal ion-induced effects can be realized through special structures of the cell membrane, the metal ion receptors, although other mechanisms, as, for example, competition for Ca-binding sites in the channel forming proteins, cannot be excluded.     

Isolation and Characterization of Two Momordins,Ribosome-Inactivating Proteins from the Seeds of Bitter Gourd (Momordica charantia)

Cathepsins L and L-like (58 kDa) proteinase from mackerel were purified to electrophoretical homogeneity by Concanavalin A-Sepharose and Econo-Pac S chromatographies. The molecular weights of cathepsins L and L-like proteinase were 30,000 and 58,000, and the optimal pH for the hydrolysis of Z-Phe-Arg-MCA (benzyloxycarbonyl-L-phenylalanyl-L-arginine-7-[4-methyl] coumarylamide) were 5.0 and 5.5, respectively. The stability of both purified proteinases at various pHs was low, when the pH was above 7.0. According to the substrate specificity analysis, these proteinases hydrolyzed Z-Phe-Arg-MCA and Z-Arg-Arg-MCA, but did not hydrolyze Z-Arg-MCA and L-Arg-MCA. The activities of these two proteinases were effectively activated by cysteine and dithiothreitol. Their thiol-dependent proteolytic activity against Z-Phe-Arg-MCA was strongly inhibited by E-64 (trans-epoxysuccinyl-L-leucylamido[4-guanidino]butane), antipain, chymostatin, iodoacetic acid, and leupeptin, but not inhibited by pepstatin or phenylmethane sulfonyl floride. The inactivation rate constants (K_D) of cathepsins L and L-like proteinases at 50°C were 5.1 × 10^?5 and 6.9 × 10^?4 s^?1, respectively. K⁺, Na⁺, Mg⁺, and Sr⁺ did not affect them, while Zn²⁺, Cd²⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺, Fe²⁺, and Fe³⁺ inhibited the activity of the purified cathepsins L and L-like proteinase.     

Uptake of Metal Ions by Rhizopus arrhizus Biomass

Rhizopus arrhizus biomass was found to absorb a variety of different metal cations and anions but did not absorb alkali metal ions. The amount of uptake of the cations was directly related to ionic radii of La³⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ba²⁺, Hg²⁺, Pb²⁺, UO₂²⁺, and Ag⁺. The uptake of all the cations is consistent with absorption of the metals by sites in the biomass containing phosphate, carboxylate, and other functional groups. The uptake of the molybdate and vanadate anions was strongly pH dependent, and it is proposed that the uptake mechanism involves electrostatic attraction to positively charged functional groups.     

Prevention by Ce3+ of DNA destruction caused by Hg2+ in fish intestine

The interactions between Hg²⁺, Ce³⁺, and the mixuure of Ce³⁺ and Hg²⁺, and DNA from fish intestine in vitro were investigated by using absorption spectrum and fluorescence emission spectrum. The ultraviolet absorption spectra indicated that the addition of Hg²⁺, Ce³⁺, and the mixture of Ce³⁺ and Hg²⁺ to DNA generated an obviously hypochromic effect. Meanwhile, the peak of DNA at 205.2 nm blue-shifted and at 258.2 nm red-shifted. The size of the hypochromic effect and the peak shift of DNA by metal ion treatments was Hg²⁺>Hg²⁺+Ce³⁺>Ce³⁺. The fluorescence emission spectra showed that with the addition of Hg²⁺, Ce³⁺, and the mixture of Ce³⁺ and Hg²⁺ the emission peak at about 416.2 nm of DNA did not obviously change, but the intensity reduced gradually and the sequence was Hg²⁺>Hg²⁺+Ce²⁺>Ce³⁺. Hg²⁺, Ce³⁺, and the mixture of Ce³⁺ and Hg²⁺ had 1.12, 0.19, and 0.41 binding sites to DNA, respectively; the fluorescence quenching of DNA caused by the metal ions all attributed to static quenching. The binding constants (K _A ) of binding siees were 8.98×10⁴ L/mol and 1.02×10⁴ L/mol, 5.12×10⁴ L/mol and 1.10×10³ L/mol, 6.66×10⁴ L/mol and 2.36×10³ L/mol, respectively. The results showed that Ce³⁺ could relieve the destruction of Hg²⁺ on the DNA structure.     

A new fluorescent chemosensor for Hg2+ in aqueous solution

We prepared an aminothiourea‐derived Schiff base (DA) as a fluorescent chemosensor for Hg²⁺ ions. Addition of 1 equiv of Hg²⁺ ions to the aqueous solution of DA gave rise to an obvious fluorescence enhancement and the subsequent addition of more Hg²⁺ induced gradual fluorescence quenching. Other competing ions, including Pb²⁺, Cd²⁺, Cr³⁺, Zn²⁺, Fe²⁺, Co³⁺, Ni²⁺, Ca²⁺, Mg²⁺, K⁺ and Na⁺, did not induce any distinct fluorescence changes, indicating that DA can selectively detect Hg²⁺ ions in aqueous solution. Copyright © 2012 John Wiley & Sons, Ltd.     

Activation of K+ channels by lanthanum contributes to the block of transmitter release in chick and rat sympathetic neurons

Summary We studied the effects of lanthanum (La³⁺) on the release of ³H-norepinephrine(³H-NE), intracellular Ca²⁺ concentration, and voltage clamped Ca²⁺ and K⁺ currents in cultured sympathetic neurons. La³⁺ (0.1 to 10 m) produced concentration-dependent inhibition of depolarization induced Ca²⁺ influx and ³H-NE release. La³⁺ was more potent and more efficacious in blocking ³H-NE release than the Ca²⁺-channel blockers cadmium and verapamil, which never blocked more than 70% of the release. At 3 m, La³⁺ produced a complete block of the electrically stimulated rise in intracellular free Ca²⁺ ([Ca²⁺] _i ) in the cell body and the growth cone. The stimulation-evoked release of ³H-NE was also completely blocked by 3 m La³⁺. However, 3 m La³⁺ produced only a partial block of voltage clamped Ca²⁺ current (I _Ca). Following La³⁺ (10 m) treatment ³H-NE release could be evoked by high K⁺ stimulation of neurons which were refractory to electrical stimulation. La³⁺ (1 m) increased the hyperpolarization activated, 4-aminopyridine (4-AP) sensitive, transient K⁺ current (I _A ) with little effect on the late outward current elicited from depolarized holding potentials. We conclude that the effective block of electrically stimulated ³H-NE release is a result of the unique ability of La³⁺ to activate a stabilizing, outward K⁺ current at the same concentration that it blocks inward Ca²⁺ current.